Hydrogenolysis of biorefinery corncob lignin into aromatic phenols over activated carbon-supported nickel

Shuizhong Wang a, Wa Gaoa, Ling-Ping Xiao*ab, Jia Shia, Run-Cang Sunab and Guoyong Song*a
aBeijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing 100083, China. E-mail: lpxiao@bjfu.edu.cn; songg@bjfu.edu.cn
bCenter for Lignocellulose Science and Engineering, Liaoning Key Laboratory of Pulp and Paper Engineering, Dalian Polytechnic University, Dalian 116034, China

Received 17th July 2018 , Accepted 21st September 2018

First published on 21st September 2018


Lignin is the most abundant renewable resource for production of bio-aromatic chemicals. However, the catalytic depolymerisation of lignin into phenolic monomers is still challenging due to its intrinsic heterogeneity. Herein, we report that Ni/AC can serve as an efficient and stable catalyst in the hydrogenolysis of biorefinery corncob lignin into low-molecular-weight aromatics. This catalyst showed high activity towards reductive fragmentation of hydroxycinnamic esters and β-O-4 linkages through scission by reaction with hydrogen to cleave C–O bonds. Under optimal conditions, this catalytic system produced 31% selectivity towards unsaturated substituents containing coumarate and ferulate derivatives, which afforded mono-aromatic phenols (up to 12.1 wt%) derived from hydroxycinnamic acid moieties (8.1 wt%) and β-O-4 units (2.7 wt%), respectively. The use of non-precious metals to develop highly efficient heterogeneous catalysts makes this approach of great interest in the production of aromatic phenols for the valorisation of industrial lignin.


1. Introduction

Tremendous biorefinery efforts have been made to use lignocellulosic biomass for the production of fuels and chemicals which can reduce our dependence on fossil fuels.1,2 Though designed applications for cellulose and hemicelluloses already exist in the form of the current industrial as well as biorefinery complexes of pulp and paper, sustainable ways to valorise the lignin fraction are yet to be established, due to its intrinsic heterogeneity.3,4 This constitutes a major drawback in the economic viability of a biorefinery, which requires valorisation of all fractions to be effective. Lignin is the most abundant natural source of renewable aromatic carbon on earth and is now well recognized as an important starting point for future production of bio-aromatic chemicals.5 One of the keys for lignin valorisation is to develop a new catalytic method for lignin fragmentation into phenolic monomers which can then either be directly used in chemical applications or undergo further upgradation to bulk chemicals and fuels.6–14 The monolignols of lignin are connected via ether and carbon–carbon (C–C) bonds, including the common linkages of β-O-4, β-β, and β-5 in native lignin.3,15 The dissociation energy of ether bonds is weaker than that of C–C bonds.16,17 Thereby, aiming at the transformation of lignin macromolecules into chemicals and fuels, the cleavage of ether bonds by chemical and physical methods is the most attractive point.18,19 However, the massive lignins obtained from the biorefinery industry possess more refractory C–C bonds than native lignin due to the irreversible condensation reactions.12,13,20 Moreover, the annual output of biorefinery lignin is much higher than the annual consumption. Hence, seeking an effective means to utilize the biorefinery lignin is an urgent problem.

Recently, it has been revealed that catalytic hydrogenolysis is a promising avenue to valorize lignin into low-molecular-weight and value added co-products. These procedures usually use a heterogeneous catalyst based on precious (Pd,21–35 Pt,30–32,34,36–39 Ru,24,25,30–35,40–44 Rh,30,34,35,39,45 Ir,32,39 and Re46,47) or non-precious metals (Ni,25,31,35,48–64 RANEY® Ni,65–70 Mo,63,64,71–77 W,32,64,78 Co,47,79 Cu,33,80–85 and Fe58,86,87). It has been reported that alkali lignin obtained from soda pulping of wheat straw could be depolymerised to phenolic monomers with the use of a metal (Ti, Mo, Nb, W) nitride catalyst (19 wt%) and CuMgAlOx catalyst (23 wt%) at 300 °C in supercritical ethanol.88,89 Wang and Xu described the catalytic hydrogenolysis of lignosulfonate into phenols, affording high lignin conversion (>60%) and selectivity for 4-propylguaiacol and 4-ethyl-guaiacol (75–95%) with Ni-based catalysts under 5 MPa pressure of H2.54 Kraft lignin was completely degraded into C6–C10 chemicals (1640 mg g−1) and C6–C11 molecules (1390 mg g−1), including alcohols, esters, monophenols, benzyl alcohols and arenes, with a Mo-based catalyst in supercritical ethanol.73 Similarly, Heeres and Barta's group used a sulphided NiW/AC catalyst to depolymerize kraft lignin into aromatic monomers (28 wt%) with a high selectivity (76%) toward alkyl phenolics and guaiacolics in supercritical methanol.64 In these cases, the catalytic disassembly of lignin into aromatic compounds demands supercritical conditions or higher reaction pressure. Milder reaction conditions should be explored to utilize the massive lignin for reducing the additional energy input.

