Dual catalysis by homogeneous/heterogeneous ruthenium species

Abstract

In this Chem issue, Han et al. devised a system relying on tandem heterogeneous and homogeneous catalysis by ruthenium species for the CO2 hydrogenation to hydrocarbons with remarkable efficiency. In particular, 5-atom (or longer) chain hydrocarbons were obtained with remarkably high selectivity (71.1%). In this Chem issue, Han et al. devised a system relying on tandem heterogeneous and homogeneous catalysis by ruthenium species for the CO2 hydrogenation to hydrocarbons with remarkable efficiency. In particular, 5-atom (or longer) chain hydrocarbons were obtained with remarkably high selectivity (71.1%). For many years, the distinction between homogeneous and heterogeneous catalysis has been clear, and the two separated fields have mostly evolved along parallel pathways, taken forward by key experts with specific competences. However, such a clear-cut of the two areas has always felt somehow preposterous, because both fields share common ground and challenges, attempting to unravel and steer reaction mechanisms by control of metal coordination and/or environment. Moreover, the homo/hetero distinction often does not find truly justification, as in many catalytic reactions the nature of the active site is uncertain. Just as an example, in the well-known Suzuki-Miyaura C–C coupling by Pd, the active metal species was greatly debated in the past, and still today there is no irrefutable consensus on whether the reaction is catalyzed by homogeneous Pd complex, by heterogeneous Pd nanoparticles formed in situ, or by both species.1Beletskaya I.P. Alonso F. Tyurin V. The Suzuki-Miyaura reaction after the Nobel prize.Coord. Chem. Rev. 2019; 385: 137-173Crossref Scopus (145) Google Scholar New research trends have reassessed the homogeneous versus heterogeneous catalytic approaches, incorporating both concepts into the development of single “catalytic ensembles,” whereby the specific catalytic cycle relies on tandem action by two task-specific homogeneous and heterogeneous phases.2Liu Y.-Y. Liang D. Lu L.-Q. Xiao W.-J. Practical heterogeneous photoredox/nickel dual catalysis for C-N and C-O coupling reactions.Chem. Commun. (Camb.). 2019; 55: 4853-4856Crossref PubMed Google Scholar The resultant synergy paves the way for allowing important chemical conversions with higher efficiency, and in the recent Chem article, it is not by chance that Han et al. chose a yin and yang symbol to graphically represent the core of their work, symbolizing two independent (and sometimes divergent) approaches that can complement each other toward ambitious chemical processes. The article targets the catalytic realization of one of the great challenges in chemical reactions, namely the transformation of CO2 into chemicals and liquid fuels.3Vogt C. Monai M. Kramer G.J. Weckhuysen B.M. The renaissance of the Sabatier reaction and its applications on Earth and in space.Nat. Catal. 2019; 2: 188-197Crossref Scopus (169) Google Scholar Here, the conversion of CO2 to hydrocarbons of various chain lengths is realized by recurring to a double ruthenium catalyst: homogeneous RuCl3, after being converted in situ in the corresponding Ru carbonyl iodide complex, is able to convert CO2 to CO under mild conditions and in the presence of Li halogenides (LiCl used as cocatalyst and LiI as promoter). Sequentially, the as-formed CO undergoes Fischer-Tropsch hydrogenation to CnH2n+2 products catalyzed by Ru(0) nanoparticles, remarkably achieving high hydrocarbons (C5+ paraffines) with more than 70% yield in one reactor.4Cui M. Qian Q. Zhang J. Wang Y. Asare Bediako B.B. Liu H. Han B. Liquid fuel synthesis via CO2 hydrogenation by coupling homogeneous and heterogeneous catalysis.Chem. 2020; 7https://doi.org/10.1016/j.chempr.2020.12.005Abstract Full Text Full Text PDF PubMed Scopus (8) Google Scholar The work is an ingenious development of the well-known Fischer-Tropsch for making higher hydrocarbons, recognizing that in conventional Fischer-Tropsch catalysis the presence of CO2 hinders the production of higher hydrocarbons. To overcome this limitation, examples with bifunctional heterogeneous catalysts have been developed in recent years able to transform CO2 into CO or MeOH, followed by FT or methanol-to-olefin steps (MTO). The contact state of the two functionalities is pivotal to control activity, selectivity, and even the stability of the catalysts.5Gao P. Li S. Bu X. Dang S. Liu Z. Wang