Glucose conversion to 5-hydroxymethylfurfural on zirconia: Tuning surface sites by calcination temperatures
Graphical abstract
Introduction
The decreasing petroleum reserves and growing environmental concerns about greenhouse gas emissions have promoted research on biomass as a raw material for the production of chemicals and other fuel substitutes for gas or oil [[1], [2], [3]]. Developing economic, efficient, and environmentally friendly technologies is a promising but at the same time challenging strategy for biomass transformation. 5-Hydroxylmethylfurfural (HMF), a kind of furan derivatives obtained from biomass, has gained much attention recently since it could be a potential substitute for petroleum-derived intermediate products during the manufacture of polymers and fuels [4]. As a chemical platform, a number of important C-6 compounds can be synthesized by HMF, such as alkoxymethylfurfurals, 2,5-furandicarboxylic acid, 5-hydroxymethylfuroic acid, 2,5-dimethylfuran [5]. Some important non-furanic compounds, namely levulinic acid, formic acid, adipic acid, 1,6-hexanediol, etc. can also be produced from HMF [1,[5], [6], [7]].
Dehydration of hexose sugars such as glucose and fructose is considered to be a promising way to produce HMF [5,8]. Though high HMF yields from fructose have been reported by many researchers, it is not an ideal feedstock for HMF production because of the high cost [9,10]. Glucose is supposed to be a preferred feedstock for HMF production due to its greater availability and lower cost [2]. Glucose dehydration to HMF is generally initiated by acid catalysts, such as zeolites, amorphous silica-alumina, oxides, and others. Among them, zirconia (ZrO2) has received considerable attention as both a catalyst and a catalyst support due to its favorable physicochemical properties [[11], [12], [13]]. Doping with other oxoanions to ZrO2 can create additional electron-deficient regions and may generate new acid sites and increase the strength of Brønsted acidity, thus can enhance catalytic behavior such as dehydration, hydrogenation and hydrogen exchange [12,14].
Jiménez-Morales et al. studied the dehydration of glucose to HMF in a biphasic system based on mesoporous MCM-41 containing ZrO2 and achieved an HMF yield of 23% [15]. Massa et al. studied the dehydration of glycerol to acrolein over monoclinic-ZrO2 doped with tungsten and niobium oxide and attained a 75% acrolein yield with almost complete transformation of glycerol [16]. Kuo et al. did research on the conversion of cellulose to HMF with a catalyst of mesoporous ZrO2 nanocatalysts in ionic liquid systems, which showed that the crystallinities may affect the acidities of catalysts [17]. It also showed that crystalline mesoporous ZrO2 nanoparticles, in either the tetragonal or the monoclinic phase, exhibited higher HMF yields than amorphous mesoporous ZrO2 nanoparticles because of the existence of a relatively strong acidity [17]. López et al. found that the increase in the catalytic activity of esterification and transesterification coincided with the formation of polymeric tungsten species in the presence of the tetragonal phase of the ZrO2 support [18]. Ginjupalli et al. found that pure ZrO2 polymorphs showed mainly weak and moderate acid strength, and the acidity of monoclinic-ZrO2 was higher than that of tetragonal-ZrO2, slightly differently from other authors [17,19].
Different calcination temperatures during the catalyst preparation may cause different physicochemical properties of catalysts, such as crystalline structure, molecular structure of the metal-oxide overlayer, acid density and strength, which in turn results in different catalytic activities [18,[20], [21], [22], [23]]. Nakano et al. reported that different pretreatment temperatures may cause different surface properties of ZrO2 and cause different catalytic activities in the isomerization of 1-butene. The acidic, basic, oxidizing, and reducing properties of ZrO2 were independent of each other over different calcination temperatures [12]. Cortés-Jácome et al. also found that different calcination temperatures may lead to different structures and electric valence of the W atoms of tungstate ZrO2, thus caused different catalytic activities in n-hexane isomerization [14]. The structural chemistry of ZrO2 has been studied by Ho, Livage et al. [11,24]. However, the relationship between the calcination temperature and the catalytic performance of ZrO2 based on structural transformation is yet to be understood.
In the present work, ZrO2 was prepared at different calcination temperatures. The formation of ZrO2 by dehydration of Zr(OH)4 was studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). ZrO2 calcined at various temperatures was characterized by N2 adsorption, X-ray diffraction (XRD) and Raman spectroscopy. The acidity of ZrO2 was studied by temperature-programmed desorption of ammonia (NH3-TPD) and solid-state nuclear magnetic resonance (NMR) spectroscopy. Their catalytic performance has been tested in the dehydration of glucose.
Section snippets
Catalyst preparation
1.2 g of Zr(OH)4 was added to 20 mL of deionized water. The mixture was then put into an ultrasonic bath for 4 h and stirred for 15 h. The resulting materials were dried at 80 °C for 6 h and calcined for 4 h at 300, 400, 700 or 900 °C.
Characterization of the catalyst
Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of Zr(OH)4 was obtained on a TGA/DSC STARe system (Thermo Fisher Scientific) in a nitrogen atmosphere. The sample was heated from 35 to 1000 °C at a ramp rate of 5 °C/min. Powder X-ray
Physicochemical characteristics of ZrO2 calcined at different temperatures
In this research, ZrO2 remains XRD amorphous when the calcination temperature is 300 °C (Figs. 1 and S1). It was reported that the structure of amorphous ZrO2 is a two-dimensional model with a thin plate consisting of a layer of zirconium atoms between two oxygen layers [24]. Since amorphous metal oxides are thermodynamically metastable at elevated temperature, crystallization is unavoidable during thermal treatment [26]. It presents a tetragonal-ZrO2 (t-ZrO2) polymorph (JCPDS card no. 79-1763)
Conclusion
ZrO2 was synthesized at different calcination temperatures. ZrO2 is in an amorphous state when the calcination temperature is 300 °C, and it starts to crystallize with increasing calcination temperature, which causes a decreasing BET surface area and an increasing crystallite size. It is mainly present in the metastable tetragonal-ZrO2 polymorph at 400 °C and rapidly transforms into the thermodynamically stable monoclinic-ZrO2 polymorph when the calcination temperature is higher than 700 °C.
Acknowledgements
We thank the MicroscopeFacility, the Center for Analytical Biotechnology, and the Department of Molecular Sciences at Macquarie University for instrumental uses and the staff for technical support. We also appreciate the financial support from Australian Research Council Discovery Projects (DP150103842 and DP180104010) and the SOAR Fellowship of the University of Sydney.
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