Oxidative dehydrogenation of ethane and propane : How far from commercial implementation?

Aerosol processing is long known and implemented industrially to obtain various types of divided materials and nanomaterials. The atomisation of a liquid solution or suspension produces a mist of aerosol droplets which can then be transformed via a diversity of processes including spray-drying, spray pyrolysis, flame spray pyrolysis, thermal decomposition, micronisation, gas atomisation, etc. The attractive technical features of these aerosol processes make them highly interesting for the continuous, large scale, and tailored production of heterogeneous catalysts. Indeed, during aerosol processing, each liquid droplet undergoes well-controlled physical and chemical transformations, allowing for example to dry and aggregate pre-existing solid particles or to synthesise new micro- or nanoparticles from mixtures of molecular or colloidal precursors. In the last two decades, more advanced reactive aerosol processes have emerged as innovative means to synthesise tailored-made nanomaterials with tunable surface properties, textures, compositions, etc. In particular, the “aerosol-assisted sol–gel” process (AASG) has demonstrated tremendous potential for the preparation of high-performance heterogeneous catalysts. The method is mainly based on the low-cost, scalable, and environmentally benign sol–gel chemistry process, often coupled with the evaporation-induced self-assembly (EISA) concept. It allows producing micronic or submicronic, inorganic or hybrid organic–inorganic particles bearing tuneable and calibrated porous structures at different scales. In addition, pre-formed nanoparticles can be easily incorporated or formed in a “one-pot” bottom-up approach within the porous inorganic or hybrid spheres produced by such spray drying method. Thus, multifunctional catalysts with tailored catalytic activities can be prepared in a relatively simple way. This account is an overview of aerosol processed heterogeneous catalysts which demonstrated interesting performance in various relevant chemical reactions like isomerisation, hydrogenation, olefin metathesis, pollutant total oxidation, selective oxidation, CO2 methanation, etc. A short survey of patents and industrial applications is also presented. Our objective is to demonstrate the tremendous possibilities offered by the coupling between bottom up synthesis routes and these aerosol processing technologies which will most probably represent a major route of innovation in the mushrooming field of catalyst preparation research.

Aerosol processing: a wind of innovation in the field of advanced heterogeneous catalysts

Effects of framework and near surface composition of quinternary, phase-pure M1 MoVTeNb oxide catalysts on their catalytic performance in selective oxidation of propane to acrylic acid have been studied. The catalysts were prepared by hydrothermal synthesis, spray-drying, and superheated water vapor treatment. Electron microscopy, chemical analysis, nitrogen physisorption, and in situ photoelectron spectroscopy have been used to characterize the materials. The yield of acrylic acid normalized to the specific surface area of the catalyst increases with decreasing percentage of Mo and increasing molar ratio of Te/V at the surface. The metal stoichiometry at the surface differs from the stoichiometry in the crystalline bulk and changes in response to the composition of the gas phase. In situ valence band spectroscopy at 623 K in the presence of all reactants revealed a substantial covalent character of the metal−oxygen bonds in M1. The surface restructuring under formation of V- and Te-containing clusters an...

Dynamics of the MoVTeNb Oxide M1 Phase in Propane Oxidation

The reaction network in propane oxidation over phase-pure MoVTeNb M1 oxide catalysts

Based on experimental observations about structure and dynamics of the reactive surface of the M1 phase some considerations are made about the nature and size of active sites. In analyzing the stoichiometry of the reactions following activation of propane it occurs that for dehydrogenation small sites are desirable being just sufficient to re-activate oxygen without kinetic hindrance. For deeper oxidation to acrylic acid (AA) the sites should be larger to accommodate all redox equivalents and oxygen species required for one transformation. A qualitative model for size and composition of the active site is made. It is likely that the active site consists on a VxOy species of strictly 2-D nature. This follows the structural suggestion for the active site on the a–b-plane of the M1 structure. The role of Te in moderating the active site is discussed. The suggestions are discussed in comparing requirements for oxidative dehydrogenation of propane (ODP) with those for AA synthesis.

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Active Sites for Propane Oxidation: Some Generic Considerations

This work presents a detailed investigation of the preparation routine for the multi-metal oxide Mo 1 V 0.30 Te 0.23 Nb 0.125 O x used as catalyst for the selective oxidation of propane to acrylic acid. In situ Raman spectroscopy on the initial aqueous polyoxometalate solution prepared from ammonium heptamolybdate, ammonium metavanadate and hexaoxotelluric acid reveals the coexistence of Anderson-type anions [TeM 6 O 24 ] n − , M = Mo, V; n  ≥ 6 and protonated decavanadate species [H x V 10 O 28 ] (6− x )− . Raman analysis showed that the monomeric motif of the Anderson-type tellurate is preserved after addition of the Nb precursor and the subsequent spray-drying process. Calcination of the X-ray amorphous spray-dried material in air at 548 K seems to be the essential step, leading to a re-arrangement of the tellurate building blocks, generating nanocrystalline precursors of the phases finally established during treatment in helium at 873 K.

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P. Beato

Papers

Analysis of structural transformations during the synthesis of a MoVTeNb mixed oxide catalyst

Ein neuartiger mikrostrukturierter Quarzreaktor für kinetische und in situ spektroskopische Untersuchungen