DOI: 10.1002/cssc.201100313

Can We Afford to Waste Carbon Dioxide? Carbon Dioxideas a Valuable Source of Carbon for the Production of LightOlefinsGabriele Centi,*[a] Gaetano Iaquaniello,[b] and Siglinda Perathoner*[a]

1. Introduction

The projections for baseline CO2 emissions without measuresto limit them (business-as-usual), estimate a value of around57 Gt CO2 for year 2050.[1, 2] In a “Blue Map” scenario, the emis-sions should be reduced to 14 Gt CO2 from the current valueof around 30 Gt CO2. Assuming as a more realistic target avalue of 35–40 Gt CO2 for 2050, it is necessary to cut about20 Gt of CO2 emissions in the next four decades. This goal im-plies a combination of efforts ranging from saving energy tousing alternative energy sources (e.g. , nuclear, renewables),but also requires a contribution by CCS (carbon capture andsequestration) technologies, estimated to be around 15–20 %of the cut of the emissions of CO2. A McKinsey report[3] esti-mated the global potential of CCS at 3.6 Gt a�1, and in Europeat 0.4 Gt a�1—around 20 % of the total European abatementpotential in 2030. This means that large amounts of CO2 willbe progressively available in the future. It is thus in our interestto consider CO2 as a valuable chemical, and not as waste.[4–8]

CO2 is already used in relatively large amounts in somechemical processes, mainly urea synthesis and salicylic acidproduction (overall about 110 Mt). Direct (non-chemical) usesof CO2 for enhanced oil and coal-bed methane recoveries canbe applied in some specific cases because of geological andlocal conditions. Other direct-use possibilities (e.g. , refrigerants,food production, plant growing stimulants) are well-estab-lished, but offer limited possibilities for further growth. There-fore, the conversion of CO2 to fuels and chemicals (carbon cap-ture and recycling; CCR) emerges as a valuable possibility toexploit a low-cost (even negative-value) raw material andreduce its impact on the environment. Due to socio-politicalpressure, the development of a resource-efficient and low-carbon economy is becoming a major driver for the chemicalindustry worldwide, especially in more sensitive areas such as

Europe. The Europe 2020 policy initiative, for example, fostersa shift towards a resource-efficient and low-carbon economy.

Therefore, it is necessary to consider the strategies for usingCO2 from the viewpoint of resource efficiency. There are twomain driving factors in reusing CO2: (i) the product/processvalue, and (ii) the economics associated to carbon taxes. CO2 isan economic raw material to produce some valuable materials,for example polycarbonate and fine chemicals. These processesmay be economical even without carbon taxes. In the secondcase, the exploitability of the reaction (e.g. , methanol synthe-sis) is instead very dependent on carbon taxes, but the largermarket (for fuel uses) indicates a potential contribution to thecontrol of CO2 emissions not present in the first case.

For example, polycarbonate is the most important large-volume example for CO2-based polymers. The global poly-carbonate market in 2009 was 2.9 million tonnes, and is ex-pected to grow at a rate of 3–5 %. Assuming that about 10–15 % of the global production could originate from new pro-cesses based on CO2 by the year 2020, the impact on the re-duction of CO2 emissions will be of the order of 0.1–0.2 Mt a�1,that is, about 0.005 % of the estimated amount of CO2 avail-able from CCS. The potential for methanol is significantly

[a] Prof. G. Centi, Prof. S. PerathonerDipartimento di Chimica Industriale ed Ingegneria dei MaterialiUniversity of Messina and INSTM/CASPEV.le F. Stagno D’Alcontres 31, 98166 Messina (Italy)Fax: (+ 39) 090 391518E-mail : centi@unime.it

perathon@unime.it

[b] Dr. G. IaquanielloTecnimont KT S.p.A.Viale Castello della Magliana 75, 00148 Rome (Italy)

Concerns about climate change have increased the amount ofactivity on carbon capture and sequestration (CCS) as one ofthe solutions to the problem of rising levels of CO2 in the tro-posphere, while the reuse of CO2 (carbon capture and recy-cling; CCR) has only recently received more attention. CCR isfocused on the possibility of using CO2 as a cheap (or evennegative-value) raw material. This Concept paper analyzes thispossibility from a different perspective: In a sustainable vision,can we afford to waste CO2 as a source of carbon in a chang-ing world faced with a fast depletion of natural carbon sources

and in need of a low-carbon, resource-efficient economy? Oneof the points emerging from this discussion concerns the useof CO2 for the production of olefins (substituting into or inte-grating with current energy-intensive methodologies that startfrom oil or syngas from other fossil fuel resources) if H2 fromrenewable resources were available at competitive costs. Thisoffers an opportunity to accelerate the introduction of renewa-ble energy into the chemical production chain, and thus to im-prove resource efficiency in this important manufacturingsector.

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larger, but the implementation possibilities are highly depend-ant on the economics associated to carbon-taxes and renewa-ble energy.

