Kinetics Using Water-soluble Catalysts

The form of rate model described by Eq. (1) was found to represent the data satisfactorily. This model is derived from the well-known mechanism of hydroformylation [20] assuming addition of hydrogen to acyl rhodium species as a rate-determining step  

B = partial pressure of carbon monoxide [MPa] C = concentration of catalyst [lmol nr3] 

D = concentration of olefin [lmol nr3] k = reaction rate constant Klt K2, K3, a = constants 

At 1500 rpm, the rate vs. ela (aqueous phase hold-up) shows a maximum. For kinetic control, the rate is expected to vary linearly with catalyst loading. However, in a case where the reaction occurs essentially at the liquid-liquid interphase, it would depend on the liquid-liquid interfacial area even though liquid-liquid mass transfer is not rate-limiting. For ela 0.4, phase inversion occurs and the in- 

Hydroformylation of ethylene using a water-soluble Rh-TPPTS catalyst system has been investigated [22] using a toluene-water solvent system at 353 K. The effect of TPPTS concentration on rate passes through a maximum of at a P/Rh ratio of 8 1. The effect of the catalyst precursor concentration on the rate of reaction first increases and above a certain concentration it remains constant. The effect of aqueous-phase hold-up shows a maximum in the rate (ela = 0.4). The apparent reaction orders for the partial pressure of hydrogen and ethylene were found to be 1 and zero respectively. A strong inhibition in the rate with an increase in Pco was observed. 

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Hydroformylation of olefins using water-soluble catalysts in two-phase systems has been extensively studied [22, 23], the role of different types of water-soluble ligands and metal complexes in the activity, selectivity, and stability has been discussed for a variety of olefinic substrates. A few case studies in which kinetics and rate behavior have been addressed are reviewed here. 

Several modifications of the water-soluble catalysts using co-solvents (cf. Section 4.3 and [14]), micelle forming reagents (Section 4.5 and [15]), super-critical C02-water biphasic system (cf. Section 7.4 and [16]), SAPC (Section 4.7 and [17]), and catalyst binding ligands (interfacial catalysis) [18, 24] have been proposed to overcome the lower rates observed in biphasic catalysis due to poor solubilites of reactants in water. So far endeavors were centered on innovating novel catalyst and development of the existing systems. However, limited information is available on the kinetics of biphasic hydroformylation. 

An investigation on the kinetic aspects of hydroformylation of 1-hexene using water soluble Rh-TPPTS complex catalyst in a biphasic medium using ethylene glycol as a cosolvent is presented. The effect of reaction parameters such as, partial pressure of CO, partial pressure of hydrogen and olefin concentration on the activity were studied at 353, K. A kinetic model has also been proposed to predict the observed rate data. Condensation of heptanals with ethylene glycol led to the formation of acetal derivatives, which were isolated and characterised. 

The objective of this work was to investigate kinetics of hydroformylation of 1-hexene using water soluble Rh-TPPTS catalyst in a two phase system in presence of a co-solvent like ethylene glycol and to develop a suitable rate equation to explain the observed trends. Several preliminary experiments on hydroformylation of 1-hexene using water soluble [Rh(COD)Cl]2/ TPPTS catalyst were carried out at 353, K. The reaction can be shown as ... 

Pd(II) catalysts have been widely used for aerobic oxidation of alcohols. The catalytic systems Pd(OAc)2-(CH3)2SO [14] and Pd(OAc)2-pyridine [15] oxidize allylic and benzylic alcohols to the corresponding aldehydes and ketones. Secondary aliphatic alcohols, with relatively high water solubility, have been oxidized to the corresponding ketones by air at high pressure, at 100 °C in water, by using a water-soluble bathophenanthroline disulfonate palladium complex [PhenS Pd(OAc)2] [5d]. The Pd catalyst has also been successfully used for aerobic oxidative kinetic resolution of secondary alcohols, using (-)-sparteine [16]. 

