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The Functionalization of PES/SAPO-34 Mixed Matrix Membrane with [emim][Tf2N] Ionic Liquid to Improve CO2/N2 Separation Properties

Jonathan S. Cardoso
Zhi Lin
Paulo Brito
3,4 and
Licínio M. Gando-Ferreira
CIEPQPF, Department of Chemical Engineering, Faculty of Sciences and Technology, University of Coimbra, Pólo II, Rua Sílvio Lima, 3030-790 Coimbra, Portugal
CICECO, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
Mountain Research Centre (CIMO), Polytechnic Institute of Bragança, Campus de Santa Apolónia, 5300-253 Bragança, Portugal
Associate Laboratory for Sustainability and Technology in Mountains Regions (SusTEC), Polytechnic Institute of Bragança, Campus de Santa Apolónia, 5300-253 Bragança, Portugal
Authors to whom correspondence should be addressed.
Inorganics 2023, 11(11), 447;
Submission received: 17 October 2023 / Revised: 3 November 2023 / Accepted: 15 November 2023 / Published: 20 November 2023
(This article belongs to the Special Issue Inorganic Composites for Gas Separation)


The use of ionic liquid [emim][Tf2N] as an additive in polyethersulphone (PES) and nano-sized silico-aluminophosphate-34 (SAPO-34) mixed matrix membrane was studied through the incorporation of different amounts of [emim][Tf2N] in the membrane composition, as presented in this work, varying from 10 to 40 wt%. Through gas permeation tests using CO2 and N2, the membrane composition containing 20 wt% [emim][Tf2N] led to the highest increase in CO2 permeability and CO2/N2 selectivity. The use of low concentrations of additive (10–20 wt%) promoted a state called antiplasticization; in this state, the permeability was even more regulated by the kinetic diameter of the species which, in this work, permitted achieving a higher CO2/N2 selectivity while increasing the CO2 permeability until an optimal condition. [emim][Tf2N] also promoted a better dispersion of SAPO-34 particles and an increase in the flexibility of the polymeric matrix when compared to a film with the same composition without [emim][Tf2N]. Moreover, the characterizations corroborated that the inclusion of [emim][Tf2N] increased the zeolite dispersion and improved the polymer/zeolite compatibility and membrane flexibility, characterized by a decrease in glass transition temperature, which helped in the fabrication process while presenting a similar thermal resistance and hydrophilicity as neat PES membrane, without affecting the membrane structure, as indicated by FTIR and a contact angle analysis.

Graphical Abstract

1. Introduction

Pressure Swing Adsorption (PSA), Temperature Swing Adsorption (TSA), and cryogenic distillation are industrially used in CO2 separation. However, those processes require a high energy demand and construction area, solvent post-treatment, and complex design; with this, the use of membranes in gas separation continues to be an attractive process, since it provides a lower energy consumption, is modulable, requires smaller processing area, does not require the regeneration of solvents, and can be connected to traditional processes. Mixed matrix membranes have a wide combination of polymers and fillers, showing a permeability and selectivity above the Robeson upper limit. Polyethersulfone (PES), polyimide (PI), polysulfone (PSf), and Matrimid 5218 are the common polymer matrixes, while zeolite 4A, carbon molecular sieves, and SAPO-34 are the most common fillers. Silicoaluminophosphate-34 (SAPO-34) is used for gas separation because of its pore size (0.38 nm), which is near the kinetic diameter of gases like H2 (0.29 nm), CO2 (0.33 nm), N2 (0.36 nm), CO (0.37 nm), and CH4 (0.38 nm). The incorporation of an inorganic filler in a polymeric matrix is intended to increase thermal stability and selectivity when compared to neat polymeric membranes, while keeping the characteristics of organic membranes in terms of high permeability and versatility [1,2,3,4,5,6,7,8].
However, the use of fillers in mixed matrix membranes presents a drawback. The compatibility between the polymeric matrix and the filler material is crucial. Some fillers may chemically interact with the polymer, causing degradation or undesired chemical reactions. Moreover, achieving a good dispersion and adhesion of the filler within the polymer matrix is essential. Poor adhesion can result in the formation of voids or weak interfaces, reducing the effectiveness of the filler in enhancing the membrane properties. The compatibility between the inorganic dispersed material and the organic dispersive media can lead to interfacial voids between the polymer and zeolite, creating non-selective defects that lower the separation performance of the membrane [9,10,11,12,13,14]. The use of different methods to improve the polymer/filler interaction are reported in the literature, with surface modification being the most common. It is intended to modify the filler surface to enhance compatibility with the polymer by adding functional groups or coatings to minimize chemical reactions; as presented in Table 1, several studies intend to use different surface modifiers to improve the separation performance in CO2/CH4 and CO2/N2. The use of additives in the membrane composition should improve the zeolite–polymer matrix interaction, increase dispersion, and improve separation performance [15,16,17].
Due to its dual nature, ionic liquids can help to enhance the interaction in a mixed matrix membrane. [emim][Tf2N] is intended as one of ionic liquid additives to membranes since it presents high CO2 adsorption, is chemically stable, presents a low melting point, and a cation and anion composed of organic structures, allowing it easy interaction with organic and inorganic compounds [14,23,24,25,26,27,28].
This work aims to study the influence of different concentrations of [emim][Tf2N] varying between 10 and 40 wt%, in polymer mass, in a fixed composition of PES/SAPO-34 mixed matrix membrane. Ideal CO2 and N2 permeabilities and CO2/N2 selectivities are obtained and compared to reach an optimal condition. The neat PES, mixed matrix PES/SAPO-34 and modified mixed matrix PES/SAPO-34/[emim][Tf2N] were characterized by assets of its structure, thermal properties, and hydrophilicity, which are vital to explain how the use of [emim][Tf2N] can interfere with membrane characteristics and performance.
The addition of different amounts of [emim][Tf2N] led to an optimal condition, 20 wt% of [emim][Tf2N], without compromising the MMM general structure and characteristics. The inclusion of a plasticizer additive in MMM preparation promoted an antiplasticization phase in low additive concentrations, between 10 and 20 wt%, resulting in a higher CO2/N2 selectivity when adding 20 wt% of [emim][Tf2N] than other modified PES/SAPO-34 membranes. However, the permeability of all species is lower than the ones in the literature for CO2/N2 separation. This fact may be derived from the increase in filler/polymer compatibility since the reduction in interfacial voids leads to lower permeabilities while increasing selectivity.

