Active centers in Mo–V–Nb–Te–Ox (amm)oxidation catalysts

Abstract

The catalytically active centers of the MoVNbTeOx system for propane ammoxidation to acrylonitrile have been identified. The catalytic system is comprised of three crystalline phases: orthorhombic Mo7.8V1.2NbTe0.94O28.9 (M1) (Pba2: a=21.1337Å; b=26.6440Å; c=4.01415Å; z=4), pseudo-hexagonal Mo4.67V1.33Te1.82O19.82 (M2) (Pmm2: a=12.6294Å; b=7.29156Å; c=4.02010Å; z=4) and a trace of monoclinic TeMo5O16 (P21/C: a=10.0349Å; b=14.430Å; c=8.1599Å; β=90.781°; z=1). The catalytically active and selective centers reside on the surface of the basal plane of the M1 phase and are comprised of an assembly of five metal oxide octahedra (2V0.325+/Mo0.686+, 1V0.624+/Mo0.385+, 2Mo0.56+/Mo0.55+) and two tellurium–oxygen sites (2Te0.944+), which are stabilized and structurally isolated from each other (site isolation) by four Nb5+ centers, each surrounded by five molybdenum–oxygen octahedra. The V5+ surface sites, distinguished through their (V5+O↔4+VO) resonance structure, are the paraffin activating sites capable of methylene-H abstraction; the Te4+ sites (lone pair of electrons) for the α-H abstraction of the chemisorbed propylene molecule, once formed; and the adjacent Mo6+ sites for the NH insertion into the π-allylic surface intermediate. Herewith, all key catalytic elements needed to transform propane directly to acrylonitrile are contained, strategically arranged and within bonding distance of each other, at the active center of the M1 phase.Based on the metal site distribution probabilities at the active center 44% of them are computed to be active and selective for propane ammoxidation, 46% inactive for propane but active for propylene ammoxidation and 10% are waste forming sites. A maximum theoretical acrylonitrile selectivity of 81% is predicted on this premise (not achieved as yet experimentally). Under mild operating conditions, the M1 phase alone suffices to effectively convert propane directly to acrylonitrile. Under demanding conditions symbiosis between the M1 and M2 phases occurs, with the latter serving as a co-catalyst or mop-up phase to the former, transforming unconverted, desorbed propylene to acrylonitrile. The M2 phase is incapable of propane activation, lacking V5+ sites, but is a good propylene ammoxidation catalyst. A maximum acrylonitrile yield from propane of 61.8% (86% conversion, 72% selectivity at 420° C) was achieved with a nominal catalyst composition of Mo0.6V0.187Te0.14Nb0.085Ox, identified by combinatorial methodology, and is comprised of 60% M1, 40% M2 and a trace of TeMo5O16.