N2 Separation

Oct 12, 2010 - Jennifer J. Williams,† Andrew D. Wiersum,‡,† Nigel A. Seaton,§ and Tina Düren*,†. Institute for Materials and Processes, School of ...
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J. Phys. Chem. C 2010, 114, 18538–18547

Effect of Surface Group Functionalization on the CO2/N2 Separation Properties of MCM-41: A Grand-Canonical Monte Carlo Simulation Study Jennifer J. Williams,† Andrew D. Wiersum,‡,† Nigel A. Seaton,§ and Tina Du¨ren*,† Institute for Materials and Processes, School of Engineering, The UniVersity of Edinburgh, Edinburgh EH9 3JL, United Kingdom, and UniVersity of Surrey, Guildford, Surrey GU2 7XH, United Kingdom ReceiVed: June 14, 2010; ReVised Manuscript ReceiVed: September 13, 2010

Mesoporous adsorbents such as MCM-41 are promising materials for use in the separation stage of carbon capture and storage (CCS). Using molecular simulation, we investigated the effectiveness of functionalizing MCM-41 with organic surface groups for CO2/N2 separation from a typical power-plant flue-gas stream. The adsorption isotherms for pure CO2 and ternary flue-gas mixtures were determined in functionalized MCM-41 using grand-canonical Monte Carlo (GCMC) simulations. Both CO2 uptake capacities and CO2/N2 selectivities were studied in order to predict the performance of functionalized materials in the removal of CO2 from flue gases. The effects of pore size, surface group concentration, alkyl chain length, surface group rigidity, and concentration of polar amino moieties on adsorption were studied in detail. Surface functionalization of MCM41 resulted in materials exhibiting a wide range of CO2/N2 selectivities. Insight from these simulations allowed for the design of a tailor-made surface group encompassing favorable characteristics that was predicted to perform significantly better than the unfunctionalized MCM-41 material in carbon-capture applications. Introduction Recent predictions on climate change and the threat of global warming have stimulated scientific research into finding means of decreasing CO2 emissions into the atmosphere. Much attention has focused on capturing CO2 at large point sources such as carbon-based power stations where large quantities of CO2 are emitted in flue-gas streams. Here, the main challenge lies in separating CO2 from N2. Existing technologies for fluegas separations include absorption, adsorption, and membranebased separations. Currently, absorption is widely used for this purpose, but it has the disadvantage that the amine-based solvents used are toxic, very reactive, and highly energy intensive to regenerate.1 Adsorption-based separations using pressure-swing adsorption (PSA) represent a viable and costeffective alternative because of the milder operating conditions and lower energy demand involved. However, the use of adsorption for CO2 sequestration is dependent on identifying adsorbents with high CO2 selectivity and capacity that are capable of separating CO2 from N2-rich streams to a high degree of purity. This has led to extensive research, both computational and experimental, into adsorbents including activated carbons,2 silica gel,3 zeolites,4-6 and metal-organic frameworks.7-9 This article concentrates on MCM-41, one of a family of porous materials, known as periodic mesoporous silicas (PMSs), discovered by the Mobil Oil Corporation in the early 1990s.10 PMSs are prepared from a silica source in a surfactant solution in which the surfactant molecules serve as a template for the porous material. The silica source polymerizes around the template, forming a stable silica framework that ultimately provides the pore walls, taking virtually any pore shape provided by the template. * Address correspondence to this author. E-mail: [email protected]. Tel.: (+44) 131 650 4856. Fax: (+44) 131 650 6551. † The University of Edinburgh. ‡ Current address: Laboratoire Chimie Provence, CNRS, Universite´ d’Aix-Marseille I, II et III, F-13397 Marseille Cedex 20, France. § University of Surrey.

