Porous Materials: Synthesis, Characterizations, and ApplicationsView this Special Issue
Research Article | Open Access
Evaluation of Optimal Pore Size of (3-Aminopropyl)triethoxysilane Grafted MCM-41 for Improved CO2 Adsorption
An array of new MCM-41 with substantially larger average pore diameters was synthesized through adding 1,3,5-trimethylbenzene (TMB) as the swelling agent to explore the effect of pore size on final adsorbent properties. The pore expanded MCM-41 was also grafted with (3-Aminopropyl)triethoxysilane (APTES) to determine the optimal pore size for CO2 adsorption. The pore-expanded mesoporous MCM-41s showed relatively less structural regularity but significant increments of pore diameter (4.64 to 7.50 nm); the fraction of mesopore volume also illustrated an increase. The adsorption heat values were correlated with the order of the adsorption capacities for pore expanded MCM-41s. After amine functionalization, the adsorption capacities and heat values showed a significant increase. APTES-grafted pore-expanded MCM-41s depicted a high potential for CO2 capture regardless of the major drawback of the high energy required for regeneration.
Fossil fuels will remain in abundant provision well into the 21st century and contribute to the high standard of living enjoyed by the industrialized world. However, the environmental and economic threats caused by possible climate change which is commonly referred to as the greenhouse effect make their future uncertain . In order to reduce global warming and enjoy the benefits of fossil fuels, CO2 adsorption and separation in a cost-effective and environmentally friendly way is one of the most promising means [2, 3].
Several groups [4–9] proposed to immobilize amines onto the mesoporous materials to capture CO2. Mesoporous silica such as MCM , SBA [10, 11], MSU , silica foam , and KIT-6  types have attracted great attention of a large community of researchers for gas separation applications. They found that the CO2 adsorption rate and capacity are chiefly determined on not only amine content but also the support porosity. Moreover, the two factors are not thoroughly independent since supports with higher pore sizes could load more amines. Among many well-known mesoporous materials, MCM-41 offers many advantages in terms of rapid adsorption kinetics, easy-to-design pore structure, large surface area, and low energy required for regeneration [15–17]. Additionally, MCM-41 possesses a large amount of Si-OH on its surface which provides abundant reaction sites and Si in the framework of silica could be replaced by other atoms, which makes the modification easier .
Various methods have been developed for the synthesis of MCM-41 silica with larger pore sizes [19–22]. Conventional MCM-41 silica exhibits a typical pore size within the range of 3 to 4 nm. Corma et al.  obtained MCM-41 with the pore size of 6 nm to 7 nm after rising the synthesis temperature (408–448 K). Zhao et al.  also concluded that the temperature could affect the pore size. Wang and Kabe  modified the pH to increase the pore size from 3.83 nm to 5.27 nm. Expanders (e.g., alkanes , amines , and trimethylbenzene ) also have been used for the pore expansion of mesoporous materials. Wang and Yang  expanded the pore diameter of SBA-15 through adding TMB as swelling agent to 7.6 nm and after amine grafted, the pore diameter reduced to 5.3 nm. The CO2 adsorption capacity of APTES grafted SBA-15 reached 1.6 mmol/g. Loganathan et al. [19, 28] prepared pore expanded MCM-41 by varying the amount of ammonia (the required preparation materials). As the amount of ammonia decreased, the pore size and volume of MCM-41 increased. The CO2 capacity of pore expanded MCM-41 was as much as 1.2 mmol/g. However, valuable information obtained shows that pore expansion easily led to a decrease of the mesostructure order. When the pore size is larger than 5 nm, the nanostructures become unstable and the pore walls cannot support three-dimensional skeleton; it is likely to cause the structural collapse. The difficulty to modify the pores increases with the presence of swelling agent. Additionally, most papers only found that pore enlargement could enhance the CO2 adsorption performance [4, 22, 29, 30], but few work study the relationship between the pore size and CO2 adsorption behavior.
