Solventen in membraansynthese en hun effect op NF/RO performantie: van conventionele organische solventen naar ionische vloeistoffen

Membrane technology has grown significantly over the last decades and is used in a broad range of applications nowadays. Nanofiltration (NF) and reverse osmosis (RO) are applied for the separation of low molecular weight components (< 1000 Da) and salts from the feed stream. The main part of the commercial NF and RO membranes are either integrally skinned asymmetric (ISA) or interfacially polymerized thin film composite (TFC) membranes. Polyamide (PA) TFC membranes are the standard in aqueous NF and RO applications, thanks to their very thin, dense top layer, able to form hydrogen bonds with water. For solvent-resistant nanofiltration (SRNF) applications, mainly ISA membranes are applied currently, which are very simple and fast to prepare. Unfortunately, they often suffer from rather low solvent permeances, associated with their thicker selective layer compared to that of TFC membranes. Therefore, the application of TFC membranes in SRNF is currently intensively investigated. The solvents used in the synthesis and post-treatment of (SR)NF and RO membranes have an impact on several aspects of the preparation process, like monomer and polymer solubility, monomer diffusion coefficients, solvent exchange rate and degree of swelling of the membrane. Therefore, they largely influence the chemical and morphological properties of the resulting membranes, and can thus significantly improve their performance. However, in interfacial polymerization, very similar solvents have always been applied to prepare the top layer, limiting the potential to obtain an optimized performance. Solvent post-treatments are often applied to improve this performance after synthesis, but the mechanism behind these treatments is still largely unclear. Therefore, in this PhD, the importance of the solvent type in interfacial polymerization and in post-synthesis solvent treatments was investigated. This resulted in important improvements in both the synthesis procedures and in membrane performance. The first part of this thesis focused on the potential to use ionic liquids (ILs) as reaction medium in interfacial polymerization, by replacing either the standard hexane or aqueous phase by an IL. As the physicochemical properties of ILs differed largely from those of conventional solvents, their use affected top layer formation in several ways. The replacement of hexane by an IL led to multiple advantages in the synthesis process. Not only the concentration of the amine monomer used for top layer formation could be reduced drastically, also the addition of commonly used additives could be omitted. By recycling the IL for use in consecutive interfacial polymerization cycles, the mass intensity of the top layer formation process decreased with 64%, resulting in a 52% lower mass intensity compared to the conventional interfacial polymerization. Also the residual acyl chloride monomer in the IL after top layer formation could be recycled, as the IL protected it from hydrolysis by lowering the reactivity of dissolved water molecules. Since the top layers formed via the IL-based interfacial polymerization were thinner, smoother, more hydrophilic, and showed a higher free volume size, they obtained a higher permeance and a significantly lower colloidal and organic fouling tendency. In the second part of this research, post-synthesis solvent treatments of both TFC and ISA membranes were studied in detail to further enhance membrane performances. Solvent activation of TFC membranes is a frequently used technique to improve the RO and SRNF performance of this type of membranes. It generally results in a drastic increase in permeance, while no decrease in selectivity is observed. Despite the clear benefits of this solvent treatment, the mechanism behind it still has been unclear. In this work, the occurrence of PA oligomer leaching from the top layer during solvent activation was proven, and an attempt was made to further optimize the leaching process. Since a similar treatment could possibly have a comparable effect on other types of membranes than these TFC membranes, the influence of a solvent treatment on the morphology and performance of ISA polyimide (PI) membranes was also investigated. The membrane was first cross-linked chemically to enable the use of harsh organic solvents. As this type of membranes is totally composed of preformed, high molecular weight polymers, no oligomeric fragments could leach during the treatment, and therefore, no increase in