key: cord-0942385-j0ui67mz
authors: Papanikolaou, Georgia; Centi, Gabriele; Perathoner, Siglinda; Lanzafame, Paola
title: Catalysis for e-Chemistry: Need and Gaps for a Future De-Fossilized Chemical Production, with Focus on the Role of Complex (Direct) Syntheses by Electrocatalysis
date: 2022-04-24
journal: ACS catalysis
DOI: 10.1021/acscatal.2c00099
sha: 1651ee6b134e5362bf87cff8f0f8df2515485f3b
doc_id: 942385
cord_uid: j0ui67mz

The prospects, needs and limits in current approaches in catalysis to accelerate the transition to e-chemistry, where this term indicates a fossil fuel-free chemical production, are discussed. It is suggested that e-chemistry is a necessary element of the transformation to meet the targets of net zero emissions by year 2050 and that this conversion from the current petrochemistry is feasible. However, the acceleration of the development of catalytic technologies based on the use of renewable energy sources (indicated as reactive catalysis) is necessary, evidencing that these are part of a system of changes and thus should be assessed from this perspective. However, it is perceived that the current studies in the area are not properly addressing the needs to develop the catalytic technologies required for e-chemistry, presenting a series of relevant aspects and directions in which research should be focused to develop the framework system transformation necessary to implement e-chemistry.

The research interest on catalysis based on the direct use of renewable energy sources -RES (photo-, electro-and plasma-catalysis as the main methodologies) is fast rising. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] As index of the growing of activities, the papers with electrocatalysis as keyword raised from about 30 in year 2000 to nearly 600 in 2021, and those containing the term photocatalysis from about 40 to nearly 1000 per year in the same period. However, most of the research activity focused on only few molecules (CO2, N2, H2O, and few biobased chemicals). On the other hand, meeting the targets fixed at political level (for example, reach a net zero greenhouse gas emission target by year 2050, as committed by EU) would require a more disruptive effort than developing some catalytic processes driven directly from RES. 10 It is necessary to consider the possibility to substitute largely, if not entirely, the use of fossil fuels (FF) both as energy and carbon source, the latter particularly for chemical production. The concept of e-chemistry indicates that this (almost) FFfree chemical production, which can be identified as part of the overall transformation necessary, can meet the targets of net zero emissions (NZE) for year 2050 and beyond. Implementing echemistry (and associated e-refinery) concept will address some important societal challenges which are facing to realize a resilient and sustainable future: i) overcome intermittency of RES, ii) implement a world-economy based on the distribution and long-distance transport of renewable energy, iii) develop CO2 neutral or even carbon-negative technologies to supply the goods and energy necessary for the society, and iv) realize a de-fossilized chemical industry. e-Chemistry is particularly associated to the last objective but is also closely related to the others.

To realize an e-chemistry requires a combination of actions, as the direct electrification of the operations in chemical industry using FF as energy input (for example, most of the furnaces in chemical processes use FF to provide the heat necessary 4 for operations) [11] [12] [13] [14] the closure of the carbon cycle in chemical production by introducing novel technologies for the molecular reuse of waste and end-of-life chemicals (from CO2 to end-of-life chemicals as recycled plastics) [15] [16] [17] the realization of novel chemical processes using RES as energy input for the process, where electro-, photo-and plasma-catalysis represent the main technological options. 8, [18] [19] [20] [21] Catalysis plays a crucial role in several of these novel technologies allowing not only to develop innovative routes with a large decrease in the carbon footprint, up to over 90%, but also realize process intensification with thus reduction of fixed costs and better suitability to a distributed model of chemical production. 8, [22] [23] [24] This combination of drastic reduction in the carbon footprint, use of alternative raw materials to FF and process intensification (with associated impacts such as the possibility to develop distributed production modes as well as faster and more flexible industrialization by parallel units) is the reading key to analyze the potential of catalytic technologies based on the use of RES, rather than limiting to purely economic considerations.

However, addressing this challenge for catalysis requires also to reconsider the fundamental bases of catalysis science and technology. We introduced the term reactive catalysis to differentiate photo-, electro-and plasma-catalysis from the conventional thermal catalysis. 10 In the latter case, energy in the form of heat is provided to overcome the activation energy, while in reactive catalysis already highly energetic species (electron, holes, radicals, vibrationally-excited species) are generated by application of an electrical potential, by light irradiation or by generation of a non-thermal plasma. The fundamental modes of operations, and consequently the design aspects for the catalysts are different. It is thus necessary to use novel methodologies to understand and develop reactive catalysis, and not just continue to use those developed for thermal catalysis. 5 e-Chemistry thus requires a revolutionary rather than an evolutionary approach, in terms of capability to integrate, in a holistic perspective, many aspects from fundamental to applied chemistry and engineering for the deep transition required to move to a new FF-free sustainable future. Facing the transformation to a NZE society requires radical system shifts (concerted) in the same direction, 25, 26 requiring thus novel assessment modes for their evaluation. 27 A concerted synergy between the development of novel routes and technologies and the parallel changes in the economic, industrial and societal systems is necessary, implying also to create the pathways by which this concerted mechanism can be achieved. Therefore, a framework assessment of the transformation is required rather than to assess single specific technologies. 28 For example, an analysis of the status of the studies on the economics of CO2 utilization 29 clearly reveals the limit of application of economic assessment models not properly consider the on-going deep transition and thus not based on a framework assessment. This is also one of the reasons why very contrasting opinions exists on the opportunity or not to develop new routes to electrify the chemical production, and how to properly rank the priorities.

