Production of Hydrogen Peroxide in Liquid CO2. 1. Design, Synthesis

2824

Ind. Eng. Chem. Res. 1999, 38, 2824-2832

Production of Hydrogen Peroxide in Liquid CO2. 1. Design, Synthesis, and Phase Behavior of CO2-Miscible Anthraquinones Dan Haˆ ncu and Eric J. Beckman* Chemical Engineering Department, University of Pittsburgh, 1249 Benedum Hall, Pittsburgh, Pennsylvania 15261

H2O2 production via sequential hydrogenation-oxidation of anthraquinones (AQs) represents a potentially efficient process application of liquid or supercritical CO2 (ScCO2). First, the use of CO2 as the organic solvent in the process will eliminate the mass-transfer limitation during the hydrogenation and oxidation and also the contamination of the aqueous phase during isolation of hydrogen peroxide. Further, the hydrogen peroxide can be recovered from CO2 solution without depressurization by liquid-liquid extraction, helping to minimize the energy cost. The primary obstacle to use CO2 as a working fluid in the process is the poor solubility of conventional AQs in CO2. Therefore, we designed and generated CO2-philic AQs suitable to be used in the production of hydrogen peroxide. Both mono- and difunctionalized anthraquinones (FAQs) were synthesized by attaching CO2-philic polymers chains (-CF(CF3)CF2O-) either to mono- or diaminoanthraquinones or to (hydroxymethyl)anthraquinone. All FAQs synthesized are highly soluble in CO2 and present liquid-liquid phase behavior with minimum miscibility pressure between 170 and 210 bar. Cloud-point pressures were shifted to lower values by using nonhydrogen bonding linkers between AQ block and CO2-philic tails or by increasing the CO2-philic content of FAQs. Introduction Carbon dioxide has elicited significant technological interest over the past decade given that it is one of the very few organic solvents that is not regulated as a volatile organic chemical (VOC) and is nonflammable, inexpensive, and relatively nontoxic.1 As such, CO2 has been applied extensively in food processing and as the blowing agent in so-called “green” foaming processes. Although CO2 has been proposed as a general purpose “green” replacement for organic solvents in chemical processes, we would propose application targets which exhibit certain process characteristics which render CO2 use particularly favorable: (a) Liquid-Liquid Extraction vs Water.2,3 In any liquid-liquid extraction between organic and aqueous phases, the aqueous phase will be contaminated to some degree by the organic phase forcing problematic (dilute solution of the organic phase in water) remediation to allow discharge of the water to the environment or reuse. This category could more generally include contact between any hydrophilic and hydrophobic phase and thus include inverse emulsion polymerization as well. (b) Gaseous Reactants in Liquid Solutions.4,5 In many cases, the use of gaseous reactants in liquid systems creates a transport-limited reaction, given that the solubility of gases in most liquids is poor at low to moderate pressures. On the other hand, gases such as hydrogen and oxygen are completely miscible with CO2 above CO2’s critical temperature of 31 °C. Thus, reactions that are diffusionally limited in conventional liquids could be operated under kinetic control if run in CO2. * To whom correspondence should be addressed. Telephone: (412) 624-9630. Fax: (412) 624-9639. E-mail: [email protected].

(c) “Unavoidable” Emissions.6 Processes such as foaming and solvent-based coating, where generation of the product is accompanied by the inevitable elimination of solvent during processing, can be rendered more environmentally sound if carbon dioxide is employed as the primary solvent. (d) Food/Pharmaceutical Processing.7,8 Here, use of CO2 reduces long-term liability from trace solvent exposure and also eases regulatory hurdles accompanying the approval process for new food or pharmaceutical products. (e) CO2 as a Raw Material.9 Because CO2 is inexpensive, nontoxic, and nonflammable, it is an ideal raw material for the introduction of carbonyl groups if it can be properly activated. (f) Reversible Plasticization of Thermally Labile Polymers.10,11 Polymers are usually processed at high temperature, although the extrusion of thermally labile polymers (as in powder coating formulation) produces significant waste. Using CO2 as a plasticizer allows processing at lower temperatures, while CO2 is readily removed from the final product. Constraints to the Economical Use of CO2. Use of CO2 as the primary solvent in processes which include one or more of the application targets described above can produce better products in a more environmentally sound fashion. Clearly, however, if use of CO2 in a process leads to poor product quality, extraordinary energy usage, or higher ultimate product cost, any environmental advantage gained via elimination of conventional liquid solvents is lost. Thus, one should consider a series of process constraints when employing CO2 that will help to minimize capital and operating costs while maintaining the environmental advantages inherent to the use of CO2 as the primary solvent: (a) Minimize Operating Pressure via the Use of “CO2-philic” Materials. One of the primary disadvan-

10.1021/ie980738d CCC: $18.00 © 1999 American Chemical Society Published on Web 06/06/1999

