The Role of Protons and Hydrides in the Catalytic Hydrogenolysis of Guaiacol at the Ruthenium Nanoparticle–Water Interface

Abstract

The mechanistic roles of free hydronium ions, surface hydrides, and interfacial protons during guaiacol hydrodeoxygenation (HDO) on ruthenium nanoparticles have been established. As guaiacol adsorbs on Ru, it loses its strong aromaticity and undergoes a rapid H-shift from its hydroxyl to meta carbons (in relation to its hydroxyl group), causing adsorbed enol and keto surface isomers to exist in chemical equilibrium. HDO occurs via a hydridic H-adatom (H*) attack on the enol, followed by a kinetically relevant C–O bond rupture step, during which water shuttles the hydroxyl proton, enabling its intramolecular attack on the methoxy, evolving to a highly charged [Ru(s)–(C6H5O–)···(H+)···OCH3]† transition state. The competing hydrogenation (HYD) begins with a rapid H* attack on the keto form, before a second, kinetically relevant H* attack without proton involvement. Water, despite shifting the thermodynamics toward the more polar surface keto, promotes HDO to a much greater extent than HYD, because of its dual catalytic roles in reducing the activation free energies(i) it mobilizes the hydroxyl proton of partially saturated guaiacol (Brønsted acid) and functions cooperatively with the Ru metal surface (base) in rupturing the C–O bond and stabilizing the resulting cationic carbon-ring fragment and (ii) water layers solvate the charged [Ru(s)–(C6H5O–)···(H+)···OCH3]† transition state. Free hydronium ions do catalyze a separate homogeneous enol–keto isomerization, but this reaction is kinetically unrelated to HDO catalysis. This mechanistic picture explains the strong effects of a polar protic solvent in hydrodeoxygenation, highlighting the requirements of surface hydrides and interfacial protons acting in tandem to complete a HDO turnover and the cooperative role of the protic solvent and the metal surface in breaking the aromaticity and preferentially stabilizing charged transition states.

Catalyst choice impacts aromatic monomer yields and selectivity in hydrogen-free reductive catalytic fractionation

“…Water has been observed in previous studies to significantly affect the rate or selectivity of hydrogenation reactions; for example, water can decrease the magnitude of the enthalpy of adsorption of organic reactants 32 or provide new pathways for hydrogen/proton transfer. 33,34…”

Catalytic Effects of Chemisorbed Sulfur on Pyridine and Cyclohexene Hydrogenation on Pd and Pt Clusters

“…Platinum group metals and their sulfides are known to be effective catalysts for hydrodenitrogenation, , hydrodesulfurization, , and hydrodeoxygenation. − Previous studies on simultaneous pyridine hydrodenitrogenation and thiophene hydrodesulfurization have shown that sulfided Pt/C and Pd/C catalysts exhibited higher HDN-to-HDS selectivities (up to an order of magnitude, 553 K) than sulfided Ru/C, Rh/C, NiMo/Al 2 O 3 , and CoMo/Al 2 O 3 catalysts . The pyridine HDN reactivities on Pt/C, Rh/C, Ru/C, Pd/C, NiMo/Al 2 O 3 , and CoMo/Al 2 O 3 catalysts decrease in the order of 12.3:7.5:4.5:4.0:1.3:1.0, which was ascribed to their different metal–sulfur binding energies and heats of bulk sulfide formation, , as these thermodynamic properties dictate the identities and electronic charges of the reactive hydrogen species, their catalytic involvements, and in turn rates.…”

Low temperature catalytic hydrodeoxygenation of lignin-derived phenols to cyclohexanols over the Ru/SBA-15 catalyst

“…Phenol would form its keto isomer in water and firstly hydrogenated to cyclohexanone (Ea 1 = 10.88 kJ mol −1 ), then cyclohexanone would be further hydrogenated to cyclohexanol (Ea 2 = 7.38 kJ mol −1 ). 24,37,41 Compared with the results reported for phenol hydrogenation in aqueous phase, Ru/SBA-15 gave a lower apparent activation energy in both of the two progress. 42,43 The zero-order character of phenol hydrogenation suggested the weak adsorption and fast desorption of the molecules reacted on Ru/SBA-15 catalyst, which explained high activity of hydrogenation and stability of the recycling reactions without carbon deposition on the catalyst.…”

“…[47][48][49][50] In addition, the absence of Brønsted acid sites on SBA-15 will lead to the inactivity for the dehydration of cyclohexanol and promote the hydrogenation of monophenols to stay in alcohols. 51 The cleavage of C aryl -OCH 3 was easier than C aryl O-CH 3 for the phenyl was a better electron attraction group compared with methyl for the C aryl -O-CH 3 ether bonds, 24 and the adsorption on Ru species could lower the activation energies of the C aryl -OCH 3 . 15 Similarly, C alkyl -OCH 3 was more difficult to cleavage compared with C aryl -OCH 3 for the steric hindrance and electronic effect will restrain the cleavage of C alkyl -OCH 3 bond and result in a higher temperature to achieve this process.…”

“…It was reported that guaiacol would loss aromaticity and form its keto isomer due to the incipient isomerization in water, and enol and keto isomers coexist even without catalysts. 24 In addition, water as a polar solvent could promote both hydrogenation and demethoxylation of methoxy-substituted monophenols compared with non-polar solvents as found from density functional theory (DFT) studies of phenol. 25,26 However, there was relatively less reports for the HDO of lignin-derived monophenols at low temperature (<100 C) (Table S1 †).…”