Ultrasonic Synthesis of Benzaldehyde from Benzyl Alcohol Using

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Ultrasonic Synthesis of Benzaldehyde from Benzyl Alcohol Using H2O2: Role of Ultrasound Naresh N. Mahamuni, Parag R. Gogate, and Aniruddha B. Pandit* Chemical Engineering DiVision, UICT, Matunga, Mumbai 400019, India

Ultrasound was used in the presence of hydrogen peroxide as the oxidant to synthesize benzaldehyde from benzyl alcohol. A commercially available homogeneous catalyst (heteropolyacid) and a phase-transfer catalyst (PTC) were used for the reaction. The effects of various parameters, such as solvent, amount of PTC, bulk temperature, mechanical agitation, concentration of hydrogen peroxide, amount of aqueous phase, amount of homogeneous catalyst, and benzyl alcohol concentration, on the extent of reaction have been studied. The kinetics of the reaction has also been studied. Enhancements in rates and selectivity of the benzaldehyde were observed in the presence of ultrasound as compared to their silent counterpart. The increased rates of reaction and selectivity are explained on the basis of ultrasonically generated cavitational effects. The work presented here clearly highlights the methodology to be followed for investigating the optimum extent of intensification achieved as a result of the use of ultrasound as compared to the conventional methods and should be followed by the design engineers to investigate a particular application in question. 1. Introduction Ultrasound has been used in the synthesis of many organic chemicals for years. Many chemicals such as acyloins, aldehydes, alkanes, amides, amines, aryl compounds, cyanides, cyclic compounds, carbenes, organometallic compounds, esters, epoxides, halides, ketones, lactones, nitroxides, olefins, pyrozolines, quinones, sulfones, etc., have been synthesized in the presence of ultrasound.1 The presence of ultrasound has shown to enhance the rates of reactions by manifolds. In the presence of ultrasound, reactions have also been reported to occur at less severe conditions and in the presence of less expensive and less active materials as catalysts as compared to their silent counterparts (approach without the use of ultrasound). It has also been reported that the presence of ultrasound reduces the induction period of some reactions2 and also possibly favors the radical reactions in chemistry.3 Although the use of ultrasound has been widespread among chemists for many years, no exact reasons are known for the enhancement of the rates of reactions in the presence of ultrasound. Various mechanisms such as a single electron-transfer mechanism, an increase in the mass-transfer coefficients because of an increase in the contact surface areas due to efficient mixing in the case of heterogeneous reactions,4 stresses created in the solution after implosion of cavities, very high temperatures and pressures reached during cavitational collapses, and transient electrical discharges during collapses are held responsible for the spectacular effects observed as a result of ultrasound. It has also been reported that the use of ultrasound, during the preparation of various catalysts used for the reactions, causes favorable changes in their morphology, size, and size distribution and the nature of the catalysts (amorphous or crystalline). This, in turn, results in the intensification of chemical reactions.5,6 It has also been speculated that the free radicals produced as a result of the cavitation initiate many reactions in the bulk, thus enhancing the rates of reactions.7,8 The presence of a supercritical region at the interface of the solution and bubble (cavity) is also supposed to enhance the rates of some reactions.9 * To whom correspondence should be addressed. Tel.: +91-222414-5616. Fax: +91-22-2414-5614. E-mail: [email protected]

Benzaldehyde is a very important organic intermediate used in flavors such as almond and cherry and in various fragrances for soaps and toiletries. It is also used in the manufacture of pharmaceuticals such as ampicillin, pesticides such as dibenzoquat, dyes such as triphenylmethane green, and perfumes and flavorimg agents such as cinnamaldehyde, amyl cinnamaldehyde, hexyl cinnamaldehyde, etc. It is also used in the manufacture of fireproof structural foam ferrocene polymers, for example, phenol benzaldehyde resins, etc. It is generally prepared either by hydrolysis of benzal chloride or by oxidation of toluene.10 Oxidation of toluene is the preferred process used in industry. Because the prices of toluene have increased a lot in the past decade as a result of an increase in the petroleum prices, the benzal chloride route is becoming economically competitive with that of oxidation of toluene. However, benzaldehyde from benzal chloride contains chloride impurities. Such a benzaldehyde is not suitable for use in the pharmaceutical industry. So, many new processes for benzaldehyde synthesis are being developed. There are many reports in the literature elucidating the synthesis of benzaldehyde from benzyl alcohol.11-23 However, in most of these reports, stress had been laid upon the preparation of different catalysts to increase the rates and selectivity of benzaldehyde formation. There had not been many efforts on the process development front so as to increase the rates and selectivity of benzaldehyde formation via the oxidation route using novel techniques such as sonication. Ultrasound is known to increase the rates of reactions as well as selectivity of the desired products in many cases. Hence, an attempt was made to investigate the effect of ultrasound on the oxidation route of synthesis of benzaldehyde. Hydrogen peroxide has been selected as an oxidizing agent because of its high oxygen availability, good oxidizing power, and environmentally clean nature. Because the reaction is biphasic in nature, a commercially available phase-transfer catalyst (PTC; Aliquat336, i.e., methyl tricaprylammonium chloride) and a homogeneous catalyst (dodecatungstophosphoric acid) have been used. An attempt has also been made to determine the mechanism of possible process intensification through the use of ultrasound. The approach used here is novel in the sense that it clearly quantifies the extent of intensification achieved as a result of the use of sonication (ultrasound) by systematic investigation

