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Polyvinylamine containing adsorbent by radiation induced grafting of N-vinylformamide onto UHMWPE films and hydrolysis for CO2 capture Tomzch Rojek, Lorenz Gubler, Mohamed Mahmoud Nasef, and Ebrahim Abouzari-Lotf Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 03 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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Industrial & Engineering Chemistry Research

Polyvinylamine containing adsorbent by radiation induced grafting of N-vinylformamide onto UHMWPE films and hydrolysis for CO2 capture

T. Rojek1, L. Gubler1, M. M. Nasef 2,3 and E. Abouzari-Lotf3 1

Electrochemistry Laboratory, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland

2

Malaysia-Japan International Institute of Technology (MJIIT), Universiti Teknologi

Malaysia, Jalan Sultan Yahya Petra, 54100, Kuala Lumpur, Malaysia 3

Center of Hydrogen Energy, Institute of Future Energy, Universiti Teknologi Malaysia,

Jalan Sultan Yahya Petra, 54000 Kuala Lumpur, Malaysia.

ABSTRACT: A facile method involving radiation induced grafting of N-vinylformamide (NVF) into the microporous structure of ultrahigh molecular weight polyethylene (UHMWPE) film followed by hydrolysis was used to prepare polyvinylamine (PVAm) containing adsorbent for CO2 capturing. The grafting parameters such as solvent type, monomer concentration, absorbed dose and reaction time were varied to control the grafting yield (GY%). The degree of hydrolysis of the grafted poly(N-vinylformamide) and density of the formed amine groups were evaluated. The chemical composition and morphology of PVAm modified films were studied using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM), respectively. The distribution of amine groups across the films was monitored by energy dispersive X-ray spectroscopy (EDX). The static CO2 adsorption characteristic of PVAm modified film (from pure CO2) with a GY of 108% was found to be promising and reached a value of 48.6 mg/g at 25oC and 1 bar. The 1 ACS Paragon Plus Environment


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breakthrough measurements of PVAm modified film showed an effective CO2 adsorption from binary mixtures with N2 without any significant loss in the performance after six adsorption/desorption cycles.

Keyword: Modification with radiation induced grafting, N-vinylformamide; microporous UHMWPE film; polyvinylamine containing adsorbent, CO2 adsorption.

1. INTRODUCTION The increase of CO2 emissions resulting mainly from burning of fossil fuel has reached a critical level causing a drastic impact on the climate change and the global warming. Particularly, the CO2 concentration hit a record value of 400 ppm in 2015 and is expected to reach 550 ppm by 2050.1-2 Therefore, reduction of CO2 emission has become a matter of great concern worldwide. Moreover, effective strategies for capturing and storage of CO2 are highly demanded to combat this trend. Amine-containing solid adsorbents have recently received a wide spread attention in CO2 capture applications due to their tunable structures and availability of wide range of modifiable inorganic and polymeric substrates and low regeneration energy.3-4,5 Particularly, these adsorbents selectively adsorb atmospheric CO2 at ambient conditions and desorb it by heating to about 100 oC.6 Previous studies on these adsorbents repeatedly used chemical or physical immobilization method to introduce amine groups to various substrates such as porous carbon structures,7 silica,8-9 zeolites10, polymeric6,

11-12

and organic/inorganic

supports.9, 13 However, chemical immobilization in which amine is supported on substrate by covalent

bonds

is

highly

favored

to

minimize

amine

leaching

during

the

adsorption/desorption cycles.5

2 ACS Paragon Plus Environment

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A number of studies reported functionalization of fibrous or porous polymer substrates with amine group by chemical modification for low temperature and humidity-aided adsorption operations.11,

