Search Results
Abstract
Two natural zeolites from Iranian deposits (clinoptilolite and natrolite) were characterized and their ability for adsorption of iodide from nuclear wastewaters was evaluated. The adsorption behavior was studied on natural and modified zeolites by g-spectrometry using 131I as radiotracer. Adsorption isotherms and distribution coefficient (Kd) were measured. The results showed that clinoptilolite is a more promising zeolite for removal of iodide compared to natrolite. Furthermore, the adsorption was higher in silver, lead and thallium forms, whereas the lowest desorption was observed in lead modified zeolite.
Abstract
Zeolites chemically modified with 1, 4 or 6 M aqueous solutions of NaOH were studied by DTA, TG and ETA (emanation thermal analysis) in the temperature range 201–200°C. The structural changes in the modified zeolites at room temperature and in the modified zeolites annealed at 1000°C were studied by XRD analysis. Thermal analysis demonstrated dehydration, dehydroxylation, structural changes and a glass transition. A gradual loss in crystallinity of the chemically modified zeolites was also observed. XRD analysis revealed structural changes caused by chemical treatment and also by annealing.
The adsorption of argon and nitrogen on a series of MFI-type zeolites (silicalite-I (Si/Al>1000) and HZSM-5 (16<Si/Al<120)) was studied by isothermal microcalorimetry, volumetry and neutron diffraction.
include SAPO molecular sieves [ 6 , 7 ], Y zeolite [ 8 , 9 ], β zeolite [ 10 , 11 ], SO 4 2− /ZrO 2 [ 12 , 13 ] and heteropolyacids [ 14 , 15 ], among which, the three-dimensional 12-membered ring β zeolite is known as one of the most effective
Introduction Detailed characterization of high-silica zeolites is crucial for understanding of these interesting materials with unique properties which offer possibility to employ them in wide range of important catalytic and
NH4Y and NH4LaY-type zeolite catalysts were prepared by cyclic ion-exchange of a synthetic Linde Y-zeolite. The release of ammonia and water were followed by evolved gas analysis (automatic thermogastitrimetric equipment) as well as with a continuous selective water detector.
Calorimetric measurements of the heat of adsorption of CO2 on zeolites with variable content of mono- and divalent cations lead to common conclusions. High initial heats (up to 120 kJ·mol−1 for NaA), generally associated with a slow and activated rate of adsorption, are found for high contents of Na+, Li+ or Ca2+. They are attributed to a limited number of chemisorption sites (0.3 per α cage in NaA).
Abstract
A new method for the determination of aluminum and silicon has been developed for zeolite catalysts. In contrast to previous methods, thermal neutrons are used for the analysis of both elements, and cadmium absorbers are not needed. The silicon determination utilizes a one-hour irradiation to observe the31Si produced by the (n, 
) reaction of30Si. A 15-second irradiation is used for the27Al(n, )28Al reaction. The28Al activity is corrected for the contribution from the28Si(n,p)28Al reaction by using the analyzed weight of silicon in the sample and the data for a silicon standard irradiated simultaneously with the zeolite and the aluminum standard. The quantitation limits are 0.012 g for silicon and 3.3×10–5 g for aluminum. Sodium presents a significant interference, but this element can be removed by taking advantage of the ion exchange properties of these materials.
Abstract
The synthetic pyrethroide insecticide (R, S)-α-cyano-3-phenoxybenzyl-IR, S-cis,trans-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanocarboxylate in solid form could be enclosed in and therefore stabilized by natural clinoptilolitic zeolite. The resulting host-guest complex exhibited characteristics different from those of the starting compounds and also their physical mixtures. The product was more stabile against the tested solvents (distilled water and acetone) and also against thermal decomposition, which suggests its possible use as a slowly-releasing insecticide.
Abstract
The thermal and structural properties of two parent NaY zeolites and of those modified by ion exchange (ReNaY, HNaY, FeNaY) were investigated by simultaneous thermal analysis (TG-DTA-DTG) and by X-ray diffraction spectroscopy. Both the intracrystalline water and the zeolite framework were in our attention. The impurities (Fe) located in the lattice as well as the ions which entered by ion exchange (Re, H, Fe) influence the properties of the zeolites. The values of the activation energy of the dehydration process prove that the water molecules are more strongly bonded in all modified samples than in the parent ones. As compared to the NaY zeolites, an increased thermal stability, of about 100°C was revealed for ReNaY or of about 180°C for HNaY, and a decreased stability, of about 50°C, for FeNaY samples. The temperature at which the lattice break-down beginsT amf, estimated by following the X-ray diffraction patterns for samples heated in air at temperatures from 300 to 1100°C, is the temperature which may be related to the structural characteristics of the zeolites, i.e., to the lattice constant of the uncalcined materials. The XRD studies reveal the heterogeneity of the crystallites constituting the zeolite material from both the point of view of the lattice constant values and the thermal stability. As the temperatureT amf, generally, does not coincide with the temperature of the first exothermic peak,T 1, of the DTA curve, we suggest the temperatureT amf to be taken as an unambiguous measure of the thermal stability.