10.1021/ie0503601 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/12/2005

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Figure 1. Experimental setup.

of the process, i.e., experiments in the absence of agitation/ ultrasound, in the presence of agitation alone, in the presence of ultrasound alone, and in the presence of ultrasound as well as agitation to check for synergism. The approach and methodology used in the present work have to be followed by the design engineers to decide the path for investigation of any particular reaction in question. 2. Experimental Section Figure 1 shows the experimental setup. Various materials and equipment used in the experimental methodology have been described in the following sections: 2.1. Materials. Benzyl alcohol, benzoic acid, catalyst dodecatungstophosphoric acid (all GR grade), and benzaldehyde (synthesis grade) were procured from Merck Ltd., Mumbai, India. Methylene chloride, ethylene chloride, toluene, and n-heptane (all AR grade) were procured from SD Fine-Chem Pvt. Ltd., Mumbai, India. 50% H2O2 (w/v) (purified grade) and 30% H2O2 (w/v) (AR grade) were procured from Merck Ltd., Mumbai, India, and SD Fine-Chem Pvt. Ltd., Mumbai, India, respectively. The phase-transfer catalyst (PTC) methyl trioctyl(tricapryl)ammonium chloride (Aliquat-336) was obtained from Dishman Ltd., Ahmedabad, India. Demineralized water was used to prepare lower concentrations of hydrogen peroxide. Demineralized water used in all of the experimentation, and analysis was obtained using the Milli-Q Gradient System from Millipore. 2.2. Equipments. The reactions were carried out in a threeneck, round-bottomed flask of 100-mL capacity. An ultrasonic bath (Supersonics) was used for irradiation with ultrasound. The frequency of the ultrasound from the bath was 22.0 kHz. It had a rated power output of 120 W, but because of a lower calorimetric efficiency of ∼34.7%, the actual power dissipation was just ∼42 W, as calculated from the calorimetric studies. Out of this, only 6.7% power was dissipated to the reaction mixture. In some set of experiments where agitation has been considered, the reactor was equipped with a mechanical agitator (six-blade, pitched-blade turbine made up of glass) whose speed can be controlled manually. 2.3. Experimental Procedure. Initially, an organic phase containing a predetermined concentration of benzyl alcohol was prepared in a specified solvent. A predetermined amount of the PTC (Aliquat-336) was then added to it. The whole solution was then transferred to the glass reactor. A total of 1 mL of demineralized water was taken in a 50-mL glass beaker. A predetermined amount of the homogeneous catalyst (dodecatungstophosphoric acid) was added to it, followed by the addition of the proper amount of a hydrogen peroxide solution. The solution was magnetically stirred for 1 min to ensure the dissolution of the catalyst in the hydrogen peroxide solution.

The contents of the beaker was immediately transferred to the glass reactor, which formed a separate top layer over the already present organic layer (because its density was lower than that of the organic layer) in the reactor. The glass reactor was clamped centrally inside the ultrasonic bath (the cavitational activity has been found to be maximum at the center of the ultrasonic bath24). The level of the water in the bath was kept at 4-5 mm above the level of the reactant contents of the glass reactor. For irradiation, a cycle of 25 min of irradiation of “on”, followed by 10 min of silence, was used. Reaction occurs only when either ultrasound is present or mechanical agitation is present. Reaction does not occur when ultrasound or agitation is not present under the operating conditions of temperature and pressure used in the present work. So, reaction does not occur in the silent mode. Hence, this was equivalent to continuous irradiation of the reaction mixture with ultrasound for 125 min. Three sets of experiments, as mentioned in the following, have been carried out in the present work: (1) Some experiments were carried out only in the presence of mechanical agitation. (2) Some experiments were carried out only in the presence of ultrasound. (3) Some experiments were carried in the presence of both mechanical agitation and ultrasound. This was done to compare the extent of yields obtained in various situations. The 1 and 3 types of experiments were used to show the efficacy of using ultrasound over a conventional system. Samples of 0.8 mL each were taken out at regular intervals of 25 min from the organic layer. Anhydrous sodium sulfate was added to the sample to remove any dissolved water and arrest the reaction. All of the experiments were carried out for 125 min. All of the experiments were carried out twice and repeated more times in the case of suspicious values. The error in the values is within (5%. 2.4. Experimental Analysis. The samples were analyzed in the presence of an internal standard for benzyl alcohol, benzaldehyde, and intermediates formed during the reaction using a gas chromatograph (GC). A Chemito 8510 gas-liquid chromatograph with a flame ionization detector was used for the analysis. A stainless steel column (1/2 in. × 2 m) packed with 5% SE-30 supported on chromosorb W (HP) of 80-100 mesh size was used. Synthetic mixtures were prepared and used for identification calibration and quantification. Some samples were also analyzed on a high-performance liquid chromatograph from Jasco Corp., Tokyo, Japan, for identification of byproducts. An ODS Hypersil column from Thermo Electron Corp. was used. Some samples from the aqueous layer were also analyzed on a GC using a glass column packed with 4% carbowax. 3. Results and Discussion Initially, experiments were carried out using nearly 8.2% benzyl alcohol (w/v) in methylene chloride and 0.38 g of the PTC (Aliquat-336). The organic layer was 30 mL in quantity. The aqueous layer consisted of 20 mL of a known concentration of H2O2 and 0.38 g of the homogeneous catalyst (dodecatungstophosphoric acid). The basic reaction mechanism is that the hydrogen peroxide molecules react with the homogeneous catalyst to produce active oxidizing species in an aqueous layer. The PTC transfers these active oxidizing species to the organic layer. Ultrasound or agitation aids the rate of transfer of the oxidizing species as a result of the turbulence generated in the system. A similar analogy of an increase in the rate of mass transfer in the presence of ultrasound for gas-liquid operation was reported in our earlier work.25 These oxidizing species oxidize the benzyl alcohol to benzaldehyde and benzoic acid depending upon the operating conditions in the organic layer.

