24 J Phys Chem B 2004 108 20465-20470 (original) (raw)
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Langmuir, 2014
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 Alenriched 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 Role of Sub-Colloidal ( Nano-Sized ) Precursor Species in Zeolites Crystallization
2013
A comparison of different approaches of the formation and evolution of sub-colloidal precursor species during crystallization of MFI-type zeolites showed that, although the formation of sub-colloidal precursor species is almost the same in both homogeneous [initially clear (alumino)silicate solutions] and heterogeneous [(alumino)silicate hydrogels], their further evolution strongly depends on the composition of the reaction mixture, especially presence (heterogeneous systems) or absence (homogeneous systems) of the solid phase as well as presence/absence of aluminium. These differences have been showed and explained through the analysis of relevant processes of crystallization of MFI-type zeolites.
Crystallization of Zeolites from Organo-Silicic Colloids
Inorganic Chemistry, 2006
As shown recently, the networks of mesoporous high-surface-area silicates and zeolites undergo a deep depolymerization process in glycerol, near 200°C. Within 1 h, X-ray diffraction analysis amorphous gels are obtained. However, some local ordering subsists as demonstrated by a striking similarity between the silicon and aluminum high-resolution solid-state NMR spectra before and after the reaction. The residual organization could be investigated indirectly in studying the recrystallization of these gels in the presence or absence of structure-directing agents. Were this attempt successful, the way should be opened for the synthesis of molecular sieves starting from gels obtained from naturally occurring zeolites. Here, it will be shown that an amorphous gel obtained from HZSM-5 recovers the initial long-range structure of the parent material in a few hours at 85°C in the presence of an aqueous solution of tetrapropyl ammonium (TPA) or NH 3. The recrystallization of HY requires the presence of tetramethylammonium, but about 25% of the crystallization is obtained rapidly (=1day) at 80°C with ammonia. Hypotheses about the preorganized structural units are presented. The value of the Si−O−Si angle in the silica cluster seems to be of paramount importance.
Mechanism of Zeolite A Nanocrystal Growth from Colloids at Room Temperature
Science, 1999
reactions (such as condensation of the sol Mechanism of Zeolite A particles); both of these results could cause the aggregation. The XRD pattern of this Nanocrystal Growth from sample shows the presence of an amorphous phase (Fig. 2A). Further confirmation of the Colloids a t Room Temperature Svetlana Mintova,'s3 Norman H. Olson? Valentin Valtchev? Thomas Bein1* The formation and growth of crystal nuclei of zeolite A from clear solutions at room temperature were studied with low-dose, high-resolution transmission electron microscopy in field emission mode and with in situ dynamic light scattering. Single zeolite A crystals nucleated in amorphous gel particles of 40 to 8 0 nanometers within 3 days at room temperature. The resulting nanoscale single crystals (10 to 30 nanometers) were embedded in the amorphous gel particles. The gel particles were consumed during further crystalgrowth at room temperature, forming a colloidal suspension ofzeolite A nanocrystals of 40 to 8 0 nanometers. On heating this suspension at 80°C, solution-mediated transport resulted in additional substantial crystal growth.
Zeolites, 1997
studies have been conducted at 25, 60, and 80°C with a clear solution batch composition that produces zeolite NaA. Aluminosilicate precursor material was added to promote the synthesis and to evaluate the effects of adding initial-bred nuclei that were created by a precursor "seed" reaction. It is shown that the initial-bred nuclei promoted the crystallization rate, that the crystalline mass produced was increased when using the initial-bred nuclei, and that the nucleation of a second population was not inhibited by the addition of initial-bred nuclei as seeds. 0 Elsevier Science inc.
