CH4 separation

CH4 separation

Journal Pre-proof Biomass Derived Carboxylated Carbon Nanosheets Blended Polyetherimide Membranes for Enhanced CO2/CH4 Separation Mohd Yusuf Khan, Abu...

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Journal Pre-proof Biomass Derived Carboxylated Carbon Nanosheets Blended Polyetherimide Membranes for Enhanced CO2/CH4 Separation Mohd Yusuf Khan, Abuzar Khan, Jimoh K. Adewole, Mohd Naim, Shaik Inayath Basha, Md. Abdul Aziz PII:

S1875-5100(20)30010-X

DOI:

https://doi.org/10.1016/j.jngse.2020.103156

Reference:

JNGSE 103156

To appear in:

Journal of Natural Gas Science and Engineering

Received Date: 16 October 2019 Revised Date:

6 January 2020

Accepted Date: 8 January 2020

Please cite this article as: Khan, M.Y., Khan, A., Adewole, J.K., Naim, M., Basha, S.I., Aziz, M.A., Biomass Derived Carboxylated Carbon Nanosheets Blended Polyetherimide Membranes for Enhanced CO2/CH4 Separation, Journal of Natural Gas Science & Engineering, https://doi.org/10.1016/ j.jngse.2020.103156. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Graphical Abstract

Biomass Derived Carboxylated Carbon Nanosheets Blended Polyetherimide Membranes for Enhanced CO2/CH4 Separation Mohd Yusuf Khana*, Abuzar Khana, Jimoh K. Adewoleb, Mohd Naimc, Shaik Inayath Bashad, Md. Abdul Aziz*a a

Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran 31261, Saudi Arabia b Membrane Science and Engineering Laboratory, Process Engineering Department, International Maritime College, Sohar, Sultanate of Oman c School of Chemical Engineering, University Sains Malaysia d Civil and Environmental Engineering Department, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran 31261, Saudi Arabia E-mail: [email protected]; [email protected]

Abstract: The fillers/nanofillers combinations with ideal interfacial morphologies could adjust the performance of pristine polymeric membranes and enhance the gas separation performances. In this study, biomass-derived 2D carboxyl functionalized nanocarbon (nanosheets) were produced from pyrolysis of the rain/monkey pod tree (Samanea saman) leaves. The X-ray diffraction, Raman spectroscopy, and X-Ray photoelectron spectroscopy (XPS) confirmed the chemical structure and carboxyl functionalization of carbon nanosheets (CCNS). The carboxyl-functionalized nanosheets blended into polyetherimide (PEI) solutions was used to fabricate mixed matrix membranes (MMMs) through modified dry/wet phase inversion method using different wt. % [0.0%, 0.25%, 0.5% and 1.0% of CCNS] of fillers into the pristine PEI NMP solutions. Surface and cross-sectional micrographs discerned the defects and dispersion of CCNS into the polymer matrix. Finally, the transport properties of pure gases (CH4 and CO2) were evaluated using constant pressure/variable volume apparatus. The gas separation studies showed improved permeance and selectivity of MMMs [(PCO2 = 1.84 GPU, (α = CO2/CH4) = 42.73] than pristine PEI membranes [PCO2 = 1.29 GPU, (α = CO2/CH4) = 23.30] under identical conditions of pressure and temperature. Keywords: Carbon nanosheets; Biomass derived carbon; Polyetherimide; CO2/CH4.

