CHEMSUSCHEM CONCEPTS DOI: 10.1002/cssc.201300926

CO2 Recycling: A Key Strategy to Introduce Green Energy in the Chemical Production Chain Siglinda Perathoner* and Gabriele Centi[a] The introduction of renewable energy in the chemical production chain is a key strategic factor both to realize a sustainable, resource-efficient, low-carbon economy and society and to drive innovation and competiveness in the chemical production. This Concept discusses this concept in terms of motivations, perspectives, and impact as well as technical barriers to achieve this goal. It is shown how an important element to realize this scenario is to foster the paths converting carbon di-

oxide (CO2) into feedstock for the chemical/process industry, which is one of the most efficient methods to rapidly introduce renewable energy into the chemical production chain. Some of the possible options to proceed in this direction are discussed, with focus on the technical barriers and enabling factors such as catalysis. The tight interconnection between CO2 management and the use of renewable energy is evidenced.

Introduction Realizing a sustainable, resource-efficient, and low-carbon economy is a major current challenge for society.[1] It is undoubtedly the role of the chemical industry to achieve this goal.[2] Even if large progresses in this direction have been made over the last two decades, a systemic change in the way energy and raw materials are used is necessary in a world with finite resources and a rapidly growing population. The challenge of resource and energy efficiency in chemical production, however, may become a winning opportunity for increasing competitiveness and innovation.[3–5] A key element to achieve this goal is to introduce renewable energy (RE) in the chemical production chain to reduce the carbon and environmental footprint. There are two possible routes to introduce RE in the process and chemical industries: (i) an indirect path using biomass, and (ii) a direct use of RE sources such as solar, wind, or hydro energy. We limit the discussion here to the second case, although biofuels have received considerable attention in the last decade, notwithstanding the concerns related to the impact on environment and greenhouse gas (GHG) emissions.[6, 7] Costcompetiveness of the use of biofuels as energy source for the process industry is still critical in the absence of specific subsidies. Differently, biomass will play a relevant role in reducing the use of fossil fuels when used to produce raw materials for large-scale production of chemicals.[8] Examples are the production of (i) ethylene from bioethanol,[9] (ii) acrylic acid from glycerol,[10] and (iii) 5-hydroxymethylfurfural (5-HMF) from C6 [a] Prof. S. Perathoner, Prof. G. Centi Dipartimento di Ingegneria Elettronica, Chimica ed Ingegneria Industriale University of Messina and INSTM/CASPE V.le F. Stagno D’Alcontres 31, 98166 Messina (Italy) E-mail: [email protected] Part of a Special Issue on “The Chemistry of Energy Conversion and Storage“. To view the complete issue, visit: http://onlinelibrary.wiley.com/doi/10.1002/cssc.v7.5/issuetoc.

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sugars.[11] Although various issues in using bio-based raw materials still exist, ranging from uncertainty in price and constant supply to the limited cases of drop-in chemicals in addition to the general high cost of production,[12] estimations point out that they will significantly contribute to meet future targets of carbon footprint. Sustainable process industry through resource and energy efficiency (SPIRE),[4] a public–private partnership lead by the European Chemical Industry Council, aims to reduce use of nonrenewable, primary raw material in process industry by 20 % as well as the use of fossil energy by 30 % by the year 2030 with regard to current levels. Different scenarios for a bio-based economy exist,[13, 14] some of them assuming a large penetration of bio-economy. However, there is a growing agreement that the use alone of biomass will not allow to realize the cited targets and that it is necessary to integrate directly RE sources (solar, wind, hydro, etc.) in the chemical production chain. The role of biomass in enabling a new scenario for the chemical production is analyzed elsewhere;[15] we focus herein on the concept that recycling CO2 is a key strategy to mediate the use of green energy in the chemical production chain. Direct use of electrical energy, the main output of current technologies for RE production (solar, wind, hydro, etc.), is limited. Heat is the main source of energy utilized in chemical industrial processes, except for a few exceptions (chlorine and soda production, for example).[2] Less than about 5–10 % of the fossil-fuel input (as energy and raw material) can be substituted by direct use of electrical energy from RE sources to reduce the carbon footprint. There are many motivations (social and political) to force an increased use of RE in process and chemical industry:[16] (i) recent developments in RE technologies making them competitive in comparison with the use of fossil fuels; (ii) cost of production largely related to fixed capital costs rather than variable costs, which are more difficult to estimate long term; ChemSusChem 2014, 7, 1274 – 1282

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CHEMSUSCHEM CONCEPTS (iii) renewable portfolio standard mandate in many states and countries as well as various energy conservation measures; (iv) legislation on climate changes, including current and future taxation; (v) limits on carbon emission and other environmental and political concerns, among others. It is thus necessary to take a step forward by developing effective methods by which RE could be integrated into the chemical production chain. The discussion is focused herein on recycling CO2 as a key strategy to mediate the use of green energy in combination with biomass.[15] Many reviews exist on the bio-based economy and the role of biomass for future chemical industry.[17–27] Often, however, the various paths are not assessed in terms of specific contribution to realize a sustainable, resource-efficient, low-carbon chemical production.

