Superhydrophobic ceramic hollow fibre membranes for trapping carbon dioxide from natural gas via the membrane contactor system

The membrane contactor system is one of the most important technologies to trap CO2 from natural gas. To apply this technology, hollow fibre membranes with a superhydrophobic surface must be used, where membranes were prepared from kaolin clay through the phase inversion/sintering technique and modified by three types of fluoroalkylsilane (FAS) molecules (C6, C8, C10) at different immersion times (6, 24, 48,72 h) to capture CO2 from natural gas via contacting the gas-liquid system. The kaolin was chosen due to its abundant availability at an affordable price as well as the high amount of the hydroxyl (OH) group in the surface which easily reacts with FAS during the grafting process. Superhydrophobicity was distinguished by Fourier transform infrared (FTIR), scanning electron microscopy (SEM), liquid entry pressure of water (LEPw) measurement, and contact angle (CA). The lowest pore size of the grafted membrane obtained for C8 was about 1.32 μm; it was considered the perfect target for high membrane resistance. The chosen superhydrophobic kaolin membrane was tested for carbon dioxide (CO2) capture via the membrane contactor system. With increasing time of immersion, the hydrophobicity phenomena rose gradually until superhydrophobicity property was obtained. Forty-eight hours was proven as sufficient time to obtain the desired superhydrophobicity property to avoid wetting pores of the membranes. Besides, the perfect type of FAS for separating CO2 was C8 based on the sufficient LEPw and contact angle. The reduction of pH was observed after testing the performance of using a membrane contactor to separate CO2 by using water as absorbent where pH value decreased from 6.6 to 4.3 within 1 h, which concludes the success of the gas-liquid system into removing CO2 from natural gas.


