Methanolysis fractionation and catalytic conversion of poplar wood

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Methanolysis fractionation and catalytic conversion of poplar wood toward methyl levulinate, phenolics, and glucose Qiaolong Zhai, Chung-Yun Hse, Feng Long, Todd F. Shupe, Fei Wang, Jianchun Jiang, and Junming Xu J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.9b03806 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 22, 2019

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Journal of Agricultural and Food Chemistry

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Methanolysis fractionation and catalytic conversion of poplar

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wood toward methyl levulinate, phenolics, and glucose.

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Qiaolong Zhai, a Chung-yun Hse, c Feng Long, a Todd F. Shupe d, Fei Wang, a Jianchun Jiang, a and Junming Xua,b*

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a

5

Material, Jiangsu Province; National Engineering Lab. for Biomass Chemical Utilization; Key and Open Lab. on Forest

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Chemical Engineering, SFA, Nanjing 210042, China

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b

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Nanjing, 210037, China.

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c

Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry; Key Lab. of Biomass Energy and

Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University,

United States Department of Agriculture (USDA) Forest Service, Southern Research Station, Pineville, Louisiana

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71360, United States

11

d

12

ab

13

Abstract

14

In the present study, methanolysis of poplar biomass was conducted for the selective transformation of

15

hemicellulose and lignin, which leads to methyl glycosides (mainly C5 glycosides) and lignin fragments in the

16

liquefied products, which can be separated according to their difference in hydrophilicity. The distribution of

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methyl glycosides and delignification was dependent on the presence of acid catalysts and reaction temperatures.

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The obtained lignin fraction was separated into solid lignin fragments and liquid lignin oil according to their

19

molecular weight distribution. Subsequently, directional conversion of methyl C5 glycosides into methyl levulinate

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was performed with dimethoxymethane/methanol as the co-solvent. A yield of 12 to 30 % of methyl levulinate

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yield (based on the methyl glycoside) was achieved under the condition. The remaining cellulose-rich substrate

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showed enhanced susceptibility to enzymatic hydrolysis, resulting in the yield of glucose above 70%. Overall, the

Wood Science Consulting, LLC, Baton Rouge, Louisiana 70816, United States *Corresponding author: Tel. +86 2585482478; Fax: 025-85482485 E-mail: [email protected].

ACS Paragon Plus Environment


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described strategy shows practical implications for the effective valorization of biomass.

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Keywords: Methanolysis; Glycoside; Phenolics; Enzymatic hydrolysis; Methyl levulinate.

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1. Introduction

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Widely distributed and highly abundant lignocellulosic biomass has been considered as a promising source of

27

energy, biofuels and biochemicals.1, 2 Lignocellulose has a complex and strongly interlinked structure consisting of

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three main biopolymers, cellulose, hemicellulose and lignin in the plant cell wall. Chemical, biological, or physical

29

constraints and their inter-relations must be overcome before the conversion of various feedstocks into valuable

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chemicals and materials can be achieved.3 The efficient fractionation of biomass into its individual components

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and efficient utilization of each component is optimized via bio-refinery processes for the valorization of the entire

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biomass. 4, 5, 6 Specifically, cellulose is considered to be the most economical because it can be depolymerized into

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monosaccharides (glucose), and then converted to bioethanol, biobutanol, or other chemicals using chemical or

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fermentation strategies.6, 7 The released sugars from hemicellulose can also be used as substrates for fermentation

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or anaerobic digestion. Alternatively, sugars can be converted into various value-added chemicals such as furfural

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or levulinic acid/ester by catalytic conversion. 8-10 Lignin, a complex heterogeneous natural polymer with different

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phenylpropane units linked by various C-O and C-C bonds (i.e., β-O-4, β-β, β-5, and β-1) and has received much

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attention in the production of various phenolics and materials.11

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Currently, biomass refinery methods have attracted much attention for the efficient fractionation of

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lignocellulose biomass. Numerous approaches have been investigated for the pre-treatment of lignocellulose, such

