In twenty-five years molecular sieve zeolites have substantially impacted adsorption and catalytic process technology throughout the chemical process industries; provided timely solutions to energy and environmental problems; and grown to over a hundred million dollar industry worldwide. The evolution in zeolite materials with improved or novel properties has strongly influenced the expansion of their applications, and provided new flexibility in the design of products and processes.
Zeolite Molecular Sieves Breck Pdf Download
A method for collecting an isotopically representative sample of CO2 from an air stream using a zeolite molecular sieve is described. A robust sampling system was designed and developed for use in the field that includes reusable molecular sieve cartridges, a lightweight pump, and a portable infrared gas analyzer (IRGA). The system was tested using international isotopic standards (13C and 14C). Results showed that CO2 could be trapped and recovered for both δ13C and 14C analysis by isotope ratio mass spectrometry (IRMS) and accelerator mass spectrometry (AMS), respectively, without any contamination, fractionation, or memory effect. The system was primarily designed for use in carbon isotope studies of ecosystem respiration, with potential for use in other applications that require CO2 collection from air.
Si/Al ratios below about 3 correspond to natural zeolites and some synthetic zeolites such as A-type and X-type zeolites. They are useful as ion-exchange agents because of their high ion-exchange capacity. Commercially available molecular sieve adsorbents often belong to this group.
As of December 2018, the framework structures of 253 different zeolites or their analogues are known, nearly 200 of which can only be synthesized artificially. For each structure, the International Zeolite Association (IZA) gives a three-letter code called framework type code (FTC).[4] For example, the major molecular sieves, 3A, 4A and 5A, are all LTA (Linde Type A). Most commercially available natural zeolites are of the MOR, HEU or ANA-types.
The term molecular sieve refers to a particular property of these materials, i.e., the ability to selectively sort molecules based primarily on a size exclusion process. This is due to a very regular pore structure of molecular dimensions. The maximum size of the molecular or ionic species that can enter the pores of a zeolite is controlled by the dimensions of the channels. These are conventionally defined by the ring size of the aperture, where, for example, the term "eight-ring" refers to a closed-loop that is built from eight tetrahedrally coordinated silicon (or aluminium) atoms and eight oxygen atoms. These rings are not always perfectly symmetrical due to a variety of causes, including strain induced by the bonding between units that are needed to produce the overall structure or coordination of some of the oxygen atoms of the rings to cations within the structure. Therefore, the pores in many zeolites are not cylindrical.
Zeolite-based oxygen concentrator systems are widely used to produce medical-grade oxygen. The zeolite is used as a molecular sieve to create purified oxygen from air using its ability to trap impurities, in a process involving the adsorption of nitrogen, leaving highly purified oxygen and up to 5% argon.
Zeolites are crystalline aluminosilicates of the alkaline and alkaline-earth metals. They possess many desirable ion- exchange, molecular sieving, and catalytic properties, which make them valuable mineral commodities. Synthetic zeolites have been used for over 25 yr in commercial processes, but only recently have natural zeolites been viewed as potentially valuable mineral commodities. Because of this interest, the Bureau of Mines undertook this report to discuss the occurrence, synthesis, mineralogy, economics, and uses of these versatile minerals.
Molecular sieves are materials that, because of their internal structure, can selectively adsorb molecules according to their size and/or shape. All zeolites are molecular sieves, but not all molecular sieves are zeolites. Activated carbon, activated clays, alumina powder, and silica gels are examples of molecular sieves that are not zeolites.
However, the apertures can serve as molecular sieves for selective catalysis inside the cavities. Molecules that can enter the cavities and be catalyzed include oxygen, ammonia, hydrogen sulfide, methanol, sulfur dioxide, straight-chain hydrocarbons, and water.
Miller stated that because the apertures of the truncated octahedral polyhedral framework are large enough to permit virtually any molecule to enter the cavity, they do not function as molecular sieves. These large apertures, however, allow metal atoms to enter the cavity and substitute for alkaline and alkaline-earth metals in the structure. A wide range of molecules (including cyclopentane, benzene, fluorine compounds, high-molecular-weight hydrocarbons, alkyl aromatics) can enter the cavity to participate in reactions that take place on or near these catalytic metals (fig. 8).
formula of the sodium form as NanAlnSi96-nO19216H2O, with n less than 27 but typically equal to 3. Whereas a structure with tetrahedral Al and corresponding exchange sites shows ion-exchange and reversible dehydration expected of an ideal zeolite, the Al-free structure does not show ion-exchange and is hydrophobic and organophilic: thus silicalite is a molecular sieve but not a zeolite.
