Aqueous Phase Synthesis of 5-Hydroxymethylfurfural from Glucose over Large Pore Mesoporous Zirconium Phosphates: Effect of Calcination Temperature.

For a solid acid-catalyzed dehydration of biomass-derived carbohydrates into useful furan derivatives, a suitable porous solid acid catalyst having an optimum acidic density and its strength is required to avoid cascade reactions in biomass conversion processes. A large-pore mesoporous zirconium phosphate (m-ZrP) was prepared hydrothermally using P123 as a template in water solvent, which resulted in a higher pore diameter (>9 nm) having wormhole-like pore structures with balanced Lewis (L) to Brönsted (B) acid sites. The effects of calcination temperature (500-800 °C) on the textural, acidic/basic, and structural properties of the m-ZrP with its catalytic performance for glucose dehydration to 5-hydroxymethylfurfural (HMF) were investigated in a pure water media as a green and sustainable alternative solvent. The larger number of L and B acid sites and basic sites with their appropriate strengths were clearly related with a better catalytic performance in terms of glucose conversion and HMF yield. The strong L acid and basic sites in the m-ZrP efficiently promoted the glucose isomerization to fructose, which dehydrated exclusively on the weak B acid sites resulting in a maximum conversion of glucose (83.8%) and HMF yield (46.6%). The adjusted acidic and basic sites with large mesopore sizes make the m-ZrP yield a higher reaction rate (2.78 mmol gcat -1 h-1) and turnover frequency (11.68/h) for conversion of glucose to HMF, which showed higher catalytic activity than those of a small-pore m-ZrP and other mesoporous heterogeneous and homogeneous acid catalysts.

Introduction

Global warming due to an increase in CO2 emissions is creating disastrous phenomena, and alternative renewable chemicals and fuels have gained popularity due to their sustainability, low contributions to carbon cycles, and lower emissions of greenhouse gases.[1] The use of CO2-neutral biomass as a raw material represents an appealing solution with unprecedented impact on energy security. Because biomass-derived carbohydrates are abundant and widely distributed, the production of biofuels and fine chemicals from carbohydrate biomass is expected to be more sustainable than that of lipid biomass.[2−5] The main drawback of carbohydrate biomass as a feedstock for industrial application is the presence of excess oxygen (40–45 wt %) within the molecular structures.[6] There are three methods, fermentative conversion, hydrogenolysis, and dehydration, for removing oxygen from carbohydrate biomass, and the dehydration method is one of the most attractive and promising approaches because it directly converts the carbohydrate biomass into furanic compounds without reducing the carbon atom number of the feedstock.[4,7] Furan derivatives, namely 5-hydroxymethylfurfural (HMF) and furfural, have been identified as key biorefining intermediates for the production of biofuels, solvents, fine chemicals, polymers, and plastics.[8] HMF is a more multifunctional molecule than furfural because it consists of both aldehyde and hydroxyl functional groups in a furan ring structure. HMF is also a good starting material for the synthesis of precursors of dialdehydes, ethers, amino alcohols, pharmaceuticals, and macrocyclic compounds.[9] HMF and its derivatives listed as top 10 biomass-derived platform chemicals[10,11] are present in a variety of food sources, especially in heat-treated foods, which has a calculated consumption rate below 5–150 mg per person.[4] However, HMF is synthesized mainly by the dehydration of the expensive and limited fructose resource in nature that involves the loss of three water molecules. These problems drive industrial attention to utilizing the most abundant and cheapest carbohydrate of glucose (main building block of biomass) for the HMF synthesis.[12] As a result, the one-pot synthesis of HMF from glucose has the considerable attention of many research groups.

The one-pot synthesis of HMF from glucose is generally carried out by the copresence of basic or Lewis and Brönsted acid catalysts, wherein the glucose isomerizes into fructose on the base or Lewis acid catalysts followed by fructose dehydration into HMF preferably on the Brönsted acid catalysts. Therefore, the combination of the basic and/or Lewis with Brönsted acid sites in solid acid catalysts has been demonstrated successfully for the conversion of glucose into HMF.[13−17] For instance, one-pot HMF synthesis from glucose has been reported over the combination of hydrotalcites (solid base) with Amberlyst-15 (solid acid) in the presence of N,N-dimethylformamide (DMF);[13] however, HMF separation seems to be problematic during distillation steps due to the high boiling point (153 °C) of DMF. Moreover, a number of solid acid catalysts such as inorganic metal oxides, metal phosphates, functionalized silica, zeolites, and heteropoly acids have been studied to produce HMF from glucose in water, organic solvents, biphasic and mixed solvents, ionic liquids, and supercritical water.[18] However, the side reactions such as rehydration of HMF to levulinic acid and formation of humins (arising from the oligomerization/polymerization of glucose itself and HMF) were usually reported during acid-catalyzed glucose conversion, especially in aqueous media, due to unbalanced Lewis (L) and Brönsted (B) acid sites.[19] Therefore, it is necessary to develop a suitable solid acid catalyst having an optimal acid site density to avoid the cascade reactions. Among the solid acid catalysts, metal phosphates are well-known for their cooperation between L and B acid sites in carbohydrate biomass conversion.[19−21] Moreover, HMF selectivity is well explained as a function of B acid site concentration and its acidic strength.[19−22]

Among the family of metal phosphates, zirconium phosphate (ZrP) having an appropriate amount of B to L acid sites showed significant activity for dehydration of fructose, glucose, and xylose in water, organic, or biphasic solvents.[19−21,23−27] However, the glucose conversion into HMF in the presence of exclusively water solvent over ZrP has not been studied much until now. Because water as a green solvent included in an original biomass has the ability to solubilize both glucose and HMF, aqueous phase conversion of carbohydrates to chemicals is a promising technique.[15,28] Asghari et al.[23] found 72.3% conversion of glucose with 23.5% HMF yield in subcritical water at 240 °C. Weingarten et al.[21] compared ZrP and tin phosphate at 40% glucose conversion, where ZrP showed higher selectivity of HMF (37.5%) than tin phosphate (23.9%). Therefore, the selectivity of HMF, particularly in water solvent, needs significant further improvement.[19,29] The catalytic properties of solid acid catalysts including ZrP strongly depend on its textural, acidic, and structural properties, which in turn are significantly influenced by its synthesis protocol.[19−21,23,26,30] However, the surface area and porosity of the synthesized ZrP is not satisfactory, and thus organic amines,[24] cetyltrimethylammonium bromide,[25] and Pluronic F127[27] have been used as a template to enhance the textural properties of ZrP. Although the template-assisted ZrP exhibited an appreciable surface area (120–316 m2/g), small pore diameters in the range of 2–6 nm were reported.[24,25,27] However, the mesoporous materials, particularly transition metal oxides obtained using block copolymers (e.g., Pluronic P123), typically showed a larger pore diameter of 5–14 nm than those achieved with organic amines or ammonium templates.[31,32] Hence, to enhance the pore size, we have chosen P123 and water to synthesize mesoporous ZrP catalysts (m-ZrP).

