Solvent Extraction of Ethanol from Aqueous Solutions. II. Linear, Branched, and Ring-Containing Alcohol Solvents

Ind. Eng. Chem. Res. 2005, 44, 6797-6803 6797

Richard D. Offeman,* Serena K. Stephenson, George H. Robertson, and William J. Orts U.S. Department of Agriculture, Western Regional Research Center, 800 Buchanan Street, Albany, California 94710

Distribution coefficients have been measured for the partitioning of ethanol and water from aqueous mixtures into 57 different alcohol solvents. The study has focused on the effects of systematic variations in chemical structure of the alcohols. Factors found to be important include chain length and hydroxyl position for the x-alcohols (i.e., 1-heptanol through 1-dodecanol, 2-heptanol through 2-undecanol, etc.), branch structure (e.g., methyl, ethyl, n-propyl, i-propyl, etc.) for a branch located on the hydroxyl carbon, location of the branch relative to the hydroxyl carbon, and multiple branching.

Introduction this is the reason to search for solvents that will chemically react, hydrogen bond, or complex preferen- - Liquid liquid solvent extraction has long been of tially with the compound to be extracted. interest in recovering ethanol from dilute aqueous Many classes of chemicals have been evaluated as mixtures.1 It has the potential to be a more energy- solvents for the recovery of ethanol from dilute aqueous efficient alternative to distillation.1-4 Fermentation of mixtures. Roddy15 summarized his data in a general grain- and biomass-derived sugars is a major and ranking of solvent classes in order of increasing K , rapidly growing source of ethanol. In fermentation DE the ethanol distribution coefficient: hydrocarbons ) systems, the ethanol product is inhibitory to the micro- < < < < < organisms producing it,5,6 so gains in yield and fer- halocarbons ethers ketones amines esters alcohols ) phosphates, though specific exceptions are menter productivity can be realized by continuous 16 13 removal of the ethanol produced.2 Continuous solvent noted. Munson and King and Cabral made use of Lewis-acid/Lewis-base concepts to rank selectivity for extraction of ethanol-producing fermentation systems < < < has been demonstrated by a number of researchers.7-12 a given KDE as hydrocarbons ethers ketones amines < esters < alcohols < carboxylic acids. Souissi For successful implementation, many criteria must be 17 met by a potential extraction solvent. These criteria and Thyrion used the Hansen extension of the Hilde- include extraction performance, chemical stability, eco- brand solubility parameter for polar (δp) and hydrogen nomical product recovery and solvent regeneration, bonding (δh) contributions, observing that higher KDE minimization of solvent losses, safety and environ- values occur when a solvent’s δp and δh values are close mental risk from emissions, and, in the case of fermen- to those of ethanol. tation systems, biocompatibility with ethanol-producing Predictive models have been applied for solvent microorganisms.13 screening purposes. Poling et al.18 and Prausnitz and For the purpose of screening a variety of possible Tavares19 provide excellent overviews of the forms and extraction solvents, extraction performance can be evolution of a variety of thermodynamic models. The measured by two characteristics at the operating condi- thermodynamic group contribution models such as tions of interest: distribution coefficient KDE and sepa- UNIFAC and ASOG have been used in attempts to ration factor R.14 The equilibrium distribution coefficient estimate liquid-liquid equilibria (LLE) to predict the 20-26 for ethanol is defined as KDE ) [EtOH]org/[EtOH]aq, the extractive performance of solvents. The original ratio of the weight percent of ethanol in the organic UNIFAC model27 with LLE parameter tables,28 how- phase to the weight percent of ethanol in the aqueous ever, does not take positional information into account. phase. KDW, the equilibrium distribution coefficient for An examination of the literature data for alcohol water, is defined similarly: KDW ) [H2O]org/[H2O]aq. The solvents shows that positional information such as the separation factor is R)KDE/KDW, or the ratio of ethanol location of the hydroxyl group and its proximity to alkyl to water in the organic phase to that in the aqueous branches has a strong effect on the distribution coef- phase. The distribution coefficient indexes the solvent’s ficients and the separation factor, as has been noted by capacity for the extracted component, while the separa- others.15,16 Later extensions of the models add or modify tion factor is the solvent’s selectivity for one component UNIFAC groups to include some structural information over another. Unfortunately, it is often observed that (e.g., ref 29), but the accuracy of the current group solvents with a high separation factor generally have contribution methods is still inadequate for isomers.30 low ethanol distribution coefficients, and vice versa. A quantum chemistry approach, COSMO-RS,31 holds 14 King explains this effect in terms of solubility param- promise for predicting LLE information. Unlike the eters for physically interacting solvents and notes that group contribution models, a quantum chemical calculation is made for each specific molecule; hence, * To whom correspondence should be addressed. Tel.: (510) structural information is automatically included. Once 559-6458. Fax: (510) 559-5818. E-mail: [email protected]. the parameters for a molecule are calculated, estimation

