Understanding metal carbide-based catalysts for alternate routes to valuable chemicals

Abstract

Depleting fossil fuel reserves and adverse environmental effects of current crude-oil-based processes have governed the development of sustainable energy resources. Biomass and natural gas are promising alternate sources for precursors used in chemical industry. Biomass upgrading is limited by its high oxygen content which reduces its energy density and brings forth significant challenges in its downstream processing. Chemistries that eliminate oxygen selectively while keeping the carbon backbone intact are required for development of technologies for conversion of low-quality, low-price waste product, biomass, to high-value specialty chemicals. Non-oxidative direct conversion of methane, major component of abundant natural gas reserves, to aromatics faces intrinsic thermodynamic constraints. This dissertation reports on (i) the kinetic, mechanistic, and site requirement studies performed on low temperature hydrodeoxygenation of biomass precursors and (ii) a novel polyfunctional catalyst formulation addressing the persistent thermodynamic limitations in high temperature methane dehydroaromatization. Transient kinetic measurements and temperature-programmed-surface reactions were utilized to establish accumulation of oxygen during vapor phase anisole hydrodeoxygenation (HDO) on molybdenum carbide (Mo2C) catalysts at 423 K and atmospheric pressure resulting in suppressed hydrogenation functionality of Mo2C. Kinetic studies on as-synthesized Mo2C (without ambient exposure prior to kinetic measurements) and oxygen treated Mo2C (oxygen incorporation of O:Mobulk ~ 0.075) demonstrated that oxygen only reduces the number of anisole HDO active sites at these low O* concentrations. Anisole HDO reactions on as-synthesized Mo2C and oxygen-treated Mo2C (O:Mobulk ≈ 0.076 – 0.276) resulted in varying benzene and phenol selectivity elucidating that O* content can be used to tune the product selectivity in hydrodeoxygenation reactions on transition metal carbides. These changes in catalytic reactivity were plausibly ascribed to the formation of MoOx/MoOxCy species that disrupt ensembles required for selective aromatic C-O bond cleavage. In-situ CO chemical titration was developed as an operando technique to obtain an accurate count of active sites and thus estimate turnover frequency for anisole HDO reactions on Mo2C catalysts (1.1±0.3 x 10-3 mol molMo-1 s-1). As-synthesized molybdenum carbide showed >98% selectivity towards deoxygenated products and stable chemical conversion for >30 h time-on-stream for vapor phase hydrodeoxygenation of acetic acid at low temperature (403 K) and atmospheric pressure. Space time variation experiments explicated the sequential reaction pathway for acetic acid deoxygenation on Mo2C. Kinetic studies established that the catalytic sites for H2 and acetic acid activation are distinct on Mo2C. Temperature programmed surface reaction (TPSR) with hydrogen post acetic acid HDO reaction evinced the catalyst surface evolution due to oxygen and carbon deposition. A comparison of the results in this thesis with prior reports suggested that the identity of the feed oxygenate determined its proficiency for heteroatom accumulation on/in fresh carbidic materials. 2,2-dimethylpropanoic acid (DMPA) was used as a selective titrant to estimate the catalytic site densities and calculate a turnover frequency (TOF) of (9 ± 1) × 10−4 mol s−1 molDMPA−1 for acetic acid HDO on Mo2C. Catalyst characterization using chemical transient experiments, high-angle annular dark-field imaging (HAADF-STEM), and Raman spectroscopy evidenced the formation of molybdenum carbide nanoclusters inside zeolite pores on high temperature (973K) methane exposure of MoO3/H-ZSM-5 physical mixtures air treated at 973 K. Coupling the catalytic function of MoCx/ZSM-5 with the hydrogen absorptive function of Zr metal in polyfunctional formulations resulted in circumvention of thermodynamic barriers to methane dehydroaromatization without perturbing the reaction pathways and aromatics product selectivity (70 % benzene and 20% naphthalene). Addition of Zr metal to MoCx/ZSM-5 in the form of staged-bed, stratified-bed, and interpellet physical mixtures effectively scavenges H2 formed in the catalyst bed, thereby, enhancing single-pass benzene + naphthalene yield to 14-16% compared to 8% in formulations without zirconium. Isothermal treatment of the MoCx/ZSM-5 + Zr formulation in helium post-reaction resulted in desorption of absorbed hydrogen and regeneration of the Zr absorbent leading to partial regeneration of the polyfunctional catalyst formulation yielding above equilibrium methane conversions in multiple reaction-regeneration cycles. The critical role of dispersive/diffusive H2 transport in lab-scale methane DHA experiments was demonstrated through a detailed reaction-transport model capturing the interplay of kinetic, diffusive, and convective length scales. Current catalytic technologies are faced with new challenges due to shift in available feedstocks towards chemically diverse and renewable sources. This research addresses the challenges in large-scale deployment of biomass and methane upgrading chemistries at fundamental and applied levels by examining the concepts of deoxygenation and C-H activation on catalytic surfaces.