The previous report showed that ferric chloride (FeCl
3) catalyst has a activity for formation of tetrachlorophthalic anhydride under vapor phase catalytic chlorination of phthalic anhydride (PAA). However under this condition was caused the liberation of large amount of FeCl
3 by sublimation from the carrier and chlorinating decomposition was also prominent. In order to improve these defect, the effect of addition of various alkali metal salts to FeCl
3-fire resisting stone catalyst was investigated. It was found that the addition of 0.4-1.0 mol of KCl per 1.0 mol of FeCl
3 was effective to prevent the sublimation of FeCl
3 and decreased the chlorinating decomposition activity considerably. By use of FeCl
3-KCl-fire resisting stone catalyst at 270°C, on the rate of supply of PAA 0.015 mol/hr and Cl
2/PAA=30±5 molar ratio, the yield of tetrachlorophthalic anhydride was above 85 mol% and that of hexachlorobenzene below 6 mol %.
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