Catalytic upcycling paper sludge for the recovery of minerals and production of renewable high-grade biofuels and bio-based chemicals

University of Groningen

Catalytic upcycling paper sludge for the recovery of minerals and production of renewable

high-grade biofuels and bio-based chemicals

He, Songbo; Bijl, Anton; Rohrbach, Leon; Yuan, Qingqing; Sukmayanda Santosa, Dian;

Wang, Zhiwen; Jan Heeres, Hero; Brem, Gerrit

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Chemical Engineering Journal DOI:

10.1016/j.cej.2021.129714

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Publication date: 2021

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He, S., Bijl, A., Rohrbach, L., Yuan, Q., Sukmayanda Santosa, D., Wang, Z., Jan Heeres, H., & Brem, G. (2021). Catalytic upcycling paper sludge for the recovery of minerals and production of renewable high-grade biofuels and bio-based chemicals. Chemical Engineering Journal, 420(Part 1), [129714].

https://doi.org/10.1016/j.cej.2021.129714

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E

fficient Conversion of Glucose to 5‑Hydroxymethylfurfural over a

Sn-Modi

fied SAPO-34 Zeolite Catalyst

Xiangbo Song, Jun Yue, Yuting Zhu, Chengyan Wen, Lungang Chen, Qiying Liu, Longlong Ma,

and Chenguang Wang

*

Cite This:Ind. Eng. Chem. Res. 2021, 60, 5838−5851 Read Online

ACCESS

*

ABSTRACT: Efficient and one-pot conversion of biomass-derived carbohydrate into highly value-added 5-hydroxymethylfurfural (HMF) is a crucial reaction step for the valorization of biomass resources toward bio-based chemicals and fuels. In this work, a series of Sn/SAPO-34 catalysts were prepared through impregnation and evaluated in glucose conversion to HMF. The physicochemical properties of Sn/SAPO-34 catalysts were systematically characterized by SEM, XRD, N2physisorption, XPS, solid-state119Sn,29Si,

and 31_{P NMR, XRF, UV−vis, NH}

3-TPD, and pyridine-FTIR techniques. It was demonstrated that controlling the Sn loading

amount could facilely adjust the acid strength and acid amount of the SAPO-34 zeolite. The incorporation of Sn species could induce the formation of a tetrahedrally coordinated Sn4+_{site and Sn-OH site to improve the amounts of Brønsted and Lewis acid}

sites of the catalyst. However, modification with sufficiently high Sn loading could decrease the acid strength and performance of the catalyst owing to its structure damage. The 5%Sn/SAPO-34 catalyst (i.e., Sn loading calculated based on the mass ratio of SnCl4·

5H2O as a precursor to the parent SAPO-34) was found to exhibit the superior catalytic performance under mild conditions and

could afford an HMF yield of 64.4% at 98.5% glucose conversion in a biphasic 35 wt % NaCl-H2O/tetrahydrofuran (THF) system at

150°C within 1.5 h. Additionally, a catalytic reaction pathway was proposed, involving the adsorption of glucose molecules by the −Cl group on the catalyst via a hydrogen bond, followed by glucose isomerization to fructose over the Lewis acid Sn4+_{and Al}3+_sites

and,finally, fructose dehydration to HMF catalyzed by the Brønsted acid Sn-OH and Si-OH-Al sites. The activity of the catalyst decreased due to the leaching of the active site Sn after several consecutive cycles. This work provides insights into the improvement in the Sn-containing zeolite catalyst for tandem conversion of glucose to HMF.

1. INTRODUCTION

The increasing fossil energy uses and demands, as well as the aggravation of the associated environmental pollution, urge a substantial effort for the search of alternative renewable resources. Biomass, as the only carbon-containing renewable feedstock, can be directly used to produce advanced liquid fuels and bio-based chemicals. To well address the sustainable development of the chemical industry toward a better society, it is of vital importance to (partially) substitute conventional fossil fuel and petro-based chemicals with bio-based prod-ucts.1,2 In recent years, more and more attention has been given to liquid phase conversion of biomass, aiming at producing highly value-added platform compounds.3,4 5-Hydroxymethylfurfural (HMF), a versatile furan

building-block molecule, is used in the further synthesis of various bulk chemicals and polymer materials.5,6 These include, for example, synthesizing fuel additives like 2,5-dimethylfuran (DMF) via the hydrogenation of HMF,7 the bio-based polymer monomer 2,5-furandicarboxylic acid (FDCA) via HMF oxidation,8 and liquid alkane fuels via aldol-condensa-tion.9 Additionally, HMF can be directly obtained from Received: March 22, 2021

Revised: March 30, 2021 Accepted: April 3, 2021 Published: April 14, 2021

Downloaded via UNIV GRONINGEN on May 4, 2021 at 08:59:43 (UTC).

dehydration of biomass-based carbohydrates such as fructose, glucose, mannitose, galactose, sucrose, starch, and cellu-lose.10,11 Therefore, HMF is identified to be a key bridge between renewable biomass resources and the petrochemical industry.

Glucose, a basic monomer of cellulose, is the most abundant monosaccharide in nature and a cheaper hexose with respect to fructose.12 Synthesis of HMF with glucose as a starting material is thus considered as an effective way for highly efficient utilization of biomass resources. However, the conversion of glucose to HMF is more challenging than fructose since glucose with a stable pyran ring structure needs to be isomerizedfirst into fructose under catalysis of a Lewis acid, followed by dehydration of fructose to generate HMF through Brønsted acid catalysis.13The isomerization process of glucose to fructose is a vital step in synthesizing HMF with high yields. Accordingly, to improve the efficiency of glucose conversion to HMF, the strategy of combining Lewis and Brønsted acid catalysts was extensively developed.14−16 A typical work was reported by Davis and his co-workers,16 where they first revealed that the Sn-Beta zeolite was particularly effective for glucose isomerization to fructose owing to its Lewis acidic Sn4+ site, and 56.9% yield of HMF with a glucose conversion of 72% could be achieved in a 35 wt % NaCl-H2O/THF biphasic system after combining with HCl

as a dehydration catalyst. Meanwhile, Gallo et al.15conducted a similar research study combining Sn-Beta with Amberlyst-70 as a cocatalyst for glucose conversion in the γ-valerolactone (GVL) solvent and obtained 58.8% HMF yield at 92% glucose conversion, further supporting the excellent isomerization performance of the Sn-Beta catalyst. Additionally, Dumesic and co-workers17used homogeneous AlCl3as the Lewis acid

and HCl as the Brønsted acid for glucose conversion and found a 61.9% yield of HMF with a glucose conversion of 91% in the biphasic water/2-sec-butylphenol system. In addition, aqueous phase transformation of glucose to HMF and levulinic acid combining homogeneous (HCl) and heterogeneous (several kinds of zeolites) catalysts was developed by Ordóñez et al.,18 obtaining 41% selectivity of HMF by combining the Beta zeolite and 200 ppm of HCl catalysts at 140°C for 5 h as well as 34% selectivity of levulinic acid by combining the Beta zeolite and 400 ppm of HCl catalysts at 140 °C for 24 h. Recently, Guo et al.19employed the same catalytic system for the synthesis of HMF from glucose in the biphasic water/ MIBK (methylisobutyl ketone) system, and a 66.2% yield of HMF could be achieved in an efficient slug flow capillary microreactor by further adding 20 wt % NaCl in the aqueous phase. Although the synergy catalysis between Lewis and Brønsted acid catalysts can afford a good HMF yield from glucose, the (partial) use of homogeneous catalysts presents some drawbacks like the difficult or tedious catalyst recovery and the corrosive nature of mineral acid catalysts, possibly limiting their further application potential.

