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CO2 adsorption on amine functionalized MCM-41: Effect of bi-modal porous structure
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Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–5
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CO2 adsorption on amine functionalized MCM-41: Effect of bi-modal porous structure M. Gholami∗, M.R. Talaie1, S.F. Aghamiri Department of Chemical Engineering, Faculty of Engineering, University of Isfahan, P.O. Box 81746-73441 Isfahan, Iran
a r t i c l e
i n f o
Article history: Received 16 March 2015 Revised 16 July 2015 Accepted 20 July 2015 Available online xxx Keywords: BPS-MCM-41 Amine grafted CO2 adsorption Taguchi method
a b s t r a c t In this study, a series of experiments was carried out to find the optimum condition for grafting 3-[2-(2Aminoethylamino)ethylamino]propyltrimethoxysilane (here after TRI) on bi-modal porous structure MCM41 (here after BPS-MCM-41). The Taguchi experimental design L9 orthogonal array (OA, three factors in three levels) was applied to investigate the effect of temperature, water to solid support ratio, and TRI to solid support ratio on the amine grafting and CO2 adsorption performance of amine grafted BPS-MCM-41 structure. The ratio of adsorbed CO2 to consumed TRI was selected as the objective function of optimization. The optimum conditions for the BPS-MCM-41 were 85 °C, the water to support ratio of 0.3 cc/g, and the aminosilane to support ratio of 1.5 cc/g. The amine grafting of BPS-MCM-41 in this condition resulted in 2.31 mmol/g adsorption capacity. The analysis of variance (ANOVA) showed that the most significant effect on the response was exerted by the amount of TRI used, while the reaction temperature was found to be the least influential. © 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction In the past decades, the need for efficient CO2 capturing has gotten researchers to focus on improving the common capturing processes among which the adsorption has surpassed the other processes [1]. It is because of the simplicity, ease of use, and less energy demand of adsorption process. CO2 capturing can be performed physically or chemically by solid sorbents; however, since the physisorption of CO2 is drastically diminished by the presence of water vapor, and because to enhance CO2 adsorption capacity at low partial pressure is the ultimate cause, the chemisorption seems to be a more reliable choice [2]. For the chemisorption, which is carried out by the means of functionalized adsorbents, different types of functional groups such as alkali hydroxides and aminosilanes can be implanted into different solid supports such as alumina, silica gel, and mesoporous materials [2–6]. But, no matter what the functional groups and solid supports are, the adsorption capacity and functional group efficiency are of the crucial characteristics of the functionalized adsorbents. To enhance both the efficiency and capacity of the functionalized adsorbent, researchers have focused on expanding the pores of the adsorbent [6–8]. The researchers showed that the pore expansion of MCM-41 has brought
about a 50% increase in CO2 adsorption capacity on amine functionalized adsorbents [9]. Another approach which can improve the adsorbent performance is adsorbent hierarchization [10–15]. It is known that hierarchization enhances the mass transfer inside the particle [10]; therefore, it is possible that the functional groups get distributed uniformly in bi-modal structures. On the other hand, the presence of macro-size pores inside support structures can create space for multilayer functionalization of aminosilane inside support structures. In fact, besides having high surface area, having large internal volume may lead to housing a larger amount of functional groups which may lead to a higher capacity of CO2 adsorption. To investigate this opinion, optimizing the CO2 adsorption performance of amine grafted BPS-MCM41 has been opted as the objective of this research. As mentioned earlier, there are numerous research works which have focused on functionalization of pore-expanded MCM-41 by TRI. Hence, to have a comparison between the role of pore expansion and bi-modality of structure on performance of CO2 capturing, the same functional group (TRI), and the same base solid support (MCM-41) were selected in the present study. 2. Material and method
∗
Corresponding author. Tel.: +98 3117934026; fax: +98 3117932679. E-mail addresses: [email protected], [email protected] (M. Gholami), [email protected] (M.R. Talaie). 1 Present address: Nottingham University Malaysia Campus, Chemical and Environmental Engineering Department, Jalan Broga, 43500, Semnyih, Malaysia.
2.1. BPS-MCM-41 synthesis and amine grafting To synthesize BPS-MCM-41, the standard procedure used in other studies was applied with slight modification [16–20]. To do so, 4.88 g of CTAB (Merck) was dissolved in 202.5 g of deionized water.
