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Increasing Selectivity of the Hydroformylation in a Miniplant: Catalyst, Solvent, and Olefin Recycle in Two Loops J. M. Dreimann, H. Warmeling, J. N. Weimann, K. K€ unnemann, A. Behr, and A. J. Vorholt Laboratory of Chemical Process Development, Dept. of Biochemical and Chemical Engineering, Technische Universit€at Dortmund, Emil-Figge-Str. 66, D-44227 Dortmund, Germany DOI 10.1002/aic.15345 Published online June 9, 2016 in Wiley Online Library (wileyonlinelibrary.com)

The application of thermomorphic solvent systems offers the combination of homogeneous catalysis in a single phase and catalyst recovery via phase separation. To increase economic feasibility the minimization of waste streams and side reactions is desired. For this, a continuous process for the hydroformylation of 1-dodecene in the solvent system DMF/ n-decane is shown. While the Rh/Biphephos catalyst is recycled in DMF in a first loop, the n-decane and remaining olefins are separated from the product via distillation to form the second loop. In this process the need for additional solvent supply and the isomerization reaction of 1-dodecene is reduced significantly. The reaction toward internal olefins decreases from initially 15 to 3%. The stable hydroformylation process with a yield of the linear hydroformylation prodC 2016 American Institute of Chemical Engineers AIChE J, 62: uct of 55% and l/b-ratio of 95/5 is shown for 120 h. V 4377–4383, 2016 Keywords: hydroformylation, miniplant, thermomorphic solvent systems, distillation, recirculation

Introduction The hydroformylation reaction is one of the most important applications of homogeneous transition metal catalysts in the industry. As the number of annual publications is steadily growing, the knowledge about this reaction increases continuously.1–3 Reaction kinetics,4,5 catalyst recovery,6–8 alternative catalyst metals,9 asymmetric hydroformylation,10 and isomerizing hydroformylation11,12 are present research topics several groups focus on. Often the Ruhrchemie/Rh^one-Poulenc-process as the most prominent biphasic industrial hydroformylation process is the origin for further investigations. Within this process propene and synthesis gas (CO/H2) are used as substrates to yield linear butanal catalyzed by a complex consisting of a rhodium precursor and a sulfonated ligand. This sulfonated triphenylphosphine keeps the Rh-complex in an aqueous catalyst solution while the substrates are present in gaseous form and the product forms a separate liquid phase after the reaction. Due to the sophisticated process design the catalyst can be recovered nearly completely in the aqueous phase from the organic liquid product phase without additional energy demand.2,13 Unfortunately this elaborate process is limited to short chain olefins as higher olefins are insoluble in the aqueous catalyst containing phase. The application of higher olefins leads to low substrate concentration at the active catalyst. Due to lower solubility of the substrate in the aqueous catalyst phase the reaction rate becomes uneconomically slow. The application of a single liquid reaction phase can avoid this

Correspondence concerning this article should be addressed to A. J. Vorholt at [email protected]. C 2016 American Institute of Chemical Engineers V

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drawback but is usually accompanied by catalyst loss and expensive separation. The application of thermomorphic solvent systems (TMS) offers huge potential for both, high reactivity in a single liquid phase containing substrate and catalyst and good catalyst recovery through phase separation (Figure 1, loop I). In general a TMS consists of at least two solvents with different polarity. The solvents are immiscible at low temperature and form a single homogeneous reaction phase at higher temperatures. Furthermore, the catalyst is predominantly soluble in one solvent (solvent B) and the product is expected to be solved in the other solvent (solvent A).14 To achieve a complete integration of the TMS in a continuous process the separation of the non-polar solvent from the products is another issue which is not addressed in the literature yet. Furthermore, unconverted reactants can be recovered in the downstreaming process and recycled to the reactor increasing the total yield significantly. The separation and recycle of all solvents reduce the costs for raw materials on the one hand and contributes to waste reduction on the other (Figure 1, loop II). The application of TMS in highly selective linear hydroformylation of long chained alkenes was developed in our group on the substrate 1-dodecene in batch operations.6 The reaction scheme consisting of the desired linear hydroformylation product from 1-dodecene is shown in Figure 2. Furthermore longterm experiments in continuous mode were conducted to gain insight in longterm stability of the TMS and the catalyst complex. For this a CSTR and a phase separator were applied to achieve the continuous catalyst recycle (Figure 1, loop I).15,16 Afterwards our objective was to close loop II experimentally to investigate effects of the recycle of solvent and side products on the reaction setup in terms of reactivity, catalyst stability, and the biphasic system.