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as hydrothermal 12, 13, steam explosion 14, alkali methods 15 and solvolysis treatment 16-18 and have been combined

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with high pressure, ultrasound-assisted and microwave-assisted processing. Remarkable progress has been made

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in the application of solvolysis pretreatment for converting biomass and the facilitation of further upgrading due

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to an improvement in reactivity. For example, an increasing number of solvolysis studies in supercritical or


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subcritical methanol have been reported.18,

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biomass because of the lower dielectric constants as compared with water. Another advantage is that methanol

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can be recovered by simple distillation, which reduces the overall energy consumption and separation cost of the

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process. Furthermore, alcohol-derived decomposition products (e.g., hydrogen, alkoxy moieties) can quench some

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intermediates during the reaction process.21, 22 The -OH in methanol acts as a nucleophile and can attract the

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glycosidic bonds of (hemi) cellulose and acid-ester bonds of lignin compounds with an acid catalyst and can cause

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depolymerization of aromatics and polysaccharides in lignocellulose.23 Previous studies have mostly focused on

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increasing yields and characterizing properties of bio-oil. The previous studied have typically yielded unstable liquid

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products, containing various complex components (e.g., aldehydes, organic acids, esters, phenolics and oligomers);

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these products mainly contributed to the structural complexity and different properties of biomass components

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16, 23.

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derivatives and lignin fragments which can be separated by their hydrophilic differences. Due to the similar

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chemical nature of each stream, the reduced complexity of the chemical components in each product stream will

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facilitate downstream upgrading processes.

19, 20

It provides higher solubility for decomposed products from

The selective conversion of (hemi)cellulose and lignin towards their respective oligomers products sugar

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Methyl levulinate (MLA) is one of the most promising derivatives converted from carbohydrates and can be

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widely used in many fields such as medicines, solvents, organic chemistry, and fragrance. 24, 25 Numerous studies

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have reported that C6 sugars in carbohydrates of biomass can potentially be converted into methyl levulinate/

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levulinic acid with an acid catalyst.25-27 However, multiple steps involving both hydrogenation and acid catalyzed

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processes for the transformation of C5 sugars to methyl levulinate/Levulinic acid are required. Recently, the

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investigation of direct conversion of C5 sugar or furfural into methyl levulinate/ levulinic acid was proposed.28, 29.

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In this study, to further valorize the hemicellulose, we explored a one-step transformation of C5 sugar (C5 glycoside)

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derived from biomass into methyl levulinate in a dimethoxymethane/methanol co-solvent with an acid catalyst.



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Here, methanolysis treatment was used to release the hemicellulose sugar (methyl glycoside) and lignin

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fragments from poplar sawdust, leaving behind a cellulose-rich solid substrate (cellulose pulp) (Fig.1). Various acid

69

catalysts and temperatures were assessed in the methanolysis process to determine the fractionation efficiency,

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delignification and product distributions. Pure lignin fragments (including solid lignin fragments and liquid lignin

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oil) were separated from methyl glycoside in high yield with stepwise fractionation methods based on the

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hydrophilic differences. Subsequently, the hemicellulose sugar (mainly methyl C5 glycosides) with high purity were

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directly transformed to levulinic acid/ester in a dimethoxymethane/methanol mixture with an acid catalyst. The

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cellulose-rich sample was further subjected to enzymatic hydrolysis to provide a high yield of glucose. The

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described strategy achieved the fractionation and sequential utilization of all the biomass components.

76 77 78

Fig. 1. Process scheme for the selective fractionation and catalytic conversion of poplar wood for value-added chemicals.

