Green Engineering - American Chemical Society

Chapter 13

Zeolite Technologies for a Greener Environment Β.

K.

Marcus and W. E. Cormier

Downloaded by UNIV OF GUELPH LIBRARY on September 7, 2012 | http://pubs.acs.org Publication Date: November 9, 2000 | doi: 10.1021/bk-2001-0766.ch013

Zeolyst International (http:\\www.zeolyst.com) Research and Development, Conshohocken, PA 19428

Applications using zeolites of aluminosilicate and other compositions are undergoing intensive development and commercialization in diverse areas due to their process advantages. Because of their wide-range of properties, these materials are also finding applicability in industrial processes that are environmentally sensitive. A review of some of the properties of zeolites, how these properties are useful for extension into environmental areas, and some of the diverse "green" application areas where zeolites are being used are covered in this paper.

Introduction Chemical processes involving catalysis and adsorption are increasingly being designed to minimize risk to humans and the environment through the use of "greener" materials. The drivers for cleaner technologies are numerous and interwoven, and they include regulatory pressures, economic considerations, and social concerns. The regulations and laws of many countries are becoming increasingly more stringent requiring a marked decrease in effluents, wastes, and by products; fines and possible plant closures are among the penalties for non compliance. Increased legislative pressures often results in increased costs, and these economic considerations are driving companies to devise new, cheaper methods to produce materials in order to compete with third world countries that do not face similar legislative pressures. Social issues centering on the desire for a clean environment are driving consumers to demand that materials be produced in a socially and environmentally conscious manner. There are many materials being studied for use in the applications being developed to meet these requirements; one of © 2001 American Chemical Society

In Green Engineering; Anastas, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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the main classes of inorganic materials being investigated is zeolites. Applications using aluminosilicate and other composition zeolites are undergoing intensive development and commercialization due to their process and environmental advantages. Zeolites have diverse properties that allow them to be used in a variety of applications and chemical environments. In order to understand how zeolites are able to function as catalysts, adsorbents, ion exchangers, desiccants and carriers, it is necessary to explain the unique and diverse properties that zeolites possess.

Zeolite Properties Molecular sieve zeolites are a class of stable mineral and synthetic crystalline inorganic compounds characterized by the presence of an open three-dimensional oxide framework structure. This framework is composed of S1O4" and A10 " tetrahedral atoms linked at the corners of the tetrahedra via shared oxygen atoms. This leads to a net negative charge on the aluminum that is balanced by a cation. Depending on how the tedrahedral atoms are connected, different zeolites structures are formed. There are more than 40 natural zeolites, but when synthetic materials are included, more than 100 structure types have been reported since the first synthetic zeolites were produced in the 1940's. The open repeating framework of a zeolite forms a regular network of uniform, microporous openings leading to channels, cages, or cavities and results in a large internal surface area. Figure 1 shows the faujasite structure (synthetic zeolites X and Y). The pore opening of approximately 7 angstroms leads into a supercage that is approximately 12 angstroms wide; the entire internal volume is accessible through the system of repeating, three dimensionally interconnected pores and cages. The sieving characteristics of zeolites are largely determined by the size of the pore opening, while the size, shape, and dimensionality of the internal structure can lead to shape and product selectivity. In addition to the uniform pore openings and large surface areas, other zeolite properties include (Table I): thermal stability, allowing their regeneration by heat; large ion exchange capacity of the mobile charge balancing cations, leading to their use as ion exchangers and leading to the inclusion of active metals; water capacities ranging from extremely hydrophilic to hydrophobic, allowing their use as adsorbents in a variety of environments; acidity that ranges from very strong to negligible; and the ability to replace the framework silicon and aluminum atoms with other atoms leading to diverse applications. These properties of zeolites have lead to their use in a variety of application areas such as water softeners in detergents, sequesters of radioactive nucleotides, desiccants, odor removers, acid and shape selective catalysts, and in many "green" areas. 5

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Figure 1. Zeolite Faujasite Structure - Synthetic Zeolite X and Y.


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Table I. Zeolite Properties Property Channels Cavities/Cages Thermal Stability Ion Exchange Capacity Surface Area Water Capacity Properties

Value 2.2-8 Angstroms 6.6 -11.8 Angstroms 500tol000°C Up to 700 meq/100g Up to 900 m /g < 1 to ~ 25 wt.% Hydrophilic to Hydrophobic Shape Selectivity Strong to No Acidity 2

Zeolites and Green Technologies It is recognized that most of the obvious and inexpensive solutions to pollution problems have been investigated and that more creative materials and technologies will be needed in the future to meet the increasing regulatory push for cleaner technologies. Zeolite researchers and manufacturers know that there is a continuing need to develop new cost-effective materials with characteristics and properties that will be useful in emerging technologies. Some of the emerging technology areas where zeolites have been studied for use are 1) air cleanup (VOC removal, stationary and mobile deNOx and deSOx, automotive cold start), 2) land and wastewater cleanup, and 3) fine chemical process improvement (waste minimization, higher efficiency, cheaper feedstocks). Several of these areas are used as examples of the potential for using zeolites in green technologies.

