Effects of Lattice O Atom Coordination and Pore Confinement on Selectivity Limitations for Ethane Oxidative Dehydrogenation Catalyzed by Vanadium-Oxo Species

M1-phase MoVTeNb mixed oxides contain V-oxo species isolated by dispersion in the Mo-oxo framework and one-dimensional heptagonal micropores that tightly enclose C2H6 molecules. These oxides catalyze C2H6 oxidation with C2H4 selectivity much higher than V2O5 oxides containing continuous V-oxo domains without micropores. Here, effects of the structures of VOx domains and of the micropores on the selectivity are discerned using (i) measured rate constant ratios and activation barrier differences relevant to selectivity on the two oxides and (ii) density functional theory (DFT) analysis of steps mediating C-H activation in C2H6 and C2H5 radicals and unselective C-O bond formations in C2H5 radicals and C2H4 molecules on (001) surfaces of both oxides and in pores of MoVTeNb oxides. The DFT-derived values of kinetic parameters representing C2H4 selectivities and activation energy differences between C2H4 formation and C-O bond formation steps on V2O5(001) are similar to measured values. In contrast, for MoVTeNb oxides, the DFT-derived selectivity inside the pores is much higher than measurements, while that on the (001) surfaces is much lower, suggesting that measured selectivity represents contributions from C-H activations inside the pores and unselective steps inside pores as well as on (001) surfaces. The selectivity on (001) surfaces is similar in V2O5 and MoVTeNb oxides, indicating that the isolation of V-oxo domains within this surface leads to only small changes in selectivity, while the pores lead to much higher selectivity. The descriptors of the selectivity trends on such transition metal oxide surfaces are derived by examining C2H4 epoxidation and C2H6 C-H activation transition-state energies and molecule-surface van der Waals (vdW) interactions and steric forces that influence these energies on a variety of O atoms with different electronic and structural properties on (001) surfaces and inside the pores. High C2H4 selectivity requires that the O atoms in oxides exhibit lower tendency to form C-O bonds in C2H4 than to activate C-H bonds in C2H6, which depends strongly on the H atom addition energies of oxides and O atom coordination. V-2-O-V tri-coordinated and V-O-V or V-O-Mo bridging O atoms require significantly greater energy penalty than V=O terminal O atoms for the metal oxygen framework distortions required for forming C-O bonds; these distortion energies reflect steric hindrance to forming C-O bonds, which leads to higher epoxidation transition-state energy and indicates higher C2H4 selectivity in tri-coordinated and bridging O atoms. The high selectivity inside the heptagonal pores originates from the inaccessibility to terminal O atoms in addition to much stronger vdW interactions and more significant steric distortion energies in tight pores. These analyses suggest that H atom addition energies, vdW interaction energies, and catalyst distortion energies are relevant descriptors of selectivity for both intrapore and external O atoms.