Green Carbon Dioxide: Advances in CO2 Utilization

Preface

Mitigating climate change, preserving the environment, using renewable energy, and replacing fossil fuels are among the grand challenges facing our society that need new breakthrough solutions to be successfully addressed. The (re)use of carbon dioxide ( f01-math-0001 ) to produce fuels and chemicals is the common factor in these grand challenges as an effective solution to contribute to their realization. Reusing f01-math-0002 not only addresses the balance of f01-math-0003 in the Earth's atmosphere with the related negative effects on the quality of life and the environment, but represents a valuable C-source to substitute for fossil fuels. By using renewable energy sources for the conversion of f01-math-0004 , it is possible to introduce renewable energy into the production chain in a more efficient approach with respect to alternative possibilities. The products derived from the conversion of f01-math-0005 effectively integrate into the current energy and material infrastructure, thus allowing a smooth and sustainable transition to a new economy without the very large investments required to change infrastructure. As a longer-term visionary idea, it is possible to create a f01-math-0006 -economy in which it will be possible to achieve full-circle recycling of f01-math-0007 using renewable energy sources, analogous to how plants convert f01-math-0008 to sugar and f01-math-0009 , using sunlight as a source of energy through photosynthesis. Capture and conversion of f01-math-0010 to chemical feedstocks could thus provide a new route to a circular economy.

There is thus a new vision of f01-math-0011 at the industrial, societal, and scientific levels. Carbon dioxide is no longer considered a problem and even a waste to be reused, but a key element and driving factor for the sustainable future of the chemical industry. There are different routes by which f01-math-0012 can be converted to feedstocks for the chemical industry by the use of renewable energy sources, which also can be differentiated in terms of the timescale of their implementation. f01-math-0013 is a raw material for the production of base chemicals (such as light olefins), advanced materials (such as f01-math-0014 -based polymers), and fuels (often called solar fuels).

There are many opportunities and needs for fundamental R&D to realize this new f01-math-0015 economy, but it is necessary to have clear indications of the key problems to be addressed, the different possible alternative routes with their related pro/cons, and their impact on industry and society. The scope of this book is to provide to managers, engineers, and chemists, working at both R&D and decision-making levels, an overview of the status and perspectives of advanced routes for the utilization of f01-math-0016 . The book is also well-suited to prepare advanced teaching courses at the Masters or Ph.D. level, even though it is not a tutorial book. Over a thousand references provide the reader with a solid basis for deeper understanding of the topics discussed.

It is worthwhile to mention that this book reports perspectives from different countries around the world, from Europe to the US and Asia. f01-math-0017 is becoming, in fact, a primary topic of interest in all the countries of the world, although with different priorities, which are reflected here.

Chapter 1 introduces the topic with a perspective on producing solar fuels and chemicals from f01-math-0018 after having introduced the role of f01-math-0019 (re)use as an enabling element for a low-carbon economy and the efficient introduction of renewable energy into the production chain. Two examples are discussed in a more detail: (i) the production of light olefins from f01-math-0020 and (ii) the conversion of f01-math-0021 to fuels using sunlight. The final part discusses outlook for the development of artificial leaf-type solar cells, with an example of a first attempt at a photoelectrocatalytic (PEC) solar cell to go in this direction.

Chapter 2, after introducing some background aspects of f01-math-0022 characteristics and the photocatalytic chemistry on titania, focuses the discussion on the analysis of photo- and electrochemical pathways for f01-math-0023 conversion, discussing in detail the role of free radical-induced reactions related especially to the mechanism of methane (and other products) formation from f01-math-0024 during both photo- and electro-induced processes.

Chapter 3 also provides a critical analysis of the possible reduction pathways for synthesis of useful compounds from f01-math-0025 , with a focus especially on photo- and electrocatalytic routes. This chapter not only offers the readers a general overview of recent progress in the synthesis of useful compounds from f01-math-0026 but provides new insights in understanding the structure-component-activity relationships. It highlights how new nanostructured functional materials play an important role in photo- and electrocatalytic conversion of f01-math-0027 , with a series of examples showing how rather interesting results could be obtained by tuning the catalysts' characteristics.

