Elsevier

Separation and Purification Technology

Volume 192, 9 February 2018, Pages 465-474
Separation and Purification Technology

Enhancement of CO2/CH4 separation performances of 6FDA-based co-polyimides mixed matrix membranes embedded with UiO-66 nanoparticles

https://doi.org/10.1016/j.seppur.2017.10.039Get rights and content

Highlights

  • Nanoparticles (NPs, ca. 50 nm) of CO2-philic MOF UiO-66 were synthesized.

  • NPs incorporated into three 6FDA-based co-polyimides to produce MMMs with 4–23 wt% loadings.

  • MMMs characterized by XRD, SEM, TEM, FTIR, TGA and N2, CO2 and CH4adsorption.

  • Separation performance evaluated with a 50%:50% CO2:CH4 binary mixture at 35 °C.

  • CO2/CH4selectivities of 32.0, 41.9 and 57.0 with CO2 permeabilities of 1728, 108 and 43.3 Barrer.

Abstract

Metal-organic frameworks (MOFs) incorporation into mixed matrix membranes (MMMs) is gaining more attention due to the combined advantages of high separation performance and easy processability. Nanoparticles (NPs) of CO2-philic MOF UiO-66 (Zr-BDC) were synthesized with high surface area and ca. 50 nm particle size (and also for comparison with 100 and 200 nm sizes). They were incorporated into three 6FDA-based co-polyimides (namely 6FDA-BisP, 6FDA-ODA, and 6FDA-DAM), forming MMMs with loadings in the 4–23 wt% range. The NPs and MMMs were characterized accordingly by XRD, BET, SEM, TEM, FTIR, and TGA. CO2 and CH4 isotherms on the NPs were measured by a static volumetric method at the pressure up to 10 bar. Fractional free volume (FFV) was calculated using solid density, measured by pycnometer. Gas separation performance was evaluated using a feed composition of 50%:50% CO2:CH4 binary mixture at 35 °C and a pressure difference of 2 bar. The presence of UiO-66 NPs in the continuous 6FDA-BisP and 6FDA-ODA co-polyimides improved both CO2 permeability and CO2/CH4 selectivity by 50–180% and 70–220%, respectively. In the case of 6FDA-DAM MMMs, the CO2 permeability was significantly improved by 92%, while maintaining the CO2/CH4 selectivity. The best results in terms of CO2/CH4 selectivity were 41.9 for 6FDA-BisP (17 wt% filler loading, 108 Barrer of CO2), 57.0 for 6FDA-ODA (7 wt% filler loading, 43.3 Barrer of CO2) and 32.0 for 6FDA-DAM (8 wt% filler loading, 1728 Barrer of CO2).

Introduction

The number of investigations on metal-organic frameworks (MOFs) has grown rapidly in the past few years due to their promising applications in gas storage and separation. The potential application varies from the eminent purpose of natural gas sweetening and CO2 post-combustion capture to the in-house air purification. MOFs can be classified by their three-dimensional crystalline frameworks with permanent porosity, formed with metal-based clusters linked by organic ligands [1]. The infinite possibilities of metal and linker selections in the synthesis of MOFs give researchers a variety of coordination geometry choices, i.e., tetrahedral, pyramidal or bi-pyramidal, trigonal or octahedron [2]. This design flexibility allows the MOFs to be attuned to their intended purposes. Additionally, their inherent properties are remarkable advantages, such as high CO2 uptakes (e.g. HKUST-1 of 7.32 [3] and 10.71 mmol·g−1 [4], MIL-53 of 10.02 mmol·g−1 [4], MIL-100 of 9.98 mmol·g−1 [5], MIL-101 of 7.20 mmol·g−1 [6]), open porous framework structures with large accessible pore volumes, tuneable pore affinity and most importantly their relatively high chemical and thermal stabilities. Several intensive reviews on MOFs for CO2 separation [7], [8], [9], [10] have been made available, and several others [1], [2], [11] comprehensively discussed on the MOF synthesis. The incorporation of these MOFs dispersed into the polymer continuous-phase as mixed matrix membranes (MMMs) has been reported using both low flux (e.g., PSF [12], PVAc [13] and PBI [14]) and high flux (e.g., rubbery PDMS [15] and glassy 6FDA-DAM [16], [17]) polymers.

Scientific attention towards the relatively new class of highly crystalline zirconium-based MOFs, especially UiO-66 (UiO: University of Oslo) grows rapidly. UiO-66 is based on a Zr6O4(OH)4 octahedron, forming 12-fold lattices connected by the organic linker, 1,4-benzene-dicarboxylate (BDC) (Fig. 1a) [18]. This zirconium terephthalate has high surface area, of experimental values 850–1300 m2·g−1 [12], [19], [20], [21], and the theoretically accessible surface of 1021 m2·g−1 [22]. The microporous framework composes of centric octahedral cages (ca. 11 Å) each connect with eight corner tetrahedral cages (ca. 8 Å) by means of trigonal windows (ca. 6 Å). The crystal face-centered-cubic contributes to its high stability towards heat (reported between 430 and 540 °C [23], [24]), pressure [25], water [25], [26], common solvents [25], even strong acid (HCl) and base (NaOH) [24]. The UiO-66 also possesses low heat adsorption with the increase of CO2 and CH4 loading, due to its bulky and non-polar aromatic ring which sterically hinders the highly adsorptive metal cluster to adsorb the heat [22], [27]. This is another added-value feature which is very desirable for thermal stability and lower cost regeneration.