In the present study, m-ZrP was prepared hydrothermally by using P123 as a soft template using water solvent to introduce larger mesoporous structures with high surface area for an easy diffusion pathway of bulky carbohydrates along the balanced acid sites. To the best of our knowledge, the synthesis of large-pore m-ZrP and its usage as a heterogeneous solid acid catalyst for glucose dehydration into HMF via the one-pot synthesis method using exclusively pure water solvent has not been studied so far. The effects of calcination temperature on the structural, textural, and acidic/basic properties of m-ZrP have not been investigated. The synthesized m-ZrP showed an excellent activity for the synthesis of HMF from glucose in an aqueous phase using a green and sustainable alternative water solvent under mild reaction conditions for an easy separation of final products. The efficiency of m-ZrP catalyst after its recycle tests with the comparison of other acid catalysts have also been addressed.

Results and Discussion

Xu et al.[27] studied the influence of phosphorus to zirconium (P/Zr) molar ratios for fructose dehydration and found that the catalyst with a P/Zr molar ratio of ∼1 performed with excellent activity in terms of both conversion and HMF yield. Therefore, we initially selected the P/Zr molar ratio of 1.0 to study the effects of calcination temperature (500–800 °C) on the textural, acidic/basic, structural, and catalytic properties of the m-ZrP catalysts.

Textural, Surface Acidic, and Basic Properties of m-ZrP

The pore size distribution curves and N2 adsorption–desorption isotherms of the m-ZrP catalysts calcined at different temperatures are shown in Figure . The pore size distributions of the m-ZrP catalysts, irrespective of the calcination temperature, were found to be broader up to 100 nm (Figure ) and revealed the larger average pore diameters in the range of 9.9–13.7 nm by increasing the calcination temperature from 500 to 800 °C (Table ). N2 adsorption–desorption isotherms of all the calcined m-ZrP catalysts were almost identical (inset in Figure ), which corresponds to type IV for the characteristics of mesoporous materials. Both BET surface area and pore volume were found to decrease with an increase in calcination temperature (Table ). It is worth noting that the m-ZrP catalysts have improved the pore size (>9 nm) as compared to the previously reported m-ZrP, which has a lower average pore size of 2–6 nm.[24,25,27] This result clearly revealed the significance of usage of the structure-directing agent of P123 for an enhanced mesoporosity of the m-ZrP, which seems to largely help for easy diffusion with an improved selectivity of HMF.[7]

Figure 1

Pore size distributions and N2 adsorption–desorption isotherms (inset) of the m-ZrP catalysts calcined at different temperatures of 80–800 °C.

Table 1

Physicochemical Properties and Catalytic Activity of m-ZrP Catalysts Calcined at Different Temperatures

				acid sites (mmol/g)					P/Zr ratio
catalyst	surface area [m2/g]	pore volume [cm3/g]	av pore diameter [nm]	totala	Brönsted (B)b	Lewis (L)b	B/L ratio	total basic sites (mmol/g)a	XPSc	EDSd	XGlu. (%)e	YHMF (%)	YFru. (%)f
m-ZrP-80	322.3	0.61	9.1						0.92		90.0	26.1	4.0
m-ZrP-500	213.0	0.49	9.9	1.25	0.032	0.116	0.27	0.35	0.95	1.18	84.2	40.3	4.5
m-ZrP-600	197.0	0.48	10.9	1.06	0.034 (0.079)g	0.115 (0.03)g	0.29 (2.6)g	0.29	0.96 (1.1)g	1.15	81.5	43.2	5.0
m-ZrP-700	143.8	0.43	11.8	0.93	0.025	0.111	0.22	0.24	0.92	1.13	78.7	39.6	5.4
m-ZrP-800	114.5	0.37	13.7	0.44	0.008	0.063	0.13	0.13	0.91	1.22	70.0	32.7	7.2

Total acid and basic sites were calculated from NH3-TPD and CO2-TPD, respectively.

Brönsted and Lewis acid sites at 150 °C were calculated by a molar extinction coefficient method [33].

Surface P/Zr ratios were calculated by using the area of P2p divided by that of Zr3d5/2.

P/Zr ratio was calculated from SEM-EDS analysis by using the values of atomic% on the P K/Zr L.

Reaction conditions: glucose = 0.2 g; H2O = 40 g; catalyst = 0.1 g; temperature = 155 °C; reaction duration = 6 h.

Levulinic, formic, and lactic acids were also observed.

The values were for the used catalyst after 3 runs.

