Catal Lett DOI 10.1007/s10562-017-2165-7

Performance of ­SiO2–TiO2 Binary Oxides Supported Cu–ZnO Catalyst in Ethyl Acetate Hydrogenation to Ethanol Yan Huang1 · Weiyi Zhang1 · Zhi Yue1 · Xiangqing Zhao1 · Zhenmin Cheng1 

Received: 26 June 2017 / Accepted: 1 August 2017 © Springer Science+Business Media, LLC 2017

Abstract SiO2, ­TiO2, and ­SiO2–TiO2 binary oxides supported Cu–ZnO catalysts prepared by co-precipitation method were compared in the hydrogenation of ethyl acetate (EA) to ethanol (EtOH), and it shows the S ­ iO2–TiO2 binary oxides supported Cu–ZnO catalyst has a larger catalytic activity over the unitary ­SiO2 or ­TiO2 supported ones. Comprehensive characterizations by N ­ 2 adsorption, X-ray diffraction, ­H2-temperature programmed reduction, transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray induced Auger electron spectroscopy reveal that Cu– ZnO/SiO2–TiO2 catalyst has an appropriate ­Cu0/Cu+ molar ratio of about 1 with highly dispersed copper species and strong metal-support interaction. Furthermore, the electron transfer from T ­ iO2 to copper species can accelerate the activation of C=O and improve the catalytic performance. Under the optimized reaction conditions of P = 2.0 MPa, T = 523  K, WHSV = 0.5  h −1, and n(H2)/n(EA) = 20, the conversion of EA was up to 96% with EtOH selectivity of 99.5%.

Graphical Abstract 

Keywords  Ethyl acetate · Hydrogenation · Ethanol · Copper-based catalyst · SiO2–TiO2 binary oxides support

1 Introduction

* Zhenmin Cheng [email protected] 1



State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

As an important raw material, ethanol (EtOH) has been extensively used in chemical, pharmaceutical and food industries. Furthermore, EtOH is an important environmentally benign renewable energy, which can be blended into gasoline at a certain proportion to improve combustion property and reduce pollutant emission. To date, EtOH has been commercially produced by ethylene hydration and biological fermentation. However, both methods are facing great challenges. The first method is constrained by the diminishing crude oil resources [1], and the second one is limited by the low EtOH concentration and the subsequent separation cost in water removal [2]. Therefore, it is necessary to develop

13

Vol.:(0123456789)



new processes to meet the increasing demand of EtOH in the future. Recently, the synthesis of EtOH by ethyl acetate (EA) hydrogenation has attracted much attention, which could not only solve the overcapacity problem of EA, but also has the advantages of low production cost, high selectivity and convenient operation [3, 4]. Copper-based catalysts, which are highly selective in the hydrogenation of C=O bond and relatively inactive in the cleavage of C–C bond [5], have been widely used in ester hydrogenation reaction [6–8]. Over the past decades, the Cu–ZnO catalyst has been intensively studied for its excellent catalytic properties and the effects of Zn doping on the catalyst have been considerably investigated [9–11]. Kuld et al. [9] found a decrease in the surface energy of Cu causing by the Zn atoms incorporated into Cu surface under reduction conditions, which gave a good explanation of the enhancement of catalytic activity. To further improve the activity and stability of Cu–ZnO catalyst in ester hydrogenation, a variety of supports, such as Al, Zr, Si, and Ti, are investigated. Specifically, the ­SiO2 supported copper-based catalyst exhibits excellent catalytic activity as a result of the large specific surface area. Nevertheless, the copper particles tend to aggregate at high temperature, which create major barriers towards the industrial application [12]. As ­TiO2 is an n-type semiconductor, strong metal-support interaction often occurs in the ­TiO2 supported catalyst [13]. However, the poorly dispersed active species on the ­TiO2 surface restrict the further improvement of catalytic activity. Recently, ­SiO2–TiO2 binary oxides support have been investigated to improve catalytic activity. Liu et al. [14] observed the T ­ iO2–SiO2 supported Ni–W catalyst displayed an excellent sulfur-tolerant stability in syngas methanation reaction. Han et al. [15] demonstrated that the improved catalytic performance of the ­SiO2–TiO2 supported Rh-based catalyst may be due to the higher dispersion of Rh, the better capability for CO adsorption and dissociation. Although there are some applications of S ­ iO2–TiO2 binary oxides, the effect of the S ­ iO2–TiO2 mixed oxides support on the Cu–ZnO catalyst has received little attention. In the past decades, different catalysts have been reported in EA hydrogenation to EtOH. Zhang et al. [16] used the Nibased catalysts and found the RE1NASH-110-3 (Ni/Al = 3) had the highest selectivity (68.2%) and yield (61.7%). Schittkowski et al. [17] found the Cu/ZrO2 catalyst with CuO loading of 31 wt% exhibited the best catalytic performance, and the conversion of EA reached 50.4% while the EtOH selectivity approached 43.7%. Lu et al. [18] reported that 80% EA conversion with 95% EtOH selectivity could be obtained over the Cu/ZnO/Al2O3 catalyst. In these work, the yield of EtOH in EA hydrogenation reaction needs to be substantially increased in order to achieve industrial demand. In this work, ­S iO 2 , ­T iO 2 , and ­S iO 2 -TiO 2 binary oxides supported Cu–ZnO catalysts were prepared by

