Synthesis of Ceria and Sulfated Zirconia Catalysts Supported on

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Synthesis of ceria and sulfated zirconia catalysts supported on mesoporous SBA-15 toward glucose conversion to HMF in a green isopropanol-mediated system Yunlei Zhang, Qingang Xiong, Yao Chen, Meng Liu, Pei Jin, Yongsheng Yan, and Jianming Pan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04671 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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Industrial & Engineering Chemistry Research

Synthesis of ceria and sulfated zirconia catalysts supported on mesoporous SBA-15 toward glucose conversion to HMF in a green isopropanol-mediated system Yunlei Zhang,*,† Qingang Xiong,‡ Yao Chen,§ Meng Liu,† Pei Jin,† Yongsheng Yan*,† and Jianming Pan† †

Institute of Green Chemistry and Chemical Technology, School of Chemistry and

Chemical Engineering, Jiangsu University, Zhenjiang, 212013, P. R. China ‡

Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

§

School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang,

212013, P. R. China

*

Corresponding author. Email: [email protected] (Y. Zhang) [email protected] (Y. Yan)

Abstract In this work, a facile synthesis of ceria and sulfated zirconia (SZ) incorporated SBA-15 catalyst was conducted. The supported SZ reveals promising Brønsted/Lewis acid and relatively low base strength. And the base strength of catalyst is significantly enhanced by introducing highly dispersed ceria into the network of SZ incorporated SBA-15. Systematic study of catalysts activities were carried

out

in

iPrOH/DMSO

solvent

conversion

of

glucose

to

5-hydroxymethylfurfural (HMF). It is found that isomerization process of glucose to fructose was promoted by co-existed base and Lewis acid sites, and the dehydration 1

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reaction of reactively formed fructose to HMF was directed by Brønsted acid sites. Up to 66.5% of HMF yield and 70.8% of selectivity can be obtained in a 90 vol% iPrOH-mediated DMSO solvent. Through the developed reaction system in this paper, use of large amount of environmentally hazardous solvent can be avoided, and a new way to large-scale economically viable processes for biomass conservation is provided. Keywords:

Glucose;

5-Hydroxymethylfurfural;

CeO2-2SZ@SBA-15

catalysts;

Brønsted/Lewis acidity; Isopropanol-mediated DMSO solvent

1. INTRODUCTION With the increased concern on the depletion of fossil fuel reserve, utilization of renewable resources such as biomass to produce sustainable fuels and chemicals1,2 to complement

traditional

5-hydroxymethylfurfural

fuels (HMF),

and a

key

chemicals platform

becomes material

imperative. to

produce

high-performance value-added chemicals and bio-fuels due to its high chemical reactivity, has attracted great interest in last decades.3-6 Since the fructofuranoic structure is more reactive to dehydrate, so far HMF has been majorly produced from fructose.7-9 However, it is worth noting that the fructose supply is rather limited, which may hinder the large-scale synthesis of HMF. Compared with fructose, glucose can find much more existence in both nature and industry, from which the large-scale production of HMF becomes more feasible. Generally, glucose to HMF conversion includes multi-step reactions, involving glucose to fructose isomerization and the formed fructose to HMF dehydration, where 2


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a base/Lewis and acid activity, respectively, are needed.10-13 This is because direct acid-catalyzed glucose gives relatively low HMF yield as the isomerization process of glucose to fructose cannot be smoothly conducted by acid catalysis, which result in poor reaction selectivity.14-17 Therefore, bifunctional catalysis, a synthetic strategy where both the base/Lewis and acid sites are present on the same catalyst, has been adopted in recent years to improve HMF selectivity and inhibit side reactions.10,18 Moreover, bifunctional catalysts employed in one-pot reactions exhibit excellent environmentally friendly benefits, in terms of avoiding the complex isolation and purification processes for intermediate products.19-24 Hence, involved multi-catalytic processes integrated into one-pot bifunctional catalysis is promising to increase efficiency, reduce waste, and elevate profitability for HMF production from glucose. Sulfated zirconium (SZ), a sub-family of solid super-acid, is revealed to possess superior hydrothermal stability.25 It has been largely used in chemical reactions occurred under mild reaction conditions that requiring strong acidic properties.26-30 Moreover, it has an attractive catalytic feature, i.e., possessing type of Brønsted and Lewis acid sites. Thus, SZ catalysts are promising candidates to enhance catalytic activity for multi-step transformation of glucose to HMF.31-33 However, SZ catalysts are majorly synthesized with low surface area and limited control over internal porosity, restricting their application in renewable carbohydrates conversion to HMF.34,35 Thus, it is highly desirable to search for a feasible route that can introduce SZ catalysts with high surface area. Nanostructured silicas with well-defined meso-porosity have already been used as 3



high-area supporter. Therefore, nanostructured silicas may be able to provide SZ catalysts high surface area for optimal catalysis. More specifically, nanostructured silicas supported cerium oxide (CeO2) has been reported superior catalysis due to its excellent physicochemical stability, high oxygen vacancy concentration, and high oxygen mobility.36,37 Moreover, cerium can cycle easily between reduced and oxidized states (Ce3+↔Ce4+), which has been proven to be very appealing for catalysis.38,39 However, to the best of our knowledge, so far there has been no report on the combination of SZ catalyst and nanostructured silicas supported CeO2 to form a high surface area heterogeneous catalyst for efficient one-pot glucose-to-HMF conversion. Thus, part of this paper is to investigate the catalytic effects of SZ and nanostructured silicas supported CeO2 combined heterogeneous catalyst for glucose-to-HMF conversion. The other purpose of this paper is to search for a solvent with superior recyclability, as this is critical from both the sustainable and economical points of view for large-scale industrial glucose-to-HMF conversion.40-43 Conventionally, the aprotic organic solvent, especially dimethyl sulfoxide (DMSO), has been widely used for HMF production from sugars, which can produce very high HMF yield (up to 100%).44 Nevertheless, employing DMSO as a practical solvent for large-scale biomass transformation has largely been unfavored because of its toxicity as well as high boiling point. The sequence of the high boiling point of DMSO poses great challenges to the separation of products as energy-intensive isolation procedures are needed, resulting in high cost and serious loss of HMF. To mitigate the undesirable 4


