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Amine-Modified Mesoporous Silica for CO2 Adsorption: The Role of Structural Parameters Rupak Kishor* and Aloke Kumar Ghoshal Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati, 781039 Assam, India

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ABSTRACT: The present study investigated the role of structural parameters such as pore size, pore volume, and specific surface area of mesoporous silica on the CO2 sorption performance of various amine-functionalized adsorbents. A series of mesoporous silica with different structural properties KIT-6, MCM-41, SBA-15, and HV MCM-41 were synthesized and functionalized with pentaethylenehexamine (PEHA) by wet impregnation. The CO2 sorption performances of the sorbents were evaluated using high pressure gas adsorption analyzer. The sorption capacity of adsorbents follows the order: MCM-41 < HV MCM-41 < SBA-15 ≈ KIT-6 at 105 °C and 1 bar. Larger pore size reduces the mass transfer resistance, and large pore volume improves the PEHA distribution inside the pores. The high specific surface area has little impact during adsorption. Due to the 3D structure with interconnected pores, KIT-6 shows the lowest heat of regeneration (57.8 kJ/mol CO2) during adsorption. The PEHA-impregnated KIT-6 (K-60 PEHA) shows the highest sorption capacity 4.48 mol CO2/g at 105 °C and 1 bar. It also exhibit fairly stable sorption performance up to 10 adsorption/desorption cycles between in the temperature range of 90−105 °C.

1. INTRODUCTION At present, an aqueous alkanolamine (such as monoethanolamine and diethanolamine) based absorption process is widely used to capture CO2 at atmospheric condition.1 However, the large energy requirement during solvent regeneration, corrosive nature of amines toward the equipment, and most importantly the amine loss in atmosphere during regeneration necessitates looking for an alternative process.1,2 Amine-functionalized porous solid such as activated carbon3,4 zeolites,5 mesoporous silica,6−9 and metal−organic frameworks (MOFs)3,10 are found to be good candidates for CO2 capture at low pressure. However, structural instability and sharp reduction in adsorption capacity under most conditions with zeolites and MOFs require further research and improvement for said application.8 In the past few years, amine-functionalized mesoporous silica received attentions from a large community of researchers for CO2 capture because of its high structural stability, high surface area, and tunable pore size and pore volume.11,12 Xu et al.13 developed a polyethylenimine (PEI)-impregnated “molecular basket”, which showed high CO2 sorption capacity (3.0 mol CO2/kg) and selectivity. Subsequently, several amine-functionalized mesoporous silica such as MCM-41,14−16 SBA-15,17 KIT6,7,16 MCF,18,19 and aerogel20 were tested in order to have an adsorbent with improved sorption performance. It was obviously observed that the sorption capacity depends on the mesoporous structure, pore size, pore volume, and type of amine. © 2017 American Chemical Society

At present, limited work has been done to understand the role of structural parameter of support on the CO2 adsorption performance. Yan et al.19 increased the sorption capacity from 2.51 to 4.04 mol CO2/kg by increasing the pore volume from 1.51 to 1.82 cc/g of PEI-impregnated MCF at 75 °C and 0.15 bar, while Chen at al.12 achieved a sorption capacity of 4.18 mol CO2/kg at 75 °C with 60 wt % PEI loading by using threedimensional hexagonal mesoporous silica with wormhole-like pore structure as support. Moreover, Wang et al.21 improved the CO2 sorption capacity of tetraethylenepentamine (TEPA)impregnated ordered mesoporous silica from 2.31 to 3.1 mol/ kg by changing the pore size from 5.6 to 7.6 nm. It should be noted that the surface area, intrinsic pore structure, and pore volume play an important role during application. In our previous study, we found that the adsorption capacity of KIT-6 was decreased with increase in molecular weight of PEI.7 The maximum sorption capacity observed was 3.0 mol CO2/kg (Mw of PEI = 800) at 105 °C and 1 bar, although low molecular weight TEPA- and PEHA-impregnated mesoporous silica showed much higher sorption than high molecular weight PEI.14,22 Additionally, 3D support improved the sorption kinetics of amine-functionalized sorbent.9,16 Thus, the above literatures clearly indicate that the structural parameters of Received: Revised: Accepted: Published: 6078

March 1, 2017 April 20, 2017 May 3, 2017 May 3, 2017 DOI: 10.1021/acs.iecr.7b00890 Ind. Eng. Chem. Res. 2017, 56, 6078−6087

