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A catalyst-free and highly efficient approach to ozonation of benzyl alcohol to benzoic acid in a rotating packed bed
Journal of the Taiwan Institute of Chemical Engineers 103 (2019) 1–6
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A catalyst-free and highly efficient approach to ozonation of benzyl alcohol to benzoic acid in a rotating packed bed Wenqiang Gao a,b, Yao Song a, Weizhou Jiao a,∗, Youzhi Liu a a b
Shanxi Province Key Laboratory of Higee-Oriented Chemical Engineering, North University of China, Taiyuan, Shanxi 030051, China Department of Chemistry and Chemical Engineering, Lvliang University, Lvliang, Shanxi 033000, China
a r t i c l e
i n f o
Article history: Received 4 April 2019 Revised 20 June 2019 Accepted 15 July 2019 Available online 26 July 2019 Keywords: Ozone Catalyst-free Benzoic acid Rotating packed bed
a b s t r a c t In this study, benzyl alcohol was ozonized to benzoic acid in a rotating packed bed (RPB-O3 ) by a catalystfree and highly efficient approach, which is the most significant transformations in synthetic organic chemistry, and the RPB-O3 process was compared with the process in a stirred tank reactor (STR-O3 ). The results show that the RPB-O3 process produces more benzoic acid (64%) than the STR-O3 process (28%) in 30 min. The effects of different operating conditions of high gravity factor, liquid flow rate, solvent and reaction time on the yield of benzoic acid in the RPB-O3 process were investigated. The results reveal that the optimal yield is obtained using ethyl acetate as the solvent at a high gravity factor of 40, a liquid flow rate of 120 L/h, and a reaction time of 60 min. Under the optimal conditions, p-substituted benzoic acid is obtained in good to excellent yields (82–98%) in 60 min. Electron paramagnetic resonance (EPR) was performed to characterize radical species formed by the self-decomposition of ozone in nonaqueous solvents, and DMPO-· OH, DMPO-O2 · ¯ and DMPO-· CH(OH)Ph were found. The possible ozonization mechanism is proposed based on the EPR results. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Selective oxidation of benzyl alcohols is one of the most important processes in the manufacture of functional compounds [1–5], such as benzoic acid. The benzoic acid is one of the most common organic acids with wide applications in medicine, food, and chemical industries, and it is also an important precursor for the industrial synthesis of organic substances. Traditionally, KMnO4 [6], CrO3 [7] and iodosylbenzene [8] are used as oxidants, which however would cause environmental pollution due to the formation of a wide variety of pollutants. To date, many efforts have been made to develop environmentally friendly synthetic methods by using green oxidants, such as O2 [9,10], H2 O2 [11–13], oxone [14,15] and tert–butyl hydroperoxide (t-BuOOH) [16,17]. Among oxidants, although O2 has attracted considerable attention in past years and gained great progress, oxidation of alcohols to carboxylic acids with different kinds of transition metal catalysts were carried out under harsh reaction conditions [18–20]. Alcohols were selectively oxidized by H2 O2 using the polyoxometalate as catalyst. Oxone, in the presence of a catalytic amount of iron (II) sulfate/graphite oxide, oxidize efficiently alcohols into their corresponding carboxylic acids or ketone compounds at room
∗
Corresponding author. E-mail address: [email protected] (W. Jiao).
temperature in short reaction times, and t-BuOOH, in the presence of a catalytic amount of CuBr2 , also oxidizes efficiently alcohols into their corresponding carboxylic acid. Therefore, using current methods, one cannot avoid problems associated with catalysts, such as cost, the need to recycle the catalysts, etc. Ozone is known as a powerful oxidant with a high oxidation–reduction potential of 2.08 V but no secondary pollution [21,22], making it particularly useful in wastewater treatment [23–28]. Nevertheless, there is little research on the application of ozone in the synthesis of benzoic acid. Here, we propose a catalyst-free approach for selective oxidation of benzyl alcohol to benzoic acid using ozone. However, the practical use of ozonation is limited by the low mass transfer efficiency of ozone [29], it is necessary to find a highly efficient contactor to promote the gas–liquid mass transfer. Ramshaw pioneered the concept of mass transfer intensification, developing rotating packed bed (RPB) [30,31]. In the RPB, high centrifugal acceleration is generated through high rotating speed, and thus liquid is split into the thinner liquid film or smaller droplets, which can strongly intensify gas–liquid mass transfer and thus improve reaction processes. Jiao et al. [32] reported that the gas–liquid mass transfer efficiency could be intensified by 1–2 orders of magnitude in RPB. Based on the above advantages, the RPB reactor was widely used in many fields [33–40]. However, the synthetic method for benzoic acid via ozonization process in RPB has not yet been reported. Therefore, we are evoked to develop an efficient approach to synthesis benzoic acids in RPB.
https://doi.org/10.1016/j.jtice.2019.07.007 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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W. Gao, Y. Song and W. Jiao et al. / Journal of the Taiwan Institute of Chemical Engineers 103 (2019) 1–6
Table 1 The equipment parameters of the RPB. Characteristic
Type/value
Shell/Packing material Packing type Outer diameter of packing Inner diameter of packing Axial height of packing Specific surface area of packing Density of packing Porosity of stainless steel wire meshes
Stainless steel Wire packing 75 mm 40 mm 75 mm 935.07 m2 /m3 7.9 g/cm3 0.74
In this work, we have developed a highly efficient method for the synthesis of benzoic acid by ozonization in RPB. The specific objectives of this study were (1) to propose a catalyst-free approach to the synthesis of benzoic acid in RPB; (2) to investigate the effects of factors such as the mole ratio of ozone to benzyl alcohol, high gravity factor, liquid flow rate and solvent on the yield of benzoic acid; (3) to illustrate ozonation process of benzyl alcohol with time; and (4) to propose the possible oxidation mechanism in the organic systems. 2. Experimental materials and methods
Fig. 1. Schematic diagram of the experimental setup. 1. O2 cylinder; 2. ozone generator; 3. gas flowmeter; 4. RPB; 5. liquid flowmeter; 6. pump; 7. tank; 8. tail-gas absorption bottle; 9. ozone concentration detector.
