2 Carbon Dioxide Reduction and Uses as a Chemical Feedstock Michele Aresta 1.1 Introduction The utilization of carbon dioxide(COz)as a source of carbon in synthetic chem istry has been a practice exploited at the industrial level since the second half of the 19th century for the synthesis of urea [1al and salicylic acid 1b.cl.CO,has also long been used for terest in the industrial utiliz ing ates and s A r arose after the essment of CO2 uti vely re d by several authors 2.A crit ation is also avar le 3. CO2 is ubiquitous- t can be either extracted pure from natural wells or re covered from various industrial sources.For instance,quite pure COa is recov ered from urea synthesis.Several other industries,such as those using fermen- tation or similar methods,also provide a convenient source of pure COz(above 99%)at low recovery cost,but their seasonality prevents full exploitation so tha several million tons per year(Mt year)of pure COz are vented.Today,there is a growing interest in recovering CO,from power station flue gases that contain around 14%of CO2.but the separation techniques are quite expensive [4]and are seldom applied on a large scale i51 In recent years.CO2 has also found growing application as a technological fluid in several industrial sectors.such as a cleaning fluid.in refrigeration.air and fire t a t for on solvent for n ticle sep as In t agro-chem an[6,7刀. n an 21 can t covered at the end of the application or vented to the atmosphere ost of the CO2 used for such applications is currently ex tracted from natural wells,yet it is highly desirable to substitute the extracted CO2 with that which is recovered from power stations or industrial processes. This would be in line with the need to reduce its emission into the atmosphere -a worrying accumulation since the beginning of the industrial era [8].How Bz2313125
Michele Aresta 1.1 Introduction The utilization of carbon dioxide (CO2) as a source of carbon in synthetic chemistry has been a practice exploited at the industrial level since the second half of the 19th century for the synthesis of urea [1 a] and salicylic acid [1 b, c]. CO2 has also long been used for making inorganic carbonates and pigments. A renewed interest in the industrial utilization of CO2 as a source of carbon arose after the 1973 oil crisis. The topic has been comprehensively reviewed by several authors [2]. A critical assessment of CO2 utilization is also available [3]. CO2 is ubiquitous – it can be either extracted pure from natural wells or recovered from various industrial sources. For instance, quite pure CO2 is recovered from urea synthesis. Several other industries, such as those using fermentation or similar methods, also provide a convenient source of pure CO2 (above 99%) at low recovery cost, but their seasonality prevents full exploitation so that several million tons per year (Mt year–1) of pure CO2 are vented. Today, there is a growing interest in recovering CO2 from power station flue gases that contain around 14% of CO2, but the separation techniques are quite expensive [4] and are seldom applied on a large scale [5]. In recent years, CO2 has also found growing application as a technological fluid in several industrial sectors, such as a cleaning fluid, in refrigeration, air conditioning and fire extinguishing equipment, as a solvent for reactions, as a solvent for nano-particle production, and in separation techniques and water treatment, as well as in the food and agro-chemical industries (packaging, additive to beverages, fumigant) [6, 7]. In all such technological applications, CO2 is not converted and can be recovered at the end of the application or vented to the atmosphere. Most of the CO2 used for such applications is currently extracted from natural wells, yet it is highly desirable to substitute the extracted CO2 with that which is recovered from power stations or industrial processes. This would be in line with the need to reduce its emission into the atmosphere – a worrying accumulation since the beginning of the industrial era [8]. How 1 Activation of Small Molecules. Edited by William B. Tolman Copyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 3-527-31312-5 1 Carbon Dioxide Reduction and Uses as a Chemical Feedstock
