Multifunctional zeolites

In this section, we discuss new synthetic trends to tailor and design zeohte properties in order to control and enhance their catalytic and adsorption performance. In spite of remarkable advances in the zeolite field, the types of zeoHtes with industrial applications are still Hmited to less than 20. Hence, it is important to consider the new opportunities and perspectives in this field. In this section, we first discuss the different synthetic approaches to overcome the limitations imposed by the small channel entrances of zeolites while preserving their chemical properties (Figure 8.4). Likewise, the tuning of active sites in zeolites to broaden their appH-cations is briefly discussed (multifunctional zeolites and hybrid organic-inorganic zeolites). 

Multifunctional Zeolites Efficient, multifunctional zeolite-based catalysts allowing one-step complex reactions are of great interest in the field of fine chemicals and organic industrial synthesis [58]. Recendy, catalysts based on zeolites are relevant in emerging areas of interest such as the catalytic conversion of biomass to fuels and chemicals (see Section 8.2.1.3 for illustrative examples). This field needs to develop specific multifunctional catalysts having the correct polarity (adsorption properties) and reactant accessibility (porosity), which are efficient in water or biphasic operation with reactants and products of different polarities and sizes. Hence, great opportunities for zeolites and related materials are offered in this new field [59-61]. 

Current state-of-the-art technology for the production of MIBK involves one-step liquid phase processes in trickle bed reactors at 100-160°C and 1 to 10 MPa utilizing various multifunctional catalysts including Pd, Pt, Ni or Cu supported on, metal oxides, cation exchange resins, modified ZSM5 and other zeolites with lull energy integration (2,3,4). However, the MIBK... 

The actual technology involves the formulation of multifunctional cracking catalysts which are composed of different amorphous and crystalline acid functions, and a series of additives for metal passivation, SOx removal, promotors for total combustion, and octane enhancing additives. Among them, zeolite Y is the main component controlling the activity and selectivity of the cracking catalysts. 

The unique features of zeolites, and the possibility of tuning acidic and basic sites, as well as the creation of multifunctional catalysts, open a wide field of applications in the production of fine chemicals. In this article we present new heterogeneously catalyzed processes for the synthesis of industrially relevant fragrances, flavors and aromas. The emphasis of this review article will remain mainly on solid acids. 

If the epoxide rearrangement (see chapter 15.2.1) of styrene oxide is carried out in the presence of hydrogen and by use of a bifunctional boron-pentasil zeolite catalyst having a hydrogenation component such as Cu, then 2-phenylethanol is obtained in one step. This hydro-isomerization renders high yields (> 85%) at 250 °C under the gas phase conditions. It is an example for multifunctional catalysis in a one pot-reaction, that means simultaneous rearrangement and hydrogenation. 

Bergaya F, Gatineau L and Van Damme H (1993) In Multifunctional Mesoporous Inorganic Solids (C A C Sequeira and MJ Hudson, eds), Kluwer, Dordrecht, p 19 Bernier, A, Admaiai, L F and Grange, P (,1991) Appl Catalysis, 77,269 Breck D W (1974) Zeolite Molecular Sieves, Wiley, New York... 

Monoliths allow the efficient use of small catalyst particles, such as zeolites, and are remarkably flexible with respect to their catalyst inventory. Multifunctional reactor operations such as reactive stripping and distillation are challenging applications that are not far away. They have several potential applications in oil refineries, in fhe chemical process industry, and for consumers. The industrial application of the monolithic stirrer reactor as alternatives to many slurry-t)q5e reactors in fine chemisfry has the greatest potential as a new practice involving monolithic catalysts. 

Porous structure of multifunctional mineral - carbon and zeolite-carbon sorbents... 

The combination of reaction and separation in one multifunctional membrane reactor is an interesting option. In such a reactor the membrane could be catalytically active itself, or it could serve only as a separation medium. There are several types of operation for such a reactor [33]. It could be used to separate the formed products from the reaction mixture. In this way it is possible to overcome equilibrium limitations or to improve the selectivity of the reaction. Another possibility is the controlled addition of reactant via the membrane, which might be of use in, for example, oxidation reactions or sequential reactions. The advantage of using zeolitic membranes in a membrane reactor is that they have a high thermal stability and exhibit a good selectivity. Moreover, they can be made catalytically active. 

In addition to practical applications, metal cluster-derived catalysts, particularly intrazeolite metal cluster compounds, may aid in the identification of catalytically important bonding and structural patterns and thereby further our molecular understanding of surface science and heterogeneous catalysis. The ship-in-bottle technique for the synthesis of bulky metal-mixed metal cluster compounds inside zeolites and/or interlayered minerals has gained growing attention for the purpose of obtaining catalytic precursors surrounded by the interior constraint, imposing molecular shape selectivity. Such approaches may pave the way to offer the molecular architecture of hybrid (multifunctional) tailored catalysts to achieve the desired selectivity and stability for industrial processes. 

Heterogenous hydrogenation catalysts can be combined with materials possessing other types of catalytic properties, resulting in bi- or multifunctional catalysts. For example, the catalyst used in the hydrocracking process is a combination of a noble metal, an acidic zeolite, and some other components. It leads, under the process conditions, to very effective hydrogenolysis of the long-chain components in the crude oil. ... 

The foregoing brief description of the catalytic properties, and physical, structural and surface chemistry of heteropoly oxometalates demonstrates the multifunctional capabilities of these solids in various processes. While in some respects similar in their properties and behaviour to zeolites some notable differences are evident. The properties of heteropoly oxometalates evidently depend predominantly but not entirely on the nature of the central and peripheral metal elements of the anion, but the contribution of the cation cannot be neglected. Nevertheless, there is convincing evidence that the terminal oxygen atoms of the anion play a central role in the mechanism of any catalytic process in which heteropoly oxometalates participate, and in particular in the conversion of methanol to hydrocarbons and the partial oxidation of methane. 

Type of Active Sites. - In heterogeneous catalysis the following type of actives sites can be distinguished (i) metallic, (ii) acid-base, (iii) red-ox type, and (iv) anchored metal-complex. The catalytic sites may contain one of the above types of active sites or can include several types of sites. In case of different type of sites the catalysts are bifunctional or multifunctional. For instance, Pt/Al203 and Pt/mordenite are typical bifunctional catalysts containing both metallic and acidic types of active sites. On the other hand, Pt or Pd supported on silicon carbide, nitride, or Pt/L-zeolite are mono-functional catalysts. There are important industrial reactions, such as isomerization and aromatization of linear hydrocarbons, which requires bifunctional catalysts, such as chlorinated... 

Combination of several individual reactions into one synthesis step, ie. shortening the synthesis route by using multifunctional catalysis with zeolites. 

Multifunctional catalysis, in which reactions consisting of several reaction steps are carried out by a shorter synthesis route, is becoming increasingly important in organic synthesis. Molecular sieve catalysts, too, help to combine several catalytic steps and tailor them optimally to one another [15, 18, 24], In this respect, molecular sieves like zeolites can be used as carriers for catalytically active components such as transition metals, noble metals. In addition the catalytic behaviour of these components the intrinsic acidic or basic or redox properties of the zeolites combined with shape selective feature are still present. 

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