Synthesis and characterization of brominated poly(2,6-diphenyl-p-phenylene oxide)/SiO2 nanocomposite membrane for CO2/N2 separation

Abstract

Synthesized poly(2,6-diphenyl- p -phenylene oxide) (PPPO dp ) and brominated poly(2,6-diphenyl- p -phenylene oxide) (BPPPO dp ) are identified as new membranes for CO 2 /N 2 separation and characterized by SEM, FTIR, and 1 HNMR. BPPPO dp is known as a flexible membrane with higher permselectivity ( αCO2/N2  = 30.53) than PPPO dp membranes. BPPPO and SiO 2 nanocomposite membranes display improved CO 2 permeability and CO 2 /N 2 selectivity compared to the pure PPPO dp , and BPPPO dp membranes. The CO 2 /N 2 separation mechanism in the membranes is related to the amount of gas dissolution than the amount of gas diffusion.

Graphical abstract

Introduction

One of the straightest techniques to decrease the global warming effect due to atmospheric CO 2 increase is found in CO 2 removal from gaseous streams which is produced in the production of energy from industrial processes [ 1 , 2 ]. Using membrane technology in gas separation has created great revolution compared to traditional separation processes [ 3 , 4 ].That is owing to their low cost, excellent efficiency and being trustworthy.

Recently, three popular gas separation membranes including polymeric membrane, inorganic membrane and new block polymeric membrane are used extensively for CO 2 separation. However, all of them have some defects to industrialization [ 5 ]. Highly permeable inorganic membranes suffer from brittle structure, structure defects and also high costs. Therefore, preparation, characterization, and application of inorganic separation membranes were just limited to the laboratory. While, polymeric separation membranes attract more and more attention because of the no expensive and easily fabricated materials. The modified polymeric membranes display improved permeability and CO 2 selectivity. Polymeric membranes are commonly used for CO 2 separation because of their extensive applications: separation of CO 2 /H 2 in gas refinement, separation of CO 2 /N 2 in carbon capture, separation of CO 2 /CH 4 in natural gas refinement, separation of CO 2 /O 2 in food packaging [ 6 ].

The mechanism of gas separation is distinguished by size sieving because of the rigid chain structures of the polymer matrix [ 7 ]. Polymeric membranes with great permeability and selectivity are difficult to prepare.

Poly(2,6-dimethyl- p -phenyleneoxide) (PPPO dm ) as a glassy polymeric material is introduced suitably as a gas separation membrane, because of the great permeability and suitable selectivity [ 1 ]. In particular, PPPO dm membrane with great CO 2 permeability (~ 40 bar) and great CO 2 /N 2 selectivity (~ 15) is preferred to other glassy polymers [ 8 ]. Chemically modified membranes such as brominated poly(2,6-dimethyl- p -phenyleneoxide) (BPPPO dm ) could often improve the gas permeability and selectivity [ 9 , 10 ].

Addition of nanosized fillers to the polymer matrix could improve the separation performance of polymeric materials through modifying the chain packing of the polymer [ 1 , 11 ]. Nano-scaled fillers like to be adjusted to the chain packing of glassy polymers in a way that increases the permselectivity trough creating additional selective free holes.

The SiO 2 , TiO 2 and MoS 2 nanoparticles through polymer chain interruption bring improvement in gas separation. Polymeric membrane with dispersing nanoparticles is usually challenging due to undesirable interactions between the polymeric matrix and inorganic nano fillers. The appropriate polymer selection and applying the useful inorganic nanoparticle are two essential issues to get useful polymeric membranes [ 12 ].

Among different inorganic nanoparticles, SiO 2 nanoparticles are identified as an appropriate modifier for gas separation membranes owing to their high specific surface area and the ability of chemical functionalization, high mechanical and thermal stabilities [ 12 , 13 , 14 , 15 – 16 ].

Merkel et al. introduced poly(4-methyl-2-pentyne)(PMP)/silica nanocomposite including silica NPs (30 wt%), as more effective membrane than pure PMP at gas separation [ 17 ]. Also poly(amide-6-b-ethylene oxide) (PEBAX) including 27 wt% silica NPs displayed 277 bar for CO 2 permeability of and 79 for αCO2/N2 selectivity [ 18 ].

With regard to the previous literature, here, we synthesized poly(2,6-diphenyl- p -phenylene oxide) (PPPO), brominated poly(2,6-diphenyl- p -phenylene oxide) (BPPPO), and BPPPO/SiO 2 nanocomposite as suitable porous membrane for CO 2 adsorption (Fig.  1 ).

