Article pubs.acs.org/IECR
Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX-XXX
Enhancing Degradation Resistance of Polyethylenimine for CO2 Capture with Cross-Linked Poly(vinyl alcohol) Yuxin Zhai and Steven S. C. Chuang* Department of Polymer Science, The University of Akron, 170 University Avenue, Ohio 44325, United States S Supporting Information *
ABSTRACT: One critical issue in the development of immobilized amine sorbents for CO2 capture is sorbent degradation, which leads to a significant increase in the overall CO2 capture cost. Here we report a novel approach for enhancing the degradation resistance of a branched polyethylenimine (PEI) by hydroxyl groups (i.e., −OH) of a porous crosslinked poly(vinyl alcohol) (PVA) support. The CO2 capture capacity of a PEI/PVA sorbent showed a 25% decrease and then leveled off. In contrast, the PEI/SiO2 sorbent exhibited more than a 3-fold decrease in CO2 capture capacity after exposure to a cyclic CO2 capture and oxidative degradation environment (15 vol % of CO2 in air at 130 °C). In situ infrared spectroscopic study revealed that the secondary amine is more liable to degrade than the primary amine on PEI/SiO2. On PEI/PVA, the PVA’s −OH groups interacted mainly with the secondary amine of branched PEI through a hydrogen bonding, which could contribute to enhancing the resistance of PEI against degradation. This new finding could provide new pathways in the development of low-cost and highly durable amine sorbents by using highly porous polymeric supports with −OH groups.
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INTRODUCTION
Fourier transform infrared (FTIR), Raman, liquid-state, and solid-state NMR studies showed that the primary amine site is gradually converted to a urea species in a CO2 environment, while the secondary amine site is liable to oxidize in an air environment by forming amide and imine species at the regeneration temperature of 100−130 °C.26−30 Approaches for improving oxidative degradation resistance include (i) the addition of additives such as polyethylene glycol (PEG) or cross-linker to stabilize the amine sites and (ii) the synthesis of the specific form of amine molecules.3,8,22,26,31,32 The high degradation resistance was obtained on polyamines without secondary amine sites or with longer alkyl linker length (e.g., ethyl or propyl).22,26,32 A number of studies have unraveled that H2O moisture slowed down the formation of urea from amine functional groups of the sorbents, which enhanced the sorbent stability in a CO2 environment.9,27,33 Our recent study has shown that H2O preferentially interacts with the secondary amine of tetraethylenepentamine, which has been widely used for the preparation of amine sorbents.34,35 The improvement of degradation resistance in the amine sorbent in an oxidative environment (i.e., air or O2) could be attributed to the interaction of H2O with the amine sorbents’ secondary amine site, which is more liable to oxidize than the primary amine site.
Atmospheric CO2 reached 411 ppm in 2017 and is projected to rise for the next few decades before renewable energy and energy storage technologies achieve a cost-competitive advantage.1,2 Capturing CO2 from stationary sources, such as coal-fired and natural gas-fired power plants, has been conceived as an effective strategy for decreasing the rate of increase in the concentration of atmospheric CO2. One promising cost-effective approach for controlling CO2 emission from stationary sources is the use of amine sorbents for thermal swing CO2 capture processes.3−20 Amine sorbents with inorganic or polymeric supports have been widely studied for the thermal swing CO2 capture processes, which capture CO2 at a temperature below 60 °C and regenerate the sorbent at a temperature above 100 °C.3−15 Screening studies of immobilized amines suggested that a branched polyethylenimine sorbent holds a very good promise for CO2 capture because of its low cost, high CO2 capture capacity, and thermal stability.3−8,18,21 One critical issue in the development of an effective regenerative sorbent is the durability of amine sites which have to maintain their structures under high-temperature regeneration conditions in the presence of CO2, steam, and air.8,22−24 The use of CO2 purged gas at the sorbent regeneration temperature allows the production of a high purity CO2 stream; steam further purges CO2 from sorbent bed; and air could serve as a low-cost purge gas to remove H2O vapor (steam) prior to cooling down the sorbent for CO2 capture.15,25 © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
