Zeolites based materials

Traa Y, Burger B and Weitkamp J 1999 Zeolite-based materials for the seleotive oatalytio reduotion of NOx with hydrooarbons Microporous Mesoporous Mater. 30 3-41... 

Cejka, ]. Centi, G. Perez-Pariente,). Roth, W. J., Zeolite-based materials for novel catalytic applications Opportunities, perspectives and open problems. Catal. Today 2012,179 2-15. 

An alkene mixture of industrial source (equal amounts of C9-C13 alkenes and alkanes) was used in the alkylation of benzene on three Nafion-silica catalysts with 5%, 13%, and 20% loadings.195 20% Nafion-silica showed high and stable activity and its performance exceeded that of a Y-zeolite-based material. The selectivity to 2-phenylalkanes (25%) was higher than in the Detal process using fluorinated silica-alumina but decreased somewhat with increasing Nafion content. 

Zeolite-based materials are also promising for the removal of organic compounds from industrial waste water (13). This is particularly true for chlorinated pollutants and the preferred process is based on adsorption/separation using hydrophobic molecular sieves (HMS). Compared to carbon adsorbents, HMS presents a good compromise between sorption capacities, selectivity to organics compared to water, and regenerability (vide infra, section 16.3.1.). 

The treatment of streams (gas or liquids) containing VOCs by zeolite-based materials belongs to adsorption/separation and/or catalytic oxidation. Regarding adsorption/separation alone, it concerns mainly industrial streams with relatively high VOCs concentrations. The VOCs taken up by zeolite are recovered and recycled. At low VOCs concentrations in complicated matrix, adsorption is often coupled with incineration or catalytic oxidation. 

Recently, great attention has been paid to the ambient temperature adsorption for removing sulfur compounds from natural gas, as the system is simple and is quick and easy to be started up. The major adsorbents that were used in the ambient temperature adsorption are AC-based and zeolite-based materials. The Osaka Gas mixed metal and metal oxide catalyst provides a low-temperature method of desulfurization. The catalysts (or adsorbents) is claimed to remove organic sulfur and H2S at room temperature.115... 

Zeolite materials are used commercially as shape/ size selective catalysts in the petrochemical and petroleum refining industry, and as molecular sieving separation media for gases and hydrocarbons. For both applications, zeolites are used in powder composite form such as pellets and granules. In this entry, we focus on zeolite membranes. We define zeolite membranes as a continuous phase of zeolite-based materials (pure zeolite or composite) that separate two spaces. Zeolite membranes are generally uniform thin films attached to a porous or a nonporous substrate. They can also be self-standing without a substrate. Note that we have included zeolite films and layers on nonporous substrate in this entry because we believe many of the synthesis strategies and applications reported for those nonporous substrates are easily transferred to a porous substrate to prepare a zeolite membrane. 

Most of the advanced zeolite-based materials described in the previous chapters have been synthesized very recently. For that reason not much effort has been devoted to investigation of their catalytic properties. However, at least some examples of interesting reactions or promising results are given in this chapter. 

Zeolite-based materials are extremely versatile uses include detergent manufacture, ion-exchange resins (ie, water softeners), catalytic applications in the petroleum industry, separation processes (ie, molecular sieves), and as an adsorbent for water, carbon dioxide, mercaptans, and hydrogen sulfide. 

Table 8.5 Types of zeolite-based materials for gas sensors...

Nicolaides et al. [185] have synthesized ZSM-5-based materials that are substantially amorphous or partially crystalline, obtained by varying the hydrothermal synthesis temperature from 298 K to 463 K. Ammonia adsorption micro calorimetry studies have shown that for these ZSM-5-based materials, whose crystalhnities as determined by XRD varied from 3 to 70%, the number of strong Brbnsted acid sites, i.e. those with a AHads between 120 and 140 kj mol increases with increasing XRD crystallinity. A strong correlation was observed between the catalytic activity of these zeolite-based materials in the n-hexane cracking reaction and the number of strong acid sites. How-... 

Dubey N, Rayalu SS, Labhsetwar NK, Naidu RR, ChattiRV, DevottaS (2006) Photocatalytic properties of zeolite-based materials for the photoreduction of methyl orange. Appl Catal A 303 152-157... 

Guth J-L and Kessler H 1999 Synthesis of aluminosilicate zeolites and related silica-based materials Catalysis and Zeolites, Fundamentals and Applications ed J Weitkamp and L Puppe (Berlin Springer) pp 1-52... 

Presently, the most successful adsorbents arc microporous carbons, but there is considerable interest in other possible adsorbents, mainly porous polymers, silica based xerogels or zeolite type materials. Regardless of the type of material, the above principles still apply to achieving a satisfactory storage capacity. The limiting storage uptake will be directly proportional to the accessible micropore volume per volume of storage capacity. 