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Figure 1. Concept for total integration of thermomorphic solvent systems in a continuous process.

The reversible isomerization is the most prominent side reaction in this setup. Since internal olefins are thermodynamically preferred, a yield of 20% internal olefins is obtained. In general isomerizing hydroformylation from internal olefins is suitable to form highly selective linear aldehydes.11,17,18 The application of an efficient recycling and conversion of these internal olefins from loop II (Figure 1) contributes thus to an increase of selectivity and therefore reduction of raw material expense. Two general requirements have to be fulfilled by the catalyst: fast isomerization and highly selective hydroformylation. To ensure both, a homogeneous catalyst consisting of a Rh-center and Biphephos ligand is used (Figure 2, right).12,19 Selectivity is especially industrially important due to economic feasibility of the process. Therefore, investigation and prove of concept are major objectives in this work and will be discussed extensively. This article addresses the total integration of the TMS system in a continuous miniplant process, in which the catalyst as well as the non-polar solvent and unconverted reactants are recycled in two different loops (loop I, loop II). The application of miniplant technique offers the advantage to observe longterm factors like accumulation of a component and the longterm stability of a reaction system with a minimum amount of effort. 20,21 Therefore, the existing miniplant was extended by a vacuum distillation column and substrate and solvent recycle repercussions on process performance parameters were observed. The implementation of the distillation column and therefore the recycle was conducted in three steps:

1. Independent operation of the distillation without recycling. 2. Operation of the reaction, phase separation, and distillation with batch wise recycling. 3. Operation of the reaction, phase separation, and distillation with continuous recycling.

Process Design Figure 3 shows the general setup of the miniplant. The first part (grey) shows the upstream, which consists of a continuously stirred tank reactor (CSTR, B1) and a decanter (B2). The setup is well known and approved for the hydroformylation of terminal long chain olefins.15,16,22 The hydroformylation of 1-dodecene is carried out in a 1000 mL stainless steel continuous stirred tank reactor B1, which is operated at 908C and 21 bar CO:H2 (1:1). The composition of the solvents in the reactor is set to 16 wt % 1-dodecene, 42 wt % n-decane, and 42 wt % N,N-dimethylformamide (DMF) during continuous operation (Figure 4, OPR). The substrate 1-dodecene is fed to the reactor by a piston pump (P1). By a subsequent heat exchanger, the reaction mixture is cooled to 58C. In decanter B2 the product phase carrying the main components tridecanal and n-decane and the catalyst phase consisting of DMF and the catalyst are separated. The catalyst solution being the denser liquid phase is recycled into the reactor by gear pump P3, while the product phase is transferred in container B3

Figure 2. Basic reaction scheme of the hydroformylation with the two main side reactions hydrogenation and isomerization (left), Biphephos ligand (right). 4378

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Figure 3. Process flow diagram of the miniplant.

expanding simultaneously to atmospheric pressure and the synthesis gas is withdrawn through an exhaust pipe. Since the applied recycles are designed to keep the solvents and the catalyst in the process, pumps P2 and P7 are implemented to compensate discharge caused by process sampling and non-ideal separation units. The n-decane is fed into container B5 by piston pump P7, while a constant DMF and Rh/ Biphephos concentration is ensured by piston pump P2. This process setup15 is extended by a distillation column connected to the flash container B3. The non-polar solvent n-decane is recovered in the distillate stream of the column, so that the main TMS solvent recycles are closed. Also internal olefins are gained in the distillate stream of the column. As the isomerization of the double bond is determined by its chemical

equilibrium the internal olefins are recycled to the reactor B1 to significantly decrease further isomerization. The distillation column is mainly determined by its internal diameter of 0.03 m and its structured laboratory packing with a total height of 1.2 m. This packing can be operated at low gas and low liquid loads and is suitable for vacuum distillation.23 Furthermore, four PT100 temperature sensors are applied to determine the temperature profile along the column. T1 represents the temperature in the distillate and T4 the temperature in the reboiler. T2 and T3 show the temperature at the height of 0.4 m, respectively, 0.8 m. The number of theoretical stages for the present separation task was determined by the method of Fenske-Underwood-Gilliland to a number of 14.68. The distillation is operated at a reflux ratio of 1, a pressure of

