|LEE, KYUNGTAE - University Of Delaware|
|GU, HO GEUN - University Of Delaware|
|VLACHOS, DIONISIOS - University Of Delaware|
Submitted to: ChemSusChem
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/2014
Publication Date: 12/2/2014
Publication URL: http://handle.nal.usda.gov/10113/60162
Citation: Lee, K., Gu, H., Vlachos, D.G., Mullen, C.A., Boateng, A.A. 2014. Guaiacol hydrodeoxygenation mechanism on Pt(111): Insights from density functional theory and linear free energy relations. ChemSusChem. 8:315-322.
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 prtroleum oil refineries, but first the oxygen must be removed through a process called hydrodeoxygenation. Some components of the bio-oil are more difficult to hydrodeoxygenate than others. Among the most difficult of the bio-oil components to deoxygenate are a class of compounds called guaiacols which are found in relatively high concentrations in bio-oil. In this study we learned that the most likely path for guaiacols to be hydrodeoxygenated is a very complex pathway, rather than simple breaking of carbon-oxygen chemical bonds. This information will be useful to those that are developing processes to refine bio-oil into renewable hydrocarbon fuels.
Technical Abstract: In this study density functional theory (DFT) was used to study the adsorption of guaiacol and its initial hydrodeoxygenation (HDO) reactions on Pt(111). Previously reported Brønsted–Evans–Polanyi (BEP) correlations for small open chain molecules are found to be inadequate in estimating the reaction barriers of phenolic compounds except for the side (methoxy) group carbon dehydrogenation. New BEP relations are established using DFT calculations of a select group of phenolic compounds in four reaction families; carbon dehydrogenation, i.e., C-H bond scission, demethoxylation, i.e., C-O bond scission in Cring-OA/CringO-CHx (A=H, CHx x=1,2), demethylation, i.e., O-CH3 bond scission, and O-H bond scission. The maximum absolute error of these BEPs is below 0.3 eV. These relations are applied to construct a potential energy surface of guaiacol HDO. Analysis shows that catechol is mainly produced via dehydrogenation of the methoxy functional group followed by the CHx (x<3) removal of the functional group and hydrogenation of the carbon ring. This finding differs from previous reports that suggested a direct demethylation path. Our analysis also shows that dehydroxylation and demethoxylation on Pt(111) are slow, implying that phenol is likely produced through a path other than the direct demethoxylation of guaiacol or the direct dehydroxylation of catechol followed by single hydrogenation.