Renewable Energy Scenario The interest in RE is growing rapidly. Despite the international economic crisis and other issues (ongoing trade disputes, policy uncertainty, and declining support in some key markets), the global demand for renewable energy continued to rise during 2011 and 2012. RE supplied about 20 % of the global final energy consumption by the end of 2012, with nearly half being supplied by traditional biomass.[16] Globally, wind power accounted for about 39 % of renewable power capacity added in 2012, followed by hydropower and solar photovoltaics (PVs), each accounting for approximately 26 %. However, hydropower still remains the largest contribution to modern renewable energy. In 2012, it accounted for nearly 78 % of the world’s renewable electricity generation (4759 TWh), and estimations for 2017 indicate that it will still be about 70 % of about 6400 TWh renewable electricity produced worldwide.[28] There is still a large hydropower potential remaining untapped. The global technically exploitable hydropower potential is estimated to be more than 16 400 TWh per year; for example, over 80 % of the potential is still unexploited.[29] This potential is unevenly distributed, being over 95 % in regions such as Africa where local grids are unable to accept additional hydropower production (in addition, the electrical energy cannot be transported to other potential users). On the other hand, the costs of hydropower still remain largely competitive over other solutions (wind, PV). It can vary widely depending on project details, but usually fall into a range of 50–100 US$ per MWh. Exploiting this hydropower potential through development of effective solutions to long-distance transport of the produced renewable energy offers the great opportunity to combine a cost-effective increase in generation capacity with the diversification of energy sources. The technical challenge is to develop a cost-effective solution to store and transport electrical energy. Hydropower, with respect to alternative RE sources, has the great advantage of nearly constant supply. This is a key element for the possibility to exploit the long-distance transport by chemical storage because the equipment used in this conversion (for example, to synthetize CH3OH from CO2 and H2O, as discussed later) will operate throughout the day or year. The same technology utilized to store electrical energy produced  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org by wind or sunlight will instead operate only for a fraction of a day or year, which thus results in much higher fixed-capital costs and frequent switch on/shut off, with a consequent decrease in the life time of the equipment and components. Dams for hydropower production are not exempt from an impact on environment. Maeck et al.[30a] noted recently that sediment trapping by dams creates methane emission. However, Hertwich[30b] determined by life cycle assessment (LCA) that GHG emissions from dams are on average 1 gCH4 per kWh, thus lower than emissions from power plants using fossil fuels, which are estimated to be in the range of 2–40 g(CO2 equivalent) per kWh. Grumbine and Pandit[30c] and Qui[30d] observed that the programs of increasing hydropower capacity in India and China have a significant impact on the ecosystem, but contribute towards limiting overall GHG emissions. Expanding the hydropower use has thus an environmental impact but reduces the overall impact of production or use of energy. With the limits noted above regarding intermittent RE sources, deserts and arid regions offer another large, unexploited potential for large-scale generation of RE (PV, wind). About 1 % of the desert surface of the earth would be enough, in theory, to provide all humans with energy. In addition, population density in most desert areas is comparatively low and flora and fauna are very sparse, lowering the possible environmental impact of large areas dedicated to the production of renewable energy. DESERTEC Foundation[31] is pushing the concept of RE production in deserts and arid regions, but still the critical limiting issue is the long-distance transport of the produced electrical energy (in addition to other aspects ranging from economics to PV fouling and damage by sand and dust, etc.). Chemical storage of RE could be also used as a complementary element in smart grids[32] to minimize the impact of fluctuations in solar and wind power on the stability of the electrical energy network.[33] The conversion of electrical to chemical energy, producing molecules that are easy to store[34, 35] and that can be used to feed gas turbines is thus a potential solution to long-distance transport of RE as well as to store RE locally (together with other approaches) in smart grids. Particularly for long-distance transport, the molecule should be preferably liquid at standard conditions [methanol or dimethyl ether (DME), for example] to reduce the cost of transport. In addition, the possible energy vectors have to be easy and safe to handle and preferably compatible with the actual energy infrastructure to reduce investment costs.[36, 37] Recycling CO2 to form methanol or DME using RE is thus a key element for a larger use of the latter and for moving toward a sustainable, low-carbon economy.[38, 39] Based on the previous considerations, the potential of RE to produce renewable methanol, for example, from CO2, may be assumed to be about 10 PWh per year. This electrical energy would allow producing about 1.2 Btons of methanol per year at a potentially competitive cost in comparison to that derived from fossil fuels. Even considering that only few percentages of this potential could be exploited, the amount of renewable methanol obtained by this route is large enough to have a significant impact on changing raw materials for chemical proChemSusChem 2014, 7, 1274 – 1282

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CHEMSUSCHEM CONCEPTS duction. In fact, the direct use of electrical (green) energy is not suitable for chemical production, whereas a better solution to achieve resource and energy efficiency is the use of raw materials alternative to those derived from fossil fuels. The use of products derived from CO2 as raw materials for the chemical production has the advantages (over the use of biomass, for example) of (i) using as carbon source a waste of the chemical production itself, thus with a low value (even negative in some case, when considering carbon taxes); (ii) contributing directly to a reduction of GHG emissions as well as (iii) effective introduction of renewable energy into the chemical production chain if the conversion of CO2 is achieved using renewable energy. CO2 conversion, even if in part still challenging as discussed later, is definitely simpler from a technological point of view in comparison with biomass transformation, particularly 2nd generation lignocellulosic-type materials, and when demand for the chemical sector (high-purity raw materials) is considered. In addition, methanol, for example, is a raw material already largely used in the chemical production,[40, 41] and the technologies for its handling and use are well established. Other possible products of CO2 conversion that are also considered for storing/transporting renewable energy (in particular, methane and formic acid[39]) are less suitable as raw materials for the chemical industry. This opens clear opportunities to introduce renewable energy and feedstocks.

