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Sustainable production of methyl levulinate from biomass in ionic liquid-methanol system with biomass-based catalyst
Fuel 259 (2020) 116246
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Full Length Article
Sustainable production of methyl levulinate from biomass in ionic liquidmethanol system with biomass-based catalyst
T
⁎
Xiaocong Liang, Yan Fu, Jie Chang
The Key Lab of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, No. 381, Wushan Road, Guangzhou, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Methyl levulinate Biomass Ionic liquid Catalyst
The preparation of bifunctional solid-acid catalyst and production of methyl levulinate were performed synergistically based on the fractionation of wood residue. The hydrothermal hydrolysate of wood residue was utilized for catalyst preparation using template method with sulfonic group as the Brønsted acid site and Zr4+ as Lewis acid site. Cellulose material for methyl levulinate production was fractionated by the delignification of hydrothermal residue using deep eutectic solvent. In this study, yield of methyl levulinate reached 38.7% under the optimized experiment condition. Insight gained from this work suggests a sustainable strategy for methyl levulinate production using recyclable biomass-based catalyst in ionic liquid-methanol system.
1. Introduction High-value chemicals production from biomass has been a vital issue for the utilization of renewable energy sources [1–4]. As widely used as the fuel additive and the raw materials for the manufacture of ⁎
spices, coatings, adhesives, plasticizers, pharmaceuticals and so on, methyl levulinate (ML) has been considered as a hot target product in bio-refinery [5–7]. Biomass including untreated biomass [8] and cellulose [9–11] could be more cost-effective as raw material for ML production when compared with glucose [12–14], levulinic acid [15],
Corresponding author. E-mail address: [email protected] (J. Chang).
https://doi.org/10.1016/j.fuel.2019.116246 Received 23 July 2019; Received in revised form 2 September 2019; Accepted 19 September 2019 Available online 24 September 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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treatment for biomass was performed to remove most hemicellulose in biomass while deep eutectic solvent (DES) treatment was further employed for delignification of hydrothermal residue. Hence the cellulose fraction in biomass could be retained as DES residue. According to the mean of three replicates obtained by composition analysis of DES residue with isolated lignin fraction and hydrothermal hydrolysate, the fractionation yield of cellulose, lignin (including acid-insoluble lignin and acid-insoluble lignin) and hemicellulose (based on xylan) were 94.5% wt., 90.3% wt. and 83.7% wt. after the two-step pretreatment. Besides, the hydrothermal hydrolysate was concentrated with the concentration ratio of 1:5.
furfuryl alcohol [16,17], 5-chloromethyl furfural [18] and so on [19]. At present, catalysts such as mineral-acids [10], mineral-salt [20], solidacid [21] and acidic ionic liquids [8,22] are commonly studied to facilitate the production of ML from biomass. Generally, homogeneously catalytic process tends to present a better performance for cellulose conversion considering the conversion ratio of cellulose and the yield of target product [23–25]. Brønsted acids such as sulfuric acid [10], hydrochloric acid [26] and Lewis acids such as Al2(SO4)3 [27] have been widely studied as the efficient homogeneous catalysts for ML production from cellulose. Li reported ML production from cellulose using extremely low concentration sulfuric acid of 0.01 mol/L with the yield of 50% at 210 °C for 2 h [10]. Zhou used Al2(SO4)3 to catalyze α-cellulose into ML at 180 °C for 5 h with the yield of 44% [28]. The recyclability and potential corrosion for equipment are the major disadvantages inhibiting further large-scale application with these homogeneous catalysts [29]. While heterogeneous catalysts are easy to be separated and reused. Research on ML production from cellulose with heterogeneous catalysts also got lots of attention as the potential alternative option for commercial homogeneous catalyst [30]. Meanwhile, synergy of Lewis acid and Brønsted acid in catalyst has been put forward to further explore the conversion mechanism of ML from cellulose and to improve the conversion efficiency of cellulose [31–33]. Brønsted acid works well in the conversion of cellulose into hexoses[34] and Lewis acid function effectively in following ML production from hexoses [33]. Tominaga [32] employed the mixture of Lewis acid In(OTf)3 (Indium trifluoro-methanesulfonate) and Brønsted acid 2-NSA (2-naphthalenesulfonic acid) to convert cellulose into ML with the maximum yield of 75% at 180 °C for 5 h. Zhang [31] reported the use of metal phosphotungstate with bifunctional Brønsted and Lewis acidities for cellulose conversion with the highest ML yield of 49.0% at 160 °C for 30 min under microwave heating. However, ML production from cellulose with hybrid Lewis acid (site) and Brønsted acid (site) catalytic strategy for commercial production is still in its fancy. More sustainable and cost-effective conversion method should be introduced to produce ML from cellulose or inexpensive material such as cellulose fraction of wood residue. Compared with the original lignocellulose, the fractionated cellulose material would generate less side products from pentose and lignin during ML production. Recently, fabrication of biomass chemicals within bio-refinery concept has been a hot issue and a lot studies were conducted with the cellulose fraction after biomass pretreatment [35,36]. The hemicelluloses-rich pre-hydrolysis liquor could be cost-effective material for catalyst support preparation [37,38].At present, very few studies reported ML production with biomass-derived bifunctional solid-acid catalyst in the recyclable ionic liquid-methanol system. Due to its good dissolution capacity for cellulose, ionic liquid 1-butyl-3-methyl imidazolium chloride (BmimCl), were employed to facilitate the dissolution of cellulose fraction. In this study, the catalysts for ML production were prepared using the hydrothermal hydrolyte of wood residue with template method. Sulfonic group and Zr4+ were loaded as the Brønsted acid site and Lewis acid site. The cellulose for ML production was fractionated from the hydrothermal residue of biomass. This study is performed to develop a potential low-cost and sustainable strategy for ML production from biomass.
