Surface interactions and quantum kinetic molecular sieving for H2 and D2 adsorption on a mixed metal-organic framework material

A rational strategy has been used to immobilize open metal sites in ultramicroporosity for stronger binding of multiple H 2 molecules per unsaturated metal site for H 2 storage applications. The synthesis and structure of a mixed zinc/copper metal-organic framework material Zn 3(BDC) 3[Cu(Pyen)] .(DMF) 5(H 2O) 5 (H 2BDC = 1,4 benzenedicarboxylic acid and PyenH 2 = 5-methyl-4-oxo-1,4-dihydro-pyridine-3-carbaldehyde) is reported. Desolvation provides a bimodal porous structure Zn 3(BDC) 3[Cu(Pyen)] (M'MOF 1) with narrow porosity (<0.56 nm) and an array of pores in the bc crystallographic plane where the adsorbate-adsorbent interactions are maximized by both the presence of open copper centers and overlap of the potential energy fields from pore walls. The H 2 and D 2 adsorption isotherms for M'MOF 1 at 77.3 and 87.3 K were reversible with virtually no hysteresis. Methods for determination of the isosteric enthalpies of H 2 and D 2 adsorption were compared. A virial model gave the best agreement (average deviation <1 standard deviation) with the isotherm data. This was used in conjunction with the van't Hoff isochore giving isosteric enthalpies at zero surface coverage of 12.29 +/- 0.53 and 12.44 +/- 0.50 kJ mol (-1) for H 2 and D 2 adsorption, respectively. This is the highest value so far observed for hydrogen adsorption on a porous material. The enthalpy of adsorption, decreases with increasing amount adsorbed to 9.5 kJ mol (-1) at approximately 1.9 mmol g (-1) (2 H 2 or D 2 molecules per Cu corresponding to adsorption on both sides of planar Cu open centers) and is virtually unchanged in the range 1.9-3.6 mmol g (-1). Virial analysis of isotherms at 87.3 K is also consistent with two H 2 or D 2 molecules being bound to each open Cu center. The adsorption kinetics follow a double exponential model, corresponding to diffusion along two types of pores, a slow component with high activation energy (13.35 +/- 0.59 kJ mol (-1)) for the narrow pores and a faster component with low activation energy (8.56 +/- 0.41 kJ mol (-1)). The D 2 adsorption kinetic constants for both components were significantly faster than the corresponding H 2 kinetics for specific pressure increments and had slightly lower activation energies than the corresponding values for H 2 adsorption. The kD 2/ kH 2 ratio for the slow component was 1.62 +/- 0.07, while the fast component was 1.38 +/- 0.04 at 77.3 K, and the corresponding ratios were smaller at 87.3 K. These observations of kinetic isotope quantum molecular sieving in porous materials are due to the larger zero-point energy for the lighter H 2, resulting in slower adsorption kinetics compared with the heavier D 2. The results show that a combination of open metal centers and confinement in ultramicroporosity leads to a high enthalpy for H 2 adsorption over a wide range of surface coverage and quantum effects influence diffusion of H 2 and D 2 in pores in M'MOF 1.

Selective Gas Sorption within a Dynamic Metal-Organic Framework

A microporous metal-organic framework 1 Co(NDC)(4,4'-Bipy)(0.5).G(x) (NDC = 2,6-naphthalenedicarboxylate; 4,4'-Bipy = 4,4'-bipyridine; G = guest molecules) was synthesized and structurally characterized of a doubly interpenetrated primitive cubic net. To make use of the framework flexibility, 1 was activated at temperatures of 150 and 200 degrees C to form 1a and 1b, respectively, exhibiting highly selective sorption behaviors of hydrogen over nitrogen-gas molecules.

A Triply Interpenetrated Microporous Metal−Organic Framework for Selective Sorption of Gas Molecules

A microporous metal-organic framework Zn(ADC)(4,4'-Bpe)(0.5).xG [1; ADC = 4,4'-azobenzenedicarboxylate, 4,4'-Bpe = trans-bis(4-pyridyl)ethylene, G = guest molecules] with a triply interpenetrative primitive cubic net was synthesized and characterized. With pores of about 3.4 x 3.4 A, the activated 1a exhibits highly selective sorption behavior toward H(2)/N(2), H(2)/CO, and CO(2)/CH(4).