Herbaceous lignin, consisting of GSH units, possesses non-canonical subunits (p-coumaric acid and ferulic acid moieties) and is connected with hemicelluloses via ether and ester bonds in the plant cell wall.90,91 In our previous reports on the MoOx/CNT-catalysed hydrogenolysis of enzymatic mild acidolysis lignins (EMALs) derived from wood and herbaceous crop, it was demonstrated that a Mo-based catalyst showed high activity and selectivity toward phenolic compounds having an unsaturated substituent especially for the reductive fragmentation of lignin from herbaceous plants.74 We also presented a comprehensive study on the catalytic degradation of biorefinery corncob lignin derived from an alkali pretreatment process to obtain methyl coumarate and methyl ferulate with zinc molybdate (ZnMoO4) catalyst supported on MCM-41.75 The results indicated that the ZnMoO4 catalyst could selectively release non-canonical subunits from biorefinery corncob lignin with minor aromatic monomers derived from β-O-4 units. Based on those results, the primary objective of this work was to continue exploring the product distribution and selectivity towards the C–O scissions from β-O-4 units and p-coumaric acid (pCA) as well as ferulic acid (FA) moieties. Thus, one of the most promising catalytic materials (5 wt% Ni/AC) was tested as the catalyst for the hydrogenolysis of biorefinery corncob lignin into aromatic phenols.

2. Materials and methods

2.1. Materials

All chemical reagents used were analytical grade or the best available. Biorefinery corncob lignin was obtained from Shandong Longlive Bio-Technology Co., Ltd, China.75,92 Biomass compositional analysis was performed using the standard NREL laboratory analytical procedure (NREL/TP-510-42618)93 as described previously.94,95

2.2. Catalyst preparation and characterization

5 wt% Ni/AC was prepared by incipient-wetness impregnation according to the pervious literature.55 Ni(NO3)2·6H2O was used as the precursor for the catalyst. Following the impregnation, the solid was dried at 100 °C overnight and reduced in 10% H2/N2 at 400 °C for 4 h.

ICP-OES was performed by using an ICP optima 8X00 instrument. Powder XRD patterns of samples were recorded by using a Rigaku Miniflex-600 operated at 40 kV voltage and 15 mA current with CuKα radiation (λ = 0.15406 nm). XPS was performed by using a scanning X-ray microprobe (PHI 5000 Verasa, ULAC-PHI, Inc.) using Al Kα radiation, and the C1s peak at BE = 284.8 eV was used as the internal standard.

2.3. Lignin depolymerisation procedure

In a typical procedure, biorefinery corncob lignin (50 mg), 5 wt% Ni/AC catalyst (10 mg), and solvent (10 mL) together with a magnetic stirrer were placed inside a 100 mL glass liner. Then the stainless-steel batch autoclave was sealed and sequentially flushed 3 times with pure N2 and H2, respectively. After this, the reactor was pressurized up to 3 MPa H2. The reaction system was heated up to the desired temperature with a heating rate of approximately 1.5 °C min−1. Once the target temperature was achieved, the system was maintained at this temperature for 4 h. Subsequently, the reaction mixture was separated through filtration and the solid residue was washed with methanol. The oily lignin product was extracted with dichloromethane (CH2Cl2) and subsequently the solvent was removed under vacuum (<40 °C). The acquired oily lignin product was dissolved in 5 mL CH2Cl2, which contained the internal standard (tetradecane). Then the oily lignin products were analyzed by GC-MS and GC after filtration using a PTFE filter (0.45 mm) as described previously.75 Fig. 1 depicts the workup procedure of the catalytic hydrogenolysis of lignin.
image file: c8se00359a-f1.tif
Fig. 1 The post-processing flowchart of catalytic hydrogenolysis of biorefinery corncob lignin with Ni/AC catalyst.

For the recycling experiments, the first run was performed with a fresh catalyst. After the reaction, the spent catalyst was first recovered by a magnet and then washed with MeOH and reused directly for the next cycle under the same reaction conditions.

