Influence of Tight Confinement on Selective Oxidative Dehydrogenation of Ethane on MoVTeNb Mixed Oxides

Abstract

M1 phase MoVTeNb mixed oxides exhibit unique catalytic properties that lead to high C2H4 yields in oxidative conversion of C2H6 at moderate temperatures. The role of the heptagonal channel micropores of the M1 phase in regulating reactivity and selectivity is assessed here using reactant size-dependent kinetic probes and density functional theory (DFT) treatments for C2H6 and cyclohexane (C6H12) activations inside and outside the micropores. The sizes of C2H6 and the micropores suggest a tight guest–host fit, but C6H12 cannot access intrapore sites. Measured C2H6 to C6H12 activation rate ratios on MoVTeNbO are much higher than those measured on nonmicroporous vanadium oxides (VOx/SiO2) and estimated by DFT on external surfaces, suggesting that most C2H6 activations on MoVTeNbO occur inside the micropores under typical conditions. C2H6 exhibits higher activation energy than C6H12 on VOx/SiO2, consistent with the corresponding C–H bond strengths; the activation energy difference between C2H6 and C6H12 is lower on MoVTeNbO because micropores stabilize C–H activation transition states through van der Waals interactions. Product selectivities for C2H6 and C6H12 suggest that the ability of VOx/SiO2 to activate C–H bonds and resist O-insertion in products is similar to the external surfaces of MoVTeNbO, but the micropores in the latter oxides are more selective for C–H activation. DFT calculations show that the tight confinement in micropores hinders the C–O contact necessary for O-insertion. These insights provide guidance for utilizing shapes and sizes of confining voids to mitigate selectivity limitations dictated by thermodynamics of sequential oxidation reactions and electronic properties of redox catalysts.