Therefore, evaluating the optimal pore size is of great importance to synthesis amine-functionalization adsorbents for improved CO2 adsorption. According to this, in this paper, a series of pore-expanded MCM-41s with increasing pore size were used as supports and APTES which contains three amine groups per organic chain was preferred as the grafted species to obtain superior capacity. Well-uniformed pore-expanded MCM-41s were examined; the effect of pore size on CO2 adsorption capacity, surface grafted, and adsorption heat was also investigated.
2. Materials and Method
The reagents of Tetraethylorthosilicate (TEOS) used for the synthesis of pore-expanded MCM41 was purchased from Tianjin Fu Chen Chemical Reagents Factory; the chemical formula is C8H20O4Si and the molar mass is 208.33 g/mol, AR. Hexadecyltrimethyl ammonium bromide (CTAB) with the molar mass of 364.446 g/mol was used as the template agent and the chemical formula is C19H42BrN. 1,3,5-trimethylbenzene (TMB) was chosen as the swelling agent and produced by Tianjin Guangfu Fine Chemical Research Institute, AR. Ethanol (95% ACS grade), toluene (99% ACS grade), and ammonia (25% ACS grade) were bought from Beijing Chemical Works. (3-Aminopropyl)triethoxysilane (APTES) obtained from J&K Scientific Ltd. was used as the amino silane coupling agent.
Figure 1 illustrates the procedure for the synthesis of pore-expanded MCM-41 and a schematic diagram of the amine grafting. The periodic mesoporous MCM-41 was prepared in the presence of CTAB and TEOS according to the procedure reported by Cai et al.  and was further expanded through adding swelling agent. MCM-41-r0, MCM-41-r2, MCM-41-r4, MCM-41-r6, and MCM-41-r8 were successfully synthesized and r means the molar ratio between swelling agent and template agent.
The five pore-expanded MCM-41s were used as supports and APTES was chosen as the amine base to synthesize the amine grafted MCM-41s. Toluene was added to the flask as the reaction solvent, and then excess APTES was added; thereafter, measured MCM-41 was put into the flask; the mixed solution was placed in a magnetic stirrer at 348 K for 20 hours. After cooling the solution to room temperature, the product was recovered by filtration on a Buchner funnel, washed with water, and dried in air at ambient temperature to acquire the amine-grafted materials. The materials were named as MCM-41-r-NH2 s.
2.2. Characterization and CO2 Adsorption Measurements
The crystal structures of all adsorbents were characterized by X-ray diffraction (XRD) on D8 ADVANCE (Bruker, German) diffractometer operating at 40 kV and 30 mA with CuKα radiation (0.1541 nm). The XRD diffraction patterns were taken in the 2θ range of 1.3°–10° at a scan speed of 0.5°·min−1.
Pore diameter, volume, and surface area of the samples synthesized were evaluated via N2 physical adsorption analysis through ASAP2020 (Micromeritics, USA) automatic adsorption system. The N2 adsorption data was recorded at the liquid N2 temperature (77 K). The surface area and the pore size distribution were calculated by the BET and BJH equations. The total pore volume was estimated from the amount of adsorbed N2 at the partial pressure = 0.99.
Thermal gravimetric analysis (TA Instruments, USA) was applied to determine the amount of amine loading and the thermal stability of adsorbent. From a series of experiments, it was determined to treat the materials at 373 K for a period of 60 min in N2 to remove all the water and CO2 adsorbed from the air. After the initial heat treatment, a heating rate of 10 K·min−1 up to 973 K in N2 was conducted to calculate the real amine impregnated on subtracts.
CO2 adsorption measurements were also performed on thermal gravimetric analysis under atmosphere pressure. A mixture gas of 10% CO2 and 90% N2 was used as the model flue gas. In each run, the adsorbents were loaded into an alumina sample pan and afterward pretreated at 283 K for 1 hour in N2 to remove the adsorbed moisture, and then the adsorbents were cooled to the adsorption temperature of 308 K prior to their exposure to the mixture gas. The desorption run was conducted in a pure N2 flow at 373 K to achieve complete desorption.
