Caprolactam: the cornerstone of the nylon world

Caprolactam is the core monomer of raw nylon 6 (polyamide 6). Nylon 6 occupies an important position in the fields of textile fibers (fishing nets, tire cords, sewing threads), engineering plastics (automotive parts, injection molded parts) and other fields due to its excellent strength-to-weight ratio, chemical stability, wear resistance and thermal stability [1][3]. Derivatives of Caprolactam are found everywhere, from the clothes on your body to the delicate components in your car’s engine compartment.

The pain point of traditional craftsmanship: the dilemma of ammonium sulfate

The traditional Caprolactam production route uses benzene or toluene as raw material and goes through three major steps: cyclohexanone synthesis, oximation, and Beckmann rearrangement. Among them, the Beckmann rearrangement relies on fuming sulfuric acid or concentrated sulfuric acid as the reaction medium, and the subsequent neutralization process will inevitably produce a large amount of ammonium sulfate by-product [3].

Although ammonium sulfate can be used as fertilizer, its value is much lower than Caprolactam, and its handling and storage bring additional environmental and economic burdens. Therefore, the “Caprolactam process without ammonium sulfate by-products” has been a dream of the chemical industry for many years [1].

Green breakthrough: TS?1 molecular sieve and ammoximation

Ammonia oximation is a key step in the green synthesis of Caprolactam. In traditional processes, the preparation of cyclohexanone oxime itself is accompanied by the generation of a large amount of ammonium sulfate. The emergence of TS-1 molecular sieve has completely changed this situation. The invention of titanium silica molecular sieve TS?1 is a key milestone in this green revolution. TS?1 introduces titanium into the molecular sieve framework of the MFI structure, uses hydrogen peroxide as the oxidant, and realizes the direct ammoximation reaction of cyclohexanone, ammonia and hydrogen peroxide under mild conditions (about 80°C) [1][2].

In terms of the reaction mechanism, the Ti active site of TS?1 first activates H?O? to generate Ti–OOH active species, and then oxidizes ammonia to hydroxylamine, which is then condensed with cyclohexanone to generate cyclohexanone oxime [2]. The only by-product of this process is water, completely avoiding the formation of ammonium sulfate.

Enichem took the lead in realizing the industrialization of this process and built a 12,000 tons/year demonstration unit in Porto Marghera [1]. The global production capacity of cyclohexanone oxime using this technology has exceeded 5 million tons/year [2].

Vapor-Phase Beckmann Rearrangement: Say Goodbye to Oleum

TS?1 solves the problem of salt by-products in the oximation process, while gas-phase Beckmann rearrangement challenges the rearrangement process itself [1][3].

Traditional liquid-phase rearrangement relies on fuming sulfuric acid, while the gas-phase process allows cyclohexanone oxime to pass through a solid acid catalyst bed above 300°C under normal pressure and be directly converted into Caprolactam [3].

The core of the catalyst is not the traditional acidic sites, but the silanol nests on the high-silica MFI molecular sieve (Silicalite-1, or S-1) [1][2]. This type of nearly neutral, weakly acidic site can efficiently catalyze rearrangement reactions without causing side reactions such as excessive cleavage and polymerization [2]. Alcoholic solvents such as methanol can selectively passivate the silanol groups on the outer surface, inhibit side reactions, and increase the selectivity to more than 92% [1].

Sumitomo Chemical took the lead in completing the industrialization of this process, and achieved excellent results in both fixed-bed and fluidized-bed reactors [1][3].

Catalyst testing and evaluation

The catalyst performance evaluation of Caprolactam mainly focuses on two major indicators: conversion rate and selectivity. The Ti content and coordination state of TS?1 affect the activation ability of H?O?; the types of silanol groups of S?1 molecular sieve (terminal silanol groups, ortho-position silanyl hydroxyl groups, and silanol hydroxyl nests) determine the difference in catalytic performance [2].

Infrared spectroscopy (FT?IR) can characterize the type of silanol, and spectroscopic techniques such as XAFS, UV?Vis, and Raman can monitor changes in the coordination environment of the Ti active center [2]. Catalyst deactivation is mainly caused by carbon deposition, loss of Ti species or dissolution of silicon skeleton, and the activity can usually be restored through roasting regeneration [1][2].

Notes and future prospects

Although the green process of Caprolactam has broad prospects, the following issues still need to be paid attention to: the dissolution of the skeleton silicon of TS?1 in alkaline ammonia solution may lead to irreversible deactivation [1]; H?O? is ineffectively decomposed under alkaline conditions and reduces the utilization rate [2]; the hydration of cyclohexene to prepare cyclohexanone is limited by thermodynamic equilibrium, and the single-pass conversion rate is only less than 10% [2].

Looking to the future, multifunctional catalyst design (such as hydration-dehydrogenation tandem catalysis) is expected to break through equilibrium limitations [2]; molecular sieve morphology control and selective exposure of crystal faces can improve the accessibility of active sites; artificial intelligence and machine learning-assisted catalyst screening will accelerate the development of new materials [2].

References

[1]Ichihashi H, Sato H. The development of new heterogeneous catalytic processes for the production of ε-caprolactam[J]. Applied Catalysis A: General, 2001, 221(1-2): 359-366. https://doi.org/10.1016/S0926-860X(01)00887-0

[2]Wang H, Qin M, Wu Q, et al. Zeolite Catalysts for Green Production of Caprolactam[J]. Industrial & Engineering Chemistry Research, 2022, 62(5): 2217-2224. https://doi.org/10.1021/acs.iecr.2c01693

[3]Dahlhoff G, Niederer J P M, Hoelderich W F. ε-Caprolactam: New by-product free synthesis routes[J]. Catalysis Reviews: Science and Engineering, 2001, 43(4): 381-441. https://doi.org/10.1081/CR-120001808