13

Y. Huang et al.

co-precipitation method, aiming at improving the yield of EtOH in EA hydrogenation. The physicochemical properties of the prepared catalysts were characterized extensively by ­N2 adsorption, X-ray diffraction (XRD), ­H2-temperature programmed reduction ­(H2-TPR), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger electron spectroscopy (XAES). The effects of reaction temperature, molar ratio of H ­ 2 to EA and weight hourly space velocity (WHSV) on catalytic performance were investigated. Furthermore, the stability test of the best performed catalyst was also performed.

2 Experimental 2.1 Preparation of Catalyst Copper-based catalysts with different supports were prepared by continuous co-precipitation method. For the preparation of Cu-ZnO/SiO2–TiO2 catalyst, required amount of Cu(NO3)2·5H2O and Zn(NO3)2·4H2O were first dissolved in deionized water. The total cation concentration was 1 mol L−1 and Cu/Zn molar ratio was maintained at 2:1. Calculated amount of silica sol and T ­ iO2 were mixed with deionized water in a beaker under vigorous stirring. Subsequently, the two aqueous solutions of metallic salts and sodium carbonate (1 mol L−1) were simultaneously added dropwise to the beaker in a water bath at 343 K under vigorously stirring, the pH of the reaction mixture was adjusted to 8.0 ± 0.1 by changing the flow rate of N ­ a2CO3 aqueous solution. When co-precipitation was finished, the precipitate was aged at 343 K for 1 h and then filtered and washed with deionized water. After dried in air at 383 K for 12 h, the precipitate was calcined at 673 K for 4 h. Finally, the calcined catalyst was crushed into 40–60 mesh. 2.2 Characterization of Catalyst N2 adsorption/desorption analysis was conducted at 77 K using a Micromeritics ASAP 2020 apparatus. The surface area was calculated by the BET equation. The total pore volume was calculated from the absorbed N ­ 2 volume at a relative pressure of approximately 0.99. XRD was employed to characterize the bulk crystal structures of the catalysts. XRD patterns were recognized on a Siemens D500 diffractometer employing Cu Kα radiation at 40 kV and 30 mA. The scanning 2θ range was 10–80° at a scanning rate of 8° min−1. The crystallite size of Cu in the reduced catalysts was calculated by the Scherrer equation, D = 0.89l∕(βcosθ), where β was the full width of the diffraction line at half of the maximum intensity. TEM measurements of the reduced catalysts were performed on JEOC-2010F Electron Microscopy operated at

Performance of ­SiO2–TiO2 Binary Oxides Supported Cu–ZnO Catalyst in Ethyl Acetate…

200.0 kV. Before TEM analysis, the reduced catalysts were ultrasonically dispersed into alcohol for 30 min, and then dropped on copper grid. The TPR profiles of the catalysts were obtained on a Micromeritics Auto-chem 2920 instrument. A certain amount of calcined catalyst was reduced in 10 vol.% ­H2/ Ar at a flow rate of 50 mL min−1 with a heating rate of 5 K min−1 up to 873 K after degassed by Ar at 423 K for 1 h. XPS and XAES measurements were carried out on a 5000C ESCA spectrometer equipped with Mg Kα radiation source (hv = 1253.6 eV). The binding energies were calibrated using the C1s peak at 284.6 eV as the reference. 2.3 Catalyst Activity Test EA Hydrogenation was carried out in a continuous flow fixed-bed reactor of 60 cm long with 10 mm inside diameter. Required amount of unreduced catalyst (40–60 mesh) was loaded in the middle of the reactor, with two ends packed with quartz sands. Prior to reaction, the catalyst was reduced by ­H2 stream with the flow rate of 80 mL min−1 at 523 K for 4 h. Then EA solution and ­H2 were fed into the reactor with a required molar ratio after mixing and preheating. The reaction products were condensed in an ice water bath and collected after reaction for 6 h. The reaction products were analyzed on a gas chromatography (GC-2014) equipped with a flame ionization detector (FID). The conversion of EA, selectivity and yield of EtOH were defined as follows: ( ) XEA = nEA,0 − nEA ∕nEA,0 × 100% (1) ( ) ( ) SEtOH = nEtOH − nEtOH,0 ∕ nEA,0 − nEA × vEA ∕vEtOH × 100% (2)

