ε‐Caprolactamium Hydrogen Sulfate: An Ionic Liquid Used for Decades in the Large‐Scale Production of ε‐Caprolactam

Abstract

Ionic liquids [1] are among the solvents [2] which can be used by molecular designers [3] to fine tune the performance of chemical systems in order to facilitate high atom economy [4] for chemical reactions or facile separation of products from reagents or catalyst, [5] all constituting key parts of green chemistry. [6] In particular, they can provide control at the molecular level of the vapor pressure of the medium, [7] a key issue of process safety and environmental protection. The ultimate success of these solvents depends on whether they will be used commercially in small-or larger-scale applications. Of course, a profitable larger-scale process will convince others to explore and use the same or similar media for different applications. Ionic liquids are starting to leave the academic laboratories 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 sulfuric acid and oleum as described [9] and conclude now that the caprolactam process [10] is in fact the largest-scale industrial technology that has been using an ionic liquid, caprolactamium hydrogen sulfate (3), as the reaction medium for decades. We report here the physical characterization of this ionic liquid, including its remarkable capability to keep the vapor pressure of 12 % dissolved sulfur trioxide below 10 kPa even at 140 8C, allowing its safe use in large-scale processes for decades. The mechanism of the Beckmann rearrangement of cyclohexanone oxime (1) to e-caprolactam (2) has been also investigated in 18 O-labeled sulfuric acid, which revealed that the migration of the oxygen function is intermolecular. The cause of the accelerating effect of excess sulfur trioxide in the Beckmann rearrangement in oleum is discussed.The commercial processes of the Beckmann rearrangement of 1 to 2 are performed in the presence of oleum.[10] The reaction is carried out in the so-called "rearrangement mixture" obtained from mixing oleum with molten 1. As the reaction proceeds to complete conversion of 1 with a remarkable 99.5 % selectivity to 2, an appropriate amount of the rearrangement mixture is removed for neutralization with ammonia to produce the final product 2 and (NH 4 ) 2 SO 4 . The molar ratio of the oleum to 1, calculated as ([, is generally greater than one because the dissolved sulfur trioxide plays an important role by significantly increasing the rate of the reaction.[11] As the Beckmann rearrangement of 1 to 2 is very exothermic, [10] the control of the vapor pressure of SO 3 has therefore been a key safety issue.The reaction medium, traditionally called the rearrangement mixture, can be prepared by dissolving 2 in sulfuric acid or oleum. Importantly, note that the addition of one equivalent of 2 to 100 % sulfuric acid results in the formation of a colorless solid at room temperature, which becomes a viscous liquid around 60 8C. Although its viscosity decreases further by increasing the temperature, it was no...

Organocatalyzed Beckmann Rearrangement of Cyclohexanone Oxime in a Microchemical System

“…As we know that COX and CPL are all protonated to form the corresponding hydrosulfate in the oleum phase. 30 The results of reaction enthalpy here indicate that the interaction between COX, CPL, and TFA is much lower than that between COX, CPL, and oleum. In addition, the reduced reaction enthalpy can decrease the heat released during the rearrangement and improve the safety.…”

“…The acetonitrile content in the reaction mixture is about 82 wt %, and the molar concentration of COX is about 0.12 mol/L, similar to the literature system. 30 As shown in Figure 3a, the reaction rate is greatly enhanced in the microreactor. The conversion of COX can be as high as 92% within 100 s at the temperature of 130°C…”

Solvents from nature

“…15 While the observed low vapor pressure of dissolved SO 3 could be the result of strong interactions between the caprolactamium hydrogen sulfate and sulfur trioxide, the formation of an other anion such as [HS 2 O 7 ]seems also plausible:…”

Catalytic transfer hydrogenation in γ-valerolactone-based ionic liquids

“…Friedel-Crafts acylation, 9 hydroformylation, 10 and Beckmann-rearrangement. 11 Hydrogenation reaction as one of the most important and widely studied transition metal catalyzed homogeneous transformation 12 was also performed in ionic liquids. 13, 14 The 35 catalytic transfer hydrogenation of organic substrates is a safer and more energy-efficient alternative operation without using hydrogen gas and can generally be performed in common organic solvents.…”