The preparation of sol–gel materials doped with ionic liquids and trialkyl phosphine oxides for Yttrium(III) uptake

Abstract

A new material (IL923SGs) composed of ionic liquids and trialkyl phosphine oxides (Cyanex 923) for Y(III) uptake was prepared via a sol–gel method. The hydrophobic ionic liquid 1-octyl-3-methylimidazolium hexafluorophosphate (C₈mim⁺PF₆⁻) was used as solvent medium and pore templating material. The extraction of Y(III) by IL923SGs was mainly due to the complexation of metal ions with Cyanex 923 doped in the solid silica. Ionic liquid was stably doped into the silica gel matrix providing a diffusion medium for Cyanex 923, and this will result in higher removal efficiencies and excellent stability for metal ions separation. IL923SGs were also easily regenerated and reused in the subsequent removal of Y(III) in four cycles.

Introduction

In recent years, doping of functional molecules in sol–gels has been attracted much attention due to potential applications in numerous areas such as optics, sensors, catalysis and separations [1], [2], [3], [4], [5], [6]. The use of functionalized solid support extractants to separate metal ions and organic compounds from aqueous media is gaining popularity [7]. Extractants doped in sol–gel materials for toxic metal separation have been reduced waste generation and developed as an alternative to the well-known solvent extraction method [8], [9]. The mild reaction conditions of sol–gel process allow these doped molecules retain their chemical activities [10]. Metal ions in solution can diffuse into the porous sol–gel matrixes and chemically interact with dopant molecules. The sol–gel materials can be physically or chemically functionalized. The physical functionalization can be achieved by incorporation of organic reagent in the matrix when it is solidified. Sol–gel silicas physically functionalized with different organic reagents such as crown ether and 1,5-diphenylcarbazide (DPC) have been prepared and applied for extraction and separation of metals [11], [12]. However, extractants doped in solid silica are difficult to contact with metals in aqueous solution. Abundant extractants are required in the sol–gel process for removing most of metal ions from aqueous solution. Makote et al. [13] reported a sol–gel glass immobilized crown ethers and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EtMeIm⁺Tf₂N⁻), a room-temperature ionic liquid, for sequestration of Sr²⁺. The partition coefficient of sol–gel glass doped with crown ether and EtMeIm⁺Tf₂N⁻ was higher than that without EtMeIm⁺Tf₂N⁻ because of the diffusion of crown ether in EtMeIm⁺Tf₂N⁻ was much faster than in the solid silica. Methanol used in the preparation of this sol–gel glass was volatile and relatively toxic. Ionic liquids have received much interest recently for their unique physicochemical properties, such as negligible vapor pressure, high thermal stability, tunable viscosity [14], [15]. Therefore, they have been extensively used as environmentally friendly organic solvent alternatives for various metal ions extraction [16], [17], [18], [19].

Cyanex 923, a mixture of trialkyl phosphine oxides, has been proved to be effective in solvent extraction processes (Fig. 1) [20], [21]. Separation of Y(III) from lanthanides(III) is difficult because their chemical properties are similar. Cyanex 923 has been used for the extraction of Y(III) and some lanthanides(III) from thiocyanate, nitrate, chloride, sulphate, and phosphate [22], [23], [24], [25]. The major advantage of Cyanex 923 is that Cyanex 923 is a liquid even at low ambient temperatures and completely miscible with most of the commonly used organic diluents [20]. Recently, our group has studied the separation of Sc(III) from Y(III), La(III) and Yb(III) in C₈mim⁺PF₆⁻ containing Cyanex 925 (mixture of branched chain alkylated phosphine oxides) similar to Cyanex 923 [26]. The results indicated that C₈mim⁺PF₆⁻ and Cyanex 925 system can be used to separate of Sc(III) from RE(III) by simply adjusting aqueous acidity.

In the current study, we report a simple approach produced a macro-mesoporous silica gel that contain a random, bicontinuous pore structure. This new method without using any volatile organic solvents has been developed to encapsulate ionic liquid and Cyanex 923 into silica gel. These sol–gel sorbents should be more stable under harsh environments because of the ceramic net works. The synthesis and characterization of ionic liquid and Cyanex 923 doped sol–gel sorbents, kinetics of Y(III) uptake, possible mechanism and regeneration of sorbents are discussed. The results clearly show that ionic liquid and Cyanex 923 doped sol–gel sorbents are suitable for Y(III) uptake.