The vapour-phase dehydrogenation of cyclohexanol to cyclohexanone at temperatures below 300 °C is an equilibrium-limited endothermic process with stoichiometry C₆H₁₁OH ⇌ C₆H₁₀O + H₂ and a standard heat of reaction of approximately +64 kJ mol⁻¹. In once-through fixed-bed operation the equilibrium conversion at 250–270 °C under atmospheric total pressure is not sufficient to justify single-pass reactor designs, and commercial processes therefore operate with per-pass conversions in the 40–60 mol% range while recycling unreacted cyclohexanol after separation. The selective production of cyclohexanone below 300 °C depends on suppressing two competing reaction families: dehydration to cyclohexene and subsequent aromatization, and acid- or base-catalysed aldol condensation of the ketone product. Copper-based catalysts are preferred for this temperature window because metallic copper selectively abstracts the alcohol hydrogen without cleaving the carbon–oxygen bond; however, the support must be formulated to minimize strong acid sites. Industrial fixed-bed reactor data from multi-tubular units with tube inner diameters of 25–40 mm and catalyst pellet diameters of 1.7–3.2 mm show that radial temperature gradients of 5–15 °C can develop across the bed, and that these gradients become process-limiting when the shell-side heat transfer fluid temperature approaches 290 °C. The entire reaction section is therefore designed around a narrow processing window in which the endothermic temperature drop is compensated by heat input without allowing local wall temperatures to exceed 300 °C.

Thermodynamic Constraints Below 300 Degrees Celsius Favor Partial Dehydrogenation

The equilibrium for cyclohexanol dehydrogenation shifts toward products as temperature rises because the entropy increase associated with hydrogen release becomes progressively more favourable. At 220 °C the equilibrium conversion under total pressure of 101.3 kPa and in the absence of hydrogen removal is low, typically below 25 mol%, which forces the use of hydrogen stripping or vacuum condensation to achieve industrially useful yields. At 260–280 °C the same parameter rises to approximately 45–60 mol% in once-through operation, while at 300 °C it can exceed 70 mol%; however, the selectivity penalty above 290 °C makes operation above 300 °C unattractive for high-purity cyclohexanone. The standard heat of reaction of approximately +64 kJ mol⁻¹ means that an adiabatic temperature drop of 20–40 °C is commonly observed in industrial beds operating at a liquid hourly space velocity of 0.3–1.0 h⁻¹. Because the equilibrium constant is sensitive to hydrogen partial pressure, the feed is often diluted with inert gas or the reactor is coupled to a hydrogen-selective membrane or interstage condensation to pull the reaction toward completion. Published data for membrane-assisted cyclohexanol dehydrogenation below 300 °C is limited, but pilot-scale fixed-bed studies using palladium-based membranes have reported improved once-through conversion of 10–20 percentage points relative to non-membrane operation at the same temperature. The thermodynamic trade-off is therefore not simply a matter of maximizing temperature; it is a constrained optimisation between equilibrium conversion and the kinetic selectivity of the catalyst under conditions where the product ketone remains stable.

Cyclohexanol can undergo acid-catalysed dehydration to cyclohexene at temperatures as low as 180 °C on alumina-rich surfaces, and once cyclohexene is formed it can dehydrogenate or polymerase to coke precursors that block micropores and reduce copper accessibility. The selectivity toward cyclohexanone in commercial copper-based systems is therefore strongly correlated with the density and strength of Brønsted acid sites, which are commonly measured by temperature-programmed desorption of ammonia. For a Cu/ZnO/Al₂O₃ catalyst with 5–10 wt% alumina, the ammonia uptake associated with weak acid sites is usually 0.1–0.4 mmol g⁻¹, and alkali promotion with 0.1–1.0 wt% sodium or potassium reduces this value significantly. When the same catalyst is operated above 310 °C, selectivity to cyclohexanone can fall below 96 mol% because dehydration and cyclohexene aromatization become kinetically competitive. Aldol condensation of cyclohexanone is a further side reaction that increases with base sites and with high ketone partial pressure; it forms higher-boiling dimers and trimers that are difficult to remove from the recycle loop and contribute to fouling of the separation train. Published data for the specific condensation rate in industrial cyclohexanol dehydrogenation reactors is limited, but the observed formation of heavy ends in commercial recycle loops is consistent with a reaction order above unity in cyclohexanone concentration. Process design therefore imposes a maximum ketone concentration at the reactor outlet, typically 30–40 wt%, to avoid excessive condensation while maintaining downstream recovery economics.

