Protic Ionic Liquids Based on Oligomeric Anions [(HSO4)(H2SO4)x]− (x = 0, 1, or 2) for a Clean ϵ-Caprolactam Synthesis*
Karolina Matuszek A , Alina Brzeczek-Szafran B , Dominika Kobus B , Douglas R. MacFarlane A D , Małgorzata Swadźba-Kwaśny C and Anna Chrobok B DA School of Chemistry, Monash University, Clayton, Vic. 3800, Australia.
B Department of Organic Chemical Technology and Petrochemistry, Silesian University of Technology, Krzywoustego 4, 44-100 Gliwice, Poland.
C The QUILL Research Centre, School of Chemistry and Chemical Engineering, The Queen’s University of Belfast, Belfast, BT9 5AG, UK.
D Corresponding authors. Email: douglas.macfarlane@monash.edu; anna.chrobok@polsl.pl
Australian Journal of Chemistry 72(2) 130-138 https://doi.org/10.1071/CH18384
Submitted: 3 August 2018 Accepted: 27 November 2018 Published: 9 January 2019
Abstract
Inexpensive Brønsted acidic ionic liquids, suitable for industrial-scale catalysis, are reported as reaction media and catalysts for the Beckmann rearrangement of cyclohexanone oxime to ϵ-caprolactam. A family of protic ionic liquids was synthesised from nitrogen bases (1-methylimidazole, N,N,N-triethylamine, N-methylpyrrolidine, 2-picoline) and sulfuric acid by proton transfer in a simple, inexpensive, solvent-free, one-step process. The density, viscosity, conductivity, and ionicity of the synthesised ionic liquids were determined. Variation in the molar ratio of sulfuric acid (χH2SO4 = 0.67 and 0.75) was used to tune the acidity of these protic ionic liquids, which showed extremely high catalytic activity in the Beckmann rearrangement of cyclohexanone oxime to ϵ-caprolactam. Both the structure of the cation and the sulfuric acid molar ratio strongly affect the rearrangement of cyclohexanone oxime. The most active ionic liquid, based on the 1-metyhylimidazolium cation, χH2SO4 = 0.75, afforded high conversion of oxime combined with very good selectivity under mild conditions (110°C, 15 min). The product could be extracted from the reaction mixture, eliminating the need for the neutralisation step that exists in conventional processes. The combination of affordable catalyst and process advantages leads to a greener alternative, competitive against existent industrial applications.
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