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2. Experimental

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2.1 Material

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Poplar wood (Populus L) was obtained from Nanjing, China, passed through a 250-425 µm sieve (60-80mesh)

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and then dried overnight at 105 °C. The composition (wt. %) of the poplar wood was determined following the

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NREL/TP-510-42618 and the cellulose, hemicellulose, and lignin contents were 47.14, 22.86 and 23.73 wt.%,

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respectively. Methanol (99%), dimethoxymethane (99%), methyl levulinate, H2SO4 (98%), H3PO4 (85%), HCl (37%),


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p-TsOH (98 %), HPW (Phosphotungstic acid) and sulfamic acid (99.3%) were purchased from Nanjing Kermel

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Chemical Reagent Co., Ltd. Methyl-α-D-glucopyranoside (≥99%), methyl-β-D-glucopyranoside (≥99%), methyl-α-D-

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xylopyranoside (≥99%), and methyl-β-D-xylopyranoside (≥99%) were purchased from Sigma-Aldrich. Amberlyst 70

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catalyst (a solid acidic resin catalyst) was purchased from Dow Chemical.

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2.2 The methanolysis fractionation of poplar biomass

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The methanol solvothermal fractionation was carried out in a high-pressure reactor. In each run, 10 g of poplar

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sawdust, 100 mL of solvent methanol (solid/liquid ratio of poplar to methanol was 1:10 (w/w)), and a certain

92

amount of acid catalyst mixture were loaded into the reactor. The reactor was sealed and flushed three times with

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N2 to remove air. The mixtures were heated to 140-200 °C and then the temperature was kept constant for 40min.

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After the reaction, the reaction vessel was cooled to room temperature. Then, the insoluble solid residues were

95

separated by a filtration step from the liquid mixture and then dried overnight at 105 °C.

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The liquid mixture consisted sugars and lignin fragments along with the methanol solvent. The stepwise

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separation of the liquid mixture was as follows: deionized water was added to the filtrate, and the mixture was

98

evaporated under a negative pressure at 40 °C to remove half of the methanol solvent, leading to the precipitation

99

of solid lignin 1# from the mixture. Repeat the previous step and then completely distill all methanol solvent in the

100

mixture, and precipitate solid lignin 2#. Next, ethyl acetate was added to the remaining deionized water and

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products mixture, which was separated into an ethyl acetate-soluble phase and a water-soluble phase using a

102

separatory funnel. Finally, methyl glycoside and liquid lignin oil products could be obtained from the aqueous

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solution phase and ethyl acetate phase respectively.

104

(

Conversion (wt %) = 1 ―

𝑚𝑠

105

Yield of methyl glycoside (wt%) =

106

Yield of lignin oil (wt%) =

(𝑚

)

𝑚0 × 100%

(𝑚

𝑜𝑖𝑙

𝑔𝑙𝑦

)

𝑚0 × 100%

)

𝑚0 × 100%



107

Yield of methyl xyloside (wt%) =

(

(𝑚

𝑚𝐿𝐷

108

Delignification (wt%) = 1 ―

109

(Hemi)cellulose retention (wt%) = (

𝑥𝑦𝑙

)

𝑚0 × 100%

)

𝑚𝐿0 × 100%

𝑚 𝐻𝐷

𝑚𝐻0) × 100%

110

𝑚0 is the original mass of the poplar, 𝑚𝑠 is the mass of cellulose-rich substrate after the methanolysis process,

111

𝑚𝑔𝑙𝑦, 𝑚𝑜𝑖𝑙 are the mass of the methyl glycoside and the lignin oil, respectively. 𝑚𝑥𝑦𝑙 is the mass of the methyl

112

xyloside based on the HPLC result. 𝑚𝐶0,𝑚𝐻0, 𝑚𝐿0 are the mass of the original cellulose, hemicellulose and lignin in

113

feedstock, respectively, and 𝑚𝐶𝐷, 𝑚𝐻𝐷 𝑚𝐿𝐷are the mass of cellulose, hemicellulose and lignin, respectively, in the

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cellulose-rich substrate.