Khdhayyer et al. [28] have recently published their findings regarding the incorporation of UiO-66 into the highly permeable polymer of intrinsic microporosity (PIM-1). The CO2 permeability was increased to 7610 Barrer, obtaining a 60% improvement with 23 wt% of UiO-66 loading. The CO2/CH4 selectivity, however, decreased with loading more than 9 wt%. Castarlenas et al. [12] reported H2/CH4 and CO2/CH4 separation with UiO-66 MMMs, where the H2/CH4 selectivity improved by 6.5% in polysulfone Udel® 3500-P and 7.7% in polyimide Matrimid® with 32 wt% loading. Remarkable H2 permeability improvements of 475% and 148% were recorded for the stated MMMs, respectively. They also reported a 3-fold CO2 permeability enhancement in the CO2/CH4 mixed gas separation, while the selectivity increased by 21% and 31%, respectively for Udel® 3500-P (32 wt% UiO-66) and Matrimid® (16 wt% UiO-66). Nik et al. [19] optimized 6FDA-ODA gas separation performance by incorporating 25 wt% of the MOF. They improved the CO2 permeability by 3.5 folds while maintaining the CO2/CH4 selectivity. Anjum et al. [21] also obtained an enhancement in membrane CO2/CH4 separation performance when embedding 30 wt% UiO-66 in polyimide Matrimid®. Shen et al. [29] utilized polyether block amide (PEBAX MH 1657) for their CO2/N2 binary gas MMM and achieved the best selectivity with 7.5 wt% UiO-66 loading. The selectivity and CO2 permeability were improved by 31% and 73%, respectively. Higher loading addition, unfortunately, decreased the CO2/N2 selectivity, even to a lower performance than that of the base polymer. Several publications have been made on UiO-66 MMMs for different applications, such as pervaporation [26], nanofiltration [30] and reverse osmosis [31].

We aimed to enhance CO2/CH4 gas separation of low and high fluxes 6FDA-polyimides, by making MMMs with different loadings of MOF UiO-66 nanoparticles. The nanoparticles, 50, 100 and 200 nm in size, were incorporated into three types of 6FDA based copolyimides with different aromatic diamine moieties, namely 6FDA-BisP, 6FDA-ODA and 6FDA-DAM. The chemical structures of the glassy polyimides are presented in Fig. 1b–d.

Section snippets

UiO-66 syntheses

The synthesis of the UiO-66 nanoparticles (ca. 50 nm in size) was conducted accordingly to the literature [32], at 1–1 M ratio of zirconium (IV) chloride (ZrCl4, ≥99.5% trace metal basis, Sigma-Aldrich) to benzene-1,4-dicarboxylic acid (BDC, 98%, Sigma-Aldrich) in N,N-dimethylformamide (DMF, ≥99.9%, Sigma-Aldrich) with a small addition of water. Commonly, 1.71 mmol (0.40 g) of ZrCl4 was dissolved in 100 mL of DMF at room temperature by sonication, before the addition of equimolar BDC (0.28 g)

Filler characterization

UiO-66 nanoparticles with size ca. 50 nm and BET surface area of 951 ± 14 m2·g−1, close to the accessible theoretical surface of 1021 m2·g−1 [22], were synthesized. Fig. 2a (inserted) shows the XRD pattern of the UiO-66 in good agreement with the literature [24]. Fig. 2a corresponds to the TGA characterization, where the negligible weight loss below 100 °C is suggested to be an initial solvent loss, while the latter drop until 300 °C is attributed to the dehydration of the Zr6O4(OH)4 nodes to Zr

Conclusion

We report the successful synthesis of high surface area Zr-based MOF UiO-66, with a uniform nanoparticle size of ca. 50 nm, appropriate crystallinity, and excellent thermal stability, as well as the fabrication of UiO-66 mixed matrix membranes with three 6FDA-based co-polyimides. Upon obtaining excellent MOF-polymer interaction with ca. 50 nm UiO-66 (and less agglomeration than using 100 and 200 nm particles), the presence of the MOF contributed to the increase of the membrane free volumes. The

Acknowledgements

The authors acknowledge the financial support of EACEA/European Commission, within the “Erasmus Mundus Doctorate in Membrane Engineering – EUDIME” (ERASMUS MUNDUS Programme 2009-2013, FPA n. 2011-0014, SGA n. 2012-1719), Operational Programme Prague – Competitiveness (CZ.2.16/3.1.00/24501), “National Program of Sustainability“ of Czech Republic, (NPU I LO1613) MSMT-43760/2015 and (MAT2016-77290-R) from the Spanish MINECO and FEDER, the Aragón Government (DGA, T05) and the European Social Fund.

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