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1. Introduction Since decades, natural gas (NG) has been a popular source of energy with continuous increasing worldwide demand (Berg, et., 998; George, et., 2016). The raw NG is often adulterated with impurities such as CO2 (acid gas) during production that needs to be removed. Therefore, it is highly desired to develop competent technologies that can treat large volume CO2 (mainly removal of CO2 from CH4) to improve NG calorific value and reduce the gas pipeline corrosion(George et al., 2016; Wang et al., 2016). The commonly employed approaches to remove CO2 are cryogenic distillation, adsorption, and membrane separation (Ahmad et., 2008; Dalane et.,2017; Mahajan et.al, 2000). However, membrane technology is an attractive and promising method and has an edge over conventional technologies due to its high energy-efficiency, simple design with high flexibility, small footprint and low environment impacts (Adewole et.al, 2017; Adewole, et.al, 2013). The polymeric membranes are the most applied membranes for the gas separations due to their high selectivity and low throughput (Qahtani et.al, 2018; Baker et.al, 2014). However, the major constraint is permeability-selectivity trade-off despite excellent processability (Qiu et al., 2011; Robeson et.al, 2015; Zhang et al., 2019). On the contrary inorganic membranes exhibit outclass performances under harsh operating conditions (high temperatures and pressures) (Yun et.al, 2011). The major setbacks associated with inorganic membranes are difficulty in fabrication and high production cost (Khakpay et.al, 2017; Koros et.al, 2017; Rangnekar et.al, 2015; Zhang et.al, 2015). Mixed-matrix membranes (MMMs) consisting of polymer-inorganic hybrid acquire the collegial advantages of the polymer and inorganic phase (Chung et.al, 2007; Aljundi et.al, 2018). Furthermore, the movement of permeating molecules through polymeric membranes is controlled by the permeability of the molecules through the membranes. The permeability is the product of the diffusivity coefficient and solubility coefficient (Adewole et.al, 2017). Diffusivity coefficient is a kinetic parameter that measures the mobility of individual molecules through the polymer chains. The solubility is a thermodynamic parameter that provides information on the sorption uptake in the membranes (Adewole et al., 2013). One of the strategies to develop a membrane, which 2

surpass the permeability-selectivity trade-off, is by increasing the selectivity without decreasing the permeability. The diffusivity can be improved by incorporating inorganic materials with molecular sieve capability. The solubility can be enhanced by choosing a polymer that possesses functional groups such as carboxyl, amine, oxygen ether, and hydroxyl (Adewole et al., 2016). Solubility can also be improved by functionalizing the inorganic fillers with any of these functional groups. Therefore, a mixed matrix approach for membranes fabrication is adopted in this study to separate CO2 from CH4. MMMs comprising chiefly of 2D nanomaterials (as filler particles) offer tremendous opportunities to overcome the hitches of pure polymer based or inorganic membranes (Koros et al., 2017). Moreover, the use of functionalized fillers can provide some synergistic effects which results in the overall enhancement of the MMMs. Therefore, the research on 2D nanomaterials to produce advanced MMMs with high permeability and selectivity is on continuous rise (Cong et al., 2018; Zhu et al., 2018). The recent use of 2D nanomaterials comprising Mxene nanosheets (Ding et al., 2018; Lukatskaya et al., 2013; Urbankowski et al., 2016), metal–organic framework (MOF) nanosheets (Islamoglu et al., 2017; Peng et al., 2014; Rodenas et al., 2015; Ullah et al., 2019), zeolites (Varoon et al., 2011) and graphene-based 2D materials (graphene oxide, and reduced graphene oxide) (Gin et al., 2011; Wang et al., 2016; Wong et al., 2019; Zhou et al., 2017) has led to a substantial growth of molecular sieving membranes with unique apertures for energy-efficient gas separation (Cong et al., 2018; Ma et al., 2019). The MMMs blended with carboxylated nanosheets of graphene oxide (GO) into polymer matrix displayed improved CO2 permeability and selectivity for gas separations applications (Kim et al., 2019, Li, et al., 2015). The CO2 selectivity of the MMMs enhanced even using small amount of GO nanosheets into polymeric membranes without significant affecting the CO2 permeability. It was largely the presence of oxygen rich functional groups (-OH, epoxide and -COOH) on GO, which are responsible for hydrogen bonding interactions between GO–GO nanosheets and GO-polymer and high CO2 sorption capability of GO nanosheets (Karunakaran et al., 2015). The carbon derived from biomass precursor could be inexpensive and ecofriendly as compared to carbon from traditional sources (like coal, pitch and phenolic resins) since, the biomass are biowastes which are often cheap, renewable and 3

