Eco-friendly Method for Efficient Conversion of Cellulose into Levulinic

Feb 6, 2017 - An eco-friendly method for efficient conversion of cellulose and other carbohydrates into levulinic acid in pure water with cellulase-mi...
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Research Article pubs.acs.org/journal/ascecg

Eco-friendly Method for Efficient Conversion of Cellulose into Levulinic Acid in Pure Water with Cellulase-Mimetic Solid Acid Catalyst Feng Shen,† Richard L. Smith, Jr.,‡ Luyang Li,† Lulu Yan,† and Xinhua Qi*,† †

Agro-Environmental Protection Institute, Chinese Academy of Agricultural Sciences, No. 31, Fukang Road, Nankai District, Tianjin 300191, China ‡ Research Center of Supercritical Fluid Technology, Tohoku University, 6-6-11 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan S Supporting Information *

ABSTRACT: Microcrystalline cellulose could be effectively converted into levulinic acid in pure water at 180 °C in 12 h without additives in a maximum yield of 51.5% with a cellulase-mimetic solid acid catalyst prepared without the use of sulfuric acid. Ball-milling pretreatment of cellulose improved levulinic acid yields by only a few percent, showing that the cellulose binding sites (−Cl) and catalytic sites (−SO3H) of the catalyst are key to the activity of the catalyst. The spent catalyst could be regenerated with H2O2 solution after recycling for 5 times to maintain more than 95% of its catalytic activity. Glucose used as starting material under the same reaction conditions and with the same cellulase-mimetic solid acid gave a yield of 61.5% levulinic acid. The conversion route for carbohydrates to levulinic acid in pure water with the biomimetic catalyst prepared with a H2SO4-free method provides an environmentally friendly method for producing biobased-platform chemicals from renewable resources. KEYWORDS: Carbohydrate, Bifunctional catalyst, Sulfuric acid-free preparation, Hydrogen peroxide regeneration, Biomass conversion

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INTRODUCTION Levulinic acid (C5H8O3), which has been identified by the US Department of Energy as a platform chemical for biorefineries,1 is an important intermediate for production of chemicals, resins, polymers, and fuel additives.2 For instance, levulinic acid can be transformed into γ-valerolactone,3−5 which has been frequently used as a green solvent or as a food or fuel additive. Levulinic acid can be oxidized into succinic acid,6 which is a feedstock for producing 1,4-butanediol, γ-butyrolactone, and maleic anhydride.7 Levulinic acid can be synthesized by acidcatalyzed hydrolysis of carbohydrates;8−10 however, the use of cellulose as a feedstock is considered to be essential for developing efficient biorefineries. Mineral acids, including H2SO4 and HCl, have been traditionally used as homogeneous catalysts to produce levulinic acid from cellulose.11,12 Although these methods are effective, they have many technological issues related to equipment corrosion, environmental pollution, and energy requirements for recycle.13 Solid acid catalysts are attractive for conversion of cellulose into levulinic acid because they avoid these problems. Some solid acid catalysts have been shown to be effective for conversion of cellulose into levulinic acid in organic solvents. For example, a yield of more than 69% levulinic acid can be obtained from cellulose with Amberlyst 70 in 90 wt % γ-valerolactone/10 wt % water mixtures,14 mainly due to the solubility of cellulose in γ-valerolactone, which © 2017 American Chemical Society

improves the mass transfer of cellulose molecule to the active sites in the resin. However, the production of levulinic acid from cellulose with solid acid catalysts in pure water is highly challenging, since the solubility of cellulose in pure water is essentially zero. Hydrothermal reaction environments can increase the solubility of cellulose in water due to the selfionization of water that promotes hydrolysis. For example, Wang et al. used sulfated TiO2 as solid acid to produce levulinic acid from cellulose in water at 240 °C to obtain yields of 27.2%.15 Amberlyst 70, which is a polymeric resin containing −SO3H groups, has been employed as a solid acid to convert cellulose into levulinic acid in pure water with 28% yields.16 Other solid catalysts, such as Nafion SAC 13 (Nafion polymer on amorphous silica),17 zirconium dioxide,18 and Ru-Al-SBA15,19 have also been used to produce levulinic acid, but the yields of levulinic acid in pure water are generally unsatisfactory. The primary reason why low levulinic acid yields are obtained in cellulose−water systems is because both cellulose and solid acid catalysts are insoluble in water so that there is poor accessibility of the catalytic sites to the reactive position of the β-1,4-glucan linkage in the cellulose substrate.20,21 Received: November 15, 2016 Revised: January 18, 2017 Published: February 6, 2017 2421