2. Results and Discussion

2.1. SAPO-34 Synthesis and Characterization

Figure 1 presents the XRD of the synthesized SAPO-34 sample in a 24 h reaction time and compared to a Chabazite (CHA) standard to verify the purity of the sample obtained. CHA is considered to be the standard for SAPO-34 XRD characterization by the International Zeolite Association (IZA). Both CHA and SAPO-34 present the same tetrahedral structure coordinated with four oxygen, each of which forms a bridge between neighbouring tetrahedral atoms, the combination of which results in a zeolite structure with the same planes of dispersion.
SAPO-34 was synthetized using the methodology described in Section 2.2. The resulting SAPO-34 sample presented a high purity in a 24 h reaction time according to the CHA standard diffractogram used as comparison criteria; this methodology follows the proceedings of the International Zeolite Association (IZA)
A low Si/Al molar ratio (≤0.15) leads to the formation of other structures (SAPO-5 or AlPO-34) due to Si content not being enough to substitute the Al and P atoms in the tetrahydric structure. However, a high Si/Al content leads to a low CO2 adsorption [21,29]. Table 2 presents that the XRF analysis revealed a Si/Al ratio of 0.29 for the post-synthesized SAPO-34 sample.
The adsorption–desorption nitrogen isotherm is presented in Figure 2 and the textural properties of the zeolite in Table 3 that allow for a better understanding of the zeolite characteristics according to the porous size and total area.
The adsorption–desorption nitrogen isotherms presented in Figure 3 show a type I isotherm which is characteristic for microporous materials such as SAPO-34. The extent of adsorption increases with pressure until saturation is reached, at which point, no further adsorption occurs. The presence of a small hysteresis at a high relative pressure can also be noted, indicating a low-level mesoporosity.
The textural properties in Table 3 show that the samples prepared have a large amount of micropores with a high BET area (SBET) and a Vmic comparable to those in the literature for SAPO-34. Furthermore, the low external area in the textural properties is due the nanosized crystal structure [30,31,32]. Figure 3 and Figure 4 show the DLS and SEM analysis, respectively, for the SAPO-34 sample to characterize the crystal size and distribution.
The results obtained through Dynamic Light Scattering (DLS) show a range of crystal size distribution with two main sizes, related to 100 nm and 1 µm, respectively. In addition, the average particle size obtained was 200 nm. This size is suitable to be used as filler for the mixed matrix membranes without causing a high agglomerate of crystals during the polymer dissolution promoted by crystals ≤100 nm, as discussed by Wu et al. 2019 [6].
Figure 5 presents the SEM analysis for the SAPO-34 sample with an agglomerate of particles, as described by the DLS analysis, a range of particle sizes with an average of nanosized crystals can be observed by an agglomerate of cubic shaped crystals.

2.2. Gas Permeation Tests

The films were submitted to the permeation test and the permeability and selectivity obtained using Equations (1) and (2). Table 4 and Figure 5 present the average value of permeability and selectivity for each film prepared. Figure 6 exemplifies the interaction between [emim][Tf2N] and zeolite surface. Figure 7 presents a comparison of the results obtained in this work with previous results in the literature organized in Table 1 for CO2/N2 separation using SAPO-34 in mixed matrix membranes plotted in a Robeson upper bound graph.
In Table 4, when comparing the neat PES and PES/SAPO-34 membranes, a reduction in CO2 and N2 permeabilities is noted. The addition of zeolites with molecular sieving properties causes an increase in selectivity due to their intrinsic properties, while the permeabilities of gases decrease through the increase in chain rigidification, extended diffusion pathways of penetrants through the membrane or blockage of zeolite pores.
The incorporation of different amounts of [emim][Tf2N] in the PES/SAPO-34 membrane presented an increase in CO2 permeability due the preferential site for CO2 adsorption when using [emim][Tf2N], as presented in Figure 6, helping the migration of CO2 throughout the membrane. The balance between polymer and plasticizer can result in a higher selectivity when compared to the PES/SAPO-34 membrane. In Table 4 and Figure 5, [emim][Tf2N] improved the separation performance through helping to disperse the zeolite crystals more efficiently through the membrane and anchor the zeolite particles on the membrane, allowing CO2 to permeate easily while still retaining N2 efficiently. That behaviour can be observed in CO2/N2 selectivity for 10 wt% and 20 wt% of [emim][Tf2N]. This behaviour is explained by Maeda et al., 1987, who evaluated the influence of additives in polymeric and mixed matrix membranes. The use of certain additives in low concentrations, of the order of 10 to 20% by weight, delays the segmental motions of the polymer accompanied by a suppression of the secondary relaxation mechanisms characteristic of polymers like polysulphone due to a loss in free volume; this phenomenon is called antiplasticization. Mahajan et al., 2002, later observed the same tendency [33,34]. This loss volume led to a higher blockage of N2 due its bigger kinetic diameter when compared to CO2. Moreover, the use of an additive with high CO2 adsorption promoted an increase in CO2 permeability, even through antiplasticization phase.
A further increase in the amount of [emim][Tf2N], 30–40 wt%, affects the polymer chain mobility, which increases the permeability of all species, reducing the selectivity. With that, the optimal condition stablished for this membrane composition is the addition of 20 wt% of [emim][Tf2N], which presents the best balance between an increase in the permeability of species while increasing the selectivity.
The chain rigidification can be analysed by the Glass Transition Temperature (Tg) presented and explained further in Section 2.3, while dealing with the characterization of the MMMs obtained. However, the addition of molecular sieving materials extends the diffusion pathway in all cases, if the molecular sieve mechanism is the limiting process, since it forces the molecules to travel a longer path than in the neat polymer membrane. This longer path through the polymer (solution–diffusion mechanism) and the material pore structure (molecular sieve mechanism) results in a longer permeation time, leading to a lower permeability, although the properties of the molecular sieve material led to an increase in selectivity by retaining larger molecules more efficiently. This affirmation can be observed when comparing the results for neat PES and PES/SAPO-34 due to the reduction in permeability for all species. Moreover, a pore blockage can be caused by the interaction between the polymer chain or additives and the particle surface reducing the accessibility of the pores, affecting the permeability of species according to the kinect diameter of the gases used [22,35].
The Robeson upper limit graph presented in Figure 7 shows that the PES/SAPO/[emim][Tf2N]20 membrane prepared presented a higher CO2/N2 selectivity than the PES/SAPO-34 membranes in the literature due the inclusion of [emim][Tf2N], which helped in the compatibility between zeolite and polymeric media in the optimal condition of 20 wt% of [emim][Tf2N]. Moreover, the membranes prepared presented a lower CO2 permeability than that presented in the literature. This fact derives from the reduction in interfacial voids through the use of strategies to mitigate this factor, mainly by using a drying methodology with temperature near the Tg of the membranes, as explained by Mahajan, 2002 [34], and the use of [emim][Tf2N], which reduced uncontrolled permeation across the membrane.

