In the last years conjugated linoleic acids (CLAs), especially the two isomers cis-9,trans-11- and trans-10,cis-12-octadecadienoic acid, proved to have many positive effects for physical health (e.g. anticarcinogensis, antidiabetic effects, reduction of body fat). For that reason CLA could play an important role as dietary supplement and, thus, for the development of functional food. In natural food products CLA can be found only in marginal portions in beef and dairy products. Therefore, the chemical production of the physiologically important CLAs by isomerization of linoleic acid (cis-9,cis-12-octadecadienoic acid) is a challenging task, even in the presence of heterogeneous catalysts. However, under a hydrogen atmosphere other possible metal-catalyzed reactions, for example the consecutive hydrogenations to oleic acid/stearic acid, have to be avoided. Therefore Ag and Au catalysts can reach high selectivity for the title reaction in presence of hydrogen, based on weakly bound hydrogen acting as non-stoichiometric agent. Under inert gas conditions Ru catalysts can be used as well after being pretreated with hydrogen.
Discontinuous experiments were done in a batch reactor. Beside those experiments continuous experiments were done using a computer-controlled trickle-bed reactor. Before the products could be analyzed by offline gas chromatography the samples were silylated.
Ag catalysts showed good results for the isomerization of linoleic acid at the batch-reactor experiments, because of their marginal chemisorption of hydrogen, so a selectivity up to 73% to the CLAs at 95% conversion could be reached with a yield up to 34% to the two target CLAs. In comparison to ruthenium catalysts, with which similar results can be achieved (selectivity to CLAs of 83% at 85% conversion and 33% yield to the two target CLAs) with Ag a direct operation under hydrogen is possible. Au showed lower conversion than Ag, but even here selectivities to CLAs up to 73% were possible. Within the group of CLA mainly the two target products of commercial interest cis-9,trans-11- and trans-10,cis-12- CLA were built, but also high amounts of undesired trans-9,trans-11-CLA.
With the first time implementation of a continuous procedure at this reaction system with an Ag catalyst all parameters, which lowered the hydrogen to linoleic acid proportion (higher flow rate of solution or concentration as well as lower flow rate of hydrogen) decreased the conversion but increased the selectivity to CLAs (up to 74%). It was also shown, that even Ru catalysts can be used under continuous conditions (77% conversion, 74% selectivity to CLAs), if they are pretreated with hydrogen before the operation. In both cases a yield up to 20% to the two target CLAs was reached. | English |