Novel Preparation Methods Of Polycyclic And Heterocyclic Aromatic Dicarboxylic Acids

Terephthalic acid(TA),2,6-naphthalenedicarboxylic acid(2,6-NDA)and 2,5-furandicarboxylic acid(FDCA)are three kinds of aromatic dicarboxylic acid monomers of polyester materials with monocyclic,polycyclic and heterocyclic structures,respectively.Polyethylene terephthalate(PET)synthesized by TA and ethylene glycol polycondensation are mainly used as fibers,and some are used for bottle flakes.The polycondensation products of 2,6-NDA and FDCA and ethylene glycol are polyethylene 2,6-naphthalene dicarboxylate(PEN)and polyethylene 2,5-furandicarboxylate(PEF)that are mainly used for special polyester bottle flakes,having good gas barrier properties in common.They are suitable for beer bottles,carbonated beverage bottles,food and pharmaceutical bottles,and air-tight polyester films.These three aromatic dicarboxylic acids are prepared by the similar MC(Mid-Century)liquid-phase air oxidation process,using the homogeneous cobalt-manganese-bromine catalysts,and acetic acid as the solvent,but the reaction conditions and results are different.An industrial unit producing 2,6-NDA with an annual capacity of 27,000 ton has been built in the United States by Amoco company in the 1990s,but its technology has not been further promoted.The main problem is that the raw material 2,6-dimethylnaphthalene(2,6-DMN)is prepared by multi-step reaction using o-xylene and butadiene as raw materials with an high cost.FDCA has become a research focus in recent years due to the use of bio-based raw materials such as fructose.The bottleneck of industrialization is still the high cost of its raw material,5-hydroxymethylfurfural(HMF).Therefore,looking for a more economical source of raw materials,exploring suitable preparation routes,comparing the oxidation performance and conditions of di-substituents of different aromatic ring structures under MC conditions is of great significance,not only for the industrial development of 2,6-NDA and FDCA but also for deeply understanding the liquid-phase oxidation of methyl aromatic hydrocarbons with different structures.The intermediate washing oil of coal tar and relatively inexpensive 5-methylfurfural(MF)were used as raw materials to prepare 2,6-NDA and FDCA respectively.The purpose of this paper was to find novel preparation methods to reduce the cost of raw materials for parparing 2,6-NDA and FDCA.Aiming at 2,6-NDA,the intermediate washing oil as the raw material was sequentially subjected to vacuum distillation,melt crystallization,and solution crystallization,and 2,6-DMN with a purity of 99.7%was achieved.The obtained 2,6-DMN was catalytically oxidized into 2,6-NDA in the MC process and the crude 2,6-NDA from oxidation was further hydrorefined to obtain purified 2,6-NDA with a purity of 99.9%.For FDCA,the relatively low-priced 5-methylfurfural(MF)was used for the first time to replace the HMF commonly used in the literature as the raw material,and the liquid-phase catalytic oxidation of MF under the MC conditions was performed to obtain FDCA with a purity of 99.6%.At the same time,the effects of various operation variables on the preparation process were examined experimentally to determine the appropriate operation conditions.Additionally,the key solubility data of the raw material and product crystals were measured,and the intrinsic kinetics of the liquid-phase oxidation of MF to FDCA was measured These investigations laid the research foundation of the chemical engineering for further industrialization.The main work of this article includes the following aspects:1.New 2,6-NDA preparation method.The intermediate washing oil distillate from coal tar containing 11.6%2,6-DMN was used as the raw materials,and the concentration of 2,6-DMN was enhanced to 30.9%by vacuum distillation,and then the purity of 2,6-DMN was further improved to 99.7%via one melt crystallization and three solution crystallization operations.The total yield of 2,6-DMN was 28.4%.Subsequently,the liquid-phase oxidation of 2,6-DMN was performed and the crude 2,6-NDA was obtained under conditions of 170 to 210? and 1.7 to 2.9 MPa using cobalt acetate,manganese acetate,and hydrogen bromide as the catalyst and acetic acid as the solvent.The effects of operating mode,reaction temperature,pressure,feed rate,and catalyst concentration on the yield and purity of 2,6-NDA and side reactions were investigated.Finally,the activated carbon-supported palladium(Pd/C)catalyst was used for further hydrorefining crude 2,6-NDA to increase the purity from 98.5%to 99.9%.The appropriate operating conditions for the separation and purification of washing oil,liquid-phase oxidation of 2,6-DMN,and hydrorefining of 2,6-NDA were determined by experiments.2.New route of FDCA preparation.The effects of temperature,pressure,catalyst concentration and composition,water content,solute/solvent ratio,and co-catalyst