Development of poly(ethylene glycol) diacrylate membrane for facilitated CO2/N2 separation

Carbon dioxide (CO2) is responsible for approximately 80% of greenhouse gases emission that is the main source to global climate change causing notable environmental impacts. Poly (ethylene glycol) diacrylate (PEGDA) have polar PEG repeating units, which provide a strong affinity towards carbon dioxide (CO2) molecules has been blended with 3-aminopropyltrimethoxysilane (APTMS) to synthesize membrane for CO2/nitrogen (N2) separation. The new synthesized membrane is studied for potential applications in gas separation and to be implemented in control CO2 emission. APTMS is also used to delay the diffusion between polymer and solvent. In this study, concentration of polymer of PEGDA and casting solvent of APTMS in terms of mol ratio from a range of 0.9:1.1 to 1.3:0.7 is discussed. Based on the results, PEGDA membrane shows best gas separation performance at mol ratio of PEGDA to APTMS of 1:1 where the permeance for both CO2/N2, and CO2/N2 selectivity are 75.21±0.15 GPU, 22.95±0.05 GPU and 3.28±0.12, respectively. An optimal aminosilane/polymer reaction ratio benefits the gas separation performance of the membrane due to the affinity of the membrane towards CO2 and formation of different membrane surface morphology.


Introduction
Global climate change has been an arising issue in the recent decade prompting organizations such as Intergovernmental Panel on Climate Change into taking proper course of resolving the issue or mitigating the effects of climate change [1]. CO2 as the primary anthropogenic GHGs emanated from power plants and industrial processes, requires proper constraint or mitigation method to control its emission. Identifying the potential of CCS, this method is executed as a major strategy to resolve the issue [2]. Among three main approaches of CCS, post-combustion capture is favourably practiced given its cost-effective and compatibility for retrofitting applications compared to oxy-fuel combustion and pre-combustion capture [3]. In comparison between conventional CO2 separation technologies, membrane separation remains favourable in industrial application [4] because of its simple modular design allows process arrangement for retrofitting purposes in industrial and cost optimization in future applications. Membrane technology also requires less maintenance; thus, proving it a cost-effective and energy-efficient technology [5]. However, the energy requirement for this technology depends on the flue gases' partial pressure while the performance of membrane is dependent on multiple parameters such as polymer concentration and casting solution concentration [6]. The principle characteristic of membranes in gas separation applications is to control the permeation of different species limited by their performance, known as Robeson limit (permeability must be sacrificed for selectivity and vice versa) [7]. It is always challenging to have a reproducible and upscalable method to fulfill the needs. Recently, novel and straightforward strategies have been developed to fabricate high-performance membranes [8][9][10][11].
PEGDA polymer have the potential of good mechanical and diffusive properties to be fabricated as gas separation membrane [12]. Increasing PEGDA content in the fabrication of membrane had increased the permeability of CO2 compared to N2 due to the strong affinity existing between polar EG repeating units and CO2 molecules [13]. The enhancement in the mechanical properties of this membrane had significantly increased the CO2/N2 selectivity due to the increase in cross-linking density that reduces the pore formation and affects gas separation performance [14].
APTMS are used as crosslinking agents for membrane curing where the synthesized membrane displayed higher crosslinking density, anti-swelling ability and excellent combination between the separation layer and support layer [15]. The increase in APTMS concentration during membrane synthesis had been reduced the formation of macrovoids in the phase inversion process by delaying the mutual diffusion of polymer and solvent [16]. More resistant towards pore formation; thus, smaller pore size of membrane, and higher permeability of CO2 compared to N2 based on their respective activation energy required [17].
Therefore, the main aim of this research is to fabricate PEGDA membrane for CO2/N2 separation using different polymer and solution concentration to optimize membrane's performance. Up to date, many works have been conducted to identify the effects of polymer concentration of various polymers on the gas separation performance. However, no research on the effects of APTMS concentration has been conducted. Therefore, this research is conduct to understand the relationship between the effects of both polymer (PEGDA) and solvent (APTMS) concentration on the structure and properties of membrane. Besides that, the suitability of APTMS as a casting solvent is to be identified through this experimental work by comparing its advantages and disadvantages exhibited by the membrane synthesized.

Fabrication of PEGDA Membrane
Membranes were synthesized by dry-phase inversion. The casting solution of PEGDA membranes were prepared by mixing a mole ratio of 2 of PEGDA and APTMS in 50 wt % chloroform solution for 24 hours at 25 °C. The casting solution was then casted onto a glass plate at a thickness of 150 μm and allowed to evaporate slowly at 25 °C for 2 days in ambient temperature. Next, the neat membrane was placed in a vacuum oven at 40 °C for 24 hours as the final drying stage. Finally, the resulting membrane was peeled off the glass plate and kept in a desiccator for experiment and performance testing [18]. The concentrations of PEGDA and APTMS are tabulated in Table 1.