10.1021/ie0500321 This article not subject to U.S. Copyright. Published 2005 by the American Chemical Society Published on Web 06/30/2005 6798 Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 of phase behavior for multicomponent mixtures is increased slope. The R values for the two 5-alcohols are relatively rapid. above, but with a slope parallel to, the 3- and 4-alcohols, Positional information is used in a correlation based and the R value of the one 6-alcohol is slightly higher on steric shielding of the hydroxyl oxygen by Munson than those of the 5-alcohols and is the highest of any of and King.16 On the basis of a cone-angle approach, they the alcohols studied. show a trend toward higher selectivity with increased The branched alcohols data is very scattered, with steric hindrance of the hydroxyl group, but several some data points falling above, and some below, the exceptions can be noted. All these approaches can make linear compounds with the same hydroxyl position. To qualitative predictions about solvent performance, but illustrate, for the eight branched 3-alcohols, two fall in the end, experimental data are still needed for final above, three on, and three below a line fitting the data solvent selection. for the linear 3-alcohols. Our objectives were to employ the solvent screening The ring compounds compare poorly to their nearest 32 method described in a companion paper to determine equivalent alkyl alcohols. For instance, 3-methylcyclo- the ethanol extractive performance of a wide range of hexanol (R)10.2) might be compared to 4-heptanol (R alcohol solvents, with focus on the systematic correlation ) 20.1) based on number of carbons and branching on to the chemical structure of the solvents. The factors the hydroxyl carbon. Similarly, 3-phenyl-1-propanol explored were the molecular weight, the location of the (9.3) might be compared to 1-nonanol (12.6). One pos- hydroxyl group, the location of branching relative to the sible reason for the comparatively poor performance of hydroxyl group, the type of branch (e.g., methyl, ethyl, the ring compounds is the lower mobility of the ring n-propyl, i-propyl, etc.), and the inclusion of an aromatic carbons compared to the alkyl chain carbons. or cyclohexyl ring. Alcohol solvents that have the greatest R for a given KDE are the 4-, 5-, and 6-unbranched-alcohols and some Experimental Section of the branched 3- and 4-alcohols such as 2-methyl-3- pentanol (but not 3-methyl-3-pentanol), 2,2-dimethyl- The extractions were carried out at 33 °C with an 3-hexanol, and 2,6-dimethyl-4-heptanol. aqueous-to-organic phase volume ratio of 2:1 and total The characteristic tradeoff between the separation liquid volumes of 7.5 mL. The mixtures were emulsified 14 factor and KDE has been discussed by King and multiple times at the extraction temperature to ensure Barton,38 who were able to derive this behavior from that the extraction system had reached equilibrium. The the Scatchard-Hildebrand equation for regular solu- mixtures were centrifuged at the extraction tem- tions, relating activity coefficients to molar volumes and perature, and the composition of each phase was solubility parameters. However, since hydrogen bonding analyzed by gas chromatography using an internal and other interactions are certainly taking place in the standard method. Distribution coefficients and separa- ethanol-water-polar solvent systems investigated here, tion factors were calculated from the compositions of the relationship between R and KDE is certainly more each phase. Specific details may be found in the complex than that of regular solutions. companion paper.32 Figure 2 presents the data for the unbranched alkyl The solvents that were investigated are listed in Table x-alcohols. The separation factor and the distribution 1. CAS number, source, purity, density, and solubility coefficients for ethanol and water are plotted against in water are included. Ethanol was 200 proof anhydrous the inverse of the molecular weight of each solvent. This grade, from Aaper Alcohol and Chemical Co. Anhydrous augments the work of Murphy et al.20 that showed K 1-butanol from Aldrich, 99.95%, was the organic-phase DE and KDW for the unbranched 1-alcohols are related to diluent and aqueous-phase internal standard. The the concentration of the solvent hydroxyl groups present. organic-phase internal standard, 1-hexanol, was anhy- In Figure 2a, each family of alcohols shows a linear drous 99.49% from Aldrich. Distilled water was used relationship of R with 1000/MW. The 1-alcohols are in all solutions. constant vs 1000/MW, and the other families have slopes that become steeper with movement of the Results and Discussion hydroxyl group toward the middle of the molecule. The largest R is that for 6-undecanol. Interestingly, a single Separation factors, R, and ethanol distribution coef- line represents KDE quite well for all six families of ficients, KDE, obtained for all 57 alcohols at 5 wt % initial alcohols (Figure 2b), with KDE increasing with 1000/MW aqueous ethanol concentration [EtOH]°aq and 33 °C are (or decreasing with lengthening alkyl chain). Evidently, listed in Table 2. Where multiple runs were carried out, KDE is proportional to the concentration of solvent the data were averaged. In this table, the alkyl alcohols hydroxyl groups, as indexed by 1000/MW, and is not are grouped as 1-alcohols, 2-alcohols, etc., followed by strongly affected by the position of the hydroxyl group ring compounds grouped as cyclohexyl rings and phenyl in the molecule. On the other hand, while KDW in Figure rings. An example best illustrates the indexing conven- 2c also increases with 1000/MW, each isomer family has tion for the alkyl alcohols: 2-methyl-2-nonanol can be a distinctly different curve with the 1-alcohols above the found in the “2-me-2-alc” row and the “non” column. 2-alcohols, which are above the 3-alcohols, and so on. Trends in the R vs KDE data become evident in Figure The largest effect is seen in moving from the 2-alcohols 1. The data show that the unbranched alkyl alcohols to the 3-alcohols. Hence, KDW is affected more strongly behave in a generally linear manner within each family, than KDE by the position of the hydroxyl group and and R increases with the distance of the hydroxyl from decreases as the hydroxyl group moves toward the the 1-position. For the 1-alcohols, R is nearly constant middle of the molecule. The consequence (Figure 2a) for vs KDE; for the 2-alcohols, R decreases with a small slope R, the ratio of KDE to KDW, is that it increases as the as KDE increases; for the 3-alcohols, R decreases with a hydroxyl moves toward the middle of the molecule. The markedly larger slope as KDE increases. The 4-alcohols largest difference is between the 2-alcohols and the behave similarly to the 3-alcohols, though with a slightly 3-alcohols. Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 6799

