Fuel 259 (2020) 116246

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Fuel

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Full Length Article Sustainable production of methyl levulinate from biomass in ionic liquid- 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

GRAPHICAL ABSTRACT

ARTICLE INFO ABSTRACT

Keywords: The preparation of bifunctional solid-acid catalyst and production of methyl levulinate were performed sy- Methyl levulinate nergistically based on the fractionation of wood residue. The hydrothermal hydrolysate of wood residue was Biomass utilized for catalyst preparation using template method with sulfonic group as the Brønsted acid site and Zr4+ as Ionic liquid Lewis acid site. material for methyl levulinate production was fractionated by the delignification of Catalyst 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 spices, coatings, adhesives, plasticizers, pharmaceuticals and so on, methyl levulinate (ML) has been considered as a hot target product in High-value chemicals production from biomass has been a vital bio-refinery [5–7]. Biomass including untreated biomass [8] and cel- issue for the utilization of renewable energy sources [1–4]. As widely lulose [9–11] could be more cost-effective as raw material for ML used as the fuel additive and the raw materials for the manufacture of production when compared with [12–14], [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. X. Liang, et al. Fuel 259 (2020) 116246 [16,17], 5-chloromethyl [18] and so on [19]. treatment for biomass was performed to remove most hemicellulose in At present, catalysts such as mineral-acids [10], mineral-salt [20], solid- biomass while deep eutectic solvent (DES) treatment was further em- acid [21] and acidic ionic liquids [8,22] are commonly studied to fa- ployed for delignification of hydrothermal residue. Hence the cellulose cilitate the production of ML from biomass. fraction in biomass could be retained as DES residue. According to the Generally, homogeneously catalytic process tends to present a better mean of three replicates obtained by composition analysis of DES re- performance for cellulose conversion considering the conversion ratio sidue with isolated lignin fraction and hydrothermal hydrolysate, the of cellulose and the yield of target product [23–25]. Brønsted acids such fractionation yield of cellulose, lignin (including acid-insoluble lignin as [10], [26] and Lewis acids such as and acid-insoluble lignin) and hemicellulose (based on xylan) were

Al2(SO4)3 [27] have been widely studied as the efficient homogeneous 94.5% wt., 90.3% wt. and 83.7% wt. after the two-step pretreatment. catalysts for ML production from cellulose. Li reported ML production Besides, the hydrothermal hydrolysate was concentrated with the from cellulose using extremely low concentration sulfuric acid of concentration ratio of 1:5. 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 2.2. Preparation and characterization of catalysts of 44% [28]. The recyclability and potential corrosion for equipment are the major disadvantages inhibiting further large-scale application Biomass-based solid acid catalysts were prepared using template with these homogeneous catalysts [29]. While heterogeneous catalysts method and the concentrated hydrothermal hydrolysate was used as are easy to be separated and reused. Research on ML production from carbon supplier to prepare support. Template Al@SiO2 microspheres cellulose with heterogeneous catalysts also got lots of attention as the were prepared with impregnation method [40]. The support of catalysts potential alternative option for commercial homogeneous catalyst [30]. was prepared at 200 °C for 10 h with Al@SiO2 microspheres and con- Meanwhile, synergy of Lewis acid and Brønsted acid in catalyst has centrated hydrolysate in the ratio of 1 g/15 mL. After filtration and been put forward to further explore the conversion mechanism of ML washing, the resulting filter residue was heated in tube furnace at ni- from cellulose and to improve the conversion efficiency of cellulose trogen condition (850 °C, 5 h). The resulting solid was soaked in 10% [31–33]. Brønsted acid works well in the conversion of cellulose into wt. hydrofluoric acid overnight. The support was obtained after filtra- [34] and Lewis acid function effectively in following ML pro- tion, washing and oven-drying for the resulting mixture. Catalysts were duction from hexoses [33]. Tominaga [32] employed the mixture of prepared by sulfonation (S), impregnation (I) or sulfonation-impreg-

