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Acid catalysis is recognised as the most reliable industrial ì way to convert hydrocarbons into various products (methanol to olefins and aromatics, glycerol to olefins, methane dehydroaromatization, isomerization, etc.). In this concern, ZSM-5 is extensively used, also because of peculiar type of active sites of such zeolite. In this paper, a fine tuning of the acidity content and strength of ZSM-5 was investigated through a combination of post- synthesis treatments: desilication via alkaline solution and wet impregnation with Mg2+ions followed by calcination. Results show that the insertion of Mg2+has a large influence on the distribution and strength of Brønsted and Lewis acid sites of the hierarchical ZSM-5, as well as an improved crystallinity with respect to the desilicated-one. The hierarchical ZSM-5 zeolite incorporating Mg-species was tested in Methanol- to-olefins reaction and the reduction of acid strength of catalyst increased the selectivity toward light olefins (mainly propylene) as well as it suppressed oligomerisation reactions delaying the coke formation.

The influence of small inorganic cations (Na+ ions in this case) on the core@shell silica nanoparticles, formed by the room temperature hydrolysis of TEOS in aqueous TPAOH (subsequent addition of NaOH – SA) and NaOH/TPAOH (direct addition of NaOH – DA) solutions was investigated by DLS, AFM, pH measurements, 29Si-NMR and 23Na-NMR. The obtained results showed that the smaller Na+ ions, displace parts of the larger TPA+ ions from the shells of the core@shell primary silica nanoparticles (PNPs), by the exchange process. This causes exposition of TPA-free terminal –OH and –O- groups to the solution and thus, enables the binding of the collided nanoparticles by the Si-O-Si bonds (aggregation). The PNPs in the aggregates, formed above the critical aggregation concentration (CAC) of silicate species are separated by the “bulky” shell (TPA-polysilicate anions) that prevents the close contacts between the nanoparticles cores and thus, their coalescence. When Na+ ions are present during the formation of the nanoparticles, a part of the Na+ ions is incorporated to the nanoparticle core. Remove of the TPA+ ions from the shells of the nanoparticles, formed below the CAC, enables close contacts of the nanoparticles cores and thus, their coalescences inside the aggregates. These findings show that the selective removal of the TPA+ ions from the nanoparticle shell enables the control of the composition and size of the precursor species and thus, represent a good basis for the engineering of nanosized (alumino)silicate microporous and mesoporous materials.

Pure and Co

Dual templating approach, using hexadecyltrimethylammonium bromide (CTAB), was employed in an attempt to synthesize hierarchical ZSM-5 zeolite. Amount of mesoporogen and the duration of aging of the precursor were varied. Majority of the synthesis routes resulted in phase separation, yielding separate ZSM-5 and amorphous mesoporous material. The relative amounts of the two phases were dependent on the CTAB amount ratio and also significantly on the duration of precursor aging before CTAB addition. One particular combination of the two factors led to the formation of a homogeneous hierarchical form of ZSM-5 with leafy morphology, consisting of intergrown thin crystalline sheets which formed flower-like structures. The hierarchical ZSM-5 possessed significant microporous (≈95 m2/g) and highly developed mesoporous surface (≈470 m2/g), with a relatively broad distribution of mesopore sizes (

Pure and Co3+-doped BaAl2O4 [Ba(Al1–xCox)2O4, x = 0, 0.0077, 0.0379] powder samples were prepared by a facile hydrothermal route. Elemental analyses by static secondary ion mass spectrometry (SIMS), X-ray absorption spectroscopy (XAS) measurements at the Co K- edge, and X-ray diffraction studies were fully correlated, thus addressing a complete description of the structural complexity of Co3+- doped BaAl2O4 powder. Powder X-ray diffraction (PXRD) patterns indicated that prepared samples were nanocrystalline with a hexagonal P63 symmetry. The X-ray absorption near-edge structure (XANES) measurements revealed the presence of cobalt in a +3 oxidation state, while the rarely documented, tetrahedral symmetry around Co3+ was extracted from the extended X-ray absorption fine structure (EXAFS) oscillation patterns. Rietveld structure refinements showed that Co3+ preferentially substitutes Al3+ at tetrahedral Al3 sites of the BaAl2O4 host lattice, whereas the (Al3)O4 tetrahedra remain rather regular with Co3+–O distances ranging from 1.73(9) to 1.74(9) Å. The underlying magneto- structural features were unraveled through axial and rhombic zero-field splitting (ZFS) terms. The increased substitution of Al3+ by Co3+ at Al3 sites leads to an increase of the axial ZFS terms in Co3+-doped BaAl2O4 powder from 10.8 to 26.3 K, whereas the rhombic ZFS parameters across the series change in the range from 2.7 to 10.4 K, showing a considerable increase of anisotropy together with the values of the anisotropic g- tensor components flowing from 1.7 to 2.5. We defined the line between the Co3+ doping limit and influenced magneto- structural characteristics, thus enabling the design of strategy to control the ZFS terms’ contributions to magnetic anisotropy within Co3+-doped BaAl2O4 powder.