To avoid these drawbacks, solid acid catalysts with both Lewis and Brønsted acid sites are highly recommended for tandem one-pot conversion of glucose to HMF, owing to its easy recovery and low corrosion.12,20,21 So far, solid acid catalysts have been extensively developed for HMF synthesis from glucose, including a zeolite,22 metal oxide,23 metal phosphate,21 heteropolyacids,24 and so on. Among various solid catalysts, a zeolite is considered as a very attractive catalyst for glucose conversion to HMF, owing to its available Lewis and Brønsted acid sites, good hydrothermal stability, and

large specific surface area.25Importantly, the acidic properties of the zeolite can be readily tuned by doping transition metal ions (e.g., Zr,26Nb,27Cr,25and Sn28). At present, most of the studies on the catalytic glucose conversion over zeolite catalysts were mainly achieved with FAU and BEA structures.20,29FAU and BEA zeolites have the suitable pore structure for the molecular size of glucose. Glucose molecules could enter into the inner pore of the catalyst for the dehydration reaction, which was more conducive to make full use of the catalytic active sites inside the pore of the zeolite, thus improving its catalytic efficiency. However, the probability of coking increased when the glucose molecules entered the inner pore channels of the zeolite, which easily caused pore blockage, thus decreasing its catalytic activity. The SAPO-34 zeolite with a CHA structure has a smaller pore size than FAU and BEA zeolites. Glucose with the size of 8.6 Å was larger than the pore size (3.8 Å) of the SAPO-34 zeolite.30 In principle, the pore size (3.8 Å) of SAPO-34 would hinder the glucose molecules entering into the inner channel of the SAPO-34 zeolite for the dehydration reaction, which was not conducive to the effective glucose conversion. At the same time, it also could avoid the coking caused by glucose molecules entering the inner channel to block the pore channel and active sites of the catalyst. According to the previous reports, the zeolite has flexible frameworks, and the reaction conditions used may have allowed the SAPO-34 framework to flex, thus exposing the internal acid sites to sugar molecules.31,32Accordingly, sugar molecules were also possibly able to access the interior acid sites of SAPO-34.33Moreover, the SAPO-34 zeolite possesses both Brønsted and Lewis acid sites, and it could thus be a feasible catalyst for carbohydrate dehydration to HMF.34,35Because of the mild acidity of native SAPO-34, the HMF yield from carbohydrate conversion is not satisfactory. For instance, Sun et al.34 synthesized a Fe- and Mg-containing SAPO-34 zeolite catalyst for fructose dehy-dration to HMF, giving a moderate HMF yield of 55% under a harsh temperature condition of 170°C for 2.5 h. To improve the acidity of SAPO-34, Liu et al.35synthesized a sulfonic acid-functionalized SAPO-34 catalyst by grafting the sulfonic group, and the HMF yield from fructose was increased to 72.0% at 160°C for 45 min, compared with the case of 38% HMF yield over SAPO-34. Throughout the above studies, the main focus is on enhancing the catalytic dehydration performance of the modified SAPO-34 catalyst. However, the tunability of Brønsted and Lewis acid sites and the synergistic catalysis effect of both acids over the SAPO-34 zeolite catalyst were not further studied during the process.

In this work, we prepared a novel and efficient Sn-modified SAPO-34 zeolite catalyst through incorporation of less toxic Sn to effectively tune its acidity and Brønsted and Lewis (B/L) acid ratio, aiming at achieving the synergistic catalysis effect for one-pot conversion of glucose to HMF over the Sn/SAPO-34 catalyst (which to the best of our knowledge has not been reported yet). The effect of various affecting parameters including Sn loading, solvent system, reaction temperature, and time was comprehensively tested. To elucidate the effect of Sn incorporation on the performance of the modified SAPO-34 zeolite in glucose transformation, the physicochemical proper-ties of Sn/SAPO-34 catalysts were systematically characterized by SEM, XRD, N2physisorption, XPS, solid-state 119Sn,29Si,

and 31P NMR, XRF, UV−vis, NH3-TPD, and pyridine-FTIR

technologies. Meanwhile, a plausible catalytic reaction pathway over the Sn/SAPO-34 zeolite was proposed based on catalyst

characterization and experimental results. Finally, the reus-ability of the catalyst was tested.

2. EXPERIMENTAL SECTION

2.1. Materials and Reagents. The parent SAPO-34 zeolite (GR) was obtained from Tianjin Nankai University Catalyst Co., Ltd., and directly used as the catalyst support. SnCl₄·5H₂O (99.0%) was purchased from Tianjin Fuchen Chemical Reagent Factory. LiCl (AR, ≥97%), KCl (AR, ≥99.5%), NaCl (AR, ≥99.5%), and NaBr (AR, ≥99%) were purchased from Shanghai Aladdin Industrial Corporation. Hydrochloric acid (HCl, 36 wt %) and nitric acid (HNO₃, 65− 68 wt %) were supplied by Tianjin Damao Chemical Reagent Factory. D-Glucose (GR), tetrahydrofuran (THF, 99%),

dimethylformamide (DMF, 99.5%), dimethyl sulfoxide (DMSO, 99%), tin(IV) acetate (99%), and 1,4-dioxine (99%) were purchased from Shanghai Macklin Biochemical Co., Ltd., and used without further purification. Deionized water was prepared via an ultrapure water system (Jinan Taiping-M Environmental Protection Equipment Co. Ltd) in our lab. The stainless steel autoclave reactor was supplied by Anhui Kemi Machinery Technology Co., Ltd.

