FOURIER-TRANSFORM INFRARED SPECTROSCOPIC STUDY OF THE MECHANISM OF HETEROGENEOUS RHODIUM-CATALYZED METHANOL CARBONYLATION

Rhodium complexes show high activity in homogenous carbonylations of a lcohols and hydrocarboxylations of alkenes to carboxylic acids in the presence of alkyl halides. Analogously to liquid phase reactions, rhod ium on different supports (active carbon, zeolite, SiO2, Al2O3, polyme rs, etc.) is the most active and selective among transition metal cata lysts for vapor phase carbonylations; very good results were obtained with rhodium on active carbon. In this paper Fourier transform infrare d (FT-IR) transmission spectroscopy was used to verify the validity of homogeneous methanol carbonylation mechanisms for heterogeneous phase applications. FT-IR spectra of adsorbed species obtained on RhCl3/Cab -O-Sil 5 catalyst (rhodium content 2.85%) pressed into a self-supporti ng pellet and treated successively with CO, CH3I and CH3OH, gave valua ble information on the reaction mechanism. The catalytically active co mplexes ([Rh(CO)(2)X(2)](-) where X = Cl, I or more correctly [Rh(CO)( 2)X(x)L(y)](-) where L=OH- or H2O and x + y=2) were formed from suppor ted RHCl(3) . 3H(2)O, CO and CH3I under relatively mild conditions: p( CO) approximate to 1 bar, p(Mel)=5 mbar, T=305-353 K. The absorption b and appearing at 1717-1723 cm(-1) after CH3I adsorption on the catalys t surface is close to the acetyl C=O stretching frequency (1708-1711 c m(-1)) obtained under homogeneous conditions, which proves the similar ity of reaction mechanisms in both liquid and vapor phases.