Figure 4. Liquid–liquid equilibrium of the system DMF/n-decane/1-dodecene,25 OPR: composition in the reactor B1 and decanter B2, OPD: composition in phase separator B4. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Table 1. Reaction Condition in the Miniplant Reaction parameters Pressure Reaction temperature Stirrer Separation temperature CO/H2-ratio Catalyst (substrate/metal) Ligand Metal/ligand-ratio Solvent system V_1-dodecene V_n-decane V_Make-up V_feed;column pcolumn pmax,reboiler Reflux ratio

21 bar 908C 750 rpm 58C 1:1 Rh(acac)(CO)2 (4000:1) Biphephos (Figure 2) 1:5 Decane/DMF 32 mL/h 88 mL/h 2,5 mL/h (wDMF 5 0.9893, wRh(acac)(CO)2 5 0.0001, wBiphephos 5 0.0107) 120 mL/h 10 mbar 500 W 1

10 mbar and a feed stream of 120 mL/h entering the column at 50% height of the packing. The liquid hold-up in the reboiler is 200 mL. During the operation the distillate temperature is controlled by the supplied heat in the reboiler. This controlling technique offers the advantage of fast start up behavior.24 Since the vapor pressure of the aldehydes is the lowest in the light product phase, the tridecanal and b-aldehydes are obtained in the bottom stream. Further n-decane and the discharged DMF of the catalyst solution are the main components in the distillate stream. As these both components form a heterogeneous azeotrope at operation conditions (Figure 4, OPD), the obtained liquid phases are separated in a subsequent decanter (B4) again. The volume of the decanter is set to 550 mL. While the small DMF phase (3 wt % of total distillate stream) is trapped in this decanter, the n-decane and the olefins are pumped into vessel B5 via gear pump P5. Referring to the second step of implementation, the presented investigation focuses on the combination of reaction, phase separation and subsequent distillation. Herein two liquid streams from the distillate and one bottom stream are obtained. The bottom stream is rich in the linear product and the distillate is rich in the solvent n-decane, respectively, terminal and internal C12-olefins. The distillate stream containing internal olefins and n-decane is recycled here in the second step to the reactor manually. Finally in the third step the obtained solvents and recovered olefins are recycled into the reactor B1 continuously using piston pump P6. The substrate (1-dodecene) input is adjusted during the operation to achieve a constant supply of olefins into the reactor. The implementation of flow sensors and periodic sampling allow for the exact calculation of mass balances and stable process operation. Therefore PID controller are designed for the control of the hold-up in vessels B3 and B5 and a maximum liquid hold-up in B4 is achieved by the combination of a level maximum indicator and a gear pump. All applied operation conditions are summarized in Table 1.

Results The progress of investigation is presented in three consecutive steps, starting with the determination of the distillate temperature and composition. Afterwards the experimental combination of TMS system and distillation is shown. Addi4380

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tionally the distillate is manually added to the substrate to show the feasibility of the recycling. Finally the light distillate stream was recycled to the reactor continuously. Also the purification of the product tridecanal is achieved with this concept but is not further optimized in this work. Step I: The investigation starts with the determination of the distillate temperature for continuous operation regarding total solvent and olefin recovery. Therefore, in Figure 5 the composition of the non-polar distillate phase over various distillate temperatures is shown. While the mass fraction of n-decane in the non-polar phase decreases with increasing distillate temperature the mass fraction of the olefins increase. This can be explained by the increasing fraction of the higher boiling alkenes in the steam that partially displaces the n-decane and the azeotropic composition of 3.5 wt % DMF, respectively. Operating the distillation column continuously at a distillate temperature of 538C, results in distillate compositions shown in Figure 6, left. The distillate stream consists of the non-polar solvent n-decane (85 wt %), the polar solvent DMF (4 wt %), and 1-dodecene (5 wt %) and its isomers (11 wt %). Furthermore, the resulting temperature profile along the height of the column is depicted in Figure 6, right. After a start-up time of 2 h a steady state of the distillation process is achieved. The major temperature increase over the column is observed in the stripping section while the rectifying section is relatively uniform (DT 208C). This profile can be accounted to the large difference in boiling temperature between the substrates and the product tridecanal, which shows high separation factors and supports the separation via distillation. Additionally the bottom stream of the distillation column is shown in Figure 7. The Operation of the reboiler at a temperature of 1368C leads to a composition of 72 wt % tridecanal and 7 wt % of branched aldehydes. Furthermore, the amount of the olefins is reduced to traces whereas high thermal stress and oxygen contamination in the reboiler leads to the formation of side products. Here 4 wt % of the tridecanoic acid and 15 wt % of aldol condensates are detected. In the presented distillation process, the n-decane and olefins are completely recovered in the distillate stream and the bottom stream consists of aldehydes and their consecutive products (>98 wt %).

