Crystallization Behavior of Zeolite Beta in OSDA-Free, Seed-Assisted Synthesis

Abstract

Recent reports on the organic structure-directing agent (OSDA)-free synthesis of some zeolites with the aid of seed crystals have opened a new way to the robust and environmentally friendly production of industrially valuable zeolites. However, the details on the crystallization behavior as well as the role of the seeds have not been fully clarified yet. In this study, the crystallization process of zeolite beta in the OSDA-free, seed-embedded Na+−aluminosilicate gel system, which never yields beta in the absence of the seeds, is investigated in detail. The XRD and TEM studies of the solid aluminosilicate products in the course of the hydrothermal treatment suggest that the crystallization of zeolite beta proceeds on the outer surface of amorphous aluminosilicates. The Raman spectroscopy, solid-state 27Al and 23Na MAS NMR and high-energy XRD analyses of seeded and nonseeded amorphous materials just before crystallization reveal that the beta seeds induce no major changes in their structures, implying that the nucleation of beta does not occur directly from the amorphous phase. The intermediate addition of the seeds after prehydrothermal treatment of a nonseeded gel enhances the crystallization rate and results in the increased number of beta crystals with smaller size. It is elucidated that, during the hydrothermal treatment, the beta seeds embedded in the gel provide crystal growth surface after they are exposed and/or released to the liquid phase by partial dissolution of the amorphous aluminosilicates. These findings provide a promising approach to the designed syntheses of valuable zeolites in the completely OSDA-free system.

Consequences of secondary zeolite growth on catalytic performance in DMTO studied over DDR and CHA

“…44 Authors showed further that it is possible to synthesize zeolites using seeds of other topology, which have common building units. The mechanisms proposed by Xiao et al 21,43 and Okubo et al 20 for the SDA-free synthesis of zeolites, in principle, hold well for this work . However, considering the very small amount of seeds used for zeolite synthesis in this work, we tentatively suggest that dissolution of seeds proceeds to much smaller aggregates in comparison to previously mentioned works, giving rise to nuclei which act as a core to build new zeolite crystals.…”

“…For example, Yu et al 19 reported ultrafast synthesis of SAPO-34 in the presence of seeds in 10 min. The faster formation of products is attributed to the fact that addition of seeds suppresses the necessity of a nucleation step either by partial dissolution of seeds to provide growth surface for new crystals 20 or via a core-shell growth mechanism. 21 Naturally, fast crystal growth might lead to the formation of some framework imperfections, which in turn can affect physical and chemical (read as catalytic) properties.…”

Rational synthesis of Beta zeolite with improved quality by decreasing crystallization temperature in organotemplate-free route

“…Later, Majano et al also reported seeded synthesis of Beta zeolite by the use of a little Beta nanocrystals containing small amount of organic template in the absence of additional organic templates [37]. Subsequently, Kamimura et al demonstrated the effects of initial gel composition, quantity of seed crystals, and crystallinity of the seeds, as well as the Si/Al ratios on the crystallization behavior of the Beta phase [38,39]. Xie et al have demonstrated the core-shell structure mechanism on the formation of Beta zeolite from seeds-directed synthesis (Beta-SDS) [40].…”

Direct Synthesis of Functional Zeolitic Materials

“…High-silica ferrierite, with Si/Al ratio close to 15, was first synthesized in OSDA-free conditions using CDO crystals as seeding [209]. Mordenite is an important catalyst in several petrochemical processes (such as hydrocracking, reforming, alkylation, among other), Reproduced from [207].…”

“…Interestingly, nucleation and crystallization mechanisms on OSDA-free conditions in the presence of zeolite seeds were studied in detail by these researchers. OSDA-free beta zeolite was first selected as the model case for the study [207]. From detailed characterization measurements, Okubo et al proposed a mechanism based on different steps for the crystallization of OSDA-free beta material (see scheme in Figure 14).…”