Fig. 1

The monomer structure of poly(2,6-diphenyl- p -phenylene oxide) (PPPO dp ) ( a ); and brominated poly(2,6-diphenyl- p -phenylene oxide) (BPPPO dp ) ( b ); and three-dimensional model of poly(2,6-diphenyl- p -phenylene oxide) (BPPPO dp ) (c)

Experimental

Materials

PPO dm ( M n 25,000, polydispersity 2.0), N,N,Nʹ,Nʹ tetramethylethylenediamine (TMEDA, 99%), 2,6-diphenylphenol (98%), bromine (Br 2 , 99.5%), chloroform (CHCl 3 , 99.8%), ethanol (99.8%), methanol (99.8%), anhydrous hydrazine (98%), 1,2-dichlorobenzene (98.5%), chlorosulfonic acid (99%), anhydrous magnesium sulfate (99.5%), glacial acetic acid (99.8%) and silica nanopowders (SiO 2 , 99.5%, 10 nm) and copper (I) chloride (CuCl, 98.5%) from Aldrich.

Synthesis of PPPOdp

The synthesis of PPPO dp was performed according to reported procedure [ 19 ]. In a 100 cc flask, CuCl (0.041 g), TMEDA (0.031 g), and anhydrous magnesium sulfate (2 g) are added to 1,2-dichlorobenzene (35 cc). The mixture is heated in an oil bath at 65 °C under magnetic stirring. Oxygen is bubbled into the mixture for 15 min. After green color observation in solution, 40 cc of 2,6-diphenylphenol solution is added dropwise during 20 min and made a dark red color immediately. After 24 h, several drops of anhydrous hydrazine are added to the solution to remove byproducts. The inorganic solids are filtered, and the solution is added dropwise to methanol (400 cc) including hydrazine and the solution is shaken for several hours. Then produced polymer is filtered and resolved in chloroform (40 cc) and precipitated in methanol (400 cc). The 3 g of polymer with M n  ∼ 120,000 is collected by filtration and dried at 80 °C under vacuum for 24 h. The density of PPPO is ∼ 1.11 g cm −3 measured by an excluded volume method at room temperature.

Synthesis of BPPPOdp

For synthesis of BPPPO, PPPO (5 g) and CHCl 3 (50 cc) are added to a 100 cc flask. The mixture is stirred with a magnetic stirrer. The 20 cc of bromine solution included Br 2 (10 cc) and CHCl 3 (10 cc), is added dropwise to the mixture during 30 min under Argon atmosphere. After 1 h stirring at room temperature, 800 cc ethanol is added under mechanical stirring and the BPPPO is precipitated out. The produced polymer is filtered and dried at room temperature under vacuum. The extent of bromination is 80–95% from NMR spectrum. The density of BPPPO is ∼ 1.43 g cm −3 measured by the excluded volume method at room temperature with M n  ∼ 180,000.

Membrane preparation

The PPPO dp , BPPO dp , and BPPPO dp /SiO 2 nanocomposite membranes are fabricated. The solution of PPPO (3 wt%, in CHCl 3 ) is cast on glass plate at room temperature. Similarly, the BPPPO membrane is prepared.

The BPPPO/silica membrane is prepared as follows: 0.3 g BPPPO is dissolved in 5 cc CHCl 3 under stirring. Silica NPs (10 nm) is added slowly to the BPPPO solution. The mixture is vigorously stirred with 1150 rpm for 15 min to dispersion. Then mixture is cast onto a plate at room temperature to dry. After drying, the resulting membranes, with 55–85 µm thickness, are peeled off and stored in a desiccator for testing.

Adding nanoparticles to glassy polymer membranes increases the gas permeability through disturbed polymer chain packing and then increase free volume of membranes.

Gas separation experiments

The membranes gas permeability is examined using a constant-volume setup (Fig.  2 ). The test setup is made of two compartment stainless steel cells in a way that a flat circular membrane with an effective area of 10.45 cm 2 , and a reticular steel support are placed between two flanges and held by O-rings to avoid gas leakage. The experiments are started by vacating the cell up to − 0.1 bar for sending out any remaining gases before using feeding gases, which is to avoid further measurement errors. The rate of permeation (Barrer) is calculated from the pressure changes on the permeate side versus time, according to the following equation (1 Barrer = 10 −10  cm 3 (STP)-cm/cm 2 s cmHg):

P=273×1010760VLATp2×7614.7dpdt

Fig. 2

Schematic illustration of the constant-volume variable-pressure permeation test setup

where V refers to the volume of the permeate side in cm, L and A represent membrane thickness (cm) and the effective area (cm 2 ), and T is the absolute temperature. p 2 is the feed pressure of the upstream in bar and d p /d t is the slope of permeate pressure change versus time, in its steady state (linear) region.

