Critical Factors Driving the High Volumetric Uptake of Methane in Cu₃(btc)₂.
A thorough experimental and computational study has been carried out to elucidate the mechanistic reasons for the high volumetric uptake of methane in the metal-organic framework Cu3(btc)2 (btc(3-) = 1,3,5-benzenetricarboxylate; HKUST-1). Methane adsorption data measured at several temperatures for Cu3(btc)2, and its isostructural analogue Cr3(btc)2, show that there is little difference in volumetric adsorption capacity when the metal center is changed. In situ neutron powder diffraction data obtained for both materials were used to locate four CD4 adsorption sites that fill sequentially. This data unequivocally shows that primary adsorption sites around, and within, the small octahedral cage in the structure are favored over the exposed Cu(2+) or Cr(2+) cations. These results are supported by an exhaustive parallel computational study, and contradict results recently reported using a time-resolved diffraction structure envelope (TRDSE) method. Moreover, the computational study reveals that strong methane binding at the open metal sites is largely due to methane-methane interactions with adjacent molecules adsorbed at the primary sites instead of an electronic interaction with the metal center. Simulated methane adsorption isotherms for Cu3(btc)2 are shown to exhibit excellent agreement with experimental isotherms, allowing for additional simulations that show that modifications to the metal center, ligand, or even tuning the overall binding enthalpy would not improve the working capacity for methane storage over that measured for Cu3(btc)2 itself.
Center for Neutron Research, National Institute of Standards and Technology , Gaithersburg, Maryland 20899, United States.|Department of Materials Science and Engineering, University of Maryland , College Park, Maryland 20742, United States.
Zeric
Z
Hulvey
Bess
B
Vlaisavljevich
Jarad A
JA
Mason
Ehud
E
Tsivion
Center for Neutron Research, National Institute of Standards and Technology , Gaithersburg, Maryland 20899, United States.|Department of Chemistry, Georgetown University , Washington, D.C. 20057, United States.
Timothy P
TP
Dougherty
Eric D
ED
Bloch
Martin
M
Head-Gordon
Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL) , Lausanne, Switzerland.
Berend
B
Smit
Jeffrey R
JR
Long
Center for Neutron Research, National Institute of Standards and Technology , Gaithersburg, Maryland 20899, United States.|Department of Chemical and Biomolecular Engineering, University of Delaware , Newark, Delaware 19716, United States.
Craig M
CM
Brown