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

Calculations on the Cu24(m-BDC)24 (m-BDC = 1,3-benzenedicarboxylate) 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 CH4 molecules, the 12 strongest binding CH4 molecules are those to the coordination unsaturated sites (CUS) to the inwardly directed Cu(+2) centers via agostic interactions. The next 12 CH4 molecules are less tightly bound followed by the next 16 CH4 molecules with average binding energies of 8.27, 7.88, and 7.36 kcal/mol per CH4, respectively. A section of the Cu24(m-BDC)24 cage was taken with the formula Cu4(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 Cu24(m-BDC)24 POM is predicted to lose 16, 12, and 12 CH4 molecules at 67, 123, and 171 °C, respectively. The 40CH4@Cu24(m-BDC)24 cage, which is isostructural to the main cavity of HKUST-1 with 40 CH4 molecules inside, is predicted to have a loading of 224 cm3(STP) cm-3 at 1 bar.