Hydrodeoxygenation of Lignin-Derived Phenols - MDPI

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Hydrodeoxygenation of Lignin-Derived Phenols: From Fundamental Studies towards Industrial Applications Päivi Mäki-Arvela and Dmitry Yu. Murzin * Johan Gadolin Process Chemistry Centre, Åbo Akademi University, 20500 Turku/Åbo, Finland; [email protected] * Correspondence: [email protected] Received: 7 August 2017; Accepted: 22 August 2017; Published: 7 September 2017

Abstract: Hydrodeoxygenation (HDO) of bio-oils, lignin and their model compounds is summarized in this review. The main emphasis is put on elucidating the reaction network, catalyst stability and time-on-stream behavior, in order to better understand the prerequisite for industrial utilization of biomass in HDO to produce fuels and chemicals. The results have shown that more oxygenated feedstock, selection of temperature and pressure as well as presence of certain catalyst poisons or co-feed have a prominent role in the HDO of real biomass. Theoretical considerations, such as density function theory (DFT) calculations, were also considered, giving scientific background for the further development of HDO of real biomass. Keywords: hydrodeoxygenation; bio-oil; guaiacol; lignin; heterogeneous catalyst; fuel

1. Introduction Lignocellulosic biomass, which is abundant, and does not compete with the food chain, has mainly been used for production of pulp and paper via the Kraft process, in which lignin and hemicellulose have been conventionally burnt to generate energy. New transformation processes should be developed in order to utilize the above-mentioned fractions more efficiently. The research in biomass transformation to produce bio-oil via flash pyrolysis [1] and utilize different lignin fractions to produce chemicals and fuels [2,3] has been very intensive in the recent years in part due to the depleting fossil based feedstock and strive towards carbon neutral fuels and chemicals. Further transformation of these fractions, can, however, be demanding, since bio-oil is unstable, exhibiting low pH, whereas lignin is a complex polyphenolic polymer difficult to transform selectively. One of the most important research area in transformation of bio-oil [4] and lignin [2,3] is catalytic hydrodeoxygenation (HDO) during which fuels or suitable blending agents are produced including benzene, toluene as well as cycloalkane and–ketones. Several reviews have already been published on HDO of lignin based phenols [5–15]. In these works, the main emphasis has been on different catalysts [5–8,11], deactivation [6,8,9], mechanism [6,8,10], kinetics [6]. Some specific topics, such as the use of carbide catalysts [12] and production of jet fuel from lignin [7] have been recently reviewed. Bio-oil HDO including the patent literature was reviewed by Zacher et al. in 2014 [13]. The recent trends in HDO of bio-oils have been summarized since this research area is highly important and very many new studies are constantly appearing [16–64]. Although HDO of phenolic compounds has already been investigated during several decades [65,66] and was shown to be a promising method to upgrade bio-oil, it is still challenging. The main challenges are catalyst deactivation, polymerization of phenolic compounds at elevated temperatures and the use of real bio-oils which contain various compounds and catalyst poisons.

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In this review, the main emphasis is in depth elucidation of the reaction network in hydrodeoxygenation of phenolic model compounds, especially focusing on which types of model molecules have been used for HDO and comparison of their reactivity. The experimental data will be correlated with the theoretical results using density functional theory and bond dissociation energies for different functional groups. In addition, to bring the HDO process closer to industry, several relevant aspects, such as catalyst deactivation, recycling and reuse have to be properly addressed. Typically, in continuous HDO of phenolic compounds deactivation of catalyst is significant, therefore relevant literature has been described. It is also of utmost importance for industrial processes to know exactly the mass balances and in particular formation of various undesired side products including gaseous ones and coke. The aim in this review is to report the main trends in the HDO of lignin derived model molecules, bio-oil and lignin per se. Special attention was put on the effect of temperature, pressure, metal and support selection. In addition, a reaction network during the HDO of model molecules will be discussed and reactivity of different model molecules in a single feed studies over the same catalyst will be compared. Continuous operation, catalyst reuse and deactivation will also be discussed in depth in order to relate the current state-of-art more closely to industrial applicability. 2. Bio-Oil and Lignin Properties and Need for Upgrading 2.1. Bio-Oil Properties Bio-oil is a product, which is most commonly produced via fast pyrolysis, in which the biomass is treated in the temperature range of 300–600 ◦ C and short residence time from 1 s to 2 s [15]. In this process biomass is deconstructed giving char as well as liquid, gases. The liquid fraction called bio-oil contains typically a large amount of water, varying in the range of 10–30 wt %, and different chemicals including aldehydes, acids, carbohydrates, phenolics, ketones, alcohols, etc. [15,67]. The composition of bio-oil varies significantly depending on the raw material. Bio-oils being typically non-stable, have a high viscosity, acidity (pH = 2.8–3.8) and low energy content or heating value due to high oxygen content [47,64]. Typically, bio-oils can contain up to 40 wt % oxygen [15], requiring upgrading during which different reactions occur, such as cracking, decarbonylation decarboxylation, hydrogenation, hydrocracking and hydrodeoxygenation. The main aim in hydrodeoxygenation is to reduce the oxygen content in bio-oil, especially present in aromatic hydroxyl and methoxy containing molecules. The HDO reactions of these molecules (e.g., guaiacol), are, however, challenging due to their low stability which causes catalyst deactivation [48]. 2.2. Structure of Different Lignin Types Properties of Organosolv, Kraft and Soda Lignin Organosolv lignin can be prepared under mild extraction conditions using different solvents, such as an ethanol/water mixture at 200 ◦ C [68]. Sulfur-free organosolv lignin suitable for catalytic transformation [69] has a lower average molecular weight compared to other lignin [1]. The organosolv lignin utilized by Kasakov et al. exhibited seven interconnected monolignols in its structure [2]. On the other hand, kraft and soda lignin is more condensed [1]. This can be explained by the preparation method involving alkaline reactions, including degradation of aryl-ethyl bonds and condensation because of carbanions addition to quinine methide during precipitation [1]. 3. HDO of Lignin-Derived Model Compounds, Simulated and Real Bio-Oil and Lignin In the HDO of bio-oil the main target is to decrease the oxygen content of the product and to improve the product heating value. It has been reported that the heating value of, for example, wheat based bio-oil is 21.2 MJ/kg on the dry basis [35] while the corresponding value for yellow poplar based bio-oil is 17.3 MJ/kg dry basis [4]. In the latter work the heating value of the processed

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maximally to 22.8 MJ/kg over Ni/SBA-15 catalyst during HDO at 300 ◦ C 3 of 42  under 30 bar hydrogen being 24% better than in the feed. Furthermore, van Krevelan plot showing the O/C molar ratio of different products gives an idea of how HDO decreases oxygen content during  H/O and O/C molar ratio of different products gives an idea of how HDO decreases oxygen content the process (Figure 1). For example, when the light oil fraction in wheat based pyrolysis oil containing  during the process (Figure 1). For example, when the light oil fraction in wheat based pyrolysis ◦ Chydrogen,  57%  water  was 57% hydrodeoxygenated  with  different  catalysts  at  250 catalysts °C  under  bar  oil containing water was hydrodeoxygenated with different at80  250 under 80 its  bar molar  O/C  ratio  decreased  to  0.22  using  Ni/ZrO   as  a  catalyst.  study  with  bio‐oil  hydrogen, its molar O/C ratio decreased to 0.222using Ni/ZrO2In  asanother  a catalyst. In another studyfrom  with yellow  the  O/C  ratio  decreased  0.8  to from 0.2  using  Pt/C  as  a Pt/C catalyst  When  bio-oil poplar  from yellow poplar thewas  O/C ratio wasfrom  decreased 0.8 to 0.2 using as a [4].  catalyst [4]. considering  the  fuel  properties  after  HDO,  it  should  be  kept  in  mind  that  even  cyclohexanol,  When considering the fuel properties after HDO, it should be kept in mind that even cyclohexanol, cyclohexanone and cyclohexane are valuable products which can be blended with petroleum [70].  cyclohexanone and cyclohexane are valuable products which can be blended with petroleum [70]. Furthermore,  Furthermore,for  forexample,  example,cyclohexane,  cyclohexane,toluene  tolueneand  andxylene  xyleneexhibit  exhibitsuitable  suitablevapor  vaporpressures  pressuresand  and carbon number to be used in gasoline [62].  carbon number to be used in gasoline [62]. bio-oil was increased Catalysts 2017, 7, 265   

  Figure1.  1.Van  VanKrevelan  Krevelanplot of  plot ofthe  thelight  lightphase  phaseof  ofpyrolysis  pyrolysisoil  oilprepared from  prepared fromwheat  wheatstraw  strawand  andits  its Figure  upgraded products using different catalysts at 250 ◦ C under 80 bar hydrogen [35]. upgraded products using different catalysts at 250 °C under 80 bar hydrogen [35]. 

In the recent review [7] results from HDO of model molecules, simulated and real bio‐oils and  In the recent review [7] results from HDO of model molecules, simulated and real bio-oils and lignin were summarized. Model molecules have been selected among the three phenols derived from  lignin were summarized. Model molecules have been selected among the three phenols derived from lignin,  p‐coumaryl alcohol, alcohol,  coniferyl  alcohol  sinapyl  alcohol  2).  The  main  lignin, namely  namely p-coumaryl coniferyl alcohol and and  sinapyl alcohol (Figure(Figure  2). The main emphasis emphasis  has on been  put  on  of  different  the  proposed  reaction  pathways  and  has been put reactivity ofreactivity  different molecules, themolecules,  proposed reaction pathways and thermodynamics thermodynamics of different reactions.  of different reactions.

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Catalysts 2017, 7, 265   

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HO  Catalysts 2017, 7, 265 

HO

HO

HO

HO

HO

CH3H3C

O OH

OH

CH3H3C

O

4 of 42 

O

CH3

O OH

O

CH3

O

Figure 2. The main phenols from lignin adapted from reference [7]. 

OH from lignin adapted fromOH OHThe main phenols Figure 2. reference [7].

   

Figure 2. The main phenols from lignin adapted from reference [7].  3.1. Conversion and Main Products in the HDO of Lignin‐Derived Model Molecules 

3.1. Conversion and Main Products in the HDO of Lignin-Derived Model Molecules

Conversion values and the main products in HDO of lignin derived model molecules are shown  3.1. Conversion and Main Products in the HDO of Lignin‐Derived Model Molecules 

HDO (%) Y HDOY(%)

Conversion values and the main HDOis of lignin derived model molecules are shown in  Tables  1  and  2  and  Figure  3.  For products guaiacol, in which  one  OF  the  most  studied  model  molecule,  Conversion values and the main products in HDO of lignin derived model molecules are shown  in Tables 1 andboth  2 and Figureand  3. hydroxyl  For guaiacol, which is one to  OFobtain  the most studied modelunder  molecule, containing  methoxy  group,  it  is  possible  complete  conversion  in  Tables  1  and  2  and  Figure  3.  For  which  is  one  OF  most complete studied  model  molecule,  either  high  temperatures  [50]  or  at guaiacol,  slightly  under  high  hydrogen  pressure    containing both methoxy and hydroxyl group,lower  it is temperatures  possible tothe  obtain conversion under containing  both  methoxy  and  hydroxyl  group,  it  is  possible  to  obtain  complete  conversion  under  (Table 1,  entry 1)  [46].  However, in  most  of  the  cases  lower  conversion  levels have  been  obtained  either high temperatures [50] or at slightly lower temperatures under high hydrogen pressure (Table 1, either  high  temperatures  [50]  or  at  slightly  lower  temperatures  under  high  hydrogen  pressure    entry[26,33,38,58]. Complete deoxygenation for guaiacol is also difficult, since these yields vary between  1) [46]. However, in most of the cases lower conversion levels have been obtained [26,33,38,58]. (Table 1,  entry 1)  [46].  However, in  most  of  the  cases  lower  conversion  levels have  been  obtained  10–75%, except for a high HDO yield with cyclohexane as the main product obtained over Ni/SiO 2‐ Complete deoxygenation for guaiacol is also difficult, since these yields vary between 10–75%, [26,33,38,58]. Complete deoxygenation for guaiacol is also difficult, since these yields vary between  ZrO 2 at 300 °C under 50 bar (Figure 3). When, however, the methoxy group is in position 3, HDO is  except for a high HDO yield with cyclohexane as the main product obtained over Ni/SiO22-ZrO 2 10–75%, except for a high HDO yield with cyclohexane as the main product obtained over Ni/SiO ‐ easier, most probably due to less steric hindrance (Table 1, entry 2). The reasons for this difficulty to  ◦ C under 50 bar (Figure 3). When, however, the methoxy group is in position 3, HDO is at 300 ZrO2 at 300 °C under 50 bar (Figure 3). When, however, the methoxy group is in position 3, HDO is  deoxygenate guaiacol and other model molecules are discussed in Section 3.3.  easier, most probably due to less steric hindrance (Table 1, entry 2). The reasons for this difficulty to easier, most probably due to less steric hindrance (Table 1, entry 2). The reasons for this difficulty to  deoxygenate guaiacol and other model molecules are discussed in Section 3.3.  deoxygenate guaiacol and other model molecules are discussed in Section guaiacol3.3. 100

1.

100 80

1.

15. 11.

80 60

12. 11.

60 40

12.

15.6.

7.

6. 13. 4. 13. 16. 4.

20 0 0

2.

250

200

250

8.

18. 19. 20. 18. 19. 9. 20.

8.

9.

14. 3. 5.

3.

5. 10. 17.

200

2.

14. 7.

16.

40 20

phenol anisole guaiacol phenol anisole

10.

300

350

400

450

300

350

400

450

17. Temperature (°C) Temperature (°C)

 

Figure  3.  Hydrodeoxygenation  (HDO)  yield  in  phenol,  anisole  and  guaiacol  HDO  over  different    catalysts  as  a  function  of  temperature.  Notation:  For  phenol  1.  CoNx,  20  bar,  cyclohexane  [63],  2.  Figure 3. Hydrodeoxygenation (HDO) yield in phenol, anisole and guaiacol HDO over different Figure  3.  Hydrodeoxygenation  (HDO)  yield  in  phenol,  anisole  and  guaiacol  HDO  over  different  MoO3/ZrO2,  60  bar,  benzene  [39],  3.  MoO3/ZrO2,  60  bar,  benzene  [39],  4.  ReOx/CNFox,  50  bar,  catalysts as aas  function of of  temperature. Notation:For  For phenol 1. CoNx, 20cyclohexane  bar, cyclohexane [63], catalysts  a  function  temperature.  Notation:  phenol  1.  CoNx,  20  bar,  [63],  2.  cyclohexane [37], 5. MoO3, 1 bar, benzene [36], for anisole 6. ReOx/CNFox, 50 bar, cyclohexane [37],  2. MoO /ZrO , 60 bar, benzene [39], 3. MoO /ZrO , 60 bar, benzene [39], 4. ReOx/CNFox, 50 3 3/ZrO 22,  60  bar,  benzene  [39],  33.  3 2 bar,  benzene  [39],  4.  ReOx/CNFox,  50  bar,  bar, MoO MoO 3/ZrO 2,  60  3, 1 bar, benzene [36], 8. MoO /ZrO 2, 60 bar, benzene [39], 9. Pt‐Sn/CNF/Inconel, 1 bar, benzene  7. MoO cyclohexane [37], 5. MoO3 , 13, 1 bar, benzene [36], for anisole 6. ReOx/CNFox, 50 bar, cyclohexane [37],  bar, benzene [36], for anisole 6. ReOx/CNFox, 50 bar, cyclohexane [37], cyclohexane [37], 5. MoO [48], 10. Ni‐catalyst, 1.7 bar, benzene [50], for guaiacol: 11. Pt/Alumina silicate, 30 bar, cyclohexane  7. MoO 13, 1 bar, benzene [36], 8. MoO bar, benzene [36], 8. MoO /ZrO [39], 9. S Pt-Sn/CNF/Inconel, 3/ZrO23, 60 bar, benzene [39], 9. Pt‐Sn/CNF/Inconel, 1 bar, benzene  7. MoO 3 , 12.  2 , 60 bar, 2,  100  bar,  main  product  cyclohexanol  S  = benzene 50%,  cyclohexane  =  25%  [47],  13.  [49],  Ni/ZrO [48], 10. Ni‐catalyst, 1.7 bar, benzene [50], for guaiacol: 11. Pt/Alumina silicate, 30 bar, cyclohexane  1 bar,Rh/ZrO benzene [48], 10. Ni-catalyst, 1.7 bar, benzene [50],3, for guaiacol: 11. Pt/Alumina 2+aluminosilicate,  40  bar,  cyclohexane  [45],  14.  MoO 1  bar,  cyclohexane,  [36],  15.  Ni/SiOsilicate, 2‐ 100  12. bar, Ni/ZrO main  product  cyclohexanol  S  =  50%,  cyclohexane  S  =  25%  [47],  13.  [49],  12.  Ni/ZrO2, [49], 30 bar, cyclohexane , 100 bar, main product cyclohexanol S = 50%, cyclohexane 2, 50 bar, cyclohexane [62], 16. CoMoS, 40 bar, benzene main product [46], 17. Mo 2 N, 50 bar, no  ZrO 2 40  bar,  cyclohexane  [45],  14.  MoO3,  1  bar,  cyclohexane,  [36],  15.  Ni/SiO2‐ Rh/ZrO HDO [58], 18. Fe/SiO S = 25% [47],2+aluminosilicate,  13. Rh/ZrO2, 1 bar, benzene and toluene [33], 19. Pt‐Sn/CNF/Inconel, 1 bar, toluene [48], 20.  2 +aluminosilicate, 40 bar, cyclohexane [45], 14. MoO3 , 1 bar, cyclohexane, [36], 2N, 50 bar, no  ZrO2, 50 bar, cyclohexane [62], 16. CoMoS, 40 bar, benzene main product [46], 17. Mo Ni‐catalyst, 1.7 bar, benzene and toluene [50]. Notation: Inside the area surrounded by the curve, the  15. Ni/SiO 2 -ZrO2 , 50 bar, cyclohexane [62], 16. CoMoS, 40 bar, benzene main product [46], 17. Mo2 N, HDO [58], 18. Fe/SiO2, 1 bar, benzene and toluene [33], 19. Pt‐Sn/CNF/Inconel, 1 bar, toluene [48], 20.  main products are cycloalkanes.  50 bar, no HDO [58], 18. Fe/SiO2 , 1 bar, benzene and toluene [33], 19. Pt-Sn/CNF/Inconel, 1 bar, Ni‐catalyst, 1.7 bar, benzene and toluene [50]. Notation: Inside the area surrounded by the curve, the  toluene [48], 20. Ni-catalyst, 1.7 bar, benzene and toluene [50]. Notation: Inside the area surrounded by main products are cycloalkanes. 

the curve, the main products are cycloalkanes.