2.4. Lignin and oily lignin product analysis

A GC-MS (Shimadzu GCMS-QP2010SE) instrument, equipped with a HP-5 MS (30 m length × 250 mm I.D. × 0.25 mm film thickness, Agilent) capillary column, was employed to analyze the oily lignin product. Helium (He, 99.9999%) was used as a carrier gas with a constant column flow of 3 mL min−1. The injector, interface and ion source temperatures were set at 250 °C, 280 °C and 200 °C, respectively. The split ratio was 50[thin space (1/6-em)]:[thin space (1/6-em)]1 and the scan model was applied to identify the aromatic monomers from 50 m z−1 to 700 m z−1. The oven temperature was programmed from 50 °C to 280 °C at 8 °C min−1, and held at 50 °C and 280 °C for 3 min and 5 min, respectively. The GC (Shimadzu GC-2010) was also equipped with a HP-5 column and a FID, and high purity nitrogen was used as a carrier gas with a constant column flow of 1.39 mL min−1. The injector temperature and the detection temperature (FID) were held constant at 250 °C and 290 °C, respectively. The split ratio was 20[thin space (1/6-em)]:[thin space (1/6-em)]1 and the temperature programming was consistent with the GC-MS. The identification and quantification of lignin monomers in the oily product were performed by comparison with authentic samples acquired from commercial purchase or independent synthesis. The monomer yield was calculated from the following equation:
 
image file: c8se00359a-t1.tif(1)

The average molecular weights (Mw) of acetylated biorefinery corncob lignin and the oily lignin product were detected by Gel Permeation Chromatography (GPC) according to our previous papers.74,75 2D HSQC NMR spectra of the biorefinery corncob lignin and oily lignin product were obtained on a Bruker AVIII 400 MHz spectrometer fitted with a 5 mm gradient probe with inverse geometry (proton coils closest to the sample) at 25 °C using 50 mg of lignin material in 0.5 mL of DMSO-d6.74

3. Results and discussion

3.1. Catalytic hydrogenolysis of biorefinery corncob lignin

Biorefinery corncob lignin was used as the substrate for the catalytic hydrogenolysis in the present study. The compositional analysis of the biomass indicated that this lignin has a high purity with a total lignin content of 90.6% (Klason lignin and acid-soluble lignin). Besides S, G, and H units with a ratio of 4[thin space (1/6-em)]:[thin space (1/6-em)]1.3[thin space (1/6-em)]:[thin space (1/6-em)]1, pCA and FA units with a ratio of 2.3 were also found in the 2D HSQC NMR spectra (Fig. 3b and c). This lignin sample mainly contained 8.7% β-O-4 linkages (A, labeled in blue in Fig. 3b).

On treatment of biorefinery corncob lignin with 20 wt% loading of Ni/AC at 240 °C and 3 MPa H2 in MeOH for 4 h in a stainless-steel batch autoclave, a brown soluble oily lignin product was obtained after extraction with CH2Cl2 (Table 1, entry 1 and Fig. S1a). This catalytic hydrogenolysis reaction afforded a yield of 12.1 wt% aromatic monomers, and the detailed monomer distribution is depicted in Table S1. This process produced mainly S, G, and H units derived phenols, including propyl/propenyl guaiacol and syrinol (2.7 wt%); mono-phenols of ethyl/vinyl phenol and guaiacol (4.5 wt%) and methyl coumarate/ferulate and their derivatives (3.6 wt%) were also detected, which were generated from hydroxycinnamic acids (pCA and FA) by a decarboxylation reaction.52 These results were different from those of our previous work on selective fragmentation of biorefinery corncob lignin into p-hydroxycinnamic esters with a ZnMoO4/MCM-41 catalytic system.75 This may be due to the fact that the late transition metal Ni exhibited higher catalytic activity than the early transition metal Mo for lignin hydrogenation.55,96 Interestingly, unsaturated phenols (31%) were also detected in the present Ni/AC catalytic system, which are in accordance with recent results on the catalytic lignin depolymerisation of Miscanthus49 and catalytic degradation of organosolv lignin isolated from poplar.48 These unsaturated products may be due to the solvolysis of lignin into aromatic monomers without further hydrogenation under the current conditions.57