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2.3 Catalytic conversion of methyl glycoside

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Further conversion of methyl glycoside (mainly methyl C5 glycoside) was conducted as described below. The

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mixtures comprising 1.5 g of methyl glycoside reactants, 3.0 g of Amberlyst 70 and 30 g co-solvent methanol/DMM

118

(w/w, 15/15) were loaded into a 50 mL high-pressure reactor. The reactor was sealed and flushed threefold with

119

pressurized N2 to remove air. After, the reactor was heated to 140-170 °C, which was held constant for 120 min

120

with 350 rpm stirring. Once the reaction time was achieved, the autoclave was cooled using flowing water until

121

the temperature dropped below 30 °C. The solid products were separated with filter paper from the liquid mixture

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and thrice washed with methanol, then dried at 105°C overnight. The weight of the coke (solid polymer) was the

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weight of the solid insoluble matter minus the weight of the catalyst (2g).

124

Yield of methyl levulinate (wt%) =

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𝑚𝑚𝑙𝑎 is the mass of the methyl levulinate based the GC result.

126

2.4 Enzymatic hydrolysis

(𝑚

𝑚𝑙𝑎

)

𝑚𝑔𝑙𝑦 × 100%

127

Enzymatic hydrolysis of the cellulose-rich substrate was conducted at 50 °C on a shaking bed incubator at 250

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r/min. Two grams of cellulose-rich substrate, 10 mL acetate buffer (pH=5), and 10 FPU/g glucan was loaded into a


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15 mL glass vial. In addition, 1 mL tetracycline chloride solution (40 mg/mL) was added to prevent microbial growth.

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In some cases, a stir bar was added and vortexed with the substrate to facilitate the defibrillation and dispersion

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of cellulose. 200 μL of liquid was sampled at specific intervals and diluted to 1 mL to determine the sugar yield by

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HPLC.

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Yield of glucose (wt%) =

(𝑚

𝑔𝑙𝑢

)

𝑚𝐶𝐷 × 100%

134

𝑚𝑔𝑙𝑢 is the mass of the glucose based the result of the HPLC analysis.

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2.5 Analytical methods

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The quality of the cellulose rich substrates was analyzed by chemical composition analysis, scanning electron

137

microscope (SEM) and X-ray diffraction (XRD). Using the two-step hydrolysis method for the chemical composition

138

analysis based on NREL/TP-510-42618. Each sample was tested at least three times and the mean values are

139

reported. The SEM (5–10 kV accelerated voltage) was used to observe the morphology of the samples with

140

different acid treatment. The SEM specimens were gold-plated before analysis. The crystallinity analysis was

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conducted by XRD with a XRD-6000 X-ray diffract meter (monochromatic Cu/Ka radiation, the diffracted intensity

142

(2θ) varied from 10 to 60°with a step of 0.02°). The crystallinity index (CrI) was determined by the following

143

formulate. 30

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CrI% = (I002 ― I𝑎𝑚)/I002 × 100%

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I002 is the peak intensity of the 002 plane at about 2θ = 22.4°, and Iam represents the peak intensity for the

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amorphous cellulose portion at a about 2θ = 18.0°.

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The released sugars (methyl glycoside) were analyzed via GC-MS and HPLC for characterization. A quantitative

148

analysis of the methyl glycosides (methyl a-D-xylopyranoside, methyl β-D-xylopyranoside, methyl a-D-

149

glucopyranoside and methyl β-D-glucopyranoside) was conducted using an HPLC instrument equipped with a Bio-

150

Rad Aminex HPX-87H (300 × 7.8 mm) column and a refractive index detector. 5 mM H2SO4 was used as the mobile



151

phase at the flow rate of 0.5 mL/min. A quantitative analysis of the methyl glycosides was based on an external

152

standard. The standard curves of concentration of methyl xyloside and methyl glucoside were shown in Fig. S1.

