Continuous, Fast, and Safe Aerobic Oxidation of 2-Ethylhexanal

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Continuous, Fast, and Safe Aerobic Oxidation of 2‑Ethylhexanal: Pushing the Limits of the Simple Tube Reactor for a Gas/Liquid Reaction Laurent Vanoye,† Jiadi Wang,† Mertxe Pablos,† Régis Philippe,† Claude de Bellefon,† and Alain Favre-Réguillon*,†,‡ †

Laboratoire de Génie des Procédés Catalytiques, UMR 5285, CPE Lyon, 43 bld du 11 nov. 1918, 69100 Villeurbanne, France Conservatoire National des Arts et Métiers, CASER-SITI, 292 rue Saint Martin, 75003 Paris, France

‡

S Supporting Information *

demonstrated to be a valuable alternative for the safe and efficient chemical processing21−24 and among them aerobic oxidation of aldehydes.25,26 While this reaction could be performed without neither catalysts nor radical initiators, the reaction rate was greatly increased using metal ions as catalysts, such as Mn(II) and pure oxygen, inside segmented flow reactors.27,28 Other advantages of continuous microreactors are the small inventory of potentially hazardous chemicals and intermediate species and their large surface to volume ratio which, besides its interest for mass and heat transfers, could contribute to inhibit gas-phase free-radical reactions by recombination on the reactor walls.21 The selectivity of the oxidation process is known to be highly dependent on the aldehyde structure.27−32 While high selectivity was observed with linear aliphatic aldehyde,26 selectivity below 80% toward the corresponding carboxylic acid was obtained for the target reaction, i.e., the oxidation of 2ethylhexanal 3.12,25,27,30 The synergistic use of alkali metals salts (i.e., sodium 2-ethylhexanoate) or organic carboxylate salt (i.e., ionic liquid) and Mn(II) as catalyst can improve the selectivity up to 98% while maintaining good productivity of the process.27,28 Under these conditions, total conversion of the aldehyde 3 was achieved using flow chemistry, and the selectivity was kept high. Herein, a study about further intensification of the aldehyde oxidation to the corresponding carboxylic acid using a simple tube continuous-flow reactor that used cheap, disposable PFA tubing and undergoing a G-L segmented flow (Taylor flow) is reported. Also, we sought to use this reactor for the visual monitoring of the flow. An increase of the internal diameter (i.d.) of PFA tubing (from 1 mm to 1.65 mm) was the only modification to our lab scale microreactor.25,27 For higher i.d., regular Taylor flow could not be easily obtained within the range of the flow rates used. The productivity of this simple experimental setup was pushed to its limits using tools such as oxygen mass transfer, catalyst loading, and concentration in order to maximize the acid yield under safe conditions with a high purity of the product.

ABSTRACT: A continuous-flow microreactor is applied for the selective aerobic neat oxidation of 2-ethylhexanal. Under 7.5 bar of O2 and 10 ppm of Mn(II) as catalyst, a production of up to 130 g/h of 2-ethylhexanoic acid can be obtained with a PFA tubing of 7 m (Ø 1.65 mm, reactor volume ca. 15 mL). The synergistic use of alkali metal salts and Mn(II) as catalyst improve the selectivity up to 94% under those conditions. We show that the productivity of this simple tube microreactor is limited by the thermal management.

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INTRODUCTION 2-Ethylhexanoic acid 1 is an important chemical intermediate used in the production of synthetic lubricants, film plasticizers for polyvinyl butyral, stabilizers for PVC as well as oil additives, and functional fluids like automotive coolants.1,2 Moreover, metal salts of 2-ethylhexanoic acid are used in wood preservatives, as catalyst for polyurethane synthesis, as metal soaps in paint dryers, and in various other applications.3 This product belongs to the C8 compounds family and is obtained through the aldol condensation of butyraldehyde 2.4,5 An overview of the industrial synthetic pathway to 1 is shown in Scheme 1. According to the patent literature, the oxidation of 2ethylhexanal 3 to 2-ethylhexanoic acid 1 is typically performed in a gas sparged stirred tank or gas lift bubble column reactors with air or enriched air as the gas phase using molecular oxygen as the terminal oxidant and with the optional presence of a catalyst.6−9 However, this oxidation is strongly exothermic (ΔrH = −287 kJ/mol, adiabatic temperature rise of 1065 K for pure aldehyde) and proceeds via highly reactive free radical species; thus an efficient temperature control is claimed to be critical for the selectivity, the productivity, and the safety of the process.6−9 This intrinsically fast reaction presents gas−liquid mass transfer limitation in most industrial reactors.6−11 As a matter of fact, even at the lab scale, the productivities published in recent articles on aerobic oxidation are affected by the gas to liquid oxygen mass transfer rate.12,13 In order to reach the chemical regime and improve the productivity, advanced microreactors can be used. They are known to provide better mass and heat transfers than classical vessels.14−20 Thus, continuous flow micro- and millimetric reactors were © XXXX American Chemical Society

