Selective hydrogenation with copper catalysts: II. Kinetics

To explain the unusually high selectivity of copper catalysts toward linolenate, model compounds were hydrogenated (150 C and atmospheric pressure) and the reaction products analyzed. Products varied depending upon location of the double bonds. Monoenes were not reduced by copper chromite except when the double bond was next to a carboxyl group. Dienes with isolated double bonds also were not reduced. Binary mixtures of model compounds were hydrogenated with copper chromite. From the composition of the initial and final products, a competitive rate ratio of the two compounds was determined. Esters with conjugated double bonds reacted faster than esters containing methylene interrupted double bonds. Kinetic data on the hydrogenation of linolenate indicated conjugation of the double bonds. Simulation of the kinetic data gave competitive reaction rates for the different isomers formed. Presented at the AOCS Meeting, New York, October 1968. No. Utiliz. Res. Dev. Div., ARS, USDA.     

Selective hydrogenation of soybean oil: IV. Fatty acid isomers formed with copper catalysts

Two samples of soybean oil hydrogenated with copper-containing catalysts at 170 and 200 C were analyzed for their natural and isomeric fatty acids. Methyl esters of the hydrogenated oils were separated into saturates, monoenes, dienes and trienes by countercurrent distribution between acetonitrile and pentane-hexane. Monoenes were further separated intocis- andtrans-isomers on a silver-saturated resin column. Double bond location in these fractions was determined by a microozonolysis-pyrolysis technique. The diene fraction was separated with an argentation countercurrent distribution method, and linoleate was identified by infrared, ozonolysis and alkaliisomerization data. The double bonds in thecis-monoenes were located in the 9-position almost exclusively. However, the double bonds in thetrans-monoene were quite scattered with 10- and 11-isomers predominating. About 86% to 92% of the dienes consisted of linoleate as measured by alkali isomerization. Other isomers identified as minor components includecis,trans andtrans, trans conjugated dienes and dienes whose double bonds are separated by more than one methylene group. No. Utiliz. Res. Dev. Div., ARS, USDA.     

Reaction of cyclopropene esters with hydrogenation catalysts

Methyl esters of stereulic and malvalic acids were heated under nitrogen in the presence of various hydrogenation catalysts, and the effect on the cyclopropene moiety was established. Similar tests were conducted with cottonseed oil, which contains glycerides of sterculic and malvalic acids. Palladium catalysts, and some palladium compounds which were tested, readily gave cyclopropene-free products, while nickel and platinum catalysts did not. Palladium catalysts freshly activated with hydrogen were not as active as those freed of adsorbed hydrogen. The catalysts could be reused. Heating cottonseed oil (0.73% cyclopropenes, calculated as trimalvalin) with 0.02% palladium, as a 10% palladium-on-carbon catalyst, gave a cyclopropene-free oil after 2 hr at 150 C. The treated oil was unaltered in appearance, and the noncyclopropene components were unaffected. Heating methyl sterculate with palladium catalyst produced a mixture of unsaturated condensation products and a number of unsaturated monoesters of practically unchanged molecular weight. The palladium treatment was shown to cleave the cyclopropene ring and produce methyl and methylene substituted fatty acid groups. Presented at the AOCS-AACC Joint Meeting, Washington, D.C., April 1968.     

Selective hydrogenation with copper catalysts: III. Hydrogen addition and isomerization

β-Eleostearate was found to be reduced by 1,6 addition of hydrogen. Because of the extensive isomerization of conjugated trienes during hydrogenation, the occurrence of 1,2 and 1,4 addition reactions could not be proven. Conjugated dienes were reduced by both 1,2 and 1,4 addition of hydrogen. The double bond distribution in the products formed from linoleate, linolenate and their isomers was consistent with the assumption that the double bonds in polyunsaturated fatty esters conjugate and then add hydrogen. Extensive isomerization (positional and geometric) of the conjugated double bond systems occurred during hydrogenation. Monoenes were not isomerized under similar conditions of hydrogenation. Since double bond distribution in monoenes formed from linoleate and alkali-isomerized linoleate was identical, indications are that conjugation precedes hydrogenation. Presented in part at the symposium “Hydrogenation Process,” Division of Industrial Engineering Chemistry, 157th American Chemical Society Meeting, Minneapolis, April 1968. No. Utiliz. Res. Dev. Div., ARS, USDA.     

