SBA-15-supported ionic liquid compounds containing silver salts: Novel mesoporous π-complexing sorbents for separating polyunsaturated fatty acid methyl esters

doi:10.1016/j.micromeso.2008.07.017

Microporous and Mesoporous Materials

Volume 117, Issues 1–2, 1 January 2009, Pages 436-443

https://doi.org/10.1016/j.micromeso.2008.07.017 Get rights and content

Abstract

Novel π-complexing sorbents were prepared by covalently immobilizing ionic liquids (ILs) onto mesoporous SBA-15 using a one-pot sol–gel process followed by coating these SBA-15-supported IL compounds with silver salts. The mesoporous π-complexing sorbents were characterized by small angle X-ray scattering (SAXS), FTIR, TEM, SEM, nitrogen adsorption desorption isotherm, NMR, and nitrogen elemental analysis. Two advantages were obtained using these novel mesoporous π-complexing sorbents versus the traditional π-complexing sorbents formed by directly anchoring silver salts onto silica gel. (1) Higher extraction capacities were found. The extraction capacity for the polyunsaturated fatty acid methyl ester (PUFAME), methyl all-cis-5,8,11,14,17-eicosapentaenoate (20:5 or EPA), was 195 mg/g sorbent using the mesoporous AgBF₄/SBA-15 · IL · PF₆ sorbent. The capacity decreased to 121 mg/g sorbent with microporous complexing sorbent AgBF₄/SiO₂ · IL · PF₆. (2) Better reusability was also achieved. The supported IL phase immobilized and retained silver salt on SBA-15 due to the interaction between the ionic liquid’s imidazolium cations and silver ions. Eight successive sorption runs with the AgBF₄/SBA-15 · IL · PF₆ sorbent showed a satisfactory reusability. The traditional π-complexing sorbent has a silver salt directly anchored on silica without the supported ionic liquid phase. Higher silver leaching into organic solution occurred from the AgBF₄/SBA-15 sorbent determined by ICP-AES. The combined percentage (wt%) of the omega-3 PUFAMEs: 20:5 and methyl all-cis-4,7,10,13,16,19-docosahexaenoate (22:6 or DHA) stripped from the AgBF₄/SBA-15 · IL · PF₆ by 1-hexene was significantly enriched from 18% in the original cod liver oil to 90.5%.

Introduction

Ordered mesoporous materials show large BET surface areas, high porosities, controllable and narrowly distributed pore sizes, and ordered pore arrangements. These materials are known to be promising candidates for applications in catalysis [1], sorbents [2] and sensors [3]. In particular, the development of functionalized mesoporous materials for the separation of heavy metal ions [4], organics [2], dyes [5], light hydrocarbons [6], gases [7], radionuclides [8], and protein [9] has generated considerable interest. Among the variety of adsorption applications, the preparation of highly effective and selective sorbents for separation and enrichment of commercially valuable natural biomolecules is clearly the most promising. Organic groups (or functional ligands) can be introduced onto meso-structured materials by post-grafting [3] or one-pot sol–gel incorporation [10]. Recently, a family of mesoporous silicas (SBA) was synthesized by using neutral organic triblock copolymers as a structure-directing template [11]. These new materials have larger pores and thicker pore walls compared with the microporous silica gel [12]. The large pore sizes of these mesoporous sorbents would generate high extraction capacities for the large biomolecules.

Ionic liquids (ILs) have been immobilized onto solid supports in order to minimize the amount of ILs used, avoid the use of organic solvents, and easily recover the catalyst. Supported ionic liquid phases (SILP) possess the advantages of both ionic liquids and heterogeneous supports. SILP viability has been confirmed by a number of studies [13] of ionic liquids supported by silica gel and their potential catalytic applications. Analytical chemistry applications of SILP for sample preparation or extractive separation were also reported [14]. Ionic liquid compounds have recently been immobilized onto SBA-15 to take advantage of its excellent chemical and thermal stability, high porosity, and large surface area. Lai et al reported the synthesis of ionic liquid-functionalized SBA-15 mesoporous materials as heterogeneous catalysts for the Knoevenagel condensation [15]. Han and coworkers studied the solvent-free Heck reaction catalyzed by Pd catalyst supported on SBA-15 via an ionic liquid [16]. Chen et al. investigated the preparation of sol–gel materials doped with ionic liquids and trialkyl phosphine oxides for Yttrium(III) uptake [17].