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Figure 2. Effect of the solvent on benzaldehyde formation.

3.1. Effect of the Solvent. The type of solvent plays an important role in the extent of completion of the reaction, and hence it was decided to study the effect of the type of solvent. Four different solvents, viz., methylene chloride (boiling point 40 °C), ethylene chloride (boiling point 83 °C), n-heptane (boiling point 98.4 °C), and toluene (boiling point 110.6 °C), which had been used in earlier studies,26 have been selected for optimization purposes. As shown in Figure 2, it has been observed that the maximum amount of benzaldehyde is formed when methylene chloride is used as the solvent, whereas the use of ethylene chloride resulted in more yield as compared to toluene and n-heptane. The abovementioned preferential order was similar to the order of the solvents with lower boiling points; that is, methylene chloride has the lowest boiling point (and, hence, higher vapor pressure), whereas toluene has the highest. For liquids with higher vapor pressure, the ease of cavity generation is higher, possibly resulting in a higher number of cavities for the given power input in the system. Although the cavitational collapse generated in vaporous cavities (generated in low-boiling-point solvents) is less violent than that in water, the generated intensity of the cavitational collapse is sufficient to produce microagitation, which enhances the transfer of the oxidizing species from the aqueous layer to the reactant in the organic phase. The high vapor pressure of the medium also reduces the cavitational threshold acoustic intensity, and, hence, for a fixed energy input rate, the numbers of cavitational events are higher in the case of a high-vapor-pressure medium (low-boiling-point medium). The only exception, here, is toluene. Though the boiling point of toluene is higher than the boiling point of n-heptane, a slightly higher amount of product is formed in the case of toluene as the medium. This may be attributed to the fact that toluene can itself be oxidized to benzaldehyde in small quantities in cavitating conditions.27 It is also important to note here that, in addition to the vapor pressure of the medium, the surface tension, dissolved gas content of the medium, and viscosity of the medium also affect the extent of cavitation generated in the reactor. Dissolved gas content was air in this case, which was similar for all of the solvents. Also, as shown in Table 1, the difference in the surface tension and viscosity of the various solvents is marginal and bubble dynamics studies clearly indicate that the extent of cavitational intensity is nearly the same over the range of surface tension and viscosity of the different solvents considered in the present work.28 So, the effect of other properties has been neglected as compared to the dominant effect produced by the vapor pressure. The boiling point is indicative of the vapor

Table 1. Properties of Solvents Used in the Present Work solvent

surface tension (dyne/cm)

viscosity (cP)

dichloromethane n-heptane toluene

27.2 20.5 27.8

0.41 0.39 0.5

pressure of the medium, so the observed effects have been explained in terms of the boiling points of the various solvents. It can also be seen that, in the absence of any solvent, an extremely small amount of benzaldehyde was formed. This is because benzyl alcohol, which has a boiling point of ∼205 °C at atmospheric pressure, is extremely difficult to cavitate. Hence, the effect of ultrasound is likely to be marginal and, hence, there is no increase in the interfacial area between the organic and aqueous layers. In the case of the presence of solvent, the formation of a significant amount of benzaldehyde is observed as a result of an increase in the interfacial area created from the collapse of cavities. The case of the absence of solvent also proves that the reaction is mainly taking place in the organic phase and not in the aqueous phase. This is because, despite nearly 4% solubility of benzyl alcohol in water at room temperature and pressure, there is hardly any formation of benzaldehyde. 3.2. Effect of the Concentration of H2O2. Independent experiments were carried out to study the effect of the concentration of hydrogen peroxide on benzaldehyde formation. 50% and 30% H2O2 were commercially available, and 15% and 3.75% H2O2 were prepared by diluting 30% H2O2 with the appropriate amount of demineralized water. The results are shown in Figure 3a. From Figure 3a, one can observe that there is an optimum concentration of ∼15% H2O2 at which a maximum amount of benzaldehyde (278.3 mg) is formed. Initially, with an increase in the concentration of hydrogen peroxide from 0% to 15%, the amount of benzaldehyde increases from almost negligible to 278.3 mg, whereas a further increase in the hydrogen peroxide concentration to 50% results in a decrease in benzaldehyde formation (128.75 mg). This can be explained by the fact that, for the formation of benzaldehyde, the transfer of active oxidizing species from the aqueous layer to the organic layer is required. This transfer is done by the PTC. In presence of ultrasound, the interfacial area available to the PTC for the transfer of active oxidizing species is increased. The intense microagitation caused by violent collapse of the cavities produced by ultrasound results in a large amount of increase in the interfacial area between the aqueous and organic layers.


