Hydrogenation of Itaconic Acid in Micellar Solutions: Catalyst

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Kinetics, Catalysis, and Reaction Engineering

Hydrogenation of itaconic acid in micellar solutions: catalyst recycling with cloud point extraction? Marcel Schmidt, Saskia Schreiber, Luise Franz, Hauke Langhoff, Ashkan Farhang, Moritz Horstmann, Hans-Joachim Drexler, Detlef Heller, and Michael Schwarze Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03313 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 18, 2018

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Industrial & Engineering Chemistry Research

Hydrogenation of itaconic acid in micellar solutions: catalyst recycling with cloud point extraction? Marcel Schmidt1, Saskia Schreiber1, Luise Franz1, Hauke Langhoff1, Ashkan Farhang1, Moritz Horstmann2, Hans-Joachim Drexler2, Detlef Heller2, Michael Schwarze1,3* 1

Technische Universität Berlin, Department of Chemistry, Sekr. TC-8, Strasse des 17. Juni

124, D-10623 Berlin 2

Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Strasse 29a,

D-18059 Rostock 3

Technische Universität Berlin, Department of Process Engineering, Sekr. TK-01, Strasse des

17. Juni 135, D-10623 Berlin *[email protected] ABSTRACT: The biggest problem in homogeneous catalysis is the efficient recycling of the catalyst in its active form. In this contribution, cloud-point extraction (CPE), a methodology already known from different applications, is adapted for the recycling of homogeneous catalysts from micellar solutions. About 96% of a homogeneously dissolved Rh/BPPM catalyst complex is recycled by CPE from aqueous micellar solutions of the nonionic surfactant NP8 and reused in subsequent reactions. The hydrogenation of itaconic acid is studied as a benchmark reaction to proof this alternative concept showing that it is feasible for catalyst recycling. However, severe catalyst deactivation takes place for substrates with carboxylic acid groups, which needs consideration during substrate selection. KEYWORDS: Hydrogenation, Micellar Catalysis, TX-100, Cloud Point Extraction, Recycling

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. Introduction In homogeneous catalysis, metal complexes are applied to carry out chemical reactions under mild operating conditions. As all reactants are dissolved in the same phase, catalyst recycling is usually a problem and could be done by distillation of the product or by nanofiltration of the reaction mixture.1 However, too high temperatures usually lead to catalyst deactivation, and nanofiltration requires solvent resistant membranes and high pressures. In some single cases, the productivity of the catalyst is high enough to disclaim catalyst recycling, e.g., the synthesis of the herbicide (S)-Metolachlor.2 But, in most cases, catalyst recycling is mandatory to run a chemical process economically. Different methods have been developed to recycle homogeneous catalysts, based on their immobilization in another phase. The new phase is either a solid or a second not miscible solvent.3 With the launch of the Ruhrchemie/Rhône-Poulenc-Process (RCH/RP), where a rhodium complex catalyst is immobilized in an aqueous phase applying an excess of the water-soluble ligand sodium triphenylphosphine trisulfonate (TPPTS), biphasic catalysis became famous.4 To overcome limitations of substrate solubility and mass transport as well as to drive the reactions more environmentally friendly, several alternative reaction media have been exploited in homogeneous catalysis. The use of cyclodextrin-based reaction media has been investigated to solve mass transfer limitation in aqueous biphasic catalysis.5,6 Moreover, several successful examples for catalysis in surfactant-based multiphase systems were reviewed by Dwars et al.7, La Sorella et al.8, and Schwarze et al.9 Recently, Lipshutz et al. have proposed different rules for organic transformations in micellar media.10 The advantages of surfactant-based systems are in the different possibilities for catalyst recycling, e.g., by extraction of the product with an organic solvent11, by micellar-enhanced ultrafiltration (MEUF) of the catalyst complex12,13, by phase separation of product and catalyst containing phases14, and by cloud point extraction of the catalyst (Figure 1).


