Industrial utilization of carbon dioxide (CO2)

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Industrial utilization of carbon dioxide (CO2) M. A r e s t a and A. D i b e n e d e t t o, University of Bari, Italy Abstract: The chapter presents the various aspects of the utilization of CO2 (technological, chemical, biotechnological) together with an analysis of the benefits derived from such practice. Conditions for correct use of CO2 are defined, and the potential of each technology is highlighted in terms of reducing emission into the atmosphere and lowering energy and/or material consumption, either directly (recycling of carbon) or indirectly, e.g. when the use of CO2 reduces the emission of products having a much higher climate change power (CCP) than CO2 itself. The potential utilization of CO2 as a tool to store excess or intermittent energies is also discussed, and the production of chemicals or energy products is presented, highlighting existing barriers to a full exploitation. The potential of enhanced fixation into aquatic biomass as a means of recyling CO2 and replacing fossil carbon in the production of chemicals or fuels for the transport sector is discussed. Emphasis is placed on the requirement for research into the potential for CO2 utilization to contribute to the reduction of its accumulation in the atmosphere. Key words: CO2 utilization, industrial-technological, biotechnological, eletrochemical, photoelectrochemical, chemicals, energy products, CO2 sources, the conditions for utilizing CO2.

14.1

Introduction

The utilization of CO2 for the synthesis of chemicals has its roots in the origins of the chemical industry, with applications such as the synthesis of soda Solvay (Na2CO3, 1861),1 salycilic acid (1869)2 and urea (1870),3 a process which is more than 140 years old now. CO2 has also long been used for the synthesis of pigments (Group 2 carbonates with the general formula MCO3) and specialty inorganic carbonates. Interestingly, all such applications are thermal reactions which do not need a catalyst. Almost a century went by between these pioneering applications and the development of the first catalytic industrial application of CO2 in 1972, namely the copolymerization of CO2 and olefins such as propene (1972).4 Interest in the chemistry of CO2 was sustained during the 1970s by the desire to understand how nature converts around 200 GtC/y in the carbon cycle. Several fundamental studies, aimed at the elucidation of the role of metals in the activation and conversion of CO2, were inspired by the discovery of the first CO2–transition metal complex, namely (PCy3)2NiCO2,5 and the demonstration that the co-ordination of CO2 to a metal centre was able 377 © Woodhead Publishing Limited, 2010

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to promote its reduction to CO under very mild conditions, instead of the harsh conditions required by the free molecule.6 Since then, the interaction of CO2 with several metal systems having different electron densities at the metal centre has been thoroughly investigated7 and several modes of coordination of CO2 have been demonstrated.8 At the same time, the use of CO2–metal complexes as catalysts in the functionalization of organic substrates was attempted and several new reactions were discovered.9 The use of transition metal complexes as electrocatalysts was also investigated10 and some interesting electrosyntheses of chemicals and pharmaceuticals were discovered10, 11a which are still of great interest today.11b Such great academic effort was not, however, followed by a real industrial interest, either because there was no apparent ‘easy’ use of CO2 or because the conditions for pushing towards a change in well-established production processes were not in place. This caused a decrease in academic interest in the chemistry of CO2 which survived in the late 1990s and early 2000s mainly due to the dedication of a few research groups around the world, a minority compared to the large number active during the 1970s and 1980s. One noteworthy development in the years from 1980 to 2000 was the appearance of a new aspect of CO2 utilization which could have a strong role in the future: the new technological applications of CO2, mainly of supercritical (sc) CO2. Sc-CO2 has been used profitably and successfully in a number of cases (dry-washing, extraction, fluid in circuits, solvent and reagent), showing a potential contribution to the reduction of the impact of chemicals on climate change. The recent increase in the price of oil and the new understanding of the need to reducing the climate change impact of the chemical and energy industry in general, and the emission of CO2 in particular, has sparked a renewed interest in issues such as the use of renewable sources of energy and alternative feedstock for the chemical industry, two issues that somehow merge into the enhanced industrial utilization of CO2 and its fixation in aquatic biomass. These are new areas of huge potential for C-recycling with reduction of CO2 emissions. Moreover, the idea of using CO2 conversion as a tool for energy storage is attracting much research attention. Overall, there is now a great deal of interest in assessing the potential for ‘CO2 utilization’, not only in chemical processes but, in general, as a tool for reducing the emissions that have a major impact on climate change.

14.2

The conditions for using carbon dioxide (CO2)

Utilization of CO2 with the aim of reducing its immission into the atmosphere (whether technological, biological or chemical) must meet three essential requirements:

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1. the new process based on CO2 must reduce the overall CO2 emission compared to the process currently on stream; 2. the new process must be safer and more eco-friendly than the old one; 3. the new process or application must be economically viable. These are absolute ‘musts’: there is no point in using CO2 in ways that increase its emission, are less healthy or more risky than current usage and which impact more negatively on the environment or involve a higher economic cost. In sum, the new process must be economically, energetically, environmentally and socially viable. We will now discuss a few examples in order to highlight some specific aspects of these concepts. The reduction of the overall CO2 emission in a given application of CO2 is not easily quantified. Such an objective means that both the energy and mass balance must be minimized in the new process, and this is not a trivial operation. Reducing energy implies control of several process parameters, such as temperature, pressure, energy use (quality and quantity) in general, and post-reaction operations, namely separation, isolation, purification, etc. Mass control requires more direct (fewer steps), effective (high yield) and selective (product entropy control) processes, with waste (gas, liquid, solid) minimized at source, and with lower loss of atoms and a higher degree of C-atoms utilization. The above considerations are valid whatever use one wishes to make of CO2. Implementing safer and eco-compatible processes is of key importance in any field of application. Avoiding toxic reagents and by-products will reduce the cost of processing while producing a lower environmental impact, in terms of end-of-pipe treatments, storage and disposal cost. CO2 is not a toxic substance, but it is important to remember that it may become an asphyxiating agent at concentrations above 10 %. Therefore, under controlled conditions of utilization, it does not give rise to serious concern as safe conditions are easily created and implemented. Usually there is no need to work at very high pressure, the highest (30–40 MPa) being used when sc-CO2 is used as solvent and/or reagent. The new process must be economically acceptable for exploitation. This aspect is also quite complex as the correct term of reference must be considered. The actual mode of carrying out an operation (chemical synthesis or any other industrial process) generates waste and, in general, an environmental loading that has given rise to a number of regulatory actions aimed at saving natural resources for future utilization. Comparison of methodologies or technologies must be performed taking into account the ‘global cost’ of the good or service, comprising not only the production cost (e.g., the cost of a chemical at the level of the production plant), but also all accessory costs linked to the production and utilization of that good or service. A life-cycle analysis (LCA) comparing all steps involved in the production–use–disposal

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of the good is necessary. Hence the real evaluation of a good or service is obtained by summing the contribution of the following operations: (extraction of primary materials from natural resources + treatment and disposal of produced waste) + (production of raw materials or intermediates + associated waste treatment and disposal) + (production of the good or service + associated waste treatment and disposal) + (utilization costs + associated waste treatment and disposal) + disposal of the useless good or service. It is most likely that the last two terms in the sum above will be the same whatever the origin of the product and the production process as they are associated with the use of the good under consideration. Conversely, the first three terms will strongly depend on the raw material used and the production route, on which also depends the presence of trace elements that can become important and play a key role in the use–disposal of the good. It is only by comparing the values of all the parameters listed above that a sound comparison of two technologies can be made, and this requires a Life Cycle Assessment (LCA). Therefore, when considering CO2, it is important to state the context in which we are locating the emission and to carry out a cost-analysis with respect to CO2 availability.