Co. Ltd., Shandong, China). The gas mixture of oxygen and ozone was measured by the gas flow meter at a flow rate of 100 L/h. The gas and liquid streams were sufficiently contacted in RPB, and then ozone reacted with benzyl alcohol. The reaction solution flowed to the tank for cycling due to gravity and the remaining gas was vented through the tail-gas absorption bottle filled with KI solution. The experiments were carried out at 20 ± 2 °C.
2.1. Chemicals and equipment
2.4. STR experiments
Analytic grade chemicals benzyl alcohols were purchased from Shanghai Macklin Biochemical Co., Ltd., China. All solvents were commercially available from Tianjin Tianli Chemical Reagent Co., Ltd., China, and without further purification in this study. The concentration of ozone in the gaseous phases was determined by the wall mounted ozone concentration detector (UV-2200C, ZiBo ZHIPRER Automation Technology Co., LTD. China). The formation of activate radicals was determined by electron paramagnetic resonance (EPR) (MiniSpcope MS 50 0 0, Magnettech, Germany) with 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO) as an electron capturing reagent. The components were quantitatively determined by gas chromatography (GC-7900, Techcomp LTD. China). Benzoic acid was prepared in a RPB, provided by the Research Center of Shanxi Province for High Gravity Chemical Engineering and Technology (Shanxi, China). The schematic diagram of the RPB can be found in the literature [41], and its main parameters are shown in Table 1.
In STR experiments, 500 mL of benzyl alcohol (55.6 mmol/L) in ethyl acetate was added in a 10 0 0 mL flask under magnetic stirring at 50 0–120 0 rpm, which was used to simulate a STR. Ozone was generated from oxygen by an ozone generator, and the gas mixture of oxygen and ozone (60 mg/L) was introduced through an aeration stone with a diameter of 25 mm and a length of 30 mm and measured by the gas flow meter at a flow rate of 100 L/h. The experiments were carried out at 20 ± 2 °C. The optimal rotation rotate speed of the STR was determined to be 1200 rpm. 2.5. Analyses The concentrations of all reactants and the corresponding products were analyzed by gas chromatography, and the conversion of reactant was calculated by the following equation:
X= 2.2. Experimental design The RPB-O3 process was compared with the STR-O3 process to determine whether RPB is superior to STR in ozonation of benzyl alcohol to benzoic acid, and then the effects of solvent, and the mole ratio of ozone to benzyl alcohol on the yield of benzoic acid were investigated in RPB in order to optimize the reaction conditions. After that, the effect of liquid flow rate on the yield of benzoic acid under the condition of different high gravity factors, as well as the effect of high gravity factor under different liquid flow rates, was investigated to illustrate the relationship between high gravity factor and liquid flow rate on the yield of benzoic acid. Finally, EPR was performed to elucidate the ozonation mechanism. 2.3. RPB experiments The experimental setup is schematically shown in Fig. 1. A typical experimental procedure was as follows: 500 mL of benzyl alcohol (55.6 mmol/L) in ethyl acetate was placed in a tank and then introduced into the RPB. The liquid through a liquid distributor converged from the inner edge to the outer edge of the packing under the centrifugal force. Ozone was generated from oxygen by an ozone generator (Shandong Lvbang Photoelectric Equipment
C0 − Ct × 100% C0
(1)
where X is the conversion of reactant; C0 is the initial reactant concentration in the solution (mg/L); Ct is the concentration of reactant at t of reaction time (mg/L1 ). The yield of product was calculated by the following equation:
Y =
MR Ct × × 100% C0 MP
(2)
where Y is the yield of product; C0 is the initial reactant concentration in the solution (mg/L); Ct is the concentration of product at t of reaction time (mg/L); MR is the molar mass of reactant (g/mol); MP is the molar mass of product (g/mol). It should be noted that the high gravity factor β was used to characterize the strength of the high gravity field, which is calculated as follows [42]:
β=
ω2 r g
=
N2 r 900
(3)
where ω is the angular velocity of the rotation of rotor (s−1 ); r is the rotor average radius, m; g is gravitational acceleration (9.8 m/s2 ); and N is rotor speed (r/min). These parameters could be controlled by adjusting the frequency of the converter.
W. Gao, Y. Song and W. Jiao et al. / Journal of the Taiwan Institute of Chemical Engineers 103 (2019) 1–6
Fig. 2. Effect of reaction solvents on yield of benzoic acid. Reaction condition: high gravity factor of 40, liquid flow rate of 100 L/h, CO3 = 60 mg/L, T = 20 ± 2 °C, t = 30 min.
