Continuous Hydrogenation of Ethyl Levulinate to γ-Valerolactone over

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Continuous Hydrogenation of Ethyl Levulinate to γ‑Valerolactone over Cu-Zn/ZrO2 Catalyst with Alumina Binder Xiangjin Kong,*,† Shuxiang Wu,† Yuzhou Jin,† Liying Liu,‡ and Junhai Liu*,† †

Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, China ‡ School of Mathematics Science, Liaocheng University, Liaocheng 252059, China ABSTRACT: Cu-Zn/ZrO2 catalyst with an alumina binder was built for catalytic hydrogenation of ethyl levulinate to γvalerolactone. N2 adsorption−desorption, X-ray powder diffraction, Fourier transform infrared spectroscopy, NH3 temperatureprogrammed desorption, H2 temperature-programmed reduction, and N2O decomposition were used to characterize the catalysts. The obtained results exhibited that porosity and acidity of the catalysts were strongly influenced by the binder; the most outstanding catalytic performance (96.4% ethyl levulinate conversion and 94.0% selectivity for γ-valerolactone) was observed on the catalyst with 20 wt % alumina binder for 30 h of time on stream. The good activity of this catalyst was due to its relatively good Cu species dispersion and proper acidity.

1. INTRODUCTION The catalytic transformation of renewable feedstock to platform chemicals has attracted much attention with the depleting fossil resources in the past two decades.1−4 γ-Valerolactone (GVL), as one of the chemicals, has large potential to be used in biofuels production, intermediate in the fine chemicals synthesis, and so on.5−7 GVL can be synthesized by hydrogenation and subsequent dehydration of levulinic acid (LA) in a hydrogen atmosphere over metal catalysts. Generally, noble metal supported catalysts are used for this transformation.8 Although desirable yield of GVL was achieved using supported noble metal catalysts, the obvious drawback of being high cost limited the practical application of these catalysts in large-scale GVL production. Besides, most of these nanocatalysts showed slow deactivation due to carbon deposition, acid-assisted metal loss, and structural changes of supports. Therefore, the development of cheap, highly active, stable, and readily recyclable catalysts remains an urgent problem in the present studies. Among all the base metal catalysts studied since then, Cu based catalysts were found to be the most effective candidates for this reaction.9−12 For example, Yan recently developed an efficient Cu-Cr catalyst, over which a 88.2% yield of GVL was obtained.13,14 However, the chromium promoter is harmful to the environment. Several chromium free catalysts such as CuFe, Cu-ZrO2, and Cu-SiO2 were also developed for the production of GVL, in which a ZrO2 supported Cu catalyst is of great interest because of its good thermal stability.12−15 The interaction between the Cu-center and the support has vital influence on the product distribution and the target product yield.16 Although a quantity of pioneering studies were reported, most of the obtained catalysts generally required severe reaction conditions, suffered from poor yield or low time-on-stream performance. Therefore, to establish an efficient catalyst for the synthesis of GVL is highly desirable. In general, the Cu based catalysts are often used in powder form, and if these Cu based catalysts were to be used in the © XXXX American Chemical Society

industrial scale, it must to be shaped into bodies with the addition of binder, which always has a significant impact on the ultimate activity of the catalyst.17−19 Thus, the effect of binder on catalytic properties of these Cu based catalyst for synthesis of ethyl levulinate from γ-valerolactone should be manifested. However, as far as we know, there are few reports about this. LA was the most used initial material for the production of GVL, which can be obtained from cellulosic feedstocks using sulfuric acid as the catalyst. However, LA is still a highly costly raw material for GVL production due to the need to use lime to neutralize acid in the product solution and the energy-intensive distillation for the separation of LA.20 In addition, leaching of copper was also observed which was caused by the acid attack of LA to the metal catalyst.21 Generally, furfuryl alcohol (FAL) is easily produced by hydrogenation of furfural, and furfural can be produced from hemicellulose-rich biomass.22,23 It has been proved that ethyl levulinate can be effectively obtained through ethanolysis of FAL over acidic catalysts.24−26 Every year, hundreds of thousands of furfural were produced; thus EL was considered to be a good candidate starting material for GVL. In general, alumina is among the most commonly employed binder.17 It has been found that the Zn oxide not only could improve the dispersion of Cu particles but also acts as an active site for hydrogen spillover. Moreover, the interaction between Cu and Zn oxide causes an electron transfer from Zn oxide to Cu metal that occurs as Cu0 and Cu+ species.27 Thus, in the present studies, Cu-Zn instead of pure Cu was used as the active species for hydrogenation of ethyl levulinate to γvalerolactone. With the aim to establish a continuous efficient synthesis process for GVL in a fixed-bed reactor, EL was applied as starting material and a new kind of Cu-Zn/ZrO2 with alumina binder catalysts were developed in the present studies. The catalysts were characterized by several methods to Received: July 22, 2017 Revised: October 3, 2017 Published: October 10, 2017 A