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Catalysts 2017, 7, x FOR PEER REVIEW  Table 

Entry

Table 1. Single model molecule feed HDO results. Notation: Y denotes yield and S selectivity. X is conversion.  Catalyst T (◦ C) P (bar) Solvent Conversion % (Time, h) HDO Selectivity

Reactant

Entry 

1. Single model molecule feed HDO results. Notation: Y denotes yield and S selectivity. X is conversion.

Rh/MCM-36

Reactant 

Catalyst 

T (°C) 

Fe/SiO2 Rh/MCM‐36  Fe/SiO2 

Mo N

Mo2N 2

Mo Mo 2C/CNF  2 C/CNF Mo 2C/CNF  Mo C/CNF 2

OH

1 1 

H3C

40

P (bar) 

250  400

40 

400 



Mo2 N Mo2N 

250

300 300  300 

1 50

50 

300

300  300 350  350

50  40  55 

50 40 55

 

HDO Selectivity  Note  Reference  (Time, h)  Ybenzene+toluene = 71% Co-feed with H2 , CO2 , decane  X = 69%, 80 min  X = 92%29% hydrocarbons    [26]  after 21 min time-on-stream CO, H2 O Ybenzene+toluene = 71% after 21  ‐  X = 92%  Co‐feed with H 2, CO2, CO, H2O  [33]  X = 11%min time‐on‐stream  Yphenol = 8% Mainly phenol and decalin X = 11%  catechol 14,500 s Yphenol = 8%  Ycatechol = 2.5% decalin  Mainly phenol and catechol  [38]  14,500 s  Ycatechol = 2.5% 

decalin

decalin 

X = 40% 

X = 40% Yphenol = 22% 

Yheavy = 12%  dodecane X = 98% (4 h)  X = 98% (4 h)Sphenol = 48%  dodecane  Yphenol = 38%  dodecane  dodecane X = 100%  X = 100% Ycreo = 13%, 6 h  WHSV = 4 h−1  1 WHSV = 4 h− Xgua = 100%  DODGua = 62%  1‐octanol  1-octanolXoctanol = 30% Xgua = 100%after 80 h TOS  Xoctanol = 30% after 78 h TOS after 78 h TOS 

250  250

100  100

CoMoS  CoMoS

300  300

40 

40

‐ 

-

X = 100% 

X = 100%

Ni‐catalyst  Ni-catalyst

400  400

1.7  1.7

‐ 

-

100% 

100%

Ni‐SiO2‐ZrO2 

300 

Ni-SiO2 -ZrO2 Pt/C 

Pt/C

Rh/ZrO2 + SiAl  Pt/alumina  silicate 

Rh/ZrO2 + SiAl Ru/C 

50   

300 300 

8 h  X = 100 

 

50 1 

‐ 

X = 87% 

58– 300 310 

40 

250 

30 

58–310

400 

40 

1

40

Note

29% hydrocarbons

Solvent 

Ni/ZrO 2  Ni/ZrO 2

O

guaiacol guaiacol 

decane Conversion %  X = 69%, 80 min

5 of 42 

decane 

X = 100% 

hexadecane 

76 (1 h) 

‐ 

YHDO = 50%  Yphenol = 36%  Stoluene = 32%  Sbenzene = 30% 

Ycyclohexane = 97 

8h

Yphenol = 22% Yheavy = 12%

[58] 

Sphenol = 48%

 

Yphenol = 38% Ycreo = 13%, 6 h

Pt/alumina silicate

250

30

hexadecane

76 (1 h)

Ru/C

400

40

-

W/F = 0.067 h−1

[58] [56]

[54] 

[54]

YHDO = 50% H2S in the feed, fixed bed  [46]  H2 S in the feed, fixed bed Yphenol = 36%

[46]

Stoluene = 32%

 

[50] 

 

[62] 

[50]

Sbenzene = 30% Ycyclohexane = 97

 

Ycyclohexane = 64%

 

[62]

[31]

[49] 

Temperature programmed [16]  experiment

[45]

Ycyclohexan = 75%

[49]

Ybenzene = 69%

[16]

X = 34%

Catalysts 2017, 7, 265; doi:10.3390/catal7090265 

[38]

[47]

W/F = 0.30 g cat h−1 /ggua

Ybenzene = 69% 

[33]

DODGua =Trickle bed, poisons  62% Trickle bed,[47]  poisons after 80 h TOS

Ycyclohexan = 75% 

X = 34% 

[26]

[56] 

X = 100 SPhenol = 40%  Scyclopentanone = 18%  Most stable among Pt, Ru, Rh  [31]  SPhenol = 40% and Pd/C catalysts, MB = 70%  Scatechol = 5%  Most stable among Pt, Scyclopentanone = 18% W/F = 0.30 g cat h−1/ggua  Ru, Rh and Pd/C X = 87% Temperature programmed  Scatechol = 5% Ycyclohexane = 64%  [45] = 70% catalysts, MB experiment 

decane W/F = 0.067 h−1  X = 100%

Reference

www.mdpi.com/journal/catalysts 

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Table 1. Cont. Catalysts 2017, 7, 265    Catalysts 2017, 7, 265  Catalysts 2017, 7, 265    Entry Reactant Catalysts 2017, 7, 265   

T (◦ C)

Catalyst

P (bar)

Solvent

Conversion % (Time, h)

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HDO Selectivity

Note6 of 42 

Reference

OH OH OH OH 2 2 

2 2  2 

CH3 CH CH33 O CH O O 3     4-methoxyphenol O 4‐methoxyphenol    4‐methoxyphenol  4‐methoxyphenol  4‐methoxyphenol 

3  3 3 3  3 

25 wt %  25 wt % 25 wt %  25 wt %  3/ZrO2  MoO 25 wt %  MoO 3 /ZrO 2 33/ZrO 22   MoO /ZrO MoO MoO3/ZrO2  Mo2Mo C  2 C Mo Mo22C  C  Mo2C 

O O O H3C O HH33CC H3C  

    

anisole  anisole anisole  anisole  anisole 

CH3 CH CH33 CH3 O O O O

O O O CH 3 O CH CH 33 CH3



4 4 4  4 

CoNx  CoNx CoNx  CoNx  CoNx 

O O H3C O   HH33CC O    H3C 1,3,5‐trimethoxybenzene   

200  200 20  200  20  200  20  200  20 

20 dodecane  dodecaneX = 100% (2 h)  X = 100% Ycyclohexane = 95%  (2 h) Ycyclohexane =No sintering  95%

320  320 320  320  320 

1

1  1 1  1 

dodecane  dodecane  dodecane 

 

    

150  150 1.1  1.1 150  1.1  150  1.1  150  1.1 

‐  ‐ ‐  ‐ 

Ni catalyst Ni catalyst  Ni catalyst  Ni catalyst  Ni catalyst 

350  350 1.7  1.7 350  1.7  350  1.7  350  1.7 

 

Pt/H‐Beta  Pt/H-Beta Pt/H‐Beta  Pt/H‐Beta  Pt/H‐Beta 

400  400 400  400  400 

‐  ‐ ‐  ‐ 

1  1 1  1 

1

Rh/MCM‐36  250  40  Rh/MCM‐36  Rh/MCM‐36  250  250 40  40  Rh/MCM-36 250  Rh/MCM‐36  250  40 

40

-

    

-

X = 100% (2 h)  X = 100% (2 h)  X = 100% (2 h) 

Ycyclohexane = 95%  Ycyclohexane = 95%  Ycyclohexane = 95% 

No sintering  No sintering  No sintering 

S = 41%, aromatic   X = 41%  X S = 41%, aromatic compounds  =S = 41%, aromatic compounds  41% X = 41%     X = 41%  S = 41%, aromatic compounds  compounds X = 41%  S = 41%, aromatic compounds    Time‐on‐stream  Time‐on‐stream  Time‐on‐stream  Scyclohexane = 20%  Time-on-streamScyclohexane = 20%  60,000 s Scyclohexane =CO co‐feed  20% Scyclohexane = 20%  60,000 s  Time‐on‐stream  60,000 s  CO co‐feed  60,000 s  CO co‐feed  Sbenzene = 80%  X = 3.5% Sbenzene = 80% Scyclohexane = 20%  Sbenzene = 80%  Sbenzene = 80%  X = 3.5%  60,000 s  CO co‐feed  X = 3.5%  X = 3.5%  Sbenzene = 80%  Yphenol = 38% Yphenol = 38%  X = 3.5%  Yphenol = 38%  h WHSV = 0.54 h WHSV = 0.54Yphenol = 38%  WHSV = 0.54 h  WHSV = 0.54 h  Ybenzene = 22%   Ybenzene = 22%  Yphenol = 38%  Ybenzene = 22%     Ybenzene = 22%  X = 100% X = 100%  WHSV = 0.54 h  X = 100%  X = 100%  Ycresol = 27%  Ycresol = 27% Ybenzene = 22%    Ycresol = 27%  Ycresol = 27%  X = 100%  X = 98%,    Ycresol = 27%  X = 98%,Ybenzene = 48%  X = 98%,  X = 98%,    Ybenzene = 48%      Ybenzene = 48%  W/F = 0.2  X = 98%,  W/F = 0.2Ybenzene = 48%  W/F = 0.2  W/F = 0.2   Ybenzene = 48%    W/F = 0.2 

decane  X = 7%, 80 min  SHDO = 62%  decane  SHDO = 62%  decane  X = 7%, 80 min  SHDO = 62%  decane X = 7%, 80 min  X = 7%, 80 min decane  X = 7%, 80 min  SHDO = 62% 

SHDO = 62%

 

    

[63]  No sintering [63]  [63]  [63] 

[51]  [51]  [51]  [51]  [44]  CO co-feed [44]  [44] 

[63]

[51] [44]

[44]  [50]  [50]  [50]  [50] 

[50]

[24]  [24]  [24]  [24] 

[24]

[26]  [26]  [26]  [26] 

[26]

1,3,5-trimethoxybenzene 1,3,5‐trimethoxybenzene  1,3,5‐trimethoxybenzene  OH 1,3,5‐trimethoxybenzene  OH OH

4

4  4 4  4 

O H3C OO HH33CC O H3C

OH

O OO CH 3 CH O CH 33 CH3

 

    

syringol  syringol  syringol  Catalysts 2017, 7, 265    syringol 5  syringol  5 5  5 

CoNx  CoNx  CoNx  CoNx 

CoNx

CoNx  CoNx  CoNx  CoNx 

OH

200  200  200  200  200  200  200  200 

CoNx

5

MoO3/ZrO2,   

20  20  20  20 

200 350 

MoO3 /ZrO2 , phenol

200

20  20  20  20 

dodecane  dodecane  dodecane  dodecane 

X = 44% (2 h)  X = 44% (2 h)  X = 44% (2 h)  h) 20 dodecane X = 44% (2 h)  X = 44% (2Yguaiacol = 10%  Yguaiacol = 10%  Yguaiacol = 10%  Yguaiacol = 10%  dodecane  X = 100% (2 h)  Ycyclohexane = 91%  dodecane  X = 100% (2 h)  Ycyclohexane = 91%  dodecane  X = 100% (2 h)  Ycyclohexane = 91%  dodecane  X = 100% (2 h)  Ycyclohexane = 91% 

20 60 

350

Ycyclohexane = 9%  Ycyclohexane = 9%  Ycyclohexane = 9%  Ycyclohexane = 9%  Ycyclohexane = 9%  

dodecane n‐decane 

60

X = 100% (2 h)  

    

Yguaiacol = 10%

    

[63]  

[39] 

Ybenzene = 97

[39]

 

phenol 

OH Ycyclohexane = 52% 

HO 6 

CoNx    catechol 

OH

200 

20 

dodecane 

X = 100% (2 h) 

  Ycyclohexanol = 62% 

[63]

7 of 42  [63]  [63]  [63]  [63] 

Ycyclohexane = 91%

Ybenzene = 97 

n-decane

 

[63]  [63]  [63]  [63] 

[63] 

Catalysts 2017, 7, 265    Catalysts 2017, 7, 265    Catalysts 2017, 7, 265    Catalysts 2017, 7, 265

7 of 42  7 of 42  7 of 42 

OH OH OH

Entry

6 6  6  6 

Reactant  



T (◦ C)

60  60  60 

CoNx CoNx  CoNx  CoNx 

 

200  200 20  200  20  200  20 

   

CoNx  CoNx CoNx  CoNx 

     

Catalysts 2017, 7, 265 o‐cresol    o‐cresol  o-cresol o‐cresol 

Solvent

  1. Cont. Table    

Ybenzene = 97  Ybenzene = 97  Ybenzene = 97 

Conversion % (Time, h)

     

HDO Selectivity

Note

Ycyclohexane = 52%  Ycyclohexane = 52% Ycyclohexane = 52%  (2 h) 20 dodecane  dodecane X = 100% (2 h) X = 100% Ycyclohexane = 52%    dodecane  X = 100% (2 h)    dodecane  X = 100% (2 h)    Ycyclohexanol = 62%  Ycyclohexanol = 62% Ycyclohexanol = 62%  Ycyclohexanol = 62%  Ycyclohexane = 45%  Ycyclohexane = 45%  Ycyclohexane = 45% 

200  20  200  200 20  200  20 

[39]  [39]  [39] 

[63]  [63]  [63] 

Reference

[63]

Ycyclohexane = 45%

dodecane  X = 100% (2 h)    [63]  20 dodecane  dodecaneX = 100% (2 h)  X = 100% (2 h)   [63]  dodecane  X = 100% (2 h)    [63]  Ycyclohexanol = 43%  Ycyclohexanol = 43%  Ycyclohexanol = 43% Ycyclohexanol = 43%  CO co‐feedTemperature  150  1.2  ‐    toluene  [59]  Mo2C  CO co‐feedTemperature  programmed reaction  150  1.2  ‐    toluene  [59]  Mo2C  CO co-feedTemperature CO co‐feedTemperature  programmed reaction  8 h  Mo C 150 1.2 toluene 150  1.2  ‐    toluene  [59]  reaction Mo2C  2 programmed programmed reaction  Ni/SiO2‐ZrO2  300  50    Stoluene = 93%    [62]  8 h  X = 100%  Ni/SiO2‐ZrO2  300  50    Stoluene = 93%    [62]  8 h  8h X = 100%  Ni/SiO Ni/SiO 2‐ZrO 2    Stoluene = 93%  [62]  300 50  50 Stoluene = 93%   2 -ZrO 2 300  8 h  X = 100%  X = 100% Ni/SiO2‐ZrO2  300  50    Stoluene = 93%    [62]  8 h  X = 100%  8 h Stoluene = 93%  Ni/SiO2‐ZrO2  300  50      [62]  8 h  8 of 42  X = 100%  Ni/SiO 300 50  50 Stoluene = 93%   2 -ZrO 2 300  Ni/SiO 2‐ZrO 2    Stoluene = 93%  [62]  X = 100% X = 100% 

OH  OH  1,3‐dihydroxybenzene  OH  1,3-dihydroxybenzene 1,3‐dihydroxybenzene  OH 1,3‐dihydroxybenzene  OH CH3 OH CH3 CH3



n‐decane  n‐decane  n‐decane 

P (bar)

OH OH

HO HO HO

catechol  OH catechol  OH OH

8 8 

350  350  350 

Catalyst

phenol    phenol    OH phenol 

catechol catechol 

7  7 7  7 

MoO3/ZrO2,    MoO3/ZrO2,    MoO3/ZrO2,   

7 of 40

[63]

[59] [62]

[62]

OH

9 9 

H3C

CoNx  CoNx

200 

200 20 

CoNx 

200 

20 

20

dodecane  Yisopropylcyclohexane = 99.2%  dodecaneX = 100% (2 h) X = 100% (2 h) Yisopropylcyclohexane =  99.2%

[63] 

dodecane 

[63] 

CH3 

4-isopropylphenol 4‐isopropylphenol 

OH

10 

CH3 CH3

X = 100% (2 h) 

 

3‐isopropylphenol 

OH

Hydrogen  40 

1 h 

Yisopropylcyclohexane = 99.9% 

 

[63]

Catalysts 2017, 7, 265   

8 of 42 

OH Catalysts 2017, 7, 265

8 of 40



CoNx 

200 

20 

dodecane 

X = 100% (2 h) 

Yisopropylcyclohexane = 99.2% 

 

[63] 

