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Song, J., Li, Y., Zhang, X., Zhang, D., Jiang, Z., and Wang, X. (2020). "Mesoporous heteropolyacid nanorods for heterogeneous catalysis in polysaccharide conversion," BioRes. 15(1), 240-264.

Abstract

Mesoporous heteropolyacid (HPA) nanorods having a composition of [C16H33N(CH3)3]xH3-xPW12O40 ((CTA)xH3-xPW, x = 1, 2, and 3) were synthesized by surfactant encapsulation and were evaluated for their catalytic activity in cellulose hydrolysis. The (CTA)H2PW nanorods were found to be most active with 57.2% yield of 5-hydroxymethylfurfural (5-HMF) at ~100% conversion in water/methyl isobutyl ketone (MIBK) biphase, which was higher than (CTA)H2PW nanosphere at 140 °C for 11 h. The yields of 5-HMF and glucose were obtained as 4.5% and 54.3% at 160 °C for 8 h in water system, respectively. (CTA)H2PW nanorods showed higher tolerance to such feedstocks as lignocellulose, i.e. corn straw with 19.8% and 8.3% yields for glucose and xylose at 35.4% conversion in water. Moreover, (CTA)H2PW nanorods showed higher stability and long duration with ten times reuse. (CTA)H2PW nanorods presented higher efficiency and reusability in conversion of cellulosic biomass.


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Mesoporous Heteropolyacid Nanorods for Heterogeneous Catalysis in Polysaccharide Conversion

Jiaying Song, Yiming Li, Xueyan Zhang, Dan Zhang, Zijiang Jiang,* and Xiaohong Wang *

Mesoporous heteropolyacid (HPA) nanorods having a composition of [C16H33N(CH3)3]xH3-xPW12O40 ((CTA)xH3-xPW, x = 1, 2, and 3) were synthesized by surfactant encapsulation and were evaluated for their catalytic activity in cellulose hydrolysis. The (CTA)H2PW nanorods were found to be most active with 57.2% yield of 5-hydroxymethylfurfural (5-HMF) at ~100% conversion in water/methyl isobutyl ketone (MIBK) biphase, which was higher than (CTA)H2PW nanosphere at 140 °C for 11 h. The yields of 5-HMF and glucose were obtained as 4.5% and 54.3% at 160 °C for 8 h in water system, respectively. (CTA)H2PW nanorods showed higher tolerance to such feedstocks as lignocellulose, i.e. corn straw with 19.8% and 8.3% yields for glucose and xylose at 35.4% conversion in water. Moreover, (CTA)H2PW nanorods showed higher stability and long duration with ten times reuse. (CTA)H2PW nanorods presented higher efficiency and reusability in conversion of cellulosic biomass.

Keywords: Heteropolyacid; Mesoporous; Nanorod; Hydrolysis; Cellulose; 5-HMF

Contact information: Key Lab of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, P. R. China;

* Corresponding author: wangxh665@nenu.edu.cn

INTRODUCTION

5-Hydroxymethylfurfural (5-HMF) is an important bioplatform chemical that has been produced from the conversion of monosaccharides or polysaccharides (Delidovich et al. 2014). Directly transforming cellulose to 5-HMF might be more economical and energy efficient without competing with the edible sugars. The process contains three cascade reactions: hydrolysis to glucose, isomerization to fructose, and final dehydration to 5-HMF in the presence of acidic catalysts (Li et al. 2018). Compared with homogeneous acids (HCl or H2SO4), heterogeneous catalysts might be more environmentally friendly with easy separation, recyclability, and no corrosion (Jing et al. 2018). The difficulty in mass-transfer between solid catalysts and solid cellulose might hinder their application in 5-HMF production.

