Catalytic conversion of microcrystalline cellulose to glucose and 5

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Catalytic conversion of microcrystalline cellulose to glucose and 5-hydroxymethylfurfural over a niobic acid catalyst. Zhe Wen, Linhao Yu, fuhang mai, Zewei Ma, Hong Chen, and Yongdan Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03824 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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Industrial & Engineering Chemistry Research

Catalytic Conversion of Microcrystalline Cellulose to Glucose and 5-Hydroxymethylfurfural over a Niobic Acid Catalyst Zhe Wen,† Linhao Yu,† Fuhang Mai,† Zewei Ma,† Hong Chen,*,‡ and Yongdan Li†,§ †Collaborative

Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin Key

Laboratory of Applied Catalysis Science and Technology, State Key Laboratory of Chemical Engineering (Tianjin University), School of Chemical Engineering, Tianjin University, Tianjin 300072, China ‡School

of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China

§Department

of Chemical and Metallurgical Engineering, School of Chemical Engineering,

Aalto University, Espoo, 02150, Finland *Corresponding Author’s E-mail: [email protected]

ABSTRACT: The catalytic conversion of microcrystalline cellulose is examined over a niobic acid catalyst (NBO) prepared by alkali fusion method. The catalytic activity of NBO is significantly affected by the reaction conditions and further acid treatment. The highest yield of HMF as 7.0 mol% and 17.7 mol% yield of glucose at cellulose conversion of 54.2% are achieved

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at 230 oC for 2 h. The H2SO4-treated (NBS) and H3PO4-treated (NBP) samples both show greatly improved performance over NBO, due to the introduction of strong acid sites and the increase of specific surface area. NBS gives a glucose yield of 26.0 mol% and a HMF yield of 14.1 mol%, while NBP exhibits a glucose yield of 21.4 mol% and a HMF yield of 15.5 mol%. NBO shows excellent recyclability for the catalytic hydrolysis of cellulose. The recovered NBS and NBP samples can be regenerated by calcination and subsequent acid treatment.

KEYWORDS: Microcrystalline cellulose; Glucose; 5-Hydroxymethylfurfural (HMF); Niobic acid; Catalyst preparation

1. INTRODUCTION Cellulose is a linear polysaccharide composed of β-D-anhydroglucopyranose units linked with β-1,4-glycosidic bonds.1, 2 As the most abundant inedible carbon source in nature and the major form of photosynthetically fixed carbon, cellulose shows great potential to substitute the traditional fossil fuels for the production of biofuels and valuable chemicals.3,

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Hydrolytic

cleavage of β-1,4-glycosidic bond leads to the generation of soluble oligosaccharides and glucose. And the formed glucose is subsequently transformed into valuable chemicals, e.g. HMF, levulinic acid and polyols in acidic conditions.5, 6 Homogeneous acids especially mineral acids demonstrate excellent catalytic activity for cellulose hydrolysis.7,

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However, the processes

generally suffer from low energy efficiency, difficulty in catalyst recovery, reactor corrosion and production of acid residue.9 In recent decades, much research has concentrated on the development of solid acid catalysts to overcome the challenges.10 Nevertheless, compared with the homogeneous acid catalysts, solid acid catalysts generally exhibit lower activity and stability,


