A Simple Strategy To Improve PEI Dispersion on MCM-48 with Long

Jun 3, 2019 - Another route to improve the amine efficiency of supported PEI is to focus on enhancing the .... compared with MCM-48-C, which may be du...
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Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 10975−10983

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A Simple Strategy To Improve PEI Dispersion on MCM-48 with LongAlkyl Chains Template for Efficient CO2 Adsorption Xingchi Qian,† Junhao Yang,† Zhaoyang Fei,† Qing Liu,*,† Zhuxiu Zhang,† Xian Chen,† Jihai Tang,†,‡ Mifen Cui,† and Xu Qiao†,‡ †

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College of Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing 210009, P. R. China ‡ Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing 210009, P. R. China ABSTRACT: Polyethylenimine (PEI) impregnated MCM48 samples without calcination (MCM-48-W) whose pores are covered with cetyltrimethylammonium bromide (CTAB with long-alkyl chains) were verified to be more efficient CO2 adsorbents than PEI impregnated conventional MCM-48 samples. The samples were characterized by thermogravimetric analysis (TGA), X-ray powder diffraction (XRD), nitrogen adsorption−desorption, and Fourier transform infrared spectroscopy (FTIR) analysis. Also, CO2 adsorption behaviors including adsorption capacity, adsorption thermodynamics, and adsorbent stability were studied to reveal the CO2 adsorption performance on as-synthesized PEI supported materials. The experimental results indicated that the CO2 adsorption and amine efficiency of MCM-48-W were always higher than those of conventional MCM-48 samples at the same PEI loading. MCM-48-W impregnated with 40 wt % PEI (MCM-48-W(40)) exhibited 2.59 mmol·g−1 of CO2 adsorption capacity (6.9 mmol CO2/g PEI), the highest amine efficiency ever reported for MCM-48 impregnated PEI in pure CO2, which may be because the existence of long-alkyl chains improved the dispersion of PEI loading. We presented a simple strategy with the advantages of less consumption of PEI and omission of the calcination step for the improvement of PEI dispersion on MCM-48 with long-alkyl chains template for high efficiency CO2 adsorption.

1. INTRODUCTION With the exploding increase in the combustion of fossil fuels, significant amounts of CO2 are emitted into atmosphere, which affects the climate and contributes to global warming.1−3 And the concentration can be predicted to reach 550 ppm in 2050, conservatively.4 To effectively ease the climate change problem brought by greenhouse gas (mainly CO2), carbon capture technology is urgently needed for attention.5−7 As one of the commercial technologies, chemical absorption by liquid amines including monoethanolamine, diethanolamine, methyldiethanolamine, and triethanolamine has been widely studied for CO2 separation.8 However, energy consumption associated with the regeneration and the equipment protection of the absorbents are problematic for absorption. Carbon capture utilizing solid adsorbents is regarded as a promising strategy such as convenient operation, low energy consumption, outstanding cycle stability, and high economic value.9 Because of the high efficiency and economy of separation, zeolite 13X as an adsorbent was widely used to separate CO2 utilizing the PSA and TSA processes.10,11 In order to further improve the CO2 adsorption capacity, heterogeneous mesoporous silicas supports such as MCM-48, MCM-41, SBA-15, and KIT-6 impregnated with amine groups were applied for CO2 adsorption.12−16 Significant work was © 2019 American Chemical Society

devoted to the impregnation of heavier amine species, namely, polyethylenimine (PEI), because of its high adsorption capacity, good thermal stability, and regeneration performance.17 Son et al.18 prepared MCM-41, MCM-48, SBA-15, SBA-16, and KIT-6 with 50% PEI loading adsorbents, and they exhibited 2.52, 2.70, 2.89, 2.93, and 3.07 mmol·g−1 of CO2 adsorption capacities, respectively, under pure CO2 at 75 °C. KIT-6 with 50 wt % PEI loading maintained stability during a 900 min adsorption−desorption cycle without deterioration. Sharma et al.19 developed 50 wt % PEI-MCM-48 sample with CO2 capacity of 2.59 mmol·g−1 under atmospheric pressure at 80 °C. It is well-known that 1 mol of CO2 molecule combines with 2 mol of amino group in the absence of moisture while 1 mol of CO2 molecule reacts with 1 mol of amino group in the existence of moisture.20 However, amine efficiencies reported by the most researches were much lower than this theoretical maximum CO2 adsorption capacity, which may be associated with uneven dispersion of amine groups and subjected to CO2 diffusional limitations in porous amine-based materials.21 Received: Revised: Accepted: Published: 10975