Table 1. Solvents Investigated solvent CAS source purity, % density, g/cm3 (°C) sol. in water, wt % BP, °C Unbranched Alcohols 1-heptanol 111-70-6 Aldrich 99.96 0.822 (20)33 0.16734 17633 1-octanol 111-87-5 Aldrich 99.93 0.826 (25)33 0.05434 19633 1-nonanol 143-08-8 Aldrich 99.5 0.827 (20)33 0.01434 21333 1-decanol 112-30-1 Fluka 99.3 0.830 (20)33 0.003734 23133 1-undecanol 112-42-5 Aldrich 99.4 0.830 (20)33 0.001934 24333 1-dodecanol 112-53-8 Aldrich 98.52 0.831 (24)33 0.000434 25933 2-hexanol 626-93-7 Aldrich 99.8 0.816 (20)33 1.3734 14033 2-heptanol 543-49-7 Aldrich 99.2 0.817 (20)33 0.32734 15933 2-octanol 123-96-6 Aldrich 97.7 0.819 (20)33 0.11234 18033 2-nonanol 628-99-9 Fluka 99.8 0.847 (20)33 0.02634 19333 2-decanol 1120-06-5 Aldrich 99.0 0.825 (20)33 0.017234 21133 2-undecanol 1653-30-1 Aldrich 98.8 0.827 (19)33 0.019834 22833 3-pentanol 584-02-1 Aldrich 99.8 0.820 (20)33 5.234 11633 3-hexanol 623-37-0 Aldrich 99.2 0.818 (20)33 1.6134 13533 3-heptanol 589-82-2 Aldrich 99.2 0.823 (20)33 0.434 15733 3-octanol 589-98-0 Aldrich 98.5 0.826 (20)33 0.13834 17133 3-decanol 1565-81-7 ChemSampCo 97.0 0.82735 4-heptanol 589-55-9 ChemSampCo 98.3 0.818 (20)33 0.4734 15633 4-octanol 589-62-8 ChemSampCo 99.3 0.819 (20)33 1.5334 17633 4-nonanol 5932-79-6 ChemSampCo 97.1 0.828 (20)33 0.037434 19233 4-decanol 2051-31-2 ChemSampCo 98.7 0.826 (20)33 0.016836 21033 5-nonanol 623-93-8 Fluka 99.8 0.836 (20)33 0.04634 19333 5-undecanol 37493-70-2 ChemSampCo 96.5 0.829 (20)33 22933 6-undecanol 23708-56-7 ChemSampCo 96.5 0.833 (20)33 22833 Branched Alcohols 2-methyl-1-butanol 137-32-6 Aldrich 99.1 0.815 (25)33 2.9734 12833 3,3-dimethyl-1-butanol 624-95-3 Aldrich 98.8 0.844 (15)33 0.75634 14333 2-ethyl-1-butanol 97-95-0 Aldrich 99.9 0.833 (20)33 0.434 14733 2-methyl-1-pentanol 105-30-6 Aldrich 98.7 0.826 (20)33 0.634 14933 3-methyl-1-pentanol 589-35-5 Aldrich 99.7 0.824 (20)33 0.4334 15333 4-methyl-1-pentanol 626-89-1 Aldrich 99.6 0.813 (20)33 0.7634 15233 2-propyl-1-pentanol 58175-57-8 Aldrich 99.9 0.830 (25)37 0.11a36 2-ethyl-1-hexanol 104-76-7 Eastman 99.84 0.832 (25)33 0.08834 18533 2-butyl-1-octanol 3913-02-8 Aldrich 97.9 0.833 (25)37 145-14937 2-methyl-2-pentanol 590-36-3 Aldrich 99.5 0.835 (16)33 3.2434 12133 4-methyl-2-pentanol 108-11-2 Aldrich 99.0 0.807 (20)33 1.6434 13233 2-methyl-2-hexanol 625-23-0 Aldrich 99.1 0.812 (20)33 9.734 14333 6-methyl-2-heptanol 4730-22-7 ChemSampCo 97.0 0.803 (25)37 0.171336 17433 2-methyl-2-nonanol 10297-57-1 ChemSampCo 96.9 2-methyl-3-pentanol 565-67-3 Aldrich 99.5 0.824 (20)33 2.034 12733 3-methyl-3-pentanol 77-74-7 Aldrich 99.7 0.829 (20)33 4.334 12233 2,2-dimethyl-3-pentanol 3970-62-5 Aldrich 98.4 0.825 (20)33 0.8234 13533 2,4-dimethyl-3-pentanol 600-36-2 Aldrich 99.6 0.829 (20)33 0.734 13933 3-ethyl-3-pentanol 597-49-9 Aldrich 99.7 0.841 (22)33 1.734 14233 2-methyl-3-hexanol 617-29-8 Aldrich 99.0 0.841 (20)33 0.5734 14333 2,2-dimethyl-3-hexanol 4209-90-9 ChemSampCo 98.1 0.834 (20)33 0.43**36 15633 3,7-dimethyl-3-octanol 78-69-3 Aldrich 98.6 0.826 (25)37 0.04**36 2-methyl-4-heptanol 21570-35-4 ChemSampCo 98.8 0.821 (20)33 0.16134 16433 4-methyl-4-heptanol 598-01-6 ChemSampCo 95.2 0.825 (20)33 0.33734 16133 2,6-dimethyl-4-heptanolb 108-82-7 Acros 90 0.811 (20)33 0.044534 17433 4-propyl-4-heptanol 2198-72-3 ChemSampCo 98.6 0.834 (21)33 0.046**36 19133 Ring Alcohols 1-methylcyclohexanol 590-67-0 Aldrich 98.6 0.919 (20)33 0.55834 15533 3-methylcyclohexanol 591-23-1 Aldrich 99.5 0.914 (25)37 0.59934 16733 2-ethylcyclohexanol 3760-20-1 Aldrich 99.1 0.906 (25)37 0.20234 18133 3-phenyl-1-propanol 122-97-4 Aldrich 99.5 0.995 (25)33 0.56834 23533 1-phenyl-2-propanol 14898-87-4 Aldrich 99.8 0.973 (25)37 219-22137 2-phenyl-2-propanol 617-94-7 Aldrich 98.8 0.973 (20)33 0.71434 20233 2-phenyl-2-butanol 1565-75-9 Aldrich 98.8 0.977 (25)37 a Estimate from correlation with boiling point. b Balance is 4,6-dimethyl-2-heptanol.