MCM-41 presents one-dimensional channel-like pores arranged regularly in a hexagonal array, whereas on the atomic level, the silica structure is largely amorphous. By using different surfactants, the diameter of the pores can be controlled from 13 to 100 Å.11 The material has a high surface area, approximately 1000 m2/g, and a pore volume in the range of 0.8 cm3/g.12 In addition to the tunable pore size, the pore surface of MCM-41 (and other PMSs) can readily be modified by being lined with organic groups either by co-condensation of triethoxysiloxanes or, postsynthesis, by grafting of the desired triethoxysiloxane to the surface of the pure-silica material.13-15 The large surface area and uniform mesopores, as well as the ability to tailor the pore size and to functionalize the pore wall, therefore make MCM-41 an ideal candidate for the design of tailor-made adsorbents. This has led to extensive experimental research in this field.16-19 In particular, a number of experimental studies have reported the surface functionalization of silica materials with amine-bearing functional groups to improve CO2 adsorption properties: Leal et al.20 studied the uptake of CO2 on silica gel functionalized with 3-aminopropyl groups and reported that the material was capable of adsorbing 10 STP cm3 of dry CO2 per gram. Hicks et al.21 synthesized a covalently tethered hyperbranched aminosilica material that could reversibly bind CO2 from flue gas and had a capacity of 3.1 mmol of CO2 per gram of material at 25 °C. Knowles et al.22 grafted 3-aminopropyltrimethoxysilane onto a hexagonal mesoporous silica support and reported an adsorption capacity of 1.59 mmol/g from a 90% CO2/Ar gas feed at 20 °C. Huang et al.23 determined a CO2 adsorption capacity of 1.14 mmol/g for aminopropyl-grafted MCM-48 using a dry 5% CO2/N2 mixture. Bhagiyalakshmi et al.24 synthesized mesoporous MCM-41, MCM-48, and SBA15 and grafted the materials with 3-chloropropyl amine. The authors reported CO2 adsorption capacities of 1.7, 1.5, and 1.1 mmol/g for SBA-15, MCM-41, and MCM-48 functionalized materials, respectively, at 25 °C. Chang et al.25 studied CO2 adsorption in SBA-15 grafted with γ-aminopropyl surface groups and showed that the material could adsorb 9-18 mg of

10.1021/jp105464u  2010 American Chemical Society Published on Web 10/12/2010

Effect of MCM-41 Functionalization on CO2/N2 Separation CO2 per gram. Hiyoshi et al.26 studied the CO2 adsorption performance of various amines grafted onto SBA-15. They reported adsorption capacities of 0.52, 0.87, and 1.10 mmol/g for monoamino-, diamino-, and triamino-grafted SBA-15, respectively. Diamine-grafted SBA-15 was reported to show an enhanced adsorption capacity of 44 mg/g.27 Harlick et al.28,29 and Serna-Guerrero et al.30 grafted 3-[2-(2-aminoethylamino)ethylamino]propyl trimethoxysilane onto MCM-41 and poreexpanded MCM-41. They found that the CO2 adsorption capacity of the pore-expanded material was significantly higher than that of the conventional MCM-41 and reported an adsorption capacity of 2.65 mmol/g at 25 °C and 1.0 atm for the optimal functionalized, pore-expanded material. Recent experimental adsorption measurements on this triamine-functionalized pore-expanded MCM-4131 reported CO2/N2 molar selectivities of up to 300. This summary demonstrates the huge potential for MCM-41 materials in gas-separation applications when functionalized with an appropriate surface group. Experimental synthesis and functionalization, however, is laborintensive and usually focuses on the investigation of a small number of the vast range of possible functional groups. It is here that molecular simulations come into their own as a fast and inexpensive alternative to high-throughput experimental synthesis and characterization techniques. Molecular simulation is now recognized as a powerful tool in the design of new materials and is a technique particularly suited to the field of adsorbent design. In this work, we used molecular simulations to study a large number of possible functionalized MCM-41 materials and to relate how the structural features of surface groups influence their adsorption capabilities for carbon-capture applications. The pure-CO2 and ternary-mixture adsorption behaviors of CO2, N2, and O2, the major constituents of flue gases, were simulated to determine the effectiveness of MCM-41 in CO2 separation applications. Simulations of flue-gas mixtures, as opposed to pure gases, provide information not only on the CO2 capacity of a material but also on how selective a material is for one component of a mixture over the other(s). Selectivity is a crucial parameter in the design of new adsorbents for separation applications: A material might have a very high CO2 capacity, but it will be ineffective in separation applications if it does not adsorb CO2 selectively, that is, in preference to other gaseous components. Here, we study the adsorption of a ternary fluegas mixture typical of a dry stream originating from a coalfired power plant with a molar composition of CO2/N2/O2 ) 14:81:5.32 The CO2/N2 selectivity is given by

