One-Step Encapsulation of TBAB in ZIF-8 for CO₂ Fixation: Revealing the Synergistic Mechanism between TBAB and ZIF-8

In this study, tetrabutylammonium bromide (TBAB) with a cationic radius of 4.94 Å was encapsulated in ZIF-8 with a pore size of 3.4 Å by a one-step method. The TBAB@ZIF-8 catalyst showed superior catalytic activity on the reaction of CO₂ with propylene oxide (PO) than TBAB and the zeolitic imidazolate framework, respectively, showing a synergistic effect. The synergist effect was related with the compensation effect between the adsorption of CO₂ and the loop-opening of PO, where ZIF-8 provided the active sites for CO₂ adsorption and Br^- in TBAB promoted the opening of the loop of PO. The catalytic performance of TBAB@ZIF-8 illustrated a volcano curve with the increase of the TBAB content. The addition of TBAB could have resulted in a positive effect on the loop-opening of PO. However, excessive TBAB hindered mass transfer and reduced the CO₂ adsorption of ZIF-8, which was certified by the CO₂ adsorption test. Diffuse reflectance infrared Fourier transform spectroscopy results showed that TBAB@ZIF-8 catalysts proceed in a new reaction pathway and that the rate-determining step of the reaction was the opening of the loop of PO. These results provided a guideline for the development of immobilization of homogeneous catalysts for CO₂ fixation.