Catalytic Transformation of Lignocellulosic Biomass

DOI: 10.1002/cssc.201800967

Full Papers

Catalytic Transformation of Lignocellulosic Biomass into Arenes, 5-Hydroxymethylfurfural, and Furfural Tianye Guo, Xiangcheng Li, Xiaohui Liu, Yong Guo, and Yanqin Wang*[a] The complete transformation of lignocellulosic biomass into valuable platform chemicals is of great significance. Herein, a catalytic process for the upgrading of lignocellulose to arenes, 5-hydroxymethylfurfural (HMF), and furfural is reported. Firstly, the lignin fraction in lignocellulosic biomass is selectively converted into lignin oil (82.9 mol % yield of lignin monomers from birch wood) over a Pd/C catalyst and then further hydrodeoxygenated to arenes in catalytic hydrogen-transfer reactions over a Ru/Nb2O5 catalyst. High yields of C7–C9 hydrocarbons (95.6 mol %) with 85.6 wt % selectivity to arenes based on lignin oil are achieved owing to the synergistic effect between Ru and Nb2O5, which enables direct hydrogenolysis of the Caromatic@OH bond in phenolics. Secondly, the cellulose and

hemicellulose fractions in the Pd/C-containing solid residue, as well as methylated C5 sugars produced during the stripping of lignin, are converted into HMF and furfural with a total yield of up to 24.5 wt % (based on the amount of birch wood) in a THF/concentrated seawater (ca. 30 wt % salts) biphasic reaction system. Here, seawater played a key role in the conversion of cellulose and hemicellulose into HMF and furfural, respectively; more importantly, it made the separation and reuse of the Pd/ C catalyst easier. With this catalytic process, the complete and efficient transformation of lignocellulose into highly valueadded products with recycling of each catalyst and solvent has been realized.

Introduction Lignocellulosic biomass is a promising renewable feedstock for the production of chemicals and fuels in light of the extensive use of fossil fuels associated with increasingly serious environmental problems.[1, 2] The composition of lignocellulosic biomass is quite complex and comprises lignin (15–30 wt %; the only large-volume renewable source of aromatic chemicals), cellulose (35–45 wt %; the most abundant renewable source of C6 sugars), and hemicellulose (16–33 wt %; renewable source of C5 and C6 sugars).[3] The catalytic conversion of lignocellulosic biomass into biochemicals and fuels is of great significance; however, its recalcitrance to chemical transformation makes complete conversion of these three components of lignocellulosic biomass a major challenge. Considerable efforts have been devoted to directly converting lignocellulosic biomass into highly value added products.[4–8] Li et al. developed a carbon-supported Ni/W2C catalyst for direct catalytic conversion of the cellulose, hemicellulose, and lignin components of lignocellulosic biomass into ethylene glycol and monophenols with relatively high yields.[4] Ma and co-workers used layered LiTaMoO6 and Ru/C in aqueous phosphoric acid medium to [a] T. Guo, X. Li, Dr. X. Liu, Dr. Y. Guo, Prof. Y. Wang Shanghai Key Laboratory of Functional Materials Chemistry, Research Institute of Industrial Catalysis; School of Chemistry and Molecular Engineering East China University of Science and Technology No. 130 Meilong Road, Shanghai 200237 (P. R. China) E-mail: [email protected] Supporting Information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.1002/cssc.201800967.

ChemSusChem 2018, 11, 2758 – 2765

convert lignocellulosic biomass into gasoline-range alkanes (hexanes and pentanes), monophenols, and related hydrocarbons.[5] Wang and co-workers realized the direct hydrodeoxygenation of raw woods to give liquid alkanes with high yield over a multifunctional Pt/NbOPO4 catalyst, which not only converts cellulose and hemicellulose into hexane and pentane, but, more significantly, also transforms lignin fractions into alkyl cyclohexanes.[6] All these systems are efficient, but they face the challenge of difficult product separation (wide distribution of monophenols) or loss of biomass functionality. Therefore, the separate and efficient conversion of cellulose, hemicellulose, and lignin is still of interest. Numerous studies have been done on (hemi)cellulose conversion.[9] Examples of products from the transformation of cellulose and hemicellulose in lignocellulosic biomass include sugars (e.g., glucose and xylose),[10, 11] furan and its derivatives (e.g., 5-hydroxymethylfurfural (HMF), furfural, and levulinic acid),[12–14] and polyols (e.g., sorbitol and xylitol).[15, 16] Most works on lignin utilization are focused on its depolymerization.[2, 17–21] Examples of methods for transformation of lignin include direct hydrolysis or acidolysis to monophenols,[22–25] oxidative processes leading to syringyl- and guaiacylderived ketones or aldehydes,[26–32] reductive depolymerization to monophenols,[33–49] and recently developed formaldehydestabilized strategies producing guaiacyl and syringyl monomers.[50, 51] Tandem organosolv fractionation and transition metal-catalyzed transfer hydrogenolysis, often referred to as the lignin-first approach[52] or catalytic fractionation, prevents reactive monomers liberated by C@O bond cleavage from recondensing and forming strong C@C bonds. Among the stud-

2758

T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers ied catalyst systems, Pd/C and metal triflates[42, 43] cooperate very well, and up to 55 wt % lignin monomers can be obtained from the lignin fraction of birch wood (23.8 wt %) owing to their synergistic effect. Although the above-mentioned methods could efficiently depolymerize lignin and produce oxygencontaining compounds with a wide distribution, their subsequent separation and purification is extremely challenging and highly energy consuming. On the one hand, it is better to obtain value-added aromatic hydrocarbons from lignin, as has been realized recently over a Ru/NbOx catalyst,[53–55] because lignin is the only large-volume renewable source of aromatic chemicals, and arenes can be directly used as aviation fuels and fuel additives. On the other hand, after fractionation and depolymerization of lignin from lignocellulose, the solid catalyst always coexists with cellulose and hemicellulose in a solid residue, and the separation of the catalyst and complete transformation of (hemi)cellulose into value-added chemicals are the key to recovery and sustainability. Moreover, some approaches circumvent this problem by caging the catalyst[37] or separating fractionation and metal catalysis by performing the reactions in a continuous-flow mode.[56, 57] Recently, we found that seawater can catalyze the transformation of cellulose and hemicellulose into HMF and furfural without addition of acid catalyst,[58] and this would make the recovery of the catalyst from solid residues and the total transformation of lignocellusic biomass into chemicals or fuels possible. Herein, we report a novel catalyst system for the stepwise conversion of lignocellulosic biomass into aromatic hydrocarbons, HMF, and furfural. Pd/C and Yb(OTf)3 catalysts are combined to depolymerize lignin fragments from lignocellulosic biomass with high yields of alkyl methoxy phenols, and then the lignin oils are converted into aromatic hydrocarbons with high selectivity by catalytic transfer hydrogenation over a Ru/Nb2O5 catalyst. Moreover, a THF/concentrated seawater biphasic system (ca. 30 wt % salts) was used to convert the (hemi)cellulose-rich residue and methylated C5 sugars to HMF and furfural with separation of the Pd/C catalyst for recycling, and thus full valorization of lignocellulosic biomass was enabled (Scheme 1).