DOI: 10.1021/acssuschemeng.6b02765 ACS Sustainable Chem. Eng. 2017, 5, 2421−2427

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(TEOS) was used as the cross-linker, and Pluronic F127 triblock copolymer (EO106PO70EO106, Mw = 12600) was used as the amphiphilic surfactant. The silica/carbon composite was prepared with an evaporation-induced triconstituent coassembly method. After the template was removed, the −SH groups on the obtained mesoporous material were oxidized to form −SO3H groups via H2O2. The binding domains (−Cl) derived from sucralose and the catalytic domains (−SO3H) of the catalyst were formed without the use of concentrated H2SO4 or fuming sulfuric acid. The mesoporous structure of the prepared solid acid should facilitate access of hydrolyzed intermediates with inner catalytic sites and should improve the catalytic activity of the solid acid. The prepared mesoporous cellulase-mimetic solid acid catalysts were evaluated by considering the conversion of cellulose into levulinic acid in pure water.

Cellulase has high selectivity and is able to effectively hydrolyze cellulose into glucose, but its low stability, high cost, and one-off consumption property limits its industrial application.22 Structurally, cellulase has a cellulose-binding domain and a catalytic domain (−COOH and −OH groups in amino acids).23−25 The cellulose binding domain adsorbs cellulose, while the catalytic domain promotes cellulose hydrolysis. Shuai and Pan,26 who explored the catalytic mechanism of this natural enzyme, prepared a cellulase-mimetic solid acid catalyst by sulfonating commercial chloromethyl polystyrene resin. The obtained catalyst contained both chloride (−Cl) and sulfonic (−SO3H) groups that acted as cellulose-binding domains and catalytic domains, respectively. Working in the same way as cellulase, the cellulose-binding domain (−Cl groups) of the solid catalyst adsorbs cellulose via strong specific interactions, and the adsorbed cellulose is subsequently hydrolyzed by the catalytic domains (−SO3H groups). With this robust and recyclable cellulase-mimetic solid acid, cellulose could be hydrolyzed into glucose with a maximum yield of 93% at 120 °C in 10 h. Another cellulasemimetic solid acid was prepared by partially substituting the chlorine atoms in the chloromethyl polystyrene resin with sulfonic groups, and levulinic acid yields of 33.1% and 65.5% were obtained from cellulose in water and 90 wt % γvalerolactone/10 wt % water mixtures, respectively.27 In this method, several steps were applied for −SO3H functionalization by treating in sequence with thiourea, NOH, H2SO4, and H2O2. Hu et al. synthesized a cellulase-mimetic solid acid catalyst containing −Cl and −SO3H by sulfonating sucralose based carbon with H2SO4,28 and they obtained glucose yields of 55% from ionic liquid pretreated cellulose within 24 h at 120 °C. However, concentrated H2SO4 was used for the −SO3H functionalization of the catalysts, which negates some of the benefits of the solid acid catalyst compared with the use of mineral acids. For example, a cellulase-mimetic solid acid catalyst having boronic and sulfonic groups for cellulose hydrolysis was prepared from naphthalene-1-boronic acid through Friedel−Crafts polymerization and then sulfonation in multiple steps with concentrated sulfuric acid and fuming sulfuric acid.29 The synthesis process can be considered to be complex, and hazardous fuming sulfuric acid is also necessary. In this work, a silica/carbon porous composite bearing −Cl and −SO3H was synthesized as a cellulase-mimetic solid acid catalyst for the purpose of producing levulinic acid from cellulose in pure water. Scheme 1 shows the synthesis steps for preparing the cellulase-mimetic solid acid: (3-mercaptopropyl)trimethoxysilane (MTOS) was used as the −SO3H precursor, sucralose was used as the −Cl precursor, tetraethyl orthosilicate

