Synthesis of binderless zeolite X microspheres and their CO₂ adsorption properties

Binderless zeolite NaX microspheres were readily prepared by using chitosan-assisted synthesis. The synthesis was conducted by hydrothermal treatment of silica/chitosan hybrid microspheres formed from a silica sol-chitosan mixture solution in a sodium aluminate alkaline solution, followed by calcination to remove the chitosan. The obtained microspheres are composed of compactly packed and intergrown octahedral particles with a particle size of ca. 500-750 nm, and exhibit an uniform particle size of ca. 1.3 mm, medium crashing strength and a surface area of 931 m²/g that is greater than that of the conventionally prepared zeolite X powders. CO₂, CH ₄ and N₂ adsorption under both ambient and elevated pressures to 1000 kPa at 298 K on the binderless NaX microspheres and Ca ²⁺ and K⁺-exchanged ones were measured. Separation of CO₂/CH₄ and CO₂/N₂ mixtures on the CaX microspheres in breakthrough experiments was performed. The binderless NaX microspheres have larger CO₂ adsorption capacity than reported zeolite X powders and the binderless CaX and KX microspheres. The CaX microspheres have the highest ideal separation factors for CO₂ from CH₄ and N₂ under mild conditions. The separation factors of CO₂/N₂ and CO₂/CH₄ mixtures on CaX microspheres are 79 and 12, respectively.