Acrylonitrile (ACN, CAS 107-13-1): Properties, Grades and Industrial Uses

Acrylonitrile (ACN, CAS 107-13-1) is a light yellow, sharp-odour unsaturated nitrile with molecular formula CH₂=CH-C≡N and relative molecular mass 53.06 g/mol. At 101.3 kPa, the boiling point is 77.3 °C, the freezing point is -83.5 °C, and the liquid density at 20 °C is 0.806 g/cm³. Vapor pressure at 20 °C is 11.5 kPa, and water solubility is limited to 7.35 g/100 mL. The closed-cup flash point is -1 °C; flammable limits in air range from 3.0 vol% to 17.0 vol%, and the autoignition temperature is reported at 481 °C. Occupational exposure is controlled under 29 CFR 1910.1000 and NIOSH REL at 2 ppm as an 8-h time-weighted average.

Industrial production is dominated by propylene ammoxidation over bismuth molybdate or antimony-iron mixed oxide catalysts at 400–500 °C and 50–200 kPa in fluidized-bed reactors. The net reaction CH₂=CHCH₃ + NH₃ + 1.5 O₂ → CH₂=CHCN + 3 H₂O produces acetonitrile and hydrogen cyanide as by-products. Because the boiling-point difference between ACN and acetonitrile is only 4.3 °C, separation requires extractive distillation with water or a polar solvent after ammonia removal and HCN recovery. Once-through propylene conversion is commonly in the 70–80% range with ACN selectivity of 80–85% depending on catalyst age and reactor temperature profile.

What Distinguishes Polymer-Grade Acrylonitrile from Technical-Grade Material?

Commercial ACN is supplied primarily as technical grade and polymer grade, with the main differences located in acetonitrile, water, hydrogen cyanide, acetone, and nonvolatile residue. Polymer-grade material is typically specified at 99.8 wt% minimum purity, acetonitrile below 100 ppm, water below 0.1 wt%, hydrogen cyanide below 5 ppm, and acetone below 50 ppm. Technical-grade material may permit acetonitrile up to 500 ppm, water up to 0.5 wt%, and hydrogen cyanide up to 10 ppm. The inhibitor package is commonly 35–50 ppm monomethyl ether hydroquinone (MEHQ), with dissolved oxygen acting as a co-stabilizer. Distillation range is measured by ASTM D1078, density by ASTM D4052, water by ASTM D1364, and colour by ASTM D1209. Trace acetonitrile and acetone are determined by gas chromatography with flame ionization detection; hydrogen cyanide is determined by titration or ion-selective electrode after alkaline trapping.

Supplier specifications are not identical because impurity partitioning depends on ammoxidation catalyst age, quench-system pH, and distillation column configuration. Published data for specific supplier limits outside contract specifications are limited.

In bulk storage, the oxygen-dependent inhibition mechanism creates a process conflict between inert blanketing and polymerization control. MEHQ does not function effectively in the absence of dissolved oxygen; complete nitrogen blanketing can therefore permit polymer formation at the vapour-liquid interface even when MEHQ is present at 35–50 ppm. Production-scale vessels are typically fabricated from 304L or 316L stainless steel and exhibit preferential fouling at vent condenser inlets, level-instrument nozzles, and transfer-line low points when inhibitor depletion occurs. Storage at 5–15 °C is commonly specified to reduce inhibitor consumption. The vapour space must be maintained outside the 3.0–17.0 vol% flammable range. If inerting is used, oxygen concentration must be controlled above the minimum required for MEHQ function but below the limiting oxygen concentration, a balance maintained by continuous oxygen analysers and split-range inerting valves. Pressure/vacuum relief devices are sized to API 2000, and flame arrestors are installed in vent lines. Published data for a universal oxygen setpoint are limited because inhibitor consumption varies with trace metal content, water content, and storage temperature. The NFPA 704 rating for ACN is health 3, flammability 3, reactivity 2.

ACN is incompatible with strong oxidizers, strong bases, concentrated acids, and initiators such as peroxides and azo compounds. Thermal decomposition products include hydrogen cyanide, carbon monoxide, and nitrogen oxides. Transfer pumps in production service are frequently sealless canned or magnetic-drive designs to reduce fugitive emissions and seal leakage. Condensers on storage tanks use chilled water at approximately 7 °C to recover monomer; persistent polymer scale at condenser inlets indicates local loss of dissolved oxygen rather than bulk inhibitor exhaustion.

