Thallium-based heterogeneous catalysts

Heterogeneous catalysts containing thallium species have heen developed over the years to promote a number of different transformations. Table 20.1 shows examples for the use of thallium-based solid catalysts in different organic transformations. 

The presence of thallium(0) led to an increase in activity and selectivity of metallic palladium catalysts supported on silica in aldose oxidation reactions. However, silica-supported thallium(0) had no activity by itself (entry 3). ° Similarly, the bimetallic catalyst platinum-thallium/ZSM-5, prepared by impregnation of thallium sulfate and chloroplatinic acid on Zeolite Socony Mobil-5 (ZSM-5), showed greater selectivity in propane aromatisation and almost the same catalytic activity as monometallic thallium/ZSM-5 (entry 4). Similar comparison of vanadium-caesium-copper and vanadium-caesium-copper-thallium catalysts supported on TiOa.SiC demonstrated that addition of thallium improved the catalytic activity in partial oxidation of p-tert-butyltoluene to p-tert-butyl-benzaldehyde (entry The application of solid-supported thallium-based catalysts in different processes includes (a) iron-thallium catalysts in carbon monoxide hydrogenations to form hydrocarbons and alcohols, and catalytic reforming of 

2 Tl203/Si02 Friedel-Crafts-type benzylation of aromatic compounds 48, 49 

5 V-Cs-Cu-Tl/TiOi Sic Partial oxidation of p-tert-butyltoluene to p-tert-butyl-benzaldehyde 52, 53 

McKillop and E. C. Taylor, in Organic Synthesis by Oxidation with Metal Compounds, ed. W. J. Miss, De Jonge and R. H. I. Cornelis, Plenum, New York, 1986, p. 695. 

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Table 20.1 Summary of some applications of thallium-based heterogeneous catalysts.

Karski and co-workers found that thallium acts also as a promoter and prevents the poisoning of the palladium particle with oxygen. Indeed, a bimetallic system which contained 5 wt% of Th led to 100% selectivity to gluconic acid at 95% conversion [117]. On their side, Boimeman et al. reported that charcoal-supported Pd-Pt catalysts prepared from a colloidal solution of Pd-Pt/NOct4Cl exhibit a superior activity and selectivity than industrial heterogeneous Pd-Pt-based catalysts [118]. 

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