In both cases, the driving force is the possibility to use acheap (or even negative-value) raw material, although the costof CO2 as a raw material is determined by the cost of its recov-ery minus the costs avoided because it is not emitted (carbontaxes, if applicable). The difference between these two is thevalue of the product. In products with a high added value(polycarbonate, fine chemicals) the cost of CO2 as raw materialis a relevant, but not critical factor. On the contrary, it is thecritical factor in converting CO2 back to fuels. Therefore, evenif this second route can have a more effective contribution tothe control of climate changes, its applicability dependsstrongly on socio-political conditions, that is, taxes on CO2

emissions.

2. Discussion

2.1. Reuse of CO2, climate change goals, and resource effi-ciency

In the approaches discussed above, the discussion about CO2

uses cannot be separated from the discussion about climatechange goals, the latter determining the carbon taxes (or, alter-natively, the subsidies for reducing CO2 emissions) and, in turn,the “value” of CO2 as raw material. However, it should bestressed that this is a limited vision on the problem, and it ismore correct to view this problem in the general context of re-source efficiency.

The carbon resources of the chemical industry are limited.Over 85 % of the world’s energy need is currently satisfiedthrough fossil fuels, and an estimation by the InternationalEnergy Agency indicates that they will remain the main energysource up to 2050, and possibly beyond.[9] Changing theenergy infrastructure requires very large investments and, thus,the timescale for the substitution of fossil fuels is very long. Ina baseline scenario for 2050, the use of primary energy rises by84 %, with the demand for liquid fuels (mainly from oil) risingby 57 %. In a Blue Map scenario, the primary energy use risesby 32 % with demand for liquid fuels falling by 4 %.[1] In a real-istic intermediate scenario, it can be assumed that oil demandincreases by 20–30 % relative to current use. When combiningthis result with the estimated oil resources [although large un-certainties exist around these estimates, with an average peakoil production (Hubbert peak) forecast between 2015 and2030] the price of oil can be expected to reach a value of$150–200 per barrel, or more. In a high-price scenario, the USEnergy Information Administration (EIA) estimates that in 2035the price of oil may reach ca. $210 per barrel.[10] Figure 1shows the price trend for a barrel of oil in the last years. Thereare large fluctuations due to geo-political problems and eco-nomic crises,[11] but by extrapolating the trend of the lastdecade it is possible to estimate a price ranging between$150–200 per barrel, or more. In conclusion, it is likely that theaverage price of oil will double in the next decades comparedto the average price in the past decade.

The production of chemicals is highly dependent on oil as araw material. Figure 1 also shows the ratio between the aver-age costs of ethylene and propylene (the two most importantbase raw materials for petrochemistry) relative to the price ofoil. These also fluctuate due to a number of factors, and moregenerally it should be considered that the prices of raw materi-als are influenced by many aspects, from geographical area tospot and contract prices, production problems, supply/demand balance, trade flow, and others. The ratio between theprices of olefins and oil (Figure 1) slightly decreased during thelast decade due to contraction of the economy, a surplus ofolefins, and improvements in the efficiency and utilization ca-pacity of plants, but is expected to stabilize to a value ofaround 15 in the next years. Notwithstanding that, the pricesof ethylene and propylene follow closely the same trend as oil,so it could be estimated that the cost for base raw materialsfor petrochemistry will nearly double in the next two decades.

Thus, the questions are whether or not the increase of theprice of oil as source of carbon for producing chemicals mightbe compensated for by other carbon sources, and whether ornot these alternative carbon sources can be effective elementsfor mitigating the increases in the prices of olefins.

While natural gas (NG) is also a potential resource, its chemi-cal use today is essentially limited to the syngas (CO/H2) route.The price of NG is closely linked to the price of oil. It is thus ex-pected that the costs of olefins from methane via syngas(direct methane oxidative coupling to C2–C3 alkanes/alkeneshas never reached the application level) or from ethane–pro-pane dehydrogenation (or oxidative dehydrogenation) will alsodouble in the next decade. In other words, there is not a de-coupling of production costs when changing the raw material,and instead, owing to the higher energy intensity when pro-ducing olefins from NG compared to oil, such a change willprobably have the opposite effect, that is, reduced incentivesto produce ethylene and propylene from NG. Thus, this routewill probably be limited to a few specific applications.

The price of coal is less dependent on the price of oil, alsobecause of the larger reserves available, but the low hydrogencontent of coal and the high costs of purification make its useto produce chemicals not the best solution for a sustainable

Figure 1. Five-year trends for the average price of oil and the ratios betweenthe average prices of ethylene or propylene relative to oil.

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G. Centi, G. Iaquaniello, S. Perathoner

development. In regions such as Europe producing chemicalsfrom coal is too expensive. EIA forecasts do not indicate alarge increase in coal production except for PR China, where itwill be used mainly for energy, although coal-to-chemicals(CTC) technologies are developing quickly. Since 2004, PRChina has embarked on a structured programme to establish acoal-based transformation industry. All processes are very ex-pensive, which significantly limits the spreading of the technol-ogies. On a global scale, CTC technologies will account for lessthan about 1–2 % of olefin production in the next two de-cades.[12]

