Hydrogenation of acrylonitrile to propionitrile

Operations are conducted with a once-through conversion of SO per cent and adiponitrile molar selectivity approaches 92 per cent Side reactions consist essentially of the hydrogenation of acrylonitrile to propionitrile and the polymerization of acrylonitrile, to which must be added the production of acrylamides, acrylates, bydroxypropionitrile and oxydipropionitrile. 

Nickel catalysts supported on alumina that has been modified by lead (5%) using a controlled surface reaction to ensure that the lead poison correctly binds at the most active sites on nickel (Pb-Ni/A Os) have been developed for the selective hydrogenation of acrylonitrile to propionitrile. This catalyst suppresses overreduction to propylamine, as well as the formation of secondary and tertiary amine byproducts. This surface reaction process uses the reduction of tetraethyllead with hydrogen adsorbed on the nickel to generate the required lead(O). ... 

A drastic example of this phenomenon is encountered in the cathodic hydrodimerization 76>8°1 of acrylonitrile to adiponitrile. This can be accomplished in very high yield in a concentrated solution of a tetraalkylammonium tosylate in water. Practically no propionitrile, the product of hydrogen addition, is formed. The reaction is believed to occur via formation of the acrylonitrile anion radical (6), which then attacks a second molecule of acrylonitrile. Further reduction of the resulting anion radical (7) followed by protonation of the dianion gives adiponitrile (Eqs. (21), (22) and (23) ). 

Along with acrylonitrile (ACN), propionitrile (PPN) was also produced. Heahng to 400 °C in nitrogen activated the catalysts. High surface area MgO produced the best yields of acrylonitrile and propionitrile but favored the hydrogenated nitrile. The low surface area MgO was considerably less effechve at hydrogenahon and... 

Cyanoethylation Reactions (Michael-Type Additions). Most compoimds with a labile hydrogen atom can add on the double bond of acrylonitrile to form cyanoethyl groups that is, the primary products are 3-substituted propionitriles. 

Small amounts of propionitrile and bis(cyanoethyl) ether are formed as by-products. The hydrogen ions are formed from water at the anode and pass to the cathode through a membrane. The catholyte that is continuously recirculated in the cell consists of a mixture of acrylonitrile, water, and a tetraalkylammonium salt the anolyte is recirculated aqueous sulfuric acid. A quantity of catholyte is continuously removed for recovery of adiponitrile and unreacted acrylonitrile the latter is fed back to the catholyte with fresh acrylonitrile. Oxygen that is produced at the anodes is vented and water is added to the circulating anolyte to replace the water that is lost through electrolysis. The operating temperature of the cell is ca 50—60°C. Current densities are 0.25-1.5 A/cm (see Electrochemical processing). 

First, we should consider the role of the tetraalkylammonium ion in the hydrodimerization reaction. Certainly the ions are essential to the process in their absence (but with for example a lithium or sodium salt as the electrolyte) the reduction of acrylonitrile leads only to propionitrile and a critical concentration of R4N is necessary to obtain a good yield of adiponitrile. This critical concentration decreases along the series (CH3)4N > (C2H5)4N > (C4H9)4N and with the latter can be as low at 0.01%. It may also be noted that the presence of these ions suppresses hydrogen evolution and there is evidence that they adsorb on the cathode surface. This led to the proposal that the adsorption of the tetraalkylammonium ion produces a layer at the electrode surface which is relatively aprotic. In this layer anionic coupling can occur because protonation is slow compared with that in the bulk solution. 

By the mid 1970s it was clear that the hydrodimerization of acrylonitrile could be run very effectively with only a low concentration of tetraalkylammonium ion and that a saturated solution of acrylonitrile in an aqueous buffer was an appropriate medium. In such circumstances, it seemed likely that the electrolysis could be run in an undivided cell without the additional complication of an anode depolarizer. Such a system was first reported by Phillips Petroleum. They ran a pilot plant with an undivided cell consisting of a lead cathode and a steel anode a very simple electrolyte, 6% acrylonitrile and 0.03% tetrabutylammon-ium salt in aqueous dipotassium hydrogen phosphate, was employed and the anode reaction was oxygen evolution. The yield of adiponitrile remained above 90% and the chief by-products were propionitrile and trimer. No base-initiated chemistry was observed due to the use of an effective buffer. 

Two rather different approaches to the catalytic dimerisation of acrylonitrile have also been explored. In the first approach, a rathenium catalyst was used in the presence of hydrogen, at temperatures in the range 200-350°C and pressures of 1-3 bar. Adiponitrile was produced directly, but unfortunately, was accompanied by an equimolar amount of propionitrile, formed by direct hydrogenation of the acrylonitrile ... 

Tretyakov and Filimonov (219) describe a coordinative interaction between benzonitrile and aprotic sites on magnesium oxide, and Zecchina et al. (256) came to the same conclusion for the adsorption of propionitrile, benzonitrile, and acrylonitrile on a chromia-silica catalyst. Chapman and Hair (257) observed an additional chemical transformation of benzonitrile on alumina-containing surfaces, which they describe as an oxidation. Knozinger and Krietenbrink (255) have shown that acetonitrile is hydrolyzed on alumina by basic OH- ions, even at temperatures below 100°C. This reaction may be described as shown in Scheme 2. The surface acetamide (V) is subsequently transformed into a surface acetate at higher temperatures. Additional reactions on alumina are a dissociative adsorption and polymerizations (255) analogous to those observed for hydrogen cyanide by Low and Ramamurthy (258), and a dissociative adsorption. Thus, acetonitrile must certainly be refused as a probe molecule and specific poison. 

These studies together showed that it was possible to run the process effectively with low acrylonitrile and tetraalkylammonium ion concentrations. In such circumstances the anode depolarizer would seem unnecessary and, indeed, Phillips have run a pilot plant with an undivided cell, lead cathode and steel anode, and a very simple electrolyte consisting of dipotassium hydrogen phosphate (1.5 m), acrylonitrile (6%) and a tetrabutylammonium salt (0.03%) in water the anode reaction is again oxygen evolution. The yield of adiponitrile is over 90% and the major byproducts were propionitrile and trimer. 

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