| As an important neurotransmitter and phospholipid precursor, L-a-glycerophosphocholine (GPC) can support human health through a variety of mechanisms, it has important roles in the nervous system, reproductive system and endocrine system, et al. GPC has myriad functions for improving people’s cognitive ability, resisting muscle atrophy and aging, speeding up the recovery of brain damage caused by surgery. As a medicine product and functional food, the most acceptable method for preparing GPC is to remove fatty acyl from natural lecithin (natural source of phosphatidylcholine (PC)) by transesterification. However, there are many problems existed in the conventional inorganic base-catalyzed transesterification, such as the more side reactions, serious emulsification, difficulties in separation, high quality requirement for the materials, and the large amount of wastewater produced during the processes. In this paper, the organic base catalysts were used instead of the inorganic ones to prepare GPC, because these catalysts were accessible, and the reactions could performed under mild conditions. The reaction conditions were studied in details for each kind of catalyst, and the competition in the process, the reaction kinetics and thermodynamics rules were analyzed, in order to provide a reference for predicting reaction process for industrial applications and a theoretical support for in-depth understanding of such reactions.Series of low boiling point organic amines were used in the catalytic transesterification of natural PC for preparing GPC, the n-propylamine, isopropylamine and tert-butylamine with better activities among the amines were selected and used in the deacylation reaction. It was found that the activity of tertiary amines were poor; and for the isomers, the branched amines showed better activity than the normal amines; for normal alkylamines, the activity decreased with the increase of carbon numbers. The n-propylamine, isopropylamine and tert-butylamine showed similar laws in the deacylation process:when the amount of catalyst was2.4%, concentration of phosphatidylcholine (PC) was0.05-0.10mol/L, reaction temperature was60℃, reaction time was260minutes, both the conversion of PC and yield of GPC were higher, and the reaction activation energy was minimum catalyzed by isopropylamine. In addition, the low boiling point amines can recovered by distillation combined with the recovery of methanol, they could get similar effect to the fresh catalyst after repeatedly used. GC and1H NMR were used to analyze the product, it was indicated that the product had high purity and there was no side reaction between the catalyst and other materials. The mechanism and kinetics of the transesterification catalyzed by the low boiling point amines were studied. It was find that the reaction can be better described by the alkoxy anion mechanism, and the second-order kinetics model deduced from the mechanism was in good agreement with the experimental results.The transesterification catalyzed by quaternary ammonium base phase transfer catalyst was studied, and the choline hydroxide showed good performance in the process, then the reaction conditions of the transesterification catalyzed by tetrabutylammonium hydroxide and choline hydroxide were studied comparatively. With tetrabutylammonium hydroxide, PC reacted completely after260minutes under a stirring speed of500rpm, catalyst amount of0.015mol/L, reaction temperature of60℃, and molar ratio of methanol to PC was30:1. With the same conditions, when choline hydroxide used as catalyst, PC could react completely under the catalyst amount of0.012mol/L. This kind of catalyst reduced the reaction temperature and shortened the reaction time, especially for choline hydroxide, which used as a food additive, it can not affect the safe use of GPC even a small amount of residue in the product. The mechanism and dynamic model of the transesterification catalyzed by quaternary ammonium base phase transfer catalyst were discussed, it was found that the catalysis process can be described by the Starks extraction model, and the pseudo-first-order kinetics model associated with the experimental data well, the reaction activation energies were32.0and30.5kJ/mol respectively with tetrabutylammonium hydroxide and choline hydroxide.The immobilization of quaternary ammonium base phase transfer catalyst was researched, and a series of quaternary ammonium base resins were prepared from chloromethyl polystyrene and different tertiary amine, the resins were attempted to be used in the catalytic transesterification for preparing GPC from natural PC. Both the resin preparation conditions and the reaction conditions of the transesterification were optimized. The results demonstrated that the resin had better catalytic activity under the conditions of:1,4-dioxane as solvent, triethylamine as aminating agent, temperature was60℃and amination time was3h. With the prepared TEABR resin, the conversion of PC can reach99%, while the yield of GPC was88.3%with the conditions of:the stirring speed was675rpm, amount of catalyst was80g/L, reaction temperature was60℃, concentration of PC was0.15mol/L, and reaction time was210minutes. The use of quaternary ammonium resin as a promising heterogeneous catalyst simplified the craft and minimized the problems existed in the homogeneous catalytic process, and the activity and stability of the catalyst were almost invariant after repeated10times.Thermodynamic behavior of the transesterification between PC and methanol was carried out. The results showed that the first step of the transesterification for generating lysophosphatidylcholine (LPC) was endothermic, and higher temperature favored the progress of the reaction; while this step did not occur spontaneously because the AGθ(T) was positive. The second step for generating GPC from LPC was exothermic, and lower temperature favored this reaction; while the AGeθ(T) was negative, this step can be carried out spontaneously. Elevating the reaction temperature to start the first reaction was the key step during the transesterification, so higher temperature and alchols concentration favored the formation of GPC.The heterogeneous reaction kinetics of the transesterification catalyzed by TEABR resin was studied, and the kinetic model was proposed respectively when methanol adsorption, surface reaction and product desorption as the rate-determining step. From the models the kinetic parameters were estimated, the results demonstrated that the experimental data fit the model better than others when the surface reaction was the rate-determining step, and this model was reliable enough to provide an important guidance for the industrial production of GPC. |
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