Thermo-chemical biomass-to-chemicals (BTC) transformationroutes are also costly and strongly impact the environment,[13]

for example through emissions from purification of the gasesproduced during gasification (direct or indirect, via fast pyroly-sis) and through the intensive agriculture necessary to producethe biomass (e.g. , emissions of the greenhouse gas N2O fromfertilizers). The huge capital investment with technological risk(an important barrier to commercialization), the costs of bio-mass as a raw material and related logistics, and syngas clean-ing are critical elements.[14] This route is also not estimated tocontribute more than a few percent to chemical production.Forecasts have estimated that in 2030 up to 15–20 % of chemi-cal production could be based on biomass, but these apply todirect substitution products, for example fats and oils for thedetergent and cosmetic industries, sugars for products such asglues or food additives, protein derivatives, and some new bio-renewables, such as lactic acid and polylactic acid, succinicacid, and several others.[15] Base raw materials such as olefinscould be produced through the dehydration of bioethanol toethylene. Currently, the capacity is about 1 Mt but its applica-bility is limited to specific countries where very cheap bioetha-nol is available, such as Brazil.[16]

The different routes to olefins are summarized in Figure 2,showing how the traditional methods for light olefin produc-tion integrate new processing routes using new or alternativefeedstocks. Dashed lines indicate processes that are still beingdeveloped. Current production of light olefins mostly involvessteam cracking of either light hydrocarbon liquids ornatural gas. The choice between gas or liquid feed-stock depends on a number of factors, including thedesired ethylene/propylene ratio, feedstock availabili-ty, cost, and by-product utilization. A refinery’s fluidcatalytic cracking (FCC) unit has long been a sourceof propylene. In recent years, FCC units have beenmodified to increase propylene production. The useof lighter cracker feeds, favoring ethylene production,has recently pushed the need for “deliberate” pro-duction of propylene by dehydrogenation of pro-pane.

Such deliberate ethylene/propylene mixtures cansince be recently produced from different sources viapartial oxidation to synthesis gas (syngas), methanolproduction from syngas, and finally conversion intolight olefins through the methanol-to-olefins (MTO)process. The MTO process will be mainly applied inareas with limited access to conventional olefin feed-

stocks, and/or to profit from a significantly cheaper feedstock.We have not considered the production of light olefins viametathesis, although it is an important component of the sce-nario because it is essentially a process to redistribute carbonlength in olefin production.

Figure 3 displays the world markets for ethylene and propyl-ene, with a breakdown of main production methods, for 2010and a forecast for 2020.[17, 18] The light olefin market willexpand by about 25 %, reaching about 250 Mt. Although thereis an expansion of production from alternative raw materialsand processes, steam cracking and refinery (FCC) productionwill remain dominant.

This analysis, although including many uncertainties relatedto the difficulty of making forecasts in a fast-changing world,shows that there is a combination of factors indicating thatwhen faced with a strong increase in the cost of carbon sour-ces for chemical production in the next two decades, there aremany constraints on the use of alternative sources of carbon,particularly for the production of base raw materials such asC2–C3 olefins. On the other hand, it is necessary to widen thenumber of possible sources to produce these base chemicals,because this will moderate the increase of their costs whilemaintaining the current value chain structure . This is an im-portant element to maintain competiveness or the production

Figure 2. Production routes for light olefins, from different raw materials.The dashed lines indicate processes under development.

Figure 3. World market for ethylene plus propylene with the breakdown of the main pro-duction methods (year 2010 and forecasts for year 2020).

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Can We Afford to Waste Carbon Dioxide?

itself, and avoid large investments related to a value change inpetrochemistry. In this scenario, it is thus interesting to evalu-ate the techno-economic feasibility for producing C2–C3 ole-fins from CO2, and avoid wasting this relevant carbon source.

2.2. Current methods and perspectives of olefin production

Ethylene and propylene are the building blocks of petrochem-istry, but their production is the most energy-consuming pro-cess in the chemical industry.[19] Steam cracking accounted forabout 3 EJ of primary energy use (due to the combustion offossil fuels and excluding the energy content of products) andnearly 200 million tonnes of CO2 emissions (due to the com-bustion of fossil fuels).[20] The pyrolysis section of a naphthasteam cracker alone consumes approximately 65 % of the totalprocess energy and accounts for approximately 75 % of thetotal exergy loss.

The market outlook for light olefins (about 190 milliontonnes currently, Figure 3), after the contraction in 2009, willbe again positive for 2010–2014 and incentives exist for olefinproduction away from the traditional sources.[21] In addition,the FCC production of light olefins is expected to slow downsignificantly, mainly in response to changing specifications forgasoline. In steam cracking, ethylene is the preferred product,but the demand for propylene is growing faster than that forethylene. Therefore, the current production of olefins is not inline with the forecast market demand.

The production of olefins is one of the major contributors(for the chemical production) to the emissions of CO2. For ex-ample, Table 1 reports specific emission factors (Mt CO2/Mt ethylene) for ethylene production in Germany. On average,ethylene production in Germany amounts to about 5 000–6 000 Mt per year, which produces about 10 000 Mt CO2 peryear of. In view of recent political pressure to reduce CO2 emis-sions, particularly in Germany, it is evident that the reductionof CO2 emissions from olefin production in the chemical indus-try is becoming a pressing factor. Instead of viewing the costsrelated to carbon taxes as a penalty, it is better to consider itas a market opportunity for reusing CO2 as a valuable carbonsource, also enabling the smart storage of H2 produced fromrenewable sources, as discussed later.

