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, epsilon-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 epsilon-caprolactam using a family of designed bifunctional, heterogeneous, nanoporous catalysts containing isolated acidic and redox sites, which smoothly convert cyclohexanone to epsilon-caprolactam with selectivities in the range 65-78% in air and ammonia at 80 degrees C. The catalysts are microporous (pore diameter 7.3 angstrom) aluminophosphates in which small fractions of the (AlO45-)-O-vertical bar vertical bar vertical bar and p(v)O(4)(3-) tetrahedra constituting the 4-connected open framework are replaced by (CoPO45-)-P-vertical bar vertical bar vertical bar and (SiO44-)-O-IV 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 approximate to 80% or the intermediate oxime, formed from NH2OH, which is produced in 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.