Selective hydrogenation with copper catalysts: V. Kinetics and mechanism at high pressure

The mechanism of hydrogenation at 900~950 psi with copper-chromite catalyst was investigated with pure methyl esters as well as their mixtures. A comparison of double bond distribution intrans-monoenes formed during hydrogenation of linoleate and alkali-conjugated linoleate revealed that 85~95% of the double bonds in linoleate conjugated prior to hydrogenation. The mode of hydrogen addition to conjugated triene and diene at high pressure is similar to that at low pressure but positional and geometric isomerizations of unreduced conjugated esters were less at high pressure. Geometric isomerization of methyl linoleate and linolenate was considerable at high pressure whereas it was negligible at low pressure. The absence of conjugated products during hydrogenation of polyunsaturated fatty acid esters resulted from their high reactivity. Conjugated dienes are 12 times more reactive than the triene, methyl linolenate, and 31 times more reactive than the diene, methyl linoleate. The products of methyl linolenate hydrogenation were the same as those predicted by the conjugation mechanism. Presented at the 70th Annual Meeting of the American Oil Chemists' Society, San Francisco, April 29~May 3, 1979.     

Selective hydrogenation with copper catalysts: I. Isolation and identification of isomers formed during hydrogenation of linolenate

Methyl linolenate was hydrogenated with 10% copper chromite catalyst at 150 C and atmospheric hydrogen pressure. The product was separated into monoene, diene and triene fractions by countercurrent distribution. These fractions were further separated into various geometrical isomers. The double bond location in the various fractions was determined by reductive ozonolysis. Double bonds in bothcis andtrans monoene fractions, as well as incis,trans andtrans,trans conjugated dienes, were extensively isomerized. A monoene containing vinylic unsaturation was one of the major products. The nonconjugated dienes were mostly dienes whose double bonds were widely separated. Results are explained on the basis of conjugation of the double bonds in linolenate followed by hydrogen addition. Presented in part at the symposium “Hydrogenation Process,” Division of Industrial Engineering Chemistry, 157th American Chemical Society Meeting, Minneapolis, April 1968. No. Utiliz. Res. Dev. Div., ARS, USDA.     

Selective hydrogenation of soybean oil: XI. Trialkyl silane-activated copper catalysts

Addition of triethyl silane to copper stearate resulted in an active heterogenous catalyst for the hydrogenation of soybean oil. The linolenate selectivity of this catalyst (K_Le/K_Lo=2.4 to 3.9) was much lower than that obtained with copper chromite (8.4). Unlike copper-chromite catalyst, triethyl silane-activated copper formed stearate during hydrogenation. Both silica and alumina increased catalyst activity. Linolenate selectivity improved slightly in the presence of alumina.     

Selective hydrogenation of soybean oil: VII. Poisons and inhibitors for copper catalysts

Hydrogenation rates for the catalytic reduction of soybean oil with a copper-on-silica catalyst increased when the oil was re-refined and bleached in the laboratory. Purification of the re-refined and bleached oil by passage through alumina further enhanced hydrogenation rates. Since these observations suggested that poisons were present in the oil, the effect of minor components of soybean oil upon the activity of copper catalysts was investigated. Free fatty acids, monoglycerides, β-carotene, phosphoric acid, sodium soaps, phosphatides, glycerine, choline, ethanolamine, water, pheophytin, and pyrrole all reduced hydrogenation rates when added to the oil. Organic sulfur added to the oil was a more effective catalyst inhibitor than inorganic sulfur added to the gas. Catalyst activity was affected adversely when iron was added to the oil as a soap or when deposited on the catalyst during its preparation. Squalene, copper soaps, and carbon monoxide had no influence on the activity of the catalyst. Aging of soybean oil also had no effect. There was no significant change in either selectivity or formation oftrans or conjugated diene isomer when these additives were added to the oil.     

