This invention relates to a process for the catalytic hydrogenation of plant-based steroid compounds. More particularly, this invention relates to the metal-catalyzed hydrogenation of phytosterols and to the recovery and regeneration of the metal catalyst species used in such metal-catalyzed hydrogenation.
Steroids are macromolecular organic compounds that, structurally, comprise a cyclopentanephenanthrene skeleton. Many of these compounds are alcohols and are identified as sterols. When combined to form esters, these compounds are called xe2x80x9cceridesxe2x80x9d.
Cholesterol, a sterol, has been known for some time. It has been reputed as being involved in the formation of biliary calculi, a theory which has been confirmed by proof of its involvement in circulatory disorders, and more particularly in the hardening of the arteries.
Nonesterified cholesterol is the main constituent of fatty substances having an animal origin. These animal fats are present in most of our foodstuffs and constitute an important source of cholesterol. When ingested in excess, they may breakdown, releasing cholesterol and the fatty acids which, in turn, may cause serious cardiovascular diseases.
However, fatty substances having a plant origin do not contain cholesterol, but instead comprise a mixture of cholesterol-like materials called phytosterols, i.e., they are unsaturated, plant-based compounds comprising the cyclopentanephenanthrene skeleton. The best known of such compounds include: stigmasterol, sitosterol and ergosterol.
The structure of the molecule on which the animal and plant steroids is based is shown below: 
The C 3 carbon atom is hydroxyl-substituted. The carbon atom at position 10 bears a methyl group.
The stereochemistry of the substituents at C 3 and C 10 is important. The steroid ring is a planar structure and, for cholesterol, the two groups at C 3 and C 10 lie above the plane of the ring, i.e., they are in the xcex2 conformation. The hydrogen atom at position C 6 (see the double bond between C 5 and C 6) lies below the plane, i.e., it is in the a position. Cholesterol""s phytosterol look-alike, sitosterol, is substantially identical to cholesterol but, in addition bears an ethyl group at C 24.
In the reduction of these steroid-based structures, it is important to retain the stereochemical conformation of the starting material. Such reductions have typically been carried out with varying degrees of success, using hydrogen in the presence of supported metallic catalysts. See, for example, U.S. Pat. No. 3,865,853. Such catalysts include iron, cobalt, nickel, palladium, platinum, copper, silver and gold.
As disclosed in U.S. Pat. No. 6,147,235, steroids have been hydrogenated in the presence of catalytic amounts of nickel black, Raney Nickel, and nickel metal embedded in inorganic supports. However, these catalysts have not proven sufficiently selective for hydrogenation of the steroid double bond.
The ""235 patent also shows that steroids have been hydrogenated using noble metal catalysts, principally platinum and palladium. According to this patent, these catalysts have been supported on inorganic substrates, on carbon or used as metal blacks. The best conversions have been obtained when such noble metal catalysts are bound to activated carbon. The noble metal of choice is palladium and one of the most successful of the catalyst species has been shown to be palladium on carbon.
However, prior art catalysts used for the reduction of steroids have proven to be disadvantageous because of difficulties of separation of the catalyst powder from the reaction mass after the hydrogenation process, the inflammability of the catalyst and the catalyst is not recyclable.
As shown on U.S. Pat. No. 6,147,235 palladium bound to an organic substrate (a polyolefin or halo polyolefin) also has been used successfully. The process for manufacturing such a catalyst is difficult and the expense of such a catalyst species has limited its use.
It is the object of the present invention to provide a process for catalytically hydrogenating unsaturated phytosterol compounds. The catalyst used for such hydrogenation is an alumina-supported transition metal selected from the group consisting of nickel, palladium, platinum, ruthenium, rhodium and mixtures thereof.
The process of the present invention is as follows.
An admixture of a solvent, an alumina-supported transition metal catalyst and a material comprising at least one of the unsaturated phytosterol compounds is first formed. This admixture is hydrogenated for a time and at a temperature and pressure sufficient to cause the hydrogenation of at least one of said unsaturated phytosterol compounds.
The alumina-supported transition metal catalyst is next separated from the reaction (the hydrogenated) mixture that was first formed. This separated alumina-supported transition metal catalyst is then admixed with a new mixture containing a solvent and a material comprising at least one unsaturated phytosterol compound. The resulting reaction mixture is hydrogenated for a time and at a temperature and pressure sufficient to cause the hydrogenation of at least one of said unsaturated phytosterol compound. The alumina-supported transition metal catalyst is then separated from the hydrogenated mixture. It may be used again for subsequent hydrogenations of unsaturated phytosterol compounds.
The hydrogenation process disclosed herein can also be applied to the hydrogenation of fatty acids derived from phytosterols, e.g., tall oil, vegetable oil and their distillation products.