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The composition analysis of methyl glycoside, lignin oil and methyl levulinate was performed using GC-MS

154

equipped with a HP-5 column with a flame ionization detector (FID). The sugars (containing methyl glycoside and

155

methyl levulinate) and lignin were dissolved in the methanol and tetrahydrofuran, respectively. The carrier gas,

156

He, was used with a flow rate of 2.0 mL/min, and the injection port temperature was 250°C. The oven temperature

157

program settings were as follows: the column temperature was heated to 90°C at a rate of 5 °C/min and then

158

heated to 250 at a rate of 10°C/min and held for 20min. The detector temperature was set to 250°C. A quantitative

159

analysis of methyl levulinate and furfural was determined using GC on a capillary column with a flame ionization

160

detector. Helium was used as the carrier gas with a flow rate of 1.0 mL/min. The internal standard was n-octanol

161

(Fig. S2) (standard curve y=2.06543x–0.02107, R2 =0.9998).

162

The fractioned lignin fragments containing lignin1#, 2# and lignin oil were analyzed by GPC, TG and 2D NMR

163

HSQC. The molecular weights of lignin fragments were measured using Waters 2695 GPC with a manually packed

164

column. The mobile phase was tetrahydrofuran with an injection volume of 20μL and a flow rate of 1 mL/min.

165

Polystyrene was used as an internal standard. The thermal properties of lignin fragments were studied using TGA

166

(PerkinElmer, Waltham, MA). Appropriately 10 mg of the sample was placed into the pan of the instrument and

167

heated from room temperatures to 700 °C with a heating rate of 10 °C/min. The lignin fragments (90 mg) were

168

dissolved in DMSO-d6 (0.5mL) for 2D HSQC NMR characterization to analyze the specific structures of the complex

169

compounds. The 2D-HSQC NMR spectra were measured by a Bruker DRX 500 NMR spectrometer. The spectral

170

widths were 8.5 and 120 ppm for the 1H and 13C dimensions, respectively.

171

3. Result and Discussion

172

3.1 The selective transformation of hemicellulose and lignin fractions in poplar wood


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Solvolysis fractionation of poplar sawdust was evaluated as a method to extract the structural components of

174

hemicellulose and lignin from biomass. In this study, the methanolysis pretreatment of poplar sawdust was

175

performed for directional fractionation and conversion of hemicellulose and lignin to methyl glycosides (mainly C5

176

glycoside) and lignin fragments. The methyl glycoside and lignin fragments were stepwise separated based on their

177

difference in hydrophilicity. We investigated the catalytic performance of various liquid acids (H2SO4, HCl and H3PO4)

178

and solid acid (HPW, C7H7SO3H, and p-TsOH) to evaluate their abilities of delignification and the transformation of

179

the target products. Table 1 summarizes the lignin fragments and methyl glycoside yield, product distributions and

180

delignification of the reactions involving different acids in 160 °C for 30min. (The amount of acid catalyst was

181

optimized as in Table.S1).

182

The reaction without acid failed to convert poplar into the expected products and yielded a low delignification

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degree of 18.04%. The use of sulfuric acid extracted 74.21% of lignin and obtained greater methyl glycosides

184

(mainly C5 sugars) and a yield of 24.85% (based on the entire poplar feedstock). In comparison, HCl and H3PO4

185

performed poorly for the degradation of poplar. HCl and H3PO4 were less effective in extracting lignin and

186

hemicellulose, and the delignification was 50.25% and 46.93%, respectively, and the yield of methyl glucosides

187

were 16.77% and 15.64%, respectively, with 10 times the concentration of sulfuric acid equivalent. The use of HPW,

188

a heteropoly acid, has been proven to be an efficient catalyst for the depolymerization of cellulose in an alcohol

189

medium.31 However, the delignification with HPW was low, which may be attributed to the complex and compact

190

structure of the substrate to hinder its acid hydrolysis under relatively mild conditions. C7H7SO3H efficiently

191

catalyzed the conversion of carbohydrate fractions with 22.48% of methyl glycoside yield, and the effect of

192

delignification was not notable. The presence of p-TsOH showed efficient extraction of lignin and the

193

transformation of hemicellulose, with a higher delignification degree (69.56%) and methyl glycosides (25.92%).

