Understanding The Fascinating Origins of CO2 Adsorption and Dynamics in MOFs

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• 作者：Shoushun Chen ; Bryan E.G. Lucier ; Paul D. Boyle ; Yining Huang
• 刊名：Chemistry of Materials
• 出版年：2016
• 出版时间：August 23, 2016
• 年：2016
• 卷：28
• 期：16
• 页码：5829-5846
• 全文大小：995K
• 年卷期：0
• ISSN：1520-5002

文摘

Metal–organic frameworks (MOFs) have shown great promise for the adsorption and separation of gases, including the greenhouse gas CO₂. In order to improve performance and realize practical applications for MOFs as CO₂ adsorbents, a deeper understanding of the number and type of CO₂ adsorption mechanisms must be unlocked, along with fine details of CO₂ motion within MOFs. Using several complementary characterization methods is a promising protocol for comprehensively investigating the various host–guest interactions between MOFs and CO₂. In this work, a combination of solid state NMR (SSNMR) and single crystal X-ray diffraction (SCXRD) has been utilized to reveal both the location and dynamics of adsorbed CO₂ within the related PbSDB and CdSDB MOFs, as well as to probe the role of metal centers in CO₂ adsorption. ¹³C SSNMR experiments targeting CO₂ reveal the number of unique adsorption sites and the types of CO₂ dynamics present, as well as their associated motional rates and angles. ¹¹¹Cd and ²⁰⁷Pb SSNMR methods are used to probe the influence of CO₂ adsorption on the MOF metal centers, and also to investigate the possibility of any metal–guest interactions. SCXRD experiments yield the exact locations and occupancies of adsorbed CO₂ in both MOFs; by pairing this information with SSNMR data, a comprehensive model of CO₂ adsorption and dynamics in PbSDB and CdSDB has been established. Both MOFs share a common adsorption site in the V-shaped “π-pocket” formed by the phenyl rings of an individual V-shaped organic linker, while CdSDB also features an additional π-pocket adsorption site arising from the phenyl rings of two linkers joined by Cd. SCXRD and SSNMR data indicate that CO₂ adsorbed at the SDB-based π pocket in both MOFs exhibits a local rotation or “wobbling” at an individual adsorption site, as well as a nonlocalized jumping or “hopping” between symmetry-equivalent adsorption sites. The combined analysis of SCXRD and SSNMR data has the potential to yield rich information regarding guest dynamics, adsorption locations, and host–guest interactions in many MOFs.