Results and Discussion Catalytic conversion of lignin-derived oils (lignin oils) into aromatic hydrocarbons through hydrogen transfer reactions The fractionation and depolymerization of lignocellulosic biomass were performed over a Pd/C catalyst with the addition of Yb(OTf)3, as done in a previous report.[42, 43] High mass/molar yields of lignin monomers, calculated on the basis of the lignin component, were obtained from birch wood, beech wood, corn stalks, and pine wood (55.8/82.9, 40.4/74.7, 56.6/93.8, and 23.1/62.5 %, respectively; see Table 1). Previously, Ru/NbOx was found to be an excellent deoxygenation catalyst for conversion of lignin into aromatic hydrocarbons by the direct deoxygenation (DDO) route, owing to the synergistic effect between NbOx and Ru species.[53–55] Here, the Ru/Nb2O5 catalyst is also efficient and can convert birch-wood lignin oil into aromatic hydrocarbons with high yields of C7–C9 hydrocarbons (38.2 wt %) and high selectivity to arenes (85.6 wt %) at 250 8C for 20 h; the corresponding carbon yield of C7–C9 hydrocarbons is 95.6 mol %, based on the monomer in birch-wood lignin oil. Moreover, the reaction temperature and the dosage of iPrOH have a great influence on product yield and distribution (see below). The influence of the reaction temperature on the conversion of lignin oils (birch) and the yields of aromatic hydrocarbons was investigated (Table 2). At relatively low temperature (230 8C), the yield of C7–C9 aromatic hydrocarbons is only 27.7 wt %, with 6.2 wt % C7–C9 cycloalkanes, owing to the coexistence of DDO and hydrogenation reaction routes at low temperature. Reactions at higher temperatures (250 and 270 8C) lead to increased yields of C7–C9 aromatic hydrocarbons (32.7 and 33.2 wt %, respectively) with a remarkable decrease in the yields of C7–C9 cycloalkanes (3.6 and 1.6 wt %, respectively), which suggests that high temperature benefits the direct cleavage of C@O bonds (DDO route). Notably, at 250 8C, carbon yields of C7–C9 hydrocarbons of about 95.6 % based on lignin monomers in lignin oil and 79.3 % based on lignin monomers in birch wood (Table 2 and Figure S2 in the Supporting Infor-

Scheme 1. Schematic of full conversion of lignocellulosic biomass into aromatic hydrocarbons, HMF, and furfural.

ChemSusChem 2018, 11, 2758 – 2765

www.chemsuschem.org

2759


Full Papers Table 1. Yields of lignin oil and lignin monomer from various lignocellulosic biomass.[a]

Biomass

Lignin monomers in biomass[b] [mmol]

Mass yield Lignin oil[c] [wt %]

Lignin monomers[d] [wt %]

Lignin monomers[e] [wt %]

Molar yield Lignin monomers[f] [% (mmol)]

Methylated C5 sugars[g] [mol %]

birch wood beech wood corn stalks pine wood

0.82 0.87 0.48 0.64

20.1 22.6 15.3 17.6

11.5 10.8 7.7 6.8

55.8 40.4 56.6 23.1

82.9 74.7 93.8 62.5

27.3 20.9 32.1 22.6

(0.68) (0.65) (0.45) (0.40)

[a] Reaction conditions: lignocellulosic biomass (1 g), 5 wt % Pd/C (0.1 g), Yb(OTf)3 (10 mg), methanol (20 mL), 2 MPa H2, 200 8C, 2 h. [b] The mass of lignin monomers determined by the NBO method; the moles of lignin monomers were calculated on the basis of 1 g of lignocellulosic biomass. [c] Based on 1 g of lignocellulosic biomass. [d] The mass of obtained lignin monomers was determined by GC-MS and GC; the yield of lignin monomers is based on 1 g of lignocellulosic biomass. [e] The yield of lignin monomers was based on Klason lignin contents of 20.6, 26.7, 13.6, and 29.4 wt % from 1 g birch wood, beech wood, corn stalks, or pine wood (Table S1). [f] The molar yield of lignin monomer is based on the moles of lignin monomers in 1 g of lignocellulosic biomass. [g] The yield of methylated C5 sugars is based on the moles of pentose units in lignocellulosic biomass. The sugar compositions of lignocellulosic biomass were based on monomeric sugars detected after two-stage acid hydrolysis and analyzed by HPLC with an external standard (Table S1).

Table 2. Effect of reaction temperature on the yield of products from birch-wood lignin oil.[a]

T [8C]

Yield of products[b] [wt %] C7–C9 aromatic hydrocarbons

C7–C9 cycloalkanes total

230 250 270

3.5 4.2 4.6

13.1 14.5 17.1

11.1 14.0 11.5

27.7 32.7 33.2

others

Total[b] [wt %]

total

1.5 0.4 0.2

2.5 1.7 1.0

2.1 1.5 0.4

6.2 3.6 1.6

1.0 1.9 1.8

34.9 38.2 36.6

Selectivity to aromatic hydrocarbons[c]

Molar yield of C7–C9 hydrocarbons[d]

[wt %]

[mol %]

79.4 85.6 90.7

89.8 95.6 91.8

[a] Reaction conditions: lignin oil (0.1 g, birch wood), 2 % Ru/Nb2O5 (0.2 g), iPrOH (1.0 g), H2O (14 mL), cyclohexane (5 mL), 20 h. [b] Based on the mass of lignin oil. [c] Based on the yield of total hydrocarbons obtained from lignin oil. [c] Based on the moles of lignin monomers in lignin oil.

