The kinetics of gas-phase propene hydroformylation over a supported ionic liquid-phase (SILP) rhodium catalyst David G. Hanna, Sankaranarayanapilla Shylesh, Sebastian Werner, Alexis T. Bell ⇑ Department of Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, CA 94720, USA article info Article history: Received 7 March 2012 Revised 11 May 2012 Accepted 12 May 2012 Available online 25 June 2012 Keywords: Propene Hydroformylation Butanal Ionic liquid Rhodium SILP Kinetics abstract An investigation of the kinetics of propene hydroformylation in the gas phase has been conducted over a silica-supported Rh–sulfoxantphos complex stabilized by the ionic liquid [bmim][OctSO 4 ]. The reaction temperature was found to have a strong effect on the kinetics of n- and iso-butanal formation. For both products, it was observed that increasing the temperature decreased the apparent activation energy, altered the reaction orders with respect to reactants, and decreased the molar ratio of n- to iso-butanal. The observed changes in the kinetics are discussed in terms of the generally accepted mechanism for ole- fin hydroformylation and are attributed to a change in the rate-determining step (RDS). It is concluded that at low temperature, the RDS is alkene insertion into an Rh–H bond but becomes the oxidative addi- tion of H 2 at high temperature. The change in the RDS is rationalized in terms of a change in the elemen- tary step with the largest Gibbs free energy of activation (DG à ). A greater loss in entropy for the oxidative addition of H 2 over alkene insertion causes the DG à of the oxidative addition to be greater than the DG à of alkene insertion at high temperature. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction The liquid-phase hydroformylation of alkenes to aldehydes, cat- alyzed by Rh complexes, is used extensively to produce intermedi- ates for a number of commodity chemicals (e.g., plasticizers, detergents, etc.) [1]. Two of the ongoing challenges with the use of homogeneous Rh complexes are separation of the organic prod- ucts from the catalyst and loss of Rh due to leaching [2]. Biphasic systems such as the Ruhrchemie/Rhône-Poulenc process are cur- rently used to facilitate product separation and reduce Rh leaching for propene hydroformylation [1]. The reaction is carried out in an aqueous phase containing the solubilized Rh complex, whereas an organic phase is used to extract the product aldehyde. Although satisfactory for propene hydroformylation, this process is not suit- able for longer chain olefins due to decreased alkene solubility in the aqueous phase [2,3]. Another disadvantage of such systems is the cost for recovering the alkene from the organic phase [3]. To circumvent the issues associated with using biphasic sys- tems, a number of authors have examined the possibility of dis- persing the active Rh complex in a liquid contained within the pores of a high surface area solid or grafting the complex to the surface of a support material [4–7]. In the early 1980s, Gerritsen and coworkers showed that molten triphenlyphosphine could be used as a solvent for HRh(CO)(PPh 3 ) supported on c-alumina and silica [8]. Subsequent work by Davis and coworkers led to the development of supported aqueous-phase (SAP) catalysts in which an aqueous solution of a Rh complex is supported on microporous glass [9]. Such catalysts proved effective for the hydroformylation of water-insoluble alkenes in nonpolar solvents [10]. However, subsequent work revealed that SAPs are not suitable for gas-phase hydroformylation due to evaporation of the aqueous phase [11]. Since ionic liquids have negligible vapor pressure, Mehnert and coworkers developed supported ionic liquid catalysts (SILCs) by impregnating a solution of Rh-tppts in [bmim][BF 4 ] or [bmim][PF 6 ] into a 1-n-butyl-3-methyl imidazolium modified silica gel support [12]. In later studies conducted by Wasserscheid and coworkers, analogous supported ionic liquid-phase (SILP) catalysts were found to be exceptionally stable and active under continuous gas-phase reaction conditions [13,14]. The stability, high activity, and facile separation of the products from the Rh complex makes SILP cata- lysts particularly promising for the commercial application of pro- pene hydroformylation. While previous studies of SILP catalysts have suggested that the Rh complex is homogeneously dispersed in an IL film [12–15], our recent study using 31 P MAS NMR and in situ FT-IR suggests that interactions of the IL and metal complex with the support are re- quired to yield a stable catalysts [16]. As shown in Scheme 1, the phosphine ligand, sulfoxantphos (SX), interacts with the slightly acidic silanol groups via the lone pair of electrons on phosphorus (conformation A) or in an ion–dipole interaction with the charged sulfonate group (conformation B). Similarly, the IL ([bmim][Oct- 0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2012.05.011 ⇑ Corresponding author. Fax: +1 510 642 4778. E-mail address: bell@cchem.berkeley.edu (A.T. Bell). Journal of Catalysis 292 (2012) 166–172 Contents lists available at SciVerse ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat