Process for the preparation of alpha, beta unsaturated aldehydes by oxidation of alcohols in the presence of a liquid phase

A process for the preparation of alpha, beta unsaturated aldehydes of general formula (I)

• wherein R1, R2 and R3, independently of one another, are selected from
• hydrogen;
• C1-C6-alkyl, which optionally carry 1, 2, 3, or 4 identical or different substituents which are selected from NO2, CN, halogen, C1-C6 alkoxy, (C1-C6-alkoxy)carbonyl, C1-C6 acyl, C1-C6 acyloxy and aryl;
• and C2-C6-alkenyl, which optionally carry 1, 2, 3, or 4 identical or different substituents which are selected from NO2, CN, halogen, C1-C6 alkoxy, (C1-C6-alkoxy)carbonyl, C1-C6 acyl, C1-C6 acyloxy and aryl;
• by oxidation of alcohols of general formula (II)
• wherein R1, R2 and R3 have the meaning as given above
• in the presence of a catalyst and in the presence of a liquid phase,
• wherein the liquid phase contains at least 25 weight-% water based on the total weight of the liquid phase, determined at a temperature of 20° C. and a pressure of 1 bar and
• wherein the oxidant is oxygen, and
• wherein the catalyst comprises a catalytically active metal on a support and the catalytically active metal is located mainly in the outer shell of the catalyst, wherein the outer shell of the catalyst is the space from the outer surface of the catalyst to a depth of X from the outer surface of the catalyst, wherein X is 15 % of the distance from the outer surface of the catalyst to the center of the catalyst.