Understanding Methane Adsorption in Porous Aromatic Frameworks: An FTIR, Raman, and Theoretical Combined Study

We present a vibrational study of PAF-302, belonging to the class of porous aromatic frameworks (PAFs), recently synthesized and applied in several applications involving gas adsorption. The precursor, tetrakis­(4-bromophenyl) methane (TBPM), and the polymer were studied with FTIR and Raman spectroscopies to investigate the structure of PAF-302, whereas the system after methane adsorption was studied by FTIR, also varying the CH<sub>4</sub> loading, to get some hints on the strength of the interactions with adsorbed methane. Theoretical calculations of the harmonic frequencies of TBPM, methane, and methane/aromatic model systems were performed at high theory level (MP2 with extended basis set) to support the assignment of vibrational bands and to estimate the interactions causing the observed frequency shifts upon methane adsorption. The analysis shows that the polymerization process is essentially complete and that the adsorbed CH<sub>4</sub> molecules interact with two phenyl rings, though stronger interactions can be envisaged. The computed interaction energies are compatible with the isosteric heats of adsorption previously measured for methane in PAF-302. A Grand Canonical Monte Carlo (GCMC) approach was used to simulate CH<sub>4</sub> adsorption isotherms at different temperatures (87–115 K) and in the 0–0.020 bar pressure range, thus allowing us to estimate the loading of methane in the FTIR adsorption study.