Gas Separation Performance Test
To study the gas separation performance, a gas permeation tests was conducted based on the previous published work [19]. Both flow rates of the permeate and retentate streams in and out of the permeation cell were measured using the bubble gas flow meters during the gas permeation test. Three samples for each fabricated membrane were collected and tested. In this work, standard error of the readings were reported to ensure reproducibility of the data.

Scanning Electron Microscopy (SEM).
Hitachi S3500N scanning electron microscope with a Schottky type thermal field emitter (SU-70 model) was used to scan the microstructure of the membranes surfaces. The analysis was carried out in vacuum conditions (1x10 -8 Pa) at an accelerating voltage of 15 kV and a secondary electron reception angle of 30.4°. The membrane samples were coated with a layer of gold approx. 2 nm thick using BAL-TEC SCD 050 Sputter Coater from Quorum Technologies to increase conductivity and prevent charge build-up [20]. Analyses of the membranes' surface morphology were carried out at different zooming rate and higher resolution electrographs to obtain clear images. Three samples were carried out for the SEM characterization to ensure reproducibility of the experimental data.

Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR).
A schematic of multiple reflections of internal reflection element (IRE) are illustrated in Figure 1 [21, 22]. The spectra of the membranes were recorded by a Thermo Scientific FTIR model (NICOLET iS10, USA) using diamond crystal over the wavenumber range of 4000-400 cm -1 with 32 scans.

The Effects of PEGDA and APTMS Concentration
The main aim of this study is to investigate the effect of the concentration of PEGDA and APTMS on the membrane structure and gas separation performance of PEGDA membrane. According to Ghadimi et al. (2014), the increase in the PEGDA content allows higher permeance of CO2 through the PEGDA membrane, attributed to the dipolar-quadrupolar interactions induced by the strong affinity between the polar ethylene glycol repeating units on the membrane's structure and CO2 molecules. The chemical affinity increases the gas permeance of CO2 through the membrane dramatically compared to N2 gases thus the membrane presents with a relatively high CO2/N2 selectivity [23]. According to Ko et al. (2011), the increment in the concentration of APTMS increases the affinity of the membrane towards CO2 which confirmed through the reaction between CO2 and primary amine, APTMS to form carbamate through a two-step zwitterion mechanism and then regeneration via zwitterion to produce APTMS, as illustrated in Figure 2  the methoxy groups of APTMS has high affinity for CO2 to provide sites of interaction for CO2 solvation; thus, improving CO2 gas permeance [25].

Figure 2.
The expected mechanism reaction between CO2 and primary amines [24].  Figure 3, the carbonyl group of the carboxylic acid (C=O) functional group is represented by the strong absorption peaks measured at 1713.97 cm -1 while the peaks at 1190 cm -1 which justified the presence of acrylic bond (C-O) attributed to the stretching of acrylate groups [26]. Both functional groups (C=O and C-O) are ester groups. Interestingly, the disappearance of characteristic peaks of twisting vibration of the acrylic bond (CH2=CH2) and the deformation of CH2=CH bond (i.e., particularly the peaks at 812 cm -1 and 1410 cm -1 ) as presented in the spectra practically indicated the complete reaction conversion between PEGDA and APTMS for membranes [26].

ATR-FTIR Analysis
M2 depicted the highest absorbance in relative to the absorption peaks of the PEGDA membrane compared to M1, M3, M4 and M5, as presented in table 2. The decrease in absorbance from M2 to M1 is due to the low mechanical stability of APTMS as polymer concentration of APTMS increases. This causes a lower absorption in IR radiation; thus, a lower absorption peak has been demonstrated in figure  3. Additionally, the reduction in the absorbance peaks from M2 to M5 have been explained by the SEM analysis conducted where the wrinkle formation showed in M3, M4 and M5 resulted lower absorption peaks, and reflection of the evanescent wave [27].   M1 shows a rough surface layer with formation of small molecular size defects, as observed in Figure 4a. Higher concentration of APTMS than PEGDA in M1 exhibits low mechanical stability where the evaporation of solvent forms the molecular size defects at the membrane's surface as the structural integrity of the membrane surface is weaken [28].

