Polyurethane /Ionic Silica Xerogel Composites for CO2 Capture

Abstract

Capturing carbon dioxide (CO₂) from exhaust gases is an important strategy to prevent climate change. There is a great interest in developing novel CO₂ sorbents. Thus, a series of polyurethane (PU) / silica xerogels functionalized with RTILs (bmim Cl and bmim TF₂N) composites were prepared and characterized. PU matrix was reinforced with functionalized silica xerogels in the range of 0.5-20 wt%. PU / functionalized silica xerogels were characterized by NMR, FTIR, DSC, TGA, DMTA and FESEM. CO₂ sorption capacity and reusability were assessed by the pressure-decay technique at 298.15 K and 1 bar. Results showed that the filler aggregation in PU matrix promoted the reduction of mechanical properties. However, addition of silica xerogels functionalized with RTILs in PU matrix led to increased CO₂ uptake. CO₂ sorption capacity tends to increase with the incorporation of silica xerogels functionalized with RTILs in PU matrix. The best CO₂ sorption value was found for PU/SX-[Bmim]-[TF₂N] 0.5 composite (48.5 mgCO₂/g at 298.15 K and 1 bar). Moreover, the PU/SX-[Bmim]-[TF₂N] 0.5 composite showed reuse capacity and higher CO₂ sorption value as compared to other reported composites.

Keywords:
Ionic silica gel; polyurethane composites; CO₂ capture

1. Introduction

Increased fossil fuel energy consumption with industrial development leads to high greenhouse gas emissions (GHG)¹. There are evidences that the increase of GHG in the atmosphere, mainly carbon dioxide (CO₂) resulted in climate change¹. Carbon capture and storage (CCS) technologies are considered important strategy to both reduce CO₂ emission and the global warming problem^1,2. Thus, the novel CO₂ sorbents synthesis are of great interest in this field^3,4.

Mixed matrix membrane (MMM) consist of a dispersed inorganic material within an organic polymer continuous matrix ⁵. MMMs containing zeolitic imidazolate framework (ZIF)^6-8, silica ^4,8, metal organic frameworks (MOFs) ⁹, nickel oxide nanoparticles¹⁰, Zeolite ¹¹, ZnO Nanoparticle ¹², alumina nanoparticles ¹³, TiO₂ nanocomposite ¹⁴ have been studied to improve the membrane separation properties.

Inorganic particle/room-temperature ionic liquids (RTILs) composite ^5,15-17 has also been examined by researchers to improve the properties of MMMs. Room temperature ionic liquids (RTILs) are salts composed by an organic cation and inorganic or organic anion presenting melting point below 100ºC ^18-24. RTILs are potential solvents for CO₂ capture because their unique properties ^22,25-29. such as negligible vapor pressure, non-flammability, high thermal stability, tenability and selective CO₂ separation^{19,22,23,25,27,30}

Polyurethane /silica composites have been obtained to improve polymer properties ^31-37. PU is a class of versatile polymers with potential to use in gas separation due to their low price, thermal stability, high mechanical properties and appropriate permeability^8,10. Urethane group is the major repeating unit in PUs. However, other groups such as esters, urea, ethers and aromatic can also be present in the PU structure ^38,39. Silica incorporation into PU matrix may cause improvements in both mechanical and thermal properties, as well as gas separation properties of PU ¹³.

This study investigated the effect of silica xerogel functionalized with different RTILs incorporation (1-Butyl-3-Methylimidazolium Chloride - bmim Cl and 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide - bmim TF₂N) on both thermal and mechanical properties, as well as CO₂ sorption capacity of PU.

2. Experimental

2.1 Waterborne polyurethane (WPU) synthesis

WPU synthesis was performed using experimental procedures described in literature^40,41. Initially, polyol (MM = 1000 g/mol, Noxeller, Brasil) and dimethylol propionic acid (DMPA, 99%, Perstorp, Sweden) were charged into a five-necked flask and heated until melting. Then, 0.1% wt of dibutyltin dilaurate (DBTDL Miracema-Nuodex Ind, Brasil) as catalyst and Isophorone diisocyanate (IPDI, Merck, USA) were poured into the same reaction flask and stirred at 80ºC for 60 min to obtain NCO-terminated PU prepolymer. The NCO/OH molar ratio of 1.7 was used in the reaction. In the next step, the reaction temperature was reduced to 55ºC for neutralization of carboxylic groups (-COOH) present in DMPA by adding trimethylamine (TEA, Perstorp, Sweden) (1.1 molar ratio). Finally, free NCO content (%NCO) was determined by titration with dibutylamine (Bayer, USA) and neutralized by chain extension addition in water (hydrazine, Merck, USA). The solid content of final dispersion was 35 wt%.