Received: November 2, 2015

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DOI: 10.1021/acs.oprd.5b00359 Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development

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Scheme 1. Overview of the Industrial Synthetic Routes to 2-Ethylhexanoic Acid 1a

a

Reaction type: (a) hydroformylation; (b) aldol condensation; (c) hydrogenation; (d) oxidation.

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EXPERIMENTAL EQUIPMENT Analytical grade heptane and tetradecane (Aldrich) was used. 2Ethylhexanal 3 (96%) (Aldrich or Acros) was used as obtained from the supplier and stored at 4 °C under N2 with protection from light. Mn(II) 2-ethylhexanoate (40% in mineral spirits) was provided by STREM and used as obtained. Pressurized oxygen (99.995%) was provided by Messer. Figure 1 shows the experimental setup. A PFA tubing (Upchurch Scientific) (internal diameter of 1.65 mm with

Figure 2. Initial rate as the function of catalyst loading. Reaction conditions: 2-ethylhexanal 3 0.8 M in heptane, O2 5 bar, O2/aldehyde molar ratio ≥5, PFA tubing 1 m, i.d. 1.65 mm, 25 °C, catalyst Mn(II) 2-ethyl hexanoate, sodium 2-ethylhexanoate 2 wt %, and tetradecane 1 wt % as internal standard. The tendency curve is only displayed to help reading. See also Figure S3 in the Supporting Information. Figure 1. Experimental setup for the neat aldehyde aerobic oxidation (O2 excess 99.5%) while maintaining a high selectivity (94%) toward the carboxylic acid. Under these conditions, a production of nearly 1 mol (0.93 M, i.e., 134 g) of 2-ethylhexanoic 3 acid per hour was achieved with a simple tube reactor in PFA. The selectivity could be increased by decreasing the amount of catalyst in order to lower the heat flux from the reaction. But to maintain a quantitative conversion, the inlet gas and liquid flow rates were divided by 2 to rise the residence time. Under those conditions, the productivity was divided by 2, but a selectivity higher than 97% was obtained (Table 1, entry 6).

Table 1. Aerobic Oxidation of Neat 2-Ethylhexanal 3 Using Single Tube Microreactora

entry

catalyst (ppm)

O2 (bar)

conv. (%)b

select. (%)b

1 2c 3d 4 5e 6f

0 10 200 10 10 5

6 6 6 7.5 7.5 7.5

9 95 69 >99.5 52 >99.5

95 95 56 94 94 97

General conditions: Inlet O2 flow: 200 NCCM, inlet aldehyde flow: 2.5 mL/min., neat 2-ethylhexanal 3 containing sodium 2-ethylhexanoate (2 wt %), tetradecane as internal standard (1 wt %), tube length: 6.7 m, id: 1.65 mm, water bath at 25 °C. bMean value of at least three samples spaced 2 min. cSee movie No. 1. dVaporization of the liquid phase was observed in the first 10 cm, see movie No. 2. e Water bath at 5 °C. fG and L flow rate divided by 2. a

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CONCLUSION These experiments demonstrate the capability of simple tube microreactor (total volume of 15 mL, overall footprint of half the size of standard laboratory fume hood) to perform continuous reactions under explosive conditions. A daily production of 3.22 kg of 2-ethylhexanoic acid with a purity of 94% could be reached without further purification nor solvent separation with simple, cheap, and readily available materials and equipment. Such productivity could not be considered using a classical 10 L lab-scale batch reactor for safety reasons due to the explosive gas mixture,33,34 the adiabatic temperature rise of 1065 K and the required heat removal capability of about 700 W. The use of PFA tubing allows the visual control of the flow associate to a low permeability to liquids, gases, organic vapors, and wide operating temperature range (−240 °C to +260 °C). However, the relatively low heat transfer coefficient of PFA (0.195 W/m·K) does not allow efficient management of the reaction heat flux. Thus, the catalyst amount had to be decreased to maintain the high conversion and selectivity of the process. Higher oxygen pressure and total flow velocity associated with a better heat management integration would help to improve the productivity by reaching a more favorable process window.