mation) were obtained, which indicate that Ru/Nb2O5 is a powNb2O5 catalyst is demonstrated by the maximum total mass erful catalyst for hydrodeoxygenation and that this reaction yield of hydrodeoxygenated products of 39.8 wt % with corresystem is capable of highly selective conversion of lignin monsponding carbon yield of C7–C9 hydrocarbons of about 98.8 % omers into aromatic hydrocarbons. based on lignin monomers in lignin oil (Figure 1). Besides the reaction temperature, the dosage of iPrOH is also a key factor for the high-yield production of C7–C9 aromatic hydrocarbons. A product yield of 28.1 wt % (ca. 76.1 % carbon yield of C7–C9 hydrocarbons based on lignin monomers in lignin oil) was obtained with a mass ratio of iPrOH to lignin oil of 5, that is, the dosage of iPrOH is not sufficient to convert lignin oil. At a mass ratio of 10, the products are composed of 32.7 wt % C7–C9 aromatic hydrocarbons, 3.6 wt % C7–C9 cycloalkanes, and 1.9 wt % dimers; notably, the C7–C9 aromatic hydrocarbons account for 85.6 wt % of the total hydrodeoxygenated products. Further increasing the mass ratio of iPrOH to lignin-oil from 10 to 80 leads to an increase of total mass yield from 38.2 to 39.8 wt %. However, generation of C7–C9 cycloalkanes increases (from 3.6 to 21.7 wt %), and the amount of C7–C9 aromatic hydrocarbons decreases gradually from 32.7 to 16.8 wt % owing to the excessive hydrogenation of aromatic hydrocarbons. These results indicate that the dosage of iPrOH should be in a proper range for Figure 1. Influence of iPrOH dosage on the conversion of lignin oil. Reaction conditions: achieving high yields of aromatic hydrocarbons. The lignin oil (0.1 g), 2 % Ru/Nb2O5 (0.2 g), iPrOH with 14 mL H2O and 5 mL cyclohexane, powerful hydrodeoxygenation ability of the Ru/ 250 8C, 20 h. ChemSusChem 2018, 11, 2758 – 2765

www.chemsuschem.org

2760


Full Papers Under the optimal reaction conditions (reaction temperature of 250 8C, iPrOH/lignin oil mass ratio of 10), conversion of other lignin oils was also investigated with the Ru/Nb2O5 catalyst (Table S2). All lignin oils could be efficiently converted. The mass yields of C7–C9 aromatic hydrocarbons/hydrodeoxygenated products from birch-wood, beech-wood, corn-stalk, and pine-wood lignin oils were 32.7/38.2, 28.5/31.4, 26.5/31.5, and 21.3/25.3 wt %, respectively, and the corresponding molar and carbon yields of C7–C9 hydrocarbons were all greater than 95.6 and 76.1 %, respectively (loss of carbon through decarbonylation or C@C bond cleavage during the HDO process), among which birch-wood lignin oil gave the highest yield of C7–C9 aromatic hydrocarbons (32.7 wt %).

Catalytic conversion of the (hemi)cellulose-rich solid residue and methylated C5 sugars into HMF and furfural The solid residue is mainly composed of cellulose and about half the amount of hemicellulose (Table S1). In the process of catalytic depolymerization of lignocellulosic biomass, recycling the Pd/C catalyst is a challenge because it ends up in the solid (hemi)cellulose-rich residue.[42, 43] Recently, we found that cellulose can be efficiently converted into 5-hydroxymethylfurfural (HMF) by an acid-free reaction system[58] composed only of THF and concentrated seawater (ca. 30 wt % salts; the detailed components are listed in Table S3). NaCl in seawater promotes

the hydrolysis of cellulose, isomerization of glucose, and dehydration of fructose in a highly selective manner. These residues and methylated C5 sugars formed during the depolymerization of lignocellulosic biomass could be efficiently converted, and the molar yields of HMF/furfural from birch wood, beech wood, corn stalks, and pine wood were 43.3/40.8, 44.2/41.4, 49.7/42.1, 46.3/37.0 % (Table 3 and Table S4), respectively, which are similar to those obtained by using CrCl3[12] and mixed Sn-Mont and NbOPO4 catalysts.[13] This acid-free method not only separates the Pd/C catalyst, but also converts cellulose and hemicellulose components into HMF and furfural, so that full use of cellulose and hemicellulose components of lignocellulosic biomass is achieved. Moreover, it may be more simple, efficient, and economically attractive for the utilization of cellulose than enzymatic catalysis (hydrolysis of cellulose to glucose),[59] which is time-consuming and requires product purification. Hence, our catalytic process enables full valorization of the raw lignocellulosic biomass, and the reusability of catalysts is also significant, as described below. After the conversion of the (hemi)cellulose-rich solid residue into HMF and furfural, the Pd/C catalyst was recovered by filtration, washed with methanol, dried under vacuum (50 8C, 12 h), and then directly used for depolymerization of lignocellulose in the next cycle (step 1). The Pd/C catalyst can be reused for three cycles, albeit with a decrease in catalytic activity: the yield of lignin oil decreases to 15.4 wt % after the first run and 10.6 wt % after the second run (Table 4, step 1). How-

Table 3. Yields of HMF and furfural obtained from various sources of lignocellulosic biomass after lignin depolymerization reaction. Entry

Source

Glucan [wt %]

Xylan [wt %]

Molar yield[a] [mol %] HMF[c] furfural[c]

furfural

1 2 3 4

birch wood beech wood corn stalks pine wood

52.6 39.0 37.6 41.2

18.7 19.4 16.4 11.9

43.3 44.2 49.7 46.3

16.8 12.4 18.9 14.6

24.0 29.0 23.2 22.4

[d]

total furfural

[e]

40.8 41.4 42.1 37.0

Mass yield[b] [wt %] HMF furfural

total

18.9 14.1 14.7 15.5

24.5 20.4 19.4 18.3

5.6 6.3 4.7 2.8

[a] Based on the moles of hexose(pentose) units in lignocellulosic biomass (Table S1). [b] Based on the mass of lignocellulosic biomass. [c] Obtained from the solid residue. Reaction conditions: solid residue (ca. 0.7 g) obtained from lignin depolymerization reaction of 1 g of lignocellulosic biomass, 0.5 MPa N2, 5 mL of concentrated seawater (ca. 30 wt % salts), 30 mL of THF, 200 8C, 5 h. [d] Obtained from methylated C5 sugars. Reaction conditions: methylated C5 sugars (ca. 0.1 g) obtained from the lignin depolymerization reaction of 1 g of lignocellulosic biomass, 0.5 MPa N2, concentrated seawater (2 mL, ca. 30 wt % salts), THF (12 mL), 180 8C, 3 h. [e] Obtained from solid residue and methylated C5 sugars (both from lignin depolymerization reaction of 1 g of lignocellulosic biomass).

