Zeolites are microporous, crystalline aluminosilicates. They are distinguished by a series of special properties: they have a defined cavity system with opening sizes of 0.3 to 0.9 nm; and they are cation exchangers. In the H form, they have a high solid-state acidity. Their hydrophobic character can be controlled by the ratio of silicon to aluminum; and they exhibit high thermal stability.
Synthetic zeolites are used at the present time, for example, as adsorbents for separating processes, as replacements for phosphates in detergents, and as catalysts in petrochemical processes. Moreover, their potential for use in environmentally beneficial technologies is high.
As selective heterogeneous catalysts, they help in producing valuable organic products more selectively with the use of less energy and with fewer undesirable by-products. Natural raw materials such as crude oil and natural gas or secondary raw materials such as bioalcohol, which can only be used incompletely at the present time, can be converted to a much greater extent to useful products with the help of these catalysts. Zeolites furthermore find use as catalysts for the removal of nitrogen oxides from exhaust gases, as energy storers and for the energy-saving separation of materials.
The synthesis of zeolites of the pentasil family, which have a high silica content, was described for the first time in 1967 by Argauer and Landolt (See, U.S. Pat. No. 3,702,886).
The preparation of these, materials succeeded, however, only if organic structure guiding compounds were added to the synthesis mixture. Generally, tetraalkylammonium compounds, such as tetrapropylammonium bromide were used for this purpose. In subsequent years, it was discovered that it is possible to carry out this synthesis with a number of other organic substances such as secondary amines, alcohols, ethers, heterocyclic compounds, and ketones.
All these various known methods for the synthesis of zeolites have a series of serious disadvantages, which preclude environmentally safe production on an industrial scale.
The organic materials used are toxic and easily flammable.
Since the synthesis must be carried out under hydrothermal conditions and at a high pressure, generally in autoclaves, an escape of these organic materials into the atmosphere can never be completely prevented.
As a result, these known methods of producing zeolites are hazardous both for the operating personnel and for the environment surrounding the production site. The effluent from the production of zeolites also contains toxic materials and therefore requires expensive and careful disposal in order to prevent contamination of the environment. Moreover, the organic portions present in the lattice must be removed by combustion at high temperatures. These organic materials, possibly decomposed or including combustion products, thus reach the ambient atmosphere. Moreover, this removal of organic products by combustion can cause additional damage to the lattice structure of the zeolite catalyst and thus impair its catalytic properties.
All of these disadvantages have contributed to the fact that the industrial production of this useful catalyst has up until now been carried out only in small batches.
In recent years, some new production methods have been disclosed in the patent literature, in which the use of these organic materials could be omitted (See, e.g., U.S. Pat. No. 4,257,885). However, these production methods proceed very slowly (several days) and generally only incompletely to the desired product. Moreover, the appearance of undesirable secondary phases generally can not be excluded.