Catalytic vapor phase ammoxidation reaction of o-xylene was investigated. The activity of the vanadium pentoxide (V
2O
5)-pumice catalyst to the formation of o-phthalonitrile was low, and gave mainly phthalimide. Therefore the effect of addition of various additives on V
2O
5 was investigated. It was found that the addition of P
2O
5 to V
2O
5 gave remarkable improvement in the activity of formation of o-ph-thalonitrile. Namely, V
2O
5-P
2O
5-fire-resisted stone catalyst under the reacting condition (molar ratio of P
2O
5/V
2O
5=0.25, reaction temperature 480°c, gas space velocity 790 cc/cc. hr (NTP) NH
3/o-xylene ca. 8 moles) yielded ca. 50 mole% o-phthal- onitrile and ca. 12 mole % of phthalimide. An increase in the activity for formation of o-phthalonitrile by the addition of P
2O
5 was probably brought by the lowering of activity for the oxidative decomposition of ammonia by the catalyst and of the excessive oxidation of o-xylene and by the increasing activity for the formation of o-phthalonitrile by the reaction of oxidation product of o-xylene with ammonia. Also, V
2O
5-P
2O
5-fire-resisted stone catalyst showed the high activity for the oxidative decomposition of ammonia at the initial period of reaction, which was gradually decreased to a fixed value with the time of reaction, while the rate of formation of o-phthalonitrile was low at the beginning, then reached to a fixed higher value after certain period.
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