Abstract
Multicomponent Bi-Mo-Fe-Co oxide catalysts prepared via flame spray pyrolysis were tested for selective propylene oxidation, showing high conversion (>70%) and selectivity (>85%) for acrolein and acrylic acid at temperatures of 330 °C. During extended time-on-stream tests (5–7 days), the catalysts retained high activity while undergoing diverse structural changes. This was evident on: (a) the atomic scale, using powder X-ray diffraction, Raman spectroscopy, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy; and (b) the microscopic scale, using synchrotron X-ray nanotomography, including full-field holotomography, scanning X-ray fluorescence, and absorption contrast imaging. On the atomic scale, sintering, coke formation, growth, and transformation of active and spectator components were observed. On the microscopic scale, the catalyst life cycle was studied at various stages through noninvasive imaging of a ~50-µm grain with 100-nm resolution. Variation of catalyst synthesis parameters led to the formation of notably different structural compositions after reaction. Mobile bismuth species formed agglomerates of several hundred nanometres and segregated within the catalyst interior. This appeared to facilitate the formation of different active phases and induce selectivity for acrolein and acrylic acid. The combined multiscale approach here is generally applicable for deconvolution of complex catalyst systems. This is an important step to bridge model two-component catalysts with more relevant but complex multicomponent catalysts.
Highlights
The selective oxidation of propylene to acrolein and acrylic acid are important reactions in the chemical industry
The fresh catalysts are denoted as flame spray pyrolysis (FSP)-1F and FSP-2F and those used in the catalytic tests as FSP-1R
Flame spray pyrolysis offers an attractive route for the preparation of multicomponent catalyst systems such as Bi-Mo-Fe-Co oxides
Summary
The selective oxidation of propylene to acrolein and acrylic acid are important reactions in the chemical industry. During FSP, liquid metal precursor solutions are sprayed into a flame and exposed to temperatures up to 3000 K for a short residence time (ms-regime), resulting in the formation of small nanocrystalline metal oxide particles [15,17] With this one-step method, Schuh et al could directly access pure bismuth molybdates including the metastable β-Bi2 Mo2 O9 high-temperature phase without further calcination [11], resulting in catalysts with comparatively large surface areas (up to 45 m2 g–1 ), excellent propylene conversion, and acrolein selectivity. As the working catalyst appears to depend on the good mixing of phases, FSP may offer a highly potent method for the synthesis of multicomponent bismuth-molybdate-based systems Such catalysts exhibit a great deal of structural complexity, with a multiphasic mixture of closely interacting species. Standardised catalyst testing must be coupled with detailed characterisation of the catalyst structure through spectroscopic and microscopic analysis
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