(654d) Influence of Confinement in Pores of M1 Phase Mixed Oxides on Selective Oxidative Dehydrogenation of Ethane

Oxidative dehydrogenation (ODH) of small alkanes provides an important energy-efficient route to the production of alkenes that constitute essential building blocks of the chemical industry. Mixed bulk oxides of Mo, V, Nb, Te and other elements (Sb, Ta, W), arranged in an orthorhombic crystalline form, known as the M1 phase, catalyze ethane ODH with exceptionally high selectivity to ethylene at low reaction temperatures. These unique catalytic properties of M1 phase oxides are attributed to specific catalytic sites on the external surfaces¹ or to the one dimensional pores² formed from metal-oxide rings with seven metal atoms that exist in the crystal structure of the M1 phase. Here, we assess the role of the heptagonal micropores of M1 phase in regulating reactivity and selectivity by means of reactant size dependent kinetic probes and density functional theory (DFT) treatments for C₂H₆ and cyclohexane (C₆H₁₂) activations inside and outside the micropores.³ The oxides were synthesized using hydrothermal methods and characterized using elemental analysis, X-ray diffraction measurements, electron microscopy and N₂ physisorption measurements to quantify intrapore and external surface areas.

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 (VO_x/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 VO_x/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 VO_x/SiO­_2. In contrast, both catalysts exhibit similar C₆H₁₂ oxidation product selectivities. These trends suggest that the ability of VO_x/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.

References

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3. 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.