For example, a recent paper by Ueckerdt et al. 30 discussing the pro and cons of e-fuels (where this term indicates the synthetic fuels produced from electricity and CO2 via water electrolysis)

argued that "e-fuels' versatility is counterbalanced by their fragile climate effectiveness, high costs and uncertain availability". We feel that this conclusion on the negative aspects of e-fuels, and the preferences about alternative solutions, derives from a series of assumptions. In particular that i) it is necessary to produce H2 via electrolysis and separate and concentrate/purify CO2, ii) e-fuels can be produced only by power-to-X technologies (i.e., producing H2 by electrolysis and its use for CO2 conversion by thermocatalytic process), and iii) renewable electricity should be that in excess with respect to other uses and with an intermittent production. We feel that these limitations 6 will be overcome within the next decades, as discussed later in this manuscript. When a deep transition occurs as that on-going, evaluations are strongly depending on the scenario assumptions and the capability to consider the possible technological developments, despite the uncertainty associated to them. Most of the scenario analyses are limited from this capability, especially when system changes are occurring as those ongoing.

On the other hand, making estimations, including the economic aspects, without considering properly the technological developments was proven to lead to uncorrected indications. This is well demonstrated by failing predictions of the cost of electricity produced by PV and wind, that about two decades ago was estimated about 5-10 higher than the effective cost currently available.

Analyzing literature studies on the economics of CO2 utilization 29 to produce CH4 and CH3OH

(mainly by power-to-X technologies) we remarked the presence of a very spread range of calculated costs, much broader than the possible uncertainty in cost estimations. This indicates how to predict that a technology like the production of e-fuels will not have a role in the future scenario, based on only cost estimations, can be very dangerous. On the other hand, this evidences the difficulties in making an estimation on technologies still to be fully developed.

Although all predictions about the future scenarios are strongly dependent on many assumption, quite difficult to proven and with a high degree of uncertain, we believe that the conclusions made by Ueckerdt et al. 30 on negligible climate mitigation effectiveness of e-fuels depend on adopting a pessimistic scenario strongly affected from a cost analysis taken from literature and not considering in the right perspective the potential of technological development and innovation. In general, we could remark the need to i) account the deep transformation occurring and ii) identify the scientific and technological gaps to overcome in order to implement this transformation.

From the catalysis perspective, it is thus necessary to understand the directions and trends 7 offering new opportunities, but also to identify properly the limits and gaps as well as the crucial issues to overcome. [31] [32] [33] Also in terms of catalysis technologies, it is necessary to address the limitations remarked above, for example the possibility to have photoelectrocatalytic (PEC) cells which are able at the same time to 1. convert directly CO2 from diluted streams without need of concentration/purification (by integrating suitable membranes, for example, 34 or combining with integrated electrochemical retention of CO2) 35 , 2. operate directly with solar light 24h (by integrating in the PEC cell a redox storage for a temporal decoupling of the redox processes requiring light and those for the production of efuels) 36, 37 3. produce directly in a single cell the e-fuels/e-chemicals from CO2, water and light. 32 Research in these directions as well on the fundamentals aspects which differentiate reactive from thermal catalysis is still quite limited. By a proper intensification and focus of the R&D, we believe that within 10-15 years technologies overcoming the above limitations become feasible. However, it is essential that research will address the proper gaps and limits, with a better focus on crucial aspects to solve. 10 Also scenario analyses, when a deep transformation is involved, should be focused at identifying the future technologies and to assess the possible directions from this perspective, and whether the existing scientific and technological gaps can be bridged.

Scope and Limits. This perspective paper aims to contribute in the analysis of the prospects, limits and gaps from the viewpoint of the needs of scientific and technological developments in the field of catalytic technologies to realize an e-chemistry. This analysis is preceded by a short assess of the feasibility and timing to realize an e-chemistry as a necessary element of the transformation to meet the NZE targets by year 2050, when a proper R&D effort will be dedicated.

However, the aim is not to discuss in depth the different opinions in literature regarding the need to transform petrochemistry into e-chemistry.

Discussion on the needs to realize an e-chemistry will not specifically address the issue of scalability of the technologies discussed. In fact, the scope of this paper is oriented to a long-term vision of the future technologies needed to realize an e-chemistry (unexplored reaction pathways) rather that to analyze gaps in the currently available technologies. It is important to identify these future technologies to prepare all the scientific and technological bases for their realization, whether a deep discussion of technology readiness/development is related to current technologies.

