Browsing by Subject "MOFs"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    C-H Activation via Direct Oxidative Routes over Molecular Metal-oxo Species Situated in Metal-Organic Frameworks
    (2021-07) Simons, Matthew
    Metal Organic Frameworks (MOFs), crystalline materials composed of inorganic nodes connected by organic linker molecules, afford an opportunity to synthesize new biomimetic catalysts engendering the oxidative activation of light alkanes, opening new pathways for the enhancement of underutilized chemical feedstocks. We aim to demonstrate in this dissertation the ability of Fe(II) centers, bearing a similar geometric and electronic structure to sites found in non-heme enzymes, situated in the trimeric iron-oxo nodes of a family of MOFs to activate light alkanes at near ambient temperatures. The identity and quantity of the active site was determined using in situ X-ray Absorption and ex situ Mössbauer Spectroscopy, in concert with in situ chemical titrations. Reaction kinetics, measured by varying reactant concentration and temperature using a recirculating batch reactor, are consistent with the rate limiting reaction of this Fe(II) site with the oxidant, N2O, to form a highly reactive Fe(IV)=O species (k = 1.2-0.8 x10-6 mol molFe(II)-1 kPaN2O-1 s-1 at 378 K) capable of activating C-H bonds homolytically, in agreement with Density Functional Theory calculations that predict subsequent radical-mediated pathways for upgrading propane and methane. A major challenge in this chemistry is resisting the over-oxidation of desired products formed by these pathways, which is often both kinetically and thermodynamically favored. We evince, using in situ Infrared Spectroscopy, that methanol, the desired product of the oxidation of methane, is stabilized as methoxy groups on the MOF through reactions with surface hydroxyl species. Pursuant to this, we added a zeolite (H-ZSM-5, Si/Al = 11.5) in inter- and intra- pellet mixtures with the MOF, observing monotonic increases in methanol selectivity with increasing ratio and proximity of zeolitic H+ to MOF-based Fe(II) sites, signaling increased amounts of methanol being dehydrated and protected within the zeolite. This work demonstrates (i) the radical-rebound mechanism commonly invoked in this chemistry is insufficient to explain the reactivity of these systems, (ii) the selectivity controlling steps involve both chemical and physical rate phenomena, and (iii) offers a strategy to mitigate over-oxidation in these and other similar systems.
  • Thumbnail Image
    Item
    Tailoring the Microstructure of 2D Molecular Sieve Materials for Thin Film Applications
    (2018-05) Shete, Meera
    Zeolites and metal organic frameworks (MOFs) are microporous materials, with pores of molecular dimensions, that are of interest in a variety of applications including catalysis, adsorption, ion-exchange, separation membranes etc. With a global need of developing clean energy resources and reducing the carbon footprint of existing processes, they are being increasingly sought after as catalysts for the conversion of biomass to chemicals and fuels, in separation membranes to replace the existing energy intensive industrial separations with clean energy-efficient processes and for capture and storage of carbon dioxide. Their performance in these applications depends mainly on their pore size but also on our ability to tune their microstructure (crystal morphology and size, orientation, phase purity, defect densities etc.) as desired for an optimum performance. Recent advances in synthesis of molecular sieve materials have resulted in the development of advanced morphologies such as hierarchical materials, core-shell catalysts, two-dimensional nanosheets and thin films. However, a lot of the reports in the literature focus on conventional crystals and studies focusing on nanoscale crystal growth control are still in their infancy. This dissertation focuses on developing synthetic methods that will enable us to tailor the microstructure of 2D molecular sieve materials at a nanoscale approaching single-unit-cell dimensions with a goal of optimizing their performance in thin film applications. A novel coating technique was applied to isolate 2D MFI zeolite nanosheets and form monolayer coatings on versatile supports such as Si wafers. Using this as a prototype, growth conditions were developed that lead to unprecedented control of zeolite MFI growth at a scale approaching single-unit-cell dimensions. It was demonstrated that these growth conditions are robust enough and can be used to grow zeolite MFI crystals of varied sizes and morphology. It also enabled us to precisely control the microstructure of MFI thin films leading to the development of a material that had one of the lowest reported dielectric constant. Furthermore, the nanoscale growth control also allowed us to tailor the design of hierarchical catalysts by controllably thickening the zeolite domains in them and open opportunities to design multifunctional catalysts. A scalable and direct synthesis of Cu(BDC) MOF nanosheets was developed. Hybrid nanocomposites incorporating the MOF nanosheets in polymer matrices were fabricated which demonstrated significantly improved performance for CO2/CH4 separation.