Dissolution Behavior and Varied Mesoporosity of Zeolites by NH₄F Etching

Abstract

The mesopores formation in zeolite crystals has long been considered to occur through the stochastic hydrolysis and removal of framework atoms. Here, we investigate the NH 4 F etching of representative small, medium, and large pore zeolites and show that the zeolite dissolution behavior, therefore the mesopore formation probability, is dominated by zeolite architecture at both nano-and subnano scales. At the nano-scale, the hidden mosaics of zeolite structure predetermine the spatio-temporal dissolution of the framework, hence the size, shape, location, and orientation of the mesopores. At the sub-nano scale, the intrinsic micropore size and connectivity jointly determine the diffusivity of reactant and dissolved products. As a result, the dissolution propensity varies from removing small framework fragments to consuming nanodomains and up to full digestion of the outmost part of zeolite crystals. The new knowledge will lead to new understanding of zeolite dissolution behavior and new adapted strategies for tailoring hierarchical zeolites.

Tailor-Made Nanoseeds for the Synthesis of Zeolites with Nanosized Dimensions

“…This is because the seeds are prepared by permanent removal of framework Si and Al atoms. Especially for those zeolites with medium or small pores, or large-pore zeolites with only a one-dimensional channel system, such as MFI, MOR, TON, the dissolution of these zeolite frameworks follows a reversed layer-by-layer mechanism. , In this case, the particle size of the dissolved products can be controlled conveniently through the tuning of etching time, etchant concentration, etc. , Also, for this reason, we believe this protocol can be broadly applied to many systems for the preparation of the initial seeds for the synthesis of nanosized zeolites, as this method has the capacity to turn large zeolite crystals into nanosized grains with preserved crystallinity, provided that the type of zeolites can be dissolved following the reversed layer-by-layer manner. In the present work, this is confirmed by the disassembly of both mordenite and ZSM-5 of an aggregative nature (Figures a and a).…”

Impact of residual sodium cations in azonia-spiro templates on the formation of large and extra-large pore zeolites

“…Besides, the rough surface morphology of the MTW zeolites indicated that the star-like shape might have formed by subgrains, suggesting that nucleation occurs simultaneously at multiple positions in the gel. It is highly possible that the interfaces between these subgrains contain defects, creating interfacial tension [50]. By contrast, MTW zeolite with an elongate and smooth morphology was formed with another small flexible molecular triethylmethylammonium template [47].…”

In situhierarchical pore engineering in small pore zeoliteviamethanol-mediated NH4F etching

“…19 However, it is noteworthy that the etching processes using NH 4 F aqueous solution are signicantly controlled by the diffusion of active etching species, which are formed in the solution aer the hydrolysis of NH 4 F. Besides, the already existing active species in NH 4 F aqueous solutions tend to cluster together through hydrogen bonding with H 2 O and NH 4 + , which further increases the diffusion resistance and limits the accessibility into the zeolite's interior. 23 This difficult diffusion leads to the characteristic behaviours where etching typically initiates at the outer surface and subsequently progresses into the interior of the zeolite crystal. As a result, a signicant concentration gradient of etching species forms within the zeolite.…”

“…This, in turn, results in a highly irregular pore structure and a less ordered mesoporous system with pore size distributions ranging from 10 nm to 100 nm. 15,23,24 The diffusion resistance is especially signicant in smallpore zeolites such as LTA and CHA zeolites, which have narrow intrinsic 8-membered ring windows (0.38 nm). 23,25 In literature, various etching parameters have been carefully attempted, such as temperature, liquid-solid ratio, concentration, and etching time, 26 however, the etching process in smallpore SSZ-13 zeolites remains primarily on the outside surface and does not affect the internal frameworks.…”

“…Similar results have also been observed in LTA and TON zeolites. 23 Therefore, creating hierarchical structures for small-pore zeolites using post-treatment methods while protecting the surface porous framework is still a major challenge, and there is a high demand for solutions that can result in uniform and controllable pore engineering.…”