Anion-pillared MOFs materials for carbon dioxide capture

The global warming crisis caused by the increasing concentration of greenhouse gases such as carbon dioxide has become the focus of global concern. In response to this challenge, a variety of carbon capture technologies have been developed to reduce CO2 emissions. In recent years, carbon capture technology based on porous adsorbent materials has attracted extensive interest from researchers around the world because of its unique advantages. Metal-organic frameworks (MOFs), a class of crystalline porous materials have emerged as promising candidates for carbon dioxide capture applications owing to their highly ordered and tunable pore structures. Anion-pillared metal-organic frameworks (APMOFs) in particular, have garnered significant interest in recent years from researchers in this field. This is due to the presence of anion-pillared ligands with abundant hydrogen bond acceptors in their structure. A combination of the chemical versatility, modular design, ultrahigh porosity, pore size and geometry, high surface area and diverse functionality of the APMOFs allows for high adsorption capacity, selectivity and efficient capture of carbon dioxide through synergistic effects. Based on the adsorption performance and material stability, APMOFs can be categorized into four distinct generations. The first-generation APMOFs possess high adsorption capacity and selectivity; however, their practical utility is limited by inadequate thermal, chemical, and aqueous stability. In contrast, the second-generation APMOFs address stability issues by incorporating anion column braces characterized by high nucleophilicity. This modification substantially enhances thermal and chemical stability but results in a higher adsorption capacity only in the low-pressure range and an elevated adsorption heat for carbon dioxide, consequently increasing regeneration costs. The third-generation APMOFs are interspersed with pore distribution spanning 4.5-6 angstrom and a more uniform adsorption site due to structural alterations. These characteristics lead to enhanced adsorption capacity, a more moderate adsorption heat profile, and sustained high CO2 selectivity. However, these materials suffer from inadequate thermal and aqueous stability, which restricts their practical applicability. To overcome these limitations, the fourth-generation APMOFs employ a dual-pore structure featuring an icosahedral cage (similar to 8.5 angstrom) and a tetrahedral cage (similar to 4 angstrom), along with ligands possessing increased coordination bonds, exceptional adsorption capacity and selectivity and also demonstrates outstanding thermal stability and resistance to acidic conditions, which aligns closely with the performance prerequisites for practical applications. This paper provides a comprehensive review of the research progress made over the last decade on carbon dioxide capture utilizing four generations of APMOFs. We systematically present the progress made in terms of CO2 adsorption capacity, adsorption enthalpy, selectivity, thermal stability, and chemical stability for each of these generations. After about 10 years of continuous efforts, researchers have developed APMOFs materials that have not only excellent separation properties, but also robust thermal and chemical stability, and achieved a series of encouraging breakthrough results. Herein, we concluded that the separation performance and stability of APMOFs can be effectively improved by avoiding open metal sites, introducing multi-dentate ligands and ligands with strong coordination ability, as well as constructing composite pores, which provide useful guidance for the development of separation materials in the future. However, several challenges persist in the practical implementation of CO2 capture. These hurdles primarily encompass the high costs associated with materials, as well as the need for advancements in large-scale material synthesis techniques and molding technologies. Future solutions to these issues will among others be to develop APMOF materials with exceptional separation performance, robust stability, and costeffectiveness, which will provide a novel technical option within the current landscape of CO2 capture methods.