Chapter 4 focuses the discussion on the analysis of the reaction mechanisms of heterogeneous catalytic hydrogenation of f01-math-0028 to produce products such as methane, methanol, and higher hydrocarbons. In f01-math-0029 methanation, f01-math-0030 is the key intermediate for methanation. In methanol synthesis, two possible pathways are discussed in detail: (i) direct hydrogenation of f01-math-0031 via formate and (ii) the reduction of f01-math-0032 to CO with subsequent hydrogenation to methanol. Depending upon the partial pressure of CO and f01-math-0033 , either the hydrogenation of f01-math-0034 species or the formation of f01-math-0035 can be rate-limiting for methanol formation. The mechanism of formation of higher alcohols may proceed through the reaction of CO insertion with hydrocarbon intermediates f01-math-0036 or through a direct nondissociative hydrogenation of f01-math-0037 . In the hydrogenation of f01-math-0038 through a modified Fischer–Tropsch synthesis (FTS) process, the different effects of carbon dioxide on Co- and Fe-based catalysts are analyzed, showing also how the nature of the catalyst itself changes, switching from CO to f01-math-0039 feed. This chapter thus gives valuable insights on how to design new catalysts for these reactions.

Chapter 5 analyzes in detail the recent developments in the metal oxide catalysts for the direct synthesis of organic carbonates such as dimethyl carbonate (DMC) from alcohol and f01-math-0040 . Ceria, zirconia, and related materials can catalyze the reaction with high selectivity under the conditions of the reaction without additives. Surface monodentate monoalkyl carbonate species are important intermediates. The yield is generally very low because of the equilibrium limitation. Combination of the reaction with organic dehydrating agents such as nitriles has been applied in order to overcome the equilibrium control. About 50% maximum methanol-based yield of DMC can be obtained when benzonitrile is used as a dehydrating agent. This chapter also analyzes future challenges for the design of catalysts and for the use of dehydrating agents to suppress the catalyst deactivation and the side reactions involving the dehydrating agents and the hydrated products.

Chapter 6 discusses in detail the theory and application of the STEP (solar thermal electrochemical production) process for the utilization of f01-math-0041 via electrosynthesis of energetic molecules at solar energy efficiency greater than any photovoltaic conversion efficiency. In STEP the efficient formation of metals, fuels, and chlorine and carbon capture is driven by solar thermal-heated endothermic electrolyses of concentrated reactants occurring at a voltage below that of the room temperature energy stored in the products. As one example, f01-math-0042 is reduced to either fuels or storable carbon at solar efficiency over 50% due to a synergy of efficient solar thermal absorption and electrochemical conversion at high temperature and reactant concentration. Other examples include STEP iron production, which prevents the emission of f01-math-0043 occurring in conventional iron production, STEP hydrogen via efficient solar water splitting, and STEP production of chlorine, sodium, and magnesium.

Chapter 7 analyzes the electrochemical reduction of f01-math-0044 in organic solvents used as the electrolyte medium, with a focus on understanding the effects of various parameters on electrolytic conversion of f01-math-0045 : Electrode materials, current density, potential, and temperature are examined, with methanol as electrolyte. A methanol-based electrolyte shows many advantages in the electrocatalytic reduction of f01-math-0046 over other aqueous and nonaqueous solvents. f01-math-0047 is completely miscible with methanol, and its solubility in methanol is five times higher than in water. The concentration of f01-math-0048 can be increased as liquid f01-math-0049 is made in a methanol electrolyte by increasing the electrolytic pressure. The faradaic efficiency of reduction products mainly depends on nature of the electrolyte. The strategy for achieving selective formation of hydrocarbons is also discussed.

