co2 reactive higth pressure

J. of Supercritical Fluids 28 (2004) 121191

Supercritical and near-critical CO2 in greenchemical synthesis and processing

Eric J. Beckman

Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, PA 15260, USAReceived 19 September 2002; received in revised form 3 February 2003; accepted 24 February 2003

Abstract

Carbon dioxide is often promoted as a sustainable solvent, as CO2 is non-flammable, exhibits a relatively low toxicity and isnaturally abundant. However, injudicious use of carbon dioxide in a process or product can reduce rather than enhance overallsustainability. This review specifically examines the use of CO2 to create greener processes and products, with a focus on researchand commercialization efforts performed since 1995. The literature reveals that use of CO2 has permeated almost all facets ofthe chemical industry and that careful application of CO2 technology can result in products (and processes) that are cleaner, lessexpensive and of higher quality. 2003 Elsevier B.V. All rights reserved.

Keywords: Carbon dioxide; Toxicity; Technology

1. Introduction

The use of carbon dioxide as a solvent or raw ma-terial has been investigated somewhat continuously inacademia and/or industry since 1950; interest in theuse of CO2 in these roles has intensified during thepast 20 years as large-scale plants using CO2 havebeen brought on line. While supercritical fluids in gen-eral exhibit interesting physical properties [1], spe-cific interest in CO2 is magnified by its perceivedgreen propertiescarbon dioxide is non-flammable,relatively non-toxic, and relatively inert. In addition,unlike water, the supercritical regime of CO2 is read-ily accessible, given its critical temperature of only304 K.

Tel.: +1-412-624-9641 fax: +1-412-624-9639.E-mail address: [email protected] (E.J. Beckman).

Whereas the use of carbon dioxide as raw ma-terial or solvent could produce product (property)advantages, process (chemistry) advantages, cost ad-vantages, or safety advantages, in this review we willfocus explicitly on uses of CO2 that provide practicalimprovements (as defined in Section 1.7) to the sus-tainability (or green-ness) of a product or process.Carbon dioxide is often promoted as a green solvent,and its use in this role has permeated throughout thechemical and materials research communities. Herewe describe recent advances that are both fundamentaland significant.

In summary, rather than present a comprehensivereview of CO2-based technology, here we focus onuses of CO2 that are relatively new and appear toprovide green advantages. It should be noted thatthere are examples provided in this paper where aCO2-based process is not particularly green, yet is

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122 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121191

generating interest because it produces better qualityproduct than conventional alternatives.

1.1. Physical properties of CO2

The pVT properties of CO2 have been known sincethe 1930s [2]; extensive data sets are available in theliterature and on the web in the form of correlations ofdensity, viscosity, dielectric constant, etc., as functionsof temperature and pressure [3]. CO2s critical pres-sure (and hence its vapor pressure in the near-criticalor liquid regime) is significantly higher than analogousvalues for alkane, fluoroalkane or hydrofluoroalkanefluids. CO2s anomalously high critical pressure is butone result of the effect that CO2s strong quadrupolemoment exerts on its physical properties. While thehigh critical pressure is problematic, the most unfor-tunate outcome of the effect of quadrupole momenton physical properties was the premise, first advancedduring the late 1960s, that CO2 might prove to be asolvent whose strength would rival or surpass that ofalkanes and ketones [4]. Because early models em-ployed to calculate CO2s solvent power relied on adirect relationship between the Hildebrandt solubilityparameter () and the square root of the critical pres-sure [(Pc)1/2], the solubility parameter of CO2 wasover-predicted by 20100%, leading to early inflatedclaims as to the potential for using CO2 to replaceconventional organic solvents.

1.2. Environmental and safety advantages to use ofCO2 in chemical processes

Carbon dioxide is non-flammable, a significantsafety advantage in using it as a solvent. It is alsonaturally abundant, with a TLV (threshold limit valuefor airborne concentration at 298 K to which it isbelieved that nearly all workers may be repeatedlyexposed day after day without adverse effects) of5000 ppm [5], rendering it less toxic than many otherorganic solvents (acetone, by comparison, has a TLVof 750 ppm, pentane is 600 ppm, chloroform is 10ppm [5]). Carbon dioxide is relatively inert towardsreactive compounds, another process/environmentaladvantage (byproducts owing to side reactions withCO2 are relatively rare), but CO2s relative inert-ness should not be confused with complete inertness.For example, an attempt to conduct a hydrogenation

in CO2 over a platinum catalyst at 303 K will un-doubtedly lead to the production of CO, which couldpoison the catalyst [6]. The same reaction run over apalladium catalyst under the same conditions will bycontrast produce lesser amounts of CO as a byproduct[7] and hence knowledge of CO2s reactivity is vitalto its use in green chemistry.

Carbon dioxide is clearly a greenhouse gas, butit is also a naturally abundant material. Like water,if CO2 can be withdrawn from the environment, em-ployed in a process, then returned to the environmentclean, no environmental detriment accrues. How-ever, while CO2 could in theory be extracted fromthe atmosphere (or the stack gas of a combustionbased power plant), most of the CO2 employed in pro-cesses today is collected from the effluent of ammo-nia plants or derived from naturally occurring deposits(e.g. tertiary oil recovery as practiced in the US [8]).Because industrially available CO2 is derived fromman-made sources, if CO2 can be isolated within aprocess one could consider this a form of sequestra-tion, although the sequestered volumes would not behigh. Ultimately, one should consider the source ofCO2 used in a process in order to adequately judgethe sustainability of the process.

CO2s combination of high TLV and high va-por pressure means that residual CO2 left behind insubstrates is not a concern with respect to humanexposurethe same can certainly not be said to be truefor many man-made and naturally-occurring organiccompounds. Because there is effectively no liabilitydue to residual CO2 in materials following process-ing, CO2 is not considered a solvent requiring processre-evaluation by the US FDA. Only water also enjoysthis special situation. Indeed, most of the commercialoperations employing CO2 as a solvent were initiatedto take advantage of CO2s particular advantages inproducts designed for intimate human contact (suchas food), or CO2s non-VOC designation (such as thefoaming of thermoplastics). The recent commercial-ization of fabric cleaning using CO2 benefits bothfrom CO2s advantages in human-contact applicationsand situations where emissions appear unavoidable.

The simultaneous use of both hydrogen and oxygenin a reaction is obviously problematic from a safetystandpoint, given that H2/O2 mixtures are explosiveover a broad concentration range. Addition of CO2to mixtures of H2 and O2 expands the non-explosive

E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121191 123

regime (in the gas phase), more so than if either N2 orwater vapor was added [9]. At this point it is not clearto what extent the non-explosive regime will expandfurther as one raises the density of the mixture (andhence the heat capacity).

In a final intriguing note regarding safety advan-tages inherent to use of CO2 as a solvent, DuPontscientists [10] discovered that addition of CO2 totetrafluoroethylene enhances the stability of that no-toriously difficult-to-handle monomer, although theexact mechanism for the enhanced stability has notbeen published. What has been revealed is that addi-tion of CO2 to TFE vapor inhibits runaway decom-position and explosion of the monomer. In addition,the CO2/TFE mixture behaves like an azeotrope, inthat boiling of a mixture of the two does not signif-icantly change the concentration of either the liquidor the vapor. According to the DuPont patent [10],this azeotrope-like behavior persists over a wideconcentration range, behavior that is quite unlike thatof typical azeotropic mixtures. The enhanced safetyof CO2/TFE mixtures relative to pure TFE is one ofthe reasons that DuPont constructed a semi-workspolymerization plant employing CO2 as solvent forthe generation of fluoropolymers.

1.3. Environmental and safety disadvantagesinherent to use of CO2 in a process

Because CO2s vapor pressure at room temperatureis >60 bar, use of CO2 in a process clearly requireshigh-pressure equipment, creating a potential safetyhazard relative to the same process operated at one at-mosphere operation. In addition, uncontrolled releaseof large quantities of carbon dioxide can asphyxiatebystanders owing to air displacement. These issueshave not impeded the commercialization of CO2-basedprocesses nor is it likely they will do so in the future. Itshould be remembered that the low density polyethy-lene polymerization process, first commercialized inthe 1940s and still in operation today [11], runs con-tinuously at 20003000 bar and 520 K with a highlyflammable component and hence, safe operation of a100200 bar CO2-based plant is readily achievable us-ing current technology. Operating an exothermic re-action in a high-pressure environment is accompaniedby additional safety concerns versus the analogous re-action run at one atmosphere.

Whether to use liquid or supercritical CO2 is achoice that actually involves safety as well as chem-istry considerations. While use of supercritical CO2almost always involves use of higher pressure (toachieve the same solubility of a given substrate asin the liquid case), other factors should also be con-sidered. First, supercritical CO2 will exhibit a highercompressibility than liquid CO2, and hence the su-percritical fluid will be better able to absorb excessheat evolved from an exothermic reaction whose ratesuddenly exceeds typical operating conditions. Onthe other hand, use of saturated liquid CO2 (in thepresence of the vapor phase) would allow boiling tobe used as a means to absorb excess heat. Use ofsupercritical CO2 (versus liquid) could avoid compli-cations owing to a phase separation occurring upona departure from established temperature or pressureconditions within a given reactor. For example, ifone is employing a mixture of oxygen, substrate, andliquid CO2 in a particular process, a sudden drop inpressure owing to a perturbation in the process couldlead to formation of a flammable gaseous phaseuseof a supercritical mixture could avoid this problemas no vaporliquid separation will be encountered.Indeed, it should also be remembered that the Tcof a mixture of CO2 and other materials will differfrom that of pure CO2 (see, e.g. Ref. [12] for usefulcorrelations) and hence T-p conditions sufficient forsupercritical operation with pure CO2 may create aliquid in the case of the mixture.