The synthesis of high silica zeolites in many cases implies the usage of organic structural direction agents (SDA). However, to manifest their functionalities, the SDA occluded inside the channels of the as-synthesized structure should be removed, usually by a high temperature treatment (calcination). In this paper, the positron annihilation lifetime spectroscopy (PALS) was used to monitor the development of accessible spaces, their sizes and distributions in MFI zeolites, ZSM-5 and silicalite-1 in order to give an additional insight in the process of the SDA removal. For that purpose, a conventional PALS setup with 22Na positron source was applied. It was established that there is a pronounced difference between positron annihilation data for these two zeolites of the same structural type. The samples were additionally analysed by X-ray powder diffraction at room temperature with a crystal structure refinement and thermogravimetry.

Inorganic materials that enable a link between the storage and release of molecular oxygen offer a fertile ground in continuous quest for the applications that can potentially reduce energy consumption and thus minimize adverse effects on the environment. Herein, we address reversible intake/release of an oxygen within the BaAl2O4 material as evidenced by unexpected magnetic ordering. Magnetic measurements unveil that an oxygen is stored in the form of condensed matter, creating a kind of low dimensional, chain-like assembly within the tunnels of BaAl2O4 structure. We demonstrate that oxygen is adsorbed simply by staying in air, at ambient conditions, and released relatively quickly by staying in the He or other gas atmosphere of several millibars pressure even at 300 K.

Powder samples of pure BaAl

Powder samples of pure BaAl2O4 and doped with 4.9 atom % Eu in relation to Ba were prepared by a hydrothermal route. The samples were characterized by X-ray diffraction, 151Eu Mössbauer spectroscopy, synchrotron-based X-ray absorption spectroscopy at the Ba L3- and Eu L3- edges, and photoluminescence measurements. Diffraction lines were broadened, indicating that the samples were nanocrystallline.The samples possessed a hexagonal crystal structure, space group P63. 151Eu Mössbauer spectroscopy revealed the presence of Eu in the 3+ oxidation state. The same information on the Eu oxidation state was also obtained by the Eu L3-edge X-ray absorption near-edge structure of the doped sample. Extended X-ray absorption fine structure showed an Eu3+ ion substituted for Ba2+ on the Ba2 site in the BaAl2O4 host structure, with charge compensation by an interstitial O in the vicinity of the Ba2 site. That was confirmed by a Rietveld structure refinement for the Eu-doped BaAl2O4 sample. Analysis of the diffraction line broadening for the prepared samples was performed simultaneously with the structure refinement. Both the dopant Eu3+ and the interstitial O acted as defects in the host BaAl2O4 lattice, which increased the lattice strain from 0.02% for pure BaAl2O4 to 0.17% for the Eu-doped sample. Crystallite sizes in the samples increased with Eu doping from 32 nm for pure BaAl2O4 to 36 nm for Eu-doped BaAl2O4. This could likely be related to the increase in the diffusion rate of the cations in the sample when a part of the Ba2+ cation content was exchanged with smaller Eu3+ cations. The Eu- doped BaAl2O4 sample exhibited red photoluminescence under excitation with λexc = 308 nm. The observed emission spectrum indicated that Eu3+ ions occupied the Ba site with lower symmetry in the doped sample.

Following the assumption that the crucial processes governing the formation, properties and evolution of the core(amorphous silica)@shell(organocations) nanoparticles take place during short-time, room-temperature (rt) stirring/aging of the homogeneous reaction mixtures (HmRMs) formed by hydrolysis of TEOS (tetraethyl orthosilicate) in solutions of Org(OH)n, we investigated these processes by various experimental methods (pH, ionic conductivity, 29Si-NMR, dynamic light scattering and atomic force microscopy). The analysis of the data obtained by detail and careful investigation of the "model" HmRMs having the starting chemical composition: xTEOS:0.25TPAOH:20H2O (TPAOH = tetrapropylammonium hydroxide; x = 0.05 – 1), offer some new elements for the understanding of the mechanisms of formation and rt evolution of the core@shell silica nanoparticles: (1) There is a resolute evidence of the formation of the stable, about 1.2 nm sized core(amorphous SiO2)@shell(TPA+ ions) nanoparticles below the critical aggregation concentration (CAC). (2) Due to the intensive particulate processes (growth, aggregation, disaggregation, dissolution) which take place during the rt aging of the investigated HmRMs, the equilibrated core@shell silica nanoparticles do not exist as individual primary ones, but as the aggregates (about 2 nm to about 20 nm), composed of 1 – 2 nm sized "primary" nanoparticles. (3) In spite of the most frequent meaning that the nanoparticle shell is composed of the "free" TPA+ ions adsorbed on the surface of the nanoparticle core, the results of this study show that the nanoparticle shell can be formed mainly by attachment of the polysilicate anions (silicate oligomers), associated with TPA+ ions, on the surfaces of the nanoparticles cores.

