Mitigation of CO2 by Chemical Conversion

Reviews

Mitigation of CO2 by Chemical Conversion: PlausibleChemical Reactions and Promising Products

Xu Xiaoding† and J. A. Moulijn*

Section of Industrial Catalysis, Faculty of Chemical Engineering and Materials Science,Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands

Received July 26, 1995. Revised Manuscript Received November 29, 1995X

A critical literature analysis was conducted on the viable usage of CO2 in the framework ofthe attempts to reduce the emissions of CO2 into the atmosphere from various processes or therate of increase of the concentration of CO2 in the atmosphere. Applications based on the physicalproperties of CO2 are summarized first. Major examples are applications in supercriticalextraction, enhanced oil recovery, and use as inert gas in fire extinguishing and safety applicationin industry. The various possibilities of use in chemical applications are systematically discussed.CO2 can react with several hydrocarbons and nitrogen-containing compounds (NH3, amines,imines). It can be used as a weak acid or as an oxidizing agent. It can be reducedelectrochemically, photochemically, or chemically or by a syngas route. A variety of productscan be manufactured from CO2, e.g., acids, alcohols, esters, lactones, carbamates, urethanes,urea derivatives, various copolymers, and polymers. In particular, polycarbonates are attractiveproducts. Nevertheless, currently, less than 1% of the CO2 emitted is used in chemical reactions.To reduce the emission of CO2 substantially, only those reactions by which CO2 is used to producebulk chemicals are relevant, whereas those useful in the manufacture of fine chemicals are notimportant in this respect. CO2 conversion to MeOH represents one of the most important optionsin CO2 mitigation. MeOH can be used as additive to fuels or as a substitute for motor fuels. Anincreasingly important application is in the production of MTBE (methyl tert-butyl ether), DMC(dimethyl carbonate), or DME (dimethyl ether) which are major components in modern gasolineto boost octane number. An interesting application of MeOH is the usage as fuel for cars via insitu decomposition into syngas; this results in enhancement of the energy efficiency. FromMeOHseveral useful products can be made. Another reaction of importance that has been commercial-ized is the so-called dry reforming in which methane reacts with CO2, giving synthesis gas. Someprocesses are commercialized and the technology is mature. An example is a process in whicha mixture of biomass and fossil fuels is converted into methanol and carbon which is stored. Itis obvious that thermodynamically most processes using CO2 are not favorable for CO2 mitigation.This does not imply that processes are never feasible. Special situations do exist where the usageof CO2 in chemical applications is attractive. Examples are processes where CO2 is available ata high temperature, where an external heat source is present without a good outlet for the heat,processes where CO2 leads to the right stoichiometry or reaction environment. In these cases ahigh potential for an efficient process exists. An elegant example is “chemical cooling” of hotgases in the chemical process industry. A promising application is the direct substitution ofcertain chemicals by a reaction product of CO2; an example is the replacement of the hazardousphosgene by urea, which is produced from CO2, in the production of organic carbonates. A trivialcase constitutes processes based on renewable energy sources; these processes, although perhapsinefficient, can lead to a reduction of CO2 emissions.

Contents

Abstract 11. Introduction 21.1. Natural Carbon Sources 31.2. CO2 Emission Sources 31.3. Present Applications of CO2 41.4. Targets of CO2 Mitigation 4

2. Possible Strategies of Reducing CO2Buildup in the Atmosphere

4

2.1. Increase of the Energy Efficiency orShift of Primary Energy Source

4

2.2. Storage or Disposal of CO2 52.2.1. CO2 Separation 52.2.2. CO2 Storage in Aquifers 52.2.3. Underground CO2 Storage 5

2.3. Usage of CO2 53. Chemical Reactions of CO2 53.1. Introduction 53.1.1. Thermodynamics 63.1.2. CO2 Reactivity 8

3.2. Utilization as an Acid 83.3. Utilization as an Oxidizing Agent 8

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3.3.1. Carbon Gasification 93.3.2. Oxidation of Alkyl Groups in

Aromatic Compounds9

3.3.3. Oxidative Dehydrogenation ofHydrocarbons

9

3.3.4. Oxidation or Combustion of Metals 103.4. Reduction of CO2 103.4.1. Electrochemical Reduction 103.4.2. Catalytic Reduction with H2 103.4.3. Photoreduction of CO2 12

3.5. Reactions with Compounds HavingActivated Hydrogen Atoms

12

3.6. Reactions with Hydrocarbons withoutActivated Hydrogen Atoms

13

3.6.1. Saturated Hydrocarbons 133.6.2. Unsaturated Hydrocarbons 133.6.3. Reactions with Other Substituted

Hydrocarbons15

3.7. Production of Polycarbonates 153.8. Reaction with Epoxides 153.9. Reaction with Alcohols and Ethers 163.10. Reaction with Nitrogen Compounds 173.10.1.Reaction with NH3 173.10.2.Reactions with Amines and Imines 17

3.11. Reactions with Sulfur Compounds 183.12. Substitution for Other Chemicals 183.13. CO2 as a Carbon Source 194. Bioconversion 195. Concluding Remarks 19

1. Introduction

CO2 is a major source for the so-called greenhouseeffect (the effect of an increase in the absorption ofradiation energy caused by the existence of gases in theatmosphere), leading possibly to changes in climate.1-7

CO2 content in the atmosphere is increasing at a rateof ca. 1% per year; from 250 ppm of the preindustrialperiod to a present level of ca. 400 ppm (315 ppm in1958, 340 ppm in 1984). The enhancement of CO2content is mainly caused by the combustion of fossilfuels and deforestation. The energy sector which is thelargest source of CO2 emission per industrial sector isresponsible for about 25% of the global CO2 emission.Moreover, deforestation of the tropic forests contributesup to 10-30% of enhancement of CO2 and volcanic gasesalso contribute significantly to the increasing CO2 level.CO2 is mainly emitted diluted in flue gases. Exhaledair contains ca. 4% CO2, while exhaust gases from petrolengines contain about 13%. It should be noted that alsosources exist containing CO2 in much higher concentra-tions. Examples are exhaust gases of several chemicalprocesses.7

In the sense of greenhouse effect, it is fair to pointout, that CO2 is not the only gas contributing to thegreenhouse effect. Other gases, in particular, chloro-fluorohydrocarbons (CFC), NOx, and CH4, are alsogreenhouse gases. Due to their longer lifetimes andstronger greenhouse effect (shown as the “relativeradioactive forcing” and “global warming potential”,

GWP), some of them, e.g., N2O, CFCs, and CH4, per unitmass are much more harmful to the environment thanCO2 (Table 1).8 Global warming potential defines thetime-integrated warming effect due to the instantaneousrelease of unit mass of a given gas in today’s atmo-sphere, relative to that of CO2. Therefore, the contribu-tion of each greenhouse gas to global warming may beevaluated as the product of GWP and the amount of thegas emitted. Table 1 shows that CO2 is the leasteffective greenhouse gas per kilogram emitted, but itscontribution to global warming (indicated by the “totalhuman contribution to radiative forcing”) is the mostsignificant due to the large amount released to theatmosphere.8

The increase of the CO2 content in the atmosphere ispartly balanced by the photosynthesis of plants andorganisms, estimated to be 100 Gt/y (1 Gt ) 109 tons),and the respiration processes of living animals. It isnot surprising that at a higher atmospheric CO2 contentphotosynthesis is promoted. Figure 1 illustrates thisfor the photosynthesis of creosote bush for three differ-ent light intensities.1 Of course, this phenomenonopposes global warming. Furthermore, aquifers, lakes,rivers, oceans, and seas, are a large sink for CO2 (3.5-5.5 Gt of C as CO2 per year).9-12 CO2 can be taken upby the oceans as carbonates or dissolved CO2, as coralreefs in shallow sea banks, as clathrates (compoundswith the formula of CO2‚nH2O, n ) 5.75), and as liquidat high depth.Figure 2 shows the worldwide mass balance of CO2.9

It appears that human activity contributes only asmall fraction to the flow rate of CO2 into the atmo-sphere. Due to the balancing effect of photosynthesis,conversion, and accumulation in seas, oceans, etc., theincrease of CO2 content in the atmosphere is actuallymuch less than might be expected from the amount ofCO2 emitted in the world. Figure 3 shows the CO2

sources and its cycle via fossil fuels, biomass, andproduction of chemicals.10

The total man-made carbon dioxide emission in theatmosphere has been estimated to be 6-8 Gt carbon, ofwhich 5 Gt is from burning of fossil fuels and the restfrom land use conversions and deforestation.3,7,9,11 Table

† Tel.: 015-2784358. Fax: 015-2784452. E-mail: [email protected]. NL.

X Abstract published in Advance ACS Abstracts, February 1, 1996.

Figure 1. Effect of increasing CO2 concentration on netphotosynthesis of creosote bush at 301 K at high (2000),intermediate (1000), and low(500) photon flux densities (inµeinstein m-2 s-1).

306 Energy & Fuels, Vol. 10, No. 2, 1996 Reviews

2 gives the sectional energy consumption in some WestEuropean countries in 1984.13

Major CO2 emission comes from private sources, inparticular, heating of homes and transport. Also heat-ing of public buildings and industrial transport areamong the major CO2 emission sources. Further large

sources constitute power stations and industry, includ-ing oil refineries.1.1. Natural Carbon Sources. Carbon is the

precursor of CO2. Carbon and/or its compounds arepresent in all segments of the planet earth.9,10 Figure4 shows the distribution. The amount of carbon presentin the atmosphere is relatively very modest.By far the most part of carbon is present as terrestrial

carbon. In fact calcite, CaCO3, is the most abundantcarbon-containing mineral. Other important mineralscontaining carbon are dolomite (CaMg(CO3)2), magne-site (MgCO3), ankerite (Ca(Fe,Mg)(CO3)2), FeCO3, Mn-CO3, and ZnCO3. Even when the amount of fossilenergy resources would be diminished significantly dueto the consumption, it is clear that carbon present asCO2 in the atmosphere still will be a small fraction ofthe total carbon inventory.1.2. CO2 Emission Sources. CO2 sources can be

subdivided in concentrated and diluted streams.14,15

Examples of concentrated CO2 sources are naturalCO2 reservoirs (they contain up to 97% of CO2) andexhaust gases from a wide variety of industrial plants.Examples of the latter are plants for processing ofnatural gas, production of ethanol by fermentation, andproduction of bulk chemicals such as ammonia andethylene oxide. Also quantitatively important is theinorganic industry, notably for the production of cement.The major examples of dilute CO2 sources are flue

gases from electric power generation and the blastfurnace, and exhaust gases from cars, trucks, and buses.Electric power generation is the largest source of CO2emission in industry. It contributes to around 25% ofthe global CO2 emission and is responsible for 14% ofthe estimated global warming in the 1980s. This figurecould grow to 30% by the year 2030.