14.3

The carbon dioxide (CO2) sources and its value

CO2 can be obtained from several sources at different prices. Up to now, natural deposits (wells) have been exploited. The cost of extracting CO2 from the well is relatively low (15–20 Euros per tonne) and the purity can be very high (> 99 %), with toxic compounds absent. This means that such extracted CO2 is very useful, possibly also as a beverage additive or food preservative. CO2 extracted from natural wells has also been used for purposes which do not require a high purity of the gas: for example, in enhanced oil recovery (EOR). It would seem logical that the extraction of CO2 should be stopped and recovered CO2 (from power generation plants or industrial processes) should be used instead. However, such captured CO2 is characterized by a different degree of purity and may require deep purification operations prior to application in the food industry. CO2 recovered from power plant flue gases may be contaminated with SOx, NOy, non- or partially-combusted toxic chemicals; CO2 from chemical industrial plants may be accompanied by chemicals typical of the process in which it is generated; while CO2 from fermentation reactors may have the purity required for food applications. The purification steps will affect the cost of CO2. In principle, products characterized by different degrees of purity may have different applications. The market price of CO2 depends on the geographic area and can be very variable; as high as $400/t12 if food quality CO2 is requested. The utilization

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of CO2 recovered from fermentation reactors may be very convenient as its purity is very high and, in principle, contaminants should not be present. The drawback of such a source is the seasonality of raw materials, a barrier that can be circumvented by some means. Therefore, two scenarios for the use of CO2 are foreseeable: 1. CO2 is recovered from industrial and energy sources: this will imply that the energy, materials and economic costs of the recovery must be taken into account in assessing the cost and environmental impact of a new process based on CO2. 2. CO2 is available on the market because its capture from flue gases has been implemented on a large scale or new technologies provide concentrated sources of CO2 (IGCC): recovered CO2 is, thus, a waste product requiring disposal: its utilization produces a benefit both in terms of carbon recycling and added value of the derived products. The difference between the two scenarios above is the economic and energy cost of the recovery of CO2, and this is of key importance in a LCA study. Large-scale utilization of CO2 and large-scale recovery of CO2 can be seen as intrinsically linked. In this chapter, it will be assumed that large amounts of CO2 are available because CO2 separation has been implemented on a large scale. In this scenario, the utilization of CO2 has to be compared with its disposal: any one of the uses described below: (i) will produce an economic benefit compared to disposal which itself entails an economic cost, and (ii) may turn out to be energetically more convenient than disposal, the latter being in all cases an energy-consuming technology (because of the energy required for the liquefaction of CO2, transportation, housing).

14.4

Technological uses of carbon dioxide (CO2)

The use of CO2 as a technological fluid includes all those applications in which CO2 is not converted into other chemicals. A list of such uses is given in Table 14.113 together with the overall market. Although this option does not convert CO2 into storable or disposable materials, and it is usually the case that at the end of the application CO2 is vented (it is only rarely recovered and recycled), nevertheless the use of CO2 as technological fluid may contribute to the reduction in the impact on climate change. The benefit comes from the fact that CO2 is a substitute for other chemicals such as chlorofluorocarbons (CFC), which have much higher climate change power (CCP) (Table 14.2), or chemicals that require energy for their production or which produce waste with a strong environmental impact upon use. The former case refers, for example, to the use of CO2 as a fluid in air-conditioners, the latter to its use as a fumigant or for water treatment, among others.

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Developments and innovation in CCS technology Table 14.1 Technological uses of CO2: 20 Mt/y Technology

Application

Food industry Fumigant Antifire Mechanical industry Electronics Dry washing Fluid in circuits Water treatment Extraction

Additive to beverages Food packaging, dry-ice Extraction of aromas, caffeine ... Antibacterial agent for cereals Extinguishers Moulding, cutting, soldering Cleaning fluid Cleaning fluid Air conditioning pH control of process waters Enhanced oil recovery (EOR) Extraction of bio-oil from biomass

Table 14.2 Comparison of the climate change power of some CFC with that of CO2 (100 y) Chemical

CCP

Chemical

CCP

Carbon dioxide R134a R22

1 1430 1700

CFC-12 CFC-11 HCF-23

8500 4000 14 800

Let us now consider some practical applications to clarify the concepts above. (i) The production and use of CFC causes the emission into the atmosphere of such chemicals. The estimated amount of CCl2F2 (CFC-12, Table 14.2) released into the atmosphere ranged at the end of the 1970s from 420 (estimated by the Chemical Manufacturers Association–CMA) to 500 kt/y evaluated by direct atmosphere monitoring.14 The substitution of equivalent amounts of CO2 for such chemicals produces a great benefit in terms of CCP reduction, considering that the CCP of CCl2F2 is 8500 times that of CO2 (Table 14.2). (ii) Other typical cases are the substitution of (i) perchloroethene (C2Cl4, PERC, ca. 3 Mt/y) in dry cleaning or (ii) fluorinated cooling gases (e.g., R134) in fixed or mobile air conditioners (A/C). Supercritical CO2 (sc-CO2) is now finding several applications.15 It is largely used in cleaning machines15a as a substitute for PERC. PERC is a highly energy-intensive chemical and its synthesis is highly polluting because of the production of chlorinated waste. If we consider current practice to be a stationary state (with the use of PERC as dry-cleaning agent) and take into consideration only the replacement of PERC lost in running the existing equipment, the replacement of the annual loss of PERC16, 17 is equivalent to 2 Mt/y CO2: this is the minimum estimate for the reduction of CO2 emission as a result of substituting PERC with CO2

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as cleaning agent. Obviously, should we consider the total energy balance for making PERC and running the existing equipment together with the associated waste, then the balance is much more positive in favour of the substitution of PERC with CO2. Similarly, the use of CO2 as a fluid in automobile or fixed A/C would avoid the effect of 3.3 kt/y of lost R-134 (an average of 0.06 kg/y times 55 Mcars circulating), equivalent to 4.7 MtCO2eq. The IPCC estimate for the impact of emissions from A/C in buildings is 0.6 GtCeq or 2.2 GtCO2eq. Therefore, the use of CO2 as fluid in A/C would significantly contribute to reducing the overall CCP, a result that can contribute to a reduction in CO2 emmission into the atmosphere. The figures above are a demonstration of the direct and indirect benefits associated with the utilization of CO2, benefits that are often hidden and difficult to discover. (iii) The use of CO2 as a fumigant avoids the use either of other pharmaceuticals, which would have a complex molecular structure and would generate a lot of waste for their production, or of chemicals that are highly toxic, such as methylbromide, cyanidric acid, methylisocyanide, formaldehyde, sulphonylfluoride, etc.). The production of chemicals or pharmaceuticals usually has an associated waste production in the range 5–250 t of waste per tonne of product. This waste production is known as the E-factor of a given product (Table 14.3).18 Therefore, assuming that a compound used as an anti-parasite or anti-fungal has an associated organic waste production with an average composition equal to C4 and an E-factor of 70 (we consider a simple molecule), for each tonne of marketed product roughly 12.3 kt of CO2 will be emitted. It is clear that the use of CO2 instead of such a chemical (assuming that the two compounds have the same specific efficacy) is a much better solution and on that also substantially reduces the CO2 emmission into the atmosphere if CO2 is vented after use. (iv) When CO2 is used for basic water treatment, it is usually a substitute for sulphuric acid (H2SO4). The emission factor of H2SO4 is of the order of 5 kg SO2 and 0.3 kg SO3 per tH2SO4, while the entire process looks to be exoergonic (–1.1 GJ/t), assuming that: (i) all the released energy produced in the hydration of SO3 is recovered in the form of steam, and (ii) the use of sulphur obtained from the desulphurization of hydrocarbons is considered Table 14.3 The E-factor of several kinds of chemicals Industrial application