high selectivity toward the acid were observed. Then, 10 ppm of Mn(II) was used as catalyst. Under these conditions, a regular and stable Taylor flow was seen in the first few centimeters of the PFA tubing (Figure 3 upper picture and movie No. 1 in the Supporting Information). Bubble shrinking was also noticed along the PFA tubing due to O2 consumption. This implies a decrease of the linear velocity of the liquid flow that could impact oxygen mass transfer rate. Inlet O2 and aldehyde flows were thus optimized to obtain a conversion up to 95%. Under these conditions, a selectivity of 95% toward carboxylic acid could be obtained (Table 1, entry 2 and Figure 4, upper chromatogram). The temperature rise inside the PFA tubing induced by the oxidation was below 20 °C (controlled with a thermocouple in a dedicated experiment) and has almost no influence on the selectivity of the process.29 The first attempt to increase the productivity was an increase in the catalytic amount of Mn(II), while the inlet gas and liquid flow rates were kept constant. However, using 200 ppm of Mn(II), a completely different flow pattern was noticed (Figure 3, bottom picture and movie No. 2 in the Supporting Information) as well as significantly lower conversion and selectivity (Table 1, entry 3). Within the first 6 cm, a regular and stable Taylor flow was observed followed by a rapid disappearance of the liquid slug in less than 1 cm. Reappearance C



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Figure 3. Top view of the PFA tubing layout and visualization of the segmented G/L flow during the aerobic oxidation of 2-ethylhexanal 3. Reaction conditions: neat 2-ethylhexanal 3 + 2 wt % of sodium 2-ethylhexanoate, 6 bar of O2, PFA tube length: 6.7 m, i.d.: 1.65 mm, water bath at 25 °C, 10 or 200 ppm, respectively, of Mn(II) for the upper and for the lower picture (images taken from the movies, see the Supporting Information).

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liquid flow rate), and calculations of heat transfer resistance distribution at the reactor wall (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ Figure 4. Comparison of GC chromatogram obtained during the neat oxidation of 2-ethylhexanal 3 with 10 ppm of Mn(II) (top) and with 200 ppm of Mn(II) (down). Experimental conditions: see Table 1.

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REFERENCES

(1) Kohlpaintner, C.; Schulte, M.; Falbe, J.; Lappe, P.; Weber, J. Aldehydes, Aliphatic and Araliphatic. In Ullmann’s Encyclopedia of Industrial Chemistry, 6th ed.; Wiley-VCH: 2003; Vol. 2, pp 65−99. (2) BASF and PETRONAS Chemicals Group have built a new world-scale production plant for 2-ethylhexanoic acid with a total annual capacity of 30 000 t in Kuantan, Malaysia. The plant is expected to start production in the fourth quarter of 2016. https://www.basf. com/en/company/news-and-media/news-releases/2015/02/p-15146.html (accessed Dec 15, 2015). (3) Raju, R.; Prasad, K. Tetrahedron 2012, 68, 1341−1349. (4) Riemenschneider, W. Carboxylic Acids, Aliphatic. In Ullmann’s Encyclopedia of Industrial Chemistry, 6th ed.; Wiley-VCH: 2003; Vol. 6, pp 493−508. (5) Seki, T.; Grunwaldt, J.-D.; Baiker, A. Chem. Commun. 2007, 3562−3564. (6) Litz, L. M.; Santalone, J. J. Improved gas-liquid mixer. Eur. Patent No. 0 374 967, Dec. 23, 1988. (7) Weise, M. K.; Adis, M. Enhanced gas-liquid reactions. Eur. Patent No. 0 439 013, Jan 3, 1990. (8) Kingsley, J. P.; Litz, L. M.; Lacopelli, F. S.; Weise, M. K. Process for producing organic acids. Eur. Patent No. 0 792 865, Feb 27, 1996. (9) Koch, H. F.; Reed, R. P. Oxidation process. Patent WO No. 01/ 303 46111, June 28, 2001. (10) Sajus, L.; Séré De Roch, I. The Liquid Phase Oxydation of Aldehydes. In Liquid-Phase Oxidation, Boamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, 1980; Vol. 16, pp 89−124. (11) McNesby, J. R.; Heller, C. A. Chem. Rev. 1954, 54, 325−346. (12) Vanoye, L.; Favre-Reguillon, A.; Aloui, A.; Philippe, R.; de Bellefon, C. RSC Adv. 2013, 3, 18931−18937.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.oprd.5b00359. Movie No. 1 showing the top view of the PFA tubing during the oxidation of 2-ethylhexanal 3 with 10 ppm of Mn(II) (AVI) Movie No. 2 showing the top view of the PFA tubing during the oxidation of 2-ethylhexanal 3 with 200 ppm of Mn(II) (AVI) Experimental setup for diluted aldehyde oxidation and neat aldehyde oxidation (Figure S1 and S2). Data used for Figure 1 (gas flow rate, liquid flow rate and calculated residence time), conversion as the function of residence time and catalytic amount of Mn(II) (Figure S3), data used for Table 1 (O2 pressure, inlet gas flow rate, and D



