Development and Performance Evaluation of Cellulose Acetate- Bentonite Mixed Matrix Membranes for CO2 Separation

Membrane science is a state-of-the-art environmentally green technology that ascertains superior advantages over traditional counterparts for CO2 capture and separation. In this research, mixed matrix membranes (MMMs) comprising cellulose acetate (CA) with various loadings of bentonite (Bt) clay were fabricated by adopting the phase-inversion technique for CO2/CH4 and CO2/N2 separation. The developed pristine and MMMs were characterized for morphological, thermal, structural, and mechanical analyses. Several techniques such as scanning electron microscopy, thermogravimetric analysis, Fourier transformed infrared spectroscopy, and nano-indentation investigations revealed the promising effect of Bt clay in MMMs as compared to pristine CA membrane. Nano-indentation test identified that elastic modulus and hardness of the MMM with 1 wt. loading was increased by 64% and 200%, respectively, compared to the pristine membrane. The permeability decreased with the incorporation of Bt clay due to uniform dispersion of filler attributed to enhanced tortuosity for the gas molecules. Nevertheless, an increase in gas separation performance was observed with Bt addition up to 1 wt. loading. The opposite trend prevailed with increasing Bt concentration on the separation performance owing to filler agglomeration and voids creation. The maximum value of ideal selectivity (CO2/CH4) was achieved at 2 bar pressure with 1wt. % Bt loading, which is 79% higher than the pristine CA membrane. For CO2/N2, the ideal selectivity was 123% higher compared to the pristine membrane with 1wt. % Bt loading at 4 bar pressure.


Introduction
Nowadays, it is widely conceded and understood that CO 2 emission is the pivotal reason behind climate changes, ocean acidification, and associated global warming. Among greenhouse gases, CO 2 exhibits the most dominating role, and its anthropogenic emission results in intensifying the greenhouse effect and climate change up to 64% [1]. The CO 2 emissions in atmosphere, majorly associated with combustion activities related to fossil fuels and industrial flue gases, have increased to 61% between 1990 and 2013 [2]. Therefore, CO 2 separation is inevitable, and efforts are underway at limited scale compared to what is required.
Currently, several techniques such as absorption, cryogenic distillation, and adsorption are used frequently to separate and capture CO 2 gas. Nonetheless, the aforementioned traditional techniques have shown certain drawbacks particularly in terms of capital cost, operational complications, and limited CO 2 loading capabilities [3]. Therefore, there is a need to look for low cost, environmentally benign, high loading capacity, and easily applicable CO 2 capturing technology. Membrane technology has the potential to offer superior advantages compared to the traditionally available technologies [4]. Membrane technology not only offers lower capital cost but also the design and operational simplicity, compactness, and energy efficiency along with environmentally friendliness are associated benefits [5,6].
Membranes are developed either from inorganic or/and polymeric materials. However, combining both aforementioned materials proved to be advantageous with regard to membrane manufacturing cost and chemical and thermal stability as well as ease in processing [4,7]. The fabrication of mixed matrix membrane (MMMs) with flawless or least affected morphology is strenuous and challenging and depends on the choice of materials [8]. Subsequently, inappropriate selection of materials may lead to interfacial defects and seriously damage the separation capacity of MMMs. Therefore, successful membrane formation is subject to the selection of compatible polymer filler systems. Generally, polysulfone (PSf), polyethersulfone (PES), polyimides (PI), and cellulose acetate (CA) are commonly used polymers for membrane fabrication [9][10][11][12][13]. CA offers inexpensive manufacturing, adequate toughness and virtuous compatibility, superior fouling resistance, easy processing, and high CO 2 solubility [14].
Clay minerals are characterized as earthy, fine-grained natural soil materials, and extensively studied as inorganic filler in polymer composite industry [15]. Montmorillonite (Mt) is a frequently used clay for reinforcing polymers among others such as cloisite and kaolin clays which are also used in manufacturing polymer composites [16,17]. Bentonite is mostly Mt comprised of aluminum 2 : 1 phyllosilicate. The elementary structure of Mt unit cell comprised of a tetrahedral sheet containing silicon encircles by four oxygen atoms and an octahedral layer of aluminum surrounded by eight oxygen atoms [18]. The interactions of the organic polymer and clay particles dictate the morphology of the nanocomposites that could either be phase-separated, intercalated, and exfoliated as depicted in Figure 1. Exfoliated morphology is the most desired one and offers uniform distribution that promotes gas separation performance in MMM [19,20].
CA is extensively studied as organic phase for polymeric and nano-composite gas separation membranes; however, its potential with bentonite clay incorporation is barely reported. Thus, in the present study, bentonite MMMs are fabricated, and the effect of Bt loading on the performance of CA-Bt MMMs is presented. Afterward, the fabricated MMMs are characterized by structural, morphological, thermal, and mechanical properties. Subsequently, gas separation (CO 2 /N 2 and CO 2 /CH 4 ) study was performed for the developed membranes.