August 31, 2017 October 7, 2017 October 24, 2017 October 24, 2017 DOI: 10.1021/acs.iecr.7b03636 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Teller (BET) method. The BET surface area of PEI/SiO2 and PEI/PVA was not measured because the unreactive GA and fragmented PEI trapped in the sorbents can be drawn out under vacuum, contaminating the gas lines in the BET analyzer. The samples were measured by a field-emission scanning electron microscope (SEM, JEOL-7401), a table-top microscope (SEM, Hitachi TM-3000) equipped with an energydispersive X-ray spectrometer (EDX), and a FEI scanning transmission electron microscope (TEM), Fourier transform infrared spectrometer (FTIR, Thermo-Nicolet 6700), DXR Raman Microscope with 532 nm excitation wavelength (Raman, Thermo Fisher Scientific), and 13C solid-state NMR (Varian INOVA 750). The ethanol uptake of sorbent was obtained by soaking 1 g of sorbent with ethanol. Attrition Test. The attrition test of PEI/PVA was conducted following the guidelines of the ASTM D5757 method. The PEI/PVA were sieved with the particle size of 250 to 500 μm and loaded into the attrition column to a bed height of 1 in. The particles were fluidized for 24 h in the compressed air with a flow rate of 30 cc/s. The weight of particle loss in attrition and particles with the particle size below 250 μm was recorded. The PEI/SiO2 which has a particle size smaller than 150 μm is in the form of fine particles. These fine particles need to be further pelletized to a size around 250−500 μm for a meaningful attrition test and applications in fluidized bed CO2 capture. Thermal Swinging CO2 Capture and Sorbent Degradation Process. Sorbent particles (50 mg) were packed in a DRIFTs cell (diffuse reflectance infrared Fourier transform spectroscopy) for in situ IR study of adsorbed species during the CO2 capture and sorbent degradation process. The composition of the outlet gases was detected by using MS (Pfeiffer QMS 200). The details of the experimental system have been reported elsewhere.40 Figure 1 illustrates each CO2 capture and degradation cycle: (i) pretreatment of sorbent at 120 °C for 10 min in the presence Ar to remove preadsorbed water and CO2, (ii) CO2 adsorption at 40 °C for 10 min by flowing 15 vol % of CO2 and 85 vol % of air, (iii) Ar purge at 40 °C for 10 min to remove the gas phase CO2 and the weakly adsorbed species, (iv) TPD (temperature-programmed desorption) from 40 to 130 °C in flowing Ar with a heating rate of 5 °C/min, (v) degradation at 130 °C for 30 min in flowing 15 vol % CO2 and 85 vol % air, and (vi) cooling to 40 °C in flowing Ar. The flow rates of streams were monitored at 100 cc/min. Flowing 15 vol % of CO2 and 85 vol % of air for 10 min over the sorbents allowed the breakthrough curve to reach more than 98% of its final value. Detailed CO2 adsorption/desorption kinetics will be reported in a separate study. The switch of the flow from Ar to 15 vol % CO2 is operated by a four-port valve so that well-defined breakthrough curves of CO2, O2, Ar, and N2 can be obtained in Figure 1a. Switching of the flow from 15 vol % CO2/air to Ar allowed the removal of part of the adsorbed CO2 from the sorbent. We classified these CO2 species desorbed during Ar purging at 40 °C as weakly adsorbed CO2 and those CO2 desorbed during TPD as strongly adsorbed CO2, as illustrated in Figure S1. The CO2 capture capacity of each cycle could be obtained by the amount of CO2 released during Ar purge and TPD, shown in Figure 1a inset, which was calculated by converting the area under the CO2 profile (m/e = 44) to the number of mole of CO2 adsorbed. The IR absorption spectra of adsorbed CO2 or degraded species were obtained by log(I0/I), where I is the IR single-
In this work, we employed the cross-linked poly(vinyl alcohol) (PVA) as a support for immobilizing a branched PEI.36,37 Cross-linked PVA, which is porous and thermally stable, could provide its −OH side groups for the interaction with branch PEI’s secondary amine, which thus enhanced its oxidative resistance. In situ infrared spectroscopy (IR) employed in this study revealed that CO2 adsorbed mainly on primary amine sites of PEI/PVA sorbent and on both primary and secondary amine sites of PEI/SiO2. Enhanced degradation resistance observed on PEI/PVA can be attributed to the hydrogen-bonding interaction between the −OH of PVA and the secondary amine of the PEI.
