Polymer Formulation Used in Carbon Membrane Synthesis and Performance Evaluation

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http://myjms.mohe.gov.my/index.php/ajfas

Polymer Formulation Used in Carbon Membrane Synthesis and Performance Evaluation

Muhammad Azan¹, Liyana Amalina Adnan¹, Iylia Idris², Mohd Azmier³, Mohamad Firdaus³, Fadzil Noor Gonawan³, Mohd Khairul Nizam⁴

1 Kolej GENIUS Insan, Universiti Sains Islam Malaysia, 71800 Nilai, Negeri Sembilan, Malaysia

2 School of Chemical Engineering, College of Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia

3 School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia

4 Pusat Pengajian Diploma, Jabatan Sains dan Matematik, Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja, Johor, Malaysia

*Corresponding Author: [email protected] Accepted: 15 February 2023 | Published: 1 March 2023

DOI:https://doi.org/10.55057/ajfas.2023.4.1.3

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Abstract: Carbon membranes are known for their high performance in the separation of gases, particularly for gases with similar kinetic diameter. For this reason, the investigation of carbon membranes continues to increase their performance by tweaking the formulation and heating strategy. One of the formulation aspects is polymer selection, which plays an important role in determining the success of producing carbon membrane with acceptable performance. This is because each polymer has its own characteristics, which then determines the method to be used to synthesize the carbon membrane. Four types of polymers were identified to produce carbon membranes, which are based on non-modified polymer solutions, modified polymer or polymer solutions, non-commercial organic materials and natural polymers such as natural cellulose. This review discusses the overview performances provided by each material.

Keywords: Carbon membrane, synthesis, polymer selection, short review

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1. Introduction

The selection of polymers to produce carbon membranes highly depends on the convertibility of the polymers into carbon materials after pyrolysis and the desired configuration. The pyrolysis of polymers and its formation into carbon molecular sieves is complex and a complete understanding of the process has not yet been established (Low & Chung, 2011). Most polyimides, as thermosetting polymers, are easily preferred as the carbon membrane precursor as their performance in gas separation is above average. However, its cost of production and complexity in production are not favourable factors for commercialization or bulk production. Currently, the trend of the preferred precursors is towards the more inexpensive polymer precursors that can be produced with simple synthesis techniques. The search for competitive carbon membranes for gas

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membranes is equally contributed by the introduction of new polymer precursors besides polymers that have been modified in terms of their recipes or the addition of secondary materials. The selection becomes highly limited when advanced configurations such as hollow fiber are taken into consideration since not many polymers are able to sustain their physical form from melting during the pyrolysis.

2. Polymers used in the synthesis of carbon membrane

According to the literature, the synthesis of carbon membranes is produced by using three types of carbon-based precursors, which are 1) pristine or non-modified polymer solutions, 2) polymers that have had their solution chemically modified or physically modified by additional secondary materials; and 3) non-commercial organic materials and others such as coal (Song, Wang, Qiu, et al., 2008) and natural cellulose (Lie & Hägg, 2006). Most precursors produced from polymers are from commercial polymers. The carbon membrane morphology highly depends on the symmetrical or asymmetrical of the polymer produced during the phase inversion of synthesis.

The synthesis of carbon membranes from unmodified polymer solutions has several benefits, such as simplicity in fabrication, increasing their reproducibility, being highly cost-effective, and their performance being highly competitive with other costly precursors. Currently, progress in the development of carbon membranes with increased performance using unmodified polymers has been mostly focused on polyimide-based polymers. For instance, some polyimides that have been used to develop carbon membranes are 6FDA-based polyimides (Mueller et al., 2012)(Xu et al., 2014)(Xiaoli Ma et al., 2016) polyimide PIM-6FDA-OH (Xiaohua Ma et al., 2013), polyimide Torlon (Hosseini & Chung, 2009), Matrimid (Sazali et al., 2015), Kapton (Su & Lua, 2009)(Li et al., 2014)) and, polyimide BTDA/APB-NDA (Sim et al., 2013). Other alternative of non- polyimide polymers with inexpensive synthesis cost includes, coal (Song, Wang, Wang, et al., 2008), poly(furfuryl alcohol) (Merritt et al., 2007), cellulose acetate (He & Hägg, 2011a), phenolic resin (Katsaros et al., 1997)(Wei et al., 2007), cellophane paper (Campo et al., 2010), novolac resin (Llosa Tanco et al., 2015)) and, poly(p-phenylene oxide) (Yoshimune & Haraya, 2013)(Jaya et al., 2021)(Jaya et al., 2019).

