Solubility in Hybrid Adsorbents - American Chemical Society

Article pubs.acs.org/JPCC

Enhanced CO2 Solubility in Hybrid Adsorbents: Optimization of Solid Support and Solvent Properties for CO2 Capture Ngoc Linh Ho,† Javier Perez-Pellitero,† Fabien Porcheron,*,† and Roland J.-M. Pellenq‡,§ †

IFP Energies nouvelles, Rond point de l’échangeur de Solaize, BP 3, 69360 Solaize France Centre Interdisciplinaire des Nanosciences de Marseille, CNRS, Campus de Luminy, 13288, Marseille, cedex 09 France § Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, 02139, Cambridge MA, USA ‡

S Supporting Information *

ABSTRACT: In this study, we optimize the CO2 adsorption performance of hybrid adsorbents prepared by confining physical solvents in porous solid supports. A number of prospective solid supports and physical solvents are chosen to prepare hybrid adsorbents, and are subsequently evaluated in CO2 adsorption experiments. Generally, all the hybrid adsorbents show an enhancement of CO2 solubility compared to the bulk physical solvent. However, not all the adsorbents positively display an improvement in the CO2 adsorption performance as compared with the original solids after confining the physical solvent into the solids’ pore. The micropore blocking effect is observed in the impregnated forms of zeolite, activated carbon, silicagel, and cecagel. Furthermore, we have obtained certain requisites for a good solid support, as efficient structures should be mesoporous with large surface area. In addition, there is an optimized solvent’s size to achieve an optimized enhanced solubility. As a result, among the candidates, N-methyl-2-pyrrolidone confined in MCM-41 and alumina are identified as the most suitable hybrid adsorbents for an effective CO2-removal application.

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INTRODUCTION The control of anthropogenic CO 2 emissions to the atmosphere has become a worldwide issue over the past few years as a direct correlation between greenhouse gas emissions and climate change is now commonly accepted. Carbon capture and storage (CCS) using amine scrubbing is one of the primary solutions developed to reduce these CO2 emissions.1−5 However, this process usually suffers from high energy requirement, corrosion and solvent degradation.3 Recently, an original solution combining adsorption/absorption process applied to CO2 capture has emerged in many studies.6−8 The idea is to combine the advantages of absorption (high selectivity and capacity) and adsorption (absence of corrosion, reduced solvent loss) properties to create an efficient generation of hybrid adsorbents, prepared by confining a solvent inside a porous solid support. This technique is well regarded to have a high potential for capturing CO2 gas from a bulk flue gas. However, a challenge for this approach is to identify a suitable sorbent displaying high selectivity and CO2 adsorption capacity. Nowadays, a large number of studies have focused on synthesizing a new generation of hybrid adsorbents for capturing CO2 by confining an amine molecule within a solid support.9−12 In most of these works, the authors expected to have a strong interaction between amines and CO2, following a © 2012 American Chemical Society

similar mechanism as conventional absorption process. The advantages are that solids are simple to handle and do not give rise to corrosion problems. On the other hand, few works have investigated the use of physical solvents as these molecules usually display very low CO2 solubility in their bulk form.13 In our previous study, we have investigated the existence and the fundamentals mechanisms underlying the apparition of enhanced CO2 solubility in hybrid materials.14,15 We have selected physical solvents, as the most dramatic effects are expected for this kind of solvents which usually exhibit low CO2 capacities at low partial pressures. In addition, we expect to observe the most appreciable effects for this kind of solvents after reducing the effective volume of the solvent down to nanometric scale by confining it into a porous solid. In fact, this method induces substantial modifications of the resulting fluid structure and dynamic and physical properties compared to the bulk ones.16−19 According to the results, the N-methyl-2pyrrolidone confined in alumina adsorbent positively exhibited an enhanced CO2 solubility as compared with the raw solid and the bulk solvent. In order to gain a deeper insight, Grand Canonical Monte Carlo simulations were performed to Received: October 17, 2011 Revised: January 9, 2012 Published: January 12, 2012 3600