Market (t/y)

E-factor (twaste/tproduct)

Petrochemical Intermediates Fine chemicals Pharmaceuticals

109 > 106 105 104

0.1 0.5–1 5–100 100–250

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not to imply any energy cost.19 As CO2 has the same neutralization power as H2SO4, for each tonne of CO2 used 2.23 t of H2SO4 will be avoided together with the accompanying environmental impact caused by the sulphate accumulation in water plus the above-mentioned emission of SOx into the atmosphere. In the case that heat is not recovered (which may happen), and we take into account the energy cost of sulphur production, then the energy consumption must be considered, making the balance much more in favour of using CO2. (v) Supercritical CO2 (sc-CO2) exists above 31 °C and 7.38 MPa. Properties like density and viscosity can be modulated over a quite wide range by changing the two parameters, pressure and temperature. The ‘dense phase fluid’ has properties close to those of a non-polar organic solvent, such as pentane or dichloromethane. The benefits derived from its use are well known today,15b so that its utilization is spreading in various industrial sectors. Applications for the replacement of traditional organic solvents with sc-CO2 include: ∑ ∑

the decaffeination of coffee beans; the extraction of fragrances and essences from plants, or proteins or fatty acids and hydrocarbons from algae; ∑ the use as solvent for: reactions, crystallizations, preparation of solid thermal-sensitive pharmaceuticals having controlled size distribution; catalysis (homogeneous and heterogeneous); the synthesis and modification of polymers including perfluoropolymers, or as mobile phase for supercritical fluid chromatography (SFC), dyeing, dry-cleaning, nuclear waste treatment. A specific application that is attracting much interest is the use of sc-CO2 as solvent and reagent.

The most important feature attached to the use of sc-CO2 is that it can be easily recovered at the end of the process (by thermal decompression), recompressed and recycled. It is also worth mentioning that most waste organic solvents are usually burned: the substitution with sc-CO2 when possible will avoid large volumes of emitted CO2. In general, thus, CO2 in its technological applications substitutes species which have a strong impact either on the atmosphere or on water or soil: even if at the end of the application CO2 is vented to the atmosphere, the net result is the avoidance of substantial amounts of CO2-equivalents due to the elimination of chemicals having a high CCP, and this results in an overall mitigation of the impact on climate change.

14.5

Biological enhanced utilization

The enhanced biological utilization of CO2 encompasses the fixation of CO2 into biomass under conditions very different (industrial) from natural ones. Typical examples are: (i) the cultivation of terrestrial biomass in greenhouses

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under a CO2 concentration in the gas phase of ca. 600 ppm compared to the natural occurrence of 380 ppm; (ii) the culturing of aquatic biomass by sparging CO2 in water or under a gas phase concentration up to 150 times the natural one. It is usually only ornamental plants and vegetables that are grown in greenhouses because of the operating cost: energy crops and plants are cultivated in open areas under the atmospheric pressure of CO2. Therefore, aquatic biomass is more suitable than terrestrial for growing under high CO2 concentrations (industrial production). It should be noted that the need to decouple energy issues from both land use and food production is prompting a move away from the first-generation biofuels (or crop-derived biofuels) towards second-(use of cellulosic materials and lignine) and third-generation biofuels, including aquatic biomass. The exploitation of microalgae, macroalgae, plants or any other vegetal biomass growing in water is a strategy that may substantially contribute to the production of large volumes of biofuels and help to meet the target of 20 % substitution of transport fossil fuels with biofuels by 2020, which represents the target of several industrialized countries.20 Algae are better converters of solar energy (h = 6–8 % under natural conditions, up to 9–10 % in bioreactors) than superior plants (h = 1.5–2.2 %), and they also have a better potential for fuel production diversification. In fact, bio-oil and biodiesel, biogas, bioethanol and biohydrogen, can be produced, depending on the type of aquatic biomass used and its composition. Also, microalgae, macroalgae and plants have different compositions and can thus be used for different purposes. Table 14.4 shows the lipid accumulation capacity of two different kinds of aquatic biomass: microalgae and macroalgae. It is evident that microalgae are, in general, richer in lipids than macroalgae. Another point to consider when comparing terrestrial and aquatic biomass is the productivity per hectare. This could be quite an important factor, influencing the choice of which option to implement. Arable land is required for food production and should not be diverted to energy production. Aquatic biomass can be grown on marginal coastal areas, on desert lands close to salty water or offshore. This greatly increases its potential as source of fuels compared to terrestrial. Table 14.5 compares these two kinds of biomass, showing their different requirements. Table 14.4 Lipid accumulation capacity of some microalgae or macroalgae Species or strain Microalgae

Lipids % dry weight

Species or strain Macroalgae

Botryococcus braunii Nannoclhoropsis sp Schizodetrium sp Nitzschia sp

25–75 31–68 50–77 45–47

Codium harveyi   9–12 duthyae 12–21 fragile 21 Cladophora 12–20

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Table 14.5 Comparison of the properties of terrestrial and aquatic biomass Terrestrial biomass

Aquatic biomass

• Light efficiency 1.5–2.2 % • Requires land and water for growing • Productivity depends on soil quality (for a given plant) • Soil additives may be required (environmental and economic costs) • Biomass is generally rich in lignocellulosic components • Cereals and seed plants are mostly used • Open area more than greenhouse cultivation

• Light efficiency 6–8 % (or higher when irradiated bioreactors are used) • Richer in water • May not require land for cultivation (coastal ponds, offshore basins); can be grown in process and municipal waters • Low lignocellulose content. Lipid/ protein/polysaccharide content can be adjusted • Easy to grow in bioreactors (light– temperature adjustment); decoupling from climatic conditions

Table 14.6 Comparison of the land requirements for the production of bio-oil for different biomass Terrestrial crop

Bio-oil production (m3/ha)

Corn Soybean Canola Jatropha Coconut Oil palm

0.175 0.447 1.19 1.89 2.7 6.0

Aquatic biomass Microalgae Macroalgae

50–130 20–30

An issue linked to efficiency of light conversion is the productivity per hectare of biofuels. Table 14.6 compares the extent of land required for the production of a given quantity of biodiesel using different terrestrial or aquatic biomass. Also in this case, the use of aquatic biomass looks to be more profitable than the use of terrestrial seed-plants. Figure 14.1 shows a typical algae farm and the raceway technique of cultivating microalgae. The cultivation technique greatly depends on the selected biomass: microalgae require continuous stirring obtained with a paddle-wheel or by flowing the water medium through a succession of basins. Macroalgae can grow floating, either attached or not attached to a hard substrate. The collection or harvesting technique is quite different as well, as is the treatment of the biomass for oil extraction. Microalgae can also be grown in bioreactors installed in many different ways: flat, vertical, slanting, coiled. Solar light or artificial white light can be employed for irradiation.21

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14.1 An example of an algae farm.