14-15

This was carried out using methods such as dip coating,

interfacial polymerization, in-situ polymerization, and graft copolymerization. The latter method can be performed with catalyst-induced grafting, photo-induced grafting, plasma induced grafting and radiation induced grafting. Of all, radiation induced grafting is very promising because of its potential in controlling the level of chemical modification of polymer substrates to levels ranging from the surface to bulk without changing their inherent properties.16 Thus, radiation induced grafting has been used for preparation of various adsorbent materials for a wide range of environmental applications.17 However, the use this method for preparation of CO2 adsorbent remains scarce. Chen and co-workers prepared an adsorbent for CO2 capturing by grafting of allylamine onto polyacrylonitrile fibres.11, 14-15 Despite the good adsorption capacity, this adsorbent has serious disadvantages including the use of very hazardous allylamine monomer and the addition of chemical initiator. Another adsorbent for CO2 was prepared by radiation induced grafting of glycidyl methacrylate (GMA) onto polyethylene/polypropylene (PE/PP) nonwoven fabric followed by amination.12 In spite of the ability to introduce various amines with desired levels by the incorporation of poly(GMA) grafts into PE-PP and amination but the stability of the aminated adsorbent remained questionable. To improve environmental aspects of the adsorbent preparation and its stability, a new simplified route is proposed to introduce amine groups by replacing allylamine with Nvinylformamide (NVF) as a monomer for grafting onto ultrahigh molecular weight polyethylene (UHMWPE) porous film followed by hydrolysis. UHMWPE was chosen as a substrate due to its high chemical stability, mechanical strength, thermal resistance and reasonable cost.18 It also has the ability to form relatively stable radicals upon irradiation



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compared to normal polyethylene (PE) even in air at room temperature owing to its high crystallinity. The behavior of radiation induced radicals in UHMWPE powder and fibers has been previously reported in the literature.19-20 The objective of this study is to investigate a facile method involving RIG of NVF onto porous UHMWPE films and subsequent hydrolysis to convert the grafted PNVF chains to polyvinylamine (PVAm). The grafting yield (GY) was controlled by variation of reaction parameters. The adsorbent was mainly characterized by means of FTIR and SEM. The suitability of the obtained PVAm grafted films as an absorbent for capturing CO2 was tested under ambient conditions in static and dynamic conditions.

2. EXPERIMENTAL SECTION 2.1 Modification of UHMWPE by radiation grafting and hydrolysis The UHMWPE film modification was carried out in two steps involving radiation induced grafting followed by a hydrolysis reaction. Porous UHMWPE substrates (SureVent UPE, Millipore) with a thickness of 67 µm and a porosity of around 58 % were irradiated in air using an MeV class electron-beam facility (Leoni Studer, Däniken, Switzerland) with an absorbed dose of 100 kGy and then stored at -80°C until further use. The grafting reactions were performed in 60 mL glass reactors. The grafting solution composed of a mixture of NVF (98%, Sigma-Aldrich) monomer and a diluent system of water and alcohol, viz., methanol (99%, VWR), ethanol (ACS, Sigma-Aldrich), n-propanol (ACS, Sigma-Aldrich) or isopropanol (ACS, VWR) having a constant ratio of 1:1 (v/v). The NVF concentration was was varied from 80-100 vol%. This is because lower concentrations (e.g. 20-70 vol%) did not yield any grafting. The irradiated UHMWPE samples were introduced into the reaction mixture and the reactor was purged with nitrogen for 1 h to remove oxygen. Grafting was then performed by placing the reactor in a water bath held at a temperature of 70°C. The


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grafting yield (in wt%) was controlled by variation of the reaction parameters such as type of solvent, monomer concentration, absorbed dose and reaction time. The obtained GY data reported was an average of 3 readings in most cases. After grafting, the modified film was rinsed in the solvent used in the copolymerization step and then immersed in ethanol for at least 16 h under vigorous agitation. Subsequently, the sample was removed and dried in a vacuum oven (p < 0.2 bar) at 80°C for at least 16 h. The grafting yield (GY) also referred to as degree of grafting or graft level, was determined gravimetrically according to: (%) =

g 0 0

x 100

(1)

where, mg and m0 are the weights of the grafted and the pristine films, respectively. The thickness of the grafted samples was measured using a digital thickness gauge (MarCator 1075R, Mahr GmbH) using a flat tip probe with 4 mm diameter. Hydrolysis of formamide groups in the grafted films into amine functionalities was carried out using a 2 M NaOH solution in a round bottom flask equipped with a reflux condenser and placed in an oil bath. The reaction was performed at 80°C for various periods of time (2-12 h). Subsequently, the treated samples were rinsed in deionized water, immersed in ethanol overnight, vacuum dried at 80°C and eventually weighed. The expected amine group density ρN (mmol/g) can be calculated based on the GY, assuming complete conversion of the formamide to amine functionality according to equation 2:

N =

NVF (

(2)

)∙VAm

where, MNVF (71.08 g/mol) and MVAm (43.07 g/mol) are the molar masses of N-vinylformamide (NVF) and vinylamine (VAm), respectively. The actual amine site density was determined gravimetrically: exp

h 0

N =

VAm ∙ 0

(3)



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where, mh and m0 denote the masses of the hydrolyzed film and original film, respectively.

