Clear Aluminosilicate Solutions Research Articles

Mesoporous materials with enhanced porosity and acidity to obtain clean fuels from low-density polyethylene (LDPE) cracking

Mesoporous materials with Si/Al ratio 7, 19 and 23 have been synthesized from ZSM-5 zeolite precursors prepared from tetrapropylammonium cations-containing clear aluminosilicate solutions, in the presence of the surfactant cetyltrimethylammonium bromide, and by employing different precursors of secondary mesoporosity or mesoporogen agents, such as nanometric carbon particles (BP700 and BP2000) and the biopolymer chitosan. The synthesized materials were characterized by X-ray diffraction, FT infrared spectroscopy, thermogravimetric analysis, electron microscopy, chemical analysis and nitrogen adsorption. It has been found that the effect of the pore-forming agents depends on the Si/Al ratio of the material, it is very small for Si/Al = 7, while for Si/Al above 19, there is a substantial increase of the secondary mesoporosity due to the mesoporogen agent, being this effect more evident for the sample with Si/Al = 23. For this material, in addition to the porosity associated to pores with a narrow size distribution centered at 3.7 nm due to the surfactant, secondary mesopores with a size distribution centered at 20–30 nm for carbon nanoparticles and in the range 4–10 nm for chitosan, are also detected. The acidity of the samples increases with their Si/Al ratio, and no significant effect of the porogen agent on the acidity has been found. The catalytic properties of these materials were tested in the conversion of m-xylene and in the cracking of low-density polyethylene (LDPE). Noticeable differences in the activity and selectivity of the gas/liquid products obtained in the catalytic cracking of LDPE were observed.

Mechanism of crystallization of zeolite A microcrystals from initially clear aluminosilicate solution: A population balance analysis

Heating of the clear aluminosilicate solution (33.33 Na 2O Al 2O 3 3.33 SiO 2 400 H 2O) from the room temperature (23 °C) to that the reaction (40, 50 and 60 °C) causes a spontaneous precipitation of an amorphous aluminosilicate (gel) and simultaneous formation of zeolite nuclei in the gel matrix. Measuring of the mass, m s, of the precipitated amorphous aluminosilicate and simultaneous analysis of the change in the concentrations, C Al, of aluminium and, C Si, of silicon in the liquid phase, showed that aluminium and silicon contained in amorphous aluminosilicate represent only a small fraction of the total amounts of aluminium and silicon in the system. The amorphous aluminosilicate tends to dissolve immediately after the formation, which causes the releasing of nuclei from the gel matrix and their growth by the reaction of monomeric and/or low-molecular aluminate and silicate species from the liquid phase on the surfaces of the growing zeolite crystals. However, the growth process and thus the crystallization of zeolite A do not start immediately after beginning of heating of the reaction mixture (e.g., at the crystallization time t c=0), but later, at a time t c= t g(0)>0. An analysis of the influence of the time of heating of the reaction mixture from the room temperature (23 °C) to the reaction one (40, 50 and 60 °C) have shown that this factor has a negligible influence on the “delaying” of the crystal growth and crystallization. Hence, it was concluded that the “delaying” of crystal growth and crystallization was determined by the time needed for the formation of amorphous aluminosilicate (gel), formation of nuclei in the gel matrix and releasing of the nuclei during the dissolution of the gel matrix. The proposed mechanism of crystallization of zeolite A from clear aluminosilicate solution was confirmed by population balance analysis of the crystallization processes at 40 and 60 °C.

Effects of ultrasound on the preparation of zeolite A coatings

Preparation of zeolite 4A coatings on stainless steel substrates by direct synthesis at 60 °C from a clear aluminosilicate solution was carried out in the presence of ultrasound of 35 kHz at different times and the results were compared to those obtained by performing conventional static synthesis of zeolite 4A coatings under similar conditions. XRD and SEM analyses revealed that more nuclei formed at earlier times in the presence of ultrasound, allowing the preparation of thinner and continuous zeolite coatings in significantly shorter synthesis times. When zeolite 4A coatings were prepared by using conventional static synthesis at 60 °C, a closed layer with a thickness of about 1.5 μm could be obtained on stainless steel after a synthesis time of 10 h. Under the same experimental conditions, when sonication was applied, the layer thickness and the closure time were decreased to about 0.6 μm and 3 h, respectively. The surface roughness, which decreased initially, later started increasing upon continued nucleation and crystal growth, and showed a final decrease towards the closure of the layers. The final roughness value was smaller for the coating prepared in the presence of ultrasound.

