Cross-linked mixed matrix membranes (MMMs) consisting of amine-functionalized multi-walled carbon nanotubes and P84 polyimide for organic solvent nanofiltration (OSN) with enhanced flux
Graphical abstract
Introduction
Organic solvent nanofiltration (OSN), also known as solvent resistant nanofiltration (SRNF), is an emerging membrane technology. OSN has been proposed for a variety of industries such as food, petrochemical, semi-conductor, and especially pharmaceutical. It not only aims to separate active pharmaceutical ingredients (APIs) with a molecular weight (MW) of 200–1000 g/mol from organic solvents but also to recycle the valuable organic solvents used in pharmaceutical and chemical syntheses. In the old days, the majority of waste organic solvents were burnt and emitting carbon dioxide (CO2) and toxic gases. Driven by the growingly stringent environmental regulations and economic benefits, the recovery of waste organic solvents become vital for the pharmaceutical and chemical industries. Thus, the development of eco-friendly, cost-effective and energy-efficient OSN processes is under high demands to replace conventional energy intensive separation methods such as distillation and CO2 emitting incinerators [1], [2], [3], [4], [5], [6], [7], [8].
The heart of OSN processes is the solvent resistant nanofiltration membrane. Ideally, the OSN membranes should have a pore size of about 0.5–2.0 nm for the recovery of waste organic solvents and the purification and separation of APIs because most APIs have molecular weights of 300–1000 Da [7]. However, it is still very challenging to design such OSN membranes due to the chemical instability of conventional polymers in organic solvents [1], [2], [3], [4], [5], [6], [7], [8]. The polyimide (PI) family generally has good chemical stability, but it still faces difficulties in aggressive organic solvents like dimethylsulfoxide (DMSO), dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), etc. Various diamines have been employed to induce cross-linking reactions and improve its chemical resistance in harsh environments [9], [10], [11], [12], [13], [14], [15]. Among them, p-xylenediamine (PDA) shows effectiveness to boost the chemical resistance and separation performance of P84 membranes for pervaporation [10], [12], while 1,6-hexanediamine (HDA) for OSN applications [13], [14].
Although the diamine cross-linking method improves chemical stability and selectivity, it also causes a drastic flux decline [10], [11], [12], [13], [14], [15]. To overcome it, mixed matrix membranes (MMMs) consisting of inorganic fillers and polymer matrices have been proposed. Currently, quite a few MMMs have been fabricated for gas, pervaporation, and water applications using silica, zeolites, metal organic frameworks (MOFs), graphene oxide, and carbon nanotubes (CNTs) as fillers [16], [17], [18], [19], [20], [21], [22], [23], [24]. However, the number of publications on MMMs for OSN are limited [22], [25], [26], [27], [28], [29].
Many reports have demonstrated the benefits of using carbon nanotubes (CNTs) as nanofillers in MMMs to boost separation performance because CNTs have unique properties such as low mass density, high flexibility, effective π–π stacking interaction with aromatic compounds, low frictional coefficients on their internal surface, a large ratio of length to diameter, and existence of substantial nanochannels [30], [31], [32], [33], [34], [35]. However, the poor or unstable dispersity in common solvents has prevented CNTs from extensive usages [30], [32]. One must either modify CNTs with functional groups such as -OH, -COOH, -COH, or -NH2, or non-covalent adsorption of molecules to improves their dispersity in solvents [33], [34]. In addition, the functionalized CNTs (F-CNTs) may induce various reactions with the polymer matrices such as amidation, esterification, alkylation, silanation, or thiolation and improve their interfacial properties [34], [35], [36]. As a result, -NH2 and -OH functionalized CNTs could form a good dispersion in P84, and the resultant MMMs had uniform CNT distributions [33].
Regardless of the numerous advantages, there is only one study on F-CNTs based MMMs for OSN applications [22]. The MMMs were fabricated from a material consisting of carboxyl functionalized multi-walled carbon nanotubes (MWCNTs-COOH) and P84 with the aid of cross-linking and annealing. Compared with the pristine cross-linked P84 membrane, the cross-linked MMMs had a higher flux but a lower dye rejection. After annealing, the cross-linked MMMs exhibited a very high rejection (almost 100% to Safranin O), but a very low flux due to the trade-off relationship between flux and rejection.
Since the amine-functionalized graphene oxide (GO) had better interactions with fluoro-containing polyimide via amidation reaction [18], it inspired us to explore if one can use amine-functionalized multi-walled carbon nanotubes (NH2-MWCNTs) instead of previous MWCNTs-COOH to fabricate MMMs for OSN applications. Therefore, the objectives of this work are to (1) design a high-flux MMM comprising NH2-MWCNTs and P84 for OSN applications, (2) investigate the amidation reaction between the amine groups of NH2-MWCNTs and the imide groups of P84, and (3) study the fundamental science behind the selectivity enhancement by thermal annealing. This study may provide helpful insights to develop advanced OSN membranes for future.
Section snippets
Materials
A commercial polyimide (P84 polyimide) was acquired from HP Polymer GmbH (Austria) and used after vacuum drying overnight at 120 °C. Amine-functionalized multi-walled carbon nanotubes (NH2-MWCNTs) with an outer diameter of 10–20 nm and a length of 0.5–2 µm were ordered from Mknano (Canada) and dried in a vacuum oven overnight at 60 °C before usage. Besides, the physical and chemical properties from XPS, and the morphology of the NH2-MWCNTs are provided in Tables S1, S2 and Fig. S1. The anhydrous
Chemistry analyses
Fig. 2(a) shows the ATR-FTIR spectra of the pristine P84 membrane (M0) and NH2-MWCNT/P84 MMMs (M2 and M4) before and after the HDA modification. The non-cross-linked membranes exhibit strong absorptions at 1360 cm−1 (C–N stretching), 1718 cm−1 (C=O stretching), and 1774 cm−1 (C=O stretching), which are representative peaks of the imide groups [22]. The intensities of these peaks reduce significantly after the cross-linking modification, while new peaks appear at 1546 cm−1 (C–N stretching) and 1643 cm
Conclusion
Diamine cross-linked mixed matrix membranes (MMMs) comprising amine-functionalized carbon nanotubes (NH2-MWCNTs) and P84 polyimide for OSN have been successfully fabricated. The effects of NH2-MWCNTs incorporation into the P84 matrix as well as thermal annealing were systematically studied. The following conclusions can be drawn from this study.
- (1)
The embedment of hydrophilic NH2-MWCNTs into P84 resulted in an amidation reaction between the imide groups of P84 and the amine groups of NH2-MWCNTs. A
Acknowledgments
The authors are grateful to National Research Foundation, Prime Minister's Office, Singapore for funding this research under its Competitive Research Program for the project entitled, “Development of solvent resistant nanofiltration membranes for sustainable pharmaceutical and petrochemical manufacture”; (CRP Award no. NRF–CRP14–2014–01 (NUS Grant no.: R-279-000-466-281)). The authors would like to acknowledge valuable suggestions provided by Ms. Pegah Nazemizadeh, Dr. Guimin Shi, and Mr.
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