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Polyethylenimine Functionalized As-Synthesized KIT‑6 Adsorbent for Highly CO2/N2 Selective Separation Rupak Kishor* and Aloke Kumar Ghoshal Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati-781039, Assam, India S Supporting Information *

ABSTRACT: The present study targeted the synthesis of CO2 adsorbent by impregnation of DETA, TEPA, PEHA, and polyethylenimine in KIT-6 with a confiscated structure directing agent in minimum time and energy. The adsorbents were characterized and revealed the influence of polyethylenimine with CO2 at a wide range of temperatures. An examination by TGA confirmed that PluronicP123 present in adsorbent enhances its thermal stability and acts as a carrier for CO2 to the inner layer of polyethylenimine during adsorption. CO2 sorption capacity of the adsorbent was found to be dependent on the polyethylenimine loading, temperature, pressure, and the surfactant PluronicP123. At higher temperatures, adsorbents showed a positive impact for CO2 adsorption; however, a negative effect was exhibited in amine efficiency. The sorption capacity decreased with increasing the molecular weight of polyethylenimine following the order PEI-25K (66 mg CO2/g) < PEI-800 (114 mg CO2/g) < TEPA (124 mg CO2/g) < PEHA (139 mg CO2/g) at 75 °C. However, 60 wt % PEHA impregnated KIT-6 showed the stable sorption capacity of 170−184 mg CO2/g at 90−105 °C and 1 bar.

1. INTRODUCTION Energy insecurity and associated environmental concerns have raised serious concerns about identifying alternate, sustainable, clean energy sources with less environmental impact.1,2 Carbon dioxide, the major greenhouse gas generated from the burning of fossil fuels such as coal, petroleum oil, and natural gas, has reached a higher concentration of ∼400 ppm in the atmosphere.2 The earth’s surface temperature in 2015 was the warmest since 1980, according to independent analyses by the National Aeronautics and Space Administration (NASA) and National Oceanic and Atmospheric Administration (NOAA). Thus, to control the surface temperature, it is the need of the hour to control emissions from large anthropogenic sources. In the 21st session of the conference of the parties (COP-21) in 2015 including over 195 countries, a unanimous decision was taken to control CO2 emission in the atmosphere in scientific ways. Carbon capture and utilization (CCU) remains a promising option to control CO2 emission, which consists of two major stages.3−29 The primary stage involves CO2 capture using an efficient separation process, while the secondary stage involves the conversion of captured CO2 into various useful products with the help of effective catalysts.3−5 Removal of CO2 from flue gas using a commercially available amine-based absorption process has been receiving much interest among several authors.3,7 However, the process remains energetically unfavorable due to reduced net power output attributed to several power losses in the CO2 capture process during the amine regeneration and CO2 compression step.3,22,24 On the other hand, numerous adsorbents, especially activated carbon (AC),25 carbon nanotubes (CNTs),6 zeolite,10,11,26 metal organic frameworks (MOFs),10,16 and ordered mesoporous silica (MSs)9−16,19,20 and its amine functionalized derivatives have received great attention for CO2 adsorption under a wide range of temperatures and pressures. Mason et al.10 showed that © XXXX American Chemical Society

zeolites and most of the MOFs are highly unstable during CO2 adsorption under flue gas conditions. However, amine functionalized mesoporous silica showed a positive effect during CO2 adsorption under moist conditions as well as a high adsorption capacity at low partial pressure (∼0.15 bar).10,27−29 MS is functionalized by aminosilane grafting and the polyethylenimine wet impregnation method.12−16,28,29 Wet impregnation received special attention over grafting for its simplicity in functionalization with low time consumption as well as high adsorption capacity. Song et al.29 prepared the first polyethylenimine impregnated MS-based adsorbent. Later, several authors extensively studied the CO2 adsorption on amine functionalized adsorbent.10−19 Sayari and Belmabkhout27 showed the enhanced CO2 sorption capacity under moist conditions over dry ones as well as stable performance over large cycles. Son et al.19 studied CO2 adsorption over a wide variety of PEI-600 impregnated MCM-41, MCM-48, SBA-15, SBA-16, and KIT-6 silica. The 50 wt % PEI-600 impregnated 3D KIT-6 with large pores showed the highest adsorption capacity (135 mg CO2/g-adsorbent) in a minimum response time. In our previous report, the calcined KIT-6 modified with 50 wt % polyethylenimine (Mw: 800, 1200, and 25000) exhibits a CO2 adsorption capacity of 132 mg CO2/g, 107.8 mg CO2/g, and 105.6 mg CO2/g, respectively, at 105 °C and 1 bar.14 High energy requirements and additional environmental pollutant formation during calcination (by burning of the structure directing agent) of mesoporous silica open a new window to exploring the uncalcined one. Yue et al.16 modified the assynthesized SBA-15 with tetraethylenepentamine impregnation Received: August 17, 2016 Revised: October 19, 2016 Published: October 21, 2016 A

