Zeolite synthesis mechanism

Future applications will also be considered, in particular with respect to the use of these techniques combined with the NMR of other nuclei (Na, Al, Si) for the investigation of zeolite synthesis mechanisms (12-14). 

The major effect of new advanced techniques on catalyst structure is found in zeolite catalysis. NMR techniques, especially MASNMR, have helped to explain aluminum distribution in zeolites and to increase our understanding of critical parameters in zeolite synthesis and crystallization. MASNMR, combined with TEM, STEM, XPS, and diagnostic catalytic reaction probes, has advanced our knowledge of the critical relationship between the structure and reactivity patterns of zeolites in the chemical fuels industry. Throughout the symposium upon which this book is based, many correlations were evident between theoretical quantum mechanical calculations and the structures elucidated by these techniques. 

Z., and Chen, J. (2005) Effect of agitation on the synthesis of zeolite beta and its synthesis mechanism in absence... 

Furthermore, investigations of template decomposition in zeolites upon calcination of the as-synthesized materials have received much attention by researchers (248-250). However, the decomposition mechanisms of quaternary ammonium salts, which are commonly used as templates for zeolite synthesis, are still not well understood. Thus, MAS NMR investigations of the decomposition of quaternary ammonium ions in zeolites bear the potential of significantly improving the understanding of these processes (243,251). For this purpose, a general method was developed for the synthesis of... 

The section on crystallization comprises zeolite synthesis, kinetics and mechanism of formation, stability relationships, recrystallization processes as well as the genesis of natural zeolites. Recent advances in this field have been surveyed, and some new perspectives have been outlined in the review by E. M. Flanigen. Most of the studies in this field are still empirical because of the complexity of the systems involved. Considerable progress has been made, however, towards a better understanding of the processes and mechanisms governing zeolite crystallization. It is not unreasonable to expect that conditions for synthesizing new zeolite structure types can eventually be predicted. 

Spectroscopic Studies of Zeolite Synthesis Evidence for a Solid-State Mechanism... 

While silicate and aluminate solutions have been extensively studied using 29Si and 27A1 NMR (see Sections III,B and III,I, respectively), there is only a handful of publications that investigate mixed (Si,Al)-bearing solutions by NMR spectroscopy with a view to elucidating the mechanism of zeolite synthesis. There is thus a considerable scope for further work in this important area. 

A recent interesting application of 13C n.m.r. has been in the investigation of the role of organic base in the preparation of ZSM-type catalysts. Tetra-propylammonium ions, for example, are very effective in the synthesis of crystalline ZSM-5. The 13C n.m.r. spectra of these occluded ions have been found to be sensitive to different environments within the zeolite,122-124 and hence can serve as useful probes in understanding the role of these template-like molecules in the synthesis mechanism and, as such, provide excellent complementary information to that obtained from Si and A1... 

Kinetics of Zeolite Formation. Since the D5R synthesis mechanism, if operative, is clearly disturbed by DMSO (vide supra), we also carried out a number of time-dependent syntheses in the absence of DMSO in the hope of being able to relate the kinetics with the D5R concentration present. Starting from a molar composition of 40 Si02 1 A OjX 5 NaOH 1000 H20 X at T - 190 °C, various organics and/or seed crystals (X) were added and the solid products were characterized after various synthesis times. Properties of the products obtained when the highest crystallinities were reached are summarized in Table II. 

The observations made here are not sufficient to prove any particular synthesis mechanism. However we may speculate as to mechanisms that are consistent with the observations. The synthesis of LTA probably proceeds by formation of sodalite units from D4R. This mechanism has been postulated before(14,15). D4R are present in silicate solutions containing sodium(16), potassium(17), and TMA(18) and in aluminosilicate solutions containing TMA(19). In TMA silicate solutions the fraction of Si in D4R decreases with dilution(20) and in TMA aluminosilicate solutions D4R s decrease with decreasing Si/Al(21). D4R with strict alternation of Si and Al, as required for zeolite A, can join in only one way and this is apparently facile as no template... 

The continuing studies on the mechanisms involved in zeolite synthesis are leading to a better understanding of the controlling factors in this complicated reaction system. But more remains to be done. This also holds the promise of leading to the synthesis of new zeolitic structures. 

In the early days of XPS the first applications of the technique to zeolites were dealing with the determination of Si/Al ratios calculated using equation (23) and their comparison with bulk values [14-16]. This of course allowed to detect important compositional gradients in the surface region, a piece of information which is related to the mechanism of zeolite synthesis and which is technically important in order to monitor the concentration of Brdnsted acid sites on the external surface of the zeolite crystals. The measurement of... 

From the above results we can propose the following mechanism for hexagonal MCM-41 synthesis. At low crystallization temperature or short crystallization time a fibrous agglomerate structure is often observed by SEM on intermediate samples. The 100 and 200 reflections are not detected by XRD and the value of the specific surface area is low. This reflects the initial step of synthesis which is generally referred to the nucleation step in zeolite synthesis. After this step, the 100 and 200 reflections are present on the XRD diffraction pattern. The value of the specific surface area is between 700 and 900 m /g. The fibrous agglomerate structure disappears and crystals of MCM-41 appear. This corresponds to the crystallization step. Finally if both the synthesis temperature and time are continuously raised, a triphasic mixture MCM-41, MCM-50 and amorphous phase is identified by XRD. The... 

The study of alkaline silicate and aluminosilicate solutions is of fundamental importance for a better understanding of the mechanism of zeolite synthesis (1,2). Real progress has been realized durinq the last fifteen years thanks to the use of new techniques such as the trimethylsilylation (3,4) of polysilicic acids (followed by a chromatographic separation of the derivatives) and, even more, 29Si and 27Ai NMR spectroscopy (5-11). The latter techniques have enabled the identification of about twenty types of oligomers in silicate solutions and in some cases estimates of their respective concentrations. The investigation of alkaline aluminosilicate solutions is... 

B.D. McNicol, G.T. Pott, and K.R. Loos. Spectroscopic Studies of Zeolite Synthesis Evidence for a Solid-state Mechanism, in Molecular Series . Adv. Chem. Ser., 1973, 121, 152-161. 

The following questions can in principle be addressed with spectroscopy (1) Zeolite synthesis what are the mechanisms of ZSM-5 synthesis and how do they influence the quality of the catalyst synthesized (2) Catalyst characterization what are the structure and composition of the zeolite, and what is the configuration of the active site for methanol conversion (3) How do methanol and dimethylether interact with the active sites i.e. what species are present in the catalyst in the initial stages of methanol conversion (4) What are the subsequent reaction pathways leading to the final alkane, alkene and aromatic products (5) What causes catalyst deactivation This question concerns both the temporary deactivation associated with coke formation, which can be reversed by oxidative regeneration, and the permanent deactivation which occurs after repeated deactivation-regeneration cycles. 

Cundy, Cox 2005 Hydrothermal synthesis of zeolites reaction mechanism [50]... 

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