Effect of ion exchange and dehydration temperature on the adsorption and diffusion of gases in ETS-4

Samples of small-pore Engelhard titanium silicate ETS-4 and its Sr-exchanged variant, patented by Engelhard Corporation, were successfully synthesized in our laboratory, and the characteristic signatures of their crystal structures were confirmed by X-ray diffraction (XRD). Thermogravimetric analysis (TGA) was carried out to study their thermal behavior and stability, and crystal morphology was examined using a scanning electron microscope (SEM). The synthesized fine crystals were pelletized by pressure binding without the addition of any binder material. Equilibrium and uptake measurements for oxygen, nitrogen, and methane on Na- and Sr-exchanged ETS-4 were then carried out in a constant-volume apparatus. The equilibrium capacities of the three gases in the Na form of ETS-4 were comparable, and Sr exchange did not seem to significantly affect these capacities. However, Sr exchange appreciably increased the uptake rate of nitrogen and reduced that of methane, thereby increasing the kinetic selectivity to a promising level. Capacity and uptake measurements were also conducted on the Sr-exchanged ETS-4 dehydrated at different temperatures. It was observed that increasing dehydration temperature progressively lowered the capacities and also slowed the kinetics of adsorption. Probable reasons for the observed changes in equilibrium capacity and uptake are discussed in detail.