Introduction
The twenty-first century has observed a population increase and massive power consumption. Approximately 85% of global energy consumption comes from the use of petroleum-based solid and liquid petrol and diesel, natural gas, and coal [1]. Due to its efficiency, durability, and environmental benefits, natural gas is considered to be among the most desirable clean energy sources. As known, raw natural gas extracted from gas wells generally contain contaminants such as the carbon dioxide (CO 2 ) that need to be extracted or reduced before being transported and delivered to natural gas suppliers to (a) decrease the amount of gas transported in pipelines, and (b) increase the heating value and decrease corrosion by natural gas transportation. Also, CO 2 contribution increases global warming, as a result of harmful gas emissions from factories [2]. Consequently, it became necessary to find ways to reduce carbon dioxide emissions. Based on economic and environmental concerns, effective and acceptable CO 2 separation technology with a low-cost production and energy usage must be implemented. Until now, several post-combustion capture gas separation technologies, such as (a) absorption, (b) cryogenic distillation, (c) adsorption, and (d) membrane separation, were investigated.
The method of trapping CO 2 which is nearest to commercialization is the mechanism of absorption by columns using amine solution; nevertheless, it does have different limits, like significant operational and capital costs, complicated production processes, and environmental pollution [3]. The membrane separation method is defined as a very important technology for sweetening natural gas and trapping CO 2 from waste gases especially in remote locations where small footprints, durability, and much less maintenance are extremely coveted [4][5][6].
Membrane contactor (MC) system is one of the most effective techniques of getting rid of pollutants like CO 2 , and is a membrane system primarily used to contact two phases (gas mixture and water); to encourage the transfer of mass between these two phases, the separation of the gas by membranes is a process driven by pressure. This technology is selected as the best option of operational versatility, free flow of gas and liquid, high volume surface area, simple structure, easy scale-up, and modularity. Due to all these benefits, the membrane contactor is an excellent solution to separate CO 2 from a mixture of gases [7]. The important criteria for the membrane contactor are the porous structure and hydrophobicity of the membrane surfaces, allowing fast penetration of gas into a liquid and preventing liquid from penetration into the pores [8]. Since membrane materials play a significant role in controlling the transfer of gas through the membrane, the membrane materials need to be appropriately chosen to meet the requirements of the gas/liquid contacting processes.
For industrial gas separations, many kinds of research about membrane contactors were increasingly investigated using polymeric materials such as polypropylene (PP) [9] and polysulfone (PS) [10] to separate CO 2 from natural gas. Nevertheless, when implemented in high-pressure systems (> 50 bar), the application of polymer membrane is limited due to membrane deformation, plasticization, and continuous wetting throughout the procedure, which usually results in comparatively poor separation performance [11]. On the contrary, an inorganic membrane can provide specific high chemical and thermal stability and other advantages such as high mechanical properties, easy to begin preparation of the asymmetric structure, and long lifetime, and could have been prepared for the desired structure, such as dense or porous membranes, and attracted extensive study of ceramic membrane production and modification, including high temperatures, especially acidic or primary feed [12,13].
The main materials used to prepare ceramic membranes are metal oxides such as alumina, zirconia, and silica, which have main negative aspects; due to its high melting point (2050°C) and high thermal coefficient, the manufacturing of wide pore hollow fibre assistance for flux amplification is much more complicated with alumina; besides, these materials are very expensive, 1 kg of alumina is almost $250 [14]. The alternative solution is to use materials that have low price and the same properties such as dolomite [15], fly ash [16], kaolin [17,18], and apatite powder [19].
In this study, the material used is kaolin; it is a clay formed by the weathering, and consists of an abundant 1:1 clay mineral structure of Al 2 Si 2 O 5 (OH) 4 for every silica alumina containing large clogged particles of SiO 4 and octahedral sheet AlO 2 (OH) 4 . Since the clay layer is stable, the AlOHOSi hydrogen bonds are bound to both unbroken layers. Kaolin gains the preferred pore size for gas separation via membrane contactor. There are many reasons for using kaolin than other types of clays to fabricate hollow fibre membrane such as it has specific characteristics, has high ability for formation which can be used for many skills special for craftsmanship, can prepare variables with a wide range, used in catalytic amounts, easy to work and setup, moderate experimental conditions, yield and/or selectivity gains, and low price around $15/kg [14,20].
Other than that, there is a structure on the interparticle surface of an asymmetric system of octahedral sheets AlO 2 (OH) 4 with hydroxyl groups; therefore, kaolin is hydrophilic and avoids fouling of membranes due to the OH groups on the surface of these materials. This surface property thus prohibits it is being used directly in the gas-absorption membrane system as the process requires hydrophobic surface [21]. The suitable method to improve the properties of the surface is the modification via silane grafting. Perfluoroalkylsilane (PFAS) molecules are one of the various saline resources to modify the surfaces and make it non-adhesive; it is a self-assembled monolayer (SAM) consisting of varying lengths of perfluorocarbon chains with a formula (C n F 2 n + 1 -C 2 H 4 Si(C 2 H 5 O) 3 ), where C n F 2n+1 is a fluorocarbon chain and (C 2 H 5 O) could be a methoxy or chlorine grouping. PFAS contain silane agents for ceramic membrane hydrophobization for gas separation technologies. The silane structure consisting of at least one carbon-silicon bond (CH 3 -Si-) is referred to as organosilane [22]. This bond is very strong, achieving low surface energy and imparting hydrophobic impact after grafting on the surface of the ceramic membrane.
Nevertheless, limited researches have been published on surface modification of the kaolin membrane to hydrophobic surfaces [23][24][25]. Hubadillah et al. applied perfluorodecyltriethoxysilane for the surface modification of kaolin hollow membranes in membrane distillation [25]. Abdulhameed et al. [23] examined the modification by FAS (C8) molecules of kaolin-alumina powders. The authors succeed in optimizing the wettability resistance of the membrane by achieving high porosity to increase the gas permeation flux to 6.72 × 10 −5 mol/m 2 Pa 1 s 1 and also high contact angel 142°by grafting C8 to modify the membrane surface. The highefficiency separation of CO 2 of almost 90% was obtained by using the modified ceramic membrane [24].
This study aims to fabricate a kaolin membrane with an improving structure to superhydrophobicity towards a highperformance contacting method for liquid gas, which was also processed using the phase inversion-based extrusion method and sintered at 1300°C, followed by grafting fluoroalkylsilane (FAS) at different immersion times, to investigate the effect of changing the time on membrane hydrophobicity. The performance of varying PFAS molecules grafted on the kaolin powders was evaluated and discussed using different analysis techniques (SEM, CA, LEPw, and FTIR). Furthermore, the efficiency of carbon dioxide absorption by pH was studied after selecting the best type of grafted membrane to apply in the membrane contactor system.

