Role of Phosphorus in Transition Metal Phosphides for Cleaving Hindered C-O Bonds in 2-Methyltetrahydrofuran

Biomass can be upgraded into higher value chemicals, such as α,ω-diols, via C-O hydrogenolysis. However, this process requires the selective activation of sterically hindered C-O bonds. Here, we observe a model biomass-derived molecule, 2-methyltetrahydrofuran (MTHF). The molecule has two C-O cleavage pathways, the unhindered (²C-O) cleavage and hindered (³C-O) cleavage. Previous work examining C-O hydrogenolysis in MTHF demonstrated that an increase in P content to Ni shifts selectivity towards ³C-O cleavage. Ni₂P (P62m spacegroup) showed the greatest selectivity towards ³C-O cleavage over ²C-O cleavage in comparison to Ni₁₅P₅ and pure Ni. Alongside this, other transition metals were also compared to their pure metal counterparts to investigate selectivity trends. A shift in selectivity towards hindered ³C-O cleavage was seen for almost all transition metals investigated when P was added. Here we examine hindered ³C-O cleavage using simulated models and calculating free energy barriers (ΔG) for Ni₂P using Density Functional Theory (DFT). Here we now more closely investigate structural effects on selectivity by modeling Ni₂P in both the Pnma and Fm3m spacegroups while comparing our results to previous work to determine if structural effects play a role in selectivity. It was found that Ni₂P(010) was the optimal surface for Pnma and Ni₂P(001) was the optimal surface for Fm3m among various Miller Indices. Probe molecules when then adsorbed to the catalyst surface to determine the likely preferred binding site for MTHF C-O cleavages. During our adsorbate calculations, it was observed that the Fm3m surface restructured, rendering it unsuitable for conducting chemical reactions due to its instability. Comparing our reaction coordinate diagram to previous work, it was found that the ³C–O activation barrier was lower than ²C–O activation in Ni₂P, making ³C-O cleavage more favorable, as was seen in the prior work. Additionally, the previous relative energy difference (ΔΔE) calculated for Ni₂P P62m was 44 kJ mol-1, and our Ni₂P Pnma ΔΔE was 17 kJ mol-1. This suggests that structure does play a role in selectivity. Future work will include completing free energy difference (ΔΔG) calculations to further prove this suggestion true.