Ethane Oxidative Dehydrogenation on M1 Phase Mixed Metal Oxides

M1 Mixed bulk oxides containing Mo, V, Nb and Te exhibit unique catalytic properties that lead to high selectivity to ethylene at lower reaction temperatures than other oxidative processes. On most other oxides the selectivity tends to be higher at higher temperature because undesired reactions involving combustion of ethane and ethylene have lower activation enthalpies than the desired ethane dehydrogenation steps. These unique catalytic properties of M1 phase oxides are typically 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. We will discuss our assessments of 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 (0.4nm) 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. This conclusion that ethane dehydrogenation occurs predominantly inside the pores is further confirmed by measuring rate ratios on a variety of supported oxides at different surface density and structural and electronic properties, which all lead to rate ratios much less than unity. A variety of M1 phase MoV and MoVTeNb oxides are also tested, which all lead to rate ratios near unity or higher than unity. The relations among accessible micropore volumes and external surface areas are used to further confirm the role of micropores.

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, while C₆H₁₂ molecules react outside the pores and experience much weaker 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 selectivities to dehydrogenated hydrocarbons from C₆H₁₂. These trends suggest that the ability of VO_x/SiO₂ to activate C-H bonds and resist C-O bond formation 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 concave pores imposed steric hindrance C-O contact necessary for C-O bond formation reactions that lead to undesired combustion products while vdW interactions lower C-H activation energies. The net result of vdW and steric forces leads to the higher measured alkene selectivity.

References

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