2.2. Catalyst Preparation. Sn/SAPO-34 catalysts were prepared via the wet impregnation method. In a typical run, the desired amount of SnCl4·5H2O was dissolved in deionized

water with ultrasonic treatment. After completely dissolving, a certain amount of parent SAPO-34 zeolite was rapidly added into the above aqueous solution and stirred at room temperature for 4 h. Thereafter, it was washed with deionized water and then dried overnight at 120°C in an oven. Finally, the obtained mixture was ground intofine powder in a mortar. The loading of the Sn element was calculated on the basis of the mass ratio of SnCl4·5H2O to parent SAPO-34. For

example, the Sn loading of 5% means that 0.05 g of SnCl₄· 5H2O was added into 1.0 g of parent SAPO-34 zeolite. Sn/

SAPO-34 catalysts modified with different Sn loadings (e.g., 5, 10, and 15%) were designated as 5%Sn/SAPO-34, 10%Sn/ SAPO-34, and 15%Sn/SAPO-34, respectively. The preparation process for 5%HCl/SAPO-34 and 5%HNO3/SAPO-34 zeolite

catalysts was the same as the above method.

2.3. Catalyst Characterization. The surface area, pore volume, and pore size of the fresh and Sn-modified SAPO-34 zeolite catalysts were measured by the standard nitrogen adsorption−desorption isotherms at −196 °C on a Quantach-rome automated physisorption analyzer (QUADRASORB SI). Before nitrogen physisorption, samples were degassed at 300 °C for 6 h under vacuum. The specific surface area of samples was obtained by the Brunauer−Emmett−Teller (BET) method. The structural and surface morphologies of fresh and Sn-modified SAPO-34 catalyst samples were analyzed by a coldfield emission scanning electron microscope (FE-SEM, S-4800). Element contents in the samples were determined by X-ray fluorescence (XRF) analysis employing an AxiosmAX Petro spectrometer. Power X-ray diffraction (XRD) measure-ment was conducted to analyze the crystallinity of samples using an X’Pert-Pro MPD diffractometer (PW 3040/60) that was operated at a current of 40 mA and a voltage of 40 kV with Cu-Kα radiation (0.154 nm). X-ray photoelectron spectrosco-py (XPS) measurement was evaluated by an ESCALAB 250Xi spectrometer (Thermo Fisher), and the binding energy of C1s signal at 284.8 eV was taken as the reference for energy calibration. The NH₃-TPD profiles of the fresh and Sn-modified SAPO-34 zeolites were detected on the

temperature-programmed desorption instrument (AutoChem II 2920, Micromeritics) equipped with a thermal conductivity detector (TCD). First, the sample (0.1 g) was pretreated at 400°C for 1 h with a Heflow rate of 30 mL/min. Then, it was cooled down to 100°C and adsorbed NH3 at 100°C for 1 h until

saturation. Subsequently, He with aflow rate of 30 mL/min was used to purge the sample for 1 h to remove physically adsorbed ammonia. Finally, the sample was further heated to 600°C at a rate of 10 °C/min under a constant He flow of 30 mL/min. The amount of NH3desorbed from the sample was

monitored by a thermal conductivity detector. The Brønsted and Lewis acid sites of the sample were measured by pyridine-absorbed Fourier transform infrared spectroscopy (Py-FTIR). The Py-FTIR spectra data were recorded on the Nicolet 6700 spectrometer with a resolution of 4 cm−1. Initially, the sample was pressed to form a small disk. Then, the small disk was treated at 350 °C for 2 h under the vacuum condition to remove the physically adsorbed water. After that, pyridine vapor was introduced to make it adsorb on the sample at room temperature. Finally, the degassing of the sample was carried out at different temperatures of 30, 150, and 300 °C. The diffuse reflectance ultraviolet−visible (UV−vis) spectra of fresh and Sn-modified SAPO-34 zeolite samples were recorded on a PerkinElmer Lambda 650 UV−vis spectrophotometer in the range of 200−800 nm, with BaSO₄as a reference. The solid-state119Sn,29Si, and31P magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of samples were recorded on a Bruker AV 400 spectrometer at resonance frequencies of 149.5, 79.4, and 161.9 MHz, respectively. The spinning rates were set at 8, 5, and 10 kHz for119Sn,29Si, and31P MAS NMR testing, respectively. The chemical shifts of 119Sn were referred to tetramethyltin. The pH value of the solution was determined by the METTLER TOLEDO pH meter (FiveEasy Plus). The Cl leaching from the catalyst into the reaction solution was determined by an ion chromatography (IP) analyzer (883 Basic IC plus). The Sn leaching from the catalyst into the reaction solution was determined by an inductively coupled plasma optical emission spectrometer (ICP-OES) instrument (PerkinElmer OPTIMA 8000DV).

2.4. HMF Synthesis from Glucose. The experiments for glucose conversion to HMF were mainly investigated in a miscible H2O/THF single-phase system over the Sn/SAPO-34

catalyst. In a typically run, a known amount of glucose and catalyst was charged into a stainless steel autoclave reactor (100 mL) equipped with a mechanical stirrer, which contained a H2O-THF solvent with a volume ratio of 2:18. Then, the

reaction was processed in a nitrogen atmosphere under the desired temperature of 140−170 °C for a certain time. After the completion of reaction, the reactor was quenched quickly in water (ca. 28°C) to ambient temperature. Subsequently, the obtained mixture was diluted with distilled water andfiltered with a sand-core funnel to separate the liquid product and solid catalyst or possible humins. Finally, the collected liquid sample was further filtered through a 0.45 μm filter membrane and subsequently measured with high-performance liquid chroma-tography (HPLC Waters e2695, vide inf ra).

2.5. Analytics and Definitions. The concentrations of glucose, fructose, HMF, levulinic acid (LA), and formic acid (FA) were monitored by a UV detector (Waters 2489) and refractive index detector (Waters 410) with a Shodex SH1011 sugar column in the HPLC instrument. The column oven temperature was set at 50°C with 5 mM sulfuric acid aqueous solution as the mobile phase at aflow rate of 0.5 mL/min. The

glucose conversion, product yield, and carbon balance are calculated based on the following equations:

=i − ×

k

jjjjj y_{zzzzz

glucose conversion (mol%) 1 moles of final glucose

moles of initial glucose 100% ₍₁₎

= ×

product yield (mol%)

moles of products obtained

moles of initial glucose 100% (2)

= ×

carbon balance (%)

sum of moles of carbon in identified products moles of carbon in glucose converted

100% (3)

With regard to the calculation of activation energy, the conversion of glucose into HMF was assumed to be a first-order reaction process. Therefore, the reaction rate (r) can be obtained by the following equation:

= = − · r C t k C d d glucose glucose ₍₄₎

where k and Cglucose are the reaction rate constant and the

concentration of glucose, respectively. The activation energy and pre-exponential factor can be calculated based on the Arrhenius equation as follows:

= − ·i k jjj y_{zzz k A E R T ln( ) ln( ) ( / )_a 1 (5)

where Ea, A, R, and T represent the activation energy,

pre-exponential factor, gas constant (8.314 J/mol), and absolute temperature, respectively.