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Figure 1. Concepts for product and catalyst separation after reaction in micellar reaction medium with metal complex catalyst. Recently, the rhodium catalyzed hydroformylation of 1-dodecene as a model substrate for long-chain olefins was carried out in a mini-plant operating for 200 h using a microemulsion system with 1-dodecene being the oil phase and the reactant at the same time, where catalyst recycling was done by phase separation.15 This example clearly shows that surfactant systems are alternative reaction media in chemical processes. Although there are many successful examples in academia for micellar catalysis, the application of surfactant-based multiphase systems as solvents is linked to different challenges, because the selected media has an impact on the reaction performance and the recycling afterward. Most of the applied metal complexes used in homogeneous catalysis are hydrophobic, and a simple method for its separation based on a micellar reaction medium is cloud point extraction (CPE). In CPE, micellar solutions of nonionic surfactants will separate into a surfactant-rich and a water-rich phase when heated above the cloud-point temperature (CPT), which depends on the selected surfactant and the composition of the reaction mixture. For a successful separation of hydrophobic catalysts and products via CPE, the product should be hydrophilic to avoid its accumulation in the waterrich phase after CPE. In this contribution, cloud point extraction is investigated as an alternative method for catalyst recycling after hydrogenation of itaconic acid in a micellar reaction medium.



2. Experimental Section 2.1 Chemicals Chemicals, which were used without further purification, are listed in the Supporting Information. 2.2 Determination of the Cloud-Point The investigations on the cloud point were carried out in a water bath, where the temperature was adjusted with a thermostat. The samples were prepared in graduated and lockable test tubes, placed into the water bath and the temperature was increased in 1 °C steps in a temperature range from 25 °C to 90 °C. After reaching thermal equilibrium for each temperature step, the turbidity of the mixture was observed to determine the cloud point. Moreover, the volume of the water- and surfactant-rich phase was measured to calculate the corresponding volume fractions. 2.3 Hydrogenation experiments Initially, the catalyst complex was prepared in Schlenk tubes, weighing the ligand and the rhodium precursor, evacuating and flushing the tube three times with argon and closing it with a septum. Afterward, 5 mL of the solvent (water for TPPTS, methanol for TPP and BPPM) was added through the septum, and the catalyst solution was stirred overnight. The complex formation was indicated by a color change from yellow to orange/red. The hydrogenation experiments were carried out in a 100 mL double-walled glass reactor to adjust the temperature with an external thermostat. The experimental setup is shown in the Supporting Information. The reactants, solvents, and additives were weighed into the reactor. After evacuation and flushing with nitrogen three times, a constant nitrogen pressure of 1.1 bar was introduced to the reactor. Under stirring at 800 rpm, the catalyst solution was injected with a syringe through a septum, and the temperature was adjusted to the desired reaction temperature. After reaching thermal equilibrium, the stirrer speed was increased to 1200 rpm and stopped. Nitrogen was replaced with hydrogen (1.1 bar), and the stirrer was turned on to start the reaction. The hydrogen pressure remained constant at 1.1 bar and the consumed volume of hydrogen, which is proportional to the conversion of the substrate, was measured with a mass flow meter. The enantioselectivity was determined by gas chromatography.


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2.4 Cloud point extraction for catalyst recycling For hydrogenation experiments with subsequent catalyst recycling via cloud point extraction, the reaction mixture was prepared as described above. After the first run, the stirrer was stopped, and the reaction mixture was heated to the corresponding cloud point. Within minutes, a water-rich and surfactant-rich phase is obtained, and 50 mL of the water-rich phase with the product was separated. After cooldown of the reactor to the reaction temperature, further reactant (1 g) was dissolved in water (50 mL) and injected with a syringe to the reaction mixture. The final reaction volume was kept constant during the entire recycling procedure. The reaction was again started as described in the previous section. 2.5 Hydrogenation experiments without product separation Consecutive hydrogenation experiments were carried out to prove the stability of the catalyst. For the first run, the reaction mixture was prepared as described above. For the second until the fifth run, 5 mL of the reaction mixture was always withdrawn to dissolve further substrate and re-added to continue the reaction. 2.6 Determination of rhodium and phosphorous The concentration of rhodium and phosphorus in the water-rich phase was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) using a Varian ICPOES 715 ES instrument. The rhodium and phosphorus concentrations were measured at a wavelength of 369.2 and 213.6 nm, respectively. Calibration of the setup was done with standard solutions. The concentration of rhodium and phosphorus in the surfactant-rich phase was obtained from the mass balance taking into account the initial weight. Due to the error of reading of the volume fraction, a standard deviation of 3% can be assumed for the rhodium and phosphorus content in the surfactant-rich phase. 2.7 Determination of enantioselectivity The enantioselectivity of the product was determined by gas chromatography. Therefore, a sample from the aqueous phase was taken and extracted with ethyl acetate. To remove all catalyst components, the extracted phase was flushed through a glass pipette filled with silica. The residue was treated with 2 M (trimethylsilyl)diazomethane solution to transform the acid into the corresponding methyl ester. Afterward, 1 µL of the solution was injected in the gas



