Performance of Density Functional Theory for Predicting Methane-to-Methanol Conversion by a Tri-Copper Complex

Efficient, low-temperature, and catalytic methane-to-methanol conversion (MMC) is of great interest, as methanol can be used as a liquid fuel and is a value-adding intermediate in the petrochemical industry. MMC can be achieved through direct C-H activation or via oxidation in strongly acidic media using noble metal catalysts. However, these processes are expensive and generally have low selectivity for methanol. In contrast, copper-exchanged zeolites can facilitate methane oxidation stoichiometrically under mild conditions with high selectivities for methanol. Approaches for achieving catalytic MMC on copper-exchanged zeolites have recently been developed. A better understanding of this process is required in order to facilitate the design of more efficient catalysts. In this work, we benchmark the performance of density functional theory (DFT) for modeling the MMC pathway by a tri-copper complex, [Cu3O3(H2O)(6)](2+). This complex is reminiscent of [Cu3O3](2+) proposed as the active-site motif in the zeolite mordenite (MOR). Using the newly developed open-shell version of the domain-based local pair natural orbitals coupled-cluster theory, DLPNO-CCSD(T), extrapolated to the complete basis set limit, as a benchmark, we found that inclusion of dispersion corrections results in only marginal improvement of the mean absolute deviations (MADs), whereas 20-30% Hartree-Fock exchange in the DFT functional leads to more improved results. Of the 31 functionals tested, MN15 and omega B97X perform best with. a mean absolute deviation of 1.2 and 1.9 kcal/mol, respectively. These functionals also faithfully reproduce the overall energy landscape of the MMC catalytic cycle. Double-hybrid functionals also perform very well. These findings are invariant to the level at which the geometries of the involved species were optimized. Although the benchmark DFT calculations were carried out with quadruple-zeta polarized basis sets, good accuracy can still be obtained at the triple-zeta level. We have also observed that scalar-relativistic effects significantly alter the calculated energetics of the individual steps in MMC cycle, even though the performances of the functionals are similar at the relativistic and non-relativistic levels.