Material
Powder clay of kaolin (Al 2 O 3 2SiO 2 2H 2 O) is with the size of a particle between 2 and-3 μm; the supplier is Kaolin Sdn., Bhd. In the whole sample, the chemical composition determined in the clay sample was presented as a relative percentage of the components representing oxides such as 50wt.% silica, 34 wt.% aluminum, 2.5 wt.% k 2 O, and iron oxide, titanium, magnesium oxide, respectively. 99.5% of methyl-2-pyrrolidone (NMP) from HPLC grade, Rathbone, and polyethersulfone (PESf) purchased from Amoco Chemicals were employed to prepare the suspension. Arlacel P135 (CRODA) was used as a solvent, for absorbent and coagulant, and tap water was used as the liquid. Ethanol (Merck, Germany) was used for the surface grafting, and fluoroalkylsilane (FAS) was supplied from Manchester Organics at different chain bonds as cleared below:

Preparation of spinning dope
The hollow fibre kaolin membrane was processed using an extrusion/sintering technique based on inversion. First of all, materials were weighed (kaolin, NMP, PESF, Arlacel). A total of 54 wt.% of NMP solvent and Arlacel 1 wt.% were mixed and stirred to dissolve these materials. Then added kaolin 40 wt.%, and gradually blended for homogeneous mixing with continuous blending. The dispersal was rolled for 2 days in planetary ball mill NQM-2 to ensure good dispersion of kaolin powder. After that, about 5 wt.% specific quantity of PESf was applied. The mixture was milled for 2 days to get uniform dope suspension. The next step was the vacuum degassing process of the (dope) suspension in order to get rid the bubbles that formed during the milling process.

Preparation precursor kaolin membrane via phase inversion technique
The technique of phase inversion, common in the manufacturing of ceramic membrane, has also been adopted, confirming that it is an efficient technique to the production of low-cost ceramic membrane with an asymmetric structure. The degassed (dope) suspension moved to the stainless steel syringes extruded is a tube in orifice spinneret with an external diameter of approximately 3 mm and an internal diameter of 1.5 mm utilizing syringe pumps (PHD 2000, Harvard Apparatus) at a steady speed of 10 ml/min and a temperature of 25°C with 10 ml/min bore fluid (tap water) as the internal coagulant bath to allow a complete phase inversion process. The air gap was maintained constant at 5 cm to run the extrusion process easily and efficiently because greater air gap distance enabled the dope suspension flow to become unstable due to the high viscosity of the dope. The hollow fibre precursor was held in water to extract the residual solvents for 24 h and then washed with water. The green fibres would then be cut to 25 cm and dried at ambient temperature for 2-3 days with good ventilation. Figure 1 illustrates the phase inversion-based extrusion method schematic diagram for the processing of kaolin hollow fibre membrane.

Sintering process
The sintering process was used to integrate the precursor of a strong kaolin hollow fibre by a rising degree of heat. To sinter the precursor, the tubular furnace was used (XY-1700 MAGNA) at high temperature. At first, the precursors were sintered at room temperature, then the temperature kept increasing to 600°C at a rate of 2°C/min and maintained for 2 h to get rid of the remaining liquid compounds, the residual organic components, and the dispersant. Then, increase the temperature to a specified temperature of 1300°C at a rate of 5°C/min and set for 4 h. The last step is to cool the furnace at a rate of 5°C/min down to ambient temperature.