chromatograph, packed with a Lipodex E column. The oven temperature was constant at 90 °C. 3. Results and Discussion The asymmetric hydrogenation of itaconic acid (IA) and dimethyl itaconate (DMI) is an important, homogeneously catalyzed reaction, used as a model reaction for testing new catalysts for hydrogenation.16–20 Dependent on the applied catalyst system, the corresponding (R)- and (S)-product can be obtained (see Figure 2). In order to get a high enantioselectivity (2S,4S)-1-tert-butoxycarbonyl-4-diphenylphosphino-2-(diphenylphosphinomethyl)pyrrolidine (BPPM) was chosen as the chiral ligand for asymmetric hydrogenation experiments,21 combined with [Rh(COD)(µ2-Cl)]2 or [Rh(COD)2]OTf as the rhodium metal precursor. To separate the homogeneous catalyst complex simultaneously from the product during CPE, it is necessary to avoid product accumulation in the surfactant-rich phase. Hence, IA was selected as the substrate because IA is very polar with a water solubility of about 80 g L-1.22 For itaconic acid, it can be expected that after CPE, the catalyst is in the surfactant-rich phase and the hydrogenated product methyl succinic acid accumulates in the water-rich phase. The product is a solid, which is stable under reaction conditions, but can decompose at much higher temperatures.

Figure 2. Rhodium catalyzed hydrogenation of itaconic acid (IA) and dimethyl itaconate (DMI) to give the corresponding (R) and (S)-product. 3.1 Investigation on the Cloud Point Initially, impacts on the cloud point were investigated to identify the optimal conditions for the combination of reaction and subsequent catalyst recycling. Particularly, the type of surfactant and its concentration, as well as the concentration of itaconic acid and the selected temperature on CPE, were investigated for the aimed separation process, which was already reviewed by Hinze and Pramauro.23 The chemical structures of the investigated surfactants are shown in Figure 3.


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Figure 3. Chemical structures of the investigated surfactants. Figure 4 shows that the cloud point depends rather on the type of surfactant than its concentration, although there is a minimum in the temperature profile for each surfactant. With increasing hydrophilic character of the surfactant, the cloud-point is shifted to higher temperatures. The more hydrophilic the surfactant, the higher is the required energy (temperature) to change the conformation of the ethoxy units, reducing the water solubility of the surfactant. As a result, the surfactant forms its own phase, indicated by turbidity of the solution. The knowledge of the CPT is very important to link the chemical reaction and the separation to each other. When the reaction is performed semi-continuously, the temperature for separation should be close to the reaction temperature to limit the time for additional heating- and cooling steps. From the surfactant screening, TX-114, NP5, NP8, and MO 13/50 show a lower CPT and might be suitable for a combined reaction and separation process. Often TX-100 is used as a surfactant in catalytic reactions24,25 with good performance but the CPT of TX-100 is about three higher than for TX-114; therefore, it was not considered in these investigations. As well, MO 13/70 shows a CPT above 70 °C, which is not suitable to combine the reaction and separation process, since catalyst deactivation could occur at high separation temperatures.



