Kinetics, Selectivity, and Deactivation in the Aldol Condensation of Acetaldehyde on Anatase Titanium Dioxide

Abstract

Metal oxide catalysts are active for aldol condensation reactions of aldehydes and ketones but typically exhibit rapid deactivation. In this study, aldol condensation with dehydration of acetaldehyde to produce crotonaldehyde was catalyzed by oxidized TiO2 anatase. Particular emphasis was placed on determination of product selectivities and reaction kinetics as a function of both conversion and the extent of catalyst deactivation. Condensation of acetaldehyde is rapid and selective on anatase TiO2, with turnover frequencies exceeding 0.03 s−1 and crotonaldehyde selectivities approaching 100%. The main side reactions generating volatile products are: (a) hydrogenation of the reactant and product aldehydes, (b) a secondary cross-esterification between crotonaldehyde and acetaldehyde to form ethyl crotonate, and (c) secondary condensations involving crotonaldehyde. Catalyst deactivation was observed to affect both the rate of reaction and the product selectivities, particularly at low values of time-on-stream. Secondary condensations that deposit nonvolatile organic species on the catalyst surface are responsible for the initial deactivation of the catalyst. The catalyst mass increase during the course of reaction was observed to be directly proportional to the rate of deactivation. Selectivity patterns were impacted by deactivation in a manner that could not be explained solely by the changing conversion levels associated with the deactivation process. We conclude that deactivation during aldol condensation on the anatase polymorph of titania alters both activity and selectivity of the active sites. As a result, care must be taken to account for catalyst deactivation when comparing both catalytic activities and selectivities.

Acetaldehyde adsorption on TiO2: Influence of NO2 preliminary adsorption

“…According to the literature [13] [14][16] [30], acetaldehyde adsorption scarcely leads to the adsorption of single acetaldehyde molecules on TiO 2 surface, but mostly produces 2-butenal through β-aldolization and dehydration reactions. Such reactive adsorption indirectly promotes acetaldehyde adsorption on TiO 2 surface by consuming adsorbed acetaldehyde.…”

Improving the selectivity to C4 products in the aldol condensation of acetaldehyde in ethanol over faujasite zeolites

“…Specifically, the aldol condensation of acetaldehyde has been investigated in several previous studies as an important intermediate reaction in the ethanol to BD process. In these studies [28][29][30][31][32] metal oxides, hydrotalcite, and amino acid have been used as catalysts, which exhibit varying extents of selectivity to C4 products. In this contribution, we have investigated the use of faujasite zeolites for the selective aldol condensation of acetaldehyde to C4 products, trying to minimize the excessive M a n u s c r i p t 5 sulfate octadecahydrate, and potassium nitrate (≥99.0%).…”

Base-Catalyzed Reactions in Biomass Conversion: Reaction Mechanisms and Catalyst Deactivation

“…A rapid deactivation is also observed when the active phase is supported on carbonaceous materials, with less than 15 % of conversion in second cycles [80]. Deactivation affects both the rate of reaction and the product selectivities, particularly at low values of time on stream [81]. Despite the relevance of this phenomenon, there is no good agreement on the reasons for and the mechanisms of deactivation.…”