Mix and match: The pentagonal [Mo(6)O(21)](n-) polyoxomolybdate building block assembles with other sources of Mo, V, and Sb ions to form an orthorhombic Mo-V-Sb oxide. The first single-crystal X-ray analysis of an orthorhombic Mo-V-based oxide, a promising catalyst for light alkane selective oxidation known as the "M1 phase", revealed the structure of the compound.

Crystalline microporous oxides such as zeolites are indispensable materials in various applications ranging from industrial processes to everyday life, such as catalysts, ion-exchange materials, and molecular sieves.[1] Most of them contain tetrahedrally coordinated metal atoms, but octahedrally coordinated metal centers have recently attracted much attention as building blocks of crystalline microporous metal oxides.[2] Manganese oxides (pyrolusite, hollandite, todorokite, and romanechite) with micropores are the only crystalline porous materials based solely on octahedra (octahedral molecular sieves). These manganese oxides contain microtunnel pores consisting of {MnO 6 } octahedra that share edges and corners.[3]Here we describe a novel type of octahedral molecular sieve, namely, crystalline orthorhombic Mo 3 VO x (x = 11.2), in which the microchannel is constructed by seven-membered rings of corner-sharing MO 6 (M = Mo or V) octahedra. It is isostructural to orthorhombic MoVNbTeO compounds, [4] which are very active and selective oxidation catalysts for light alkanes.[5] These mixed metal oxides have a layered orthorhombic structure with a slab composed of six-and seven-membered rings of corner-sharing {MO 6 } octahedra and pentagonal {(M)M 5 O 27 } units with a {MO 7 } pentagonal bipyramid and five edge-sharing {MO 6 } octahedra, where M is Mo, V, or Nb. The layered six-and seven-membered rings form channel structures. The Te atom is believed to be located both in the six-and seven-membered rings [4] and block the channel. Recently, we succeeded in preparing an orthorhombic Mo 3 VO x compound that contains only Mo and V, [6] in which the channel is expected not to be blocked (Figure 1). [7] Aperture diameters of the seven-and six-membered rings are estimated to be about 0.33-0.37 nm and about 0.25-0.28 nm, respectively. [8] The orthorhombic Mo 3 VO x mixed-metal oxide was synthesized from a reaction mixture of ammonium heptamolybdate (NH 4 ) 6 Mo 7 O 24 ·4 H 2 O, and vanadyl sulfate VOSO 4 ·n H 2 O (Mo/V 4:1) in H 2 O under hydrothermal conditions.[6] The crude material contained an amorphous phase as a byproduct, which was removed by washing the products with an aqueous solution of oxalic acid. Water and NH 3 in the micropores were removed by calcination under air without collapse of the structure, as confirmed by thermogravimetry (TG), temperature-programmed desorption (TPD), and X-ray diffraction studies. The TG data and TPD revealed a weight loss in the range of 320-460 K corresponding to water evaporation and

In this work, we structurally characterize defects, grain boundaries, and intergrowth phases observed in various Mo-V-O materials using aberration-corrected high-angle annular dark-field (HAADF) imaging within a scanning transmission electron microscope (STEM). Atomic-level imaging of these preparations clearly shows domains of the orthorhombic M1-type phase intergrown with the trigonal phase. Idealized models based on HAADF imaging indicate that atomic-scale registry at the domain boundaries can be seamless with several possible trigonal and M1-type unit cell orientation relationships. The alignment of two trigonal domains separated by an M1-type domain or vice versa can be predicted by identifying the number of rows/columns of parallel symmetry operators. Intergrowths of the M1 catalyst with the M2 phase or with the Mo 5 O 14 -type phase have not been observed. The resolution enhancements provided by aberration-correction have provided new insights to the understanding of phase equilibria of complex Mo-V-O materials. This study exemplifies the utility of STEM for the characterization of local structure at crystalline phase boundaries.propane | oxidation | catalyst | acrylonitrile | bronze S elective catalytic oxidation of light hydrocarbons is of tremendous commercial importance for the production of a variety of key industrial organic chemicals and intermediates. Existing processes, valued in the billions of US dollars per annum, are predominantly based on oxidation of olefin feeds; however, there are significant economic and environmental benefits to replacement by more energy-and carbon-efficient paraffin-fed processes. Pursuit of active and selective catalysts for use in such replacement processes is a currently very active area of research. Mixed-metal oxides in the system Mo-V-Nb-Te-O, when prepared under mildly reducing conditions, may consist of one or more of several network "bronze" structures with mixed valences for Mo and V (1-4). Materials from this system show promise as catalysts for selective oxidation of ethane to ethylene, propane to acrylic acid, and, in the presence of cofed ammonia, propane ammoxidation to acrylonitrile. One phase in particular, commonly designated as "M1," has been found to be essential as a catalyst component for selective paraffin oxidation, and under some conditions seems to be promoted by coexistence with another phase designated as "M2" (1-18). Our interest in chemical and structural inhomogeneities observed in this system is driven by the apparent need for composite M1/M2 phase coexistence to realize the most active, selective, and stable catalysts.The M1 phase has an orthorhombic unit cell with a structure comprised of a network of interconnected pentagonal fMo 6 O 21 g rings joined together by linking octahedra, resulting in the formation of nanoscale hexagonal and heptagonal channels (4,14,(19)(20)(21). The M2 phase is belongs to the hexagonal tungsten bronze family, but exhibits a small orthorhombic distortion (4). In its pure form, the M2 phase is efficient in ...

High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) imaging was used to study various hydrothermally prepared MoVO phases with either M1-type orthorhombic symmetry, Mo5O14-type tetragonal symmetry, or trigonal symmetry. For the M1-type phase, HAADF analysis suggests atomic positions consistent with existing M1 models having mixed Mo and V occupancy in the octahedral sites linking the pentagonal units. For the V-substituted Mo5O14 phase, contrast analysis provides evidence of V occupation primarily in the linking octahedra that interconnect the pentagonal {Mo6O21} units. Selected-area electron diffraction (SAED) studies suggest that the structure of the Mo5O14 analog is likely to be tetragonal, rather than orthorhombically distorted as has previously been proposed. HAADF analysis of the trigonal phase provides reasonable support for previously reported structural models. High-resolution images provide strong support for the existence of atomic disorder within the octahedral sites connecting the three heptagonal channels that characterize this structure.

We demonstrated that the sorption behavior of orthorhombic Mo-V oxide (Mo(29)V(11)O(112-x)) is continually and reversibly tunable by redox treatment.