Zeolites structural defects

In addition to existing or potential framewoik vacancies, structural defects may also be present in as-synthesized zeolites (ex. Beta zeolite). Structural defects also lead to mesopores formation during dealumination. If such defects are characterized by specific orientations, the mesopores created can present similar orientations (this is the case for dealuminated fluoride medium synthesized Beta). 

Zeolite structures sometimes remain unsolved for a long time, because of either their complexity, the minute size of the crystallites or the presence of defects or impurities. One extreme example of stacking disorder is provided by zeolite beta [1,2], Different stacking sequences give rise to two polymorphs (A and B) in zeolite beta that always coexist in very small domains in the same crystal. Not only do the small domains make the peaks in the powder X-ray diffraction pattern broad and thereby exacerbate the reflection overlap problem, but the presence of stacking faults also gives rise to other features in the diffraction pattern that further complicate structure solution. 

During the last decade large progresses have been performed in the so much difficult art of zeolites synthesis. As a consequence, the amounts of structural defects and chemical impurities have been reduced in zeolite samples (crystallites of larger sizes and well-defined morphology have been synthesized ). At the same time, the zeolite sorption capacities increase. Such an observation is well illustrated by the sorption... 

The bulkier 1,3,5 TIPB showed about one order of magnitude faster diffusion in Si-MCM-41 than in NaX zeolite (16). Moreover, contrary to prevalent expectation, this molecule shows higher diffusivity than the smaller xylene isomers. These observations are indicative of the diffusion occurring in larger cylindrical pores (mesopores) of the Si-MCM-41 sample. However, diffusivity values of 1,3,5 TIPB, as well as PFTBA, are of the 10 9 cm2/s order, which is more typical of diffusion in zeolites and other microporous materials. The relatively slow diffusion of these molecules in the larger mesopores could be related to some hindrance effects resulting from structural defects and/or from the presence of extra-framework materials in the cylindrical mesopores. 

It now seems that the Loewenstein rule is obeyed by all known zeolites, at least on a spatially averaged basis. It is of course possible that occasional A1—O—A1 linkages are present as structural defects. 

Clathrasils are host/guest complexes comprised of covalent guest molecules entrapped within cages formed by a silica host framework (1, 2). Like all zeolitic materials, clathrasils have enormous potential as advanced optical and electronic materials whose composite character permits synthetic manipulation of both the molecular structure of the guest species and the extended structure of the host framework (3, 4). Like other zeolites, however, clathrasils also suffer severe handicaps as advanced materials due to a reluctance to form large single crystals and a tendency to form stoichiometrically and structurally defective crystals (5 -10). 

On the other hand, many syntheses procedures are carried out using fluoride anions as a substitute for hydroxide ions, resulting in zeolites or related material with higher crystal sizes, and lower structural defects with respect to standard procedures via OH- [97-115], Temperature is also a significant factor. Generally, the temperatures used are below 350°C. Besides, high values of temperatures yield more condensed phase species. The pH, reaction time and stirring of the reaction mixture are also important factors. 

Some experimental studies point out that the diffusion rate of pure hydrocarbons decreases with the coke content in the zeolite [6-7]. Theoretical approaches by the percolation theory simulate the accessibility of active sites, and the deactivation as a function of time on stream [8], or coke content [9], for different pore networks. The percolation concepts allow one to take into account the change in the zeolite porous structure by coke. Nevertheless, the kinetics of coke deposition and a good representation of the pore network are required for the development of these models. The knowledge of zeolite structure is not easily acquired for an equilibrium catalyst which contains impurity and structural defects. 

Under our experimental conditions, complete removal of the carbonaceous residues leads to the appearance of structure defects and/or short-range amorphization of the zeolitic framework, not detectable by X-ray diffraction. The coke produced during ortho-xylene cracking is the most difficult to remove and its elimination leads to pronounced modifications of the structure and of the crystallinity, doubtless because of its greater poly aromatic (or graphitic) character. 