S(CO2 /N2) )

xCO2 /yCO2 xN2 /yN2

(1)

where xi is the mole fraction of component i in the adsorbed phase and yi is the mole fraction of component i in the bulk phase. With the insight gained from molecular simulations, we identify structural features of adsorbents that enhance CO2 separation capabilities and use this insight to design functional groups through a bottom-up approach. The most promising surface groups are identified as candidate materials for experimental synthesis. Simulation Method Creation of the Model Pores. The atomistic model pore used for grand-canonical Monte Carlo (GCMC) simulations was created using a kinetic Monte Carlo (kMC) simulation. The kMC

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Figure 1. A 2 × 2 × 2 unit cell of the 42-Å MCM-41 model pore. Key: yellow, Si; red, O; white, H.

technique mimics, in an atomistic simulation, the hydrothermal synthesis of siliceous MCM-41 and results in realistic representations of MCM-41, as has been shown previously.33 Although the full details of the kMC technique can be found elsewhere,33 the steps involved are as follows: (1) The synthesis starts with an aqueous solution containing the silicic precursors and a surfactant. (2) The surfactant forms rodlike micelles. These micelles arrange in a hexagonal array, and the silica framework is built around them through aggregation and polymerization of the silica monomers. (3) Once the network is formed, the surfactant is removed through calcination (a simulated rise in temperature), and the network is allowed to relax and adjust as the temperature of the simulation is gradually lowered. (4) The space that was previously occupied by the micelle becomes the pore, and the periodically replicated simulation volume assumes the shape of a parallelepiped. In this work, two atomistic model pores were used. The first pore, having a diameter of 42 Å and illustrated in Figure 1, is the same model pore as used in the work of Schumacher et al.33 The pore diameters cited in this work are the average geometric diameters of the MCM-41 pores obtained by performing a random walk along the pore wall with a probe molecule 3.3 Å in diameter. This pore is a good starting point for our simulations, as the agreement between the simulated adsorption properties of the model pore and experimental isotherms for the synthesized material have already been demonstrated.33 To investigate the effect of surface groups on CO2 adsorption in smaller pores, a second pore with a diameter of 19 Å was produced. This pore size is of interest as it is toward the lower limit of pore diameters with which MCM-41 can be synthesized and also represents the regime where the surface groups occupy a large fraction of the available pore space. In such a small pore, surface groups from opposite sides of the pore wall effectively bridge the pore space, creating an adsorbent that is microporous rather than mesoporous. The unit-cell lattice vectors of both model pores are given in Table S1 in the Supporting Information. To investigate the factors that influence the adsorption properties of functionalized MCM-41, the pore was modified by the introduction of organic surface groups. The procedure used for this purpose is as follows: Organic surface groups are introduced to the model pores generated by the kMC simulation by substituting the silanol (OH) groups on the surface of the pure-silica model with the desired organic functional group. Initially, the silanol groups to be substituted are chosen randomly, regardless of possible steric hindrance of the introduced surface groups. This generates some unrealistic surface group configurations of very high energy. To generate a more realistic distribution of surface groups, the positions of the surface groups are swapped in a Monte Carlo scheme. The

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Figure 2. Surface groups studied. Silicon atoms show attachment to the MCM-41 pore wall. [OH] ) unmodified silanol group, [ETH] ) ethyl, [BUT] ) butyl, [AM] ) amino, [AM_MET] ) methylamine, [AM_ET] ) ethylamine, [AM_PROP] ) propylamine, [AM_BUT] ) butylamine, [AM_PENT] ) pentylamine, [PH] ) phenyl, [PH_F] ) 4-fluorophenyl, [PH_CL] ) 4-chlorophenyl, [PH_BR] ) 4-bromophenyl, [PH_I] ) 4-iodophenyl, [PH_F2] ) 3,5-difluorophenyl, [PH_NH2] ) 3,5-diaminophenyl, [BIPH_NH2] ) 3,3′,5′,5-tetraamino-4,1′-biphenyl, [TERPH_NH2] ) 3,3′,3′′,5′′,5′,5-hexaamino-4,1′:4′,1′′-terphenyl, [BIPH] ) biphenyl, [TERPH] ) terphenyl.