3
Fig. 4. The yield of benzoic acid in RPB and STR at different reaction times. Reaction condition: high gravity factor of 40, liquid flow rate of 100 L/h, CO3 = 60 mg/L, T = 20 ± 2 °C, t = 30 min.
solubility of ozone in water. The solubility of ozone in water is only 12 mg/L as determined by the iodometric method, which is much lower than that in organic solvents (Fig. 3). Therefore, EA is determined to be the most suitable solvent. 3.2. Comparison experiments
3. Results and discussion
Two experiments were carried out in STR (STR-O3 ) and RPB (RPB-O3 ) respectively to synthesize benzoic acid using ethyl acetate as the solvent. As shown in Fig. 4, the yield of benzoic acid increases as the reaction time increases from 10 min to 30 min in both STR-O3 and RPB-O3 experiments. Compared with the STR-O3 process, the RPB-O3 process results in obviously higher reactivity for the synthesis of benzoic acid and consequently an increase in the yield of benzoic acid increased from 28% to 64% within 30 min. Thus, there may be a synergistic effect of RPB and O3 due to the enhancement of the mass transfer efficiency of ozone in RPB. The reason is that the interfacial area of gas and liquid was increased and the resistance of ozone mass transfer was decreased by reducing the thickness of liquid films and the size of droplets in RPB. The volumetric gas-liquid mass-transfer coefficients could be enhanced by 1–2 orders of magnitude in RPB [43]. Thus, it is concluded that the ozonation process can be improved using the high-gravity technology.
3.1. Effects of reaction solvents
3.3. Effect of the mole ratio of ozone to benzyl alcohol
The effects of several commonly used reaction solvents, such as ethyl acetate (EA), dichloromethane (DCM), dichloroethane (DCE), chloroform (CHCl3 ), acetonitrile (CH3 CN), methanol (CH3 OH) and ethanol (EtOH), on the yield of benzoic acid was evaluated. As shown in Fig. 2, EA is the most suitable solvent with a 64% yield of benzoic acid, followed by CH3 CN with a 49.4% yield of benzoic acid, and then by other solvents with a yield ranging from 28% to 38%. It is also noteworthy that EA is one of the most popular and important industrial solvents with low toxicity. The solubility of ozone in different solvents was determined by the iodometric method. As shown in Fig. 3, the solubility of ozone in ethyl acetate in RPB is higher than that in DCM, DCE, CHCl3 and CH3 CN. Alcohol is an effective quencher for hydroxyl radicals, thus resulting in a slow proceeding of the reaction. Besides, the reaction is carried out under different pH conditions, and benzoic acid is obtained with a lower yield (3–5%), due to insolubility of reactants and low
The effect of the mole ratio of ozone to benzyl alcohol on the reaction was investigated by adjusting the concentration of the gaseous phase. As shown in Fig. 5, the yield of benzoic acid increases from 0% to 63% with the increase of the mole ratio from 0 to 2. Clearly, a higher mole ratio of ozone to benzyl alcohol allows for higher ozonation of benzyl alcohol for the synthesis of benzoic acid in RPB, which can be attributed to the higher driving force for the mass transfer of ozone at a higher mole ratio of ozone to benzyl alcohol.
Fig. 3. The concentration of ozone in different solvents in RPB. Determination conditions: high gravity factor of 40, liquid flow rate of 100 L/h, CO3 = 60 mg/L, T = 20 ± 2 °C, t = 30 min.
3.4. Effect of high gravity factor Fig. 6 shows that the yield of benzoic acid by O3 oxidation at a flow rate of 80, 100 and 120 L/h increases as the high gravity factor of RPB increases from 10 to 40, and then decreases slightly with the high gravity factor. Thus, the optimal value of β is determined
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Fig. 5. The effect of the mole ration of ozone to benzyl alcohol on the yield of benzoic acid. Reaction condition: liquid flow rate of 100 L/h, high gravity factor of 40, T = 20 ± 2 °C, t = 30 min.
Fig. 6. The effect of high gravity factor on the yield of benzoic acid. Reaction condition: CO3 = 60 mg/L, T = 20 ± 2 °C, t = 30 min.
to be 40 in this study. This is because increasing the high gravity factor in RPB caused the turbulence of gas and liquid, and thus the gas/liquid interface can be rapidly renewed due to the decrease of the size of liquid droplets and the thickness of liquid films. Nevertheless, the gas-liquid contact time decreases as the high gravity factor increases [44], resulting in a decrease in the mass transfer of O3 and consequently a decrease in the yield of benzoic acid at a high gravity factor higher than 40. It is also observed that the yield of benzoic acid increases as the liquid flow rate increases from 80 L/h to 120 L/h at the same high gravity factor. 3.5. Effect of liquid flow rate Fig. 7 shows that the yield of benzoic acid at a high gravity factor of 20, 30 and 40 increases rapidly with the increase of the liquid flow rate from 60 to 120 L/h, which can be attributed to the increase of the contact surface area between gas and liquid phases. However, it decreases slowly as the liquid flow rate increases from 120 to 160 L/h in 30 min at a high gravity factor of 40. As the liquid flow rate continues to increase, the mass transfer efficiency decreases slightly because of the limited shearing capacity of the
Fig. 7. The effect of liquid flow rate on the yield of benzoic acid. Reaction condition: CO3 =60 mg/L, T = 20 ± 2 °C, t = 30 min.