Different ways have been proposed to prepare zeolite membranes. A layer of a zeolite structure can be synthesized on a porous alumina or Vycor glass support [27, 28]. Another way is to allow zeolite crystals to grow on a support and then to plug the intercrystalline pores with a dense matrix [29], However, these two ways often lead to defects which strongly decrease the performance of the resulting membrane. A different approach consists in the direct synthesis of a thin (but fragile) unsupported monolithic zeolite membrane [30]. Recent papers have reported on the preparation of zeolite composite membranes by hydrothermal synthesis of a zeolite structure in (or on) a porous substrate [31-34]. These membranes can act as molecular sieve separators (Fig. 2), suggesting that dcfcct-frcc materials can be prepared in this way. The control of the thickness of the separative layer seems to be the key for the future of zeolite membranes. 

The reaction of zeolites with an aqueous fluorosilicate solution under relatively mild conditions has been shown to yield zeolites with silicon enriched frameworks which are essentially free of structural defects (1). As a result of the treatment, the framework topologies of the respective zeolites are relatively unchanged, but the zeolite compositions which are produced either do not occur naturally or are not synthesized directly. The fluorosi1icate treatment process has been termed "Secondary Synthesis". 

Zeolite ZSM-20 with a rather constant Si/Al ratio of 5 only crystallizes when the initial Al concentration is sufficient. TEA+ together with the Na+ ions directly act as counterions to neutralize the Al-ffamework negative charges and very few Si-O-R (R = Na or TEA) structural defects are generated. A higher initial Al concentration does not affect the final composition of the material but markedly increases the yield. 

The results indicate that both NH4,TMA-fl and NH.,K-L are de-aluminated upon fluorination. Strong supporting evidence comes from framework I. R. data where the shifts in band position to higher wave numbers are as much as 20 cm-1. However, there is no evidence of structure stabilization. Also McBain water adsorption data give no indication of surface hydrophobicity. Therefore, it is likely that structure defects are formed in these two zeolites as a result of dealumination and cause low thermal stability. 

Experimental information on LASs is poorer and complicated by uncertainties and contradictions of experimental data. Thus, even in the case of zeolites which were investigated in a great number of experimental works, there is still no certainty about the nature of LASs. Our knowledge of active sites formed by low-coordinated transition-metal ions and other surface structural defects is even less definite. 

The experimental methods for measurement of transport and self-difiiision in zeolite crystals (and in other microporous materials) are reviewed. Large discrepartcies between different techniques are commonly observed and appear to be related to the scale of the measurements, suggesting that structural defects may be more important than is generally believed. 

The molecular field distribution within the channels must be investigated, taking into consideration the structure of the zeolite, and the calculation of the potential energy of interaction between the zeolite and particular molecules must be made. These investigations would be assisted greatly by spectroscopic studies which would make it possible to establish the nature of the zeolite surface, the presence and the nature of structural defects, and the state of the adsorbed molecules. 

Since the first 29xe NMR study of xenon adsorbed on a zeolite, this technique has been shown to be of interest for the investigation of the distribution and the size of supported metal particles, the quantitative distribution of phases chemisorbed on these particles, the dimensions of the void spaces of zeolites, the detection of structure defects, the location of cations and the effect of electric fields they create [1,2], We report here some typical applications related to the study of the location and the electronic structure of the cations. 

The insertion of Ti in the zeolite framework was accompanied by a significant decrease in A1 content (Table 1). However, there was no stoichiometric process between A1 removal and Ti insertion. Moreover, it was found that the treatment of Ig of an aluminum containing beta zeolite with a 75 ml of 3 x 10 M oxalic acid solution decreased the Si/Al ratio from its original value of 30 to 85 due to A1 extraction. Attempts to incorporate Ti into other zeolites like ZSM-12 and mordenite were not successful. Interestingly, the extraction of A1 from these zeolite structures was also unsuccessful with oxalic acid solutions with comparable concentrations. However, preliminary data show that siliceous mesoporous molecular sieves (MCM-41 and HMS) treated similarly with ammonium titanyl oxalate solutions exhibit good epoxidation activity. It is inferred that the presence of framework cations that can be extracted by oxalate species and/or the presence of defect sites in the parent zeolite is a requisite for the subsequent incorporation of titanium. 

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