A Comparative Study of Basic, Amphoteric, and Acidic Catalysts in the Oxidative Coupling of Methanol and Ethanol for Acrolein Production

Abstract

The impact of acid/base properties (determined by adsorption microcalorimetry) of various catalysts on the cross-aldolization of acetaldehyde and formaldehyde leading to acrolein was methodically studied in oxidizing conditions starting from a mixture of methanol and ethanol. The aldol condensation and further dehydration to acrolein were carried out on catalysts presenting various acid/base properties (MgO, Mg-Al oxides, Mg/SiO , NbP, and heteropolyanions on silica, HPA/SiO ). Thermodynamic calculations revealed that cross-aldolization is always favored compared with self-aldolization of acetaldehyde, which leads to crotonaldehyde formation. The presence of strong basic sites is shown to be necessary, but a too high amount drastically increases CO production. On strong acid sites, production of acrolein and carbon oxides (CO ) does not increase with temperature. The optimal catalyst for this process should be amphoteric with a balanced acid/base cooperation of medium strength sites and a small amount (<100 μmol g ) of very strong basic sites (Q >150 kJ mol ).

Review on Alternative Route to Acrolein through Oxidative Coupling of Alcohols

“…In a first series of spinels, where Al 2 O 3 /MgO ratios were varying, they obtained the best acrolein production with 1Al 2 O 3 ,1MgO with 27% acrolein (at 285 • C and GHSV 5000 h −1 ). Calculation of the number of strong basic to strong acidic sites ratio confirmed the previous results obtained by Lilic et al ( [38,39]), underlining the detrimental effect of an excess of strong basicity compared to strong acidity [40].…”

“…Though surprisingly, although both formaldehyde and acetaldehyde are coproduced, the selfaldolization product of acetaldehyde (crotonaldehyde) was detected only in minute quantities. The first thought was that this product was not thermodynamically favored as supported by thermodynamic calculations [38]. However, the difference between selfaldolization and cross-aldolization is not very large either.…”

“…Experimental work to validate the proof of concept has been carried out over several years by Dubois, Auroux, Capron and co-workers [23,[36][37][38][39][40][41][42]. As explained above, the ultimate goal in this reaction is to have a single reactor to minimize the capital cost (at least not higher than the propylene oxidation process).…”

“…For the oxidative coupling of alcohols, where aldol condensation of formaldehyde and acetaldehyde is the consecutive step, influence of acid/base properties of catalysts has been studied by Lilic et al and Folliard et al [38][39][40][41][42]. In these experiments, the goal was to better understand the reaction mechanism, not to get the best results.…”

“…A second study performed over basic, amphoteric and acidic catalysts led to the conclusion that aldol condensation of acetaldehyde and formaldehyde to acrolein in oxidizing condition was taking place on both acidic and basic sites [38]. The authors underlined that basic sites allowed to increase acrolein yield but also increased CO + CO 2 production, while on acidic sites neither acrolein nor COx yield increased.…”