For a carbon membrane synthesis to be successful, the polymer precursor requires certain chemical properties in order to maintain structural integrity during high temperature treatment. Polymers such as polyimide do not have such a drawback in adapting themselves during the thermal conversion since they are thermostable polymers. Thermoplastic polymers, such as polyacrylonitrile, poly(p-phenylene oxide), and polyetherimide, which constitute the majority of the lower cost polymer precursors for carbon membrane synthesis, require proper thermostabilization and will melt under thermal stress.To overcome this situation, thermostabilization is conducted in which the polymer is heated close to its melting temperature under oxygen-enriched air. The oxidation will sustain the polymer structural integrity during the carbonization. The polyetherimide requires a small amount of poly (vinylpyrrolidone) addition to sustain the thermostabilization (Barbosa-Coutinho et al., 2003).

In the event that the thermoplastic polymers cannot be thermostabilized, a dip-coating technique is used by utilising ceramic-based porous supports to produce thin-film carbon membranes. In this

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Another issue in synthesizing carbon membranes using polymer precursors is that the polymers should be non-graphitizable at high temperatures. This is because graphitization will promote pore enclosure (Chen & Yang, 1994). Polymers with simpler structures, such as poly(p-phenylene oxide) (Yoshimune & Haraya, 2013), and poly(furfuryl) alcohol resin (Zhong et al., 2014) were preferred in some literature. Polymers of simpler structures produced smaller micropores than those of more complex structures, which have a lot of functional groups (Kyotani, 2000). Another reason for the preference is that a simple molecular structure helps in understanding the carbon formation mechanism from fundamental experimental and atomistic simulation studies (Sedigh et al., 1999)(A. F. Ismail & Li, 2008). A more complex structure also contributes to tight packing of the polymer chain, which lowers the gas permeation properties (Zhang et al., 2015). Most of the complex polymers, such as polyimides, are laborious to synthesize. Reports of non-modified polymers utilized in developing carbon membranes are listed in Table 1. The italic numbers represent the unit in GPU and non-italic in Barrer.

3. Polymide as precursor

Cost of materials, availability of materials, configuration of the membrane, synthesis and methods, and performance are all things that need to be thought about when making carbon membranes for commercial use.Based on the literature surveyed, most of the carbon membranes produced were synthesized using polyimides such as Matrimid, 6FDA-based polyimides, and Kapton. The selection of polyimide precursors to produce carbon membranes is motivated by their thermal stability and structures that can be tailored from the permutation of diverse dianhydride and diamine moieties (Low & Chung, 2011). Besides that, the high glass transition temperature of polyimides reflects high rigidity caused by the presence of imide rings of the polyimides (Xiao et al., 2005). This property is also reflected by the high glass transition temperatures (Tseng et al., 2011). Carbon membrane porosity highly depends on the amount of gas evolved during the pyrolysis, where structural stability and rigidity is important to prevent deformation and the pore from collapsing as it inhibits intersegmental packing and segmental mobility (Zhou et al., 2003)(Kita et al., 1997)(Xiaohua Ma et al., 2013)(Bhuwania et al., 2014). As can be seen from the Table 2.4 as well, polyimide based carbon membranes delivered both high permeability and permselectivity, while the non-polyimide based carbon membranes delivered competitive permselectivity but lower permeability, particularly for O2/N2 and CO2/CH4 separation performance.

Development of carbon membrane using polyimides is costly, complicated and time consuming (Kiyono et al., 2010). This would also compromise the reproducibility, quality and performance of the carbon membrane. Due to its synthesis challenges, most of the polyimide-based carbon membranes were produced in a form of thin films.