21 Carbon Dioxide Reduction and Uses as a Chemical Feedstock much CO is used in the chemical industry or other applications?Close to 110 Mtco,/y are either converted into chemicals [9]such as urea (70 Mtco, year).inorganic carbonates and pigments (around 30 Mtco,year)or used as additives to Co in the synthesis of methanol (6 Mtco,year-).Other chemicals such as salicylic acid (20 ktco,year)and propylene carbonate (a few kilotons per year)comprise a minor share of the market.In addition,18 Mtco,year are used 7]as technological fluids,and in the food and agro-chemical indus. tries (see above).Among industrial uses,the synthesis of urea and salicylic acid are purely thermal pro esses.the latter being influenced by the nature of the e original phenolate reacted with CO.Cor versely,both the oxides and,mor re impor rtantly.the synthesis of methanol ar driven by catalyst The developmer new catalytic not equ es the kn ledge of t properties of etal sys ly co and,thus,if it sho to redu its emission/a on into the atmosph uld ered as a technology for the control of global warming is under assessment [2e].As effective reduction of CO2 emission into the atmosphere requires the elimination of a few gigatons per year (1 Gt=10t).technologies for disposal in natural fields appear better suited [10].Nevertheless,such technologies are now seldom applied and are limited,in the best of cases.to megaton-scale geological disposal;others (e.g.ocean disposal)have yet to be demonstrated.At the mo ment.the best app oach to reducing co.emission into the atmosphere would be to make a selection of a number of technologies,each able to reduce the emission by a fraction of gigaton per year.According to this perspective,the uti- lization of over 130 Mtv ar-1 of CO2 c ould well re ent a technology that could significantly contribute to the reduction of atm spheric loading ng the at its so ustainabl by reey p a better use of energy and car sused in chemical o fraction not repre sented only by the amount of fixed CO2 one must consider the whole reaction cycle based on the emission [11.Life cycle assessment (LCA)is the only meth odology that can give an answer to such a question [12]. In the following sections.the CO,molecule will be considered.Its interaction with metal centers and the conversion paths that already find utilization or may find industrial exploitation in the near future are discussed,with emphasis on the most useful processes
much CO2 is used in the chemical industry or other applications? Close to 110 MtCO2 /y are either converted into chemicals [9] such as urea (70 MtCO2 year–1), inorganic carbonates and pigments (around 30 MtCO2 year–1) or used as additives to CO in the synthesis of methanol (6 MtCO2 year–1). Other chemicals such as salicylic acid (20 ktCO2 year–1) and propylene carbonate (a few kilotons per year) comprise a minor share of the market. In addition, 18 MtCO2 year–1 are used [7] as technological fluids, and in the food and agro-chemical industries (see above). Among industrial uses, the synthesis of urea and salicylic acid are purely thermal processes, the latter being influenced by the nature of the Group 1 cation of the original phenolate reacted with CO2. Conversely, both the carboxylation of epoxides and, more importantly, the synthesis of methanol are driven by catalysts, mainly metal systems. The development of new catalytic conversions of CO2 requires the knowledge of the properties of metal systems. Whether or not the utilization of CO2 can effectively contribute to reducing its emission/accumulation into the atmosphere and, thus, if it should be considered as a technology for the control of global warming is under assessment [2 e]. As effective reduction of CO2 emission into the atmosphere requires the elimination of a few gigatons per year (1 Gt = 109 t), technologies for disposal in natural fields appear better suited [10]. Nevertheless, such technologies are now seldom applied and are limited, in the best of cases, to megaton-scale geological disposal; others (e.g. ocean disposal) have yet to be demonstrated. At the moment, the best approach to reducing CO2 emission into the atmosphere would be to make a selection of a number of technologies, each able to reduce the emission by a fraction of gigaton per year. According to this perspective, the utilization of over 130 Mt year–1 of CO2 could well represent a technology that could significantly contribute to the reduction of atmospheric loading by recycling carbon and reducing the emission at its source. Such sustainable production technologies would reduce waste and make a better use of energy and carbon. The evaluation of how much CO2 is avoided when it is used in chemical or technological processes is not a simple task. The avoided fraction is not represented only by the amount of fixed CO2 – one must consider the whole reaction cycle based on the emission [11]. Life cycle assessment (LCA) is the only methodology that can give an answer to such a question [12]. In the following sections, the CO2 molecule will be considered. Its interaction with metal centers and the conversion paths that already find utilization or may find industrial exploitation in the near future are discussed, with emphasis on the most useful processes. 2 1 Carbon Dioxide Reduction and Uses as a Chemical Feedstock