http://dx.doi.org/10.1016/j.jtice.2015.07.021 1876-1070/© 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: M. Gholami et al., CO2 adsorption on amine functionalized MCM-41: Effect of bi-modal porous structure, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.07.021
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M. Gholami et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–5 Table 1 Functionalization design of experiment. Run
Temperature (°C)
Water to support ratio (cc/g)
Aminosilane to support ratio (cc/g)
1 2 3 4 5 6 7 8 9
70 70 70 85 85 85 100 100 100
0.1 0.3 0.5 0.1 0.3 0.5 0.1 0.3 0.5
1.5 3.0 4.5 3.0 4.5 1.5 4.5 1.5 3.0
After attaining a clear solution, 9.76 g of carbon black nanoparticle (YongFeng, average particle size: 29 nm, surface area 118 m2 /g) which had been already degased in 200 °C for 2 h was added to the solution as the solid template to create larger pores. For the complete dispersion of carbon nanoparticles in the solution, the mixture was mixed by ultrasonic homogenizer (Sonopuls HD2200 Sonicator, Bandelin) for 30 minutes at 5 cycles and 35% power. Then, 22.73 cc of NH4 OH (25%, Merck) was added and stirred for 10 min; following that, 20 cc of TEOS (Merck) was added drop by drop during 30 min to reach CTAB 0.15, H2 O 126, NH4 OH 1.64 and TEOS 1 molar ratios. The solution was then sealed and stirred for 3 h in the ambient temperature. Lastly, the suspension was put in the oven at 100 °C for 48 h. Eventually, the final product was filtered and washed with deionized water and dried in 80 °C overnight. Calcination was performed through a one degree per minute temperature rise up to 550 °C, and after reaching 550 °C, the status quo was maintained for 5 h. It should be noted that to extract the carbon black from the silica structure, the calcination was performed under air flow. Amine grafting was done by using the procedure offered in Harlick and Sayari’s [9]. Following their procedure, 1 g of BPS-MCM-41 was dispersed in 75 ml toluene (Merck). To make sure that there was no residual water in toluene, it was dried using commercial 3A zeolite prior to use. After 30 min of vigorous mixing, the measured amount of deionized water was added to the mixture, and the suspension was mixed for 3 h. The glass flask, then, was submerged in silicon oil bath at a certain temperature, and subsequently, the measured amount of TRI (Sigma-Aldrich) was added under stirring, and held under full reflux for 16 h. Then, the material was filtered and washed with a copious amount of toluene followed by normal pentane. Finally, the filtered solid dried at 80 °C in a circulating oven for 1 h.
Fig. 1. Schematic diagram of closed loop volumetric apparatus (1: loading cell, 2: Adsorption Chamber, 3: circulating pump, 4–11: ball and needle valve, 12: RH and temperature sensor, 13: pressure transmitter, 14: RTD temperature sensor).
and analyze the results, the MINITAB® Release 14.1 (Minitab Inc.) is used. 2.3. Characterization The structural properties of BPS-MCM-41 were determined by XRD (X-ray Diffractometer, D8ADVANCE, Bruker, Germany, X-Ray Tube Anode: Cu Wavelength: 1.5406 A˚ (Cu Kα ), Filter: Ni), and nitrogen adsorption/desorption at 77 K (BELsorp-mini II). The surface area was determined by the BET method, and the pore size distribution was calculated using BJH method. The pore volume was calculated based on the amount of liquid nitrogen adsorbed at P/P0=0.995. The CHNS elemental analyzer (LECO 932) was used to measure the amount of TRI grafted inside the BPS-MCM-41. Prior to CHNS test, the sample was degased under 0.01 bar absolute pressure at 100 °C to remove any gaseous adsorbed contaminants. By using the CHNS test, the mass percentages of elemental nitrogen, hydrogen and carbon in the adsorbent sample are provided. 2.4. CO2 adsorption experiment The equilibrium measurement of CO2 adsorption capacity is done by the means of a home-made closed loop volumetric apparatus. A schematic diagram of the apparatus is shown in Fig. 1. The system consists of two cells, a pressure transmitter, a variable-speed circulating pump, an RH sensor, and an RTD temperature sensor. The gases which were used in this study were purchased from Farafan Gas Company. The specification of the gases is as follows: Carbon dioxide (high purity grade, 99.995%) and nitrogen (high purity grade, 99.995%).
2.2. The optimization of CO2 adsorption on amine grafted BPS-MCM-41
3. Result and discussion
As mentioned earlier, the most significant parameters for grafting aminosilanes in mesoporous adsorbents are reaction temperature, water to solid ratio, and aminosilane to solid ratio. Harlick and Sayari have stated that the temperature of 85 °C, water to solid ratio of 0.3 cc/g, and aminosilane to solid ratio of 3 cc/g form the optimum condition of aminosilane grafting on pore-expanded MCM-41 [9]. As the characteristics of BPS-MCM-41 and pore-expanded MCM-41 are different, the optimum condition, in which the pore-expanded MCM41 was functionalized, is not necessarily the optimum condition required for BPS-MCM-41 functionalization; nevertheless, it is not expected to be so different, either. Therefore, to find the optimum condition, it is necessary to run some experiments in the range of conditions reported by Harlick and Sayari [9]. Although the full factorial method is the most reliable approach to find the full effect of parameters on the results of the experiment, there are statistical methods by using which the number of experiments can be reduced [21]. Thus, the Taguchi method (L9 orthogonal array) is used in this study to reduce the number of experiments. Table 1 summarizes different parameters and the levels for each parameter. To design the experiment
3.1. Characterization of BPS-MCM-41 and TRI-BPS-MCM-41 The XRD patterns of BPS-MCM-41 and TRI-BPS-MCM-41 are shown in Fig. 2. This figure shows that BPS-MCM-41 and TRI-BPSMCM-41 have low structural order. The adsorption and desorption isotherms of nitrogen are shown in Fig. 3. As shown in this figure, for BPS-MCM-41, there are two rises in adsorption isotherm. The first one belongs to the micellar pores, and the second to the larger pores which were created because of the vacant places of carbon black nanoparticles. The adsorption/desorption curves of TRI-BPS-MCM-41 show that micellar pores were almost filled or blocked by TRI functional groups, and there is a small rise at high partial pressure due to capillary condensation in larger pores. The hysteresis between adsorption and desorption curves at the second rise confirms that there is distribution in pore size at that range. This fact is thoroughly justified in Fig. 4. The quantitative characteristics of BPS-MCM-41 and TRI-BPSMCM-41, determined by nitrogen adsorption/desorption technique, are shown in Table 2.