Zeolite beta films synthesized from basic and near-neutral precursor solutions and gels

Zeolite Beta films were grown on silicon wafers from clear aluminosilicate solutions in alkaline media or from aluminium free gels in near-neutral nonalkaline (F −) media. Colloidal zeolite Beta seeds were preliminary deposited onto the substrates by either electrostatic adsorption or spin coating. The seeded wafers were then vertically placed in polypropylene reactors or autoclaves and hydrothermally treated with the synthesis solutions or gels at 100 °C or 150 °C. The products crystallized in the bulk solution and the corresponding zeolite films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), dynamic light scattering (DLS) and thermogravimetric analysis (TGA). In addition, the crystal orientation along the films depth was studied by grazing incidence X-ray diffraction (GID) using synchothron X-ray radiation. The BEA films prepared from clear solutions in alkaline media were relatively homogeneous, built up by crystals with a size of ca. 80 nm and with an overall thickness of ∼200 nm. The zeolite Beta films prepared in fluoride media were very rough with a thickness exceeding 20 μm and were composed of well-faceted uniform 2.5–3 μm truncated square dipyramidal crystals. The different composition of the films in terms of aluminium content resulted in different hydrophilicity/hydrophobicity of the zeolite Beta crystals building up the films. The GID analysis showed that the crystals of the zeolite Beta films grown from the two types of precursor synthesis mixtures were randomly oriented.

Crystal Growth Behavior of Zeolites Elucidated by Atomic Force Microscopy

Seeded growths of faujasite and zeolite A were investigated by atomic force microscopy (AFM). For seeded growth, clear aluminosilicate solutions (90Na2O:xAl2O3:9SiO2:5760H2O, where x = 0.1–1.0), which are useful for faujasite or zeolite A growth on seeded faujasite or zeolite A, respectively, were prepared at 353 K. Changes at the same position were observed along with growth period, and it was revealed that crystal growth behaviors of faujasite and zeolite A were different in the same aluminosilicate solution. Corresponding aluminosilicate solutions were characterized by Raman spectroscopy. By combining the results of both AFM and Raman spectroscopy, the behavior of aluminosilicate species incorporating into the solid phase from the liquid one was discussed. Key precursors of crystal growth for faujasite and zeolite A were proposed to be six-membered and four-membered rings, respectively.

Nucleation Rates of Zeolite A Crystals from Clear Aluminosilicate Solutions.

In order to understand the crystallization conditions of zeolite A, induction periods for nucleation from clear aluminosilicate solutions were measured and correlated with operating variables. The induction periods were found to decrease with supersaturation of reactant solutions and were represented by two lines when they were plotted against the supersaturation on a semilog ordinate system at given temperature and given NaOH concentration. At higher supersaturations, amorphous phases precipitated, while zeolite A crystals were obtained at lower supersaturations.For precipitation of zeolite A, the induction periods decreased with the NaOH concentrations, indicating that the crystallization rate of zeolite A was accelerated by the presence of NaOH. The boundary between the precipitation of zeolite A and that of amorphous phases was found to depend on the NaOH concentration and the ratio of [Al3+] to [NaOH] as well as saturation ratios.Finally, an attempt was made to determine the activation energy for nucleation and the surface energy of zeolite A on the basis of measured induction periods.

Lower temperatures for the preparation of thinner zeolite A coatings

The possibility of preparing thin zeolite 4A coatings on quartz and stainless steel substrates by direct synthesis from a clear aluminosilicate solution has been investigated. In this context, the effects that originate from the synthesis temperature on the thickness of the zeolite coatings have been determined. The nucleation curves and the crystal growth rates pertaining to the coatings prepared at different temperatures have been obtained by taking into account the variation of the numbers and sizes of the zeolite particles and the sizes of the largest crystals on the substrates, respectively, with respect to time. The apparent activation energies corresponding to nucleation and crystal growth have been estimated from these data. It has been determined that the thickness of closed zeolite 4A layers may be decreased at relatively lower synthesis temperatures in case relatively longer synthesis times are employed. The reduction obtained in the thickness of the zeolite coatings with decreasing temperature is due to the rate of nucleation being less temperature sensitive than the rate of crystal growth. The crystal growth activation energies corresponding to the synthesis of zeolite 4A coatings on stainless steel plates are noticeably higher than those obtained when quartz plates are employed while the apparent activation energies for nucleation on these two substrates are quite close. Zeolite 4A coatings of 1 and 0.7 μm thickness have been prepared on quartz and stainless steel substrates, respectively, at 45°C.

