Production of Zeolites

Most zeolites are synthesized from proportional mixtures of sodium aluminate and sodium silicate and, in some cases, colloidal silica. The procedure is different to that for the preparation of silica alumina catalysts because zeolites form under alkaline conditions as the silica/alumina co-gel crystallizes in the presence of hydroxyl ions. The zeolite type formed depends on the proportion of silicate and aluminate in the solution and the reaction temperature and pressure. The time taken for zeolite crystals to form can range from a few hours to several days. Seeds or templates are often added to induce formation of the appropriate crystalline product. Knowhow is very important, and precise details for a specific preparation are not always published. 

Y-zeohte is relatively easy to produce requiring only the addition of finely divided silica to seed the ciystalhzation of the saturated solutiom The method originally used by Filtrol, the company that supplied the first catalysts to Hou-dry, was to produce pure Y-zeolite by the conversion of kaolin clay. The first step was to form metakaolin by dehydrating the kaolin above 600°C. The dehydrated kaolin then contained activated alumina that could be extracted with acid to give the appropriate silica/alumina ratio. Following extraction the clay was aged in caustic soda solution until pure Y-zeolite crystals formed. The success of the procedure depended on the use of pure kaolin and careful temperature control. 

An improved synthesis of Y zeolite within a matrix of kaolin was introduced by Engelhard. Kaolin was calcined to about 1000 C to produce a sih-ca/alumina spinel, but without forming mullite. The spinel contained less active alumina that meta-kaolin. The spinel mixture was then slurried with kaolin and seed particles before being spray-dried to form micro-spheres. These are then initially aged for some time in caustic solution and then heated to 80 C during which time the Y-zeolite crystallizes. The supernatant liquid from the crystallization stage still contains sodium silicate, and the strength of the kaolin matrix 

Zeolite syntheses start from alkaline aqueous mixtures of aluminum and silicon compounds. The reactions are sometimes carried out at atmospheric pressure but more often in a high-pressure autoclave. The controlled crystallization of a particular zeolite requires careful control of the concentration and stoichiometry of the reaction partners, the temperature, and the shearing energy of the stirrer. After mixing of the liquid phase and formation of a gel, a transition of the gel phase in to the liquid aqueous phase occurs, whereby crystalline zeolites are formed from the amorphous particles. 

The silicon-rich pentasils are mainly synthesized in the presence of organic cations. Their open structures seem to be formed around hydrated cations or other cations such as NRJ. In particular, templates such as tetrapropylammonium hydroxide are used, and are of decisive importance for the crystallization of the zeolite structures. The C, H, and N of the tertiary ammonium cation is removed in the subsequent calcination of the microcrystalhne product. 

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ILLUSTRATION 8.2 DETERMINATION OF HOLDING TIME AND REACTOR SIZE REQUIREMENTS FOR THE PRODUCTION OF ZEOLITE A IN A BATCH REACTOR... 

Two points are emphasized (i) zeolites can be successfully operated at the same or higher severities (with respect to P/O (feed alkane/alkene) ratio and OS V (alkene space velocity)) than the liquid acids (ii) the productivities of zeolite catalysts (i.e., the total amount of alkylate produced per mass of catalyst) are roughly the same as of that of sulfuric acid. If the intrinsic activities of zeolites (which have 0.5-3 mmol of acid sites per gram) are compared with that of sulfuric acid (which has 20 mmol of acid sites per gram), zeolites outperform sulfuric acid. Nevertheless, the price of a zeolite catalyst and the high costs of... 

Typically, for production of zeolites (Figure 4.8), a silicon source such as sodium silicate and an aluminum source such as sodium aluminate, are prepared in solutions containing sodium and water contents as required for the formation of the respective zeolite [6], These solutions are mixed in a reactor and reacted at temperatures typically in the range between 80 and 200°C. The reaction time may vary from hours to days, and for reactions at temperatures > 100°C the reactions have to... 

