Preparation of zeolites
Zeolites are synthesized by crystallization from reactive forms of silicon, aluminum, sodium, sodium hydroxide and organic template at 90-180°C and 1-10 atm pressure. The pH is maintained higher than 10. Seed crystals are added to the reactor to initiate the crystallization process. Typical crystallization time varies in the range from 16-36 h. Organic template is added to facilitate formation of pores and supercages. Typically organic template is an organic amine or alkyl ammonium compound. Crystallization of the gels proceeds around the template molecular mold producing the porous network. The synthesis of various zeolites is achieved by varying the synthesis conditions such as temperature, pH, crystallization time, order of mixing and amount of Si, Al, Na, and H2O. Laboratory preparation of NaX Zeolite and ZSM-5 zeolite is discussed below.
Preparation of ZSM-5 zeolite [1]
Aluminum nitrate and colloidal silica are added to a stirred mixture of tetrapropyl ammonium bromide and sodium hydroxide solution to give a hydrogel. Then, the hydrogel was transferred to a stainless-steel autoclave with a Teflon lining and placed in an oven for appropriate periods. After the completion of crystallization under autogeneous pressure, the autoclave is cooled down, samples are washed and dried at 120°C for 24 h. Finally, the sample is calcined at 500°C for 16 h to remove the organic base occluded in the zeolite framework, protonated in hydrochloric acid solution at room temperature for 24 h, and then again dried at 393 K.
Preparation of NaX Zeolite [2]
The sodium silicate and sodium aluminate are prepared separately. The silica gel and aluminum isopropoxide is used as starting materials for silicon and aluminum respectively. The sodium silicate is prepared by adding silica gel, sodium hydroxide, and deionized water to a plastic beaker stirred until the solids are completely dissolved. The sodium aluminate solution is prepared simultaneously by adding aluminum isopropoxide, sodium hydroxide, and deionized water. The mixture is stirred below 80°C until the solids are dissolved to form a clear gel and the mixture is cooled to room temperature. Then the aluminate solution is added to the silicate solution with additional amount of water. The final mixture is stirred until homogenous and then placed in an oven for 24h at 90°C. After 24 h, the mixture is cooled to room temperature giving white zeolite crystals. The crystals are washed thoroughly with water, filtered and air-dried.
Properties of zeolites
- High surface area and ordered pore structure
High surface area and ordered pore structure of zeolites result in their unique adsorption properties. Zeolites are characterized by large surface area because of its highly porous nature. The surface area of zeolites is in the range of 600-800 m2/g. As a result of high surface area zeolites can adsorb large quantities of adsorbate depending on adsorbate size, aperture size, temperature and surface acidity of zeolites.
Zeolites have aperture or pore diameter of the order of molecular dimension therefore molecules having diameter of the same order or larger than pore diameter or aperture are excluded from entering the pores or super cages. Since larger molecules are excluded, preferential adsorption and reaction can be done using zeolites. For examples separation of O2 and N2 in air can be done using and 13 X-NaX zeolites.
- Acidity:
- Thermal stability:
- Shape selectivity:
The OH bridging a framework of silicon to a framework of aluminum acts as the Bronsted acid site. Coordinately unsaturated Al sites give rise to Lewis acidity. Acidity in zeolites increases with decreasing Si: Al ratios because acid sites are associated with Al ions. Bronsted and Lewis acid sites play important roles in various catalytic reactions involving hydrocarbons. Zeolites are used in catalytic cracking reaction in petroleum industry.
Most of the zeolites are stable upto 400°C. Stability increases with increasing silica content. Introduction of rare earth cations in zeolites result in stability upto 800°C
Unique pore structure of zeolites results in its high shape selective properties. Shape selectivity results due to
– geometric restrictions on accesses of reactants to the zeolite framework
– geometric restrictions on diffusion of reactants in or diffusion of products out from catalysts