High Efficient Hydrogenation of Lignin-Derived Monophenols ... - MDPI

catalysts Article

High Efficient Hydrogenation of Lignin-Derived Monophenols to Cyclohexanols over Pd/γ-Al2O3 under Mild Conditions Jian Yi, Yiping Luo, Ting He, Zhicheng Jiang, Jianmei Li * and Changwei Hu * Received: 4 December 2015; Accepted: 31 December 2015; Published: 13 January 2016 Academic Editor: Rafael Luque Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China; [email protected] (J.Y.); [email protected] (Y.L.); [email protected] (T.H.); [email protected] (Z.J.) * Correspondence: [email protected] (J.L.); [email protected] (C.H.); Tel./Fax: +86-28-8541-5608 (J.L.); +86-28-8541-1105 (C.H.)

Abstract: The catalytic hydrogenation of lignin-derived monophenols with high efficiency and selectivity is important for the sustainable production of chemicals and fuels. Here, Pd/γ-Al2 O3 was prepared via impregnation and used as catalyst for the hydrogenation of phenols to cyclohexanols under mild conditions in aqueous solution. 3 wt. % Pd/γ-Al2 O3 exhibited good catalytic activity for the selective hydrogenation of 4-ethylphenol into 4-ethylcyclohexanol, and a conversion of 100% with selectivity of 98.9% was achieved at 60 ˝ C for 12 h. Other lignin-derived monophenolic model compounds such as 4-methyl phenol and 4-propyl phenol could be hydrogenated into cyclohexanols selectively under optimal conditions. Moreover, the Pd/γ-Al2 O3 catalyst displayed good activity for the hydrogenation of the mixture of monophenols directly derived from raw biomass system to cyclohexanols as the main products, and was favorable for the depolymerization of lignin oligomers under milder conditions. Pd/γ-Al2 O3 catalyst showed good water resistance and stability after recycling four times. This result might provide a promising approach to selectively producing cyclohexanol directly from raw biomass material under mild conditions in aqueous solutions. Keywords: 4-ethylphenol; lignin; hydrogenation; 4-ethylcyclohexanol; Pd/γ-Al2 O3 catalyst

1. Introduction As the only renewable resource which can be converted into carbon-based fuels and chemicals, biomass is recognized as the optimal feedstock to replace the exhausted fossil fuels [1]. Lignin, constituting 15–30 wt. % and carrying about 40% of its energy of biomass, represents an ideal renewable source of aromatics for the production of value-added chemicals [2,3]. Currently, many efforts have been made to depolymerize lignin to monophenols because of its high degree of polymerization and complex structure. Hydrogenolysis, catalytic cracking, and solvothermal treatment were the main methods adopted [4,5], and a satisfied yield of monophenols was obtained [6,7]. A high yield of monophenols was achieved by Zhao et al. via the catalytic hydrogenation process [8]. Very recently, Van den Bosch et al. reported the selective delignification of lignocelluloses in methanol through simultaneous solvolysis and catalytic hydrogenolysis, resulting in a lignin oil consisting of above 50% monophenols [9]. In our previous work [10], selective conversion of lignin in corncob residue resulted in 24.3 wt. % of monophenols. Among the obtained monophenol products, 4-ethylphenol was the common and predominant product. Therefore, the further conversion of 4-ethylphenol to produce value-added chemical intermediates and fuels is becoming necessary and crucial.

Catalysts 2016, 6, 12; doi:10.3390/catal6010012

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Cyclohexanol derivatives are important value-added chemical intermediates, which are widely applied to the fields of pesticide, pharmacy and cosmetics. Cyclohexanol derivatives can be produced from the hydrogenation of phenols. Much effort had been directed towards the hydrogenation of phenols over the decades in the gas or liquid phase. In the case of gas phase hydrogenation, high temperatures (120–300 ˝ C) are usually required [11–16]. However, the formation of coke due to high temperatures in the reaction process causes the deactivation of catalysts, which involves the limitation of circulation and further utilization. Additionally, high temperatures require more energy consumption and thus an increase in cost. Recently, much attention has been focused on the liquid phase hydrogenation of phenols [8,17–25]. For example, Huang et al. [26] prepared a high-performance PdRu bimetallic catalyst on mesoporous silica nanoparticles for the hydrogenation of phenol in CH2 Cl2 solvent. Rode et al. [27] reported that a charcoal-supported rhodium catalyst was highly active for the ring hydrogenation of phenol and cresols under scCO2 , and high conversions of phenol were achieved under such conditions as 328 K, 10 MPa H2 and 12 MPa CO2 . Although high catalytic efficiency was achieved, some environmentally unfriendly organic solvents and tough reaction conditions were indispensable. Compared to organic solvent, water is much greener in the view of environmental protection [28–31], although most of the catalysts could deactivate in aqueous solutions. Moreover, most of the reports involving the cyclohexanol production usually employed commercial phenols as starting material. There are limited studies about the production of cyclohexanol directly from lignin in raw biomass. Here, Pd/γ-Al2 O3 was prepared via impregnation and used as a catalyst for the selective hydrogenation of 4-ethylphenol into cyclohexanols in an aqueous phase under mild conditions. In addition, the catalytic system was further applied to other phenolic model compounds and liquid mixture with monophenols directly from the lignin in raw biomass material. 2. Results and Discussion 2.1. Catalyst Characterization The surface area, pore structure parameters and the actual Pd loading of all the samples are summarized in Table 1. The average pore diameter of all the samples ranged from 82.0 Å to 92.5 Å, confirming that all the samples possessed a mesoporous structure. The BET surface area and pore volume of samples decreased distinctly by the loading of palladium on the bare support. Both the BET surface area and pore volume gradually decreased with increasing Pd loading from 1 to 5 wt. %, whereas the average pore diameter gradually increased. It seemed that more porous space inside the support was occupied, and the smaller pores were blocked, with an increasing amount of palladium introduced. Meanwhile, the blocking of smaller pores gave rise to the enlarging of average pore diameters. The ICP-AES results suggest that most of the actual Pd loadings are slightly less than or close to the controlled Pd loadings. Table 1. BET and ICP-AES analyses of the catalysts.

a

Sample

Surface Area (m2 /g)

Pore Volumes (cm3 /g)

Average pore diameter (Å)

Pd a (wt. %)

γ-Al2 O3 1Pd/γ-Al2 O3 b 2Pd/γ-Al2 O3 3Pd/γ-Al2 O3 5Pd/γ-Al2 O3

230 223 218 197 174

0.471 0.468 0.463 0.423 0.403

82.0 83.1 84.7 86.0 92.5

0.71 1.79 3.05 4.54

Total Pd loadings determined by ICP; b 1 wt. % Pd/γ-Al2 O3 was abbreviated to 1Pd/γ-Al2 O3 and other samples were also abbreviated analogously.

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Figure 1A shows the XRD patterns of samples with different treatments. Characteristic diffraction peaks of γ-Al2 O3 were observed at 2θ of 37.2˝ , 42.4˝ , 45.5˝ and 67.3˝ . The XRD pattern of γ-Al2 O3 Catalysts 2016, 6, 12  3 of 12  impregnated with PdCl2 aqueous solution (UC-Pd/γ-Al2 O3 ) showed no difference from γ-Al 2 O3 . ˝ ˝ After calcinations at 500 C for 4 h, two diffraction peaks corresponding to PdO at 2θ of 33.8 and 54.9˝ detected, indicating that PdCl were detected, indicating that2 was transformed into PdO by calcination in air. The diffraction peaks of  PdCl2 was transformed into PdO by calcination in air. The diffraction ˝ inpatterns  Pd/γ‐Al 2O3  at  2θ  of  39.9°  and  46.4°  the  XRD  be  assigned  the  (111),  (200)  planes  of    peaks of Pd/γ-Al2 O3 at 2θ of 39.9˝ in  and 46.4 the XRDcan  patterns can beto  assigned to the (111), (200) face‐centered cubic (fcc) crystallographic structure of metallic palladium, respectively [32]. This indicated  planes of face-centered cubic (fcc) crystallographic structure of metallic palladium, respectively [32]. that PdO was reduced to Pd° via the reduction treatment. The characteristic diffraction peaks of metallic  This indicated that PdO was reduced to Pd0 via the reduction treatment. The characteristic diffraction Pd can be observed in the XRD patterns of Pd/γ‐Al 3 catalysts with varied Pd loadings (see Figure 1B).  peaks of metallic Pd can be observed in the XRD2Opatterns of Pd/γ-Al2 O3 catalysts with varied Pd The  average  particle  sizes  of  Pd  particles  were  calculated  from  the  Scherrer from equation.  With  loadings (see Figure 1B). The average particle sizes of Pd particles were calculated the Scherrer increasing Pd loading to 5 wt. %, the average particle sizes of Pd particles increased from 9 to 16 nm,  equation. With increasing Pd loading to 5 wt. %, the average particle sizes of Pd particles increased resulting in a lower surface area and a lower degree of dispersion. This result was consistent with that  from 9 to 16 nm, resulting in a lower surface area and a lower degree of dispersion. This result was of the BET analyses.  consistent with that of the BET analyses. A

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Figure  1. XRD XRD patterns patterns ofof the the  samples.  (A):  γ‐Al 2O:3:  γ‐Al22O O33 support; support; (b) (b) UC-3 UC‐3 wt. wt. %% Pd/γ-Al Pd/γ‐Al22O33: :   Figure 1. samples. (A): (a)(a)  γ-Al 2O 3 γ-Al γ‐Al γ-Al22O O3 3support was only impregnated with aqueous liquid of PdCl support was only impregnated with aqueous liquid of PdCl22;; (c) UR‐3 wt. % Pd/γ‐Al (c) UR-3 wt. % Pd/γ-Al22O33: : UC‐Pd/γ‐Al 2O3: UR‐Pd/γ‐Al 2O3 catalyst  UC-Pd/γ-Al22OO33 catalyst was calcined at 500 °C for 4 h; (d) 3 wt. % Pd/γ‐Al catalyst was calcined at 500 ˝ C for 4 h; (d) 3 wt. % Pd/γ-Al 2 O3 : UR-Pd/γ-Al 2 O3 O3Pd/γ-Al : Pd/γ‐Al22O O33: catalyst recycled after  was reduced with H catalyst was reduced2 at 450 °C for 2 h; (e) used 3 wt. % Pd/γ‐Al with H2 at 450 ˝ C for 2 h; (e) used 3 wt. 2% Pd/γ-Al2 O3 catalyst 2O3.  on γ-Al2 O3 . reaction; (B): patterns of catalysts with different Pd loading on γ‐Al recycled after reaction; (B): patterns of catalysts with different Pd loading

The XPS spectra of calcined (UR‐Pd/γ‐Al O3) and reduced (Pd/γ‐Al 2O3) samples are shown in  The XPS spectra of calcined (UR-Pd/γ-Al22O 3 ) and reduced (Pd/γ-Al2 O3 ) samples are shown in 5/2 and 3d3/2 binding energies were given in Table S1 along with a quantitative  Figure 2. The Pd 3d Figure 2. The Pd 3d5/2 and 3d3/2 binding energies were given in Table S1 along with a quantitative estimation of the surface elements. The XPS spectra of the calcined sample exhibited two prominent  estimation of the surface elements. The XPS spectra of the calcined sample exhibited two prominent 2+ 3d5/2 and Pd2+ 3d3/2, respectively [30,33]. This implied  peaks at 336.8 and 342.2 eV, assigning to Pd 2+ 3d peaks at 336.8 and 342.2 eV, assigning to Pd2+ 3d 5/2 and Pd 3/2 , respectively [30,33]. This implied that  PdO  was  the  primary  oxidation  state  of  palladium  on  UR‐Pd/γ‐Al2O O3.  In  addition,  two  small  that PdO was the primary oxidation state of palladium on UR-Pd/γ-Al 2 3 . In addition, two small peaks  observed  at  335.1  (Pd°  3d 5/2 )  and  340.3  eV  (Pd°  3d 3/2 )  indicated  the  peaks observed at 335.1 (Pd0 3d5/2 ) and 340.3 eV (Pd0 3d3/2 ) indicated thepresence  presence of  of metallic  metallic Pd,  Pd, possibly attributing to the partial decomposition of PdCl possibly attributing to the partial decomposition of PdCl22 during calcination. The XPS spectra of the  during calcination. The XPS spectra of the 2+ reduced sample indicated that significant changes occurred in the relative amounts of Pd° and Pd reduced sample indicated that significant changes occurred in the relative amounts of Pd0 and Pd2+. . 2+ For the reduced sample, the percentage of Pd For the reduced sample, the percentage of Pd2+ decreased markedly, and Pd° was the predominant  decreased markedly, and Pd0 was the predominant 2+ species (ca. 60%). However, a small amount of Pd species (ca. 60%). However, a small amount of Pd2+ was also detected after hydrogen reduction.  was also detected after hydrogen reduction.

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UR-Pd/r-Al2O3

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345

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335

330

B.E.(eV)

 

Figure 2. XPS spectra of samples.

Figure 2. XPS spectra of samples.  2.2. The Selective Hydrogenation of 4-Ethylphenol

2.2.The Selective Hydrogenation of 4‐Ethylphenol 

2.2.1. Effect of Reaction Temperature and Time on the Hydrogenation of 4-Ethylphenol

Aqueous-phase hydrogenation of 4-ethylphenol was conducted at different reaction temperatures 2.2.1. Effect of Reaction Temperature and Time on the Hydrogenation of 4‐Ethylphenol  ˝ (40–120 C) (see Figure 3). 4-Ethylphenol was found to be converted to 4-ethylcyclohexanone and

4-ethylcyclohexanol with the catalysis of 5 wt. % Pd/γ-Al2 O3 at a very low temperature, and the Aqueous‐phase hydrogenation of 4‐ethylphenol was conducted at different reaction temperatures  of 4-ethylphenol reached up towas  94.6% at 40 ˝to  C. Higher temperatures than 60 ˝ C led to the (40–120  conversion °C)  (see  Figure  3).  4‐Ethylphenol  found  be  converted  to  4‐ethylcyclohexanone  and    complete conversion of 4-ethylphenol. This result might provide an energy saving and lower cost 4‐ethylcyclohexanol  with  the  catalysis  of  5  wt.  %  Pd/γ‐Al2O3  at  a  very  low  temperature,  and  the  approach for the hydrogenation of lignin model compounds. conversion of 4‐ethylphenol reached up to 94.6% at 40 °C. Higher temperatures than 60 °C led to the  4-Ethylcyclohexanone and 4-ethylcyclohexanol were the main products detected in the complete conversion of 4‐ethylphenol. This result might provide an energy saving and lower cost  hydrogenation processes at various reaction temperatures. However, the selectivity to the product was greatly dependent on the reaction temperature. The selectivities to 4-ethylcyclohexanone and approach for the hydrogenation of lignin model compounds.  4-ethylcyclohexanol were 51%4‐ethylcyclohexanol  and 29% at 40 ˝ C, respectively. With increasing temperature, the in  the  4‐Ethylcyclohexanone  and  were  the  main  products  detected  selectivity to 4-ethylcyclohexanone was markedly decreased to only 2% at 120 ˝ C. The selectivity to hydrogenation processes at various reaction temperatures. However, the selectivity to the product was  4-ethylcyclohexanol remained around 66% at a temperature of 60 ˝ C or higher. The total selectivity greatly  to dependent  on  the  reaction  temperature. was The  to  4‐ethylcyclohexanone  and    4-ethylcyclohexanone and 4-ethylcyclohexanol firstselectivities  increased, reached up to 100% at 60 ˝ C, 4‐ethylcyclohexanol  were  51%  and  29%  40  °C,  respectively.  With  increasing  temperature,  the  and was then gradually decreased withat  increasing reaction temperature. The carbon balance of ˝C 4-ethylphenol hydrogenation showed a similar tendency, reaching a maximum of 100% at 60 selectivity to 4‐ethylcyclohexanone was markedly decreased to only 2% at 120 °C. The selectivity to  (see Table 2). Only 68% products were identified at 120 ˝ C. To further analyze the reaction 4‐ethylcyclohexanol remained around 66% at a temperature of 60 °C or higher. The total selectivity to  products, the liquid products were further conducted by LC-MS analysis (see Figure S1). Besides 4‐ethylcyclohexanone and 4‐ethylcyclohexanol was first increased, reached up to 100% at 60 °C, and  4-ethylcyclohexanone and 4-ethylcyclohexanol, some products with a molecular weight from 200 to was  then  decreased  increasing  reaction  temperature.  The increased carbon with balance  of    400 gradually  (about 2 to 3 benzene rings)with  were also detected, and their amount was gradually ˝ 4‐ethylphenol hydrogenation showed a similar tendency, reaching a maximum of 100% at 60 °C (see  increasing temperature from 80 to 120 C. This indicated that dimers or polymers could be formed at higher temperatures even in the hydrogenation system, which led to the decrease of total selectivity. Table 2). Only 68% products were identified at 120 °C. To further analyze the reaction products, the  Full conversion and afurther  total selectivity of 100% was 5 wt. % Pd/γ-Al 2 O3 catalyst liquid  products  were  conducted  by  achievable LC‐MS  over analysis  (see  Figure  S1). at Besides    ˝ 60 C for 6 h. 4‐ethylcyclohexanone and 4‐ethylcyclohexanol, some products with a molecular weight from 200 to 400  (about  2  to  3  benzene  rings)  were  also  detected,  and  their  amount  was  gradually  increased  with  increasing temperature from 80 to 120 °C. This indicated that dimers or polymers could be formed at  higher temperatures even in the hydrogenation system, which led to the decrease of total selectivity.  Full conversion and a total selectivity of 100% was achievable over 5 wt. % Pd/γ‐Al2O3 catalyst at    60 °C for 6 h. 

