Incorporation into zeolite frameworks

Elements such as B, Ga, P and Ge can substitute for Si and A1 in zeolitic frameworks. In naturally-occurring borosilicates B is usually present in trigonal coordination, but four-coordinated (tetrahedral) B is found in some minerals and in synthetic boro- and boroaluminosilicates. Boron can be incorporated into zeolitic frameworks during synthesis, provided that the concentration of aluminium species, favoured by the solid, is very low. (B,Si)-zeolites cannot be prepared from synthesis mixtures which are rich in aluminium. Protonic forms of borosilicate zeolites are less acidic than their aluminosilicate counterparts (1-4). but are active in catalyzing a variety of organic reactions, such as cracking, isomerization of xylene, dealkylation of arylbenzenes, alkylation and disproportionation of toluene and the conversion of methanol to hydrocarbons (5-11). It is now clear that the catalytic activity of borosilicates is actually due to traces of aluminium in the framework (6). However, controlled substitution of boron allows fine tuning of channel apertures and is useful for shape-selective sorption and catalysis. 

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

The 1980s saw major developments in secondary synthesis and modification chemistry of zeolites. SUicon-enriched frameworks of over a dozen zeolites were described using methods of (i) thermochemical modification (prolonged steaming) with or without subsequent acid extraction, (ii) mild aqueous ammonium fluorosilicate chemistry, (iii) high-temperature treatment with silicon tetrachloride and (iv) low-temperature treatment with fluorine gas. Similarly, framework metal substitution using mild aqueous ammonium fluorometaUate chemistry was reported to incorporate iron, titanium, chromium and tin into zeolite frameworks by secondary synthesis techniques. 

When treated under the proper conditions, aluminum can be removed from zeolites without loss of crystallinity (20). It has recently been demonstrated that for zeolite Y the extra-lattice aluminum is incorporated into the framework when the sample is treated with KOH (21). We sought to determine if dealuminated zeolite frameworks are reactive, and are isomorphously substituted when treated with borate salts in basic medium. 

We have earlier addressed the problem of the post-synthesis insertion of aluminium in zeolites ZSM-5 (12) and Y (Hamdan, H. Sulikowski, B. Klinowski, J. T.Phvs.Chem.. (in press)). The substitution of gallium in silicalite-n has also been achieved (13). It was therefore of considerable interest to establish whether boron can also be incorporated into silicate frameworks after the completion of synthesis. We report isomorphous substitution of boron into zeolite ZSM-5 by mild hydrothermal treatment with borate species. 

In addition to hydrothermal procedures, incorporation of Ti into zeolites framework may be achieved by different post-synthesis modifications such as the treatment of alumino- or... 

We prepared boron substituted mordenite by direct synthesis from gel precursors and by post- synthetic substitution into dealuminated mordenite. Direct substitution is favored in aluminum deficient gels, but exacting crystallization requirements for mordenite formation limit the amount of boron that can be incorporated into the framework structure. Higher substitution levels were achieved using a post-synthetic treatment. Boron substituted zeolite Y could not be prepared by a similar direct synthetic method, but post-synthetic methods were effective at providing low substitution levels. This demonstrates the more general utility of post-synthetic substitution methods. The hexane cracking activity of... 

Heteroatoms (B, Al, Fe, Ga, and Ti) may be incorporated into the framework of high-silica and all-silica materials in the presence of fluoride as well, giving rise to active acid catalysts. Usually, transition metal ions will hydrolyse to form hydroxide or oxide precipitates in a high-pH solution. Therefore, there is a limitation to the content of transition metals in heteroatom-substituted zeolites. However, this limitation can be significantly increased by using fluoride during the synthesis because fluoride can coordinate to the transition metal atoms to form stable complex, which will help transition metal atoms incorporate into the framework of zeolites. 

The Co-exchanged zeolites were not effective catalysts for the oxidation of cyclohexane. The cobalt exchanged ions were not stabilized enough by the zeolite interactions and part of these cations were released in the oxidation medium. Thus, we decided to explore the activity of P-zeolites in which cobalt ions were incorporated into the framework. We hoped that the incorporation would increase the stability of the cation within the solid. We studied the catalytic activities of cobalt substituted P-zeolites containing aluminium (Co-Al-BEA) and boron (Co-B-BEA) towards the oxidation of cyclohexane into adipic acid. 

The photocatalytic activity of transition metal oxides incorporated into the framework of zeolites, and of transition metal ions exchanged within the zeolite cavities, has been reviewed in relation to the reduction of CO2 with H2O to give CH4 and. CH30H. ... 

As a result from this early work we thought that zeolite Beta crystals can grow without the incorporation of a trivalent element (Al, Ga, B, Fe,...) provided that Ti is incorporated into the framework. This was actually supported by the fact that, as mentioned above, Ti-Beta samples with Si/Al ratios higher than 100 have no Al in its outer shell, which means that in the last steps of its crystallization these samples grow without Al incorporation. Unfortunately, we were unable to synthesize Ti-Beta in the absence of aluminium (or other T3+), the conclusion being that a trivalente element is necessary for zeolite Beta to nucleate. 

NH -TPD reveals that the crystallization process involves a post-synthesis insertion of gallium atoms into the zeolite framework (Fig 3b). After a crystallization time of 24 h only about 50 % of the gallium present in the sample is actually incorporated into the framework (Fig. 5), although the... 

B MAS NMR yields quantitative information about the incorporation of boron into zeolite frameworks. H MAS NMR and IR spectroscopy show that OH groups introduced into the framework by boron substitution are non-acidic. 2D proton spin diffusion measurements of the zeolite SAPO-5 reveal that defect OH groups are adjacent to acidic bridging hydroxyl groups and do not exist in an amorphous phase. Strongly adsorbed water molecules in mildly steamed zeolites H-Y can be explained by Lewis sites. 

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