Abstract
Owing to its excellent catalytic performance, ZSM-5 zeolite has been extensively used in petrochemical industries. However, when ZSM-5 zeolite is used in various catalytic reactions, even its composition remains the same, its activity and selectivity may still vary due to, for example, the type of alumina distribution (namely, the eggshell type, eggwhite type or the uniform distribution type), crystalline size, and morphology (namely, the single crystalline or the crystalline aggregate). These factors are closely correlated to the starting materials and synthesis conditions applied during the synthesis of the ZSM-5 zeolite. While there exists many patents and reports on the study of the synthesis of ZSM-5 zeolite, few were focus on the impact of starting materials and synthesis conditions on the distribution of aluminum atoms and the morphology of the synthesized crystal, and there exists many discrepancies in the existing literatures. In this work, we investigated the effects of starting materials and synthesis conditions on the synthesis of ZSM-5 zeolite with varied crystalline sizes ranging from ca. 0.01 to 100μm. A variety of different analytical methods were used to characterize the physical properties of the synthesized materials, including surface survey, structural examination, depth distribution and composition analyses. Moreover, an in-depth investigation on the "Controlled Surface-Crystallization Process" was also performed to provide the criteria for synthesizing high-performance ZSM-5 zeolite. This investigation was based on the concept of seeding crystallization and cooperated the exact determination of equilibrium concentration of silica. A crystallization procedure of the pure silica-layer with MFI structure participated in the ZSM-5 synthesis process. This procedure not only shifted the acidic site from the external surface to the inside of ZSM-5 zeolite, but also retained the structural consistency and communicated with each other. The results of this work not only provided valuable information on the surface modification of ZSM-5 zeolite, but also could referred to other zeolitic catalysts. It was found that the ZSM-5 zeolites, synthesized with different starting materials, had different alumina distribution and morphology. For those synthesized using a high concentration of starting material and OH- anion, such as those synthesized by colloid silica and sodium aluminate, possessed the morphology of crystal-aggregates, which consisted of spherical particles of various sizes. A uniform alumina distribution was found for these crystal-aggregates. On the other hand, ZSM-5 zeolites synthesized using the sodium silicate and aluminum sulfate showed both single-crystal and crystal- aggregate morphologies. The alumina distribution of the single-crystal exhibited as the combination of the eggwhite and yolk types, whereas the crystal-aggregates exhibited as the uniform distribution type. However, the alumina distribution, crystalline size distribution and the morphology were found to vary markedly with the kind and nature of the additive. For example, in the presence of potassium fluoride additive, the morphology of the ZSM-5 zeolite using colloid silica and sodium aluminate as starting materials tended to change from the crystal-aggregates to the single-crystal type. Meanwhile, the alumina distribution would alter from the uniform distribution to the eggwhite type. On the other hand, those synthesized using sodium silicate and aluminum sulfate as starting materials would turn from the single-crystal and/or the crystal-aggregates to only the single-crystal type. In this case, some single-crystals with strip- and rod-like morphology were evident. The alumina distribution however maintained as the combination of the eggwhite and yolk type. Besides, in the presence of urea additive, the morphology of the ZSM-5 zeolite using colloid silica and sodium aluminate as starting materials tended to change from the crystal-aggregates to the single-crystal type with a plane hexagonal form and/or plane hexagonal twin form. Meanwhile, the alumina distribution would alter from the uniform distribution to the combination of the eggshell and eggwhite type. On the other hand, those synthesized using sodium silicate and aluminum sulfate as starting materials would turn from the single-crystal and/or the crystal-aggregates to only the crystal-aggregates type. Meanwhile, the alumina distribution would alter to the uniform distribution. Finally, for ZSM-5 zeolites that were subjected to the "Controlled Surface-Crystallization Process", a notable difference was found in both the amount of surface aluminum atoms and the depth distribution curve of the silica to alumina ratio. The post-synthesis treatment not only served to decrease the amount of aluminum atoms on the external surface of the ZSM-5 zeolite, but also shifted the depth distribution curve of the silica to alumina ratio toward a higher value.