Amine wetting evaluation on hydrophobic silane modified polyvinylidene fluoride/silicoaluminophosphate zeolite membrane for membrane gas absorption

https://doi.org/10.1016/j.jngse.2018.08.003Get rights and content

Highlights

  • Hydrophobicity and LEP of silane modified PVDF/SAPO-34 membranes were improved.

  • CO2 absorption flux into water was increased up to 25%.

  • SAPO-34 zeolite prevented membrane swelling and reduced membrane wetting by amine.

  • Interaction between fluoroalkyl silane and amine groups induced membrane wetting.

Abstract

Membrane wetting in membrane gas absorption is undesirable as it reduces the gas transfer in wet pores. This work aims to understand the improvement of membrane hydrophobicity using fluoroalkyl silane and its effects on membrane properties, separation performance and membrane wetting by amine. Polyvinylidene fluoride/silicoaluminophosphate zeolite membranes were prepared using non-solvent induced phase separation technique and then immersed into a mixture of (3,3,3-trifluoropropyl) trimethoxysilane and ethanol solution (volume ratio of 1: 100) for 20 min. The post-modification resulted in a slight improvement of water contact angle and liquid entry pressure. Hence, the highest CO2 absorption flux of 4.48 ± 0.36 × 10−4 mol.m−2. s−1 was achieved using silanated membrane, about 25 % of improvement in comparison to the neat membrane. The absorption flux of humid gas was slightly reduced (-16 %) due to pore wetting. Fluoroalkyl silane could be used to prevent water wetting, resulting improvement of CO2 absorption. The membranes were immersed in diethanolamine solution (2 M) for 50 h before characterization using goniometer, Fourier transform infrared spectroscopy and scanning electron microscope. The incorporation of zeolite reduced membrane wetting because membrane swelling could be prevented. The unfavorable interaction between fluoroalkyl and amine groups induced severe membrane wetting by amine.

Introduction

Amine dominates the market up to 90 % of CO2 capture of for the absorption systems (Zhao et al., 2016). Besides blending different types of absorbent (Li and Zhang, 2018; Li and Zhang et al., 2018), the cost and absorber size can be greatly reduced using a macroporous membrane which provides a large surface area to transfer CO2 into the absorbent circulated at the other side of membrane. Although the field tests of membrane gas absorption in many countries have shown positive results, (Falk-Pedersen and Dannström, 1997; Feron and Jansen, 1995, Scholes et al., 2014, Li et al., 2013), the tested membrane gas absorption systems have not been commercialized up to date due to the concerns on membrane wetting for long operations. Membrane wetting is a condition where the membrane pores are filled up with liquid hence cause serious reduction of CO2 transportation.

Hydrophobic polymers including polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polypropylene (PP), poly (ether ether ketone) (PEEK) and more had been widely used to develop the hydrophobic membranes with minimum wetting to achieve a high mass transfer rate through air-filled pores. Besides conventional pore formers such as phosphoric acid, glycerol, polyethylene glycol (PEG) and lithium chloride (LiCl), different types of inorganic particles including montmorillonite (Rezaei et al., 2015), zeolite socony mobil-5 (ZSM-5) (Shirzadeh-Gharacheh and Rahbari-Sisakht, 2016), hydrophobic CaCO3 (Fosi-Kofal et al., 2016), graphene (Wu et al., 2016), polyhedral oligmeric silsesquioxane (POSS) (Ahmad et al., 2017) and silico-alumino-phosphate-34 (SAPO-34) zeolite (Ahmad et al., 2016b) were blended into hydrophobic polymers to improve membrane properties such as surface hydrophobicity, porosity, pore size and gas affinity. These mixed matrix membranes could be further modified using hydrophobic silane in order to achieve superhydrophobicity (Hamzah and Leo, 2016). In the past, researchers had tried different methods to improve membrane hydrophobicity up to superhydrophobic state in the combat of membrane wetting. A thin layer of porous polyethylene (low density) was coated on poly (vinylidenefluoride) (PVDF) hollow fibers to create superhydrophobic surface (Ahmad et al., 2013). The coated PVDF membrane exhibited an improved CO2 flux in membrane gas absorption compared to the neat PVDF hollow fibers. On the other hand, superhydrophobic polyether ether ketone (PEEK) hollow fibers were produced by chemical modification of PEEK hollow fibers using a functional perfluoro oligomer (Li et al., 2013). Superhydrophobic ceramic membranes were also applied in membrane gas absorption (Abdulhameed et al., 2017; Yu et al., 2015; An et al., 2015). Different types of fluoroakyl silane were used to convert these hydrophilic ceramic membrane into superhydrophobic membranes. Although silane grafting increased the mass transfer resistance of ceramic membranes slightly, the superhydrophobic ceramic membranes were less fouled by the dust in flue gas in comparison to PP membrane. However, periodic drying was still practiced during the field test of superhydrophobic membrane in order to reduce membrane wetting (An et al., 2015).