CO2 Utilization: A New Scenario for Chemical Production The previous discussion has introduced the concept that CO2 recycling, driven by the need to use unexploited RE sources, opens new opportunities to use this path to better integrate the use of renewable energy and feedstocks in the chemical production chain as well as to progressively substitute the use of fossil fuels. In this vision, CO2, although derived from fossil fuels, is recycled as carbon source for the chemical industry; thus, it can be considered as an alternative raw material for the reduction of the dependence of the chemical and energy industry on fossil fuels.[42] The question is whether or not this vision for a CO2 economy is feasible. Actual chemical production is currently oil centric. Natural gas (NG) is used, but this essentially limited to the production of H2 (the main use in chemical production is for ammonia— about 135 Mt production per year—and its related value chain) and syngas, mainly converted to methanol—about 65 Mt per year—and its related value chain. Coal has still limited usage for chemical production, but China is actively pushing its use through syngas production and production of methanol and then olefins. The expanded production of shale gas, notwithstanding the large environmental concerns that have limited the production in areas such as Europe for now, resulted in a decoupling of the natural gas price market costs from that of oil in some areas such as the USA. In the USA (but not in Europe, where the impact of shale gas on NG costs is less relevant), chemical companies are actively studying the new opportunities created by the large availability of shale gas  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org (methane, ethane) to expand production capacity for ethylene, ethylene derivatives (i.e., polyethylene, polyvinyl chloride, etc.), ammonia, methanol, propylene, and chlorine.[43] This analysis (maintained brief for conciseness) suggests that world chemical production is progressively moving from an oil-centric common vision to different regional-based systems, with the USA pushing the chemical production centered on the use of shale gas. China focused on coal utilization whereas the Middle East is fostering the use of oil owing to the low local costs. Europe, to remain competitive in this global competition, needs to foster the use of alternative raw materials. The use of waste biomass and CO2 are the only two possible solutions; these play a central role in the SPIRE roadmap,[4] around which the vision of future for the chemical industry in Europe is centered. The feasibility of CO2 economy thus translates into the question of practicability of chemical production based on the use of alternative (to fossil fuels) raw materials for chemical production. History teaches[2, 44] that this could be a game change. Industry should be prepared that oil will not remain available (and at low cost) for the future. Nikolai Kondratieff was an economist that predicted the existence of cycles in economy. The ongoing economic crisis may presage a 6th Kondratieff cycle. Five characteristics for a new Kondratieff cycle seem to be fulfilled:[45] - potential for further exploitation of old innovations that have driven the start of a new cycle is exhausted after around 40–60 years - high level of excess financial capital (vs. physical capital) - period of severe recession (period of radical change) - social/institutional transformations - new technologies overcoming macroeconomic bottlenecks This new 6th, “green” Kondratieff cycle will be characterized by global structural changes in the economy with a crucial reorganization of the energy infrastructure,[45] for which the switch to REs will largely influence the market. Kondratieff’s economic theory, when applied to chemical production,[2] indicates that a fast transition to new raw materials and energy is at the beginning of each new cycle, the new one just starting. The building blocks for petrochemistry are H2, syngas, light olefins, and aromatics (Figure 1). Using renewable H2 (produced, for example, by water electrolysis using electrical energy from renewable sources), CO2 as carbon source in integration with platform molecules derived from biomass, and lignin (or other sources) to produce aromatics, it is potentially possible to completely avoid the use of fossil fuels for all chemical productions. Figure 1 illustrates schematically this concept. To keep the Figure readable, only the relevant alternative production paths for building blocks as well as some examples of specific intermediates are reported. Emphasis is on the paths for drop-in products, especially building blocks. Note that drop-in products avoid the need of large investments required to change the production and to introduce new molecules/products on the market.

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Figure 1. Simplified flowchart of current petrochemical production with an outline of the possible new scenario for a sustainable chemical production based on the reuse of CO2 and of biomass to produce both raw materials (building blocks) and specific intermediates.

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CHEMSUSCHEM CONCEPTS Benner et al.[46] analyzed the feasibility of some aspects of this new scenario for chemical production, in particular the impact of the production of (i) ammonia using H2 produced by water electrolysis (using RE) or from biomass gasification, (ii) light olefins from bioethanol, and (iii) aromatics from lignin or bioethanol. The introduction of these alternative processes allows reducing GHG emissions from 50 % to 100 % compared to the conventional processes to produce ammonia, olefins, and aromatics (BTX). Also, the economic assessment of these technologies is positive, even if preliminary. Light olefins could be also produced from CO2 using modified Fischer–Tropsch (FT) catalysts and renewable H2. Centi et al.[47] have specifically analyzed this reaction in terms of techno-economic feasibility. When renewable H2 is available at costs that are less than about 2–3 US$ per kg, producing light olefins from CO2 may be economically attractive, even if more detailed studies would be required. The analysis of the cost of production of renewable H2[39] indicates that this is a feasible target within about 5–10 years. US National Renewable Energy Laboratory has a target H2 production at plant gate (from wind) of < 2.00 US$ per gasoline gallon equivalent (gge) H2 (e.g., < 2 US$ per kg H2) for 2017; best actual results are around 2 US$ per kg H2.[39] Interestingly, the cited study by Benner et al.[46] reports similar targets for enabling the production of ammonia using renewable H2. In particular, the authors indicate that the production of low-carbon ammonia (e.g., by using H2 produced with RE rather than from NG) can compete on price with conventional ammonia production provided that the decrease in capital costs is realized and the carbon price (actually around 30 E per ton CO2 equiv in Europe) is considered. H2 produced from biomass gasification is also an alternative feasible possibility.[46] Converting CO2 to methanol to trade RE produced at low costs in remote areas (from hydropower, for example) is another feasible option. Barbato et al.[48] recently performed a techno-economic assessment of the reuse of CO2 to produce methanol using renewable H2. The remote methanol production from CO2 is currently economically competitive (cost of production around 300–350 E per ton, in the same range of actual market costs) for a cost of H2 produced using hydropower of 0.140 E per normal cubic meter (Nm3) H2 (a realistic value in some remote areas). The study also indicates that when using a conventional hydrocarbon-based technology about 75 % of the production costs are related to variable operating costs (VOCs), whereas in the proposed concept the situation is opposite. The depreciation cost of capital investment is playing a key role in determining the production cost (around 70 %). This is an important element for the success of the technology. VOCs, largely depending on the future of fossil fuels, are unpredictable in the long term, whereas capital investment could be well estimated. The investment of venture capital is favored, which enables a faster introduction of the technology. CO2 could be derived from large-volume sources of rather pure CO2 in refinery and chemical processes (ammonia production, ethylene oxide production, gas processing, H2 production, liquefied natural gas, Fischer–Tropsch synthesis) as well as from biorefineries (ethanol production).[49] Over  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org 500 Mtons per year of CO2 with these characteristics are available. There are a number of challenges to be solved to enable this technology on a large scale, from improved electrolyzers to specifically tailored methanol catalysts to use directly CO2 and H2 instead of syngas with limited contents of CO2 (around 3 % in conventional plants) and the development of microreactors for an intensified and energy-efficient distributed methanol production. Nevertheless, data indicate that it is possible within a reasonably short time (about 5 years) to implement the technology commercially. CO2 is also used as co-raw material in the synthesis of various interesting polymers, in particular polycarbonates and polyurethanes,[38, 50, 51] as well as in the synthesis of various chemicals for fine and specialty sectors.[52–54]