Volatile Organic Chemicals (VOCs) Historically, zeolites have been used as desiccants because they are hydrophilic and have excellent water capacities. They are capable of maintaining a very low water concentration in closed systems even at low partial pressures. One application area where this is useful is in insulated glass windows, where zeolites are used to entrap the moisture left between the panes during manufacture. However, through the years, there has been an increase in the number of zeolites available with high silica to alumina (Si/Al ) ratios. These materials are either directly synthesized, as in the case of silicalite (very high silica ZSM-5), or materials that have been dealuminated using hydrothermal or chemical means. These siliceous materials are thermally stable to temperatures in excess of 900°C and are more hydrophobic in 2


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nature. This hydrophobic property has lead to their use in applications where organic vapors must be removed from air streams and into areas where carbon has been the traditional adsorbent. Zeolites, unlike carbon, are especially effective where the stream is dilute, where the contact time is short or when water is present; they are also non-combustible and can be regenerated on site. The use of mixtures combining different zeolites and/or carbon is being investigated, and there are several companies marketing systems containing zeolites for the removal of VOCs.

Nitrogen Oxides (NOx) One important area of environmental research is the reduction of atmospheric nitrogen oxide gases that cause photochemical smog. Stationary sources of NOx include power plants and chemical operations, and the reduction technology that has been used in these sources is selective catalytic reduction (SCR). SCR uses ammonia as the reductant but is temperature limited. NOx conversions are increased at higher temperatures, and therefore, higher temperature catalysts and new process methods are being developed to further decrease NOx. As regulations have tightened on the amount of NO that can be emitted and the types of streams that have to be treated, the temperature limits for various catalysts have improved, and new materials to meet the regulations have been introduced. Figure 2 (/) shows the operating temperature windows for SCR catalyst formulations. In the temperature range below 400°C, precious metal catalysts are effective. Vanadia/titania catalysts are effective to approximately 425°C but are poisoned when sulfur is present in the gas stream. At higher temperatures, zeolite catalysts such as metal containing ZSM-5 and mordenite are effective. In these materials, the zeolite ion exchange property has been utilized to exchange copper into the zeolitic structure, where it is available for the reaction. The use of zeolites has extended the range over which it is possible to have effective deNO systems and offers several advantages. These advantages include: dust resistance, allowing their use in coal fired boiler applications; temperature resistance, allowing their use in higher temperature streams that could not be treated previously; increased selectivity at higher temperatures since they do not oxidize ammonia to NO , and they are less expensive than precious metals. An emerging extension technology is N O conversion under lean burn (oxidizing) conditions in which hydrocarbons are used as the reductant in place of ammonia. The hydrocarbons used include methane and ethane that come from the fuel source. This results in a system that is cheaper and requires no special handling of the reductant. The materials of choice in this application are copper or cobalt ion exchanged ZSM-5 and cobalt exchanged beta zeolite (2,3). By removing the need for ammonia in the deNO system, it is possible to extend the application to mobile emission sources such as diesel automobiles, and much research has been done in the area (4,5). Diesel engines are lean burn and are oxygen rich. They produce less C 0 than a gasoline car and are more efficient. However, the traditional precious metal three-way catalyst is not effective in removing the N O x

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ι Zeolite Catalyst V205/T102

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\ Modified Pt Catalyst Pt Catalyst

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Operating temperature windows for different SCR catalyst formulations. 20

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Temperature (°C) Heck, R.M. and Farrauto, R.J., "Catalytic Air Polution Control: Commercial Technology", Van Nostrand Reinhold, 1995.

Figure 2. Operating Temperature Windows for NOx Conversion Catalysts. (Reproducedfrom Reference 1. Copyright 1995 VanNostrand Reinhold.)


165 formed. The zeolites being researched for this application include copper and cobalt ion exchanged ZSM-5 and beta.

Nitrous Oxide (N 0)


2

Nitrous oxide is a powerful greenhouse gas because it absorbs in the infrared region. It is produced in large quantities in nitric acid plants and in adipic acid plants; adipic acid being used in the production of nylon. At higher temperatures, a variety of metal exchanged zeolites will decompose N 0 (

Green Engineering - American Chemical Society

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