1.4. Chemical advantages to use of CO2 as a solvent

Carbon dioxide can provide not only environmen-tal advantages, but also chemical advantages when ap-plied strategically, as described below.

1.4.1. CO2 cannot be oxidizedIn essence, carbon dioxide is the result of complete

oxidation of organic compounds; it is therefore partic-ularly useful as a solvent in oxidation reactions. Useof almost any organic solvent in a reaction using airor O2 as the oxidant (the least expensive and mostatom-efficient route) will lead to formation of byprod-ucts owing to reaction of O2 and the solvent. Indeed,the commercial anthraquinone process used to gener-ate H2O2 requires the removal and regeneration (orincineration) of substantial volumes of such solvent


byproducts [13]. Oxidation reactions in CO2 have con-sequently been investigated extensively over the pastdecade (see Section 2.8).

Because CO2 is inert towards oxidation and is alsonon-flammable, CO2 is one of the very few organicsolvents that could be considered for the direct reac-tion of hydrogen and oxygen to form hydrogen per-oxide [14]. This process has been under investigationfor over two decades, yet traditional organic solventsare not sufficiently inert/safe, while water exhibits pro-ductivity disadvantages.

1.4.2. CO2 is benign and hence cross-contaminationof the other phase during liquidliquid extraction isnot really contamination

There are a number of large-scale chemicalprocesses that employ biphasic (waterorganic)mixturesH2O2 production and hydroformylation oflow molecular weight alkenes are but two examples[13]. In any contact between aqueous and organicphases, some cross-contamination is inevitable. Theaqueous phase will require subsequent remediationto eliminate the organic contamination, while the or-ganic phase may require drying to allow further usein the process.

While CO2 will contaminate an aqueous phaseupon contact in a process, a mixture of CO2 andwater clearly does not require remediation (the CO2phase may, of course, require drying for further use).Consequently, CO2 exhibits a particular advantage inprocesses where a biphasic reaction or liquidliquidextraction against water is required. Eckert et al.[15] have, for example, investigated the use of phasetransfer catalysts in CO2/water mixtures. Further, thecoffee decaffeination process employs a liquidliquidextraction between CO2 and water to recover theextracted caffeine [16].

1.4.3. CO2 is an aprotic solventClearly, CO2 can be employed without penalty in

cases where labile protons could interfere with thereaction.

1.4.4. CO2 is generally immune to free radicalchemistry

Because carbon dioxide does not support chaintransfer to solvent during free-radically initiated poly-merization, it is an ideal solvent for use in such

polymerizations, despite the fact that it is typically apoor solvent for high molecular weight polymers. Inchain transfer, a growing chain (with a terminal rad-ical) abstracts a hydrogen from a solvent molecule,terminating the first chain. The solvent-based radi-cal may or may not support further initiation, andhence chain transfer to solvent can lead to diminishedmolecular weight and diminished polymerization rate.Research conducted during the 1990s (primarily byDeSimone et al.) showed that CO2 does not supportchain transfer, as it is inert towards polymer-basedfree radicals [17]. Other researchers have examinedsmall-molecule free radical chemistry in CO2 to beviable as well [18]. Indeed, it is likely that most of thepolymerizations currently conducted by DuPont in itssemi-works facility are precipitation polymerizations,where the improved control over molecular weightand the enhanced safety inherent to use of TFE/CO2mixtures (see Section 1.2) more than makes up for anydifficulties caused by polymer precipitation duringthe reactions.

1.4.5. CO2 is miscible with gases in all proportionsabove 304 K

The rate of most processes where a gas reacts witha liquid is limited by the rate at which the gas diffusesto the active site (either within a catalyst particle orsimply to the liquid reactant). Gases, such as hydro-gen and oxygen, are poorly soluble in organic liquidsand water and hence in many two- and three-phasereactors, the rate is limited specifically by the rate atwhich the gas diffuses across the gasliquid interface.

Although phase separation envelopes exist withgases at lower temperatures, liquid CO2 can absorbmuch higher quantities of H2 or O2 than typicalorganic solvents or water [19]. Hence, one can elim-inate the dependence of the rate on gas transportinto the liquid phase by employing CO2. Althoughconventional wisdom might claim that this effect isachieved only through creation of a single phase (ofCO2, gaseous reactant and liquid substrate), recentwork in the literature shows that one can achieve highgas solubility and hence high rate while remainingtwo-phase (see Section 2).

It should be remembered that CO2 will exhibit totalmiscibility with gases >304 K only if those gases alsoexhibit critical temperatures 304 K. This includescommonly used reactant gases such as H2, O2 and CO,


for example. Further, addition of any third component(here, a gas such as H2 or CO) to a mixture of CO2,substrate (and catalyst, perhaps) will alter the phasebehavior of the mixture. Because commonly used re-actant gases, such as H2, O2 and CO, exhibit low criti-cal temperatures [12], at typical reaction temperatures(273373 K), the density of these gases, even underrelatively high pressures used to compress CO2, willbe quite low (more gas-like than liquid-like). As such,we expect these gases to behave as non-solvents to-wards the substrate and/or catalyst [20]. Thus, additionof large amounts of reactant gas to the mixture maysolve one problem (diffusion limitations) and createanother (phase separation).

1.4.6. CO2 exhibits solvent properties that allowmiscibility with both fluorous and organic materials

Carbon dioxide is miscible with a variety of lowmolecular weight organic liquids, as well as withmany common fluorous (perfluorinated) solvents. Theliterature has shown previously that one can create ahomogeneous mixture of certain fluorous and organicliquids at one temperature, where phase separationoccurs upon a temperature increase or decrease. Re-cently, Eckert et al. has shown that one can employCO2 as a phase separation trigger in much thesame waythe addition of CO2 (at pressures as lowas 2030 bar) to a mixture of organic and fluorousliquids creates a homogeneous single phase, whileremoval (through depressurization) returns the systemto a two-component, two-phase system [21].

1.4.7. CO2 exhibits a liquid viscosity only 1/10 thatof water

At liquid-like densities, CO2s viscosity is only 1/10that of water and hence Reynolds numbers (VD/,where V is fluid velocity, is density and is theviscosity) for flowing CO2 will be approximately tentimes those for conventional fluids at comparable fluidvelocity. Because convective heat transfer is usually astrong function of Reynolds number, heat transfer ina CO2 mixture can be expected to be excellent. Onthe other hand, CO2s physical properties also leadto significant natural convection causing problems insome coatings processes. The extent to which naturalconvection is an issue is directly related to the magni-tude of the Grashof number [22], which itself scales as2/2. Because CO2 exhibits a liquid-like density and

a gas-like viscosity, Grashof numbers for CO2-basedprocesses can be significantly higher than for analo-gous liquid processes.

The surface tension in carbon dioxide is much lowerthan that for conventional organic solvents and thediffusivity of solutes is expected to be considerablyhigher, owing to CO2s low viscosity. Consequently,CO2 would be expected to wet and penetrate com-plex geometries better than simple liquids. Further, so-lutes would be expected to diffuse faster within cata-lyst pores where CO2 is the solvent than in analogoussystems using conventional liquids.

1.5. Chemical disadvantages to use of CO2as solvent

Carbon dioxide exhibits some inherent disadvan-tages where chemistry is concerned; some of these areunique to CO2 while others are common to any num-ber of solvents.

1.5.1. CO2 exhibits a relatively high criticalpressure and vapor pressure

As mentioned above, CO2 exhibits high critical andvapor pressures; these characteristics guarantee highercapital costs for a CO2-based process relative to oneusing a conventional solvent, as well as the need forspecialized equipment for laboratory work. Exother-mic reactions pose special problems for operation inCO2, given that high pressure is the baseline situation.

1.5.2. CO2 exhibits a low dielectric constantCarbon dioxide exhibits a dielectric constant of

1.5 in the liquid state; supercritical CO2 will exhibitvalues generally between 1.1 and 1.5, depending upondensity. This low dielectric can be both a processdisadvantage and a chemistry disadvantage. Somereactions, for example, require polar solvents for bestresults. Further, low dielectric constant also suggestspoor solvent power, and hence solubility in CO2 canrequire much higher pressures for certain classes ofsolute than more polar compressible fluids (fluoro-form, for example, which exhibits a liquid dielectricof 10). On the other hand, the thermodynamic inter-action between CO2 and non-polar methylene groupsis not particularly favorable and hence, ethane is oftena better solvent for hydrocarbons than CO2.


1.5.3. CO2 is a Lewis acidCarbon dioxide will react with strong bases (amines,

phosphines, alkyl anions) [23]. When attempting touse amines as reactants, this can be a serious disadvan-tage, in that carbamate formation can slow the rate ofthe intended reaction and can also alter the solubilitycharacteristics of the substrate. While alkyl-functionalprimary and secondary amines react readily with CO2,tertiary amines are non-reactive. Further, the pres-ence of electron-withdrawing groups in close prox-imity to the nitrogen atom (as in anilines) preventsformation of carbamates between CO2 and such com-pounds. Carbon dioxide will also react (not surpris-ingly) with metal alkoxides, metal alkyls, and metalhydrides.