Experimental
2.1. Materials. Cellulose acetate (CA, M n 30,000) and tetrahydrofuran (THF, ≥99.5%) were purchased from Sigma-Aldrich. Bentonite clay was provided by Marshal trading company, Pakistan, and was used as received.

2.2.
Optimization of CA Concentration. The defect-free membrane morphology is subject to the dissolution of optimized polymer concentration in solvent. Therefore, five dope-solutions with varying CA concentration (5, 10, 15, and 20 wt. %) in THF were prepared at ambient conditions. The dope-solution was left for stirring for 24 hours to completely dissolve CA in THF, followed by degassing for another 24 hours to eliminate any entrapped air bubbles. The viscosities of the degassed solutions were determined using rheometer by measuring the viscosity change against the time, whilst all other parameters were kept constant.

Dope-Solution Preparation.
Prior to preparing the dopesolution, CA was dried overnight at 100°C to remove the moisture content, whereas THF and bentonite were used as received. The dried CA was dissolved in THF to formulate 13% w/w polymer solution and fabricate the pristine polymeric membrane.
For MMM, the desired 0.5 wt. % Bt was dispersed in THF by stirring for 60 minutes to ensure uniform dispersion at room temperature. A 5 wt. % of CA solution was added in the dispersed mixture for priming purposes and magnetically stirred for 3 hours. The remaining amount of CA was poured gradually to get complete polymer dissolution whilst keeping stirring rate of 650 rpm for 24 hours. Subsequently, the prepared CA-Bt solution was allowed to degas for 12 hours at ambient conditions. A similar procedure was repeated to synthesize the rest of MMMs. The synthesized pristine and MMMs comprised of 0.5 wt. %, 1 wt. %, 1.5 wt. %, and 2 wt. % loading of bentonite are coded as M1, M2, M3, M4, and M5, respectively.

Membrane Development.
The CA-Bt MMMs were fabricated through the dry-wet phase inversion technique. The tape casting unit with adjustable film thickness was used, and the casted membranes were kept at ambient temperature for 24 hours prior to immersion in distilled water for a day to encounter solvent exchange. The developed pristine and MMMs were dried in a vacuum oven in order to evaporate the residual solvent at 70°C for 12 hours.

Characterization of the Developed Pristine and MMMs.
The dry-wet phase inversion method was adopted to fabricate CA-Bt MMMs, and this influenced the membrane structure. Therefore, the morphology of the casted membranes was studied through SEM analysis.
The possible functional groups attached to the developed membranes were identified by FTIR spectroscopy (Perkin Elmer 1650). The spectra of all the membranes were obtained in the range of 400 to 4000 cm -1 under the transmission mode.
TGA (Perkin Elmer, STA6000) was used to determine the thermal stability of CA-Bt MMMs. The thermal behavior was analyzed in the presence of nitrogen environment by using a heating rate of 10°C/min for a temperature range of 30 to 800°C.
2.6. Pure Gas Permeation Test. The permeation behavior of developed MMMs was evaluated by using pure N 2 , CO 2 , and CH 4 gases at ambient temperature, whilst at varied pressure from 2 to 6 bars. Prior to carry out permeation tests, the system was evacuated from air gases by using vacuum pump. The flow rate of the membrane permeated gases was measured by using soap bubble flowmeter. This can be done simply by recording the time taken by the bubble to move from zero to a certain higher level 2 Advances in Polymer Technology [21]. Figure 2 presents the schematic diagram of the gas permeation rig used in this study. The standard equations (1) and (2) as provided are used to calculate the permeability and selectivity of the permeated gases, respectively [15].
Polymer Clay

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where P a is the gas permeability (barrer), Q represents the volumetric flow rate (cm 3 /s), l is the membrane thickness, A corresponds to effective surface area (cm 2 ), ΔP is the transmembrane pressure (cm.Hg), and T is the temperature (K). On the other hand, selectivity is represented by α ab for gas a to b.

