Search Results
Abstract
Alkylation of benzene with 1-alkenes (C6–C18) was performed in liquid phase at 160–200 °C over beta zeolites with Si/A molar ratio 12.5–75.0. With increasing alkyl chain length, the conversion of 1-alkene decreased. The alkylation activity decreased with increasing Si/Al ratio of beta-zeolite. The increase of Si/Al molar ratio was connected with the enhancement of the portion of Lewis acid sites in total acidity of beta zeolites, resulting in greater dimerization/oligomerization activity and consequently in more rapid deactivation of beta-zeolites.
Abstract
The catalytic properties of a novel MFI-type zeolite with different SiO2/Al2O3 ratio in the dehydration of glucose to levulinic acid (LA) were investigated in this work. The results demonstrate the strength of acidic sites and the mesoporosity of the zeolites have significant effects on LA formation.
Abstract
Competitive uptake of Cs+, Ba2+ and Zn2+ ions from aqueous medium by zeolite-3A is studied by the radiotracer technique. Suppression Cs+ uptake in presence of Ba2+ is the maximum amongst the competitive uptakes. The simultaneous uptakes of the individual cations are considered in the light of the zeolite pore size, hydration effects and mobilities of the competing cations.
Abstract
Zeolite rocks from different locations in Bulgaria are characterized in geological-mineralogical respect and as ion exchangers for cesium and strontium radionuclides. The basic equilibrium and kinetic exchange parameters are determined. The influence of the mineralogical and chemical composition of the zeolite rocks on the ion exchange properties is shown.
Abstract
Thermodesorption is here considered for its possibility of giving access to the microporosity of adsorbents. The requirements of this application (good separation of successive desorption steps, good control of the desorption pressure and temperature throughout the sample, possibility of a safe kinetic analysis of each step) are here fulfilled by carrying out the thermodesorption in the Controlled transformation Rate Thermal Analysis (CRTA) mode. The method is applied to 4 zeolites (3A, 4A, 5A and 13X) and a well characterized charcoal, from −25 to 325°C, after pre-adsorption of water.
Abstract
Different Nd-zeolites were prepared from the original NaX and NaY zeolites by ion exchange. The hydrated and thermally activated (at 550 °C) samples obtained were irradiated with 
-rays of 1.5 and 10.0 Mrad. The unirradiated and irradiated samples were characterized mainly by X-ray diffraction and tested for catalytic activity in cumene cracking. The X-ray diffraction patterns indicated a slight decrease of crystallinity after irradiation. However, the irradiated samples exhibited higher catalytic activities than unirradiated ones. -Irradiated hydrated zeolites were found to possess comparable activities, whereas the irradiated dehydrated samples were more active. Higher irradiation doses resulted in more active dehydrated zeolites than those irradiated with a lower dose. The observed higher activity was attributed to the formation of tricoordinate aluminium atoms in the zeolite structure, leading to increase of the number of acidic sites and consequently to a catalytic activity.
Abstract
The sorption of Pb(II) from aqueous solution using NKF-5 zeolite was investigated by batch technique under ambient conditions. The NKF-5 zeolite sample was characterized by using FTIR and X-ray powder diffraction in detail. The sorption of Pb(II) was investigated as a function of pH, ionic strength, foreign ions, and humic substances. The results indicated that the sorption of Pb(II) on NKF-5 zeolite was strongly dependent on pH. The sorption was dependent on ionic strength at low pH, but independent of ionic strength at high pH. At low pH, the sorption of Pb(II) was dominated by outer-sphere surface complexation and ion exchange with H+ on NKF-5 zeolite surfaces, whereas inner-sphere surface complexation was the main sorption mechanism at high pH. From the experimental results, one can conclude that NKF-5 zeolite has good potentialities for cost-effective preconcentration of Pb(II) from large volumes of aqueous solutions.
Abstract
Four different samples of NdX, NdY, NdNH4X and NdNH4Y-zeolites were prepared by ionexchange methods. DTA and XRD analyses have been carried out for the samples. The thermally activated zeolites were irradiated by 1.5 and 10.0 Mrad 
-rays. The catalytic activities of these samples were tested in dehydration of isopropanol. The results of DTA indicated that all samples showed endothermic peaks at about 215 °C related to the release of physically adsorbed water and exothermic peaks at 850–950 °C indicating the collapses of the zeolite. The X-ray analysis revealed that the exchange of sodium by neodymium or ammonium followed by neodymium ions did not change the crystal structure but some decrease in the crystallinity was observed. The catalytic activities of these zeolites were measured in dehydration of isopropanol as a function of temperature. It was found that the activity of the prepared Nd-zeolites depends on the crystallinity of zeolites and on the condensation products formed on catalyst surface. However, the irradiated samples exhibited higher catalytic activities in isopropanol conversion than the unirradiated ones. The observed higher activity for irradiated samples was attributed to the increase of the number of acidic centers responsible for dehydration of alcohols. These centers were formed as a result of the formation of tricoordinate aluminium atoms in -irradiated zeolites.
Abstract
The paper presents physico-chemical properties of mixed adsorbents in the clinoptylolite (mordenite)/SiO2 system containing 30, 50, 80 mass% zeolite. Adsorption capacity towards polar (water, butanol) and non-polar (n-octane) substances as well as total surface heterogeneity (energetic and geometrical) were determined. Desorption energy distribution functions as well as fractal dimensions were also determined and compared with the low-temperature nitrogen adsorption data. Irregular shapes of the curves q=f(E d) as well as large values of volumetric fractal dimensions (D f~2.6) revealed heterogeneous properties of the zeolite/SiO2 system surfaces. Addition of zeolite increases total heterogeneity of the material.
Zeolites for nuclear waste treatment: Co, Ni, Zn uptake into synthetic faujasites X & Y
I. pH effects, calcination, elution and encapsulation studies
Abstract
Synthetic zeolites X & Y have been examined for their abilities to take in Co, Ni and Zn radioisotopes. Distribution coefficients (K d ) have been determined, with and without competing cations, and at various pH values. Elution studies on zeolites containing Co, Ni and Zn radioisotopes considered the effect of acid and alkaline conditions, calcination and cement encapsulation. Leach rates arising from these experiments have been calculated. The general conclusions can be drawn are that zeolites X & Y are suitable for decontamination of aqueous wastes containing Co, Ni and Zn. Cement encapsulation is appropriate but no advantage accrues from pre-calcination. Some comments on the likely mechanism of leaching losses have been made.
