Polyallylamine and NaOH as a novel binder to pelletize amine-functionalized mesoporous silicas for CO2 capture

Microporous and Mesoporous Materials 197 (2014) 278–287

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Polyallylamine and NaOH as a novel binder to pelletize amine-functionalized mesoporous silicas for CO2 capture Worasaung Klinthong, Chih-Hung Huang, Chung-Sung Tan ⇑ Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 17 February 2014 Received in revised form 3 June 2014 Accepted 26 June 2014 Available online 5 July 2014 Keywords: Pellet Binder Polyallylamine Amine-functionalized mesoporous silicas CO2 capture

a b s t r a c t A binder solution containing polyallylamine (PAA) and NaOH is proposed to construct pellets from powdered amine-functionalized mesoporous silica, thereby providing active sites for CO2 capture. Various powdered amine-functionalized adsorbents were prepared and pelletized, including 3-aminopropyltriethoxysilane-functionalized MCM-41 obtained through post-modification and direct synthesis, and polyethylenimine-loaded MCM-41 obtained through impregnation. The effects of the concentrations of PAA and NaOH on the strength, durability, and CO2 adsorption capacity of the pellets were evaluated, as the anhydrous and humid CO2 adsorption behavior and cyclic thermal stability. The pellets prepared after mixing the powdered adsorbents with an aqueous solution of 3 wt% PAA and 2 wt% NaOH exhibited the CO2 adsorption capacity slightly lower than the powdered adsorbent, a recovery of greater than 90% of the powdered adsorbents was observed, while their mechanical strength was over 0.4 MPa and the weight could be retained over 90% in durability tests. Moreover, the pelletized adsorbents possessed the high thermal stability in cyclic adsorption/desorption. As a result, the proposed binder formula can be used to provide pelletized amine-functionalized adsorbents for CO2 capture from power plants. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Carbon dioxide is a key global warming gas and its capture and sequestration is an important issue that has drawn global attention. Although the use of alkanolamines as chemical absorbents is currently one of the most viable means of capturing CO2, this technology has several drawbacks, including high degrees of corrosion and expensive regeneration of the absorbents [1,2]. The application of mesoporous silicas might possibly overcome these drawbacks, because their adsorption capacities toward CO2 can be enhanced by loading alkaline groups, such as amines, onto their internal surfaces [3]. In general, the CO2 adsorption performances of amine-based silica adsorbents are influenced by several factors, including the loading method, the nature of the amine, the surface silanol content, the loading conditions, etc. [4–12]. Although alkanolamines impregnated onto mesoporous silicas can exhibit extremely high CO2 adsorption capacities, they often lack thermal stability during desorption and encounter a large resistance to diffusion [9,12,13]. Several approaches have been proposed to overcome these thermal limitations, including the covalent grafting of aminosilanes, especially 3-aminopropyltriethoxysilane (APS), onto porous silicas through post-modification ⇑ Corresponding author. Tel.: +886 3572 1189; fax: +886 3572 1684. E-mail address: [email protected] (C.-S. Tan). http://dx.doi.org/10.1016/j.micromeso.2014.06.030 1387-1811/Ó 2014 Elsevier Inc. All rights reserved.

[4,5,8,14] or direct synthesis [15–17], ring-opening polymerization of amine monomers onto oxide support (hyperbranched aminosilica materials) [18,19], and the impregnation of polyamine onto porous supports. Silylation of aminosilane-grafted adsorbents onto the intrachannel surfaces of a template-removed mesoporous silica provided a material that exhibited a comparatively higher adsorption rate and higher stability in cyclic runs relative to those of alkanolamine-impregnated adsorbents [11]. Polyethylenimine (PEI), a sterically bulky branched polymer having a repeating unit featuring amino groups and a low heat of adsorption, has generally been loaded into porous silicas through impregnation [20–25]. It has been reported that high PEI-loaded adsorbents significantly enhance the CO2 adsorption capacity while decreasing of pore characteristics; thus, a balance exists between the adsorption event supplying additional active sites on one hand, yet blocking up the channels on the other. Moreover, PEI-loaded adsorbents can exhibit high thermal stability over numerous regeneration cycles at moderate temperatures (110–145 °C) [20]. Although these amine-functionalized powdered materials are proven to be effective adsorbents for CO2 capture, they cannot be used directly in large-scale applications because their small particle sizes have several practical drawbacks, especially a large pressure drop in gas–solid systems [26]. With a view toward industrial applications, pelletized adsorbents are preferred over powdered forms, especially they used in a fixed bed adsorber.

