In Situ Formation of Wilkinson-Type Hydroformylation Catalysts: Insights into the Structure, Stability, and Kinetics of Triphenylphosphine- and Xantphos-Modified Rh/SiO 2 Sankaranarayanapillai Shylesh, David Hanna, Anton Mlinar, Xü e ́ -Qia ̅ n Kǒ ng, Jeffrey A. Reimer, and Alexis T. Bell* Department of Chemical and Bimolecular Engineering University of California, Berkeley, Berkeley, California 94720, United States * S Supporting Information 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/SiO 2 ) 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 (PPh 3 ) 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/SiO 2 is nearly identical to those for sulfoxantphos-modified rhodium- containing supported ionic liquid phase (SX-Rh SILP) catalysts. In-situ FTIR and solid-state 31 P MAS NMR characterization provide evidence for the formation of HRh(CO) n (PPh 3 ) 4-n species on PPh 3 -modified Rh/SiO 2 , and HRh(CO) 2 (X) species on xantphos-modified Rh/SiO 2 . 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. KEYWORDS: rhodium, xantphos, phosphine ligands, butanal, hydroformylation 1. INTRODUCTION The hydroformylation of alkenes to produce aldehydes is carried out on a large scale using an aqueous solution of a Rh- tppts complex (tppts = tri(m-sulfonyl)triphenylphosphine trisodium salt). 1-3 Although this process is highly effective, it is limited to alkenes with fewer than four carbon atoms because of the low solubility of higher-molecular-weight alkenes. The need to separate the reaction products from the catalyst solution presents an additional challenge for carrying out alkene hydroformylation using homogeneous complexes. 4 Heteroge- nization of homogeneous catalysts on solid supports can easily mitigate these problems and enable alkene hydroformylation to be carried out in the gas phase or in the liquid phase without need for a solvent. 5,6 Several attempts have been made to achieve this goal. Davis and co-workers have shown that a supported aqueous phase catalyst can be prepared by dispersing tppts to Rh(acac)(CO) 2 dissolved in a thin aqueous layer within the pores of a high-surface-area silica. 7 Exposure of this impregnated solid to synthesis gas (H 2 /CO = 1) at atmospheric pressure and room temperature produced an active catalyst for propene hydroformylation. Wasserscheid and co-workers have shown that supported ionic liquid phase (SILP) catalysts can be prepared by dispersing a solution of the catalyst complex in an ionic liquid (IL) as a thin film on the internal surface of a porous solid. 8-11 The advantage of using ILs over water is the near-zero vapor pressure of ILs, which virtually eliminates the loss of solvent under reaction conditions. Such SILP catalysts have been demonstrated to be highly effective for propene hydroformylation. The factors influencing the activity and stability of Rh-based SILP catalysts for the gas-phase hydro- formylation of propene have been discussed by Shylesh et al. 12 Evidence from in situ FTIR and 31 P MAS NMR suggests that interactions of the IL and metal complex with the support are required to yield a stable catalyst. Catalyst activity is found to be a strong function of IL composition, as well as ligand-to-Rh ratio, IL loading, and temperature of support pretreatment. Ding and co-workers have reported a different approach for producing an active catalyst for propene hydroformylation. In this case, silica-supported Rh nanoparticles are promoted with PPh 3 , 13,14 and Kim et al. have recently shown that the highest activity for PPh 3 -modified Rh/SiO 2 is achieved for PPh 3 /Rh = 15. 15 Because of the similarities in activation energies, partial and total pressure dependences of the reaction rates for PPh 3 - modi fied Rh/SiO 2 and homogeneous HRh(CO)(PPh 3 ) 3 catalysts suggest that the active species in both systems are identical. Received: November 16, 2012 Revised: January 14, 2013 Published: January 15, 2013 Research Article pubs.acs.org/acscatalysis © 2013 American Chemical Society 348 dx.doi.org/10.1021/cs3007445 | ACS Catal. 2013, 3, 348-357