Aqueous solution and vapor phase adsorption of oxygenates onto zeolites

Abstract

The ability of zeolites to discriminate between molecules on the basis of size and functionality gives them the potential to be effective adsorbents and membrane materials for purification of biomass-derived chemicals and fuels. Since molecules from biomass are polyfuntional and non-volatile, it is necessary to decouple the interactions that drive aqueous adsorption of oxygenates onto zeolites in order to develop efficient zeolite-based separations for biomass processing. In this dissertation, the roles of adsorbent structural and chemical composition and adsorbate functionality are explored through the systematic development of aqueous and vapor adsorption isotherms of C2-C6 oxygenates on small (FER), medium (MWW, MFI, BEA), and large (MOR, FAU) pore zeolites as well as on hierarchical microporous-mesoporous materials (MCM-36, 3DOm-MFI, and SBA-15). Ambient temperature Henry’s constants (Kads) for aqueous diol and triol adsorption on silicalite-1 (aluminum-free MFI) increase exponentially with carbon number demonstrating that confinement of the adsorbate in the zeolite pores is a primary driving force for adsorption. This conclusion is supported by a monotonic decrease in propylene glycol Kads values with an increase in adsorbent pore size, and by a comparison of propylene glycol Kads values on MWW and MFI and their hierarchical counterparts (MCM-36 and 3DOm-MFI, respectively) that shows that propylene glycol preferentially adsorbs in the micropores of hierarchical materials. A comparison of diol and triol adsorption on silicalite-1 demonstrates that increasing the number of hydroxyl groups causes a decrease in adsorption affinity, and this phenomenon is probed by comparing Henry’s constants for aqueous adsorption of C3 polyfunctional molecules onto zeolites with their octanol-water partition coefficients, Kow, which were calculated using the prevalent ClogP group contribution method. It was found that Kads increases linearly with Kow for these adsorbates on H-ZSM-5 (aluminum-containing MFI), FAU, BEA, and ITQ-1 (MWW) at 278 K regardless of interactions in the bulk phase as measured by the solution activity coefficient. Exceptions to the correlation established between Kads and Kow are the adsorption of 1,2,!-triols with carbon number greater than three on H-ZSM-5 and adsorption of all oxygenates studied on FER, which we postulate is due to a shift in the adsorption configuration with adsorbate/zeolite structure which cannot be captured by Kow alone. The effect of zeolite defects on oxygenate adsorption was isolated through the development of vapor and aqueous adsorption isotherms on silicalite-1 materials that vary in structural and surface properties. Silicalite-1 crystals prepared through alkaline-synthesis, alkaline synthesis with steaming post-treatment, and fluoride synthesis routes are confirmed as crystalline MFI by SEM and XRD and are shown to contain ∼8.5 to 0 silanol defects per unit cell by 29Si MAS, 1H MAS, and 1H-29Si CPMAS NMR. A hysteresis in the Ar 87 K adsorption isotherm at 10−3 P/P0 evolves with a decrease in silanol defects, and, through features in the XRD and 29Si MAS NMR spectra, it is postulated that the hysteresis is the result of an orthorhombic-monoclinic symmetry shift with decreasing silanol defect density. Gravimetric and aqueous solution measurements reveal that propylene glycol adsorption at 333 K is promoted by silanol defects, with a maximum 20-fold increase observed for aqueous adsorption in the Henry’s Law regime with an increase from ∼0 to 8.5 silanols per unit cell. A comparison of vapor and aqueous propylene glycol adsorption on defect-free silicalite-1 at 333 K, both of which exhibit the Type V character, indicates that water enhances adsorption by a factor of 2 in the Henry’s Law regime, which is in agreement with simulations reported in the literature. Kads values for aqueous C2-C4 polyol adsorption at 298 K are shown to have a linear dependence on the silanol defect density, which indicates that these molecules preferentially interact with silanol defects. i