Table 1. Potential of Selected Gases To Contribute to the Greenhouse Effect8

CO2 CH4 N2O CFC-11 CFC-12

concentration (ppm)preindustrial (1750-1800) 280 0.8 0.288 0 0current (1990) 353 1.72 0.310 28 × 10-5 4.84 × 10-6

lifetime in atmosphere (y) 10 150 60 130rel radiative forcingper mol 1 21 206 12400 15800per kg 1 58 270 4500 7100

current human-made emission (Mt/y) 26000 300 6 0.3 0.4rate of accumulation in the atmosphere10-4 ppm/y 18000 150 8 0.095 0.17%/y 0.5 0.9 0.3 4 4

rel global warming potentialt ) 20 y 1 63 270 4500 7100t ) 100 y 1 21 290 3500 7300t ) 500 y 1 0 9 190 1500 4500

total human contribution to radiative forcing (%) 55 15 6 17

Figure 2. Inventories and flow rates of CO2.9 The numbersin rectangles represent the inventories and the other numbersthe flow rate (net values, except the quasi equilibriumatmosphere T ocean).

Figure 3. The CO2 cycle.

Table 2. Sectional Use of Energy in Some EuropeanCountries in 1984 (%)13

buildings

private commercial transport industry rest

Denmark 23 9 17 31 20France 22 13 19 31 15FRG 25 11 22 34 8Greece 20 10 27 43Ireland 33 12 22 33Italy 25 10 18 42 4UK 25 13 22 35 5Belgium 29 12 53 6Holland 28 13 41 18

Figure 4. Carbon sources in the nature.

Reviews Energy & Fuels, Vol. 10, No. 2, 1996 307

The present CO2 sources used in the US industriesare quantified in Table 3.15Presently, many power stations in the world are using

coal as the energy source; the exhaust gas contains 13up to 27% CO2. This is rather low for a profitableprocess. Due to the high expenses in enrichment andseparation, CO2 from flue gas of power stations hashardly been used as a source for CO2. To our knowl-edge, only one pilot plant (100 ton/day) was tested inUSA for the separation of CO2 from flue gases of energysector (Chapter 1, ref 14).Deforestation, especially burning of tropic forests, and

respiration of living matters also lead to dilute CO2sources which nevertheless contribute significantly tothe global CO2 emission.1.3. Present Applications of CO2. CO2 usage can

be divided into two groups: those using the physicalproperties and those using the chemicalproperties.9,10,14-25 It should be borne in mind that onthe one hand the former use does not contribute directlyto the reduction of CO2 emission, but on the other hand,consumption of other materials might be decreased. Thelatter will, in fact, often lead indirectly to CO2 reduction.The first category, making use of its physical proper-

ties, includes the beverage industry (ca. 0.75 Mt/y), 14enhanced oil recovery (EOR), and supercritical (SC) CO2extraction and cleaning.Because CO2 is a safe gas, it is used as a protective

gas (in chemical or steel industries, in food reservation,in welding, etc.) and even as fire extinguisher.9,10,14,15Its favorable diffusion properties are the basis forapplication as blowing agent in polymer processingindustries. It is also used as a pesticide to kill insectsin fumigation.15CO2 used in EOR (ca. 6-7 Mt/y)14 represents one of

the largest potential applications of CO2. This applica-tion is expected to grow in the near future. It does notlead to a consumption of CO2 eventually, but due to theextra amount of oil produced, it is economically feasiblein many cases.9,14-17 The benefits of using CO2 in EORstems from the reduction of the viscosity due to mixingwith CO2.Supercritical CO2 extraction is also a major use of CO2

at present.9,10,14,15 It is used in the production ofdecaffeinated coffee, in the extraction of soybeans andhops, in pharmaceutical industry to recover activesubstances, in the cosmetic industry to extract fats andethereal oils, and in food industry to isolate spices,aromas and essences (Eisenbach, W. O., ref 9, pp 371-388 and the references therein). In some cases, lessenergy is consumed if supercritical CO2 extraction isused, compared with other options. Moreover, super-critical CO2 may replace, e.g., in spray painting, solventswhich contribute to global warming, e.g., volatile organ-

ics or CFCs. In this sense, the use of supercritical CO2is environmentally beneficial.9,20Dry ice (solid CO2) is used in refrigeration (1.5 Mt/

y),14 especially in refrigerated railcars or trailers, tosubstitute CFCs.9,10,14,15 This replacement is importantin combating the greenhouse effect as well as the ozonedepletion problem.In the other category, CO2 is a reactant. In organic

chemical industry, it is used in the synthesis of urea,methanol, salicylic acid, and cyclic organic carbon-ates.9,10,14,21 In inorganic chemical industry, it is usedto manufacture Na2CO3 or NaHCO3 (e.g., the Solvayprocess), CaCO3, and other carbonates (e.g., lead orbarium carbonates). Moreover, it is consumed in bio-mass production and in greenhouses to stimulate thegrowth of plants. It is also used as an acid in waterpurification or in neutralization processes.15Figure 5 shows the major applications of CO2 in USA.9

Most applications are nonchemical.Totally, only 0.7-1.0% of the CO2 produced is used

at present. The consumption in the production ofchemicals is in the order of 0.1% (ca. 50 Mt/y).1.4. Targets of CO2 Mitigation. According to the

European Union, the CO2 emission should be broughtback to the level of 1990 in the year 2000. Othercountries, e.g., USA, Japan, and Canada, also aim atstabilization of CO2 emission at the level of 1990 in2000. The long-term objective is to achieve a balancedCO2 production and uptake, energy saving, and the useof renewable energies. The Netherlands aims at sta-bilizing CO2 emission in 1994-1995 and at achieving areduction of CO2 by 3-5% by the year 2000.

2. Possible Strategies of Reducing CO2 Buildupin the Atmosphere

In principle, three strategies are possible, viz., reduc-tion of the amount of CO2 produced, usage of CO2, andstorage of CO2.2.1. Increase of the Energy Efficiency or Shift

of Primary Energy Source. In a sense, the CO2problem is an energy problem. The increase of energyefficiency of the primary energy carrier is the mostimportant in reducing CO2 emission. Also, the type offossil fuel used is important. For example, the ∆Hvalues for the combustion of carbon (graphite), naturalgas (CH4), and C2H4 to water and CO2 are respectively-393, -929.1, and -662.3 kJ/mol carbon, i.e., much lessCO2 is produced when fossil fuels other than carbon areused. The replacement of C-rich energy carrier (coal)by other less C-rich fossil fuels (oil or natural gas) is anoption that leads relatively easily to reduction of CO2

Table 3. Present CO2 Sources Used in the USIndustries15

industry contribution (%)

ammonia production 35oil and gas refineries 20geological formations 20ethanol production 12chemicals manufacturing 7flue gases or cogeneration 3alcohol production 2others 1total 100

Figure 5. Major uses of CO2 in USA.9 The shaded area refersto the expected increase in usage.


emission. However, in practice, for obvious reasons, itis a wise policy to diversify the use of energy sources.Although combustion of fossil fuels is responsible for ca.two-thirds of the world’s CO2 production due to humanactivities, the usage of CO2 in these exhaust gases,however, due to its low concentration, often is economi-cally less feasible.12 An exception might be thoseexhaust gases that are at a high temperature.26

Currently electricity is produced at 30-40% efficiency.If the best available technologies were used this wouldincrease to about 60%. Thus, emission of CO2 wouldbe halved.27 It can be estimated that replacement ofoil by CH4 may result in 25% reduction of CO2 emission.It is important to point out that this is a route that leadsto direct and efficient reduction of CO2 emission.Moreover, the fraction of CO2 emission reduction achiev-able is much larger than most other routes. However,as stated before, from a practical point of view, thisoption might be of limited value.In many countries, proposals have been formulated

to reduce CO2 emission and economic evaluations havebeen made. Examples are the following. For USA, ithas been estimated that carbon emission reduction of10% is possible, provided an increased energy efficiencyand some substitution of natural gas for coal in theelectricity sector are achieved (with a net economicbenefit of US $230/ton by the year 2000).28 It has beenclaimed that an increase of the end-use efficiency andthe use of renewable energy sources in Sweden’s districtheating and electricity sectors can result in 35% emis-sion reductions and a net benefit of US $40/ton by theyear 2010.28 Of course, the correctness of these predic-tions remains to be proven.2.2. Storage or Disposal of CO2. 2.2.1. CO2

Separation. For any process aimed at reducing theemission to the atmosphere of CO2 produced, CO2 hasto be captured and separated or at least the exhaustgases have to be enriched, which considerably adds tothe cost of the process. The costs are of course depend-ent on the source. For instance, in the case of flue gasfrom coal-fired plants, due to the relatively low CO2concentration, the separation will be expensive. More-over, there are various sulfur and nitrogen compoundsin the coal. During coal combustion, they will beconverted into SOx and NOx compounds, which areusually catalyst poisons in downstream chemical pro-cesses. This generates additional costs.Several separation technologies are available:29 (i)

gas/liquid scrubbing systems; (ii) gas/solid adsorptionsystems; (iii) cryogenic (fractionation) techniques; and(iv) membrane separation technology.Separation by solvent absorption systems (e.g., by

using monoethanolamine, MEA) or by membrane tech-nology appears to be the most viable technique.14 Aninteresting approach in power generation is combustionof fossil fuels in an atmosphere of pure oxygen (fromprior air separation) and recycled CO2: in this way theCO2 produced is much more concentrated (>90%); thecost of such a process was estimated to be in the rangeof $15-100/t of CO2.5 The air separation/flue gasrecycling process is reported to be the most energy-efficient process analyzed, requiring 26-31% of the coalheating value, while the CO2 recovery is close to 100%.The other processes studied require over 50% of the coalheating value, and the resulting cost of electricity

production increases by over 80%.12 For practicalapplications, these numbers are obviously prohibitivelyhigh. Membrane technology is basically in the earlystage of development. When a breakthrough occurs inthe near future, membrane technology might well bethe basis for feasible CO2 separation processes.2.2.2. CO2 Storage in Aquifers. Aquifers, espe-

cially seas and oceans, constitute an important CO2 sinkand buffer9,11-13 (cf. Figure 2); the amount of CO2 thatcan be stored additionally is in principle vast. It canbe achieved by increasing microalgae biomass or byfixing CO2 as CaCO3-like coral reefs. It has even beensuggested to add nutrients such as iron to ocean surfacewater for increasing the production of microorganisms.It was estimated that treatment of the Antarctic oceanswith iron might allow the phytoplankton to convert allthe available nutrients into new organic matter, thiswould amount to 0.3-1.7 Gt of carbon. It was estimatedthat the Antarctic Ocean, equatorial Pacific, and sub-arctic Pacific have a potential extra uptake of 2.8 Gt ofC/y.30CO2 can be injected into 700-1000 m, or even to 3000

m, deep ocean water (as liquid CO2). As to the forma-tion of coral reefs, it was estimated that in the shallowsea banks near Japan, the CO2 consumed to formcarbonates may amount to 3% of the CO2 emitted bythat country. Of course, CO2 may also form (bi)-carbonates or it may dissolve in water or it may formclathrates. According to some authors,7,11 storage inoceans is an important method to combat the CO2problem, although the ecological impact of injectinglarge amount of CO2 into sea and ocean water is stilluncertain. Moreover, it is not clear how the cations, e.g.,iron or calcium, can be provided in a sensible way.2.2.3. Underground CO2 Storage. Depleted oil or

gas fields are often porous. It is possible to store a largeamount of CO2 into those underground reservoirs6 (totalcapacity estimated to be ca. 80-300 Gt of C in ex-hausted gas wells and 40-200 Gt of C in exhausted oilwells).31 These quantities are very high. However, itremains to be seen if this storage can provide a long-term fixation of CO2.It was also suggested to use large quantities of

natural underground brine deposits to store CO2. Thisapproach appears less practical due to the lack of a largecalcium source. Of course, lime can be obtained, fromdecomposition of CaCO3, which in turn can be used asa CO2 adsorbent or calcium source. It is needless to saythat such a process does not lead to a reduced emissionof CO2.2.3. Usage of CO2. In principle, CO2 can be utilized

in a large number of processes. In the above, applica-tions were summarized based on the physical propertiesof CO2. In recent years the potential application of CO2as a chemical feedstock has received a lot of attention.In the next chapter the potential application as achemical feedstock is discussed systematically.

