Chemical Engineering Science 66 (2011) 6039–6047

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Chemical Engineering Science

0009-25

doi:10.1

n Corr

E-m

antoniu

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Room temperature ionic liquids for propyne–propylene separations:Solubility behaviors and selectivity investigation

Jelliarko Palgunadi a, Antonius Indarto b,n

a Department of Chemistry and Research Institute of Basic Sciences, Kyung Hee University, 1 Hoegi-dong, Dongdaemun-gu, Seoul 130-701, Republic of Koreab Department of Chemical Engineering, Institut Teknologi Bandung, Labtek X, Kampus ITB, Jalan Ganesha 10, Bandung 40132, Indonesia

a r t i c l e i n f o

Article history:

Received 25 May 2011

Received in revised form

3 August 2011

Accepted 15 August 2011Available online 23 August 2011

Keywords:

Propyne

Propylene

Separation

Solubility

Selectivity

Thermochemistry

09/$ - see front matter & 2011 Elsevier Ltd. A

016/j.ces.2011.08.030

esponding author. Tel.: þ62 821 1616 4676.

ail addresses: jelliarko@gmail.com (J. Palguna

s.indarto@che.itb.ac.id (A. Indarto).

a b s t r a c t

Room temperature ionic liquids (RTILs) have been discovered as very promising media for acetylene–

ethylene separation. In this work, propyne and propylene solubility behaviors in dialkylimidazolium-

based RTILs bearing various kinds of anions have been investigated for the feasible extraction process of

propyne from propylene mixture. Solubility–molar volume relationship supported by thermodynamic

analysis indicated that physical absorption mechanism plays dominantly in determining the propylene

solubility. Meanwhile, unlike the acetylene solubility, which is controlled almost exclusively by

hydrogen bonding interaction (chemical interaction) between the anion of RTIL and the solute, the

solubility of propyne is a result from a tradeoff between chemical and physical interactions. Generally

speaking, the trend of the ideal absorption selectivity for propyne over propylene is close to the

acetylene–ethylene case where the higher the hydrogen bond basicity and the smaller the molar

volume of RTIL results in the greater the selectivity for propyne.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Along with ethylene, propylene (C3H6) also belongs to veryimportant building blocks in chemical and polymer industries.Propylene is also used in gasoline production and heatinggas. Today, propylene is mostly obtained as a byproduct fromsteam cracking of naphtha in ethylene production where a smallamount of acetylene and propyne (methylacetylene) are alwaysco-produced (Eisele and Killpack, 2008; Buckl and Meiswinkel,2008). Methylacetylene (C3H4) exists in equilibrium with itsisomer, allene (propadiene) and they form a mixture calledMAPD. Although in majority propyne (in a mixture with allene)has been used as combustion fuel for welding, it can also be usedas a building block for organic synthesis to conveniently producenon-saturated oxygenates, e.g. alkynyl ketones (Sims andPautard-Cooper, 1996). Once, propyne has been considered as aprospective and less toxic rocket propellant (Burkhardt et al.,2004).

After separation of C2 fraction, cracked hydrocarbons of C3, C4–5,and heavier fractions must undergo different steps of separation.Similar to acetylene, propyne is an undesirable substance, whichmust be eliminated from a propylene feed because it can interfere

ll rights reserved.

di),

with the catalytic polymerization of propylene. A common indus-trial technique to remove minute amount of propyne is selectivehydrogenation using a supported Pd or Pt catalyst (Jackson andCasey, 1995; Liu et al., 1997; Kennedy et al., 2004). However,when the concentration of propyne is appreciably valuable, Liquidextraction method using organic solvents may be applied torecover propyne. Linde and Shell have developed such kind ofprocess though it has not been commercially applied yet (Buckland Meiswinkel, 2008). Liquid extraction technique can be inte-grated into the fractionation units where olefins, alkynes, andparaffins generated from naphtha pyrolysis are separatedcontinuously.

Room temperature ionic liquids (RTILs), salts melting below100 1C are potentially attractive and novel solvents for chemicalseparations including for gases purifications and storage due totheir unique physicochemical properties resulted from the varia-tions or the modifications of the cations and the anions, such astemperature stability, negligible vapor pressure, tunable hydro-phylicity/hydrophobicity, and miscibility with hydrocarbons(Wilkes et al., 2008). Carbon dioxide and some light hydrocarbonsare found to be low to moderately soluble in conventional RTILs(Jacquemin et al., 2006; Finotello et al., 2008). Meanwhile, sometoxic and reactive gases, such as SO2 (Shiflett and Yokozeki, 2010),PF3, and BF3 (Tempel et al., 2008) have been discovered to behighly soluble in the same types of RTILs. An ionic liquid, even asimple one, such as dialkylimidazolium halide is a complexstructure carrying several sites whose are capable to specifically

J. Palgunadi, A. Indarto / Chemical Engineering Science 66 (2011) 6039–60476040

respond and interact with a guest molecule. Differences in thesolubility capacity of those gases in the RTILs could arise from thedifferent nature of interactions between the gas and RTIL mole-cules (Crowhurst et al., 2003).

Acetylene has been found to be highly soluble in RTILs havingbasic anions providing opportunity to easily extract this simplealkyne from much less soluble ethylene (Palgunadi et al., 2010).Therefore, it is of a great interest to also investigate the capacityand solubility behavior of propylene and propyne as well as theirseparation selectivity in various kinds of RTILs. Sixteen dialkyli-midazolium-based RTILs having variations on the alkyl chainlength at cation and on anion types ranging from neutral torelatively basic anions were selected to evaluate the effect ofRTIL’s structure on the gas solubility. Based on this study, suitableRTILs can be selected for the effective extraction of propyne frompropylene.