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Table 1: List of non-modified polymers used in the development of carbon membranes

Poly(phenylene oxide) (PPO) Poly(furfuryl alcohol) (PFA) Single-coat supported (assisted by RF/F-127 interlayer coat) Kapton (PMDA/ODA) Polyetherimide (PEI) Polyetherimide (PEI) Hollow fiber Matrimid (BTDA-DAPI) Polymer

Single-coat supported Single-coat supportedUnsupported thin film Unsupported thin film Single-coat supported Single-coat supported Unsupported thin film Configuration

16.3 165.7 1012.9 75.4 𝑷𝑯𝟐(Barrer/GPU)

347 73.84 5.51 11.97 𝜶𝑯𝟐𝑵𝟐⁄

0.6 22.8 4.75 109 81 123.03 191 𝑷𝑶𝟐(Barrer/GPU)

12.5 10.15 15.83 51.15 2.09 7.66 173.64 𝜶𝑶𝟐𝑵𝟐⁄

368 1321.44 1.5 1046 302 304.4 12 457.06 𝑷𝑪𝑶𝟐(Barrer/GPU)

138 65.94 134 27.6 6.27 63.93 76 72.03 𝜶𝑪𝑶𝟐𝑪𝑯𝟒⁄

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(Lee et al.,2006) (Kiyono et al.,2010) (Song, Wang, Qiu, et al., 2008) (Li et al.,2014) (Su & Lua, 2009) (Tseng et al.,2012) (Itta et al.,2010) (Sedigh et al.,1999) (Vu et al.,2002) (Rungta et al.,2015) (Tin et al., 2004) Reference

Table 1: Cont.

Phenol formaldehyde resin(PFR) PIM-6FDA-OH ODPA-DAI Poly(phthalazinone ether sulfone ketone) (PPESK) Matrimid P84 polypyrrolone BTDA/APB/TrisAPB-NDA Cellulose Cellophane paper Torlon Polymer

Single-coat supported Unsupported film Unsupported thin film Unsupported thin film Unsupported thin film Multi-coatssupported Multi-coatssupported Unsupported thin film Unsupported thin film Unsupported thin film Unsupported thin film Unsupported thin film Configuration

4.57 5100 2177 118.35 1340 1100 108.1 390.1 𝑷𝑯𝟐(Barrer/GPU)

31.80 23.61 128.06 187.86 614.68 478.26 1081 126.66 𝜶𝑯𝟐𝑵𝟐⁄

0.3 2300 149 116 4.45 35.8 357.26 54 3.5 8.39 𝑷𝑶𝟐(Barrer/GPU)

2.30 10.65 8.76 7.8 25.65 16.42 6.4 12.86 17.50 2.72 𝜶𝑶𝟐𝑵𝟐⁄

0.42 556 344 107.38 120.12 1085 43.1 𝑷𝑪𝑶𝟐(Barrer/GPU)

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3.30 92.67 54 67.95 98.4 52 18.33 𝜶𝑪𝑶𝟐𝑪𝑯𝟒⁄

(Wei et al.,2007) (Wang et al.,1996) (Xiaohua Ma et al., 2013) (Xiao et al.,2005) (Zhang, Wang, Liu, et al., 2006) (Sazali et al.,2018) (N. H. Ismail et al., 2018) (Kita et al.,1997) (Sim et al.,2013) (Lie & Hägg, 2006) (Campo et al.,2010) (Hosseini &Chung, 2009) Reference

There have been some efforts to introduce non-polyimide polymers to improve carbon membrane production cost effectiveness. Cellulose acetate, poly (acrylonitrile), polyaramides, and poly (p- phenylene oxide) are examples of polymer precursors that have been successfully developed as hollow fiber carbon membranes. David and coworkers (David & Ismail, 2003) produced hollow fiber carbon membrane using poly(acrylonitrile) with O2/N2 permselectivity of 3.7 to 1.7 and permeance of 50 to 300 GPU. The values of the permselectivity suggested that the transport of the gas through the carbon membrane is highly influenced by Knudsen diffusion. This is also supported by their report that the permeance was affected by the feed pressure. Their works are more focused towards characterization to fully understand the structural development of the membrane during heat treatment.

The continuation of work in carbon membrane development can be seen through the work of Soffer and coworkers (Saggy, 1987) that produced hollow fiber carbon membranes and was then optimized by He and Hagg (He & Hägg, 2011b) for CO2/N2 separation. The performance could be optimized by adjusting the strategy (temperature and heating rate) and heating atmosphere. More works are needed to improve the performance to surpass the 2008 Robeson’s upper bound.

