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Developing Innovative Synthetic Technologies of Industrial Relevance Based on Carbon Dioxide as Raw Material Michele Aresta,* Angela Dibenedetto, and Immacolata Tommasi Universita` degli Studi di Bari, Dipartimento di Chimica and Centro METEA, Via Celso Ulpiani 27, 70126-Bari, Italy Received October 25, 2000. Revised Manuscript Received January 15, 2001
The reduction of carbon dioxide emission into the atmosphere requires the implementation of several convergent technologies in different production sectors. The chemical industry can contribute to the issue with innovative synthetic technologies, which implement the principles of atom-economy, dematerialization, energy saving, and raw material diversification with carbon recycling. Such methodologies based on carbon dioxide utilization merge two issues, namely, avoiding the production of CO2 and carbon recycling. In this paper, some options are discussed, such as the synthesis of carboxylic acids, organic carbonates, and carbamates/isocyanates. Synthetic processes of methanol from either CO2 or CO are compared. The results of application of LCA to selected cases are discussed.
Introduction
Scheme 1. Amount of CO2 Fixed per t Reacted
The implementation of innovative synthetic technologies based on CO2 can considerably contribute to the reduction of carbon dioxide emission. The avoided amount is not simply represented by the amount of fixed CO2. New synthetic methodologies implementing the principles of atom economy, solvent shift, waste minimization at source, use of less noxious materials, and carbon recycling, contribute to CO2 reduction in many different ways, which cannot be discovered and quantified by simply looking at the stoichiometry of a reaction. As we have previously discussed,1 the assessment of the exact amount of avoided CO2 requires an integrated approach which takes into consideration several parameters, as shown in Scheme 1. However, only if the energy of each reagent, product, and reaction step is estimated for the conventional synthetic technology and the innovative one based on CO2, will it be possible to evaluate the amount of avoided CO2 per reacted ton, that can be lower or higher than one ton, depending on the process considered. To compare a conventional synthetic technology with one based on CO2 is not, thus, a trivial exercise. As an example, one can take into account the simplest case, the synthesis of formic acid from CO2 (eq 1), which has 100% atom efficiency.
H2 + CO2 f HCOOH(l) ∆Hr° ) - 31.2 kJ/mol; ∆Gr° ) 33.0 kJ/mol (1) Formic acid, a product on the market, is actually prepared using various technologies, one of which is * Corresponding author. Fax: +39 080 544 2429. E-mail: miares@ tin.it. (1) Aresta, M.; Tommasi, I. Energy Convers. Manage. 1997, 38, S373-S378.
reported in eqs 2a-d. To evaluate the exact amount of CO2 avoided, implementing the synthesis based on CO2 (eq 1), it is necessary to evaluate how much CO2 has not been produced because of the technology shift, in addition to the amount fixed. In other words, to make a correct estimate, the conventional technology and the innovative one based on CO2 must be compared consid-
10.1021/ef000242k CCC: $20.00 © 2001 American Chemical Society Published on Web 02/28/2001
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Table 1. Comparison of the Energy Requirement of LNG Extraction for Syngas Production and CO2 Recovery from Flue Gases According to Various Technologies process
energy in kW h kg -1
natural gas extraction CO2 recovery by refrigeration CO2 recovery using membrane separation CO2 recovery using MEA
13.8a 0.88b 0.79b 0.60b
The application of the Environmental Life Cycle Assessment to this case is useful for highlighting the difference of using CO or CO2 as the carbon source. Discussion
a Database SIMAPRO SP4. b Halmann, M. M.; Steinberg, M. Greenhouse gas-CO2 mitigation; Lewis Publisher: p 135.
ering their “total energy”. A correct estimate of the amount of all chemicals involved in the reaction must also be done, and the difference of waste processing evaluated for the two processes. Thermodynamics and kinetics of the processes must also be taken into account as they play an important role in industrial processes. If one considers the synthesis of formic acid according to eqs 2a-c and compares it with the synthesis based on CO2 (eq 1), it comes out that the thermodynamics of the direct synthesis is less favorable than that of the indirect reaction. Nevertheless, one has to consider that the former process does not require the CO synthesis and a considerable amount of energy can be saved relevant to LNG or coal extraction and processing (see Table 1), supposed that a CO2-free source of hydrogen were available.