2.2. Preparation and characterization of catalysts Biomass-based solid acid catalysts were prepared using template method and the concentrated hydrothermal hydrolysate was used as carbon supplier to prepare support. Template Al@SiO2 microspheres were prepared with impregnation method [40]. The support of catalysts was prepared at 200 °C for 10 h with Al@SiO2 microspheres and concentrated hydrolysate in the ratio of 1 g/15 mL. After filtration and washing, the resulting filter residue was heated in tube furnace at nitrogen condition (850 °C, 5 h). The resulting solid was soaked in 10% wt. hydrofluoric acid overnight. The support was obtained after filtration, washing and oven-drying for the resulting mixture. Catalysts were prepared by sulfonation (S), impregnation (I) or sulfonation-impregnation (SI) of support using concentrated sulfuric acid as sulfonating agent and zirconium sulfate as supported substance [41]. In addition, the number 10, 20, 30 and 40 displayed after the catalyst code SI or I represents for the percentage amount of zirconium sulfate loaded on the support. FT-IR measurement was performed with KBr tablet method using a Tensor 27 spectrometer within 400–2000 cm−1 to determine the chemical groups of catalysts. And the XRD spectra of catalysts were obtained on a D8 ADVANCE X-ray powder diffractometer (Bruker, Germany) with the 2ϴ ranging 5-60°. The microstructure of prepared catalysts was obtained with a scanning electron microscope (Merlin SEM ZEISS, Germany) and transmission electron microscope (JEM1400 Plus, Germany). 2.3. Production of methyl levulinate, catalyst recovery and IL recovery 0.5 g oven-dried DES residue (used as cellulose fraction) was used in each experiment for the production of ML. Certain amount of methanol, ionic liquid BmimCl, deionized water, catalyst and DES residue were mixed homogeneously and added in a hydrothermal reactor with configured reaction condition (Scheme shown in Fig. 1). Reacting product was filtered and washed with ethyl acetate. The resulting two-phase filtrate contained ionic liquid (IL) phase A and ethyl acetate phase A. IL phase A was further extracted with ethyl acetate to obtain ethyl acetate phase B and IL phase B. The IL phase B was rotary evaporated to recover BmimCl. Meanwhile, residue A resulted by ethyl acetate washing was further washed with methanol to get methanol phase and residue B. Liquid mixture containing methanol phase, ethyl acetate phase A and ethyl acetate phase B were mixed for GC–MS (Shimadzu QP 2010-plus, equipped with hp-5 ms column) analysis and the quantitation of ML. Residue B was oven-dried before hexose content analysis. Catalysts were recovered after the hydrolysis of remaining cellulose in residue B using dilute sulfuric acid. Filter residue obtained after acid hydrolysis was heating in tube furnace at 650 °C for 2 h under nitrogen atmosphere before the recycling of catalyst.
2. Experimental section 2.1. Materials and biomass pretreatment Dewaxed wood powder of eucalyptus globulus wood residue was employed for biomass fractionation with the size of 40–60 mesh. Reagent-grade chemicals such as methanol, zirconium sulfate and so on were obtained from Sigma-Aldrich. 1-butyl-3-methyl imidazolium chloride (BmimCl, 99% purity) was supplied by Lanzhou Yulu Fine Chemical Co. LTD. The fractionation of biomass was performed with a two-step pretreatment as previous reported [39]. Briefly, hydrothermal
2.4. Measurement of methyl levulinate Product ML was quantified by gas chromatography (Agilent 6890, equipped with HP-5 column and FID). Conversion ratio of cellulose R and molar yield of ML Y were calculated as follows: 2
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Fig. 1. Scheme of methyl levulinate production and separation.
R=
WO × 100% WR
(1)
Y=
N1 × 100% N2
(2)
in Fig. 3, the catalytic performance of sulfonated catalyst S and sulfonated-impregnated catalysts were remarkably better than that of impregnated catalysts. Cellulose conversion ratio of 78.4% and ML yield of 7.6% was obtained using sulfonated catalyst S under the adoptive experiment condition. With the increasing Zr(SO4)2 loading of SI (sulfonation-impregnation) catalysts from 10% to 40%, the resulting cellulose conversion ratio and ML yield increased at the beginning and decreased after Zr(SO4)2 loading surpassing 20%. The highest cellulose conversion ratio 82.5% was resulted using SI-10 catalyst and the highest ML yield 9.4% was obtained using SI-20 catalyst. Obviously, a higher Zr(SO4)2 loading of catalyst would result in a reduction of specific surface area and further coverage of sulfonic acid sites. Hence the contact chances between sulfonic acid (Brønsted acid) sites and reactant would be decreased, which would negatively affect the hydrolysis of cellulose, formation of the important intermediate methyl glucoside and its subsequent dehydration into 5-methoxymethylfurfural. However, the generation of ML by the hydrolysis of 5-methoxymethylfurfural got significantly promoted due to the further introduction of Lewis acid site Zr4+ with these sulfonated catalysts as their Zr(SO4)2 loading elevated from 0 to 20%. Thus the conversion of cellulose to ML was facilitated with these sulfonated-impregnated catalysts when compared with that of sulfonated catalyst S. Since the conversion ratio of cellulose resulted by catalyst SI-10 (82.5%) and catalyst SI-20 (82.3%) were approaching, ML yield by catalyst SI-20 (9.4%) was significantly higher than that of catalyst SI-10 (7.9%). Therefore, catalyst SI-20 seemed more suitable for ML production and was chosen to further study the influence of experimental parameters and the conversion mechanism of cellulose fraction in BmimCl-methanol system.
