ANÁLISIS E INTERPRETACIÓN DE LOS RESULTADOS
4.1 Objetivo y finalidad
In addition to modifying current industrial processes to be compatible with renewable feedstocks or new synthetic routes that use CO2 to form C1 products, there is also a need to develop alternative,
direct conversion routes for the production of important chemicals. These co-called “dream reac- tions” would consume renewable energy sources and materials (ideally those that can be harvested from the air, e.g. CO2, H2O, and N2) and directly convert them to value-added products and es-
sential molecular functional groups, thereby circumventing multistep reaction sequences involving separation and purification of reagents. The high reactivity and excellent atomic-level design con- trol of homogeneous catalysts can be exploited to develop pathways and analyze reaction networks for these complex “dream reactions,” which largely require multi-electron redox processes and se- quential bond breaking/forming events. The reductive functionalization of CO2 has led to recent
efforts and successes, which must be pursued and reinforced. For instance, pathways enabling the formation of C-C bonds to give C2+ products directly from CO2 have still to be established. Fur-
thermore, these concepts can be transposed to the valorization of N2 and NOx for the formation of N-containing chemicals to improve the environmental footprint of agrochemicals.
To realize these processes, more effective strategies for the activation of small molecules, generally, as well as the reversible activation of strong bonds (e.g. C-O, C-N, C-C, C-H, and N-N) are needed. Furthermore, while H2 is a suitable energy carrier (or reductant) in the short
term, the direct use of electrons and/or photons is highly desirable in the longer term to minimize infrastructure and facilitate decentralized production routes. To this end, it is crucial to develop efficient photocatalysts and electrocatalysts able to harvest and store the energy of photons and electrons in chemical bonds. It will also be important to ensure closed cycles not only for carbon, but also for nitrogen, in particular, and other critical elements such as phosphorus and sulfur. Coordination of such research efforts across fields, e.g. those mentioned in Sections1, 2,5, and7, will be important for efficient technological development.
A short-term goal (∼5 years) related to this future research need concerns the development of novel catalytic reactions for the multicomponent coupling of CO2 and H2 or of ammonia to value-
added, industrially relevant chemicals. At the horizon 2030, the potential of replacing H2 with H2O
and ammonia with N2 or NOx by using electrolytic or photolytic catalysts could be developed from
exploration to validation. Beyond 10 years, the shortcutting concepts learned regarding conversion of carbon and nitrogen feedstocks could be translated to other feedstocks, e.g. those containing phosphorus, sulfur, and halogens.
6.4
Conclusion
Homogeneous catalysis is a key element in our current chemical industry and is essential for the tran- sition to a sustainable future. Immediate action can be takenby adapting existing technologies to use renewable feedstocks, e.g. the essential “power molecules” originating from electrolysis or co-electrolysis based on renewable energy (e.g. “green” hydrogen or carbon monoxide) and capital- izing on the proven potential of organometallic catalysts to convert carbon dioxide (CO2) directly
into value-added chemicals. The lighthouse project exemplified in Figure6.3 provides an illustra- tive example of an immediately possible fossil-free development. However, the long-term success of the transition to renewables will rely on new and disruptive approaches for accelerating catalyst development and designing catalytic processes from the molecular to the system level. Ultimately, this will lead to novel technologies that directly utilize electrons and photons to drive chemical transformations of renewable feedstocks via novel pathways, thus providing a new paradigm in the chemical industry. The scientific challenges and goals outlined in this report will require co- ordinated efforts of interdisciplinary teams involving academic and industrial partnerships at the international level.
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Section 7
Inspiration from biological processes
Serena DeBeer (Max Planck Institute for Chemical Energy Conversion) Huub de Groot (Leiden University)7.1
Importance of subject
Over billions of years of evolution, biological systems have optimized cellular energy conversion processes. Hence, as the scientific community searches for sustainable solutions to the world’s energy challenge, nature has the potential to provide real answers. For one, nature uses earth- abundant metallocofactors within responsive protein matrices [1] to enable challenging chemical conversions. Both the metal active sites, as well as the hierarchical protein structure, can provide important chemistry and engineering lessons that may be broadly translated in all areas of cataly- sis. Additionally, nature has evolved photosynthetic pathways that provide inspiration for various forms of artificial photosynthesis technologies: (i) Bioinspired artificial systems can directly use sunlight together with CO2, H2O, or N2 for the synthesis of essential molecules. (ii) Engineered
photosynthetic organisms can directly produce target fuels and all chemicals from sunlight. (iii) Non-photosynthetic organisms can serve as catalysts in biohybrid systems, in which sunlight is har- vested and provided to the organism by either photoelectrochemical or photovoltaic components.
7.2
State of the art and scientific challenges
Nature possesses a remarkable ability to activate small molecules under ambient conditions by uti- lizing earth abundant transition metal active sites within responsive protein matrices that optimize electron and proton transfer processes. In fact, many of the reactions of interest to a renewably- powered future that are identified in this report (Sections 1,2, and 5) possess equivalent reactions in nature. Water is oxidized by the Mn4O5Ca oxygen-evolving complex of photosystem II [2], dini-
by Ni/Fe-containing carbon monoxide dehydrogenase [4]. Furthermore, responsive protein matrices have evolved over billions of years to optimize essential energy conversion processes, such as energy and electron transfers by porphyrin-type cofactors in photosynthetic conversion and respiration.