CO2 has been shown to react reversibly with anumber of enzymes (lysine residues, specifically),leading to low activity in the presence of CO2 (al-though activity returns to normal following removalof the enzyme from the CO2-rich environment) [24].Because carbamate formation is reversible, even athigh pressure, researchers have employed CO2 as aprotecting group for amines [25] and hence, CO2s re-activity with amines can be an advantage as well as adisadvantage. Finally, because CO2 reacts readily withcarbanions to form relatively unreactive carboxylates,anionic polymerization cannot be conducted in carbondioxide.

1.5.4. CO2 can be hydrogenated in the presence ofnoble metal catalysts to produce CO

If one is trying to hydrogenate a substrate in CO2over a heterogeneous platinum catalyst, production ofCO will poison the catalyst and produce toxic byprod-ucts. Unfortunately, this reaction takes place at rela-tively mild temperatures [6]. There has been a certaindegree of controversy recently as to whether the samereaction occurs over palladium catalysts. For exam-ple, Hancu and Beckman [14] demonstrated that hy-drogenations could be carried out successfully in CO2(over palladium), although it should be noted that thehydrogenation in question was very fast and was con-ducted at 298 K. Subramaniam et al. [26] was ableto successfully conduct a hydrogenation reaction overpalladium in a continuous reactor; no loss in catalystactivity was observed over a period of 12 days. Bycontrast, Brennecke and Hutchensen [27] found thata palladium catalyst de-activated rapidly during batch

hydrogenations in CO2. Subramanian [28] recently in-vestigated these apparent contradictions and found thathigher temperatures (>343 K) and greater residencetimes (such as would be found in batch reactions) dolead to the formation of CO which ultimately poisonsthe catalyst. This is an area where further researchwork is certainly merited, given the potential impor-tance of hydrogenation reactions.

In addition to CO, it is likely that some formatecould be created through hydrogenation of CO2 overnoble metals; formate has been observed during ho-mogeneous catalysis [29] and could theoretically formunder heterogeneous conditions as well.

1.5.5. Dense CO2 produces low pH (2.85) uponcontact with water

Carbon dioxide dissolves in water at molar con-centrations [30] at moderate pressures (


1.5.6. CO2 is a weak solvent (low polarizability perunit volume, low cohesive energy density)

This is perhaps CO2s greatest flaw, in that itsinability to solvate compounds of interest (hencerequiring uneconomically high process pressures)has greatly inhibited its commercial use. This is-sue will be discussed in more detail in Section3.3.

1.5.7. CO2 poisons Ziegler-type polymerizationcatalysts

CO2 will terminate olefin polymerizations that em-ploy classical Ziegler (titanium halide) catalysts, hencepreventing such polymerizations from being carriedout in carbon dioxide.

1.6. How we will approach our analysis

Reaction schemes will be critiqued on their abil-ity to provide a more sustainable process compared toexisting technology, using the 12 principles of greenchemistry as a basis for judgments on sustainability.The basic principles of green chemistry have beenoutlined by Anastas and Warner [34] and are listedbelow:

1) Prevention (alter process schemes and chemicalpathways to prevent the generation of waste,rather than remediate waste once formed).

2) Atom economy.3) Less hazardous chemical synthesis.4) Designing safer chemicals.5) Safer solvents and auxiliaries (create and employ

solvents and process aids that, if emitted to the en-vironment, exhibit a lower impact than currentlyused materials).

6) Design for energy efficiency.7) Use of renewable feedstocks.8) Reduce derivatives.9) Catalysis (create catalysts that are more selective

than current analogs and which therefore producelower volumes of byproducts during reactions).

10) Design for degradation.11) Real-time analysis for pollution prevention.12) Inherently safer chemistry for accident preven-

tion.

If one examines the properties of CO2 and its manyproposed applications, several common trends appear

vis--vis the twelve principles shown above. CO2 hasbeen proposed as a benign alternative to common or-ganic solvents, and hence principle (5) comes intoplay. If one assumes that some proportion of the or-ganic solvent that is employed in any chemical pro-cess will be emitted to the environment, then replace-ment of that solvent with CO2 is a mode of prevention(principle 1), as CO2 emissions are less problematic.The toxicity of CO2 is lower than for many organicsolvents (principle 4) and is naturally abundant (prin-ciple 7).

It should be noted that while use of CO2 is within thescope of several of the principles of green chemistry,improper or ill-considered process design could leadto egregious violation of some of the others. Indeed,if use of CO2 as solvent leads to higher energy con-sumption or an inherently unsafe process, then someof the 12 principles will be followed while others areviolated. Judgment of the net benefit must be done ona case-by-case basis.

Finally, the source of CO2 used in any processshould be considered within the framework of the12 principles of green chemistry. CO2 is naturallyabundant, yet CO2 employed in an industrial processis typically not captured from the atmosphere. Car-bon dioxide is a byproduct (of sizeable volume) ofthe commercial ammonia process [13] and much ofthe commercially available CO2 is derived from thissource (after purification). CO2 can also be capturedfrom fermentation processes, yet this is not generallypracticed commercially (owing to CO2s low currentvalue). Large deposits of CO2 exist naturally in theUS; these are currently tapped for use in tertiary re-covery of petroleum in older fields in West Texas andOklahoma [8]. Hence, if we examine the source ofCO2, we can come to different conclusions of CO2sworthiness as a benign solvent. If, for example, CO2generated by the ammonia process is employed, thenone could consider this as pollution prevention, asthis CO2 would otherwise be emitted to the atmo-sphere. If we employ CO2 from natural deposits, thiscould be construed as anti-sequestration, as this CO2would ordinarily remain underground. If CO2 couldbe captured from the atmosphere (or power plantflue gas) in an energy efficient and economic man-ner, then used in a process, this would likely be thebest source with respect to the 12 principles of greenchemistry.


1.7. Process design using supercritical fluids: areCO2-based plants inherently uneconomical?

The number of processing plants operating world-wide that employ supercritical CO2 is slightly above100 and growing steadily [35]. Most of the currentplants use CO2 to process food in some way (extrac-tion or fractionation), yet other types of plants havebeen or are being brought on stream (e.g. fluoropoly-mer synthesis by DuPont, hydrogenation by ThomasSwan, coatings by Union Carbide, polyurethane pro-cessing by Crain Industries). Despite this steadygrowth, there is a general sense (or unease) withinboth the academic and industrial communities thatthere are elements connected to the design and con-struction of CO2-based plants that effectively blockgreater use of the technology.

Several authors have reviewed aspects of processdesign and costing of supercritical plants [36]; thesereviews typically focus on a specific industry. For ex-ample, Perrut reports that for the case of extraction, therelative cost of a supercritical plant scales as (V*Q)1/4,where V is the column volume and Q the flow rate.This is consistent with what we report in Section 1.7.1,where minimizing equipment size and flow rate willhelp to minimize process cost.

Each of the authors who has reviewed processdesign using supercritical CO2 emphasizes that oneneeds access to the relevant fundamental parametersin order to complete and optimize the design. Suchparameters include both the relevant thermodynamicmodel for the mixture(s) in question with the ap-propriate binary interaction parameters, reaction data(rate constants, heats of reaction, Ahrrenius constants)and transport constants (densities, diffusivities andviscosities). Note that these parameters are exactly thesame as would be required to design a one-atmosphereprocess and hence there is nothing inherently foreignabout a CO2-based process that inhibits design andcosting. Indeed, high pressure alone is not sufficientto explain the perceived difficulty of CO2-based pro-cess scale-up, given that hydroformylation operates at200300 bar at large scale, while low density polyethy-lene is produced at over 2000 bar. If one has accessto the necessary basic information, one can employsoftware such as ASPEN to accomplish the processdesign and ICARUS to handle the costing (the authorhas carried this out successfully with colleagues).

Hence, we must conclude that, if the inhibition inthe scale-up of CO2-based processes is real rather thanperceived, then it must be due to a lack of the fun-damental parameters needed for process design, plusother factors that would inhibit the commercializationof any new technology. For example, it is relativelydifficult at present to predict the effect of molecularstructure on phase behavior in CO2 of molecules thatexhibit any substantial degree of complexity. Carbondioxide exhibits both non-polar tendencies (low di-electric constant) and polar properties (Lewis acidity,strong quadrupole moment) and hence predictions ofphase behavior are not straightforward (as in the caseof alkanes or alkenes). Recent work [37] has shownthat the statistical associating fluid theory (SAFT) canprovide good descriptions of the phase behavior ofcomplex mixtures including CO2, yet the complexityof this model and/or lack of suitable parameters maycurrently limit its use industrially. Group contributionmodels have been applied to CO2 solutions somewhatnarrowly, generally targeting a single class of solutes[38]. What appears to be needed is a means to easilypredict the properties of mixtures involving CO2, suchthat confident predictions of process requirements andcosts can be made using conventional process softwaresuch as ASPEN.

1.7.1. Operating a process economically with CO2:heuristics

While use of CO2 as a solvent is often considered tobe green, operation of any process at high pressuretypically involves higher costs than the analogous pro-cess operated at one atmosphere. If such a process isconsidered green, but cannot be created and operatedeconomically, then the process will be of academic in-terest only and its potential green benefits unrealized.There are some simple rules of thumb that one canuse to render the cost of a CO2-based process as lowas possible.

1.7.1.1. Operate at high concentration. One way inwhich to minimize the cost of a CO2-based processis to minimize the size of the equipment. Given thatCO2 is typically proposed as a solvent (rather than areactant), the most obvious means by which to min-imize equipment size is to minimize the amount ofsolvent (CO2) flowing through the process. Conse-quently, one should try to choose or design substrates


Fig. 1. Solid-fluid phase behavior [1]: CO2naphthalene.

such that they exhibit high solubility in CO2. In ad-dition, those processes where CO2 is employed as theminor component (use of CO2 as a plasticizer in poly-mer processing, for example) are likely to be favoredeconomically.

