Study On Reaction And Mass Transfer In Polycarbonate Synthesis Process Catalyzed By Quaternary Ammonium Hydroxides

The industrial processes for the production of Bisphenol-A polycarbonate(PC)by the non-phosgene melt transesterification method mainly used the composite alkali metal catalytic system.However,this catalytic system is inclined to cause serious side reactions,such as branching and crosslinking,which can result in lower quality of PC.And due to the limitation of mass transfer on the melt polycondensation process in the traditional polycondensation reactor,it is still a challenge to obtain PC with high molecular weight in a short reaction time.In order to prepare high-quality PC efficiently,both melt transesterification reaction and polycondensation process between bisphenol-A(BPA)and diphenyl carbonate(DPC)have been investigated in detail by the combination of computational simulations and experiments in this work.This research mainly included the activity analysis of transesterification catalysts,reaction mechanism and kinetics of transesterification process,characteristics of reaction and mass transfer in polycondensation process,synthesis of PC with high molecular weight,modeling of continuous polycondensation process and optimal design of novel PC falling-film reactor.The results should provide new catalyst,reaction kinetics and design strategy of new-type polycondensation reactor for manufacturing polycarbonate.Density functional theory(DFT)calculation was firstly used to investigate the effect of alkyl chain length on catalytic performance of quaternary ammonium hydroxides(TMAOH,TEAOH and TBAOH)at the molecular level.PC synthetic experiments were aslso carried out using different catalyst combinations.DFT results revealed that TBAOH exhibited a higher catalytic activity,which was mainly attributed to the fact that the active sites of quaternary ammonium hydroxides were located on the hydroxyl,presenting the nucleophilicity.The hydroxyl was stabilized by the weak hydrogen bond with quaternary ammonium cation.Furthermore,the energy gap of quaternary ammonium hydroxides increases gradually with the increased alkyl chain length,leading to a lower binding energy between cations and anions.Thus,the quaternary ammonium hydroxides with higher alkyl chain length promoted a higher binding energy between reactants(DPC and BPA)and hydroxyl.Additionally,the experimental results indicated that the high-quality PC possessing high viscosity average molecular weight in the range of 30000-50000 and containing few by-products could be obtained through the melt transesterification and polycondensation process using quaternary ammonium hydroxides as catalyst.In particular,as the alkyl chain length of quaternary ammonium hydroxides prolonged,the reaction rates of the transesterification process rised and the molecular weight of prepared PC was also increased,which suggested the higher catalytic activity of quaternary ammonium hydroxides with longer alkyl chain length,it is in good agreement with the trend of DFT calculated results.Then,on the basis of the nucleophilic substitution,the catalytic reaction mechanisms of melt transesterification between BPA and DPC were proposed.The basic catalyst could be considered as the nucleophile to directly attack the carbonyl,and it could also promote the dissociation of BPA.The formed BPA also acted as the nucleophile to attack the carbonyl.This synergistic effect could accelerate the transesterification process.Based on the proposed catalytic mechanism,the functional group kinetic model and molecular species kinetic model were developed,respectively.Particularly,both models could well predict the experimental concentration variation of the reactants,phenol and PC oligomers in a range of 155-175 ℃with catalyst TEAOH/BPA of(1.05-13.02)×10-5(mol/mol).The thin film experiments with the thickness range of 0.085-0.68 mm were performed to study the reaction and mass transfer characteristics of PC polycondensation process in temperature range of 250-280 ℃ and pressure range of 10-1000 Pa.According to the quasi-steady state assumption,the overall apparent reaction kinetic model coupled with reaction and mass transfer were developed.The reaction rate constants,equilibrium constants and mass transfer coefficients were obtained with PC molecular weight in the range of 9000-63400.Using the penetration theory,the diffusion coefficients of phenol in melt polycarbonate were determined with the value of(0.87-2.59)×10-10 m2/s and the equilibrium constants were 0.3-0.55 at 250-280 ℃.The obtained mass transfer parameters were further confirmed by the continuous polycondensation disk-reactor model of polycarbonate.Finally,the two-phase steady state model of disk-reactor and the falling-film reactor model for the continuous PC polycondensation process were built by combining the characteristics of the flow and mass transfer in the different-type polycondensation reactor.The two-phase steady state modeling results showed that the mass transfer coefficient with the corresponding value more than 0.27 s-1 had no obvious influence on the polymerization degree of PC products in the range of 250-290 ℃ and 10-200 Pa,when the residence time was in range of 80-160 min with catalyst TEAOH/BPA of(0.77-3.87)×10-4(mol/mol).Additionally,the calculated values of PC polycondensation process were in good agreement with the reported experimental ones in the falling-film pilot reactor.A kind of new-type falling-film polycondensation reactor matching the characteristic of reaction and mass transfer was preliminary designed,which could implement the viscosity average molecular weight increase from 9700 to 48700 within 51 min at temperature 280 ℃,pressure 50 Pa,catalyst TEAOH/BPA of 1.3×10-4(mol/mol).