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Figure 3. Effect of reaction temperature on the conversion of 4-ethylphenol and the selectivity Figure 3. Effect of reaction temperature on the conversion of 4‐ethylphenol and the selectivity to the  to the products. Reaction conditions: 25 mL H2 O, 0.82 mmol 4-ethylphenol, 0.20 g 2Pd/γ-Al 2 O3 , 2, 6 h.  products. Reaction conditions: 25 mL H 2O, 0.82 mmol 4‐ethylphenol, 0.20 g Pd/γ‐Al O3, 2 MPa H 2 MPa H2 , 6 h.

2O3 at different reaction  Table 2. Carbon balance of 4‐ethylphenol hydrogenation on 5 wt. % Pd/γ‐Al Table 2. Carbon balance of 4-ethylphenol hydrogenation on 5 wt. % Pd/γ-Al2 O3 at different reaction ˝ temperature and on Pd/γ‐Al 2O3 with different Pd loading at 60 °C.  temperature and on Pd/γ-Al 2 O3 with different Pd loading at 60 C.

a

a T/°C  T/˝ C  a Carbon balance/% Carbon balance/% Pd loading/wt. % bb Pd loading/wt. %  Carbon balance/% Carbon balance/%

40 40  60 60  80 80  100 79.679.6 100100  86.8 86.8 70.9 0 0  1 1  2 2  3 100 100 100

‐ 

100 

100 

100 120 70.9  68.9 3  5 100 100 

120  68.9  5  100 

Reaction conditions: 25 mL H2 O, 0.82 mmol 4-ethylphenol, 0.20 g 5 wt. % Pd/γ-Al2 O3 , 2 MPa H2 , 6 h; b Reaction conditions: 25 mL  Reaction conditions: 25 mL H 2O, 0.82 mmol 4‐ethylphenol, 0.20 g 5 wt. % Pd/γ‐Al 2O˝3, 2 MPa H2, 6 h.    H2 O, 0.82 mmol 4-ethylphenol, 0.20 g Pd/γ-Al 2 O3 , 2 MPa H2 , 12 h, 60 C.

a

 Reaction conditions: 25 mL H2O, 0.82 mmol 4‐ethylphenol, 0.20 g Pd/γ‐Al2O3, 2 MPa H2, 12 h, 60 °C. 

b

The effect of reaction time on the aqueous-phase hydrogenation of 4-ethylphenol over 5 wt. %

The effect of reaction time on the aqueous‐phase hydrogenation of 4‐ethylphenol over 5 wt. %  Pd/γ-Al2 O3 is illustrated in Figure S2. In a short reaction time of 2 h, only about 53.7% of 4-ethylphenol Pd/γ‐Al2was O3 is illustrated in Figure S2. In a short reaction time of 2 h, only about 53.7% of 4‐ethylphenol  converted to 4-ethylcyclohexanone and 4-ethylcyclohexanol with selectivities of 84.9% and 15.1%, respectively. When the reaction time was 6 h, the conversion of 4-ethylphenol significantly increased was converted to 4‐ethylcyclohexanone and 4‐ethylcyclohexanol with selectivities of 84.9% and 15.1%,  to 100%. By further prolonging the reaction time from 6 to 14 h, the selectivity to 4-ethylcyclohexanol respectively. When the reaction time was 6 h, the conversion of 4‐ethylphenol significantly increased to  was gradually increased, while the selectivity to 4-ethylcyclohexanone decreased significantly. This 100%. By further prolonging the reaction time from 6 to 14 h, the selectivity to 4‐ethylcyclohexanol was  indicate that prolonging reaction time could facilitate the conversion of 4-ethylphenol and promote gradually increased, while the selectivity to 4‐ethylcyclohexanone decreased significantly. This indicate  the selectivity to 4-ethylcyclohexanol. When the reaction time was 12 h, 4-ethylphenol was converted that prolonging reaction time could facilitate the conversion of 4‐ethylphenol and promote the selectivity  completely to 4-ethylcyclohexanol with a selectivity of 98.3%. to  4‐ethylcyclohexanol.  When  the  reaction  time  was  12  h,  4‐ethylphenol  was  converted  completely  to    2.2.2. Effect of Pd Loading on the Hydrogenation of 4-Ethylphenol 4‐ethylcyclohexanol with a selectivity of 98.3%.  To investigate the effect of Pd loading on γ-Al2 O3 , a series of reactions were conducted in the presence of Pd/γ-Al2 O3 with different Pd loadings (see Figure 4). When a bare γ-Al2 O3 catalyst (0 wt. % 2.2.2. Effect of Pd Loading on the Hydrogenation of 4‐Ethylphenol  Pd/γ-Al2 O3 ) was used, 4-ethylphenol was almost unconverted. The conversion of 4-ethylphenol was significantly enhanced with Pd loading increasing from 0 to 3 wt. %. A conversion of 100% was To investigate the effect of Pd loading on γ‐Al 2O3, a series of reactions were conducted in the presence  ˝ obtained over 3 wt. % Pd/γ-Al2 O3 for 12 h at 60 C. This implied that Pd/γ-Al 2 O3 was an effective of Pd/γ‐Al 2O3 with different Pd loadings (see Figure 4). When a bare γ‐Al 2O3 catalyst (0 wt. % Pd/γ‐Al 2O3)  catalyst for 4-ethylphenol hydrogenation. In addition, different Pd loadings on γ-Al2 O3 significantly was used, 4‐ethylphenol was almost unconverted. The conversion of 4‐ethylphenol was significantly  influence the selectivity to hydrogenation products of 4-ethylphenol. With increasing Pd loading, the enhanced  with  Pd  loading  increasing  from  0 decreased, to  3  wt. while %.  A conversion of 100% was obtained over    selectivity to 4-ethylcyclohexanone gradually the selectivity to 4-ethylcyclohexanol 3  wt.  % greatly Pd/γ‐Al 2O3  for  h  at  60  °C. toThis  implied  that was Pd/γ‐Al 2O3 when was 3 an  for    increased. A12  selectivity of 99% 4-ethylcyclohexanol achieved wt. effective  % Pd/γ-Alcatalyst  2 O3 was used. When 5 wt. % Pd/γ-Al2 O3 was employed, the conversion of 4-ethylphenol remained at 4‐ethylphenol hydrogenation. In addition, different Pd loadings on γ‐Al 2O3 significantly influence  100%. All the carbon balances remained at 100% when Pd loading increased from 1 to 5 wt. % (see the  selectivity  to  hydrogenation  products  of  4‐ethylphenol.  With  increasing  Pd  loading,  the  Table 2). Based on the result of the XRD, it was found that higher Pd loading led to a larger particle of selectivity to 4‐ethylcyclohexanone gradually decreased, while the selectivity to 4‐ethylcyclohexanol  catalyst, which further resulted in better catalytic activity for the production of 4-ethylcyclohexanol.

greatly increased. A selectivity of 99% to 4‐ethylcyclohexanol was achieved when 3 wt. % Pd/γ‐Al2O3  was used. When 5 wt. % Pd/γ‐Al2O3 was employed, the conversion of 4‐ethylphenol remained at 100%.  All the carbon balances remained at 100% when Pd loading increased from 1 to 5 wt. % (see Table 2).  Based on the result of the XRD, it was found that higher Pd loading led to a larger particle of catalyst, 

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This result is similar to that in the literature. [34] Thus, the selective hydrogenation of 4-ethylphenol was achievable over a 3 wt. % Pd/γ-Al2 O3 catalyst with a conversion of 100% and a selectivity of 99%. Compared to a commercial Pd catalyst such as Pd/C in the literature [29], 3 wt. % Pd/γ-Al2 O3 showed better catalytic activity for phenol hydrogenation to cyclohexanol under mild conditions in an Catalysts 2016, 6, 12  aqueous solution.

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Loading (%)

 

Figure 4. Effect of Pd loading on the hydrogenation of 4-ethylphenol. Reaction conditions: 25 mL H2 O,

Figure 4. Effect of Pd loading on the hydrogenation of 4‐ethylphenol. Reaction conditions: 25 mL H 2O,  0.82 mmol 4-ethylphenol, 0.20g Pd/γ-Al2 O3 , 2 MPa H2 , 12 h, 60 ˝ C. 0.82 mmol 4‐ethylphenol, 0.20g Pd/γ‐Al2O3, 2 MPa H2, 12 h, 60 °C.  2.3. Hydrogenation of Other Substituted Phenols

2.3. Hydrogenation of Other Substituted Phenols 

The aqueous phase hydrogenation of phenols with different substituent groups was performed over a 3 wt. % Pd/γ-Al2 O3 catalyst. Under the optimized conditions (60 ˝ C, 2 MPa H2 , 12 h), a range The aqueous phase hydrogenation of phenols with different substituent groups was performed  of bio-derived phenols with electron-donating groups such as –CH3 , CH3 CH2 , –(CH2 )2 CH3 and over a 3 wt. % Pd/γ‐Al2O3 catalyst. Under the optimized conditions (60 °C, 2 MPa H 2, 12 h), a range  –CH(CH3 )2 were selectively hydrogenated into cyclohexanols with both high conversion of substituted of  bio‐derived  phenols  with  electron‐donating  groups  such  as  –CH 3,  CH3CH2,  –(CH2)2CH3  and    phenols and high selectivity to products (see Table 3). Unexpectedly, the chosen chlorophenols –CH(CHexhibited 3)2  were  selectively  hydrogenated  into  cyclohexanols  with  both  high  conversion  of  full conversion under this reaction condition. The products of chlorophenol hydrogenation werephenols  cyclohexanone cyclohexanol with selectivities depending on both the different substituted  and and high  selectivity  to different products  (see  Table  3).  Unexpectedly,  the  chosen  substituted position of –Cl and the number of –Cl. This result indicated that dechlorination occurred chlorophenols exhibited full conversion under this reaction condition. The products of chlorophenol  in the reaction process. According to the reports [35,36], it was found that the –Cl was very difficult to hydrogenation were cyclohexanone and cyclohexanol with different selectivities depending on both  remove from a benzene ring of chlorophenol. Therefore, this work might provide a possible pathway the  different  substituted  position  of in–Cl  and  the of number  of  –Cl.  This  result  indicated  that  to remove the –Cl of chlorophenol the treatment chlorophenols as pollutants [35]. However, dechlorination occurred in the reaction process. According to the reports [35,36], it was found that the    guaiacol showed a much lower conversion, which might have been caused by one of the following. On the one hand, the methoxy group displayed electrophilic property when it was connected with –Cl was very difficult to remove from a benzene ring of chlorophenol. Therefore, this work might provide  a benzene ring in an elimination or addition reaction, and thus could have decreased the electron a possible pathway to remove the –Cl of chlorophenol in the treatment of chlorophenols as pollutants [35].  density of the benzene ring and made the ring more stable. On the other hand, a probable mechanism However, guaiacol showed a much lower conversion, which might have been caused by one of the  for the hydrogenation of phenols on Pd-based catalysts indicated that phenols were adsorbed and following.  On  the  one  hand, [37], the  methoxy  displayed  electrophilic  when  activated on the support while H2 was group  activated on the Pd and induced the property  hydrogenation of it  was  connected with a benzene ring in an elimination or addition reaction, and thus could have decreased  adsorbed phenols. If the adsorption of phenols occurred through the hydroxyl, the methoxy group in ortho-position may have increased steric hindrance of the reaction such that guaiacol had more trouble the electron density of the benzene ring and made the ring more stable. On the other hand, a probable  beingfor  hydrogenated than the other did on  [19,37]. mechanism  the  hydrogenation  of phenols phenols  Pd‐based  catalysts  indicated  that  phenols  were 

adsorbed  and  activated  on  the  support  [37],  while  H2  was  activated  on  the  Pd  and  induced  the  hydrogenation of adsorbed phenols. If the adsorption of phenols occurred through the hydroxyl, the  methoxy  group  in  ortho‐position  may  have  increased  steric  hindrance  of  the  reaction  such  that  guaiacol had more trouble being hydrogenated than the other phenols did [19,37].  Table 3. The catalytic efficiency of the catalysts on the hydrogenation of different lignin‐derived monophenols a.  Entry 

Reactant 

Conversion (%)

Products Selectivity (%) Selectivity (%) 