Fig. 8. The effect of reaction time on the reaction. Reaction condition: β = 40, liquid flow rate of 120 L/h, CO3 = 60 mg/L, T = 20 ± 2 °C.
packing material [45]. It is also noted that the high gravity factor has a great effect on the yield at the same liquid flow rate. Thus, the optimal liquid flow rate is determined to be 120 L/h at a high gravity factor of 40. 3.6. Effect of reaction time on the reaction As shown in Fig. 8, the conversion of benzyl alcohol and the yield of benzoic acid increase, but the yield of benzaldehyde first increases and then decreases. Benzyl alcohol is oxidized to benzaldehyde, which is then oxidized to benzoic acid. It is noted that the conversion of benzyl alcohol to benzaldehyde is higher than that of benzaldehyde to benzoic acid within the first 30 min, so that the yield of benzaldehyde increases with time. However, the opposite is observed at a reaction time longer than 30 min, so that the yield of benzaldehyde decreases with time. The yield of benzoic acid continues to increase to 94% in 60 min. 3.7. Effect of substituent groups on the reaction As shown in Fig. 9, the ozonization of substrates with methoxy or nitro groups proceeds quite smoothly to give benzoic acid
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benzyl alcohol. Some key reactions proposed for the loss of O3 in this work are shown in Eqs. (4)–(13) [24,47,48].·OH is derived from the self-decomposition of ozone with water. The reaction of hydroxyalkyl radicals generated in Eq. (4) with O2 produces superoxide radicals in Eqs. (5)–(7), which act as a powerful reductant and react with O3 to generate O3 ·− in Eq. (8). Superoxide radical is a well-recognized chain carrier in the decomposition of ozone, and O3 ·− reacts with H+ to obtain HO3 ·, followed by dissociation to give OH in Eqs. (9) and (10). OH oxidizes benzaldehyde by hydrogen atom abstraction to obtain benzoyl radical in Eq. (11), followed by further reactions with O2 to give benzoic acid in Eqs. (12) and (13).
Fig. 9. The ozonization of p-substituted benzyl alcohols. Reaction condition: β = 40, liquid flow rate of 120 L/h, CO3 = 60 mg/L, T = 20 ± 2 °C.
PhCH2 OH + · OH → PhC˙ HOH
(4)
PhC˙ HOH + O2 → PhCH(OH )OO˙
(5)
PhCH(OH )OO˙ → PhCH + HO˙ 2
(6)
+ HO˙ 2 O·− 2 +H
(7)
·− O·− 2 + O3 → O2 + O3
(8)
· + O·− 3 + H → HO3
(9)
HO·3 → O2 + · OH
(10)
PhCHO + · OH → PhC˙ O
(11)
PhC˙ O + O2 → PhC(O )OO˙
(12)
PhC(O )OO˙ + PhCHO → PhCOOH
(13)
4. Conclusions
Fig. 10. EPR spectra of three kinds of the radical species.
in good to excellent yields in 60 min. However, the yield of pmethoxybenzoic acid is always higher than that of benzoic acid, because the reactant containing a methoxy substituent as the electron-donating substituent would speed up the oxidation process, and the yield of p-methoxybenzoic acid is increased to 98% in 60 min. However, when the nitro group is introduced into the reactant, the yield of p-nitrobenzoic acid is obviously lower than that of benzoic acid is reduced to 82% in 60 min, because the nitro group as the electron-withdrawing substituent can slow down the oxidation process. 3.8. Ozonation mechanism of benzyl alcohol EPR was performed to detect new species from decomposition of ozone [46]. However, it is very difficult to hold and then measure these short-lived species. A classical spin-trapping agent, 5,5-dimethylpyrroline-oxide (DMPO), was used to capture free radicals. Interestingly, DMPO-· OH, DMPO-O2 · ¯ and DMPO-· CH(OH)Ph are observed, as shown in Fig. 10. The typical quadruple peaks indicate the presence of DMPO-·OH; while DMPO-O2 · ¯ and DMPO· CH(OH)Ph show six peaks. According to EPR experiments, we proposed a possible mechanism for the synthesis of benzoic acid by ozone oxidation of
In this work, benzoic acid has been successfully synthesized via ozonation of benzyl alcohol in RPB by a catalyst-free and highly efficient approach. The results reveal that the optimal yield is obtained using ethyl acetate as the solvent at a high gravity factor of 40, a liquid flow rate of 120 L/h, and a reaction time of 60 min. Under the optimum conditions, para-substituted benzoic acids are obtained in good to excellent yields (82–98%) in 60 min. With the increasingly serious environmental pollution, and green chemistry should be promoted. Although ozone is a green oxidant, its production cost is very high. The use of ozonation is also limited by the low solubility and mass transfer efficiency of ozone. The highgravity technology has the potential to enhance ozonation and has the advantages of mild reaction conditions, catalyst-free, and high ozone utilization, etc. Further investigation is being undertaken in our laboratory for scaled-up production of benzoic acid. However, the high-gravity technology should be further developed in order to address several challenges, including shorter residence time distribution and significant amplifications. Acknowledgements This work was supported by the Natural Science Foundation of China (U1610106) and Shanxi Excellent Talent Science and Technology Innovation project (201705D211011), Specialized Research Fund for Sanjin Scholars Program of Shanxi Province (201707) and North University of China Fund for Distinguished Young Scholars (201701).