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impacts of DMSO, Qi et al. utilized the acetone/DMSO mixture as the reaction solvent for fructose dehydration to HMF,45 in which 70% mass fraction of DMSO can be substituted by acetone. However, different from fructose, the capacity of acetone to dissolve glucose is known to be very low, which limits further substitution to DMSO. Isopropanol (iPrOH) is an environmentally friendly alcohol solvent with features of relatively low boiling point (82.2 oC), easy recovery, and good solubility of sugars.46,47 Moreover, iPrOH can be easily produced from biomass-derived glucose by fermentation, which makes it a green medium for the conversion of glucose to HMF. Therefore, employing iPrOH as cost-efficient and green co-solvent to substitute a reasonable amount of DMSO seems to be a promising way to reduce the undesirable impacts of DMSO. Thus, this paper is also devoted to study in detail the performance of iPrOH as a co-solvent for HMF production from glucose. In this study, a catalytic system with thermally robust CeO2-SZ@SBA-15 catalysts possessing high surface area and adjustable acid-base strength was synthesized and used in the iPrOH-DMSO co-solvents for efficient conversion of glucose to HMF. In the following, the detailed synthesis procedures for CeO2-SZ@SBA-15 catalysts are described first. Then, the characteristics of the synthesized catalysts and their performance are presented and discussed. Finally, the performance and recyclability of the iPrOH-DMSO co-solvents for the conversion of glucose to HMF are demonstrated.

2. EXPERIMENTAL SECTION 2.1. Synthesis of SZ@SBA-15 catalysts. 5



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All reagents were obtained from commercial suppliers. The procedure for mesoporous SBA-15 synthesis follows Zhao’s process.48 The sulfated zirconium grafted SBA-15 catalysts, i.e., SZ@SBA-15, was operated based on ref. 31 with some modifications. Briefly, 1.0 g SBA-15 was dried at 300 °C for 4.0 h, then added to 5.85g of 70% zirconium propoxide in propanol in 60 mL of anhydrous hexane. The mixture was refluxed at 69 °C overnight, filtered, and washed three times with hexane to remove any unreacted precursor. The obtained products were rehydrated in 30 mL of double distilled water (DDW) under stirring for 4.0 h, filtered, dried at 80 °C overnight, and the obtained product was named 1 ML-Zr@SBA-15. Same procedure was repeated two and three times exactly to synthesize 2 ML-Zr@SBA-15 and 3 ML-Zr@SBA-15 samples. To introduce Brønsted acid sites, 0.2 g of previous step obtained sample was further reacted with 2.0 mL of 0.075 M sulphuric acid for 5.0 h, then filtered and dried at 80 °C overnight. Afterwards, the solids were calcined in muffle under 550 °C for 3.0 h. And the catalysts were named 1, 2, 3SZ@SBA-15. 2.2. Synthesis of CeO2-2SZ@SBA-15 catalysts by wet impregnation method. The cerium doped 2SZ@SBA-15 catalyst was prepared by impregnating 2SZ@SBA-15

with

an

aqueous

solution containing

requisite

amount

of

Ce(NO3)3·6H2O. Typically, 0.2 g 2SZ@SBA-15 was initially dispersed in 10 mL DDW containing 0.03 g Ce(NO3)3·6H2O under magnetic stirring at room temperature. After stirred for 2.0 h, the sample was dried at 110 °C in an oven overnight, and then calcined at 450 °C for 2.0 h. The final catalyst was renamed 5 wt% CeO2-2SZ@SBA-15, where 5 wt% represents the quality fraction of Ce species to 6


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2SZ@SBA-15. As well, 3 wt% CeO2-2SZ@SBA-15 and 8 wt% CeO2-2SZ@SBA-15 were synthesized by the same procedure. For comparison, catalysts of 5 wt% CeO2@SBA-15 and 5 wt% CeO2-2 ML-Zr@SBA-15 (denoted as 5 wt% CeO2-2MZ@SBA-15) was also obtained under the same experimental condition as the 5 wt% CeO2-2SZ@SBA-15 but replacing sulfated zirconium grafted SBA-15 with pure SBA-15 and 2 ML-Zr@SBA-15, respectively. Moreover, SBA-15-free 5%wt CeO2-SZ catalyst was also synthesized. During the preparation process, bulk SZ material was first obtained according to ref. 25. Then calculated amount of Ce species was introduced to synthesize 5%wt CeO2-SZ catalyst, following the above mentioned procedure. 2.3. Catalyst characterization. Scanning electron microscopy (SEM) images were carried out with an electron microscope equipped with a field emission electron gun and energy dispersive spectrometer (EDS). To collect transmission electron microscope (TEM) images, the sample dispersed in ethanol was disposed onto copper grids for the TEM test. X-ray diffraction patterns (XRD) was performed on samples using a Bruker D8 powder diffractometer (Cu Kα). A scanning rate of 8°/min was employed. N2 physisorption experiment was performed to measure BET specific surface area and porous structural parameter. X-ray photoelectron spectroscopy (XPS) experiment was carried out on a Thermo ESCALAB 250 to test the surface chemical composition of samples. Inductively, Coupled Plasma Optical Emission Spectrometry (ICP-OES, VISTA-MPX) and vario EL III elemental analyzer were used to analyze the synthesized samples 7



