(465e) Size Selective Oxidation of Alkanes By Microporous Oxides

MoVTeNb mixed oxides, with a particular crystal phase called the M1 phase, are among catalysts that provide the highest selectivity to desired C₂H₄ products in C₂H₆ oxidative dehydrogenation (ODH) with the least amounts of combustion products (CO, CO₂).¹ The origin of their selective catalysis is typically attributed to electronic properties of their lattice oxygens.² The M1 phase contains network arrangement of the corner and edge shared octahedra that form pentagonal, hexagonal and heptagonal channels. The heptagonal channels have dimensions comparable to kinetic diameters of linear alkanes (~ 0.4 nm).³ Recently, Ueda et al. reported analogous crystalline materials containing only Mo and V elements and, by varying crystallite sizes, showed that C₂H₆ molecules react within heptagonal channels, while C₃ oxygenates react on external surfaces.⁴ Here, we probe the origin of selective catalysis of these materials by examining oxidations ethane (C₂H₆) molecules that can enter the heptagonal channels, and cyclohexane (C₆H₁₂) molecules that are too big to enter these channels. We measure the C₂H₆/C₆H₁₂ rate ratios and selectivity trends on MoVTeNb oxides and compare them with silica supported vanadium oxides (VO_x/SiO₂) without micropores. Such comparisons lead us to conclude that confinement within micropores plays a significant role in selective C₂H₆ODH catalysis.

Materials and methods

MoVTeNb mixed oxide samples were prepared by a hydrothermal method,⁵ followed by thermal treatment in flowing Helium. The crystal structures were determined using X-ray diffraction (XRD) and microstructural details were examined using electron microscopy. VO_x/SiO₂ (41%wt.) samples were prepared by wet impregnation of ammonium metavanadate and followed by thermal treatments, flowing dry air. Surface areas and elemental compositions of oxides were measured using nitrogen physisorption methods and inductively coupled plasma atomic emission spectra, respectively. Samples were diluted with SiO₂to eliminate heat and mass transfer limitations during reactions. Catalytic alkane oxidation experiments were carried out in quartz plug flow reactors and the products were measured via online gas chromatography.

Results and discussion

The XRD pattern of MoVTeNbO is consistent with crystalline oxides with majority M1 phase. Electron micrographs reveal rod-like structures of crystalline oxides typical of the M1 phase (~1 µm length and 0.2-0.4 µm diameter) with a small fraction of much smaller particles. The catalytic sites of VO_x/SiO₂, a catalyst without micropores, are equally accessible to C₂H₆ and C₆H₁₂ (Figure 1a) In contrast, the micropores of MoVTeNbO allow access to ethane, leading to rates from pores and external surfaces (r1_ext + r1_pore; Figure 1a), while restricting C₆H₁₂ rates to external surfaces (r2_ext; Figure 1a). Therefore, the C₂H₆/C₆H₁₂ ODH rate ratio on MoVTeNbO relative to that on VOx/SiO₂ reflects the contribution of micropores to C₂H₆ conversion. The measured rate ratio is 52 times higher for MoVTeNbO than that of VO_x/SiO₂, suggesting that most of the C₂H₆ ODH reactions on MoVTeNb occur within the pores (Figure 1b). The C₂H₆ oxidation product selectivity to C₂H₄, at similar conversions, is much higher on MoVTeNb than on VOx/SiO­₂ (Figure 2a), while both catalysts exhibit similar C₆H₁₂ oxidation product selectivity to cycloalkenes and COx (Figure 2b). These trends confirm that only the pores, and not the electronic properties of external surfaces, are responsible to high alkene selectivity. The dependence of rates on reactant pressures are consistent with Mars-van-Krevelen redox cycles. Activation enthalpies for measured rate constants are lower for C₂H₆ than for C₆H₁₂, in spite of the weaker C-H bonds in the latter reactant, which is consistent with the role of van der Waals (vdW) interactions within pores in stabilizing the C₂H₆ activation transition states. We conclude that the high C₂H₄ selectivity arises from steric restriction to secondary reactions in the heptagonal channels. Our preliminary DFT calculations confirm that such vdW stablizations and steric restrictions are prevalent in the micropores.⁶

Figure 1. (a) Scheme representing size selectivity of alkane oxidation on MoVTeNb oxides and VO_x/SiO₂ using C₂H₆/C₆H₁₂ rate ratios. (b) Measured rate ratios. [Reaction conditions: 30 cc/min; 3 kPa C₂H₆ or C₆H₁₂; 3 kPa O₂; 648 K; Conversions: VO_x/SiO₂ - 5% (C₆H₁₂) and 0.4 % (C₂H₆); MoVTeNbO - 8% (C₆H₁₂) and 1.7% (C₂H₆)]

Figure 2. (a) C₂H₆ and (b)C₆H₁₂ product selectivity on the two oxides. [Reaction conditions: 30 cc/min; 3 kPa C₂H₆ or C₆H₁₂; 3 kPa O₂; 648 K; Conversions: VO_x/SiO₂ - 5% (C₆H₁₂) and 0.4 % (C₂H₆); MoVTeNbO - 8% (C₆H₁₂) and 1.7% (C₂H₆)]

Financial support from Tufts University and its Faculty Research Fund are gratefully acknowledged.

References

1. Botella, P.; López Nieto, J. M.; Solsona, B.; Mifsud, A.; Márquez, F. J. Catal. 2002, 209, 445-55.

2. Getsoian, A. B.; Zhai, Z.; Bell, A. T. J. Am. Chem. Soc. 2014, 136, 13684-97.

3. Ishikawa, S.; Ueda, W. Catal. Sci. Technol. 2016, 6, 617-29.

4. Ishikawa, S.; Yi, X.; Murayama, T.; Ueda, W. Appl. Catal., A 2014, 474, 10-17.

5. Gaffney, A. M.; Song, R, US 7074956 B2, B2, 2004.

6. Cheng, M.-J.; Goddard, W. A. Top. Catal. 2016, 59, 1506-17.