Hierarchical Sn-Beta Zeolite Catalyst for the ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Hierarchical Sn-Beta Zeolite Catalyst for the Conversion of Sugars to Alkyl Lactates Jian Zhang,† Liang Wang,*,† Guoxiong Wang,† Fang Chen,† Jie Zhu,† Chengtao Wang,† Chaoqun Bian,† Shuxiang Pan,† and Feng-Shou Xiao*,†,‡ †

Key Laboratory of Applied Chemistry of Zhejiang Province, Zhejiang University, Hangzhou 310028, China Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Zhejiang University, Hangzhou 310027, China

‡

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S Supporting Information *

ABSTRACT: Enhancing the yield of alkyl lactate from sugars is in great demand but challengeable to accomplish. Here we report a facile but efficient route to make this by employing a hierarchical and Sn-containing Beta zeolite (Sn-Beta-H). The hierarporosity in the Sn-Beta-H facilitates the conversion of sugars into important intermediates for producing alkyl lactate in the competition with undesirable side reactions, thus outperforming the conventional Sn-zeolite catalysts in yielding alkyl lactate from a wide scope of sugars. Particularly, the yield of alkyl lactate could reach as high as 72.1% from sucrose. Importantly, the Sn-Beta-H is stable and can be easily recycled for five times with constant catalytic performances. KEYWORDS: Biomass conversion, Hierarchical zeolite, Alkyl lactate, Heterogeneous catalyst

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synthesis,29 and hydrodeoxygenation of biomass.30−38 More recently, several studies have reported the synthesis of hierarchical stannosilicates as heterogeneous catalysts,18,25 where the hierarchical porosity is usually formed by posttreatments using acid/base leaching. In these cases, the mesoporosity is randomly formed, and it is challengeable to control the hierarchical pore size and volume.18,26 Additionally, it is well-known that the hierarchical porosity facilitates the formation of alkyl lactate, but the role of the hierarchical structure is still uncertain. Recently, we developed a new methodology for one-pot synthesis of hierarchical zeolites templated with cationic polymers (polydiallyldimethylammonium chloride), where the three-dimensionally interconnected mesopores have been formed.39 Herein, we employed this new method to synthesize hierarchically porous Sn-Beta zeolite with the three-dimensionally interconnected mesopores. By functionalization with Sn, we found that the hierarchical Sn-Beta (Sn-Beta-H) favors the formation of desired products (e.g., fructose, methyl fructoside, dihydroxyacetone, lactic acid, glyceraldehyde, erythrose, glycolaldehyde, alkyl lactate) at the start of the reaction from glucose substrate, all of which can be further transformed into alkyl lactate. In contrast, the Sn-Beta-C gave lower activity and selectivity than Sn-Beta-H to form these desired products, but favors the formation of undesired products (e.g., oligomer,

INTRODUCTION With increasing demand of sustainable development, one important route for biomass conversion is the transformation of glucose into lactic acid derivatives, which can act as feedstock for the production of biodegradable plastics and a wide range of fine chemicals.1−4 However, current production of lactic acid derivatives is mainly through the fermentation process, which has drawbacks such as relatively high production cost, difficulty in regeneration of the biocatalysts, and the formation of salt waste.3 Recently, a heterogeneously catalytic process has been developed for the production of alkyl lactates from sugars, which is regarded as a promising route because of the significant advantages of relatively low production cost and avoidance of salt waste, compared with fermentation process.3 Many Lewis acidic catalysts such as Ti-, Zr-, and Sn-containing Beta and MWW zeolites have successfully catalyzed the conversion of various mono- and disaccharides to lactic acid or alkyl lactate.5−20 In these cases, the Sn-containing zeolite catalysts (e.g., Sn-MWW6 and Sn-Beta14) have outperformed other catalysts in alkyl lactate yields, but the yields are unsatisfactory yet. Considering the complex reaction pathways in sugar conversion, enhancing the selectivity/yield of alkyl lactate is still challenging. Zeolites with hierarchical porous structures containing both micropores and mesopores have been developed, which have been denoted as hierarchical zeolites.21−27 Compared with the conventional zeolites, the hierarchical zeolite based catalysts have exhibited unusual catalytic performances in many reactions, including hydrodesulfurization,28 Fischer−Tropsch © 2017 American Chemical Society

