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

Abstract

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 zincsilicate hemimorphite (15 wt%), have been studied for the one-step conversion of ethanol to 1,3butadiene. 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 1,3butadiene 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 Zn 2+ sites associated with the hemimorphite component of the catalyst, and probably also by a loss in Zn 2+ sites due to the reduction to Zn 0 during catalysis. This reduction induces an imbalance between Hf 4+ and Zn 2+ sites, which changes catalyst selectivity. Regeneration by calcination with air removes coke and re-oxidizes a fraction of Zn 0 back to Zn 2+ , but it does not fully re-establish the original Zn 2+ /Hf 4+ balance.

Properties and activity of Zn–Ta-TUD-1 in the Lebedev process

“…In fact, the necessity of basic or redox sites for converting ethanol first to acetaldehyde before BD has often been highlighted. 33,36,90 However, in our previous publication, we reported that CO 2 -TPD of Zn and Tacontaining catalysts revealed little correlation between the basic properties of our samples and their performances; 37 the inadequacy of CO 2 for probing sites active in the Lebedev process reported by other scholars may be the cause. 77,91 H 2 -TPR was also used to characterize the properties of catalysts capable of dehydrogenation reactions, as their reducibility can be related to activity.…”

“…Interestingly, we did not observe the passivation of stronger acid sites caused by the presence of Zn as reported by other scholars on Zn-Hf and Zn-Zr catalysts. 28,36,81 Instead, the Zn-TUD-1 possessed more strong sites than Ta-TUD-1. The significantly different preparation methods used may explain this discrepancy, as other works have generally added Zn subsequently.…”

“…5,101,102 Understanding the deactivation mechanism would assist the design of more resistant catalysts. While recent work has provided precious insights, 15,35,36 catalyst deactivation has not been fully understood, in part due to the multiplicity of materials and reaction conditions used.…”

“…Coking has been identify major as source of deactivation during the conversion of ethanol to BD over many different catalysts. 19,24,[34][35][36] Moreover, our team previously reported that calcination under air regenerated the catalytic activity of Zn-Ta-TUD-1-a possible sign of deactivation by coking. 37,102 Coke deposits occurring in the Lebedev process on ZTT-1 were quantified by the TGA of spent catalysts after 1.5-72 hours on stream at 400°C.…”

“…15 Sintering and changes in the active phase oxidation state have also been proposed. 31,32,36 As Taifan et al proved, 32 the deactivation mechanism depends on the type of catalyst used. Hence, the individual study of catalytic systems appears necessary.…”