where WO and WR are the mass of hexose in cellulose fraction and the oven-dried residue B, N1 is the amount of ML in product and N2 is the amount of hexose in the cellulose fraction. 3. Results and discussion 3.1. Comparison between different catalysts Comparisons between catalysts were performed with characterization results and catalytic performance for ML production. The FT-IR spectra of catalysts are shown in Fig. 2a. Apart from the absorption peaks of C=O stretching vibration (1719 cm−1), C=C stretching vibration (1595 cm−1) and out-of-plane bending vibration of cis-form sp2 C–H in the FT-IR spectra of each catalyst, peaks for the stretching vibration of C-S (620 cm−1), asymmetry S = O (1190 cm−1) and symmetric S = O (1095 cm−1) were capable to prove the introduction of sulfonic acid group for the five kinds of sulfonated catalysts. While the stretching vibration of asymmetry and symmetric S=O in SO42- were observed at 1170 cm−1 and 1080 cm−1 for the eight kinds of Zr(SO4)2impregnated catalysts. Besides, the out-of-plane bending vibration of carboxylate O–H (950 cm−1) and trans-form sp2 C–H (990 cm−1) would also illustrate the diversity of carbon functional groups in these catalysts. Meanwhile, the diffraction peak of Zr(SO4)2 appeared in the XRD spectra of catalysts (Fig. 2b) when the Zr(SO4)2 loading of catalyst exceeded 20%. And the peak intensity increased with elevatory Zr (SO4)2 loading. That is, agglomerated crystals of Zr(SO4)2 began to appear on the catalyst after Zr(SO4)2 loading became more than 20%. The FT-IR results were generally in accord with the TEM and SEM results (Fig. 2c) of these catalysts. And these catalysts generally present individually spherical and partially concatenated spherical shape with hollow structure. Diameter of single spherical structure is between 100 nm and 500 nm. And more agglomeration appeared on the surface of the catalyst as Zr(SO4)2 loading increased. Structural differences between these catalysts would lead to their differentiated catalytic performance for cellulose conversion. As shown
3.2. Influence of experimental factors Results in section 3.1 indicate that structural characteristic of catalysts would notably affect cellulose conversion and ML formation. Major factors such as temperature, time, catalyst dosage and ionic liquid dosage also have a significant influence on the reaction rate and equilibrium of the entire conversion process. Hence a detailed study on the effect of these major factors was further performed. It can be seen from Fig. 4a that ML yield was significantly increased (14.3%–38.7%) as experimental temperature rose from 180 °C to 3
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Fig. 2. Characterization of prepared catalysts (a-FT-IR spectra, b-XRD spectra, c-SEM and TEM images).
conversion process of cellulose into ML could be thoroughly conducted. According to above analysis, optimum conversion temperature of cellulose into ML was chosen as 220 °C with catalyst SI-20. Influence of reaction time on ML production is shown in Fig. 4b. It can be seen that cellulose conversion ratio of ~60% with ML yield of 5.9% was obtained as reaction time reached 15 min. While chemical equilibrium of cellulose conversion process has not been reached during the reacting period of 15 min. Cellulose conversion ratio and ML yield gradually increased to 100% and 38.7% as reaction time extended to 75 min. Afterwards, cellulose conversion ratio and ML yield hardly changed as reaction prolonged. Considering the energy consumption, duration of conversion experiment should better not exceed 75 min. Furthermore, the use of catalyst SI-20 in cellulose conversion experiment would significantly promote the formation of ML as shown in Fig. 4c. The resulting ML yield was 2.1% and conversion ratio of cellulose was 91.5% without using catalyst SI-20. However, conversion ratio of cellulose lifted from 92.1% to 100% and ML yield increased from 9.8% to 38.7% as dosage of catalyst SI-20 increased from 5% to 20%. While ML yield was slightly increased to 39.0% as catalyst dosage reached 30%. Hence a catalyst dosage of 20% would make the active sites of the catalyst well-utilized for ML production. And it’s not feasible to accelerate the conversion rate by increasing catalyst dosage. Moreover, an excessive catalyst dosage would not significantly increase the selectivity for methyl levulinate formation. Previous results have proven that ionic liquid BmimCl played an important role in the depolymerization of cellulose into small molecules, generating sufficient substrate that can be contacted and catalyzed by catalyst for subsequent conversion. Hence the influence of BmimCl dosage on the conversion process was also studied. As can be seen in Fig. 4d, conversion ratio of cellulose and ML yield were about 80% and 20% using methanol-catalyst system without BmimCl. During the rising process of BmimCl dosage from 10 fold (mass of cellulose fraction) to 20 fold, conversion ratio of cellulose and ML yield gradually increased within the range of 92.7%-100% and 28.4%-38.7%. Besides, it can be seen that the excessive dosage of BmimCl would inhibit the formation of ML without the inhibition for cellulose conversion. A higher dosage of BmimCl in solvent system would increase the viscosity
Fig. 3. Catalytic performance comparisons between different catalysts. Reaction conditions: cellulose fraction (0.5 g), methanol (7.5 g), ionic liquid (10 g), catalyst (0.1 g), deionized water (3 wt%), temperature (200 °C), time (60 min).
220 °C, while ML yield gradually decreased as temperature further increased. Main reason for this relationship is that the exorbitant experiment temperature would promote the formation of dimethyl ether due to the Brønsted acid sites-catalyzed dehydration of methanol, which would inhibit the formation of methyl glucoside [14]. Meanwhile, conversion ratio of cellulose got enhanced gradually from 88.7% to 100% as temperature rose from 180 °C to 220 °C. And the conversion ratio of cellulose was maintained at 100% as temperature further went up. This result indicates that the use of ionic liquid is somehow favorable for the depolymerization of cellulose. Although methanol the reactant and solvent for ML production reduced due to enhanced side reaction as temperature increased, synergistic effect of acid sites on catalyst and the hydrogen bond of ionic liquid BmimCl would maintain the efficient depolymerization for cellulose by the isomerization and dehydration of intermediate products [42]. And the efficient depolymerization of cellulose laid foundation for the sufficient contact between the acidic sites on catalyst and oligosaccharides. Thus the entire 4
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Fig. 4. Influence of Reaction temperature (a), Reaction time (b), Catalyst dosage (c) and ionic liquid dosage (d) on catalytic performance. Reaction conditions: a, cellulose fraction (0.5 g), methanol (7.5 g), ionic liquid (10 g), catalyst (0.1 g), deionized water (3 wt%), time (75 min). b, cellulose fraction (0.5 g), methanol (7.5 g), ionic liquid (10 g), catalyst (0.1 g), deionized water (3 wt%), temperature (220 °C). c, cellulose fraction (0.5 g), methanol (7.5 g), ionic liquid (10 g), catalyst (wt.%, based on cellulose fraction), deionized water (3 wt%), time (75 min), temperature (220 °C). d, cellulose fraction (0.5 g), methanol (7.5 g), ionic liquid (fold, based on cellulose fraction), catalyst (0.1 g), deionized water (3 wt%), time (75 min), temperature (220 °C). Table 1 Affiliation of product peaks in GC–MS results of liquid mixture.