Molecular sieving refers to the selective adsorption of cations within a sorbent, based on physical dimensions and charge distribution. Molecular sieving depends on the characteristics of both the sorbent and the material being adsorbed. Zeolites are well suited for molecular sieving because they possess an open, yet uniform crystalline framework that has a narrow pore size distribution. This contrasts with other types of molecular sieves, such as silica gels or activated carbon, which have a wider range of pore size distributions (fig. 15). This property makes zeolites more size selective than other molecular sieves.
Diffusion is the migration of sorbate within the crystal. It affects selective adsorption, desiccation, molecular sieving, and catalysis. Diffusion in zeolites is very complicated. Diffusivity, which is affected by molecule size and shape and framework topology, has proven to be extremely difficult to predict. Maurer notes that with broader knowledge of zeolite diffusion processes, it has become clear that currently no general model for diffusivity prediction exists. Review of the available literature suggests that these data must, for the foreseeable future, be obtained empirically. Barrer lists the variables that influence these rates:
Plank reported that the invention of zeolite cracking catalysts was in response to a need for a stable, selective catalyst of high activity. He looked for a catalyst whose active sites were located in controlled pores not much larger than the molecules to be cracked. Both molecular sieve zeolites and silica-alumina gels were considered, but stream regeneration significantly lowered the activity of the gel formulation while not affecting zeolite.
According to Mumpton, zeolites were first used 2,000 yr ago for building stones. The ion-exchange capability of zeolites was first investigated about 100 yr ago; the molecular sieving capability for separating gases, 40 yr ago; and the first commercial uses of synthetic zeolites, 30 yr ago. Breck reported that, in 25 yr, zeolites had achieved worldwide recognition as evidenced by the appearance of about 1,000 publications a year, and the recognition of 40 natural zeolite minerals and over 150 synthetic zeolites. Still, in 1977, only 10% of the known natural zeolites had commercial applications and fewer than 10% of the synthetic zeolites were successfully marketed.
Flanigen reported that the major use of natural zeolites is in bulk mineral applications: in Europe in the building and construction industry, where proximity to building location makes them cost effective, and in the Far East as filler in the paper industry, largely because of the unavailability of alternate mineral resources. A modest market for zeolite minerals has developed as a molecular sieve adsorbent in acid gas drying in the natural gas industry, in NH4 removal in water treatment systems by ion exchange, and in the production of oxygen and nitrogen via adsorptive air separation, especially in Japan. In general, however, their penetration into molecular sieve applications has been quite limited. Zeolite applications are summarized as follows:
The specificity of clinoptilolite for ammonium has already been proven in water treatment, particularly for acquaculture, This specificity also may be useful for environmental control during uranium processing. Its purpose would be to adsorb ammonia that has contaminated ground water as the result of the leaching of uranium ores.The natural zeolites have a pronounced selectivity for molecules with permanent dipoles, such as water. Reducing the silica-alumina ratio during synthesis or through acid leaching decreases the water absorption capacity, as shown in figure 17. Silicalite, the ZSM-5 molecular sieve polymorph, has no aluminum, has no active sites, and is hydrophobic. ZSM-5 can be formulated to contain enough aluminum to be active but little enough to remain hydrophobic. Such zeolites, then, can remove trace organics such as n-hexane from water but have greater stability during regeneration than the competing carbon.
Besides the well-documented uses of zeolites as a desiccant, ammonium remover, and gas stream dryer, Garg and Ausikaitis reported the use of zeolites in energy-efficient adsorption cycle for removing large amounts of water (up to approximately 20%) from process streams. The adsorption cycle and the unique properties of zeolite molecular sieves can be used to dehydrate water-organic azeotropes. The dehydration of the ethanol-water azeotrope can be accomplished with less capital and lower energy costs [less than 2,000 Btu/gal (560 kJ/L)] using zeolites than with conventional azeotropic distillation methods. They claimed that this technology extends the use of molecular sieve adsorptive dehydration to the removal of over 20 wt % water from organic admixtures, and is capable, for example, of dehydrating the ethanol-water azeotrope from 190 proof (7.58 wt % water) to 199 proof ethanol. 2ff7e9595c
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