Zeolites economic importance

Sewage treatment has both ecological and economical importance. Ammonium and phosphate ions are the primary inorganic contaminants in municipal wastewater and these ionic compounds are responsible for the phenomenon of eutrophication. In common sewage treatment, ammonium is removed by biological denitrification and phosphates are precipitated with the help of aluminum and iron salts [56], However, the use of low-cost natural zeolites has been found promising for... 

If the cations are hydrogen ions (see Section 3.2.4.), guest molecules may add to them to give, for example, hydronium, ammonium, oxonium, or carbenium cations. The latter two may rearrange, and then decompose or dissociate to give product(s) which can leave the zeolite. Oxonium ions in particular are central to the most economically important processes of petrochemical industry. In simpler words, hydrogen zeolites are very important catalysts. 

Zeolites are microporous crystalline aluminosilicates built from corner-connected TO4 (T = Si or Al) tetrahedra. Their micropores with uniform dimensions give rise to molecular sieve properties. Tetracoordinated aluminum atoms lead to ion-exchange properties and make internal acid sites possible. They are economically important as detergent builders, as adsorbents, and, because zeolites have high thermal stability, as catalysts in the petrochemical industry. They are also considered to have a high potential as catalysts in the synthesis of fine chemicals. 

Zeolites are microporous aluminosilicates that have interesting molecular sieve properties and contain microenvironments giving rise to a wide range of interesting chemical reactions. Because of the economic importance of these materials, scientists have been carrying out computational studies on both the... 

There are about forty-five natural zeolites. They form in a number of relatively low temperature geologic environments. Gas pockets in basalt and other volcanic rocks may contain dramatic crystal groups of zeolites. Economically more important are the fine-grained zeolites such as clinoptilolite (Na, K)AlSi50i2 3H2O formed by the alteration of fine-grained volcanic... 

Over the past ten years, remarkable and rapid progress has been made in the use of zeolites, particularly pentasil zeolites in the organic synthesis of intermediates and fine chemicals [14 - 24]. This new area represents a second promising development in zeolite catalysis in addition to refinery technology and petrochemistry, which continues to be of great economic importance. 

So far we have dealt with weU-defined, intuitively obvious phases gas, liquid solid. But chemical engineers deal with other, less intuitive phases of considerable technical interest and economic importance, for example gases and vapors adsorbed onto solids. Figure 11.21 shows, as an example the equilibrium curve (always called an adsorption isotherm or isotherm in the adsorption literature) for nitrogen adsorbed on zeolite at a temperature well below room temperature, but far above the critical temperature of nitrogen (126.2 K). 

The aromatization of liquefied petroleum gases (LPG) has been investigated for more than a decade due to its economical and strategic importance for the exploitation of natural gas reserves and valorization of light hydrocarbons obtained from petroleum refining. Commercially, these reactions using gallium modified ZSM-5 zeolite catalysts are known as Cyclar process, developed jointly by UOP and BP [1]. 

Zeolites are mesoporous materials that are crystalline in nature. The simplest zeolites are made up of Al and/or Si and O atoms. Also known as molecular sieves, they find use as drying agents because they are very hygroscopic, but from an economic standpoint they are of greatest importance as size-selective catalysts in various reactions involving... 

Cymene or isopropyltoluene is produced via alkylation of toluene with propylene. Cymene is an important intermediate in the production of cresol, and it is also used as an industrial solvent. Again, for both environmental and economic reasons, the use of zeolitic materials for this conversion has been studied. For example, Flockhart et al. have used zeolite Y to effect this reaction (7). They observed that the state of the zeolite, including its degree of ion-exchange and the temperature at which it was calcined, strongly affected the distribution of cymene isomers obtained. In order to enhance the selectivity to para-cymene, the direct precursor to para-cresol, various studies have focused on the use of surface modified zeolites, for example, ZSM-5 materials, including those produced by chemical vapor deposition (CVD) of silicate esters. These species serve to reduce surface acidity and change limit diffusion within the crystal. 

This paper is focused on commercially important syntheses of aromatic ketones and ethers with application in pharmaceuticals, agrochemicals, perfumery, etc. It will be demonstrated using several examples that clean processes using zeolite catalysts can be substituted economically for existing polluting processes. Limitations in the use of zeolite catalysts will also be emphasized, other solid catalysts leading, in certain cases, to much better results in terms of selectivity and stability. 

The most important consequence of restricted transition state selectivity is that ZSM-5 and many other medium-pore zeolites deactivate much slower than most other crystalline and amorphous catalysts. The difference is not trivial. In most acid catalyzed reactions large-pore zeolites deactivate within minutes or in hours, whereas the activity of ZSM-5 ranges from weeks to years. Most of the coke in large-pore zeolites is formed within the pores. In ZSM-5 most of the coke is deposited on the outer surface of the crystals like an eggshell over an egg [23] because coke precursors cannot form in the pores of pentasil molecular sieves. The resistance of ZSM-5 to coking makes a number of industrial processes economical. 

---