2.3. CO2 to olefins: An opportunity to store solar energy

The upgrading of CO2 to valuable chemicals or fuels needsenergy, in direct or indirect form (by reaction with higher-energy molecules). CO2 is, with water, the end product of the

combustion of any carbon source and thus lies in a potential-energy well. The standard energy of formation of CO2 is�393.51 kJ mol�1 (gas). The addition of a third oxygen atom toCO2 is an exothermic process (DF8f for CO3

2� (aq) =

�675.23 kJ mol�1), but the removal of oxygen necessary to goto any kind of fuel (by definition a fuel should lie at a higherenergy level than the final product, that is, CO2) is an endo-thermic process. The standard energy of formation of CO (g) is�110.53 kJ mol�1, those of CH3OH (g) and CH4 (g) are �200.67and �74.4 kJ mol�1, respectively, and those of ethylene andpropylene +50.28 and +20.41 kJ mol�1, respectively.

The energy necessary for these endothermic processes mustclearly be derived from renewable energy sources, becauseotherwise the process is not sustainable. It has been pointedout earlier that converting CO2 to fuels is a great opportunityto efficiently store and transport solar or other renewableenergy sources.[5] Today, the energy produced from renewablesources is still a limited fraction of the total energy consump-tion, and thus the energy produced could be inserted in thegrid without major problems. However, the need to storeenergy produced from renewable source for a more rationaluse (during peak hours) has already started to become a prob-lem. Large investments are planned to store the electricalenergy produced in large-field wind and solar plants. Batteriesare not efficient in terms of energy-density and cost-effective-ness, and other methods such as pumping water into moun-tain basins also show a low efficiency and are applicable toonly some areas, to avoid long-distance energy transport. Stor-age in the form of chemical energy is still the preferredmethod, resulting in high-density energy vectors that are easyto store and transport. For this reason, CO2-derived productssuch as methanol and longer-carbon-chain alcohols or hydro-carbons have been indicated as valuable vectors to efficientlystore and transport solar energy.[5]

When considering the C2–C3 olefins in previous sections, itis evident that these products, due to their large energy of for-mation, offer an excellent opportunity to store solar energyand incorporate it in the value chain for producing chemicals,instead of that for energy. As indicated in the Introduction, astrong effort is being made today to reconsider the productionof chemicals and make a transition towards a resource-effi-cient, low-carbon economy. The large energy of formation ofolefins also explains why the actual formation process is themost energy-consuming process in chemical industry, withsuch large CO2 emissions. Currently, most of the produced eth-ylene and propylene is used to produce polymers, either di-rectly (i.e. , for polyethylene and polypropylene; polypropyleneproduction accounts for more than 60 % of total world propyl-ene consumption) or indirectly (e.g. , some important propyl-ene products are acrylonitrile, propylene oxide, acrylic acid,and cumene, which are also mainly used to produce polymers).With respect to products such as polycarbonate, the market ispotentially much bigger because ethylene and propylene areat the basis of most of current chemistry, and when producedfrom this alternative source there can be a perfect insertioninto the actual production chain, avoiding the massive invest-ments required when new production chains are proposed.

Table 1. Specific emission factors (Mt CO2/Mt ethylene) in ethylene pro-duction from different sources in Germany.

Source Process Fuel Electricity (indirect) Total

Gasoil 0.24 1.58 0.04 1.86LPG 0.03 1.27 0.03 1.32Naphtha 0.02 1.47 0.03 1.53Refinery off-gases 0.03 1.19 0.93 1.24

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Producing ethylene and propylene from CO2 can thus also beviewed as a way to capture CO2 for longer periods of time,compared to the much shorter life cycles of CO2-based fuels.

The impact of producing olefins from CO2 in terms of miti-gating climate change is therefore very high, because it(i) avoids CO2 emissions and oil consumption related to theircurrent method of production; (ii) reuses waste CO2 ; and (iii) s-tores CO2 in the form of products with relatively long life-cycles, such as polymers.

2.4. Producing ethylene and propylene from CO2

Producing olefins from CO2 requires H2. Ethylene and propyl-ene have a positive standard energy of formation with respectto H2, but water forms in the reaction [H2O(g): �285.8 kJ mol�1]and thus in essence the process does not need extra energywith respect to that required to produce H2. Therefore, froman energetic point of view the energy efficiency of the processis related to the energy efficiency of H2 production. The pro-cess for olefin synthesis from CO2 may be described as thecombination of a stage of reverse water–gas shift (rWGS):

CO2þH2 Ð COþH2O ð1Þ

and a consecutive Fischer–Tropsch (FT) synthesis stage:

COþH2 ! CnH2nþCnH2nþ2þH2OþCO2; ðn ¼ 1, 2, . . .Þ ð2Þ

The FT catalyst should be modified to minimize the forma-tion of alkanes, especially methane, and as much as possibleselectively produce C2–C3 olefins. The two stages may becombined, but water should preferably be removed in situ toshift equilibrium and avoid inhibition of the FT catalyst.

