Merging Thermomorphic Solvent Systems and ... - ACS Publications

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Merging Thermomorphic Solvent Systems and Organic Solvent Nanofiltration for Hybrid Catalyst Recovery in a Hydroformylation Process Jens M. Dreimann,† Frank Hoffmann,† Mirko Skiborowski,‡ Arno Behr,† and Andreas J. Vorholt*,† †

Laboratory of Chemical Process Development, Department of Biochemical and Chemical Engineering, Technische Universität Dortmund, Emil-Figge-Straße 66, D-44227 Dortmund, Germany ‡ Laboratory of Fluid Separations, Department of Biochemical and Chemical Engineering, Technische Universität Dortmund, Emil-Figge-Straße 70, D-44227 Dortmund, Germany S Supporting Information *

ABSTRACT: The application of homogeneous transition metal catalysts offers various advantages for chemical processes, such as mild reaction conditions and high selectivity. The main drawback is the difficult recovery of these precious catalysts, so that a small loss of catalyst can cause economic insufficiency of a chemical process. Our approach for overcoming this challenge is the application of two different catalyst recovery techniques, which are combined in a so-called hybrid separation process. Here, a thermomorphic solvent system is used for the recovery of the precious rhodium catalyst in a first stage, and a subsequent organic solvent nanofiltration unit is used in a second stage.

1. INTRODUCTION Hydroformylation is one of the most important industrial applications of homogeneous catalysts with an annual production capacity of 107 t. Although rhodium catalyzed hydroformylation of short chained olefins is well-known and established, for example, in the Ruhrchemie/Rhône-Poulencprocess, usually industrial processes for long chained olefins (>C5) are operated with cobalt catalysts.1 On the one hand the application of rhodium catalysts offers the advantage of operating at lower catalyst loading due to higher activity and higher selectivity through highly flexible ligand modification. On the other hand nearly complete catalyst recovery of precious rhodium is mandatory for an economically feasible process.2 To make the hydroformylation of long chained olefins via rhodium catalysts accessible for industrial application, an advanced separation and recycling of the precious catalyst is required. Several approaches for the recovery of homogeneous transition metal catalysts, such as phase transfer catalysts, organic solvent nanofiltration, thermomorphic multicomponent solvent systems, or micellar catalysis have been presented in the literature so far.3−8 Thermomorphic multicomponent solvent (TMS) systems are mainly characterized by a highly temperature dependent miscibility gap of two different solvents.9 Exploitation of this temperature sensitivity facilitates the avoidance of mass transport limitations during reaction, operating in a single homogeneous phase at elevated temperatures, while separation is afterward performed by means of phase separation, forming a © XXXX American Chemical Society

polar catalyst-rich phase and a nonpolar product-rich phase at lower temperatures. Although product and catalyst are separated conveniently by a shift in temperature, usually catalyst leaching cannot be avoided completely. While even a small loss of precious catalyst leads to poor profitability there is a limited effort that can be spent on separation to result in an economically viable process.10 At first, this concept of catalyst recovery was applied in a hydrogenation reaction by Bergbreiter et al.11 and in a hydrosilylation reaction by Behr et al.12 However, catalyst separation via organic solvent nanofiltration (OSN), which is based on differences in affinity between the membrane material and permeating components as well as size exclusion, facilitates a highly selective separation of the catalyst and product through a membrane. The feed solution is separated into a permeate stream rich in product and a retentate stream rich in catalyst.13,14 Several reports are available combining different types of reactions (such as hydrogenation, oxidation, or coupling reactions) and organic solvent nanofiltration.15−19 Also reports about the recovery of Rh-based hydroformylation catalysts via OSN are available in the literature, focusing on different parameters such as ligand type, membrane material, or operation mode. Shaharun et al. showed the separation of the Rh/triphenylphosphine catalyst Received: Revised: Accepted: Published: A

November 2, 2016 January 3, 2017 January 19, 2017 January 19, 2017 DOI: 10.1021/acs.iecr.6b04249 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Research Note

Industrial & Engineering Chemistry Research

Figure 1. Hydroformylation reaction of 1-dodecene.

detailed overview of the various accomplishments obtained by TMS systems and OSN. Since both TMS and OSN are not sufficient as stand-alone separation for a homogeneously catalyzed process, a hybrid separation sequence of both these techniques is promising. Up to now the third stage of the TMS process (Table 1) shows a compromise of 64% l-aldehyde yield and rather low Rhleaching (1%/h of the Rh-loading in the process) within 200 h of operation. However, the application of OSN membranes shows even better performance in terms of catalyst recovery with an Rh-leaching of 0.5%/h (Table 1, OSN, stage II, 96% rejection of rhodium) but a stable process operation using the desired Rh/Biphephos catalyst complex was not achieved. Combining a TMS system and OSN in a way that the nonpolar product phase of the TMS system is separated via OSN produces a stable process operation, while the recovery of the homogeneous catalyst is increased significantly.31 For the separation in the decanter, the temperature of the incoming stream is reduced, such that phase separation takes place and both leaving phases are at reduced temperature (c.f. Figure 2). For the OSN membrane the pressure in the incoming stream is kept constant, while there is a pressure gradient over the membrane, such that only the permeate is at a reduced (atmospheric) pressure, while the retentate remains at the high feed pressure.

