(253b) Methyl Formate Hydrogenation to Higher Hydrocarbons over Mn- and Ti-Oxide Promoted Cobalt Catalysts

Authors: Andrew Raub, Motahare Athariboroujeny, Norbert Kruse

Methyl formate, the simplest of the carboxylic esters, has been considered since the early 1990s as a potential platform molecule for C1 chemistry¹. Recently, we have established that it is possible to produce an array of chain-lengthened paraffins from methyl formate via a Fischer-Tropsch akin process utilizing cobalt-based catalysts at atmospheric pressure conditions. The fact that this molecule contains CO and H₂ in a 1:1 ratio makes it a potentially suitable feedstock for the preparation of hydrocarbon chains. In addition, methyl formate’s low toxicity and relatively high boiling point (32 ^oC) make the molecule easy to transport and store. With further exploration, methyl formate hydrogenation may also provide a novel route for the synthesis of olefins and oxygen-functionalized hydrocarbons. In this work, the basic catalytic science of methyl formate hydrogenation over cobalt catalysts promoted by Mn- and Ti-oxides will be investigated. Manganese and titanium were selected due to their documented promoter effects observed for cobalt-based catalysts studied under conventional Fischer-Tropsch synthesis conditions^{2, 3} . This paper will provide kinetic and mechanistic insight into the atmospheric pressure hydrogenation of methyl formate using transient response methods.

Chemical Transient Kinetics (CTK), as applied in the present studies, relies upon the abrupt switching between a non-reactive (H₂ + Ar) and a reactive gas mixture (H₂ + He + methyl formate) while maintaining constant molecular flow conditions at the inlet to a catalyst bed. The initial construction of the catalytically active surface, termed the “build-up”, occurs when reactants are admitted to the gas phase over a freshly reduced catalyst surface. Likewise, the removal of reactants from the gas phase results in a “scavenging” of the surface as residual adsorbed species are hydrogenated and desorb. Observation of the build-up and scavenging behaviors for a catalytic system provides mechanistic insight unattainable from a steady-state kinetic analysis. Integration of calibrated surface flows allows for time-dependent carbon and oxygen coverages on the catalyst surface to be determined at different stages of the reaction.

During the build-up of the methyl formate hydrogenation reaction, using a Co-MnO_x catalyst under atmospheric pressure conditions at 220 ^oC, alkanes are observed to form in the sequence of their carbon numbers. With the exception of methane formation, which is the first product observed in the present study, chain-lengthened alkanes appear once methyl formate decomposes to produce CO in the gas phase. The time-correlation of chain lengthening initiation and CO appearance in the gas phase places a strong argument in favor of a CO insertion mechanism being in operation. Differing from earlier studies on methyl formate hydrogenation in our lab, in which only alkanes were observed during methyl formate hydrogenation over Co/MgO catalysts⁴, we also observe olefins. Remarkably, the delay time for olefin formation is longer than that for alkanes. Methanol production is also observed, time-delayed relative to the appearance of gaseous CO. The formation of methanol from methyl formate under mild conditions using heterogeneous promoted cobalt catalysts has not been previously reported.

As for the reaction mechanism, we hypothesize that formyl groups (resulting from the dissociation of methyl formate) adsorbed on the catalyst surface undergo decarbonylation to yield CO, which subsequently inserts into the bonds of surface alkoxy or hydroxyl groups. The resulting carboxylate-type species may then be hydrogenated to produce alkanes/alkenes or turned into a (C+1)-alkoxy homologue subject to another CO insertion, and therefore, chain lengthening. Such a scenario would also be able to account for the formation of oxygenated products.

References

1. Lee, J. S.; Kim, J. C.; Kim, Y. G., Methyl Formate as a New Building Block in C₁ Chemistry. Applied Catalysis 1990, 57 (1), 1-30.
2. Dinse, A.; Aigner, M.; Ulbrich, M.; Johnson, G. R.; Bell, A. T., Effects of Mn promotion on the activity and selectivity of Co/SiO2 for Fischer-Tropsch Synthesis. Journal of Catalysis 2012, 288, 104-114.
3. Morales, F.; de Smit, E.; de Groot, F. M. F.; Visser, T.; Weckhuysen, B. M., Effects of manganese oxide promoter on the CO and H-2 adsorption properties of titania-supported cobalt Fischer-Tropsch catalysts. Journal of Catalysis 2007, 246 (1), 91-99.
4. Iablokov, V.; Kruse, N., Discovery of a Fischer-Tropsch Hybrid Reaction: Hydrogenation of Methylformate to Long-Chain Hydrocarbons with Anderson-Schulz-Flory Chain Length Distribution. Chemcatchem 2019, 11 (4), 1200-1204.