Received: November 29, 2016 Revised: January 21, 2017 Published: March 6, 2017 3123

DOI: 10.1021/acssuschemeng.6b02881 ACS Sustainable Chem. Eng. 2017, 5, 3123−3131

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ACS Sustainable Chemistry & Engineering

Figure 1. (A) Proposed procedures for preparation of the Sn-Beta-H; (B) XRD patterns of (a) Beta-H, (b) Sn-Beta-H, (c) Beta-C, and (d) Sn-BetaC (inset: enlarged view); (C) N2 sorption isotherms of the Sn-Beta-H (Inset: pore size distribution curve of Sn-Beta-H). glass tube. Notably, this procudure should be performed in a glovebox to avoid the influence of water. The mixture was calcnied at 550 °C for 6 h (ramp rate at 5 °C/min) under vacuum and then for another 6 h in flowing air (30 sccm) to obtain the final product. Synthesis of SnO2. The bulky SnO2 was obtained by calcining the SnCl4·5H2O at 500 °C for 4 h in air (ramp rate at 5 °C/min). Characterizations. X-ray diffraction (XRD) patterns were collected on a Rigaku D/MAX 2550 diffractometer with Cu Kα radiation (λ = 1.5418 Å). Sn contents were characterized by inductively coupled plasma (ICP) analysis over Perkin-Elmer 3300DV (see Supporting Information for details, Table S1). Nitrogen sorption isotherms were measured using a Micromeritics ASAP 2020 system. XPS spectra were collected by a Thermo ESCALAB 250 with Al Kα radiation at θ = 90° for the X-ray sources; the binding energy were calibrated using the C 1s peak at 284.9 eV. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) images were performed on a JEM-2100F electron microscoe (JEOL, Japan) with an acceleration voltage of 200 kV. The 119Sn solidstate magic angle spinning nuclear magnetic resonance (MAS NMR) experiments were performed on a Bruker Avance III spectrometer at resonance frequencies of 149.5 MHz and with a sample spinning rate of 8 kHz. Diffuse reflectance ultraviolet−visible (UV−vis) spectra of dehydrated samples were recorded against BaSO4 in the region of 200−1000 nm on a Perkin-Elmer Lambda 20 spectrophotometer. IR spectra were recorded using a Nicolet is50 FT-IR spectrometer equipped with a MCT/A detector and ZeSe windows and a high temperature reaction chamber. The samples were pretreated at 300 °C for 2 h before tests. The deuterated acetonitrile stream was introduced into the system with a flow of Ar carrier gas (30 sccm) with the reactant partial pressure in the range of 100−120 mbar. The samples were treated with deuterated acetonitrile for 30 min to make sure that the Lewis acid sites in the samples were fully coordinated with the acetonitrile. Then Ar carrier gas (30 sccm) was inleted to remove the excess deuterated acetonitrile, after which the spectra were recorded. Catalytic Tests. The conversion of saccharides to methyl lactate over Sn catalysts were performed in a high-pressure autoclave with a magnetic stirrer with stirring rate at 900 rpm. As a typical run, 225 mg of saccharide and 160 mg of catalyst were mixed in 175 mmol of alcohol. The autoclave was purged with N2 for three times to remove the air and charged with N2 to make sure that the pressure was 1 MPa under reaction temperature. The autoclave was heated to desired temperature in an oil bath (the temperature was mesured by a

coke). This phenomenon is reasonably assigned to the mesoporosity in the catalyst, which should be favorable for the fast formation of Sn-glucose intermediate with bulky diameter, followed by fast transformation via retro-aldol reaction to cleavage the C−C bond.