3. RESULTS AND DISCUSSION

3.1. Catalyst Characterization. 3.1.1. Morphology of Catalysts. The SEM images of fresh and Sn-modified SAPO-34 zeolite catalysts are shown inFigure 1. For the fresh SAPO-34 sample, a hexahedral structure with a regular morphology and smooth surface was observed (Figure 1a). Also, its particle distribution was uniform, consistent with the morphology of SAPO-34 reported in the literature.36 However, when the SAPO-34 zeolite was modified with 5, 10, and 15% SnCl4·

5H₂O solutions (with respect to the mass of parent SAPO-34 zeolite), the morphology and particle size of SAPO-34 catalysts all changed obviously. The surface of the modified SAPO-34 zeolite became rougher accompanied by a decrease in structural regularity (Figure 1b−d). As previously re-ported,37,38 when a zeolite was treated with acidic aqueous

Figure 1.SEM images of fresh and Sn-modified SAPO-34 zeolite samples: (a) fresh SAPO-34, (b) 5%Sn/SAPO-34, (c) 10%Sn/SAPO-34, and (d) 15%Sn/SAPO-34. In panels (a)−(d), two images with different magnifications are shown.

Figure 2.XRD patterns of the SAPO-34 zeolite catalyst before and after modifying with Sn. Panel (b) is the partial enlargement of panel (a). Asterisks (*) in panel (a) represent characteristic diffraction peaks for the SAPO-34 zeolite.

solution under vigorous stirring conditions, the dealuminiza-tion of the zeolite would occur. Subsequently, we measured the pH of 5, 10, and 15% SnCl₄·5H₂O aqueous solutions. As expected, all solutions exhibited a low pH value, representing strong acidity (as shown inTable S1). Such an acidic solution environment could cause the occurrence of dealuminization of the SAPO-34 zeolite.37

The elemental content of fresh and Sn-modified SAPO-34 zeolites was analyzed with an XRF spectrometer. As presented in Figure S1 and Table S2, the content of chlorine and tin elements in Sn-modified catalysts increased gradually with increasing SnCl4·5H2O concentration used in SAPO-34

treatment. The molar ratio of Si to Al in the zeolite slightly increased when SAPO-34 was treated with 5, 10, and 15% SnCl4·5H2O aqueous solutions, indicating that dealuminization

of the SAPO-34 zeolite might have occurred during the treatment process.

3.1.2. Textural Properties of Catalysts. To understand the effect of Sn modification on the structure of the SAPO-34 zeolite, powder XRD patterns of the fresh and Sn-modified SAPO-34 zeolite samples were collected as presented inFigure 2. Similar characteristic diffraction peaks that originated from the SAPO-34 zeolite were observed at 2θ = 9.5°, 12.9°, 16.0°, 20.6°, 26.0°, and 31.0° in 5%Sn/SAPO-34 and 10%Sn/SAPO-34 samples but with different peak intensities, indicating that both of them still maintained the typical CHA topological structure.34,39 However, most of diffraction peaks in the 15% Sn/SAPO-34 sample disappeared, and the main characteristic diffraction peaks were weakened dramatically. This result demonstrated that modification with highly concentrated Sn solutions could greatly damage the structure integrity of SAPO-34 owing to its strong acidity.40 Moreover, the characteristic diffraction peak for Sn-modified samples exhibited slight shifts toward a high angle compared with that of the fresh one (Figure 2b). The reason could be that the introduction of external metal Sn ions caused an expansion of the zeolite framework.41,42

The textural properties of the fresh and Sn-modified zeolite samples were further analyzed by using N₂ adsorption/ desorption isotherms. The isotherms of these samples are presented inFigure S2. All the tested samples exhibited type I isotherms without an obvious hysteresis loop, demonstrating the mainly microporous properties of the samples, in line with the literature report.43The corresponding specific surface area, pore volume, and average pore diameter of these samples are summarized inTable 1. It could be clearly observed that with increasing concentration of SnCl4·5H2O solution, slight

increases in the average pore diameter of samples were detected, while the specific surface area and pore volume were significantly decreased especially for the 15%Sn/SAPO-34 sample. It was possibly ascribed to the fact that the partial

collapse of the zeolite framework caused the structure damage of SAPO-34 when modified with highly concentrated SnCl4·

5H₂O solution. This was well consistent with the SEM and XRD results (Figures 1and2).

3.1.3. State of Tin Species. To investigate the existing state of Sn species in Sn/SAPO-34 catalysts, the XPS spectra of the Sn-modified SAPO-34 zeolite and SnO2 samples were

collected (Figure S3). It can be seen that typical double peaks in the Sn 3d region with binding energies of 496.1 and 487.5 eV were observed for Sn-modified SAPO-34 samples, respectively corresponding to 3d3/2and 3d5/2for the Sn species

with the tetrahedral coordination state.44However, the binding energies of the SnO₂sample were identified at 494.7 and 486.2 eV, ascribed to 3d_3/2 and 3d_5/2 of octahedral coordination Sn,45respectively. These results indicate that Sn species in the Sn/SAPO-34 zeolite catalyst existed in the form of tetrahedral (instead of octahedral) coordination state. Diffuse reflectance UV−vis analysis was employed to further identify the coordination state of Sn species in the Sn-modified SAPO-34 zeolite. As presented in Figure S4, a similar absorption band centered at 210 nm was observed in Sn-modified SAPO-34 samples, which was associated with ligand-to-metal charge transfer from O2− to Sn4+.46 This strong absorption band is generally recognized as the active site Sn4+ _{in the zeolite}

framework in the form of tetrahedral coordination.47,48For the fresh SAPO-34 sample, almost no apparent absorption band was detected. These results reveal that Sn atoms were incorporated into the SAPO-34 framework in the form of tetrahedral coordination,49consistent with the XPS results.

To provide direct evidence for the Sn coordination state, the Sn-modified SAPO-34 samples were measured by the 119_Sn

MAS NMR technique (Figure 3). Results show that a main peak at −604 ppm could be detected for the SnO2 sample,

which corresponded to the Sn species with octahedral coordination.42 For the Sn-modified samples (5%Sn/SAPO-34 and 10%Sn/SAPO-(5%Sn/SAPO-34), a weak signal peak at ca.−420 ppm could be observed, which was assigned to the Sn species in the zeolite framework with tetrahedral coordination state.42 However, the corresponding peak of the 15%Sn/SAPO-34 sample could not be detected at ca. −420 ppm due to the structural damage. Additionally, according to the report by Corma et al.,50because of the low natural abundance of 8.6% of119_{Sn, the detected signal of the resonance peak of}119_{Sn was}

relatively weak. The 119Sn MAS NMR results further demonstrate that Sn species of Sn-modified SAPO-34 were tetrahedrally coordinated in the zeolite framework. The 29_Si

and 31P MAS NMR spectra of the fresh and Sn-modified SAPO-34 zeolite catalysts were further measured to detect the structure variation of SAPO-34 caused by Sn incorporation (Figure S5). The characteristic peaks of silicon for the samples modified with 5, 10, and 15% SnCl4·5H2O solutions were

