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Liquid Phase Oxidation of Toluene to Benzaldehyde by Air Hemanl V. Borgaonkar, SanJay R. Raverkar, and SampatraJB. Chandalla' Department of Chemical TechnokJgy, Universffy of Bombay, Mtun@, Bombay 400 0 19, India
Liquid-phase oxidation of toluene by air in acetic acid medium, with cobalt acetate catalyst and sodium bromide or paraldehyde as promoter, has been investigated with a view to developing a process for the production of benzaldehyde with benzoic acid as a coproduct. The effects of various process parameters such as temperature, period of reaction, presswe, and concentrations of the substrate, catalyst, and promoter on the overall conversion and yield of benzaldehyde were studied. Up to 10% conversion, the use of either sodium bromide or paraldehyde as a promoter gave more than 90% yield of benzaldehyde. However, at high conversions, the yield of benzaldehyde decreased markedly. Under suitable conditions. 40% yield of benzaldehyde was obtained at a conversion of 20% in the presence of sodium bromide.
Introduction Benzaldehyde is widely used in dyestuff, perfumary, and pharmaceutical industries. In the latter industry it is used for the manufacture of intermediates for chloramphenicol, analgin, ephedrin, and ampicillin etc. For the production of benzaldehyde, particularly for the grade conforming to pharmaceutical and perfumary standards, which is required to be free from compounds containing chlorine in the nucleus, the vapor phase (Ray and Mukherjee, 1983) oxidation of toluene is well established. However, in this process, the conversion per pass is restricted to about 15% and the yield of benzaldehyde is generally not more than 70%, with carbon dioxide as the main byproduct. Further, the low concentration of toluene in the toluene-air feed mixture poses problems of recovery. Therefore, with the increasing cost of toluene, the process does not appear to be attractive. The liquid-phase oxidation of toluene by air mostly proceeds to the carboxylic acid, and the amount of benzaldehyde obtained is generally very low, though aldehyde is an intermediate product. Morimoto and Ogata (1967), Fields and Meyerson (1968), and Kamiya (1968) have reported that high yield of benzaldehyde may be obtained, if oxidation is carried out in acetic acid medium with cobalt acetate as a catalyst and sodium bromide as a promoter. This process also suffers from the disadvantage of relatively low yield. Further, the recovery of benzaldehyde from the reaction mixture containing acetic acid, toluene, and benzoic acid would be quite involved. However, unlike vapor phhase oxidation, no carbon dioxide is formed, and the benzoic acid obtained as a byproduct may be marketed. The information available on the process is very limited and is found mostly in the patent literature. In this paper, the effects of various process parameters on the reaction rate and yield of benzaldehyde are reported. Experimental Procedure The flow diagram of the experimental unit has been reported previously (Kamath and Chandalia, 1973). The experiments were conducted in a batch manner in a bubble column stainless steel reactor (44mm i.d., 330 mm long) fitted with an air sparger, a thermometer pocket, a pressure gauge, and a water condenser. The reactor was heated by a nichrome wire element and the temperature was controlled by a Variac. The reaction was carried out in a medium of acetic acid using cobalt acetate as a catalyst and sodium bromide or paraldehyde 8s a promoter. The presence of sodium bromide in the reaction mixture is known to cause severe corrosion, and stainless steel is not suitable as a material of construction. Therefore, when
sodium bromide was used as a promoter, a glass tube was inserted in the stainleas steel reactor tube so that the liquid containing sodium bromide did not come in contact with the stainless steel tube. The vapors emerging from the reaction mixture did come in contact with stainless steel, but since the vapors were essentially free from brominecontaining compounds, it did not pose a problem. A compressor was used for passing air continuously through the reactor, and the air flow rate and pressure were controlled by means of needle valves at the inlet and outlet of the reactor. A rotameter was fitted at the exit to allow measurement of the air flow rate.