Table 1. Cont. H3C Entry

CH3 

Reactant 4‐isopropylphenol 

T (◦ C)

Catalyst

P (bar)

Solvent

Conversion % (Time, h)

HDO Selectivity

Note

Reference

OH

1010 

CH3 CH3

CoNx  CoNx

200 

200 20 

dodecane  20

Pt/AC(N)  Pt/AC(N)

280 

40  Hydrogen 40 water  280 Loade at Loade at RT

X = 100% (2 h)  dodecane

Yisopropylcyclohexane = 99.9%  X = 100% (2 h)

Yisopropylcyclohexane =   99.9%

[63] 

[63]

[57] 

[57]

 

3-isopropylphenol 3‐isopropylphenol 

OH

Hydrogen 

1111 

1 h 

water X = 100% 

1h X = 100%

Ypropylcyclo‐hexane = 97%  Ypropylcyclo-hexane   = 97%

RT 

CH3  Catalysts 2017, 7, 265    4-propylphenol 4‐propylphenol  OH

12 

9 of 42 

CH3

Pt/AC(N) 

280 

O Pt/AC(N) Pd/C + HSM‐5 

12

CH3

 

4-propylguaiacol 4‐propylguaiacol 

OH

Hydrogen  40  water  Loade at  Hydrogen 40 RT 

water

280 240 

Pd/C + HSM-5

6 h  X = 100% 

50 

  Loade at RT

240

X = 86%, 4 h 

50

Ypropylcyclo‐hexane = 65  Ypropylcyclopentane = 5 

6h X = 100% Ypropenylcyclohexane = 75% 

X = 86%, 4 h

 

[57] 

Ypropylcyclo-hexane = 65 Ypropylcyclopentane = 5  

[57] [27] 

Ypropenylcyclohexane = 75%

[27]

CH3 O

13 

Pt/AC(N) 

O CH3

  4‐(2‐propenal)‐guaiacol 

CH3 O

OH

CH3 O

280 

Hydrogen  40  Loade at  RT 

water 

6 h  X = 100% 

Ypropylcyclo‐hexane = 65  Ypropylcyclo‐pentane = 6 

 

[57] 

OH OH

CH CH33 O O

Catalysts 2017, 7, 265

Pd/C + HSM‐5  Pd/C + HSM‐5 

240  240 

50  50 

  

X = 86%, 4 h  X = 86%, 4 h 

Ypropenylcyclohexane = 75%  Ypropenylcyclohexane = 75% 

  

9 of 40

[27]  [27] 

Table 1. Cont. CH CH33 Entry

Reactant 4‐propylguaiacol  4‐propylguaiacol 

OH OH

  

T (◦ C)

Catalyst

P (bar)

Solvent

Conversion % (Time, h)

HDO Selectivity

Note

Reference

CH CH33 O O

1313  13 

Pt/AC(N)  Pt/AC(N) Pt/AC(N) 

280  280 

280

Hydrogen  Hydrogen  40  40  Hydrogen 40 water  water  Loade at  Loade at RT Loade at  RT  RT 

water

Pt/AC(N)  Pt/AC(N)  Pt/AC(N)

280  280 

Hydrogen  Hydrogen  40  40  Hydrogen 40 water  water  280 Loade at  Loade at  Loade at RT RT  RT 

water

O O CH CH33

6 h  6 h  X = 100%  X = 100% 

Ypropylcyclo‐hexane = 65  6h Ypropylcyclo‐hexane = 65  Ypropylcyclo‐pentane = 6  X = 100% Ypropylcyclo‐pentane = 6 

Ypropylcyclo-hexane = 65    Ypropylcyclo-pentane = 6

[57]  [57] 

[57]

6 h  6 h  X = 97%  X = 97% 

Ypropylcyclo‐hexane = 58  6h Ypropylcyclo‐hexane = 58  Ypropylcyclopentane = 10  Ypropylcyclopentane = 10  X = 97%

Ypropylcyclo-hexane = 58    Ypropylcyclopentane = 10

[57]  [57] 

[57]

(1 h) X = 100%  (1 h) X = 100% 

Ypropylcyclo‐hexane = 17%  Ypropylcyclo‐hexane = 17% 

  

4-(2-propenal)-guaiacol 4‐(2‐propenal)‐guaiacol  4‐(2‐propenal)‐guaiacol 

CH CH33

OH OH

CH CH33

O O

O O

14  1414 

CH CH22

  

4-propenalsyringol 4‐propenalsyringol  4‐propenalsyringol  Catalysts 2017, 7, 265    15  15 

250  250  Pt/Silica  alumina 

OH O

30  30 

hexadecane  hexadecane 

Ydihydroeugenol = 75% 

Pt/Silica alumina

250

30

hexadecane

  

10 of 42  [49]  [49] 

Ypropylcyclo-hexane = 17%

(1 h) X = 100%

CH3

[49] Ydihydroeugenol = 75%

8 h 

15 Ni/SiO2‐ZrO2 

CH2

300 

Ni/SiO2 -ZrO2

50 

300

 

50

Spropylcyclohexane = 98%  8h

[62] 

Spropylcyclohexane = 98%

X = 100% 

 

 

[62]

X = 100%

eugenol eugenol 

O

16 

O OH

CH3

Ni/SiO2‐ZrO2 

300 

50 

dodecane 

(5)  Xvan = 83% 

Smethylcyclo‐hexane = 65%  Scyclohexane = 31 

  

[62] 

Pd/TiO2‐N‐C 

150 

10 

water 

X = 91% (2 h) 

Y 2‐methoxy‐4‐methylphenol =  78% 

 

[29] 

 

Vanillin   

 

Pd‐

Catalysts 2017, 7, 265   

10 of 42  Pt/Silica  alumina 

OH O

Ydihydroeugenol = 75% 

CH3 8 h 

Catalysts 2017, 7, 265 Ni/SiO22‐ZrO22 

300 

50 

10 of 40

 

Spropylcyclohexane = 98% 

 

[62] 

Table 1. Cont. X = 100% 

CH2 Reactant eugenol 

Entry

 

T (◦ C)

Catalyst

P (bar)

Solvent

Conversion % (Time, h)

HDO Selectivity

Note

Reference

O

16 16 

Ni/SiO Ni/SiO 22‐ZrO 22  2 -ZrO 2 300 

CH3

O OH

300 50 

50 dodecane  dodecane

150

10

(5) Smethylcyclo-hexane = 65% Smethylcyclo‐hexane = 65%     Xvan = 83% Scyclohexane = 31 Scyclohexane = 31 

(5)  Xvan = 83% 

[62] 

[62]

 

Vanillin Vanillin   

Pd/TiO -N-C

2 Pd/TiO22‐N‐C 

 

 

 

 

 

150 

Pd-carbonaceous Pd‐ microspheres

carbonaceous  Mo2 C/AC microspheres  Mo22C/AC 

10 

90 90 

100

100 

water

water 

5.5 5.5 

10

10 

Y 2‐methoxy‐4‐methylphenol =  X = 91% (2 h) Y 2-methoxy-4-methylphenol = 78%

X = 91% (2 h) 

78% 

water

X = 99% (3 h)

water 

X = 99% (3 h) 

water 

X = 100% (3 h) 

water

Sp‐creosol = 50% 

X = 100% (3 h)

Sp‐creosol = 60% 

 

[29] 

Sp-creosol = 50%

[60]

 

[60] 

 

[25] 

Sp-creosol = 60%

[29]

[25]

HO X ≥ 99% 

17 17 

O

ZnCl 22  ZnCl 2

CH3

 

Y1,4-dimethoxy-phenol Y1,4‐dimethoxy‐phenol = 60%    = 60%

MeOH MeOH 

 

8 h 

OH

X ≥ 99% [21] [21] 

8h

 

Catalysts 2017, 7, 265    alcohol Vanillyl Vanillyl alcohol 

11 of 42  CH3

8 h 

18

18 

O

Ni/SiO2‐ZrO2 

Ni/SiO2 -ZrO2

CH3

300 

300

50 

50

 

Spropylcyclohexane = 99%  Spropylcyclohexane  = 99%

 

19 

[62] 

X = 100% 

transanthole transanthole  O

8h

X = 100% CH3

O

Pt/Al2O3 

325 



 

X = 75%  W/F = 1 kg  cat.s/mol 

Sphenol = 87%  Sbenzene = 20% 

H2/O = 23 

[61] 

CoNx 

200 

20 

dodecane 

X = 100% 

Ycyclohexane = 92% 

 

[63] 

CoNx 

200 

20 

dodecane 

C = 100% 

Ycyclohexane = 99.9% 

 

[63] 

  phenylacetate 

O 20    Biphenyl ether  O

21 

HO

OH  

4,4‐dihydroxydiphenyl  ether  OH

H3C

O

[62]

Catalysts 2017, 7, 265  Catalysts 2017, 7, 265     Catalysts 2017, 7, 265 

11 of 42  11 of 42  11 of 42 

Catalysts 2017, 7, 265

18  18  18 

11 of 40

CH CH333 CH

8 h  8 h  8 h 

O O O

Ni/SiO Ni/SiO222‐ZrO 2‐ZrO ‐ZrO222 2   Ni/SiO

CH CH333 CH

Entry

   

Catalyst

2O33  O Pt/Al Pt/Al 2O Pt/Al 22O 3 3 2 Pt/Al 3

O O O

50  50  50 

   

Table 1. Cont.

T (◦ C)

325  325  325 325 

P (bar)

5  5  5 

5

Solvent

   

   

phenylacetate phenylacetate  phenylacetate  phenylacetate 

Spropylcyclohexane = 99%  Spropylcyclohexane = 99%  Spropylcyclohexane = 99% 

   

[62]  [62]  [62] 

X = 100%  X = 100%  X = 100% 

transanthole  Reactant transanthole  transanthole  O CH O CH333 O CH

19  19 19  19 

300  300  300 

Conversion % (Time, h) X = 75%  X = 75%  X = 75%  Sphenol = 87%  X = 75% Sphenol = 87%  Sphenol = 87%  W/F = 1 kg  W/F = 1 kg  W/F = 1 kg  Sbenzene = 20%  W/F = 1 kg cat.s/mol Sbenzene = 20%  Sbenzene = 20%  cat.s/mol  cat.s/mol  cat.s/mol 

HDO Selectivity

Sphenol = 87%H2/O = 23  H22/O = 23  2/O = 23  H Sbenzene = 20%

Note

Reference

[61]  H2 /O[61]  = 23 [61] 

[61]

O O O 20  20 20  20 

CoNx  CoNx CoNx  CoNx 

200  20  200  200 20  20  200 

dodecane  X = 100%  20 dodecane  dodecane X = 100%  dodecane  X = 100% 

Ycyclohexane = 92%  X = 100% Ycyclohexane = 92%  Ycyclohexane = 92%     Ycyclohexane = 92% 

[63]  [63]  [63] 

[63]

CoNx CoNx  CoNx  CoNx 

200  20  200  200 20  20  200 

20 dodecane  dodecane C = 100%  dodecane  C = 100%  dodecane  C = 100% 

C = 100% Ycyclohexane = 99.9%  Ycyclohexane = 99.9% Ycyclohexane = 99.9%    Ycyclohexane = 99.9% 

[63]  [63]  [63] 

[63]

Zn/Pd/C  Zn/Pd/C  Zn/Pd/C  Zn/Pd/C

150  21  150  150 21  21  150 

21

Y2‐methoxy‐4‐propylphenol =  Y2‐methoxy‐4‐propylphenol =  Y2‐methoxy‐4‐propylphenol =  Y2-methoxy-4-propylphenol 2h 85%  85%  85%      = 85% X = 100%Yguaiacol = 85%  Yguaiacol = 85%  Yguaiacol = 85%  Yguaiacol = 85%

[21]  [21]  [21] 

[21]

   

Biphenyl ether Biphenyl ether  Biphenyl ether  Biphenyl ether  O O O O

21 21  21  21 

H O O HO O HH

OH OH OH    OH

4,4-dihydroxydiphenyl 4,4‐dihydroxydiphenyl  4,4‐dihydroxydiphenyl  4,4‐dihydroxydiphenyl  ether ether  ether  ether  H C 3C H HH 3C O 33C O O O

OH OH OH OH

O O O O

22  22  22 22 

H O O H O HH O

OH OH OH OH O O O CH O CH 3 CH CH 333

   

   

dimer  dimer dimer  dimer 

   

MeOH  MeOH  MeOH  MeOH

2 h  2 h  2 h  X = 100%  X = 100%  X = 100% 

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Table 2. Comparative single feed HDO results using one catalyst. Notation: Yi, j is yield in which i is the reactant and j product, X denotes conversion and S selectivity. Comparison of the Reactivity

Catalyst

T (◦ C)

P (bar)

Phenol, anisole, guaiacol

MoO3 /ZrO2

350

Phenol, cresol, anisole, guaiacol, diphenyl ether

Guaiacol, anisole, phenol, o-cresol

Anisole, phenylacetate

Anisole, guaiacol

MoO3

10ReOx /CNFox

Pt/Al2 O3

Pt-Sn/CNF/Inconel

320

300

325

400

Conversion % (Time, h)

Products

Notation

Reference

60

Xphe = 48% Xanisole = 55% Xgua = 51%

Yphe, benzene = 46% Yani, benzene = 40% Ygua, phe = 23%

Trickle bed

[39]

1

-

Xphe = 29% Xani = 79% Xgua = 74% Xcre = 49% Xdip = 83%

Yphe, benzene = 44% Yani, benzene = 31% Ygua, phenol = 31% Ycre, toluene = 49% Ydip, benzene = 72%

With phenol the solvent is mesitylene

[36]

dodecane

(4 h) Xphe = 90% Xanisole = 100% Xgua = 100% Xcre = 44%

Yphe, cyclohexane = 51% Ygua, cyclohexane = 65% Yani, cyclohexane = 79% Ycre, methylcyclohexane = 18% Ycre, toluene = 16%

5

50 h time-on-stream Xani = 50% Xpheac = 58%

Spheac, benzylalcohol = 83% Spheac, benzene = 16% Sani, benzylalcohol = 55% Sani, benzene = 12

1

125 min TOS W/F = 2 g cat/g react h−1 Xgua = 97% Xani = 67%

Y gua, toluene = 63% Y ani, benzene = 38%

50

Solvent

[37]

H2 /O = 23.2

[61]

[48]

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For methoxylated benzenes, such as anisole, and trimethoxybenzene (Table 1, entries 3 and 4), For methoxylated benzenes, such as anisole, and trimethoxybenzene (Table 1, entries 3 and 4), it  For methoxylated benzenes, such as anisole, and trimethoxybenzene (Table 1, entries 3 and 4), it  it is easier to achieve HDO compared to guaiacol. For example, selectivity to anisole HDO products is easier to achieve HDO compared to guaiacol. For example, selectivity to anisole HDO products  is easier to achieve HDO compared to guaiacol. For example, selectivity to anisole HDO products  was 100% at relatively mild temperature of 150 ◦ C under atmospheric pressure over Mo2 C catalyst was 100% at relatively mild temperature of 150 °C under atmospheric pressure over Mo 2C catalyst  was 100% at relatively mild temperature of 150 °C under atmospheric pressure over Mo 2C catalyst  (Table 3, entry 3) [44]. (Table 3, entry 3) [44].  (Table 3, entry 3) [44]. 