Please cite this article as: M. Gholami et al., CO2 adsorption on amine functionalized MCM-41: Effect of bi-modal porous structure, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.07.021
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Fig. 2. The XRD pattern of BPS-MCM-41. Fig. 4. Pore size distribution of BPS-MCM-41 and TRI- BPS-MCM-41.
Fig. 3. N2 adsorption desorption curve at 77 K. Table 2 The structural properties of BPS-MCM-41.
2
as (BET) (m /gr) Total pore volume (cm3 /gr) Mean pore diameter (nm) ap (BJH plot) (m2 /gr) vp (BJH plot) (cm3 /gr) rpeak (BJH plot) (nm)
BPS-MCM-41
TRI-BPS-MCM-41
726 1.47 8.09 678 1.39 1.22
48.4 0.243 20.054 45.3 0.240 1.22
3.2. Optimization of amine grafting on BPS-MCM-41 using Taguchi method Based on the conditions mentioned in Table 1, the adsorbent was functionalized. The results of CHNS test and CO2 adsorption capacity are presented in Table 3. To analyze the results of Taguchi method,
the analysis of variance (ANOVA) was used. Table 4 shows the ANOVA statistical terms for the amount of adsorbed CO2 per gram of used aminosilane. Since it is the aminosilane group which adsorbs the CO2 , in order to maximize the efficiency of aminosilane used, the ratio of adsorbed CO2 per gram of used aminosilane was selected as the objective function of optimization. The F-ratio in ANOVA table is a reliable criterion for distinguishing the important factors from those of less significance. Higher values for the calculated F-ratio means a greater influence of the factor on the experiment outcome. As shown in Table 4, the amount of aminosilane added has the most noticeable effect, while the temperature seems to be the least effective on the performance of the functionalization. The last column of this table represents the contribution of each parameter to the results of the experiment. By using the ANOVA procedure, the optimum condition of functionalization was obtained as follows: Temperature, 85 ˚C; water to support ration, 0.3 cc/g; and aminosilane to support ratio, 1.5 cc/g. The ANOVA prediction of response at optimum condition is 3.162 mmol of adsorbed CO2 per gram of used aminosilane. These results show that the optimum condition of BPS-MCM-41 functionalization is almost the same as the one for pore-expanded MCM41 functionalization. The only difference lies in the amount of aminosilane. Because the obtained optimum condition was not present in Table 1, to verify the prediction of ANOVA procedure, the adsorbent was functionalized in the obtained optimum condition. The mass ratio of optimized amine grafted adsorbent to support ratio was 2.04; the amount of nitrogen in adsorbent was 6.69 mmol of N per gram; the CO2 adsorption capacity was 2.39 mmol/g; the amount of adsorbed CO2 per gram of used aminosilane was 3.164, which is in excellent agreement with the prediction of ANOVA procedure; and finally the amine group efficiency (adsorbed CO2 /N) was 0.36.
Table 3 Results of CHNS test and CO2 adsorption capacity. run
Amine grafted adsorbent/support (g/g)
N (mmol/g)
a
Adsorbed CO2 (mmol/g)
b
Adsorbed CO2 (mmol CO2 per gram of used aminosilane)
CO2/N
1 2 3 4 5 6 7 8 9
1.54 2.50 3.47 1.74 2.73 2.12 1.67 1.97 3.22
5.86 7.81 8.74 5.82 7.56 6.45 5.39 6.33 7.63
2.11 2.64 2.11 2.17 2.10 2.09 1.82 2.31 1.50
2.10 2.14 1.58 1.22 1.86 2.86 0.65 2.94 1.57
0.36 0.34 0.24 0.37 0.28 0.32 0.34 0.37 0.20
a b
The CO2 adsorption done at 0.3 bar CO2 partial pressure and 25 ˚C. Objective function of optimization experiments.
