Vapor phase hydrodeoxygenation of lignin-derived phenolic monomers to aromatics on transition metal carbides under ambient pressure

Abstract

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.