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Aqueous Solution and Vapor Phase Adsorption of Oxygenates onto Zeolites

Elizabeth Mallon Ph.D. Thesis Defense
Chemical Engineering and Materials Science Department
IPrime Programs: Nanostructural Materials and Processes &
Coatings Process Fundamentals
October 19, 2012, University of MinnesotaEMallon.jpg


Advisors: Michael Tsapatsis and Aditya Bhan


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Abstract
The ability of zeolites, crystalline aluminosilicates with sub-nanometer pore dimensions, 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 polyfunctional 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 presentation, 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). As has been shown with alkanes, Henry's constants (Kads) for diol and triol adsorption on MFI, BEA, and MWW increase exponentially with carbon number and decrease with increasing zeolite pore size which points to dispersion forces between the adsorbate and the lipophilic zeolite pore wall as a primary driving force for adsorption. It was found that Kads for aqueous adsorption of C2-C6 oxygenates on MFI, FAU, BEA, and MWW is linearly correlated with the octanol-water partition coefficient, Kow, demonstrating that Kow is an accurate predictor of aqueous oxygenate adsorption onto zeolites. A single linear correlation between Kads and Kow is not exhibited for adsorption on FER, which we postulate is due to steric hindrance between the branched adsorbates and the narrow channels of FER. Vapor and aqueous solution adsorption measurements on silicalite-1 (aluminum-free MFI) materials varying from 0 to 8.5 silanol defects per unit cell reveal that propylene glycol adsorption is promoted by silanol defects and that water enhances adsorption by a factor of 2 in the Henry's Law regime. These results enable the prediction of separation selectivities of oxygen-containing compounds on zeolite adsorbents.

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