Gas-Expanded Liquids and Near-Critical Media - American Chemical

Chapter 12

Sulfur Trioxide Containing Caprolactamium Hydrogen Sulfate: An Expanded Ionic Liquid for Large-Scale Production of ε- Caprolactam 1

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Downloaded by TUFTS UNIV on December 2, 2014 | http://pubs.acs.org Publication Date: January 6, 2009 | doi: 10.1021/bk-2009-1006.ch012

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I. T. Horváth , V. Fábos , D. Lantos , A. Bodor , A . - M . Bálint , L. T. Mika , O. E . Sielcken , and A. D. Cuiper 2

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1Institute of Chemistry, Eötvös University, Pázmány Péter sétány 1/A, H-1117 Budapest, Hungary DSM Research, Geleen, The Netherlands 2

The Beckmann rearrangement of cyclohexanone oxime to ε -caprolactam in the presence of oleum proceeds in an ionic liquid, the caprolactamium hydrogen sulfate. We report its remarkable capability to keep the vapor pressure of dissolved sulfur trioxide below 10 kPa even at 140°C, allowing its safe use in large scale processes for a long-time.

Controlling the vapor pressure of hazardous chemicals (7) is perhaps the most important safety and environmental challenge during their use in storage, transportation, chemical reactions, and processes. While their replacement would be the best solution to mitigate risk, it is frequently difficult and sometimes not possible to find an alternative chemical with a similar performance profile. One of the possible approaches to lower the risks of toxic gases is the use of appropriate solvents, which lower the vapor pressure by participating in reversible chemical reactions. For example, the vapor pressure of hydrogen fluoride and pyridine mixtures is very low, but it is high enough to release HF to catalyze chemical reactions (2). Ionic liquids (3) are among the solvents (4) which can be used by molecular designers (5) to fine tune the performance of chemical systems. In particular, they can provide molecular level control of the vapor pressure of the medium (5), a key issue of process safety and environmental protection (7). Ionic liquids © 2009 American Chemical Society

In Gas-Expanded Liquids and Near-Critical Media; Hutchenson, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

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236 are starting to leave academic labs and find their way into a wide variety of industrial applications (8). We have been investigating the mechanism of the Beckmann rearrangement of cyclohexanone oxime (1) to e-caprolactam (2) in oleum as described in literature (9) and concluded that the caprolactam process is in fact the largest scale industrial technology that has been using an ionic liquid, the caprolactamium hydrogensulfate (3) (10% as the reaction medium for decades. We report here that the vapor pressure of 12% dissolved sulfur trioxide in 3 is below 10 kPa even at 140°C, allowing its safe use in large scale processes for decades. The commercial processes of the Beckmann rearrangement of 1 to 2 are performed in the so-called "rearrangement mixture" (11) in the presence of oleum. As the reaction proceeds to complete conversion of 1 in 99.5% selectivity to 2, an appropriate amount of the rearrangement mixture is removed and neutralized with ammonia to produce 2 and (NH ) S0 . The molar ratio of the oleum to 1, calculated as ([H S0 ]+[S0 ])/[1], is generally higher than one because the dissolved sulfur trioxide significantly increases the rate of the Beckmann rearrangement (12). Since the reaction is very exothermic (//), the control of the vapor pressure of S0 has been a key safety issue. The reaction medium, traditionally called "rearrangement mixture", can be prepared by dissolving 2 in oleum. It should be noted that the addition of one equivalent of 2 to 100% sulfuric acid results in the formation of the caprolactamium hydrogen sulfate (3), a colorless solid at room temperature, which becomes a viscous liquid around 60°C. The viscosity of 3 decreases by the addition of S 0 and becomes easy flowing at 12% S0 , even at room temperature. While the density of 3 decreases by increasing the temperature, its ion conductivity increases, as expected (10). In comparison to other ionic liquids (13-19), the density of 2-based ionic liquids are similar, but their conductivity is noticeably lower. *H- and C-NMR measurements have revealed that the carbonyl group of e-caprolactam (2) is protonated to form a caprolactamium hydrogensulfate (3) as the main species: It is important to emphasize that ionic liquids are considered as greener solvents because their vapor pressure is extremely low (6). The temperaturedependent vapor pressures of 2/100% H S 0 (1/1), 2/100%H SO /SO (3.3/4.7/1), and 100% H S0 /S0 (12/1) were measured (Figure 1.) (10). The vapor pressure measurements were performed in a 25 mL Hasteloy-C Parr reactor connected to a Rosemount® Hasteloy-C-276 digital pressure gauge. The system was validated by measuring the vapor pressure of methanol at different temperatures and comparing the experimental values to literature data. Surprisingly, the vapor pressures of e-caprolactam (2) based ionic liquids are much lower above 70°C than that of oleum, which explains why the rearrangement mixture has been used without any serious problems for decades. While the observed low vapor pressure of dissolved S0 could be the result of 4

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Figure 1. Temperature-dependent vapor pressures of2/H S0 =l:l (D); 2/H SOyS0 =3.3:4.7:l (O); and H SO/S0 =12:1 (A) (Reproduced with permission from reference 10. Copyright 2008 Wiley-VCH Verlag GmbH and Co.)

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239 strong interactions between 3 and sulfur trioxide, the formation of another anion such as [HS 0 ]" in 4 (20) seems also plausible: In conclusion, various mixtures of 2 and sulfuric acid or oleum must be considered as ionic liquids, which could hold S 0 very strongly up to 140°C and therefore provide a safe environment for a very exothermic reaction. 2

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Gas-Expanded Liquids and Near-Critical Media - American Chemical

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