environmentally friendly (Gao et.al., 2017). Moreover, there is no study on biomassderived carbon (from monkey pod tree), as 2D fillers in MMMs for improving the gas separation ability. Therefore, in this study, CCNS was synthesized from leaves of monkey pod tree (Samanea saman), which is found in almost every part of the world. The obtained CCNS were exploited as blends into PEI membranes for the separation of CO2 from CH4. This is the first effort so far to synthesize and investigate the characteristics of 2D nanocarbon obtained from monkey pod tree for gas separation applications. The effect of different loadings of CCNS on physicochemical, morphological and CO2/CH4 separation properties were well investigated and compared to the pristine polymeric membrane.

2. Experimental 2.1. Materials Most of the chemicals were procured from Sigma-Aldrich except ethanol, which was received from Merck. Polyetherimide [melt index 9 g/10 min (337 °C/6.6kg)] was used to fabricate the pristine and mixed matrix membranes. A less volatile solvent N-methyl-2pyrrolidone (NMP) (anhydrous, 99.5%) with analytical grade was used, and tetrahydrofuran (anhydrous, ≥ 99.9%, inhibitor-free) was the primary volatile solvent, while ethanol (absolute, ≥ 99.8%) was picked as a non-solvent additive. Sodium bicarbonate (anhydrous, ACS reagent, ≥ 99.7%), Sulfuric acid (ACS reagent, 95.098.0%), Hydrochloric acid (ACS reagent, 37%), Nitric acid (ACS reagent, 70.0%) were purchased from Sigma-Aldrich. Rain/monkey pod tree leaves were acquired from the west region of Mominpur, Keshabpur, Jessore, Bangladesh. The high purity (99.995%) methane (CH4) and carbon dioxide (CO2 = 99.9%) were obtained from Abdullah Hashim Company, Dammam, KSA. 2.2. Synthesis of biomass-derived carbon nanosheets (CNS) The carbon nanosheets were produced from the monkey pod (Samanea saman) tree leaves. The leaves were thoroughly washed with tap water followed by deionized water (DI) to eliminate the dust and exotic particles. Later, the cleaned leaves were dried at 100 °C in an electric oven for 24 h. A household grinder used to micronize the dried leaves. The micronized powder was filtered through sieves (100-µm) to get a particle size of 4

≤100 µm. The obtained powder and NaHCO3 in a mass ratio of 1:4 was blended in a mortar and pestle to obtain the homogenous mixture. The homogenous mixture was pyrolyzed under N2 atmosphere in a tube furnace at 850 °C for 5 h. Finally, the resultant product was washed twice with 0.5 M HCl and thrice with DI water and dried overnight at 60 °C in an oven to acquire the carbon nanosheets. 2.3.

Functionalization of CNS with carboxylic acid (CCNS)