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2825 Table 1. Solubility of Anthraquinones in Supercritical CO2 anthraquinone 9,10-anthraquinone 1-amino-2-methyl-AQ 1-(methylamino)-AQ 1,4-diamino-2-methoxy-AQ 1,4-bis(methylamino)-AQ 1,4-bis(n-butylamino)-AQ 1,4-bis(n-octylamino)-AQ 1,4-bis(n-dodecylamino)-AQ 1,4-bis(n-hexadecylamino)-AQ 1,4-bis(n-octadecylamino)-AQ

Figure 1. Anthraquinone-anthrahydroquinone process of generation of hydrogen peroxide.

tages to the use of CO2 in a process is that relatively high pressures are needed, and it is well established that capital costs rise as the operating pressure of a process rises. Further, carbon dioxide is generally considered to be a relatively poor solvent. Our group12 and others13 have employed so-called CO2-philic compounds, i.e., materials which exhibit high solubilities in CO2 at relatively low pressures, to lower the pressure required to operate a given process (or use of CO2-philic compounds can also be said to reduce the size of needed equipment, by allowing higher solubility of required compounds at a given pressure). (b) Eliminate Large Pressure Drops. In many discussions of supercritical fluid technology, mention is made that compounds can be easily recovered from solution via simple depressurization. While this is true, use of large pressure drops to recover products from solution is accompanied by a significant operating cost penalty due to the need to recompress the gas. Consequently, products should be recovered from solution with the lowest pressure drops possible. (c) Employ Continuous Processing. Naturally, use of continuous processing permits use of smaller equipment, which, in the case of operation at high pressure, greatly reduces capital cost. (d) Minimize the CO2 Flow Rate. Clearly, minimizing the flow rate of CO2 (assuming that CO2 here is merely a solvent or diluent) reduces the equipment size. However, if the presence of CO2 in the system greatly reduces the viscosity of the working fluid, then use of extra CO2 may pay for itself in the form of lower pumping costs. (e) Recycle CO2-philic Materials. Although CO2philic compounds can greatly lower the operating pressure and/or equipment size of a CO2-based process, they are almost always more expensive than their CO2phobic, conventional analogues. As such, it is important to recycle as much of the CO2-philic material as possible to minimize the impact of its cost on the overall cost of the process. Potential Advantages for Use of CO2 in the Anthraquinone-Anthrahydroquinone (AQ-AQH2). Process for the Generation of Hydrogen Peroxide. Generation of hydrogen peroxide in the anthraquinoneanthrahydroquinone (AQ-AQH2) process represents an ideal target application for the use of CO2 as a primary solvent. In the conventional process.14-16 (Figure 1),

solubility pressure temp (mM) (bar) (K) reference 0.96 0.13 0.19 0.015 0.045 0.055 0.060 0.035 0.01 0.005 0.05 0.001 0.049

219 200 200 404 200 200 126 126 126 200 600 200 600

318 313.15 313.15 300.0 310.0 320.0 330 330 330 330 330 330 345

22 23 23 24 24 24 24 24 24 25

2-alkylanthraquinone (usually ethyl or amyl) is dissolved in an organic solvent (typically a mixture of an aromatic hydrocarbon and a long-chain alcohol17) and reacted with hydrogen over a palladium catalyst18 at mild temperatures (30-50 °C) in a three-phase reactor. Following hydrogenation, the anthrahydroquinone is fed to the second reactor where it is oxidized (in a two-phase reactor) back to anthraquinone, also producing hydrogen peroxide. The hydrogen peroxide is stripped into water via a countercurrent liquid-liquid extraction. The organic phase, containing primarily AQ, is recycled to the first reactor to complete the cycle. Replacement of the conventional aromatic-alcohol solvent mixture in the AQ-AQH2 process for manufacturing hydrogen peroxide with carbon dioxide is advantageous for the following reasons: 1. Elimination of the organic solvent via replacement by CO2 eliminates the contamination of the aqueous product with trace organics. 2. Use of CO2 as the working fluid eliminates the gasphase and mass-transfer limitations in both the hydrogenation19 and oxidation20 reactors, allowing for potential kinetic control of the reactions and thus higher throughputs. If kinetic control can be achieved, one could move to plug-flow operation in the hydrogenation reactor, minimizing or eliminating the deep hydrogenation reactions which are known to degrade the anthraquinone.21 3. In a CO2-based AQ-AQH2 process, the CO2 travels in a loop (where pressure is relatively constant), and the product can be recovered without a large pressure drop. This satisfies one of the process constraints mentioned above. 4. Continuous processing is feasible, thus conforming to another of the important constraints described in the previous section. The primary obstacle to the use of CO2 as the working fluid in H2O2 production from the AQ-AQH2 system is that conventional 2-alkyl-AQs exhibit poor (Table 1) to negligible solubility in carbon dioxide at pressures up to 200 bar. Thus, the initial focus of our research has been to design and generate CO2-philic analogues of 2-ethyl-AQ that would support the production of H2O2 via sequential hydrogenation and oxidation. Success in this endeavor thus allows us to conform to several of the other process constraints listed in the previous section: 1. The operating pressure can be reduced, or the concentration of AQ in the system increased at constant pressure, via use of the CO2-philic AQ. 2. In the AQ-AQH2-based process for H2O2 generation, the AQ is continuously recycled, and thus the