Heteropolyacids (HPAs) are acidic catalysts with strong Brønsted acidity; they have been widely used in cellulose conversion (Geboers et al. 2010; Fan et al. 2011; Chambon et al. 2011; Palkovits et al. 2011; Sun et al. 2012; Qu et al. 2012; Deng et al. 2012; Tsubaki et al. 2013; Xie et al. 2014; Zhao et al. 2014; Zhao et al. 2015; Sun et al. 2015a; Kumar et al. 2016; Zhao et al. 2018a; Zhang et al. 2018; Almohalla et al. 2018; Zhang et al. 2019). To overcome the mass-transfer difficulty, surfactant-type HPAs were developed and have shown benefits including concentrating substrates and providing special surroundings by their hydrophobic tail and hydrophilic head (Li et al. 2004; Tang et al. 2012; Lü et al. 2010; Zhang et al. 2010; Cheng et al. 2011; Zhao et al. 2011; Zhang et al. 2011; Zheng et al. 2013; Lu et al. 2015; Sun et al. 2015b; Sun et al. 2016; Zhang et al. 2016a). The compounds [C16H33N(CH3)3]H2PW12O40 ((CTA)H2PW), Cr[(DS)H2PW12O40]3 (DS is the abbreviation of dodecyl sulfate), (CTA)H3PW11CrO39, (CTA)H4PW11TiO40, [HOCH2CH2N(CH3)3]H2PW12O40 and [HOCH2CH2N(CH3)3] H4AlW12O40 all showed good activity in cellulose transformation to glucose, 5-HMF, or levulinic acid (LA), which the best yields of 5-HMF, LA and glucose were obtained as 75.0%, 74.8%, and 75.9% upon [HOCH2CH2N(CH3)3]H2PW12O40 and [HOCH2CH2N(CH3)3]H4AlW12O40, respectively (Zhang et al. 2016a; Sun et al. 2016). However, these surfactant-type HPAs did not exhibit porous characteristics, which could provide more reacting room for transformations. Various solid hybrids with porous structure have been found to be active for cellulose to 5-HMF (Zhang et al. 2016b; Zhang et al. 2017; Zhao et al. 2018b). In this concept, surfactant-type HPAs with porous structure would be designable.

Surfactant-type HPAs had been developed to fabricate different architectures with tunable morphologies including one-dimensional wires (Kang et al. 2004) and fibers (Carraro et al. 2008), two-dimensional thin-films (Liu et al. 2002; Bao et al. 2009) and disks (Nisar et al. 2009a), and three-dimensional vesicles (Zhang et al. 2008; Bu et al. 2009), spheres (Li et al. 2007; Nisar et al. 2009b), tubes (Ritchie et al. 2009), flowers (Nisar et al. 2009c), and cone (Nisar et al. 2011). A surfactant-type HPA with nanorod morphology and porous properties was designable due to the well-matched linear shape and porous cavity for interactions between active sites and cellulose (Zhang et al. 2016c).

Based on the above, surfactant-type HPAs (CTA)xH3-xPW nanorod with mesoporous structure had been synthesized through simply controlling their initial usages. Such nanorod showed enhanced activity in cellulose hydrolysis either in water or in H2O/MIBK biphase. Higher efficiency can be attributed to their one-dimensional morphology, which is a good match to linear cellulose, porous structure favoring for the reaction. Moreover, (CTA)xH3-xPW nanorod showed easily separation compared to their spherical species. The cellulose or even lignocellulose was hydrolyzed into 5-HMF or glucose in H2O/MIBK biphase or in water systems upon (CTA)H2PW due to its stronger Brønsted acidity and special nanorod morphology, which also showed higher stability and longer duration.

EXPERIMENTAL

Materials

Microcrystalline cellulose (white, average particle size 50 μm) was obtained from Beijing InnoChem Science & Technology Co., Ltd. All other reagents were of AR grade and used without further purification. The 3,5-dinitrosalicylic acid (DNS) reagent was prepared according to ref (Cowan et al. 2001). H3PW12O40 was prepared based on Duan et al. (2013).

Characterization

The elemental analysis was obtained using a Leeman Plasma Spec (I) ICP-ES and a P-E 2400 CHN elemental analyzer. FTIR spectra were recorded on a Nicolet Magna 560 IR spectrometer (KBr discs) in the 4000 to 400 cm-1 region. X-ray diffraction (XRD) patterns of the catalysts were carried out using a Japan Rigaku Dmax 2000 X-ray diffractometer with Cu Kα radiation (λ = 0.154178 nm) (Rigaku Corporation, Japan). UV-vis spectra (200-600 nm) were recorded on a Cary 500 UV-vis-NIR spectrophotometer. DR-UV-vis spectra (200-600 nm) were obtained on a UV-2600 UV-vis spectrophotometer (Shimadzu). The 31P NMR spectra of the catalysts were achieved with a Bruker AM 400 spectrometer at 161.9 MHz. SEM images were determined by a SU8010 scanning electron microscope. The energy dispersive X-ray analysis (EDX) was performed to calculate the contents of P, W, O, N, and C elements. TEM micrographs were recorded on a Hitachi H-600 transmission electron microscope. Brunauer-Emmett-Teller (BET) specific surface area, total pore volume and average pore width of the catalysts were determined by a N2 physisorption experiment using an automatic gas adsorption system (ASAP 2020, Micromeritics Instrument Corp, USA) at -196 °C. The sample was degassed at 90 °C for 8 h before the physical adsorption of N2. The electro-potential variation was measured with an instrument of ZDJ-4B automatic potentiometric titration, using a BestLab Non-aqueous pH Titration electrode (Shanghai, China).

