Catalysts zeolite-based

Several groups investigated over-exchanged Fe-ZSM-5 by X-Ray absorption spectroscopy and supported unanimously the formation and catalytic relevance of the binuclear Fe-O-Fe sites [75-77]. The EXAFS spectra obtained in these studies 

For HC-SCR, this correlation resulted in strong support for a contribution of both isolated and oligomeric Fe 0x0 sites to the reaction rates observed at low temperatures. At high temperatures, the oligomers catalyze the oxidation of the hydrocarbon reactant very effectively, therefore, the best catalysts for HC-SCR contain iron only in small quantities. The results for NH3-SCR are summarized in 

The surprisingly good correlations between site abundance and SCR activity stiU leave a number of questions unanswered. As already mentioned, isolated sites is a quantity lumped of at least three species detectable by EPR. The redox properties of these sites in the typical feeds (NH3-SCR and HC-SCR) were found to be very different, octahedrally/distorted tetrahedrally coordinated isolated sites being more prone to reduction than tetrahedrally coordinated sites under conditions where oligomers withstood reduction completely [87], As SCR most likely requires Fe to be in the -1-3 state in order to activate the reductant (cf. Chaps. 8 and 9), 

The importance of acidity for NH3-SCR has been discussed also with respect to Fe zeolite catalysts. A favorable role of acidity is a priori plausible because this tends to increase the local concentration of the ammonia reductant near the active sites. The more fundamental question is, however, if an acidic function is part of the active sites as, for instance, in the sites driving the reaction cycle proposed by T0psoe [61-63] for V205/Ti02 catalysts. From a comparison of SCR activities measured with Fe in nonacidic and acidic zeolite supports, Schwidder et al. concluded that acidity favors the reaction without being an essential ingredient of the active site and hence the reaction mechanism [100]. A more recent study of Brandenberger et al. arrived at similar conclusions [101], which are at variance with some earlier proposals, e.g., in [83]. 

The technical relevance of Fe zeolites is related to their potential to catalyze the fast SCR reaction (Eq. 7.2) rather than to their activity in standard SCR. Fast SCR is a rather facile reaction which has been proposed to proceed without any involvement of Fe sites [102,103]. This has been confirmed in [88], but it has been shown at the same time that Fe zeolites offer sites which accelerate the reaction dramatically. Standard SCR and fast SCR are stoichiometrically related to each other The former (Eq. 7.1) results when NO oxidation (Eq. 7.3) is added to fast SCR (Eq. 7.2). 

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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. 

The hydroisomerization of heavy linear alkanes is of a great interest in petroleum industry. Indeed, the transformation of long chain n-alkanes into branched alkanes allows to improve the low temperature performances of diesel or lubricating oils [1-3]. On bifunctional Pt-exchanged zeolite catalysts, n-CK, transformed into monobranched isomers, multibranched isomers and cracking products [4], The HBEA zeolite based catalyst was more selective for isomerization than those containing MCM-22 or HZSM-5 zeolites [4], This was explained on one hand by a rapid diffusion of the reaction intermediates inside the large HBEA channels, and on the other hand by the very small crystallites size of this zeolite (0.02 pm). 

The use of zeolite-base catalysts have only emerged relatively recently. Mobil olefin to gasoline/disHllate (MOGD) developed by Mobil and conversion of olefin to disHllate (COD) of PetroSA use ZSM-5 zeolite to convert olefins to gasoHne and... 

Zeolite based catalysts for linear alkylbenzene production dehydrogenation of long chain alkanes and benzene alkylation. Catal. Today, 38, 243-247. 

Kawase, M., Nomura, K., Nagamori, Y., and Kinishita, J. (2001) High silica content zeolite based catalyst. U.S. Patent 6, 207,605. 

Zeolites are not typically employed in converting these classes of compounds, but their impact on zeolitic catalyst performance in FCC and hydrocracking, coupled with the occasional use of zeolite based catalysts to assist in their conver-... 

Chapter 3 outlines zeolite synthesis, modification and the manufacturing of zeolite-based catalysts and adsorbents. Extensive patent references are given to provide the reader with a historical perspective. Some of the pitfalls associated with the operation of synthesis and manufacturing units are also described. 

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. 

A major industry shift to zeolite-based catalyst systems is expected to lower production costs and improve product yield. 

The mechanism of fast SCR over a zeolite-based catalyst has also been addressed by Sachtler and co-workers using an IR technique [69, 70]. They concluded that nitrogen is produced through fast decomposition of ammonium nitrite (the hydrated form of nitrosamide), vhich is formed from equimolar NO/NO2 feeds via N2O3 and its reaction vith water and ammonia ... 