Mechanical Analysis Using Nano-Indentation Technique.
The nano-indentation test was performed by adopting continuous stiffness measurement technique. The aforementioned test records the displacement of pyramidal diamond at the sample surface produced by a dynamic load. The test in this study was carried out with load of indentation around 100 mN and applied at least at 38 points on each mixed matrix membrane. The nano-indentation permits measuring the mechanical properties, hardness, and reduced Young's modulus of thin membranes. The schematic diagram of studied membrane is shown in Figure 3.

Results and Discussion
3.1. Optimization of CA Concentration. Figure 4 presents the viscosity curve for a binary dope solution comprised of 5 to 20 wt. % CA in THF solvent. An upward trend in the viscosity curve is observed with increase in CA composition in dope solution. Initially, the viscosity increased gradually with CA concentration. Nonetheless, the curve becomes steeper beyond a critical polymer concentration. This critical polymer concertation can be found by extrapolating the linear sections of the curve and then intercepting at a point as shown in Figure 4. This method of determining critical polymer concentration from the viscosity curve has been used frequently and regarded as a valid approach in membrane development technique [22].
The aforementioned method was used here, and critical dope viscosity was determined at 13 wt. % CA concentration. The critical dope viscosity is associated with the polymer chain entanglements at a particular polymer concentration in dope solution. Below this critical dope viscosity point, polymer chains exhibit loose packing and hence welcome the intrusion of nonsolvent to create voids at the surface. The membrane formation with the dope solution below critical polymer concentration forms porous morphology with weak mechanical properties. In the contrary, above the critical dope viscosity, polymer chains demonstrate remarkable entanglements that impede nonsolvent diffusion through the surface forming dense, void-free structure. The membrane formation above critical dope viscosity produces nonporous and mechanically strong morphology [23]. Therefore, it is important to know the critical viscosity of polymer solutions since it a balance between highly dense and porous structures in membranes to achieve optimum separation performance [22].