Table 1 Product distribution, average molecular weight (Mw), and monomer yield of the catalytic depolymerisation of biorefinery corncob lignin with different solventsa
Entry Catalyst Solvent Mw (g mol−1) Monomer yieldb (wt%) Total monomers yield (wt%)
Ethyl/vinyl phenol and guaiacol Methyl coumarate/ferulate and their derivatives Propyl/propenyl guaiacol and syrinol Others
a Reaction conditions unless specified otherwise: lignin (50 mg), Ni/AC catalyst (10 mg), solvent (10 mL), 240 °C, 4 h, H2 (3 MPa).b Monomer yield is based on the total lignin content.c Reaction performed under N2 (0 MPa).
1 Ni/AC MeOH 1181 4.5 3.6 2.7 1.3 12.1
2 Ni/AC EtOH 1231 2.1 2.3 1.7 2.3 8.4
3 Ni/AC iPrOH 878 3.3 1.4 1.9 3.7 10.3
4 Ni/AC THF 836 2.1 0.5 2.3 0.9 5.8
5 Ni/AC Dioxane 1000 0.6 0 1.0 1.5 3.1
6 Ni/AC H2O 352 2.7 0 1.3 2.5 6.5
7 None MeOH 1654 3.8 0.7 0.5 1.2 6.2
8 AC MeOH 1027 3.2 0.3 0.2 1.6 5.3
9c Ni/AC MeOH 846 3.2 1.2 1.0 1.2 6.6


Acetylation of the oily lignin product and analysis by GPC showed a significant decrease in the average molecular weight (Mw 1181 g mol−1) relative to the raw lignin (Mw 4410 g mol−1) (Table 1, entry 1; Fig. 2). The structural properties of the obtained oily lignin product were analyzed by 2D HSQC NMR (Fig. 3), and the assignments are summarized in Table S5. The signals of Aα (δC/δH 71.6/4.85 ppm) and Aβ (δC/δH 86.6/4.09 ppm) corresponding to benzylic alcohols and secondary alkyl protons of β-O-4 linkages were no longer observed after the catalytic reaction (Fig. 3b and e). This result suggested the complete scission of C–O–C linkages in biorefinery corncob lignin under the current reaction conditions. Moreover, the C–H signal peaks of lignin monomer compounds were detected and assigned, including 4-ethyl-phenol/guaiacol (H1/G1), methyl 3-(4-hydroxyphenyl)propanoate (H3), methyl coumarate (H4), methyl 3-(4-hydroxy-3-methoxyphenyl)propanoate (G3), methyl ferulate (G4), 2-methoxy-4-propylphenol (G5), and 2,6-dimethoxy-4-propylphenol (S1) (Fig. 3d and f). Vinyl phenol and guaiacol were only detected at the signal peak of H/G2(7) at δC/δH 136.1/6.72 ppm. These results correspond well to the lignin monomer products of GC-MS analysis.


image file: c8se00359a-f2.tif
Fig. 2 The molecular weight distribution of biorefinery corncob lignin (red), oily lignin product with (green) and without the Ni/AC catalyst (black).

image file: c8se00359a-f3.tif
Fig. 3 2D HSQC NMR spectra of (a–c) biorefinery corncob lignin and (d–f) Ni/AC-catalysed oily lignin product (reaction conditions from Table 1, entry 1).

3.2. Effect of reaction solvent

A series of solvents were screened in the catalytic hydrogenolysis of biorefinery corncob lignin in the presence of Ni/AC catalyst at 240 °C, 4 h, and 3 MPa H2, and the results are summarized in Table 1 and Table S1. Analogous to the case of MeOH, the catalytic hydrogenolysis reaction in EtOH and iPrOH proceeded to give mono-phenolic compounds in 8.4 wt% and 10.3 wt% yields, respectively. In the case of THF, the yield of monomers decreased to 5.8 wt% (Table 1, entry 4). When dioxane (Table 1, entry 5 and Table S1) and H2O (Table 1, entry 6 and Table S1) were used instead of MeOH, multiple phenolic monomers were obtained in 3.1 wt% and 6.5 wt%, respectively, with no observable coumarate and ferulate derivatives.

A control experiment which involved heating the lignin sample in MeOH under 3 MPa of H2 without the catalyst was performed, from which a dark oily lignin product (Fig. S1b) with a high molecular weight of 1654 g mol−1 was obtained (Table 1, entry 7 and Table S1), demonstrating that lignin may undergo severe condensation. Analysis of 2D HSQC NMR spectra revealed that the β-O-4 linkages still remained after the control reaction as we previously observed.75 These results suggested that the Ni/AC catalyst is essential for the cleavage of β-O-4 linkages. When the catalytic reactions were carried out using the catalyst support of activated carbon (AC) or under an ambient pressure of N2, the yields of both lignin monomers significantly dropped (Table 1, entries 8 and 9; Table S1). In these cases, poor selectivities towards coumarate and ferulate derivatives were observed. These results demonstrated that the Ni/AC catalyst could achieve the selective reductive fragmentation of hydroxycinnamic esters and β-O-4 linkages through scission by reaction with hydrogen to cleave C–O bonds.