Lewis acid In(OTf)3 (Indium trifluoro-methanesulfonate) and Brønsted nation (SI) of support using concentrated sulfuric acid as sulfonating acid 2-NSA (2-naphthalenesulfonic acid) to convert cellulose into ML agent and zirconium sulfate as supported substance [41]. In addition, with the maximum yield of 75% at 180 °C for 5 h. Zhang [31] reported the number 10, 20, 30 and 40 displayed after the catalyst code SI or I the use of metal phosphotungstate with bifunctional Brønsted and represents for the percentage amount of zirconium sulfate loaded on the Lewis acidities for cellulose conversion with the highest ML yield of support. 49.0% at 160 °C for 30 min under microwave heating. However, ML FT-IR measurement was performed with KBr tablet method using a − production from cellulose with hybrid Lewis acid (site) and Brønsted Tensor 27 spectrometer within 400–2000 cm 1 to determine the che- acid (site) catalytic strategy for commercial production is still in its mical groups of catalysts. And the XRD spectra of catalysts were ob- fancy. More sustainable and cost-effective conversion method should be tained on a D8 ADVANCE X-ray powder diffractometer (Bruker, introduced to produce ML from cellulose or inexpensive material such Germany) with the 2ϴ ranging 5-60°. The microstructure of prepared as cellulose fraction of wood residue. Compared with the original lig- catalysts was obtained with a scanning electron microscope (Merlin nocellulose, the fractionated cellulose material would generate less side SEM ZEISS, Germany) and transmission electron microscope (JEM- products from pentose and lignin during ML production. 1400 Plus, Germany). Recently, fabrication of biomass chemicals within bio-refinery concept has been a hot issue and a lot studies were conducted with the 2.3. Production of methyl levulinate, catalyst recovery and IL recovery cellulose fraction after biomass pretreatment [35,36]. The hemi- -rich pre-hydrolysis liquor could be cost-effective material for 0.5 g oven-dried DES residue (used as cellulose fraction) was used in catalyst support preparation [37,38].At present, very few studies re- each experiment for the production of ML. Certain amount of methanol, ported ML production with biomass-derived bifunctional solid-acid ionic liquid BmimCl, deionized water, catalyst and DES residue were catalyst in the recyclable ionic liquid-methanol system. Due to its good mixed homogeneously and added in a hydrothermal reactor with con- dissolution capacity for cellulose, ionic liquid 1-butyl-3-methyl imida- figured reaction condition (Scheme shown in Fig. 1). Reacting product zolium chloride (BmimCl), were employed to facilitate the dissolution was filtered and washed with ethyl acetate. The resulting two-phase of cellulose fraction. In this study, the catalysts for ML production were filtrate contained ionic liquid (IL) phase A and ethyl acetate phase A. IL prepared using the hydrothermal hydrolyte of wood residue with phase A was further extracted with ethyl acetate to obtain ethyl acetate template method. Sulfonic group and Zr4+ were loaded as the Brønsted phase B and IL phase B. The IL phase B was rotary evaporated to recover acid site and Lewis acid site. The cellulose for ML production was BmimCl. Meanwhile, residue A resulted by ethyl acetate washing was fractionated from the hydrothermal residue of biomass. This study is further washed with methanol to get methanol phase and residue B. performed to develop a potential low-cost and sustainable strategy for Liquid mixture containing methanol phase, ethyl acetate phase A and ML production from biomass. 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. 2. Experimental section Residue B was oven-dried before content analysis. Catalysts were recovered after the hydrolysis of remaining cellulose in residue B 2.1. Materials and biomass pretreatment using dilute sulfuric acid. Filter residue obtained after acid hydrolysis was heating in tube furnace at 650 °C for 2 h under nitrogen atmosphere Dewaxed wood powder of eucalyptus globulus wood residue was before the recycling of catalyst. employed for biomass fractionation with the size of 40–60 mesh. Reagent-grade chemicals such as methanol, zirconium sulfate and so on 2.4. Measurement of methyl levulinate were obtained from Sigma-Aldrich. 1-butyl-3-methyl imidazolium chloride (BmimCl, 99% purity) was supplied by Lanzhou Yulu Fine Product ML was quantified by gas chromatography (Agilent 6890, Chemical Co. LTD. The fractionation of biomass was performed with a equipped with HP-5 column and FID). Conversion ratio of cellulose R two-step pretreatment as previous reported [39]. Briefly, hydrothermal and molar yield of ML Y were calculated as follows:

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Fig. 1. Scheme of methyl levulinate production and separation.

