Immobilization of homogeneous Rh-P complex on ordered mesoporous silica with different spacers and their application in hydroformylation of olefins

Prof. Dehua He, Tsinghua University, 6 May 2015, 13.30-14.30

Immobilization of homogeneous Rh-P complex on ordered mesoporous silica with
different spacers and their application in hydroformylation of olefins

Dehua HE (h-index of 21 as of April 2017)

Department of Chemistry, Tsinghua University, Beijing,10084 China

Abstract

Hydroformylation of olefins to aldehydes/alcohols is an important and famous industrial process,
and the catalysts for the industrial hydroformylation are mainly homogenous rhodium complexes, for
example the famous Wilkinson’s catalyst (RhCl(PPh3)3). However, homogeneous catalytic processes
suffer from complicated catalyst separation. The immobilization of homogeneous catalysts is therefore
a research interest, no matter to inorganic supports or to organic supports (mainly polymers). Organic
polymers are often swollen in solvents and they also possess of low thermostability. Therefore, many
inorganic materials are also used for homogeneous catalyst anchor. Ordered silica-based mesoporous
molecular sieves have offered us the opportunities to heterogenize various effective organometallic
catalysts inside the pores via multi-step assemble, which gratefully benefits from their high specific
surface area, uniform pore size distribution, high pore volume capacity, large enough pores (in the
range of 2 nm to 50 nm), easy surface modification, solvo-, and thermo- stabilities.
In traditional immobilized catalysts, metal composition is stiffly anchored to the support through
short organic groups as spacer. Due to the active sites being localized, reactant molecules are
supposed to approach to the solid surface, and mass transfer thus may affect the reaction efficiency.
Thus, the traditional immobilized catalysts usually reveal decreased activities than the homogeneous
counterparts.
This lecture will talk one of strategies for immobilizing homogeneous catalysts and its
application in the immobilization of homogeneous Rh-P complex. In this strategy, n-alkyls (carbon
number n = 1, 3, 5, 8, 11) were used as the spacer for connecting support (ordered mesoporous silica
SBA-15 or MCM-41) and Rh-P complex, and the flexibility of spacers were favor to free the motion
of anchored Rh complex, so as to increase the substrate activation efficiency. The catalysts were
characterized by X-ray powder diffraction (XRD), isothermal N2 sorption analysis, inductive coupling
plasma-atomic emission spectroscopy (ICP-AES). The multi-step-assemble 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 octene hydroformylation to nonyl
aldehydes. Rh leaching in the reaction liquids was determined by ICP-MS, which was found
satisfyingly low along with successful catalyst cycles. With the increase of alkyl spacer length, the
specific activity and stability of the immobilized catalyst were both increased. The catalyst prepared
by the longest alkyl spacer (n = 11, C11) revealed the activity comparable to the homogeneous
counterpart and resistance to the by-product poisoning.