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Energy & Fuels and showed an adsorption capacity of 173 mg CO2/g at 75 °C and 1 bar with 65 wt % loading. However, Wang et al.18 incorporated the additional surfactant during polyethylenimine impregnation in calcined SBA-15. The incorporated surfactant facilitates CO2 diffusion in the deeper layers of polyethylenimine. Thus, a review of the literature suggests that further research is needed to understand the CO2 adsorption performance, thermal stability, type of amine, and recyclability during practical application of polyethylenimine impregnated assynthesized mesoporous silica over a calcined one. Therefore, the present study targeted the merits of uncalcined KIT-6, possibly for the first time in the literature, to get better CO2 adsorption performance. The uncalcined KIT-6 being used as an adsorbent in the synthesis process is less cumbersome as it bypasses the calcination step. The as-synthesized (ASK) is functionalized with a different polyethylenimine (Scheme 1)

2.2. Preparation of Adsorbent. The 3D KIT-6 was synthesized by following an earlier reported procedure.12 In brief, 4.0 g of P123 was dissolved in a mixture of 7.9 g of hydrochloric acid and 144 g of water. After it became homogeneous, 4.0 g of n-butanol was added in the solution and mixed for an hour. Further, 8.6 g of TEOS was added in the solution and stirred for 24 h. The temperature of the solution was maintained at 38−40 °C during the synthesis process. The molar ratio of the TEOS/P123/HCl/H2O/n-butanol solution mixture was 1:0.017:1.83:195:1.31. The resulting solution was transferred to a Teflon coated autoclave and hydrothermally treated at 100 °C for 24 h. Finally, the solution was filtered and washed with water. The assynthesized KIT-6 (ASK) was obtained after drying at 100 °C. Polyethylenimine was incorporated in ASK through the wet impregnation method13,15 as shown in Scheme 2. The amount of

Scheme 2. Schematic of Polyethylenimine Impregnation and CO2 Reaction Mechanism

Scheme 1. Molecular Structure of Different Polyethylenimine

polyethylenimine was calculated using the equation “x” (wt %) = (wt of polyethylenimine × 100)/(wt of polyethylenimine + wt of ASK). Initially, 1.0 g of TEPA was stirred for 30 min in 50 mL of ethanol. After it turned homogeneous, 1.0 g of dry ASK was added and refluxed at 80 °C for 6 h. Solid product was obtained after evaporation of ethanol and subsequent drying at 70 °C in a vacuum for 6 h. The resulting adsorbent was stored for further analysis after drying for an hour in a hot air oven at 100 °C. A similar procedure was followed for DETA and PEHA impregnation in ASK. PEI-800 and PEI-25K were separately dispersed in methanol, and after the addition of ASK, the solution was mixed for 6 h at room temperature. Other steps were similar to those discussed above. The resulting adsorbent was denoted as “x” TEPA, “x” PEHA, “x” PEI-800, and “x” PEI-25K, where “x” (in wt %) represents the loading of polyethylenemine in ASK. 2.3. Material Characterization. The textural properties of synthesized adsorbent were analyzed by N2 adsorption/desorption isotherm. Before analysis, the sample was degassed for 2 h at 100 °C, and N2 adsorption was studied in a volumetric adsorption apparatus (Quantachrome, AutosorbiQ) at −196 °C. The specific surface area (SBET) was calculated using the Brunauer−Emmett−Teller (BET) method in a 0.05−0.30 relative pressure (P/P0) range. Pore size distribution was calculated from the N2 desorption branch using the Barrett−Joyner−Halenda (BJH) method. Total pore volume (Vt) was analyzed form maximum N2 volume adsorption capacity at 0.99 relative pressure. Structural properties of the adsorbent were analyzed through powder X-ray diffraction spectra (Bruker D8 advance diffractometer) using Cu Kα (λ = 0.154250 nm) radiation between 0.5 and 5°. Thermal properties of the adsorbent were performed in a thermogravimetric (TG, NETZSCH TG 209F1 Libra) analyzer in the temperature range 30−800 °C with a 10 °C/min heating rate in an N2 atmosphere. The molecular composition of the adsorbent was identified by Fourier transform infrared (FT-IR, PerkinElmer) spectrometry in attenuated total reflection (ATR) mode in the wavenumber range 4000−500 cm−1. Apportionment of polyethylenimine on the KIT-6 was analyzed by transmission electron microscopy (TEM; JEOL, JEM2100) at 200 kV. Molecular apportionment in amine functionalized adsorbent was analyzed by surface mapping of

through the wet impregnation method and is characterized by different analytical techniques that include N2 adsorption/ desorption isotherms, X-ray diffraction, and thermogravimetric analysis. Further, the different polyethylenimine loaded adsorbents are subjected to CO2 adsorption over a wide range of temperatures in a volumetric adsorption apparatus. The effect of a structure directing agent present in the functionalized adsorbent was understood from the CO2 adsorption performance. The work shows improved thermal stability of the amine functionalized ASK and higher CO2 adsorption due to increased CO2 diffusion into the inner layers of polyethylenemine.