Table 1. Texture Properties of the Fresh and Sn-Modified SAPO-34 Zeolite Samples entry catalyst S_BETa_(m2_{/g) S}

microb(m2/g) Smesoc(m2/g) Vporesd(cm3/g) Vmicroe(cm3/g) Vmesof(cm3/g) pore sizeg(nm)

1 SAPO-34 568.2 537.2 31.0 0.294 0.264 0.030 2.07

2 5%Sn/SAPO-34 428.3 407.5 20.8 0.215 0.190 0.025 2.01

3 10%Sn/SAPO-34 255.9 255.9 19.9 0.139 0.114 0.025 2.18

4 15%Sn/SAPO-34 118.4 96.9 21.5 0.072 0.047 0.025 2.44

a_{The BET surface area was obtained by the nitrogen adsorption isotherm.}b_{The surface area of the micropore was obtained by the t-plot method.} c_{Surface area of mesopore = S}

BET− Smicro.dThe volume of the pore was obtained by the single-point desorption method.eThe volume of the micropore was obtained by the t-plot method.fVolume of mesopore = Vpores− Vmicro.gThe average pore size was obtained from the desorption average pore diameter (4V/A by BET).

gradually shifted from −71.32 ppm to −78.45, −81.46, and −82.12 ppm, respectively (Figure S5a). Similar peak shift phenomena were also observed in the31P MAS NMR spectra of the Sn-modified catalysts (Figure S5b). This may be mainly attributed to the fact that the removal of aluminum in SAPO-34 followed by the incorporation of Sn atoms affected the chemical environment of silicon and phosphorus atoms connected with the nearest Al atoms (Scheme 1). Moreover, the characteristic peaks of silicon for Sn-modified SAPO-34 became broader than that of the fresh one (Figure S5a), indicating that the amorphous silicon species were formed after modification,51 which was possibly caused by the structural damage of SAPO-34, which is well in line with XRD analysis (Figure 2).

3.1.4. The Acidity of Sn/SAPO-34 Zeolite Catalysts. Owing to the process of glucose conversion to HMF closely related to the acid properties of the catalyst, the acidity of the fresh and Sn-modified SAPO-34 zeolite catalysts was measured by NH₃ -TPD (Figure 4). Generally, the strength of the acid site of the catalyst could be evaluated by NH₃ desorption temperature from the TPD profile. The strong, moderate, and weak acid

sites correspond to the peaks of HN₃ desorbed at a high temperature of 500−700 °C, at a moderate temperature of 300−500 °C, and at a low temperature of 100−300 °C, respectively.12As presented inFigure 6, two main desorption peaks of HN₃ were detected in the temperature regions of 150−250 °C and 350−500 °C in TPD profiles. The weak acid site stemming from P-OH was in the unsaturated coordination with AlO4tetrahedra.52The moderate acid site was caused by

the incorporation of a Si atom into the zeolite framework.53 However, the intensity of the desorption peak for the moderate acid site was weakened and gradually shifted to a lower temperature with increasing concentration of SnCl4·5H2O

solution, indicating that the strength of the acid site of the SAPO-34 zeolite could be weakened after Sn modification, especially for the case with high Sn loading. This was mainly due to the structural damage of the SAPO-34 zeolite caused by treatment with a high concentration of SnCl₄·5H₂O solution, well in line with the results of XRD analysis.

The Py-FTIR spectrum was used to further infer the Brønsted and Lewis acid sites of the fresh and Sn-modified SAPO-34 zeolite catalysts, as depicted inFigure 5(cf.Table S3

for the detailed data). Apparently, four obvious peaks could be

Figure 3.119_{Sn MAS NMR spectra of SnO}

2and Sn-modified SAPO-34 zeolite catalysts.

Scheme 1. Possible Enhancement Pathway for Brønsted and Lewis Acid Sites of the SAPO-34 Zeolite via Sn(IV) Incorporation

Figure 4.NH3-TPD profiles of the fresh and Sn-modified SAPO-34 zeolite samples.

detected in all the tested catalyst samples. The characteristic peak at around 1545 cm−1was assigned to pyridine adsorbed on Brønsted acid sites (B),54 and the peak at 1488 cm−1 belonged to an overlap of Brønsted and Lewis acid sites (B + L).55 Moreover, Lewis acid sites (L) adsorbed by pyridine were detected at the peaks of 1595 and 1442 cm−1.56 In addition, as presented inTable S3, modification with different Sn loadings exhibited a great impact on the acidity properties of the SAPO-34 zeolite. Both the numbers of Brønsted and Lewis acid sites of 5%Sn/SAPO-34 and 10%Sn/SAPO-34 catalysts were obviously improved (Table S3), which was mainly ascribed to the fact that the incorporation of Sn atoms induced the formation of tetrahedrally coordinated Sn4+ sites (Lewis acid site) and Sn-OH (Brønsted acid site),57,58 as illustrated inScheme 1. This can be supported by the literature findings; for instance, Sedighi et al.59

investigated the performance of MeSAPO-34 (Me = Fe, Co, Ni, La, and Ce) catalysts in converting methanol to light olefins and found that the metal ions incorporated into the SAPO-34 zeolite could significantly affect its acidity. However, the number of Brønsted and Lewis acid sites was decreased upon further increase in the Sn loading as seen in the 15%Sn/SAPO-34 catalyst (Table S3), which was mainly caused by the structural damage of SAPO-34 treated with a higher concentration of SnCl4·5H2O solution. This phenomenon is also evidenced by

XRD and NH₃-TPD analyses (Figures 2 and 4). The above results illustrate that the number of Brønsted and Lewis acid sites of the SAPO-34 zeolite could be facilely adjusted by controlling the Sn loading amount during the catalyst preparation process, and the Sn/SAPO-34 zeolite thus could be a potential bifunctional solid acid catalyst for effective conversion of glucose to HMF.

3.2. Catalytic Performance of Catalysts. 3.2.1. Con-version of Glucose over the Sn/SAPO-34 Catalyst. As known, tetrahydrofuran (THF) as a bio-based solvent could be obtained from biomass through the decarbonylation and hydrogenation of furfural (a product of biomass-based xylose dehydration).60 In addition, its merits of low toxicity and boiling point make it a good solvent for conversion of carbohydrate to value-added chemicals.20 However, the yield of HMF obtained in the monophasic THF system was generally lower than that in the miscible H₂O/THF system possibly due to the low solubility of single THF toward

sugars.61Therefore, a water-lean solvent system composed of THF and a certain amount of water was used for glucose conversion in this part. To elucidate the effect of Sn modification on the catalytic performance of the SAPO-34 zeolite, Sn/SAPO-34 catalysts with different Sn loadings were evaluated for the glucose conversion to HMF. As shown in

