Kinetic and mechanistic studies for the direct conversion of syngas to ethanol

The direct conversion of syngas to ethanol (STE) is a promising route to ethanol from fossil and non-fossil carbon resources. Rh-based catalysts offer the most promising results so far. However, the accomplished yields and rates still must be improved before industrial applications become viable. Owing to the complexity of the reaction network and the catalyst's behavior under high pressure reaction conditions, the entire complexity of this reaction at process relevant conditions has not been unraveled so far. The current study develops a comprehensive picture of the STE reaction network and reaction-induced changes of Rh/SiO2 catalysts at industrially relevant pressures, temperature, time scale and catalyst complexity. Moreover, a reaction network is derived that describes key influences of promoting elements Mn and Fe, including the dynamics of surface restructuring. The study investigates Rh/SiO2 catalysts in the absence and presence of Mn or Fe or both. Reaction mechanism and kinetics were studied under high pressure conditions for extended time scales with repeatedly tested standard conditions to assess catalyst aging. The physicochemical bulk- and surface properties of all catalysts were comprehensively analyzed prior to reaction as well as after different reaction times and atmospheres. Catalytic measurements that included variations of reactant partial pressures, temperature, contact time as well as cofeed and dropout experiments revealed a reaction network for Rh/SiO2. Key features of the reaction network include a CO insertion mechanism as the only plausible C-C-coupling step and the existence of two adsorbed CO* species with different reactivity – one being hydrogenated and dissociated and the other performing CO insertion. The necessity of two different CO* types implies the existence of two active sites that might be different in nature. While hydrogenation reactions and C-O bond breaking were assigned to metallic Rh, CO insertion possibly occurs on dynamically formed non-metallic Rh species or clusters. A dynamic behavior of Rh metal particle surfaces is observed, which strongly depends on CO partial pressure and time on stream. The reaction network proposed for Rh/SiO2 proves to be generic in nature and describes major aspects of the reaction network also for RhMn/SiO2, RhFe/SiO2, and RhMnFe/SiO2. The following key features were identified for the promoting elements: (i) Mn stabilizes the CHxCHO* intermediate resulting in enhanced ethanol, acetaldehyde, acetic acid yields, (ii) Fe accelerates hydrogenation rates in general and methanol formation in particular, and (iii) over RhMnFe/SiO2, the combination of both effects achieves the highest ethanol selectivities among the tested catalysts. The performed catalyst characterization also yields a comprehensive picture of the catalyst's structure as well as reaction-induced reversible and irreversible changes. Sintering and agglomeration strongly depend on reaction atmosphere, time on stream and promoting elements. If present, Fe exists in close proximity to Rh and strongly modifies the reactivity of Rh. In contrast, Mn appears to serve the role as oxidic support phase which reduces Rh mobility and thus prevents sintering and agglomeration. The derived insights provide a comprehensive picture of Rh/Fe/Mn based catalysts under realistic operation conditions and pave the way to the development of improved catalysts and reactor concepts.

Author dc:creator

• Bauer, Julia Ulrike

Advisors dc:contributor.advisor

• Rosowski, Frank
• Krähnert, Ralph

Licence dc:rights.uri

• https://creativecommons.org/licenses/by/4.0/

Language dc:language.iso

en

Identifier URI

http://dx.doi.org/10.14279/depositonce-11359

OAI identifier oai:identifier

oai:depositonce.tu-berlin.de:11303/12540