Figure 5. Composition of the distillate stream at various distillate temperatures.


V_feed;column 5 120 mL=h; pcolumn 5 10 mbar, wfeed,n-decane 5 0.64, wfeed,int.-olefins 5 0.06, wfeed,1-dodecene 5 0.06, wfeed,DMF 5 0.07, wfeed,tridecanal 5 0.14. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]


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Figure 6. Composition of the non-polar distillate (left), Temperature profile of the distillation column (right). V_feed;column 5 120 mL=h; pcolumn 5 10 mbar, wfeed,n-decane 5 0.64, wfeed,int.-olefins 5 0.06, wfeed,1-dodecene 5 0.06,wfeed,DMF 5 0.07, wfeed,tridecanal 5 0.14. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Focusing on the recycling approach the distillate stream is applied in subsequent investigations and recycled to the reactor. Step II: After a stable distillation process is achieved in the first step, the hydroformylation in the TMS system and the distillation unit are interconnected, entering the second phase of the process development. Figure 8 shows the reaction performance of the continuous operation. For this a feed stream into the reactor of 120 mL/h is applied, while the composition is set to 27.6 wt % 1-dodecene and 72.4 wt % n-decane within the first 25 h. During this procedure a constant yield of the linear tridecanal of 58% at constant l/b-ratio of 97/3 is achieved. Furthermore, a yield of internal olefins of 17% is detected. The non-polar phase containing the product is fed from the phase separator into the distillation column continuously. Temperatures in the distillation column oscillate in intervals of 58C as a result of the process control (Figure 9, right). Therefore also the distillate stream is varying in composition, e.g., 1-dodecene between 5 and 10 wt % (Figure 9, left). It is illustrated that the average bottom temperature decreases slightly from around 140 to 1308C during operation time because a part of the 1-dodecene was accumulating in the rectifying section of

Figure 7. Composition of the bottom stream leaving the distillation column. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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the column and therefore pushing down the boiling temperature of the mixture. After a stable operation of reactor, decanter, and distillation column is shown, the solvent and substrate recycle is closed manually and the light distillate phase is fed into the reactor after 25 h. The influence on conversion and yields for linear and branched aldehydes are shown in Figure 8. In detail 10% of the terminal 1-dodecene is replaced by internal olefins. Due to the higher amount of 1-dodecene isomers and their lower reactivity toward hydroformylation the overall conversion decreases slightly for around 2.5%. Feeding of isomers is however not affecting the selectivity toward linear aldehydes and the feasibility of isomer recycle is therefore shown. Step III: Since the combination of TMS and distillation is a promising approach for the total recovery of catalyst, solvents and substrate, the continuous recycle of the distillate stream is striven in the third and last step of this process development. Therefore, catalyst activity and selectivity is presented in Figure 10, considering yields of obtained products in the reactor. Since, the recovery of solvents and olefins directly effects the hydroformylation reaction, the composition of the light distillate stream is shown in Figure 11. Finally, mass flow and composition of the product stream leaving the process is given to show the process capacity. During the start-up, hydroformylation (55%) and isomerization (18%) reactions show similar

Figure 8. Conversion and yields of linear and branched aldehydes after the manual recycle of solvent, 1-dodecene and substrate isomers (25 h).