Figure 6, a low HMF yield of 19.0% with 69.6% of glucose

conversion was obtained over the SAPO-34 catalyst in the miscible H₂O-THF system at 170°C with a reaction time of 2 h. However, the HMF yield and glucose conversion could be remarkably improved to 51.7 and 98.9%, respectively, over the 5%Sn/SAPO-34 catalyst under otherwise the same reaction conditions, indicating the positive effect of Sn incorporation on the performance of the SAPO-34 zeolite for glucose conversion. According to Py-FTIR (Figure 5 and Table S3) and NH3-TPD (Figure 4) analyses, this phenomenon could be

ascribed to the enhancement in Brønsted and Lewis acid sites of the catalyst as well as the suitable acid strength after Sn modification. The catalytic performance of the zeolite catalyst on the conversion of glucose to HMF is strongly correlated with its acidic properties,62 and 5%Sn/SAPO-34 possessed much more Lewis acid for the isomerization of glucose to fructose and a bit more Brønsted acid for the dehydration of fructose to HMF compared with the fresh SAPO-34 (Table S3). Nevertheless, when using the 10%Sn/SAPO-34 zeolite as a catalyst, the yield of HMF decreased to 41.7%. Meanwhile, the corresponding yield of the by-product LA increased to 1.8% (compared with the 51.7% HMF yield and 1.0% LA yield obtained over the 5%Sn/SAPO-34 catalyst). This could be explained by the further increase in the amount of Brønsted and Lewis acid sites of the catalyst (Figure 5 andTable S3), which induced more side reactions such as the rehydration of HMF to LA and the polymerization of HMF itself or with sugars and intermediates to form humins.63 The HMF yield further decreased to 34.8% when the 15%Sn/SAPO-34 catalyst was used. Meanwhile, the glucose conversion and LA yield were lowered to 91.5 and 1.2%, respectively. This was mainly attributed to the structural damage of a catalyst treated with such a high concentration of SnCl₄·5H₂O solution (as evidenced by XRD analysis in Figure 2), leading to the

Figure 5.Pyridine-adsorbed FTIR spectra of fresh and Sn-modified SAPO-34 zeolite samples.

Figure 6.Conversion of glucose to HMF over Sn-modified SAPO-34 zeolite catalysts. Reaction conditions: 5 wt % glucose, substrate-to-catalyst weight ratio of 1:1, 20 mL of solvent (volume ratio of H2O to THF at 1:9), 170°C, 2.0 h, and nitrogen atmosphere. The amount of glucose is relative to the total solvent.

decrease in Brønsted and Lewis acid sites compared with 5% Sn/SAPO-34 (Table S3). Fewer Lewis acid sites might decrease the glucose isomerization efficiency, thus resulting in less fructose produced, and at the same time, the decrease of Brønsted acid sites could cause less HMF formation from fructose.

3.2.2. Effect of Solvent System. The use of an appropriate solvent system is conducive to effectively improve the HMF selectivity from biomass-based carbohydrates due to the solvent effect suppressing side reactions.64 The catalytic performance of 5%Sn/SAPO-34 for conversion of glucose to HMF was further investigated in several solvent systems. As presented in Figure 7, the catalyst exhibited poor selectivity

toward HMF in the pure water system, where a low HMF yield of 9.2% was obtained along with 95.6% glucose conversion at 170°C for 2.0 h. This is because the formed HMF was highly unstable in water medium in the presence of an acid catalyst, which facilely underwent a deep decomposition to form levulinic acid (26.9% in yield) and was consumed by other side reactions like polymerization and condensation involving HMF.65

Nevertheless, replacement of water with organic solvents such as DMSO, DMF, THF, and 1,4-dioxane as a reaction medium could significantly improve the HMF yield. For example, HMF yields of 30.9, 40.8, 39.8, and 24.5% were obtained in 1,4-dioxane, DMSO, DMF, and THF, respectively, and the corresponding LA yields were 3.2, 0.7, 0.0, and 1.7% (Figure 7). Among these solvents, HMF was obtained in a lower yield in DMF, owing to the fact that DMF contains amino groups that possibly neutralized the acidic sites of the catalyst.66 A moderate HMF yield of 40.8% was obtained in DMSO. This is because the fructofuranose intermediates were preferred to form in DMSO, which was more conducive to the formation of HMF.67However, DMSO as a high boiling point solvent is less favored to use when it comes to the separation and purification of HMF, potentially causing a high production cost. The low yield of HMF but high glucose conversion was obtained in the single 1,4-dioxane and THF. This is because their poor solubility toward sugars resulted in the glucose molecules together in large amounts during the reaction

process, which increased the probability of side reactions occurring. This is well in line with thefindings of Tsapatsis and co-workers.68 The biomass-derived THF with a low boiling point is likely to be a good candidate as a solvent, allowing for its renewability and facile separation of HMF. To improve the solubility of THF toward glucose, a certain amount of deionized water was added into THF. As expected, the HMF yield was significantly increased from 24.5% in the single THF system to 51.7% in the miscible H2O-THF system, indicating

that the 5%Sn/SAPO-34 zeolite is a good bifunctional solid catalyst for one-pot conversion of glucose to HMF in the miscible H2O-THF system. Additionally, the effect of water

content on the glucose conversion to HMF was investigated, and results are summarized inTable S4.

3.2.3. Effect of Reaction Temperature and Time. The reaction temperature and time greatly affect the conversion pathway of carbohydrates and yield of the target product.69 Their influence on the glucose conversion and HMF production was thus systematically investigated in the miscible H₂O-THF system (Figure 8). Results show that with reaction temperature increasing, the glucose conversion was signi fi-cantly accelerated for a given reaction time. The yield of fructose isomerized from glucose decreased gradually with increasing reaction temperature and time (Figure 8a) because of its further conversion to HMF. The appearance of fructose corroborated that the presence of the Lewis acid site on the catalyst drove the glucose isomerization toward fructose, followed by dehydration to HMF over the Brønsted acid site. As given inFigure 8b, the yield of HMF could be improved by increasing the reaction temperature and time. Under low temperature conditions (e.g., 140 and 150°C), the HMF yield tended to increase gradually at longer reaction times (tested up to 180 min). Similarly, the yield of LA derived from the rehydration of HMF also increased (Figure 8c). However, further increasing the reaction temperature to 170 °C, the HMF yield showed an increasing trend in thefirst 60 min and then decreased with further prolonging of the reaction time. This isfirst attributed to the fact that when HMF accumulated to a certain concentration level, side reactions such as rehydration and polymerization involving HMF became more accelerated. In addition, higher temperature could accelerate the production of by-products like levulinic acid and unknown polymers or humins. This could be confirmed by the remarkable increase in LA yield (Figure 8c). Although unknown polymers and humins are difficult to identify by the present HPLC techniques, its extent of formation can be proved by the fact that the color of reaction solution varied from brown to black at the increased reaction time (Figure S6). In addition, as high as 58.2% HMF yield could be achieved under the optimized conditions of 150°C within 1.5 h. To better understand the HMF production from glucose over the 5%Sn/SAPO-34 zeolite catalyst in the H₂O-THF system, the reaction kinetics for glucose conversion to HMF werefitted with a first-order reaction rate equation based on previous reports showing that the reaction kinetics for the conversion of glucose to HMF were applicable forfitting with a first-order reaction rate equation.70,71