Fig. 5. GC–MS spectra of liquid products.
of solvent and reduce the recant (methanol) concentration, which would restrict the alcoholysis of cellulose and the formation of ML from intermediate products. Compared with recent studies on ML production, the use of biomassderived bifunctional catalyst in ionic liquid-methanol provided competitive results in ML yield and cellulose conversion. For example, Li [21] obtained a maximum ML yield of 27.0% from commercial cellulose with zirconium oxide-loaded zeolite. Feng [43] reported the ML yield of 30.8% from bamboo liquefaction product catalyzed with dilute sulphuric acid. Chang [35] used metal sulfate as the catalyst to convert straw stalk and resulted in a ML yield of 20.2%. Nevertheless, the
NO.
Retention time
Affiliation
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
3.1 3.5 4.2 5.8 7.1 7.7 8.1 9.2 10.8 10.9 11.5 13.1 13.9 15.3 16.1 16.6 18.7 25.1 25.8 27.9 28.4
furfural methyl formate 3-furaldehyde 2-Methoxytetrahydrofuran methyl levulinate 2-cyclohexen-1-one Ethyl 3,3-diethoxypropionate Butanedioic acid, hydroxy-, dimethyl ester Decanoic acid, 3-hydroxy-, methyl ester Levulinic acid Acetic acid, methyl ester 2H-pyran-2-one, 4-methoxy-6-methylPentanoic acid, 3-hydroxy-4-methyl-, ethyl ester Propanoic acid, 2-methyl-, methyl ester levoglucosenone β-methyl glucoside citric acid trimethyl ester methyl 4-O-methyl-α-D-mannopyranosideuronate methyl 4-hydroxycinnamate ferulic acid methyl ester hexadecanoic acid methyl ester
conversion mechanism and conversion route of cellulose into ML in this catalytic system is pivotal to further improve the selectively of ML production. 3.3. Mechanism and route for methyl levulinate production According to the GC–MS spectra in Fig. 5, it can be seen that a 5
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Fig. 6. Recycling performance of catalyst (a) and ionic liquid (b). Reaction conditions: cellulose fraction (0.5 g), methanol (7.5 g), ionic liquid (10 g), catalyst (0.1 g), deionized water (3 wt%), time (75 min), temperature (220 °C).
batches recycling of ionic liquid BmimCl (Fig. 6b). And the yield of methyl levulinate hardly got reduced and was maintained above 36.6%. Thus we can conclude that ionic liquid BmimCl can be recycled effectively no fewer than 5 batches.
significant amount of ML was existed in the collected product mixture for analysis. Meanwhile, as listed in Table 1, a series of side reactions occurred during the alcoholysis of cellulose. Side reactions would compete with the major reactions by consuming methanol and intermediate products. During the conversion process of cellulose, monosaccharide would get converted into aldehyde or ketone such as furfural and furan by dehydration. Further reactions such as alcoholysis, dehydration and replacement by methoxy group would occur due to the presence of methanol. Relatively chemically stable products such as esters and glycoside would be generated from these reactive intermediates and by-products. Hence the products obtained after the catalyzed alcoholysis of cellulose fraction can be divided into four categories, including aldehydes-ketones, esters, acids and others. Combined with listed results and literature [43], pathways for the conversion of cellulose into ML using the catalytic system are summarized (Fig. S1). Firstly, cellulose macromolecule got hydrolyzed and glucose was produced due to the hydrogen bond formed by Cl- of BmimCl and sulfonic acid sites on catalyst. The glucose was further catalyzed and methyl glucoside was formed after alcoholysis, which was further converted to 5-methoxymethyl furfural after dehydration. Finally, the target product ML and by-product methyl formate was resulted by the hydrolyzation of 5-methoxymethylfurfural. In addition, the fructose isomerized from glucose would get dehydrated to form 5HMF (5-hydroxymethyl furfural), which can be dehydrated into 5methoxymethyl furfural. Thus the target product ML can be obtained by following hydrolyzation of 5-methoxymethyl furfural. Meanwhile, 5HMF can also be hydrolyzed into levulinic acid. ML and levulinic acid in the resulting product can be converted into each other before the final conversion equilibrium.
4. Conclusion Sustainable production of methyl levulinate from biomass was proven feasible and efficient using biomass-based bifunctional catalyst in ionic liquid-methanol system. Ionic liquid BmimCl played an important role in the depolymerization of cellulose into small molecules. BmimCl and the prepared biomass bifunctional catalyst showed good recyclability after 5 batches of recycling. Previous research focused on the screening of catalyst for methyl levulinate production, while this work could be a considerable reference for the sufficient utilization of biomass for catalysts preparation and the efficient production of methyl levulinate. Acknowledgement Thanks for the financial support of National Key Research and Development Program of China (2018YFB1501404), National Key Research and Development Program of China (2017YFD0601003) and Science and Technology Planning Project of Guangdong Province, China (2017A010104005). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.116246.
3.4. Recycling of catalyst and ionic liquid
References
Efficient recycling of catalyst and ionic liquid is pivotal for practical production of bio-chemicals. Hence the recycling performance of catalyst SI-20 and ionic liquid BmimCl was determined. It can be seen in Fig. 6a that the conversion ratio of cellulose got no reduction as catalyst SI-20 was recycled for 5 batches. Meanwhile, ML yield reduced from 38.7% (fresh catalyst) to 35.5% after one batch of catalyst recycling. This was resulted due to the loss of Zr(SO4)2 component agglomerated on the surface of catalyst. A decrease in Zr4+ (Lewis acid) sites affected the catalytic hydrolysis process of 5-methoxymethylfurfural to ML. However, slight affection on ML yield was generated during subsequent recycling of catalyst. Yield of target product decreased to 33.1% as the catalyst was recycled for 5 batches. Catalyst SI-20 still maintained an acceptable catalytic activity for the conversion of cellulose to ML. Reduction in catalytic activity of catalyst SI-20 was not that significant during 5 batches of recycling considering the coking of catalyst. Besides, conversion ratio of cellulose maintained at 100% during the 5
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7
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Full Length Article
Sustainable production of methyl levulinate from biomass in ionic liquidmethanol system with biomass-based catalyst
T
⁎
Xiaocong Liang, Yan Fu, Jie Chang
The Key Lab of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, No. 381, Wushan Road, Guangzhou, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Methyl levulinate Biomass Ionic liquid Catalyst
The preparation of bifunctional solid-acid catalyst and production of methyl levulinate were performed synergistically based on the fractionation of wood residue. The hydrothermal hydrolysate of wood residue was utilized for catalyst preparation using template method with sulfonic group as the Brønsted acid site and Zr4+ as Lewis acid site. Cellulose material for methyl levulinate production was fractionated by the delignification of hydrothermal residue using deep eutectic solvent. In this study, yield of methyl levulinate reached 38.7% under the optimized experiment condition. Insight gained from this work suggests a sustainable strategy for methyl levulinate production using recyclable biomass-based catalyst in ionic liquid-methanol system.
