PRODUCT REVIEW Synthetic Wax Esters and Diesters from Crambe and Limnanthes Seed Oils H. J. Nieschlag,' G. F. Spencer, R. V. Madrigal, and J. A. Rothfus Northern Regional Research Center, Agricultural Research Service, U.S. Department Of Agriculfure, Peoria. Illinois 6 1604
Richard V. Madrigal has been with the Agricultural Research Seruice for 18 years. Preuiously he was with the Glidden Company for 4 years. He holds a bachelor's degree from DePaul University. At the present time, he is engaged in characterization of unusual components i n plant seed oils.
Henry J. Nieschlag is a research chemist in the Horticultural and Special Crops Laboratory of the Northern Regional Research Center, a laboratory of the Agricultural Research Service, US. Department of Agriculture. His work inuolues chemical modifications of new crop seed oils in order to dkcouer and develop new products of industrial utility. He received his bachelor's degree from Northwestern Uniuersity and his master$ degree from Bradley University. Prior to joining the Northern Center, he was employed by Commercial Solvents Corporation
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Gayland F. Spencer grdduated from Bradley University, Peoria, Illinois, i n 1969 andhas bee'n engaged in the analysis and identification of lipid constituents.
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J. A. Rothfus leads Horticultural and Special Crops Evaluation Research at the Northern Regional Research Center investigating all aspects of plant resources utilization. Educated at Drake Uniuersity and the Uniuersity of Illinois, Dr. Rothfus has been involved in biochemical and natural products research for over 15 years.
Wax esters similar to those of sperm whale oil were prepared from two promising agricultural commodities: crambe oil, a domestic source of erucic acid, and limnanthes oil, which is essentially free of conjugatable unsaturation. Reduction of crambe acids with Adkins-type catalysts gave >85% monoenoic acids. Dienoic wax esters and monoenoic alcohols were produced at 300 OC and 1500-1600 psig hydrogen with minimal double bond isomerization or saturation. Controlled hydrogenation gave a 70% yield of wax ester. The remaining near stoichiometric mixture of acid and alcohol was esterified to give an all wax ester product. Dienoic wax ester analogues were also synthesized from low-polyene crambe acids with 2,2,4,4-tetramethyl-l,3-~yclobutanediol and 2,2dimethyl-l,3-propanediol to give added ester functionality.
Introduction Liquid wax esters have been important in commerce since 1712 when New England whalers accidently caught their first sperm whale. In more recent times, sulfurized sperm whale oil served as an excellent additive in many critical lubricant applications. In 1970, the sperm whale was classified as an endangered species and imports of its products were banned in December 1971. The concomitant need of substitutes for this unique material prompted these studies. Miwa and Wolff (1962,1963) originally demonstrated that crambe (Crambe abyssinica) and limnanthes ( L i m n a n t h e s douglasii) seed oils, two promising agricultural raw materials, could be converted to liquid wax esters similar to those of sperm oil (Spencer and Tallent, 1973). Both oils are advantageous starting materials: crambe due to its high erucic acid content and limnanthes because its oil is essentially devoid of conjugatible unsaturation (Phillips et al., 1971). Furthermore, crambe has been grown in major agricultural areas of both the United States and Canada. Already shown to be a hardy plant that can flourish even in moderately cold climates, crambe is a well-tested agronomically viable crop that can provide ma;y useful products (Nieschlag and Wolff, 1971). Limnanthes, though somewhat less developed as a new crop, is especially well suited for early spring production in the Pacific Northwest (Miller et al., 1964). Miwa and Wolff (1963) made alcohols from crambe oil by sodium reduction of glycerides; esterification with the corresponding acids yielded wax esters. Earlier, Adkins and his colleagues (1954) demonstrated that catalytic hydrogenolysis was a preferred method of making fatty alcohols. Extensive research extending this early work has been reviewed by Bertsch et al. (1964). Adkins and others isolated wax ester by-products during high-pressure catalytic hydrogenations (Bertsch et al., 1969; Pantula and Achaya, 1964). Our initial attempts to prepare waxes by hydrogenolysis were complicated by propyl by-products formed by thermal decomposition of glycerol (Spencer et al., 1974). Use of free acids, rather than glyceride