Chapter 8 analyzes the conversion of f01-math-0050 to synthetic fuels via a thermochemical process, particularly the reforming of f01-math-0051 with hydrocarbons to form syngas. Aspects discussed include catalyst selection, possible operation, and potential application. In addition, research approaches for the conversion of syngas to methanol, DME, and alkane fuel (which is commonly known as gas-to-liquid or GTL) are also analyzed.

Chapter 9 discusses in detail the photocatalytic reduction of f01-math-0052 with water on f01-math-0053 -based nanocomposite photocatalysts. In particular, it is shown how the rate of f01-math-0054 conversion can be improved by several means: (i) incorporation of metal or metal ion species such as copper to enhance electron trapping and transfer to the catalyst surface; (ii) application of a large-surface-area support, such as mesoporous silica, to enhance better dispersion of f01-math-0055 nanoparticles and increase reactive surface sites; (iii) doping with nonmetal ions such as iodine in the lattice of f01-math-0056 to improve the visible light response and charge carrier separation; and (iv) pretreatment of the f01-math-0057 catalyst in a reducing environment like helium to create surface defects to enhance f01-math-0058 adsorption and activation. Combinations of these different strategies may result in synergistic effects and much higher f01-math-0059 conversion efficiency. The final section also provides recommendations for future studies.

Recent updates on the photocatalytic mechanism of f01-math-0060 reduction, with focus on novel carbon-based AgBr nanocomposites, are discussed in Chapter 10. Aspects analyzed include the efficiency of photocatalytic reduction of f01-math-0061 and stability under visible light ( f01-math-0062 420 nm). Carbon-based AgBr nanocomposites were successfully prepared by a deposition-precipitation method in the presence of cetyltrimethylammonium bromide (CTAB). The photocatalytic reduction of f01-math-0063 on carbon-based AgBr nanocomposites irradiated by visible light gives as main products methane, methanol, ethanol, and CO. The photocatalytic efficiency for f01-math-0064 reduction is compared with that of AgBr supported on different materials such as carbon materials, f01-math-0065 , and zeolites.

While Chapters 1–10 look mainly in a medium-long term R&D perspective, it is necessary to have practical solutions also for the short term, because the climate changes associated with the increase in greenhouse gas (GHG) emissions have already started to become an issue in several countries, with an intensification of extreme weather events. Chapter 11 thus is focused on a topic different from those discussed in the other chapters. It provides an analysis of the state of the art in enhanced oil recovery (EOR) and carbon capture and sequestration (CCS) and their role in providing a stable energy supply and reduction in f01-math-0066 emissions. EOR increases oil production by using f01-math-0067 , thus achieving both a stable energy supply and f01-math-0068 reduction simultaneously. In contrast, CCS reduces f01-math-0069 emissions even for non-oil producers. This chapter provides the background, fundamental mechanisms, and challenges associated with EOR and CCS, and shows that there are still several issues that need to be resolved, including recovery or storage efficiency, the cost of f01-math-0070 capture, transport, and injection, and the f01-math-0071 leakage risk. More research is required on fundamental mechanisms of the dynamics of EOR and CCS to allow significant improvements in the efficiency and safety of these techniques.

This book thus provides an overview on the topics of f01-math-0072 (re)use from different perspectives, with strong focus on aspects related to industrial perspectives, catalyst design, and reaction mechanisms. Most of the contributions are related to photo- and electrocatalytic conversion of f01-math-0073 , because these are considered the new directions for achieving a sustainable use of f01-math-0074 , and the basis for realizing over the long term artificial leaf-type (artificial photosynthesis) devices.

The editors are very grateful to all the authors for their authoritative participation in this book. A special thanks goes to Dr. Maria D. Salazar-Villalpando, formerly of the National Energy Technology Laboratory (NETL-DoE, US), who originally initiated this book, inviting all authors to contribute the different chapters.

