Propylene and Propane Solubility in Imidazolium ... - ACS Publications

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Propylene and Propane Solubility in Imidazolium, Pyridinium, and Tetralkylammonium Based Ionic Liquids Containing a Silver Salt Marcos Fallanza, Alfredo Ortiz, Daniel Gorri, and Inmaculada Ortiz* Department of Chemical Engineering and Inorganic Chemistry, University of Cantabria, Avenida de los Castros s/n. 39005 Santander, Cantabria, Spain S Supporting Information *

ABSTRACT: The gas solubility of propane and propylene in seven ionic liquids, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4), 1-hexyl-3-methylimidazolium tetrafluoroborate (HMImBF4), 1-octyl-3-methylimidazolium tetrafluoroborate (OMImBF4), 3-methylimidazolium nitrate (BMImNO3), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonylimide) (BMImTf2 N), methyltrioctylammonium bis(trifluoromethylsulfonylimide) (MOOONTf2N), and butyltrimethylammonium bis(trifluoromethylsulfonylimide) (BMMMN Tf2N), is reported. The equilibrium isotherms of both pure gases were measured in the pure ionic liquids and in presence of a silver salt containing the same anion of the ionic liquid in a range of concentration of (0 to 0.77) mol·kg−1 IL at temperatures between (288 and 308) K and pressures ranging from (0 to 700) kPa. Henry’s law constant values for physical solubility as well as the characteristic parameters for chemical solubility such as chemical equilibrium constants and enthalpies of the chemical reactions between silver cations and propylene are reported. Based upon the experimental results, ionic liquids based on imidazolium cations with less and shorter alkyl substituents improve the selective separation of propylene from these mixtures. Regarding to the structure of the anion it was gathered that ionic liquids with the BF4− anion, combined with the AgBF4 silver salt, provided the best results in terms of olefin capacity and selectivity. In this article we provide valuable data that evidence that the separation of propane/ propylene gas mixtures by reactive absorption could represent an efficient alternative to the traditional separation process based on cryogenic distillation and serve for the new process design.

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INTRODUCTION Light olefins such as ethylene and propylene are very important to petrochemical industries because they are utilized as main building blocks for many essential chemicals and products for industrial and domestic consumption. For nearly seven decades, the separation of olefins such as ethylene and propylene from refinery catalytic cracker off-gases and effluent streams has been performed by cryogenic distillation which is a highly energyintensive process.1,2 In the past few years, many researchers have paid attention to developing new alternatives to solve the economic and environmental problems associated to the traditional technology. Among a number of alternatives, the application of reactive absorption using silver salts combined with membrane technology to the separation of C3H8/C3H6 gas mixtures has been considered a potential alternative to overcome the drawbacks associated to the cryogenic distillation.3−6 The separation performance is mostly associated with the ability of olefins to react selective and reversibly with silver cations Ag+, by a π-complexation mechanism.7−10 Room temperature ionic liquids (RTILs) attract increasing attention as potential substitutes for conventional solvents because in addition to their well-known and remarkable properties such as negligible vapor pressure which allow they to perform gas © 2013 American Chemical Society

separations without solvent losses or gas stream pollution, and their high thermal and chemical stability, they present higher affinity for olefinic compounds compared to their corresponding saturated hydrocarbons and at the same time they provide stability to the metal cation dissolved inside.11−13 In addition, they are considered as “designer solvents” since a large number of possible combinations of cations and anions allows for a tunability of their properties to specific applications. Therefore it is possible to select an appropriate ionic liquid for each separation task; however it is critical to have available gas solubility data in ionic liquids with different structures in order to select the most suitable one to carry out the separation process. Unfortunately, although the number of studies related to gas solubility on ionic liquids is steeply growing,14−17 publications regarding the investigation of the thermodynamic data of C3H6 and C3H8 in Ag+-IL systems is still very scarce.18−26 In previous works Ortiz et al.27−29 reported the physicochemical characteristics of the reactive system, including Received: December 26, 2012 Accepted: June 9, 2013 Published: July 9, 2013 2147

