Driving forces for adsorption of polyols onto zeolites from aqueous solutions

Ambient temperature adsorption isotherms have been developed for C ₂-C₆ diols and triols on small (FER), medium (MWW, MFI, BEA), and large (MOR, FAU) pore zeolites as well as on ordered mesoporous materials (MCM-36, 3DOm-MFI, and SBA-15) using gravimetry. Henry's constants for diol and triol adsorption on silicalite-1 increase exponentially with carbon number demonstrating that confinement of the adsorbate in the zeolite pores is the primary driving force for adsorption. This conclusion is supported by results for propylene glycol adsorption at low coverages on materials differing in topology and chemical composition. It is shown that adsorption decreases with an increase in the adsorbent pore size, and aluminum content only has a marginal effect. Comparison of diol and triol adsorption on silicalite-1 shows that increasing the number of hydroxyl groups causes a decrease in the Henry's constant possibly due to a change of the configuration of the adsorbate in the zeolite pores, while the location of the hydroxyl groups does not have a significant effect. Overall, this study provides evidence that polyol adsorption is primarily a function of dispersion forces that are derived from the fit of the adsorbate in the adsorbent pores. These findings could have an impact on the separation and catalytic conversion of oxygenates in the processing of biomass to chemicals and fuels.