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EXPERIMENTAL SECTION

Materials. Sucralose, sucrose, cellobiose, glucose, starch, and Amberlyst-15 were obtained from Beijing J&K Co., Ltd. (Beijing, China). Poly(ethylene oxide)-b-poly(propylene oxide)-b-poly( e t h y l e n e o x i d e ) tr i b l o c k c o po l y m e r s P l u r o n i c F1 2 7 (EO106PO70EO106, Mw = 12 600), tetraethyl orthosilicate (TEOS), (3-mercaptopropyl)trimethoxysilane (MTOS), cellulose (Avicel), and levulinic acid were obtained from Sigma-Aldrich (St. Louis, MO, USA). Other chemicals and analytical reagents were obtained from Shuanghe Ziyang Technology Co., Ltd. (Tianjing, China). Preparation of the catalyst. The synthesis route of the biomimetic solid acid is shown in Scheme 1. In a typical synthesis procedure, 3 g of F127 was dissolved in 20 mL of ethanol with 0.15 mL of HCl (37%). Then, 3 g of sucralose with 5 mL of water was added. The mixture was treated with an ultrasonic bath until a clear solution was obtained. After that, 3 mL of TEOS and 2 mL of MTOS were added and stirred for 1 h. Next, the mixture was evaporated in a fume hood at room temperature for 48 h to afford a gel. Then, thermopolymerization was carried out in a muffle furnace under a N2 atmosphere at 200 °C for 15 h. The obtained solid was ground into a fine powder and washed with boiling ethanol for 24 h to remove the template F127. Finally, 2 g of the dried sample was oxidized with 150 mL of H2O2 (38%). The as-prepared catalyst was denoted as SASO3H. A control sample was synthesized in the same way but using sucrose as the carbon precursor instead of sucralose, and the sample was denoted as SO-SO3H. Characterization of the catalyst. The Cl and S content of the catalysts was determined with inductively coupled plasma spectroscopy (ICP-9000, Thermo Jarrell-Ash Corp., USA). The specific surface areas and pore volume/diameters were determined by BET and BJH methods (Micromeritics, ASAP 2020/Tristar 3000, USA), respectively. X-ray photoelectron spectroscopy (XPS) analysis of the solids was performed with a Thermo Scientific Escalab 250 instrument. The average size of catalyst particles was characterized with a laser particle analyzer (Zetasizer NANO ZS90). Catalytic procedure. Typically, 0.10 g of catalyst and 0.05 g of cellulose were added into 5 mL of water and heated in a steel autoclave (10 mL) at 180 °C. The catalytic reaction was conducted by magnetic stirring at 1000 r/min for 12 h and was terminated by rapidly cooling the steel autoclave in ice−water for 20 min. The liquid fraction of the mixture was filtered through a membrane filter (0.22 μm), and the products in the filtrate were quantified with HPLC. HPLC analysis was carried out on a Waters ACQUITY UPLC H-CLASS liquid chromatography system equipped with an RI detector connected to a SHODEX SH1011 column. The mobile phase was 5 mM H2SO4 with a flow rate of 0.5 mL/min. The column and RI detector temperatures were 50 and 35 °C, respectively. The concentration of each product was calculated by area integral according to an external standard method. Typical HPLC chromatograms for the solid acidcatalyzed hydrolysis of cellulose are shown in Figure S1.

Scheme 1. Synthesis of Mesoporous Cellulase-Mimetic Solid Acid

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DOI: 10.1021/acssuschemeng.6b02765 ACS Sustainable Chem. Eng. 2017, 5, 2421−2427

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ACS Sustainable Chemistry & Engineering Yields of glucose and levulinic acid (mol %) were calculated according to the following formula:

yield (mol%) =

mol of product × 100 mol of glucose unit in feedstock

ICP-AES measurements revealed that the SA-SO3H had 2.2% (w/w) of Cl (Table 1). It has been reported that the −Cl group with its strong electronegativity improves the binding ability of solid acids to cellulose by hydrogen-bond interactions.21 To investigate the binding capacity of −Cl in SA-SO3H to cellulose, adsorption experiments were performed with cellobiose as a model compound instead of cellulose, since cellulose is insoluble in water. Additionally, cellobiose is frequently employed as a model substrate to test the adsorption ability of solid acids for cellulose.21,28,37 As shown in Figure 1,