chemical composition. The types of Brønsted/Lewis acid sites and acid-base strength were identified by in-suit pyridine adsorption FT-IR and temperature-programmed desorption (TPD) experiments, respectively. The details of test procedure was presented in our previous study.4 2.4. Catalytic reaction for one-pot glucose-to-HMF. To evaluate the catalytic performance of synthesized catalysts, one-pot glucose-to-HMF conversion was performed in iPrOH-mediated DMSO solvent, as depicted in Fig. 1. Typically, glucose (50 mg), 5 wt% CeO2-2SZ@SBA-15 (20 mg), iPrOH (4.5 mL), and DMSO (0.5 mL) were added into a 25 mL graduated pyrex glass tube. The mixture was purged with argon, sealed, and then placed in a preheated oil bath at 120 oC with vigorous stirring for 6.0 h. When the reaction was finished, the system was cooled to room temperature, and the catalyst was separated from the system by filtration and centrifugation, then washed with a mixture of deionized water and ethanol and dried in a vacuum oven for next cycle. Three times were repeated for the catalytic reaction to achieve HMF yield, selectivity and substrate conversion. Prior to analysis, the mixture was first diluted with 95% ethanol, then filtered with a 0.22 lm syringe filter. HMF is analyzed by 1200 Agilent HPLC equipped with Agilent TC-C18 (2) Column (4.6×250 mm, 5.0 µm) and Uv detector at 283 nm. The temperature of the column remained at 25 oC, where the mobile phase is methanol-water (7:3, vol/vol) at a flow rate of 0.7 mL min-1. Fructose and glucose are analyzed by 1200 Agilent HPLC fitted with a refractive index detector (RID) and Biorad column HPX 87H (7.8 mm × 300 mm). During this 8


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process, the column temperature remained constant at 65 ºC, while the applied mobile phase is 0.001 M H2SO4 at the flow rate of 0.55 mL min-1. Standard curves of fructose, glucose and HMF are exhibited in Supplementary Fig. S1, and their HPLC chromatograms are listed in Supplementary Fig. S2. The HMF yield, selectivity, and substrate conversion were calculated as HMF yield (mol%) =

n1 × 100 , n0

(1)

moles of reacted substrate × 100 moles of initial substrate , moles of initial substrate-moles of final substrate = × 100 moles of initial substrate Substrate conversion (%) =

HMF selectivity (%) =

moles of HMF obtained × 100 , moles of reacted substrate

(2)

(3)

n1 and n0 stands for the mole number of carbon atoms of HMF product and loaded substrate, respectively. The identification and structural characterization of products were performed on the HPLC-ESI-MS/MS system consisted of Agilent 1200 system (Thermo Fisher Scientific, America) with an electrospray ionization (ESI) source. The MS/MS parameters are as follows: positive mode; ESI source voltage, 5.0 kV; capillary voltage, 36 V; sheath gas flow rate, 40 arb; aux gas flow rate, 5 arb; sweep gas flow rate, 0 arb; capillary temperature, 300 oC; and scan range, 50-500 m/z.

3. RESULTS AND DISCUSSION 3.1. Characterization of the physicochemical properties of synthesized catalysts. TEM pictures and low-angle powder XRD spectra of SBA-15 support, SZ-grafted SBA-15 and CeO2-2SZ@SBA-15 are illustrated in Fig. 2. The pristine SBA-15 displays 2-dimensional porous structures with well-ordered hexagonal arrays of 9



mesopores. Moreover, highly distributed mesoporous cavities can be seen on the external surface of SBA-15 support (insert in Fig. 2a), indicating its high permeability. Compared with pristine SBA-15, well-ordered porous structures are preserved during the preparation of sulfated zirconium grafted SBA-15 catalysts, as shown in Fig. 2b-d. In addition, with the increased incorporation of Zr content (Table 1), the thickness of mesopore walls gradually increased. To our delight, no obvious crystalline zirconia deposits during the sulfated zirconium grafting cycles, suggesting the layer-by-layer growth mode. TEM images of 2SZ@SBA-15 grafted with different content of cerium species are displayed in Fig. 2e-g. Same with sulfated zirconium grafting cycles, no pore blocks can be observed and the ordered porous structures remained intact upon CeO2 grafting process. Corresponding low-angle XRD spectra are exhibited in Fig. 2h. A main peak at 2θ of approximate 0.8° and two weaker ones at 2θ of around 1.7° and 1.9°, corresponding to the (100), (110) and (200) reflections can be seen from the low-angle XRD spectra.48,49 Combined with previous reported results,50 TEM and XRD characters suggest the obtained catalysts retain ordered porous structures with typical hexagonal P6m symmetry. SEM pictures of SBA-15 support, SZ-grafted SBA-15 and CeO2-2SZ@SBA-15 are demonstrated in Fig. 3a-g. Both pristine SBA-15 and its supported catalysts show short-rod form with length at 360 × 1000 nm, suggesting sulfated zirconium and CeO2 were incorporated into the network of SBA-15 framework. EDS results and SEM mapping pictures in Supplementary Fig. S3 show a homogeneous distribution of 10