DOI: 10.1021/acs.energyfuels.7b02140 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels investigate the relationship of physicochemical properties and catalytic activity as presented in the following section.

Selectivity (mol %) moles of EL transformed to the respective product = × 100% moles of EL converted (2)

2. EXPERIMENTAL SECTION 2.1. Materials. Ethyl levulinate and ZrO2 were supplied by Shanghai Macklin Biochenical Co., Ltd., China. Cu(NO3)2·6H2O, Zn(NO3)2·6H2O (AR), and Na2CO3 were purchased from Tianjin Guangfu Technology Development Co., Ltd., China. 2.2. Catalyst Preparation. The catalyst was prepared by a coprecipitation kneading method. The Cu weight percent in the catalyst is 20%, and the Zn weight percent in the catalyst is 10%. The ratio of Cu:Zn 2:1 was chosen according to previous studies.28 Cu-Zn/ ZrO2-m%Al2O3 (m = the weight percent of Al2O3 in the catalyst) was prepared by kneading a mixture of ZrO2 and pseudo-boehmite (the precursor of Al2O3 binder) together with the precipitates of Cu-Zn carbonates or hydroxides according to our previous studies.29 For example, Cu-Zn/ZrO2-20%Al2O3 was prepared as follows. 19.33 g of Cu(NO3)2·3H2O and 11.90 g of Zn(NO3)2·6H2O were dissolved in 200 mL of H2O, and 300 mL of an aqueous solution of 15.26 g of Na2CO3 was added dropwise into a beaker. The mixture was aged for 2 h and then filtered, washed, and dried at 110 °C for 6 h. The dried solids were mixed with a mixture of 8.36 g of pseudo-boehmite and 13.93 g of ZrO2 and molded into bars using an extruder. The bars were dried at 110 °C for 6 h and calcined at 550 °C for 4 h. 2.3. Catalyst Characterization. N2 adsorption−desorption was measured via an Autosorb-IQ-C. The catalysts were degassed at 200 °C for 3 h under vacuum before liquid N2 adsorption, and the isotherms were taken at 77 K. X-ray powder diffraction (XRD) patterns of the catalysts were recorded on a PERSEE XD3 diffractometer via a model (D8, Advance, Bruker, Germany) with Cu−Kα radiation (λ = 15.42 nm, 36 kV and 20 mA) and a Ni filter scanning at 4° per min ranging 20−70°. The characteristic absorption bands were obtained via FT-IR, using the KBr method. The catalysts were reduced in a tube furnace at 300 °C for 4 h at atmospheric H2 pressure before XRD and FT-IR analysis. NH3 temperatureprogrammed desorption (NH3-TPD), temperature-programmed reduction (TPR), and N2O decomposition were conducted on a TP5080 automatic chem-adsorption instrument. In a typical experiment for NH3-TPD, the catalyst was activated at 300 °C for 2 h by passage of helium gas. Then, it was saturated with NH3 and He mixture gas at 100 °C for 1 h and subsequently flushed with He gas at 100 °C for 1 h to remove physiosorbed NH3. Finally, the TPD of catalyst was performed at a heating rate of 10 °C/min to 800 °C. Prior to TPR studies, the catalysts sample was treated at 300 °C for 1 h by passage of helium gas. When cooling to room temperature, the sample was exposed to H2−He for 0.5 h and then TPR analysis was carried out from room temperature to 700 °C at a heating rate of 10 °C/min. Cu dispersion and surface area were measured by N2O decomposition. The catalyst was pretreated in a He flow (50 mL/min) and heated at 100 °C for 0.5 h. Then, the catalysts were reduced with a flow mixture of 5% H2-Ar at a heating rate of 10 °C/min up to 550 °C, and then oxidation of the catalyst was conducted at 60 °C for 1 h in a flowing N2O. 2.4. Catalytic Reaction. The present reaction was performed in a fixed-bed reactor, which was loaded with about 4.0 g of cylinder catalyst particles (with a diameter of 1 mm and height about 1−2 mm). The catalyst was reduced at 300 °C in a hydrogen stream at atmospheric H2 pressure for 4 h before use. A solution of EL (10 wt % EL in 1,4-dioxane) was dosed into the reactor at a flow rate of 0.1 mL/ min. The reaction product was collected every hour and then analyzed using an offline gas chromatograph with a SE-30 capillary column (30 m in length) and selected ethyl laurate as an internal standard. The components were analyzed by using GC−MS with an HP-1 capillary column (30 m × 0.25 mm, 0.2 μm film thickness) equipped with an ion trap MS detector. Conversion and selectivity toward GVL and the byproducts are calculated according to eqs 1 and 2.