Factors Enabling This New Scenario Political support to enable this new scenario for the chemical production is necessary. The European Parliament has approved a roadmap for moving to a low-carbon economy until 2050.[55] The roadmap is one of the long-term policy plans put forward under the Resource Efficient Europe flagship initiative intended to bring the EU on route to the use sustainable resources. The roadmap specifies for 2030 a 40 % reduction in GHG emissions compared to 1990 and over 80 % reduction for 2050. These are very demanding targets. Reusing CO2 to synthesize methanol (or other chemicals) using unexploited RE resources has many advantages: - It is one of the most effective methods to reduce GHG emissions. Geological storage of CO2 has an effectiveness factor of about 0.5 when considering the energy need to capture and store CO2. The effectiveness of CO2 reuse (incorporating renewable energy in the conversion process) is large, a factor of over 10–20. The exact values depend on the time scale considered because each recycle introduces RE in the system.[56] - Trading RE through the conversion of CO2 contributes to energy security by diversification of the sources and valorization of the local production. - Reusing CO2 fosters innovation in the chemical industry and preserves production and jobs. Biofuel production and consumption has soared over the last decade, largely because of subsidies and mandated use of biofuels to address concerns related to environment, energy security, and rural development. On average, the use of bioenergy allows to save about 50 % of CO2 emissions with respect to fossil fuels, but many parameters affect LCA of the use of bioenergy.[57–60] Methanol produced from CO2 and RE is more effective than biofuels in reducing the environmental footprint of carbon. Carbon debt and payback time are often used as fairly simple short-term indicators of the relative merits and demerits of different bioenergy feedstocks. Payback time (for example, the time delay before the emissions from bioenergy ChemSusChem 2014, 7, 1274 – 1282

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systems will have reached a breakeven point compared that of the fossil-fuel systems) ranges from 20 to over 100 years.[60] For renewable methanol it may be estimated to be lower than 10 years. Renewable methanol (or equivalent fuels) obtained from CO2/RE offer a faster and more effective approach to reduce GHG, impact on the environment, and energy security. Thus, there are good motivations to demand political incentives to push this route, making, for example, renewable methanol equivalent to biofuels. In terms of social impact, recycling CO2 by using RE and to foster a new green (low-carbon) scenario for chemical production has a good social acceptance. In a longer-term vision, CO2 reuse is part of the path to develop artificial-leaf-like devices (solar-derived fuel cells). These devices will be able to capture CO2 and convert it in the presence of water and solar light to chemicals such as methanol or other products (even if most of the actual discussion on artificial-leaf devices is limited to H2 production rather than to CO2 conversion).[61–63] Artificial-leaf devices are part of t visionary attractive ideas for society. The concept of a grid-connected distributed energy production (economy 3.0) has gained good acceptance by young people, who see it in analogy to internet. Distributed systems limit the possibility of a centralized control, of monopoly-like situations, and increase competiveness, thus maintaining a low the energy price.[64] On the other hand, the distributed energy systems are considered to be the necessary direction to proceed to reduce the local impact of centralized energy generation on the environment.

- decrease in fixed capital costs for electrolyzers (< E 300 per kW of electric energy (kWe)) - development of large electrolyzer stacks - in a longer term substitution of Pt in electrolyzers and reduction of the overpotential The efficiency losses in polymer electrolyte membrane (PEM) electrolyzer performance can be attributed to three aspects: (i) electrical resistance, (ii) membrane resistance, and (iii) kinetic overpotential. The first two aspects can be improved by developing improved cell engineering and membrane materials, whereas the latter requires a better understanding of the surface processes and modifications occurring during electrolysis (for example, by using an advanced in situ characterization). Other requirements for electrolyzers are efficient operation under variable load and stability against frequent power interruption. Catalysts for CO2 conversion also have to be improved. Although catalysts for methanol synthesis are well established on an industrial scale (now at the 4th generation) and it is known that the presence of small amounts of CO2 in the feed (typically around 3 %) promotes performance, the use of pure CO2 requires improved catalysts.[65] There are two main challenges: (i) CO2/H2 mixtures are stronger oxidizing agents than CO/H2 (syngas) mixtures, thus modifying the active state of the catalyst during reaction, and (ii) water, which forms through the reverse water gas shift (rWGS) reaction, inhibits the catalyst activity. The productivity of catalysts using a CO2/H2 feed is

Technical Challenges Enabling the new scenario for chemical production faces many technical challenges. Although discussion is focused herein on CO2, Figure 1 clearly shows a scenario that integrates biomass and CO2/RE with fossil fuels. The part related to biomass is discussed elsewhere.[15] Figure 2 reports a roadmap for the technical challenges discussed below related to CO2 recycling to green energy and its introduction in chemical production. The currently high challenge to enable the reuse of CO2 represents a possibility to produce economically renewable H2, which has to meet some targets: - stability of operation under pressure (> 30 bar) - improvement in the current energy efficiency (> 75–80 %) Figure 2. Indicative roadmap for CO2 recycling to introduce green energy in the chemical production.

