(570aj) Hydroformylation by Using Rhodium Tethered On Selectively Functionalized Silica and High-Pressure IR Study

ABSTRACT: In this study, selective surface functionalization was performed with amine functional groups by post toluene reflux condition over MCM-41. This material was used as a support for rhodium immobilization. The selective functionalized MCM-41-based heterogeneous catalysts were applied toward the 1-octene hydroformylation. MCM-41 materials with amine and Rh complexes in the intra-pores show a higher selectivity for linear aldehydes. The mesopore, the steric bulky triphenylphosphine ligand greatly improved the selectivity of the less hindered linear aldehydes over the branched aldehyde. The active species involved in the hydroformylation has been studied by in situ FT-IR. The active species containing Rh(CO)₂(PPh₃)₂ and HRh(CO)(Ph3P)3 were involved in the hydroformylation of 1-octene over the heterogeneous catalyst systems.

INTRODUCTION: Immobilization of rhodium complexes on the mesoporous supports has been widely studied in the last several decades. Also the study on the surface functionalization of mesoporous materials has grown sharply. Among these useful functional groups, amino groups represent many functions for metal ion extraction, sensor and catalyst. It is possible that the functionalized mesoporous silicas can be used in hydroformylation reactions through immobilization of metal complexes. An alkene is easily converted to a linear and/or branched aldehyde by addition of CO/H₂ under Co or Rh catalysts. Between the linear and branched aldehydes, the linear aldehyde is much more commercially useful, and as such, its selective production would be very advantageous.

EXPERIMENTAL: In this study, selective surface functionalization was performed with amine functional groups by post toluene reflux condition over MCM-41. This material was used as a support for rhodium immobilization. It was expected that, in the hydroformylation of 1-octene, it was shown different catalytic behavior in comparison with homogeneous rhodium catalyst. The active species involved in the hydroformylation has been studied by in situ FT-IR. MCM-41 was prepared according to the reference. The calcined MCM-41 was pretreated with bulky diethoxydiphenylsilane (Ph₂Si(OEt)₂) to graft a more accessible external surface. Modification of the internal surface (in pores) was then performed with amino propylmethyldimethoxysilane (AEAPMDMS) [4]. Catalysts was added to a SUS-316 autoclave reactor (80 mL) containing THF and 1-octene. Subsequently, the autoclave was sealed and flushed with nitrogen gas. The syn-gas (CO/H₂ =1) was pressurized up to 20 bar in the reactor, and then the reactor was heated to 140 ^oC. The reactor was kept at that temperature, and the syn-gas was filled up from a reservoir to maintain the pressure of 20 bar. After the completion of the reaction, the resulting products were analyzed by gas chromatography.

RESULTS and DISCUSSION: The homogeneous and the selective functionalized MCM-41-based heterogeneous catalysts were applied toward the 1-octene hydroformylation. The amount of immobilized rhodium on the support was 1.42 wt%. The yield of nonyl aldehyde measured with gas chromatography was 5.7 wt% over MCM-41/AEAPMDMS/Rh. The catalyst of MCM-41/Ph₂Si(OEt)₂/ AEAPMDMS/Rh with TPP was shown the yield of 23.8 wt%. When the ligand was added to the rhodium immobilized aminated MCM-41, the yield of nonyl aldehyde was higher than that of rhodium immobilized aminated MCM-41. It seems that, electron donating amine groups take effect in the hydroformylation reaction during alkene insertion. The TPP as ligand could stabilize rhodium metal center as active site for 1-octene hydroformylation. The l/b of the aldehydes produced with the Rh homogeneous and MCM-41/AEAPMDMS / Rh (non-selective) systems was 1.6 and 1.7, respectively. However, significantly, when selective functionalized materials were used for 1-octene hydroformylation, the l/b rapidly increased to 3.6. It seems that the selectively passivated materials with amine and Rh complexes in the intra-pores show a higher selectivity for linear aldehydes in the 1-octene hydroformylation. The mesopore (pore size 2.0 ~ 3.0 nm), the steric bulky triphenylphosphine ligand greatly improved the selectivity of the less hindered linear aldehydes over the branched aldehyde. It has been reported that the active species was HRh(CO)₂(PPh₃)₂ in the hydroformylation of lower olefins catalyzed by homogeneous catalyst including Rh?Ph3P system. In order to confirm the active species in our heterogeneous catalyst system, in situ FT-IR examinations were performed at 20 atm of CO/H₂ pressure. In the in situ FT-IR spectra of MCM-41/AEAPMDMS/Rh catalyst. No signal except at 1978 cm^-1 was detected over MCM-41/AEAPMDMS/Rh. The IR absorption at 1978 cm^-1 could be assigned to rhodium carbonyl dimer. In case of in situ IR measurement of MCM-41/AEAPMDMS/Rh with TPP system, the absorptions at 1945 and 2003 cm^-1 were observed. It is worth to note that the activity of 1-octene hydroformylation of MCM-41/AEAPMDMS/Rh with TPP system is much higher than that of the catalyst system without TPP. From the above in situ IR results, it was clearly that the active species containing Rh(CO)₂(PPh₃)₂ and HRh(CO)(Ph₃P)₃ were involved in the hydroformylation of octenes over the heterogeneous catalyst systems.

ACKNOWLEDGEMENTS: This work was supported by a Korea Science and Engineering Foundation (KOSEF) grant, funded by the Korea government(MOST) (No. R01-2007-000-20144-0).

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