Reaction sequence in hydrogenation of piperylene over skeletal nickel catalyst

Abstract

The reaction mechanisms operating in the chain growth to C3+ primary alcohols and in the formation of ketones, secondary alcohols, methyl esters, ethers, and hydrocarbons during higher alcohol synthesis (HAS) over high-temperature modified methanol catalysts have been investigated by the temperature-programmed surface reaction (TPSR) technique. Experiments with linear and branched C4 alcohols, aldehydes, and acids over an unpromoted ZnCr oxide sample have indicated a series of major catalyst functions, namely aldol-like condensation (also with oxygen retention reversal), decarboxylation and decarboxylative condensations, hydrogenation-dehydrogenation, dehydration and hydrolysis, along with isomerization and cracking. TPSR experiments with linear C4 molecules over K-promoted ZnCr oxide have demonstrated the effects of alkali addition on the catalyst functions. The relevance of these functions and of the associated chemical reactions during HAS has been discussed in the light of catalytic tests performed under real synthesis conditions. The results are supportive of a mechanism of chain growth to C3+ primary alcohols based on a sequence of aldolic condensations of aldehydes, which do not operate over 2-methyl species. Formation of ketones under TPSR conditions is explained by decarboxylative condensation reactions of aldehydic and carboxylate species, as well as by aldol-like condensation reactions with oxygen retention reversal. Secondary alcohols detected in the products of the synthesis are formed by hydrogenation of ketones. Methyl esters and ethers are produced in the synthesis by alcoholysis of carboxylate and alkoxide species, respectively. Decarboxylation of carboxylate species, along with dehydration, may also play a role in the formation of hydrocarbons during HAS.