Heterogeneously Catalysed Aldol Reactions in Supercritical Carbon Dioxide as Innovative and Non-Flammable Reaction Medium

Abstract

Aldol reactions of several aldehydes have been investigated over acidic and basic catalysts in supercritical carbon dioxide at 180 bar and 100°C. Both acidic (Amberlyst-15, tungstosilicic acid (TSA) on SiO 2 and MCM-41) and basic (hydrotalcite) materials showed interesting performance in this preliminary study under the entitled reaction conditions. Small and linear aldehydes, such as propanal, butanal, pentanal and hexanal, react more efficiently than the branched 3-methylbutanal, which is converted much slower. Whereas Amberlyst-15 showed the highest conversion based on the catalyst mass, tungstosilicic acid-based catalysts were significantly better if the rates were related to the number of acidic sites ([1000 h -1 ). The rate depends both on the dispersion and the kind of support. Strikingly, tungstosilicic acid (TSA) on MCM-41 was also an effective catalysts for the selective C=C double bond hydrogenation of 2-butenal and is therefore a potential catalyst for the ''one-pot'' synthesis of 2-ethyl-2-hexenal and 2-ethylhexanal via combined hydrogenation and aldol reaction from 2-butenal. A number of characterisation techniques, such as temperatureprogrammed desorption of ammonia (NH 3 -TPD), transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), etc. were used to get an insight into the catalyst structure, which support a high dispersion and strong acidity of the tungsten based species on silica and MCM-41.

Direct synthesis of 2-ethylhexanol via n-butanal aldol condensation–hydrogenation reaction integration over a Ni/Ce-Al2O3 bifunctional catalyst

“…Unfortunately very few studies on n-butanal self-condensation catalyzed by a solid acid catalyst have been published in the literature. 12,13 Therefore, it is significant to develop a solid acid catalyst with an industrial application prospect.…”

Silica‐immobilized acid ionic liquid: An efficient catalyst for pentanal self‐condensation

“…Although liquid alkali catalyst has the advantages of low cost, easy availability, mild reaction conditions, and high yield of product, it also suffers from problems such as difficult recovery and high cost for treatment of alkali‐containing wastewater. Several kinds of heterogeneous catalysts have been investigated in petanal self‐condensation in order to conquer the above drawbacks, including inorganic solid base catalysts, organic solid base catalysts, and solid acid catalysts . Among them, the aminopropyl functionalized chitosan and TiO 2 showed better catalytic performance.…”

Synergistic catalysis of acid–base bifunctional ionic liquids for pentanal self‐condensation reaction

“…However, it also suffers from some obvious drawbacks such as difficulty in catalyst recovery, environmental pollution and high cost for the treatment of the alkali wastewater. Several kinds of heterogeneous catalysts have been investigated in pentanal self‐condensation to conquer the earlier‐mentioned drawbacks, including inorganic solid base catalysts, organic solid base catalysts, solid acid catalysts . The aminopropyl functionalized chitosan and titanium dioxide (TiO 2 ) showed better catalytic performance in these catalysts.…”