Design And Study Of Mo (V) Te (Nb) O Catalytic Active Phases For Propane Selective Oxidation To Acrolein

Propane selective oxidation is of high importance in nature gas industry. The studies in crystal structure and related properties are the hot topics and "seven pillars" in fundamental research of catalysis. The researches that focused on propane selective oxidation not only provide novel catalytic materials but also elucidate some new phenomenon and concepts in catalysis.There are still many ambiguities in crystal structure of MoVTe(Nb)O mixed oxides as the most efficient catalysts, such as the interaction between M1 and M2 phase:The symbiosis between M1 and M2 phase, that is the presence of M2 phase promotes the production of acrylic acid (AA) or acrylonitrile (ACN) of M1 phase; M2 phase replenishes the Mo/Te loss in Ml phase and alleviates its deactivation. On the other side, propane selective oxidation to acrolein in high selectivity is a difficult research topic in this century.Firstly, this work studied the relationship between the electronic structure and crystal structure of M1 phase and found out that there's strong O-M…O alteration along the c axis which resulted in the coupling of d electrons. The coupled d electrons were prone to locate at V sites rather than Mo sites. M3, M7, M8 and M11 sites preferred the location of d electrons. The high redox activity and reversibility of V5+/V4+pair resulted in the formation of surface VOX active sites for deep oxidation. On the other side, the doping of V emerged oxygen vacancy in the bulk and boosted the bulk oxygen diffusion rate that caused the over-oxidation. While M4, M5, M9 and M10 sites were prone to present oxygen vacancy and acted as C-C active sites to produce formaldehyde and acetaldehyde.Secondly, the crystal structure and surface reconstruction of M2 phase were studied to interpret the interaction between M1 and M2 phases. Results showed that the surface of M2 phase reconstructed to nanocrystalline VOMoO4, because Te4+ species in the neighboring two TeO3E tetrahedral units accepted two electron from the Mo(V)O6 interconnections and disproportionated to Te6+and Te0 species on the surface. Te6+species are enriched on the topmost surface and Te0 species sublimed from M2 grains in the form of Te and MoTe2 to replenish Mo/Te loss in M1 phase by either spillover through grain boundaries or solid-gas-solid deposition. While those Te6+species should be more active than Te4+species activating a C-H bond and efficiently converted propylene intermediate that migrated from M1 grains and formed AA and ACN by (amm)oxidation. This was interpreted as the nature of M1/M2 phases'symbiosis. At the same time, the Te depleted in the sublayer and reconstructed to nanocrystalline VOMoO4.Having known the decent crystal structure of M2 phase, the content of intrinsic oxygen vacancy was intended to be tuned to study the roles of oxygen diffusion pathways in the catalytic performance. Results showed that the addition of nitric acid and oxidative atmosphere could tune the content of intrinsic oxygen vacancy with different coordination modes. These methods controlled three anisotropic oxygen diffusion pathways in the bulk lattice of M2 phase:ⅰ) inter-pyramid vertex anion hopping between fluctuated 12f Wyckoff 03 terminal sites along [0110] direction; ii) ab-plane intra/inter-pyramid anion-vacancy wagging involving 12f Wyckoff 02 bridge sites; iii) intra-pyramid "cog-wheel" type pitching between 02 and 03 sites. The content of oxygen vacancy neighboring to V controlled the rates of all anisotropic diffusion pathways, and content of oxygen vacancy at terminal sites controlled the diffusion rate of pathway iii) that can change surface reaction pathway from C-H activation to C-C bond cracking-oxidation.Due to the strong redox ability of V5+/V4+pair, the large content of V in the catalyst may result in the over-oxidation of acrolein. Hence, we introduce traces of V into the lattice of monoclinic TeMo5O16 to induce the structural relaxation. The structural relaxation changed the distribution of Mo-O bond lengths and the d electron as well as its coupling. The d electrons in the framework on one side localized at Mo(3) and Mo(5) centered octahedral units, and filled the lower t2g levels from Eg of 0.45 eV to 0.23 eV. The relaxation on the other side facilitated the formation of oxygen vacancy in Mo(3)-O-Mo(5) and Mo(2)-O-Mo(4) bonds. The relaxation occurred not only in bulk but also on the surface, and formed the redox sites of (Mo5+-O-Mo5+)/(Mo6+-O-Mo6+) with electron donor-acceptor and fast oxygen transfer abilities. Those active sites on the surface highly promoted the surface redox reactions.We also found high content of V doping in monoclinic TeMo5O16 active phase induced a phase transition to orthorhombic TeMo5O16. After investigating the phase transition temperature of TeMo5O16, we succeeded in synthesizing pure orthorhombic TeMo5O16. Studies also showed that the channel structure of orthorhombic TeMo5O16 acted as an oxygen reservior and played an important role producing acrolein. We found out the nature of the oxygen species in the channel and proposed a mechanism of oxygen transfer and reservation. Upon those ideas derived from previous studies, we designed novel catalyst in two ways:i) increasing the oxygen reservation;ⅱ) symbiosis between M2 and orthorhombic TeMo5O16 phases. The catalytic performance of as-synthesized TeCrδMo5-δO16 and M2/TeMo5O16 catalysts reached a world class yield of acrolein (20.1% and 21.4%).