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Pelleting can not only improve the strength of a material but also decrease the cost of its transportation, operation, and storage [27]. A pellet possessing high mechanical strength and durability can avoid damage and the formation of fines or fragments and, thereby, prevent increased pressure drop in a fixed bed adsorber. Binders play important roles in pelleting systems; they are applied to increase the mechanical strength of the pellets. The binders can be either inorganic or organic materials, including bentonite, various polymeric materials, etc. [28]. Polymers are organic binders that can be adsorbed onto pore surface and functional groups on surface. A network is generated by the addition of a polymeric binder, which functions as a bridge linking the particles together. The polymer chains adsorbed on the particles become entangled, thereby tightening the structure and bringing the particles even closer together [29]. Polyallylamine (PAA) is a representative polyamine featuring a long allyl chain and a large mass-fraction of basic primary amino groups. PAA can link particles through the cohesive forces among themselves and adhesive forces between PAA and particles, making it a particularly good candidate for use as a binder material. In particular, the basic primary amino groups also allow the adsorption of CO2 while minimizing thermal mass and providing good oxidative stability [30]. With the goal of practical CO2 capture, mesoporous supports modified with various amines as well as their CO2 adsorption performance have been studied systematically; although the pelleting of powders has also been investigated extensively, the application of amine-functionalized pelletized materials has been reported rarely. Sharma et al. [31] prepared powdered and pelletized mesoporous silicas that were further impregnated with PEI; the pore characteristics and CO2 adsorption performance were then investigated. The pore characteristics of mesoporous silicas decreased dramatically after pelleting, making them less effective for PEI impregnation. As a result, loading amines onto supports prior to pelletizing is proposed herein as a means to more thoroughly apply the pore structures of mesoporous silica. Moreover, the use of amine-containing polymers as binders can make up for the loss of CO2 adsorption capacity after pelleting. Therefore, the objective of this study was to pelletize amine-functionalized mesoporous silicas using a binder solution containing PAA and NaOH. Various powdered amine-functionalized adsorbents were used as modeled samples in this study, including APS-functionalized MCM-41 prepared through post-modification and direct synthesis [16], and PEI-loaded MCM-41 prepared through impregnation. A high recovery of the CO2 adsorption capacity and high mechanical strength were observed after pelleting. To verify the applicability of pelletized amine-functionalized adsorbents, the CO2 adsorption under anhydrous and humid conditions and the cyclic CO2 adsorption/ desorption stability were also examined in this study. 2. Experimental 2.1. Synthesis of powdered adsorbents Four types of powdered adsorbents were prepared: APS-grafted calcined MCM-41 prepared in refluxing toluene and in supercritical (SC) propane; as-synthesized APS-functionalized MCM-41 prepared through direct synthesis; and PEI-loaded MCM-41 prepared through impregnation. 2.1.1. APS-grafted calcined MCM-41 prepared in refluxing toluene The APS-grafted calcined MCM-41 material was prepared in refluxing toluene following the procedures reported by Chang et al. [5]. Commercial calcined MCM-41 (MCM-41, Sigma–Aldrich; 1 g) was dispersed in anhydrous toluene (50 mL) and stirred for 30 min at room temperature. APS (10 mL, Sigma–Aldrich) was

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added and the resulting mixture was heated under reflux at 100 °C for 16 h. The suspended solid product was filtered off, washed with anhydrous ethanol (EtOH, 500 mL), and then dried at 70 °C overnight in the open air to give APS-MCM(R). 2.1.2. APS-grafted calcined MCM-41 prepared in SC propane The APS-grafted calcined MCM-41 material was prepared in SC propane following the procedures reported by Huang et al. [14]. MCM-41 (0.88 g) was dried under vacuum at 150 °C for 12 h and then placed into a 50-mL reactor along with APS (8.80 mL). Pressurized propane was fed into the reactor until a desired pressure of 11.0 MPa was reached. The temperature was increased to 100 °C and the grafting was performed for 16 h. The solid product was washed with anhydrous EtOH (500 mL), and then dried at 70 °C overnight in the open air to give APS-MCM(S). 2.1.3. As-synthesized APS-functionalized MCM-41 prepared through direct synthesis The as-synthesized APS-functionalized MCM-41 material was prepared through direct synthesis following the procedures reported by Klinthong et al. [16]. The pH of a solution of cetyltrimethylammonium bromide (CTAB, Sigma–Aldrich; 3.79 g) in deionized water (200 g) was increased to 13 through the addition of tetramethylammonium hydroxide (TMAOH, Alfa Aesar; 5.65 g). The silica sources, tetraethyl orthosilicate (TEOS, Acros) and APS, were then added such that the molar composition of the mixture was 0.5 TEOS:0.5 APS:0.12 CTAB:0.36 TMAOH:130 H2O. The solution was stirred vigorously for 2 h at ambient temperature and then kept statically in a Teflon autoclave at 100 °C for 4 days. The solids were filtered off, washed with deionized water (3 L) and EtOH (500 mL), and then dried at 70 °C to give APS/MCM. 2.1.4. PEI-loaded MCM-41 through impregnation MCM-41 (4 g) was added into a stirred solution of PEI (4 g) in methanol (80 g). After stirring and heating the mixture under reflux for 8 h, the liquid part of the mixture was evaporated off at 80 °C and then the solid part was dried at 100 °C for 1 h, giving PEI/MCM. 2.2. Synthesis of pelletized adsorbents 2.2.1. Formula of binder solution for pellet production Initially, the formula of the binder solution, containing PAA (molecular weight: 17,000; Sigma–Aldrich) and NaOH (Acros), was investigated using APS-MCM(R) as the powdered adsorbent. The binder solutions were prepared by diluting 20% PAA solution with deionized water to give 2.0–5.0% PAA solutions. NaOH was added to obtain 0.5–3.0% NaOH in the PAA solutions. An aliquot of the binder solution (1.0 g) was added to APS-MCM(R) (1.0 g) and stirred for 5 min. The mixture was poured into a mold and dried at 80 °C for 2 h. The resulting white cylindrical pellets of APS-MCM(R), herein named APS-MCM(R)-P, where P denotes the pelletized product, were obtained with a diameter of 5.0 mm and a height of 2.0 mm (Fig. 1). 2.2.2. Pellet production of powdered adsorbents A powdered adsorbent (APS-MCM(S), APS/MCM, or PEI/MCM; 1.0 g) was pelletized using a binder solution (1.0 g) having a selected formula. The mixture was poured into the mold and dried at 80 °C for 2 h. The resulting pelletized products are named herein as APS-MCM(S)-P, APS/MCM-P, and PEI/MCM-P. 2.3. Characterization The N2 physical adsorption/desorption isotherms of the samples were measured at 196 °C using a Micromeritics Tristar 3000

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2.5. CO2 adsorption

Fig. 1. Typical morphology of pelletized adsorbents.

analyzer; dehydration was performed by evacuating the samples at 150 °C overnight. The pore size distributions of the samples were determined through Barrett–Joyner–Halenda (BJH) analysis. The morphologies of the samples were characterized using a JSM-5600 scanning electron microscope; the sample was adhered to a carbon tape and coated with a thin layer of gold prior to measurement. Elemental analysis (EA) of each sample was performed using a Heraeus Vario EL III-CHNS instrument. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed using an SDT Q600 apparatus; each sample was heated under an air flow of 50 mL/min from room temperature to 100 °C at a heating rate of 10 °C/min, and then the temperature was maintained at 100 °C for 1 h; finally, the sample was heated from 100 to 800 °C at a heating rate of 10 °C/min.

