Selective C–O bond cleavage is an essential step in upgrading biomass-derived feedstock into more valuable chemicals. Metal phosphides can selectively cleave the C−(O,N,S) bonds while leaving other C–C bonds intact to increase the yield of high value chemicals. These materials, in contrast to pure transition metals, also show significantly higher selectivity towards cleaving the sterically hindered 3C–O bonds over the unhindered 2C–O bonds. For example, the measured turnover rate of 3C–O cleavage in 2-methyltetrahydrofuran (2-MTHF) over 12 nm Ni2P catalyst is 3 times faster than that of 2C–O cleavage, near 50 times increase in selectivity compared to pure 4 nm Ni. Density functional theory (DFT) calculations were used, in concert with high-pressure kinetics, to investigate the mechanism of C–O bond cleavage in 2-MTHF over NixPy and to identify the active site as well as the reactive intermediates that undergo C–O bond cleavage. The observed equilibrated mixture of 2-MTHF and 2-methylfuran at all conditions of this study indicates that the dehydrogenation steps are quasi-equilibrated and C–O activation is the kinetically relevant step, analogous to alkanes hydrogenolysis where C–C activation occurs in partially dehydrogenated intermediates. The half order dependence on H2 pressure ([H2]1/2) of the measured ratio of 3C–O to 2C–O activation rates suggests that the reactive intermediates that cleave 3C–O and 2C–O bonds differ by one H atom. Both 3C–O and 2C–O cleavage turnover rates, however, are inhibited by CO pressure to the same extent, indicating that both reactions occur on identical sites. DFT-predicted barriers show that C–O activation occurs when the C atom vicinal to the C–O bond is fully dehydrogenated (C5H9O* for 3C–O; C5H8O* for 2C–O) and the selectivity towards 3C–O cleavage decreases with increasing the P:Ni ratio, in agreement with the measured data at 543 K. For example, the relative enthalpy barrier of 3C–O to 2C–O activation is −34 kJ mol−1 on Ni2P and −11 kJ mol−1 on Ni12P5, while 2C–O activation is far more favorable on Ni by 48 kJ mol−1. Despite this significant increase in selectivity on NixPy, the phosphorous atoms do not participate in binding the reactive intermediates and the same Ni ensembles catalyze both reactions. The electronegative P atoms, however, alter the electronic nature of the Ni atoms and break the extended Ni ensembles (average Ni−Ni distances increase from 2.5 Å on Ni to 3.7 Å on Ni2P) with increasing the P:Ni ratio.

Preferential activation of the sterically hindered C–O bond within 2-methyltetrahydrofuran over nickel phosphide catalysts

2019 Fall National Meeting

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