A number of recent studies have dealt with the effect of CO2

on FT synthesis. CO2 can be activated by suitable promotersfor hydrocarbon synthesis at low temperature.[22] There is in-creased carbon utilization in FT synthesis using CO2-rich syngasfeeds.[23] A low potassium content is suitable for increasing thehydrocarbon yield. Iron catalysts promoted with equalamounts of zinc and copper have higher CO and CO2 conver-sion rates and decreased CH4 selectivity.[22] Xu et al. originallyreported the effect of potassium in CO2 hydrogenation (at lowpressure, 2 MPa) over iron-based FT catalysts (Fe-Mn/silicalite-2).[24] The doping with potassium increases the CO2 conversion,the amount of hydrocarbons, and the selectivity to light ole-fins, while decreasing the amount of methane (Table 2).

Sai Prasad et al. used an Fe/Cu/Al/K catalyst at 300 8C and1 MPa pressure to convert a CO/CO2/H2 syngas,[25] and reportedca. 85 % selectivity to alkene in the C2–C4 fraction (about 40 %of the whole hydrocarbon fraction). Zhao et al. studied the ki-netics of light alkene syntheses by CO2 hydrogenation over Fe-Ni catalysts.[26] When using a Co/g-Al2O3 FT catalyst, but moretypical FT reaction conditions (P = 20 bar, T = 220 8C), heavierhydrocarbons were observed (>70 % in C5+).[27] The presenceof CO2 in the feed stream had a negative effect on catalyst sta-bility and on the formation of heavy hydrocarbons. Dorneret al. also did not find light olefins when studying CO2 hydro-

genation over cobalt-based catalysts under typical FT synthesisconditions.[28] Clearly, the olefins are readsorbed and furtherconverted in these conditions, and to optimize their formationa tuning of both the catalyst and the reaction conditions isnecessary.

Yao et al. recently discussed FT synthesis using H2/CO/CO2

syngas mixtures over a cobalt catalyst.[29] Two feed gases, H2/CO/CO2 = 2:1:0 and H2/CO/CO2 = 3:0:1, were mixed in variousproportions, thus varying the ratio of CO, CO2, and H2 stoichio-metrically. The results showed that CO and CO2 mixtures canbe used as feed for a cobalt catalyst. A comparison of FT re-sults using different syngas mixtures (CO2/H2, CO2/CO/H2, andCO/H2) showed that:

* CO2 can be hydrogenated along with CO in the FT reactorover cobalt, especially in the case of a high CO2 content. Inspite of the fact that cobalt catalysts are not active for thewater–gas shift reaction, the rate of hydrocarbon production ismaximized at an intermediate CO/CO2/H2 composition.

* Hydrogenation of CO2 or a CO/CO2 mixture leads to a typ-ical Anderson–Schulz–Flory distribution. A CO feed exhibits thetypical two-alpha distribution while CO2 and CO2-rich feedsonly exhibit a single-alpha distribution.

Jiang et al. used a zeolite capsule catalyst with a core–shell(Fe/SiO2–Silicalite-1) structure for the direct synthesis of lightalkenes from syngas.[30] This zeolite capsule catalyst exhibitedexcellent selectivities compared to a traditional FT catalyst,suppressing the formation of undesired long-chain hydrocar-bons.

Light olefins can also be obtained via conversion of metha-nol/DME on multifunctional catalysts, instead of through theFT reaction. Kang et al. used an Fe-Cu-K catalysts supported onZSM-5 to improve selective olefin production.[31] An Fe-Cu-K/ZSM-5 catalysts with a low Si/Al ratio (25) was superior toother catalysts, offering a better C2–C4 selectivity and higherolefin/(olefin+paraffin) ratio, possibly due to the facile forma-tion of iron carbide. Park et al. instead used a dual-bed reactorapproach, with an Fe-Cu-Al based FT catalyst in the first stageand a ZSM-5 cracking catalysts in the second stage.[32] The for-mation of olefins was enhanced by doping the first catalystwith potassium. A 52 % selectivity to C2–C4 hydrocarbon richin olefins (77 % selectivity) was reported.

In summary, 30–40 % yields to light olefins (C2–C4, with typi-cal relative ratio of 0.37:0.36:0.27 in C2/C3/C4 olefin distribu-tion) have been reported on modified FT or methanol/zeolitecatalysts, typically operating at lower pressures than those re-

Table 2. Effect of potassium on the performances of Fe-Mn/silicalite-2catalysts for CO2 hydrogenation.[a] Adapted from Xu et al.[24]

K2O[wt %]

CO2 conversion[%]

Hydrocarbons[mol %]

Selectivity toC2–C4 [wt %]

CH4

[wt %]

0.0 44.1 75.8 54.8 37.81.0 46.8 77.9 60.3 32.95.0 49.4 79.7 65.3 27.9

10.0 50.2 81.6 68.3 25.1

[a] T = 347 8C, P = 2.0 MPa, Q = 1200 h�1, CO2/H2 = 1:3

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quired for FT or methanol syntheses. Although these results,being out of the scope of this work, are not exhaustive theydo give a detailed state-of-the-art analysis ; evidence that it ispossible to modify FT or methanol catalysts to selectively formlight olefins, although actual data are still not satisfactory. Anumber of patents also appeared recently, evidencing industri-al interest in this approach.[33] Many aspects in catalyst designhave been not optimized, such as the mean size of metallicparticles, the type of support, the use of bimetallic catalysts orother dopants, the reaction conditions. It is thus reasonable toexpect further optimization with a target in selective synthesisof light olefins over 80 % at higher productivity.

