Ethanol conversion into 1,3-butadiene over a mixed Hf-Zn catalyst: A study of the reaction pathway and catalyst deactivation

Some fundamental and practical aspects of a Hf-Zn catalyst, with a nominal composition of 3.0 wt% Hf and 9.3 wt% Zn, and prepared as a physical mixture of Hf/SiO2 (85 wt%) and the zinc-silicate hemimorphite (15 wt %), have been studied for the one-step conversion of ethanol to 1,3-butadiene. The elucidation of the main reactions leading to 1,3-butadiene and by-products was made by means of kinetic curves and catalytic tests where intermediates were individually fed. In addition, the convenience of by-product separation from unreacted ethanol in an industrial process was studied by performing experiments where ethanol was co-fed with intermediates. The causes of catalyst deactivation and the impact on catalyst structure and performance of regeneration by calcination were also investigated. According to our results, the pathway to 1,3-butadiene over the Hf-Zn catalyst includes ethanol dehydrogenation, acetaldehyde aldol condensation, crotonaldehyde reduction with ethanol, and crotyl alcohol dehydration. The recycling of the by-products butanal, acetone and 1-butanol into the reactor should be avoided, as the first two are converted to heavy compounds by aldol condensation reactions, while 1-butanol dehydration leads to butenes, which are difficult to separate from 1,3-butadiene. The suppression of diethyl ether formation from ethanol by recycling to extinction is possible. It has been found that catalyst deactivation is mainly caused by the retention of oxygenated aromatic-type coke species, preferentially formed on the dehydrogenating Zn2+ sites associated with the hemimorphite component of the catalyst, and probably also by a loss in Zn2+ sites due to the reduction to Zn degrees during catalysis. This reduction induces an imbalance between Hf4+ and Zn2+ sites, which changes catalyst selectivity. Regeneration by calcination with air removes coke and re-oxidizes a fraction of Zn degrees back to Zn2+, but it does not fully re-establish the original Zn2+/Hf4+ balance.