2.4. Mechanical properties The mechanical properties of all prepared pellets were determined by testing their mechanical strength and durability. A universal testing machine was used to quickly measure the mechanical strength of the pellets. The pellets were kept between the two anvils of a strength machine and then an increasing load was applied at a constant rate along the direction vertical to the pellet, until cracking or breaking was observed. The applied load at fracture was recorded as the mechanical strength of the tested pellets. The durability of the pellets was determined through drop resistance tests, in which pellets were dropped four times from a height of 1.85 m onto a metal plate. The durability was calculated as the percentage of retained weight after dropping, relative to the initial weight [32].

The TGA/DSC analyzer was employed to examine the CO2 adsorption performance of the powdered and pelletized adsorbents. Helium, a premixed gas containing 5–40 vol% CO2 in N2, and pure CO2 were purchased from Boclh Industrial Gases (Taiwan). The powdered and pelletized adsorbents were pretreated and dehydrated by heating from room temperature to 120 °C with a ramping rate of 10 °C/min; afterward the temperature was maintained at 120 °C for 6 h under a helium flow. CO2 adsorption was then performed by purging a gas stream containing CO2 for 7 h, which was found to be sufficient to reach equilibrium that was determined by a mass change within 0.0005 mg. The measured CO2 adsorption capacities were found to be reproduced with a deviation less than 5%. The CO2 adsorption capacity was calculated from the mass gain after exposure of the sample to the gas. The recovery of CO2 adsorption capacity of pelletized adsorbents was calculated as

Recovery ð%Þ ¼

CO2 adsorption capacity of pelletized adsorbent CO2 adsorption capacity of powdered adsorbent  100 ð1Þ

The effect of the adsorption temperature was investigated under anhydrous 15% CO2 from 35 to 105 °C and the effect of CO2 concentration was tested in a CO2 concentration range of 5% to 100%. The comparison of CO2 desorption between the powdered and pelletized adsorbents was also conducted. The CO2 desorption was performed at 120 °C under N2 flow. The results and discussion of CO2 desorption for APS-MCM(R), PEI/MCM, APS-MCM(R)-P and PEI/MCM-P are given in Supplementary data. The adsorption of CO2 in a packed bed adsorber was also performed to evaluate the CO2 adsorption performance of the prepared pellets (Fig. 2), because the operations under a humid gas are also needed to evaluate the effect of moisture on CO2 adsorption capacity that cannot be tested in the TGA/DSC measurement. A pelletized adsorbent was loaded into a Pyrex-glass adsorber that was wound with heating tape. Three thermal couples, located at the inlet, outlet, and inside the cell, were used to monitor the temperatures during operation. The temperature could be controlled to within ±0.3 °C. The inlet gas flow rates were controlled using a mass flow controller (Brooks Instruments, 5850E). The CO2 concentration in the gas was measured using a NDIR CO2 analyzer (Drager, Polytron Transmitter IR CO2), letting the gas bypass the adsorber in each run. The reliable measurement range of the analyzer was up to 30% CO2, with a resolution of 0.01%. Prior to the CO2 entering the adsorber, the loaded sample was dehydrated under N2 at a flow rate of

Mass flow controller

Regulator T Thermocouple Regulator T

Cell

T Hygrometer CO2/N2

N2

Water saturator CO2 analyzer Fig. 2. Schematic representation of the packed-bed adsorber.

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10 cm3/min at 120 °C for 12 h. No temperature variation was observed between the inlet and exit of the adsorber. After the adsorption had reached equilibrium, thermal regeneration of the sample was performed by purging the adsorber with N2 at 120 °C. Using this approach, the loaded sample was completely regenerated. To study the effect of water vapor, the sample was treated with a humid N2 stream prior to adsorption of CO2. The humid N2 gas (relative humidity: 74%) was prepared by passing N2 through a water saturator that had been placed in a constant-temperature oven. After the humidity of the gas stream, determined using a hygrometer (Lutron Electronic Enterprise, HT-3009), had plateaued, the humid 15% CO2 in N2 gas was passed into the adsorber until the CO2 concentration in the effluent stream remained unchanged. Afterward, thermal regeneration was performed under humid N2 at 120 °C for 2 h. For the cyclic stability test, 10 cycles of CO2 adsorption/desorption were performed using a temperature swing operation under both anhydrous and humid 15% CO2 at 35 or 95 °C for adsorption and at 120 °C for desorption, as described above.