2. Experimental section

2.1. RTILs synthesis

Dialkylimidazolium-based RTILs bearing alkylphosphite anddialkylphosphate anion ([RMIm][RHPO3] and [RMIm][R2PO4])(where R¼–CH3, –C2H5, and –C4H9) were prepared from thereaction of 1-methylimidazole with the corresponding dialkylphosphite and trialkyl phosphate, respectively (Kuhlmann et al.,2007). Dialkylimidazolium alkylsulfates ([RMIm][RSO4], whereR¼–CH3, and –C2H5) were prepared by reacting 1-methylimida-zole with dialkyl sulfate (Holbrey et al., 2002). Dialkylimidazo-lium tetrafluoroborate and bis(trifluoromethylsulfonyl)imide arecommercially available from Aldrich Co. and were used asreceived. Purities of the RTILs were found to be better than 97%as deduced from the proton NMR analysis (400 MHz Bruker NMRspectrometer). These ionic liquids were dried at 70 1C under areduced pressure (�10–2 Pa) at least for 4 h prior to use. Watercontents were below 300 ppm as determined by Karl Fischermoisture titrator (MKC-520, KEM Co. Ltd.). The chemical struc-tures of RTILs used in this study are given in Fig. 1.

Fig. 1. The structures and nomenclat

2.2. Solubility measurement

Propylene and propyne was purchased from Sin Yang Gas, Koreaand Aldrich Chemicals Co, respectively. Solubility evaluations ofpropylene or propyne in RTILs at 303–333 K were carried out atpressures close to atmosphere (�170 kPa) based on the isochoricsaturation method using a similar apparatus as described in theliterature elsewhere (Jacquemin et al., 2006).

An in-house stainless steel apparatus consisting of a gas reservoir(GR) and an equilibrium cell (EC) was constructed and placed in anisothermal oven (controlled with a K-type thermocouple with anaccuracy of 70.1 K; allowed operation temperature from 303 to343 K) as illustrated in Figure S.1 (Supporting Information). Theapparatus was equipped with K-type thermocouples and with apressure transducer for measuring pressures close to atmosphericpressure (OMEGA Engineering PX309-030AI, 0–207 kPa, accuracy0.25% full scale). Volume of the EC (VEC¼100.070.1 cm3) wasmeasured by filling it with distilled water at room temperature.Volumes of the GR and the rest of the system (VGRþ rest¼87.070.1 cm3) were determined from N2 expansion employing the pVTrelation.

In a typical experiment, a known quantity of a RTIL (6–9 cm3)was loaded into the sample container and degassed undervacuum (�10–2 Pa) at 70 1C at least for 8 h prior to each test. Ata specified oven temperature, the valve connecting EC and GR wasclosed to separate these vessels. The gas was fed from the gassupply cylinder to the GR and the amount of the gas in the GR atequilibrium was calculated from the pVT relation. The gas wasthen brought into contact with the absorbent in the EC byopening the valve connecting EC–GR and the absorbent was thenstirred vigorously to facilitate the absorption until it reachedequilibrium. The amount of dissolved gas was calculated from thedifference between the initial gas concentration in the GR and theconcentration in the gaseous phase after equilibration. Anothernew equilibrium of absorption at a specified temperature can beachieved by setting the oven temperature.

Gaseous solute concentrations were calculated using the gasvirial equation of state truncated after the second term employingthe second virial coefficients (Dymond et al., 2002). The uncer-tainty in all reported Henry’s law constants obtained from the

ure system of RTILs under study.

J. Palgunadi, A. Indarto / Chemical Engineering Science 66 (2011) 6039–6047 6041

error of the slope has been estimated to be within 2.0%. Details ofthe measurement apparatus, and data reductions are provided inthe Supporting Information.

3. Results and discussion

3.1. Solubility behaviors of propylene–propyne

The solubilization of a solute in a solvent is controlled by twodifferent thermodynamic factors. The first one is the formation ofcavities within the solvent to accommodate the solute molecules.The second one is the chemical interaction of the inserted solutemolecules with the solvent molecules (Gwinner et al., 2006). The firstfactor requires energy to break the solvent–solvent interaction. Basedon regular solution model, this interaction can be described asHildebrand solubility parameter or cohesive parameter (d). Accord-ingly, if the first thermodynamic step is dominant in controlling thesolubility of gaseous solute, the solubility parameter for solvent (d1)can be used to determine the solubility behavior of a gas.

To demonstrate the importance of the first thermodynamicstep in solubilization and also to predict the solubility behavior ofCO2 and some other gases in dialkylimidazolium-based RTILs atlow pressures, Camper et al. proposed simplified regular solutiontheory (RST) (Camper et al., 2005). Fundamental aspects andapproaches can be found in their recent publications (Camperet al., 2005, 2006a). Based on RST, the gas solubility in a physicalsolvent can be solely determined from solubility parameters (di)as long as Henry’s law is valid, which means solubility at pressureclose to atmosphere:

ln½KH� ¼ aþbðd1�d2Þ2

ð1Þ

In Camper’s works (Camper et al., 2006a), the Kapustin–Skiiequation was used to estimate the lattice energy density of RTILsand later this value was used to calculate the solubility parameterof RTILs (d1). This approach results in a specific relation ofsolubility parameter (d1) as a function of molar volume of RTIL

Table 1Solubility of propylene (expressed in Henry’s law constant) in several RTILs at various

