Mordenite preparation

Sorption. The sorption properties of aluminum-deficient mordenite are strongly affected by the dealumination procedure used and by the degree of dealumination. Materials prepared by procedures that do not involve high temperature treatments show a relatively high sorption capacity for water (15,70), due to the presence of silanol groups, which are hydrophilic centers. However, aluminum-deficient mordenite zeolites prepared by methods requiring heat treatment show a lower sorption capacity for water due to fewer silanol groups. This was shown by Chen (71), who studied the sorption properties of aluminum-deficient mordenite prepared by the two-step method. 

The amount of boron substitution achieved using this post-synthetic method is approximately six times higher than the most heavily substituted mordenite prepared by direct synthesis from gels (vide supra). If this post-synthetic treatment... 

Figure 7. Single pulse excitation and cross polarization 2 Si NMR spectra of boron mordenite prepared from dealuminated mordenite.

Heats for H-Zeolon, a commercial H-mordenite, fell well below the curve for H-mordenites prepared in our laboratory. The procedure in making H-Zeolon from its Na-form is not known tojis. 

Catalytic systems containing molybdenum continue to receive considerable attention in the literature. Molybdenum, however, is one of the most difficult elements to ion exchange into zeolites (refs. 1,2) although several other successful methods of preparation are reported in the literature (refs. 3-7). Perhaps due to the difficulty of preparation, there are only a few reports of catalytic reactions over Mo-zeolites in the literature. The objective of the present study was to investigate the reaction of some simple olefins over Mo-mordenite prepared by vapour phase adsorption of MoClg into the hydrogen and sodium forms of this zeolite. 

Fig. 56. Mordenites. Mordenites prepared with (a) 3.7, (b) 8.0, and (c) 10.2 SbCVg temperature-dependent protonic conductivities for relative humidities of 20 % (filled triangle), 60 % (filled square), 70 % (empty circle), and 80 % (empty square) [94H2].

Experiments were carried out using isotopically labelled methanol (97% 0) and ethanol (98% purchased from MSD Isotopes. Anhydrous isobutanol was purchased from Aldrich Chemical Co., Inc. and contained the natural abimdances of orygen isotopes, i.e. 99.8% and 0.2% O. Nafion-H was obtained fi om C. G. Processing, Inc. and Amberlyst resins were provided by Rohm and Haas. The 2SM-5 zeolite was provided by Mobil Research Development Corp. H-Mordenite, montmorillonite K-10, and silica-alumina 980 were obtained firom Norton, Aldrich, and Davison, respectively. y-AIumina was prepared from Catapal-B fi om Vista. 

The three-function model introduced in the preceding section has been established on an H-mordenite (HMOR) supported cobalt—palladium catalyst [12], For the sake of demonstration, model catalysts with a unique function, i.e. FI, F2 or F3, (Figure 5.1), were prepared to separately give evidence of the major role of each active site (Figure 5.1). Let us note that three functions does not necessarily mean three different active sites, but in the case of CoPd/HMOR material, three different sites were identified. 

Exchanged mordenites (CoPd/HMOR, Co/HMOR, Pd/HMOR) were purchased from the Institut Regional des Materiaux Avances (I.R.M.A.) , located in Ploemeur (France). They were prepared according to Hamon et ai s patent [23] by exchanging a NH4-mordenite with the appropriate amount of metallic precursors, respectively, cobalt(II) acetate and Pd(NH3)4Cl2. 

The following example illustrates the preparation of a hydrogen fluoride-modified mordenite ... 

Zeolite samples (NaY. Na-mordenite and Na-ZSM-5) were prepared in Research Institute for Petroleum and Hydrocarbon Gases in Bratislava. A mesoporous alumina, the carrier for reforming catalyst was used. Porosity of pure mesoporous alumina evaluated by t-plot method did not show the presence of micropores within the range of accuracy of 0.001 cm3/g. Mixtures of zeolites with mesoporous alumina were prepared on the base of dried samples in 5% steps. The prepared mixtures of alumina with zeolite were homogenized in vibration mill. 

The zeolites-chitosan composites were prepared by adding a known amount of zeolite (X, Y, or mordenite) into a 3 % chitosan solution in 1 % aqueous acetic acid. The zeolite powder was dispersed in the chitosan solution and stirred at room temperature during 1-2 hours. The gelling procedures were later carried out like as in the absence of zeolites. 

Figure 2. Scanning electron microscopy of (a) zeolite X-chitosan, (b) zeolite Y-chitosan and (c) mordenite-chitosan composites prepared by encapsulation of zeolites during the gelling of chitosan.

These microporous crystalline materials possess a framework consisting of AIO4 and SiC>4 tetrahedra linked to each other by the oxygen atoms at the comer points of each tetrahedron. The tetrahedral connections lead to the formation of a three-dimensional structure having pores, channels, and cavities of uniform size and dimensions that are similar to those of small molecules. Depending on the arrangement of the tetrahedral connections, which is influenced by the method used for their preparation, several predictable structures may be obtained. The most commonly used zeolites for synthetic transformations include large-pore zeolites, such as zeolites X, Y, Beta, or mordenite, medium-pore zeolites, such as ZSM-5, and small-pore zeolites such as zeolite A (Table I). The latter, whose pore diameters are between 0.3... 

As Ti is incorporated in the silicate lattice, the volume of the unit cell expands (consistent with the flexible geometry of the ZSM-5 lattice) (75), but beyond a certain limit, it cannot expand further, and Ti is ejected from the framework, forming extraframework Ti species. Although no theoretical value exists for such a maximum limit in such small crystals, it depends on the type of silicate structure (MFI, beta, MCM, mordenite, Y, etc.) and the extent of defects therein, the latter depending to a limited extent on the preparation procedure. Because of the metastable positions of Ti ions in such locations, they can expand their geometry and coordination number when required (for example, in the presence of adsorbates such as H20, NH3, H2O2, etc.). Such an expansion in coordination number has, indeed, been observed recently (see Section II.B.2). The strain imposed on such 5- and 6-fold coordinated Ti ions by the demand of the framework for four bonds with tetrahedral orientation may possibly account for their remarkable catalytic properties. In fact, the protein moiety in certain metalloproteins imposes such a strain on the active metal center leading to their extraordinary catalytic properties (76). 

The preparation methods of aluminum-deficient zeolites are reviewed. These methods are divided in three categories (a) thermal or hydrothermal dealumination (b) chemical dea-lumination and (c) combination of thermal and chemical dealumination. The preparation of aluminum-deficient Y and mordenite zeolites is discussed. The structure and physico-chemical characteristics of aluminum-deficient zeolites are reviewed. Results obtained with some of the more modern methods of investigation are presented. The structure, stability, sorption properties, infrared spectra, acid strength distribution and catalytic properties of these zeolites are discussed. 

Combination of thermal and chemical dealumination. This is a two-step method which was applied in the preparation of aluminum-deficient mordenite (4,5) and Y zeolites (28,29). In some instances the two-step treatment was repeated on the same material, in order to obtain a higher degree of dealumination (5,28). 

The formation of such bonds during the heat treatment of dealuminated mordenite has also been suggested by Rubinshtein et al. (72-74), in some instances without the intermediate formation of SiOH groups. The hydrophobic nature of the zeolite also increases with progressive dealumination. Chen (71) has shown that aluminum-deficient mordenite zeolites with SiO /Al O ratios over 80 absorb little or no water at low pressure. These highly silicious zeolites are truly hydrophobic and in this respect are similar to highly silicious zeolites prepared by direct synthesis (e.g. ZSM-5) (75). 

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