Browsing by Subject "hydrodeoxygenation"

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    Evolution in structure and function of transition metal carbides during the catalytic activation of C-O, C-H, and C-C bonds
    (2017-08) Sullivan, Mark
    Transition metals such as Mo or W can form interstitial carbides and related interstitial formulations by occlusion of atoms such as C*, H*, O* and N*. Diverse combinations of bulk and surface stoichiometries catalyze a multitude of reactions ranging from entirely metallic hydrogenolysis and (de)hydrogenation to entirely acidic dehydration. Transition metal carbides exhibit extreme oxophilicity, and oxygen-modification results in persistent suppression of metallic characteristics and genesis of acidic properties. The carbidic penchant for cleaving C-O bonds makes these formulations promising catalysts for hydrodeoxygenation or COx activation chemistries. Bulk structure and catalytic function modification resulting from varied synthesis and reaction conditions necessitates in situ studies to quantify the effects of catalyst evolution. Isopropanol (IPA) and acetone reactions with and without O2 co-feed at 354 - 430 K were used to quantify the effects of surface oxidation on acidic and metallic characteristics of MoCx and WCx catalysts including a-Mo2C, b-Mo2C, WC, and W2C. Exposure of a freshly synthesized carbide to O* (O2, IPA or acetone) initiated suppression of metallic and basic functionality (de/hydrogenation and carbonyl condensation, respectively). Dehydration rates per gram could be reversibly tuned by over an order of magnitude using an O2 co-feed over MoCx and WCx catalysts by reversibly creating Brønsted acid sites on the carbide surface without altering bulk carbidic structure as evinced by X-ray diffraction. Oxidation decreased surface areas by an order of magnitude from ~68 to 9 m2 g-1. MoCx and WCx catalysts exhibited IPA-saturated surfaces with zero-order dependence upon IPA pressure from 0.05 – 5 kPa, and intrinsic activation energies over all MoCx and WCx catalysts under all reaction conditions ranged from 89 – 116 kJ mol-1. Brønsted acid site densities were assessed using in situ 2,6-di-tert-butylpyridine (DTBP) titrations; DTBP-normalized turnover frequency (TOF) values for MoCx and WCx catalysts were all within the range of 0.07 – 0.31 propylene s-1 (DTBP site)-1 while dehydration rates per gram varied by nearly two orders of magnitude. IPA dehydration proceeded via an E2 elimination mechanism limited by β-hydrogen scission as evinced by a kinetic isotope effect (KIE) of ~1.8. Carbonyls (acetone and propanal) are deoxygenated over H2-activated Mo2C via sequential C=O hydrogenation to equilibrium and subsequent rate-determining IPA dehydration over Brønsted acid sites at 369 K that is kinetically independent of H2 pressure from 10 – 82 kPa. Mo2C catalyzes propane dehydrogenation and hydrogenolysis with >95% selectivity to dehydrogenation in absence of H2, >95% selectivity to hydrogenolysis to CH4 with H2 co-feed, or >80% selectivity to CO via reforming with H2/CO2 co- feed at 823 K. The fraction of oxidized (O*) and carbidic (*) catalytically active surface sites can be assessed from measured effluent PCO2/PCO ratios due to an established reverse water gas shift equilibrium under steady state reaction conditions. Dehydrogenation rates can be quantitatively described using a two-site reverse Horiuti-Polyani mechanism over three distinct site pairings: (i) O*-O*, (ii) O*-*, and (iii) *-*, as evidenced by distinct dehydrogenation rate constants and activation energies in absence of co-feed, in presence of CO2 co-feed, and in presence of combined H2/CO2 co-feed. O*-* site pairs exhibited the highest dehydrogenation rate constant per gram of catalyst (kO*-* = 0.078 μmol s-1 gcat-1 kPaC3H8-1 > kO*-O* ~ k*-*). Propane dehydrogenation can be used as a probe to rigorously model transient evolution of the extent of surface oxidation and to extract kinetic dehydrogenation rate constants with near quantitative agreement with rate parameters obtained from steady state modeling (kO*-* = 0.065 μmol s-1 gcat-1 kPaC3H8-1).
