Mechanism of Guaiacol Hydrodeoxygenation on Cu (111): Insights from Density Functional Theory Studies

Destiny Konadu,Evans Adei,Nora Henriette De Leeuw,Richard Tia,Caroline Rosemyya Kwawu

doi:10.3390/catal11040523

Abstract

Understanding the mechanism of the catalytic upgrade of bio-oils via the process of hydrodeoxygenation (HDO) is desirable to produce targeted oxygen-deficient bio-fuels. We have used calculations based on the density functional theory to investigate the reaction mechanism of HDO of guaiacol over Cu (111) surface in the presence of H2, leading to the formation of catechol and anisole. Our analysis of the thermodynamics and kinetics involved in the reaction process shows that catechol is produced via direct demethylation, followed by dehydrogenation of –OH and re-hydrogenation of catecholate in a concerted fashion. The de-methylation step is found to be the rate-limiting step for catechol production with a barrier of 1.97 eV. Formation of anisole will also proceed via the direct dehydroxylation of guaiacol followed by hydrogenation. Here, the rate-limiting step is the dehydroxylation step with an energy barrier of 2.07 eV. Thermodynamically, catechol formation is favored while anisole formation is not favored due to the weaker interaction seen between anisole and the Cu (111) surface, where the binding energies of guaiacol, catechol, and anisole are -1.90 eV, −2.18 eV, and −0.72 eV, respectively. The stepwise barriers also show that the Cu (111) surface favors catechol formation over anisole as the rate-limiting barrier is higher for anisole production. For catechol, the overall reaction is downhill, implying that this reaction path is thermodynamically and kinetically preferred and that anisole, if formed, will more easily transform.

Highlights

We have investigated the hydrodeoxygenation mechanism of guaiacol on the Cu
Our results indicate that the transformation of guaiacol into catechol will occur via a direct demethylation pathway, in the following sequence: demethylation, dehydrogenation of hydroxyl, and concerted hydrogenation into a product, it would be desirable if the barrier for demethylation could be lowered
We should note, that the energy barriers calculated in this work could be considered as a worstcase scenario, as defects on an experimental Cu (111) surface, e.g., surface vacancies and lowcoordinated edge and corner sites, would be more reactive leading to lower energy barriers

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Hydrodeoxygenation (HDO) is considered a critical reaction in the oxygen removal from pyrolyzed lignin, leading to hydrocarbon fuels and high valueadded chemicals (such as benzene, toluene, and other aromatics) [4,5]. Solid-phase catalysts used for bio-oil upgrading should improve the selectivity towards desired products, but should provide an alternative lower energy pathway in the depolymerization reaction [7]. Several reaction pathways have been proposed for HDO and the reactions that accompany HDO [9] Understanding of these reaction pathways have led to studies focused on investigations of various individual model compounds over different catalysts, due to the complexity of the bulk lignin polymer [10]. Cu (111) surface to understand the reaction pathway leading to desired HDO products, such as catechol and anisole, with the aim of identifying an efficient copper surface for the upgrade of bio-oil into hydrocarbon fuels

Computational Details

Adsorption of Desired Compounds and Their Charge Density Difference Plots

Mechanism of Catechol Formation

Mechanism of Anisole Formation

Conclusions