Zeolite induction period

As far as we know,the influence of the viscosity of the medium on the size and distribution of the ZSM-5 crystallites has never been reported in the literature. The strong dependence of the induction period required for a zeolite nucleus to form, on alkalinity (pH), has been emphasized by Barrer (36). Yet, ZSM-5 zeolites appear to behave rather differently than other Al-rich zeolites. The 0H /Si02 ratio was recognized by Rollmann as the... 

As already stated, isomerization on zeolite HY was always accompanied by disproportionation, even at 180 C. With time on stream, Yo,. Np increases, because these heavy products are most efficiently held by the fresh catalyst. It is an interesting result that, at 180 °C, the yield of naphthalene passed through a maximiun as well. Obviously, under appropriate reaction conditions, the disproportiotuOion of methylnaphthalenes in zeolite HY exhibits an induction period, as does the disproportionation of ethylbenzene in large pore zeolites [39,40]. 

Zeolite crystallization can be interpreted in terms of a ripening mechanism. The initially formed gel consists of amorphous dispersed particles of the order of 100-300 A in size. Growth of these particles to approximately 1000 A occurs during the induction period after which zeolite crystals appear imbedded in the amorphous gel matrix. This is especially evident in electron microscopic studies of gel solids (66,88). Ciric comments on the observation of growing crystals imbedded in gel particles which, as the crystals grow, tend to shrink together, resulting in coalescence (74)-... 

Synthesis of Zeolite X. Zeolite X was crystallized from a batch of overall composition 4 Na20-Al203-5 SiO2-200 H20 at 90° C (8). The crystallization curve is shown in Figure 7. After an induction period of 2.4 hours, zeolite X was formed rapidly as a single phase at a conversion... 

Figure 9. Dependence of conversion rate and induction period on temperature for zeolite A from a batch composition 2.5 NOtO-AUOr-l-7 Si0r 150 H2O...

It was possible for two of the systems chosen that the nucleation and crystallization activation energies could be determined separately by distinguishing the induction period and crystal growth period in the overall crystallization process. Of the two hypotheses proposed for zeolite crystallization, in the gel phase or from the solution phase, the data support the latter hypothesis for crystal growth with the crystal-liquid surface enhancing the nucleation process in seeded systems. The precise mechanism of nucleation in unseeded systems remains to be determined. 

The crystallization of zeolites from alkaline aluminosilicate gels was studied by luminescence and Raman spectroscopy. Trace amounts of Fe3+ ions substituted for Al3+in the tetrahedral aluminosilicate gel framework exhibit characteristic phosphorescence spectra, which have been used to follow the buildup of the zeolite framework. Phosphorescence spectra of exchanged Eui+ cations and Raman spectra of (CH N+ cations present in the solid phase of the gel indicate that no zeolitic cages exist in this phase during the induction period. Raman spectra of the liquid phase of the gel system show only the presence of Si02-(0H)2 and Al(OH)a anions. Our results demonstrate that crystallization of zeolites occurs within the solid phase of the gel, which is believed to consist of amorphous tetrahedral alumino-... 

In Linde A and sodalite syntheses the signal grew to about 20 times its initial intensity. In other systems, such as faujasite, the increase was somewhat smaller. The increase seemed to depend upon the Si/Al ratio of the resultant zeolite crystals—i.e., the smallest increase occurred for mordenite crystallizations having an Si/Al ratio of 5 (for Linde A and sodalite Si/Al = 1). No Fe3+ phosphorescence was observed in the liquid phase of the gel. In three experiments carried out under identical conditions Fe3+ phosphorescence studies of the growth kinetics gave identical results (induction periods equal within 5%, Fe3+ intensity increase on crystallization equal within 10%). 

Breck (1) was the first to investigate the reaction in the hydrothermal formation of zeolites. He found that there is always some delay before crystallization starts. This so-called induction period can be reduced by raising the temperature or alkalinity of the reaction batch (2). As Sand (8) reported in 1968 in connection with the formation of mordenite, the nature of the Si02 material also has a decisive influence on the reaction and the nature of the zeolite crystals. The induction period as a nucleation phase is discussed by Domine and Quobex (4) in connection with kinetic investigations relating to mordenite formation. 