Because of the amphoteric nature of zirconia-based catalysts,[15] the amounts and strengths of acid and basic sites on the m-ZrP catalysts were determined by NH3- and CO2-TPD. Figure (a) shows a broad desorption profile of NH3 ranging from 50 to 540 °C for all of the m-ZrP catalysts. It indicates that a higher heat of adsorption of ammonia over L acid sites compared to that over B acid sites due to the copresence of those acid sites.[19] The solid acid strengths were not changed much while increasing the calcination temperatures of the m-ZrP catalysts by revealing the NH3 desorption at weak (∼130 °C) and moderate to strong (∼300–540 °C) strengths of acid sites. The m-ZrP catalysts also displayed medium and strong basic strengths appearing at the separate desorption temperatures of ∼300–380 and ∼620 °C as shown in Figure S1 from the CO2-TPD analysis; however, significantly lower basic sites than the corresponding total acid sites were clearly observed as summarized in Table . The concentrations of total acid and basic sites, which correlate with the surface area of the m-ZrP catalysts, were steadily decreased with increasing calcination temperatures from 1.25 mmol/g on m-ZrP-500 to 0.44 mmol/g on m-ZrP-800. It is well-known that both L and B acid sites are required for the conversion of glucose to HMF, and thus, the natures of the acid sites were distinguished by FT-IR spectroscopy of the adsorbed pyridine (Py-IR). The Py-IR spectra of the m-ZrP catalysts exhibited the characteristic absorption bands at 1540 cm–1 for the pyridine molecules coordinated to B acid sites and bands at 1448 cm–1 for the covalently bonded pyridine molecules with L acid sites (Figure b).[20] The representative catalytically active B and L acid sites on the m-ZrP catalyst are also shown in the inset of Figure b. The coexistence of another strong absorption band at ∼1490 cm–1 can be generally assigned to the combination of B and L acid sites.[33] By increasing the calcination temperatures (mainly from 600 to 800 °C) on the m-ZrP catalysts, the concentrations of both B and L acid sites were found to decrease (Table ), although the decrease of B acid sites was more than that of L acid sites, which showed that L acid sites were stronger than B acid sites. The strengths of L and B acid sites were further confirmed by varying the pyridine desorption temperatures from 150 to 350 °C as shown in Figure S2. The L acid sites were observed to be strong enough as compared to B acid sites because the former acid sites, regardless of calcination temperature, were present even after heating at 350 °C. The B/L ratios were decreased continuously with increasing pyridine desorption temperatures from 150 to 350 °C, although m-ZrP-500 and m-ZrP-600 catalysts retained their maximum acid sites (∼0.15) up to 350 °C in comparison with m-ZrP-700 and m-ZrP-800 catalysts as shown in Figure . It suggests the stable preservation of the active acid sites on m-ZrP-500 and m-ZrP-600, which are responsible for enhanced catalytic activity as well. The stronger L acid sites in the m-ZrP catalysts could promote glucose isomerization to fructose, which can be dehydrated exclusively on the weak B acid sites with an optimal acid strength to maximize HMF yield.[7] The B and L acid sites calculated from the molar extinction coefficient method for bands at 1540 and 1448 cm–1, respectively, are summarized in Table . The maximum B (∼0.034 mmol/g) and L (∼0.116 mmol/g) acid sites were observed for m-ZrP-500 and m-ZrP-600, which were found to be slightly smaller than the total acidic sites measured by NH3-TPD due to the different proton affinity and basicity of ammonia and pyridine molecules.[33] A very lower amount of B acid sites in m-ZrP-800 (0.008 mmol/g) may be due to the surface hydroxyl groups condensed (caused P–O–P bonds) at higher temperature.[23] The results also showed that the amounts of L acid sites, irrespective of calcination temperature, were significantly higher (∼3–4-times) than those of B acid sites. The B/L ratios on the m-ZrP catalysts also decreased in the following order of calcination temperatures, 500 ≈ 600 > 700 > 800, as summarized in Table .

Figure 2

(a) NH3-TPD profiles and (b) pyridine-adsorbed FT-IR spectra with illustrations of Brönsted (B) and Lewis (L) acid sites (inset) of the m-ZrP catalysts calcined at 500–800 °C.

Figure 3

Comparison of Brönsted (B) and Lewis (L) acid sites and Brönsted (B) to Lewis (L) acid site ratios with respect to pyridine desorption temperatures on the m-ZrP catalysts calcined at 500–800 °C.

Morphology and Surface Structures of m-ZrP

TEM microscopic images of the calcined m-ZrP catalysts showing spherical morphology with aggregates of particles (Figure S3). The particles were found to have sizes in the range of ∼10–25 nm. The high-resolution TEM (HR-TEM) images displayed wormhole-like pore structures (Figure a), which strongly suggests their highly amorphous natures.[22] Similar wormhole-like pore structures were also observed by CTAB-assisted synthesis of the m-ZrP.[34] The HR-TEM results of all the calcined m-ZrP catalysts at 500–800 °C showed almost similar uniform morphologies and porous structures. As expected, the PXRD patterns displayed no distinct characteristic peaks of the Zr and P crystallites by only showing two broad peaks in the 2θ ranges of 15–40° and 40–70° (Figure S4). The crystalline phases for either ZrP and/or zirconia are not observed in the PXRD pattern of the m-ZrP catalyst even calcined at 800 °C, which supports the amorphous nature of the framework walls of the m-ZrP.

Figure 4

(a) HR-TEM images, (b) FT-Raman (i) and FT-IR (ii) spectra, and (c) XPS spectra of the m-ZrP catalysts calcined at different temperatures.

The surface phosphate species of the m-ZrP catalysts was differentiated by using FT-Raman spectroscopy. It is generally accepted that the bending and stretching vibrations of phosphate groups occur at 400–700 and 1000–1200 cm–1, respectively.[35] As shown in Figure b(i), all calcined m-ZrP catalysts displayed one intense band assigned to the stretching PO2 vibration mode at around 1040 cm–1 while merging other phosphate bands in the regions of 850–1150 cm–1 and another broad band at ∼440 cm–1 for the bending vibration of PO43–.[36] The surface phosphate species seems to be insignificantly changed when the calcination temperatures of the m-ZrP catalysts increased from 500 to 800 °C. However, the more intense peak at 1040 cm–1 on m-ZrP-600 suggests the well-developed zirconium phosphate phases. The calculated PO2/PO43– ratios were on the order of 8.3 (m-ZrP-600) > 7.2 (m-ZrP-500) > 6.9 (m-ZrP-80) > 6.8 (m-ZrP-700) > 6.4 (m-ZrP-800). The FT-Raman results were also confirmed by the FT-IR results. The FT-IR spectra exhibited the broad bands centered at ∼1070, 756, and 537 cm–1 (Figure b(ii)). The band at ∼1070 cm–1 is attributed to the stretching vibration of the P–O–Zr bond.[37,38] After the calcinations of m-ZrP from 500 to 800 °C, the band in the P–O stretching region (1000–1200 cm–1) shifted toward higher wave numbers by ∼50 cm–1, which indicate that the P–O bonds in the tetrahedron sites become more covalent states. The presence of a weak band at ∼756 cm–1 corresponds to the asymmetric vibration of bridging P–O–P (polyphosphate) bonds.[34] The shoulder at 633 and band at 537 cm–1 could be attributed to deformation modes of P–O–H and PO4.[39] The band shifts at 1070 cm–1, and a relatively larger band intensity at 537 cm–1 can be possibly attributed to the generation of surface defect sites related with the acidic characters, especially on m-ZrP-500 and m-ZrP-600, which are in line with the results of FT-Raman analysis as well.