2.2 Silica xerogels functionalized with RTILs synthesis

Silica xerogels functionalized with RTILs were synthesized according to procedures adapted from literature^42,43. In a typical preparation, 25 mg RTIL, 2.28 mmol TEOS (Merck, 98%, USA), PVA (Dinâmica)(4.64 g/L), NaF (Synth, 99%, Brasil) (0.20g/L) and 6.86 mmol water were mixed and cooled until gelation. The gels formed were kept at 35 ºC for 1 day and washed with solvent. Finally, silica xerogels were dried at 35 ºC for 1 day. The RTILs structures used in order to obtain silica xerogels are imidazolium-based ILs with two different anions as shown in Fig. 1. A silica xerogel sample (SX) was also synthesized without RTIL. Silica xerogels functionalized with RTIL were labeled as SX- RTIL. For example, SX-[bmim][Cl] means silica xerogel containing 1-Butyl-3-methylimidazolium chloride IL.

Figure 1
RTILs structures used for silica xerogels synthesis

2.3 PU composites preparation

PU/ionic silica xerogel composites were prepared by addition of silica xerogel functionalized with bmim Cl or bmim TF₂N into the WPU dispersion. PU matrix was reinforced with functionalized xerogels in the range of 0.5-20 wt% (see Table 1). In a typical preparation, mixtures were placed in ultraturrax mixer (IKA T18 Basic) during 5 min at 10,000 rpm. Finally, films around 70 µm thick were produced. The films were dried at 35 ºC during 120 min.

Table 1
PU composite compositions

PU composite	Silica xerogel content (wt%)
PU/ SX-[Bmim] [Cl] 0.5	0.5
PU/SX-[Bmim] [Cl] 5	5
PU/ SX-[Bmim] [Cl] 20	20
PU/SX-[Bmim] [TF2N] 0.5	0.5
PU/SX-[Bmim] [TF2N] 5	5
PU/SX-[Bmim] [TF2N] 20	20

2.4 PU composites characterization

Specific surface area, pore volume and pore diameter of silica xerogels were determined by Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively using NOVA 4200e. Prior to measurements, the samples were degassed in vacuum at 125 ºC for 6h. The structural elucidation of silica xerogels was carried out by solid state NMR (SS NMR) techniques. ¹³C MAS spectra were acquired with a 7 T (300 MHz) AVANCE III Bruker spectrometer operating respectively at 75 MHz (¹³C), equipped with a BBO probe head. The films and silica xerogels were characterized by Fourier transform infrared spectroscopy (FTIR Perkin Elmer spectrometer model Spectrum100, using UATR from 4000 at 650 cm^-1), 16 scans were performed for each sample and the resolution was 4. Differential Scanning Calorimetry (DSC) thermograms were attained using TA Instruments model Q20 equipment. Temperature range from -90 to 20 ºC with a heating rate of 20 ºC min^-1 was used under N₂ atmosphere. Analyses were performed in triplicate. Thermogravimetric analyses were performed using SDT equipment (TA Instruments model Q600). Temperature range was from 25 to 800 ºC with a heating rate of 20 ºC /min under constant N₂ flow. Analyses were performed in triplicate. Films with a thickness close to 0.15 mm, length 12 mm, and a width of approximately 7.0 mm were used to perform the stress 9 strain tests. All tests were carried out at 25 ºC with on DMTA equipment (model Q800, TA Instruments) with 1 N/min. Young moduli of materials were determined according to procedure described elsewhere (ASTM D638). The analyses were carried out in triplicate. Morphology of PU/ionic silica xerogel composites was investigated by field emission scanning electron microscopy (FESEM) using FEI Inspect F50 equipment in secondary electrons (SE) mode. Samples were place into a stub and covered with a thin gold layer (15-20nm).

2.5 CO₂ sorption measurements

CO₂ sorption capacity was determined using a dual-chamber gas sorption cell by pressure-decay technique previously described in detail ^44-47. Experiments were carried out in triplicate. Samples (Ws≈1g) were previously degassed under vacuum (10^-3 mbar) at 298.15K during 1h. CO₂ sorption measurements were carried out at 25º C (298.15 K) and 1 Bar.

Recycle experiments were performed by repeating the sorption/desorption cycles six times at 1 bar and 25 ºC (298.15 K) with desorption following each cycle under vacuum (10^-3 mbar) at 298.15K during 1h.

3. Results and Discussion

SX-[bmim][TF₂N] showed higher surface area (343 m²g⁻¹) and pore volume (0.24 cm³) compared to SX - [bmim] [Cl] (surface area = 116 m² g⁻¹, pore volume = 0.10 cm³). However, SX - [bmim] [Cl] presented a pore diameter (1.63 nm) higher than SX-[bmim][TF₂N] (1.41 nm). This behavior may be related to bulky anion of [bmim][TF₂N] (See Fig.1) and the form as it is organized on the silica surface. ¹³C CPMAS NMR spectra of silica xerogel are shown in Fig.2. All samples presented chemical shifts from CH₃, CH₂ and OCH₂ groups at 15; 29 and 57-59 ppm, respectively. SX-[bmim][TF₂N] and SX - [bmim] [Cl] samples showed additional chemical shifts that reveal the presence of IL, more specifically, the aromatic ring carbons at 120-130 ppm and the aliphatic chain chemical shifts between 20-40 ppm⁴⁸.

