Effect of hydrocarbon tail-groups of transition metal alkoxide based amphiphilic catalysts on transesterification

Abstract

In liquid/liquid/solid (L/L/S) systems pertinent to two immiscible reactant liquids mixed with a solid catalyst, the reaction efficacy depends on the mass transfer limitations at the L/L/S phase boundary. Formation of an emulsion in such a system will likely reduce the mass transfer barrier significantly. The stability of such an emulsion system depends on the hydrophilicity of the head group of the catalytic emulsifier toward the more polar liquid reactant and the hydrophobicity of the tail group toward the more nonpolar liquid reactant. This study looks at the effect of the alkyl groups with varying carbon numbers in titanium alkoxide as a catalyst that also has emulsification (amphiphilic) properties to transesterify triglycerides in alcohols. All forms of oligomeric titanium alkoxides tested were highly basic. Those with smaller alkoxide groups (lower carbon numbers) tended to be more basic than those with higher carbon numbers. The chirality did not affect the degree of basicity of the alkoxides. The maximum ester yield noticed was 64.25% (with 63.85% selectivity towards transesterification) with titanium methoxide after 3 hours of reaction. It was observed that higher the number of carbon atoms in the tail group the lower the catalytic ability of the amphiphile towards transesterification. It is expected that longer the carbon-chain in the tail group stronger the emulsification ability of the amphiphile in oil-in-alcohol systems. However, when looking at the efficacy of the amphiphile for the combined emulsification and catalytic ability, it is apparent that the length of the alkoxide group needs to be compromised.