3. Chemical Reactions of CO2

3.1. Introduction. CO2 is one of the cheapest andmost abundant carbon-containing raw materials in theworld. Therefore, it is logical that it is considered as apotential building block for carbon-carbon chains or asa competitive carbon source in chemistry or in chemicalindustry.9,23 However, CO2 is rather inert and its


reactions are energetically highly unfavorable. Ofcourse, in principle the first problem can be solved bydeveloping a good catalyst. The second point is ofthermodynamic origin and not solvable by catalysis. Inthe following it will be shown that this is in general true,but when certain special conditions exist (high T or P,“free” energy available, need for CO2 for chemicalreasons) a practical process might be feasible.3.1.1. Thermodynamics. Table 4 shows the ∆H°

and ∆G° values for several interesting exothermic CO2reactions.21-23

At first sight it might be surprising that so manyexothermic conversions exist in view of the high stabilityof CO2. However, when the Gibbs free energy ∆G° isconsidered it is clear that most of these reactions areassociated with highly positive ∆G° values and, as aconsequence, thermodynamically not favored.It is interesting to compare CO2 with CO because

often CO2 is present mixed with CO or it is (partially)converted into CO. In Table 5, ∆H° and ∆G° values forsome exothermic CO reactions are shown.22Although many exothermic reactions of CO2 (and CO)

do exist, it appears that for all the reactions in Tables4 and 5, the Gibbs free energy values are less favor-

able: they are all more positive than the corresponding∆H° values; only a few reactions have both negative ∆G°and ∆H° values. Upon closer examination of Table 4 itis apparent that all cases where ∆G° < 0, correspondto hydrogenations, or reactions leading to productscontaining C-O bonds. The reason for the relativelyfavorable values of ∆G° for hydrogenation reactions isthat water is produced. As hydrogen has to be producedat the cost of energy input, in fact, none of thesereactions is favorable for CO2 mitigation. So, these datamight lead to a too optimistic view. In Table 5 also afew reactions with negative ∆G° are given which do notinvlove hydrogenation. Inspection of the reaction equa-tions reveals that no C-O bonds are broken and, as aconsequence, the penalty for breaking of these bondsdoes not exist.Tables 4 and 5 refer to exothermic reactions. Of

course also CO2 reactions with a positive ∆H° do exist.Table 6 gives some examples.It is no surprise that these reactions are associated

with highly positive ∆G° values and are, as a conse-quence, not favorable.When producing a certain compound, it is interesting

to compare CO and CO2 as potential feedstocks. In

Table 4. Enthalpy and Gibbs Free Energy Changesa in Some Exothermic Reactions Involving Carbon Dioxide21-23

eq reactions ∆H° (kJ/mol) ∆G° (kJ/mol)

1 CO2(g) + H2(g) f HCOOH(l) -31.0 +34.32 CO2(g) + 2H2(g) f HCHO(g) + H2O(l) -11.7 +46.63 CO2(g) + 3H2(g) f CH3OH(l) + H2O(l) -137.8 -10.74 CO2(g) + 4H2(g) f CH4(g) + 2H2O(l) -259.9 -132.45 2CO2(g) + H2(g) f (COOH)2(l) -39.3 +85.36 2CO2(g) + 6H2(g) f CH3OCH3(g) + 3H2O(l) -264.9 -38.07 CO2(g) + H2(g) + CH3OH(l) f HCOOCH3(l) + H2O(l) -31.8 +25.88 CO2(g) + H2(g) + CH3OH(l) f CH3COOH (l) + H2O(l) -135.4 -63.69 CO2(g) + 3H2(g) + CH3OH(l) f C2H5OH(l) + 2H2O(l) -221.6 -88.910 CO2(g) + H2(g) + NH3(g) f HCONH2(l) + H2O(l) -103.0 +7.211 CO2(g) + CH4(g) f CH3COOH(l) -13.3 +58.112 CO2(g) + CH4(g) + H2(g) f CH3CHO(l) + H2O(l) -14.6 +74.413 CO2(g) + CH4(g) + 2H2(g) f (CH3)2CO(l) + H2O(l) -70.5 +51.214 CO2(g) + C2H2(g) + H2(g) f CH2dCHCOOH(l) -223.6 -115.015 CO2(g) + C2H4(g) f CH2dCHCOOH(l) -49.1 +26.216 CO2(g) + C2H4(g) + H2(g) f C2H5COOH(l) -166.6 -56.617 CO2(g) + C2H4(g) + 2H2(g) f C2H5CHO(l) + H2O(l) -171.1 -44.418 CO2(g) + C6H6(l) f C6H5COOH(l) -21.6 +30.519 CO2(g) + C6H5OH(l) f m-C6H4(OH)COOH(l) -6.6 +46.9

a Aspen was used in calculating the data.

Table 5. Enthalpy and Gibbs Free Energy Changesa in Some Exothermic Reactions Involving CO22


20 CO(g) + 2H2(g) f CH3OH(l) -131.6 -29.921 CO(g) + H2O(l) f HCOOH(l) -24.8 +15.122 2CO(g) + 2H2O(l) f (COOH)2(l) -26.9 +46.923 CO(g) + CH3OH(l) f HCOOCH3(l) -25.6 +6.624 CO(g) + CH3OH(l) f CH3COOH(l) -129.2 -82.825 CO(g) + NH3(g) f HCONH2(l) -96.8 -12.026 CO(g) + C2H2(g) + H2O(l) f CH2dCHCOOH(l) -217.4 -134.227 CO(g) + C2H4(g) + H2(g) f C2H5CHO(l) -164.9 -63.6


Table 6. Enthalpy and Gibbs Free Energy Changesa in Some CO2-Involving Endothermic Reactions


28 CO2(g) + CH2dCH2(g) f CH2CH2O(l) + CO(g) +152.9 +177.329 CO2(g) + C(s) f 2CO(g) +172.6 +119.930 3CO2(g) + CH4(g) f 4CO (g) + 2H2O(l) +235.1 +209.231 CO2(g) + CH4(g) f 2CO(g) + 2H2(g) +247.5 +170.832 CO2(g) + 2CH4(g) f C2H6(g) + CO(g) + H2O(l) +58.8 +88.033 2CO2(g) + 2CH4(g) f C2H4(g) + 2CO(g) + 2H2O(l) +189.7 +208.334 CO2(g) + C2H4(g) f C2H4O(g) + CO(g) +178.0 +176.0



Table 7 enthalpy and Gibbs free energy changes of someinteresting and relatively favorable reactions startingfrom CO or CO2 are compared. It is obvious that thereaction enthalpies for the production of the sameproduct from either CO or CO2 are comparable, althoughin most cases CO is favored compared to CO2.Coming back to the comparison of CO and CO2, it is

appropriate to state that when the total productionprocess is considered CO is much more favorable thanCO2. For example, in the formic acid production in theformer case H2O is consumed whereas in the latter caseH2 is the reactant. The production of H2 requiresenergy, whereas H2O is “free”. It is obvious that theenergy required is provided by the coreactant, be it H2or a hydrocarbon.From the foregoing it should not be concluded that

in all cases reactions based on CO2 or CO are onlyfeasible in those cases where ∆G° has a reasonablevalue. In practice favorable conditions (T, pressure) canbe chosen or, as is done in partial oxidation, reactionscan be coupled. In the latter case, at favorable condi-tions, endothermic and exothermic reactions are carriedout simultaneously and, the endothermic reaction occurswhile the heat of reaction is produced by the exothermicoxidation.Figure 6 shows the ∆G° value for the reaction CO2(g)

+ 3H2(g) ) MeOH(l) + H2O(l) as a function of the totalpressure. At all pressures ∆G is negative and aspressure increases it becomes more negative; i.e., thereaction becomes more favorable, thermodynamically.A similar case is the reaction for formate from CO2 andH2 which has a positive ∆G° while an increase in CO2or H2 pressure shifts the equilibrium favorably.Figure 7 shows the influence of temperature on the

reaction of CH4 and C2H6 with CO2 to syngas. Theformer reaction is called “dry reforming”. It appearsthat as temperature increases the Gibbs free energies

for the respective reactions change from positive tonegative. At 900 K the reaction between CO2 and CH4giving synthesis gas is associated with a ∆G° value ofzero. For the reaction between CO2 and C2H6 thisapplies at 800 K. So, when CO2 is available at hightemperature (>800 K) a reaction with CH4 or C2H6 ispossible, thermodynamically. As this will lead to coolingof the mixture the term “chemical cooling” has beenused. Many process streams do exist where this prin-ciple might be used successfully. A second advantagein practice of the production of synthesis gas from CO2and CH4 is the ratio of H2 and CO produced (1:1); inmany practical applications steam reforming of CH4gives the wrong ratio (3:1), i.e., too much H2. When CO2instead of H2O is used, the ratio can be tuned withoutthe necessity to carry out a separate water gas shift.A similar situation has been found for the reaction

between CO2, H2, and amines producing formamides.In summary, although CO2 is the end product of

energy-producing processes such as combustion, manychemical reactions using CO2 are thermodynamicallyfeasible. However, in many reactions a coreactant ispresent, for instance hydrogen which reacts into water.The stability of water is the reason that overall ∆G°becomes negative for many hydrogenation reactions.3,23Also, reactions do exist in which no C-O bonds arebroken. This class contains reactions with favorable ∆Hand ∆G values.From a thermodynamic point of view, it is clear that

in general for a useful application of CO2 a specialcondition must exist. Examples are the following. CO2can be present at very high temperature or pressure.Heat might be available without cost so that chemicalcooling is feasible. There might be a surplus of hydro-gen without an attractive outlet. The chemistry of adesired reaction might require CO2, for example, inmethanol production.

Table 7. Comparison of Enthalpy and Gibbs Free Energy Changesa in Some CO2 and CO Reactions


1 CO2(g) + H2(g) f HCOOH(l) -31.0 +34.321 CO(g) + H2O(l) f HCOOH(l) -24.8 +15.13 CO2(g) + 3H2(g) f CH3OH(l) + H2O(l) -137.8 -10.720 CO(g) + 2H2(g) f CH3OH(l) -131.6 -29.98 CO2(g) + H2(g) + CH3OH(l) f CH3COOH(l) + H2O(l) -135.4 -63.624 CO(g) + CH3OH(l) f CH3COOH(l) -129.2 -82.814 CO2(g) + C2H2(g) + H2(g) f CH2dCHCOOH(l) -223.6 -115.026 CO(g) + C2H2(g) + H2O(l) f CH2dCHCOOH(l) -217.4 -134.27 CO2(g) + H2(g) + CH3OH(l) f HCOOCH3(l) + H2O(l) -31.8 +25.823 CO(g) + CH3OH(l) f HCOOCH3(l) -25.6 +6.6


Figure 6. Gibbs free energy for CO2 hydrogenation tomethanol as a function of reaction pressure.

