Abstract
Gas-phase ethylene hydroformylation to propanal was investigated in a fixed-bed reactor on a metal–organic framework catalyst functionalized with ligand-coordinated Rh centers. The hydrogenation product ethane was observed in small amounts (selectivity up to 5 %) as the only side product, most particularly at the highest investigated temperature of 120 °C. At the maximum performance, the ethylene conversion and the propanal selectivity amounted to 90 % and 99 %, respectively, in the absence of a solvent. A low-conversion experimental data served as a basis for microkinetic model construction employing the same hydroformylation mechanism as on a homogeneous catalyst, i.e., Wilkinson’s dissociative mechanism, with an additional hydrogenation catalytic cycle included. The proposal of this mechanism was supported by DRIFTS measurements in the presence of the hydroformylation reactants. The model and its parameters were statistically significant and exhibited a good correspondence to the experimentally observed propanal and ethane outlet flow rates. Activation energies of 65 kJ mol−1 and 88 kJ mol−1 were estimated for ethylene insertion and ethane reductive elimination, respectively. Ethylene insertion, propanal reductive elimination and hydrogen oxidative addition were found to be the most kinetically relevant steps for hydroformylation, while ethane reductive elimination was essentially rate determining for hydrogenation. The same elementary steps were identified as kinetically relevant for (liquid-phase) homogeneously catalyzed hydroformylation in our previous work, which supports the hypothesis that the hydroformylation reaction mechanism is the same in both cases.