Comparative Study of Zeolite L Etching with Ammonium Fluoride and Ammonium Bifluoride Solutions

Abstract

A commercial zeolite L was subjected to chemical etching with aqueous solutions of NH 4 F and NH 4 HF 2 to generate secondary porosity. The derived samples were characterized by X-ray diffraction, N 2 adsorption, scanning and transmission electron microscopy, 27 Al MAS NMR, thermogravimetric analysis, and their acid sites were quantified by FTIR spectroscopy of adsorbed basic probe molecules. The catalytic properties of the pristine and modified zeolites were evaluated in the dealkylation of 1,3,5-triisopropylbenzene. Tuning the etching conditions allowed us to generate hierarchical zeolites without losing micropore volume. Furthermore, both fluoride etchants did not change the acid properties of the zeolite substantially, which is confirmed by chemical composition anlysis (ICP-AES) and by FTIR spectroscopy of pyridine. The dissolution rate of zeolite L was much faster with NH 4 HF 2 than NH 4 F. The set of experimental data shows that the 1 wt % NH 4 HF 2 solution is an efficient etchant able to generate mesopores without affecting the parent zeolite properties as lower amounts of fluoride source are required than with concentrated (20-40 wt %) NH 4 F solutions. Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff))

Regulation of the Si/Al ratios and Al distributions of zeolites and their impact on properties

“…However, the pH of the solution needs to be carefully controlled to prevent the framework collapse when hydrogen fluoride (HF) is used. Besides HF, NH 4 F, 102,118,119 NaF, 119,120 NaHF 2 , 119,121 and NH 4 HF 2 102,119 have also been used as the source of fluoride. Moreover, dealumination by For HF 2can also generate the secondary porosity.…”

“…Moreover, dealumination by For HF 2can also generate the secondary porosity. 102,119,120,122 (NH 4 ) 2 SiF 6 is also a widely used dealumination agent, 123 which can not only extract the aluminium from the framework but also can serve as an extraneous source of silicon that can fill up the vacancies created in the framework by the extraction of aluminium. [123][124][125][126][127] For example, Yokoi and co-workers tuned the Si/Al ratio of the Al-rich Beta to 73 and removed more than 40% of the Al atoms of MCM-22 by treating with (NH 4 ) 2 SiF 6 solution.…”

“…The foundation of the "top-down" strategy is that the tension of the tetrahedral sites (T-sites) in the framework of a zeolite is not uniform throughout the framework and depends on its position, which makes it possible to selectively remove Si (desilication) or Al (dealumination). [100][101][102][103] The routes for the dealumination of zeolites via post-modi-cation include acid, uoride, steam, and salt treatment. During dealumination by acid treatment, Al species with high tension are dissolved using an acid.…”

Preparation of hierarchical SSZ-13 by NH4F etching

“…[48,49] Thus, the deactivation is slower, with the sample having a higher external surface due to introduced mesopores Previous studies of NH 4 F or HF buffered with NH 4 F etching focused on medium-pore (10MR) zeolites like MFI and FER and large-pore (12MR) zeolites like FAU, LTL, MOR, and Beta. [42,[50][51][52][53] Compared with previous studies, the dissolution of the small-pore SSZ-13 shows some differences. For instance, Qin et al also used 40 w/% NH 4 F solutions to etch ZSM-5 upon similar experimental conditions.…”

Busting the efficiency of SAPO-34 catalysts for the methanol-to-olefin conversion by post-synthesis methods

“…This is possible by employing the bi-fluoride anion as an etching agent, achieved by mixing HF and NH 4 F. [42] Later, the same group reported the unbiased chemical etching based on the sole use of NH 4 F. [43,44] The fluoride post-etching method is very flexible and controllable for regulating both the density and size of the secondary pore system. [45][46][47][48][49] The unbiased chemical etching of zeolite framework is particularly appropriate for the etching SAPO-type materials. In 2016, Chen et al applied the fluoride post-synthesis route to fabricate hierarchical SAPO-34 zeolites.…”