Framework of zeolite

The discovery that Ti(IV) incorporated into the framework of zeolites produces an outstanding oxidation catalyst stimulated the incorporation of other... 

The frameworks of zeolites are formed by fully connected Si04 and A104 tetrahedra linked by shared oxygen atoms as shown in Figure 4.1 (top) for a Faujasite-type zeolite. Faujasite is a zeolitic mineral, which can be found in nature. Synthetic Faujasite-type zeolites are of particular importance in zeolite catalysis as we will see below. 

Figure 1.3. Framework of zeolite L. Upper Top view, perpendicular to the c axis, displayed as stick (left) and as van der Waals (right) representation with a dye molecule entering the zeolite channel. Lower Side view of a channel along the c axis, without bridging oxygen atoms.

Smith, J.V. (ed.) (2000) Microporous and Other Framework Materials with Zeolite-Type Structures, Suhvolume A Tetrahedral Frameworks of Zeolites,... 

Man, P.P. and Klinowski, J. (1988) Quadrupole nutabon A1 NMR studies of isomorphous subsbtubon of aluminum in the framework of zeolite Y. Chem. Phys. Lett., 147, 581-584. 

The typical unit cell content of zeolite L is (K,Na)gAi9Si27072.nH20 and its Si/AI ratio varies in the range of 2.6 - 3.5 [1-4]. Takaishi recently determined the distribution of Al atoms in the framework of zeolite L ly analyzing Si-MAS-NMR spectra. He thereby deduced five kinds of extra-framework cation sites as shown in Fig. 1., and estimated the relative strengths of their cation affinities [5]. 

The incorporation of Al atoms into the framework of zeolites occurs in a tetrahedral oxygen coordination and leads to negative framework charges. These framework charges are compensated by protons in acidic hydroxyl groups or by extra-framework cations such as Li , Na, Cs", Mg ", etc. Accordingly, these surface sites are responsible for the chemical behavior of zeolites in separation processes and in catalysis (199,200). 

It is well known that the elements in framework of zeolite molecular sieves greatly influence the properties and behaviors of these materials [1-3], The introduction of heteroatoms into the framework has become one of most active fields in study of zeolites. The investigations were mostly focused on the methods to introduce heteroatoms into the framework (for examples, hydrothermal synthesis and post-synthesis), the mechanisms for incorporations, the effect of heteroatoms on the acid-base properties and the catalytic features of modified samples [1-10]. Relatively less attention was paid to the effect of treatment process on the porous properties of samples although the incorporation of heteroatoms, especially by the so-called post-synthesis, frequently changes the distribution of pore size. Recently, we incorporated Al, Ga and B atoms into zeolites (3 by the post-synthesis in an alkaline medium named alumination, galliation and boronation, respectively. It was found that different trivalent elements inserted into the [3 framework at quite different level. The heteroatoms with unsuitable atom size and poor stability in framework were less introduced, leading to that a considerable amount of framework silicon were dissolved under the action of base and the mesopores in zeolite crystal were developed. As a typical case, the boronation of zeolites (3 and the accompanied formation of mesopores are reported in the present paper. 

Another promising approach to the study of microporosity of zeolites involves the measurement of the isotropic 13C NMR chemical shift which, as has been shown in the studies of the tacticity of polymers, is highly sensitive to the environment of the nucleus. In the first study of this kind, Boxhoorn et al. (329) observed that the C-3 carbon resonance from the tetrapropylam-monium cation enclathrated in the framework of zeolite ZSM-5 in the course of synthesis is split into two components of equal intensity. The reason for this is that the cation is located at the cross-section of the two nonequivalent... 

Breck has reviewed the early literature where Ga3+, P5+, and Ge4 were potentially incorporated into a few zeolite structures via a primary synthesis route (2). Evidence has also been presented to show that the small amounts of Fe3+, typically present in both natural and synthetic zeolites, are located in framework tetrahedral positions (3). A more recent review of "isomorphic substitution" in zeolites, via primary synthesis methods, speculates on the potential Impact of such substitutions on catalysis (4). The vast majority of work has been related to the high silica zeolites, particularly of the ZSM-5 type. Another approach to substitution of metal atoms into the open frameworks of zeolite structures has been to replace the typical silica alumina gel with gels containing other metal atoms. This concept has resulted in numerous unique molecular sieve compositions containing aluminum and phosphorus 5 silicon, aluminum and phosphorus (6) and with... 

The desire to synthesize molecular sieve compositions containing other than the typical silicon and aluminum atoms is evidenced by the large number of efforts, primarily in the primary synthesis area. The Secondary Synthesis process has now been extended to include substitution of both Fe3t and T1 + into the frameworks of a number of zeolites. This paper will describe substitution of iron or titanium ions into the frameworks of zeolites Y, L, W, mordenite, ZSM-5 and LZ-202. Zeolites Y, L, W and mordenite were obtained from Union Carbide Corporation. Zeolon, a synthetic mordenite, was obtained from The Norton Company. ZSM-5 was synthesized according to the procedures described by Argauer et al., (8). LZ-202 is an omega type zeolite, synthesized without the... 