Please cite this article as: M. Gholami et al., CO2 adsorption on amine functionalized MCM-41: Effect of bi-modal porous structure, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.07.021
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M. Gholami et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–5 Table 4 The ANOVA table for adsorbed CO2 per gram of used aminosilane. parameter
DOF
Sum of Squares (SS)
Variance
F ratio
Percent
Temperature (°C) Water to support ratio (cc/g) Aminosilane to support ratio (cc/g) Other/ error Total
2 2 2 2 8
0.113 1.53 2.674 0.007 4.324
0.056 0.765 1.337 0.003
13.724 185.145 323.515
2.431 35.183 61.621 0.765 100%
Table 5 The comparison of results and outcomes of other TRI amine grafted porous supports. Porous support
Nitrogen content (mmol/g)
Ads. Temp (°C)
CO2 partial pressure (bar)
Dry adsorption capacity (mmol/g)
CO2 /N ratio
References
BPS-MCM-41 BPS-MCM-41 BPS-MCM-41∗ Aerogel (dry graft) Aerogel – 300/95 Boiled SBA-15 PE-MCM-41 PE-MCM-41 SBA-15 PE-MCM-41 MSF SBA-15 Silica nanotubes PE-MCM-41 Meso. pore silica
6.69 7.56 7.81 4.13 7.4 5.8 7.95 7.8 5.21 4.33 4 3.68 4.93 7.9 5.18
25 25 25 25 25 60 25 70 45 50 75 60 25 25 25
0.3 0.05 0.3 1 1 0.15 0.05 0.05 1 1 0.15 0.15 1 1 1
2.39 1.74 2.64 1.64 2.61 1.8 2.65 2.28 1.75 2.43 1.3 2.41 2.23 2.75 1.74
0.36 0.23 0.34 0.40 0.35 0.31 0.33 0.29 0.34 0.56 0.33 0.65 0.45 0.35 0.34
Present study [20] Present study [14] [14] [22] [9] [23] [24,25] [26] [27] [28] [29] [30] [31]
∗
Table 3 run 2.
Fig. 5. Adsorption isotherm of CO2 on TRI-PE-MCM-41 and TRI-BPS-MCM-41 at 25 ˚C.
The reasonable results of present study were compared to the performance of TRI grafted sorbents in Table 5. The results of this table indicate that the adsorption capacity of TRI-BPS-MCM-41 is comparable to the reported sorbents which are prepared via aminosilane grafting. However, the optimized aminosilane to support ratio which was used in grafting procedure is among the lowest of the reported data. To have a better comparison between CO2 adsorption performance of TRI-PE-MCM-41 [32] and TRI-BPS-MCM-41, the adsorption isotherm of CO2 on both adsorbents is presented in Fig. 5. As it is shown in this figure, the adsorption isotherms are almost the same at low to moderate (0.3 bar) partial pressure; however, the TRI-PEMCM-41 shows a higher capacity at a higher pressure. It should be noted that the amount of aminosilane used to functionalize BPSMCM-41 is 50% of the amount of aminosilane used to functionalize TRI-PE-MCM-41. 4. Conclusion In this study, TRI grafting on BPS-MCM-41 was optimized and characterized by XRD, CHNS, and N2 adsorption/desorption techniques. The characterizations showed that both solid support and functionalized adsorbent have low structural order, and due to func-
tionalization, the most of the internal volume of BPS-MCM-41 was filled with TRI functional groups. The optimum condition for grafting TRI on BPS-MCM-41was found using Taguchi experimental design (T: 85 ˚C, Water to support ratio: 0.3 cc/g, and TRI to support ratio: 1.5 cc/g). The ANOVA showed that the amount of TRI used had the most remarkable effect on the response, while the least significant one was of the reaction temperature. The optimum condition, found for grafting BPS-MCM-41, is almost the same as the optimized condition which has been found for pore-expanded MCM-41 functionalization, with the only difference being the amount of TRI used. The result showed that CO2 adsorption capacity of TRI-BPS-MCM-41 is almost the same as capacity of TRI-PE-MCM-41 at low to moderate CO2 partial pressure proving that beside the surface area, the internal volume of solid support is also influential on amine loading and CO2 adsorption capacity. Acknowledgment The authors appreciate the financial support from NIGC (Grant No: 190141). References [1] Choi S, Drese JH, Jones CW. Adsorbent materials for carbon dioxide capture from large anthropogenic point sources. ChemSusChem 2009;2(9):796–854. [2] Ello AS, de Souza LKC, Trokourey A, Jaroniec M. Development of microporous carbons for CO2 capture by KOH activation of African palm shells. J CO2 Util 2013(0). [3] Leal O, Bolivar C, Ovalles C, Garcia JJ, Espidel Y. Reversible adsorption of carbon dioxide on amine surface-bonded silica gel. Inorg Chim Acta 1995;240(1–2):183– 9. [4] Chao K-J, Klinthong W, Tan C-S. CO2 Adsorption ability and thermal stability of amines supported on mesoporous silica SBA-15 and fumed silica. J Chin Chem Soc 2013 n/a-n/a. [5] De Canck E, Ascoop I, Sayari A, Van Der Voort P. Periodic mesoporous organosilicas functionalized with a wide variety of amines for CO2 adsorption. Phys Chem Chem Phys 2013;15(24):9792–9. [6] Loganathan S, Tikmani M, Ghoshal AK. Novel pore-expanded MCM-41 for CO2 capture: synthesis and characterization. Langmuir 2013;29(10):3491–9. [7] Belmabkhout Y, Sayari A. Effect of pore expansion and amine functionalization of mesoporous silica on CO2 adsorption over a wide range of conditions. Adsorption 2009;15(3):318–28. [8] Mizutani M, Yamada Y, Yano K. Pore-expansion of monodisperse mesoporous silica spheres by a novel surfactant exchange method. Chem Commun 2007;0(11):1172–4.
