Impact of Design on the Activity of ZrO 2 Catalysts in Cellulose Hydrolysis-Dehydration to Glucose and 5-Hydroxymethylfurfural

: The one-pot hydrolysis-dehydration of activated microcrystalline cellulose was studied in pure hydrothermal water at 453 K over ZrO 2 catalysts produced by thermodegradation, microwave treatment, mechanical activation, and sol–gel methods and spent without any co-catalyst. ZrO 2 prepared by microwave treatment was more active compared to ones derived by other methods. The catalyst calcination temperature also impacted reactivity. The cellulose conversion increased simultaneously with acidity and S BET , which in turn were set by the preparation method and calcination temperature. Phase composition did not affect the activity. Yields of glucose and 5-HMF reaching 18 and 15%, respectively, were over the most promising ZrO 2 prepared by microwave treatment at 593 K. To our knowledge, this ZrO 2 sample provided the highest activity in terms of TOF values (15.1 mmol g − 1 h − 1 ) compared to the pure ZrO 2 systems reported elsewhere. High stability of ZrO 2 derived by microwave irradiation was shown in ﬁve reaction runs.


Introduction
Cellulose is well known to be the most abundant natural plant polymer. In recent years, it has been proposed for use as a raw material for chemical and material sciences [1][2][3]. Significant interest in cellulose transformations has been affected by the serious negative influence of traditional fossil resources such as oil and coal on the environment and human health. New catalytic approaches have been developed for the production of various chemicals and biofuels from inedible cellulose [4][5][6][7][8][9][10][11][12][13][14]. 5-Hydroxymethylfurfural (5-HMF) is one of the main platform molecules which can be obtained from cellulose. 5-HMF and its derivatives can become an alternative feedstock which could replace non-renewable sources. Applying 5-HMF in the production of polymers, motor oils, solvents, fuel additives, drugs and chemical reagents seems to be promising [13,[15][16][17]. In 2014, the commercial production of 5-HMF via the acid-catalysed dehydration of fructose was established [18]. Another perspective compound for industry is glucose. Nowadays, it is traditionally produced by enzymatic or acid hydrolysis of starch or sucrose [19][20][21].
Traditional homogeneous catalysts have some disadvantages in the synthesis of glucose and 5-HMF. Thus, catalyst recycling and separation from reaction mixtures is quite challenging. Mineral acids could have a corrosive effect. The use of heterogeneous catalysts makes it possible to overcome the most serious drawbacks of homogeneous catalytic systems. Moreover, some solid systems, such as oxides of zirconium (IV) or titanium (IV), are quite stable under hydrothermal conditions, which are applied when processing water-insoluble cellulose. The crystal structure of this polysaccharide makes the use of   The aim of this work was to investigate the impact of catalyst design by different techniques (thermodegradation, microwave heating, mechanical activation, and sol-gel method) on the catalytic properties of ZrO 2 . The influence of acidity, phase composition, and textural properties, which depended on the preparation method on the catalytic activity of ZrO 2 were studied in the hydrolysis-dehydration of cellulose to glucose and 5-HMF. The efficiency of zirconia catalytic systems was revealed in pure water without any co-catalysts. The best synthesis method was proposed.

Catalyst Preparation
Information on ZrO 2 synthesis techniques and parameters can be found in Table 2. To prepare zirconia materials by microwave treatment, 1.5 g of ZrO(NO 3 ) 2 ·H 2 O or ZrCO 3 (OH) 2 ·xH 2 O precursor were placed into a quartz cell and exposed to a microwave irradiation in a microwave oven PYRO (Milestone, Sorisole (BG), Italy). Treatment time and oven power were varied to prepare different ZrO 2 samples [48] (Table 2).

Sol-Gel Method (SG)
To prepare the ZrO 2 -SG sample, zirconium hydroxide was precipitated from a solution of zirconium (IV) tetraisopropoxide in anhydrous isopropyl alcohol by dropwise addition of ethanol (96%) under Ar atmosphere. The precipitate was kept for 7 days under the liquor, then separated by decantation and calcined at 873 K for 8 h.