Powder BaAl2O4 samples doped with 0 and 1.76 at.% Cr in relation to Al were hydrothermally prepared. Both samples were characterized by X-ray diffraction (XRD) and synchrotron based X-ray absorption spectroscopy (XAS) at the Cr K- and the Ba L3-edge. Diffraction patterns indicated that samples were nanocrystalline with a hexagonal crystal structure, space group P63. Chromium doping of barium aluminate caused an increase of the unit-cell volume and diffraction line broadening. The doped sample contained a small amount of an impurity phase, namely BaCrO4. Analyzed Cr K-edge X-ray absorption near edge structure (XANES) for the doped sample showed the presence of chromium in 6+ and 3+ oxidation states: Cr6+ being characteristic for chromium in the impurity phase BaCrO4, while Cr3+ participated in the formation of the doped phase BaAl2O4:Cr. Extended X-ray absorption fine structure (EXAFS) suggested an unusual tetrahedral coordination of Cr3+ ions within the BaAl2O4 host phase. The structure of samples was refined by the Rietveld method, simultaneously with the analysis of diffraction line broadening. Rietveld structure refinement showed that in doping the Cr3+ ions likely substituted for Al3+ ions on Al1 tetrahedral sites of barium aluminate crystal lattice. Crystallite sizes in the samples decreased with chromium doping, from 32 nm for pure BaAl2O4 to 24 nm for Cr-doped BaAl2O4. The dopant Cr3+ cations acted as defects in the barium aluminate structure that increased lattice strain from 0.02% for pure BaAl2O4 to 0.14% for doped BaAl2O4, and disturbed the crystallites to grow.

The microstructure of the starting hydrogel has extremely important role in the synthesis of the microporous materials. Due to the fact that very limited number of experimental methods (techniques) can be used for gel analysis, there are still a lot of missing information about the processes on molecular level, which occur before and during the nuclei formation. In this paper, various methods were used in characterization of pretreated (aged) hydrogel before the process of its hydrothermal transformation to zeolite A. The results obtained from different techniques (FTIR, PALS, BET, DTG) show the changes in chemical composition and in microstructure of the solid phase, indicating that the structure of the ordered phase (potential nuclei) within gel matrix (completely amorphous according to PXRD patterns) is rather of different structural units typical for FAU and SOD (or their mixture) than the final product (LTA) only!

Crystallization of zeolite ZSM-5 from a diluted heterogeneous system (12.5Na2O-Al2O3-8TPABr-60SiO2-4000H2O) was investigated by various experimental methods such as, X-ray diffraction (XRD), electron diffraction (ED), infrared spectroscopy (FTIR), X-ray fluorescence (XRF), scanning-electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), thermo-gravimetric analysis (TG-DTG), particle size analysis (PSA) and Inductive Coupling Plasma (ICP) Emission Spectrometry. The crystallization process is characterized by a very long “induction period” (95 % of the entire reaction time) and very fast transformation (5 % of the entire reaction time) of amorphous to crystalline phase (zeolite ZSM-5) at the end of crystallization process. Analysis of the obtained results have shown that the crystallization process takes place by a chain of processes: (i) formation of “primary” amorphous aluminosilicate precursor (gel) at room temperature, (ii) formation of “secondary” amorphous aluminosilicate precursor (“worm-like” particles – WLPs), (iii) formation of “tertiary” amorphous aluminosilicate precursor (condensed aggregates – CAs) by aggregation of the WLPs and densification (condensation) of aggregates and (iv) formation of nuclei and their growth in the matrixes of CAs ; these processes result in the formation of fully crystalline zeolite ZSM-5 in the form of polycrystalline aggregates.