H. Zhong L. Qiu M. Yang C. Cai J. et al.Direct conversion of CO2 into liquid fuels with high selectivity over a bifunctional catalyst.Nat. Chem. 2017; 9: 1019-1024Crossref PubMed Scopus (463) Google Scholar In the present contribution by Han et al., the utilization of a catalytic package featuring a homogeneous and a heterogeneous species allows to operate at considerably milder conditions and with improved yields, and this article will be a key demonstration on the huge potential of combining these two fields of catalysis, which can hold the key to unlock new technologies.6Cui X. Li W. Ryabchuk P. Junge K. Beller M. Bridging homogeneous and heterogeneous catalysis by heterogeneous single-metal-site catalysts.Nat. Catal. 2018; 1: 385-397Crossref Scopus (383) Google Scholar The work raises new important scientific questions: what is the mechanism that yields to such improved performance? What is the fate of the homogeneous catalyst under reaction conditions? What is the exact role of the Li+? More precisely, does it intervene in the heterogeneous catalytic cycle or in the homogeneous one (or in both)? MeOH formation is reported to be detrimental in the reaction:7Monai M. Melchionna M. Fornasiero P. From metal to metal-free catalysts: Routes to sustainable chemistry.in: Advances in Catalysis. Volume 63. Academic Press, 2018: 1-73Google Scholar what is the mechanism with which it is formed? While this fascinating and important work provides many insights into a very modern chemical process, it also opens several new doors to further deepen our understanding of the process and inspire new combined homogeneous/heterogeneous catalysis for other conversion. In the short, run, an answer to some of the above questions may rely on the use of operando spectroscopy techniques, which are becoming ubiquitous in many fields of catalysis such as for example electrocatalysis.8Carbonio E.A. Velasco-Velez J.-J. Schlögl R. Knop-Gericke A. Perspective—outlook on operando photoelectron and absorption spectroscopy to probe catalysts at the solid-liquid electrochemical Interface.J. Electrochem. Soc. 2020; 167: 054509Crossref Scopus (13) Google Scholar A second question is how the present findings can be extended to other metal complexes/heterogeneous catalyst combinations, in order to replace high risk metals with more renewable alternatives e.g., Li with Na and Ru with Ni, while retaining similar performances. Mechanistic insights and theoretical modeling can help lead the way, noting that one of the frontier methodologies in catalysis is to join experimental and theoretical efforts for upgrading or developing existing catalysts.9Seh Z.W. Kibsgaard J. Dickens C.F. Chorkendorff I. Nørskov J.K. Jaramillo T.F. Combining theory and experiment in electrocatalysis: Insights into materials design.Science. 2017; 355https://doi.org/10.1126/science.aad4998Crossref PubMed Scopus (4165) Google Scholar, 10Mateo D. Cerrillo J.L. Durini S. Gascon J. Fundamentals and applications of photo-thermal catalysis.Chem. Soc. Rev. 2021; 50: 2173-2210Crossref PubMed Google Scholar, 11Neese F. High-level spectroscopy, quantum chemistry, and catalysis: not just a passing fad.Angew. Chem. Int. Ed. 2017; 56: 11003-11010Crossref PubMed Scopus (21) Google Scholar Hence, apart from the notable results obtained from a practical point of view, namely the invention of a tandem catalytic system based on Ru species that allows a key chemical transformation, the work by Han et al. is relevant as being one of the powerful examples of how rational unification of two fields of catalysis, which have been kept for too long separated, holds the key for new sustainable solutions. We expect a rapid flourishing in the forthcoming years of this contemporary (and more holistic) philosophy for setting new state-of-the-art in catalysis. Liquid fuel synthesis via CO2 hydrogenation by coupling homogeneous and heterogeneous catalysisCui et al.ChemDecember 30, 2020In BriefLiquid fuel can be high-efficiently produced via CO2 hydrogenation by coupling homogeneous RuCl3-catalyzed RWGS reaction and heterogeneous Ru0-accelerated FTS reaction. The reaction proceeded at the lowest temperature reported so far and reached the highest C5+ selectivity to date. The TOF of the CO2 hydrogenation is comparable with that of the reported Ru0-catalyzed FTS reaction using syngas. The outstanding reaction results lie in elegant synergy of the catalytic components and the tandem reactions. Full-Text PDF Open Archive