Roughly 90% of the world’s cyclohexane production feeds a single supply chain: the manufacture of nylon.1Journal of Catalysis. Catalytic conversions in green aqueous media: Highly efficient biphasic hydrogenation of benzene to cyclohexane catalyzed by Rh/TPPTS complexes Millions of tonnes of benzene are converted into cyclohexane each year, and from there the molecule is funneled through a handful of chemical steps that ultimately yield the polyamide fibers and plastics found in everything from car parts to carpet. But that dominant role tends to overshadow a set of smaller, equally interesting industrial uses that stretch from rubber manufacturing to atmospheric research.
From Benzene to Cyclohexane
Cyclohexane does not come from crude oil in ready-to-use form. Almost all of it is produced by hydrogenating benzene, a reaction that adds three molecules of hydrogen gas across benzene’s ring to give a fully saturated six-carbon cycle. The process runs over metal catalysts at elevated temperatures and pressures, and it has been refined over decades to reach very high yields. Because cyclohexane and benzene have nearly identical boiling points, any unreacted benzene left behind creates an azeotrope that is notoriously difficult to separate by simple distillation. Specialized techniques like extractive distillation with deep eutectic solvents have been developed specifically to tackle this purification bottleneck.2Processes. Separation of Benzene-Cyclohexane Azeotropes Via Extractive Distillation Using Deep Eutectic Solvents as Entrainers
Getting the purity right matters because the downstream chemistry is sensitive to trace contaminants. High-purity cyclohexane is the starting gun for the entire nylon value chain, so refineries and chemical plants invest heavily in this separation step before the molecule moves on to oxidation.
The Oxidation Step That Makes Everything Possible
The first major transformation cyclohexane undergoes on its way to nylon is oxidation to a mixture called KA oil, shorthand for the ketone-alcohol pair of cyclohexanone and cyclohexanol. This is one of the largest-volume catalytic oxidation processes in the chemical industry, and it is also one of the trickiest. Industrial plants typically run the reaction at above 150 °C and above 10 atmospheres of pressure, using molecular oxygen and a homogeneous catalyst. Even under these harsh conditions, only about 4–6% of the cyclohexane in the reactor actually converts per pass, with selectivity toward KA oil hovering around 70–85%.3Open Chemistry. Ionic liquids modified cobalt/ZSM-5 as a highly efficient catalyst for enhancing the selectivity towards KA oil in the aerobic oxidation of cyclohexane
Those numbers sound terrible compared to most textbook reactions, and in a sense they are. The reason plants keep conversion so low is to prevent over-oxidation: push the reaction too hard and the desired cyclohexanone and cyclohexanol break down into a soup of acids, esters, and other byproducts that are expensive to remove. So the industry accepts low single-pass conversion and recycles the unconverted cyclohexane back into the reactor, running the loop continuously. A great deal of catalysis research still focuses on improving this step, because even a small bump in selectivity at slightly higher conversion could save enormous amounts of energy and feedstock across the billions of kilograms produced annually.4ChemCatChem. Fe─Co Synergism in the Heterogeneous Catalytic Conversion of Cyclohexane to KA Oil
The Adipic Acid Route to Nylon 6,6
Once KA oil has been produced, it splits into two major pathways depending on which type of nylon the plant is making. One of the most important leads to adipic acid, a six-carbon dicarboxylic acid that is one of the two building blocks of nylon 6,6 (the other is hexamethylenediamine). Adipic acid is made by further oxidizing KA oil, typically with nitric acid. The cyclohexanone and cyclohexanol in the KA oil both convert to adipic acid in this step, which is why the industry does not bother separating the two components beforehand.
Nylon 6,6 is an important thermoplastic that shows up across a wide range of everyday products. Automotive manufacturers use it for under-the-hood components, connectors, and structural brackets because it holds up well under heat and mechanical stress. The electronics industry relies on it for housings and insulating parts. And the textile sector turns it into fibers for everything from activewear to industrial conveyor belts.5Industrial & Engineering Chemistry Research. Assessment of Nylon-66 Depolymerization for Circular Economy: Kinetic Modeling, Purification, and Sustainable Design When you trace any of those products back to their chemical origin, you eventually arrive at a tank of cyclohexane being bubbled with air.