Table 3. Homolytic bond dissociation energies forlignin  lignin derived model molecules calculated by Table  energies  for  Table 3. 3. Homolytic  Homolytic bond  bond dissociation  dissociation  for  lignin derived  derived model  model molecules  molecules calculated  calculated by  by  ◦ C energies  B3LYP/6–311 G(d,p) level theory at 320 in the gas phase adapted from reference [36] and comparison B3LYP/6–311  B3LYP/6–311 G(d,p)  G(d,p) level  level theory  theory at  at 320  320 °C  °C in  in the  the gas  gas phase  phase adapted  adapted from  from reference  reference [36]  [36] and  and  withcomparison with other bond dissociation energies from the literature. Bond dissociation energies are  other bond dissociation energies from the literature. Bond dissociation energies are given comparison with other bond dissociation energies from the literature. Bond dissociation energies are  given in kJ/mol.  in kJ/mol. given in kJ/mol.  Monolignol  Monolignol Monolignol  Bond  Bond Bond 

Guaiacol  Guaiacol Phenol M‐Cresol  M-Cresol Anisole  Anisole Guaiacol  Phenol  Phenol  M‐Cresol  Anisole    

Ph-OH Ph‐OH  Ph‐OH  Ph-OCH 33  Ph‐OCH Ph‐OCH Ph-O—CH33  Ph‐O—CH 3  Ph‐O—CH Ph-OPh 3  Ph‐OPh  Ph‐OPh  aliphatic C-O aliphatic C‐O  aliphatic C‐O 

472 472  472  384 384  384  218 218  -218  -‐ ‐  ‐ ‐ 

481 481  481  397 397  397  205 205  205  ‐  - ‐  ‐ ‐ 

469 469  469  ‐ ‐ ‐ ‐  ‐ ‐ ‐ ‐ 

469 469  469  ‐ ‐ ‐ ‐  ‐ ‐ ‐ ‐ 

‐ ‐ 384 384  384  238 238  238       

Diphenyl  Diphenyl  Diphenyl Ether  Ether  Ether O O

   ‐ -‐  ‐ -‐  ‐ ‐  280 280  280    

   ‐ ‐  ‐ ‐  ‐ ‐  ‐ ‐ [43] 339 339 [43]  339 [43] 

Phenol  HDO  giving  high  conversions  of  cyclohexane  at  Phenol HDO is is  also giving high and highyields  yields cyclohexane at moderate Phenol  HDO  is also  also  giving  high conversions conversions and  and high  high  yields  of of cyclohexane  at moderate  moderate  temperatures under relatively high hydrogen pressures (Table 1, entry 5) [63]. On the other hand,  temperatures under relatively high hydrogen pressures (Table 1, entry 5) [63]. On the other hand, temperatures under relatively high hydrogen pressures (Table 1, entry 5) [63]. On the other hand,  dihydroxylated catechol was more difficult to deoxygenate compared to phenol, since this molecule  dihydroxylated catechol was more difficult to deoxygenate compared to phenol, since this molecule dihydroxylated catechol was more difficult to deoxygenate compared to phenol, since this molecule  exhibited sterical restrictions and strong adsorption containing two hydroxyl groups in the vicinity  exhibited sterical restrictions and strong adsorption containing two hydroxyl groups in the vicinity  exhibited sterical restrictions and strong adsorption containing two hydroxyl groups in the vicinity of each other (see Section 3.5.5). It was also observed that when the other hydroxyl was in the meta  of each other (see Section 3.5.5). It was also observed that when the other hydroxyl was in the meta  of each other (see Section 3.5.5). It was also observed that when the other hydroxyl was in the meta position, degree of HDO was much higher (Table 1, entry 7). The products in the HDO of guaiacol,  position, degree of HDO was much higher (Table 1, entry 7). The products in the HDO of guaiacol,  position, degree of HDO was much higher (Table 1, entry 7). The products in the HDO of guaiacol, phenol  phenol and  and anisole  anisole are  are mainly  mainly aromatic  aromatic compounds,  compounds, such  such as  as benzene  benzene and  and toluene  toluene at  at high  high  phenol and anisolewhereas  are mainly aromatic compounds, suchbelow  as benzene andover  toluene at highexhibiting  temperatures, temperatures,  cyclohexane  can  be  formed  340  °C  catalysts  temperatures,  whereas  cyclohexane  can  be  ◦formed  below  340  °C  over  catalysts  exhibiting  whereas cyclohexane can be formed below 340 C over catalysts exhibiting hydrogenation activity, hydrogenation activity, such as supported Ni, Rh, ReOx, CoNx and MoO 3 [36,37,43,62,63].  hydrogenation activity, such as supported Ni, Rh, ReOx, CoNx and MoO 3 [36,37,43,62,63].  such as supported Ni, Rh, ReOx, CoNx and MoO3 [36,37,43,62,63]. Monooxygenated o‐cresol with a methyl group in ortho position is easy to deoxygenate (Table  Monooxygenated o‐cresol with a methyl group in ortho position is easy to deoxygenate (Table  1, entry 8) [62]. For molecules containing three oxygenated groups, such as vanillin, maximally only  Monooxygenated o-cresol with a methyl group in ortho position is easy to deoxygenate (Table 1, 1, entry 8) [62]. For molecules containing three oxygenated groups, such as vanillin, maximally only  2‐ZrO2  only entry65% selectivity towards methylcyclohexane was obtained at 300 °C under 50 bar over Ni/SiO 8) [62]. For molecules containing three oxygenated groups, such as vanillin, maximally 65% selectivity towards methylcyclohexane was obtained at 300 °C under 50 bar over Ni/SiO 2‐ZrO2  ◦ catalyst [62].  65% selectivity towards methylcyclohexane was obtained at 300 C under 50 bar over Ni/SiO2 -ZrO2 catalyst [62].  The propyl substituted phenols, including 4‐isopropyl‐, and 4‐propylphenol easily underwent  The propyl substituted phenols, including 4‐isopropyl‐, and 4‐propylphenol easily underwent  catalyst [62]. hydrogenation and deoxygenation (Table 1, entries 1 and 9–11), whereas 4‐propylguaiacol and 4‐(2‐ hydrogenation and deoxygenation (Table 1, entries 1 and 9–11), whereas 4‐propylguaiacol and 4‐(2‐ The propyl substituted phenols, including 4-isopropyl-, and 4-propylphenol easily underwent propenyl)‐guaiacol gave slightly lower HDO degree due to presence of three oxygenated substituents  propenyl)‐guaiacol gave slightly lower HDO degree due to presence of three oxygenated substituents  hydrogenation and deoxygenation (Table 1, entries 1 and 9–11), whereas 4-propylguaiacol and (Table 1, entries 12 and 13). Noteworthy is also that 4‐propensyringol was transformed not only to 4‐ (Table 1, entries 12 and 13). Noteworthy is also that 4‐propensyringol was transformed not only to 4‐ 4-(2-propenyl)-guaiacol gave slightly lower HDO degree due toin presence ofdeoxygenation  three oxygenated propylcyclohexane,  propylcyclohexane, but also  but also to 4‐propylcyclopentane  to 4‐propylcyclopentane indicating  indicating that  that  in addition  addition to  to  deoxygenation  substituents (Table 1, entries 12 and 13). Noteworthy is also that 4-propensyringol was transformed and ring hydrogenation in this case ring contraction occurred (Table 1, entry 14).  and ring hydrogenation in this case ring contraction occurred (Table 1, entry 14).  not onlyFor  to 4-propylcyclohexane, but also to°C  4-propylcyclopentane indicating that in addition to eugenol  bar  the  however,  For  eugenol HDO  HDO under  under 30  30  bar at  at 250  250  °C  the main  main product  product was,  was,  however, dihydroeugenol  dihydroeugenol  showing  difficulties  to  hydrogenate  the  phenyl  ring  at  these  conditions  (Table  1,  15)  deoxygenation and ring hydrogenation in this case ring contraction occurred (Table 1, entry 14). showing  difficulties  to  hydrogenate  the  phenyl  ring  at  these  conditions  (Table  1, entry  entry  15) [49],  [49],  ◦ whereas at elevated temperature and pressure the main product was 4‐propylcyclohexane [62]. The  For eugenol HDO under 30 bar at 250 C the main product was, however, dihydroeugenol whereas at elevated temperature and pressure the main product was 4‐propylcyclohexane [62]. The  same  was  HDO  which  maximally  gave  selectivity  towards  same difficulties was  valid  valid tofor  for  HDO  of  of  vanillin  vanillin  which  maximally  gave  65%  65%  (Table selectivity  towards  showing hydrogenate the phenyl ring at these conditions 1, entry 15) [49], methylcyclohexane at 300 °C under 50 bar over Ni/SiO 2‐ZrO2 catalyst (Table 1, entry 16). The main  methylcyclohexane at 300 °C under 50 bar over Ni/SiO 2‐ZrOproduct 2 catalyst (Table 1, entry 16). The main  whereas at elevated temperature and pressure the main was 4-propylcyclohexane [62]. product from  vanillyl alcohol HDO is  1,4‐dimethoxyphenol using  ZnCl2 as a catalyst in  methanol  product from  vanillyl alcohol HDO is  1,4‐dimethoxyphenol using  2 as a catalyst in  methanol  The same was valid for HDO of vanillin which maximallyZnCl gave 65% selectivity towards (Table 1, entry 17) [21]. Transanthole contains both propyl and methoxy substituents and it is easy to  (Table 1, entry 17) [21]. Transanthole contains both propyl and methoxy substituents and it is easy to  methylcyclohexane at 300 ◦ C under 50 bar over Ni/SiO2 -ZrO2 catalyst (Table 1, entry 16). The main deoxygenate (Table 1, entry 18). Phenylacetate formed in bio‐oil via a reaction between acetic acid  deoxygenate (Table 1, entry 18). Phenylacetate formed in bio‐oil via a reaction between acetic acid  product from vanillyl alcohol HDO is 1,4-dimethoxyphenol using ZnCl 2 as a catalyst in methanol and phenol underwent HDO at 325 °C under 5 bar hydrogen over Pt/Al 2O3 catalyst [61]. At elevated  and phenol underwent HDO at 325 °C under 5 bar hydrogen over Pt/Al 2O3 catalyst [61]. At elevated  (Table 1, entry 17) [21]. Transanthole contains both propyl and methoxy substituents and it is 1,  easy to temperature  due  to  thermodynamic  limitations  the  main  product  was  phenol  temperature  due  to  thermodynamic  limitations  the  main  product  was  phenol (Figure  (Figure 4, 4, Table  Table  1,     deoxygenate (Table 1, entry 18). Phenylacetate formed in bio-oil via a reaction between acetic acid entry 19) [61].  entry 19) [61].  ◦ and phenol underwent HDO at 325 C under 5 bar hydrogen over Pt/Al2 O3 catalyst [61]. At elevated temperature due to thermodynamic limitations the main product was phenol (Figure 4, Table 1, entry 19) [61].    Catalysts 2017, 7, 265; doi:10.3390/catal7090265 Catalysts 2017, 7, 265; doi:10.3390/catal7090265

www.mdpi.com/journal/catalysts  www.mdpi.com/journal/catalysts 

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Table 4. Comparison of the HDO using mixtures or bio-oil. Notation: X is conversion, Y yield and S selectivity. Comparison of the Reactivity

Catalyst

T (◦ C)

P (bar)

Solvent

Conversion % (Time, h)

HDO Selectivity

Reference

25 wt % Guaiacol, 25 wt % o-cresol, 50 wt % phenol mixture

NiFe/H-Beta

300

16

water

(4 h) Xphe = 50% Xcre = 68% Xgua = 88%

Ycycloalkane = 22% Yarom. HC = 29%

[40]

Bio-oil

Pt/C

350

78% HDO, reuse, Van Krevelan

[30]

Phenol, vanillin, cresol, eugenol, trans-anethole

Ni/SiO2 -ZrO2

300

50

dodecane

Xphe = 100% Xtransanethole = 100% Xeug = 100% Xo-cre = 100%

Scyclohexane = 98% Smethylcyclohexane = 93%e

[62]

Bio-oil

Ni/SiO2 -ZrO2

300

50

Dodecane

(8 h) 86% of identified compounds transformed to cyclic hydrocarbons

Yphenolic = 8 wt % Yarom. HC = 8 wt % Ycyclic alkanes = 55 wt %

[62]

Guaiacol, eugenol

Pt/

250

30

hexadecane

(1 h) Xgua = 88% Xeug = 100%

Ydihydroeugenol = 80% Ycyclohexane = 85%

[49]

m-cresol, anisole, 1,2-dimethoxybenzene, guaiacol

Supercritical EtOH

Sbenzene = 66% Mo2 C

280

1.14

-

Stoluene = 27%

[42]

Scyclohexane = 5% Yield 13 g/100 g oil in water and organic phase

[35]

ethanol

Y = 22 wt %

[30]

ethanol

Deoxygenation degree 54.9%

[4]

20

ethanol

Ylignin monomer = 51% HDO degree = 84%

[3]

320

hexadecane

Shydrocarbon = 70%

[2]

Ni/SiO2

250

80 at 25 ◦ C

Water 57%

Bio-oil

Pd/C

350

30

Bio-oil

Ni/SBA-15

300

30

Soda-lignin

CuMgAlOx + NiO/SiO2 containing Ni2 P

340

Organosolv lignin

Ni/H-BEA

20

Bio-oil

Xmixture = 94%

X = 100%, 6 h

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  Figure 4. Results from kinetic results of the gas-phase HDO of phenyl acetate over Pt/Al2 O3 catalyst Figure 4. Results from kinetic results of the gas‐phase HDO of phenyl acetate over Pt/Al2O3 catalyst  at 325 ◦ C under 5 bar with H2 /O ratio of 23 [61]. at 325 °C under 5 bar with H2/O ratio of 23 [61]. 

Dimeric asas  biphenyl ether andand  the corresponding 4,4-dihydroxydiphenyl ether Dimeric compounds, compounds, such such  biphenyl  ether  the  corresponding  4,4‐dihydroxydiphenyl  can be also easily deoxygenated to cycloalkanes (Table 1, entries 20 and 21), whereas a synthetic ether can be also easily deoxygenated to cycloalkanes (Table 1, entries 20 and 21), whereas a synthetic  β-O-4 lignin dimer, guaiacylglycerol-β-4-O-guaiacyl could only be transformed to 2-methoxyphenol ‐O‐4 lignin dimer, guaiacylglycerol‐ ‐4‐O‐guaiacyl could only be transformed to 2‐methoxyphenol  and guaiacol via  via breaking  breakingthe  theether  ether linkage in HDO  its HDO reaction performed with Zn/Pd/C and  guaiacol  linkage  in  its  reaction  performed  with  Zn/Pd/C  system  ◦ C in methanol [21]. The minor product was 3-(3-methoxysystem under 21 bar hydrogen at 150 under  21  bar  hydrogen  at  150  °C  in  methanol  [21].  The  minor  product  was  3‐(3‐methoxy‐4‐ 4-hydroxyphenyl)-1-propanol. Synergy between Pd and Zn promoted the bond cleavage and a further hydroxyphenyl)‐1‐propanol. Synergy between Pd and Zn promoted the bond cleavage and a further  reaction of the monomeric units on the catalyst surface with spillover hydrogen. reaction of the monomeric units on the catalyst surface with spillover hydrogen.  3.2. Comparative Studies of a Single Feed Reactant over the Same Catalysts 3.2. Comparative Studies of a Single Feed Reactant over the Same Catalysts    From HDO of different model compounds, comprising guaiacol, anisole, phenol and o-cresol From HDO of different model compounds, comprising guaiacol, anisole, phenol and o‐cresol  with a single reactant over the same catalyst it is possible to compare their relative reactivity, with a single reactant over the same catalyst it is possible to compare their relative reactivity, which  which is of high interest for industrial applications (Table 2) [36,37,39,48]. Results over ReOx/CNF is of high interest for industrial applications (Table 2) [36,37,39,48]. Results over ReOx/CNF catalyst  catalyst at elevated temperature, 300 ◦ C and pressure of 50 bar showed that conversion of four at elevated temperature, 300 °C and pressure of 50 bar showed that conversion of four different model  different model molecules decreased in the following order: guaiacol = anisole > phenol > cresol molecules decreased in the following order: guaiacol = anisole > phenol > cresol (Figure 5) [37]. In the  (Figure 5) [37]. In the work of Prasomsri et al. [36] the corresponding reactivity was the following: work  of Prasomsri  et al. [36]  the  corresponding reactivity  was  the  following: diphenyl > anisole >  diphenyl > anisole > guaiacol > phenol > o-cresol [36]. Thus, dehydroxylation of the phenyl ring is guaiacol > phenol > o‐cresol [36]. Thus, dehydroxylation of the phenyl ring is difficult [37,39,48,61].  difficult [37,39,48,61]. Reactivity of anisole higher than guaiacol reported in [39] similarly to the work Reactivity of anisole higher than guaiacol reported in [39] similarly to the work of Prasomsri et al.  of Prasomsri et al. [36], can be explained by a higher bond dissociation energy required for the aromatic [36],  can  be  explained  by  a  higher  bond  dissociation  energy  required  for  the  aromatic  hydroxyl  hydroxyl compared to methoxyl (Table 3). For o-cresol the preferred route is demethoxylation followed compared to methoxyl (Table 3). For o‐cresol the preferred route is demethoxylation followed by ring  by ring hydrogenation [37]. Only when HDO was performed at 400 ◦ C over Pt-Sn/CNF/Inconel hydrogenation  [37].  Only  when  HDO  was  performed  at  400  °C  over  Pt‐Sn/CNF/Inconel  nickel‐ nickel-(Inconel is chromium-based superalloy), guaiacol reactivity was higher than for anisole probably (Inconel is chromium‐based superalloy), guaiacol reactivity was higher than for anisole probably due  due to a very high temperature [48]. Biphenyl with an ether bond is easy to break, as experimentally to a very high temperature [48]. Biphenyl with an ether bond is easy to break, as experimentally and  and theoretically shown [36]. theoretically shown [36]. 

 

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Conversion/ Yield (mol-%)

100

80

60

40

20

0 0

20

40

60

80 100 120 140 160 180 200 220 240

Time (min)

 

(a)

Conversion/ Yield (mol-%)

100

80

60

40

20

0 0

20

40

60

80 100 120 140 160 180 200 220 240

Time (min) (b) Figure 5. Cont.

 

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100

80

60

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20

0 0

20

40

60

80 100 120 140 160 180 200 220 240

Time (min)

 

(c)

Conversion/ Yield (mol-%)

100

80

60

40

20

0 0

20

40

60

80 100 120 140 160 180 200 220 240

Time (min)

 

(d) Figure 5. Comparative HDO of different model compounds over 10 wt % ReOx/CNF in dodecane at  Figure 5. Comparative HDO of different model compounds over 10 wt % ReOx/CNF in dodecane at 300 °C under 5 MPa hydrogen adapted from reference [37], HDO of (a) guaiacol; (b) phenol; (c) anisole  300 ◦ C under 5 MPa hydrogen adapted from reference [37], HDO of (a) guaiacol; (b) phenol; (c) anisole and (d) o‐cresol. Notation: (■) conversion, (+) phenol, (x) benzene, (◊) toluene, (o) cyclohexane, (▲)  and (d) o-cresol. Notation: () conversion, (+) phenol, (x) benzene, (♦) toluene, (o) cyclohexane, methoxycyclohexane, () cyclohexanol, () cyclohexene, (●) lights.  (N) methoxycyclohexane, (∆) cyclohexanol, (∇) cyclohexene, ( ) lights.