Preparation of zeolite coatings by direct heating of the substrates

A novel method is proposed for the preparation of zeolite coatings on substrates, which suppresses the reaction in the bulk and promotes that on the substrate by applying a temperature difference between the reaction mixture and the substrate. The substrate is heated directly, with a soldering resistance in this study, while the reaction mixture is kept at a lower temperature by means of a water bath. The method is tested for the case of zeolite 4A synthesis on stainless steel plates from a clear aluminosilicate solution. As a result of suppressed activity, the composition of the solution remains nearly constant and the solution side of the coating always experiences a lower temperature. As a result, the phase transformations of the metastable zeolites can be delayed for extremely long periods of time, depending on the volume of the reaction mixture. The deposition of crystals from the solution onto the surface of the coating is also avoided. Crystalline coatings of zeolite 4A could be prepared by the appropriate selection of the temperatures of the water bath and the substrate. A final treatment, which involves an increase in the temperature of the water bath, may be applied to the coatings at the end of their preparation, in order to remove the sparse amount of impurities observed at certain synthesis times and to increase the crystallinities when necessary. The mass of the zeolite coated on the substrate is shown to increase significantly with respect to that obtained by conventional synthesis when the proposed method is utilized. The method can thus be useful for some applications where the employment of relatively thicker zeolite coatings of metastable phases may be desired.

Investigation of the influence of seeding on the crystallization of zeolite A in the membrane-type reactor

The mutual interactions between zeolite A seed crystals, added to a clear aluminosilicate solution (seed-side), and an amorphous aluminosilicate gel (gel-side), were studied by crystallization of zeolite A. The seed-side in a membrane reactor was separated from the gel-side by a membrane (made of nylon-66) having average pore size openings of 0.8 μm. The membrane allows `free' passage of soluble species, while keeping the solids separate. An increase of the amount of zeolite formed by growth of the seed and a simultaneous decrease of the amount of zeolite crystallized in the gel-side of the reactor indicate that the reactive species formed by dissolution of the gel pass from the gel-side through the membrane to the seed-side of the reactor. Particulate properties of the zeolite formed in the seed-side of the reactor depend considerably on the reaction time needed for complete transformation of the gel into the zeolite, and thus on the amount of seed added to the seed-side of the reactor as well as on the agglomeration processes that occur simultaneously with the crystallization process. On the other hand, the particulate properties of zeolite A formed in the gel-side of the reactor do not depend on the amount of seed added to the seed-side of the reactor. Hence, it was concluded that the crystallization of zeolite A in the gel-side of the reactor occurs by the growth of nuclei `released' from the gel dissolved during the crystallization.

Study of the crystallization of zeolite NaA by quasi-elastic light-scattering spectroscopy and electron microscopy

A study of the crystallization of the molecular sieve zeolite NaA from clear aluminosilicate solutions is reported. It was determined by powder X-ray diffraction that zeolite NaA was the only phase in the solid precipitate. The syntheses were monitored in situ by quasi-elastic light scattering spectroscopy. Characteristics of the products were examined by scanning and transmission electron microscopy. Our data show that nuclei formed from the clear aluminosilicate solution, that crystal growth occurred from the solution, that crystal growth was accelerated at elevated temperatures, and that aging the solution at room temperature before raising the synthesis temperature increased both the number of nuclei that formed and also the crystal growth rate for aging times up to a limit. Adding crushed seed crystal fragments to an unaged synthesis solution produced similar results to aging the solution, including reduced induction times, increased crystal growth rates, and polycrystalline product formation.

Nucleation and crystal growth of analcime from clear aluminosilicate solutions

Hydrothermal syntheses of the zeolitic mineral analcime have been carried out in clear aluminosilicate solutions with batch composition Al2O3:84SiO2:87Na2O:2560H2O in the temperature range 130–160 °C and under autogenous pressures. Observation of the changes of the crystal sizes with time indicated that the analcime crystals grew at a constant rate, dependent on the synthesis temperature, and that the rate decreased when the particles settled to the bottom of the autoclave. At that point in the synthesis, a second population of nuclei was observed to form and grow in the solution above the settled crystals. It was demonstrated that when pure forms of silica were used in these syntheses, fewer crystals were formed. These results supported the concept that nucleus formation is promoted by impurities in the silica source.

Formation of bimodal zeolite crystal populations from clear synthesis solutions

Several recent reports on molecular-sieve zeolite synthesis from clear aluminosilicate solutions have indicated that bimodal crystal-size distributions were produced. This curious result was attributed to agitation or heterogeneous nucleation in one case. It is suggested by results presented here that bimodal crystal-size distributions may be the result of ageing the clear solutions at room temperature.