Mcntasty el al. [35] and others [13, 36] have measured methane uptakes on zeolites. These materials, such as the 4A, 5A and 13X zeolites, have methane uptakes which are lower than would be predicted using the above relationship. This suggests that either the zeolite cavity is more attractive to 77 K nitrogen than a carbon pore, or methane at 298 K, 3.4 MPa, is attracted more to a carbon pore than a zeolite. The latter proposition is supported by the modeling of Cracknel et al. [37, 38], who show that methane densities in silica cavities will be lower than for the equivalent size parallel slit shaped pore of their model carbon. Results reported by Ventura [39] for silica xerogels lead to a similar conclusion. Thus, porous silica adsorbents with equivalent nitrogen derived micropore volumes to carbons adsorb and deliver less methane. For delivery of 150 V./V a silica based adsorbent would requne a micropore volume in excess of 0.70 ml per ml of packed vessel volume. 

The second method used to reduce exliaust emissions incorporates postcombustion devices in the form of soot and/or ceramic catalytic converters. Some catalysts currently employ zeolite-based hydrocarbon-trapping materials acting as molecular sieves that can adsorb hydrocarbons at low temperatures and release them at high temperatures, when the catalyst operates with higher efficiency. Advances have been made in soot reduction through adoption of soot filters that chemically convert CO and unburned hydrocarbons into harmless CO, and water vapor, while trapping carbon particles in their ceramic honeycomb walls. Both soot filters and diesel catalysts remove more than 80 percent of carbon particulates from the exliatist, and reduce by more than 90 percent emissions of CO and hydrocarbons. 

Another recent new application of a microporous materials in oil refining is the use of zeolite beta as a solid acid system for paraffin alkylation [3]. This zeolite based catalyst, which is operated in a slurry phase reactor, also contains small amounts of Pt or Pd to facilitate catalyst regeneration. Although promising, this novel solid acid catalyst system, has not as yet been applied commercially. 

Cationic and neutral dyes have the tendency to adsorb at the inner and at the outer surface of the zeolite crystals. It is to be expected that the affinity of molecules to the coat and the base area differs. The coat and the base area of a good zeolite L material are nicely illustrated on the left and right side, respectively, of Figure 1.7. The number of molecules needed to form a monolayer nD on a cylinder of surface Az is... 

Smith and coworkers recently proposed a specific and novel mineral-based solution to the problem of dilution and diffusion of prebiotic reactants. They have suggested [132-134] the uptake of organics within the micron-sized three-dimensional cross-linked network of pores found to exist within the top 50 xm, or so, of alumina-depleted, silica-rich weathered feldspar surfaces. These surfaces incorporate cavities typically about 0.5 pm in diameter along with cross inter-connections of about 0.2 pm. The nominal area of the weathered feldspar surface is apparently multiplied by a factor of about 130 arising from this network. The similarity of these pores to the catalytic sites in zeolite-type materials is pointedly mentioned. 

The most commonly employed crystalline materials for liquid adsorptive separations are zeolite-based structured materials. Depending on the specific components and their structural framework, crystalline materials can be zeoUtes (silica, alumina), silicalite (silica) or AlPO-based molecular sieves (alumina, phosphoms oxide). Faujasites (X, Y) and other zeolites (A, ZSM-5, beta, mordenite, etc.) are the most popular materials. This is due to their narrow pore size distribution and the ability to tune or adjust their physicochemical properties, particularly their acidic-basic properties, by the ion exchange of cations, changing the Si02/Al203 ratio and varying the water content. These techniques are described and discussed in Chapter 2. By adjusting the properties almost an infinite number of zeolite materials and desorbent combinations can be studied. 

A second area that will be important in the future is the continued development of MOFs and ZlFs [152]. Much as the discovery of AlP04-based materials revolutionized the catalyhc use of zeolites when only aluminosilicates were known, MOFs and ZlFs have the potential to revolutionize low temperature processes such as oxidations and organic reachons [153]. Newly discovered materials along these same lines are covalent organic frameworks, the so-called COFs [154]. These materials have similar channels to those known for MOFs and ZlFs but tend to have higher thermal stability. 

Zeolites have an enormous impact on our daily lives, both directly and indirectly. For example, upstream hydrocarbons such as aromatics and olefins are produced using zeolite catalysts. The aromatics or olefins are then separated from the reaction mixtures using zeolite adsorbents. The purified components produced by these zeolite-based methods are then used in downstream processes to produce products that we use daily, such as clothes, furniture, foods, construchon materials and materials to build roads, automobile parts, fuels, gasoline, etc. In addihon to the indirect impacts mentioned above, zeolites also have a direct impact on our daily lives. For example, zeolites are used as builders in detergent formulations. 

There are, however, two limitations associated with preparation and application of zeolite based catalysts. First, hydrothermal syntheses Umit the extent to which zeolites can be tailored with respect to intended appUcation. Many recipes involving metals that are interesting in terms of catalysis lead to disruption of the balance needed for template-directed pore formation rather than phase separation that produces macroscopic domains of zeoUte and metal oxide without incorporating the metal into the zeohte. When this happens, the benefits of catalysis in confined chambers are lost. Second, hydrothermal synthesis of zeoHtic, silicate based soHds is also currently Hmited to microporous materials. While the wonderfully useful molecular sieving abihty is derived precisely from this property, it also Hmits the sizes of substrates that can access catalyst sites as weU as mass transfer rates of substrates and products to and from internal active sites. 

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