Design of a ‘‘green’’ one-step catalytic production of ␧-caprolactam (precursor of nylon-6) John Meurig Thomas*†‡ and Robert Raja§ *Department of Materials Science, University of Cambridge, Cambridge CB2 3QZ, United Kingdom; †Davy Faraday Research Laboratory, Royal Institution of Great Britain, 21 Albemarle Street, London W1S 4BS, United Kingdom; and §Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom Communicated by John D. Roberts, California Institute of Technology, Pasadena, CA, August 11, 2005 (received for review June 1, 2005) The ever-increasing industrial demand for nylon-6 (polycaprolac- tam) necessitates the development of environmentally benign methods of producing its precursor, ␧-caprolactam, from cyclohex- anone. 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 ␧-caprolactam using a family of designed bifunctional, heterogeneous, nanoporous catalysts containing isolated acidic and redox sites, which smoothly convert cyclohexanone to ␧-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 III 5؊ V 3؊ small fractions of the Al O4 and P O4 tetrahedra constituting III 5؊ the 4-connected open framework are replaced by Co PO4 and IV 4؊ Si O4 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 Ϸ80% for the intermediate oxime, formed from NH2OH, which is pro- Scheme 1. duced 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. depletes the ozone layer), are single-site heterogeneous catalysts (SSHCs). hydroxylamine ͉ single-site heterogeneous catalysts (SSHC) ͉ SSHCs are those in which the active centers are spatially ammoximation isolated from one another [and uniformly distributed through the solid (5)] such that each site has the same energy of interaction between it and the incoming reactant (6). Moreover, ␧ precursor to nylon-6, -caprolactam, is manufactured on a that energy remains constant when all of the sites participate in Amassive scale through the agency of two currently favored catalytic turnover. SSHCs possess the supreme advantage of methods, each of which starts from cyclohexanone, 1 (Scheme 1). combining the merits of conventional homogeneous and heter- In one, the oxidant that forms the intermediate cyclohexanone ogeneous catalysts: they facilitate the separation of products oxime, 2, is hydroxylamine sulfate, and ammonia is used to from reactants as well as the recyclability of the catalyst, and, in neutralize the liberated acid. In the other, a far less environ- addition, the products of reaction are molecularly sharply de- mentally aggressive oxidant, aqueous H2O2 is used in conjunc- fined just as with the best traditional single-site homogeneous tion with a solid redox catalyst, a titanosilicate known as TS-1 catalysts and all enzymes. (Fig. 1) to ammoximize the ketone. However, both methods Whereas homogeneous catalysts are seldom bifunctional [a entail the use of oleum to effect the Beckmann rearrangement recent, unusual example involving allosteric regulation has, of the oxime to the lactam, 3, and the former method generates very however, been reported (7)], it is feasible, as we show below, to large quantities of low-value ammonium sulfate as by-product. design single-site, open-structure catalysts in which Bro¨nsted Here, we describe a route that produces 3 without generating acidic sites are well separated from one another and from redox unwanted ammonium sulfate in an environmentally benign, sites in a nanoporous solid. solvent-free manner using air as oxidant and nanoporous solid Previously, we have described (1, 8) the many practical catalysts. These nanoporous acid catalysts (1, 2) are bifunctional advantages of designing open-structure SSHCs where, in effect, because they also have within them isolated redox centers (CoIII, one may judiciously distribute isolated active centers over a MnIII,orFeIII ions tetrahedrally coordinated to oxygen) (3) at large, 3D (internal) solid surface. Because essentially all of the which air (or oxygen) in the presence of ammonia forms atoms that constitute the bulk of the open-structure solids are hydroxylamine in situ. Because the open structure of the bifunc- simultaneously at their internal surface, they are freely accessi- tional catalysts contain pores large enough to facilitate the ble to reactant species, and ingress and egress of reactants and diffusion of reactants, intermediates, and products within them, products freely occurs, provided the pore dimensions of such the parent ketone is sequentially and smoothly converted to the structures exceed those of the participating molecules. More- lactam when it, air, and ammonia are brought into contact with over, such open-structure solids, typified by Fig. 1, enable all of one another within the solid catalysts. The solids designed for this purpose, like those that we have used to effect other environmentally benign