2.2 Characterization of PVAm modified film Infrared spectra were collected in transmission mode (Bruker Vertex 70) in vacuum in the range between 4000 and 400 cm-1. Scanning electron micrographs were collected with a field emission scanning electron microscope (Carl Zeiss Ultra), FESEM coupled with an EDX accessory (EDAX TSL AMETEK). Prior to the measurements samples were stained with chloride by immersing in 0.1 M HCl and washing in deionized water, and sputtered with gold using a sputter coater (Leica EM SCD500). The Innova® Atomic Force Microscope (BRUKER) was used for surface morphology evaluation of pristine and functionalized samples in tapping mode. An etched silicon probe of RTESPA-CP (BRUKER) with a nominal force constant of 40 N/m and a nominal resonance frequency of 300 kHz was used. Surface roughness is characterized in terms of roughness average of surface measured microscopic peaks and valleys (Ra) and root mean square roughness (Rq) using the NanoScope analysis 1.7 software. The surface porosity was calculated based on the roughness data using the same software.

2.3 Testing CO2 adsorption under static conditions The adsorption measurements with pure CO2 on PVAm modified films were performed using a Rubotherm gravimetric-densimetric gas sorption apparatus (Bochum, Germany) composed mainly of a gas dosing unit containing a network of valves, mass flowmeters and temperature and pressure sensors controlled by a data acquisition system and commercial software. The gas flow and pressure were controlled by a static gas dosing unit (i.e. no gas flow when the set point pressure is reached). The pressure range was varied in the range 0.2-1.0 bar. The


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adsorbent sample is placed in a basket suspended by a permanent magnet through an electromagnet. The cell in which the basket is housed is then closed and evacuated until a constant MSB signal is reached (i.e. no more weight change of the sample). The adsorbent sample was exposed to pure and dry CO2 at a flow rate of 100 ml/min at the desired temperature, which is controlled by an electrical thermal jacket and internal heat exchange and was maintained at 25 oC.

Figure 1. Schematic diagram of fixed bed column adsorption system.

2.4 Testing CO2 adsorption under dynamic conditions The CO2 gas adsorption on PVAm modified films was carried out at 25oC in a fixed bed column adsorption system (Figure 1). Breakthrough curves characterizing CO2 adsorption performance of the new adsorbent were obtained using a packed bed comprising stacked adsorbent discs forming a bed capped with glass wool for the top and bottom of a stainless steel column (50 mm height and 10 mm internal diameter). The adsorbent samples were dried at 60oC overnight prior to loading. A CO2/N2 gas mixture with 10% CO2 concentration was fed to the column at a pressure of 1 bar. The concentration of CO2 at the outlet of the bed was continuously monitored using an online CO2 analyzer (model 906, Quantec Instr.,) with



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operating range of 0-50% (0.01% resolution), which is also used to to check the initial CO2 concentration at the inlet. The thermal regeneration of adsorbent was carried out after CO2 adsorption reached equilibrium by heating the column to 80oC and flushing it with pure N2 until the CO2 signal measured by the CO2 analyzer was zero. The gas desorption time was 15 minutes.

3. RESULTS AND DISCUSSION 3.1 Effects of reaction parameters on grafting yield Initial investigation of grafting of NVF onto UHMWPE films showed that no grafting occurred below 80% monomer concentration. This left the range for concentration envelop too small and made it difficult to run a full systematic optimization study. Thus, the focus was made on 80-100 vol% NVF concentration range that led to limited values of GY.

3.1.1 Effect of solvent Grafting of NVF onto porous UHMWPE film was performed in four different solvents: methanol, ethanol, n-propanol and isopropanol mixed with water at a constant ratio of 1:1 (v/v). The grafting level increased with the increase in size of the alcohol alkyl group according to the following order: methanol < ethanol < n-propanol < isopropanol as depicted in Figure 2. Particularly, the highest grafting yield (GY) was recorded when an isopropanol/H2O mixture was used for diluting NVF. Isopropanol/H2O mixture was also reported

to

enhance

RIG

of

styrene

onto

poly(tetrafluoroethylene-co-

hexafluoropropylnene)(FEP) film.21 This observation suggests that isopropanol, which has a branched higher alkyl group with secondary hydrogen, enhanced the monomer diffusion to the grafting sites compared to the first two solvent mixtures with smaller alkyl groups and n-


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propanol having a linear chain. Thus, the isopropanol/water mixture was selected as a diluent for the rest of the experiments.