Analytical Procedure The major products of reaction were the carboxylic acid and the aldehyde. The reaction mixture was allowed to cool to room temperature and was filtered to remove some of the insoluble carboxylic acid. The aldehyde present in the filtrate was analyzed by oxidation with silver oxide, which is formed in situ by reaction of silver nitrate with sodium hydroxide (argentometric method). The necessary correction for the silver nitrate consumed by the reaction with sodium bromide present in the reaction mixture was made by performing a blank titration without addition of sodium hydroxide solution to the sample of reaction mixture. The carboxylic acid present in the filtrate was analyzed by diluting a sample with water and extracting the aqueous solution with ether. The combined ether extracts were washed with aqueous sodium hydroxide solution to extract the carboxylic acid as sodium salt. The aqueous layer was neutralized with mineral acid to precipitate the carboxylic acid. The latter was dried and weighed. In the case where paraldehyde was used as a promoter, it was ascertained that all the paraldehyde got oxidized to acetic acid under the reaction conditions and the same was not present in the reaction mixture. In a few typical runs, the unreacted toluene in the product mixture was analyzed by gas-liquid chromatography. The ether solution after removal of the carboxylic acid was used for the purpose. A 2 m long stainless steel column containing 15% Apizion-L on Chromosorb-W was used. The oven temperature was varied from 75 to 180 "C at the rate of 4 OC/min. Injector temperature was kept at 195 "C and thermal conductivity detector temperature was kept at 210 "C. Results and Discussion Definitions. Conversion to product: amount of reactant consumed for the formation of a product divided by
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Table I. Oxidation of Toluene with Paraldehyde Promotera overall conv to yield of run time, conv, aldehyde, aldehyde, mol % % no. h mol % 9.7 9.7 100 1 0.5 2 1 19.0 5.8 30.7 nil 39.6 nil 3 1.5 92.5 nil nil 4 2.0 5b 2.0 4.8 4.8 100 5.2 19.2 6b 5.0 27.2 a Toluene, 28.56% w/v; paraldehyde, 0.486 g-mol/L; CoAc, 0.167 g-mol/L; air flow rate, 3 L/min; pressure, 10 kg/cm2; temperature, 110 "C. bCoAc, 0.04 g-mol/L; pressure, 12 kg/cm2; temperature, 120 "C.
the amount of reactant taken. Yield or selectivity: the amount of reactant consumed for the formation of a particular product divided by the total amount of reactant consumed. Overall conversion: the overall conversion is defined as the ratio of total moles of reactant reacted to the moles of reactant taken. It is well-known that in consecutive reactions the yield of intermediate product will decrease as the overall conversion increases. Therefore, in the following discussion, the yield of benzaldehyde is compared only at similar level of conversion. Benzaldehyde and benzoic acid were the only major products of reaction. Other byproducts such as benzyl alcohol or benzyl acetate were not obtained. Therefore, the yield of benzaldehyde could be found by dividing the moles of benzaldehyde formed by the moles of benzaldehyde and benzoic acid. Oxidation of Toluene with Paraldehyde as a Promoter. When the oxidation of toluene was carried out with paraldehyde as a promoter with a view to obtaining benzaldehyde in high yield, it was found that almost quantitative yield of benzaldehyde WBS obtained up to 10% level of conversion. At higher conversions, the yield decreased markedly, and above 40% conversion no aldehyde was obtained (Table I). For a comparable cobalt acetate concentration, the reaction rate was less when paraldehyde was used as compared to that when sodium bromide was
