Kinetics and mechanism of deoxygenation reactions over proton-form and molybdenum-modified zeolite catalysts

Abstract

The depletion of fossil fuel resources and the environmental consequences of their use have dictated the development of new sources of energy that are both sustainable and economical. Biomass has emerged as a renewable carbon feedstock that can be used to produce chemicals and fuels traditionally obtained from petroleum. The oxygen content of biomass prohibits its use without modification because oxygenated hydrocarbons are non-volatile and have lower energy content. Chemical processes that eliminate oxygen and keep the carbon backbone intact are required for the development of biomass as a viable chemical feedstock. This dissertation reports on the kinetic and mechanistic studies conducted on high and low temperature catalytic processes for deoxygenation of biomass precursors to produce high-value chemicals and fuels. Low temperature, steady state reaction studies of acetic acid and ethanol were used to identify co-adsorbed acetic acid/ethanol dimers as surface intermediates within specific elementary steps involved in the esterification of acetic acid with ethanol on zeolites. A reaction mechanism involving two dominating surface species, an inactive ethanol dimeric species adsorbed on Brønsted sites inhibiting ester formation and a co-adsorbed complex of acetic acid and ethanol on the active site reacting to produce ethyl acetate, is shown to describe the reaction rate as a function of temperature (323 - 383 K), acetic acid (0.5 - 6.0 kPa), and ethanol (5.0 - 13.0 kPa) partial pressure on proton-form BEA, FER, MFI, and MOR zeolites. Measured differences in rates as a function of zeolite structure and the rigorous interpretation of these differences in terms of esterification rate and equilibrium constants is presented to show that the intrinsic rate constant for the activation of the co-adsorbed complex increases in the order FER < MOR < MFI < BEA. High temperature co-processing of acetic acid, formic acid, or carbon dioxide with methane (CH3COOH/CH4 = 0.04-0.10, HCOOH/CH4 = 0.01-0.03, CO2/CH4 = 0.01-0.03) on Mo/H-ZSM-5 formulations at 950 K and atmospheric pressure in an effort to couple deoxygenation and dehydrogenation reaction sequences results instead in a two-zone, stratified bed reactor configuration consisting of upstream oxygenate/CH4 reforming and downstream CH4 dehydroaromatization. X-ray absorption spectroscopy and chemical transient experiments show that molybdenum carbide is formed inside zeolite micropores during CH4 reactions. The addition of an oxygenate co-feed causes oxidation of the active molybdenum carbide catalyst while producing CO and H2 until completely converted. Forward rates of C6H6 synthesis are unperturbed by the introduction of an oxygenate co-feed after rigorously accounting for the thermodynamic reversibility caused by the H2 produced in oxygenate reforming reactions and the fraction of the active catalyst deemed unavailable for CH4 dehydroaromatization. All effects of co-processing C1-2 oxygenates and molecular H2 with CH4 can be interpreted in terms of an approach to equilibrium. Co-processing H2O, CO2, or light (C1-2, C/Heff < 0.25) oxygenates with CH4 at 950 K over Mo/H-ZSM-5 catalysts results in complete fragmentation of the oxygenate and CO as the sole oxygen-containing product. The C/Heff accounts for removal of O as CO and describes the net C6H6 and total hydrocarbon synthesis rates at varying (0.0-0.10) C1-2 oxygenate and H2 to CH4 co-feed ratios. Co-processing larger (C3-5, C/Heff &#8805; 0.25) oxygenates with CH4 results in incomplete fragmentation of the co-fed oxygenate and preferential pathways of C6H6 synthesis that exclude CH4 incorporation. This results in greater net C6H6 synthesis rates than would be predicted from observations made when co-processing C1-2 oxygenates. Catalytic technologies have served a crucial role in processing petroleum feedstocks and are faced with new challenges as the feedstock shifts to chemically diverse but renewable biomass sources. This research addresses these challenges at fundamental and applied levels as it offers the potential to convert readily available biomass to commodity chemicals and fuels while simultaneously examining the elementary concepts of deoxygenation reactions on catalytic surfaces.