dx.doi.org/10.1021/je301368x | J. Chem. Eng. Data 2013, 58, 2147−2153

Journal of Chemical & Engineering Data

Article

Table 1. Chemical Structures of the Ionic Liquids and Silver Salts Studied in This Work

tetrafluoroborate (OMImBF4) (CAS number 244193-52-0), 3-methylimidazolium nitrate (BMImNO3) (CAS number 179075-88-8), 1-buthyl-3-methylimidazolium bis(trifluoromethylsulfonylimide) (BMImTf2N) (CAS number 17489983-3), methyltrioctylammonium bis(trifluoromethylsulfonylimide) (MOOONTf2N) (CAS number 375395-33-8), and butyltrimethylammonium bis(trifluoromethylsulfonylimide) (BMMMN Tf2N) (CAS number 258273-75-5) supplied by Iolitec, with a minimum purity of 99 % and residual halide content less than 500 ppm. The silver salts used in this work are silver tetrafluoroborate (CAS number 14104-20-2) of 99 % purity purchased from Apollo Scientific Ltd., silver nitrate (CAS number 7761-88-8) of 99 % purity, and silver bis(trifluoromethylsulfonylimide) prepared in our lab from the reaction between silver oxide (CAS number 20667-12-3) and bis(trifluoromethylsulfonylimide) (CAS number 82113-65-3) supplied by Sigma-Aldrich. Synthesis of AgTf2N. The AgTf2N salt was prepared in our laboratory from the reaction between silver oxide and bis(trifluoromethylsulfonylimide) following a synthesis route

thermodynamic and kinetic data of the complexation reaction between propylene and silver cations in the ionic liquids BMImBF4 and BMPyBF4. Thus, in this paper we extended our studies to describe the gas−liquid equilibrium behavior of propane and propylene into seven additional ionic liquids with different structures in absence and in presence of silver cations dissolved inside. The chemical structures of the seven ionic liquids used in this work (and two previously reported) are collected in Table 1. Experimental data were obtained at pressures between (0 and 700) kPa, different temperatures in the range (288 to 308) K and silver concentrations from (0 to 0.77) mol·kg−1 IL (see also Supporting Information).

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EXPERIMENTAL SECTION Materials. Propylene and propane gas were purchased from Praxair with a minimum purity of 99.5 %. The ionic liquids selected in this work were 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) (CAS number 143314-16-3), 1hexyl-3-methylimidazolium tetrafluoroborate (HMImBF4) (CAS number 244193-50-8), 1-octyl-3-methylimidazolium 2148



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which is rather similar to earlier literature procedures.20,30 For this preparation route, pure silver oxide (Ag2O) is dissolved in an aqueous solution of H[Tf2N], and the reaction takes place under stirring at room temperature for 8 h. Afterward the mixture was filtered in order to remove possible solid impurities or unreacted Ag2O particles, and subsequently the Ag[Tf2N] silver salt was isolated as a white solid by evaporation under vacuum (p = 0.1 kPa) at room temperature and dark conditions. Gas Solubility Measurements. In this work, we measured the gas solubility of C3H8 and C3H6 in the ionic liquids EMImBF4, HMImBF4, OMImBF4, BMImNO3, BMImTf2N, MOOONTf2N, and BMMMNTf2N using an autoclave and the isochoric saturation technique. This method calculates the amount of gas absorbed by knowing the total reactor volume, subtracting the amount of gas which is in the vapor phase from the total amount of gas added. More details about the experimental procedure can be found in our previous works.27,29 Gas solubilities were obtained for the pure ionic liquids at different pressures between (0 and 700) kPa and temperatures ranging from (288 to 318) K. Furthermore, the effect of the addition of a suitable silver salt (with the same anion as the ionic liquid) in the range (0 to 0.77) mol·kg−1 of ionic liquid on the solubility of both gases has been analyzed, and the reaction parameters (equilibrium constants and enthalpies of reaction) between the silver cations and propylene were determined. The silver concentration in the ionic liquids was periodically checked, observing that it remained constant for periods longer than one year, proving the stability of the silver ions in the reaction media. Each experiment was performed twice, and the experimental error was determined. The weighted standard deviation, defined by eq 1, was calculated leading to values of σw < 2 %, concluding that the experiments were replicable. σ=

