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)] center dot(DMF)(5)(H2O)(5) (H2BDC = 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 similar to 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.