DFT Studies of the Effects of Trivalent Cations (B, Al, Ga) on Au(I)-Exchanged Zeolites for Catalytic CO 2 Cycloaddition with Light Epoxides: The Interplay between Catalyst Acidity and Epoxide Reactivity

In this work, the reaction between CO2 and light epoxides (EO, PO, and IBO) over isomorphously substituted Au-[M]-ZSM-5 and Au-[M]-MCM-22 (M = B, Al, and Ga) was investigated using DFT at the M06-L/6-31G(d,p) level. Isomorphous substitutions strongly affect catalyst acidity of the Au(I) cation. The acidity increases in the order [B]- < [Al]- < [Ga]-zeolites. For all substitutions, MCM-22 consistently exhibits higher acidity than ZSM-5, resulting in stronger adsorption energies of epoxides on the Au(I) cation. The reaction proceeds via a single-step mechanism in which Au promotes epoxide ring-opening and C–O bond formation with CO2. Methyl electron-donating groups increased the nucleophilicity of the epoxide oxygen, resulting in stronger interaction between the epoxide and the Lewis active site. Consequently, the activation free energy decreases in the order EO > PO > IBO. For the IBO system, the substituent effect outweighs variations in the Au-site acidity across different zeolites. Although the Au(I) cations in the two zeolites exhibit different acidities, the activation free energies for IBO/CO2 conversion are nearly identical. The SISSO algorithm was applied using the molecular orbital (MO) energies of the epoxides together with their adsorption free energies to derive a mathematical model for predicting the activation free energy of CO2 cycloaddition with light epoxides over the selected zeolites. A SISSO-based model reveals the utilization of epoxide MO energies and adsorption energies for accurately predicting activation free energies for this reaction. The SISSO-based equation shows excellent agreement with DFT results (R2 = 1.00, RMSE = 0.8 kcal mol–1).