Study On Synthesis Of Benzaldehyde And Its Derivatives By Indirect Electrooxidation With On-Line Ultrasound

As an important species of organic compound, benzaldehyde and its derivatives were widely used in synthesis of many kinds of chemicals, such as Pharmaceuticals, spiceries, cosmetics, dyes, pesticide, etc. It has high annual demands, promising market and good economic benefits. Studying theoretically and practically on the methods for its preparation has been all along a significant project for the chemist, since it has both good economic benefits and high academic values.In this paper, benzaldehyde and its derivatives were prepared by electrosynthetic method with on-line ultrasound; meanwhile the course and the mechanisms were studied. In the research, toluene and xylene were employed as raw materials, Mn(III)/Mn( II) , Ce(IV)/Ce(III) and Mn( II )/MnO₂ as oxidative medias, respectively. Corresponding oxidants Mn(III), Ce(IV), MnO₂ were electrolyzed through sonoelectrooxidation method, and were applied to oxidate the raw materials to prepare benzaldehyde and its derivatives under the ultrasonic radiation. Sonoelectrosynthesis is a method in which the electrolysis-synthesis technique and the ultrasonic technique were well combined, and it took full advantage of ultrasonic functions such as well-stirring, emulsification and effects of cleaning, purging, depassivating to the electrodes' surface; meanwhile it made the best of electrolytic technique's merits such as high efficiency, low energy consumption, high selectivity, warm reaction conditions and general equipments. Thus oily phase and water phase were contacted sufficiently, and mass transfer was promoted, which resulted in less reaction time and high product yields. This method overcame the weak points of traditional organic synthetic technique; moreover it strengthened the primary electrosynthetic technique. As only electron agent aside from the reactants took part in the reaction, it was not only a new effective and energysaving technics in the fine chemical engineering area, but also a clean synthetic way with nearly no pollutions to the environment, and that it was an advanced field of sonoelectrochemistry nowadays.In order to detect product's quality and to adjust the synthetic conditions, UV and RP- HPLC methods for the determination of the target product were established according to the components of products in the electrosynthesis course, by which target product were able to be simply, fast and accurately determined without separation in advance. And satisfactory results were obtained when they were applied to the determined of the product's content. By comparing the two analytical methods, it was found that RP-HPLC method had a wide linear range, and a high recovery, furthermore it was applicable to content determination of tolualdehyde and maleic acid. Therefore, all the target products on the experiment were analyzed by RP-HPLC method.When ultrasound was brought into the course of electrooxidating Mn( II) to Mn(III) , it could continuously swash and clean the electrode surface, cleared away the air bubbles around the electrode and maintained its activity. As a result, mass transfer rate was increased, while oxidative potential was decreased, and current efficiency was promoted. What's more, it made the electrolytic reaction time less and the concentration of sulfuric acid lower than those by mechanical stirring. All mentioned above indicated a special function of ultrasound. By optimizing the electrolytic conditions in the experiment, best reaction conditions for the electrooxidation of Mn( II) to Mn(III) were given as : Electrolytic time was 4100s, current density was 0.09A/cm² , concentration of sulfuric acid was 5.5mol/L, concentration of manganese sulfate was 0.4mol/L, electrolytic temperature was 20℃, ultrasonic frequency was 59kHz, and ultrasonic output power was 175W. In this case, the current efficiency of electrolysis was up to 84.79%. When ultrasound was employed in the heterogeneous reaction of preparation for benzaldehyde and three species of tolualdedhyde, the oxidant solution could be reproduced and utilized repeatedly by electrolysis with little changes of the effect that less than 4% and 2% decrease in the current efficiency and benzaldehyde's yield respectively. Conditions of benzaldehyde's preparation by oxidizing toluene in the research were obtained as follows: reaction temperature was 60℃, mole ratio of toluene and Mn(II) was 4:1, concentration of sulfuric acid was 7.0mol/L, ultrasonic frequency was 59kHz, and ultrasonic output power was 175W. While the method was applied to prepare p-,m-,o-tolualdehyde by oxidizing corresponding xylene, the effect of ultrasound was more remarkable. As it could reduce the spatial block effect between the two methyl group in the molecule, concentration of sulfuric acid needed was decreased, thus it could met the demands in the reaction without adjusting the acid concentration. Moreover, product's yield was apparently improved compared with stirring. The yield of p-tolualdehyde, m-tolualdehyde, and o-tolualdehyde was increased by 4.3%, 19.0% and 20.7%, respectively.In the experiment it was found that the higher ultrasonic frequency was always apt to be chosen whether in the electrolysis or in the oxidation. This might be because the ultrasonic frequency of 59kHz was more close to the natural frequency of the air bubble, which caused a most effective energy coupling of the two, thus the function of ultrasound could be fully carried out. The effect of ultrasonic power fluctuated in the electrolytic course, this might result from that the effect of ultrasonic power had a stronger effect to the microjet and the shockflow caused by cavitation of ultrasound than that to the subeffect such as emulsification. Meanwhile, experimental results showed that it didn't change the oxidative mechanism of Mn(II) in the electrolysis when the ultrasound was introduced in. Mn(II) didn't directly exchange electrons with Pb electrode, but exchanged electrons with Pb(IV) created on the Pb electrode surface, that was, Mn(II) was oxidized