High-resolution XPS spectra of the m-ZrP catalysts showed the characteristic peaks for P2p, Zr3d5/2, and O1s in their corresponding binding energies of ∼134, 183, and 532 eV, respectively. As shown in Figure c, no appreciable changes of surface properties of zirconium and phosphorus species were observed when increasing the calcination temperatures of the m-ZrP catalysts. The presence of the P2p peak at 134 eV indicates that phosphorus existed in a P5+ oxidation state on the m-ZrP catalysts.[40] The Zr3d consist of two peaks; one intense peak at 183.4 eV for 3d5/2 and another peak at 185.8 eV for 3d3/2, which were characteristic of Zr4+.[41] The O1s peak might be fitted by three components situated at binding energies of ∼529.8, 531.8 and ∼534 eV.[40] The intense O1s peak at 531.8 eV, irrespective of calcination temperature, corresponds to oxygen combined with phosphorus; the broadness of the O1s peak at a lower binding energy (∼529.8 eV) showed oxygen atoms of the zirconia framework, and a higher binding energy (∼534 eV) is for oxygen atoms attributed to carbon.[40] The higher intensity of the O1s peak at 531.8 eV in all m-ZrP catalysts indicates that the O–P species is dominant as compared to that of the O–Zr species. Interestingly, the relatively larger peak intensity of 529.8 eV on m-ZrP-500 and m-ZrP-600 suggests the well-structured zirconium phosphate phases compared to that of m-ZrP-800, which were also supported by the results of FT-Raman and FT-IR results. There were no significant differences in the surface atomic compositions of P/Zr ratios before (80 °C) and after calcination (500–800 °C). Comparisons of XPS surface data with SEM-EDS could allow for obtaining insights about the distribution of Zr and P elements. As shown in Figure S5, the EDS pattern at different positions confirmed the uniform distribution of Zr and P elements by overlapping P peak with the Zr peak for all of the calcined catalysts.[37] In addition, the obtained surface molar ratios of P/Zr are near 1 (Table ), which fairly matched with the theoretical molar ratio predetermined in the preparation procedures. We believe that those well-structured amorphous zirconium phosphate phases are responsible for the larger amounts of total acid sites with higher B/L ratios on m-ZrP-500 and m-ZrP-600.

Catalytic Activity: Effect of Calcination Temperature of m-ZrP

For the influence of calcination temperature on the activity of the m-ZrP catalysts to be studied, all catalysts calcined in the range of 80–800 °C were evaluated for the dehydration of glucose with water solvent, and the results are summarized in Table . The conversion of glucose decreased with increasing calcination temperatures. As-synthesized m-ZrP-80 displayed a higher glucose conversion of 90%, which decreased to 70% with increasing calcination temperature up to 800 °C under the reaction conditions with 0.1 g of catalyst at 155 °C for 6 h. In contrast, an increased yield of HMF was observed (43.2%) until the calcination temperature of 600 °C (m-ZrP-600). Upon further increasing the calcination temperature up to 800 °C, a decrease in HMF yield was observed (32.7%) on m-ZrP-800 by showing a maximum HMF yield on m-ZrP-600. On the other hand, the yield for fructose showed continuously increasing trends from 4.0 to 7.2% with an increase in the calcination temperature.

Regardless of calcination temperature, trace amounts of undesirable byproducts, namely levulinic, formic, and lactic acids and humins, were also observed. The humins are both water-soluble and -insoluble (both have been considered as humic substances). The color of the final reaction mixtures was found to be pale yellow (Table S1), which indicates that a significant amount of black humic substances was not formed under the reaction conditions employed.[42] The quantification of these unwanted byproducts and carbon mass balance was not fully achieved in this study. We assumed that the yield for the organic acids may be significantly lower as a very dilute concentration of glucose reactant (0.5 wt %) used in this study, and thus, a lower amount of HMF available in the reaction mixture can be used for further rehydration reactions.[19] As per the HPLC peak area, however, the amounts of the organic acids formed varied significantly according to calcination temperature. For instance, the m-ZrP-80 resulted in a considerably higher amount of lactic acid formation than that of calcined m-ZrP catalysts. It is reported that a preferential cascade reaction of carbohydrates in aqueous media over L acid sites and the presence of P123 surfactant may result in lactic acid.[43,44] On the other hand, the m-ZrP-500 and m-ZrP-600 displayed an equal amount of levulinic and formic acids, whereas the m-ZrP-700 and m-ZrP-800 exhibited a higher amount of formic acid formation than that of levulinic acid. The formation of organic acids, which was significantly varied with calcination temperature, seems to depend on the structural and textural properties of the m-ZrP catalysts besides the nature of acidic/basic sites. The levulinic and formic acids may be produced from HMF rehydration over B acid sites.[22] For the cascade reaction to be confirmed, a comparative experiment was carried out for the fructose dehydration over homogeneous H2SO4 catalyst under the same reaction conditions. We observed much higher yield for levulinic (44%) and formic (13%) acids with very low HMF yield (<1%), which confirms that the weaker B acid sites seem to be suitable for a selective formation of HMF.

Many literature studies reported that glucose can also be directly dehydrated to HMF over B acids alone. However, the cascade reaction (i.e., isomerization followed by subsequent dehydration) has a lower activation barrier, and thus, it has been known to be faster than the direct dehydration.[7] Therefore, we believed that the m-ZrP followed the cascade reaction, whereby L acid and/or basic sites promoted the isomerization of glucose to fructose, which can be dehydrated to HMF on the B acid sites. To understand the structure–activity relationships, we have compared the properties of the m-ZrP catalysts calcined at 500–800 °C. The larger variations in all structural, textural, and acidic/basic properties as well as catalytic activity were found by indicating the significant influences of calcination temperature (Table ). The correlations between acidic characters in terms of L and B acid site concentrations, B/L ratios, and activities on all the calcined m-ZrP catalysts revealed that the glucose was isomerized into fructose and is mainly catalyzed by the strong L acid sites, whereas dehydration of fructose to HMF may be catalyzed by both B and L acid sites but mostly weak B acid sites (Figure a). Besides L acid sites, glucose conversion was correlated with the amount of total acidic and basic sites as well (Table ). Furthermore, the m-ZrP catalysts calcined in the range of 500–700 °C exhibited a higher HMF yield (40–43.2%), which was fairly correlated to both B acid site concentrations and B/L ratios (Figure b). Although the B acid sites of m-ZrP-800 were found to be much lower, it also exhibited an appreciable yield for HMF (32.7%), which may be attributed to the possibility of regeneration of some hydroxyl groups via the hydrolysis of condensed P–O–P bonds in water media.[23] These results clearly indicate a significant influence of the calcination temperatures of the surface properties of the m-ZrP catalysts, and it can alter the acid site concentrations along with total acidity besides the textural and structural properties of the m-ZrP catalysts. Though among all the m-ZrP catalysts the HMF yield was found to be higher on m-ZrP-600, there was not much difference in the conversion of glucose over m-ZrP-500 and m-ZrP-600. Therefore, m-ZrP-600 was selected for further studies to optimize the amount of water, reaction time, catalyst concentration, and temperature to achieve the maximum yield for HMF as well as glucose conversion.

Figure 5

Correlation of (a) glucose conversion with Lewis (L) acid sites and HMF yield with Brönsted (B) acid sites and (b) HMF yield with B/L ratios over the m-ZrP catalysts calcined in the range of 500–800 °C.

