Mordenite bifunctional catalysts

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. 

As it is generally the case with bifunctional catalysis processes, the balance between hydrogenating and acid functions determines for a large part the catalyst activity. This was quantitatively shown for series of bifunctional catalysts constituted by mechanical mixtures of a well dispersed Pt/Alumina catalyst and of mordenite samples differing by their acidity and their porosity (25). The balance between hydrogenating and acid functions was taken as nPt/nH+ the ratio between the number of accessible platinum atoms and the number of protonic sites determined by pyridine adsorption. 

The hydrogen form of mordenite is an extremely active catalyst in paraffin hydrocarbon isomerization as well. For example, 50% of n-pentane at 280 °C and 30 atm undergoes isomerization to produce isopentane. To attain the same degree of hydrocarbon isomerization with H-mor-denite as compared with bifunctional catalysts involving Pt, Pd, and Ni per AI2O3 or zeolite, the temperature should be 70°-120° lower 14). 

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. 

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... 

Platinum supported on zeolites such as mordenite is also active and the isomerisation of longer-chain alkanes is achieved with such catalysts, under mild conditions. The essential details of the process are illustrated in Scheme 8.19, with the metal function being to establish equilibrium between alkanes, hydrogen and a low level of olefins. The olefin content must be sufficient to allow isomerisation but sufficiently diluted by alkanes to prevent rapid oligomerisation and coking of the catalyst. The performance of the bifunctional catalyst is improved by the introduction of secondary mesoporosity in the mordenite catalyst, to enhance molecular transport between active site and gas stream, and by the generation of extra-framework aluminium sites that promote Lewis acid catalysed reactions. 

It was shown that solid-state ion exchange is also a suitable route to preparation of active acidic or bifunctional catalysts. Introduction of Ca or Mg into mordenite [21] or La " into Y-type zeolite, mordenite or ZSM-5 [22] by solid-state reaction yielded, after brief contact with small amounts of water, acidic zeolite catalysts which were, for instance, active in disproportionation and/or dealkylation of ethylbenzene or in cracking of n-decane [43]. The contact with water was essential to generate, after solid-state ion exchange, acidic Brpnsted centres (compare, for instance. Figure 2). In the case of solid-state exchange between LaClj and NH -Y an almost 100% exchange was achieved in a one-step procedure, and the hydrated La-Y reaction product exhibited a catalytic performance (selectivity in ethylbenzene disporportionation, time-onstream behaviour) comparable to or even better than that of a conventionally produced La-Y (96) catalyst [22,23]. In fact, compared to the case of NH -Y the introduction of La " " by solid-state reaction proceeded less easily and was frequently lower than 100% with H-ZSM-5 or H-MOR. 

It is believed that the isomerization of ethylbenzene to xylenes occur only on bifunctional catalysts, and hydrogenated intermediates are required (190). Selective catalysts for canying out this reaction have to maintain a good balance between the H-DM and the add function. Zeolites, with strong acid functions such as in mordenite or ZSM-5 actively isomerize cycloolefinic intermediates but also catalyze ring opening reactions which lead to a decrease in the formation of xylenes. However, the mild add nature of SAPOS make them specially useful for ethylbenzene isomerization. In this sense, 0.4-0.6 %wt Pt on SAPO-11 and SAPO-5 catalysts were used to isomerize mixtures of ethylbenzene and meta-xylene. Both catalysts produce near-complete xylene equilibration. SAPO-11 was more selective for producing xylenes than SAPO-5 (191). 

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