Figure 4. CPT as a function of surfactant concentration for different nonionic surfactants. The impact of itaconic acid concentration on the CPT was also studied in the range of 0 – 80 g L-1. For an itaconic acid concentration of 10 g L-1, the CPT is the same as for the pure surfactant. For an itaconic acid concentration of 40 and 80 g L-1, the CPT shifts to higher temperatures due to the hydrophilic character of itaconic acid. Hence, the polarity of the water phases decreases, which leads to a higher solubility of the surfactant and, thus, to a higher CPT. The shift is in the range of 5 to 10 °C. To avoid rigorous CPT shifts by high substrate concentrations, the following reactions were done with an itaconic acid concentration of 10 g L-1. During the separation process, the micellar solution is separated into a water-rich phase and a surfactant-rich phase. For the combined semi-continuous process, a huge water-rich phase is desired to extract most of the hydrophilic product in a single step. Hence, the impact of the temperature on the volume fraction of the surfactant-rich phase ΦSRP was investigated. As the surfactant becomes more hydrophobic with increasing temperature, the volume of the surfactant-rich phase decreases with increasing temperature, which is shown in Figure 5. The volume fraction of the surfactant-rich phase decreases below 10 %, making these surfactants ACS Paragon Plus Environment

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applicable for a subsequent quantitative separation of the product. Also here, the more hydrophobic surfactant shows a better performance because a lower volume fraction is already obtained at lower temperatures.

Figure 5. Volume fraction of the surfactant-rich phase (csurfactant = 50 g L-1, Tsep = 30 min) as a function of the temperature in the presence of dissolved itaconic acid (cIA = 10 g L-1). 3.2 Proof of concept To prove whether or not the concept of CPE is applicable for catalyst recycling in homogeneous catalysis, a Rh(I) complex [Rh(cod)Cl]2 (cod = 1,5-cyclooctadiene) with varied hydrophilic and hydrophobic ligands (see Figure 6) was dissolved in micellar solutions of the nonionic surfactant NP8 forming the corresponding catalyst complex and forwarded to CPE.



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Figure 6. Chemical structures of the investigated ligands. The surfactant NP8 was chosen as the benchmark, since promising results were obtained from CPT and volume fraction. Furthermore, the impact of the rhodium to ligand ratio was investigated towards the distribution of the applied catalyst complex. The rhodium and phosphorus concentrations in the single phases after CPE were determined by ICP-OES. The calculated rhodium and phosphorus content in the surfactant-rich phase for the individual systems are summarized in Table 1. To avoid matrix effects during analysis, the concentration of rhodium and phosphorus in the surfactant-rich phase was obtained from the mass balance taking into account the initial weight. Table 1. Rhodium and phosphorus content in the surfactant-rich phase after CPE (csurfactant = 5 g L-1, c[Rh(COD)µ2-Cl]2 = 0.2 mmol L-1). TCPE

RhSRP

PSRP

[°C]

[wt%]

[wt%]

NP8

50

96.4

99.8

1:2.3

NP8

50

98.0

96.4

Rh/TPPTS

1:3.2

NP8

50

20.6

10.8

4

Rh/BPPM

1:2.2

NP8

50

98.7

98.5

5

Rh/BPPM

1:3.3

NP8

50

98.7

99.5

Entry

Catalyst

[Rh]:[Ligand]

Surfactant

1

Rh/BPPM

1:1.1

2

Rh/TPP

3

The data in Table 1 shows that CPE can be used to recycle a homogeneous catalyst from micellar solutions. Obviously, the hydrophobic ligands TPP and BPPM lead to a rhodium and phosphorous content in the surfactant-rich phase above 96 %, which is visualized in Figure 7. The surfactant-rich phase is at the bottom of the flask due to the higher density. As expected, the rhodium content in the surfactant-rich phase decreases significantly to 20.6 % when using the hydrophilic ligand TPPTS (see Figure 7). Due to the formed hydrophilic catalyst complex with TPPTS as the ligand, the water solubility is drastically increased, leading to the low ACS Paragon Plus Environment

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rhodium content in the surfactant-rich phase. Furthermore, the phosphorus content in the surfactant-rich phase is expectedly low with 10.8 %. Nevertheless, significant amounts of rhodium and phosphorus are in the surfactant-rich phase due to the surface active properties of the sulfonated ligand, which is in accordance with the literature.26 Hence, the ligand and catalyst complex behaves similar to the surfactant and the solubility in the surfactant-rich phase is not negligible.