80 70

Grafting yield (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 50 40 30 20 10 0 MeOH/H2O

EtOH/H2O

n-PrOH/H2O

i-PrOH/H2O

Figure 2. Variation of grafting yield with different solvents for grafting of NVF onto porous UHMWPE films. Reaction conditions: 80 vol% NVF concentration, 100 kGy absorbed dose, 20 vol% solvent at alcohol/H2O 1:1 (v/v) ratio, 2 h reaction time and 70 oC temperature. 3.1.2. Effect of monomer concentration Figure 3 shows variation of the grafting yield with the monomer concentration. It can be seen that the GY increases with the increase in monomer concentration reaching a value of 110% when pure monomer was used. This trend can be attributed to the increase in the monomer diffusion to the grafting sites supplying abundant monomer molecules for the grafting reaction. A similar GY increasing trend with monomer concentration was observed for grafting of styrene onto UHMWPE powder.18



120 100 Grafting yield [wt%]

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80 60 40 20 0 70

75

80

85

90

95

100

Monomer concentration [vol%]

Figure 3. Variation of grafting yield with monomer concentration for grafting of NVF onto UHMWPE films. Reaction conditions: 100 kGy absorbed dose, 20, 10 and 0 vol% iPrOH/H2O mixture having a ratio of 1:1 (v/v), 2 h reaction time and 70oC temperature 3.1.3. Effect of absorbed dose Variation of the absorbed dose on the grafting yield is shown in Figure 4. As can be seen, the

GY increased linearly with the increase in dose from 20 to 100 kGy at all investigated concentrations. This is due to the increase in the number of radicals generated in the polymer film and involvement of more radicals in the initiation reaction. Similar linear dosedependent trends were observed for RIGP of styrene onto PE22 and polyethylene terephthalate (PET).23 It was reported that UHMWPE is a highly crystalline polymer and radicals upon irradiation are formed in the crystalline phase and a fraction of these radicals move to the amorphous areas and possibly to the crystallites surfaces to initiate the grafting reaction. The ratio of radicals existing in the crystalline part to those that migrate to the amorphous part is governed by the irradiation dose and irradiation as well as the reaction temperature.24 It can be confirmed that the GY is not only dependent on the solvent type and monomer concentration but also on the absorbed dose.


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120 100

Grafting level [wt%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 40 80 vol% NVF 90 vol% NVF 100 vol% NVF

20 0 0

20

40

60

80

100

120

Absorbed dose (kGy)

Figure 4. Variation of grafting yield with absorbed dose for grafting of NVF onto UHMWPE films at different monomer concentrations. Reaction conditions: i-PrOH/H2O 1:1 (v/v) constant solvent ratio, 70oC temperature and 2 h reaction time.

3.1.4 Effect of reaction time Variation of grafting level with reaction time is shown in Figure 5. The grafting rate increased rapidly with the increase in the reaction time from 0.15 to 4 h beyond which the GY levelled off. The initial increase in GY is due to the increase in the diffusion of NVF monomer through film surface layers leading to bulk grafting and high GY. As the reaction further proceeded to 8 h a slight decrease in GY took place. This observation is likely to be due to mutual annihilation of graft growing chains and homopolymerization of NVF monomer that leads an increase in the viscosity and subsequent reduction in the monomer diffusion. Similar decreasing trends in kinetics were obtained for grafting of 4-vinlypyridine and sodium styrene sulfonate onto poly(vinylidene fluoride) (PVDF) films where the optimum time was found to be 4 h and 24 for these systems, respectively.25-26



80 70 Grafting yield [wt%]

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60 50 40 30 20 10 0 0

2

4

6

8

10

Reaction time [h]

Figure 5. Variation of grafting yield with reaction time for grafting of NVF onto UHMWPE films. Reaction conditions: 100 kGy absorbed dose, 80 vol% NVF concentration, iPrOH/H2O 1:1 (v/v) constant solvent ratio and 70oC, temperature.

