Mechanism and Kinetics of Isobutene Formation from Ethanol and Acetone over Zn_xZr_yO_z

Abstract

Isobutene is a specialty chemical used in the production of fuel additives, polymers, and other high-value products. While normally produced by steam cracking of petroleum naphtha, there is increasing interest in identifying routes to synthesizing isobutene from biomass-derived compounds, such as ethanol and acetone. Recent work has shown that zinc−zirconium mixed oxides are effective and selective catalysts for producing isobutene from ethanol. However, the reaction pathway, the roles of acidic and basic sites, and the role of water in promoting stability and selectivity are not yet clearly defined. In this study, a series of zinc− zirconium mixed oxides with tunable acid−base properties were synthesized and characterized with XRD, Raman spectroscopy, BET, CO 2 -TPD, NH 3 -TPD, and IR DRIFTS of adsorbed pyridine in order to probe the roles of acid and base sites for each step in the ethanol-to-isobutene reaction pathway. The observed reaction kinetics, supported by modeling of these kinetics, suggest that the reaction of ethanol to isobutene proceeds via a five-step sequence. Ethanol first undergoes dehydrogenation to acetaldehyde, which is then oxidized to acetic acid. This product undergoes ketonization to produce acetone, which dimerizes to form diacetone alcohol. The latter product either decomposes directly to isobutene and acetic acid or produces these products by dehydration to mesityl oxide and subsequent hydrolysis. The acetic acid formed undergoes ketonization to produce additional acetone. The dispersion of zinc oxide on zirconia was found to produce a balance between Lewis acidic and basic sites that prevent the loss of ethanol via dehydration to ethylene and promote the cascade reactions of ethanol and acetone to isobutene. Water, while inhibiting both isobutene and mesityl oxide formation, improves isobutene selectivity by suppressing side reactions such as unimolecular dehydration, acetone decomposition, and deactivation due to coke formation.

Production and Polymerization of Biobased Acrylates and Analogs

“…[ 100 ] Bell recently investigated the mechanism of this reaction and found that acetic acid is an intermediate in the ethanol to isobutene route, as well as a byproduct that is readily recycled ( Figure ). [ 101 ] The Wang group has patented both approaches and the subsequent conversion of isobutene to MAA. [ 102 ]…”

Fluid Characteristics of Biodiesel Produced from Palm Oil with Various Initial Water Contents

“…Eze et al [15] and Alptekin and Canalci [16] found that water, which can be produced from the mixing process of a strong alkaline catalyst and methanol, can cause a saponification phenomenon once it is mixed with the raw oil prior to transesterification. Therefore, the formation of OH − radicals from the dissociation of water might have promoted the ensuing transesterification reaction of the raw oil with an adequate amount of water added [17]. Moreover, the water content would have facilitated the hydrolysis of the raw oil's fatty acids [18], leading to the production of free fatty acids and thus increasing the acid value [19].…”

La‐H‐zeolites: efficient catalysts for acetic acid ketonic decarboxylation and esterification

“…Acetone conversion to isobutene takes place predominantly on the Lewis acid sites. 52 In the first stage, aldol condensation between an acetone molecule adsorbed on the Lewis site and an acetone molecule from the gaseous phase take place to produce a diacetone alcohol. The next pathway consists of a ⊎-scission reaction of diacetone alcohol, with formation of isobutene and acetic acid molecules.…”