Influence of Catalyst Acid/Base Properties in Acrolein Production by Oxidative Coupling of Ethanol and Methanol

Acrolein is the simplest form of unsaturated aldehyde and is widely used in the chemical industry as an intermediate for the synthesis of a number of chemicals including methionine, acrylic acid, biocides, fragrances and many more. In the past, Acrolein was produced industrially by aldolization of formaldehyde and acetaldehyde. Later, a preparation method based on propylene oxidation was adopted and widely researched. Recently, reference to bulging interest in more diverse raw materials and the necessity to curtail greenhouse emissions, and the need to optimize the supply of raw reactants, a new production process beginning with renewable feedstock such as ethanol, glycerol, and methanol have been considered.
Glycerol dehydration has been adopted as an alternative for the production of Acrolein. This comes with several benefits because glycerol is an ubiquitous co-product of the oleochemical industry as well as biodiesel production. Glycerol dehydration also comes with some drawbacks. Glycerol as a coproduct of oleochemicals and biodiesel is normally available in the ratio of about 10 wt% of oil used. This implies that for a large biodiesel plant of 250kt/year, only 25kt of glycerol will be produced, which will subsequently be turned into 10kt/year Acrolein assuming 70 mol% yield. This fits well the demand for fine chemicals, but not the large markets of the commodity chemicals.
For this reason, there remains the need to come up with alternative processes to synthesize Acrolein or acrylic acid cheaply, implementing a route that will eventually use renewable resources. A team of researchers under the guidance of Aline Auroux from the University of Lyon (IRCELYON) in France and Jean-Luc Dubois at Arkema, prepared a series of catalysts by impregnating a silica support with oxide precursors to undertake the direct oxidative coupling of methanol and ethanol, involving alcohols oxidation to aldehydes and cross-condensation of acetaldehyde and formaldehyde to generate Acrolein. The addition of the basic oxides of group I and II of the periodic table to the silica support allowed for the modulation of basicity and acidity on the surface of the catalyst. Their research work is published in ChemSusChem and was identified as a very important paper.
The research team decoupled the reaction in two steps; oxidation and the aldolization, by applying two consecutive reactors in a bid to evaluate the role of acid-base attributes of silica-supported oxide catalysts. They performed the oxidation of a blend of ethanol and methanol to acetaldehyde and formaldehyde over a FeMoOx catalyst. The product was then transferred to a subsequent reactor without intermediate separation. This is where aldol condensation as well as dehydration to Acrolein was done over supported oxides. The researchers then investigated the effects of the acid-base attributes on the selectivity towards Acrolein under oxidizing conditions.
Characterization of the catalysts exhibited unique physicochemical attributes that can be referenced to their catalytic performance for the Acrolein synthesis under oxidizing conditions. In a bid to improve selectivity to Acrolein, the authors needed to decrease carbon dioxide production by enhancing the acidity of the aldolization catalysts. The balance between acidity and basicity enhanced the selectivity of Acrolein. For this reason, fine-tuning the acid-base surface attributes was important for improving the Acrolein selectivity and to curtail carbon dioxide production referenced to over-oxidation.
The authors observed that the magnesium oxide supported on silicon dioxide catalyst was the most active for Acrolein production under oxidizing conditions due to the presence of acidic and basic sites. Acrolein production was observed to diminish when the acidic sites were poisoned by ammonia flow implying that their presence was necessary for the Acrolein yield. Acrolein, ethanol, acetaldehyde, methanol carbon dioxide and carbon monoxide were the main quantified products detected in both catalysts tests. Their study concluded although catalysts synthesized offered promising yields of Acrolein, further research and development is required to make for a feasible process.