Conversion (mol %) =

moles of EL transformed × 100% moles of EL fed

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. Table 1. shows N2 sorption characteristics of Cu-Zn/ZrO2-Al2O3 catalysts. The Table 1. N2 Physisorption Characterization of Cu-Zn/ZrO2Al2O3 Catalysts catalyst

surface areaa (m2/g)

total pore volumeb (cm3/g)

Cu-Zn/ZrO2-10%Al2O3 Cu-Zn/ZrO2-20%Al2O3 Cu-Zn/ZrO2-30%Al2O3 Cu-Zn/ZrO2-40%Al2O3 Cu-Zn/ZrO2-50%Al2O3

36 63 74 108 126

0.21 0.34 0.43 0.56 0.66

a

Specific surface area calculated by the BET method. bThe pore volume was evaluated at P/P0 = 0.99.

surface area plays an important role in providing active sites for adsorption of reactants. From Table 1, it can be found that the specific surface areas and the pore volume of Cu-Zn/ZrO2Al2O3 catalysts increased from 36 to 126 m2/g with the increase of Al2O3 loading content. Figure 1 depicts the N2 adsorption−

Figure 1. N2 adsorption−desorption isotherms of (a) Cu-Zn/ZrO210%Al2O3, (b) Cu-Zn/ZrO2-20%Al2O3, (c) Cu-Zn/ZrO2-30%Al2O3, (d) Cu-Zn/ZrO2-40%Al2O3, (e) Cu-Zn/ZrO2-50%Al2O3.

desorption isotherms of Cu-Zn/ZrO2-Al2O3 catalysts at 77 K. As seen in Figure 1, the uptake of N2 was enlarged with the increase of Al2O3 content, which was consistent with the variation of specific surface area and pore volume. This may probably be caused by the quite better porosity of Al2O3 than ZrO2. The XRD patterns of Cu-Zn/ZrO2-Al2O3 catalysts are shown in Figure 2. Strong diffractions peaks at 28.5°, 31.5°, and 40.7° were exhibited in the XRD patterns of the catalysts, which indicates that ZrO2 presented in the catalyst mainly in the monoclinic phase.30,31 With the enhancement of Al2O3 content in the catalysts, the intensity of ZrO2 was significantly weaken.

(1) B

DOI: 10.1021/acs.energyfuels.7b02140 Energy Fuels XXXX, XXX, XXX−XXX

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at 1640 cm−1 can be assigned to the water vibrational modes and the bands at 1560 and 3450 cm−1 were attributed to the hydroxyl vibrational modes.35−37 The NH3-TPD curves of Cu-Zn/ZrO2-Al2O3 catalysts are shown in Figure 4, while NH3-TPD analysis of catalysts is listed

Figure 2. XRD patterns of (a) Cu-Zn/ZrO2-10%Al2O3, (b) Cu-Zn/ ZrO2-20%Al2O3, (c) Cu-Zn/ZrO2-30%Al2O3, (d) Cu-Zn/ZrO2-40% Al2O3, (e) Cu-Zn/ZrO2-50%Al2O3..