by using conventional polyimide membranes achieving catalyst rejection greater than 93%.20,21 Priske et al. showed the combination of 1-octene and 1-dodecene hydroformylation and catalyst recovery by OSN in three consecutive batch experiments, in which catalyst activity remained constant and catalyst recovery higher than 99% was achieved.22 Industrial relevance of this separation technique is demonstrated by several Evonik patents claiming catalyst recovery of Rh-catalysts modified by bidentate ligands after hydroformylation in batch and continuous operation.23−27 Laborious molecular weight enlargement is presented in the literature to improve catalyst recovery, achieving rejection higher than 99.9%.4,28−30 Total recovery of the transition metal catalyst is desired for maximal profitability, which usually is a crucial objective in the development of processes using homogeneous catalysts. To address this issue, the concept of hybrid separation processes has recently been theoretically discussed for the recovery of transition metal catalysts by Dreimann et al.31 Insufficient catalyst recovery achieved by a single separation unit can be compensated by a sophisticated combination of two different separation units with this concept. The term hybrid separation process is used when at least two different separation techniques are combined to solve a common separation task.32 Synergetic effects can be used to overcome limitations of the single techniques and overall process performance can be increased, while each separation unit is operated at optimal conditions.33 A hybrid process that sequentially improves catalyst recovery presumably enables highly efficient and economically feasible chemical processes using precious homogeneous catalysts. Basically, the development of processes based on homogeneous catalysts can be structured into three stages: i. separation and recycling in batch operation ii. continuous catalyst recycling iii. steady-state operation with catalyst replenishment. Within this work the highly selective hydroformylation of the terminal C12-olefin with synthesis gas (CO/H2) to the linear (l-)tridecanal using a Rh/Biphephos catalyst complex is under investigation (Figure 1). Besides the linear aldehyde, also branched (b-)aldehydes as well as internal olefins are obtained as side products. In our previous publications we showed the suitability of two independent separation techniques for the recovery of rhodium catalysts in every development stage. Table 1 provides a

2. EXPERIMENTAL SECTION 2.1. Miniplant Operation. According to Figure 2 the TMS system and the OSN membrane are combined in a continuous process, in which the catalyst concentration of the product phase is subsequently reduced. While separation efficiency is improved by the combination of the different separation units, new challenges arise from the combination of the operating windows of the different technologies. To decouple these and keep the process flexible, the hold-up tank between the decanter and the OSN membrane plays a vital role. While the previously determined optimized process conditions such as temperatures and pressure are kept in accordance with the prior process development (Table 1), flow rates are adjusted to ensure a suitable residence time in the reactor as well as turbulent flow in the membrane module. These crucial process parameters are given in Table 2. In the reactor a 90 °C temperature and 20 bar synthesis gas pressure are mandatory for sufficient hydroformylation rates within 1 h B

DOI: 10.1021/acs.iecr.6b04249 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Research Note

