The kinetics of water desorption from porous glasses

The kinetics of water desorption from porous glasses silica gel and porous aluminosilicates were followed through the TG and DTG methods. In all cases only one thermodesorption peak appeared. The kinetic parameters were determined by standard nonisothermal methods. The activation energy is constant and independent of the coverage degree and pore diameter in the system porous glass-water. The functionE(θ) were determined for the silica gel and porous aluminosilicatesE=E ₀+a exp (-bθ). The parametersE ₀,a andb depend on the SiO₂/Al₂O₃ ratio and on the distribution of active centers on the surface.     

Calorimetric study of room temperature adsorption of N2O and CO on Cu(II)-exchanged ZSM5 zeolites

Copper ion-exchanged ZSM5 zeolites have been prepared with different cooper loadings from under- to over-exchanged levels. The adsorptions of N₂O and CO at 303 K have been studied using calorimetric method and infrared spectroscopy. The samples were additionally characterised by ammonia adsorption at 423 K. The active sites for both N₂O and CO are Cu(I) ions, which were formed as a result of pre-treatment in vacuum at 673 K.

Room temperature adsorption of nitrous oxide at low equilibrium pressures (up to 66.7 Pa) resulted in small amounts of chemisorbed N₂O (<0.2 molecule per one Cu ion). Differential heats of N₂O adsorption between 80 and 30 kJ/mol were obtained. Differential heats of CO adsorption between 140 and 40 kJ/mol were obtained. The obtained amounts of chemisorbed species in the investigated systems and the values of differential heats of both nitrous oxide and carbon monoxide demonstrate the dependence on the copper content.     

Kinetics of isotope exchange between [3-(iodophenyl)methyl]guanidine (mIBG) and [131I]-iodide

The reaction of isotope exchange between [3-(iodophenyl)methyl]guanidine, mIBG, and [¹³¹]-iodide in relatively concentrated solutions, in the presence of different ammonium salts, in a closed system, over the temperature range from 130 to 150°C, has been investigated. The reaction occurs either with (NH₄)₂SO₄ or CH₃COOH, which indicates that the reaction goes through some intermediate stages. Kinetic studies show the influence of the additives. The activation energies for the reaction with (NH₄)₂SO₄/H₂O, (NH₄)₂SO₄/CH₃COOH and CH₃COOH are 121.1, 115.1 and 84.5 kJ·mol^–1, respectively.     

Phase transition in hexacelsian at about 580 K

Temperature-induced structure and microstructure changes in hexacelsians (BaAl₂Si₂O₈) that have been synthesised from the Ba-exchanged LTA and FAU zeolites (hexacelsian_LTA and hexacelsian_FAU) show that the phase transition near 580?K exists only in hexacelsian_LTA. The X-ray powder diffraction method has been used to follow the evolution of the structure during the phase transition, as described here. The excess thermodynamic quantities Gibbs free energy (G), entropy (S) and enthalpy (H) are obtained through the Landau theory of phase transition. The constants of proportionality between the G and ordering parameter (Q) are: h?=??170345?J?mol^?1, a?=??66.6?J?mol^?1?K^?1 and b?=??410534?J?mol^?1. The abrupt change in the trigonal distortion of the single six-member tetrahedral [SiO₄]^4? and [AlO₄]^5? ring near 580?K is responsible for the phase transition. The phase transition is non-convergent, ferroelastic, pure and proper.     

The lithium- and sodium-enhanced transformation of Ba-exchanged zeolite LTA into celsian phase

The Li⁺- and Na⁺-doped hexacelsians (HC) synthesized from Ba-LTA synthetic zeolite as precursors were used for preparation of monoclinic celsians (MC). The doped pure MC and mixture HC/MC species were obtained by thermally induced polymorphous transformation at 1200 °C. Synthesized-doped MC have been characterized by X-ray powder diffraction and spectroscopic (infrared, Raman and ²⁹Si magic angle spinning nuclear magnetic resonance) methods. The obtained results suggest that in all investigated samples Na⁺ or Li⁺ dopants were incorporated in MC crystal structures during thermal transformation of a zeolite. It has been shown that HC→MC transformation depends on molar fractions of alkali cations. These findings combined with used temperature/time conditions could help in optimization of MC synthesis route.     