3.2. Properties of the grafted samples 3.2.1 Variation of thickness with degree of grafting Since the target to have highest GY to achieve maximum amine content upon hydrolysis, the sample with 108% GY that was obtained at the maximum dose of 100 kGy, NVF concentration of 100 vol%, time of 2 h and temperature of 70oC was selected. Sample with 67% GY that was obtained by lowering NVF concentration to lowest value (80 vol%) with 100 kGy, 2 h and 70oC) was chosen, whereas, sample 42% was (obtained at 60 kGy, 80 vol% NVF in i-PrOH/H2O 1:1 (v/v), 2 h and 70oC.) chosen to set the lowest GY limit taking same the monomer concentration into consideration (80 vol%). Figure 6 shows the variation of film thickness with grafting level in PNVF grafted samples. The thickness of the grafted samples increased from 70 to 90 micron with the increase in GY from 42% to 108%. Since grafting mainly occurs in the amorphous region and the growing graft chains expand the amorphous region of the film, it can be suggested that


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grafting took place mainly on the inner surface of the porous structure and also on the surface of samples having a GY in the range of 42-108% causing gradual reduction of the pore size.

Table 1. Variation of theoretical and experimental amine densities with grafting yield Grafting yield, GY (wt%)

Grafting conditions

42

- 60 kGy, 80 vol% NVF in i-PrOH/H2O 1:1 (v/v), 2 h and 70oC. - 100 kGy, 80 vol% NVF in i-PrOH/H2O 1:1 (v/v), 2 h and 70oC. - 100 kGy, 100 vol% NVF, 2 h and 70oC.

67

108

Theoretical amine site density N (mmol/g) 4.7

Experimental amine exp site density N (mmol/g) 4.3

Difference (%) -9.5

6.7

6.4

-4.8

9.1

8.9

-3.2

100 90 80 70 60 Thickness [um]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50 40 30 20 10 0 0

10

20

30

40

50

60

70

80

90

100 110 120

Grafting yield [wt%]

Figure 6. Variation of film thickness with grafting yield in PNVF grafted samples.

3.2.2 Amine sites density Table 1 shows the variations taking place in the theoretical and experimental of amine site density after hydrolysis reaction. As can be seen, the amine site density both theoretical and experimental increased with the increase in GY as a result of the presence of more PNVF



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grafts. The difference observed between the experimental and expected amine site density results is the loss of a small fraction of the graft component during hydrolysis. Figure 7 shows typical spectra of pristine UHMWPE film, PNVF grafted UHMWPE film and corresponding PVAm containing film hydrolyzed at different reaction times. The incorporation of PNVF grafts to UHMWPE film is confirmed by the appearance of a number of new peaks such as the peak at 1660 cm-1 of amide I, which is assigned to C=O stretching vibration. The new peak at 766, 1130 cm-1, 1252 and 1532 cm-1 are assigned to amide V (NH out-of-plane bending), amide III (2° amide C-N stretching of cis and trans form) and amide II (2° amide N-H bending and C-N stretching) respectively. The characteristic peaks assigned for the grafted PNVF film is a complete agreement with literature.27 The hydrolysis of amide to amine is confirmed by a parallel continuous reduction in the intensities of the amide peaks, most notably the amide I peak at 1660 cm-1, and appearance of a broad band at 1590 cm-1 (1° amine N-H2 bending) with increase in the reaction time until a complete peak disappearance was observed after 12 h of treatment with alkaline solution at 80oC. This suggests that a 100% hydrolysis was reached with a reaction time of 12 h. Based on the FTIR results an illustrative mechanism for preparation of the adsorbent film synthesized in this study shown in Figure 8 can be proposed.


CH2 rock.

Amide V

band ?

Comb.

CH2 bend.

δ(NH2)

Amide II

Amide III trans-cis

UPE-g-NVF 12h

Absorbance (a.u.)

UPE-g-NVF 6h

UPE-g-NVF 2h detector saturated

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Amide I

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UPE-g-NVF UPE

2000

1800

1600

1400

1200

1000

800

600

Wavenumber (cm-1)

Figure 7. FTIR spectra of pristine UHMWPE film, PNVF grafted UHMWPE film and corresponding PVAm films hydrolyzed for different reaction times. Important band assignments are indicated.

Figure 8. Mechanism for preparation of the adsorbent film containing amine groups by radiation induced grafting of NVF and hydrolysis.

3.2.3 Surface properties of grafted samples The grainy and crackly surface structure with many lined cracks alongside the surface bridged with fibrils is depicted in Figure 9 (A1 and A2). The incorporation of the PNVF graft component took place in the porous structure of the film and on the surfaces. The former is indicated by the decrease in the void space between fibrils whereas the latter is evident from the thickening of grainy surface as shown in Figure 9 (B1 and B2) for a sample having a



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grafting yield of 67%. The thickness of sample increased from 69 to ~80 micron. The increase in the GY to 108% further narrowed the void spaces between fibrils to a great extent and led to a rather thicker grainy surface structure (Figure 9, C1 and C2) with the thickness of the grafted film increased to 80 micron. This result is consistent with the observation made from Figure 7 and suggests that grafting decreases the porosity of the UHMWPE film.