The above discussion was mainly focused on electrocatalysis. Other catalytic technologies using RES, such as photo-and plasma-catalysis, or catalysis with microwave or other radiations such as magnetic heating will be also important to implement an e-chemistry. However, the latter two technologies (microwave or magnetic heating) change the mechanism of heating, but the process remains a thermal process, with all the related intrinsic thermodynamic limits. Photo-and plasma-catalysis instead share with electro-catalysis the presence of quite reactive species, as electrons, holes, radicals and for this reason lumped together as reactive catalysis, in contrast with the traditional thermal catalysis. 10 However, from an application perspective, electrocatalysis has a more mature stage of development, and advantages of higher productivities, efficiencies and process intensification. 8 In addition, it will take advantages of the increasing experience on fuel cells and electrolyzers, as well of commercial electrocatalytic processes (chloro-soda, adiponitrile, etc.). Finally, by using stacks it is possible to obtain high productivities per reactor volume. All these aspects will make the scale-up of the electrocatalytic processes faster. Electrocatalysis likely will be thus the first technology to be applied industrially for the development of new routes for echemistry. We have thus focused discussion here on electrocatalysis, but the relevance of other 9 routes (in particular photo-and plasma-catalysis) need to be taken into account. However, it is also necessary to note that a proper comparative analysis of electro-, photo-and plasma-catalysis, and of pro/cons of these different technologies, is largely missing in literature. Moreover, data are reported in a way hard to be properly compared, for example in terms of productivity, energy efficiency, selectivity. Thus, an effort in this direction is necessary.

Note finally that we do not address here the role of bio-based chemicals in the development of a FFs-free chemical production, because has been already discussed elsewhere. [38] [39] [40] However, the bioroutes have often a still significant footprint and are not designed in most cases to integrate RES in the production. 41, 42 Thus, a transition to a fossil-free e-chemistry for a NZE target would require to reconsider these routes in terms of integration with RES and with technologies, such as electrocatalysis (as discussed later), representing the link to effectively integrate RES in the process.

Biocatalysis instead will certainly play a significant role in the future e-chemistry, although the challenges of i) performance, ii) process costs and iii) process intensification must be solved to expand from dominant pharmaceutical and fine chemical applications to the production of bulkchemicals. [38] [39] [40] Also in this case, the synergy and interface with electrocatalysis and other catalytic methods based on the use of RES have to be analysed to define the optimal paths for the future echemistry. However, this is a largely unexplored area.

There are many different opinions on whether an e-chemistry can be feasible, and when it could be effectively implemented on a large scale. Regarding timing, a question is when we would consider realized the implementation of the transition from petro-to e-chemistry.

The timing of a transition is when the new technologies (for e-chemistry) prevail over those traditional (currently in use) at the stage of planning new investments in industrial chemical production. In fact, every massive change of a production system requires time to be completed and there is an unavoidable period in which the new and old technologies will operate in parallel.

The transition is realized when the novel technologies will start to be introduced for new plants.

The further years are due to the time necessary to complete the switchover.

FFs are significantly decreasing. Already large consulting companies such as McKinsey, as commented later, evidence the reduced attractiveness to invest in current petrochemistry processes based on FFs. 43 Deloitte, another major consulting company, also advised about the changing petrochemicals landscape and that petrochemicals industry is at a crossroads of major structural shifts. 44 Despite the great uncertainty in predicting future, these signs and indications from consulting companies allow us to assume that for year 2050 the still likely use of FFs will be mainly a consequence of to this switchover period rather than of the profitable use of the current petrochemistry technologies (or slightly improved), especially in investing in new plants.

Understanding this difference is crucial for a proper scenario analysis and prediction of the technological landscape in year 2050, particularly in geographical regions pushing the NZE transition, such as Europe. However, a prerequisite to realize this scenario is that the R&D investment should be increased, with the identification also of the implementation mechanisms able to focus the R&D activities on the key fundamental and technological aspects leading to an acceleration of the innovation.

An often-posed question concerns the availability of the needed green electricity for the transformation of petro-to e-chemistry and its cost. The McKinsey report "Pluggin in: What electrification can do for industry" 13 indicates that "renewables could produce more than half of the world's electricity by 2035, at lower prices than fossil-fuel generation". Many other reports are in line with this estimation. FFs are no longer considered as a privileged energy source in terms of costs, but their still dominating monopoly character determines a market controlled by other factors than those associated to a truly competitive economy, as shown in the second half of 2021.

Thus, to overcome the dominant use of FFs is a strategic direction and not only relevant to reduce greenhouse gas emissions. 45 Substituting the use of FFs is thus the result of different converging elements, from cost advantage, drastic cut in greenhouse gas (GG) emissions (and associated avoided costs) and improved energy geopolitics. It is the combination of these elements making irreversible the transition, and thus also petrochemistry should reinvent itself in this direction.

McKinsey, a major consulting company, already some years ago indicated that petrochemistry has lost the window of opportunity as advantaged feedstock provided and therefore it needs to reinvent themselves. 43 In petrochemistry, less than half of the FFs input (accounting for > 90% of input of the chemical sector) is used as feedstock (e.g., as carbon source), while the remaining part to produce the necessary energy for the chemical processes. Global fossil fuel consumption corresponds about 137,000 TWh and by considering that around 14% and 8% of total primary demand for oil and gas, respectively, is related to chemical production, 46 the latter uses globally account for about 12,000 TWh equivalent of FFs. Global amount of electricity generated from renewables (in 2021) was estimated in 8300 TWh, 47 but with the introduction of the new technologies for e-chemistry it is expected to increase the efficiency of energy use, thus reducing the energy consumption. 47 Clearly, actual renewable energy production is already in use for different applications, but by year 12 2050 the production of RES is expected to increase by a factor of 3-4 arriving up to 90% renewable share in electricity, with also a reduction in the total final energy consumption from 378 EJ (in 2018) to 348 EJ (in 2050). 48 In addition, the introduction of technologies such as that (indicated above) of PEC devices with integrated redox storage, to enable their continuous use overcoming intermittency of RES (one of the current main drawbacks), offers an additional path to produce efuels/e-chemicals using directly solar light.

