Self-catalyzed cycloaddition of CO2 and epoxides over covalent organic frameworks without adding solvent and co-catalyst

Abstract

The cycloaddition of CO2 and epoxides to synthesize cyclic carbonates is a key strategy for CO2 utilization, though heterogeneous catalysts often suffer from instability. Covalent organic frameworks (COFs) present a compelling alternative due to their excellent textural properties and abundant Lewis basic sites. Herein, triazine-based COFs (PC-COFs) were synthesized by optimizing reaction time and temperature and were applied to catalyze the CO2 cycloaddition with epichlorohydrin (ECH) under solvent-free conditions, Instead of necessity of adding homogeneous co-catalyst, this study reveals a synergistic self-catalysis mechanism, where the carbonate product adsorbed on the Lewis basic PC-COF surface forms catalytic pairs with Lewis acidic carbonates, significantly accelerating the reaction. After five cycles, catalytic activity increased by 35% from 56.2% to 91.4%, and stabilizing over seven cycles. Under optimal reaction conditions, PC-COF-50-30 demonstrated outstanding catalytic performance, with a 98.7% ECH conversion, 97.6% selectivity to ECH carbonate and a CO2 conversion rate of 9.0 g‧g−1·h−1. This work provides a valuable example of high-performance CO2 cycloaddition catalysts and a strategy to achieve enhanced catalytic efficiency through product-catalyst synergy.

1. Introduction

With the worsening of global warming, carbon dioxide (CO2) as the main greenhouse gas, its emission reduction and resource utilization have become the focus of international attention [1,2]. The chemical conversion of CO2 into value-added products has attracted much attention. Captured CO2 can be efficiently converted into a variety of valuable products such as polyurethane, formic acid, dimethyl carbonate and cyclocarbonate [3—6]. Moreover, the sustainable capture of carbon dioxide by cycloaddition reaction with epoxides to produce high-yield and highly selective cyclocarbonates is considered to be an atomic-economic method.

Numerous heterogeneous catalysts have been developed for the CO2 cycloaddition reactions, including Schiff base complexes [7], organoboron catalysts [8,9], cobaloxime complexes [10,11] and metal-organic frameworks (MOFs), in which the MOFs exhibit excellent catalytic performance due to their inherent metal sites [12,13]. However, relatively weak coordination bonds between the metal nodes and organic ligands in MOF materials result in the poor stability during the reaction process [14,15]. Miralda et al. [16] investigated the catalytic performance of zeolitic imidazole framework-8 (ZIF-8) and aminated ZIF-8 catalysts in the cycloaddition reaction between CO2 and epichlorohydrin (ECH). Though both ZIF-8 and the functionalized ZIF-8 displayed good activity in this reaction, the catalytic activity decreased by approximately 50% after one cycle. A series of ZIF materials were prepared by controlling the water content in the solvent, in which ZIF-(80)-Co exhibited excellent catalytic performance in the cycloaddition reaction of CO2 and ECH [17]. However, the catalytic activity decreased by approximately 20% after four cycles due to the disruption of Co―N bonds.

In contrast, covalent organic frameworks (COFs) represent a class of porous crystalline materials constructed through strong covalent bonds [18,19] These materials have gained significant attention in catalysis due to their tunable porous structures, high density of heteroatoms, and exceptional chemical and thermal stability [20—22]. COF-based materials also offer promising potential for CO2 conversion, however, most COFs-based catalysts require long reaction time or co-catalyst in CO2 cycloaddition reaction [23,24]. For example, a nitrogen-rich CP-COF has been used for the CO2 cycloaddition reaction, reaching a full conversion of ECH at 110 ◦C, while this process requires the addition of homogeneous tetrabutylammonium bromide as co-catalyst [25]. The abundant nitrogen sites on the pore walls of COFs enhance CO2 affinity and adsorption capacity [26], while the lack of Lewis acidity may hamper its catalytic activity. Moreover, COFs can also be modified by incorporating metal sites, which act as the Lewis acid sites that promote the cycloaddition. It has been demonstrated that the use of metal-loaded COFs in CO2 cycloaddition reactions under mild conditions, achieving excellent catalytic performance [27,28]. However, the metal-incorporated COFs may face the problems including complex synthesis procedures as well as potential metal leaching.