oils, preserved glycerol and avoided undesired components. The procedures described in this paper allow synthesis of wax esters a t reasonable temperatures and pressures with minimal double bond isomerization or saturation. Prior to the wax ester synthesis, crambe acids were first partially hydrogenated to avoid the instability inherent in compounds containing conjugatable unsaturation. Unsaturation in limnanthes acids is neither conjugated nor conjugatable and thus a partial hydrogenation pretreatment is not necessary. Hydrogenolysis of the low-polyene crambe acids or limnanthes acids yields the corresponding alcohols, some of which react with acids present to form wax esters. Ester synthesis during hydrogenolysis is incomplete, so remaining acids and alcohols must be esterified in order to prepare a pure wax ester mixture. This synthesis is summarized in Scheme I which applies to crambe oil. Experimental Section Materials and General Methods. Refined and bleached crambe oil was prepared in pilot-plant quantities (Mustakas
et al., 1965). Limnanthes oil was obtained on a smaller scale by hexane extraction of ground seed. Powdered Girdler type catalysts (Chemetron Corp., Louisville, Ky.) were G-22 (barium promoted copper chromite), T-988 (16%copper, 25% cadmium, and 32% chromium), and T-1057 (40.8% CuO, 19% CdO, 14.3% Cr203). Two commercial diols, 2,2,4,4-tetramethyl-1,3-cyclobutanediol (mixture of isomers) and 2,2-dimethyl-1,3-propanediol (97%) (Aldrich Chemical Co., Milwaukee, Wis.), were used as received. Hydrogenations were done in a 1-L stainless-steel Magne Dash bomb (Autoclave Engineers, Erie, Pa.). Fatty acids (without solvents) and catalysts were thoroughly mixed and added to the bomb. After flushing with hydrogen to remove air, the bomb was pressurized to 100 psig at room temperature and then adjusted to the proper hydrogen pressure after reaching temperature. When hydrogenation was complete, the bomb was cooled to 75-100 "C and most of the reaction mixture was removed via the sampling tube under hydrogen pressure. This procedure eliminated the task of pouring the hot mixture out of a heavy bomb and avoided losses from foamy reaction mixtures being carried out the hydrogen vent line on release of pressure. Filtration of the mixture through celite gave a clear darkcolored product. Esterifications were conducted in a 2-L single-necked flask fitted with a Bidwell-Sterling distilling trap and a condenser. Mixtures to be esterified were refluxed in an equal volume of toluene with 1%p-toluenesulfonic acid as catalyst unless indicated otherwise. Water was collected azeotropically. The mixture was then cooled, washed with distilled water, and stripped to dryness. Fatty Acid Isolation. Fatty acids were obtained from both Crambe abyssinica and L i m n a n t h e s douglasii seed oils by resin-catalyzed hydrolysis of the oil, using an adaptation of the method of Sutton and Moore (1953). Seed oil, Amberlite 252 (3%by weight of oil; Rohm and Haas, Philadelphia, Pa.), Penn-Drake Petrasol 742 emulsifier (0.3% of the oil; Pennsylvania Refining Co., Butler, Pa.), and a volume of water equal to that of the oil were stirred and heated to 90 "C for 6 h. The reaction mixture was cooled overnight. The aqueous layer and catalyst were siphoned off and any emulsions were broken with saturated aqueous sodium chloride. The oil layer was re-treated with fresh water, emulsifier, and regenerated resin. The extent of hydrolysis was followed by TLC on precoated F-254 Silica Gel G plates in hexane-ethyl ether-glacial acetic acid (7030:2) using the original oil and crambe free fatty acids as standards. Spots were visualized by spraying with sulfuric acid-dichromate solution followed by heating. Hydrolysis was usually complete after three treatments; the triglyceride spot was no longer detected by TLC. Limnanthes acids were prepared on a laboratory scale (3-kg batches), while crambe oil was hydrolyzed in pilot-scale quantities (30-kg batches). GLC analysis of both fatty acid mixtures and low-polyene crambe acids (to be described later) are shown in Table I. Polyene Reduction. Crambe acids (700 mL) were partially Ind. Eng. Chem., Prod. Res. Dev., Vol. 16,No. 3, 1977
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Scheme I CH,-0-
1
-R
1 0 CH-0-C--R
CH,-OH (+ ’L 5% saturated
acids) where x = 0-6 v = 1-3 = 7-13
Partial Hydrogenation
>
d
Hydrogenolysis
Esterification
CH,(CH,), CH,CH=CH(CH,),COOCH,(CH,),CH=CHCH,(CH2)xCH, Wax esters hydrogenated a t 200-300 psig and 200 “C using 35 g of G-22 catalyst. The minimum temperature for reaction was 190 “C. Usually about 5 h was required to lower the polyene content to