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(13) Vanoye, L.; Pablos, M.; de Bellefon, C.; Favre-Reguillon, A. Adv. Synth. Catal. 2015, 357, 739−746. (14) Neuenschwander, U.; Jensen, K. F. Ind. Eng. Chem. Res. 2014, 53, 601−608. (15) He, Z.; Jamison, T. F. Angew. Chem., Int. Ed. 2014, 53, 3353− 3357. (16) Cantillo, D.; Kappe, C. O. J. Org. Chem. 2013, 78, 10567− 10571. (17) Pieber, B.; Kappe, C. O. Green Chem. 2013, 15, 320−324. (18) Zaborenko, N.; Bedore, M. W.; Jamison, T. F.; Jensen, K. F. Org. Process Res. Dev. 2011, 15, 131−139. (19) Leclerc, A.; Alame, M.; Schweich, D.; Pouteau, P.; Delattre, C.; de Bellefon, C. Lab Chip 2008, 8, 814−817. (20) Inoue, T.; Schmidt, M. A.; Jensen, K. F. Ind. Eng. Chem. Res. 2007, 46, 1153−1160. (21) Gemoets, H. P. L.; Su, Y.; Shang, M.; Hessel, V.; Luque, R.; Noel, T. Chem. Soc. Rev., 10.1039/C5CS00447K. (22) Ley, S. V.; Fitzpatrick, D. E.; Myers, R. M.; Battilocchio, C.; Ingham, R. J. Angew. Chem., Int. Ed. 2015, 54, 10122−10136. (23) Gutmann, B.; Cantillo, D.; Kappe, C. O. Angew. Chem., Int. Ed. 2015, 54, 6688−6728. (24) Hessel, V.; Kralisch, D.; Kockmann, N.; Noël, T.; Wang, Q. ChemSusChem 2013, 6, 746−789. (25) Vanoye, L.; Aloui, A.; Pablos, M.; Philippe, R.; Percheron, A.; Favre-Réguillon, A.; De Bellefon, C. Org. Lett. 2013, 15, 5978−5981. (26) Baumeister, T.; Kitzler, H.; Obermaier, K.; Zikeli, S.; Röder, T. Org. Process Res. Dev. 2015, 19, 1576−1579. (27) Vanoye, L.; Pablos, M.; Smith, N.; de Bellefon, C.; FavreReguillon, A. RSC Adv. 2014, 4, 57159−57163. (28) Favre-Réguillon, A.; Vanoye, L.; De Bellefon, C. Prococédé et préparation d’acides carboxyliques. France priority patent application No. FR1455588, June 18, 2014. (29) Dedicated experiments were performed to measure the temperature inside the PFA tubing. A thermocouple was introduced by the outlet of a 1 m PFA tubing under the conditions listed in Table 1. The thermocouple is then moved progressively along the tube to determine the temperature of the different zone. The continuous maximum service temperature of PFA tubing is 260 °C. The melting range of PFA is 300−310 °C. (30) Lehtinen, C.; Brunow, G. Org. Process Res. Dev. 2000, 4, 544− 549. (31) Lehtinen, C.; Nevalainen, V.; Brunow, G. Tetrahedron 2000, 56, 9375−9382. (32) Lehtinen, C.; Nevalainen, V.; Brunow, G. Tetrahedron 2001, 57, 4741−4751. (33) Tschirschwitz, R.; Schröder, V.; Brandes, E.; Krause, U. J. Loss Prev. Process Ind. 2015, 36, 562−568. (34) Osterberg, P. M.; Niemeier, J. K.; Welch, C. J.; Hawkins, J. M.; Martinelli, J. R.; Johnson, T. E.; Root, T. W.; Stahl, S. S. Org. Process Res. Dev. 2015, 19, 1537−1543.

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Continuous, Fast, and Safe Aerobic Oxidation of 2-Ethylhexanal

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