Zeolite synthesis units

Framework infrared has also been used to look for the formation of a zeolite during synthesis. Since many of the secondary building units can be detected in the infrared spectrum, it is possible to see zeolite formation at very early stages in the synthesis. In fact, zeolite formation can be detected in the infrared before crystallinity is observed by X-ray diffraction. Most of the reported work has been done by sampling the zeolite synthesis at various stages, isolating the soUds and measuring infrared spectra of the dried samples. 

Chapter 3 outlines zeolite synthesis, modification and the manufacturing of zeolite-based catalysts and adsorbents. Extensive patent references are given to provide the reader with a historical perspective. Some of the pitfalls associated with the operation of synthesis and manufacturing units are also described. 

Dr. E.M. Flanigen is a world expert on zeolite synthesis. In the early years she was first to synthesize high silica Y with silica/alumina rations above 4.0, first to remove aluminum from zeolite lattices without loss of structure, and was responsible for identification and evaluation of the myriad of samples from Union Carbide s investigation of sedimentary zeolite deposits in Western United States. Additionally, I would like to acknowledge her assistance in... 

Fig. 3.3 Relationship between unit cell size and framework al content 3.2.2.2 MFI and BEA zeolite synthesis...

One-step direct synthesis of the highly stable mesoporous silica-based material is also possible. MMS-H[209] has a structure analogous to that of MCM-48 but which contains zeolite building units. A mixture of CTAB and Brij30 was used as template for the mesopores. The use of TPAOH without the assistance of NaOH helps to introduce zeolite secondary building units, as well as the direct formation of acidity after removal of the template. This material was also found to possess superior thermal, hydrothermal, steam, and mechanical stabilities. 

It seems reasonable to suppose that the construction of the nucleus of a zeolite crystal may involve a more complex assembly process than is necessary for simpler substances whose unit cells are smaller and contain far fewer atoms, although there should be no difference in basic principles. The nature of the nucleation process in zeolite systems is also complicated by the nature of zeolite synthesis sols. These contain observable solid phases (amorphous gel, crystals) and components (cations, anions) in true solution. However, there is frequently also a colloidal component, invisible to the naked eye, which itself may be amorphous or crystalline. 

There are thus groups who rely on more conventional autoclaves also for parallelized studies, but the transition to conventional zeolite synthesis are not sharply defined. At hte-company an autoclave system is used which allows larger sample volumes (Fig. 3). 16 inserts are used in one autoclave housing. These inserts can be filled by a dispensing robot and simultaneously placed in a heating unit. Centrifugation in a module, which is adapted to the autoclave set, is used for product isolation. From the centrifuge module the product is transferred to an XRD sample holder where it can automatically be analyzed. 

Zeolites are widely used as acid catalysts, especially in the petrochemical industry. Zeolites have several attractive properties such as high surface area, adjustable pore size, hydrophilicity, acidity, and high thermal and chemical stability. In order to fully benefit from the unique sorption and shape-selectivity effects in zeolite micropores in absence of diffusion limitation, the diffusion path length inside the zeolite particle should be very short, such as, e.g., in zeolite nanocrystals. An advantageous pore architecture for catalytic conversion consists of short micropores connected by meso- or macropore network [1]. Reported mesoporous materials obtained from zeolite precursor units as building blocks present a better thermal and hydrothermal stability but also a higher acidity when compared with amorphous mesoporous analogues [2-6]. Alternative approaches to introduce microporosity in walls of mesoporous materials are zeolitization of the walls under hydrothermal conditions and zeolite synthesis in the presence of carbon nanoparticles as templates to create mesopores inside the zeolite bodies [7,8]. 

Initially, the number of nuclei formed per unit time is expected to increase. As nuleation and growth is assumed to consume the same precursor species, both processes at a given stage will start to compete. The nucleation rate will therefore pass through a maximum in time and then decline, in zeolite synthesis many experimental S-shaped crystallization curves have been observed (16,18). Zdhanov and Samulevich advanced a method for analysis of the nucleation and growth part of such curves (25). 

SAXS measurements are normally performed in situ and therefore require access to synchrotron radiation [21, 23, 27, 46]. SAXS is probably one of the most used in situ techniques for the study of zeolite synthesis. In situ cells, similar to those for WAXS or XRD measurements, can easily cope with elevated synthesis temperatures and pressures. It has been used to determine the activation energy of nucleation, where two independent studies show that it is around 70-85 kj mol [47, 48]. Also, three distinct particle sizes were observed in the reaction mixtures, being primary units of approximately 2.8 nm, their aggregates, and the actual zeolite crystals [47]. Other mechanistic studies include the effect of the precursor molecule, the chemical nature of which appears to be important in... 

Zeolite synthesis can also be approached from classical nucleation-crystalli-zation theory [105]. A viable nucleus is estimated to have a size between one and eight unit cells, depending on the structure type and the experimental synthesis conditions [106]. However, because of crystallization from high surface area systems, the energetics of zeolite nucleation can be significantly different from that of more dense phases [107]. Advanced techniques such as HRTEM and cryo-TEM are the basis of the experimental information about zeolite nucleation [108]. 

The zeolite synthesis solution provides a multitude of small oligomeric molecules that can display very different interactions with a selected template molecule. A specific complex with template molecule will have unique stability. Because of the equilibria between the oligomeric units, crystallization will consume all molecules from the mother liquid to form this particular complex through the recombination of particular oligomers formed. 

Zhang H, Guo Q, Ren L, Yang C, Zhu L, Meng X, et al. Organotemplate-free synthesis of high-sUica ferrierite zeolite induced by CDO-structure zeolite building units. J Mater Chem 2011 21 9494-7. 

Zeolite IZA structure code Typical unit cell composition Si02/Al203 range by synthesis Dimensionality of channel system Pore apertures (nm)... 

The initial transition of dissolved silicate molecules into solid nanoparticles is perhaps the least explored step in the synthesis of zeolites. One impediment to understanding this mysterious step is the poorly elucidated molecular composition of dissolved particles. The major mechanistic ideas for the formation of zeolites approach these structures differently i) many researchers believe that secondary building units (SBU) must be present to form initial nanoslabs [1,2] ii) some others prioritize the role of monomers to feed artificially introduced crystal nuclei or assume that even these nuclei form via appropriate aggregation of monomers [3] iii) silicate solutions are also frequently viewed as random mixtures of various siloxane polymers which condense first into an irregular gel configuration which can rearrange subsequently into a desired crystal nucleus at appropriate conditions [4,5],... 

Irrespectively of the iron content, the applied synthesis procedure yielded highly crystalline microporous products i.e. the Fe-ZSM-22 zeolite. No contamination with other microporous phases or unreacted amorphous material was detected. The SEM analysis revealed that size and morphology of the crystals depended on the Si/Fe ratio. The ZSM-22 samples poor in Fe (Si/Fe=150) consisted of rice-like isolated crystals up to 5 p. On the other hand the preparation with a high iron content (Fe=27, 36) consisted of agglomerates of very small (<0.5 p) poorly defined crystals. The incorporation of Fe3+ into the framework positions was confirmed by XRD - an increase of the unit cell parameters with the increase in the number of the Fe atoms introduced into the framework was observed, and by IR - the Si-OH-Fe band at 3620 cm 1 appeared in the spectra of activated Fe-TON samples. 

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