Hydrogenation heterogeneous catalysed

Ru(II)-TPPTS to the corresponding unsaturated alcohols in biphasic mode. If one compares the reaction times until full conversion, it becomes clear that the reaction rate correlates with the solubility of the substrate in the aqueous phase, as expected. The latter decreases with increasing chain length or branching of the chain at the C3-atom. In contrast to heterogeneously catalysed hydrogenations of o , d-unsaturated aldehydes, the steric hindrance of substituents at the C3-atom only plays a minor role in the coordination mode of the substrate at the metal centre, since selectivity differences from croton-aldehyde to citral are marginal. 

A catalytic amount of hydrobromic acid with an excess of hydrogen peroxide is found to be an effective reagent for the facile regeneration of carbonyl compounds from their 1,3-dithiane and 1,3-dithiolane derivatives.136 Bromide-assisted oxidation of substituted phenols with hydrogen peroxide, catalysed by heterogenous WO42-ions, resulted in the formation of p-quinols and their ethers in almost quantitative yields.137... 

The commonest example of. v>w-addition is the heterogeneously catalysed hydrogenation of an alkene, using either Ni, Pt or Pd, or in the case of the reduction from an alkyne to an alkene, using the Lindlar catalyst. The reduction of an alkene can also be achieved using diimide. 

Of more interest mechanistically though is the liquid nature of the molten phase present in the interstices of the inert porous support. It is now recognized that, deliberately or accidentally, a number of heterogeneously catalysed processes involve a supported liquid phase (SLP) rather than a solid catalyst. SLP catalysts are reviewed by Villadsen and Livbjerg. These include the vanadium-based sulphuric acid catalysts and, of comparable antiquity, the Deacon catalysts for oxidizing hydrogen chloride, where mixtures of copper and potassium chlorides can form melts in the catalyst support under reaction conditions. Thus in addition to diflfusional restrictions arising from pellet pore structure any... 

The heterogeneously catalysed conversion of eth3me (acetylene) into vinyl chloride (chloroethene) by the addition of hydrogen chloride is a reaction important for the manufacture of pol3rvinyl chloride (PVC) [18, 213, 248, 249]. Mercuric chloride, supported on activated carbon, is often used as a catalyst commercially [18, 213, 248-252] but, unfortunately, this catalyst rapidly deactivates. The high rate of deactivation, together with the toxicity associated with mercury compounds, has prompted a search for alternative catalysts [213,248,249,251,252]. 

To summarize, transient kinetic experiments are an established and valuable tool in the investigation of heterogeneously catalysed gas phase reactions. For liquid-phase systems, transient studies are much more rare than for gas-phase systems. It is probably related to slower dynamics and the fact that the intrinsic kinetic phenomena can be obscured by mass transfer effects and catalyst deactivation. As an illustration (Figure 8.11) we will consider three-phase continuous hydrogenation of an organic compound leading to two products over a metal catalyst on a structured support (knitted silica). 

Itoh N, Akiha T, Sato T (2005) Preparation of thin palladium composite membrane tube by a CVD technique and its hydrogen permselectivity. Catal Today 104 231-237 Kleinert A, Grubert G, Pan X, Hamel C, Seidel-Morgenstern A, Caro J (2005) Compatibility of hydrogen transfer via Pd-membranes with the rates of heterogeneously catalysed steam reforming. Catal Today 104 267-273... 

C., and Rooney, D.W. (2004) Heterogeneously catalysed selective hydrogenation reactions in ionic liquids. Green Chem., 5, 448-453. 

Heterogeneous catalysis occurs when the catalyst is in a different phase to the reaction mixture. For example, the decomposition of aqueous hydrogen peroxide catalysed by manganese(IV) oxide. 

Aqueous solutions of hydrogen peroxide decompose slowly the decomposition is catalysed by alkalis, by light and by heterogeneous catalysts, for example dust, platinum black and manganese... 

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