2. EXPERIMENTAL SECTION 2.1. Materials. The structure directing agent Pluronic P123 triblock copolymer (PEO20PPO70PEO20; Mw = 5800 Da, Aldrich) and silica source tetraethoxysilane (TEOS, Aldrich), hydrochloric acid (35%; Merck), n-butanol (Merck), methanol (Merck), and ethanol (Merck) were purchased. Diethylenetriamine (DETA, Mw = 103, Merck), tetraethylenepentamine (TEPA, Mw = 189, density: 0.998 g/ mL, Aldrich), and pentaethylenehexamine (PEHA, Mw 232, density: 1.003 g/mL, Merck) as a low molecular weight; polyethylenimine (PEI-800, Mw = 800, density: 1.05 g/mL, Aldrich); and polyethylenimine (PEI25K, Mw = 25 K, density: 1.03 mL/g, Aldrich) were purchased. All compounds were used without any further purification. All solutions were prepared with Millipore purified water. B

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Energy & Fuels the adsorbent using a field emission scanning electron microscopy (FESEM, Zeiss Model: Sigma) micrograph. Heat of the CO2 reaction with adsorbent was analyzed using differential scanning calorimetry (DSC, Mettler Toledo) at 75 °C. The sample (7−10 mg) was encapsulated in an aluminum pan and mounted in a DSC analyzer. The sample was isothermally heated at 100 °C for 1 h in the presence of N2 with a 40 mL/min flow rate and 5 °C/min heating rate. Further, the sample was cooled to a 75 °C reaction temperature at −5 °C/min and maintained for an hour. After the sample equilibrated at the reaction temperature, purge gas N2 was switched off and pure CO2 was passed at 100 mL/min. The CO2/N2 adsorption performance of the adsorbent was performed in an volumetric adsorption apparatus (Quantachrome, iSorbHP1-XKRLSPN100). In adsorption analysis, ∼0.5 g of the adsorbent was kept in the sample holder fitted with an adsorption apparatus. Before analysis, the sample was degassed at 100 °C for 2 h in an ultrahigh vacuum to remove the pre- adsorbed moisture and other gases. During analysis, the temperature was controlled by an external water circulator bath (30 °C) and furnace (45−120 °C). During CO2 cyclic performance, the adsorbent was degassed in ultrahigh vacuum at 100 °C for 0.5 h after each cycle.

Figure 2. Variation of surface area and pore volume of different polyethylenimine impregnated adsorbents (a) TEPA, (b) PEHA, (c) PEI-600, and (d) PEI-25K with concentration.

3. RESULT AND DISCUSSION 3.1. Characterization of Sorbents. N2 adsorption/ desorption isotherms (at −196 °C) and corresponding pore size of pure ASK and shown in Figure 1a,b. The ASK clearly

gets filled with increasing the polyethylenimine concentration. After ∼50 wt %, it completely fills the pore of ASK. Since the Vt of ASK is 0.86 cc/g as mentioned above, it can accommodate a maximum 46 wt % polyethylenimine inside the pores. Thus, in the case of polyethylenimine (50 to 60 wt %) concentration, the excess amount covers only the external surface of ASK. The maximum amount of polyethylenimine retained in ASK is found to be 60 wt %. In the case of 70 wt % TEPA and PEHA, adsorbents become a gel type with a light yellow color. With 50 wt % PEI-25K, most of the amine is retained outside the porous channels. It is possibly because of the large size of PEI-25K molecules compared to ASK pore size. Low angle powder X-ray diffraction spectra of TEPAimpregnated as-synthesized KIT-6 are shown in Figure 3.

Figure 1. N2 adsorption/desorption (−196 °C) of as-synthesized (■) and calcined KIT-6 (●).

exhibited a type IV isotherm (Figure 1a) as per IUPAC classification with an H1 hysteresis loop, indicating its uniform mesoporous structure.30 The SBET, Wd, and Vt of ASK are 394 m2/g, 6.6 nm, and 0.86 cc/g, respectively. After calcination at 550 °C for 5 h, mesoporous KIT-6 has the same structural properties and pore size (Figure 1b). The SBET, Wd, and Vt of KIT-6 are 857 m2/g, 6.6 nm, and 1.25 cc/g, respectively. Even after impregnation, the adsorbent shows a type IV isotherm, but N2 adsorption capacity gradually decreases with increasing polyethylenimine concentration (Figure S1). The SBET and Vt of all the adsorbents decrease gradually with an increase in polyethylenimine concentration in the ASK (Figure 2).13,31,32 It indicates that the Vt of ASK gradually gets filled with an increase in polyethylenimine concentration. The pore size of adsorbents does not change significantly; however, the corresponding volume adsorbed is gradually decreased (Figure S1). After ∼50 wt % polyethylenimine impregnation, pore size distribution becomes flat. It indicates that the pore gradually

Figure 3. X-ray diffraction spectra of TEPA-impregnated assynthesized KIT-6.

