Dealumination fluorination

More recently, dealumination was achieved by fluorination of zeolites at ambient temperature with a dilute fluorine-in-air stream, followed by high-temperature calcination (102). The suggested reaction mechanism involves the formation of different aluminum-fluorine compounds along with zeolites containing hydroxyl and fluorine nests. During the high-temperature calcination, it is assumed that silica insertion occurs, similar to the scheme in Figure IB. 

Partially proton-exchanged Na faujasite X, in turn, is the best catalyst for selective monochlorination with tert-butyl hypochlorite.258 NaX, NaY, and NaKL zeolites used in the chlorination of toluene with sulfuryl chloride undergo rapid deactivation because of the accumulation of polychlorinated toluenes in the pores of the catalysts and dealumination.259, 260 Direct electrophilic fluorination of aromatics can be effected by using Selectfluor in the presence of triflic acid.261 Electrophilic fluorination may also be carried out by R2NF and R3N+FA reagents.262 Elemental fluorine may also act as a powerful electrophile in acidic media (sulfuric acid, trifluoroacetic acid, or formic acid), but monosubstituted aromatics give isomeric mixtures.263-265... 

Dilute fluorine gas (0-20%) can be used to treat zeolites at near-ambient temperature and pressure. Most of the resulting materials retain very high crystallinity even after 600°C postcalcination for two hours. Both framework infrared spectra and X-ray powder diffraction patterns clearly show structural dealumination and stabilization. The hydrophobic nature of the fluorine-treated and 600sC-calcined material is shown by a low water adsorption capacity and selective adsorption of n-butanol from a 1 vol.% n-butanol-water solution. Fluorination also changes the catalytic activity of the zeolite as measured by an n-butane cracking method. 

The spectrum for the 600°C-calcined fluorine-treated erionite sample shows substantial shifts in band positions, but band sharpening is less obvious. The bands at 1082, 792, 578, 470 and 438 cm- are shifted to 1098, 814, 585, 477 and 444 cm , respectively, after fluorine treatment and 600°c calcination. The large shifts observed are evidence of dealumination. The splitting of the 1082 cm-1 band into a doublet located at 1098 and 1085 cm-1 and some degree of band sharpening imply structure stabilization for fluorine treated erionite. 

Finally, I. R. spectra for fluorine-treated and then 500°C-calcined nh4,K-L and NH4,TMA-fl (Figures 2c and 2d) show an upshift of band positions, but loss of spectral resolution. Thus, the I. R. results indicate that the two zeolites undergo structural dealumination by fluorine treatment and subsequent calcination, but both show no evidence of structure stabilization. 

The results indicate that both NH4,TMA-fl and NH.,K-L are de-aluminated upon fluorination. Strong supporting evidence comes from framework I. R. data where the shifts in band position to higher wave numbers are as much as 20 cm-1. However, there is no evidence of structure stabilization. Also McBain water adsorption data give no indication of surface hydrophobicity. Therefore, it is likely that structure defects are formed in these two zeolites as a result of dealumination and cause low thermal stability. 

High degrees of dealumination are difficult to achieve using AHFS compared with those obtained via hydrothermal treatment, because of loss of framework crystallinity. The characterisation techniques used here have shown that silicon enrichment occurs during the AHFS treatment, leading to higher bulk Si/Al ratios. 7a1 MAS NMR appears to show the presence of aluminium species other than those teU a- or octahedrally co-ordinated. These may be the fluorinated aluminium species mentioned in earlier works. The textural properties of AHFS treated zeolites are not changed relative to the parent material in contrast to the steam dealuminated zeolites, where the introduction of secondary mesopores occurs. 

Additional reflections at 20 = 18,43 21,26 30,30 appear in the X-ray diffractions patterns of the samples with high degree of dealumination. These reflections correspond to extraframework aluminium species containing fluorine, which, as reported in [3], are not washed out from zeolite cavities even by repeated washing due to their strong interaction with the zeolite and dispersion in the crystallite bulk. 

The research of adsorption properties has shown, that adsorption isotherms of benzene for dealuminated (NH4)2SiF6 zeolite lay below adsorption isotherms of benzene for NaY obtained even at more high adsorption temperatures. In a fig. 1 are shown adsorption isotherms for a sample 2. It is characteristic that at different temperatures (180, 200, 230 C) isotherms are not divided. Apparently, it is connected that so a little benzene is kept on a surface, that the distinction in adsorption at given temperature interval is not fixed. Probably, the samples contain impurities of complex compounds of aluminium with fluorine, that is agreed XRD data. Also relative concentration of defect sites of structure is increased during dealumination, that also can affect on adsorption property. 

Direct fluorination of zeolites has been reported as a method for increasing their acidity for hydrocarbon cracking reactions. Lok et al. treated various zeolites with fluorine gas and reported zeolite dealumination and stabilisation and in certain cases an increase in n-butane cracking activity.20 It appears that the optimum fluorine content for maximum cracking activity is about 1% m/m. 

NMR spectroscopy has been used to study the effect of leaching agent in the dealumination of stabilised Y zeolites. The influence of surface acidity of modified Y zeolites has been investigated by MAS NMR spectroscopy. " The dealumination of Nd-, Sm-, Gd- and Dy-containing Y zeolites has been investigated by A1 and Si NMR spectroscopy. The interaction between CuCl and HY zeolite has been studied using A1 and Si NMR spectroscopy. The reaction between HY zeolite and molecular fluorine has been followed by and... 

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