Zeolite dehydration

Bulk amounts of elements were determined by atomic absorption spectrophotometry. The amount of framework A1 was determined by Al MAS NMR. The acidic properties of the metallosilicates were determined by IR and NH3-TPD measurements. Before the IR measurements, the sample wafer was evacuated at 773 K for 1.5 h. In the observation of pyridine adsorbed on metallosilicates, the sample wafer was exposed to pyridine vapor (1.3 kPa) at 423 K for 1 h, then was evacuated at the same temperature for 1 h. All IR spectra were recorded at room temperature. NH3-TPD experiments were performed using a quadrupole mass spectrometer as a detector for ammonia desorbed. The sample zeolite dehydrated at 773 K for 1 h was brought into contact with a 21 kPa of NH3 gas at 423 K for 0.5 h, then evacuated at the same temperature for 1 h. The samples were cooled to room temperature, and the spectra obtained at a heating rate of 10 K min from 314 to 848 K. 

Figure 4 ESR spectrum (low eld components) of CuNaY zeolites dehydrated at 800° C and taken at room temperature in the X-hand as a function of degree of exchange (numbers on the left)...

Zeolite Dehydrated zeolite Rehydrated zeolite (0 = 1) [Al(acac)3]... 

Figure 1. (a) The zeolite dehydration apparatus (b) the zeolite reduction apparatus... 

Ni-exchanged zeolites Dehydration at Location of nickel in DR/IS Garbowski et al. 

Molecular sieves50 is the term first given to zeolites dehydrated by heating in vacuum at about 350°C. However, there are now other materials, such as phosphorus substituted zeolites, microporous silicas, and A1P04 frameworks the latter are made hydrothermally in the presence of quaternary ammonium salts, like zeolites. 

C. Naccache Figure 1 shows that rehydration results in a large decrease of spin concentration. For example, as can be determined in this figure (dotted line corresponds to sample activated at 450° and then rehydrated), LaY zeolite dehydrated at 450° gives about 1.5 X 10 positive radical ions, while after rehydration the number of radical ions is only 1.1 X 10 . The decrease in electron-acceptor sites resulting from rehydration of the zeolite is demonstrated. 

Figure 5. Chemical shift versus xenon concentration for zeolite dehydrated at 400°C ...

Several of the zeolite dehydration equilibria have been delineated recently in simple aqueous solution, using classic hydrothermal techniques. Equilib-brium temperatures for the simple dehydration ... 

Figure 7. Reflectance spectra of NiHY zeolites dehydrated at 773 K.

Figure 2.3 Dehydration of alcohols using zeolites dehydration of (1) butan-2-ol on zeolite-X (2) n-butan-l-ol on zeolite-X (3) butan-l-ol on zeolite-A (4) butan-2-ol on zeolite-A...

Zeolite dehydration, its reversibility, water substitution for other molecules, having dimensions not larger than the characteristic pore openings of the material, interactions between host structure and guest molecule, surface properties (acidity and adsorption capability), the occurrence of solid-solid phase transitions, thermal stability, and so on, are all research subjects in which thermal analysis can make an invaluable contribution. Thermal analysis has been used extensively since its appearance to determine some of the peculiar properties of the zeolites, so that several hundred papers on this subject are present in the technical literature. 

Also, DSC is an useful tool for studying zeolite dehydration, either for obtaining its distinctive thermal profile, reproducing those of DTA or DTG, or especially for measuring heat amounts connected with water removal. 

Figure 10.12. N2 adsorption isotherm (in molecules/unit cell) at 25 °C, for LixAgyLSX zeolites dehydrated in vacuo at 450 ° C. This shows that the addition of increasing amounts of Ag results in a change in the generai aspect of isotherm toward that of the nearly fully Ag+-exchanged material (Hutson and Yang, 2000a with permission).

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