Structural Basis of CO2 Adsorption in a Flexible Metal-Organic Framework Material

Abstract

This paper reports on the structural basis of CO2 adsorption in a representative model of flexible metal-organic framework (MOF) material, Ni(1,2-bis(4-pyridyl)ethylene)[Ni(CN)4] (NiBpene or PICNIC-60). NiBpene exhibits a CO2 sorption isotherm with characteristic hysteresis and features on the desorption branch that can be associated with discrete structural changes. Various gas adsorption effects on the structure are demonstrated for CO2 with respect to N2, CH4 and H2 under static and flowing gas pressure conditions. For this complex material, a combination of crystal structure determination and density functional theory (DFT) is needed to make any real progress in explaining the observed structural transitions during adsorption/desorption. Possible enhancements of CO2 gas adsorption under supercritical pressure conditions are considered, together with the implications for future exploitation. In situ operando small-angle neutron and X-ray scattering, neutron diffraction and X-ray diffraction under relevant gas pressure and flow conditions are discussed with respect to previous studies, including ex situ, a priori single-crystal X-ray diffraction structure determination. The results show how this flexible MOF material responds structurally during CO2 adsorption; single or dual gas flow results for structural change remain similar to the static (Sieverts) adsorption case, and supercritical CO2 adsorption results in enhanced gas uptake. Insights are drawn for this representative flexible MOF with implications for future flexible MOF sorbent design.

Density Functional Theory Study of the Structure of the Pillared Hofmann Compound Ni(3-Methyl-4,4′-bipyridine)[Ni(CN)4] (Ni-BpyMe or PICNIC-21)

“…In Figure , we compare the residual pores in the DFT structures for Ni-BPyMe with our earlier results for Ni-Bpene . We also calculated whether there was room for a CO 2 molecule to fit inside the pores in each case, using the minimal van der Waals radii tabulated in ref .…”

“…We found that the inclusion of dispersion interactions was essential to reproduce the structure of Ni-BpyMe. In ref , we found that the Hirshfeld form of dispersion with partial charges − worked very well for the structure of Ni-Bpene; thus, the same form of dispersion was used here. We used an isotropic Hubbard U correction of 5 eV on Ni.…”

“…In a previous work, 23 we used experimental measurements and density-functional-theory (DFT) calculations 24 to investigate in detail the structural changes in Ni-Bpene upon CO 2 adsorption. We concluded that rotations of the Bpene ligands occur as the structure goes from low CO 2 filling to maximum CO 2 uptake, along with a substantial expansion of the unit cell.…”

Improved Predictive Tools for Structural Properties of Metal–Organic Frameworks

“…Kohn-Sham density functional theory (KS-DFT) is the most popular way to calculate structural parameters of solids [15][16][17][18][19]. The accuracy of a KS-DFT calculation of a system depends on the choice of exchange-correlation (XC) functional.…”

Enhanced CO2 Adsorption on Nitrogen-Doped Carbon Materials by Salt and Base Co-Activation Method

“…Carbon dioxide adsorption especially by porous materials has become a hot research topic because these materials possess many advantages such as low energy requirements, quick and convenient processes of adsorption and desorption compared with chemical absorption [2]. In this context, many porous materials including zeolites [3,4,5], other inorganic molecular sieves [6,7,8,9,10,11,12,13], metal-organic frameworks [14,15,16,17,18,19] and carbon-based materials [20,21,22,23,24] have been investigated.…”