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existed elements. As summarized in Table 1, the Zr atom percent for the CeO2-2SZ@SBA-15 catalysts decreases with increasing the Ce content of the seeding solution used, suggesting an increased thickness of introduced ceria layer. However, no characteristic diffraction peaks of zirconia and ceria can be observed from wide-angle XRD test in Fig. 3h, indicating the highly dispersed nature of supported sulfated zirconium and CeO2 nanoparticles, accord well with TEM results. N2 adsorption-desorption isotherms in Fig. 4a reveal that both pristine SBA-15 and its supported catalysts exhibit type IV isotherms with H1-type hysteresis loops, indicating their mesoporous structure. As the grafted sulfated zirconium is increased, there is a decrease in N2 adsorption capacity. The diameter of mesopore also experience a concomitant reduction for sulfated zirconium grafted SBA-15 (Fig. 4b). When compared with SZ-grafted SBA-15, the porous structural parameter for catalysts coated with different cerium contents, showed similar decreasing trend. Notably, the detailed porous structural parameter of pristine SBA-15 and its supported catalysts are summarized in Table 2. Results show that after incorporated with SZ and CeO2 nanoparticles, the surface area, pore volume, and pore size of obtained catalysts decreased dramatically, compared with SBA-15. Characterizations of the obtained 5 wt% CeO2@SBA-15 and 5 wt% CeO2-2MZ@SBA-15 are provided in Supplementary Fig. S4 and S5, which further confirmed the successful incorporation of Ce species into mesoporous SBA-15 and 2 ML-Zr@SBA-15 supports. As indicated in Fig. 5a, the XPS revealed the surface composition of the obtained catalysts including their original SBA-15 support. New peak of Zr 3d appears in 2 11



ML-Zr@SBA-15 sample, when compared with SBA-15, indicating the successful incorporation of Zr into the scaffold of SBA-15. Another new peak of S 2p at 169.3 eV occurring with 2SZ@SBA-15, which revealed the successful introduction of sulfonate groups that coordinated within the grafted zirconia layers,51 during the sulfate-induced sintering process. After the CeO2 grafting, a new peak from Ce 3d appears, confirming the successful introduction of cerium species into the scaffold of 2SZ@SBA-15. O 1s, Si 2p, Zr 3d, S 2p and Ce 3d high-resolution XPS spectra for SBA-15 and relevant catalysts are displayed in Fig. 5b-f. As shown in Fig. 5b, binding energy of Si 2p spectra, shifted to a lower domain after the consecutive Zr deposition, possibly ascribed to the exponential decay in SBA-15 support intensity during the grafting process.52 Moreover, O 1s spectrum of 2SZ@SBA-15 not only experiences a shift to a lower binding energy, but also reveals new peak at 530.4 eV, due to the grafted sulfated zirconium (Fig. 5c). The O 1s binding energy of 5 wt% CeO2-2SZ@SBA-15 shifts to a higher domain due to the introduction of lattice oxygen (O2- in the CeO2).53 The Ce 3d XPS spectra of 5 wt% CeO2-2SZ@SBA-15 can be assigned to the 3d 3/2 spin-orbit states (labeled u) and 3d 5/2 states (labeled v), as presented in Fig. 5f. The bands v, v2, v3, u, u2 and u3 are attributed to Ce4+, whereas v1 and u1 are from Ce3+.53,54 The co-existence of Ce3+/Ce4+ oxidation states on the surface of the 5 wt% CeO2-2SZ@SBA-15 catalyst could be detected, indicating that the catalyst surfaces are not fully oxidized. These characteristics are very important in terms of the catalytic performance in glucose conversion, which would be studied in details as 12


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follows. In addition, the results in Fig. 5b, 5d and 5e show that there are no obvious binding energy shifts of Si 2p, Zr 3d and S 2p for 5 wt% CeO2-2SZ@SBA-15 and the 2SZ@SBA-15, suggesting that the chemical states of these species in the 5 wt% CeO2-2SZ@SBA-15 are the same as those in the 2SZ@SBA-15. NH3-TPD and CO2-TPD methods were employed to calculate the acid and base sites strength of the obtained catalysts. As shown in Supplementary Fig. S6, the synthesized catalysts were proven to be thermally stable before 600 oC. Thus, carbon dioxide and ammonia temperature-programmed desorption tests were performed at a linear heating rate of 10 oC min-1 up to 600 oC. TPD curves in Fig. 6 were integrated to calculate acid and base strength of SBA-15 supported catalysts, which are exhibited in Table 3. Results show that grafted SZ conformal monolayers exhibit varied acid/base sites strength, with 2SZ@SBA-15 possess the highest acidity at 0.42 mmol g-1 and basicity loading at 0.03 mmol g-1, respectively. Bulk and surface content of Zr increases in linear with initial ZrO2 thickness, while both the S content and S:Zr ratio fall from 2 to 3 SZ monolayer, indicating that the terminating zirconia layer undergoes sulfation other than other relevant layers.52 Thus, the slight decrease of acid densities from second to third monolayer of zirconia on the support SBA-15 was caused by the formation of lumps of zirconia crystals and pore-blocking, leading to a reduction in the accessibility of sulfate ions coordinated with zirconia monolayers. Also, the variation of base densities was consistent with that for acid densities. These results synergistically suggest that a ZrO2 bilayer effectively optimized acid-base strength. After the introduction of cerium, catalysts acidity experiences a significant 13



