Dual-role iron catalyst for covalent triazine framework synthesis and efficient CO₂ cycloaddition under mild conditions

Abstract

A pivotal yet often overlooked strategy in sustainable catalysis is the use of a single species for both catalyst synthesis and reaction mediation, thereby minimizing waste and resource consumption. Here, we demonstrate this integrative principle by employing an earth-abundant iron catalyst dually for constructing a covalent triazine framework (CTF) and for catalyzing CO2 cycloaddition to epoxides under mild conditions. Direct condensation of melamine and dipicolinic acid, as commercially available precursors, in the presence of a catalytic amount of FeCl3 under relatively mild conditions leads to the formation of an Fe-based catalyst (Fe-CTF-3) with remarkable surface area and pore volume of 683 m2 g-1 and 1.34 cm3 g-1, respectively. The role of the Fe catalyst, along with its loading and reaction time on CTF formation, was systematically investigated and discussed in the manuscript. It was found that the Fe centers control supersaturation, followed by nucleation and subsequent growth of the polymeric counterpart. All materials were characterized using various techniques to obtain accurate information about their physical, chemical, and textural properties. Fe-CTF-3 also displayed an exceptional turnover number (TONc up to 3760) toward various types of epoxides under optimized conditions: 0.025 mol% catalyst, 0.125 mol% TBAB, 10 bar CO2 at 70 °C within a short reaction time. To elucidate the distinct contributions of each catalytic component, a series of control experiments was performed under identical conditions. The catalyst was also recycled for 6 subsequent runs. In addition to the high catalytic activity of Fe-CTF-3 at an exceptionally low loading and its dual function in both catalyst synthesis and carbon dioxide conversion, it is noteworthy that reusable iron-catalyzed carbon dioxide coupling reactions have been rarely reported. The high catalytic performance of Fe-CTF-3 is attributed to the high surface area organic framework, which facilitates mass transfer and substrate diffusion, as well as the high nitrogen content that, in concert with uniformly dispersed Fe active sites, enhances CO2 adsorption capacity. This study introduces a novel design strategy for multifunctional catalysts based on earth-abundant metals, enabling integrated pathways for material synthesis and application.