Triple tandem catalysis for single-pass conversion of syngas to ethanol
Synthesis of ethanol from different carbon resources via syngas (CO/H2) is highly attractive but challenging. In a recent article (https://www.nature.com/articles/s41467-020-14672-8), we report a triple tandem catalysis system, which affords an ethanol selectivity as high as 90% from syngas.
Syngas is one of the most important C1-chemistry platforms, which can be produced from carbon resources including natural (shale) gas, coal, biomass, carbon-containing waste and carbon dioxide. Syngas chemistry plays a crucial role in the current supply of energy and chemical feedstocks. Ethanol is an ideal fuel additive, a promising hydrogen carrier and a versatile building-block for chemicals. The current production of ethanol relies on the fermentation of sugars, but this process suffers from the competition with food supply, the high energy consumption in product separation and purification, and the limited efficiency and ethanol selectivity. The chemical synthesis of ethanol from syngas is one of the most attractive route.
Many studies have been devoted to the direct conversion of syngas into ethanol. This reaction is complicated because of many elementary steps involved, which include H2 dissociation, CO dissociative and non-dissociative activation, formation of adsorbed CO, CHx and CHxO intermediates, C–C coupling between different intermediates, and formation of products. Many reaction channels coexist during this process. Although much effort has been devoted to designing bi- or multi-component catalysts such as Rh–Mn, Cu–Co and Cu–Fe catalysts, the ethanol selectivity is lower than 60% even at limited CO conversions. Syngas can also be transformed into ethanol through indirect routes via methanol or dimethyl ether (DME) synthesis, followed by carbonylation with CO to form acetic acid or methyl acetate, and subsequent hydrogenation of acetic acid or methyl acetate. These indirect routes suffer from multiple processes and energy-consuming during product separation/purification in each process. Hence, the development of new methodologies for direct conversion of syngas to ethanol with high selectivity is a very attractive but highly challenging.
We have succeeded in develop new tandem catalytic systems for direct synthesis of ethanol with high ethanol selectivity. We found that the integration of methanol synthesis, methanol carbonylation and acetic acid hydrogenation in tandem in one reactor could accomplish the highly selective conversion of syngas into ethanol. We have demonstrated that the K+–ZnO–ZrO2 catalyzed syngas conversion to methanol and the H-MOR functioned for methanol carbonylation to acetic acid, which was then hydrogenated to ethanol over the Pt–Sn/SiC catalyst. In the combination of K+−ZnO−ZrO2│HMOR−DA−12MR│Pt−Sn/SiC,
the ethanol selectivity reached 90% and 81% at CO conversions of 0.7% and 4.0% at 503 and 543 K, respectively. At a CO conversion of ~7.0%, the ethanol selectivity could be sustained at 70%.
We have demonstrated that to keep the high selectivity of each step by carefully designing the corresponding catalyst is the key to obtaining high ethanol selectivity. The modification of ZnO−ZrO2 with K+ decreases the acidity, thus improving CH3OH selectivity. The selective removal of Brønsted acid sites in 12-MR channels while keeping the density of Brønsted acid sites in 8-MR is the main reason for the enhancement in acetic acid selectivity by using the HMOR−DA−12MR. The presence of Sn not only promotes the formation of Pt–Sn alloy, and keeps the chemical status of Pt at Ptδ+, thus improves the catalytic performances of acetic acid hydrogenation to ethanol.
The interplay between different steps and catalysts is crucial for this tandem catalysis. We discover that methanol synthesis is the rate-determining step in this tandem process, and to keep a sufficiently high CO/CH3OH ratio is crucial for methanol carbonylation with CO to acetic acid, which determines C–C coupling. Too higher a CO conversion in the first step, i.e. methanol synthesis, would lead to a lower CO/CH3OH ratio and may be detrimental to the carbonylation reaction on H-MOR–DA–12MR zeolite. The complete separation of the three catalysts is also important to the selective formation of ethanol. The K+−ZnO−ZrO2, HMOR−DA−12MR and Pt−Sn/SiC catalysts should be separated in sequence by proper amounts of quartz wool. The significant decrease in the amount of quartz wool or the height of quartz-wool bed decreased the selectivity of ethanol and promoted the formation of ethylene, which came from the dehydration of ethanol. The use of mixed granules of three catalysts or grinding fine powers of the three components led to the disappearance of ethanol, and ethylene became the major products.
We have revealed that the compatibility of catalysts in syngas stream, which contains both H2 and CO plays key role for the methanol carbonylation and acetic acid hydrogenation reactions. The presence of H2 improves the stability of H-MOR zeolite for methanol carbonylation by suppressing the carbon deposition. The co-existence of CO requires careful design of catalysts for acetic acid hydrogenation to avoid the poisoning effect of CO. The introducing of Sn into Pt/SiC not only promotes the selective hydrogenation, but also decreases the CO chemisorption to avoid the poison of CO on Pt surfaces, and thus improve the activity of acetic acid hydrogenation. The present work not only presents a promising catalytic system for high-selective conversion of syngas into ethanol but also offers a method of controlling reaction selectivity by decoupling a complicated and uncontrollable reaction into well-controlled tandem reactions.