(778g) Deoxygenation of Alkanols, Alkanals and Phenolic Monomers On Metal and Solid Acid Catalysts

Selective removal of excess oxygen from biomass-derived molecules is a formidable challenge for the efficient formation of fuels and chemicals. This study demonstrates how the rates of different deoxygenation pathways, such as decarbonylation, C-O hydrogenolysis and dehydration are controlled by the identity of the metal function and, in some cases, by the synergistic presence of acid sites within molecular diffusion distance.  Specifically, we examine reactions of alkanols, alkanals and phenolic monomers.

Alkanols and alkanals react predominantly via decarbonylation, forming CO on monofunctional catalysts based on Group VIII metals (e.g. Ru, Pt and Ir with similar cluster sizes of 0.6-0.8 nm, 473-523 K, 1-3 MPa H₂) at turnover rates that are highest on Ru and lowest on Ir. Alkanol-alkanal equilibration is not complete and turnover rates for C-O hydrogenolysis (to form alkanes and H₂O) are ~10 times smaller than for decarbonylation on these catalysts. Methane is also formed through the methanation reaction of CO with H₂, generating H₂O. Chemisorbed CO (CO*) and hydrogen (H*) cover cluster surfaces during reaction, causing  the inhibition of decarbonylation by the CO product and lowering turnover rates as conversion increases or CO is added to reactants. CO* hydrogenation on Ru decreases these inhibition effects by converting CO to CH₄, albeit with concomitant loss of H₂ and formation of low-value products. 

Decarbonylation rates and selectivities are much lower on monofunctional Cu catalysts than on Ru, Pt, and Ir catalysts and C-O hydrogenolysis rates are ~10-fold higher than decarbonylation rates on Cu.  Alkanol-alkanal equilibration is reached at all residence times on Cu catalysts and the effects of H₂ and 1-butanol pressures on C-O hydrogenolysis rate indicate that the most abundant surface intermediate (MASI) is an alcohol-derived species. C-O hydrogenolysis, which can be carried out selectively over Cu/SiO₂, is preferable to decarbonylation as a means of deoxygenation for the production of fuels, because the carbon length of the starting molecule is preserved, and excess depletion of hydrogen through methanation of the decarbonylation product, CO, is avoided.

C-O cleavage reactions of alkanols and alkanals can be observed on surfaces of γ-Al₂O₃ powders when an ad-mixed metal catalyst (Cu/SiO₂) is used to provide hydrogenation-dehydrogenation sites. 1-butanol can be converted to butane through the dehydration/hydrogenation pathway with a 5-fold increase in turnover rates per Cu sites, when γ-Al₂O₃ is mixed with Cu/SiO₂ (at a mass ratio of γ-Al₂O₃ to Cu/SiO₂ equal to 0.05). Bimolecular dehydration, resulting in the formation of dibutyl ether, becomes a competitive pathway over Lewis acid sites with a 60% selectivity (40% selectivity is towards butane and butenes) at 523 K and 1 MPa H₂. Kinetic studies show that the surface of γ-Al₂O₃ is covered by alkoxide, alkene and water species, and alkenes can act as intermediates for the formation of the ether product on Lewis acid sites. Increasing the hydrogen pressure to 2 MPa decreases the ether selectivity to 40%, demonstrating that selectivity towards the monomolecular and bimolecular dehydration products can be controlled by changing the alkene surface coverage through H₂ pressure or the ratio of Cu to Lewis acid sites. Dehydration turnover rates of alkanols measured on zeolites (e.g. H-FAU, H-BEA) show that the selectivity of bimolecular and monomolecular dehydration products are influenced by the size of microporous voids of these materials through solvation and diffusion effects. Metal-acid site proximity was changed by changing the ratio of metal to acid sites in zeolites containing encapsulated metal (Cu) clusters.  Metal sites in close proximity to acid sites result in increased monomolecular dehydration selectivities because alkene intermediates are hydrogenated prior to secondary bimolecular reactions in the acid domain.

 Bifunctional (metal along with acid sites) catalytic hydrodeoxygenation can also be utilized to convert lignin-derived phenolic monomers to cyclic and acyclic alkanes. Transition metals, such as Pt and Ru can carry out the hydrodeoxygenation of phenol, anisole and guaiacol when physically mixed with solid acids, γ-Al₂O₃ and zeolite H-BEA. Kinetic studies show that the degree of unsaturation in the cyclic products can be controlled by the H₂ pressure and while metal-acid site proximity does not have significant effects on deoxygenation turnover rates, it can alter the product selectivities when secondary acid catalyzed reactions take place at rates comparable to diffusion rates in the acid domains.