In a transition path to 2050, FFs substitution in chemical production can be first realized by replacing their use as energy source (the so-called electrification of the chemical production) and then as carbon source, by introducing technologies for efficient closure of the carbon cycle coupled with a rational use of biobased resources. 49 The carbon recycle introduces energy efficiency, besides a better use of resources. The energy efficiency in recycling CO2 to methanol, for example, is potentially up to over 80%, 50 while considering the different steps necessary to produce methanol from oil and the losses related to the extraction and transport of oil, the energy efficiency from fossils is on the average lower than 50%. 51, 52 This is not the efficiency of the single process of methanol production (what often considered), but the global efficiency which accounts for the many steps necessary to arrive to methanol with the current route.

Converting CO2 directly to acetic acid by an electrochemical process would further reduce the number of steps (a main industrial route involves the carbonylation of methanol) and improve the overall energy efficiency of the system. 53 The minimum process energy divided by the total process energy input is about 27% for acetic acid in the conventional route, 54 while the electrochemical route potentially can reach energy efficiencies above 60%, drastically lowering the carbon footprint. This concept is exemplified in Figure 1 reporting a Grassmann-type diagram of indicative comparison of exergy in the multistep conventional process to produce acetic acid 13 using FF sources and the direct electrocatalytic route of CO2 conversion to acetic acid with in-situ water electrolysis. 55 The difference between energy input (as sum of raw materials and fuel input) and the final work potential of the target product represents the sum of internal and external exergy losses, and those related to steam export, which can be hard to be utilized in distributed approaches.

This resulting decrease of the carbon footprint is higher than 70%.

The great potential of these e-technologies is to introduce process intensification by reducing the number of steps, potentially lowering fixed and operative costs, and introducing new modalities of production with a better use of the local resources (distributed production). The full value chain of chemical production and the strong nexus with refinery would be changed in passing to an echemistry model. This is the perspective required for the analysis of the feasibility and impact.

We may conclude that in the frame of the proposed high tech scenario, the potential to substitute FFs in the chemical production with renewable energy and alternative C sources exists. This objective would correspond to a better use of the resources changing from a linear to a circular economy model. The impact is potentially to lower rather than increase the production costs as instead often claimed. Saygin and Gielen, 56 for example, estimated that "achieving full decarbonisation in this sector will increase energy and feedstock costs by more than 35%". tech scenario, where an intensified R&D will allow to solve current technological limitations, the 14 innovation which brings from the transition to an e-chemistry will result in an overall reduction of the costs, in addition to an increased sustainability and reduced impact on the environment. 55 In the past, when a proper stimulus of R&D was given, for example when the strong limitations on car exhaust emissions (Clean Air Act) were introduced around year 1970, the results were largely beyond expectations. Car exhaust treatment technology realized what was indicated as impossible to achieve. In addition, the current petrochemistry based on olefins was largely realized in few years around year 1960, due to a series of converging driving elements. 23 Also in this case, transformation occurred much faster than predicted. Therefore, history teaches how major technological changes, as those necessary to implement an e-chemistry, may occur despite many negative predictions. The scenarios for 2050 predict completely different situations in terms of reduction of GHG. We believe that it would be better to analyze what are the gaps and limits to implement a possible scenario, rather than to long discuss whether could be realized, using different assumptions all difficult to prove to be the most correct.

The key question thus remains how to develop the technologies which are needed, accelerating their discovery and industrial implementation. Catalysis, being a key element in most of these technologies, should follow the same trend and thus the question is how to accelerate the progress necessary in catalysis for e-chemistry, rather than to discuss whether this petro-to e-chemistry transition is feasible. We believe that many elements indicate that an irreversible transition towards the realization of an e-chemistry is already started.