The sharp diffraction peak corresponding to the d211 plane at 2θ = 0.85° indicates that ASK is highly ordered and cubical in nature and is in agreement with previous reports.33,34 After impregnation, peak intensity gradually decreases with an increase in TEPA concentration in the adsorbent, indicating a gradual decrease of X-ray scattering toward the silica wall.13,16,35 The peak corresponding to the d211 plane has completely disappeared above 50 wt % impregnation of TEPA. This is because of the complete surface coverage of ASK with TEPA. C

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Energy & Fuels Figure 4 shows the electron micrograph of ASK before and after polyethylenimine impregnation. FESEM micrograph

S2d). This confirms that TEPA, PEHA PEI-800, and PEI-25K were inserted in the mesopores of ASK during impregnation. ATR spectra of TEPA impregnated ASK adsorbents are shown in Figure 5. The formation of KIT-6 is confirmed by

Figure 4. FESEM spectra of polyethylenimine impregnated (a) ASK, (c) 60 TEPA, (d) 60 PEHA, (e) 50 PEI-800, and (f) 40 PEI-25K adsorbents and (b) a TEM micrograph of ASK.

Figure 5. Attenuated total reflection (ATR) spectra of polyethylenimine impregnated ASK.

major characteristic bands of silica around 1050 cm−1, 960 cm−1, and 795 cm−1 corresponding to a symmetric stretching vibration (Si−O−Si), free surface silanol group (Si−OH), and asymmetric stretching vibration of Si−O−Si of the mesoporous KIT-6.12,36 The symmetric and asymmetric vibrations of primary amine (−NH2) present in the TEPA impregnated adsorbent are confirmed by peaks at 3420−3190 and 1580 cm−1, respectively, in the spectra.23,37 However, the presence of secondary amine (−NH−) is confirmed by the peak at 1660 cm−1.23,37 The sharp bands in the range of 2936−2820 cm−1 and 1460 cm−1 represent the stretching vibration of CH2 and bending vibration of CH present in the TEPA skeletal.11 The peak around 1350 cm−1 is for carbamate formation by chemical reaction between CO2 and the amine group and that at 1315 cm−1 for the stretching vibration of −NC present in TEPA.23,37 With the percentage increase in TEPA content, the peak area gradually increases. Similar ATR spectra are observed for PEHA, PEI-800, and PEI-25K impregnated ASK, as shown in Figure 5. This is mainly for similar elemental compounds used in the preparation of amine functionalized ASK. The thermal stability of ASK and polyethylenimine impregnated 60 TEPA, 60 PEHA, 50 PEI-800, and 40 PEI25K adsorbents are shown in Figure 6. The DTA curve of the adsorbent suggests that the total weight loss occurs in two different stages. The initial weight loss up to ∼130 °C is mainly for physically and chemically preadsorbed CO2, moisture, and other gases.32 The major weight loss in ASK between 150−500 °C is due to thermal degradation of Pluronic P123. It is

clearly shows that ASK is a colloidal particle with a ∼1−2 μm size (Figure 4a).19 The TEM micrograph shows that the ASK contains highly ordered and interconnected pore channels (Figure 4b), corroborating the results from the X-ray diffraction spectra.34 Three major peaks are observed in EDX spectra of ASK, confirming the presence of elemental components Si, C, and O of ASK (Figure S2a).The mapping shows no uniform elemental distribution over the surface (Figure S2c). The sharp peak of silica (Si) in the spectra confirms its presence as a major component of ASK. The trace amount of aluminum (Al) present in the EDX spectra is mainly for the base material (Al foil) used in analysis. However, there was no structural change observed after different polyethylenimine impregnations in ASK, as confirmed by the FESEM micrograph of 60 TEPA, 60 PEHA, 50 PEI-800, and 40 PEI-25K adsorbents (Figure 4), although most of the ASK particles are agglomerated with each other after polyethylenimine impregnation. With an increase in the molecular weight of polyethylenimine, the agglomeration of ASK solid particles is increased along with an increase in the surface concentration of polyethylenimine.36 In the case of highly branched PEI (Mw = 25 000), part of the PEI molecule gets inserted in the pore and part remains outside the pore. It is due to an increase in agglomeration of ASK solid particles with 40 PEI-25K adsorbent. The additional peak of nitrogen (N) in EDX spectra confirms the presence of TEPA in the adsorbent (Figure S2 b). However, the TEM micrograph of 60 TEPA clearly shows that pores of ASK are filled with TEPA (Figure D

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nimine have low CO2 sorption capacity; however the polyethylenimine and mesoporous silica composite improved the sorption capacity.16,19 For preliminary screening, a series of polyethylenimine (Scheme 1) impregnated ASKs with different loadings in the range from 20 to 70 wt % were synthesized and subjected to CO2 adsorption performance at 75 °C. In the case of diethylenetriamine (HN(CH2CH2NH2)2) impregnated ASK based adsorbent, most of the amine was leached out from the adsorbent during drying and degassing of the adsorbent. It was confirmed by the initial and final weight difference of the adsorbent. The above observation confirms that diethylenetriamine impregnated ASK based adsorbent has very low thermal stability39 and is found to be not suitable for CO2 separation during real practical application. In the case of TEPA, 60 wt % maximum loading was obtained over ASK, but with a further increase in concentration, the adsorbent turned into a yellow gel. Similar behavior was also observed with PEHA. In the case of 50 PEI-25K, most of the amine was retained outside the ASK, as shown in Figure 7. It was mainly for the highly branched and ultra-large molecular size of PEI (Mw = 25K).