3. Results and Discussion
3.1. Material Characterization
The Nitrogen adsorption isotherms observed from Figure 2 of MCM-41-r0 and MCM-41-r2 exhibit a typical type IV adsorption-desorption isotherm according to the IUPAC classification, which demonstrates the characteristic of mesoporous materials. The capillary condensation in mesopores of MCM-41-r2 in mesopores occurred at a higher relative pressure due to their larger pore diameter compared with conventional MCM-41. The pore diameter of MCM-41-r2 increased significantly to 5.12 nm. The pore volume of MCM-41-r2 was around 1.00 cm3, similar to the value of MCM-41-r0. The pore-expanded MCM-41-r2 showed comparably lower pore volume of 771 m2/g but the value is not far from that of conventional MCM-41 (865 m2/g). The pore size of MCM-41-r2 increased significantly but the mesostructure order decreased as a result of pore expansion. In order to continuously enlarge the pore size of MCM-41, more swelling agent was required to add into the mixture; however, excessive swelling agent may destroy the mesostructure.
Due to a slight loss of the mesostructure during the pore-expansion process, the intensity of diffraction peaks of MCM-41-r2 was lower than that of MCM-41-r0  as demonstrated in Figure 3 which showed the XRD patterns of MCM-41-r0 and MCM-41-r2. The highest peaks observed shifted to lower 2θ from 1.9503° to 1.8124°, and the smaller 2θ related to the large cell parameters.
As shown in Figure 3, all the pore-expanded MCM-41s were studied to explore the effect of swelling agent on structure characteristics. In the process of pore-expanding, when increasing the amount of TMB, TMB molecules congregate to form “big oil particle” and TMB added to the reaction gel can be solubilized inside the hydrophobic regions of micelles. The hydrophobic solvate interaction of the aromatic molecule with the hydrocarbon tails is analogous to the hydrophilic solvate interaction of water with the charged head groups of CTAB. This caused an increase in the micelle diameter and an enlargement of the pore size. The TMB between the surfactant molecules of the micelles can interact with the ammonium cations of the surfactants through its π-electrons. It caused an additional increase in the micelles and consequently an enlargement of the pore size . It is clear that the peaks between 1° and 2° can still be observed when the ratios of TMB/CTAB are less than 8; however, the intensity of the peaks decreased greatly. The pattern had a clear (100) peak at 1.9503°; the sample prepared using a TMB/CTAB ratio of 2 exhibited a d(100) spacing of 5.12 nm compared with 4.64 nm in the absence of TMB. No peak was detected for samples synthesized with the TMB/CTAB ratio of 8, indicating that a disordered pore structure was obtained. From inspection of these XRD patterns, a conclusion is drawn that the structural ordering of the obtained materials decreases as the ratio of TMB/CTAB increases.
Here, = , is the adsorbent constant, β is the affinity coefficient, is the amount of the liquid adsorbate at relative pressure of , and is the micropore volume. The results are shown in Table 1, with the increase of the molar ratio between swelling agent and template agent; both mesopore volume and percentages of mesopore volume were increasing which proved that the pore expanded mesoporous MCM-41s were synthesized successfully.
|: surface area calculated using BET equation.|
: total pore volume estimated at relative pressure.
: micropore volume is determined from the Dubinin-Radushkevich equation.
: subtraction of micropore volume from total volume.
: fraction of mesopore volume.
: the average pore diameter.
Table 1 also summarizes the textural properties calculated from N2 adsorption isotherms. As the swelling agent increased, the pore sizes rose slightly from 4.64 nm to 7.50 nm. In the process of crystallization reaction, the TMB molecules entered the surfactant micelles, and, consequently, the pore sizes were expanded. The increase of TMB resulted in the concomitant increase of pore size, but when the amount increased consecutively, the dissolution degrees were inconsistent in the micelle due to the effect of hydrophobic; the structural order of the hexagonal mesophase would be destroyed  and foam-like structures would be obtained . The destroyed mesostructure made the change of surface area and pore volume irregular. As a result, the surface area and pore volume showed no significant change. Although calcined MCM-41-r8 showed structural disordering, it exhibited relatively narrow pore size distribution. Therefore, it was suitable to immobilize more APTES inside the pores.
