Bifunctional metal/zeolite catalysts

The use of bifunctional metal/zeolite catalysts for the conversion of synthesis gas (carbon monoxide and hydrogen) to gasoline range hydrocarbons has recently attracted much attention. For example, the combination of metal oxides with the medium pore ( 6A) zeolite ZSM-5 and the use of a metal nitrate impregnated ZSM-5 catalyst have been shown to produce gasoline range hydrocar-... 

In addition to this, solid acid catalysts can also be used in the hydroisomerization cracking of heavy paraffins, or as co-catalysts in Fischer-Tropsch processes. In the first case, it could also be possible to transform inexpensive refinery cuts with a low octane number (heavy paraffins, n-Cg 20) to fuel-grade gasoline (C4-C7) using bifunctional metal/acid catalysts. In the last case, by combining zeolites with platinum-promoted tungstate modified zirconia, hybrid catalysts provide a promising way to obtain clean synthetic liquid fuels from coal or natural gas. 

Industrial metal-zeolite catalysts undergo a bifunctional, monomolecular mechanism [1-5, 7]. Carbenium ions are the critical reaction intermediates to complete chain reactions. In the zeolite channels, carbenium ions likely exist as an absorbed alkoxyl species, rather than as free-moving charged ions [8], Figure 14.2 illustrates the accepted reaction mechanism, using hexanes as an example. 

Metal containing zeolites are of great interest as practical catalysts for petrochemical reactions. Such catalysts are usually considered to be bifunctional, with the zeolite framework acting as a support for the metal centers. In order to optimize reaction conditions and parameters in the processes for which metal-zeolite catalysts are used, it i s necessary to understand the chemistry of the support and the metal and their interaction. 

Noble metal zeolite catalysts are used in various processes, most of them occurring through bifunctional hydrogenating/acid catalysis. One exception, however, is the selective aromatization of n-alkanes (e.g. n-hexane into benzene) proceeding through monofunctional metal catalysis. Indeed the PtLTL catalyst used commercially does not present any protonic sites. 

Thus, study of the kinetics of n-pentane isomerization on H-mordenite leads to the conclusion that the mechanism of the reaction in question is different from that of isomerization on bifunctional and metal-zeolite catalysts. This difference lies in the manner of carbonium ion formation. With bifunctional catalysts, carbonium ion originates with the attachment of a proton to the olefin molecule, while with H-mordenite it originates as a result of splitting off hydride ion from the saturated molecule of the starting hydrocarbon by mordenite proton, as has been suggested by the above reaction scheme. 

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. 

One-Pot Multistep Synthesis of Ketones on Bifunctional Zeolite Catalysts. One-pot multistep reactions constitute an elegant and efficient way to decrease the number of chemical and separation steps, hence, to develop greener synthesis processes. Bifunctional metal-acidic or metal-basic zeolite catalysts, which can be prepared easily with the desired properties (e.g., distribution of the... 

The paper deals with some new data concerning the state of the metal after reduction and the catalytic functions of zeolite catalysts containing nickel and platinum. By using the molecular sieve selectivity in the hydrogenation of mesitylene it has been proved that metal (platinum) is contained in the volume of the zeolite crystal. The temperature dependence of the formation of nickel crystals was investigated. The aluminosilicate structure and the zeolite composition influence mainly the formation of the metal surface which determines the catalytic activity. In the hydrocracking of cumene and disproportionation of toluene a bifunctional action of catalysts has been established. Hydrogen retarded the reaction. 

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. 

In 1949, the development of a catalyst based on a combination of platinum and an acidic component (e.g. A1203, A1C13) allowed the use of lower reaction temperatures than with the early catalysts.6 However, problems were still encountered with chlorine corrosion. In the 1960s, Universal Oil discovered that the addition of rhenium to a bifunctional Pt/Al203 catalyst resulted in slower deactivation by carbon deposition, and other dopants have since been found to modify the catalyst acidity and resistance to poisons, e.g. Cl, Sn, Ir. More recently, catalysts based on zeolites and noble metals have been shown to be more resistant to nitrogen and sulphur compounds, while giving a high activity and selectivity to branched alkanes. 

