Selective Butanol Synthesis over Rhodium−Molybdenum Catalysts Supported in Ordered Mesoporous Silica

Rhodium catalysts were synthesized in ordered mesoporous silica FSM-16 using Rh/Al heteropolyacid anions and/or [Rh(COD)2]+ complex. The hydroformylation activity of propene was compared to impregnated Rh/FSM-16 catalysts prepared from rhodium chloride. The catalysts were very selective to produce butanols due to the effect of two-dimensional mesoporous reaction space (effective internal pore diameter 27 Å). The selectivity to butanols (n-butanol and i-butanol) was as much as >98% over [RhMo6O18(OH)6]3-/FSM-16 catalysts and 73% over RhCl3/FSM-16 catalysts. Under the hydroformylation reaction conditions at 433 K and 60 kPa, 22−28 Å of supported nanoparticles were present based on Rh K-edge extended X-ray absorption fine structure (EXAFS) and high-resolution transmission electron microscopy (HR TEM) measurements. Rh metallic nanoparticles atomically mixed with Mo atoms and distorted heteropolyacid species coexisted in the [RhMo6O18(OH)6]3-/FSM-16 catalysts based on Rh and Mo K-edge EXAFS and HR TEM measurements. The population of metallic nanoparticles increased as the metal loading amount decreased from 5.2 to 0.22 wt % Rh. Thus, metallic nanoparticles in FSM-16 were essential for the butanol synthesis and Mo played an additional promoter role to increase the selectivity further. A reaction mechanism was proposed in which the metallic nanoparticle surface was predominantly adsorbed with CO and multiple-step hydrogenations of oxygenate intermediates proceeded to form butanols during slow diffusion in the two-dimensional mesoporous space.