Omega-3 (or n-3) fatty acids are polyunsaturated fatty acids (PUFAs) found in fish oil and vegetable sources. Omega-3 fatty acids are classified as essential because they cannot be synthesized in the body [18]. They must be obtained from food. Some nutritionally important omega-3 fatty acids are α-linolenic acid (18:3, ALA), eicosapentaenoic acid (20:5 or EPA), and docosahexaenoic acid (22:6 or DHA) (Table 1). These ω-3 long chain polyunsaturated fatty acids help prevent cardiovascular disease, hypertension, inflammatory and autoimmune disorders, depression and certain disrupted neurological functions [19], [20], [21], [22]. Consequently, the production of ω-3 fatty acid concentrates continues to be a topic of interest for both the pharmaceutical and health food industries. Growing public awareness of the nutritional benefits of seafood and their ω-3 PUFAs is expected to increase the ω-3 PUFA market demand. Fish oils are complex triglyceride mixtures containing fatty acids with varying chain lengths and degrees of unsaturation. Therefore, separation of individual fatty acids is difficult during the production of highly concentrated ω-3 components. Furthermore, commercial production of fish oil concentrates with high PUFAMEs 20:5 and 22:6 contents now poses a major challenge for food scientists, analytical scientists and biotechnologists engaged in this research area.

It is possible to separate fatty acids or fatty acid methyl esters according to their degrees of unsaturation using the so-called π-complexing sorbents [23], [24] due to the specific and reversible complexation between silver ions and double bonds in unsaturated fatty acids or their methyl esters. Adsorption chromatography over layers or columns of silica gel impregnated with silver nitrate has been utilized to isolate PUFAs [25]. However, the retention of silver on silica gel was impaired because of the direct and non-covalent AgNO₃ grafting on silica gel. Methyl or ethyl esters of polyunsaturated fatty acids have been enriched by solid-phase extraction over aminopropyl-bonded silica columns [26] and bonded benzenesulfonic acid resins after complexation to silver ions [27] However, the reuse of the these solid extraction phases was not pursued in these studies. The application of mesoporous π-complexing sorbents containing a supported ionic liquid phase has not been previously reported. Here we report both the synthesis of new π-complexing sorbents (SBA-15-supported IL compounds containing silver salts) and their use in highly effective and selective extractions and enrichments of PUFAMEs from cod liver oil.

Section snippets

Synthesis of ionic liquid-modified silanes

The detailed synthesis procedures for the ionic liquid-modified silanes and their ¹H NMR, ¹³C NMR, FTIR characterizations were reported in the Supporting Information. The preparation of microporous π-complexing sorbent, AgBF₄/SiO₂ · IL · PF₆, was also described in the Supporting Information. The preparation of fatty acid methyl esters from cod liver oil and their quantification by gas chromatography–flame ionization detection (GC–FID) were reported in detail in the Supporting Information.

Synthesis of unmodified SBA-15

According

Synthesis of the mesoporous π-complexing sorbents

Hydrophobic ionic liquids containing silver salts were previously found to be ideal extraction phases for the separation and enrichment of PUFAMEs from fish oil [28]. Hydrophobic ionic liquids, such as 1-hexyl-3-methylimidazolium hexafluorophosphate, [hmim][PF₆], showed a strong silver salt retention without significant silver ion leaching occurring during the extraction or stripping. Moreover, AgBF₄ suspended in [hmim][PF₆] exhibited approximately two orders of magnitude higher extraction

Conclusions

Novel ionic liquid-modified mesoporous π-complexing sorbents have been developed which draw on the strengths of both supported ionic liquids and functionalized mesoporous material concepts. The sorbents were successfully applied to extract and enrich long chain omega-3 PUFAMEs from cod liver oil with high selectivities and capacities. Efforts to expand these novel sorbents to other analytes such as polyunsaturated free fatty acids and polyunsaturated triglycerides are currently under way in our

Acknowledgements

Partial support of this work by the Department of Energy, DE-FG36-06G086025 is gratefully acknowledged. We also appreciate Dr. Alicia Beatty and Nilantha Bandara’s help on the TGA tests.

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SBA-15-supported ionic liquid compounds containing silver salts: Novel mesoporous π-complexing sorbents for separating polyunsaturated fatty acid methyl esters

Abstract

Introduction

Section snippets

Synthesis of ionic liquid-modified silanes

Synthesis of unmodified SBA-15

Synthesis of the mesoporous π-complexing sorbents

Conclusions

Acknowledgements

Chem. Mater.

J. Mater. Chem.

Chem. Mater.

Science

J. Am. Chem. Soc.

Chem. Mater.

J. Nutr. Rev.

Am. J. Med.

J. Am. Oil Chem. Soc.

Lipids

J. Lipid Res.

Sep. Sci. Technol.

Chem. Mater.

Psychiatry Clin. Neurosci.

J. Mater. Chem.

J. Catal.

Micropor. Mesopor. Mater.

Micropor. Mesopor. Mater.

Langmuir

Ind. Eng. Chem. Res.

Ind. Eng. Chem. Res.

Electrochem. Commun.

Micropor. Mesopor. Mater.