Figure 3 describes the effect on R of branching at the C8 data shows that tert-butyl gives a higher R than hydroxyl carbon. The branching types are none, methyl, isobutyl. The C7 data shows that n-propyl and isopropyl ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, have similar values of R. In order of increasing R for and n-pentyl. To separate the branch structure effect the C6 - C8 solvents, none , methyl , ethyl e n-propyl from the strong molecular weight effect shown in Figure ≈ isopropyl e n-butyl ≈ isobutyl e tert-butyl for a given 2, comparisons are made for different alcohols with the molecular weight. The largest difference in R occurs same number of carbons. Note that, since molecular between methyl and ethyl for total carbon numbers >5. weight is constant, a growth in branch size means a For the C9s and C10s, there is additional improvement decrease in the length of the main chain. In general, R in R for propyl and even butyl, but for the C6s-C8s, increases with the number of carbons in the branch. The the improvement in R is small above ethyl. Finally, as 6800 Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005

Table 2. Separation Factors and Ethanol Partition Coefficients (r/KDE) for Solvents Investigated but pent hex hept oct non dec undec dodec 1-alc 12.4/0.88 12.3/0.73 12.6/0.63 12.5/0.57 12.0/0.47 12.2/0.45 2-me-1-alc 11.6/1.16 13.7/0.92 3-me-1-alc 11.8/0.93 4-me-1-alc xa 11.4/0.95 2-et-1-alc 15.6/0.96 19.5/0.69 2-pr-1-alc x 18.6/0.61 2-bu-1-alc x x 24.0/0.36 3,3-dime-1-alc 13.3/1.08 2-alc 12.2/1.03 13.5/0.86 15.4/0.77 14.6/0.61 15.1/0.55 15.2/0.46 2-me-2-alc 9.2/1.05 12.1/0.97 15.3/0.60 4-me-2-alc x 13.0/1.02 6-me-2-alc x x x 14.6/0.72 3-alc x 12.3/1.29 17.1/1.06 19.3/0.86 21.8/0.74 22.0/0.51 2-me-3-alc x 17.2/1.13 19.8/0.80 3-me-3-alc x 9.8/1.21 3-et-3-alc x 13.5/0.99 2,2-dime-3-alc x 20.1/0.81 24.1/0.62 2,4-dime-3-alc x 19.0/0.85 3,7-dime-3-alc xxxx18.9/0.57 4-alc x x x 20.1/0.76 22.8/0.64 23.8/0.55 25.0/0.45 2-me-4-alc x x x 22.8/0.61 4-me-4-alc x x x 16.5/0.68 4-pr-4-alc x x x 24.1/0.45 2,6-dime-4-alc x x x 27.6/0.35 5-alc xxxxx25.0/0.54 27.3/0.40 6-alc xxxxxxx28.2/0.40

