In Situ Formation of Wilkinson-Type Hydroformylation Catalysts: Insights into the Structure, Stability, and Kinetics of Triphenylphosphine- and Xantphos-Modified Rh/SiO₂

Abstract

An investigation has been carried out to identify the effects of catalyst preparation on the activity, selectivity, and stability of phosphine-modified rhodium/silica catalysts (Rh/SiO2) for propene hydroformylation. High selectivity to aldehydes was achieved, without the formation of propane or butanol. Catalyst activity and selectivity was found to depend strongly on the nature and concentration of the phosphine ligands and the amount of rhodium dispersed on the silica support. Screening of different ligands showed that a bidentate xantphos (X) ligand was ∼2-fold more active than the monodentate phosphine ligand (PPh3) screened at a ligand-to-rhodium ratio of 15:1. Investigation of the effects of reaction temperature, reactant partial pressures, and phosphine-to-rhodium ratio indicates that the kinetics of propene hydroformylation over X-promoted Rh/SiO2 is nearly identical to those for sulfoxantphos-modified rhodium-containing supported ionic liquid phase (SX-Rh SILP) catalysts. In-situ FTIR and solid-state 31P MAS NMR characterization provide evidence for the formation of HRh(CO) n (PPh3)4–n species on PPh3-modified Rh/SiO2, and HRh(CO)2(X) species on xantphos-modified Rh/SiO2. The high catalytic activity observed over rhodium-containing silica catalysts is attributed to formation of Rh(I)(CO)2 species by the process of corrosive chemisorption of Rh nanoparticles by CO and the subsequent ligation of phosphine ligands to the dicarbonyl species. Evidence is also presented suggesting that the active form of the catalyst resides on the surface of the Rh nanoparticles.

Rhodium Nanoparticles as Precursors for the Preparation of an Efficient and Recyclable Hydroformylation Catalyst

“…The kinetic curve for the first reaction using the Fe 3 O 4 @SiO 2 ‐N(CH 2 PPh 2 ) 2 Rh catalyst showed a clearly pronounced induction period of 1–2 h during which active catalytic species were likely formed from the rhodium precursor (Figure 2). According to Shylesh et al.,6f rhodium NPs can undergo corrosive chemisorption in the presence of carbon monoxide under hydroformylation conditions, which leads to a weakening of RhRh bonds owing to the higher strength of RhCO bonds. This may suggest that molecular rhodium species are formed in situ, residing on the support due to the interactions with the phosphine ligands grafted on the silica matrix.…”

“…The most common supports used are silica,7 clays,8 and active carbon,9 although other materials have also been tested 10. Only a few papers have been published on the immobilization of rhodium NPs on solid supports such as silica,6f, 11 carbon,6c and hypercrosslinked polystyrene 12. In most cases, no experimental proof that the hydroformylation reaction occurs on the surface of metal NPs has been provided.…”

Support Functionalization with a Phosphine‐Containing Hyperbranched Polymer: A Strategy to Enhance Phosphine Grafting and Metal Loading in a Hydroformylation Catalyst

“…The intensity of the band at ñ = 1208 cm À1 decreased, but this was also the case for the other phenyl ring modes,w hich makes it difficult to ascertain if this decrease in intensity is because of the reduction of the diphenylphosphine oxide under the reaction conditions or is ac onsequence of the decrease of the number of supported metal NPs. If we consider that the Rh NPs are the precursors of the active molecular speciesf ormedi nsitu as suggested before, [25] ad ecrease in the number of Rh NPs on the support through their corrosive chemisorption in the presence of CO is expected. The vibrational mode associated with PÀCr ing stretching in the graftedl igand at ñ = 1095 cm À1 (Figure4b) was no longer observed after the immobilization of Rh NPs on Fe 3 O 4 @SiO 2 -HYP-N(CH 2 PPh 2 ) 2 (Figure 4c)o ri nt he spent catalyst (Figure 4d), which does not allow us to draw conclusions about the interaction between Pa nd the Rh surface.…”

“…Such activation could occurt hrough the corrosive chemisorption of Rh NPs in the presenceo fC O. [25] The formation of relatively strong RhÀCO bonds andt he weakening of the RhÀRh bonds could result in the appearance of molecular Rh speciesi mmobilized on the solid surface by the interaction with phosphine ligandsg rafted on the silica matrix.N otably,n oc hanges in the regio-and chemoselectivity of the reactionw ere observed in the recycling experiments (Tables 3a nd 4), which indicates that the nature of the catalytically active species remains essentially unchanged.…”

Effect of lengthening alkyl spacer on hydroformylation performance of tethered-phosphine modified Rh/SiO2 catalyst

“…Five bands at 2071, 2039, 2005, 1991 and 1948 cm -1 appeared in the DPPPTS-Rh/SiO2. The bands at 2071 and 1991 cm -1 have been assigned to ee-HRh(CO)2(DPPPTS)2/SiO2 (i.e., tethered ee-HRh(CO)2 (DPPPTS)2 on SiO2), while the bands at 2005 and 1948 cm -1 have been assigned to ea-HRh(CO)2(DPPPTS)2/SiO2 [10][11][12].The band at 2039 cm -1 has been assigned to linear adsorbed CO, as Rh-CO [13][14]. This band has been significantly red-shifted compared with the heterogeneous Rh/SiO2 catalyst [15][16][17], because of the weaker C-O bond resulting from π* back-donation from the Rh to the CO in the presence of the tethered-phosphine.…”