1.2 Properties of the CO2 Molecule 121 Molecular Geometry In its ground state,linear and belongs point This makes the onpolar,although it the vectors associated with charge separation in the C-O bonds are equal in intensity and opposite in direction (Scheme 1.1). 0-C-046c8=-040=c6s-4+80=C-0*66.C=0 Scheme 1.1 Polarity of the CO2 molecule in its ground state. Nevertheless,CO2 maintains all the characteristics of a species containing po- lar bonds,with two sites that behave quite differently.The carbon atom is elec- trophilic,while the oxygen atoms are nucleophilic.Consequently,CO2 often re- quires bifunctional catalysis [13]for its activation or conversion.It is noteworthy that the electrophilicity of carbon is higher than the nucleophilicity of each of the oxygen atoms,so CO2 prevalently behaves as an electrophile. The Walsh diagram [14](Fig.1.1)shows the energy of the molecular orbitals in the ground and excited states.Any distortion of the molecule from linearity causes the variation of the molecular energy and C-O bond length,due to the repulsive i eraction of tha ording to the plan which the .L ling of the w日y any ex 0 molecule or int causes the population o (LUMO)will caus a1 Co. equently,electronicall excited COz the radi al ar 1C0 or the adduct of Co with an electron-rich specics.such as B-CO.will have a bent ge ometry with the O-C-O angle close to 133,a value that minimizes the electron repulsion and the molecular energy.This is clearly shown by solid-state structur- al determinations-in all forms in which the carbon atom of CO2 is bonded to a third atom,the O-C-O angle is close to 133. 1.2.2.1 Vibrational Table 1.1 shows infrared (IR)and Raman data [15]of gaseous and solid COz Due to its nonpolar character,in the ground state the symmetric stretching of the C=O bond is not IR active.In the Raman spectrum this vibration is found at 1285-1388 cm.The IR properties of the molecule are used for many pur. poses,including the quantification of the amount of COz in the atmosphere(by
1.2 Properties of the CO2 Molecule 1.2.1 Molecular Geometry In its ground state, CO2, a 16e– molecule, is linear and belongs to the Dh point group. This makes the molecule nonpolar, although it contains two polar C–O bonds; the vectors associated with charge separation in the C–O bonds are equal in intensity and opposite in direction (Scheme 1.1). Nevertheless, CO2 maintains all the characteristics of a species containing polar bonds, with two sites that behave quite differently. The carbon atom is electrophilic, while the oxygen atoms are nucleophilic. Consequently, CO2 often requires bifunctional catalysis [13] for its activation or conversion. It is noteworthy that the electrophilicity of carbon is higher than the nucleophilicity of each of the oxygen atoms, so CO2 prevalently behaves as an electrophile. The Walsh diagram [14] (Fig. 1.1) shows the energy of the molecular orbitals in the ground and excited states. Any distortion of the molecule from linearity causes the variation of the molecular energy and C–O bond length, due to the repulsive interactions generated among electrons. Figure 1.1 also shows that the energy of the molecular orbitals varies according to the plane in which the bending of the molecule occurs. In a similar way, any excitation of the molecule or interaction with electron donors that causes the population of the lowest unoccupied molecular orbital (LUMO) will also cause a distortion of CO2 from linearity. Consequently, electronically excited CO2, the radical anion CO2 – or the adduct of CO2 with an electron-rich species, such as B-CO2, will have a bent geometry with the O–C–O angle close to 133, a value that minimizes the electron repulsion and the molecular energy. This is clearly shown by solid-state structural determinations – in all forms in which the carbon atom of CO2 is bonded to a third atom, the O–C–O angle is close to 133. 1.2.2 Spectroscopic Properties 1.2.2.1 Vibrational Table 1.1 shows infrared (IR) and Raman data [15] of gaseous and solid CO2. Due to its nonpolar character, in the ground state the symmetric stretching of the C=O bond is not IR active. In the Raman spectrum this vibration is found at 1285–1388 cm–1. The IR properties of the molecule are used for many purposes, including the quantification of the amount of CO2 in the atmosphere (by 1.2 Properties of the CO2 Molecule 3 Scheme 1.1 Polarity of the CO2 molecule in its ground state.
