Molybdenum Disulfide: Catalytic Dehydrogenation and the Role of Structural DisorderPublic Deposited
MoS2 is a ubiquitous catalyst for hydrogenation and hydrogenolysis reactions for hydrotreating of petroleum sources for the removal of sulfur and nitrogen containing compounds. Through exploration of this activity, structural models for MoS2 as a series of stacked S-Mo-S platelets held together by Van der Waals forces has been developed. It has been determined that reactivity takes place at the edges of these platelets and that the large basal planes are inert towards catalytic activity. Studies into the interaction between the MoS2 edge and various small molecules have determined a working knowledge of which intermediates are likely to be stabilized on the MoS2 surface for subsequent reactions and a literature review has resulted in the identification of three main aspects of MoS2 interaction with substrates of relevance in this thesis: interaction of MoS2 with heteroatom containing species, stabilization of carbonyl containing intermediates, and stabilization of CO. There have been limited applications of MoS2 to synthetic reactions of industrial importance. This work accomplishes two related objectives: demonstration that MoS2 is a poor catalyst for carbonylation of alcohols in the absence of promoters, but is very effective for alcohol dehydrogenation to aldehydes and esters. And (2) development of structure-function relationships providing insight into the mechanism for ester formation. These correlations indicate that acetaldehyde is formed on the MoS2 edges, while ethyl acetate synthesis takes place via hemiacetal formation away from the MoS2 edges. X-ray absorption spectroscopy and diffuse reflectance infrared Fourier Transform spectroscopy are used to measure structural disorder and correlate with reactivity measured via condensed phase batch reactions and gas phase flow reactions. Use of MoS2 as a dehydrogenation catalyst has been extended to formation of alkenes from light alkanes, specifically isobutane dehydrogenation to isobutene. Promising initial results indicate that MoS2 may be a good catalyst for isobutane dehydrogenation, particularly with deliberate pre-oxidation of the catalyst. Pre-oxidation converts the Mo (IV) to Mo(VI). Upon exposure to reaction conditions which contain high partial pressures of H2, the catalyst is re-reduced to Mo(V) and Mo(IV) centers. Study and optimization of this reaction is continuing to maximize conversion and selectivity and increase mechanistic understanding. Characterization is ongoing to determine the active phase of these pre-oxidized MoS2 catalysts and the role that structural disorder may play in formation of the active phase. Future work on the use of MoS2 catalysts for methanol steam reforming for hydrogen production is proposed.