(113a) Silica Supported Amine Catalysts: Co-Operative Acid-Base Interactions for the Self Condensation of n-Butanal

Alcohols are important class of products that find application as intermediates in organic synthesis for agrochemicals, detergents, plasticizers, or as oxygen containing compounds blended into fuels. In most of the cases, these valuable materials are produced in separate steps from terminal alkenes, namely by a hydroformylation followed by aldol condensation of the resulting aldehyde to higher aldehydes and its subsequent hydrogenation to alcohols [1]. We have recently reported supported catalysts for gas phase hydroformylation of propene to butanal [2]; whereas the objective of the present work was to develop a CO-tolerant C-C bond-forming catalyst that can be used for the tandem conversion of propene to 2-ethylhexenal. Conventionally, n-butanal is quantitatively converted to 2-ethylhexenal using 2% sodium hydroxide solution or using basic ion exchange resins [3]. To mitigate the problems related to homogenous catalysts, we synthesized and characterized amorphous as well as mesoporous silica-supported primary, secondary and tertiary amine catalyst for the gas phase self-condensation of n-butanal to 2-ethyl hexenal. The activity and stability of amine-supported catalysts are strongly influenced on the nature of amine species, concentration of amine groups, support pore size, density of silanol groups and the spatial separation between the acid and amine groups. Silica-supported secondary amines were nearly five times more active than the primary amine, whereas tertiary amines showed negligible catalytic rates in the self-condensation of n-butanal to 2-ethylhexenal. Secondary amines in combination with the silanol groups provide the best acid-base pair and the role of weakly acidic silanol groups was elucidated by blocking the silanol groups, as well as by replacing the silanol groups by organic acids of different pK_a. The mechanism and kinetics of aldol condensation over primary, secondary and tertiary amines are also discussed.

[1] C. Kohlpaintner, M. Schulte, J. Falbe, P. Lappe, J. Weber, Ullmann's Encyclopedia of Industrial Chemistry, 2008, Wiley-VCH, Weinheim.

[2] S. Shylesh, D. Hanna, S. Werner, A. T. Bell, ACS Catal. 2 (2012) 487-493.

[3] F. King, G. J. Kelly, Catal. Today 73 (2002) 75-81.