The production modes in petrochemistry are rigidly hierarchical with few building blocks (mainly light olefins, aromatics and syngas/methanol) requiring a sequence of steps, often many, to obtain the final chemicals for industrial (polymers, synthetic fibres and rubbers, solvents, etc.) or consumer uses (detergents, drugs, fertilizers and pesticides, paints, etc.). This production scheme is largely associated to the concept of scale economy, e.g., the need to develop large-scale centralized plants and thermal processes. 23 This model of chemical production has many limits, from the significant local impact on the environment to the intrinsic low flexibility and adaptability instead required due to an uncertain future. Chemical plants are designed for a utilization factor typically above 90% to operate economically. The global ethylene production plants decreased to an average 82-83% in the last years and it is predicted to remain lower than 90% in the next decade. 60 In these conditions, economic margins for the production are very low, or even negative. This is a general situation for petrochemical production (ended the windows of opportunity) 43 and it will be accentuated in the future, needing to change the production model from centralized (few very large sites) to a distributed model, more flexible and strongly reducing costs and impact of transport/distributions. 60 In addition, a distributed model offers the integration with the local resources rather than along the value chain (creating also new opportunities for symbiosis and investments). All these elements create a competitive environment based on innovation. A distributed production requires efficient small-medium scale plants well integrated with the territory and the local resources/needs, with a modular plant scheme allowing faster time to market and great flexibility of operations. e-Chemistry technologies should have these characteristics. Therefore, the remark often made that it is not possible to produce large-scale building blocks as ethylene (typical size of steam crackers to produce ethylene goes from 200 to over 1000 ktons/y of ethylene) by electrocatalysis (or other routes based on RESs) is mispresented. In e-chemistry the model of production is changed with the target of small-medium scale productions tailored for the local production needs. It avoids distribution/transport on a large-scale. The aim is the direct conversion to the final product (or at least to strongly reduce the number of steps), avoiding fragmentation of the production in a long sequence of steps. Ethylene is a raw material for polymers (polyethylene), but also the building block for a large series of other chemicals (used mainly for other polymers) such as vinyl chloride, ethylene oxide, vinyl acetate, etc. Thus, technologies for the direct ethylene production should be used for polyethylene manufacture, while other chemicals derived from ethylene should be ideally produced directly rather than via ethylene as intermediate.

Many well-established large-scale chemicals are produced with a sequence of steps which can be potentially drastically reduced. As an example, phenol production is realized commercially with the conversion of benzene to cumene, which is converted to cumene hydroperoxide then decomposed to phenol and acetone. However, the yield is low, about 8% (on the whole process), due to the critical step of the intermediate cumene hydroperoxide production. The direct one-step synthesis of phenol from benzene and H2O2 is potentially competitive but suffering of hydrogen peroxide cost. 61 H2O2 could be produced electrochemically and thus a novel electrochemical route to directly produce phenol from benzene is potentially feasible. It may be a hybrid system with production of H2O2 electrocatalytic combined with in-situ catalytic conversion, or better by direct electrocatalytic benzene hydroxylation using hydroxyl species generated at the anode. There are many open questions, from the type of electrolyte and electrode to use to the control of multiple hydroxylation (phenol is more reactive than benzene towards the insertion of a further hydroxyl groups, but there are strategies to control this issue by inhibiting the further reactivity of phenol). 61 However, studies in the field are extremely limited. The production of H2O2 by electrocatalytic routes is a topic of growing interest, 62, 63 but not the direct electrocatalytic hydroxylation of benzene, or the hydroxylation of other substrates. However, papers are available on the catalytic hydroxylation of benzene with H2O2. [64] [65] [66] [67] Benzene direct electrocatalytic hydroxylation is one of the routes to explore for an e-chemistry, but which has been not yet investigated. Benzene current source derives from FFs, mainly as a product of the refinery reforming process. However, in a future e-chemistry it could be produced from lignin. 68 The example above introduces the concept of extending the use of electrocatalysis by addressing complex syntheses of chemicals by combining electrocatalysis of small molecules (CO2, H2O, N2, CH4, the latter from biogas sources) and of biobased molecules. The integration of these two separate worlds is the grand challenge for e-chemistry and e-refinery. 69 Most of the electrocatalysis studies instead address simple reactions as the two-electron reduction of CO2 to CO or to HCOOH. [70] [71] [72] [73] Also in CO2 electrocatalytic conversion, the challenge is instead to realize complex conversions of CO2 leading to multi-carbon (C2+) products and to the intermediates needed to build a petrochemistry-equivalent framework, as commented above.

Target is to produce directly both the base chemicals which can be used then as raw materials for the current chemical production (light olefins, for example) or even better produce directly (in onestep) more complex molecules.

The direct synthesis of multi-carbon products is a topic of growing interest in CO2 electroreduction, 33,74-76 but mainly from an academic perspective rather than as part of a strategy to build the new e-chemistry. For example, being intrinsically simpler the electroreduction of CO2 to CO than the formation of C2+ products, a large part of the studies focused to the CO2 electroreduction to CO, with the justification that the performances (Faradaic yield, productivity) are better, and CO could be then used in combination with H2 to make a variety of other chemicals, via methanol or Fischer-Tropsch catalytic processes. In general, the productivity of CO2RR to C2+ products (especially C2+ hydrocarbons) is still low, although significant progresses have been made recently and now Faradaic selectivity and productivity/current density (to ethylene and ethanol, in particular) have reach levels in some cases which allow to consider a possible industrialization. [74] [75] [76] However, still a gap exists between production of syngas by co-electrolysis on solid oxide electrolyzer cell (SOEC). The latter is a better solution with respect to the separate production of CO from CO2 and H2 from H2O in PEM-type electrolyzers, although SOEC still shows relevant issues in terms of cost/performances, reliability and durability. 77 After producing the syngas, with eventual adjustment of the CO/H2 ratio, a compression and heating could be necessary, then one or more thermocatalytic steps of conversion could be necessary. Two routes could be possible to obtain olefin: via Fischer-Tropsch (FT) process (in the modified version FT to olefins, although performances are still unsatisfactory notwithstanding the recent progresses) 79 or via intermediate methanol synthesis followed by methanol to olefin conversion (followed by further C4+ cracking unit). 80 In both routes a broad distribution of products is obtained. If the overall efficiency, especially energetic, is accounted for these multistep processes, and the needs of complex downstream separation units which also makes costly the development of small-scale distributed applications, the direct electrocatalytic production of olefins appears as a preferable route. 81 However, the multistep power-to-olefin route (syngas by co-electrolysis, then thermocatalytic steps) is more mature in terms of implementation. In this case, a decrease of the carbon footprint could be realized through electrification of the thermocatalytic process, e.g. using electricity to provide the heat of reaction. 82 A wider range of possibilities exists (than forming C2+ production from CO2) which can be used to build new routes for e-chemistry. An example is the electrocatalytic reductive coupling of CO2 to oxalic acid, 83, 84 which can be further electro-reduced to a range of valuable chemicals for polymerization like glycolic acid, creating a new C2 value chain from CO2. [85] [86] [87] Further reduction of glycolic acid may lead to produce ethylene glycol (a main intermediate for polyesters), thus a new path with respect to the current one via ethylene and ethylene oxide. This electrocatalytic chemistry is investigated in the EU project OCEAN (Oxalic acid from CO2 using electrochemistry at demonstration scale, grant 767798). Oxalic acid can be electrocatalytic reduced to glyoxylic acid and further to tartaric acid, 88 opening a new path to C4 chemistry from CO2.

