Probing Adsorption Interactions in Metal–Organic Frameworks using X-ray Spectroscopy

Abstract

ABSTRACT:We explore the local electronic signatures of molecular adsorption at coordinatively unsaturated binding sites in the metal−organic framework Mg-MOF-74 using X-ray spectroscopy and first-principles calculations. In situ measurements at the Mg K-edge reveal distinct pre-edge absorption features associated with the unique, open coordination of the Mg sites which are suppressed upon adsorption of CO 2 and N,N′-dimethylformamide. Density functional theory shows that these spectral changes arise from modifications of local symmetry around the Mg sites upon gas uptake and are strongly dependent on the metal−adsorbate binding strength. The expanded MOF Mg 2 (dobpdc) displays the same behavior upon adsorption of CO 2 and N,N′-dimethylethylenediamine. Similar sensitivity to local symmetry is expected for any open metal site, making X-ray spectroscopy an ideal tool for examining adsorption in such MOFs. Qualitative agreement between ambient-temperature experimental and 0 K theoretical spectra is good, with minor discrepancies thought to result from framework vibrational motion. ■ INTRODUCTIONMetal−organic frameworks (MOFs) that feature internal pore surfaces replete with open metal sitesfive-coordinate metal cations known to behave as Lewis acidsare presently among the most promising solid adsorbents for selectively removing CO 2 from dry gas mixtures. Open metal sites preferentially adsorb CO 2 over N 2 or CH 4 due to its larger quadrupole moment and greater polarizability. The moderate heat of adsorption of CO 2 onto such sites produces selective, highcapacity adsorbents that can be easily regenerated via temperature or vacuum swing methods. The MOF-74 series of MOFs have been widely studied for their high performance in both gas separation and hydrogen storage applications.1−5 In particular, Mg-MOF-74 (CPO-27-Mg, Mg 2 (dobdc) (dobdc 4− = 2,5-dioxido-1,4-benzenedicarboxylate)) is a promising adsorbent for selective CO 2 adsorption due to its large CO 2 capacity and high selectivity for CO 2 over other gases.6−9 This MOF features a unique five-coordinate square-pyramidal geometry around the Mg sites; unlike other square-pyramidal Mg complexes, 10,11 the organic framework in Mg-MOF-74 constrains the bond angles at the Mg sites and ensures that a significant portion of the Mg coordination sphere is available for adsorbing other species. The Mg sites can therefore be considered quasi-octahedral, with the apical site vacant. To date, characterization efforts aimed at a microscopic understanding of interactions between gases and the adsorbent have consisted primarily of low-temperature X-ray and neutron diffraction experiments, 12−14 NMR spectroscopy, 15 infrared spectroscopy, 7 inelastic neutron scattering, 16 and theoretical studies. 7,17,18 These studies have determined a tilted, end-on binding configuration for CO 2 , but insight into any changes to the electronic structure of the MOF upon CO 2 adsorption have so far come solely from theoretical calculations. Experimental examination of these changes would vali...

Cooperative insertion of CO2 in diamine-appended metal-organic frameworks

“…4c) 22 , and all observed spectral changes were accurately reproduced by computed spectra. From the NEXAFS spectra, the new pre-edge peak at 402.3 eV arises solely from the carbamate nitrogen and is a clear signature of carbamate insertion into the metal-nitrogen bond.…”

Theoretical study of conformational disorder and selective adsorption of small organic molecules in the flexible metal-organic framework material MIL-53-Fe

“…8 While significant progress has been made in the rational design of new MOF materials with various topologies and chemical compositions in recent years, [9][10] generally, there is still a lack of molecular level information on the mechanisms of adsorption and sieving. Experimentally, highresolution powder X-ray diffraction (XRD), 11 neutron powder diffraction 12 and X-ray absorption fine structure spectroscopy 13 offer the potential to resolve the position of the guest in the host and the nature of their chemical interactions. However, due to the softness of some MOF materials, it is rather difficult to fully resolve the adsorption pattern of organic molecules inside a MOF.…”

Understanding Small‐Molecule Interactions in Metal–Organic Frameworks: Coupling Experiment with Theory

“…[ 75 ] It was hypothesized that the lack of agreement was the result of variations in the lattice parameters from the as-prepared samples, highlighting the sensitivity of these calculations to atomic structure. [ 75,103 ] A wide range of in situ spectroscopic methodologies such as IR and Raman, [ 104,105 ] inelastic neutron scattering (INS), [ 106 ] NMR, [ 67 ] and others [107][108][109] are highly sensitive to molecular interactions in porous media, and they have been used to successfully characterize various guest-framework interactions. While many of these techniques directly probe small-molecule dynamics related to rotations, vibrations, and diffusion, the resulting spectra can also be used to extract binding confi gurations, binding enthalpies, and even loading levels.…”

“…Spectra, simulated using a DFT-based protocol and compared with the experimental spectra, proved essential in understanding the variations in the local electronic environment around the open metal site with adsorption. [ 107 ] …”