1. Introduction High-value chemicals production from biomass has been a vital issue for the utilization of renewable energy sources [1–4]. As widely used as the fuel additive and the raw materials for the manufacture of ⁎
spices, coatings, adhesives, plasticizers, pharmaceuticals and so on, methyl levulinate (ML) has been considered as a hot target product in bio-refinery [5–7]. Biomass including untreated biomass [8] and cellulose [9–11] could be more cost-effective as raw material for ML production when compared with glucose [12–14], levulinic acid [15],
Corresponding author. E-mail address: [email protected] (J. Chang).
https://doi.org/10.1016/j.fuel.2019.116246 Received 23 July 2019; Received in revised form 2 September 2019; Accepted 19 September 2019 Available online 24 September 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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treatment for biomass was performed to remove most hemicellulose in biomass while deep eutectic solvent (DES) treatment was further employed for delignification of hydrothermal residue. Hence the cellulose fraction in biomass could be retained as DES residue. According to the mean of three replicates obtained by composition analysis of DES residue with isolated lignin fraction and hydrothermal hydrolysate, the fractionation yield of cellulose, lignin (including acid-insoluble lignin and acid-insoluble lignin) and hemicellulose (based on xylan) were 94.5% wt., 90.3% wt. and 83.7% wt. after the two-step pretreatment. Besides, the hydrothermal hydrolysate was concentrated with the concentration ratio of 1:5.
furfuryl alcohol [16,17], 5-chloromethyl furfural [18] and so on [19]. At present, catalysts such as mineral-acids [10], mineral-salt [20], solidacid [21] and acidic ionic liquids [8,22] are commonly studied to facilitate the production of ML from biomass. Generally, homogeneously catalytic process tends to present a better performance for cellulose conversion considering the conversion ratio of cellulose and the yield of target product [23–25]. Brønsted acids such as sulfuric acid [10], hydrochloric acid [26] and Lewis acids such as Al2(SO4)3 [27] have been widely studied as the efficient homogeneous catalysts for ML production from cellulose. Li reported ML production from cellulose using extremely low concentration sulfuric acid of 0.01 mol/L with the yield of 50% at 210 °C for 2 h [10]. Zhou used Al2(SO4)3 to catalyze α-cellulose into ML at 180 °C for 5 h with the yield of 44% [28]. The recyclability and potential corrosion for equipment are the major disadvantages inhibiting further large-scale application with these homogeneous catalysts [29]. While heterogeneous catalysts are easy to be separated and reused. Research on ML production from cellulose with heterogeneous catalysts also got lots of attention as the potential alternative option for commercial homogeneous catalyst [30]. Meanwhile, synergy of Lewis acid and Brønsted acid in catalyst has been put forward to further explore the conversion mechanism of ML from cellulose and to improve the conversion efficiency of cellulose [31–33]. Brønsted acid works well in the conversion of cellulose into hexoses[34] and Lewis acid function effectively in following ML production from hexoses [33]. Tominaga [32] employed the mixture of Lewis acid In(OTf)3 (Indium trifluoro-methanesulfonate) and Brønsted acid 2-NSA (2-naphthalenesulfonic acid) to convert cellulose into ML with the maximum yield of 75% at 180 °C for 5 h. Zhang [31] reported the use of metal phosphotungstate with bifunctional Brønsted and Lewis acidities for cellulose conversion with the highest ML yield of 49.0% at 160 °C for 30 min under microwave heating. However, ML production from cellulose with hybrid Lewis acid (site) and Brønsted acid (site) catalytic strategy for commercial production is still in its fancy. More sustainable and cost-effective conversion method should be introduced to produce ML from cellulose or inexpensive material such as cellulose fraction of wood residue. Compared with the original lignocellulose, the fractionated cellulose material would generate less side products from pentose and lignin during ML production. Recently, fabrication of biomass chemicals within bio-refinery concept has been a hot issue and a lot studies were conducted with the cellulose fraction after biomass pretreatment [35,36]. The hemicelluloses-rich pre-hydrolysis liquor could be cost-effective material for catalyst support preparation [37,38].At present, very few studies reported ML production with biomass-derived bifunctional solid-acid catalyst in the recyclable ionic liquid-methanol system. Due to its good dissolution capacity for cellulose, ionic liquid 1-butyl-3-methyl imidazolium chloride (BmimCl), were employed to facilitate the dissolution of cellulose fraction. In this study, the catalysts for ML production were prepared using the hydrothermal hydrolyte of wood residue with template method. Sulfonic group and Zr4+ were loaded as the Brønsted acid site and Lewis acid site. The cellulose for ML production was fractionated from the hydrothermal residue of biomass. This study is performed to develop a potential low-cost and sustainable strategy for ML production from biomass.
