(425b) Identifying Effects of Phosphorous in Transition Metal Phosphides for Selective Hydrogenolysis of Hindered C–O Bonds

For biomass-derived molecules, 2-methyltetrahydrofuran (MTHF) is a model compound that can be used to contrast hindered and unhindered C–O bond activation. MTHF contains a furan ring whose O atom has neighboring secondary (²C–O) and tertiary (³C–O) carbons. Supported Ni catalysts activate the unhindered ²C–O bond at a rate 10× that for hindered ³C–O bonds in MTHF, as supported by DFT-derived ΔH_act values on Ni(111) surfaces that are 47 kJ mol^−1 lower for ²C–O bond activation. This selectivity shifts toward ³C–O activation as phosphorous is incorporated, with ΔH_act for ³C–O bond activations that are 11 kJ mol^−1 and 29 kJ mol^−1 lower on Ni₁₂P₅ and Ni₂P, respectively [1]. It remains unclear how P causes these shifts in selectivity, which could occur because of geometric (separation of Ni centers) or electronic effects (withdraw of e^− from Ni). Here, we use DFT calculations to study the effect of P incorporation into other transition metals using surfaces isostructural to Ni(111) and Ni₂P(001) to decouple electronic from geometric effects. Our results show that incorporating P on a series of transition metals consistently shifts selectivities toward ³C–O activation, reflected by the difference in the ²C–O and ³C–O bond activation enthalpies (ΔΔH) and free energies (ΔΔG): Ni₂P, Pd₂P, Rh₂P Fe₂P and Co₂P all have more positive values of ΔΔH and ΔΔG than their pure metal counterparts (Fig. 1). The most dramatic effects of P incorporation are observed in Ni₂P and Pd₂P, and only those two surfaces had lower ³C–O activation barriers than those for ²C–O. These data begin to provide insights into the geometric and electronic effects of P incorporation which appear to cooperatively increase the selectivity toward hindered ³C–O bonds.

References

[1] ACS Catal. 2018, 8, 7141–7157.