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EXPERIMENTAL SECTION

Materials. All reagents were used without further purification. Polydiallyldimethylammonium chloride (PDADMA) (1−2 × 105, 20 wt %), deuterated acetonitrile, and tetraethylammonium hydroxide (TEAOH, 35 wt % in water) were purchased from Sigma-Aldrich Co. Fumed silica was purchased from Shenyang Chemical Co. Tetraethyl orthosilicate, NaOH, NaAlO2, methanol, ethanol, n-butanol, glucose, fructose, sucrose, maltose monohydrate, maltotriose, and inulin were purchased from Sinopharm Chemical Reagent Co. SnCl4·5H2O was bought from Aladdin Chemical Reagent Co. SnMeCl3 was obtained from J&K China Chemical Co., Ltd. Methods. Synthesis of Hierarchical Beta (Beta-H). As a typical run, 0.08 g of NaAlO2 and 0.3 g of NaOH were dissolved in 13 mL of H2O, followed by addition of 2 g of PDADMA. After stirring to form a clear solution, 0.94 g of fumed silica was added and stirred for another 12 h. Then, the gel was transferred into an autoclave for crystallization at 180 °C for 96 h. After filtrating, washing with a large amount of water, drying under 80 °C, calcining at 550 °C for 5 h to remove the organic template, the hierarchical Beta zeolite was obtained, which was denoted as Beta-H. Synthesis of Conventional Beta (Beta-C). The conventional Beta zeolite (Beta-C) was synthesized using TEAOH as template. As a typical run, 0.3 g of NaAlO2 and 0.16 g of NaOH were dissolved into 12.64 g of H2O. Then, 19.36 g of TEAOH and 4.8 g of fumed silica was added into the liquor. After stirring for 5 h, the gel was transferred into an autoclave for crystallization at 140 °C for 96 h. After filtrating, washing with large amount of water, drying at 80 °C, calcining at 550 °C for 5 h to remove the organic template, the Beta zeolite was obtained, which was denoted as Beta-C. Synthesis of Sn-Beta Zeolites. The Sn-Beta-H and Sn-Beta-C were synthesized from similar route. As a typical run, the Al atoms in the Beta zeolite framework were removed by HNO3 (13 M) at 100 °C for overnight. After washing with a large amount of water and drying in vaccam under 150 °C for 10 h, a desired amount of Sn precursor were added. After grinding for 20 min, the mixture was transferred into a 3124


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Figure 2. (A) SEM image and (B) TEM image of Sn-Beta-H, (C and D) HR-TEM image of regions 1 and 2 in panel B.

used for synthesizing Sn zeolites.6−8,40,41 As proposed procedures in Figure 1A, hierarchical aluminosilicate Beta zeolite (Beta-H) was synthesized using polydiallyldimethylammonium chloride (PDADMA) as a template.39 After calcination to remove the template, the Beta-H zeolite was treated with HNO3 (13 M) for 12 h to remove most of Al species in the framework. Then, Sn sites were incorporated into the treated zeolite by mixing the SnMeCl3 precursor, followed by calcination in a tubular reactor at 550 °C for 6 h under vacuum. After calcining at 550 °C for another 6 h in flowing air, the final product was obtained, which was denoted as Sn-BetaH. By ICP analysis, the atomic ratio of Si/Sn was about 50. For comparison, the conventional Sn-Beta (Sn-Beta-C) catalyst was synthesized with similar method but use of conventional Beta (Beta-C). Figure 1B shows XRD patterns of Beta-H, Sn-Beta-H, BetaC, and Sn-Beta-C, giving typical peaks associated with *BEA structure with good crystallinity. Notably, the peaks of SnO2 crystal are undetectable in the XRD patterns of Sn-Beta-H and Sn-Beta-C, suggesting the high dispersion of Sn sites. Figure 1C shows nitrogen sorption isotherms of Sn-Beta-H, exhibiting two steep steps in the P/P0 < 0.01 and 0.60 < P/P0 < 0.90 regions, ascribed to the filling of the micropore volumes and capillary condensation in the mesopores, indicating the presence of both microporosity and mesoporosity in the sample.39 Interestingly, the nitrogen uptake in the relative pressure (P/P0) from 0.20 to 0.95 is very high, indicating a huge amount of mesoporous volume. After calculation, it is found that the pore volume of Sn-Beta-H is as high as 1.05 cm3/g. In contrast, the pore volume of Sn-Beta-C is only 0.27 cm3/g. These results are summarized in Table S2.