3. Results and discussion 3.1. Formula of binder solution To formulate the composition of the binder solution, the APSMCM(R) samples were pelletized and then tested for their recovery of CO2 adsorption capacity and mechanical properties. The effects of the PAA in the binder solution without containing NaOH on the recovery of CO2 adsorption capacity, the mechanical strength, and the durability of APS-MCM(R)-P were investigated first. Increasing the PAA concentration from 2% to 5% (Table 1) resulted in increased mechanical strength (from 0 to 0.06 MPa). Although PAA possesses amino groups that could react with CO2 [30], the recovery of CO2 adsorption capacity was decreased with PAA concentration (from 111.4% to 85.2%). This behavior was possible due to the formation of a PAA network that hindered and/or interacted with the grafted APS on MCM-41, thereby decreasing the number of free active sites on APS-MCM(R). Notably, small amounts of PAA (2–3%) could increase the recovery of CO2 adsorption capacity such that it reached over 100% due to the increase of free primary amine active sites supplied by the added PAA. Indeed, the primary amines have been reported to be the highly active groups to CO2 adsorption by comparing with the secondary and tertiary amines [33]; thus, PAA is believed to be a more suitable binder because it also participates in CO2 adsorption. Since 2% PAA could not form pellets, the 3% PAA was selected as the lowest PAA concentration for further pellet development. For comparison, polyvinyl alcohol

Table 1 Formula of binder solution for pelletizing APS-MCM(R). PAA (%)

NaOH (%)

Recoverya (%)

Mechanical strength (MPa)

Durabilityb (%)

Characteristic

0 2 3

– 0 0 0.5 1 2 3 0 0

– 111.4 104.5 98.9 94.3 90.9 55.7 86.4 85.2

– – 0.03 0.09 0.30 0.45 0.50 0.05 0.06

– – 0 0 62.42 90.60 93.00 0 0

Powder Powder Pellet Pellet Pellet Pellet Pellet Pellet Pellet

4 5 a

CO2 Recovery ð%Þ ¼ CO 2

b

weight of pelletized adsorbent after tested Durability ð%Þ ¼ weight  100. of pelletized adsorbent before tested

adsorption capacity of pelletized adsorbent adsorption capacity of powdered adsorbent

 100.

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(PVA), a polymer commonly employed as a binder, was also tested in place of PAA to pelletize APS-MCM(R). With the comparable mechanical strength, the pellets obtained using 3% PVA exhibited a recovery of CO2 adsorption capacity approximately 15% lower than that obtained when using PAA, highlighting the beneficial effect of the amino groups of PAA on the CO2 adsorption. Although the addition of PAA could enhance the CO2 adsorption capacity because of the added amino groups, the mechanical strength of the resulting pellets obtained by the 3% PAA should be further improved for practical use, because a mechanical strength of 0.03 MPa is not acceptable, usually 0.2 MPa is required. To improve the mechanical strength of the pelletized adsorbent, the different amounts of NaOH were added into the binder solution containing 3% PAA. When the NaOH concentration in the 3% PAA binder solution was increased, the mechanical strength and durability of the APS-MCM(R)-P samples were both improved, but the recovery of CO2 adsorption capacity was decreased significantly (Table 1). The improved mechanical properties arose from the strong interaction between the binder materials and the APSMCM(R) particles; these interactions resulted, however, in the weaker CO2 adsorption capacities because they decreased the number of free amine active sites on APS-MCM(R). For comparison, only 2% NaOH aqueous solution without PAA was also tested to reveal the effect of added NaOH on the pelletized APS-MCM(R); in this case, APS-MCM(R) could not be pelletized. It is therefore speculated that NaOH might act as a linker, leading to enhancement of the network strength through ensuring strong interactions between the binder and the powder. Considering the recovery of CO2 adsorption capacity and mechanical properties, the optimal formula of the binder solution was 3% PAA and 2% NaOH because the prepared APS-MCM(R)-P exhibited a CO2 recovery of 90.9%, a mechanical strength of 0.45 MPa, and a durability of 90.6% that fit the criteria for practical use [34]. To validate the versatility of the binder solution for various adsorbents, this formula was further applied to produce pellets from other powdered adsorbents, namely APS-MCM(S), APS/MCM, and PEI/MCM. Characterization data, mechanical properties, and CO2 adsorption performances of these prepared pellets are described below. 3.2. Characterization of powdered and pelletized adsorbents The pore characteristics and mechanical properties of the various powdered and pelletized adsorbents are listed in Table 2. The MCM-41 sample possessed a surface area of 1164.9 m2/g, a pore volume of 0.78 cm3/g, and a pore diameter of 4.0 nm. The pore characteristics decreased after amines, including APS and PEI, were functionalized to the surface of MCM-41. The grafting of APS was through the covalent silylation between the silanol (SiOH) or silane (SiOR) groups of APS and the SiOH groups of MCM-41. The decrease in the pore characteristics of the APS-grafted MCM-41 sample prepared using SC propane was greater than that of the sample prepared using toluene, presumably because more APS could be introduced into the pore spaces in SC propane [14]. When PEI was loaded on MCM-41, the pore volume and diameter could not be determined at all and a significant decline in the pore surface was observed, presumably because the pores of MCM-41 were fully filled by the loaded PEI. The APS/MCM sample synthesized through direct synthesis possessed a low surface area and low pore volume because the surfactant CTAB occluded the pores [16]. After all the adsorbents – namely APS-MCM(R), APS-MCM(S), APS/MCM, and PEI/MCM – had been pelletized, significant losses in surface area, pore volume, and pore diameter were observed because of networks formed through strong interactions between the binders and the particles. For APS-MCM(R) and APS-MCM(S), large decreases in the pore characteristics were observed after pelleting,

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Table 2 Pore characteristics, mechanical properties, and N contents of powdered and pelletized adsorbents. Samplea

SBET (m2/g)

Vdes (cm3/g)

D (nm)

N content (mmol N/g)

Mechanical strength (MPa)

Durability (%)

CO2 adsorption capacity (mmol/g)