oxidations, such as the conversion of Abbreviations: SSHC, single-site heterogeneous catalyst; AlPO, aluminophosphate. cyclohexane in air to adipic acid with 65% selectivity (4) (thereby ‡To whom correspondence should be addressed: E-mail: [email protected]. circumventing the production of N2O, a greenhouse gas that also © 2005 by The National Academy of Sciences of the USA 13732–13736 ͉ PNAS ͉ September 27, 2005 ͉ vol. 102 ͉ no. 39 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0506907102 Downloaded by guest on September 26, 2021 CHEMISTRY Fig. 1. Structural drawing and projected image of redox and acid active centers in silicalite. (A) Silicalite, SiO2, is a synthetic polymorph of silica possessing channels of pore-diameter 5.5 Å. (B) TS-1 (titanosilicalite number 1) is silicalite containing a few TiIV ions as redox centers in place of some of the SiIV ions that occupy the tetrahedral sites. (C) Bro¨nsted acid centers 'Al-O(H)-Si' are created in silicalite when AlIII ions (shown in green) replace some of the SiIV ions (yellow) and a loosely attached proton (white) is bound to a neighboring framework oxygen atom. (D) High-resolution micrograph of the silicalite structure imaged along the [010] direction. Inset shows computed image. the conventional spectroscopic, scattering, and diffraction tech- either by chemical intuition (13) or by a computational algorithm niques regularly deployed (1, 8) by solid-state chemists and (12)]. Three typical AlPOs are shown in Fig. 2. Of these AlPO-5, physicists to be used for the identification and characterization which is the key structure exploited in this work, and AlPO-36, of all of the atomic detail of the active centers at the (internal) which we used as the basis of a preliminary study (14), are surface. The open-structure solids exploited by us are all frame- relatively readily prepared. The third structure, however, exists, work-substituted microporous aluminophosphates (AlPOs). as yet, only in silico; but its thermodynamic stability, lattice energy, internal pore volume [using the Connolly method (15)], Materials and Methods and pore dimensions are comparable with those of AlPO-5 (10). Numerous distinct structural types of open-structure AlPOs Bro¨nsted acid sites are introduced into the AlPOs in two ways: III exist (9), exhibiting diameters of pores ranging from 3.8 to 7.5 Å either by isomorphously replacing some of the Al ions by II II II V and many different kinds of pore and channel systems. The AlPO divalent (M) ones (Zn ,Mg ,Co , etc.) or by replacing the P IV ' III V' family may be regarded as a collection of structural polymorphs ions with Si . In other words, a fraction of the Al –O–P linkages is replaced by 'MII–O(H)–PV' or by 'AlIII– of microporous, crystalline silica, like silicalite-1 (Fig. 1). ' IV' Whereas in silicalite, SiIV ions tenant each tetrahedral site (T), O(H) Si , the proton that is loosely attached to the bridging 4Ϫ oxygen being the locus of the Bro¨nsted acid center. Experimen- and in the open network the SiO tetrahedra are connected 4 tally, this process is done during synthesis (as described in ref. 16 three-dimensionally in a corner-sharing fashion, in the AlPOs 5Ϫ 3Ϫ and outlined below). The degree of isomorphous substitution of there is an alternation of AlO4 and PO4 tetrahedra. In each the AlIII by MII and of PV by SiIV ions is deliberately kept low, case the ion at the T site is connected to its neighboring T site up to Ϸ4 atom %, to maximize the isolation of the individual by bridging oxygens. The most recent theoretical and computa- acidic sites. Redox centers [in this work, they are predominantly tional analyses (10), employing advances in tiling theory (11), CoIII in tetrahedral coordination and in some catalysts MnIII or reveal that, in principle, 887 distinct kinds of open structures, FeIII ions also tetrahedrally linked to bridging oxygens (a rare based on 3D networks of TO4 tetrahedra, exist (with unit cell situation for these ions in inorganic oxides)] are introduced 3 volumes less than Ϸ30,000 Å ). However, very many of these are simply by exposing the as-synthesized CoII (or MnII or FeII)- not chemically plausible because of restrictions imposed by framework-substituted AlPO solid to dry oxygen at 550°C for bonding theory. Nevertheless, several hundred AlPO structures prolonged periods. [We considered the option of producing should exist, and nearly 100 have already been prepared by using redox sites in siliceous analogues of zeolites (instead of AlPOs), so-called structure-directing (template) organic molecules (12) but a TiIV tetrahedral site in a silica as in the titanosilicate TS-1 that are added to the nutrient gel from which the AlPO, or its (or AlPO) framework does not function as a redox center with framework-substituted analogues (see below), crystallize.