Figure 3. (a) Effect of the concentration of H2O2 on benzaldehyde formation. (b) Effect of ultrasound on the concentration of H2O2.

Because the boiling point of hydrogen peroxide is ∼150 °C and the boiling point of water is 100 °C, it is easier to cavitate water than a hydrogen peroxide solution. So, as the water content of the aqueous layer increases, the number of cavitational events occurring per unit dissipated power increases. More cavitation means more microagitation and more transfer of active oxidizing species to the organic layer. However, at the same time, the amount of active oxidizing species formed is also important. The amount of active oxidizing species formed depends on the concentration of hydrogen peroxide. At 15% H2O2, large amount of oxidizing species are formed as compared to the case with lower amount of hydrogen peroxide and they are transferred to the organic layer at a higher rate because of the large number of cavitational events (creating a large interfacial area). This is reflected in the higher amount of benzaldehyde formation (278.3 mg). In contrast, in the case of 3.75% H2O2, though a large number of cavitational events are occurring, a smaller number of active oxidizing species are formed as a result of the lower concentration of hydrogen peroxide. This was reflected in the low amount of benzaldehyde formed in the presence of 3.75% H2O2 (197.2 mg). In the case of 30% and 50% H2O2, though a higher number of active oxidizing species are formed, because of the lower number of cavitational events (because of the lower water content), they cannot be transferred at higher rates to the organic layer (because the interfacial area is low). This is reflected in a lower amount of benzaldehyde formed in these cases (Figure 3a). To prove the above explanation, the effect of ultrasound on different concentrations of hydrogen peroxide was studied. The results are shown in Figure 3b. From this figure, one can see that the molar concentrations of 50% and 30% H2O2 have increased under ultrasound, whereas they remained nearly the

same for 15% and 3.75% H2O2 (for a detailed explanation, please refer to Appendix 1). The greater changes observed in the case of the hydrogen peroxide concentration as 30% H2O2 can be attributed to a higher cavitational intensity, which leads to the formation of a higher amount of hydrogen peroxide in the case of 30% H2O2 inside the bubble than in the case of 50% H2O2.29 Also, because of the higher collapse temperatures generated in the case of 30% H2O2 concentration, the dissociation reaction is favored, resulting in an immediate decrease in the H2O2 concentration. The net result is an instantaneous increase in the concentration and a subsequent decrease because of the higher rates of the dissociation reaction. From Figure 3a, one can also observe that, in the absence of H2O2, there was no formation of benzaldehyde. This shows that there is no formation of active oxidizing species in the aqueous layer without the presence of H2O2 though ultrasound is present. Thus, the quantity of hydrogen peroxide generated from irradiation of water29 is unable to bring out any oxidation reaction for the formation of the desired product, benzaldehyde. Also, in the presence of mechanical agitation alone, a maximum formation of benzaldehyde was observed for 30% H2O2 (143 mg at 800 rpm), whereas in the presence of ultrasound, the maximum amount of benzaldehyde formation was observed at 15% H2O2 (278.3 mg). Thus, one can say that the presence of ultrasound reduces the necessity of using high concentrations of reagents or, in other words, increases the utilization of the oxidants/reagents. 3.3. Effect of the PTC. Two sets of experiments were carried out to see the effect of the PTC. In first set, 50% H2O2 was used as the oxidizing agent, and in the second set, 15% H2O2 was used. This was done to identify the role of ultrasound in enhancing the formation of the product (the extent of cavitation

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Figure 4. Effect of the PTC on benzaldehyde formation.

Figure 5. Effect of the temperature on benzaldehyde formation.

and its role in transferring the oxidizing species is a maximum at 15% H2O2 and a minimum at 50% H2O2). The results have been depicted in Figure 4.

intensity and frequency irradiations to confirm whether the use of ultrasound can completely eliminate the requirement of the catalyst.

From Figure 4, one can see that, in the case of 50% H2O2, nearly the same amount of benzaldehyde was formed (128.75 mg in the case of 0.38 g of the PTC and 125.4 mg in the case of 0.76 g of the PTC), although the amount of the PTC has been increased by 100%. However, it was also observed that the conversion has increased by ∼1.7 times, possibly indicating that benzyl alcohol is getting further oxidized to benzoic acid. In the case of 15% H2O2, as the amount of the PTC increased from 0 to 0.1 to 0.38 g, the amount of benzaldehyde formed increased from 12.7 to 146.8 to 278.3 mg, respectively. This increase in the amount of benzaldehyde formed when the amount of the PTC was increased can be explained from the fact that, as the amount of the PTC is increased, the amount of active oxidizing species transferred to the organic layer from the aqueous layer is increased (cavitational activity is a maximum in the case of 15% H2O2).

