The cleavage of C-O bonds in phenol, catechol, and guaiacol has been explored with mono-and dualfunctional catalysts containing Ni and/or HZSM-5 in the aqueous phase. The aromatic ring of phenol is hydrogenated in the first step, and the C-O bond of the resulting cyclohexanol is dehydrated in sequence. The initial turnover frequency (TOF) of phenol hydrodeoxygenation increases in parallel with the acid site concentration irrespective of the concentration of the accessible surface Ni atoms. For catechol and guaiacol conversion, Ni catalyzes the hydrogenolysis of the C-O bonds in addition to arene hydrogenation. For catechol, the hydrogenation of the aromatic ring and the hydrogenolysis of the phenolic -OH group occur in parallel with a ratio of 8 : 1. The saturated cyclohexane-1,2-diol can be further dehydrated over HZSM-5 or hydrogenolyzed on Ni to complete hydrodeoxygenation. Guaiacol undergoes primarily hydrogenolysis (75%) to phenol via demethoxylation, and the hydrogenation route accounts for only 25%. This is attributed to the steric effects arising from the adjacent sp 3 hybrid O-CH 3 group. 2-Methoxycyclohexanol (from guaiacol hydrogenation) reacts further either via hydrogenolysis by Ni to cyclohexanol or via acid catalyzed demethoxylation and rearrangement steps followed by the subsequent hydrogenation of the intermediately formed olefins. On Ni/HZSM-5, the hydrodeoxygenation activities are much higher for the phenolic monomers than for their respective saturated analogues, pointing to the importance of sp 2 orbitals. The presence of proximal acid sites increases the activities of Ni in the presence of H 2 by a synergistic action.

Improved synthetic approaches for preparing small-sized Ni nanoparticles (d=3 nm) supported on HBEA zeolite have been explored and compared with the traditional impregnation method. The formation of surface nickel silicate/aluminate involved in the two precipitation processes are inferred to lead to the stronger interaction between the metal and the support. The lower Brønsted acid concentrations of these two Ni/HBEA catalysts compared with the parent zeolite caused by the partial exchange of Brønsted acid sites by Ni(2+) cations do not influence the hydrodeoxygenation rates, but alter the product selectivity. Higher initial rates and higher stability have been achieved with these optimized catalysts for the hydrodeoxygenation of stearic acid and microalgae oil. Small metal particles facilitate high initial catalytic activity in the fresh sample and size uniformity ensures high catalyst stability.

Aqueous phase catalytic phenol hydroalkylation and hydrodeoxygenation have been explored using Pd/C combined with zeolite H-BEA and La-BEA catalysts in the presence of H2. The individual steps of phenol hydrogenation, cyclohexanol dehydration, or alkylation with phenol were individually investigated to gain insight into the relative rates in the cascade reactions of phenol hydroalkylation. The hydroalkylation rate, determined by the concentrations of phenol and cyclohexanol in phenol hydroalkylation, required the hydrogenation rate to be relatively slow. The optimized H+/Pd ratio was 21, which allowed achieving comparable cyclohexanol formation rates via phenol hydrogenation and consumption rates from alkylation with phenol in phenol hydroalkylation. La-BEA was shown to be more selective for hydroalkylation than H-BEA in combination with Pd/C, because cyclohexanol dehydration was retarded selectively compared to alkylation of phenol. This indicates that dehydration is solely catalyzed by Brønsted acid sites, while alkylation can be achieved in the presence of La3+ cations.

Liquid-phase dehydration of 1-octadecanol, which is intermediately formed during the hydrodeoxygenation of microalgae oil on zeolite H-BEA, has been studied, combining experiment and theory. Both the OH group and the alkyl chain of 1-octadecanol interact with zeolite Brønsted acid sites, inducing inefficient utilization in the presence of high acid-site concentrations. The parallel intramolecular and intermolecular dehydration pathways, leading to octadecene and dioctadecyl ether, have different activation energies and pass through different reaction intermediates. The formation of surface alkoxides is the rate-limiting step in the intramolecular dehydration, whereas the intermolecular dehydration proceeds via a bulky dimer intermediate, occurring preferentially at the pore mouth or outer surface of zeolite crystallites. Despite the main contribution of Brønsted acid sites toward both dehydration pathways, Lewis acid sites are also active to form dioctadecyl ether.