Rhodium–Phosphite SILP Catalysis for the Highly Selective Hydroformylation of Mixed C4 Feedstocks

Abstract

The hydroformylation of alkenes catalyzed by dissolved rhodium complexes is not only one of the largest applications of homogeneous catalysis in industry, but also an established benchmark reaction for testing immobilization concepts for homogeneous catalysts. In recent years, ionic liquids (ILs) as non-aqueous solvents for liquid–liquid biphasic hydroformylation catalysis have been the subject of intensive study. Important features of ILs compared to the industrial aqueous–organic biphasic catalysis (Ruhrchemie–Rh ne–Poulenc process), are their much better solubility for higher alkenes and their compatibility with phosphite ligands, which readily decompose by hydrolysis in water. Despite these attractive features, we know of no largescale industrial application of ionic liquids in biphasic hydroformylation catalysis to date. Two important drawbacks of the biphasic ionic liquid systems are the relatively high amounts of expensive IL that are required and its intrinsically high viscosity, which leads to slow mass transport between the two liquid phases. To overcome these limitations, we, among others, have in recent years developed the supported ionic liquid phase (SILP) concept. SILP materials are prepared by dispersing a solution of the catalyst complex in an ionic liquid as a thin, physisorbed film on the large internal surface area of a porous solid material. Since the film thickness of the ionic liquid is within the nanometer range, diffusion problems are minimized by the extremely small diffusion distances. Excellent ionic liquid utilization is achieved; that is, the same catalytic performance can be achieved with a much smaller total IL amount compared to liquid–liquid biphasic systems. Because ionic liquids typically have extremely low vapor pressures, catalysis with SILP materials is particularly attractive in continuous gas-phase contact. During catalysis the immobilized catalytic ionic liquid film comes into contact solely with gaseous reactants and products. For the continuous gas-phase hydroformylation of pure 1-alkene feedstock, such as, propene and 1-butene, this concept has been demonstrated quite successfully with good catalytic activity (turnover frequencies (TOFs) up to 500 h 1 in the case of propene and 564 h 1 in the case of 1-butene) and excellent catalyst stability (up to 200 h time-on-stream in the case of propene and 120 h in the case of 1-butene) as was demonstrated using a Rh-SILP catalyst modified with the sulfonated phosphine ligand sulfoxantphos (1). The sulfoxantphos–rhodium catalyst is, however, unable to react with internal alkenes such as 2butenes in either hydroformylation or isomerization. Thus, to convert 1-butene and 2-butenes from a mixed technical C4 feedstock from steam-cracker into the desired linear pentanal, a different catalyst system is required. Rhodium–phosphite complexes are known to be capable of selective isomerization/hydroformylation activity, which converts internal alkenes in a classical monophase homogeneous catalysis into linear aldehydes with good to excellent selectivity. Most of these ligands, however, are highly airand moisture-sensitive, making it difficult to handle and use them in large quantities and a real challenge to recycle rhodium– phosphite systems. Herein, we show how the new diphosphite ligand 2 in form of a SILP catalyst system is applied in the continuous gas-phase hydroformylation of an industrial mixed C4 feedstock as illustrated in Scheme 1. Synthesizing 2 and using it in