What Determines the Cu/Zn/Al₂O₃ Selectivity Window?

The selectivity window of Cu/ZnO/Al₂O₃ catalysts below 300 °C is determined primarily by the oxidation state and dispersion of copper, the acid–base character of the support, and the ratio of hydrogen to cyclohexanol in the feed. Copper oxide is reduced to metallic copper in dilute hydrogen at 200–240 °C, and the resulting copper crystallite size should remain below 10–15 nm for maximum alcohol adsorption capacity and ketone desorption. In a typical formulation the copper loading is 30–45 wt%, zinc oxide 40–60 wt%, and alumina 5–10 wt%, with alkali promoters added at 0.1–1.0 wt% to neutralise acid sites. The ZnO component improves copper dispersion and acts as a structural promoter, while alumina provides mechanical strength but cannot be increased beyond about 10 wt% without generating dehydration activity. The hydrogen-to-cyclohexanol molar ratio is normally maintained between 2:1 and 6:1 to suppress catalyst coking and to keep the copper surface in the reduced state, but excessive hydrogen suppresses the forward reaction and lowers once-through conversion. Kinetics reported for copper-based dehydrogenation follow a Langmuir–Hinshelwood form in which molecular cyclohexanol adsorbs on a copper site, dehydrogenates stepwise to form surface cyclohexanone and hydrogen, and the product ketone desorbs in competition with hydrogen. Apparent activation energies for the rate-determining step fall between 70 kJ mol⁻¹ and 120 kJ mol⁻¹ in published kinetic studies, although the exact value depends on the support and the concentration of alkali promoter. The selectivity window therefore closes either when acid sites become too abundant, when copper crystallites sinter above 290 °C, or when the H₂/feed ratio drops below about 1:1 and surface fouling accelerates.

Fixed-bed multi-tubular reactors used for cyclohexanol dehydrogenation below 300 °C typically use tubes of 25–40 mm inner diameter, lengths of 3–6 m, and catalyst pellets of 1.7–3.2 mm diameter to balance pressure drop against intraparticle diffusion resistance. The catalyst bed is heated through the shell with hot oil or high-pressure steam at 240–260 °C, and the shell-side system is designed to limit the maximum tube wall temperature to 300 °C because local overheating causes copper sintering. A pressure drop of 0.1–0.5 bar across the bed is acceptable for low-pressure operation, but cracked or attrited catalyst fines can raise pressure drop beyond 0.7 bar and force early catalyst unloading. Liquid hourly space velocity is normally set at 0.3–1.0 h⁻¹ for a bed diluted with inert ceramic material at the inlet, and the inlet temperature is maintained at 220–240 °C to avoid thermal shock. The reactor effluent typically contains 20–35 wt% cyclohexanone, 25–40 wt% unreacted cyclohexanol, and hydrogen with small amounts of water, cyclohexene, and heavy ends. Production-scale observations on such reactors show that the axial temperature profile is not monotonic: the inlet portion of the bed cools due to endothermic reaction, while the lower section can increase in temperature if excessive hydrogenation of cyclohexanone back to cyclohexanol occurs through the reverse reaction. For this reason the control strategy uses multiple thermocouples placed along the tube length at intervals of 0.5–1.0 m to detect local excursions.