Catalytic Activity: Amount of Solvent and Catalyst, Reaction Time, and Temperature

The dehydration reaction of glucose was further studied with different water amounts of 20, 30, and 40 g over 0.1 g of m-ZrP-600 catalyst at 155 °C for 2 h. The glucose conversion was steadily increased with an increase in the amount of water (Figure a). At 20 g of water, the conversion of glucose was 47.2%, which was enhanced up to 60.7% by increasing the water to 40 g. However, HMF yield was slightly decreased upon increasing the amount of water that may be due to solvation effects.[20] The reported studies revealed that exposure of the solid acid catalyst to a polar solvent such as water can possibly change the intrinsic nature of the acid surface due to solvation effects, although L acid sites in aqueous media have still been well-known active sites for isomerization of glucose to fructose.[20] The conversion of glucose was significantly higher at 40 g of water, and yet the HMF yield was relatively comparable with the amount of water studied; thus, 40 g of water was chosen as an optimum amount for the rest of the studies. It is important to mention that organic solvents like DMSO and biphasic systems, namely water/methyl isobutyl ketone (MIBK), showed a promising HMF yield (>50%);[18] however, complicated separation of HMF from organic solvents and formation of aldolic products, such as aliphatic ketone derivatives, from MIBK over acid-catalyzed condensation reactions were reported as difficult and energy-consuming processes.[45] For the disadvantages associated with organic solvents and HMF-extracting solvents (i.e., MIBK) to be overcome, pure water can be used as a green and sustainable alternative solvent for HMF production, although it showed a relatively lower HMF yield. As shown in Figure b, the reaction time variation study showed an exponential increase in the conversion of glucose from 25.5% for 1 h to 80.2% for 6 h. Similarly, a substantial enhancement in the yield of HMF was observed by increasing the reaction time from 25.5% for 1 h to 45.1% for 5 h. However, because no further improvement in the yield of HMF was observed after 6 h, all further experiments were performed for 6 h.

Figure 6

Conversion of glucose and yield of HMF at various (a) water amounts, (b) reaction times, (c) catalyst amounts, and (d) reaction temperatures using m-ZrP-600. Reaction conditions: glucose = 0.2 g. (a) time = 2 h; (a, b) m-ZrP-600 catalyst = 0.1 g; (b–d) H2O = 40 g; (a–c) temperature = 155 °C; (c, d) reaction duration = 6 h; (d) m-ZrP-600 catalyst = 0.125 g.

By increasing the catalyst amounts from 0.075 to 0.15 g, the conversion of glucose was increased from 70.8 to 87.8% (Figure c). Similarly, the HMF yield was also increased from 41.5 to 46.1% by increasing the catalyst amounts; however, by further increasing the catalyst amount up to 0.15 g, the yield was slightly decreased to 45.3%. Therefore, further studies have been carried out with 0.125 g of catalyst. A higher amount of the catalyst increased the availability of acid sites, which favor the accessibility of a higher number of glucose molecules to the acid sites and thus enhanced the conversion and yield as well. However, at a much larger amount of catalyst, some cascade reactions can possibly occur by further reaction of HMF. The effects of reaction temperatures were studied in the temperature range of 145–165 °C as shown in Figure d. The progress of the glucose consumption and HMF formation was monitored by HPLC chromatograms that was evidenced by a successive significant decrease of glucose concentration (at ∼8.6 min) with a moderate decrease in the peak intensity (at ∼35.9 min) assigned to HMF as shown in Figure S6. The results showed a significant increase in glucose conversion from 59 to 92.6% with a slight decrease in HMF yield (49.2–42.4%) by increasing the temperature from 145 to 165 °C. The yield for HMF was relatively higher (49.2%) at 145 °C, whereas the conversion of glucose was found to be lower (59%) compared to either results at 155 or 165 °C. Nevertheless, at 155 °C, both glucose conversion (82.6%) and HMF yield (46.1%) were fairly comparable to those obtained at 165 °C (92.6%) and 145 °C (49.2%), respectively.

Catalytic Activity: Effect of Type of Carbohydrates and P/Zr Molar Ratios

Besides the glucose carbohydrate as a reactant, the production of HMF was also compared with fructose and sucrose carbohydrates. The results showed the successive increase in the conversion of fructose to sucrose (95–100%) under identical reaction conditions (Figure ). The increase in fructose conversion was mainly due to a higher reactivity of ketohexoses (i.e., fructose) than that of aldohexoses (i.e., glucose). The complete conversion of sucrose, a disaccharide of glucose and fructose units, can be explained by the easy hydrolysis activity of sucrose to glucose and fructose. These results are consistent with those of previous reports in the literature.[19,22,32] However, it is interesting to note that the HMF yield from glucose (46.1%) is ∼2-times higher than that of the obtained yield from fructose (23.5%). On the other hand, the HMF yield from sucrose was intermediate (32%) to those of the glucose and fructose reactants (Figure ). The higher yield from both glucose and sucrose carbohydrates can be attributed to the presence of the balanced B to L acid site ratio of ∼0.3 on m-ZrP-600, which can favorably catalyze glucose over fructose as previously reported by other researchers.[19,22] Moreover, as we discussed earlier, the weaker B acid sites have been known to be more selective toward HMF formation from fructose, and the stronger L acid sites promote the degradation and polymerization of fructose and/or HMF to unwanted byproducts, which resulted in a very low yield of HMF from fructose. Under the same reaction parameters, a comparative experiment of fructose dehydration over Lewis acid (ZrOCl2) confirmed the formation of black humic substances with lactic and levulinic acids besides a very low yield for HMF (∼4%). Glucose conversion to HMF involves an additional isomerization step to fructose, which was efficiently catalyzed by L acid and basic sites. Sucrose can be easily hydrolyzed into glucose and fructose units, both of which are simultaneously dehydrated to HMF, and thus, HMF yield from sucrose is intermediate to those of glucose and fructose. Our results clearly envisaged that glucose could be converted to HMF as efficiently as fructose if a suitable amount of B and L acid sites are present in the solid acid catalysts. Thus, from an industrial point of view, glucose as a feedstock, owing to its availability and main building block of biomass, is more economical than fructose for the production of HMF and other platform chemicals.

Figure 7

Conversion of carbohydrates and their corresponding yields for HMF. Reaction conditions: carbohydrate = 0.2 g; H2O = 40 g; m-ZrP-600 catalyst = 0.125 g; temperature = 155 °C; time = 6 h.