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which are mainly attributed to the slow mass transfer and the limited amount of acidic active sites.11, 12 In addition, most solid acid catalysts cannot retain the activity in water or other highly polar media for the deactivation of acid sites.13 Therefore, development of solid acid catalysts with good water-tolerance and high acid exchange capacity promotes the efficient utilization of cellulose.14 Niobic acid (Nb2O5.nH2O) exhibits excellent water-tolerance and strong protonic acidity (Ho ≤ -5.6), which is equivalent to that of 70% H2SO4.15, 16 Brønsted and Lewis acid sites exit simultaneously on the surface of niobic acid.17 Lewis acid sites are produced from the highly polarized NbO4 tetrahedra associated with OH groups, while Brønsted acid sites from the NbO6 octahedra associated with OH groups.18 Niobic acid exhibits high efficiency for a number of acid-catalyzed reactions when H2O participates in or is produced, e.g. dehydration19, hydration20, esterification21, alkylation22, isomerization23, etc. The application of niobic acid in saccharide conversion by far mainly focuses on the conversion of fructose, glucose, sucrose and inulin. Carniti and co-workers24 examined the generation of HMF from fructose dehydration over a commercial niobic acid catalyst in water medium under initial 0.2-0.6 MPa of N2 in a fix-bed reactor. HMF yields of 18-20% were achieved at 100 oC in 70 h with no levulinic acid formation. Reche et al.25 employed a niobic acid catalyst prepared by precipitation method using niobium pentachloride as precursor for the generation of HMF with glucose and fructose as substrates in water at 80-110 oC. The conversion of glucose was 50% at 110 oC for 6 h, leading to only 6% yield of HMF. Nevertheless, the HMF yield was 11% with fructose conversion of 90% at the same reaction conditions. They proposed that the poor selectivity of HMF could be mainly resulted in the formation of humin and HMF rehydration during the reaction. Sumiya and coworkers26 studied the production of glucose and fructose form sucrose hydrolysis over a bulk and


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a SBA-15-supported niobic acid catalysts in a batch reactor at 80 oC. The TON (the mole ratio of converted sucrose to Nb) over the bulk niobic acid was only 0.28, while the value over the SBA15-supported niobic acid was much higher as 4.36. The poor activity of the bulk niobic acid could be resulted in the low strength and limited Brønsted acid sites. Marzo and co-workers27 examined the commercially available niobic acid for the catalytic hydrolysis of sucrose, maltose and cellobiose under mild conditions, i.e. 50-80 oC in water with a fixed-bed continuous reactor. Niobic acid showed a low, below 10%, conversion of sucrose to glucose and fructose, while no activity for converting maltose and cellobiose. As generalized, HMF yield over niobic acid has been below 20% in water medium and can be improved in biphasic systems, such as 2butanol/water, methyl isobutyl ketone/water, tetrahydrofuran/water, dimethyl sulfoxide/water, etc.28 Yang and co-workers29 examined the catalytic activities of niobic acid catalysts for the dehydration of mono- and polysaccharide to HMF in a 2-butanol/water biphasic system at 160 oC.

HMF yields of 89%, 49% and 54% were obtained from fructose, glucose and inulin,

respectively, with reaction time of 50, 110 and 140 min. However, niobic acid showed poor performance for cellulose hydrolysis in previous works. Hara and co-workers30, 31 investigated the purchased niobic acid catalyst for cellulose hydrolysis. They reported that niobic acid was unable to hydrolyze cellulose at 100 oC. However, further works are still necessary to examine niobic acid as catalyst for the cellulose hydrolysis at relatively high reaction temperatures. Herein, we present the hydrolysis of cellulose over niobic acid catalyst prepared by alkali fusion method in water medium in reaction temperature range of 200-280 oC. Effects of reaction time, catalyst amount as well as acid treatment of catalyst are also examined in detail. 2. EXPERIMENTAL


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2.1. Chemicals The microcrystalline cellulose (d=25 μm) acquired from Aladdin Industrial Corporation was dried at 110 oC for 20 h before used. The analytic grade chemicals including niobium oxide, potassium hydroxide, acetic acid, phosphate acid, sulfate acid, anhydrous glucose, HMF, furfural and sulfate acid were purchased from Tianjin Guangfu Company, which were directly used without further purification. 2.2. Catalyst preparation The niobic acid in this work was synthesized using previously reported alkali fusion method17 with niobium oxide as the precursor and denoted as NBO. Briefly, 10 g of niobium oxide and 14 g of potassium hydroxide were mixed and ground in an agate mortar. The mixture was then loaded in a nickel crucible and calcinated at 450 oC for 3 h in air. The obtained molten salt was dissolved in 500 mL H2O. The mixture was then filtrated to get the filtrate. Glacial acetic acid was slowly dropped in the obtained filtrate under vigorously stirring until pH value was 5. The obtained solid-liquid mixture was filtrated again after ageing for 2 h. The filter cake was collected and dried at 120 oC overnight and NBO was obtained as white powder. The acidtreated samples were carried out by dipping 3 g of NBO in 10 mL H3PO4 and H2SO4 with concentration of 1 mol/L for 48 h. The H3PO4- and H2SO4-treated samples were obtained and denoted as NBP and NBS after treatment, respectively. 2.3. Catalyst characterization The XRD patterns were recorded with a D8 Focus diffractometer with a Cu-Kα radiation source (40 kV, 40 mA ). The range of the 2θ was 20 to 80o and the scanning rate was 8°/min.