As shown in Table 14.7, the content of lipids may vary over a large range for the same strain, depending on the culture conditions to which micro- and macroalgae usually adapt quite easily. The cellular composition may make a specific strain more or less suitable for the production of a kind of energy vector. Algae richer in lipids are better suited for the production of biodiesel, while algae richer in starch may be used for alcoholic fermentation to produce ethanol and a high protein content, and starch is ideal for biogas production. The adaptation to the growing conditions means that the organisms can be easily manipulated and their lipid content, as well as the protein or starch content, can be adjusted either by genetic modification or, more simply, by physical stress manipulation, i.e. by regulating the N or Ci content of the cultures. Another point of interest is that algal bio-oil rarely comprises a single type of fatty acid; more frequently, the lipid fraction of algae (both microand macroalgae) contains a large variety of fatty acids, as shown in Table 14.8. Nevertheless, as shown in Table 14.7, such distribution can be driven by controlling the CO2 concentration in the culture. Macroalgae (seaweeds) have so far attracted attention only as an agent for bioremediation and waste water treatment and, partly, as a source of chemicals. Their potential for energy production has been only marginally explored.21 As Table 14.4 shows, macroalgae have on average a lower lipid content than microalgae, the best performance reaching 20–30 % lipids. Nevertheless, the lower growing and harvesting costs make them very interesting compared to microalgae as a source for energy or chemicals. Both micro- and macroalgae are rich in chemicals that can be extracted using a series of different technologies, as shown in Table 14.9. Compounds that can be extracted from micro- and macroalgae are: ∑ ∑

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% CO2 in 14:0 16:0 16:1 18:0 18:1 18:2 20:0 20:4 20:5 the gas-phase

Total FAMEs

Control 0.038 Enhanced 10.0

29.1 ± 4.3 55.5 ± 3.7

5.2 ± 1.7 5.0 ± 1.1

9.4 ± 1.6 0.9 ± 0.2 16.2 ± 2.0 0.8 ± 0.1

0.5 ± 0.2 0.5 ± 0.3

5.9 ± 1.8 5.9 ± 1.2 0.2 ± 0.1 11.0 ± 1.6 19.2 ± 2.5 0.3 ± 0.1

0.5 ± 0.2 1.1 ± 0.2

0.6 ± 0.2 1.6 ± 0.4

Developments and innovation in CCS technology

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Table 14.7 Influence of the CO2 concentration on the distribution of fatty acids in Chaetomorpha l. cultured at ambient conditions and under a high concentration (10 % in the gas phase) of CO2

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Table 14.8 Distribution of fatty acids in lipids present in some macroalgae Fatty acid

Species and relative percentage of organic compounds

Number of carbon atoms/number of unsaturated bonds

Ulva lactuca

Enteromorpha compressa

Padiva pavonica

Laurencia obtuse

Saturated C12 Æ C20

15.0 %

19.6 %

23.4 %

30.15 %

Monounsaturated C14 Æ C20

18.7 %

12.3 %

25.8 %

9 %

Polyunsaturated C16/2 Æ C16/4 C18/2 Æ C18/4, C20/2

66.3 %

68.1 %

50.8 %

60.9 %

Table 14.9 Use of a cascade of technologies for a full use of biomass Very soft Soft Non-destructive Semi-destructive technologies technologies

Hard Destructive technologies

Extraction of molecules with a complex molecular structure; molecular and polymeric compounds for special applications

Breaking of natural complex structures and production of very simple chemicals (CO–H2) that can be used for making new complex molecular compounds (chemicals and fuels) again

Breaking of complex structures Production of energy products or simple chemicals: this is the case with lignine that can be used for the synthesis of phenolic compounds upon hydrolytic treatment.

∑ ∑

polymers (polysaccharides, starch, poly-beta-hydroxybutiric acid); peptides, toxins, aminoacids, steroids, essential oils such as geraniolgeranyl formate or acetate- cytronellol-nonanol-eucalyptol; ∑ pigments, such as chlorophylls, carotenoids, xantophylls; ∑ amines, inorganic compounds. The existence of significant differences in levels of entropy means that the extraction of a product is not always economically viable. However, the fact that algal organisms are able to produce concentrations of a type of substance when subjected to stress may help to reduce the entropy, increasing the concentration of a desired product in the biomass. It is, in fact, possible to grow selected types of algae for the production of, for example, astaxanthine or carotenoids, that are chemicals with high added value. Several of the substances listed above similarly have a high added value that makes their production using this approach economically viable. The implementation of the cascade of technologies allows the extraction, using the most appropriate technology, of substances with a complex structure in addition to the use of the biomass, for example, for the production of more simple molecules such as H2 and CO (Syngas).

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Table 14.10 shows the differences between macro- and microalgae. The former may afford a much larger productivity and require only very simple growing technologies. Their harvesting is also very simple and low-cost. All together, such positive aspects may compensate for the lower lipid productivity typical of macro- compared to microalgae. In summary, aquatic biomass represents a much larger variety of raw materials compared to fossil fuels and their potential needs to be fully exploited by defining the most appropriate transformation routes and the most suitable technologies. In order to make the most advantageous use of aquatic biomass it is necessary to integrate the existing expertise in the area of aquatic biomass cultivation with nanotechnologies, process intensification and the production of new nanosized materials in a single process. The biorefinery approach is now considered worldwide to be the most suitable for genuine exploitation of the potential of aquatic biomass. The way to evaluate the real energy or economic potential of aquatic biomass is the application of LCA 22 following Fig. 14.2 on page 391. LCA allows us to calculate the energy and substance-flow in the entire process and to establish the real potential of biomass for chemicals and energy production. All in all, aquatic biomass is an interesting source for chemicals and energy that requires accurate investigation in order to discover its full potential. It must nevertheless be emphasized that the fluctuation of the price of fossil carbon (coal, oil, gas) does not favour the implementation of the production of biodiesel from aquatic biomass. With the oil price below 120 US$/barrel it is not economic to produce biodiesel with such biotechnology. Should the concept of biorefinery enter into operation, it will be possible to develop an economically sustainable industry of fuels and chemicals production from aquatic biomass. Should this happen on a large scale, it can be foreseen that quite significant masses of CO2 will be recycled with a considerable reduction in its emmission into the atmosphere. Meeting the target of producing 20 % of transport diesel from biomass will, in fact, avoid the emission of roughly 1 Gt of CO2 per year. More on algae can be found in Chapter 15.

