MoVW mixed metal oxides catalysts for acrylic acid production: from industrial catalysts to model studies

Abstract

MoVW) 5 O 14 -type oxides were identified as the active and selective components in industrial acrylic acid catalysts. Tungsten is suggested to play an important role as a structural promoter in the formation and stabilization of this oxide. Vanadium is responsible for high catalytic activities but is detrimental for the stability of this oxide at the necessary high concentrations for optimum catalytic performance. The activity of mixed MoVW oxide catalysts for methanol, propene, and acrolein partial oxidation could be considerably improved, when the amount of the (MoVW) 5 O 14 -type oxide was increased by thermal annealing. A model is proposed on the basis of the correlation between Raman wavenumber and bond order and degree of reduction, which explains the observed different selectivities of MoO 3)x and the (MoVW) 5 O 14 -type oxides in terms of metal-oxygen bond strengths, i.e. oxygen basicity and oxygen lability, respectively. According to this model, the (MoVW) 5 O 14 mixed oxide catalyses partial oxidation because of its intermediate C-H activation and oxygen releasing oxygen functionalities. However, these (MoVW) 5 O 14 -type industrial oxidation catalysts are heterogeneous and highly complex systems. Their physicochemical characterization also revealed that their chemical bulk and surface compositions vary with thermal activation and oxygen potential. A core-shell model is suggested to describe the active catalyst state, the shell providing a high number of active centers, the core high electronic conductivity and ion mobility. The fact that the surface composition of such catalysts is considerably different from their bulk compositions, most probably implies that the ''molecular structure'' at their surface differs too considerably from their bulk crystal structure. Hence, the posed question about the active catalyst structure and its relation to its catalytic performance cannot unambiguously be explained by the crystallographic structure, but still remains unsolved.

Active Sites for Propane Oxidation: Some Generic Considerations

“…The in situ analytical results described above and the concept of site isolation preclude, however, the view that a reactive dimer ''antenna site'' may be connected to the bulk of the M1 phase acting as collective electron storage unit. Such an idea [73] put forward for other mixed metal species contradicts the site isolation concept as it would not allow to control the timing of substrate oxidation and oxygen reduction and thus violate the concept of minimization of reactive species for maximising selectivity. The exchange of electrophilic oxygen for redox equivalents is a facile process [56,[74][75][76] when the site can easily change its oxidation state without having to break bonds to many neighbouring atoms.…”

Systematic study on the influence of the morphology of α-MoO3 in the selective oxidation of propylene

“…The method is, however, particularly attractive for (mixed) molybdenum oxides, which have high potential as catalysts in the selective oxidation of hydrocarbons [17][18][19][20][21][22]. Various preparation routes are known for the hydrothermal synthesis of molybdenum trioxide in the shape of ribbons, rods or nanobelts [12,[23][24][25][26][27][28].…”

Heterogeneously catalysed partial oxidation of acrolein to acrylic acid—structure, function and dynamics of the V–Mo–W mixed oxides

“…In many publications it is described as the phase responsible for the catalytic activity towards ACR oxidation to AA. 21,34,43,46,52,53 As shown in the recent sections the reoxidation of the reduced catalysts and the treatment under oxidising/reactive conditions always force the formation of Mo 5 O 14 which is stable in dependency on the particle size. 43 The decomposition of Mo 5 O 14 only sets in at temperatures 4430 1C.…”

“…Slight anomalies in the structures lead to small surface energies and allow the transport of O 2À ions through the phase boundary. 18 Often the so-called core-shell or cherrylike model are found in the literature: 21 on the surface there is a phase that provides a large number of active centres. The bulk phase is characterised by high electronic conductivity and ion mobility.…”