3.4. Effect of the Temperature. Continuous ultrasonic irradiation resulted in an increase in the temperature of the solution from ∼29.5 to 38 °C (average 34 °C) over the reaction period of 125 min for all of the runs because it was not possible to operate under conditions of constant temperature because of the limitations of the experimental setup. Whether the increase in the temperature was playing a role in the observed increase in the amount of benzaldehde formed in the presence of ultrasound needed to be checked, and hence the effect of the bulk liquid temperature on benzaldehyde formation with 50% H2O2 was further investigated. The experiments were carried out only in the presence of ultrasound by maintaining the temperature of the water in bath at a predetermined value (15 and 39 °C). The aim of these experiments was to separate the effect of ultrasound and the effect of the rise in temperature due to ultrasound in the overall observed intensification of the reaction. The effect of the temperature on hydrogen peroxide is that hydrogen peroxide is decomposed at a higher rate at higher temperature and at a lower rate at lower temperature. This effect is more pronounced at higher concentrations of hydrogen peroxide.30 Thus, to quantify the role played by the temperature in our case, experiments have been carried out using 50% H2O2, where the extent of the increase in the rate of dissociation due to higher operating temperature will be more as compared to that using 15% H2O2. The results are shown in Figure 5.

One should also note that an extremely low amount of benzaldehyde (∼12.5 mg) is formed in the absence of the PTC and in the presence of ultrasound. There was a large amount of benzaldehyde formed (142.6 mg) in the presence of the PTC and stirring and in the absence of ultrasound. This proves that the presence of ultrasound cannot replace the PTC for the reaction considered in the present work, though it is difficult to generalize this fact. It only aids the functioning of the PTC by creating a large interfacial area for contact between the immiscible layers. More studies are indeed required with higher


Figure 6. Effect of the catalyst on benzaldehyde formation.

It can be observed from Figure 5 that only a small amount of benzaldehyde (∼17 mg) was formed in the case of lower temperature and that too after an induction period. A larger amount (∼36 mg) of benzaldehyde was formed when the temperature was increased to 39 °C, but this amount of benzaldehyde is still far less than the maximum amount of benzaldehyde formed in the presence of ultrasound (128.75 mg) and uncontrolled operation of the temperature with an initial temperature of 30 °C. Thus, it can be seen that the presence of higher temperature is not the sole reason for the observed increase in the amount of benzaldehyde formed in the presence of ultrasound though the higher temperature definitely favors the formation of benzaldehyde. Because the boiling point of methylene chloride is ∼40 °C, the cavitational collapse occurring in the organic layer is cushioned to a larger extent at this temperature of the bulk liquid. So, there exists an optimum temperature at ∼30 °C (initial temperature), at which the maximum amount of benzaldehyde was formed (128.75 mg). It must be noted that this is the temperature at which we observe maximum cavitational activity in water, i.e., in the aqueous layer.31 This further strengthens our assumption that the microagitation caused by ultrasound in the aqueous layer and the interfacial turbulance caused by the same are playing the dominant role in the observed increase in benzaldehyde formation. 3.5. Effect of a Homogeneous Catalyst. In section 3.3, it was concluded that the PTC is absolutely necessary for the formation of a significant amount of benzaldehyde. It is also important to investigate whether the presence of a homogeneous catalyst is necessary or not. The oxidizing agent used in this stage of the investigation was 15% H2O2. The results are shown in Figure 6. One can see that, as the amount of catalyst decreases from 0.38 to 0.1 g, the amount of benzaldehyde formed decreases from 278.29 to 171.73 mg. It was also observed that, in the absence of catalyst, there was no benzaldehyde formation. Thus, one can conclude that the presence of ultrasound cannot replace the catalyst and the catalyst is a must. The decrease in benzaldehyde formation with a decrease in the amount of catalyst can easily be explained by the fact that, as the amount of catalyst decreases, the amount of active oxidizing species formed also decreases. Ultrasound is not having any effect on the catalyst. This was proved by the study in which the aqueous layer containing the catalyst was irradiated with ultrasound for 5 min or even for 30 min before addition to the glass reactor.

Use of this preirradiated aqueous solution in the oxidation of benzyl alcohol in the presence of ultrasound did not improve the formation of benzaldehyde, indicating that continuous use of ultrasound for efficient mixing and transfer at the microlevel is essential. 3.6. Effect of Agitation. It has been observed in the previous sections that, in the presence of ultrasound, the amount of benzaldehyde formed was more than that in the absence of ultrasound. The observed effects have been attributed to enhanced mixing at the microlevel in the presence of ultrasound (in the absence of agitation). The same effect may also be obtained using mechanical agitation though it gives macrolevel mixing/mass transfer. To conclusively establish this possibility, the effect of mechanical agitation at two different speeds, viz., 300 and 800 rpm, at room temperature (30-31 °C) was studied in the absence of ultrasound. The results along with the results of ultrasonic experiments (in the absence of agitation) are given in Figure 7 for the sake of comparison. One can see that, in the absence of mechanical agitation as well as sonication, only 14.25 mg of benzaldehyde was formed using 50% H2O2 in 125 min. When only mechanical agitation at 300 rpm was used, it increased to 48.17 mg, which was 3.38 times more than that in the absence of mechanical agitation and ultrasound. This means that there is a role played by agitation to improve the benzaldehyde formation. Because this is a biphasic reaction, the presence of mechanical agitation increases the interfacial area of contact between the two phases. However, this happens only at the macrolevel and not at the microlevel. A further increase in the speed of agitation to 800 rpm, however, did not result in any further increase in benzaldehyde formation, indicating the limitation of macrolevel mixing/mass transfer. When both mechanical agitation at 300 rpm and sonication were used, benzaldehyde formation was increased to 116.63 mg, which was marginally lower than that when ultrasound alone was used (128.75 mg). This indicates that ultrasonic irradiation and agitation in combination does not give synergistic effects. Thus, one can see that in the presence of ultrasound, benzaldehyde formation has increased 9.04 times that in the absence of both mechanical agitation and sonication. If a comparison is made between benzaldehyde formation in the presence of sonication alone and that in the presence of mechanical agitation alone at 300 rpm, one can observe an increase of ∼2.67 times. One can also compare the effect of agitation at different concentrations of hydrogen peroxide, viz., 30% H2O2, 15% H2O2, 3.75% H2O2, etc., with the effect of ultrasound on

