SNIA Viscosa Process

Snia-Viscosa developed an s-caprolactam process in 1958 based on the use of toluene as the raw material, which was licensed in 1962. The oxidation of toluene has been studied since at least 1892, when the first products obtained were benzaldehyde and benzoic acid. Catalysts based on platinum, vanadium pent-oxide, and various other oxides have been described  

Weiss and Downs described the use of vanadium pentoxide for the catalytic oxidation of toluene and naphthalene. Subsequently, Graver suggested a mixed oxide catalyst derived from uranium oxide and molybdenum trioxide in molar ratios ranging from 3-13 1. Copper oxide was also suggested as a possible promoter. When using the vanadium pentoxide catalyst, benzoic acid was the main product at temperatures below 400°C, with some benzaldehyde formed at higher temperatures. Selectivity was only about 50% at 5% conversion. 

The Snia-Viscosa process now uses benzoic acid as an intermediate in caprolactam production. The oxidation of toluene in the liquid phase is still used in modem processes, with selectivity greater than 90%, at about 30% conversion Typical operating conditions are at temperatures around HO C, and pressures in the range 8-10 bar, using a cobalt acetate catalyst. 

The benzoic acid is now hydrogenated to cyclohexane eaiboxylie acid with a palladium/carbon catalyst at 170°C and 10-19 bar, whereas originally a nickel catalyst was used. Reaction with nitrosyl sulfuric acid in olenm solntion at 80°C then gives e-caprolactam. In the original Snia-Viscosa process, abont 25% more ammonium sulfate was formed than in the conventional proeess. However, solvent extraction of the eaprolactam from the add solntion nsing alkyl phenols does avoid the prodnetion of the ammonium salt. 

Catalytic oxidation of toluene with air to benzoic acid  

The oxidation of toluene with air is carried out in the liquid phase using a cobalt catalyst at 160 to 170°C and 0.8 to 1.0 MPa (8 to 10 bar). The yield is well above 90% of theoretical. The gases leaving the reactor contain mainly nitrogen with small amounts of oxygen, carbon dioxide and carbon monoxide. The are cooled to 7 to 8°C in order to recover unreacted toluene. The water from the reaction is removed in a separator drum and toluene is recycled to the reactor264. 

Most of the toluene is removed by distillation, and the remaining concentrated solution is sent to a rectification column. The lower-boiling intermediates and the remaining toluene are removed at the top and reused264. 

Benzoic acid in the vapor is removed from the rectification column as a side stream. The high-boiling by-products leave the column as a residue. The benzoic acid is suitable for hydrogenation without any further purification264. 

The reaction in Eq. (21.19) is carried out in the liquid phase in the presence of a palladium-on-graphite catalyst. A series of stirred reactors are used with a temperature of 170°C and a pressure of 1.0 to 1.17 MPa (10 to 17 bar). Conversion is 99.9%264 

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E. Dodgen, "Continuous Nitration of Cellulose SNIA Viscosa Process," in Symposium on Processing Propellants, Explosives and Ingredients, ADPA, Washington, D.C., 1977, p. 4.2-1. 

SNIA Viscosa process for manufacturing caprolactam from toluene... 

Other industrial methods for manofacturing laurolactam 12.4 4.1 SNIA Viscosa processes... 

Since its discovery some 55 years ago, the synthesis of caprolactam has been the subject of intense research and development. Interest in alternative routes continues today and current activities receiving a lot of attention are carbon monoxide-based routes under development by DSM, EniChem and DuPont [32]. Numerous routes using a variety of feedstocks have been patented and many have been piloted, however, only seven have actually been commercialized. The first was the process developed by I. G. Farben based on Schlack s chemistry known today as the Rashig or conventional route. Other commercial routes are the CAPROPOL process, the BASF process, the DSM-HPO process, the Allied process, the Toray PNC process, and the SNIA Viscosa process. 

There are seven commercial processes for producing caprolactam the Ras-hig (conventional) process, CAPROPOL process, BASF process, DSM-HPO process. Allied process, Toray PNC process, and the SNIA Viscosa process. Two of these, the CAPROPOL and BASF processes, utilize pure oxygen and are described in Chapter 5. Hydrogen is used to produce cyclohexane for all but the SNIA Viscosa process which uses toluene as a feedstock. The hydrogenation of benzene to cyclohexane has been described earlier in this chapter. The CAPROPOL, BASF, DSM-HPO, and SNIA Viscosa processes all involve hydrogenation in downstream steps. The hydrogenations are discussed here. 

Figure 15 Process sequence for the SNIA Viscosa process.

Among the industrially produced lactams, e-caprolactam has by far the highest production capacity due to its important role as monomer in the polyamide business. There exist several synthetic routes to produce e-caprolactam. The most important one starts from benzene (Scheme 5.3.7). Benzene is hydrogenated in a first step to cyclohexane, followed by oxidation of the latter to a mixture of cyclohexanone and cydohexanol. This mixture is then reacted with NH2OH to give cyclohexanone oxime, which is converted under add catalysis in a so-called Beckmann rearrangement reaction to e-caprolactam. Alternative routes try to avoid the oxime intermediate (UCC peracetic add process via e-caprolactone), try to avoid the cydohexanone intermediate (e.g., DuPont process converting cydohexane directly into the oxime intermediate by reaction with nitric add), or start from toluene (Snia-Viscosa process). 