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The FTIR spectra were measured on a Nicolet Nexus-870 instrument by the KBr method. The SEM micrographs were recorded with a Hitachi S-4800. NH3-TPD analysis was conducted with an Auto Chem II 2920 analyzer. The IR spectra of the three catalyst samples after pyridine absorption were measured using a Nicolet 6700 instrument. The surface area and porosity of the catalysts were performed on a Quantachrome Autosorb-1 apparatus. A titration method reported by Onda et al.32 was used to determine the amount of the Brønsted acid sites of the samples. Briefly, the mixture of NaOH solution (20 mL, 0.01 mol/L) and sample (0.04 g) was stirred at ambient temperature for 2 h. The supernatant was collected after centrifugal separation and then titrated with HCl solution (0.01 mol/L) with phenolphthalein as indicator. 2.4. Cellulose conversion reaction and product analysis The hydrolysis of cellulose was conducted using a 300 mL high-pressure reactor (MS250H-C276, Anhui Kemi Machinery Technology Co., LTD.). In a typical reaction, microcrystalline cellulose (1.0 g), catalyst (0.1-0.5 g) and H2O (100 mL) were added into the reactor, which was then sealed and purged with nitrogen for five times. Then the reactor was heated at the predefined temperature (100-280 oC) for the specified time (1-4 h) with stirring at 650 rpm. After the reaction, the reactor was cooled down in a water bath to room temperature. The reaction mixture was filtrated and the obtained residue was dried at 120 oC overnight and weighed. The products in liquid sample were qualitatively and quantitatively identified by HPLC (Agilent 1200) equipped with a Bio-Rad Aminex HPX-87C Carbohydrate Column (300 mm×7.8 mm) and a refractive index detector by standard curve method. The flow rate of mobile phase (H2SO4 solution, 0.005 mol/L) was 1 mL/min and the column temperature was 65 oC. The retention times of glucose, HMF and furfural were 5.4, 21.6 and 23.6 min, respectively. The


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cellulose conversion, the yield of product and the selectivity of product were calculated as follows:

Conversion (%) =

Yield (mol%) =

Selectivity (%) =

minitial cellulose + mcatalyst - mrecovered residue minitial cellulose ncarbon in the product ncarbon in initial cellulose Yieldproduct Conversion

¡ Á 100%

¡ Á100%

¡ Á 100%

(1)

(2)

(3)

3. RESULTS 3.1. Conversion of cellulose Table 1 gives the results of the catalytic performance of NBO. Cellulose showed indistinctive depolymerization at a conversion lower than 5% with neither glucose nor HMF formation when the reaction temperature was below 150 oC. Glucose and HMF were produced at temperatures above 180 oC. The conversion of cellulose increased from 6.7% to 86.1% as the temperature increased from 180 to 250 oC, and remained at the same value with the further increased temperature to 280 oC. The HMF yield firstly increased with the increasing temperature and achieved the highest value of 7.0 mol% at 230 oC (Table 1, Entry 5) and then decreased with the further increased temperature. The glucose yield presented the same trend with that of HMF, while the maximum value of 21.0 mol% was achieved at 250 oC (Table 1, Entry 6). The HMF yield decreased to 0.1 mol% as the temperature further increased to 280 oC, while the glucose yield decreased to 15.1 mol%. Trace amount of furfural was generated accompanied with glucose and HMF. Higher temperature favored the production of furfural,