Chemical Tests

The acidic strength was measured by the titration according to the previous literature (Pizzio and Blanco 2007). The concentrations of 5-HMF and LA were determined on Agilent Technologies 7820A GC system fitted with an Agilent J&W Advanced capillary GC column (Shanghai, China). The concentration of glucose was measured in the aqueous phase by HPLC equipped with a refractive index detector (Shimadzu LC-10A, HPX-87H column) column at 35 °C. The mobile phase was H2O with a flow rate of 0.5 mL/min at 75 °C. The concentration of xylose was measured by HPLC equipped with a refractive index detector using a UltimateXB-NH2 (4.6 mm×150 mm, 5 μm) column at 35 °C. The mobile phase was H2O/acetonitrile (1/4 v/v) with a flow rate of 1.0 mL/min. The error bars were obtained as the standard deviation of three measurements, which were calculated based on the following:

 .

Methods

50 mL of hexadecyltrimethylammonium bromide (CTAB) aqueous solution (20 mM) was added into 50 mL of H3PW12O40 (HPW, 20 mM) with stirring. The white precipitate formed immediately. Then the mixture was kept still for 24 h. The white precipitate was separated and dried under vacuum at 50 C. Then it was calcined at 200 C for 3 h, 72.3% yield. The other hybrids (CTA)xH3-xPW with different x values were synthesized using the same procedure except using MCTAB: MHPW = 2: 1 and 3: 1, instead.

Catalytic Procedure

For the hydrolysis of cellulose into the 5-HMF in the double solvent MIBK/H2O system, the mixture cellulose (0.1 g), and the catalyst (0.09 mmol) were added to H2O (0.5 mL) and MIBK (5 mL). Then they were placed in a steel autoclave for 11 h at 140 °C under magnetic stirring (300 rpm). The reaction was stopped by rapidly cooling the reactor in an ice bath at 0 C for 30 min. After the temperature reached room temperature, the reaction solution formed three layers: the first layer was the organic phase which contained the desired product 5-HMF. The second layer was the aqueous phase that contained fructose and glucose. The third layer was solid which contained unreacted cellulose and the catalyst. The mixture was centrifuged, then washed three times with water and ethanol, and dried under vacuum at 60 °C for 24 h. Repeated experiments only increased some fresh cellulose to 0.1 g as the initial amount due to the inability to separate the catalyst in the mixture from the unreacted cellulose and by-products humin by centrifugation. The total amount of the catalyst leaching after ten cycles of experiments was detected by UV-vis spectroscopy. Cellulose conversions (wt %) were determined by the difference between the cellulose weight before and after the reaction. The quantitative analyse of 5-HMF was determined by Gas Chromatograph (GC).

For the hydrolysis of cellulose in water, a mixture of cellulose (0.1 g) and catalyst (0.09 mmol) was added into water (7 mL). Then the mixture was heated at 160 °C in a steel autoclave lined with Teflon for 8 h with stirring (300 rpm). The reaction was stopped by rapidly cooling the reactor in an ice bath at 0 °C. After the temperature reached room temperature, the mixture was centrifuged to separate the catalyst and unreacted cellulose. Cellulose conversions (wt%) were determined by the change in cellulose weight before and after the reaction. 5-HMF was determined by HPLC (high-performance liquid chromatography). The concentration of glucose was measured in the aqueous phase by HPLC equipped with a refractive index detector (Shimadzu LC-10A, HPX-87H column) column at 35 °C. In both two systems, the conversion of sucrose, cellobiose, starch and corn straw was done in the same procedure as cellulose conversion.

Total Reducing Sugars (TRS) Analysis

A mixture that contained 2 mL of the DNS reagent and 1 mL of the reaction sample was heated for 2 min in a boiling water bath, then cooled to room temperature with flowing water, and mixed with deionized water to a volume of 25 mL. The color intensity of the mixture was measured in a UV757CRT model spectrophotometer at 540 nm. The concentration of total reducing sugars was calculated based on a standard curve obtained with glucose. (Cowan et al. 2001)

Adsorption Experiment

In order to confirm the adsorption capacity of the catalyst to cellulose, the adsorption experiment was conducted. During the adsorption experiment, a mixture of cellulose (0.1 g) and the catalyst (0.09 mmol) were mixed together in H2O (5.5 mL) in a steel autoclave lined with Teflon at a 140 °C for 1 h. The powder solids were removed and dried in a vacuum of 60 C. Then it was used to determine the adsorption of cellulose upon (CTA)H2PW nanorod by IR spectroscopy.

