Controlling Cooperative CO2 Adsorption in Diamine-Appended Mg2(dobpdc) Metal–Organic Frameworks

In the transition to a clean-energy future, CO2 separations will play a critical role in mitigating current greenhouse gas emissions and facilitating conversion to cleaner-burning and renewable fuels. New materials with high selectivities for CO2 adsorption, large CO2 removal capacities, and low regeneration energies are needed to achieve these separations efficiently at scale. Here, we present a detailed investigation of nine diamine-appended variants of the metal–organic framework Mg2(dobpdc) (dobpdc4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) that feature step-shaped CO2 adsorption isotherms resulting from cooperative and reversible insertion of CO2 into metal–amine bonds to form ammonium carbamate chains. Small modifications to the diamine structure are found to shift the threshold pressure for cooperative CO2 adsorption by over 4 orders of magnitude at a given temperature, and the observed trends are rationalized on the basis of crystal structures of the isostructural zinc frameworks obtained from in situ single-crystal X-ray diffraction experiments. The structure–activity relationships derived from these results can be leveraged to tailor adsorbents to the conditions of a given CO2 separation process. The unparalleled versatility of these materials, coupled with their high CO2 capacities and low projected energy costs, highlights their potential as next-generation adsorbents for a wide array of CO2 separations.

在向清洁能源未来转型的进程中，二氧化碳（CO₂）分离技术对于缓解当前温室气体排放、促进向清洁燃烧燃料与可再生能源的转化具有关键作用。要实现高效规模化的CO₂分离，亟需开发兼具高CO₂吸附选择性、大CO₂脱除容量与低再生能耗的新型材料。本研究针对九种经二胺修饰的金属有机框架（metal–organic framework, MOF）Mg₂(dobpdc)（其中dobpdc⁴⁻代表4,4′-二氧代联苯-3,3′-二羧酸根）变体展开了详尽的实验探究。该类材料因CO₂协同可逆插入金属-胺键以形成氨基甲酸铵链，故而呈现出阶梯状CO₂吸附等温线。研究表明，仅对二胺分子结构进行微小修饰，即可在给定温度下将协同CO₂吸附的阈值压力调控范围跨越四个数量级；结合原位单晶X射线衍射（in situ single-crystal X-ray diffraction）实验获得的同构锌基框架晶体结构，可对该变化趋势给出合理解释。从上述研究结果中推导得到的构效关系（structure–activity relationships），可用于针对特定CO₂分离工艺的工况条件定制适配的吸附剂。这类材料兼具无与伦比的多功能性、高CO₂吸附容量与较低的预计能源成本，充分彰显了其作为适用于多种CO₂分离场景的下一代吸附剂的应用潜力。