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    Understanding metal carbide-based catalysts for alternate routes to valuable chemicals
    (2019-02) Kumar, Anurag
    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.
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    Vapor phase hydrodeoxygenation of lignin-derived phenolic monomers to aromatics on transition metal carbides under ambient pressure
    (2016-08) Chen, Cha-Jung
    Lignin is a sustainable source to produce aromatics such as benzene, toluene, and xylenes (BTX). Vapor phase hydrodeoxygenation (HDO) of depolymerized lignin monomers can directly upgrade pyrolysis vapor without processing corrosive and viscous bio-oil. Selective cleavage of Ar–O bonds, however, is challenging because Ar–O bonds are strong (422-468 kJ mol-1). Severe reaction conditions of high H2 pressure (~1–5 MPa) and high temperatures (~473–723 K) thus limit the yields of BTX from HDO of lignin pyrolyzates by successive hydrogenation of the aromatic ring or direct hydrogenolysis of C–C bonds. This dissertation reports kinetics, mechanism, and in situ chemical titration studies on HDO of lignin-derived phenolic compounds on molybdenum and tungsten carbide formulations for selective synthesis of benzene and toluene under ambient H2 pressure and low temperatures (420–553 K). High aromatics yield (>90%, benzene and toluene) was obtained from vapor phase HDO of phenolic compound mixtures containing m-cresol, anisole, 1,2-dimethoxybenzene, and guaiacol over Mo2C under atmospheric pressure at 533–553 K, even with H2 to phenolic compound molar ratios of ~3,300. Toluene selectivity increased proportionately (4%–66%) to m-cresol content in HDO of phenolic compound mixtures (molar composition: 0%–70%) at quantitative conversion. Low selectivity to cyclohexane and methylcyclohexane (<10%) across the conversions investigated (18–94%) demonstrates that undesired successive hydrogenation reactions of aromatics over Mo2C were inhibited, presumably due to in situ oxygen modification, as inferred from titration studies of aromatic hydrogenation reactions using methanol and water as titrants. Kinetic studies of anisole and m-cresol HDO on molybdenum and tungsten carbide formulations show that high benzene and toluene selectivity (>80% C6+ selectivity) are a result of selective cleavage of aromatic-oxygen bonds (Ar–OH and Ar–OCH3). The same reactant dependencies, zero order on oxygenate pressure and half order on H2 pressure, for both benzene and toluene synthesis from anisole and m-cresol HDO, respectively, demonstrates that two distinct sites are involved in HDO of phenolic compounds on molybdenum and tungsten carbides. In situ CO titration studies under reaction conditions showed that metallic sites are required for selective HDO of phenolic compounds. Oxygenate-modified Mo2C catalysts were prepared by pretreating fresh Mo2C catalysts in 1 kPa of O2, H2O, and CO2 at 333 K and were employed to study the effect of oxygenate-modification on the metal-like function of Mo2C using m-cresol HDO as a probe reaction. Molecular oxygen was found to have a higher propensity to deposit oxygen (O/Mobulk before HDO = 0.23 ± 0.02) on fresh Mo2C compared to CO2 and H2O (O/Mobulk before HDO ~ 0.036) as assessed from temperature-programmed surface reactions with H2 before m-cresol HDO. Oxygen adsorbed in amounts exceeding ~ 0.06 ± 0.01 of O/Mobulk was found to poison the active sites for toluene synthesis and the effect of adsorbed oxygen on turnover frequency of toluene synthesis was found to be agnostic to the source of oxygen, as inferred from in situ CO titration and m-cresol HDO reactions on fresh and oxygenate-modified Mo2C catalysts.