One-Step Catalytic Production of -Caprolactam (Precursor Of

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. [The O2 (or air) as oxidant, unlike the situation that obtains with H2O2 choice of the appropriate structure-directing agent is made as oxidant.]

Thomas and Raja PNAS ͉ September 27, 2005 ͉ vol. 102 ͉ no. 39 ͉ 13733 Downloaded by guest on September 26, 2021 Fig. 2. Three microporous AlPOs. (A) AlPO-5, which has pores of diameter 7.3 Å. (B) AlPO-36, in which the pore-opening is elliptical (7.5 ϫ 6.5 Å). (C) The hypothetical AlPO structure known as 2࿝103 (see ref. 10). This structure contains a 1D channel system, similar to that of AlPO-5, with pore diameter of 7.5 Å.

The appearance or disappearance of Bro¨nsted acidity after Typical procedures and crystallization conditions for the synthesis and subsequent treatments, including catalysis, is mon- synthesis of the family of eight bifunctional catalysts that we itored (17) by Fourier transform infrared stretching and bending studied are given below. In outline, the procedure was as follows: frequencies when the solid acid is exposed to test molecules the phosphorous source (85% H3PO4) and H2O were first gently (typically H2O, NH3,N2,orCH3CN); and x-ray absorption fine stirred in a Teflon-lined beaker, to which the aluminum source structure (near edge, x-ray absorption near edge structure, and [Al(OH)3] was added slowly. The two metal sources (redox and extended x-ray absorption fine structure) chart (18) the conver- acid components) were first dissolved in water and then added II III sion of Co to Co , both by shifts in K-edge and in bond lengths. slowly to the previously prepared Al–H3PO4 mixture. The tem- The CoII–O bond length in an AlPO host is close to 1.93 Å, and plate (structure-directing agent) was then introduced, drop-wise, that of CoIII–O is 1.82 Å. Extended x-ray absorption fine under vigorous stirring, and the gel was aged for Ϸ1 h at room structure studies also provide direct proof of site isolation in temperature, before being sealed in a Teflon-lined stainless steel these single-site catalysts. Electron-stimulated x-ray (K)- autoclave and heated to the desired temperature, under auto- emission maps recorded in a high-resolution microscope are also geneous pressure, for the required amount of time. The solid used to verify site isolation of the CoIII and SiIV ions in the product was isolated by filtration (after crystallization), washed bifunctional catalysts. with deionized water, and dried in air (90°C). The as-prepared Extensive measurements, as well as ab initio quantum com- product was calcined at 550°C, first in nitrogen for 4 h and then putational studies (19–22), reveal that the Bro¨nsted acidity of a in dry oxygen for 16 h. particular kind of isolated site varies as a function of the actual Phase purity of the catalyst was monitored by a combination open structure in a manner that is rather complicated. For of powder x-ray diffractograms and the element distribution example, tetrahedrally bound MgII ions in MAlPO-5 and also in maps as seen in electron micrographs. The precise stoichiometry other MAlPO frameworks (23) generate stronger Bro¨nsted acid (with an error of approximately Ϯ3 ϫ 10Ϫ3) was determined by sites than MgII ions in MAlPO-36. It seems (22) that a combi- inductively coupled plasma (metal) analysis. nation of the ionic radius and electronegativity of the MgII ion The single-step liquid-phase ammoximation of cyclohexanone is a key determinant for the acidity of MgAlPO framework. and its subsequent Beckmann rearrangement to ␧-caprolactam Likewise, the redox strength of an isolated T-site (CoIII,MnIII, was carried out in a high-pressure, poly(ether-ether-ketone)- or FeIII) in MAlPO frameworks is also a function of the actual lined, stainless steel catalytic reactor (Cambridge Reactor De- structure type, and even the percentage of the substituting MII sign, Cambridge, U.K.; 100 ml). Then, 0.5 g of the bifunctional, ion incorporated into an AlPO framework that may be raised (by microporous AlPO catalyst, previously calcined in dry O2 at III calcination in O2) to a higher oxidation state. M is also 550°C and stored under inert conditions (nitrogen or argon), was governed, in a manner that is not yet clear, by the nature of the transferred to the catalytic reactor (using a robotically controlled structure type. In CoAlPO-18, for example, all of the CoII ions catalyst delivery unit) containing Ϸ5 g of cyclohexanone (Al- introduced substitutionally to the framework may be converted drich; Ͼ99.8% pure), Ϸ14.6gofNH3, and 0.5 g of the internal to tetrahedrally coordinated CoIII, whereas in CoAlPO-36 only standard (mesitylene). The reactor was sealed, and its contents II Ϸ50% of the Co in the as-prepared acid catalyst may be raised were inertized (three times) with dry N2 before reaction. The (24) to a high-spin CoIII state, thereby yielding a bifunctional, contents of the reactor were stirred (1,200 rpm) and heated to isolated acidic and redox sites, catalyst. 80°C. Dry air (35 bar) was pressurized into the reaction vessel

