Critical Factors Driving the High Volumetric Uptake of Methane in Cu₃(btc)₂

Abstract

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.

Porous Metal-Organic Frameworks: Promising Materials for Methane Storage

“…In the past two decades, great endeavors have been dedicated to developing new MOF materials for high-capacity methane storage. [16][17][18][19][20][21][22][23][24] To date, the highest volumetric methane storage and working capacity for all reported MOFs at 65 bar and room temperature (RT) are about 270 and 200 cm 3 (STP) cm À3 , respectively. [25][26][27] Despite significant progress, there are still no MOF materials whose methane storage capacities can fulfill the new DOE targets at RT.…”

Strategies for Overcoming Defects of HKUST‐1 and Its Relevant Applications

“…As a benchmark of methane adsorption (200 cm 3 cm −3 at 80 bar, 298 K), HKUST‐1 shows its remarkable performance which could be an alternative material to traditional methane compression technology. HKUST‐1 was considered as the most hopeful CH 4 uptake material according to the research of Hulvey et al where they found the methane–methane interaction is the main factor explaining its enhanced bond at the copper metal sites …”

“…Hulvey et al thought it was the interaction between methane molecules and not the electronic interaction resulting in the strong methane binding. It is useless to change the center metal atom when the structure did not change, which was proved by the adsorption data of replacing Cu 2+ with Cr 2+ . A recent research creatively succeeded in preparing a type of compact, flexible, and high hardness of HKUST‐1 and surprisingly found it had an outstanding methane adsorption uptake.…”

Renaissance of the Methane Adsorbents

“…The combination of experiments and calculations revealed that the high methane uptake and deliverable capacity of HKUST‐1 is mainly contributed to the CH 4 −CH 4 interactions with the effect of adsorbed compounds rather than its OMS . Such CH 4 −CH 4 interactions can be further amplified in the confinement space to increase the methane packing density in the pores and improve the methane storage performance.…”