Figure 4.8 Industrial production of zeolites. (Reprinted from Winnacker-Kuchler Chemische Technik, Prozesse und Produkte, Vol. 3, D. Kern, N. Schall, W. Schmidt, R. Schmoll, J. Schtirtz, Siliciumverbindungen, pp. 849-850. Copyright 2005. With permission from Wiley-VCH.)...

This relationship between the water to sodium oxide molar ratio and the type of zeolite formed was not previously known. For example, U.S. patents issued to Milton (JL, 7), show a water to sodium oxide molar ratio of from 35 to 200 for the production of zeolite A and a water to sodium oxide molar ratio of from 35 to 60 for the production of zeolite X. If anything, this would imply that the reaction mixture for preparing zeolite A should have a higher water to sodium oxide molar ratio than the reaction mixture for preparing zeolite X. In U.S. patent 3,119,659, the water to sodium oxide molar ratio for the production of zeolite A is from 30 to 60. None of the above examples show that the water to sodium oxide molar ratio should be higher for making zeolite X than for making zeolite A. 

Several other anhydrous calcium aluminosilicates are known, including grossular or garnet (C3AS3), which is a high-pressure phase, various dehydration products of zeolites, and various products formed metastably by crystallization from melts or glasses. Most are too acid in composition to be of clear relevance to cement chemistry, but some of the devitrification products, especially those with compositions near to CA and structures similar to those of nepheline (Na3KAl4Si40i6) or kalsilite (KAlSiOj (Y4), are of possible interest in relation to the formation of calcium aluminate cements. 

Detergency. The largest-scale industrial production of zeolites is that devoted to the production of the sodium form of zeolite A (LTA) for use in the detergent industry. This uses currently in excess of 0.5 million metric toimes per annum worldwide. Zeolite A (NaA) is added to washing powders and other household detergents or cleansing powders as a water softener (described by the detergent industry as a builder). It... 

Zeolite A, the most important phosphate substitute, became the highly demanded builder worldwide. The worldwide production of zeolite A increased in western Europe in the 1990s. Determent builder zeolites represent the largest application field for zeolite. Almost 90 percent of zeolites produced worldwide (or 215,000 tons/year) in 2003 were used for detergents. Meanwhile, production capacities for detergent-grade zeolites have largely surpassed the demand [4]. 

Volatile transition metal complexes have been widely used for the production of zeolite-encaged metal particles or organometallic compounds (222,223). The resulting catalysts are active for a wide range of reactions (224) and in some cases are superior to other preparations. For those metals... 

The question remains why the other components, principally branched paraffins, are converted at all. Several explanations can be offered, none completely satisfactory. Not all the palladium is inside the zeolite cages but may be partially on external surfaces and nonzeolite components, amorphous material which is either the residue of incomplete crystallization or the product of zeolite decomposition in subsequent treatments. Since x-ray crystallinity is uniformly high, the amorphous component should be quite small. Branched paraffins can penetrate the zeolite surface far enough to be cracked. High temperature alters the selective adsorption properties of the zeolite, which were observed at low temperature. Offretite intergrowths provide enough surface in larger diameter pores partially to convert branched and cyclic molecules. There is some truth in all of these but we prefer the latter. 

There is little statistical data regarding the total production of zeolites. Since their market introduction in the 1950 s the capacity and consumption of zeolites has steadily increased. This increase has accelerated in recent years due to their utilization in detergents (see Table 5.1-1). 

Industrial interest in these materials has been driven by their excellent ion-exchange properties in the hydrated state, and by the exciting adsorption and catalytic properties exhibited by their dehydrated forms. Indeed, it is for this reason that many of the most eye-catching synthetic discoveries have been made in industrial laboratories, such as those of Mobil, Union Carbide (now UOP), British Petroleum and Exxon. Table 18.2 summarizes some of the most important actual and prospective applications. Commercial production of zeolites was approximately one million tons in 1999, most of which was accounted for by the applications in catalytic cracking, xylene isomerization, and detergency. This pattern is gradually changing, however, as new applications are developed. 

