Toward Environmentally Benign Oxidations: Bulk Mixed Mo‐V‐(Te‐Nb)‐O M1 Phase Catalysts for the Selective Ammoxidation of Propane

Abstract

Selective oxidation catalysis is of vital importance to society, being used to provide about a quarter of all organic chemicals and intermediates used to make consumer goods and the industrial products that rely on them, [1] and is key to many proposed green chemical processes targeted to cut back harmful emissions.[2] As we look to the future, selective oxidation catalysis increasingly holds the promise of eliminating or at least dramatically curbing pollution from chemical and refining processes by employing abundant and environmentally friendly alkanes instead of olefins and aromatics.[3] The goal to use less expensive and environmentally benign feedstocks has spawned the development of new and improved heterogeneous catalysts for C2-C4 alkane conversions. In the selective oxidation and ammoxidation of C3 feedstocks, significant efforts are underway to use propane feeds in processes that produce acrylic acid and acrylonitrile to replace processes that have traditionally relied on propene feeds catalyzed by promoted (multicomponent) bismuth molybdates for the production of acrolein and acrylonitrile. The production of acrylic acid (currently from acrolein) and acrylonitrile has reached a scale that approaches the equivalent of nearly 1 kg for every human being on Earth each year.The bulk mixed-metal Mo-V-Te-Nb oxide is the most promising catalyst system reported to date for the selective ammoxidation of propane to acrylonitrile and oxidation of propane to acrylic acid. The current method of production of acrylonitrile is based on the ammoxidation of more expensive, less abundant propene. The Mo-V-Te-Nb-O catalyst synthesized by the dry-up method was originally discovered by Mitsubishi Kasei corporation.[4] The catalyst consists of two major phases, denoted as M1 and M2, of which the former is active and selective for the direct ammoxidation of propane. [5][6][7][8] Recently, the crystal structure of the M1 phase, which has the Mo-V-Te-Nb-O composition, was refined by using a combination of transmission electron microscopy (TEM), neutron powder diffraction (NPD), and synchrotron X-ray powder diffraction (S-XPD) techniques. [9][10][11] The orthorhombic unit cell of the Mo-V-Te-Nb-O M1 phase (space group Pba2) contains 13 crystallographically distinct metal lattice positions. Several metal sites were found to form solid solutions of Mo VI,V and V V,IV. Nb could not be distinguished from Mo as a result of their similar X-ray and neutron scattering factors and was tentatively assigned to a unique pentagonal-bipyramidal site. A structural study involving a Tacontaining variant of the M1 phase suggested that both Ta and Mo occupy the pentagonal-bipyramidal sites.[11] Te occupies the hexagonal channel sites, and some evidence was obtained in high-resolution TEM studies that it may also reside partially in heptagonal channels. [9][10][11] On the other hand, the M1 phases with Mo-V-O and Mo-V-Te-O compositions represent a simple and well-defined model system for elucidating the structure-reactivity/selectivity relationsh...

Propane Ammoxidation Over the Mo–V–Te–Nb–O M1 Phase: Reactivity of Surface Cations in Hydrogen Abstraction Steps

“…Each cluster was separated from its neighboring images by *11 Å of vacuum in the x, y, and z directions. In the bulk M1 phase, the V and Mo cations are terminated by oxo groups on opposite sides of the bulk ab planes [6,17], but the nature of the surface ab plane termination by oxo groups remains a subject of on-going research [29]. In this study, V and Te were capped by oxygen forming vanadyl (V=O) and telluryl (Te=O) moieties, while the Mo=O groups were present on the opposite side of the cluster.…”

“…Its basal ab plane exhibits 13 crystallographically distinct metal lattice sites (S1-S13) [6]. The 11 lattice sites include 10 octahedral Mo and V sites (S1-S8, S10, S11) and 1 pentagonal bipyramidal Nb site (S9), while the remaining 2 sites are occupied by Te=O species intercalated into hexagonal (S12) and heptagonal (S13) channels [17]. Recent studies of the M1 phase combining high resolution TEM (transmission electron microscopy), synchrotron X-ray, and powder neutron diffraction methods have shown that V, Mo, and Te are present in the 4?/5?, 5?/6?…”

“…=O and other cations to abstract H and in the facility of H removal from the different cation sites. Indeed, supported vanadia (VO x ) is a well-known oxidation and oxidative dehydrogenation (ODH) catalyst [23][24][25][26][27][28], while the presence of Te and Nb has been shown to be crucial to the high activity and selectivity of the M1 phase [17]. However, the mechanistic roles of the various cations in the Mo-V-Te-Nb-O M1 phase are far from clear, and fundamental understanding of its significantly higher activity and selectivity in propane ammoxidation to acrylonitrile than the Mo-V-O or Mo-V-Te-O M1 phases remains lacking, in no small part because of the complex structure of the M1 phase.…”

Propane Ammoxidation over Mo–V–Te–Nb–O M1 Phase Investigated by DFT: Elementary Steps of Ammonia Adsorption, Activation and NH Insertion into π-Allyl Intermediate

“…The proposed active center is outlined by dashed lines the x, y, and z directions. In the bulk M1 phase, the V and Mo cations are capped by oxo groups on opposite sides of the bulk ab planes [1,9], whereas the nature of the surface ab plane termination by oxo groups remains a subject of on-going research [11]. The eight equatorial oxygen atoms in each truncated ab plane were terminated with hydrogen atoms and both the O and H atoms were fixed at a bond length of 0.96 Å during geometry optimization, while all of the other atoms in the cluster models and all the atoms in the adsorbates were allowed to relaxed.…”

“…Direct propane ammoxidation to acrylonitrile catalyzed by the bulk mixed metal oxides has gained considerable attention in recent years [1][2][3] because it is an environmentally benign and potentially more economical process that relies on the abundant propane feedstock than the current process of propene ammoxidation. Among all mixed metal oxides, the bulk mixed Mo-V-Te-Nb-O system is particularly promising because of its significant activity and selectivity in converting propane to acrylonitrile [4] compared to other Mo-V-O based catalysts.…”