Oils as viscous fluids cannot be selectively hydrogenated by standard hydrogenating techniques. Because of the high viscosities their transport to a catalyzer is limited. Also if they are thinned to a lower viscosity by solvents, the usual heterogeneous metal catalyzers, which are applied to a highly porous matrix of carrier material, cannot be used to advantage. The necessarily large catalytic surface area is achieved by the high dispersion of the metal into the pores of the carrier. Since by the slow diffusion of the long chained compounds into the pores a complete hydrogenation of all double bonds—in the case of edible oils for example, triglycerides of stearic acid—takes place, and these products block the pores and therewith the catalyzer.
The possibility of working with strong dilution is of little sense, since first the solvent has to be again expensively separated, and in the case of edible oils a health danger cannot be excluded. A technical solution for the hydrogenation of edible oils has been found in the use of nickel-carrier catalyzers where the metal is applied to a very fine grained catalyzer carrier, which again has to be expensively practically entirely separated by filtration. This also leads to a considerable loss of partially hydrogenated oil. As the carrier, diatomaceous earth, silicon dioxide or aluminum oxide is used. The hydrogenation typically is carried out at 170-200° C. in a stirred kettle, with the temperature of the highly exothermal reaction being not well controllable. The making of the catalyzer as a fixed bed reactor is possible, but is not thought to make sense technically, since with the highly exothermal reaction it is necessary in sequence to execute three steps for the required temperature control. [http://www.soci.org/SCI/publications/2001/pdf/pb95.pdf, TRENDS IN THE DEVELOPMENT OF EDIBLE OIL HYDROGENATION CATALYSTS, W T Koetsier and M C Lok, Unichema International (now Synetix), Emmerich, Germany and Billingham, UK 1998, ISSN 1353-114X].
All in all the exchange of material in the hydrogenation of oils imposes a high demand on the reactor [Veldsink, et al. Heterogeneous Hydrogenation of Vegetable Oils: A Literature Review, Catal. Rev.-Sci. Eng., 39 (1997) 253-318]. Since the hydrogenation represents a “fast” reaction not only the oil to be hydrogenated but also the hydrogen must be quickly delivered in sufficient amounts to the catalyzer. The balance of water/metal hydride at the catalyzer moreover influences the isomerization at the catalyzer. With good supply of hydrogen there results from the selective hydrogenation of the naturally appearing cis-double bond compounds fewer trans-compounds as byproducts of the isomerization. These not naturally appearing trans-compounds are thought to be of health concern and also change the product physically, since they have higher melting points and therefore lead to a higher “solid fat content”.
To improve the supply of hydrogen at the catalyzer, a membrane reactor made as a three phase reactor with inorganic membranes was tested. The oil to be hydrogenated was pumped along the catalytically active outer surface of a porous membrane and hydrogen was supplied from the membrane rear side. [Veldsink, Selective Hydrogenation of Sunflower Oil in a Three-Phase Catalytic Membrane Reactor, JAOCS 78 (2001) 443-446]. With this reactor indeed the transport problems were reduced, but it did not succeed to make available sufficiently large catalytic membrane surfaces per reactor surface. Therefore reaction times of up to several hundred hours had to be used. By increasing the reaction temperature the reaction time can indeed be shortened, however it is known that the degree of isomerization increases with the temperature and a higher trans-content must be taken into account. Another possibility for carrying out the selective hydrogenation of oils under good control of the conditions has been found by Illinitch et al. [Illinitch et al. Nanosized Palladium Loaded Catalytic Membrane: Preparation and Cis-Trans Selectivity in Hydrogenation of Sunflower Oil, Stud. Surf. Sci. Catal. 118 (1998) 55-61]. Here the inner surface of a porous polyamid membrane of nylon-6 is catalytically activated with palladium and the oil to be hydrogenated together with dissolved hydrogen is pumped through the pore system of the membrane. In comparison to a batch reactor with a 2% palladium catalyzer carried on active charcoal, an about 2% smaller 18:1 trans-value of about 12% was found with a reduction of 18:2 concentration from about 62 to about 33%. This was taken as a confirmation of the theory that less isomerization to trans-compounds is found in forced passage through catalytically activated membrane pores. The membrane of nylon-6 was activated by means of a benzene solution of palladium salt Pd3(OAc)6 or Pd6Cl12. With an aqueous or acetone solution of H2PdCl4 only fragile, mechanically unstable, membranes were obtained.
In the technical carrying out of the hydrogenation optimized catalyzers are used which must elaborately and with loss of product again be entirely separated since they contain poisonous nickel or other noble metals. The reaction for the selective partial hydrogenation moreover because of the highly exothermal reaction cannot be easily controlled and the trans-content of the partially hydrated product is too high. Fixed bed reactors and membrane reactors with hydrogen supply to the rear side of the membrane offer no alternatives, since they are either too elaborate (fixed bed) or too small amounts of catalyzer per membrane reactor volume can be brought to bear. The possibility of catalytically activating the interior pore system of the membranes and to use that with a forced flow through the membranes was indeed demonstrated, but the investigated material could only be catalytically activated with toxic solvents (benzene). The possibility of building the carried catalyzer into a porous membrane and to thereby obtain the reaction capability was not considered. Moreover the solvents for nylon-6 are limited. Therefore in U.S. Pat. No. 4,702,840 (Pall Corp.) it was disclosed that the preferred solvent is formic acid. Other solvents coming into question were fluid aliphatic acids such as acetic acid and propionic acid, and further phenols (including halogenated phenols), inorganic acids such as hydrochloric acid and sulfuric acid or saturated alcoholic solutions of alcohol soluble salts such as CaCl2, MgCl2, LiCl and other OH group carrying solvents such as halogenated alcohols. None of these solvents are distinguished by environmentally friendly characteristics, and several are downright toxic, only to be used with limitations and high costs. Moreover there exists the danger that carried catalyzers are not compatible with the acids or that the content of salts in the casting solution inadmissibly changes the reactivity and selectivity of the catalyzers.