Benzoic Acid. Ben2oic acid is manufactured from toluene by oxidation in the liquid phase using air and a cobalt catalyst. Typical conditions are 308—790 kPa (30—100 psi) and 130—160°C. The cmde product is purified by distillation, crystallization, or both. Yields are generally >90 mol%, and product purity is generally >99%. Kalama Chemical Company, the largest producer, converts about half of its production to phenol, but most producers consider the most economic process for phenol to be peroxidation of cumene. Other uses of benzoic acid are for the manufacture of benzoyl chloride, of plasticizers such as butyl benzoate, and of sodium benzoate for use in preservatives. In Italy, Snia Viscosa uses benzoic acid as raw material for the production of caprolactam, and subsequendy nylon-6, by the sequence shown below. 

In the Hquid-phase process, both benzaldehyde and benzoic acid are recovered. This process was iatroduced and developed ia the late 1950s by the Dow Chemical Company, as a part of their toluene-to-phenol process, and by Snia Viscosa for their toluene-to-caprolactam process. The benzaldehyde recovered from the Hquid-phase air oxidation of toluene may be purified by either batch or continuous distillation. Liquid-phase air oxidation of toluene is covered more fully (see Benzoic acid). 

In the United States all other processes have been completely phased out and virtually all benzoic acid is manufactured by the continuous hquid-phase air oxidation of toluene. In the late 1950s and the early 1960s both Dow Chemical and Snia Viscosa constmcted faciUties for Hquid-phase toluene oxidation because of large requirements for benzoic acid in the production of phenol and caprolactam. Benzoic acid, its salts, and esters are very useful and find appHcation in medicinals, food and industrial preservatives, cosmetics, resins, plasticizers, dyestuffs, and fibers. 

Caprolactam. At the same time that Dow was constmcting toluene to phenol plants, Snia Viscosa (28—30) introduced two processes for the manufacture of caprolactam (qv) from benzoic acid. The earlier process produced ammonium sulfate as a by-product, but the latter process did not. In either process benzoic acid is hydrogenated to cyclohexanecarboxyHc acid [98-89-5] which then reacts with nitrosylsulfuric acid to form caprolactam [105-60-2]. 

Snia Viscosa. Catalytic air oxidation of toluene gives benzoic acid (qv) in ca 90% yield. The benzoic acid is hydrogenated over a palladium catalyst to cyclohexanecarboxyhc acid [98-89-5]. This is converted directiy to cmde caprolactam by nitrosation with nitrosylsulfuric acid, which is produced by conventional absorption of NO in oleum. Normally, the reaction mass is neutralized with ammonia to form 4 kg ammonium sulfate per kilogram of caprolactam (16). In a no-sulfate version of the process, the reaction mass is diluted with water and is extracted with an alkylphenol solvent. The aqueous phase is decomposed by thermal means for recovery of sulfur dioxide, which is recycled (17). The basic process chemistry is as follows ... 

SNIA Viscosa developed a toluene-based process to produce caprolactam in 1960. The steps in the SNIA process are listed below and shown in Figure... 

In a newer process revision, caprolactam is extracted from the sulfuric acid solution with alkylphenols and then stripped with water. In this way the Snia Viscosa route can be a salt-free process. With a 50% conversion of cyclohexanecarboxylic acid, the selectivity to caprolactam is 90%46. 

The process originally proposed by SNIA Viscosa starts with cydododecatriene converted selectively to mono-ozoukie by the action of ozone at 20°C in a system of solvents which extracts the ozonide formed as soon as it appears in the medium, thus preventing the formation of polyozonides. 

Another process also proposed by SNIA Viscosa is based on the technique employed by this company to manufacture caprolactam from toluene. It comprises the following steps ... 

Toluene is the starting material of a commercial process developed by SNIA Viscosa [128] (process 3, Figure 2.11). It involves the reactions shown in the following scheme ... 

Snia Viscosa (Italy) operate an entirely different process, wherein benzoic acid is hydrogenated to cyclohexanecarboxylic acid, which gives caprolactam by reaction with nitrosyl hydrogen sulphate ... 

The main field of application for benzoic acid is the production of phenol however, the significance of this phenol route has declined in recent years. Additionally, benzoic acid is used in the production of benzoyl chloride and sodium benzoate. In Italy, benzoic acid is used as the raw material for the production of 8-caprolactam, in a process developed by Snia Viscosa, This involves hydrogenating benzoic add at 170 °C and 15 bar over a palladium catalyst, purifying the cyclohexane carboxylic acid by distillation followed by its reaction with nitrosyl-sulfuric acid to yield 8-caprolactam. 

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