Permeability data of CO 2 and N 2 is measured for three times, and the average values are reported. The membranes CO 2 /N 2 ideal selectivity ( αCO2/N2 ) is determined as the ration of CO 2 permeation to N 2 permeation using the following equation: α=PCO2/PN2 ; where PCO2 and P B are related to the permeabilities of pure CO 2 and N 2 , respectively [ 20 , 21 – 22 ].

Characterization

Scanning electron microscope (SEM) (Philips 505) is used to detect the morphology and particle dispersion in all three membranes. The SEM is used at 10 kV operation voltage. FTIR spectra of pure PPPO dp , and BPPO dp are obtained with a FTIR spectrometer (Thermoscientific Nicolet 6700) in the range of 4000–400 cm −1 . The solution of PPPO dp , and BPPO dp in CDCl 3 (~ 2% w) are used for 1 HNMR analyses on a Bruker Advance DRX-400 spectrometer.

Result and discussion

Characterization of PPPO, BPPPO, and BPPPO/SiO2 nanocomposite and their membranes

Differential scanning calorimetry (DSC)

Thermal properties of the white crystalline PPPO dp , and yellowish crystalline BPPPO dp are investigated by DSC analysis (Table  1 ). To eliminate the influences of less favorable crystal phases are formed at 248 °C, 288 °C for PPPO, BPPPO, respectively. Higher Tg of BPPPO dp is related to three bulky bromine groups in each monomer unit. The second heating run as their melting point are considered at 470 and > 500 °C for PPPO and BPPPO that confirm their crystalline structures.

Table 1

DSC testing results of poly(2,6-diphenyl- p -phenylene oxide) (PPPO dp ); and brominated poly(2,6-diphenyl- p -phenylene oxide) (BPPPO dp )

Sample

Mn

Solvent

Approximate density (g/cc)

Tg (°C)

Tm (°C)

PPPO

~ 120,000

CHCl3

~ 1.11

248

470

BPPPO

~ 180,000

CHCl3

~ 1.33

288

− 500

The scanning electron microscope image (SEM)

The gas transport properties strongly depend on the membranes morphology. The cross section morphology of the neat PPPO dp , BPPPO dp , and BPPPO dp /silica nanocomposite membranes are investigated via SEM images (Fig.  3 ). Figure  3 a show porous structure for PPPO dp membrane while Fig.  3 b displays optimized porous structure with fewer surface defects for its relatively brominated membrane (BPPPO dp ). Adding silica nanoparticles to the brominated membrane (BPPPO dp ) propel it to an agglomerated BPPPO dp /silica nanocomposite membrane (Fig.  3 c). The nanocomposite membranes with their hydrophilic characterization of silica nanoparticles prefer to agglomerate. Hence BPPPO dp /silica nanocomposite membrane seems to appear more selective due to its high density.

Fig. 3

The SEM image of poly(2,6-diphenyl- p -phenylene oxide) (PPPO dp ) ( a ), brominated poly(2,6-diphenyl- p -phenylene oxide) (BPPPO dp ) ( b ), and BPPPOdp/SiO 2 nanocomposite membranes ( c )

FT-IR

Functional groups on the surface of poly(2,6-diphenyl- p -phenylene oxide) PPPO dp and brominated poly(2,6-diphenyl- p -phenylene oxide) (BPPPO dp ) are investigated by means of FTIR spectra (Fig.  4 ). The observed peaks at 1400–1600 cm −1 are assigned to the C–C stretching bands of the aromatic rings. PPPO dp reveals that some of the CH stretching of aromatic rings at 3025 cm −1 (Fig.  4 a). In addition, the observed peaks at 695 cm −1 , and 1147 cm −1 corresponds to out of plane bending aromatic CH and C–O stretching bond.

Fig. 4

The FT-IR of poly(2,6-diphenyl- p -phenylene oxide) (PPPO dp ) ( a ), and brominated poly(2,6-diphenyl- p -phenylene oxide) (BPPPO dp ) ( b )

As mentioned before, the observed peaks at 1400, 1600 cm −1 illustrate aromatic CC stretching, 3058 cm −1 clarify aromatic CH stretching, and 1183 cm −1 show out of plane bending aromatic CH bond for brominated poly(2,6-diphenyl- p -phenylene oxide) (BPPPO dp ). Also, monitored peaks around 1030–1050 and 1183 cm −1 imply C–Br and C–O stretching bond, respectively.

