Organic compound modification of CeO₂ and 2-cyanopyridine hybrid catalyst in carbonate synthesis from CO₂ and alcohols

The effect of surface modification of CeO₂ and 2-cyanopyridine hybrid catalyst with organic compounds was investigated by using dialkyl carbonate synthesis from CO₂ and alcohols. Phenol is the useful modifier for CeO₂ and 2-cyanopyridine hybrid catalyst without drastic decrease of the activity. Based on the characterizations of phenol-modified CeO₂ and 2-cyanopyridine hybrid catalyst such as TG-DTA, GC, FTIR and UV–vis, phenol and 2-cyanopyridine were adsorbed on CeO₂ at the same time and the adsorption ratio to the surface Ce on CeO₂ is estimated to be 0.11 and 0.12, respectively (2-cyanopyridine/Ce on CeO₂ surface = 0.12, phenol/Ce on CeO₂ surface = 0.11). The reactivity of alcohols was changed by the addition of phenol due to the steric hindrance of phenol adsorbed on CeO₂. The effect of the steric hindrance was large at the C2 and C3 positions of alcohols, which corresponds to the C2 and C3 positions of phenol. The reactivity was drastically decreased in the cases of C2 position-branched alcohols compared with the case without phenol addition, and the yield ratio (The yield without phenol/the yield with phenol) increased: 61 in the case of 2-ethyl-1-butanol, 9.9 in the case of 2-methyl-1-butanol, and 3.7 in the case of 1-butanol. From the DFT calculations, phenol is adsorbed nearby 2-cyanopyridine adspecies by the π-π interaction between the aromatic rings of these chemicals to decrease the reaction space over CeO₂, resulting in the reactivity decrease of the steric alcohols, particularly the alcohols with substituents at the C2 position. Therefore, phenol can restrict the reaction field of CeO₂ and 2-cyanopyridine hybrid catalyst by the steric hindrance, enabling the control of the reactivity of alcohols in the synthesis of dialkyl carbonates from CO₂ and alcohols.