swapping of groups is continued until the energy reaches a local minimum. At this point, the distribution of surface groups is such that most have low potential energy. The swapping of surface groups for silanol groups is limited to those silanol sites lining the pore wall. The scheme therefore simulates postsynthesis rather than in situ functionalization where surface groups can become entrapped within microcavities of the silica pore wall. The maximum achievable functionalization of surface groups is equal to the number of silanol groups lining the pore wall. In this work, the surface group loading is defined as the ratio of substituted OH groups to the total number of OH groups lining the pore wall, expressed as a percentage. Full details of the kMC technique used for functionalization of the pore wall can be found in refs 34 and 35. The unmodified MCM-41 material is itself selective for CO2, as the quadrupolar CO2 molecules are attracted to the polar pore wall containing Si-OH silanol groups. To increase adsorption of CO2, the surface groups introduced must be more attractive to CO2 than the pore wall itself. Our simulation study focused on surface groups bearing polar moieties such as amino and halo groups. Here, favorable Coloumbic interactions between the polar moieties and quadrupolar CO2 molecules are expected to increase the affinity of the adsorbent toward CO2. Figure 2 shows a summary of all surface groups investigated. GCMC Simulations. The Monte Carlo simulations were performed in the grand-canonical ensemble where the MCM-

Williams et al. 41 is in contact with a bulk-gas reservoir that fixes the temperature and the chemical potential of each component. All simulations were carried out at 298 K. An in-house GCMC code was used to calculate adsorption isotherms of pure CO2 as well as of ternary mixtures of CO2, N2 and O2. At each external gas pressure, a total of 1.5 × 107 Monte Carlo steps were used to generate the average number of gas molecules in the simulation cell. Full details of the GCMC technique used are given in ref 36. In our simulations, the MCM-41 structure was considered to be rigid, and the silicon and oxygen atoms of the silica structure were kept fixed throughout the GCMC runs. The surface groups consisted of either alkyl chains or aromatic rings bearing polar moieties anchored to the pore wall. The covalent bonds of the alkyl chains were fully flexible, allowing for bond bending and torsion. Chain molecules were handled using a dual-cutoff configurational-bias Monte Carlo method.37 Aromatic rings were assumed to be rigid, but C-C-Si and O-Si-C bonds were allowed to bend such that rings were free to rotate about the Si-C bond. For all surface groups, CH2 groups were represented by a united-atom approach (i.e., consisted of one pseudoatom), but the hydrogen atoms of NH2 moieties were modeled explicitly. The interaction between the NH2 moiety and CO2 was assumed to be strictly physical. Although there is evidence of carbamate formation and therefore chemisorption,25,38,39 IR studies have shown that physisorption, rather than chemisorption, is the dominant mechanism, except at low pressure.38,40 The predominance of physisorption is supported by the observation that amine-functionalized materials can be regenerated by purging with an inert gas without heating.41 Our own GCMC simulation studies of physisorption in propylamine-functionalized MCM-41 replicated the shape of the experimental CO2 isotherms very well,36 thus further supporting the validity of this assumption. The parametrization of the potentials for the organic surface groups was adopted from the optimized potential for liquid simulations (OPLS) force field42-45 and the MM2 force field.46 The van der Waals interaction between atoms of the adsorbent, surface groups, and gas molecules were represented by the Lennard-Jones potential. The Lorentz-Berthelot mixing rules were applied for combinations of different species. The silicon and hydrogen atoms of the silica structure, although bearing a charge for the calculation of Coulombic interactions, were assigned zero values for the Lennard-Jones parameters. Here, the interaction of the adsorbate gas molecule or surface group with the silica framework was instead described by an effective Lennard-Jones potential for the oxygen atoms; this potential is transferable between different silica materials.47 Long-range electrostatic interactions were calculated using the Ewald technique where molecules were assigned effective point charges.48 All intermolecular interactions beyond a cutoff radius of 5σij were neglected, where σij is the combined LennardJones parameter σ for fluid i and j. Potential parameters for CO2, N2, and O2 adsorbates were taken from the literature.49 Each of these molecules was modeled as a three-site particle. Effective point charges were used to model the quadrupole moment as shown in Figure 3. The full list of potential parameters used in the simulations is given in the Supporting Information. Results and Discussion As a starting point, the adsorption of pure CO2 in functionalized MCM-41 materials was studied using the 42-Å-diameter pore. Surface groups that were identified as enhancing the CO2

Effect of MCM-41 Functionalization on CO2/N2 Separation

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Figure 3. Charges assigned to the three-site adsorbate molecules. Q is an effective point charge used to model the quadrupole moment in N2 and O2. Figure 5. (a) Snapshot of 50%-propylamine-functionalized MCM-41 (pore diameter ) 42 Å). (b) Snapshot of a single propylamine surface group illustrating how the alkyl chains bend such that the terminal amino moiety interacts with the pore surface rather than projecting into the pore center. Key: yellow, Si; red, O; white, H; dark blue, N; light blue, C.

