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[ Pobierz caÅ‚ość w formacie PDF ] for two zeolites, namely Pd/H-ZSM-5 (MFI) and Pd/H-L (LTL) [139]. It has furthermore been demonstrated for zeolite EU-1 (EUO) that the amount of noble metal introduced into the pores does not influence the selectivity of the shape-selective hydrocracking of butylcyclohexane [139], at least not within a reasonable range. In conclusion, for the characterization of large-pore molecular sieves, the Spaciousness Index is the method of choice. The carbocation intermediates, which govern the selectivity of hydrocracking of C10 cycloalkanes, seem to be ideally suited for exploring the space available in the whole range of 12-membered-ring materials [2]. 4.4 Are Monofunctional or Bifunctional Forms of Molecular Sieve Catalysts More Suitable? To make a molecular sieve catalyst bifunctional, a strong hydrogenation/de- hydrogenation component has to be incorporated into the pores of its acidic form. In the vast majority of cases, either platinum or palladium have been used for this purpose. The catalytic experiments have to be conducted in a hy- drogen atmosphere. Obvious questions that have to be asked, are: a) Does the noble metal under the conditions of the test reaction develop an undesired catalytic activity of its own, viz. for metal-catalyzed skele- tal isomerization and hydrogenolysis? Such side reactions could falsify the selectivities of the reactions via carbocations described in Sect. 4.3. b) Does the noble metal introduced into the pores influence their effective width, i.e., are the zeolite pores slightly narrowed by the metal clusters generated? As a matter of experience, the answer to question a) is almost always fa- vorable: Isomerization (cf. the Modified Constraint Index, Sect. 4.3.1) and hydrocracking (cf. the Spaciousness Index, Sect. 4.3.2) via the bifunctional route are usually significantly faster than the reactions on the noble metal. This is particularly so for hydrocarbons with as many as ten carbon atoms like n-decane (CI") and butylcyclohexane (SI). The typical temperature range æ% of their conversion on bifunctional zeolite catalysts is around 250 Cor below, whereas their skeletal isomerization and hydrogenolysis on the noble metal æ% begin to occur at or even above ca. 300 C. It is, hence, generally easy to de- Characterization of the Pore Size of Molecular Sieves 149 termine CI" and SI without disturbances by reactions on the metal. Since the risk of undesired reactions on the metal is lower for palladium than for plat- inum (this holds especially for skeletal isomerization and to a lesser extent for hydrogenolysis), the use of palladium, typically with a loading of 0.3 wt.- % on the dry zeolite, as hydrogenation/dehydrogenation component inside the pores has been recommended [2]. An answer to question b) can immediately be obtained from a quantita- tive estimation of the metal cluster density inside the pores of zeolites, which are typically used as catalysts, i.e., faujasite (FAU) and ZSM-5 (MFI). If intro- duced into the pores via ion exchange with the tetraamine complex and after an adequate activation of the ion-exchanged zeolite (see vol 3 of this book se- ries, Chap. 4 Preparation of Metal Clusters in Zeolites ), the metal will reside in its zero-valent state in the form of small clusters located inside supercavi- ties of faujasite or pore intersections of ZSM-5. An estimation for palladium gives cluster sizes of ca. 60 Pdatoms and25 Pdatoms inside a supercavity of faujasite and a pore intersection of ZSM-5, respectively. For a typical palla- dium loading of 0.3 wt.- % it is estimated that one out of ca. 1550 supercavities in faujasite and one out of ca. 625 pore intersections in ZSM-5 are filled with a palladium cluster. It is obvious from these figures that, in typical bifunc- tional zeolites, with their low metal loadings, the vast majority of cavities and pore intersections are totally unaltered and free from metal atoms. Hence the risk of narrowing the zeolite pores by incorporating the noble metal can be considered as negligible. Among the important advantages of hydrocarbon reactions on bifunc- tional catalysts in a hydrogen atmosphere is the lack of self-poisoning by coke formation and the absence of catalyst deactivation. By contrast, the formation of carbonaceous deposits is very frequently encountered during hydrocar- [ Pobierz caÅ‚ość w formacie PDF ] |