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Figure 7. Effect of agitation on benzaldehyde formation.

benzaldehyde formation at these concentrations (Figure 7). One can see that, at 30% H2O2, the amount of benzaldehyde formed has increased 1.84 times in the presence of sonication compared to that in the presence of mechanical agitation alone at 300 rpm. These values are 2.55 and 1.35 times increases respectively in the case of 15% H2O2 and 3.75% H2O2. Thus, it can be concluded that the maximum increase in product formation is observed for 15% H2O2 (corresponding to maximum cavitational activity at 15% H2O2). The increase in benzaldehyde formation in the presence of ultrasound alone as compared to mechanical agitation alone can be explained easily on the basis of an increased interfacial area between the two layers in the presence of ultrasound. One must note that, in the presence of mechanical agitation, only bulk movement of the liquid layers is taking place and there is no proper mixing because of the absence of baffles, whereas sonication causes agitation at the microscopic level, and hence there is a much larger increase in benzaldehyde formation as a result of a large increase in the interfacial area (formation of fine emulsion). We must note that the velocities of the jets formed in the aqueous solutions upon collapse of the cavities are >170 m/s.32 Such high-velocity jets cause a great deal of turbulence at microscopic levels, causing effective microagitation and a large increase in interfacial areas. Also, one can note that higher conversions were achieved in the presence of mechanical agitation alone (no ultrasound) as compared to the case of no agitation and also no ultrasound but the selectivity of benzaldehyde was very low, whereas slightly lower conversions were obtained in the presence of ultrasound alone (i.e., in the absence of agitation) but selectivities for benzaldehyde were higher. This can be explained by the fact that, because of bulk mixing caused by mechanical agitation, there were certain pockets in the liquid where mixing between two layers was taking place. However, at these points, because of nonuniform transfer of active oxidizing species, overoxidation of benzyl alcohol to benzoic acid was taking place, whereas in the presence of ultrasound alone, due to uniform agitation at the microscopic level, uniform transfer of active oxidizing species was taking place and hence overoxidation of benzyl alcohol to benzoic acid was inhibited. This resulted in higher selectivities for benzaldehyde formation in the presence of ultrasound alone. These results clearly indicate that ultrasonic irradiation can achieve much more effective product formation (improved

selectivity of the desired product because of uniform mixing at the microscopic level) as compared to mechanical agitation. It is also important to note here that 4 orders of magnitude more power is put into the liquid when ultrasound was used as compared to when mechanical agitation was used. We have calculated the amount of power consumed in both cases (see Appendix II). The power consumed in the case of ultrasound is 1.39 × 10-2 W/mL, whereas it is 5.9 × 10-6 W/mL for 300 rpm. 3.7. Effect of the Benzyl Alcohol Concentration. It is very important always to know the kinetics of any chemical reaction, and hence kinetic investigation for benzyl alcohol oxidation to benzaldehyde in the presence of sonication was undertaken. The effect of the benzyl alcohol concentration on the formation of benzaldehyde has been studied. The results are shown in Figure 8. The effects of three different benzyl alcohol concentrations, viz., 14.44, 28.9, and 43.33 mM, were studied. From Figure 8, one can see that, as the concentration of benzyl alcohol increases, there is an increase in the amount of benzaldehyde formed. A total of 221.9, 278.3, and 283.1 mg of benzaldehyde were formed in the cases of 14.44, 28.9, and 43.33 mM benzyl alcohol, respectively. One also observes that, at higher concentration, the amount of benzaldehyde formed has more or less remained the same. This can be explained as follows: Because the amount of active oxidizing species formed and transferred to the organic phase remains the same in all three cases, when the concentration of benzyl alcohol in the organic layer was lower than those of active oxidizing species, a lower amount of product benzaldehyde was formed. When the benzyl alcohol concentration increased in the organic layer, all of the active oxidizing species were consumed to form benzaldehyde and the amount of benzaldehyde increased. Up to this point, the benzyl alcohol concentration is the controlling factor and hence any increase in the concentration results in a corresponding increase in the production of benzaldehyde. Any further increase in the concentration of benzyl alcohol does not increase the amount of benzaldehyde formed because now the rate of transfer of active oxidizing species becomes rate controlling. One can also observe here that the percent conversion decreased with an increase in the concentration of benzyl alcohol although the amount of benzaldehyde has increased. This is because, since only ∼4.0% benzyl alcohol is soluble in water,10


Figure 8. Effect of the benzyl alcohol concentration on benzaldehyde formation.