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. 

Metal incorporation into the zeolite using metal loaded seed materials. The combination of catalyst metal with zeolite catalyst is one of the most intriguing subjects for bifunctional catalysis. The achievement of prominent effect of the seed crystals on the crystallization of ZSM-34 type catalyst induced an idea that the seed material on which a catalyst metal had been supported previously would also be effective for rapid crystallization. 

The reformation of lower paraffins to aromatics has been studied for about 20 yr by using zeolite catalysts. Recently, an excellent review was published of lower alkane transformation to aromatics on ZSM-5 zeolites [2]. From the studies of the mechanism of this reaction, it has been suggested that the bifunctional catalysts, having solid acidity and dehydrogenation activity, can effectively promote the aromatization of lower paraffins[3-6]. It has been reported that ZSM-5 and ZSM-11 are excellent solid acid catalysts [7] and the transition metals [8], Ga [9] and Zn [9] show high dehydrogenation activity in this reaction. In the case of bifunctional... 

Selective synthesis of cyclohexylcyclohexanone on bifunctional zeolite catalysts. Influence of the metal and of the pore structure... 

Bifunctional Pt or Pd zeolite catalysts (with large or average pore sizes) can catalyze in one pot the transformation of cyclohexanone into cydohexyl-cyclohexanone which requires three successive steps catalyzed by acid sites aldoUsation and dehydration or by metal sites hydrogenation. Pd catalysts are more selective than Pt catalysts, for palladium catalyzes preferentially the hydrogenation of C=C double bonds (compared to the C=0 bonds). PdHFAU zeolites because of their large pores and of their tridirectional pore system are the most active and selective catalysts. With these catalysts the formation of... 

Some well-known types of bifunctional zeolitic catalysts are zeolites in the H-forrn containing noble metal crystallites... 

Although GEMS is a more difficult technique experimentally than normal Mdssbauer spectroscopy it offers many exciting possibilities for the study of Mdssbauer isotopes on or near the surface. It could be a useful technique to study the properties of bifunctional zeolite catalysts and metal-impregnated zeolite catalysts. 

Incorporation of metals or metal oxides into zeolite cavities leads to the formation of nanosized clusters exhibiting different catalytic properties from the bulk materials. These metal particles are usually introduced into zeolite channels through ion-exchange followed by reduction or oxidation/reduction to get their final dispersions. Metal clusters can also be formed via zeolite impregnation by corresponding azides from methanolic solutions followed by thermal decomposition. " Catalytic activities of the bifunctional or basic catalysts prepared using these methods can be successfully combined with shape-selective properties of parent zeolites. 

It may be, however, that no single phase zeolite catalyst is able to fulfil the role, and there has also been interest in bifunctional lean burn-deNOx catalysts where the zeolite is mixed with another catalyst. A zeolitic catalyst may be used together with a supported Pt/Al203 catalyst that is more active for NOx reduction at low temperatures. Other examples include Mn203/Ce-ZSM-5, where the manganese oxide catalyses NO oxidation to NO2, which reacts rapidly over the metal-containing zeolite, and Pt/H-ZSM-5, where the acid sites activate the hydrocarbon and promote its reaction with the NOx-... 

Whereas the Mobil process starts with syn gas based methyl alcohol, Olah s studies were an extension of the previously discussed electrophilic functionalization of methane and does not involve any zeolite-type catalysts. It was found that bifunctional acidic-basic catalysts such as tungsten oxide on alumina or related supported transition metal oxides or oxyfluorides such as alumina or related supported transition metal oxides or oxyfluorides such as tantalum or zirconium oxyfluoride are capable of condensing methyl chloride, methyl alcohol (dimethyl ether), methyl mercaptan (dimethyl sulfide), primarily to ethylene (and propylene) (equation 65) . 

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