Selective hydrogenation of soybean oil in the presence of copper catalysts

Many investigators associate the poor keeping properties of soybean oil with its linolenic acid content. On the other hand the high linoleic acid content is a desired property from a nutritional point of view. We have therefore developed a process for the preferential reduction of the linolenic acid content by selective hydrogenation. Conventional catalysts for the hydrogenation of fats have a rather low selectivity in this respect. When linolenic acid in soybean oil is hardened (e.g., with a nickel catalyst), most of the linoleic acid is converted into less unsaturated acids. It was found that linolenic acid is hydrogenated much more preferentially in the presence of copper catalysts than in that of nickel and other hydrogenation catalysts. At a linolenic acid content of 2%, soybean oil hardened with nickel catalyst contained about 28% linoleic acid, whereas with copper catalyst the hardened soybean oil contained 49% linoleic acid. By means of our process it is possible to manufacture a good keepable oil of, e.g., I.V. 115 and containing 1% linolenic acid and 46% linoleic acid. The storage stability of this product is comparable with that of sunflower-seed oil. A liquid phase yield of 86% is obtained after winterization at 5C for 18 hr. The high selectivity for linolenate reduction of copper catalysts must be ascribed to the copper part of the catalyst. Investigations into the structure of the catalyst indicate that the active center consists of copper metal crystallites; whether these centers are promoted by the carrier or traces of other substances is under investigation.     

Selective hydrogenation of soybean oil. II. Copper-chromium catalysts

Soybean oil was partially hydrogenated at 170 and 200C with 0.5 and 0.1% copper-chromium catalysts, respectively. The reaction proceeded selectively at both temperatures, although selectivity was better at the lower temperature. Both commercial and laboratory-prepared catalysts reduced the linolenic acid to less than 1% and with selectivity ratios (K_Le/K_Lo) ranging from 6 to 13. Since stearate did not increase, linoleate selectivity (K_Lo/K_Ol) was extremely high. About 80% or more of the original linoleic acid remained in the hydrogenated products as measured by the alkali-isomerization method. More conjugated dienes were formed at 200 than at 170C.     

Selective liquid-phase hydrogenation of 2,6-dinitrotoluene with platinum catalysts

The kinetics of the consecutive liquid-phase hydrogenation of 2,6-dinitrotoluene to 2-amino-6-nitrotoluene and 2,6-diaminotoluene was studied in ethanol with 0·5% Pt/Al₂O₃ as a catalyst using a stirred tank slurry reactor in the temperature and pressure ranges of 313 to 348 K and 0·5 to 10 MPa, respectively. The intrinsic kinetics of the consecutive reaction can be described by a Langmuir-Hinshelwood type model with non-competitive adsorption of organic species and hydrogen on the catalyst surface. At 313 K a maximum yield for the intermediate 2-amino-6-nitrotoluene of over 95% can be achieved if heat and mass transfer effects are eliminated.     

Selective hydrogenation of soybean oil with sodium borohydride-reduced catalysts

The reaction of metallic salts in aqueous solution with sodium borohydride produces finely divided metals that are catalytically active for hydrogenation. Salts of nickel, cobalt, palladium and platinum give active catalysts for the selective hydrogenation of soybean oil. Iron and silver salts, when reduced with sodium borohydride, show no activity at 200C and atmospheric hydrogen pressure. The cobalt catalyst produces the least amount of stearate. Incorporation of palladium, platinum, copper or chromium up to 2% enhance the activity of the nickel catalyst. Copper and chromium salts, when reduced together, form catalysts that hydrogenate linolenyl groups in soybean oil seven times more rapidly than linoleyl groups. No stearate formation is observed with these binary catalysts. Presented at the AOCS Meeting, Houston, April 1965. No. Utiliz. Res. Dev. Div., ARS, USDA.     