Figure 9. Kinetics of the benzyl alcohol oxidation process.

a constant amount of benzyl alcohol is dissolved into the aqueous solution in all three cases. Because we are analyzing the samples from the organic layer with GC, we observe a continuous decrease in the concentration of benzyl alcohol. This decrease consists of a constant chunk due to dissolution of benzyl alcohol into the aqueous solution and the continuous decrease in benzyl alcohol due to conversion to benzaldehyde, which we were measuring directly. The solubility of benzaldehyde in aqueous solutions is very low (0.4% only10). So, it does not dissolve into the aqueous layer. That was precisely the reason we have analyzed our results in terms of the amount of benzaldehyde formed and not in terms of the conversion of benzyl alcohol. Some experiments had been done to check whether benzyl alcohol was really dissolved in 15% H2O2 or not? We found that, in fact, some amount of benzyl alcohol (∼4%) indeed dissolved in hydrogen peroxide. However, it was not possible to find the extent of dissolution of benzyl alcohol in hydrogen peroxide because the remaining benzyl alcohol was present in the form of a fine stable emulsion. Because the amount of benzyl alcohol dissolved in the aqueous phase was constant, the profile of the change of the concentration of benzyl alcohol with time remains unaffected and we were able to find the order of the benzyl alcohol oxidation reaction and the initial rate constant of the reaction. This is shown in

Scheme 1

Figure 9. The ultrasonic oxidation of benzyl alcohol is a firstorder reaction with respect to benzyl alcohol, and the rate constant of reaction is 0.0038 min-1. The reaction scheme is given in Scheme 1. 3.8. Effect of the Amount of Aqueous Layer. It has been observed in earlier sections that there was some role played by microagitation caused by ultrasonic cavitation in the organic layer (section 3.1) and microagitation caused by ultrasonic cavitation in the aqueous layer (section 3.2). There were some implications in the results that microagitation in the aqueous layer was playing a relatively dominant role. Because greater than 300% molar excess of hydrogen peroxide had been used with respect to benzyl alcohol in our experiments, it was decided to study the effect of variation of the amount of the aqueous layer on the amount of benzaldehyde formation. To keep the power density (watts per milliliter) constant in all cases, the

106


Figure 10. Effect of the amount of aqueous layer on benzaldehyde formation (R ) amount of organic layer/amount of aqueous layer).

total volume of the reaction mixture was kept constant as 40 mL in all cases. The concentration of benzyl alcohol in the organic layer was also constant. Only the amount of the aqueous layer was varied (varied R) while keeping its concentration constant at 15% H2O2. The results are shown in Figure 10. One can see that, as the amount of the aqueous layer was increased from 10 to 20 to 30 mL, i.e., when R was varied from 3.0 to 1 to 0.33 (R ) amount of organic layer/amount of aqueous layer; total volume 40 mL), the maximum amount of benzaldehyde formed has increased from 228.99 to 400.34 to 641.57 mg, respectively. This clearly indicates that, as the amount of the aqueous layer increases at a constant concentration in both the organic and aqueous layers, more and more active organic species are transferred to the organic layer by the PTC as a result of increased microagitation by ultrasound. This is reflected in the amount of benzaldehyde formed. Thus, as the amount of the aqueous layer increases, the cavitational zone increases, and there are a higher number of cavities formed per unit of power dissipated in the aqueous layer. Thus, it has been conclusively established that it is the microagitation and that too in the aqueous layer that is playing a dominant role in the oxidation of benzyl alcohol to benzaldehyde under ultrasound. Because we did not observe any byproducts besides benzoic acid in our experiments, both in the presence of mechanical agitation alone and with ultrasound alone, cavitation is not possibly affecting the chemistry of the reaction but is aiding the chemical reaction with its powerful physical effects. 4. Conclusions From this detailed study of ultrasonic oxidation of benzyl alcohol with hydrogen peroxide in the presence of a commercially available homogeneous catalyst and a PTC, the following conclusions can be established: 1. It is possible to oxidize benzyl alcohol to benzaldehyde at room temperature in the presence of ultrasound. 2. The rate and selectivity of benzaldehyde formation is more in the presence of ultrasound than in the presence of mechanical agitation alone. The combination of ultrasound and stirring does not result in synergistic effects. 3. The presence of PTC and catalyst is absolutely necessary. Thus, ultrasound cannot replace the PTC and catalyst (at least in the present work though, the case of higher intensity and higher frequency ultrasound might be different), but it increases the effectiveness of the catalyst.