Figure 7. Gibbs free energy as a function of reactiontemperature for (1) CO2(g) + CH4(g) ) 2CO(g) + 2H2(g), (2)2CO2(g) + C2H6(g) ) 4CO(g) + 3H2(g).


3.1.2. CO2 Reactivity. Catalysis will play a crucialrole in CO2 conversion reactions.10 Development ofsuitable catalysts represents a major challenge for thecatalysis community. Possible interactions betweenCO2 and a substrate (S) on a transition metal (M) areschematically shown in the Figure 8.10

As shown in this figure, the formation of S-CO2,product between CO2 and a substrate S, can proceedeither via S insertion, followed by CO2 insertion, or viceversa; due to the coordination to the active center (M),the activation energy might be lowered for the reaction.There are three possible bonding modes for CO2 (cf.

Figure 9), namely, pure carbon coordination (I), pureoxygen coordination (II), and mixed oxygen-carboncoordination (III). Recent results indicate that, ingeneral, the CO2 molecule prefers to adopt either amixed carbon-oxygen coordination (III) or a pureoxygen coordination (II) to the metal center.14

Coordination to metal centers may lead to furtherchemical reactions.10,32-34 A large number of studies inliterature describe the coordination chemistry ofCO2.9,10,21,22,24,35,36 Figure 10 illustrates the wide varietyof metal-mediated reactions in which CO2 can take part.However, many of the reactions are stoichiometrical,requiring expensive organometallic complexes, and oftenthe reactions involve difficult conditions. Various usefulproducts, fine chemicals, intermediates in pharmaceuti-cal and food industries can be produced. However, theseapplications, if they may lead to industrial processes,only consume very small amounts of CO2 (typically

below 10-100 kt/y). They would never lead to substan-tial reduction of CO2 content in the atmosphere.Other features of CO2 chemistry make use of its

acidity and oxidizing abilities.15,37

3.2. Utilization as an Acid. CO2 is an acidic oxide;when it is dissolved in water, either as bicarbonate orcarbonate,14 it is slightly acidic. This weak acidity canbe used in neutralization processes,15 e.g., in purificationof water from swimming pools. Due to its weak acidity,the pH value it can reach is limited (from pH of 12-13to 6-9). The use of CO2 in neutralization has its merits.It is a clean acid; it does not lead to environmentalproblems associated with strong acids, e.g., H2SO4 orHCl.15 Moreover, it is less harmful or even harmlessto the enzymes used in processes involving microorgan-isms or enzymes.15 In this type of applications low costtechnology is a must. In principle, a high concentrationof CO2 is not needed and an application of CO2-containing exhaust gas might be feasible without priorseparation step. Indeed, it has been reported that fluegas from coal combustion can be used directly forneutralization purposes without purification or concen-tration.15 Of course, for most applications coal gas istoo dirty to apply as acid and purification is absolutelynecessary. The acid properties of CO2 can be improvedby converting it in an organic acid, e.g., formic acid.CO2 is used to prepare various carbonates. Besides

CaCO3, NaHCO3, and Na2CO3, it is used to preparecarbonates of La3+, Nd3+, Sm3+, Eu3+, Gd3+, Dy3+, andHo3+ by reacting an aqueous solution of the correspond-ing oxide, M2O3, with supercritical CO2.38 At a tem-perature of 300-320 K and a pressure of 70-250 bar,the yield was reported to be >95%.3.3. Utilization as an Oxidizing Agent. CO2 can

be viewed as an oxygen carrier.37 Figure 11 shows somepossibilities. CO2 can replace oxygen or air in oxidationprocesses. Compared with air or oxygen, CO2 is a mildoxidizing agent. Numerous reactions exist in which, inprinciple, CO2 can be used as an oxidizing agent. Insome cases, this use may have beneficial results, e.g., abetter selectivity to intermediate oxidation products inchemical industry. It is interesting to note that the NOx

content in the flue gases from combustion of heatinggas is reduced by injection of CO2 into the gas upstreamof the burners.39 Although it might be thought that thisis due to a different reaction kinetics, it is mainly causedby the lowering of the temperature.Examples of the use of CO2 as oxidizing agent are

given later on.

Figure 8. Possible routes of interaction between CO2 and asubstrate (S) on a transition metal (M).

Figure 9. Various modes of CO2 coordination to a metalcenter.

Figure 10. Stoichiometric and catalytic reactions involvingCO2 and alkanes or alkenes.10

Figure 11. CO2 as an oxidant.


3.3.1. Carbon Gasification. If carbon is reactedwith CO2, CO is obtained in the reverse Boudouardreaction:

This highly endothermic reaction is widely carried out.It is a crucial reaction in coal gasification, which isnormally carried out autothermally; i.e. the heat re-quired is produced by partial oxidation of the coal. Thisreaction is not limited to coal; in general, all carbon-aceous materials can be gasified into CO which is auseful product. A modern example is gasification ofwaste material. Underground coal gasification is stud-ied on a scale near to demonstration scale. It issometimes defended that for coal utilization under-ground gasification is the technology of the future.Because of the large scale, the potential amounts of CO2involved are enormous. Carbon gasification is alsocarried out in the manufacture of active carbon to obtainthe desirable porosity.When the CO2 source is at high temperature, “chemi-

cal cooling” (use of its chemical potential) is possible.An example of using CO2 in chemical cooling is anadvanced iron ore reduction process, where a large gasflow containing 15% CO2 and H2O has to be cooled fromca. 1900 K to 1100 K, because it is to be used in thereduction of iron ore, which gets sticky above 1100 K.40It is considered to inject coal particles in this streamresulting in (i) an increased reducing power and (ii) alowering of the temperature.Another example is already shown in Figure 7. If

high-temperature CO2 is used in the oxidation of al-kanes, the reactions are much more favorable.Although, at present, no technology is available to

separate CO2 from the flue gas at high temperature, wethink that in the near future membrane separation byinorganic membranes will probably provide the possibleroute to use high-temperature CO2, ceramic membraneshave been developed which can be used for gas separa-tions at much higher temperatures, e.g., 673-873 K,than the polymer membranes.41,42

3.3.2. Oxidation of Alkyl Groups in AromaticCompounds. Alkylaromatic compounds can react withCO2 in the presence of catalysts containing Pd andoxides of La, Sm, Y, and Ce to give dealkylated deriva-tives and synthesis gas.43 For toluene and CO2 (1:1),the reaction proceeds at 673 K over a Pd-La2O3-Al2O3catalyst to give benzene with 90% selectivity.

This reaction might be a good example for a processbased on a membrane reactor.3.3.3. Oxidative Dehydrogenation of Hydrocar-

bons. Hydrocarbons can be dehydrogenated in severalways.44-47 An often applied method is oxidative dehy-drogenation. An example is the oxidative coupling ofCH4 which has drawn a lot of attention.45 The desiredreaction usually is

As complex radical chain reactions are involved, andthe intermediates are much more reactive than thereactant, CH4, it is no surprise that selectivities incombination with high yields are disappointing up tothe present. Total combustion occurs, to a significantextent. One may also think of using CO2. Indeed, itwas reported that by using CO2/O2 mixtures instead ofoxygen, the selectivity for ethane and ethylene inoxidative coupling of methane can be enhanced.44,45Moreover, CO, a valuable product is produced. PbO/MgO catalyst was considered to be the best systemstudied so far,45 although its activity is still too low forpractical use.

The thermodynamic data show that the equilibrium isnot favorable, not even at 1098 K.Another example is the oxidative dehydrogenation of

propane into propylene:46,47

Essentially, the heat of reaction needed is provided bythe formation of H2O. An alternative is the use of CO2rather than O2:

In the reaction using K-Cr-Mn on silica catalysts,also ethylene, CO, and H2 are produced. This reaction,though rather highly endothermic, has potential merits.It is, in general, difficult to reach high selectivities inoxidative dehydrogenation. In fact, one of the undesiredbyproducts is CO2. When CO2 is used instead of air oroxygen, CO2 production is suppressed. It is not illogicalto expect that use of CO2, which is a mild oxidant, maylead to higher selectivities in this type of reactions.Alkanes can be dehydrogenated (aromatized) into

aromatics.46,47Usually air is used, and water, CO and cracking

products, lower alkanes, or alkenes are the products. Itwas reported46,47 that CO2 can be used in the dehydro-genation or aromatization of lower alkanes (e.g., ofpropane) over Ga- or Zn-doped H-ZSM-5 catalysts. Itwas interesting to find that the presence of CO2 sup-presses cracking products and deactivation of the cata-lyst by coking.Ethane was reported to be active in the dehydroge-

nation by CO2 over the same catalysts.44

It is clear that all these CO2 based oxidative dehy-drogenation reactions are thermodynamically unfavor-able, in contrast to O2-based analogous processes. Asthe former are highly endothermic and the latter highlyexothermic, a mixture of O2 and CO2 might lead to afeasible process. The alternative is to add heat froman external source or to start with a high-temperaturereactant.

CO2 + C ) 2CO ∆H° ) +172.6 kJ/mol CO2 (29)

PhCH3(l) + CO2(g) f C6H6(l) + H2(g) + 2CO(g)∆H° ) +210.1 kJ/mol, ∆G° ) +130.5 kJ/mol (35)

2CH4(g) + O2(g) f C2H4(g) + 2H2O(l)∆H° ) -424.7 kJ/mol, ∆G° ) -306.3 kJ/mol (36)

CO2(g) + 2CH4(g) f C2H6(g) + CO(g) + H2O(g)∆H°1098 K ) 196.7 kJ/mol CH4, ∆G°1098 K )

70.0 kJ/mol CH4 (32)

C3H8(g) + 1/2O2(g) f C3H6(g) + H2O(l)∆H° ) -165.0 kJ/mol, ∆G° ) -151.0 kJ/mol (37)

C3H8(g) + CO2(g) f CO(g) + H2O(l) + C3H6(g)∆H° ) 118.2 kJ/mol, ∆G° ) 105.5 kJ/mol (38)

C2H6(g) + CO2(g) f C2H4(g) + CO(g) + H2O(l)∆H° ) +842.2 kJ/mol, ∆G° ) +120.3 kJ/mol (39)


3.3.4. Oxidation or Combustion of Metals. Com-bustion of metals, e.g., Mg or Li, is used to generateenergy under special conditions, e.g., in space ships.48When using oxygen, these reactions are very fast andhighly exothermic, and, as a consequence, favorableconditions are present for a runaway. The use of CO2,a mild oxygen carrier, is in this respect preferable overthat of air.48

