Mesoporous materials as host for an entrapped enzyme

Three mesoporous materials, silica, alumina and titania, were used as host for a lipase and the enzyme-loaded particles were employed as catalyst for esterification of caprylic acid using a mixture of glycerol and water as reaction medium. The reaction proceeded well with all three oxides but alumina gave considerably higher conversion than the other two. Hydrophobized silica gave an even higher degree of esterification. The degree of esterification obtained is believed to depend on the microenvironment of the enzyme. When alumina, which is positively charged under the conditions used, and hydrophobized silica are used as host material, the negatively charged lipase can be assumed to be adsorbed at the walls of the pores. The water activity is believed to be lower at the solid surface than in the middle of the pores, where the enzyme is situated when silica and alumina are used as host material. It is shown that the lipase is not irreversibly entrapped in the pores of the mesoporous materials. When the particles are removed by filtration after completed reaction and subsequently washed with an aqueous buffer, the enzyme is leached out. The lipase can be immobilized in the pores, however, by cross-linking in situ inside the pores using glutaraldehyde as cross-linking agent. Mesoporous materials loaded with cross-linked lipase can be reused several times with only marginal loss of activity.