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CHEMSUSCHEM CONCEPTS about one third of that possible with an optimal feed composition. A partial solution is to perform the process in two consecutive reactors, with an intermediate removal of water produced after the first rWGS reactor. It is also possible to perform the reaction in a membrane reactor, where a water-permselective membrane is used for in situ removal of water formed during the rWGS reaction. However, the preferable solution is to develop specific catalysts for direct methanol synthesis using a CO2/H2 feed. A further challenge is to develop an efficient process based on microreactors because small-scale production (commonly the case for methanol synthesis from CO2 using RE) would require intensification of the process and improved energy efficiency. Catalysts have to be also designed to operate with microreactors. Furthermore, these microplants can be shut down and started up fast, as is necessary for a better coupling with intermittent renewable energy sources. Synthesis of DME rather than methanol is preferable in some cases, for which a multi-component catalyst [for example, composed of a core based on a methanol-synthesis catalyst and a shell based on a dehydration catalysts (an oxide or zeolite with controlled acidity)] will be attractive. Development of catalysts for CO2 methanation is necessary for applications in which methane (rather than other molecules such as methanol) is the target compound (power-to-gas, for storage of excess electrical energy produced by wind),[66] even if methane is not the preferable raw material for the chemical industry. The reversible storage of H2 in liquid form by reaction with CO2 to yield formic acid[67, 68] is an option to transport renewable H2 produced in remote areas. Most efficient catalysts are homogeneous, and their cost and productivity (per volume of reactor, rather than turnover) has to be improved. Engineering of large-scale processes has to be developed, whereas labscale demonstration devices work well. There are also other options to produce syngas from CO2/ H2O using renewable energy. The use of concentrated solar power (CSP) coupled with a two-step cycle for splitting H2O and CO2 via metal oxide redox reactions is an option explored also on the solar-reactor-prototype scale (3–10 kW power level).[69] There are several challenges for scale-up, which have to be solved, starting from the stability of the materials and still low effective productivity to finally the cost of the devices. There is also the need for fundamental studies of heat and mass-transport phenomena and of materials development in these high-temperature solar thermochemical processes to preserve the pore structure in oxides subjected to high-temperature (> 800–1000 8C) redox cycles. CSP offers also interesting opportunities to improve energy balance (and thus reduce GHG emissions) in highly endothermic reactions, such as CO2 reforming and steam cracking, as well as in biomass conversion,[70] but they are not discussed herein because we feel that it is unlikely that these approaches will contribute significantly to a new scenario in chemical production, if not in niche applications. On a medium-long term, the main challenges are to develop a cost-effective production of renewable H2, either by photoelectrochemical approaches (we do not feel that the photo 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org chemical approach is suited because there is a need to produce H2 and O2 separately, both for safety reasons and to avoid separation costs)[61] or by other possible approaches, biological or CSP.[37, 56, 71–74] Production of H2 from renewable sources derived from agricultural or other waste streams offers the possibility to contribute to the production capacity with low GHG emissions, thus increasing the flexibility and improving the economics of distributed and semicentralized reforming.[75] Reforming of aqueous-phase-oxygenated hydrocarbons has interesting potential for renewable H2 production from organic molecules (glycerol, polyols, alcohols, sugars, organic acids) derived from agricultural and food-process industries (waste). The major challenge is the development of inexpensive and stable catalysts with high conversion efficiencies. Biomass gasification can utilize renewable feedstocks derived from agricultural and/ or forestry residues, but these processes also generate a variety of gaseous, and in some cases also liquid phase, co-products. Purification of the products still remains a challenge. Partial oxidation of supercritical water generates clean H2, but requires a significant energy input to attain the temperatures and pressures above a mixture’s thermodynamic critical point. Biological H2 (bio-H2) production by fermentation is attractive, but still the rates of H2 production are not enough for practical application. It is necessary to increase the rates and yields of H2 by optimization of bioreactor designs for rapid removal of H2 and CO2 to maintain low H2 partial pressures and to develop as well as improve genetic modification of H2-synthesizing microorganisms. Dark-fermentation processes produce a mixed biogas containing primarily H2 and CO2 and in lesser amounts CH4, CO, and/or hydrogen sulfide (H2S). Costly purification will thus be necessary. Photoelectrochemical (PEC) cells for solar H2 production[76] appear to be the potentially most promising solution, although data are still too limited for reliable assessments. Nevertheless, comparing the H2 productivity per h and m2 of irradiated surface area, PEC productivity is larger than that of CSP and bioH2, even if the quantum efficiency of the PEC devices is still largely lower than the target of 10 %. Quantum efficiency is the conventional parameter considered but not appropriate because sunlight cannot be considered a classical reactant in terms of costs. Productivity per unit cost and unit volume of the device (including depreciation) are the appropriate parameters to consider, but no data are typically reported on these aspects. It is thus necessary to unify the way in which experimental results are reported and analyzed in this growing area of devices using solar light. A number of strategies are under development to improve PEC performances of photoelectrode materials, including: (i) doping for enhanced visible-light absorption in a widebandgap semiconductor or promoting charge transport in a narrow-bandgap semiconductor, respectively; (ii) surface treatment for removing segregation phases or surface states; (iii) electrocatalysts for decreasing overpotentials; (iv) morphology control for enhancing light absorption and shortening transfer distance of minority carriers; (v) other methods, such as sensitization, passivating layer, and band-structure engineering using heterojunction structures, among others. Durability ChemSusChem 2014, 7, 1274 – 1282

