Alkanal transfer hydrogenation catalyzed by solid Brønsted acid sites

Abstract

Catalytic pathway and requirements for transfer hydrogenation of n-alkanals (CnH2nO, n=3–6) on Brønsted acid sites (H+) immobilized in microporous MFI and FAU crystalline structures or dispersed on H4SiW12O40 polyoxometalate clusters are established by isolating its rates from those of the various concomitant catalytic cycles. Transfer hydrogenation of alkanals involves a kinetically-relevant, inter-molecular hydride transfer step from substituted tetralins or cyclohexadienes produced from the parallel alkanal coupling and ring closure reactions as the hydride donor (R′H2) to protonated alkanals (RCH2CHOH+) as the hydride acceptor, via a bi-molecular transition state with a shared hydride ion, (RCH2CHOH+--H−–R′H+)‡. The rate constants for the inter-molecular hydride transfer step correlate directly to the hydride ion affinity difference between the carbenium ions of the H-donors (R′H+) and the protonated alkanals (RCH2CHOH+). As a result, smaller alkanals with higher hydride ion affinities are more effective in abstracting hydride ions and in transfer hydrogenation (C4>C5>C6). Propanal is an exception, as it is less effective in transfer hydrogenation than butanal. The deviation of propanal from the reactivity trend is apparently caused by its smaller transition state for hydride transfer, which is solvated to a lesser extent in FAU cages. The transfer hydrogenation occurs much more effectively on partially confined H+ sites in FAU structures than in smaller pore MFI or unconfined H4SiW12O40 polyoxometalate clusters, an indication that FAU solvates and stabilizes the bulky transition state of hydride transfer via van der Waals interactions. These effects of local site structures and the thermochemical properties of reactant determine the reactivity of alkanal transfer hydrogenation and thus selectivity ratio of alkenes, dienes, aromatics, and larger oxygenates during deoxygenation catalysis.