thermometer in the oil bath). After reaction, the autoclave was tansferred into an ice bath to stop the reaction. The products were separated and anlyzed by a LC-10AT liquid chromotography and a Shimadzu GC-2014C gas chromotography. The carbon balance was above 85% over the Sn-Beta-H and Sn-Beta-C. The recyclability of the catalysts were studied by separating the catalyst after reaction, washing with acetone, drying under 80 °C, calcining at 550 °C for 5 h, and then using in the next run. In the kinetic study, the points of sugar conversion/production yield as a function of time at the initial reaction were performed, followed by making the straight lines with the best fitting of the points. The carbon balance was over 95% in all tests. Then, the TOFs were calculated on the basis of the slopes (k) of these straight lines according to eq 1: TOF = (msub /mSn) × k

(1)

where msub is the moles of substrate in the system before the reaction, mSn is the moles of Sn atom in the system, and k is the slope of the straight lines. The productivities were calculated on the basis of the slopes (k) of these straight lines according to eq 2:

productivity = (mpro /mSn) × k

(2)

where mpro is the moles of poducts in the system after the reaction, mSn is the moles of Sn atom in the system, and k is the slope of the straight lines. The adsorption of glucose was performed by stirring the catalyst (dried under vacuum at 150 °C for 5 h) in a glucose aqueous solution with initial concentration at 5 mg/mL. The changes in glucose concentration of the liquor was analyzed by liquid chromotography.

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RESULTS AND DISSCUSION Synthesis and Characterizations. The Sn-Beta-H catalyst was synthesized from a combined strategy of both dealumination and Sn incorporation, which have been extensively 3125


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Figure 3. (A) STEM image and the corresponding (B) O, (C) Si, and (D) Sn elemental map of Sn-Beta-H.

Figure 2A shows SEM image of Sn-Beta-H, giving uniform particles with diameter ranging at 500−900 nm. Notably, there are abundant mesopores in the Sn-Beta-H, as confirmed by the TEM image (Figure 2B). By high-resolution TEM characterizations (Figures 2C,D), it is observed that the mesopores are mainly distributed in the size of 6−13 nm, in good agreement with the results from N2 sorption isotherms (Figure 1C). These results demonstrate that the hierarchial structure of Beta-H is well maintained during the synthesis of Sn-Beta-H. In comparison, the mesopores are nearly undetectable on the SEM and TEM images of Sn-Beta-C (Figure S1). In addition, it is difficult to observe SnO2 crystals in the HR-TEM images of both Sn-Beta-H and Sn-Beta-C, suggesting high dispersion of Sn sites. This result is further confirmed by the EDS mapping analysis, where the Sn is uniformly distributed on the zeolite crystals (Figure 3, Figure S2). Figure 4A shows 119Sn MAS NMR spectra of Sn-Beta-H sample under different conditions. 119Sn MAS NMR spectrum of the dehydrated sample gives a signal at −425 ppm assigned to the tetrahedral coordinated Sn site,40,42 which indicates that SnO2 crystal (−610 ppm) is absent.40 After hydration, this sample gives a signal at −682 ppm, which is attributed to the framework Sn coordinated with two additional water molecules, forming octahedral Sn species.40 These results confirm that the Sn sites in Sn-Beta-H are exclusively located in the framework. Figure 4B shows the UV−visible spectra of dehydrated SnBeta-H, Sn-Beta-C, and SnO2. The Sn-Beta-H and Sn-Beta-C give the peaks at 222 nm, which is attributed to tetrahedral coordinated Sn sites in the zeolite framework.8 In contrast, the SnO2 gives a broad peak at 230−290 nm, assigning to typical hexa-coordinated Sn. Figure 4C gives XPS spectra of Sn-Beta-H and Sn-Beta-C with Sn 3d3/2 and 3d5/2 binding energies at

Figure 4. (A) 119Sn NMR spectra of (a) hydrated and (b) dehydrated Sn-Beta-H; (B) UV spectra of (a) Sn-Beta-H, (b) Sn-Beta-C, and (c) SnO2; (C) XPS spectra of (a) Sn-Beta-H and (b)Sn-Beta-C; (D) deuterated acetonitrile-adsorption IR spectra of (a) Sn-Beta-H, (b) SnBeta-C, and (c) SnO2 (magnified 10 times).