The effect of P to Zr molar ratio on the catalytic activity of m-ZrP has been studied by varying the P/Zr ratios to 0.75, 1.0 (previous reference m-ZrP catalyst), and 1.25. As the L and B acid site concentrations were decreased with an increase in P content, both conversion of glucose and HMF yield decreased, which indicating that the acid sites of the m-ZrP could be controlled by varying the P contents, although the B/L ratio lies in a similar range (Table ). The decrease of B acid sites possibly due to the formation of a polyphosphate (P–O–P) group by sacrificing P–OH groups and Lewis acid sites may be due to the domination of the O–P species as compared to that of the O–Zr species.[33,34] The phosphorus-rich m-ZrP catalyst (P/Zr = 1.25) having lower L (0.087 mmol/g) and B (0.027 mmol/g) acid sites showed a lower glucose conversion of 73.3% and HMF yield of 40%, which increased to 83.8 and 46.6%, respectively, with a decrease of P content (P/Zr = 0.75). The reference catalyst (P/Zr = 1) also had comparable textural and acidic properties along with catalytic activity (Table ) to those of the catalyst having a P/Zr = 0.75. Figure S7 shows the N2 adsorption–desorption isotherms (a), pore size distribution profiles (b), NH3-TPD profiles (c), and pyridine-adsorbed FT-IR spectra (d) of the varied P/Zr molar ratios after being calcined at 600 °C. The isotherms of all three catalysts were observed to be of type IV (Figure S7a), typical for mesoporous materials. The BET surface areas of the m-ZrP catalysts were observed to decrease with an increase of P contents, whereas the reverse trend was found for both pore volume and pore diameter as well (Table ). The pore size distribution of the m-ZrP catalysts (Figure S7b) showed the presence of wider pore distributions with a larger pore diameter in the range of 9.4–11.6 nm (Table ).

Table 2

Physicochemical Properties and Catalytic Activity of the m-ZrP (Calcined at 600 °C) with Different P/Zr Molar Ratios

P/Zr molar ratio	0.75	1.0	1.25
BET surface area (m2/g)	211.6	197.0	192.6
pore volume (cm3/g)	0.47	0.48	0.60
av pore diameter (nm)	9.4	10.9	11.6
total acid sites (mmol/g)a	1.00	1.10	1.39
Brönsted acid sites (mmol/g)b	0.039	0.034	0.027
Lewis acid sites (mmol/g)b	0.118	0.115	0.087
B/L ratio	0.33	0.29	0.31
glucose conversion (%)c	83.8	82.6	73.3
HMF yield (%)	46.6	46.1	40.0

Total acid sites were calculated from NH3-TPD.

Brönsted (B) and Lewis (L) acid sites at 150 °C were calculated by the molar extinction coefficient method.

Reaction conditions: glucose = 0.2 g; H2O = 40 g; catalyst = 0.125 g; temperature = 155 °C; reaction duration = 6 h.

Catalytic Activity: Reusability of m-ZrP-600

The reusability of m-ZrP-600 was examined by carrying out consecutive recycle reaction runs. After the reaction, the catalyst was separated from the reaction mixture, washed 3 times with deionized water followed by with acetone (∼40 mL in each time) to remove the adhered reactant and product molecules from the catalyst surface and thermally activated at 500 °C before the next reaction cycle with fresh reactants. The small quantity of formed humic substances was deposited on the catalyst surfaces after the recycle reaction, which tended to become a dark brown color (inset of Figure ). As shown in the HPLC chromatogram (Figure S8), the catalyst exhibited a similar yield for HMF, although the glucose conversion showed a slight decrease monotonously from 82.6 to 74.8% until three recycle runs (Figure ). The loss of activity may be due to the changes of surface property of the m-ZrP, and thus pyridine-IR and XPS were further analyzed. Pyridine-IR spectra of the used catalyst (after 3 runs) showed the presence of both B and L acid sites up to 250 °C (Figure S9a); however, an increase in B acid sites from 0.034 (fresh) to 0.079 mmol/g (used) and a significant decrease in L acid sites from 0.115 (fresh) to 0.03 mmol/g (used) were observed (Table ). This observation suggests that the active L acid sites converted to B acid sites in water solvent during the successive runs. XPS spectra of fresh and used (after 3 runs) m-ZrP-600 catalysts (Figure S9b) clearly showed the peaks for P2p, Zr3d, and O1s; although the corresponding peaks of used catalyst were observed at slightly lower binding energy with broadness relative to that of fresh m-ZrP-600 catalyst.

Figure 8

Reusability study for m-ZrP-600. Reaction conditions: glucose concentration = 0.5 wt %; catalyst = 62.5 wt %; temperature = 155 °C; time = 6 h.

Catalytic Activity: Comparison of Catalytic Performance with Other Catalysts

Some heterogeneous mesoporous metal oxides such as Al2O3, ZrO2, and W/ZrO2 and representative homogeneous catalysts such as H2SO4, H3PO4, and ZrOCl2 have been further tested for the dehydration of glucose to HMF in water media with a view to compare the activity of m-ZrP-600 under optimized reaction conditions. The dehydration of glucose over a series of selected homogeneous catalysts showed significant variations in glucose conversion (Table ). For instance, H2SO4 and H3PO4, which are strong B acids, displayed lower conversions of glucose (9.3–17.7%) as well as HMF yields (3.5–5.4%). On the other hand, the Lewis acid metal salt of ZrOCl2 showed the highest glucose conversion of 98.5% with a very low HMF yield (2.7%). The reaction mixtures obtained were found to be dark-brown with black colors (Table S1), which indicates that side cascade reactions on the L acid sites were more favorable. These results clearly confirm that the glucose was converted efficiently by L acid sites, whereas the stronger B acids (H2SO4) are not efficient for the glucose conversion. The m-ZrO2, an amphoteric metal oxide, showed a lower acidity (0.55 mmol/g) and thus resulted in lower activity (36.2%) with HMF yield (12.4%), whereas m-Al2O3, Lewis acid, having 0.29 mmol/g of total acid sites[46] could slightly improve both the glucose conversion (41.6%) and HMF yield (15.6%) (Table ). The heterogeneous-catalyzed glucose dehydration solution appeared to be a pale yellow to pale brown color (Table S1). To confirm the effects of pore size of the m-ZrP, we performed the glucose dehydration reaction over the zirconium phosphate having two different pore sizes at which the m-ZrP with a large pore size of 10.9 nm exhibited the highest glucose conversion (82.6%) with HMF yield (46.1%) than that of a small pore of 3.9 nm (72.1% conversion and 31.2% yield). The activity variations of the small (Figure S10a) and large (Figure S10b) pore m-ZrP clearly distinguished the effects of mesoporosity on the diffusions of reactants and products. The superior performance of the large pore m-ZrP clearly indicated that the solid acids having a large pore size with suitable amounts of B and L acid sites facilitated the easy diffusion of glucose and HMF molecules under the present reaction conditions. A similar observation was also reported by Dutta et al.[37] The pore size distribution of the m-ZrP catalyst with small pores showed the presence of a narrow pore size distribution with intrinsic mesoporous structures (Figure S11). Although a very low HMF yield of 3.9%, with lactic acid beside unconverted fructose (∼16%), was observed on m-W/ZrO2, the L acid sites of tungstated zirconia catalyst were reported to be highly stable, being not diminished in aqueous media;[47] thus, comparable glucose conversion was obtained. Representative HPLC chromatograms of glucose dehydration to HMF on the different heterogeneous mesoporous catalysts are shown in Figure S12. A plausible mechanism for HMF formation via fructose from isomerization of glucose is demonstrated in Scheme . As shown in the inset of Figure b, the m-ZrP catalysts have both Lewis (Zr+) and Brönsted (P–OH) acid sites; therefore, we believe that both acid sites may take part in the reaction. The strong L acid sites of m-ZrP have the potential to initiate glucose isomerization via an intramolecular hydride shift[15] and then stepwise dehydration of fructose to HMF on the weak B acid sites as depicted in Scheme . It is also important to mention that the basic sites of the m-ZrP catalysts may also take part in the isomerization of glucose into fructose via the proton transfer mechanism, which was clearly described by Osatiashtiani et al.[15]