Chemical
Analysis. FTIR was used to analyze the characteristic peaks appeared in Bt, pure CA, and CA-Bt MMM spectra and are depicted in Figure 5. The spectrum of Bt nanoparticles exhibits a characteristic bands at 1043, 1713, and 2850 cm -1 and is associated with Al-O-Si deformation, carbonyl groups stretching, and C-H bonding stretching [1].
For the developed membranes, the peaks appearing below 900 cm -1 represent C-O vibrations in alcohols which might be due to the presence of residual THF in the membrane [23]. The peak at 901 cm -1 is attributed to the β-links in the pristine CA membrane [24]. On the other hand, bands at 1033 cm -1 , 1124, and 1159 cm -1 are attributed to symmetric and asymmetric C-O vibrations in secondary alcohol [3]. The bands observed at 1218 cm -1 , 1367 cm -1 , and 1437 cm -1 correspond to C-C-O stretching, CH 3 deformation, and H-C-H vibrations, respectively [25]. The characteristic C=O stretching vibration in CA was observed at 1733 cm -1 [25]. Similarly, the peak observed around 3000 cm -1 represents the mild effects of carboxylic acid [26]. These observations signify the presence of CA-Bt interactions in the synthesized membrane structures.
3.3. Morphological Analysis. Gas transport properties across the MMM strongly depend on the structural morphology of resultant membranes. SEM was used to analyze the morphology and structure of the pure CA membrane and MMMs, as well as the interfacial interactions of the CA and Bt particles. Figure 6 shows the cryo-fractured membrane cross-sections. Generally, clay particles owing to small size are invisible in SEM images unless form agglomerates.
Furthermore, the synthesized membranes exhibited dense morphology with the presence of certain voids. For gas separation membranes, the dense morphology is always preferred as it provides superior separation performance compared to its counterpart porous morphology [27]. The membrane (M2) showed dense morphology with fewer voids, which are attributed to the evaporation of THF traces during membrane drying. M3 membrane presented the uniform dense and voids free morphology. These observations anticipate the uniform Bt dispersion in CA, which ascertained dense, voids free membrane  Advances in Polymer Technology morphology. For M4 and M5 membranes, clay agglomeration led to the increased void formation, which consequently decreases the separation performance of these membranes. It is also observed that the void size and void frequency increase with increasing clay loading. The aforementioned observations are in agreement with previously reported studies on the clay agglomeration in the polymer matrix [15,17,28].
3.4. Thermal Stability Analysis. The thermal stability of pristine CA and CA-Bt MMMs at various Bt loadings is shown in Figure 7. For simplicity, the decomposition temperature is considered at 10% weight loss. The thermogram showed no weight loss below 200°C, which is attributed to complete solvent removal from the synthesized membranes. The decomposition temperature for the pristine CA membrane is 235.27°C. The thermal stability increased with the addition of Bt. This improvement is attributed to the inherent thermal characteristics of Bt, as the latter absorbed the greater amount of heat. In addition, nano-fillers settle in the polymer chains and impede the interchain segmental motion, which consequently, enhance the energy needed for polymer chain movement [29]. In comparison to pristine membrane, the addition of 0.5, 1, 1.5, and 2 wt. % Bt increased the decomposition temperature to 290.14, 244.41, 243.52, and 240.76°C, respectively. Among composite membranes, the maximum thermal stability achieved is with 0.5 wt. % clay, which is attributed by the uniform dispersion of the clay platelets.
A similar trend was observed by Liang in montmorillonite (Mt) filled PES membranes [28]. The addition of Mt (2 to 20 wt. %) consistently improved the thermal stability of the MMMs. Herrra-Alonso also observed similar findings with Cloisite Na + in PBMA where the onset degradation temperature increased from 222 to 260°C with the addition of clay (0-5 wt. %) [30]. PMMA/clay nanocomposite also showed similar trend; thermal stability improved owing to the higher content of alumino-silicate in Mt. The improvement in decomposition temperature is due to layered morphology of clay which restricted the oxygen gas' diffusion into polymer phase. The heat provided is absorbed by the clay platelets, eventually delaying the decomposition process [31].

Nano-Indentation Analysis
3.5.1. Load Displacement Curves. The average penetration depth at peak load for the loading-unloading cycle for the pristine membrane is 195 μm. With the introduction of clay in 0.5 wt. % loading, the displacement depth decreases to approximately 145 μm, which is about 25% lower than the pristine CA membrane. The decreasing trend continued until 1.5 wt. % Bt loading due to the increase in impermeable stiffer clay platelets. Nevertheless, at higher bentonite loading (2 wt. %), the indentation depth of MMM is decreased to 26% lower than the pristine CA membrane. The degree of the dispersion of clay might increase the resistance to plastic deformation contributing to a lower depth of indentation.
A constant load of 100 mN was applied on the membrane samples, and unloading of this force imparts creep region as shown by hysteresis in Figure 8. Creep is the change in depth overtime where the force is kept constant, and a new limit is reached immediately beforehand. After that creeping area, the indenter starts to unload. As the unloading curves do not overlap with the loading curves, it implies that the developed membranes are not completely elastic but also shows plastic behavior [32]. For all the developed membranes, the loading and unloading curves appeared to be uninterrupted and stable [33].

Indentation Hardness. The indentation hardness (H)
gauges the developed membranes' resistance against plastic or permanent deformation [34]. Theoretically, the hardness is an intrinsic property of materials and varies by the change in contact depth [35]. As anticipated, the addition of bentonite clay to cellulose acetate has a significant effect on nanohardness.
In general, the rise in clay loading greatly improved the membranes' mechanical properties [36]. It is noticed that hardness increases with the rise in bentonite composition in the developed membranes as depicted in Table 1. Hence, proper dispersion can cause the membrane to become harder and to resist the penetration of indenter into the sample. For M4 and M5 membranes, dramatic increase in hardness is observed. The clay particles agglomerated at this much loading as depicted in SEM micrographs; the nano-indenter might hitting the agglomerated area. Hence, an exponential increase in hardness is observed for membranes incorporating Bt loading beyond 1.5 wt. % loading.