CO2 is used, in special cases, in catalyst calcination,to prevent the active phase from oxidation to too highan oxidation state or in special treatment, e.g., in themanufacture of uranium fuel particles.In all these examples, use is made of the mildly

oxidizing capacity compared to oxygen and air. It is alsosuccessfully used in applications where the reducingcapacity of a gas flow has to be decreased. An exampleis MeOH synthesis, where the catalyst is based oncopper.49-54 The optimal oxidation state has beensuggested to involve slightly oxidized Cu or Cu+,52,54 andsynthesis gas without CO2 and H2O deactivates thecatalyst by reducing copper essentially to Cu0. There-fore, CO2 is added to the reactant mixture in order toprevent the complete reduction of Cu oxide to metalliccopper.50

3.4. Reduction of CO2. Although the previousparagraphs, which discuss CO2 as an oxidant, deal withreactions in which CO2 is reduced, reactions directlyaimed toward CO2 reduction are treated separately.Reduction of CO2 can proceed electrochemically orchemically.3.4.1. Electrochemical Reduction. Electrochemi-

cal reduction can be carried out catalytically or non-catalytically. Regardless of the route,9,14,22,55,56 in mostcases, the major products obtained are hydrogen, oxy-gen, CO, formic acid, formaldehyde, methanol, andmethane. In some cases, more complex molecules, suchas oxalic, succinic, adipic, and suberic acids, i.e., dicar-boxylic acid, HOOC-(CH2)n-COOH, with n ) 0, 2, 4,and 6, respectively, can be obtained.9,55,56 Some can beused as a building block in C1 chemistry. The selectivi-ties reported are not high. Moreover, an overpotentialis needed for the reaction to proceed. Electrocatalyticreduction deserves further study since it increases theefficiency of the process. There are reports of electrore-duction processes which use solar energy as an energysource. These processes are promising since they userenewable energy. It should be pointed out that mostproducts from electrochemical reduction can also bemade by catalyzed chemical reduction routes. In manycases, the productivity of catalytic chemical reductionsis much higher than that of the corresponding electrore-duction. So, we conclude that only when cheap electric-ity is available, electrochemical processes can be attrac-tive.3.4.2. Catalytic Reduction with H2. With respect

to catalytic reduction of CO2 or CO, hydrogen is the mostcommonly used reducing agent.32 The reactions, whichbelong to the class of so-called C1 chemistry, are wellstudied and technically viable and, in some cases,economically feasible.33,35,49-51 The economic feasibilityof many processes depends strongly on fuel prices.Several routes can be proposed to use CO2 in hydro-

gen reduction: (1) conversion to products via synthesisgas, (2) direct hydrogenation into useful products, and(3) reactions to other products via MeOH.

3.4.2.1. Conversion to Products via SyngasRoute. CO2 can be converted first to synthesis gas, CO/H2, followed by reactions of the class of C1 chemistry,Fischer-Tropsch reaction, MeOH synthesis, methana-tion, etc.Synthesis gas can be produced via several reactions.

The most important are steam reforming of CH4, CO2reforming of CH4, partial oxidation of CH4 and partialoxidation of coal.

Actually, in the partial oxidation of methane, thereaction proceeds via two steps: the first step ismethane oxidation which is usually a combination oftwo reactions in series (eqs 42 and 43), followed bymethane reforming in the steam and CO2 produced (eqs40 and 31):

So, methane reforming with CO2 is clearly a largelyapplied reaction in industry.

It should be mentioned that in steam reforming ofmethane (eq 40), which is the most commonly used, asyngas mixture with a H2/CO ratio of 3 is produced,while the other two reactions produce syngas with a H2/CO ratio of 1. For MeOH synthesis, the ratio shouldbe ca. 2 which is usually achieved via the water-gas shift(WGS) reaction (45) or partial oxidation.

The WGS reaction often plays a role in reactionsinvolving CO and H2. It is interesting to analyze thereactions occurring in terms of the products obtained.The byproduct formed in oxidation reactions is eitherCO2 or water. The latter is not desired because in thatcase expensive hydrogen is converted into water.The WGS reaction is a crucial reaction in ammonia

and MeOH synthesis, forming large amounts of CO2.Also in other syngas reactions, e.g., the synthesis of

steam reforming of methane

H2O(l) + CH4(g) ) CO(g) + 3H2(g)∆H° ) +229.7 kJ/mol, ∆G° ) +161.5 kJ/mol (40)

CO2 reforming of methane57,58 (“dry reforming”)

CO2(g) + CH4(g) ) 2CO(g) + 2H2(g)∆H° ) +223.5 kJ/mol, ∆G° ) +170.8 kJ/mol (31)

partial oxidation of methane

CH4(g) + 1/2O2(g) ) CO(g) + 2H2(g)∆H° ) -59.7 kJ/mol, ∆G° ) -86.5 kJ/mol (41)

2CH4(g) + O2(g) f 2CO(g) + 4H2O(l) ∆H° ) -638.5 kJ/mol CH4, ∆G° ) -562.7 kJ/mol CH4 (42)

2CO(g) + O2(g) f 2CO2(g) ∆H° ) -283.2 kJ/mol CO, ∆G° ) -257.3 kJ/mol CO (43)

steam gasification of coal

C(s) + H2O(l) ) CO(g) + H2(g)∆H° ) +178.7 kJ/mol, ∆G° ) +100.7 kJ/mol (44)

WGS reaction:

CO(g) + H2O(l) f CO2(g) + H2(g)∆H° ) +6.2 kJ/mol, ∆G° ) -19.2 kJ/mol (45)


higher alcohols, syngas with a H2/CO ratio of less than2 is preferred and the WGS reaction can be applied totune the syngas composition.49 It is not necessary toapply the WGS reaction in order to arrive at the desiredratio. Syngas of the right composition can be producedfrom methane directly by combining CO2 and H2Oreforming. In such a way large amounts of CO2 areconsumed while at the same time the WGS reaction isavoided, which reduces process cost.In CO2 conversion processes, as mentioned above,

thermodynamics are in general not favorable andprocess schemes leading to a net CO2 consumption areonly possible in special cases. It is not surprising thatseveral processes based on renewable energy carriershave been suggested.Methane can be produced in a hydropyrolyzer in

which a carbonaceous feedstock (e.g., biomass) can beconverted to methane.59-63 The reactions for hydro-genolysis of biomass, coal (bituminous) and oil, respec-tively, are as follows:62

Hydrogen can be produced by catalytic decompositionof methane at 1273-1373 K:62,63

After separation, the hydrogen can be used in thehydrogenation processes, whereas the carbon can beused as a fuel or stored permanently or for futureuse.62,63 The combination of these reactions with meth-ane reforming in CO2 and syngas reactions can lead tolow or zero CO2 emission processes for electricitygeneration and production of chemicals, e.g., fuel metha-nol.59 Promising processes are the HYDROCARB andCARNOL processes.62,63 In the former process biomassand fossil fuels are converted simultaneously producingmethanol and carbon black, which is stored.62 Figure12 illustrates the HYDROCARB process.In this process, an essentially zero emission of CO2

is realized by removing carbon from the cycle. Whenthe carbon is stored, of course, an energy penaltyexists.62,63Figure 13 shows the CARNOL process.63The CARNOL process is similar to the HYDROCARB

process in that they both contain processes of methanepyrolysis and methanol synthesis. The feedstock inCARNOL process is methane instead of biomass and

fossil fuels, and instead of hydrogasification, CO2 gas-ification is applied. 50% of the carbon produced isstored and, as a consequence, prevented from enteringthe atmosphere.63Table 8 shows the methane efficiency and CO2 emis-

sion per mol MeOH produced for the four processes.63From the viewpoint of CO2 emission, the CARNOLprocess is superior to steam reforming, CO2 reforming,or a combination of steam/CO2 reforming.63 Of course,its CH4 efficiency is lower than the other processes, sincecarbon is produced and stored. The separation of thecarbon produced is less difficult as compared to otherprocesses. Carbon can be sold as a natural commodity.3.4.2.2. Direct CO2 Hydrogenation. CO2 can be

directly hydrogenated into several products.32,49,64,65 Aninteresting example is methanol synthesis.

Actually, in practice CO2 (ca. 5%) is mixed withsyngas in the synthesis of methanol.9By the reverse reaction of MeOH synthesis, viz.,

methanol decomposition,66 syngas can be obtained insitu. For small-scale applications of syngas this oftenis a convenient route.67 Another interesting applicationof methanol decomposition is the following. If syngasfrom MeOH decomposition is injected into a diesel orgasoline engine, the combustion efficiency can be greatlyenhanced.66 The energy needed for MeOH decomposi-tion can be provided by heat exchanging with the hotexhaust gas from the engines.MeOH, either pure or mixed with other fuels, can be

used as fuel for engines.66-71 It was reported that usingMeOH as a fuel less environmentally harmful gases,e.g., hydrocarbons, SOx, and NOx are produced, althoughformaldehyde emission is increased.67,68,71Note that most of the engines used in testing the

environmental effect of fuels were designed for usingfuels other than methanol.68,71 Engines designed spe-cially for using methanol will show even better perfor-mance. Methanol as fuel has also disadvantages. Dueto its lower heat capacity (compared with gasoline) alarger tank is needed. Methanol is more corrosive thangasoline and as a consequence, measures to prevent

Figure 12. HYDROCARB process.

CH1.44O0.66 + 1.94H2 ) CH4 + 0.66H2O (46)

CH0.8O0.08 + 1.68H2 f CH4 + 0.08H2O (47)

CH1.7 + 1.15H2 f CH4 (48)

CH4 f C + 2H2 (49)

Figure 13. CARNOL process.

Table 8. Comparison of Various MeOH ProductionProcesses from Methane63

process

CH4 efficiency(mol MeOH/mol CH4)

CO2 emission(mol CO2/mol MeOH)

steam reforming (eq 50)H2O + CH4 f CH3OH + H2 0.95 1.05

CO2 reforming (eq 51)CO2 + CH4 + 0.33H2 f 1.33CH3OH 1.039 0.962

steam + CO2 reforming (eq 52)1.5CH4 + H2O + 0.5CO2 f 2CH3OH 1.01 0.99

CARNOL process (eq 53)0.5CO2 + CH4 f 0.5C + CH3OH 0.845 0.684

CO2(g) + 3H2(g) f CH3OH(l) + H2O(l)∆H° ) -137.8 kJ/mol, ∆G° ) -10.7 kJ/mol (3)


corrosion should be taken.66-68,71 The methanol con-sumption is increasing tremendously, mainly becauseof changes in the composition of the gasoline pool. Itshould be stressed that MeOH used as fuel or fueladditive is one of the most important potential chemicalusage of CO2 by volume.26,68MeOH can be produced from hydrocarbons for which

there is no outlet. For example, it provides a means touse the hydrocarbons in remote oil production sites orrefineries. In many cases, natural gas is a byproductof oil winning and for convenience the natural gas isflared, leading to CO2 emission.67 An elegant solutionis to convert the useless hydrocarbons via synthesis gasinto methane which can be easily and safely stored andshipped.67The simplest example of direct hydrogenation of CO2

is the production of methane.

Of course, economically, this reaction is not attrac-tive: hydrogen is more expensive than methane.Fischer-Tropsch reactions are also possible:49,72

Besides these products, other products, such as alde-hydes, ethers, acids, and esters, can be synthe-sized.9,33,34,72 In the production of oxygenates comparedto hydrocarbons, less hydrogen is consumed and thethermodynamics is more favorable. This is illustratedfor formic acid and formaldehyde:

In the presence of H2 and HX (X ) OH, OM, OR, NR2),CO2 can be converted to HCOX, derivatives of formicacid: 9,10,33

Some of these products are quite useful, e.g., estersor amides of formic acid.3.4.2.3. Reactions to Other Products via MeOH.