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Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
A catalyst-free and highly efficient approach to ozonation of benzyl alcohol to benzoic acid in a rotating packed bed Wenqiang Gao a,b, Yao Song a, Weizhou Jiao a,∗, Youzhi Liu a a b
Shanxi Province Key Laboratory of Higee-Oriented Chemical Engineering, North University of China, Taiyuan, Shanxi 030051, China Department of Chemistry and Chemical Engineering, Lvliang University, Lvliang, Shanxi 033000, China
a r t i c l e
i n f o
Article history: Received 4 April 2019 Revised 20 June 2019 Accepted 15 July 2019 Available online 26 July 2019 Keywords: Ozone Catalyst-free Benzoic acid Rotating packed bed
a b s t r a c t In this study, benzyl alcohol was ozonized to benzoic acid in a rotating packed bed (RPB-O3 ) by a catalystfree and highly efficient approach, which is the most significant transformations in synthetic organic chemistry, and the RPB-O3 process was compared with the process in a stirred tank reactor (STR-O3 ). The results show that the RPB-O3 process produces more benzoic acid (64%) than the STR-O3 process (28%) in 30 min. The effects of different operating conditions of high gravity factor, liquid flow rate, solvent and reaction time on the yield of benzoic acid in the RPB-O3 process were investigated. The results reveal that the optimal yield is obtained using ethyl acetate as the solvent at a high gravity factor of 40, a liquid flow rate of 120 L/h, and a reaction time of 60 min. Under the optimal conditions, p-substituted benzoic acid is obtained in good to excellent yields (82–98%) in 60 min. Electron paramagnetic resonance (EPR) was performed to characterize radical species formed by the self-decomposition of ozone in nonaqueous solvents, and DMPO-· OH, DMPO-O2 · ¯ and DMPO-· CH(OH)Ph were found. The possible ozonization mechanism is proposed based on the EPR results. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Selective oxidation of benzyl alcohols is one of the most important processes in the manufacture of functional compounds [1–5], such as benzoic acid. The benzoic acid is one of the most common organic acids with wide applications in medicine, food, and chemical industries, and it is also an important precursor for the industrial synthesis of organic substances. Traditionally, KMnO4 [6], CrO3 [7] and iodosylbenzene [8] are used as oxidants, which however would cause environmental pollution due to the formation of a wide variety of pollutants. To date, many efforts have been made to develop environmentally friendly synthetic methods by using green oxidants, such as O2 [9,10], H2 O2 [11–13], oxone [14,15] and tert–butyl hydroperoxide (t-BuOOH) [16,17]. Among oxidants, although O2 has attracted considerable attention in past years and gained great progress, oxidation of alcohols to carboxylic acids with different kinds of transition metal catalysts were carried out under harsh reaction conditions [18–20]. Alcohols were selectively oxidized by H2 O2 using the polyoxometalate as catalyst. Oxone, in the presence of a catalytic amount of iron (II) sulfate/graphite oxide, oxidize efficiently alcohols into their corresponding carboxylic acids or ketone compounds at room
∗
Corresponding author. E-mail address: [email protected] (W. Jiao).
temperature in short reaction times, and t-BuOOH, in the presence of a catalytic amount of CuBr2 , also oxidizes efficiently alcohols into their corresponding carboxylic acid. Therefore, using current methods, one cannot avoid problems associated with catalysts, such as cost, the need to recycle the catalysts, etc. Ozone is known as a powerful oxidant with a high oxidation–reduction potential of 2.08 V but no secondary pollution [21,22], making it particularly useful in wastewater treatment [23–28]. Nevertheless, there is little research on the application of ozone in the synthesis of benzoic acid. Here, we propose a catalyst-free approach for selective oxidation of benzyl alcohol to benzoic acid using ozone. However, the practical use of ozonation is limited by the low mass transfer efficiency of ozone [29], it is necessary to find a highly efficient contactor to promote the gas–liquid mass transfer. Ramshaw pioneered the concept of mass transfer intensification, developing rotating packed bed (RPB) [30,31]. In the RPB, high centrifugal acceleration is generated through high rotating speed, and thus liquid is split into the thinner liquid film or smaller droplets, which can strongly intensify gas–liquid mass transfer and thus improve reaction processes. Jiao et al. [32] reported that the gas–liquid mass transfer efficiency could be intensified by 1–2 orders of magnitude in RPB. Based on the above advantages, the RPB reactor was widely used in many fields [33–40]. However, the synthetic method for benzoic acid via ozonization process in RPB has not yet been reported. Therefore, we are evoked to develop an efficient approach to synthesis benzoic acids in RPB.
https://doi.org/10.1016/j.jtice.2019.07.007 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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W. Gao, Y. Song and W. Jiao et al. / Journal of the Taiwan Institute of Chemical Engineers 103 (2019) 1–6
Table 1 The equipment parameters of the RPB. Characteristic
Type/value
Shell/Packing material Packing type Outer diameter of packing Inner diameter of packing Axial height of packing Specific surface area of packing Density of packing Porosity of stainless steel wire meshes
Stainless steel Wire packing 75 mm 40 mm 75 mm 935.07 m2 /m3 7.9 g/cm3 0.74
In this work, we have developed a highly efficient method for the synthesis of benzoic acid by ozonization in RPB. The specific objectives of this study were (1) to propose a catalyst-free approach to the synthesis of benzoic acid in RPB; (2) to investigate the effects of factors such as the mole ratio of ozone to benzyl alcohol, high gravity factor, liquid flow rate and solvent on the yield of benzoic acid; (3) to illustrate ozonation process of benzyl alcohol with time; and (4) to propose the possible oxidation mechanism in the organic systems. 2. Experimental materials and methods
Fig. 1. Schematic diagram of the experimental setup. 1. O2 cylinder; 2. ozone generator; 3. gas flowmeter; 4. RPB; 5. liquid flowmeter; 6. pump; 7. tank; 8. tail-gas absorption bottle; 9. ozone concentration detector.
