Recently discovered multi-component Mo-V-Te-Nb-O catalysts have shown great promise in propane (amm)oxidation to acrylic acid or acrylonitrile[1]. These mixed-phase catalysts contain orthorhombic (M1 phase) and hexagonal (M2 phase) structures. The M1 and M2 crystal structures and their catalytic behavior in propane oxidation to acrylic acid were studied in detail earlier [2]. In this work we focus on propane and propylene ammoxidation to acrylonitrile. The best catalytic performance in propane ammoxidation to acrylonitrile (up to 62% yield) was reported for Mo-V-Te-Nb-O catalyst consisting of 60% M1 and 40% M2 [3]. It was proposed that propane ammoxidation to acrylonitrile occurs in two steps through the intermediate propylene formation, where the M1 phase is responsible for propane oxidative dehydrogenation to propylene and its further ammoxidation to acrylonitrile, while the M2 phase is more efficient in propylene ammoxidation to acrylonitrile [4]. However, the effect of the M1/M2 composition and the synergy between these two phases in propane ammoxidation are still poorly understood.

In this study we report the phase composition (XRD, TEM), microstructure (TEM, STEM) and catalytic behavior of 3-component Mo-V-Te-O and 4-component Mo-V-Te-Nb-O catalysts containing pure M1, M2 phases and their mixtures. The effects of various synthesis parameters (precursor concentration, solution pH, time) and catalyst activation (temperature of calcination, gas flow rate and heating rate) on the formation of the M1 and M2 phases, their stability and catalytic behavior were systematically investigated. In order to better understand the effect of the M1/M2 composition and the synergy between these two phases, we investigated both propane and propylene ammoxidation over the M1 and M2 phases. The pure M1 phase exhibited the optimum yield of 40 mol. % acrylonitrile during propane ammoxidation, whereas the M2 phase exhibited 30 mol. % yield of acrylonitrile during propylene ammoxidation. The synergistic effect between the M1 and M2 phases was investigated for model catalysts containing 50/50, 60/40 and 70/30 M1/M2 phase ratios. The catalytic behavior of this model system was compared with that of the conventional Mo-V-Te-Nb catalyst containing the dominant M1 and minor M2 phase.

References:

1.T. Ushikubo, K. Oshima, A. Kayo, T. Umezawa, K. Kiyono, I. Sawaki, European Patent 529853 (1992), Mitsubishi Chemical Corporation, Tokyo, Japan;

2.J.N. Al-Saeedi, V.K. Vasudevan, V.V. Guliants, Cat. Comm. 4(2003)537-542; V.V. Guliants, R.Bhandari, B. Swaminathan, V.K. Vasudevan, H.H. Brongersma, A. Knoester, A.M.Gaffney and S. Han, J. Phys. Chem., 109 (2005) 24046-24055;

3.R.K. Grasselli, D.J.Buttrey, P.DeSanto, J.D.Burrington, C.G. Lugmair, A.F. Volpe, T.Weingand, Cat.Today, 91-92 (2004) 251-258;

4.R.K. Grasselli, Cat. Today, 99 (2005) 23-31;