Synthesis gas to MeOH is the only alcohol synthesisreaction with both a high selectivity (>99%) and a highproductivity (>1 g MeOH/g catalyst‚h).49,50,72 All theother alcohol products suffer from a lack of productivityand/or selectivity.49 MeOH is a bulk product which iswidely used in chemical industry.73 Many useful prod-ucts can be produced from methanol.73 Figure 14summarizes the major possible reactions into usefulproducts from MeOH.The carbonylation of methanol is currently the most

important synthetic route to acetic acid.The reaction of MeOH and formic acid may lead to

methyl formate:

Higher alcohols can be manufactured by reaction withsyngas.49 This reaction can be viewed as a combinationwith Fischer-Tropsch reactions.72 With the phase-outof the tetraalkyllead-type antiknocking agents in petrol,the octane pool needs to be modified by the addition ofeither alcohols or esters, e.g., MTBE. Both additionsneed a large amount of MeOH because either methanolis used as such or it is a reactant in the production ofthe additive.67-71 In some countries, e.g., in Germany,3-5% MeOH is added to the gasoline to enhance theoctane number.49Using the Mobil MTG (methanol-to-gasoline) process,

MeOH can be converted to gasoline. This is a routewhich can produce gasoline from nonoil sources.49,66,72,73Via dehydrogenation, formaldehyde can be produced:73

It is clear that methanol is a building block inchemical industry with a lot of potential.73 MeOH isthe first choice in the recent Japanese national programfor CO2 conversion.743.4.3. Photoreduction of CO2. The energy needed

for photoreduction of CO29,75-78 is solar energy or

another form of light. It is too early to assess thepotential of photoreduction. Only a few publicationshave appeared. Kisch et al.75 reported the heteroge-neous photocatalysis of CO2 to formate by irradiationof light with λ g 290 nm. Osaka et al.76 describedphotocatalytic fixation of CO2 in oxoglutaric acid toisocitric acid. Matsuoka et al.77 reported the photore-duction of CO2 to formic acid at irradiation of light withλ g 290 nm. Since renewable energy79,80 is used for theCO2 conversion, they certainly lead to CO2 mitigation.The problem, however, is the selectivity of the reactionsand in particular, the limited efficiency.3.5. Reactions with Compounds Having Acti-

vated Hydrogen Atoms. Compounds with activatedhydrogen in the molecule can react with CO2 by inser-tion of CO2 into the C-H bond.10,22 For example,acetophenone reacts with CO2 in the presence of phe-nolates to yield carboxylic acid (DMK ) dimethylketone):10

This reaction is interesting due to its mild reactionconditions and wide applications. For example, citric

CO2(g) + 4H2(g) f CH4(g) + 2H2O(l)∆H° ) -259.9 kJ/mol, ∆G° ) -132.4 kJ/mol (4)

nCO2 + (3n + 1)H2 f CnH2n+2 + 2nH2O (54)

nCO2 + 3nH2 f CnH2n + 2nH2O (55)

CO2(g) + H2(g) f HCOOH(l)∆H° ) -31.0 kJ/mol, ∆G° ) +34.3 kJ/mol (1)

CO2(g) + 2H2(g) f HCHO(g) + H2O(l)∆H° ) -11.7 kJ/mol, ∆G° ) +46.6 kJ/mol (2)

H2 + CO2 + HX f HCOX + H2O (56)

CH3OH(l) + HCOOH(l) f HCOOCH3(l) + H2O(l)∆H° ) -309.5 kJ/mol, ∆G° ) -8.5 kJ/mol (57)

Figure 14. Scope of production of chemicals via MeOH-basedroutes.

CH3OH f HCHO + H2 (58)

O O O

PhCCH3 + CO2PhOK

DMKPhCCH2COOK PhCCH2COOH (59)


acid can be made via three stages from acetone (DMF) dimethylfuran):9

The reaction of sodium phenolate with CO2, the so-called Kolbe-Schmitt process, has been used in theindustrial synthesis of salicylic acid since 1874:9,22

The product is mainly used in the production ofaspirin, the most popular drug of all. In 1980 in USA,the production of salicylic acid amounted to 25 kt/y.Carboxylation of hydrocarbons with activated hydro-

gen has received a lot of attention. Figure 15 showsthat a wide variety of products can be made.223.6. Reactions with Hydrocarbons without Ac-

tivated Hydrogen Atoms. In contrast to activated-hydrogen-containing hydrocarbons, paraffins which donot contain activated hydrogen can be carboxylated onlyunder drastic conditions. Of course, unsaturated hy-drocarbons are more reactive than saturated ones.

3.6.1. Saturated Hydrocarbons. CO2 methanereforming is probably the most interesting reaction withsaturated hydrocarbons.13,57,58

It has been commercialized under the name SPARGprocess. Levy et al.81 described a 20 kW reforming/methanation installation based on a solar furnace, forproviding the energy for heating the CO2 and for theenthalpy of the endothermic reforming reaction.CO2 reforming is not restricted to methane; higher

hydrocarbons may also be converted:

Moreover, as stated before, in principle, other prod-ucts may be produced:

In view of the low reactivity of both CO2 and CH4, ahigh temperature is called for and an acceptable selec-tivity can probably not be reached. Moreover, thermo-dynamics is not favorable.It has been suggested to carry out cracking of hydro-

carbons in the presence of CO2.9,10,22 The process takesplace at elevated temperatures, typically at 800-900K. Radicals are formed and in this reactive environ-ment CO2 might show reactivity and interesting prod-ucts could be formed, e.g., various lower carboxylicacids.9,10,22

An original suggestion is to use underground reser-voirs as reactor. Under such conditions temperaturesof say 333 K and pressures up to 130 bar exist, allowingthe synthesis of oxygenates.In principle, it is possible to use supercritical liquid

CO2 to extract the oxygenates formed and to bring themto the surface for further concentration.9 Due to thelarge reactor volume and, as a consequence, the longresidence time, the conversion might be acceptable.However, the costs for product extraction and purifica-tion may not be favorable. To the same category belongsmicrobiologically enhanced oil recovery.823.6.2. Unsaturated Hydrocarbons. The reactions

of CO2 with unsaturated hydrocarbons can be subdi-vided into9,10,22,83 reactions with monoalkenes, dienes,cycloalkenes, alkynes, and other substituted hydrocar-bons.3.6.2.1. Monoalkenes. CO2 can react with monoalk-

enes, e.g., ethylene and propylene, at 743-753 Kcatalyzed by triethylamine and calcium chloride, leadingto, respectively, ethylene carbonate and propylene car-bonates:9,22

Both products are very useful. For example, propy-lene carbonate is a tonnage petrochemical solvent;ethylene carbonate has also an excellent potential.10

Figure 15. Some compounds with active hydrogen and theircarboxylation products.22

O

CH2–COOH OH

CNCH2–COOH

C O + CO2

CH3

CH3

PhONa

DMF

[H+]

HCN

(60)

HO–C–COOH NaOOCCH2CCH2COONa

NaOOCCH2CCH2COONa

ONa

COONa

OH

COOH

OHCO2 +[H+]

(61)

CO2(g) + CH4(g) f 2CO(g) + 2H2(g)∆H° ) +247.5 kJ/mol, ∆G° ) +170.8 kJ/mol (31)

nCO2 + CnH2n+2 f 2nCO + (n + 1)H2 (62)

CO2(g) + CH4(g) f CH3COOH(l)∆H° ) -13.3 kJ/mol, ∆G° ) +58.1 kJ/mol (11)

CnH2n+2 + CO2 f CmH2m+1COOH (m , n) (63)

CO2(g) + C2H4(g) f CH2dCHCOOH(l) (15)

CH3CHdCH2 + CO2 f CH2(COOH)CHdCH2 (64)


Other organic carbonates also may have applicationsas solvents or additives.Lapidus et al.84 reported a Ru-catalyzed reaction of

ethylene and CO2; ethanol, propionic acid, and its ethylester were formed in yields of up to 38% at 450 K and700 atm.

Ethylene, propylene, 1-butene, 2-butene, and 1-pen-tene can react similarly.85 However, many of thesereactions cannot be carried out catalytically, but onlystoichiometrically. A challenging goal in CO2 chemistryis the development of catalysts for these conversions.3.6.2.2. Dienes. In a great number of stoichiometric

reactions, 1,3-dienes proved to be more reactive thanmonoenes. Reactions with dienes are illustrated by thatof isoprene:

Apparently, CO2 insertion into the dimer occurs.86Two esters and a pyrone are formed from allene in

the presence of a Pd catalyst as shown below.10

Ni and Ru compounds catalyze the formation ofδ-lactones, though only in low yield (ca. 6%).10The large variety of new lactones presented in this

section may become of interest as fine chemicals whichmay be useful, e.g., in the production of fragrances. Agreat number of natural compounds are known whichhave a similar lactone structure.10 Both in plants, forinstance, the aromatic components of black tea, straw-berries, or coconuts, and in animal products, for ex-ample, in beef, pork, and butter fat, comparable lactonesare found. After alcoholic fermentation, numerouslactones act as flavorings in whiskey, rum, beer, andwine. The application of lactones as herbicides orpharmaceuticals is also under investigation.10Many attempts of reacting dienes, other than buta-

diene, with CO2 failed. However, copolymerization ofbutadiene and CO2 with isoprene or piperylene yieldednovel lactones,10 as shown in Figure 16, though theyields were low (around 5%).These products, substituted δ-valerolactone (lactones

of substituted 5-hydroxypentanoic acid) might be inter-esting chemicals in pharmaceutical industry.Butadiene can also react with CO2 and an epoxide

catalytically. The reactions with ethylene or propyleneepoxide are shown below. The byproducts are products

of the reactions between butadiene and CO2 (lactones),and the octadienyl esters.9,10

Copolymer of styrene and a conjugated diene, hydro-genated in the presence of CO2, is used to introduce acarboxylic groups, predominantly on the diene units(carboxylated polymers).3.6.2.3. Cycloalkenes. Cyclic methylenecyclopro-

panes are more reactive than linear alkenes, leading toγ-lactones by using Pd catalysts.10

CH2dCH2 + CO298[Ru]

CH3CH2CO2CH2CH3 +CH3CH2OH + CH3CH2COOH (65)

H2CdCHsCMedCH2 + CO2 f

Me2CdC(CO2H)CH2CHdCMeCHdCH2 +Me2CdC(CO2H)CH2CHdCHCMedCH2 (66)

O

O

O

O

O O

CH2 C CH2 + CO2

(67)

[cat]

Figure 16. Linkage modes of isoprene and piperylene withbutadiene and CO2.