Elastic Modulus.
The elastic modulus of a particular membrane depends on the contact depth of the indenter and is presented in Table 1. The elastic modulus of the synthesized membranes decreases with increasing contact depth [32]. The elastic modulus increased linearly with increasing clay concentration in the CA matrix, which shows the brittle behavior of the MMMs. The presence of clay particles inside in CA chains makes them stiffer to resist chain orientation and imparts plasticity in the composite membrane. With an increase in clay concentration in MMMs, the modulus is higher than the pristine CA membrane, and therefore, MMM has more resistance to elastic deformation as stress is applied [32].

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3.6. Gas Permeation Performance. The permeation of CO 2 , N 2 , and CH 4 gases in CA and CA/bentonite clay MMMs was investigated at room temperature. The gas permeation tests were carried out in a constant pressure setup at 2, 4, and 6 bar.
3.6.1. Effect of Feed Pressure on MMM Performance. Figure 9 represents the CO 2 , CH 4 , and N 2 permeability of the developed pristine and MMMs at various Bt loadings. For the pris-tine CA membrane, with increasing CO 2 feed pressure, the permeance decreased at all pressures. This attribute of CA is associated with its glassy nature as the stiff polymer chains as well as the pendant group of CA hamper the close chain packing and enhance the available free volume. Therefore, with the rise in incident gas pressure, the polymer chains shift from freely packed to closely packed structure, thereby suppress the available free volume and restrict the gas molecules diffusion through the membrane.