January 28, 2019 April 21, 2019 June 3, 2019 June 3, 2019 DOI: 10.1021/acs.iecr.9b00545 Ind. Eng. Chem. Res. 2019, 58, 10975−10983

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Industrial & Engineering Chemistry Research

at 60 °C. The sample was labeled as MCM-48-C after calcination at 550 °C for 5 h under air environment to remove CTAB, and the sample was marked as MCM-48-W without calcination. 2.1.2. Amine Functionalization. A series of PEI-supported materials containing 10, 20, 30, 40, and 50 wt % PEI were prepared by impregnation. Typically, a certain amount of PEI was added into 50 mL of ethanol, and the solution was dissolved at room temperature for 30 min. After that, a required amount of mesoporous materials was added under the condition of stirring at 25 °C until the solvent evaporated. Finally, the sample was transferred into a vacuum oven at 60 °C for 6 h. As shown in Figure 1, amine functionalized materials were denoted as MCM-48-C(x)/MCM-48-W(x), where x represents the weight percent of PEI loading.

To mitigate the negative effect of low amine efficiency on CO2 adsorption capacity, one strategy to improve amine efficiency is to incorporate additives such as surfactants or polymers into PEI impregnated porous materials.22−24 Xu and co-workers25 found that MCM-41 samples impregnated with PEG and PEI showed CO2 adsorption capacity of 1.75 mmol· g−1 using pure CO2 at 75 °C, higher than the adsorbents impregnated with PEI solely (1.56 mmol·g−1). Wang et al.26 revealed that the introduction of additive molecules into PEI impregnated porous silica materials can significantly increase the CO2 adsorption capacity compared to the adsorbents with just PEI loading. Another route to improve the amine efficiency of supported PEI is to focus on enhancing the pore structure of mesoporous silica supports.27 Heydari-Gorji et al.28 developed pore-expanded MCM-41 using N,Ndimethyldecylamine (DMDA) as a pore-expanding agent, and the surface of MCM-41 sample is covered with long-alkyl chains. They pointed out that pore-expanded MCM-41 exhibited higher CO2 adsorption efficiency than PEI modified on the corresponding calcined MCM-41 at the same PEI loading. In addition, an ingenious amine double-functionalized adsorbent based on the impregnation of previously grafted materials was prepared for CO2 adsorption.29,30 To achieve the high efficiency of the amino groups, Sanz’s group30 developed a novel adsorbent, aminopropyltrimethoxysilane (AP), that was adopted to graft on the pore-expanded SBA-15, followed by impregnation with PEI. CO2 adsorption capacity of PESBA-AP-PEI (30) was up to 2.53 mmol·g−1, typically higher than the CO2 adsorption capacity of 1.83 mmol·g−1 gained from PE-SBA-PEI (30) at 45 °C and 1 bar. In this work, we present a simple strategy that has the advantages of convenient operation, low energy consumption, and high economy, with the purpose of the improvement of CO2 adsorption efficiency. A series of MCM-48 samples without calcination (MCM-48-W) whose pores are covered with cetyltrimethylammonium bromide (CTAB with long-alkyl chains) impregnated with PEI were developed. As comparison, conventional PEI modified MCM-48-C samples whose templates are removed by calcination were also synthesized. The samples were characterized by thermogravimetric analysis (TGA), X-ray powder diffraction (XRD), nitrogen adsorption−desorption, and Fourier transform infrared spectroscopy (FTIR) analysis. Also, CO2 adsorption behaviors including adsorption capacity, adsorption thermodynamics, and adsorbent stability were studied to reveal the CO2 adsorption performance of as-synthesized PEI supported materials.

Figure 1. Formation process of as-prepared samples.

