Diffusion coefficient of ketones in liquid media within mesopores

Abstract

The liquid-phase diffusion coefficient of several ketones within mesopores was estimated by measuring the change in UV absorbance of mesoporous plates immersed in a solvent. Monolithic plates of silica gel with different mesopore sizes at ca. 4 and 10 nm were used for the measurement after their pore surface had been modified with trimethylsilyl groups. In the modified silica plates, the diffusion coefficient of ketones, D_p, decreases with increasing the molecular size of the solvent, whereas little dependence is observed on the molecular size of solutes. This indicates that diffusivity of the solute in the mesopores is determined by the diffusivity of the solvent. A zigzag tendency (i.e. the D_p in alkane solvents with an odd number of carbons is larger than those with an even number of carbons) was observed in the small pores with a diameter of ca. 4 nm. The zigzag in D_p can explain a similar zigzag trend in the reaction rate observed only in the small mesopores of the Raney nickel catalyst with a diameter of 3.8 nm in the hydrogenation of several ketones.