The Oxygenate-Mediated Conversion of COx to Hydrocarbons─On the Role of Zeolites in Tandem Catalysis

Decentralized chemical plants close to circular carbon sources will play an important role in shaping the postfossil society. This scenario calls for carbon technologies which valorize CO2 and CO with renewable H2 and utilize process intensification approaches. The single-reactor tandem reaction approach to convert COx to hydrocarbons via oxygenate intermediates offers clear benefits in terms of improved thermodynamics and energy efficiency. Simultaneously, challenges and complexity in terms of catalyst material and mechanism, reactor, and process gaps have to be addressed. While the separate processes, namely methanol synthesis and methanol to hydrocarbons, are commercialized and extensively discussed, this review focuses on the zeolite/zeotype function in the oxygenate-mediated conversion of COx to hydrocarbons. Use of shape-selective zeolite/zeotype catalysts enables the selective production of fuel components as well as key intermediates for the chemical industry, such as BTX, gasoline, light olefins, and C3+ alkanes. In contrast to the separate processes which use methanol as a platform, this review examines the potential of methanol, dimethyl ether, and ketene as possible oxygenate intermediates in separate chapters. We explore the connection between literature on the individual reactions for converting oxygenates and the tandem reaction, so as to identify transferable knowledge from the individual processes which could drive progress in the intensification of the tandem process. This encompasses a multiscale approach, from molecule (mechanism, oxygenate molecule), to catalyst, to reactor configuration, and finally to process level. Finally, we present our perspectives on related emerging technologies, outstanding challenges, and potential directions for future research.

Recent advances in the routes and catalysts for ethanol synthesis from syngas

Ethanol, as one of the important bulk chemicals, is widely used in modern society. It can be produced by fermentation of sugar, petroleum refining, or conversion of syngas (CO/H₂). Among these approaches, conversion of syngas to ethanol (STE) is the most environmentally friendly and economical process. Although considerable progress has been made in STE conversion, control of CO activation and C-C growth remains a great challenge. This review highlights recent advances in the routes and catalysts employed in STE technology. The catalyst designs and pathway designs are summarized and analysed for the direct and indirect STE routes, respectively. In the direct STE routes (i.e., one-step synthesis of ethanol from syngas), modified catalysts of methanol synthesis, modified catalysts of Fischer-Tropsch synthesis, Mo-based catalysts, noble metal catalysts and multifunctional catalysts are systematically reviewed based on their catalyst designs. Further, in the indirect STE routes (i.e., multi-step processes for ethanol synthesis from syngas via methanol/dimethyl ether as intermediates), carbonylation of methanol/dimethyl ether followed by hydrogenation, and coupling of methanol with CO to form dimethyl oxalate followed by hydrogenation, are outlined according to their pathway designs. The goal of this review is to provide a comprehensive perspective on STE technology and inspire the invention of new catalysts and pathway designs in the near future.

Recent advances in the direct conversion of syngas to oxygenates

Direct synthesis of oxygenates from syngas is a promising way to utilize non-petroleum carbon resources because the oxygenate products serve as precursors for the downstream production of fuels and value-added chemicals.

Tandem catalysts for the conversion of methanol to aromatics with excellent selectivity and stability

A schematic diagram of the construction methods and reaction paths of different catalysts.

Single-pass transformation of syngas into ethanol with high selectivity by triple tandem catalysis

Synthesis of ethanol from non-petroleum carbon resources via syngas (a mixture of H₂ and CO) is an important but challenging research target. The current conversion of syngas to ethanol suffers from low selectivity or multiple processes with high energy consumption. Here, we report a high-selective conversion of syngas into ethanol by a triple tandem catalysis. An efficient trifunctional tandem system composed of potassium-modified ZnO–ZrO₂, modified zeolite mordenite and Pt–Sn/SiC working compatibly in syngas stream in one reactor can afford ethanol with a selectivity of 90%. We demonstrate that the K⁺–ZnO–ZrO₂ catalyses syngas conversion to methanol and the mordenite with eight-membered ring channels functions for methanol carbonylation to acetic acid, which is then hydrogenated to ethanol over the Pt–Sn/SiC catalyst. The present work offers an effective methodology leading to high selective conversion by decoupling a single-catalyst-based complicated and uncontrollable reaction into well-controlled multi-steps in tandem in one reactor.

Direct synthesis of ethanol from non-petroleum carbon resources via syngas (CO/H₂) is a highly attractive but challenging target. Here, the authors report a triple tandem catalytic system for single-pass conversion of syngas into ethanol with selectivity as high as 90%.