Elucidation of the role of potassium fluoride in the chemical and physical nature of ZSM-5 zeolite

This investigation synthesized the ZSM-5 zeolite using sodium silicate, aluminum sulfate and tetrapropylammonium bromide. In addition to examining how the KF additive affected the crystal structure, crystallinity, crystal size and crystal morphology by XRPD and SEM measurements, this work investigated how the KF additive affected the specific surface area, Al contents and zeolite framework through BET, ICP-MS, SIMS, 29Si, 27Al-MAS NMR and 29Si, 27Al-CP MAS NMR measurements. Experimental results indicated that adding an appropriate amount of KF did not affect the product yield, crystal structure, Al contents or the specific surface area of the synthesized ZSM-5 zeolite. However, adding KF slightly degraded the crystallinity of the product and significantly increased crystal size. High concentrations of KF additive also changed the morphology to a mixture of rod-type and strip-type single crystals. Furthermore, the KF additive could lead the non-framework six-coordinate Al species to transfer to the framework four-coordinate Al species in the synthetic process of ZSM-5 zeolite, and induced further defects in the zeolite framework, i.e., the Q3(0Al) and Q3(1Al) units belonging to the SiO− (or SiOH) defect centers.