Carbon monoxide hydrogenation over alkali-promoted Rh/Al2O3, Rh/V2O3/SiO2 and Rh/ThO2/SiO2

Abstract

Addition of Li, Na, K and Cs to a 1.5 wt% Rh/Al,O, catalyst resulted in a decrease of the hydrocarbon formation rate, while the methanol formation rate was almost unaffected. The overall result is an increased oxygenate selectivity at a decreased total activity. The oxo-selectivity increased in the order unpromoted < Li < Na< K < Cs. The results suggest that the main role of the alkali promoter is to decrease the carbon monoxide dissociation reaction.Rh/SiO, catalysts promoted with ThO, or V,O, had higher C&-oxygenate selectivities and a higher activity than a Rh/Si02 catalyst. Addition of Li, Na, K and Cs did not increase the oxoselectivity of the ThO,-and V,O,-promoted Rh/SiO, catalysts. Independent of the kind of alkali added, it mainly decreased the activity and lowered the hydrogenation rate, suggesting that the added alkali blocked the special perimeter sites which are held responsible for the ThO, and V,O,, promotion effect or that the added alkali formed a mixed oxide with the promoter oxide. Alkali ions, therefore, did not improve the catalytic behaviour of the ThO,-and V,O,-promoted Rh/SiO, catalysts.

EXAFS and FT-IR Characterization of Mn and Li Promoted Titania-Supported Rh Catalysts for CO Hydrogenation

“…It is clear that the introduction of a promoter shifts the reduction of Rh to higher temperatures, which could be the result of an intimate contact between Rh and the promoters on the surface. A similar increase in the reduction temperature for promoted Rh has also been observed by others. , …”

“…A similar increase in the reduction temperature for promoted Rh has also been observed by others. 8,22 The reduction behavior observed by XANES and the particle size estimated by EXAFS are consistent with the TPR profiles shown in Figure 5. From the literature, 5,42 the low-temperature peak at the TPR curve can be attributed to the reduction of surface Rh oxide species and the smaller high-temperature peak to the reduction of bulk Rh species.…”

“…The product distribution is strongly dependent on the reaction conditions and the presence of promoters. The use of alkali promoters has been shown to be crucial for shifting selectivities toward the thermodynamically less favored products, such as ethanol and C 2 + oxygenates. − The role of the alkali promoters is to enhance oxygenate formation by suppressing the hydrogenation activity of Rh, which can lead to C 1 products (methane and methanol). , This suppression has to be balanced with C–C bond formation so that selectivity to C 1 products is minimized, but sufficient hydrogenation activity is present to form C 2 + oxygenates …”

Recent Advances and Perspectives in Catalyst Design for Converting Syngas to Higher Alcohols

“…26,74 It was reported that when using the 1.5 wt % Rh/Al 2 O 3 catalyst, the selectivity of C 2+ oxygenates was increased in the following order: unpromoted < Li < Na < K ≤ Cs. 75 However, the addition of alkali promoters usually decreased the overall activity, owing to the physical blockage of Rh surface sites, as reported by Chuang and co-workers, where the activity decreased following the order unpromoted > Li > K > Cs. 74 There is still controversy about whether the electronic state of the Rh metal surface is altered by alkali promoters or not.…”

“…Adding alkali promoters to Rh-based catalysts facilitated the formation of C 2+ oxygenates and suppressed the production of alkanes, e.g., methane. It is widely accepted that the main role of alkali promoters is to suppress the dissociation of adsorbed CO by blocking the Rh sites or by creating new active sites at the Rh–alkali interface, thus keeping the surface CH x and undissociated CO/CHO species in balance. , It was reported that when using the 1.5 wt % Rh/Al 2 O 3 catalyst, the selectivity of C 2+ oxygenates was increased in the following order: unpromoted < Li < Na < K ≤ Cs . However, the addition of alkali promoters usually decreased the overall activity, owing to the physical blockage of Rh surface sites, as reported by Chuang and co-workers, where the activity decreased following the order unpromoted > Li > K > Cs …”