The Acrylonitrile Route to Adiponitrile: From ACN to Nylon 6,6 is a multi-stage industrial sequence in which propylene-derived acrylonitrile (ACN) is electrochemically coupled to adiponitrile (ADN), hydrogenated to hexamethylenediamine (HMD), and reacted with adipic acid to form the 1:1 salt that is polymerised to polyamide 6,6. Propylene ammoxidation is carried out in fluidised-bed reactors over bismuth molybdate or iron antimony oxide catalysts at 400–500 °C and near-atmospheric pressure; typical ACN selectivity is 80–85% at propylene conversion above 95%. The ACN route to ADN is practised on a production scale but is not the dominant global ADN source; it is electricity-intensive and remains relevant where low-cost power and ACN logistics are available.

What Limits Current Efficiency in the Electrochemical ACN Dimerization Cell?

ADN formation proceeds by cathodic reduction of two ACN molecules in an aqueous catholyte. The overall stoichiometry 2 CH₂=CH-CN + 2e⁻ + 2H⁺ → NC(CH₂)₄CN requires controlled proton availability; excess alkalinity favours Michael addition side products and oligomer formation. Public patent examples describe a divided cell with a cadmium or lead cathode, a dimensionally stable oxygen-evolving anode, and a perfluorosulfonic acid cation-exchange membrane separating the organic catholyte from dilute sulfuric acid anolyte. Catholyte temperature is maintained at 40–60 °C, and pH is kept in the range 7–9. The use of a quaternary ammonium salt as catholyte additive, typically 0.2–1.0 wt%, improves electrode wetting and suppresses hydrogen evolution.

Current density is the central process variable. Published examples report 1,000–5,000 A m⁻², with cell voltage 3–5 V and energy consumption values in the range 2,500–3,500 kWh t⁻¹ of ADN depending on membrane condition and electrode gap. At high current density, local pH rises near the cathode and propionitrile selectivity increases to 4–10%, while bis(cyanoethyl) ether and higher oligomers accumulate in the catholyte loop. A production cell therefore balances ACN conversion per pass, commonly 30–60%, against selectivity to ADN of 85–92%. These ranges are extracted from public literature; full licensor data for current cells are limited.

Separation of the catholyte is performed by phase separation and vacuum distillation. Unconverted ACN is recovered and returned to the electrochemical feed, while propionitrile is purged as a by-product. Oligomeric fouling on the cathode and membrane is controlled by catholyte filtration, periodic polarity reversal, or dilute acid washing; the exact protocol depends on the cell supplier. ADN is distilled to a purity of 99.0–99.9 wt% before hydrogenation, because nitrile by-products and quaternary ammonium residues poison the downstream hydrogenation catalyst.

Adiponitrile Hydrogenation and Hexamethylenediamine Purification

Adiponitrile hydrogenation is performed over Raney nickel or supported cobalt catalysts at high pressure. Published industrial descriptions indicate temperatures of 100–160 °C and hydrogen pressures of 20–35 MPa; ammonia is co-fed at a molar ratio to nitrile of 5–15 to suppress secondary amine formation. The reaction is carried out in fixed-bed trickle reactors or slurry reactors with external catalyst separation. ADN conversion exceeds 99% and selectivity to HMD is controlled by the ammonia partial pressure and catalyst activation state. The main intermediates are 6-aminocapronitrile and the cyclic by-product hexamethyleneimine, both of which must be removed to avoid colour and end-group defects in the final polyamide.

After depressurisation and ammonia recovery, the hydrogenation product is distilled under vacuum. HMD is isolated as a colourless liquid with a specification of 99.6–99.9 wt%; residual water, 6-aminocapronitrile, and hexamethyleneimine are closely monitored because even trace secondary amines reduce nylon 6,6 pH stability and thermal stability. The purified HMD is stored under an inert gas and kept below 50 °C to limit CO₂ absorption, which would form carbamate species and shift the nylon salt stoichiometry.

When the ACN Route Is Operated in Parallel with Butadiene Hydrocyanation

Within an integrated site, the ACN route is compared with butadiene hydrocyanation for ADN capacity. Butadiene hydrocyanation uses HCN and a homogeneous nickel catalyst in a two-stage process and is generally regarded as more atom-economical for large-scale ADN; the ACN route avoids direct HCN addition in the ADN step but consumes two moles of ACN per mole ADN and significant electricity. Full energy comparisons between the two routes are constrained by limited licensor data. The ACN route has a smaller installed base and is more sensitive to power-price volatility; however, it can be embedded in an ACN-producing petrochemical complex to avoid external ADN supply. Published data for specific electrochemical cell configurations are limited, and site-specific electricity, steam, and nitrogen logistics determine the final cost ranking.

Controlling Nylon 6,6 Salt Stoichiometry and Polycondensation

Nylon 6,6 is produced by reacting HMD with adipic acid in water to form hexamethylenediammonium adipate, commonly known as AH salt. The salt is controlled at a 1.000 ± 0.005 molar ratio of diamine to diacid; a 50 wt% aqueous solution typically shows pH 7.6–8.2 when measured at 25 °C. Off-ratio salt creates end-group imbalance. Excess diamine raises reactive amine ends and can cause thermal crosslinking, gel particles, and yellowing; excess adipic acid lowers molecular weight and increases carboxylic acid ends. The salt solution is concentrated to 50–70 wt% under an oxygen-poor atmosphere to prevent oxidative colour formation.

Polycondensation uses a two-stage autoclave or continuous reactor train. The salt solution is heated to 220–280 °C while steam is vented; after reaching atmospheric pressure, vacuum finishing is applied at an absolute pressure of 1–10 kPa. Molecular weight is tracked by viscosity number under ISO 307:2019 or by relative viscosity under ASTM D789-18; injection-moulding grades typically show viscosity numbers in the range 140–180 cm³ g⁻¹. Antioxidants, heat stabilisers, or TiO₂ delustrant are added during or after the finishing stage. Residual moisture must be below 0.2 wt% before melt processing; at relative humidity above 60%, pre-drying is mandatory.

The following analytical methods are applied to ACN-route nylon 6,6 to verify that the upstream ACN-to-ADN process has not introduced nitrogen-containing spoilers or end-group anomalies.

Processing of ACN-route nylon 6,6 on injection-moulding lines requires barrel temperatures of 270–300 °C, mould temperatures of 60–100 °C, and screw residence times below 8–10 min. The polymer is dried to 0.08–0.15 wt% moisture before feeding; melt temperature and residence time are the main variables because thermal degradation above 290 °C leads to chain branching, gel formation, and loss of tensile elongation measured under ISO 527-2:2012. Molded parts are tested for tensile stress at yield, density, and moisture content; these values are used as lot-release criteria. Avoid combination with boiling water or strong acids during service because polyamide 6,6 hydrolyses at elevated temperature, and the hydrolysis rate increases with decreasing pH under closed system conditions.