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Industrial & Engineering Chemistry Research

was added in the solution and refluxed for 6 h at 70 °C to improve the diffusion of PEHA inside the pore. The slurry was dried at 70 °C to complete volatilization of methanol. Then, PEHA-impregnated sample was dried at 100 °C for an hour. The resulting sample was denoted as K-xPEHA, where x represents the amount of PEHA loaded in the adsorbent. The value of x was calculated as followd: x = (wt of PEHA)/(wt of PEHA + 1 g of KIT-6). A similar synthesis procedure was also used to synthesize the adsorbents M-xPEHA, S-xPEHA, and HV M-xPEHA using MCM-41, SBA-15, and HV MCM-41 mesoporous silica, respectively. 2.4. Characterization of Adsorbent. X-ray powder diffraction (XRD) analysis was performed in a Bruker D8 advance diffractometer using Cu Kα radiation operating at 40 kV and 40 mA. The XRD spectra were obtained in the range of 0.5° ≤ 2θ ≤ 10°. Morphology of mesoporousous silica was analyzed by field-emission scanning electron microscopy (FESEM, Zeiss-Sigma, 4 kV) and transmission electron microscopy (TEM, JEOL-JEM 2100, 200 kV). Thermal stabilities of adsorbent were analyzed by thermogravimetric analysis (TGA, NETZSCH TG 209F1 Libra) in the temperature range of 30−800 °C with a heating rate of 10 °C/min under N2 atmosphere. Structural parameters of mesoporous silica were analyzed by N2 adsorption/desorption isotherm. Before analysis, sample was degassed at 120 °C to remove the moisture and preadsorbed gases. N2 isotherms were collected using volumetric gas adsorption instrument (Quantachrome, Autosorb iQ) at −196 °C. The specific surface area (SBET) was analyzed by Brunauer−Emmett−Teller method in relative pressure (P/P0) range of 0.05−0.30. Pore size (dBJH) was analyzed by the Barrett−Joyner−Halenda (BJH) method with desorption branch. Total pore volume (Vt) of mesoporous silica was analyzed from the maximum N2 volume (at P/P0 ≈ 0.99) adsorbed. Heat evolved during CO2 adsorption was analyzed by differential scanning calorimeter (DSC, Mettler Toledo). Before starting the analysis, a small pinhole was made in the pan cap. Sample (∼0.7 mg) was encapsulated in an aluminum pan and mounted in DSC analyzer. In the first stage, the sample was heated to 100 °C with 5 °C/min heating rate in N2 atmosphere with 40 mL/min flow rate and maintained for 90 min. During this stage, adsorbent was made free from preadsorbed moisture and CO2. Furthermore, the sample was cooled to 90 °C with −5 °C/min cooling rate and maintained for 30 min to achieve a uniform temperature of the sample during the second stage. Then, N2 was interchanged with CO2 with 100 mL/min flow rate and maintained for 30 min. 2.5. CO2/N2 Adsorption Performance. CO2 adsorption performance of the adsorbent was performed using highpressure volumetric gas adsorption (Quantachrome iSorbHP1XKRLSPN100) analyzer in the temperature range of 30−120 °C. In a typical measurement process, ∼500 mg of sorbent was placed in a sample holder and fitted with analysis port. Before analysis, the sorbent was degassed at 100 °C for an hour to remove the pre-adsorbed gas and then filled with He gas. During analysis, temperature of the sample was maintained by circulator bath (30−45 °C) and external heating mantle (60− 120 °C). The CO2 uptake was calculated on the basis of the change in the volume of gas. The stability of adsorbent was analyzed on the basis of change in adsorption capacity over period of time. During cyclic performance, sorbent was degassed after each cycle at 100 °C for 30 min under vacuum.

mesoporous silica directly influence the performance of adsorbent but there is no any such consolidate report are available. In this work, we have synthesized four different types of mesoporous silica, namely, MCM-41, SBA-15, KIT-6, and HV MCM-41 (high pore volume MCM-41), having different structural properties such as specific surface area, pore volume, pore size, and morphology. Furthermore, they were functionalized with PEHA and subjected to CO2 sorption studies. The oligomeric amine PEHA was chosen for the functionalization of mesoporous silica because of its high amine group content per unit mass and low viscosity that allows the CO2 molecule to diffuse easily in the inner layer of PEHA and in turn improves the sorption performance. We further studied the role of parameters such as pore size, pore volume, surface area, and structure on the CO2 adsorption performance and its regeneration.