dx.doi.org/10.1021/jp2099625 | J. Phys. Chem. C 2012, 116, 3600−3607

The Journal of Physical Chemistry C

Article

Merck), octamethylcyclotetrasiloxane (OMCTS, Merck), triethyl phosphate (TEP, Merck), 1,3-dimethyl-3,4,5,6, tetrahydro-2-pirimidinone (DMPU, Sigma-Aldrich), 1,3,5-trimethylhexahydro-1.3.5-triazine (TMTZ, Sigma-Aldrich), 1,3-dimethyl2 imidazolidinone (DMI, Sigma-Aldrich), and 1,2-difluorobenzene (DFB, Sigma-Aldrich), are used as the impregnated agents. Hybrid Adsorbents Preparation. The hybrid adsorbents are prepared by confining a solvent on a solid support with a full pore loading using the “wet impregnation” method.20 The adsorbent is dehydrated at 350 °C for 2 h to drive out water and trapped gas. A concentration of the physical solvent corresponding to a targeted percentage of the pore volume of the solid support is mixed with 5-fold mass of ethanol. The adsorbent is then slowly wetted with the above solution until completely submerged. After that, the mixture of solvent/ ethanol/solid is placed in a rotary evaporator flask and ethanol is removed by heating the flask in a 50 °C oil bath. The mixture is rotated during the evaporation step to achieve a homogeneous dispersion of the solvent into the pore of the support solid. Finally, the obtained hybrid adsorbent is exposed to dry air overnight to remove excess solution and entrapped gas. Solubility Measurements. Figure 1 presents a schematic diagram of the experimental apparatus used for measuring CO2

interpret the CO2 solubility behavior in a modeled system of hybrid adsorbent. As a result, the microscopic mechanisms underlying the apparition of enhanced solubility were clearly identified as the presence of solvent molecules favors the layering of CO2 within the pore. We found that, to achieve a maximized CO2 adsorption capacity, an ideal hybrid system should have a weak solvent-solid interaction but a strong solvent-CO2 affinity. Moreover, we showed that the size of the confined solvent has a large influence on the adsorption performance as a maximum CO2 solubility was attained for an optimal size in the hybrid system. According to the simulation results, the solvent layers build pseudo-micropores inside the mesoporous solid, enabling more CO2 molecules to be absorbed under greater influence of spatial confinement and surface interaction. As early stated, evidence of enhanced solubility in hybrid adsorbents were presented both in our experiments and simulations. However, a good hybrid adsorbent requires an appropriate selection of both the support and the physical solvent, aiming to create a good dispersion and distribution of guest solvent molecules on the porous support, yielding a high CO2 selectivity and adsorption capacity. Therefore, in this study, a set of different solid supports with a wide range of porosity and different physical solvent types are screened in order to optimize the efficiency of hybrid adsorbents for CO2 capture.

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EXPERIMENTAL SECTION Materials. Commercial mesoporous alumina (Axens/ Procatalyse), activated carbon (Ceca), cecagel (Ceca), silicagel (Crosfield), and 13-X zeolite (Ceca), are chosen as impregnated supports in our study. Furthermore, we also pursue the use of “model” adsorbents like cylindrical-pore MCM-41 and SBA-15. Since these solids are not commercially available, we operate a synthesis following the subsequent procedure. Mesoporous MCM-41 is synthesized by using ionic surfactant, hexa-decyltrimethylammonium bromide (CTAMBr), as a templating agent. Initially, 5.1 g of CTAMBr are dissolved in 133 mL of water and 144.35 g of Ammoniac (NH4OH, 25%), stirred at ambient temperature for 10 min. Subsequently, 22.32 g of tetraethyl orthosilicate (TEOS) are fast added. The mixture is gradually heated to 35 °C and held at that set point for 48 h under stirring. The adsorbents are then recovered by filtration and washed with 600 mL of deionized water, then with 200 mL of ethanol. Finally, these sorbents are dried at 100 °C overnight and calcinated at 550 °C for 4 h. Structural characterizations of the MCM-41 adsorbents are reported in Figure S1 of the Supporting Information. Mesoporous SBA-15 is synthesized by using a triblock copolymer, EO20-PO70-EO20 (Pluronic P123). Initially, 10 g of P123 is added to 380 mL of 1.6 M HCl, stirred at ambient temperature for 1 h. Afterward, 21.3 g of TEOS is added. The solution is aged for 24 h at 35 °C. The mixture is gradually heated to 100 °C and held at that set point for 24 h under stirring. The adsorbents are then recovered by filtration and washed with 600 mL of deionized water, then with 200 mL of ethanol. At the last stage, these sorbents are dried at 100 °C overnight and calcinated at 550 °C for 4 h. Structural characterizations of the SBA-15 adsorbents are reported in Figure S2 of the Supporting Information. A number of prospective physical solvents, propylene carbonate (PC, Merck), N-methyl-2-pyrrolidone (NMP,