The CNS nanosheets. (0.2 g) was added into a 200 mL solution of concentrated H2SO4 and HNO3 (3:1 by volume), and the suspension was sonicated for about 8 h for carboxylic group functionalization. Later, the solution was diluted with the surplus amount of water and left undisturbed for 8 h (Aziz et al., 2008; Peng et al., 2017). settled down the CNS nanosheets due to gravity. The upper part of the diluted solution was decanted, and the process was repeated several times. The settled carboxyl functionalized CCNS was filtered and washed several times with DI water followed by drying at 60 °C for 24 h. 2.4. Membrane fabrication method Gas separation membranes are often prepared using the phase inversion method. This method can produce both dense (non-porous) membranes as well as porous membranes. The type of membrane produced depends on a lot of factors including (but not limited to) the type of polymer, solvent, non-solvent, free standing time, and the concentration of polymer solution. Herein, we optimized the conventional dry/wet phase inversion method to attain the better dispersion of CCNS nanofiller to make defect-less membranes. The distinction between membranes obtained from conventional and modified dry/wet phase method is shown in Fig.1. The membrane obtained from conventional dry/wet phase inversion method (Mazinania et al., 2017) showed shrinkage behavior (Fig.1 A), which was not observed when the casted membrane was allowed to stand for 1 h at 50 °C before immersing into water bath i.e. modified dry/wet phase inversion method (Fig.1 B). Therefore, all the membranes used in this study were fabricated by this method. The asymmetric nature of the membrane can be easily distinguished by observing the two sides of the membrane (Fig.1 C&D). The same membrane was later dried at 100 °C and heated upto 180 °C (Fig.1 C&D) to ensure complete removal of moisture and high boiling solvent. Furthermore, the effect of heating (at 180 °C) on the 5

morphology and the pore structure of the membranes revealed that the heat treatment had minimal effect on the morphology and the pore structure of the membranes (Fig.1 E&F). 2.5. Fabrication of pristine and mixed matrix membranes Pure PEI beads and plant derived CCNS were dried at 100 °C for 24 h to eliminate traces of adsorbed molecules before casting membranes. In this study, one pristine and threemixed matrix membranes (MMMs) were fabricated with the thicknesses between 150 ±5 µm (fabricated through modified dry/wet phase inversion method as mentioned above). The ratio of polymer and polymer filler to solvent for all the membranes was kept identical (20% PEI or PEI + Filler to 80 wt. % solvent or mixture of the solvents). The three different wt. % (0.25%, 0.50, and 1.0%) of (CCNS) was blended into the pure PEI solution maintaining 99.75, 99.50 and 99.00 % of PEI respectively. The CCNS was dispersed into NMP with probe sonicator (15 min) to ensure proper dispersion. 10 wt. % of PEI was added to the dispersed (CCNS) and the solution was stirred for 30 minutes at 60 °C. Finally, remaining (90 wt. %) of PEI was added to the above solution and stirred at the same temperature for 2 h to ascertain complete PEI dissolution. The degassing was performed in ultra-sonication bath to remove remaining bubbles and solution was left at room temperature for about 1 h. The pristine PEI and MMMs were fabricated onto a flat, smooth and clean, and glass plate. The solution was spread to a uniform thickness by pneumatic force using a casting knife. The spread solution was kept at 50 °C for about 1 h in convection oven. Then, the casted membranes were immersed into the water for coagulation and kept immersed in the bath for overnight. To ensure the complete removal of NMP solvent from the membranes, the membranes were immersed in 80% ethanol and pure ethanol for 1 h, respectively. Finally, the membranes were dried in an electric oven at 100 °C (6h) followed by 180 °C (12h) to remove the residual moisture and solvent respectively. The general schematic of membrane fabrication is depicted in Scheme 1.

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Fig.1. Photographs and images of Membranes casted by different methods (A) Membrane casted by conventional phase inversion method, (B) Membrane casted by modified dry/wet phase inversion method (after keeping at 50 °C in the oven for 1 h) before immersing into H2O bath, (C & D) Opposite sides of the same membrane casted by modified dry/wet phase inversion method after immersing into H2O bath and drying and (E & F) FESEM images of the membrane (C&D) before (E) and after heating (F) at 180 °C.