2.3. Characterization of Mixed Matrix PES/SAPO-34 Membranes

The PES/SAPO-34 membranes were prepared according to the methodology used to denote the influence of [emim][Tf2N] on the separation performance. According to the gas permeation results obtained, the addition of 20 wt% of [emim][Tf2N] in membrane fabrication promotes the best ideal separation performance in this composition. DMTA and TGA analyses for PES, PES/SAPO-34 membrane, and PES/SAPO-34/[emim][Tf2N]20 are depicted in Figure 8 and Figure 9, respectively.
The DMTA analysis, in Figure 8, presents the Glass Transition Temperature (Tg) at a 1 Hz frequency of 234 °C, 219 °C, and 170 °C for PES, PES/SAPO-34, and PES/SAPO-34/[emim][Tf2N]20, respectively. Those temperature express the flexibility degree of the membranes, showing that the inclusion of SAPO-34 does not lead to a rigidified polymer/zeolite interface, since those rigidified interfaces would lead to a higher Tg than neat PES. Moreover, the inclusion of [emim][Tf2N] reduces chain rigidity and improves flexibility, lowering the membrane Tg to better accommodate with the solvent evaporation. A lower Tg expresses a more flexible membrane, which can better accommodate rigid particles when compared to a more rigid membrane [34,36].
Furthermore, the TGA analysis in Figure 9 shows that the degradation temperature for PES is 565 °C, while the one for the PES/SAPO-34 is 581 °C. This is due to the addition of the zeolite particle improving the thermal stability of the membrane. Moreover, the degradation temperature for the PES/SAPO-34/[emim][Tf2N]20 membrane presents the first degradation temperature in 463 °C for [emim][Tf2N] [37,38,39,40] with a subsequent degradation of PES at 569 °C, which is comparable to neat PES. [emim][Tf2N] functions as a linking agent to reduce the stress from the interaction between the SAPO-34 crystals and bulk PES as explained by Mahajan et al., 2002 [34]. Figure 10 presents the FTIR analysis for neat PES, PES/SAPO-34, PES/SAPO-34/[emim][Tf2N]20, and [emim][Tf2N].
The three peaks observed at 1576–1577 cm−1, 1482–1484 cm−1, and 1405–1406 cm−1 indicate the presence of benzene rings. The two peaks at 1320–1321 cm−1 and 1296–1297 cm−1 indicate the presence of ether function. The two peaks at 1144–1146 cm−1 and 1100–1101 cm−1 indicate the presence of a sulfone group. All spectra are in accordance with the FTIR spectra for PES in the literature [41,42] and implies that the addition of SAPO-34 or the use of ILs does not change the membrane matrix.
Moreover, the appearance of peaks in 1351 cm−1, 1192 cm−1, 1055 cm−1, and others dislocated peaks like 618.3 cm−1 and 508.4 cm−1 in the PES/SAPO-34/[emim][Tf2N]20 membrane can be related to the inclusion of [emim][Tf2N] in the membrane, since those peaks also appear in the FTIR for [emim][Tf2N] substance. However, it is notorious in Figure 10 that the inclusion of [emim][Tf2N] presents no influence in the overall polymer chain structure.
A contact angle analysis was conducted in ambient conditions and is presented in Table 5 with the average values for neat PES, PES/SAPO-34, and PES/SAPO-34/[emim][Tf2N]20, respectively.
All membranes show a slightly hydrophilic characteristics, which is in accordance with the literature [43]. This is more pronounced with the addition of the [emim][Tf2N] due to its hydrophilicity. It is a drawback in gas separation due to the adsorption of water that can decrease the membrane separation efficiency, degrade the zeolite, and reduce the operation time of the membrane.
An SEM analysis for neat PES, PES/SAPO-34, and PES/SAPO-34/[emim][Tf2N]20 membranes is presented in Figure 11, Figure 12 and Figure 13, respectively, and a zoom in on the particles in Figure 14 for a better understanding of the particle dispersion.
Figure 11a present the SEM analysis of the neat PES membrane cross-section and surface. The dense PES membrane presents no imperfections and a similar homogeneity through the membrane. Figure 12a presents the SEM analysis of the PES/SAPO-34 membrane cross-section, and the agglomeration of crystals along the polymeric matrix could allow gases to permeate through the interface of the crystals. Figure 13a presents the SEM analysis of the PES/SAPO-34/[emim][Tf2N]20 membrane cross-section with more disperse crystals in the polymeric matrix. Figure 12b and Figure 13b present a similar surface for both membranes. Figure 14a presents a zoom in of the particle agglomeration, which could cause a reduction in selectivity due the presence of voids between particles. The use of [emim][Tf2N] helped to reduce crystal agglomeration, as presented in Figure 14b. Higher dispersion diminishes imperfections and increases the separation performance of the membrane.
Table 4 and Figure 5 combined with the SEM analysis in Figure 14 show how a well-dispersed filler can affect the membrane separation characteristics and how the use of ionic liquids can improve the separation performance due to an increase in the interaction between the zeolite and polymer, the reduced formation of aggregates, increased dispersion of the zeolite in the polymeric matrix, increased membrane flexibility to better accommodate particles, and preferential interaction with CO2 to increase permeability.