CO + 2H2 f CH3OH
(2a)
CH3OH + CO f HC(O)OCH3
(2b)
HC(O)OCH3 + H2O f HCOOH + CH3OH (2c) 2CO + 2H2 + H2O(g) f HCOOH(l) + CH3OH(l) ∆Hr° ) - 201 kJ/mol, ∆Gr° ) - 25 kJ/mol (2d) Moreover, the correct comparison of the two processes requires that the fate of methanol is considered. If it is recovered, will represent a credit for the process, but the energy of recovery has to be considered in the energy balance. Conversely, if it is discarded, it will generate back CO2. Formic acid can be synthesized in SC-CO2 which can act at the same time as solvent and reagent. This approach may introduce further advantages due to the fact that organic solvents are totally excluded. However, this simple exercise shows that the exact comparison of two synthetic processes can be made only by using a methodology like Life Cycle Assessment, LCA, that demands an exhaustive basis of process data, including emission, energy, and mass data.2 The target compounds considered in this paper are the following: organic carbonates, carboxylic acids (longchain aliphatic, or mono- and di-carboxylic aromatic), carbamates, and isocyanates, i.e., chemicals that have a large market (>1Mt/y). They are today prepared using quite energy- and material-intensive synthetic methodologies. More ecoefficient and economic pathways could be used instead, including those based on carbon dioxide. A case of great interest is the methanol synthesis, which could allow the recycling of huge amounts of CO2. (2) Aresta, M. and Galatola, M., J. Cleaner Production 1999, 7, 187193.
Monomeric (linear, 1 or cyclic, 2) and polymeric organic carbonates 3 have a market of ca.1.8 Mt/y, bisphenol A-polycarbonate being by far the largest commercialized product.
However, the large majority of the production of non polymeric carbonates does not reach the market, as they have a captive use as solvents and internal commodities.3a Carbonates are mostly produced by the old phosgenation of alcohols, (eq 3):
2ROH + COCl2 f (RO)2CO + 2HCl
(3)
a technology that has a positive aspect in the high reactivity of phosgene, and drawbacks in the high energy requirement for the synthesis of chlorine and CO (considering the LNG extraction and processing), the limitation to the transport of phosgene, the safety measures required for the use of phosgene, the chloride and halogenated solvents end-production, which demand an ad hoc treatment. These aspects may be a barrier to the further enlargement of the phosgene technology exploitation and to a many-purpose use. As a matter of fact, the limitation to the transport of phosgene concentrates its use at its production site and has encouraged the building-up of small-medium units3b that can be mounted at the customer site. This does not represent the solution to all problems, and has prompted several industries to develop phosgene-free technologies. However, alternative syntheses of dimethyl carbonate (DMC) are now on stream based on the oxidativecarbonylation of methanol (ENIChem4 and Ube5 processes). The synthetic approaches proposed in this paper consider the reaction of alcohols (eq 4) or ethers (eq 5) with carbon dioxide, the alcoholysis of urea (eq 6), the reaction of ketals with carbon dioxide and alcohols (eq 7), and the oxidative-carboxylation of olefins (eq 8).
2 ROH + CO2 f (RO)2CO + H2O
(4)
R2O + CO2 f (RO)2CO
(5)
(H2N)2CO + 2ROH f (RO)2CO + 2NH3
(6)
RR′C(OCH2CH2O) + CO2 + 2R′′OH f RR′CO + HOCH2CH2OH + (R′′O)2CO (7) RHCdCH2 + 1/2O2 + CO2 f RHC-CH2OC(O)O (8) All these syntheses are characterized by a high atom efficiency, the only byproduct being water. This is true also for reaction 7, in which the ketone and glycol can
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Figure 1. LCA of alternative synthetic pathways for the synthesis of DMC. The synthesis of methanol is not considered.
be reconverted into the ketal, which is used as promoter of the reaction of carbon dioxide with alcohol. Reactions 5 and 7 avoid that water is formed in the reaction medium together with the organic carbonate, an issue that can be crucial for equilibrium shift. The thermodynamics of reactions based on CO2 is, in general, less favorable than that of reactions based on CO,11 but the difference is not such to exclude a priori processes based on CO2. This becomes particularly true if the latter have better environmental performances. A preliminary Life Cycle Assessment, LCA, study performed considering reactions 3, 4, and 6 has clearly shown that reaction 3 has an impact factor much higher than reactions 4 and 6, (Figure 1) the key steps responsible for such high impact being the energy required for the synthesis of CO and chlorine, and the production of chloride.6,10 We have already described in detail in ref 2 the LCA methodology used. Long chain mono- and di-carboxylic acids, 4 and 5 respectively, are high quality products with a major use in polymer chemistry.