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Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–5
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CO2 adsorption on amine functionalized MCM-41: Effect of bi-modal porous structure M. Gholami∗, M.R. Talaie1, S.F. Aghamiri Department of Chemical Engineering, Faculty of Engineering, University of Isfahan, P.O. Box 81746-73441 Isfahan, Iran
a r t i c l e
i n f o
Article history: Received 16 March 2015 Revised 16 July 2015 Accepted 20 July 2015 Available online xxx Keywords: BPS-MCM-41 Amine grafted CO2 adsorption Taguchi method
a b s t r a c t In this study, a series of experiments was carried out to find the optimum condition for grafting 3-[2-(2Aminoethylamino)ethylamino]propyltrimethoxysilane (here after TRI) on bi-modal porous structure MCM41 (here after BPS-MCM-41). The Taguchi experimental design L9 orthogonal array (OA, three factors in three levels) was applied to investigate the effect of temperature, water to solid support ratio, and TRI to solid support ratio on the amine grafting and CO2 adsorption performance of amine grafted BPS-MCM-41 structure. The ratio of adsorbed CO2 to consumed TRI was selected as the objective function of optimization. The optimum conditions for the BPS-MCM-41 were 85 °C, the water to support ratio of 0.3 cc/g, and the aminosilane to support ratio of 1.5 cc/g. The amine grafting of BPS-MCM-41 in this condition resulted in 2.31 mmol/g adsorption capacity. The analysis of variance (ANOVA) showed that the most significant effect on the response was exerted by the amount of TRI used, while the reaction temperature was found to be the least influential. © 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction In the past decades, the need for efficient CO2 capturing has gotten researchers to focus on improving the common capturing processes among which the adsorption has surpassed the other processes [1]. It is because of the simplicity, ease of use, and less energy demand of adsorption process. CO2 capturing can be performed physically or chemically by solid sorbents; however, since the physisorption of CO2 is drastically diminished by the presence of water vapor, and because to enhance CO2 adsorption capacity at low partial pressure is the ultimate cause, the chemisorption seems to be a more reliable choice [2]. For the chemisorption, which is carried out by the means of functionalized adsorbents, different types of functional groups such as alkali hydroxides and aminosilanes can be implanted into different solid supports such as alumina, silica gel, and mesoporous materials [2–6]. But, no matter what the functional groups and solid supports are, the adsorption capacity and functional group efficiency are of the crucial characteristics of the functionalized adsorbents. To enhance both the efficiency and capacity of the functionalized adsorbent, researchers have focused on expanding the pores of the adsorbent [6–8]. The researchers showed that the pore expansion of MCM-41 has brought
about a 50% increase in CO2 adsorption capacity on amine functionalized adsorbents [9]. Another approach which can improve the adsorbent performance is adsorbent hierarchization [10–15]. It is known that hierarchization enhances the mass transfer inside the particle [10]; therefore, it is possible that the functional groups get distributed uniformly in bi-modal structures. On the other hand, the presence of macro-size pores inside support structures can create space for multilayer functionalization of aminosilane inside support structures. In fact, besides having high surface area, having large internal volume may lead to housing a larger amount of functional groups which may lead to a higher capacity of CO2 adsorption. To investigate this opinion, optimizing the CO2 adsorption performance of amine grafted BPS-MCM41 has been opted as the objective of this research. As mentioned earlier, there are numerous research works which have focused on functionalization of pore-expanded MCM-41 by TRI. Hence, to have a comparison between the role of pore expansion and bi-modality of structure on performance of CO2 capturing, the same functional group (TRI), and the same base solid support (MCM-41) were selected in the present study. 2. Material and method
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Corresponding author. Tel.: +98 3117934026; fax: +98 3117932679. E-mail addresses: [email protected], [email protected] (M. Gholami), [email protected] (M.R. Talaie). 1 Present address: Nottingham University Malaysia Campus, Chemical and Environmental Engineering Department, Jalan Broga, 43500, Semnyih, Malaysia.
2.1. BPS-MCM-41 synthesis and amine grafting To synthesize BPS-MCM-41, the standard procedure used in other studies was applied with slight modification [16–20]. To do so, 4.88 g of CTAB (Merck) was dissolved in 202.5 g of deionized water.