Figure 4. Effect of chain length on the adsorption of CO2 in alkylamino-modified MCM-41 (pore diameter ) 42 Å) at 50% surface group coverage at 298 K: (9) unmodified and (O) amine-, (0) methylamine-, (2) ethylamine-, (]) propylamine-, and (∆) pentylaminemodified. Note that the symbols for the amine-modified material are mostly obscured by the symbols for the propylamine-modified material.

uptake of the material were then investigated further by studying adsorption of flue-gas mixtures to determine the selectivity of the adsorbent. The starting point for our simulations was a series of alkylamine surface groups where the alkyl chain length was varied from zero to five carbons. The amine, methylamine, ethylamine, propylamine, and pentylamine surface groups simulated are shown in Figure 2. This series of surface groups was chosen because amine-based solvents are widely used for CO2 capture by absorption. Considering a series of surface groups with successively longer alkyl chains enables the investigation of the effect of projecting the polar amino moiety progressively farther into the pore space. It was expected that longer chains would maximize the interactions between the CO2 molecules and the terminal amino moieties, resulting in an increase in CO2 uptake. Interestingly, this was found not to be the case. The CO2 adsorption isotherms for each of the alkylamine surface groups and the unmodified MCM-41 material are shown in Figure 4. Here and elsewhere (unless stated otherwise), the calculated errors were within the symbol size. The isotherms show that, in the low-pressure region (15 bar), each of the

alkylamine chains, with the exception of methylamine, actually resulted in a smaller CO2 uptake than was found for the unmodified MCM-41 material. This effect is even more pronounced when gravimetric rather than volumetric uptake is considered, because of the additional molecular weight of the longer surface groups. The expected trend of increased CO2 adsorption with increasing alkyl chain length was therefore not realized. Rather, the dominant effect appeared to be a reduction in pore volume and, therefore, a reduction in CO2 capacity at higher pressures for the larger surface groups. At first sight, this behavior is counterintuitive because surface groups bearing amine moieties are expected to increase the affinity of the adsorbent toward CO2. In the case of the amine surface group, the lower-than-expected CO2 uptake can be explained in terms of the position of the group relative to the pore wall. In a chain consisting of a single link, the polar group is not projected into the pore space where it can interact with CO2. Rather, the molecule is anchored in recesses in the amorphous pore wall, where it is largely inaccessible to CO2 molecules. However, the decrease in CO2 uptake relative to the unmodified material was also observed for surface groups having chains consisting of two to five links, which should be projected well clear of the pore wall. This decrease in CO2 affinity can be rationalized by examining the orientation of the surface molecules from different snapshots saved during the GCMC simulation of 50%-substituted propylamine MCM-41 (Figure 5a,b). Here, the adsorbate molecules are omitted for clarity. Figure 5 shows that the most common orientation of the polar amino moiety is bent backward toward the silanol groups of the pore surface. This results in a bent propyl chain being exposed to the adsorbates rather than the polar terminal moiety. In such an orientation, the amino moiety cannot come into close contact with the adsorptive gases occupying the pore space, and the overall affinity of the pore surface for CO2 is reduced. Moreover, the section of apolar alkyl chain screens portions of the polar silanol pore wall thus further reducing the affinity of the adsorbent for CO2. Any polar effect conferred by the amino

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Williams et al. TABLE 1: Selectivities S(CO2/N2) at 1 and 10 bar and 298 K for MCM-41 Functionalized with Different Surface Groupsa pore diameter ) 42 Å pore diameter ) 19 Å

Figure 6. Effect of surface group concentration on the adsorption of CO2 in methylamine-modified MCM-41 (pore diameter ) 42 Å) at 298 K: (9) unmodified and (0) 25%-, (2) 50%-, (]) 75%-, and (∆) 100%-functionalized.