Improvement superhydrophobicity of kaolin hollow fibre membrane by FAS
The wetting of the membrane is determined by the membrane and absorption liquid properties; as mentioned earlier, the surface of kaolin hollow fibre membranes is hydrophilic. To prevent the membrane wetting, the modification for surface properties can be applied by using FAS materials to improve the characterization of the surface. Three t ypes of fluoroalkylsilane chain (C6, C8, C10) were grafted to the kaolin membrane surface. The process of modification occurred by the reaction between two groups (hydroxyl which is available in the membrane); these groups were located essentially vertically to the tetrahedral sheet of another layer until intense hydrogen bonds were formed and the groups of ethoxy (O-Et) for organosilane (silane groups in FAS compounds), via the Si-O-Si bond, the excessive Si-OH of the chemisorbed silane on the membrane surface, construct additional links with the surrounding silane compounds as shown in Fig. 2. To complete the grafting process, the hydrolysis procedure should be applied by putting the membrane in a flask and filling it with 200 ml of ethanol and the rest with water to rebuild the OH group that was lost during the sintering process and then leave the membrane for one evening; after that, wash it with tap water and dry it at temperature around 100°C for 1 day. Then cut the membrane lengthwise (10 cm) and immersed it in a 0.02 mol/L FAS solution using ethanol 0.98 ml at room temperature for 6, 24, 48, and 72 h to allow proper interaction.
Most widely used technique of grafting silane agent is immersion due to easy implementation and high efficiency for membrane hydrophobization. As a result, the chemisorbed groups become strongly connected and packed; in the end, a thinner and standardized film will form [26]. The molecules on the membrane surface are completely coated by chemical bonding and intermolecular interaction, which improves the hydrophobic membrane surface's chemical and mechanical stability. Finally, the grafted membranes were cleaned with distilled water and dried all night in an oven at 100°C. As cleared from Fig. 2, the effect of hydrophobization was very successful in changing the properties of the surfaces as the contact angle changes from 20°(hydrophilic) to 144°(hydrophobic).

Contact angle measurement
Contact angle (CA) measurement is an effective way to identify the conversion of water droplet size after modification Fig. 1 The phase inversion-based extrusion method schematic diagram for the processing of hollow fibre surfaces from hydrophilicity to superhydrophobicity. This technique is applied by measuring the angle of droplet water attached to the modified membrane surfaces, to clarify the changes which occurred due to the grafting process by CA (OCA 15EC, DataPhysics, Germany), using sessile drop property. With a very high-resolution camera, it is possible to obtain highly accurate results by placing points of distilled water on different positions of an outside surface to calculate the average results to reduce errors that occur during the test.

Liquid entry pressure of water
To identify the gas permeation of the kaolin membrane, the liquid entry pressure of water (LEPw) technique was applied. A diaphragm pump was used to inject distilled water into the lumen sides of the hollow fibre membranes. The pressure started increasing at a rate of 0.5 bar, and the pressure was registered as LEPw at which the first water droplets appear on the shield outer side of the kaolin hollow fibre.

Scanning electron microscope
Scanning electron microscopy (SEM) technique of the kaolin membrane was applied with the use of SEM (Hitachi TM3000). This technique had been used to examine membrane morphology changes for kaolin surfaces and section crossings during the grafting process. To create a smoother cross-section surface, the hollow fibres were carefully cut. Before processing, the split fibres were put on a disc for spraying with such a simple gold film. At different magnifications, the SEM micrographs had been obtained of the crosssections and the shell side surfaces.
The membrane surface's average pore size distribution was calculated via SEM images using ImageJ software (Java 1.8.0_112) [27]. The segmentation of the pore shape was measured using image thresholding. SEM image was turned into white and dark regions, representing particles and pores, respectively. After the pores were outlined, the pore size was estimated by assuming cylindrical porous texture.

Fourier transform infrared
Fourier transform infrared (FTIR) spectra were obtained by using a spectrophotometer (IR tracer 100 Shimadzu) fitted with a singular-reflection PIKE Horizontally Attenuated Total wavelength adapter. More than 1000 scans over a spectrum of 600-4000 cm −1 were averaged with FTIR spectra. Membranes were dried at room temperature in a vacuum oven at 40°C after testing. The FTIR technique was used to confirm the presence of the silane function group on the membrane surface and also to estimate the efficacy of the grafting operation.