Figure 1. Schematic of the experimental device used to measure CO2 adsorption isotherms.

solubility. The adsorption experiment system, consisting of a reactor connected upstream to a gas reservoir, is integrated within an oven to regulate the operating temperature and is designed to operate at pressures ranging from vacuum up to 10 bar and at temperatures up to 120 °C. First, 5 g of adsorbent are loaded into the reactor cell. Before starting the experiments, the cell and the gas reservoir are vacuumed and the system is programmed to achieve a steady temperature of T = 40 °C. CO2 is then injected into the gas reservoir. The valve between the reactor and the reservoir is opened and remained that way during the adsorption time. To proceed to the next step of the isotherm, this valve is then closed and a small amount of CO2 is admitted to the reservoir. This procedure is repeated in a pressure domain ranging from 0 to 3 bar to determine the complete CO2 adsorption isotherm. Pressures and temperatures within the cell and the reservoir are recorded every 1 s. CO2 solubility in bulk solvents is obtained using a high throughput screening experimental device described elsewhere.5 More details on the preparation step, as well as the solubility measurements, can be referred to in our previous works.5,14,15 3601



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RESULTS AND DISCUSSION Screening of Solid Support. In this part, a number of prospective solid supports, described in Table 1 (Vμ, Vm, and Table 1. Surface Area and Volume Properties of the Raw Solid Supports Using N2 Sorption and Mercury Porosimetry adsorbent alumina cecagel silicagel activated carbon MCM-41 SBA-15 zeolite 13-X

BET surface area (m2/g)

Vμ (t-plot) (cm3/g)

Vm (B.J.H.) (cm3/g)

VM (cm3/ g)

%Vμ

278 700 671 1609

0.000 0.283 0.233 0.705

0.752 0.126 0.105 0.677

0.009 0.003 0.032 0.800

0 69 63 32

1013 697 585

0.000 0.031 0.235

0.793 0.868 0.000

0.007 0.062 0.065

0 3 78

Figure 3. CO2 adsorption isotherms at T = 40 °C for the 100%-NMPalumina solid (Δ), -MCM-41 (◊), -SBA-15 (□), -silicagel (▲), -cecagel (●), -activated carbon (■), and -13-X zeolite (⧫) hybrid adsorbents.

VM are the microporous, mesoporous, and macroporous volumes, respectively), are chosen to prepare hybrid adsorbents by confining NMP solvent with a full pore occupation. CO2 adsorption isotherms are obtained at T = 40 °C and pressures ranging from 0 to 3 bar. The results are presented in Figure 2