Table 14.10 Performance of micro- and macroalgae Parameters

Microalgae

Macroalgae

Growing season Productivity (dw) Lipid content Production cost Heat value (GJ t–1) Energy cost

250–280 d 33–50 t ha–1 20–75 % dw 100/5000 US$ t–1 dw 21 GJ t–1 dw 0.56 $ MJ–1

210–240 d 10–70 t ha–1 0.3–32 % dw 100 $ t–1 dw 12.2–20 GJ t–1 dw 0.05–0.6 $ MJ–1

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Industrial utilization of carbon dioxide (CO2) MWh Fuel

1 – Power plant

kg CO2

kg CO2 Separation and transport

2 – CO2 separation and transport

MJ

kg CO2

3 – CO2 distribution

MJ

4 – Algae production

MJ

5 – Conversion technology

MJ

6 – Net energy produced

MJ

kg CO2 Type of algae kg algae Nutrients

391

Conversion technology Type of algae kg algae Conversion technology kg CO2 kg algae Nutrients

14.2 Representation of the flow-chart for the LCA of the production of biofuels from biomass.

14.6

Carbon dioxide (CO2) conversion as ‘storage’ of excess electric energy or intermittent energies

One of the major problems with the production of electric energy is that technologies for its easy storage are lacking. Also, intermittent energies are often not used as it is neither practical nor convenient to convert them according to their availability. In both cases a solution could be represented by their conversion into chemical energy and then using such a form of energy when and where necessary. A form of chemical energy would be represented by reduced forms of CO2: methane or liquid hydrocarbons being the most suitable as they are already exploited in transportation media. Therefore, in this section the potential conversion of CO2 into energy-rich products by thermal or electrochemical or electrocatalyzed reactions will be discussed. The electrochemical or electrocatalytic reduction10, 11, 23–28 of CO2 to other C1 or Cn molecules (such as CO, CH3OH, CH4, C2H5OH, C2H4, Cnolefins, Cn-hydrocarbons, Cn-alcohols, etc.) as a way to store energy29 or its fixation into chemicals such as formic acid30 has long been investigated. Recent years have seen a revamping of such reactions because of the need to reduce the emmission of CO2 into the atmosphere: the use of excess electric energy for making energy products from CO2 would be exactly the same as using electric energy for pumping water uphill during the night for making

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electric energy during the day using the hydro-power of falling water, a practice used in hydro-power stations. The conversion of CO2 into energychemicals can be considered, thus, as a form of ‘storage of excess electric energy’ or else of ‘intermittent energies’, which are not usually exploited because they are not suited to continuous generation of energy as requested by most process uses. An analysis will be made of possible processes with the aim of answering the question: how close are we to the exploitation of such conversion of CO2?

14.6.1 Thermal processes Thermal energy must be generated in ways that minimize CO2 emission. Fossil fuel carbon cannot possibly be used to this end. High-temperature gases recovered from industrial processes such as stainless steel plants or from cement plants could represent a solution. The associated heat could be used to produce steam, used in turn to produce electricity or directly for running reactions. Direct use of concentrated solar energy would surely represent an elegant solution. For example, fused salts (obtained by using solar energy) would represent a valuable source of heat for running chemical reductions of CO2. A field of 1 ha of fused salts (Na/K nitrates) would produce a stable temperature of up to 823 K that is suited for running reactions such as those shown in Equation 14.1

CH4 + CO2 Æ 2H2 + 2CO

[14.1a]



C2H6 + 2CO2 Æ 3H2 + 4CO

[14.1b]

Alternatively, long-chain hydrocarbons, which do not find an application as energy vectors, can be reacted with CO2 to produce Syngas (Equation 14.2) used for the production of fuels:

CnH2n + 2 + nCO2 Æ 2nCO + (n + 1)H2

[14.2]

Equations 14.1b and 14.2 show that the longer the C-chain the lower the ratio H2/CO, and this is not good for the quality of the Syngas. In order to improve that ratio, the dry reforming of HC can be combined with wet reforming (Equation 14.3) or with the partial oxidation of methane (Equation 14.4)

CH4 + H2O Æ CO + 3H2

[14.3]



CH4 + 1/2O2 Æ CO + 2H2

[14.4]

This also improves the heat balance, being exothermic (see below). Such an approach may be useful to recycle HCs with a low or high number of carbons which currently have no use.

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14.6.2 Electrochemical conversion of CO2 One of the main issues with this approach is that, despite all efforts made so far, there is not yet an established procedure that selectively and efficiently creates a product. Table 14.11 lists the potential for multi-electron reduction of CO2. It is noticeable that the mono-electron reduction of CO2 in non-protic media to produce the radical-anion CO2– has a E° = –2.2 V (VHE), much higher than the multi-electron reduction in protic media. Although, in principle, any reduced form of CO2 would be of interest, nevertheless some species such as formic acid (easily and selectively prepared from CO2 and hydrogen under mild conditions30) and oxalate31 have little application as energy vectors and are of no interest from the energetic point of view. Although methane is abundant in nature and does not seem to be the first priority, nevertheless it is often formed as a product of total reduction of CO2. Therefore, it seems quite sound to confine the discussion to the following molecules: CO, methanol, ethanol (or other alcohols), olefins and HCn. Some general considerations are necessary before we look at each of the products short-listed above and describe the state-of-the-art and identify remaining barriers to their exploitation. It must be said that currently there is no selective and efficient means of forming any of the products above so a genuine breakthrough is highly desirable. The key parameters to be taken into consideration for the electrochemical reduction of CO2 are: ∑ the support solvent and the species reduced; ∑ the electrodes; ∑ the eventual use of electrocatalysts; ∑ the competing processes and the faradic efficiency. The support solvent Clearly, the electroreduction reaction is intended to take place in water and not in an organic solvent. This approach is necessary to reduce the cost of operation. In water, CO2 can exist as such or in the form of hydrogen Table 14.11 Electrode-potential (V vs SHE) for some multi-electron reductions of CO2 Reaction and product

E°

Eeq (pH = 7)

CO2 + 2H+ + 2e– Æ HCOOH CO2 + 2H+ + 2e– Æ CO + H2O CO2 + 4H+ + 4e– Æ CH2O CO2 + 6H+ + 6e– Æ CH3OH + H2O CO2 + 8H+ + 8e– Æ CH4 + 2H2O 2CO2 + 2H+ + 2e– Æ (COOH)2

–0.199 –0.103 –0.028 +0.031 +0.169 –0.49

–0.61 –0.52 –0.44 –0.38 –0.25 –0.90

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carbonate, HCO3– or carbonate, CO32–. There is experimental evidence which indicates that CO2 is reduced32 but not hydrogen carbonate or carbonate. This raises the problem of the solubility of CO2 in water33 and of equilibria which govern the existence of the various forms as a function of the pH of the solution, as depicted in Equations 14.5–14.7.

CO2 (g) + H2O Æ H2O·CO2 (l) HCO3–



H2O·CO2 (l) + H2O Æ



HCO3– + H2O Æ CO32– + H3O+

[14.5] +

+ H 3O

[14.6] [14.7]

The solubility of CO2 is represented by the fraction of ‘free’ CO2 and not by the other species (HCO3–, CO32–) into which CO2 may be converted by reaction with water. Table 14.12 shows the solubility of CO2 in several solvents. It is clear that water is not the best solvent at 0.1 MPa, due to its polarity. Table 14.12 also shows that the solubility increases with pressure. Therefore, one may conduct the electrolysis under pressure. Such an operation would help to increase the concentration of CO2 in the water solution, but would also increase the operating costs. This is a point that must be taken into consideration. The use of organic solvents would not be the best solution due to their market cost, the need to replace the loss (by oxidation, decomposition, evaporation) and pollution issues. Therefore, the development of efficient technologies is necessary in order to use water as solvent with high and selective conversion of CO2. The electrodes Several massive metals have been used as solid electrodes.34 The behaviour of the various metals has been seen to be in some way dependent on their Table 14.12 Solubility of CO2 in several solvents and the influence of pressure1 Solvent

Solubility, 293 K mL/mL at 0.1 MPa

Solubility, 293 K mL/mL at 2.0 MPa

Water Methanol E DMF Acetone B T

0.89 4.20 2.87 5.10 6.98 2.54 2.42

13 110 104.8

1

71.16 57.91

Solubility of Inorganic and Organic Compounds, H. Stephen and T. Stephen, Part 1 and 2, Pergamon Press, London, UK, 1963.

