Highly Selective Quantum Sieving of D2 from H2 by a Metal–Organic Framework As Determined by Gas Manometry and Infrared Spectroscopy

The quantum sieving effect between D2 and H2 is examined for a series of metal–organic frameworks (MOFs) over the temperature range 77–150 K. Isothermal adsorption measurements demonstrate a consistently larger isosteric heat of adsorption for D2 vs H2, with the largest difference being 1.4 kJ/mol in the case of Ni-MOF-74. This leads to a low-pressure selectivity for this material that increases from 1.5 at 150 K to 5.0 at 77 K. Idealized adsorption solution theory indicates that the selectivity decreases with increasing pressure, but remains well above unity at ambient pressure. Infrared measurements on different MOF materials show a strong correlation between selectivity and the frequency of the adsorbed H2 translational band. This confirms that the separation is predominantly due to the difference in the zero-point energies of the adsorbed isotopologues

Insights into the Anomalous Vibrational Frequency Shifts of CO 2 Adsorbed to Metal Sites in Microporous Frameworks

Diffuse reflectance infrared (IR) spectroscopy was used to study the structure and dynamics of H2 and CO2 adsorbed within the isostructural metalorganic frameworks M2L (M = Mg, Mn, Fe, Co, Zn; L = 2,5-dioxidobenzene-1,4-dicarboxylate) referred to as M-MOF-74 and CPO-27-M. Cluster models of the primary adsorption site were excised from periodic models that were optimized using plane-wave density functional theory at the PerdewBurkeErnzerhof (PBE) level. Models incorporating an adsorbed H2 or CO2 were optimized using dispersion-corrected density functional theory (DFT), and the anharmonic vibrational frequencies of the adsorbate were calculated using the discrete variable representation method. The calculated vibrational frequency shifts reveal the same trend among the M2L materials as those observed experimentally and provide insight into the origins of these shifts. Our experimental spectra of adsorbed CO2 confirm a unique blue shift of the v(3) mode for molecules adsorbed in Mg2L, while the frameworks assembled from transition metals induce a red shift. By shifting the focus to the CO2 local vibrational modes, a deeper insight into the influence of back bonding (metal d-electron density donation into CO2 pi* orbitals) is revealed; for Mg2L there is a near-complete cancellation of the opposing local mode contributions to the observed frequency shift. Additional spectral features in the CO2 v(3) region are assigned to (1) the v(3) mode of the (CO2)-C-13 isotopologue, (2) a combination mode involving a v(2) excitation, and (3) librational sidebands arising from center-of-mass motion of the adsorbed molecule on the surface. Interestingly, below 100 K we observe the appearance of a new band that is distinct from the primary v(3) band observed at room temperature. This band is attributed to an alternate, localized orientation of CO2 adsorbed to the metal site, which is supported by the DFT model

Hydrogen Uptake on Coordinatively Unsaturated Metal Sites in VSB-5: Strong Binding Affinity Leading to High-Temperature D2/H2 Selectivity

We examine the adsorption of hydrogen and deuterium into the nanoporous nickel phosphate, VSB-5. On the basis of gas sorption analysis, VSB-5 exhibits one of the highest measured H2 heats of adsorption (HOA) for hydrogen (16 kJ/mol) yet reported. This high HOA is consistent with an unusually large red shift in the Q(1) and Q(0) hydrogen vibrational modes as measured with in situ infrared spectroscopy. The HOA for D2 is measured to be 2 kJ/mol higher than that for H2. “Ideal adsorbed solution theory” analysis of H2 and D2 isotherms provides selectivities above 4 for deuterium at 140 K, suggesting that VSB-5 is a promising adsorbent for pressure-swing adsorption-type separations of hydrogen isotopes