Table 3

Conversion of Glucose and Yield of HMF with Various Homogeneous and Heterogeneous Acid Catalystsa

catalyst	nature of acid sites	Sg (m2/g)	Dp (nm)	total acid sites (mmol/g)b	XGlu. (%)	YHMF (%)
blank					8.8	7.4
ZrOCl2c	Lewis				98.5	2.7
H2SO4c	Brönsted				17.7	5.4
H3PO4	Brönsted				9.3	3.5
m-ZrO2	amphoteric	22.4	3.3	0.55	36.2	12.4
m-Al2O3	Lewis	253.0d	6.0d	0.29d	41.6	15.6
m-W/ZrO2e	Lewis + Brönsted	52.3	5.7	0.61	83.4	3.9
m-ZrPf	Lewis + Brönsted	125.7	3.9	1.02	72.1	31.2
m-ZrP	Lewis + Brönsted	197.0	10.9	1.10	82.6	46.1

Reaction conditions: glucose = 0.2 g; H2O = 40 g; catalyst = 0.125 g; temperature = 155 °C; time = 6 h.

Total acidic sites were calculated from the results of NH3-TPD.

Unidentified products also exist.

Data were obtained from our previously reported work,[46] where Sg stands for the BET surface area and Dp for the average pore diameter.

Zirconia was loaded with 10 wt % W.

Small pore m-ZrP was used for a comparative study with m-ZrP.

Scheme 1

Plausible Reaction Mechanisms for One-Pot Synthesis of HMF from Glucose in Aqueous Media via Isomerization of Glucose to Fructose over Strong Lewis Acid Sites (Zr+) and Stepwise Dehydration of Fructose to HMF on Weak Brönsted Acid Sites (P–OH) of the m-ZrP Catalyst

The kinetic profile of the dehydration of glucose (Figure S13) showed a continuous decrease in the concentration of glucose with reaction time. The glucose concentration [glucose, mmol] was calculated by subtracting the consumed concentration from the initial concentration of glucose substrate. The initial reaction rate at 45 min (mmol gcat–1 h–1) showed the activity of the m-ZrP calcined at different temperatures in the order of 3.26 (m-ZrP-500) > 2.78 (m-ZrP-600) > 2.16 (m-ZrP-700) > 1.34 (m-ZrP-800), which were found to be significantly higher as compared to the reported bilayer sulfated zirconia grafted on SBA-15 (∼0.2 mmol gcat–1 h–1).[16] Turnover frequency (TOF) values were calculated by dividing the rates for HMF formation by the total concentrations of B and L acid sites. The TOF of m-ZrP-600 was found to be 11.68/h, which was very high as compared to the previously reported ZrP (1.62/h) and titanium phosphate (1.08/h) for fructose and inulin dehydration[48] and zirconia (4.7/h)[49] and sulfated zirconia (∼0.065/h)[15] for glucose dehydration in aqueous media. A higher TOF of 26.77/h was also observed over ZrP (∼80% fructose conversion) at 3.35 MPa under a subcritical water environment at 240 °C[23] that may be due to high reactivity of fructose besides the high temperature and pressure. Though a low concentration of glucose with a relatively higher reaction time was used in our study, HMF yield and glucose conversion over m-ZrP in an aqueous phase were observed to be higher due to its larger pore size and higher acid sites with proper B/L ratios as compared to previously reported heterogeneous solid acid catalysts such as Nb4W4,[17] SnP,[21] sulfated ZrO2 (S-ZrO2),[15,50] Ti–Cl (TiO2 prepared from chloride precursor),[50] TiO2 nanotubes (NT)[51] or TiO2 nanoparticles (NP),[52] and Nb2O5[22] or H3PO4/Nb2O5·nH2O[53] as summarized in Figure and Table S2. Overall, m-ZrP can be one of the appealing catalysts for the environmentally benign synthesis of HMF from glucose due to its high reaction rate with stability, optimal acid sites of mesopores, and reproducible in an aqueous phase using water as a green and sustainable alternative solvent.

Figure 9

HMF yield versus total acidic sites (mmol/g) on the various heterogeneous solid acid catalysts compared with the present m-ZrP catalysts in an aqueous phase synthesis of 5-hydroxymethylfurfural from glucose.

Conclusions

Large mesoporous zirconium phosphate catalysts (m-ZrP) with wide pore size distribution up to 100 nm were prepared by a simple hydrothermal method and calcined at 500–800 °C, which were evaluated for the dehydration of glucose to 5-hydroxymethylfurfural (HMF) in pure water as an environmentally benign green solvent. The calcination temperatures of the m-ZrP catalysts have significant influences on the textural, acidic/basic and surface properties as well as the activity of the m-ZrP catalysts. Our results clearly revealed the significances of the usage of P123 with pure water resulted into the formation of large mesoporous structures and more number of accessible Lewis (L) and Brönsted (B) acid sites, which facilitated easy diffusion pathways of carbohydrates and for HMF possibly as compared to the small pore m-ZrP. The m-ZrP catalyst having a higher concentration of acidic and basic sites with suitable strengths showed a higher glucose conversion and HMF yield. The study also revealed that glucose can be converted to HMF as efficiently as fructose if a suitable amount of L and B acid sites are present on the catalyst surfaces. The reusability test of m-ZrP-600 (calcined at 600 °C) showed similar yields for HMF until three recycle runs. Among the various homogeneous (H2SO4 and ZrOCl2) and mesoporous heterogeneous catalysts (Al2O3, ZrO2 and W/ZrO2) studied, the m-ZrP-600 exhibited the highest glucose conversion and HMF yield, which can provide an efficient and sustainable process for both the catalyst synthesis and production of HMF from glucose in water media.