1Carbon Dioxide Reduction and UsessChemical Feedstock ev Orbitals 8 homo 8-8 4, 2b 363, 90°120°150°180° Czv o-c-0 angle Dh Fig.1.1 Walsh diagram for COa:the energy of the MOs changes with the molecular geometry Table 1.1 IR and Raman data for CO2 Symco Bending Asymeo Gaseous 1285-1388(Raman) 667 2349 Aqueous solution 2342 Solid 660.653 2344 nondispersive IR).and as a diagnostic tool for identifying CO2 and its mode of bonding in metal systems. 1.2.2.2UV.Vis The UV-Vis spectrum [16]of gaseous CO2 presents absorption bands of various intensities in the range 1700-3000A.The UV-Vis spectrum has not been used as extensively as the IR. 1.3C-Nuclear Magnetic Resonance (NMR) Co2 dissolved in nance at 16 ppm solve clos t012 5 ppm [17]
nondispersive IR), and as a diagnostic tool for identifying CO2 and its mode of bonding in metal systems. 1.2.2.2 UV-Vis The UV-Vis spectrum [16] of gaseous CO2 presents absorption bands of various intensities in the range 1700–3000 Å. The UV-Vis spectrum has not been used as extensively as the IR. 1.2.2.3 13C-Nuclear Magnetic Resonance (NMR) CO2 dissolved in a nonpolar solvent such as benzene or toluene shows a resonance at 126 ppm. In aqueous solutions the resonance is close to 125 ppm [17] 4 1 Carbon Dioxide Reduction and Uses as a Chemical Feedstock Fig. 1.1 Walsh diagram for CO2: the energy of the MOs changes with the molecular geometry. Table 1.1 IR and Raman data for CO2 SymC=O Bending AsymC=O Gaseous 1285–1388 (Raman) 667 2349 Aqueous solution 2342 Solid 660, 653 2344
and can be used for the quantification of free CO2.The C-NMR resonance is often used as a diagnostic tool for identifying the CO2 moiety in a compound. 12 3 Energy Data and reaction kinetics relevant to co conversion Co. is,with vater the ther odynamic end-p duct of the n of mate rials bon and hyd In fact,CO2 is the most ther stable of all ca arho bin neutral a contain the 0 T stability of CO has g ed the will render the use of CO2 for the synthesis of chemicals inconvenient.The "in ertness"of CO2 is important with respect to oxidants such as O2:indeed.CO behaves as a great combustion regulator or suppressor.Conversely,there are a number of reactions in which there is no need for an external energy supply. because the coreagent brings enough energy for the reaction with CO2 to occur at room temperature or lower (e.g.the reaction of CO2 with hydroxide,amines or olefins).It is important to distinguish the thermodynamic from the kinetic aspects of reactions involving CO,.In fact.quite exergonic reactions.such as Table 1.2 Energy of formation of some chemicals relevant to CO chemistry Compound AH?(k]mol-) AG?(k]mol-') s￥calK- co (g) -110.53 -1372 -393.5 -394.4 Coz() -386 HCO (aq) -689.93 -603.3 38.1 -285.8 H,0 18 _1307 -11292 -219.1 -204.9 1115 18.8 -361.4 12 HCOOH (g) -378.4 -50.3 Co, -333.6 CH:OH ( -239.1 -166.6 126.8 CHOH (g) -201.5 -162.6 239.8
and can be used for the quantification of free CO2. The 13C-NMR resonance is often used as a diagnostic tool for identifying the CO2 moiety in a compound. 1.2.3 Energy Data and Reaction Kinetics Relevant to CO2 Conversion CO2 is, with water, the thermodynamic end-product of the combustion of materials containing carbon and hydrogen. In fact, CO2 is the most thermodynamically stable of all carbon-containing binary neutral species (Table 1.2). Carbonates, both organic and inorganic, that contain the “CO3” moiety are even more stable than CO2. The stability of CO2 has generated the common belief that it is “nonreactive” and that any transformation of it will require an energy input that will render the use of CO2 for the synthesis of chemicals inconvenient. The “inertness” of CO2 is important with respect to oxidants such as O2; indeed, CO2 behaves as a great combustion regulator or suppressor. Conversely, there are a number of reactions in which there is no need for an external energy supply, because the coreagent brings enough energy for the reaction with CO2 to occur at room temperature or lower (e.g. the reaction of CO2 with hydroxide, amines or olefins). It is important to distinguish the thermodynamic from the kinetic aspects of reactions involving CO2. In fact, quite exergonic reactions, such as 1.2 Properties of the CO2 Molecule 5 Table 1.2 Energy of formation of some chemicals relevant to CO2 chemistry [18] Compound Hf 0 (kJ mol–1) Gf 0 (kJ mol–1) Sf 0 (cal K–1) CO (g) –110.53 –137.2 197.7 CO2 (g) –393.51 –394.4 213.8 CO2 (l) –386 CO2 (aq) –413.26 CO3 2– (aq) –675.23 CaO (s) –634.92 HCO3 – (aq) –689.93 –603.3 38.1 H2O (l) –285.83 H2O (g) –241.83 CaCO3 (s) (calcite) –1207.6 –1129.1 91.7 CaCO3 (s) (aragonite) –1207.8 –1128.2 88 COCl2 (g) –219.1 –204.9 283 CS2 (l) 89 64.6 151.3 CS2 (g) 116.6 67.1 237.8 HCN (l) 108.9 125.0 112.8 HCN (g) 135.1 124.7 201.8 CH2O (g) –108.6 –102.5 218.8 HCOOH (l) –424.7 –361.4 129 HCOOH (g) –378.6 129 CH4 (g) –74.4 –50.3 186.3 CH3Cl (g) –81.9 H2NCONH2 (s) –333.6 CH3OH (l) –239.1 –166.6 126.8 CH3OH (g) –201.5 –162.6 239.8