Method for the prevention of post oxidation of methacrolein

A method for the prevention of post oxidation of methacrolein prepared by the vapor-phase oxidation of isobutylene, tertiary butanol or methallyl alcohol with a molecular oxygen-containing gas in the presence of a molybdenum-bismuth-iron catalyst, by employing a vertical shell-and-tube reactor which has an upper empty column portion, reaction tubes and a lower empty column portion successively in order from an inlet thereof, the improvement comprising sparging a cooling gas at a linear velocity of 2-8 times the linear velocity of the reaction product gas as expressed in terms of its velocity at the lower empty column portion, into the lower empty column portion near the outlet of the reaction tubes, whereby the temperatures of the reaction product gas is reduced to 200.degree.-300.degree. C., the cooling gas being at least one of the following gases:

• (A) an inert inlet gas selected from the group consisting of nitrogen, carbon dioxide and steam or mixtures thereof;
• (B) a recirculated reaction gas which is a portion of a gas after removing a reaction product from the outlet gas of the reactor and which can be recirculated into the reactor;
• (C) a mixed gas of the inert gas (A) with air;
• or (D) a mixed gas of the recirculated reaction gas (B) with air;
• thereby preventing post oxidation of the methacrolein.