Development of a simple method for predicting CO2 enhancement of H2 gas solubility in ionic liquids

• H2 solubility in ionic liquid (IL) by CO2 was evaluated with enhancement ratio (ER).
• ER correlation uses CO2 molality, Henry constants and IL solubility parameter.
• Predictive model developed for H2–CO2–IL systems.
• H2 enhancement of 20% per unit CO2 molality predicted for IL that dissolve cellulose.

The solubility of hydrogen in an ionic liquid (IL) is important for hydrogenolysis and hydrogenation used in biomass transformations. In this work, we formulate an enhancement ratio (ER) for H2–CO2–IL systems that is defined in terms of the ternary system H2 molality, the binary H2 molality at constant temperature and the isofugacity condition. Literature data were used to demonstrate the utility of the definition for ER and to develop a predictive model for ionic liquids that dissolve cellulose. The ER is greater than one when CO2 can be used to increase H2 solubility in the IL. Although the solubility of H2 in many IL can be enhanced by CO2, for some ILs the presence of CO2 reduces H2 solubility in the IL. The ER for [hmim][Tf2N] was found to be less than one at low CO2 concentrations. Temperature affected the ER of ionic liquids in different ways with some IL exhibiting maximum hydrogen solubility ([bmim][PF6]) and others having an ER that monotonically increased with temperature ([Tf2N]-containing IL). Correlation of the ER with literature data is proposed in terms of Henry's constant of H2 and CO2 and the solubility parameter of the ionic liquid. The correlation was applied to six ionic liquids at conditions of 313–453 K and at CO2 molalities up to 5 mol/kg to give a sum of squares of the residuals of 1.4 × 10−3 in ΔmH2. Solubility enhancement ratios for H2 gas by CO2 for ionic liquids that dissolve cellulose were estimated with the predictive model to be as high as 1.2 per unit molality CO2 in the IL. Vapor–liquid equilibrium measurements of mixed gases (H2–CO2, CO–CO2, O2–CO2) with ionic liquids that dissolve biomass are urgently needed to promote development of methods for transforming biomass into biofuels and chemicals.

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