Sustainable Catalytic Alcohol Synthesis from Hydrogen and Carbon Dioxide

Abstract

The catalytic conversion of sustainably produced syngas (carbon monoxide and hydrogen) into higher alcohols constitutes a desirable pathway of storing renewable energy in chemicals. However, up to today there has been no catalyst identified that would allow an efficient and direct conversion of syngas into higher alcohols. This work investigates the catalytic hydrogenation of CO to C2-oxygenates over supported Rh catalysts. Structure sensitivity for the Rh-catalyzed CO hydrogenation was studied over a set of Rh/SiO2 catalysts that were prepared via wet impregnation technique yielding Rh particle sizes between 2 and 5 nm. Characterization of the Rh particles via H2-chemisorption shows an expected reverse correlation between the concentration of under-coordinated edge/corner sites and the particle size. Further, results from N2-chemisorption show an optimum in the surface concentration of B5 coordination sites for a Rh particle size of around 4 nm. Catalytic reaction tests in CO/H2 atmosphere reveal a pronounced dependence of the total CO turn-over frequency on the particle size. Small particles (≤ 2 nm) are around one order of magnitude less active than larger particles (> 5 nm). Here it is shown that the state of low activity in small particles is linked to a prohibitively high CO adsorbate coverage which originates from a high concentration of under-coordinated surface sites. For larger particles the fraction of such under-coordinated sites declines significantly which leads to a lower CO coverage on the metal surface. As a result, the accessible metal surface area increases and the apparent reaction rate of CO hydrogenation is higher over larger particles. Further, a pronounced particle size dependent pressure sensitivity is observed for the catalytic activity. Small particles are found to be nearly pressure independent in their activity whereas larger particles become clearly more active with increasing CO/H2 reactant pressure. This observation of particle size dependent reaction kinetics underlines a structure sensitivity of the CO coverage on the Rh surface.   Results from transient response experiments and infrared spectroscopy studies on CO-chemisorbed catalysts confirm the presence of a prohibitively high CO coverage on Rh as well as an intensification of the binding strength between the metal surface and CO adsorbates for small Rh particles. However, the presence of such a high CO coverage also leads to an enhanced probability of C + CO coupling reactions, which results in favorable high selectivity levels of C2-oxygenate formation.Results from catalytic reactor studies on the synthesis and conversion of oxygenates suggest that only a high steady state CO coverage on the Rh surface protects the formed oxygenates from further reactions. In essence, this explains why any oxygenate formation can take place at all. Effects from different support materials on the Rh-catalyzed CO hydrogenation were investigated over silica or zirconia supported Rh particles. Hereby, the catalytic productivity when using zirconia supported Rh particles is more than one order of magnitude higher than for Rh/SiO2 while showing similar particle sizes.Temperature programmed hydrogenation (TPH) experiments underline that the CO dissociation on the metal surface is rate limiting. TPH studies after low-temperature CO pre-adsorption reveal a H-assisted pathway of CO activation through methoxide species that form on the Rh surface. This mechanism is also likely in operation during steady state reaction at higher temperatures. The observation of such a CO activation involving hydrogen reconciles with previous studies that have observed an H/D-isotope effect for Rh-catalyzed CO hydrogenation; despite CO dissociation being the rate limiting step. Further, the presence of a more strongly bound CO adlayer that forms at reaction temperature conditions was identified. Such a stabilized CO adlayer stretches the activation of CO towards higher temperatures. As a result, the detection of eventual low-temperature reaction pathways such as the methoxide-mediated CO dissociation is obscured. Knowing this process helps to understand an apparent contradiction in the methane formation after CO pre-adsorption at different temperatures. Exposing the catalyst to CO at high-temperature conditions shifts the methane formation to higher temperatures in comparison to CO pre-adsorption at ambient temperature. The impact of utilizing mixed CO2/CO carbon oxide gas feeds for the hydrogenation to C2-oxygenates was studied during catalytic reaction tests using silica or zirconia supported Rh catalysts. Changes to the catalytic results suggest a partial oxidation of Rh and/or an altered CO adsorbate coverage in the presence of elevated CO2 concentrations (> 10 vol%). Investigation of the relationship between the space velocity during CO/H2 reaction and the relative product distribution between acetaldehyde and ethanol shows that acetaldehyde is the primarily formed C2-oxygenate for both catalysts. One main pathway to ethanol is likely to proceed through the consecutive hydrogenation of an acetyl-like intermediate. Different relative product distributions between the two primary C2-oxygenates are observed under comparable conversion levels over silica or zirconia supported Rh. Utilizing ZrO2 as support material results in the preferable hydrogenation of C2-oxygenate precursor adsorbates towards ethanol. Further, an unexpected non-linear behavior for the productivity of Rh on ZrO2 is observed when varying the residence time towards higher conversion levels. Possible underlying principles for such an effect are discussed.