RTIL KH/105 Pa C3H6

303.1 K 308.1 K 313.1 K

[DMIM][MeHPO3] 161.5 (0.1)a – 186.1 (0.2)

[EMIM][MeHPO3] 103.7b (0.3) 110.0 (0.9) 117.0 (0.4)

[BMIM][MeHPO3] – 60.4c (1.0) 68.8 (0.1)

[EMIM][EtHPO3] – 102.9 (0.1) 110.0 (2.0)

[BMIM][BuHPO3] 30.4 (0.2) 33.4 (0.5) 37.2 (0.6)

[DMIM][Me2PO4] 121.4d (0.1) 128.9 (0.4) 137.4 (0.3)

[EMIM][Me2PO4] 85.1 (0.2) 94.2 (1.2) 100.1 (0.9)

[EMIM][Et2PO4] – 46.1e (0.1) 49.6 (0.2)

[BMIM][Bu2PO4] – 23.0f (0.5) 24.6 (1.0)

[DMIM][MeSO4] – 231.2g (0.1) 241.4 (0.3)

[EMIM][MeSO4] 158.1h (1.3) 164.3 (0.8) 173.7 (2.3)

[EMIM][EtSO4] 94.7 (0.3) 101.3 (1.4) 112.8 (1.7)

[BMIM][MeSO4] – 94.8i (0.8) 97.9 (0.3)

[EMIM][BF4] – 160.5j (0.2) 168.2 (0.2)

[BMIM][BF4] 74.1 (0.3) 80.4 (0.9) 88.7 (0.2)

[EMIM][Tf2N] 33.0 (0.4) 36.1 (0.5) 39.7 (0.5)

a Values in parenthesis are the percent absolute deviation from the fit given in Tabb 305.1 K.c 307.8 K.d 303.8 K.e 308.5 K.f 309.0 K.g 309.5 K.h 304.1 K.i 309.3 K.j 309.4 K.

(Eq. (2)):

d1p1

V4=3m

!1=2

ð2Þ

After lumping d2 into a constant to give Eq. (3), the combina-tion of Eqs. (2) and (3) gives Eq. (4) as the simplified model of RSTwhere A, B, and Bn are constants that depend on the temperatureand the type of gas:

ln½KH� ¼ AþBðd1Þ2

ð3Þ

ln KH½ � ¼ AþBn

V4=3m

!ð4Þ

From Eq. (4), a linear relationship must exist for RST to be validover the gas–RTIL–temperature combination. This relationshipalso indicates that a solvent with a greater molar volume pos-sesses a smaller solubility parameter and hence a smaller cohesionenergy leading to a higher solubility of solute molecules.

The propylene and propyne solubilities (expressed in Henry’slaw constant) in various kinds of dialkylimidazolium-based RTILsare listed in Tables 1 and 2, respectively.

Selective separation of gases using a liquid absorbent, such asRTILs is dependent on the solubility behavior of each gas. As detailedin Tables 1 and 2, solubility of propylene is much lower than that ofpropyne and significantly increases with the size of RTILs. Highersolubility of propyne over propylene is not unexpected since likeacetylene, propyne bears one acidic proton with pKa�14 (protonfrom C–H sp hybridization) capable to specifically interact with thebasic site on a RTIL, thus, enhancing the solubility. The solubility ofsome light olefins, such as ethylene and butylene have been sug-gested to be largely determined by a physical absorption mechanism(Camper et al., 2004, 2005, 2006a).

To figure out the solubility behavior of propylene and propyne,simplified regular solution theory (RST) model suggested by Camperet al. (2006b) has been applied. Fig. 2 shows the data plots ofpropylene solubilities (in Henry’s law constants) and the inverse

temperatures.

318.1 K 323.1 K 328.1 K 333.1 K

203.1 (1.8) 208.6 (2.0) 228.0 (0.5) 241.7 (0.2)

124.4 (1.1) 135.1 (0.7) 143.9 (0.9) 149.9 (0.6)

75.7 (0.2) 82.3 (0.3) 86.8 (2.1) 94.6 (0.4)

123.4 (0.6) 134.6 (0.3) 144.4 (1.7) 161.0 (0.1)

40.7 (0.5) 44.2 (0.1) 47.5 (1.0) 52.3 (0.7)

146.6 (0.2) 156.5 (0.2) 167.1 (0.2) 178.5 (0.3)

108.8 (0.3) 118.5 (0.9) 126.1 (0.2) 134.3 (0.1)

53.8 (0.5) 57.6 (0.4) 62.6 (0.1) 68.2 (0.3)

27.0 (0.3) 29.2 (0.8) 31.9 (0.5) 34.7 (0.7)

257.4 (0.2) 274.5 (0.2) 282.2 (2.9) 306.7 (0.1)

190.1 (0.1) 208.7 (3.1) 212.9 (0.8) 225.3 (0.7)

120.9 (1.3) 126.7 (0.6) 132.8 (2.1) 145.7 (1.5)

104.3 (0.10) 112.3 (1.0) 118.5 (0.4) 127.8 (0.1)

179.5 (0.2) 191.6 (0.2) 204.5 (0.2) 218.5 (0.2)

96.6 (0.5) 103.4 (0.2) 110.9 (0.3) 118.9 (0.1)

44.2 (1.7) 47.0 (0.1) 50.0 (1.3) 54.7 (0.7)

le 5.

Table 2Solubility of propyne (expressed in Henry’s law constant) in several RTILs at various temperatures.