Co. Ltd., Shandong, China). The gas mixture of oxygen and ozone was measured by the gas flow meter at a flow rate of 100 L/h. The gas and liquid streams were sufficiently contacted in RPB, and then ozone reacted with benzyl alcohol. The reaction solution flowed to the tank for cycling due to gravity and the remaining gas was vented through the tail-gas absorption bottle filled with KI solution. The experiments were carried out at 20 ± 2 °C.
2.1. Chemicals and equipment
2.4. STR experiments
Analytic grade chemicals benzyl alcohols were purchased from Shanghai Macklin Biochemical Co., Ltd., China. All solvents were commercially available from Tianjin Tianli Chemical Reagent Co., Ltd., China, and without further purification in this study. The concentration of ozone in the gaseous phases was determined by the wall mounted ozone concentration detector (UV-2200C, ZiBo ZHIPRER Automation Technology Co., LTD. China). The formation of activate radicals was determined by electron paramagnetic resonance (EPR) (MiniSpcope MS 50 0 0, Magnettech, Germany) with 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO) as an electron capturing reagent. The components were quantitatively determined by gas chromatography (GC-7900, Techcomp LTD. China). Benzoic acid was prepared in a RPB, provided by the Research Center of Shanxi Province for High Gravity Chemical Engineering and Technology (Shanxi, China). The schematic diagram of the RPB can be found in the literature [41], and its main parameters are shown in Table 1.
In STR experiments, 500 mL of benzyl alcohol (55.6 mmol/L) in ethyl acetate was added in a 10 0 0 mL flask under magnetic stirring at 50 0–120 0 rpm, which was used to simulate a STR. Ozone was generated from oxygen by an ozone generator, and the gas mixture of oxygen and ozone (60 mg/L) was introduced through an aeration stone with a diameter of 25 mm and a length of 30 mm and measured by the gas flow meter at a flow rate of 100 L/h. The experiments were carried out at 20 ± 2 °C. The optimal rotation rotate speed of the STR was determined to be 1200 rpm. 2.5. Analyses The concentrations of all reactants and the corresponding products were analyzed by gas chromatography, and the conversion of reactant was calculated by the following equation:
X= 2.2. Experimental design The RPB-O3 process was compared with the STR-O3 process to determine whether RPB is superior to STR in ozonation of benzyl alcohol to benzoic acid, and then the effects of solvent, and the mole ratio of ozone to benzyl alcohol on the yield of benzoic acid were investigated in RPB in order to optimize the reaction conditions. After that, the effect of liquid flow rate on the yield of benzoic acid under the condition of different high gravity factors, as well as the effect of high gravity factor under different liquid flow rates, was investigated to illustrate the relationship between high gravity factor and liquid flow rate on the yield of benzoic acid. Finally, EPR was performed to elucidate the ozonation mechanism. 2.3. RPB experiments The experimental setup is schematically shown in Fig. 1. A typical experimental procedure was as follows: 500 mL of benzyl alcohol (55.6 mmol/L) in ethyl acetate was placed in a tank and then introduced into the RPB. The liquid through a liquid distributor converged from the inner edge to the outer edge of the packing under the centrifugal force. Ozone was generated from oxygen by an ozone generator (Shandong Lvbang Photoelectric Equipment
C0 − Ct × 100% C0
(1)
where X is the conversion of reactant; C0 is the initial reactant concentration in the solution (mg/L); Ct is the concentration of reactant at t of reaction time (mg/L1 ). The yield of product was calculated by the following equation:
Y =
MR Ct × × 100% C0 MP
(2)
where Y is the yield of product; C0 is the initial reactant concentration in the solution (mg/L); Ct is the concentration of product at t of reaction time (mg/L); MR is the molar mass of reactant (g/mol); MP is the molar mass of product (g/mol). It should be noted that the high gravity factor β was used to characterize the strength of the high gravity field, which is calculated as follows [42]:
β=
ω2 r g
=
N2 r 900
(3)
where ω is the angular velocity of the rotation of rotor (s−1 ); r is the rotor average radius, m; g is gravitational acceleration (9.8 m/s2 ); and N is rotor speed (r/min). These parameters could be controlled by adjusting the frequency of the converter.
W. Gao, Y. Song and W. Jiao et al. / Journal of the Taiwan Institute of Chemical Engineers 103 (2019) 1–6
Fig. 2. Effect of reaction solvents on yield of benzoic acid. Reaction condition: high gravity factor of 40, liquid flow rate of 100 L/h, CO3 = 60 mg/L, T = 20 ± 2 °C, t = 30 min.