RESULTS AND DISCUSSION

Characterization of the Catalysts

From the results of the elemental analysis (Table 1), the molar ratio of W and P in catalysts was P: W = 1: 12, corresponding to that with Keggin structure of PW12O403-. The contents of C, H and N were inherent with the calculated values indicating the formula as (CTA)xH3-xPW12O40 (x = 1-3).

Figure 1 shows the IR spectra of the (CTA)xH3-xPW nanorods, giving the four characteristic peaks similar to their parent H3PW12O40 (1075, 976, 899, and 796 cm-1) (Deltcheff et al. 1983), indicating that the catalysts maintained the original heteropolyacid structure during reaction with CTAB. In addition, peaks of C-H stretching vibration at 2916 and 2845 cm-1, and C-N at 1467 cm-1 confirmed the presence of CTA+ in the hybrids. The characteristic peaks belonging to PW12O403- anion shifted depending on the CTA+ contents, showing the existence of anion-cation interaction between the polyoxometalate anion and CTA+. The Keggin structure of (CTA)xH3-xPW (x = 1-3) nanorod was further determined by DR-UV-vis spectroscopy, which presented one peak at 267 nm assigned to O → W change transition (Fig. 2).

Fig. 1. IR spectra of (a) H3PW12O40, (b) (CTA)H2PW nanorod, (c) (CTA)2HPW nanorod, (d) (CTA)3PW nanorod, and (e) (CTA)H2PW nanorod after the reaction

Table 1. Elemental Analysis, Total Acidity and Hydrolysis Activity a of H3PW12O40 and (CTA)xH3-xPW Nanorod

The XRD patterns of H3PW12O40 and (CTA)xH3-xPW showed that all exhibited typical X-ray diffractograms of Keggin anion at 10.2°, 20.6°, 25.1°, 34.5°, and 53.7°, indicating the retain of the structure after reacting with CTAB (Fig. 3). And slight shifts for XRD of (CTA)xH3-xPW determined the existence of (CTA)xH3-xPW and no physical mixture of PW12O403- anion and CTAB (Wang et al. 2016).

The 31P MAS NMR spectrum of the (CTA)H2PW nanorod gave one peak at -17.99 ppm (Fig. 4a) with some shift compared to its parent H3PW12O40 (-15.6 ppm). This result confirmed the formation of (CTA)H2PW nanorod and no physical mixture of CTA+ and H3PW12O40.

Fig. 2. DR-UV-vis spectra of the catalysts. (a) H3PW12O40, (b) (CTA)H2PW nanorod, (c) (CTA)2HPW nanorod, and (d) (CTA)3PW nanorod

Fig. 3. XRD patterns of (CTA)xH3-xPW nanorod. (a) H3PW12O40, (b) (CTA)H2PW nanorod, (c) (CTA)2HPW nanorod, (d) (CTA)3PW nanorod, and (e) (CTA)H2PW nanorod after the reaction

.

Fig. 4. 31P MAS NMR spectrum of the (CTA)H2PW nanorod (a) before the reaction, and (b) after the reaction

The SEM images of (CTA)H2PW with different concentrations of CTAB and H3PW12O40 determined the synthetic procedure of nanorods (Fig. 5). At lower concentrations, CTAB firstly reacted with H3PW12O40 through ion exchanging reaction to form micellar sphere with size of 100 nm (Fig. 5a). Increasing their concentrations both to 15 mM, the micellar sphere grew bigger and then self-assembled at 20 mM to nanorods with diameters from 300 nm to 400 nm (Fig. 5b, 5c). When concentrations were further increased to 25 mM, the regular nanorod of (CTA)H2PW was the main product in SEM with diameter of 500 nm (Fig. 5d). Furthermore, the ordered mesoporous structure of (CTA)H2PW nanorod was determined by TEM (Fig. 6a). Combined with SEM and TEM, the self-assembly performance of CTAB and H3PW12O40 was as follows: (1) CTA+ reacted with H3PW12O40 through ion-exchanging to (CTA)H2PW single molecule; (2) (CTA)H2PW self-assembled to form micellar sphere at lower concentrations; and (3) The obtained micelles were further stacked to form cylinders that were attached together as nanorod. The nanorod presented an ordered mesoporous structure (Scheme 1). From the IR and XRD of samples with different initial concentrations (Fig. 7), it could be concluded that such samples all showed the Keggin structure. This indicated that CTAreacted with H3PW12O40 firstly in a stoichiometric manner to (CTA)H2PW, then it self-assembled into nanosphere or nanorod depending to the initial usages of CTAB and H3PW12O40. The SEM images of (CTA)2HPW nanorod and (CTA)3PW nanorod are given in Fig. 8.

