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PROCESS SELECTION:
Benzaldehyde is an industrially the most important aromatic aldehyde. It is used in the production of dyes, pharmaceuticals and perfumeries. The physical and chemical properties of the raw material and the product are discussed in detail. Hydrolysis of benzalchloride or the partial oxidation of toluene (vapor or liquid phase) principally produces benzaldehyde. Some other process for the manufacture of benzaldehyde include the reaction of benzene and co the oxidation or dehydrogenation of benzyl alcohol and catalyzed oxidation of styrene with hydrochlorite.
Benzaldehyde can also be manufactured by the hydrolysis of mixture of benzyl chloride and benzalchloride in oil HNO3 using vanadium pentoxide as a catalyst or in the presence of hexamethylene tetramine. These process have no industrial importance in the production of benzaldehyde are more likely to be used to make certain unclear substituted derivatives. The commercial process for the production of benzaldehyde are:
1) LIQUID PHASE OXIDATION ROUTE:
This route involves oxidation of liquid toluene to benzaldehyde in the presence of homogeneous catalyst. Manganese dioxide with sulphuric acid has been used as the most suitable catalyst. A 14% yield of benzaldehyde has been claimed. When the cobalt is used as a catalyst at 3-atm pressure, the total conversion of benzoic acid and benzaldehyde was found to be 40%. Benzaldehyde is separated by fractional distillation.
C6H5CH3+ O2 ---------------->C6H5CHO + H2O dH= 92 Kcal / mole at 250C.
2) VAPOUR PHASE OXIDATION ROUTE:
Vapour phase oxidation provides chloride free benzaldehyde with a minimum amount of by product formation. Toluene is directly oxidized by air in the vapour phase at atmospheric pressure in the presence of a catalyst maintained at above 4000C. In this route extensive work was carried out using different catalyst. The most selection catalyst appears to be Vanadium pentoxide promoted with potassium sulphate and promoted uranium oxide catalyst. Both catalysts given approximately 60% yields of benzaldehyde based upon toluene. There is a varying amount of by product formation during oxidation. The principle by products are Benzoic acid, carbon dioxide, Anthraquinone, carbon monoxide and water.
3) ELECTROCHEMICAL ROUTE:
In a single stage process an emulsion of toluene in 60% sulphuric acid is oxidized electrolytically to benzaldehyde with lead electrodes. Mn2(SO4)4.5H2O is used as the oxygen carried with CaSO4 as the promoter. Unreacted toluene and the electrolytes are recycled. In a two-stage process, the first stage involves electrolytic oxidation of manganous sulphate to manganic sulphate paste. In the second stage, manganic sulphate is reacted with toluene in the reactor and benzaldehyde is formed. The manganous sulphate formed is regenerated electrolytically.
4) CHLORINATION OF TOLUENE:
In this process chlorine is reacted with toluene in presence of light to give benzalchloride, benzylchloride as intermediate and then hydrolysed benzalchloride gives benzaldehyde. Product found from this process is chlorinated on small scale. The chlorination method is cheapest and gives chlorine free benzaldehyde. Considering the different process and studying it for manufacture of benzaldehyde do not give high percentage of yield and unwanted materials are also produced in a large amount.
The electrochemical route is also expensive as it uses electricity and catalyst in large amount. There is also one process i.e. chlorination of toluene using catalyst. The catalyst used is PCl3 handling and storage of this catalyst creates problems and therefore the process is not considered. The process described here is the photochlorination of toluene, which gives a good yield. From this process we get two major by product such as 30% HCl and benzoic acid and also some sodium hypochlorate, which can be sold in market. Raw material used is chlorine comes in tonnes and therefore, this process is selected for manufacturing of benzaldehyde.