employed. In the range of 20 to 40% conversion, the yield of benzaldehyde was also more in the case of sodium bromide. Hence sodium bromide was preferred to paraldehyde in the subsequent work. Oxidation of Toluene with Sodium Bromide as a Promoter. In this case also the yield of benzaldehyde was very high (above go%), if the conversion was restricted to 10%. The yield decreased markedly with the increase in conversion. About 40% yield of benzaldehyde was obtained at conversion levels of about 20 to 25%. The presence of bromine-containingcompounds in the reaction mixture poses corrosion problems, and stainless steel is not suitable as a material of construction. In industrial practice, titanium-lined or glass-lined vessels may be used. Effect of Concentration of Toluene in the Reaction Mixture. When the concentration of toluene was decreased from 64.32 to 42.88% (w/v of the reaction mixture), for a given period of reaction the overall conversion of toluene remained almost the same but the yield of benzaldehyde increased significantly. When the concentration of toluene was further reduced to 21.44% w/v, the overall conversion of toluene increased markedly, but at the same level of conversion the yield of benzaldehyde did not change significantly (Table 11). Effect of Temperature. Most of the work was carried out at 110 "C. When the temperature was varied to 180 "C, there was no significant effect on the yield of benzaldehyde (run no. 10 and 11, Table 111). Effect of Pressure. When the pressure was increased from 1 to 11 kg/cm2 (absolute), the rate of overall conversion of toluene increased because a relatively shorter time of reaction was required to obtain similar conversion. The yield of benzaldehyde also increased considerably in this range. With further increase in the pressure to 21 kg/cm2, the overall conversion increased significantly, while the yield of benzaldehyde was almost the same. Therefore, the pressure in the range of 10-20 kg/cm2 may be chosen for the process (runs 1 to 7, Table 111). Effect of Period of Reaction. Kinetic data were obtained by varying the period of reaction from 0.5 to 2 h for 28.56% w/v concentration of toluene at 10 kg/cm2
Table 11. Effect of Toluene Concentration' concn of overall conv to yield of toluene, % w/v reaction conv of of reaction time, toluene, aldehyde, aldehyde, run no. mixture h mol % mol % % 1 64.32 2.5 26.2 7.3 27.9 2 42.88 2.5 26.9 9.8 36.4 6.4 12.8 3 42.88 5.0 49.9 4 21.44 2.5 45.0 5.8 11.4 9.1 35.0 5 28.56 0.75 26.0 'Reaction volume, 300 mL; cobalt acetate, 0.02 g-mol/L; sodium bromide, 0.16 g-mol/L; temperature, 110 "C; air pressure, 10 kg/cm2; air flow rate, 3 L/min. Table 111. Effect of Pressure a n d Temperaturea period of overall conv to yield of reaction, conv of benzaldehyde, benzaldehyde, temp, press., "C h toluene. % % ke/cm2 % l b 1.0 110 2 19.2 5.6 29.1 110 5 33.6 4.2 12.5 2 3.5 30.7 4.5 14.6 3 6.0 110 3.5 4 11.0 110 2 24.6 9.5 38.6 5b 8.0 110 2 33.2 6.8 20.6 6 21.0 110 1.5 27.9 9.9 35.5 7b 11.0 110 0.5 20.2 8.0 40.0 8 11.0 95 3.5 33.5 10.7 31.9 96 11.0 110 1.0 35.5 7.9 22.2 180 0.25 16.6 7.8 46.9 1Ob 11.0 l l b 11.0 110 0.33 16.0 8.0 50.0 'Reaction volume, 300 mL; toluene concentration, 42.88% w/v of reaction mixture; temperature, 110 'C; air flow rate, 3 L/min; cobalt acetate, 0.02 g-mol/L; sodium bromide, 0.16 g-mol/L. Concentration of toluene, 28.56% w/v. run no.
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Table IV. Effect of Concentration of Cobalt Acetate and Sodium Bromidea cobalt sodium bromide acetate concn, run concn, g-mol/L no. g-mol/L 1 0.02 0.08 2 0.02 0.16 3 0.01 0.16 46 0.05 0.16 56 0.02 0.16 6b 0.5 nil 7b 0.05 0.16 86 0.5 nil OReaction volume, 300 mL; toluene, 42.88% toluene concentration. cAssuming the validity
overall SP conv of conv to yield of reaction reaction benzaldehyde, period, toluene, benzaldehyde, rate (hl): mol % mol % % g-mol/(h L) h 3.5 25.1 1.6 6.4 0.337 2 24.6 9.5 38.6 0.578 0.319 3.5 23.8 6.4 26.9 1.5 42.7 8.3 19.3 0.883 1.5 40.8 6.6 16.2 0.843 3 62.0 0.5 0.9 0.640 2 58.0 9.2 15.8 0.899 0.5 14.2 2.4 16.7 0.880 w/v; temperature, 110 "C; air pressure, 10 kg/cm2; air flow rate, 3 L/min. b28.56% w/v of zeroth-order rate expression from zero time instead of from 0.5 h.