Kinetics of hydrogenation of conjugated triene and diene with nickel,palladium, platinum and copper-chromite catalysts

A mixture of methyl linoleate and alkali-conjugated methyl linoleate was reduced with nickel, palladium, platinum and copper-chromite catalysts. The course of hydrogenation was followed by gas liquid chromatography of samples withdrawn at intervals. Relative rate constants of reactants and inermediates were calculated by a computer. Conjugated linoleate was 10–18 times more reactive than methyl linoleate with all catalysts except platinum, which showed no selectivity at 60 C. At 150 C conjugated diene reacted four times faster than methyl linoleate with platinum catalyst. A conjugated diene-to-stearate shunt was observed with palladium and platinum catalysts. When β-eleostearate was hydrogenated with the same catalysts, 50–97% of the triene was reduced directly to monoene with all catalysts except copper chromite, which selectively reduced conjugated triene to conjugated diene. On the basis of present kinetic data and previous knowledge about the mode of hydrogen addition to conjugated systems, a scheme has been proposed to account for the products formed during hydrogenation of methyl linolenate. ARS, USDA.     

Selective hydrogenation of soybean oil. VI. Copper-on-silica gel catalysts

The preparation of copper-on-silica gel catalysts containing 15% and 20% copper is described. These catalysts can be reused three times without appreciable loss of activity. Their activity compares favorably with the highly active 5% copper-on-silica gel catalyst previously reported. Higher copper catalysts are somewhat less selective for the reduction of linolenate in soybean oil than 5% copper-on-silica gel, but these copper catalysts have greater activity, better reuse characteristics, and selectivity comparable to commercial copper-chromite catalysts. No. Mark. and Nutr. Res. Div., ARS, USDA.     

Isomerization phenomena during hydrogenation of methyl oleate and methyl elaidate over nickel-silica catalysts

Methyl oleate was hydrogenated at temperatures varying from 50–175 C over three nickel-silica catalysts of different pore-size distribution. Methyl elaidate was reduced over one of these catalysts at temperatures between 75–150 C. From the detailed double bond distributions information was obtained on transport phenomena in the pores of the catalyst. It was established that the migration of the double bond in methyl oleate proceeds with a stepwise mechanism, and evidence was obtained that the double bond in methyl elaidate migrates significantly faster than that in methyl oleate, while the rate of hydrogenation of these esters was equal. Thetrans-cis ratio of the geometrical isomers which are formed by double bond migration varies strongly during hydrogenation.     

Selective hydrogenation of soybean oil: XII. Trialkyl aluminum-copper stearate homogeneous catalysts

The reaction of copper stearate with triethylaluminum (TEAL) formed a soluble catalyst that promoted the selective hydrogenation of the linolenyl groups in soybean oil. This homogeneous catalyst was more active than copper-chromite. The activity was enhanced by the addition of silica, alumina or titania. Ethyl alcohol accelerated the hydrogenation when it was added in small amounts and retarded hydrogenation when increased amounts were added. More active and, in some cases, more selective catalysts were formed when TEAL was replaced by trialkylaluminum compounds containing longer chain length in the alkyl groups. Among other organometallics tested, diethylmagnesium and diisobutylaluminum ethoxide formed catalysts with activity comparable to heterogeneous catalysts (K_Le/K_Lo=2.8~5.2) was less than that obtained with copper-chromite (12~14), but greater than that of commercially used nickel catalysts (2). Isomerization, as measured by the percentage oftrans isomers formed, was similar to that of heterogeneous copper catalysts (%trans/ΔIV=0.6~0.7). Presented at the AOCS meeting in New Orleans, May 1981.     

Transfer hydrogenation and transfer hydrogenolysis: XII. Selective hydrogenation of fatty acid methyl esters by various hydrogen donors

The selective hydrogenation of methyl linoleate was studied using various organic compounds as hydrogen sources in the presence of homogeneous and metallic palladium catalysts. Complete selectivity to monoenes and relatively little formation of isolatedtrans double bonds were realized by the hydrogen transfer from L-ascorbic acid at 47% conversion of starting material to hydrogenation products. The hydrogenation bytrans-1,2-cyclohexanediol catalyzed by RuH₂(PPh₃)₄ also showed rather high selectivity tocis-monoenes. In the reaction catalyzed by RuH₂(PPh₃)₄, also showed rather high selectivity tocis-monoenes. In the reaction catalyzed by RuH₂(PPh₃)₄, the presence of these hydroxy compounds increased the isomerization of methyl elaidate tocis-monoenes.