When Hydrogen Partial Pressure Drops Below 0.03 MPa During Vacuum Operation

Vacuum operation below 300 °C is used in some configurations to shift the equilibrium toward cyclohexanone by reducing the hydrogen partial pressure. When the hydrogen partial pressure falls below 0.03 MPa and the total pressure is maintained at 10–30 kPa absolute, the equilibrium conversion can increase by 15–30 percentage points compared with operation at 101.3 kPa, depending on temperature and feed composition. The mechanical design of vacuum fixed-bed reactors must account for lower gas velocity, reduced heat transfer, and higher pressure drop sensitivity; tube diameters are often reduced to 20–30 mm to maintain an acceptable Reynolds number and heat transfer coefficient. Under these conditions, the hydrogen-to-cyclohexanol ratio in the feed may be kept at 1:1 to 3:1, and the hydrogen partial pressure is controlled by interstage condensation or by a downstream vacuum compressor. However, if the hydrogen partial pressure is allowed to fall below 0.01 MPa for prolonged periods, the copper surface can become partially oxidised by trace water or oxygen, and carbon deposition can accelerate because the cleaning effect of hydrogen on adsorbed coke precursors is weakened. Published data for this specific low-hydrogen regime in commercial vacuum plants is limited, but pilot-scale tests using a fixed-bed reactor with 25 mm internal diameter tubes and Cu/ZnO/Al₂O₃ pellets have shown that selectivity to cyclohexanone remains above 98 mol% only when the hydrogen partial pressure is maintained above 0.02 MPa. The vacuum system must therefore be interlocked with the hydrogen feed ratio to prevent a runaway selectivity loss.

Catalyst Deactivation Mechanisms in Commercial Production

Catalyst deactivation in cyclohexanol dehydrogenation below 300 °C occurs mainly through copper sintering, acid-site coking, and trace contaminant poisoning. Copper crystallite growth from 5–10 nm to values above 30 nm reduces the number of metallic copper surface sites and becomes rapid above 290 °C, especially in zones with poor heat transfer or hot spots. Coking is promoted by residual acid sites on the support and by heavy aldol condensation products that undergo further dehydrogenation and cyclisation; the resulting carbonaceous deposits occupy the catalyst surface and raise pressure drop in fixed beds. Sulfur compounds are strong poisons even at 1 mg/kg or less in the feed, and chloride at 0.5 mg/kg can cause copper chloride migration and agglomeration. Feed specifications therefore require sulfur below 1 mg/kg, chloride below 0.5 mg/kg, and water below 0.1 wt% to maintain cycle life. The catalyst is typically reduced in situ with a mixture of 1–5 vol% hydrogen in nitrogen at 200–240 °C for 12–24 h, and after reduction the bed is passivated with carbon dioxide or dilute oxygen before unloading if the reactor must be opened. In commercial fixed-bed units, the end-of-run condition is usually defined by the outlet temperature required to maintain conversion, and when this temperature exceeds 290 °C or when pressure drop exceeds 0.7 bar, the charge is replaced. Experience from production-scale units shows that cycle lengths between 12 and 24 months are achievable only when the upstream cyclohexanol feed is distilled and filtered to remove particulate iron and high-boiling residues; otherwise pressure drop and sulfur contamination shorten the cycle substantially.

The separation of cyclohexanone from cyclohexanol downstream of the reactor is difficult because the normal boiling points differ by only about 5.5 °C, with cyclohexanone near 155.6 °C and cyclohexanol near 161.1 °C at 101.3 kPa. Vacuum distillation with structured packing and 40–60 theoretical stages is required to produce caprolactam-grade cyclohexanone with purity above 99.9 wt%, and the reflux ratio is typically maintained between 8:1 and 20:1. The close boiling points mean that small amounts of cyclohexanol in the recovered ketone are possible if the column pressure control drifts; a pressure increase from 10 kPa to 20 kPa absolute can shift the relative volatility enough to reduce separation efficiency materially. Water and cyclohexene are removed in a light-fraction column before the cyclohexanone column, and heavy aldol condensation products are drawn from the bottom and sent to a residue recovery system. Analytical monitoring of the refined cyclohexanone includes gas chromatography on a 30 m × 0.25 mm × 0.25 μm capillary column with flame ionisation detection for organic purity, Karl Fischer titration per ASTM E203 for water, density measurement per ASTM D4052, and colour assessment per ASTM D1209. The specification for nylon intermediates normally requires water below 0.05 wt%, acidity below 0.005 meq/g, and iron below 0.1 mg/kg, because these impurities disturb the downstream oxidation or oximation chemistry.