The Editors

G. Centi and S. Perathoner

May, 2013

Acknowledgments

The Editors and all the authors contributing to the book wish to express their sincere thanks to Maria D. Salazar-Villalpando, who started this editorial project.

Contributors

Mudar Abou Asi School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, China

S. Assabumrungrat Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Thailand

Gabriele Centi Department of Electronic Engineering, Industrial Chemistry and Engineering, CASPE/INSTM and ERIC, University of Messina, Messina, Italy

Burtron H. Davis University of Kentucky, Center for Applied Energy Research, Lexington, Kentucky, USA

G.R. Dey Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India

K. Faungnawakij National Nanotechnology Center (NANOTEC), Pathumthani, Thailand

Muthu K. Gnanamani University of Kentucky, Center for Applied Energy Research, Lexington, Kentucky, USA

Chun He School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, China; Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou, China

Shuichiro Hirai Department of Mechanical and Control Engineering (Research Center for Carbon Recycling and Energy), Tokyo Institute of Technology, Tokyo, Japan

Masayoshi Honda Department of Applied Chemistry, School of Engineering, Tohoku University, Sendai, Miyagi, Japan

Boxun Hu Department of Chemistry, University of Connecticut, Storrs, CT

Yanling Huang School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, China

Gary Jacobs University of Kentucky, Center for Applied Energy Research, Lexington, Kentucky, USA

Satoshi Kaneco Department of Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu, Mie, Japan

Hideyuki Katsumata Department of Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu, Mie, Japan

M. Kumaravel Department of Chemistry and Applied Chemistry, PSG College of Technology, Peelamedu, Coimbatore, Tamilnadu, India

N. Laosiripojana The Joint Graduate School of Energy and Environment, King Mongkut's University of Technology Thonburi, Thailand

Ying Li Mechanical Engineering Department, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin

Stuart Licht Department of Chemistry, George Washington University, Washington, DC, USA

Wenping Ma University of Kentucky, Center for Applied Energy Research, Lexington, Kentucky, USA

M. Murugananthan Department of Chemistry and Applied Chemistry, PSG College of Technology, Peelamedu, Coimbatore, Tamilnadu, India

Yoshinao Nakagawa Department of Applied Chemistry, School of Engineering, Tohoku University, Sendai, Miyagi, Japan

V. R. Rao Pendyala University of Kentucky, Center for Applied Energy Research, Lexington, Kentucky, USA

Siglinda Perathoner Department of Electronic Engineering, Industrial Chemistry and Engineering, CASPE/INSTM and ERIC, University of Messina, Italy

Dong Shu Base of Production, Education & Research on Energy Storage and Power Battery of Guangdong Higher Education Institutes, School of Chemistry and Environment, South China Normal University, Guangzhou, China

Steven L. Suib Department of Chemistry, University of Connecticut, Connecticut, USA

Tohru Suzuki Environmental Preservation Center, Mie University, Tsu, Mie, Japan

Keiichi Tomishige Department of Applied Chemistry, School of Engineering, Tohoku University, Sendai, Miyagi, Japan

Shohji Tsushima Department of Mechanical and Control Engineering (Research Center for Carbon Recycling and Energy) Tokyo Institute of Technology, Tokyo, Japan

Suguru Uemura Department of Mechanical and Control Engineering (Research Center for Carbon Recycling and Energy) Tokyo Institute of Technology, Tokyo, Japan

Ya Xiong School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, China; Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou, China

Zuocheng Xu School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, China

Jingling Yang School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, China

Qiong Zhang School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, China

Linfei Zhu School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, China

Chapter 1

Perspectives and State of the Art in Producing Solar Fuels and Chemicals from CO2