3. Materials and Methods

3.1. Materials

The polymer used as the media for the membrane in this study was polyethersulfone (PES) from Goodfellow, transparent, with a 3 mm granular size and molecular weight of 58,000 g/mol. The inorganic filler SAPO-34 was prepared using aluminium isopropoxide (≥99% purity), phosphoric acid (85%), colloidal silica HS-40, morpholine (≥99% purity), and tetraethylammonium hydroxide (TEAOH) solution (35 wt% in water) purchased from Sigma-Aldrich (St. Louis, MO, USA). PES and SAPO-34 were dried overnight at 105 °C and 200 °C, respectively, prior to use. The solvent used for membrane preparation and casting was N-methyl-pyrroline (NMP) (≥99.8% purity) from Roth. The ionic liquid used was [emim][Tf2N] (≥98%) from Sigma-Aldrich was selected for its high thermal stability and CO2 adsorption characteristics. It presents a decomposition temperature of 360 °C and was used without further purification.

3.2. Synthesis and Characterization of SAPO-34

The SAPO-34 sample was synthesized using a gel with the molar composition 1.0 Al2O3: 1.0 P2O5: 0.6 SiO2: 1.5 Morpholine: 0.5 Tetraethylammonium hydroxide (TEAOH): 70 H2O. The zeolite was prepared using a dry-gel methodology with 1:1 mass ratio of dry mass/water, heated at 200 °C for 24 h; this condition was determined by a previous study [44]. The produced mixture was then centrifuged, washed, dried at 105 °C in an oven, and calcined at 560 °C for 8 h. The samples were characterized via Powder X-Ray Diffraction (PXRD) in a PANalytical Empyrean diffractometer (PANalytical, Malvern, UK) using a Bragg–Brentano geometry and CuKα X-radiation, an Energy Dispersive X-ray spectrometry (EDS) on a Hitachi SU-70 microscope (Hitachi, Tokyo, Japan) equipped with EDS system, laser diffraction spectroscopy (LDS) in a Malvern Masterziser 2000 from Malvern Instruments (Malvern, UK) and N2 adsorption and desorption in a NOVAtouch LX4 from Quantachrome (Boynton Beach, FL, USA).

3.3. Synthesis of Mixed Matrix PES/SAPO-34 Membranes

PES/SAPO-34 membranes were prepared according to the mass composition of 20 wt% of PES and 20 wt% of SAPO-34, based on polymer mass, in NMP. The SAPO-34 zeolite was added and stirred with N-methyl-2-pyrrolidone (NMP) for 1 h and in an ultrasonic bath for 4 h. Then, a small quantity of PES was added to the NMP/SAPO-34 solution and mixed for 1 h at 80 °C. The remaining PES was added and mixed for 3 h at 80 °C, and the solution was stirred overnight at room temperature. The solutions were degassed for 4 h in an ultrasonic bath and casted with a knife of 300 µm of initial thickness in a glass plate, dried at 80 °C for 2 h under vacuum and at 200 °C for 20 h under vacuum. The PES/SAPO-34/[emim][Tf2N] membrane synthesis followed the same procedure using 10, 20, 30, and 40%w/w, respectively, of IL added in the NMP-SAPO-34 solution step.

3.4. Gas Permeations Tests

Gas permeation experiments were conducted using pure gases to determine their ideal permeability and diffusivity coefficients and the results were compared to each other in a permeability unit called Barrer ( 10 10   cm STP 3 × cm / cm 2 × s × cmHg ). A schematic representation of the permeation system is shown in Figure 15 The membranes were cut into 47 mm disks and mounted on a steel permeation cell and evaluated at feed pressure of 10 bar at room temperature. The ideal permeabilities were obtained according to Equation (1).
P i = V s l T A M B Δ p T S T P p S T P A d p d t
The permeability of a certain gas (Pi) is related to the thickness of the film (l), the fixed volume of the permeate (Vs), which is measured as standard. The ambient temperature (TAMB), the difference of pressure between permeate and retentate (Δp), the permeation area (A), and temperature and pressure in normal conditions (TSTP and PSTP). The ideal selectivity results (αab) are represented by the ratio between the permeability (P) of two pure gases, a and b, respectively, measured separately under the same conditions, in the same membrane to evaluate the separation performance, as presented in Equation (2).
α a b = P a P b

3.5. Characterization of Mixed Matrix PES/SAPO-34 Membranes

The samples were characterized by a Dynamic Mechanical and Thermal Analysis (DMTA) in Triton Technology equipment, Leicestershire, UK, model Tritec 2000, with heating rate of 2 °C/min in 1 and 10 Hz frequencies for all samples to verify the glass transition temperature, a Thermogravimetric Analysis (TGA) in a TA Instruments TGA Q500 with an N2 flow of 10 mL/min at 10 °C/min to obtain the degradation temperatures, and Fourier-transform infrared spectroscopy (FTIR) was used to compare the presence or absence of chemical components and alterations. A total attenuated reflectance non-destructive technique (ATR) was used in the range of 4000–550 cm−1 in a Perkin Elmer spectrometer model Spectrum Two FT-IR with detector DTGS and a KBr beam-splitter, resolution of 4.0 cm−1, 64 accumulations, and 50 N of constant applied force, Scanning Electron Microscopy (SEM) on a Hitachi S-4100 microscope to compare the membrane structure, morphology, and zeolite dispersibility, and a Contact Angle analysis using a Dataphysics equipment, model Contact Angle System OCA, from Filderstadt, Germany.

