(315h) Tuning CO2 Adsorption in Mixed-Metal Metal?Organic Frameworks

With the dramatic increase in the annual growth of greenhouse gases over the last 50 years, CO₂ separations have become a critical component in climate change mitigation strategies. While many adsorbents show promising results, diamine-appended metal−organic frameworks (MOFs) stand out due to their large CO₂ uptake/removal capacity and low regeneration energies. One problem facing these frameworks, preventing their applications at large scales is their lack of tunability for their adsorption thermodynamics. Although the employment of different metals and diamines in these frameworks do alter the adsorption thermodynamics, they do not allow for continuous tuning that might be required for targeting particular gas streams. Here, we employ the use of mixed-metal metal−organic frameworks in the well-known e-2-M₂(dobpdc) framework (e-2 = N-ethylethylenediamine; M = Mg, Ni; dobpdc^4- = 4,4’-dioxidobiphenyl-3,3’-dicarboxylate) in the hopes of tuning the CO₂ adsorption step over a range of temperatures under isobaric conditions. This study also aims to understand the distribution of metals in the Ni_xMg_(2-x)(dobpdc) framework and any differences in the previously reported cooperative adsorption mechanism for the e-2-Mg₂(dobpdc) MOF via solid-state NMR.

Solid-state magic angle spinning NMR results on the non-diamine appended, ¹³CO₂-dosed M₂(dobpdc) MOF suggests that the distribution of metals is roughly homogeneous on the scale of hundreds of nanometers. Additionally, ICP-OES analysis of the synthesized Ni_xMg_(2-x)(dobpdc) MOF suggests that there is little to no preferential incorporation of one metal over the other. NMR results for the diamine-appended, ¹³CO₂-dosed e-2-Ni_xMg_(2-x)(dobpdc) MOF indicates that the adsorption mechanism is slightly altered in the mixed-metal framework. Finally, thermogravimetric analysis revealed that fine tuning of the adsorption temperatures for varying ratios of metal in the Ni_xMg_(2-x)(dobpdc) MOF is feasible and might be applicable for a variety of other metals. Our work thus opens the door for the preparation of designer mixed-metal MOF adsorbents for a wide range of CO₂ separations.

Acknowledgement – This work was supported by the College of Chemistry at the University of California, Berkeley under the supervision of Alexander C. Forse and Jeffrey A. Reimer.