Green solvents for the fabrication of polymer inclusion membranes (PIMs)

doi:10.1016/j.seppur.2019.116486

Separation and Purification Technology

Volume 239, 15 May 2020, 116486

https://doi.org/10.1016/j.seppur.2019.116486 Get rights and content

Highlights

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Fabrication of polymer inclusion membranes (PIMs) using green solvents was assessed.
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Tetrahydrofuran (THF) could be replaced by ethyl acetate or 2-methylTHF.
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2-MethylTHF can be used to fabricate poly(vinyl chloride) (PVC)-based PIMs.
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Ethyl acetate dissolves poly(vinylidene fluoride-co-hexafluoropropylene)
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PIMs prepared with green solvents exhibited similar extraction performance.

Abstract

Chemical separation based on polymer inclusion membranes (PIMs) is a “Green Chemistry” alternative to solvent extraction by drastically reducing the use of toxic and volatile solvents which are used predominantly in PIM fabrication. This paper thus assesses the suitability of non-hazardous and renewably sourced “green” solvents (i.e., acetone, ethyl acetate, 2-methyltetrahydrofuran, and dihydrolevoglucosenone (Cyrene^TM)) for the fabrication of PIMs composed of the most commonly used polymers (i.e., poly(vinyl chloride) (PVC), cellulose triacetate and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)) and extractants (i.e., di-(2-ethylhexyl) phosphoric acid and Aliquat 336). It is demonstrated that PVC- and PVDF-HFP-based PIMs fabricated with 2-methyltetrahydrofuran and ethyl acetate, respectively, exhibit extraction performance and stability similar to those of their counterparts fabricated with the conventional solvent tetrahydrofuran, even though their surface morphologies were slightly different. This result indicates that “green” solvents are a viable alternative to conventional “non-green” solvents used in the fabrication of PVC- and PVDF-HFP-based PIMs.

Graphical abstract

Introduction

The use of solvents is ubiquitous in chemistry as they are vital to many chemistry processes in research and industry [1]. However, it is important to recognise the environmental hazards, toxicity, and non-sustainable sourcing associated with current solvent usage. Although solvent use cannot be entirely eliminated, recent research is being focused on finding “greener” alternatives to current solvents [2], [3].

Solvent extraction is a method commonly used in industry for the separation and recovery of a wide range of chemical species, although it involves copious volumes of solvents that are often toxic, flammable and expensive [4]. Separation based on the use of polymer inclusion membranes (PIMs), however, has proven to be a suitable alternative to conventional solvent extraction [4], [5]. The use of PIMs for the extraction and transport of metal ions and some organic compounds has grown exponentially in recent years mainly due to their high selectivity, durability and reusability [6], [7].

PIMs are composed of a base polymer, with the most common being poly(vinyl chloride) (PVC), cellulose triacetate (CTA) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), and an extractant (also known as carrier) with the most common being Aliquat 336 (a mixture of quaternary alkylammonium chlorides) and di-(2-ethylhexyl) phosphoric acid (D2EHPA). In some cases, a plasticizer (e.g., 2-nitrophenyloctylether (NPOE)) or a modifier (e.g., 1-tetradecanol) can also be included in the membrane composition to improve the membrane’s performance. The casting method commonly used for the preparation of PIMs involves the dissolution of all these components into a small volume of an appropriate solvent (i.e., tetrahydrofuran (THF) when PVC or PVDF-HFP is used as a base polymer, or dichloromethane (DCM) when CTA is used). This mixture is then poured into a cast (e.g., a Petri dish or a glass ring placed on a flat glass plate) and the solvent is allowed to evaporate slowly over a suitable period of time (e.g., 24 h). The resultant PIM is considered successful when it is flexible, self-standing and transparent, indicating a good compatibility between the membrane components.

Even though the application of PIMs for the extraction and separation of a wide range of analytes, as an alternative to solvent extraction, eliminates the need for the use of large volumes of solvents, small volumes of hazardous and non-sustainable solvents are still required for the PIM fabrication. Hence, it is of great interest to replace these solvents with “greener” and safer alternatives (i.e., renewably sourced, non-hazardous), especially considering the potential of these membranes for industrial applications at a large scale.

Following the recommendations reported by Prat et al. [2] and Pena-Pereira et al. [3], who have ranked a wide range of solvents in terms of their health, safety and environmental hazards, the solvents acetone, ethyl acetate, 2-methyltetrahydrofuran (2-methylTHF) and dihydrolevoglucosenone (Cyrene^TM) were selected as prospective greener alternatives to THF and DCM to dissolve PIM components.

Each of these greener alternatives was screened for their suitability in the preparation of PIMs with the most frequently used compositions, i.e., containing PVC, PVDF-HFP or CTA as the base polymer and D2EHPA or Aliquat 336 as the extractant. Those membranes cast successfully were then compared to PIMs prepared using conventional solvents in terms of their ability to extract Zn(II) or SCN⁻, for those membranes containing D2EHPA or Aliquat 336 as an extractant, respectively. Their stability and surface morphology were also studied.

Section snippets

Reagents and solutions

PIMs were prepared by weighting given masses of D2EHPA (97%), Aliquat® 336 (88.2–93.0%), NPOE (>99%), 1-tetradecanol (97%), PVC (high molecular weight), and PVDF-HFP (MW 400,000), all purchased from Sigma Aldrich, as well as CTA, purchased from Acros Organics-Thermo Fisher Scientific. These components were dissolved in THF (without stabiliser, 99.75%, VWR), acetone (99%, VWR), 2-methylTHF (>99%, Sigma Aldrich), ethyl acetate (99%, Chem Supply), or dihydrolevoglucosenone (Cyrene^TM, >99%, Circa

Green solvents screening

Acetone, 2-methylTHF, and ethyl acetate were selected as alternative volatile solvents for the present study based on their sustainability (i.e., produced from renewable sources), non-hazardousness and for being known as greener alternatives to THF or DCM [2], [3]. Cyrene^TM [9] was also included in this study even though it is not volatile, since it is a bio-based alternative to DCM, and to the best of our knowledge its ability to dissolve the base polymers commonly used in PIMs is unknown.

Conclusions

Renewable and non-hazardous solvents, namely acetone, ethyl acetate, 2-methylTHF and Cyrene^TM, were screened as greener alternatives to THF and DCM for the casting of PIMs. Ethyl acetate and 2-methylTHF were found to be suitable solvents for the preparation of PVDF-HFP- and PVC-based PIMs, respectively, containing either D2EHPA or Aliquat 336 as the extractant. Similar extraction performance and stability was observed when comparing PIMs prepared using the conventional solvent THF with their

CRediT authorship contribution statement

Clayton A. Carner: Investigation, Writing - original draft. Charles F. Croft: Validation, Formal analysis, Investigation, Writing - review & editing. Spas D. Kolev: Resources, Writing - review & editing, Supervision, Funding acquisition. M. Inês G.S. Almeida: Conceptualization, Methodology, Validation, Formal analysis, Writing - review & editing, Supervision, Project administration.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors wish to thank Dr Anastasios Polyzos from the School of Chemistry, The University of Melbourne, for valuable suggestions, Circa Group for kindly providing Cyrene^TM, and Dr Tian Zheng from the Materials Characterisation and Fabrication Platform at The University of Melbourne for her assistance with the AFM imaging.

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¹: These authors contributed equally to this work.

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Green solvents for the fabrication of polymer inclusion membranes (PIMs)

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Reagents and solutions

Green solvents screening

Conclusions

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgements

J. Membr. Sci.

J. Membr. Sci.

J. Membr. Sci.

Talanta

J. Membr. Sci.

J. Membr. Sci.