Selectivity determinants for dual function catalysts: applied to methanol selective oxidation on iron molybdate

Bowker, Michael; House, Matthew Peter; Alshehri, Abdulmohsen Ali; Brookes, Catherine; Gibson, Emma K.; Wells, Peter P.

doi:10.1179/2055075815y.0000000002

Cited by 19 publications

(35 citation statements)

References 20 publications

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“…CO production is observed at higher temperatures, reaching its peak at approximately 230 °C; similar behaviour was observed for previous Mobased systems, though formaldehyde production peaks at higher temperatures for MoO x /Fe 2 O 3 [18,19]. It is suggested that isolated cation sites are responsible for CO generation [18,20,24,25]; surface VO x is present in sufficient quantity to preclude multiple neighbouring Fe sites ( : this can be ascribed to the shell-core process, which is known to enhance surface area [18,19].…”

Section: Resultssupporting

confidence: 82%

VOx/Fe2O3 Shell–Core Catalysts for the Selective Oxidation of Methanol to Formaldehyde

et al. 2017

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Efficient oxidation catalysts are important in many current industrial processes, including the selective oxidation of methanol to formaldehyde. Vanadium-containing catalysts have been shown to be effective selective oxidation catalysts for certain reactions, and research continues to examine their applicability to other reactions of interest. Several VO x /Fe 2 O 3 shell–core catalysts with varying VO x coverage have been produced to investigate the stability of VO x monolayers and their selectivity for methanol oxidation. Catalyst formation proceeds via a clear progression of distinct surface species produced during catalyst calcination. At 300 °C the selective VO x overlayer has formed; by 500 °C a sandwich layer of FeVO 4 arises between the VO x shell and the Fe 2 O 3 core, inhibiting iron cation participation in the catalysis and enhancing catalyst selectivity. The resulting catalysts, comprising a shell–subshell–core system of VO x /FeVO 4 /Fe 2 O 3 , possess good catalytic activity and selectivity to formaldehyde. Electronic supplementary material The online version of this article (10.1007/s11244-017-0873-2) contains supplementary material, which is available to authorized users.

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Section: Resultssupporting

confidence: 82%

VOx/Fe2O3 Shell–Core Catalysts for the Selective Oxidation of Methanol to Formaldehyde

et al. 2017

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“…Although widely valued for its efficacy in the reaction, there is a surprising lack of knowledge regarding the catalyst surface layer in Fe 2 (MoO 4 ) 3 , and more importantly how it reacts with the incoming methanol reactant under high pressure conditions. This has been a topic of great interest, with many authors able to identify the phases which are formed through reduction and regeneration of the catalyst [3,[36][37][38], but unable to designate these intermediate phases to a defined reaction mechanism.…”

Section: Initial Investigations Into the Mechanism And Reaction Site mentioning

confidence: 99%

“…A recent paper from our research team reports a simple quantative model to describe the behavior of bi-cationic systems [36], comparing selectivity as a function of Mo loading on Fe2O3 (Figures 19 and 20). Surface doped materials are commended for their use in learning about the active configuration and its relationship to high selectivity.…”

Section: Initial Investigations Into the Mechanism And Reaction Site mentioning

confidence: 99%

Catalysts for the Selective Oxidation of Methanol

2016

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Abstract:In industry, one of the main catalysts typically employed for the selective oxidation of methanol to formaldehyde is a multi-component oxide containing both bulk Fe 2 (MoO 4 ) 3 and excess MoO 3 . It is thought that the excess MoO 3 primarily acts to replace any molybdenum lost through sublimation at elevated temperatures, therefore preventing the formation of an unselective Fe 2 O 3 phase. With both oxide phases present however, debate has arisen regarding the active component of the catalyst. Work here highlights how catalyst surfaces are significantly different from bulk structures, a difference crucial for catalyst performance. Specifically, Mo has been isolated at the surface as the active surface species. This leaves the role of the Fe in the catalyst enigmatic, with many theories postulated for its requirement. It has been suggested that the supporting Fe molybdate phase enables lattice oxygen transfer to the surface, to help prevent the selectivity loss which would occur in the resulting oxygen deficit environment. To assess this phenomenon in further detail, anaerobic reaction with methanol has been adopted to evaluate the performance of the catalyst under reducing conditions.

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“…5 below, and show clear segregation of Mo to the surface of the particle. This has also subsequently been reproduced by high resolution TEM/EDAX analysis [24,25].…”

Section: Rule 3 Preferential Cation Segregation Occurs and Is Crucialmentioning

confidence: 66%

“…What can be seen here is that the catalyst starts operating at very high conversion and selectivity at 250°C, but the conversion drops quickly to *20 % and then more slowly during the course of the experiment, reaching 10 %. The selectivity drops to *50 %, and the main non-selective product is CO, most likely due to the appearance of some Fe in the surface layers [24]. However, the most important point here is that many monolayers equivalent of oxygen can be removed from the lattice, and this can only occur by bulk to surface diffusion.…”

Section: Rule 2 Bulk Oxygen Can Become Active Surface Oxygenmentioning

confidence: 92%

Rules for Selective Oxidation Exemplified by Methanol Selective Oxidation on Iron Molybdate Catalysts

Bowker

2015

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A number of simple rules are proposed which dictate for high selectivity to formaldehyde during methanol oxidation on iron molybdate catalysts. The reaction is of the Mars-van Krevelen type, that is, it is surface lattice oxygen that is the active species. However, we also show that the material is quite a flexible one, in that the bulk oxygen anions become mobile enough at moderate temperatures to exchange rapidly with the surface vacancies that are created during the reaction. Further, and essential to high selectivity, is the fact that Mo is preferentially segregated to the surface layer of the catalyst, with no Fe present there. Finally, it can also be shown that the important oxidation state for the cations at the surface is the highest one (6 ? for Mo), with lower oxidation states being detrimental to selectivity. It is likely that these basic rules are also important for a range of other catalytic processes/catalysts.

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Selectivity determinants for dual function catalysts: applied to methanol selective oxidation on iron molybdate

Cited by 19 publications

References 20 publications

VOx/Fe2O3 Shell–Core Catalysts for the Selective Oxidation of Methanol to Formaldehyde

VOx/Fe2O3 Shell–Core Catalysts for the Selective Oxidation of Methanol to Formaldehyde

Catalysts for the Selective Oxidation of Methanol

Rules for Selective Oxidation Exemplified by Methanol Selective Oxidation on Iron Molybdate Catalysts

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