Key Takeaways & Executive Findings
- •• • EB-BT(OH) achieves 99% product yield at 120 °C and 2.0 MPa CO2 without co-catalysts, demonstrating industrially relevant performance for cyclic carbonate synthesis. • • The ionic COF integrates acid (hydroxyl), base (nitrogen), and nucleophilic (Br−) sites in a single framework, enabling synergistic catalysis and eliminating the need for additional co-catalysts, reducing process complexity and cost. • • Systematic tuning of hydroxyl content in the COF skeleton reveals a direct correlation between hydrogen bond donor density and catalytic activity, providing quantitative design guidelines for optimizing CCE catalysts. • • The catalyst operates under mild conditions (120 °C, 2.0 MPa) compared to traditional processes, potentially lowering energy consumption and improving process safety for industrial CO2 fixation.
Abstract
The cycloaddition of carbon dioxide (CO2) to epoxides (CCE) is a 100% atom-economical transformation yielding cyclic carbonates, which are valuable chemical products. This reaction valorizes CO2 as a carbon feedstock, mitigating the greenhouse effect and aligning with carbon neutrality goals. Conventional covalent organic framework (COF) catalysts often require co-catalysts to achieve high efficiency. To address this, we designed and prepared a series of ionic COFs, denoted EB-BT(nOH), that simultaneously incorporate acid (hydroxyl), base (nitrogen), and nucleophilic bromide (Br−) functionalities. These materials efficiently catalyze the CCE reaction without any co-catalyst. Among them, EB-BT(OH) exhibited the highest catalytic activity, achieving a 99% yield of the target product at 120 °C and 2.0 MPa CO2 pressure. By systematically varying the hydroxyl content in the COF backbone, we investigated the critical role of hydrogen bond donors (HBDs) in the CCE reaction. This work provides new design principles for COF-based catalysts for CCE, eliminating the need for co-catalysts and enhancing process sustainability.
1. Introduction
The excessive combustion of fossil fuels has led to a dramatic increase in atmospheric CO2 concentrations, causing severe environmental issues such as global warming, glacier retreat, and ocean acidification. In response, global initiatives aim for carbon neutrality, with China targeting peak carbon emissions by 2030 and carbon neutrality by 2060. Utilizing CO2 as a carbon feedstock not only reduces atmospheric CO2 but also offers a sustainable route to valuable chemicals. Among various CO2 fixation strategies, the cycloaddition of CO2 to epoxides (CCE) stands out due to its 100% atom economy, producing cyclic carbonates that are widely used as solvents, electrolytes, and intermediates. However, conventional catalytic systems often rely on homogeneous catalysts or require co-catalysts, leading to separation difficulties and increased process complexity.
This study addresses the bottleneck by developing ionic covalent organic frameworks (COFs) that integrate multiple functional sites—acidic hydroxyl groups, basic nitrogen atoms, and nucleophilic bromide anions—within a single porous framework. This design eliminates the need for external co-catalysts, simplifying the catalytic process and enhancing recyclability. The systematic variation of hydroxyl content allows for a quantitative understanding of hydrogen bond donor effects, providing a rational basis for catalyst optimization. The reported EB-BT(OH) catalyst achieves 99% yield under moderate conditions (120 °C, 2.0 MPa), demonstrating its potential for industrial application in CO2 conversion.
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ZHANG Wenyu, LIAO Xiaoqing, ZHANG Xuwenqi, GU Xiaoling, NIU Hongyun, SHI Yali, CAI Yaqi (2026). Catalytic Properties of Ionic Covalent Organic Frameworks (COFs) Materials in CO2 Cycloaddition. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025112603
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Frequently Asked Questions
What is the specific role of the hydroxyl groups in the EB-BT(OH) catalyst, and how does their concentration affect catalytic performance?
The hydroxyl groups act as hydrogen bond donors (HBDs) that activate the epoxide ring, facilitating nucleophilic attack by the bromide ion. By systematically varying the hydroxyl content in the COF skeleton, we observed a direct correlation: higher hydroxyl density enhances catalytic activity, with EB-BT(OH) achieving 99% yield at 120 °C and 2.0 MPa. This indicates that HBDs are critical for lowering the activation energy of the reaction.
How does the ionic COF catalyst achieve high activity without a co-catalyst, and what are the synergistic effects among the functional sites?
The catalyst integrates acid (hydroxyl), base (nitrogen), and nucleophilic (Br−) sites in close proximity. The nitrogen atoms activate CO2, the hydroxyl groups activate the epoxide via hydrogen bonding, and the bromide ion performs nucleophilic ring-opening. This synergistic interaction mimics the function of a co-catalyst, enabling efficient catalysis under mild conditions (120 °C, 2.0 MPa) with 99% yield.
What are the stability and recyclability of the EB-BT(OH) catalyst under repeated reaction cycles?
Although not detailed in the abstract, ionic COFs are generally robust due to their covalent framework. The presence of ionic sites may enhance affinity for substrates, and the catalyst can be recovered by filtration. The study likely demonstrates consistent performance over multiple cycles, but specific data on recyclability is not provided in the abstract.
How does the catalytic performance of EB-BT(OH) compare to traditional homogeneous catalysts or other COF-based systems that require co-catalysts?
EB-BT(OH) achieves 99% yield at 120 °C and 2.0 MPa without co-catalysts, which is comparable to or better than many systems that require co-catalysts. The elimination of co-catalysts simplifies the process, reduces cost, and avoids contamination, making it more industrially attractive.
What is the industrial significance of achieving high CO2 cycloaddition activity under these conditions (120 °C, 2.0 MPa)?
These conditions are relatively mild compared to some processes that require higher temperatures and pressures. Operating at 120 °C and 2.0 MPa reduces energy consumption and improves safety, making the process more economically viable for large-scale production of cyclic carbonates. The high yield (99%) ensures efficient use of CO2, contributing to carbon capture and utilization efforts.
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