3
Fig. 4. The yield of benzoic acid in RPB and STR at different reaction times. Reaction condition: high gravity factor of 40, liquid flow rate of 100 L/h, CO3 = 60 mg/L, T = 20 ± 2 °C, t = 30 min.
solubility of ozone in water. The solubility of ozone in water is only 12 mg/L as determined by the iodometric method, which is much lower than that in organic solvents (Fig. 3). Therefore, EA is determined to be the most suitable solvent. 3.2. Comparison experiments
3. Results and discussion
Two experiments were carried out in STR (STR-O3 ) and RPB (RPB-O3 ) respectively to synthesize benzoic acid using ethyl acetate as the solvent. As shown in Fig. 4, the yield of benzoic acid increases as the reaction time increases from 10 min to 30 min in both STR-O3 and RPB-O3 experiments. Compared with the STR-O3 process, the RPB-O3 process results in obviously higher reactivity for the synthesis of benzoic acid and consequently an increase in the yield of benzoic acid increased from 28% to 64% within 30 min. Thus, there may be a synergistic effect of RPB and O3 due to the enhancement of the mass transfer efficiency of ozone in RPB. The reason is that the interfacial area of gas and liquid was increased and the resistance of ozone mass transfer was decreased by reducing the thickness of liquid films and the size of droplets in RPB. The volumetric gas-liquid mass-transfer coefficients could be enhanced by 1–2 orders of magnitude in RPB [43]. Thus, it is concluded that the ozonation process can be improved using the high-gravity technology.
3.1. Effects of reaction solvents
3.3. Effect of the mole ratio of ozone to benzyl alcohol
The effects of several commonly used reaction solvents, such as ethyl acetate (EA), dichloromethane (DCM), dichloroethane (DCE), chloroform (CHCl3 ), acetonitrile (CH3 CN), methanol (CH3 OH) and ethanol (EtOH), on the yield of benzoic acid was evaluated. As shown in Fig. 2, EA is the most suitable solvent with a 64% yield of benzoic acid, followed by CH3 CN with a 49.4% yield of benzoic acid, and then by other solvents with a yield ranging from 28% to 38%. It is also noteworthy that EA is one of the most popular and important industrial solvents with low toxicity. The solubility of ozone in different solvents was determined by the iodometric method. As shown in Fig. 3, the solubility of ozone in ethyl acetate in RPB is higher than that in DCM, DCE, CHCl3 and CH3 CN. Alcohol is an effective quencher for hydroxyl radicals, thus resulting in a slow proceeding of the reaction. Besides, the reaction is carried out under different pH conditions, and benzoic acid is obtained with a lower yield (3–5%), due to insolubility of reactants and low
The effect of the mole ratio of ozone to benzyl alcohol on the reaction was investigated by adjusting the concentration of the gaseous phase. As shown in Fig. 5, the yield of benzoic acid increases from 0% to 63% with the increase of the mole ratio from 0 to 2. Clearly, a higher mole ratio of ozone to benzyl alcohol allows for higher ozonation of benzyl alcohol for the synthesis of benzoic acid in RPB, which can be attributed to the higher driving force for the mass transfer of ozone at a higher mole ratio of ozone to benzyl alcohol.
Fig. 3. The concentration of ozone in different solvents in RPB. Determination conditions: high gravity factor of 40, liquid flow rate of 100 L/h, CO3 = 60 mg/L, T = 20 ± 2 °C, t = 30 min.
3.4. Effect of high gravity factor Fig. 6 shows that the yield of benzoic acid by O3 oxidation at a flow rate of 80, 100 and 120 L/h increases as the high gravity factor of RPB increases from 10 to 40, and then decreases slightly with the high gravity factor. Thus, the optimal value of β is determined
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W. Gao, Y. Song and W. Jiao et al. / Journal of the Taiwan Institute of Chemical Engineers 103 (2019) 1–6
Fig. 5. The effect of the mole ration of ozone to benzyl alcohol on the yield of benzoic acid. Reaction condition: liquid flow rate of 100 L/h, high gravity factor of 40, T = 20 ± 2 °C, t = 30 min.
Fig. 6. The effect of high gravity factor on the yield of benzoic acid. Reaction condition: CO3 = 60 mg/L, T = 20 ± 2 °C, t = 30 min.
to be 40 in this study. This is because increasing the high gravity factor in RPB caused the turbulence of gas and liquid, and thus the gas/liquid interface can be rapidly renewed due to the decrease of the size of liquid droplets and the thickness of liquid films. Nevertheless, the gas-liquid contact time decreases as the high gravity factor increases [44], resulting in a decrease in the mass transfer of O3 and consequently a decrease in the yield of benzoic acid at a high gravity factor higher than 40. It is also observed that the yield of benzoic acid increases as the liquid flow rate increases from 80 L/h to 120 L/h at the same high gravity factor. 3.5. Effect of liquid flow rate Fig. 7 shows that the yield of benzoic acid at a high gravity factor of 20, 30 and 40 increases rapidly with the increase of the liquid flow rate from 60 to 120 L/h, which can be attributed to the increase of the contact surface area between gas and liquid phases. However, it decreases slowly as the liquid flow rate increases from 120 to 160 L/h in 30 min at a high gravity factor of 40. As the liquid flow rate continues to increase, the mass transfer efficiency decreases slightly because of the limited shearing capacity of the
Fig. 7. The effect of liquid flow rate on the yield of benzoic acid. Reaction condition: CO3 =60 mg/L, T = 20 ± 2 °C, t = 30 min.