Another aspect of this issue is reflected in the typi-cal phase behavior of compounds in CO2 (see Figs. 1and 2). Note that in the typical phase diagram ofa crystalline solid in CO2, an essentially pure solidphase exists in equilibrium with a solution. Given thatthe solid phase cannot be processed, one obviouslymakes use of the solution, where naturally CO2 is

Fig. 2. Liquidliquid phase behavior [1]: CO2hexane.

the major component. For the case of liquidliquidphase behavior, a CO2-rich phase exists in equilib-rium with a substrate-rich phase. However, becauseCO2 has been shown to lower the viscosity of solu-tions substantially, one can actually pump and processthe substrate rich phase. Further, one can operate atlower pressure in addition to at higher concentration.Consequently, it may be beneficial to employ systemswhere liquidliquid phase behavior occurs rather thanliquid solid. Efficient operation of a process is botheconomically favorable and more environmentallyfriendly.


1.7.1.2. Operate at as low a pressure as possible.Operation of a process at high pressure is more expen-sive than at one atmosphere, owing to equipment de-sign and construction, as well as the additional safetyfeatures that are necessary. Further, the capital cost ofa high-pressure process is not linear with pressure be-cause the pressure ratings of certain vital equipment(flanges, for example) are available in discrete steps(60 and 100 bar, for example). In addition, the numberof companies with experience in high-pressure pro-cess design drops dramatically as the operating pres-sure rises above 200 bar.

Clearly, these caveats strongly recommend oper-ating at the lowest pressure possible. One means bywhich to accomplish this is in the chemical designof reactants and/or substrates. It has been knownfor a number of years that certain functional groupsare more CO2-philic (thermodynamically moreCO2-friendly) than others. Use of CO2-philic func-tional groups in the design of substrates or catalystscan greatly lower the needed operating pressure, al-though it should be remembered that their use couldeasily raise raw material costs.

Given that carbon dioxide is a relatively feeblesolvent, a classic technique for lowering operatingpressure (or raising operating concentration) is toemploy co-solvents. Methanol and ethanol are mostcommonly used [1,39], but a wide range of organicsolvents has been employed in this fashion, usually atconcentrations


in the complete precipitation of any dissolved mate-rial, rendering product recovery easy. This may betrue, but use of such a route for product recoveryraises costs, as one must then either recompress theCO2 prior to re-use or compress make-up CO2. Asgas compression is energy-intensive and expensive, agreener route to product recovery is desirable.

One example of product recovery without a high-pressure drop is liquidliquid extraction against water.A liquidliquid extraction between an organic andaqueous phase inevitably cross-contaminates thephases, normally requiring remediation of one, andprobably both phases. In the case of a waterCO2 ex-traction, however, the inevitable cross-contaminationis benign (carbonated water!). Indeed, the CO2-basedcoffee decaffeination process employs a waterCO2extraction to recover the caffeine, allowing the CO2to move in loop at relatively constant pressure (seeFig. 3). Further, the cross-contamination here is actua-lly beneficial, as the low pH in the CO2-contaminatedwater allows for a higher partition coefficient for caf-feine, while the water-contaminated CO2 is a betterextractant for caffeine than pure CO2. Beckman andHancu also employed a liquidliquid extraction, herefor the recovery of H2O2 synthesized in CO2 [14].

Fig. 3. Process schematic for coffee decaffeination using CO2 [1].

1.7.1.4. Operate the process continuously if possible.The rationale for operating in a continuous mode isthat the equipment can be smaller while maintaininghigh productivity. While this is usually straightforwardfor liquid substrates, it can be much more difficult forthe processing of solids at high pressure. Indeed, therecurrently does not exist a viable means for introducingand removing solids continuously from a high pressure(100 bar +) process. Those commercial CO2-basedprocesses that employ solids use either batch orsemi-batch mode. An example of the latter is the coffeedecaffeination process, where dual extraction columnsare employed, such that one is in extraction modewhile the other is being emptied and re-filled [16].

In the late 1980s, Chiang et al. at the Universityof Pittsburgh developed a process (LICADO) for thecleaning of coal that employed a biphasic mixture ofCO2 and water [44]. Here, the coal was introduced tothe process continuously as a slurry in water. If theuse of a water slurry of solid substrate is tolerable,this is a useful means by which to introduce solidscontinuously into a high-pressure process.

A clever example of the use of phase behav-ior trends to accomplish continuous processing, aswell as to recover products without large pressure


drops, is shown by Charpentier et al. [45] in theexamination of the continuous polymerization offluorinated monomers in carbon dioxide. Here, themonomers are soluble in CO2 (as are many vinylmonomers) while the polymers are insoluble (also arelatively general trend). Thus, monomer can be con-tinuously recycled through the continuously stirredtank reactor while the polymer precipitates and iscollected.

1.7.1.5. Recover and reuse homogeneous catalystsand CO2-philes. The discovery of CO2-philes inthe early 1990s allowed for the exploration of anumber of processes in CO2 that had been hereto-fore untenable owing to CO2s feeble solvent power.Highly CO2-soluble surfactants and catalyst lig-ands became available, leading to a number of im-portant discoveries regarding chemistry in carbondioxide. However, the new CO2-philes are signifi-cantly more expensive than their CO2-phobic coun-terparts and hence it is important to the economicsof a CO2-based process that any CO2-philes usedin the process be recycled as extensively as possi-ble. Note that recycle of CO2-philes not only makesgood economic sense, but is also more sustainablethan the case where the CO2-philes are simply dis-posed.

Recovery and recycle of homogeneous catalysts isimportant whenever such catalysts are employed be-cause the metals employed in such catalysts are typi-cally expensive. In the case of a CO2-based process,the ligands are also likely to be expensive (they mustbe designed to exhibit high CO2 solubility) and hencethe need for effective catalyst recycle is even moreimportant.

In summary, attention must always be paid to theeconomic viability of processes employing CO2 asreactant and/or solventwhile CO2-based processesare generally thought to be green, their benefits willnever be realized if the cost of such processes dwarfsconventional analogs.

1.7.2. Where would process improvements enhanceopportunities for green chemistry in CO2?

As in the previous section, examples described hereare not directly related to green chemistry, but solutionof such problems would greatly enhance the viabilityof CO2-based processes and are hence intimately tied

to green chemistry in carbon dioxide. For example,there remains no truly efficient means by which toinject and remove granular solids from a high-pressuresystem (screw feeders have been tried with limitedsuccess). There are clearly a number of areas (foodprocessing) where continuous injection and removalof solids would greatly enhance the economic via-bility of a CO2-based process, yet lack of the me-chanical means by which to accomplish this relegatesthe process to batch or semi-batch operation. Notethat the chemical basis for continuous polyurethanefoam production using liquid CO2 as the blowingagent (see Section 3.5.2) was established in the early1960s, whereas commercialization only occurred af-ter development of the proper equipment in the early1990s.

Over the past decade, there has been significantacademic and industrial interest in cleaning processesusing CO2cleaning of metal parts, electronics com-ponents, and fabrics. CO2 is ideally suited to suchapplications owing to its low viscosity and environ-mentally benign nature, yet mechanical issues com-plicate application of CO2 to these processes. Foreach of these applications, individual pieces mustbe rapidly inserted into a high pressure chamber, thechamber sealed and pressurized, the piece cleaned,the then chamber depressurized and emptied. At oneatmosphere, such an operation is trivially simple toconduct and easy to scale (cost per part drops aschamber volume rises). The opposite is currently truefor high-pressure operation; scale-up is non-trivialand the cost of the system rises rapidly as the size ofthe chamber rises. More efficient piecework oper-ations at high pressure will not only render cleaningoperations less expensive, but also coating and fabricdying operations. Finally, many proposed CO2-basedprocesses (including spin coating, lithography anddeveloping, free meniscus coating) that are under ex-amination in academic/industrial laboratories wouldbenefit greatly from breakthroughs in the design ofequipment designed to efficiently transfer parts in andout of high-pressure environments.

1.8. Scope of this report

This report will focus on CO2-based processeswhere chemical reactions are taking place (i.e. greenchemistry) or materials are being processed to create


viable products. Research conducted over the previous5 years (1997present) will be emphasized.

Needless to say, this focus will eliminate discussionof processes that contain only separations unit oper-ations (example: extractions and cleaning). This doesnot mean that such processes are unimportanton thecontrary, several have been commercialized, includingextraction of caffeine from coffee beans and tea leaves,certain acids from hops and various components fromspice plants [36,46]. In addition, CO2-based chromato-graphic instruments have been commercialized at boththe analytical and preparative scale [47].

Clearly, a continuing challenge to the reader whois interested or actively involved in research involv-ing CO2 as a solvent is Can the use of CO2 createnew products, eliminate waste, save energy, and/or en-hance safety to the point where the costs of the productare reduced and a more sustainable process created?The new DuPont fluoropolymer facility may be thefirst example of this, as the use of CO2 has eliminatedthe need for fluorinated solvents, has made workingwith some of the monomers safer and produces prod-uct with better properties than the traditional emulsionprocess.