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giving the highest yield of 3.3 mol% at 280 oC. The results revealed that the reaction at 230 oC gave an overall yield of glucose and HMF as 24.7 mol% with an overall selectivity of 45.5%. Nevertheless, the overall yield of glucose and HMF at 250 oC was 23.6 mol% with an overall selectivity of 27.5%. Taking the energy efficiency into consideration, 230 oC was recognized as the optimum reaction temperature for further study. Trace amounts of cellobiose, fructose and formic acid with total yield of 1.4 mol% were also identified in the reaction solution. Besides, oligosaccharides hard to identify were also formed during the hydrolysis process. Furthermore, the hydrothermal conversion of cellulose without catalyst was examined under the same reaction conditions (230 oC, 2 h). Only a conversion of cellulose 12.4% was obtained, leading to a HMF yield as 2.3 mol% (Table 1, Entry 11). The effects of reaction time and catalyst amount are examined in detail. The conversion of cellulose increased from 35.6% to 76.9% as the reaction time prolonged from 1 to 4 h, leading to the increase of glucose yield from 4.1 mol% to 23.1 mol%. The HMF yield firstly increased with the prolonging time and the maximum value of 7.0 mol% was achieved at 2 h (Table 1, Entry 5) and then decreased at further prolonging time. The overall selectivity of glucose and HMF showed the same trend with that of HMF yield. However, the catalyst amount had negligible effect on the cellulose conversion and the glucose yield. Nevertheless, the HMF yield decreased from 7.0 mol% to 2.0 mol% with the increase of the catalyst amount from 0.1 to 0.5 g. The performances of the NBS and NBP catalysts were measured at 230 oC for 2 h with 0.1 g catalyst. As Figure 1 shown, the catalytic activities of NBS and NBP were improved significantly. NBS exhibited higher activity than NBO, providing 80.8% conversion of cellulose with HMF and glucose yields of 14.1 mol% and 26.0 mol%, respectively. For NBP, the


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conversion of cellulose was slightly lower than that over NBS as 77.9% and the glucose yield was 21.4 mol%. However, NBP gave a higher HMF yield as 15.5 mol%. 3.2. Catalyst Characterization Figure 2 exhibits the XRD patterns of NBO, NBS and NBP. The XRD pattern of NBO reveals two broad diffraction peaks with weak intensities at 20-40o and 40-60o, characterizing amorphous structure, which could function as an acid catalyst33. The amorphous structure remains stable during acid treatment in both sulfate acid and phosphate acid as depicted in the same figure. As the FTIR spectrum of NBO in Figure 3 shown, the peak at 623 cm-1 is resulted in the stretching vibration of -Nb=O, while the peaks at 1092 and 918 cm-1 may be ascribed to the stretching vibration of -Nb-O. The peaks at 1631 and 1334 cm-1 can be assigned to the stretching and bending vibration of -OH or H2O adsorbed on the surface of the sample. The broad peak centered at 3429 cm-1 is probably assigned to the stretching vibration of -OH in Nb-OH or H2O. The bands at 1413 and 1251 cm-1 are respectively attributable to the deformation vibration of OH and bending vibration of C-O in carboxylic group, which is probably introduced by acetic acid. Compared to that of NBO, there are some obvious difference in the spectra of NBS and NBP. As shown in the spectrum of NBS, the peaks at 1206, 1081 and 814 cm-1 are attributable to the stretching vibration of S-O and S=O. The additional peak at 1067 cm-1 in the spectrum of NBP is attributable to the asymmetrical stretching vibration of the P-O bond. The peaks attributable to the stretching vibration of -OH shift to 3417 and 3409 cm-1 in NBS and NBP, respectively, indicating the introduction of S-OH and P-OH onto NBS and NBP.


