Edward J. Creyghton

Stereoselective Meerwein–Ponndorf–Verley and Oppenauer reactions catalysed by zeolite BEA

Abstract Zeolite BEA has been studied as catalyst in the Meerwein–Ponndorf–Verley and Oppenauer (MPVO) reactions of substituted cyclohexanones and cyclohexanols. In the MPV reduction of 4- tert -butylcyclohexanone to 4- tert -butylcyclohexanol a high stereoselectivity (>95%) to the thermodynamically less stable cis -isomer was obtained while in the complementary Oppenauer oxidation the cis -alcohol was preferentially converted. This stereoselectivity is explained by transition-state selectivity imposed by the zeolite structure. The catalytic activity is related to Lewis-acid aluminium sites which are located in the micropores. FT-IR results indicate that these sites are related to aluminium atoms which are only partially bonded to the framework. The mechanism is proposed to involve a six-membered transition state in which both the alcohol and the carbonyl are coordinated to the same aluminium.

Vapour-phase hydrodehalogenation of chlorobenzene over platinum/H-BEA zeolite

Abstract Platinum/H-BEA zeolite and Pt/Al 2 O 3 were studied as catalysts in the vapour-phase hydrodehalogenation of chlorobenzene. Both catalysts show high activity in the hydrogenolysis of the carbonhalogen bond. Deactivation of platinum/H-BEA is ascribed to acid catalyzed oligomerization reactions and coke formation. This is supported by quantitative DSC analyses and by extraction/GC-MS experiments. Replacement of Bronsted acid sites in platinum/H-BEA by sodium ions results in a diminished coke formation and an improved stability.

Shape-selective hydrogenation and hydrogen transfer reactions over zeolite catalysts

Abstract A review of the application of zeolite systems in shape-selective hydrogenation and hydrogen transfer reactions is presented. Two different types of catalytic systems are discussed. The first consists of metal clusters or coordination complexes encapsulated in the micropores of a zeolite employing hydrogen as the reductant. Recent developments, such as the application of zeolite-containing composite catalysts are included. In the second system, zeolites are applied as catalyst in hydrogen-transfer reactions using a secondary alcohol as hydrogen donor in the Meerwein–Ponndorf–Verley reduction.

CATALYTIC TESTING OF TIO2/PLATINUM/SILICALITE-1 COMPOSITES

The synthesis, characterisation and testing of a composite catalyst, consisting of TiO2-supported platinum catalyst particles covered with a 0.8–1.3 µm thick silicalite-1 layer, are described. The composite shows mass transport selectivity, which is demonstrated by the high ratios of the initial conversion rates in the competitive hydrogenation of a linear and a dibranched alkene, reaching average values of 35 at 100 °C. At the temperatures applied, adsorption of the alkene is dominant and hydrogen supply to the catalytic sites is relatively small. As a result the double-bond migration is more pronounced than with an uncovered platinum catalyst. At the catalytic sites of the composites steric constraints are observed, which lead to regioselectivity in the hydrogenation of long-chain alkenes. A linear alkene with a terminal double bond is converted preferentially over an isomer with an internal double bond.

Beta-type zeolites as selective and regenerable catalysts in the Meerwein-Ponndorf-Verley reduction of carbonyl compounds

Zeolite Beta, in the Al–form as well as in the Al–free, Ti–containing form, appears to be a selective and regenerable catalyst in the Meerwein–Ponndorf–Verley and Oppenauer (MPVO) reactions. In the liquid–phase MPV reduction of 4–tert–butylcyclohexanone with secondary alcohols, both catalysts display a high stereoselectivity to cis–4–tert–butylcyclohexanol, the isomer of industrial relevance. This stereoselectivity can be explained by considering the two transition states inside the pores of zeolite Beta. By using (S)–2–butanol as the reductant enantioselective reduction of phenylacetone was observed. 4–methylcyclohexanone was studied as the substrate in the gas–phase MPV reduction. Catalyst deactivation is much more pronounced with the acidic Al–Beta catalyst than with the non–acidic Ti–Beta.