Fig. 5. SEM images of CTAB reacting with H3PW12O40 with different concentrations of (a) 10 mM, (b) 15 mM, (c) 20 mM, and (d) 25 mM

Scheme 1. The synthetic procedures for (CTA)xH3-xPW nanorod

Fig. 6. The TEM image of (CTA)H2PW nanorod at concentrations of 20 mM (a), EDX pattern (b), and compositional EDX mapping of the (CTA)H2PW nanorod (c)

Fig. 7. IR spectra of and XRD patterns of CTAB reacting with H3PW12O40 with different concentrations of (a) 10 mM, (b) 15 mM, (c) 20 mM, and (d) 25 mM

 

Fig. 8. The SEM image of (CTA)2HPW nanorod (a) and EDX pattern (b), and the SEM image of (CTA)3PW nanorod (c) and EDX pattern (d)

The energy dispersive X-ray spectroscopy (EDX) measurement (Fig. 6b) showed that the (CTA)H2PW nanorod had no other impurities from the W, C, O, N, and P elements. The molar ratio of P and W obtained from the Fig. 6b was about 1:12, being consistent with the ICP elemental analysis, indicating that the (CTA)H2PW nanorod maintained the Keggin structure. And the molar ratio of W to C was 12:19, indicating the formula of nanorod as (CTA)H2PW, which is coherent with elementary analysis. The energy dispersive spectroscopy (EDX) mapping (Fig. 6c) revealed a uniform dispersion of the component elements W, C, O, N, and P, demonstrating the strong interaction between CTA+ and H3PW12O40.

The N2 adsorption-desorption isotherm and the pore size distribution of Barrett-Joyner-Halenda (BJH) for (CTA)H2PW nanorod are given in Fig. 9. The sample exhibited a typical type IV isotherm and the relative pressure P/P0 had a very pronounced H1 type hysteresis loop at 0.4 to 0.8, indicating that the prepared nanorod had a mesoporous structure. The pore size distribution of Barrett-Joyner-Halenda (BJH) indicated that the pore size of the catalyst was mainly distributed at 2.5 to 3 nm. The surface area of (CTA)H2PW nanorod was 80.3 m2/g.

Fig. 9. (a) Nitrogen adsorption-desorption isotherm of (CTA)H2PW nanorod, and (b) its BJH pore size distribution

Effect of Different Catalysts on Cellulose Hydrolysis

Firstly, the different catalysts were subjected to screening tests for cellulose hydrolysis in aqueous medium (Fig. 10). It could be clearly seen that without any catalyst, cellulose was hardly converted under the reaction conditions employed. In a previous report, micellar (CTA)H2PW showed certain activity in cellulose hydrolysis, which presented 39.3% yield of glucose at 44.1% conversion in water at 170 °C for 8 h (Zhao 2011). It could be seen that the conversion of cellulose was improved to 64.8% in the presence of (CTA)H2PW nanorods, while the yield of glucose also was increased to 54.3%. This improvement was attributed to its wire-like morphology (Zhang et al. 2016c) and higher surface area. The conversion of cellulose and yield of glucose also depended on the composition of the catalysts as (CTA)H2PW > (CTA)2HPW > (CTA)3PW (Table 1), which was coherent with their acidic contents as (CTA)H2PW (1.41 mmol/g) > (CTA)2HPW (0.72 mmol/g) > (CTA)3PW (0.04 mmol/g).

Fig. 10. Different catalysts on cellulose hydrolysis in water. Reaction conditions as 0.1 g of cellulose, catalyst (0.09 mmol), water (7 ml), 160 °C for 8 h