13734 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0506907102 Thomas and Raja Downloaded by guest on September 26, 2021 Table 1. Catalytic low-temperature solvent-free production of ␧-caprolactam: Performance of a series of bifunctional AIPO-5 catalysts Product selectivity, mole % Conv mole, Microporous bifunctional catalyst* TON† % Oxime ␧-caprolactam Others

Mn0.02Mg0.02Al0.96PO4 61.5 68.3 5.7 77.9 16.4 Fe0.02Mg0.02Al0.96PO4 64.8 71.9 11.5 72.0 16.3 Co0.02Mg0.02Al0.96PO4 53.6 57.5 3.2 65.5 35.2 Mn0.02Zn0.02Al0.96PO4 97.3 61.5 6.3 65.0 28.6 Fe0.02Zn0.02Al0.96PO4 109.5 66.9 3.7 61.2 35.1

Reaction conditions: Cyclohexanone Х 5 g; catalyst ϵ 0.50 g; air ϵ 35 bar; NH3 Х 14.6 g; mesitylene (internal standard) Х 0.5 g; T ϵ 353 K; t ϭ 8 h; others ϵ high molecular weight conjugated products formed through aldol condensation of cyclohexanone. *The absolute values of the elemental composition have error limits of Ϯ3 ϫ 10Ϫ3. † Ϫ1 TON ϭ [(molsubstr)(molmetal) ] where molsubstr ϭ moles of cyclohexanone converted.

and, by using minirobot liquid and gas sampling valves, small 'AlIIIϪO(H)-SiIV', it is seen (Table 2) that all of the three aliquots (0.1 ␮l) of liquid and gas samples were removed to study redox centers tested in the bifunctional catalyst are good pro- the kinetics of the reaction without perturbing the pressure in the ducers of the oxime, the selectivities ranging from a low of 63% reactor. The composition both of liquid and gaseous products with FeIIISiIVAlPO-5 to a high of 87% with MnIIISiIVAlPO-5. was continuously monitored by using an online computer- Doubtless, subtle variations in the compositions of these cata- controlled system, which is linked to a gas chromatograph and lysts could result in further improved selectivities and conver- liquid chromatography mass spectrometer (Shimadzu QP 8000). sions. It is noteworthy that FeIIIZnIIAlPO-5 and MnIIIZnII- The products were analyzed (using mesitylene as the internal AlPO-5 catalysts display good performance so far as the standard) by gas chromatography (GC; Varian Model 3400 CX) production of ␧-caprolactam is concerned; and there is scope