2826 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999

added cost due to redesign of the AQ for use in CO2 is minimized. The financial impact of anthraquinone redesign is further reduced if use of CO2 as the process solvent diminishes anthraquinone degradation, as described above. 3. As described below, our strategy for the generation of highly CO2-soluble anthraquinones results in formation of materials which are either liquids or amorphous solids at room temperature. Consequently, the phase behavior of these materials in CO2 is of the liquid-liquid type, which allows us to work at very concentrated regimes where the CO2 functions more as a viscosityreducing diluent than a solvent. This allows one to minimize the CO2 flow rate in the system, satisfying another of the process constraints listed above. Theory The thermodynamic criterion for two materials to form a solution is that the Gibbs free energy of mixing, ∆GM, is negative. The energetics of the mixing process reflected in ∆HM, the enthalpy of mixing, are governed by the molecular interactions between the components of the solutions: solvent-solvent, solute-solvent, and solute-solute interactions. The balance of these interactions can be expressed by the interchange energy, ω, defined as26

ω ) z[Γij(r,T) - 1/2(Γii(r,T) + Γjj(r,T))]

(1)

where z is the coordination number and Γi,j is the attractive intermolecular pair-potential energy between components i and j, including contributions due to induction, dispersion, and dipole-dipole, dipole-quadruple, quadruple-quadruple interactions as well as specific interactions. While attractive interactions between like molecules (either solute-solute or solventsolvent interactions) lead to positive contribution to ∆HM and therefore to ∆GM, solute-solvent attractive interactions favor mixing by lowering the value of both ∆HM and ∆GM. The entropy of mixing, ∆SM, incorporates both the combinatorial entropy of mixing (∆SM(comb)) and contribution due to the change of the free volume during mixing. According to the Flory-Huggins theory, the combinatorial entropy of mixing per volume of solution, V, can be expressed as27

[

]

φ1 ln φ1 φ2 ln φ2 ∆SM )+ RV V1 V2

(2)

where φi are the volume fractions of the components in the mixture and Vi are the molar volumes of the pure components. As φi < 1, ∆SM(comb) is always positive, favoring the mixing process by lowering ∆GM. An increase of the molecular mass of the polymer clearly leads to larger molar volume Vi and thus a smaller entropic contribution to ∆GM. Both ∆SM and ∆HM are also affected by the change in the free volume, as reflected by ∆VM, the volume change on mixing. For most polymer solutions, ∆VM is negative, leading to negative contributions to ∆SM and ∆HM. Because the two contributions do not typically cancel, the final expression of ∆GM contains a positive term unfavorable to the mixing process.27 Using the same strategy employed to synthesize other CO2-philic ligands,3 surfactants,28 or graft copolymer dispersants,29 a family of functionalized anthtraquino-

Figure 2. General chemical formula of functionalized anthraquinone ((1)Kr ) poly(perfluoropropylene oxide) polymer).

