Method for producing aldehydes

A Heterogeneous Hydroformylation process for the preparation of aldehydes comprising reacting an olefin selected from the group consisting of mono-olefins, conjugated and non-conjugated polyolefins, cyclo-olefins and derivatives of said olefins with carbon monoxide and hydrogen at temperatures of 20 to 150° C. and pressures of 0.1 to 20 MPa in the presence of a non-aqueous ionic liquid of the formula (Q+)aAa−and at least one rhodium compound, wherein Q+is a singly charged ammonium cation optionally substituted by organic groups or the equivalent of a multiply charged ammonium cation optionally substituted by organic groups, Aa−is the anion of a sulfonated or carboxylated triester of phosphorous acid and a is an integer equal to or greater than 1 and wherein the aldehyde-containing product phase and the catalyst phase comprising rhodium and ionic liquid are separated from one another by phase separation and the catalyst phase is at least partly recycled to the hydroformylation process.