2.2. Material Characterizations. TGA (Mettler Toledo TGA/DSC 3+) was carried out in order to ascertain the amount of the CTAB and PEI content in adsorbents under a 40 mL min−1 oxygen flow. For this purpose, the temperature of the sample was increased to 550 °C at a heating rate of 5 °C min−1. The crystallinity of the adsorbents was tested by XRD using a SmartLab powder diffractometer with Cu Kα radiation (λ = 0.154 06 nm). The textural properties of the samples including the BET surface and the pore size were measured from N2 sorption utilizing a BETSORP-mini (BEL, Japan) analyzer. FTIR in the range of 500−4000 cm−1 was recorded on a Nicolet iS50 IR spectrometer. CO2 Adsorption Measurements. The CO2 adsorption capacities were tested by the TGA unit. A certain amount sample weight was loaded into an alumina sample pan and activated under pure N2 flow at a rate of 40 mL·min−1 at 100 °C for 30 min. The CO2 adsorption process occurred at 25 °C under pure CO2 flow at a rate of 40 mL·min−1. The CO2 adsorption process in the flue gas was measured using a Hidden mass spectrometer with a packed-bed column (length = 10 cm, inner diameter = 1.0 cm). The CO2 adsorption capacity of samples at 50 °C (CO2/N2 is 15/85) was calculated by CO2 breakthrough curves. Also, the impact of moisture and SO2 (100 ppm, N2 as equilibrium gas) on CO2 adsorption was investigated. The moisture was introduced by

2. EXPERIMENTAL SECTION 2.1. Materials and Preparation. All reagents were obtained from Aladdin including tetraethyl orthosilicate (TEOS, 98%), sodium hydroxide (NaOH, 97%), sodium fluoride (NaF, 98%), and cationic−neutral surfactant cetyltrimethylammonium bromide (CTAB, 99%), polyethylenimine (PEI, MW = 600, 99%). 2.1.1. Synthesis of MCM-48. MCM-48 was synthesized using CTAB as the structure-directing template and TEOS as the silica source.31 Typically, 20 mL of TEOS was put into deionized water (112 mL) under vigorous stirring for 30 min. Then, solid NaOH (1.8 g) was added directly into the solution. Next, NaF powder (2.0 g) was added into the mixture and 8 g of CTAB was introduced in the mixture under stirring at room temperature for 1 h. After being collected for crystallization at 100 °C for 72 h, the samples were filtered and dried overnight 10976

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Figure 2. TG analysis of MCM-48-C and MCM-48-W samples.

N2 flow passing through a water saturator which was put into a constant temperature water bath (30 °C). The CO2 adsorption capacities of adsorbents are calculated by eq 1.32 Ä É 1 ÅÅ t c0 − c ÑÑÑÑ T0 1 q = ÅÅÅÅ Q dt ÑÑ M ÅÅÇ 0 1 − c ÑÑÖ T Vm (1)

T0 is 273 K, T is the adsorption temperature, and Vm (cm3· mmol−1) is the molar volume of gas. PEI efficiency was obtained from CO2 adsorption capacity and PEI contents. The CO2 adsorption capacity and PEI contents were calculated from TG curves. PEI efficiency = CO2 adsorption capacity/PEI contents. N efficiency (mmol CO2/mmol N), defined as the number of moles of CO2 captured per mass unit divided by the moles of N per mass unit, is an important index to evaluate solid amine adsorbent. N efficiency was obtained from CO2 adsorption capacity and N contents; N contents were calculated from PEI contents, N contents = PEI contents/N

∫

where q (mmol·g−1) is the CO2 adsorption capacity in the presence of 15% CO2; M (g) represents the weight of sample, Q (cm3·s−1) represents the flow rate of mixture gas, c0 (%) and c (%) represent CO2 initial concentration and effluent concentration, respectively, t (s) represents adsorption time, 10977

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Industrial & Engineering Chemistry Research contents per PEI, N efficiency = CO2 adsorption capacity/N contents. Adsorption heat was calculated from CO2 adsorption and DSC curves. The CO2 adsorption heat of adsorbent (kJ·mol−1) was obtained from the integrated area below the DSC curves and CO2 adsorption capacity,33 as follows: adsorption heat = heat/CO2 adsorption capacity. The stability of MCM-48 samples was carried out using TGA. The samples were up to saturated CO2 adsorption at 25 °C and 1 atm, and then the desorption process occurred under N2 flow (40 mL·min−1) at 100 °C for 30 min, to complete a cycle.

