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Surface interactions and quantum kinetic molecular sieving for H-2 and D-2 adsorption on a mixed metal-organic framework material

Chen, Banglin and Zhao, Xuebo and Putkham, Apipong and Hong, Kunlun and Lobkovsky, Emil B. and Hurtado, Eric J. and Fletcher, Ashleigh J. and Thomas, K. Mark (2008) Surface interactions and quantum kinetic molecular sieving for H-2 and D-2 adsorption on a mixed metal-organic framework material. Journal of the American Chemical Society, 130 (20). pp. 6411-6423. ISSN 0002-7863

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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.