W R =×O 100% in Fig. 3, the catalytic performance of sulfonated catalyst S and sulfo- WR (1) nated-impregnated catalysts were remarkably better than that of im- pregnated catalysts. Cellulose conversion ratio of 78.4% and ML yield N Y =×1 100% of 7.6% was obtained using sulfonated catalyst S under the adoptive N2 (2) experiment condition. With the increasing Zr(SO4)2 loading of SI (sul- fonation-impregnation) catalysts from 10% to 40%, the resulting cel- where W and W are the mass of hexose in cellulose fraction and the O R lulose conversion ratio and ML yield increased at the beginning and oven-dried residue B, N is the amount of ML in product and N is the 1 2 decreased after Zr(SO ) loading surpassing 20%. The highest cellulose amount of hexose in the cellulose fraction. 4 2 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

3. Results and discussion higher Zr(SO4)2 loading of catalyst would result in a reduction of spe- cific surface area and further coverage of sulfonic acid sites. Hence the 3.1. Comparison between different catalysts contact chances between sulfonic acid (Brønsted acid) sites and re- actant would be decreased, which would negatively affect the hydro- Comparisons between catalysts were performed with characteriza- lysis of cellulose, formation of the important intermediate methyl glu- tion results and catalytic performance for ML production. The FT-IR coside and its subsequent dehydration into 5-. spectra of catalysts are shown in Fig. 2a. Apart from the absorption However, the generation of ML by the hydrolysis of 5-methox- −1 peaks of C=O stretching vibration (1719 cm ), C=C stretching vi- ymethylfurfural got significantly promoted due to the further in- −1 2 bration (1595 cm ) and out-of-plane bending vibration of cis-form sp troduction of Lewis acid site Zr4+ with these sulfonated catalysts as – C H in the FT-IR spectra of each catalyst, peaks for the stretching vi- their Zr(SO4)2 loading elevated from 0 to 20%. Thus the conversion of −1 −1 bration of C-S (620 cm ), asymmetry S = O (1190 cm ) and sym- cellulose to ML was facilitated with these sulfonated-impregnated cat- −1 metric S = O (1095 cm ) were capable to prove the introduction of alysts when compared with that of sulfonated catalyst S. Since the sulfonic acid group for the five kinds of sulfonated catalysts. While the conversion ratio of cellulose resulted by catalyst SI-10 (82.5%) and 2- stretching vibration of asymmetry and symmetric S=O in SO4 were catalyst SI-20 (82.3%) were approaching, ML yield by catalyst SI-20 −1 −1 observed at 1170 cm and 1080 cm for the eight kinds of Zr(SO4)2- (9.4%) was significantly higher than that of catalyst SI-10 (7.9%). impregnated catalysts. Besides, the out-of-plane bending vibration of Therefore, catalyst SI-20 seemed more suitable for ML production and −1 2 −1 carboxylate O–H (950 cm ) and trans-form sp C–H (990 cm ) would was chosen to further study the influence of experimental parameters also illustrate the diversity of carbon functional groups in these cata- and the conversion mechanism of cellulose fraction in BmimCl-me- ff lysts. Meanwhile, the di raction peak of Zr(SO4)2 appeared in the XRD thanol system. 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 3.2. Influence of experimental factors 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 re- Results in section 3.1 indicate that structural characteristic of cat- sults (Fig. 2c) of these catalysts. And these catalysts generally present alysts would notably affect cellulose conversion and ML formation. individually spherical and partially concatenated spherical shape with Major factors such as temperature, time, catalyst dosage and ionic li- hollow structure. Diameter of single spherical structure is between quid dosage also have a significant influence on the reaction rate and 100 nm and 500 nm. And more agglomeration appeared on the surface equilibrium of the entire conversion process. Hence a detailed study on of the catalyst as Zr(SO4)2 loading increased. the effect of these major factors was further performed. Structural differences between these catalysts would lead to their It can be seen from Fig. 4a that ML yield was significantly increased differentiated catalytic performance for cellulose conversion. As shown (14.3%–38.7%) as experimental temperature rose from 180 °C to

