Abstract

The desilicated form of the ZSM-5 was prepared by a process which involved the selective removal of silicon atoms from the parent sodium form zeolite in an aqueous basic solution of sodium carbonate. If the resulting material was activated at 700C̕, its pore size was larger than that of the parent zeolite. The sample is called DS for: Desilication and Stabilization. The desilication treatment has the effect of lowering the Si/Al ratio, while keeping the zeolite structure essentially unmodified. The silicate material which had been removed was determined to be in large proportion sodium orthosilicate. Ion-exchange of the Na-DZSM-5 with ammonium chloride gives us the more acidic form H-DZSM-5. An amount of 5 wt.% sodium orthosilicate is reinserted on the H-DZSM-5 zeolite and with a stepwise heating treatment to 700C̕ for 16-18 hours, the pore size decreases to 4.6Å. This process is termed DRS, which stands for: Desilication, Reinsertion and Stabilization. To get this pore narrowing, there is a reaction that occurs between the zeolite's and the orthosilicate's silanols during the thermal treatment. This reaction results from dehydroxylation and formation of siloxane bonds. Selective adsorptive tests with the BTX aromatics and catalytic runs performed with methanol at various reaction temperatures, confirm the enlargement due to the DS treatment, and the pore narrowing as the result of the DRS treatment. With the goal of better understanding the mechanism involved in the DRS process and to improve the success rate, we passed the sodium orthosilicate through an acid resin and then an amount of 8 wt.% of the resulting orthosilicic acid was reinserted into the H-DZSM-5. The sample was then activated to 700C̕ for a period of 16-18 hours and a measure of the median pore size gave us 4.4Å with an improved reproducibility. The reaction temperature between the zeolite's and the orthosilicic silanols was determined with the Differential Scanning Calorimetry/Thermal Gravimetric Analysis (DSC/TGA) instrument to be ca. 600C̕. The product of this reaction gives us the formation of new siloxane bonds which are responsible for pore narrowing. Selective adsorption tests with n-heptane and isooctane, catalytic runs performed with methanol at various reaction temperature, the determination of the constraint index and the internal space, confirmed the pore narrowing as the result of the DRS treatment.