Membrane contactor setup
In membrane contactor setup, kaolin hollow fibre membranes with a length of 10 cm were arranged into a tube made from stainless steel with a diameter of 1 cm. Before starting the operation of this process, especially before pumping, the flow rate of CO 2 should be ascertained through the control unit (Cole-Parmer model 75211-15). Pressure should also be maintained by pressure gauge gas. Gas pressure should be less than liquid pressure (for gas 1 bar and liquid 1.2 bars) to avoid formation gas bubbling into liquid. After making sure to adjust these requirements, the operation of the membrane contactor begins. Through the beginning flow, both water and mixture of gases in opposite directions absorb CO 2 starting via water from the gas mixture through the porous membrane, and due to the superhydrophobic nature of the kaolin membrane surface after grafting, the droplet of water was carried onto the CO 2 and outside the membrane to collect the special tank for this acidic solution as shown in Fig. 3. Fig. 2 Scheme of the grafting process by PFAS compounds Fig. 3 Schematic of the integrated method by using a water as absorber in membrane contactor method to complete CO 2 separation from natural gas

Hydrophobicity property measurement
Determining the efficacy of the kaolin membrane grafting procedure, the membrane's wettability characteristics were assessed in terms of the contact angle value. Figure 4 illustrates the contact angles (CA) for the membrane of both original and grafted with three types of FAS agents (C6, C8, and C10) at different times. Figure 4 a demonstrates a pristine kaolin membrane called a hydrophilic membrane. This appears to mean that the contact angle (0°< < 90°), as well as the surface of the membrane, have a wetting phenomenon and this effect has an impact on the efficiency of the CO 2 separation due to the hydroxyl content of the membrane surfaces. A research carried out by [28] showed that the pristine membrane was super hydrophilic without any changes when the contact angle was equal to zero or hydrophilic when the contact angle was less than 90°.
After the grafting process, the contact angle values of all sample powder exceeded 90°; this verified that the grafted kaolin membrane had hydrophobicity. After 6 h of grafting, the nature of C6 and C8 were changed to ultra-hydrophobic with a contact angle around 131°but C10 is still just hydrophobic. With increasing time of grafting to 24 h, the surface membrane continues to change more to ultra-hydrophobic with contact angles 142°a nd 137°for C8 and C6, respectively, but for C10, around 128°. At 48 h for C6, the contact angle was around 150°compared to C8 162°, and C10 around 138°as shown in Fig. 4b.
When the time of grafting increased to 72 h, the behavior of the membrane changed to be superhydrophobic with a contact angle of more than 150°and more than 160°for C6 and C8, respectively, but for membrane grafted by C10 at this time, ultra-hydrophobic surface was obtained. This concludes that superhydrophobicity can be obtained at 48 h for C8 agent because it is the first grafted kaolin membrane that has a phenomenon of superhydrophobicity due to the highest value of CA that is in excellent accordance with the results of Lu's work [29]. Some factors make fluorodecyltriethoxysilane C8 a perfect type to get the superhydrophobic nature of the kaolin hollow fibre membrane. At first, the surface density and particle area density of the kaolin membrane for C8 were factored and were highest among the other kinds of FAS. As a result, the contact angle was also highest. Also, both roughness surface and density reached the highest value, which leads to making the contact angle largest, and this point was the main reason for the various contact angles between the samples of the FAS agent. Furthermore, C8 molecules had longer hydrophilic chains which covered a much higher surface.
So, there is a correlation between the conditions of modification (time and kind of FAS agent) which have a significant influence on the resulting degree of hydrophobicity. This interpretation is in good accordance with the work of the research groups Lee and Hoon [30,31] who also establish that increasing the grafting time results in increased water contact angle value, which has also been obtained from this study.