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CHEMSUSCHEM CONCEPTS is another big challenge, which can be addressed by optimizing the photoelectrode, electrocatalyst, and electrolyte at the same time. Inverse fuel cells, which are able to convert CO2 to methanol or other chemicals, represent another medium-term challenge, although still scarcely addressed up to now. In a direct methanol fuel call (DMFC), methanol is converted to CO2 at the anode with production of protons and electrons that reduce O2 to water at the cathode. Electrical energy is produced. In an inverse methanol fuel cell the process is formally inverted, by feeding (renewable) electrical energy, water, and CO2 to produce methanol at the anode and O2 by water oxidation at the cathode. The type of electrocatalysts on both sides of the cell are completely different and have to be developed, even if there are common aspects in the cell engineering with DMFCs. By tuning the CO2 reduction electrocatalyst, different products from methanol can be formed. By substituting the electrocatalyst on the cathode side with a photoactive catalyst (an O2 evolution photocatalyst) and with a semiconductor element able to generate the photocurrent necessary to drive the reduction of CO2 using the proton generated on the cathode, the cell becomes an artificial-leaf-like device able to use sunlight to reduce CO2 to methanol (or other chemicals) in the presence of water. Different types of configurations for these solar-driven fuel cells are under development, which incorporates research on molecular and hybrid as well as solid catalysts for CO2 reduction and O2 evolution.[61–63, 77–79] There are many challenges to be addressed to realize with good efficiency this process, from challenges in electrocatalysts to understanding of the coupling of the fast processes of charge separation with the slower (at least three orders of magnitude) processes of catalytic reduction and charge transfer. There is thus the need to understand this chemistry and how material design can control these processes. Transport processes and reactions at the interface are often dominating the overall performances. Thus, the development of single components should proceed in parallel with device development, not separate as is currently the case. In addition, most of the current developments are not directly comparable and, therefore, a first challenge is to enable their comparison on solid grounds and using common evaluation parameters, such as cost effectiveness, performances (in terms of rate of solar fuel production under comparable conditions), durability and robustness, sustainability (including the use of critical raw materials), and scalability. The advanced materials and processes for energy application joint alliance (AMPEA) coordinates energy research with a goal for a low-carbon Europe[80] and is an example of the activities in this direction; by harnessing and integrating materials science and process innovation for high performance sustainable energy technologies the longterm competitiveness of the European industry is enhanced.

Conclusions The introduction of RE in the chemical production chain is a key strategic factor to realize a sustainable, resource-efficient,  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org low-carbon economy and society and to drive innovation and competiveness in chemical production. This paper discusses this concept in terms of motivations, perspectives, and impact as well as technical barriers to achieve this goal. Both the use of biomass (particularly waste) and reuse of CO2 are two key elements enabling a new scenario for the chemical production; however, we have focused the discussion herein on the latter because it is one of the most efficient methods to rapidly introduce renewable energy into the chemical production chain as well as to trade and import renewable energy from further away. It also demonstrates the role of chemistry to enable a sustainable future for renewable energy. Other aspects, and particularly an analysis of the state-of-theart on catalysts to enable this challenge have been discussed elsewhere;[39] the discussion herein focused on strategic aspects and motivations, with an analysis of also the role of politics and society and the impact for the future of chemical production. It is thus a medium- to long-term scenario. For this reason, some of the critical technical challenges and targets are discussed herein (with emphasis on catalysis because of its critical contribution) for the development of a roadmap to proceed in this direction and define new scenarios for chemical production in integration with other roadmaps, such as the cited SPIRE roadmap.[4] A book is in preparation to discuss in more detail the various aspects outlined here.[8] Keywords: carbon dioxide · chemical production · industrial chemistry · reusability · renewable energy [1] European Commission, A Resource-Efficient Europe—Flagship Initiative Under the Europe 2020 Strategy, COM(2011). Jan. 2011. Accessed on Aug. 16th, 2013: http://ec.europa.eu/resource-efficient-europe/pdf/resource_efficient_europe_en.pdf. [2] Sustainable Industrial Chemistry (Eds.: F. Cavani, G. Centi, S. Perathoner, F. Trifir), Wiley-VCH, Weinheim, Germany, 2009. [3] CEFIC (The European Chemical Industry Council), European Chemistry for Growth. Unlocking a Competitive, Low Carbon and Energy Efficient Future, April 2013. Accessed on Aug. 16th, 2013: http://www.cefic.org/ Documents/PolicyCentre/Energy-Roadmap-The%20Report-Europeanchemistry-for-growth.pdf. [4] SPIRE roadmap, July 2013. Accessed on Aug. 16th, 2013: http:// www.spire2030.eu/uploads/Modules/Documents/spire-roadmap_broch_ july2013_pbp.pdf. [5] G. Centi, Chim. Ind. (Milan, Italy) 2012, 94, 64. [6] S. Zinoviev, F. Mueller-Langer, P. Das, N. Bertero, P. Fornasiero, M. Kaltschmitt, G. Centi, S. Miertus, ChemSusChem 2010, 3, 1106. [7] X. Yan, O. R. Inderwildi, D. A. King, Energy Environ. Sci. 2010, 3, 190. [8] Green Energy and Resources for the Chemical Industry (Eds.: G. Centi, E. D’Hooghe, S. Perathoner), De Gruyter, Berlin 2014, in preparation. [9] M. Zhang, Y. Yu, Ind. Eng. Chem. Res. 2013, 52, 9505. [10] N. Bohmer, T. Roussiere, M. Kuba, S. A. Schunk, Comb. Chem. High Throughput Screening 2012, 15, 123. [11] R.-J. van Putten, J. C. van der Waal, E. de Jong, C. B. Rasrendra, H. J. Heeres, J. G. de Vries, Chem. Rev. 2013, 113, 1499. [12] Catalysis for Renewables: From Feedstock to Energy Production (Eds.: G. Centi, R. A. van Santen), Wiley-VCH, Weinheim, Germany, 2007. [13] L. Landeweerd, M. Surette, C. van Driel, Interface Focus 2011, 1, 189. [14] European Commission, Bio-based economy in Europe: State of play and future potential, May 2011. Accessed on Aug. 16th, 2013: http://ec.europa.eu/research/consultations/bioeconomy/bio-based-economy-foreurope-part1.pdf. [15] P. Lanzafame, G. Centi, S. Perathoner, Chem. Soc. Rev. 2014, DOI: 10.1039/c3cs60396b.