When Acrylonitrile Is Polymerized into Acrylic Fiber Precursor

Solution polymerization of acrylonitrile is used for polyacrylonitrile (PAN) fibre and carbon fibre precursor. The monomer is copolymerized with 2–5 mol% methyl acrylate and itaconic acid to control solubility, cyclization, and oxidation behaviour. In dimethyl sulfoxide or sodium thiocyanate solutions at 50–70 °C, azobisisobutyronitrile initiator at 0.1–0.5 wt% based on monomer produces a dope with 20–25 wt% solids and dynamic viscosity 30–100 Pa·s at 50 °C. Dope filtration through 5–10 µm sintered metal filters is required before spinning; gel particles above 5 µm cause filament breakage and spinneret plate fouling. Wet-spinning through spinnerets of 0.05–0.12 mm hole diameter into 20–50 wt% sodium thiocyanate or dimethyl sulfoxide/water coagulation baths forms filaments that are drawn at 3–10× ratios in hot water at 95–98 °C.

For carbon fibre precursor, total ash must be held below 10 ppm and sodium below 1 ppm to avoid defects after oxidation stabilization at 200–300 °C and carbonization at 1000–1500 °C under nitrogen. Molecular weight is usually 80,000–170,000 g/mol with polydispersity 1.8–3.0. Spinneret plate fouling and gel-particle breakage are observed when inhibitor carryover in the monomer feed exceeds 1 ppm. Residual unreacted ACN in spinning dope must be stripped below 1 ppm to reduce workplace exposure in the spinning line.

For ABS resin manufacture, emulsion grafting of styrene and acrylonitrile onto polybutadiene latex is conducted with 20–30 wt% ACN in the styrene-acrylonitrile copolymer phase. Rubber particle size is controlled at 0.1–0.5 µm before grafting; graft efficiency and phase inversion govern impact strength and gloss. Residual ACN in the final SAN matrix is stripped to 10 ppm or lower. Food-contact grades are evaluated under 21 CFR 180.22 with a migration limit of 0.05 mg/kg for residual acrylonitrile. Compounding on a 40:1 L/D co-rotating twin-screw extruder with barrel vacuum -0.08 MPa is used to remove unreacted monomer; hot runner temperatures above 260 °C accelerate plate-out on mould surfaces. Melt flow rate of injection-moulding ABS is measured by ISO 1133-1:2022 at 220 °C and 10 kg; tensile properties are tested by ASTM D638.

Adiponitrile Routes from Acrylonitrile: Catalyst Selectivity and By-Product Control

Electrochemical hydrodimerization of ACN to adiponitrile is performed in an aqueous catholyte containing quaternary ammonium salts at pH 7–10. Current densities of 1–3 kA/m² and ACN concentrations of 10–20 wt% are typical. Selectivity to adiponitrile can reach 85–90%; by-products include propionitrile and bis(2-cyanoethyl) ether. Electrode materials are selected to suppress hydrogen evolution and maintain cell voltage in the 4–6 V range. Modern nylon 6,6 capacity relies largely on butadiene hydrocyanation because of lower energy requirements; ACN-based electrodimerization is retained only where electrochemical power costs are favourable. Published data for specific cell configurations are limited, particularly for membrane separator stability and long-term selectivity drift.

Why Nitrile Rubber Specifications Require Low Acetonitrile and Hydrogen Cyanide Levels

Nitrile rubber (NBR) is produced by emulsion copolymerization of butadiene and ACN. Bound ACN content ranges from 18 wt% to 50 wt%. Low-ACN grades at 18–20 wt% are specified for low-temperature flexibility; medium grades at 28–33 wt% balance oil resistance and compression set; high-ACN grades at 45–50 wt% are selected for aromatic hydrocarbon service. Cold-polymerized NBR is initiated by a redox system at 5–30 °C. Mooney viscosity ML 1+4 at 100 °C ranges from 30 to 100 MU. Acetonitrile above 100 ppm in the ACN feed retards radical entry and broadens molecular weight distribution, causing coagulum formation in stirred-tank reactors of 10–40 m³. The polymerization is stopped at 60–70% conversion with a shortstop addition; crumb drying is carried out at 80–100 °C. Vulcanization behaviour is assessed by ASTM D2084 or ISO 6502; tensile properties by ASTM D412; oil immersion by ASTM D471. Hydrogen cyanide in monomer feed must remain below 5 ppm because it can complex with redox initiator metals and disrupt latex stability.

Acrylamide production from ACN via catalytic hydration uses copper-based heterogeneous catalysts or nitrile hydratase biocatalysts. Chemical hydration is operated at 70–100 °C; enzymatic hydration at 5–25 °C can exceed 99% conversion with 98% selectivity. Residual ACN is stripped to <1 ppm in the finished monomer. The resulting polyacrylamides for municipal water treatment and enhanced oil recovery are produced with molecular weights of 10–25×10⁶ g/mol and anionic charge densities of 10–50 mol%. Fixed-bed biocatalytic reactors operate at low ACN concentration and require tight temperature control to prevent inactivation of the nitrile hydratase enzyme. Published data for residual ACN in finished flocculants is limited.