Catalyst Characterization
The texture properties of the catalysts were revealed by low-temperature nitrogen adsorption at 77 K (ASAP-2400, Micromeritics, GA, USA). The X-ray of the catalysts were carried out using a Bruker D8 Advanced diffractometer diffractometer (Bruker, Germany) with Cu-K α (λ = 1.5418 Å) radiation. The POLYCRYSTAL program package was used to determine the unit lattice constants by the least squares method [50]. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) applied for chemical analysis was carried out using a PERKIN-ELMER OPTIMA 4300 instrument. Catalyst characterization techniques are described in detail in our previous papers [24][25][26].
To evaluate the acidic properties of the catalysts, the pH values of the mixtures of catalyst with water were determined. To evaluate this parameter, a 100 mg of ZrO 2 sample were added to Milli-Q water (10 mL) purged with argon for 10 min; then the glass cell was closed. The pH values were measured with an Anion 4100 pH-meter (Anion, Novosibirsk, Russia) under vigorous stirring and constant argon blowing until a constant pH value was reached.

Mechanical Activation and Characterization of Cellulose
Cellulose was treated in a planetary mill Pulverizette 5 (Fristch, Germany) to activate the substrate. Optical microscope Zeiss-Axiostar plus (Zeiss-Axiostar, Germany) and XRD diffractometer Bruker D8 Advanced (Bruker, Germany) were used to evaluate a particle size and crystallinity degree of the polysaccharide, respectively. The average length of activated cellulose particles was 13 ± 6 µm, crystallinity index was 35-55%. Cellulose activation and characterization techniques are presented in our previous works [24,36,51].

Catalytic Tests
Testing the stability of ZrO 2 catalysts was carried out under hydrothermal conditions in a high-pressure autoclave purchased from Autoclave Engineers, USA. The treatment conditions were 453 K, 1 MPa of argon and vigorous stirring of 1000 rpm. A weighed portion of the catalyst equal to 10 g·L −1 was added to 45 mL of Milli-Q water placed into the reactor. The reactor was purged with argon, heated to 453 K, and held for 5 h. The Zr content in the reaction medium after hydrothermal treatment was measured by an ICP-AES spectrometer Optima 4300 DV (PerkinElmer Inc., Shelton, CT, USA).
Catalytic tests were carried out in a high-pressure autoclave (Autoclave Engineers, Erie, PA, USA). Cellulose transformation conditions were 453 K, 1 MPa of Ar, 1000 rpm. To control the composition of the reaction mixtures by HPLC, a~1 mL portion of the reaction solution was collected in 0, 1, 2, 3, 5 and 7 h. The volume of the reaction mixture samples did not influence on the cellulose processing. The experimental mixtures were investigated by HPLC Shimadzu Prominence LC-20 (Shimadzu, Kyoto, Japan). HPLC apparatus was equipped by RI detector, Rezex RPM-Monosaccharide Pb 2+ and Rezex ROA-Organic Acids columns (Phenomenex, Torrance, CA, USA), 300 × 5.0 mm) thermostated at 343 and 313 K, respectively. Total organic carbon balance (TOC) was controlled using Multi N/C 2100S TOC equipment (Analytik Jena, Jena, Germany). More detailed information about cellulose hydrolysis-dehydration experimental techniques, as well as analytic methods of reaction mixtures by HPLC and TOC can be found in our previous papers [23,24,36,52].
Product yields were revealed by the formula [26]: where Y-a product yield, mol%, C pr -a product concentration, mol·L −1 , V-the reaction mixture volume, L, Nc-ratio of carbon in a product and glucose unit (1-for glucose, fructose, 5-HMF, etc.), M gly -molar weight of glucose unit in cellulose, 162 g·mol −1 , m Cellcellulose weighted, g.