Figure 1. Physical solubility of C3H8 and C3H6 in EMImBF4.

in Table 2. In this table we include some previously published data29,31 to facilitate the analysis and comparison. From Table 2 it can be observed that in the nine ionic liquids and for both gases the gas solubility decreases with increasing the temperature of the system. This dependence with temperature can be described using an Arrhenius type equation (eq 3).32 H(T ) = H0e−ΔHsol / RT

where H is the Henry’s law constant at a given temperature −1 (mol·kg−1 IL ·kPa ), H0 is the pre-exponential factor, ΔHsol is the solvation enthalpy (kJ·mol −1 ), R is the gas constant (kJ·mol−1·K−1), and T is the temperature (K). Therefore, the logarithm of the H values of propylene and propane were plotted against the reciprocal of the temperature (Figure 2), showing a linear relationship. From this linearization, the solubility parameters such as the preexponential factor (H0) and the standard solvation enthalpy for every IL were obtained (Table 3). Gas Solubility in Ag+-RTILs Media. To improve the capacity and selectivity of absorption for propylene, silver cations as complexation agent were added to the ionic liquid, and the effect of the temperature, silver concentration, and pressure in the separation performance was analyzed. The isotherms of propylene solubility with the silver reactive media exhibited a nonlinear trend with pressure as shown in Figure 3 for the case of EMImBF4-silver salt mixtures. As previously expected, the data showed positive influences of both, the gas pressure as well as the silver concentration on the solubility of propylene. However, for the system BMImNO3−AgNO3 the propylene solubility showed a linear dependence with pressure, providing the same solubility values as the pure BMImNO3. This fact indicates that the system containing NO3− resulted ineffectively to carry out the reactive absorption of propylene which is in good agreement with the results obtained by Galán18 for ethylene absorption. However, the solubility of propylene increases with decreasing the temperature, showing that the absorption of propylene is an exothermic process which can be enhanced at low temperatures (Figure 4). The equilibrium constants (Keq1, Keq2) for the complexation reactions (reactions a and b) depend on the temperature and can be described by the van’t Hoff equation.33

n ′ )/Cexp]2 ∑i = 1 [(Cexp − Cexp

n−1

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(1)

RESULTS AND DISCUSSION Physical Solubility in RTILs. To further examine the role of physical solubility in the absorption equilibrium curves, silver-free ionic liquids were used as absorption media. In previous works it was observed that the equilibrium curves for propylene and propane generated with the silver-free ionic liquids showed an ideal Henry’s law behavior in the studied range of pressures. Furthermore it was noted from solubility data that there was a higher affinity of RTILs for propylene than for propane. As a representative, Figure 1 shows solubility values obtained in this work using pure EMImBF4 as absorbing liquid. Results obtained with this ionic liquid follow the general observed trend, where the gas solubility increases with pressure (linearly) and decreases with temperature and propylene is slightly more soluble than propane. In this work we use Henry’s law has been defined according to the following equation: Hi =

Ci Pi

(3)

(2) −1 mol·kg−1 IL ·kPa ,

where Hi is the Henry’s law constant in Ci is the concentration of the gas in the liquid phase in mol·kg−1 IL , and pi is the gas partial pressure in kPa. The Henry’s law constants for both gases at different temperatures are collected 2149



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−1 Table 2. Henry’s Constants (mol·kg−1 IL ·kPa ) of C3H8 and C3H6 in the Ionic Liquids under Study at Different Temperatures