The electrocatalytic production of acetate/acetic acid from CO2 53,89 is also opening new possibilities, not only for the use of acetic acid itself (as solvent) but also to produce electrocatalytically a range of interesting products. One of them is the combination of the electrocatalytic production of acetate from CO2 and its reaction with (bio)ethanol to form ethylacetate, a green solvent of large use. The electrocatalytic reactor is used to produce ethylacetate both at the anode side by anodic oxidation of ethanol, and at the cathode side, through in-situ catalytic reduction of CO2 to acetate which reacts with ethanol to form ethylacetate. Therefore, the same product is produced at both electrode sides. This is an interesting example of coupling CO2 electroreduction chemistry to the use of chemicals from biorefinery. This electrocatalytic chemistry is investigated in the frame of EU project DECADE (Distributed chemicals and fuels production from CO2 in photoelectrocatalytic devices, grant 862030). Another possibility is to explore a similar chemistry but finalized to the direct synthesis of vinyl acetate.

No studies in this direction are available, but in an old patent 90 an electrochemical process to produce vinyl acetate from ethylene and anolyte acetic acid is claimed.

There is a much wider range of possibilities, for example to develop paired or tandem electrocatalytic conversions. 91 This concept of competitive chemisorption to enhance selectivity in electrocatalytic reactions has a more general validity in electrocatalysis.

Electrocarboxylation of olefins and diolefins 104 (ethylene and butadiene, especially, both which can be produced from bioethanol) 105 or of aromatics is another route of interest to build an echemistry. 106 ,107 A variety of valuable intermediates could be produced by this method.

When other reactants are present, for example nitrate, the spectrum of the possible products further enlarges. One example is the electrochemical synthesis of glycine from oxalic acid and nitrate, 108 rather than the electroreduction of oxalic acid to glycolic and glyoxylic acids discussed before. Glycine is one of the base aminoacids. Figure 2 summarizes the concept that by combining primary electrocatalytic reactions of small molecules (by using final products and active intermediates, in a combined approach), to the electrocatalytic conversion of bio-derived products, it is possible to produce a large range of products forming the skeleton of a new chemical production. 69, 109 Reactions to explore include i) direct coupling of in-situ generated 23 intermediates, ii) tandem reactions using in-situ generated chemicals, and iii) the eventual coupling between electro-and heterogeneous catalysis.

By properly combining the primary reactions of electro-conversion, a larger range of products can be derived, potentially meeting the needs for a novel e-chemistry in combination with the other paths described before. This approach extends the concept of tandem electrocatalytic processes discussed before.