487.9 and 496.3 eV, which are obviously different from the peaks of SnO2 in the literature.43,44 Figure 4D shows deuterated acetonitrile-adsorption IR spectra of various samples, which is a general technique to characterize Lewis acidic zeolites.15,45 The Sn-Beta-H and Sn-Beta-C exhibit similar IR spectra with the 3126


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Figure 5. (A) Reaction formula for conversion of glucose to methyl lactate and methyl vinylglycolate; (B) conversion of glucose (blue column) or yield of methyl lactate (green column) and methyl vinylglycolate (yellow column) over various Sn-containing catalysts. Reaction conditions: 225 mg of glucose, 160 mg of catalyst, 8 g of methanol, 160 °C, 1 MPa N2, 20 h; (C) yield of methyl lactate and methyl vinylglycolate over Sn-Beta-H (blue and yellow) and Sn-Beta-C (pink and lite green). Reaction conditions: 225 mg of saccharide, 160 mg of catalyst, 8 g of methanol, 160 °C, 1 MPa N2, 20 h.

bands at ∼2308, ∼2316, and 2268−2276 cm−1 regions, assigning to the adsorption of acetonitrile on the closed, open Sn sites, and silanol groups, respectively.48 Notably, the band of open Sn sites (∼2316 cm−1) are very significant in the IR spectra (Figure S3), different from the conventional Snzeolite,15 which should be due to the use of SnMeCl3 precursor. The rich open Sn sites should be favorable for the reactions.40,41,46 These results indicate similar Sn type and acidity on Sn-Beta-H and Sn-Beta-C. In contrast, these bands are absent in the SnO2 sample. Catalytic Conversion of Sugars to Alkyl Lactates. Figure 5 shows catalytic performance in the conversion of sugars in methanol to methyl lactate over various catalysts in an autoclave reactor (Figure S4). In the reactions from glucose feedstock (Figure 5B), all Sn catalysts are active for the reaction. SnO2 and SnCl4 exhibit methyl lactate yields at 4.0 and 16.1%, the poor yields should be due to the lack of tetrahedral coordinated Sn sites.3 The conventional Sn-Beta-C catalyst exhibits methyl lactate yield at 41.8%, which is very similar to those in literature.3,6 Interestingly, the Sn-Beta-H gives lactate yield at 52.5%, which is significantly enhanced compared with Sn-Beta-C catalyst (Table S3). It is generally known that the glucose-to-alkyl lactate proceeds multiple steps of retro-condensation of glucose to dihydroxyacetone and isomerization of dihydroxyacetone (Figure S5) with alcohol to alkyl lactate (Scheme 1).3,18,47 To understand the origin of higher methyl lactate yield on SnBeta-H than that on Sn-Beta-C, we studied the catalytic performances of both catalysts in the conversion of glucose and dihydroxyacetone. Turnover frequency values (TOFs) calculated from the initial reaction rates as the number of molecules