Figure 1. Effect of period of reaction: (0) disappearance of toluene; (0)conversion to acid; (A)conversion to benzaldehyde; (0)yield of benzaldehyde; reaction conditions; toluene, 28.56% w/v; cobalt acetate, 0.02 g-mol/L; sodium bromide, 0.16 g-mol/L; reaction volume, 300 mL; pressure, 10 kg/cm2; air flow rate, 3 L/min; temperature, 110 OC.
pressure and 110 "C (Figure 1). Kinetic data were also obtained at 7 kg/cm2 pressure (Figure 2). The pressure of 7 kg/cm2 was chosen because glass-lined reactors are normally designed to withstand pressures around that range.
Interpretation of the Kinetic Data It was observed from the kinetic data that up to 0.5 h the average rate for disappearance of toluene (rl) is greater than the average reaction rate for the formation of benzoic acid (r2),implying that, initially, the rate of formation of benzaldehyde is more than its disappearance, and thus benzaldehyde accumulates. It is interesting to note that with the further increase in the reaction period from 0.5 to 2 h, when overall conversion of toluene was varied from 20 to 55%, the conversion to benzaldehyde was almost unaffected. This indicated that during this time interval, the rate of formation of benzaldehyde from toluene was almost equal to its rate of disappearance to benzoic acid. Thus, the yield of benzaldehyde decreased markedly with the increase in the overall conversion of toluene. Though a detailed kinetic analysis was not carried out, it appeared that for most of the time during the run (i.e., 0.5 to 2 h), the rate of disappearance of toluene was zeroth order with respect to toluene. A similar observation has been made by Kamiya (1968) for the oxidation of toluene
Figure 2. Effect of period of reaction: (0) disappearance of toluene; (0) conversion to acid; (A)conversion to benzaldehyde; (0) yield of benzaldehyde; reaction volume, 300 mL; toluene, 28.56% w/v; cobalt acetate, 0.02 g-mol/L; sodium bromide 0.16 g-mol/L; pressure, 7 kg/cm2; temperature, 110 "C; air flow rate, 3 L/min.
and by Ravens (1959) for the oxidation to toluic acid in the presence of cobalt acetate and sodium bromide as a catalyst system. Effect of Sodium Bromide and Cobalt Acetate. When the concentration of sodium bromide was increased from 0.08 g-mol/L to 0.16 g-mol/L, it was seen that the overall conversion of toluene as well as yield of benzaldehyde increased markedly (runs 1 and 2, Table IV). Assuming the validity of the zeroth-order rate expression from zero time (which is strictly not true) instead of from 0.5 h, the specific reaction rate (k,)was calculated (runs 1 and 2, Table IV). I t appears that the specific reaction rate for disappearance of toluene (k,) is directly proportional to the concentration of sodium bromide. Still higher concentraion of sodium bromide could not be used because of the limited solubility of sodium bromide in the reaction mixture at the operating temperature. In the absence of bromide, both overall conversion of toluene as well as yield of benzaldehyde were found to decrease drastically (runs 6 and 8, Table IV). When cobalt acetate concentration was varied from 0.01 to 0.02, k1 increased almost in a linear manner with the concentration of cobalt acetate. The yield of benzaldehyde also increased significantly, indicating that k2 did not increase to the same extent as k l (runs 2 and 3, Table IV). With further increase in cobalt acetate concentration to 0.05 g-mol/L (run no. 4, Table IV), there was not effect
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on reaction rate as well as yield of benzaldehyde, implying that in this range k1 and k2 are independent of cobalt acetate concentration. Oxidation of p -Methoxytoluene to p -Anisaldehyde. The process for oxidation of toluene to benzaldehyde could be extended to the oxidation of p-methoxytoluene for the production of p-anisaldehyde. Thus p-methoxytoluene was oxidized in a medium of acetic acid, with cobalt acetate (0.02 g-mol/L) as a catalyst and sodium bromide (0.16 g-mol/L) as a promoter at 10 kg/cm2 pressure and 125 "C temperature to obtain almost quantitative yield of panisaldehyde, when the overall conversion of p-methoxytoluene was restricted to about 20%.
rates were obtained in the former case. In the range of 20 to 40% conversion, the yield of benzaldehyde was also higher with sodium bromide than that obtained with paraldehyde. Under suitable conditions (toluene, 28.56 % w/v of reaction mixture; solvent, acetic acid; cobalt acetate, 0.02 g-mol/L; sodium bromide, 0.16 g-mol/L; temperature, 110 O C ; pressure 10 kg/cm2; air flow rate 3 L/min) it was possible to get 40% yield of benzaldehyde, provided the overall conversion of toluene was restricted to 20%. Registry No. Toluene, 108-88-3; benzaldehyde, 100-52-7; sodium bromide, 7647-15-6;cobalt acetate, 71-48-7;paraldehyde, 123-63-7;p-methoxytoluene, 104-93-8;p-anisaldehyde, 123-11-5.
