Vapours competing reactions

A characteristic of catalysis processes is that a variety of compounds may catalyse a particular reaction, but only one or two of these catalysts show enough selectivity, activity and stability to warrant use in an industrial process. Selectivity is the ability of a catalyst to increase the relative rate of formation of a desired product when two or more competing reactions may occur. For modification of the direction of a reaction, mixed catalysts consisting of two compounds both with moderate to good catalytic activity have been developed. For example, the vapour phase oxidation of alcohols to aldehydes and ketones involves a mixed a- Fe203/ M0O3 catalyst rather than a single oxide. 

The toluene vapour-flow technique has been applied with success to a number of decomposition studies of halogen compounds. Where there are no significant competing reactions this technique has provided information about the primary bond-breaking process. The results of such studies have been reviewed by Szwarc (refs. 623-625) and more recently by Kerr and these are summarised in Table 9 for chlorine and bromine compounds. 

One-shot techniques have become feasible with the availability of suitable catalysts to control the rate of the various competing reactions. Thus polyol, isocyanate and chain extender can be simultaneously blended and by the use of such catalysts as stannous octoate it is possible to promote the NCO/OH reaction to a rate similar to that of the NCO/NH2 reaction. The one-shot process has the disadvantage of having free isocyanate at the casting stage which presents a vapour hazard. 

Olefins can be prepared by the dehydrogenation of paraffins, dehydration of alcohols, or decomposition of ethers and halides, if vapours of these substrates are passed over metals or metal oxides at elevated temperatures (300-600°C). Dehydration reactions have been most widely studied and by careful selection of the catalyst and the reaction conditions the direction and stereochemistry of elimination can be controlled. However, dehydration often has to compete with dehydrogenation, and isomerisation of olefinic products by the acidic sites on the catalyst can reduce the synthetic utility of these reactions. Most frequently alumina has been used as the catalyst and the advantages and complexities of the method are amply illustrated by the dehydration of alcohols. Surface-catalysed eliminations have been the subject of several reviews "". ... 

Phosphoric acid is produced in the reaction between phosphorus vapour and steam (4.21). Although high yields can be obtained at 650-800°C using catalysts, the process cannot compete commercially... 

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