nes is designed and generated by attaching perfluoroether tails to either amino- or hydroxyanthraquinones. The general chemical formula of a FAQ is illustrated in Figure 2. In this work, we report the phase behavior of functionalized anthraquinones (FAQs) in liquid CO2. The length of CO2-philic tails, the number and topology of the tails on the anthraquinone rings, and the nature of the linker are the structural variables that are taken into consideration to explore the effect of structure on solubility. Experimental Section Materials. Oligomers of hexafluoropropylene monofunctionalized with a terminal carboxylic group (Krytox functional fluids, FSL (FW ) 2500), FSM (FW ) 5000), FSH (FW ) 7500), DuPont), thionyl chloride (99.9%, Aldrich), N,N-dimethylformamide (Aldrich), perfluoro2,5,8-trimethyl-3,6,9-trioxadecanoyl fluoride (Lancaster), 1-aminoanthraquinone (Aldrich, 97%), 2-aminoanthraquinone (Aldrich, technical grade), 1,2-diaminoanthraquinone (Aldrich), 1,4-diaminoanthraquinone (Aldrich, 85%), 2,6-diaminoanthraquinone (Aldrich, 97%), and 2-(hydroxymethyl)anthraquinone (Aldrich, 97%) were used as received. Perfluoro-1,3-dimethylcyclohexane (Aldrich, 80%) and 1,1,2-trichlorotrifluoroethane (Aldrich, 99+%) were distilled and dried over 4A molecular sieves. Synthesis of Fluoroether Acid Chloride (KrCOCl). In a typical experiment, 5 mmol of poly(perfluoropropylene oxide) monofunctionalized with a terminal carboxylic group (FW ) 2500, 5000, 7500), 50 cm3 of perfluoro-1,3-dimethylcyclohexane, 25 mmol of thionyl chloride (2.97 g, 1.82 mL), and 10 mmol of N,Ndimethylformamide (0.73 g, 0.77 mL) were charged to a one-neck flask equipped with a dry ice condenser. The reaction mixture was heated at reflux (T ∼ 82 °C) for 6 h while stirring vigorously under a blanket of nitrogen. After reaction, the phases were separated and the solvent was removed under vacuum at 75-80 °C. The product was characterized by the disappearance of the carboxylic group peak at 1775 cm-1 and the appearance of the acid chloride peak at 1805 cm-1 in the FTIR spectrum. Synthesis of Monoamido-Functionalized Anthraquinones (1,(2)-(Kr-CONH)-AQ). Typically, 2 mmol of poly(perfluoropropylene oxide) acid chloride (FW ) 2500, 5000, and 7500) and 0.892 g (4 mmol) of aminoanthraquinone (1- or 2-amino-anthraquinone) were heated at 100 °C under a nitrogen atmosphere. After 5 h of reaction, the reaction mixture was dissolved in 50 cm3 of perfluoro-1,3-dimethylcyclohexane, the

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2827 Table 2. Structural Characterization of Functionalized Anthraquinones (FAQs) E

AQ

KrCOCla FW

productb,c

IR (cm-1)

NMR (ppm)

1 2 3 4 5 6 7 8 9 10 11 12

2-NH2-AQ 2-NH2-AQ 2-NH2-AQ 1-NH2-AQ 1-NH(Me)-AQ 1,2-NH2-AQ 1,4-NH2-AQ 2,6-NH2-AQ 2-(CH2OH)-AQ 2-(CH2OH)-AQ 2-(CH2OH)-AQ 2-(CH2OH)-AQ

2500 5000 7500 5000 5000 2500 2500 2500 700 2500 5000 7500

2-(2500-NHCO)-AQ 2-(5000-CONH)-AQ 2-(7500-CONH)-AQ 1-(5000-CONH)-AQ 1-(5000-CON(Me))-AQ 1,2-Twin(2500)-AQ 1,4-Twin(2500)-AQ 2.6-Twin(2500)-AQ 2-(700-COO-CH2)-AQ 2-(2500-COO-CH2)-AQ 2-(5000-COO-CH2)-AQ 2-(7500-COO-CH2)-AQ

1539, 1589, 1680, 1737, 3321 1538, 1589, 1675, 1738, 3325 1540, 1593, 1675, 1738, 3321 1419, 1523, 1587, 1677, 1744, 3105 1425, 1592, 1675, 1740, 3040, 3068 1532, 1599, 1677, 1724, 1739 1518, 1597, 1653, 1749 1583, 1594, 1678, 1722, 3320 1440, 1592, 1674, 1778, 3045, 3065, 3321 1441, 1593, 1674, 1779, 3046, 3065, 3322 1443, 1593, 1675, 1780, 3046, 3066, 3323 1441, 1593, 1675, 1779, 3046, 3066, 3323

7.25, 8.09, 10.15 7.29, 8.08, 10.21 7.32, 8.15, 10.05 8.15-8.30, 9.09 2.09, 7.28, 8.26, 8.44 8.11, 9.16, 10.00 7.23, 8.32, 9.16 8-8.30, 10.33 2.08, 5.45, 8.24 2.11, 5.46, 8.23 2.09, 5.45, 8.25 2.04, 5.43, 8.29

a Kr ) poly(perfluoropropylene oxide) polymer. b Number in the chemical formula indicates the molecular weight of the poly(perfluoropropylene oxide) tail (Kr). c Twin denotes the class of difunctionalized FAQs.