Due to its higher efficiency (CTA)H2PW was used as the main catalyst in cellulose hydrolysis in water. The reaction conditions were optimized as usage of catalyst, volume of water, reaction temperature, reaction time, and different usage of cellulose (Fig. 11). It could be seen that the amount of catalyst increased from 0.05 mmol to 0.09 mmol (Fig. 11a). The conversion of cellulose and the yield of glucose were increased from 42.1% and 34.1% to 64.8% and 54.3%, respectively. This increase could be attributed to an increase in the active sites of the catalyst. However, when the amount of catalyst was increased to 0.1 mmol, the yield of glucose decreased to 47.2%, while conversion was not increased significantly. This decrease was attributable to the higher usage of catalyst, leading to further decomposition of glucose. Water was an essential reactant for the hydrolysis of cellulose. The yield of TRS and glucose increased with added volume of water from 4 mL to 7 mL (Fig. 11b). However, further increasing usage of water decreased the efficiency, which might be due to a reduction in acidic sites. Increasing the reaction time could increase the conversion rate, the yield of glucose increasing first and then decreasing, and the glucose yield reached the maximum at 8 h (Fig. 11c). As the temperature was increased from 140 to 160 °C, the conversion rate and glucose yield increased from 41.6% and 30.7% to 64.8% and 54.3%, respectively. The cellulose usages was increased from 0.1 g to 0.3 g with lowing yields, the usage of 0.1 g was chosen to be the optimum usage (Fig. 11d). In summary, the reaction conditions were optimized to be in 7 mL water at 160 °C for 8 h upon 0.09 mmol (CTA)H2PW nanorod and 0.1 g of cellulose to give 54.3% yield of glucose at 64.8% cellulose conversion (Fig. 11).

Fig. 11. The effect of (CTA)H2PW nanorod under different reaction conditions: (a) usage of catalyst, (b) the volume of water, (c) temperature verse reaction time, (d) usage of cellulose, and hydrolysis for different feedstocks upon (CTA)H2PW (e)

Other polysaccharides (sucrose, cellobiose, and starch), and lignocellulose (corn straw) were also achieved using (CTA)H2PW nanorod as a catalyst under various reaction conditions (Fig. 11e). The yields of glucose were obtained as high as 47.6%, 89.4%, and 91.2%, corresponding to sucrose (containing 49.6% yield of fructose), cellobiose, and starch at ~100% conversion under various reaction conditions. (CTA)H2PW nanorod showed potential for hydrolysis of polysaccharides to glucose. Beside polysaccharides, lignocellulose as corn straw had been used as feedstocks to produce value-added chemicals upon (CTA)H2PW nanorod. The carbohydrates and lignin contents in corn straw samples were determined by the National Renewable Energy Laboratory (NREL) standard analytical procedure (Yang et al. 2019). The corn straw raw material contained cellulose 38.6%, hemicellulose 22.7%, and lignin 16.8%. Upon addition of (CTA)H2PW nanorods, 19.8% yield of glucose and 8.3% yield of xylose at 35.4% conversion were obtained at 150 °C for 12 h. Up to this point, the best yield of glucose directly from Fenton-modified corn straw was achieved as 47.2% upon the ultralow enzyme combined with maleic acid and aluminum chloride under the reaction conditions as 45 °C for 48 h (Yang et al. 2019).

HMF is an important platform chemical, which has been produced directly from cellulose and lignocellulose in water or in water/MIBK biphase (Delidovich et al. 2016). So far, the best yields of HMF were 75.0% from cellulose and 27.6% from lignocellulose upon (HOCH2CH2N(CH3)3)H2PW12O40 catalysts in water/MIBK system under the reaction conditions as 8 h at 140 °C (Zhang 2016c). And (CTA)H2PW nanosphere showed some activity in hydrolysis of cellulose in water. In order to obtain 5-HMF, hydrolysis of cellulose was done in water/MIBK biphase in the presence of (CTA)H2PW nanorod (Fig. 12). It could be seen without any catalyst, there was only 3.8% conversion for cellulose, and no glucose or 5-HMF were found. In the presence of H3PW12O40, 91.2% conversion was obtained with yields for 5-HMF and glucose as 47.2 and 14.2%, indicating the essential of Brønsted acid sites in cellulose hydrolysis. The dependence of cellulose conversion and HMF yield on the Brønsted acidity further demonstrated this: (CTA)H2PW nanorod (Con. ~100%, glucose yield 17.6%, and 5-HMF yield 57.2%) > (CTA)2HPW nanorod (60.9%, 13.2%, and 29.1%) > (CTA)3PW nanorod (32.7%, 7.9%, and 14.3%). Compared to (CTA)H2PW nanosphere, the conversion and HMF yield increased from 72.9% to ~100% and 36.3% to 57.2%, respectively. This showed that the morphology of nanorod might improve the conversion as well as 5-HMF yield. Another contribution was that the adsorption of cellulose and protection of 5-HMF were improved by (CTA)H2PW nanorods (Fig. 13). (CTA)H2PW nanorods presented higher activity than (CTA)H2PW nanospheres did. The production of HMF from cellulose had been achieved in the H2O/MIBK system, including AlCl3·6H2O with 37.0% yield (Feng et al. 2018), HCl with 33.7% yield (Sweygers et al. 2018), ChCl/H-Y with 55.0% yield (Peela et al. 2019), NbPO-pH7 with 16.2% yield (Zhang et al. 2015), PBnNH3Cl with 40.0% yield (Cao et al. 2015), betaine and HCOOH with 26.0% yield (Delbecq et al. 2017), and ZnCl2 with 80.6% yield (Zhang et al. 2016). On the other hand, the production of glucose from cellulose in water was achieved to give less than 32.1% yield upon some solid catalysts as TiO2/SO42− (Zhang et al. 2019), Nb/ZrO2 (Gromov et al. 2017), and sulfonated carbon (Li et al. 2017). It could be concluded that (CTA)H2PW nanorod played a significant role in cellulose hydrolysis, which was more active than most catalysts.