2.2. Preparation and characterization of catalysts Biomass-based solid acid catalysts were prepared using template method and the concentrated hydrothermal hydrolysate was used as carbon supplier to prepare support. Template Al@SiO2 microspheres were prepared with impregnation method [40]. The support of catalysts was prepared at 200 °C for 10 h with Al@SiO2 microspheres and concentrated hydrolysate in the ratio of 1 g/15 mL. After filtration and washing, the resulting filter residue was heated in tube furnace at nitrogen condition (850 °C, 5 h). The resulting solid was soaked in 10% wt. hydrofluoric acid overnight. The support was obtained after filtration, washing and oven-drying for the resulting mixture. Catalysts were prepared by sulfonation (S), impregnation (I) or sulfonation-impregnation (SI) of support using concentrated sulfuric acid as sulfonating agent and zirconium sulfate as supported substance [41]. In addition, the number 10, 20, 30 and 40 displayed after the catalyst code SI or I represents for the percentage amount of zirconium sulfate loaded on the support. FT-IR measurement was performed with KBr tablet method using a Tensor 27 spectrometer within 400–2000 cm−1 to determine the chemical groups of catalysts. And the XRD spectra of catalysts were obtained on a D8 ADVANCE X-ray powder diffractometer (Bruker, Germany) with the 2ϴ ranging 5-60°. The microstructure of prepared catalysts was obtained with a scanning electron microscope (Merlin SEM ZEISS, Germany) and transmission electron microscope (JEM1400 Plus, Germany). 2.3. Production of methyl levulinate, catalyst recovery and IL recovery 0.5 g oven-dried DES residue (used as cellulose fraction) was used in each experiment for the production of ML. Certain amount of methanol, ionic liquid BmimCl, deionized water, catalyst and DES residue were mixed homogeneously and added in a hydrothermal reactor with configured reaction condition (Scheme shown in Fig. 1). Reacting product was filtered and washed with ethyl acetate. The resulting two-phase filtrate contained ionic liquid (IL) phase A and ethyl acetate phase A. IL phase A was further extracted with ethyl acetate to obtain ethyl acetate phase B and IL phase B. The IL phase B was rotary evaporated to recover BmimCl. Meanwhile, residue A resulted by ethyl acetate washing was further washed with methanol to get methanol phase and residue B. Liquid mixture containing methanol phase, ethyl acetate phase A and ethyl acetate phase B were mixed for GC–MS (Shimadzu QP 2010-plus, equipped with hp-5 ms column) analysis and the quantitation of ML. Residue B was oven-dried before hexose content analysis. Catalysts were recovered after the hydrolysis of remaining cellulose in residue B using dilute sulfuric acid. Filter residue obtained after acid hydrolysis was heating in tube furnace at 650 °C for 2 h under nitrogen atmosphere before the recycling of catalyst.
2. Experimental section 2.1. Materials and biomass pretreatment Dewaxed wood powder of eucalyptus globulus wood residue was employed for biomass fractionation with the size of 40–60 mesh. Reagent-grade chemicals such as methanol, zirconium sulfate and so on were obtained from Sigma-Aldrich. 1-butyl-3-methyl imidazolium chloride (BmimCl, 99% purity) was supplied by Lanzhou Yulu Fine Chemical Co. LTD. The fractionation of biomass was performed with a two-step pretreatment as previous reported [39]. Briefly, hydrothermal
2.4. Measurement of methyl levulinate Product ML was quantified by gas chromatography (Agilent 6890, equipped with HP-5 column and FID). Conversion ratio of cellulose R and molar yield of ML Y were calculated as follows: 2
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Fig. 1. Scheme of methyl levulinate production and separation.
R=
WO × 100% WR
(1)
Y=
N1 × 100% N2
(2)
in Fig. 3, the catalytic performance of sulfonated catalyst S and sulfonated-impregnated catalysts were remarkably better than that of impregnated catalysts. Cellulose conversion ratio of 78.4% and ML yield of 7.6% was obtained using sulfonated catalyst S under the adoptive experiment condition. With the increasing Zr(SO4)2 loading of SI (sulfonation-impregnation) catalysts from 10% to 40%, the resulting cellulose conversion ratio and ML yield increased at the beginning and decreased after Zr(SO4)2 loading surpassing 20%. The highest cellulose conversion ratio 82.5% was resulted using SI-10 catalyst and the highest ML yield 9.4% was obtained using SI-20 catalyst. Obviously, a higher Zr(SO4)2 loading of catalyst would result in a reduction of specific surface area and further coverage of sulfonic acid sites. Hence the contact chances between sulfonic acid (Brønsted acid) sites and reactant would be decreased, which would negatively affect the hydrolysis of cellulose, formation of the important intermediate methyl glucoside and its subsequent dehydration into 5-methoxymethylfurfural. However, the generation of ML by the hydrolysis of 5-methoxymethylfurfural got significantly promoted due to the further introduction of Lewis acid site Zr4+ with these sulfonated catalysts as their Zr(SO4)2 loading elevated from 0 to 20%. Thus the conversion of cellulose to ML was facilitated with these sulfonated-impregnated catalysts when compared with that of sulfonated catalyst S. Since the conversion ratio of cellulose resulted by catalyst SI-10 (82.5%) and catalyst SI-20 (82.3%) were approaching, ML yield by catalyst SI-20 (9.4%) was significantly higher than that of catalyst SI-10 (7.9%). Therefore, catalyst SI-20 seemed more suitable for ML production and was chosen to further study the influence of experimental parameters and the conversion mechanism of cellulose fraction in BmimCl-methanol system.