4. Conclusions

To study the influence of ionic liquids as additives into mixed matrix membranes, [emim][Tf2N] was added in situ to prepare similar mixed matrix membranes. Concentrations varying from 10 to 40 wt%, according to polymer mass, were used in the membrane preparation to compare the influence of [emim][Tf2N] concentrations in MMM and obtain an optimal condition.
The results showed that PES/SAPO-34 membrane containing 20 wt% of [emim][Tf2N] presented the highest separation performance among the tested membranes due to an increase in the dispersion of the crystals in the polymeric matrix, helped by a better interaction of zeolite–polymer, as well as keeping the interaction during the solvent removal, while promoting an increase in CO2 permeation and the selectivity of the PES/SAPO-34/[emim][Tf2N]20 membrane. This increase in separation performance is due the reduction in N2 permeability by the antiplasticization state, which reduces the polymer chain mobility and free volume. Since this antiplasticization phase occurs at low concentrations of additives (10–20 wt%), the permeation of species in the membrane is even more regulated by kinetic diameter, and the use of a high CO2 solubility additive explains how CO2 permeation was not affected. However, a further increase in [emim][Tf2N] in PES/SAPO-34 mixed matrix membrane, 30 wt% or higher, resulted in the higher permeation of all species due to the high influence in the polymer chain mobility and, consequently, a reduction in CO2/N2 selectivity.
Moreover, the use of ionic liquid does not affect the general structure of the polymer, as presented by tge FTIR and contact angle analysis, only delaying the segmental motion of the polymer and the suppression of the secondary relaxation mechanism and free volume with low concentrations of additive. Furthermore, it keeps the thermal resistance, as observed with TGA characterization. However, [emim][Tf2N] promoted more stable interaction and increased dispersion of the zeolite in the polymer, as can be observed in the SEM analysis, while it improved the membrane flexibility related to a reduction in the membrane Tg when compared to the neat PES and PES/SAPO-34 membranes.
In addition, other ionic liquids and deep eutectic solvents are also planned to be evaluated and compared as more eco-friendly and less expensive alternatives, to understand the effects of those in the separation performance of PES/SAPO-34.

Author Contributions

Conceptualization, J.S.C.; Z.L.; P.B. and L.M.G.-F.; investigation, J.S.C.; resources, Z.L.; P.B. and L.M.G.-F.; writing—original draft preparation, J.S.C.; writing—review and editing, J.S.C.; Z.L.; P.B. and L.M.G.-F.; visualization, J.S.C.; supervision, Z.L.; P.B. and L.M.G.-F.; project administration, Z.L.; P.B. and L.M.G.-F.; funding acquisition, J.S.C.; Z.L.; P.B. and L.M.G.-F. All authors have read and agreed to the published version of the manuscript.


The authors gratefully acknowledge the fundings from the Strategic Project of CIEPQPF (UIDB/00102/2020), CICECO-Aveiro Institute of Materials (UIDB/50 011/2020, UIDP/50 011/2020 & LA/P/0 0 06/2020), CIMO (UIDB/00690/2020 and UIDP/00690/2020) and SusTEC (LA/P/0007/2021), financed by Fundação para a Ciência e Tecnologia (FCT) through national funds. J. S. Cardoso is also grateful for the financial support of the FCT through the PhD grant (SFRH/BD/148170/2019).

Data Availability Statement

All data underlying the results are available as part of the article and no additional source data are required.


The authors gratefully acknowledge the support from CIEPQPF, University of Coimbra, CICECO—Aveiro Institute of Materials, CIMO—Montain Research Centre, SusTEC—Associate Laboratory for Sustainability and Technology in Mountains Regions and FCT—Fundação para a Ciência e Tecnologia.

Conflicts of Interest

The authors declare no conflict of interest.