In a mixture of guaiacol and eugenol a slightly higher conversion levels were obtained for HDO  In a mixture of guaiacol and eugenol a slightly higher conversion levels were obtained with the latter molecule at 250 °C under 30 MPa over Pt/C and Pt/aluminosilicate catalysts [49]. The  for HDO with the latter molecule at 250 ◦ C under 30 MPa over Pt/C and Pt/aluminosilicate main product with Pt/SA was 2‐methoxy‐4‐propylphenol, whereas 2‐methoxy‐4‐propylcyclohexanol  catalysts [49]. The main product with Pt/SA was 2-methoxy-4-propylphenol, whereas 2-methoxy-4was mainly generated over Pt/C [49]. It is also interesting that when dehydroxylation occurred over  propylcyclohexanol was mainly generated over Pt/C [49]. It is also interesting that when an acidic Pt supported on silica alumina, the reaction did not proceed to ring hydrogenation contrary  dehydroxylation occurred over an acidic Pt supported on silica alumina, the reaction did not proceed to Pt/C.  to ring hydrogenation contrary to Pt/C. Temperature  is  one  of  the  main  parameters  affecting  product  distribution  in  the  HDO  of  phenolic mixtures. This was visible, for example, in the HDO of mixtures at high temperature, above  350 °C, when aromatic compounds were the main products [39,48], whereas below this temperature 

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Temperature is one of the main parameters affecting product distribution in the HDO of phenolic mixtures. This was visible, for example, in the HDO of mixtures at high temperature, above 350 ◦ C, when aromatic compounds were the main products [39,48], whereas below this temperature non-aromatic cyclic hydrocarbons are also formed [36,37]. For example, in guaiacol HDO, phenol was the main product in accordance with a higher bond dissociation energy for Ar-OH compared to that of Ar-OCH3 [39] (Table 3). In case of o-cresol HDO the main products were methylcyclohexane and toluene requiring dehydration with a high bond dissociation energy for Ar–OH probably also explaining a low conversion of o-cresol. In the gas-phase HDO of a mixture of model compounds, m-cresol, anisole, dimethoxybenzene and guaiacol at 280 ◦ C under 114 kPa over Mo2 C catalyst, phenol was seen to decrease with an increase of conversion. Two other main products were benzene and toluene. These results indicated that aromatic –OCH3 bond was preferentially cleaved from guaiacol prior to the cleavage of aromatic –OH group [42]. These results were in accordance with the computational chemistry calculations of the bond dissociation energies being for Ar-OCH3 and Ar-OH in guaiacol equal to 376 kJ/mol and 456 kJ/mol [36]. 3.3. Reaction Pathways and the Thermodynamic Feasibility of Different Reactions during HDO of Model Compounds Since several different reactions occur during HDO of model compounds one of the main aims in using them is to elucidate the reaction pathways during HDO [32,37,48–50]. In addition to kinetic studies, also adsorption studies [48], theoretical DFT (density function theory) calculations [22,71–75] and bond dissociation energy calculations [36] have been performed. 3.3.1. Kinetic Studies Elucidating the Reaction Pathways during HDO of Model Compounds Guaiacol hydrodeoxygenation can proceed via either demethoxylation pathway forming in the first step phenol at 300 ◦ C under 50 bar hydrogen over ReOx/CNF (Figures 5 and 6) [37] and mainly phenol over Ni-catalysts at 350 ◦ C (Figure 7) [50] or alternatively via dehydroxylation pathway forming first phenol and subsequently cyclohexanol and finally cyclohexane at moderate temperature, when hydrogenation is thermodynamically feasible, e.g., over Pt/silica aluminate at 250 ◦ C under 30 bar hydrogen [49] and above 250 ◦ C over an Rh catalyst (Figure 8) [45]. Phenol was also the main product in guaiacol HDO at 400 ◦ C under atmospheric conditions over Co/Al-MCM-41 [34]. These results clearly show that both the metal type and reaction conditions, i.e., pressure and temperature, are decisive for the product distribution, for example, towards ring hydrogenation or HDO products. On acidic catalysts catechol is formed as the major product from guaiacol and anisole having low reactivity for further transformations [32]. Substantial amounts of catechol were also formed over Pt/C catalyst in HDO of guaiacol at 300 ◦ C under atmospheric pressure (Figure 9) [22]. Further reaction is very slow due to strong adsorption on the catalyst surface, in line with DFT calculations (see Section 3.3.2) [22]. It is also interesting to note that catechol formation was more prominent in HDO of guaiacol over Ru/C catalyst at 350 ◦ C than at 400 ◦ C, which was probably also related to temperature dependence of adsorption, namely low desorption of catechol at lower temperature [16]. According to a DFT study for Ru(0001) surface it was concluded that catecholate is formed prior to phenolate, finally yielding phenol as the main product [74].

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12

CH 3

+ CH4 ME CH3

OH CH 3

ME + CH4 ‐ H2 OH

CH 3 O

OH

DMO +H2

‐ CH3OH 1

CH3

+3 H2

+H2

HYD

‐ H2O 10

9

11

OH

+5 H2

+ H2 ‐ H2O

2 +H2

4

3

HYD

5 ‐ CH3OH DMO

+3 H2 +H2 ‐ H2O

DDO

7

+H2

CH 3

+H2

O

HYD

6

CH 3 O

+3 H2 8

 

Figure 6. Reaction scheme for guaiacol HDO at 300 °C under 50 bar hydrogen over ReOx/CNF catalyst  ◦ adapted  from  reference  [37].  Notation:  1.  guaiacol,  2,  phenol,  3.  cyclohexanol,  5.  cyclohexene,  5,  cyclohexane,  6.  methoxybenzene,  7.  benzene,  8.  methoxycyclohexanol,  9.  O‐cresol,  10.  toluene,  methylcyclohexne and 11. xylene. DMO denotes demethoxylation, DDO direct deoxygenation, ME  methanation and HYD hydrogenation. 

Figure 6. Reaction scheme for guaiacol HDO at 300 C under 50 bar hydrogen over ReOx/CNF catalyst adapted from reference [37]. Notation: 1. guaiacol, 2, phenol, 3. cyclohexanol, 5. cyclohexene, 5, cyclohexane, 6. methoxybenzene, 7. benzene, 8. methoxycyclohexanol, 9. O-cresol, 10. toluene, methylcyclohexne and 11. xylene. DMO denotes demethoxylation, DDO direct deoxygenation, ME methanation and HYD hydrogenation. Catalysts 2017, 7, x FOR PEER REVIEW   

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Figure 7. Proposed reaction network for guaiacol hydrodeoxygenation over Ni-based catalysts at 350 ◦ C [50]. Figure 7. Proposed reaction network for guaiacol hydrodeoxygenation over Ni‐based catalysts at 350 °C [50]. 

 

 

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Figure 8. Guaiacol HDO over bifunctional metal supported acidic catalysts [45].  Figure 8. Guaiacol HDO over bifunctional metal supported acidic catalysts [45]. Figure 8. Guaiacol HDO over bifunctional metal supported acidic catalysts [45]. 

  Figure 9. Kinetic for guaiacol HDO over Pt/C catalyst at 300 ◦ C under atmospheric pressure [22]. Figure 9. Kinetic for guaiacol HDO over Pt/C catalyst at 300 °C under atmospheric pressure [22]. 

 

In Figure 9. Kinetic for guaiacol HDO over Pt/C catalyst at 300 °C under atmospheric pressure [22].  case of anisole the reaction rate is very high due to demethoxylation occurring more easily In case of anisole the reaction rate is very high due to demethoxylation occurring more easily for  for anisole compared to guaiacol, most probably due to a strong adsorption of the hydroxyl group anisole  compared  to  guaiacol,  most  probably  due  to  a  strong  adsorption  of  the  hydroxyl  group  In case of anisole the reaction rate is very high due to demethoxylation occurring more easily for  lowering demethoxylation rate of guaiacol [37]. Phenol dehydroxylation is rather slow due to a high lowering demethoxylation rate of guaiacol [37]. Phenol dehydroxylation is rather slow due to a high  anisole  compared  to  guaiacol,  most  to  a  strong  adsorption  of  the  hydroxyl  group  bond dissociation energy required forprobably  phenolic due  hydroxyl group as already mentioned. In addition, bond dissociation energy required for phenolic hydroxyl group as already mentioned. In addition,  lowering demethoxylation rate of guaiacol [37]. Phenol dehydroxylation is rather slow due to a high  dehydroxylation rate is diminished for methyl substituted phenols due to steric hindrance. dehydroxylation rate is diminished for methyl substituted phenols due to steric hindrance.  bond dissociation energy required for phenolic hydroxyl group as already mentioned. In addition,  In eugenol HDO the following reaction network was proposed over Pd supported on Al2 O3 , In  eugenol  HDO  the  following  reaction  dehydroxylation rate is diminished for methyl substituted phenols due to steric hindrance.  ◦ network  was  proposed  over  Pd  supported  on  Al2O3,  SiO 2 , C and alumina silicate catalysts at 250 C under 30 bar hydrogen in hexadecane (Figure 10): SiO2In  , C and alumina silicate catalysts at 250 °C under 30 bar hydrogen in hexadecane (Figure 10):  eugenol forming HDO  the  following  reaction  network  was followed proposed by over  supported  on  Aland 2O3,  hydrogenation 2-methoxy-4-propylcyclohexanol itsPd  demethoxylation hydrogenation  forming  2‐methoxy‐4‐propylcyclohexanol  followed  by  its  demethoxylation  and  SiO2, C and alumina silicate catalysts at 250 °C under 30 bar hydrogen in hexadecane (Figure 10):  hydrolysis forming 2-hydroxy-4-propylcyclohexanol and methanol [49]. Over acidic sites dehydration hydrolysis  forming  2‐hydroxy‐4‐propylcyclohexanol  and  methanol  [49].  Over  acidic  sites  hydrogenation  forming  2‐methoxy‐4‐propylcyclohexanol  followed toby  demethoxylation  and  of hydroxypropylcyclohexanol can happen with further hydrogenation theits  corresponding saturated dehydration  of  hydroxypropylcyclohexanol  can  happen  with  further  hydrogenation  to  the  hydrolysis  forming  2‐hydroxy‐4‐propylcyclohexanol  and  methanol  [49].  Over  acidic  sites  compound. In addition, dealkylation can occur on the acidic sites causing cleavage of the propyl corresponding saturated compound. In addition, dealkylation can occur on the acidic sites causing  dehydration  of  hydroxypropylcyclohexanol  happen  with  further  hydrogenation  the  chain, finally forming cyclohexanol. These resultscan  show also difficulties of deoxygenation over to  Pd/C. cleavage  of  the  propyl  chain,  finally  forming  cyclohexanol.  These  results  show  also  difficulties  of  corresponding saturated compound. In addition, dealkylation can occur on the acidic sites causing  In addition, HDO of 2-methyl-4-propylphenol was tested using a mixture of Pd/C–H-ZSM-5 giving a deoxygenation over Pd/C. In addition, HDO of 2‐methyl‐4‐propylphenol was tested using a mixture  cleavage  of  the  propyl  chain,  finally  These [27] results  show  also  difficulties  of  relatively high conversion and yield forforming  mainly cyclohexanol.  propylcyclohexene showing that some acidity is of Pd/C–H‐ZSM‐5 giving a relatively high conversion and yield for mainly propylcyclohexene [27]  deoxygenation over Pd/C. In addition, HDO of 2‐methyl‐4‐propylphenol was tested using a mixture  needed for HDO. In eugenol HDO the best selectivity towards the desired product was achieved using showing  that  some  acidity  is  needed  for  HDO.  In  eugenol  HDO  the  best  selectivity  towards  the  of Pd/C–H‐ZSM‐5 giving a relatively high conversion and yield for mainly propylcyclohexene [27]  Pd/C and H-ZSM-5 (Figure 11). HDO activity increased with acidity increase. The kinetic diameter showing  that  some  acidity  is  needed  for  HDO.  In  eugenol  HDO  the  best  selectivity  towards  the   

 

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desired product was achieved using Pd/C and H‐ZSM‐5 (Figure 11). HDO activity increased with  acidity increase. The kinetic diameter of eugenol is, however, smaller than the pore size of H‐ZSM‐5.  of eugenol is, however, smaller than the pore size of H-ZSM-5. Alkali leached H-ZSM-5 exhibiting Alkali leached H‐ZSM‐5 exhibiting mesopores was used in eugenol HDO giving 60% selectivity to  mesopores was used in eugenol HDO giving 60% selectivity to hydrocarbons at 240 ◦ C under 50 bar hydrocarbons at 240 °C under 50 bar hydrogen [27]. It was stated by Zhang et al. [27] that the key  hydrogen [27]. It was stated by Zhang et al. [27] that the key factors inhibiting transformations of factors inhibiting transformations of polysubstituted phenols are sterical hindrance, for example, the  polysubstituted phenols are sterical hindrance, for example, the bulkiness of the alkyl substituent in bulkiness of the alkyl substituent in the aromatic ring.  the aromatic ring.

  Figure 10. Reaction scheme for eugenol HDO over various Pd catalysts supported on Al2 O3 , Figure  10. aluminosilicate Reaction  scheme  for  eugenol  over  at various  Pd incatalysts  supported  on  Al2O3,  SiO2,  SiO2 , carbon and under 30 bar HDO  hydrogen 250 ◦ C hexadecane as a solvent carbon and aluminosilicate under 30 bar hydrogen at 250 °C in hexadecane as a solvent adapted from  adapted from reference [49]. Notation: 1. Eugenol, 2. Isoeugenol, 3. 2-methoxy-4-propylphenol, reference  [49].  Notation:  1.  5. Eugenol,  2.  Isoeugenol,  3.  2‐methoxy‐4‐propylphenol,  4.  2‐methoxy‐4‐ 4. 2-methoxy-4-propylcyclohexanol, 3-propylmethoxycyclohexane, 6. 4-methyl-2-methoxyphenol, propylcyclohexanol,  5.  3‐propylmethoxycyclohexane,  6.  4‐methyl‐2‐methoxyphenol,  7.  4‐ 7. 4-propylphenol, 8. 4-propylcyclohexanol and 9. propylcyclohexane. propylphenol, 8. 4‐propylcyclohexanol and 9. propylcyclohexane. 

In vanillin HDO full deoxygenation was not achieved at 150 ◦ C under 10 bar hydrogen in In vanillin HDO full deoxygenation was not achieved at 150 °C under 10 bar hydrogen in water  water over mesoporous Pd/TiO2 -CNx giving the main product 2-methoxy-4-methylphenol with 78% over  mesoporous  Pd/TiO2‐CNx  giving  the  main  product  2‐methoxy‐4‐methylphenol  with  78%  selectivity at 91% conversion [29]. The reaction proceeded via hydrogenation of the aldehyde group selectivity at 91% conversion [29]. The reaction proceeded via hydrogenation of the aldehyde group  followed by dehydroxylation of the formed alcohol (Figure 12). The pressure applied was not sufficient followed  by  dehydroxylation  of  the  formed  alcohol  (Figure  12).  The  pressure  applied  was  not  to hydrogenate the phenyl ring while acidity was not enough to catalyze deoxygenation. sufficient to hydrogenate the phenyl ring while acidity was not enough to catalyze deoxygenation.  Cleavage of lignin β-O-4-link model molecules has been experimentally investigated over Cleavage  of  lignin  ‐O‐4‐link  model  molecules  has  been  experimentally  investigated  over  CuO/γ-Al2 O3 catalyst at 150 ◦ C under 25 bar hydrogen. The experimental results have been confirmed CuO/ ‐Al2O3 catalyst at 150 °C under 25 bar hydrogen. The experimental results have been confirmed  by calculating the bond dissociation energies of the corresponding compounds [53]. by calculating the bond dissociation energies of the corresponding compounds [53]. 

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Figure 11. Conversion and product selectivity in HDO of eugenol at 240 ◦ C under 50 bar hydrogen Figure 11. Conversion and product selectivity in HDO of eugenol at 240 °C under 50 bar hydrogen  after Figure 11. Conversion and product selectivity in HDO of eugenol at 240 °C under 50 bar hydrogen  4 h over Pd/C catalyst together with HZSM-5 with different acidity [27]. after 4 h over Pd/C catalyst together with HZSM‐5 with different acidity [27].  after 4 h over Pd/C catalyst together with HZSM‐5 with different acidity [27]. 