The diffractions lines at 43.3° and 50.4° were detected after the catalysts were reduced, which could be assigned to the crystals of Cu0.15 It was also found from the main diffraction line (43.3°) that the intensity of Cu was strengthened with the increase of Al2O3 content on the XRD patterns. It indicated that the reduction of Cu was influenced by the doped Al2O3. In addition, no diffractions peaks of pseudo-boehmite was observed on these patterns. It was reported that pseudoboehmite could be transformated to γ-Al2O3 by the removal of water at about 447 °C.32 In the present studies, the catalyst bars were calcined at 550 °C. Therefore, the alumina binder probably existed in the catalyst in the γ-Al2O3 phase. The FT-IR spectra of the catalysts are depicted in Figure 3. The characteristic absorption bands of ZnO and CuO stretching vibrations were observed at 420 and 500 cm−1, respectively.33,34 The characteristic absorption bands of Al−O bonds (around 740 cm−1) and Zr−O bonds (around 1410 cm−1) were also observed from the FT-IR spectra.20 The band

Figure 4. NH3-TPD profiles of (a) Cu-Zn/ZrO2-10%Al2O3, (b) CuZn/ZrO2-20%Al2O3, (c) Cu-Zn/ZrO2-30%Al2O3, (d) Cu-Zn/ZrO240%Al2O3, (e) Cu-Zn/ZrO2-50%Al2O3.

Table 2. NH3-TPD Analysis of Cu-Zn/ZrO2-Al2O3 Catalysts catalyst

temperature (°C)

acidity (mmol/g)

Cu-Zn/ZrO2-10%Al2O3 Cu-Zn/ZrO2-20%Al2O3 Cu-Zn/ZrO2-30%Al2O3 Cu-Zn/ZrO2-40%Al2O3 Cu-Zn/ZrO2-50%Al2O3

146.42 150.81 156.06 153.89 152.69

0.219 0.434 0.501 0.588 0.789

in Table 2. Only one desorption peak at around 100−200 °C was found for all catalysts, and with the increases of Al2O3 content, the peak area evidently increased. Table 2 also displays that, when Al2O3 content increases from 10% to 50%, the catalyst acidity is increasing from 0.219 to 0.789 mmol/g. Clearly, this result was caused by the high acidity intensity of Al2O3 than that of ZrO2.38,39 TPR analysis was used to study the influence of added Al2O3 on the reducibility of the active metallic phase. As shown in Figure 5, a reduction peak was observed at the temperature of about 300 °C for all of the samples. The TPR peak of low Al2O3 containing catalyst was much broader than that of high Al2O3 containing catalyst. From Table 3, it can be seen that, with increase of Al2O3 content from 10% to 50%, the uptake of H2 reduces from 110.7 to 85.3 mmol/g. This probably indicates the formation of CuAl2O4, which is a surface spinel species. Compared to CuO, CuAl2O4 is more difficult to be reduced,40 which revealed the catalysts were more and more difficult to be reduced with the addition of Al2O3. Copper percentage dispersion and area are calculated by a N2O decomposition method, which is listed in Table 3. The dispersion and metal area increased to 29% and 102 m2 g−1 Cu, respectively, over the 20% Al2O3 doped catalyst. This is probably due to that the doped Al2O3 increased the catalyst’s BET surface area as shown

Figure 3. FT-IR spectra of (a) Cu-Zn/ZrO2-10%Al2O3, (b) Cu-Zn/ ZrO2-20%Al2O3, (c) Cu-Zn/ZrO2-30%Al2O3, (d) Cu-Zn/ZrO2-40% Al2O3, (e) Cu-Zn/ZrO2-50%Al2O3. C

DOI: 10.1021/acs.energyfuels.7b02140 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

both metal sites and acid sites are needed during the transformation of EL to GVL. As shown in Table 4, the doping of Al2O3 binder in the catalyst plays an important role in the activity of the catalysts. With the increase of Al2O3 loading to 20 wt %, the conversion of EL was increased from 65.8% to 96.4%, while the selectivity toward GVL increased from 88.2% to 94.0%. A further increase in Al2O3 binder leads to a decreases in both the conversion of EL and selectivity toward GVL. The reaction results are well correlated with the measurements by the XRD, TPR, and N2O decomposition methods. The higher dispersion of Cu species accounts for the much improved catalytic performance of CuZn/ZrO2-20%Al2O3 than Cu-Zn/ZrO2-10%Al2O3. The decrease in conversion of the EL beyond 20 wt % of Al2O3 is caused by the loss of availability of Cu active sites, as shown in Table 3. The selectivity toward GVL rose from 88.2% to 94.0% with the increase of Al2O3 loading to 20 wt % and decreased from 94.0% to 83.0% at higher Al2O3 loadings. It is well-known that the protonation of GVL is the initial step of the acid-catalyzed ring-opening of GVL to produce 1-pentanol. Thus, the decrease in the selectivity of GVL over the catalyst beyond 20 wt % of Al2O3 is probably due to the increased acidic sites on the catalyst. Noncatalytic reaction for comparison purposes was also carried out, and the obtained results demonstrated that hydrogenation of EL to GVL could not be conducted without any catalyst, which demonstrated that the catalysts are indispensible to the present reaction. In addition, when compared with previous studies,12−15 it was found that the Cu-Zn/ZrO2-20%Al2O3 exhibits better activity than most of the previously reported catalysts. 3.3. Stability of Catalyst. The stability of Cu-Zn/ZrO220%Al2O3 was evaluated for 30 h; the obtained results are depicted in Figure 6. As can be found, conversion of EL and selectivity toward GVL remained at around 96.0% and 94.0%, respectively. These results demonstrated that Cu-Zn/ZrO2-20% Al2O3 displayed an excellent time-on-stream performance for continuous synthesis of GVL.

