Study On Static Synthesis And Crystallization Mechanism Of ZSM-35 And Fe-ZSM-35 Zeolite

ZSM-35 zeolite (FER) shows excellent properties in skeletal isomerization and absorption because of the unique channels. Iron-substituted ZSM-35 (Fe-ZSM-35) zeolite modifies the physicochemical properties, like acidity strength. ZSM-35 zeolite has been widely used in petroleum refining, petrochemistry and fine chemical industries. Understanding the process of synthesis and crystallization mechanism of ZSM-35 zeolite and Fe-ZSM-35 zeolite are a very interesting but challenging topic. In this paper, pure ZSM-35 zeolite was synthesized in a static hydrothermal system. Fe-ZSM-35 zeolite was synthesized by the novel and effective sub-step crystallization of synthetic method, which can promote the insertion of iron in the ZSM-35 framework. The crystallization mechanisms of ZSM-35 zeolite and Fe-ZSM-35 zeolite have been studied through UV Raman and Resonance Raman spectroscopies in combination with X-ray powder diffraction patterns and UV-vis diffuse reflectance spectroscopy. The following progresses have been made:Using cyclohexylamine as the template and Si-sol as the Si source pure ZSM-35 zeolite was synthesized in hydrothermal system at 220℃. When the initial gel contains a certain amount of potassium ions, the mutual role of potassium and sodium ions inhibits the formation of mordenite. Well crystallized products were formed with n(K+)/n(K++Na+)=0.3.Optimum regions of the synthetic conditions and crystallization mechanism were studied by X-ray diffraction and UV-Raman spectroscopy. The building units in the precursors were identified as five-membered and six-membered silicate rings. The intensity of the Raman band at 450 cm-1 increases in the beginning of crystallization and the new bands at 421,312, and 215 cm-1 appear with consuming the amorphous gel, which turned out to be the aggregation of these ring species through hydrothermal treatment.Fe-ZSM-35 zeolite was synthesized in a static hydrothermal system by sub-step crystallization of synthetic method, which consists of rational chosen the iron-containing silicate clusters with proper coordination environment, condensation with the semi-crystallized ZSM-35 fragments, and futher crystallization at 220℃using cyclohexylamine as the template and Si-sol as the Si source. This method promotes the insertion of tetrahedrally coordinated iron in the zeolite framework. It is advanced that high content of iron, absent Fe2O3 phase, short crystallization time, easy operation and high degree of crystallization. The resonance Raman bands of the framework irons in Fe-ZSM-35 zeolite are detected and assigned. The evolutions of both the zeolite framework and iron species were identified selectively by using different excitation laser lines. It was found that the precursors contain five-membered and six-membered silicate rings and iron atoms in distorted tetrahedral coordination in the early synthesis stage. These species connected with each other to form the nuclei of FER structure. Finally, the nuclei via consuming amorphous and tetrahedrally coordinated iron atoms completely formed Fe-ZSM-35 crystals.