From the table, the tabulation between the non-polyimide, polyimide can be plot against the Robeson’s upperbound as shown in Figure 1. The performances of non-polyimide polymers are not always inferior to those of polyimides, as demonstrated by phenol-formaldehyde separation in H2/N2 (Wang et al., 1996), poly(phthalazinone ether sulfone ketone (PPESK) in O2/N2 separation (Zhang, Wang, Zhang, et al., 2006), and poly(phenylene oxide) (Lee et al., 2006) in CO2/CH4

separation which are very close to the polyimide-based carbon membrane performance. The phenol-formaldehyde is thermosetting polymer which enable it to be pyrolyzed at higher temperature (800-950 °C) without collapsing the pores and deformed (Wang et al., 1996)(Xiaohua Ma et al., 2013). The PPESK is not only due to its low cost; it also has good solubility, high char yield after pyrolysis, and thermal resistance. It is a thermoplastic polymer which requires oxidative stabilization to improve its polymer structure through cross-linking, which inhibits the growth of microcrystals during the pyrolysis and enables the formation of carbon membranes at high pyrolysis temperatures with a large amount of pore structure (Zhang, Wang, Zhang, et al., 2006).

This indicates that more work is required to search for more non-polyimide polymers and optimized for the higher separation performance.

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4. Polymer and their performance overview as carbon membranes

The distribution of performance of carbon membranes from their early development until recently is demonstrated in the plots (Figure 2.5). To identify the carbon membrane that meets the requirements, Robeson's upperbound and commercially attractive regions have been added. The figure indicates that the H2/N2 was dominated by the unsupported thin film configuration (10), followed by hollow fiber (3) and single-coated support (3). In this case, the modified polymers dominate the performance as compared to polyimide-based and non-modified polymeric membranes. According to the figure as well, there are also modified polymers that perform lower than the non-modified polymers but mostly are above the upper boundary. Because of their high crosslinking structure, polyimides perform mostly well, as shown in the above of the Robeson's upper boundary.

The O2/N2 separation performance reported in the work related to carbon membranes is overwhelming as compared to H2/N2 and CO2/CH4 separation. Different from the H2/N2

development, most of the carbon membranes developed in this department have performances located above the Robeson’s upperbound. The configuration was obviously dominated by unsupported thin films. The competition between the modified polymers and non-polyimide and polyimide-based carbon membranes is close to each other. Overall, the polyimide-based carbon membranes have higher permeability and, at the same time, higher selectivity as compared to non- polyimide-based carbon membranes. This is mostly because the polyimide precursor has a more rigid polymer structure, which makes it more selective. This is shown by the fact that its glass transition temperature is very high (Low & Chung, 2011) (Xiao et al., 2005) (Tseng et al., 2011).

Referring to the performance surpassing the upperboundary and within the commercially attractive region, the dominance of polyimide based carbon membranes was very obvious in CO2/CH4

separation where most of the configurations were unsupported thin films. The modified polymers mostly consisted of the addition of zeolite such as SAPO-34, ZSM-5, zeolite KY and zeolite L.

Other configurations were also available, but only in a minority. Poly(p-phenylene oxide) and phenolic resin in a single-coated supported configuration are the only non-modified polymers that exist above the upper boundary and within the commercially appealing region.The polyimide- based carbon membranes deliver both high permeability and selectivity. The modified polymers offer competitive performance against the polyimide-based carbon membranes.

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Figure 1: Tabulation of carbon membrane a) H2/N2 b) O2/N2 c) CO2/CH4 performances against Robeson’s 2008 upperbounds and attractive regions for non-polyimide (green), polyimide (red) and modified polymers (blue) as

precursors (Jaya, 2018)

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Figure 1: Continued

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Figure 1: Continued

5. Conclusion

The review shows that many types of polymers have been used in the development of carbon membranes. Each polymer showed a significant difference in terms of performance as compared to each other. Aspects such as cost need to be considered in the synthesis of carbon membranes as it is not a vital factor to have higher performance. Some polymers without any modification showed superior performance as compared to polymers with complicated and advanced modifications when they are converted into carbon membranes. This polymer can be further

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improved in terms of heating strategy to produce performance on par or higher than polymer with modification or costlier production.

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