2.6. Evaluation of Gas Permeation Performance Constant pressure/variable volume system was used to evaluate the transport properties of pure gases (CH4 and CO2). The apparatus consists of three components, a permeation cell, a flow controller and soap-film bubble flowmeter. The flow controller and soap-film bubble flowmeter was connected on the upper and downstream side of the steam respectively. The permeation cell was placed in an oven with controlled and the desired temperature. The entire system was evacuated by vacuum pump for 2 h before each experiment to take off all possible residual gases in the system. The pure gas 7

CH4(99.995%) or CO2 (99.9%) was tested in the order of CH4 and CO2 and repeated three times for each membrane. The flow rate/flux of gases was monitored by bubble flowmeter. The gas permeance (P/l) of the membranes at steady-state condition was determined using the below equation (1):

P 22, 414 p1 1 dV = l A ( p 2 − p1 ) RT dt

(1)

Where, A denotes the area of membrane (cm2), p 2 and p1 represent feed (upstream) and permeate (downstream) pressures, respectively. R and T indicate is universal gas constant (6236.56cm3cmHg/molK) and the absolute temperature in Kelvin (K), while dV/dt is the volumetric displacement of the soap-film in the bubble flowmeter (cm3/s) and 22,414 is the number of cm3 STP of penetrant per mole. The active permeation area of each membrane was 12.25 cm2. The permeance (P/l) values are reported in gas permeation unit (GPU), which is defined as 1 GPU = 1 × 10−6 cm3(STP)/(cm2·s·cmHg). The separation factor or ideal selectivity (αA/B) was calculated by Equation 2, taking the ratio of permeances of CO2 to CH4 as shown below:

αA /B =

PA / l A PB / l B

(2)

2.7. Structural and morphological characterizations The 2D nanocarbon powder samples were characterized for crystallinity and phase structure using X-ray diffractometer (Rigaku MiniFlex) which was operated from 2θ = 5° to 70° at a scan rate of 0.05 degree/min. The Cu Kά (λ = 0.15406 nm) was used as radiation source. Raman spectra of the samples were acquired from 500-2500 cm1

with the excitation wavelength of 532 nm on a Horiba iHR320 Raman spectrometer

which was equipped with CCD detector and a green laser (300 mW). BET (Micromeritics ChemiSorb 2750) was used for surface area measurement and pore size calculation of the resultant carbon nanosheets. The XPS equipped with an Al K-alpha micro-focusing X-ray

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monochromator (ESCALAB 250Xi XPS Microprobe, Thermo Scientific, USA) was used to analyze the chemical structure and carboxyl functionalization of CCNS. Field Emission Scanning Electron Microscope (FESEM, Tescan Lyra-3 Dual Beam instrument) equipped with an Energy Dispersion Spectrometer (EDX, Oxford Instruments) discerned the morphological features, confirmed the constituent elements and their ratio in the membranes. The pristine and MMMs samples were analyzed for thermo gravimetric analysis (TGA) using Mettler Toledo instrument. The samples were heated from 30 °C to 700 °C at a heating rate of 5 °C/min. All the experiments were conducted under argon atmosphere with 20 ml/min flux. Fourier transform infraredattenuated total reflectance spectrophotometry (FTIR-ATR) spectra of pristine and MMMs were recorded on Nicolet 6700 spectrometer.

Scheme 1. Mixed Matrix Membranes (MMMs) fabrication scheme.

3. Results and Discussions Field-emission scanning electron microscopy (FE-SEM) of biomass-derived carboxyl functionalized carbon nanosheets (CCNS) was conducted to analyze the surface morphological features of the as-synthesized carbon nanosheets. The typical FE-SEM images represented in Fig. 2 (A-B) noticeably confirm sheet like morphology of resultant carbon possessing a highly ordered sheet like structures. The expanded view of FE-SEM 9

(Fig. 2B) revealed detailed nanosheet structure, which confirms that the size of each nanosheet is around ~200-500 nm with the thickness of about ~5-10 nm.

Fig. 2. FE-SEM images of functionalized carbon obtained from monkey pod tree. (A) Low magnification, (B) high-magnification, (C) energy dispersive X-ray spectroscopy (EDS) and (D & E) elemental mapping of the functionalized carbon. The energy dispersive X-ray spectroscopy (EDS) was used to confirm the chemical content of the nanosheet sample confirming the existence of C and O (Fig. 2C). The high intensity peak beside C and O between 1-2 ekV in Fig.2 (C) corresponds to Aluminum tape used to disperse the sample for FESEM analysis. The purple and yellow color spectrum in elemental mapping depicted in Fig. 2 (D & E) respectively, denote the presence of oxygen (O), and carbon (C) in homogeneous fashion. The homogeneous distribution of oxygen validates the claim of functionalization of carbon throughout the carbon matrix.