Influence of the simultaneous change of batch alkalinity ( A = [Na 2 O/H 2 O] b ) and total batch silica concentration ( tsc = [SiO 2 /H 2 O] b ) of the reaction mixture, at two characteristic batch molar ratios, y ≡ [SiO 2 /Al 2 O 3 ] b (1.3 and 2.0), on the duration of crystallization, reaction yield and the particulate properties (size and shape) of the crystalline end product (zeolite A) was investigated. Sharp decrease of the duration of crystallization process with simultaneous increase of A and tsc is caused by simultaneous increase of the number of nuclei formed in gel matrix, rate of their release from the gel matrix and their growth with the increase of the alkalinity of the reaction mixture. Although the reaction yield, Y R , is proportional to the total batch silica concentration, tsc , the ratio Y R / tsc slightly decreases with increasing alkalinity as a consequence of the increasing solubility of zeolite A with the increase of the alkalinity. Specific number of crystals (number of crystals per unit mass of the crystalline end products), N s increases with simultaneous increase of A and tsc as a consequence of the increase of the number of nuclei formed in the gel matrix with increase of alkalinity of the reaction mixture. On the other hand, a complex dependence of the crystal size distribution on A , tsc and y , is a consequence of influence of the mentioned chemical factors not only on the number of nuclei, but also on their distribution in the gel matrix. Four different morphological forms of zeolite A, namely, (i) regular cubic crystals with “sharp” edges and apexes, (ii) cubic crystals with truncated edges, (iii) cubic crystals with “rounded” edges and apexes and (iv) pseudo-spherical, face-less particles are obtained during hydrothermal treatment of investigated hydrogels. Influence of the chemical factors on the formation of particular morphological forms of zeolite A are discussed and the possible mechanisms of their formation are considered.

In this work, positron annihilation lifetime spectroscopy was used for the structural characterisation of porous materials prepared from amorphous zeolite precursors acting as the starting materials and hexadecyltrimethylammonium bromide acting as a mesoporous template. The best fits of the obtained lifetime spectra provide a four-component exponential decomposition. The lifetime values of the long-lived components point to the presence of mesoporous voids. Infrared and X-ray analyses show that the obtained materials consist partly of a crystalline zeolite structure.

The applicability of positron lifetime (LT) spectroscopy to the study of progress of formation of Secondary Building Units (SBU) in gels yielding in FAU and LTA type zeolites was investigated. Samples were prepared from aluminosilicate gels with various degrees of local structural order. LT measurements were performed at room temperature in air and in vacuum. Coexistence of annihilation modi with long lifetime components was shown; a correlation with precursors of nucleation and type of exchanged ions was also indicated.

Changes of the concentrations, CAl, of aluminum and, CSi of silicon in the liquid phase as well as of the dimension, Lm, of the largest crystals were measured during crystallization of zeolite A from the amorphous aluminosilicate precursors dispersed in alkaline solutions containing different amounts of Na+ ions. Analysis of the measured data have shown that the growth rate of zeolite A crystals is size-independent, and that Na+ ions, together with aluminate and silicate anions from the liquid phase take part in the surface reaction of the crystal growth of zeolite A. Kinetic equations which describe the change of Lm as a function of the change in concentrations of Na, Al and Si in the liquid phase are derived and evaluated by correlation with the measured changes of Lm.

Kinetics of nucleation, crystal growth and entire process of crystallization of zeolite A in the presence of different sodium salts were analyzed by population balance method. Both the rate of crystallization and rate of crystal growth of zeolite A increase with increasing concentration of sodium sulfate dissolved in the liquid phase of the crystallizing system, but equimolar amounts (with respect to Na + ions) of different sodium salts influences the rate of crystal growth only on the extent which can be attributed to experimental errors characteristic for this type of experiments. Thus, it was concluded that only sodium ions and not the associated anions influence the rate of crystal growth of zeolite A. Insensibility of the particulate properties of the crystalline end products on the presence of different anions supports this conclusion was explained by the “memory”effect of specially prepared amorphous aluminosilicate gel precursors.

The principle of the “memory” effect of amorphous aluminosilicate gel precursors, namely that crystal size distribution in the crystalline end product depends exclusively on the number and distribution of nuclei in the gel matrix, previously evidenced theoretically by simulation of zeolite crystallization under various conditions, was evaluated experimentally by analysis of the data obtained from specially designed kinetic experiments of zeolite crystallization. It was found that particulate properties (specific number of crystals, crystal size distribution, size of the largest zeolite crystals) considerably depend on the conditions of gel preparation, and thus on the specific number of nuclei and their distribution in the gel matrix. On the other hand, despite considerable differences in the rates of nucleation and crystal growth during crystallization from appropriate precursor, the particulate properties of the crystalline end products as well as number and distribution of nuclei in the gel matrix are not affected, or are affected very slightly by the crystallization condition, and thus by the rates of the relevant critical process (gel dissolution, nucleation, crystal growth). Hence, it was shown that invariability of the size distribution of zeolite crystals in the crystalline end products (zeolite) crystallized from the amorphous aluminosilicate precursors prepared in the same way, but differently treated prior and during crystallization is a consequence of the constancy in the number and distribution of nuclei in the matrix of appropriate gel precursor. This result is in accordance with the result obtained previously by simulation, and thus the principle of the “memory” effect of amorphous aluminosilicate gel precursors was evidenced not only theoretically but also experimentally.