where WO and WR are the mass of hexose in cellulose fraction and the oven-dried residue B, N1 is the amount of ML in product and N2 is the amount of hexose in the cellulose fraction. 3. Results and discussion 3.1. Comparison between different catalysts Comparisons between catalysts were performed with characterization results and catalytic performance for ML production. The FT-IR spectra of catalysts are shown in Fig. 2a. Apart from the absorption peaks of C=O stretching vibration (1719 cm−1), C=C stretching vibration (1595 cm−1) and out-of-plane bending vibration of cis-form sp2 C–H in the FT-IR spectra of each catalyst, peaks for the stretching vibration of C-S (620 cm−1), asymmetry S = O (1190 cm−1) and symmetric S = O (1095 cm−1) were capable to prove the introduction of sulfonic acid group for the five kinds of sulfonated catalysts. While the stretching vibration of asymmetry and symmetric S=O in SO42- were observed at 1170 cm−1 and 1080 cm−1 for the eight kinds of Zr(SO4)2impregnated catalysts. Besides, the out-of-plane bending vibration of carboxylate O–H (950 cm−1) and trans-form sp2 C–H (990 cm−1) would also illustrate the diversity of carbon functional groups in these catalysts. Meanwhile, the diffraction peak of Zr(SO4)2 appeared in the XRD spectra of catalysts (Fig. 2b) when the Zr(SO4)2 loading of catalyst exceeded 20%. And the peak intensity increased with elevatory Zr (SO4)2 loading. That is, agglomerated crystals of Zr(SO4)2 began to appear on the catalyst after Zr(SO4)2 loading became more than 20%. The FT-IR results were generally in accord with the TEM and SEM results (Fig. 2c) of these catalysts. And these catalysts generally present individually spherical and partially concatenated spherical shape with hollow structure. Diameter of single spherical structure is between 100 nm and 500 nm. And more agglomeration appeared on the surface of the catalyst as Zr(SO4)2 loading increased. Structural differences between these catalysts would lead to their differentiated catalytic performance for cellulose conversion. As shown
3.2. Influence of experimental factors Results in section 3.1 indicate that structural characteristic of catalysts would notably affect cellulose conversion and ML formation. Major factors such as temperature, time, catalyst dosage and ionic liquid dosage also have a significant influence on the reaction rate and equilibrium of the entire conversion process. Hence a detailed study on the effect of these major factors was further performed. It can be seen from Fig. 4a that ML yield was significantly increased (14.3%–38.7%) as experimental temperature rose from 180 °C to 3
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Fig. 2. Characterization of prepared catalysts (a-FT-IR spectra, b-XRD spectra, c-SEM and TEM images).
conversion process of cellulose into ML could be thoroughly conducted. According to above analysis, optimum conversion temperature of cellulose into ML was chosen as 220 °C with catalyst SI-20. Influence of reaction time on ML production is shown in Fig. 4b. It can be seen that cellulose conversion ratio of ~60% with ML yield of 5.9% was obtained as reaction time reached 15 min. While chemical equilibrium of cellulose conversion process has not been reached during the reacting period of 15 min. Cellulose conversion ratio and ML yield gradually increased to 100% and 38.7% as reaction time extended to 75 min. Afterwards, cellulose conversion ratio and ML yield hardly changed as reaction prolonged. Considering the energy consumption, duration of conversion experiment should better not exceed 75 min. Furthermore, the use of catalyst SI-20 in cellulose conversion experiment would significantly promote the formation of ML as shown in Fig. 4c. The resulting ML yield was 2.1% and conversion ratio of cellulose was 91.5% without using catalyst SI-20. However, conversion ratio of cellulose lifted from 92.1% to 100% and ML yield increased from 9.8% to 38.7% as dosage of catalyst SI-20 increased from 5% to 20%. While ML yield was slightly increased to 39.0% as catalyst dosage reached 30%. Hence a catalyst dosage of 20% would make the active sites of the catalyst well-utilized for ML production. And it’s not feasible to accelerate the conversion rate by increasing catalyst dosage. Moreover, an excessive catalyst dosage would not significantly increase the selectivity for methyl levulinate formation. Previous results have proven that ionic liquid BmimCl played an important role in the depolymerization of cellulose into small molecules, generating sufficient substrate that can be contacted and catalyzed by catalyst for subsequent conversion. Hence the influence of BmimCl dosage on the conversion process was also studied. As can be seen in Fig. 4d, conversion ratio of cellulose and ML yield were about 80% and 20% using methanol-catalyst system without BmimCl. During the rising process of BmimCl dosage from 10 fold (mass of cellulose fraction) to 20 fold, conversion ratio of cellulose and ML yield gradually increased within the range of 92.7%-100% and 28.4%-38.7%. Besides, it can be seen that the excessive dosage of BmimCl would inhibit the formation of ML without the inhibition for cellulose conversion. A higher dosage of BmimCl in solvent system would increase the viscosity
Fig. 3. Catalytic performance comparisons between different catalysts. Reaction conditions: cellulose fraction (0.5 g), methanol (7.5 g), ionic liquid (10 g), catalyst (0.1 g), deionized water (3 wt%), temperature (200 °C), time (60 min).
220 °C, while ML yield gradually decreased as temperature further increased. Main reason for this relationship is that the exorbitant experiment temperature would promote the formation of dimethyl ether due to the Brønsted acid sites-catalyzed dehydration of methanol, which would inhibit the formation of methyl glucoside [14]. Meanwhile, conversion ratio of cellulose got enhanced gradually from 88.7% to 100% as temperature rose from 180 °C to 220 °C. And the conversion ratio of cellulose was maintained at 100% as temperature further went up. This result indicates that the use of ionic liquid is somehow favorable for the depolymerization of cellulose. Although methanol the reactant and solvent for ML production reduced due to enhanced side reaction as temperature increased, synergistic effect of acid sites on catalyst and the hydrogen bond of ionic liquid BmimCl would maintain the efficient depolymerization for cellulose by the isomerization and dehydration of intermediate products [42]. And the efficient depolymerization of cellulose laid foundation for the sufficient contact between the acidic sites on catalyst and oligosaccharides. Thus the entire 4
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Fig. 4. Influence of Reaction temperature (a), Reaction time (b), Catalyst dosage (c) and ionic liquid dosage (d) on catalytic performance. Reaction conditions: a, cellulose fraction (0.5 g), methanol (7.5 g), ionic liquid (10 g), catalyst (0.1 g), deionized water (3 wt%), time (75 min). b, cellulose fraction (0.5 g), methanol (7.5 g), ionic liquid (10 g), catalyst (0.1 g), deionized water (3 wt%), temperature (220 °C). c, cellulose fraction (0.5 g), methanol (7.5 g), ionic liquid (10 g), catalyst (wt.%, based on cellulose fraction), deionized water (3 wt%), time (75 min), temperature (220 °C). d, cellulose fraction (0.5 g), methanol (7.5 g), ionic liquid (fold, based on cellulose fraction), catalyst (0.1 g), deionized water (3 wt%), time (75 min), temperature (220 °C). Table 1 Affiliation of product peaks in GC–MS results of liquid mixture.
