Nonuniform Chain-Length-Dependent Diffusion of Short 1-Alcohols in SAPO-34 in Liquid Phase

Liquid-phase diffusion of 1-alcohols in SAPO-34 was explored by batch experimentation. The uptake of pure and binary mixtures of 1-alcohols, dissolved in tert-butanol, was obtained for C1-C8 1-alcohols at temperatures between 25 and 80 degrees C, concentrations varying between 0.5 and 10 wt %, and crystal sizes between 7.5 and 20 mu m. The experimental uptake data were fitted with an intracrystalline diffusion model and a linear driving force model. The intracrystalline diffusion coefficient showed a nonuniform stepwise decrease with chain length, ranging from 10(-12) m(2)/s for methanol to 10(-20) m(2)/s for 1-pentanol. No effect of the external concentration on the intracrystalline diffusion coefficient was observed. Variation of the crystal size showed that the intracrystalline diffusion is the rate-limiting step. On the basis of the Arrhenius equation, the activation energies of diffusion of ethanol, 1-propanol, and 1-butanol were determined, being, respectively, 27.8, 47.8, and 471 kJ/mol. Co-diffusion occurred in the uptake of binary mixtures of methanol/ethanol, methanol/1-propanol, and ethanol/1-propanol, where mutual effects could be noticed. From this experimental work, it could be concluded that the small dimensions of the SAPO-34 framework generate a very sterically hindered diffusion of 1-alcohols into the crystals, resulting in a chain-length-dependent behavior, interesting to obtain efficient kinetic-based separations.