Search

This work demonstrates the development of a hierarchical mesoporous hybrid ZIF-8 with 1,2,4-triazole (T), ZIF-8T, as a heterogeneous catalyst for CO2cycloaddition to epoxides under a solvent- and cocatalyst-free conditions at ambient pressure. The hybrid ZIF-8T catalyst was prepared via a facile one-pot synthesis method using a mixed-ligand of 2-methyl imidazole (2-Mim) and T. The contents of the introduced T ligand in ZIF-8T were controlled by adjusting a molar ratio of T: 2-Mim ligand. Interestingly, the ZIF-8T with 84.9% of T contents showed a hierarchical hollow mesopore properties and it exhibited excellent catalytic performance with 100% conversion of epichlorohydrin with high selectivity (>99%) under mild conditions (80 °C, 1 bar CO2). In addition, ZIF-8T showed high catalytic activity toward various epoxide substrates. This significant activity of ZIF-8T was attributed to the synergistic effect of the abundance of Lewis acid/base sites derived from unsaturated Zn node and N species in T and 2-Mim ligand, along with the inherent properties of hierarchical mesoporosity. Various reaction parameters, including the effect of the reaction temperature, time, and catalyst loading, were investigated to optimize the reaction condition. Additionally, reaction kinetics and the reusability test of the ZIF-8T catalyst were conducted. Based on the results obtained, a plausible reaction mechanism for CO2–epoxide cycloaddition catalyzed by the ZIF-8T catalyst was also predicted.

The efforts to achieve sustainable chemical conversion and efficient CO2utilization are critical to addressing climate change and environmental sustainability, and therefore the development of effective CO2conversion catalysts is crucial. In this study, novel acid-base bifunctional catalysts, MIL-101-SO3H@Atz and MIL-101-SO3H@DAtz, were successfully synthesized using MIL-101-SO3H as an acid support material through post-synthetic modification with 3-amino-1,2,4-triazole (Atz) and 3,5-diamino-1,2,4-triazole (DAtz) for CO2conversion reaction. The incorporation of the amine species into MIL-101-SO3H created unique synergistic effects with dual acid-base catalytic sites, facilitating the efficient conversion of CO2and epoxides into cyclic carbonates. Among the catalysts, MIL-101- SO3H@DAtz demonstrated superior catalytic performance with achieving 100% epichlorohydrin (ECH) conversion with over 99% selectivity to cyclic carbonate with high selectivity of > 99% under the ambient condition of 1 bar CO2without any co-catalyst or solvent. The elucidated CO2cycloaddition reaction mechanism highlights the synergistic effects of acid-base functionalities and CO2-philic Lewis bases as well as hydrogen bonding donor groups, providing valuable insights into the factors contributing to superior catalytic performance. The simulated flue gas (15% CO2/85% N2, v/v) was also conducted using MIL-101-SO3H@DAtz catalyst. Additionally, the MIL-101-SO3H@DAtz catalyst exhibited remarkably robust structural stability and recyclability, maintaining its activity over multiple cycles.

Size-controlled hybrid ZIFs (core-shell ZIF-67@ZIF-8, ZIF-8@ZIF-67, and bimetallic Zn/Co-ZIF-8) with the same particle size were synthesized by a seed-mediated methodology by adjusting the particle size of core ZIFs. The synthesized hybrid ZIFs was tested as heterogeneous catalysts for the CO2cycloaddition to epoxides using pure CO2gas and mixed gas (85 % CO2/15% N2) without any co-catalyst and solvent. The effect of the composition of shell and core ZIFs on the catalytic performance of CO2cycloaddition reaction to epoxides were investigated. Furthermore, the correlation between structural defects and catalytic activity in ZIF catalysts along with their particle sizes was investigated by calculating the ratio of DXRD/DSEM. Both core-shell type ZIF-67@ZIF-8 and ZIF-8@ZIF-67 exhibited better catalyst performance of CO2cycloaddition reaction than other ZIF catalysts. Among the ZIF catalysts, ZIF-67@ZIF-8 exhibited the highest catalytic performance for epoxide conversion owing to its abundant Lewis acid-base properties and relatively large window size of ZIF-8 shell with appropriate binding force between epoxide and CO2. It showed good recyclability for CO2cycloaddition reaction without a structural collapse during ten recycles. The structural stability of ZIF-67@ZIF-8 was examined in detail by a hot filtering test, leaching test, and TGA analysis. The mechanism of CO2cycloaddition reaction was proposed and the effect of the shell composition in core-shell type ZIFs on the binding energy with epoxide substrate also was studied based on DFT calculations.

Amine-functionalized bimetallic zeolitic imidazolate framework-8 (Co/Zn-ZIF-A) with 3-amino-1,2,4-triazole (Atz) was synthesized by a post-synthetic modification and utilized as a heterogeneous catalyst for the synthesis of cyclic carbonates from CO2and epoxides. Different amine contents in Co/Zn-ZIF-A materials were controlled by adjusting the ligand exchange reaction time of the of 2-mIM with Atz. Co/Zn-ZIF-A-24 h with the highest amine contents (32% of Atz) achieved an excellent result of 99% conversion of epichlorohydrin with high selectivity of > 99% after 24 h without co-catalyst and solvent under mild conditions (80 oC, 1 bar CO2). The enhanced catalytic activity is attributed to the acid-base synergistic effect by the combination of bimetallic systems consisting of Zn and Co as Lewis acid sites and the abundant Lewis bases sites of -NH2, -NH groups from Atz ligand. A kinetic study of the cycloaddition reaction over Co/Zn-ZIF-A-24 h confirmed an approximately first-order dependence on the epichlorohydrin concentration and CO2pressure with an activation energy of approximately 39.5 kJ mol-1. Co/Zn-ZIF-A-24 h catalyst showed good reusability with no obvious loss in catalytic activity during five recycling runs and its structural stability was also confirmed. A plausible reaction mechanism of Co/Zn-ZIF-A catalyst for CO2-epoxide cycloaddition was proposed.