Polyurethane /Ionic Silica Xerogel Composites for CO 2 Capture

Capturing carbon dioxide (CO 2 ) from exhaust gases is an important strategy to prevent climate change. There is a great interest in developing novel CO 2 sorbents. Thus, a series of polyurethane (PU) / silica xerogels functionalized with RTILs (bmim Cl and bmim TF 2 N) 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. CO 2 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 CO 2 uptake. CO 2 sorption capacity tends to increase with the incorporation of silica xerogels functionalized with RTILs in PU matrix. The best CO 2 sorption value was found for PU/SX-[Bmim]-[TF 2 N] 0.5 composite (48.5 mgCO 2 /g at 298.15 K and 1 bar). Moreover, the PU/SX-[Bmim]-[TF 2 N] 0.5 composite showed reuse capacity and higher CO 2 sorption value as compared to other reported composites.


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 (CO 2 ) resulted in climate change 1 . Carbon capture and storage (CCS) technologies are considered important strategy to both reduce CO 2 emission and the global warming problem 1,2 . Thus, the novel CO 2 sorbents synthesis are of great interest in this field 3,4 .

PU composites preparation
PU/ionic silica xerogel composites were prepared by addition of silica xerogel functionalized with bmim Cl or bmim TF 2 N 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 °C during 120 min.

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 °C for 6h. The structural elucidation of silica xerogels was carried out by solid state NMR (SS NMR) techniques. 13 C MAS spectra were acquired with a 7 T (300 MHz) AVANCE III Bruker spectrometer operating respectively at 75 MHz ( 13 C), 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 °C with a heating rate of 20 °C min -1 was used under N 2 atmosphere. Analyses were performed in triplicate. Thermogravimetric analyses were performed using SDT equipment (TA Instruments model Q600). Temperature range was from 25 to 800 °C with a heating rate of 20 °C /min under constant N 2 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 °C 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).
Recycle experiments were performed by repeating the sorption/desorption cycles six times at 1 bar and 25 °C (298.15 K) with desorption following each cycle under vacuum (10 -3 mbar) at 298.15K during 1h.  Fig.1) and the form as it is organized on the silica surface. 13 C CPMAS NMR spectra of silica xerogel are shown in Fig.2. All samples presented chemical shifts from CH 3 , CH 2 and OCH 2 groups at 15; 29 and 57-59 ppm, respectively.

SX-[bmim][TF 2 N] 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 ppm 48 .
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    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 .
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 50°C and 150°C is related to water evaporation. The second stage is associated mainly to degradation of hard segments (urethane bonds) [58][59][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 . 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 °C. 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 chains 64 .
In this work, PU/SX-[Bmim] [TF 2 N] composites were chosen to study the effects of filler addition in PU matrix on mechanical properties due to higher CO 2 sorption capacity compared to PU (see Fig. 7). Tensile properties and Young moduli are presented in Figs 7 and Table 3. PU/SX-[Bmim] [TF 2 N] 0.5 was selected to recyclability study due to higher CO 2 sorption capacity compared to all other samples. CO 2 sorption capacity was increased in the first four recycles, probably due to remaining moisture.   Mechanical analysis revealed that both filler addition and increased content in PU matrix results in mechanical properties reduction (See Fig.7 CO 2 sorption results are presented in Fig. 8. PU showed a CO 2 sorption capacity of 25 mgCO 2 /g due to the polar groups present into PU structure which may promote CO 2 affinity 65,66 . CO 2 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 CO 2 capture processes. 17

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 CO 2 sorption capacity increase. CO 2 sorption values were higher for PU composites prepared from silica xerogels functionalized with fluorinated RTIL. The best CO 2 sorption capacity was found for PU/ SX-[Bmim] [TF 2 N] 0.5 (48.5 mgCO 2 /g). Furthermore, PU composites CO 2 sorption/desorption cycle results showed both stability and reuse capacity in CO 2 capture processes.