moiety is lost because of the conformational flexibility of the chain species and the attraction of the terminal amino moiety to the pore wall. This bending effect was observed for the ethylamine, propylamine, butylamine, and pentylamine surface groups, that is, for all surface groups longer than one carbon. At any one time in a simulation, a fraction of the surface groups will assume an unfavorable “bent” configuration. Rather than facilitating CO2 adsorption by projecting the terminal amino moieties away from the pore wall, the longer alkyl chains confer greater flexibility to the surface group and facilitate bending. This effect has also been observed experimentally: Harlick et al.28,29 reported lower-than-expected CO2 adsorption in MCM41 grafted with 3-[2-(2-aminoethylamino)ethylamino]propyl surface groups. They suggested that the amine-CO2 interaction was hindered by amine hydrogen bonding between surface hydroxyl groups or between amine groups. In the case of the methylamine surface group containing only a single carbon in the alkyl chain, chain bending could not occur. The terminal amino moiety is projected away from the pore wall and a short distance into the pore, where it can maximize its interactions with the adsorbate molecules and, therefore, increase the CO2 uptake. To confirm the idea that the attractiveness of the amino moiety to the pore surface combined with chain bending negated the effect of polar amino moieties, simulations were carried out that compared the effect of alkyl groups with analogous alkylamine derivatives of similar chain length [e.g., butyl (C4H9) and propylamine (C3H6NH2)]. The resulting isotherms (not shown) showed no significant difference in the CO2 adsorption capacities between analogous pairs of surface groups, thus confirming that the bending of the chains negates any benefit from the terminal amino moiety. The only exception, as expected, was observed in the case of methylamine and ethyl, where the methylamine increased CO2 uptake relative to the ethyl surface group. Having identified that functionalization with methylamine improved the CO2 uptake of the unmodified MCM-41, we then attempted to further improve the CO2 adsorption of this material by modifying the degree of functionalization, that is, the extent of coverage of the pore wall. In general, if a surface group influences the adsorption capabilities of a material, then this effect will be related to the number of surface groups present. CO2 adsorption isotherms were simulated for methylaminefunctionalized MCM-41 at 25%, 50%, 75%, and 100% functionalization using the 42-Å-diameter model pore. Figure 6 illustrates the adsorption of CO2 in MCM-41 materials with different concentrations of methylamine surface groups compared to the unmodified MCM-41 material. The material with the lowest surface group functionalization (25%) did not improve CO2 adsorption relative to the unmodified

unmodified MCM-41 propylamine phenyl difluorophenyl fluorophenyl chlorophenyl bromophenyl iodophenyl diaminophenyl a

1 bar

10 bar

1 bar

10 bar

7.0 8.7 10.9 12.3 9.9 10.8 9.0 11.6 16.5

6.4 7.2 8.7 8.9 9.1 8.9 9.7 9.6 13.9

9.0 10.0 12.4 12.7 12.3 13.2 14.2 14.3 16.9

8.5 9.3 12.1 12.8 13.4 14.6 14.9 15.3 18.0

50% functionalization.

material. The CO2 adsorption isotherm for this material was very similar to that of unmodified MCM-41. At higher pressures, a marked improvement was observed for surface group loadings of 50%, 75%, and 100%, demonstrating that, at sufficient concentrations, the methylamine surface group is more attractive to CO2 than the polar silanol wall. At 100% functionalization, CO2 adsorption was significantly improved, and no evidence of pore filling was observed over the pressure range simulated. This is a result of using very small surface groups; in contrast to the bulkier surface groups presented later, these surface groups occupy only a small amount of the pore volume even at high percentages of functionalization. The gravimetric uptake of the methylamino-functionalized materials (Figure S1 in the Supporting Information) follows the same trend as the volumetric uptake shown in Figure 6. The replacement of a hydroxyl group with CH2NH2 causes only a small increase in the mass of the adsorbent. For 100% surface group functionalization, for example, the mass increase due to surface group functionalization is less than 2.5% of the total weight of the adsorbent. The simulations carried out on alkylamine chains have demonstrated that polar moieties do improve CO2 adsorption when supported by a stiff surface group that is not prone to bending. To improve the CO2 uptake further over the moderate improvements obtained with methylamine surface groups, a series of monophenyl surface groups functionalized with different polar moieties was evaluated. The key difference between these surface groups and the alkylamine groups considered previously is that the aromatic rings are inherently stiff. This avoids the bending problems observed with flexible surface groups. By using surface groups where a polar moiety is attached to carbon 4 (where carbon 1 is the point of attachment to the pore wall), the polar moiety is projected directly into the center of the pore space such that contact between the adsorptive species and polar moieties should be enhanced. The functionalized monophenyl surface groups studied occupy more space than the simple alkyl chains considered previously and could potentially lead to a significant decrease in the pore volume and, therefore, a reduction in the CO2 capacity of the functionalized material. To explore this effect, we additionally studied adsorption of the flue-gas mixture in a smaller, 19-Å pore. The selectivities at 1 and 10 bar for each of the materials are reported in Table 1. In addition, the selectivity of the flexible propylamine surface group was also simulated for comparison. It should be noted that, although the simulations were of ternary mixtures of CO2, N2, and O2, the amount of O2 adsorbed was very low (only 1 or 2 molecules were present in the simulation). The results for O2 are therefore not reported.