Table 4. Recycling tests of catalytic depolymerization of lignocellulose (step 1) and conversion of (hemi)cellulose-rich solid residue into HMF and furfural (step 2). Cycle Step 1[a] mass yield molar yield lignin oil[c] lignin monomers[c] lignin monomers[d] lignin monomers in lignin oil[e] lignin monomers[f] [wt %] [wt % (mmol)] [wt %] [wt %] [mol %]

Step 2[b] molar yield[g] mass yield[h] HMF furfural HMF furfural total [mol %] [mol %] [wt %] [wt %] [wt %]

1st 2nd 3rd

43.3 43.6 42.9

20.1 15.4 10.6

11.5 (0.68) 8.6 (0.50) 5.8 (0.34)

55.8 41.7 28.2

57.1 55.7 55.1

82.9 61.0 41.5

40.8 41.8 41.4

18.9 19.1 18.7

5.6 5.7 5.7

24.5 24.8 24.4

[a] Reaction conditions: birch wood (1 g), 5 wt % Pd/C (0.1 g), Yb(OTf)3 (10 mg), methanol (20 mL), 2 MPa H2, 200 8C, 2 h. [b] Reaction conditions: solid residue obtained from the lignin depolymerization reaction of 1 g of lignocellulosic biomass, concentrated seawater (5 mL, ca. 30 wt % salts), THF (30 mL), 200 8C, 5 h. [c] Based on the mass of lignocellulosic biomass. [d] The yield of lignin monomers is based on a Klason lignin content of 20.6 wt % from 1 g of birch wood. [e] Based on the mass of lignin oil. [f] The molar yield of lignin monomer is based on the moles of lignin monomers in lignocellulosic biomass. [g] Based on the moles of hexose (pentose) units in lignocellulosic biomass. [h] Based on the mass of hexose (pentose) units in lignocellulosic biomass.

ChemSusChem 2018, 11, 2758 – 2765

www.chemsuschem.org

2761


Full Papers ever, the content of lignin monomers in the lignin oil still remains at 55.1 % after the third run, which indicates incomplete depolymerization of lignocellulose, and the partial decrease in the catalytic activity of the Pd/C catalyst could be due to loss of palladium during the cycles (decreased from 4.7 to 3.1 wt % according to ICP analysis; Table S5) and slight growth of Pd particles (Figure S3). Further investigations aimed at enhancing the stability of Pd/C catalyst are ongoing. During the conversion of the (hemi)cellulose-rich solid residue and methylated C5 sugars (step 2), the upper organic phase was separated after the reaction and the aqueous phase (concentrated seawater) was directly used in the next run without further modification. After two successive reaction cycles in concentrated seawater, the yield of HMF and furfural still remained 42.9 and 41.4 %, respectively (Table 4, step 2), without any obvious changes; thus, this THF/seawater system has excellent reusability. However, the weight of the used Pd/C catalyst of about 0.11 g after the reaction is higher than that of the fresh Pd/C catalyst (0.1 g). This result indicates carbon deposition on the Pd/C catalyst, which also leads to partial loss of catalytic activity. Recycling tests of catalytic depolymerization of lignocellulose were also conducted (ten times). Methanol and THF could be recycled, although slight losses occurred during the rotaryevaporation process. Meanwhile, about 82 wt % of the Yb(OTf)3 catalyst could be recycled from the reaction system, and its catalytic activity was well maintained in the next run (Table S6). The stability of the Ru/Nb2O5 catalyst for the conversion of lignin oil from each cycle in Table 4 was tested in three recycling runs (Table 4, step 3; Figure 2). After each reaction, the used Ru/Nb2O5 catalyst was isolated, washed with ethanol and ethyl acetate, and then dried under vacuum (50 8C, 12 h) before the next run. For the third run, the mass yields of C7–C9 aromatic hydrocarbons and C7–C9 cycloalkanes of 31.4 and 4.0 wt %, respectively, which were similar to those obtained in the first run (32.7 and 3.6 wt %, respectively), implied excellent activity and stability of the Ru/Nb2O5 catalyst for the hydrodeoxygenation reaction. The cycling tests in Table 4 and Figure 2 confirm that this catalyst system based on a Pd/C catalyst, concentrated seawater, and a Ru/Nb2O5 catalyst has high

Figure 2. Recycling tests of lignin-oil conversion over 2 % Ru/Nb2O5 catalyst (step 3). Reaction conditions: lignin oil (0.1 g, from Table 4), 2 % Ru/Nb2O5 (0.2 g), iPrOH with 14 mL H2O and 5 mL cyclohexane, 250 8C, 20 h.

stability for continuous conversion of lignocellulose into aromatic hydrocarbons, HMF, and furfural. The catalytic process Figure 3 shows a schematic flowsheet of the overall catalytic process for the conversion of lignin into aromatic hydrocarbons as well as cellulose and hemicellulose to HMF and furfural, which realizes full valorization of the raw lignocellulosic biomass and easy catalyst recycling. The key to this process is the acid-free conversion of (hemi)cellulose, which makes separation and recycling of the Pd/C catalyst feasible. Firstly, ligninderived lignin oils can be obtained with high selectivity from lignocellulosic biomass (step 1), owing to the synergistic effect between Yb(OTf)3 and Pd/C catalysts, which can efficiently cleave the lignin–carbohydrate intralinkages (b-O-4, a-O-4, and O-4 ether linkages).[42, 43] Secondly, an acid-free, simple, and highly efficient THF/concentrated seawater biphasic system can be used to convert cellulose and hemicellulose fractions in the solid residue, as well as methylated C5 sugars, into HMF and furfural (step 2). Moreover, the problem of Pd/C catalyst

Figure 3. Flowsheet of full utilization of lignocellulosic biomass to give aromatic hydrocarbons, HMF, and furfural.

