Kinetics of Biphasic Hydroformylation

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). 

The kinetics of low-carbon olefins, ethylene [21] and propylene [22], in aqueous systems was reported and different rate models proposed. Wachsen et al. [13] proved that aqueous biphasic hydroformylation of propylene took place at the interfacial region, in contrast to two preliminary kinetic models that incorporate mass transport. 

The biphasic hydroformylation of 1-octene was studied in the presence of ethanol as a co-solvent and a proposed kinetic rate expression was nearly identical to that of the homogeneous system [10, 23]. A further kinetic study of this biphasic hydroformylation system was conducted by Lekhal et al. [6] to analyze the experimental data by coupling kinetics to a pseudo-homogeneous gas-liquid-liquid macroscopic conservation model the authors proved that gas-hquid mass transfer was the only limitation. 

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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. 

Interfacial kinetics of biphasic hydroformylation of 1-dodecene catalyzed by water-soluble rhodium complex have been studied by a combined numerical and experimental approach [54]. 

Purwanto and Delmas [10] proposed the addition of co-solvent (ethanol) to enhance the solubility of 1-octene in the aqueous phase so that the overall reaction rate was increased, and their kinetic study led to a rate model similar to that in homogeneous liquid systems consistently from the point of view of bulk reaction mechanism. Chaudhari et al. [11] reported the improvement of the hydroformylation rate by addition of a small amount of PPhj to the biphasic system to enrich the effective catalyst species at the liquid-liquid interface. Kalck et al. [12] tested two more approaches to improve the mass transfer rate of biphasic hydroformylation of 1-octene and 1-decene with catalyst precursor [Rh2(/i-S Bu)2(CO)2(TPPTS)3j use the phase-transfer agent /i-cyclodextrin to transport the substrate into the aqueous phase to react there (see Section 2.2.3.2.2), and the supported aqueous-phase (SAP) catalyst to increase the reaction area due to the high specific surface area of porous silica (see Section 2.6). The improved conversion and TOF gave informative suggestions for the reaction mechanisms. 

Quite new ideas for the reactor design of aqueous multiphase fluid/fluid reactions have been reported by researchers from Oxeno. In packed tubular reactors and under unconventional reaction conditions they observed very high space-time yields which increased the rate compared with conventional operation by a factor of 10 due to a combination of mass transfer area and kinetics [29]. Thus the old question of aqueous-biphase hydroformylation "Where does the reaction takes place " - i.e., at the interphase or the bulk of the liquid phase [23,56h] - is again questionable, at least under the conditions (packed tubular reactors, other hydrodynamic conditions, in mini plants, and in the unusual,and costly presence of ethylene glycol) and not in harsh industrial operation. The considerable reduction of the laminar boundary layer in highly loaded packed tubular reactors increases the mass transfer coefficients, thus Oxeno claim the successful hydroformylation of 1-octene [25a,26,29c,49a,49e,58d,58f], The search for a new reactor design may also include operation in microreactors [59]. 

To estimate costs for the liquid-liquid biphasic hydroformylation using ionic liquids, a process was designed for the production of 100,000 tons per year of nonanal. The use of ionic liquids in hydroformylation catalysis is a fairly new technology and exact kinetic data are scarce, thus the TOFs reported for the Rh-sulfoxantphos system [80] have been used to determine catalyst inventory and reactor dimensions. In a similar way the plant design for the SILP process for a production capacity of 100,000 tons per year of butanal has been derived based on preliminary literature results [68]. The process flow sheets for both process variations are shown in Figures 7.12 and 7.13. 

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]. 

The kinetics of the above-mentioned biphasic hydroformylation of allyl alcohol (see eq. (6)) were described by the rate eq. (12) [20]... 

There is still a dispute among experts as to the place in which the biphasic aqueous reaction actually takes place, although it is very probably not the bulk of the liquid but the interfacial layer between the aqueous and organic phases. In the case of aqueous biphasic hydroformylation, this question has been decided by methods of reaction modeling and comparison with experimentally proven facts, thus leading to scale-up rules and appropriate kinetic models as a basis for optimal reactor design [34]. 

Kinetics of the 1-octene hydroformylation in aqueous biphasic media with the complex catalyst [RhCl(l,5-cod)Cl]2/TPPTS was also reported in [7]. Thereby, the reaction rate was also described by a mechanistic model (Eq. 14) assuming the addition of the olefin to the active catalyst to be the rate limiting step. 

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). 

Obviously, the reason for the low space-time yields of the biphasic hydroformylation reaction is in some way related to the low solubility of the higher alkenes in the catalyst phase. However, this finding does not necessarily imply that the catalytic reaction takes place in the bulk phase of the catalyst solution (concerning kinetics, see [22] and Section 6.1.2). 

Kinetics in biphasic catalysis using ethylene glycol as a co-solvent in the hydroformylation of 1-hexene... 

Several chemical engineering factors affect the biphasic hydroformylation of 1-dodecene in a gas-hquid-Hquid three-phase reachon system. In previous research [1], effects of temperature, total pressure, H2/CO molar raho, catalyst and ligand concentration, olefin concentration, surfactant concentration, and organic/ aqueous-phase volume ratio on the hydroformylation kinetics were studied with... 

The kinetics of hydroformylation of styrene using HRhCO(TPPTS)3 catalyst in a biphasic system with various co-solvents was investigated by Nair [31]. N-Methyl-pyrrolidone (NM P) and ethanol were found to enhance the rate by 7-9-fold compared to that in the absence of any co-solvent. However, with ethanol as a co-solvent, the aldehyde selectivity observed was 80% with acetals as side products. With NMP cosolvent the aldehyde selectivity was > 99%. A kinetic study at 373 K revealed that the rate was first-order dependent on catalyst concentration, fractional-order on CO, and first-order tending to zero-order on styrene concentration. 

As mentioned earlier, in the Ruhrchemie-Rhone Poulenc process for propene hydroformylation the pH of the aqueous phase is kept between 5 and 6. This seems to be an optimum in order to avoid acid- and base-catalyzed side reactions of aldehydes and degradation of TPPTS. Nevertheless, it has been observed in this [93] and in many other cases [38,94-96,104,128,131] that the [RhH(CO)(P)3] (P = water-soluble phosphine) catalysts work more actively at higher pH. This is unusual for a reaction in which (seemingly) no charged species are involved. For example, in 1-octene hydroformylation with [ RhCl(COD) 2] + TPPTS catalyst in a biphasic medium the rates increased by two- to five-fold when the pH was changed from 7 to 10 [93,96]. In the same detailed kinetic studies [93,96] it was also established that the rate of 1-octene hydroformylation was a significantly different function of reaction parameters such as catalyst concentration, CO and hydrogen pressure at pH 7 than at pH 10. 

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