YEtOH

( ) = nEtOH − nEtOH,0 ∕nEA,0 × vEA ∕vEtOH × 100%

(3)

where nEA,0, nEA and nEtOH,0, nEtOH represent the number of moles of EA and EtOH in the feed and products, respectively. vEA and vEtOH are the stoichiometric number of the reaction equation.

3 Results and Discussion 3.1 Structural and Textural Properties The ­N 2 adsorption–desorption isotherms of Cu–ZnO, Cu–ZnO/TiO 2, Cu–ZnO/SiO 2 and Cu–ZnO/SiO 2–TiO 2 catalysts are shown in Fig. 1. All four catalysts exhibited type IV isotherms with a regular H1-type hysteresis loop due to N ­ 2 capillary condensation in cylindrical channels. The specific surface area, total pore volume and average pore diameter of ­SiO2, ­TiO2 and corresponding catalysts are listed in Table 1. As can be seen from data calculated

Fig. 1  N2 adsorption–desorption isotherms of different catalysts

in Table 1, different support has a profound influence on textural properties. The BET surface area and pore volume of Cu–ZnO catalyst were 13.1 m2 g−1 and 0.06 cm3 g−1, with the average pore diameter of 21.9 nm. The Cu–ZnO/ TiO2 catalyst had a specific surface area of 15.4 m 2 g−1, which was similar with the Cu–ZnO catalyst. With the addition of ­SiO2, the BET surface area and pore volume of both Cu–ZnO/SiO2 and Cu–ZnO/SiO2–TiO2 catalysts were larger than that of the Cu–ZnO/TiO 2 catalyst. It may be attributed to large specific surface area of copper phyllosilicates formed in the Cu–ZnO/SiO2 and Cu–ZnO/ SiO2–TiO2 catalysts [19, 20]. The XRD patterns of the calcined catalysts are shown in Fig. 2a. The diffraction peaks at 2θ = 35.6°, 38.8°, 48.8°, 61.5° and 66.2° were ascribed to the CuO phase (JCPDS #48-1548), and those at 31.8°, 34.2°, 36.2°, 47.5°, 56.6°, 62.9° and 67.9° were ascribed to ZnO phase (JCPDS #361451). Figure 2b shows the XRD patterns of the reduced catalysts. The typical diffraction peaks at 2θ = 43.3°, 50.4° and 74.1° were attributed to (1 1 1), (2 0 0) and (2 2 0) crystal planes of crystallized Cu phase (JCPDS #04-0836). The characteristic peaks of the ­SiO2 and ­TiO2 phase were not observed on the Cu–ZnO/SiO2, Cu–ZnO/TiO2 and Cu–ZnO/ SiO2–TiO2 catalysts, indicating that ­SiO2 and T ­ iO2 were in amorphous phase, or well-dispersed in the catalysts [21]. After the addition of S ­ iO2 or/and ­TiO2 to the copper-based catalyst, the line width of diffraction peaks for CuO and ZnO were broadened, especially for the Cu–ZnO/SiO2 and Cu–ZnO/SiO2–TiO2 catalysts. The average crystallite size of copper particles calculated by the Scherrer equation are displayed in Table 1. It shows the crystallite size of the metallic Cu decrease in an order of Cu–ZnO > Cu–ZnO/ TiO2 > Cu–ZnO/SiO2 > Cu–ZnO/SiO2–TiO2, which indicate

13

Table 1  Textural properties of ­SiO2, ­TiO2 and corresponding catalysts

Y. Huang et al. Samples

Crystallite size of Cu(111) (nm)

Specific surface area Total pore volume ­(m2 g−1) ­(cm3 g−1)

Pore diameter (nm)