With respect to unmodified solids, as can be clearly observed from the results, not only does 13-X zeolite exhibit the highest capacity but it also possesses the strongest strength of adsorption as indicated by the largest initial slope of the adsorption isotherm. At PCO2 = 1 bar, the CO2 adsorption capacity reaches values as high as 110 mg of CO2 per gram of zeolite. From a structural point of view, 13-X zeolite contains Na+ cations which are distributed preferably on sites (SIII) located inside α-cages. Hence, the main quadrupole interaction between CO2 and the Na+ cations is reinforced by the electrostatic effect and the close vicinity between CO2 and Na+ cations, reasonably explaining the remarkable CO2 adsorption capacity of 13-X zeolite. The raw activated carbon also exhibits considerable adsorption capacity. This is possibly due to its large, accessible internal surface, and large pore volume, facilitating a high loading of CO2 nonpolar molecules. Regarding other original solids, alumina and silica-based adsorbents (cecagel, silicagel, SBA-15, and MCM-41) only display moderate CO2 adsorption capacities. A direct link can be clearly identified between the trend of CO2 adsorption capacity and the solid’s BET surface area. Yet, it is necessary to note that, in spite of its high surface area (1013 m2/g), the mesoporous MCM-41 solid presents a rather poor CO2 adsorption capacity in the pressure range conditions sampled in this work. This exception can be explained by a weaker CO2MCM-41 interaction characterized from the lower initial slope of the CO2 adsorption isotherm. When considering hybrid adsorbents, we observe that only few supports positively display an enhancement of CO2 removal performance after confining the physical solvent into the pores of the solid. A markedly adverse effect could be observed in the impregnated forms of zeolite, activated carbon, silicagel, and cecagel. This phenomenon can be correlated to the decreasing effective volume of micropores in these solid structures. In fact, the blockage of micropores, a key factor governing the physisorption of CO2, by solvent molecules, may to a great extent prevent CO2 molecules to access the entire porosity of the solid material. As clearly illustrated in Figure 4 and Table 1, when the percentage of microporous volume increases, the CO2 adsorption capacity decreases similarly. An apparent evidence of this effect can be seen in the case of

Figure 2. CO2 adsorption isotherms at T = 40 °C for the raw alumina solid (Δ), the raw MCM-41 (◊), the raw SBA-15 (□), the raw silicagel (▲), the raw cecagel (●), the raw activated carbon (■), and the raw 13-X zeolite (⧫).

for raw adsorbents and in Figure 3 for hybrid adsorbents. CO2 solubility is usually expressed as gram of CO2 per gram of solid. However, when comparing such different systems (i.e., raw solid support, bulk solvent, hybrid adsorbent) it is more useful to express the solubility in terms of gram of CO2 per cm3 of porous volume.14 The corresponding quantity is obtained for the case of the bulk solvent by calculating the mass of CO2 absorbed per cm3 of liquid. The CO2 adsorption capacity of the original solids increases with the order of alumina < MCM-41 < silicagel ≈ SBA-15 < cecagel < activated carbon < 13-X zeolite. However, after NMP impregnation, a totally different picture emerges as CO2 adsorption capacity at low pressure range (0−0.5 bar) follows 13-X zeolite < silicagel ≈ cecagel < activated carbon < MCM41 ≈ SBA-15 < alumina. In the case of pressures above P = 1 bar, CO2 adsorption capacities of MCM-41 and SBA-15 remarkably surpasses the one of alumina. 3602



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Figure 4. Comparison between CO2 adsorption on the NMP-hybrid adsorbent (■), the raw adsorbent (▲) and the bulk solvent (○) using (a) activated carbon support, (b) silicagel support, (c) cecagel support, (d) 13-X zeolite support.

slighter increase in CO2 removal capacity is observed as compared with the raw support. It must be noted that the adsorption isotherms of SBA-15 and MCM-41 follow the same trend, implying similar solid−fluid interaction strength. Thus, the reason behind the adverse effect of its adsorption performance should come from the existence of micropores in SBA-15. As discussed earlier, this phenomenon occurs due to the intrusion of solvent molecules into the pores of SBA-15, blocking the micropores on the surface of the walls. The CO2 adsorption performance of various mesoporous adsorbents are compared in Figure 6. According to these results, it is interesting to see that the slope of the adsorption isotherm for hybrid alumina appears to be much steeper than that of hybrid MCM-41, especially in the low pressure range (Figure 5b). It suggests a higher strength of adsorption corresponding well with a stronger interaction between CO2 molecules and the surface of hybrid alumina. However, hybrid MCM-41 exhibits the most appreciable enhancement of CO2 solubility when the pressure increases. This behavior is directly correlated to the large surface area of MCM-41 which permits the adsorption of a greater amount of CO2 on the solid surface. Appropriate solid supports should therefore possess large surface areas and be strictly mesoporous as the presence of micropores induces pore blocking effect while the existence of macropores does not affect the bulk solvent structure.