In each of the following sections, recent researchon various aspects of green chemistry using CO2 willbe summarized. Whereas much of the published workin this area emanates from academic groups, it shouldbe noted that some industrial concerns have also beenquite active. Industry quite naturally tends to patentbefore they publish and consequently a patent searchwas conducted for the period 19962001 where find-ing the term supercritical in either the patent title orabstract was employed as the criteria defining a hit.This search produced 450 hits for the time period inquestion. Well over half of these patents describedinventions where CO2 is used as the solvent in natu-ral product extractions or cleaning. Of the remainder,academic inventors filed nearly half. In addition, asearch using CO2 or carbon dioxide in title or ab-stract (without supercritical) produced 1500 additionalhits, although the vast majority of these did not in-volve use of CO2 as a solvent. For each of the sectionson CO2-based research, a paragraph is appended thatdescribes industrial activity (as described in patents)that is significant but not expressly mentioned inthe main body of the section. Without question,the most active industrial entities (in producing US

patents) on use of supercritical fluids in green chem-istry/processing during 19962001 were DuPont,Micell Inc. and Thomas Swan (UK). Not surprisingly,each of these companies also has supported majorcommercialization efforts in CO2-based chemistry andprocessing (DuPontpolymerization of fluoropoly-mers in CO2; Micelldry cleaning in CO2; ThomasSwanhydrogenations and alkylations in CO2). Allthree have strong research ties to universities.

1.9. A note on cleaning using CO2

There has been substantial effort made by boththe academic and industrial community to employcarbon dioxide in the cleaning of clothing, me-chanical parts and the surface of microelectronicscomponents. Whereas this report will not explicitlyaddress the state of the art in cleaning using CO2,it will evaluate several technological issues that aresignificant to the advancement of CO2-based clean-ing.

For example, although carbon dioxide is not aparticularly strong solvent (see Section 3.3), it willreadily solubilize low molecular weight, volatile,non-polar compounds. If the contamination to beremoved using CO2 falls into this category, thenno additional fundamental science is required, andthe economics of the design and construction of theequipment will determine whether the technology ispracticed. Breakthroughs in the design of high pres-sure cleaning equipment that could rapidly processindividual parts would greatly help to promote use ofCO2 as a cleaning solvent.

CO2 is a weak solvent and hence, cleaning thatrequires the solubilization of polar, inorganic or highmolecular weight material will require the use of CO2-soluble auxiliaries (surfactants, chelating agents). Thediscovery that certain fluorinated compounds areCO2-philic during the early 1990s allow for rapidadvancement in the design of such auxiliaries and adiscussion of the design of such auxiliaries is includedin this report. For the future, the design of CO2-philicauxiliaries must likely include non-fluorinated build-ing blocks, as fluorinated materials are expensive andsome (the fluoroalkyl sulfonate family) are environ-mentally suspect [48].

For the case of microelectronics processing, clean-ing is accompanied by the need to perform chemistry


(photolithography, etching). These topics are includedin Section 3.11 and Section 3.12.

Fabric cleaning has recently been commercializedby two groups in the US (Micell, Inc., and GlobalTechnologies/DryWash). Major issues confrontingthese groups in the future include design of inex-pensive surfactants that clean effectively in CO2, thedesign of high pressure cleaning equipment that ren-ders the process cost-competitive and competitionfrom other benign cleaning technologies (such asthe use of high flash point alkanes, silicones and wa-ter). The use of silicones (Green Earth [49]) seemsto present significant competition, as these materialsare promoted as being more benign than PERC (theyare, if TLV is any indication), they are used at oneatmosphere (hence, equipment is relatively inexpen-sive) and their use is backed by some large, relativelywealthy corporations (GE for silicone production,Procter and Gamble for surfactant production [49]).Indeed, even the design of more efficient conventionaldry cleaning equipment (i.e. that using perchloroethy-lene (PERC) as the solvent) represents a commercialchallenge [50]; the volume of PERC used by drycleaners in the US has dropped dramatically over thepast decade primarily owing to the use of tighterequipment (lower fugitive losses during cleaning).Indeed, significant consolidation occurred in theCO2-based dry cleaning industry during early 2002.Chart Industries, Inc., a member of the DryWashconsortium, decided to exit the CO2-based dry clean-ing business [51] after several years of disappointinggrowth ($126,000 net sales in 2001); the connectionto the consortium was maintained by some of theiremployees as a spin-out company (Cool Clean). CoolClean recently purchased the Hangers franchisingoperation from Micell. Finally, intellectual propertyissues could complicate the use of carbon dioxidein fabric cleaning. Unilever, for example, has fileda number of patents (and continuations in part, etc.)on the use of surfactants in CO2 for the purpose offabric cleaning [52], as well as on the general processwhere CO2 plus a surfactant is employed in fabriccleaning.

In summary, this report will include several issuesimportant to future cleaning applications for CO2,namely the design of effective, low-cost auxiliariesand the design of lower cost equipment for use in partscleaning.

1.10. The effect of regulation on use of CO2 in greenchemistry and chemical processing

The extent to which conventional solvents are reg-ulated will have a profound effect on the extent towhich CO2 is used as a solvent in the future. Forexample, we can examine the recent history of chlo-rofluorocarbons (vis--vis CO2). Chlorofluorocarbons(CFCs) were preferred as solvents for cleaning be-cause they are non-flammable, relatively non-toxic(TLV of chlorodifluoromethane is 1000 ppm [5]), andinexpensive. As a result of research performed duringthe 1970s and 1980s, it became apparent that CFCscontributed to the chemical erosion of the strato-spheric ozone layer, leading to the Montreal Protocolsthat outlined a timetable for the withdrawal of CFCsfrom use as solvents (and refrigerants, etc.). Carbondioxide is often described as a potential substitute forCFCs in cleaning (and also refrigeration). BecauseCFCs exhibited a number of highly favorable proper-ties, without the regulation restricting their use, it isnot likely that CO2 would have ever been consideredas a viable competitor.

Although CFCs represent a somewhat extreme case,regulation does exert more subtle effects on the useof CO2. This is most often seen when comparing thepluses and minuses of using conventional solvents touse of carbon dioxide. From an engineering perspec-tive, carbon dioxide is nearly always more difficult toemploy as a solvent because one needs high-pressureequipment. Consequently, the extent to which a par-ticular solvent is regulated and hence, the obstacles tothe use of such a solvent in a chemical process, cantip the scales either in favor or against use of CO2. Forexample, acetone is not currently on the list of com-pounds that require reporting under section 313 of theEmergency Planning and Community Right-to-KnowAct (EPCRA, also known as the Toxics Release Inven-tory (TRI) [53]). Neither is it listed as a HazardousAir Pollutant [54] by the Office of Air Quality Plan-ning and Standards at the US EPA. Consequently, ifa manufacturer was currently using carbon tetrachlo-ride, for example, in a process where some of thesolvent was emitted to the atmosphere, a natural ap-proach to greening the process might be to first deter-mine whether acetone could be substituted for carbontetrachloride (the latter is included on both the TRIand classified as a hazardous air pollutant). Naturally,


use of acetone in place of carbon tetrachloride wouldlikely not involve any changes to the equipment usedin the process, while use of CO2 would most certainlyrequire equipment re-design. One manifestation of asystematic approach to choosing alternative solventsbased on environmental considerations is SAGE, thesolvent alternative guide, a web-based interactive tool[55a]. Carbon dioxide is indeed one of the possiblechoices that might result from an interactive session onSAGE, depending upon inputs, but no economic cal-culations are performed. An excellent description ofthe industrial perspective on choosing solvents givenboth physical property and regulatory constraints maybe found in Ref. [55b].

As shown above, current regulations affect applica-tion of CO2 by rendering some conventional solventsbetter or worse (from the cost of complying with cur-rent regulations) than carbon dioxide. In addition, itis possible to envision how future regulations mightalso affect the use of CO2 in green processing. Giventhat CO2 has been determined to play a role in globalclimate change, it is conceivable that the emissionof CO2 to the atmosphere will be regulated in thefuture. Consequently, a number of companies havebegun instituting trading credits in CO2 emissions,primarily on an internal basis. In these systems, CO2is assigned a negative value and thus use of CO2 asa raw material allows one to theoretically reduce thecost of the process or product. If this practice becomeswidespread (owing to future regulation on CO2 emis-sions) it will likely spur research and development onprocesses or products that consume CO2.

Another area where future regulation could greatlyimpact the use of CO2 is if restrictions are placedon the use of various fluorinated materials. Certainfluorinated materials have been found to be highlyCO2-soluble (see Section 2.4.1 and Section 3.3) andhence these materials have been applied in the designof highly CO2-soluble auxiliaries (surfactants andchelating agents). To date, the expense of fluorinatedcompounds has greatly limited their use in commer-cial CO2 technology, yet there are applications areas(such as microelectronics) where the cost of fluori-nated compounds will not be an impediment to com-mercial use of CO2 processing. However, it has beenreported recently that certain fluorinated surfactantspersist in the environment, causing concern within theenvironmental and public health communities. The

EPA has proposed a significant new use rule (SNUR)for perfluorooctanesulfonic acid and closely relatedcompounds [48] requiring manufacturers to notifyEPA at least 90 days before commencing the manu-facture or import of these materials for a significantnew use. This may be expanded to include perfluori-nated carboxylic acids (and their precursors) as well.If the use of fluorinated compounds is restricted in thefuture, it could limit the use of CO2 in certain areas ofapplication. Needless to say, design of non-fluorinatedCO2-philic compounds would therefore become apriority in advancing the state of the science.