SEM Analysis
The SEM analysis of M2 (Figure 4b) shows a smooth surface layer. The polymer support provided by the concentration of PEGDA in M2 is able to prevent formation of molecular size defects on the membrane surface compared to M1. The hydrophilic properties exhibited by PEGDA is not significant to form wrinkles on the membrane surface as concentration of PEGDA of M2 is lower as compared to M3, M4 and M5 [29].
M3 presented with wrinkle formations on the membrane's top surface as observed in Figure 4c. The hydrophilic properties exhibited by PEGDA causes the diffusion of water vapour during air-drying of membrane to induce interfacial tension with the PEGDA at the membrane surface. The increase in the PEGDA concentration from M2 to M3 results in a stronger diffusion of water vapour through the membrane that caused folding of the membrane surface structure into a wrinkle formation [30]. Therefore, significant wrinkle formations are also observed on the membrane surface of both M4 and  Figure 4d and Figure 4e, respectively, as the PEGDA concentration increased from M3 to M4 to M5 [27].  in the form of carbamate species produced during the reaction between CO2 and APTMS that is present in the membrane affected the gas permeance of CO2; therefore, the gas separation performance towards CO2 of M2 is lower than of M1 [31]. However, the decrease in CO2 gas permeance from M2 to M5 can be interpreted as the inferior effect of the dipolar-quadrupolar interactions induced between CO2 and PEGDA compared to the affinity exhibited by the intermediate reaction between CO2 and APTMS to form carbamate [32]. The methoxy groups present in APTMS also verified the high affinity between CO2 and APTMS to provide sites of CO2 interaction; hence, increasing the gas permeance of CO2 of the membrane as APTMS concentration increases from M5 to M2 [33]. Besides that, based on the FTIR analysis conducted, the absorption peaks recorded for C=O and C-O stretch are the highest in M2 compared to M1, M3, M4 and M5 due to the low mechanical stability in M1 and the wrinkle formations in M3, M4 and M5. Therefore, the higher absorption peaks proves the stronger dipole moment between M2 and CO2 which is attributed to the increase in gas permeance.

Membrane Performance towards N2.
Based on Figure 6, the N2 permeance is the lowest for M2 (22.95±0.05 GPU) when the ratio of PEGDA to APTMS is 1:1 as compared to M1 (23.47±0.11 GPU), M3 (23.36±0.210 GPU), M4 (24.19±0.21 GPU) and M5 (24.79±0.18 GPU). The increment of N2 permeance from M2 to M1 is a result of the formation of molecular size defects of both 49.1 μm and 50.7 μm that provided sites for gas permeation thus presented with higher permeation of N2 gases [23]. As for membrane M3, M4 and M5 have higher gas permeance compared to M1 due to their surface morphology where the wrinkles formed on the top surface provides more interlayer is involved in the N2 gas permeation across the membrane thus increasing the N2 gas permeance [34].   [35]. This increasing in CO2/N2 selectivity from M5 to M2 is attributed to the affinity between CO2 and CO2-affinitive moieties in APTMS (methoxy and amine groups), which is relatively stronger than the affinity between PEGDA and CO2 as the concentration of APTMS increases from M5 to M2; thus, M5 exhibits a lower CO2/N2 selectivity as compared to M2 [36].  In summary, M2 that has the highest CO2 permeance (75.21±0.15 GPU) is presented with the highest CO2/N2 selectivity (3.278±0.12) and also the lowest N2 gas permeance (22.95±0.05 GPU) as compared to M1, M3, M4 and M5, as illustrated in figures 5 and 6. This is due to the highest absorption peaks for both C=O and C-O stretch that represents the carbonyl group, as shown in table 2. Therefore, the higher reactivity of the membrane towards CO2 is attributed to the high CO2 gas permeance of M2. Besides that, the concentration of PEGDA to APTMS of M2 presents a smooth membrane surface which minimizes the gas permeance of N2 as compared to M1 that have molecular size defects formation and M3, M4, M5 that have wrinkle formation on the membrane surface, which subsequently increases the N2 gas permeance across the membrane [30]. The increase in APTMS concentration from M5 to M2 increases the affinity of the membrane towards CO2 due to the higher presence of amine and methoxy group which has a stronger effect compared to the dipolar-quadrupolar interactions between PEG and CO2. However, the increment of concentration is limited by the adsorption of CO2 that affects the gas permeance of CO2 [28]. Therefore, the optimized concentration of PEGDA to APTMS is determined at a mol ratio of 1:1 of M2.
Furthermore, the performance of the present work has been compared to the high performance membranes, as tabulated in table 3. It is clearly showed that the new synthesized PEGDA membrane need a further modification suggested by developing hollow fiber membrane similar to Liang et al. (2017) and Yong et al. (2013). Similarly, Naderi et al. (2018) proposed the addition of ethylene oxide group that could affect the overall gas permeability and selectivity because it has a high affinity towards CO2 molecules through quadrupolar interactions [11]. * GPU ** barrer

Conclusion
The main objective of this study is to synthesize effective gas separation membranes for CO2 /N2 gas separation. Firstly, the neat polymeric membranes PEGDA were fabricated. The performance in regards of permeance towards CO2 and N2, and CO2/N2 selectivity confirmed that the concentration of PEGDA and APTMS correlated to the membranes' structure and surface morphology as well as the Lewis acidbase interaction towards CO2. In term of membrane separation performances, the optimal mol ratio of PEGDA and APTMS at 1:1 (M2) exhibited the highest CO2/N2 selectivity of (3.278±0.12) with CO2 permeance (75.21±0.15 GPU) and N2 permeance (22.95±0.05 GPU). Therefore, PEGDA to APTMS concentration of 1:1 mol ratio is recommended for the fabrication of PEGDA membrane to be utilized in the industrial application for CO2/N2 separation and efficiently reduced CO2 emission into the atmosphere thus mitigate the effect of global warming.