zeolite, where the highest CO2 adsorption capacity achieved in its raw form has dramatically dropped to the lowest as compared with other adsorbents after impregnation, totally matching with the presence of a microporous structure and the smaller average pore diameter of zeolite. Still, in comparison with the bulk NMP solvent, the adsorption capacity of these hybrid adsorbents appears to be slightly better. Furthermore, for activated carbon, the shape of the adsorption isotherm in the hybrid adsorbent well resembles the one in the bulk fluid thereby indicating that absorption mechanism now prevails in the system. It can be explained from the fact that activated carbon contains large amount of macroporous volume (0.8 cm3/g), causing the solvent confined in this region to behave mostly as the bulk fluid. For this reason the efficiency of the hybrid adsorbent does not vary appreciably from the bulk one. On the contrary, solids possessing mainly mesoporosity in their textural properties, for example, alumina, SBA-15, and MCM-41, appear to be appropriate candidates for impregnation (see Figure 5). Among these mesoporous solids, SBA-15 adsorbent possesses the largest specific pore volume of 0.995 cm3/g, with a small amount of micropores volume within its structure (0.031 cm3/g), resulting in a higher CO2 adsorption capacity in the raw form. As seen in Figure 6, the CO2 adsorption capacity at PCO2 = 1 bar of SBA-15 is 1.05 and 1.90 times higher than MCM-41 and alumina support, respectively. Nevertheless, in its impregnated form, only a 3603



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Figure 5. Comparison between CO2 adsorption on the NMP-hybrid adsorbent (■), the raw adsorbent (▲), and the bulk solvent (○) using (a) MCM-41 support, (b) alumina support, and (c) SBA-15 support.

capacity in their raw form. Therefore, the experimental results suggest that the enhancement of CO2 solubility would be maximized for solids displaying weak adsorption capacity in their unmodified form. As obtained from our CO2 adsorption experiment, MCM-41 and alumina are the most suitable supports for impregnation. The synthesized hybrid MCM-41 and alumina adsorbents do not only show a significant improvement in the adsorption performance over the raw solid, but also display a markedly enhanced CO2 solubility compared to that of bulk solvent. We will now confine a set of different solvents in MCM-41 and alumina to study the effect of the physical solvent on the hybrid adsorbent properties. Screening of Physical Solvents. A number of prospective physical solvents, as listed in Table 2, are fully loaded into the mesoporosity of MCM-41 and alumina using wet impregnation method. The spherical equivalent molecular sizes of these solvents are computed from optimized geometries obtained using Material studio software.21 Initially, geometrical optimizations are performed at the level of Density Functional Theory (DFT) for different solvent molecules. The Becke Lee−Yang−Parr (BLYP) functional is then used in combination with triple numerical polarized (TNP) basis sets. All calculations in this stage are conducted with the DMol3 package integrated in the MS suite. The performance of these hybrid adsorbents is subsequently evaluated in terms of CO2

Figure 6. CO2 solubility in various mesoporous supports (blank), mesoporous hybrid adsorbents (barrier) and bulk solvent (horizontal stripes) at PCO2 = 1 bar and T = 40 °C.

Furthermore, the enhanced solubility effect is to a greater extent noticeable for adsorbents originally possessing the lowest adsorption capacity, such as alumina or MCM-41. On the other hand, solvent impregnation appears to be of little help for solids which are intrinsically good adsorbents, for instance zeolite and activated carbon which have shown satisfactory CO2 adsorption 3604



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samples very close to the one of nonimpregnated MCM-41. However, at moderate and higher pressure, the sequence in terms of CO2 removal capacity follows OMCTS < PC ∼ TEP < TMTZ < DMPU < DMI < DFB < NMP. It can be noticed that the CO2 solubility has increased with respect to all the solvents but that only NMP proves its best efficiency after impregnation as 100%-NMP-MCM-41 exhibits a positive improvement in the adsorption capacity over the raw MCM-41 support. To further understand the underlying mechanism, it is necessary to have a better perspective on the interaction between the basic surface of adsorbents and the two types of adsorbed molecules: solvent and CO2. There are two possible factors which can explain the synergetic effect of hybrid MCM41, one is the high surface area with uniform mesoporous channels of MCM-41 and the other is the impact of the confined solvent. The results showing the effect of various solvent sizes on CO2 solubility at a partial pressure of CO2 ranging from 0 to 3 bar are presented in Figure 8.