into Mn(III) by Pb(IV), which was reproduced on the electrode surface after reduced. Under ultrasonic radiation, electrolytic potential was diminished; in contrast, electrolytic yield and current efficiency were promoted effectively.In the study, the procedures and mechanisms of preparing Ce(IV) oxidative solution and of producing compounds like benzaldehyde by oxidation of toluene and its derivatives with Ce(IV) were discussed. In undivided electrobath, cathode would reduce the Ce(IV) just created on the anode, which caused a decease of the electrolytic yields. Therefore, divided electrobath was employed, and a dielectrode system with PbO₂/Pb electrode as anode while pure Pb electrode as cathode was chosen. Affecting rules of different factors to the current efficiency or the electrolytic yield were found out through orthogonal experiments and solofactor experiments. With integrated consideration of current efficiency, electrolytic yield, electrical efficiency, spatio-temporal efficiency and so on, suitable conditions for galvanoplastics electrooxidation of Ce(III) were as follows: ultrasonic frequency was 59kHz, ultrasonic power was 200W, current density was 0.06A/cm², electricity passed through was 1.0F/mol, anodic electrolyte was composed of 0.4mol/L Ce(III) and 0.5mol/L H₂SO₄, cathodic electrolyte was merely 0.5mol/L H₂SO₄. On these conditions, both the current efficiency and electrolytic yield of electrooxidating Ce(III) to Ce(IV) could reach 87.93%. Moreover, on the occasion that ultrasonic frequency was 59kHz, ultrasonic power was 200W, reaction temperature was 75℃, mole ratio of toluene or xylene and Ce(IV) was 3:1, concentration of sulfuric acid was 7.5mol/L(4.0mol/L for xylene), the product yield could be up to 95.78%, 84.7%,81.8% and 73.5% respectively, when Ce(IV) was utilized to oxidize toluene or certain xylene for preparation of benzaldehdye and p-,m-,o-tolualdehyde. Meanwhile, mechanisms of electrode reaction was studied by cyclic voltammetry, and it was found that Ce(III) was mainly oxidized into Ce(IV) by releasing electrons directly on the electrode surface, which was proved by polarization curves subsequently.Comparing Ce(IV) /Ce(III) with Mn(III)/Mn( II) oxidative medias, we learned that:â‘ A better results could be achieved using Ce(III) than using Mn(III) as oxidant to prepare benzaldehyde and tolualdehyde from toluene or xylene with the product's yield 10%～15% higher;â‘¡Mechanisms of the two electrooxidation reaction were different from each other. Ce(IV) was directly oxidized from Ce(III), but Mn(III) was produced by indirect electrooxidation of Mn( II), and concentration of sulfuric acid needed in the former was only one tenth of that in the latter;â‘¢Different electrobathes were employed. More complicated divided type was used in the former, while undivided type was used in the latter;â‘£In the electrooxidation of Ce(III) to Ce(IV), as 1.0F/mol electricity passed through 5.6 hours of time were needed, which was 4.7 times of that in Mn( II) to Mn(III). Therefore, with integrated consideration of the situations mentioned above, if the cathode reaction could be sufficiently utilized, preparation of benzaldehyde and tolualdehyde from toluene and xylene by electrooxidation with Ce(IV) /Ce(III) as oxidative media would be more advantageous.Concerning to the problems in electrooxidation of Ce(III) to Ce(IV) that undivided electrolysis had a lower yield while divided electrolysis consumed more energy, a new paired electrolytic system was set up, that was, ion membrane paired electrolysis for preparations of Ce(IV) coupling with succinic acid with on-line ultrasound. Experimental results showed that, the new paired electrolytic system was completely applicable; moreover, it was of high current efficiency and electrolytic yield (or transformation ratio) either on the anode or on the cathode. The average current efficiency was 92.71% and 87.81% on the cathode and the anode, respectively. Thus the total current efficiency was up to 180.52%. The percent of maleic acid transformed into succinic acid reached 92.09%, and the bath potential decreased by 0.25v compared with single electrooxidation of Ce(III). Additionally, the electrolytic product arisen from the cathode was proved to be succinic acid with purity more than 99.0% by melting point test, IR spectrometry and purity test, etc.In the experiment, preparation conditions of particle MnO₂ by electrolyzing MnSO₄ acid solution with two kinds of electrode made of different materials were optimized and compared, effect mechanisms of different factors were discussed, and diameter, specific surface area and crystal phase of the particle MnO₂ was all determined by several means such as laser diameter analysis, IR and XRD.In the experiment it was found that the particle diameter of MnO₂ produced by grid Pt electrode was much smaller than that produced by PbO₂-Pb electrode. The specific surface area of the former was approximately 54 times as that of the latter. Particles with a diameter under 1 urn accounted to 80% above in the former, while it was not detected in the latter. From the test results, it was learned that, electrolytic product of grid Pt electrode was a mixture crystal ofα-MnO₂ andγ-MnO₂, however,γ-MnO₂ crystal was the major product. Therefore, it was used in oxidizing toluene to prepare benzaldehyde and compared with analytical reagent MnO₂. The results showed electrolytic MnO₂ was of higher oxidative activity, which resulted in a higher yield of benzaldehyde.Through the experiment it was found that no matter which one was employed for oxidizing xylene to prepare tolualdehyde, the same phenomenon turned up. That was, the sequence of oxidative reaction time needed in preparing the three kinds of tolualdehyde was p-tolualdehyde < m-tolualdehyde < o-tolualdehyde, while the sequence of the product's yield was p-tolualdehyde > m-tolualdehyde > o-tolualdehyde. It indicted that paraxylene was most easily oxidized, and then was the metaxylene, while the orthaxylene was the last. What's more, yield of the target product was apparently promoted when ultrasound was imposed on.