Before conducting more research on membrane gas absorption using membranes with various types of hydrophobic enhancement, it is important to understand the wetting mechanisms. Several groups of researchers concluded that the wetting of PP and PVDF membrane was actually triggered by absorbent diffusion and membrane swelling (Lv et al., 2010; Mahmud et al., 2004). The effects of hydrophobicity improvement on membrane wetting in amine absorbent remains unclear since most the wetting evaluation usually involves water contact angle measurement on the fresh membranes only without further details (Ahmad et al., 2010, 2012). The improvement of CO2 flux within the short operation duration could be related to morphology improvement instead of wetting reduction. In this work, the major aim is to study the modification effects of hydrophobic silane on the mixed matrix membrane used in membrane gas absorption. PVDF/SAPO-34 membrane developed in our previous work (Ahmad et al., 2016b) was modified using fluoroakyl silane to improve membrane hydrophobicity. The membranes were further characterized and tested to understand the effects of silane. The membranes with hydrophobic enhancement were also evaluated in amine wetting test. This test is helpful to generate some preliminary knowledge regarding the effects of hydrophobicity enhancement on membrane wetting by amine.

Section snippets

Membrane preparation

The hydrophobic and macroporous PVDF and PVDF/SAPO-34 membranes were prepared using non-solvent induced phase separation (NIPS) method. SAPO-34 zeolite was first hydrothermally synthesized based on the previous work (Junaidi et al., 2014). SAPO-34 zeolite particles were further dispersed in the solvent, N-methyl-2-pyrrolidone (NMP) (Emplura, Merck Chemical) under sonication (Ahmad et al., 2016b). In order to achieve a homogeneous solution, PVDF powder (Solef® PVDF 6010/1001) was dried at 60 °C

The effects of hydrophobic silane on membrane characteristics

The effects of silane modification on membrane hydrophobicity was first studied using water contact angle. The water contact angles for all the prepared membranes signified that hydrophobic membranes with water contact angle more than 90 ° were successfully synthesized as shown in Table 2. Using double coagulation baths in NIPS method, nearly superhydrophobic P-0 membrane with a high water contact angle of 141.1 ± 1.2 ° was successfully produced. However, the incorporation of hydrophilic

Conclusions

PVDF membranes incorporated with SAPO-34 zeolite were successfully fabricated and post-modified using fluoroalkyl silane. The post-modified PVDF/SAPO-34 membrane showed slightly improved CO2 absorption flux. This work is important to understand the effects of a common membrane modifier, hydrophobic silane on membrane properties, separation performance and amine wetting. The following major conclusions can be drawn from this work:

  • (1)

    Silanation enhanced the membrane hydrophobicity slightly although

Acknowledgements

The authors would like to acknowledge Ministry of Education, Malaysia for providing LRGS (304/PJKIMIA/6050296) and MyPhD scholarship. The authors would also like to thank Universiti Sains Malaysia for the facilities and equipment (RUI 1001/PJKIMIA/8014060).

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