Catalyst Characterization
The samples of ZrO 2 catalysts were prepared from different precursors by thermodegradation (TD), microwave treatment (MW), mechanical activation (MA), microwave treatment combined with mechanical activation (MA + MW), and the sol-gel method (SG). A description of the catalyst synthesis methods can be found in Section 2.2 and Table 2.
The textural parameters of the catalysts were determined by low temperature nitrogen adsorption (Table 3). In general, the prepared catalysts were characterized by the absence of micropores and an insignificant volume of mesopores (except for ZrO 2 -T-723, ZrO 2 -W-7, ZrO 2 -MA-W, and ZrO 2 -MA-N). The specific surface area varied in a wide range of 6-134 m 2 ·g −1 . Specific surface area and total pore volume decreased linearly with increasing temperature of calcination (Table 3, lines 1 and 2). Samples ZrO 2 -T-873, ZrO 2 -W-5, ZrO 2 -W-6, ZrO 2 -MA-W, and ZrO 2 -SG, which were subjected to thermal or microwave treatment at 873 K, have a low specific surface area of 6-14 m 2 ·g −1 .
The phase composition of the catalysts was studied by XRD. Dependence on the catalyst preparation method is observed (Table 3). Thus, zirconium oxides ZrO 2 -MA-C and ZrO 2 -MA-N prepared by mechanical activation contain a tetragonal phase, the phase composition does not depend on the zirconium oxide precursor (Table 3, lines 10 and 11). ZrO 2 -MA-W prepared by mechanical activation, followed by microwave treatment has a monoclinic structure (Table 3, line 9). Sample ZrO 2 -T-723 (Table 3, line 1) prepared by thermal decomposition contains equal proportions of monoclinic and tetragonal ZrO 2 phases. It should be noted that the monoclinic structure is more thermally stable than the tetragonal one. Samples prepared by microwave treatment turned out to be X-ray amorphous (Table 3, lines [3][4][5][6][7][8]. Complement studies were carried out on the ZrO 2 -W-7 sample, which was additionally calcined for 1 h at 723 K and 853 K. During this heat treatment, recrystallization accompanied by an increase in the crystallite size occurs. The X-ray diffraction pattern of the additionally calcined sample shows lines of tetragonal and monoclinic zirconium oxide (Supplementary Information, SI, Figure S1). The content of the monoclinic phase is 38% and 56%, and the tetragonal phase is 62% and 44% when ZrO 2 -W-7 sample calcined at 723 K and 853 K, respectively. We assume that pristine ZrO 2 -W-7 sample prepared by microwave treatment at 593 K could contain small monoclinic structures.
Main Admixture The acidic properties of zirconium oxide catalysts were estimated by measuring the pH of the mixtures of ZrO 2 with water (pH ZrO2 ) (Table 3). Previously, a correlation between total amount of acidic groups on the surface of solid catalysts and the pH at point of zero charge, as well as the pH of aqueous catalyst slurry was demonstrated [53]. The catalysts prepared by thermodegradation (Table 3, lines 1, 2) and mechanical activation (Table 3, lines 10, 11) showed moderate acidity values; pH ZrO2 was in the range of 6.0-7.7. pH ZrO2 varied in a wider range for the samples prepared by microwave treatment. ZrO 2 -W-1 sample made using short processing time and lower microwave irradiation power had a pH value of 6.5 (Table 3, line 3). On the other hand, the increase in preparation time and micro-oven power (Table 3, Table 3, lines 6, 7, 9), pH ZrO2 gains significantly up to 5.8-7.4. The sample ZrO 2 -SG prepared by the sol-gel method had a weakly alkaline pH ZrO2 value of 8.7 (Table 3, line 12).