MCM-41 APS-MCM(R) APS-MCM(R)-P APS-MCM(S) APS-MCM(S)-P APS/MCM APS/MCM-P PEI/MCM PEI/MCM-P

1164.9 298.0 56.6 185.0 35.2 25.3 4.2 1.2 0.5

0.78 0.30 0.11 0.24 0.08 0.02 0.01 – –

4.0 3.6 3.4 3.4 3.6 – – – –

– 2.42b 3.53 2.69 3.89 3.46b 4.54 10.63 11.65

– – 0.45 – 0.60 – 0.70 – 0.46

– – 90.6 – 96.8 – 98.8 – 90.7

– 0.88b,c 0.80c 1.11c 1.01c 1.18b,c 1.10c 2.61d 2.36d

a APS-MCM(R): APS-grafted MCM-41 prepared in refluxing toluene; APS-MCM(S): APS-grafted MCM-41 prepared in SC propane; APS/MCM: as-synthesized APS-functionalized MCM-41 prepared through direct synthesis; PEI/MCM: PEI-loaded MCM-41 through impregnation; P: pelletized adsorbent. b Data obtained from our previous work [16]. c CO2 adsorption under 15% CO2 in N2 at 35 °C. d CO2 adsorption under 15% CO2 in N2 at 95 °C.

due to serious pore blockage upon formation of the binder network. This finding is in accordance with the results of Sharma et al. [31]. SEM images of APS-MCM(R)-P, APS-MCM(S)-P, APS/MCM-P, and PEI/MCM-P are presented in Fig. 3. Fine particles are clearly observed in the SEM images of APS-MCM(R), APS-MCM(S), and PEI/MCM, whereas large particles are evident for APS/MCM (Fig. S1). For APS-MCM(R), APS-MCM(S) and PEI/MCM, the amine could be modified into intrachannel surface of fine MCM-41 particles without structural change. In contrast, APS chain added in the mixture during direct synthesis of APS/MCM could adopt a random orientation, even become part of the silica matrix, leading to the formation of large particles [16]. After pelletized, the particle aggregations are observed in the SEM image of APS-MCM(R), APS-MCM(S), and PEI/MCM (Fig. 3a, b and d). The fine particles of APS-MCM(R), APS-MCM(S) and PEI/MCM were easily aggregated. On the other hand, the large particles of APS/MCM were difficult to be aggregated. Therefore, fibrous-like networks covering the particles are observed in the SEM image of APS/MCM-P (Fig. 3c). These networks might be formed from PAA and NaOH, leading to the improvement in mechanical strength. The mechanical strength and durability of the pelletized adsorbents are also listed in Table 2; they were all over 0.45 MPa and 90.6%, respectively. As a result, the mechanical properties of these prepared pelletized adsorbents fit the criteria for practical use [34]. This finding is in accordance with the results of the formula tests, indicating that the formula was adequate for preparing the proposed amine-functionalized mesoporous silicas. The results of TGA of the powdered and pelletized adsorbents are presented in Fig. 4 and Table S1. The weight loss percentages of MCM-41 were 5.4 wt% below 100 °C (from physisorbed water), 0.6 wt% between 100 and 350 °C (from hydrogen-bonded H2O), and 1.1 wt% above 350 °C (from condensed SiOH groups forming siloxane units on the surface of MCM-41) [16]. For APS-MCM(R), the decomposition and desorption of APS were evident by the exothermic peak at 300 °C (Fig. 4a) and the 15.91 wt% weight loss that occurred at temperatures above 100 °C. The exothermic peak of APS-MCM(S) (Fig. 4b) – related to the decomposition and desorption of APS – appeared at 302 °C with a weight loss of 15.21 wt% at temperatures above 100 °C. The combined decomposition of CTAB and APS resulted in two broad exothermic peaks for APS/ MCM at 277 and 352 °C (Fig. 4c) and a weight loss at temperatures above 100 °C of 55.81 wt%. For PEI/MCM, it can be seen from Fig. 4d that the decomposition and desorption of PEI were evident by two exothermic peaks at 196 and 323 °C and a weight loss of 48.15 wt% occurred at temperatures above 100 °C. Based on these information, the weight of PEI loaded on PEI/MCM was 48.15 wt% which

was sufficiently close to the weight ratio of 50% in the preparation. After adsorbents had been pelletized, the temperatures at which the exothermic peaks appeared changed to 327 °C for APSMCM(R)-P, 303 °C for APS-MCM(S)-P, 340 and 439 °C for APS/ MCM-P, and 218, 302, and 489 °C for PEI/MCM-P (Fig. 4). Notably, the exothermic peaks of all of the pelletized samples were broader and shifted to higher temperatures. The broader peaks presumably derived from decomposition of the binders and amines; the shifting of peaks to higher temperature revealed the improved thermal resistance of the amine that could interact strongly with the binder. EA data for the pelletized adsorbents are listed in Table 2. Notably, one mole of APS corresponds to one mole of N. For the samples prepared using the different post-modification methods, the N contents of APS-MCM(R) and APS-MCM(S) were 2.42 and 2.69 mmol N/g, respectively. The N content of APS-MCM(S) was larger than that of APS-MCM(R) because the low solvent viscosity and high diffusivity of APS in SC propane induced the grafting of more APS onto the MCM-41 surface. For the sample prepared through direct synthesis, the N content of APS/MCM reached as high as 3.46 mmol N/g, derived from the N of both CTAB and APS [16]. The highest N content (10.63 mmol N/g) was exhibited by PEI/MCM, presumably because PEI, a liquid polyamine, could be loaded fully into the pore volume space of MCM-41. After pelleting the powdered adsorbents, the N contents of APS-MCM(R)-P, APSMCM(S)-P, APS/MCM-P, and PEI/MCM-P all increased to 3.53, 3.89, 4.54, and 11.65 mmol N/g, respectively. These increases in N contents of approximately 10–45% on the pelletized adsorbents resulted from the amino groups of PAA. 3.3. CO2 adsorption 3.3.1. Pelletized APS-functionalized MCM-41 The CO2 adsorption capacity of the APS-functionalized adsorbents in both powder and pellet forms are illustrated in Fig. 5. The CO2 adsorption was measured under 15% CO2 in N2 at various adsorption temperatures. The APS/MCM and APS/MCM-P samples possessed the highest CO2 adsorption capacities among these powdered and pelletized APS-functionalized MCM-41 samples, presumably because they contained the greatest N contents (Table 2). The N content increased, but the CO2 adsorption capacity decreased, after pelleting these powdered adsorbents; this behavior was due to the loss of free amine active sites and pore mouth blockage through the network formation and particle aggregation, as described above. The recovery of the CO2 adsorption capacity was greater than 90% for all of the pelletized APS-functionalized MCM-41 samples. The APS/MCM-P possessed a higher recovery

