455534 Process Intensification of Aqueous Biphasic Hydroformylation of Mid Chain Olefin 1-Octene in a Jet Loop Reactor Creating Large Interfacial Areas
Helge Warmeling, Arno Behr and Andreas J. Vorholt*
Chair of Technical Chemistry, Department of Biochemical and Chemical Engineering
Technical University of Dortmund, Dortmund, DE-44227, Germany
The hydroformylation is an atom-economic, homogenously catalyzed reaction where an alkene is converted under syngas pressure to form linear and branched aldehydes following figure 1. Most commonly the reaction is catalyzed by rhodium complexes modified with steric phosphine ligands to increase reactivity and selectivity. 
Fig. 1. Reaction scheme of the hydroformylation of an terminal alkene with prominent side reactions like isomerization and hydrogenation to alkanes.
Due to the high market price and even higher price volatility of rhodium, an efficient catalyst recycle is eminent for an economic process. A possible solution is to immobilize the rhodium in an aqueous second phase by the use of the ligand triphenylphosphine trisulfonate (TPPTS). Substrates diffuse into the aqueous catalyst phase and react at the catalyst complex. Products can subsequently be separated by a simple decantation step in an organic phase. This recycle concept works exceptional for short-chain substrates but has its limitations for alkenes with C6+ because the substrate solubility in the catalyst phase decreases significantly with increasing chain length. While the majority of studies working on this limitation heavily focus on the use of chemical additives like surfactants, co-solvents or phase transfer promoters to increase solubility or interfacial surface only seldom procedural approaches were investigated.  This work therefore focuses on an additive-free procedural approach to enhance the reaction rates of the hydroformylation of mid-chain olefin 1-octene in a phase contacting jet loop reactor.
Jet loop reactors (JLR) are a promising tool in the intensification of catalytic multiphase reactions as they provide large interfacial areas due to intense mixing and therefore excellent mass transfer characteristics. The overcoming of mass transfer limitations can be of significant economic interest for large scale industrial applications. In contrast to other phase contacting reactor concepts the jet loop reactor also offers a high energy efficiency which makes it especially interesting in process intensification projects. 
The principle of operation is based on the partial withdrawal and reinjection of the reaction medium through a jet nozzle as can be seen in figure 2. The recycle stream sucks gaseous media via impulse transfer into the jet stream and disperses it into the liquid phases to achieve a fine gas-liquid-liquid distribution. The small gas bubbles coagulate in an outer reactor ring called the riser and add additional circulation momentum following the principle of an airlift pump. The reactor itself features no moving parts and is hence particularly suitable for high pressure applications like the hydroformylation.
Fig. 2. Scheme of the jet loop reactor concept in biphasic homogenously catalyzed hydroformylation of terminal alkenes. Intense mixing and therefore high internal interfacial area is created through a partial external circulation of the reaction medium through a nozzle.
In this work the jet loop reactor is compared to a standard reactor type of the chemical industry, the stirred tank reactor (STR). Both reactor types were build up on miniplant scale and were successively optimized. Reaction conditions like temperature, pressure and syngas composition were kept in standard industrial range while catalyst and ligand concentration were investigated in preliminary tests. Surprisingly the reaction rates showed an inhibitory effect in regards of catalyst concentration and the highest catalytic activity and productivity were found at very low rhodium and ligand concentrations (cRh=0.00025 kmol/m³aq) with peak turn over frequencies (TOF) of up to 12500 in the JLR and 4500 in the STR. Due to increased leaching the investigation was continued with higher catalyst concentrations of cRh=0.0005 kmol/m³aq and a ligand to rhodium ratio of 10.
While the jet loop reactor reached high
turnover frequencies up to 5500 h-1 and space time yields (STY) up
to 55.5∙10-2 mol/m³h the stirred tank
reactor only achieved lower TOFs of up to 2200 h-1 but with a
satisfying STY of up to 44∙10-2 mol/m³h as can be seen in Fig.
3. With a
3-pitched blade gassing stirrer the performance of the STR was improved up to a TOF of 2600 h-1 with a space time yield of 55.5∙10-2 mol/m³h. The results show that in contrast to the STR the JLR is able to achieve excellent phase dispersion even with relatively small amounts of aqueous catalyst phase which results in a higher catalytic activity achieved with about a tenth of the specific energy input (0.27 kW/m³ compared to 2.2 kW/m³).
Fig. 3. Catalytic activity (TOF) and productivity (STY) of the aqueous biphasic hydroformylation of 1-octene in a jet loop reactor compared to a stirred tank reactor at different stirrer speeds.
The results point out the potential of the reactor setup compared to standard reactor types and also academic studies using additives published in the literature. The direct connection between specific energy input, created interfacial area and reaction rate is currently subject of investigation. Based on calculations of the mass transfer coefficients for all compounds, mass transfer resistance as the limiting step can be excluded due to the relatively slow reaction rates compared to substrate diffusion. The authors propose the creation of surface near film volume with elevated substrate concentration as the cause of rate acceleration.
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