(1)

For the recycle experiments, mixtures were centrifuged after reaction, and the catalytic solids were washed twice with ethanol and then washed twice again with ultrapure water. After the solids were dried in an oven at 80 °C for 12 h, they were reused in the next reaction. Adsorption experiments. Centrifuge tubes (2 mL) were each filled with 1 mL of water, and then a mixture of cellobiose (100 mg) and SA-SO3H or SO-SO3H (100 mg) was added and the contents were stirred at room temperature (20−120 min). Then, the mixtures were filtered and the cellobiose in the filter was quantified with HPLC as described above. The amount of cellobiose adsorbed by the SASO3H or SO-SO3H was calculated by subtracting the amounts in the supernatants from the amounts initially added.

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RESULTS AND DISCUSSION Characterization of the prepared catalysts. The silica/ carbon composite catalyst, denoted as SA-SO3H, was prepared by an evaporation-induced coassembly method (EISA).30−32 Pluronic F127, with a structure of [(PEO)x(PPO)y(PEO)x] (x = 106, y = 70), was employed as the surfactant. MTOS with TEOS, F127, and sucralose self-assembles into gels with PPO chains in their center (Scheme 1). After evaporation of solvent, the gel was polymerized and F127 was removed, followed by oxidation of −SH to −SO3H with H2O2. The average sizes of SA-SO3H and reference catalyst SO−SO3H were 2.3 μm and 1.9 μm, respectively. To verify whether the as-prepared solid acid (SA-SO3H) had cellulase-mimetic characteristic domains, chemical structures of the SA-SO3H were analyzed. ICP-AES measurement showed that the SA-SO3H contained 3.51% (w/ w) of S (Table 1). XPS analysis indicated that all sulfur on the

Figure 1. Adsorption curves of cellobiose by SO-SO3H and SA-SO3H solid acids in pure water (0.10 g of catalyst, 0.10 g of cellobiose, 1 mL of water).

approximately 15% of cellobiose was adsorbed by the SA-SO3H catalyst within 100 min. These results are in agreement with those of Zuo et al., who used sulfonated chloromethyl polystyrene resin to adsorb cellobiose.27 The control sample (SO-SO3H), which was prepared from sucrose in the same way as that from sucralose, was free of −Cl groups (Table 1), and adsorbed only 5% of cellobiose within 120 min. Another control solid catalyst (SA) was prepared according to the same method with the cellulase-mimetic solid acid (SA-SO3H), but without the addition of bisulfite precursor (MTOS). Experimental results showed that negligible amounts of product formed as analyzed with HPLC when SA was used to catalyze cellulose at 180 °C for 12 h in water (data not shown), meaning that −Cl groups in the solid sample had no catalytic activity for the hydrolysis. The results show that the −Cl groups do not participate in other catalytic steps of the reaction. Thus, −Cl most likely facilitates the access of cellulose chains to the solid acid catalyst. Results demonstrated that the asprepared solid acid SA-SO3H had similar characteristics as those of cellulase, as the solid acid contained both carbohydrate-binding sites (−Cl) and carbohydrate-catalytic sites (−SO3H). N2 adsorption/desorption isotherms and the pore size distribution of the SA-SO3H sample and the control sample SO-SO3H are shown in Figures S3 and S4, respectively. BET analysis revealed that SA-SO3H was mesoporous (average pore diameter 8.0 nm) with a total surface area of 482 m2 /g (Table 1). Porous structures facilitate the transport of hydrolyzed oligomers into inner active sites, thus improving catalytic activity. The advantages of mesoporous materials as catalyst supports for cellulose hydrolysis have been demonstrated.38−41 Structure characterization showed that SO-SO3H had almost the same surface chemistry properties as the SA-SO3H catalyst, except that it was free of −Cl groups.

