Aqueous solution and vapor phase adsorption of oxygenates onto zeolites
2012-11
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Aqueous solution and vapor phase adsorption of oxygenates onto zeolites
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2012-11
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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.
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University of Minnesota Ph.D. dissertation. November 2012. Major: Chemical Engineering. Advisors: Michael Tsapatsis and Aditya Bhan. 1 computer file (PDF); x, 124 pages, appendices A-C.
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Mallon, Elizabeth Emma. (2012). Aqueous solution and vapor phase adsorption of oxygenates onto zeolites. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/144009.
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