Figure 6. (a) Thermogravimetric analysis (TGA) and (b) differential thermal analysis (DTA) of pure and polyethylenimine impregnated assynthesized KIT-6 in a N2 atmosphere with a 10 °C/min heating rate.

confirmed from the DTA analysis that Pluronic P123 is completely degraded around 400 °C (Figure 6b). Further weight loss in ASK at higher temperatures is for surface dehydroxylation.31 The total amount of P123 present in ASK is ca. 30%. In Figure 6, for 60 TEPA and 60 PEHA, the weight loss occurred in 150−350 °C, but for 50 PEI-800 and 40 PEI25K, the weight loss occurred in 150−400 °C. And for 60 TEPA and 60 PEHA, two weight losses occurred in 250−300 °C, but for 50 PEI-800 and 40 PEI-25K, the weight loss peak occurred approximately at 350 °C. The major weight loss between 150−350 °C in adsorbent is attributable to the volatilization and decomposition of TEPA and PEHA molecules.11,13,29 However, degradation temperature is nearly 350 °C with polyethylenimine impregnated ASK. The thermal stability of adsorbents follows the order TEPA < PEHA < PEI800 < PEI-25K; i.e., stability increases with the increasing molecular weight of polyethylenimine.38,39 As observed earlier, the thermal stability of polyethylenimine impregnated calcined MCM-41 (∼200 °C),29 SBA-15 (∼200 °C),13 and KIT-6 (∼220 °C)19 is significantly lower than that of the ASK (350 °C) based adsorbent. Thus, the presence of Pluronic P123 significantly improves the thermal stability of the ASK based adsorbent by forming the Pluronic P123 and polyethylenimine composite. 3.2. CO2 Adsorption Analysis. The synergistic effects of as-synthesized mesoporous silica with wide varieties of polyethylenimine present in the adsorbent on CO2 sorption capacity have spurred the recent interest in exploring this hybrid material for designing the high CO2/N2 selective adsorbent. The pure mesoporous silica as well as polyethyle-

Figure 7. (a) CO2 adsorption capacity and (b) different polyethylenimine efficiency of adsorbents.

The CO2 sorption performances of all different quantities of PEI impregnated ASK are summarized in Figure 7. The CO2 sorption capacity gradually increases with an increase in amine loading. The maximum CO2 sorption capacity is about 136, 139, 114, and 64 mg of CO2/g of 60 TEPA, 60 PEHA, 50 PEI800, and 40 PEI-25K adsorbent, respectively, at 75 °C and 1 bar. In PEI-800, 50 wt % impregnated adsorbent shows maximum adsorption capacity (114 mg CO2/g adsorbent); however, 40 PEI-25K shows the maximum sorption of 64 mg of CO2/g adsorbent. Further, an increase in polyethylenimine concentration results in the sharp reduction of CO2 adsorption E

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Figure 8. CO2 adsorption performance of (a) 60 TEPA, (b) 60 PEHA, (c) 50 PEI-800, and (d) 40 PEI-25K adsorbent at different temperatures.

highly branched polyethylenimine (PEI-800 and PEI-25K) is a combination of primary, secondary, and tertiary amines. The ratios of primary, secondary, and tertiary amine are changed from 42:33:25 (PEI-800) to 31:39:30 (PEI 25K) when the molecular weight is increased from 800 to 25 000 Da.43 Primary and secondary amines can react with dry CO2 to form carbamates through the formation of zwitterionic intermediates.44,45 The zwitterionic mechanism was elaborated by Caplow45 and other several authors.27,36,37,44 However, tertiary amine reacts only in aqueous conditions and reflects very low adsorption capacity.44,45 Ko et al.44 have shown that the CO2 sorption capacity and rate of adsorption follow the order tertiary amine < secondary amine < primary amine. Thus, the reduction in primary and secondary amine in polyethylenimine with an increase in molecular weight consequently reduces the CO2 sorption capacity of the adsorbent. 3.3. Effect of Adsorption Temperature and CO2 Partial Pressure. In order to understand the effect of temperature on sorption performance, the adsorption isotherm experiments were performed between 30 and 105 °C with a 15 °C interval for pure CO2, and the results are depicted in Figure 8. The CO2 adsorption capacity increased with increasing adsorption temperature from 30 to 105 °C in all of the adsorbent. Similar adsorption isotherm trends were also observed for different polyethylenimine impregnated adsorbents such as calcined mesoporous silica (MCM-41, SBA-15, and MCF),10,13,27,35,40 activated carbon,25 and metal organic frameworks.10,17 The CO2 sorption capacity sharply increased at low partial pressure (less than 0.1 bar) in all of the polyethylenimine functionalized ASK and then approached a saturation with a further increase in pressure. The sharp increase in adsorption capacity at low partial pressure is mainly for the chemisorption between CO2 and amine sites present in the adsorb-

