Theoretical Study of Methane Storage in Cu₂₄(m-BDC)₂₄

Abstract

Calculations on the Cu 24 (m-BDC) 24 (m-BDC = 1,3benzenedicarboxylate) polyoxometalate (POM) cage with 0, 12, 24, and 40 methane molecules inside were made using the M06 exchange/correlation functional. During filling of the cage with 40 CH 4 molecules, the 12 strongest binding CH 4 molecules are those to the coordination unsaturated sites (CUS) to the inwardly directed Cu(+2) centers via agostic interactions. The next 12 CH 4 molecules are less tightly bound followed by the next 16 CH 4 molecules with average binding energies of 8.27, 7.88, and 7.36 kcal/mol per CH 4 , respectively. A section of the Cu 24 (m-BDC) 24 cage was taken with the formula Cu 4 (m-BDC)(BC) 6 (BC = benezenecarboxylate) in order to estimate zero-point, thermal, and entropy corrections of the larger cage. Estimating free energies at 1 bar, the Cu 24 (m-BDC) 24 POM is predicted to lose 16, 12, and 12 CH 4 molecules at 67, 123, and 171 °C, respectively. The 40CH 4 @Cu 24 (m-BDC) 24 cage, which is isostructural to the main cavity of HKUST-1 with 40 CH 4 molecules inside, is predicted to have a loading of 224 cm 3 (STP) cm −3 at 1 bar.

Theoretical investigation of C1–C4 hydrocarbons adsorption and separation in a porous metallocavitand

“…The CH 4 molecule strongly binds to PPX with a Δ E value of −17.9 kcal mol −1 . The CH 4 uptake capacity of PPX is exceptionally higher than many of the reported systems such as MOF-5 (−4.51 kcal mol −1 ), 25 NI-MOF-74 (−7.78 kcal mol −1 ), 26 Cu 24 ( m -BDC) 24 (−8.27 kcal mol −1 ), 27 TpPa1 COF (−5.39 kcal mol −1 ), 28 Graphdiyne (−5.18 kcal mol −1 ) 29 etc. The ultrahigh binding energy of CH 4 in PPX is due to the strong interaction between the 6s levels of Au( i ) and 2a 1 orbital of CH 4 and the charge transfer through level hybridization from CH 4 to Au( i ) and not via Au( i ) unoccupied orbitals.…”