Subsequently, an Arrhenius plot was generated by using the reaction rate constants at different reaction temperatures, as shown in

Figure 9. The activation energy of 100.9 kJ/mol was obtained in this work, which was comparable with the results obtained in a previous report.71 For instance, Hu et al.71 reported an

Figure 7. Conversion of glucose to HMF over 5%Sn/SAPO-34 in various solvents. Reaction conditions: 5 wt % glucose, substrate-to-5% Sn/SAPO-34 catalyst weight ratio of 1:1, 20 mL of solvent, 170°C, 2.0 h, and nitrogen atmosphere. The volume ratio of H2O to THF is maintained at 1:9 in the H2O-THF system.

activation energy of 97.4 kJ/mol for glucose conversion with the H-Beta zeolite in [Bmim]Cl medium.

3.2.4. Catalytic Reaction Pathway for Glucose Conversion over Sn/SAPO-34. To preliminarily understand the reaction pathway for glucose conversion to HMF catalyzed by Sn/

SAPO-34, some comparative experiments were carried out over several modified SAPO-34 catalysts (e.g., treated by different acid solutions), including 5%Sn/SAPO-34, 5%HCl/ SAPO-34, and 5%HNO3/SAPO-34 (cf. detailed results in

Table S5). It can be seen that the catalytic performance of 5% Sn/SAPO-34 (58.2% HMF yield;Table S5, entry 2) for HMF production was obviously superior to the others. Meanwhile, the catalytic performance of 5%HCl/SAPO-34 (34.6% HMF yield;Table S5, entry 3) was remarkably higher than that of 5% HNO₃/SAPO-34 (1.4% HMF yield; Table S5, entry 4) and SAPO-34 catalysts (2.3% HMF yield;Table S5, entry 1) but lower than that of 5%Sn/SAPO-34. Accordingly, it can be speculated that in addition to the fact that the incorporated Sn played an important role in affecting the catalytic performance of the modified SAPO-34 zeolite, the introduced Cl element might also have a certain influence on its catalytic performance in this process (Table S5, entry 5). As reported in previous studies,72,73researchers investigated the introduction of the Cl element into a solid acid catalyst for the catalytic conversion of carbohydrate to levulinic acid and pointed out that Cl in the catalyst as an electronegative group could interact with carbohydrate molecules through hydrogen bonding, narrowing the distance between the substrate and active sites of the catalyst. It is well in line with the result obtained in the present work. Based on the current results of reaction testing, catalyst

Figure 8.Effect of reaction temperature and time on the (a) glucose conversion and fructose yield, (b) HMF yield, (c) LA yield, and (d) carbon balance over the 5%Sn/SAPO-34 catalyst. Reaction conditions: 5 wt % glucose, substrate-to-catalyst weight ratio of 1:1, 20 mL of solvent (volume ratio of H2O to THF at 1:9), and nitrogen atmosphere.

Figure 9.Arrhenius plot for glucose conversion to HMF with the 5% Sn/SAPO-34 zeolite as the catalyst.

characterization, and the literature study,72−75 a plausible catalytic reaction pathway for the conversion of glucose to HMF over the Sn/SAPO-34 catalyst containing Cl (Figure S1) was proposed. As illustrated in Scheme 2, the Sn/SAPO-34 catalyst first adsorbed glucose molecules via hydrogen bonds between the −Cl group (an electronegative group) on the catalyst and the −OH group of glucose, which might be capable of keeping the glucose molecules with great affinity to the active sites of the catalyst.72 Then, the adsorbed glucose molecule was effectively isomerized to the fructose molecule under catalysis of the Lewis acid Sn4+ _{(stemmed from the}

incorporation of Sn) and Al3+ (originated from the parent SAPO-34) sites in Sn/SAPO-34 (Scheme 1). Because of the introduction of tetrahedrally coordinated Sn4+_{sites with high}

isomerization activity,74 the catalytic efficiency of the Sn/ SAPO-34 catalyst for glucose to fructose was thus greatly enhanced (as evidenced byFigure 6). In the end, the produced fructose was subsequently dehydrated to form HMF catalyzed by the Brønsted acid sites Si-OH-Al (originated from the parent SAPO-34) and Sn-OH (stemmed from the incorpo-ration of Sn) in the Sn/SAPO-34 catalyst.

3.2.5. Enhancement of HMF Yield in the Biphasic System. It is known that the use of inorganic salts (e.g., NaCl) can promote the formation of two phases for the miscible H₂ O-THF system due to the salting-out effect and extract HMF from the aqueous phase to the organic phase, suppressing the side reactions of HMF to some extent, further improving the HMF selectivity.76 Accordingly, to investigate the effect of inorganic salts on the glucose conversion and HMF yield over the 5%Sn/SAPO-34 catalyst, experiments were further

conducted in the H₂O-THF system by adding several inorganic salts such as LiCl, KCl, NaCl, and NaBr. As shown inTable 2, with the addition of inorganic salts into the reaction medium, both the HMF yield and glucose conversion were improved compared with the blank case (no salt addition, entry 1), indicating that the addition of inorganic salts was beneficial to improve the HMF yield owing to the salting-out effect. Among of them, NaCl salt could afford the highest HMF yield, well in line with the previous reports suggesting that NaCl could generate the highest extraction coefficient.

3.2.6. Reusability of Catalysts. Good reusability of catalysts is of great practical significance for effectively lowering the production cost of HMF. Therefore, the reusability of the 5% Sn/SAPO-34 catalyst in the glucose conversion was tested in the miscible H2O-THF system. After each run, the catalyst was

recovered from the reaction mixture viafiltration and washed with deionized water for at least three times, followed by drying in an oven at 120 °C overnight to remove water adsorbed in the catalyst. Then, the recovered catalyst was added to a H2O-THF system containing fresh glucose for the

next cycle reaction. Results of catalyst testing during four successive reaction runs are summarized inFigure 10, where it can be seen that there is a somewhat decrease in the HMF yield and glucose conversion, especially for the second run. XRD analysis showed that similar characteristic diffraction peaks were observed for both fresh and spent 5%Sn/SAPO-34 catalysts (Figure S7), indicating that the structure of the catalyst was (almost) unchanged. The decrease in catalyst activity after several consecutive runs was mainly attributed to the Sn leaching in the catalyst (Tables S6 and S7). In addition, Scheme 2. Proposed Catalytic Reaction Pathway for the Conversion of Glucose into HMF over the Sn/SAPO-34 Catalyst

