A facile method for promoting activities of ordered mesoporous silica-anchored Rh–P complex catalysts in 1-octene hydroformylation

Abstract

This work deals with the promotion of immobilized Rh catalyst activities in olefin hydroformylation by lengthening the alkyl spacers and choosing an active Rh precursor. The flexibility of long chain alkyls was used to free the motion of the anchored Rh complex in confining porous spaces so as to increase the substrate activation efficiency. RhCl(CO)(PPh₃)₂ was anchored to mesoporous silicas SBA-15 and MCM-41 through n-alkyl (carbon number n = 1, 3, 5, 8, 11) as spacer. The catalysts were characterized by X-ray powder diffraction (XRD), isothermal N₂ sorption analysis and inductive coupling plasma-atomic emission spectroscopy (ICP-AES). The multi-step-assembly of the complex into surface-organic modified mesoporous silica was monitored and evidenced by infrared spectroscopy (IR). The prepared Rh-immobilized catalysts were applied in 1-octene hydroformylation to nonyl aldehydes. Rh leaching in the reaction solutions was determined by ICP-MS, which was found to be satisfyingly low along with successful catalyst cycles. With the increase of the alkyl spacer length, the specific activity (shown as turnover number, TON) and stability of the immobilized catalyst were both increased. The catalyst prepared by the longest alkyl spacer (n = 11, C₁₁) revealed activity comparable to the homogeneous counterpart and resistance to the by-product poisoning. RhCl(CO)(PPh₃)₂-supported catalysts were also prepared by impregnating RhCl(CO)(PPh₃)₂ into SBA-15 and MCM-41 without the presence of diphenylphosphinoalkyl, the prepared catalysts showed a sharp activity decrease within only two catalyst cycles, owing to the significant active species leaching.