Cyclohexene hydrogenation kinetics

The cyclohexene hydrogenation is a well-studied process especially in conventional trickle-bed reactors (see original citations in [11,12]) and thus serves well as a model reaction. In particular, flow-pattern maps were derived and kinetics were determined. In addition, mass transfer can be analysed quantitatively for new reactor concepts and processing conditions, as overall mass transfer coefficients were determined and energy dissipations are known. In lieu of benchmarking micro-reactor performance to that of conventional equipment such as trickle-bed reactors, such a knowledge base facilitates proper, reliable and detailed comparison. 

Kinetic data on cyclohexene hydrogenation catalyzed by RhClL species (n = 1, 2, or 3 L = p-dimethylaminophenyl phosphines) were interpreted in terms of active dimer catalysts (cf. 1), possibly involving coordination through nitrogen as well as phosphorus (82). 

This kinetic equation is applied to the observed kinetic curves obtained in cyclohexene hydrogenation (model reaction) following the molecular hydrogen consumption. Of note, the present kinetic equation provides the value of fe2obs and not kj. However, the real value of the rate constant k2 can be obtained easily using the relationship k2 = k2obs x S/C, where S/C is the substrate/catalyst molar ratio (the catalyst is given as the number of metaUic moles employed). 

Phosphinecarbonyl complexes of cobalt have long been known to act as hydrogenation catalysts. In a recent study involving cyclohexene the kinetics of its hydrogenation by the complex [CoH(CO)2(PBun3)2] were studied. Unlike the systems described above, the carbonyl complexes generally require elevated temperature and pressure. The proposed mechanism is given in Scheme 6. 

Cyclohexene hydrogenation is a well-studied process that serves as model reaction to evaluate performance of gas-liquid reactors because it is a fast process causing mass transfer limitations for many reactors [277,278]. Processing at room temperature and atmospheric pressure reduces the technical expenditure for experiments so that the cyclohexene hydrogenation is accepted as a simple and general method for mass transfer evaluation. Flow-pattern maps and kinetics were determined for conventional fixed-bed reactors as well as overall mass transfer coefficients and energy dissipation. In this way, mass transfer can be analyzed quantitatively for new reactor concepts and processing conditions. Besides mass transfer, heat transfer is an issue, as the reaction is exothermic. Hot spot formation should be suppressed as these would decrease selectivity and catalytic activity [277]. 

The generally accepted mechanism for alkene hydrogenation (Figure 1) is mainly due to Halpern and is supported by careful kinetic and spectroscopic studies of cyclohexene hydrogenation. The dominant path of the cycle is inside the dotted line. 

Catalysis of cyclohexene hydrogenation has been studied extensively both in the vapour and liquid phases on platinum ", palladium and other metallic surfaces. Here the kinetics of the cyclohexene hydrogenation on platinum have been considered lu terms of the specific activities of samples of silica-supported platinum, previously characterised by hydrogen chemisorption. Particular attention has been paid to the structure sensitivity-insensitivity of the reaction and how this varies as carbonaceous overlayers are built up on the catalysts with increasing reaction time. 

The results of the study on the hydrogenation of different functional groups were summarized [194]. Here complexes of rhodium, palladium and nickel fixed on balls of densely cross-linked macroporous polystyrene of HAD-4 grade, on which an-thranilic acid residues were bonded, were used as catalysts. Special attention was paid to the kinetic study of cyclohexene hydrogenation by rhodium(+) derivatives and to elucidate the reaction mechanism using D2. The influence of diffusion restrictions on the reaction rate was discussed, in particular, that of the transport of hydrogen to pores and through the gas-liquid interface. 

X. Su, K. Y. Rung, J. Lahtinen, Y. R. Shen, G. A. Somorjai, 1-3 and 1-4 cyclohexadiene reaction intermediates in cyclohexene hydrogenation and dehydrogenation on Pt(lll) crystal surface a combined reaction kinetics and surface vibrational spectroscopy study using sum frequency generations, J. Mol. Catal. A 1999, 141, 9-19. 

Among the cases in which this type of kinetics have been observed are the addition of hydrogen chloride to 2-methyl-1-butene, 2-methyl-2-butene, 1-mefliylcyclopentene, and cyclohexene. The addition of hydrogen bromide to cyclopentene also follows a third-order rate expression. The transition state associated with the third-order rate expression involves proton transfer to the alkene from one hydrogen halide molecule and capture of the halide ion from the second ... 

Models of this type were used successfully in the interpretation of the kinetic data of Maennig and Kolbel as well as of kinetic data obtained for the hydrogenation of a-methylstyrene and cyclohexene. 

GL 16] [R 12] [P 15] As excess of cyclohexene was used, the kinetics were zero order for this species concentration and first order with respect to hydrogen [11]. For this pseudo-first-order reaction, a volumetric rate constant of 16 s was determined, considering the catalyst surface area of 0.57 m g and the catalyst loading density of1g cm. ... 

Using the titanocene-catalyzed co-hydrogenation of cyclohexene, we have studied the kinetics of the polymerization of a number of primary silanes ( 20 ). The rate law was found to be ... 

The hydrogens within the octahedral olefin-dihydride intermediate are transferred consecutively with overall cis addition, and the rate-determining step (k9) is olefin insertion to give the alkyl- hydride. Kinetic and thermodynamic parameters for nearly all the steps of Fig. 1 have been estimated for the cyclohexene system. Because the insertion reaction is generally believed to require a cis disposition of the hydride and olefin... 

Muetterties has suggested that the dimeric hydride [RhH(P OiPr 3)2]2 catalyzes alkene and alkyne hydrogenation via dinuclear intermediates [91]. However, no kinetic evidence has been reported to prove the integrity of the catalysts during the reactions. On the other hand, studies of the kinetics of the hydrogenation of cyclohexene catalyzed by the heterodinuclear complexes [H(CO) (PPh3)2Ru((u-bim)M(diene)] (M = Rh, Ir bim=2,2 -biimidazolate) suggested that the full catalytic cycle involves dinuclear intermediates [92]. 

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