capacity, which is attributed to the reduction of accessible amine sites during adsorption.19 In addition to that, a lower amount of PEI-25K loading compared to TEPA, PEHA, and PEI-800 inside the pores of ASK also reduces the CO2 sorption capacity of the adsorbent. The polyethylenimine efficiency is defined as the ratio of the amount of CO2 adsorbed to the polyethylenimine present in the adsorbent after impregnation.41 It indirectly indicates the CO2 adsorption behavior toward amine moieties by chemisorption with increasing concentration of polyethylenimine. The 20 TEPA shows the highest efficiency (ca. 281 mg CO2/gTEPA) which decreases with an increase in its concentration in the adsorbent, as shown in Figure 7b. Similar trends are also observed for PEHA, PEI-800, and PEI-25K impregnated adsorbents. However, an increase in loading and molecular weight reduces the polyethylenimine efficiency.21,41,42 At low amine loading, the porosity of the adsorbent is higher compared to higher amine loading (Figure 2), which provides better dispersion of the polyethylenimine inside the porous channels and reduces the CO2 transfer resistance toward amines sites.39,42 Additionally, CO2 diffusional resistance increases with increasing the molecular weight of polyethylenimine and reduces the CO2 interaction with amine molecules present in the lower layer of polyethylenimine.32,39,42 It is interesting to observe that the adsorption capacity increases with increasing amine group numbers in TEPA to PEHA. However, an increase in high molecular weight of the polyethylenimine in the adsorbent results in a drastic reduction of sorption capacity (Figure 7). The reduction in adsorption capacity with increasing molecular weight of the polyethylenimine can be understood on the basis of its molecular structure and CO2 reaction mechanism. TEPA and PEHA consist of primary and secondary amine groups (Scheme 1). However, F

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Energy & Fuels Table 1. Comparison of Polyethylenimine Impregnated As-Synthesized KIT-6 with Other Hybrid Adsorbent adsorbent

polyethylenimine

loading (wt %)

T (°C)

PCO2 (bar)

methoda

qe (mg CO2/g)

Y-zeolites MCM-41 SBA-15 disorder MS KIT-6 MCM-41 SBA-15 MCM-48 PE-SBA-15 ASK SBA-15 ASK MCM-41 KIT-6 MCM-48 nanoporous carbon CNT SBA-15 SBA-16 PE-SBA-15 PTNTs nanosilica SBA-15 KIT-6 ASK ASK

TEPA TEPA TEPA TEPA TEPA TEPA (grafting) TEPA (grafting) TEPA (grafting) TEPA TEPA PEHA PEHA PEI, Mw = 600 PEI, Mw = 600 PEI, Mw = 600 PEI, Mw = 600 PEI, Mn = 600 PEI, Mw = 423 PEI, Mw = 600 PEI, Mw = 800 PEI, Mw = 600 PEI, Mw = 600 PEI, Mw = 800 PEI, Mw = 25000 PEI, Mw = 800 PEI, Mw = 25000

50% 60% 50% 50% 50%

60 70 100 100 60 25 25 25 45 105 80 105 75 75 75 75 70 75 75 45 100 75 75 105 75 105

0.5 0.15 1 1 0.1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

GA FBR GC MB GC TGA TGA TGA TGA Vol. GC Vol. TGA TGA TGA TGA GC GC TGA TGA TGA TGA Vol. Vol. Vol. Vol.

146.8 107.8 151 193.8 127.6 43 30 30 94.9 157 154.2 184.3 111 129 119 48 111.3 154 129 80.5 130.8 138 89.8 105.6 146 74

30% 60% 50% 60% 50% 50% 50% 50% 60% 50% 30% 50% 50% 50% 50% 50% 40%

ref. 11 46 41 47 15 48 48 48 28 this 49 this 19 19 19 50 51 13 19 28 52 32 35 14 this this

work work

work work

a GA, gas analyzer; MB, microbalance, GC, gas chromatography; TGA, thermogravimetric analyzer; Vol., volumetric adsorption; FBR, fluidized bed reactor.