ChemSusChem 2018, 11, 2758 – 2765

www.chemsuschem.org

2762


Full Papers recycling caused by difficult separation after lignin depolymerization was solved. Finally, the Ru/Nb2O5 catalyst shows excellent performance in the transfer hydrodeoxygenation of ligninderived lignin oils to aromatic hydrocarbons with iPrOH as hydrogen donor (step 3). This catalyst system not only has high potential for continuous conversion of raw lignocellulose into aromatic hydrocarbons, HMF, and furfural, but also avoids the use of acid catalysts, which reduces environmental pollution and opens a new efficient and green pathway for the full valorization of lignocellulosic biomass.

Depolymerization of lignocellulosic biomass

Conclusions

Separation of lignin oil

We have demonstrated an efficient catalytic process for the production of aromatic hydrocarbons, HMF, and furfural from lignocellulosic biomass. A Ru/Nb2O5 catalyst catalyzes the selective and direct cleavage of the Caromatic@O bonds of lignin monomers (derived from a lignin fraction depolymerized by Pd/C and Yb(OTf)3 catalysts) to aromatic hydrocarbons by using iPrOH as the hydrogen source. The biphasic THF/concentrated seawater reaction system used in this study not only exhibited high performance for the conversion of (hemi)celluloserich residues into HMF and furfural, but also showed remarkable advantages in recycling of the Pd/C catalyst. This whole catalytic approach is capable of converting lignocellulosic biomass into aromatic hydrocarbons, HMF, and furfural, and it provides a highly industrially promising route for the production of chemicals and fuels from biomass.

The mixture after biomass depolymerization was filtered to remove Pd/C and (hemi)cellulose solid residue, and methanol in the liquid phase was removed by rotary evaporation. Then the residues were extracted with water and ethyl acetate to obtain an aqueous phase (mainly containing methylated C5 sugars) and an ethyl acetate phase (mainly containing lignin monomers). After removal of ethyl acetate by rotary evaporation, the lignin oil was obtained. The lignin oil was diluted with ethyl acetate (10 mL) and directly analyzed by GC-MS (Agilent 7890A-5975C) and GC (Agilent 7890B). The yields of products are defined as follows [Eqs. (1)–(5); LM = lignin monomer; LO = lignin oil]: Mass yield of LO ½wt %A ¼ weight of LO produced > 100 weight of starting biomass

ð1Þ

Mass yield of LM ½wt %A ¼ weight of LM > 100 weight of starting biomass

Experimental Section

ð2Þ

Mass yield of LM ½wt %A ¼

Chemicals Pd(NO3)2 and RuCl3 solutions were purchased from Heraeus Materials Technology Shanghai Co., Ltd. Activated carbon L3S was purchased from SCM Industrial Chemical Co., Ltd. 5-Hydroxymethylfurfural (HMF, > 98 %) was purchased from Alfa Aesar Chemical Reagent Company. All other chemicals were chemically pure and purchased from Shanghai Chemicals Company. The ball-milled lignocellulosic biomass samples were prepared by using a laboratory ball mill (QM-3SP04). Lignocellulosic biomass (ca. 2 g) was charged into the grinding cell, and ball milling was performed at a frequency of 50 Hz with 6 mm agate balls for 12 h. Raw seawater was obtained from the distant sea of Qidong City, Jiangsu Province (P. R. China) and used without purification. Concentrated seawater was obtained from as-received seawater by rotary evaporation.

Catalyst preparation The 5 wt % Pd/C catalyst was prepared by a typical wetness impregnation method. Typically, Pd(NO3)2 (1 mL of a 0.1 g mL@1 solution) was dispersed with activated carbon (2 g), and the resulting material was dried at 50 8C for 12 h, reduced in a 10 % H2/Ar flow at 200 8C for 2 h, and then purged with N2 for 2 h. Nb2O5 was synthesized according to published procedures.[60] The Ru-based catalysts were prepared by the incipient wetness impregnation method with appropriate amounts of an aqueous solution of RuCl3. The product was dried at 100 8C for 12 h, reduced in a 10 % H2/Ar flow at 400 8C for 3 h, and then purged with N2 for 2 h until room temperature was reached. The metal loading was 2 wt %. ChemSusChem 2018, 11, 2758 – 2765

In a typical depolymerization reaction, ball-milled lignocellulosic biomass (1 g), 5 wt % Pd/C (0.1 g), Yb(OTf)3 (10 mg), and methanol (20 mL) were put in a Teflon-lined stainless steel autoclave (50 mL) equipped with a temperature-controlled heating jacket and magnetic stirrer, and the reactor was sealed, purged with nitrogen, and then filled with hydrogen. The hydrogen pressure was set to 2 MPa and the reaction mixture was heated to 200 8C with continuous stirring at 700 rpm for 2 h. After the reaction, the reactor was cooled to room temperature with cooling water.

www.chemsuschem.org

weight of LM > 100 weight of Klason lignin in starting biomass

ð3Þ

LM content in LO ½wt %A ¼ weight of LM > 100 weight of LO produced

ð4Þ

Molar yield of LM ½mol %A ¼ mol of LM produced > 100 mol of LM in starting biomass

ð5Þ

Isolation of methylated C5 sugars and recycling of Yb(OTf)3 To separate methylated C5 sugars and Yb(OTf)3, water was removed from the aqueous phase by rotary evaporation under vacuum, methylated C5 sugars were redissolved in methanol (5 mL), and the remaining Yb(OTf)3 was directly used in the next cycle. Following methanol removal by rotary evaporation under vacuum, methylated C5 sugars were obtained.