HOOC(CH2)nCH3 4
cleavage of cyclic alkanes/alkenes (eq 10), which afford the formation of the two carboxylic functionalities. ox
HOH2C(CH2)nCH2OH 98 HOOC(CH2)nCOOH
(9)
Such processes are neither atom-efficient nor selective. Such a nonconvergent and destructive synthetic approach, with high waste production, is a major drawback that cancels out the advantages of the use of heterogeneous catalysis. An analogous situation exists for long chain mono-carboxylic acids which find a wide application as additives, and in the production of surfactants. The use of dienes and carbon dioxide as substrates (eq 11) brings in the great advantages of atom economy and selectivity.7,12
HOOC(CH2)nCOOH 5
Presently, di-carboxylic acids are synthesized by oxidation of di-alcohols (eq 9) or by oxidative ring (3) (a) Aresta, M.; Dibenedetto, A. Proceedings in GHGT-5, Cairns, Australia, August 13-16, 2000; (b) Aresta, M.; Quaranta, E. ChemTech 1997, 27 (3), 32. (4) (a) Romano, U.; Vimercate, F.; Rivetti, S.; Di Muzio, N. 1982. U.S. Patent 4,318,862, (ENIChem); (b) Romano, U. 1989. E.P. Patent 365,083, (ENIChem); (c) Di Muzio, N.; Fusi, C.; Rivetti, F.; Sasselli, G. 1991. E.P. Patent 460,732, (ENIChem); (d) Paret, G.; Donati, G.; Ghirardini, M. 1991 E.P. Patent 460,735, (ENIChem); (e) Dreni, D.; Rivetti, F.; Delledonne, D. 1994. U.S. Patent 5,322,958, (ENIChem); (f) Rivetti, F.; Romano, U.; Garrone, G.; Ghirardini, M. 1995. E.P. Patent 634,390, (ENIChem). (5) (a) Matsuzaki, T.; Shimamura, T.; Toriyahara, Y.; Yamasaki, Y. 1993. Eur. Patent Appl. E.P. 565,076; Chem Abstr. 1994, 120, 194496f; (b) INFO Chimie Magazine, April 1999, 407, 31. (6) Aresta, M.; Magarelli, A.; Tommasi, I. Science of the Total Environment, in press. (7) Aresta, M.; Quaranta, E.; Tommasi, I. New J. Chem. 1994, 18, 133-142. (8) Union Carbide and Carbon, U.S. Patent 2,238,474, 1941; Shell Devel, U.S. Patent 2,404,438, 1946; Celanese Corp. of America, U.S. Patent 2,578,841, 1951. (9) (a) Aresta, M.; Dibenedetto, A.; Tommasi, I. Appl. Organomet. Chem. 2000, 14, 799-802; (b) Jacobson S. E. Eur. Pat. Appl. E.P. 118,248, 1984; Chem. Abstr. 1985, 102, 6460w; Eur. Pat. Appl. E.P. 117,147, 1984; Chem. Abstr. 1985, 102, 24461b; (c) Aresta, M.; Quaranta, E.; Ciccarese, A. J. Mol. Catal. 1987, 41, 355-359. (10) Aresta, M.; Caroppo, A.; Magarelli, A.; Dibenedetto, A. Proceedings in GHGT-5, Cairns, Australia, August 13-16, 2000.
Such methodology has been advantageously applied to the synthesis of lactones13 and can be also used for the synthesis of either di- or mono-carboxylic acids, by changing the catalyst and controlling the reaction conditions. C-5 or C-9 mono-carboxylic or C-10 dicarboxylic acids can be prepared in mild conditions from butadiene and carbon dioxide. Longer chains can be also built up, according to the metal used as catalyst (Scheme 2). The thermodynamics of such reactions is favorable. The reaction takes place under nondrastic conditions and is controlled through the use of specific ligands on the metal center, the solvent, and the pressure.12,13 (11) (a) Aresta, M. Energy Technologies for reducing emission of greenhouse gasessProceedings of an experts' seminar Paris 1989, Vol. 1, p 599 OCDE-OECD.; (b) Blok, K.; Hendriks, C. A.; Turkenburg, W. C. Decarbonisation of fossil fuel. IEA-GHG R&D Programme Report n° PH 2/2 March 1996 - U.K. (12) Dinjus, E. Proceedings of ICCDU-IV, Bari, Italy, September 7-11, 1997, KL6. (13) Behr, A.; Ebbinghaus, P.; Heite, M. Appl. Organomet. Chem., in press; and Proceedings of ICCDU-V, GDCH Publishers: 1999.
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Scheme 2. Metal Catalysis in butadiene-CO2 Chemistry for the Synthesis of Cn-carboxylates.
Aromatic mono- and di-carboxylic acids are nowadays prepared using drastic oxidation reactions (eq 12) with low atom efficiency.
The side chain oxidation of alkyl-aromatic compounds (eq 13), which may yield different types of oxygenated organic chemicals, pertains to a large number of industrial intermediates addressed to the manufacture of plastics, fibers, pharmaceuticals, agrochemicals, etc.