ChemSusChem 2014, 7, 1274 – 1282

1281

CHEMSUSCHEM CONCEPTS [16] a) United Nations Industrial Development Organization (UNIDO), Renewable Energy in Industrial Applications. An assessment of the 2050 potential, 2010. Accessed on Aug. 19th, 2013: http://www.unido.org/fileadmin/user_media/Services/Energy_and_Climate_Change/Energy_Efficiency/Renewables_%20Industrial_%20Applications.pdf; b) Renewable Energy Policy network for the 21st Century (REN21), Renewables 2013 Global Status Report, 2013. Accessed on Aug. 19th, 2013: http:// www.ren21.net/Portals/0/documents/Resources/GSR/2013/GSR2013_ highres.pdf. [17] J. P. S. Aniceto, I. Portugal, C. M. Silva, ChemSusChem 2012, 5, 1358. [18] P. Frenzel, S. Fayyaz, R. Hillerbrand, A. Pfennig, Chem. Eng. Technol. 2013, 36, 233. [19] J. A. Moulijn, I. V. Babich, ACS Symp. Series 2011, 1088, 65. [20] P. Y. Dapsens, C. Mondelli, J. Perez-Ramirez, ACS Catal. 2012, 2, 1487. [21] P. N. R. Vennestrøm, C. M. Osmundsen, C. H. Christensen, E. Taarning, Angew. Chem. Int. Ed. 2011, 50, 10502. [22] J. ten Dam, U. Hanefeld, ChemSusChem 2011, 4, 1017. [23] C. H. Christensen, J. Rass-Hansen, C. C. Marsden, E. Taarning, K. Egeblad, ChemSusChem 2008, 1, 283. [24] G. W. Huber, A. Corma, Angew. Chem. Int. Ed. 2007, 46, 7184. [25] A. Corma, S. Iborra, A. Velty, Chem. Rev. 2007, 107, 2411. [26] a) P. Gallezot, Chem.Soc. Rev. 2012, 41, 1538; b) P. Gallezot, ChemSusChem 2008, 1, 734. [27] G. Centi, P. Lanzafame, S. Perathoner, Catal. Today 2011, 167, 14. [28] International Energy Agency (IEA), Renewable Energy 2012. Medium-term Market Report, 2012. Accessed on Aug. 19th, 2013: https://www.iea.org/ publications/freepublications/publication/MTrenew2012_web.pdf. [29] World Energy Council, 2010 Survey of Energy Resources, 2010. Accessed on Aug. 19th, 2013: http://www.worldenergy.org/documents/ser_2010_ report_1.pdf. [30] a) A. Maeck, T. DelSontro, D. F. McGinnis, H. Fischer, S. Flury, M. Schmidt, P. Fietzek, A. Lorke, Environ. Sci. Technol. 2013, 47, 8130; b) E. G. Hertwich, Environ. Sci. Technol. 2013, 47, 9604; c) R. E. Grumbine, M. K. Pandit, Science 2013, 339, 36; d) J. Qiu, Science 2012, 336, 288. [31] DESERTEC Foundation web site. Accessed on Aug. 19th, 2013: http:// www.desertec.org/concept/. [32] T. Samad, S. Kiliccote, Comp. Chem. Eng. 2012, 47, 76. [33] Institute for Energy Research, Germany’s Green Energy Destabilizing Electric Grids Wind, Jan. 2013. Accessed on Nov. 17th, 2013: http:// www.instituteforenergyresearch.org/2013/01/23/germanys-greenenergy-destabilizing-electric-grids/. [34] R. Schlçgl, ChemSusChem 2010, 3, 209. [35] Chemical Energy Storage (Ed.: R. Schlçgl), De Gruyter, Berlin, 2012. [36] G. Centi, S. Perathoner, Greenhouse Gases Sci. Technol. 2011, 1, 21. [37] G. Centi, S. Perathoner, ChemSusChem 2010, 3, 195. [38] E. A. Quadrelli, G. Centi, J.-L. Duplan, S. Perathoner, ChemSusChem 2011, 4, 1194. [39] G. Centi, E. A. Quadrelli, S. Perathoner, Energy Environ. Sci. 2013, 6, 1711. [40] G. A. Olah, A. Goeppert, G. K. Prakash Surya, J. Org. Chem. 2009, 74, 487. [41] G. A. Olah, A. Goeppert, G. K. Surya Prakash, Beyond Oil and Gas: The Methanol Economy, 2nd ed., Wiley-VCH, Weinheim, Germany, 2009. [42] G. Centi, S. Perathoner, Catal. Today 2009, 148, 191. [43] American Chemistry Council, Shale Gas, Competitiveness, and New US Chemical Industry Investment: An Analysis Based on Announced Projects, May 2013, Accessed on Aug. 19th, 2013: http://chemistrytoenergy.com/ sites/chemistrytoenergy.com/files/shale-gas-full-study.pdf. [44] R. C. Valencia, The Future of the Chemical Industry by 2050, Wiley-VCH, Weinheim, 2013. [45] Allianz Global Investors, The “Green” Kondratieff, 2013. Accessed on Aug. 20th, 2013: https://www.allianzglobalinvestors.de/cms-out/kapitalmarktanalyse/docs/pdf-eng/analysis-and-trends-the-green-kondratieff.pdf. [46] J. Benner, M. van Lieshout, H.y Croezen, A long term view on the sustainable production of ammonia, olefins and aromatics in the European region, Publication code: 12.3581.16, CE Delft Pub: Delft, The Netherlands, Jan. 2012. Accessed on Aug. 20th, 2013: http://www.ce.nl/?go = home.downloadPub&id = 1221&file = CE_Delft_finalreport_ 1329899563.pdf. [47] G. Centi, G. Iaquaniello, S. Perathoner, ChemSusChem 2011, 4, 1265.