Figure 5. TPR profiles of Cu-Zn/ZrO2-Al2O3 catalysts: (a) Cu-Zn/ ZrO2-10%Al2O3, (b) Cu-Zn/ZrO2-20%Al2O3, (c) Cu-Zn/ZrO2-30% Al2O3, (d) Cu-Zn/ZrO2-40%Al2O3, (e) Cu-Zn/ZrO2-50%Al2O3.

Table 3. TPR Analysis and Copper Dispersion of Cu-Zn/ ZrO2-Al2O3 Catalysts catalyst

H2 uptakea (mmol/g)

Cu dispersionb (%)

metal areab (m2 g−1 Cu)

Cu-Zn/ZrO2-10%Al2O3 Cu-Zn/ZrO2-20%Al2O3 Cu-Zn/ZrO2-30%Al2O3 Cu-Zn/ZrO2-40%Al2O3 Cu-Zn/ZrO2-50%Al2O3

110.7 102.9 98.6 86.9 85.3

23 29 24 19 13

85 102 100 84 72

a

Calculated from temperature-programmed reduction. bDetermined from N2O decomposition values.

in Table 1, which facilitated the dispersion of Cu active sites. However, beyond this, dispersion and metal areas are decreased with an increase in Al2O3 loading. 3.2. Catalyst Activity. On the basis of the GC−MS of the reaction mixture and previous studies, the proposed reaction pathways of EL hydrogenation to GVL are shown in Scheme 1.21 EL undergoes selective catalytic hydrogenation to afford ethyl 4-hydroxypentanoate over the metal sites, and then some ethyl 4-hydroxypentanoate undergoes dehydration to GVL over the acid sites. According to the reaction pathway, it is sure that

4. CONCLUSIONS The influence of alumina binder on Cu-Zn/ZrO2 catalyst for continuous hydrogenation of ethyl levulinate to γ-valerolactone in a fixed-bed reactor was investigated. The amount of surface acid sites and the dispersion properties of Cu particles were influenced by the addition of alumina binder. Among the

Scheme 1. Proposed Reaction Pathways of Ethyl Levulinate Hydrogenation

D

DOI: 10.1021/acs.energyfuels.7b02140 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 4. Effect of Different Al2O3 Contents on Hydrogenation of EL to GVLa selectivity (%)

a

catalyst

conversion (%)

GVL

1-pentanol

unidentified

Cu-Zn/ZrO2 Cu-Zn/ZrO2-10%Al2O3 Cu-Zn/ZrO2-20%Al2O3 Cu-Zn/ZrO2-30%Al2O3 Cu-Zn/ZrO2-40%Al2O3 Cu-Zn/ZrO2-50%Al2O3 Cu-Zn/100% Al2O3