The Caprolactam Route to Nylon 6
The other major branch from KA oil leads to caprolactam, the ring-shaped monomer that opens up and polymerizes into nylon 6. The chemistry here is a bit different. First, cyclohexanone from the KA oil is reacted with hydroxylamine to form cyclohexanone oxime. That oxime then undergoes a reaction called the Beckmann rearrangement, in which the ring expands from six atoms to seven and a nitrogen atom is inserted, producing epsilon-caprolactam.6PubMed. Hydrogen bond driven chemical reactions: Beckmann rearrangement of cyclohexanone oxime into epsilon-caprolactam in supercritical water
Industrially, the Beckmann rearrangement is usually carried out using concentrated sulfuric acid or oleum as a catalyst, but this generates large amounts of ammonium sulfate as a byproduct, which creates disposal headaches. Researchers have been exploring solid-acid catalysts that can run the rearrangement at temperatures as low as 130 °C, potentially cutting out the acid waste stream.7ACS Catalysis. Understanding the Role of Molecular Diffusion and Catalytic Selectivity in Liquid-Phase Beckmann Rearrangement Others have tested entirely different reaction media, including trifluoroacetic acid in acetonitrile, reporting high yields and selectivity for caprolactam.8PubMed Central. A New Sustainable Multistep Catalytic Process from Benzene to Caprolactam: Amination, Hydroximation and Beckmann Rearrangement Promoted and Catalyzed by Trifluoroacetic Acid
Caprolactam then undergoes ring-opening polymerization: the seven-membered ring cracks open and the molecules link end to end into long chains of nylon 6. This polymer is widely used in carpet fibers, packaging films, and engineering plastics. Together with nylon 6,6, these two polyamides account for about 90% of all polyamide production worldwide, and cyclohexane is the common ancestor of both.1Journal of Catalysis. Catalytic conversions in green aqueous media: Highly efficient biphasic hydrogenation of benzene to cyclohexane catalyzed by Rh/TPPTS complexes
Cyclohexane as a Polymerization Solvent
Outside the nylon supply chain, cyclohexane plays a quieter but significant role as a solvent, particularly in the production of synthetic rubbers and specialty polymers. Its appeal comes from being nonpolar, relatively inert under the conditions used for many catalytic polymerizations, and easy to remove afterward because of its moderate boiling point (about 81 °C). In the manufacture of polybutadiene, a key synthetic rubber used in tires and impact-resistant plastics, cyclohexane is not just a passive bystander. Studies of neodymium-based catalysts for the polymerization of 1,3-butadiene have shown that the presence of cyclohexane in the reaction medium has a marked effect on the molecular weight of the resulting polybutadiene.9European Polymer Journal. Solvent effect in cis-1,4 polymerization of 1,3-butadiene by a catalyst based on neodymium The solvent influences how the growing polymer chains fold and interact with the catalyst, which in turn controls chain length and the physical properties of the final rubber.
Cyclohexane also shows up as a solvent and reaction medium in adhesive formulations, certain paint strippers, and some specialty coating processes. In laboratory settings, it is a common nonpolar solvent for recrystallizations and extractions. These uses collectively represent a much smaller slice of total consumption than nylon feedstock, but they illustrate why cyclohexane remains a workhorse chemical rather than a one-trick molecule.
Why the Low Conversion Rate Matters More Than You’d Think
The 4–6% single-pass conversion during cyclohexane oxidation mentioned earlier is worth a closer look because it shapes the economics and environmental footprint of the entire nylon chain. A plant processing cyclohexane on an industrial scale has to heat and compress enormous volumes of material, only a small fraction of which reacts on each pass. The unconverted cyclohexane must be cooled, separated from the KA oil, and pumped back to the front of the reactor. That recycling loop requires energy, heat exchangers, distillation columns, and compressors, all of which add cost and carbon emissions.
This is why so much catalysis research is aimed at nudging conversion upward without sacrificing selectivity.3Open Chemistry. Ionic liquids modified cobalt/ZSM-5 as a highly efficient catalyst for enhancing the selectivity towards KA oil in the aerobic oxidation of cyclohexane Novel catalyst systems using ionic-liquid-modified zeolites, bimetallic iron-cobalt formulations, and other designs aim to get higher selectivity toward KA oil at slightly elevated conversion levels.4ChemCatChem. Fe─Co Synergism in the Heterogeneous Catalytic Conversion of Cyclohexane to KA Oil Even moving from 5% to 8% conversion at the same selectivity would mean dramatically less unreacted cyclohexane cycling through the plant, translating into meaningful savings in energy, equipment wear, and greenhouse gas output. For a chemical produced on the scale of millions of tonnes per year, small percentage gains ripple out into large real-world effects.