OOH

O

OOH

O

OOH

O

O

O

+ CO2[cat] (68)

+

O

O

O

O

O

O

O

O

O

O O

CO2

CO2

CO2

[A]

[B]

[B,C]

[A]

[B]

(69)

(70)

(71)

[A] = Pd(dba)2/PPh3[B] = Pd(dppe)2[C] = Pd(PPh3)4

(dba = dibenzylidineacetone)(dppe = 1,2-bis(diphenylphosphino)ethane)

+


The reaction of unsubstituted methylenecyclopropanewith CO2 leads to 3-methyl-2-buten-4-olide in yields upto 80%.10

3.6.2.4. Alkynes. CO2 can react with alkynes (e.g.,1-butyne, propyne), producing unsaturated organic car-bonates. Thus, 1-hexyne reacts with CO2 producing 4,6-dibutyl-2-pyrone.10

With other substituted alkynes, substituted pyronesare obtained. Alkynes with internal triple bonds, suchas 3-hexyne or 4-octyne, can react similarly, whilephenyl and diphenyl acetylene did not react, possiblyfor steric reasons.Derien et al.87 reported that the incorporation of CO2

into nonactivated alkynes gives R,â-unsaturated acids.3.6.3. Reactions with Other Substituted Hydro-

carbons. CO2 is able to react with many other substi-tuted hydrocarbons.9,10,22 For example, reactions withpolymers giving carboxy-terminated polymers (func-tionalization of polymers) were reviewed by Quirk etal.88 The products of CO2 with hydrocarbons may leadto polymers with special properties, carboxylic acids,esters, lactones, etc.Amatore et al.89 reported the electrochemical reaction

between CO2 and bromobenzene, catalyzed by NiII-(dppe)Cl2 (dppe ) 1,2-bis(diphenylphosphino)ethane),which after hydrolysis leads to benzoic acid nearlyquantitatively.

3.7. Production of Polycarbonates. CO2 can reactwith R,ω-dibromo compounds and the potassium salt of

diols by the promotion of crown ethers; polycarbonateswith the structure shown below are the products: 22

A number of crown ethers can be used; those havingan 18-membered ring were found to give the bestresults.22Synthesis of polycarbonates represents, in principle,

a method for long-term CO2 fixation as organic poly-mers.83 Of great interest is the polymerization ofpropylene carbonate to yield high molecular weightpolymers. These should be long lasting and finallybiodegradable and should have very large markets.83These polymers can also be strengthened by inorganiccarbonates. It is also possible to synthesize polycar-bonates by CO2 reaction with epoxides, which will bedescribed in the following section.Polycarbonates are appealing in the sense that they

constitute a type of chemical change of CO2 molecules.The best compound in this respect would be the poly-carbonate,

. A higher density of CO2 is not conceivable. We,however, expect that this polymer will not be stableenough under ambient conditions.3.8. Reaction with Epoxides. CO2 can form co-

polymers with epoxides9,22,83 under mild conditionscatalyzed by organometallic catalyst systems. Theproduct is a high molecular weight aliphatic polycar-bonate. The reactions are given below.

Many epoxides are copolymerizable. Figure 17 showssome of them.22Many catalyst systems can catalyze this reaction. It

was reported that using aluminum porphyrin catalystor diethylzinc-water catalyst, the copolymer has a verynarrow molecular weight.22 The synthesis of macro-molecules with controlled molecular weight is of impor-tance for molecular design of polymeric materials. Thisreaction has been claimed to be the first example of thesynthesis of an alternative copolymer with a narrowmolecular weight distribution.9High molecular weight alternating copolymer of CO2

and propylene oxide or ethylene oxide may be moldedto sheets or film by pressing it at 413-423 K.9,83 Lowpermeability of oxygen is one of the characteristics ofthis material. Because of the high oxygen content they

O O

O O

O O

O O

O O

O O

(72)

copentamers

codimer

cotrimers

cotetramers

[1]

(1)

(1)

(1)

+ CO2

[Pd]

[Pd]

O

n-Bu

O

n-Bu

CO2+ 2n-BuC CH (73)[Ni]

CO2 + C6H5Br + 2e f C6H5COO- + Br- (74)

C(CH3)2

CH2OCOOROCOCH2

CH2–,–CH2,R =

CH2X + nKO–R–OKnXCH2

CO2

crown ethers

X = Cl, Br

+ 2nKXn

(75)

C( O) On

OO O

O

O

O

O–C–O

O

CH2–CHR + CO2 (CH2–CHR–O–C–O)

O

[cat]

[cat]

+ CO2

+ nCO2n

(76)

(77)

(78)

n

n


possess a low heat of combustion and decompose at arelatively low temperature.9 Their highly regular struc-ture is reflected in the fact that decomposition takesplace within a narrow temperature range and a simpleproduct spectrum is observed.9,83 The product of thethermal decomposition of CO2-propylene oxide copoly-mer, for example, at 453 K is exclusively propylenecarbonate. The aliphatic polycopolymer was confirmedto be biodegradable, which makes it applicable incontrolled release drug systems.9 Those long-lastingbiodegradable polymers should have large markets.CO2 can also react with epoxides to form cyclic organic

carbonates:10

Chemische Werke Huls adopted this reaction for anindustrial process, and plants using the Huls processwere built in Germany and Romania.10 At room tem-perature ethylene carbonate is a solid which melts at310 K and boils at 520 K. Propylene carbonate has alsoan exceptionally high boiling point (516 K).10 These twocyclic carbonates are used as high-boiling solvents fornatural and synthetic polymers such as lignin, celluloseester, nylon, and PVC. The ability of these solvents todissolve a wide range of substances has led to theirextensive use in the production of polyacrylic fibers andpaints.10It is possible to produce vinylene carbonate via

chlorinating and subsequently dehydrochlorinating.10

The reaction for linear carbonates is illustrated by thesynthesis of 1,2-propanediol formates from CO2, H2, andpropylene oxide.33

Other examples of epoxides that can form cycliccarbonates with CO2 are styrene epoxide and epichlo-rohydrin (1-chloro-2,3-epoxypropane). The diepoxide ofbutadiene leads to erythritol (HOCH2[CH(OH)]2CH2OH)dicarbonate. Vinylethylene carbonate is formed fromthe monoepoxide in the presence of KOH:10

The major applications of cyclic carbonates are il-lustrated in Figure 18.9Many useful products, which are otherwise difficult

to prepare, can be made from cyclic carbonates.3.9. Reaction with Alcohols and Ethers. Alcohols

can react with CO2 in the presence of a metal alkoxide;the general reaction equation is22

Alkyl formates can be produced, catalyzed by severaltransition metal complexes and tertiary amines under25 atm of CO2 and H2 at 413 K:22,33

Typical catalysts are Pd(diphos)2 and RuH2(PPh3)4,while Et3N is a representative amine cocatalyst.33Epoxides can be considered as a type of cyclic ethers,

whose reactions with CO2 have already been described.Other ethers can also react with CO2.CO2 and ethyl vinyl ether yield a polymeric product

containing 23% ester linkage when heated at 340 K withaluminum acetylacetonate or aluminum alkoxide as acatalyst. Vinyl methyl ether gives a copolymer with CO2when heated at 350 K without any added catalyst.22

Figure 17. Cyclic ethers for copolymerization with CO2 byan organozinc catalyst system.22

O O

O

R

O

CH2–CH–R + CO2 (79)[cat]

(R = H, CH3)

OO O O O O

O O

Cl

O

+ Cl2

–HCl

–HCl(80)

CO2 + H2 + H2CO

CH

CH3

O

H2C

HCO

CH + H2C

CH3

OH HO

O

CH + H2C

HCO

CH3

CH

CH3

OCH

OO

HCO

+ H2C

O

CH + H2C

CH3

O HO

CH2

CH

CH3

OH

(81)

Figure 18. Chemistry based on carbonates.

CH2

OCH CH CH2 + CO2

OC

O

CHCH2 CH CH2

O

(82)polymers

[cat]

O

ROH + CO2 ROCOH (83)R′OM

ROH + CO2 + H2 f HCOOR + H2O R )Me, Et, Pr (84)

CH2 CH + CO2

OCH3

CH CH2

OCH3

C

O

CH2 CH

OCH3

O CH

OCH3

CH2

(85)x

y


This type of copolymers might be used in the produc-tion of blends of poly(vinyl methyl ether) (PVME) andpolycarbonates.3.10. Reaction with Nitrogen Compounds. 3.10.1.

Reaction with NH3. CO2 can react with variousnitrogen-containing compounds, e.g., ammonia andamines.9,10,22 The best known example is urea produc-tion:

In fact, this process belongs to bulk chemical produc-tion (1985: 42 Mt/y).10 Urea or (alkyl-) substitutedureas can be further reacted to produce derivatives.223.10.2. Reactions with Amines and Imines. Ali-

phatic amines can be carboxylated by CO2; R-aminoacids are obtained.22

Ureas can be synthesized from CO2 and variousamines with a number of phosphorous compounds inthe presence of tertiary amines under mild conditions.

Many symmetrical ureas can be similarly synthesized(cf. Table 9).22The reaction is influenced by the basicity of the amine

and the steric hindrance at the nitrogen atom of theamine; the latter may explain the failure of similarreactions with dialkylamines.Diamine can react further with CO2 with various

N-phosphonium salts of pyridine, giving polyurea:9

Table 10 shows some of the diamines that can reactin polycondensation reaction.22Some diamines may form copolymers with CO2 and

acrylonitrile (CH2dCHsCN). Thus when heated at393-433 K in the presence of triethylenediamine,N(C2H4)3N, CO2, and acrylonitrile form a copolymerwith the following structure:22

Ternary copolymerization is also possible such as thatof CO2, cyclic phosphonite and acrylic compounds (e.g.,acrylonitrile), yielding a 1:1:1 copolymer with molecularweight of ca. 20000.22

It was reported that various carbamates, compoundswith an -O-C(O)-N- bond, can be obtained from theCO2 reaction with amines. Table 11 gives some ex-amples of the cyclic carbamates.22

Reactions may take place between CO2, an epoxide,and an amine, with carbamate and amino alcohol (thedecarboxylation product of the former) as the products.22Table 12 shows some examples.

Table 9. Symmetrical Ureas from CO2 and Amines22

aminea yield of symmetrical ureas (%)b

aniline 85o-toluidine 74isopropylamine 23cyclohexylamine 27N-methylaniline 0diphenylamine 0diethylamine 0

a HOP(OC6H5)2 ) amine ) 50 mmol in 40 mL of pyridine; T )313 K; time ) 4 h. b Based on the amine used.

2NH3 + CO2 f H2O + NH2CONH2 (86)

NH2

RCH2NH2 + CO2

H2O2 (30%)

283–323 K, HClD,L-RCHCOOH (87)

O

CO2 + RNH2 + HOP(OC6H5)2Pyr

313 K

RNHCNHR + C6H5OH + (OH)2POC6H5 (88)

NH(CH2)nNHCO (89)NH2(CH2)nNH2 + CO2

(PhO)2POH

m

CH2 CH2 O CNCH2

CH2

CH2

CH2

N C

O

O CH2 CH

CN

CH2 CH

CN

O

(90)

x 1–x

Table 10. Yields of Polycondensation of CO2 and VariousDiamines22

diamine yield (%)

NH2 CH2 NH2

100

NH2 O NH2

100

NH2 O C (CH3)2

2

100

NH2 SO2 NH2

86

NH2 NH2

100

NH2CH2

CH2NH2

46

a Using diphenyl phosphate in pyridine at 20 atm CO2 at 673K for 4 h.