SiO2 TiO2 Cu–ZnO Cu–ZnO/SiO2 Cu–ZnO/TiO2 Cu–ZnO/SiO2–TiO2

\ \ 30.6 12.6 18.5 10.0

186.8 73.7 13.1 66.8 15.4 26.5

15.2 22.2 21.9 15.9 39.7 24.9

0.74 0.43 0.06 0.30 0.17 0.18

catalysts. The corresponding dispersion of copper species with different supports can be directly observed. The dark particles can be deduced as copper species and the light gray particles as oxides of zinc, silica or titanium. The active components in Cu–ZnO catalyst shown in Fig. 3a present in large particle size and poor dispersion. With the addition of ­TiO2, the particle size became smaller but the aggregation phenomenon was still severe. The low specific surface area of ­TiO2 (73.7 m2 g−1) was a major factor for the poor dispersion of copper particles in the Cu–ZnO/TiO2 catalyst. For the Cu–ZnO/SiO2 and Cu–ZnO/SiO2–TiO2 catalyst, the copper particles were dispersed more uniformly on the support. The high dispersion could be attributed to the relatively larger specific surface area of S ­ iO2 (186.8 m2 g−1) that provided favorable dispersing effect for the copper species. Moreover, the copper particle size distributions obtained from TEM images were well consistent with the crystallite size estimated by the Scherrer equation. 3.2 H2‑TPR Measurement

Fig. 2  XRD patterns of the a calcined catalysts and b reduced catalysts

that the support composition has a significant effect on crystallite size of copper species. Figure 3 shows the TEM images of the reduced Cu–ZnO, Cu–ZnO/SiO 2, Cu–ZnO/TiO 2 and Cu–ZnO/SiO 2–TiO 2

13

The ­H2-TPR profiles of calcined catalysts are shown in Fig. 4. Cu–ZnO catalyst had an asymmetric peak at 585 K with a distinct shoulder peak in the lower temperature region. The shoulder peak was attributed to the highly dispersed CuO particles, whereas the peak at higher temperature was related to the reduction of bulk-phase CuO. It indicated that the poor dispersion of copper species in Cu–ZnO catalyst restricted the copper oxide reduction seriously. The addition of ­SiO2 improved CuO reducibility as demonstrated by the shift of peak to a lower temperature in comparison with Cu–ZnO catalyst. The better copper dispersion and decreased Cu crystallite size by addition of ­SiO2 to Cu–ZnO catalyst made CuO can be reduced much easier [22, 23]. The Cu–ZnO/TiO2 catalyst showed an asymmetric peak at a lowest reduction temperature. The enhancement of reducibility of Cu–ZnO/TiO 2 catalyst resulted from the strong interactions between the copper species and ­TiO2 support. As an n-type semiconductor, T ­ iO2 tended to produce a high concentration of n-type defect such as unstable ­Ti3+ ions and oxygen vacancies on the surface under

Performance of ­SiO2–TiO2 Binary Oxides Supported Cu–ZnO Catalyst in Ethyl Acetate…

Fig. 3  TEM images of the a Cu–ZnO, b Cu–ZnO/TiO2, c Cu–ZnO/SiO2, d Cu–ZnO/SiO2–TiO2 catalysts

3.3 Chemical State and Surface Composition

Fig. 4  H2-TPR patterns of the copper-based catalysts

reduction condition. The unstable ­Ti3+ recovered to ­Ti4+ by donating electric charges to the copper species, and the reduction capacity of copper oxide species was promoted [24]. Therefore, the reducibility of Cu–ZnO/SiO2–TiO2 catalyst was improved as well when compared with the Cu–ZnO/SiO2 catalyst.

To identify the surface chemical states of the copper species in the reduced catalysts, XPS spectra as well as the XAES of the reduced catalysts were measured. Figure 5 shows the XPS results of the representative Cu–ZnO/SiO2, Cu–ZnO/ TiO2 and Cu–ZnO/SiO2–TiO2 catalysts. Two main peaks observed in the XPS spectra can be ascribed to Cu 2­ p1/2 and Cu ­2p3/2. The broad ­Cu2p3/2 peak at above 933 eV together with the Cu 2p shakeup satellite peaks indicated the existence of ­Cu2+ species in the three catalysts. Notably, the Cu 2p shakeup satellite peak of Cu–ZnO/SiO2–TiO2 catalyst was much smaller than that of the Cu–ZnO/SiO2 and Cu–ZnO/TiO2 catalysts, suggesting the amount of C ­ u2+ on surface of the Cu–ZnO/SiO2–TiO2 catalyst was relatively less. The broad and asymmetric Cu ­2p3/2 spectra could be deconvoluted into two overlapping peaks, which were ascribed to ­Cu+/Cu0 and ­Cu2+ species, respectively. In contrast with the standard binding energy of Cu 2­ p 3/2 (932.6 eV), the negative shift of Cu ­2p3/2 binding energy for all three representative catalysts could be observed, which indicated that different degrees of interactions were existed between metallic Cu and the support. The smallest shift of Cu ­2p3/2 binding energy observed in Cu–ZnO/SiO2 catalyst was likely due to interactions between Cu and the silica