T = 288 K C3H8 −4

EMImBF4 BMImBF429 HMImBF431 OMImBF431 BMImNO3 BMImTf2N31 BMPyBF429 MOOONTf2N BMMMNTf2N

2.5·10 3.7·10−4 7.9·10−4 9.4·10−4 3.8·10−4 7.5·10−4 4.0·10−4 3.0·10−3 7.4·10−4

T = 298 K C3H6

C3H8

−4

−4

4.9·10 8.5·10−4 1.4·10−3 1.5·10−3 6.6·10−4 1.0·10−3 8.7·10−4 3.0·10−3 1.2·10−3

1.5·10 2.3·10−4 5.6·10−4 7.8·10−4 2.4·10−4 5.2·10−4 2.9·10−4 2.1·10−3 5.0·10−4

T = 308 K C3H6 −4

3.8·10 5.7·10−4 1.1·10−3 1.1·10−3 5.4·10−4 6.6·10−4 6.7·10−4 2.2·10−3 9.3·10−4

C3H8 −4

1.0·10 1.8·10−4 4.2·10−4 6.4·10−4 1.9·10−4 4.0·10−4 2.0·10−4 1.4·10−3 3.9·10−4

C3H6 2.8·10−4 4.6·10−4 7.8·10−4 8.8·10−4 3.6·10−4 5.2·10−4 5.4·10−4 1.6·10−3 6.7·10−4

Figure 2. (A) C3H8 and (B) C3H6 Henry’s constant as a function of the temperature.

where KEq is the equilibrium constant in kgIL·mol−1, T is the temperature (K), ΔHr is the enthalpy of reaction (kJ·mol−1), and R is the gas constant (kJ·mol−1·K−1). To successfully describe the absorption process in our previous work, an equilibrium model based on the formation of two organometallic complexes between silver cations and propylene was proposed.27 The unknown parameters such as the enthalpies of reactions (ΔHr1 and ΔHr2) as well as the

KEq1

Ag + + C3H6IL ←→ Ag +(C3H6)IL

(a)

KEq2

Ag +(C3H6)IL + C3H6IL ←→ Ag +(C3H6)2IL d ln KEq d(1/T )

=−

ΔHr R

(b)

(4) 2150



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Table 3. Solubility Parameters of C3H8 and C3H6 in Pure Ionic Liquids C3H8 EMImBF4 BMImBF429 HMImBF4 OMImBF4 BMImNO3 BMImTf2N BMPyBF429 MOOONTf2N BMMMNTf2N

C3H6

−1 H0/(mol·kg−1 IL ·kPa )

ΔHsol/(kJ·mol−1)

−1 H0/(mol·kg−1 IL ·kPa )

ΔHsol (kJ·mol−1)

2.74·10−10 4.92·10−9 5.86·10−8 2.59·10−6 6.86·10−9 3.69·10−8 1.54·10−8 3.64·10−8 4.17·10−8

−32.8 −26.8 −22.7 −14.1 −26.1 −23.7 −24.3 −27.1 −23.4

8.42·10−8 6.16·10−8 2.81·10−7 3.64·10−7 7.33·10−8 3.39·10−8 6.74·10−7 1.27·10−7 1.30·10−7

−20.8 −22.8 −20.4 −19.9 −21.9 −24.6 −17.1 −24.1 −21.9

accuracy of the simulation results is presented in the parity graph shown in Figure 5. Predicted concentration values (Csim)

Figure 3. Equilibrium isotherms of C3H8 and C3H6 at 298 K in EMImBF4. ●, experimental; , model fitting.

Figure 5. Parity graph of propylene solubilities for simulated and experimental equilibrium values.

are plotted versus experimental data (Cexp) obtained under the conditions studied in this work. All of the results of Csim fall within the interval Cexp ± 10 % Cexp, showing good agreement between experimental and simulated results. Next the model was run using the estimated parameters. The values of the estimated model parameters are shown in Table 4. Table 4. Enthalpies of Complexation and Equilibrium Constants for Different Temperatures

EMImBF4 BMImBF429 HMImBF4 OMImBF4 BMImTf2N BMPyBF429 BMMMNTf2N

Figure 4. Equilibrium isotherms of C3H6 in EMImBF4 with 0.77 mol·kg−1 IL AgBF4: ●, experimental; , model fitting.

equilibrium constants at 298 K (KEq,1(298K), KEq,2(298K)) were estimated using the experimental data series and the parameter estimation tool of the software Aspen Custom Modeler. A comparison was made between simulated and experimental values (as representative, see Figures 3 and 4). The weighted standard deviation between experimental and simulated values of the reactive gas absorption was calculated as σw = 3 %. The

KEq,1 (298K)/ (kgIL·mol−1)

KEq,2(298K)/ (kgIL·mol−1)

ΔHr1/ (kJ·mol−1)

ΔHr2/ (kJ·mol−1)