3.3. Effect of reaction temperature and time

The effect of reaction temperature and time on the Ni/AC-catalytic hydrogenolysis of biorefinery corncob lignin was investigated (Fig. 4b and c, and Tables S2 and S3). Increasing the reaction temperature led to an increase of the monomer yield of lignin oil until 240 °C. Further increase of the temperature to 280 °C caused the yields of monomer phenols to decrease (10.7 wt%), probably due to the slight recondensation of the resulting monomers at higher reaction temperatures. A similar parabolic trend was also observed at different reaction times (Fig. 4c). In the case of shorter reaction times, the reductive fragmentation gave lower yields of monomers (2 h, 8.7 wt%; 3 h, 10.7 wt%) in comparison with those from a 4 h reaction time (12.1 wt%) (Fig. 4c, Table S3). Prolonging the reaction time to 5 h slightly led to a decreased monomer yield (11.1 wt%). The molecular weight remained constant in the range of 1123–1231 g mol−1 (Fig. S4).
image file: c8se00359a-f4.tif
Fig. 4 Effects of (a) reaction solvent, (b) reaction temperature, (c) reaction time, and (d) run time on the catalytic hydrogenolysis of biorefinery corncob lignin with Ni/AC. Reaction conditions for (a): lignin (50 mg), Ni/AC catalyst (10 mg), solvent (10 mL), 240 °C, 4 h, H2 (3 MPa). Reaction conditions for (b): lignin (50 mg), Ni/AC catalyst (10 mg), MeOH (10 mL), 4 h, H2 (3 MPa). Reaction conditions for (c): lignin (50 mg), Ni/AC catalyst (10 mg), MeOH (10 mL), 240 °C, H2 (3 MPa). Reaction conditions for (d): lignin (50 mg), Ni/AC catalyst (10 mg), MeOH (10 mL), 240 °C, 4 h, H2 (3 MPa).

3.4. Catalyst reusability

Finally, the reusability and stability of the Ni/AC catalyst were investigated. After the reaction, the catalyst could be easily separated from the reaction medium with an external magnet (Fig. S2). The spent catalyst was then washed with MeOH three times and used directly for the subsequent cycle under the same reaction conditions as described for the first run time. As shown in Fig. 4d, the monomer yield was almost the same after six successive run times. The ICP-OES analysis of the reused catalyst shows only less than 0.5% Ni leaching in the reaction solution, indicating a good reuse performance of Ni/AC for lignin hydrogenolysis. Notably, the proportion of unsaturated aromatic compounds slightly increased as the run times increased, which indicated a decrease in the catalytic hydrogenation activity of Ni/AC. To further illustrate this issue, we characterized the used Ni/AC using XRD and XPS. Both XRD pattern (Fig. S5) and Ni 2p XPS results (Fig. S6) are slightly modified compared to those of the fresh catalyst, which indicates that the Ni species in Ni/AC changed to some degree after the catalytic reaction run. Moreover, the decrease in the catalytic hydrogenation activity of Ni/AC may be attributed to the mass loss of the catalyst in the catalyst recovery process and catalyst aggregation as well as the blockage of catalytic active sites by the cokes produced under high temperature reaction conditions.97,98 Overall, this result indicated that the catalyst could be reused without significant loss of its catalytic activity.

4. Conclusions

In summary, earth-abundant Ni catalyst supported on activated carbon (Ni/AC) exhibits high activity in the catalytic hydrogenolysis of biorefinery corncob lignin. We have demonstrated that the Ni/AC catalyst could achieve selective reductive fragmentation of hydroxycinnamic esters and β-O-4 linkages through scission by reaction with hydrogen to cleave C–O bonds. Under optimal conditions, this catalytic system produced 31% selectivity towards unsaturated substituents containing coumarate and ferulate derivatives. The parameters such as reaction solvent, reaction temperature and time have a significant influence on both the activity and selectivity of the biorefinery lignin hydrogenolysis. Finally, the Ni/AC catalyst could be readily collected from the reaction solution by an external magnet and reused up to six run times without loss of its catalytic reactivity. Thus, the strategy of design and application of non-precious metal catalysts might offer an alternative way to develop highly efficient heterogeneous catalysts for the valorisation of biorefinery lignin.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities (No. 2018ZY01, 2017ZY25) and the National Natural Science Foundation of China (No. 21506013, 21776020), and the National Program for Thousand Young Talents of China. W. Gao thanks the National Training Program of Innovation and Entrepreneurship for Undergraduates (No. 201610022053, S201610022082).

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8se00359a
These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2018