http://dx.doi.org/10.1016/j.jtice.2015.07.021 1876-1070/© 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: M. Gholami et al., CO2 adsorption on amine functionalized MCM-41: Effect of bi-modal porous structure, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.07.021
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M. Gholami et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–5 Table 1 Functionalization design of experiment. Run
Temperature (°C)
Water to support ratio (cc/g)
Aminosilane to support ratio (cc/g)
1 2 3 4 5 6 7 8 9
70 70 70 85 85 85 100 100 100
0.1 0.3 0.5 0.1 0.3 0.5 0.1 0.3 0.5
1.5 3.0 4.5 3.0 4.5 1.5 4.5 1.5 3.0
After attaining a clear solution, 9.76 g of carbon black nanoparticle (YongFeng, average particle size: 29 nm, surface area 118 m2 /g) which had been already degased in 200 °C for 2 h was added to the solution as the solid template to create larger pores. For the complete dispersion of carbon nanoparticles in the solution, the mixture was mixed by ultrasonic homogenizer (Sonopuls HD2200 Sonicator, Bandelin) for 30 minutes at 5 cycles and 35% power. Then, 22.73 cc of NH4 OH (25%, Merck) was added and stirred for 10 min; following that, 20 cc of TEOS (Merck) was added drop by drop during 30 min to reach CTAB 0.15, H2 O 126, NH4 OH 1.64 and TEOS 1 molar ratios. The solution was then sealed and stirred for 3 h in the ambient temperature. Lastly, the suspension was put in the oven at 100 °C for 48 h. Eventually, the final product was filtered and washed with deionized water and dried in 80 °C overnight. Calcination was performed through a one degree per minute temperature rise up to 550 °C, and after reaching 550 °C, the status quo was maintained for 5 h. It should be noted that to extract the carbon black from the silica structure, the calcination was performed under air flow. Amine grafting was done by using the procedure offered in Harlick and Sayari’s [9]. Following their procedure, 1 g of BPS-MCM-41 was dispersed in 75 ml toluene (Merck). To make sure that there was no residual water in toluene, it was dried using commercial 3A zeolite prior to use. After 30 min of vigorous mixing, the measured amount of deionized water was added to the mixture, and the suspension was mixed for 3 h. The glass flask, then, was submerged in silicon oil bath at a certain temperature, and subsequently, the measured amount of TRI (Sigma-Aldrich) was added under stirring, and held under full reflux for 16 h. Then, the material was filtered and washed with a copious amount of toluene followed by normal pentane. Finally, the filtered solid dried at 80 °C in a circulating oven for 1 h.
Fig. 1. Schematic diagram of closed loop volumetric apparatus (1: loading cell, 2: Adsorption Chamber, 3: circulating pump, 4–11: ball and needle valve, 12: RH and temperature sensor, 13: pressure transmitter, 14: RTD temperature sensor).
and analyze the results, the MINITAB® Release 14.1 (Minitab Inc.) is used. 2.3. Characterization The structural properties of BPS-MCM-41 were determined by XRD (X-ray Diffractometer, D8ADVANCE, Bruker, Germany, X-Ray Tube Anode: Cu Wavelength: 1.5406 A˚ (Cu Kα ), Filter: Ni), and nitrogen adsorption/desorption at 77 K (BELsorp-mini II). The surface area was determined by the BET method, and the pore size distribution was calculated using BJH method. The pore volume was calculated based on the amount of liquid nitrogen adsorbed at P/P0=0.995. The CHNS elemental analyzer (LECO 932) was used to measure the amount of TRI grafted inside the BPS-MCM-41. Prior to CHNS test, the sample was degased under 0.01 bar absolute pressure at 100 °C to remove any gaseous adsorbed contaminants. By using the CHNS test, the mass percentages of elemental nitrogen, hydrogen and carbon in the adsorbent sample are provided. 2.4. CO2 adsorption experiment The equilibrium measurement of CO2 adsorption capacity is done by the means of a home-made closed loop volumetric apparatus. A schematic diagram of the apparatus is shown in Fig. 1. The system consists of two cells, a pressure transmitter, a variable-speed circulating pump, an RH sensor, and an RTD temperature sensor. The gases which were used in this study were purchased from Farafan Gas Company. The specification of the gases is as follows: Carbon dioxide (high purity grade, 99.995%) and nitrogen (high purity grade, 99.995%).
2.2. The optimization of CO2 adsorption on amine grafted BPS-MCM-41
3. Result and discussion
As mentioned earlier, the most significant parameters for grafting aminosilanes in mesoporous adsorbents are reaction temperature, water to solid ratio, and aminosilane to solid ratio. Harlick and Sayari have stated that the temperature of 85 °C, water to solid ratio of 0.3 cc/g, and aminosilane to solid ratio of 3 cc/g form the optimum condition of aminosilane grafting on pore-expanded MCM-41 [9]. As the characteristics of BPS-MCM-41 and pore-expanded MCM-41 are different, the optimum condition, in which the pore-expanded MCM41 was functionalized, is not necessarily the optimum condition required for BPS-MCM-41 functionalization; nevertheless, it is not expected to be so different, either. Therefore, to find the optimum condition, it is necessary to run some experiments in the range of conditions reported by Harlick and Sayari [9]. Although the full factorial method is the most reliable approach to find the full effect of parameters on the results of the experiment, there are statistical methods by using which the number of experiments can be reduced [21]. Thus, the Taguchi method (L9 orthogonal array) is used in this study to reduce the number of experiments. Table 1 summarizes different parameters and the levels for each parameter. To design the experiment
3.1. Characterization of BPS-MCM-41 and TRI-BPS-MCM-41 The XRD patterns of BPS-MCM-41 and TRI-BPS-MCM-41 are shown in Fig. 2. This figure shows that BPS-MCM-41 and TRI-BPSMCM-41 have low structural order. The adsorption and desorption isotherms of nitrogen are shown in Fig. 3. As shown in this figure, for BPS-MCM-41, there are two rises in adsorption isotherm. The first one belongs to the micellar pores, and the second to the larger pores which were created because of the vacant places of carbon black nanoparticles. The adsorption/desorption curves of TRI-BPS-MCM-41 show that micellar pores were almost filled or blocked by TRI functional groups, and there is a small rise at high partial pressure due to capillary condensation in larger pores. The hysteresis between adsorption and desorption curves at the second rise confirms that there is distribution in pore size at that range. This fact is thoroughly justified in Fig. 4. The quantitative characteristics of BPS-MCM-41 and TRI-BPSMCM-41, determined by nitrogen adsorption/desorption technique, are shown in Table 2.
