Hydrophobicity, fluorinated zeolites

DAY zeolites obtained by fluorination of Y zeolites are also hydrophobic (102). This is due to the low concentration of OH groups in these materials as shown by their I.R. spectra. 

The importance of recognizing and dealing with zeolite synthesis as a kinetic process that involves the isolation of metastable phases is pointed out in this book in a variety of ways. An examination of the extensive scientific and patent literature on zeolite synthesis rapidly convinces one that a lack of understanding of this point has been a major bottleneck in the characterization of zeolite chemical and physical properties. The zeolite properties are defined not only by synthesis parameters, but also by treatment following synthesis for example, most synthesis treatment of zeolites with fluorine can be used to modify hydrophobicity drastically and increase catalytic activity for n-butane cracking. 

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. 

In contrast, the 500°C-calcined fluorine-treated NH4,K-L and NH4,TMA-fl show about the same capacity loss ( 15 vol %) for water (at 4.6 torr) as for oxygen (at 100 torr). Therefore, these two zeolites apparently did not develop hydrophobicity after fluorination. 

A simple shake test with aqueous 1 vol % n-butanol solution has been developed to test the selective adsorption of organic molecules over water molecules by a hydrophobic and organophilic adsorbent (26). Table V gives the results for severed, fluorine-treated zeolites and their untreated counterparts. The data clearly indicate that fluorine treatment of LZ-105, H-zeolon and NH4Y materials substantially increases their selectivity for n-butanol over water. 

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. 

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