New multifunctional probe for testing outer surface activity of zeolites: application to surface-located platinum clusters and acid sites

Abstract Allyl 3,5-di-tert-butylphenyl ether is being developed as a multifunctional probe for testing the outer surface activity of zeolites. Its use is demonstrated by the hydrogenation of the double bond over platinum/H-BEA, in which comparison with various non zeolite-based platinum catalysts enabled the proportion of platinum clusters located on the outer surface to be quantified. The Bronsted acid-catalysed Claisen rearrangement of the probe molecule, followed by the cyclization of the primary product to 4,6-di-tert-butyl-2-methyldihydrobenzofuran illustrates the use of the probe to provide information regarding the outer surface acidity of H-MOR and H-BEA. This test reaction proved also to be useful for the investigation of the effects of various modifications of H-MOR on the outer surface acidity.

Meerwein-Ponndorf-Verley and Oppenauer reactions catalysed by heterogeneous catalysts

Summary Meerwein-Ponndorf-Verley and Oppenauer reactions (MPVO) are catalysed by metal oxides which possess surface basicity or Lewis acidity. Recent developments include the application of basic alkali or alkaline earth exchanged X-type zeolites and the Lewis-acid zeolites BEA and [Ti]-BEA. The BEA catalysts show high stereoselectivity, as a result of restricted transition state selectivity, in the MPV reduction of substituted alkylcyclohexanones with i-PrOH.

Synthesis of Pt clusters in zeolite BEA. Effect of reduction rate on cluster size and location

The relation between the overall reduction rate of the intermediate PtO species in zeolite BEA, created upon calcination of the tetraammineplatinum(II) exchanged zeolite, and the size and location of the Pt clusters formed has been studied. When the reduction is carried out in excess hydrogen, small clusters (⩽ 15 A) are formed which are homogeneously distributed over the zeolite matrix. This is explained by the rapid and homogeneous formation of a high number of nucleation sites (Pto atoms) which results in a rapid formation of small Pt aggregates. When the supply of hydrogen is rate determining, a larger average Pt cluster size, a non-uniform cluster size distribution and a larger fraction of clusters on the external surface is obtained. In this case the reduction starts at the external surface and gradually proceeds towards the core of the zeolite crystals. This non-homogeneous formation of nucleation sites results in the enhanced probability of collisions between neutral Pt atoms and aggregates going from the core of the zeolite crystal to the external surface. This, together with steric considerations, explains the increase in the amount of clusters located on the external surface. Pt atoms which collide with full grown clusters, located in the zeolite matrix, have a lower sticking probability because growth beyond the spatial limits requires partial destruction of the zeolite lattice. Migration to the external surface, however, provides the possibility of unrestricted growth.

Regioselective hydrogenation of alkenes over Pt-loaded zeolite BEA

The hydrogenation of dec-1-ene and of (E)-dec-5-ene has been studied over platinum/Na-BEA and non-zeolitic platinum catalysts. Provided that the solvent does not compete with the substrates for sorption in the zeolite, and that the external surface platinum is deactivated with a bulky phosphine, dec-1-ene is hydrogenated 18 times faster than (E)-dec-5-ene, whereas the ratio is only ca. 2 for platinum on non-microporous supports. This regioselectivity is explained by steric constraints imposed by the microporous structure of the zeolite. The hydrogenation rate over platinum/Na-BEA is much lower than that observed over the amorphous supported platinum catalysts; further, for the zeolite-based catalyst, approximately first-order kinetics are found compared with zero-order for the amorphous catalysts. Both effects can be explained in terms of alkene coverage of the Pt sites in the two types of catalyst.

Stereoselective reduction of 4-tert-butylcyclohexanone to cis-4-tert-butylcyclohexanol catalysed by zeolite BEA

Zeolite BEA is found to be the first Stereoselective and regenerable heterogeneous catalyst for the Meerwein–Ponndorf–Verley reduction of 4-tert-butylcyclohexanone to cis-4-tert-butylcyclohexanol.