3. RESULTS AND DISCUSSION 3.1. Material Characterizations. 3.1.1. TG Analysis. TG analysis in oxygen was carried out in order to ascertain the

Figure 4. (a) N2 adsorption−desorption isotherms and (b) pore size distributions of MCM-48 samples. MCM-48(40) and MCM-48(50) cannot be measured out.

Figure 3. XRD patterns of MCM-48 samples.

amount of the CTAB in MCM-48-W. As clearly shown in Figure 2, there was no weight loss on MCM-48-C after heating above 100 °C. On the contrary, MCM-48-W exhibited about a 39 wt % weight loss, which may verify the presence of the CTAB in the absorbents after washing with deionized water and have a substantial effect on PEI loading and CO2 adsorption performance. The existence of CTAB may improve the disperison of PEI loading. Also, the weight loss curves of PEI impregnated in adsorbents were also presented in Figure 2. The weight loss of MCM-48-W(x) was clearly higher than MCM-48-C(x) at the same PEI loading, which may be attributed to the oxidation of extra CTAB. As listed in Table 2, the calculated PEI content was closed to the theoretical amount. The actual PEI content of MCM-48-C(x) was 9, 18, 32, 42, and 47 wt %, respectively, and 10, 20, 27, 37, and 46 wt % for MCM-48-W(x). 3.1.2. XRD Analysis. The powder X-ray diffractograms of MCM-48-C and MCM-48-W samples are depicted in Figure 3. MCM-48-C exhibited Ia3d cubic structure due to the existence of four major diffraction peaks (211) and (220), (420) and (332). The characteristic diffraction peaks (211) and (220) also can be observed between 2° and 3.5° for MCM-48-W, which suggested that the MCM-48-W sample is still the ordered material.34 What’s more, the peak of MCM-48-W in the range of 4.5−5 disappeared, which may be attributed to the fact that the organic template reduced scattering contrast between the pore channel and silica wall and led to the reduction of peak intensity.35 Figure 3 also exhibits the XRD

Table 1. Physicochemical Parameters of As-Made MCM-48 Samples sample MCM-48-C MCM-48-W MCM-48-C(10) MCM-48-W(10) MCM-48-C(20) MCM-48-W(20) MCM-48-C(30) MCM-48-W(30) MCM-48-C(40) MCM-48-W(40) MCM-48-C(50) MCM-48-W(50)

BET surface area (m2·g−1)

pore volume (cm3·g−1)

Dp (nm)

1039 533 585 206 529 163 149 35 13 3

0.82 0.31 0.38 0.15 0.37 0.14 0.14 0.18 0.04 0.01

3.7 3.4 3.8 3.3 3.7 2.8 3.5 2.2

patterns of MCM-48-C and MCM-48-W with 10−50 wt % of PEI loading. The intensity of major diffraction peaks (211) decreased with the increase in the amount of PEI loading, which can be ascribed to the loading of PEI into both the pore channels and the surface of the MCM-48 samples.25 3.1.3. N 2 Physisorption Analysis. N 2 physisorption isotherms and pore size distributions of MCM-48-C and MCM-48-W samples at 77 K are shown in Figure 4. As shown in Figure 4a, MCM-48-C and MCM-48-W samples displayed a 10978

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Figure 5. FTIR spectra for MCM-48 samples. Figure 7. CO2 adsorption capacity and amine efficiency on MCM-48 samples at 25 °C and 1 atm.

