To separate an oxidation reaction product and an oxidation catalyst from an oxidation reaction mixture in high efficiency without causing the decomposition of the oxidation catalyst.
A substrate (e.g. hydrocarbon) is oxidized by using an imide compound such as N-hydroxyphthalimide expressed by the formula (1) (R1 and R2 are each H or a substituent such as a halogen atom or bonded together to form a double bond or an aromatic or non-aromatic 5 to 12-membered ring; X is O or OH; (n) is 1-3) as an oxidation catalyst. The oxidation reaction product (e.g. adipic acid, benzoic acid or butenediol) and the oxidation catalyst are separated from the obtained reaction mixture by using an aqueous solvent containing at least water (e.g. water) and a non-aqueous solvent (e.g. methyl benzoate or benzonitrile) separable from the aqueous solvent.
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