Figure 9. SEM images of a) pristine UHMWPE film and PNVF grafted UHMWPE films with b) 67% GY and c) 108% GY at two different magnifications (2k and 10k).

Table 2. Surface morphology parameters of original and modified films Samples

Roughness factors (nm) Surface porosity (%) Ra Rq Original 262 332 57 GY = 42% 237 324 40 GY = 67% 193 243 31 GY = 108% 134 174 20

Surface area µm2 (in 20 µm ×20 µm) 508 533 490 461


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Figure 10 shows the changes in the surface of pristine porous UHMWPE film and grafted films with different grafting yields. The pristine UHMWPE film is found not to have a smooth surface as indicated by the roughness value of around 262 nm. However, it can be clearly seen that the surface properties improved and became more uniform with the increase in GY as the difference between peaks and valleys in the range of micrometers (z axes in Figure 1) is reduced to nanometer scale. At GY of 41%, a marginal decrease in the roughness is observed and Rq almost remains in the same range as shown in Table 2. However, the Ra and Rq values decrease by around 48% when GY increased to 108%. Moreover, it is obvious that grafting took place in the vicinity of pores and led to partial filling and subsequent reduction of the porosity of the UHMWPE film. Particularly, the surface porosity of the sample having GY of 108% is reduced by around 65%.

Figure 10. Tapping mode AFM images of original UPE (a) and functionalized substrates with different GY of: (b) 41.4, (c) 67%, and (d) 108%. 17 ACS Paragon Plus Environment


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3.2.4 Micro-structure and functional group distribution

Figure 11 shows SEM image of a cross-section of PVAm-containing film having GY of 108% and EDX mappings for carbon and chlorine (obtained from staining with HCl solution and yielding NH3+/Cl-) together with their profile across the membrane. As can be seen, the distribution of carbon suggests that the porosity of UHMWPE support is not homogeneous. The inverse distribution of chlorine relative to carbon may indicate higher porosity in the interior (core) of UHMWPE support, resulting in higher grafting yield in the centre of the film. The inhomogeneous distribution of grafting in the direction of film thickness may be also attributed possible radical deactivation of some radicals took place near film surface by O2 in air.

A

B

C

D

Figure 11 SEM image a cross-section of A) film having GY of 108% and EDX mappings for B) carbon and C) chlorine in the same grafted film together with D) their profiling across the membrane.


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Table 3. Variation of CO2 adsorption capacity and amine site occupancy with amine site density at 1 bar Amine site CO2 adsorption Amine site density capacity @ 1bara occupancyb (mmol/g) (mg/g) (mmol/g) (%) 4.3 18.3 0.42 9.7 6.4 32.6 0.74 11.6 8.9 48.6 1.10 12.4 a Adsorption values are obtained from first adsorption/desorption cycle. b Number of CO2 molecules per amine site is based on experimental amine site density.

3.3 Static CO2 adsorption 3.3.1 Effect of pressure Figure 12 shows the effect of variation of pressure on CO2 adsorption capacity at 25°C for samples having various grafting yields whereas the data for the adsorption capacity and amine site occupancy versus amine site density at 1 bar is presented in Table 3. It can be seen that CO2 adsorption capacity of all samples increased sharply when an initial pressure of 0.2 bar was applied beyond which it tended to slow down gradually and level off. The initial steep slope indicates that the new adsorbent has a strong affinity for CO2 molecules. The shape of adsorption isotherms of Type 1 is due to the chemisorption and reflects the presence of a possible monolayer adsorption that can be represented by the Langmuir model. The CO2 adsorption capacity also increased with the increase of GY (amine site) in the pressure range of 0-1 bar. The sample with GY of 108% showed the highest CO2 adsorption capacity of 48.6 mg/g at 25°C and 1 bar. This is due to the increase in the number of CO2 adoption sites as a result of the rise in the content of amine groups originated from fully hydrolysed amide groups as indicated by not only the reduction in porosity and surface area of UHMWPE film with the increase in GY (shown in Table 2) but also the increase in the thickness with the incorporation of PVAm grafts. This trend is going along with previous studies reported in literature.28-29a,b However, one can observe from Table 3 that the amine occupancy, which represents the adsorption efficiency, is in the range of 9.7-12.4%. These values are not



