Infrared after dealumination

Fig.21. Infrared spectra in the OH stretching region ofY-type zeolite after dealumination via steam treatment. Calcined in A moist air at 813 K B self-steam at 813 K C moist air at 1033 K D self-steam at 1033 K [411]...

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. 

Gasoline selectivity is influenced by both the method of dealumination and steam treatment and, hence, depends on framework acid sites and on presence of extraframework material. Both framework and extraframework sites contribute to the overall zeolite acidity. Framework Al(IV) acid sites are associated with Bronsted acidity whereas extraframework Al(VI) acid sites are associated with Lewis acidity(21). Calcined AFS samples are reported (22) to contain greater Bronsted acidity than USY samples from infrared characterization and to have stronger acidity as measured by ammonia desorption. As a result, the stronger acidity of calcined AFS samples cracks hexadecane to lower molecular weight products than USY. After steaming, the acidities of both AFS and USY are reduced to similar levels and lead to similar gasoline selectivities which are increased relative to the calcined zeolites. 

Ceckiewicz studied the methanol conversion between 25 and 400 C on zeolite H-T (SiOj/AljOj molar ratio of 7.4) with different degrees of decationization and dealumination. By Fourier transform infrared spectroscopy (FT-IR) he found that CH3OH molecules interact principally with the most active 3,600 cm OH groups corresponding to Al-OH bonds. No transformation of methanol occurred at 25 C. The dehydration of methanol to dimethyl ether was the most important reaction between 200 and 300 C. The conversion of methanol to hydrocarbons was evident at temperatures higher than 300 C. The conversion of methanol was less than 100% in all the cases. After... 

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