A critical analysis was carried out for the purpose of understanding the role of subcolloidal (nanosized) (alumino)silicate precursor species in the early stage of crystallization of zeolites in heterogeneous systems (hydrogels). The formation and evolution of these subcolloidal species in both the solid and the liquid phases were investigated by various experimental methods such a scanning electron microscopy (SEM, FE-SEM), transmission electron microscopy, atomic force microscopy, particle size analysis, pH measurement, atomic absorption spectroscopy, and dynamic light scattering, after careful separation of intermediates from reaction mixture by two-step centrifugation treatment. The results revealed that a chain of processes (i) the formation of low-molecular-weight (LMW) silicate species, by dissolution of Al-enriched amorphous silica, and their aggregation into about 3 nm sized primary precursor species (PPSs), (ii) the formation of larger (∼3 to ∼15 nm sized) silicate precursor species (LSPSs) by a rapid aggregation/coalescence of PPSs, (iii) the formation of "gel" (primary amorphous precursor) by a random aggregation of LSPSs at room temperature, and (iv) the formation of the worm-like particles (secondary amorphous precursor) occurred in the solid phase during heating of the reaction mixture (hydrogel) from room temperature to 170 °C. It is interesting that almost the same processes occur in the liquid phase but with decreased rate according to the relative low concentration of LMW silicate species. With the above described findings, it is highly expected that the manipulation of crystallization pathway through controlling the formation/evolution of precursor species in the initial stage of the process can be achieved.

The kinetics of the crystal growth of zeolite A were measured during the crystallization of zeolite A at 80°C from an amorphous aluminosilicate precursor (1.03Na 2 O·Al 2 O 3 ·2.38SiO 2 ·1.66H 2 O) dispersed in 1.2 M (system I), 1.4 M (system II), 1.6 M (system III), 1.8 M (system IV) and 2 M (system V) NaOH solutions. Analysis of the experimental results (changes in the concentrations C Al of aluminum and C Si of silicon in the liquid phase as well as of the dimension L m of the largest crystals of zeolite A during crystallization) shows that the growth rate of zeolite A crystals is independent of size, and that the growth is governed by the reaction of monomeric and/or low-molecular aluminate, silicate and aluminosilicate anions from the liquid phase on the surfaces of the growing zeolite crystals. The crystal growth takes place in accordance with the model of Davies and Jones for growth and dissolution, and taking into consideration the particularities of crystallizing zeolite systems, the kinetics of crystal growth of the zeolites can be defined as d L/ d t c =k g [C Al −C Al ∗][C Si −C Si ∗], where C Al ∗ and C Si ∗ are the concentrations of aluminum and silicon in the liquid phase which correspond to the solubility of zeolite A at given crystallization conditions, and k g is the rate constant of the linear growth of zeolite A crystals.

Positron annihilation (PA) techniques became, by now, routinely used nuclear techniques with applications which range from structural investigations of materials to biological samples and medical applications in positron emission tomography (PET). As at present, both PA spectroscopy and PET could profit from an improvement in the fast coincidence technique, it was investigated how far a fully digitized positron lifetime spectrometer (with full event storage capability for the simultaneous recording of time and energy information of the γ -rays) could offer advantages over a conventional setup employing fast analogue nuclear electronics. The additional information allows off-line analysis which could be particularly useful in separating various annihilation channels, like the positronium contribution, recently recognized to offer a new “ window of observation” .

Crystallization–transformation processes in the reaction mixture: (4Na2O–0.1667Al2O3–100SiO2–4000H2O) seeded with 4 wt% of 260 nm silicalite-1 seeds, at 170 °C were investigated by various experimental methods such as, X-ray diffraction (XRD), electron diffraction (ED), scanning-electron microscopy (SEM), transmission electron microscopy (TEM), particle size analysis (PSD), pH measurement, atomic absorption spectrometry (AAS) and dynamic light scattering (DLS). Analysis of the obtained results have shown that the crystallization–transformation takes place by a chain of processes: (i) dissolution of amorphous silica (used as silica source), and formation of 10–50 nm precursor species (PSs), (ii) spending of the PSs for both the limited growth of zeolite ZSM-5 on the surfaces of silicalite-1 seed crystals and formation of worm-like particles (WLPs), (iii) formation of condensed aggregates (CAs) by aggregation/condensation of the WLPs; a part of the composite particles, composed of the silicalite-1 core and ZSM-5 shell are incorporated into CAs, (iv) nucleation of keatite followed by solution-mediated transformation of the condensed WLPs into lamellar keatite, (v) nucleation of α-quartz followed by solution-mediated transformation of keatite into α-quartz; a part of the nuclei of α-quartz are formed on the surface of the core–shell silicalite-1–ZSM-5 composite species and another part of the nuclei are formed in the vicinity of the dissolving keatite and (vi) dissolution of the silicalite-1 core and a major part of the ZSM-5 “interlayer” of the composite particles (α-quartz shell-ZSM-5 “interlayer”-silicalite-1 core) and formation of hollow α-quartz particles with a thin ZSM-5 layer on the inner surfaces of the hollow α-quartz particles. Occurrences of all the mentioned processes are rationally analyzed and explained on the basis of the existing and newly obtained knowledge.

