Theoretical investigation of the carbonyl-ene reaction between encapsulated formaldehyde and propylene over M-Cu-BTC paddlewheels (M= Be, Mg, and Ca): A DFT study

Formaldehyde is a VOC gas that plays a key role in air pollution. To limit emissions into the environment, the utilization of this waste as a raw material is a promising way. In this work, the M06-L functional calculation was used to investigate the structure, electronic properties, and catalytic activity of group IIA metals (Be, Mg, and Ca) partial substitution on Cu-BTC paddlewheels for formaldehyde encapsulation and carbonyl-ene reaction with propylene. Formaldehyde is absorbed by the metal center of the paddlewheel via its oxygen atom. The adsorption of formaldehyde on the substituted metal sites increased as compared to the parent Cu-BTC which can facilitate formaldehyde to react with propylene. The adsorption free energies are predicted to be −15.1 (Be–Cu-BTC), −14.7 (Mg–Cu-BTC), and −14.5 (Ca–Cu-BTC) kcal mol⁻¹, respectively. The substituted metal has a slight effect on the Lewis acidity of the Cu ion in the paddlewheel. The adsorption free energy of formaldehyde, similar to that found in the pristine Cu-BTC, is observed. For the carbonyl-ene reaction, the reaction is proposed via a single step involving the C–C bond formation between two reactants and one hydrogen of propylene methyl group moves to formaldehyde oxygen, simultaneously. It was found that the substituted metals do not affect the catalytic performance of the Cu center for this reaction. The activation energies for the reaction at the Cu center are in the range of 22.0–23.4 kcal mol⁻¹, which are slightly different from Cu-BTC (21.5 kcal mol⁻¹). Interestingly, the catalytic activity of this reaction on the substituted metal is greater than that on the Cu center. The catalytic activities are in the order Be–Cu-BTC (13.3 kcal mol⁻¹) > Mg–Cu-BTC (15.9 kcal mol⁻¹) > Ca–Cu-BTC (17.8 kcal mol⁻¹). Among them, the Be site of the bimetallic Be–Cu-BTC paddlewheel is predicted as a promising candidate catalyst.