Bifunctional zeolitic catalysts

Under the operating conditions, the reaction intermediates (w-hexenes and i-hexenes in n-hexane isomerization) are thermodynamically very adverse, hence appear only as traces in the products. These intermediates (which are generally olefinic) are highly reactive in acid catalysis, which explains that the rates of bifunctional catalysis transformations are relatively high. The activity, stability, and selectivity of bifunctional zeolite catalysts depend mainly on three parameters the zeolite pore structure, the balance between hydrogenating and acid functions, and their intimacy. In most of the commercial processes, the balance is in favor of the hydrogenation function, that is, the transformations are limited by the acid function. 

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... 

Dewaxing is the final example of a reaction illustrated here with possibly multiple restricted transition state shape selectivity effects. Bifunctional zeolitic catalysts... 

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],... 

Heterogeneous catalysts are solid materials that sometimes consist of the bulk material itself, for example, acid zeolite catalysts [10] or fused catalysts [11], Or in other cases of an active component or components deposited, as a rule, on a highly developed area support, for example, silica, alumina, carbon or in some cases a zeolite. The function of the support is to enhance the catalyst properties, for example, the stability of the active component or components, or in some cases to be even included in the catalytic reaction, for example, by providing acidic sites in bifunctional zeolite catalysts [10],... 

Bifunctional zeolite catalysts are used in various commercial processes light alkane hydroisomerization (chapter 7), hydrocracking (chapter 6),hydrodewaxing (chapter 8), light alkane aromatization and hydroisomerization of the C8 aromatic cut (chapter 9). The hydrogenation/dehydrogenation components included in zeolite catalysts can be very different and located in different positions ... 

The Cyclar process converts C3 and G paraffins to aromatics via a bifunctional zeolitic catalyst (25). Typical aromatic yields in Table 3.11 are 70 wt-%, with 6 wt-% hydrogen yields. The light hydrocarbon products can be used as fuel for the plant. Cyclar can be considered for remote locations as an alternative to flaring LPG, or when refrigerating LPG for shipment is not economical. 

Bifunctional zeolite catalysts such as platinum loaded acid zeolite catalysts are applied in several petroleum refinery operations, designated as hydroconversion processes isomerisation of light n htha, iso-dewaxing and hydrocracking of heavy fractions [4]. Most experimental investigations in academic laboratories are typically performed with pure model components or simple mixtures thereof as feedstock, and using reaction conditions under which the hydrocarbon compounds are in the vapor phase. Industrial hydroconversion processes are mostly run under three phase, or even in some cases under liquid phase conditions and with feedstocks that are extremely complex mixtures of large numbers of different hydrocarbon compounds [4]. 

The mechanism of n-alkane transformation over bifunctional zeolite catalysts has traditionally been assumed to occur through a sequence of steps (41) ... 

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

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

Mono- as well as bifunctional zeolite catalysts are applied in the multistep conversions of ... 

The most probable way of forming benzene is from a Cg oligomer, obtained from C3 condensation, but not through primary carbocations. The critical step would be the formation of the protonated cyclobutane ring, however, the four member ring cycloalkane has been claimed to occur as an intermediate during the isomerization of alkanes on bifunctional zeolite catalysts... 

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. 

Finally, we shall discuss two examples that demonstrate the shape selectivity of bifunctional zeolite catalysts. Thus the difiusivity of trans-2-hutene in zeolite CaA is 200 times higher than that of cis-2-butene. Doping with Pt allows selective hydrogenation of fmns-2-butene to be carried out [T35]. Also of interest is shape selective hydrogenation on [Pt]ZSM-5, which is compared to hydrogenation on a conventional supported Pt catalyst in Table 7-8. With the zeolite catalyst, hydrogenation of the unbranched alkene is favored. 

Bifunctional zeolitic catalysts are also used for hydrocracking, in which the heaviest fraction of crude oil is cracked in the presence of hydrogen to give... 

Scheme 8.19 Hydroisomerisation of w-hexane over bifunctional zeolite catalysts.

Hierarchical micro/mesopore structure can be obtained by forming composite from delaminated layered-stmcture zeohte and a mesoporous MTS material. At the same space time the bifunctional zeolite catalyst, having hierarchical micro/mesoporous stractme, show lower n-C hydroconversion activity and higher hydroisomerization selectivity than the corresponding microporous zeolite catalyst. 

Thus, to reduce the density of Bronsted acid sites in the bifunctional zeolite catalysts, introduction of both Ca + and Pd was carried out, and this was done either simultaneously (method A) or successively (method B). Best results were obtained by method B. As an example. Fig. 62 shows the conversion of ethylbenzene and the yields of ethylcyclohexane, xylenes, dimethylcyclohexanes, alkanes, benzene and diethylbenzenes over a Pd,Ca,H-ZSM-5 catalyst prepared by a two-step solid-state ion exchange. In a first step, CaCl2 was incorporated via SSIE, followed by a second step, viz., SSIE of PdCl2 into Ca,H-ZSM-5 obtained in the first step. 

The direct synthesis of bifunctional zeolitic catalysts creates new opportunities for the application of shape selective materials in heterogeneous catalysis. 

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