Figure 2
¹³C MAS spectra for silica xerogels.

FTIR analysis results for functionalized silica xerogels, PU and PU composites are shown in Fig.3 (a-b). In functionalized silica xerogels spectra revealed characteristic silica and RTIL bands at around 3305 cm^-1 (-OH group), 1635 cm-1 (Si-OH and H-O-H), 1050 cm⁻¹ (Si-O-Si) and 790 cm⁻¹ (Si-O) and 1634 (C=N imidazole)^49-51. PU formation was observed by means of characteristic PU bands ^52-54 at around 2936 - 2840 cm⁻¹ (C-H), 1532 cm⁻¹ (HN), 1246 cm⁻¹ (C-N and C-O of urethane), 1100 cm⁻¹ (C-O-C), 3350 cm⁻¹ (N-H of bonded hydrogen) and 1727 cm⁻¹ (C=O). FTIR analysis also showed that band area at 3322 cm^-1 tends to increase with both the incorporation and concentration of fillers in PU matrix, indicating an increase in hydrogen bond formation in the presence of fillers ^52,53,55.

Figure 3
FTIR spectra for functionalized silica xerogels, PU and PU composites.

PU and PU composites FESEM images clearly show that fillers are unevenly dispersed in the PU matrix (Fig.4). Moreover, filler aggregation tends to increase with filler concentration increase in PU matrix. Filler aggregation in the polymer matrix may promote the reduction of mechanical properties ^56,57.

Figure 4
SEM micrographs: a) PU, b) PU/SX-[Bmim] [Cl] 0.5, c) PU/SX-[Bmim] [Cl] 5, d) PU/SX-[Bmim] [Cl] 20; e) PU/SX-[Bmim] [TF₂N] 0.5, f) PU/SX-[Bmim] [TF₂N] 5 and PU/SX-[Bmim] [TF₂N] 20.

PU and PU composite thermal stability was investigated by TGA. PU and PU composite TG and DTG curves are shown in Fig. 5. All samples presented three typical degradation stages (Fig.5). The first weight loss between 50ºC and 150ºC is related to water evaporation. The second stage is associated mainly to degradation of hard segments (urethane bonds) ^58-60 and the third stage is attributed mainly to decomposition of soft segments (polyol) ⁶¹. PU and PU composite degradation temperatures (T_onset) of two stages are given in Table 2. TGA analysis reveals that the addition of functionalized silica xerogels in PU matrix results in a degradation temperatures decrease (T_onset) as seen in Table 2. Interactions between hydroxyl groups present in the filler structure and PU hard segments can generate a deleterious effect on PU composites thermal stability. This effect might lead to breaking urethane and urea bonds of hard segments ^62,63.

Table 2
TGA and DSC data for PU and PU composites

sample	T1 onset (ºC)	T2 onset (ºC)	Tg (ºC)
PU	270 ± 1.2	356 ± 1.0	-40.0 ± 0.9
PU/ SX-[Bmim] [Cl] 0.5	181 ± 1.5	266 ± 1.3	-35.9 ± 0.4
PU/SX-[Bmim] [Cl] 5	194 ± 1.3	323 ± 1.0	-35.5± 0.3
PU/ SX-[Bmim] [Cl] 20	209 ± 2.0	325 ± 1.7	-30.3 ± 0.5
PU/SX-[Bmim] [TF2N] 0.5	203 ± 2.2	316 ± 1.8	-35.0 ± 0.6
PU/SX-[Bmim] [TF2N] 5	204 ± 0.8	320 ± 1.4	-32.9 ± 0.3
PU/SX-[Bmim] [TF2N] 20	206 ± 1.3	323 ± 1.2	-30.8 ± 0.6

Figure 5
PU and PU composites: a) TG and b) DTG thermograms.

PU and PU composites DSC thermograms are shown in Fig. 6. PU showed a glass transition temperature (Tg) related to the soft domain at - 40.0 ºC. PU composite DSC thermograms exhibited significant changes in Tg compared to PU. Tg tends to rise with filler concentration increase in PU matrix as seen in Table 2. According to these results, the hydrogen bonding increase observed by FTIR after the filler addition may be restricting the mobility of PU chains⁶⁴.

Figure 6
DSC thermograms obtained for PU and PU composites.

