(702c) Microkinetic Elucidation of the Ethylene Hydroformylation Mechanism on a Heterogenized Rh-Based MOF Catalyst

Hydroformylation converts alkenes, in the presence of syngas (CO + H₂), into aldehydes with an additional carbon atom and remains the largest homogeneously catalyzed process industrially. As homogeneous catalysis is associated with several disadvantages, such as difficult catalyst separation and recycling, the development of heterogeneous hydroformylation catalysts, e.g., via immobilization of the homogeneous variant on a solid support, is currently a hot topic. In this respect, ethylene hydroformylation has been experimentally investigated on a Metal Organic Framework (MOF) catalyst functionalized with ligand-modified Rh coordination centers and via microkinetic modelling. The reaction proceeds in the gas phase, allowing for continuous reactor operation with no solvent. At the maximum performance, ethylene conversions of 90% and propanal selectivities of 99% were obtained simultaneously, with ethane as the side-product, and the catalyst has shown stability after 10 days of time-on-stream. The microkinetic model accounted for Wilkinson’s dissociative mechanism, see Figure 1.a, which is widely-accepted for homogeneously catalyzed hydroformylation, to which a hydrogenation pathway was added. The model was regressed to the experimental data, which led to significant model parameter estimates, i.e., the activation energies and reaction enthalpies of the considered elementary steps. The ethylene conversions and propanal selectivities were well-reproduced by the model, see Figure 1.b and Figure 1.c. Activation energies around 80 kJ mol^-1 were estimated for ethylene insertion and ethane reductive elimination, respectively. A sensitivity analysis indicated that the most kinetically relevant steps of the hydroformylation pathway were ethylene insertion, hydrogen oxidative addition and propanal reductive elimination, while ethane reductive elimination was essentially rate-determining in the hydrogenation pathway. As the experimentally obtained product spectrum is very similar and the same kinetically relevant elementary steps were identified as in the homogeneously catalyzed hydroformylation in our previous work, it can be concluded that there are major similarities between the reaction mechanisms on both catalysts.