2. Reactions using gases

In the following sections, recent significant researchand development on the use of CO2 as solvent (or rawmaterial) to aid in the greening of various classes ofreaction or material processing will be discussed. Inthis section, the use of gaseous reactants (H2, CO, O2)in CO2 will be described.

2.1. Hydrogenation

Hydrogenation is widely used in industry at scalesranging from grams per year to tons per hour [56]. Hy-drogenation is conducted at large scale in either the gasor liquid phase; further, while gas phase reactions areperformed over a solid catalyst (heterogeneous cataly-sis), liquid phase reactions are conducted in either two(homogeneous catalyst, liquid and gas each present)or three (heterogeneous catalyst, liquid and gas eachpresent) phase modes. Finally, heterogeneous cataly-sis is conducted in batch, continuous slurry and fixedbed reactor configurations, although the latter is lesscommon than the former two.

Despite the broad range of potential reactor config-urations and reactions, we can, by examining the 12principles of green chemistry described previously,make some general comments as to how the use ofsupercritical fluids (CO2 primarily) can enhance (andpossibly detract from) the sustainability and eco-nomic viability of a hydrogenation process. We willrestrict this discussion to those hydrogenations cur-rently carried out in the liquid phaseaddition of asupercritical solvent to a gas-phase reaction will sim-ply dilute the reactant concentrations, reducing the


rate significantly. With some exceptions (describedbelow), it is not likely that use of a supercritical sol-vent will enhance either the economic viability or thesustainability of a gas-phase hydrogenation.

Two areas where addition of CO2 might benefit agas-phase hydrogenation are flammability and catalystdefouling; addition of CO2 to a mixture of hydrogenand a substrate will enlarge the non-flammable region,while CO2 could help to prevent catalyst fouling bydissolving compounds that contribute to coke forma-tion [57].

2.2. Liquid-phase hydrogenations: advantages to useof supercritical solvents

A number of hydrogenations (synthesis of unsatu-rated fatty acids, reduction of fatty esters to alcohols)are conducted commercially in organic solvent and re-placement of these solvents with benign carbon diox-ide will reduce both liability (reduced flammability,potential toxicity issues) and the potential for VOCemissions owing to fugitive losses. In addition, use ofany supercritical fluid in a liquid-phase hydrogenationprocess can significantly alter the relative importanceof fundamental processes governing the rate expres-sion. In a three-phase hydrogenation, the rate can begoverned purely by the kinetics of the reaction, butmore likely will depend on the rate at which hydro-gen diffuses from the gas phase to the active sites onthe catalyst. The overall rate of transport is itself gov-erned by three resistances in series: (1) the resistanceto transport of H2 across the gasliquid interface; (2)the resistance to transport of H2 through the liquid tothe surface of the catalyst; and finally (3) resistance totransport of H2 within the pores of the catalyst. Giventhat the overall rate is related to the sum of the resis-tances in series [58], one term can easily dominate theexpression for the overall rate. Use of a supercriticalfluid solvent (as opposed to a traditional liquid) elim-inates the gasliquid interface, as low Tc gases suchas H2, O2 and CO are completely miscible with flu-ids above their critical point. However, this does notnecessarily mean that the reaction will be kineticallycontrolled, as one must deal with the remaining tworesistances to transport (bulk liquid to solid surface,interpore diffusion). Because the diffusion constantis embedded in each of these resistances, the use ofa supercritical fluid can also aid in their elimination,

although simply switching from a conventional liquidto a supercritical fluid solvent for hydrogenation byno means guarantees that the reaction rate will dependsolely on the underlying kinetics.

It should be noted that significant effort is expendedin hydrogenation reactor design to ensure that H2 iswell dispersed in the liquid phaseeffective sparg-ing greatly increases the contact surface area betweenthe phases and hence the rate at which H2 diffusesinto the liquid. If use of a supercritical fluid allowsfor a reactor redesign (for example, plug-flow versuscontinuous-stirred tank given that gas sparging is un-necessary), then it may be possible to enhance theselectivity of the reaction through reactor design im-provement, reducing waste.

Indeed, selectivity is a major concern in any chemi-cal processhydrogenation is no exception. It is wellknown that solvents affect the yield and selectivity ofvarious hydrogenation reactions where one very use-ful, although fallible, generality is that in a series ofsolvents, the extremes in selectivity will be found atthe extremes of the dielectric constant. . . [56]. Thesupercritical fluids most often employed as hydro-genation solvents, propane and CO2, exhibit dielectricconstants at the lower end of the scale (1.51.7) andwe might expect to see an effect on selectivity if apolar solvent is replaced by CO2. In addition, thephysical properties of supercritical fluids are readilyvaried over a significant range through changes topressure and temperature and it may be possible to af-fect selectivity by altering these variables. Finally, theaddition of CO2 or operation above the critical pointof the reactant mixture could aid in coke removalfrom the catalyst, prolonging its life or maintainingfavorable selectivity [57]. Clearly, enhancing selec-tivity of a reaction will ultimately reduce the volumeof byproducts generated and potentially the volumeof waste emanating from a particular process.

Hydrogenation is generally exothermic and remov-ing heat from the process is thus more of a problemthan injecting heat [59]. In this case, the use of a super-critical fluid may or may not be advantageous. Liquidsare useful as heat transfer fluids in that one can employthe heat of vaporization to absorb excess heat. Convec-tive heat transfer, which will depend upon both fluidvelocity and fluid physical properties, may or may notbe more successful in a supercritical fluid, dependingupon the exact conditions. For example, the magnitude


of heat transfer is related both to the Prandtl numberand Reynolds number [22]; Prandtl numbers for SCFsare typically lower than for liquids, while the Reynoldsnumber for an SCF could be quite a bit higher (giventhat kinematic viscosity for SCFs is high) at constantvelocity. Heat removal is important, in that inabilityto effectively remove heat could lead to loss of selec-tivity. Liquid CO2 could be useful in this regard, asboiling is often employed as a means by which to ab-sorb excess heat, although it must be remembered thatCO2s heat of vaporization is relatively low.

2.3. Heterogeneous hydrogenation in CO2

As mentioned above, the key green driving forcebehind the use of a supercritical solvent rather thanan organic solvent in a heterogeneous reaction is theelimination of transport resistance (owing to diffusionof the gas across the liquidvapor boundary) and po-tentially a more efficient reaction. Ease of separationof products from reactants is also often mentioned, butnot typically evaluated. Indeed, products and reactantsmay be more easily separated in the conventional ana-log via a simple distillation. Baiker [60] has reviewedprogress in heterogeneous reactions in supercriticalfluids up to 1999; we will focus on key discoveriesprior to 1999 and significant strides made since then.

Harrod et al. [61] have successfully performedthe hydrogenation of fats and oils using supercriticalpropane; propane was employed to allow for solubil-ity of both the substrates (whose solubility in CO2is poor) and hydrogen, which is completely misci-ble with any supercritical fluid. The homogeneouspropane/H2/substrate mixture was fed into a packedbed containing a commercial Pd catalystextremelyhigh reaction rates were indeed achieved (gasliquidtransport resistance being eliminated) and the concen-tration of trans fatty acids (an undesirable byproduct)was reduced. Hence, the green advantages to thisreaction would include reduced waste content andsmaller, more efficient reactors. However, the use ofpropane is problematic, and it is not clear whetherthe process advantages due to faster reaction ratebalance the disadvantages deriving from use of aflammable solvent and the problems inherent tohigh-pressure process design/development. Further,the catalyst deactivated quickly, an important problemfor both economic and sustainable reasons [57,59].

Tacke et al. [62] also investigated the hydrogena-tion of fats and oils (over a supported Pd catalyst),although they employed CO2 as the supercriticalsolvent. Again, rates were shown to be significantlyhigher in the supercritical case (6-fold increase inspace-time yields) and selectivity and catalyst lifetimewere also improved. Each of these features contributesto enhancing the green potential of the process, whilethe need for high pressure operation detracts bothfrom the cost and the sustainability (energy, unit op-eration complexity). Macher and Holmquist [63] alsoexamined the hydrogenation of an oil in supercriticalpropane; similar results to those found by Harrod wereobtained. King et al. [64] examined the hydrogena-tion of vegetable oil and fatty acid esters over nickelcatalysts using both CO2 and propane as supercriticalsolvents and under conditions where either one ortwo fluid phases existed in the reactor. This approachis interesting, as it ultimately could prove a usefulengineering solution to the problem of solubilizingsubstrates in CO2 at moderate operating pressures.

Indeed, Chouchi et al. [65] recently examined thehydrogenation of pinene (over Pd/C) in supercriticalCO2. They found that the rate of the reaction was sig-nificantly faster in the two-phase regime (i.e. lowerpressures) than when the pressure was raised to thepoint where only a single fluid phase existed. The rea-son for this seems clear; the Chouchi study was per-formed by charging a known amount of each of theingredients to the reactor, then pressurizing with CO2.The partitioning of compounds between phases (in thetwo-phase system) must have been such that the con-centration of reactants in the lower phase was higherthan under single-phase conditions. In other words,raising the pressure to create a single phase simply di-luted the reactants, lowering the rate. Note that the con-centration of CO2 in the lower phase (in the two phasesystem) was likely to be substantial, as CO2 should in-teract favorably with a volatile, low molecular weightcompound, such as pinene. Further, the concentra-tion of hydrogen in the lower phase must also havebeen substantial to support the high rate observed, andhence we see that CO2 can swell an organic substratesignificantly and carry substantial amounts of hydro-gen into a swollen liquid phase. CO2 could thereforefunction as a reversible diluent, much in the sameway that it is employed as a reversible plasticizer inpolymer science [66]. In this case, addition of CO2


at relatively low pressures would enhance solubilityof H2 in the substrate, raising rates while not impact-ing process costs precipitously. Even safety could beimproved, as previous work has shown that additionof CO2 to a mixture of hydrogen and air expands thenon-explosive regime more so than addition of nitro-gen [9]. As such, a sudden leak in the reactor, lead-ing to a mixture of CO2, air and hydrogen would stillbe safer than the same case where nitrogen was beingused as the pressure-transmitting fluid. Use of CO2 insuch reactions could thus be green, safe and practical.