RTIL KH/105 Pa C3H4

303.1 K 308.1 K 313.1 K 318.1 K 323.1 K 328.1 K 333.1 K

[DMIM][MeHPO3] 8.6b (0.2)a 8.8c (0.1) 11.3 (0.3) 13.1 (0.1) 15.0 (0.1) 17.2 (0.3) 19.5 (0.2)

[EMIM][MeHPO3] 6.6b (0.1) 7.5 (0.1) 8.7 (0.3) 10.1 (0.1) 11.6 (0.3) 13.2 (0.2) 15.1 (0.1)

[BMIM][MeHPO3] – 6.5 (0.1) 7.5 (0.8) 8.6 (0.5) 10.0 (1.1) 11.2 (0.3) 12.7 (0.4)

[EMIM][EtHPO3] – 6.6d (0.6) 7.6 (1.0) 8.8 (0.5) 10.3 (1.5) 11.5 (0.3) 13.1 (0.2)

[BMIM][BuHPO3] 4.4 (0.1) 5.1 (0.1) 5.9 (0.3) 6.7 (0.1) 7.7 (0.3) 8.7 (0.2) 9.8 (0.1)

[DMIM][Me2PO4] – 7.6e (0.1) 8.6 (0.2) 10.0 (0.0) 11.5 (0.3) 13.1 (0.2) 15.0 (0.1)

[EMIM][Me2PO4] 5.2 (0.4) 6.1 (0.4) 7.1 (0.3) 8.4 (0.8) 9.5 (0.2) 11.0 (0.2) 12.5 (0.1)

[EMIM][Et2PO4] – 5.2f (0.4) 5.8 (0.5) 6.8 (0.2) 7.8 (0.2) 9.0 (0.4) 10.2 (0.3)

[BMIM][Bu2PO4] – 4.0g (0.1) 4.6 (0.8) 5.3 (0.5) 6.0 (0.6) 6.7 (1.0) 7.7 (0.2)

[DMIM][MeSO4] – 15.9h (0.6) 17.1 (0.1) 19.6 (0.8) 22.1 (0.0) 25.2 (0.1) 28.8 (0.4)

[EMIM][MeSO4] 10.2b (0.4) 11.3 (0.8) 13.1 (0.1) 15.0 (0.2) 17.1 (0.1) 19.4 (0.1) 21.9 (0.2)

[EMIM][EtSO4] 7.1 (0.5) 8.2 (0.2) 9.4 (0.6) 10.9 (0.3) 12.6 (1.2) 14.2 (0.4) 16.0 (0.5)

[BMIM][MeSO4] – 8.2i (0.2) 9.2 (0.5) 10.6 (0.1) 12.1 (0.1) 13.7 (0.2) 15.4 (0.2)

[EMIM][BF4] – 12.6 (0.1) 14.6 (0.0) 16.9 (0.2) 19.2 (0.1) 21.8 (0.1) 24.6 (0.1)

[BMIM][BF4] 7.3 (0.1) 8.5 (0.1) 9.7 (0.1) 11.2 (0.6) 12.7 (0.2) 14.3 (0.6) 16.2 (0.5)

[EMIM][Tf2N] 6.7 (0.1) 7.6 (0.3) 8.5 (0.6) 9.7 (0.1) 10.9 (0.3) 12.1 (0.3) 13.5 (0.1)

a Values in parenthesis are the percent absolute deviation from the fit given in Table 5.b 304.1 K.c 305.1 K.d 308.0 K.e 309.0 K.f 309.2 K.g 307.8 K.h 310.1 K.i 308.8 K.

R2 = 0.95

R2 = 0.72

0.0

1.0

2.0

3.0

4.0

5.0

6.0

3.0E-04 5.0E-04 7.0E-04 9.0E-04 1.1E-03 1.3E-03

ln( K

H /

105

Pa)

Vm -4/3 RTIL

C3H4

C3H6

Fig. 2. Plots of RTIL molar volumes versus natural logarithmic values of Henry’s

law constant of propylene and propyene at 313 K.

Table 3Volume molar of RTILs and solubility of propylene and propyne expressed in

volume-based unit at 313.1 K and 0.1 MPa.

RTIL Vm

(cm3 mol–1)v/v (cm3

solute/cm3solvent) 313.1 K–0.1 MPa

Propylene Propyne

[DMIM][MeHPO3] 155.0 0.89 16.09

[EMIM][MeHPO3] 170.4 1.31 19.82

[BMIM][MeHPO3] 205.5 1.88 19.75

[EMIM][EtHPO3] 188.2 1.26 20.93

[BMIM][BuHPO3] 255.9 2.80 20.94

[DMIM][Me2PO4] 176.3 1.07 19.24

[EMIM][Me2PO4] 192.0 1.35 21.88

[EMIM][Et2PO4] 231.8 2.28 23.21

[BMIM][Bu2PO4] 328.7 3.34 22.02

[DMIM][MeSO4] 156.6 0.69 10.26

[EMIM][MeSO4] 172.3 0.88 12.55

[EMIM][EtSO4] 190.6 1.20 15.98

[BMIM][MeSO4] 206.9 1.30 15.28

[EMIM][BF4] 158.4 0.98 11.97

[BMIM][BF4] 186.8 1.57 15.80

[EMIM][Tf2N] 259.2 2.58 13.31

J. Palgunadi, A. Indarto / Chemical Engineering Science 66 (2011) 6039–60476042

molar volumes of RTILs. As expected, a molar-volume dependencysimilarly found for ethylene (Palgunadi et al., 2010) also for methaneand carbon dioxide (Palgunadi et al., 2009; Finotello et al., 2008)exists for the propylene solubilities. This relationship may indicatethat a physical absorption mechanism is preeminent in determiningthe solubility of propylene in RTILs (Scovazzo et al., 2004).

