Surface Interaction and Quantum Kinetic Molecular Sieving for H2 and D2 Adsorption on a Mixed-Metal-Organic Framework Material
- University of Texas, Pan American, Edinburg, TX
- University of Newcastle upon Tyne
- ORNL
- Cornell University
A rational strategy has been used to immobilize open metal sites in ultramicroporosity for stronger binding of multiple H{sub 2} molecules per unsaturated metal site for H{sub 2} storage applications. The synthesis and structure of a mixed zinc/copper metal/organic framework material Zn{sub 3}(BDC){sub 3}[Cu(Pyen)] {center_dot} (DMF){sub 5}(H{sub 2}O){sub 5} (H{sub 2}BDC = 1,4 benzenedicarboxylic acid and PyenH{sub 2} = 5-methyl-4-oxo-1,4-dihydro-pyridine-3-carbaldehyde) is reported. Desolvation provides a bimodal porous structure Zn{sub 3}(BDC){sub 3}[Cu(Pyen)] (M{prime}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{sub 2} and D{sub 2} adsorption isotherms for M{prime}MOF 1 at 77.3 and 87.3 K were reversible with virtually no hysteresis. Methods for determination of the isosteric enthalpies of H{sub 2} and D{sub 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{sup -1} for H{sub 2} and D{sub 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{sup -1} at {approx} 1.9 mmol g{sup -1} (2 H{sub 2} or D{sub 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{sup -1}. Virial analysis of isotherms at 87.3 K is also consistent with two H{sub 2} or D{sub 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{sup -1}) for the narrow pores and a faster component with low activation energy (8.56 0.41 kJ mol{sup -1}). The D{sub 2} adsorption kinetic constants for both components were significantly faster than the corresponding H{sub 2} kinetics for specific pressure increments and had slightly lower activation energies than the corresponding values for H{sub 2} adsorption. The kD{sub 2}/kH{sub 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{sub 2}, resulting in slower adsorption kinetics compared with the heavier D{sub 2}. The results show that a combination of open metal centers and confinement in ultramicroporosity leads to a high enthalpy for H{sub 2} adsorption over a wide range of surface coverage and quantum effects influence diffusion of H{sub 2} and D{sub 2} in pores in M{prime}MOF 1.
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Center for Nanophase Materials Sciences (CNMS)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- DE-AC05-00OR22725
- OSTI ID:
- 935730
- Journal Information:
- Journal of the American Chemical Society, Vol. 120, Issue 20; ISSN 0002-7863
- Country of Publication:
- United States
- Language:
- English
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