Nuclear magnetic resonance (NMR)

In 1 HNMR spectrum of poly(2,6-diphenyl- p -phenylene oxide) (PPPO dp ) seen some peaks at 6.28 (2H, C 6 H 2 O), 6.96, 7.1–7.3 ppm (10H, 2 C 6 H 5 ) (Fig.  5 a). While, brominated poly(2,6-diphenyl- p -phenylene oxide) (BPPPO dp ) demonstrate some peaks at 6.32 (1H, C 6 HOBr), 6.75 and 7.4–7.8 ppm (8H, 2 C 6 H 4 Br) (Fig.  5 b). As seen, decrease of the number of hydrogen atoms in brominated poly(2,6-diphenyl- p -phenylene oxide) (BPPPO dp ) could confirm the new structure from brominating (Fig.  5 b).

Fig. 5

The NMR spectrum of poly(2,6-diphenyl- p -phenylene oxide) (PPPO dp ) ( a ), and brominated poly(2,6-diphenyl- p -phenylene oxide) (BPPPO dp ) ( b )

Gas separation performance test

Three prepared membranes of PPPO dp , BPPPO dp , BPPPO dp /SiO 2 nanocomposite are used in the gas separation test (Table  2 ). According to previous works, the permeability of pure gas is measured in a constant-volume variable-pressure unit and the permselectivity ( α ) is determined from the following formula: α  =  P A / P B ; where P A and P B are related to the permeabilities of pure gases A and B, respectively (Fig.  2 , Table  2 ) [ 20 , 21 – 22 ].

Table 2

Gas separation performance of poly(2,6-diphenyl- p -phenylene oxide) (PPPO dp ), brominated poly(2,6-diphenyl- p -phenylene oxide) (BPPPO dp ), and BPPPOdp/SiO 2 nanocomposite

Membranes

PCO2 (bar)

PN2 (bar)

αCO2/N2

PPPOdp

68

4.8

14.17

BPPPOdp

58

1.9

30.53

BPPPO/nano-silica

85

2.1

40.48

The brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPPO dp ) flexible membrane is identified with lower permeability of CO 2 ( PCO2  = 58 Barrer) but higher selectivity ( αCO2/N2  = 30.53) than poly(2,6-dimethyl-1,4-phenylene oxide) (PPPO dm ) membranes with permeability of CO 2 ( PCO2  = 68 Barrer) and selectivity ( αCO2/N2  = 14.7) (Table  2 , Fig.  6 ). Thus, BPPPO dp is selected as a beginning material for manufacture process of polymer/SiO 2 nanocomposite membrane. The BPPPO dp /SiO 2 nanocomposite membrane including 10 wt% silica NPs present a greatly enhanced CO 2 permeability in ( PCO2  = 85 Barrer) and ( αCO2/N2  = 40.48). Hence they are found more effective than poly(4-methyl-2-pentyne)(PMP)/silica nanocomposite with permeability and selectivity for large organic molecules [ 17 ]. Meanwhile poly(amide-6-b-ethylene oxide)/silica nanocomposite displayed 277 bar permeability of CO 2 and αCO2/N2  = 79 [ 18 ].

Fig. 6

The permselectivity diagram ( αCO2/N2 ) of poly(2,6-diphenyl- p -phenylene oxide) (PPPO dp ) (blue), brominated poly(2,6-diphenyl- p -phenylene oxide) (BPPPO dp ) (orange), and BPPPOdp/SiO 2 nanocomposite membranes (gray) (according to the calculation in Table  2 )

The study of mechanism demonstrates the silica nanoparticles owing to their incompatibility with polymer chains produce some gaps in the structure of nanocomposite, which results in enhanced permeability [ 19 ]. The surface of silica NPs with polar silano groups displayed low compatibility with the BPPO dp matrix that causes silica particles to aggregate in the membrane. A narrow gap around the silica NPs formed due to lack of tight interaction between polymer chains and silica NPs, which made high gas diffusivity and permeability (Fig.  7 ).

Fig. 7

Illustration of nanogap formation in the BPPOdp/silica nanocomposite membranes

Conclusion

Poly(2,6-diphenyl- p -phenylene oxide) (PPPO dp ), brominated poly(2,6-diphenyl- p -phenylene oxide) (BPPPO dp ), and BPPPOdp/SiO 2 nanocomposite are synthesized and compared together in CO 2 permeability and CO 2 /N 2 permselectivity. BPPPO dp membranes are known to be more efficient than PPPO dp through higher CO 2 /N 2 permselectivity. The BPPPO dp /SiO 2 nanocomposite membrane with the highest CO 2 permeability and CO 2 /N 2 permselectivity is introduced as the most effective membrane. The CO 2 permeability increase through silica addition to the membrane.

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