Methane and carbon dioxide storage in a porous van der Waals crystal

The first molecular crystal showing permanent porosity, made by of tris-o-phenylenedioxycyclotriphosphazene TPP molecules, is presented. The empty-pore hexagonal structure was solved after isolation of diffraction-quality single crystals and showed no residual electron density in the large unoccupied volume, shaped as straight nanochannels with a minimum diameter of 4.6 Å. Weak intermolecular interactions consolidate molecular stacks along the c axis (repeat period of 10.16 Å) and layers on the a–b plane (a=11.45 Å), ensuring the stability of the assembly. We showed the remarkable sorption properties of the porous molecular crystal with respect to important gases that participate in a network of soft interactions. Methane and carbon dioxide could be incorporated with high efficiency in the novel adducts (60% and 100% occupation, respectively, of the available sites, guest–host molar fraction of up to 1.25) with high selectivity over other gases. Unprecedented observations of the gases in van der Waals crystals and their topology could be provided by fast MAS NMR spectroscopy. The impressive upfield shifts caused by the aromatic ring currents of the host on gas molecules at the van der Waals contacts provided an unconventional tool for measuring the intermolecular gas-host distances and ricognizing the specific interactions that contribute to the overall stabilization. The multiple receptors lining the channel walls and the tight fit with the guests recall the specificity of biological nanochannels.