Ring Compounds 3-methylcyclohexanol 10.2/0.86 1-methylcyclohexanol 10.0/0.93 2-ethylcyclohexanol 15.1/0.67 3-phenyl-1-propanol 9.3/0.65 2-phenyl-2-propanol 11.5/0.75 2-phenyl-2-butanol 15.1/0.61 1-phenyl-2-propanol 8.9/0.70 a An x indicates a compound that cannot exist, or that is named incorrectly. For example, 6-butanol does not exist; 4-hexanol should be named 3-hexanol. methyl data. As in Figure 3, it can be seen that the methyl group has a smaller effect on R than either ethyl or n-propyl. Figure 5 describes partitioning by the multibranched alcohols, with R plotted vs KDE, and compares several multibranched alcohols to unbranched alcohols of the same molecular weight. The unbranched alcohols are marked with an asterisk symbol. Compounds of the same molecular weight are designated by the same symbol shape. The naming convention used in the legend is based on the groups attached to the hydroxyl carbon. A pair of examples best illustrate this: “Me, nPr, nPr” represents the molecule 4-methyl-4-heptanol; “H, iBu, iBu” represents 2,6-dimethyl-4-heptanol. Table 3 Figure 1. Separation factor, R, and ethanol distribution coef- lists IUPAC names and chemical structures for the data ficient, KDE, obtained for all 57 alcohols at 5 wt % initial aqueous points in Figure 5. ethanol concentration [EtOH]° and 33 °C. Linear alcohols are aq The general trend of R to increase and KDE to decrease shown as solid symbols, branched alcohols are shown as open as the number of carbons increases follows the behavior symbols, and ring alcohols are shown as dashes. The number indicates the position of the hydroxyl group. shown in Figure 2 for the unbranched alcohols. For each molecular weight, as expected from Figures 3 and 4, also noted for the unbranched alcohols (Figure 2), R methyl branches are less effective than larger branches increases with molecular weight (or decreases with in increasing R. For the C10 alcohols, “nPr, nPr, nPr” 1000/MW) for all except the 1-alcohols. > “Me, Et, iHex” > “Me, Me, nHept”, which is 4-propyl- The effect on R of branch position relative to the 4-octanol > 3,7-dimethyl-3-octanol > 2-methyl-2-nonanol. hydroxyl carbon is shown in Figure 4. The data sets for For the C7 alcohols, “Et, Et, Et” > “Me, Me, nBu”, and C6 and C8 alcohols are examined. The C6 methyl for the C5s, “Me, Et, Et” > “Me, Me, nPr”. progression shows decreasing R in the order 1 > 0 > 2 Branch structures of the same molecular weight that > 3 carbons away from the hydroxyl carbon. The C6 and contain i-butyl have a higher R, but a lower KDE, than C8 ethyl progressions show decreasing R in the order 0 that of n-butyl. Similarly, t-butyl gives a slightly higher > 1 carbons away from the hydroxyl carbon, as does R but lower KDE, than that of n-butyl. On the other n-propyl for the C8 series, a reversal of the order in the hand, i-propyl gives a slightly lower value of R, and a Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 6801

Figure 4. Influence of the position and structure of the branch on the separation factor (R) for selected single-branched alcohols.

improvement in R occurs when a hydrogen is replaced with a methyl or a methyl is replaced with an ethyl group. A steric hindrance approach to correlating performance of the alkyl alcohols was attempted using the “cone angle” method. Munson and King16 nominally followed this approach, with mixed results; the method they used to calculate cone angles is not clear. Here, we define the cone angle for an alcohol solvent to have an apex centered at the hydroxyl oxygen, its height axis aligned with the C-O bond, and a swept volume that contains all the alkyl carbon and hydrogen atoms within the cone formed by rotation about the C-O bond, using R the van der Waals radius of the groups. Cone angles Figure 2. (a) Separation factor ( ) for the unbranched x-alcohols 39 and its relation to molecular weight. (b) Ethanol distribution were taken from Datta and Majumdar and adjusted - coefficient (KDE) for the unbranched x-alcohols and its relation to to the C O average bond length of 1.43 Å for the cone molecular weight. (c) Water distribution coefficient (KDW) for the apex length. This approach failed to adequately cor- unbranched x-alcohols and its relation to molecular weight. relate our data. This may be due to the assumption in the cone angle calculation that alkyl groups are in the staggered conformation; hence, all alkyl groups longer than ethyl have the same cone angle as ethyl. Depend- ing on the solvent environment, other conformations will minimize the free energy; hence, steric effects will likely have some dependence on the concentration of ethanol and water in the organic phase.

