(222ak) Relevant Phase Equilibria for the Hydroformylation of Long Chain Olefins
Since the 1940’s the Rh- or Co-catalyzed hydroformylation is the most important pathway to produce aldehydes in the chemical industry. Annually, more than 10 mio. metric tons were produced with help of this process (Marcus, et al., 2013). Today’s state of the art process for hydroformylation of short chain alkenes is the heterogeneous Rh-catalyzed hydroformylation in aqueous media under mild reaction conditions named the Ruhrchemie/Rhône-Poulenc oxo process. This process has several advantages: The application of Rh as a catalyst provides an excellent selectivity to the desired product, the linear aldehyde. Furthermore, the required extreme low catalyst loss is affected by catalyst immobilization in the aqueous phase. This is absolutely essential due to Rh’s extreme high price. After the reaction took place product and catalyst can be separated easily by a phase separation step. Based on the mild reaction conditions (low temperatures and moderate pressures) the energy consumption is very low. Though, if the alkene’s chain length is increasing this process is no longer applicable having in mind that the water solubility of the long chain alkenes is decreasing dramatically.
Long chain alkenes are hydroformylated until today under harsh conditions with help of homogenous Co-catalyzed processes like the EXXON or BASF process (Robert, et al., 2012). However, this process has several disadvantages: The energy consumption is very high because of high pressures (up to 30 MPa) and high temperatures (up to 180°C) (Falbe & Bahrmann, 1984). The Co-catalyst shows in contrast to Rh a poor selectivity to the desired product, the linear aldehyde. Therefore often complicated product purification is needed. Also the catalyst recovery is complex and the loss of catalyst cannot be prevented, what makes a catalyst make up stream necessary.
One alternative to avoid the harsh conditions beside others (addition of co-solvent or supercritical fluid or ionic liquids) is the addition of a surfactant in order to solubilize the polar catalyst in the apolar environment (Haumann, et al., 2002), (Ünveren & Schomäcker, 2006). The hydroformylation should take place in the Winsor III region were the contact between aqueous regions and oil rich regions is maximal. Therefore the product achievement is maximal as well. With lowering the temperature catalyst and product can be separated easily with a phase separation process analogue to the Ruhrchemie/Rhône-Poulenc oxo process. In any case the knowledge of the phase behavior is necessary for application of these systems.
Therefore for basic understanding the phase behavior of the system water + otaethyleneglycolmonododecylether (C12E8) + 1-dodecene was studied (Schrader, et al., 2013), here the Winsor III phase region occurs in a for the reaction optimal temperature range. However, the application of a highly purified surfactant is not applicable in a technical process due to the enormous high price (about 250$ per g). Therefore, in industry technical grade surfactants have to be used. These technical grade surfactants are always a mixture of different surfactant molecules of different ethoxylate chain lengths and/or different carbonyl chain length. Also a large amount of not converted fatty alcohol can be enclosed in the technical grade surfactant. The influence of the ethoxylate chain length was studied by means of the so called Kahlweit’s fish at a constant oil/water ratio α=0.5 at different surfactant weight fractions γ. Beside this, the fish was measured for a system containing the technical grade surfactant Genapol X080® and technical grade 1-dodecene. The major component of this technical grade surfactant (beside not converted fatty alcohols and surfactants with different alkyl and ethoxylate chain length) is C12E8. The three phase area is shifted to much higher temperatures (temperatures differ up to 30°C) thus additional cooling could be necessary in a technical process.
Furthermore, the phase prism in a range between 30°C and 90°C was detected for the system containing C12E8 + pure 1-dodecene + water and Genapol X080® + technical grade 1-dodecene + water. When technical grade surfactant is used at high temperatures and high surfactant concentrations a four phase equilibrium was found where the micro emulsion splits into two phases.
The last part of the contribution is highlighting the influence of the product and the catalyst on the phase equilibrium. It is obvious that this data is crucial for optimized reaction control. The influence of the product n-tridecyl aldehyde on the phase equilibrium is investigated at three different portions of n-tridecyl aldehyde in the oil rich phase with help of the Kahlweit’s fish. Aside, this catalyst influence on the phase behavior was investigated, too. Literature
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