LTL-Zeolite

Several supported Pt catalysts were prepared. The supports used are Mg hydrotalcite (HT), SiC>2, amorphous Si02-Al203 (ASA) and LTL zeolite. The support characteristics are shown in Table 1. 

LTL zeolites have pores in a single direction along the [001] axis27. The pores are constructed of cages with a diameter of 13A, and are interconnected with 12 membered rings with a diameter of 7.1 A. 

The LTL zeolites have a Si/Al ratio of 3.027. Based upon the crystallographic formula of LTL ([K9(H20)2i][Al9Si27072]), the number of Bronsted acid sites in the LTL with a K/Al = 0.47 is 1.76 mmol/g. In the case of the LTL [0.82], this is 0.57 mmol/g. The LTL zeolites with K/A1>1 have no Bronsted acid sites, and after calcination at elevated temperatures, the excess K is probably present as KOH or K20. Increased amounts of K results in a more basic support, with an increased electron-richness on the support oxygen atoms. ... 

Establishing the presence of internal diffusion limitations is less trivial. An easy check would be to decrease the crystallite size. If internal diffusion limitations are absent, this would not affect the activity. However, the internal diffusion is related to the pore-size. The catalyst particles of the catalyst with the smallest pores, the LTL zeolite, are constructed of several... 

If the Pt/SiC>2 [big] is compared with LTL [0.47, big], than the particles are the largest on the LTL [0.47] support, but the roll-over mechanism is much more important on the Pt/SiC>2 support. Probably, the freedom of the CP is limited within the constraints of the pores in the LTL zeolite and rolling-over is sterically hindered. 

This concept of zeolites as enzyme mimics was used by Derouane and Vanderveken (59) to explain the selective aromatization of n-hexane on Pt/LTL catalysts confinement effects combined with the unique pore structure of LTL zeolite would be responsible for the fast and selective conversion of n-hexane to benzene. 

The performance of platinum supported in LTL zeolite catalyst is well illustrated by data of Lane et al. [23] for conversion of -hexane in the presence of excess H2 at 330-440°C and atmospheric pressure (Table 4) Primary products were observed to form both from one-six and one-five ring closure, giving benzene and methylcyclohexane, respectively. The catalyst is remarkable for its high benzene selectivity, which increases with increasing conversion of n-hexane because some of the primary products are further converted into benzene For example, ultimate benzene selectivities (defined as the hexanes converted to benzene divided by the hexanes converted to benzene and light, C1-C5, hydrocarbons) as high as 93% were observed at 420°C. 

Several explanations have been advanced for the unique performance of Pt/LTL zeolite catalysts. There is a consensus that dehydrocyclization is catalyzed by the platinum clusters alone, with the support providing no catalytic sites [23,38,40]. The support must be nonacidic to prevent acid-catalyzed isomerization and hydrocracking as side reactions [41-43] The aromatic selectivity increases with the basicity of the LTL zeolite support. The interaction of the platinum clusters with the basic support has been suggested to result in an increase in the... 

The results suggest that it may be fhiitful to search for reactions for which supported metal clusters have catalytic properties superior to those of conventional supported metals. The important opportunity in catalysis may be to find reactions for which the activity or selectivity of supported metal clusters is superior to those of conventional supported metals. The high selectivity of Pt/LTL zeolite catalysts for paraffin dehydrocyclization, which is now exploited... 

The reduced Pt/K-LTL zeolite has been studied by EXAFS at RT, before and after CO adrttission [%M2]. After reduction, very small platinttm metal particles were present consisting of five to six atoms. The CO admission, at RT, leads to complete decortqrosition of the platinum metal particles arrd the formation of a platinrtm carboxyl clrrster most probably stabilized by the zeolite walls. The platinrrm carbortyl cluster just fits irtside the pores of the zeolite LTL. 

LTL Zeolite structure, acronym for Linde-type L zeolite. ... 

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