cerium on the main and side reactions during the liquid phase oxidation of MF were investigated systematically.The influence of the substitute activity and ring stability on the product yield was evaluated.Experimental results showed that the main by-products of MF oxidation were maleic acid,fumaric acid,2-furoic acid,brominated products,CO and CO2(COx).In the range of 130-170?,the product yield was more than 60%and product purity was greater than 99.3%for the MF oxidation The selectivity of FDCA reached a maximum of 75.1%at 150? with the Co/Mn/Br concentration of 2000/200/2400 ppm.The low water content(0-5 wt%)in the solvent had little effect on the yield and purity of FDCA,but increasing the water content from 7.5 wt%to 10 wt%could significantly inhibit the main reaction,and promote side reactions,and reduce the yield of FDCA.The bond energies of the substituents were analyzed through quantitative calculations,and the oxidation paths and oxidation mechanisms of the substituents were further proposed.It was confirmed that the reactivity order of the functional groups was:hydroxymethyl group>aldehyde group>methyl group.The oxidation experiments of HMF,MF and 2,5-dimethylfuran confirmed that the more active the substituents were,the fewer side reactions were,and the higher the FDCA yield was.By comparing the oxidation experiments of 2,5-dimethylfuran,2,6-DMN and p-xylene,it was found that the stability of the furan ring was much weaker than that of the benzene ring and naphthalene ring,which intensified the ring opening,decarboxylation,and decarbonylation in the MF oxidation,reducing the yield of the main product FDCA.These conclusions could provide a reference for the process optimization of preparing aromatic carboxylic acids by liquid-phase catalytic oxidation.3.MF oxidation kinetics.Based on the radical mechanism and the composition of oxidation products,the detailed reaction pathways were proposed,and a fractional kinetic model of the liquid-phase catalytic oxidation of MF to FDCA was developed.Experiments showed that HMF was easily polycondensed and esterified in acetic acid solvent at 140?,while MF could exist stably.The effects of the initial substrate concentration,reaction temperature,and catalyst concentration on the MF oxidation process were investigated by batch experiments.The kinetic model parameters were regressed by fitting data.The results showed that the oxidation of 5-methyl-2-furanoic acid to 5-formylfuran-2-carboxylic acid was the controlling step of the tandem reactions.The activation energies of each step were in the range of 27.9-56.0 kJ/mol,among which the activation energy of 5-methyl-2-furanoic acid oxidation to 5-formylfuran-2-carboxylic acid was the largest,indicating that the temperature change had a more significant effect on the oxidation of methyl groups than the oxidation of aldehyde groups.The kinetic model was tested by the semi-continuous MF oxidation experiments.The results showed that the kinetic model obtained by batch experiments could be used to predict the results of semi-continuous experiments.4.Measurement of basic data.In order to obtain the basic data of the crystallization purification process of the raw materials and the reaction crystallization of the product in the oxidation process,the equilibrium method was used to measure the solubility of 2,6-DMN in the range from 286.7 to 341.7 K in isobutanol,ethyl acetate,acetic acid n-propyl ester,n-heptane,cyclohexane and 2,2,4-trimethylpentane solvents,and we also measured the solubility of 2,6-DMN in the range from 300.8 to 353.7 K in formic acid aqueous solution(water content:0-20 wt%)and acetic acid aqueous solution(water content:0-20 wt%).?-h equation and NRTL model were used to regress the experimental data and model parameters.The experimental results showed that the solubility order of 2,6-DMN in different solvents was:n-propyl acetate>ethyl acetate>cyclohexane>2,2,4-trimethylpentane>n-heptane>isopropanol>acetic acid>formic acid.Using the same method,the solubility of FDCA in acetic acid aqueous solution(water content:0-30 wt%),methanol aqueous solution(water content:0-30 wt%),and ethanol aqueous solution(water content:0-30 wt%)were obtained.The NRTL model was used to regress the solubility data and the model's binary interaction parameters were optimized by the least-square method.The Van't Hoff equation was used to obtain the relevant thermal parameters.The experimental results showed that the solubility order of FDCA in different solvents was:methanol>ethanol>acetic acid>water.As the water content increased from 0 to 30 wt%,the solubility of FDCA in aqueous methanol and ethanol solutions continued to decrease.The solubility of FDCA in the mixed "acetic acid+water" solvent was greater than the solubility of pure acetic acid and pure water,and the solubility of FDCA in acetic acid solution containing 70 wt% acetic acid was the largest.