In all these cases, which are feasible from an industrial perspective as explored in the cited EU projects , it is necessary to develop both improved electrocatalysts and novel scalable electro-reactors and units. As commented before, they push chemical reaction engineering to explore new directions, but going beyond the current approaches. This is an opportunity beginning to be explored [ , ], being a major change with respect to engineering of conventional plants for chemical production.
Process intensification is also a necessary step to move to distributed production. We refer here only to process intensification related to the use of the electrocatalysis approach, because this is an aspect scarcely considered [ 66 ]. There are at least two aspects to mention. The actual dominant process of ammonia synthesis starts from methane as H 2 source and requires several steps to arrive to ammonia. In the electrocatalytic approach direct synthesis , water is the source of H 2 , and thus in a single electrocatalytic reactor N 2 and H 2 O are converted to NH 3.
There are thus game-charger advantages in this approach: i large reduction in the process steps, ii milder operations, iii elimination of the use of fossil fuels, iv suitability for distributed production avoiding impact of large-scale processes and relevant local impact, cost and impact of transporting ammonia , and v use for the chemical storage of excess renewable energy. It is necessary to develop electrocatalysts based on earth-abundant materials and use flow reactors with easy, non-energy-intensive, recover of ammonia [ ], and especially designed to obtain higher productivities per geometrical surface area of the electrode.
Most of the data reported in literature for high Faradaic efficiency were obtained at extremely low productivities, or current densities. There are other issues, however, which have been not considered up to now.
The current industrial ammonia synthesis technology starting from methane uses air as N 2 source, because the oxygen is then consumed in the process. A membrane for N 2 and O 2 separation should be integrated with the electrocatalytic unit, but still some residual oxygen will be present. Therefore, the challenge not considered up to now is how to develop electrocatalysts for ammonia synthesis from N 2 and H 2 O operating efficiently in the presence of some residual O 2 after air separation.
The problem is thus significantly different from current studies on the reaction [ , , ], where the only issue considered is a high Faradaic selectivity to NH 3.
Alternative Energy Sources and Technologies. Process Design and Operation. Editors: Martín, Mariano (Ed.) Free Preview. Maximizes reader insights into the. Request PDF on ResearchGate | Alternative energy sources and technologies: Process design and operation | Presenting a comprehensive analysis of the use .
The problem of oxygen contamination, the recovery of ammonia and flow reactor operations, the design to minimize ammonia crossover through the membrane [ ], a compact scalable reactor design, the use of low-cost scalable synthesis of electrocatalysts, stability under operation are some of the problems to consider from an engineering perspective, which are not separated from the development of the electrocatalysts, and may instead determine different selections, as remarked before for water splitting catalysts.
Another relevant example of possible significant reduction in the process steps, and carbon footprint, is given by the direct electrocatalytic synthesis of acetic acid or acetate from CO 2 [ 99 , , , ]. Acetic acid is also a large-volume chemical about 14 Mt. Current production routes use fossil sources, apart from few based on fermentation processes.
The current main industrial route is a multi-step process, via production of syngas from methane, production of methanol, and carbonylation of the latter. The key aspect is also in this case how to control selectivity and favor reactions leading to C-C bond formation [ 99 ], but considering that often mechanistic studies provide contrasting results, because they do not consider the role of surface coverage or reactants and products, electrolyte in determining the paths of electrocatalytic transformations [ 87 , ], the use of different reactor configurations in the presence or not of an electrolyte of support [ ] , and the dependence on the potential applied [ 82 ].
There are thus several fundamental open questions to understand, but requiring a broader approach than the limited often currently used. On the other hand, new value chains and opportunities will derive from these studies. A second and different possibility for process intensification is related to the application of the electrocatalytic approach to the production of chemicals from biosources.
Here the point is that in these processes, both catalytic oxidation and reduction steps are often present. FDCA derives from fructose as raw materials via intermediate HMF formation, while ethylene glycol still derives from fossil sources via ethylene and ethylene oxide. Interesting is that the two monomers for PEF could be synthetized one by oxidation and one by reduction of platform biomolecules HMF and xylose deriving from cellulose and hemicellulose, the main components of biomass.
TERRA project thus explores the development and scale-up of a tandem electrocatalytic reactor, schematically shown in Fig. The approach avoids the need of oxidizing and reducing agents for the two redox reactions and the process intensification decreases the process steps and allow better energetic integrations. There are, on the other hand, a series of technological aspects to solve, besides to develop the electrocatalysts allowing high selectivity and stability.
One of the crucial issues is the fact that the optimal temperature of operation could be not the same for the two sides of the electrocatalytic reactor. Inside the TERRA project, a technological solution to decouple the temperature of the two sides of the reaction within a certain range has been developed, although cannot be discussed in detail here.
This is a new innovation aspect not present in conventional electrochemical approaches, but opening a range of new possibilities in coupling redox reactions. Moreover it represent an example of how the need to solve critical questions from the application side pushes the development of new engineering solutions, which, on the other hand, open new fields of application. In fact, a range of new possibilities opens in the area of biomass valorization using electrocatalytic approaches.
An interesting example is that coupled redox reactions are present in the synthesis of other biobased monomers such as adipic acid, one of the most important of the commercially available aliphatic dicarboxylic acids [ 17 , ]. A major route for its industrial production is based on the hydrogenation of benzene to cyclohexane, which is then oxidized to a mixture of cyclohexanol and cyclohexanone indicated as KA oil. KA oil is then oxidized with nitric acid producing N 2 O as byproduct, a powerful greenhouse gas Fig.
The full process thus involves many steps, uses toxic chemicals benzene , produces N 2 O as byproduct, and requires both reducing agents H 2 and oxidants O 2 , HNO 3.
The new electrocatalytic process starts from a safer raw material glucose and uses a single electrocatalytic reactor, with a relatively simple downstream separation. The entire process, avoiding the use of reductants and oxidants, significantly decreases the carbon footprint, greenhouse gas emissions and the environmental impact.
In addition, the process is well suited for distributed production allowing thus to develop novel business models. There are many novel electrocatalytic, reactor engineering and process technology aspects to develop for enabling the implementation of the process, but which can open novel market opportunities for various other applications.
Many aspects, from economic to sustainability, social and political, are inducing a radical transition in both the energy and chemical production systems. This creates a push for new chemical reaction technologies and associated engineering aspects. We have identified in this review two main aspects on which focus the discussion: i the development of alternative carbon sources and ii the integration of renewable energy in the chemical production. These areas cannot be considered just an extension of the current ones.
Often these aspects are still underestimated. Moving in the indicated directions will produce radical changes in the way production is made, requiring thus new fundamentals and applied engineering approaches. In conclusion, we hope to have demonstrated that exists a push for new chemical reaction technologies deriving from energy and chemistry in transition. However, the speed to which this transition will be enabled will also depend on the capability to have a broader and integrated view on the problems.
Some of the needs have been discussed here. Chemistry future: priorities and opportunities from the sustainability perspective. New catalytic materials for energy and chemistry in transition.
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