Fabrication and modification of cellulose acetate based mixed matrix membrane: Gas separation and physical properties

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Abstract

[Cellulose acetate (CA)-blend-multi walled carbon nano tubes (MWCNTs)] mixed matrix membranes (MMMs), [CA/polyethylene glycol (PEG)/MWCNTs] and [CA/styrene butadiene rubber (SBR)/MWCNTs] blend MMMs were prepared by solution casting method for gas separation applications using Tetrahydrofuran (THF) as solvent. Both raw-MWCNTs (R-MWCNTs) and functionalized carboxylic-MWCNTs (C-MWCNTs) were used in membrane preparation. The MWCNTs loading ratio and pressure effects on the gas separation performance of prepared membranes were investigated for pure He, N2, CH4 and CO2 gases. Results indicated that utilizing C-MWCNT instead of R-MWCNTs in membrane fabrication has better performance and (CO2/CH4) and (CO2/N2) selectivity reached to 21.81 and 13.74 from 13.41 and 9.33 at 0.65 wt% of MWCNTs loading respectively. The effects of PEG and SBR on the gas transport performance and mechanical properties were also investigated. The highest CO2/CH4 selectivity at 2 bar pressure was reached to 53.98 for [CA/PEG/C-MWCNT] and 43.91 for [CA/SBR/C-MWCNT] blend MMMs at 0.5 wt% and 2 wt% MWCNTs loading ratio respectively. Moreover, increase of feed pressure led to membrane gas permeability and gas pair selectivity improvement for almost all prepared membranes. The mechanical properties analysis exhibited tensile modules improvement with increasing MWCNTs loading ratio and utilizing polymer blending.

Introduction

Recently membrane gas separation technology has emerged as a major unit operation technique over the traditional gas separation procedures like cryogenic distillation and pressure swing adsorption. Therefore some kinds of material have been utilized for these membranes in the last two decades [1], [2]. Among them, polymeric membranes have been used in a wide range of industrial applications such as CO2/N2 and CO2/CH4 gas separation processes [3]. These membranes have some remarkable advantages such as low energy costs, environmental endurance, simplicity of operations and good thermal and mechanical properties [4], [5]. Despite of these advantages of polymeric membranes, there is significant limitation in the newer membranes development for gas separation applications which is called upper bound Robeson trade-off [6]. Despite of this polymeric membranes limitation, they are still good candidates for membrane preparing but more view points are necessary to improve the gas transport properties above the Robeson upper bound [7]. To eliminate this limitation, combination of both desirable properties of inorganic and polymeric materials has been accomplished by mixed matrix membranes (MMMs) [8]. Their structure includes an inorganic material in the form of micro or nano particles introduced into a polymeric matrix [9]. Some of these inorganic materials are meso-porous molecular Sieves [10], zeolite [11], [12], silica [13], [14], carbon molecular sieves (CMS) [15], [16], [17] and carbon nanotubes (CNTs) [18], [19], [20]. Among these inorganic materials, CNTs have attracted more research, due to their small size, high length to diameter ratio, cylindrical structure and prominent mechanical, electrical, physical and chemical properties [21], [22], [23], [24], [25]. As invented by Iijima [26], [27], carbon nanotubes can be synthesized in two forms: (1) single-walled carbon nanotubes (SWCNTs), (2) multi-walled carbon nanotubes (MWCNTs). Hinds et al. [28] prepared carbon nanotube membranes applying aligned MWCNTs. Kim et al. [29] synthesized membranes including functionalized carbon nanotube embedded in commercial polysulfone (PSF) matrix. With increase in weight fraction of carbon nanotubes up to 5%wt the He and CO2 permeability was enhanced 30% and N2 and CH4 permeability increased 40% and 60%, respectively at 4 atm. Also Aroon et al. [25] fabricated polyimide (PI)/chitosan functionalized multi-walled carbon nanotubes MMMs. They found that at 1 wt% functionalized MWCNTs, the CO2 and CH4 permeabilities increase by 20.48 and 0.71 Barrer, respectively. In this study, cellulose acetate/raw-MWCNT (CA/R-MWCNT) MMMs were prepared at different wt% of CNTs loading. CA, has been widely used for micro-filtration, reverse osmosis, and gas separation technologies [30], [31], [32] due to its good biocompatibility, good toughness, high potential flux and low price [33], [34], [35]. Actually Combining two different methods for making gas separation membranes were utilized: (1) the using MMMs, (2) applying polymer blending membranes. A few researches worked on like this combining. So the effects of MWCNTs loading ratio and feed pressure on the gas transport properties of the prepared membranes at room temperature were studied. Also for improvement of mechanical properties and gas separation performance of MMMs, a new kinds of MMMs including a second rubbery polymer in their polymeric matrix, CA/PEG1000/C-MWCNTs and CA/SBR/C-MWCNTs, (which is named blend MMMs) has been prepared. PEG is a rubbery polymer with polar groups in its main chains and high flexibility which is known as a good solvent for acidic gases [36]. Moreover, SBR is a rubbery polymer with high permeability and appropriate abrasion resistance and aging stability. So, using these polymers blends can make a good combination of supporting characteristic of glassy polymers and high permeability provided by rubbery polymers [37], [38], [39]. The pure He, N2, CH4 and CO2 gases were used in membrane characterization.

Section snippets

Materials

CA, PEG1000 and THF as the base of polymer matrix, the second polymer and the solvent, respectively, were purchased from Merck. SBR as the second polymer were purchased from Bandar Imam Petrochemical (BIP), Iran. R-MWCNT with the diameter of 16 nm, the length of 10 μm, specific surface area of 133 m2/g and the mean pores diameter of 110.70 Å and C-MWCNT (COOH-MWCNT) with the diameter of 8 nm, the length of 10 μm, specific surface area of 110.23 m2/g and the mean pores diameter of 98.11 Å were provided

Gas permeation results

Studies on the gas separation properties of prepared membranes can be divided to two characterization classes: (i) loading ratio effects and (ii) feed pressure effects.

Conclusion

R-MWCNTs and C-MWCNTs loading ratio and pressure effects on the gas separation properties of polymeric membranes based on CA were investigated. However, applying both of R-MWCNTs and C-MWCNTs, leads to permeability improvement for test gases. Membranes filled with C-MWCNTs indicated better performance especially for CO2/CH4 gas pair separation. The highest selectivity for CO2/CH4 at 2 bar pressure was achieved 21.81, for 0.65 wt% of C-MWCNTs in prepared MMMs. Also the effects of using second

Acknowledgment

The authors gratefully acknowledge Arak University for the financial support during this research.

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