Please cite this article as: M. Gholami et al., CO2 adsorption on amine functionalized MCM-41: Effect of bi-modal porous structure, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.07.021
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M. Gholami et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–5
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Fig. 2. The XRD pattern of BPS-MCM-41. Fig. 4. Pore size distribution of BPS-MCM-41 and TRI- BPS-MCM-41.
Fig. 3. N2 adsorption desorption curve at 77 K. Table 2 The structural properties of BPS-MCM-41.
2
as (BET) (m /gr) Total pore volume (cm3 /gr) Mean pore diameter (nm) ap (BJH plot) (m2 /gr) vp (BJH plot) (cm3 /gr) rpeak (BJH plot) (nm)
BPS-MCM-41
TRI-BPS-MCM-41
726 1.47 8.09 678 1.39 1.22
48.4 0.243 20.054 45.3 0.240 1.22
3.2. Optimization of amine grafting on BPS-MCM-41 using Taguchi method Based on the conditions mentioned in Table 1, the adsorbent was functionalized. The results of CHNS test and CO2 adsorption capacity are presented in Table 3. To analyze the results of Taguchi method,
the analysis of variance (ANOVA) was used. Table 4 shows the ANOVA statistical terms for the amount of adsorbed CO2 per gram of used aminosilane. Since it is the aminosilane group which adsorbs the CO2 , in order to maximize the efficiency of aminosilane used, the ratio of adsorbed CO2 per gram of used aminosilane was selected as the objective function of optimization. The F-ratio in ANOVA table is a reliable criterion for distinguishing the important factors from those of less significance. Higher values for the calculated F-ratio means a greater influence of the factor on the experiment outcome. As shown in Table 4, the amount of aminosilane added has the most noticeable effect, while the temperature seems to be the least effective on the performance of the functionalization. The last column of this table represents the contribution of each parameter to the results of the experiment. By using the ANOVA procedure, the optimum condition of functionalization was obtained as follows: Temperature, 85 ˚C; water to support ration, 0.3 cc/g; and aminosilane to support ratio, 1.5 cc/g. The ANOVA prediction of response at optimum condition is 3.162 mmol of adsorbed CO2 per gram of used aminosilane. These results show that the optimum condition of BPS-MCM-41 functionalization is almost the same as the one for pore-expanded MCM41 functionalization. The only difference lies in the amount of aminosilane. Because the obtained optimum condition was not present in Table 1, to verify the prediction of ANOVA procedure, the adsorbent was functionalized in the obtained optimum condition. The mass ratio of optimized amine grafted adsorbent to support ratio was 2.04; the amount of nitrogen in adsorbent was 6.69 mmol of N per gram; the CO2 adsorption capacity was 2.39 mmol/g; the amount of adsorbed CO2 per gram of used aminosilane was 3.164, which is in excellent agreement with the prediction of ANOVA procedure; and finally the amine group efficiency (adsorbed CO2 /N) was 0.36.
Table 3 Results of CHNS test and CO2 adsorption capacity. run
Amine grafted adsorbent/support (g/g)
N (mmol/g)
a
Adsorbed CO2 (mmol/g)
b
Adsorbed CO2 (mmol CO2 per gram of used aminosilane)
CO2/N
1 2 3 4 5 6 7 8 9
1.54 2.50 3.47 1.74 2.73 2.12 1.67 1.97 3.22
5.86 7.81 8.74 5.82 7.56 6.45 5.39 6.33 7.63
2.11 2.64 2.11 2.17 2.10 2.09 1.82 2.31 1.50
2.10 2.14 1.58 1.22 1.86 2.86 0.65 2.94 1.57
0.36 0.34 0.24 0.37 0.28 0.32 0.34 0.37 0.20
a b
The CO2 adsorption done at 0.3 bar CO2 partial pressure and 25 ˚C. Objective function of optimization experiments.
