Application of Prussian Blue Analogue‐Derived Mn−Co Catalysts in the Hydrogenation of CO to Higher Alcohols

Abstract

MnxCo3‐x[Co(CN)6]2 Prussian blue analogues were synthesized with Mn : Co ratios in the range from 7 : 10 to 1 : 11 and pyrolyzed at 600 °C to obtain a highly nitrogen‐ and oxygen‐functionalized carbon matrix with embedded reduced Mn and Co species. These catalyst precursors were applied in the CO hydrogenation to higher alcohols at 260 °C and a pressure of 60 bar using a H2/CO ratio of 1. Metallic Co0 formed during pyrolysis was partially transformed into Co2C under reaction conditions. With increasing Co content CO conversion increased up to 10.6 % reaching a total alcohol selectivity of 19 %. Gas chromatograms revealed the expected formation of primary short‐chain alcohols, but also of secondary alcohols, acetic acid and propionaldehyde indicating olefin hydration, carbonylation and hydroformylation as reaction pathways, respectively. The obtained hydrocarbon fractions had a very high olefinicity, which is beneficial for both olefin hydration to secondary alcohols catalyzed by adsorbed carboxylic acids and for hydroformylation. Whereas the carbide‐based reaction pathway and the reductive hydroformylation are assumed to occur at the Co2C/Co0 interface, carbonylation is presumably catalyzed by an additional Co‐based active site. Thus, a unique class of multifunctional catalysts was obtained with highly promising properties bridging the gap between heterogeneous and homogeneous catalysis.

On the role of potassium in Prussian blue analogue-derived Mn–Co catalysts applied in the CO hydrogenation to higher alcohols

“…16 According to the proposed reaction network, olefin hydration to secondary alcohols is an essential reaction mechanism for HAS over PBA-based catalysts. 4 Furthermore, a high CO 2 selectivity was observed for all investigated catalysts due to the WGSR. This was rather unexpected, because these Cu-free catalysts had been investigated with the goal to lower the CO 2 selectivity.…”

“…Both reactions are followed by hydrogenation to the corresponding long-chain alcohol (eqn (6) and (8)). 4,12–14 H 3 COH + CO → H 3 CCOOHBoth acetic acid and propionaldehyde have been observed in the obtained chromatograms while analyzing the effluent with a customized gas chromatographic (GC) application. Furthermore, the product gas stream was collected in a cooling trap and investigated by GC-MS supporting these mechanisms.…”

“…In addition, strongly anchored molecular Co-based active sites resembling the Co carbonyl hydride complex in their catalytic carbonylation properties are assumed to be formed under reaction conditions resulting in this intertwined reaction network. 4 According to Zeng et al , 7 who applied a Co-based PBA in the hydrogen evolution (HER) and oxygen reduction reaction (ORR), the active molecular Co site can be described as a Co/N-doped carbon (Co–N–C) site. Based on the high electrochemical performance of the investigated catalysts, the authors concluded that Co diffuses into the formed carbon shell forming bonds with carbon and nitrogen.…”

“…The solid-state reactions also contribute to the observed structural changes of the catalysts derived from PBA-based precursors during pyrolysis and under HAS conditions. 4…”

“…Still, PBA-based catalysts for the CO hydrogenation to higher alcohols are a rather new system and have not been investigated in great detail. 4 Especially the role of K in the product distribution and in the WGSR is of much interest. Thus, this study aims at lowering the K content by two different methods and at investigating its influence on the structural properties of the PBA-derived samples in their different states and on the catalytic properties.…”