Conclusions

The availability of a large number of alcohol solvents with diverse chemical structures has led to the op- portunity for a systematic examination of the effects of solvent chemical structure on partitioning and separation factor for the extraction of ethanol from dilute aqueous mixtures. For unbranched alcohols, the separation factor increases as the hydroxyl group is moved toward the middle of the chain, with the largest effect seen when Figure 3. Influence of the structure of the branch on the going from the 2-alcohols to the 3-alcohols. The separa- separation factor (R) for the single-branched alcohols with the branch on the hydroxyl carbon. Comparisons are made for different tion factor also increases with molecular weight, except alcohols with the same number of carbons. in the case of the 1-alcohols, where it is constant. KDE is primarily a function of molecular weight (or concen- higher KDE, than that of n-propyl, which is the reverse tration of solvent hydroxyl groups), decreasing as the of the effect for the butyl groups. The effect in these chain length grows. KDW, however, is a function of both latter two pairings is small. The relative order of the molecular weight and position of the solvent hydroxyl branch structures in terms of the effect on R is the same group. It decreases as chain length grows and as the as that shown in Figure 3. It seems clear that the largest hydroxyl moves toward the middle of the chain. 6802 Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005

Table 3. IUPAC Names and Chemical Structures for Solvents Shown in Figure 5

Figure 5. Comparison of separation factor (R) and ethanol distribution coefficient (KDE) for multibranched and unbranched alcohols. The three moieties, R1,R2, and R3, attached to the hydroxyl carbon are listed in the legend, and the alcohols are grouped by number of carbons. Straight-chain alcohols are denoted by *. When the branch group is attached to the hydroxyl carbon and the molecular weight of the alcohol is held constant, the effect of the structure of the branch group is to increase the separation factor as the molecular weight of the branch group increases. The largest effect is seen when going from methyl to ethyl. For the C6- C8 solvents, the gain in separation factor when going to propyl or butyl is small, and there is little difference in effect for normal structures vs iso-structures, though a modest increase when going to the tert-structure is seen. For the C9-C11 solvents, the separation factor continues to increase with the molecular weight of the branch group, though more gradually than that seen between methyl and ethyl. Only limited data could be gathered on the effect of branch position relative to the hydroxyl carbon because of the limited number of alcohols available to us with predict. Higher R is also favored here by the more highly ethyl or larger chains branching off carbons at least one branched butyl moiety: i-butyl > n-butyl and tert-butyl removed from the hydroxyl carbon. However, for a > n-butyl. The reverse is seen for propyl where: n- methyl branch, the position yielding the highest separa- propyl > i-propyl, though this effect is small. tion factor is one removed from the hydroxyl carbon. For The ring-containing alcohols that were studied gener- an ethyl or n-propyl branch, this position is on the ally showed R values lower than those of the 1-alcohols hydroxyl carbon. Methyl has a much smaller influence for a given KDE. At best, two (2-ethylcyclohexanol and than ethyl on separation factor. 2-phenyl-2-propanol) matched the performance of the Multibranched alcohols are described in terms of the 2-alcohols. Both of these have ethyl groups near the three moieties attached to the hydroxyl carbon. Looking hydroxyl carbon. at trends while keeping the molecular weight of the total The tradeoff of the separation factor with KDE is molecule constant, the separation factor is increased and evidenced in the composite data for the 57 alcohols KDE is decreased when the three moieties each have at studied. The alcohols that gave the highest separation least two carbons, as compared to having a methyl or factor for a given KDE were the 4-, 5-, and 6-unbranched- hydrogen and longer corresponding chains. An exception alcohols and some of the branched 3- and 4-alcohols such to this is 3-ethyl-3-pentanol (Et, Et, Et), which has a as 2-methyl-3-pentanol (but not 3-methyl-3-pentanol), lower R and higher KDE than this generalization would 2,2-dimethyl-3-hexanol, and 2,6-dimethyl-4-heptanol. Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 6803

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