OH

1 

OH

100 

‐ 

‐ 

100 

However, guaiacol showed a much lower conversion, which might have been caused by one of the  However, guaiacol showed a much lower conversion, which might have been caused by one of the  following.  On  the  one  hand,  the  methoxy  group  displayed  electrophilic  property  when  it  was  following.  On  the  one  hand,  the  methoxy  group  displayed  electrophilic  property  when  it  was  However, guaiacol showed a much lower conversion, which might have been caused by one of the  However, guaiacol showed a much lower conversion, which might have been caused by one of the  However, guaiacol showed a much lower conversion, which might have been caused by one of the  following.  On  the  one  hand,  the  methoxy  group  displayed  electrophilic  property  when  it  was  following.  hand,  methoxy  group  displayed  electrophilic  property  when  it  following.  On  the  one  hand,  the  methoxy  group  displayed  electrophilic  property  when  it  was  was  following.  On On  the the  one one  hand,  the the  methoxy  group  displayed  electrophilic  property  when  it  was  was  following.  following.  On  On  the  the  one  one  hand,  hand,  the  the  methoxy  methoxy  group  group  displayed  displayed  electrophilic  electrophilic  property  property  when  when  it  it  connected with a benzene ring in an elimination or addition reaction, and thus could have decreased  connected with a benzene ring in an elimination or addition reaction, and thus could have decreased  following.  On  the  one  hand,  the  methoxy  group  displayed  electrophilic  property  when  it  was  was  following.  following.  On  On  the  the  one  one  hand,  hand,  the  the  methoxy  methoxy  group  group  displayed  displayed  electrophilic  electrophilic  property  property  when  when  it  it  was  was  connected with a benzene ring in an elimination or addition reaction, and thus could have decreased  connected with a benzene ring in an elimination or addition reaction, and thus could have decreased  connected with a benzene ring in an elimination or addition reaction, and thus could have decreased  connected with a benzene ring in an elimination or addition reaction, and thus could have decreased  connected with a benzene ring in an elimination or addition reaction, and thus could have decreased  connected with a benzene ring in an elimination or addition reaction, and thus could have decreased  the electron density of the benzene ring and made the ring more stable. On the other hand, a probable  the electron density of the benzene ring and made the ring more stable. On the other hand, a probable  connected with a benzene ring in an elimination or addition reaction, and thus could have decreased  connected with a benzene ring in an elimination or addition reaction, and thus could have decreased  connected with a benzene ring in an elimination or addition reaction, and thus could have decreased  the electron density of the benzene ring and made the ring more stable. On the other hand, a probable  the electron density of the benzene ring and made the ring more stable. On the other hand, a probable  the electron density of the benzene ring and made the ring more stable. On the other hand, a probable  the electron density of the benzene ring and made the ring more stable. On the other hand, a probable  the electron density of the benzene ring and made the ring more stable. On the other hand, a probable  the electron density of the benzene ring and made the ring more stable. On the other hand, a probable  mechanism  for  the  hydrogenation  of  phenols  on  Pd‐based  catalysts  indicated  that  phenols  were  mechanism  for  the  hydrogenation  of  phenols  on  Pd‐based  catalysts  indicated  that  phenols  were  the electron density of the benzene ring and made the ring more stable. On the other hand, a probable  the electron density of the benzene ring and made the ring more stable. On the other hand, a probable  the electron density of the benzene ring and made the ring more stable. On the other hand, a probable  mechanism  for  the  hydrogenation  of  phenols  on  Pd‐based  catalysts  indicated  that  phenols  were  mechanism  hydrogenation  of  Pd‐based  catalysts  indicated  that  phenols  were  mechanism  for  the  hydrogenation  of  phenols  phenols  on  Pd‐based  catalysts  indicated  that  phenols  were  mechanism  for for  the the  hydrogenation  of  phenols  phenols  on on  Pd‐based  catalysts  indicated  that  phenols  were  mechanism  mechanism  for  for  the  the  hydrogenation  hydrogenation  of  of  phenols  on  on  Pd‐based  Pd‐based  catalysts  catalysts  indicated  indicated  that  that  phenols  phenols  were  were  adsorbed  and  activated  on  the  support  [37],  while  H 2  was  activated  on  the  Pd  and  induced  the  adsorbed  and  activated  on  the  support  [37],  while  H 2   was  activated  on  the  Pd  and  induced  the  mechanism  for  the  hydrogenation  of  phenols  on  Pd‐based  catalysts  indicated  that  phenols  were  mechanism  mechanism  for  for  the  the  hydrogenation  hydrogenation  of  of  phenols  phenols  on  on  Pd‐based  Pd‐based  catalysts  catalysts  indicated  indicated  that  that  phenols  phenols  were  were  adsorbed  and  activated  on  the  support  [37],  while  H 2 H was  activated  on  the  Pd  and  induced  the  adsorbed  and  activated  on  the  support  [37],  while  2  was  activated  on  the  Pd  and  induced  the  Catalysts 2016, 6, 12 7 of 13 adsorbed  and  activated  on  the  support  [37],  while  H 2  was  activated  on  the  Pd  and  induced  the  adsorbed  and and  activated  on  on  the the  support  [37],  while  H22   H was  activated  on  on  the the  Pd Pd  and and  induced  the  adsorbed  adsorbed  and  activated  activated  on  the  support  support  [37],  [37],  while  while  H was  2  was  activated  activated  on  the  Pd  and  induced  induced  the  the  hydrogenation of adsorbed phenols. If the adsorption of phenols occurred through the hydroxyl, the  hydrogenation of adsorbed phenols. If the adsorption of phenols occurred through the hydroxyl, the  adsorbed  and  activated  on  the  support  [37],  while  H 2  was  activated  on  the  Pd  and  induced  the  adsorbed  adsorbed  and  and  activated  activated  on  on  the  the  support  support  [37],  [37],  while  while  H H 22   was  was  activated  activated  on  on  the  the  Pd  Pd  and  and  induced  induced  the  the  hydrogenation of adsorbed phenols. If the adsorption of phenols occurred through the hydroxyl, the  hydrogenation of adsorbed phenols. If the adsorption of phenols occurred through the hydroxyl, the  hydrogenation of adsorbed phenols. If the adsorption of phenols occurred through the hydroxyl, the  hydrogenation of adsorbed phenols. If the adsorption of phenols occurred through the hydroxyl, the  hydrogenation of adsorbed phenols. If the adsorption of phenols occurred through the hydroxyl, the  hydrogenation of adsorbed phenols. If the adsorption of phenols occurred through the hydroxyl, the  methoxy  group  in  ortho‐position  may  have  increased  steric  hindrance  of  the  reaction  such  that  methoxy  group  in  ortho‐position  may  have  increased  steric  hindrance  of  the  reaction  such  that  hydrogenation of adsorbed phenols. If the adsorption of phenols occurred through the hydroxyl, the  hydrogenation of adsorbed phenols. If the adsorption of phenols occurred through the hydroxyl, the  hydrogenation of adsorbed phenols. If the adsorption of phenols occurred through the hydroxyl, the  methoxy  group  in  ortho‐position  may  have  increased  steric  hindrance  of  the  reaction  such  that  methoxy  group  in  may  have  increased  steric  hindrance  of  reaction  such  that  methoxy  group  in  ortho‐position  ortho‐position  may  have  increased  steric  hindrance  of  the  the  reaction  such  that  methoxy  group  in  ortho‐position  may  have  increased  steric  hindrance  of  the  the  reaction  such  that  methoxy  methoxy  group  in  ortho‐position  in  may  may  have  have  increased  steric  steric  hindrance  hindrance  of  of  reaction  reaction  such  such  that  that  Table 3.group  The catalytic efficiency of the increased  catalysts onhindrance  the hydrogenation of different guaiacol had more trouble being hydrogenated than the other phenols did [19,37].  guaiacol had more trouble being hydrogenated than the other phenols did [19,37].  methoxy  group  in  ortho‐position  ortho‐position  may  have  increased  steric  hindrance  of  the  the  reaction  such  that  methoxy  methoxy  group  group  in  in  ortho‐position  ortho‐position  may  may  have  have  increased  increased  steric  steric  hindrance  of  of  the  the  reaction  reaction  such  such  that  that  guaiacol had more trouble being hydrogenated than the other phenols did [19,37].  guaiacol had more trouble being hydrogenated than the other phenols did [19,37].  guaiacol had more trouble being hydrogenated than the other phenols did [19,37].  guaiacol had more trouble being hydrogenated than the other phenols did [19,37].  a. guaiacol had more trouble being hydrogenated than the other phenols did [19,37].  guaiacol had more trouble being hydrogenated than the other phenols did [19,37].  lignin-derived monophenols guaiacol had more trouble being hydrogenated than the other phenols did [19,37].  guaiacol had more trouble being hydrogenated than the other phenols did [19,37].  guaiacol had more trouble being hydrogenated than the other phenols did [19,37].  a.  a.  Table 3. The catalytic efficiency of the catalysts on the hydrogenation of different lignin‐derived monophenols  Table 3. The catalytic efficiency of the catalysts on the hydrogenation of different lignin‐derived monophenols  a.  a.  Table 3. The catalytic efficiency of the catalysts on the hydrogenation of different lignin‐derived monophenols  Table 3. The catalytic efficiency of the catalysts on the hydrogenation of different lignin‐derived monophenols  a.  a.  Table 3. The catalytic efficiency of the catalysts on the hydrogenation of different lignin‐derived monophenols  Table 3. The catalytic efficiency of the catalysts on the hydrogenation of different lignin‐derived monophenols  a.  a Products Table 3. The catalytic efficiency of the catalysts on the hydrogenation of different lignin‐derived monophenols  Table 3. The catalytic efficiency of the catalysts on the hydrogenation of different lignin‐derived monophenols  aa.  Table 3. The catalytic efficiency of the catalysts on the hydrogenation of different lignin‐derived monophenols  Table 3. The catalytic efficiency of the catalysts on the hydrogenation of different lignin‐derived monophenols  Table 3. The catalytic efficiency of the catalysts on the hydrogenation of different lignin‐derived monophenols  .  a. .  Entry Conversion (%) Reactant Products Products Products Products Entry  Reactant  Conversion (%) Entry  Reactant  Conversion (%) Products Selectivity (%) Selectivity (%)Products Products Entry  Reactant  Conversion (%) Entry  Reactant  Conversion (%) Products Selectivity Selectivity (%)  (%) Selectivity (%)  Entry  Reactant  Reactant  Conversion (%) Conversion (%) Selectivity Entry  Reactant  Conversion (%) Products Products Products Selectivity (%)(%) Selectivity (%)  Selectivity (%) Selectivity (%)  Entry  Entry  Reactant  Conversion (%) Selectivity (%) Selectivity (%)  Selectivity (%) Selectivity (%)  OH OH Entry  Reactant  Reactant  Conversion (%) Selectivity OH OH Entry  Entry  Reactant  Conversion (%) Conversion (%) Selectivity (%) (%) Selectivity (%)  Selectivity (%)  OH OH OH OH Selectivity (%) Selectivity (%)  Selectivity Selectivity (%) (%) Selectivity (%)  Selectivity (%)  OH OH OH OH 1  100  ‐  ‐  100  ‐  ‐  100  OH OH OH OH 1 1  100 100 1  1  100 100  ‐  ‐ ‐  ‐  ‐ ‐  100 100  100  OH OH OH OH OH OH 1  100  100  1  1  100 100  ‐  ‐  ‐  ‐  100 100              100  1  100  ‐  ‐  OH 1  1  1  100  100 100  ‐ ‐  ‐  ‐ ‐  ‐  100  100 100  OH   OH   OH OH OH   OH OH   OH OH OH OH     100 100      OH     OH OH 2  100  ‐  ‐  OH 2  100  ‐  ‐  100 100  ‐  ‐  - ‐ ‐  2  2 2  100 - ‐ ‐  100 OH OH OH 100 100  OH OH OH 2  100  100  2  2  100 100  ‐  ‐  ‐  100 100              100  2  100  ‐  ‐  OH OH OH OH 2  2  2  100  100 100  ‐  ‐  ‐ ‐  ‐  ‐  ‐  100  100 100            OH OH OH OH OH OH OH OH            OH  OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH 100  ‐  ‐  3  100  ‐  ‐  100  OH OH OH OH 3  3  100  ‐  ‐  100 100  3  100  ‐ ‐  ‐ ‐  100  OH OH OH OH 3  100  100  3 3  100 - ‐  100 3  3  100 100  ‐  ‐  - ‐  100 100  OH OH OH OH OH OH 100  ‐  ‐  100          3  100  ‐  ‐  3  3  100  100  ‐ ‐  ‐ ‐  100  100 100          OH OH OH OH      OH   OH OH OH       OH OH OH OH     OH     OH OH OH 4  100  ‐  ‐  4  4  100 100  ‐  ‐  ‐  ‐  100 100  OH OH OH 100  4  100  ‐ ‐  ‐ ‐  100  OH OH OH 4  100  100  4  4  HO 100 100  ‐  ‐  - ‐  HO HO 100 100  HO          4  100  ‐  ‐  ‐  100  100 100 4 4  HO HO HO HO       100  ‐ O O ‐ ‐  ‐  HO HO 100  4  4  HO HO 100  100  ‐  ‐  100  OH   100  OH OH  OH     OH  OH   OH  OH  O O HO HO HO HO O OH   OH  HO HO HO OH   OH  HO O HO HO OH OH 90.2  9.8  90.2  5  100  OH OH O O 9.8  5  5  100 100  5  100  90.2  OH OH OH 90.2  O 9.8 9.8  OH OH OH OO 5  100  9.8  90.2  5  5  100 100  9.8  9.8  90.2 90.2              90.2  5  100  9.8  Catalysts 2016, 6, 12  7 of 12  Catalysts 2016, 6, 12  7 of 12  5 100 9.8 90.2 7 of 12  5  100  9.8  90.2  5  5  100  100  9.8  9.8  90.2  90.2  Catalysts 2016, 6, 12  7 of 12    Catalysts 2016, 6, 12        Catalysts 2016, 6, 12  7 of 12  Catalysts 2016, 6, 12  Catalysts 2016, 6, 12  7 of 12  7 of 12              Catalysts 2016, 6, 12  7 of 12      Catalysts 2016, 6, 12  7 of 12  Catalysts 2016, 6, 12  7 of 12  7 of 12  OH OH 7 of 12  OH OH   Catalysts 2016, 6, 12    Catalysts 2016, 6, 12  OH OH OH OH OH OH OH OH OH OH   100  ‐  ‐  100  100  ‐  ‐  100       OH 6 6 6  6 6  100 100 OH 100  100 100  ‐  ‐ ‐  ‐  ‐ ‐  100 100  OH OH 6  100  ‐ ‐  ‐ ‐       6  100  100    OH OH  OH 100  OH OH  OH   6  6  100 100  100  OH   OH   OH   OH  100  Catalysts 2016, 6, 12  Catalysts 2016, 6, 12  7 of 12  7 of 12  6  100  ‐ ‐  ‐ ‐            OH OH OH OH   6  100  ‐ ‐  ‐  ‐ ‐  ‐  6  6  100 100  100 100    100  OH OH OH OH OH  OH OH   OH 92.9  ‐  ‐  100    OH    OH    92.9  100    100  7 7  92.9  100  7 7  7  92.9  ‐ ‐  ‐ ‐  ‐ ‐  - ‐ ‐  7 7  92.9 100 OH OH OHOHOHOH OH OH OH OH 100  92.9  ‐ -‐  ‐ ‐  7  92.9  92.9  100  100  7  92.9  ‐  ‐  100         100 100  7  92.9  ‐  ‐  ‐  6  6  100 100  OH  OH  OH  OH 7  92.9  ‐ ‐  O‐  7  7  92.9 92.9  100 100  ‐ O OO ‐ ‐  ‐            100  OH     OH OH OH       Catalysts 2016, 6, 12  Catalysts 2016, 6, 12  7 of 12  OH OH OH OH O OH OH O O     OH OH OH OH OH 8  100  21.7  78.3  7 of 12  8  100  21.7  78.3  OH O OH        78.3  OH    8  100  21.7  78.3  8  8  100 100  21.7  78.3  O 100  21.7  OH OH  OH   100 21.7 78.3 8 8  8  100  21.7  21.7  78.3  78.3  Cl OH OH Cl O OH O     8  7  Cl Cl OH     100 92.9  21.7  78.3  7  92.9  ‐  ‐  O 21.7  ‐  ‐  100 100  8  100  78.3  Cl ClOH OH    OH   OH 78.3   OH    OH   8  100  8  8  Cl 100 100  21.7 21.7  78.3 78.3  OH Cl O O 21.7  OH  OH OH Cl OH OH   O O OH OH 6  6  ClCl ClOH 100  100  ‐  ‐  ‐  ‐  100  100  Cl   Cl       O OH O O OH   OH   Cl   Cl   OH 80.3  100  19.7  80.3  100  19.7  O O OH OH  O Cl     ClOH ClOH  19.7  OH OH OH 80.3  9 9  100  19.7  O 80.3  9 9  9  100  OH ClOH OH 80.3  9  100  19.7  OH OH 80.3  80.3  9  100  100  19.7  19.7  OO   O   80.3  OH OH  OH   19.7  Cl 9  100  9 9  100 80.3 19.7 8  8  21.7 21.7  78.3 78.3  Cl Cl   Cl   100 100  19.7          19.7         80.3  9  100  80.3  9  7  100 92.9  OH OH 9  Cl Cl OH 100    OH  19.7              7  92.9  ‐  ‐ O   O 80.3  ‐  80.3  ‐  100 19.7  100    OH OH OHOH OHOHOH    OO     O OH   OH   OH O OH OH O OH   O O OH OH   OH          34.4  10  100  65.7  34.4  10 10  100  65.7  O OH OH OH Cl OH 100  65.7  34.4  OH ClOH 10  100  65.7  34.4  OO OH OH OH O 80.3  80.3  9  10  9  19.7  19.7  10  100  10  100  100  65.7  65.7  34.4  34.4  OO   O   65.7  OH OH  OH   34.4  10  100  65.7  34.4          Cl   Cl   10  100 100  65.7  34.4  100 65.7 34.4 1010  8  8  ClCl ClCl 21.7  21.7  78.3  78.3       65.7       34.4  Cl   Cl   100  10  100 100  65.7  34.4  O   10  ClCl Cl 65.7  34.4  O OH OH Cl OH OH    Cl           Cl   O OH Cl O Cl Cl OH OH OH OH      Cl OH OH O OH     O O OH OH     OH OHOCH OCH     Cl OH Cl3OCH   OCH OCH3OCH OH OH O 3OH 3 OH O 3OCH O 3 3 58.4  OH 3OCH Cl Cl OCHCl OCH OCH    Cl3 3  O 11  ClCl OH 56.9  58.4  OH 41.6  3OCH 11 11  56.9  41.6  O 3OCH O OH33OCH OH33 OCH OCH OCH33OCH3 OCH OCH OO 56.9  58.4  41.6  3 3 58.4  OH 11  56.9  41.6  OH OH OH OOCH OH OH OCH OCH OCH Cl Cl 3 3 3 11  56.9  58.4  41.6  10  10  100  100  65.7  65.7  34.4  34.4  11  11  56.9  56.9  58.4  58.4  41.6  41.6  OCH3 OCH3 OCH3 11  56.9  58.4  41.6  80.3  80.3  9  9  100  100  19.7  19.7  OCH OCH   3   OCH OCH33OCH OCH33OCH OCH 33OCH  3 58.4  11  56.9  41.6         3       41.6  11 56.9 58.4 41.6 11  56.9  11  11  Cl Cl 56.9 56.9  58.4  58.4      58.4     g, 0.056 mmol Pd ),    41.6 41.6        Cl    a Reaction conditions: reactant (0.82 mmol), H Cl a Reaction conditions: reactant (0.82 mmol), H 2O (25 mL), 3 wt. % Pd/γ‐Al 2O3 (0.200 2O (25 mL), 3 wt. % Pd/γ‐Al 2O3 (0.200 g, 0.056 mmol Pd ),  a Reaction conditions: reactant (0.82 mmol), H     a Reaction conditions: reactant (0.82 mmol), H   O O OH OH   2O (25 mL), 3 wt. % Pd/γ‐Al 2O 3 (0.200 g, 0.056 mmol Pd ),  OH OH g, 0.056 mmol Pd ),  2 O (25 mL), 3 wt. % Pd/γ‐Al 2 O 3  (0.200 aa Reaction conditions: reactant (0.82 mmol), H a OH OH   22O (25 mL), 3 wt. % Pd/γ‐Al 22O g, 0.056 mmol Pd ),    g, 0.056 mmol Pd ),   Reaction conditions: reactant (0.82 mmol), H O (25 2O (25 mL), 3 wt. % Pd/γ‐Al mL), 3 wt. % Pd/γ‐Al O33 (0.200  (0.200 2O3 (0.200 g, 0.056 mmol Pd ),    a Reaction conditions: reactant (0.82 mmol), H        Reaction conditions: reactant (0.82 mmol), H 2O (25 mL), 3 wt. % Pd/γ‐Al 2O3 (0.200 OH g, 0.056 mmol Pd ),  60 °C, 2 MPa H 2, 12 h, reaction mixture stirred at 500 rpm. a Reaction conditions: reactant (0.82 mmol), H 60 °C, 2 MPa H 2, 12 h, reaction mixture stirred at 500 rpm. O  3OCH O  3 OH 3 OCH3OCH3 OCH 3OCH 2O (25 mL), 3 wt. % Pd/γ‐Al 2O3 (0.200 OCH g, 0.056 mmol Pd ),  2, 12 h, reaction mixture stirred at 500 rpm. 2, 12 h, reaction mixture stirred at 500 rpm. aa60 °C, 2 MPa H    a60 °C, 2 MPa H   11  11  56.9  56.9  58.4  58.4  41.6 41.6  a60 °C, 2 MPa H  Reaction conditions: reactant (0.82 mmol), H 22O (25 mL), 3 wt. % Pd/γ‐Al 2 O 3  (0.200 g, 0.056 mmol Pd ),   Reaction conditions: reactant (0.82 mmol), H O (25 mL), 3 wt. % Pd/γ‐Al 2 O 3  (0.200 g, 0.056 mmol Pd ),  60 °C, 2 MPa H 2 , 12 h, reaction mixture stirred at 500 rpm.  Reaction conditions: reactant (0.82 mmol), H 2 O (25 mL), 3 wt. % Pd/γ‐Al 2 O 3  (0.200 g, 0.056 mmol Pd ),  60 °C, 2 MPa H 2 , 12 h, reaction mixture stirred at 500 rpm. 2 , 12 h, reaction mixture stirred at 500 rpm. Reaction reactant (0.82 mmol), H2 O (25 mL), 3 wt.  % Pd/γ-Al 60 ˝ C, 10  10 conditions: 100 100  65.7 65.7  34.4 34.4  2 O3 (0.200 g, 0.056 mmol Pd), 60 °C, 2 MPa H 2, 12 h, reaction mixture stirred at 500 rpm. 60 °C, 2 MPa H 2, 12 h, reaction mixture stirred at 500 rpm.        260 °C, 2 MPa H MPa H2 , 12 2h, reaction mixture stirred at 500 rpm. 60 °C, 2 MPa H 60 °C, 2 MPa H 2, 12 h, reaction mixture stirred at 500 rpm. , 12 h, reaction mixture stirred at 500 rpm.           2, 12 h, reaction mixture stirred at 500 rpm.     2.4. The Probable Reaction Pathway of 4‐Ethylphenol Hydrogenation  2.4. The Probable Reaction Pathway of 4‐Ethylphenol Hydrogenation  Cl Cl Cl   Cl   2.4. The Probable Reaction Pathway of 4‐Ethylphenol Hydrogenation  2.4. The Probable Reaction Pathway of 4‐Ethylphenol Hydrogenation  a Reaction conditions: reactant (0.82 mmol), H a Reaction conditions: reactant (0.82 mmol), H O mL), 3 wt. % Pd/γ‐Al O OH OH 2.4. The Probable Reaction Pathway of 4‐Ethylphenol Hydrogenation  2O (25 2O (25 mL), 3 wt. % Pd/γ‐Al 2O3 (0.200 2O 3 (0.200 g, 0.056 mmol Pd ),  OH OHg, 0.056 mmol Pd ),  2.4. The Probable Reaction Pathway of 4‐Ethylphenol Hydrogenation  2.4. The Probable Reaction Pathway of 4‐Ethylphenol Hydrogenation  2.4. The Probable Reaction Pathway of 4‐Ethylphenol Hydrogenation  OCH OCH OCH OCH     OCH OCH To  study  the  probable  reaction  pathway  of  4‐ethylphenol  hydrogenation,  series  of kinetic  kinetic  3 3 3 To  study  the  probable  reaction  pathway  of  4‐ethylphenol  hydrogenation,  series  of kinetic  kinetic  3 3 2.4. The Probable Reaction Pathway of 4‐Ethylphenol Hydrogenation  2.4. The Probable Reaction Pathway of 4‐Ethylphenol Hydrogenation  60 °C, 2 MPa H 2, 12 h, reaction mixture stirred at 500 rpm. 2, 12 h, reaction mixture stirred at 500 rpm. 2.4. The60 °C, 2 MPa H Probable Reaction Pathway of 4-Ethylphenol 2.4. The Probable Reaction Pathway of 4‐Ethylphenol Hydrogenation  To  study  the  probable  reaction  pathway  of  4‐ethylphenol  hydrogenation,  a a 41.6  series  of  To  study  probable  reaction  pathway  of Hydrogenation 4‐ethylphenol  hydrogenation,  a a 3series  of  11  11 the  56.9  56.9  58.4  58.4  41.6 