Figure 7. CPE with NP8 as the surfactant (left: Rh/BPPM, entry 1, table 1; middle: Rh/TPP, entry 2, table 1; right: Rh/TPPTS, entry 3, table 1). Moreover, the impact of the rhodium to ligand ratio on the catalyst distribution in micellar solutions after CPE was investigated. Increasing the amount of BPPM, enrichment of rhodium in the surfactant phase can even be increased slightly. The phosphorus content in the surfactant-rich phase remains constant at a high level. Based on these results, the proof of concept for CPE of homogeneously dissolved metal catalysts was successful and this methodology is a promising tool for recycling of homogeneous catalysts from micellar solutions. After showing that the separation concept works, a combined reaction and separation process was investigated.



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3.3 Hydrogenation of itaconic acid in aqueous micellar solutions After investigation of catalyst partitioning after CPE and the influence of the surfactant on CPE, the catalytic reaction was studied in micellar solutions and pure methanol. As mentioned above, itaconic acid was selected as the model substrate and based on earlier investigations and experiences25, (2S,4S)-BPPM was selected as the enantioselective ligand that preferentially leads to the formation of (S)-methyl succinic acid. In polar protic solvents, such as methanol, the reaction is very fast and shows a good enantioselectivity of about 95% enantiomeric excess (ee). Hereby, the hydrogenation of itaconic acid is much faster than the hydrogenation of its esters as shown in the Supporting Information. As the recycling of the dissolved rhodium catalyst from an organic solvent is difficult to achieve, the organic solvent methanol was replaced by a micellar reaction medium of the nonionic surfactant NP8 as the benchmark surfactant, so that CPE can be used to recycle the homogeneous catalyst complex after hydrogenation reaction. Additionally, the anionic surfactant

sodium

dodecyl

sulfate

(SDS)

and

the

cationic

surfactant

dodecyltrimethylammonium bromide (DTAB) were tested to compare the reaction performance with the nonionic one. SDS and DTAB are often used as benchmark surfactants in micellar catalysis. The conversion plots are shown in Figure 8.


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Figure 8. Conversion plot for the hydrogenation of IA in micellar solutions of different surfactants. Experimental conditions: cIA = 10 g L-1, c[Rh(COD)2]OTf = 0.24 mmol L-1, Rh:BPPM ratio = 1:1.1, T = 25 °C, phydrogen = 0.11 MPa, n = 1200 min-1, V = 100 mL, cNP8 = 50 g L-1, cSDS = cDTAB = 10xCMC. The hydrogenation in the aqueous micellar solution proceeds on a comparable time scale than in methanol. Together with the high ee of 92-96% for NP8 and SDS, it is a proof of the successful transfer of this reaction from an organic solvent to water. Interestingly, the reaction performance of the anionic surfactant SDS and the nonionic surfactant NP8 outperform the cationic surfactant DTAB so that DTAB was not further considered in the investigations. The main reason for this lower activity of DTAB is still unclear. We assume that it is not due to the size of the micelles as SDS micelles are the smallest and NP8 micelles are the largest. DTAB micelles are between. One possibility might be the location of the catalyst complex inside the micelles. In the case of SDS, the cationic rhodium complex might be located near the head groups close to the surrounding water phase leading to a faster reaction. In the case of DTAB, the complex might be located in the core of the micelles, which is too hydrophobic for the substrate. Although only NP8 as the nonionic surfactant will be appropriate for catalyst ACS Paragon Plus Environment