To remove water during the reaction, possible solutions arethe catalytic distillation or (preferably) the use of inorganicwater permselective membranes. Suitable and recoverablemembranes are those based on hydrophilic nanopore zeolite(NaA) films over a ceramic tubular support and developed forpervaporation of water–ethanol solutions.[34] Membranes origi-nally intended for other applications have been proposed forFT synthesis, to counter the inhibiting effect of the byproductH2O.[35] Another possible use of membranes is optimization ofthe distribution of reactants along the axial profile of fixed-bedreactors, for example by using a hydrogen-permselective mem-brane.[36] Removing water and controlling the COx/H2 ratio inthe reactor are important to optimize the selectivity and pro-ductivity to light olefins. Therefore, reactor optimization is afurther aspect to consider, in addition to optimal catalystdesign and reaction conditions, to improve the selective syn-thesis of light olefins in CO2 hydrogenation.

2.5. Renewable H2

As mentioned earlier, H2 used for converting CO2 to light ole-fins should be produced using renewable energy sources, oth-erwise the process is not sustainable. Although H2 producedfrom biomass-derived products (e.g. , glycerol, the byproductof biodiesel production via transesterification of vegetable oils)can also be considered as renewable,[37] we consider here onlydirect methods for producing hydrogen using renewableenergy sources.

The production of H2 by electrolysis is well-established.Herein, the electrical energy is derived from photovoltaic (PV)cells or other non-carbon (renewable) energy sources. Today,the efficiency of the PV-electrolysis system is optimized, and bymatching the voltage and maximum power output of the PVmodule to the operating voltage of proton exchange mem-brane (PEM) electrolyzers,[38] efficiencies up to about 12 % canbe obtained.[39] An advantage is that H2 can be producedunder pressure in modern electrolyzers, while other routes(presented below) mainly produce H2 at atmospheric pressure,and thus may require its compression before use. PEM waterelectrolysis technology offers a safe and efficient way to pro-duce electrolytic hydrogen and oxygen from renewable energysources. Stack efficiencies close to 80 % have been obtainedoperating at high (1 A cm�2) current densities using low-costelectrodes and high operating pressures (up to 130 bar).[40]

PEM electrolyzers have several advantages compared to the

well-established alkaline technology: (i) no corrosive electro-lytes are used, (ii) PEMs enable differential pressure operation,(iii) direct leveraging of PEM fuel cell advances, and (iv) theycan be better integrated with solar and wind power. Therehave already been many developments that have led to reduc-tions of the costs of stacks.[41] These include (i) catalyst optimi-zation (50 % loading reduction on anode, >90 % reduction oncathode), (ii) optimized design of electrolyzer cell, and (iii) 90 %cost reduction of the MEAs (membrane–electrode assemblies)by fabricating chemically etched supports. Stability duringmore than 60 000 h of operation has been demonstrated in acommercial stack.[41] The electricity and feedstock are the keycost components for hydrogen generation (Figure 4). The DoE

(US Department of Energy) targets for distributed water elec-trolysis are summarized in Table 3. They include distributedbaseline costs (compression, storage, dispensing). These costs

would increase if renewable energy would be considered. In-dustrial electricity was considered at an average price of$0.04 kWh�1 in these estimates, but this value clearly dependson many factors. In Germany, 2011 solar PV tariffs range fromabout $0.32 to 0.41 kWh�1, that is, more expensive by a factorof about 10 compared to the average price of industrial elec-tricity. However, the costs for solar PV are falling rapidly due toreductions in material costs and increases in efficiency. In addi-tion, off-peak electrical energy from different non-carbon sour-ces is also available at competitive costs. When consideringthe incidence of electricity on H2 production cost, it is possibleto estimate the cost of renewable H2, currently around $8–10 kg(H2)�1, as decreasing to $5–6 kg(H2)�1 in year 2020; about3–4 times higher than DoE targets.

The costs may be lowered if cheap excess electrical energywere available, for example that produced at night by nuclear

Figure 4. Key cost component in hydrogen generation by PEM electrolyzers.BOP: balance-of-plant components. Adapted from Ref. [41c] .

Table 3. US DoE targets for distributed water electrolysis.

Year Hydrogen Electyrolyzer capital Electrolyzer efficiency[a]

cost [$ kg(H2)�1] cost [$ kg(H2)�1] LHV [%] HHV [%]

2006 4.80 1.20 62 732012 3.70 0.70 69 822017–2020 2.0–4.0 0.30 74 87

[a] LHV: lower heating value; HHV: higher heating value.