In tandem or paired reactions, the concept is the valorisation of both cathodic and anodic reactions to form added-value products. In water splitting, but similarly in most studies on CO2 or N2 electroreduction (CO2RR and NRR, respectively), O2 is produced at the anode site: the oxygen is most of the practical cases cannot be used, and is released to atmosphere, and even when used, it is a low-value chemical. In addition, oxygen evolution reaction (OER) is a four-electron reaction and it is typically the slow step of the electrocatalytic process, which determines most of the energy losses due to overpotential. Moreover, to produce H2O2 by water oxidation instead O2 (a two rather than a four electron reaction) is an interesting target. 62, 63 H2O2 is a valuable oxidant in many selective oxidation processes (as discussed later) and in environmental applications. There is thus a rising interest on this process which represents not only an alternative to OER but also a synthesis process for H2O2 as industrial selective oxidant, even if attempts to produce H2O2 electrocatalytically are known from decades. 110 H2O2 is already production in Mtons scale by anthraquinone route as selective oxidant in commercial processes (propylene oxide, caprolactam, catechol and hydroquinone syntheses), for soil and water remediation uses and as a versatile bleaching agent. 23 H2O2 formation is half the reaction of the most studied oxygen evolution reaction (OER), the anodic reaction in water electrolysis systems. 111 However, recent studies, for example on single-atom electrocatalysts, are in contrast with these mechanistic indications. 112 Recent advances in the understanding and design of the gas-liquidsolid three-phase architecture have led to significant progresses in obtaining a high selectivity (83-99% current efficiency) combined with high current density and stability. 113 By using a superhydrophobic natural air diffusion electrode (NADE) to improve the oxygen diffusion coefficient at the cathode as compared to the normal gas diffusion electrode (GDE) system, Zhang et al. 114 showed that it is possible to largely increase H2O2 production rate and oxygen utilization efficiency. These examples show how the deep understanding of the engineering aspects at the electrode nanoscale is the crucial factor in determining the behaviour, and not only the nature, of the electrode itself. 115 Quite similar aspects are also strongly determining the behaviour in NRR. 116 Produce H2O2 rather than O2 in water splitting or other electrocatalytic reduction reactions (CO2RR and NRR) has thus the advantages of making a 2erather than a 4eprocess (with advantages in terms of rate and reduced overpotential), and obtain a higher value product. Pd δ+ clusters (Pd3 δ+ and Pd4 δ+ ) onto mildly oxidized carbon nanotubes (containing controlled defects) were recently shown to allow nearly 100% selectivity in H2O2 formation with low overpotential and high mass activity. 117 Therefore, significant progresses have been made recently on the electrocatalytic production of H2O2. [118] [119] [120] [121] [122] However, even if H2O2 is used in many chemical applications, the volume of the potential commercial market is largely lower with respect to that it would be needed for potentially very large-scale reactions as H2 production by water electrolysis, CO2RR and NRR.

In addition, in many applications of H2O2, for example in its industrial use in selective oxidation reactions, the concentration and solvent requirements do not fit well with those produced in the electrocatalytic H2O2 synthesis. 23 To find an alternative optimal reaction to OER remains thus still a challenge. Some further aspects will be discussed later. Note also that the increasing trend to operate in non-protic solvents to limit side reaction of H2 formation in CO2RR and NRR further stress the need to identify valuable anodic substitutes to OER and H2O2 production as well.

Coupling between electro-and heterogeneous catalysis is another area of interest to remark.

The development of hybrid catalysts/reactors combining electro-and heterogeneous (or eventually homogeneous) catalysis is an area still largely unexplored which represents a specific opportunity.

On the contrary, hybrid electrocatalytic/biocatalytic systems have been more systematically explored. 123 In addition, direct or mediator-assisted electrocatalysis is possible. The use of redox mediators (typically organic compounds, but not limited to them) is common in organic electrosynthesis. 126 In the indirect electrocatalysis the electron transfer step is shifted from a heterogeneous process occurring at an electrode to a homogeneous process using an electrochemically generated reagent issues of separation and stability that often become the discriminating elements for industrial processes. 128 On the other hand, by using heterogenized mediators, and by designing the electrocatalytic reactor with integrated membranes it is possible to overcome, in principle, these issues and exploit the benefits of mediated electrochemical reactions.

Above discussion evidenced a series of key directions to build the new catalysis required by e-chemistry. It is useful to summarize them to remark some of the areas on which R&D should 27 be pushed:

1. The development of PEC devices integrated with redox mediators that allow to make the direct solar-to-fuels/chemicals conversion with continuous (24h) operations, because the redox mediators store the energy when light is present on one electrode side, allowing to operate on the other electrode side in electrocatalytic continuous modes. The spatiotemporal decoupling of the processes in PEC devices overcomes the limits of intermittency (with associated costs and issues) offering also a new possibility to design the devices and to overcome other limits such as insufficient current density for industrial operations.

2. The study of the benzene direct electrocatalytic hydroxylation to phenol, which can be also considered a model for a wider range of novel e-chemistry routes.

3. Exploring more systematically the novel electrocatalytic routes by coupling the products of the 10. Investigating with a broader approach the direct synthesis of ammonia (or derivatives, such as the synthesis of amination products) from N2, another emerging electrocatalytic path to both fertilizers (perhaps the largest volume chemical) or N-containing chemicals. 134, 135 Here it was also earlier evidenced 10,31 that the large research effort is improving too slowly from a more practical perspective, in part due to the use of approaches which are not accounting the difference between electro-and thermal catalysis. Methodologies (including theoretical) which are not able to catch the intrinsic difference in electrocatalysis are typically used. The result is to "demonstrate" a very large range of (contradictory) mechanistic features for relatively analogous electrocatalytic results. The use of different approaches, including biomimetic based on multi-electron/proton simultaneous transfer (requiring a different nature of the active sites)

is a direction offering new clues to design electrocatalysts from a different perspective. 136 11. Intensifying the investigation on the electrocatalytic production of H2O2. 110 Besides to the commercial interest of hydrogen peroxide as commented before, it represents half of the 30 reaction of the most studied OER, the anodic reaction in water electrolysis systems. It is thus a two-electron path alternative to four-electron path to produce O2 at the anodic part of the cell.

It is a lower overpotential reaction, requiring less electrons and giving an added-value product, contrasted, however, from the easier decomposition. to organic electrosynthesis to the production of bulk chemicals by electrocatalysis is a challenge, which often requires to turn the type of approach used up to now.

