Bifunctional Zeolite Catalysts

Sai Prasad, P.S., Bae, J.W., Kang, S.H., Lee, Y.J., Jtm, K.W., 2008. Single-step synthesis of DME from syngas on Cu-ZnO-Al203/zeolite bifunctional catalysts the superiority of ferrierite over the other zeolites. Fuel Processing and Technology 89,1281-1286. 

Hydroisomerization of n-hexadecane on Pt/HBEA bifunctional catalysts effect of the zeolite crystallites size on the reaction scheme. 

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 objective of this work is to determine the influence of the porous structure (size and shape) and acidity (number and strength of the acid sites) on isomerization selectivity during the conversion of ethylbenzene on bifunctional catalysts PLAI2O3/ 10 MR zeolite. The transformation of EB was carried out on intimate mixtures of Pt/Al203 (PtA) and 10 MR zeolites (ZSM-5, ZSM-22, Ferrierite, EU-1) catalysts and compared to Mordenite reference catalyst activity. 

There are several examples of one-pot reactions with bifunctional catalysts. Thus, using a bifunctional Ru/HY catalyst, water solutions of corn starch (25 wt.%) have been hydrolyzed on acidic sites of the Y-type zeolite, and glucose formed transiently was hydrogenated on ruthenium to a mixture of sorbitol (96%), mannitol (1%), and xylitol (2%) [68]. Similarly a one-pot process for the hydrolysis and hydrogenation of inulin to sorbitol and mannitol has been achieved with Ru/C catalysts where the carbon support was preoxidized to generate acidic sites [69]. Ribeiro and Schuchardt [70] have succeeded in converting fructose into furan-2,5-dicarboxylic acid with 99% selectivity at 72% conversion in a one-pot reaction... 

After a short description of the main features of zeolites, the significant contribution of zeolite catalysts in green chemistry will be shown in examples of commercial or the potential processes of refining, petrochemicals, and fine chemicals involving acid or metal acid bifunctional catalysts. 

Noble metals (e.g., Pt) can be introduced within the micropores of zeolites by exchange with a complex cation (e.g., Pt(NH3)4 ) followed by calcination and reduction. This mode of introduction generally leads to very small clusters of Pt (high Pt dispersion) located within the micropores. Pt supported on acid zeolites are used as bifunctional catalysts in many commercial processes. The desired transformations involve a series of catalytic and diffusion (D) steps, as shown in n-hexane isomerization over Pt acidic zeolite (Equation 12.1). 

Octadecane hydroprocessing behavior of Pt-containing bifunctional catalysts with TON and MTT framework types was compared, as illustrated in Figure 13.31 [28]. While the two zeolitic catalysts showed similar activities, the selectivity vs conversion performances were different. At any given conversion, the selectivity to dibranched isomers was lower and the selectivity to mono-branched isomers... 

Coke formation during xylene isomerization has been studied using in situ infrared spectroscopy [78]. A study done on EB isomerization with a bifunctional catalyst containing EUO zeolite indicated that poor initial selectivity of the catalyst improves after a period of fast deactivation, during which micropores are blocked [79]. 

In addition to large-scale industrial applications, solid acids, such as amorphous silica-alumina, zeolites, heteropoly acids, and sulfated zirconia, are also versatile catalysts in various hydrocarbon transformations. Zeolites are useful catalysts in fine-chemical production (Friedel-Crafts reactions, heterosubstitution).165-168 Heteropoly compounds have already found industrial application in Japan, for example, in the manufacture of butanols through the hydration of butenes.169 These are water tolerant, versatile solid-phase catalysts and may be used in both acidic and oxidation processes, and operate as bifunctional catalysts in combination with noble metals.158,170-174 Sulfated zirconia and its modified versions are promising candidates for industrial processes if the problem of deactivation/reactivation is solved.175-178... 

Feolite catalysts modified by transition metals are interesting and difficult subjects to study. In one of the first studies of zeolites as catalysts, Rabo and co-workers (1) used a zeolite catalyst containing 0.5% platinum for isomerization of n-paraffins. In this reaction the metal-zeolite system acted as a typical representative of the bifunctional catalysts. Studies of zeolites modified by transition metals ( 2, 3, Jf) showed that their polyfunctional properties are determined by the structural and chemical properties of the zeolite and by the state of the metal in it. In this paper we discuss new data on the metal state after reduction as well as the catalytic functions of zeolite catalysts containing nickel and platinum. 

Once the multi-step reaction sequence is properly chosen, the bifunctional catalytic system has to be defined and prepared. The most widely diffused heterogeneous bifunctional catalysts are obtained by associating redox sites with acid-base sites. However, in some cases, a unique site may catalyse both redox and acid successive reaction steps. It is worth noting that the number of examples of bifunctional catalysis carried out on microporous or mesoporous molecular sieves is not so large in the open and patent literature. Indeed, whenever it is possible and mainly in industrial patents, amorphous porous inorganic oxides (e.g. j -AEOi, SiC>2 gels or mixed oxides) are preferred to zeolite or zeotype materials because of their better commercial availability, their lower cost (especially with respect to ordered mesoporous materials) and their better accessibility to bulky reactant fine chemicals (especially when zeolitic materials are used). Nevertheless, in some cases, as it will be shown, the use of ordered and well-structured molecular sieves leads to unique performances. 

Otherwise, by impregnating a Pd precursor onto a basic K-exchanged FAU zeolite a highly selective bifunctional catalyst is obtained for the low-pressure one-step synthesis of 2-ethylhexanal (a component of perfumes and fragrances) from M-butyraldehydc and H2 in a fixed-bed reactor/12,131 Under optimum reaction... 

When metal centers act in conjunction with acid sites on the zeolite, bifunctional catalysis can occur (e.g., Pd/HY). This type of catalysis is used mainly for the hydrocracking and isomerization of long-chain n-alkanes. For example, the rates of formation of 2- and 5-methylnonane isomers obtained from n-decane isomerization over bifunctional zeolite catalysts depend on the size and structure of the zeolites used. This reaction has been developed as a test reaction to characterize zeolite structures (17-19). 

Another important zeolite catalyst is the so-called bifunctional catalyst. The thermal reduction of zeolites previously exchanged with metals is the method currently used for the preparation of bifunctional catalysts for hydrocarbon conversion. The bifunctional zeolite catalysts are composed of both acidic sites and metal clusters. The preparation methods of these catalysts encompass three steps ion exchange, calcinations, and reduction, (Section 3.2.1.4) [123,127],... 

Rapid crystallization would overcome the disadvantages of slow crystallization and, more significantly, hetero elements could be incorporated inside the crystals. Metal-incorporated zeolitic materials serve as bifunctional catalysts, exhibiting properties of both metal catalysts and zeolite catalysts. 

Silico-alumina zeolites are an important class of catalyst, serving both as solid acids and as supports in bifunctional catalysts. The acidity of the zeolite can be modified by substituting a heteroatom for the Si or A1 atoms in the zeolitic framework. Whenever a framework substitution is attempted, the first question is always whether the heteroatom is indeed in the framework, or instead exists as an extra-framework species. Then, if it can be demonstrated that the heteroatom is in the framework, the question arises as to the exact crystallographic site in the lattice where the substitution has occurred. Detailed knowledge of the site (the so-called T-site in a zeolite) is needed for a complete characterization of the catalyst. 

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... 

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