Synergistic Effects of Lewis Acid–Base Pair Sites─Hf-MOFs with Functional Groups as Distinguished Catalysts for the Cycloaddition of Epoxides with CO₂

Abstract

The incorporation of Lewis acid–base sites in catalysts has been considered as a significant approach to fabricating bifunctional catalysts with efficient catalytic activity for CO2 fixation. In this paper, a series of Hafnium-based metal–organic frameworks (Hf-MOFs), NU-912(Hf) and NU-912–X(Hf)–X (X = −NH2, −Br, −CN, and −I) derivatives assembled by Lewis acidic Hf6(μ3–O)4(μ3–OH)4(H2O)4(OH)4 (Hf6) clusters and Lewis base-attached organic linkers, are successfully synthesized by a facile ligand functionalization method. These isostructural Hf-MOFs, which exhibit diamond channels of 1.3 nm diameter, great chemical stability, and CO2 adsorption capacity, have been evaluated as catalysts for the CO2 cycloaddition reaction with epoxides. Catalytic experiments reveal that the micropore environments of these MOFs have an outstanding impact on catalytic activity. Remarkably, NU-912(Hf)–I serves as an efficient heterogeneous catalyst for this catalytic reaction under mild conditions due to the high density of Lewis acid Hf6 cluster centers and strong Lewis base functional groups, surpassing most of the reported MOF-based catalysts.

Design and synthesis of functionalized defective MOFs for catalytic conversion of CO2 to cyclic carbonates under green conditions

“…In particular, Hf-MOFs are an ideal candidate material for the study of the CO 2 cycloaddition reaction. 19 In addition, the dissociation enthalpy of the Hf–O bond is 802 kJ mol −1 , which is higher than that of the Zr–O bond (776 kJ mol −1 ), making Hf–OH more acidic than Zr–OH. 20 Klet et al 21 evaluated the Brønsted acidity in Zr and Hf organic scaffolds by electrochemical pH titration and determined the p K α values of three typical proton types in MOFs: μ 3 -OH, M–(OH) 2 , and M–OH (M = Zr, Hf).…”

Zn-MOF as a Single Catalyst with Dual Lewis Acidic and Basic Reaction Sites for CO2 Fixation

“…As evidenced from theoretical and experimental studies, MOF with exposed nitrogen-donor groups, namely amine, pyridine, imidazole, triazole, and tetrazole, effectively catalyze CO 2 conversion to various value-added cyclic carbonates . To date, plenty of MOF and MOF based catalysts have been synthesized to produce cyclic carbonates through the cycloaddition of CO 2 with epoxides. , However, most of the MOF-based catalysts that have been used so far involve challenging conditions for reaction, including, high temperature and pressure. , Besides this, several MOF exhibited appreciable catalytic activity at lower temperatures and pressure, − but longer reaction times were required, limiting their practical applicability . An ideal MOF catalyst for CO 2 conversion must have excellent catalytic activity at ambient conditions (atmospheric pressure and lower temperature) with good chemical stability and recyclability .…”

Experimental and Theoretical Investigation of Interfacial Engineering in Fe2O3/NiFe2O4 Heterostructures toward the Cycloaddition of CO2 with Styrene Oxide

“…13−15 For example, SC can be generated by the cycloaddition reaction between CO 2 and SO via the ring-opening of SO, insertion of CO 2 , and intramolecular cyclization. 16 In 1967, the synthesis of cyclic carbonates via the cycloaddition reaction was realized. 17 Since then, continuously increased attention has been focused on this area from both academia and industry.…”