enabling a higher temperature (90−105 °C) beneficial for CO2 adsorption. However, further increasing the temperature to 120 °C leads to a sharp decrease of adsorption capacity in low partial pressure (less than 0.2 bar) because CO2 desorption is more favorable than adsorption (Figure S3). The N2 adsorption isotherm of 60 TEPA, 60 PEHA, 50 PEI800, and 40 PEI-25K adsorbent at 90 °C is shown in Figure S4. The ultralow N2 sorption capacity at 1 bar of 60 TEPA (∼0.2 mg N2/g), 60 PEHA (∼0.1 mg N2/g), 50 PEI-800 (∼0.0 mg N2/g), and 40 PEI-25K (0.0 mg N2/g) compared to CO2 indicates that synthesized adsorbents have very high selectivity toward CO2 in the flue gas of a large anthropogenic source, where N2 is the primary component of the mixture. In order to understand the effect of Pluronic P123 in polyethylenimine impregnated ASK, we performed CO 2 adsorption on 50 wt % PEI-800 impregnated calcined KIT-6 at different temperatures, viz., 75, 90, and 105 °C. The adsorption capacity of PEI-800 impregnated KIT-6 is about 108, 118, and 132 mg of CO2/g-adsorbent respectively at 75, 90, and 105 °C and 1 bar as shown in Figure S5. Goeppert et al.39 have shown that CO2 adsorption capacity increases with the temperature (up to 100 °C) of different polyethylenimine impregnated nanostructured silica. It is interesting to note that the sorption capacity of 50 PEI-800 is 146 mg of CO2/gadsorbent, and it is higher than 50 wt % PEI-800 impregnated calcined KIT-6 as well as pure PEI-800.41 It indicates that P123 has an important role in CO2 adsorption. The nonionic P123 contains both the hydrophilic poly(ethylene glycol)chain (PEO) and hydrophobic poly(propylene glycol) chain (PPO). During KIT-6 synthesis, PPO is present in the core, while PEO is in the shell of the micelle.53 After synthesis, PEO

ent.11,12,23,27,36 A further increase in adsorption capacity at higher partial pressure can be for the diffusion of CO2 inside the amine layer present in the adsorbent. The maximum adsorption capacity of 60 TEPA, 60 PEHA, 50 PEI-800, and 40 PEI-25K is 138−157, 170−185, 139−147, and 71−74 mg of CO2/g-adsorbent, respectively, between 90 and 105 °C and at 1 bar. The CO2 adsorbed amounts on some PEI modified adsorbents as reported in the literature are summarized in Table 1. The results suggest that the adsorption capacities of polyethylenimine impregnated ASK show better adsorption performance than earlier reported adsorbents except disordered mesoporous silica. Similar adsorption isotherm trends were also observed in the literature with different polyethylenimine impregnated MS,10,40,41 AC,26 and MOFs.10,17 The low temperature range is thermodynamically more favorable for the CO2 adsorption and the high temperature, for desorption.23,26 Additionally, adsorption capacity is also proportional to the exposed amine sites for CO2 in polyethylenimine functionalized adsorbent.31,33,42 The TEM micrograph and N 2 adsorption isotherm clearly show polyethylenimine filled porous channels of ASK during impregnation. At low temperatures, CO2 reacts with exposed amine sites present on the adsorbent surface.13,32,42 Increasing trends of CO2 sorption capacity indicate the increase in number of exposed amine sites with an increase in adsorption temperature.13,14,32 An increase in adsorption temperature reduces the viscosity and thereby increases the intermolecular distance between polyethylenimine molecules that exposes maximum amine sites for CO2 adsorption in the inner layer of polyethylenimine.39,41,42 Additionally, it also reduces the CO2 diffusional resistance in the inner layer of polyethylenimine, G

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mol CO2, respectively, which is much lower than that in 30% MEA solution (198.24 kJ/mol CO2).56 3.5. Cyclic Adsorption Performance. Stable sorption capacity during cyclic adsorption/desorption performance over a wide range of temperature and pressure is the desirable requirement for a good industrial adsorbent. An extensive cyclic adsorption/desorption (Ads/Des) performance of the adsorbents was performed at 90 °C/100 °C and 105 °C/100 °C. Figure 10 shows the cyclic performance of the 60 TEPA, 60

is found in the porous channel of KIT-6. During impregnation, the PEO chain present on the surface gets inserted in the polyethylenimine layer as shown in Scheme 2 and enhances the dispersion of polyethylenemine. During adsorption, PEO acts as a CO2 carrier facilitating transport into the inner layer of polyethylenimine 18 (Scheme 2b) and reduces the CO 2 diffusional resistance during adsorption (Figure S6). It also increases the amine utilization ratio from 132 to 145 mg of CO2/g PEI-800. 3.4. The Enthalpy of CO2 Adsorption. The isosteric heat of adsorption (ΔHq) at a fixed adsorption capacity was calculated from the Clausius−Clapeyron equation: ln(p) = [(ΔHq)/RT] + C; where C, R, p, and T represent the constant, gas constant, applied pressure at constant sorption capacity, and temperature, respectively.31 The ΔHq value was evaluated from the slope of the linear plot between ln(p) vs 1/T at the same adsorbed quantity. The plots of different polyethylenimine impregnated adsorbents are depicted in Figure 9. The ΔHq was

Figure 10. CO2 adsorption performances at (a) 90 °C adsorption/100 °C desorption and (b) 105 °C adsorption/100 °C desorption of different adsorbents. Figure 9. Variation of ln(p) with 1/T of different adsorbent (■, 60 TEPA; ●, 60 PEHA; ▲, 50 PEI-800; ◊, 40 PEI-25K) by Clausius− Clapeyron equation.