Please cite this article as: M. Gholami et al., CO2 adsorption on amine functionalized MCM-41: Effect of bi-modal porous structure, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.07.021
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M. Gholami et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–5 Table 4 The ANOVA table for adsorbed CO2 per gram of used aminosilane. parameter
DOF
Sum of Squares (SS)
Variance
F ratio
Percent
Temperature (°C) Water to support ratio (cc/g) Aminosilane to support ratio (cc/g) Other/ error Total
2 2 2 2 8
0.113 1.53 2.674 0.007 4.324
0.056 0.765 1.337 0.003
13.724 185.145 323.515
2.431 35.183 61.621 0.765 100%
Table 5 The comparison of results and outcomes of other TRI amine grafted porous supports. Porous support
Nitrogen content (mmol/g)
Ads. Temp (°C)
CO2 partial pressure (bar)
Dry adsorption capacity (mmol/g)
CO2 /N ratio
References
BPS-MCM-41 BPS-MCM-41 BPS-MCM-41∗ Aerogel (dry graft) Aerogel – 300/95 Boiled SBA-15 PE-MCM-41 PE-MCM-41 SBA-15 PE-MCM-41 MSF SBA-15 Silica nanotubes PE-MCM-41 Meso. pore silica
6.69 7.56 7.81 4.13 7.4 5.8 7.95 7.8 5.21 4.33 4 3.68 4.93 7.9 5.18
25 25 25 25 25 60 25 70 45 50 75 60 25 25 25
0.3 0.05 0.3 1 1 0.15 0.05 0.05 1 1 0.15 0.15 1 1 1
2.39 1.74 2.64 1.64 2.61 1.8 2.65 2.28 1.75 2.43 1.3 2.41 2.23 2.75 1.74
0.36 0.23 0.34 0.40 0.35 0.31 0.33 0.29 0.34 0.56 0.33 0.65 0.45 0.35 0.34
Present study [20] Present study [14] [14] [22] [9] [23] [24,25] [26] [27] [28] [29] [30] [31]
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Table 3 run 2.
Fig. 5. Adsorption isotherm of CO2 on TRI-PE-MCM-41 and TRI-BPS-MCM-41 at 25 ˚C.
The reasonable results of present study were compared to the performance of TRI grafted sorbents in Table 5. The results of this table indicate that the adsorption capacity of TRI-BPS-MCM-41 is comparable to the reported sorbents which are prepared via aminosilane grafting. However, the optimized aminosilane to support ratio which was used in grafting procedure is among the lowest of the reported data. To have a better comparison between CO2 adsorption performance of TRI-PE-MCM-41 [32] and TRI-BPS-MCM-41, the adsorption isotherm of CO2 on both adsorbents is presented in Fig. 5. As it is shown in this figure, the adsorption isotherms are almost the same at low to moderate (0.3 bar) partial pressure; however, the TRI-PEMCM-41 shows a higher capacity at a higher pressure. It should be noted that the amount of aminosilane used to functionalize BPSMCM-41 is 50% of the amount of aminosilane used to functionalize TRI-PE-MCM-41. 4. Conclusion In this study, TRI grafting on BPS-MCM-41 was optimized and characterized by XRD, CHNS, and N2 adsorption/desorption techniques. The characterizations showed that both solid support and functionalized adsorbent have low structural order, and due to func-
tionalization, the most of the internal volume of BPS-MCM-41 was filled with TRI functional groups. The optimum condition for grafting TRI on BPS-MCM-41was found using Taguchi experimental design (T: 85 ˚C, Water to support ratio: 0.3 cc/g, and TRI to support ratio: 1.5 cc/g). The ANOVA showed that the amount of TRI used had the most remarkable effect on the response, while the least significant one was of the reaction temperature. The optimum condition, found for grafting BPS-MCM-41, is almost the same as the optimized condition which has been found for pore-expanded MCM-41 functionalization, with the only difference being the amount of TRI used. The result showed that CO2 adsorption capacity of TRI-BPS-MCM-41 is almost the same as capacity of TRI-PE-MCM-41 at low to moderate CO2 partial pressure proving that beside the surface area, the internal volume of solid support is also influential on amine loading and CO2 adsorption capacity. Acknowledgment The authors appreciate the financial support from NIGC (Grant No: 190141). References [1] Choi S, Drese JH, Jones CW. Adsorbent materials for carbon dioxide capture from large anthropogenic point sources. ChemSusChem 2009;2(9):796–854. [2] Ello AS, de Souza LKC, Trokourey A, Jaroniec M. Development of microporous carbons for CO2 capture by KOH activation of African palm shells. J CO2 Util 2013(0). [3] Leal O, Bolivar C, Ovalles C, Garcia JJ, Espidel Y. Reversible adsorption of carbon dioxide on amine surface-bonded silica gel. Inorg Chim Acta 1995;240(1–2):183– 9. [4] Chao K-J, Klinthong W, Tan C-S. CO2 Adsorption ability and thermal stability of amines supported on mesoporous silica SBA-15 and fumed silica. J Chin Chem Soc 2013 n/a-n/a. [5] De Canck E, Ascoop I, Sayari A, Van Der Voort P. Periodic mesoporous organosilicas functionalized with a wide variety of amines for CO2 adsorption. Phys Chem Chem Phys 2013;15(24):9792–9. [6] Loganathan S, Tikmani M, Ghoshal AK. Novel pore-expanded MCM-41 for CO2 capture: synthesis and characterization. Langmuir 2013;29(10):3491–9. [7] Belmabkhout Y, Sayari A. Effect of pore expansion and amine functionalization of mesoporous silica on CO2 adsorption over a wide range of conditions. Adsorption 2009;15(3):318–28. [8] Mizutani M, Yamada Y, Yano K. Pore-expansion of monodisperse mesoporous silica spheres by a novel surfactant exchange method. Chem Commun 2007;0(11):1172–4.
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Please cite this article as: M. Gholami et al., CO2 adsorption on amine functionalized MCM-41: Effect of bi-modal porous structure, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.07.021