Figure 6. CO2 adsorption profiles on MCM-48 samples at 25 °C and 1 atm. 10979

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Industrial & Engineering Chemistry Research Table 2. CO2 Adsorption Capacity Calculated from TGA Curves at 25 °C and 1 atm sample

PEI content (wt %)

adsorption capacity (mmol·g−1)

mmol CO2/mmol N

mmol CO2/g PEI

adsorption heat (kJ·mol−1)

MCM-48-C MCM-48-W MCM-48-C(10) MCM-48-W(10) MCM-48-C(20) MCM-48-W(20) MCM-48-C(30) MCM-48-W(30) MCM-48-C(40) MCM-48-W(40) MCM-48-C(50) MCM-48-W(50)

0 0 9 10 18 20 32 27 42 37 47 46

0.44 0.52 1.06 1.51 1.42 2.04 1.82 2.35 2.16 2.59 2.11 2.53

0.50 0.65 0.33 0.44 0.24 0.36 0.22 0.30 0.19 0.24

11.6 15.0 7.6 10.1 5.6 8.4 5.1 6.9 4.4 5.5

63 56 62 59 63 61 61 60 58 60

Table 3. CO2 Adsorption on MCM-48 under Similar Conditions from the Literature support

amine

amine amount (wt %)

temperature (°C)

adsorption capacity (mmol·g−1)

ref

MCM-48 MCM-48 MCM-48 MCM-48 MCM-48 MCM-48 MCM-48 MCM-48 Cu-MCM-48 MCM-48 MCM-48 MCM-48 MCM-48 MCM-48 MCM-48-C MCM-48-W

NQ-62 PEHA/DEA APTS APTS APTS APTS 3-CPA PEI HPA TREN PEI

43 50 54 54 50 51 30 50

APTS PEI PEI PEI

62 50 40 40

25 25 25 25 25 25 25 80 25 25 75 25 0 80 25 25

1.59 0.51 0.64 0.76 1.68 2.05 1.10 2.59 1.59 0.04 2.70 0.82 2.10 2.14 2.16 2.59

15 39 36 8 16 12 40 19 41 34 18 13 42 43 this work this work

50

CH2 stretching modes, while the absorption band at 1475 cm−1 can be assigned to a C−H stretch,37 which may be due to the existence of carbon chain of CTAB and PEI. Moreover, ammonium groups of MCM-48-C(40) and MCM-48-W(40) can also be observed by the IR bonds that appeared at 1569 cm−1 owing to the PEI loading.34 The IR bonds observed at 2926, 2850, and 1475 cm−1 can verify the existence of CTAB in MCM-48-W, corresponding to the results concluded from TG analysis, XRD analysis, and N2 physisorption analysis. 3.2. CO2 Adsorption. 3.2.1. CO2 Adsorption Capacity. CO2 adsorption profiles of MCM-48-C(x) and MCM-48W(x) samples are illustrated in Figure 6, and the calculated adsorption capacities are shown in Figure 7. All adsorption processes were carried out under 25 °C and 1 atm conditions. As listed in Table 2, the MCM-48-C, MCM-48-C(10), MCM48-C(20), MCM-48-C(30), MCM-48-C(40), MCM-48-C(50) showed CO2 adsorption capacities of 0.44, 1.06, 1.4, 1.82, 2.16, and 2.1 mmol·g−1, respectively, while MCM-48-W, MCM-48W(10), MCM-48-W(20), MCM-48-W(30), MCM-48-W(40), MCM-48-W(50) exhibited 0.52, 1.5, 2.04, 2.35, 2.59, and 2.53 mmol·g−1. With the increase of PEI loading, the CO 2 adsorption capacities of MCM-48-C and MCM-48-W samples increased significantly, which may be attributed to the addition of density of amine groups. Both MCM-48-C and MCM-48-W samples achieved the maximum CO2 adsorption capacity at the 40 wt % PEI loading. The decrease of CO2 adsorption capacity at 50 wt % PEI loading may be due to the blockage of part of

typical type IV adsorption isotherm, which is characteristic of mesoporous material with cubic structure.36 What’s more, a significant decrease in N2 adsorption was observed for the MCM-48-W compared with MCM-48-C, which may be due to the occupation of CTAB. The average pore size and pore volume of MCM-48 samples were obtained from pore size distributions shown in Figure 4b. The pore size distributions of samples ranged from 2 to 4 nm, and all of the samples owned uniform pore sizes. As listed in Table 1, the surface area and the pore volume of MCM-48-C were 1039 m2·g−1 and 0.82 cm3·g−1, obviously higher than those of MCM-48-W (533 m2· g−1 and 0.31 cm3·g−1). Similar trends were found after PEI impregnation. The BET surface area and the pore volume of MCM-48 samples continuously reduced with the increase of the amount of PEI loading. However, low specific surface area and the pore volume may not represent low CO2 adsorption capacity. On the contrary, the existence of CTAB may promote the dispersion of PEI and improved the amine efficiency. When PEI was introduced, the PEI connected with the CTAB and the inner surface of MCM-48 through van der Waals forces or hydrogen bonds. New intertwined network structure was formed, which promoted the dispersion of PEI and improved the amine efficiency.28−30 3.1.4. FTIR Analysis. The IR spectra of MCM-48-C, MCM48-W, MCM-48-C(40), and MCM-48-W(40) are shown in Figure 5. The IR bands at 2926 and 2850 cm−1 of MCM-48-W, MCM-48-C(40), and MCM-48-W(40) were attributed to the 10980

