Browsing by Subject "heterogeneous catalysis"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    A Controllable Synthesis and Catalytic Behavior Study of Micro- and Mesoporous Zeolites Made by Rotational Intergrowths
    (2018-07) Xu, Dandan
    Zeolites are considered as one of the key heterogeneous catalysts in refinery operations and petrochemistry processes. Researchers have been actively exploring new designs or concepts to foster the applications of zeolites in other fields. To improve the accessibility of active sites and reduce the diffusion limitations within micropores, various hierarchically organized pore systems have been architected based on the conventional microporous zeolites. Advances in zeolite synthesis and hierarchical structure design could bring about opportunities for new catalytic applications as well as improvements of current technological processes. However, the multilevel pore architectures complicate the understanding of their catalytic behaviors. Therefore, this dissertation has addressed the attempts on understanding the structure—activity relationship in hierarchical zeolites for maximal catalytic advantages and future rational catalyst design. Self-pillared pentasil (SPP) zeolite was selected as a prototype of hierarchical zeolites, and its formation mechanism was studied at the beginning of this dissertation. Different strategies were then developed to tailor the SPP zeolite structure and mesoporosity. Catalytic behavior of SPP zeolites was explored through studies of model reactions (i.e., benzyl alcohol self-etherification and alkylation with mesitylene). Key structural attributes to selectivity and catalytic efficiency were determined through kinetic studies. A quantitative analysis towards the catalytic contributions and diffusion resistance could assist the practical applications of SPP zeolites and other similar hierarchical zeolites in the future.
  • Thumbnail Image
    Item
    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.