2. EXPERIMENTAL SECTION 2.1. Materials. Amphiphilic triblock copolymer pluronic P123 (EO20−PO70−EO20, Sigma), cetrimide (CTAB, Merck), tetraethyl orthosilicate (TEOS, Sigma), and pentaethylenehexamine (PEHA, Merck) were purchased. Hydrochloric acid (HCl 25%), ammonia solution (NH3, 25%), 1-butanol (BuOH), and ethanol (EtOH) were purchased from Merck. In all the experiments, Millipore purified water were used. 2.2. Synthesis of Mesoporous Silica. KIT-6 was synthesized by a previously reported procedure.11 In a typical synthesis process, pluronic P123 (4.0 g) was dissolved in mixture of HCl (6.7 mL) and water (144.0 mL) using a magnetic stirrer at 40 °C. After the solution became homogeneous, BuOH (4.0 g) was added and mixed for an hour. The silica source, TEOS (8.6 g), was added in the solution and stirred for 24 h. The resulting solution was transferred in a Teflon-lined autoclave and aged for 24 h at 100 °C. The white solid product was filtered, washed with water, and dried at 100 °C for 24 h in hot air oven. Surfactant-free KIT-6 was obtained after calcination at 550 °C for 5 h. SBA-15 was synthesized with the following procedure.17 In a typical synthesis, pluronic P123 (4 g) was dissolved in a solution of 144 mL of 1.7 N HCl at 40 °C. After complete dissolution, 9.2 mL of TEOS was added in the solution and mixed for 24 h. Then, the solution was transferred in a Teflonlined autoclave and hydrothermally treated at 100 °C for 24 h. White solid product obtained after filtration was washed with water and dried at 100 °C. Surfactant-free SBA-15 was obtained after calcination at 550 °C for 5 h. MCM-41 and HV MCM-41 were synthesized from the earlier reported procedure.15 In typical synthesis process, CTAB (2 g) was dissolved in water (120 mL). When CTAB was completely dissolved, NH3 (9 mL) was added in the solution and mixed for an hour. Furthermore, TEOS (10 mL) was added in the solution and mixed for 12 h at room temperature. The resulting product was filtered, washed with water, and dried at 100 °C for 24 h. Surfactant-free MCM-41 was obtained after calcination at 550 °C for 5 h. HV MCM-41 was synthesized by changing the concentration of NH3 from 9 to 1 mL in the synthesis solution. Other synthesis steps were similar to those explained for MCM-41. 2.3. Synthesis of PEHA Impregnated Adsorbent. KIT-6 was functionalized by wet impregnation method.21 In a typical synthesis process, a specific amount of PEHA was dissolved in methanol (50 mL). After complete dissolution, 1 g of KIT-6 6079

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Figure 1. XRD spectra of (a) KIT-6, (b) SBA-15, (c) MCM-41, and HV MCM-41 mesoporous silica.

consist of highly ordered uniform mesoporous channels.25,26 The channel breadths of KIT-6 and SBA-15 are greater than that of MCM-41. It indicates that the KIT-6 and SBA-15 have larger pore size than that of MCM-41. However; HV MCM-41 is disordered in nature, similar to mesoporous silica foam.18 After PEHA impregnation, the morphology of different mesoporous silica is changed significantly. After 60 wt % PEHA impregnation, KIT-6 shows agglomeration (Figure 2b). Similar behavior is also observed with SBA-15, MCM-41, and HV MCM-41 (Figure 2d,f,h). From this, it can be inferred that the major part of PEHA gets inserted in the mesopores and partially covers the external surface of silica because the maximum loading capacity of PEHA (density = 0.95 g/cm3) is in excess of the maximum uptake capacity (Vt = 1.25 cc/g). It leads to the pronounced agglomeration in mesoporous silica.19 The above phenomena also corroborates results from the XRD analysis discussed earlier. N2 adsorption/desorption isotherm of pure mesoporous silica KIT-6, SBA-15, MCM-41, and HV MCM-41 are shown in Figure 4. The structural properties of mesoporous silicas are summarized in Table 1. All the mesoporous silica show a type IV isotherm as per IUPAC classification.27 This is the basic characteristic of mesoporous material. The specific surface areas are 857, 853, 1492, and 986 m2/g for KIT-6, SBA-15, MCM-41, and HV MCM-41, respectively. KIT-6 and SBA-15 show a pronounced capillary condensation step at 0.6 < P/P0 < 0.9. This is due to the uniform internal mesoporosity.27 MCM-41 and HV MCM-41 show a small step increment in N2 adsorbed volume at low relative pressure 0.1 < P/P0 < 0.4. This is due to the small internal pores present in MCM-41 and HV MCM41.27 However, HV MCM-41 shows a sharp increase in N2 volume adsorbed in high relative pressure 0.9 < P/P0 < 0.99 (Figure 4 d), which can be ascribed to the large textural mesopores.27,28 Pore size distribution clearly shows two types of pore size in HV-MCM-41 (2.2 and 34 nm). However,

