(654d) Influence of Confinement in Pores of M1 Phase Mixed Oxides on Selective Oxidative Dehydrogenation of Ethane
AIChE Annual Meeting
Thursday, November 1, 2018 - 1:30pm to 1:50pm
The sizes of C2H6 and the micropores (0.4nm) suggest a tight guest-host fit but C6H12 cannot access intrapore sites because of their much larger size (0.6 nm). Thus, C2H6 molecules have the opportunity to react in micropores as well as external surfaces of the M1 phase crystallites, but C6H12 molecules are restricted to external surfaces. Measured C2H6 to C6H12 activation rate ratios on MoVTeNb oxides are more than two orders of magnitude larger than those measured on non-microporous vanadium oxides (VOx/SiO2) and those estimated by DFT on external surfaces of both oxides, suggesting that most C2H6 activations on MoVTeNb oxides occur inside the micropores under typical conditions. C2H6 exhibits higher activation energy than C6H12 on VOx/SiO2, consistent with the C-H bond strengths in the two reactants. In spite of the significant stronger C-H bonds, the activation energies for C2H6 are similar to C6H12 on MoVTeNb oxides because micropores stabilize C-H activation transition states through van der Waals (vdW) interactions. The tightness of confinement and vdW stabilization are confirmed by slightly endothermic (representing slight steric repulsion from pore walls) and significantly exothermic C2H6 adsorption form DFT methods without vdW and with rigorous vdW functionals, respectively.
The selectivity to C2H4 in C2H6 oxidation is much higher on MoVTeNb than on VOx/SiO2. In contrast, both catalysts exhibit similar C6H12 oxidation product selectivities. These trends 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 MoVTeNb oxides but the micropores in MoVTeNb oxides are more selective for C-H activation. DFT calculations show that the tight confinement in micropores hinders C-O contact necessary for O-insertion and vdW interactions lower C-H activation energies, which leads to the measured higher alkene selectivity.
These effects of heptagonal pores of M1 phase on reactivity and selectivity were confirmed for MoVTeNb oxides synthesized at different hydrothermal conditions and modified using post-synthesis treatments, as well as for MoV oxides without Te and Nb. The higher fraction of accessible micropore volume on MoV oxides than MoVTeNb oxides led to greater rates and rate ratios, which further confirms a direct role of heptagonal pores in controlling rates and selectivity to alkene products.
- Zhu, Y.; Sushko, P. V.; Melzer, D.; Jensen, E.; Kovarik, L.; Ophus, C.; Sanchez-Sanchez, M.; Lercher, J. A.; Browning, N. D. Formation of Oxygen Radical Sites on MoVNbTeOx by Cooperative Electron Redistribution. J. Am. Chem. Soc. 2017, 139, 12342-12345.
- Ishikawa, S.; Ueda, W. Microporous Crystalline Mo-V Mixed Oxides for Selective Oxidations. Catal. Sci. Technol. 2016, 6, 617-629.
- Leelavathi, A.; Liu, Y.; Ezenwa, S.; Dang, Y.; Suib, S. L.; Deshlahra P. Influence of Tight Confinement on Selective Oxidative Dehydrogenation of Ethane on MoVTeNb Mixed Oxides. Submitted to J. Am. Chem. Soc.