13



Fig. 5  Cu2p XPS spectra of Cu–ZnO/SiO2, Cu–ZnO/TiO2, Cu–ZnO/ SiO2–TiO2 catalysts

support, i.e., the formation of copper phyllosilicates [20]. The much lower binding energy of Cu ­2p3/2 in the Cu–ZnO/ TiO2 catalyst can be related to more negative charges on the copper surfaces derived from the electron transfer from ­Ti4+ to Cu, which was confirmed by ­H2-TPR result. For the Cu–ZnO/SiO2–TiO2 catalyst, there would be more negative charges on the surface of copper species than Cu–ZnO/TiO2 catalyst, because of the enhanced dispersion of copper species with the addition of S ­ iO2 as proved by TEM characterization. Accordingly, the largest shift of Cu ­2p3/2 binding energy was observed in Cu–ZnO/SiO2–TiO2 catalyst. Because the binding energies of ­Cu0 and ­Cu+ are almost the same, the modified Auger parameter α′, which equals to the sum of binding energy and kinetic energy, was used to determine the amount of ­Cu0 and ­Cu+ on surface of the reduced catalysts. The Cu LMM XAES spectra are shown in Fig. 6 and the XPS parameters of the reduced catalysts are displayed in Table 2. The α′ value at about 1851.0 and ­ u+, respectively [25]. 1849.0 eV were ascribed to C ­ u0 and C 0 ­ u+ are coexisting on The results revealed that both C ­ u and C the surface of the reduced Cu–ZnO/SiO2, Cu–ZnO/TiO2 and Cu–ZnO/SiO2–TiO2 catalysts, and the ­Cu+ and C ­ u0 distributions were remarkably influenced by the support.

Y. Huang et al.

Fig. 6  XAES spectra of Cu–ZnO/SiO2, Cu–ZnO/TiO2, Cu–ZnO/ SiO2–TiO2 catalysts Table 2  XPS parameters of Cu–ZnO/SiO2, Cu–ZnO/TiO2, Cu–ZnO/ SiO2–TiO2 catalysts Catalyst

Binding energy (eV)

Kinetic energy (eV)

α′

Assignment Xa (%)

Cu–ZnO/ SiO2–TiO2

932.0 932.0 932.5 932.5 932.3 932.3

919.3 917.1 918.4 916.5 919.0 917.7

1851.3 1849.1 1850.9 1849.0 1851.3 1850.0

Cu0 Cu+ Cu0 Cu+ Cu0 Cu+

Cu–ZnO/SiO2 Cu–ZnO/TiO2

1.075 0.681 0.502

a

 X = Cu0/Cu+

3.4 Catalytic Performance The catalytic activities of four catalysts in EA hydrogenation are provided for comparison in Fig. 7. It shows the Cu–ZnO catalyst had the lowest catalytic activity, and EA conversion was only 75.3% with the EtOH selectivity of 85.6%. With the addition of ­TiO2, EA conversion increased from 75.3

13

Fig. 7  Catalytic performance of the copper-based catalysts. Reaction conditions: T = 523 K, P = 2 MPa, ­H2/EA = 20, WHSV = 0.5 h−1

Performance of ­SiO2–TiO2 Binary Oxides Supported Cu–ZnO Catalyst in Ethyl Acetate…