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The SEM images of the three catalyst samples are depicted in Figure 4. NBO exhibits an aggregated structure of irregularly shaped particles. NBS remains almost the same morphological characteristics as those of NBO, while NBP exhibits highly dense structure on the surface. Figure 5 depicts the NH3-TPD profiles of the three catalyst samples. NBO shows two desorption peaks centered at 214 and 372 oC, indicating the presence of weak and medium strong acid sites. NBP exhibits two desorption peaks at 229 and 495 oC, demonstrating the weak and strong acid sites, respectively. NBS exhibits one strengthened peak at 236 oC and two weak peaks at 400 and 459 oC. In addition, this sample exhibits a strong peak at 618 oC, which represents quite a large amount of acid sites with high strength and does not appear in the curves of the other two samples. Figure 6 exhibits the IR spectra of the three catalyst samples after pyridine absorption. The results indicated the presence of both Brønsted acid sites (B) at 1544 cm-1 and Lewis acid sites (L) at 1446 and 1569 cm-1 on the surface of the catalysts. The peak at 1492 cm-1 is assigned to both Brønsted and Lewis acid sites associated with pyridine. According to the absorption peak areas, the amounts of Brønsted and Lewis acid sites on NBO are less than those on NBS and NBP. The amount ratios of Brønsted to Lewis acid sites on NBO, NBS and NBP are 0.93, 1.00 and 0.88, respectively. Table 2 provides the textural properties and surface acidity measurements of NBO, NBS and NBP. The surface areas of the samples increase from 65.3 m2/g to 137.0 and 275.3 m2/g after acid treatment with sulfate acid and phosphate acid, respectively. The pore volume also increases after acid treatment, while the average pore diameter is essentially maintained. The amounts of Brønsted acid sites on NBO, NBS and NBP are 0.59, 1.10 and 1.02 mmol/g, respectively. The


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amounts of Lewis acid sites on NBO, NBS and NBP are calculated to be 0.64, 1.10 and 1.15 mmol/g, respectively. 3.3. Recyclability of the catalyst samples The catalyst sample was recovered by filtration after each run and then washed with ethanol to remove the adsorbent on the surface. Before the next consecutive run, the recovered sample was dried at 100 oC for 24 h. The recyclability of the catalyst samples were examined under the optimal conditions, i.e. 230 oC for 2 h, and shown in Figure 7. NBO showed good stability in the recycle experiments. The cellulose conversion and HMF yield remained essentially constant, while the yield of glucose decreased slightly from 17.7 mol% to 16.8 mol% during the third consecutive run. The catalytic activities of the recovered NBS and NBP decreased significantly during the second consecutive run. Decreased yields of glucose and HMF of 17.0 mol% and 7.0 mol%, respectively, were obtained over the recovered NBS. And the recovered NBP provided the yields of glucose and HMF of 20.6 mol% and 6.7 mol%, respectively. The recovered NBS and NBP samples remained basically the same catalytic activities as that of NBO during the second and the third runs. During the third run, a glucose yield as 15.7 mol% and a HMF yield as 7.0 mol% were obtained over NBS, while those over NBP were 20.9 mol% and 6.5 mol%, respectively. 4. DISCUSSION 4.1. Conversion of cellulose The catalytic activity of NBO for the cellulose hydrolysis was significantly affected by the reaction temperature. Cellulose showed indistinctive depolymerization at cellulose conversions


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lower than 5% with neither glucose nor HMF formation at temperatures below 150 oC, which were in consistency with previous reports29-31. In addition, niobic acid catalyst showed poorer catalytic activity for the production of HMF from glucose than that from fructose.25, 29 The high crystallinity and the supramolecular structure make cellulose more difficult to be converted than glucose.34 Here, cellulose can be hydrolyzed to glucose and HMF at temperatures above 180 oC. The yields of glucose and HMF firstly increased with the increase of temperature, achieving the maximum values as 21.0 mol% at 250 oC (Table 1, Entry 6) and 7.0 mol% at 230 oC (Table 1, Entry 5), respectively, then decreased with the further increased temperature. The yields of HMF and glucose decreased significantly at a too high reaction temperature of 280 oC, which was probably resulted in the formation of polymeric by-products from polymerization of HMF or oligomerization of glucose in acidic media in the temperature range35. Trace amount of furfural was generated accompanying with glucose and HMF via the proposed two parallel reactions, i.e. the loss of formaldehyde from HMF and direct formation from fructose.36 In our work, 230 oC was recognized as the optimum reaction temperature for further study because of the relatively high yield of HMF and overall selectivity towards glucose and HMF from cellulose. The conversion of cellulose and the yield of glucose increased as the reaction time prolonged from 1 to 4 h. Nevertheless, the yield of HMF firstly increased with the prolonging time and then decreased when the time exceeded 2 h. The decrease of HMF yield at prolonged reaction time was probably resulted in the generation of polymeric by-products from polymerization of HMF under acidic conditions.37 The catalyst amount had almost negligible effect on the cellulose conversion and the glucose yield. However, the HMF yield decreased with the increase of catalyst amount, indicating that NBO offered enough catalytic active sites for the cellulose hydrolysis at the mass ratio of cellulose to NBO as 10:1.