Cellulose Hydrolysis-Dehydration in the Presence of ZrO 2 Catalysts
The process of depolymerization of cellulose in an aqueous medium requires the use of rather harsh conditions, namely high temperatures and pressures. Therefore, solid catalysts are required to be highly stable in the hydrothermal reaction medium. To determine the stability of the prepared catalysts, they were subjected to hydrothermal tests at 453 K for 5 h. The amount of dissolved Zr revealed by ICP turned out to be insignificant. The values of zirconium dissolved after hydrothermal treatment were in the range of ≤1.3 × 10 −4 -1.3 × 10 −3 %. However, 1.3% of Zr dissolved in the case of processing ZrO 2 -W-1 and ZrO 2 -W-3 samples. These tests confirm high stability of the ZrO 2 catalysts.
Hydrothermal hydrolysis-dehydration of cellulose polysaccharide in the presence of ZrO 2 catalysts was carried out at 453 K under inert argon atmosphere. The results are shown in Table 4. Activity of ZrO 2 samples was studied in pure water without any co-catalysts. Glucose and 5-HMF were the major reaction products. Mannose and fructose formation during isomerization of glucose was confirmed by HPLC. Formic and levulinic acids derived during the decomposition of 5-HMF, as well as furfural produced by the side transformation of fructose were also observed by HPLC analysis (Scheme 1). The total yield of all identified by-products did not exceed 5%. The only exception was the 14.6% yield of levulinic acid achieved in the presence of ZrO 2 -W-7 in 7 h of reaction. The formation of humins was also observed under hydrothermal reaction conditions. Hydrothermal hydrolysis-dehydration of cellulose polysaccharide in the presence of ZrO2 catalysts was carried out at 453 K under inert argon atmosphere. The results are shown in Table 4. Activity of ZrO2 samples was studied in pure water without any co-catalysts. Glucose and 5-HMF were the major reaction products. Mannose and fructose formation during isomerization of glucose was confirmed by HPLC. Formic and levulinic acids derived during the decomposition of 5-HMF, as well as furfural produced by the side transformation of fructose were also observed by HPLC analysis (Scheme 1). The total yield of all identified by-products did not exceed 5%. The only exception was the 14.6% yield of levulinic acid achieved in the presence of ZrO2-W-7 in 7 h of reaction. The formation of humins was also observed under hydrothermal reaction conditions.  It is interesting to compare cellulose conversion values with both pH ZrO2 , specific surface values and phase composition of ZrO 2 samples (Figure 1).
Note that pH ZrO2 and S BET depend on preparation techniques. Thus, samples  Table 1 lines 5, 7, and Table 4

lines 5, 8). Interestingly, both ZrO 2 -W-4
and ZrO 2 -W-7 were more active compared to ZrO 2 -W-3, characterized by higher acidity but lower S BET value. Thus, activity of ZrO 2 significantly increases with increasing catalyst acidity and density of acid sites, which depends on a specific surface area. The highest cellulose depolymerization value equal to 62.4% have been demonstrated for ZrO 2 -W-7. Interestingly, the increasing temperature of microwave treatment causes decreasing activities of ZrO 2 -W series due to diminishing both acidity and S BET value. It should be emphasized, that catalysts prepared by different techniques and having similar parameters of pH ZrO2 Table 1 lines 5, 7, and Table 4

