On the Mesoporogen‐Free Synthesis of Single‐Crystalline Hierarchically Structured ZSM‐5 Zeolites in a Quasi‐Solid‐State System

Abstract

Hierarchically structured zeolites (HSZs) have gained much academic and industrial interest owing to their multiscale pore structures and consequent excellent performances in varied chemical processes. Although a number of synthetic strategies have been developed in recent years, the scalable production of HSZs single crystals with penetrating and three-dimensionally (3-D) interconnected mesopore systems but without using a mesoscale template is still a great challenge. Herein, based on a steam-assisted crystallization (SAC) method, we report a facile and scalable strategy for the synthesis of single-crystalline ZSM-5 HSZs by using only a small amount of micropore-structure-directing agents (i.e., tetrapropylammonium hydroxide). The synthesized materials exhibited high crystallinity, a large specific surface area of 468 m(2) g(-1) , and a pore volume of 0.43 cm(3) g(-1) without sacrificing the microporosity (≈0.11 cm(3) g(-1) ) in a product batch up to 11.7 g. Further, a kinetically controlled nucleation-growth mechanism is proposed for the successful synthesis of single-crystalline ZSM-5 HSZs with this novel process. As expected, compared with the conventional microporous ZSM-5 and amorphous mesoporous Al-MCM-41 counterparts, the synthesized HSZs exhibited significantly enhanced activity and stability and prolonged lifetime in model reactions, especially when bulky molecules were involved.

Surface‐Passivated Hierarchically Structured ZSM5 Zeolites: High‐Performance Shape‐Selective Catalysts for para‐Xylene Production

“…Figure a shows the XRD patterns of the synthesized HSZ series materials and microporous ZSM‐5 zeolites in the 2 θ range of 5–50°. All of them demonstrate the typical and similar diffraction patterns of MFI‐type zeolite irrespective of post‐synthesis dealumination or CLD of TEOS treatment, indicating their high crystallinity and high stability . Figure b shows their N 2 sorption isotherms and the textural properties are summarized in Table .…”

“…Accordingly, Figure a gives the NH 3 ‐TPD profiles of HSZ series and microporous ZSM‐5 and the calculated acid amounts are listed in Table . All of the HSZ series feature two desorption bands similar to that of microporous ZSM‐5, in which the high‐temperature and low‐temperature band correspond to the strong and the weak acid sites, respectively ,. Because the strong acidity of ZSM‐5 zeolites originates from the isomorphous substitution of framework Si by Al, the change of Al content and/or Al coordination state would induce the variation of acid amount and/or acid strength.…”

“…Herein, a strategy of passivating the external surface of as‐synthesized HSZ for enhanced activity and high selectivity towards p ‐X production is reported. First, HSZ was synthesized by a mesoporogen‐free synthetic route previously reported by our group . Then the external surface acidity of HSZ was fine‐tuned by dealumination in oxalic acid solution or CLD of TEOS.…”

State of the Art and Perspectives of Hierarchical Zeolites: Practical Overview of Synthesis Methods and Use in Catalysis

“…[171] Ge et al succeeded in the synthesis of single-crystalline mesoporous ZSM-5 having an abundant 3D interconnected network based on the SAC method using small amounts of TPAOH. [172] The method was also applicable for high-silica crystals. [173] Similarly, Rilyanti et al also used a TPA + lean SAC, and found that the presence of mesopores is attributed to the presence of severe defect sites.…”

Controllable fabrication of single-crystalline, ultrafine and high-silica hierarchical ZSM-5 aggregates via solid-like state conversion

“…2C), which demonstrates that the introduction of mesopores maintains the single crystal integrity. 36,37 Most importantly, these ultrane nanocrystals tend to agglomerate into the larger secondary particles with a loose structure, which may be attributed to their high surface Gibbs free energy. 38 The abundant intercrystalline mesopores in the HA-ZSM-5 zeolite were formed by aggregation of ultrane nanocrystals with sizes of 10-20 nm.…”