Microporous and Mesoporous Materials, 2007
An aqueous aluminosilicate hydrogel having oxide molar composition: 4.72Na 2 O AE Al 2 O 3 AE 1.93SiO 2 AE 254.86H 2 O was treated in two ways: (1) the hydrogel (system S NL ) was heated at 80°C until the entire amount of amorphous aluminosilicate precursor (gel) has been transformed to zeolite A, (2) the hydrogel was frozen in liquid air and freeze-dried; solid liophilizate was poured in an appropriate volume of water, and the obtained suspension (system S L ) was treated in the same way as the system S NL . The drastic treatment of hydrogel did not substantially change the distribution of nuclei in the matrix of the amorphous aluminosilicate precursor (gel) established during its precipitation. The consequence is that, in accordance with the principles of the gel ''memory'' effect, the crystal size distributions of the crystalline end product (zeolite A) obtained from both the systems (S NL and S L ) are almost the same. On the other hand, the above mentioned treatment of hydrogel changes the concentrations of aluminium and silicon in the liquid phase and caused that the rate of crystal growth is somewhat higher in system S L than in system S NL . Population balance analysis of the crystallization processes has shown that the difference of the kinetics of crystallization of zeolite A from the systems S NL and S L is in larger extent caused by the small difference in the crystal growth rate and in the smaller extent by the particulate processes determined by the specific number of nuclei N S and distribution of nuclei in the gel matrix.
Croatica Chemica Acta
There is abundant experimental evidence that most, or even all zeolite nuclei are formed in the aluminosilicate gel and/or gel/liquid interface by a linking of specific subunits during gel precipitation and/or ageing. Since the nuclei (particles of quasicrystalline phase) cannot grow inside the gel matrix, they start to grow after being »released« from the gel dissolved during the crystallization, i.e. when they are in full contact with the liquid phase (autocatalytic nucleation). Based on these findings it was assumed that the rate of autocatalytic nucleation depends on the rate of gel dissolution as well as on the number and distribution of nuclei in the gel matrix, but that crystal size distribution in the crystalline end product depends exclusively on the number and distribution of nuclei in the gel matrix and not on the crystallization conditions, or even on the treatment of aluminosilicate gel precursor prior to crystallization. This so called »memory« effect of amorphous aluminosilicate precursors was evidenced by simulation of zeolite crystallization under different conditions, using the population balance method.
Indirect Observation of Structured Incipient Zeolite Nanoparticles in Clear Precursor Solutions
Angewandte Chemie International Edition, 2008
The structure of the zeolite precursor particles present in clear synthesis solutions yielding colloidal silicalite-1 (purely siliceous zeolite with MFI-type structure) is one of the most controversial issues in modern zeolite science. Owing to the nature of the synthesis and the absence of Al, these systems are widely used as models to study the mechanism of zeolite crystallization in general. Ever since the "nanoslab" hypothesis was proposed by Martens et al. it has been the subject of a lot of criticism. The major disagreement is about the structure of the subcolloidal precursor particles present in the initial clear mixtures. Whereas Martens et al. suggest that these particles have MFI structural features, an amorphous nature of the particles has been proposed in numerous other studies. In some cases, the nanoparticles were extracted from the synthesis solutions, and concerns have been raised about possible changes in their structure or interference from the extraction procedure. Other authors base their arguments against the crystalline nature of the precursor particles on the results of solid-state NMR analysis after 29 Si enrichment. A recent NMR study on aqueous silicate solutions revealed the presence of structural units typical of the MFI structure. Other complementary techniques such as dynamic light scattering have also been used to further characterize the species present in the precursor solu-tions. Thus, despite the fact that a remarkable number of papers discussing the structure of the silicalite-1 precursor particles have already been published, the discussion is still ongoing and new publications regularly appear.
Crystallization of nanosized MEL-type zeolite from colloidal precursors
Materials Science and Engineering: C, 2002
The crystal growth of nanosized MEL-type zeolite at 90 8C from various colloidal precursor solutions was investigated. Dynamic light Ž. scattering DLS in backscattering mode monitors the evolution of particle size in synthesis solutions and purified MEL sols. All measurements were performed at original concentrations since complete penetration of the incident light through the sample is not required in backscattering geometry. In addition, the silicate species formed in the course of the condensation are identified by 29 Si NMR Ž spectroscopy. The DLS data show that upon aging of precursor solutions from 20 h to 2 weeks at RT, the precursor species with radius. of about 2 nm rearrange into species with 10-nm radius, prior to further hydrothermal treatment of the precursor colloids. Complete transformation from amorphous to MEL crystalline colloidal particles is observed after a 72-h extended hydrothermal treatment of the aged precursor solution. Increasing water content in the initial precursor solution leads to a considerable increase of the radius of the crystallites, i.e., from 100 nm to 2 mm.