It was mentioned (Palgunadi et al., 2011) that a chemical associa-tion via hydrogen bonding between the acidic protons in acetyleneand the anion of RTILs is dominant over physical interactions andalmost exlusively determines degree of acetylene solubility as evi-dently seen from a good linearity between the solute solubility andthe HBA ability of RTIL. It is also expected that the analogous situationis applicable to propyne. However, the propyne case is somewhatinteresting. Unlike acetylene absorption behavior (Palgunadi et al.,2010), the squared correlation coefficient (R2) for propyne solubility–RTIL molar volume correlation shown in Fig. 2 suggests that physicalabsorption mechanism is indeed not insignificant. In addition, anattempt to obtain a similar conclusion as applied to acetylene–RTILs

(Palgunadi et al., 2011) by plotting Henry’s law constants of propyneversus hydrogen-bond acceptor (HBA) ability of RTILs and getting alinear correlation is unsuccessful due to a very limited availability ofb (HBA) values of Kamlet–Taft parameters (Table 7).

While Henry’s coefficient is more useful to express the solubilityin the molecular perspective, direct comparisons based on thisconstant could be misleading because for an equivalent amount ofgas uptake per mass unit, an ionic liquid with relatively largermolecular formula will produce a smaller Henry’s coefficient (meansa higher solubility). It was suggested that volume-based unit shouldbe used for practical applications to compare the bulk absorptioncapacity of various types of solvent. Therefore, solubilities of propy-lene and propyne expressed in v/v (cm3

solute/cm3solvent) at 313.1 K and

0.1 MPa and their correlation with the molar volume of RTILs aregiven in Table 3 and Fig. 3, respectively.

R2 = 0.92

R2 = 0.22

0

5

10

15

20

25

350V m/cm3 mol-1

v/ v

(cm

3 solu

te/c

m3 so

lven

t)

C3H4

C3H6

200150 250 300

Fig. 3. Plots of RTIL molar volumes versus volume-based solubility of propylene

and propyene at 313 K.

Table 4Coefficients of Eq. (5)a and the percent average absolute deviation of the fit

(AAD (%)) for propylene.

RTIL B0 B1 B2 AAD (%)

[DMIM][MeHPO3] þ9.005 �1.023�103�5.132�104 0.8

[EMIM][MeHPO3] þ3.967 �1.896�103�5.167�105 0.7

[BMIM][MeHPO3] �11.47 þ1.189�104�2.185�106 0.7

[EMIM][EtHPO3] þ18.62 �6.988�103þ8.24�105 0.8

[BMIM][BuHPO3] þ5.896 þ3.97�102�3.498�105 0.5

[DMIM][Me2PO4] þ12.89 �3.711�103þ3.793�105 0.2

[EMIM][Me2PO4] þ5.631 þ9.191�102�3.887�105 0.6

[EMIM][Et2PO4] þ16.83 �6.6�103þ7.977�105 0.3

[BMIM][Bu2PO4] þ14.15 �5.182�103þ5.488�105 0.6

[DMIM][MeSO4] þ8.121 �3.916�102�1.367�105 0.6

[EMIM][MeSO4] þ5.733 9.871�102�3.646�105 1.3

[EMIM][EtSO4] þ2.755 þ2.681�103�6.492�105 1.3

[BMIM][MeSO4] þ20.49 �8.829�103þ1.204�106 0.4

[EMIM][BF4] þ14.63 �4.703�103þ5.392�105 0.2

[BMIM][BF4] þ3.373 þ2.342�103�6.258�105 0.4

[EMIM][Tf2N] þ1.482 þ3.127�103�7.644�105 0.7

a Equation of Krauss and Benson.

Table 5Coefficients of Eq. (5) and the percent average absolute deviation of the fit

(AAD (%)) for propyne.

RTIL B0 B1 B2 AAD (%)

[DMIM][MeHPO3] þ8.262 �7.413�102�3.415�105 0.2

[EMIM][MeHPO3] þ9.161 �1.474�103�2.262�105 0.2

[BMIM][MeHPO3] þ1.969 þ2.926�103�9.128�105 0.5

[EMIM][EtHPO3] þ9.565 �1.887� �103�1.487�105 0.7

[BMIM][BuHPO3] þ6.542 �2.236�102�3.992�105 0.2

[DMIM][Me2PO4] þ7.788 �5.699�102þ3.756�105 0.2

[EMIM][Me2PO4] þ8.464 �1.121�103�2.872�105 0.3

[EMIM][Et2PO4] þ7.478 �5.587�102�3.87�105 0.3

[BMIM][Bu2PO4] þ10.63 �3.079�103�7.073�104 0.5

[DMIM][MeSO4] þ21.48 �9.185�103þ1.047�106 0.3

[EMIM][MeSO4] þ11.79 �3.078�103þ5.842�104 0.2

[EMIM][EtSO4] þ8.968 �1.42�103�2.156�105 0.5

[BMIM][MeSO4] þ8.537 �1.27�103�2.223�105 0.2

[EMIM][BF4] þ1.968 þ3.316�103�9.695�105 0.1

[BMIM][BF4] þ7.113 �3.188�102�3.758�105 0.3

[EMIM][Tf2N] þ9.805 �2.427�103þ7.766�103 0.3

J. Palgunadi, A. Indarto / Chemical Engineering Science 66 (2011) 6039–6047 6043

As can be seen in Table 3, the propylene solubility seems notincrease in accordance with increasing size of molar volume of RTILs.However, data plot of propylene solubility versus molar volumeof RTILs in Fig. 3 suggests that the molar volume of RTILs has asignificant role in determining the propylene solubility. While on thecontrary, a similar regularity is not shown by propyne solubility.

