Lewis acids extra-framework aluminium

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. 

Fig. 4 shows clearly that the steaming treatment conditions have a considerable impact on the variation of the acidic properties of ZSM-5 zeolites. Notably, ZSM-5-ST only possesses residual Bronsted acidity. ZSM-5-MT exhibits a lower number of pyridine molecules chemisorbed on Bronsted acid sites at room temperature, compared to ZSM-5-P. As steaming posttreatment extracts part of the framework aluminium species (acting as Bronsted acid sites) and converts these into extra-framework aluminium, (acting as Lewis acid sites), a deerease in the number of Bronsted acid sites is expected. A close look to the band at 1545 cm of ZSM-5-MT at 573 K and 673 K reveals a higher number of pyridine molecules attached to... 

Figure 1 shows the relative concentrations of Lewis (L) and Bronsted (B) acid sites, calculated from IR spectra of ad-sorpted pyridine [22, 23], as a function of the number of framework aluminium atoms per unit cell, Nai. When Nai decreases and therefore NEFAL increases, [L]/[B] increases, meaning that the increase of extra-framework species corresponds to an increase of Lewis acidity. 

Most working zeolite catalysts are likely to have both Bronsted and Lewis acidity, the latter resulting from aluminium, either as extra-framework cations... 

The main feature of the Meerwein-Ponndorf-Verley reaction pathway involves the coordination of both reactants to the Lewis-acid metal eentre and hydride transfer from the alcohol to the earbonyl group. Aluminium or titanium alkoxides are usually effective homogeneous catalysts. With tin-Beta catalyst, cyclohexanone reduction with 2-butanol led selectively to cyclohexanol at 100 °C. Ketone conversion was >95%, whereas silicon-Beta, Sn02/Si02 and SnCl4 -5H20 were inaetive under the same experimental conditions. Therefore, the activity is likely due to tetrahedral tin in the zeolite framework, and not to extra-framework tin or to leached tin. ... 

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