lines 5, 8). Interestingly, both ZrO2-W-4 and
ZrO2-W-7 were more active compared to ZrO2-W-3, characterized by higher acidity but lower SBET value. Thus, activity of ZrO2 significantly increases with increasing catalyst acidity and density of acid sites, which depends on a specific surface area. The highest cellulose depolymerization value equal to 62.4% have been demonstrated for ZrO2-W-7. Interestingly, the increasing temperature of microwave treatment causes decreasing activities of ZrO2-W series due to diminishing both acidity and SBET value. It should be emphasized, that catalysts prepared by different techniques and having similar parameters of pHZrO2 and SBET show compatible values of the polysaccharide conversion, for example ZrO2-T-873 and ZrO2-W-6 as well as ZrO2-MA-C and ZrO2-W-5 ( Figure 1). The phase composition of ZrO2 could also influence the catalytic properties of zirconia [40]. However, in our study, the phase composition of ZrO2 ,which had similar pHZrO2 and SBET parameters, did not affect cellulose conversion significantly. For example, depolymerization degrees of the polysaccharide were 24.2-24.3% for ZrO2-MA-C with tetragonal phase and X-ray amorphous ZrO2-W-5 catalysts. Precursors of ZrO2 could also affect the textural and acid properties of the catalysts. Thus, ZrO2-MA-C and ZrO2-MA-N were prepared from ZrCO3(OH)2·xH2O and ZrO(NO3)2·H2O, respectively, under equal conditions of mechanical activation. Specific surface area and pore volume of ZrO2-MA-N (116 m 2 ·g −1 , 0.24 cm 3 g −1 ) were 4-5 times higher compared to ones of ZrO2-MA-C (22 m 2 ·g −1 , 0.06 cm 3 g −1 ). However, both ZrO2-MA-C and ZrO2-MA-N demonstrated moderate activity in terms of cellulose conversion due to similar acidity 7.2-7.5 (Table 3, lines 10,11, Table 4, lines 10,11).
Watanabe et al. [44] and Chareonlimkun et al. [40] previously reported that catalytic activity depended on both acid and base properties of catalysts. Acid centres are respon- The phase composition of ZrO 2 could also influence the catalytic properties of zirconia [40]. However, in our study, the phase composition of ZrO 2 ,which had similar pH ZrO2 and S BET parameters, did not affect cellulose conversion significantly. For example, depolymerization degrees of the polysaccharide were 24.  (Table 3,  lines 10,11, Table 4, lines 10,11).
Watanabe et al. [44] and Chareonlimkun et al. [40] previously reported that catalytic activity depended on both acid and base properties of catalysts. Acid centres are responsible for hydrolysis, while base sites catalyse isomerization of glucose to fructose, facilitating 5-HMF formation [40]. According to our results shown on Figure 2, the initial reaction rates (R) of glucose and 5-HMF formation depended on pH ZrO2 . The decrease in pH ZrO2 gains both initial rates of glucose and 5-HMF formation. However, ZrO 2 -SG samples with pH ZrO2 value of 8.7 demonstrated notable activity to glucose dehydration to 5-HMF. This may indicate that both acid and base sites of ZrO 2 are involved to reaction.
Catalysts 2021, 11, x FOR PEER REVIEW 9 of 14 sible for hydrolysis, while base sites catalyse isomerization of glucose to fructose, facilitating 5-HMF formation [40]. According to our results shown on Figure 2, the initial reaction rates (R) of glucose and 5-HMF formation depended on pHZrO2. The decrease in pHZrO2 gains both initial rates of glucose and 5-HMF formation. However, ZrO2-SG samples with pHZrO2 value of 8.7 demonstrated notable activity to glucose dehydration to 5-HMF. This may indicate that both acid and base sites of ZrO2 are involved to reaction. Summarizing the above section, the proposed catalyst ZrO2-W-7 was prepared by microwave treatment under low temperature. Thus, such a synthesis approach can be assumed to be the most promising. This catalyst preparation technique provides high acidity and specific surface areas which supply high catalytic efficiency.

Perspectives of ZrO2 Catalysts
As mentioned above, the yields of glucose and 5-HMF can reach 12.7 and 20.6%, respectively, over ZrO2 catalysts [38][39][40][41][42][43][44][45]. The main results published previously are captured in Table 1. Gliozzi et al. [42] showed 8.2% total yield of 5-HMF and furfural from softwood in the presence of silica-zirconia catalyst at 423 K. Biomass conversion was 34%. Yang et al. [45] transformed cotton cellulose over mixed ZnO-ZrO2 under hydrothermal conditions. 5-HMF yield reached 3.76% at 1.4 MPa autogenic pressure, 463 K and cellulose conversion 52.18%. Gavilà et al. [41] reported transformation of microcrystalline cellulose to 5-HMF at 453 K, pressure 3 MPa. The selectivity of the target product was ~20.6%. Watanabe et al. [44] investigated transformation of glucose and fructose in the presence of oxide catalysts at 473 K. 5-HMF yields were ~5.4% and ~15% from glucose and fructose, respectively, over zirconium oxide. Chareonlimkun et al. [40] used TiO2, ZrO2 and mixed TiO2-ZrO2 catalysts prepared by co-precipitation, sol-gel and physical mixing methods when processing different biomass resources (sugarcane, bagasse, rice husk and corn cob). ZrO2 catalyst derived via precipitation demonstrated higher activity; 4.2 and 1.8% yields of 5-HMF and glucose were reached at 523 K and 34.5 MPa. Qiao et al. [46,47] investigated cellulose hydrolysis under microwave heating in the presence of oxides at 433 K. The highest total yield of reducing sugars (TRS) was 93.6% in the presence of amorphous ZrO2. It should be noted that 0.04 mol L −1 H2SO4 co-catalyst was applied for the activity of solid ZrO2 based catalysts to be gained. Hydrolysis-dehydration of microcrystalline cellulose over NbOx/ZrO2 and pure ZrO2 catalysts was reported by [34]. Experiments were carried out at 453 K under 1 MPa pressure of Ar. Maximum yields of 5-HMF and glucose were 12.7 and 13.3%, respectively, in the presence of ZrO2. Summarizing the above section, the proposed catalyst ZrO 2 -W-7 was prepared by microwave treatment under low temperature. Thus, such a synthesis approach can be assumed to be the most promising. This catalyst preparation technique provides high acidity and specific surface areas which supply high catalytic efficiency.

