Synthesis of Zeolites
In this chapter the conditions and strategies used for synthesizing zeolites and the techniques for turning them into useful catalysts is discussed. Most zeolites are synthesized from amorphous aluminosilicate gels using inorganic and organic cations under highly basic and hydrothermal conditions. The organic cations are typically referred to as templates or organic structure direction agents (OSDAs) owing to their ability to direct the synthesis to a specific zeolitic product. The substitution of aluminium with other trivalent metals such as boron and gallium and the substitution of silicon with germanium have generated a large number of new frameworks. Besides basic hydroxide ions, fluoride has also been found to be a very effective medium for synthesizing new zeolites. In more recent years new strategies have been discovered to delaminate or directly synthesize these layers to give highly reactive catalysts. These layers can also be reassembled in different ways to form new frameworks. While most zeolites are transformed into useful catalysts by post-synthetic treatments, direct synthesis techniques such as OSDA-free seeded syntheses, low-water synthesis with fluoride to prepare high-silica compositions, and the use of crystal modifiers to change crystal size and morphology have recently been developed.