Table 2. Properties of Physical Solvents Used in This Work

adsorption isotherms at T = 40 °C and PCO2 ranging from 0 to 3 bar. Figure 7 summarizes the results of CO2 adsorption isotherms in various hybrid MCM-41 loaded with different physical

Figure 8. CO2 solubility in hybrid MCM-41 with different solvent sizes at PCO2 = 3 bar (■), PCO2 = 2 bar (◊), PCO2 = 1.5 bar (▲), PCO2 = 1 bar (○), and PCO2 = 0.1 bar (⧫).

As we can see, a similar CO2 adsorption capacity at PCO2 = 0.1 bar is observed, regardless of the confined solvent. It suggests that at low CO2 coverage, all CO2 molecules tend to adsorb on the solid surface as a result of the homogeneous structure of MCM-41 and the strong enthalpic interactions with the active sites in the narrow pores. In this case, the effect of confined solvent is negligible because of limited admission of CO2 molecules into the pore. On the other hand, by increasing PCO2, an optimum confined solvent’s size (i.e., NMP solvent), with which we can obtain a maximum CO2 solubility, is clearly observed. This phenomenon has also been identified and discussed in our previous work using MCM-41 hybrid adsorbent model.15 In fact, under confinement conditions, the adsorbed solvent molecules are accommodated, creating pseudo-micropores inside the pore of the solid support, resulting in several complex nanoscale phenomena of entropic, molecular sieving and confinement effects in the hybrid system. For instance, raw MCM-41 shows a capacity of 0.755 molecules-CO2/nm2, which is lower than the CO2 amount of molecules adsorbed in a complete monolayer (5 molecules-CO2 /nm2), hence these molecules will predominantly form a monolayer on the surface of raw MCM-41. For hybrid adsorbents, the CO2 molecules

Figure 7. CO2 adsorption isotherms at T = 40 °C for the MCM-41 solid (solid line), 100%-NMP- (■), -DFB- (□), -DMI- (⧫), -DMPU(◊), -TMTZ- (●), -TEP- (○), -PC- (▲), and -OMCTS-(Δ) -MCM41 hybrid adsorbents.

solvents. Different types of solvent exhibit different CO2 adsorption performances. At low pressure of CO2 (PCO2 < 0.1 bar), the influence of the confined solvent is not obvious, illustrated by the slopes of adsorption isotherms of all hybrid 3605



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coordination/orientation constraint during the adsorption process could probably be a reason for higher adsorption coverage. Figure 9 illustrates possible packing arrangements in

Figure 10. CO2 solubility in hybrid alumina with different solvent sizes at PCO2=3 bar (■), PCO2=2 bar (◊), PCO2=1.5 bar (▲), PCO2=1 bar (○), and PCO2=0.1 bar (⧫).

Figure 9. Schematic of possible packing arrangement of CO2 in the hybrid pore. Color code: purple, solvent; gray, CO2.