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org [48] L. Barbato, G. Iaquaniello, A. Mangiapane in CO2 : A Valuable Source of Carbon (Eds.: M. De Falco, G. Iaquaniello, G. Centi), Green Energy and Technology series, Springer, Heidelberg 2013, Chap. 4, 67. [49] G. Centi, S. Perathoner in The Role of Catalysis for the Sustainable Production of Bio-fuels and Bio-chemicals (Eds.: M. Støcker, K. Triantafyllidis), Elsevier Science Pub., Amsterdam, 2013, Chap. 13, 529. [50] X.-B. Lu, W.-M. Ren, G.-P. Wu, Acc. Chem. Res. 2012, 45, 1721. [51] M. Peters, B. Kçhler, W. Kuckshinrichs, W. Leitner, P. Markewitz, T. E. Mller, ChemSusChem 2011, 4, 1216. [52] Carbon Dioxide as Chemical Feedstock (Ed.: M. Aresta), Wiley-VCH, Weinheim, Germany, 2010. [53] M. Aresta, A. Dibenedetto, Dalton Trans. 2007, 2975. [54] T. Sakakura, J.-C. Choi, H. Yasuda, Chem. Rev. 2007, 107, 2365. [55] European Commission, Roadmap for moving to a low-carbon economy in 2050, 2013, Accessed on Aug. 20th, 2013: http://ec.europa.eu/clima/ policies/roadmap/index_en.htm. [56] G. Centi, S. Perathoner, R. Passalacqua, C. Ampelli in Carbon-Neutral Fuels and Energy Carriers (Eds.: N. Z. Muradov, T. Vezirog˘lu), CRC Press, Boca Raton, FL, 2011, Chap. 4, 291. [57] F. Cherubini, N. D. Bird, A. Cowie, G. Jungmeier, B. Schlamadinger, S. Woess-Gallasch, Resour. Conserv. Recycl. 2009, 53, 434. [58] E. Johnson, Environ. Impact Assessment Rev. 2009, 29, 165. [59] Committee on Climate Change, Is bioenergy low-carbon?, Dec. 2011. Accessed on Aug. 20th, 2013: http://downloads.theccc.org.uk.s3. amazonaws.com/Bioenergy/1463%20CCC_Bio-TP1_low-carb_FINALwith%20BkMks.pdf. [60] Institute European Environmental Policy, The GHG emissions intensity of bioenergy. Does bioenergy have a role to play in reducing Europe’s GHG emissions?, Oct. 2012, Accessed on Aug. 20th, 2013: http://www.ieep.eu/assets/1008/IEEP_-_The_GHG_Emissions_ Intensity_of_Bioenergy_ -_October_2012.pdf. [61] S. Bensaid, G. Centi, E. Garrone, S. Perathoner, G. Saracco, ChemSusChem 2012, 5, 500. [62] J. Barber, P. D. Tran, J. R. Soc. Interface 2013, 10, 20120984. [63] D. G. Nocera, Acc. Chem. Res. 2012, 45, 767. [64] J. Farrell, Democratizing the Electricity System: A Vision for the 21 st Century Grid, Institute for Local Self Reliance Pub.: Washington DC, US, 6/ 2011. Accessed on Nov. 17th, 2013: http://www.ilsr.org/wp-content/uploads/2011/06/democratizing-electricity-system.pdf. [65] G. Centi, S. Perathoner in CO2 : A Valuable Source of Carbon (Eds.: M. De Falco, G. Iaquaniello, G. Centi), Green Energy and Technology series, Springer, Heidelberg, 2013, Chap. 9, 14. [66] M. Specht, J. Brellochs, V. Frick, B. Strmer, U. Zuberbhler, M. Sterner, G. Waldstein, Erdçl Erdgas Kohle 2010, 126, 342. [67] F. Jo, ChemSusChem 2008, 1, 805. [68] M. Grasemann, G. Laurenczy, Energy Environ. Sci. 2012, 5, 8171. [69] A. Steinfeld, Annu. Rev. Heat Transfer 2012, 15, 2555. [70] A. Nzihou, G. Flamant, B. Stanmore, Energy 2012, 42, 121. [71] S. Koumi Ngoh, D. Njomo, Renewable Sustainable Energy Rev. 2012, 16, 6782. [72] M. Pagliaro, A. G. Konstandopoulos, R. Ciriminna, G. Palmisano, Energy Env. Science 2010, 3, 279. [73] K. Sivula, Chimia 2013, 67, 155. [74] S. J. Burgess, B. Tamburic, F. Zemichael, K. Hellgardt, P. J. Nixon, Adv. Appl. Microbiol. 2011, 75, 71. [75] D. B. Levin, R. Chahine, Int. J. Hydrogen Energy 2010, 35, 4962. [76] Z. Li, W. Luo, M. g. Zhang, J. Feng, Z. Zou, Energy Environ. Sci. 2013, 6, 347. [77] A. Thapper, S. Styring, G. Saracco, A. W. Rutherford, B. Robert, A. Magnuson, W. Lubitz, A. Llobet, P. Kurz, A. Holzwarth, S. Fiechter, H. de Groot, S. Campagna, A. Braun, H. Bercegol, V. Artero, Green 2013, 3, 43. [78] A. J. Cowan, J. R. Durrant, Chem. Soc. Rev. 2013, 42, 2281. [79] Y. Tachibana, L. Vayssieres, J. R. Durrant, Nature Photonics 2012, 6, 511. [80] Advanced Materials and Processes for Energy Application (AMPEA), web site 2013. Accessed on Aug. 22th, 2013: http://www.eera-set.eu/index.php?index = 78. Received: August 29, 2013 Revised: November 16, 2013 Published online on March 5, 2014

ChemSusChem 2014, 7, 1274 – 1282

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