Bertucco [67] and later Devetta [68], also showedthe advantages of using a multi-phase system in theirwork on the hydrogenation of an unsaturated ketoneover a Pd/alumina catalyst. These researchers foundthat one could eliminate transport resistance while op-erating in the three-phase (solid catalyst plus liquidplus gas) regime. Here again, the fact that CO2s pres-ence in the lower liquid phase greatly enhances the sol-ubility of hydrogen in the liquid (substrate plus CO2)allows one to eliminate transport resistance withoutthe need to apply pressure high enough to create onephase. Consequently, one could conceivably render thereaction more efficient (and hence less wasteful) andeconomically practical by using moderate pressures.

Arai et al. examined the hydrogenation of unsat-urated aldehydes in both CO2 and ethanol over aPt/Al2O3 catalyst [69]. The selectivity of the reac-tion towards unsaturated alcohol in CO2 was signif-icantly better than that in ethanol; while increasingthe pressure in the CO2 case improved selectivity,the opposite occurred when increasing the hydrogenpressure in the ethanol analog. Indeed, here is a casewhere the use of CO2 appears to enhance selectiv-ity, and thus reduce waste in a reaction versus theliquid analog. It is not clear from the discussionby Arai whether this improvement in selectivity isenough to offset the difficulties involved in scalingup a high-pressure process and whether the energyinput to the CO2-based analog is more or less thanthe liquid case. Interestingly, Arai did not observethe rapid catalyst deactivation formerly observed byMinder et al. [70] during hydrogenation in CO2 overa platinum catalyst. Minders results were readilyexplained by formation of CO and other poison-ing species owing to the hydrogenation of CO2 it-self; it is unclear why Arai was able to avoid thisproblem.

Poliakoff et al. [71] have evaluated the efficiencyof hydrogenation of a wide variety of substrates in su-percritical fluids (propane and CO2) over a Pd catalystin a continuous flow reactor. Substrates included aro-matic alcohols, aldehydes, ketones, unsaturated cyclicethers, nitro compounds, oximes and Schiff bases. Re-actions were conducted at temperatures ranging from360 to 670 K at pressures between 80 and 120 bar.All of the substrates examined could be hydrogenatedto some extent, with measured space-time yields ex-ceeding 2105 kg h1 m3 for the hydrogenation ofcyclohexene. Given the high temperatures employed,the relatively low pressure, the presence of significantamounts of hydrogen and the low volatility of someof the substrates employed, it is highly likely thattwo or more phases existed in the reactor during theinitial phases of the process. CO2s density will notbe liquid-like at these pressures and temperatures,while hydrogen will act as a non-solvent owing to itslow critical temperature (and hence low reduced den-sity at the reaction conditions). Poliakoff examinedthe phase behavior in the cyclohexene-to-cyclohexanesystem and indeed found that multiple phases existinitially, while a single phase forms near the end ofthe reaction. Single-phase behavior results becausethe temperature increases to a point above the criticaltemperatures of both cyclohexene and cyclohexane.Whereas Poliakoff demonstrated the breadth of con-tinuous hydrogenation in CO2, lack of comparisonswith traditional hydrogenation reactions make it dif-ficult to judge whether the technology will ultimatelybe deemed green. Catalyst lifetime, for example,is not mentionedrapid loss in activity could renderthis technology less than adequate from both greenand financial perspectives. If CO2-based hydrogena-tion allows for elimination of significant volumes ofsolvent without greatly increasing energy or catalystdemand, then this technology could ultimately beboth economically successful and green.

Subramanian et al. [26] also examined the hydro-genation of cyclohexene to cyclohexane (over Pd/C)in supercritical CO2, although under conditions wherethe system remained single phase throughout the re-action and the temperature was held at a constant 343K. The reaction remained stable over periods exceed-ing 20 h and catalyst activity was maintained at a highlevel by pretreating the cyclohexene feed to removedeleterious peroxides. No CO or formate formation


was observed. While this work does not suggest asto how or why such reactions could be consideredgreen, it does demonstrate that stable (with respectto temperature and pressure) catalytic hydrogenationin a continuous reactor using CO2 as solvent is read-ily achievable. Again, the assumption here is thatuse of CO2 will eliminate the gasliquid interface,rendering the reaction more efficient and potentiallyless wasteful. Subramaniam has authored a compre-hensive review on process design issues inherent tocatalytic processes performed in carbon dioxide [59];interested readers should consult this paper.

Hancu and Beckman [72] examined the hydrogena-tion of oxygen (production of H2O2) in CO2 underboth liquid and supercritical conditions. Hydrogenperoxide is currently produced via hydrogenation(over a Pd supported catalyst), then oxidation of a2-alkyl anthraquinone (AQ) in an organic solvent (seeFig. 4). Whereas H2O2 is widely accepted as a greenoxidant, the process by which it is manufactured ex-hibits a number of less-than-green attributes. First, useof the organic solvent (coupled with the liquidliquidextraction against water used to recover the product)creates a significant contamination issue, one that iscurrently remedied using energy-intensive distillation.Further, because each of the reactions are transportcontrolled (again, by the rate of diffusion of H2 orO2 from the gas to liquid phase), CSTRs (continuousstirred tank reactors) are used, allowing for a rangeof anthraquinone residence times and hence overhydrogenation of the AQ to form waste byproducts.

Fig. 4. Schematic of the anthraquinone route to hydrogen peroxide[15].

Gelbein [73] has estimated that one-third of the costof H2O2 can be tied directly to anthraquinone andsolvent make-up/regeneration; 1.5 million poundsof anthraquinone and 15 million pounds of solvent areproduced each year simply to support consumptionin the AQ process for producing hydrogen peroxide.

Hancu first examined the use of CO2 as the or-ganic solvent in the anthraquinone process by gen-erating a highly CO2-soluble analog to conventionalalkyl anthraquinones (alkyl AQs exhibit solubilitiesin CO2 that are three orders of magnitude belowwhat is employed in the commercial process). Thesefluoroether-functional AQs exhibited complete misci-bility with CO2; maximum miscibility pressures weresensitive functions of anthraquinone composition andtopology. Hancu showed that kinetic control couldbe obtained in both the hydrogenation and oxidationreactions using CO2 as the solvent. Here, use of CO2eliminates the need for the distillation train, as con-tamination of the aqueous phase by solvent and otherbyproducts is not an issue. Further, while the solventin the conventional process is prone to both hydro-genation and oxidation, this is not the case for theCO2 analog.

Despite the promising laboratory results, Hancusprocess in its original state exhibited a criticaleconomic flaw, yet one that could be corrected givenrecent results. The fluoroether-functional AQ will besignificantly more expensive than an alkyl AQ andpressures required to maintain a homogeneous mix-ture will be high, despite the use of the CO2-philicAQ. If, however, we examine the results of Bertucco,Chouchi and Devetta [65,67,68], it is clear that an al-ternative route exists where one could take advantageof the green aspects of CO2 use while minimizing theAQ cost issues and reducing the operating pressure.The works cited in the previous sentence show that itis quite possible that one does not need to achieve asingle phase of hydrogen, CO2 and substrate to elim-inate gasliquid diffusional limitations to reaction. Ingasliquid reaction systems, often the primary resis-tance to transport is the low solubility of the reactantgases in the liquid phase and slow diffusion acrossthe interface. The high degree of swelling of a sub-strate by CO2 can allow for significant increases inhydrogen solubility in the liquid phase, while the lowviscosity of carbon dioxide enhances diffusion rates.Thus, it is quite likely that one could derivatize an


anthraquinone with an inexpensive oligomer (such asa short chain polypropylene oxide or silicone) thatwould (a) not raise cost significantly; (b) transformthe crystalline, high melting alkyl AQ to a low melting(or amorphous) derivatized AQ that would; (c) swellsignificantly with CO2 at moderate pressures (343 K seem to greatly acceler-ate the process. Longer residence times (as would beexperienced in batch reactors or CSTRs) also enhancethe rate of poisoning.

2.4. Homogeneous hydrogenation in CO2

2.4.1. CO2-soluble catalyst designClearly, the most pressing issue one must deal

with to conduct a homogeneous hydrogenation in asupercritical fluid is that of catalyst and substrate sol-ubility. Carbon dioxide is without question the mostpopular solvent of those with a readily accessible(


with CO2 itselfsuch side reactions can be inhibitedthrough proper catalyst design or choice of operatingconditions.

2.4.2. Engineering rationale for homogeneousversus heterogeneous catalysis

In homogeneous hydrogenation, the catalyst hasbeen designed such that it is soluble in the liquidphase; the ligands of the catalyst are usually con-structed to produce high selectivity to product. Therationale for conducting homogeneous hydrogenationreactions in CO2 has three primary thrusts, (1) thatoperation in CO2 eliminates the need for organic sol-vent; (2) operation in CO2 eliminates the gasliquidinterface and hence allows for kinetic control over thereaction; and (3) use of CO2 will alter the selectivityof the reaction (hopefully for the better). Much of therecent work on homogeneous hydrogenation has beendirected at asymmetric synthesis, with the generalhypothesis that use of CO2 could possibly alter theenantioselectivity of the reactions concerned.