65.8 74.3 96.4 89.7 87.5 83.7 70.2

88.2 91.3 94.0 93.0 92.7 90.9 83.0

9.8 6.2 4.3 5.0 5.2 6.2 12.4

2.0 2.5 1.7 2.0 2.1 2.9 4.6

Reaction conditions: 10 wt % EL, reaction temperature 240 °C, atmospheric H2 pressure, feeding rate 0.1 mL/min. Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T. The path forward for biofuels and biomaterials. Science 2006, 311, 484−489. (3) Huber, G. W.; Corma, A. Synergies between bio- and oil refineries for the production of fuels from biomass. Angew. Chem., Int. Ed. 2007, 46, 7184−7201. (4) Chheda, J. N.; Huber, G. W.; Dumesic, J. A. Liquid-phase catalytic processing of biomass-derived oxygenated hydrocarbons to fuels and chemicals. Angew. Chem., Int. Ed. 2007, 46, 7164−7183. (5) Testa, M. L.; Corbel-Demailly, L.; La Parola, V.; Venezia, A. M.; Pinel, C. Effect of Au on Pd supported over HMS and Ti doped HMS as catalysts for the hydrogenation of levulinic acid to γ-valerolactone. Catal. Today 2015, 257, 291−296. (6) Horváth, I. T.; Mehdi, H.; Fábos, V.; Boda, L.; Mika, L. T. γValerolactone a sustainable liquid for energy and carbon-based chemicals. Green Chem. 2008, 10, 238−242. (7) Sudhakar, M.; Kantam, M. L.; Jaya, V. S.; Kishore, R.; Ramanujachary, K. V.; Venugopal, A. Hydroxyapatite as a novel support for Ru in the hydrogenation of levulinic acid to γvalerolactone. Catal. Commun. 2014, 50, 101−104. (8) Upare, P. P.; Lee, J. M.; Hwang, D. W.; Halligudi, S. B.; Hwang, Y. K.; Chang, J. S. Selective hydrogenation of levulinic acid to γvalerolactone over carbon-supported noble metal catalysts. J. Ind. Eng. Chem. 2011, 17, 287−292. (9) Jiang, K.; Sheng, D.; Zhang, Z. H.; Fu, J.; Hou, Z. Y.; Lu, X. Y. Hydrogenation of levulinic acid to γ-valerolactone in dioxane over mixed MgO-Al2O3 supported Ni catalyst. Catal. Today 2016, 274, 55− 59. (10) Putrakumar, B.; Nagaraju, N.; Kumar, V. P.; Chary, K. V. R. Hydrogenation of levulinic acid to γ-valerolactone over copper catalysts supported on γ-Al2O3. Catal. Today 2015, 250, 209−217. (11) Upare, P. P.; Lee, J. M.; Hwang, Y. K.; Hwang, D. W.; Lee, J. H.; Halligudi, S. B.; Hwang, J. S.; Chang, J. S. Direct Hydrocyclization of biomass-derived levulinic acid to 2-methyltetrahydrofuran over nanocomposite copper/silica catalysts. ChemSusChem 2011, 4, 1749−1752. (12) Hengne, A. M.; Rode, C. V. Cu-ZrO2 nanocomposite catalyst for selective hydrogenation of levulinic acid and its ester to γvalerolactone. Green Chem. 2012, 14, 1064−1072. (13) Yan, K.; Chen, A. C. Efficient hydrogenation of biomass-derived furfural and levulinic acid on the facilely synthesized noble-metal-free Cu-Cr catalyst. Energy 2013, 58, 357−363. (14) Yan, K.; Liao, j. y.; Wu, X.; Xie, X. M. A noble-metal free Cucatalyst derived from hydrotalcite for highly efficient hydrogenation of biomass-derived furfural and levulinic acid. RSC Adv. 2013, 3, 3853− 3856. (15) Balla, P.; Perupogu, V.; Vanama, P. K.; Komandur, V. R. C. Hydrogenation of biomass-derived levulinic acid to γ-valerolactone over copper catalysts supported on ZrO2. J. Chem. Technol. Biotechnol. 2016, 91, 769−776. (16) Yan, K.; Liu, Y. Q.; Lu, Y. R.; Chai, J. J.; Sun, L. P. Catalytic application of layered double hydroxide-derived catalysts for the conversion of biomass-derived molecules. Catal. Sci. Technol. 2017, 7, 1622−1645.

Figure 6. Time-on-stream catalytic performance of Cu-Zn/ZrO2-20% Al2O3 catalyst.

catalysts studied, Cu-Zn/ZrO2-20%Al2O3 exhibits the best catalytic performance, and a conversion value of 96.0% and selectivity toward γ-valerolactone of 94% were obtained for 30 h of time on stream.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.K.). *E-mail: [email protected]. (J.L.) ORCID

Xiangjin Kong: 0000-0001-5291-4507 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grant No. 21406103), the Natural Science Foundation of Shandong Province (Grant No. ZR2015BM014), and the Foundation of Liaocheng University (Grant No. 318011303).

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REFERENCES

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Energy & Fuels

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DOI: 10.1021/acs.energyfuels.7b02140 Energy Fuels XXXX, XXX, XXX−XXX