decrease from 0.42 mmol g-1 for 2SZ@SBA-15 to 0.15 mmol g-1 for 8 wt% CeO2-2SZ@SBA-15, possibly due to the coverage of acid sites during the CeO2 grafting. However, the introduced cerium significantly enhances the basicity from 0.03 mmol g-1 for 2SZ@SBA-15 to 0.14 mmol g-1 for 8 wt% CeO2-2SZ@SBA-15. As we know, in the one-pot glucose to HMF conversion, Lewis and Brønsted acid sites are necessary for the sequential glucose isomerization and fructose dehydration processes. Therefore, infrared spectroscopic studies of pyridine adsorption on catalysts surfaces in Fig. 7 make possible to distinguish between Brønsted (1542 cm-1) and Lewis acid sites (1447 cm-1). Pyridine-desorption results show a switch from Lewis to Brønsted acidity with increased ZrO2 conformal monolayers. It is noted that the Brønsted/Lewis acid ratio increases along with each ZrO2 grafting cycle, suggesting that the interfacial zirconia appears more chemically inert than the initial ZrO2 layers to form Lewis acidity.55 Thus, from the results of TPD and pyridine-IR studies, it could be concluded that the growth of 2SZ conformal monolayers on SBA-15 template endowed catalyst with the highest acid-base densities and optimized balance of Brønsted/Lewis acidity. Moreover, the introduced CeO2 nanoparticles reveal no significant change in the balance of Brønsted/Lewis acidity, as shown in Table 3. 3.2. Catalytic activities of catalysts. Performance of obtained catalysts was evaluated in the iPrOH-mediated DMSO solvent one-pot conversion of glucose to HMF. To optimize the activity of developed catalytic system, effects of reaction temperature and time, the ratio of iPrOH for the 14


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substitution of DMSO, and the amount of catalyst for representative 1SZ@SBA-15 were first evaluated. Typically, 20 mg 1SZ@SBA-15 was added to pure DMSO (5.0 mL) containing glucose (50 mg), and the reaction was operated under 90-140 oC for 1-10 h. As shown in Fig. 8a, within the reaction time of 6.0 h, elevated temperature from 90 to 120 oC can greatly enhance the HMF yield. Nevertheless, prolongation of the reaction temperatures to 130 and 140 oC are not favorable for further enhancing the HMF yield. Highest HMF yield at 41.2% was achieved under 120 oC and 6.0 h. Nevertheless, glucose conversion always improves with elevated reaction temperature or time (Supplementary Fig. S7(a)). Furthermore, as depicted in Supplementary Fig. S7(b), fructose yield decreases over optimized reaction time or temperature. These results indicate that increases in reaction time or temperature may result in the etherification reaction of HMF and formation of some undesired by-products such as soluble polymers and insoluble humins.56,57 Therefore, the following catalytic reactions were carried out under 120 oC for 6.0 h. Then, iPrOH was added as a partial substitution for DMSO. As demonstrated in Fig. 8b, the added iPrOH does not impede the glucose conversion and HMF yield obviously, even if the iPrOH content is increased to be 90 vol%. Moreover, HMF yield remains acceptable up to the substitution level at 95%. However, the HMF yield significantly drops to approximately 18.7% in a pure iPrOH solvent system, which was mainly attributed to the formation of some byproducts when excessive iPrOH was used.46 It is noticeable that addition of 5 vol% DMSO significantly increases the HMF yield from 18.7% to 29.8%, suggesting that this polar aprotic solvent has 15



positively affected the glucose-to-HMF reaction, which might be attributed to its excellent dissolving ability and advantageous properties for HMF formation in the dehydration process of sugars.8,55 Results indicate that iPrOH can be used as an effective co-solvent with DMSO for glucose dehydration to HMF, which suited well to the development of a green and economical solvent system. The influence of other parameters on catalytic effectiveness was then discussed and the reaction was set at an iPrOH/DMSO mixture with the volume ratio of 9:1. The amount of representative 1SZ@SBA-15 ranging from 5-50 mg was investigated to discuss their effect on corresponding catalytic performances (Fig. 8c), while other experimental parameters remain constant as listed above. It could be concluded that the percentage of glucose conversion and HMF yield increase linearly from 61.2 to 78.9% and 19.9 to 38.5%, respectively, as the amount of the catalyst increases from 5 to 20 mg. Although the further increase in catalyst amount from 20 to 50 mg also results in a higher glucose conversion of 92.5%, decrease of HMF yield is occurred, indicating that side-reaction of HMF to some other byproducts could also be enhanced with overloaded catalyst. Consequently, it was suggested that 20 mg of representative 1SZ@SBA-15 was enough for catalyzing 50 mg of glucose substrate. Then, subsequent experiments were carried out with 120 oC, 6.0 h, 20 mg of catalyst to discuss the other parameters. Theoretically, transformation of glucose to HMF involves the processes of glucose-to-fructose isomerization and fructose-to-HMF dehydration, and each procedure needs catalysts with different active sites. To understand the role of 16


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catalysts comprehensively during each step, the synthesized catalysts with varied acid and base strengths were employed for fructose dehydration to HMF, under the optimized conditions, i.e., 120 oC, 6.0 h, 20 mg catalyst, 50 mg substrate in a mixture of 4.5 mL iPrOH and 0.5 mL DMSO. It is evident in Fig. 8d that in the iPrOH/DMSO solvent blank control experiment can obtain the HMF yield as high as 37.5%, indicating the fructose to HMF dehydration can be easily performed. After the addition of 5 wt% CeO2@SBA-15 catalyst, no improvement of HMF yield is observed, suggesting the introduced Ce species reveal no advantageous property in improving the fructose-to-HMF dehydration. Also, no obvious changes for fructose conversion and HMF yield can be observed in the presence of Ce species doped unsulfated zirconia@SBA catalyst, i.e., 5 wt% CeO2-2MZ@SBA-15 with no Brønsted active sites. By the grafted SZ monolayer, however, it is observed that the catalytic activity could be greatly enhanced, and 2SZ@SBA-15 with the highest acidity yields HMF yield at 91.5% and selectivity at 92.5%. In addition, it should be noted that there was no obvious relation between the increased basicity and catalytic performances. Moreover, the HMF yield decreases to 77.6% for catalyst 8 wt% CeO2-2SZ@SBA-15 with the lowest acid sites strength, suggested that acidic active sites are the key factor for fructose dehydration to HMF and the base sites reveal no means in promoting HMF yield and selectivity. Catalysts performances for glucose conversion to HMF are shown in Fig. 8e. It is clearly seen that the blank reference exhibits HMF yield at 9.6%, much lower than blank control experiment of fructose to HMF (ca. 37.5%). Results in Fig. 8d 17