4. The physical effect of ultrasound, viz., microagitation caused by intense cavitational collapse and associated turbulence, is the controlling mechanism for the observed intensification of the oxidation process. 5. Less concentrated reagents are required when reactions are carried out in the presence of ultrasound than in the presence of mechanical agitation. Acknowledgment The authors acknowledge the funding of the Department of Science & Technology, New Delhi, India, for the research work through a project entitled “Application of cavitation to chemical processing: Establishing scale-up strategies using industrially important reactions”, and N.N.M. also acknowledges the support of UGC through a fellowship. Appendix I Bond dissociation energies of various bonds are given below:33 series no.

bond

∆H°298.2K (kcal/mol)

1. 2. 3. 4. 5.

H-OH H-O O-O HO-OH HO2-H

119 102 119.1 51 90

The reactions involving bond dissociations such as entries 1-3 take place at high temperature because they require high energy. These reactions are given as follows:

H2O f H0 + OH0 O2 f O0 + O0 OH0 f O0 + H0 The reactions involving bond dissociations such as entries 4 and 5 take place at medium temperatures because they require comparatively low energies. These reactions are given as follows:

H2O2 f 2OH0 H2O2 f HO20 + H0


When the H2O2 content of the cavities is lower, the average Cp value10 of the bubble content is more because Cp of H2O2 (g) is 1.352 J/g‚K and Cp of H2O(g) is 1.865 J/g‚K. Because of the high Cp value of the bubble content, the temperature reached in such a bubble upon collapse is very high as compared to the bubble whose content is higher in H2O2. (1) Case of 30% H2O2: At this concentration of hydrogen peroxide, the bubble content may be in such a proportion (H2O2:H2O:O2) that collapse of the bubble is able to produce a large amount of H2O2 (g), which is then dissolved into the bulk liquid. So, an increase in the concentration of hydrogen peroxide was observed initially. Because now the concentration of hydrogen peroxide is increased a lot, the composition of the bubble content changes. It now contains more hydrogen peroxide. This decreases the average Cp value of the bubble content. Hence, bubble collapse is less violent now. This causes no formation of hydrogen peroxide but decomposition of hydrogen peroxide. These two processes, viz., decomposition of hydrogen peroxide at high concentration and formation of hydrogen peroxide at low concentration, continue until an equilibrium concentration of hydrogen peroxide, at which the rate of decomposition and the rate of formation are equal, is reached. The greater changes in the concentration in the case of 30% H2O2 can be explained as follows: Because the water content of the cavitation bubble at 30% H2O2 is more than the water content of the bubble at 50% H2O2, a higher amount of hydrogen peroxide is formed in the case of 30% H2O2 inside the bubble than in the case of 50% H2O2. This is reflected in a greater increase in the concentration of hydrogen peroxide, which is totally decomposed in the very next cycle of bubble formation and collapse. For 30% hydrogen peroxide, this equilibrium is reached in 45 min. The equilibrium concentration in the case of 30% H2O2 is 1.29 times the initial concentration of hydrogen peroxide. (2) Case of 50% H2O2: The same explanation as above is true for 50% hydrogen peroxide also. Because here the initial concentration is more, more time is required to achieve the equilibrium concentration. Equilibrium is reached in 60 min. The equilibrium concentration here is 1.14 times the initial concentration of hydrogen peroxide. (3) Case of 15% H2O2: Here the composition of the bubble is lower in hydrogen peroxide. Because of this, the average Cp value of the bubble content is very high. So, the temperatures reached inside the bubble at the time of collapse are very high. The collapse temperatures reached in this case are higher than that in the case of 30% H2O2, so decomposition of H2O and H2O2 to oxygen is more dominating in this case. So, a very small amount of hydrogen peroxide is formed and a large amount of oxygen is produced. Hence, a very small increase in the concentration of hydrogen peroxide is observed. Because the initial concentration of hydrogen peroxide is low, lower time is required to achieve the equilibrium concentration than in earlier cases. The equilibrium concentration is reached in just 15 min. The equilibrium concentration is 1.04 times the initial concentration. (4) Case of 3.75% H2O2: The same explanation as above is true for this case also. The equilibrium concentration is 1.03 times the initial concentration of hydrogen peroxide. The equilibrium concentration is reached in less than 15 min. Appendix II From Perry’s Chemical Engineer’s Handbook,34 the power consumed in turbulent flow by a mechanical agitator is given by

P ) NpFN3d5 where P ) power consumed (W), Np ) power number (characteristic of the type of agitator) ) 1.25, F ) density of the liquid (kg/m3), N ) speed of agitation (rps), d ) diameter of the agitator (m). (1) The power consumed by the mechanical agitator at 300 rpm is

P ) {1.25 × 1000 × 53(1.8 × 10-2)5}/L P ) 2.95 × 10-4 W So, power consumed per unit volume ) P/volume of the reaction mixture

) 2.95 × 10-4/50 ) 5.91 × 10-6 W/mL (2) The actual power consumed by the ultrasonic bath is

P)

mCp dT time

where m ) mass of the liquid (kg), Cp ) specific heat of the liquid (kJ/kg‚K), dT ) rise in temperature, and time (s).

P ) (50 × 4.184 × 3)/900 P ) 0.6973 W So, power consumed per unit volume ) P/volume of the reaction mixture

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ReceiVed for reView March 18, 2005 ReVised manuscript receiVed October 11, 2005 Accepted October 18, 2005 IE0503601

Ultrasonic Synthesis of Benzaldehyde from Benzyl Alcohol Using

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