different from most of the published adsorption values, which are lower than 0.5%.28 The lower amine occupancy is likely due to the nature of the adsorbent, which is in a form of a film (UHMWPE support) having low surface area with inhomogeneous porosity that is higher in the core and has irregular pore structures. This led to higher GY in the core of the film and in the vicinity of the pores as shown form SEM and EDX images (Figures 9 and 11). It can suggested that CO2 adsorption, which is dominated by chemisorption takes place on the amine sites occluded on in the vicinity of the pores and film surfaces, which is controlled by the amine density. and most likely enhanced by the presence of free volume remaining after incorporation of amine grafts (20% porosity as shown in Table 2). Moreover, CO2 adsorption capacity is a function of amine content, i.e. grafting yield. It can be concluded that radiation induced grafting of NVF onto porous UHMWPE films and subsequent hydrolysis reaction is highly effective in imparting CO2 adsorption characteristic and converting the grafted film into CO2 adsorbent by the incorporation of polyvinylamine grafts using a shorter route.

60.00 Adsorption capacity (mgl/g)

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50.00 40.00 30.00 20.00 10.00 42%

67%

108%

0.00 0.00

0.20

0.40

0.60

0.80

1.00

1.20

Pressure (bar)

Figure 12. Adsorption isotherms of CO2 on radiation grafted amine containing adsorbents with various grafting yields (42, 67 and 108 %) at 25°C and 0% humidity.


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3.3.2 Effect of temperature The effect of temperature variation the adsorption capacity of CO2 on PVAm containing adsorbent with 108% GY at 0–1 bar is shown in Figure 13. The CO2 adsorption capacity decreased with the increase in the temperature from 25 to 55 oC at all pressure levels. The optimum temperature for CO2 adsorption on the new adsorbent film was found to be 25 °C within the temperature range studied. The inverse effect of temperature on CO2 adsorption can be attributed to two main thermodynamic and dynamic factors affecting the adsorption capacity in the dynamic adsorption process.30 The thermodynamics of the exothermic nature of the adsorption process causes a reversible reaction when the temperature rises.31 Since the adsorbent has a porous structure, the thermodynamic effect is more prominent than the dynamic effect. Similar behavior was reported for adsorption of pure CO2 on fibrous aminecontaining adsorbent obtained by radiation grafting copolymerization of allylamine onto polyacrylonitrile fibers.11 The authors suggested that the decrease in CO2 adsorption capacity with the rise in temperature is due to the increase in the rate of decomposition of ammonium carbonate that lowered the adsorption capacity. The noticeable decrease in CO2 adsorption capacity and the relatively small temperature differential for adsorption/desorption are likely to be very useful for the minimization of the energy penalty for adsorbent regeneration.32



60 50 Adsorption capacity (mg/g)

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40 30 20 10

25 oC

40oC

55oC

0 0.00

0.20

0.40

0.60

0.80

1.00

1.20

Pressure (bar)

Figure 13. Adsorption isotherms of CO2 on radiation grafted amine containing adsorbents at various temperatures. Graft yield in adsorbent is 108% and gas has 0% humidity.

3.4 Dynamic CO2 adsorption The dynamic performance of PVAm containing adsorbent was evaluated using a feed CO2/N2 mixture containing 10% CO2 and compared with the adsorption/desorption regeneration cycles obtained from the fixed bed flow system at 25 oC, 1 bar and dry condition. The variation of the CO concentration at the outlet of the fixed bed relative to that at the inlet is 2

used for determining the CO2 adsorption breakthrough curve for 6 adsorption/desorption (regeneration) cycles and the obtained data are plotted in Figure 14. The adsorbent demonstrates a breakthrough time of 2 minutes whereas the exhaustive point at which the CO2 concentration at outlet equals to that at the inlet is at 15 minutes. A typical desorption time was 15 minutes at which a complete desorption process indicating that the CO2 was almost released from spent adsorbent after heat treatment at 80 oC. A longer desorption time of 30 minutes is needed for ion exchange resin (Lewatit VP OC 1065, Lanxess) containing benzylamine at 200

o

C,33 and amine-containing polyacrylonitrile fibrous adsorbent.11

Desorption times of 20 and 60 minutes were also reported for amine adsorbent based on


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polypropylene microfibers at 100 oC34 and amine-based nano-fibrillated cellulose at 90 oC,35 respectively. These results suggest that the new adsorbent has a reasonably fast regeneration time. 1.2 1.0 0.8 C/Co

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0.6 Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle5 Cycle 6

0.4 0.2 0.0 0

200

400

600

800

1000

Time (s)

Figure 14. Breakthrough curve for CO2 adsorption on amine-grafted adsorbent at: 10% CO2 in CO2/N2 mixture (inlet feed), 1 bar, 25oC and 0% humidity.

