Process for preparing aldehydes and separation of the catalyst system by membrane separation

A process for preparing aldehydes, said process comprising at least

• a hydroformylation through the reaction of C2 to C20 olefins with syngas in the presence of a homogeneously dissolved catalyst system that comprises at least Co or Rh, in at least one reaction zone, yielding a liquid reaction output containing the product, wherein the partial pressure fraction of CO or H2 in the hydroformylation constitutes not more than 75% of the total gas pressure, the total gas pressure being the sum of the pressures occurring from all the gaseous substances present, and
• a membrane separation for separating the homogeneously dissolved catalyst system, wherein,
• before the membrane separation, a gas exchange with CO or H2 is carried out, as a result of which the partial pressure fraction of CO or H2 constitutes more than 80% of the total gas pressure.