The different solubility behavior of propyne from that ofacetylene may be interpreted as a consequence of increasingshort-range, van deer Waals interactions due to the presence of anon polar domain (methyl group) in propyne. Alternatively, a smallfraction of propadiene existing in equilibrium with propyne maybehave similarly to propylene and reduce the hydrogen bondingeffect. In any of those two plausible explanations, the solubility ofpropyne seems to be controlled by a tradeoff between solute–solvent chemical and physical interactions.

3.2. Thermodynamics of absorption

Thermodynamics of absorption is important information to under-stand the solubilization behavior. The enthalpy of absorption can beassociated with the molecular interaction between CO2 and RTIL, andthe entropy of absorption is related to the solvent organizationsurrounding the solute or degree of disorder (Jacquemin et al.,2006). To derive the thermodynamic information, the relationshipbetween the solubility data of each hydrocarbon and temperatureof absorption is expressed using the Krause and Benson equation(Krause and Benson, 1989):

ln½KHðTÞ=105 Pa� ¼Xn

i ¼ 0

BiðT=KÞ�ið5Þ

and the optimized coefficients derived from linear regression ofmultiple variables operation are listed in Tables 4 and 5.

Gibbs free energies, enthalpies, and entropies of absorption forpropylene and propyne in various RTILs are determined using thefollowing equations and their values at several discrete tempera-tures are displayed in Table 6:

DGsolv ¼ RT lnKHðT,pÞ

p0

� �ð6Þ

DHsolv ¼ R@ lnðKHðT ,pÞ=p0Þ

@ð1=TÞ

� �p

ð7Þ

DSsolv ¼DsolH�DsolG

T

� �ð8Þ

As shown in Table 6, the more negative enthalpy of propyneabsorption than that of propylene clearly suggests that the specificinteraction of propyne and RTIL is much stronger than that of

propylene–RTIL. It is worth noting that beside hydrogen bonding,a specific but weaker interaction of XH � � �ppropyne (where Xcan be a proton acceptor such as O, N, or halogen atoms) alsopotentially presents (Sanchez-Garcıa et al., 2004; Sundararajanand Ramanathan, 2007; Mardyukov et al., 2009).

Less negative values of enthalpy of propylene absorption (from�10 to �17 kJ mol–1) and larger Henry’s law constant relative tothose of propyne are indicative that the physical absorption ofpropylene takes place dominantly. Surprisingly, as displayed inFig. 4, a simple linear correlation between the natural logarithmicvalues of propyne solubility versus enthalpies of absorption is indeedabsent suggesting that enthalpy is not the major factor in determin-ing the propyne solubility. Recalling the existance of a moderatelylinear correlation of data plot of propyne solubility versus molarvolume of RTILs in Fig. 2, it can be suggested that the specificchemical as well as non-specific physical interactions of propyne–RTIL play synergically in controlling the propyne solubility.

To observe the effect of entropy in the solubilization, thesevalues are presented in the multiplication with temperature. Ascan be seen from Table 6, the entropies for propylene solubiliza-tion are much greater than the enthalpies. This suggests thatentropy factor dominantly controls degree of propylene solubility.On the other hand, the entropies and enthalpies for propyne

Table 6Gibbs free energy, enthalpy, and entropy of solvation for propylene and propyne in various 1,3-dialkylimidazolium-based RTILs at several temperatures (the values are

consistent with p0¼0.1 MPa).

T (K) DGsolv (kJ mol–1) DHsolv (kJ mol�1) TDSsolv (kJ mol�1) DGsolv (kJ mol�1) DHsolv (kJ mol–1) TDSsolv (kJ mol–1)

[DMIM][MeHPO3] Propylene (C3H6) Propyne (C3H4)