Table 1. Characterization of the As-Prepared Functionalized Solid Acid Catalysts from Sucralose (SA) or Sucrose (SO) Starting Materials

sample

S (w/w)%

Cl (w/w)%

SBET (m2/g)

Vtotal (cm3/g)

Vmeso (cm3/g)

Average pore diameter (nm)

SA-SO3H SO-SO3H

3.51 3.56

2.20 0

482 459

0.63 0.55

0.47 0.40

8.0 7.2

material before oxidation was in its lowest valence state (163.25 eV) attributable to −SH groups (Figure S2). In contrast, after oxidation by H2O2, all sulfur was present in its highest valence state (168.51 eV), indicating that all −SH groups were completely oxidized to −SO3H groups.28 In previously published literature, −SO3H groups grafted onto carbonaceous solids have been shown to be effective for transforming biomass.33−35 In comparison with other porous solid acids synthesized by the EISA method,30,31 a −SO3H precursor (MTOS) was employed in this work that alleviated the postsulfonation process with concentrated sulfuric acid or fuming sulfuric acid. Although MTOS as the bisulfite precursor is toxic and is a potential environmental hazard, it reacts completely with the cross-linker (TEOS) by sol−gel reaction without creating waste. Zhong et al. reported that this type of solid acid can be mass produced via a modified EISA method with standard rotary evaporation equipment.36 2423

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ACS Sustainable Chemistry & Engineering Levulinic acid production by SA-SO3H and SO-SO3H catalysts. The direct conversion of cellulose into levulinic acid was performed in aqueous solutions. Negligible levulinic acid and glucose formed in water throughout the reaction in the absence of catalyst, even though the temperature was maintained at 180 °C for 15 h (blank experiment, Figure S1a). As shown in Figure 2, the yields of levulinic acid were high in the presence of the solid acid catalysts compared with their

SO3H via hydrogen bonds between hthe ydroxyl groups (−OH) of cellulose and the electronegative groups (−Cl) of SA-SO3H, and are hydrolyzed into oligosaccharides by the adjacent −SO3H groups. Next, the formed oligosaccharides can enter into the mesopores of the solid acid43 and undergo further hydrolysis to glucose. After that, the generated glucose is transformed into 5-HMF, followed by a rehydration step to generate levulinic acid by the −SO3H groups. In the absence of catalyst, the dissolution of cellulose in the water under the reaction conditions is extremely limited. When the SA-SO3H catalyst is added, the dissolved cellulose is converted into levulinic acid via the intermediates glucose and 5-HMF, which promote the continuous dissolution and conversion of cellulose. Thus, cellulose dissolution is probably the ratedetermining step in the mechanism. The temperature dependence of the conversion of cellulose to levulinic acid with SA-SO3H catalyst was investigated. As shown in Figure 3(a), when the reaction time was 12 h, levulinic acid was not formed at temperatures below 120 °C and glucose was the main product when the temperature was below 160 °C. The yields of glucose and levulinic acid at 160 °C were 23.0% and 6.11%, respectively. At temperatures of 180 and 200 °C, the yield of levulinic acid dramatically increased to 39.7% and 43.1%, respectively. On the contrary, for these conditions, the yield of glucose gradually decreased to 8.9% and 2.5%, respectively. Thus, for a fixed reaction time (12 h), higher temperatures favored conversion of cellulose into levulinic acid by the SA-SO3H solid acid catalyst. The effect of SA-SO3H solid acid catalyst/cellulose ratio on levulinic acid yield from cellulose was investigated. As shown in Figure 3(b), when the dosage of the catalyst was low for a catalyst/cellulose mass ratio of 0.5, there were a limited number of acid sites available for catalytic hydrolysis of cellulose into glucose, and there were an insufficient number of active sites available to efficiently convert the formed glucose into levulinic acid. Thus, in this case, glucose was the dominant product. When the catalyst/cellulose ratio increased from 0.5 to 4, the yield of levulinic acid increased gradually from 10.3% to 51.5% for a 12 h reaction time, which can be ascribed to the increased number of catalytic sites. However, when the catalyst/cellulose ratio was further increased to 5, the yield of levulinic acid slightly decreased to 49.4%. This may be ascribed to the excess −SO3H sites that promote not only the transformation of intermediate 5-HMF into levulinic acid, but also the degradation of 5-HMF into humins.45,46 The presence of humins was apparent by the characteristic color change of the

