Among C3 olefin derivatives, acrylonitrile is a base monomer whose nitrile side group imparts resistance to polar solvents, controlled oxygen permeability, and high polarity in copolymers. How Acrylonitrile Is Made: The Sohio Ammoxidation Route from Propylene and Ammonia is the primary industrial process, accounting for nearly all global capacity. The molecule is a colorless, volatile liquid at ambient conditions: molar mass 53.06 g mol⁻¹, boiling point 77.3 °C at 101.3 kPa, flash point −1 °C closed cup, vapor pressure 11.3 kPa at 20 °C, and density 0.806 g cm⁻³ at 20 °C. The single-stage heterogeneously catalyzed route uses chemical-grade propylene, anhydrous ammonia, and air as feedstocks. Its dominance is rooted in raw material availability, lower feedstock cost, and continuous fluid-bed operation.

Why Did the Sohio Ammoxidation Route Replace Acetylene Hydrocyanation?

Before ammoxidation, acrylonitrile was produced through acetylene hydrocyanation over cuprous chloride in aqueous hydrochloric acid at 80–90 °C and roughly 1–2 bar(g). Acetylene feed required on-site generation or transport with acetone absorbent, and hydrogen cyanide handling raised safety complexity. The Sohio process, first commercialized by Standard Oil of Ohio in 1960, collapsed cyanide addition and dehydrogenation into one oxidation step over a bismuth molybdate catalyst. Process operating costs were lower because propylene from steam cracking was abundant and air substituted for pure oxygen. In modern practice, the Sohio route achieves propylene conversion of 97.5–98.5% at molar ammonia-to-propylene ratios of 1.05–1.20 and air-to-propylene ratios of 9.5–10.5. Selectivity to acrylonitrile is typically 78–82%.

The net vapor-phase chemistry is represented by CH₂=CHCH₃ + NH₃ + 1.5 O₂ → CH₂=CHCN + 3 H₂O. The reaction is highly exothermic, with a heat of reaction of approximately −515 kJ mol⁻¹ under standard conditions. Competing pathways include oxidative dehydrogenation of propylene to acrolein, ammonia oxidation to nitrogen and water, and deep oxidation of hydrocarbons to carbon monoxide and carbon dioxide. Hydrogen cyanide and acetonitrile are formed by parallel oxidation of ammonia/propylene fragments. High selectivity depends on lattice oxygen mobility in the catalyst, short vapor-phase residence time, and limiting non-selective gas-phase radical reactions. The dense fluid bed suppresses hot spots because catalyst particles act as a heat-transfer medium and because back-mixing distributes the reaction exotherm across the bed.

Fluid-Bed Bismuth Molybdate Catalysts and Heat-Flux Distribution

Commercial Sohio catalysts are multicomponent bismuth molybdates on silica carriers, often promoted with iron, cobalt, nickel, potassium, or antimony. The active phase operates in a redox cycle: surface oxygen abstracts hydrogen from propylene, ammonia is activated at molybdenum sites, and gas-phase oxygen replenishes lattice vacancies. Catalyst particle size typically spans 45–125 μm to maintain turbulent fluidization without excessive elutriation. Superficial gas velocity is normally 0.25–0.50 m s⁻¹, with dense-phase bed density around 600–800 kg m⁻³. Reactor temperature is maintained at 430–450 °C at 0.5–2.0 bar(g). Below 420 °C conversion falls because lattice oxygen mobility declines; above 470 °C carbon oxide selectivity increases and catalyst attrition accelerates. The internal temperature profile is controlled by vertical steam-generation coils immersed in the dense bed. Steam generated from the exotherm is typically medium-pressure utility steam used elsewhere in purification.

Reactor mechanical design for Sohio trains uses a refractory-lined carbon steel shell with internal cyclones arranged in the freeboard. The feed distributor plate is designed for a pressure drop of 3–5 kPa to ensure uniform gas distribution across the bed. Reactor effluent passes through two-stage cyclones with dip legs that return entrained catalyst to the dense bed. Catalyst attrition is a continuous operational boundary: excessive superficial velocity increases fines generation, while insufficient velocity causes defluidized zones and local overheating. Bed level, pressure differential, cyclone pressure drop, and steam drum level are monitored continuously. Production-scale failures typically present as afterburning in the freeboard when hydrocarbon-rich bubbles break through the bed, or as localized reduction of catalyst oxidation state that depresses acrylonitrile selectivity and raises acrolein yield.