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283

(a)

(b)

(c)

(d)

Fig. 3. SEM images of pelletized adsorbents (left: 60; right: 3500): (a) APS-MCM(R)-P, (b) APS-MCM(S)-P, (c) APS/MCM-P and (d) PEI/MCM-P.

of its CO2 adsorption capacity because most of the APS was coated on the surface of the template-occluded support for the powdered APS/MCM, thereby facilitating the access of CO2 to amine after pelleting. In contrast, the APS units were grafted onto the intrachannel surfaces of both APS-MCM(R)-P and APS-MCM(S)-P, whereas their pore mouths were blocked by the binder network after pelleting, as evidenced by the large decreases in the surface area and pore volume and the absence of a measurable pore diameter (Table 2). As a result, the location of amine influenced CO2 recovery of pelletized adsorbents. The effect of the adsorption temperature on the CO2 adsorption capacity under 15% CO2 in N2 at 35–75 °C is also illustrated in Fig. 5. The CO2 adsorption capacities of both the powdered and pelletized APS-functionalized MCM-41 samples decreased upon increasing the adsorption temperature. Based on its

exothermic adsorption behavior, more CO2 could be adsorbed at lower temperatures. The CO2 adsorption capacities of APS-MCM(R)-P, APS-MCM(S)P, and APS/MCM-P under various CO2 concentrations (from 5% to 100%) are displayed in Fig. 6. The CO2 adsorption capacities of APS/MCM-P were higher than those of both APS-MCM(R)-P and APS-MCM(S)-P at all concentrations. The CO2 adsorption capacities increased nonlinearly upon increasing the CO2 concentration in the gas mixtures. A steep increase in the CO2 adsorption capacity occurred at CO2 concentrations of less than 15%, while a small increase occurred at concentrations of greater than 15%. The former steep curve at low CO2 concentration was caused by quite strong interactions between CO2 and the amino groups; the latter small increase was ascribed to the physical adsorption of CO2.

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(a) 8

o

300 C 100

(b) 8 o

302 C

100

4

4 0

0

Weight (%)

8

o

327 C

100

4

-8 75

APS-MCM(S) -12 8

o

303 C

100

4

Heat flow (W/g)

APS-MCM(R) -8

Heat flow (W/g)

75

Weight (%)

-4 -4

0 0 -4 75

75

-4

-8

APS-MCM(R)-P -8 0

200

400

600

APS-MCM(S)-P -12

800

0

o

400

o

800

277 C

4

o

o

352 C

(d) 8

196 C

100

4

o

75

600

Temperature ( C) (c) 8

100

200

o

Temperature ( C)

323 C

75

0

0

-4

APS/MCM

100

8

o

340 C

o

439 C 75

4 0

Weight (%)

Weight (%)

25

-12

50 -8 25

PEI/MCM 218 C

100

14 o

302 C

75

7

o

489 C

-4 50

-12

o

Heat flow (W/g)

-8

-4

Heat flow (W/g)

50

0 50

-8 APS/MCM-P

25 0

200

400

600

-12 800

o

Temperature ( C)

-7 PEI/MCM-P

25 0

200

400

600

-14 800

o

Temperature ( C)

1.2

0.8

APS-MCM(R) APS-MCM(S) APS/MCM APS-MCM(R)-P APS-MCM(S)-P APS/MCM-P

0.4

0.0 30

40

50

60

o

Temperature ( C)

CO2 adsorption capacity (mmol/g)

CO2 adsorption capacity (mmol/g)

Fig. 4. Thermogravimetric analyses of powdered and pelletized adsorbents.

1.6

1.2

0.8 APS-MCM(R)-P APS-MCM(S)-P APS/MCM-P Langmuir

0.4

0.0 0

20

40

60

80

100

CO2 concentration (%)

Fig. 5. CO2 adsorption capacities of powdered and pelletized APS-functionalized MCM-41 under anhydrous flows with 15% CO2 in N2 at various temperatures.

Fig. 6. CO2 adsorption capacities of pelletized APS-functionalized MCM-41 under various CO2 concentrations at 35 °C.

The CO2 adsorption capacities of these three samples were aptly correlated by a Langmuir adsorption isotherm with a value of r2 of greater than 0.997. The values of the constants qm and b in the Langmuir equation are listed in Table 3. The lowest value of constant b was that of 0.303 L/mmol for APS/MCM-P, indicating its lower heat of adsorption relative to those of APS-MCM(R)-P and APS-MCM(S)-P.