3 X. Liang, et al. Fuel 259 (2020) 116246

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 cel- lulose 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 ex- periment would significantly promote the formation of ML as shown in Fig. 4c. The resulting ML yield was 2.1% and conversion ratio of cel- lulose was 91.5% without using catalyst SI-20. However, conversion Fig. 3. Catalytic performance comparisons between different catalysts. ratio of cellulose lifted from 92.1% to 100% and ML yield increased 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 from 9.8% to 38.7% as dosage of catalyst SI-20 increased from 5% to (60 min). 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 220 °C, while ML yield gradually decreased as temperature further in- to accelerate the conversion rate by increasing catalyst dosage. More- creased. Main reason for this relationship is that the exorbitant ex- over, an excessive catalyst dosage would not significantly increase the periment temperature would promote the formation of dimethyl selectivity for methyl levulinate formation. due to the Brønsted acid sites-catalyzed dehydration of methanol, Previous results have proven that ionic liquid BmimCl played an which would inhibit the formation of methyl glucoside [14]. Mean- important role in the depolymerization of cellulose into small mole- while, conversion ratio of cellulose got enhanced gradually from 88.7% cules, generating sufficient substrate that can be contacted and cata- to 100% as temperature rose from 180 °C to 220 °C. And the conversion lyzed by catalyst for subsequent conversion. Hence the influence of ratio of cellulose was maintained at 100% as temperature further went BmimCl dosage on the conversion process was also studied. As can be up. This result indicates that the use of ionic liquid is somehow favor- seen in Fig. 4d, conversion ratio of cellulose and ML yield were about able for the depolymerization of cellulose. Although methanol the re- 80% and 20% using methanol-catalyst system without BmimCl. During actant and solvent for ML production reduced due to enhanced side the rising process of BmimCl dosage from 10 fold (mass of cellulose reaction as temperature increased, synergistic effect of acid sites on fraction) to 20 fold, conversion ratio of cellulose and ML yield gradually catalyst and the hydrogen bond of ionic liquid BmimCl would maintain increased within the range of 92.7%-100% and 28.4%-38.7%. Besides, the efficient depolymerization for cellulose by the isomerization and it can be seen that the excessive dosage of BmimCl would inhibit the dehydration of intermediate products [42]. And the efficient depoly- formation of ML without the inhibition for cellulose conversion. A merization of cellulose laid foundation for the sufficient contact be- higher dosage of BmimCl in solvent system would increase the viscosity tween the acidic sites on catalyst and oligosaccharides. Thus the entire

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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.

