Catalytic ketonization over metal oxide catalysts. XIII. Comparative measurements of activity of oxides of 32 chemical elements in ketonization of propanoic acid

Highlights

• A systematic study of ketonization of propanoic acid in the presence of oxide catalysts was performed.

• The oxides of U, Th, Ce, Zr, Mn proved to be the most active.

• No synergistic effect was noted in binary oxide systems.

• Potassium ions significantly decrease the activity of the uranium oxide catalyst, but not that of thorium.

• Although Th and U catalysts are equally active, the use of the former is strongly recommended due to its much lower radioactivity.

Abstract

The ketonization of propanoic acid in the presence of many metal oxides deposited on the surface of inorganic supports leads to the formation of 3-pentanone as the main product. This paper is the first to investigate simple oxides of Ag, Bi, Cd, Cu, In, Pb, Re, Cr, Mg, Zn, Ca, Ga, Sr, Ba, Al, Eu, Gd, V, Co, Fe, La, Mn, Zr, Ce, Th and U in the ketonization of propanoic acid under the same conditions. Such experiments allowed us to compare the activities of these oxides. Our results clearly show that the studied oxides can be divided into three groups: slightly active, fairly active and highly active. The last group consists of catalysts containing oxides of: manganese, zirconium, cerium, thorium, and uranium. It has also been checked whether any binary catalytic system containing two of these very active oxides shows a higher activity than the single oxide systems. However, no synergistic effect was noted between these oxides. Moreover, the influence of the addition of an alkali metal cation (K⁺) to the two most active catalysts on their activities has been studied. The results of these experiments indicate that the addition of potassium cations to uranium oxide significantly decreases its activity in this reaction due to the formation of potassium uranates, whereas thorium oxide is much more resistant to alkali metal ion poisoning. Another reason why thorium catalysts are better for practical implementation is their significantly lower radioactivity.

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.