Reaction Time. The rate of formation of to at reflux temperature is illustrated in Figure 2. The rate was estimated by taking samples from the reaction mixture at various times and examining them by x-ray diffraction. The crystallization is characterized by an unusually long induction period of about 13 hr, which is followed by a relatively slow crystal growth. No alteration of the zeolite was observed during an extra 50 hr reaction time after crystallization had been completed. 

A number of simple and inexpensive materials catalytically promote the cobalt-carbonylation (Reaction 2) in aqueous solution. These include ion-exchange resins, zeolites, or special types of activated carbon. Formation of the active catalyst in a separate reactor is thus economically feasible. The mechanism of this catalysis has not yet been elucidated and seems to differ for each promoter mentioned. After an induction period during which the cobalt fed to the reactor is partially retained by the promoter, fully active materials have absorbed cobalt carbonyl anion Co(CO)4 (ion exchange resins), Co2+ cation (zeolites), or a mixture of Co2+, cobalt carbonyl hydride, and cluster-type cobalt carbonyls (activated carbon). This can be shown by analytical studies (extraction, titration, and IR studies) of active material withdrawn from the reactor. 

The analysis of the kinetics of crystallization of different types of zeolites from aluminosilicate gels points to the conclusion that the crystallization takes place by the simultaneous growth of the constant number N0 of nuclei-I present in the system at the very start of the crystallization process and the number Na of nuclei-II released from the gel disolved during the crystallization process. Some characteristics of the crystallization systems such as the duration of the "induction period", the shortening of the "induction period" and the increase of the crystallization rate, respectively, with the gel ageing and the bimodal size distributions in the specific cases have been discussed and explained in relation to the ratio Na/N0 of particles (nuclei)-II and particles (nuclei)-I present in the crystallizing systems. 

It is well known that the low-temperature ageing of aluminosilicate gel precursor markedly influences the course of zeolite crystallization at the appropriate temperature (1-10). The primary effects of the gel ageing are the shortening of the induction period and the acceleration of the crystallization process (1-5), but in some cases the gel ageing also influences the type(s) of zeolite(s) formed (1,6,7,10). 

The induction periods of both zeolite X and zeolite Na-Pc shortens and the maximal yield of zeolite X increases, respectively, with the increased time of gel ageing. All kinetics of zeolite X and zeolite Na-Pc, respectively, can be mathematically expressed by the simple kinetic equation (5,23-26),... 

The shortening of the induction periods of the crystallization of both zeolite X and zeolite Na-Pc is most probably the consequence of the increase in the number of nuclei-I of both zeolites with the ageing of the gel. 

Initiation of Zeolite Crystallization. An induction period (tj) is always observed at zeolite crystallization, and is accepted to be the period necessary for the formation of zeolite nuclei (20). The inverse value of induction period, 1/t, is called the rate of nucleation. The crystallization curves were plotted as the degree of crystallization (estimated by the XRD method) versus synthesis time. The values of tj were obtained by extrapolating the time when crystal growth started. 

Phenomenology of the crystallization. The conversion versus time curves obtained at three different temperatures are shown in Figure 1. With the synthesis procedure used, the sigmoid curves were characterized by shorter induction periods than the traditional method (11,12). As expected, temperature had a strong effect on the rate of crystallization. The overall crystallization rates may be approximated by the reciprocal of the times of half conversion. From these values an apparent activation energy of 22 1 kcal/mol was obtained. With respect to literature data, this value exceeds that reported, for instance, for zeolite Na-X (1,4) but compares well with the 19.8 kcal/mol found for ZSM-11 (13). 

Once the high crystallization temperature is reached, zeolite crystallization will start after an induction period. During this induction period the dissolution of the gel phase continues, leading to the formation of clusters of polysilicates or aluminosilicates that become stable above a certain critical dimension (e.g. about 10 A for zeolite A and about 20 A for zeolite ZSM-5), and crystallization commences. The course of nucleation and crystallization of zeolites can generally be described by a characteristic S-shaped crystallization curve, in which the amount of crystalline material is plotted against crystallization time. Depending on the... 

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