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

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.

The catalytic hydrogenolysis of compounds derived from guaiacol on the Cu (111) surface: mechanisms from DFT studies

“…Our earlier work 30 has shown that catechol is produced from guaiacol via direct demethylation into catecholate, followed by the concerted re-hydrogenation of catecholate into catechol. Anisole formation via the direct dehydroxylation of guaiacol followed by hydrogenation was found to be less favoured on Cu (111), both kinetically and thermodynamically.…”

“…We observed that the C aryl -OH cleavage into anisole is more challenging than the C akyl -O bond cleavage into catechol, hence leading to the preferred formation of catechol over anisole. 30 To better understand, and thus be able to control, the catalytic deoxygenation reaction of phenolics on Cu (111) into pure hydrocarbon end-products, e.g. benzene, we have studied the detailed surface reaction mechanisms involved in the upgrade of the oxygenated phenolic compounds formed from guaiacol.…”

“…The energetics from our earlier work show that the ratelimiting steps for guaiacol transformation have energy barriers of 2.0 eV to form catechol and 2.1 eV to form anisole, with reaction rates of the order of 99.6 Â 10 À23 s À1 and 1.95 Â 10 À23 s À1 , respectively, where catechol formation is 99 times faster than anisole formation on the Cu (111) surface. 30 There, we have shown that the rate-limiting steps in the formation of catechol from guaiacol, and subsequently phenol and benzene from guaiacol feedstock are the same, as the direct demethylation step into catecholate is the slowest step. The kinetic steps after catechol and anisole transformations are less than 2.0 eV, making the catechol and anisole formation the most challenging steps in the guaiacol upgrade.…”

“…[14][15][16][17][18][19][20][21][22][23] Using phenol, guaiacol and other types of substituted phenols, 8,9,24 a few studies have addressed HDO via a theoretical approach, usually using calculations based on the density functional theory (DFT). 17,19,[25][26][27][28] For example, guaiacol HDO has been studied over the Ru (0001), Pt (111) and Cu (111) surfaces, 15,29,30 whereas the dissociation of phenol to phenoxy has been investigated on Pt, 26 Rh, 26 Pd 25 and Fe 25 surfaces. Our previous DFT studies have shown that on the Cu (111) surface, guaiacol upgraded by one mole of H 2(g) produces preferentially catechol over anisole with reaction rates of the order of 99.6 Â 10 À23 s À1 and 1.95 Â 10 À23 s À1 , respectively, where catechol formation is 99 times favoured over anisole formation on the Cu (111) surface.…”

Doping an Oxophilic Metal into a Metal Carbide: Unravelling the Synergy between the Microstructure of the Catalyst and Its Activity and Selectivity for Hydrodeoxygenation

“…In comparison with metal catalysts like Cu and Pt, Mo 2 C has shown better deoxygenation ability. Recently, Konadu et al have reported a high electronic energy barrier of 200 kJ/mol for the dehydroxylation step from guaiacol, whereas it is more feasible on Mo 2 C with a low barrier of 78 kJ/mol (provided in Table S1). On the Pt(111) surface as well, high barriers of 240 and 244 kJ/mol for dehydroxylation and demethoxylation reaction steps, respectively, were observed .…”