Influence of Mn Promotion on CO Hydrogenation over Rh/CNTs Catalyst

Formation, dynamics, and long-term stability of Mn- and Fe-promoted Rh/SiO2 catalysts in CO hydrogenation

Fe servers as an electronic modifier on Rh/SiO2 through in situ RhFe nanoalloy formation, whereas Mn is more likely a structural modifier and does not substantially change Rh's intrinsic product spectrum.

Unveiling the Structure Sensitivity for Direct Conversion of Syngas to C2-Oxygenates with a Multicomponent-Promoted Rh Catalyst

Abstract

Mn and Li promoted Rh catalysts supported on SiO2 with a thin TiO2 layer were synthesized by stepwise incipient wetness impregnation approach. The thin TiO2 layer on the surface of SiO2 was proved to stabilize those small Rh nanoparticles and hinder their agglomeration. The reducibility of Rh on these catalysts depends on Rh particle size as well as the position of manganese oxide, and large Rh nanoparticles with MnO on Rh nanoparticles can be only reduced at an elevated temperature. Catalyst with large Rh particles exhibits a higher CO conversion and higher products selectivity towards long chain hydrocarbons and C2-oxygenates at the expense of decreasing methane formation than a similar catalyst with smaller Rh particles. This was attributed to the synergistic effect of Mn and Li promotion and molar ratio between Rh0 and Rhδ+ sites on the surface of Rh nanoparticles. Moreover, Rh nanoparticles on MnO are proved to be more efficient in promoting hydrogenation of acetaldehyde to ethanol than its counterpart with MnO on Rh nanoparticles. Finally, in order to target high C2-oxygenates selectivity, low reaction temperature together with a low H2/CO ratio in the feed is recommended.

Graphic Abstract

CO2 methanation on the catalyst of Ni/MCM-41 promoted with CeO2

CO₂ as a raw feed combined with renewable hydrogen for the production of useful chemicals and alternative energy products is one of the solutions to environmental and energy problems. In this study, a series of Ni-xCeO₂/MCM-41 catalysts with a nickel content of 20wt% were prepared through deposition precipitation method for CO₂ methanation. Different characterization methods, including BET, XRD, TEM, SEM, H₂-TPR and H₂-TPD were applied to help explore the influence mechanism of CeO₂ on Ni/MCM-41 in CO₂ methanation. It was found that all CeO₂-promoted catalysts exhibited enhanced catalytic activity when compared to Ni/MCM-41. The catalyst modified with 20wt% CeO₂ showed the best catalytic performance, with CO₂ conversion and CH₄ selectivity of 85.6% and 99.8%, respectively, at the temperature of 380°C under atmospheric pressure. The synergetic effects among Ni⁰ active sites, the promoter and the support, including nickel dispersion improvement and increased CO₂ adsorption sites due to the addition of CeO₂, were considered as important factors for high reactivity of the promoted catalysts. The stability test showed that the promoted catalyst maintained its high reactivity after 30h.

Status and prospects in higher alcohols synthesis from syngas

Higher alcohols are important compounds with widespread applications in the chemical, pharmaceutical and energy sectors. Currently, they are mainly produced by sugar fermentation (ethanol and isobutanol) or hydration of petroleum-derived alkenes (heavier alcohols), but their direct synthesis from syngas (CO + H₂) would comprise a more environmentally-friendly, versatile and economical alternative. Research efforts in this reaction, initiated in the 1930s, have fluctuated along with the oil price and have considerably increased in the last decade due to the interest to exploit shale gas and renewable resources to obtain the gaseous feedstock. Nevertheless, no catalytic system reported to date has performed sufficiently well to justify an industrial implementation. Since the design of an efficient catalyst would strongly benefit from the establishment of synthesis-structure-function relationships and a deeper understanding of the reaction mechanism, this review comprehensively overviews syngas-based higher alcohols synthesis in three main sections, highlighting the advances recently made and the challenges that remain open and stimulate upcoming research activities. The first part critically summarises the formulations and methods applied in the preparation of the four main classes of materials, i.e., Rh-based, Mo-based, modified Fischer-Tropsch and modified methanol synthesis catalysts. The second overviews the molecular-level insights derived from microkinetic and theoretical studies, drawing links to the mechanisms of Fischer-Tropsch and methanol syntheses. Finally, concepts proposed to improve the efficiency of reactors and separation units as well as to utilise CO₂ and recycle side-products in the process are described in the third section.

Effect of Mn doping on the activity and stability of Cu–SiO2catalysts for the hydrogenation of methyl acetate to ethanol

A series ofxMn–Cu–SiO₂catalysts with different manganese contents were prepared by an ammonia-evaporation method for methyl acetate hydrogenation.

Promotional effects of Cr and Fe on Rh/SiO2 catalyst for the preparation of ethanol from CO hydrogenation

The mode of contact between Rh and Cr is different to that between Rh and Fe.