Table 2. Effect of Alkali Salt on Glucose Conversion over the 5%Sn/SAPO-34 Catalysta mole yield (%)

entry inorganic salt LA HMF fructose glucose conversion (%) carbon balance (%)

1 blank 1.23± 0.36 58.20± 1.67 3.53± 0.56 93.82± 0.78 69.99

2 LiCl 0.74± 0.11 60.99± 0.93 98.14± 0.45 63.59

3 KCl 0.81± 0.23 61.50± 1.35 99.06± 0.31 64.33

4 NaCl 0.69± 0.09 64.43± 1.21 98.53± 0.57 65.04

5 NaBr 0.75± 0.10 62.03± 0.89 98.64± 0.26 64.34

a_{Reaction conditions: 5 wt % glucose, substrate-to-catalyst weight ratio of 1:1, 35 wt % alkali salts, 20 mL of solvent (volume ratio of H}

2O to THF at 1:9), 150°C, 1.5 h, and nitrogen atmosphere. The amount of added alkali salts is relative to the volume of water.

the deposition of undesired polymers or humins on the active sites of the catalyst (Figures S8 and S9) might also lead to the decrease in catalyst activity.

3.3. Comparison with Other Heterogeneous Catalytic Systems. The catalytic performance of the Sn/SAPO-34 zeolite for glucose conversion to HMF was compared with other metal-modified zeolite catalysts in a similar reaction system. The results are summarized in Table 3, where the HMF yield obtained by Sn/SAPO-34 was higher than most of the presented catalysts, and the reaction conditions required for glucose conversion over the Sn/SAPO-34 catalyst are relatively milder than the others in a similar solvent system. Although the Nb-Mont catalyst (entry 7) exhibited superior performance than Sn/SAPO-34 (entry 8), achieving 70.5% HMF yield, Nb as a noble metal can lead to a high catalyst cost. Additionally, Sn/SAPO-34 gave a better result than Sn-Beta (entry 1) in the same NaCl-H₂O/THF biphasic system. However, an additional HCl catalyst was required for the one-pot glucose conversion to HMF over the Sn-Beta zeolite, achieving an HMF yield of 56.9%. The Sn/SAPO-34 zeolite thus exhibited a comparable catalytic performance, which could be an attractive catalyst for glucose conversion to HMF.

4. CONCLUSIONS

An effective Sn-modified SAPO-34 catalyst was prepared and evaluated in the one-pot conversion of glucose to HMF. The 5%Sn/SAPO-34 catalyst appeared to have the superior performance, over which a good HMF yield of 64.4% at

98.5% of glucose conversion could be achieved at 150 °C within 1.5 h in the H2O-THF system with NaCl addition. The

good performance of the catalyst was mainly ascribed to the decrease in acid strength of the catalyst and the synergistic catalysis effect stemmed from the appropriate ratio of Brønsted site to Lewis acid site after Sn modification. A reaction pathway for the glucose conversion to HMF over the Sn/SAPO-34 catalyst was proposed, including, first, the adsorption of glucose by the−Cl group on the catalyst via a hydrogen bond, then glucose isomerization to fructose over the Lewis acid Sn4+ and Al3+_{sites, and eventually, dehydration of fructose to HMF}

catalyzed by the Brønsted acid Si-OH-Al and Sn-OH sites. Moreover, the catalyst could be reused for at least four consecutive cycles without significant loss in catalytic activity. This work provides insights into the improvement in the tin-containing zeolite catalyst for tandem conversion of lignocellulosic carbohydrate to value-added chemicals.

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*

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.iecr.1c01121.

pH value of SnCl₄·5H₂O solutions (Table S1); element content of catalysts (Figure S1 and Table S2); nitrogen adsorption/desorption isotherms, XPS spectra, UV−vis spectra, and29Si and31P MAS NMR spectra of catalysts (Figures S2−S5); acid properties of catalysts (Table S3); effect of water content on glucose conversion (Table S4); color change of reaction solution (Figure S6); glucose conversion over different modified SAPO-34 catalysts (Table S5); XRD patterns of catalysts (Figure S7); element composition of catalysts (Table S6); leaching test of Sn and Cl (Table S7); photograph comparison for catalysts (Figure S8); TG analysis of catalysts (Figure S9) (PDF)

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Corresponding Author

Chenguang Wang − CAS Key Laboratory of Renewable Energy, Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China; orcid.org/0000-0002-1950-006X; Phone: +86-020-3702-9721; Email:wangcg@ ms.giec.ac.cn

Authors

Xiangbo Song − CAS Key Laboratory of Renewable Energy, Guangdong Provincial Key Laboratory of New and

Figure 10. Reusability of the 5%Sn/SAPO-34 catalyst in glucose conversion to HMF. Reaction conditions: 5 wt % glucose, substrate-to-catalyst weight ratio of 1:1, 20 mL of solvent (volume ratio of H2O to THF at 1:9), 150°C, 1.5 h, and nitrogen atmosphere.

Table 3. Results for the Glucose Conversion to HMF over Different Heterogeneous Catalysts

entry catalyst solvent temp. (°C) time (min) glucose conversion (%) HMF yield (%) ref.

1 Sn-Beta/HCl NaCl-H2O/THF 180 70 79.0 56.9 16

2 Sn-Mont THF/DMSO 160 180 98.4 53.5 77

3a Sn-MCM-41 DMSO 150 60 88.0 45.0 41

4 Cu-Cr/ZSM-5 DMSO 140 240 62.7 52.6 78

5 Al-MCM-41 NaCl-H2O/MIBK 195 30 98.0 63.0 79

6 AlNb/SBA-15 H2O/MIBK 170 360 93.4 55.7 80

7 Nb-Mont NaCl-H2O/MIBK 170 180 99.0 70.5 61

8 Sn/SAPO-34 NaCl-H2O/THF 150 90 98.5 64.4 this work

Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China; University of Chinese Academy of Sciences, Beijing 100049, P. R. China

Jun Yue − Department of Chemical Engineering, Engineering and Technology Institute Groningen, University of Groningen, 9747 AG Groningen, The Netherlands; orcid.org/0000-0003-4043-0737

Yuting Zhu − CAS Key Laboratory of Renewable Energy, Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China; University of Chinese Academy of Sciences, Beijing 100049, P. R. China

Chengyan Wen − School of Energy and Environment, Southeast University, Nanjing 210009, P. R. China Lungang Chen − CAS Key Laboratory of Renewable Energy,

Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China

Qiying Liu − CAS Key Laboratory of Renewable Energy, Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China; orcid.org/0000-0002-4957-7930

Longlong Ma − CAS Key Laboratory of Renewable Energy, Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China; orcid.org/0000-0003-3848-5149

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.iecr.1c01121

Notes

The authors declare no competingfinancial interest.

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This work isfinancially supported by the National Key R&D Program of China (2018YFB1501500), National Natural Science Foundation of China (no. 51776206), and the Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01N092).

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