514 406 54.9 54 185 196 38.6

7.38 5.89 0.57 0.76 8.41 31.1 10.4

−16.1 −13.1 −64.8 −80.1 −18.4 −21.3 −29.4

−18.2 −40.8 −149 −64.4 −47.5 −51.6 −38.9

From the comparison between the equilibrium constants of the systems based on the BMIm cation shown in Table 4 it is possible to compare the effect of the anion in the separation performance. As previously discussed the system based on the NO3− anion did not offer any enhancement when adding the silver salt to the IL. On the contrary, although the system based 2151



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on the Tf2N− anion showed an important increase in the solubility of propylene in presence of the silver salt AgTf2N, the ionic liquid BMImBF4 combined with AgBF4− provided the best results in terms of propylene capacity and thermodynamic selectivity (eq 5). ⎛ CC3H6T ⎞ ⎜ ⎟ ⎝ CC3H8T ⎠ selectivity = ⎛ pC3H6 ⎞ ⎜p ⎟ ⎝ C3H8 ⎠

containing the same anion than the ionic liquid. Experiments were carried out at temperatures between (288 and 308) K and pressures ranging from (0 to 700) kPa. The parameters that characterize the physical solubility as well as the characteristic parameters of the chemical equilibrium such as equilibrium constants and enthalpies of reaction are reported showing that both the physical dissolution of both gases in the ILs and the complexation reaction between propylene and silver cations are exothermic processes. Moreover, it was observed that the increment of the number and length of substituents of the ionic liquids has a negative effect in the separation performance. Furthermore, based upon the experimental results it can be concluded that the IL containing the silver salt with the BF4− anion provided the best results in terms of olefin capacity and separation selectivity compared to the anions Tf2N− and NO3− that was proven not to be effective to carry out the reactive absorption of the olefin. The results reported in this manuscript evidence that from a thermodynamic point of view the separation of propane/propylene gas mixtures by reactive absorption could represent an efficient alternative to the traditional separation process based on cryogenic distillation, and we provide the equilibrium parameters required for the design of the new process.

(5)

Figure 6 shows the effect of the pressure on the thermodynamic selectivity. The selectivity reached the highest

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ASSOCIATED CONTENT

S Supporting Information *

Experimental solubilities of propylene and propane in different ionic liquids at different temperatures, pressures, and silver concentrations. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 6. Effect of the pressure on the thermodynamic selectivity with a [Ag+] = 0.77 mol·kg−1 IL and 298 K.

value at the lowest gas partial pressure because the physical solubility effects were dominated by the chemical complexation effects. On the contrary, at partial pressures over 300 kPa, the selectivity starts to level out because under these conditions the silver sites were becoming saturated. As it can be seen in Figure 6 for the ionic liquids containing the same anion (EMImBF4, BMImBF4, HMImBF4, and OMImBF4) the selectivity increases when the length of the alkyl substituents in the cation decreases. This is because shorter alkyl chains provide lower physical solubility of both gases, and therefore the separation process is dominated by the effect of the chemical reaction. Therefore, based upon the experimental results, ionic liquids based on imidazolium cations with less and shorter alkyl substituents improve the selective separation of these mixtures. Regarding the structure of the anion it was gathered that ionic liquids with BF4− anion, combined with the AgBF4 silver salt, provided the best results in terms of olefin capacity and selectivity.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +34942201585. Fax: +34942201591. E-mail: ortizi@ unican.es. Funding

This research has been funded by the Spanish Ministry of Ministry of Economy and Competitiveness (Projects CTQ2008-00690/PPQ and ENE2010-15585). Marcos Fallanza also thanks MINECO for the FPI fellowship. Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS This manuscript reports G-L equilibrium data of the absorption of propane and propylene gases in (i) seven ILs with different types of cation, anion, and number and length of alkyl substituents, and (ii) in the reactive media consisting of the solutions of a suitable silver salt in a concentration range of (0 to 0.77) mol·kg−1 IL in the mentioned ILs. We have analyzed the effect of the structure of the IL on the propane/propylene selectivity. First of all the physical solubility was evaluated for all the ionic liquids, and subsequently the equilibrium isotherms of both gases were measured in presence of a silver salt 2152



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