Perspectives of ZrO 2 Catalysts
As mentioned above, the yields of glucose and 5-HMF can reach 12.7 and 20.6%, respectively, over ZrO 2 catalysts [38][39][40][41][42][43][44][45]. The main results published previously are captured in Table 1. Gliozzi et al. [42] showed 8.2% total yield of 5-HMF and furfural from softwood in the presence of silica-zirconia catalyst at 423 K. Biomass conversion was 34%. Yang et al. [45] transformed cotton cellulose over mixed ZnO-ZrO 2 under hydrothermal conditions. 5-HMF yield reached 3.76% at 1.4 MPa autogenic pressure, 463 K and cellulose conversion 52.18%. Gavilà et al. [41] reported transformation of microcrystalline cellulose to 5-HMF at 453 K, pressure 3 MPa. The selectivity of the target product was~20.6%. Watanabe et al. [44] investigated transformation of glucose and fructose in the presence of oxide catalysts at 473 K. 5-HMF yields were~5.4% and~15% from glucose and fructose, respectively, over zirconium oxide. Chareonlimkun et al. [40] used TiO 2 , ZrO 2 and mixed TiO 2 -ZrO 2 catalysts prepared by co-precipitation, sol-gel and physical mixing methods when processing different biomass resources (sugarcane, bagasse, rice husk and corn cob). ZrO 2 catalyst derived via precipitation demonstrated higher activity; 4.2 and 1.8% yields of 5-HMF and glucose were reached at 523 K and 34.5 MPa. Qiao et al. [46,47] investigated cellulose hydrolysis under microwave heating in the presence of oxides at 433 K. The highest total yield of reducing sugars (TRS) was 93.6% in the presence of amorphous ZrO 2 . It should be noted that 0.04 mol L −1 H 2 SO 4 co-catalyst was applied for the activity of solid ZrO 2 based catalysts to be gained. Hydrolysis-dehydration of microcrystalline cellulose over NbO x /ZrO 2 and pure ZrO 2 catalysts was reported by [34]. Experiments were carried out at 453 K under 1 MPa pressure of Ar. Maximum yields of 5-HMF and glucose were 12.7 and 13.3%, respectively, in the presence of ZrO 2 .
According to the data obtained in this work, the most promising catalyst is ZrO 2 -W-7 prepared by microwave treatment of zirconyl oxynitrate for 3 min. High activity of the catalyst is confirmed by significant cellulose conversion degree equal to 62.4% and notable yields of glucose and 5-HMF (Table 3, line 8). Influence of ZrO 2 -W-7 catalyst loading was also investigated. Catalyst: cellulose ratio equal to 1:20 was found to be the optimum one (Table 5 and Figure 3). According to the data obtained in this work, the most promising catalyst is ZrO2-W-7 prepared by microwave treatment of zirconyl oxynitrate for 3 min. High activity of the catalyst is confirmed by significant cellulose conversion degree equal to 62.4% and notable yields of glucose and 5-HMF (Table 3, line 8). Influence of ZrO2-W-7 catalyst loading was also investigated. Catalyst: cellulose ratio equal to 1:20 was found to be the optimum one (Table 5 and Figure 3).  Catalyst: cellulose = 1:1 caused a decrease in yields of glucose and 5-HMF due to significant transformation of the target products into side products in the presence of an excess of active sites in the reaction medium; 18 and 15% yield of glucose and 5-HMF were achieved under the optimum ratio of cellulose:ZrO2-W-7 = 20:1 (Table 5).
To compare the efficiency of ZrO2 catalysts, TOF values were calculated according to the following equation [26]: where TOF is the turnover frequency, mmol·g −1 ·h −1 , CGlu+5-HMF is amount of glucose and 5-HMF formed, mmol, mcat is the catalyst amount, g, and t is the reaction time, h. The most promising ZrO2 catalyst studied in this work (TOF value 15.1 mmol·g −1 ·h −1 ) was significantly more active than the systems reported elsewhere (Table 6). Catalyst: cellulose = 1:1 caused a decrease in yields of glucose and 5-HMF due to significant transformation of the target products into side products in the presence of an excess of active sites in the reaction medium; 18 and 15% yield of glucose and 5-HMF were achieved under the optimum ratio of cellulose:ZrO 2 -W-7 = 20:1 (Table 5).
To compare the efficiency of ZrO 2 catalysts, TOF values were calculated according to the following equation [26]: where TOF is the turnover frequency, mmol·g −1 ·h −1 , C Glu+5-HMF is amount of glucose and 5-HMF formed, mmol, m cat is the catalyst amount, g, and t is the reaction time, h. The most promising ZrO 2 catalyst studied in this work (TOF value 15.1 mmol·g −1 ·h −1 ) was significantly more active than the systems reported elsewhere (Table 6). To reveal the stability of zirconia catalytic systems, ZrO 2 -W-7 was tested in five runs of cellulose hydrolysis-dehydration reaction. After each cycle, the catalyst spent was separated by centrifugation, washed several times with Milli-Q water, dried at 333 K, and reused in a new cycle. Figure 4 shows the results of catalyst reuse experiments. In five cycles, the yields of glucose and 5-HMF decreased by 5%.  To reveal the stability of zirconia catalytic systems, ZrO2-W-7 was tested in five runs of cellulose hydrolysis-dehydration reaction. After each cycle, the catalyst spent was separated by centrifugation, washed several times with Milli-Q water, dried at 333 K, and reused in a new cycle. Figure 4 shows the results of catalyst reuse experiments. In five cycles, the yields of glucose and 5-HMF decreased by 5%.

