Kinetics of hydroformylation

The kinetics of hydroformylation by phosphine- or phosphite-modified complexes is even more complex than that of the cobalt-catalyzed reaction. Depending on the reaction conditions, either alkene complexation (Scheme 7.1, 6 to 7) or oxidative addition of hydrogen (Scheme 7.1, 9 to 10) may be rate-determining. 

Surprisingly little information is available about the kinetics of hydroformylation reactions. For several decades Natta s equation served as a basic explanation however, in the last few years the application of reaction models of the Lang-muir-Hinshelwood type, even to biphasic systems, has been successfully demonstrated. This contribution (see Section 2.1.1) puts more emphasis on this area than has been usual in reviews on hydroformylation (see Section 2.1.1.3.2). In addition, the fundamentals of the oxo synthesis are discussed, along with the most important recent developments. The industrial processes in operation today are described as well. Due to its importance, the hydroformylation reaction has already been extensively reviewed elsewhere. For information beyond and in addition to this contribution, see [4, 7-12, 293]. 

Quite a number of contributions to ligand research in oxo chemistry are known (e.g., [16, 17, 23, 37, 38, 46, 49, 79, 80, 96, 113-119, 153]), as well as those in respect of other central atoms, binuclear complexes, photosensitized hydroformylations, or other starting olefins, including bioorganometallic applications (e.g., [38, 116, 120-123, 145, 146, 151]). The substitution of Na by Li, K or other cations in TPPTS-derived or other processes is claimed to be advantageous (e.g., [124, 125]). According to some observations [126] the kinetics of hydroformylations in aqueous phase may be different from those in nonaqueous media, as suspected by Chaudhari and co-workers [127]. Special aspects, mainly the behavior, control, and organization of the phases of aqueous biphasic processes, are dealt with in special papers [31, 41, 128, 129]. 

Table 3. Parameters of eq. (11) obtained for the kinetics of hydroformylation of 1-octene at two temperatures [24].

The aim of this contribution is to present a review of the current status of the kinetics of hydroformylation of olefins using water-soluble catalysis. Kinetic studies for various reaction systems and the role of ligands, pH, co-solvents, and surfactants are discussed. 

It has been reported that use of a suitable co-solvent increases the concentration of the olefin in water (catalyst) while retaining the biphasic nature of the system. It has been shown that using co-solvents like ethanol, acetonitrile, methanol, ethylene glycol, and acetone, the rate can be enhanced by several times [27, 28], However, in some cases, a lower selectivity is obtained due to interaction of the co-solvent with products (e.g., formation of acetals by the reaction of ethanol and aldehyde). The hydroformylation of 1-octene with dinuclear [Rh2(/t-SR)2(CO)2(TPPTS)2] and HRh(CO)(TPPTS)3 complex catalysts has been investigated by Monteil etal. [27], which showed that ethanol was the best co-solvent. Purwanto and Delmas [28] have reported the kinetics of hydroformylation of 1-octene using [Rh(cod)Cl]2-TPPTS catalyst in the presence of ethanol as a co-solvent in the temperature range 333-353 K. First-order dependence was observed for the effect of the concentration of catalyst and of 1-octene. The effect of partial pressure of hydrogen indicates a fractional order (0.6-0.7) and substrate inhibition was observed with partial pressure of carbon monoxide. A rate eqution was proposed (Eq. 2). 

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-... 

Kinetics of hydroformylation of styrene using HRhCO(TPPTS)3 catalyst in a bi-phasic system with various co-solvents was investigated by Nair [25]. He reported that 50% aqueous (v/v) N-methyl pyrrolidone (NMP) solution showed much better performance in comparison to ethanol as a co-solvent due to its non-reactive nature towards the aldehyde products. The rate was found to increase by seven times compared to that in the absence of any co-solvent. Kinetic study at 373 K revealed that the rate was first order dependent with catalyst concentration, fractional order with CO and first order tending to zero order with styrene concentration. 

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 ... 

As expected from the known kinetics of hydroformylation the authors found that increasing hydrogen solubility in the ionic liquid (either induced by increased hydrogen pressure or by higher hydrogen solubility) increased the reaction rate while higher CO concentration slowed the hydroformylation down. 

Biphasic hydroformylation is a typical and complicated gas-liquid-liquid reaction. Although extensive studies on catalysts, ligands, and catalytic product distributions have appeared, the reaction mechanism has not been understood sufficiently and even contradictory concepts of the site of hydroformylation reaction were developed [11, 13, 20]. Studies on the kinetics of hydroformylation of olefins are not only instructive for improvement of the catalytic complexes and ligands but also provide the basic information for design and scale-up of novel commercial reactors. The kinetics of hydroformylation of different olefins, such as ethylene, propylene, 1-hexene, 1-octene, and 1-dodecene, using homogeneous or supported catalysts has been reported in the literature. However, the results on the kinetics of hydroformylation in aqueous biphasic systems are rather limited and up to now no universally accepted intrinsic biphasic kinetic model has been derived, because of the unelucidated reaction mechanism and complicated effects of multiphase mass transfer (see also Section 2.4.1.1.2). 

This observation is somewhat in contrast to the results of Kandanarachchi and coworkers [33], who studied the kinetics of hydroformylation of triglycerides with oleic and linoleic functionalities as a model for soybean oil. In the presence of a homogeneous Rh catalyst modified with PPhj and P(OPh)j, comparable initial rates and TOFs were observed with both ligands. In comparison to individual fatty... 

Exchanging the Rh/TPPTS complex with an anion exchange resin resulted in a stable heterogenized catalyst for the hydroformylation of alkenes. The kinetics of hydroformylation of 1 -hexene using this catalyst was investigated. The rate was found to be first-order dependent on the catalyst, 1-hexene concentration, and H2 partial pressure. A maximum in the rate with increasing partial pressure of carbon monoxide was observed [87]. 

The kinetics of hydroformylaion in ScCO has been studied using empiri-cal/semiempirical rate equations. The kinetics of hydroformylation of propylene in ScCOj (homogeneous catalyst) was studied by Guo and Akgerman [75] using the empirical equation proposed by Comils [76]... 

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