To  study  the  probable  reaction  pathway  of  hydrogenation,  a  of  study  probable  reaction  pathway  of  4‐ethylphenol  hydrogenation,  a  series  of  kinetic  To To  study  the the  probable  reaction  pathway  of 4‐ethylphenol  4‐ethylphenol  hydrogenation,  a series  series  of kinetic  kinetic  experiments were conducted over 3 wt. % Pd/γ‐Al O33 (see Figure 5). The conversion of 4‐ethylphenol   (see Figure 5). The conversion of 4‐ethylphenol  experiments were conducted over 3 wt. % Pd/γ‐Al 2O32 (see Figure 5). The conversion of 4‐ethylphenol  To  study  the  probable  reaction  pathway  of  4‐ethylphenol  hydrogenation,  a  series  of  kinetic  experiments were conducted over 3 wt. % Pd/γ‐Al 2 (see Figure 5). The conversion of 4‐ethylphenol  O experiments were conducted over 3 wt. % Pd/γ‐Al 2 O 3         To  study  the  probable  reaction  pathway  of  4‐ethylphenol  hydrogenation,  a  series  of  kinetic  To  study  the  probable  reaction  pathway  of  4‐ethylphenol  hydrogenation,  a a series  of  ToTo  study thethe  probable reaction pathway of222O 4-ethylphenol hydrogenation, series of kinetic  kinetic study  probable  reaction  pathway  of  hydrogenation,  a  series  of  kinetic  experiments were conducted over 3 wt. % Pd/γ‐Al 33 (see Figure 5). The conversion of 4‐ethylphenol  experiments were conducted over 3 wt. % Pd/γ‐Al 24‐ethylphenol  O3 (see Figure 5). The conversion of 4‐ethylphenol  experiments were conducted over 3 wt. % Pd/γ‐Al O 3 (see Figure 5). The conversion of 4‐ethylphenol  2.4. The Probable Reaction Pathway of 4‐Ethylphenol Hydrogenation  2.4. The Probable Reaction Pathway of 4‐Ethylphenol Hydrogenation  gradually increased with prolonging reaction time, and 4‐ethylphenol was almost converted when  gradually increased with prolonging reaction time, and 4‐ethylphenol was almost converted when  a Reaction conditions: reactant (0.82 mmol), H a Reaction conditions: reactant (0.82 mmol), H experiments were conducted over 3 wt. % Pd/γ‐Al 2O 3 (see Figure 5). The conversion of 4‐ethylphenol  2O (25 2O (25 mL), 3 wt. % Pd/γ‐Al mL), 3 wt. % Pd/γ‐Al 2O3 (0.200 2O3 (0.200 g, 0.056 mmol Pd ),  g, 0.056 mmol Pd ),  gradually increased with prolonging reaction time, and 4‐ethylphenol was almost converted when  gradually increased with prolonging reaction time, and 4‐ethylphenol was almost converted when  experiments were conducted over 3 wt. % Pd/γ‐Al 22O 33 (see Figure 5). The conversion of 4‐ethylphenol  experiments were conducted over 3 wt. % Pd/γ‐Al O experiments were conducted over 3 wt. % Pd/γ-Al O (see Figure 5). The conversion of 4-ethylphenol experiments were conducted over 3 wt. % Pd/γ‐Al 2 (see Figure 5). The conversion of 4‐ethylphenol  O 3 (see Figure 5). The conversion of 4‐ethylphenol  gradually increased with prolonging reaction time, and 4‐ethylphenol was almost converted when  gradually increased with prolonging reaction time, and 4‐ethylphenol was almost converted when  2 3 gradually increased with prolonging reaction time, and 4‐ethylphenol was almost converted when  the reaction was performed for 10 h. After 12 h, the conversion of 4‐ethylphenol reached up to 100%.  the reaction was performed for 10 h. After 12 h, the conversion of 4‐ethylphenol reached up to 100%.      gradually increased with prolonging reaction time, and 4‐ethylphenol was almost converted when  60 °C, 2 MPa H 2probable  , 12 h, reaction mixture stirred at 500 rpm. 2probable  , 12 h, reaction mixture stirred at 500 rpm. the reaction was performed for 10 h. After 12 h, the conversion of 4‐ethylphenol reached up to 100%.  the reaction was performed for 10 h. After 12 h, the conversion of 4‐ethylphenol reached up to 100%.  To 60 °C, 2 MPa H To  study  study  the the  reaction  reaction  pathway  pathway  of  4‐ethylphenol  of and 4‐ethylphenol  hydrogenation,  hydrogenation,  a  series  a  series  of  kinetic  of when kinetic  gradually increased with prolonging reaction time, and 4‐ethylphenol was almost converted when  gradually increased with prolonging reaction time, and 4‐ethylphenol was almost converted when  gradually increased with prolonging reaction time, and 4‐ethylphenol was almost converted when  gradually increased with prolonging reaction time, 4-ethylphenol was almost converted the reaction was performed for 10 h. After 12 h, the conversion of 4‐ethylphenol reached up to 100%.  the reaction was performed for 10 h. After 12 h, the conversion of 4‐ethylphenol reached up to 100%.  the reaction was performed for 10 h. After 12 h, the conversion of 4‐ethylphenol reached up to 100%.  At the initial stage of the reaction, the predominant products were 4‐ethylcyclohexanone, and the  At the initial stage of the reaction, the predominant products were 4‐ethylcyclohexanone, and the  the reaction was performed for 10 h. After 12 h, the conversion of 4‐ethylphenol reached up to 100%.  At the initial stage of the reaction, the predominant products were 4‐ethylcyclohexanone, and the  At the initial stage of the reaction, the predominant products were 4‐ethylcyclohexanone, and the  experiments were conducted over 3 wt. % Pd/γ‐Al experiments were conducted over 3 wt. % Pd/γ‐Al 2 O 3 2  (see Figure 5). The conversion of 4‐ethylphenol  O 3  (see Figure 5). The conversion of 4‐ethylphenol  the reaction was performed for 10 h. After 12 h, the conversion of 4‐ethylphenol reached up to 100%.  the reaction was performed for 10 h. After 12 h, the conversion of 4‐ethylphenol reached up to 100%.  the reaction was performed for 10 h. After 12 h, the conversion of 4‐ethylphenol reached up to 100%.  theAt the initial stage of the reaction, the predominant products were 4‐ethylcyclohexanone, and the  reaction was performed for 10 h. After 12 h, the conversion of 4-ethylphenol reached up to 100%. At the initial stage of the reaction, the predominant products were 4‐ethylcyclohexanone, and the  At the initial stage of the reaction, the predominant products were 4‐ethylcyclohexanone, and the  detected  4‐ethylcyclohexanol  was  very  limited.  With  prolonging  the  reaction  time,  the  amount  detected  4‐ethylcyclohexanol  was  very  limited.  With  prolonging  the  reaction  time,  the  amount  of  of   of    2.4. The Probable Reaction Pathway of 4‐Ethylphenol Hydrogenation  2.4. The Probable Reaction Pathway of 4‐Ethylphenol Hydrogenation  At the initial stage of the reaction, the predominant products were 4‐ethylcyclohexanone, and the  detected  4‐ethylcyclohexanol  was  very  limited.  With  prolonging  the  reaction  time,  the  amount  detected  4‐ethylcyclohexanol  was  very  limited.  With  prolonging  the  reaction  time,  the  amount  of  gradually increased with prolonging reaction time, and 4‐ethylphenol was almost converted when  gradually increased with prolonging reaction time, and 4‐ethylphenol was almost converted when  At the initial stage of the reaction, the predominant products were 4‐ethylcyclohexanone, and the  At the initial stage of the reaction, the predominant products were 4‐ethylcyclohexanone, and the  At the initial stage of the reaction, the predominant products were 4‐ethylcyclohexanone, and the  At the initial stage of the reaction, the predominant products were 4-ethylcyclohexanone, and the detected  4‐ethylcyclohexanol  was  very  limited.  With  prolonging  the  reaction  time,  the  amount  of    of     detected  4‐ethylcyclohexanol  was  very  limited.  With  prolonging  the  reaction  time,  the  amount  detected  4‐ethylcyclohexanol  was  very  limited.  With  prolonging  the  reaction  time,  the  amount  of  4‐ethylcyclohexanone was gradually reduced before 6 h, and afterward sharply decreased. On the  4‐ethylcyclohexanone was gradually reduced before 6 h, and afterward sharply decreased. On the  detected  4‐ethylcyclohexanol  was  was  very  very  limited.  limited.  With  With  prolonging  prolonging  the  the  reaction  reaction  time,  time,  the  the  amount  amount  of  of     4‐ethylcyclohexanone was gradually reduced before 6 h, and afterward sharply decreased. On the  4‐ethylcyclohexanone was gradually reduced before 6 h, and afterward sharply decreased. On the  the reaction was performed for 10 h. After 12 h, the conversion of 4‐ethylphenol reached up to 100%.  the reaction was performed for 10 h. After 12 h, the conversion of 4‐ethylphenol reached up to 100%.  detected  4‐ethylcyclohexanol  detected  4‐ethylcyclohexanol  was  very  limited.  With  prolonging  the  reaction  time,  amount  of  To To  study  study  the the  probable  probable  reaction  reaction  pathway  pathway  of  4‐ethylphenol  of  4‐ethylphenol  hydrogenation,  hydrogenation,  a time,  series  a the  series  of amount  kinetic  of  kinetic  detected  4‐ethylcyclohexanol  was  very  limited.  With  prolonging  the  reaction  the  detected 4-ethylcyclohexanol was very limited. With prolonging the reaction time, the amount of  of    4‐ethylcyclohexanone was gradually reduced before 6 h, and afterward sharply decreased. On the  4‐ethylcyclohexanone was gradually reduced before 6 h, and afterward sharply decreased. On the  4‐ethylcyclohexanone was gradually reduced before 6 h, and afterward sharply decreased. On the  contrary, the amount of 4‐ethylcyclohexanol gradually increased before 6 h, and afterward sharply  contrary, the amount of 4‐ethylcyclohexanol gradually increased before 6 h, and afterward sharply  4‐ethylcyclohexanone was gradually reduced before 6 h, and afterward sharply decreased. On the  contrary, the amount of 4‐ethylcyclohexanol gradually increased before 6 h, and afterward sharply  contrary, the amount of 4‐ethylcyclohexanol gradually increased before 6 h, and afterward sharply  At the initial stage of the reaction, the predominant products were 4‐ethylcyclohexanone, and the  At the initial stage of the reaction, the predominant products were 4‐ethylcyclohexanone, and the  4‐ethylcyclohexanone was gradually reduced before 6 h, and afterward sharply decreased. On the  4‐ethylcyclohexanone was gradually reduced before 6 h, and afterward sharply decreased. On the  experiments were conducted over 3 wt. % Pd/γ‐Al experiments were conducted over 3 wt. % Pd/γ‐Al 2O32 (see Figure 5). The conversion of 4‐ethylphenol  O63 (see Figure 5). The conversion of 4‐ethylphenol  4‐ethylcyclohexanone was gradually reduced before 6 h, and afterward sharply decreased. On the  4-ethylcyclohexanone was gradually reduced before h, and afterward sharply decreased. On the contrary, the amount of 4‐ethylcyclohexanol gradually increased before 6 h, and afterward sharply  contrary, the amount of 4‐ethylcyclohexanol gradually increased before 6 h, and afterward sharply  contrary, the amount of 4‐ethylcyclohexanol gradually increased before 6 h, and afterward sharply  improved. Nearly all of the 4‐ethylphenol was converted to 4‐ethylcyclohexanol after 12 h. This result  improved. Nearly all of the 4‐ethylphenol was converted to 4‐ethylcyclohexanol after 12 h. This result  contrary, the amount of 4‐ethylcyclohexanol gradually increased before 6 h, and afterward sharply  improved. Nearly all of the 4‐ethylphenol was converted to 4‐ethylcyclohexanol after 12 h. This result  improved. Nearly all of the 4‐ethylphenol was converted to 4‐ethylcyclohexanol after 12 h. This result  detected  detected  4‐ethylcyclohexanol  4‐ethylcyclohexanol  was  was  very  very  limited.  limited.  With  With  prolonging  prolonging  the  the  reaction  reaction  time,  time,  the the  amount  amount  of   of    contrary, the amount of 4‐ethylcyclohexanol gradually increased before 6 h, and afterward sharply  contrary, the amount of 4‐ethylcyclohexanol gradually increased before 6 h, and afterward sharply  gradually increased with prolonging reaction time, and 4‐ethylphenol was almost converted when  gradually increased with prolonging reaction time, and 4‐ethylphenol was almost converted when  contrary, the amount of 4‐ethylcyclohexanol gradually increased before 6 h, and afterward sharply  contrary, the amount of 4-ethylcyclohexanol gradually increased before 6 h, and afterward sharply improved. Nearly all of the 4‐ethylphenol was converted to 4‐ethylcyclohexanol after 12 h. This result  improved. Nearly all of the 4‐ethylphenol was converted to 4‐ethylcyclohexanol after 12 h. This result  improved. Nearly all of the 4‐ethylphenol was converted to 4‐ethylcyclohexanol after 12 h. This result  suggested that 4‐ethylphenol was firstly hydrogenated to 4‐ethylcyclohexanone as an intermediate, and  suggested that 4‐ethylphenol was firstly hydrogenated to 4‐ethylcyclohexanone as an intermediate, and  improved. Nearly all of the 4‐ethylphenol was converted to 4‐ethylcyclohexanol after 12 h. This result  suggested that 4‐ethylphenol was firstly hydrogenated to 4‐ethylcyclohexanone as an intermediate, and  suggested that 4‐ethylphenol was firstly hydrogenated to 4‐ethylcyclohexanone as an intermediate, and  4‐ethylcyclohexanone was gradually reduced before 6 h, and afterward sharply decreased. On the  4‐ethylcyclohexanone was gradually reduced before 6 h, and afterward sharply decreased. On the  improved. Nearly all of the 4‐ethylphenol was converted to 4‐ethylcyclohexanol after 12 h. This result  improved. Nearly all of the 4‐ethylphenol was converted to 4‐ethylcyclohexanol after 12 h. This result  the reaction was performed for 10 h. After 12 h, the conversion of 4‐ethylphenol reached up to 100%.  the reaction was performed for 10 h. After 12 h, the conversion of 4‐ethylphenol reached up to 100%.  improved. Nearly all of the 4‐ethylphenol was converted to 4‐ethylcyclohexanol after 12 h. This result  improved. Nearly all of the 4-ethylphenol was converted to 4-ethylcyclohexanol after 12 h. This result suggested that 4‐ethylphenol was firstly hydrogenated to 4‐ethylcyclohexanone as an intermediate, and  suggested that 4‐ethylphenol was firstly hydrogenated to 4‐ethylcyclohexanone as an intermediate, and  suggested that 4‐ethylphenol was firstly hydrogenated to 4‐ethylcyclohexanone as an intermediate, and  then 4‐ethylcyclohexanone was further hydrogenated to produce 4‐ethylcyclohexanol. Some prior  then 4‐ethylcyclohexanone was further hydrogenated to produce 4‐ethylcyclohexanol. Some prior  suggested that 4‐ethylphenol was firstly hydrogenated to 4‐ethylcyclohexanone as an intermediate, and  then 4‐ethylcyclohexanone was further hydrogenated to produce 4‐ethylcyclohexanol. Some prior  then 4‐ethylcyclohexanone was further hydrogenated to produce 4‐ethylcyclohexanol. Some prior  contrary, the amount of 4‐ethylcyclohexanol gradually increased before 6 h, and afterward sharply  contrary, the amount of 4‐ethylcyclohexanol gradually increased before 6 h, and afterward sharply  suggested that 4‐ethylphenol was firstly hydrogenated to 4‐ethylcyclohexanone as an intermediate, and  suggested that 4‐ethylphenol was firstly hydrogenated to 4‐ethylcyclohexanone as an intermediate, and  At the initial stage of the reaction, the predominant products were 4‐ethylcyclohexanone, and the  At the initial stage of the reaction, the predominant products were 4‐ethylcyclohexanone, and the  suggested that 4‐ethylphenol was firstly hydrogenated to 4‐ethylcyclohexanone as an intermediate, and  suggested that 4-ethylphenol was firstly hydrogenated to 4-ethylcyclohexanone as an intermediate, then 4‐ethylcyclohexanone was further hydrogenated to produce 4‐ethylcyclohexanol. Some prior  then 4‐ethylcyclohexanone was further hydrogenated to produce 4‐ethylcyclohexanol. Some prior  then 4‐ethylcyclohexanone was further hydrogenated to produce 4‐ethylcyclohexanol. Some prior  research indicates that the hydrogenation of phenols proceeded mainly in a sequential manner [13,32].  research indicates that the hydrogenation of phenols proceeded mainly in a sequential manner [13,32].  then 4‐ethylcyclohexanone was further hydrogenated to produce 4‐ethylcyclohexanol. Some prior  research indicates that the hydrogenation of phenols proceeded mainly in a sequential manner [13,32].  research indicates that the hydrogenation of phenols proceeded mainly in a sequential manner [13,32].  improved. Nearly all of the 4‐ethylphenol was converted to 4‐ethylcyclohexanol after 12 h. This result  improved. Nearly all of the 4‐ethylphenol was converted to 4‐ethylcyclohexanol after 12 h. This result  then 4‐ethylcyclohexanone was further hydrogenated to produce 4‐ethylcyclohexanol. Some prior  then 4‐ethylcyclohexanone was further hydrogenated to produce 4‐ethylcyclohexanol. Some prior  detected  detected  4‐ethylcyclohexanol  4‐ethylcyclohexanol  was  was  very  very  limited.  limited.  With  With  prolonging  prolonging  the 4-ethylcyclohexanol. the  reaction  reaction  time,  time,  the the  amount  amount  of   of    then 4‐ethylcyclohexanone was further hydrogenated to produce 4‐ethylcyclohexanol. Some prior  and then 4-ethylcyclohexanone was further hydrogenated to produce Some prior research indicates that the hydrogenation of phenols proceeded mainly in a sequential manner [13,32].  research indicates that the hydrogenation of phenols proceeded mainly in a sequential manner [13,32].  research indicates that the hydrogenation of phenols proceeded mainly in a sequential manner [13,32].  The  benzene  ring  of phenols  phenols  was  partially  hydrogenated  to an  an  enol  in the  the  first  step,  which  was  The  benzene  ring  of phenols  phenols  was  partially  hydrogenated  to an  an  enol  in the  the  first  step,  which  was  research indicates that the hydrogenation of phenols proceeded mainly in a sequential manner [13,32].  The  benzene  ring  of  was  partially  hydrogenated  to  enol  in  first  step,  which  was  The  benzene  ring  of  was  partially  hydrogenated  to  enol  in  first  step,  which  was  suggested that 4‐ethylphenol was firstly hydrogenated to 4‐ethylcyclohexanone as an intermediate, and  suggested that 4‐ethylphenol was firstly hydrogenated to 4‐ethylcyclohexanone as an intermediate, and  research indicates that the hydrogenation of phenols proceeded mainly in a sequential manner [13,32].  research indicates that the hydrogenation of phenols proceeded mainly in a sequential manner [13,32].  4‐ethylcyclohexanone was gradually reduced before 6 h, and afterward sharply decreased. On the  4‐ethylcyclohexanone was gradually reduced before 6 h, and afterward sharply decreased. On the  research indicates that the hydrogenation of phenols proceeded mainly in a sequential manner [13,32].  research indicates that the hydrogenation of phenols proceeded mainly in athe  sequential manner [13,32]. The  benzene  ring  of  phenols  was  partially  hydrogenated  to  an  enol  in  first  step,  which  was  The  benzene  ring  of  phenols  was  partially  hydrogenated  to  an  enol  in  the  first  step,  which  was  The  benzene  ring  of  phenols  was  partially  hydrogenated  to  an  enol  in  the  first  step,  which  was  unstable  and  isomerizes  rapidly  to  form  cyclohexanone;  and  cyclohexanone  can  be  converted  into  unstable  and  isomerizes  rapidly  to  form  cyclohexanone;  and  cyclohexanone  can  be  converted  into  The  benzene  ring  of  phenols  phenols  was  partially  hydrogenated  to  an cyclohexanone  enol  in  in  the  the  can  first  step,  which  was  was  unstable  and  isomerizes  rapidly  to partially  form  cyclohexanone;  and  can  be  converted  into  unstable  and  isomerizes  rapidly  to  form  cyclohexanone;  and  cyclohexanone  be  converted  into  then 4‐ethylcyclohexanone was further hydrogenated to produce 4‐ethylcyclohexanol. Some prior  then 4‐ethylcyclohexanone was further hydrogenated to produce 4‐ethylcyclohexanol. Some prior  The  benzene  ring  of  was  partially  hydrogenated  to  an  enol  first  step,  which  The  benzene  ring  of  phenols  was  partially  hydrogenated  to  an  enol  in  the  first  step,  which  was  contrary, the amount of 4‐ethylcyclohexanol gradually increased before 6 h, and afterward sharply  contrary, the amount of 4‐ethylcyclohexanol gradually increased before 6 h, and afterward sharply  The  benzene  ring  of  rapidly  phenols  was  hydrogenated  to cyclohexanone  an  enol  in  the  first  step,  which  was  unstable  and  isomerizes  to  form  cyclohexanone;  and  cyclohexanone  can  be  converted  into  unstable  and  isomerizes  rapidly  to  form  cyclohexanone;  and  can  be  converted  into  unstable  and  isomerizes  rapidly  to  form  cyclohexanone;  and  cyclohexanone  can  be  converted  into  cyclohexanol  in  further  hydrogenation  process  [32].  Thus,  it was  was  inferred  that  4‐ethylphenol  was  cyclohexanol  in a  a  further  hydrogenation  process  [32].  Thus,  it was  was  inferred  that  4‐ethylphenol  was  unstable  and  isomerizes  isomerizes  rapidly  to  form  form  cyclohexanone;  cyclohexanone;  and  cyclohexanone  can  be  converted  into  cyclohexanol  in  a a further  further  hydrogenation  process  [32].  Thus,  it  inferred  that  4‐ethylphenol  was  cyclohexanol  in  further  hydrogenation  process  [32].  Thus,  it  inferred  that  4‐ethylphenol  was  research indicates that the hydrogenation of phenols proceeded mainly in a sequential manner [13,32].  research indicates that the hydrogenation of phenols proceeded mainly in a sequential manner [13,32].  unstable  and  rapidly  to  and  cyclohexanone  can  be  into  unstable  and  isomerizes  rapidly  to  form  cyclohexanone;  and  cyclohexanone  can  be converted  converted  into  improved. Nearly all of the 4‐ethylphenol was converted to 4‐ethylcyclohexanol after 12 h. This result  improved. Nearly all of the 4‐ethylphenol was converted to 4‐ethylcyclohexanol after 12 h. This result  unstable  and  isomerizes  rapidly  to  form  cyclohexanone;  and  cyclohexanone  can  be  converted  into  cyclohexanol  in  a  further  hydrogenation  process  [32].  Thus,  it  was  inferred  that  4‐ethylphenol  was  cyclohexanol  in  a  hydrogenation  process  [32].  Thus,  it  was  inferred  that  4‐ethylphenol  was  cyclohexanol  in  a  further  hydrogenation  process  [32].  Thus,  it  was  inferred  that  4‐ethylphenol  was  hydrogenated  to  an  enol  and  rapidly  isomerized  to  form  4‐ethylcyclohexanone  in  the  first  step,  and  hydrogenated  to  an  enol  and  rapidly  isomerized  to  form  4‐ethylcyclohexanone  in  the  first  step,  and      cyclohexanol  in  a  further  hydrogenation  process  [32].  Thus,  it  was  inferred  that  4‐ethylphenol  was  hydrogenated  to  an  enol  and  rapidly  isomerized  to  form  4‐ethylcyclohexanone  in  the  first  step,  and  hydrogenated  to a  an  enol  and  rapidly  isomerized  to  form  4‐ethylcyclohexanone  in  the  first  step,  and  The  The  benzene  benzene  ring  ring  of  phenols  of  phenols  was  was  partially  partially  hydrogenated  hydrogenated  to  to  enol  enol  in  the  in that  the  first  first  step,  step,  which  which  was  was  cyclohexanol  in  further  hydrogenation  process  [32].  Thus,  it  was  inferred  4‐ethylphenol  was  cyclohexanol  in  a  further  hydrogenation  process  [32].  Thus,  it an  was  inferred  that  4‐ethylphenol  suggested that 4‐ethylphenol was firstly hydrogenated to 4‐ethylcyclohexanone as an intermediate, and  suggested that 4‐ethylphenol was firstly hydrogenated to 4‐ethylcyclohexanone as an intermediate, and  cyclohexanol  in  a  further  hydrogenation  process  [32].  Thus,  it an  was  inferred  that  4‐ethylphenol  was  hydrogenated  to  an  enol  and  rapidly  isomerized  to  form  4‐ethylcyclohexanone  in  the  first  step,  and         hydrogenated  to  an  enol  and  rapidly  isomerized  to  form  4‐ethylcyclohexanone  in  the  first  step,  and  hydrogenated  to  an  enol  and  rapidly  isomerized  to  form  4‐ethylcyclohexanone  in  the  first  step,  and  4‐ethylcyclohexanone was further hydrogenated into 4‐ethylcyclohexanol in a subsequent process.  4‐ethylcyclohexanone was further hydrogenated into 4‐ethylcyclohexanol in a subsequent process.  hydrogenated  to  an  an  enol  enol  and  and  rapidly  isomerized  to  form  form  4‐ethylcyclohexanone  4‐ethylcyclohexanone  in can  the  first  step,  and  and  4‐ethylcyclohexanone was further hydrogenated into 4‐ethylcyclohexanol in a subsequent process.  4‐ethylcyclohexanone was further hydrogenated into 4‐ethylcyclohexanol in a subsequent process.  unstable  unstable  and  and  isomerizes  rapidly  rapidly  to  form  to  form  cyclohexanone;  cyclohexanone;  and  and  cyclohexanone  cyclohexanone  can  be  converted  converted  into  into  hydrogenated  to  rapidly  isomerized  to  in  first  step,        hydrogenated  to  isomerizes  an  enol  and  rapidly  isomerized  to  form  4‐ethylcyclohexanone  in the  the  first  step,  and  then 4‐ethylcyclohexanone was further hydrogenated to produce 4‐ethylcyclohexanol. Some prior  then 4‐ethylcyclohexanone was further hydrogenated to produce 4‐ethylcyclohexanol. Some prior  hydrogenated  to  an  enol  and  rapidly  isomerized  to  form  4‐ethylcyclohexanone  in  be  the  first  step,  and  4‐ethylcyclohexanone was further hydrogenated into 4‐ethylcyclohexanol in a subsequent process.  4‐ethylcyclohexanone was further hydrogenated into 4‐ethylcyclohexanol in a subsequent process.  4‐ethylcyclohexanone was further hydrogenated into 4‐ethylcyclohexanol in a subsequent process.  4‐ethylcyclohexanone was further hydrogenated into 4‐ethylcyclohexanol in a subsequent process.  cyclohexanol  cyclohexanol  in  a  in further  a  further  hydrogenation  hydrogenation  process  process  [32].  [32].  Thus,  Thus,  it  was  it  was  inferred  inferred  that  that  4‐ethylphenol  4‐ethylphenol  was  was  4‐ethylcyclohexanone was further hydrogenated into 4‐ethylcyclohexanol in a subsequent process.  4‐ethylcyclohexanone was further hydrogenated into 4‐ethylcyclohexanol in a subsequent process.  research indicates that the hydrogenation of phenols proceeded mainly in a sequential manner [13,32].  research indicates that the hydrogenation of phenols proceeded mainly in a sequential manner [13,32].  4‐ethylcyclohexanone was further hydrogenated into 4‐ethylcyclohexanol in a subsequent process.  hydrogenated  hydrogenated  to  an  to  an  enol  enol  and  and  rapidly  rapidly  isomerized  isomerized  to  form  to  form  4‐ethylcyclohexanone  4‐ethylcyclohexanone  in  the  in  the  first  first  step,  step,  and  and      The  The  benzene  benzene  ring  ring  of  phenols  of  phenols  was  was  partially  partially  hydrogenated  hydrogenated  to  an  to  an  enol  enol  in  the  in  the  first  first  step,  step,  which  which  was  was 