3. RESULTS AND DISCUSSION 3.1. Characterization of Mesoporous Silica. The XRD patterns of synthesized mesoporous silica are shown in Figure 1. Samples exhibited peaks at approximately 2θ of 0.95° (d211), 0.90° (d100), 2.6° (d100), and 2.2° (d100) for KIT-6, SBA-15, MCM-41, and HV MCM-41, respectively.11,15,23 The d211 and d100 planes represent the cubic Ia3d (KIT-6) and hexagonal p6mm (SBA-15 and MCM-41) structures, respectively. It is observed that the diffraction peak intensity of HV MCM-41 is much lower than that of the MCM-41 (Figure 1). Lower peak intensity of HV MCM-41 is due to its disordered nature as observed by TEM micrograph. After impregnation, the intensity of characteristic diffraction peaks gradually decreases with the increase in PEHA concentration. A sample diffraction peak for PEHA-impregnated KIT-6 is shown in Figure S1. Hammond et al.24 suggested that the diffraction intensity is associated with the scattering intensity toward the silica wall. However, Sanz-Pérez et al.23 reported that in case of TEPA- and PEI-impregnated SBA-15 the reduction in diffraction peak intensity is due to the filling of pores with organic compounds which avoids the X-ray diffraction through the mesoporous structure. This suggests that the gradual reduction in peak intensity of PEHAimpregnated KIT-6 is because of the filling of pore with PEHA with increase in its concentration. The structure and morphology of pure mesoporous silica is investigated by electron micrograph as shown in the Figures 2 and 3. FESEM micrograph of KIT-6, SBA-15, MCM-41 and HV MCM-41 (Figure 2a,c,e,g) clearly shows the synthesized mesoporous silica exhibiting as a particle (∼1−2 μm), bundle of rope (∼1−2 μm), plate-type (∼1 μm), and particles (∼2−6 μm), respectively. The TEM micrograph of different mesoporous silicas are shown in Figure 3. The TEM micrograph indicates that the KIT-6, SBA-15, and MCM-41 6080

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6. A sharp weight loss is observed between 150 and 350 °C in all the PEHA-impregnated adsorbents. This is mainly for the decomposition and volatilization of amine present in KIT6.16,21,22 3.2. CO2 Adsorption Performance. 3.2.1. Effect of PEHA Loading. The CO2 adsorption performance of sorbents with different loadings of PEHA ranging from 30 to 70 wt % are measured at both 30 and 75 °C and shown in Figure 7. The CO2 sorption capacity of bare KIT-6 is 0.64 and 0.28 mol/kg at 30 and 75 °C, respectively. After PEHA impregnation (K-x PEHA), sorption capacity of KIT-6 is sharply improved. The CO2 sorption capacity is increased with increase in amine loading, and the maximum sorption capacities are 2.44 mol/kg (40 wt %) and 3.41 mol/kg (60 wt %) at 30 and 75 °C, respectively. With further increase in PEHA concentration, the reduction in CO2 sorption capacity is due to the pore blocking and filling effect of PEHA molecules.16,29 At low concentration (until 40 wt %), PEHA molecules are homogeneously distributed inside the porous channels.30 An increase in PEHA concentration in the KIT-6 increases the amine loading, but simultaneously reduces the active amine sites for CO2 interaction by forming layers inside the channels.30 At 75 °C, the CO2 sorption capacity of adsorbent is increased until 60 wt % PEHA loading (Figure 7). With increase in temperature, viscosity of PEHA is decreased, and the spacing between molecules is increased.14 It exposes more amine sites for CO2 interaction. The resulting phenomenon is the enhancement of the sorption capacity of the K-xPEHA at 75 °C. Theoretically, the maximum amount of PEHA that can be loaded in KIT-6 is 55.4 wt %. Thus, the excess amount of PEHA loading in the optimum adsorbent (K-60 PEHA) maybe present on the external surface of the KIT-6. The sorption performance of PEHA-impregnated different mesoporous silica at 75 °C is shown in Figure S4. The sorption capacity gradually increases with increase in amine loading up to 60 wt % with MCM-41, SBA-15, and HV MCM-41 as observed above with KIT-6. A further increase in concentration reduced the sorption capacity. PEHA present in the adsorbent is the active ingredient for CO2 separation in the adsorbent. The amine efficiency (the adsorbed CO2 capacity based on per kg of amine) is summarized in Figure 7b. The amine efficiency gradually decreases with increase in PEHA loading. The 30 wt % PEHA (K-30 PEHA) adsorbent shows loading of around 6.74 mol CO2/kg PEHA at 75 °C and 1 bar. With an increase in concentration to 60 wt % PEHA, efficiency is reduced to 2.27 mol CO2/kg PEHA. At higher amine loading, more amine sites are inside the porous KIT-6, which promotes agglomeration as well as layer formation inside the pore29 and in turn reduces the active amine sites in KIT-6. 3.2.2. Effect of Temperature and Partial Pressure on CO2 Adsorption. The influence of temperature on the CO2 adsorption performance of K-60 PEHA was investigated in the temperatures ranging from 30 to 120 °C. The CO2 adsorption isotherm is depicted in Figure 8a. The CO2 sorption capacity is increased with an increase in adsorption temperature up to 105 °C. The maximum sorption capacity of K-60 PEHA lies between 1.9 molCO2/kg-adsorbent at 30 °C to 4.45 molCO2/kg-adsorbent at 105 °C. A further increase in temperature (viz. 120 °C) reduced the sorption capacity of the adsorbent. This suggests that the CO2 sorption in PEHAimpregnated KIT-6 is more favorable at higher temperatures until 105 °C. Usually, temperature influences the CO2

