Difference of Oxidation Mechanism between Light C3–C4 Alkane and Alkene over Mullite YMn₂O₅ Oxides’ Catalyst

Abstract

Revealing the catalytic oxidation mechanism of volatile organic compounds (VOCs) is insightful for the development of efficient catalysts. However, because of the complicated interactions and a large number of intermediate species during the reactions, the analysis of the entire reaction mechanism (including the activation modes of reactant molecules and the rate-limiting step) remains a great challenge. Herein, the YMn2O5 mullite catalyst was proposed to demonstrate how to distinguish the deep oxidation difference among C3–C4 alkanes and olefins via combining experiments and theoretical calculations. The YMn2O5 catalyst prepared via the hydrothermal method displayed a superior catalytic behavior with a low T90 temperature (C3H8 at 250 °C; C3H6, C4H10, and C4H8 less than 200 °C) (1000 ppm of C3–C4 and 10% O2 balanced with He, WHSV = 30 000 mL/g·h). The catalytic activity remained the same after continuous reaction for 100 h at 275 °C for each reactant. Overall, the YMn2O5 mullite catalyst exhibits excellent durability with no activity declines for 400 h. Combined with TPD, DRIFTS, XPS, and DFT analysis, surface oxygen species were found to be active for the oxidation. Owing to the difference of the HOMO induced partial charge distributions between alkanes and alkenes, the dehydrogenation of the end-site C–H of propane is the first step prior to the crucial conversion of acrylate over surface labile oxygen in an octahedral ligand unit. For propene oxidation, the CC double bond is preferentially attacked by two surface oxygen atoms belonging to the octahedral and pyramid ligand units with the crucial step of acetate decomposition. These findings provide insights into the oxide catalyst design toward the complicated VOCs oxidation from a fundamental point of view.

Amorphous Boron Dispersed in LaCoO3 with Large Oxygen Vacancies for Efficient Catalytic Propane Oxidation

“…The strong peak at 2968 cm −1 , which belongs to C−H vibration, is usually associated with gaseous C 3 H 8 [50] . During 0–29 min of the absorption process, CH 2 or CH 3 species (2931 and 2873 cm −1 ) appear rapidly, indicating the terminal methyl species of C 3 H 8 is attacked by surface active oxygen species [51] . Meanwhile, the C 3 H 8 molecule absorbed on the sample is easy to dehydrogenate and decompose to form methyl (1376 cm −1 ), allyl (1455 cm −1 ), carboxyl (1471 cm −1 ) groups and so on [52–54] .…”

Earth‐Abundant Transition Metal‐Based Mullite‐Type Oxide Catalysts for Heterogeneous Oxidation Reactions

“…Such diversity of mullite‐type compounds offers a unique platform for tuning their chemical composition and electronic structure to generate interesting physical and chemical properties, e.g., magnetism and ferroelectricity in multiferroic AMn 2 O 5 and AFeMnO 5 materials, [ 54–57 ] photocatalysis in Bi 2 Fe 4 O 9 , [ 58 ] and recently oxidation catalysis and electrocatalysis in AMn 2 O 5 . [ 38–41,44–46 ]…”

“…HT synthesis causes crystal growth at lower temperatures than the CP method, which results in high‐surface‐area materials desirable for heterogeneous catalysis and has been successfully used to prepare mullite oxides. [ 43,44,61–64 ] To synthesize AMn 2 O 5 , the metal salt precursors of A, Mn, and KMnO 4 are mixed in a molar ratio of 5:7:3 in DI water and then a precipitating agent (NaOH or KOH) is added dropwise. The HT treatment is conducted at 180–200 °C for 24–48 h in an autoclave under autogenous pressure.…”

“…The HT treatment is conducted at 180–200 °C for 24–48 h in an autoclave under autogenous pressure. [ 43,44,61–64 ] The mullite oxide catalysts prepared by this method exhibit high specific surface area with narrow particle size distribution and well‐controlled morphologies. For example, Ma et al synthesized SmMn 2 O 5 with different morphologies such as nanorods and nanoparticles by varying the pH and HT reaction time.…”

“…Thus, the pursuit of cost‐effective, highly active, and thermally stable catalysts has led to the formulation of mullite‐type oxides for oxidation catalysis and electrocatalysis. [ 38–46 ]…”