Kinetic Requirements for Catalytic Deoxygenation of Alkanals on Solid Brønsted Acid Sites

Alkanals undergo catalytic deoxygenation on solid Brønsted acid catalysts (e.g. acidic zeolites and polyoxometalate clusters), producing light alkenes, alkadienes, or heavy aromatic species, under atmospheric pressure and moderate temperature (473-673 K). The details about the reaction mechanisms and kinetics, however, remain ambiguous. In this thesis, C3-C6 n-alkanals were used as model reactants in combined kinetic measurement, chemical titration, and infrared spectroscopic experiments, to investigate the mechanisms and kinetic requirements for alkanal deoxygenation on solid Brønsted acid sites (H+). The reaction network for alkanal (CnH2nO) deoxygenation consists of three kinetically coupled primary reaction pathways: (i) intermolecular C=C bond formation, a bimolecular pathway lengthening the carbon chain via aldol condensation and dehydration and forming larger alkenals (e.g. C2nH4n-2O and C3nH6n-4O), (ii) intramolecular C=C bond formation, an unimolecular pathway evolving alkene (CnH2n) via transfer hydrogenation and dehydration while preserving the carbon backbone, and (iii) isomerization-dehydration, another unimolecular pathway directly ejecting a water molecule from the alkanal and producing alkadiene (CnH2n-2). The secondary cyclization-dehydration following pathway (i) produces aromatic species which provide the H atoms required for the alkanal transfer hydrogenation during pathway (ii); pathway (i) is also kinetically coupled with pathway (iii) as both pathways share a bimolecular surface intermediate and the accessibility of acid-base site pairs dictates the selectivities between these two pathways. In pathway (ii), the kinetic relevant alkanal transfer hydrogenation step proceeds via hydride ion transfer from a hydrocarbon (e.g. aromatic products) as the H-donor to the protonated alkanal as the H-acceptor, upon forming a bimolecular carbocation transition state. The hydride transfer is favored on H+ sites under confinement of molecular dimension because the confined structure solvates and stabilizes the transition state and lowers the activation barrier. The rate of the transfer hydrogenation is dictated by the hydride ion affinity difference between the H-donor-acceptor pair. The chain length of the alkanal reactant determines the stability of the enol tautomer and the hydride ion affinity of the protonated alkanal, as a result, dictating the rates and selectivities of these deoxygenation pathways. This mechanistic knowledge on the multiple catalytic cycles and their kinetic and thermodynamic requirements provides the framework for predicting the rates for larger oxygenates and hydrocarbons during alkanal deoxygenation and could guide the design of catalyst structures to enable tuning of rates and product distributions.Ph.D.2019-07-11 00:00:0