Mechanistic analysis of the role of metal oxophilicity in the hydrodeoxygenation of anisole

Abstract

A combined experimental and theoretical comparative study of the hydrodeoxygenation (HDO) of anisole was conducted over Pt, Ru, and Fe metals. In the experimental part, an inert silica support was used to directly compare the catalytic activity and selectivity of the three metals at 375 ºC under H 2 flow at atmospheric pressure. In parallel, for density functional theory (DFT) calculations the close-packed Pt(111), Ru(0001), and Fe(110) surfaces were employed to compare the possible mechanisms on these metals. It was observed that over Pt/SiO 2 and Ru/SiO 2 catalysts, both phenol and benzene were the major products in a phenol/benzene ratio that decreased with the level of conversion. By contrast, over the Fe/SiO 2 catalyst, no phenol formation was detected, even at low conversions. The DFT results show that over all the three metal surfaces the dehydrogenation at the-CH 3 side group occurs before the CO bond breaking. This removal of H atoms from the-CH 3 group facilitates the activation of the aliphatic C alkyl-O bond. Therefore, it can be concluded that a common intermediate for the three metals is a surface phenoxy and the significant differences between the three metals is related to the reactivity of this surface phenoxy. That is, over Pt(111) and Ru(0001) the phenoxy intermediate is hydrogenated to phenol, which in turn, can undergo further HDO to form benzene. This result is in agreement with the experiments over Pt/SiO 2 and Ru/SiO 2 catalysts. Over these catalysts, both phenol and benzene are major products, with the selectivity to benzene increasing with conversion at the expense of phenol. In contrast, over the Fe(110) surface, the strong metal oxophilicity makes the direct cleavage of the CO bond in the surface phenoxy easier than

Catalytic Upgrading of Biomass Model Compounds: Novel Approaches and Lessons Learnt from Traditional Hydrodeoxygenation – a Review

“…These results was further supported by a DFT (Density Functional Theory) study which suggested that the HDO process of phenol on Fe (110) surface followed the DDO reaction pathways (Figure , Pathway 1) . Tan et al . attributed the direct cleavage of C−O bonds over Fe catalyst to the strong oxyphilicity of Fe metal.…”

Controlled deposition of titanium oxide overcoats by non-hydrolytic sol gel for improved catalyst selectivity and stability

“…We investigated the performances of our bifunctional platinum/ acid sites in the hydrodeoxygenation of oxygenated aromatics derived from the lignin fraction of biomass [72][73][74][75][76][77][78][79][80][81][82][83]. of acid sites close to metal nanoparticles has been shown to favor CAO bond cleavage and therefore deoxygenation.…”

NiMoS on alumina-USY zeolites for hydrotreating lignin dimers: effect of support acidity and cleavage of C–C bonds

“…Several catalysts (mono-bimetallic and bifunctional) containing noble metals (Pt, Pd, Rh, Ru, Re, and Ir), metal-suldes, carbides, nitrides, and phosphides of various supports (carbon, Al 2 O 3 , ZrO 2 , TiO 2 , ZrO 2 -SiO 2 , SiO 2 -Al 2 O 3 , zeolites, mesoporous silicates, etc.) have been studied extensively for HDO of phenols, [5][6][7][8][9][10][11][12] anisole, 9,[13][14][15][16] substituted phenols (cresols, guaiacols, syringols, eugenols, etc.) 6,9,[17][18][19][20][21][22][23][24][25][26][27][28][29][30] and lignin dimers.…”

“…6,37 Nevertheless, by tuning the intrinsic energy barriers for HYD versus direct deoxygenation (DDO) on different surfaces one can alter the selectivity to bio-arenes. 14,21 But noble metals are not stable in the presence of sulfur which makes them less favorable for upgrading Kra (1-3 wt% S) and lignosulfonate (4-8 wt% S) lignin. 38 Also, the cost of such metals hinders scale up.…”