Acrylonitrile Synthesis
 

Scott Strikmiller
Louisiana State University

Acrylonitrile, vinyl cyanide, CH₂=CHCN, is a colorless man-made liquid with a slightly pungent odor. It was first prepared in 1893 by a French Chemist, Ch. Moreau, who dehydrated ethylene cyanohydrin with phosphorus pentoxide. Its prominent use came shortly before World War II in connection with the development of copolymers for use in oil-resistant rubber. Since that time the demand for acrylonitrile has dramatically increased due largely to acrylic fibers, first introduced commercially in 1950 by DuPont under the trademark Orlon. Acrylonitrile has a boiling point of 77.3^oC, a freezing point of -83.55^oC, and a vapor pressure of 100 mm Hg at 23.6^oC.In 1977, the U.S. Federal Drug Administration declared acrylonitrile to be an indirect food additive and banned its use in beverage containers and other food-packaging applications. The Environmental Protection Agency(EPA) and the German MAK commission has classified acrylonitrile to be a human carcinogen.
 

The DuPont process for the production of acrylonitrile involved the catalytic addition of hydrocyanic acid to acetylene.

This commercial reaction was carried out at 80^oC in dilute hydrochloric acid containing cuprous chloride. Although the yield from this reaction was good, the raw materials were relatively expensive and some undesirable impurities were difficult to remove. The cuprous chloride catalyst used in this process required frequent regeneration and expense. DuPont eventually abandoned this procedure in 1970.

In 1947, Allied Chemical and Dye Corporation patented the manufacture of acrylonitrile via an ammoxidation process of propylene but it was not until 1960 when Standard Oil of Ohio(Sohio) developed the first commercially viable catalyst for this process. Today, over 90% of acrylonitrile production in the world is based on the Sohio process. In this process, propylene, oxygen, and ammonia are catalytically converted directly to acrylonitrile using a fluidized-bed reactor operated at temperatures between 400^oC and 500^oC and gauge pressures between 0.3 and 2 bar.

Acrylonitrile can be used to produce a wide variety of products. Such products are adiponitrile(used in production of Nylon 66), ABS and SAN resins(automobile plastics), acrylic fibers, acrylamide and nitrile elastomers(oil-resistant rubber). Nearly 60% of the acrylonitrile produced is used to make acrylic fibers. These acrylic fibers find use primarily in wearing apparel and in home furnishings such as carpets and draperies. By 2001, the demand for acrylonitrile is expected to be 3.8 billion pounds. Following is a list of the dominant producers of acrylonitrile in the United States.

                 Producer                                                                                    1997 Capacity*

                   BP Chemicals, Green Lake, TX                                                          1,000
                   BP Chemicals, Lima, OH                                                                     500
                   Cytec Industries, Avondale, LA                                                            475
                   DuPont, Beaumont, TX                                                                        385
                   Monsanto, Alvin, TX                                                                            500
                   Sterling Chemicals, Texas City, TX                                                       700

                   * millions of pounds of acrylonitrile produced per year

Production

In the Sohio process, propylene, oxygen, ammonia, and water are fed to a fluidized-bed reactor containing a complex metal-oxide catalyst. Small amounts of water are fed to the reactor to increase the selectivity of acrylonitrile and decrease the temperature within the reactor(Gates 381). The reactor temperature is in the range of 400^oC to 500^oC and the residence time is a few seconds. The pressure of the reactor is 0.3-2 bar and the yield is 80-90 percent depending on the catalyst used(Languardt 179). This process is highly selective and requires no recycling. Each of the reactants are fed in their respective stoichiometric ratios. The by-products formed during the ammoxidation reaction are hydrogen cyanide and acetonitrile, which are separated during the distillation process.

Catalysts

The Sohio process utilizes a series of complex metal-oxide catalysts.These catalysts are oxide combinations or compounds containing at least two different metals, one of which coming from the later row 5a elements. The second metal is almost always a transition metal(Gates 349). Ammoxidation catalysts are highly selective for partial oxidation. Some of the best-known ammoxidation catalysts are BI₂O₃+MoO₃ and UO₃+Sb₂O₄. BI₂O₃ has low activity and causes complete oxidation and MoO₃ has good selectivity and lower activity than BI₂O₃.  The first of the commercial ammoxidation catalysts was bismuth phosphomolybdate(Bi₉PMo₁₂O₅₂). In 1972, Sohio announced an ammoxidation catalyst with superior properties(higher selectivity). This catalyst has the composition of M₈²⁺Fe₃³⁺Bi³⁺(MoO₄)₁₂O₂^2- where M²⁺ is Ni, Co, or Mg(Gates 378). The present-day catalyst implemented by Sohio is their patented catalyst 41, which is another multicomponent molybdate complex having the composition of  Co₆²⁺Ni₂²⁺Fe₃³⁺Bi³⁺(MoO₄)₂(Gates 380).
 

Catalyst Production and Regeneration

The metal oxide catalyst used in the ammoxidation process for the production of acrylonitrile are usually prepared by precipitation from solution. In this process, an aqueous metal salt solution, consisting of bismuth nitrate and ammonium molybdate, is rapidly mixed to form product precipitates. Depending on Bi/Mo ratio, pH, and the time of interaction, these precipitates are filtered, washed, dried, and heated to temperatures of  500^oC-800^oC to form the final catalyst. The catalyst is then sent to a grinder to obtain catalyst particles ranging from 10-100 microns.

During the ammoxidation reaction process, small particles tend to fuse together to form much larger ones. This process steadily deactivates the catalyst and reduces its activity. Catalyst activity can also be reduced during the reaction process by losses of oxygen attached to the bismuth molecule. Reoxidation of the catalyst occurs by adsorption and dissociation of oxygen on anion vacancies associated with molybdenum. Anion vacancies on the bismuth are reoxidized by transfer of oxygen from the oxidized molybdenum(Satterfield 276). Catalyst lifetime in the fluidized-bed reactor is approximately 1 year.
 

References

1. Gates, Bruce C.;Katzer, James R.;Schuit, G.C.A.,Chemistry of Catalytic Processes. McGraw-Hill Book Company. New York, 1979.

2. Satterfield, Charles N., Heterogenous Catalysis in Industrial Practice, Second Edition. Krieger Publishing Company. Malabar, Florida, 1996.

3. Languardt, Patrick W., "Acrylonitrile". Ullman's Encyclopedia of Industrial Chemistry, Fifth Edition: Vol. A1.

4. BP Lima Chemicals. "Acrylonitrile Manufacturing". www.bplima.com/acrtour.html

5. Chemical Information Services, Inc., "Acrylonitrile". July 14, 1997. www.chemexpo.com/news/PROFILE970721.cfm

6. Pierce R.;Patterson. W.R., Catalysis and Chemical Processes. John Wiley and Sons. New York, 1981.