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electronic configuration, i.e. whether or not they use d-electrons (transition metals or sp-metals).35 The extended work done during the 1970s and 1980s has been summarized in reviews and papers.36–39 Zn, Au and Ag seem to drive the reaction towards the formation of CO, while Cu has good properties for the formation of HCs, alcohols and ethene. Recently, more sophisticated electrodes were used, such as polycrystalline materials,40, 41 supported metals (on polymeric substrates: 35, 42) or porous electrodes 43 or high-throughput (HT) gas diffusion electrodes.44 In general, the nature of the electrode strongly influences the formation of the reduced species as it drives the adsorption of species which can behave as intermediates. An issue to consider is that often the electrodes are consumed during operation, with an impact on operational cost and efficiency. The eventual use of electrocatalysts In order to improve the selectivity of the reduction process and to reduce the effect of electrode passivation which may increase the electrochemical reduction potential, electrocatalysts such as transition metal systems, either homogeneous or heterogeneous, have been used.45 In these systems, the interaction of the substrate is not with the electrode surface but with the catalyst in solution: therefore, the electrode transfers electrons to the substrate through the catalyst. A partial modification of this strategy is the deposition of the catalyst on the electrode: in this way, it is possible to use as cathode a material (carbon electrode) that alone would not be active in the electroreduction but, because it is covered with an active catalyst such as a metal porphyrin.46 results in a quite effective system. Co and Ni complexes have been effectively used as electrocatalysts. The use of suspended particles of a metal is also a technique used for improving the yield and selectivity: Cu has been particularly investigated for its attitude to act as catalyst when coupled with Zn and Pb electrodes.47 The competing processes and the faradic efficiency As the reduction of CO2 is carried out in water, one must expect that the reduction of the proton to dihydrogen can be a competing process that may reduce the selectivity and the yield of reduced CO2. Hydrogen can also play a role as an electrosorbed species Had that could address the formation of the reduced species of CO2. Had can drive the formation of formic acid, or the interaction of adsorbed species (HCOOad) with a product such as methanol to produce acetic acid (Equation 14.8)

HCOOad + CH3OH Æ CH3COOH + OHad

[14.8]

All the above factors influence the CO2 reduction: there is, in fact, not yet

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a process that presents a high efficiency and a high selectivity towards a single product. In the follow-up to this section, each of the above short-listed chemicals will be considered in detail. Carbon monoxide CO can be used for the production of energy by combustion with dioxygen even if the associated energy is quite modest. Table 14.13 shows the free energy of formation of several C1 species and their combustion heat. It is clear that burning CO affords only a limited amount of heat if compared to other vectors. Nevertheless, such species must be considered as it is easily formed under several working conditions, either alone (photocathode made of p-InP or p-GaAs48, Pt–Pd–Rh alloy49) or together with H2.50 In several other cases, CO is formed in more complex mixtures containing formic acid33, 41, 44f . Mixtures of CO and H2 are of interest as they can be either burned to produce thermal energy or used for the synthesis of HCn. Methanol, ethanol (or other Cn alcohols) Methanol and, more rarely, other Cn oxygenates are formed at variable concentrations under several working conditions. The products40, 43 must be separated from water, and the mixtures obtained are such that they still demand a large energy input for their fractionation so that their best use is the production of thermal energy. As reported above, the search for active catalytic electrodes for CO2 reduction has led to new discoveries that produce real breakthroughs in this area. For example, RuO2 deposited on conductive diamond (boron-doped diamond)51 is very active in such a reduction process.

Table 14.13 Free energy of formation and combustion heat of C1 molecules1 Compound

DG°f (kJ/mol)

DG°comb, (kJ/mol)

CO2g HCOOHl COg CH2Ol CH3OHl CH4g

–394 –361 –137 –102 –166 –51

–262 –257 –560 –714 –881

1

Handbook of Chemistry and Physics, 41st edn, CRC Press, Boca Raton, FL.

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Olefins Ethene is formed very frequently when Cu-cathodes are used,42, 52 more often in mixture with other species (such as H2 or methane) than as a single product. Nevertheless, it may reach a quite interesting concentration in the range 3253 to 80 %.42 Superior olefins have also been found in some specific conditions: they are found more frequently in thermal reductions of CO2 (see below) but are not common products of electrochemical reductions. Hydrocarbons, HCn Methane is a frequent product of the reduction of CO2. It can be the only product54a,b or it can be generated in mixture with other hydrocarbons,55 CO,32, 56, 57 ethene40, 51, 52, 58 or formic acid.44f, 56 More recently, evidence for the formation of long-chain HC has been reported, but this approach is still in its infancy.59

14.6.3 Photocatalytic reduction of CO2 The use of light harvesting systems able to generate a ‘hole+ –electron’ separation (Fig. 14.3) has been long investigated60, 61 as a tool for photocatalytic reduction of CO2. The barriers to exploitation were the low efficiency of the semiconductors used (h < 0.1 %) under solar light irradiation, the low selectivity of the catalytic system and the need to use sacrificial organic species for the oxidation. The latter makes no sense as often high-value organics (C3-alcohols, other more complex molecules) are oxidized for producing lower value organic molecules (methane, methanol, CO, etc.). Recently, a new interest in photocatalysis for CO2 reduction has developed, essentially because of the discovery of the properties of TiO2-derived catalysts62, 63 and of new transition metal systems.64, 65, 66, 67 The discovery and development of active supramolecular systems has given a strong impetus to such an

hn

CO2 Æ Reduced forms

h+

H2O Æ 1/2O2 + 2H+ + 2e–

e–

14.3 Use of semiconductors for CO2 reduction in water under solar light irradiation.

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approach.68, 69 Also, new heterogeneous species have shown an interesting activity in water, splitting into H2 and O2,70 a reaction that could be coupled with CO2 reduction. The barriers to overcome are still those listed above: use of solar light, use of water oxidation coupled to CO2 reduction, avoiding organic compounds oxidation. The new systems seem to have a better performance than old ones, and turnover numbers higher than 200 have been reported.68 Still, several improvements to the photocatalytic systems are required because they may find practical application, mainly in relation to the stability, selectivity, utilization with visible light and dioxygen generation from water as oxidized species. The photoelectrochemistry of CO2 is further discussed in Chapter 17.

14.7

Production of chemicals

The key issue is that the new process must be economically and energetically viable. This means that the energy used or/and the waste production must be lower for the CO2-based process compared to existing processes. It should be emphasized, to frame correctly the issue of CO2 utilization, that what is of real interest is not the amount of CO2 that can be used in chemical applications, but more exactly the contribution that the innovative technologies based on CO2 may make to the reduction of the CO2 emission. It is clear that any product made from CO2, after it is used, will release CO2. This makes the ‘storage’ potential of a number of chemicals made from CO2 is very low and not attractive, except for copolymers such as polycarbonates and polyurethanes which can store CO2 for decades. Therefore, the major contribution to CO2 emission control that the chemical use of CO2 can make comes from the development of innovative synthetic routes based on CO2 which may reduce the production of waste via more direct syntheses71 that use less energy and save carbon.