Experimental Section

Preparation of m-ZrP Catalysts

The m-ZrP catalysts were prepared from aqueous solutions of zirconium and phosphate precursors (zirconium oxychloride (99%), Kanto Chemical Co.; orthophosphoric acid (85% in water), Duksan Pure Chemicals Co.) using P123 (Sigma-Aldrich) as a nonionic polymeric structure-directing agent. In a typical procedure,[32] P123 (5 g) was dissolved in a solution of deionized water (∼80 g) and orthophosphoric acid (∼5 g) under continuous stirring. An aqueous solution of zirconium oxychloride (16.1 g) was added to the above solution. The colloidal precipitate formed was stirred at room temperature (∼23 °C) for 3 h. The whole mixtures were then heated in a polypropylene bottle at ∼95 °C for 3 days. After cooling, the white material obtained was filtered, washed thoroughly with deionized water, dried at 80 °C, and calcined in the range of 500–800 °C. The resulting white sample was designated as m-ZrP–T, where T represents the calcination temperature (°C). The concentration of the orthophosphoric acid was varied in the range of 0.025–0.075 M to obtain m-ZrP-600 with different phosphorus to zirconium (0.75–1.25) molar ratios. Moreover, to compare the catalytic activities, a small pore mesoporous ZrP (P/Zr ratio = 1) was prepared with Pluronic F127 using the EISA process followed by being calcined at 600 °C.[37] The mesoporous metal oxides such as zirconia (m-ZrO2),[54] tungstated zirconia (m-W/ZrO2),[55] and alumina (m-Al2O3)[46] were also prepared for comparative studies as described in the literature.

Characterization of m-ZrP Catalysts

Specific surface area, pore volume, and average pore diameter were obtained from N2 adsorption–desorption isotherms at −196 °C by a Micromeritics TRISTAR-3000 after in situ pretreating the catalyst at 200 °C for 6 h. The specific surface area and pore size were calculated by the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively.

Temperature-programmed desorption of NH3 (NH3-TPD) was used to estimate total surface acidity by a BELCAT-M instrument. NH3 is a strong nonsite-specific base (pKb ∼ 5) that adsorbed on both B and L acid sites. In a typical procedure, the catalyst was pretreated at 500 °C for 2 h and then cooled to 50 °C. NH3 gas (99.99%) was introduced to the catalyst for 30 min at a flow rate of 50 mL/min followed by purging for 15 min. The catalyst was then heated at a rate of 10 °C/min up to 700 °C, and the amount of desorbed NH3 was measured. CO2-TPD was also carried out to obtain basic sites on m-ZrP catalysts by following the same procedure as mentioned in NH3-TPD. The Fourier-transform infrared (FT-IR) spectroscopy of adsorbed pyridine (Py-IR) was also used to differentiate B and L acid sites on the catalyst by a Nicolet 6700 FTIR spectrometer (Thermo Fisher Scientific). In a typical procedure,[56] ∼30 mg of a thin catalyst pellet was pretreated at 350 °C with a ramping rate of 10 °C/min under vacuum conditions. Subsequently, 5 μL of pyridine was injected into the static vacuum system by using a microsyringe for adsorption of pyridine to the catalyst for 10 min followed by desorption of physisorbed pyridine for ∼30 min. The Py-IR spectra were recorded at different temperatures after in situ heating from 150 to 350 °C. The B and L acid site concentrations and B/L ratios were calculated from the characteristic peak areas measured after pyridine desorption at 150 °C by the molar extinction coefficient method as described in the literature.[33]

Transmission electron microscopy (TEM) images of the catalysts were obtained with a TECNAI G2 instrument operated at an accelerating voltage of 200 kV. Powder X-ray diffraction (PXRD) patterns of all calcined catalysts were obtained using an X’PertPowder (PANalytical) operated at 40 kV with a Cu Kα radiation of 0.1540 nm in the diffraction range of 2θ = 5–70° at a scanning rate of 4°/min. Raman spectrum of the powder catalyst was obtained by a Bruker FRA 160/S FT-Raman spectrophotometer equipped with a 300 mW Nd3+:YAG laser emitting at 1064 nm. The Raman spectra were recorded with 64 scans with a resolution of 4 cm–1. Fourier-transform infrared (FT-IR) spectra were recorded on a PerkinElmer Frontier IR spectrometer in the range of 400–4000 cm–1 with a resolution of 4 cm–1. The electronic states and surface concentrations of zirconium and phosphorus species on m-ZrP were recorded using X-ray photoelectron spectroscopy (XPS) with an ESCALAB MK-II instrument. The Al Kα X-ray source having 1486.8 eV energy under a working pressure of 10–7 Pa with a resolution of 0.05 eV was adopted to analyze the Zr3d5/2 and P2p peaks. The binding energy (BE) of zirconium and phosphorus species was adjusted with a reference BE of C1s (284.4 eV). The ratios of P/Zr were calculated to verify the surface concentrations of zirconium and phosphorus species. The distribution of Zr and P elements at different positions on m-ZrP were done using a field emission scanning electron microscope (FE-SEM) combined with energy-dispersive X-ray spectroscopy (EDS) (JEOL, JSM-7600F).

Activity Measurement for the Dehydration Reaction

The dehydration reaction of glucose was carried out using a liquid phase batch reactor. Then, 0.5 wt % glucose solution (a lower concentration was taken deliberately to minimize humin byproduct formation) using deionized water, and the required amount of m-ZrP was charged in a 100 mL Pyrex glass vessel. The batch reactor was purged with nitrogen, and reactant mixtures were magnetically stirred at 500 rpm at a temperature of 155 ± 3 °C for 6 h. After the reaction, the mixture was filtered prior to analysis on a YL9100 HPLC system (Young Lin) equipped with an Aminex HPX-87H column (300 × 4.6 mm) and a refractive index detector (RID). The column temperature was maintained at 35 °C and 0.01 N H2SO4 solution was used as mobile phase with a flow rate of 0.6 mL/min. Known concentrations of both glucose and HMF were used as standards, and multipoint calibration curves were used for quantification. The conversion of glucose and yield for HMF were calculated by applying the following equations:[57] The glucose conversion and HMF yield were reproducible in the range of ±4% variation.