Gabriele Centi and Siglinda Perathoner

1.1 Introduction

1.1.1 GHG Impact Values of Pathways of CO2 Chemical Recycling

1.1.2 CO2 Recycling and Energy Vectors

1.2 Solar Fuels and Chemicals From CO2

1.2.1 Routes for Converting CO2 to Fuels

1.2.2 H2 Production Using Renewable Energy

1.2.3 Converting CO2 to Base Chemicals

1.2.4 Routes to Solar Fuels

1.3 Toward Artificial Leaves

1.3.1 PEC Cells for CO2 Conversion

1.4 Conclusions

Acknowledgments

References

1.1 Introduction

The last United Nations Climate Change Conference (COP17/CMP7, Durban, Dec. 2011) and the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report under preparation [1], two of the actual reference points regarding the strategies for the reduction of greenhouse gas (GHG) emissions, are still dedicating minor attention to the question of reusing CO2. We discuss here how reusing CO2 is a key element in strategies for a sustainable development as well as a nonnegligible mitigation option for addressing the issue of climate change. There is a somewhat rigid separation between the discussion on the reduction of the emissions of GHG, based mainly on the introduction of renewable or alternative sources of energy and on the increase of efficiency in the use/production of energy, and the strategies for cutting current GHG emissions, based essentially only on the carbon capture and sequestration (CCS) option. The use of carbon dioxide as a valuable raw material is considered a minor/negligible contribution for the issue of climate change and thus not a priority to address.

The World Energy Outlook 2010 [2] report prepared by the International Energy Agency (IEA) has discussed different options and scenarios for GHG emissions, proposing a reduction of CO2 emissions in the 2.3–4.0 Gt·y-1 range within one decade (by the year 2021) and in the 10.8–15.4 Gt·y-1 range in two decades (by the year 2031) with respect to the business-as-usual scenario. About 20% of this reduction would derive from CCS. According to this estimation, about 400–800 Mt·y-1 of CO2 in a decade and about 2100–3000 Mt·y-1 of CO2 in two decades will be captured. The McKinsey report [3] estimated the global potential of CCS at 3.6 Gt·y-1 and the potential in Europe at 0.4 Gt·y-1—around 20% of the total European abatement potential in 2030.

With these large volumes of CO2 as raw material at zero or even negative cost (the reuse of CO2 avoids the costs of sequestration and transport, up to about 40–50% of the CCS cost, depending on the distance of the sequestration site from the place of emission of CO2) soon becoming available, there are clear opportunities for the utilization of CO2. In addition to direct use, many possibilities exist for its conversion to other chemicals, in addition to already-existing industrial processes.

A number of recent articles, reviews, and books have addressed the different options for converting CO2 [4–15]. Scientific and industrial initiatives toward the chemical utilization of CO2 have increased substantially over the last few years [6a], and there is increasing attention to the use of CO2 to produce

Advanced materials (for example, a pilot plant opened at Bayers Chempark in Leverkusen, Germany in February 2011 to produce high-quality plastics—polyurethanes— based on CO2 [16]);

Fine chemicals [5, 10, 11, 14] (for example, DNV is developing the large-scale electrochemical reduction of carbon dioxide to formate salts and formic acid [17]);

Fuels [6, 9, 15] (for example, Carbon Recycling International started in September 2011 in Svartsengi, Iceland, a plant for producing 5Mt·y-1 of methanol from CO2 using H2 produced electrolytically from renewable energy sources—geothermal, wind, etc. [18]).

The chemical transformation of CO2 is a dynamic field of research, in which many industrial initiatives also thrive, even if it is not always straightforward to grasp the real opportunities and limitations of each option. CO2 utilization as a raw material is expected in a short- to medium-term perspective to continue its progression, with several new products coming onto the market (e.g., polycarbonates). In the long term, CO2 recycling can become a key element of sustainable carbon-resource management in chemical and energy companies, combined with curbing consumption. CO2 can also become a strategic molecule for the progressive introduction of renewable energy resources into the chemical and energy chain, thus helping to slowly lessen our consumption of fossil fuels. Thus the prospects for large-scale utilization [6a] indicate that CO2 recycling can become an important component of the strategy portfolio necessary for curbing CO2 emissions (with an estimated potential impact of hundred millions of tons of CO2 recycling, similar to the impact of CCS) and at the heart of strategies for sustainable chemical, energy, and process industries, for a resource and energy efficiency development, for example, as a key enabling technology and backbone for the resource-efficient Europe flagship initiative of the Europe 2020 Strategy [19].