suggested fructose to HMF dehydration can be easily performed in iPrOH/DMSO solvent. And we assume that the lower HMF yield is aroused by glucose isomerization to fructose could hardly be processed in the blank control experiment. Although glucose conversion is significantly enhanced in the presence of 5 wt% CeO2@SBA-15 and 5 wt% CeO2-2MZ@SBA-15 catalysts, there were no obvious difference in their HMF yields. After the addition of 1SZ@SBA-15 with both base/Lewis and acid sites, HMF yield (38.5%) and selectivity (48.8%) could be improved. The Brønsted/Lewis acid and base active sites have been previously reported to work synergistically for one-pot glucose to HMF conversion, in which isomerization process of glucose to fructose was promoted by co-existed base and Lewis acid sites, and the dehydration reaction of reactively formed fructose to HMF was directed by Brønsted acid sites.22,31 Thus, among the SBA-15 supported SZ catalyst, 2SZ@SBA-15 with both optimum base and Lewis sites reveals the superior glucose conversion (89.3%), HMF yield (57.3%) and HMF selectivity (64.2%), compared with 1SZ@SBA-15 and 3SZ@SBA-15. In the fructose-to-HMF conversion, 2SZ@SBA-15 and 3 wt% CeO2-2SZ@SBA-15 catalysts reveal similar HMF yield and selectivity, as shown in Fig. 8d. However, the HMF yield and selectivity from glucose

could be further improved

to 61.2% and 66.7% via 3 wt%

CeO2-2SZ@SBA-15. Moreover, catalysts characterizations have confirmed that the introduction of cerium species into the network of 2SZ@SBA-15 reveal no significant change in the ratio of Brønsted/Lewis acidity. Therefore, improved catalytic performance of 3 wt% CeO2-2SZ@SBA-15 in the glucose-to-HMF conversion was 18


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ascribed to the improvement of their base sites strength. Although the highest base sites strength is observed with 8 wt% CeO2-2SZ@SBA-15, its insufficient acid active sites also impeded its catalytic performance in glucose dehydration process for dehydration of formed fructose to HMF. As such, our results indicated that the highest HMF

yield

(66.5%)

and

selectivity

(70.8%)

are

achieved

for

5

wt%

CeO2-2SZ@SBA-15, suggesting its optimum acid-base strength for one-pot glucose to HMF conversion, which was superior in glucose conversion and HMF production, when compared with other reported heterogeneous catalysts (Supplementary Table S1). Furthermore, the ESI(+)-MS spectra in Supplementary Fig. S8 reveal the characteristic fragment ions at m/z 127, supporting the structure of HMF product in the fructose and glucose conversion systems. The performances of SBA-15-free 5 %wt CeO2-SZ catalyst for fructose and glucose conversion were also included in Figure 8d and 8e. Although the catalytic behavior of 5%wt CeO2-SZ@SBA-15 and bulk 5%wt CeO2-SZ catalyst was similar, 5 %wt CeO2-SZ@SBA-15 revealed a rise in 5-HMF production from both fructose and glucose. This difference may reflect the greater dispersion of high-area SZ thin film and cerium species into the mesopore network of porous SBA-15 delivered a far superior performance of Brønsted acidity for fructose dehydration, and the concomitant increase in Lewis/base character to drive glucose isomerization compared to bulk 5 %wt CeO2-SZ. The results indicated SBA-15 scaffold played an important role in promoting 5-HMF formation. Several solvents were also tested as the co-solvent with DMSO for HMF production from glucose conversion under the optimized reaction conditions. It can be 19



Page 20 of 41

clearly seen in Fig. 8f, although water holds great advantages owing to its ecological superiority and technological convenience, it was not considered as an effective co-solvent in the dehydration reaction, as the water/DMSO mixture gives HMF a low yield of 6.7%. The existence of excessive water in fact not only disturbs the equilibrium of glucose-to-HMF dehydration process, but also accelerates the rehydration of HMF to byproducts.59 Similarly, although the glucose conversion could be significantly improved in methanol (MeOH) and ethanol (EtOH) mediated systems, their lower HMF yields are mainly attributed to the fact that the reaction conditions are also suitable for the etherification, which also resulted in the formation of large amount HMF-alcohol ether by-products according to previous studies.46,60,61 It is noticeable that the HMF yield is higher than that obtained with the use of MeOH and EtOH, upon using acetone, 1-propanol (1-PrOH), iPrOH, 1-butanol (1-BuOH), isobutanol (iBuOH), and tert-butanol (tBuOH) as cosolvents, suggesting the etherification reaction can be effectively hampered in these reaction systems. In addition,

tetrahydrofuran

(THF)/DMSO,

pure

DMSO

and

1-butyl-3-methyl-imidazolium chloride ([BMIM]-Cl)/DMSO reaction systems could further improve the glucose conversion and HMF yield. And the stability of HMF in these mediated systems without glucose was examined. As a control, HMF (1.0 mmol) and 5 wt% CeO2-2SZ@SBA-15 (20 mg) were added to the tested solvent mixtures of co-solvent (1.8 mL) and DMSO (0.2 mL) (Supplementary Fig. S9) in the absence of glucose for 6 h at 120 oC. It is evident that lowest HMF percent remains in water/DMSO mixture, and the THF/DMSO, pure DMSO and [BMIM]-Cl/DMSO 20