14.7.1 Synthesis of intermediates and fine chemicals The introduction of the carboxylic moiety ‘–COO’ into an organic substrate in a single step using a catalytic process is one of the most challenging reactions in synthetic chemistry. In principle, one can imagine inserting CO2 into a C–E bond, where E is another element such as H, C, O, N, etc. So far, only one process has been implemented at the industrial level, namely, the carboxylation of epoxides to yield molecular organic carbonates or, better, polymers (Fig. 14.4) that is, in effect, an insertion of CO2 into a C–O bond. Polycarbonates (and polyurethanes) have a market of a few Mt/y and are examples of long-living products derived from CO2. Such materials can thus be considered as examples of an economically viable chemical ‘storage’

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O H2N C NH2

ONa/K

HO

H3COH

COONa/K O

O

O

O

O

O

O O

O

A

HCOOH RNH2

n

O

D H2

CO COOH COOH H2C = CH2

O

O

O2

B

O

HOOC

COO R

Br C e –, H +

O O

N H

O

N H

H N 3

OR¢ C

O O

COOH

HOOC

O

RNH O

COOH

COOH

R

RC ∫ CR

RNH2 + R¢X

HCONHR CnH2n+2 CnH2n

O

O RO C OR

ROH

399

O OH

n

14.4 Some uses of CO2 in synthetic chemistry. Reactions framed in A are on stream, those in B are under advanced investigation: both do not require an energy input as the whole CO2 moiety is incorporated into the final product through an exoergonic reaction. C and D reactions demand energy.

of CO2. Polycarbonates can be obtained by reacting the epoxide and CO2 in the presence of the appropriate catalyst. Al–porphyrin complexes, 72 the first to be discovered, have recently been used in processes that are on stream. Several other metal-systems (Zn, Cr, Mo, Ru, etc.) compounds in a liquid73 or supercritical phase74 have been intensively investigated with the aim of developing more active and selective systems that may afford a regular alternate insertion of the two co-monomers, avoiding the preferential polymerization of the epoxide. Among molecular carbonates, dimethylcarbonate (DMC), diethylcarbonate (DEC), diallylcarbonate (DAC) and diphenylcarbonate (DPC) are the most interesting compounds. DMC has a market of ca 0.1 Mt/y and is used in several different applications in the chemical, pharmaceutical and electronic industries. Indeed, its total production is one order of magnitude larger and

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most of it does not reach the market as it is used for the preparation of polymers. A potential new use of DMC or DEC is as an additive to gasoline for which more than 30 Mt/y would be necessary. However, the actual synthetic methodology, which is based on phosgene, cannot be expanded to meet such a demand. This demand also cannot be met by the new processes based on the oxidative carbonylation of methanol (the ENIChem75 or UBE Process76) due to some drawbacks that limit the upscaling of the plants. The direct carboxylation of methanol (route d, Fig. 14.5) is now under investigation. Recent studies have shown that the reaction suffers thermodynamic limitations and requires the elimination of water to give acceptable conversion of the alcohol.77 As reported above, the availability of large volumes of recovered CO2 may sustain the development of new synthetic technologies based on it, assuming that efficient methodologies are developed that are somehow able to overcome the thermodynamic and kinetic issues related to the free energy of formation of CO2 (DG = –394 kJ/mol). Such a low free energy value could suggest, at a glance, that the conversion of CO2 may require a large energy input. In fact, the carboxylation reactions in which the entire –COO moiety is incorporated into a product (organic or inorganic) with an increase of the C/H ratio, are either exoergonic or are characterized by a very limited energy demand. In fact, CO2 reacts promptly at room or lower temperature with electron-rich species (such as olefins, dienes, amines, hydroxo groups, carbanions, etc.) to create chemicals that are currently produced through complex reaction pathways characterized by a high E-factor. The direct CO2 + 2NH3 e –H2O COCl2 + RR≤NH a

H2NC(O)NH2

RR≤NCOCl

+2ROH –2NH3

a¢ ROH CO2 + R¢R≤NH + RX

i –HX

O R¢R¢¢NC OR h ROH

g

f

b + R¢R≤NH – ROH +ROH –R¢R≤NH c

RO RO

C

O

CO2 d + –H2O 2ROH

R¢=H

R≤NCO

14.5 Synthesis of carbamates, carbonates and isocyanates based on CO2 and their inter-conversion through trans-esterification reactions.

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synthetic methodology based on CO2 may produce a reduction of the emission, because the new route is characterized by higher selectivity and yield.78 It is true, on the other hand, that processes that utilize CO2 may present high kinetic barriers so that, although they are exoergonic, in order to have an appreciable reaction rate it will be necessary to work at high temperature. An example is given by the conversion of silicates into carbonates (natural weathering of silicates), an exoergonic reaction with a very slow kinetics at room temperature: an appreciable reaction rate is shown close to 1000 K. Figure 14.5 shows an interesting network of reactions based on CO2 aimed at substituting phosgene (COCl2), a toxic species with a world market higher than 8 Mt/y, now banned in several countries. The synthesis of carbamates (route i) is of great interest, as carbamates are intermediates for the production of isocyanates (monomers for polymers) and carbonates. The utilization of urea (route f) is also of great interest for the synthesis of carbonates and urethanes.79, 80 The reaction of CO2 with olefins (Fig. 14.4) is of interest as it may be considered as a direct route to acrylates that are monomers for large market polymers (> 2 Mt/y). Despite recent achievements,81 several issues must still be addressed in order to develop the reaction to an application level. The reaction with dienes has been for long investigated82 and the reaction mechanism at Pd-centres (dimerization of butadiene followed by the insertion of CO2 and the release of the carboxylated product) is well known. Also, such knowledge has prompted some research groups to develop very selective syntheses of six-membered lactones used as fragrances: specific ancillary ligands have been identified that may control the entropy of product distribution.83 Such coupling of dienes with CO2 is interesting also because, by changing the metal centre (from Pd to Rh), the diene can be trimerized before CO2 insertion, addressing, thus, the production of long-chain carboxylic acids used as biodegradable emulsifiers or detergents (market of the order of 10 Mt/y) (Fig. 14.6). All the above reactions occur under mild conditions and are thus exoergonic. Remaining with the issue of producing organic acids from CO2, a process of great interest is the synthesis of formic acid (Fig. 14.4) via the direct interaction of CO2 and dihydrogen.84 Formic acid finds substantial use in the chemical industry and is of interest as hydrogen carrier as it can be easily decomposed back to H2 and CO2 by contact with a metal.84b Such direct synthesis is more eco-friendly than existing technologies.71 The use of CO2 as mild oxidant or dehydrogenating agent85 can be of great interest as it would produce hydrogen and useful olefins by converting long-chain hydrocarbons with a limited use. The direct carboxylation of saturated or unsaturated organic substrates is also a process of interest. Either radiations86 or active catalysts87 in ionic liquids are used. The direct functionalization of hydrocarbons requires the activation of C–H bonds: it

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Lx—Pd CO2

+3

Rh—Lx M = Rh

M=Pd (Ru, Ni)

C

Lx—Pd

Rh—Lx

O

O

O



O

O

O

O

O [MLx]

O C

O

CO2

O

O

O

O

C

O

O

O

A. Behr, Chem. Ber., 1984, 272, 29

O

Rh—Lx M. Aresta, New J. Chem, 1994, 18, 133

14.6 Coupling of butadiene with CO2 under the catalytic action of different metal systems. The correct choice of the ancillary ligands may control the entropy of product distribution.

is foreseeable that as knowledge in the area of C–H activation advances, so the synthesis of carboxylic acids based on the use of CO2 will progress, ultimately reaching the level of application.