  1. Robeson, L.M. The Upper Bound Revisited. J. Memb. Sci. 2008, 320, 390–400. [Google Scholar] [CrossRef]
  2. Li, S.; Falconer, J.L.; Noble, R.D. Improved SAPO-34 Membranes for CO2/CH4 Separations. Adv. Mater. 2006, 18, 2601–2603. [Google Scholar] [CrossRef]
  3. Usman, M. Recent Progress of SAPO-34 Zeolite Membranes for CO2 Separation: A Review. Membranes 2022, 12, 507. [Google Scholar] [CrossRef] [PubMed]
  4. Robeson, L.M. Correlation of Separation Factor versus Permeability for Polymeric Membranes. J. Memb. Sci. 1991, 62, 165–185. [Google Scholar] [CrossRef]
  5. Robeson, L.M. Polymer Membranes for Gas Separation. Curr. Opin. Solid State Mater. Sci. 1999, 4, 549–552. [Google Scholar] [CrossRef]
  6. Wu, T.; Liu, Y.; Kumakiri, I.; Tanaka, K.; Chen, X.; Kita, H. Preparation and Permeation Properties of PeSU-Based Mixed Matrix Membranes with Nano-Sized CHA Zeolites. J. Chem. Eng. Jpn. 2019, 52, 514–520. [Google Scholar] [CrossRef]
  7. Cardoso, J.S.; Lin, Z.; Brito, P.; Gando-Ferreira, L.M. Surface Functionalized SAPO-34 for Mixed Matrix Membranes in CO2/CH4 and CO2/N2. Sep. Purif. Rev. 2023, 1–14. [Google Scholar] [CrossRef]
  8. Comesaña-Gándara, B.; Chen, J.; Bezzu, C.G.; Carta, M.; Rose, I.; Ferrari, M.C.; Esposito, E.; Fuoco, A.; Jansen, J.C.; McKeown, N.B. Redefining the Robeson Upper Bounds for CO2/CH4 and CO2/N2 Separations Using a Series of Ultrapermeable Benzotriptycene-Based Polymers of Intrinsic Microporosity. Energy Environ. Sci. 2019, 12, 2733–2740. [Google Scholar] [CrossRef]
  9. Huang, Y.; Yu, M.; Carreon, M.A.; Song, Z.; Zhou, R.; Li, S.; Feng, X.; Zhou, S.J.; Zong, Z.; Meyer, H.S. SAPO-34 Membranes for N2/CH4 Separation: Preparation, Characterization, Separation Performance and Economic Evaluation. J. Memb. Sci. 2015, 487, 141–151. [Google Scholar] [CrossRef]
  10. Mannan, H.A.; Nasir, R.; Mukhtar, H.; Mohshim, D.F.; Shaharun, M.S. 11—Role of Ionic Liquids in Eliminating Interfacial Defects in Mixed Matrix Membranes. In Interfaces in Particle and Fibre Reinforced Composites; Goh, K.L., Rangika, A.M.K., De Silva, R.T., Thomas, S., Eds.; Woodhead Publishing: Sawston, UK, 2020; pp. 269–309. ISBN 978-0-08-102665-6. [Google Scholar]
  11. Tan, X.; Robijns, S.; Thür, R.; Ke, Q.; De Witte, N.; Lamaire, A.; Li, Y.; Aslam, I.; Van Havere, D.; Donckels, T.; et al. Truly Combining the Advantages of Polymeric and Zeolite Membranes for Gas Separations. Science 2022, 378, 1189–1194. [Google Scholar] [CrossRef]
  12. Ahmad, N.N.R.; Mukhtar, H.; Mohshim, D.F.; Nasir, R.; Man, Z. Effect of Different Organic Amino Cations on SAPO-34 for PES/SAPO-34 Mixed Matrix Membranes toward CO2/CH4 Separation. J. Appl. Polym. Sci. 2016, 133, 43387. [Google Scholar] [CrossRef]
  13. Liu, Y.; Takata, K.; Kita, H.; Tanaka, K. Investigation of Gas Diffusivity and Solubility of PES Based Mixed Matrix Membranes Using Commercial SAPO-34 Zeolite. Trans. Mat. Res. Soc. Jpn. 2021, 46, 39–43. [Google Scholar] [CrossRef]
  14. Ahmad, N.N.R.; Leo, C.P.; Mohammad, A.W.; Shaari, N.; Ang, W.L. Recent Progress in the Development of Ionic Liquid-Based Mixed Matrix Membrane for CO2 Separation: A Review. Int. J. Energy Res. 2021, 45, 9800–9830. [Google Scholar]
  15. Oral, E.E.; Yilmaz, L.; Kalipcilar, H. Effect of Gas Permeation Temperature and Annealing Procedure on the Performance of Binary and Ternary Mixed Matrix Membranes of Polyethersulfone, SAPO-34, and 2-Hydroxy 5-Methyl Aniline. J. Appl. Polym. Sci. 2014, 131, 8498–8505. [Google Scholar] [CrossRef]
  16. Ismail, A.F.; Kusworo, T.D.; Mustafa, A. Enhanced Gas Permeation Performance of Polyethersulfone Mixed Matrix Hollow Fiber Membranes Using Novel Dynasylan Ameo Silane Agent. J. Memb. Sci. 2008, 319, 306–312. [Google Scholar] [CrossRef]
  17. Junaidi, M.U.M.; Khoo, C.P.; Leo, C.P.; Ahmad, A.L. The Effects of Solvents on the Modification of SAPO-34 Zeolite Using 3-Aminopropyl Trimethoxy Silane for the Preparation of Asymmetric Polysulfone Mixed Matrix Membrane in the Application of CO2 Separation. Microporous Mesoporous Mater. 2014, 192, 52–59. [Google Scholar] [CrossRef]
  18. Suer, M.G.; Baç, N.; Yilmaz, L. Gas Permeation Characteristics of Polymer-Zeolite Mixed Matrix Membranes. J. Memb. Sci. 1994, 91, 77–86. [Google Scholar] [CrossRef]
  19. Venna, S.R.; Carreon, M.A. Amino-Functionalized SAPO-34 Membranes for CO2/CH4 and CO2/N2 Separation. Langmuir 2011, 27, 2888–2894. [Google Scholar] [CrossRef]
  20. Lixiong, Z.; Mengdong, J.; Enze, M. Synthesis of SAPO-34/Ceramic Composite Membranes. Stud. Surf. Sci. Catal. 1997, 105, 2211–2216. [Google Scholar] [CrossRef]
  21. Makertihartha, I.G.B.N.; Kencana, K.S.; Dwiputra, T.R.; Khoiruddin, K.; Mukti, R.R.; Wenten, I.G. Silica Supported SAPO-34 Membranes for CO2/N2 Separation. Microporous Mesoporous Mater. 2020, 298, 110068. [Google Scholar] [CrossRef]
  22. Cakal, U.; Yilmaz, L.; Kalipcilar, H. Effect of Feed Gas Composition on the Separation of CO2/CH4 Mixtures by PES-SAPO 34-HMA Mixed Matrix Membranes. J. Memb. Sci. 2012, 417–418, 45–51. [Google Scholar] [CrossRef]
  23. Mohshim, D.F.; Mukhtar, H.; Man, Z. The Effect of Incorporating Ionic Liquid into Polyethersulfone-SAPO34 Based Mixed Matrix Membrane on CO2 Gas Separation Performance. Sep. Purif. Technol. 2014, 135, 252–258. [Google Scholar] [CrossRef]