At present, these oxidation reaction, namely to obtain the correspondent aromatic acids, are commercially carried out both in the gas and liquid phase. Gas-phase processes adopt fixed or fluidized bed, on V2O5-based catalysts, with air as oxidant, at temperatures ranging from 350 to 550 °C. Liquid-phase oxidation routes, usually with metal salt catalysts dissolved, adopt milder reaction conditions, 110-180 °C, under pressure up to 2 MPa. Soluble acetates or naphthenates of Co, Mn, or Mo are generally used with bromine-based cocatalysts, with carboxylic acids, mainly acetic acid, added as solvents. These technologies are used for the synthesis of benzoic acid, phthalic acids, and phthalic anhydride, which account for more than 16 Mt/y. The processes
present relevant objections for what concerns the formation of polluting effluents, the use of corrosive conditions due to the aggressivity of the reaction medium employed, the low or moderate selectivity in the synthesis of partially oxygenated intermediates, such as ketones and aldehydes, the low or moderate conversion, due to the necessity to maintain the reaction system outside the explosion limits. In addition some of these processes are not continuous. Such reaction could be advantageously carried out in SC-CO2 which allows waste reduction via a better reaction conditions control. The oxidation of olefins with dioxygen is a process that has found a seldom industrial exploitation as in the synthesis of ethylene oxide.8 In most other cases, peroxides are used as oxidants with higher costs. The oxidative carboxylation of olefins using dioxygen and CO2 (eq 14) may lead to the development of new technologies.9
The use of SC-CO2 may add some more benefits for what concerns critical aspects difficult to drive in solution, namely, the control of the reaction selectivity9a (one- or two-oxygen transfer to the substrate), yield, working conditions, eventual radical lifetime control. Moreover, the use of SC-CO2 offers a unique opportunity of using carbon dioxide as solvent and reagent. Another area of application for carbon dioxide is its use in the synthesis of methanol that is actually
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Figure 2. Comparison of different methanol synthetic routes.
synthesized from syngas (eq 15).
CO + 2H2 f CH3OH
Dry reforming of LNG (eq 19)
Methanol may have a large market as fuel or intermediate and its synthesis from carbon dioxide attracts the attention and interest of several research groups all over the world in order to define its potential. The use of carbon dioxide (eq 16) has as major drawback, the higher consumption of dihydrogen with respect to CO.
CO2 + 3H2 f CH3OH + H2O
CH4 + CO2 f 2CO + 2H2
(15)
(16)
However, higher selectivity and better yields may counterbalance such an apparent negative point. We have performed a LCA study of the possible synthetic pathways of methanol,10 by using the methodology reported in ref 2. Recycling CO2 is by far less polluting and energy demanding than producing CO (Table 1), the extraction of LNG and steam reforming being very energy intensive. The key issue for developing a CO2-based synthesis of methanol is the hydrogen production. WGSR (eq 18), which converts carbon monoxide produced in the WGR (eq 17a) or in steam reforming of LNG (eq 17b) into CO2,
C + H2O f CO + H2
(17a)
CH4 + H2O f CO + 3H2
(17b)
CO + H2O f CO2 + H2
(18)
from the point of view of controlling the CO2 emission, is not a useful tool. Therefore, alternative sources of H2 (renewables, biomass) must be considered in order to make the CO2-based process advantageous from the point of view of energetics and costs.
(19)
is considered with attention for its potential application in the conversion of methane into liquid fuels at the LGN extraction well. The moment being, if one combines eq 17b, which produces an excess of dihydrogen with respect to the stoichiometry of the synthesis of methanol from syngas (eq 15), with the use of recovered CO2 (eq 16), to optimize the atom and energy use, finds that the use of a mixture CO/CO2 ) 2 (molar ratio) is the best choice for reducing the environmental impact and the energy use (Figure 2). Should a source of dihydrogen alternative to WGR or steam reforming become available, the conversion of recovered CO2 into methanol would become the most economic, energy-saving, and environmentally safe synthetic procedure. Studies in this direction are in progress in order to evaluate the contribution that the use of recovered heat may give to the CO2-free production of H2, through water-electrolysis.14 Conclusions All the synthetic methodologies described above respond to the enviro-economic principles. They are characterized by a high level of innovation and may promote the corporate image of the chemical industry as they couple the reduction of carbon dioxide emission and waste minimization at source with possible recycling of carbon. Their implementation could be on a time scale short to medium, according to the process considered, and may produce benefits for the chemical industry. Thermodynamics of processes must be considered with great attention for the correct definition of the potential of the new processes. LCA is necessary in order to assess the benefits of implementing innovative technologies with respect to conventional ones. EF000242K (14) Aresta, M.; Caroppo, A.; Dibenedetto, A. forthcoming paper and RUCADI Report, EU BRITE BRRT CT98-5089, 2000.
