Olefin hydroformylation is a major catalytic transformation of olefins, achieved at industrial scale, generally using Rh based soluble catalytic species aiming at linear aldehyde synthesis. Figure 1 depicts the overall and simplified reaction scheme where several by-products can be generated (branched aldehydes, hydrogenated compounds, internal C=C migration, etc.) depending on the initial olefin substrate and on the synthesis conditions due to syngas pressure.

Reactions can be achieved either in solvent free mixtures or solvent diluted ones; toxic toluene being widely used. Moreover, the ligand selection, that must be in large excess, is also tricky either for the catalyst stability or the hydroformylation enantio-selectivity. Ligand (a phosphine such as PPh₃ or a phosphite such as biphephos, etc.) is in-situ associated with Rh catalytic precursor to create the active species. Even with high yields and enantioselectivity, the final mixture is quite complex to separate aiming at the recovery of the catalytic system for recycling and to the simultaneous extraction of the target product/by-products.

With respect to toluene and material resistance we have widely studied the cross-flow OSN separation of such media with PDMS membranes (Pervap 4060, Sulzer®, Puramem flux Evonik®, 10-40 bar). The paper provides an overview of results and discussion dealing with the following items:

For a given mixture and single step OSN:

• in toluene as well as in solvent free mixtures, the catalyst/aldehyde selectivity depends strongly on the ligand chemical nature appearing as a main parameter to control
• solvent free mixtures are much more viscous than toluene ones impacting significantly the OSN flux; moreover, the catalyst/aldehyde selectivity is modified compared to the one in presence of toluene
• addition of small amount of solvent to initial solvent free media can be a tool to modify the ligand rejection and thus the feasibility of catalyst recycling with respect to its stability, however the environmental impact of each useful solvent is different (LCA)For multiple steps’ OSN

For multiple steps’ OSN:

• Regardless of the mixture, single step separations are not efficient enough to simultaneously reach the two goals explained above and membrane cascades with internal recycling appeared promisin; however, combination with distillation is still required to reach high purity but with a lowered energy consumption compared to only distillation separation
• The cascade designs are different for solvent free and diluted mixtures, but also depend on the ligand chemical nature and overall mixture composition even when quite similar

Acknowledgments:

This study has been supported by the French ANR: MemChem and Cyrano projects.

MRB thanks KAUST for invitation at 9^th OSN 2024 conference.