The results from the ACE FEB unit showed clearly that the low activity of the MAB catalyst would produce high slurry oil yields in present day FCC configurations, optimized for Y-zeolite based catalysts. Some adjustments of the FCC hardware would certainly be necessary to fully exploit the new catalyst selectivity potential. 

The effect of the Si/Al ratio of H-ZSM5 zeolite-based catalysts on surface acidity and on selectivity in the transformation of methanol into hydrocarbons has been studied using adsorption microcalorimetry of ammonia and tert-butylamine. The observed increase in light olefins selectivity and decrease in methanol conversion with increasing Si/Al ratio was explained by a decrease in total acidity [237]. 

In this section the methods described in Sections III and IV are applied to derive a dynamic numerical model of the SCR of NO-N02 with NH3 over a commercial V205/W03/Ti02 extruded monolith catalyst. The extension of the same dynamic model to a zeolite-based catalyst is currently in progress (Chatterjee et al., 2007). 

In zeolite-based catalysts, the Lewis acidity is related to the existence of extra-framework A1 (EFAL) species formed during the zeolite dealumination process [18], It occurs frequently in zeolite activation, for example, during the calcination process... 

Simultaneously scientists at Esso Research and Engineering and Mobil Oil were working with X based catalysts [33-35]. Mobil Oil introduced the first zeolite based catalysts for cracking gas oils in 1962 using rare earth exchanged X in a silica-alumina matrix. This replaced the older silica-alumina catalysts. When we made Y available, the Y based catalysts largely replaced the X based catalysts in this application. 

A number of zeolite-based catalysts are active for the dimerization of ethylene. The major products are n-butenes (1-butene, tram-2-butene, m-2-butene), i.e.,... 

A patent (184) describes zeolite-based catalysts for the direct production of ethylbenzene from butadiene, i.e.,... 

To the author s knowledge, there are at present no major industrial processes which could be strictly defined as nonacid catalysis that make use of zeolite-based catalysts. This is in contrast to acid catalysis where zeolites continue to make an impact. Technically, a number of zeolite-based catalysts for reactions, such as Wacker chemistry and olefin or diolefin oligomerization reactions, appear to be quite attractive, and it is almost certainly economic factors that have limited further development. 

Cumene manufacture consumed about 10 percent (2.2 billion lb) of the propylene used for chemicals in the United States in 1998. It is prepared in near stoichiometric yield from propylene and benzene with acidic catalysts (scheme below). Many catalysts have been used commercially, but most cumene is made using a solid phosphoric acid catalyst. Recently, there has been a major industry shift to zeolite-based catalyst. The new process has better catalyst productivity and also eliminates the environmental waste from spent phosphoric acid catalyst. It significantly improves the product yield and lowers the production cost. Cumene is used almost exclusively as feed to the cumene oxidation process, which has phenol and acetone as its coproducts. 

Alkylation. In the field of alkylation of benzene with ethene zeolite-based catalysts are used for the past 20 years, replacing the conventional A1C13- and BF3-on-alumina based processes. Here the question in case of a new plant is not whether a zeolite-based process will be selected but rather which one to choose. The Mobil-Badger process uses ZSM-5 as the catalyst and is the most widely applied though recently other zeolites (Y, Beta and MCM-22) have come to the fore. 

Direct dehydroisomerisation (DHI) of n-butane into isobutene over bifunctional zeolite-based catalysts represents a potential new route for the generation of isobutene utilising cheap n-butane feedstock. Isobutene is used worldwide for production of methyl tert-butyl ether (MTBE) and polyisobutylene. It is currently obtained via extraction from refinery/cracker C4 streams or via conversions of isobutane (in one step) or n-butane (in two steps).1,2 Isobutene can also be produced via the isomerisation of n-butenes,3 although there is no evidence that this is practised commercially.2,3... 

The key feature of the adequate transition element seems to be in this ability to stabilize both electrophiles and nucleophiles within the same complex. Extra ligands may or may not increase the reactivity of one of these two species and therefore makethe design of a transition metal zeolite based catalyst within the same ease or difficulty as their homogeneous analogues. 

The efficiency of a Vanadium trap additive is illustrated in Figure 5.8 (Si reflects the presence of the zeolite based catalyst while La and V are present on the same additive particle, the V-trap) and the catalytic effect demonstrated in Figure 5.9. The effectiveness of V-traps is particularly difficult to test in the laboratory, because the level of vanadium mobility in commercial units is difficult to simulate, and the competitive reaction to form sulfate is not taken into account by most laboratory testing. 

Small quantities of sulfur and water will severely reduce isomerization performance and at moderate levels will permanently deactivate chlorinated alumina (that cannot be regenerated) or sulfated zirconia based catalysts. Traditional zeolite based catalysts are also lacking sulfur tolerance, but to a lesser extent. 

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