Hexafluorosilicate, ammonium

Similarly, the addition of triethylaluminum to the lithium reagent leads to a high a-regioselectivity (eq 4). A stereocontrolled s)ui-thesis of anh-homoallylic alcohols is possible by preparing the corresponding allylic boronic ester and subsequent reaction with an aldehyde. A stereoselective elimination allows the preparation of either ( )- or (Z)-dienes (eq 5). The isomeric reagent (1) is a useful reagent for the preparation of 2-substituted polyfunctional allylic silanes (eq 6).  

Ayalon-Chass, D. Ehlinger, E. Magnus, P., J. Chetn. Soc., Chetn. Commun. 1977, 772. 

Physical Data d 2.01 gcm decomposes prior to melting odorless. 

Form Supplied in white crystalline solid, widely available. 

Handling, Storage, and Precautions store in plastic/poly airtight containers, in a cool, dry place, away from acids and oxidizers. Unsuitable for glass or ceramic containers. Stable compound decomposes upon heating to HF, ammonia, and nitrogen oxide (NOjc) gases. Handle with care in well-ventilated area. Irritant to skin and mucus membranes, however, acute and chronic toxicity of this substance is not fully known. 

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Cryolite, see Sodium hexafluoroaluminate Cryptohalite, see Ammonium hexafluorosilicate Cupric and cuprous, see under Copper Cuprite, see Copper(I) oxide... 

AMMO 2.5 EC , cypermetlu-in, 13 Ammonia, 13 Ammonium acetate, 13 Ammonium arsenate, 13 Ammonium benzoate, 13 Ammonium bicarbonate, 13 Ammonium bifluoride, 14 Ammonium bisulfite, 14 Ammonium carbamate, 14 Ammonium carbonate, 14 Ammonium chloride, 14 Ammonium chlorplatmate, 14 Ammonium clu omate, 14 Ammonium citrate, 14 Ammonium diclu omate, 14 Ammonium fluoride, 14 Ammonium fomiate, 15 Ammonium hexafluorosilicate, 15 Ammonium hydroxide, 15 Ammonium metavanadate, 15 Ammonium molybdate, 15 Ammonium nitrate, 15 Ammonium oxalate, 15 Ammonium perfluorooctanoate, 15 Ammonium persulfate, 15 Ammonium phosphate, 15 Ammonium picrate, 16 Ammonium salicylate, 16... 

Evidence that something more than a proper Alf distribution is needed to create strong acidity first came from the experiments of Beyerlein et al. (7). Using the acid-catalyzed conversion of isobutane as a measure of strong acidity, they found that a dealuminated zeolite prepared by treatment with ammonium hexafluorosilicate (AHF) exhibited much less carbonium ion activity than might be expected, based on the number of Alf atoms. This treatment leaves very little extraframework Al in the zeolite. When the sample was mildly steamed, the activity became considerably greater. The authors concluded that the enhanced acidity was a result of a synergism between the framework Bronsted sites and the Lewis sites associated with extraframework aluminum. 

Y-type zeolite prepared by steaming a, Y-type zeolite dealuminated with ammonium hexafluorosilicate , after La3+ exchange to level of maximum activity , ZSM-20 g, after La3+ exchange to level of maximum activity v, as synthesized zeolite Y , after exchange to level of maximum activity , HLa-X. Reproduced with permission from Ref. 20. 

The initial rate of 2-MN acetylation depends on the framework Si/Al ratio of the zeolite catalyst.[27] For a series of dealuminated BEA samples (by treatment with hydrochloric acid or with ammonium hexafluorosilicate), the acetylation rate passes through a maximum for a number of framework A1 atoms per unit cell (/VA() between 1.5 and 2.0 (Si/Al ratio between 30 and 40). The activity of the protonic sites (i.e. the TOF) increases significantly with Si/Al from 420 h 1 for Si/Al = 15 to 2650 h 1 for Si/Al = 90. It should be noted that similar TOF values could be expected from the next nearest neighbour (NNN) model. Indeed all the framework A1 atoms of the zeolite (hence all the corresponding protonic acid sites) are isolated for Si/Al ratio 10.5. Therefore the acid strength of the protonic sites is then maximal as well as their activity.[57,58] This was furthermore found for m-xylcnc isomerization over the same series of BEA zeolites.1271 This increase in TOF for... 

Dealumination with ammonium hexafluorosilicate, developed by Skeels and Breck47, was mainly applied to HFAU zeolites. NH4 zeolites are treated with solutions of (NH4)2SiF6 under controlled pH conditions at 80°C. Aluminium is removed from the... 

In accord with this proposal, the removal of these EFAL species by acid treatment (17) or with ammonium hexafluorosilicate causes a significant decrease in acidity and catalytic activity. 

In a series of zeolite samples resulting from acid dealumination of HBEA15, a maximum in acetylation and isomerization activities was found for a Si/Al ratio between 20 and 35 (30). This maximum in activity can be explained by the opposite effect of the increase with Si/Al of the ease of desorption of the bulky and polar acetylation products and of the decrease in the number of protonic sites. The selective elimination of Lewis acid sites by treatment of HBEA15 with ammonium hexafluorosilicate allows one to confirm that these sites do not directly participate in acylation reactions over zeolites (30). 

Chemical dealumination involves reaction of the zeolite framework with any one of a variety of reagents(2). In this work, zeolites were reacted with ammonium hexafluorosilicate in aqueous solution(9-12) to prepare dealuminated products. Aluminum was extracted from the zeolite framework and removed from the crystal as a soluble fluoroaluminate complex the resulting lattice vacancies are believed to be filled by silicon in solution. Composition profiles of chemically dealuminated zeolites (AFS) are homogeneous and indicate the entire crystal is accessible for dealumination(13). Sorption data indicate that AFS zeolites do not possess a secondary pore system although pore blockage may occur due to occlusion of fluoroaluminate species(13). 