Figure 2. FESEM images of (a) KIT-6, (b) K-60 PEHA, (c) SBA-15, (d) S-60 PEHA, (e) MCM-41, (f) M-60 PEHA, (g) HV MCM-41, and (h) HV M-60 PEHA.

MCM-41 consists of uniform internal pores (2.2 nm). The large difference in maximum volume adsorption capacity between MCM-41 (Vt = 0.91 cc/g) and HV MCM-41 (Vt = 2.15 cc/g) is due to the difference in pore volume (Figure 4, Table 1). N2 adsorption/desorption of PEHA-impregnated KIT-6 is shown in Figure S2, and the variation of its textural properties with PEHA loading is summarized in Figure 5. As expected, the N2 sorption capacity gradually decreases with increase in PEHA loading in KIT-6. The reduction in N2 adsorption indicates the extent of pore filling during impregnation.18−20 In the case of low PEHA concentration (until 40 wt %), SBET and Vt are reduced sharply and further slightly. It indicates that some of the pores were completely filled with PEHA molecules. After 50−60 wt % PEHA impregnation, SBET and Vt of KIT-6 are nearly zero (Figure 5). This indicates that PEHA completely filled the pores of KIT-6 and that a major portion covered the external surface of KIT-6. Similar adsorption behavior is also observed with M-60 PEHA, S-60 PEHA, and HV M-60 PEHA, and their physical properties are summarized in Figure S3. Thermogravimetric weight loss curves of pure and PEHAimpregnated KIT-6 are shown in Figure 6. All the samples shows some weight loss below 100 °C, which is associated with pre-adsorbed moisture and gases.21,22 With further increase in temperature until ∼150 °C, no weight loss is observed in KIT6081

DOI: 10.1021/acs.iecr.7b00890 Ind. Eng. Chem. Res. 2017, 56, 6078−6087

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Figure 3. TEM images of (a) KIT-6, (b) SBA-15, (c) MCM-41, and (d) HV MCM-41 mesoporous silica.

Figure 4. N2 adsorption/desorption (at −196 °C) and corresponding BJH pore size distribution: (a) KIT-6, (b) SBA-15, (c) MCM-41, and (d) HV MCM-41 mesoporous silica.

cally less favorable for CO2 adsorption due to its exothermic nature.31 However, the rate of CO2 adsorption kinetics increases with an increase in temperature.32 However, enhanced temperature reduces the viscosity of PEI phase, and CO2 molecule diffuses more easily in the PEI layer to contact more and more amine sites.22,29,31 The enhancement in CO2 adsorption capacity of K-60 PEHA until 105 °C indicates that the diffusion of CO2 in PEHA phase is predominant. Similar adsorption behavior was also reported in the literatures with

Table 1. Structural Parameters of Mesoporous Silica sample

SBET (m2/g)

d(BJH) (nm)

Vt (cc/g)

KIT-6 SBA-15 MCM-41 HV MCM-41

857 853 1492 986

6.6 6.6 2.2 2.2 and 34

1.25 1.23 0.91 2.15

adsorption capacity of PEI-impregnated adsorbent in two different ways. An increase in temperature is thermodynami6082

DOI: 10.1021/acs.iecr.7b00890 Ind. Eng. Chem. Res. 2017, 56, 6078−6087

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Industrial & Engineering Chemistry Research

Figure 5. Change in specific surface area (black □) and pore volume (blue ○) with PEHA loading in KIT-6.