Analysis of lignin monomers by the alkaline nitrobenzene oxidation (NBO) method Lignin monomers were analyzed by the NBO method.[50] In a typical reaction, birch wood (40 mg) was mixed with nitrobenzene (0.4 mL) and 2 m NaOH (7 mL) and the mixture heated at 170 8C for 2 h. Afterwards, the reactor was cooled in ice–water and freshly

2763


Full Papers prepared ethyl vanillin (3-ethoxy-4-hydroxybenzaldehyde; 1 mL of a 5 mmol mL@1 solution in 0.1 m NaOH) was added to the reaction mixture as internal standard. The mixture was transferred to a 100 mL separating funnel and washed with dichloromethane (3 V 15 mL). The remaining aqueous layer was acidified with 2 m HCl until the pH was below 3.0 and extracted twice with dichloromethane (20 mL) and diethyl ether (20 mL). The combined organic layer was washed with deionized water (20 mL) and dried with Na2SO4. After filtration, the filtrate was collected in a 100 mL pearshaped flask and dried under reduced pressure. For the trimethylsilyl derivatization step, NBO products were washed with pyridine (3 V 200 mL) into a GC vial and N,O-bis(trimethylsilyl)trifluoroacetamide (150 mL) was added. The mixture was heated to 50 8C for 30 min. The silylated NBO products were analyzed by GC-MS (Agilent 7890A GC-MS equipped with a 30 m V 250 mm HP-5 capillary column) to identify the products by comparison of their peak retention times and mass spectra with those of authentic compounds. The identified products were quantified by GC-FID (Agilent 7890B) by using the same column. The initial column temperature of 150 8C was held for 10 min and then raised at 5 8C min@1 to 280 8C (held for 20 min).

Hydrodeoxygenation of lignin oils The hydrodeoxygenation of lignin oil was conducted in a 50 mL stainless steel autoclave. In a typical run, lignin oil (0.10 g), 2 % Ru/ Nb2O5 (0.20 g), deionized water (14 mL), cyclohexane (5 mL), andpropan-2-ol (1.0 g) were charged to the autoclave. After purging air from the reactor with N2 three times, the reaction was carried out at 250 8C in the presence of N2 with a magnetic stirring speed of 700 rpm. After the reaction, the reactor was cooled to room temperature with cooling water. The organic products were extracted with ethyl acetate and analyzed by a GC (Agilent 7890B) and GC-MS (Agilent 7890A-5975C). The yields of liquid alkanes were determined by adding n-tridecane as internal standard. During the cycling hydrodeoxygenation experiments on lignin oil, 2 % Ru/Nb2O5 was removed by centrifugation, dried under vacuum (50 8C, 12 h), then directly used in the next run. Mass yields of aromatic hydrocarbons were defined as follows [Eq. (6); AHCs = aromatic hydrocarbons; LO = lignin oil]: Mass yield of AHCs ½wt %A ¼ weight of aromatic hydrocarbons produced > 100 weight of lignin oil input

ð6Þ

temperature-controlled heating jacket and magnetic stirring. Owing to the high corrosiveness of the seawater-based mixture, Teflon-lined reactors are recommended. In a typical run, solid residue (ca. 0.7 g) obtained from the lignin depolymerization reaction of lignocellulosic biomass (1 g), concentrated seawater (5 mL, ca. 30 wt % salts), and THF (30 mL) were added to the autoclave. The autoclave was sealed, and the mixture was stirred at 700 rpm. N2 gas (0.5 MPa) was used for purging air from the reactor and keeping the solvent in the liquid phase. When the reactor temperature reached 200 8C, zero time was recorded. Then, the reactor was held at this temperature for 5 h. After the reaction, the mixtures were cooled quickly, the Pd/C catalyst was recovered by filtration, and the filtrate was taken for analysis.

Dehydration reaction of methylated C5 sugars The methylated C5 sugars were placed a Teflon-lined stainless steel autoclave (50 mL) equipped with a temperature-controlled heating-jacket and magnetic stirring. In a typical run, methylated C5 sugars (ca. 0.1 g) obtained from the lignin depolymerization reaction of lignocellulosic biomass (1 g), concentrated seawater (2 mL, ca. 30 wt % salts), and THF (12 mL) were placed a Teflon-lined stainless steel autoclave (50 mL) equipped with a temperature-controlled heating-jacket and magnetic stirring. The autoclave was sealed, and the mixture was stirred at 700 rpm. N2 gas (0.5 MPa) was used to purge air from the reactor and keep the solvent in liquid phase. When the reactor temperature reached 180 8C, zero time was recorded. Then, the reactor was held at this temperature for 3 h.

Analysis of methylated C5 sugars, HMF, and furfural The methylated C5 sugars were analyzed by HPLC (Agilent, 1200 series) with a Shodex SUGAR SC1011 column (8 V 300 mm) and a refractive-index detector (AgilentG1362A) by using high-purity water as mobile phase at a flow rate of 0.8 mL min@1. An autosampler (Agilent G1329A) was used to enhance the reproducibility. The products were quantified by using an external standard. HMF and furfural were analyzed by HPLC (Agilent 1200 Series) with an XDBC18 column (Eclipse USA) and quantified with an UV detector (Agilent G1314B) at 254 nm. The eluent with a flow rate of 0.6 mL min@1 was methanol/water (20/80 v/v). An autosampler (Agilent G1329A) was used to enhance the reproducibility. The quantification of products was performed by using an external standard. Yields were calculated as follows [Eqs. (9) and (10)]:

Molar and carbon yields of C7–C9 hydrocarbons were defined as follows [Eqs. (7) and (8); HCs = hydrocarbons; LM = lignin monomers; LO = lignin oil]:

Yield of methylated C5 sugars ½mol %A ¼

Molar yield of C7 @ C9 HCs ½mol %A ¼

Yield of HMF ðfurfuralÞ ½mol %A ¼

mol of C7 @ C9 HCs produced > 100 mol of C7 @ C9 monomers in LO Carbon yield of C7 @ C9 HCs ½%A ¼ carbon of C7 @ C9 HCs produced > 100 carbon from LM in LO

ð7Þ

ð10Þ

Acknowledgements

The solid residue collected after lignin depolymerization was put in a Teflon-lined stainless steel autoclave (100 mL) equipped with a www.chemsuschem.org

moles of HMFðfurfuralÞ produced > 100 moles of hexose ðpentoseÞ units in starting biomass

ð9Þ

ð8Þ

Dehydration reaction of (hemi)cellulose

ChemSusChem 2018, 11, 2758 – 2765

moles of methylated C5 sugars produced > 100 moles of pentose units in starting biomass

This work was supported financially by the NSFC of China (No. 91545103), the Fundamental Research Funds for the Central Universities (222201718003, 222201817022), and the Science and Technology Commission of Shanghai Municipality (10dz2220500).