Pressure resistance measurement
To assess the wettability resistance of the membrane, liquid entry pressure of water (LEPw) was carried out for kaolin membranes after 6, 24, 48, and 72 h of modification (Fig. 5). In the membrane contactor systems, wetting resistance is critical as the membranes with higher LEPw will avoid liquid penetration into the pores until the pressure difference is surpassed. As illustrated, the best kind of grafted membrane was grafting with agent C8, which has an exposure for 48 h because the pressure at this point was 2 bars, and this means, at this point of pressure, the membrane has excellent features that prevent water penetration through the outside surface of the kaolin membrane inside the lumen. These findings are also in sync with the results presented by [32]. This high pressure is correlated with the high contact angle~160°, which means if the contact angle increases, the LEPw value also increases and vice versa, providing a good air pocket that produces sufficient air pockets on the membrane surface, and improving water penetration resistance. Besides, LEPw is increased by decreasing the diameter of the membrane pores, as indicated by [33].
The low value of LEPw was for C10, and this is due to the different structures of grafting molecules. But as indicated from the figure after increasing grafting time more than 48 h, the value of LEPw decreased; there are several reasons for this decline, such as the amount of hydroxyl group lowering with increasing the grafting time, as a result of a long time of interaction between the OH group on kaolin surface and silica group from FAS agent which led to losing the amount of OH in contrast to previous period of time of grating, when the amount of OH was plentiful. Furthermore, the CO 2 absorption of membrane fluxes decreased with increased FAS coating period; in addition to increasing the grafting time, the gas permeation was decreased. This concludes the pore size of the membrane is more significant than the initial time, and the value of LEPw is reduced. At last, the pores of the membrane wetting made the quality of the membrane low. Studies conducted by Rácz et al. [34] have reported the factors which affect the value of hydrophobicity and pressure resistance of water and the major relation between these two measurements. Morphology of the fabricated membranes Figure 6 shows the scanning electron microscope (SEM) to investigate the microstructure of both the outer layer and the cross-section of the kaolin membranes for pristine and grafted membranes with a variety for FAS agents at 48 h which was selected as the appropriate time to achieve superhydrophobicity. A high porosity sponge-like structure emanating from the internal surface of the pristine kaolin fibre could be obtained based on the SEM cross-sectional images as showed in Fig. 6  (A1, A2). The inner diameter of the kaolin membrane is 1.82 mm and the thickness of the wall is 1.35 mm. Meanwhile, the membrane substrate possessed asymmetric structure which comprised of the sponge-like voids and macrovoids. This finding was also following the earlier research of Abdulhameed et al.; the authors have already reported that the reason for the presence of the sponge-like formation is due to the gradual precipitation during contact with the dope suspension of the non-solvent (water), the infrequent shape of kaolin, which looked like flakes that affect the exchange between water and solvent, then leads to the formation of the sponge structure [18]. It is worth to mention the study by Hubadillah et al.; they have observed the perfect particle content to use for membrane manufacturing if the kaolin content from ceramic suspensions is 40%, for several reasons such as high viscosity, which decreases the creation of finger-like voids and creates sponge structure [35], where the reduction in kaolin content below 35-40% leads in an extremely low viscosity and thus a reduced formation of a sponge-like structure, which was the same behavior found in this study.
After the grafting process, some changes occurred in the microstructure of membranes. Figures 6 b, c, and d show the grafted membranes with three types of FAS (C6, C8, C10) respectively at immersion time (48 h). As mentioned in Fig. 6 (b1-d1), there is a slight gradual increase in dimensions (inner diameter and wall thicknesses), even the pore size of the kaolin membrane showed significant improvement at the sponge-like structure, due to the use of the FAS agent. In Fig. 6 (b2) which used C6 molecules, the size of pores is closer to more than A2 due to the reaction of OH groups on membrane surfaces with silane groups to transform surface to hydrophobic. Figure 6 c represented the grafted kaolin membrane by the C8 agent. The inner diameter and wall thickness became 1.89 mm and 1.42 mm, respectively. The cross-section of the pore becomes more closed, and the surface was denser as shown in Fig. 6 (c2, c3). It means that the membrane surface is fully covered with fluoroalkylsilane and the top layer was created by the grafted C8, which has a shallow thickness (1.42 mm) due to the polymer binder in the precursor fibres. It is worth noting that the obtained results in this study are in line with the results found by [36]. Figure 6d shows the grafting of FAS agent C10 at 48 h. The inner diameter increased to 2.8 mm and the wall thickness to 1.62 mm. It implies the interaction of FAS agent bonds with inner diameter and thickness was directly proportional. There are no big differences between the outer surfaces for all different grafting membranes, even in non-grafted ones, as shown in Fig. 6 (a-d) which means the FAS is affected in the inner more than the outer surface.