The concept of Green Carbon Dioxide [20], considering CO2 not a devil molecule (a problem or even a possible reuse of a waste) but a key element for sustainable strategies of energy and chemical companies, is thus the new emerging vision that we emphasize in this chapter, because the new strategies toward resource and energy efficiency development need to be advanced in both the industrial and scientific communities. The concept of solar fuels is a key part of this vision [21–26], but we will not limit the discussion on the state of the art and perspectives to this area, because CO2 recycling is an enabling element for a low-carbon economy and the efficient introduction of renewable energy in the production chain. This chapter thus analyzes these aspects and describes the opportunities offered by CO2 recycling and solar fuels in this more general vision and context.

1.1.1 GHG Impact Values of Pathways of CO2 Chemical Recycling

For a correct evaluation of the real impact of recycling CO2 via chemical conversion with respect to alternative options such as storage (CCS) or even direct use in applications such as enhanced oil recovery (EOR), which, however, can be applied only in specific locations, food use, and intensive agriculture (to enrich the atmosphere in greenhouses), etc. it is necessary to discuss the impact value of the chemical recycling of CO2.

The GHG impact value (GIV) indicates the effective amount of CO2 eliminated from contribution to the GHG effect (over a given time frame, for example, 20 years) on a life-cycle assessment (LCA) basis. For example, for CO2 storage (CCS) the energy necessary for the recovery, transport, and storage of CO2 must be calculated for each ton of stored CO2. GIV for CCS clearly depends on a number of factors, from the type and composition of the emissions, to the distance of the capture site from the storage site, the modalities of transport, etc. Detailed studies are not available, but on average, it is a realistic estimate that around 0.5–0.6 tons of CO2-equivalent energy is necessary for the capture, transport, and storage of 1 ton of CO2 sequestrated [21, 27]. In fact, capture with the amine-absorption technologies (the most used today) accounts for about 0.2 tons of CO2 and transport/storage accounts for an additional 0.3–0.4 tons of CO2 (per ton of CO2 sequestrated). These are average values, because in many places, such as in various areas of Europe, it is necessary to transport CO2 for over 150–200 km and pipelines are not available. The storage will be long term, and thus over a time frame of 20 years the average GIV for CCS will be around 0.4–0.5.

There are different options for the reuse of CO2. We may roughly distinguish two main routes for reusing CO2 to produce commercially valuable products, apart from the routes involving bacteria and microorganisms:

1. Those reactions incorporating the whole CO2 moiety in organic or inorganic backbones;

2. Those involving the rupture of one or more of the C–O bonds.

This classification is important in term of energy balance and applications. The first class of reactions (both organic and inorganic) is not energy intensive and sometimes may also occur spontaneously, although with low kinetics, as in the production of inorganic carbonates. The second class, reactions involving the cleavage of the C–O bond, is energy intensive and requires the use of reducing agents, typically H sources such as H2. For a CO2 resource- and energy-efficient management, the energy necessary for these reactions should derive from renewable (solar, wind, geothermal energy, etc.), or at least from non-carbon-based (nuclear energy) sources or, eventually, waste-energy sources.