Quench, Ammonia Neutralization, and Recovery of Aqueous Acrylonitrile

Reactor gas exits at approximately 430 °C and is quenched with water in a venturi and quench column to 80–90 °C. Unreacted ammonia is neutralized with sulfuric acid in the quench circuit, forming ammonium sulfate as a byproduct. The quench liquor contains acrylonitrile, acetonitrile, hydrogen cyanide, acrolein, and heavy organics. Insoluble carbon oxides and nitrogen are withdrawn as off-gas and incinerated in a thermal oxidizer at 850–1100 °C with selective catalytic reduction for NOx control. Hydrogen cyanide is stripped first because of its 25.7 °C boiling point and either burned or recovered as sodium cyanide. Acetonitrile is separated as a byproduct by extractive distillation after hydrogen cyanide removal. The remaining aqueous acrylonitrile is directed to dehydration.

The dehydration sequence relies on the low-boiling acrylonitrile-water azeotrope at 70.6 °C and approximately 88 wt% acrylonitrile at atmospheric pressure. Aqueous crude acrylonitrile is fractionated to recover the azeotropic distillate, followed by final drying and high-purity distillation. Trace water is removed to below 0.4 wt%. Low-boiling impurities such as hydrogen cyanide, acrolein, and acetonitrile are rejected in side-draw or purge streams. The purified acrylonitrile is condensed at 20–30 °C and transferred to storage under polymer-grade protocols. Recovery column and storage tank materials are stainless steel because acidic impurities and water can initiate corrosion in carbon steel.

What Purification Thresholds Define Fiber-Grade Acrylonitrile?

Fiber-grade acrylonitrile is controlled by impurity limits rather than single distillation cuts. Water must be below 0.4 wt% because moisture hydrolyzes acrylonitrile slowly and affects cyano-group conversion in polymerization. Acetonitrile and hydrogen cyanide are controlled below 300 mg/kg and 5 mg/kg, respectively, to avoid chain-transfer effects and specks in polyacrylonitrile precursor. Monomethyl ether of hydroquinone is added at 35–50 mg/kg as a polymerization inhibitor. Color is limited to ≤ 10 Pt-Co units measured by ASTM D1209. Distillation range is verified under ASTM D1078. The table below summarizes representative polymer-grade parameters.

At production-scale terminals, polymer-grade acrylonitrile is stored in 316L stainless steel or aluminum vessels with refrigeration or internal cooling coils maintaining liquid temperature below 25 °C. Monomethyl ether of hydroquinone is effective only when dissolved oxygen remains above 5–10 mg/kg; inert-gas-only blanketing can therefore become an operational hazard by deactivating the inhibitor system. Copper and copper alloys are excluded from pumps, valves, and instrument wetted parts because copper can promote polymer formation and is attacked by trace cyanides. Acrylonitrile is flammable: lower flammability limit 3.0 vol%, upper flammability limit 17 vol%, closed-cup flash point −1 °C, and autoignition temperature 481 °C. These boundaries require storage-area electrical classification and vapor detection.

When High-Nitrile Barrier Resins Demand Controlled Acrylonitrile Distribution

Acrylonitrile is copolymerized with butadiene, styrene, and other vinyl monomers to produce ABS and SAN resins. In ABS, bound acrylonitrile typically ranges from 15–35 wt% of the styrene-acrylonitrile matrix. Higher acrylonitrile content raises heat deflection temperature and polar solvent resistance, but it reduces melt processability. Processors specify melt flow rate by ISO 1133-1:2022 and tensile properties by ASTM D638-14. Nitrile rubber grades contain 18–50% bound acrylonitrile; increasing content shifts Shore hardness and oil resistance upward. High-nitrile barrier resins containing approximately 70–75 mol% acrylonitrile are used in packaging where oxygen permeability must remain low.

Polyacrylonitrile precursor for carbon fiber is a high-purity copolymer that usually contains 1–3 wt% methyl acrylate or itaconic acid to control cyclization exotherm. Oxidative stabilization is performed in forced-air ovens at 200–300 °C, followed by carbonization in nitrogen at 1000–1500 °C. The Sohio-derived monomer must have low particulate content and tight hydrogen cyanide and acetonitrile limits because ionic and chain-transfer impurities affect fiber diameter uniformity and tensile strength. Batch-to-batch purity variations within the polymer-grade range can shift precursor viscosity stability and require corrective adjustment of polymerization initiator feed.