Aluminium can be isomorphously substituted for silicon in the framework of zeolite Y by hydrothermal treatment of the dealuminated (ultrastabilised) zeolite with aqueous solutions of strong bases at elevated temperatures. The extent and efficiency of the reaction depend on the temperature, duration of treatment and especially on the kind and concentration of the basic solution. The degree of crystallinity and the thermal stability of the products are primarily controlled by the composition of the parent material. 29Si and 27A1 magic-angle-spinning NMR (MAS NMR) indicates that the extent of realumination is determined by the number of available tetrahedral Si(OAl) sites. 

Competitive roles of Na+ and TEA+ ions in forming and stabilizing frameworks of zeolites ZSM-20 and Beta... 

The framework of zeolite ZSM-48 can be oriented by hexamethonium (HM++) templates in various gel systems having defined Al concentrations. The HM++ ions... 

One approach is to incorporate Lewis acids into, for example, zeolites or me-soporous silicas [141]. For example, incorporation of Sn(IV) into the framework of zeolite beta afforded a heterogeneous water-tolerant Lewis acid [142]. It proved to be an effective catalyst for the intramolecular carbonyl-ene reaction of citronellal to isopulegol [143] (Fig. 2.43) in batch or fixed bed operation. Hydrogenation of the latter affords menthol (Fig. 2.43). 

The first breakthrough was provided by Flanigen et al. [7] who, playing on the similarity 2 Si4+ -o-A13+ -f P5+, synthesized microporous aluminophosphates (hereafter noted AlPOs) with structures related to those of zeolites. The structural studies [8] showed however a striking difference between the two families. As already mentioned, the framework of zeolites is built up exclusively from connected tetrahedra which can accept small amounts (<10% of substitution) of other metals, whereas in aluminophosphates and homologous gallophosphates, Al and Ga polyhedra can adopt five and sixfold coordinations, which change [9] the connectivity of the framework, and therefore the shape of the windows. 

Our calculation is based on the Burchartl.01-Dreiding2.21 force field that combines the Burchart force field [8], which is used to treat the frameworks of zeolites, and Dreiding II... 

On the other hand, information about the framework of zeolite can be obtained form the Si-MAS-NMR spectrum. In the case of the high silica-type zeolite used here, the Si/metal ratio in the zeolite framework is determined from the relative values of Si(OM) and Si(lM). As is seen in Table IV, this ratio becomes larger in comparison with that of the fresh catalyst for all spent catalysts, thus indicating that some active components in the form of A1 or Ga are being eliminated from the framework by the reaction and regeneration cycles. Based on this finding, the major... 

The measured enthalpies of adsorption of alkanes into zeolites are very large, and can be calculated from this formula [13]. The dielectric constants for the open networks formed by the aluminium silicate frameworks of zeolites can be related to the known and measured dielectric and optical properties of bulk quartz. The calculation assiunes that the zeolite framework tessellates a hyperbolic surface as described in Chapter 2. The polarisabilities of different adsorbate alkanes are also known. When the calculations are carried out for a whole range of alkanes as adsorbate molecules and for different zeolites (with differing pore structure and size) the agreement between measured and predicted heats of adsorption is excellent (cf. Fig. 3.2). The results depend... 

The frameworks of zeolites offretite and erionite consist of layers of linked cancrinite and gmelinite cages arranged so that in the case of erionite the orientation of alternate layers blocks the 12MR chaimels whereas in offretite the channel is elear (7, 8). 

As one can see from XRD data the modification of start zeolite does not result in change of a degree crystallinity (the initial intensity of reflexes is saved), and comparison of unit sell parameters allows to say about an invariance its structure. Some difference in a parameter for a sample H-BS is connected to interaction of atoms boron in a crystalline framework of zeolite. 

The open frameworks of zeolites are slightly less stable than the corresponding condensed structures [15,16] into which they will transform during severe thermal treatment. Nevertheless, the difference in energy between a-quartz, the most stable polymorph of silica, and siliceous faujasite, one of the most open and least stable, is only about 15 kj mol k The extensive occurrence of aluminosilicate zeolites and their widespread utility in industry therefore depend heavily upon both the strengths of their T-O bonds (e.g. Si-O 466 kJ mol ), which render them stable with respect to framework rearrangement. The challenge with many of the newer materials is that their stability with respect to transformation into alternative condensed structures is considerably lower and they frequently collapse on dehydration or other means of activation. It is for this reason that only a small subset of the many open-framework families of materials can be rendered truly nanoporous,... 

---