Uniform Si-CHA Zeolite Layers Formed by a Selective Sonication-Assisted Deposition Method

摘要

Zeolites are crystalline aluminosilicate materials employed in a wide range of applications, including catalysis, separation, water softening, and adsorption. The rigid molecular-sized pores within zeolites are desirable for use in separations because they enable the discrimination and separation of gas molecules on the basis of their size and/or shape.This intrinsic molecular-sifting property also holds promise for the selective isolation of CO2 from mixtures, for example, the CO2/N2/H2O mixture that results from postcombustion processes and the CO2/CH4/H2O mixture in the natural-gas stream.Zeolite pore sizes corresponding to eight-mem-bered rings (8-MRs) lie in the range suitable for CO2 separation, as they are a little larger than CO2 and smaller than or similar in size to N2 or CH4. Specifically, the kinetic diameters of CO2, N2, and CH4 are 0.33, 0.364, and 0.38 nm, respectively, whereas the maximum free dimension of 8-MRs is about 0.43 nm.Multiple 8-MR zeolite membranes have been fabricated in an attempt to capture CO2 from mixtures.Specifically, 8-MR zeolite and zeolite-like membranes, such as DDR (decadodecasil 3R) and SAPO-34 (CHA type, but comprised of silicon, aluminum, phosphate, and oxygen)membranes, respectively, have exhibited high performance for CO2 separation. The pore apertures of pure-silica CHA-type zeolites (Si-CHA zeolites) are about 0.370 x 0.417 nm~2,which enables the discrimination of CO2 and N2 according to their size. Although the separation of CO2 from N2 is possible by size exclusion with 8-MR pore apertures, the separation of CO2 from H2O is more challenging owing to the smaller molecular size of H2O (0.265 nm).[2i To minimize H2O flux through CHA membranes, their hydrophilicity, which is presumably due to Al constituents in the CHA framework, should be minimized by the use of an all-silica CHA zeolite.