Preparation, characterization and catalytic hydroformylation properties of carbon nanotubes-supported Rh–phosphine catalyst Original Research Article

Two kinds of carbon nanotubes grown catalytically, as a novel material for catalyst carrier, were prepared and characterized. Propene hydroformylation catalyzed by the Rh–phosphine complex catalysts supported by carbon nanotubes was investigated, and compared to that catalyzed by the Rh–phosphine complex catalysts supported by SiO2 (a silica gel), TDX-601 (a carbon molecular sieve), AC (an active carbon), and GDX-102 (a polymer carrier). Activity assay of the catalysts showed that the carbon nanotubes-supported Rh–phosphine complex catalysts displayed not only high activity of propene conversion but also excellent regioselectivity to the product butylaldehyde. Under the reaction conditions of 393 K, 1.0 MPa, C3H6/CO/H2 = 1/1/1 (v/v), GHSV = 9000 ml (STP) h−1 (g catal.)−1, P/Rh = 9–12 (molar ratio), and Rh-loading at 0.1 mmol Rh (g carrier)−1, the molar ratio of normal/branched (n/i) aldehydes reached 12–13 at a turnover frequency (TOF) of 0.12 s−1, corresponding to propene conversion of ∼32%. The characterization by using TEM, HRTEM, XRD, Raman, XPS, BET and temperature programmed desorption (TPD) methods indicated that the carbon nanofibers prepared were quite even nanotubes with the outer diameters at 15–20 nm and the inner diameters (i.e., pore diameters) at ∼3 nm. Each tube wall was constructed of many layers of carbon with graphite-like platelets in a cross-section orientation of ‘parallel type’ or ‘fishbone type’; their C (1 s) electron binding energy was about 0.5 eV lower than that of graphite. These results, together with the results of comparative studies of the Rh–phosphine complex catalysts supported by several other carriers, implied strongly that the tubular channels with the inner diameter of ∼3 nm in the carbon nanostructures and its hydrophobic surface consisting of six-membered C-rings played important roles in enhancing the activity of propene hydroformylation, especially the regioselectivity of butylaldehyde on the Rh–phosphine complex catalysts supported by them.