Design of a “green” one-step catalytic production of ε-caprolactam (precursor of nylon-6)

The ever-increasing industrial demand for nylon-6 (polycaprolactam) necessitates the development of environmentally benign methods of producing its precursor, ε-caprolactam, from cyclohexanone. It is currently manufactured in two popular double-step processes, each of which uses highly aggressive reagents, and each generates substantial quantities of largely unwanted ammonium sulfate as by-product. Here we describe a viable laboratory-scale, single-step, solvent-free process of producing ε-caprolactam using a family of designed bifunctional, heterogeneous, nanoporous catalysts containing isolated acidic and redox sites, which smoothly convert cyclohexanone to ε-caprolactam with selectivities in the range 65–78% in air and ammonia at 80°C. The catalysts are microporous (pore diameter 7.3 Å) aluminophosphates in which small fractions of the\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\setlength{\oddsidemargin}{-69pt}\begin{document}\begin{equation*}{\mathrm{Al}}^{{\mathrm{III}}}{\mathrm{O}}_{4}^{5-}\end{equation*}\end{document} and\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\setlength{\oddsidemargin}{-69pt}\begin{document}\begin{equation*}{\mathrm{P}}^{{\mathrm{V}}}{\mathrm{O}}_{4}^{3-}\end{equation*}\end{document} tetrahedra constituting the 4-connected open framework are replaced by\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\setlength{\oddsidemargin}{-69pt}\begin{document}\begin{equation*}{\mathrm{Co}}^{{\mathrm{III}}}{\mathrm{PO}}_{4}^{5-}\end{equation*}\end{document} and\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\setlength{\oddsidemargin}{-69pt}\begin{document}\begin{equation*}{\mathrm{Si}}^{{\mathrm{IV}}}{\mathrm{O}}_{4}^{4-}\end{equation*}\end{document} tetrahedra, which become the loci of the redox and acidic centers, respectively. The catalysts may be further optimized, and already may be so designed as to generate selectivities of ≈80% for the intermediate oxime, formed from NH2 OH, which is producedin situ within the pore system. The advantages of such designed heterogeneous catalysts, and their application to a range of other chemical conversions, are also adumbrated.