| Equilibrium-limited reaction is a common phenomenon in nature, it limited the final conversion of a reaction under determined conditions. Common Equilibrium-limited reactions seen in industry production include esterification, acetalizaion, nitration reaction, etc. It is a main challenge facing organic chemistry production regarding how to improve the yield, increase the raw material conversion and decrease the energy consumption in downstream purification process.In this work, we propose the fabrication of novel composite "sandwich-like" catalytically active membrane using the technique of immersion phase inversion for the conversion enhancement of equilibrium-limited liquid phase reaction in a catalytically active pervaporation membrane reactor. First, the membrane preparation conditions were optimized to improve the catalytic activity from the aspect of membrane structure. Then, using the dehydration of butanol-water solution as a test solution, the separation performance of the composite catalytic membrane was evaluated in a pervaporation experiment. Finally, using the esterification between n-butanol and acetic acid as a test reaction, we investigated the reaction-separation coupling performances for catalytically active membrane reactor with dense catalytic layer (dCAMR), catalytically active membrane reactor with porous catalytic layer (pCAMR) and traditional inert membrane reactor (IMR), respectively. The mass transfer behavior of all the components in the membrane reactor was also evaluated. We concluded that the using of pCAMR can further enhance the conversion displacement, the final conversion of the reaction in pCAMR was higher than that in dCAMR and IMR under the same conditions.When optimizing the porous structure of the catalytic layer, we found that the PVA concentration of the casting solution played a key role in determining the final membrane structure, that is:when lower PVA concentration of 5 wt.% was used, a nodule-like surface structure would formed, when higher PVA concentration of 10 wt.% was used, a sponge-like surface structure would formed. The optimal catalyst concentration for casting solution is 10 wt.% and it tends to form a dense structure when catalyst concentration of casting solution is higher than 10 wt.%. Through the catalytic esterification between n-butanol and acetic acid, we compared the catalytic activity between different catalysts:catalytic membrane with dense structure= catalytic membrane with nodule-like structure (PVA concentration 5 wt.%)> free catalyst beads> catalytic membrane with sponge-like structure (PVA concentration 10 wt.%). A Q-H kinetic model was also developed based on the kinetic data and the model results fits the experimental results very well.Through the dehydration of n-butanol/water solution, we found that the separation factor in pCAMR is smaller than that in dCAMR and IMR, this means the coating of a catalytic layer on the selective layer using the technique of immersion phase inversion would cause some mechanic damages to the selective layer, thus the separation performance decreased. However, most importantly, the overall mass transfer resistance in pCAMR is comparable to that in IMR, this results indicated that the using of high porous catalytic layer of the catalytic membrane could offer the benefits of better accessibility of the catalytic sites, lower mass transfer resistance of the membrane. The mass transfer resistance in dCAMR was the highest when compared to that in pCAMR and IMR because a dense membrane was used in dCAMR.In the reaction-separation coupling experiments, we found a similar concentration behaviors for all the components of the reaction for IMR, dCAMR and pCARM, that is:with increasing the reaction time, the concentration of acetic acid and n-butanol decreased synchronously, the concentration of ester and water increased at first, but deviated after a time period, the concentration of ester continued to increase and the concentration of water started to decrease, this is when the water removal rate was higher than water forming rate. The acetic acid conversion increased with increasing the reaction time, there was no Equilibrium-limitation in both three reactors because the separation of byproduct water out of the reactor. And the acid conversion increased with increasing the reaction temperature. The total flux behaviors in three reactor was also similar:at the beginning of the reaction, the flux increased, ant then it started to decrease after some time period.In pCAMR under 85℃, it was found that there is no water found in the reactor after 28 hours of reaction, but we can still collect water in the permeate of the experiment, this phenomenon demonstrated that water was removed as soon as it was formed in the catalytic membrane, a real "in situ" product removal was achieved. When further decreased the catalyst loading from 4.5 g/L to 2.25 g/L for the pCAMR, this "zero water concentration time" shortened to 23.5 hours.It was demonstrated when comparing the conversion enhancement between three reactors that the use of catalytically active membrane with highly porous catalytic layer would further the conversion displacement and thus a higher acid conversion could be achieved under same time period:the conversion in pCAMR achieved as high as 85% at 35.5 hour while the conversion in IMR was only about 75%.To demonstrate that the immersion phase inversion is a versatile way to fabricate of catalytically active membrane with highly porous catalytic layer, we also tested its feasibility for the immobilization of other catalysts and reaction systems. We prepared an ion-exchange resin/PVA/PES composite catalytic membrane to enhance the conversion of esterification of n-butanol and acetic acid, batch reaction results show that its catalytic activity is comparable to that of catalyst beads. The result of esterification reactions in pervaporation membrane reactors (PVMRs) shows that the acetic acid conversion reaches 91.4% in 20 h at 85℃ by using the catalytically active membrane, while the equilibrium conversion is around 71.9% under the same condition. The catalytic activity and structure stability of the catalytically active membrane are further confirmed by consecutive experiments and SEM analyses.A composite catalytically active membrane immobilized with Candida rugosa lipase has also been prepared by immersion phase inversion technique for enzymatic synthesis of lauryl stearate in a pervaporation membrane reactor. SEM images showed that a "sandwich-like" membrane structure with a porous lipase-PVA catalytic layer uniformly coated on a polyvinyl alcohol (PVA)/ polyethersulfone (PES) bilayer was obtained. The membrane was proved to exhibit superior thermal stability, pH stability and reusability than free lipase under similar conditions. In the case of pervaporation coupled synthesis of lauryl stearate, benefited from in-situ water removal by the membrane, a conversion enhancement of approximately 40% was achieved in comparison to the equilibrium conversion obtained in batch reactors. In addition to conversion enhancement, it was also found that excess water removal by the catalytically active membrane appears to improve activity of the lipase immobilized. |
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