excess aminoanthraquinone was removed by filtration, and the solvent then evaporated under vacuum. The product was washed several times with acetone. The chemical structure of the product was established by its 1H NMR and IR spectra. (entries 1-5 in Table 2). Synthesis of Diamido-Functionalized Anthraquinones (x,y-Twin(2500)-AQ). In a typical procedure, 0.472 g (2 mmol) of diaminoanthraquinone (1,2-, 1,4-, or 2,6-diaminoanthraquinone) and 4 mmol of fluoroether acid chloride were heated at 100 °C in the presence of 0.36 mL (0.356 g, 4.5 mmol) of pyridine under a nitrogen atmosphere. After 5 h of reaction, 50 mL of perfluoro1,3-dimethylcyclohexane was added to the mixture, and the pyridinium chloride and excess pyridine were removed with 5% aqueous HCl. After addition of 10 cm3 of benzene to help remove water emulsified during the acid wash, the solvent was evaporated under vacuum. The chemical structure of the product was established by its 1H NMR and IR spectra (entries 6-8 in Table 2). Synthesis of Fluoroether Ester Anthraquinones (2-(Kr-COO-CH2)-AQ). A total of 3.5 mmol of fluoroether acid chloride was added dropwise to a mixture of 0.953 g (4 mmol) of 2-(hydroxymethyl)-anthraquinone and 0.32 mL (0.31 g, 4 mmols) of pyridine. The reaction mixture was mixed for 10-15 min at room temperature, after which 30 cm3 of 1,1,2-trifluorotrichloroethane was added and the mixture was refluxed for 3 h. Following completion of the reaction, pyridinium chloride (white salt) formed in the reaction was removed by vacuum filtration. Excess pyridine was removed by washing three times with a 5% HCl solution, and the solvent (along with water emulsified during the washing) was removed by evaporation under vacuum in the presence of 5 mL of benzene. The product was identified by the appearance of the ester peak at 1780-1785 cm-1 and the disappearance of the acid chloride peak at 1806 cm-1 in the FTIR spectrum along with the disappearance of the OH peak at 4.7 ppm in the 1H NMR spectrum (entries 9-12 in Table 2). Characterization. All 1H NMR spectra were recorded on a Bruker DMX 300 instrument where FAQs were dissolved in 1,1,2-trichlorotrifluoroethane. Infrared spectra were obtained on a Mattson Genesis II FTIR spectrometer, and the samples were prepared as a thin film between two NaCl windows. All spectra were referenced to the 1601 cm-1 band of a thin polystyrene film. Phase Behavior Measurements. The phase diagrams of FAQs were determined in a high-pressure, variable-volume view cell (D. B. Robinson and Associates) as described previously.3

Figure 3. Functionalized anthraquinones (FAQ; Abbreviations as defined in Table 2).

Results and Discussion All functionalized anthraquinones synthesized are listed in Table 2, and their chemical structures are illustrated in Figure 3. Amide groups (entries 1-8 in Table 2) were identified by the peak at 1720-1740 cm-1 (CdO stretch) in the IR spectrum and by the peak at 9-10.2 ppm (amidic H) in the 1H NMR spectrum. Ester groups (entries 9-12 in Table 2) were characterized by the appearance of the peak at 1780-1785 cm-1 in the

2828 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999

Figure 4. Effect of tail length on the phase behavior of 2-(KrCOO-CH2)-AQ in CO2 (data measured at T ) 25 °C; abbreviations as defined in Table 2): (2) 2-(700-COO-CH2)-AQ, (b) 2-(2500-COOCH2)-AQ, (1) 2-(5000-COO-CH2)-AQ, (9) 2-(7500-COO-CH2)-AQ.

Figure 5. Effect of tail length on the phase behavior of 2-(KrCONH)-AQ in CO2 (data measured at T ) 25 °C; abbreviations as defined in Table 2): (9) 2-(2500-CONH)-AQ, (2) 2-(5000CONH)-AQ, (b) 2-(7500-CONH)-AQ.

mid-IR because of the CdO stretch. Anthraquinone groups were identified by the peaks at 1650-1680 cm-1 (CdO stretch) and at 1590 cm-1 (aromatic rings) in the IR spectrum and by the multiplets at 7.30 and 8-8.30 ppm (aromatic protons) in the 1H NMR spectrum. As with many CO2-philic amphiphiles3,29 previously studied by our group, our functionalized anthraquinones consist of four structural blocks: a CO2-philic polymer (1) connected through a spacer (2) and a linker (3) to a relatively CO2-phobic head (anthraquinone rings) (4). The affinity of the molecules for CO2 and, therefore, the enthalpy of mixing can be varied either by modifying the chemical identity of the spacer or linker, or by changing the positions of the tails on the aromatic rings. Further, the “CO2-philicity” of the molecule can also be enhanced by increasing the molecular weight of the CO2-philic perfluoroether or the number of perfluoroether tails attached to the anthraquinone block. Tail Length. Increasing the molecular weight of the CO2-philic tail (perfluoroether) of FAQs has a 2-fold effect: more numerous CO2-philic polymer-CO2 interactions result in a negative contribution to ∆HM, while a larger value of molar volume of the polymer reduces ∆SM(comb). For low molecular weight fluoroether tails, the enthalpic effect appears to dominate, and the cloudpoint pressures therefore shift to lower values as molecular weight increases. Starting with a certain chain length, the mixing process becomes entropically controlled and the gain in CO2-philicity due to a higher fraction of CO2-philic groups is overcome by the less favorable entropy of mixing owing to longer tail length. The global effect is a decrease in solubility (an increase in cloud-point pressure) for higher molecular weight tails. Therefore, one would expect to observe an optimum chain length for which the cloud-point pressures reach a minimum. Figure 4 depicts a portion of the cloud-point pressure curves for a series of FAQs having perfluoroether tails of different lengths (FW ) 700, 2500, 5000, and 7500) connected to the 2 position of the anthraquinone rings