  24. Nasir, R.; Ahmad, N.N.R.; Mukhtar, H.; Mohshim, D.F. Effect of Ionic Liquid Inclusion and Amino-Functionalized SAPO-34 on the Performance of Mixed Matrix Membranes for CO2/CH4 separation. J. Environ. Chem. Eng. 2018, 6, 2363–2368. [Google Scholar] [CrossRef]
  25. Mannan, H.A.; Mukhtar, H.; Shahrun, M.S.; Bustam, M.A.; Man, Z.; Bakar, M.Z.A. Effect of [EMIM][Tf2N] Ionic Liquid on Ionic Liquid-Polymeric Membrane (ILPM) for CO2/CH4 Separation. Procedia Eng. 2016, 148, 25–29. [Google Scholar] [CrossRef]
  26. Ding, S.S.; Li, X.; Ding, S.S.; Zhang, W.; Guo, R.; Zhang, J. Ionic Liquid-Decorated Nanocages for Cooperative CO2 Transport in Mixed Matrix Membranes. Sep. Purif. Technol. 2020, 239, 116539. [Google Scholar] [CrossRef]
  27. Cadena, C.; Anthony, J.L.; Shah, J.K.; Morrow, T.I.; Brennecke, J.F.; Maginn, E.J. Why Is CO2 so Soluble in Imidazolium-Based Ionic Liquids? J. Am. Chem. Soc. 2004, 126, 5300–5308. [Google Scholar] [CrossRef]
  28. Zhang, X.; Zhang, X.; Dong, H.; Zhao, Z.; Zhang, S.; Huang, Y. Carbon Capture with Ionic Liquids: Overview and Progress. Energy Environ. Sci. 2012, 5, 6668–6681. [Google Scholar] [CrossRef]
  29. Li, S.; Falconer, J.L.; Noble, R.D. SAPO-34 Membranes for CO2/CH4 Separations: Effect of Si/Al Ratio. Microporous Mesoporous Mater. 2008, 110, 310–317. [Google Scholar] [CrossRef]
  30. Yang, H.; Liu, X.; Lu, G.; Wang, Y. Synthesis of SAPO-34 Nanoplates via Hydrothermal Method. Microporous Mesoporous Mater. 2016, 225, 144–153. [Google Scholar] [CrossRef]
  31. Li, Z.; Martínez-Triguero, J.; Yu, J.; Corma, A. Conversion of Methanol to Olefins: Stabilization of Nanosized SAPO-34 by Hydrothermal Treatment. J. Catal. 2015, 329, 379–388. [Google Scholar] [CrossRef]
  32. Sena, F.C.; De Souza, B.F.; De Almeida, N.C.; Cardoso, J.S.; Fernandes, L.D. Influence of Framework Composition over SAPO-34 and MeAPSO-34 Acidity. Appl. Catal. A Gen. 2011, 406, 59–62. [Google Scholar] [CrossRef]
  33. Maeda, Y.; Paul, D.R. Effect of Antiplasticization on Selectivity and Productivity of Gas Separation Membranes. J. Memb. Sci. 1987, 30, 1–9. [Google Scholar] [CrossRef]
  34. Mahajan, R.; Burns, R.; Schaeffer, M.; Koros, W.J. Challenges in Forming Successful Mixed Matrix Membranes with Rigid Polymeric Materials. J. Appl. Polym. Sci. 2002, 86, 881–890. [Google Scholar] [CrossRef]
  35. Şen, D.; Kalipçilar, H.; Yilmaz, L. Development of Polycarbonate Based Zeolite 4A Filled Mixed Matrix Gas Separation Membranes. J. Memb. Sci. 2007, 303, 194–203. [Google Scholar] [CrossRef]
  36. Naderi, A.; Yong, W.F.; Xiao, Y.; Chung, T.S.; Weber, M.; Maletzko, C. Effects of Chemical Structure on Gas Transport Properties of Polyethersulfone Polymers. Polymer 2018, 135, 76–84. [Google Scholar] [CrossRef]
  37. Liu, S.H.; Chen, C.C.; Zhang, B.; Wu, J.H. Fire and Explosion Hazards of 1-Ethyl-3-Methylimidazolium Bis(Trifluoromethylsulfonyl)Imide. RSC Adv. 2020, 10, 22468–22479. [Google Scholar] [CrossRef] [PubMed]
  38. Rogers, R.D.; Seddon, K.R. Ionic Liquids—Solvents of the Future? Science 2003, 302, 792–793. [Google Scholar] [CrossRef]
  39. Forsyth, S.A.; Pringle, J.M.; MacFarlane, D.R. Ionic Liquids—An Overview. Aust. J. Chem. 2004, 57, 113–119. [Google Scholar] [CrossRef]
  40. Anthony, J.L.; Anderson, J.L.; Maginn, E.J.; Brennecke, J.F. Anion Effects on Gas Solubility in Ionic Liquids. J. Phys. Chem. B 2005, 109, 6366–6374. [Google Scholar] [CrossRef]
  41. Noel Jacob, K.; Senthil Kumar, S.; Thanigaivelan, A.; Tarun, M.; Mohan, D. Sulfonated Polyethersulfone-Based Membranes for Metal Ion Removal via a Hybrid Process. J. Mater. Sci. 2014, 49, 114–122. [Google Scholar] [CrossRef]
  42. Alenazi, N.; Hussein, M.; Alamry, K.; Asiri, A. Nanocomposite-Based Aminated Polyethersulfone and Carboxylate Activated Carbon for Environmental Application. A Real Sample Analysis. C 2018, 4, 30. [Google Scholar] [CrossRef]
  43. Ran, F. Polyethersulfone Membrane. In Encyclopedia of Membranes; Springer: Berlin/Heidelberg, Germany, 2015; pp. 1–2. [Google Scholar]
  44. Cardoso, J.S.; Fonseca, J.P.; Lin, Z.; Brito, P.; Gando-Ferreira, L.M. Optimization and Performance Studies of PES/SAPO-34 Membranes for CO2/N2 Gas Separation. Microporous Mesoporous Mater. 2023, 364, 112845. [Google Scholar] [CrossRef]
Figure 1. XRD for SAPO-34 sample and CHA standard.
Figure 1. XRD for SAPO-34 sample and CHA standard.
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Figure 2. Adsorption–desorption nitrogen isotherms for SAPO-34 sample.
Figure 2. Adsorption–desorption nitrogen isotherms for SAPO-34 sample.
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Figure 3. DLS for SAPO-34 sample.
Figure 3. DLS for SAPO-34 sample.
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Figure 4. SEM for SAPO-34 sample.
Figure 4. SEM for SAPO-34 sample.
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Figure 5. Comparison between CO2 permeability, N2 permeability, and CO2/N2 selectivity according to [emim][Tf2N] content.
Figure 5. Comparison between CO2 permeability, N2 permeability, and CO2/N2 selectivity according to [emim][Tf2N] content.