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EXPERIMENTAL SECTION Materials. Poly(vinyl alcohol) (PVA, Mw = 75 000 g/mol, ChemicalStore.com), EPON-826 (EPON, Miller-Stephenson), amorphous silica (SiO2, Rhodia Chemicals), hydrochloric acid (HCl, 37%, Chemical Store, The University of Akron), glutaraldehyde (GA, 25 wt %, Sigma-Aldrich), polyethylenimine (PEI, Mn = 60 000 g/mol, Mw = 750 000 g/mol, 50% aqueous solution, Sigma-Aldrich), linear polyethylenimine (LPEI, Mn = 5000, Sigma-Aldrich), decylamine (DEC, 95%, Sigma-Aldrich) polyethylene glycol (PEG, Mw = 200 g/mol, 99.5% Sigma-Aldrich) were used as received. Preparation of Amine Adsorbents. Porous PVA support was prepared by templating and phase inversion.36,38 PVA (100 g, 15% aqueous solution) was mixed homogeneously with GA (3 g, 25% aqueous solution) and kept at 100 °C for 2 h. PEG (250 g, 99.5%) was kept at 100 °C for 20 min and then slowly added to the PVA/GA solution under stirring at 100 °C to generate a gel. The resulting gel was pulverized in a blender for 5 min before phase inversion, which was carried out in 800 mL of acetone (pH = 1, adjusted by 1 M HCl) under constant stirring at 55 °C for 30 min. After the filtration, the agglomerated particles with the particle size of 10−50 μm were washed in acetone three times and then dried at room temperature. The PVA was partially cross-linked with GA during the phase inversion process to obtain insoluble porous PVA particles. The molar ratio of −OH to aldehyde was 22:1. The PEI/SiO2 and PEI/PVA sorbents were prepared by impregnating PEI (2.0 g, 50% aqueous solution), EPON (0.14 g), PEG (0.7 g), and ethanol (5 g) onto amorphous SiO2 (2 g) and porous PVA (2 g) supports to obtain the amine loading of 26 wt %. The amine groups from PEI were reacted with the epoxy groups from EPON to enhance the stability of PEI in the CO2 adsorption/desorption process.3,8 The molar ratio of amine to epoxy groups is 24:1. The low-PEI/PVA sorbent was prepared by impregnating PEI (0.65 g, 50% aqueous solution) and PEG (0.65 g) on 1 g of PVA support. The theoretical amine loadings for PEI/PVA and low-PEI/PVA were 5.84 and 4.12 mmol [N]/g [sorbent], respectively. The 10% PVA/GA solution was cast by a doctor blade on a Mylar film and phase inversed in 200 mL of acetone (pH = 1, adjusted by 1 M HCl). The molar ratio of −OH to aldehyde was 10:1. The PEI aqueous solution was impregnated on PVA/ GA membrane and dried at 100 °C to obtain a PEI/PVA membrane with amine loading of 5.23 mmol [N]/g [sorbent]. The amine thin films (thickness, 16 μm) of L-PEI, PEI, and L-PEI/DEC (molar ratio of NH: NH2 = 2:1) on an aluminum surface were prepared with a 5 wt % amine/EtOH solution.39 Characterization. The surface area of SiO2 and PVA supports was measured by Surface Area and Porosity Analyzer (micromeritics ASAP 2020) using the Brunauer−Emmett− B
DOI: 10.1021/acs.iecr.7b03636 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 1. (a) MS intensity profiles and (b) temperature profile during the CO2 capture and degradation on PEI/PVA. Inset: MS intensity profiles during CO2 capture. Figure 2. (i-a, i-b, ii-a, ii-b) TEM images of supports and sorbents; (i-c, ii-c) SEM images of sorbents; (i-d, ii-d) optical microscope (magnification, 10×) of sorbents on metal grids. Inset: nitrogen mapping in EDX.