313.1 13.6 �11.2 �24.8 6.3 �24.3 �30.6

323.1 14.4 �11.2 �25.6 7.3 �23.7 �31.0

333.1 15.2 �11.1 �26.3 8.2 �23.2 �31.4

[EMIM][MeHPO3] Propylene Propyne

313.1 12.4 �11.7 �24.1 5.6 �24.3 �29.9

323.1 13.1 �10.8 �23.9 6.5 �23.9 �30.4

333.1 13.9 �10.0 �23.9 7.5 �23.6 �31.1

[BMIM][MeHPO3] Propylene Propyne

313.1 11.0 �17.2 �28.2 5.2 �24.1 �29.3

323.1 11.8 �13.6 �25.4 6.1 �22.6 �28.7

333.1 12.6 �10.2 �22.8 7.0 �21.2 �28.2

[EMIM][EtHPO3] Propylene Propyne

313.1 12.3 �14.4 �26.7 5.3 �23.6 �28.9

323.1 13.1 �15.7 �28.8 6.2 �23.3 �29.5

333.1 14.0 �17.0 �31.0 7.1 �23.1 �30.2

[BMIM][BuHPO3] Propylene Propyne

313.1 9.4 �15.3 �24.7 4.6 �23.1 �27.7

323.1 10.1 �14.7 �24.8 5.5 �22.4 �27.9

333.1 10.9 �14.2 �25.1 6.3 �21.8 �28.1

[DMIM][Me2PO4] Propylene Propyne

313.1 12.8 �10.7 �23.5 5.6 �24.7 �30.3

323.1 13.5 �11.3 �24.8 6.5 �24.1 �30.6

333.1 14.3 �11.9 �26.2 7.5 �23.5 �31.0

[EMIM][Me2PO4] Propylene Propyne

313.1 12.0 �13.0 �25.0 5.1 �24.6 �29.7

323.1 12.8 �12.4 �25.2 6.0 �24.1 �30.1

333.1 13.5 �11.8 �25.3 7.0 �23.7 �30.7

[EMIM][Et2PO4] Propylene Propyne

313.1 10.1 �12.5 �22.6 4.6 �25.2 �29.8

323.1 10.9 �13.8 �24.7 5.5 �24.6 �30.1

333.1 11.7 �15.1 �26.8 6.4 �24.0 �30.4

[BMIM][Bu2PO4] Propylene Propyne

313.1 8.3 �13.9 �22.2 4.0 �21.8 �25.8

323.1 9.1 �14.9 �24.0 4.8 �22.0 �26.8

333.1 9.8 �15.7 �25.5 5.6 �22.1 27.7

[DMIM][MeSO4] Propylene Propyne

313.1 14.3 �10.5 �24.8 7.4 �20.8 �28.2

323.1 15.1 �10.3 �25.4 8.3 �22.5 �30.8

333.1 15.8 �10.1 �25.9 9.3 �24.1 �33.4

[EMIM][MeSO4] Propylene Propyne

313.1 13.5 �11.2 �24.7 6.7 �22.5 �29.2

323.1 14.2 �10.6 �24.8 7.6 �22.6 �30.2

333.1 15.0 �10.0 �25.0 8.5 �22.7 �31.2

[EMIM][EtSO4] Propylene Propyne

313.1 12.2 �12.2 �24.4 5.8 �23.3 �29.1

323.1 13.0 �11.1 �24.1 6.7 �22.9 �29.6

333.1 13.7 �10.1 �23.8 7.7 �22.6 �30.3

[BMIM][MeSO4] Propylene Propyne

313.1 11.9 �9.5 �21.4 5.8 �22.4 �28.2

323.1 12.6 �11.5 �24.1 6.7 �22.0 �28.7

333.1 13.4 �13.3 �26.7 7.5 �21.7 �29.2

J. Palgunadi, A. Indarto / Chemical Engineering Science 66 (2011) 6039–60476044

Table 6 (continued )

T (K) DGsolv (kJ mol–1) DHsolv (kJ mol�1) TDSsolv (kJ mol�1) DGsolv (kJ mol�1) DHsolv (kJ mol–1) TDSsolv (kJ mol–1)

[EMIM][BF4] Propylene Propyne

313.1 13.3 �10.5 �23.8 7.0 �23.9 �30.9

323.1 14.1 �11.4 �25.5 7.9 �22.3 �30.2

333.1 14.9 �12.2 �27.1 8.8 �20.8 �29.6

[BMIM][BF4] Propylene Propyne

313.1 11.6 �13.8 �25.4 5.9 �22.6 �28.5

323.1 12.4 �12.7 �25.1 6.8 �22.0 �28.8

333.1 13.2 �11.8 �25.0 7.7 �21.4 �29.1

[EMIM][TF2N] Propylene Propyne

313.1 9.6 �14.6 �24.2 5.6 �19.8 �25.4

323.1 10.3 �13.3 �23.6 6.4 �19.8 �26.2

333.1 11.0 �12.2 �23.2 7.2 �19.8 �27.0

R2 = 0.05R2 = 0.42

1.0

2.0

3.0

4.0

5.0

6.0

5.0 10.0 15.0 20.0 25.0 30.0

ln( K

H /

105

Pa)

−ΔHsol /kJ mol-1

C3H4

C3H6

Fig. 4. Plots of enthalpies of absorption versus natural logarithmic values of

Henry’s law constant of propylene and propyne at 313 K.

Fig. 5. Optimized geometry showing the molecular interaction between propy-

lene (a) or propyne (b) and [EMIM][MeHPO3]. All values are in Angstrom unit.

J. Palgunadi, A. Indarto / Chemical Engineering Science 66 (2011) 6039–6047 6045

solubilization are relatively comparable. This may indicate thatboth thermodynamic factors affect the propyne absorption. Thisthermodynamic analysis is in good agreement with the findingsthat the solubility of propylene is mostly dependent on thesolubility parameter (solvent-solvent cohesive energy) closelyrelated with physical absorption mechanism. Meanwhile, thesolubilization of propyne is determined by both physical andchemical absorption (hydrogen bonding interaction) mechanisms.

In order to support the above investigation result and thethermodynamic analysis, ab initio molecular calculations usingGaussian 03 package software were also carried out. Fig. 5 showsthe optimized geometries of propylene and propyne when inter-acting with [EMIM][MeHPO3] at B3LYP/6-31þG(d) level of theory.

Fig. 5 clearly shows that propylene as well as propyne is locatedclose to the anion of the RTIL. The acidic protons of those hydro-carbons (H sp2 in propylene and H sp in propyne) interact and formhydrogen bonding with the most basic site of the anion, the oxygenatom. The figure also indicates that the H-bond of propyne-anion(2.01 A) is much shorter than that of propylene-anion (2.32 A)implying stronger interaction. In addition, the theoretical calcula-tion of enthalpy (H) and Gibbs (G) differences between propyleneand propyne interaction supports the experimental value shown inTable 6. Taking an example of data at 313 K for propylene orpropyne—[EMIM][MeHPO3], the experimental enthalpy differencebetween propylene and propyne ðDHC3H6

�DHC3H4Þ, 12.6 kJ mol–1,

closed to 11.4 kJ mol–1 by theoretical calculation. The similar

J. Palgunadi, A. Indarto / Chemical Engineering Science 66 (2011) 6039–60476046

situation was found for G where Gibbs’ energy differencesðDGC3H6

2DGC3H4Þ are 6.8 kJ mol–1 (experiment) and 7.9 kJ mol–1

(theory). A complete assessment of the experimental and theore-tical comparison study is undergoing.