transformed per hour per Sn atom are presented in Table 1 and Table 2. Clearly, the TOFs of Sn-Beta-H and Sn-Beta-C in conversion of dihydroxyacetone are much higher than those in conversion of glucose and fructose, demonstrating the conversion of hexose to triose is a rate-control step in the glucose-to-alkyl lactate process. Notably, the Sn-Beta-H catalyst gives TOF (Figure S6) for the glucose conversion at 27.4 h−1, which is nearly twice that over Sn-Beta-C catalyst (14.8 h−1). A similar phenomenon was also observed in the conversion of fructose, where Sn-Beta-H is more active (TOF at 28.1 h−1) than that over Sn-Beta-C (TOF at 15.5 h−1). Considering that Sn-Beta-H and Sn-Beta-C have the same *BEA framework, very similar Sn sites, and almost the same Si/Sn ratio, the high catalytic activity of Sn-Beta-H should reasonably attributed to the hierarchical structure instead of other factors. Although it is well-known that the micropore size of *BEA framework is enough for the diffusion of glucose and fructose molecules, we assume that the presence of mesopores might benefit the diffusion. Figure S7 shows the dependences of glucose concentration change on time in a glucose adsorption test over Sn-Beta-H and Sn-Beta-C catalysts. The data clearly show that Sn-Beta-H exhibits faster adsorption rate and higher capacity than Sn-Beta-C. This phenomenon should be attributed to the mesopores in Sn-Beta-H, where the enrichment of glucose in Sn-Beta-H favors the fast conversion. In the conversion of glucose and fructose, side reaction of glucose or fructose condensation to oligomers always spontaneously occurred at reaction temperature (160 °C) even without any catalysts,48,49 which competes with the desired glucose conversion to important intermediates for producing methyl lactate (e.g., C2−C4 sugars and glucoside). 3127


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ACS Sustainable Chemistry & Engineering Scheme 1. Proposed Reaction Pathways in the Conversion of Glucose to Methyl Lactate

Table 1. Kinetic Values in Glucose and Fructose Conversion over Sn-Beta-H and Sn-Beta-Ca glucose

fructose

catalyst

TOF (h−1)b

productivity (h−1)c

sel. to desired products (%)d

sel. to byproducts (%)e

TOF (h−1)b

productivity (h−1)f



Sn-Beta-H Sn-Beta-C

27.4 14.8

23.3 11.0

85.0 74.3

15.0 25.7

28.1 15.5

24.2 11.7

86.1 75.4

13.9 24.6

Reaction conditions: 1 mmol of substrate, 32 mg of catalyst, 6.4 g of CH3OH, 160 °C, 1 MPa N2. bmmolsubstrate converted mmolSn−1 h−1; mmolintermediate and product mmolSn−1 h−1, the intermediates and products include fructose, methyl fructoside, dihydroxyacetone, lactic acid, glyceraldehyde, erythrose, glycolaldehyde, 1,1-dimethoxy-acetone, 1,1,2,2-tetramethoxy-propane, and 1,1,2-trimethoxy-2-propanol; dcalculated from (productivity/TOF) × 100%; ecalculated from (1− productivity/TOF) × 100%; fmmolintermediate and product mmolSn−1 h−1, the intermediates include methyl fructoside, dihydroxyacetone, lactic acid, glyceraldehyde, erythrose, glycolaldehyde, 1,1-dimethoxy-acetone, 1,1,2,2-tetramethoxy-propane, and 1,1,2-trimethoxy-2-propanol. a c

To understand the role of hierarchical structure in influencing the pathways of glucose and fructose conversion, productivities

calculated from the initial yields as the number of desired intermediate molecules formed per hour per Sn atom are 3128


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ACS Sustainable Chemistry & Engineering Table 2. Kinetic Values in the Conversion of Dihydroxyacetone over Sn-Beta-H and Sn-Beta-Ca dihydroxyacetone

Catalyst

TOF (h−1)b

productivity (h−1)c



Sn-Beta-H Sn-Beta-C

118.0 102.7

111.2 97.0

94.2 94.4

5.8 5.6

a

Reaction conditions: 1 mmol of substrate, 32 mg of catalyst, 6.4 g of CH3OH, 160 °C, 1 MPa N2. bmmoldihydroxyacetone converted mmolSn−1 h−1; c mmolintermediate and product yield mmolSn−1 h−1, intermediates include glyceraldehyde, lactic acid, 1,1-dimethoxy-acetone, 1,1,2,2-tetramethoxy-propane, and 1,1,2-trimethoxy-2-propanol; dcalculated from (productivity/TOF) × 100%; ecalculated from (1 − productivity/ TOF) × 100%.