  Figure 12. Reaction scheme for HDO of vanillin under 10 bar hydrogen in water at 70–150 ◦ C over Figure 12. Reaction scheme for HDO of vanillin under 10 bar hydrogen in water at 70–150 °C over  Pd/TiO from reference [29]. Notation (a) vanillin; (b) 4-hydroxymethyl-2-methoxyphenol 2 adapted Pd/TiO 2  adapted  from  reference  [29].  Notation  (a)  vanillin;  (b)  4‐hydroxymethyl‐2‐methoxyphenol  Figure 12. Reaction scheme for HDO of vanillin under 10 bar hydrogen in water at 70–150 °C over  and (c) 2-methoxy-4-methylphenol. and (c) 2‐methoxy‐4‐methylphenol.  Pd/TiO2  adapted  from  reference  [29].  Notation  (a)  vanillin;  (b)  4‐hydroxymethyl‐2‐methoxyphenol  and (c) 2‐methoxy‐4‐methylphenol.  3.3.2. Different Reactions and Their Thermodynamic Feasibility in HDO of Model Compounds 

 

 

3.3.2. Different Reactions and Their Thermodynamic Feasibility in HDO of Model Compounds

Thermodynamic feasibility of different reactions occurring during HDO of lignin based model  3.3.2. Different Reactions and Their Thermodynamic Feasibility in HDO of Model Compounds  Thermodynamic feasibility of different reactions occurring during HDO of lignin based model molecules together with the specific catalyst properties are summarized below. During HDO several  molecules together with the specific catalyst properties are summarized below. During HDO several Thermodynamic feasibility of different reactions occurring during HDO of lignin based model  reactions, such as demethoxylation, hydrogenation, transalkylation, hydrogenolysis, demethylation,  reactions, such as demethoxylation, hydrogenation, transalkylation, hydrogenolysis, demethylation, ring opening, decarbonylation, oligomerisation can occur.  molecules together with the specific catalyst properties are summarized below. During HDO several  ring opening, decarbonylation, oligomerisation can occur. Demethoxylation is one of the main desired reactions for HDO occurring typically at relatively  reactions, such as demethoxylation, hydrogenation, transalkylation, hydrogenolysis, demethylation,  Demethoxylation is one the main desired reactions forHDO  HDO occurring typically of  at 1,3,5‐ relatively high  temperature  prior  to of dehydroxylation  during  guaiacol  [50].  Demethoxylation  ring opening, decarbonylation, oligomerisation can occur.  high trimethoxybenzene followed by ring hydrogenation occurred over Rh/MCM‐36 at 250 °C under 40  temperature prior to dehydroxylation during guaiacol HDO [50]. Demethoxylation of Demethoxylation is one of the main desired reactions for HDO occurring typically at relatively  bar hydrogen in decane [26]. It was concluded in [34] that demethoxylation was catalyzed by metal  1,3,5-trimethoxybenzene followed by ring hydrogenation occurred over Rh/MCM-36 at 250 ◦ C under high  temperature  prior  to  dehydroxylation  during  guaiacol  HDO  [50].  Demethoxylation  of  1,3,5‐ 40 barsites using Co‐ and Ni‐catalysts supported with Al‐MCM‐41 at 400 °C under atmospheric pressure  hydrogen in decane [26]. It was concluded in [34] that demethoxylation was catalyzed by metal trimethoxybenzene followed by ring hydrogenation occurred over Rh/MCM‐36 at 250 °C under 40  [34]. Cyclopentanone formation occurred during HDO of guaiacol over Pt/C catalyst at 300 °C under  sites using Co- and Ni-catalysts supported with Al-MCM-41 at 400 ◦ C under atmospheric pressure [34]. bar hydrogen in decane [26]. It was concluded in [34] that demethoxylation was catalyzed by metal  Cyclopentanone formation occurred during HDO of guaiacol over Pt/C catalyst at 300 ◦ C under sites using Co‐ and Ni‐catalysts supported with Al‐MCM‐41 at 400 °C under atmospheric pressure  [34]. Cyclopentanone formation occurred during HDO of guaiacol over Pt/C catalyst at 300 °C under 

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atmospheric pressure [31]. The proposed reaction network for guaiacol HDO proposed over Pt/C is  atmospheric pressure [31]. The proposed reaction network for guaiacol HDO proposed over Pt/C is shown in Figure 13a. Cyclopentanone was formed via hydroxycycloalkene intermediate and its ring  shown in Figure 13a. Cyclopentanone was formed via hydroxycycloalkene intermediate and its ring opening [41]. Slow demethoxylation of guaiacol observed experimentally was in agreement with DFT  opening [41]. Slow demethoxylation of guaiacol observed experimentally was in agreement with DFT calculations for HDO on Pt(111) surface (Figure 13b) [22] leading to formation of large amounts of  calculations for HDO on Pt(111) surface (Figure 13b) [22] leading to formation of large amounts of catechol. Formation of cyclopentanone was confirmed by DFT calculations made for guaiacol HDO  catechol. Formation of cyclopentanone was confirmed by DFT calculations made for guaiacol HDO over Pt(111). This product can be formed by partial hydrogenation of the aromatic ring in guaiacol  over Pt(111). This product can be formed by partial hydrogenation of the aromatic ring in guaiacol followed  by  formation  of  alpha  diketone  which  undergoes  decarbonylation  and  ring  closing  [22].  followed by formation of alpha diketone which undergoes decarbonylation and ring Analogous  results  were  reported  by  Lee  et  al.  [68]  who  performed  DFT  calculations  for closing Pt(111)  [22]. Analogous results were reported by Lee et al. [68] who performed DFT calculations for Pt(111) surface. surface. 

  (a) 

  (b)  Figure 13. (a) Reaction scheme for HDO of guaiacol over Pt/C catalyst in the temperature range of 

Figure 13. (a) Reaction scheme for HDO of guaiacol over Pt/C catalyst in the temperature range 275–325  °C  under  atmospheric  pressure  over  Pt/C  and  (b)  energy  levels  for  reaction  coordinate  of 275–325 ◦ C under atmospheric pressure over Pt/C and (b) energy levels for reaction coordinate calculated with DFT for Pt(111) surface [22].  calculated with DFT for Pt(111) surface [22].

Demethoxylation of dihydroeugenol resulting in formation of 4‐propylphenol requires acidity  (Figure 10) [49]. In temperature programmed HDO of guaiacol at 280 °C over Rh/ZrO 2 together with  Demethoxylation of dihydroeugenol resulting in formation of 4-propylphenol requires acidity ◦ aluminosilicate  under  40  bar  hydrogen  in  decane  complete  demethoxylation  of  guaiacol  was  (Figure 10) [49]. In temperature programmed HDO of guaiacol at 280 C over Rh/ZrO2 together with aluminosilicate under 40 bar hydrogen in decane complete demethoxylation of guaiacol was achieved at 280 ◦ C showing clearly its easiness for breaking this bond (Figure 14) (Table 3) [45]. It was also stated in [37] that ReOx/CNF is a promising catalyst favoring C-O bond cleavage and forming

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achieved at 280 °C showing clearly its easiness for breaking this bond (Figure 14) (Table 3) [45]. It was  24 of 40 also stated in [37] that ReOx/CNF is a promising catalyst favoring C‐O bond cleavage and forming  phenol at 300 °C under 50 bar hydrogen (Figures 5 and 6) without breaking C‐C bond being thus  hydrogen efficient and carbon atom economical catalyst. Demethoxylation was faster compared to  phenol at 300 ◦ C under 50 bar hydrogen (Figures 5 and 6) without breaking C-C bond being thus transalkylation  in  and continuous  HDO economical of  a  phenylacetate  anisole  mixture  at  325  °C compared under  5  bar  hydrogen efficient carbon atom catalyst.and  Demethoxylation was faster to ◦ hydrogen  over inPt/C  catalyst HDO [61]  of and  thus  it  was  proposed  anisole  is  converted  transalkylation continuous a phenylacetate and anisolethat  mixture at 325 C under 5more  bar preferentially to phenol than to m‐cresol.  hydrogen over Pt/C catalyst [61] and thus it was proposed that anisole is converted more preferentially to phenol than to m-cresol. Catalysts 2017, 7, 265

a)

b)

  Figure 14. (a) Temperature and pressure profiles during HDO of guaiacol; (b) yields of different Figure  14.  (a)  Temperature  and  pressure  profiles  during  HDO  of  guaiacol;  (b)  yields  of  different  products in HDO of guaiacol as a function of temperature, solvent decane [45]. products in HDO of guaiacol as a function of temperature, solvent decane [45]. 

Dehydroxylation ring is thermodynamically more demanding thandemanding  demethoxylation Dehydroxylation of the of  phenyl the  phenyl  ring  is  thermodynamically  more  than  (Table 3) [22,36]. Substantial amounts of catechol were formed from guaiacol prior to phenol formation demethoxylation (Table 3) [22,36]. Substantial amounts of catechol were formed from guaiacol prior  showing apparent difficulties of direct phenol formation from guaiacol at 300 ◦ C over Pt/C [22]. to phenol formation showing apparent difficulties of direct phenol formation from guaiacol at 300 °C  Typically demethoxylation of bulky 1,3,5-trimethoxybenzene was rather slow even over mesoporous over Pt/C [22]. Typically demethoxylation of bulky 1,3,5‐trimethoxybenzene was rather slow even  Rh/MCM-36 at 250 ◦ C under 40 bar hydrogen due to the sterical hindrance, despite the fact that it is over mesoporous Rh/MCM‐36 at 250 °C under 40 bar hydrogen due to the sterical hindrance, despite  easy to break the methoxy bond compared to the hydroxyl bond connected to the phenyl ring [26]. the fact that it is easy to break the methoxy bond compared to the hydroxyl bond connected to the  Substantial demethylation has been observed over Ni-, Fe-based catalysts [50] as well as over phenyl ring [26].  Mo2 S and CoMoS catalysts [46] in HDO of guaiacol at relatively high temperatures. Methanation was also observed in HDO of guaiacol over CoMo/ZrO2 catalyst at 300 ◦ C under 50 bar hydrogen [46] and Co/Al-MCM-41 at 400 ◦ C under 1 bar [34]. In the latter work it was concluded that demethylation

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occurs only on the acid sites especially with high catalyst weight to flow rate [34]. Methanation could however, be inhibited in guaiacol HDO via transalkylation increasing the carbon efficiency [34,50]. On the other hand, a very low methanation activity occurred in HDO of anisole at 320 ◦ C under 1 bar using MoO3 /ZrO2 catalyst [51]. It is also noteworthy that substantial methanation occurred in HDO of guaiacol and anisole over Pt-Sn/CNF [48]. In addition, methanation is especially promoted by Rh and Ru [76]. Methanation in anisole HDO increased with increasing temperature over Mo2 C [43]. Phenol and methanol were the primary products in anisole HDO over Mo2 C, whereas methanol deoxygenated to methane at higher temperatures [43]. It was thus concluded that Mo2 C is capable for breaking phenolic C-O bond in anisole at low temperature. Demethylation of guaiacol involves C(sp3)-Oar and is from the thermodynamic point of view the easiest reaction compared to, e.g., phenol, via C(sp2)-OMe cleavage [46]. Non-promoted MoS2 catalyst was beneficial for demethylation [46]. 2-methylphenol is formed over CoMoS/Al2 O3 via adsorption of the methoxy oxygen group followed by methylation of the phenyl ring. The deoxygenation of methoxyphenol is more difficult than that of phenol [77]. Ring hydrogenation is thermodynamically favored during HDO of phenolic compounds under high pressure using relatively low temperatures [27,37,45,47–49,62,63]. Such ring hydrogenation makes the overall process more expensive because of hydrogen consumption. Typically hydrogenation during HDO is favored over supported Ni catalysts [47,62], CoNx [63], Rh/ZrO2 with the mixture of aluminosilicates [43], Pt/alumina [49] and even with MoO3 [36]. If the target is to perform HDO to produce BTX products, lower hydrogen consumption can be achieved under high temperatures [16,34]. For example, no ring hydrogenation occurred in HDO of guaiacol and anisole at 400 ◦ C and 1 bar over Pt-Sn/CNF [48]. At the same temperature nickel and iron catalysts were also inefficient for hydrogenation [33,50]. Even when using 60 bar at 350 ◦ C in anisole and phenol HDO, the main product was benzene over MoO3 /ZrO2 catalyst showing clearly thermodynamic limitations at this temperature [39]. Ring hydrogenation in HDO of 4-propylphenol occurred with very high selectivity over Pt/AC at 280 ◦ C under 40 bar hydrogen giving propylcyclohexane as the main product [57]. It was also pointed out that under these conditions using water as a solvent, protons can be formed increasing acidity of the reaction media [57]. Slightly lower yields of propylcyclohexane were achieved in HDO of 4-propylguaiacol, since Pt/AC catalyst promoted also formation of propylcyclopentane [57]. In eugenol HDO, ring hydrogenation occurred after demethoxylation over supported Pd catalysts under 30 bar hydrogen at 250 ◦ C in hexadecane as a solvent [49]. It should, however, be noticed that ring hydrogenation could occur already prior to dehydroxylation [49]. Noteworthy is that 10 bar hydrogen was not sufficient to facilitate ring hydrogenation during vanillin HDO over Pd/CNF catalyst in the temperature range of 70–150 ◦ C in water [29]. Acidic catalysts are active in HDO facilitating often complete conversion of phenolic compounds, but not hydrogenation [25,55], for example, vanillin transformation to p-creosol [25]. Transalkylation occurring after demethylation is beneficial for carbon efficiency [50] and it is favored by acidic catalysts [32]. Several transalkylated C7–C12 products were formed in guaiacol HDO at 400 ◦ C under atmospheric pressure due to acidity of Co-Ni-Al-MCM-41 [34]. MoO3 /ZrO2 promoted also transalkylation in anisole HDO at 350 ◦ C under 60 bar hydrogen [39]. It was also proposed based on the product distribution that transalkylation occurred parallel with hydrogenolysis [39]. On the other hand, no transalkylation occurred with Mo2 C in HDO of a mixture of anisole, m-cresol, dimethoxybenzene and guaiacol at 280 ◦ C under 114 kPa [42]. These results suggest that temperature is one of the decisive factors determining the transalkylation activity. A low alkylation ability was, however, observed in HDO of guaiacol at 400 ◦ C over Pt-Sn/CNF catalyst exhibiting low acidity (Figure 15) [48].

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O

26 of 40 21 of 42 

CH 3

OH

+H2 ‐H2O

+ O

CH 3

O

CH3 CH 3

CH3

CH 3

+H2 ‐CH4

OH

+H2 ‐H2O

Figure 15. Reaction scheme for HDO and transalkylation of anisole at 400 ◦ C under 1 bar over Figure  15.  Reaction  scheme  for  HDO  and  transalkylation  of  anisole  at  400  °C  under  1  bar  over    Pt-Sn/CNF adapted from ref. [48]. Pt‐Sn/CNF adapted from ref. [48]. 

 

Oligomerisation [25] formation of gaseous products are also as evidenced from several Oligomerisation  [25] and and  formation  of  gaseous  products  are typical also  typical  as  evidenced  from  HDO studies regarding lignin derived phenolics. As a result mass balance is very often below 100%. several HDO studies regarding lignin derived phenolics. As a result mass balance is very often below  For example, in vanillin HDO, performed over Mo2 C/AC2catalyst under 20 bar hydrogen at 100 ◦ C 100%. For example, in vanillin HDO, performed over Mo C/AC catalyst under 20 bar hydrogen at  oligomerisation of vanillin occurred causing incomplete mass balance [25]. Analogously in guaiacol 100 °C oligomerisation of vanillin occurred causing incomplete mass balance [25]. Analogously in  HDO the mass balance was not complete when performing HDO over Mo2 C/CNF in the temperature guaiacol HDO the mass balance was not complete when performing HDO over Mo 2C/CNF in the  ◦ range of 300–375 C under 55 bar hydrogen. The highest mass balance closure of ca. 96% was obtained temperature range of 300–375 °C under 55 bar hydrogen. The highest mass balance closure of ca. 96%  at 375 ◦ C [54]. It was stated [35] that in order to avoid polymerisation, stirring of the bio-oil should be was obtained at 375 °C [54]. It was stated [35] that in order to avoid polymerisation, stirring of the  started during its heating. In bio-oil HDO CO2 was the main product due to the presence of carboxylic bio‐oil should be started during its heating. In bio‐oil HDO CO 2  was the main product due to the  acids in the feed [35]. When comparing gaseous composition after bio-oil HDO it could be seen that presence of carboxylic acids in the feed [35]. When comparing gaseous composition after bio‐oil HDO  Ru/C produced more methane from CO2 than supported Ni-catalysts, in line with 80% more hydrogen it could be seen that Ru/C produced more methane from CO 2 than supported Ni‐catalysts, in line  consumption [35]. with 80% more hydrogen consumption [35].  3.4. HDO of Simulated and Real Bio-Oils and Lignin 3.4. HDO of Simulated and Real Bio‐Oils and Lignin  3.4.1. HDO of Simulated Bio-Oils 3.4.1. HDO of Simulated Bio‐Oils  Mixtures of different phenolic compounds to simulate bio-oil have been used in HDO (Figure 16, Mixtures  of  different  phenolic  compounds  to  simulate  bio‐oil  have  been  used  in  HDO    Table 4) [40,62]. The main products in the HDO of simulated bio-oil containing phenol, o-cresol (Figure 16, Table 4) [40,62]. The main products in the HDO of simulated bio‐oil containing phenol, o‐ and guaiacol over Ni- and Ni-Fe/H-Beta catalysts at 300 ◦ C during 4 h under 16 bar hydrogen cresol and guaiacol over Ni‐ and Ni‐Fe/H‐Beta catalysts at 300 °C during 4 h under 16 bar hydrogen  were oxygenated aromatic compounds [40]. The lowest amount of oxygenates was produced over were oxygenated aromatic compounds [40]. The lowest amount of oxygenates was produced over  NiFe-H-Beta catalysts along with substantial amounts of cycloalkanes. It was also stated that water as NiFe‐H‐Beta catalysts along with substantial amounts of cycloalkanes. It was also stated that water  a solvent is beneficial for HDO, whereas methanol diminished selectivity towards hydrocarbons [40]. as a solvent is beneficial for HDO, whereas methanol diminished selectivity towards hydrocarbons  The proposed reaction network for HDO of this simulated bio-oil is shown in Figure 17. It contains [40].  The  proposed  reaction  network  for  HDO  of  this  simulated  bio‐oil  is  shown  in  Figure  17.  It  in addition to hydrogenation and deoxygenation also ring contraction of 1,2-cyclohexanediol to contains  in  addition  to  hydrogenation  and  deoxygenation  also  ring  contraction  of  1,2‐ cyclopentanol over acidic Ni-Fe/H-Beta catalyst. In HDO of a mixture of phenol, cresol, eugenol and cyclohexanediol to cyclopentanol over acidic Ni‐Fe/H‐Beta catalyst. In HDO of a mixture of phenol,  trans-anethole over Ni/SiO2 -ZrO2 at 300 ◦ C under 50 bar during 8 h it was observed that this catalyst cresol,  eugenol  and  trans‐anethole  over  Ni/SiO2‐ZrO2  at  300  °C  under  50  bar  during  8  h  it  was  was rather efficient towards hydrocarbons with 62% selectivity to cyclohexane at 91% conversion [62]. observed  that  this  catalyst  was  rather  efficient  towards  hydrocarbons  with  62%  selectivity  to  Gas-phase pyrolytic products contain also hydrogen, CO2 , CO and water vapours. Guaiacol HDO was cyclohexane at 91% conversion [62]. Gas‐phase pyrolytic products contain also hydrogen, CO 2, CO  investigated using a co-feed of the simulated pyrolysis vapors [33]. The effect of gaseous co-feed is and  water  vapours.  Guaiacol  HDO  was  investigated  using  a  co‐feed  of  the  simulated  pyrolysis  discussed, however, in Section 3.5.6. vapors [33]. The effect of gaseous co‐feed is discussed, however, in Section 3.5.6. 

