Glycine–nitrate combustion synthesis of CuO–ZnO–ZrO₂ catalysts for methanol synthesis from CO₂ hydrogenation

Abstract

A series of CuO–ZnO–ZrO₂ (CZZ) catalysts were synthesized by a glycine–nitrate combustion method and characterized by XRD, BET, N₂O chemisorption, SEM and TPR techniques. The results show that the physicochemical properties of the catalysts are strongly influenced by the fuel content used in the combustion process. The dispersion of CuO exhibits an inverse-volcano variation trend with an increase in the glycine amount from 50% to 150% of the stoichiometry. The relationship between physiochemical properties and the fuel content is discussed in detail in terms of combustion temperature. The catalytic performance for the synthesis of methanol from CO₂ hydrogenation was examined. The CZZ catalyst exhibits an optimum catalytic activity when 50% of stoichiometric amount of glycine was used. The turnover frequency has been calculated for various CZZ catalysts, and it reveals that the catalytic activities depend not only on the surface area of metallic copper but also on the phase state of ZrO₂.

Introduction

It is well known that carbon dioxide is the most important greenhouse gas. With the increase in carbon dioxide concentration in the atmosphere, global warming problems are becoming more and more serious in recent years. Conversion of carbon dioxide to useful chemicals and fuels is one of the most promising ways to mitigate the problem. Most of the existing research focuses on methanol synthesis from CO₂ because methanol is a common chemical feedstock for several important chemicals and a potential alternative energy to fossil fuels [1], [2].

CuO–ZnO–Al₂O₃ catalysts have been widely used for methanol synthesis from syngas (CO + H₂); however, they exhibited a poor catalytic performance for the hydrogenation of CO₂ [3], [4]. The reason can be ascribed to the negative effect of water on the rate of methanol formation and the strong hydrophilic characteristic of alumina [3]. Zirconia-supported copper catalysts, which show an interesting catalytic behavior for CO₂ hydrogenation, have been well documented [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Among them, CuO–ZnO–ZrO₂ (CZZ) or modified-CZZ has gained an increasing interest for its high activity in recent years [3], [4], [8], [9], [10], [11], [12], [13], [14]. Apart from catalyst compositions, preparation methods have a considerable influence on the catalytic performance [3], [15], [16]. Numerous methods such as co-precipitation [7], [8], [9], [10], [14], impregnation [17] and sol–gel [5], [18] have been developed to prepare copper-based oxide catalysts. However, these methods are pH sensitive, time consuming or restricted by the deviation from stoichiometry and the requirement of expensive precursors [15], [19], [20]. Compared with those traditional methods, the combustion synthesis method, based on the principles of the propellant chemistry, has many advantages such as precise stoichiometric ratio, homogeneous component, low cost and short reaction time. Therefore, it has been an attractive technique for the synthesis of metal oxide powders in recent years [13], [21], [22], [23], [24], [25], [26]. In the combustion synthesis process, a thermally induced redox reaction takes place between an oxidant and a fuel. In general, the metal nitrates acting as cation sources are used as oxidants, whereas organic compounds such as citric acid, urea and glycine are employed as fuels [27], [28], [29]. The combustion characteristics are closely related to the selection of fuel. A good fuel should react non-violently and act as a complexant for metal cations [27]. Glycine, one of the simple amino acids, is known to act as a complexing agent for a number of metal ions since it has a carboxylic acid group at one end and amino group at the other end. Such types of zwitterionic character of a glycine molecule can effectively complex metal ions of varying ionic sizes, which helps in preventing their selective precipitation and maintaining compositional homogeneity among the constituents [27], [28]. Therefore, glycine-nitrate combustion synthesis has been a very popular combustion method for the preparation of metal oxide powders [22], [27], [28]. Arena et al. [13] prepared CZZ catalyst with a combustion method and investigated its catalytic action for methanol synthesis via CO₂ hydrogenation. However, the fuel of oxalic-di-hydrazide employed in their work is toxic (carcinogenic), and no related discussion about the combustion reaction was reported.

In this paper, we report the synthesis of CZZ catalysts using the glycine-nitrate combustion method and examine their catalytic properties for methanol synthesis from CO₂ hydrogenation. The combustion reactions are analyzed in terms of propellant chemistry, and the combustion behaviors are described as well. The prepared CZZ catalysts have been characterized by XRD, BET, SEM, H₂-TPR and reactive N₂O adsorption techniques. Since fuel content is an important parameter in combustion synthesis, the effect of different glycine amounts on the properties of the derived CZZ catalysts is discussed in detail. Furthermore, the catalytic performances of the CZZ catalysts are discussed in relation to the results of physicochemical characterization.