permeance was observed here. Instead, the permeance drastically decreased and the retention increased after immersion in DMF, caused by densification of the membrane skin layer. The degree of densification was related to the polymer-solvent affinity, resulting in a varying degree of swelling and subsequent reorganization of the polymer chains. Besides the possibility to establish more energetically favorable interchain interactions during this reorganization, densification was also driven by extra cross-linking during immersion, due to a facilitated contact between the solvated, flexible polymer chains and partly unreacted cross-linker molecules. This simple treatment could transform ultrafiltration membranes into highly permeable membranes with selectivities in the (SR)NF range, showing an up to 400% higher solvent permeance compared to commercial SRNF membranes.CHAPTER 1 Introduction and scope of the thesis 1 1.1 Membrane technology 2 1.1.1 Classification 2 1.1.2 Performance 3 1.2 Pressure-driven membrane processes 4 1.2.1 Classification 4 1.2.2 Transport mechanism 4 1.2.3 Applications 5 1.3 Preparation of thin film composite membranes 6 1.3.1 Phase inversion 6 1.3.2 Cross-linking 7 1.3.3 Interfacial polymerization 8 1.3.4 Post-treatments 13 1.4 Characterization of the interfacial polymerization mechanism and top layer properties 14 1.4.1 In-situ techniques 14 1.4.2 Simulations 14 1.4.3 Post-synthesis characterizations 15 1.5 Ionic liquids 16 1.5.1 Properties 16 1.5.2 Synthesis 17 1.5.3 Applications 18 1.6 Thesis objectives and outline 18 CHAPTER 2 Preparation of high-performance thin film composite membranes using ionic liquids as the organic reaction medium Fundamental study 21 2.1 Introduction 23 2.2 Experimental 24 2.2.1 Materials 24 2.2.2 Membrane synthesis 25 2.2.3 Membrane performance 25 2.2.4 Membrane characterization 26 2.2.5 MPD mass transfer 27 2.2.6 MPD solubility in the organic phase 27 2.3 Results and discussion 28 2.3.1 Influence of synthesis conditions 28 2.3.2 Further top layer characterization 38 2.3.3 RO performance and fouling tendency 41 2.4 Conclusions 43 2.5 Acknowledgements 44 CHAPTER 3 Preparation of high-performance thin film composite membranes using ionic liquids as the organic reaction medium Optimization 45 3.1 Introduction 47 3.2 Experimental 48 3.2.1 Materials 48 3.2.2 Membrane synthesis 49 3.2.3 Membrane performance 49 3.2.4 Membrane characterization 49 3.2.5 Other measurements 50 3.2.6 Mass and solvent intensity 50 3.3 Results and discussion 50 3.3.1 Ionic liquid drying 50 3.3.2 Reaction time 52 3.3.3 Rinsing time 54 3.3.4 TFC membrane drying 55 3.3.5 Ionic liquid recycling 57 3.3.6 Mass and solvent intensity 62 3.4 Conclusions 62 3.5 Acknowledgements 63 CHAPTER 4 Preparation of high-performance thin film composite membranes by replacing the aqueous phase in interfacial polymerization by an ionic liquid 65 4.1 Introduction 67 4.2 Experimental 68 4.2.1 Materials 68 4.2.2 Membrane synthesis 69 4.2.3 Membrane performance 70 4.2.4 Membrane characterization 70 4.2.5 TMC mass transfer 70 4.3 Results and discussion 70 4.3.1 Determination of the optimal monomer concentrations 71 4.3.2 Position of the reaction zone 71 4.3.3 Further top layer characterization 75 4.3.4 Improvement of the performance 79 4.4 Conclusions 82 4.5 Acknowledgements 83 CHAPTER 5 Solvent activation on interfacially polymerized SRNF membranes: elucidation of the mechanism 85 5.1 Introduction 87 5.2 Experimental 88 5.2.1 Materials 88 5.2.2 Membrane synthesis 88 5.2.3 Membrane performance 89 5.2.4 Membrane characterization 89 5.2.5 Characterization of PA oligomers 90 5.3 Results and discussion 90 5.3.1 Fundamental understanding 90 5.3.2 Optimization of solvent activation effect 96 5.4 Conclusions 99 5.5 Acknowledgements 99 CHAPTER 6 Transformation of cross-linked polyimide UF membranes into highly permeable SRNF membranes via solvent annealing 101 6.1 Introduction 103 6.2 Experimental 104 6.2.1 Materials 104 6.2.2 Membrane synthesis 104 6.2.3 Membrane performance 104 6.2.4 Membrane characterization 105 6.2.5 Interaction parameters 105 6.3 Results and discussion 106 6.3.1 Principle 106 6.3.2 Solvent type 107 6.3.3 Immersion time 112 6.3.4 Degree of cross-linking 113 6.3.5 Formation of nanofiltration membranes 115 6.4 Conclusions 117 6.5 Acknowledgements 117 CHAPTER 7 General conclusions and perspectives 119 7.1 General conclusions 120 7.1.1 ILs as solvent in interfacial polymerization 120 7.1.2 Post-synthesis solvent treatments 121 7.2 Future prospects and challenges 123 7.2.1 RO applications 123 7.2.2 SRNF applications 124nrpages: 166status: publishe