Dehydration of sorbitol to isosorbide in melted phase with propyl-sulfonic functionalized SBA-15: Influence of catalyst hydrophobization

Abstract

Propyl sulfonic acid functionalized mesostructured SBA-15 silicas have been used to promote the dehydration of sorbitol to isosorbide, in melted phase, under atmospheric pressure. These materials were prepared with different acid capacities and modified with different types of methylsilane functionalities, looking for an enhancement of the catalytic activity of these solid acids in the studied reaction. These two strategies seemed not only to condition the way in which reactants interact with the surface and the catalytic acid sites, i.e. through variations in the hydrophilic/hydrophobic balance, but a change in the acid strength of the catalytic sites is also influenced. Results indicate that increasing the acid loading slowed down the first reaction stage (sorbitol to sorbitan) but boosted the second one (sorbitan to isosorbide), because the interaction of the same sorbitol molecule with more than one acid site difficult its conversion, but increase acid strength, favoring the second dehydration step. Similarly, the organic modification of the solid acid SBA-15-based materials with hydrophobic methylsilane functionalities moieties reduced the affinity of the sorbitol substrate for the catalyst surface, but it also enhanced acid strength as well as the accesibility of sulfonic acids, slowing down the first step, but enhancing the second one. Despite of the influence of both assayed strategies on the catalytic performance of the tested materials is opposite in both dehydration steps, the positive influence of the same on the second reaction stage was proved to be dominant, because of being the limiting step on the conversion of sorbitol to isosorbide. In this way, using highly acid loaded sulfonic acid functionalized SBA-15 modified with surface grafted methylsilanes can be considered the right combination to maximize isosorbide production using these catalysts.