Elsevier

Journal of Catalysis

Volume 307, November 2013, Pages 1-17
Journal of Catalysis

Effect of the ZrO2 phase on the structure and behavior of supported Cu catalysts for ethanol conversion

https://doi.org/10.1016/j.jcat.2013.06.022Get rights and content

Highlights

  • Cu catalysts supported on am-, m-, and t-ZrO2 for selective ethanol conversion.

  • Cu/m-ZrO2 catalyst shows a high selectivity to ethyl acetate.

  • The high rate of ethyl acetate is defined by oxygen mobility from bulk ZrO2 to Cu.

  • A high rate for acetaldehyde formation is observed for Cu/t-ZrO2 catalyst.

Abstract

The effect of amorphous (am-), monoclinic (m-), and tetragonal (t-) ZrO2 phase on the physicochemical and catalytic properties of supported Cu catalysts for ethanol conversion was studied. The electronic parameters of Cu/ZrO2 were determined by in situ XAS, and the surface properties of Cu/ZrO2 were defined by XPS and DRIFTS of CO-adsorbed. The results demonstrated that the kind of ZrO2 phase plays a key role in the determination of structure and catalytic properties of Cu/ZrO2 catalysts predetermined by the interface at Cu/ZrO2. The electron transfer between support and Cu surface, caused by the oxygen vacancies at m-ZrO2 and am-ZrO2, is responsible for the active sites for acetaldehyde and ethyl acetate formation. The highest selectivity to ethyl acetate for Cu/m-ZrO2 catalyst up to 513 K was caused by the optimal ratio of Cu0/Cu+ species and the high density of basic sites (O2−) associated with the oxygen mobility from the bulk m-ZrO2.

Graphical abstract

Effect of ZrO2 phases on the formation of acetaldehyde and ethyl acetate from ethanol over Cu/ZrO2 catalysts.

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Introduction

The establishment targets for reducing greenhouse gas emissions that contribute to the global warming should have a significant increase in the consumption of biofuels in coming years, especially ethanol. This requires a substantial increase in ethanol production and, in this sense, encourages research and development of new raw materials for ethanol, such as lignocellulosic biomass, and the construction of integrated bio-refineries, a similar concept to that of oil refineries. Attention also turned to chemical ethanol or ethanol chemistry, as the way to increase the chemical production using a green chemistry route employing renewable resources [1].

Acetaldehyde is a product of ethanol dehydrogenation, which can be used as an intermediate in the production of acetic acid, acetic anhydride, ethyl acetate, butyraldehyde, crotonaldehyde, n-butanol, and many other chemicals. Ethyl acetate as one of the main products of ethanol dehydrogenation is an environmentally friendly organic solvent, which can be used for organic synthesis, printing inks, adhesives, and cosmetics.

Copper-based catalysts were found to be excellent catalysts for ethanol dehydrogenation because of their ability to maintain the C–C bond intact while dehydrogenating the CO–H bond [2]. Some investigations have been focused on the effect of support material on secondary product selectivity. In addition to simple dehydrogenation of ethanol, dehydrogenative coupling between the product acetaldehyde and the ethanol could also occur to produce ethyl acetate and hydrogen. Studies have shown that ethyl acetate can be obtained directly from ethanol employing specific heterogeneous catalysts [3], [4], [5], [6], [7], [8]. Iwasa and Takezawa [9] examined SiO2, ZrO2, Al2O3, MgO, and ZnO as support for Cu-based catalysts and found that the selectivity to acetaldehyde or ethyl acetate depends strongly on the support kind. As the ethanol dehydrogenation is an exothermic reaction, there is a formation of hot spots in the reactor which leads to sintering the Cu particles and to decrease in the catalyst activity [8]. ZrO2 has been described as preventive element for sintering of Cu crystallites under the reaction conditions and therefore was considered as a structural promoter and as an alternative support [10], [11]. Recently, rice husk ash (RHA) composed of amorphous silica has been used as a carrier for supported copper catalysts prepared by ion-exchanged, which showed very high Cu dispersion and stability in ethanol dehydrogenation to acetaldehyde at reaction temperature of 483–548 K [12].

Zirconium oxide has received considerable attention as a catalyst and support due to its unique properties; the preservation of both acid and basic sites, on the one hand, and reducing and oxidizing properties, on the other hand. As a result, the use of zirconia rather than silica or alumina as a structural promoter or more frequently as a support material has attracted considerable interest for a number of catalytic applications in the recent years. In particular, it has been found that the activity of Cu/ZrO2 catalysts is strongly dependent on the phase structure of ZrO2. Within this context, it was reported [13] that Cu catalysts supported over m-ZrO2 are more active in methanol synthesis than catalysts with the same Cu surface density deposited on t-ZrO2. The origin of these differences was explained by the presence of higher concentration of anionic defects on m-ZrO2 than on t-ZrO2 [14]. The different spacing and symmetry of the Zr–O and –OH bonds at t-ZrO2 and m-ZrO2 are considered to play key roles in determining the dispersion of the active metal component and the catalytic properties of Cu/ZrO2 system [14].