Figure 7. Effect of PEHA loading on (a) CO2 adsorption capacity and (b) amine efficiency of KIT-6.

HV M-60PEHA, and S-60PEHA, respectively. The difference in CO2 adsorption capacity with same PEHA loading clearly suggests that the support plays an important role during designing the efficient adsorbent. Wang et al.21 improved the CO2 sorption capacity from 2.39 mol/kg to 3.10 mol/kg by tailoring the pore size from 5.6 to 7.6 nm of mesoporous silica with constant TEPA loading. However, adsorbents with shorter pore length outperformed during adsorption/desorption kinetics over longer channel materials.36 Principally, the supports with high surface area, large pore diameter, high pore volume, and three-dimensional structure are favorable for CO2 separation as well as chemical reaction. High pore volume provides high amine loading,17,20,36,37 and large pores provide an easy pathway for CO2 molecules toward active amine sites during adsorption.21 The specific surface area of the MCM-41 (SBET = 1492 m2/g) is much higher than those of KIT-6, SBA-15, and HV MCM-41 (Table 1). However, its sorption capacity is lower than others throughout the temperature and pressure ranges of the experiments. SBET of M-60 PEHA is much lower than those of the HV M-60 PEHA, K-60 PEHA, and S-60 PEHA (Figure S3). The lowest SBET of MCM-41 may provide the minimum number of amine sites during adsorption. Therefore, we note that the surface area of adsorbent is not the only significant factor during designing the amine-functionalized CO2 adsorbent by wet impregnation. As listed in Table 1, the pore size of MCM-41 (2.2 nm) is lower than those of the other mesoporous silica (SBA-15, KIT6, and HV MCM-41). The sorption capacity of M-60 PEHA is 1.59 mol/kg at 30 °C and 1 bar. It is much lower than the other PEHA-impregnated mesoporous silica (Figure 8). Sorption capacity of the adsorbents at 30° and 1 bar follows the order MCM-41 < KIT-6 < SBA-15 < HV MCM-41. The least value for MCM-41 should be attributed to the smallest pore size.21 Dao et al.37 showed the CO2 sorption capacity 5.91 mol/kg at

Figure 6. TGA analysis of pure and PEHA-impregnated mesoporous KIT-6 between 30 and 800 °C in the presence of N2 atmosphere.

TEPA- and PEI-impregnated MCM-41, SBA-15, and KIT-6 (below 75 °C).16,30,32−34 It is further observed that the CO2 sorption capacity of K-60 PEHA starts decreasing at low partial pressure (ca. until 0.2 bar) with an increase in temperature from 90 to 105 °C (Figure 8). An increase in temperature to 120 °C reduced the CO2 adsorption capacity of adsorbent until 1 bar. Zhao et al.35 reported the adsorption performance of TEPA-impregnated SBA-15 at low partial pressure (10% CO2 feed gas) and 1 bar in the temperature range of 30−100 °C. They observed that the sorption capacity of adsorbent is sharply reduced with increase in temperature from 75 to 100 °C at 0.1 bar CO2 partial pressure. However, the change in sorption capacity for the same sorbent at 1 bar was minimal in the temperature range of 75− 100 °C. In the present study, the similar phenomena is observed for K-60 PEHA between 90 and 105 °C. However, an increase in temperature beyond 90 °C enhances the adsorption kinetics rate but would not be beneficial for CO2 separation in K-60 PEHA at low partial pressure (below 0.2 bar). The CO2 adsorption performance of other mesoporous silicas with the same loading (60 wt % PEHA) at different temperatures are shown in Figure 8b−d. The adsorption capacity gradually increases with increase in adsorption temperature and fall in the ranges of (1.59−4) mol/kg, (2.94−4.3) mol/kg, and (2.28−4.50) mol/kg of M-60PEHA, 6083

DOI: 10.1021/acs.iecr.7b00890 Ind. Eng. Chem. Res. 2017, 56, 6078−6087

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Industrial & Engineering Chemistry Research

Figure 8. Effect of adsorption temperature on the CO2 adsorption performance of with pressure (a) K-60 PEHA, (b) S-60 PEHA, (c) M-60 PEHA, and (d) HVM-60 PEHA.