3.3. Solubility–selectivity

A successful separation is strongly dependent on the solubility–selectivity of one component from the others. The ideal selectivitiesof propyne over propylene at several temperatures in various RTILsunder study are listed in Table 7. The ideal solubility–selectivitytoward propyne is the ratio of the individual solubility of propyneand propylene. Two definitions for selectivity are used in this paper.One is based on the ratio of Henry’s law constants of propylene overpropyne (SKH¼KHC3H6/KHC3H4). Another one is based on volumeratio, which is expressed as the volume ratio (data in Table 3)of dissolved propylene over propyne in a specified volume of RTIL(Sv/v¼v/v C3H4/v/v C3H6). Although the selectivity values from thetwo definitions are slightly different, their trends agree to each other.

Table 7 shows that the RTIL possessing relatively high tomedium hydrogen bond basicity (HBA ability), such as dialkyli-midazolium with [R2PO4], [RHPO3], or [RSO4] anion gives sela-tively higher selectivity compared to the RTIL possessing lowerbasicity or neutral anion, like [BF4]– or [Tf2N]–. However, longeralkyl chain bonded on the cation or the anion notably decreasesthe selectivity (especially when the alkyl chain grows from C2 toC4) due to the increase of molar volume of RTILs resulting in thedramatic increase of particularly propylene solubility. The solu-bilities of those hydrocarbons and the ideal absorption selectivityare influenced by the temperature of absorption. Both are reducedwith the increase of temperature. Nevertheless, this work alsoshows that the absorption selectivity for propyne/propylenemixture in various RTILs under study are relatively low for aneffective propyne/propylene separation process.

Previously, it was shown that higher acetylene absorptionselectivity over ethylene is observed for RTIL possessing smallermolar volume and greater HBA ability (Palgunadi et al., 2011). Asa result, [DMIM][MeHPO3], which is the smallest and the mostbasic RTIL from the list shows the highest selectivity. In thepropylene–propyne case, the selectivity for propyne absorption isfound to have a similar trend to that of acetylene–ethylene casealthough the physical and chemical absorption mechanisms play

Table 7Ideal absorption selectivity of propyne over propylene at several temperatures.

RTIL b (HBA) (SKH)a Sv/vb

313.1 K 323.1 K 333.1 K 313.1 K

[DMIM][MeHPO3] – 16 14 12 18

[EMIM][MeHPO3] 1.000 13 12 10 15

[BMIM][MeHPO3] – 9 8 7 11

[EMIM][EtHPO3] – 14 13 12 17

[BMIM][BuHPO3] – 6 6 5 7

[DMIM][Me2PO4] – 16 14 12 18

[EMIM][Me2PO4] 1.000 14 12 11 16

[EMIM][Et2PO4] – 9 7 7 10

[BMIM][Bu2PO4] – 5 5 5 7

[DMIM][MeSO4] – 14 12 11 15

[EMIM][MeSO4] 0.610 13 12 10 14

[EMIM][EtSO4] 0.710 12 10 9 13

[BMIM][MeSO4] – 11 9 8 12

[EMIM][BF4] – 12 10 9 12

[BMIM][BF4] 0.376 9 8 7 10

[EMIM][Tf2N] 0.100 5 4 4 5

a SKH¼KHC3H6/KHC3H4.b Sv/v¼v/v C3H4/v/vC3H6.

simultaneously in controlling the propyne solubility. It can beseen that the highest ideal absorption selectivity is obtained with[DMIM][MeHPO3] or [DMIM][Me2PO4].

4. Conclusions and outlook

Solubility investigation reveals that the solubility of propyne invarious dialkylimidazolium-based RTILs are controlled by both thephysical and chemical absorption mechanism. Whereas, the phy-sical absorption seems to be more dominant in determining thepropylene solubility. Better tools and technique of analysis perhapscan reveal in more detail the solubility behaviors of propylene andespecially propyne in RTILs. A smaller RTIL with higher hydrogenbond basicity results in a better separation for propyne frompropylene. The highest ideal absorption selectivity for propyne isobtained for [DMIM][MeHPO3] and [DMIM][Me2PO4] due to theirrelatively small molar volumes and higher hydrogen-bond basicity.

Because the absorption selectivities for propyne over propy-lene in various kinds of conventional RTILs have been found to berelatively low, task-specific ILs system, which are able to largelyabsorb propyne while keeping the propylene solubility low havebeen introduced. Recently, it was demonstrated that a very highseparation factor for propyne over propylene can be achieved if asmall concentration of copper chloride (CuCl) is dissolved in[DMIM][MeHPO3] (Kim et al., 2010). Fast atomic bombardmentmass spectrocopy (FAB-MS) indicates the presences of severalspecies of copper–[MeHPO3] complex, which may strongly inter-act with propyne. Based on the ab initio calculations, complexcoordination of propyne to copper is improbable but the specificinteraction between propyne and the anion via hydrogen bondingis impressively enhanced. On the contrary, the solubility ofpropylene is found to be independent on the addition of CuCl.Such a solubility difference leads to the improvement of the idealabsorption selectivity of propyne over propylene up to 14 timeshigher than in [DMIM][MeHPO3] only. Importantly, the propyneabsorption is reversible giving an opportunity for a processintensification.

Appendix A. Supporting materials

Supplementary data associated with this article can be foundin the online version at doi:10.1016/j.ces.2011.08.030.

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