Sustainable production of bio-propionic acid: Facet-mediated C-O bond cleavage of Fe3O4 nano-crystallites

Abstract

The disclosure of structure-dependent activity is always of great significance to rationalize the design of catalyst for achieving an excellent catalytic performance. Three clearly shaped and structured α-Fe2O3 crystallites (nano-truncated hexagonal bipyramids (THB), nano-cubes (QC) and nano-spheres (RC)) were used as precursors to successfully synthesize their counterparts of three Fe3O4 nano-crystallites, marked as THB-R, QC-R and RC-R, respectively, which remained consistent in shape. HRTEM images reveal that the three as-synthesized Fe3O4 nano-crystallites expose different facets, in which (311) plane is exclusively exposed in RC-R, and the mixed planes of (111) and (222) are exposed in THB-R and QC-R. DFT calculations reveal that the Fe site located at the exposed (311) plane is more preferential to selectively adsorb α‑hydroxyl group (α-C-OH) in lactic acid (LA) molecule than the other two exposed planes of (111) and (222) on basis of adsorption energy of LA molecule, leading to a high activity on CO cleavage. The thermoanalysis of sample further indicates that the Fe site in RC-R is covered with less organic polymer or coke, than QC-R and THB-R, endowing an excellent activity and stability during the catalytic reaction. At 380 °C and LA LHSV of 16 h−1 under the inert atmosphere, the catalyst offered a satisfactory propionic acid selectivity of nearly 70%, and its catalytic performance hardly decayed within 34 h on stream.