Zeolite beta (ZB) is a well-known acid catalyst with applications in the isomerization of waxes and the Friedel Crafts reactions (alkylation and acylation).beta zeolite It is also used as a molecular sieve, adsorbent and detergent builder.beta zeolite The structure of the zeolite beta framework consists of a network of SiO4 tetrahedra and NaO3 tetrahedra arranged in a tetrahedral form with the silica atoms bonded to the NaO3 tetrahedra by strong hydrogen bonds. The presence of this network makes zeolite beta highly crystalline and gives it its unique adsorption characteristics, high pore volume and low pH stability. The zeolite beta framework can also be constructed in other structures and polymorphs. It is believed that a majority of the adsorption sites are located at the surface of the framework where it can be accessed easily by reactants.
Zeolite beta is synthesized hydrothermally from an aqueous reaction mixture containing the conventional reactive sources of SiO2, Al2 O3 and NaOH.beta zeolite The organic directing agent is imparted to the reaction mixture in the form of either a tetraethylammonium hydroxide or halide salt, with diethanolamine being used to provide an increased level of basicity necessary for the formation of zeolite beta crystals.beta zeolite Optionally, seed crystals of zeolite beta may be employed to shorten the crystallization period.
The morphology and porosity of the mesoporous zeolite beta synthesized with the organic functionalized silica as the silicon source are thoroughly characterized by scanning electron microscopy (SEM) and nitrogen adsorption isotherms.beta zeolite The specific surface areas, SBET, of the zeolites varied between 664 and 807 m2/g, but did not correlate with the SiO2/Al2 O3 ratio. The acidity of the zeolites was determined by measuring their zeta potential at 25 degC, and it was found that the amount and strength of the Bronsted and Lewis acid sites influenced the values of zeta potential.
The catalytic performance of the zeolite beta was studied in the benzylation of naphthalene with benzyl chloride using a fixed proportion of benzyl chloride and naphthalene.beta zeolite The results showed that zeolite beta synthesized with organic functionalized silica as the silicon donor was superior to the mesoporous H-beta-25 and H-beta-300 zeolites. In addition, the zeolite beta with an increase in the number of bronsted acid sites exhibited significantly better performance than the mesoporous H-beta-150. This was attributed to the improved access of reactants to the acid sites by the substitution of the bromide ions with water molecules. Consequently, the catalyst was capable of reacting at much lower temperature than the corresponding mesoporous zeolites with less acidic sites. This improvement in the benzylation activity of naphthalene with a large molecule indicates that mesoporous zeolites prepared with the organic functionalized silica as a silicon source offer significant advantages for the synthesis of complex aromatic compounds.