The high copper–zirconia interface has been considered as a key for obtaining highly active and selective copper catalysts with improved stability [15]. The surface metallic copper area as well as the kind of supported copper species is also expected to play a vital role in the catalytic systems. A correlation between the surface metallic Cu area and the catalyst activity was found [18]. New type of nanostructured Cu/ZrO2 materials featured with high component dispersion has been obtained for methanol steam reforming [16].

There are several reports in the literature concerning the ethyl acetate and acetaldehyde formation from ethanol on Cu-based catalysts [9], [17], [18]. But these reports are not focused on the active catalytic sites. To establish reliable reaction pathways models, the first step is the knowledge of the catalyst active sites, a point of vital importance to understand and predict the catalytic properties, the most ambitious goal in the field of catalysis.

ZrO2-supported Cu catalysts have been used in many others reactions such as methanol synthesis [14], steam reforming of methanol [19], [20], and water–gas shift reaction [21]. Based on the XPS and XAS experiments, it was reported the presence of Cu+ species in the CuO pathway reduction in Cu/ZrO2 catalysts [20], [22], [23]. Also, it was reported [19], [20] that quite stable Cu+ as intermediate Cu species in the CuO reduction were formed as active species in the steam reforming of methanol over Cu/ZrO2 catalysts, but there was no conclusion whether the partial copper oxidation was due to the Cu/ZrO2 interaction or to the presence of water. Although there was not directly detection of Cu+, it was proposed [8] that the presence of a certain amount of Cu+ species over Cu/ZrO2 catalyst is very important for improving the selectivity to ethyl acetate.

Understanding the chemistry occurring on the surface of oxide-supported metal clusters provides critical information for the design and development of superior heterogeneous catalysts. At the present, a few information exists in the literature about the direct relation between the electronic properties of copper-based catalysts supported on zirconia and the product selectivity in the ethanol conversion. In the present work for first time, it is attempted to demonstrate that the catalytic behaviors of Cu catalysts supported on different ZrO2 phases, such as amorphous (am-), monoclinic (m-), and tetragonal (t-), in the reaction of ethanol conversion are directly related to the electronic properties of supported copper species as well as to the specific properties of support. The study reports in detail the physicochemical characterization of Cu/ZrO2 catalysts in relation to their structure and acid–base properties as a function of the support kind. The influence of the ethanol pressure, the contact time, and the reaction temperature on the rate of ethyl acetate and acetaldehyde formation as well as on the product selectivity is clarified. To gain insight into the nature of supported copper species and supports, extensive characterization was carried out by: N2 adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance Fourier transform infrared spectroscopy of CO adsorption (DRIFTS), temperature-programmed reduction (TPR), in situ X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopy.

Section snippets

Catalyst preparation

Amorphous ZrO2 was prepared by sol–gel method described in Ref. [24]. Zirconium n-propoxide (Zr(OCH2CH2CH3)4 (10.9 ml, 70% in propanol) was dissolved in 233 ml anhydrous ethanol to obtain a 0.1 M solution. Deionized water was added dropwise to the zirconium n-propoxide solution to give water/alkoxide ratios of 32. A cloudy solution was obtained on the initial addition of water, which turned to a white suspension on complete addition. The suspension was aged at room temperature for 24 h, followed by

XRD

XRD patterns of pure ZrO2 supports and calcined Cu/ZrO2 samples are shown in Fig. 1. Intensive diffraction patterns at 2θ = 24.2°, 28.2°, 31.4°, and 34.3° of monoclinic ZrO2 crystal phase (JCPDS 37-1484) are predominant for Cu/m-ZrO2. In addition to the characteristic peaks of zirconia, XR diffractions at 2θ = 35.5° and 38.7° are detected for Cu/m-ZrO2, corresponding to crystalline CuO phase with tenorite structure (JCPDS 48-1548). The absence of crystalline CuO peaks in the XRD of Cu/am-ZrO2 and

Nature of the species in Cu/ZrO2 system

The present results show that the zirconia-supported copper species in reduced catalysts are in different oxidation states, in the form of Cu+ and Cu0, which distribution depends on the nature of ZrO2 support phase. According to the literature data [54], [55], the change in the proportion Cu0/Cu+ for copper samples supported on different zirconia phases can be attributed to several reasons that involve the following: (i) changes in the average Cu–Cu coordination number of Cu nanoparticle atoms

Conclusions

Copper catalysts supported on different ZrO2 polymorphs are found to be active catalysts for ethanol dehydrogenation to acetaldehyde or ethyl acetate depending on the kind of zirconia phase. The catalytic properties of Cu catalyst supported on am-, m-, or t-ZrO2 phase are due the specific electron density of supported copper species (Cu0 and Cu+) defined by the particle size and the interface at copper metal oxide support. The active surface sites for Cu/m-ZrO2 catalyst show that the higher

Acknowledgments

The authors gratefully acknowledge by support finance for this research from CNPq and FAPESP and the Brazilian Synchrotron Light Laboratory (LNLS) for the XAS beamline experiments (D06A-DXAS 10156) and (D04B-XAFS1). S.D. is gratefully acknowledged to Bulgarian Ministry of Education, Science and Youth (Project DTK-02/36).

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