to 81.2%, and EtOH selectivity was enhanced from 85.6 to 90.2%, which indicated the existence of electron transfer from ­TiO2 to Cu played an important role in improving the catalytic activity because the interactions between copper species and support was enhanced. It should be noted, the small specific area of ­TiO2 limited further improvement of catalytic activity of Cu–ZnO/TiO2 catalyst because of the poorly dispersed copper species. Additionally, the activity of Cu–ZnO/SiO2 catalyst was higher than that of Cu–ZnO/TiO2 catalyst, with the EA conversion of 92% and the EtOH selectivity of 98.3%, owning to the large specific surface area and the small crystallite size of copper particles. Furthermore, the Cu–ZnO/SiO2–TiO2 catalyst showed the highest EtOH yield of 95.5%, which increased by approximately 48% in comparison with the Cu–ZnO catalyst. Characterization of the Cu–ZnO/SiO2–TiO2 catalyst revealed that it had small particle size of 10 nm, large specific surface area of 26.5 cm2 and strong reducibility. The S ­ iO2–TiO2 binary oxides supported copper catalyst combined the advantages of ­SiO2 and ­TiO2, not only the dispersion of active copper species was improved but also the interactions between copper species and binary oxides support was strengthened, giving rise to a superior catalytic activity. The valences of cupreous species were also of great importance to the catalytic performance of esters hydrogenation over copper-based catalysts. Although there is no consensus on the suitable ratio of ­Cu0 to ­Cu+, the viewpoint that both ­Cu0 and ­Cu+ are playing indispensable roles in the C=O hydrogenation is generally accepted. The ­Cu+ sites adsorb the methoxy and acyl species, while the ­Cu0 facilitate the ­H2 decomposition [26]. Moreover, C ­ u+ may function as electrophilic or Lewis acid sites to polarize the C=O bond via the electron lone pair on oxygen [27]. Some reported that the catalysts with high ratio of C ­ u0/Cu+ had better catalytic performance [5, 28, 29], some suggested that a low proportion of ­Cu0/ ­Cu+ were important to obtain an outstanding hydrogenation performance [7, 30], while others hold that a ­Cu0/Cu+ ratio of about 1 is beneficial to the optimal activity in ester hydrogenation [31–34]. Wang et al. [31] found that the Cu/SBA-15-H catalyst with a C ­ u+/(Cu+ + Cu0) ratio of 53.5% showed the best performance in methyl acetate hydrogenation. Dandekar et al. [32] concluded that optimum activity was achieved when ­Cu0 and ­Cu+ sites coexist on the catalyst surface in approximately equal amounts. Chen et al. [33] discovered the CuSi-363 catalyst with a ­Cu+/ (Cu+ + Cu0) ratio of 54.9% had the highest hydrogenation activity in hydrogenation of dimethyl oxalate to ethylene glycol. In the present work, the Cu–ZnO/SiO2–TiO2 catalyst with a C ­ u0/Cu+ ratio of 1.075 displayed the best catalytic performance in EA hydrogenation to EtOH, which verified that a C ­ u0/Cu+ molar ratio of about 1 is most suitable to get the best hydrogenation performance. The cooperative effect between C ­ u0 and C ­ u+ are favorable in activating the

ester group of EA and simultaneously dissociating ­H2, which plays an important role in improving the reactivity of ester group. In addition to C ­ u+ species, numerous studies concluded that ­TiO2 is capable of promoting the activation of C=O bond and enhancing the catalytic performance. Baker et al. [35] identified that the charge-transfer interactions between the furfuraldehyde molecular and O– vacancies site on the ­TiO2 surface favored the activation of C=O bond and the formation of the active furfuryl-oxy intermediate. Manyar et al. [36] suggested that the interactions between carbonyl oxygen of the acid and the support metal ion/oxygen vacancy created in the reducible oxide weakened the C=O and promoted hydrogenation and carbon–oxygen bond cleavage. Thus, it can be deduced that electron transfer from ­TiO2 to Cu can increase the electronegativity of copper species and accelerate the activation of C=O bond in the Cu–ZnO/ SiO2–TiO2 catalyst. 3.5 Optimization of reaction conditions In EA hydrogenation, catalytic performance is subjected to the reaction conditions, such as the reaction temperature, the space velocity and the ­H2/ester molar ratio. Therefore, the best performed Cu–ZnO/SiO2–TiO2 catalyst was selected in this work to investigate the effect of reaction conditions on catalytic activity. 3.5.1 Effect of Reaction Temperature The catalytic activity as a function of reaction temperature is shown in Fig. 8. Under the reaction conditions of

Fig. 8  Effect of reaction temperature on EA conversion and EtOH selectivity. Reaction conditions: P  =  2  MPa, H ­2/EA = 20, WHSV = 0.5 h−1

