Structural Evolution of Highly Active Multicomponent Catalysts for Selective Propylene Oxidation

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.

3‐D X‐ray Nanotomography Reveals Different Carbon Deposition Mechanisms in a Single Catalyst Particle

“…However, the volume that can be investigated by APT, EM, and soft X-ray STXM tomography is not sufficient to study a complete sub-millimeter catalyst body and CFM still offers limited spatial resolution and certainly limited information depth. Hard X-ray imaging techniques provide a similar spatial resolution as soft X-ray STXM [29][30][31] but the highest information depth of all above-mentioned techniques thus enabling 3-D studies [32][33][34][35][36] of, for example, metal distribution in the macropores [37] (pore sizes > 50 nm) of an entire catalyst body [38][39][40] revealing the effect of those deposits on the catalyst's macropore space and accessibility. However, while this approach is very sensitive to metal deposits hard X-rays provide much weaker contrast for organic phases than soft X-rays because the energy dependent X-ray absorption is correlated with material density and atomic number.…”

Chemical Imaging of Mixed Metal Oxide Catalysts for Propylene Oxidation: From Model Binary Systems to Complex Multicomponent Systems

“…The spent catalyst HS: Bi−Mo−Co−Fe oxide was investigated at the hard X‐ray nanoimaging beamline ID16B of the European Synchrotron Radiation Facility (ESRF, Grenoble, France). Experimental details regarding: (i) full‐field X‐ray holotomography; (ii) scanning X‐ray fluorescence nanotomography (XRF‐CT); and (iii) scanning transmission X‐ray tomography (STXM‐CT) can be found in previous work [46] …”

“…Experimental details regarding: (i) full-field X-ray holotomography; (ii) scanning X-ray fluorescence nanotomography (XRF-CT); and (iii) scanning transmission X-ray tomography (STXM-CT) can be found in previous work. [46] The exact same catalyst particle was further investigated at the hard X-ray microprobe endstation of beamline P06 at PETRA III (DESY, Hamburg, Germany). X-rays with an incident energy of 21 keV were focused to 650 × 500 nm (h × v) beam size using KB mirrors.…”

“…MMO catalysts often show remarkable enhancements in catalytic performance through cooperation of their different structural components, and can be visualised as heterogeneous ensembles of crystalline and amorphous oxide phases intermixed on at least the micrometre scale. [6][7] Due to this high complexity, simplified particle models and characterisation of model binary systems have historically been used to illustrate and predict the interaction of individual MMO phases. [8][9][10] The present understanding of BiÀ MoÀ CoÀ Fe oxide interactions is: 1) bismuth molybdates (α-Bi 2 Mo 3 O 12 , β-Bi 2 Mo 2 O 9 and γ-Bi 2 MoO 6 ) enable selective propylene oxidation, 2) iron molybdates (FeMoO 4 /Fe 2 Mo 3 O 12 ) improve the availability of lattice oxygen at the reactive centres, and 3) cobalt molybdates (α-CoMoO 4 and β-CoMoO 4 ) act as a host structure or support phase.…”

“…Acrylic acid alone is a major commodity chemical with global production on the scale of several megatons per year. MMO catalysts often show remarkable enhancements in catalytic performance through cooperation of their different structural components, and can be visualised as heterogeneous ensembles of crystalline and amorphous oxide phases intermixed on at least the micrometre scale [6–7] . Due to this high complexity, simplified particle models and characterisation of model binary systems have historically been used to illustrate and predict the interaction of individual MMO phases [8–10] .…”