The Push Toward Bio-Based Alternatives
The overwhelming reliance on petroleum benzene as the starting material for cyclohexane, and therefore for nylon, has prompted researchers to explore whether the supply chain could be partly or fully decoupled from fossil fuels. Much of this work focuses on making adipic acid directly from renewable carbon sources, bypassing cyclohexane entirely. Biomass-derived platform molecules such as glucose, 5-hydroxymethylfurfural, and gamma-valerolactone have all been studied as potential feedstocks for adipic acid synthesis, using combinations of biocatalysis and chemical catalysis.10PubMed. Sustainable Routes for the Synthesis of Renewable Adipic Acid from Biomass Derivatives
Biotechnological approaches are also under investigation. Engineered microorganisms can, in principle, convert sugars or lignin-derived aromatics into adipic acid or its direct precursors. These bio-based routes are still largely at the laboratory or pilot scale, and none has yet matched the cost or throughput of the established petroleum-based process.11PubMed. Toward biotechnological production of adipic acid and precursors from biorenewables But the interest is genuine and growing, driven by both carbon-reduction targets and the price volatility of benzene. If any of these routes reach commercial viability, they would not necessarily eliminate cyclohexane from the nylon supply chain overnight, but they could chip away at its dominance over time, particularly for the adipic acid branch.
Caprolactam is harder to replace biologically because its synthesis involves the ring-expansion step of the Beckmann rearrangement, which is not something biological pathways do naturally. Some groups have explored fermentation routes to caprolactam precursors, but the gap between lab yields and industrial requirements remains wide. For now, cyclohexane’s position as the starting material for nylon 6 looks more secure than its role in nylon 6,6.
Cyclohexane in the Atmosphere
Not all cyclohexane ends up inside a reactor. Fugitive emissions from refineries, chemical plants, and solvent applications release the molecule into the atmosphere, where it reacts with hydroxyl radicals. Atmospheric chemists have studied these reactions in detail because cyclohexane serves as a useful model compound for understanding how larger cyclic hydrocarbons break down in polluted air. When OH radicals attack cyclohexane in the presence of nitrogen oxides, the main products are cyclohexanone and cyclohexyl nitrate, with measured formation yields of roughly 32% and 17%, respectively.12The Journal of Physical Chemistry A. Products of the Gas-Phase Reaction of OH Radicals with Cyclohexane: Reactions of the Cyclohexoxy Radical
The remaining products come from more exotic pathways: the intermediate cyclohexoxy radical can either break its ring open or rearrange internally, producing bifunctional compounds like 6-oxohexyl nitrate and hydroxylated dialdehydes. These secondary products can go on to form aerosol particles, contributing to fine particulate matter in urban and industrial air. The atmospheric fate of cyclohexane is a niche topic, but it connects the molecule’s industrial life to broader air-quality concerns, particularly in regions with dense petrochemical infrastructure.
How Cyclohexane Behaves at the Molecular Level
Cyclohexane’s usefulness as both a chemical feedstock and a solvent traces partly to its molecular shape. The six-carbon ring does not sit flat; it puckers into a three-dimensional arrangement known as the chair conformation, which is the most thermodynamically stable form. A recent large-scale computational analysis of ring structures found in small-molecule ligands across protein crystal structures reported that about 78% of cyclohexane rings sit in the chair conformation, with the rest adopting higher-energy shapes like the boat (about 7%) or twist-boat and half-chair (about 14%).13Journal of Cheminformatics. Analysis of cyclohexane, cyclopentane, and benzene conformations in ligands for PDB X-ray structures using the Hill-Reilly approach
In the liquid and gas phases relevant to industrial chemistry, cyclohexane overwhelmingly adopts the chair form, which places all substituents in a staggered arrangement and minimizes internal strain. This stability is part of why cyclohexane is relatively inert under mild conditions and only reacts when you push it with high temperatures, pressures, or aggressive catalysts. It also explains why the oxidation step described earlier is so challenging: you are trying to pry open a comfortable, low-energy molecule and stick an oxygen atom on it, and the molecule resists.
The chair conformation has practical consequences beyond reactivity. When cyclohexane derivatives are built into drug molecules or agrochemicals, the three-dimensional shape of the ring affects how the compound fits into a protein binding pocket or crosses a cell membrane. Medicinal chemists sometimes prefer cyclohexane-based scaffolds over flat aromatic rings precisely because the puckered ring fills three-dimensional space more effectively, improving binding selectivity. In that sense, the same molecular geometry that makes cyclohexane a stubborn partner in industrial oxidation makes it a versatile building block in pharmaceutical design.