The main objective of this work is to characterise the textural properties of a series of Y zeolites dealuminated by ammonium hexafluorosilicate treatment. It was observed that the fluorosilicate treatment produced a highly crystalline product with a contracted unit cell. Both textural and XRD analysis confirmed the samples to be at least 95% crystalline for dealumination degrees <50%. According to N2 adsorption only a minimal contribution of mesopores was observed. However, a notable loss of micropores accompanied by the formation of mesopores was noted for severe dealumination levels, along with a considerable structural degradation. 

In the present study, a commercial H-Y zeolite was dealuminated via the procedure described by Skeels and Breck [3,4] using ammonium hexafluorosilicate (AHFS) as the dealuminating agent under closely controlled conditions. The fluorosilicate method is attractive because it allows to produce silicon-enriched zeolites which are in principle perfectly microporous and exempt from framework defects and non-ffamework A1 species. Typically, the AHFS treatment differs from many of the dealumination methods in that it is carried out in aqueous media under relatively mild conditions [5]. 

The main objective of this work is to analyse the effect of chemical dealumination by ammonium hexafluorosilicate treatment on the textural properties of Y zeolite. 

Characterisation and Catalytic Properties of Dealuminated Zeolite-Y A Comparison of Ammonium Hexafluorosilicate and Hydrothermal Treatments... 

Zeolite beta samples with different framework and extraframework composition have been prepared by submitting the acid form of a commercial TEA-beta sample to different post-synthesis treatments, i.e. steam calcination, acid (HCl) leaching, and ammonium hexafluorosilicate (HFS) treatment. The samples were characterized by XRD, adsorption of N. at 77 K. i.r. spectroscopy with adsorbed pyridine, Si and Al MAS-NMR and XPS. Bifunctional catalysts were obtained by impregnation with 0.3 wt% Pt, and the catalytic activity for the isomerization of a simulated LSR feed (n-Cj/n-Cj, 60/40 wt%) was measured under different reaction conditions. 

Acid form of zeolite beta (Hp) was obtained from a commercial TEA-beta (Valfor CP806B-25) by calcination at 773 K for three hours to remove the template, followed by ion exchange with a 2M aqueous solution of NH4CI at 353 K for two hours, and a final calcination at 773 K for three hours. Then, the HP was dealuminated using three different procedures, i.e. steam calcination at 873 K during three hours (sample HPs,), acid treatment with HCl O.IM at reflux for two hours (sample HP, ), and ammonium hexafluorosilicate (HFS) treatment (sample HP ps2)- The latter two procedures produced almost EFAL-free beta samples. Moreover, the EFAL formed in HP during the calcination steps was also extracted with the required amount of ammonium hexafluorosilicate in order to avoid framework dealumination (sample HP fsi)- The HFS treatments were carried out in an ammonium acetate buffer at 348 K with slow addition of a 3M hexafluorosilicate solution (12 cm /h). Afterwards,... 

Dealumination and silicon-enrichment reaction of (NH SiF6 (AHFS) with zeolites Besides the hydrothermal method for preparation of ultra-stable Y zeolite (USY), Breck and Skeels[23] in 1983 invented a new secondary synthesis method for silicon-enriched zeolites. This method uses an ammonium hexafluorosilicate solution to remove the aluminum atoms from the framework structure of Y zeolite to the solution, and to insert silicon atoms back into the Al-removal vacancies in the framework so as to form a more or less perfect Y zeolite with a high Si/Al ratio. In comparison with the USY prepared by the hydrothermal method, the framework silicon-enriched Y zeolite obtained through the current technique possesses fewer framework hydroxyl vacancies, and the resulting zeolite has an ideal crystal lattice, and hence higher structural stability. Meanwhile, there... 

Min investigated the reaction mechanism of this dealumination approach.[24] Breck and Skeels had found that, in the ammonium hexafluorosilicate solution, the extraframework silicon atoms could substitute for the aluminum atoms in the crystal lattice of the zeolite framework, but the mechanism of the whole reaction process was not clear. Through 29Si,27Al-MAS-NMR and IR characterization techniques, Min and coworkers investigated the removal of A1 atoms from the zeolite framework, the chemical environment of the extraframework silicon atoms, and the species of the silicon that can be inserted into the Al-removal framework vacancies during the reaction process, and finally they proposed the reaction mechanism [Equations (6.18)-(6.25)] as follows. 

In the reaction process of dealumination and silicon-addition for zeolites with ammonium hexafluorosilicate solution, the ammonium hexafluorosilicate is first hydrolysed to form Si(OH)4, and the released F interacts with the framework Al atoms and... 

Influence of the treatment of Y zeolite by ammonium hexafluorosilicate on the physicochemical and catalytic properties application for chlororganics destruction... 

The objective of this work is to evaluate the dealumination via ammonium hexafluorosilicate treatment as an effective method for enhancing the catalytic performance of H-Y zeolite for oxidative destruction of chlorinated VOC. A series of Y zeolites with various Si/Al ratios was prepared from a commercial sample, then characterised and tested for the catalytic decomposition of chlorinated VOC (1,2-dichloroethane and trichloroethylene). In general, these modified Y zeolites exhibited a higher activity with respect to that of the parent material, the zeolite subjected to 50% dealumination resulting in the most active catalyst. This increase in activity was associated with the development of strong Bronsted acidity due to dealumination. 

Bronsted acidity. In the present study, an H-Y zeolite was dealuminated via the procedure described by Skeels and Breck [7,8] using ammonium hexafluorosilicate (AHFS) as the dealuminating agent under closely controlled conditions. 

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