through an ester linker (COO) and a methylene spacer (CH2). As expected, the pressure required to solubilize 3 mM concentration drops from 235 bar for a 700 FW tail to 185 bar for a 2500 FW tail and further to 145 bar for FW ) 5000. However, an additional increase of the length of the CO2-philic tail from FW ) 5000 to FW ) 7500 shifts the cloud-point pressures to higher values, from 145 bar to approximately 165 bar, showing that the 5000 FW fluoroether chain represents the optimum chain length. The cloud-point pressure curves shown in Figure 5 include the phase behavior of amide FAQs where the CO2-philic tails of various lengths (FW ) 2500, 5000, and 7500) are bonded to the 2 position of the AQ block through an amidic linker (NHCO). As the CO2-philic content increases, the cloud-point pressures shift to lower values but no minimum cloud-point pressure is reached, suggesting that the optimum chain length is higher than 7500. Polarity/H-Bond Ability of the Headgroup. Solute-solute interactions can be influenced either by the chemical identity of the linker or by the position of the linker on the anthraquinone rings. We designed three types of headgroups having different H-bond ability/ polarity. First, we synthesized FAQs with a secondary amidic linker in the 2 position (2-NHCO), which can form only intermolecular H bonds and one bonded to the 1 position (1-NHCO) that can form either inter- or intramolecular H bonds. Further, the substitution of the amidic proton in the secondary amide linker with a methyl group forms a tertiary amidic linker (N(CH3)CO), which is unable to form any H bonds but is more polar than the ester variant (Figure 6). Figure 7 shows the effect of headgroup polarity/Hbond ability on the cloud-point pressure curve of FAQs for which a 5000 FW fluoroether tail is bonded to the AQ block through the four different linkers: 1-NHCO; 1-N(Me)CO, 2-NHCO, and 2-COO. Interestingly, cloud points for 2-(5000-CONH)-AQ occur at pressures that are 50 bar higher than 2-(5000-COO-CH2)-AQ, suggesting that the tendency of the linker to form inter-

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2829

Figure 7. Effect of headgroup on the phase behavior FAQs in CO2 (data measured at T ) 25 °C; abbreviations as defined in Table 2): (2) 2-(5000-COO-CH2)-AQ, (1) 1-(5000-CON(Me))-AQ, (b) 1-(5000-CONH)-AQ, (9) 2-(5000-CONH)-AQ.

Figure 6. Hydrogen-bonding ability for 1- and 2-amido-FAQs (abbreviation as defined in Table 2).

molecular H bonds renders solute-solute interactions so favorable that the solubilization in CO2 is difficult. Further, substitution of the amidic proton in 1-(5000CONH)-AQ by a methyl group to form 1-(5000-CON(CH3))-AQ drops the cloud points by over 20 bar. Finally, as one decreases the polarity of the linker still further by replacing the N-substituted amide with the ester, the cloud point drops a further 20 bar, revealing a thermodynamic preference by CO2 for the less polar linker, i.e., ester linkage. As noted above, the enthalpy of mixing includes contributions from solute-solute, solute-solvent, and solvent-solvent interactions, where the latter is not an issue when altering the structure of the FAQs. Upon lengthening the tail, we primarily affect the enthalpy of mixing through an increased number of favorable solute-solvent interactions, although it might be argued that the solute-solute interactions are also affected by including an increased number of relatively weak fluoroether-fluoroether interactions. By altering the nature of the linking group from amide to ester, we also impact the enthalpy of mixing through alteration to both the solute-solute interactions and solute-solvent contributions. Replacement of the amide group with the ester

Figure 8. Effect of number of tails on the phase behavior of FAQs in CO2 (data measured at T ) 25 °C; abbreviations as defined in Table 2): (2) 1,4-Twin(2500)-AQ, (9) 1-(5000-CONH)-AQ, (b) 2-(5000-CONH)-AQ, (1) 2,6-Twin(2500)-AQ.

weakens solute-solute interactions by eliminating a source of hydrogen-bonding donation. In addition, as shown by Kazarian et al.,30 carbonyl groups can exhibit specific interactions with CO2, and thus replacement of amide by ester also renders solute-solvent interactions more favorable. Number of CO2-philic Tails. The CO2-philic content of FAQs can be increased either by increasing the length of the CO2-philic tail or by increasing the number of tails attached to the AQ block. Previous studies29 showed that increasing the number of tails increases the free volume of the polymer molecule. Therefore, the positive contribution of the free volume to ∆GM is diminished and we expect that increasing the number of tails at constant CO2-philic content would lower the cloud-point pressures of FAQs. To test our assumption, we compare in Figure 8 the phase behavior of two monoamido and two diamido

