Experimental investigation has been carried out in order to elucidate the reaction mechanism of higher alcohol (mixture of $C_1-C_6$ alcohols) formation from syngas over Cu/ZnO catalysts. $^{13}CH_3OH$ and $D_2O$ were used to investigate the behavior of $C_1$ oxygenated species and the dehydrogenation characteristics of $C_2$ oxygenated species, respectively.
In the reaction between $C_1$ oxygenated species, only methylformate was formed as a coupled product, and it was found from the $^{13}C$ labeling study that the major reaction route was methanol-formaldehyde reaction over Cu under lower CO pressure, and the methanol-CO reaction over ZnO under higher CO pressure. The major route of methylformate formation over ZnO changed from methanol-CO reaction to methanol-formaldehyde reaction with increasing temperature.
In the reaction between $C_2$ oxygenated species, many kinds of higher oxygenates were formed as coupled products. The coupling reaction was promoted markedly by the interaction of Cu and ZnO, but the dissolved copper in the ZnO lattice did not play any crucial role. It was found from the H-D exchange study that the dehydrogenation takes place mostly on the α-carbon over Cu, while on the β-carbon over ZnO. The reaction mechanism for the coupling of $C_2$ species was proposed on the basic assumptions comprised of (1) metal-oxygen bond adsorption of the reaction intermediates, (2) nucleophilic addition of β-carbon to the electrophilic α-carbon, and (3) bond cleavage at the weakest point between the two adsorption sites.
The proposed reaction mechanism could be also applied to the chain growth scheme in higher alcohol formation from syngas, and most of the product formations could be explained systematically.