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Figure 16. Conversion and selectivity to different products as a function of time on stream in HDO of  Figure 16. Conversion and selectivity to different products as a function of time on stream in HDO of Figure 16. Conversion and selectivity to different products as a function of time on stream in HDO of  C  a mixture of m‐cresol, anisole, dimethoxybenzene and guaiacol at 280 °C under 144 kPa over Mo a mixture of m-cresol, anisole, dimethoxybenzene and guaiacol at 280 ◦ C under 144 kPa over2C  Mo22C a mixture of m‐cresol, anisole, dimethoxybenzene and guaiacol at 280 °C under 144 kPa over Mo catalyst [42].  catalyst [42]. catalyst [42]. 

  Figure 17. Proposed reaction scheme for the HDO of simulated bio‐oil containing guaiacol, phenol  and  o‐cresol  as  feedstock  over  Ni‐H‐Beta  catalyst.  Notations:  DDO:  direct  deoxygenation,  HYD:    hydrogenation, DMO: demethoxylation, DME: demethylation, AL: alkylation and DHY: dehydration  Figure 17. Proposed reaction scheme for the HDO of simulated bio-oil containing guaiacol, Figure 17. Proposed reaction scheme for the HDO of simulated bio‐oil containing guaiacol, phenol  [40]. 

phenol and o-cresol as feedstock over Ni-H-Beta catalyst. Notations: DDO: deoxygenation,  direct deoxygenation, and  o‐cresol  as  feedstock  over  Ni‐H‐Beta  catalyst.  Notations:  DDO:  direct  HYD:  HYD: hydrogenation, DMO: demethoxylation, DME: demethylation, AL: alkylation and DHY: hydrogenation, DMO: demethoxylation, DME: demethylation, AL: alkylation and DHY: dehydration  dehydration [40]. [40]. 

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3.4.2. HDO of Real Bio-Oil HDO of real bio-oils [4,30,35,62] have been investigated in several publications. Bio-oil derived from yellow poplar via fast pyrolysis was hydrodeoxygenatied at 300 ◦ C in ethanol at 30 bar hydrogen [4]. The lowest O/C content was obtained with Pt/C catalyst [4]. HDO of the bio-oil prepared via fast pyrolysis of Miscanthus sinensis at 350 ◦ C and 30 bar over noble metals supported on carbon, in supercritical ethanol [30] showed deoxygenation of the heavy oil fraction of the bio-oil to 78% with the heating value of the product 27.8 MJ/kg. It is also important to note that such unstable compounds, as acids, furfural, vanillin and levoglucosan were converted to esters, ketones and saturated phenols [30]. Macromolecular products in that work were also characterized [28]. The results showed that the carbon content in the polymer increased and at the same time oxygen content decreased indicating lower amounts of phenolic and hydroxyl and methoxy groups. It was also observed that after prolonged reaction times the amount of methoxy groups decreased while phenol groups remained constant indicating that dehydroxylation of the phenol groups was not feasible under these reaction conditions [28]. On the other hand, when the bio-oil was hydrodeoxygenated over Pd/C catalyst at different temperatures varying from 250 ◦ C to 350 ◦ C phenolics started to polymerize [28]. In addition to Pt/C also Ni/SBA-15 was rather active in HDO of bio-oil giving ca. 55% deoxygenation in 1 h [4]. The main reactions occurring during HDO of bio-oil over the latter catalyst were guaiacol and 4-propylsyringol. In this case stability of the deoxygenated bio-oil was improved, since acetic acid, levoglucosan and furfural were totally converted. Gas phase products contained methane, CO, ethane and propane. The produced heavy oil fraction contained only 9.3% water with the heating value of 22.8 MJ/kg [4]. HDO of a real lignin derived bio-oil was successfully carried out over Ni/SiO2 -ZrO2 catalyst at ◦ 300 C under 50 bar hydrogen resulting in formation of 63% of cyclic hydrocarbons, out of which 8% were aromatics. In addition it was stated that most of the identified products, were cyclic hydrocarbons, such as cyclohexane, toluene and xylene exhibiting suitable vapor pressures and carbon number to be used in gasoline [62]. 3.4.3. HDO of Lignin Soda lignin HDO was investigated over various Ni/SiO2 catalysts impregnated with Ni2 P and also with CuMgAlOx [3]. The best results were obtained using a mixture of catalysts comprising Ni/SiO2 containing Ni2 P together with CuMgAlOx. At 340 ◦ C under 20 bar of hydrogen for HDO in ethanol of oxygen removal 84% was achieved with the aromatic lignin monomer yield of 51% and relatively low ring hydrogenation [3]. It was stated that the role of CuMgAlOx was to generate hydrogen during lignin ethanolysis [19] which in turn activates the reduced and passivated Ni2 P/SiO2 catalyst [78]. Complete conversion of organosolv lignin was obtained during its HDO with Ni-BEA at 320 ◦ C under 20 bar hydrogen [2]. The hydrocarbon selectivity of 70% was reported, while the amount of water and the solid residue were 21% and 4%, respectively. The reaction network includes disruption of phenolic compounds of lignin from ether linkages. Thereafter ring hydrogenation followed by deoxygenation occur over Ni-catalyst with slow hydrogenolysis. It was also concluded that at lower temperatures oligomers are formed via recombination of phenolic compounds. 3.5. Catalyst Deactivation, Regeneration and Reuse during HDO of Phenolic Compounds Catalyst deactivation and continuous HDO have been intensively investigated for phenolic compounds [31,33,42,49,50,57]. Catalyst stability, activity as a function of time-onstream [31,33,36,39,42,43,47,48,58,59,61], reuse [30,54,56–58,63], regeneration [34,36], origin of the catalyst deactivation [30], chemical composition of coke [31], thermogravimetrical analysis of the spent catalysts [62], modelling of deactivation [33] are also very important from the industrial point of view. Since the real bio-oils contain in addition to lignin derived compounds also water [59], acetic acid

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or furfural [56] as well several catalyst poisons, such as chlorine, potassium and sulphur containing compounds [47], it is very important not only to investigate HDO of model compounds but also the real feed [30] using different approaches, such as a single component feed, co-feed studies, real bio-oil [30], simulated bio-oil [40], adsorption studies [48,79] as well as continuous operation [33,36,39,42,43,48]. 3.5.1. Catalyst Reuse in HDO of Model Compounds and Bio-Oils Reuse of the catalysts in HDO of phenolic compounds has been intensively investigated [49,54,56,57,62]. Mo2 C/CNF catalyst was reused in guaiacol HDO at 350 ◦ C under 55 bar after rinsing it with diethyl ether [54]. The results were very promising showing that in three consecutive experiments more than 99% conversion was achieved. On the other hand, W2 C/CNF gave lower activity exhibiting only 48% conversion after three consecutive experiments. Mo2 C/CNF catalyst suffered from irreversible coking during three consecutive HDO experiments of guaiacol at 300 ◦ C under 40 bar [56]. The reuse of Ni/SiO2 -ZrO2 was investigated in guaiacol HDO at 300 ◦ C under 50 bar hydrogen [62]. The results showed that in four consecutive experiments the conversion decreased as follows: 100%, 91%, 90% and 87% indicating minor catalyst deactivation. The use of Pt/C is favorable in HDO of phenolic compounds in order to avoid coke formation [57]. Successful recycling was demonstrated in 4-propylphenol HDO over Pt on active carbon giving more than 99% yield of propylcyclohexane in three consecutive experiments at 280 ◦ C under 40 bar hydrogen [57]. The results revealed also that no sintering occurred. Recycling of noble metal supported catalysts in guaiacol HDO was done in three consecutive experiments, with catalyst washing with acetone and reduction at 200 ◦ C in between. The results showed minor deactivation of Ru/C and Pd/silica alumina [49]. It was confirmed that especially in HDO of eugenol the catalyst was poisoned by strongly adsorbed products causing also a brownish colour of the product. In bio-oil HDO over noble metal catalysts supported on active carbon at 300 ◦ C under 30 bar hydrogen, the catalyst deactivation was intensive during reuse [30]. It was also concluded that Ru/C deactivated mainly via fouling and blocking of active sites, whereas activity loss of Pt/C was attributed to extensive char deposition on the catalyst surface. 3.5.2. Time-on Stream Behavior in HDO of Model Compounds Time-on-stream behavior during HDO of cresol over MoO3 catalyst at 320 ◦ C at atmospheric pressure showed catalyst deactivation. When the catalyst was regenerated at 400 ◦ C via calcination, its original activity was restored, however, after each 24 h a repeated regeneration was required [36]. Promising results have been obtained in HDO of phenyl acetate over Pt/Al2 O3 at 325 ◦ C under 5 bar hydrogen. After 25 h time-on-stream a stable period of 40 h with constant activity followed [61]. Phenol formation was rapid from phenyl acetate, whereas the consecutive benzene formation was rather slow. Only minor catalyst deactivation was also observed in HDO of m-cresol, anisole, dimethoxybenzene and guaiacol mixture at 280 ◦ C under 1.5 bar pressure over Mo2 C catalyst giving mostly toluene and benzene as products (Figure 18) [42]. On the other hand, the most rapid deactivation of Mo2 C/ZrO2 as a function of time on stream was observed in HDO of guaiacol at 350 ◦ C under 60 bar hydrogen, followed by anisole and phenol showing clearly that compounds bearing two oxygen atoms caused the most intensive deactivation (Figure 18) [39]. Prominent catalyst deactivation observed in guaiacol HDO can partially be explained by a strongly bound phenate mode of guaiacol by double anchoring to the catalyst surface covering a large part of active sites [80]. Catalyst deactivation occurred also in HDO of anisole and guaiacol over Pt-Sn/CNF/Inconel as a funbction of time on stream (Figure 19) [48]. In addition, in the HDO of anisole, less water is formed than, for example, in phenol HDO. At higher temperature catalyst stability is improved in the presence of smaller amounts of adsorbed water. Catalyst deactivation in guaiacol HDO was also more intensive than in HDO of anisole over Pt-Sn/CNF catalyst at 400 ◦ C under atmospheric pressure (Figure 20) i.e., conditions favoring deactivation [48]. Guaiacol is overall more reactive than anisole at 400 ◦ C, thus more prominent catalyst

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deactivation in HDO of guaiacol compared to anisole is mainly seen from the product distribution. Catalysts 2017, 7, x FOR PEER REVIEW  25 of 42  For anisole, the main product was  benzene, whereas it was anisole for HDO of guaiacol. 60

50

Conversion (%)

"OH" 40

"-OCH3"

30

20

"-OH + -OCH3"

10

0 0

2

4

6

8

Time on stream (h)

  ◦C

Figure 18. Conversion of guaiacol, anisole and phenol in their single feed HDO at 350 under 60 bar Figure 18. Conversion of guaiacol, anisole and phenol in their single feed HDO at 350 °C under 60 bar  inin  a down flow trickle bed reactor adapted from [39]. Notation: ( ) phenol, ) anisole and (o) guaiacol. a  down  flow  trickle  bed  reactor  adapted  from  [39].  Notation:  (●) (phenol,  (■)  anisole  and  (o) 

guaiacol. 

Figure 19. Continuous hydrodeoxygenation of (a) anisole and (b) guaiacol at 400 ◦ C under atmospheric pressure over Pt-Sn/CNF/Inconel catalyst [48].

Figure  19.  Continuous  hydrodeoxygenation  of  (a)  anisole  and  (b)  guaiacol  at  400  °C  under  atmospheric pressure over Pt‐Sn/CNF/Inconel catalyst [48]. 

 

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calcination

80

Conversion (C-mol%)

31 of 40

regenerated

60

calcination

40

20

0 0

5

10

15

20

25

30

35

40

45

TOS (h)

50

 

Figure 20. Figure 20. Conversion of m‐cresol as a function of time on stream at 320 °C under 1 bar over MoO Conversion of m-cresol as a function of time on stream at 320 ◦ C under 1 bar over MoO3 3  catalyst. Regeneration of the catalyst was performed at 400 °C in oxygen for 3 h adapted from [36].  catalyst. Regeneration of the catalyst was performed at 400 ◦ C in oxygen for 3 h adapted from [36]. Catalyst  regeneration  has  also  been  investigated  in  continuous  operation  [36].  Typically,  the  catalyst is calcined in air at 400 °C in order to avoid sintering of metal‐like particles, as it was the case  Catalyst regeneration has also been investigated in continuous operation [36]. Typically, the catalyst in HDO of m‐cresol at 320 °C under atmospheric pressure over MoO 3 (Figure 21) [36]. The results  is calcined in air at 400 ◦ C in order to avoid sintering of metal-like particles, as it was the case in HDO showed clearly that already after 24 h TOS the catalyst activity declined from 50% conversion to 28%  ◦ of m-cresol at 320 C under atmospheric pressure over MoO3 (Figure 21) [36]. The results showed conversion and the intermittent calcination was required in order to keep the catalyst activity high  clearly thatenough.  already after 24 h TOS the catalyst activity declined from 50% conversion to 28% conversion and the intermittent calcination was Catalysts 2017, 7, x FOR PEER REVIEW    required in order to keep the catalyst activity high enough. 27 of 42 

Conversion/Selectivity (%)

100

50

0 0

2

4

Reaction time (h)

6

 

Figure 21. Results from HDO of guaiacol over Mo2C/CNF catalyst at 350 °C under 55 bar hydrogen.  Figure 21. Results from HDO of guaiacol over Mo2 C/CNF catalyst at 350 ◦ C under 55 bar hydrogen. Symbols: mass balance (■), conversion (●), phenol (o) and cresol (□) adapted from reference [54].  Symbols: mass balance (), conversion ( ), phenol (o) and cresol () adapted from reference [54].

3.5.3. Catalyst Selection in HDO of Phenolic Compounds  Precious  metal  catalysts  have  some  advantages  during  HDO  of  phenolic  compounds  being  highly  active  at  low  pressures  and  temperatures  and  enabling  use  of  smaller  reactors  and  other  economic benefits [61]. Metal‐like carbides have also shown activity in HDO of phenolic compounds  [56]. For nitride and carbide catalysts, neutral carbon and zirconia are preferred as supports in direct  deoxygenation of guaiacol [46]. Hydrophobic carbons are also efficient supports for bio‐oil HDO [78]  deactivating only slightly in the presence of water during bio‐oil HDO [55].  Hydrothermal  stability  of  the  catalysts  is  important  during  HDO  of  lignin  derived  phenolic 

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3.5.3. Catalyst Selection in HDO of Phenolic Compounds Precious metal catalysts have some advantages during HDO of phenolic compounds being highly active at low pressures and temperatures and enabling use of smaller reactors and other economic benefits [61]. Metal-like carbides have also shown activity in HDO of phenolic compounds [56]. For nitride and carbide catalysts, neutral carbon and zirconia are preferred as supports in direct deoxygenation of guaiacol [46]. Hydrophobic carbons are also efficient supports for bio-oil HDO [78] deactivating only slightly in the presence of water during bio-oil HDO [55]. Hydrothermal stability of the catalysts is important during HDO of lignin derived phenolic compounds. It is known that ZrO2 exhibits hydrothermal stability, whereas alumina is unstable in the presence of water under hydrothermal conditions [81]. Nitrides have been more stable against sulfides in HDO of guaiacol in a continuous operation [58]. Transition metal carbides, nitrides and phosphides require demanding preparation conditions and exhibit poor stability in HDO of phenolic compounds [37]. Performing HDO at higher temperature enhances stability of MoO3 /ZrO2 due to lower amounts of water adsorbed on the catalyst surface [39]. 3.5.4. Coking during HDO of Model Compounds Possible reasons for catalyst deactivation during HDO of phenolic compounds are coking [34] and poisoning [47], in addition to a loss of active sites. In some cases minor sintering has been observed [31]. In guaiacol HDO over Pt/C slight catalyst deactivation occurred at 325 ◦ C [31]. Coking can be explained by ring condensation favored by low pressure and high temperature [82], whereas aromatic hydrogenation is favored at lower temperature using high hydrogen pressure. Condensed ring products are more strongly adsorbed on the catalyst surface and can be desorbed only at higher temperatures [83]. Co-MCM-41 was used for HDO of guaiacol resulting in a quite rapid catalyst deactivation occurred at 400 ◦ C under atmospheric conditions [34], as these conditions promoted extensive coking. Coking during HDO is typically favored over catalysts having acidic supports, while for less acidic supports, such as zirconia and activated carbon, the catalysts retain their activity longer [58]. More intensive coking during HDO of guaiacol at 300 ◦ C was observed for Ru/C compared to Pt/C [31]. Catalyst coking has been also investigated in HDO of guaiacol and anisole [50]. Confirming that reactant nature has an effect on coking in particular more coke has been formed from guaiacol than anisole on Ni-based catalysts at relatively high temperatures, between 350–400 ◦ C [50]. Coke formation was confirmed in guaiacol HDO at 400 ◦ C under atmospheric pressure over Co/Al-MCM-41 catalyst by TGA [34]. Furthermore, it was stated that the catalyst regeneration at 450 ◦ C was not able to release all coke. From TEM images it was found that coke is located on both on acidic sites and close to Co-particles [34] leading to conclusion that coke formation depends not only on acidity, but also on the presence of metal sites [34]. Analysis of coke deposited on the spent catalyst used in HDO of guaiacol at 300 ◦ C under atmospheric pressure was performed using thermogravimetry and GC-MS [31]. The TGA results revealed that more coke was desorbed from Ru/C compared to Pt/C. Furthermore, linked ring structures such as biphenyl, aromatic compounds, e.g., benzene, and condensed rings including naphthalene and 1-methylnaphtalene were found after extraction of adsorbed species by dichloromethane not being present in the liquid product. It was additionally concluded that there was no sintering of Ru/C under HDO conditions, while minor sintering of Pt/C was observed. 3.5.5. Deactivation Due to Strong Adsorption on the Catalyst Surface Deactivation studies of different supports, such as alumina and silica have been performed by adsorption of different model molecules (phenol, anisole, guaiacol) on these supports and desorbing them at different temperatures [79]. Based on IR-studies interaction mechanisms of these molecules at higher temperatures have been proposed. The results revealed that anisole is demethylated at