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mixture are superior in stabilizing the HMF. Nevertheless, they can never be implemented as practical solvents for large-scale application in biomass transformation due to their environmental toxicity and high boiling cost. Among the other examined co-solvents, the iPrOH/DMSO solvent system was favorable where 93.3% of HMF could be achieved, largely due to the existence of bulkiness and steric hindrance for iPrOH and their decreased activity in etherification.62 Although the actual performance of our catalytic system needs to be improved further,59 the current work reflected our pre-industrial trial to develop a green and sustainable process employing cost-efficient alcohols of iPrOH as co-solvent to substitute a reasonable amount of DMSO to mitigate the environmental hazards. Meanwhile, to evaluate the recyclability, one-pot glucose to HMF conversion was studied over five cycles by 5 wt% CeO2-2SZ@SBA-15 catalyst under the optimized conditions. Fig. 9a shows the HMF yields for each run, and the results indicate that the catalyst maintained well with catalytic activities during the regeneration test. After the fifth run, we measured the BET specific surface area and porous structural parameter of recovered catalyst. Results in Fig. 9b and 9c suggest that the reused catalyst reveals slight decrease in BET specific surface area, and the ordered pore network was also retained. Low-angle powder XRD spectrum of fifthly recovered 5 wt% CeO2-2SZ@SBA-15 catalyst inset in Fig. 9c shows no loss of low-angle characteristic diffraction peak, further confirmed the retaining of porou structure. Moreover, pyridine-desorption test in Fig. 9d shows Brønsted/Lewis acid ratio remained almost unchanged when compared with fresh and the fifthly used catalyst. 21



Consequently, it can be concluded that as-synthesized 5 wt% CeO2-2SZ@SBA-15 catalyst was excellent in its physicochemical stabilities and catalytic recyclability.

4. CONCLUSIONS Sulfated zirconia and ceria supported on SBA-15 catalysts were successfully obtained via layer-by-layer grafting and wet impregnation methods. The supported SZ reveals promising Brønsted/Lewis acid and relatively low base strength, with the bilayer zirconia monolayers catalyst (i.e., 2SZ@SBA-15) exhibits the maximum surface acidity and optimized balance in Brønsted/Lewis acidity. The base strength of catalyst is significantly enhanced by introducing highly dispersed ceria into the network of SZ incorporated SBA-15. The effectiveness of CeO2-2SZ@SBA-15 catalysts were examined toward the glucose conversion to HMF in iPrOH/DMSO solvent mixture. Catalytic experiments indicate that isomerization process of glucose to fructose was promoted by co-existed base and Lewis acid sites, and the dehydration reaction of reactively formed fructose to HMF was directed by Brønsted acid sites. Up to 66.5% of HMF yield and 70.8% of selectivity were obtained with the iPrOH-for-DMSO substitution level of 90%. Overall, the employed iPrOH can be used as green and effective substitution for DMSO to minimize its environmental hazards. Moreover, the developed catalysis system presents superior recyclability, indicating its potential use in green and sustainable biomass conservation.

22


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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21606100), Natural Science Foundation of Jiangsu Province (No. BK20160498), China Postdoctoral Science Foundation (No. 2016M601723).

ASSOCIATED CONTENT Supporting Information Figure S1. Calibration plots of standard HMF (a), fructose (b) and glucose (c). Figure S2. HPLC chromatograms of fructose (a) and glucose (b) products obtained from fructose and glucose catalytic reactions in the presence of 5 wt% CeO2-2SZ@SBA-15 catalyst under optimized conditions. Figure S3. SEM-mapping images and EDS spectrum of 5 wt% CeO2-2SZ@SBA-15 catalyst. Figure S4. TEM (a) (insert: low-angle X-Ray powder diffraction patterns) and SEM (b) images, Nitrogen adsorption-desorption isotherms (c) (insert: pore size distribution) and EDS spectrum (d) of 5 wt% CeO2@SBA-15 catalyst. Figure S5. TEM (a) (insert: low-angle X-Ray powder diffraction patterns) and SEM (b) images, Nitrogen adsorption-desorption isotherms (c) (insert: pore size distribution) and EDX spectrum (d) of 5 wt% CeO2-2MZ@SBA-15 catalyst. Figure S6. TG and DSC curves of the synthesized catalysts. Figure S7. Effects of reaction temperature and time on the glucose conversion (a) and fructose yield (b) with reaction conditions: 20 mg 1SZ@SBA-15 catalyst, 50 mg glucose, 5 mL DMSO. Figure S8. ESI(+)-MS spectra of the fructose (a) and glucose (b) conversion systems recorded after 6 h reaction at 23



Page 24 of 41

120 °C. The reaction system was composed of 50 mg glucose or fructose, 20 mg 5 wt% CeO2-2SZ@SBA-15, and 5 mL iPrOH/DMSO (v:v, 9:1) solvent mixture. Figure S9. Effects of different solvents mixed with DMSO on the HMF stability in the presence of 5 wt% CeO2-2SZ@SBA-15, and the other reaction parameters were constant: [HMF] = 1.0 mmol, T= 120 oC, t= 6.0 h. Table S1. HMF production from glucose using different catalysts from literatures.