There are two typical potentially large-scale examples for the first class of reactions [6a], the production of saleable precipitated carbonate and bicarbonates or carbonates from minerals and the production of polymers incorporating CO2 units. An example of the first case is the mineralization via aqueous precipitation (MAP) process developed by Calera [28] in a 10MW demonstration unit in Moss Landing, California (US) and followed by an Australian demonstration project in Latrobe Valley, Victoria (Australia). The flue gas from fossil fuel combustion is reacted with alkaline solutions heavy in calcium and/or magnesium, such as certain minable brines, to form a stable carbonate solid with a by-product of relatively fresh water that would be suitable for desalination. When suitable brines are not readily available, an alkaline solution of sodium hydroxide must be manufactured via, for example, chemical electrolysis. Once the CO2 has been absorbed into a bicarbonate solution, it can be stored underground or transformed into a carbonate (building material). A full LCA does not exist, and also in this case the exact value depends on the specific process characteristics (the alkalinity sources, for example) and use of the final product [29]. The production of the alkaline solution is the energy-intensive step, and energy estimation indicates a GIV value of about 0.6–0.8 [30] for the production of building materials (with thus a lifetime over 20 years). This average value is similar for the other CO2 mineralization technologies, where, for example, the critical energy-intensive step is the mining and crushing of the minerals (for example, olivine) used as the raw material.

For the production of CO2-based polymers, correct LCA assessments also do not exist, and the GIV value depends on specific process characteristics that have not yet been developed on a commercial scale, apart from Asahi Kasei's phosgene-free process to produce aromatic polycarbonate starting from ethylene epoxide, bisphenol-A, and CO2 [31]. Polypropylene and polyethylene carbonate as well as polyhydroxyalkanoate (PHA) from CO2 and linear epoxides are currently developed by Novomer on a pilot-plant scale [32], while Bayer is developing on a pilot scale a process for producing polyurethane from polyether polycarbonate polyols, as already cited [16]. These are the more relevant examples of CO2-based polymers, but additional examples also exist [6a].

The difference with respect to inorganic carbonates is that these polymers will substitute for polymers derived from fossil fuel sources, although it must be noted that the weight content of CO2 ranges from about 17%wt. for aromatic polycarbonate to about 30%wt. for polyurethane and about 50%wt. for polypropylene carbonate. In addition, the use of these polymers will bring further benefits. For example, polyurethane foams are one of the most efficient insulation materials on the market today for roof and wall insulation, insulated windows and doors, and air barrier sealants. Spray polyurethane foams can cut yearly energy costs upwards of 35% with respect to alternative insulation material, but their use is still limited by their cost. The expansion of their market through larger availability at low cost from production using CO2 as raw material will thus have direct benefits in terms of use of CO2 as raw material and reduction of the use of fossil fuels and indirect benefits in terms of energy saving. Polycarbonate multiwall structures also offer a real advantage in thermal insulation. As clear as glass, polycarbonate has superior characteristics as to energy savings, safety, and practicality for civil and industrial buildings, with the use of panels in structures that need reduced weight, abundant light, and high resistance to atmospheric agents. The indirect impact on energy saving is difficult to estimate, because it will depend on market development, incentives for energy saving, etc. It may be thus given only a very approximate estimation. We consider realistic a conservative average GIV value (the lifetime of these CO2-based polymers is over 20 years) of about 2–4.