improved. Nearly all of the 4‐ethylphenol was converted to 4‐ethylcyclohexanol after 12 h. This result  suggested that 4‐ethylphenol was firstly hydrogenated to 4‐ethylcyclohexanone as an intermediate, and  then 4‐ethylcyclohexanone was further hydrogenated to produce 4‐ethylcyclohexanol. Some prior  research indicates that the hydrogenation of phenols proceeded mainly in a sequential manner [13,32].  Catalysts 2016, 6, 12 8 of 13 The  benzene  ring  of  phenols  was  partially  hydrogenated  to  an  enol  in  the  first  step,  which  was  unstable The and  isomerizes  rapidly  to  form  cyclohexanone;  and  cyclohexanone  can  be  converted  into  benzene ring of phenols was partially hydrogenated to an enol in the first step, which was cyclohexanol  in and a  further  hydrogenation  [32].  Thus,  it  was  inferred  that  4‐ethylphenol  was  unstable isomerizes rapidly to formprocess  cyclohexanone; and cyclohexanone can be converted into hydrogenated  to  an inenol  and  hydrogenation rapidly  isomerized  form  4‐ethylcyclohexanone  in  the  first  cyclohexanol a further process to  [32]. Thus, it was inferred that 4-ethylphenol wasstep,  and    hydrogenated to an enol and rapidly isomerized to form 4-ethylcyclohexanone in the first step, and 4‐ethylcyclohexanone was further hydrogenated into 4‐ethylcyclohexanol in a subsequent process.  4-ethylcyclohexanone was further hydrogenated into 4-ethylcyclohexanol in a subsequent process.