the hybrid pore. The first hypothesis consists in the continuous adsorption of CO2 molecules at the solid surface to complete the monolayer or possibly forming multilayer (Figure 9a). The second hypothesis relies on the strong competition ability upon adsorption between solid surface and pseudo-micropores created by solvent molecules (Figure 9b). This latest hypothesis is further supported by our calculated isosteric heat of adsorption (ΔH) on hybrid adsorbent.14 In fact, we showed that ΔH value decreases rapidly with increasing CO2 coverage, suggesting that CO2 molecules are more likely to be bounded by solvent molecules in the hybrid system for high loadings. Nevertheless, it should be noted that not all the additional layers of different solvents can promote the enhancement of CO2 solubility in hybrid adsorbent. Because of the change of pseudo-micropore’s size caused by the different polarity and size of the solvents, the structure of fluid layers may be wellorganized in a fashion that favors the distribution of CO2 molecules. In some cases, an optimized structure may be obtained in the hybrid system in which the solvent property (i.e., polarity) is compatible with that of surface MCM-41, resulting in a strongest potential of confinement for adsorbing CO2 in hybrid adsorbent.15 It is also a possible explanation for the case of NMP solvent confined into MCM-41 yielding the highest adsorption capacity. In addition, the same phenomenon also appears when using the alumina support as shown in Figure 10 which confirms the important role of confined solvent’s size in the adsorption performance. Yet in this case, even at low PCO2, the optimum point of solvent’s size can be clearly observed. These results hence further validate the hypothesis of synergetic effect of hybrid adsorbent. Alumina displays a heterogeneous surface resulting from purely physical surface features, such as point defects, corner and edge locations on microcrystallites, or exposure of different crystal planes,22 therefore increasing the competition for adsorption sites. As a result, at the solid surface, there is a coverage transition of CO2 molecules between energetically heterogeneous active sites. At the same time, the solvent molecules forming pseudo-micropores might act as competing active sites for CO2 molecules. Consequently, subject to pseudo-micropores’ size, CO2 molecules are not only inclined to adsorb on the surface of alumina, but are also attracted to pack in the void created by solvent molecules. In other words, the effect of confined solvent’s size on the

adsorption of CO2 in hybrid alumina should be carefully taken into consideration, especially at low pressure range. Pore size distributions obtained from N2 adsorption/ desorption isotherms showed that the mean pore diameter of alumina (dp = 10 nm) is larger than the one of MCM-41 (dp = 3.4 nm). However, the trends of CO2 adsorption performance varying with different confined solvents are similar. These results imply that the effect of solid support’s pore size is less dominant than the confined solvent’s size.

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CONCLUSIONS In this study, a screening test of solid supports is performed by impregnating NMP solvent in a number of prospective solids with a wide range of textural and structural properties (alumina, activated carbon, silicagel, cecagel, zeolite, SBA-15, and MCM41), and subsequently measuring CO2 adsorption isotherms in the synthesized hybrid adsorbents. The trend of CO 2 adsorption remarkably changes between the raw and the hybrid adsorbents. The hybrid MCM-41 replaces zeolite as the strongest adsorbent in their impregnated form. Regarding these hybrid adsorbents, interestingly not all of them positively display an enhancement of CO2 removal performance after confining the physical solvent within the solid porosity. An adverse effect could be observed in the impregnated forms of zeolite, activated carbon, silicagel, and cecagel. This effect may be correlated to the presence of micropores in these solid structures, or more precisely to the blocking effect of micropores by solvent molecules. On the other hand, solids showing mostly mesoporosity in their textural properties, for example, alumina, MCM-41, and SBA-15, appear to be appropriate supports for preparing efficient hybrid adsorbents in capturing CO2. In addition, in comparison with the bulk solvent, an enhanced solubility is obtained in all the hybrid samples. In conclusion, according to the experiments and analysis, the optimized solid supports can be obtained from those containing solely mesopores and large surface area in their structure. Among these ideal candidates, MCM-41 is proven to be the most suitable support for an effective CO2removal hybrid adsorbent. Subsequently, a screening of physical solvents is performed by confining a set of eight different molecular structures (PC, NMP, OMCTS, TEP, DMPU, TMTZ, DMI, and DFB) within MCM-41 and alumina. An optimum confined solvent’s size 3606



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(i.e., NMP solvent), with which we can obtain a maximum CO2 solubility, is clearly observed. Furthermore, this optimum point would yield the highest differential CO2 adsorption between the hybrid adsorbent and the bulk solvent. The same trends of adsorption performance varying with different confined solvent types are found for alumina and MCM-41, implying that the effect of solid support’s size and surface heterogeneity are less dominant than the confined solvent’s size. In conclusions, our studies have highlighted the appropriate properties of the solid support, as well as the ideal confined solvents for preparing efficient CO2-capturing hybrid adsorbents in terms of adsorption capacity. Work is in progress to conduct a process simulation in order to fully evaluate their application on an industrial scale.

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ASSOCIATED CONTENT

S Supporting Information *

XRD powder diffraction patterns. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

REFERENCES

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Solubility in Hybrid Adsorbents - American Chemical Society

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