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higher temperature, whereas guaiacol is bound via two oxygen atoms on Al-sites. The amounts of adsorbed species on SiO2 were much lower than those recorded for Al2 O3 in the case of phenol and anisole. Even if for guaiacol adsorption quantitative data were not achieved, it was concluded that its interaction with supports was clearly stronger than for phenol and anisole. Strong adsorption of reactants or other molecules during eugenol hydrodeoxygenation has been proposed to cause catalyst deactivation [49]. In eugenol HDO incomplete carbon balance at 250 ◦ C under 30 bar hydrogen in hexadecane over noble metal catalysts supported was ascribed strong adsorption of the products on the catalyst surface [49]. Adsorption strength of guaiacol and anisole was investigated using pulse experiments at 400 ◦ C by adsorbing these molecules on Pt-Sn/CNF catalyst [48]. It was confirmed that guaiacol is more strongly adsorbed than anisole. Furthermore, it was also stated that guaiacol and its HDO-product, catechol with two oxygen atoms, are more strongly adsorbed on the catalyst surface than their single functionalized counterparts, e.g., anisole and phenol [48]. 3.5.6. Poisons in Bio-Oil Causing Catalyst Deactivation Hydrodeoxygenation (HDO) is an important treatment method for bio-oils which have low stability and heating value as described above. Due to a complex structure of bio-oil and presence of possible catalyst poisons [47] and water, its HDO is, however, demanding. Bio-oils stemming from hemicellulose and cellulose contain also acetic acid and furfural, which have been used as co-feed in HDO of guaiacol [56]. Co-feed including also H2 O, CO2 and O2 has been used in HDO of guaiacol [33,59]. The effect of several poisons, such as chlorine, sulfur and potassium was investigated in HDO of guaiacol and 1-octanol over Ni/ZrO2 catalyst in a trickle bed reactor at 250 ◦ C under 100 bar hydrogen [47]. In these experiments, the catalyst was deactivated with 1-octanethiol, 1-chlorooctane, KCl or KNO3 . When using potassium salts a stoichiometric amount of salt compared to nickel was used. The results revealed that with sulfur present in the catalyst, the latter retained its activity during more than 4 h which was followed by a dramatic activity drop almost to zero. Effect of chlorine poisoning was more rapid decreasing the catalyst activity immediately after the start of the chlorine feed. On the other hand, a potassium poisoned catalyst exhibited high activity in guaiacol conversion, even if its deoxygenation activities towards guaiacol were 3% and 18% for respectively KCl and KNO3 in comparison with the not poisoned catalyst displaying 66% deoxygenation degree of guaiacol. It was also noteworthy that chlorine and potassium poisoning was reversible and chloride was found in the liquid product. Sulfur poisoning inhibited both hydrogenation of the aromatic ring in guaiacol as well as subsequent deoxygenation reactions over Ni/ZrO2 catalyst at 250 ◦ C under 100 bar hydrogen, whereas potassium poisoned mainly deoxygenation sites. Chlorine poisoning decreased both hydrogenation and deoxygenation activity of Ni/ZrO2 catalyst. The presence of acetic acid and furfural was investigated in HDO of guaiacol over Mo2 C/CNF catalyst in dodecane at 300 ◦ C under 40 bar hydrogen [56]. The results showed that both acetic acid and furfural were converted to a large extent, whereas guaiacol conversion was only 6% after 4 h. The main products were phenol and catechol, while without furfural and acetic acid catechol formation was much lower. Catalyst stability was investigated during HDO of m-cresol with Mo2 C using co-feeds of H2 O, CO2 and O2 [59]. The results revealed that especially toluene formation rate was retarded with adsorption of oxygen on metal like sites catalyzing HDO reaction. In the gas phase HDO of m-cresol-anisole, dimethoxybenzene and guaiacol mixture the experimental observations could be correlated with the bond dissociation energies showing that the most prominent products were toluene and benzene at 280 ◦ C over Mo2 C catalysts. This catalyst was completely inactive for hydrogenation of the products. It was stated [59] that CO and water, which are typically present in pyrolysis vapor, inhibit hydrogenation of the aromatic ring. Co-feeding CO and CO2 has been also investigated in HDO of guaiacol [33]. Direct hydrodeoxygenation of phenolic compounds was achieved over Mo2 C catalyst without ring

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hydrogenation which was also confirmed by co-feed of water and CO. In guaiacol HDO in the presence of CO, CO2 and water over Fe/SiO2 catalyst at 400 ◦ C under atmospheric pressure inhibition of guaiacol transformations to cresol was observed (Figure 22) [33]. In the vapor-phase anisole HDO, benzene was formed with high selectivity over metal-like carbides, Mo2 C and W2 C [43,44]. It was, however, observed that with co-fed CO formation of benzene during anisole HDO was suppressed over Mo2 C catalyst at 133 ◦ C under atmospheric pressure [43]. Analogously in the presence of Catalysts 2017, 7, x FOR PEER REVIEW  CO-co-feeding during anisole HDO  at 150 ◦ C over Mo2 C under atmospheric pressure benzene31 of 42  formation was suppressed and completely recovered after CO-co-feed was stopped [44]. Pt/C is active for water gas shift reaction forming carbon dioxide during HDO [84]. 2.0x10-6

1

k (kmol/s.kgcat)

1.5x10-6

1.0x10-6

5.0x10-7

2 3 4

0.0 0

50

100

150

200

Time on stream (min) Figure 22. Decrease of the kinetic constants as a function of time on stream in the HDO of guaiacol at 400 ◦ C under 1 bar over Fe/SiO2 adapted from ref. [33]. The reaction mixture Figure 22. Decrease of the kinetic constants as a function of time on stream in the HDO of guaiacol at  contained also 50% H2 , 5% CO2 , 2.5% CO and 2.5% H2 O. Notation: (1) guaiacol to products, 400 °C under 1 bar over Fe/SiO 2 adapted from ref. [33]. The reaction mixture contained also 50% H 2,  6 e−4.61·10−3 x , (2) guaiacol to cresol, line fitting : y = 4.93·10−7 e−7.29·10−3 x , line fitting: = 2, 2.85 ·10−CO  5%  CO 2.5%  and  2.5%  H2O.  Notation:  (1)  guaiacol  to  products,  line  fitting:  2.85 ∙ −3 x . ∙ . ∙ to toluene, line fitting: (3) phenol fitting: y = 1.37line fitting: ·10−7 e−4.85· 10 4.93 , and (4) cresol 10 to benzene, , line 2 guaiacol to cresol, ∙ 10 , − 3 −8 e−5.03·10 x . . ∙ y = 2.813·10   phenol to benzene, line fitting: 1.37 ∙ 10

and 4 cresol to toluene, line fitting:

2.81 ∙ 10

.





3.5.7. Carbon Balance during HDO of Model Compounds 4. Conclusions  Mass balance in HDO of phenolic model compounds has often been reported to be below Hydrodeoxygenation  of  guaiacol bio‐oils,  lignin  has  been  intensively  100% [25,31,54]. One example is HDO and  overtheir  Mo2model  C/CNFcompounds  and W2 C/CNF catalysts (Figureinvestigated in the recent years. The main findings in this area are summarized in the current work.  12) [54], where the authors also stated that the mass balance increased with increasing reaction ◦ C under Several Amodel  molecules  have  been  closure used  in above HDO 90% of  bio‐oil  including  single,  double  or  triple  temperature. relatively high mass balance was achieved at 375 55 bar oxygenated  phenolic  compounds.  Several  reaction  networks  based  on  kinetic  results  have  hydrogen [54]. Analogously to the work of Jongerius et al., mass balances vary in the range of 70–98% been  proposed  for  guaiacol  HDO.  main catalysts parameters  affecting  activity  and out product  in guaiacol HDOespecially  at 300 ◦ C under 40 bar using MoThe  (Figure 21). It also pointed 2 C/CNF distribution are temperature and pressure, type of a catalytically active metal, nature of the support  that the mass balances contained only the liquid phase and coke from the catalyst surface omitting and  its  acidity. [56]. Two  main coking pathways  during  HDO  are  either  ring  followed  by  the gaseous products Clearly [56] and release of products into the hydrogenation  gas phase decrease the deoxygenation at high pressure and temperature below 300 °C, as well as deoxygenation at elevated  mass balance calculated on the basis of the liquid phase products. In HDO of guaiacol the highest carbontemperatures, when hydrogenation is thermodynamically suppressed.  recovery in the liquid phase over noble metals supported on carbon was ca. 70% obtained The degree of deoxygenation is important in order to increase the energy density and heating  at 300 ◦ C [31]. Especially the ring opening reaction at high temperatures leads to formation of more value of the products. Typically, guaiacol is deoxygenated at elevated temperatures or under very  high pressures at temperatures below 350 °C affording HDO yields between 60–80%. One exception  in  guaiacol  HDO  is  Ni/SiO2‐ZrO2  giving  97%  cyclohexane  at  300  °C  under  50  bar.  In  addition  to  cyclohexane,  also  cyclopentanone  is  formed,  especially  over  Pt.  These  results  can  be  explained  theoretically  at  a  DFT  level,  confirming  the  reactivity  of  phenolic  compounds  decreases  with  an 

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gaseous products [18]. On the other hand, total carbon yields varied in the range of 95–98% in the HDO of different phenolic compounds at 300–350 ◦ C over MoO3 catalyst under atmospheric pressure [36]. Solid products are also formed during HDO of phenolic compounds and lignin [2,25,62]. Polymerization, occurring especially for vanillin, is intensive, decreasing the carbon balance [25], for example, in vanillin HDO at 100 ◦ C under 30 bar hydrogen over Mo2 C/AC catalyst. As a result mass balances in the range of 65–70% were reported. It was concluded by Zhang et al. [62] that the mass losses are caused by formation of coke and polymers as well as formation of volatile gaseous products. In addition large amounts of char were formed being in the range of 9–19% in the HDO of pyrolysis oil over Ni-supported catalysts at 300 ◦ C under 30 bar hydrogen [4]. Noteworthy is that formation of a solid residue in lignin HDO decreased with increasing reaction temperature indicating that at higher temperature unwanted reactions of phenolic products are retarded [2]. Formation of gaseous products is typically enhanced at higher temperatures [31,34], while at lower temperatures less gases were formed [43]. For example, in the temperature range of 147–247 ◦ C no gaseous C2–C5 products were observed in vapor phase HDO of anisole on Mo2 C and the main product was benzene formed by C-O bond cleavage [43]. In addition, low gas generation during HDO was observed at 250 ◦ C under 80 bar hydrogen over various Ni catalysts [35]. At elevated temperature, 350 ◦ C, both char and gas formation from HDO of miscanthus bio-oil was enhanced when HDO was performed over Pd/C catalyst under 30 bar of hydrogen [28]. Under these conditions char and gas yields vary in the ranges of 16% and 19–36%, respectively depending on the reaction time in the range of 30–60 min [28]. The liquid yields, composed of light and heavy oil fractions were the highest, above 80%, when HDO of bio-oil was performed over Pd/C catalyst between 250–300 ◦ C [28]. Formation of gaseous products depends also on the type of active metal in the catalyst [31]. For noble metal catalysts the amounts of gaseous products formed during guaiacol HDO increased in the following order: Pd/C < Ru/C < Rh/C < Pt/C [31]. A substantial amount of gaseous products, 50%, was formed in HDO of guaiacol at 400 ◦ C under atmospheric pressure over NiCo-Al-MCM-41 catalysts [34]. Methane formation was very extensive during HDO of guaiacol over Co/Al-MCM-41 at 400 ◦ C under atmospheric pressure and it increased with increasing ratio of catalyst weight to feed rate [34]. The gas phase product amount was was the highest with Ni/Al-MCM-41, being 78%, whereas it was the lowest (30%) for Co-MCM-41 [34]. The main gaseous product yield in the HDO of bio-oil over supported Ni-catalysts at 300 ◦ C under 30 bar hydrogen was CO being in the range of 88–95% from the gaseous products, followed by methane varying in the range of 2–7.1% [4]. It was also possible to obtain a few % of propane, ethyne and ethane [4]. Only minor amounts of methane were formed over supported Ni-catalysts in the HDO of guaiacol under 7.9 bar hydrogen in the temperature range of 350–450 ◦ C, whereas coking was rather high under these conditions over Ni-catalysts being in the range of 3–8 wt % from the total products [50]. Analogously, in the HDO of pyrolysis bio-oil from yellow polar over Ni supported catalysts at 300 ◦ C under 30 bar of hydrogen, formation of gaseous products was significant over Ni supported catalysts varying in the range of 6.6–27% depending on the support type [4]. On the other hand, low methanation degree was achieved over MoO3 /ZrO2 at 320 ◦ C under atmospheric pressure in anisole HDO showing clearly its carbon efficiency [51]. 3.5.8. Modelling of Deactivation for HDO of Model Compounds Modelling of deactivation along with kinetic modelling was performed [33] for HDO of guaiacol over Fe/SiO2 catalyst [33]. When the rate constants were plotted against time on stream it was noticed that the decrease of rate constants occurred similarly for HDO of phenol to benzene and cresol to toluene, whereas in guaiacol transalkylation to cresol the decline of the rate constant was more rapid (Figure 22) [33]. Thus, it was concluded that inhibition of guaiacol to cresol transformations may happen on different sites compared to HDO reactions.

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4. Conclusions Hydrodeoxygenation of bio-oils, lignin and their model compounds has been intensively investigated in the recent years. The main findings in this area are summarized in the current work. Several model molecules have been used in HDO of bio-oil including single, double or triple oxygenated phenolic compounds. Several reaction networks based on kinetic results have been proposed especially for guaiacol HDO. The main parameters affecting activity and product distribution are temperature and pressure, type of a catalytically active metal, nature of the support and its acidity. Two main pathways during HDO are either ring hydrogenation followed by deoxygenation at high pressure and temperature below 300 ◦ C, as well as deoxygenation at elevated temperatures, when hydrogenation is thermodynamically suppressed. The degree of deoxygenation is important in order to increase the energy density and heating value of the products. Typically, guaiacol is deoxygenated at elevated temperatures or under very high pressures at temperatures below 350 ◦ C affording HDO yields between 60–80%. One exception in guaiacol HDO is Ni/SiO2 -ZrO2 giving 97% cyclohexane at 300 ◦ C under 50 bar. In addition to cyclohexane, also cyclopentanone is formed, especially over Pt. These results can be explained theoretically at a DFT level, confirming the reactivity of phenolic compounds decreases with an increasing number of oxygen atoms. Moreover, thermodynamically the hydroxyl group connected to benzene is more difficult to cleave compared to a methoxy group. HDO of real bio-oils was also successfully demonstrated over various Ni-and Pd-supported catalysts. Unstable compounds, such as furfural, acetic acid and levoglucosan were also transformed to more stable ketones and esters. In addition, high HDO activity was obtained for lignin itself, especially over Ni/H-BEA catalyst. Catalyst deactivation studies revealed that main reason for catalytic activity decline during HDO of phenolic compounds is coking in addition to reactant polymerization. Interestingly higher mass balance closures exceeding 90% for guaiacol at 375 ◦ C could be achieved at very high temperatures, due to suppression of oxygen compounds, such as phenol over Mo2 C/CNF. Catalyst poisoning was systematically investigated showing irreversible bulk poisoning of the catalyst with sulphur, whereas chlorine poisoning was reversible. Hydrothermal stability of the catalyst is of crucial importance and suitable candidates are active carbon and ZrO2 , whereas alumina and metal carbides exhibit lower stability under catalytically relevant conditions. The HDO of phenolic compounds is challenging and requires optimization of different parameters, such as temperature, pressure and the catalyst per se in order to maximize the liquid product yield and produce desired compounds with high enough heating value and RON number. In addition, further research efforts are still needed to diminish hydrogen consumption and provide high carbon efficiency. Author Contributions: P.M-A designed the review, collected references and wrote the first draft. D.Yu.M. revised the text and contributed to the final paper. Conflicts of Interest: The authors declare no conflict of interest.

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