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Figure 10. CO2 adsorption recycle runs of MCM-48-W(40) and MCM-48-C(40) samples at 25 °C.

Results of CO2 adsorption on MCM-48 under similar adsorption conditions obtained from literature are listed in Table 3 for comparison. Both MCM-48-C(x) and MCM-48W(x) samples that we developed exhibited higher adsorption capacity than most of the MCM-48 samples reported by literature, and MCM-48-W(40) showed the highest amine efficiency among MCM-48 impregnated PEI works listed in Table 3. Son et al.18 prepared MCM-48 samples with 50 wt % PEI loading which exhibited 2.70 mmol·g−1 CO2 capture amount, close to the materials we synthesized. However, MCM-48-W(40) consumed less PEI than that developed by Son and omitted the step of calcination. MCM-48-W(x) omitted the step of calcining template while showing higher CO2 adsorption capacities compared to MCM-48-C(x) at the same PEI loading. The existence of CTAB promoted the dispersion of PEI and improved the amine efficiency, which may provide a new strategy for developing amino-functionalized adsorbents. 3.2.2. CO2 Breakthrough Curves. CO2 breakthrough curves of MCM-48 samples were investigated to simulate the actual flue gas flow at 50 °C in the presence of 15% CO2. As shown in Figure 8a, the CO2 adsorption capacities of MCM-48-C(40) and MCM-48-W(40) calculated by breakthrough curves were 1.01 and 1.54 mmol·g−1. MCM-48-W(40) showed better adsorption performance than MCM-48-C(40) even under low CO2 concentration and high temperature, which may be because the existence of CTAB promoted the dispersion of PEI and improved the amine efficiency. As actual flue gas may contain moisture, the evaluation of the effect of moisture on CO2 adsorption process of MCM-48 samples was important. MCM-48-W(40) was selected to investigate the effect of moisture (13.74%) on CO2 adsorption at 50 °C in the 15/85 of CO2/N2 mixture. CO2 breakthrough curves in the presence of dry and humid gas feed were exhibited in Figure 8b, which clearly showed the moisture had a positive effect on the CO2 adsorption for MCM-48-W(40) sample. Under dry conditions, two amines react directly with CO2 to produce carbamates through the formation of zwitterionic intermediates. However, in the presence of water, carbamates form initially and then are converted to bicarbonates.44−46 The CO2 adsorption capacity (1.81 mmol·g−1) increased with the existence of water vapor, which is probably related to the change of the formation of chemical compound between CO2 and PEI from carbamate under dry conditions (CO2/N = 0.5) to the bicarbonate (CO2/N = 1) under humid conditions.47 The existence of

Figure 8. (a) CO2 breakthrough curves on MCM-48-C(40) and MCM-48-W(40); (b) CO2 breakthrough curves under dry, humid, and SO2 conditions.

Figure 9. DSC profiles of MCM-48 samples for CO2 adsorption at 25 °C.

the pore channel by additional PEI species.17,40 At the same PEI loading, the CO2 adsorption capacities and amine efficiency of MCM-48-W(x) were always higher than MCM48-C(x) samples, which may be because the existence of CTAB improved the dispersion of PEI and enhanced the N efficiency. It was inferred that the high adsorption performance was associated with the enhancement of dispersion within the surfactant layer, which increased the amine accessibility.38 10981