Room-temperature interaction of n-hexane with ZSM-5 zeolites

In this work, room temperature interaction of n-hexane with HZSM-5 (Si/Al=20) and ion-exchanged samples containing one (CuZSM-5, FeZSM-5 and MnZSM-5) or two transition-metal cations (Fe,CuZSM-5; Cu,MnZSM-5 and Fe,MnZSM-5) was studied by microcalorimetry and TPD methods. Both differential heats and the amounts of n-hexane adsorbed per one unit cell were quantitatively determined. Higher heats of adsorption and higher amounts of adsorbed gas were found for ion-exchanged samples than for HZSM-5. The experiments of n-hexane adsorption on hydrated samples were also performed. The amounts of n-hexane adsorbed on hydrated ZSM-5 were lower in comparison with dehydrated samples, while the energies of interaction were similar.     

An analysis of elementary processes of water desorption from zeolites of type a Part II. Zeolites with bivalent counterions

DSC and EGA were used to follow the process of water desorption from synthetic MgA, ZnA and CdA zeolites. Applying a semi-empirical algorithm the complex experimental curves were resolved into elementary ones. It turned out that in zeolites with bivalent cations there are four phases of sorbed water. Sorption heats were determined for each individual phase. The NMR spectra of water on a CaA zeolite were investigated. They also point out the existence of four phases. On the basis of the results obtained a possible model of water sorption on synthetic zeolites with bivalent cations is discussed.     

Reconstructive and displacive transformations of tectosilicates

By using the thermally induced phase transformation initial zeolites were converted into pure carnegieite, stuffed derivative of cristobalite. The polymorphs obtained from Na-LTA are stoichiometric (NaAlSiO₄), since those obtained from Na-FAU zeolite are non-stoichiometric (Na_1-xAl_1-xSi_1+xO₄). Stoichiometric carnegieite have cubic structure, while non-stoichiometric carnegieite crystallized in cubic and orthorhombic forms. ²⁹Si MAS NMR spectra show a very large but expecting difference between stoichiometric and non-stoichiometric carnegieite. The spectrum of stoichiometric carnegieite has only one peak Si(4Al), while the spectrum of non-stoichiometric carnegieite consist few superimposed peaks assigned to Si(4Al), Si(3Al), Si(2Al), Si(1Al) and Si(0Al). DTA study indicates the occurrence of displacive phase transition of all synthesized carnegieite. The transition temperature is depending on silicon aluminum order: T _m=690°C for stoichiometric, T _m=565 and 660°C for non-stoichiometric, low-temperature and high-temperature carnegieite, respectively. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermally induced rings formation in aluminosilicate structures

In the present study, we performed an investigation of phase transformation of zeolite Na-GIS in temperature interval from room temperature up to 1300°C. The collapse of S4R and S8R (secondary building units in zeolite Na-GIS) and the forming of pure sodium nepheline phase, consists of S6R, was examined. During the thermal transformation of Na-GIS zeolite symmetry of rings was changed and their deformation increases. The irregularity of rings causes the appearance of defect modes in the IR spectra of amorphous precursors. Investigated phases were characterized by XRPD, IR, NMR and DSC methods. A semiempirical method AM1 was used for calculation of vibrational spectrum of nepheline.     

Strontium deficient feldspar – structure and X – ray powder diffraction line broadening analysis

Vacant Sr‐feldspar was synthesized by thermal transformation of FAU zeolite and the structure was refined (space group I2/c). Two refinement procedures were performed: with and without micro strain modelling. Reliable structure model was achieved only with including micro strain modelling. Vacancies in the Sr²⁺ positions produce micro strain. The concentration of vacancies was the highest along [011] direction. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)