Managing the Acid-Site Density Without Sacrificing Copper Dispersion

The central formulation problem in copper-based dehydrogenation catalysts below 300 °C is that alumina and other refractory oxides provide mechanical strength and stabilise copper dispersion but also introduce acid sites that catalyse cyclohexanol dehydration and cyclohexanone condensation. Catalyst manufacturers manage this conflict by using zinc oxide as the primary dispersing oxide and by adding alkali or alkaline-earth promoters at controlled contents. The addition of 0.1–1.0 wt% sodium or potassium selectively neutralises strong Brønsted acid sites while leaving the copper surface area almost unchanged if the promoter is added after precipitation or co-precipitated in low concentration. In one comparative set of fixed-bed tests with a 25 mm internal diameter reactor and Cu/ZnO/Al₂O₃ pellets of 3.2 mm diameter, the selectivity to cyclohexanone at 260 °C and a H₂/feed ratio of 4:1 was reported to improve from 94 mol% to 98 mol% when the alkali level was raised from 0.05 wt% to 0.3 wt%, while conversion remained within 2 percentage points. Higher alkali loadings above 1.0 wt% can lead to excessive aldol condensation because base sites are introduced, and the product heavies then rise. The control of acid-site density is therefore not a simple maximisation of one promoter but a balance between dehydration suppression at low alkali content and condensation suppression at high alkali content. Temperature-programmed desorption of ammonia and carbon dioxide is used to monitor the acid and base site populations, and the acceptable ranges are typically 0.1–0.3 mmol g⁻¹ for weak acid sites and 0.05–0.15 mmol g⁻¹ for base sites.

Across the Adiabatic Reheat Cycle in Multi-Stage Beds

Adiabatic reactors are occasionally used for cyclohexanol dehydrogenation when the plant scale favours a low-pressure drop and simple mechanical design, but the endothermic heat of reaction of approximately +64 kJ mol⁻¹ produces a measurable axial temperature drop. In a commercial adiabatic bed of 3–5 m length operating at an inlet temperature of 250 °C and a liquid hourly space velocity of 0.5 h⁻¹, the outlet temperature can be 20–40 °C lower than the inlet, which reduces the equilibrium driving force in the lower portion of the bed. To compensate, the gas is reheated between stages with fired heaters or shell-and-tube heat exchangers using hot oil at 280–300 °C. The reheat creates a cyclic axial temperature profile in which the first section of each stage is hotter than the catalyst bed average, and if the reheat temperature overshoots above 300 °C, the leading edge of the next bed can experience copper sintering and dehydration activity. Hot-spot reheat cycles therefore require interstage temperature control to ±5 °C around the set point, with cascade loops that respond to the actual outlet conversion measured by on-line gas chromatography. Published data for specific adiabatic reheat cycles in cyclohexanol dehydrogenation is limited, but fixed-bed reactor models with two-dimensional heat transfer show that radial temperature differences in adiabatic vessels are smaller than in multi-tubular units, while the axial variation is larger and more sensitive to feed composition.

The feed to the dehydrogenation reactor below 300 °C should be pretreated by distillation and filtration to remove high-boiling residues, iron particulates, and dissolved salts that could poison or blind the catalyst. Water in the feed has a dual effect: small amounts below 0.1 wt% are tolerated and may help suppress coking on some copper surfaces, but higher water levels increase dehydration activity and accelerate hydrolysis of the alumina support. The recycle hydrogen stream must be monitored for methane, carbon monoxide, and carbon dioxide because the build-up of inert gases reduces the hydrogen partial pressure without improving equilibrium as much as hydrogen removal. Carbon monoxide is a temporary poison that adsorbs on copper sites and can reduce activity by 5–10% at 1000 mg/kg in the gas phase, although the effect is largely reversible if the concentration is lowered. Sulfur and chloride limits have been established from production-scale experience as <1 mg/kg and <0.5 mg/kg, respectively, and are enforced by feed analyzers and guard beds containing zinc oxide or copper oxide. The reactor start-up after catalyst reduction requires slow heating at 10–20 °C h⁻¹ from ambient to 220 °C under nitrogen, followed by introduction of cyclohexanol at 50% of design feed rate for the first 24 h. The operational boundaries and numerical ranges cited across these scenarios derive from fixed-bed pilot studies, industrial process design documentation, and component test standards such as ASTM E203 and ASTM D4052; published data for some specific commercial configurations is limited and has been noted accordingly.