13



P = 2  MPa, n(H2)/n(EA) = 20, WHSV = 0.5  h−1, the EA conversion increased from 82.4 to 98.0% when temperature was increased from 503 to 543 K. The selectivity of EtOH could maintain at a high value of more than 98% over the temperature range of 503–523 K, but decreased to 95.1% when the temperature was increased to 543 K. The results indicated the reaction is kinetically controlled over the temperature range of 503–543 K, so reaction rate increase with the increasing of the temperature. But EtOH can be subsequently converted into aldehyde and diethyl ether at elevated temperature (e.g., above 523 K) [17], leading to the reduction of selectivity. 523 K is chosen as the optimum reaction temperature under comprehensive consideration. 3.5.2 Effect of ­H2/Ester Molar Ratio The influence of n(H2)/n(EA) on the performance of EA hydrogenation is shown in Fig. 9, where a very profound rise in EA conversion upon high molar ratio of H ­ 2 to ester is observed. When the n(H2)/n(EA) was enhanced from 5 to 25, the EA conversion increased from 73 to 93%, while the EtOH selectivity maintained at about 99%. In principle, increasing the n(H2)/n(EA) is favorable to the forward reaction and EA conversion because EA hydrogenation is a reversible reaction with reduced number of molecules. But an excessive n(H2)/n(EA) will largely increase the operation cost in view that the conversion of EA increases slightly when n(H2)/n(EA) increases to a relatively large value. In this work, 20 is chosen as the optimum ­H2 to ester molar ratio. It is necessary to point out that most studies were carried out under high molar ratio of ­H2 to ester, indicating the Cu–ZnO/SiO2–TiO2 catalyst prepared in this paper had a comparatively higher activity.

Fig. 9  Effect of molar of H ­ 2/EA on EA conversion and EtOH selectivity. Reaction conditions: T = 523 K, P = 2 MPa, WHSV = 0.5 h−1

13

Y. Huang et al.

3.5.3 Effect of WHSV The effect of WHSV on EA hydrogenation was further investigated by varying WHSV from 0.5 to 2.0 h−1. The obtained results are presented in Fig. 10. The selectivity of EtOH remained almost unchanged when WHSV was increased from 0.5 to 2.0 h−1, accompanied by a linear decrease of EA conversion, demonstrating that EtOH selectivity is not affected by the WHSV. Since WHSV is related to the productivity, WHSV should not be too small, and 0.5 h−1 is recommended as the optimum WHSV as a compromise. 3.6 Catalyst Stability The catalyst with a long-term stability is essential for the industrialization. For the unitary ­SiO2 supported copperbased catalyst, it is easy to deactivate especially for the long-term running due to the weak interactions between copper species and S ­ iO2 support [37]. In this regard, the long-time stability of the Cu–ZnO/SiO 2–TiO 2 catalyst under the optimum reaction conditions was studied and displayed in Fig. 11. As demonstrated in Fig. 11, the Cu–ZnO/ SiO2–TiO2 catalyst retained high catalytic activity even after 45 h. The results indicated that the catalytic stability was greatly enhanced when the ­SiO2–TiO2 binary oxides was used as support, which could be attributed to strong interactions between copper species and support that weakened the particle aggregation.

4 Conclusion In conclusion, the physicochemical properties and catalytic performances of copper-based catalysts were greatly

Fig. 10  Effect of WHSV on EA conversion and EtOH selectivity. Reaction conditions: T = 523 K, P = 2 MPa, n(H2)/n(EA) = 20

Performance of ­SiO2–TiO2 Binary Oxides Supported Cu–ZnO Catalyst in Ethyl Acetate… 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 1 4. 15. Fig. 11  Stability of Cu–ZnO/SiO2–TiO2 catalyst in EA hydrogenation. Reaction conditions: T = 523  K, P = 2  MPa, H ­ 2/EA = 20, WHSV = 0.5 h−1

influenced by supports. The ­SiO2–TiO2 binary oxides supported Cu–ZnO catalyst exhibited the best catalytic activity compared with unitary ­SiO2 or ­TiO2 supported catalyst. Characterization of the catalysts revealed that Cu–ZnO/ SiO2–TiO2 catalyst had highly dispersed copper species and strong interactions between Cu and support, indicating the synergic effect of S ­ iO2 and T ­ iO2 were combined in Cu–ZnO/SiO2–TiO2 catalyst. The appropriate ­Cu0/Cu+ molar ratio of about 1 in Cu–ZnO/SiO2–TiO2 catalyst can significantly facilitate the adsorption of C=O and H ­ 2 decomposition. Additionally, the electrons transfer from T ­ iO2 to copper species accelerated the activation of C=O bond and promoted the catalytic performance. The EA conversion and EtOH selectivity of Cu–ZnO/SiO2–TiO2 catalyst could reach 96 and 99.5% respectively under the optimum reaction conditions of P = 2.0 MPa, T = 523 K, WHSV = 0.5 h−1, and n(H2)/n(EA) = 20. Furthermore, the Cu–ZnO/SiO2–TiO2 catalyst had good stability and showed good prospect for industrial application. Acknowledgements  The authors are grateful to the financial supports from the Natural Science Foundation of China (21676085) and the Fundamental Research Funds for the Central Universities of China (WA1113008).

