Catalytic ketonization over metal oxide catalysts. XIII. Comparative measurements of activity of oxides of 32 chemical elements in ketonization of propanoic acid☆
Graphical abstract
Introduction
Various carbonyl compounds are used in organic synthesis and numerous synthetic methods have been developed for their preparation. Most of these methods are non-catalytic, batch-type processes, in which various solvents are used. Some are complex, multistep processes. From the industrial point of view, it would be beneficial to find a less expensive, easy to upscale, environmentally friendly (solvent-free), and preferably one-step (catalytic) synthesis.
In order for the process of attaining carbonyl compounds to be fairly inexpensive, both the substrates and the catalyst should not cost much. Carboxylic acids are very convenient starting materials in organic synthesis, because they commonly occur in nature and can be easily transformed. Since the 1850s the metallic salts of carboxylic acids have been used in the synthesis of ketones [1], [2] and aldehydes [3], [4]. Thermal decomposition of these salts leads to obtaining the appropriate carbonyl compounds, according to the following equations:(RCOO)2M → RC(O)R + MCO3(RCOO)2M + (HCOO)2M → 2 RCHO + 2 MCO3
Numerous carbonyl compounds have been synthesized thanks to this classical method, even those that had not been obtained otherwise [5], [6]. The literature concerning thermal decomposition of various metallic salts into ketones and aldehydes has already been reviewed [7]. However, the method became obsolete when its modification was introduced in 1895 [8]. In this modification, ketones and aldehydes are synthesised from carboxylic acids, not their salts. The method is a catalytic process carried out in the vapour phase. It is a simple, single-step, solvent-free method, which is carried out in a continuous mode under flow conditions. The method has been used in the preparation of many ketones and aldehydes [9], [10], [11], [12], [13], [14], [15], [16], [17]. A broad range of carboxylic acids, such as straight-chained [9], [18], branched [9], [17], cyclic [19], [20] or even unsaturated ones [18], [21] have been transformed into ketones in this way. In some cases, alkyl esters of carboxylic acids can replace pure acids in the ketonization reaction. This happens when a high molecular weight acid cannot vaporize without decomposing [22], [23], [24], or has a high melting point, e.g. dicarboxylic acids [25], [26], [27], [28].
Many catalysts have been studied in the synthesis of carbonyl compounds. According to the results of these studies, various metal oxides unsupported or, more often, deposited on the surface of inorganic supports are active in the ketonization of carboxylic acids. The use of the following oxides has been reported: alkaline earth metals [11], rare earth metals: Ce [9], [15] and others [29], [30], transition metals: Mn [15], [16], [21], [31], Fe [32], [33], Ti [16], [34], Cr [35], [36] and Zr [15], [37], the actinides: Th [12], [13], [38], [39] and U [40], as well as composite oxides of Ni, Co and Cu [9] and layered double hydroxides of Zn/Al and Mg/Al [41], [42]. It has been shown that a modification of a particular metal oxide (e.g. ZrO2) with alkali metal ions, such as Na+ or K+ enhances its activity in the ketonization of acetic acid [14].
Very recently a mechanism of catalytic ketonization of carboxylic acids has been studied experimentally and theoretically by DFT calculations [43]. The mechanism involving a β-ketoacid intermediate has been proposed on the basis of DFT results, as well as kinetic measurements.
Among many metal oxide catalysts tested in the ketonization reaction, a few of them have shown quite promising activity. However, in literature there is a lack of results of the activity measurements of different metal oxide catalysts performed under the same reaction conditions and from one substrate. Such studies would enable a comparison of the activities of various metal oxide catalysts and choose the most active ones. The present work fills this gap.
Section snippets
Catalyst preparation
Thirty two metal oxide catalysts deposited on the surface of inorganic supports were prepared. Three different supports were used: Al2O3 (alumina, C), SiO2 (silica, Aerosil), and TiO2 (titania, P-25), all from Degussa. The powders of these oxides were mixed with redistilled water until homogeneous slurries were obtained, and left for 24 h in closed containers at room temperature. Thus formed semi-transparent gels were dried at 313, 353 and 393 K for 24 h at each temperature. The lumps were
Results and discussion
Gas phase catalytic ketonization of propanoic acid in the presence of all of the prepared catalysts occurs according to the equation:2CH3CH2–COOH → CH3CH2–CO–CH2CH3 + CO2 + H2O
3-Pentanone (diethyl ketone) was the organic product of the reaction. The activities of catalysts containing 240 μmol cations of the chemical elements in the form of oxides supported on silica were studied at five different temperatures, i.e. 623, 648, 673, 698 and 723 K. The results indicate that pure silica exhibits a lack of
Conclusions
The activity of 32 metal oxide catalysts, each containing 240 μmol of cations of a metal deposited on the surface of silica, alumina or titania, in the ketonization of propanoic acid into 3-pentanone has been tested. This systematic study revealed that at the temperature range of 623–723 K, the studied catalysts can be divided into three groups:
- (i)
catalysts whose activity decreases steeply due to the partial reduction of their active phases to metals caused by contact with propanoic acid (oxides of
Acknowledgements
The authors wish to express their thanks to Ph.D. Ewa Iwanek (née Wilczkowska) for her helpful discussion about XRD results and valuable comments on the manuscript and to Prof. Ph.D. Krzysztof Jankowski from Chair of Analytical Chemistry, Faculty of Chemistry Warsaw University of Technology for performing ICP–OES measurements.
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Part XII—M. Gliński, A. Kozioł, D. Łomot, Z. Kaszkur, Appl. Catal. A: Gen. 323 (2007) 77–85.