Fourier transform infrared analysis
To ensure the effectiveness of the grafting and its efficacy, Fourier transform infrared (FTIR) analysis has been applied. Figure 7 presents FTIR spectra of pristine and three types of FAS agent (C6, C8, and C10) grafted on kaolin membranes at 48 h, which was selected as the best time to get superhydrophobic phenomena. Ungrafted membrane starts at band 2851 cm −1 as mentioned in Fig. 7, since the pristine kaolin surface membranes are saturated with hydroxy groups which are the main reasons why kaolin acts as hydrophilic. After the membrane modification, the spectra changed to 2880 cm −1 correlating to the OH stretching mode of the adsorbed water. The relevant vibration band perfluorinated chains were observed at the frequency range 1244-1030 cm −1 [37]. The Si-O bond vibration was observed on frequency 1119 cm −1 ; this can refer to the presence of the Si-O-X (X = kaolin) bond as a result of the chemical Si(OCH 2 CH 3 ) 3 methyl groups and the accessible hydroxyl groups on the surface of the powder content [33]. At 1084 cm −1 , the Si-OH band was observed; this band increased with increasing grafting time; it was clear the C10 has the highest intensity for this band. For Si-O-C stretching and deformation, vibrations were located at bonds between 840 cm −1 ; this result is mostly in alignment with Jeong's work [38]. The peak at around 721 cm −1 was located in all samples and was identical to Si-C stretching motion. It can be concluded that the appeared peaks confirms the occurence of the reaction between kaolin and FAS agent for all samples ks, and the highest intensity value bond was recorded for C10 followed by C8 and then C6,  Fig. 7 Kaolin hollow fibre spectra FTIR before and after grafting with three FAS groups (C6, C8, and C10) at 48 h while the lowest one was the pristine due to the presence of OH group.

Influence of membrane pore size and physiochemical properties on modified surface
Kaolin membrane pore sizes are of vital importance to the permeability of the coated membrane. Ungrafted membrane has big pores; this increases the wettability of the membrane; therefore, small pore size improved membrane surface stability. After grafting by FAS agents, many properties changed and all improvements on surface membrane were related to pore size. The smaller pore size of the membrane refers to a higher density surface and less open space over its surface. The existence of further surfaces means an increase in the quantity of OH groups available for attaching on the membrane surface. This can allow the creation of several silanol bonds with the ceramic surface to grant the membrane-improved hydrophobicity. This was verified by the steady rise in contact angle values with a decline in the size of the membrane pore as occurred after 48 h. Figure 8a illustrated the highest CA around 160°for membrane coated by C8 agent have lower pore size around 1.32 μm, meaning the best one due to the lowest pore size, the same finding has also been verified by [39,40]. At same line for the highest CA, the highest value of LEPw leads to higher stability of the membrane; as mentioned in Fig. 8b, at 2 bars, the bore size was 1.32 μm, which means with the decline of membrane pore size, LEPw rises. High membrane permeability also helps to increase the flux via the membrane, besides the pore size. Consequently, for the gas separation, ceramic membranes, noted for being highly porous, were indeed attractive.
The procedure of hydrophobization modified the physicochemical characteristics of ceramic membranes reflected by their changes in the wetness property associated with the tribological properties of a surface nanolayer, at result to change in dynamic behavior of the surface changes. In addition, improved surface of membrane strength showed that there was no strain rate activity or membrane faults due to the coating process, and gave the membrane mechanical strength to resist the necessary pressure and provide some physical bonding to the membrane due to the membrane surface's apparent change pore size. As a result of all these changes, it was observed that the coating time had a significant effect on increasing hydrophobicity, because after the grafting process, the surface properties of kaolin were changed from hydrophilic to superhydrophobic and thus prevents the physical interaction between the membrane pores and the solvents, which prevents membrane wetting, in addition the changes in surface properties such as low average pore size values of surface which is closely related with the high contact angle and LEPw