Fig. 8. The effect of reaction time on the reaction. Reaction condition: β = 40, liquid flow rate of 120 L/h, CO3 = 60 mg/L, T = 20 ± 2 °C.
packing material [45]. It is also noted that the high gravity factor has a great effect on the yield at the same liquid flow rate. Thus, the optimal liquid flow rate is determined to be 120 L/h at a high gravity factor of 40. 3.6. Effect of reaction time on the reaction As shown in Fig. 8, the conversion of benzyl alcohol and the yield of benzoic acid increase, but the yield of benzaldehyde first increases and then decreases. Benzyl alcohol is oxidized to benzaldehyde, which is then oxidized to benzoic acid. It is noted that the conversion of benzyl alcohol to benzaldehyde is higher than that of benzaldehyde to benzoic acid within the first 30 min, so that the yield of benzaldehyde increases with time. However, the opposite is observed at a reaction time longer than 30 min, so that the yield of benzaldehyde decreases with time. The yield of benzoic acid continues to increase to 94% in 60 min. 3.7. Effect of substituent groups on the reaction As shown in Fig. 9, the ozonization of substrates with methoxy or nitro groups proceeds quite smoothly to give benzoic acid
W. Gao, Y. Song and W. Jiao et al. / Journal of the Taiwan Institute of Chemical Engineers 103 (2019) 1–6
5
benzyl alcohol. Some key reactions proposed for the loss of O3 in this work are shown in Eqs. (4)–(13) [24,47,48].·OH is derived from the self-decomposition of ozone with water. The reaction of hydroxyalkyl radicals generated in Eq. (4) with O2 produces superoxide radicals in Eqs. (5)–(7), which act as a powerful reductant and react with O3 to generate O3 ·− in Eq. (8). Superoxide radical is a well-recognized chain carrier in the decomposition of ozone, and O3 ·− reacts with H+ to obtain HO3 ·, followed by dissociation to give OH in Eqs. (9) and (10). OH oxidizes benzaldehyde by hydrogen atom abstraction to obtain benzoyl radical in Eq. (11), followed by further reactions with O2 to give benzoic acid in Eqs. (12) and (13).
Fig. 9. The ozonization of p-substituted benzyl alcohols. Reaction condition: β = 40, liquid flow rate of 120 L/h, CO3 = 60 mg/L, T = 20 ± 2 °C.
PhCH2 OH + · OH → PhC˙ HOH
(4)
PhC˙ HOH + O2 → PhCH(OH )OO˙
(5)
PhCH(OH )OO˙ → PhCH + HO˙ 2
(6)
+ HO˙ 2 O·− 2 +H
(7)
·− O·− 2 + O3 → O2 + O3
(8)
· + O·− 3 + H → HO3
(9)
HO·3 → O2 + · OH
(10)
PhCHO + · OH → PhC˙ O
(11)
PhC˙ O + O2 → PhC(O )OO˙
(12)
PhC(O )OO˙ + PhCHO → PhCOOH
(13)
4. Conclusions
Fig. 10. EPR spectra of three kinds of the radical species.
in good to excellent yields in 60 min. However, the yield of pmethoxybenzoic acid is always higher than that of benzoic acid, because the reactant containing a methoxy substituent as the electron-donating substituent would speed up the oxidation process, and the yield of p-methoxybenzoic acid is increased to 98% in 60 min. However, when the nitro group is introduced into the reactant, the yield of p-nitrobenzoic acid is obviously lower than that of benzoic acid is reduced to 82% in 60 min, because the nitro group as the electron-withdrawing substituent can slow down the oxidation process. 3.8. Ozonation mechanism of benzyl alcohol EPR was performed to detect new species from decomposition of ozone [46]. However, it is very difficult to hold and then measure these short-lived species. A classical spin-trapping agent, 5,5-dimethylpyrroline-oxide (DMPO), was used to capture free radicals. Interestingly, DMPO-· OH, DMPO-O2 · ¯ and DMPO-· CH(OH)Ph are observed, as shown in Fig. 10. The typical quadruple peaks indicate the presence of DMPO-·OH; while DMPO-O2 · ¯ and DMPO· CH(OH)Ph show six peaks. According to EPR experiments, we proposed a possible mechanism for the synthesis of benzoic acid by ozone oxidation of
In this work, benzoic acid has been successfully synthesized via ozonation of benzyl alcohol in RPB by a catalyst-free and highly efficient approach. The results reveal that the optimal yield is obtained using ethyl acetate as the solvent at a high gravity factor of 40, a liquid flow rate of 120 L/h, and a reaction time of 60 min. Under the optimum conditions, para-substituted benzoic acids are obtained in good to excellent yields (82–98%) in 60 min. With the increasingly serious environmental pollution, and green chemistry should be promoted. Although ozone is a green oxidant, its production cost is very high. The use of ozonation is also limited by the low solubility and mass transfer efficiency of ozone. The highgravity technology has the potential to enhance ozonation and has the advantages of mild reaction conditions, catalyst-free, and high ozone utilization, etc. Further investigation is being undertaken in our laboratory for scaled-up production of benzoic acid. However, the high-gravity technology should be further developed in order to address several challenges, including shorter residence time distribution and significant amplifications. Acknowledgements This work was supported by the Natural Science Foundation of China (U1610106) and Shanxi Excellent Talent Science and Technology Innovation project (201705D211011), Specialized Research Fund for Sanjin Scholars Program of Shanxi Province (201707) and North University of China Fund for Distinguished Young Scholars (201701).
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