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Fig. 3. (A) Powder XRD, (B) Raman spectra, (C) Nitrogen adsorption–desorption isotherm and (D) pore-size distributions of CCNS. XRD pattern of CCNS depicted in Fig. 3 (A) indicates that pure porous carbon was obtained. The broad diffraction peak at 2θ of 23° and 43° corresponded to the (002) and (101) reflections respectively (Peng et al., 2017),(Ahammad et al., 2019) that could be assigned to the amorphous carbon (JCPDS no. 41-1487) (Lukatskaya et al., 2013). Raman-spectrum shown in Fig. 3 (B) designates two broad peaks ascribed as D and G-bands, which are characteristic peaks of graphitic carbon. The D and G-band peaks observed around ~1589 cm−1 and 1360 cm−1 regions respectively correspond to the sp2 hybridized amorphous carbon. The ratio intensity (R = IG/ID) of the G-band to the intensity of the D-band is ∼1.25, which confirms that the graphitic nature is dominating over amorphous nature (Han et al., 2011). Fig. 3 (C) depicts the N2 adsorption–desorption isotherms of the obtained CCNS measured at ˗196 °C from 0 to 1 bar. The result showed a type-II adsorption–desorption isotherm for the 11

porous nanosheets of carbon with a high N2 adsorption–desorption capacity. The specific Brunauer–Emmett–Teller (BET) surface area and mean pore diameter (Fig. 3D) of the carbon nanosheets were obtained 536.12 m2 g−1 and ~40 A° respectively. This BET surface area of carboxylated carbon nanosheets was found even more than that of CNT and graphene (Cañete-Rosales et al., 2012).

Overall Syurvey (CCNS)

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Fig. 4. (A) XPS overall survey and (B) High-resolution XPS of C1s spectra of resultant CCNS. The elemental composition and functionalization of the resultant CCNS was investigated through X-Ray photoelectron spectroscopy (XPS). The overall survey and high resolution of C1s spectra of CCNS are depicted in Fig 4. The carbon (C1s) and oxygen (O1s) peaks in the overall survey are positioned around 284.6 and 532.4 respectively (Fig.4 A). The peaks at 284.6 and 288.4 (Fig.4 B) corroborated the presence of C-C/C=C and O–C=O (carboxyl group), which were validated with the reported literature (Joseph et al., 2019; Tian et al., 2016). The presence of O–C=O (carboxyl group) confirmed the functionalization of CNS. The oxygen (O1s) peak observed around 532.4 might be due to CCNS sample or/and the glass substrate since the sample was prepared on glass substrate for XPS analysis.

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3.1. Membrane morphology

Fig. 5. (A-D) FESEM images of the upper surfaces, containing 0.0, 0.25, 0.50, and 1.0 wt. % of CCNS fillers into PEI membranes, respectively. FE-SEM spectroscopy was used to investigate the morphological attributes of the prepared pristine and MMMs. The pristine PEI membrane was white and opaque, as the wt. % of CCNS loadings increased, the membrane color changed from lighter to grey. All the membranes exhibited a homogeneous dense and integrated structure with single phase. Fig. 5 (A-D) represents the upper surface images of pristine PEI and MMMs containing different filler loadings (0.0-1.0% of CCNS) into PEI solutions. The 13

