(715e) A Combined Theoretical and Kinetic Assessment of C–O Bond Rupture Pathways within 2-Methyltetrahydrofuran over Nickel Phosphide Catalyst

The
conversion of oxygenates in biomass-derived feedstocks into more valuable fuels
and chemicals requires selective C–O bond cleavage while leaving other C–O and
C–C bonds intact. Previous studies have shown that metal phosphides (MPs) are
promising catalysts for selective C–O, C–S, and C–N bond rupture over C–C bonds
[1]. MPs also exhibit remarkably higher selectivity towards breaking sterically-hindered
C–O (³C–O) bonds compared to pure transition metals. Our
measurements indicate that Ni₂P cleaves ³C–O bonds at a
rate three times faster than ²C–O bond cleavage, and has 50 times
higher selectivity for ³C–O bond cleavage than pure Ni. The specific
mechanism behind C–O bond cleavage on Ni_xP_y,
as well as the identity of the reactive intermediate and
active site, are not fully understood despite recent studies. Here, we combine
kinetic studies and density functional theory (DFT) calculations to investigate
the mechanisms of hindered and unhindered C–O bond cleavage in 2-methyltetrahydrofuran
(2-MTHF) on Ni_xP_y
and Ni surfaces.

Previous
studies showed that C–C and C–O hydrogenolysis in
alkanes and alkanols occur via unsaturated species formed by sequential quasi-equilibrated
dehydrogenation steps [2,3]; the removal of H atoms
weakens the C–C and C–O bonds by replacing C–H bonds with C–metal bonds. The
measured ratio of products to reactants pressures during the dehydrogenation of
2-methyltetrahydrofuran (2-MTHF) to 2-methylfuran (2-MF) was constant at low
conversions (< 1%), suggesting that C–H activation steps are
quasi-equilibrated and leaving the C–O bond rupture as the kinetically relevant
step. The measured ratio of ³C–O to ²C–O bond cleavage
rates (χ) has a half order dependence on hydrogen pressure [H₂]^1/2,
indicating that the reactive intermediate for ²C–O bond rupture must
lose an additional hydrogen atom compared with the reactive intermediate for ³C–O
bond rupture; the identity of those intermediates that undergo C–O bond
cleavage can be assessed using DFT calculations.

Periodic
plane-wave DFT calculations were performed using the Vienna ab initio Simulation package (VASP) with
the RPBE exchange correlation functional and PAW potentials. The (001) and
(111) surfaces were chosen to model Ni_xP_y
and Ni catalysts, respectively, because they exhibit the lowest surface
formation energies. Vibrational frequency calculations were performed to estimate
enthalpies and free energies of each state at temperatures relevant to the
catalytic reactions. Results show that the effective enthalpy barrier (ΔH҂)
for ³C–O bond rupture is 34 kJ mol⁻¹ lower than for
²C–O on Ni₂P(001) and 21 kJ mol⁻¹
on Ni₁₂P₅(001) at 543 K. These DFT-derived enthalpy
barriers are consistent with the measured values on 12 nm Ni₂P (23 ±
9 kJ mol⁻¹) and on 19 nm Ni₁₂P₅ (25 ± 5
kJ mol⁻¹). ²C–O bond rupture on Ni(111),
however, is more favorable by 73 kJ mol⁻¹. Although C–O bonds
activations through partially dehydrogenated intermediates (i.e. ²CH₂–O,
²CH–O, and ³CH–O) have lower enthalpy barriers, the
removal of H atoms from the carbon atom vicinal to the C–O bond being broken
results in an increase in entropy, rendering ΔG҂ barrier the
lowest for the fully dehydrogenated C–O bond. The contribution of each transition
states to the measured hydrogenolysis rate depends on
their ΔG҂ values, and thus partially dehydrogenated
intermediates are unlikely to contribute to the measured rate. These findings
confirm the essential role of entropy in addition to enthalpy in determining the
rate constants. DFT-predicted ratio of ³C–O to ²C–O bond cleavage
rates χ is equal to 2.7 on Ni₂P(001) and 0.53 on Ni₁₂P₅(001)
at 543 K and 1 MPa H₂, in agreement with the measured values (~3 on
12 nm Ni₂P and ~0.4 on 19 nm Ni₁₂P₅) at low
conversions.

References

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Flaherty, D.; Hibbitts, D.; Iglesia, E., Theoretical
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[3]  
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