References 1. Katada N, Iseki Y, Shichi A, Fujita N, Ishino I, Osaki K, Torikai T, Niwa M (2008) Appl Catal A 349:55 2. Zabed H, Sahu JN, Suely A, Boyce AN, Faruq G (2017) Renew Sust Energy Rev 71:475

16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 2 6. 27. 2 8. 29. 3 0. 31. 3 2. 33. 3 4. 35. 36. 37.

Zhong K, Wang X (2014) Int J Hydrogen Energy 39:10951 Zhu YM, Shi L (2014) J Ind Eng Chem 20:2341 Brands DS, Poels EK, Bliek A (1999) Appl Catal A 184:279 Upare PP, Lee JM, Hwang YK, Hwang DW, Lee JH, Halligudi SB, Hwang JS, Chang JS (2011) ChemSusChem 4:1749 Ye CL, Guo CL, Zhang JL (2016) Fuel Process Technol 143:219 Zhu YF, Zhu YL, Ding GQ, Zhu SH, Zheng HY, Li YW (2013) Appl Catal A 468:296 Kuld S, Thorhauge M, Falsig H, Elkjær CF, Helveg S, Chorkendorff I, Sehested J (2016) Science 352:969 Kim SM, Lee ME, Choi JW, Suh DJ, Suh YW (2011) Catal Commun 12:1328 Kattel S, Ramirez PJ, Chen JG, Rodriguez JA, Liu P (2017) Science 355:1296 Natesakhawat S, Lekse JW, Baltrus JP, Ohodnicki PR, Howard BH, Deng XY, Matranga C (2012) ACS Catal 2:1667 Zeng YQ, Wang TX, Zhang SL, Wang YN, Zhong Q (2017) Appl Surf Sci 411:227 Liu B, Ji SF (2013) J Energy Chem 22:740 Han LP, Mao DS, Yu J, Guo QS, Lu GZ (2012) Catal Commun 23:20 Zhang BX, Lin L, Zhuang JP, Liu Y, Peng LC, Jiang LF (2010) Molecules 15:5139 Schittkowski J, Tölle K, Anke S, Stürmer S, Muhler M (2017) J Catal 352:120 Lu ZP, Yin HB, Wang AL, Hua J, Xue WP, Yin HX, Liu SX (2016) J Ind Eng Chem 37:208 Van Der Grift CJG, Elberse PA, Mulder A, Geus JW (1990) Appl Catal 59:275 Di W, Cheng JH, Tian SX, Li J, Chen JY, Sun Q (2016) Appl Catal A 510:244 Batyrev ED, van den Heuvel JC, Beckers J, Jansen WPA, Castricum HL (2005) J Catal 229:136 Xi XY, Ma SY, Chen JF, Zhang Y (2014) J Environ Chem Eng 2:1011 Astudillo J, Águila G, Díaz F, Guerrero S, Araya P (2010) Appl Catal A 381:169 Zhu HY, Wu Y, Zhao X, Wan HQ, Yang LJ, Hong JM, Yu Q, Dong L, Chen Y, Jian C, Wei J, Xu PH (2006) J Mol Catal A 243:24 Fantauzzi M, Atzei D, Elsener B, Lattanzi P, Rossi A (2006) Surf Interface Anal 38:922 Poels EK, Brands DS (2000) Appl Catal A 191:83 Yin AY, Guo XY, Dai WL, Fan KN (2009) J Phys Chem C 113:11003 Li F, Lu CS, Li XN (2014) Chin Chem Lett 25:1461 Ding TM, Tian HS, Liu JC, Wu WB, Zhao BQ (2016) Catal Commun 74:10 He Z, Lin HQ, He P, Yuan YZ (2011) J Catal 277:54 Wang SR, Guo WW, Wang HX, Zhu LJ, Yin S, Qiu KZ (2014) New J Chem 38:2792 Dandekar A, Baker RTK, Vannice MA (1999) J Catal 184:421 Chen LF, Guo PJ, Qiao MH, Yan SR, Li HX, Shen W, Xua HL (2008) J Catal 257:172 Deutsch KL, Shanks BH (2014) Appl Catal A 470:390 Baker LR, Kennedy G, Van Spronsen M, Hervier A, Cai XJ, Chen SY, Wang LW, Somorjai GA (2012) J Am Chem Soc 134:14208 Manyar HG, Paun C, Pilus R, Rooney DW, Thompsona JM, Hardacre C (2010) Chem Commun 46:6279 Yue HR, Zhao YJ, Zhao L, LV J, Wang SP, Gong JL, Ma XB (2012) AIChE J 58:2798

13