194

Furthermore, it was found that the retention of cellulose and hemicellulose was largely dependent on the acid



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195

used. Characterization results showed that 82.63% of cellulose and 11.33% of the hemicellulose were retained with

196

H2SO4 as the catalyst. Most of the hemicellulose was dissolved and converted to methyl C5 glycoside. In comparison,

197

the use of HCl and C7H7SO3H caused more obvious conversion of cellulose components. For example, the use of

198

HCl resulted in 38.39 % of hemicellulose and 73.34% of cellulose retained in the cellulose-rich substrate after the

199

methanolysis reaction. These results show that H2SO4 was more effective in releasing hemicellulose sugars and

200

depolymerizing lignin during the methanolysis process under relatively mild conditions. We speculate that the

201

stronger acidity of sulfuric acid it’s higher catalytic efficiency. The highly reactive protons in H2SO4 can effectively

202

activate the oxygen atoms in the structural bonds of cellulose, hemicelluloses, and lignin, and break the glycosidic

203

bonds in the units of holocellulose and C-O in the units of lignin.

204

Table 1 Catalytic performance of various acid catalysts for the conversion of poplar in methanol medium Yield of sugars (%)

Yield of lignin (%)

Acid

Delignification Conv. (%)

Catalyst

Carbohydrate retention (%)

Methyl

Methyl

Total

Solid

glucoside

yield

lignin

Lignin

a

xyloside

b

c

d

(%)

Cellulose

Hemicellulose

oil

No acid

18.24

2.25

0.14

5.31

4.74

0

18.04

95.25

87.54

HCl

40.21

9.57

4.92

16.77

10.22

2.59

50.25

73.34

38.39

H3PO4

37.06

8.93

4.07

15.64

9.72

2.2

46.93

78.83

42.51

H2SO4

52.25

12.32

3.77

24.85

13.68

4.09

74.21

82.63

14.33

HPW

45.35

11.01

4.32

18.42

12.45

3.54

52.32

76.34

23.51

C7H7SO3H

49.36

13.74

5.65

21.48

12.34

2.09

61.32

70.34

16.51

p-TSOH

51.71

12.23

4.13

25.92

13.34

3.73

69.56

78.53

18.12

205

a

206

methanol: 200 mL; time: 30min. The acid dose is based on the amount of H2SO4, 1 equivalents of p-TsOH and

Reaction conditions: temperature: 160°C; pressure: about 1.7 MPa at 160 °C; catalyst: 1.0 g; poplar: 20 g;


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207

C7H7SO3H, 10 equivalents of H3PO4 and HCl, 0.3 equivalents of HPW.

208

xylopyranoside, and methyl-β-D-xylopyranoside; Methyl glucoside contains methyl-a-D-glucopyranoside, methyl-

209

β-D-glucopyranoside. c The total yield of glycoside were calculated based on the original poplar wood. d the solid

210

lignin contained lignin 1# and lignin 2#.

211

b

Methyl xyloside contains methyl-a-D-

Table2. The methanolysis of poplar sawdust for directional conversion of hemicellulose and lignin at

212

various temperatures Yield of sugars (%)

Carbohydrate retention (%) Delignification

Temp.

a

Conv. (%)

Methyl

Methyl

Total

xyloside

glucoside

yield

(%)

Cellulose

Hemicellulose

140

25.87

5.76

0.97

7.45

46.49

93.21

56.7

160

52.25

12.32

3.77

24.85

76.21

82.63

11.33

180

69.40

15.35

9.83

36.87

85.55

64.79

7.35

200

80.45

5.51

15.43

42.16

91.34

24.81

2.67

213

a

214

and 3.2 MPa at 140, 160, 180 and 200 °C, respectively.