Figure 2. Yields of levulinic acid and glucose from cellulose in pure water by SA-SO3H (hashed) and SO-SO3H (unhashed) solid acid catalysts. (Reaction conditions: 0.10 g of catalyst, 0.05 g of cellulose, 5 mL of water, 180 °C).

values in the absence of catalyst. For SO-SO3H solid acid catalyst, the yield of levulinic acid was 11.1% for a 15 h reaction time. For SA-SO3H solid acid catalyst, the yield of levulinic acid was as high as 46.0% for a 15 h reaction time. Clearly, the yield of levulinic acid from cellulose by SA-SO3H was much higher than that of SO-SO3H. SO-SO3H has almost the same surface chemistry properties as SA-SO3H, except that SO-SO3H is free of −Cl groups (Table 1). Thus, the much higher catalytic activity of SA-SO3H with respect to SO-SO3H can be most likely attributed to the presence of −Cl groups as the binding domain. The total yield of product began to gradually decrease with prolonged reaction time, which is attributed to the complete cellulose conversion and the increase in humin formation. According to experimental results and to the literature,42−44 a reaction pathway for levulinic acid formation from cellulose in pure water by the SA-SO3H solid acid catalyst can be proposed (Scheme 2). First, cellulose becomes partially dissolved in water at the hydrothermal conditions, which leads to the formation of long-chain oligomers that may not be completely soluble in water. The solubilized cellulose chains are adsorbed by the SA-

Scheme 2. Proposed Reaction Pathway of Levulinic Acid Formation from Cellulose with SA-SO3H Solid Acid Catalyst in Pure Water

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Figure 3. Conversion of cellulose into levulinic acid in pure water: (a) Influence of temperature (Reaction conditions: 0.10 g of catalyst, 0.05 g of cellulose, 5 mL of water, 12 h) and (b) influence of solid acid catalyst/cellulose ratio (Reaction conditions: 0.05 g of cellulose, 5 mL of water, 12 h, 180 °C).

reaction mixtures from pale yellow to deep brown.47−49 Although the maximum yield of levulinic acid from cellulose herein (51.5%) is not the highest compared with published literature, this yield is high for pure water solid acid catalytic systems without additives. Formation of levulinic acid from different biomass substrates with SA-SO3H solid acid catalyst was investigated for the same reaction time. For comparison, conversions of cellulose catalyzed by Amberlyst-15 resin and SO−SO3H are shown in Table 2. As shown in Table 2, the yield of levulinic acid from

One of the attractive properties of solid acids compared with bioenzyme or liquid acids is their reusability. To examine the stability and reusability of the prepared SA-SO3H, recycle experiments of the catalyst with cellulose substrate were performed. As shown in Figure 4, the yield of levulinic acid

Table 2. Catalytic Hydrolysis of Carbohydrate Substrates in Pure Water by Different Catalystsa Yield (mol %) Catalyst

Substrate

glucose

levulinic acid

SA-SO3H SA-SO3H SA-SO3H SA-SO3H SA-SO3H Amberlyst-15 SO-SO3H

cellulose (Avicel) glucose cellobiose starch ball-milled cellulose cellulose (Avicel) cellulose (Avicel)

7.6

46.0 61.3 58.9 54.3 52.2 5.74 19.9

4.2 2.9 5.7 2.56 12.6

Figure 4. Reusability and regeneration of the SA-SO3H for catalytic conversion of cellulose to levulinic acid (Reaction conditions: 0.10 g of catalyst, 0.05 g of cellulose, 5 mL of water, 12 h, 180 °C, Catalyst regenerated with H2O2 at times 6 and 10).