NO. Retention time Affiliation

1 3.1 furfural 2 3.5 methyl formate 3 4.2 3-furaldehyde 4 5.8 2-Methoxytetrahydrofuran 5 7.1 methyl levulinate 6 7.7 2-cyclohexen-1-one 7 8.1 Ethyl 3,3-diethoxypropionate 8 9.2 Butanedioic acid, hydroxy-, dimethyl ester 9 10.8 Decanoic acid, 3-hydroxy-, methyl ester 10 10.9 Levulinic acid 11 11.5 Acetic acid, methyl ester 12 13.1 2H-pyran-2-one, 4-methoxy-6-methyl- 13 13.9 Pentanoic acid, 3-hydroxy-4-methyl-, ethyl ester 14 15.3 Propanoic acid, 2-methyl-, methyl ester 15 16.1 levoglucosenone 16 16.6 β-methyl glucoside Fig. 5. GC–MS spectra of liquid products. 17 18.7 citric acid trimethyl ester 18 25.1 methyl 4-O-methyl-α-D-mannopyranosideuronate 19 25.8 methyl 4-hydroxycinnamate of solvent and reduce the recant (methanol) concentration, which 20 27.9 ferulic acid methyl ester would restrict the alcoholysis of cellulose and the formation of ML from 21 28.4 hexadecanoic acid methyl ester intermediate products. Compared with recent studies on ML production, the use of biomass- derived bifunctional catalyst in ionic liquid-methanol provided com- conversion mechanism and conversion route of cellulose into ML in this petitive results in ML yield and cellulose conversion. For example, Li catalytic system is pivotal to further improve the selectively of ML [21] obtained a maximum ML yield of 27.0% from commercial cellu- production. lose with zirconium oxide-loaded zeolite. Feng [43] reported the ML yield of 30.8% from bamboo liquefaction product catalyzed with dilute 3.3. Mechanism and route for methyl levulinate production 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 According to the GC–MS spectra in Fig. 5, it can be seen that a

5 X. Liang, et al. Fuel 259 (2020) 116246

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). significant amount of ML was existed in the collected product mixture batches recycling of ionic liquid BmimCl (Fig. 6b). And the yield of for analysis. Meanwhile, as listed in Table 1, a series of side reactions methyl levulinate hardly got reduced and was maintained above 36.6%. occurred during the alcoholysis of cellulose. Side reactions would Thus we can conclude that ionic liquid BmimCl can be recycled effec- compete with the major reactions by consuming methanol and inter- tively no fewer than 5 batches. mediate products. During the conversion process of cellulose, mono- saccharide would get converted into or ketone such as furfural 4. Conclusion and by dehydration. Further reactions such as alcoholysis, de- hydration and replacement by methoxy group would occur due to the Sustainable production of methyl levulinate from biomass was presence of methanol. Relatively chemically stable products such as proven feasible and efficient using biomass-based bifunctional catalyst esters and glycoside would be generated from these reactive inter- in ionic liquid-methanol system. Ionic liquid BmimCl played an im- mediates and by-products. Hence the products obtained after the cat- portant role in the depolymerization of cellulose into small molecules. alyzed alcoholysis of cellulose fraction can be divided into four cate- BmimCl and the prepared biomass bifunctional catalyst showed good gories, including -ketones, esters, acids and others. recyclability after 5 batches of recycling. Previous research focused on Combined with listed results and literature [43], pathways for the the screening of catalyst for methyl levulinate production, while this conversion of cellulose into ML using the catalytic system are sum- work could be a considerable reference for the sufficient utilization of marized (Fig. S1). Firstly, cellulose macromolecule got hydrolyzed and biomass for catalysts preparation and the efficient production of methyl glucose was produced due to the hydrogen bond formed by Cl- of levulinate. BmimCl and sulfonic acid sites on catalyst. The glucose was further catalyzed and methyl glucoside was formed after alcoholysis, which Acknowledgement was further converted to 5-methoxymethyl furfural after dehydration. Finally, the target product ML and by-product methyl formate was re- Thanks for the financial support of National Key Research and sulted by the hydrolyzation of 5-methoxymethylfurfural. In addition, Development Program of China (2018YFB1501404), National Key the isomerized from glucose would get dehydrated to form 5- Research and Development Program of China (2017YFD0601003) and HMF (5-hydroxymethyl furfural), which can be dehydrated into 5- Science and Technology Planning Project of Guangdong Province, methoxymethyl furfural. Thus the target product ML can be obtained by China (2017A010104005). following hydrolyzation of 5-methoxymethyl furfural. Meanwhile, 5- HMF can also be hydrolyzed into levulinic acid. ML and levulinic acid Appendix A. Supplementary data in the resulting product can be converted into each other before the final conversion equilibrium. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.116246.

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