estimated, as presented in Table 1. In the conversion of glucose, Sn-Beta-H gives higher productivity (23.3 h−1) to desired intermediates than Sn-Beta-C (11.0 h−1). In this case, the selectivity to undesired oligomers over the Sn-Beta-H is only 15.0%, which is significantly lower than 25.7% over the SnBeta-C. A similar phenomenon was observed in the conversion of fructose (Table 1). These phenomena indicate that the mesoporosity in the Sn-Beta-H could facilitate the conversion of glucose or fructose feedstocks into important intermediates for producing alkyl lactate in the competition with undesirable side reactions. On the basis of these data and the knowledge from literature,18,50,51 the mesoporosity should facilitate the fast formation of Sn-fructose intermediate with bulky diameter, followed by fast transformation via retro-aldol reaction to cleavage the C−C bond, thus outperforming the conventional Sn-zeolite catalysts in yielding alkyl lactate. This is also supported by the data in the conversion of feedstock with small molecule (e.g., dihydroxyacetone), where the C−C bond cleavage is unnecessary, therefore, the Sn-Beta-H and Sn-BetaC exhibits comparable activity and selectivity. Furthermore, the strategy for improving alkyl lactate yields by Sn-Beta-H catalyst is extended to use other sugars (mono-, di-, and polysaccharide of fructose, sucrose, maltose monohydrate, maltotriose, and inulin, Figure 5C and Table S4). In these cases, the Sn-Beta-H gives higher methyl lactate yields than those over Sn-Beta-C catalyst. Particularly, the yield of methyl lactate from the conversion of sucrose could reach as high as 72.1%. More importantly, the Sn-Beta-H is reusable (Figure 6). After each run, the catalyst can be easily separated and regenerated by washing with acetone and calcination. By ICP and XRD analysis (Table S1, Figure S8), the used Sn-BetaH exhibits similar Si/Sn ratio and crystallinity to those of the as-synthesized catalyst. In the conversion of glucose, after recycling for 6 runs, the yield of methyl lactate (53.0%, Figure 6A) is well remained. Notably, if the Sn-Beta-H catalyst was reused without intermediate calcination, significantly decrease in activity and product selectivity was observed in the second and third run (Figure 6B). ICP analysis of the used Sn-Beta-H gives the Sn loading at 3.61 wt %, which is similar to that of the as-synthesized sample (3.70 wt %), confirming the Sn leaching is negligible. The decreased activity and selectivity might be attributed to the deposed oligomers, which covered the active sites.52,53 Further calcination of the used sample could fully regenerate the activity and product selectivity, demonstrating the good stability and recyclability of Sn-Beta-H. The universality of the Sn-Beta-H for biomass conversion combined

Figure 6. Recyclable tests of Sn-Beta-H in the conversion of glucose (conversion of glucose, blue column; yield of methyl lactate, green column; yield of methyl vinylglycolate, yellow column). (A) Catalyst was calcined after each run; (B) catalyst was calcined after the 3rd and 5th runs.

with high product yield and excellent recyclability offers a good opportunity for wide applications in the future.

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CONCLUSION In summary, we report that hierarchical Sn-Beta-H catalyst is active, selective, and recyclable for the production of alkyl lactate from sugars. The excellent performance of Sn-Beta-H is strongly related to its hierarchical structure, which facilitates the conversion of sugar feedstocks into important intermediates for producing alkyl lactate in the competition with undesirable side reactions, thus outperforming the conventional Sn-zeolite catalysts in yielding alkyl lactate. We hope that this work could be helpful for designing highly efficient heterogeneous catalysts for biomass conversion.

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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.6b02881. XRD, TEM, N2 sorption and more catalytic data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*L. Wang. Email: [email protected]. *F.-S. Xiao. E-mail: [email protected]. ORCID

Feng-Shou Xiao: 0000-0001-9744-3067 Author Contributions

J. Zhang performed the catalyst preparation, characterizations, and catalytic tests. G. Wang and F. Chen performed the TEM 3129


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characterization. C. Wang and J. Zhu participated the discussion and offered helpful suggestions. S. Pan and C. Bian performed the N2 sorption test and analyzed the data. L. Wang and F.-S. Xiao designed this study, analyzed the data, and wrote the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (91634201, U1462202, 21403192, and 91645105).

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