The results of 10 adsorption/desorption cycles for APS-MCM(R)P, APS-MCM(S)-P, and APS/MCM-P under anhydrous and humid 15% CO2 in N2 at 35 °C are displayed in Fig. 7. Notably, the CO2 adsorption capacities measured from a packed bed adsorber are consistent with those obtained by a TGA/DSC analyzer for anhydrous operation with an average deviation of 3%, indicating the reliability of the packed bed operation. The CO2 adsorption

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W. Klinthong et al. / Microporous and Mesoporous Materials 197 (2014) 278–287 Table 3 Constants of the Langmuir equationa of pelletized APS-functionalized MCM-41. qm (mmol/g)

b (L/mmol)

r2

APS-MCM(R)-P APS-MCM(S)-P APS/MCM-P

1.03 1.21 1.57

0.372 0.414 0.303

0.998 0.997 0.998

Langmuir equation: q = qmbCe/(1 + bCe).

capacities of APS-MCM(R)-P, APS-MCM(S)-P, and APS/MCM-P under anhydrous conditions were 0.75, 0.95, and 1.05 mmol/g, respectively; under humid conditions they were 0.94, 1.07, and 1.13 mmol/g, respectively. The CO2 adsorption capacities under anhydrous 15% CO2 in N2 were lower than those under humid 15% CO2 in N2. Theoretically, the adsorption of CO2 gases on amine-supported silica should follow an acid/base mechanism:

ð2Þ

RNHþ2 COO þ RNH2 ! RNHCOO þ RNHþ3

ð3Þ

RNHþ2 COO þ H2 O ! RNHCOO þ H3 Oþ

ð4Þ

CO2 adsorption capacity (mmol/g)

RNH2 þ CO2 ! RNHþ2 COO

1.2

(a)

Anhydrous condition Humid condition

0.8

0.4

0.0 2

4

6

8

10

CO2 adsorption capacity (mmol/g)

Cycle

1.2

(b)

Anhydrous condition Humid condition

0.8

0.4

0.0 2

4

6

8

10

CO2 adsorption capacity (mmol/g)

Cycle

1.2

(c)

Anhydrous condition Humid condition

0.8

0.4

0.0 2

4

3.3.2. Pelletized PEI-loaded MCM-41 PEI/MCM was also tested as the amine-loaded adsorbent through impregnation for pellet production. PEI was loaded into the intrachannels of MCM-41, stabilized through hydrogen bonding interactions between the PEI molecules and the SiOH groups on the surface of MCM-41. The CO2 adsorption capacities under 15% CO2 in N2 at different adsorption temperatures are presented in Fig. 8. The recovery of the CO2 adsorption capacity for PEI/ MCM-P was approximately 90%. This decrease in CO2 adsorption for PEI/MCM-P was due to the lower number of free amine active sites after pelleting and pore mouth blockages. The effect of the adsorption temperature on the adsorption capacities of PEI/MCM and PEI/MCM-P was examined under 15% CO2 in N2 at 35– 105 °C. At 35 °C, the CO2 adsorption capacities of PEI/MCM (0.99 mmol/g) and PEI/MCM-P (0.89 mmol/g) were lower than those of APS-MCM(S) (1.11 mmol/g), APS/MCM (1.18 mmol/g), APS-MCM(S)-P (1.01 mmol/g) and APS/MCM-P (1.10 mmol/g). This was possible due to the aggregation of high viscosity PEI resulting in the loss of amine active sites for CO2 adsorption and the increase

6

8

10

Cycle Fig. 7. Results from 10 adsorption/desorption cycles of (a) APS-MCM(R)-P, (b) APSMCM(S)-P, and (c) APS/MCM-P under anhydrous and humid 15% CO2 in N2 at 35 °C, tested in the packed bed.

3 CO2 adsorption capacity (mmol/g)

a

Sample

The RNHCOOH species and RNHCOORNH+3 ion pair are a carbamic acid and an ammonium carbamate salt, respectively. In the presence of water, CO2 can be hydrolyzed to form HCO 3 and CO2 3 , which further interact with the protonated amine to form the salts ammonium bicarbonate (RNH+3HCO 3 ) and ammonium carbonate ((RNH+3)2CO2 3 ) [9]. Proximately, one molecule of amine reacts with one molecule of CO2 in the presence of water. The influence of moisture on the CO2 adsorption capacity of amine-functionalized silicas has been investigated previously [5,7,8]. Water can have a positive or negative effect on CO2 adsorption on the amine-supported mesoporous silicas SBA-15 and MCM-41 and the pore-expanded MCM-41. The effect of water on CO2 adsorption capacity has been reported to depend on the structure of the amine as well as the nature of the porous support [7]. For comparison, the CO2 adsorption capacities of prepared pellets were also determined under 100% CO2. The CO2 adsorption capacities of APS-MCM(R)-P (0.97 mmol/g), APS-MCM(S)-P (1.13 mmol/g), and APS/MCM-P (1.44 mmol/g) were comparable with those reported powdered adsorbents for APS-grafted MCM-41 (0.83 mmol/g) [15] and APSSBA-15 (1.40 mmol/g) [10]. From 10 thermal swings of adsorption/desorption under anhydrous and humid 15% CO2 in N2 at 35 °C (Fig. 7), stable adsorption and desorption behavior was observed under both sets of conditions. The similar results have been reported in literature [14,16,35,36] as well. Thus, the CO2 adsorption capacities of APSMCM(R)-P, APS-MCM(S)-P, and APS/MCM-P during the 10 cycles appeared to be invariant, indicating the high thermal stability of APS on each sample.

PEI/MCM PEI/MCM-P 2

1

0 40

60 80 o Temperature ( C)

100

Fig. 8. CO2 adsorption capacities of PEI/MCM and PEI/MCM-P under 15% CO2 in N2 at various temperatures.

