Intracrystalline diffusivity of lignin-derived benzene derivatives in silicalite-1 crystal in aqueous-phase system

Abstract

Lignin is a renewable resource that can generate a variety of value-added benzene derivatives during hydrothermal and/or chemical hydrolysis treatment; the crucial task is to separate the targeted derivatives from the hydrolysate. This study aims to investigate the adsorption kinetics of lignin-derived benzene derivatives onto silicalite-1 crystal in an aqueous-phase system to examine the potential applicability of silicalite-1 to aqueous-phase kinetic separation. Functional groups substituted to benzene include –OH, –CH3, –CH2–CH3, –OCH3, –CHO, –COOH, and –CH2=CH2–COOH. First, the corrected diffusivity was compared among cresol isomers. Critical diameter of the isomers was found to be a key parameter, giving rise to significant differences in diffusivity. Interestingly, we also discovered a considerable difference (maximum over 4 orders of magnitude) in corrected diffusivity, even among mono- and para-substituted derivatives, although these sorbates have the same critical diameter. In particular, the carboxylic derivatives tended to exhibit extremely low diffusivities and to show S-shaped isotherms. The diffusivities were strongly affected by the melting point of the sorbates. These results indicate a strong sorbate-sorbate interaction (e.g., hydrogen-bonding between carboxylic groups) that considerably limits molecular mobility in the narrow channel. Our findings are relevant to research on silicalite-1 to aqueous-phase kinetic separation.