50 °C and 1 bar with TEPA-impregnated ultralarge pore (28 nm) and low specific surface area MSU-F (272 m2/g). The sorption capacity of K-60 PEHA is 4.48 mol CO2/kg at 105 °C and 1 bar. A similar adsorption capacity is also observed with S-60 PEHA (4.5 mol/kg at 105 °C and 1 bar). It suggests that the similar structural property (Table 1) shows similar sorption performance. Son et al.16 studied the CO2 sorption performance over TEPA-impregnated SBA-15 and KIT-6. They also observed similar adsorption behavior. The sorption capacity of HV M-60 PEHA is 2.9−4.3 mol CO2/kg at between 30 and 90 °C and 1 bar as shown in Figure 8d. HV M-60 PEHA has a higher sorption capacity than those of the other mesoporous silica MCM-41, SBA-15 and KIT-6 between 30 and 90 °C. The higher pore volume of HV MCM41 gives more uniform dispersion of PEHA inside the pores and reduces agglomeration of amine inside the adsorbent (Figure S3).17,20,36 During adsorption, more and more CO2 molecules come in contact with dispersed amine sites and improve the sorption capacity. Similar adsorption performance (∼6.1 mol CO2/kg) was also observed by Linneen et al.20 with TEPA-impregnated high pore volume (5.0 cc/g) silica aerogel. Overall, the larger pore volume increases the amine dispersion with higher loading and enhances the mass transfer of CO2 to sorption sites over that of simple mesoporous silica. With a further increase in temperature from 90 to 105 °C, the sorption capacity of HV M-60 PEHA is sparsely reduced, although the sorption capacities of K-60 PEHA and S-60 PEHA are significantly improved. This is possibly due to the more uniform structures of SBA-15 (hexagonal with larger pore) and KIT-6 (cubical with large interconnected porous channels). Structure of the support also plays an important role during adsorption kinetics. In order to understand its effect during

adsorption, a constant volume of CO2 was exposed to PEHAimpregnated adsorbent. Adsorption kinetics of different adsorbents are shown in Figure 9. The rate of adsorption

Figure 9. CO2 adsorption kinetics of PEHA-impregnated adsorbents at 75 °C.

follows the order SBA-15 < HV MCM-41 ≈ KIT-6. It suggests that the 3D cubical structure with interconnected pores improves the sorption kinetics over those of 2D cylindrical pores. The sorption kinetics of HV MCM-41 is similar to those of KIT-6. The improvement in adsorption kinetics of HV MCM-41 over those of SBA-15 can possibly be for large secondary mesopores (Table 1). 3.3. Heat of Regeneration. Replacement of a more traditional CO2 separation technique, aqueous MEA-based absorption process by amine-functionalized mesoporous silica 6084

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Industrial & Engineering Chemistry Research Table 2. Adsorption Capacity and Heat of Regeneration Performance of Adsorbents sorption capacity (mol CO2/kg) sorbent

90 °C

105 °C

sensible heat (kJ/mol CO2)

DSC (J/g)

ΔHr at 90 °C (kJ/mol CO2)

Q (kJ/mol CO2)

K-60 PEHA S-60 PEHA M-60 PEHA HV M-60 PEHA

4.08 3.97 3.26 4.3

4.48 4.5 4.0 4.07

4.4 4.6 5.6 4.2

218 224 236 275

53.4 56.4 72.7 64.2

57.8 61.0 78.3 68.4

higher than that with the amine-impregnated mesoporous silica. Thus, the CO2 adsorption performance of PEHA-impregnated mesoporous silicas MCM-41, SBA-15, KIT-6, and HV MCM41 studied in this work are better than those of the conventional MEA process in terms of energy efficiency; KIT-6 appears to be the best adsorbent among the others. 3.4. Stability Performance and Cycle Test for PEHA Impregnated KIT-6. The practical application of an adsorbent requires high sorption capacity, easy regeneration, stability in normal atmosphere, as well as stable performance during cyclic use for a long-term operation. Stability of the PEHAimpregnated KIT-6 is confirmed based on CO2 adsorption performance. The adsorbent (K-60 PEHA) was aged for 6 months, and its adsorption performance at 90−105 °C is shown in Figure 11. The K-60 PEHA showed sorption capacities of 4.0 and 4.3 mol CO2/ kg at 90 and 105 °C at 1 bar even after 6 months.

can be decided based on two major factors: (a) adsorption capacity and (b) heat of regeneration. The working capacity within 3−4 mol CO2/kg may become a good adsorbent for practical application. Gray et al.38 found that the amount of heat required (Q) for regeneration is associated with the CO2 sorption capacity of adsorbent as shown in equation: Q ≈ mCpΔT+ ΔHr, where m (kg) is the mass, Cp (kJ/kg K) is the heat capacity, ΔT (K) is the change in temperature, and ΔHr (kJ/mol) is the heat of reaction during adsorption. Zhang et al.39 estimated that the average value of specific heat capacity is 1.81 kJ/kg K of amine-functionalized mesoporous silica between 70−130 °C. The sensible heat (mCpΔT) of sorbents between 90 to 100 °C is summarized in Table 2. The amount of heat evolved (ΔHr) during CO2 adsorption is analyzed by the area under the curve of DSC exothermic peak as shown in Figure 10. Results show that the heat of regeneration of PEHA-

