Selective removal of oxygen from renewable feedstocks (i.e., alkanols, alkanals, furanic and phenolic species) poses formidable challenges for their efficient conversion to fuels and chemicals. The rates of different deoxygenation pathways (i.e., decarbonylation, C-O hydrogenolysis and dehydration) are controlled by the identity of the metal and, in some cases, by the synergistic presence of acid sites within molecular diffusion distances.  Alkanols/alkanals predominantly decarbonylate on monofunctional Group VIII metals (Ir, Pt, Ru; 473-523 K, 1-3 MPa H2). Decarbonylation of alkanols goes through equilibrated H-removal steps prior to the kinetically relevant C-H cleavage to form the acyl intermediate that undergoes C-C cleavage on Ir clusters. Decarbonylation leads to undesired carbon losses, and forms CO, which strongly inhibits the reaction by competitive adsorption on metals. These CO inhibition effects are weaker on larger clusters, because of weaker binding of CO on low-index planes, and on metals, such as Ru, that effectively scavenge chemisorbed CO (CO*) to form CH4. In such instances, however, H2 requirements are greater than for C-O hydrogenolysis routes. Cu, in contrast, catalyzes C-O hydrogenolysis with higher selectivity than decarbonylation because of its oxophilic nature, weak M-C bonds and CO* binding. C-O cleavage in alkanols occurs on solid Br�nsted acids with proximate metal sites hydrogenating the alkene products, such that the rates are >10-fold higher when solid acids are mixed with Cu-based catalysts (0.025 mass ratio H-BEA:Cu-SiO2) than on monofunctional Cu. These bifunctional pathways and site proximity requirements are explored here for more complex molecules (e.g. p-cresol and 2-methylfuran), on which metals can hydrogenate C=C bonds to form species that undergo C-O cleavage on acid sites (dehydration or ring opening/hydrolysis). The effects of acid strength (W-based polyoxometalates with different central atoms (P,Si,Al,Co)) and confinement (zeolites with different size/structure of void environments) on C-O cleavage reactivity and product selectivity were studied.

Reactivity and mechanisms of oxygen removal catalysis on metal clusters and metal-acid bifunctional systems

248th National Meeting (2014)

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