Abstract
In order to develop new porous materials for applications in gas separations such as natural gas upgrading, landfill gas processing and acetylene purification it is vital to gain understanding of host-substrate interactions at a molecular level. Herein we report a series of six isoreticular metal-organic frameworks (MOFs) for selective gas adsorption. These materials do not incorporate open metal sites and thus provide an excellent platform to investigate the effect of the incorporation of ligand functionality via amide and alkyne groups on substrate binding. By reducing the length of the linker in our previously reported MFM-136, we report much improved CO2/CH4 (50 : 50) and CO2/N2 (15 : 85) selectivity values of 20.2 and 65.4, respectively (1 bar and 273 K), in the new amide-decorated MOF, MFM-126. The CO2 separation performance of MFM-126 has been confirmed by dynamic breakthrough experiments. In situ inelastic neutron scattering and synchrotron FT-IR microspectroscopy were employed to elucidate dynamic interactions of adsorbed CO2 molecules within MFM-126. Upon changing the functionality to an alkyne group in MFM-127, the CO2 uptake decreases but the C2H2 uptake increases by 68%, leading to excellent C2H2/CO2 and C2H2/CH4 selectivities of 3.7 and 21.2, respectively. Neutron powder diffraction enabled the direct observation of the preferred binding domains in MFM-126 and MFM-127, and, to the best of our knowledge, we report the first example of acetylene binding to an alkyne moiety in a porous material, with over 50% of the acetylene observed within MFM-127 displaying interactions (<4 Å) with the alkyne functionality of the framework.
Highlights
Porous metal–organic frameworks (MOFs) have attracted considerable attention due to their promise in a wide range of applications, notably in gas adsorption, selectivity and catalysis.1–6 Modulation of organic linkers and metal nodes allows ne-tuning of properties to tailor MOFs for the application of interest
Our approach has been to focus on the role of ligand sites for substrate binding and, signi cantly, we report the rst example of binding of acetylene to the alkyne groups in a porous material at crystallographic resolution
The relative ease of synthesis of MFM-126 coupled with its high selectivity for binary mixtures of CO2/CH4 and CO2/N2, position this MOF as a viable candidate for CO2 separations
Summary
Porous metal–organic frameworks (MOFs) have attracted considerable attention due to their promise in a wide range of applications, notably in gas adsorption, selectivity and catalysis.1–6 Modulation of organic linkers and metal nodes allows ne-tuning of properties to tailor MOFs for the application of interest. We present a comprehensive investigation into the roles of functional groups, pore geometry and porosity in enhancing selective gas binding through a series of six isostructural MOFs (MFM-126–128 and MFM-136–138; Table 1).
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