Process for preparing unsaturated esters proceeding from aldehydes by direct oxidative esterification

A process for preparing methyl methacrylate, comprising:

• reacting methacrolein in a continuous oxidative esterification with oxygen and methanol which is conducted in a liquid phase in a reactor at a pressure of 2 to 100 bar in the presence of a heterogeneous catalyst comprising gold-containing nanoparticles having a particle size less than 20 nm;
• withdrawing continuously the liquid phase from the reactor and adjusting the pH after the withdrawal to a pH between 5 and 9 by addition of a basic solution outside of the reactor;
• and
• conducting at least 50% of the withdrawn liquid phase having a pH between 5 and 9 back into the reactor.