Fig. 5. GC–MS spectra of liquid products.
of solvent and reduce the recant (methanol) concentration, which would restrict the alcoholysis of cellulose and the formation of ML from intermediate products. Compared with recent studies on ML production, the use of biomassderived bifunctional catalyst in ionic liquid-methanol provided competitive results in ML yield and cellulose conversion. For example, Li [21] obtained a maximum ML yield of 27.0% from commercial cellulose with zirconium oxide-loaded zeolite. Feng [43] reported the ML yield of 30.8% from bamboo liquefaction product catalyzed with dilute sulphuric acid. Chang [35] used metal sulfate as the catalyst to convert straw stalk and resulted in a ML yield of 20.2%. Nevertheless, the
NO.
Retention time
Affiliation
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
3.1 3.5 4.2 5.8 7.1 7.7 8.1 9.2 10.8 10.9 11.5 13.1 13.9 15.3 16.1 16.6 18.7 25.1 25.8 27.9 28.4
furfural methyl formate 3-furaldehyde 2-Methoxytetrahydrofuran methyl levulinate 2-cyclohexen-1-one Ethyl 3,3-diethoxypropionate Butanedioic acid, hydroxy-, dimethyl ester Decanoic acid, 3-hydroxy-, methyl ester Levulinic acid Acetic acid, methyl ester 2H-pyran-2-one, 4-methoxy-6-methylPentanoic acid, 3-hydroxy-4-methyl-, ethyl ester Propanoic acid, 2-methyl-, methyl ester levoglucosenone β-methyl glucoside citric acid trimethyl ester methyl 4-O-methyl-α-D-mannopyranosideuronate methyl 4-hydroxycinnamate ferulic acid methyl ester hexadecanoic acid methyl ester
conversion mechanism and conversion route of cellulose into ML in this catalytic system is pivotal to further improve the selectively of ML production. 3.3. Mechanism and route for methyl levulinate production According to the GC–MS spectra in Fig. 5, it can be seen that a 5
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Fig. 6. Recycling performance of catalyst (a) and ionic liquid (b). Reaction conditions: cellulose fraction (0.5 g), methanol (7.5 g), ionic liquid (10 g), catalyst (0.1 g), deionized water (3 wt%), time (75 min), temperature (220 °C).
batches recycling of ionic liquid BmimCl (Fig. 6b). And the yield of methyl levulinate hardly got reduced and was maintained above 36.6%. Thus we can conclude that ionic liquid BmimCl can be recycled effectively no fewer than 5 batches.
significant amount of ML was existed in the collected product mixture for analysis. Meanwhile, as listed in Table 1, a series of side reactions occurred during the alcoholysis of cellulose. Side reactions would compete with the major reactions by consuming methanol and intermediate products. During the conversion process of cellulose, monosaccharide would get converted into aldehyde or ketone such as furfural and furan by dehydration. Further reactions such as alcoholysis, dehydration and replacement by methoxy group would occur due to the presence of methanol. Relatively chemically stable products such as esters and glycoside would be generated from these reactive intermediates and by-products. Hence the products obtained after the catalyzed alcoholysis of cellulose fraction can be divided into four categories, including aldehydes-ketones, esters, acids and others. Combined with listed results and literature [43], pathways for the conversion of cellulose into ML using the catalytic system are summarized (Fig. S1). Firstly, cellulose macromolecule got hydrolyzed and glucose was produced due to the hydrogen bond formed by Cl- of BmimCl and sulfonic acid sites on catalyst. The glucose was further catalyzed and methyl glucoside was formed after alcoholysis, which was further converted to 5-methoxymethyl furfural after dehydration. Finally, the target product ML and by-product methyl formate was resulted by the hydrolyzation of 5-methoxymethylfurfural. In addition, the fructose isomerized from glucose would get dehydrated to form 5HMF (5-hydroxymethyl furfural), which can be dehydrated into 5methoxymethyl furfural. Thus the target product ML can be obtained by following hydrolyzation of 5-methoxymethyl furfural. Meanwhile, 5HMF can also be hydrolyzed into levulinic acid. ML and levulinic acid in the resulting product can be converted into each other before the final conversion equilibrium.
4. Conclusion Sustainable production of methyl levulinate from biomass was proven feasible and efficient using biomass-based bifunctional catalyst in ionic liquid-methanol system. Ionic liquid BmimCl played an important role in the depolymerization of cellulose into small molecules. BmimCl and the prepared biomass bifunctional catalyst showed good recyclability after 5 batches of recycling. Previous research focused on the screening of catalyst for methyl levulinate production, while this work could be a considerable reference for the sufficient utilization of biomass for catalysts preparation and the efficient production of methyl levulinate. Acknowledgement Thanks for the financial support of National Key Research and Development Program of China (2018YFB1501404), National Key Research and Development Program of China (2017YFD0601003) and Science and Technology Planning Project of Guangdong Province, China (2017A010104005). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.116246.
3.4. Recycling of catalyst and ionic liquid
References
Efficient recycling of catalyst and ionic liquid is pivotal for practical production of bio-chemicals. Hence the recycling performance of catalyst SI-20 and ionic liquid BmimCl was determined. It can be seen in Fig. 6a that the conversion ratio of cellulose got no reduction as catalyst SI-20 was recycled for 5 batches. Meanwhile, ML yield reduced from 38.7% (fresh catalyst) to 35.5% after one batch of catalyst recycling. This was resulted due to the loss of Zr(SO4)2 component agglomerated on the surface of catalyst. A decrease in Zr4+ (Lewis acid) sites affected the catalytic hydrolysis process of 5-methoxymethylfurfural to ML. However, slight affection on ML yield was generated during subsequent recycling of catalyst. Yield of target product decreased to 33.1% as the catalyst was recycled for 5 batches. Catalyst SI-20 still maintained an acceptable catalytic activity for the conversion of cellulose to ML. Reduction in catalytic activity of catalyst SI-20 was not that significant during 5 batches of recycling considering the coking of catalyst. Besides, conversion ratio of cellulose maintained at 100% during the 5
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