Favorable results also can be attributed to the fact that this catalyst had a linear shape and a pore structure, enriched a large amount of cellulose, and then increased contact with the active site, so that the reaction proceeded efficiently.

Fig. 12. Different catalysts on cellulose hydrolysis in biphase. Reaction conditions: 0.1 g of cellulose, catalysts (0.09 mmol), water (0.5 mL), MIBK (5 mL), 140 °C, 11 h

Fig. 13. (a) IR spectra of the fresh cellulose, the unreacted cellulose, the adsorption test of cellulose by (CTA)H2PW nanosphere, and (CTA)H2PW nanorod, and (b) the yield of LA and stability of 5-HMF combined with (CTA)H2PW nanosphere and (CTA)H2PW nanorod at 140 °C

The reaction conditions were optimized as temperature, reaction time, usage of the catalyst, concentration of cellulose (Fig. 14), and molar ratio of water to MIBK (Table 2). The hydrolysis of cellulose to HMF was studied at various temperatures and reaction times, as well as the yield of HMF and cellulose conversion over time at 130 to 150 °C (Fig. 14a-c). It could be seen that the reaction time and temperature had a great influence on the hydrolysis. Below 140 °C, the yield of HMF and the conversion of cellulose increased with the increase of time. Over 140 °C, the yield of HMF and the selectivity decreased, which was also ascribed to the side-reactions being predominant under higher temperature conditions. The conversion of cellulose and the yield of HMF reached a maximum of ~100% and 57.2% at 11 h.

Fig. 14. The effect of (CTA)H2PW nanorod under different reaction conditions: (a) reaction temperature, (b) temperature verse reaction time, (c) reaction time, (d) usage of catalyst, (e) usage of cellulose, and (f) the activity of (CTA)H2PW nanorod on different substrates hydrolysis. Reaction conditions: sucrose (0.1 g), 120 °C, 5 h; cellobiose (0.1 g), 120 °C, 5 h; starch (0.1 g), 130 °C, 7 h

As the reaction time increased from 6 h to 11 h (Fig. 14b,c), such as at 140 °C, the conversion of cellulose and the yield of HMF increased from 70.1% and 33.3% to ~100% and 57.2%, respectively, prolonging the reaction time to 12 h lowed yield due to the presence of by-products. The effect of catalyst usage on cellulose hydrolysis was explored by varying the amount of catalyst from 0.06 mmol to 0.09 mmol (Fig. 14d). Unexpectedly, the more the catalyst, the higher the conversion of cellulose. The conversion of cellulose and the yield of HMF increased from 76.2% and 34.9% to ~100% and 57.2%, with the changing of the amount of (CTA)H2PW from 0.06 mmol to 0.09 mmol. Further increasing usage of catalyst to 0.1 mmol only led to decreasing trend of HMF yield, which was due to the dehydration of 5-HMF to levulinic acid. And the amount of cellulose had a certain effect on the hydrolysis of cellulose (Fig. 14e) at a range of 0.1 g to 0.3 g. The conversion of cellulose and the yield of HMF were able to reach a maxima of ~100% and 57.2% with 0.1 g cellulose. The volume ratio of H2O to MIBK at 1: 10 was more suitable for 5-HMF generation (Table 2). As a result, the optimal conditions were as follows: 0.1 g cellulose and 0.09 mmol of catalyst in H2O/MIBK = 1: 10 biphase at 140 °C for 11 h with ~100% conversion of cellulose and 57.2% yield of 5-HMF.

Table 2. The Effect of Organic Solvent on the Hydrolysis of Cellulose to Form 5- HMF

Beside cellulose, other polysaccharides including cellobiose, sucrose, and starch can be hydrolyzed in the presence of (CTA)H2PW nanorod (Fig. 14f) in H2O/MIBK biphase. It could be seen that almost all polysaccharides could be converted under different conditions. (CTA)H2PW nanorod showed 60.0% yield at 97.6% conversion for sucrose under 120 °C for 5 h, and 62.6% yield at ~100% conversion for cellobiose under 120 °C for 5 h, and 60.9% yield at 96.5% conversion for starch under 130 °C for 7 h.