Performance of CO 2 absorption
The absorption CO 2 performance should be estimated to ensure the efficiency of material used to complete the process of CO 2 stripping in the membrane contactor system through a mixture Fig. 8 a Contact angle versus surface's average pore size. b LEPw versus surface's average pore size, for grafted kaolin hollow fibre with C6, C8, C10 at 48 h of gases (CH 4 , CO 2 ) and non-solvent (water) as solute gas and absorbent gas respectively. By presenting the gas mixture pumped through the lumen side, the testing was conducted out and water was pumped through the shell side at the same time in the mode of counter-current. pH values as a function of different periods for kaolin membrane grafted by C8 were selected as the best type of FAS to provide superhydrophobic membrane at 48 h as shown in Fig. 9. Figure 9 shows the first point for reverse osmosis (RO) water which has a pH of around 6.6; this normal value for RO water, means it is almost free from CO 2 . Ten minutes later, the value of pH decreases to 5.5, and by increasing the time of using a membrane contactor system, the value of pH gradually goes down to reach 4.3 after 1 h. The reason for the decrease in pH with increasing the absorbing time was because the amount of CO 2 absorbed was increased and led to a decrease in the amount of CO 2 in a mixture of gases then made it almost pure from this impurity, and made the solution more acidic, for that, pH value decreases. These results are also in line with findings presented by Sedghi et al. [41]; the authors modified the polyethylene hollow fibres to use in the membrane contactors; they noticed that the chemical degradation of the membranes appears to be less significant in the presence of CO 2 , and this leads to a reduction of the pH of the loaded MEA solutions. In another study carried out by [42], in their studies, due to CO 2 absorption, the most significant pH decrease from 6.01 to 3.69 was obtained to separate CH 4 and CO 2 utilizing membrane contactors. Also, it should be mentioned that with increasing time of separation, the membrane was wetted and led to a decrease in the efficiency of separation. Therefore, the kaolin membrane should be maintained from time to time to prevent the wetting membrane. This concludes the success of gas-liquid membrane contactor to absorb carbon dioxide by using water as an absorber and all this happens due to using the membrane grafted by C8.

Conclusions
The kaolin powder was effectively modified from hydrophilic to superhydrophobic by using the extrusion-based phase inversion process. Coating the kaolin surface by C6, C8, and C10 was successfully applied to improve wetting resistance and separation performance in the gas-liquid contactor system for efficient CO 2 removal. The mode of anchoring FAS molecules depends on the FAS type and the time of grafting.
The grafting time is the most important for obtaining the highest hydrophobicity for ceramic materials; a hydrophobic increase is, therefore, observed by increasing the grafting time. By contact angle analysis, the highest angle obtained was 160°for C8 at 48 h, which means the superhydrophobic phenomenon was achieved. SEM examines the microstructure of the membrane, the modifications observed on the inside of the hollow fibre membrane rather than the outside surfaces, due to the impact on the surface of the FAS bonds. The higher wettability resistance has occurred, when getting LEPw value 2 bars. A reduction in membrane pore size was seen at high contact angle values for three grafted membranes. It was also noted that with the decline in membrane pore size, the LEPw value of the modified membrane rose.
In conclusion, 48 h was appropriate to avoid the wetting of membrane pores, and the best type of FAS used for CO 2 separation from the gas mixture was C8. Further studies are to fabricate superhydrophobic surfaces that have the long-term thermal and chemical stability of the superhydrophobic membrane. Also, used materials can get the superhydrophobicity at a shorter time with prevention wetting membrane, in order to be implemented in MC practice at larger scale.