3.5 Stability of PVAm containing adsorbent Figure 15 shows variation in CO2 adsorption capacity of PVAm containing adsorbent over a number of adsorption–desorption cycles. The amine-containing adsorbent showed an excellent stability as indicated by not only by the presence of similar breakthrough time but also by the lack of significant change in mass transfer zone as well as absence of loss in CO2 adsorption capacity that was observed after 6 adsorption-desorption cycles.



48.7 Adsorption capacity (mg/g)

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48.6 48.5 48.4 48.3 48.2 48.1 48.0 1

2

3 4 No. of cycles

5

6

Figure 15. CO2 adsorption capacity of PVAm containing UHMWPE porous film over a number of adsorption–desorption cycles.

4 CONCLUSIONS New CO2 adsorbent was successfully prepared by modification of porous UHMWPE films by incorporation of PVAm moiety by radiation induced grafting of NVF and subsequent hydrolysis. The content of PNVF grafts was found to be dependent on the reaction parameters. Particularly, the GY was found to be a function of the type of solvent, monomer concentration, absorbed dose and reaction time. A complete hydrolysis of the amide group was achieved in PNVF grafted samples after chemical treatment with NaOH for 12 h at 80oC as revealed by FTIR analysis. The incorporation of PVAm grafts took place in the vicinity of the voids falling alongside the line crack connected by tiny fibrils and on the surface of the grainy structure in the grafted film as suggested by SEM images. The CO2 adsorption capacity of PVAm modified adsorbent was found to be a function of GY, which is equivalent to the content of amine upon hydrolysis. The sample with GY of 108% showed the highest CO2 adsorption capacity of 48.6 mg/g when tested with pure CO2 gas at 25oC and 1 bar under static conditions. The temperature was also found to significantly affect CO2 adsorption across the whole tested pressure range. The dynamic adsorption of CO2 was evaluated using CO2/N2 mixture containing 10% CO2 at 25 oC and the breakthrough curves were established 24 ACS Paragon Plus Environment

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for 6 adsorption-desorption cycles. The PVAm modified film acted effectively as adsorbent that could be easily regenerated (within 15 min) at relatively low temperatures (80 oC) without structural degradation after 6 regeneration cycles.

ASSOCIATED CONTENT

AUTHOR INFORMATION Corresponding Author *1,2

E-mail addresses: [email protected]. Tel: +6 03 2203 1229, Fax: +6 03

2203 1229.

Author Contributions M.M. Nasef proposed collaboration and L. Gubler hosted the visit. T. Rojek, M.M. Nasef and E. Abouzari-Lotf carried out the experiments in PSI and UTM, respectively. M.M. Nasef and L. Gubler analyzed the data and wrote the manuscript.

Funding Sources Swiss National Science Foundation & Contract research grant (vot # 4C116) between UTM and MTJA.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT M. M. Nasef would like to acknowledge financial support to the visit to PSI to perform part of the work. E. Abouzari-Lotf is thankful for the financial support from contract research 25 ACS Paragon Plus Environment


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grant (Vot no. 4C116) between UTM and MTJA. T. Rojek and L. Gubler wish to acknowledge the financial support from the Swiss National Science Foundation (Grant no. 156604).

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Polyvinylamine containing adsorbent by radiation induced grafting of N-vinylformamide onto UHMWPE films and hydrolysis for CO2 capture T. Rojek1, L. Gubler1, M. M. Nasef 2,3 and E. Abouzari-Lotf3 1

Electrochemistry Laboratory, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland

2

Malaysia-Japan International Institute of Technology (MJIIT), Universiti Teknologi

Malaysia, Jalan Sultan Yahya Petra, 54100, Kuala Lumpur, Malaysia 3

Center of Hydrogen Energy, Institute of Future Energy, Universiti Teknologi Malaysia,

Jalan Sultan Yahya Petra, 54000 Kuala Lumpur, Malaysia.

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