micrographs of pristine PEI membrane surface were flat and smooth deprive of any defects. The morphology of the CCNS blended membranes was comparable to the pristine one. All the CCNS achieved excellent dispersions into PEI matrix without any apparent agglomeration and no interfacial gaps were observed between the polymeric and the CCNS particle phase, demonstrating good compatibility as shown in the Fig. 5 (A-D). Better adhesion of CCNS with PEI was anticipated due to possibility of hydrogen bonds interactions between carboxyl functionalized carbon nanosheets (CCNS) and PEI, which resulted in enhancing the separation performances of the MMMs (Yang, Chuah, Nie, & Bae, 2019). It must be noted that these results were acquired without any surface modifying agent. It could be affirmed that the “filler particles–polymer chains” contact is acceptable as corroborated by the permeation measurements (Chuah et al., 2019; Yang et al., 2019).

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Fig. 6. FE-SEM cross-sectional images of pristine PEI and 0.5 % CCNS/PEI MMMs. (A) low magnification, (B) high magnification images of pristine PEI. (C) low magnification and (D) high magnification images of 0.5 % CCNS/PEI MMMs. Inset (C) active later. The MMMs fabricated by the addition of CCNS showed a single phase and dense homogeneous morphology. Therefore, PEI/CCNS mixed membranes could be considered as a new polymer phase. This new polymer phase could be discerned from the crosssectional views of MMMs.

Therefore, SEM images were acquired from the cross-

sections of the pristine PEI and 0.5% CCNS/PEI MMMs. Fig. 6 (A&B) represents the low and high magnification cross sectional views of pristine PEI membrane. The crosssection analysis of pristine PEI membrane demonstrated uniform and clean surface without any visible defects. Fig. 6 (C&D) illustrates low and high magnification images of MMMs with 0.5% CCNS loadings, which showed that the morphology of the internal matrix was distinctly changed from pristine membrane (Fig. 6 B&D). It was observed that different loadings of CCNS were homogeneously dispersed and embedded into PEI matrix. Moreover, blending of CCNS into PEI membranes (containing upto 0.5 wt. %) demonstrated better CO2 permeance in pure gas permeation measurements. Mostly, the carboxylate functionalized nanoparticles exhibited good compatibility with the polymer due to presence of carboxylic and hydroxyl group on the surface of the carbon nanosheets, which can result in an increased interaction with the polymer (Li et al., 2018). CCNS were disseminated well on surface of the membrane and membrane morphology altered due to CCNS loadings as can be distinguished from cross sectional view of pristine and mixed matrix membrane. 3.2. Single gas permeation through pristine and MMMs The constant-pressure/variable-volume apparatus was utilized to measure the single-gas permeabilities of membranes. The complete permeation apparatus was positioned in a convection oven. The whole permeation apparatus was evacuated from both (upstream and downstream) sides to avoid any contamination of the pure permeating gases by the atmospheric air.

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Fig. 7. (A) The single gas permeance plots (CO2 and CH4), (B) Ideal selectivity of pristine PEI and MMMs with different wt. % CCNS loadings (0.25, 0.50 and 1.0), at 35 ⁰C and 1 bar, (C) Single gas permeance plots (CO2 and CH4) and (D) Ideal selectivity

trend of MMM with 0.5 % of CCNS loading at 25⁰C, 35 ⁰C and 50 ⁰C at 1 bar. The fabricated pristine and MMMs were evaluated for single pure gases (CO2 and CH4) permeabilities at 35 ⁰C and 1 bar feed pressure. The pure gases CH4 and CO2 were analyzed sequentially for each membrane with an active area of 12.25 cm2 as described in experiment section. The permeance and selectivity results obtained for CO2 and CH4 using pristine and MMMs with different CCNS loadings are illustrated in Fig. 7 (A&B). The permeation results of MMMs (CCNS as fillers) indicated a substantial increase in permeance and selectivity compared to pristine PEI membrane after incorporating CCNS. However, best results in terms of permeability-selectivity were noticed using 0.5% CCNS loadings. Small decrease in permeability and selectivity was witnessed using 1% of CCNS loading. It could be due to less dispersion of CCNS in a PEI solution, resulting the performance of the MMMs. This behavior of the MMMs can be correlated in agreement with the previous analogous studies (Castarlenas et.al, 2017).