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4.2. Effect of acid treatment After acid treatment, the activities of NBS and NBP were significantly improved. NBS gave higher cellulose conversion and glucose yield, while NBP gave a higher HMF yield under the same reaction conditions. Previous works38-40 indicated that the acidity of niobic acid could be greatly improved via acid treatment, providing acidity equivalent to strong inorganic acids at low temperatures. During the process of cellulose hydrolysis in acidic conditions, glucose is generated on the Brønsted acid sites, while HMF on the Lewis acid sites.41,

42

Results of

hydrolysis reactions indicated the different modification effects of sulfate and phosphate acids on the effective acidity of catalyst, such as nature, strength and amount of acid sites. No obvious differences were observed in the XRD patterns (Figure 2) of NBO, NBS and NBP. NH3-TPD analysis indicated that large amount of strong acid sites were introduced during the acid treatment. FTIR analysis of the samples demonstrated the introduction of S-OH and POH in NBS and NBP. S-OH and P-OH are stronger acid sites compared to Nb-OH, leading to the significantly strengthened surface acidity of NBS and NBP.43 Besides strength, the nature of acid sites has crucial effect on the product distributions. The pyridine adsorption IR results indicated that there were more Brønsted and Lewis acid sites on the surface of NBS and NBP than those of on NBO. Sulfate acid introduces a larger amount of strong Brønsted acid sites, while phosphate acid introduces more Lewis acid sites. The results of acidity measurements were in consistent with the pyridine adsorption IR results. In addition, SEM images showed that NBP exhibited highly dense structure on the surface, which were distinctly different from those of NBO and NBS. Textural properties measurements of NBO, NBS and NBP indicated that the specific surface area and pore volume increased significantly after acid treatment. In conclusion, acid treatment resulted in the enhancement of catalytic activity, for the strong acid sites were


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introduced and the specific surface area was increased during the acid treatment. Furthermore, the nature of acid sites has crucial effect on the selectivity of product, i.e. more Brønsted acid sites favors the generation of glucose and Lewis acid sites favors the generation of HMF. 4.3. Recyclability of the niobic acid catalysts NBO showed good recyclability in the recycle experiments, with only a slight loss of glucose yield after three consecutive recycles. The catalytic performances of the recovered NBS and NBP decreased significantly during the second consecutive recycle, leading to distinct decrease of glucose and HMF yields. The NBS and NBP catalysts remained basically the same catalytic activities as that of NBO in the second and the third runs, indicating that the catalytic active sites introduced during the acid treatment on the surface of NBS and NBP were removed during the reaction. The acidity measurements demonstrated that the Brønsted acidity of the recovered NBS and NBP samples after three consecutive recycles decreased to 0.58 and 0.62 mmol/g, respectively, which were almost equal to that of fresh NBO as 0.59 mmol/g. Furthermore, the regeneration of the recovered NBS and NBP catalysts can be achieved by calcination at 300 oC and subsequent acid treatment. 5. CONCLUSIONS The NBO catalyst demonstrates good performance for the production of glucose and HMF from cellulose hydrolysis at reaction temperatures above 180 oC in water. The catalytic activity of NBO was significantly affected by the reaction conditions and further acid treatment. High temperature favored the conversion of cellulose, while moderate high temperature favored the production of glucose and HMF. The highest yield of HMF as 7.0 mol% along with glucose yield as 17.7 mol% at cellulose conversion of 54.2% were achieved at 230 oC for 2 h over 0.1 g