Conclusions
Fields, E. K.; Meyerson. S. A&. Chem. Ser. 1968, No. 76(2), 395. Kamath, S. S.; Chandalia, S. B. J . Appl. Chem. Blotechnol. 1973, 23, 469. Kamiya, Y. A&. Chem. Ser. 1968, No. 76(2), 193. Morlmoto, T.; Ogata, Y. J . Chem. Soc., Sect. B 1967, 62. Ravens, D. A. S. Trans. Faraday. Soc. 1959, 55, 1768. Ray, S. K.; Mukherjee, P. N. Indian J . Technol. 1983, 21(4), 137.
Up to 10% conversion, the use of either sodium bromide or paraldehyde as a promoter gave more than 90% yield of benzaldehyde. At higher conversions, the yield of benzaldehyde decreased markedly and benzoic acid was obtained as a coproduct. The use of sodium bromide was preferred to paraldehyde because relatively higher reaction
Literature Cited
Received for review September 22, 1983 Accepted March 5, 1984
Kinetics and Efficiency of Solar Energy Storage in the Photochemical Isomerization of Norbornadlene to Quadricyclane Constantine Phlllppopoulor and John Yarangozls Laboratory of Chemlcal Process Engineering, National Technlcal University of Athens, Athens, e e e c e
The conversion of norbornadieneto quadricyclane by polychromatic radlation in a solar simulator was investigated. Parameters examined were photosensitizers, reactant and sensitizer concentrationsJnsolationpower, and temperature. The rate of conversion and the efficiency of solar energy storage were measured and have been quantitatively correlated. The most efficient sensitizers were Michler's ketone, acetophenone, and benzophenone in that order. A mixture of the sensitizers performed worse than the less efficient sensitizer, perhaps due to a mechanism of intersensitizer energy transfer. Data have been obtained by exposing the system to the actual sunlight, and repeated cycles of photochemical conversion and thermal reverslon of the reaction were made. The kinetics of the thermal reverse reaction were investigated and presented. This information is considered to be useful in the development of a photochemical solar energy storage system.
Introduction
Photochemical conversion and storage of solar energy was recently being investigated in various laboratories around the world [Hautala et al. (1979); Scharf et al. (1979);Jones et al. (1979)j. Part of this interest is directed to the valence photoisomerizations of organic molecules. In particular, the norbornadiene (N)-quadricyclane (Q) system has been identified as a promising one for the storage of solar energy [Jones et al. (1979); Philippopoulos et al. (198311. The efficiency and kinetics of conversion of norbornadiene upon monochromatic irradiation (A = 254 nm) in the presence of solvents and photosensitizers has recently been reported [Phdippopoulos et al. (1983)l. The quantum efficiency was found to be a function of norbornadiene concentration, the largest value being 0.91 in pure norbornadiene with acetophenone as sensitizer. Acetophenone was found to be the most quantum efficient sensitizer followed by benzophenone and Michler's ketone. 0196-4321/84/1223-0458~01.50/0
It was suggested that the energy transfer from the excited photosensitizer to N is inversely proportional to the difference of triplet energy between the excited molecule and the acceptor. The present paper aims at studying the kinetics and efficiency of storage of polychromatic solar radiation under controlled solar simulation in the laboratory and under conditions of exposure to the sun. Also other objectives were to study the reverse thermal reaction Q N and to carry out a number of cycles of the process to determine possible product deterioration. It is hoped that this information may be useful in the development and improvement of a viable process for photochemical solar energy storage.
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Theoretical Section
The main criterion which can be used for the screening of various proposed photochemical systems for the storage of solar energy is the overall storage efficiency, qw. As has 0 1984 American Chemical Society
