Caprolactam is an important industrial chemical that is used widely for manufacturing of polymeric material such as nylon 6. Certain industrial processes for making caprolactam are well known in the patent literature. Conventionally, caprolactam is made by first converting materials derived from petrochemical feedstock such as cyclohexane, phenol or toluene, to cyclohexanone 2, treating with hydroxylamine to produce the corresponding oxime 3 followed by an acid-induced Beckmann Rearrangement to give caprolactam 1 as shown in Scheme 1. Such a process is described in, for example, U.S. Pat. Nos. 3,914,217; 5,264,571; 4,804,754; 5,354,859 and 7,351,820.

A disadvantage of this existing technology is that large amounts of ammonium sulfate—up to 4.5 tonnes per tonne of caprolactam are produced. Much development work is concentrating on reducing or even eliminating this sulfate by-product. For example, DSM's Hydroxylamine Phosphate Oxime (HPO)-plus process has substantially reduced this sulfate by-product to 1.5 tonnes/tonne of caprolactam. [Chem. Week, 2000, 162(32), 17; Dahlhoff, G., et al., Catal. Revs., 2001, 43(4), 389; “Encyclopedic dictionary of named processes in chemical technology”, Alan E. Comyns, CRC Press, 2007, p. 172.
A more recent approach, developed by EniChem and commercialized by Sumitomo in 2003, completely eliminates the production of ammonium sulfate. The chemical reaction in this case is a so-called ammoximation reaction, whereby cyclohexane is reacted with ammonia and hydrogen peroxide at around 90° C. in the presence of a titanium silicate-2 catalyst [Reddy, J. S., et al., P., J. Mol. Catal., 1991, 69, 383. Chem. Br., 1995, 31(2), 94]. This process allows for considerable cost savings since no hydroxylamine plant is needed. However, hydrogen peroxide is expensive and must be manufactured on a large scale to provide sensible scale economies and transfer pricing.
Another improvement of this process developed by Toray Industries of Japan utilizes a photochemical process for making caprolactam from cyclohexane in the presence of nitrosyl chloride and hydrogen chloride, without the use of the oximation step. This process provides substantial capital cost savings, with the elimination of both cyclohexanone, hydroxylamine and oximation plants. However, the process requires access to low-cost power to be truly cost effective. Large scale photochemical reactors are difficult to design and require constant cleaning to remove tar-like reaction residues. [Hydrocarbon Process. Int. Ed., 1989, 68(11), 97; Dahlhoff, G., et al., Catal. Revs., 2001, 43(4), 389.]
Other notable processes developed by DSM, Shell, BASF, DuPont and Rhodia use butadiene or adiponitrile as starting material for manufacturing caprolactam. Altam, a process developed by DSM and Shell, uses butadiene and carbon monoxide feedstocks to make caprolactam without ammonium sulfate production. The process employs four steps—carbonylation, hydroformylation, reductive amination and cyclization and DSM claims has allowed cost reductions of 25-30% through simplification of plant operations and lower energy consumption.
BASF, Rhodia, and DuPont also investigated the feasibility of converting butadiene to caprolactam. Both BASF and Rhodia' processes involve the hydrogenation of adiponitrile, which can be manufactured from butadiene and hydrogen cyanide, or by electrolysis from acrylonitrile to make 6-aminocapronitrile with hexamethylenediamine as a co-product, using different operating conditions and catalysts.
Other processes for making of caprolactam are also available in the literature using starting materials other than those already mentioned. For instance, U.S. Pat. No. 2,351,939 describes a vapor phase synthesis of caprolactam from adipic acid, using a Ni catalyst in the presence of H2 and NH3, with dehydrating agents (boric and phosphoric acids). The process provided 45% of caprolactam along with 18% of HMI formed. Another synthesis of caprolactam from adipic acid is described in U.S. Publication No. US2007/0244317 where a homogeneous ruthenium catalyst was used leading to a series of products formed, including the dimethyl adipamide and 8% of caprolactam.
U.S. Pat. No. 4,800,227 describes the use of two catalysts (Pd+Ru, Rh or Re) to produce lactams from C4-C6 dicarboxylic acids.
Another process using dicarboxylic acids was described in U.S. Pat. Nos. 4,263,175 and 4,356,124 where Ru oxide or an oxide complex of Ru, Fe, Ni, Co was used to make pyrrolidone. Still another process based on the hydrogenation of dicarboxylic acids with Ru or Os in the presence of an organic phosphine is described in U.S. Publication No. US2007/0244317, which uses N-methylamine to produce N-methyl caprolactam from adipic acid. A number of other products are observed, including some caprolactam.
However, the above mentioned processes all involve using petroleum-derived chemicals or petrochemicals as a starting material. Because of the reliance of these processes on non-renewable petroleum, there is an urgent need to find processes for making chemicals from renewable sources such as biomass, as a way to reduce mankind's dependence on crude oil, to increase the use of renewable energy sources, and to reduce air and water pollution from the petrochemical industry.
Clearly, it would be advantageous to have an alternative and improved process for making caprolactam from a renewable biomass source, while providing higher yield and generating fewer by-products.