14.7.2 Synthesis of energy products In principle, CO2 can be used as source of carbon for the synthesis of compounds such as alcohols or hydrocarbons (see Fig. 14.4) which are energy products and have a market much larger than chemicals. In fact, 7–10 % of the total extracted fossil carbon is converted into chemicals, the rest being used for the generation of various forms of energy (electric, thermal, mechanical). In the energy compounds, the C/H ratio decreases upon CO2 incorporation, meaning a parallel incorporation of hydrogen. This is the key issue: the production of energy products from CO2 demands hydrogen. Two questions follow. First, why convert CO2 and not simply use hydrogen; second, where or from what should the hydrogen be produced. The answer to the first question has been given in the previous section: hydrocarbons are a much better energy vector than hydrogen: they have the correct energy density and are easy to stock and transport. The answer to the second question is: the source of hydrogen cannot be fossil carbon (neither hydrocarbons nor coal), because CO2 would be generated in its production, defeating the object of the synthetic process. An external source of hydrogen should be used (water, biomass, residual

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hydrocarbons, currently not used) that decouples the CO2 recycling in the form of a fuel from fossil carbon extraction. Recently, methanol has attracted much attention (from both industry and the scientific world) for its potential use as fuel, intermediate or hydrogen vector (use in fuel-cells). Catalysts are available characterized by 100 % selectivity and high turnover frequency (TOF). A demonstration plant (50 kg/day) using a Cu–ZnO-based catalyst with 99.9 % selectivity during 8000 h operation at 523 K and 5 MPa was built in the late 1990s in Japan.88 The process was further improved by adding silica and Pd to the catalyst.89 Such technology represents an interesting improvement with respect to Syngas conversion.90 A real breakthrough would be represented by coupling the process with the production of hydrogen from non-fossil sources. This better performance of CO2 with respect to CO, despite the higher consumption of hydrogen (Equations 14.9 and 14.10), is due to a different reaction mechanism91 that implies the direct conversion of CO2 into methanol92 without involving the reverse water gas shift reaction and the preliminary conversion of CO2 into CO. It is interesting to note that in the industrial synthesis of methanol from Syngas, CO2 is used (up to 30 % as C), and this addition reduces both the energy consumption (thermal yield 66.5 compared to 64.3 % for Syngas alone) and the CO2 emission. This reduction is due to the fact that CO2 promotes a better utilization of H293, 94 and methane with improvement of both the overall energetic and product yield (50 compared to 42.3 %).95

CO + 2H2 Æ CH3OH



CO2 + 3H2 Æ CH3OH + H2O

[14.9] [14.10]

The dry reforming of methane (or other hydrocarbons) (Equation 14.11) produces Syngas (H2–CO) used for the synthesis of methanol or gasoline. Such technology might be of practical interest as it would enforce the gasto-fuel (GTF) conversion at the well extraction site, avoiding the methane separation from CO2 and LNG distribution, reducing losses. The CO2 dry reforming (Equation 14.11) is often coupled to the steam reforming (Equation 14.12), and to the partial oxidation of methane (Equation 14.13).

CH4 + CO2 Æ 2CO + 2H2

DH0298 = + 247 kJmol–1

[14.11]



CH4 + H2O Æ CO + 3H2

DH0298 = + 206 kJmol–1

[14.12]



CH4 + 1/2O2 Æ CO + 2H2 DH0298 = – 36 kJmol–1

[14.13]

The combination of these three reactions is known as ‘tri-reforming’.94 It produces a CO/H2 ratio equal to 1.7 that is good for methanol or higher hydrocarbons synthesis.96 A key issue here is coke formation when temperatures below 1000 K are used.97 New catalysts are under development that combine

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a high efficiency with coke-formation inhibition. The catalyzed cold plasma approach to methane dry reforming98 appears to be a promising and energyefficient technology used also for the direct synthesis of oxyfuels99 and, in fact, a pilot plant for GTF conversion under plasma conditions is being operated in Alberta (Canada).100

14.8

Conclusions and future trends

The utilization of CO2 is a promising technology that may contribute to reducing the accumulation of CO2 in the atmosphere. Several ways are open that may have different potentials for CO2 mitigation. Innovations in the field of synthetic industrial chemistry may bring about the discovery of cleaner production processes based on CO2 that may reduce the overall emission of CO2 with respect to processes on stream because of more selective and less energy- and carbon-intensive methodologies. These approaches need more research on catalysts and process development, while using new reaction media. In this area, the use of CO2 as reagent and solvent may provide a significant innovation with the development of new processes with low emission trends. The use of CO2 as a technological fluid is also very attractive. Moving from the study phase to the implementation of CO2 as a fluid in air conditioners and refrigerators and extending its use in dry cleaning and as an extraction fluid may enable a reduction in the current use of a number of chemical products with high CCP thus mitigating the impact on climate change. Searching for new technological applications of dense CO2 may be very useful, assuming that the new application based on CO2 will reducte the use of fluids with a much higher CCP. Enhanced fixation in aquatic biomass is a very interesting application that may result in the production of quasi-zero-emission biofuels that may supplant fossil fuels, especially in the transportation sector, a process very much to be wished for. In this area, algal strains with a good productivity are known; their culture–collection–treatment–extraction processes must be improved so that they can provide large volumes of biofuels. The application of the biorefinery concept can be a winning strategy for reducing the overall CO2 emission with concomitant economic benefit. The thermal or electrochemical conversion of CO2 into fuels is of great interest, especially when residual energies or intermittent perennial energies can be applied to this end. This area has a large potential not yet explored. New electrocatalysts are needed and more sophisticated technologies for the production of efficient semiconductors that may use solar light for fostering the CO2 reduction in water, developing a kind of artificial photosynthesis. In sum, the use of CO2 can help to reduce the impact on climate change either directly (reduced emmission into the atmosphere) or indirectly (less

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emission of chemicals having a CCP much higher than CO2). More investment in research is needed in order to discover new applications and new technologies that may increase the actual 140 Mt/year utilization to 300–400 Mt/year avoided CO2 that is the estimated potential of the utilization technology in the medium term. Research is also necessary for the correct assessment of the potential of utilization and of the reduction of CCP. LCA studies must be developed that allow the benefits to be certified as, at the end of the day, the use of CO2 is not per se a guarantee of environmental, energetic and economic convenience: this needs to be scientifically demonstrated!

14.9

Sources of further information and advice

Several books and reviews have been published on the topic discussed in this chapter. Most significant publications are cited in the text above and listed in the references section. Readers may also refer to IPCC Reports for a more general approach to the CO2 problem. However, several other aspects of CO2 capture and storage are discussed in other chapters of this book. We also wish to recall that the fixation into biomass is a topic discussed in Chapter 15.

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