Article pubs.acs.org/IECR
High Solubilities for Methane, Ethane, Ethylene, and Propane in Trimethyloctylphosphonium Bis(2,4,4-trimethylpentyl) Phosphinate ([P8111][TMPP]) Xiangyang Liu,†,# Elia Ruiz,†,⊥ Waheed Afzal,†,§,⊗ Víctor Ferro,⊥ José Palomar,⊥ and John M. Prausnitz*,†,§ †
Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California 94720-1462, United States Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States # MOE Key Laboratory of Thermo-Fluid Science and Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China ⊥ Sección de Ingenierı ́a Quı ́mica (Departamento de Quı ́mica Fı ́sica Aplicada), Universidad Autónoma de Madrid, Cantoblanco, Madrid 28049, Spain ⊗ Institute of Chemical Engineering and Technology, University of the Punjab, Lahore 54590, Pakistan §
ABSTRACT: Solubilities are reported for methane, ethane, ethylene, and propane in trimethyloctylphosphonium bis(2,4,4trimethylpentyl) phosphinate [P8111][TMPP] from 299 to 323 K up to 4 MPa. Consistent with prediction of COSMO-RS, [P8111][TMPP] shows large solubilities for all four hydrocarbons. Contrary to solubility data in conventional ionic liquids, in [P8111][TMPP] the solubility for ethane is higher than that for ethylene. When compared with solubilities in other similar phosphonium-based ionic liquids, the cation has only a small influence on the solubilities of small hydrocarbons. Ionic liquid [P8111][TMPP] may be useful for storage or separation of small hydrocarbons.
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INTRODUCTION Separation of small hydrocarbons (carbon number ≤ 3) is of interest in the purification of natural gas and in the petrochemical industry.1 A possible separation method is provided by absorption using an ionic liquid; such liquids have desirable properties such as negligible vapor pressure, thermal stability, and high flash-point temperature. In our earlier work,2−4, we found that several gases have high solubilities in low-density ionic liquids tetraalkylphosphonium bis(2,4,4-trimethylpentyl) phosphinate. Contrary to expectation, solubilities for ethane and propane are higher, respectively, than those for ethylene and propylene in trihexyl tetradecylphosphonium bis(2,4,4-trimethylpentyl) phosphinate ([P(14)666][TMPP]2) and tetrabutylphosphonium bis(2,4,4trimethylpentyl) phosphinate ([P4444][TMPP]3); these solubilities are similar to those in a large hydrocarbon such as dodecane.2 Following our earlier work with tetraalkylphosphonium bis(2,4,4-trimethylpentyl) phosphinate, we studied the effect of different alkyl groups on the cation on gas solubility. We present solubility data for a similar ionic liquid with trimethyl and one octyl alkyl groups on the cation: trimethyloctylphosphonium bis(2,4,4-trimethylpentyl) phosphinate ([P8111][TMPP]). Our earlier work5 compared densities and viscosities of four tetraalkylphosphonium bis(2,4,4-trimethylpentyl) phosphinate ionic liquids including [P8111][TMPP]. This work reports experimental solubilities for methane, ethane, ethylene, and propane in [P8111][TMPP] from 299 to 323 K up to 4 MPa. For solvent screening, we use computer program COSMORS (Conductor-like Screening model for Realistic Solvents) to predict solubilities of small hydrocarbons in ionic liquids. We © 2013 American Chemical Society
calculated predictions from COSMO-RS for Henry’s constants for small olefin/paraffin pairs in about 700 ionic liquids comprising more than 50 cations and anions. Predictions are in qualitative agreement with experiment.
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EXPERIMENTAL SECTION All gases were supplied by Praxair or Matheson with purity ≥99.9%. Ionic liquid [P8111][TMPP] was custom-synthetized by Ionic Liquid Technologies (Io-Li-Tech) with purity >97%. The ionic liquid was further purified by drying at 373 K for 48 h under vacuum. The Karl Fischer method (Aquaastar C2000 Titrator) was used to determine the water content in [P8111][TMPP] before and after solubility measurements, all