recycling via cloud-point-extraction, this study was to emphasize that surfactant selection is crucial for the catalytic reaction and that the reaction can be done with good performance in NP8 solutions. Parameters related to the surfactant itself, e.g. surfactant structure, critical micelle concentration (CMC), solubilization capacity, size of the micelles, etc., may have an influence on the catalytic performance, which is already discussed in plenty of other investigations.27–29 The impact of the NP8 concentration is shown in Figure 9. The applied surfactant concentrations are far from the CMC for NP830 to ensure high volume fractions of the catalyst-containing surfactant-rich phase, facilitating the catalyst recycling. Typically, the higher the concentration of surfactant, the higher is the reaction rate due to an increased solubility of the substrate and, thus, higher local concentrations. However, the structure of the nonionic surfactant NP8 contains an aromatic ring and the formation of a η6-complex with rhodium is possible as shown earlier for TX-100.25,31 This will lead to a lower amount of the active rhodium species without having an influence on the ee. In both cases, an ee of about 90% was obtained, which is comparable to the enantioselectivity in methanol. Ensuring high reaction rates, 50 g L-1of the surfactant is used to conduct the recycling experiments.


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Figure 9. Conversion plot for the hydrogenation of IA in micellar solutions with different concentrations of NP8. Experimental conditions: cIA = 10 g L-1, c[Rh(COD)2]OTf = 0.24 mmol L-1, Rh:BPPM ratio = 1:1.1, T = 25 °C, phydrogen = 0.11 MPa, n = 1200 min-1, V = 100 mL. 3.4 Combined hydrogenation and CPE experiments After knowing that the hydrogenation of itaconic acid can be done in micellar solutions of the nonionic surfactant NP8 and CPE can separate the catalyst complex from micellar solutions, in the final step both parts were linked together. Combined reaction and recycling experiments were done to prove the concept of catalyst recycling rather than to optimize the reaction conditions. As shown in Figure 10, the recycling experiment was successful. After the first run, the temperature was increased to 55 °C to reach the cloud point. Waiting 10 minutes leads to a separation into a water and surfactant-rich phase and the product was removed with CPE. Fresh IA dissolved in water was added to conduct the next run. As expected, a full conversion could be achieved in the second run. However, the turnover frequency (TOF) decreases slightly from 3060 h-1 to 2830 h-1. Finally, a third run was performed, in which the reaction rate breaks drastically down and a TOF of only 160 h-1 was determined. The strong decrease of the TOF is caused by catalyst deactivation due to the ACS Paragon Plus Environment


applied IA as the substrate, which is described in detail in the following section. Due to deactivation process, a turnover number (TON) of only 648 could be obtained after three runs.

Figure 10. Recycling experiment with IA. Experimental conditions: cIA = 10 g L-1, cNP8 = 50 g L-1, c[Rh(COD)µ2-Cl]2= 0.24 mmol L-1, Rh:BPPM ratio = 1:1.1, Treaction = 25 °C, Tseparation = 55 °C, phydrogen = 0.11 MPa, n = 1200 min-1, V = 100 mL, TOF was determined from the conversion plots at X=15% (for third run at X=2%). Furthermore, the rhodium and phosphorous concentration in the aqueous phase was determined after each CPE. As expected, the CPE leads to an accumulation of the catalyst complex in the surfactant-rich phase, resulting in low rhodium concentrations in the aqueous phase of 1.5 ppm after the first run and 1.6 ppm after the second run. The phosphorous leaching into the water phase was below 1.2 ppm for both cases, showing the applicability of CPE for recycling of homogeneous catalysts. Additionally, a recycling experiment was done with DMI as the substrate to exclude the deactivation by IA, which is shown in Figure 11. In general, the TOF is lower compared to the experiments with IA. Nevertheless, for the first and second run, similar TOF could be ACS Paragon Plus Environment

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achieved around 800 h-1. In contrast, in the third run, the TOF decreases to 401 h-1, indicating also a slight catalyst deactivation. Compared to IA as the substrate, the catalyst deactivation is significantly reduced since no carboxyl groups are in the structure of DMI leading to unreactive rhodium species (see section 3.5). After three runs, a TON of 605 can be obtained.