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plants. The cost of hydrogen produced from water splittingusing nuclear technologies is around $2 kg(H2)�1.[42] Further im-provements in H2 technology could be achieved by electrolysisat high temperatures (above 200 8C), in particular by usingsolid oxide electrolysis cells (SOECs; temperatures around 950–1000 8C).[43] For low-temperature electrolysis a larger quantityof electrical energy is necessary to overcome the endothermicheat of reaction, while at high temperatures the primary elec-trical energy demand is considerably reduced and the electricallosses in the cell decrease due to lower Ohmic resistance inthe electrolyte and lower polarization losses from the electrodereactions, resulting in the kinetics of the electrolysis reactionbecoming faster. H2 costs from high-temperature electrolysishave been estimated to be in the range of $1.5–2.6 kg(H2)�1.[43]

As a comparison, the cost of large-scale biohydrogen pro-duction (from bio-oil, produced by fast pyrolysis) is estimatedto be in the range of $4–8 kg(H2)�1 for a plant size below 1000dry tonnes of biomass per day (the costs depend on the typeof starting biomass, with whole-tree the least expensive andstraw the most expensive), which decreases to $2–5 kg(H2)�1

for plant sizes above 4000–5000 tonnes per day.[44]

There are three alternative routes for the production of re-newable H2 : (i) bioroutes involving cyanobacteria or greenalgae,[45–47] (ii) high-temperature thermochemistry using con-centrated solar power (CSP),[48–50] and (iii) photo-(electro)chemical water splitting or photoelectrolysis usingsemiconductors.[51–54] All of these routes are still being devel-oped, and so there are many uncertainties at present sur-rounding the cost of H2 production. For large-scale photobio-logical H2 production the cost of H2 was estimated to be about26.4 (compression and marine transportation) cents perkWh,[45] that is, about $10 kg(H2)�1. The projected costs of H2

produced by CSP (concentrated solar power) range from 15 to20 cents per kWh, or $5.90–7.90 kg(H2)�1,[55] assuming solarthermal electricity costs of 8 cents per kWh. The US target for2017 is $3 per gge (gasoline gallon equivalent; 1 gge is about1 kg H2), and the EU target for 2020 is E3.50 kg(H2)�1.[55] Photo-electrochemical water splitting or photo-electrolysis research isstill at an earlier stage of development; too early for reliabledata on the cost of production. However, the EU NanoPECproject (Nanostructured Photoelectrodes for Energy Conver-sion), directed by Prof. Michael Gr�tzel, has a target productioncost of less than E5 kg(H2)�1 H2 near 2013.[56]

In conclusion, notwithstanding the spread of the data andoften not homogeneous comparisons, it is possible to considera renewable hydrogen price in the range of $2–10 kg(H2)�1. Byusing PEM electrolyzers, H2 can already be produced on smalland medium scales. There are opportunities for further costdecreases, particularly through optimization of the electrodesand device configuration. H2 production from biomass transfor-mation products (e.g. , bio-oil, byproducts) is possibly close tocommercialization, but the economic and sustainability incen-tives for this route (with respect to electrolysis) are limited, ifnot integrated in a biorefinery.[13, 57] Alternative routes are stillmore costly, but have a larger degree of optimization toreduce the costs of production. In our opinion, bioH2 using cy-anobacteria or green algae, and photo(electro)chemical water

splitting or photoelectrolysis have a larger potential for thefuture large-scale production of renewable H2 at competitivecosts, but intense research and development efforts are neces-sary to realize this potential.

A March 2011 report by the International Partnership for Hy-drogen and Fuel Cells in the Economy indicates a projectedhigh-volume cost of hydrogen in the $2–4 US per ggerange,[58] that is, ca. $2–4 kg(H2)�1, for 2015–2020. RenewableH2 from electrolysis using wind, solar, geothermal, or hydroenergy, and from biomass gasification or reforming are indicat-ed as near- to mid-term objectives, while H2 from CSP, micro-organisms and semiconductors are considered long-term ob-jectives (beyond 2020). The US National Renewable EnergyLaboratory (NREL) also estimates the actual cost of wind-to-hy-drogen production at around $6 kg(H2)�1, with possibilities toreduce the cost by better integration of renewable energysources and electrolyzer stacks (Wind2H2 project).

The IPHE report also highlights the availability of significantamounts of “stranded hydrogen”. This describes H2 formed asa byproduct of industrial processes that is not resold or usedwithin the plant where it is produced. In Japan, extra H2 pro-duction capacity from municipal gas companies and oil refiner-ies is estimated at 4.7 billion normal cubic meters. Thus, largeamounts of H2 that may be used for CO2 hydrogenation are al-ready available.

2.6. Process flowsheet for light olefin production from CO2

A simplified block flowsheet for the CO2-to-olefin process(CO2TO) is shown in Figure 5. Renewable H2 is produced byelectrolysis, either by PEM electrolyzers or high-temperatureSOEC. Electrical energy for the process comes from renewableenergy sources (wind, solar, geothermal, or hydro) or fromexcess electrical energy from non-carbon-based sources (e.g. ,nuclear power plants during the night). These can also supplythe heat necessary for high-temperature electrolysis, or thisheat can be derived from other sources, for example by inte-grating the high-temperature H2 production with the integrat-ed gasification combined cycle (IGCC).[59] This is an interestingopportunity, also for the possibility to valorize CO2 emitted inthe process.

Figure 5. Simplified block flowsheet for the CO2-to-olefin process.