PEHA, 50 PEI-800, and 40 PEI-25K adsorbent with pure CO2. The cyclic performance reveals that the synthesized adsorbents have a very stable sorption capacity between 90 and 105 °C adsorption temperatures. Even after 10 cycles, 60 TEPA (130− 144 mg CO2/g), 60 PEHA (165−180 mg CO2/g), 50 PEI-800 (133−142 mg CO2/g), and 40 PEI-25K (70−69 mg CO2/g) show less than 2−3% reduction in sorption capacity between 90 and 105 °C sorption temperatures. However, lower molecular weight polyethylenimine such as TEPA and PEHA impregnated calcined mesoporous silica are generally reported to have a significant loss in adsorption capacity during cyclic performance.57,58 These results suggest that the above synthesized adsorbent (60 PEHA) can become the best potential adsorbent for practical application.

found to be 89, 70, 110, and 83 kJ/mol for 60 TEPA, 60 PEHA, 50 PEI-800, and 40 PEI-25K adsorbent, respectively. The higher value of isosteric heats of adsorption indicates strong interaction (chemisorption) between CO2 and amine sites present in the adsorbent.12,40,41 The heat of adsorption of pure adsorbent such as mesoporous silica, zeolite, activated carbon, and metal organic frameworks lies between 18 and 50 kJ/ mol.25,26,33 The addition of polyethylenimine in the adsorbent increased the heat of adsorption to greater than 50 kJ/mol and reaches up to 140 kJ/mol).9,23,31,40 Gray et al.54 suggested that the heat required for regenerating the adsorbed CO2 could be predicted by the following equation: Q ≈ ΔHr + mCPΔT, where Q is the regeneration heat duity, ΔHr is hear of reaction, m is the adsorbent quantity, CP is heat capacity, and ΔT is the change in temperature. The sensible heat (mCPΔT) is much lower than ΔHr for amine functionalized mesoporous silica.55 The ΔHr is analyzed from the DSC profile (area corresponding to exothermic peak) in the presence of pure CO2 at 75 °C (Figure S7). The CO2 q of 60 TEPA, 60 PEHA, and 50 PEI-800 is 124 mg/g (2.82 mmol/g), 139 mg/g (3.15 mmol/g), and 114 mg/g (2.59 mmol/g) at 75 °C and 1 bar, respectively. The ΔHr of 60 TEPA, 60 PEHA, and 50 PEI-800 are 81 kJ/mol CO2, 61 kJ/mol CO2, and 68 kJ/

4. CONCLUSIONS In the present study, three-dimensional ASK was functionalized with TEPA, PEHA, PEI-800, and PEI-25K at different loadings using the wet impregnation method and subjected to CO2 adsorption. Increasing polyethylenimine concentration decreases the surface area and pore volume of the adsorbent by filing the porous channel of ASK. Pluronic P123 present in KIT-6 enhances the thermal stability of the polyethylenimine impregnated adsorbents. The sorption capacity of all the synthesized adsorbent (60 TEPA, 60 PEHA, 50 PEI-800, and 40 PEI-25K) is dependent on the temperature and increases with increasing temperature from 30 to 105 °C. Shorter chain H

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amine TEPA and PEHA with low molecular weight showed higher adsorption capacity than high molecular weight PEI-800 and PEI-25K. However, 60 PEHA shows a maximum sorption capacity of ca. 170−184 mg CO2/g-adsorbent at 90−105 °C and 1 bar with stable cyclic performance. The PEI-800 impregnated ASK showed ca. 10% higher CO2 sorption capacity than the calcined one. The PEO chain of Pluronic P123 present on the ASK surface facilitates the CO2 diffusion toward the inner layer of polyethylenimine during adsorption, enabling more CO2 molecules to come in contact with inner ammine sites. Polyethylenimine impregnated adsorbents developed here can be synthesized energy efficiently and more easily as compared to calcined silica. The adsorbents are also highly regenerable. Thus, high sorption capacity at low partial pressure and high temperature make ASK (particularly, 60 PEHA) a promising candidate for CO2 capture from large anthropogenic sources such as coal-based thermal power plants.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b02082. N2 adsorption/desorption at −196 °C, XRD spectra, CO2 adsorption performance of different adsorbents (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +91−361−258261. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the central instruments facility (CIF), Indian Institute of Technology Guwahati for FESEM, TEM, N 2 adsorption/desorption, and CO2 /N 2 adsorption analysis.

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REFERENCES

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