100

100

O

80

60

60 OH

40

40

20

20

0

Conversion (%)

Selectivity (%)

80

0 0

2

4

6

8

10

12

Reaction time (h)

 

Figure 5. Effect of reaction time on the conversion of 4-ethylphenol and selectivity to products. Reaction

Figure  5.  Effect  of  reaction  time  on  the  conversion  of  4‐ethylphenol  and  selectivity  to ˝ products.  conditions: reactant (0.82 mmol), H2 O (25 mL), 3 wt. % Pd/γ-Al2 O3 (0.200 g/0.056 mmol Pd), 60 C, Reaction conditions: reactant (0.82 mmol), H 2O (25 mL), 3 wt. % Pd/γ‐Al2O3 (0.200 g/0.056 mmol Pd),  2 MPa H2 . 60 °C, 2 MPa H2.   

2.5. The Hydrogenation of Phenols Directly from Raw Biomass on Pd/γ-Al2 O3 Given the good activity of the Pd/γ-Al2 O3 catalyst for the hydrogenation of phenols, the catalyst was also applied to the hydrogenation of monophenols directly derived from the raw biomass system. In our previous work [38], a liquid mixture with lignin-derived phenols was obtained from the selective degradation of lignin in pubescens via a two-step process in an ethanol solvent, and 4-ethylphenol was the primary monophenols identified. So we chose the liquid mixture as our reaction substrate for the hydrogenation of monophenols in the raw biomass system. 4-ethylcyclohexanone and 4-ethylcyclohexanol as the hydrogenation products of 4-ethylphenol were detected, and the amount of 4-ethylcyclohexanol was three times that of 4-ethylcyclohexanone. This indicated that 4-ethylphenol in a liquid mixture derived from raw biomass was able to be hydrogenated under this catalytic system. In addition, some other monophenols such as guaiacol, 4-methyl guaiacol, 2,6-dimethoxyphenol and syringaldehyde was also able to be partly hydrogenated (see Table 4). Therefore, we deem that the catalyst exhibits a good activity for the hydrogenation of phenols directly derived from a raw biomass system. Moreover, it was unexpected that the yield of 4-ethylphenol after hydrogenation treatment would greatly increase, though its hydrogenation products were definitely detected. According to our previous investigations [38], it was found that many lignin oligomers existed in the liquid mixture, since the yield of identified monophenols was less than 10%. Therefore, we speculate that the Pd/γ-Al2 O3 catalyst is favorable for the depolymerization of lignin oligomers for producing monophenols, especially 4-ethylphenol under very mild conditions. It is known that fairly high temperatures are usually required for the depolymerization of lignin to monophenols. Thus, this catalytic system might provide a promising approach to yield monophenols directly from lignin under milder conditions in an aqueous solution instead of the conventional sever reaction conditions in an organic solvent.

