Search Results

You are looking at 1 - 10 of 150 items for :

  • "molecular sieves" x
  • Refine by Access: All Content x
Clear All
Reaction Kinetics, Mechanisms and Catalysis
Authors: Cleanio L. Lima, Hélvio S. A. de Sousa, Santiago J. S. Vasconcelos, Josué M. Filho, Alcemira C. Oliveira, Francisco F. de Sousa, and Alcineia C. Oliveira

2 O 3 , clays and molecular sieves with sulfate ions gives rise to strong acid solids [ 2 , 5 ]. When zeolites are considered, a variety of techniques has been reported in the literature focused on the way to introduce SO 4 2− ions onto the zeolite

Restricted access

in SBA-15 (and MCM-41) type hexagonally structured molecular sieves as such or functionalized to bind HRP amino acid residues in comparison with the 2D immobilization onto aerosil A380. Guest–host interactions were pursued by spectrometry and thermal

Restricted access

Abstract  

We report here on the effect of nitric acid solutions on the structural stability and ion-exchange properties of a series of natural and synthetic zeolite molecular sieves. The implications of this work for the industrial application of zeolites in acidic aqueous systems are discussed.

Restricted access

Abstract  

The SiMCM-41 mesoporous molecular sieve was synthesised by the hydrothermal method. The physicochemical characterisation by infrared spectroscopy, X-ray diffraction and thermogravimetry, showed that the material presents a well-defined structure. In this study, the determination of the total acidity and relative strength of the acid sites of the SiMCM-41,was performed by desorption of adsorbed n-butylamine combined with thermogravimetric measurements. The total acidity, determined by means of the amount of amine desorbed normalised by mass of solid, was equivalent to 0.927 mmol g–1, in the temperature range from 96 to 235°C. By using the Flynn and Wall integral kinetic model, at 5, 10 and 20°C min–1 heating rates, it was determined that the activation energy to desorb n-butylamine was 90.7 kJ mol–1, in the same temperature range, evidencing that SiMCM-41 presents only weak acid sites on its surface.

Restricted access

Until recently, it has been believed that during adsorption and desorption cycles of molecular sieve pellets, the pellets have been passive participants in the process. It has lately been shown by others using X-ray diffraction technique that the molecular sieve structure undergoes expansion and contraction as a result of moisture content changes.

Restricted access

The degenerations on heat treatment and the thermal stabilities of Na-A molecular sieve zeolites of different crystallinities were studied by thermal analysis. The degenerations of the sieves were computed from the decreases of the zeolitic water content. The thermal stabilities were determined from the commencement of the exothermic peaks. Heating at 600‡ causes considerable degeneration of these sieves. The degeneration and thermal stability depend on the crystallinity of the sieve. Poorly-crystalline sieves are degenerated to a greater extent than well-crystaline ones. The thermal stabilities on poorly-crystalline sieves also decrease during heating at 600‡.

Restricted access

Abstract  

The adsorption of thorium(IV) ions on molecular sieve (13X type) powder from aqueous solutions has been studied as a function of shaking time pH, thorium ion concentration and temperature. The conditions of maximum adsorption of thorium ions obeys Langmuir and D-R isotherms over the entire concentration range studied. Thermodynamic quantities such as H, G and S have been calculated fromK D values determined at various temperatures. The results show endothermic heat of adsorption, but negative free energy value indicates that the process of thorium adsorption on molecular sieve powder is favored at high temperature. The influence of various cations and anions on thorium(IV) ion adsorption was examined. A wavelength dispersive X-ray fluorescence spectrometer was used for measuring the thorium ion concentration in solutions.

Restricted access

Abstract  

Direct anchoring of the synergistic extractant TOPO-P204 onto the inner walls of mesoporous molecular sieve generates a non-stoichiometric synergistic extractive membrane absorbent: MCM-TP which has well-spaced and structurally well defined active sites. Specifically, the feasibility of applying the MCM-TP absorbent with an absorption capacity of 0.02 mmol[Pd]/gabsorbent to the recovery of non-radioactive palladium from spent nuclear fuels has been examined showing an advantage over the liquid-liquid extraction or precipitation method. MCM-TP studied by powder XRD, TEM and29Si MAS NMR indicates that it has 35 Å regular, well-defined channels and an improved hydrothemal stability.

Restricted access

Abstract  

Mn–MCM-41 was synthesized by the hydrothermal method. The synthesized material was characterized by various physicochemical techniques such as small angle X-ray diffraction, elemental analysis, Fourier transform infrared spectroscopy and diffuse reflectance ultraviolet–visible spectroscopy. The gas-phase cyclohexanol and air reaction over Mn–MCM-41 molecular sieve was studied for the first time. The conversion is almost constant even up to 5 h, and follows a parabolic behavior with respect to temperature. The conversion reaches a maximum at 350 °C and selectivity towards cyclohexene is increased with temperature. The conversion and cyclohexene selectivity were increased with flow rate.

Restricted access

Abstract  

The separation characteristics of hydrogen-tritium mixtures by gas chromatography at 77 K have been studied using molecular sieve 4A, coated with 5 wt% vanadium in the form of V2O4, as supporting material. The performance of the column was found to be better than that observed on the conventional coated and uncoated molecular sieves in terms of reduced retention times, resolution, nature of the peak and added thermal stability of the stationary phase.

Restricted access