Positron annihilation lifetime spectroscopy has been used for observation of the free voids in low silica zeolites, of LTA and FAU structure and different crystal size, before and after their heating at different temperatures below 200 A degrees C. Sizes of crystals of the zeolite samples, as well as their structures and water contents, were characterized by X-ray diffraction, scanning electron microscopy, thermogravimetry, and particle size distribution measurements. The number of positron lifetime components as well as their values and intensities, after water removal from the zeolite structure, vary. However, changes are not uniform in all investigated zeolites. The differences are compared and discussed in the dependence of different structure types and crystal sizes of investigated zeolites.

The solid phases of: (i) freshly prepared aluminosilicate gel, (ii) the same gel aged for 48 h at either room temperature or at 40 °C and (iii) products of hydrothermal treatments (at 80 °C for 240 min) of gels (i) and (ii) were analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and positron annihilation lifetime spectroscopy (PALS). Additionally, crystal size distributions (CSD) of the crystalline end products obtained by hydrothermal treatment of (i) and (ii) were measured. By analysis of the o-Ps annihilation lifetimes τ3 and τ4, it was concluded that in all amorphous (gel) and crystalline (zeolite A) samples annihilation takes place in voids characteristic for the crystal structure of zeolite A. The number of such structural elements increases with ageing of the hydrogel, as evidenced by the increase of the o-Ps lifetime intensities I3 and I4 of the aged hydrogels. Based on the combined analysis of PALS (measuring of intensities I3 and I4, associated to lifetimes τ3 and τ4) and CSD (measuring of average crystal size and specific number of crystals) data, a correlation between the amount of (quasi)crystalline phase and the intensities I3 and I4 is assumed.

Positron annihilation spectroscopy (PAS) established itself as a routine method for structural studies of ordered substances in general, but some media present PAS results that are still puzzling. This situation is notably true for solid porous systems. This work recalls reasons for possible large discordances in the PAS results on such substances but focuses attention not only on the sample production, preparation and treatment prior and during the PAS measurements but also on the determination of the appropriate measuring conditions and establishing the optimal range of interest of considerations for obtaining descriptive parameters which provide the requested critical sensitivity. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Solid phase (gel) separated from freshly prepared sodium aluminosilicate hydrogel as well as during its hydrothermal treatment at 80 °C was analyzed by different methods such as powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential thermal gravimetry (DTG), high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). Analysis of the obtained results have shown that the freshly prepared gel is mainly composed of disc-shaped primary particles, but also partially or even fully crystalline entities were observed by AFM analysis. AFM analysis of the solids separated from the hydrogel at various stages of its hydrothermal treatment (heating at 80 °C) indicates that the particles of the partially and/or fully crystalline phase are nuclei for further crystallization of zeolite. Although it is assumed that the nuclei have structure of faujasite rather than zeolite A, further growth of zeolite A was readily explained by the fact that regardless of the “structure” of nuclei, the type of zeolite to be formed is determined by the concentrations of silicon and aluminum in the liquid phase of the crystallizing system.

Fe-exchanged samples of zeolite A were prepared and their time-dependent properties during hydrothermal crystallization were studied by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), Rutherford backscattering (RBS) and 57Fe Mossbauer spectroscopy. The relative amounts of various iron species deduced from 57Fe Mossbauer spectra of the Fe-exchanged samples are in a good correlation with the extent of ordering during the hydrothermal crystallization

Positron annihilation lifetime spectroscopy (PALS) is sensitive and proved method for material structure investigations. We used it to investigate the influence of different alkali cations on the structure of zeolite precursors in order to understand better the crystallization mechanism of zeolites. Our previous PALS investigations on Li gels show different structural features obtained for differently prepared gels, indicating the importance of the way on which cations are introduced in the gel matrix. These investigations were now extended to gels containing K, Rb and Cs cations, in correlation to their ionic radius and the way of preparation.