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ChemSusChem 2014, 7, 402-406. (60) Wang, H.; Hou, X.; Deng, T.; Wang, Y.; Mu, X.; Zhu, Y. L. Graphene oxide as a facile

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5-ethoxymethylfurfural. Green Chem. 2013, 15, 2379-2383. (61) Wang, H. L.; Deng, T. S.; Wang, Y. X.; Qi, Y. Q.; Hou, X. L.; Zhu, Y. L. Efficient catalytic system for the conversion of fructose into 5-ethoxymethylfurfural. Bioresour. Technol. 2013, 136, 394-400. (62) Zhang, J.; Xiao, Y. Y.; Zhong, Y. H.; Du, N.; Huang, X. R. Alcohol effect and the related mechanism on fructose dehydration into 5-hydroxymethylfurfural in the deep eutectic solvent of [emim]Cl/alcohol. ACS Sustainable Chem. Eng. 2015, 4, 3995-4002.

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FIGURES

Figure 1. Process flow diagram to produce HMF from glucose over ceria and sulfated zirconia incorporated mesoporous SBA-15 catalyst

33



Figure 2. TEM images of pristine SBA-15 (a), 1SZ@SBA-15 (b), 2SZ@SBA-15 (c), 3SZ@SBA-15 (d), 3 wt% CeO2-2SZ@SBA-15 (e), 5 wt% CeO2-2SZ@SBA-15 (f), 8 wt% CeO2-2SZ@SBA-15 (g), and Low-angle X-Ray powder diffraction patterns of pure SBA-15 and the synthesized catalysts (h)

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Figure 3. SEM images of SBA-15 (a), 1SZ@SBA-15 (b), 2SZ@SBA-15 (c), 3SZ@SBA-15 (d), 3 wt% CeO2-2SZ@SBA-15 (e), 5 wt% CeO2-2SZ@SBA-15 (f), 8 wt% CeO2-2SZ@SBA-15 (g), and wide angle X-Ray powder diffraction patterns of pure SBA-15 and the synthesized catalysts (h)

35



Figure 4. Nitrogen adsorption-desorption isotherms (a) and pore size distribution (b) of pristine SBA-15 and its supported catalysts

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Figure 5. (a) XPS wide spectra for pristine SBA-15 and its supported catalysts. (b-e) Si 2p, O 1s

Zr 3d, S 2p XPS spectra for pristine SBA-15, 2SZ@SBA-15 and 5 wt%

CeO2-2SZ@SBA-15 catalysts. (f) High-resolution Ce 3d spectra for 5 wt% CeO2-2SZ@SBA-15 catalyst

Figure 6. NH3 (a) and CO2 (b) TPD curves of catalysts

Figure 7. Pyridine-desorption FT-IR spectra of catalysts 37



Figure 8. (a) Effects of reaction temperature and time on glucose to HMF conversion under conditions as: 20 mg 1SZ@SBA-15 catalyst, 50 mg glucose, 5.0 mL DMSO. (b) Effects of the ratio of iPrOH for the substitution of DMSO. (c) Influence of catalyst loading on the HMF yield from glucose in the iPrOH/DMSO solvent mixture, and the other reaction parameters were constant: [glucose] = 50 mg, T= 120 oC, t= 6.0 h. (d-e) Effects of the catalysts with different acid-base strengths in fructose and glucose to 38


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HMF conversions, respectively, across both iPrOH/DMSO solvent mixture under the optimal conditions. (f) Effects of different solvents mixed with DMSO on the production of HMF from glucose catalyzed by 5 wt% CeO2-2SZ@SBA-15 catalyst

Figure 9. (a) HMF yields for each recycle test of 5 wt% CeO2-2SZ@SBA-15 catalyst. (b) Nitrogen adsorption-desorption isotherms, (c) pore size distribution (insert: low-angle X-Ray powder diffraction patterns) and (d) pyridine-desorption FT-IR spectra of the fresh 5 wt% CeO2-2SZ@SBA-15 catalyst and after the fifthly repeated usage

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Table 1. Composition of pristine SBA-15 and its supported catalysts Sample

Zr content (wt %)

Ce content (wt %)

S content (mmol g-1)

SBA-15 1SZ@SBA-15 2SZ@SBA-15 3SZ@SBA-15 3 wt% CeO2-2SZ@SBA-15 5 wt% CeO2-2SZ@SBA-15 8 wt% CeO2-2SZ@SBA-15

5.2 13.7 18.5 9.8 7.2 4.8

2.9 4.8 7.6

1.8 2.5 2.2 2.1 1.6 0.8

Table 2. Structure properties of pristine SBA-15 and its supported catalysts SBET (cm2 g-1)

DBJH (nm)

Total pore volume (cm3 g-1)

Micropore volume (cm3 g-1)

Mesopore volume (cm3 g-1)

685.2 555.8 406.6 348.5

7.2 6.1 5.3 5.0

0.94 0.75 0.58 0.46

0.07 0.06 0.05 0.03

0.71 0.52 0.42 0.35

284.1

4.8

0.41

0.03

0.30

227.7

4.4

0.32

0.02

0.22

182.4

3.1

0.25

0.01

0.13


Table 3. Acid/base properties of pristine SBA-15 and its supported catalysts Sample

Total basicity (mmol g-1)

Total acidity (mmol g-1)

B:L


0.008 0.03 0.02 0.06 0.12 0.14

0.18 0.42 0.36 0.35 0.28 0.15

0.65 0.84 0.96 0.82 0.80 0.79

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FOR TABLE OF CONTENTS USE ONLY

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Synthesis of Ceria and Sulfated Zirconia Catalysts Supported on

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