In China, zeolites A and X were first synthesized in 1959, followed by the industrial production of zeolite Y and mordenite. With the development of the zeolite industry, zeolites were applied in many fields as well in China. In the 1950s, zeolites were mainly used in drying, separation, and purification of gases. Since the 1960s, zeolites have been widely used as catalysts and catalyst supports in petroleum refining. At present, zeolites have become the most important adsorbents and catalysts in the petroleum industry. 

The history of crystalline gas adsorbents is old and starts with the mineral zeolite. Nowadays, there are successful synthetic techniques for the production of zeolites and zeotypes with controlled diameter pores. Moreover, processes able to produce porosity in pure organic and metal-organic skeletons have been developed. Typical compounds suitable for the topics of this chapter are shown in Fig. 3.4.6. They possess inner accessible spaces for gaseous guests to generate a gas inclusion state... 

Industrial production of synthetic zeolites gives fine powders (<10 tm) in their sodium exchanged form. These materials can be used as synthesized , such as for use as detergents, and Figure 16 is a schematic diagram of the production of zeolite ETA (Na form) for this purpose. The manufacture of the synthetic faujasite (FAU) zeolite Y for catalytic uses follows a similar route. 

ILLUSTRATION 8.2 Determination of Holding Time and Reactor Size Requirements for the Production of Zeolite A in a Batch Reactor... 

The production of zeolite by the hydrogel route utilizes sodium silicate solutions, which have the two commercially most available silica to sodium oxide ratios of ca. 2.4 and ca. 3.5 [15,99,103,104,117,124,128-131,114,134-142]. 

The production of zeolites is frequently performed in batch reactors. Gel precipitation and crystallization are then carried out stepwise. A sodium aluminate solution is prepared first by mixing an alumina source with caustic soda. The silica-containing solution is then dosed to the alkaline alumina-containing solution [140]. 

Fawer, M. 1996. Life Cycle Inventory for the Production of Zeolite A for Detergents. St. Gallen EMPA. 

Roland, E. 1989. Industrial Production of Zeolites. In Karge, H. G., Weitkamp, J., Eds. Zeolites as Catalysts, Sorbents, and Detergents Builders. Studies in Surface Science and Catalysis. Vol. 46. Amsterdam Elsevier, pp. 645-659. 

Y.V. Mirsky, A.Z. Dorogochinskyy, N.F. Meged and A. P. Kosolapova, Process for the Production of Zeolites in the Form of Binderless Spherical Granules, US Patent 4113843 (1978). 

The introduction of monovalent and bivalent transition metal cations into zeolites is also possible and introduces in zeolites sites with redox activity. Several of these systems have wide application in catalysis. In particular, Co-zeolites, such as Co-MFI and Co-FER, have been deeply investigated for their activity in the CH4-SCR reaction [246]. In this case the adsorption of bases such as nitriles and ammonia, followed by IR and by TPD technique, show that they act as medium-strong Lewis acid sites. The current opinion is that these sites are catalytically active for the DeNO c reaction just when they are isolated in the zeolite cavities. A recent investigation provided evidence for the deposition of part of Co ions also at the external surface of the zeolite upon cation exchanging [85] and to their likely nonnegligible catalytic activity [247]. The deposition of Co species at the external cavities can be a reason for only apparent over-exchanging (i.e., production of zeolites with Co +/AP+ atomic ratios >0.5). 

Scalability is an important issue for any material for industrial applications. Large-scale synthesis of MOFs for industrial applications was successfully performed by BASF by considering the classical solvothermal synthetic approach, which is generally used in industrial production of zeolites [26]. In large-scale synthesis (kilotons/year), the availability and cost of reactants and the space-time-yield (STY) of synthesis play a crucial role. The higher the STY value (kg of MOF per of reaction mixture per day of synthesis),... 

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