Title : Spontaneous deposition of boron oxide on a rhodium nanostructure for selective conversion of syngas to ethanol
Abstract:
Ethanol has been regarded as a crucial platform molecule for the modern chemical industry to produce valuable chemicals with a large demand. The current ethanol production strongly depends on the fermentation of grains, which competes with human food. Developing non-grain route for ethanol production is highly desirable, and multiple routes have been developed by cellulose hydrogenolysis and ethylene hydration. Besides these techniques, a trend is to produce ethanol from syngas, which is a mixture of CO and hydrogen that can be easily obtained on a large scale from coal, biomass, and natural gas. Since the 1950s, the supported Rh nanoparticles with various promoters (e.g. Mn, Fe, Cu, V, Ti, Mo, Zr, alkali metals, and rare earth metals) have been investigated in direct conversion of syngas to ethanol. However, the methane selectivity is usually higher than 50% with C2-oxygenate selectivity lower than 40% in most cases. It is still far to meet the desirable selectivity to oxygenate products because of the uncontrollable methanation.
Generally, the formation of methane is mainly due to excessive dissociation of carbon-oxygen bonds (C-O) and subsequent hydrogenation of hydrocarbons intermediates (CHx). Hindering CO dissociation and weakening the hydrogenation activity of the catalysts is supposed to reduce methane selectivity. Note that selectively blocking the specific sites for undesired side reactions on catalyst nanostructure is a promising way to improve the performances, especially for structurally-sensitive processes such as CO dissociation on Rh surfaces. However, it is a challenge for the control of catalyst structure at atomic scale.
Herein, we show that a boron oxide species could spontaneously and selectively react with the low-coordination sites on Rh nanoparticles, which are responsible for undesired methanation in the conversion of syngas to ethanol. As a result, the boron oxide modified RhMn nanoparticles on a silica support (RhMnB3.9/SiO2) exhibited oxygenate selectivity as high as 63.9% by methane selectivity reduced to 31.1%, of which 90.1% of the oxygenates are C2-oxygenates. Such an oxygenate selectivity outperforms supported RhMn catalysts, which usually exhibit selectivity of higher than 50% for undesired methane. This work offers an alternative route for ethanol production from syngas.
