Abstract
This paper presents an overview of heterogeneous partial (amm)oxidation and oxidative dehydrogenation (ODH) of hydrocarbons. The review has been voluntarily restricted to metal oxide-type catalysts, as the partial oxidation field is very broad and the number of catalysts is quite high. The main factors of solid catalysts for such reactions, designated by Grasselli as the “seven pillars”, and playing a determining role in catalytic properties, are considered to be, namely: isolation of active sites (known to be composed of ensembles of atoms), Me–O bond strength, crystalline structure, redox features, phase cooperation, multi-functionality and the nature of the surface oxygen species. Other important features and physical and chemical properties of solid catalysts, more or less related to the seven pillars, are also emphasized, including reaction sensitivity to metal oxide structure, epitaxial contact between an active phase and a second phase or its support, synergy effect between several phases, acid-base aspects, electron transfer ability, catalyst preparation and activation and reaction atmospheres, etc. Some examples are presented to illustrate the importance of these key factors. They include light alkanes (C1–C4) oxidation, ethane oxidation to ethylene and acetic acid on MoVTe(Sb)Nb-O and Nb doped NiO, propene oxidation to acrolein on BiMoCoFe-O systems, propane (amm)oxidation to (acrylonitrile) acrylic acid on MoVTe(Sb)Nb-O mixed oxides, butane oxidation to maleic anhydride on VPO: (VO)2P2O7-based catalyst, and isobutyric acid ODH to methacrylic acid on Fe hydroxyl phosphates. It is shown that active sites are composed of ensembles of atoms whose size and chemical composition depend on the reactants to be transformed (their chemical and size features) and the reaction mechanism, often of Mars and van Krevelen type. An important aspect is the fact that surface composition and surface crystalline structure vary with reaction on stream until reaching steady state, which makes characterisation of active and selective surface sites quite difficult. The use of oxidants other than O2, such as H2O2, N2O or CO2, is also briefly discussed. Based on such analysis and recent discoveries and process developments, our perspective is given.
Highlights
Introduction and General Features of PartialOxidation ReactionsIn the chemical industry, hydrocarbons’ partial oxidation reactions correspond to important processes, playing a key role in numerous industrial, environmental and energy applications [1]
In the past 30 years, direct alkane selective oxidation has been the focus of many efforts in industry and academia [3], because of its higher abundance and lower price compared to the corresponding olefins
The most used solid catalysts for oxidation reactions are metals, metal oxides or metal complexes immobilized in zeolites, silica, alumina or polymeric resins [4] and, more recently, in metal organic frameworks (MOFs) for selective and chiral oxidation catalysis [5]
Summary
Hydrocarbons’ partial oxidation reactions correspond to important processes, playing a key role in numerous industrial, environmental and energy applications [1]. Another general feature of such reactions in heterogeneous catalysis, which has appeared over the years, is that the oxide surface could be considered as living, as in a breathing motion, to allow the Mars and van Krevelen (MvK) mechanism to take place (vide infra) This feature could be determined via electrical conductivity measurements as it was observed in many cases (vide infra Section 4.2)
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