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Catalysts from Metal Clusters
167 thought to have some important advantages compared to zeolites, particularly in connection with the reactions of the heavier fractions of petroleum which cannot fit into the pores of the zeolites used for cracking. Pillared clays have pores anything up to six times the size of those in zeolites and the pore size can be tailored at will. They are made by introducing water into the spaces between the layers of the clay structure, thus increasing the layer spacing. Cationic species are then introduced and these remain after the water has been removed from the structure, these cations forming the pillars; most commonly, smectites are used as it is relatively simple to exchange the cations in these clays. It is hoped to be able to use the clays for the production of bulk chemicals from syngas. They can also be used to bind homogeneous catalysts. However, a major problem remains to be overcome: the pillared clays do not have the hydrothermal stability needed to withstand the regeneration conditions used in petroleum refining. Highfield reports that David Vaughan (then of W.R.Grace and now of Exxon) had in 1980 overcome this problem by introducing pillars based on aluminium or zirconium and that such materials have since been used successfully to refine petroleum to gasoline and diesel oil.
The Conversion
of Toluene
to Styrene
Catalytica Highlights, Vol.10, No.1, discusses a conceptual process for the production of toluene to styrene by a route involving the oxidative dimerisation of toluene to stilbene ((C H CH) ) followed by metathesis of the 6 s 5. ilbzne with ethylene to produce two molecules of styrene (C6H5CH:CH ). It would seem that the major obstac 1e is the first step: low yields are obtained and difficulties also arise with catalyst regeneration. Reference is made to work by Firuzi et al. (React. Kinet. Catal. Lett., 24(1984) 371.) These workers have investigated the kinetics of the reaction over a modified Pb-Sn oxide catalyst in 'the temperature range 773 to 843 K (conversions up to 10%) and have found that the rate-determining step is probably the interaction of the toluene molecule with the catalyst surface, leading to the formation of benzyl species.
mdC_-volume
11 No.1 -July 1984 ._.
Catalytica Highlights points to the need for high surface areas in this reaction and indicates that attempts to increase the surface area by supporting the catalyst on an inert carrier have resulted in "attrition of the active phase and excessive solids handling requirements"; whose work is referred to in connection with stability is not clear.
Catalysts
from Metal
Clusters
An item in New Scientist (5 January 1984) describes some recent work on unusual rhodium clusters by Gregory Geoffroy and a joint team from Pennsylvania and Delaware Universities. Most work on metallic clusters has involved the use of carbon monoxide ligands to stabilise the clusters. However, when carbon monoxide is used, all the available bonding sites on the metal atoms tend to be filled. This leaves little or no space for other reactant molecules to approach the metal. The aim of the work described was to replace some or all of the carbon monoxide ligands with less tightly bonded ones. This was achieved for a tetra-rhodium complex by the use of phosphorus bridging groups and four octa-1,5-diene ligands. One of the latter is attached to each of the rhodium atoms: being less strongly bonded than carbon monoxide ligands, they are more easily replaced by other ligands. The cluster is also coordinatively unsaturated. Geoffroy and his team hope that the new cluster will be catalytically active; with a smaller cluster containing only two rhodium atoms, the team earlier showed that the octa-1,5-diene ligands could be replaced by phosphorus-containing ligands in a hydrogen-containing atmosphere at room temperature.
Some New Books A number of new books of relevance to those working in the field of catalysis have recently appeared. Some of these will be reviewed in later issues of Applied Catalysis. In the meantime, some indication is given of their contents. Volumes 5 and 6 of "CatalysisScience and Technology", edited by J.R.Anderson and M.Boudart, have
The Conversion
of Toluene
to Styrene
Catalytica Highlights, Vol.10, No.1, discusses a conceptual process for the production of toluene to styrene by a route involving the oxidative dimerisation of toluene to stilbene ((C H CH) ) followed by metathesis of the 6 s 5. ilbzne with ethylene to produce two molecules of styrene (C6H5CH:CH ). It would seem that the major obstac 1e is the first step: low yields are obtained and difficulties also arise with catalyst regeneration. Reference is made to work by Firuzi et al. (React. Kinet. Catal. Lett., 24(1984) 371.) These workers have investigated the kinetics of the reaction over a modified Pb-Sn oxide catalyst in 'the temperature range 773 to 843 K (conversions up to 10%) and have found that the rate-determining step is probably the interaction of the toluene molecule with the catalyst surface, leading to the formation of benzyl species.
mdC_-volume
11 No.1 -July 1984 ._.
Catalytica Highlights points to the need for high surface areas in this reaction and indicates that attempts to increase the surface area by supporting the catalyst on an inert carrier have resulted in "attrition of the active phase and excessive solids handling requirements"; whose work is referred to in connection with stability is not clear.
Catalysts
from Metal
Clusters
An item in New Scientist (5 January 1984) describes some recent work on unusual rhodium clusters by Gregory Geoffroy and a joint team from Pennsylvania and Delaware Universities. Most work on metallic clusters has involved the use of carbon monoxide ligands to stabilise the clusters. However, when carbon monoxide is used, all the available bonding sites on the metal atoms tend to be filled. This leaves little or no space for other reactant molecules to approach the metal. The aim of the work described was to replace some or all of the carbon monoxide ligands with less tightly bonded ones. This was achieved for a tetra-rhodium complex by the use of phosphorus bridging groups and four octa-1,5-diene ligands. One of the latter is attached to each of the rhodium atoms: being less strongly bonded than carbon monoxide ligands, they are more easily replaced by other ligands. The cluster is also coordinatively unsaturated. Geoffroy and his team hope that the new cluster will be catalytically active; with a smaller cluster containing only two rhodium atoms, the team earlier showed that the octa-1,5-diene ligands could be replaced by phosphorus-containing ligands in a hydrogen-containing atmosphere at room temperature.
Some New Books A number of new books of relevance to those working in the field of catalysis have recently appeared. Some of these will be reviewed in later issues of Applied Catalysis. In the meantime, some indication is given of their contents. Volumes 5 and 6 of "CatalysisScience and Technology", edited by J.R.Anderson and M.Boudart, have