hydrogenation treatment would greatly increase, though its hydrogenation products were definitely  hydrogenation treatment would greatly increase, though its hydrogenation products were definitely  detected.  According  to  our  previous  investigations  [38],  it  was  found  that  many  lignin  oligomers  hydrogenation treatment would greatly increase, though its hydrogenation products were definitely  detected.  According  to  our  previous  investigations  [38],  it  was  found  that  many  lignin  oligomers  detected.  According  to  our  previous  investigations  [38],  it  was  found  that  many  lignin  oligomers  hydrogenation treatment would greatly increase, though its hydrogenation products were definitely  detected.  According  to  our  previous  investigations  [38],  it  was  found  that  many  lignin  oligomers  detected.  According  to  our  previous  investigations  [38],  it  was  found  that  many  lignin  oligomers  detected.  According  to  our  previous  investigations  [38],  it  was  found  that  many  lignin  oligomers  hydrogenation treatment would greatly increase, though its hydrogenation products were definitely  hydrogenation treatment would greatly increase, though its hydrogenation products were definitely  detected.  According  to  our  previous  investigations  [38],  it  was  found  that  many  lignin  oligomers  detected.  According  to  our  previous  investigations  [38],  was  found  that  many  lignin  oligomers  detected.  According  to  our  previous  investigations  [38],  it it  was  found  that  many  lignin  oligomers  existed in the liquid mixture, since the yield of identified monophenols was less than 10%. Therefore,  detected.  According  to  our  previous  investigations  [38],  it  was  found  that  many  lignin  oligomers  existed in the liquid mixture, since the yield of identified monophenols was less than 10%. Therefore,  existed in the liquid mixture, since the yield of identified monophenols was less than 10%. Therefore,  detected.  According  to  our  previous  investigations  [38],  it  was  found  that  many  lignin  oligomers  existed in the liquid mixture, since the yield of identified monophenols was less than 10%. Therefore,  existed in the liquid mixture, since the yield of identified monophenols was less than 10%. Therefore,  existed in the liquid mixture, since the yield of identified monophenols was less than 10%. Therefore,  detected.  According  to  our  previous  investigations  [38],  it  was  found  that  many  lignin  oligomers  detected.  According  to  our  previous  investigations  [38],  it  was  found  that  many  lignin  oligomers  existed in the liquid mixture, since the yield of identified monophenols was less than 10%. Therefore,  existed in the liquid mixture, since the yield of identified monophenols was less than 10%. Therefore,  existed in the liquid mixture, since the yield of identified monophenols was less than 10%. Therefore,  we speculate that the Pd/γ‐Al 2O 3 catalyst is favorable for the depolymerization of lignin oligomers  existed in the liquid mixture, since the yield of identified monophenols was less than 10%. Therefore,  we speculate that the Pd/γ‐Al 2O 3 catalyst is favorable for the depolymerization of lignin oligomers  we speculate that the Pd/γ‐Al 2O 3 catalyst is favorable for the depolymerization of lignin oligomers  existed in the liquid mixture, since the yield of identified monophenols was less than 10%. Therefore,  we speculate that the Pd/γ‐Al 23O 3 catalyst is favorable for the depolymerization of lignin oligomers  we speculate that the Pd/γ‐Al 2O 3 catalyst is favorable for the depolymerization of lignin oligomers  we speculate that the Pd/γ‐Al 2O  catalyst is favorable for the depolymerization of lignin oligomers  existed in the liquid mixture, since the yield of identified monophenols was less than 10%. Therefore,  existed in the liquid mixture, since the yield of identified monophenols was less than 10%. Therefore,  we speculate that the Pd/γ‐Al 2O 3 catalyst is favorable for the depolymerization of lignin oligomers  we speculate that the Pd/γ‐Al 2O 3 catalyst is favorable for the depolymerization of lignin oligomers  we speculate that the Pd/γ‐Al 2O 3 catalyst is favorable for the depolymerization of lignin oligomers  for producing monophenols, especially 4‐ethylphenol under very mild conditions. It is known that  we speculate that the Pd/γ‐Al 2O3 catalyst is favorable for the depolymerization of lignin oligomers  for producing monophenols, especially 4‐ethylphenol under very mild conditions. It is known that  for producing monophenols, especially 4‐ethylphenol under very mild conditions. It is known that  we speculate that the Pd/γ‐Al 3 catalyst is favorable for the depolymerization of lignin oligomers  for producing monophenols, especially 4‐ethylphenol under very mild conditions. It is known that  for producing monophenols, especially 4‐ethylphenol under very mild conditions. It is known that  for producing monophenols, especially 4‐ethylphenol under very mild conditions. It is known that  we speculate that the Pd/γ‐Al 2O 3 catalyst is favorable for the depolymerization of lignin oligomers  we speculate that the Pd/γ‐Al 2O23O  catalyst is favorable for the depolymerization of lignin oligomers  for producing monophenols, especially 4‐ethylphenol under very mild conditions. It is known that  for producing monophenols, especially 4‐ethylphenol under very mild conditions. It is known that  for producing monophenols, especially 4‐ethylphenol under very mild conditions. It is known that  fairly  high  temperatures  are  usually  required  for  the  depolymerization  of  lignin  to  monophenols.  for producing monophenols, especially 4‐ethylphenol under very mild conditions. It is known that  fairly  high  temperatures  are  usually  required  for  the  depolymerization  of  lignin  to  monophenols.  fairly  high  temperatures  are  usually  required  for  the  depolymerization  of  lignin  to  monophenols.  for producing monophenols, especially 4‐ethylphenol under very mild conditions. It is known that  fairly  high  temperatures  are  usually  required  for  the  depolymerization  of  lignin  to  monophenols.  fairly  high  temperatures  are  usually  required  for  the  depolymerization  of  lignin  to  monophenols.  fairly  high  temperatures  are  usually  required  for  the  depolymerization  of  lignin  to  monophenols.  for producing monophenols, especially 4‐ethylphenol under very mild conditions. It is known that  for producing monophenols, especially 4‐ethylphenol under very mild conditions. It is known that  fairly  high  temperatures  are  usually  required  for  the  depolymerization  of  lignin  to  monophenols.  fairly  high  temperatures  are  usually  required  for  the  depolymerization  of  lignin  to  monophenols.  fairly  high  temperatures  are  usually  required  for  the  depolymerization  of  lignin  to  monophenols.  Catalysts 2016, 6, 12 9 of 13 Thus, this catalytic system might provide a promising approach to yield monophenols directly from  fairly  high  temperatures  are  usually  required  for  the  depolymerization  of  lignin  to  monophenols.  Thus, this catalytic system might provide a promising approach to yield monophenols directly from  Thus, this catalytic system might provide a promising approach to yield monophenols directly from  fairly  high  temperatures  are  usually  required  for  the  depolymerization  of  lignin  to  monophenols.  Thus, this catalytic system might provide a promising approach to yield monophenols directly from  Thus, this catalytic system might provide a promising approach to yield monophenols directly from  Thus, this catalytic system might provide a promising approach to yield monophenols directly from  fairly  high  temperatures  are  usually  required  for  the  depolymerization  of  lignin  to  monophenols.  fairly  high  temperatures  are  usually  required  for  the  depolymerization  of  lignin  to  monophenols.  Thus, this catalytic system might provide a promising approach to yield monophenols directly from  Thus, this catalytic system might provide a promising approach to yield monophenols directly from  Thus, this catalytic system might provide a promising approach to yield monophenols directly from  lignin  under  milder  conditions  in  an  aqueous  solution  instead  of  the  conventional  sever  reaction  Thus, this catalytic system might provide a promising approach to yield monophenols directly from  lignin  under  milder  conditions  in  an  aqueous  solution  instead  of  the  conventional  sever  reaction  lignin  under  milder  conditions  in  an  aqueous  solution  instead  of  the  conventional  sever  reaction  Thus, this catalytic system might provide a promising approach to yield monophenols directly from  lignin  under  milder  conditions  in  an  aqueous  solution  instead  of  the  conventional  sever  reaction  lignin  under  milder  conditions  in  an  aqueous  solution  instead  of  the  conventional  sever  reaction  lignin  under  milder  conditions  in  an  aqueous  solution  instead  of  the  conventional  sever  reaction  Thus, this catalytic system might provide a promising approach to yield monophenols directly from  Thus, this catalytic system might provide a promising approach to yield monophenols directly from  lignin  under  milder  conditions  in  an  aqueous  solution  instead  of  the  conventional  sever  reaction  lignin  under  milder  conditions  in  an  aqueous  solution  instead  of  the  conventional  sever  reaction  lignin  under  milder  conditions  in  an  aqueous  solution  instead  of  the  conventional  sever  reaction  conditions in an organic solvent.  lignin  under  milder  conditions  in  an  aqueous  solution  instead  of  the  conventional  sever  reaction  conditions in an organic solvent.  conditions in an organic solvent.  lignin  under  milder  conditions  in  an  aqueous  solution  instead  of  the  conventional  sever  reaction  conditions in an organic solvent.  conditions in an organic solvent.  conditions in an organic solvent.  lignin  under  milder  conditions  in  an  aqueous  solution  instead  of  the  conventional  sever  reaction  lignin  under  milder  conditions  in  an  aqueous  solution  instead  of  the  conventional  sever  reaction  Table 4. The yield of monophenols before and after the catalytic hydrogenation of lignin-derived conditions in an organic solvent.  conditions in an organic solvent.  conditions in an organic solvent.  conditions in an organic solvent.  conditions in an organic solvent.  conditions in an organic solvent.  conditions in an organic solvent.  liquid mixture. Table  4.  The  yield  of  monophenols  before  and  after  the  catalytic  hydrogenation  of  lignin‐derived  Table  4.  The  yield  of  monophenols  before  and  after  the  catalytic  hydrogenation  of  lignin‐derived  Table  4.  The  yield  of  monophenols  before  and  after  the  catalytic  hydrogenation  of  lignin‐derived  Table  4.  The  yield  of  monophenols  before  and  after  the  catalytic  hydrogenation  of  lignin‐derived  Table  4.  The  yield  of  monophenols  before  and  after  the  catalytic  hydrogenation  of  lignin‐derived  Table  4.  The  yield  of  monophenols  before  and  after  the  catalytic  hydrogenation  of  lignin‐derived  Table  4.  The  yield  of  monophenols  before  and  after  the  catalytic  hydrogenation  of  lignin‐derived  Table  4.  The  yield  of  monophenols  before  and  after  the  catalytic  hydrogenation  of  lignin‐derived  Table  4.  The  yield  of  monophenols  before  and  after  the  catalytic  hydrogenation  of  lignin‐derived  liquid mixture.  liquid mixture.  liquid mixture.  Table  4.  The  yield  of  monophenols  before  and  after  the  catalytic  hydrogenation  of  lignin‐derived  liquid mixture.  Table  4.  The  yield  of  monophenols  before  and  after  the  catalytic  hydrogenation  of  lignin‐derived  liquid mixture.  liquid mixture.  Table  4.  The  yield  of  monophenols  before  and  after  the  catalytic  hydrogenation  of  lignin‐derived  Table  4.  The  yield  of  monophenols  before  and  after  the  catalytic  hydrogenation  of  lignin‐derived  liquid mixture.  liquid mixture.  liquid mixture.  Yield (mg) liquid mixture.  Entry Monophenols Hydrogenation Products (mg) liquid mixture.  liquid mixture.  liquid mixture.  Yield (mg) Yield (mg) Yield (mg) Yield (mg) Yield (mg) Yield (mg) a b Entry  Monophenols  Hydrogenation Products (mg)  Entry  Monophenols  Hydrogenation Products (mg)  Entry  Monophenols  Hydrogenation Products (mg)  Before Yield (mg) After Yield (mg) Yield (mg) Entry  Monophenols  Monophenols  Before Hydrogenation Products (mg)  Entry  Monophenols  Hydrogenation Products (mg)  aaa bb b Entry  Hydrogenation Products (mg)  Before After After Before After Yield (mg) Entry  Monophenols  Monophenols  Before Hydrogenation Products (mg)  a a a After bb b Entry  Monophenols  Hydrogenation Products (mg)  Entry  Hydrogenation Products (mg)  Before After Before After Yield (mg) Yield (mg) a b Yield (mg) a b a b Entry  Monophenols  Hydrogenation Products (mg)  Before After Beforea Afterb Before After OCH OCH OCH OCH OCH OCH Entry  Monophenols  OCH OCH 3Hydrogenation Products (mg)  333 OCH 3 3 3 Entry  Monophenols  Monophenols  Hydrogenation Products (mg)  3 Entry  Hydrogenation Products (mg)  3 OCH OCH OCH OCH OCH Before OCH OCH a a After b b OCH OCH 33 3 33 3 a b 33 3 Before After OCH OCH OCH OCH Before After After OCH Before OCH OCH 3 3 OCH OCH 3 3OH 3 3O 3 OH 3 3OH OH OO OH OH OCH OCH OCH3 OH 3 OH 3 OO O OH OH OH OCH OCH OCH 3OH 3.4  1.5  1.2  0.7  3 3O 1 1  3.4  1.5  1.2  0.7  OCHOCH OCHOCH 1 1  3.4  1.5  1.2  0.7  11  3.4 1.5 1.2 0.7 3 OH 3OH OCH 3 OH 3 O 3OH O OH 3OCH OH 1  3.4  1.5  1.2  0.7  3.4  1.5  1.2  0.7  3.4  1.5  1.2  0.7  O OH OH 3.4  1.5  1.2  0.7  3.4  1.5  1.2  0.7  1 1 1  3.4  1.5  1.2  0.7  O OH OH O OH O OH OH OH 1  1 1  3.4  1.5  1.2  0.7        3.4  1.5  1.2  0.7  1  3.4  3.4  1.5  1.5  1.2  1.2                    0.7  0.7  OCH OCH OCH OCH OCH OCH OCH OCH OCH 3 3 3   3 3 3 3 3 3 OCH OCH OCH OCH OCH OCH OCH OCH OCH 33 3   33 3 3 3 3  OCH OCH OCH OCH OCH OCH OCH OCH 3  OH    3 O OCH 3OH 3 3OH 3 3O 3 3OH   OH OH O   OH    OCH3 OH OCH3 O OCH3 OH OH c‐  cc c c  OH OO OH OCH OCH 2  3.8  2.7  2  3.8  2.7  ‐    ‐ ‐   ‐  OCH 2  3.8  2.7  ‐  ‐ cc- ‐  c  3 OH OCH 3O OCH OCH c  cc   c c OCH 3 OH OCH c OCH 3OH O OH 3 3 3 3 3 OH O OH 2  3.8  2.7  ‐  2  3.8  2.7  ‐  3.8 2.7 22 2 2  3.8  2.7  ‐    ‐  c  c c OH O OH 3.8  2.7  ‐  3.8  2.7  2  3.8  2.7  ‐ c‐     ‐ c‐ c ‐ c c  OH O OH OH O OH OH O OH 2  2 2  3.8  2.7  ‐    ‐    c  c c  c         c c     3.8  2.7  ‐  ‐  2  3.8  3.8  2.7  2.7  ‐    ‐                    ‐    ‐    OCH OCH OCH OCH OCH OCH OCH OCH OCH 333 333 333     OCH OCH OCH OCH OCH OCH OCH OCH OCH 33 3 33 3 33 3        OCH OCH OCH OCH OCH OCH OCH OCH 3 OCH 3 3 OH       3 3O 3 3OH 3 3OH OH O   OH O OH OCH OCH OCH3 OH 3 OO O 3 OH OH OH OH OH OCH OCH c OCH c c cc 3 3O 3 OH OCH3OCH OCH3OCH 3 OH OCH3OCH 3OH 4.4  3.7  3OH 3 3  4.4  3.7  ‐ ‐   ‐  ‐ ‐   ‐  3 3  4.4  3.7  ‐ c ‐  ‐ cc ‐  O OH c   c   c   c   O OH 3  4.4  3.7  4.4  3.7  3  4.4  3.7  ‐  ‐  OH O O OH 4.4  3.7  4.4  3.7  4.4  3.7  ‐ c‐ c- ‐ cc c  ‐ c‐ -c ‐ cc c  33 3 3  4.4 3.7 OH OH O OHOH OH O OH 3  4.4  3.7  ‐    ‐    c OCH OCH OCH OCH OCH OCH OCH 4.4  3.7  OCH OCH 3 3  3    33 4.4  3.7  333 3  3 3  4.4  3.7  ‐ c ‐ c‐   c    3   ‐ c ‐  ‐   c  OCH OCH OCH OCH OCH OCH OCH OCH OCH 33 3  3   3      3   33 3 OCH OCH OCH OCH OCH OCH OCH OCH 3 3  3 3    OCH 3OCH 3OCH OCH 3  3 3   3 33   OCH OCH OCH OCH 3 3 33 OCH OCH 3    3  3   3 OCH OCH OCH OCH 3OH OCH 3OH OCH OCH OCH 3 OCH OCH 3 3 OCH OCH 3 3  OH 3   3  3   ‐ c‐  OH OCH3 OH OH cc cc cc cc OCH 25.9  3.4  4 4  25.9  3.4  ‐ ‐   ‐  ‐ ‐   ‐  ‐ ‐   ‐  4 4  25.9  3.4  ‐ cc ‐  ‐ cc ‐  ‐ cc ‐  ‐ c  ‐  3 OH OCH c   c   c   c   c   c   c   c   OH 3OH 3OCH 4  25.9  3.4  25.9  3.4  4  25.9  3.4  ‐  ‐  ‐  ‐ ‐  c c c c c c OH OOO 25.9  3.4  25.9  3.4  4 4 4  25.9  3.4  ‐ c‐ cc ‐     ‐ c‐ c ‐ c    ‐ c‐ c ‐ c    ‐ c‐ c ‐ cc c  OHOH OH OCH OCH OCH 4  4 4  OOOOOO 25.9  3.4  ‐    ‐    ‐    ‐    44  25.9 3.4 c c c 3 3 3       25.9  OCH 25.9  3.4  OCH OCH 25.9  3.4  3.4  ‐ c ‐  ‐   c OOO ‐ c ‐  ‐   c  ‐ c ‐  ‐   c OH ‐ c ‐ c‐   c  3  3  3   OCH OH O OH OH OCH OH OH OCH 3  3  3   OH OH OH OH OH OH O OO OO O OCH 3  3 OH OH OH OH OCH OH OH O OO OCH OCH 27.9  49.2  2.5  12.1  5 5  27.9  49.2  2.5  12.1  5 5  27.9  49.2  2.5  12.1  OH3     3   27.9  OH O 5  27.9  49.2  2.5  12.1  27.9  49.2  2.5  12.1  5  49.2  2.5  12.1  OHOH OHOH O O O 2.5  27.9  49.2  2.5  12.1  27.9  49.2  2.5  12.1  5 5 5  27.9  49.2  12.1      OH     OH      12.1       27.9  5  49.2  2.5  5  27.9  49.2  2.5  12.1  5 27.9 49.2 2.5 12.1 5  27.9  49.2  2.5    5  27.9  49.2  2.5  12.1      a The yield of the monophenols from lignin‐derived liquid mixture in our previous work [38];  b the    a The yield of the monophenols from lignin‐derived liquid mixture in our previous work [38];  b the    a The yield of the monophenols from lignin‐derived liquid mixture in our previous work [38];  b12.1     the  a b the  a b the  a The yield of the monophenols from lignin‐derived liquid mixture in our previous work [38];        bb the       a The yield of the monophenols from lignin‐derived liquid mixture in our previous work [38];  b the  a The yield of the monophenols from lignin‐derived liquid mixture in our previous work [38];  b the  a The yield of the monophenols from lignin‐derived liquid mixture in our previous work [38];       The yield of the monophenols from lignin‐derived liquid mixture in our previous work [38];   The yield of the monophenols from lignin‐derived liquid mixture in our previous work [38];   the  ayield  b the  yield  of  monophenols  by  the  hydrogenation  of  the  lignin‐derived  liquid  mixture  obtained  in  our  of  monophenols  by  the  hydrogenation  of  the  lignin‐derived  liquid  mixture  obtained  in  our  yield  of  monophenols  by  the  hydrogenation  of  the  lignin‐derived  liquid  mixture  obtained  in  our   The yield of the monophenols from lignin‐derived liquid mixture in our previous work [38];  a b a The yield of the monophenols from lignin‐derived liquid mixture in our previous work [38];  b the  ayield  bin  yield  of  monophenols  by  the  hydrogenation  of  the  lignin‐derived  liquid  mixture  obtained  our   The yield of the monophenols from lignin‐derived liquid mixture in our previous work [38];   the  yield  of  monophenols  by  the  hydrogenation  of  the  lignin‐derived  liquid  mixture  obtained  in  our  of  monophenols  by  the  hydrogenation  of  the  lignin‐derived  liquid  mixture  obtained  in  our   The yield of the monophenols from lignin‐derived liquid mixture in our previous work [38];   the  a The b yield  of  monophenols  by  the  hydrogenation  of  the  lignin‐derived  liquid  mixture  obtained  in  our  yield  of  monophenols  by  the  hydrogenation  of  the  lignin‐derived  liquid  mixture  obtained  in  our  yield of the monophenols from lignin-derived liquid mixture in2Oour previous work [38]; the yield of yield  of  monophenols  by  the  hydrogenation  of  the  lignin‐derived  liquid  mixture  obtained  in  our  2O (25 ml), 3 wt. % Pd/γ‐Al 23  O 3  (0.200 g, 0.056 mmol Pd ), 60 °C,  previous work [38]; reactant (0.45 mmol), H 2O (25 ml), 3 wt. % Pd/γ‐Al (0.200 g, 0.056 mmol Pd ), 60 °C,  previous work [38]; reactant (0.45 mmol), H 2O (25 ml), 3 wt. % Pd/γ‐Al 2O 3  (0.200 g, 0.056 mmol Pd ), 60 °C,  previous work [38]; reactant (0.45 mmol), H yield  of  monophenols  by  the  hydrogenation  of  the  lignin‐derived  liquid  mixture  obtained  in  our  2O (25 ml), 3 wt. % Pd/γ‐Al 23 O 3 liquid  (0.200 g, 0.056 mmol Pd ), 60 °C,  previous work [38]; reactant (0.45 mmol), H yield  of  monophenols  by  the  hydrogenation  of  the  lignin‐derived  liquid  mixture  obtained  in  our  2O (25 ml), 3 wt. % Pd/γ‐Al 2O 3 obtained (0.200 g, 0.056 mmol Pd ), 60 °C,         previous work [38]; reactant (0.45 mmol), H 2O (25 ml), 3 wt. % Pd/γ‐Al 2O (0.200 g, 0.056 mmol Pd ), 60 °C,    [38]; previous work [38]; reactant (0.45 mmol), H yield  of  monophenols  by  the  hydrogenation  of  the  lignin‐derived  mixture  obtained  in  our  yield  of  monophenols  by  the  hydrogenation  of  the  lignin‐derived  liquid  mixture  obtained  in  our  monophenols by the hydrogenation of the lignin-derived liquid mixture in our previous work 2O (25 ml), 3 wt. % Pd/γ‐Al 23  O(0.200 g, 0.056 mmol Pd ), 60 °C,  (0.200 g, 0.056 mmol Pd ), 60 °C,  previous work [38]; reactant (0.45 mmol), H 2O (25 ml), 3 wt. % Pd/γ‐Al 2O 3 3  (0.200 g, 0.056 mmol Pd ), 60 °C,    previous work [38]; reactant (0.45 mmol), H 2O (25 ml), 3 wt. % Pd/γ‐Al 2O   previous work [38]; reactant (0.45 mmol), H c c c 2,12 h; reaction mixture stirred at 500 rpm;  2 MPa H 2,12 h; reaction mixture stirred at 500 rpm;   the hydrogenation products were detected in  2 MPa H 2,12 h; reaction mixture stirred at 500 rpm;   the hydrogenation products were detected in  2 MPa H 2O (25 ml), 3 wt. % Pd/γ‐Al 2O3 (0.200 g, 0.056 mmol Pd ), 60 °C,    previous work [38]; reactant (0.45 mmol), H c the hydrogenation products were detected in  c the hydrogenation products were detected in  c the hydrogenation products were detected in  2,12 h; reaction mixture stirred at 500 rpm;  2 MPa H 2Pd/γ-Al O (25 ml), 3 wt. % Pd/γ‐Al 3  (0.200 g, 0.056 mmol Pd ), 60 °C,  previous work [38]; reactant (0.45 mmol), H 2,12 h; reaction mixture stirred at 500 rpm;  2 MPa H reactant (0.45 mmol), H2 O (25 mL), 3 wt. % (0.200 g, 0.056 mmol Pd), 60 ˝ C, 2 MPa H2 , 12 2,12 h; reaction mixture stirred at 500 rpm;  2 MPa H 2O (25 ml), 3 wt. % Pd/γ‐Al 2O 3 (0.200 g, 0.056 mmol Pd ), 60 °C,  previous work [38]; reactant (0.45 mmol), H 2O (25 ml), 3 wt. % Pd/γ‐Al 2O23 O (0.200 g, 0.056 mmol Pd ), 60 °C,     h;  previous work [38]; reactant (0.45 mmol), H 2O c the hydrogenation products were detected in  c3 the hydrogenation products were detected in  c the hydrogenation products were detected in  2,12 h; reaction mixture stirred at 500 rpm;   the hydrogenation products were detected in  2 MPa H 2,12 h; reaction mixture stirred at 500 rpm;  2 MPa H 2,12 h; reaction mixture stirred at 500 rpm;  2 MPa H c the hydrogenation cproducts GC‐MS, but not quantified by GC due to the lack of standard substances.  GC‐MS, but not quantified by GC due to the lack of standard substances.  GC‐MS, but not quantified by GC due to the lack of standard substances.  2,12 h; reaction mixture stirred at 500 rpm;   the hydrogenation products were detected in  2 MPa H c reaction mixture stirred at 500 rpm; were detected in GC-MS, but not quantified c c GC‐MS, but not quantified by GC due to the lack of standard substances.  2,12 h; reaction mixture stirred at 500 rpm;   the hydrogenation products were detected in  2 MPa H GC‐MS, but not quantified by GC due to the lack of standard substances.  GC‐MS, but not quantified by GC due to the lack of standard substances.  2,12 h; reaction mixture stirred at 500 rpm;   the hydrogenation products were detected in  2 MPa H 2,12 h; reaction mixture stirred at 500 rpm;   the hydrogenation products were detected in  2 MPa H GC‐MS, but not quantified by GC due to the lack of standard substances.  GC‐MS, but not quantified by GC due to the lack of standard substances.  GC‐MS, but not quantified by GC due to the lack of standard substances.  byGC‐MS, but not quantified by GC due to the lack of standard substances.  GC due to the lack of standard substances. GC‐MS, but not quantified by GC due to the lack of standard substances.  GC‐MS, but not quantified by GC due to the lack of standard substances.  GC‐MS, but not quantified by GC due to the lack of standard substances. 

