CATALYTIC HYDROGENATION OF BENZOIC ACID TO BENZALDEHYDE

Abstract

The selective deoxygenation of aromatic acids to aldehydes is an example of "green" technology and an alternative to current aldehyde synthesis methods which are characterized by low yields and/or high amounts of waste formation. Aromatic aldehydes constitute an important group of fine chemicals that are used as intermediates in the production of pharmaceuticals such as analgetics, antipyrethics and antispasmodics and in the production of agrochemicals, in particular pyrethroid-based insecticides. Aromatic aldehydes can be produced in a variety of ways such as oxidation of alkylaromatics, halogenation of alkylaromatics followed by hydrolysis or halogenation of aromatic acids followed by hydrogenation, the so-called Rosenmund reduction. There are, however, serious drawbacks to these technologies. The yield of aldehydes is often unsatisfactory. For example, in the case of toluene oxidation a mixture of benzaldehyde, berioic acid and benzylalcohol is formed which has to be separated. Benzoic acid can be used to produce phenol but the added value of this process is rather small. Other processes require the use of corrosive reactants, which poses problems. Moreover, the production of waste in stoichiometric amounts, as in the case of the Rosentnund reduction, is undesirable because of its impact on the environment. An alternative method of preparing aromatic aldehydes is the selective deoxygenation of aromatic acids: R— COOH + H 2 R— CHO + H 2 0 ii With a suitable catalyst high aldehyde yields can be achieved in a one-step process while the formation of environmentally damaging waste is minimised. Another advantage of this process is that aromatic acids are a cheap feedstock that is they are more easily produced by oxidation of alkylaromatics than the less oxygenated aldehyde, For this reason the selective deoxygenation of benzoic acid has received much attention in industry as an alternative method for preparing aldehydes, as can be inferred from the large volume of patents (primarily on metal oxides) that has appeared over the last 20 years. Group III metals are found to present good behavior as hydrogenation catalysts when supported on highly porous materials such as alumina. Nevertheless, derived from the fact of working simultaneously with 3 phases-a solid catalyst, liquid reactants, and solvents, and gaseous hydrogen —several problems mainly of diffusional nature arise with respect hydrogen access to the liquid phase. Studying these problems in which their effects are negligible are essential before going any further. Hydrogen partial pressure is an important factor. Too fast hydrogenation induced by high hydrogen partial pressure may lead to mass transfer limitations that would result in decrease of the selectivity. Therefore a low hydrogen pressure is used and the space velocity (LHSV) is selected accordingly in order to get required conversion level. Hydrogenation reactions are a class of reactions that are valuable in the pharmaceutical and speciality chemical industry. Studies on catalytic hydrogenation reactions have been carried out in the gas phase or in the liquid phase. Nowadays attention is being led toward some liquid-phase processes of increasing importance in the chemical industry. iii Carrying out the reaction in the liquid phase, in a slurry reactor, presents important advantages with respect to the gas. phase. In liquid phase reactions, better temperature control due to higher heat capacity of the liquids, thus increasing the catalyst life and quality and uniformity of the reaction products, saving the devices and energy, higher rate of reaction per unit weight of catalyst because of the use of smaller particles, and higher conversions and selectivity attained. Hydrogenation is performed in liquid phase to save vaporization and condensation energy and at relatively low temperature. It has also been found out that for the same conversions reactions are more selective in liquid phase than the gas phase. In contrast relatively few studies exist in open literature on "vapour phase catalytic hydrogenation of benzoic acid to benzaldehyde" and no literature is available on liquid phase catalytic hydrogenation of benzoic acid. Moreover, those relevant publications provide conflicting views on the mechanism of this reaction. The present work has been carried out to invent a process for catalytic hydrogenation of benzoic acid to benzaldehyde in liquid phase. iv