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Figure 9. Composition of light distillate phase (left), temperature profile (right). V_feed;column 5120 mL=h; pcolumn5 10 mbar, reactor, decanter and column continuously interconnected. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

yields (Figure 10) compared to Figure 8, since 1-dodecene is used as substrate. Within the operating hours 10–20 the distillation column is filled with the light product phase obtained from the decanter and heated to operation temperature. After 20 h the light distillate stream of the column was recycled to the reactor, carrying the solvent n-decane as well as terminal and internal olefins. Due to the isomerization and highly selective linear hydroformylation the recycle of the internal olefins leads to a significant reduction of the isomerization of the substrate 1-dodcene, from initially 15 to 3%. Over the course of investigation (20–150 h) the yield of the linear tridecanal decreases from 64 to 51%, while a l/b-ratio of 95/5 is preserved during the total operation. These results show the presence of the active catalyst species within the reactor, since the l/b-ratio does not change over time. Furthermore, the recycle of the internal olefins lead to the decrease of linear hydroformylation, since the combination of isomerization and linear hydroformylation is slower than linear hydroformylation of the terminal olefin. Figure 11 shows the composition of the light distillate stream (80 g/h), which is recycled continuously to the reactor. Also in this diagram the first 20 h show the start-up phase of the process and the adjustment of the final distillate temperature after 40 h. The compositions of this distillate stream compared to investigations without continuous solvent recycle are equal, while oscillation can be reduced significantly in the long run. Apply-

ing the suggested distillate temperature of 538C and recycle of the distillate stream shows the decrease in isomerization between hours 20 and 30. Since this recycle influences the amount of internal olefins in the reactor and therefore in the distillation column the readjustment (Figure 11, 35 h) of the distillate temperature to 588C leads to another increase of internal olefins in the recycle stream, so that the isomerization is reduced in the reactor to an average yield of 3%. Finally a product stream (12 g/h) containing 68 wt % of the linear tridecanal is gained at the bottom of the distillation column. Furthermore, 4 wt % 1-dodecene, 4 wt % internal olefins, 2 wt % branched aldehydes, 7 wt % aldol condensates, and 12 wt % tridecanoic acid occur in this stream. Generally the product stream consists of 89% aldehydes and consecutive products of these. As aldol condensates and tridecanoic acid are formed in the reboiler due to high residence time (>10 h), high temperature (>1308C) and air (leakage at 10 mbar), shorter residence time and less thermal stress can avoid these side reactions. Therefore, the replacement of the distillation column by a thin-film evaporator can serve for a mild separation process. Nonetheless the proof that all solvents of a TMS system are recovered within a continuous process for the first

Figure 11. Distillate composition of longterm experiment. Figure 10. Conversion and yield of the long term study. 4382

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[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]


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time is presented. Additionally the isomerization reaction was reduced to a minimum by the recycle of internal olefins.

Conclusions Within this work we demonstrated the total recovery of a TMS system in the hydroformylation of 1-dodecene, leading to a process with a single substrate feed and a product stream, while just small amounts of the catalyst species were replenished. Therefore, the catalyst was recycled in the polar solvent DMF after phase separation and the solvent n-decane and terminal as well as internal olefins were recovered as distillate stream in a distillation column. Especially the recycle of internal olefins lead to the minimization of further isomerization in the reactor so that exclusively hydroformylation products were obtained in the product stream of the process. For this a distillation column was connected to the TMS process and operated at a suitable distillate temperature of 588C. The stable operation of TMS system and distillation column lead to the continuous recycle of the distillate stream. Finally an average yield of 55% of the linear aldehyde was achieved in the reactor for 120 h of continuous operation. Since side reactions as isomerization and hydrogenation were reduced to an amount of 2–5% a total selectivity of 93% of hydroformylation products in the process is promising. Therefore a distillation unit operating at mild conditions is a promising tool for further investigations.

Acknowledgments This work is part of the Sonderforschungsbereich/Transregio 63 “Integrated Chemical Processes in Liquid Multiphase Systems” (TRR63). The authors thank the Deutsche Forschungsgemeinschaft (DFG) for financial support and the Umicore AG & Co.KG for the donation of the rhodium precursor Rh(acac)(CO)2.

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