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Figure 6. Preferential adsorption of CO2.
Figure 6. Preferential adsorption of CO2.
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Figure 7. Robeson upper bound to compare different results for membranes using SAPO-34 [1,8].
Figure 7. Robeson upper bound to compare different results for membranes using SAPO-34 [1,8].
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Figure 8. DMTA analysis for PES, PES/SAPO-34, and PES/SAPO-34/[emim][Tf2N]20 at 1 Hz and 10 Hz.
Figure 8. DMTA analysis for PES, PES/SAPO-34, and PES/SAPO-34/[emim][Tf2N]20 at 1 Hz and 10 Hz.
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Figure 9. TGA analysis for (a) PES, (b) PES/SAPO-34, and (c) PES/SAPO-34/[emim][Tf2N]20.
Figure 9. TGA analysis for (a) PES, (b) PES/SAPO-34, and (c) PES/SAPO-34/[emim][Tf2N]20.
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Figure 10. FTIR for neat PES, PES/SAPO-34, PES/SAPO-34/[emim][Tf2N], and [emim][Tf2N].
Figure 10. FTIR for neat PES, PES/SAPO-34, PES/SAPO-34/[emim][Tf2N], and [emim][Tf2N].
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Figure 11. (a) Cross-section and (b) surface of neat PES membrane.
Figure 11. (a) Cross-section and (b) surface of neat PES membrane.
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Figure 12. (a) Cross-section and (b) surface of PES/SAPO-34 membrane.
Figure 12. (a) Cross-section and (b) surface of PES/SAPO-34 membrane.
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Figure 13. (a) Cross-section and (b) surface of PES/SAPO-34/[emim][Tf2N]20 membrane.
Figure 13. (a) Cross-section and (b) surface of PES/SAPO-34/[emim][Tf2N]20 membrane.
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Figure 14. Zoom in particle distribution for (a) PES/SAPO-34 and (b) PES/SAPO-34/[emim][Tf2N]20 membranes.
Figure 14. Zoom in particle distribution for (a) PES/SAPO-34 and (b) PES/SAPO-34/[emim][Tf2N]20 membranes.
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Figure 15. Schematics of the permeation system.
Figure 15. Schematics of the permeation system.
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Table 1. Comparison of CO2 permeability or permeance, CO2/CH4 and CO2/N2 selectivities in the literature.
Table 1. Comparison of CO2 permeability or permeance, CO2/CH4 and CO2/N2 selectivities in the literature.
Neat-PES2.6 barrer-18.57[18]
Neat SAPO-34
(stainless steel)
--328 GPU15929[19]
Neat SAPO-34
(porous alumina)
--892 GPU-7.09[20]
Neat SAPO-34
--6000 GPU-53[21]
Neat-PES6.7 barrer37.834.04[6]
SAPO-3420 wt%PES8.25 barrer42.634.51
SAPO-3430 wt%PES8.9 barrer48.37.08
Neat-PES4.45 barrer33.2-[22]
HMA 10%-PES0.8 barrer32.3-
SAPO-3420 wt%PES5.7 barrer37-
SAPO-34 + HMA 10%20 wt%PES1.3 barrer44.7-
HMA 4%-PES5.1 barrer39.3-[15]
SAPO-3420 wt%PES13.8 barrer32.7-
SAPO-34 + HMA 4%20 wt%PES7.8 barrer41.6-
SAPO-3420 wt%PES85.69 GPU20.67-[23]
SAPO-34 + IL 5%20 wt%PES230.81 GPU46.20-
SAPO-34 + IL 10%20 wt%PES255.69 GPU58.83-
SAPO-34 + IL 15%20 wt%PES279.26 GPU60.62-
SAPO-34 + IL 20%20 wt%PES300 GPU62.58-
SAPO-3420 wt%PES30 GPU1.3-[12]
SAPO-34 + m-EDA20 wt%PES10 GPU12.14-
SAPO-3420 wt%PES50 GPU2.5-[24]
SAPO-34 + IL20 wt%PES0.03 GPU4.9-
SAPO-34 + m-EDA + IL20 wt%PES0.09 GPU26.5-
SAPO-34 + m-HA + IL20 wt%PES0.045 GPU37.2-
Table 2. Si/Al molar ratio in original gel and crystal samples.
Table 2. Si/Al molar ratio in original gel and crystal samples.
PowderSi/Al Molar Ratio
SAPO-34 sample0.300.29
Table 3. Textural properties of the materials.
Table 3. Textural properties of the materials.
SampleSBET (m2 g−1)SLangmuir (m2 g−1)Sext (m2 g−1)Smic (m2 g−1)Vmic (mm3 g−1)
Table 4. Comparison in separation performance of PES/SAPO-34 and PES/SAPO-34/[emim][Tf2N] membranes.
Table 4. Comparison in separation performance of PES/SAPO-34 and PES/SAPO-34/[emim][Tf2N] membranes.
SampleαCO2/N2PCO2 (barrer)PN2 (barrer)
Neat PES21.35 ± 3.621.04 ± 0.0030.05 ± 0.0090
PES/SAPO-3427.31 ± 1.080.67 ± 0.040.025 ± 0.0005
PES/SAPO-34/[emim][Tf2N]1022.71 ± 2.141.64 ± 0.050.072 ± 0.009
PES/SAPO-34/[emim][Tf2N]2039.44 ± 0.752.11 ± 0.230.054 ± 0.007
PES/SAPO-34/[emim][Tf2N]3027.36 ± 3.282.04 ± 0.010.075 ± 0.020
PES/SAPO-34/[emim][Tf2N]4019.00 ± 0.702.94 ± 0.030.155 ± 0.007
Table 5. Average contact angle.
Table 5. Average contact angle.
SampleAverage Contact Angle (°)
PES70 ± 2
PES/SAPO-3471 ± 2
PES/SAPO-34/[emim][Tf2N]2061 ± 2
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Cardoso, J.S.; Lin, Z.; Brito, P.; Gando-Ferreira, L.M. The Functionalization of PES/SAPO-34 Mixed Matrix Membrane with [emim][Tf2N] Ionic Liquid to Improve CO2/N2 Separation Properties. Inorganics 2023, 11, 447.

AMA Style

Cardoso JS, Lin Z, Brito P, Gando-Ferreira LM. The Functionalization of PES/SAPO-34 Mixed Matrix Membrane with [emim][Tf2N] Ionic Liquid to Improve CO2/N2 Separation Properties. Inorganics. 2023; 11(11):447.

Chicago/Turabian Style

Cardoso, Jonathan S., Zhi Lin, Paulo Brito, and Licínio M. Gando-Ferreira. 2023. "The Functionalization of PES/SAPO-34 Mixed Matrix Membrane with [emim][Tf2N] Ionic Liquid to Improve CO2/N2 Separation Properties" Inorganics 11, no. 11: 447.

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