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The dual-sorption model explains the sorption and diffusion of gas molecules through the polymer matrix and combined Henry's law and Langmuir behavior. The former defines the dissolution in the rubbery phase, whereas the latter explains the microvoids presence in glassy polymers [37]. CO 2 solubility increases initially with pressure according to Henry's Law; nevertheless, soon become constant and independent of pressure as the voids in glassy polymer start shrinking with further pressure increase due to Langmuir's behavior. Similarly, CH 4 and N 2 exhibited a similar downward trend with increasing pressure. This behavior of the gases is consistent with the previously published literature [38,39].     Advances in Polymer Technology Moreover, the permeability of gases across membranes depends upon the solubility and diffusivity of the incident gas molecules. The former is a thermodynamic property and depends upon the gas condensability at the membrane surface, whereas latter depends upon the shape and size of the incident molecules [4,40]. Figure 9 shows that CO 2 exhibited the highest permeability for pristine and MMMs compared to other gases at all pressure range. Since, CO 2 owing to its polar nature has higher condensability as well as solubility towards the glassy CA membrane compared to CH 4 and N 2 gases. Apart from solubility, the lower kinematic diameter and linear molecular shape of CO 2 compared to other counterparts are advantageous for fast diffusion across the CA membranes [41]. That is why the permeability of CO 2 gas is higher compared to CH 4 and N 2 gases across the developed pristine and MMMs. The permeability decreased in the following order: CO 2 > CH 4 > N 2 . The decreasing trend is ascribed to the solubility difference and kinematic diameter of the gas molecules. It is also observed that the permeability of CH 4 and N 2 across developed membranes was not considerably affected by pressure changes from 2 to 6 bar. Maryam et al. developed Pebax-Clay MMMs and reported similar behavior for CH 4 and N 2 gases against incident gas pressure [42]. This is due to the fact that for most diffusion controlled incident gases, solubility and diffusion coefficient remain unaffected with increase in pressure [43,44]. For instance, the incident gases with low-adsorbing and condensing attributes remain unchanged or slightly changed with increase in transmembrane pressure. The opposite trend is associated with the penetrants having high adsorption as well as condensation; the solubility and permeability coefficients exhibit soaring effect with an increase in pressure [43].
3.6.2. Effect of Bentonite on MMM Selectivity. Bentonite clay loading may influence the gas separation efficiency of MMMs; therefore, it is pondered as a one influential param-eter in this analysis. Figure 9 shows the CO 2 permeance of the synthesized membrane with increasing filler concentration. It is observed that with the addition of bentonite, the permeability exhibited a decreasing trend at 2 bar pressure. This decreasing trend showed uniform distribution of bentonite clay in exfoliated morphology. These exfoliated clay particles act as impermeable phase and settle in polymer chains to restrict the gas diffusion pathways [45]. The aforementioned phenomenon is the reason of the permeation decrement in pristine and CA-Bt MMMs.
This decreasing trend with Bt addition is not limited to CO 2 only, but the trend is more prominent for CH 4 and N 2 gas. As mentioned earlier, diffusion is considered as the leading transport mechanism for CH 4 and N 2 gas across the membrane. Due to the higher kinematic diameter of N 2 (3.64 Å) and CH 4 (3.80 Å), the permeability is lower compared to CO 2 (3.30 Å) gas. Furthermore, since the bentonite clay particles are impermeable and stiff materials and its uniform disruption create tortuous path for the incident gas molecules and create obstruction of the large gas molecules to pass through [46]. Figures 10(a) and 10(b) present the ideal selectivity of CO 2 /CH 4 and CO 2 /N 2 at various Bt loadings. For the pristine CA membrane, the membranes at various bentonite clay loadings showed improved ideal selectivity. It is noted that the highest CO 2 /CH 4 and CO 2 /N 2 selectivity was noticed at all pressures for 1 wt. % bentonite clay. The ideal selectivity of CO 2 /N 2 and CO 2 /CH 4 selectivity increase from 4.13 and 7.33 to 6.15 and 9.07, respectively, at 2 bar.
An upward trend in the ideal selectivity of the membranes is prevailed until 1 wt. % Bt incorporation, which then reversed with further Bt addition for both CO 2 /CH 4 and CO 2 /N 2 selectivity. For CO 2 /CH 4 , the highest selectivity attained is 13.1 at 2 bar pressure, and this is 79% higher than pristine CA membranes. Moreover, the membrane with 1% loading displayed the maximum ideal selectivity (CO 2 /N 2 )  Advances in Polymer Technology in comparison to pristine and MMMs at all transmembrane pressures. Nevertheless, the maximum selectivity noticed is 9.83 at 4 bar, which is 123% higher than pure CA membranes. Above this concentration, the filler started to form agglomerates; void morphology appeared and influenced the membrane surface and hampered the separation characteristics [47,48]. Thus, any further increase beyond 1 wt. % loadings of clay particles do not significantly increase the CO 2 /CH 4 and CO 2 /N 2 selectivity of the developed MMMs. Therefore, it is concluded that the addition of Bt clay expands the gas separation performance of the MMMs compared. Hashemifard incorporated Cloisite 15 in the PEI matrix and observed similar results [16]. Ideal morphology existed below 2 wt. % of filler loading, upon which increment caused void morphology and deteriorate the MMM separation performance.

Conclusion
In this work, the potential of bentonite clay (Bt) as the inorganic filler has been investigated in cellulose acetate (CA) membranes. The phase-inversion technique was adopted for the fabrication of pristine CA membrane as well as MMMs with various Bt loadings in the CA matrix. The thermal stability of membranes improved with increasing Bt loading due to heat resistant properties of the clay particles. The load-displacement test showed that at 2% Bt loading, penetration depth decreases to 62% than that of the pristine membrane, which was further supported by the hardness test that showed a soaring trend with Bt incorporation. Furthermore, the improved elastic modulus of the developed membranes reflects the superior interfacial interactions of Bt in the CA system.

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Gas separation tests of the developed membrane exhibited a remarkable increase in CO 2 /CH 4 selectivity by the addition of Bt at constant pressure. Generally, the permeability of incident gases decreased for MMMs with reference to the pristine membrane by virtue of the tortuosity induced by the Bt platelets inside CA matrix. The MMMs at 1 wt. % Bt loading proved to be the most suitable composition and showed superior gas separation performance. Meanwhile, the ideal selectivity of CO 2 /CH 4 and CO 2 /N 2 has soared to 79% and 123%, respectively. Thus, it can be concluded that the Bt incorporation in the CA matrix has the potential to exhibit thermally, mechanically, and morphologically improved MMMs along with superior CO 2 separation characteristics.

Data Availability
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.