Various types of zeolites such as Zeolites A,X, ZSM-5, mordenite etc. are used for removing H₂O, NH₃, NO, NO₂, SO₂, CO₂ and other impurities from gas stream. In gas cleaning, zeolites are normally used for the removal of H₂O, SO₂ and CO₂ from sour natural gas stream. Zeolite 4A are used for removal of CO₂ from submarines and spacecraft. The CaA (Ca ion exchanged zeolites A) is used to adsorb H₂S from sour gas. They are also used for selective removal of NH 3 produced during gasification of coal and for removal of NH₃, SO₂, NO_x and CO₂ from air. Separation of close boiling point mixtures of alkylphenols, such as mixture of p-cresol (201.8)/2,6-xylenol (203°C) or m-cresol (202.8°C)/2,6-xylenol has been attempted using Na-X zeolites[3]. Effect of the cation on the selectivity has also been investigated. The Na-X zeolite adsorbs p-cresol and m-cresol selectively from their mixtures with 2,6-xylenol, while Ca-X and Ba-X zeolites preferentially adsorb 2,6-xylenol. The selectivity seems to be decided by the diffusion in the zeolite framework.

4. Water and waste water treatment :
   
   In water and waste water treatment zeolites are used for various purposes, mainly for water softening, ammonia removal and heavy metal removal. Zeolite is used for exchange of hard ions [Ca⁺²] with soft ions [Na⁺, H⁺] for softening water in a broad range of pH values. It is one of the oldest applications. This is particularly important in detergent industry. Zeolites can also remove dyes from the washing liquor by hetero-coagulation and adsorption. Another application of zeolites is separation of ammonia from drinking or wastewater. Ammonia in the environment originates from metabolic, agricultural and industrial processes and from water disinfection with chloramine. The zeolites are very effective for ammonia removal, due to their high selectivity for ammonium ion in the presence of competing cations, such as K⁺, Ca²⁺ or Mg²⁺. Metals having density higher than 5 g/cm³ are generally considered as heavy metals. Among the heavy metals Cd, Cr, Cu, Ni, Zn, Pb and Hg are well known with their toxicity and considered as environmentally hazardous. Zeolites have been widely explored for removal of heavy metals from natural or industrial wastewater. The removal efficiency of zeolites depends upon the type and amount of zeolite, contact time, pH, temperature, initial metal concentration as well as on presence of competitive ions.

 

Text reference

 

• H. Bartholomew and R. J. Farrauto, Fundamentals of Industrial catalytic Processes , Wiley, VCH, 2006
• J. Weitkamp and L. Puppe (ed.), Catalysis and zeolites: fundamentals and applications, Springer Verlag, 1999
• J. Cejka, A. Corma and S. Zones (eds.), Zeolites and catalysis: synthesis, reactions and applications – 2010, Wiley, 2010

 

Journal reference

1. T. Sano, Y. Kiyozumi, M. Kawamura, F. Mizukami, and H. Takaya,T. Mouri, W. Inaoka, and Y. Toida, M. Watanabe and K. Toyoda, ZEOLITES, 11(1991) 842-845
2. J. Kenneth , Jr. Balkus, T. Kieu , Ly, Journal of Chemical Education, 68 (1991) 875-877
3. A. Raychoudhuri, V.G Gaikar, Separations Technology 5 (1995) 91-96