Figure 11. CO2 sorption performance of K-60 PEHA at 90 °C (■) and 105 °C (●) after aging for 6 months. Figure 10. DSC curve of PEHA-impregnated (a) KIT-6, (b) SBA-15, (c) MCM-41, and HV MCM-41 adsorbent in the presence of CO2.

Sorption performance of adsorbent was tested for 10 adsorption/desorption cycles, and results are depicted in Figure 12. The sorption capacity of K-60 PEHA is decreased by less

impregnated adsorbents lies between 58 and 78 kJ/mol CO2 and follows the order KIT-6 < SBA-15 < HV MCM-41 < MCM-41 as shown in Table 2. The difference in regeneration energy is probably due to difference in structure properties. The lower value of Q of KIT-6 is for large interconnected pores with 3D structure and higher for small mesoporous hexagonal MCM-41. The satirically hindered primary amine present in the adsorbent reacts with CO2 by formation of carbamate. In a complete reaction, 2 mol of amines reacts with 1 mol of CO2 by the following reactions: 2(−NH2) + CO2 ↔ −NHCO−2 + −NH+3 ; 2(NH) + CO2 ↔ −NCO−2 + −NH+2 .40 A previous study has shown the CO2 regeneration energy for PEIimpregnated mesoporous silica to be in the range of 74−80 kJ/mol CO2 with the working capacity of 3−4.6 mol CO2/kg,22 although energy required in 30% MEA solution is ∼3200−5500 kJ/kg CO2 (i.e., 141−242 kJ/mol CO2),41 which is much

Figure 12. Cyclic performance of PEHA-impregnated KIT-6 (K-60 PEHA) adsorbent. 6085

DOI: 10.1021/acs.iecr.7b00890 Ind. Eng. Chem. Res. 2017, 56, 6078−6087

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than 4% in 90−105 °C adsorption/100 °C desorption and 1 bar without structural degradation (Figure S5). However, sorption capacity of K-60 PEHA is reduced by ∼17% of the initial sorption capacity in 105 °C adsorption/120 °C desorption in 10 cycles. The sorption performance of several amine-impregnated mesoporous silica is summarized in Table S1. It is observed that the K-60 PEHA shows better sorption performance than those of earlier reported adsorbents, except for high pore volume (Vt = 5.0 cc/g) silica aerogel. The marginal reduction in sorption capacity after every cycle is possibly due to amine loss via volatilization as well as urea formation.6,29

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00890. N2 adsorption/desorption at −196 °C, XRD spectra, CO2 adsorption performance of different adsorbents (PDF)

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REFERENCES

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4. CONCLUSIONS A series of PEHA-impregnated mesoporous silica MCM-41, HV MCM-41, KIT-6, and SBA-15 with different textural properties were synthesized, characterized, and utilized for CO2 separation under wide range of conditions. All the adsorbents showed adsorption capacity higher than 3 mol CO2/kg, which can make them suitable for practical application. However, PEHA-impregnated highly ordered 3D cubical KIT-6 with interconnected pores (6.6 nm) shows better sorption performance (4.48 mol/kg at 105 °C and 1 bar) than those of the others. Pore size of support plays an important role during adsorption kinetics. Increased pore size improves the sorption kinetics of adsorbent. However, 3D structures with large interconnected pores show faster sorption kinetics than those of hexagonal SBA-15. Higher pore volume improves the PEHA distribution inside the pore. However, specific surface area of sorbent has little impact during designing the adsorbent. 3D KIT-6 also shows lower regeneration energy than required with other mesoporous silica SBA-15, MCM-41, and HV MCM-41. K-60 PEHA is stable even after aging for 6 months. In addition, K-60 PEHA shows stable sorption performance even after 10 adsorption/desorption cycles at 90−105 °C. Hence, K-60 PEHA is the best of the four various adsorbents studied for CO2 adsorption both from technical and energetics point of view.

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Article

AUTHOR INFORMATION

Corresponding Author

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

Rupak Kishor: 0000-0002-5178-7697 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Central Instruments Facility (CIF), Indian Institute of Technology Guwahati, for FESEM, TEM, N2 adsorption/desorption, and CO2 adsorption analysis. 6086

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