2.6. Recyclability of the Catalyst  2.6. Recyclability of the Catalyst  2.6. Recyclability of the Catalyst  2.6. Recyclability of the Catalyst  2.6. Recyclability of the Catalyst  2.6. Recyclability of the Catalyst  2.6. Recyclability of the Catalyst  2.6. Recyclability of the Catalyst  2.6. Recyclability of the Catalyst  After reaction, the catalyst was separated by filtration from the liquid products, and then dried at  After reaction, the catalyst was separated by filtration from the liquid products, and then dried at  After reaction, the catalyst was separated by filtration from the liquid products, and then dried at  After reaction, the catalyst was separated by filtration from the liquid products, and then dried at  After reaction, the catalyst was separated by filtration from the liquid products, and then dried at  After reaction, the catalyst was separated by filtration from the liquid products, and then dried at  100 °C. After that, the catalyst was calcined in a muffle furnace at 500 °C for 4 h. Prior to the next reaction,  After reaction, the catalyst was separated by filtration from the liquid products, and then dried at  100 °C. After that, the catalyst was calcined in a muffle furnace at 500 °C for 4 h. Prior to the next reaction,  100 °C. After that, the catalyst was calcined in a muffle furnace at 500 °C for 4 h. Prior to the next reaction,  Catalysts 2016, 6, 12  After reaction, the catalyst was separated by filtration from the liquid products, and then dried at  100 °C. After that, the catalyst was calcined in a muffle furnace at 500 °C for 4 h. Prior to the next reaction,  100 °C. After that, the catalyst was calcined in a muffle furnace at 500 °C for 4 h. Prior to the next reaction,  100 °C. After that, the catalyst was calcined in a muffle furnace at 500 °C for 4 h. Prior to the next reaction,  After reaction, the catalyst was separated by filtration from the liquid products, and then dried at  After reaction, the catalyst was separated by filtration from the liquid products, and then dried at  After reaction, the catalyst was separated by filtration from the liquid products, and then9 of 12  dried 100 °C. After that, the catalyst was calcined in a muffle furnace at 500 °C for 4 h. Prior to the next reaction,  100 °C. After that, the catalyst was calcined in a muffle furnace at 500 °C for 4 h. Prior to the next reaction,  100 °C. After that, the catalyst was calcined in a muffle furnace at 500 °C for 4 h. Prior to the next reaction,  100 °C. After that, the catalyst was calcined in a muffle furnace at 500 °C for 4 h. Prior to the next reaction,  ˝ C. After that, the catalyst was calcined in a muffle furnace at 500 ˝ C for 4 h. Prior to the 100 °C. After that, the catalyst was calcined in a muffle furnace at 500 °C for 4 h. Prior to the next reaction,  100 °C. After that, the catalyst was calcined in a muffle furnace at 500 °C for 4 h. Prior to the next reaction,  100 °C. After that, the catalyst was calcined in a muffle furnace at 500 °C for 4 h. Prior to the next reaction,  at 100 2.6. Recyclability of the Catalyst  2.6. 2.6. Recyclability of the Catalyst  Recyclability of the Catalyst 2.6. Recyclability of the Catalyst  2.6. Recyclability of the Catalyst  After reaction, the catalyst was separated by filtration from the liquid products, and then dried at  After reaction, the catalyst was separated by filtration from the liquid products, and then dried at  After reaction, the catalyst was separated by filtration from the liquid products, and then dried at 

   the catalyst was pre‐reduced with H 2 at 450 °C for 2 h. As shown in Figure 6, the catalyst continued to        reaction, next the catalyst was pre-reduced with H2 at 450 ˝ C for 2 h. As shown in Figure 6, the       have      high  activity  after  three  runs.  Both  the  conversion  of  4‐ethylphenol  and  the  selectivity  to      catalyst continued to have high activity after three runs. Both the conversion of 4-ethylphenol and the 4‐ethylcyclohexanol had no significant decrease compared to the fresh catalyst. However, there was  selectivity to 4-ethylcyclohexanol had no significant decrease compared to the fresh catalyst. However, some loss in activity after the fourth and fifth cycles. In order to investigate the reason of the activity  there was some loss in activity after the fourth and fifth cycles. In order to investigate the reason of the loss,  the  catalyst  after  the  fifth  cycle  was  analyzed  by  ICP  and  XRD  (see  Figure  S3).  The  results  activity loss, the catalyst after the fifth cycle was analyzed by ICP and XRD (see Figure S3). The results indicated that the Pd loading of the catalyst decreased from 3.05 to 2.47 wt. % after five cycles, while  indicated that the Pd loading of the catalyst decreased from 3.05 to 2.47 wt. % after five cycles, while the average particle size remained unchanged. This implied that the activity loss of the catalyst was  the average particle size remained unchanged. This implied that the activity loss of the catalyst was mainly ascribed to the loss of Pd in the recycle process.  mainly ascribed to the loss of Pd in the recycle process.

Selectivity(%)

100

80

80

60

60

40

40

20

20

0

Conversion (%)

C-OH C=O

100

0 1

2

3

Recycle time

4

5

 

Figure 6. Recycle of the catalyst. Reaction conditions: reactant (0.82 mmol), H2 O (25 mL), 3 wt. % Figure 6. Recycle of the catalyst. Reaction conditions: reactant (0.82 mmol), H 2O (25 mL), 3 wt. % Pd/γ‐ ˝ C, 2 MPa H . Pd/γ-Al O (0.200 g/0.056 mmol Pd), 60 2 3 2 Al2O3 (0.200 g/0.056 mmol Pd), 60 °C, 2 MPa H2. 

3. Experimental Section  3.1. Materials  The pubescens sample (80 meshes, Huzhou, China) was washed three times with distilled water 

Catalysts 2016, 6, 12

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3. Experimental Section 3.1. Materials The pubescens sample (80 meshes, Huzhou, China) was washed three times with distilled water and dried at 110 ˝ C in an oven before use. The materials for catalyst preparation (γ-Al2 O3 , Shanxi Taiyuan Daily Chemicals Factory, Taiyuan, China; PdCl2 , Sino-Platinum Metals Co., Kunming, China) were purchased commercially. Other chemicals such as 4-ethylphenol, p-cresol, p-n-propylphenol, p-isopropylpehnol, p-vinylphenolwere were purchased from J&K SCIENTIFIC LTD (>99%, Beijing, China). Guaiacol was obtained from the Sheshan Chemical Factory (99%, Shanghai, China). Phenol, catechol, hydroquinol, o-chlorophenol, p-chlorophenol, 2,4-dichlorophenol and ethanol as a solvent were purchased from Chengdu Changlian Chemical Regent Co., LTD (>99%, Chengdu, China). All the materials were used directly from commercial resources without further purification. 3.2. Methods 3.2.1. Catalyst Preparation Pd/γ-Al2 O3 catalysts were prepared using PdCl2 as a metal precursor, employing the wetness impregnation method. Before use, γ-Al2 O3 was calcined at 350 ˝ C for 4 h. Typically, a desired amount of an aqueous solution of PdCl2 was dropped on γ-Al2 O3 support and ultrasonically dispersed for 1 h, and the slurry was then left for 24 h at room temperature. After removing the excess H2 O in a water bath at 80 ˝ C, the resulting solid was dried overnight at 100 ˝ C, and was then calcined in muffle furnace at 500 ˝ C for 4 h. Before reaction, the obtained catalyst was pre-reduced by H2 at 450 ˝ C for 2 h. 3.2.2. Catalyst Characterization N2 adsorption-desorption isotherms were measured by static N2 physisorption with a Micromeritics Tristar 3020 analyzer (Micromeritics, Atlanta, GA, USA). Before measurement, the samples were degassed at 100 ˝ C for 2 h and then evacuated at 300 ˝ C for 4 h to remove the physically adsorbed impurities under vacuum. The surface area of these samples was calculated using the Brunauer-Emmett-Teller (BET) method. The total pore volume resulted from the adsorbed N2 volume at a relative pressure of approximately 0.99. The average pore diameters and the pore size distribution were determined by the Barrett-Joyner-Halenda (BJH) method. X-Ray Diffraction (XRD) measurements of the samples were performed with a DANDONG FANGYUAN DX-1000 diffraction instrument (Dandong Fangyuan Instrument Co., Ltd., Jinan, China) using Cu-Kα radiation (λ = 0.15406 Å) with a scanning angle (2θ) from 10˝ to 80˝ . The tube voltage was 40 kV and the current was 25 mA. X-Ray photoelectron spectroscopy (XPS) was carried out using an AXIS Ultra DLD (KRATOS) spectrometer (Shimadzu, Kyoto, Japan) Al-Kα radiation. The energy scale was internally calibrated by setting the C1s peak at 284.8 eV. The samples were powdered and flaked into sheet, and kept in a vacuum drier before measurement. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to analyze the actual Pd loading of the samples using Thermo Elemental IRIS Advantage ER/S spectrometer (Thermo Elemental, MA, USA). The pump rate was 110 rpm (2.035 mL min´1 ), and the nebulizer pressure (PSI) was 27 with a low auxiliary gas rate. 0.05 g sample was dissolved in aqua regia and diluted with water to 100 mL before ICP-AES measurement. 3.2.3. Hydrogenation Reactions of Phenols The hydrogenation of phenols was carried out in a 60 mL stainless steel autoclave reactor equipped with a magnetic stirrer. In a typical run, 0.82 mmol 4-ethylphenol and 0.20 g Pd/γ-Al2 O3 were placed in the reactor with 25 mL high purity water. Air in the reactor was blown out with nitrogen three times, and the inner nitrogen was then replaced with hydrogen. The initial pressure of hydrogen was

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added to 2.0 MPa. The reactor was heated from room temperature to the designated temperature with continuous stirring at 500 rpm, and then kept in the designated temperature for a different time. The reactor was cooled down to room temperature naturally after reaction. The mixture in the autoclave was collected by simply pouring it out, and the reactor was washed with high-purity water three times. After filtration, the filtrate was extracted with ethyl acetate three times. The organic phase was analyzed by Fuli 9750 Gas Chromatography (GC) (Fuli, Taizhou, China) equipped with Flame Ionization Detector (FID) and a HP-innowax column (30 m ˆ 0.25 mm ˆ 0.25 µm) using benzyl alcohol as an internal standard. The temperature of both the detector and injector were 280 ˝ C. The oven was heated from 50 ˝ C to 250 ˝ C at a rate of 5 ˝ C/min, and then held at 250 ˝ C for 10 min. Each sample was tested three times to confirm the reproducibility of reported results. The error of the each result was less than 3%. The molar yield and selectivity of the hydrogenation product were calculated with the following equations: Mole of product Yield pmol%q “ Mole of reaction material Selectivity p%q “

Mole of product Mole of converted material

4. Conclusions In conclusion, 3 wt. % Pd/γ-Al2 O3 showed remarkable catalytic activity for the selective hydrogenation of 4-ethylphenol to cyclohexanols under mild conditions in an aqueous phase. The conversion of 4-ethylphenol reached up to 100%, and all of the 4-ethylphenol was converted to 4-ethylcyclohexanol and 4-ethylcyclohexanone with a carbon balance of 100%. High selectivity to 4-ethylcyclohexanol (98.9%) was achieved at 60 ˝ C for 12 h. Additionally, the –Cl of chlorophenols were removable during the hydrotreatment process, which might provide a possible pathway to reduce the pollution of chlorophenols. Meanwhile, the Pd catalyst is also applicable to the hydrogenation of other monophenols in raw biomass systems, and exhibits good activity and selectivity. Kinetic experiments indicated that 4-ethylphenol hydrogenation passed through the formation of 4-ethylcyclohexanone as an intermediate, and then further hydrogenated into 4-ethylcyclohexanol. The 3 wt. % Pd/γ-Al2 O3 catalyst showed good water resistance and stability after recycling four times. Due to the milder reaction temperature and green water as solvent, this result might provide a low-cost, green, and promising approach to selectively producing cyclohexanol directly from raw biomass material. Acknowledgments: This work was financially supported by the National Basic Research Program of China (973 program, No. 2013CB228103). The characterization of the residue from Analytical and Testing Center of Sichuan University are greatly appreciated. Author Contributions: J.L. and C.H. conceived and designed the experiments; J.Y., Y.L., T.H. and Z.J. performed the experiments; J.Y., J.L. and C.H. analyzed the data and wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors were not involved in the design of the study, the collection, the analyses, the interpretation of the data, the writing of the manuscript, nor the decision to publish the results.

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