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CO2 adsorption capacity (mmol/g)

of CO2 diffusion hindrance. Upon increasing temperature, the CO2 adsorption capacities increased, reaching 2.62 and 2.36 mmol/g for PEI/MCM and PEI/MCM-P, respectively, at 95 °C. These results suggest that the elevated temperature decreased the viscosity of PEI and increased of number of accessible amine sites in the adsorbent, thereby increasing the CO2 adsorption capacity. When the temperature was higher than 95 °C, Fig. 8 shows a decrease of CO2 adsorption capacity. As a result, the best CO2 adsorption capacity was obtained at 95 °C for both PEI/MCM and PEI/MCM-P. The CO2 adsorption capacities of PEI/MCM-P under different CO2 concentrations (5–100%) at 95 °C are presented in Fig. 9. Similar to the results for the pelletized APS-functionalized MCM-41, the CO2 adsorption capacities of PEI/MCM-P increased nonlinearly upon increasing the CO2 concentration in the gas mixture. Moreover, a steep increase in the CO2 adsorption capacity occurred at CO2 concentrations of less than 15% CO2, caused by the relatively strong interactions between CO2 and the amino groups of PEI; a small increase occurred at concentrations of greater than 15%, ascribed to the physical adsorption of CO2. The CO2 adsorption capacity of PEI/MCM-P correlated well with a Langmuir adsorption isotherm (r2 = 0.996) having values of the constants qm and b of 2.51 mmol/g and 0.797 L/mmol, respectively. The results of 10 adsorption/desorption cycles of PEI/MCM-P under anhydrous and humid 15% CO2 in N2 at 95 °C are presented in Fig. 10. The CO2 adsorption capacities of PEI/MCM-P under anhydrous and humid conditions were 2.35 and 2.59 mmol/g, respectively, higher than that reported for PEI-loaded pelletized MCM-41 (1.73 mmol/g), SBA-15 (1.66 mmol/g) and MCM-48 (2.14 mmol/g) [31]. Similar to the behavior of the pelletized APSfunctionalized adsorbents, the CO2 adsorption capacity of PEI/ MCM-P under humid conditions was greater than that under anhydrous conditions. The CO2 adsorption capacity of PEI/MCM-P (2.42 mmol/g) was comparable with those of other reported

2

1

Langmuir

0 0

20

40

60

80

100

CO2 concentration (%)

CO2 adsorption capacity (mmol/g)

Fig. 9. CO2 adsorption of PEI/MCM-P under various CO2 concentrations at 95 °C.

3

Anhydrous condition Humid condition

powdered PEI-loaded adsorbents under 100% CO2, such as PEIloaded KIT-6 (1.79 mmol/g) [15], PEI-loaded MCM-41 (2.52– 2.55 mmol/g) [24,37] and PEI-loaded SBA-15 (1.72–2.73 mmol/g) [10,23,38,39]. Furthermore, the results of 10 thermal swings of adsorption/desorption under anhydrous and humid 15% CO2 in N2 at 95 °C revealed that the PEI/MCM-P displayed stable adsorption and desorption behavior under both anhydrous and humid conditions. This performance was comparable to the case with powdered PEI-loaded mesoporous silicas [6,18,24,37,40], indicating that a high stability of PEI in PEI/MCM-P. According to these results achieved in this study, the proposed binder formula of 3% PAA and 2% NaOH could be employed extensively to pelletize amine-functionalized mesoporous silica adsorbents obtained from amine grafting, amine impregnating or coating with different amine contents. The prepared pellets displayed adequate mechanical properties and acceptable CO2 adsorption capacities with high recoveries of CO2 adsorption capacities, making them for practical CO2 capture from power plants.

4. Conclusions For practical use, the pelletized adsorbents instead of powdered adsorbents are generally employed to lower pressure drop in a packed bed adsorber. To make the pellets, binders are generally required to improve the mechanical strength of the pellets. A binder solution containing PAA and NaOH has shown its great promise for use in the construction of pellets from various amine-functionalized mesoporous silica powders with various types and amine contents. The binder solution featuring 3% PAA and 2% NaOH was found to be a suitable formula for pelleting amine-functionalized mesoporous silica adsorbents synthesized through grafting in toluene or SC propane, direct synthesis, or impregnation. The pelletized adsorbents possessed adequate mechanical properties – mechanical strength of greater than 0.45 MPa and durability of greater than 90% – that fit the criteria for practical use. Moreover, the prepared pellets displayed acceptable CO2 adsorption capacities with recoveries of CO2 adsorption capacities of greater than 90%, values that are comparable with those of previously reported powdered adsorbents. The PAA/NaOH binder formed a strong network with the particles to result in the improved mechanical properties. Moreover, the amino groups of PAA provided additional free active amine sites to further increase the CO2 adsorption capacity. Dynamic adsorption/desorption cycles revealed that the pelletized adsorbents possessed high thermal stability. Therefore, the pelletized amine-functionalized mesoporous silicas prepared using the proposed binder formula appear to be useful adsorbents for practical CO2 capture when applying a temperature-swing adsorption technology.

Acknowledgments The authors like to thank the ROC National Science Council (Grant Nos. NSC102-3113-P-007-007 and NSC102-EPA-F-013001), the Ministry of Economic Affairs of ROC (102-D0626), and National Tsing Hua University (Hsinchu, Taiwan, ROC) for financial support.

2

1

0 2

4

6

8

10

Cycle Fig. 10. Results from 10 adsorption/desorption cycles of PEI/MCM-P under anhydrous and humid 15% CO2 in N2 at 95 °C, tested in the packed bed.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso.2014. 06.030.

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