Microkinetic Elucidation of the Mechanism of Ethylene Hydroformylation on a Heterogenized Rh-Based MOF Catalyst

The direct partial oxidation of methane to primary oxygenates such as methanol suffers from impractically low product yields due to over-oxidation of the desired product to CO₂ under the reaction conditions necessary for methane activation [1-2]. The derivatization of methanol to oxidation-resistant methyl esters in concentrated acid solvents is a successfully demonstrated product protection strategy that can maintain high product selectivity independent of conversion extent [3]. Still, there are a number of critical shortcomings left unaddressed, including: the reliance on molecular Pt-group catalysts, economically infeasible oxidants (e.g. K₂S₂O₈), and the use of corrosive, fully concentrated acid solvents [4].

In the current contribution, new developments to some of the key shortcomings of these processes are presented, namely active heterogeneous catalyst that perform the conversion in a diluted acid medium with pressurized air as the oxidant source. Under our improved reaction conditions, facilely synthesized Co/SiO₂ and Mn/TiO₂ display competitive performance on the basis ester production rates (up to 245 and 613 µmol g_cat^-1 h^-1, respectively) with negligible metal leaching in comparison to previously reported homogeneously-catalyzed systems that employ stronger oxidants and higher methane partial pressures. [5-6]. Through a combination of characterization techniques, correlations between catalytic activity and the distribution of metal species in the catalyst can be derived. The conclusion of the contribution underscores impactful directions for further developing high-yield methane conversion processes through more holistic consideration of catalysts, reagents, and operating conditions.

[1] M. Ahlquist et al., J. Am. Chem. Soc., 2009, 131, 47, 17110-17115.

[2] A. Latimer et al., ACS Catal., 2018, 8, 8, 6894-6907.

[3] R. Periana et al., Science, 1998, 280, 5363, 560-564.

[4] A. Blankenship et al., Chimia, 2021, 75, 305-310.

[5] A. Blankenship et al., Angew Chem Int Edit, 2021, 60, 18138-18143.

[6] Y. Ji et al., ACS Catal., 2023, 13, 3896-3901.