2830 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 Table 3. Melting Points of Difunctionalized Anthraquinones x,y-(R)2-AQ33 R

1,2-AQ

1,4-AQ

2,6-AQ

-NH2 -OH -Cl -CH3

304 290

268 200 188 140

319 360 291 242

156

FAQs having the same overall composition. As expected, 1,4-Twin(2500)-AQ exhibits a cloud-point pressure of ca. 20 bar lower than that of 1-(5000-CONH)-AQ, because of a smaller free volume contribution to ∆GM. Somewhat surprisingly, 2,6-Twin(2500)-AQ shows a 50 bar shift to higher cloud-point pressures compared to its singletailed isomer 2-(5000-CONH)-AQ. In this case, we suggest that the smaller free volume contribution to ∆SM is overcome by the positive contribution to enthalpy of mixing due to attractive solute-solute interactions generated by the intermolecular H bonds occurring between 2,6-Twin(2500)-AQ molecules. The global effect is a decrease in solubility of the difunctionalized AQs as compared to the monofunctionalized variant having the same CO2-philic content. Topology of Tails. Differences in the solubility of aromatic isomers in organic solvents31 or supercritical CO2 (ScCO2)32 have been reported by many research groups. In a study of the solubility of hydroxybenzoic acids in ScCO2, Stahl32a and later Krukonis32b determined that the para isomer is 2 orders of magnitude less soluble than the ortho isomer because of its high melting point. In a similar study, Chang32d showed that p-methylnitrobenzene exhibits the poorest solubility in ScCO2 of its various isomers. This behavior was explained by the well-known inverse relationship between solubility in CO2 and the melting point. The molecules of the para isomer, which exhibits the highest molecular symmetry of the disubstituted benzenes, can stack more easily into the crystal lattice, rendering melting or dissolution more difficult. In the family of disubstituted anthraquinones, melting points are strongly dependent on the position of the functional groups on the anthraquinone rings. As shown in Table 3, the melting points of disubstituted AQs decrease in the following order: mp2,6 > mp1,2 > mp1,4. Therefore, it is not entirely surprising that we see dramatic effects of the topology of the tails on the cloud-point pressures owing to differences in the molecular symmetry/H-bond ability of different difunctional anthraquinones. Figure 9 shows the phase behavior of three FAQs in which two 2500 FW fluoroether tails were attached through amidic linkers to the AQ rings in three different configurations: 1,2, 1,4, and 2,6. 1,2-Twin(2500)-AQ and 1,4-Twin(2500)-AQ isomers are viscous liquids at room temperature, while the 2,6 variant is a solid wax, which liquifies almost immediately upon exposure to CO2. The 2,6 isomer exhibits cloud-point pressures of ca. 65 bar higher than the 1,2 and 1,4 isomers. We again suspect that both hydrogen bonding and molecular symmetry play important roles here. In the 2,6 isomer, the molecules may stack because of molecular symmetry and because of intermolecular H bonding by the secondary amide linkers. This could explain both its low solubility in CO2 (high cloud-point pressures) and its waxlike appearance at ambient conditions. The relatively high solubilities of the 1,2 and 1,4 isomers might be the result of low molecular associations due to the intramolecular H bonds which prevail between the molecules of these isomers.

Figure 9. Effect of topology on the phase behavior of Twin(2500)AQs in CO2 (data measured at T ) 25 °C; abbreviations as defined in Table 2): (1) 1,4-Twin(2500)-AQ, (2) 1,2-Twin(2500)-AQ, (b) 2,6-Twin (2500)-AQ.

Figure 10. Generalized liquid-liquid phase diagram.

The functionalized anthraquinones synthesized in this work are either liquids or amorphous solids at room temperature and liquify almost immediately upon exposure to CO2. Consequently, the phase diagrams described represent only portions of the generalized liquid-liquid-phase envelope. As illustrated in Figure 10, above the minimum miscibility pressure, Pmin, these materials are completely miscible with CO2 in all proportions and we have the option of working at dilute concentrations (the left-hand side of the phase boundary) or under concentrated conditions (the right-hand branch of the phase boundary). While the former case employs CO2 as a solvent, in the latter, CO2 functions more as a viscosity-reducing diluent whose purpose is to lower the viscosity of the working fluid to minimize pumping costs. Such behavior cannot occur with typical crystalline alkyl-functionalized AQs, as only the dilute portion of the phase boundary exists. Conclusions Fluoroether-functionalized anthraquinones synthesized by attaching CO2-philic tails to the anthraquinone block by either amide or ester linkers exhibit liquid-