The mechanism of particle formation at submicellar surfactant concentrations was established several years ago. New insight was gained into how the structure of surfactants influences the outcome of the reaction. The gap between suspension and emulsion polymerization was bridged. The mode of popularly used redox catalysts was clarified, and completely novel catalyst systems were developed. For non-styrene-like monomers, such as vinyl chloride and vinyl acetate, the kinetic picture was elucidated. Advances were made in determining the mechanism of copolymerization, in particular the effects of water-soluble monomers and of difunctional monomers. The reaction mechanism in flow-through reactors became as well understood as in batch reactors. Computer techniques clarified complex mechanisms. The study of emulsion polymerization in nonaqueous media opened new vistas. 

The same catalyst precursor, generated from [(EDTA)RuCI] which is also water soluble, was used for the hydroformylation of allylic alcohol under the same reaction conditions (//). At 50 bar and 130°C, in water as solvent, 4-hydroxybutanal was produced [Eq. (5)], together with about 2% of formaldehyde. However, the reaction proceeded further to give butane-1,4-diol by hydrogenation and y-butyrolactone as well as dihydrofuran by cyclization [Eq. (6)]. The same catalytic cycle as that proposed in Scheme 3 can be considered. A kinetic investigation revealed a first-order dependence on the ruthenium complex concentration and on the allyl alcohol... 

Substitution reactions of platinum(II) complexes containing one or more metal carbon bond(s) have a long history.217 Interest in such complexes and their reactivity is associated with their function as catalysts in synthetic procedures and in environmental applications. One important characteristic is the kinetic trans effect this is ascribed to the labilisation of a ra/is-positioned group caused by a C-bonded ligand,218 typically alkyl or aryl. In a mechanistic study in which kinetics measurements at elevated pressures were employed, cyclometallated Pt(II) complexes were used.219 The complex shown in Scheme 4 is water-soluble permitting the kinetics of substitution by a wide range of nucleophiles to be studied in aqueous solution. 

However, knowledge of the physicochemical fundamentals of catalytic biphasic reactions, their kinetics, and mass-transfer processes related to reactions where a gas phase and a second (aqueous) or even a third (organic) phase are present lags behind the successful development of industrial processes such as, for example, the hydro-formylation of propene using a water-soluble Rh-TPPTS catalyst (cf. Section 6.1). 

Fig. 2 Atmospheric-pressure hydrogenation of sodium cinnamate using Rh/6 (N = 32, R = n-CsH,) as the catalyst in water at different temperatures. The cloud point of the catalyst is 64 °C. Anti-Arrhenius kinetic behavior results due to the inversely temperature-dependent water-solubility of the nonionic phosphine [16].

The kinetics of hydroformylation of 1-octene using [Rh(cod)Cl]2 as a catalyst precursor with TPPTS as a water-soluble ligand and ethanol as a co-solvent was further studied by Deshpande et al. [14]. In this case the aqueous phase was continuous and the organic phase was in the form of dispersed droplets. The organic phase consisted of 1-octene in octane and the aqueous phase consisted of Rh/ TPPTS along with the co-solvent ethanol. The effect on the initial rate of reaction of the concentration of catalyst and of 1-octene, and of the partial pressures of hy-... 

Interestingly, the rate almost quadruples if nitrogen bases such as quinoline (Q) or aniline are used as co-catalysts. This kinetic effect may be due to several beneficial actions by the nitrogen bases on either the catalyst (faster formation and/or better stabilization) or the phase system (stabilization of the emulsion, improved solubility of BT in water). The quality of the emulsion seems to be of particular importance as shown by the fact that TPPTS, which is a worse surfactant than TPPMS, also gives worse catalytic results. Other nitrogen bases such as acridine, tetrahydro-quinoline (THQ), piperidine, and triethylamine give lower conversions to DHBT as compared with analogous reactions co-catalyzed by aniline or quinoline. 

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