Pd-catalyzed Hydrogenation Of Unsaturated Fatty Acid Methyl Esters And Mechanism Investigation

Catalyst and process conditions are two main factors that influence oils and fats hydrogenation, what's more, catalyst is the direct acting factor of them. At present, due to it's low price, oils an fats hydrogenation catalysts in the food industry is mainly for nickel-based catalysts. However, using nickel-based catalysts needs high temperature, which can lead to amounts of trans fatty acid(TFA) and poor selectivity. Even though the researches of catalyst and hydrogenation process have been lasted for one hundred years, the problem that how to effectively reduce the TFA generated in the process of oils and fats hydrogenation is still not completely solved. Some studies found that noble metal catalyst(such as palladium) has higher activity than normal metal catalyst in the hydrogenation of oils and fats, meanwhile, due to it's process conditions of low temperature and low pressure, it was widely used in fine chemical industry and pharmaceutical industry. This experiment uses palladium(Pd) as the catalyst of the hydrogenation of unsaturated vegetable oil fatty acid methyl ester and studies its hydrogenation law.Under mild condition, using PdC l2 as catalyst and hydrogen as reductant, this section investigates the hydrogenation of linoleic acid methyl ester. The content of trans fatty acid generated during linoleic acid methyl ester complete hydrogenated into methyl oleate is the main evaluation index of this reaction, the optimal conditions are as follows: under the condition of normal pressure of hydrogen(1atm), anhydrous ethanol as solvent, palladium chloride with linoleic acid methyl ester mole ratio is 0.002, the reaction temperature is 20 ?, and the stirring speed is 1500 r/min. The content of trans fatty acid in the hydrogenated product was 69.3%, the stearic acid content is 5.7%, linoleic acid methyl ester conversion rate is 100%.Using polyethylene glycol(PEG) as reducing agent to prepare Pd-PEG nanoparticles catalysts. The optimal conditions for catalyst preparation are as follows: vigorous stirring, reaction time is 2 h, catalyst type is 0.005mmol/gPd-PEG4000, reaction temperature is 75 ?.In this conditions, linoleic acid methyl ester conversion rate was 100%, among them, more than 85% into methyl oleate. In addition, using the high resolution transmission electron microscopy(HRTEM) for the catalysts prepared under different condit ions were characterized, and found that:(1) high temperature can make the catalyst deactivation during the preparation of catalyst and a part of free Pd(0) exists on the condition of A high concentration of palladium added;(2) under the appropriate cond ition of catalysts preparation, catalysts can form nanoparticles with good crystal forms and uniform dispersion;(3) using nanoparticles catalyst, which the average size of 4 nm, catalyzes the hydrogenation of linoleic acid methyl ester has a good hydrogenation selectivity and generated a low content of trans fatty acid when the iodine value is 77.Using 0.005mmol/gPd-PEG4000 as the catalyst of the hydrogenation of sunflower oil methyl ester. The optimal conditions are as follows: 10 bar pressure, sunflower oil methyl ester and palladium mole ratio of 2000, the temperature of 80 ?, stirring speed 1500 r/min. Under this condition, The content of trans fatty acid in the hydrogenated product was 27.0%, linoleic acid methyl ester conversion rate is 100%.Adding ligands to Pd- PEG nanoparticles catalysts in the catalytic sunflower oil methyl ester hydrogenation reaction. The results showed that: join the sulfur ligands would poison the catalyst and with the increase of sulfur content can make the catalyst deactivation completely. Adding nitrogen ligands or phosphine ligands during the hydrogenation of sunflower oil methyl ester, the ligand concentration has no effect on the activity and selectivity of catalysts. By adding different kinds of single or bidentate phosphine ligand can be found that contains bidentate phosphine ligands in regulating Pd- PEG catalyst selectivity can significantly superior to the single phosphorous ligands, but contains rigid bidentate phosphine ligands on selectivity of catalyst for Pd- PEG regulation effect is poorer.