In a further step, MCM-41-r0, MCM-41-r2, MCM-41-r4, MCM-41-r6, and MCM-41-r8 were grafted with APTES to study the effect of amine base on structure. As shown in Figure 5, it depicts the XRD patterns of amine-grafting pore-expanded MCM-41s compared with the original pore-expanded MCM-41; the degree of order of APTES-grafted pore-expanded MCM-41s reduced significantly as a result of grafting. It related to the presence of organosilane molecules within the pores of pore-expanded MCM-41 . For MCM-41-r4-NH2, MCM-41-r6-NH2, and MCM-41-r8-NH2, the mesostructure was not maintained. However, the influence of amine fictionalization and pore size could be concluded by the trend.
3.2.1. Influence of Pore Size on Adsorption
Figure 6 shows the CO2 adsorption capacity and the adsorption rate on MCM-41-r0 at 308 K under the mixture gas of 10% CO2/90% N2 at a flow rate of 50 mL/min. Pure Nitrogen flowed through the surface of silica in the first 336 s, giving the background absorbance. Then, 10% of CO2 in Nitrogen was substituted in the flow and adsorbed on the silica surface. The corresponding absorbance increased rapidly to reach a steady state. The maximum capacity of MCM-41-r0 calculated from the figure was 0.27 mmol/g and the largest adsorption rate was 0.30%/min.
Table 2 demonstrates the adsorption capacities and rates of the MCM-41-r samples. As seen from the table, the CO2 adsorption capacity and adsorption rate have been significantly improved with the larger pore size when . In this region, MCM-41-r4 showed the largest adsorption capacity and adsorption rate of 0.44 mmol/g and 2.45%/min, respectively, even when the surface area was the smallest, and they were 1.7 and 7.8 times more than the values of MCM-41-r0. The reasons for this apparent discrepancy were that the degree of order and crystallinity still remained high when although they showed the larger pore sizes. Physical adsorption was enhanced with increasing pore size . However, for MCM-41-r6, the physical adsorption capacity and rate reduced because of the destroyed mesostructure. For MCM-41-r8, the CO2 adsorption capacity and adsorption rate illustrated apparent increases to 0.48 mmol/g and 3.93%/min separately. MCM-41-r8 showed the highest CO2 adsorption capacity and rate, but the mesostructure was totally destroyed; the pore size and characteristics were not uniform; the adsorption mechanism was difficult to distinguish and required further research.
The adsorption heat values of pore-expanded MCM-41 samples are illustrated in Figure 7 and the results are summarized in Table 2. The corresponding values obtained from the thermodynamic analysis increased in the order of MCM-41-r0, MCM-41-r6, MCM-41-r2, MCM-41-r4, and MCM-41-r8 which are correlating with the order of the adsorption capacity; this was also consistent with a higher interaction potential between the adsorbate and adsorbent molecules with larger pore size. MCM-41-r8 showed the highest value of −26.33 kJ/mol. All the values were in the range of −27 to −15 kJ/mol, and it demonstrated that the processes belong to physical adsorption. Weak van der Walls force contributed to the adsorption capacity. The negative values of adsorption heat illustrated that the reaction was exothermic between the adsorbent and adsorbate interface.
3.2.2. Influence of Amine Functionalized to Adsorption
MCM-41-r-NH2 s were used to study the adsorption performance and calculate the adsorption heat; the adsorbents were kept under the condition of 393 K in the pure Nitrogen for 2 h; after that, switch the gas to 10% CO2/90% N2 for 5 hours to make sure there was no change of the weight.