decrease in activity expected due to Rh-leaching

up to 77% l-aldehyde yield

up to 26% l-aldehyde yield high isomerization activity

residence time, while low temperatures show good phase separation. Stable operation of the OSN unit is achieved at 30 °C and a transmembrane pressure difference of 20 bar. Furthermore, turbulent flow (Re > 2300) on the feed side of the membrane is mandatory. According to these parameters recycle streams are adjusted as given in Table 2 resulting in a DMF recycle stream from the decanter (S-4) of 140 mL/h, a n‑decane recycle stream (S-6) of 75 mL/h, and a membrane feed (S-7) of 93 L/h. DMF, Rh(acac)(CO)2, and Biphephos are replenished continuously (S-2) as determined in the third stage of catalyst recovery using TMS systems (Table 1). For process start up, the catalyst phase (DMF, Rh(acac)(CO)2, Biphephos) was preformed in the reactor at 90 °C and 20 bar synthesis gas. Furthermore, S-1 was set to 0.12 L/h to fill the process units successively. With reaching the desired volume of a unit the associated pump was put into operation at determined flow rate. In the end of the start up the OSN unit was put into operation and the hydroformylation product was obtained in the permeate stream (S-8). Here, the first drop of the permeate stream (S-8) determined the end of the start-up and meanwhile the beginning of the continuous process operation. The piping and instrumentation diagram as well as details on the membrane material and membrane dimensions are presented in the Supporting Information. 2.2. Chemicals. The substrate 1-dodecene (95%) was purchased by VWR, DMF (99%) was purchased by Acros Organics, and n-decane (94%) was purchased by Merck. The purity of each chemical was determined by gas chromatography. The chemicals were stripped with argon before use. The Rh(acac)(CO)2 precursor (39.9% Rh) was donated by Umicore and the Biphephos ligand (97%, Figure 1) was synthesized by Molisa. The OSN membrane (POL-oNf-M1_1) was purchased by PolyAn. The membrane is based on polydimethylsiloxane (PDMS) with a nominal molecular weight cut off of 400−450 g/mol. 2.3. Analysis. The composition of the liquid fractions are analyzed by gas chromatograph (Agilent gas chromatograph HP6890A), which is equipped with a capillary column (HP5, 30 m × 0.32 mm × 0.25 μm) and a flame ionization detector. The external standard quantification method was used. For calibration a deviation below 1% was obtained for each component. The amount of Rh and P are analyzed via ICP-OES (Thermo Elemental Iris Intrepid). For this, 0.23 g of a sample was prepared in a Teflon cup with 2.5 mL of nitric acid (65%) and 4 mL of sulfuric acid (96%). The digestion process was conducted in a MWS μPrep start-system microwave (MLS). Upon completion of the digestion process, the samples were treated with 2 mL of distilled water and 1 mL of H2O2 (Fisher Scientific, Optima grade, phosphorus free). The prepared samples were allowed to rest for 12 hours before measurement. The quantification limit of 3 ppm Rh and 5 ppm P is determined in the ICP-OES, for low Rh- and P-concentrations samples were concentrated by rotary evaporation.

decrease in activity after 40 h of operation decrease in activity compensated by increasing reaction time Rh-replenishment

decrease in activity compensated by permanent Rh replenishment

up to 64% l-aldehyde yield up to 70% l-aldehyde yield activity

l/b-ratio up to 99/1 decrease in selectivity compensated by permanent ligand replenishment Const. l/b-ratio of 99/1 decrease in selectivity compensated by permanent ligand replenishment

Const. l/b-ratio of 99/1 decrease in selectivity compensated by ligand replenishment up to 78% l-aldehyde yield regio-selectivity ligand replenishment

Rh-concentration 0.025 mol % Rh-concentration 0.05 mol % Rh-concentration 0.1 mol % catalyst loading

this work

Const. l/b-ratio of 97/3 decrease in selectivity after 17 h

Rh-concentration 0.05 mol % Const. l/b-ratio of 94/6

catalyst recycling in two consecutive batch runs

catalyst recycling in a continuous OSN process for 35 h Rh-concentration 0.05 mol % catalyst recycling in a continuous TMS process improved by subsequent OSN catalyst recycling in a continuous TMS process for 200 h catalyst recycling in a continuous TMS process for 60 h catalyst recycling in 30 consecutive batch runs operational mode and time scale

stage II35 stage I34

TMS

stage III31,36

TMS and OSN combined development stage

Table 1. Milestones in the Presented Process Development for the Hydroformylation of Long Chained Olefins

stage II37

OSN

stage I38


3. RESULTS AND DISCUSSION The main goals for the described process are stable reaction performance on the one hand and high catalyst recovery on the other. In Figure 3 obtained yields of l-aldehydes, b-aldehydes, and internal olefins are shown. Here the starting point of the diagram (0 h) represents the switch from start up to continuous operation. At that point of time the constant feed stream for start-up of 0.12 L/h was set to process control in a way that the C


Research Note


Figure 2. Process flowsheet.

received, which corresponds to a flux of 20.3 L/m2h (30 cm2 membrane area). After startup the yield of the desired l-aldehyde increased within 10 h from initially being 40% to 60%. This yield was described previously in a steady-state operation for the single stage application of the TMS system DMF/n-decane.31 Over the course of operation (10−50 h) the yield of the linear aldehyde increased to 70%, while the yield of branched aldehydes and internal olefins stayed at 1% and 18%, respectively. Additionally, the total amount of formed products and catalyst consumption is presented in the Supporting Information. Besides the consideration of obtained yields, the stable operation of the OSN unit and high rhodium rejection were main criteria for successful process intensification. While Figure 3 already illustrated that stable operation was reached, additionally the rate of recovery of the catalyst in the OSN unit is shown in Table 3. Here an Rh-rejection of 75%−87% can be achieved.