Production of fuels from CO2, which involves the rupture of one or more of the C–O bonds and thus the need to supply renewable energy to make the conversion sustainable, is a different case. In fact, CO2 recycling via incorporating renewable energy introduces a shorter path (in terms of time) to close the carbon cycle compared to natural cycles and an effective way to introduce renewable energy sources in the chemical/energy chain. In addition, it will reduce the use of fossil fuels for these chemical/energy uses. Let us consider the simple case of methane production from CO2, although as discussed below this is not the ideal energy vector into which carbon dioxide can be transformed. However, it is the simplest example for considering the fuel life cycle and the related GIV. If we capture CO2 from the emissions deriving from the combustion of methane, we must spend energy in the capture (similar to the CCS case), but if we use renewable energy for the conversion of CO2 to methane (as discussed below), the net effect is that we introduce renewable energy into the energy chain. Considering that (i) 0.2 tons of CO2 are necessary for capturing each ton of CO2 (in terms of CO2-equivalent energy), (ii) 0.2 tons of CO2 are associated with the loss of energy in the conversion process, and (iii) 0.1–0.2 tons of CO2 are necessary to produce the renewable energy necessary for the conversion of CO2 to fuels, we have still a positive value of saving about 0.4–0.5 tons of CO2. We must note that the carbon footprints for the different renewable resources are different and on the average not negligible, but to simplify the discussion we consider only an average value. When an excess of energy is used in the transformation, for example, to store the excess of energy in the form of chemical fuel, the impact value could be even better. Each time that a cycle is completed (capture of CO2, conversion of CO2 to CH4, for example, by using H2 produced from a renewable energy sources—photovoltaic, wind, etc., storage/transport of methane, use of methane to produce energy and CO2), there is a saving of at least 0.4–0.5 tons of CO2 per amount of CO2 sequestrated. However, the cycle could be repeated several times. In a 20-year time frame, a single molecule of CO2 is recycled virtually several thousand times, with thus a continuous mechanism of reintroduction of renewable energy. From a practical aspect, the number of cycles will depend on the cost differential with respect to use of fossil fuels, incentives in limiting GHG emissions, carbon taxes, technology development, etc. It is thus quite difficult to estimate a correct GIV, but we consider a conservative average a GIV value of about 10–12 over 20 years. There is thus a large amplification of the impact value of chemical utilization of CO2 to produce polymers or fuels, with respect to the CCS or mineralization cases, even within the limits resulting from the absence of more specific studies.

Figure 1.1 summarizes the discussed average impact value on GHG for the different routes of chemical CO2 recycling with respect to CCS and CO2 mineralization. It may be noted that this concept is the opposite of that used in the IPCC report [33] which indicated that the lifetime of the chemicals produced is too short with respect to the scale of interest in CO2 storage. Therefore, the contribution of industrial uses of captured CO2 to the mitigation of climate change is expected to be small. This statement is not correct, because the chemicals/fuels produced from the conversion of CO2 and incorporation of renewable energy have an effective impact value for the reduction of GHG emissions at least one order of magnitude higher than that for CCS. Thus the effective potential of carbon capture and recycle (CCR) technologies in GHG control is at least similar to that of CCS technologies and estimated to be around 250–350 Mt·y-1 in the short to medium term [6a]. This amount represents about 10% of the total reduction required globally and is comparable to the expected impact of CCS technologies, but with additional benefit in terms of (i) fossil fuel savings, (ii) additional energy savings (e.g., the cited insulating effect of polyurethane foams), and (iii) acceleration of the introduction of renewable energy into the chemical production and energy chains.

Figure 1.1 Average indicative impact values estimated for the different routes of chemical CO2 recycling with respect to CCS and CO2 mineralization.

1.1.2 CO2 Recycling and Energy Vectors

Solar energy is abundant, accounting for over three orders of magnitude the current global consumption of primary energy, but it must be converted to electrical, thermo-mechanical, or chemical energy to be used. Energy must be also stored/transported to be used when and where it is necessary. Storage of energy in chemical form, that is, fuels, is still the most efficient way for storage and transport of energy, in terms of energy density and cost-effectiveness [21]. Energy density in a typical fuel is about two orders of magnitude larger than in batteries, and even with the possible future developments in batteries, it will be not possible to fill this gap. This is the main reason that (chemical) energy vectors will be still dominating the future energy scenario. Even in the blue-sky scenarios of the IEA [2], chemical energy will still have a predominant role with respect to other forms (electrical, etc.) in the year 2050. Thus our society is and still will be in the future largely based on the use of liquid hydrocarbons as the energy vector. Even today, with a very limited fraction of energy produced from renewable energy sources (solar, wind, etc.), it is not possible to fully use the renewable energy produced outside peak hours. When this fraction of renewable energy will exceed about 5–10%, it will become imperative to find efficient ways to convert electrical to chemical energy [34, 35].