employing a HP-1 capillary column (25 m ϫ 0.32 mm) and flame here, as with the other six catalysts, for optimizing the stoichi- CHEMISTRY ionization detector by using a variable ramp temperature pro- ometry of each viable catalyst. gram from 80°C to 220°C. The identities of the products were The viable catalysts described here for both the lactam and first confirmed by using authenticated standards, and their NH2OH were arrived at more by accumulated chemical intuition individual response factors were determined by using the cali- and the principles and practices of solid-state chemistry (1, 5, 25, bration method. The conversions and selectivities were deter- 26) than by quantum-chemical guidance. We could have, in mined, and the yields were normalized with respect to the practice, invoked variants of the popular practical protocols response factors obtained as above. The mass of the products was embracing fast, high-yielding syntheses (27) and high-speed further measured by liquid chromatography mass spectrometry. testing (28) of catalytic performance recently adopted by others. In this study, we did not monitor directly the build-up of NH2OH However, the number of options open to such a combinatorial [as was done in our preliminary investigations (14)]. Its presence approach is forbiddingly large: several dozen candidate AlPO was indirectly established by conducting separate experiments with structures and nearly 100 siliceous zeolites (10) would have been air, ammonia, and the solid catalyst (in the absence of cyclohex- open to us. Moreover, the need to work with only scrupulously anone). Cyclohexanone was added to the above reaction mixture phase-pure solid catalysts with the considerable care and time after the solid catalyst was filtered at the end of the reaction (8 h). needed to secure such purity rendered such an approach im- The formation of cyclohexanone oxime (detected by GC) from the practicable. Our principal aim, to evolve a benign single-step, reaction mixture (above), on addition of cyclohexanone, conclu- solvent-free, low-temperature synthesis of ␧-caprolactam with- sively proved the in situ generation of NH2OH from air and out generating ammonium sulfate as a by-product, has been ammonia. achieved. A subsidiary aim, also achieved [and submitted as a patent application (29)], was to discover a good catalyst for Results and Discussion generating hydroxylamine, NH2OH, in situ within the nanopores Tables 1 and 2 summarize the results obtained with the family of of the bifunctional catalyst, because it is this oxidant (produced eight bifunctional catalysts designed by us. It is clear that, of the from NH3 and air at the redox centers) that ammoximizes the three distinct kinds of Bro¨nsted centers enumerated here, the ketone to the intermediate 2. best results in producing ␧-caprolactam arise with the MgII In summary, we have arrived at a single-step (Scheme 2) method substitution for AlIII, the selectivity of the desirable product that produces ␧-caprolactam in an ecologically responsible fashion, being Ϸ78% at an overall conversion of 68% with MnIIIMgII- in which no aggressive reagents and no solvents are used, simply air AlPO-5. Keeping the Bro¨nsted acid center the same as the oxidant and ammonia along with the designed, bifunctional

Table 2. Catalytic low-temperature, solvent-free ammoximation of cyclohexanone: Performance of three bifunctional catalysts in which the Bro¨nsted acid center is kept constant Product selectivity, mole % Conv mole, Microporous bifunctional catalyst* TON† % Oxime ␧-caprolactam Others

Mn0.02Si0.02Al0.96PO4 77.4 76.5 86.6 3.3 10.0 Co0.02Si0.02Al0.96PO4 69.8 68.2 79.6 4.7 14.7 Fe0.02Si0.02Al0.96PO4 88.0 87.5 63.2 5.5 31.2

Compare with Table 1. See Table 1 for reaction conditions. *The absolute values of the elemental composition have error limits of Ϯ3 ϫ 10Ϫ3. † Ϫ1 TON ϭ [(molsubstr)(molmetal) ] where molsubstr ϭ moles of cyclohexanone converted.

Thomas and Raja PNAS ͉ September 27, 2005 ͉ vol. 102 ͉ no. 39 ͉ 13735 Downloaded by guest on September 26, 2021 morillonite that the resulting nano-composite material may be used as a component of car engines, and with the need to invoke clean (‘‘green’’) technology, our catalysts offer a viable alternative to the existing manufacturing procedures. SSHCs have been designed by using an extension of the practices that we have evolved in recent years to effect other environmentally desirable or chemically de- manding conversions [such as the oxyfunctionalization of terminal methyl groups by air or oxygen (3, 26)]. A number of important other reactions, including the epoxidation of replenishable terpenes and esters of naturally occurring unsaturated fatty acids as well as Scheme 2. the hydrogenation of polyenes and unsaturated ketoesters, may also be effected by using appropriately designed SSHCs (6, 32).

catalyst at Ϸ80°C. With nylon-6 (polycaprolactam) now playing an We thank Drs. Wim Buijs (DSM, Heerlen, The Netherlands) and Alan Levy (Honeywell, Morristown, NJ) for helpful discussions. This work was increasing role in automobile manufacture, the Toyota Co. has supported in part by a rolling grant from the Engineering and Physical shown (30, 31) that the softening temperature of this polymer can Sciences Research Council (to J.M.T.) and by a British Petroleum be so greatly increased by incorporation of thin lamellae of mont- research grant (to R.R. and J.M.T.).

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13736 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0506907102 Thomas and Raja Downloaded by guest on September 26, 2021