Reusability of the Catalyst

Reusability was an important feature for heterogeneous catalysts while reducing costs. The way to assay life span of (CTA)H2PW nanorod in cellulose system was explored in the following way: after the first run of the reaction finished, the residues of unreacted cellulose and the catalyst were separated by centrifuge and dried on air. In order to ensure the quality of the cellulose was 100 mg, a certain amount of fresh cellulose was refilled for the next reaction. The activity of the catalyst was still present after ten repeated experiments (Fig. 15). In water system, it showed the higher yields of glucose and the cellulose conversion in ten consecutive cycles (Fig. 15a). The catalyst was recycled with a total leaching of only 9.5%. The cellulose conversion or the glucose yield was all reduced from 64.8% and 54.3% to 53.9% and 41.5%, respectively. In biphase system, the conversion of cellulose and the yield of 5-HMF were 90.5% and 46.8% after ten cycles (Fig. 15b), respectively. After ten reaction runs, the total amount of (CTA)H2PW nanorod leaching was only 9.1% of the initial usage. The stability of (CTA)H2PW nanorod after the ten reactions was determined by IR spectroscopy (Fig. 1e).

Fig. 15. The recycle tests of the catalyst under different reaction systems: (a) in water system, and (b) in biphase system.

Fig. 16. (a) SEM image of the catalyst (CTA)H2PW nanorod after the reaction, and (b) TEM image of (CTA)H2PW nanorod after the reaction

There was no difference for the IR spectrum compared to that of a fresh one, which showed that the catalyst still kept the original Keggin structure during the reaction. And the IR spectra of the fresh cellulose and unreacted cellulose, since the solid catalyst and cellulose were inseparable, the IR spectrum of unreacted cellulose was measured by background removal, which showed the same main characteristic peaks at 3276, 2890, 1650, 1432, and 1167 cm-1 (Fig. 13a). Indicating that the structure of unreacted cellulose did not change during the reaction under such conditions. From the XRD powder diffraction pattern (Fig. 3e) and 31P MAS NMR spectrum (Fig. 4b) of the catalyst after the reaction, it could be seen that the structure and composition of the catalyst was not destroyed after the reaction, indicating the excellent stability of the (CTA)H2PW nanorod during the reaction. The SEM image and the TEM image (Fig. 16a, Fig. 16b) of the catalyst (CTA)H2PW nanorod after the reaction showed no significant change in the morphology. These results clearly indicated that (CTA)H2PW nanorod had excellent stability and produced 5-HMF activity without reducing after several ten repeated uses.

CONCLUSIONS

  1. A series of novel mesoporous heteropolyacid nanorod catalysts (CTA)xH3-xPW (x = 1-3) was synthesized by surfactant encapsulation through simply controlling their initial concentration ratios of CTAB and H3PW12O40. Particularly, the (CTA)H2PW nanorod showed a high specific surface area, mesoporous structure, one-dimensional morphology, and strong Brønsted acidity.
  2. The catalytic effect was greatly improved compared to (CTA)H2PW with spherical morphology in conversion of cellulose to 5-HMF in the biphasic system with a yield of 57.2% and conversion of ~100% under the reaction conditions as 0.1 g cellulose and 0.09 mmol of catalyst in H2O/MIBK = 1: 10 biphase at 140 °C for 11 h. Meanwhile, 54.3% glucose could be obtained in hydrolysis of cellulose in water upon (CTA)H2PW nanorod. Such higher efficiency was contributed to its physical properties as linear structure with mesoporous, high specific surface area and Brønsted acidity.
  3. The (CTA)H2PW nanorod showed easy separation, higher stability, and longer duration compared to (CTA)H2PW nanosphere, which could be reused for at least ten times without significant loss of activity.
  4. (CTA)H2PW nanorod exhibited tolerance to feedstocks, which could catalyze lignocellulose of corn straw to glucose or xylose. This allowed (CTA)H2PW to be applied widely in conversion of cellulosic biomass into value-added chemical platform.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No. 51578119), the major projects of Jilin Provincial Science and Technology Department (20180414069GH).

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Article submitted: July 31, 2019; Peer review completed: October 19, 2019; Revised version received: November 4, 2019; Accepted: November 5, 2019; Published: November 13, 2019.

DOI: 10.15376/biores.15.1.240-264