Solubilità e diffusione di gas in matrici miste per la separazione e cattura di CO2

The mixed matrix membrane Polysulfone-ZIF was studied to check the improvement on the selectivity on H2/CO2, H2/CH4, H2/N2 separation with respect to the pure PSf membrane. Sorption experiments have been compared with results of permeation tests, to confirm values of solubility. Solubility was measured through the knowledge of the equilibrium pressure and Peng Robinson equation of state. Its value was unequivocally independent of how the gas uptake happened. Diffusivity values suffered from some arbitrariness because in some cases, especially for the composite membrane, the process was not Fickian. In order to overcome this, the short-time and long-time approximation were applied. Values were different from ones obtained in permeation experiments for the MMM since the kinetics is very much affected by the type of experiment. For N2, CH4, CO2 in pure PSf, the diffusion coefficients calculated from sorption experiments were similar to those evaluated from permeation, suggesting that the transport characteristic time is the same in both experiments. Helium presented a permeation kinetics faster: this can be because the helium molecule is able to permeate small preferential free volume pathways present in the polymer, inaccessible to the other larger gases. In the MMM, for N2 and CH4, the kinetics of permeation is faster than the kinetics of sorption. This can be caused by the presence of voids at the interface between polymer and filler and explains. Helium showed quite the same behaviour as in the pure PSf and this could be because the larger size of the free volume voids presents in the MMM when ZIF is added doesn't affect much its behaviour, due to its small size. About CO2, in the MMM the diffusion measured in permeation is slower than that measured in sorption. This could be due to the different chemical nature of CO2, which undergoes stronger interactions with ZIF-8.