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2831

liquid-phase behavior in carbon dioxide with a minimum miscibility pressure between 170 and 210 bar. The length of the CO2-philic tail, the position and number of the tails on the aromatic rings, and the nature of the linker are the structural parameters which influence the phase behavior of FAQs. The present work suggests that a strategy to lower the minimum miscibility pressure of CO2-FAQ system would be to attach a number of small to medium CO2-philic tails to AQ blocks in an asymmetric configuration through linking groups without acidic protons, such as esters or ethers. For a largescale application, one could replace the perfluoroether polymer tails with less expensive CO2-philic tails, such as silicones. In the next paper of this series, we evaluate the kinetics of Pd-catalyzed hydrogenation of FAQs in liquid CO2 as the first step in the anthraquinone process for generation of hydrogen peroxide. Literature Cited (1) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction: Principles and Practice, 2nd ed.; Butterworth-Heinemann Publishers: Stoneham, MA, 1994. (2) Ghenciu, E. G.; Beckman, E. J. Affinity Extraction into Carbon Dioxide. 1. Extraction of Avidin Using a Biotin-Functional Fluoroether Surfactant. Ind. Eng. Chem. Res. 1997, 36, 5366. (3) (a) Yazdi, A. V.; Beckman, E. J. Design, Synthesis, and Evaluation of Novel, Highly CO2-Soluble Chelating Agents for Removal of Metals. Ind. Eng. Chem. Res. 1996, 35, 3644. (b) Yazdi, A. V.; Beckman, E. J. Design of Highly CO2-Soluble Chelating Agents. 2. Effect of Chelate Structure and Process Parameters on Extraction Efficiency. Ind. Eng. Chem. Res. 1997, 36, 2368. (4) Jessop, P. G.; Hsiao, Y.; Ikariya, T.; Noyori, R. Homogeneous Catalysis in Supercritical Fluids: Hydrogenation of Supercritical Carbon Dioxide to Formic Acid, Alkyl Formates, and Formamides. J. Am. Chem. Soc. 1996, 118, 344. (5) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. Reactions at Supercritical Conditions: Applications and Fundamentals. AIChE J. 1995, 41, 1723. (6) Lee, C.; Hoy, K. L.; Donohue, M. D. Supercritical fluids as diluents in liquid spray application of coatings. U.S. Patent 4,923,720, 1990. (7) Khundker, S.; Dean, J. R.; Hitchen, S. M. Extraction of Ibuprofen by Supercritical Carbon Dioxide. Anal. Proc. 1993, 30, 472. (8) Brunner, G. Processing of Natural Materials by Supercritical Gases. Proceedings of the 5th Meeting on Supercritical Fluids; Institut National Polytechnique de Loraine (Atelier de reprographie) Vandœuvre Cedex: Nice, France, 1998; p 413. (9) (a) Leitner, W. Carbon Dioxide as a Raw Material: The Synthesis of Formic Acid and Its Derivatives from CO2. Angew. Chem., Int. Ed. Engl. 1995, 2207. (b) Jessop, P. G.; Hsiao, Y.; Ikariya, T.; Noyori, R. Catalytic Production of Dimethylformamide from Supercritical Carbon Dioxide. J. Am. Chem. Soc. 1994, 116, 8851. (10) McHugh, M. A.; Krukonis, V. J. Supercritical Fluids. In Encyclopedia of Polymer Science Engineering; Mark, H. E., Bikales, N. M., Overberger, C. G., Menges, G., Kroschwitz, J. I., Eds.; John Wiley & Sons: New York, 1985; Vol. 16, p 368. (11) Mandel, F. S. A Case Study in Scale-Up: Highly Loaded Polymer Systems in Supercritical Fluid Carbon Dioxide. Proceedings of the 5th Meeting on Supercritical Fluids; Institut National Polytechnique de Loraine (Atelier de reprographie) Vandœuvre Cedex: Nice, France, 1998; p 69. (12) (a) Hoefling, T. A.; Enick, R. M.; Beckman, E. J. Microemulsions in Near-Critical and Supercritcal CO2. J. Phys. Chem. 1991, 95, 7127. (b) Newman, D. A.; Hoefling, T. A.; Beitle, R. R.; Beckman, E. J.; Enick, R. M. Phase Behaviour of FluoroetherFunctional Amphiphiles in Supercritical Carbon Dioxide. J. Supercrit. Fluids 1993, 6, 205. (13) DeSimone, J. M.; Guan, Z.; Elsbernd, C. S. Synthesis of Fluoropolymers in Supercritical Carbon Dioxide. Science 1992, 257, 945. (14) Powell, R. Hydrogen Peroxide Production; Noyes Dev. Corp.: Park Ridge, NJ, 1968.

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Received for review November 23, 1998 Revised manuscript received April 5, 1999 Accepted April 20, 1999 IE980738D