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Can We Afford to Waste Carbon Dioxide?

The core of the process is the combination of rWGS andmodified FT reactions. Although these two steps may be com-bined in a single unit, it is probably preferable to have sepa-rate stages in order to allow optimization of the individual cat-alysts and reaction conditions. In both the separate and inte-grated designs, the use of inorganic membranes that arepermeo-selective to water can be considered in order to im-prove the performance of the process. In the case of separatestages, H2 may be also added in part between the two stages,while the methane, light alkanes, and >C5 hydrocarbons pro-duced in the process can be recycled here. Due to the forma-tion of water, the overall process is slightly exothermic. Afterthermal recovery, the light olefins are separated in a sequenceof columns similar to the steam cracking process, while COand CO2 are recycled to the rWGS unit.

2.7. Economic considerations for the CO2TO process

Feedstock costs in current steam cracking production process-es as well as in alternative routes via methanol account for 80–90 % of the production costs,[19b] while the difference to 100 %is the sum of fixed costs (typically around 10 %, includinglabor, operation, and maintenance), other variable costs (e.g. ,utilities such as electricity and water), capital depreciation, andother costs. In the CO2TO process the feedstock is also thedominant cost component, considering that capital-intensiveunits such as electrolyzers and others are included in the H2

production costs, and that utilities are limited due to moderatereaction conditions. Renewable H2 is clearly the dominant costfactor, because CO2 may be considered a feedstock with evena negative cost, as discussed earlier. In a preliminary economicanalysis, it is thus possible to limit the discussion to the cost ofrenewable H2 to convert CO2 to olefins.

In the FT process, the two main competing reactions are:

n COþ2n H2 Ð CnH2nþn H2O ð3Þ

2n COþn H2 Ð CnH2nþn CO2 ð4Þ

The H2/CO molar ratio in the syngas feed thus ranges be-tween 0.5 and 2, but for H2/CO ratios <2 the so-called “inverseBoudouard” reaction becomes significant, particularly in thecase of cobalt-based catalysts, which leads to catalyst deactiva-tion by soot formation:

2 COÐ CþCO2 ð5Þ

The water–gas shift (WGS) reaction [inverse of Reaction (1)]also takes place. There is therefore some flexibility in the H2/CO ratio, although the optimal value depends on the type ofcatalyst used. A typical H2/CO molar ratio used in FT, using asyngas containing 3–4 % CO2, is about 2:1. Starting from CO2,H2 is consumed for the rWGS reaction [Reaction (1)] . The stoi-chiometric overall H2/CO2 ratio to synthetize light olefins is 3:1.Since methane, light alkanes, and >C5 hydrocarbons are recy-cled, as is COx (Figure 5), as a first approximation 3 moles of H2

are consumed for each mole of CO2 converted to light olefins.

On a weight basis, about 0.43 tonne H2 is necessary per tonneof ethylene or propylene produced.

The average current ethylene and propylene prices rangebetween $1200–1400 per ton, although the price depends onmany variables as discussed above. The price tendency for thenext decade is upwards due to the increases in fossil fuelcosts, while CO2 is at least a zero-cost raw material. It may bethus estimated that for a renewable H2 cost in the $2–3 kg(H2)�1 range, the CO2TO process may be economicallycompetitive to current production methods, in addition to thesustainability advantages that the process offers. It must be re-membered that there is general tendency to apply carbontaxes on energy-intensive processes, and this will have furtheradverse effects on olefin production current methods.

3. Conclusions

This Concept paper has analyzed whether or not we can affordto waste CO2, as it is a valuable raw material in a changingworld faced with a fast depletion of natural carbon sourcesand in need of a low-carbon economy and resource efficiency.The answer is no, but it is necessary to develop the relatedtechnologies for the use of renewable energy and H2 that canenable to establish a CO2-based economy.

To focus the discussion, we have examined the case of theproduction of light olefins (the fundamental building blocks ofcurrent petrochemistry) starting from CO2, and examined thevarious elements needed to enable this “CO2TO” process indi-cated. The current production of olefins is very energy-inten-sive, and there is an interest in producing these buildingblocks by using alternative carbon sources such as CO2. Theeconomic evaluation indicates that for a renewable H2 cost of$2–3 kg(H2)�1, predicted or targeted by many institutions for2020, the process is economically valuable. Currently the costof renewable H2 is higher, but not so much higher that de-tailed studies on the possibility of produce light olefins fromCO2 should be delayed.

This process has not been considered before, but is an inter-esting example of CO2-based, resource-efficient production ofchemicals. Thus, the current view that CO2 is a valuable re-source only for specific high-value products but cannot be re-cycled or contribute to a low-carbon economy in a significantmanner should be expanded. It is also evident that producinglight olefins from CO2 can be an enabling factor for the intro-duction of renewable energy into the chemical productionchain.

Acknowledgements

This Concept paper is based on a keynote lecture presented by G.Centi at the 22nd North American Catalysis Society Meeting, De-troit, MI (USA), June 5–10, 2011.

Keywords: carbon dioxide fixation · heterogeneous catalysis ·industrial chemistry · olefination · renewable resources

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Received: June 23, 2011

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Can We Afford to Waste Carbon Dioxide?