Reaction conditions: catalyst: 1.0 g; poplar: 20 g; methanol: 200 mL; time: 30min; pressure: it was 0.9, 1.6, 2.5

Methyl C6 Glycoside

Methyl C5 Glycoside 140°C

160°C

180°C

200°C

4

6

8

10

12

14

Time/min

215 216

(a)


16

18


Methyl C6 glycosides

Methyl C5 glycosides

140°C

160°C O

HO O

O O

180°C

O

O

O

8

200°C

12

16

20

24

28

32

Time/min

217 218

(b)

methyl xyloside derivatives

200°C

180°C 160°C

Methyl-β-D-xyloside 2

4

6

8

10

12

14

16

18

20

22

Time/min

219 220

(c)

221

Fig. 2 (a) The HPLC and (b) GC-MS results of the sugars products from the methanolysis of poplar at various

222

temperatures. (c) Stability analysis of methyl-β-D-xyloside in methanol at different temperatures.

223

To further optimize methanolysis conditions, four temperatures 140, 160, 180 and 200 °C were tested, all

224

using H2SO4 as the catalyst (Table2). The goal of the methanolysis step was to maximize the conversion of

225

hemicellulose and lignin fractions to release methyl C5 glycoside and lignin fragments while retaining the maximize

226

cellulose fraction. In general, hemicellulose is the most susceptible to thermal degradation and was expected to

227

release C5 sugars via solvent thermal treatment. As expected, the yield of methyl glycoside significantly increased

228

with increasing reaction temperature from 140 to 200°C. As evidenced by the HPLC and GC-MS analysis of the


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229

products (Fig. 2. (a, b), methyl C5 glycosides were the dominate products below the reaction temperature of 180°C;

230

this can be seen according to the relative higher peak area of C5 sugars. However, a remarkable increase in cellulose

231

conversion was observed at 180 °C. Methyl C6 glycosides became the main products and more than 80% of poplar

232

was converted at 200 °C. These results indicate that in terms of the methanolysis of carbohydrates, the

233

transformation of hemicellulose mainly occurred below 160 °C and that 88.67% of hemicellulose was transformed

234

at 160°C. Meanwhile, the yield of methyl glycoside was 24.85 wt% with 13.32 wt % of methyl xyloside and 3.77%

235

of methyl glucoside based on the HPLC analysis. The conversion of cellulose started at 180 °C, and 65.19% of

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cellulose was fractioned from the substrate at 200 °C. Furthermore, various sugar-derivatives, such as furfural, 5-

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methoxymethylfurfural (MMF) and methyl levulinate, were detected in the methyl glycoside compounds, which

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were derived from further reactions of methyl glycoside.

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An investigation of the stability of methyl C5 sugar was conducted, which can provide a reference for choosing

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more suitable reaction conditions due to the tendency of hemicellulose to thermally decompose into multiple

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oxygenated compounds. Methyl a-D-xylopyranoside was used as a representative to investigate the thermal

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stability of methyl C5 sugar (Table. S2). At 160 °C, 26% of pure methyl β-D-xylopyranoside was converted. However,

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the degradation of methyl α-D-xylopyranoside significantly increased at >180 °C. More than 65% of methyl β-D-

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xylopyranoside was converted into various chemicals, such as furfural, β-methoxy-2-furanethanol and 3-ethyl-2-

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pentenoic acid methyl ester at 180°C. Therefore, taking into account the alcoholysis conversion efficiency of poplar

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wood and the stability of methyl xyloside, the methanolysis at 160 °C was selected as the reaction temperature for

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further study.

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3.2 The conversion of methyl glycoside in methanol/DMM co-solvent

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The methyl C5 glycoside from the methanolysis carbohydrate at 160°C was further converted in

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DMM/methanol co-solvent with an acid catalyst. The effect of the reaction temperature on the transformation of



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methyl glycosides (mainly C5 glycoside) into methyl levulinate was investigated (Fig.3). Methyl levulinate was the

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main product and MMF, furfural and other derivatives were formed as intermediate products. The conversion of

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methyl glycoside increased with the increase in temperature. Above 90 % of methyl xyloside was transformed at

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160 °C. It was found that the increase in temperature to

Methanolysis fractionation and catalytic conversion of poplar wood

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