Figure 8 represents the adsorption and adsorption rate of APTES grafted pore-expanded MCM-41. The maximum capacity calculated from the figure was 1.16 mmol/g and the adsorption heat was −85.79 kJ/mol. The adsorption capacity, adsorption rate, and adsorption heat of other APTES-grafted MCM-41-r samples are exhibited in Table 3. The adsorption capacities of APTES grafted MCM-41 increased significantly due to the amine groups which could react with CO2 molecules. Compared with the original CO2 adsorption capacity of 0.67 mmol/kg, the sample could absorb more CO2. The adsorption heat values were more than 40 kJ/mol, which indicates the chemical adsorption.
For all amine-grafted materials, APTES grafted pore-expanded MCM-41s exhibited much higher adsorption capacities and adsorption rates than the APTES grafted conventional MCM-41. These data also implied that the pore-expanded MCM-41 material can be grafted with a slightly higher quantity of APTES than the conventional MCM-41. This behavior was largely due to high surface density and the possible aminosilane polymerization at the pore openings. Furthermore, the dynamic adsorption performance of APTES grafted pore-expanded MCM-41s was far superior to APTES grafted MCM-41. The plateau of max adsorption rate (MCM-41-r2-NH2, MCM-41-r4-NH2, and MCM-41-r6-NH2) may be related to the mobility of the CO2 within the pore, caused by the openness of the pore.
Figure 9 and Table 3 depict the adsorption heat of APTES grafted pore-expanded MCM-41; the trend is similar to the adsorption heat of pore expanded MCM-41. From these data, it was apparent that the pore-expanded MCM-41s were capable of grafting more amine but with negative effects of more energy consumption to break the bond between aminosilane and CO2 molecules [35, 36].
The relationship between the fraction of mesopore/micropore volume and the CO2 adsorption capacity of all samples is represented in Figure 10. The CO2 adsorption capacities were increased gradually with the fraction of mesopore volume, whereas the CO2 adsorption capacities of r6 samples were exceptions. The CO2 adsorption capacities likely depend on the fraction of mesopore volume and pore size rather than the surface area and pore volume. The presence of well-developed mesopore is essential for optimum packing of the CO2 molecules at room temperature and pressure. Therefore, a strategy for well-developed mesopore structures and amine functionalization is essential to obtain superior performance for CO2 capture.
Pore-expanded MCM-41s were prepared by 1,3,5-trimethylbenzene as the swelling agent to develop promising large pore CO2 capture adsorbent materials. The design for the pore size was possible by controlling the molar ratio between the swelling agent and template agent. The highest CO2 adsorption capacity for MCM-41-r8 was 0.48 mmol/g which was 1.7 times more than the original MCM-41. In addition, MCM-41-r8-NH2 exhibited the highest adsorption capacity and adsorption rate of 1.16 mmol/g and 4.28%/min, respectively. The adsorption heat values for pore-expanded MCM-41 were in the range of −11.68 and −25.15 kJ/mol. It indicated that the reactions are exothermic and belong to physical adsorption. The adsorption heats of APETS-grafted MCM-41 and pore-expanded MCM-41 were larger than 40 kJ/mol which means chemical adsorption. More energy was needed in the regeneration process to break the bond between aminosilane and CO2 for APTES-grafted pore-expanded MCM-41s since they could not desorb all the CO2 molecules completely. In addition, the CO2 adsorption capacity was strongly affected not by surface area and pore volume but by pore size and mesopore volume fraction. Therefore, modified mesoporous MCM-41 materials with a suitable pore size for CO2 molecules are expected to be excellent solid adsorbents for achieving high CO2 adsorption capacity if regeneration energy requirements could be minimized.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors gratefully acknowledge the National Natural Science Foundation of China (91434120, 51172251), Shanxi Province Coal-based Key Scientific and Technological Project (MD2014-09, MD2014-03), and Fundamental Research Funds for the Central Universities (2014ZD06). Thanks are due to Dr. Kaixi Li for assistance with the experiments and valuable discussion.
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