X-ray amorphous Me-aluminosilicates (Me = Li, K, Na, Rb and Cs) were prepared in two ways: (A) by ion-exchange of sodium ions from Na-aluminosilicate gel with other alkaline cations (Li, K, Rb and Cs) resulting in Me-gels(A) and (B) by direct precipitation from the corresponding Me-aluminate and Me-silicate solutions resulting in Me-gels(B). The Me-aluminosilicates prepared by both methods (zeolitic forms) as well as the products of their solid-state, high-temperature transformations (ceramic forms), were characterized by different techniques such as X-ray powder diffraction (XRD), elemental analysis by X-ray dispersive spectroscopy (EDS), derivative thermogravimetry (DTG), differential scanning calorimetry (DSC) and texture analysis (BET method). Elemental analyses of prepared Me-gels(A) have shown that the exchange of sodium cations from the Na-aluminosilicate gel does not considerably change its chemical composition, structural and physical properties. On the other hand, the type of ion-exchanged cations Me + considerably influences the aluminosilicate composition in the Me-gels(B), and thus their skeleton as revealed by XRD analysis. Thermal treatment of Me-gels(A) and the Me-gels(B) (isothermal heating at 925–1250 °C for 3 h) resulted in their transformation into appropriate crystalline aluminosilicate phases having chemical composition MeAlSiO 4 or MeAlSi 2 O 6 , respectively. All the transformations are accompanied with a drastic decrease in the specific surface area caused by the transition of porous amorphous Me-aluminosilicates into more dense, non-porous crystalline phases.

Positron lifetime (LT) measurements have revealed that in porous media the annihilation pattern is probably dominated by a near-saturation level trapping both for the positron and the positronium as well, which masks the contribution from the ldquofree annihilationrdquo, i.e., annihilation from delocalised positrons. In this work we compare positron lifetime spectra of crystalline zeolites with bulk ceramic materials fabricated by sintering from the same constituents as the zeolites and analyse common features and differences between them. For the dense ceramic samples the contribution from long-living components to the annihilation pattern is substantially lower than that for the zeolites. The results are analysed with the aim to extract new knowledge for the zeolites and for the sensitivity limits for free volume studies in ceramics. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Positron annihilation acquired acceptance for structural investigation of solids but results in porous media--where positron lifetime spectroscopy (LT) reveals substantial Ps formation--were ambiguous. Data on zeolites lead to the conclusion that Ps trapping in competing "extended free volume" sites, inhomogeneous regions and grain boundaries occurs. Furthermore, positron trapping must also be considered. Systematic errors due to incomplete time range selection are discussed, significance and importance of corrections for 3[gamma]/2[gamma] counting efficiency differences are shown in practice. http://www.sciencedirect.com/science/article/B6TVT-4JYTRSS-1/1/b6007344f10f5d0b3c23cc259fe1e8cb

Kinetics of the crystal growth of zeolites A and X was measured during their crystallization, at 80 °C, from an amorphous aluminosilicate precursor dispersed in a 1.4 M NaOH solution containing different amounts of dissolved NaAlO2 or Na2SiO3. The crystallization pathway and fractions of zeolites A and X in the crystalline end product strongly depend on the composition of the liquid phase of the crystallizing system. Analyses of the changes of the concentrations of aluminum, cAl, and of silicon, cSi, in the liquid phase as well as of the dimension, Lm, of the largest crystals of zeolites A and X during crystallization, have shown that the growth rate of zeolite A crystals is size-independent, and that the growth is governed by the reaction of monomeric and/or low-molecular aluminate, silicate and aluminosilicate anions from the liquid phase on the surfaces of growing zeolite crystals. Influence of the composition of the liquid phase of the crystallizing system on the course of crystallization process and on the growth rates of zeolite A and zeolite X crystals are discussed in terms of the possible distribution of aluminate, silicate and/or aluminosilicate anions in the liquid phase., Brzine rasta kristala zeolita A i X mjerene su tijekom kristalizacije pri 80 °C iz amorfnoga alumosilikatnoga prekursora dispergiranoga u 1,4 M otopini NaOH koja je sadržavala različite količine otopljenoga NaAlO2 ili Na2SiO3. Tijek kristalizacije i frakcije zeolita A i X u produktima kristalizacije značajno ovise o sastavu tekuće faze kristalizacijskoga sustava. Analizom promjena koncentracija aluminija, cAl i silicija, cSi u tekućoj fazi te veličine Lm, najvećih kristala zeolita A i X tijekom kristalizacije, utvr|eno je da je brzina rasta kristala neovisna o njihovoj veličini te da se rast kristala odvija reakcijom monomernih i/ili niskomolekularnih aluminatnih, silikatnih i alumosilikatnih aniona iz tekuće faze na površini rastućih kristala zeolita. Utjecaj sastava tekuće faze kristalizacijskih sustava na tijek procesa kristalizacije i brzinu rasta kristala zeolita A i X razmatran je u odnosu na moguće raspodjele aluminatnih, silikatnih i alumosilikatnih aniona u tekućoj fazi.