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NBO. The catalytic activities of NBS and NBP increased significantly. Glucose yield of 26.0 mol% and HMF yield of 14.1 mol% at cellulose conversion of 80.8% were obtained over NBS, while glucose yield of 21.4 mol% and HMF yield of 15.5 mol% at cellulose conversion of 77.9% over NBP. The enhancement of catalytic activity by acid treatment was mainly resulted in the introduction of strong acid sites and the increase of specific surface area. The NBO catalyst showed excellent recyclability in the recycle experiments. The recovered NBP and NBS samples lose a substantial ratio of catalytic activity during the second consecutive recycle, but remained basically the same activities as that of NBO in the subsequent recycles. The catalytic activities of the recovered NBS and NBP samples can be regenerated by calcination and subsequent acid treatment.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Hong Chen: 0000-0002-0325-2786 Yongdan Li: 0000-0002-0430-9879 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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The financial supports from the National Natural Science Foundation of China under Contract No. 21690083, 21808163 and 21336008 are gratefully acknowledged. REFERENCES (1)

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Table 1. Hydrolysis of cellulose over NBOa Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Catalyst amount (g) 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0 0.2 0.3 0.4 0.5

Temperature (oC) 100 150 180 200 230 250 280 230 230 230 230 230 230 230 230

Time (h) 2 2 2 2 2 2 2 1 3 4 2 2 2 2 2

Glucose -b 1.6 6.0 17.7 21.0 15.1 4.1 20.5 23.1 2.3 17.9 18.0 18.3 18.7

Yield (mol%) HMF Furfural 0.1 Tracec 0.9 0.2 7.0 1.0 2.6 2.6 0.1 3.3 4.8 0.6 6.1 1.0 5.5 1.1 5.4 1.2 3.6 1.4 2.8 1.7 2.0 2.0

Conversion (%) 0.7 5.1 6.7 13.2 54.2 86.1 86.2 35.6 67.0 76.9 12.4 54.9 55.3 55.6 56.7

a

Reaction conditions: microcrystalline cellulose 1.0 g, water 100 mL, initial 0 MPa of N2, 650 rpm. b “-”means not detected. c Trace means yield less than 0.05%.


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Table 2. Textural properties and surface acidity measurements of NBO, NBS and NBP Samples NBO NBS NBP

BET (m2/g) 65.3 137.0 275.3

Pore Volume (cm3/g) 5.58E-2 1.19E-1 2.21E-1

Average Pore Diameter (nm) 3.42 3.46 3.22

Surface acidity (mmol/g) 0.59 1.10 1.02


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Figure 1. Effects of acid treatment on the conversion of cellulose and the product yields. (Reaction conditions: microcrystalline cellulose 1.0 g, catalyst 0.1 g, water 100 mL, 230 oC, 2 h, initial 0 MPa of N2, 650 rpm.)


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Figure 2. XRD patterns of NBO, NBS and NBP.


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Figure 3. FTIR spectra of NBO, NBS and NBP.


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Figure 4. SEM images of (a) NBO, (b) NBS and (c) NBP.


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Figure 5. NH3-TPD profiles of NBO, NBS and NBP.


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Figure 6. IR spectra of NBO, NBS and NBP after pyridine absorption.


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Figure 7. Recyclability of NBO, NBS and NBP for the conversion of cellulose. (Reaction conditions: microcrystalline cellulose 1.0 g, catalyst 0.1 g, water 100 mL, 230 oC, 2 h, initial 0 MPa of N2, 650 rpm.)


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Graphical abstract


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Highlights 1. A water-tolerant niobic acid catalyst is prepared. 2. Hydrolysis of microcrystalline cellulose is examined. 3. Niobic acid catalysts showed activity at reaction temperatures above 200 oC. 4. The catalytic activities of catalysts are greatly improved by acid treatment. 5. The used catalysts can be recovered by calcination and acid treatment.


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Catalytic conversion of microcrystalline cellulose to glucose and 5

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