Conclusions
The transformation of cellulose was studied in hydrothermal pure water at 453 K to efficiently derive glucose and 5-HMF. The ZrO2 catalysts prepared by microwave treatment gained higher efficiency than ones derived by thermodergadation, mechanical activation and sol-gel methods. The catalyst calcination temperature also had a significant impact on reactivity with regard to acidity and SBET. The most promising ZrO2 prepared by microwave irradiation at 593 K demonstrated high stability in five reaction runs and the highest activity (TOF 15.1 mmol·g −1 ·h −1 ) compared to the systems reported elsewhere; 18 and 15% yields of glucose and 5-HMF were reached without any co-catalyst under optimized cellulose/catalyst ratio = 20/1 (g/g).

Conclusions
The transformation of cellulose was studied in hydrothermal pure water at 453 K to efficiently derive glucose and 5-HMF. The ZrO 2 catalysts prepared by microwave treatment gained higher efficiency than ones derived by thermodergadation, mechanical activation and sol-gel methods. The catalyst calcination temperature also had a significant impact on reactivity with regard to acidity and S BET . The most promising ZrO 2 prepared by microwave irradiation at 593 K demonstrated high stability in five reaction runs and the highest activity (TOF 15.1 mmol·g −1 ·h −1 ) compared to the systems reported elsewhere; 18 and 15% yields of glucose and 5-HMF were reached without any co-catalyst under optimized cellulose/catalyst ratio = 20/1 (g/g).