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Surface Interactions and Quantum Kinetic Molecular Sieving for H2 and D2 Adsorption on a Mixed Metal−Organic Framework Material

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journal contribution
posted on 21.05.2008, 00:00 by Banglin Chen, Xuebo Zhao, Apipong Putkham, Kunlun Hong, Emil B. Lobkovsky, Eric J. Hurtado, Ashleigh J. Fletcher, K. Mark Thomas
A rational strategy has been used to immobilize open metal sites in ultramicroporosity for stronger binding of multiple H2 molecules per unsaturated metal site for H2 storage applications. The synthesis and structure of a mixed zinc/copper metal−organic framework material Zn3(BDC)3[Cu(Pyen)] ·(DMF)5(H2O)5 (H2BDC = 1,4 benzenedicarboxylic acid and PyenH2 = 5-methyl-4-oxo-1,4-dihydro-pyridine-3-carbaldehyde) is reported. Desolvation provides a bimodal porous structure Zn3(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 H2 and D2 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 H2 and D2 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 H2 and D2 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 ∼1.9 mmol g−1 (2 H2 or D2 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 H2 or D2 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 D2 adsorption kinetic constants for both components were significantly faster than the corresponding H2 kinetics for specific pressure increments and had slightly lower activation energies than the corresponding values for H2 adsorption. The kD2/kH2 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 H2, resulting in slower adsorption kinetics compared with the heavier D2. The results show that a combination of open metal centers and confinement in ultramicroporosity leads to a high enthalpy for H2 adsorption over a wide range of surface coverage and quantum effects influence diffusion of H2 and D2 in pores in M′MOF 1.

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