Highly efficient and selective hydroformylation by rhodium-based PEG@silica hybrid microreactors

This study presents the development of a practical homogeneous-heterogeneous hybrid microreactor, synthesized through a one-pot non-aqueous sol–gel process. The process begins by forming an oil-in-oil emulsion of poly(ethylene glycol) (PEG) containing a rhodium catalyst and formic acid in heptane. Upon adding a silane monomer, a non-aqueous sol–gel process occurs, resulting in the formation of the silica component of the microreactors. Unlike conventional catalyst heterogenization methods that rely on chemical or physical immobilization to anchor the catalyst to a support, this method allows the catalyst to remain fully soluble within the reaction medium inside the pores of the hybrid microreactors. This design enables the microreactor to function as a homogeneous catalyst with high reactivity and selectivity while also allowing for easy catalyst recovery through simple centrifugation, owing to the microreactor's robust structure. The framework of the hybrid microreactor is established through covalent bonding between PEG and silica species, with PEG playing a crucial role in maintaining the durability and stability of the microreactors. Additionally, PEG exists as free molecules within the hybrid network, providing a green reaction medium within the microreactor's pores and facilitating homogeneous catalysis. The Rh-based microreactor was specifically tested in hydroformylation reactions, using different PEGs with varying molecular weights to control the microreactor's reactivity and selectivity. The catalytic performance and properties of these microreactors were thoroughly characterized. Notably, the Rh/PEG200@silica microreactors exhibited a high turnover frequency (TOF) of 11,190 h⁻¹ for styrene at 120 °C, and achieved remarkable regioselectivity towards the branched aldehyde product, with a b:l ratio of 40:1 for 4-vinylbenzoic acid at room temperature.