Synthesis of 2-imidazolidinone (EU) from ethylenediamine carbamate (EDA-CA), which was produced via chemical absorption of CO2 in ethylenediamine (EDA), with a fixed-bed flow reactor was performed using CeO2 and EDA as a heterogeneous catalyst and reaction solvent, respectively. In the previously reported batch system for the same reaction, the yield of EU was low (62%) due to its sequential reaction with EDA into N,N′-bis(2-aminoethyl)urea (LU) with 38% yield in the liquid phase under the optimized reaction conditions at 413 K. In stark contrast, the flow reaction enabled the highly selective and high-yielding production of EU (94% yield) under the optimized reaction conditions at 363 K. This difference in the selectivity of EU originated from the lower ratio of the amount of EDA to that of the CeO2 catalyst in the current flow reaction system compared to the previous batch system. The highly crystallized CeO2 surface bearing both acid and base sites was found to be responsible for its high catalytic performance. The combination of the kinetics, density functional theory calculations, and adsorption experiments demonstrated that EDA-CA is preferentially adsorbed onto the surface of CeO2 even in the presence of an excess amount of EDA, which is used as a solvent, to be activated and transformed into EU. The solid-state 13C magic angle spinning/cross polarization nuclear magnetic resonance spectroscopy revealed that the CeO2 catalyst was deactivated gradually in a long-term reaction operation due to the formation and deposition of polyurea-like organic compounds on the catalyst surface.

The detailed structures of monomeric ReO x catalysts supported on the CeO 2 surface and the reaction mechanism of the deoxydehydration (DODH) reaction were investigated by density functional theory calculations. After examining various ReO x H y structures over CeO 2 without substrate adsorption, the stable structure under the experimental condition at 400 K was determined to be the Re VII O 2 species. The reaction mechanism of DODH was then investigated for the conversion of 1,4-anhydroerythritol to 2,5-dihydrofuran as a model reaction. Through the investigations of several reaction pathways, an oxygen vacancy-assisted mechanism, in which the starting structure is the Re IV O species and the oxidation state of the Re atom changes between +IV and +VI during the reaction, was postulated to be the most plausible pathway, considering the energies of the intermediates and the barrier height for the cleavage of the two C−O bonds.

The development of efficient heterogeneous catalysts for converting biomass into value-added chemical compounds remains at the forefront of catalysis research. Deoxydehydration (DODH) reaction that can transform vicinal hydroxy groups with the cis-configuration to the corresponding CC bond in a single step is one of the promising techniques, and molybdenum oxide catalysts supported on TiO2 have been reported as an effective catalyst using hydrogen as a reducing agent. Here, using density functional theory calculations, structures of monomeric and dimeric molybdenum oxide catalysts supported on anatase TiO2(101) have been determined, and we decipher the reaction mechanisms of the conversion of 1,4-anhydroerythritol to 2,5-dihydrofuran over these catalysts as a model reaction. We have found that MoO3 and Mo2O5 are the most stable structures for monomeric and dimeric species that exhibit the oxidation states of MoVI and MoV–MoVI, respectively, under the experimental conditions. For monomeric species, it is rather difficult to catalyze DODH reaction due to the instability for MoIV species and also the higher barrier for the C–O bond scission for MoV or MoVI species. For dimeric species, structures with the oxidation state of MoIV–MoV or MoV–MoV that is found in the form of Mo2O4 exhibit promising energy profiles in terms of stability and energy barrier (∼1.0 eV) for the C–O bond dissociation. Considering the experimental facts that MoIV species is responsible for the DODH reaction and Mo–Mo bond is present, the MoIV–MoV structure could be the plausible active species. Our findings would provide useful information for the catalyst design using earth-abundant and less-expensive metal-based catalysts for the DODH reaction.