2764


Full Papers Conflict of interest The authors declare no conflict of interest. Keywords: aromatic hydrocarbons · biomass · furfural · heterogeneous catalysis · lignocellulose [1] M. Besson, P. Gallezot, C. Pinel, Chem. Rev. 2014, 114, 1827 – 1870. [2] C. Z. Li, X. C. Zhao, A. Q. Wang, G. W. Huber, T. Zhang, Chem. Rev. 2015, 115, 11559 – 11624. [3] R. J. Moon, A. Martini, J. Nairn, J. Simonsen, J. Youngblood, Chem. Soc. Rev. 2011, 40, 3941 – 3994. [4] C. Z. Li, M. Y. Zheng, A. Q. Wang, T. Zhang, Energy Environ. Sci. 2012, 5, 6383 – 6390. [5] Y. Liu, L. G. Chen, T. J. Wang, Q. Zhang, C. G. Wang, J. Y. Yan, L. L. Ma, ACS Sustainable Chem. Eng. 2015, 3, 1745 – 1755. [6] Q. N. Xia, Z. J. Chen, Y. Shao, X. Q. Gong, H. F. Wang, X. H. Liu, S. F. Parker, X. Han, S. H. Yang, Y. Q. Wang, Nat. Commun. 2016, 7, 11162. [7] Y. Huang, S. Qiu, I. N. Oduro, X. Guo, Y. M. Fang, Energy Fuels 2016, 30, 9524 – 9531. [8] Z. H. Sun, G. Bottari, A. Afanasenko, M. C. A. Stuart, P. J. Deuss, B. Fridrich, K. Barta, Nat. Catal. 2018, 1, 82 – 92. [9] C. H. Zhou, X. Xia, C. X. Lin, D. S. Tong, J. Beltramini, Chem. Soc. Rev. 2011, 40, 5588 – 5617. [10] J. S. Luterbacher, J. M. Rand, D. M. Alonso, J. H. Han, J. T. Youngquist, C. T. Maravelias, B. F. Pfleger, J. A. Dumesic, Science 2014, 343, 277 – 280. [11] J. J. Wang, J. X. Xi, Y. Q. Wang, Green Chem. 2015, 17, 737 – 751. [12] S. Van de Vyver, J. Thomas, J. Geboers, S. Keyzer, M. Smet, W. Dehaen, P. A. Jacobsa, B. F. Sels, Energy Environ. Sci. 2011, 4, 3601 – 3610. [13] J. J. Wang, X. H. Liu, B. C. Hu, G. Z. Lu, Y. Q. Wang, RSC Adv. 2014, 4, 31101 – 31107. [14] J. N. Chheda, Y. Roman-Leshkov, J. A. Dumesic, Green Chem. 2007, 9, 342 – 350. [15] W. P. Deng, X. S. Tan, W. H. Fang, Q. H. Zhang, Y. Wang, Catal. Lett. 2009, 133, 167 – 174. [16] X. C. Li, T. Y. Guo, Q. N. Xia, X. H. Liu, Y. Q. Wang, ACS Sustainable Chem. Eng. 2018, 6, 4390 – 4399. [17] J. Zakzeski, P. C. A. Bruijnincx, A. L. Jongerius, B. M. Weckhuysen, Chem. Rev. 2010, 110, 3552 – 3599. [18] C. P. Xu, R. A. D. Arancon, J. Labidi, R. Luque, Chem. Soc. Rev. 2014, 43, 7485 – 7500. [19] D. M. Alonso, S. G. Wettstein, J. A. Dumesic, Chem. Soc. Rev. 2012, 41, 8075 – 8098. [20] W. Schutyser, T. Renders, S. Van den Bosch, S.-F. Koelewijin, G. T. Beckham, B. F. Sels, Chem. Soc. Rev. 2018, 47, 852 – 908. [21] Z. H. Sun, B. Fridrich, A. de Santi, S. Elangovan, K. Barta, Chem. Rev. 2018, 118, 614 – 678. [22] Z. C. Jiang, T. He, J. M. Li, C. W. Hu, Green Chem. 2014, 16, 4257 – 4265. [23] P. J. Deuss, C. S. Lancefield, A. Narani, J. G. de Vries, N. J. Westwood, K. Barta, Green Chem. 2017, 19, 2774 – 2782. [24] P. J. Deuss, C. W. Lahive, C. S. Lancefield, N. J. Westwood, P. C. J. Kamer, K. Barta, J. G. de Vries, ChemSusChem 2016, 9, 2974 – 2981. [25] R. Jastrzebski, S. Constant, C. S. Lancefield, N. J. Westwood, B. M. Weckhuysen, P. C. A. Bruijnincx, ChemSusChem 2016, 9, 2074 – 2079. [26] A. Rahimi, A. Ulbrich, J. J. Coon, S. S. Stahl, Nature 2014, 515, 249 – 252. [27] C. S. Lancefield, O. S. Ojo, F. Tran, N. J. Westwood, Angew. Chem. Int. Ed. 2015, 54, 258 – 262; Angew. Chem. 2015, 127, 260 – 264. [28] K. St-rk, N. Taccardi, A. Bçsmann, P. Wasserscheid, ChemSusChem 2010, 3, 719 – 723. [29] K. Yamamoto, T. Hosoya, K. Yoshioka, H. Miyafuji, H. Ohno, T. Yamada, ACS Sustainable Chem. Eng. 2017, 5, 10111 – 10115. [30] S. W. Liu, Z. L. Shi, L. Li, S. T. Yu, C. X. Xie, Z. Q. Song, RSC Adv. 2013, 3, 5789 – 5793. [31] I. Bosque, G. Magallanes, M. Rigoulet, M. D. K-rk-s, C. R. J. Stephenson, ACS Cent. Sci. 2017, 3, 621 – 628. [32] N. C. Luo, M. Wang, H. J. Li, J. Zhang, T. T. Hou, H. J. Chen, X. C. Zhang, J. M. Lu, F. Wang, ACS Catal. 2017, 7, 4571 – 4580.

