Promoting Molybdenum Carbide for Biofuels Upgrading

Abstract

Pyrolysis biofuels are an attractive near-term solution for reducing carbon emissions from vehicles including automobiles and jets while still being compatible with current engine technology. However, in order to be used as a “drop-in” fuel, bio-oil must be upgraded into biofuel by removing oxygen, increasing hydrocarbon chain lengths, and increasing energy density. Current catalytic processes rely on expensive noble metal catalysts, and/or are not sufficiently selective in their upgrading. Molybdenum carbide (Mo2C) is a low-cost, high surface area catalyst that is known to be active for hydrogenation and other relevant reactions and was identified as a promising candidate for use as a bio-oil upgrading catalyst. The research undertaken in this dissertation aims to investigate methods to control the activity and selectivity of the Mo2C catalyst through adding promoter metals to the surface of the Mo2C catalyst. Model compounds were selected to represent important properties of bio-oil; both acetic acid and crotonaldehyde were used as model compounds. Fe, Co, Ni, Cu, Ru, Rh, Pd, and K were screened as metal promoters of crotonaldehyde conversion. Rh, Pd, and Co did not significantly affect catalyst activity or selectivity. Ni, Cu, and K increased the selectivity to the isomerization product, while Fe increased the selectivity to the HDO products. K showed the highest selectivity to isomerization product, so it was selected for further study. A series of catalysts with increasing amounts of K promotion up to 1.1 equivalent monolayers on Mo2C were synthesized via incipient wetness and tested for their activity and selectivity in acetic acid and crotonaldehyde conversion. K promotion increased selectivity to ketonization and isomerization products, respectively, and reached a maximum effect at 0.5ML. Similarly, K increased base site concentration on the Mo2C surface, and the change in base site concentration as found to correlate with the ketonization and isomerization products’ productivities. Consequently, the base site, thought to be an exposed negatively charged C atom or an Mo-O species, was proposed as the active site for dominant product formation on Mo2C. Additionally, K promotion was found to be an effective tool to control the base site density. In initial screening, Fe showed highest selectivity to HDO products, so it was selected for further study and to compare with K promotion. A series of catalysts with increasing amounts of Fe promotion up to 1.1 equivalent monolayers on Mo2C were synthesized via incipient wetness (because it allowed to higher Fe promotion) and tested for their activity and selectivity in crotonaldehyde conversion. Fe promotion was found to decrease both acid and base site concentrations with more than 0.5ML of Fe, as well as crotonaldehyde conversion rates. Productivity of much products decreased, but correlations between active site concentrations and productivities showed which active site types were most predictive of a given product. The weak base site concentration was found to be predictive of (correlate with) productivity of butenes and butyraldehyde, and strong base site concentration was predicative of C8+ products. Though K and Fe promotion had opposite effects on the base site concentration, both sets of catalysts revealed that base sites were predictive of productivity of particular products. Overall, the contribution of this dissertation is that has shown how promoters (specifically, K and Fe) can be employed to manipulate active site concentration on Mo2C support to control selectivity of bio-oil model compound upgrading.