Table 2. Crucial Process Parameters CSTR

decanter

OSN

pR: 20 bar TR: 90 °C VR: 330 mL τR: 72 min

pS1: 20 bar TS1: 5 °C VS1: 450 mL τ:̅ 80 nub V̇ R1: 140 mL/h

Δp: 20 bar TS2: 30 °C u: 0.57 m/s Re: 2900 V̇ R2: 75 mL/h

Table 3. Catalyst Recovery via OSN time [h] RRh [%]

Figure 3. Yields in the CSTR. CSTR: wn‑decane = 0.42, wDMF = 0.42, w1‑dodecene = 0.42, p = 20 bar, CO/H2 = 1/1, TR = 90 °C, n1‑dodecene/ nRh(acac)(CO)2 = 4000/1, nBiphephos/nRh(acac)(CO)2 = 5/1. Decanter: p = 20 bar, CO/H2 = 1/1, TS1 = 5 °C. OSN: Δp = 20 bar, TS2 = 30 °C, A = 30 cm2, V̇ F = 93 L/h. Feed: V̇ Feed = 61 mL/h, w1‑dodecene = 0.28, wn‑decane = 0.72. Make-up: V̇ Make‑Up = 3.8 mL/h, wDMF = 0.9893, wRh(acac)(CO)2 = 0.0001; wBiphephos = 0.0107.

13 87

20 86

28 79

37 75

45 78

52 76

In total the rhodium concentration of 30 ppm in stream S-3 leaving the reactor was reduced by phase separation to a rhodium concentration of 3 ppm in the product stream S-5. By applying OSN as a second stage for catalyst recovery the rhodium concentration in the product stream S-8 was reduced below 1 ppm. If the definition of catalyst rejection via OSN (eq 1) for both TMS system (RRh = 87%) and OSN (RRh = 80%) is considered, a total catalyst rejection of 97.3% is reached.

obtained permeate volume was replenished continuously by feed solution (1-dodecene/n-decane). Over the course of investigation a constant permeate stream of 61 mL/h was D


Industrial & Engineering Chemistry Research RRh = 1 −

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c Rh,permeate c Rh,feed

ACKNOWLEDGMENTS This work is part of the Sonderforschungsbereich/Transregio 63 “Integrated Chemical Processes in Liquid Multiphase Systems” (TRR63). The authors would like to 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.

(1)

The significantly improved recovery of the homogeneous catalyst compared to a single stage TMS process31,36 results in an increase of the yield of tridecanal over the course of investigation. In the long run the Rh-concentration in the membrane loop will stabilize, so that the Rh-amount in the permeate stream equals the replenished Rh-amount. With this also the yield of the linear aldehyde will reach an upper threshold and therefore reach steady-state operation. After all the proof of concept for increasing catalyst recovery by a hybrid separation process consisting of a thermomorphic solvent system and organic solvent nanofiltration is presented. Optimization of the two-stage process, such as operation temperatures, catalyst replenishment, and membrane area, can even further reduce catalyst demand and improve product yield.

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4. CONCLUSIONS Within this work we presented the efficient catalyst recovery of precious rhodium catalyst via a multistage catalyst separation based on different separation techniques. Therefore, the wellknown n-decane/DMF TMS system was combined with a subsequent OSN stage. Continuous operation was shown for 50 h, while the yield of tridecanal (70%) was increased compared to a single stage TMS process. Furthermore, overall catalyst recovery of 97% was achieved which corresponds to a Rh loss of 0.25%/h of the total Rh load. Since the Rh precursor and the Biphephos ligand were replenished according to previous investigations and catalyst recovery was improved, the catalyst complex was enriched in the operated process. In the long run a stabilization of the catalyst concentration and reaction behavior in terms of product yields is expected. The postulated concept of increasing catalyst recovery by the combination of two different separation techniques was herein proven. In further investigations a longer period of operation is desired, while steady state operation is targeted. With this, optimization of the catalyst replenishment is accompanied. To achieve good catalyst dosing, knowledge of active as well as inactive catalyst species is required. For successful catalyst characterization the application of in situ analysis such as infrared spectroscopy is promising. Investigations to utilize in situ spectroscopy for process control are in progress.

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ABBREVIATIONS CSTR = continuously stirred tank reactor DMF = N,N-dimethylformamide OSN = organic solvent nanofiltration PDMS = polydimethylsiloxane S-i = stream i (1−9) TDC = tridecanal TMS = thermomorphic solvent system REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b04249. Figure S1, Piping and instrumentation diagram of the presented process; Table S1, membrane material; Figure S2, membrane module and membrane dimensions; Table S2, yields and total amounts of products (PDF)

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Research Note

AUTHOR INFORMATION

Corresponding Author

*Tel.: 0231-755-2313; [email protected]. ORCID

Andreas J. Vorholt: 0000-0001-9302-2273 Notes

The authors declare no competing financial interest. E


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