Effects of alkali-treatment on the pore structure of zeolite X

The formation of mesoporous structure of zeolite X treated with alkali solution was studied. The effects of the concentration of alkali solution used, treating temperature and time on the formation of mesoporous structure were investigated. The zeolites after alkali treatment were characterized by ICP, N₂ adsorption-desorption, SEM, and XRD. The results reveal that alkali-treatment can change the Si/Al molar ratio of zeolite X, and thus modify its properties. It was found that the Si in the framework of zeolite X is difficult to be removed when the alkali-treatment temperature is lower than 70°C, and zeolite X is prone to form macropores when used concentration of NaOH solution is higher than 0.05 mol·L^-1 and/or the alkali-treatment time is longer than 30 min. When the zeolite X was treated under the optimal conditions (using 0.05 mol·L^-1 NaOH solution at 70°C for 30 min), the mesoporous zeolite with mesoporous volume of 0.24 cm³·g^-1, pore size distributions of 2.3-8 nm and specific surface area of 443.11 m²·g^-1 can be obtained. The methylene blue adsorption tests show that the adsorption equilibrium amount of methylene blue adsorbed by alkali-treated zeolite X is larger than that by the untreated zeolite X, which confirms that a certain amount of mesopores are indeed formed in the zeolite X after alkali-treatment.