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Scheme 2. The possibility of hydrogen bonding of CCNS with PEI.

The increase in permeabilities was more evident for CO2 than CH4 in almost all the cases. These results evinced that incorporation of carboxyl functionalized carbon nanosheets could influence the diffusion selectivity of CO2. Moreover, the presence of large amounts of carboxyl on the surface of carbon nanosheets creates the possibility of hydrogen bonding with PEI matrix as shown in the Scheme 2. This hydrogen bonding could induce the alteration in structure at the interface inside the polymer matrix, which removes the possibility of interface defect formation as well as enhance the permeation of MMMs (Cong et al., 2018). More importantly, both the CO2 permeability and CO2/CH4 selectivity of MMMs were significantly enhanced after the incorporation of CCNS fillers. Furthermore, separation performances of the best MMM containing 0.5% CCNS loading was studied at different temperatures (25, 35 and 50 °C). The results obtained at different temperature keeping other parameters identical are displayed in Fig. 7 (C&D). The noticeable enhancement in CO2 permeabilities with increase in temperature was observed (Fig. 7C) in all the cases. However, selectivity trend indicated slight drop (Fig. 7D) at 50 °C. The loss of selectivity (CO2/CH4) of the membranes experience with the rise in temperature was through the Arrhenius relationship and is well documented in literature (Cheng et al., 2017).

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4. Conclusions In this study, we successfully employed a robust strategy to produce biomass-derived carbon 2D nanosheets from monkey pod tree leaves. The different wt. % of carbon nanosheets was blended as nanofiller to the PEI polymer matrix. The modified dry/wet phase inversion method was utilized to fabricate smooth and defect-free pristine PEI membrane and MMMs. In addition, transport properties (permeance and selectivity) of the fabricated membranes were evaluated and compared using constant-pressure/variablevolume apparatus for pure gases (CO2 and CH4) at 35 ⁰C and 1 bar. The blending of CCNS led to the enhancement of CO2 permeance and selectivity (CO2/CH4) of MMMs [(PCO2 = 1.84 GPU, (α =CO2/CH4) = 42.73] compared to pristine PEI membrane [PCO2 = 1.29 GPU, (α = CO2/CH4) = 23.30]. The increase in CO2 permeance observed in MMMs could be due to the possibility of H-bonding between PEI matrix with CCNS possessing –COOH group. The MMMs with 0.5% of CCNS filler showed the best performance. Moreover, the performances of the best membrane (0.5% of CCNS filler) studied at different temperatures (25, 35, 50 ⁰C) revealed that the permeance of CO2 increased with the rise in temperature. However, divergent trend in selectivity was noticed at 50 ⁰C. This robust newly developed strategy to produce MMMs using biomass-derived nanocarbon may have great prospective in CO2/CH4 separations. Acknowledgements The authors acknowledge the financial support provided by the Center of Research Excellence in Nanotechnology (CENT) and King Fahd University of Petroleum & Minerals, KSA.

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Highlights •

Synthesis and characterization of biomass-derived 2D nanocarbon (nanosheets) from monkey pod tree leaves.



An excellent dispersion of carboxylated carbon nanosheets (CCNS) into PEI matrix.



Incorporation of CCNS into PEI enhances CO2/CH4 selectivity from 23.3 to 42.7 with a comparable CO2 permeability



This low-cost robust newly developed strategy to produce MMMs using CCNS may have great prospective in CO2/CH4 separations

Declaration of Interest Statement

Name: Mohd Yusuf Khan (Ph.D.) Research Scientist (III), Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran 31261, Saudi Arabia

I have read and understood the Journal of Natural Gas Science and Engineering’s policy on declaration on interest and declare the following. This is to declare that all the authors have read the manuscript very well and do not have any conflict of interest.