Figure 11. Recycling experiment with DMI. Experimental conditions: cDMI = 10 g L-1, cNP8 = 50 g L-1, c[Rh(COD)µ2-Cl]2= 0.24 mmol L-1, Rh:BPPM ratio = 1:1.1, Treaction = 35 °C, Tseparation = 55 °C, phydrogen = 0.11 MPa, n = 1200 min-1, V = 100 mL, TOF was determined from the conversion plots at X=15%. The recycling of the homogeneous catalyst complex via CPE in its active form is possible, although the selected substrate IA leads to catalyst deactivation. Hence, the catalyst deactivation is discussed in detail in the following section. In principle, one can decide between aqueous-micellar solutions (mainly water and surfactant) and microemulsion systems (water, oil, and surfactant). It has already been shown how a homogeneous catalyst complex can be recycled from these systems, e.g. by micellar-enhanced ultrafiltration (MEUF) or phase separation.13,15,32 Both systems have advantages and disadvantages. In MEUF, the



catalyst is immobilized in surfactant micelles and recycled by membrane filtration. This is a very complex system and needs the adjustment of membrane, surfactant and operation conditions. Due to surfactant adsorption, the flux is often low and the release of the product takes too much time. Additionally, depending on the hydrophobic character of the product, it can accumulate in the micelles and is recycled together with the catalyst. In microemulsion systems, phase separation is an appropriate method, but depending on the composition of the reaction mixture and the applied temperature different phase state (1-3) are obtainable. It is possible to modify the homogeneous catalyst complex by careful selection of the ligand so that it accumulates always in the surfactant-rich phase. However, the reactants will distribute between the individual phases, so that, after phase separation, some of the product will remain in the system as shown for the Boscalid synthesis using microemulsion systems.14 In the joint project InPROMPT, this problem is partially solved as the reacting olefin is at the same time the oil-phase of the microemulsion system. The CPE, which was used as an alternative to MEUF for catalyst recycling, is limited to nonionic surfactants, but it represents the simplest system in comparison to MEUF and the phase behavior of microemulsion systems. For the right combination of catalyst, substrate, and surfactant, easy separation can be done after the reaction that results in a huge water-rich phase and a small surfactant-rich phase. In the case of a hydrophobic catalyst and a hydrophilic product, most of the product can be obtained in a single separation step due to the huge excess phase. 3.5 Catalyst stability in aqueous micellar solutions In the previous section, it was shown that the Rh/BPPM catalyzed hydrogenation of itaconic acid is possible in an aqueous micellar solution. For the development of a continuous process, catalyst stability is an important issue. To evaluate possible deactivation mechanisms, a set of experiments was performed, where after one batch run new IA was added for at least five runs. However, product isolation was avoided to exclude catalyst deactivation pathways caused by the recycling procedure. The experiments were carried out in methanol and aqueous micellar solutions of SDS (2.4 wt% = 10 x CMC). Although SDS cannot be used to recycle the catalyst via CPE, it has been quite often used in micellar catalysis and its high purity allows for studying only the catalyst behavior in micellar solutions. Moreover, undesired catalyst deactivation by the formation of an arene complex can be avoided. For comparison, the same set of experiments was done with DMI. The TOF for each run, calculated from the gas consumptions, is shown in Figure 12. The TON after the five runs was about 1100.


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Figure 12. Hydrogenation of DMI and IA in MeOH and aqueous micellar SDS solution. Reaction conditions: cDMI =cIA = 10 g L-1, c[Rh(COD)(µ2-Cl)]2,1 = 0.2 mmol L-1, Rh:BPPM ratio = 1:1.1, T = 25 °C, phydrogen = 0.11 MPa, n = 1200 min-1, V1 = 90 mL, TOF was determined from the conversion plots at X=15%. As shown in Figure 12, the TOF increases for both substrates from the first to the second run and thereafter decreases. The increase of TOF can be explained by an induction period of the catalyst complex, which is fully formed during the first run.33 After five runs, the catalyst is still active, but the initial reaction rate drops significantly, whereby the decrease in TOF for the following runs is much stronger for IA than for DMI. In addition to the calculated reaction rate, a picture of the reaction solution was taken after each run, which is shown in the Supporting Information, indicating that in the case of IA the color changed from yellow to colorless, whereby the color for DMI is almost the same after the five runs. The decoloration of the reaction mixture is a result of a side-reaction of the rhodium complex with IA forming inactive Rh-(IA)-alkyl complex shown earlier by Heller et al..34,35 As catalyst deactivation in the case of IA is a result of the carboxyl groups, it can be assumed that less catalyst deactivation by the mentioned side-reaction takes place in the case of DMI. Hence, no decoloration can be observed using DMI as the substrate. Furthermore, NMR experiments were conducted to confirm the previous assumptions. To the best of our knowledge, the only deactivation phenomenon in case of carboxyl functionalized ACS Paragon Plus Environment