In this work, PU/SX-[Bmim] [TF₂N] composites were chosen to study the effects of filler addition in PU matrix on mechanical properties due to higher CO₂ sorption capacity compared to PU (see Fig. 7). Tensile properties and Young moduli are presented in Figs 7 and Table 3. Mechanical analysis revealed that both filler addition and increased content in PU matrix results in mechanical properties reduction (See Fig.7). Young’s modulus, tensile strength, elongation-at-break are decreased after filler addition in PU matrix. Thus, the best result of mechanical properties was found for PU (Young’s modulus of 7.54, tensile strength of 2.41 MPa and elongation at a break of 133%). These results are consistent with FESEM finding and indicate that filler aggregates in polymer matrix promote increase of weak points in the PU composites leading to mechanical properties decrease ^56,57.

Figure 7
Stress/strain curves obtained for PU and PU composites.

Table 3
PU and PU composites mechanical properties

Sample	Stress (MPa)	Strain (%)	Young Moduli (MPa)
PU	2.41 ± 0.8	133 ± 0.7	7.54 ± 0.5
PU/SX-[Bmim][TF2N] 0.5	1.57 ± 0.5	107 ± 0.5	5.03 ± 0.7
PU/SX-[Bmim] [TF2N] 5	1.07 ± 0.3	94 ± 0.4	4.68 ± 0.2
PU/SX-[Bmim] [TF2N] 20	0.40 ± 0.3	50 ± 0.4	3.64 ± 0.6

CO₂ sorption results are presented in Fig. 8. PU showed a CO₂ sorption capacity of 25 mgCO₂/g due to the polar groups present into PU structure which may promote CO₂ affinity ^65,66. CO₂ sorption tends to increase with filler incorporation in PU matrix. Silica xerogel functionalization with fluorinated anion increased the affinity to CO₂ molecules compared to non-fluorinated anion (PU/ SX-[Bmim] [Cl] 0.5 = 31mgCO₂/g PU/ SX-[Bmim] [TF₂N] 0.5 = 46 mgCO₂/g). The best CO₂ sorption values were found for PU composites prepared with SX-[Bmim] [TF₂N] 0.5. This behavior is probably associated with both the higher specific surface area of SX-[Bmim] [TF₂N] (343 m² g⁻¹) compared to SX-[Bmim] [Cl] (116 m² g⁻¹) and the presence of fluorinated anion that may improve CO₂ sorption ^46,67_. PU/ SX-[Bmim] [Cl] CO₂ sorption values showed to be constant for all concentrations while PU/ SX-[Bmim] [TF₂N] CO₂ sorption capacity tends to decrease with filler concentration increase in PU matrix possibly due to high filler aggregation in the polymer matrix evidenced by FESEM images (Fig.3). PU/SX-[Bmim] [TF₂N] 0.5 composite demonstrated higher CO₂ sorption capacity (46 mgCO₂/g at 298.15 K and 1 bar) as compared to reported polyvinylidene-fluoride-hexafluoropropylene (PVDF-HFP)/ amino-silica composites⁴ ( PVDF-HFP-20 wt% AFS = 26.27 mgCO₂/g and PVDF-HFP-20 wt% ANS = 12.36 mgCO₂/g at 323.15 K and 1.01 bar).

Figure 8
PU and PU composite CO₂ sorption capacity values at 0.1 bar and 298.15 K.

PU/SX-[Bmim] [TF₂N] 0.5 was selected to recyclability study due to higher CO₂ sorption capacity compared to all other samples. CO₂ sorption capacity was increased in the first four recycles, probably due to remaining moisture. CO₂ sorption values were constant for the next cycles as seen in Fig. 9. This result evidences the reuse capacity and potential of this material for use in CO₂ capture processes.

Figure 9
CO₂ sorption/desorption tests for PU/SX-[Bmim] [TF₂N] 0.5.

3. Conclusion

Uniform distribution of filler in PU matrix is desirable to obtain PU composites with improved mechanical and thermal properties. PU composites showed lower mechanical properties than PU. However, the functionalized silica xerogels addition in PU matrix led to CO₂ sorption capacity increase. CO₂ sorption values were higher for PU composites prepared from silica xerogels functionalized with fluorinated RTIL. The best CO₂ sorption capacity was found for PU/SX-[Bmim] [TF₂N] 0.5 (48.5 mgCO₂/g). Furthermore, PU composites CO₂ sorption/desorption cycle results showed both stability and reuse capacity in CO₂ capture processes.

4. Acknowledgments

The authors would like to thank the Nokxeller Microdispersions - Waterbased Polyurethane for donating polyol and Marta Corvo for NMR spectra. Sandra Einloft thanks CNPq for research scholarship.

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Publication Dates

• Publication in this collection
  16 Sept 2019
• Date of issue
  2019

History

• Received
  18 Jan 2019
• Reviewed
  20 May 2019
• Accepted
  03 June 2019