Mechanism of dehydration of phenols on nobel metals using first-principles micokinetic modeling

Authors: GU, GEUN HO - University Of Delaware; Mullen, Charles; Boateng, Akwasi; VLACHOS, DIONISIOS - University Of Delaware

Submitted to: ACS Catalysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/30/2016
Publication Date: 4/4/2016
Publication URL: https://handle.nal.usda.gov/10113/62479
Citation: Gu, G., Mullen, C.A., Boateng, A.A., Vlachos, D.G. 2016. Mechanism of dehydration of phenols on nobel metals using first-principles micokinetic modeling. American Chemical Society (ACS) Catalysis. 6:3047-3055.

Interpretive Summary: Fast pyrolysis, the rapid heating of biomass in the absence of oxygen, can be used to convert biomass (e.g. wood, grasses and agricultural residues) to a liquid called bio-oil or pyrolysis-oil. Bio-oil is made up of many different types of oxygenated chemical compounds. It can be converted to hydrocarbons that can be refined to green drop-in fuels in existing oil refineries, but first the oxygen must be removed through a process called hydrodeoxygenation (HDO). The HDO process involves the use of a metal catalyst and hydrogen at elevated pressures and temperatures. Some components of the bio-oil are more difficult to perform HDO on than others. Among the most difficult of the bio-oil components to deoxygenate are a class of compounds called phenols, an example of which is a compound called p-cresol. If p-cresol can successfully undergo HDO, toluene, a compound useful as fuel can be formed. In this study, Density Functional Theory calculations were used to create a chemical model, called a ‘microkinetic model’ that gives insight into the chemical transformation that the molecule undergoes during conversion from this p-cresol to toluene. In this study we learned that the most promising pathways for phenol conversion involve the use of platinum rather than other metal catalyst such as Ni, Fe or Ru because the hydrogenation steps are better favored. This information will be useful to those developing processes to refine bio-oil into renewable hydrocarbon fuels.

Technical Abstract: Phenolic compounds constitute a sizable fraction of depolymerized biomass and are an ideal feedstock for the production of chemicals such as benzene and toluene. However, these compounds require catalytic upgrade via hydrodeoxygenation (HDO), a process whereby oxygen is removed as water by adding hydrogen while retaining the carbon molecular architecture. While the HDO of phenolics has been widely studied, a mechanism that is consistent with the data is still lacking. Herein, we perform first-principles microkinetic calculations for the HDO mechanism of an archetypical compound, pcresol, on Pt(111). In contrast to the general belief, and in accordance with experimental data, we show that the single metal functionality is sufficient to carry out the HDO chemistry selectively, although ring activation is necessary. However, complete hydrogenation of the ring is neither necessary nor kinetically preferred. As a result, the conversion of p-cresol to toluene follows a complex energy landscape, where ethylcyclohexanol and ethylcyclohexane are not intermediates to toluene but rather hare a common pool of intermediates with the hydrocarbons.