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impurity gases such as SOx and NOx in the flue gas poses a significant challenge to the practical application of the adsorption-based CO2 capture technologies.48 MCM-48W(40) was selected to investigate the effect of SO2 (100 ppm) on CO2 adsorption at 50 °C in the 15/85 of CO2/N2 mixture. As shown in Figure 8b, SO2 had an adverse effect on CO2 removal even at low concentration. Because the irreversibly adsorbed SO2 onto the material blocked some amine sites toward subsequent CO2 adsorption,49 MCM-48W(40) showed 1.20 mmol·g−1 of CO2 adsorption capacity in the existence of SO2, significantly lower than 1.54 mmol·g−1. 3.2.3. Adsorption Heat. In order to understand the adsorbate−adsorbent interaction, adsorption heat was calculated from CO2 adsorption and DSC curves. The CO2 adsorption heat of adsorbent (kJ·mol−1) was obtained from the integrated area below the DSC curves presented in Figure 9 and CO2 adsorption capacity listed in Table 2. The corresponding values of the adsorption heat of all samples are around 60 kJ·mol−1, close to the value (63 kJ·mol−1) calculated by Jung et al.50 using PEI-silica sorbent for CO2 adsorption, indicating that CO2 adsorption over MCM-48C(x) and MCM-48-W(x) was mainly based on chemical adsorption. 3.2.4. Adsorbent Stability. Regeneration performance of MCM-48-W(40) and MCM-48-C(40) was also studied. The CO2 adsorption process was performed at 25 °C, and the desorption was managed under pure N2 flow at a rate of 40 mL·min−1 at 100 °C for 30 min. Figure 10 shows that the CO2 adsorption capacity of MCM-48-W(40) and MCM-48-C(40) stay up to 2.50 and 2.15 mmol·g−1 during six cycle runs. No obvious loss of CO2 adsorption capacity on samples took place after several cycles, which showed good adsorption stability. Therefore, the PEI modified MCM-48 samples that we developed have excellent regeneration ability, which is significant for the potential application of MCM-48 adsorbents.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 025 83587168. Fax: +86 025 83587168. E-mail: [email protected]. ORCID

Zhaoyang Fei: 0000-0003-4048-4209 Qing Liu: 0000-0003-3844-6086 Jihai Tang: 0000-0002-8166-9958 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by National Natural Science Foundation of China (Grants 21606130, 21306089), National Key R&D Program of China (Grants 2017YFB0307304, 2017YFC0210903), Science and Technology Department of Jiangsu (Grant BY2015005-02), State Key Laboratory of Materials-Oriented Chemical Engineering (Grant ZK201610), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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REFERENCES

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4. CONCLUSIONS Herein, high dispersion of PEI loading on MCM-48-W with long-alkyl chains template was prepared successfully using an efficiency and simple strategy. As comparison, conventional PEI modified MCM-48-C samples whose templates are removed by calcination were also synthesized. The mesophases of MCM-48-C and MCM-48-W were verified by XRD and N2 adsorption/desorption isotherms. The FTIR analysis confirmed the existence of CTAB, and the TG analysis revealed that about 40 wt % of CTAB was left in the channel of MCM48-W samples, which led to low BET surface area and pore volume. Both MCM-48-C and MCM-48-W samples achieved the maximum CO2 adsorption capacity (2.16 mmol·g−1 of MCM-48-C(40) and 2.59 mmol·g−1 of MCM-48-W(40)) at the 40 wt % PEI loading. At the same PEI loading, the CO2 adsorption capacity and amine efficiency of MCM-48-W(x) were always higher than MCM-48-C(x) samples, which may be because the existence of CTAB improves the dispersion of PEI and enhances the N efficiency. Similar results occurred at low CO2 concentration and high temperature calculated from CO2 breakthrough curves. What’s more, MCM-48-W(x) samples exhibited excellent regeneration ability. All in all, MCM-48-W(x) samples without calcination showed high CO2 adsorption capacity and N efficiency, which may provide a new strategy to develop amine functionalized mesoporous adsorbents using a template with long chain. 10982

DOI: 10.1021/acs.iecr.9b00545 Ind. Eng. Chem. Res. 2019, 58, 10975−10983

Article

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DOI: 10.1021/acs.iecr.9b00545 Ind. Eng. Chem. Res. 2019, 58, 10975−10983