beam intensity of the sorbent at the end of the step (ii) (i.e., adsorbed CO2 + pret. sorbent) or the step (vi) (i.e., degraded species + pret. sorbent) and I0 is the IR single-beam intensity of the pretreated sorbent in step (i) before CO2 capture/ degradation cycles (i.e., pret. sorbent). The IR single-beam intensity is calculated by Fourier transforming an interferogram.41 Long-Term Degradation Study of PEI/PVA in the Oven. The PEI/PVA sorbents and membrane were degraded at 110 °C in air for 12, 24, or 30 h in the oven. The CO2 capture capacity of PEI/PVA after oxidative degradation was measured by the weight-change method, which consisted of (i) pretreatment of sorbent in an oven at 100 °C for 10 min to remove preadsorbed CO2 and water (Wpret.), (ii) flowing pure CO2 on the sorbent in a closed bag at 40 °C for 10 min (WCO2), and (iii) regenerating the sorbent at 100 °C for 10 min (Wreg.). The weight of sorbent before and after each step was recorded. The difference in the weight was used to calculate the CO2 capture capacity of the sorbent, followed the equation of CO2 capture capacity (mmol/g) = (WCO2 − Wreg.) × 1000/ (Wpret. × 44). The procedures were repeated five times to obtain an accurate result.
to ii-d, respectively. Impregnation resulted in the agglomeration of initial fine SiO2 and PVA particles.6 The agglomerated PEI/ PVA, which was out of focus in the optical microscope, has a larger particle size (300−500 μm) than PEI/SiO2 (50−150 μm). It is worth noting that TEM images of the sorbents in Figure 2 panels i-b and ii-b were taken from the samples which were ultrasonicated in EtOH for 30 min. A large and connected structure of PEI/PVA in Figure 2ii-b compared to PEI/SiO2 in Figure 2i-b indicates that PEI/PVA particles which have a good mechanical strength can stay intact and withstand the ultrasonication treatment. PEI/PVA also showed exceptionally high attrition resistance in the fluidized bed, which has less than 0.1 wt % of loss under a continuous fluidization attrition test for 24 h. PEI/SiO2 particles show a low attrition resistance because organic binders have to be used for fabrication of the particle greater than 250 μm. Particles with the size less than 250 μm will lead to entrainment. Sorbent particles greater than 250 μm are suitable for fluidized-bed CO2 capture processes.42,43 Table 1 summarizes the results of the characterization of PVA, SiO2, and sorbents. The cross-linked PVA support possesses an insoluble network, high porosity, and an enhanced thermal stability.36,38 The cross-linked PVA has a higher decomposition temperature than the uncross-linked PVA particles, shown in Figure S2. The EtOH uptake was applied
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RESULTS AND DISCUSSION Characterization of Amine Sorbents. Impregnating SiO2 (10−20 nm) and PVA (50−70 nm), shown in Figure 2 panels i-a and ii-a, with PEI/PEG/EPON produced PEI/SiO2 and PEI/PVA which possess both mesopores (i.e., 2−50 nm) and macropores (i.e., > 50 nm) in Figure 2 panels i-b to i-d and ii-b C
DOI: 10.1021/acs.iecr.7b03636 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research Table 1. Properties of Supports and Amine Sorbents CO2 capture capacity (mmol/g)b sorbent
surface area of support (m2/g)
amine loading (mmol [N]/ g [sorbent])a
EtOH uptake (mL/g)
PEI/SiO2 PEI/PVA
160 355
5.84 5.84
0.98 1.91
Amine efficiency (mmol CO2/ mmol N)
40 °C 70 °C 40 °C 70 °C 2.01 1.31
1.89 1.55
0.34 0.22
0.32 0.26
wt loss after 24 h attrition (wt %) 250−500 μm N/Ac