substrates in Rhodium catalysis has been studied for IA. To generalize this concept further unsaturated substrates containing carboxyl groups were added to solvate complex [Rh(PP)(MeOH)2]BF4. As PP ligand was chosen diphenylphosphinoethane (dppe) as an achiral and easy to analyze ligand. The chosen substrates should not possess phenyl rings to exclude formation of arene complexes known as further deactivation phenomena. So we added IA and 2-acetamidoacrylic acid (aH) to solvate complex [Rh(PP)(MeOH)2]BF4. In the case of IA, a fast decoloration within one hour was observed. In the 31P-NMR spectrum of the resulting product, only one species was detected. The typical small coupling constant 2JP,P = 25 Hz underlines the formation of Rh(III)-alkyl complex.34 A proposed mechanism for the deactivation is shown in the Supporting Information. We assume that due to the high stability of tridentate coordination of the substrate, the Rh(III)-alkyl complex is blocked for hydrogenation reaction. The addition of aH to the solvate complex [Rh(dppe)(MeOH)2]BF4 also showed decoloration. Following the reaction via

31

P-NMR, the putative Rh(I) substrate

complex (2JP,P = 40 Hz) completely reacts to the Rh(III)-alkyl complex within 3 hours. The crystal structure revealed a sigma Rh-C bond in the same manner as the published Rh alkyl complex.34 Whereas the crystal structure showed a dimeric complex bridged by the carbonyl group of the carboxylic function. The 31P–NMR spectrum of the dissolved complex in MeOH displayed only 8 lines (ddd) indicating only two chemical inequivalent phosphorous nuclei. Therefore we assume monomeric structures in solution stabilized by solvent molecules, see Figure 13. These results hint at a general deactivation process caused by carboxyl groups of olefins in Rh catalyzed reactions independent of the diphosphine ligand. It was found that the formation of inactive alkyl complexes requires only one carboxyl group.


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Figure 13. left: crystal structure of Rh(III) complex after oxidative addition of 2acetamidoacrylic acid. Hydrogen atoms are omitted for clarity. Right: Putative monomeric structure in solution.

4. Conclusions Aqueous micellar solutions can be used as a reaction medium for homogenously catalyzed hydrogenation reactions. A hydrophobic catalyst complex can be immobilized by surfactant micelles, and in the case of nonionic surfactants, cloud point extraction can be used to recycle >96% of the catalyst complex after the reaction. Hydrophilic products are required to avoid product accumulation in the surfactant-rich phase. Itaconic acid as a very hydrophilic reactant was used in the experiments, but severe catalyst activation due to the carboxylic acid groups was observed. By NMR, it was identified that this problem is not limited to itaconic acid and it can be found for other substrates containing carboxylic acid groups. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Michael Schwarze) SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website. Information on the applied chemicals, the setup, a proposed mechanism for deactivation, catalytic details, and X-ray analysis (PDF).



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ACKNOWLEDGEMENTS This work is part of the Collaborative Research Center "Integrated Chemical Processes in Liquid Multiphase Systems" (subproject A2) coordinated by the Technische Universität Berlin.

Financial

support

by

the

German

Research

Foundation

(Deutsche

Forschungsgemeinschaft, DFG) is gratefully acknowledged (TRR 63). REFERENCES (1)

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Hydrogenation of Itaconic Acid in Micellar Solutions: Catalyst

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