Abstract
LaAlO3 perovskites, as such and with 25% molar Al substitution by Cu, Co, or Ga, have been prepared by sol-gel methods and tested as heterogeneous catalysts in the gas-phase conversion of ethanol. LaAlO3 presented a significant acidic character, with high formation of ethylene by ethanol dehydration. B-site substitutions increased the basicity of the catalysts, favoring the dehydrogenation of ethanol to acetaldehyde. The most reducible Cu- and Co-substituted materials, characterized by easier formation of surface oxygen vacancies, promoted the self-condensation of acetaldehyde by the Tishchenko mechanism, with formation of acetone and odd-carbon number products. Aldol coupling of acetaldehyde, favored on pure and Ga-substituted LaAlO3, led to the formation of butadiene and hexadiene. The role of Ga insertion, favoring both dehydrogenation of ethylene and dehydration of higher alcohols, corresponds to an amphoteric character. The formation of olefins and diolefins on all catalysts suggests that LaAl-based materials present the most acidic character among La-perovskites.
Highlights
Ethanol is the main chemical commodity issued from renewable resources; more than90% ethanol being produced by fermentation of biomasses [1]
The distribution of products on all the catalysts of the present study indicated a significant activity towards alcohol dehydration and formation of unsaturated hydrocarbons, a trend already observed on lanthanum hydroxides and La-doped aluminas [70,100], but not yet evidenced on La-based perovskites
The replacement of a fraction of Al by Cu, Co or Ga decreases increases the basicity of LaAlO3 and favors products formed on bifunctional acidbase sites
Summary
Ethanol is the main chemical commodity issued from renewable resources; more than90% ethanol being produced by fermentation of biomasses [1]. Bioethanol production of 110 billion liters per year represents more than 70% of the global biofuel market [2]. The volatility of biofuel markets concurs with sustainable development issues in fostering alternative outlets to bioethanol production, mainly through the catalytic conversion of ethanol to valuable chemicals or intermediates, reducing the dependence of chemical industry from fossil feedstock. Dehydration of ethanol, mainly on acidic catalysts, forms diethyl ether (a useful solvent) at relatively low temperature and ethylene (the largest produced monomer for polyolefins) at higher temperature [10]. Dehydrogenation of ethanol (usually carried out on supported copper catalysts) forms acetaldehyde, an effective intermediate towards useful monomers, solvents and fuels [11,12,13]. Ethanol conversion pathways through acetaldehyde lead to the formation of butanol (a performant biofuel) by the Guerbet reaction [14,15,16,17,18,19] or of butadiene (a major component of elastomers) through the time-honored and recently revamped Ostromislensky and Lebedev processes [20,21,22,23,24,25]
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