decreased gradually from 41% to 30% when the catalyst was reused five times. There are at least two possible reasons why the catalyst was deactivated: (i) leaching of −SO3H and −Cl groups or (ii) humins being adsorbed onto catalyst active sites. The Cl and S content of the catalyst SA-SO3H after being used for five times was measured with ICP-AES and compared with fresh SA-SO3H (Table S1). It was concluded that negligible Cl and S losses occurred during reaction (Table S1). Thus, deactivation of the solid acid catalyst was not due to leaching of functional groups. The morphology of the SA-SO3H before and after recycle was observed (Figure S5). The fresh solid catalyst was brown, and it became black after five uses, indicating humins probably deposited onto the solid surface. The total surface areas of the fresh SA-SO3H and the reused catalyst for five recycles were 482 and 302 m2/g, respectively (Table S1). This result implies that the formed humins blocked the pores of the catalyst. Zuo et al.27 and Alonso et al.14 reported that H2O2 solution could effectively remove humins from solid acids. In this work, the catalyst SA-SO3H after five recycles was washed with H2O2 solution (30 wt %) at room temperature for 12 h. The color of the used SA-SO3H changed back from black to brown (Figure S5). The total surface area of the regenerated SA-SO3H catalyst increased to 419 m2/g after H2O2 treatment (Table S1). After regeneration, yields of levulinic acid increased and were comparable with those of fresh SA-SO3H (Figure 4).

a

All hydrolysis experiments were performed under the same conditions: 0.10 g of catalyst, 0.05 g of substrate, 5 mL of water, 180 °C, 12 h. Ball-milled cellulose was obtained by ball-milling of cellulose with a planetary ball mill (SFM-3, MIT corporation) for 4 h.

cellulose by the SA-SO3H was 46.0%, which is much higher than that by the commercial solid acid Amberlyst-15 and −Cl free solid acid SO-SO3H prepared in this work (5.7% and 19.9%, respectively), indicating that the SA-SO3H gave much better performance for levulinic acid production than other catalysts. For glucose as substrate, yields of levulinic acid were as high as 61.3%. Cellobiose is a disaccharide sugar consisting of two β-glucose molecules, and it is relatively easy to be hydrolyzed into glucose by acid catalysts. The levulinic acid yield from cellobiose (58.9%) was comparable to that from glucose. When starch was used as substrate, good levulinic acid yields were obtained (Table 2). It is likely that mass transfer resistance exists at the substrate−-catalyst contact. To examine this point, reaction of ball-milled cellulose with SA-SO3H catalyst was performed. As shown in Table 2, levulinic acid yields increased by 6% (Table 2) over that of as-is Avicel cellulose (46.0%). Thus, ball-milling, which reduces cellulose particle size and crystallinity, improves the cellulose accessibility to the catalyst.50 2425

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ACS Sustainable Chemistry & Engineering

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Thus, regeneration of the prepared catalyst is simple, and it can be conducted when the catalytic activity decreases in the present reaction system.

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CONCLUSIONS An eco-friendly method for efficient conversion of cellulose and other carbohydrates into levulinic acid in pure water with cellulase-mimetic solid acids was developed. The SO3H functionalized sucralose (SA) catalyst exhibits cellulase-mimetic properties and has favorable catalytic activity that is attributed to its functional groups and mesoporous structure. With SASO3H solid acid catalyst, levulinic acid was formed from untreated cellulose in pure water with yields as high as 51.5%. Ball-milling pretreatment of cellulose improved the performance of the solid acid catalyst. The prepared catalyst could be reused by H2O2 regeneration. Biomimetic catalysts have high potential for developing efficient transformation routes of lignocellulosic biomass into value-added chemicals.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02765. Figure S1, typical HPLC chromatogram for the blank experiment and SA-SO3H-catalyzed hydrolysis of cellulose; Figure S2, XPS spectra (S 2p) of SA-SO3H and SOSO3H before and after H2O2 oxidation; Figure S3, N2 adsorption/desorption isotherms of SA-SO3H and SOSO3H; Figure S4, pore size distributions of SA-SO3H and SO-SO3H; Figure S5, optical images of fresh SA-SO3H, SA-SO3H used after 5 times, and regenerated SA-SO3H; Table S1, Cl and S content of the SA-SO3H before and after use and after regeneration (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X. Qi); Tel (Fax): 86-222361-6651. ORCID

Richard L. Smith Jr.: 0000-0002-9174-7681 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial supports by the National Natural Science Foundation of China (NSFC, Grant No. 21577073), the Natural Science Foundation of Tianjin (No. 16JCQNJC05300 and No. 16JCZDJC33700), The Science and Technology Innovation Program and Elite Youth program of Chinese Academy of Agricultural Sciences (X.Q.).

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