Technical Process Ozonolysis

Using modern analytical methods, a number of transient intermediates and byproducts could be verified [19, 20]. The first step in the mechanism of ozonolysis is the 1,3-dipolar cycloaddition of the dipole ozone to the double bond of OA. A 1,2,3-trioxolane is formed, the unstable primary ozonide or molozonide. The primary ozonide collapses in a 1,3 dipolar cycloreversion to a carbonyl compound and a carbonyl oxide, the so-called Criegee zwitterion. Since OA is substituted with two diverse groups at the double bond, two different opportunities exist for the formation of carbonyl compound and carbonyl oxide. Again, a 1,3-dipolar cycloaddition of these intermediates leads to three different pairs of 1,2,4-trioxolane derivatives (cisltram), the secondary ozonides, which are more stable than the primary ones. Their oxidative cleavage results in AA and PA. 

A first patent for the technical implementation of ozonolysis of OA was claimed by Rieche [21]. According to a BASF patent filed in 1941, AA (97%) and PA (95%) are accessible by treatment of the ozonide with alkaline lye and Ag20 [22]. However, the technical manufacturing of AA succeeded first by Emery Industries in the year 1953 [23]. Their successor Emery Oleochemicals is still the worlds largest producer of AA by ozonolysis of OA with a production volume of about lOOOOt/year [9, 24]. The achievable yield of AA is 70-80% [24], based on the amount of applied OA. AA yields of more than 90% were obtained in a pilot plant [25]. The reaction is carried out in PA/water (70/30v/v) as solvent. The admixture of water reduces the formation of side products by removal of the heat of reaction via evaporative heat loss, and in addition, undesired reactive radicals are quenched by reaction with water [25, 26]. 

Ozone is applied in a concentration of about l-3vol% in air [25]. Another process uses the ozone synthesis from oxygen obtained via air separation in a closed circular flow. Ozone is then adsorbed on silica and desorbed subsequently 

The products of different quality categories of AA are available in the market under the brand EMEROX azelaic acids [4]. Products containing 79% (19% other dicarboxylic acid, 2% monocarboxylic acids) [6] and 85% AA are used for the preparation of low-temperature and polymeric type vinyl plasticizers, ester-based 

The benefits of ozonolysis are (i) its high selectivity to give a sufficiently pure AA for polymer production, (ii) the low reaction temperature of the first step (20-45°C), and (iii) moderate temperature in the oxidation step (70-120 C) [7, 23, 27]. The coupling product PA is a valuable material that can be merchandized itself (e.g., as lubricant [4]), or as described later, it may be used in the bioenzy-matic synthesis of AA in the future. 

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New interesting developments regarding technical processes for the preparation of AA such as biocatalytic oxidation of PA and the two-stage cleavage of oleic acid using H2O2 and O2 as oxidants are reported later in the text. However, ozonolysis coupled with an aerobic oxidative work-up of intermediates is the most important industrial process at the present time [5, 7]. 

Though ozonolysis is very selective and high yielding, the technical use is, according to industrial complaints [5], difficult for reasons of economy and safety. The development of ozone-free processes for oxidative C = C-cleavage is therefore a serious task in fat-chemistry research. 

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