Table 11. Examples of the Synthesis of Carbamates22

O

OPC6H5 + CH2 CH + CO2

CN

CH2 CH2 O P

C6H5

O

CH2 CH

CN

C

O

O

n

(91)


Carbamate esters can be prepared from CO2, vinylether and amines (dimethyl or diethyl).22

The reaction products of epoxide and CO2, organiccyclic carbonates, can be used in chemical synthesis;they react with ammonia and several amines, even atlow temperatures, to form carbamates:10

In the reaction with diamines, di(hydroxyethyl) car-bamates are formed which, for example, can reactfurther with urea to form polyurethanes.CO2 can also react with aziridines to form products

which possess urethane (H2NCO2C2H5) linkages. Thering-opening reaction is facilitated only by acidic re-agents, in contrast to that of epoxides which is facili-tated by both acidic and basic reagents.10 The reactionis as follows:

Ethylen- and propylenimines also react with CO2 toform soluble polymers.22 This type of reactions isundesired, leading to deterioration of the aziridines.90Aziridines are useful chemicals in paper making indus-try, textile industry, and waste water treatment. Thereaction is often very exothermic which may lead toexplosion.90 Sodium hydroxide is often used to inhibitthe reaction by neutralizing carbon dioxide in the air.3.11. Reactions with Sulfur Compounds. Many

processes exist for the conversion of H2S in wastestreams in a useful product. The most prominentprocess is the Claus process. Here, part of the H2S iscombusted in air giving SO2. CO2 can be used tosubstitute air:91

The extremely high positive values of ∆H° and ∆G°do not need further comment. Nevertheless, also in thiscase a process might be feasible by using a combinationof O2 and CO2 in a multifunctional reactor such as amembrane reactor.Direct oxidation processes exist in which H2S is

oxidized to elemental sulfur: CO2 can substitute alsoin this case air:92

CO2 can also be used in the manufacture of COS.93

CO2 is added to an aqueous solution of thioantimonatein NH4Cl to manufacture antimony pentasulfide.943.12. Substitution for Other Chemicals. Many

chemicals can be made from CO2, which can substituteother chemicals, that are harmful to the environment.This substitution can be either direct or indirect. Theidea of direct substitution is illustrated in Figure 19.25Urea, a product from CO2, can in many cases directly

replace phosgene (COCl2, with an annual market of 8Mt) in the production of organic carbonates, e.g., dim-ethyl carbonates, diethyl carbonates, diphenyl carbon-ates.9,25

The old process using phosgene may result in Cl-containing byproducts.25

By using urea, not only is the use of poisonousphosgene prevented but also the production cost isreduced.25 There is a market for dialkyl carbonates assolvents and as chemical intermediates for the produc-tion of plastics; e.g., that for dimethyl carbonate isestimated to be about 105 tons per year.31 The synthesisof isocyanates, useful intermediates in the chemicalindustry, from primary amine carbonates obtained fromCO2 (dialkylcarbonates) deserves specific attention.Another example is the use of CO2 in fumigation to

replace chemical pesticides traditionally used.15 CO2 isregistered at the US Environmental Protection Agency

Table 12. Examples of Products Formed by the Reactionof CO2, Epoxide, and Amine22

epoxide aminea carbamate (%) amino alcohol (%)

OMe2NH 6 15Et2NH 21 15n-PrNH2 15 6Ph2NH 0 0

O

Me2NH 16 2Et2NH 24 42n-PrNH2 20 21Ph2NH 0 0

O

Me2NH 42 40Et2NH 62 5n-PrNH2 54 5Ph2NH 0 0

a CO2, 50 bar, amine and epoxide, 0.2 mol each; 353 K; 22 h.b Oligomer of ethylene oxide was formed.

R2NH + CO2 + H2C CHOCH2CH3

OCH2CH3R2NC(O)OCH

CH3

(92)

308–373 K

70 h

O O

R

O

+ NH3 (93)

OH

R O

CH2CHOCNH2

(94)

CH2

N

CH2

R

CO2 + CH2CH2N CH2CH2NCO

RORn m

2H2S(g) + 4CO2(g) f 2SO2(g) + 4CO(g) + 2H2(g)∆H° ) 578.9 kJ/mol, ∆G° ) 494.8 kJ/mol (95)

Figure 19. Direct substitution opportunities for CO2 (afterLipinsky).25

2H2S + CO2 f CS2 + 2H2O (96)

CS2 + CO2 f S(g) + CO T > 800 K (97)

CS2 + CO2 f 2COS (98)

(99)

O

H2NCNH2 + 2ROH ROCOR + 2NH3

O

(100)COCl2 + 2ROH ROCOR + 2HCl

O


as a nonrestricted pesticide. Even an atmosphere of60% CO2 is sufficient to kill 100% of all insects viadesiccation caused by hyperventilation.15 Moreover,insects cannot develop a resistance to it. This applica-tion of CO2 is environmentally beneficial.An example of indirect substitution is the replacement

of CFCs. When solid CO2 is used to replace CFCs inthe in-transit refrigeration,15 not only are the CFCsreplaced by the harmless CO2, but also the harmfuleffect of CFCs on the depletion of ozone layer is reduced.Another example is the replacement of relatively

expensive TiO2 ($2000/ton) by cheap CaCO3 ($200-500/ton) in paint, paper, rubber, and ink industries.25CaCO3 is one of the most stable carbon compounds,which can be easily made from lime and CO2. Thisreplacement leads to cost savings. Currently, almosthalf (ca. 0.5 Mt/y) of the TiO2, used in the past inindustry, has been replaced by CaCO3.25 This is anexample of indirect substitution. Unfortunately, sinceenergy is needed for the decomposition of CaCO3, andthis decomposition itself produces CO2, the wholeprocess of CaCO3 production cannot contribute to thereduction of CO2.In fact, one may think of numerous cases, in which

products from CO2 may substitute one or anotherchemicals, directly or indirectly, which make the pro-cesses safer, cheaper, or environmentally better.3.13. CO2 as a Carbon Source. After splitting into

carbon and oxygen or after reduction, CO2 can be usedas a carbon source in CVD (chemical vapor deposition)synthesis of diamond and carbon fibers95 and in themanufacture of additives for lubricants (by carbon-ation).96,97Walker98 reported the synthesis of tar- and ash-free

carbon from CO via the Boudouard reaction (reversereaction of eq 29).

4. Bioconversion

CO2 can be converted biochemically or biologically; ofcourse, the most important reaction is photosynthesis:10

It was estimated that glucose is formed at a rate of 1g per hour per square meter of leaf surface and ca. 200Gt/y of glucose are produced by this process.10 Oneshould realize that this reaction uses atmospheric CO2with a very low concentration (ca. 0.4%) and solarenergy to produce organic fuel substituent with a highselectivity and a high rate at ambient temperature.Until now, none of the man-made catalysts couldcompete with this catalyst made by mother nature.It was estimated13 that in The Netherlands, each year

475 000 hectares of agricultural land will become free.Taking the capacity of photosynthesis as 3000 kgcarbon/ha annually, ca. 3% CO2 emitted can be used inphotosynthesis. This example shows that due to thelarge area of land needed, the capacity of CO2 mitigationvia bioconversion is very limited.Of course, efficient agricultural technology might

result in the sequestration of larger amounts of CO2.99

5. Concluding Remarks

CO2 emission is mainly caused by the combustion offossil fuels, and the energy sector is the major contribu-

tor to CO2 emission. In a certain sense, the CO2problem is an energy problem. Saving energy is a directmethod of reducing CO2 emission. The increase of theenergy efficiency in the energy sector is extremelyimportant in reducing CO2 emissions.The CO2 production from various fossil fuels is per

unit of energy quite different. Therefore, an option isto use a higher fraction of natural gas (methane) whichproduces the lowest amount of CO2.It is not surprising that thermodynamically the

conversion of CO2 is unfavorable. Only in specialsituations CO2 conversion is favorable in CO2 mitiga-tion. In fine chemistry several reactions with a negativeGibbs free energy exist. However, the volumes involvedare too modest. In bulk chemistry not so many favor-able reactions exist, but here the volumes involved aremuch higher. The equilibria change when the reactionconditions are changed. CO2-containing gases at severeconditions (high temperature or high pressure) can leadto an attractive process. An example is “chemicalcooling” of hot gases in the chemical process industry.In general, there is an energy penalty with CO2

reactions: energy input is needed for the reaction or forthe production of a reactant, for example hydrogen, orelectricity. The most ideal energy source is of course arenewable energy, such as solar, wind, tidal, hydro,geothermal energy, and biomasses. An example is usingCO2 in methane reforming, coupled with solar energyinput.100Carbon dioxide can react in different ways with a

large variety of compounds. The products that may beobtained are, e.g., organic carbonates, (amino-) acids,esters, lactones, amino alcohols, carbamates, urea de-rivatives, and various polymers or copolymers. Thelimited number of publications in this research areashow that this new territory is still to be exploited.Some of these products are of great technical interest.The major reactions and their products are listed inTable 13.CO2 may be utilized directly or indirectly; the latter

refers to the use of reaction products of CO2 as describedabove.Considering the potential amounts of CO2 to be

utilized chemically, the production of bulk products isrelevant whereas fine chemicals are not very interest-ing, because of the low volumes involved. MeOH andother gasoline or diesel substitutes are potential can-didates. Moreover, MeOH as engine fuel leads toenvironmental advantages over gasoline at equivalent

nCO2 + nH2O f (CH2O)n + nO2 (101)

Table 13. Reactants and Their Products in CO2Reactions

reactants products with CO2

alkane syngas, acids, esters, lactonescycloalkane acids, esters, lactonesactive-H compd acids, esters, lactonesmonoalkene acids, esters, lactonesdienea acids, esters, lactonesbcycloalkene acids, esters, lactones, (co)polymerssubstitutedhydrocarbonc

acids, esters, lactones, polycarbonates

alkyne lactones, unsaturated organic carbonatesepoxide carbonates, (co)polymers (polycarbonates)NH3 and amine symmetrical ureas, aminoacids, (co)polymersdiamine ureas, carbamates, (co)polymers (polyureas)imines carbamates, (co)polymers (urethane)

a Allenes and 1,3-dienes. b With longer C-C chain than theoriginal monomer. c Dihalogen substituted.


temperature and average speed (except for formalde-hyde emission).68,71,101Although the knowledge of mankind in biochemistry

and catalysis has improved tremendously, photosyn-thesis is still the most efficient method for CO2 conver-sion. The development in imitating photosynthesiscatalysts will certainly contribute to the reduction ofCO2 emission.Using CO2 as a reactant, many useful products can

be made. The economy of these processes, besides theprice of products and reactants, depends on the sourceof energy and/or hydrogen and on the development ofnew active catalysts. Many of the potential technologiesare already used. In countries with a (high) carbon tax,the relevant technologies could be implemented earlier.The environmental impact of many of the proposed

technologies is unanswered. Possibly, depleted oil andgas fields are attractive options for CO2 disposal thoughthese have their own unique problems and limitedapplicability.

Acknowledgment. We thank NOVEM (Nether-lands Agency for Energy and the Environment) forfinancial support and Braun Consultants in Technologyand Management in Hengelo, The Netherlands, forsubcontracting us in the NOVEM project.

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