ChemSusChem 2018, 11, 2758 – 2765

www.chemsuschem.org

[33] M. V. Galkin, J. S. M. Samec, ChemSusChem 2016, 9, 1544 – 1558. [34] L. P. Xiao, S. Z. Wang, H. L. Li, Z. W. Li, Z. J. Shi, L. Xiao, R. C. Sun, Y. M. Fang, G. Y. Song, ACS Catal. 2017, 7, 7535 – 7542. [35] H. Luo, M. M. Abu-Omar, Green Chem. 2018, 20, 745 – 753. [36] E. M. Anderson, R. Katahira, M. Reed, M. G. Resch, E. M. Karp, G. T. Beckham, Y. Rom#n-Leshkov, ACS Sustainable Chem. Eng. 2016, 4, 6940 – 6950. [37] S. Van den Bosch, T. Renders, S. Kennis, S.-F. Koelewijin, G. Van den Bossche, T. Vangeel, A. Deneyer, D. Depuydt, C. M. Courtin, J. M. Thevelein, W. Schutyser, B. F. Sels, Green Chem. 2017, 19, 3313 – 3326. [38] Q. Song, F. Wang, J. Y. Cai, Y. H. Wnag, J. J Zhang, W. Q. Yu, J. Xu, Energy Environ. Sci. 2013, 6, 994 – 1007. [39] S. Van Den Bosch, W. Schutyser, R. Vanholme, T. Driessen, S. F. Koelewijn, T. Renders, B. De Meester, W. J. J. Huijgen, W. Dehaen, C. M. Courtin, B. Lagrain, W. Boerjan, B. F. Sels, Energy Environ. Sci. 2015, 8, 1748 – 1763. [40] T. Parsell, S. Yohe, J. Degenstein, T. Jarrell, I. Klein, E. Gencer, B. Hewetson, M. Hurt, J. I. Kim, H. Choudhari, B. Saha, R. Meilan, N. Mosier, F. Ribeiro, W. N. Delgass, C. Chapple, H. I. Kenttamaa, R. Agrawal, M. M. AbuOmar, Green Chem. 2015, 17, 1492 – 1499. [41] T. H. Parsell, B. C. Owen, I. Klein, T. M. Jarrell, C. L. Marcum, L. J. Haupert, L. M. Amundon, H. I. Kentt-maa, F. Ribeiro, J. T. Miller, M. M. Abu-Omar, Chem. Sci. 2013, 4, 806 – 813. [42] X. M. Huang, O. M. Morales Gonzales, J. D. Zhu, T. I. Kor#nyi, M. D. Boot, E. J. M. Hensen, Green Chem. 2017, 19, 175 – 187. [43] X. M. Huang, J. D. Zhu, T. I. Kor#nyi, M. D. Boot, E. J. M. Hensen, ChemSusChem 2016, 9, 3262 – 3267. [44] X. M. Huang, X. H. Ouyang, B. M. S. Hendriks, O. M. M. Gonzalez, J. D. Zhu, T. I. Kor#nyl, M. D. Boot, E. J. M. Hensen, Faraday Discuss. 2017, 202, 141 – 156. [45] E. Feghali, G. Carrot, P. Thu8ry, C. Genre, T. Cantat, Energy Environ. Sci. 2015, 8, 2734 – 2743. [46] J. F. Zhang, Y. Chen, M. A. Brook, ACS Sustainable Chem. Eng. 2014, 2, 1983 – 1991. [47] P. Ferrini, R. Rinaldi, Angew. Chem. Int. Ed. 2014, 53, 8634 – 8639; Angew. Chem. 2014, 126, 8778 – 8783. [48] M. V. Galkin, A. T. Smit, E. Subbotina, K. A. Artemenko, J. Bergquist, W. J. J. Huijgen, J. S. M. Samec, ChemSusChem 2016, 9, 3280 – 3287. [49] X. Q. Si, F. Lu, J. Z. Chen, R. Lu, Q. Q. Huan, H. F. Jiang, E. Taarning, J. Xu, Green Chem. 2017, 19, 4849 – 4857. [50] L. Shuai, M. T. Amiri, Y. M. Questell-Santiago, F. H8roguel, Y. D. Li, H. Kim, R. Meilan, C. Chapple, J. Ralph, J. S. Luterbacher, Science 2016, 354, 329 – 333. [51] W. Lan, M. T. Amiri, C. M. Hunston, J. S. Luterbacher, Angew. Chem. Int. Ed. 2018, 57, 1356 – 1360; Angew. Chem. 2018, 130, 1370 – 1374. [52] T. Renders, S. Van den Bosch, S.-F. Koelewijn, W. Schutyser, B. F. Sels, Energy Environ. Sci. 2017, 10, 1551 – 1557. [53] Y. Shao, Q. N. Xia, L. Dong, X. H. Liu, X. Han, S. F. Parker, Y. Q. Cheng, L. L. Daemen, A. J. Ramirez-Cuesta, S. H. Yang, Y. Q. Wang, Nat. Commun. 2017, 8, 16104. [54] T. Y. Guo, Q. N. Xia, Y. Shao, X. H. Liu, Y. Q. Wang, Appl. Catal. A 2017, 547, 30 – 36. [55] L. Dong, L.-L. Yin, Q. N. Xia, X. H. Liu, X.-Q. Gong, Y. Q. Wang, Catal. Sci. Technol. 2018, 8, 735 – 745. [56] I. Kumaniaev, E. Subbotina, J. S-vmarker, M. Larhed, M. V. Galkin, J. S. M. Samec, Green Chem. 2017, 19, 5767 – 5771. [57] E. M. Anderson, M. L. Stone, R. Katahira, M. Reed, G. T. Beckham, Y. Rom#n-Leshkov, Joule 2017, 1, 613 – 622. [58] X. C. Li, Y. Y. Zhang, Q. N. Xia, X. H. Liu, K. H. Peng, S. H. Yang, Y. Q. Wang, Ind. Eng. Chem. Res. 2018, 57, 3545 – 3553. [59] A. Berlin, V. Maximenko, N. Gilkes, J. Saddler, Biotechnol. Bioeng. 2007, 97, 287 – 296. [60] Y. Zhang, J. J. Wang, J. W. Ren, X. H. Liu, X. C. Li, Y. J. Xia, G. Z. Lu, Y. Q. Wang, Catal. Sci. Technol. 2012, 2, 2485 – 2491.

Manuscript received: May 3, 2018 Revised manuscript received: June 10, 2018 Version of record online: July 16, 2018

2765