Changes of the concentrations of aluminium, C-Al, and of silicon, c(Si), in the liquid phase as well as of the dimension, L-m, of the largest crystals were measured during crystallization of zeolite A from the amorphous aluminosilicate precursor (1.03 Na2O . Al2O3 .2.38 SiO2 . 1.66 H2O) dispersed in alkaline solutions (1.2 or 1.4 M NaOH solution) containing or not containing dissolved Al2O3 and/or SiO2 Of appropriate concentrations. Analysis of the relationship between the concentration factors f(c) = f(c(Al),c(Si)) relevant to different growth models and the growth rate of zeolite A crystals have shown that the growth is governed by the reaction of monomeric and/or low-molecular aluminate, silicate and aluminosilicate anions from the liquid phase on the surfaces of growing zeolite crystals, and hence that the kinetics of the growth rate of zeolite A crystals may be expressed as: dL/dt(c) = k(g) [c(Al) - c(Al)(eq)][c(Si) - c(Si)(eq)] = k(g) f(c), where t(c) is the time of crystallization, c(Al)(eq) and c(Si)(eq) are the aluminium and silicon concentrations in the liquid phase, corresponding to the solubility of zeolite A under given crystallization conditions, and k(g) is the rate constant of the linear growth of zeolite A crystals.

Analyses of the distribution of nuclei in matrices of differently prepared and treated aluminosilicate gels have shown that both a specific number (number per unit mass or volume) of nuclei and their distribution f (N) in the gel matrix depend strongly on the chemical and physical conditions under which hydrogel is prepared. The distribution of nuclei in most of gel precursors is inhomogeneous (number of nuclei decreases from the “surface” of gel particles to their “interior”); the homogeneous distribution may be a special case.

Influence of the simultaneous change of batch alkalinity (A = [Na2O/H2O]b) and total batch silica concentration (tsc = [SiO2/H2O]b) of the reaction mixture, at two characteristic batch molar ratios, y = [SiO2/Al2O3]b (1.3 and 2.0), on the kinetics of the nucleation, crystal growth and kinetics of crystallization and on particulate properties of crystallized zeolite A, was investigated. It was found that the mentioned processes during crystallization from high and medium alkaline systems (0.0165 ≤ A ≤ 0.046) takes place in accordance with previously described experimental evidences, but that the anomalous kinetics of nucleation during crystallization of zeolite A from low alkaline systems (A ≤ 0.01) was not either predicted theoretically or, till recently, evidenced experimentally. Results of the analysis of distribution of nuclei in gel matrix have shown that in low alkaline systems, most of the nuclei are concentrated in the "centers" ("cores") of gel particles and rest of nuclei are distributed between the "surfaces" and "centers" of gel particles. Population balance analyzes of the crystallization processes, showed that the nucleation in the low alkaline systems takes place in two steps as a consequence of the mentioned distribution of nuclei in the gel matrix. Another consequence of the anomalous distribution of nuclei in the gel matrix is that more than 95 wt. % of zeolite A is formed by the growth of the nuclei released from the gel matrix during the first step of nucleation, and thus that the maximum rate of nucleation in the second step is reached at the crystallization time when about 80 – 90 wt. % of gel has been transformed into zeolite A. This is, at the same time, the reason that the crystal size distributions of the products obtained by crystallization of zeolite A from low-alkaline systems are characterized by two separate crystal populations.

KFKI Res. Inst. for Part. Nucl. Physics P.O.B. 49, H-1525 Budapest 114, Hungary Div. of Mater. Chem., Ruder Boskovic Institute Bijenicka 54, P.O.B. 180, Zagreb, Croatia Inst. of Isotopes, HAS, H-1525 Budapest, P.O.B. 77, Hungary Flerov Laboratory of Nuclear Reactions, Center of Applied Physics, JINR Ru-141980 Dubna, Russia Dept. of Mater. Physics, Eotvos Univ. Pazmany Peter setany 1A, H-1117 Budapest, Hungary f ICEMS, Dept. of Physics, University of Coimbra 3004-516 Coimbra, Portugal Dep. de Engenharia Quimica, Inst. Superior de Engenharia 3031-199 Coimbra, Portugal

In porous media, positron annihilation techniques (PAT) revealed the existence of annihilation modi with long lifetimes connected to structural and extrastructural free volumes acting as trapping centres for o-Ps. Our aim in this work was to study the influence of the synthesis applied for the production of the zeolite framework and the correlation between the trapping pattern and crystallization progress. PAT lifetime (LT) measurements were performed in silicalite-1 samples in various stages of crystallization and a large change was observed in the long components. The results are discussed in connection to the crystallization and the availability of free volume.