Note that in addition to direct electrocatalytic syntheses, also indirect syntheses via redox mediators (TEMPO being one of the most used) are possible. Thus, various examples of organic electrosynthesis are known, but related to electrochemistry rather than to electrocatalysis.

Addressing the challenge given from developing a full framework for e-chemistry ( Figure 2) requires making a next step in combining catalysis to electrochemistry.

Research attention should be putted on specific fundamental areas, still not enough explored, in order to create the bases for e-chemistry. These areas should complement the development of new electrocatalysts and mechanistic studies, on which research attention is mainly focused currently. Among the different aspects to develop, the following can be highlighted:

-The design of advanced electrocatalytic reactors, which strongly determine the performances and behaviour, while still most of the studies are based on too simple reactors (like H-cells) not allowing to proper translate the results to realistic reactors closer to industrial exploitability.

-The study of multiphasic electrocatalytic reactors, an area that as outlined before is crucial to explore novel electrocatalytic paths, but scarcely investigated. 141 -Understanding the difference, including in mechanistic aspects, between electrocatalysis (in general reactive catalysis) and thermal catalysis, 10 as the basis to develop new approaches and explore novel (complex) pathways. Current studies consider mainly the application of methods used for thermal catalysis to reactive catalysis, with still limited attempts to understand the 33 difference in operations and type of active centers. Dynamics of polarized interface is crucial, also in determining selectivity 33 and significant reconstruction of active nanoparticles may occur upon application of a potential during electrocatalytic operations. 142 -Electrocatalytic behaviour is determined by an intricate interplay between surface structure (both on the nano-and on the mesoscale), electrolyte effects (pH, buffer strength, ion effects) and mass transport conditions, a complex interplay still far from being completely understood. 7 A new electrocatalytic design, for electrolyte-less operations, allow to improve the behaviour by exploiting this interplay, but still systematic knowledge are limited. 143, 144 A better understanding and a rational design of three-phase boundary in electrocatalysis can lead to an enhanced control of the performances. [144] [145] [146] [147] -Addressing the selectivity challenges of electrocatalysts. In several reactions, like CO2 electroreduction, many reactions are possible at the applied potential, which includes the necessary overpotential to drive the reaction at sufficiently high rate. Developing an effective theory of selectivity in electrocatalysis is still at the infancy, even with the progresses in the field. [148] [149] [150] 

The examples presented show that the potential and feasibility to develop a new e-chemistry exists, but it is necessary to push the research to address more specifically this challenge, exploring the various possibilities indicated and integrating in a holistic view the different developments to accelerate the discovery of new paths and the identification of the best options among the different possibilities. The new e-chemistry requires to turn the approach in the development of new paths, with a push towards new direct reactions which limit as much as possible the need of multistep processes realizing direct processes in small and efficient devices based on the use of RES. This is a grand challenge, which needs to be addressed accelerating the evolution in this direction.

Although attention was given here mainly to electrocatalysis to focus the discussion, the importance of exploring also other routes of direct use of RES, such as photo-and plasmacatalysis, must be remarked again.

A general indication given is that to properly address the challenge of e-chemistry, there is the need to target complex (direct) syntheses by electrocatalysis. The question which may be posed is that what presented above is a dream (electro)catalysis and chemistry and it should be better to focus on simpler electrocatalytic reactions (two electrons, such as CO2 to CO), because more complex multielectron/proton transformations are too challenging. The reply to this question is that it was scarcely attempted to make a rational approach to catalysis complexity, identifying the ways to proceed faster in targeting complex (direct) syntheses. Not addressing this question will delay the acceleration in the progress in this area which is crucial to meet the challenge of converting petro-to e-chemistry.

In conclusion, we believe that an e-chemistry (a chemistry using RES, closure of the carbon cycle and biomass as key elements to defossilize the industrial chemical production) is feasible (assuming a high-tech scenario), and this transformation from current petrochemistry is already started and will be largely irreversible. The nexus between refinery and chemistry is already changing, as a sign of this irreversible transformation. 151 However, acceleration of this process requires to turn the approach, with a broader vision of the future. For catalysis this will require to revise some of the fundamental bases and recognize that the new methodologies essential for echemistry need to approach catalysis from a wider and different perspectives, although based on the extensive background on knowledges on catalysis developed in the last decades. 152, 153 In deep transitions, a synchronism between R&D and economics/societal changes is necessary. Thus, the 35 development of novel sustainable process systems requires thinking at a multi-scale level identifying the energy-efficient and highly integrated systems deployed within local and regional contexts. 154 While the transformation of the chemical industry to an e-chemistry is often considered to be motivated (only) by reducing the carbon-footprint, but at the "expense of higher production costs and unintended environmental burden shifting", 154 we would further remark that instead the push to innovation necessary will results in lower global costs, with a different model of chemical production, interaction with territory and society. 155 For catalysis, e-chemistry is a grand challenge, offering the possibility to rebuild its role as key technology and develop new bases for understanding. This will occur only when the significant changes in the approach to catalysis required to face this transformation are understood and introduced in the scientific practice. 

The manuscript was written through contributions of all authors which all equally contributed All authors have given approval to the final version of the manuscript.

This work was made in the frame of the ERC Synergy SCOPE (project 810182), EU DECADE 

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