Abstract
The catalytic activity of both supported and soluble molecular zirconium complexes was studied in the transesterification reaction of ethyl acrylate by butanol. Two series of catalysts were employed: three well defined silica-supported acetylacetonate and n-butoxy zirconium(IV) complexes linked to the surface by one or three siloxane bonds, ( SiO)Zr(acac)(3) (1) ( SiO)(3)Zr(acac) (2) and ( SiO)(3)Zr(O-n-Bu) (3), and their soluble polyoligosilsesquioxy analogues (c-C5H9)(7)Si8O12(CH3)(2)Zr-(acac)(3) (1'), (c-C5H9)(7)Si7O12Zr(acac) (2), and (c-C5H9)(7)Si7O12Zr(O-n-Bu) (3'). The reactivity of these complexes were compared to relevant molecular catalysts [zirconium tetraacetylacetonate, Zr(acac)(4) and zirconium tetra-n-butoxide, Zr(O-n-Bu)(4)]. Strong activity relationships between the silica-supported complexes and their polyoligosilsesquioxane analogues were established. Acetylacetonate complexes were found to be far superior to alkoxide complexes. The monopodal complexes 1 and F were found to be the most active in their respective series. Studies on the recycling of the heterogeneous catalysts showed significant degradation of activity for the acetylacetonate complexes (1 and 2) but not for the less active tripodal alkoxide catalyst, 3. Two factors are thought to contribute to the deactivation of catalyst: the lixivation of zirconium by cleavage of surface siloxide bonds and exchange reactions between acetylacetonate ligands and alcohols in the substrate/product solution. It was shown that the addition of acetylacetone to the low activity catalyst Zr(O-n-Bu)(4) produced a system that was as active as Zr(acac)(4). The applicability of ligand addition to heterogeneous systems was then studied. The addition of acetylacetone to the low activity solid catalyst 3 produced a highly active catalyst and the addition of a stoichiometric quantity of acetylacetone at each successive batch catalytic run greatly reduced catalyst deactivation for the highly active catalyst 1.