The rate of a homogeneous hydrogenation reac-tion conducted in an organic solvent or water islikely to be governed by the rate at which hydrogendiffuses across the vaporliquid interface. As such,elimination of this interface (via operation in CO2)eliminates this transport resistance. Indeed, becausethe catalyst in this case is soluble, elimination of theinterface entirely eliminates transport resistance. Toallow direct replacement of the organic solvent in ahomogeneous hydrogenation reaction with CO2, boththe catalyst and the substrate must be soluble in CO2.Consequently, the majority of the scientific effort inliterature works on homogeneous hydrogenation inCO2 is directed at synthesis of CO2-soluble analogsof conventional catalysts. Substrates must be chosenthat are CO2-soluble and hence one observes pre-dominantly model compounds employed rather thannecessarily compounds of industrial interest.

One could pose the question, if a liquid substrateis being employed, why not simply run the reactionusing the homogeneous catalyst neat, in the absenceof any solvent? The solubility of hydrogen in organicliquids is typically quite low, and hence running thehydrogenation of a neat substrate will encounter sig-nificant transport resistance (of hydrogen across the in-terface) to reaction. If carbon dioxide readily dissolvesor swells the liquid phase (catalyst and substrate), the

rate of reaction can increase owing to enhance hydro-gen concentration at the locus of reaction, despite thepresence of CO2, a diluent.

An example of the use of homogeneous catalysisto achieve an engineering goal was shown by Hancuand Beckman [14], who examined the generation ofH2O2 in CO2 directly from H2 and O2 in a singlestep using a CO2-soluble palladium catalyst. Thisprocess has been examined in industry for over twodecades, as elimination of the anthraquinone from theprocess eliminates several unit operations and greatlyreduces raw material input. If one examines Gelbeinsnumbers for the economics of H2O2 production [73],one would estimate that the using the direct routewould reduce the cost of production by over 50%, asignificant amount for a commodity process. Hancuproposed that one could generate H2O2 in CO2 (fromH2 and O2) using a soluble palladium catalyst, wherethe H2O2 is then rapidly stripped into water. Thegreen aspects of this process include elimination ofsolvent waste and anthraquinone input/byproducts,elimination of the distillation train and the associatedenergy input, and elimination of several unit oper-ations and the associated energy input. The processcould be run continuously and the product recoveredfrom CO2 without a large pressure drop, renderingthe process economics more favorable. Previous workon the direct route to H2O2 has focused on the bal-ance between safety and productivity, where most ofthe patented processes employ water as the reactionmedium to maintain safety. However, because thesolubility of H2 and O2 in water is so low, the pro-ductivity of these processes is not sufficient to meritscale-up. In addition, the Pd catalysts employed tendto catalyze degradation of H2O2 as well as forma-tion, and hence running the reaction in water doesnot lead to the desired productivity. Hancu showedthat one could employ a CO2-soluble catalyst, andhence run the reaction in CO2 without transport lim-itations and in a non-explosive concentration regimewhere rates are high. Future work is needed in thisarea with respect to optimizing catalyst performanceand lifetime, yet this is a good example of the use ofhomogeneous hydrogenation in carbon dioxide to ac-complish what are normally perceived to be processgoals.

Unlike in the previous example, in cases where aseparate aqueous phase is not present, we may be able


to take advantage of the favorable properties of CO2(with respect to hydrogenation) while avoidingsome of the negative process issues by employing agasliquid rather than one-phase system. For exam-ple, it is known that H2 is poorly soluble in mostorganic liquids and hence it is expected that a hy-drogenation in organic solvent would be transportlimited. If one knows the fundamental kinetic param-eters of the reaction, one should be able to predictat what [H2] to [substrate] ratio the reaction couldbe controlled by the underlying kinetics, and hencecalculate the target [H2] for the reaction in the pres-ence of CO2. If the substrate is a liquid, one shouldbe able to find conditions where a two-phase sys-tem (H2CO2-substrate) exists, yet where substantialamounts of hydrogen are dissolved in the lower phase.As described previously, liquidliquid phase diagramsof CO2 and larger molecules are typically asym-metric and hence operation at high concentrationsof substrate is possible at relatively lower pressures.Further, the catalyst would be required to dissolve ina mixture of (primarily) substrate and CO2, suggest-ing that one might not have to fluorinate the catalystto achieve solubility in the proper phase. Thus, byoperating in the two-phase region, one could oper-ate at lower pressure with the original catalyst whilealso eliminating the need for the organic solvent andthe transport resistance to reaction. Ideal substrateswould be those that are relatively high in molecularweight, or are polar, yet are also liquids (or low melt-ing solids, where CO2 can depress the melting point[83]).

Another interesting possibility would, in fact, in-volve functionalization of the catalyst (fluorination) toallow better solubility in CO2 while also operating inthe two-phase regime. Here, the presence of the CO2in the lower phase would serve to not only allow higherhydrogen concentrations but would also solubilize thecatalyst. Upon removal of the CO2, the catalyst wouldprecipitate, allowing recycle. This would present theCO2-based analogy to recent work by Gladysz et al.[84], where a fluorinated catalyst was developed thatwas insoluble in the reaction solvent, but dissolvedupon heating. Hence, temperature was used as the re-versible trigger to allow catalyst use and recovery. Re-cently, it has indeed been shown that CO2 itself couldalso be employed as a reversible solvation trigger[85].

2.4.3. Chemical rationale for homogeneouscatalysis

The final reason for conducting a homogeneoushydrogenation in CO2 is the premise that use of CO2would alter the selectivity of the reaction in a positiveway. Xiao, for example [86], examined the asymmet-ric hydrogenation of tiglic acid (2-methyl-2-butenoicacid) in CO2 using a ruthenium catalyst; ees (enan-tiomeric excesss) in CO2 were essentially no betterthan those found for the same reaction in methanol.Tumas [87] examined the hydrogenation of dehy-droamino acids in CO2 using a cationic rhodiumcatalysthere the fluorinated counteranion (3,5bis(trifluoromethyl phenyl) borate (BARF) or triflate)enhanced solubility of the catalyst in CO2. Tumasfound somewhat better ees for some substrates inCO2 versus hexane or methanol, but overall the per-formance of CO2 was comparable to that of the otherorganic solvents. Leitner [88] has used chiral iridiumcatalysts to perform the hydrogenation of imines inCO2. The catalysts were modified (using fluoroalkylponytails) to permit solubility in CO2. Enantiomericexcesses in CO2 were comparable to those found forthe same reaction in dichloromethane, while rateswere found to be much higher for some substrates inCO2 versus CH2Cl2.

Recently Tumas [89] and Jessop [90] explored theuse of biphasic mixtures of ionic liquids and car-bon dioxide to perform hydrogenations. Ionic liquidsare salts (typically ammonium or phosphonium) thatexhibit melting temperatures near or below roomtemperature. Ionic liquids behave as polar solvents,yet exhibit vanishingly small vapor pressures. In boththe Tumas and Jessop studies, a CO2-insoluble cat-alyst was dissolved in the ionic liquid, which is thenbrought into contact with a mixture of CO2, substrateand hydrogen. As has been shown by Brennecke [91],ionic liquids absorb large amounts of CO2 (molefractions >0.5) at pressures below 100 bar. Further,the ionic liquid does not measurably dissolve in CO2.Consequently, both Tumas and Jessop were able toconduct reactions in the ionic liquid at very high rates(the high CO2 swelling allowed for high H2 solubil-ity), where the product could be stripped from theionic liquid into CO2 and the catalyst retained in theionic liquid for recycle. Note that this is an analogy ofthe two-phase CO2/H2/substrate mixture mentionedabove, where the high swelling of the lower phase by


CO2 eliminates transport limitations while two-phaseoperation permits use of moderate pressure.

To date, the ionic liquids being explored as solventsare primarily based on imidazolium or pyridiniumcations (some work has also been conducted on phos-phonium ILs). Whereas these ionic liquids (ILs) areproposed as benign solvents (owing to their near-zerovapor pressures), it must be remembered that the tox-icity and fate (in the environment) of such materialsis currently not known. Brennecke et al. [92] haverecently observed that the toxicity of the butyl im-idazolium hexafluorophosphate salts towards Daph-nia magna is similar to that shown by benzene ordichloromethane, where toxicity of the IL did not de-pend strongly on the nature of the anion. We expectmore such studies in the future in this area. In ad-dition, because large-scale manufacturing processesfor these solvents have yet to be established, the im-pact of such processes on the environment is also notknown. In summary, the current crop of ILs may ul-timately be judged to be benign solvents or they maynot.

2.4.4. Homogeneous hydrogenation and materialsynthesis

Watkins has explored a novel means by which toapply homogeneous hydrogenation in CO2 to creationof metal nanoparticles and thin metal films. Watkinshas found that certain metal complexes exhibit mil-limolar solubility in CO2 at pressures below 100 bar.Exposure of these complexes to hydrogen under mildconditions reduces the metal to the zero valent state,inducing nucleation of pure metal. Watkins first em-ployed this reaction to create small metal particleswithin polymer monoliths [93]. The complex is addedto CO2, and this solution brought into contact withthe polymer, which swells accordingly. Hydrogen isthen introduced, which reduces the complex

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