Effect of MCM-41 Functionalization on CO2/N2 Separation All of the materials simulated, including the unmodified MCM-41 material, were selective for CO2 [i.e., S(CO2/N2) > 1]. The presence of each of the surface groups increased the selectivity toward CO2 with respect to the unmodified material. In general, the selectivity for CO2 increased with increasing polarity of the surface group, with diaminophenyl giving the highest selectivity for CO2. As CO2 has a strong quadrupole and N2 a weak one, the introduction of a polar surface group will favor the adsorption of the more polar adsorbate. The effects of the various structural features of the surface groups on the selectivity can be determined by comparing pairs of surface groups that differ only slightly. Whereas the phenyl group results in only a small improvement in the selectivity, the diaminophenyl group has a selectivity that is double that of the unmodified material, indicating that it is the amino moiety rather than the phenyl ring that is mainly responsible for the increased CO2 selectivity. The fact that phenyl does show a slightly higher selectivity than the unmodified MCM-41 indicates that the phenyl ring does increase CO2 uptake, either because it provides additional adsorption sites or because the ring’s delocalized π electrons are attractive to CO2. The similarities in the selectivities of the halo-functionalized surface groups demonstrate that the increasing polarizability and decreasing electronegativity on going down the halogen group F > Cl > Br > I did not significantly alter the interactions of the surface groups with CO2 or N2. Nor were the selectivities of these materials much improved over that of the unmodified material. It is therefore beneficial to incorporate amino rather than halogen moieties into surface groups. This can be rationalized on the basis of the structure of the polar sections of the surface group: Whereas a halo functional moiety consists of a single electronegative atom that is attractive to the positively polarized carbon atom (C δ+), the amino moiety has both a negatively polarized N atom (N δ-), which is attractive to C δ+, and positively charged hydrogen atoms (H δ+), which are attractive to O δ- of CO2. The amino moieties therefore provide both larger surface areas and stronger interaction sites for the adsorption of CO2. This stronger interaction of the CO2 molecules with the aminophenyl surface groups can, for example, be quantified by the radial distribution functions as shown in Figure S2 in the Supporting Information. The propylamine surface group also contains an amino moiety but was only slightly more selective for CO2 than the unfunctionalized material and was much less selective that the diaminophenyl group. This result fits with the idea that the alkyl chain undergoes bending such that the polar amino moiety buries itself in the pore wall. In this conformation, the surface group is not expected to improve CO2 uptake considerably. The selectivities in the smaller 19-Å pore were found to be larger than those of the corresponding 42-Å pore because, in a smaller pore, adsorbed species are forced into closer contact with the functional groups. It should be noted that pore filling, the point at which further CO2 uptake is prevented, will occur at lower pressures in smaller pores, particularly when bulky surface groups are present. Figure 7 shows the adsorption isotherms of CO2 from flue gas for three different surface groups and two different pore sizes. In Figure 7a, the volumetric adsorption isotherm is plotted. For both pore sizes and over the entire pressure range, CO2 adsorption increases in the order unmodified MCM-41 < bromophenyl < diaminophenyl. Here, the 19-Å diaminophenylfunctionalized pore showed the highest CO2 adsorption, and there was no evidence of pore filling. Figure 7b shows the adsorption isotherms plotted using a gravimetric scale, which

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Figure 7. Isotherms for the adsorption of CO2 from flue gas for unfunctionalized MCM-41 and MCM-41 functionalized with bromophenyl and diaminophenyl at 298 K. Data are shown for two different pore sizes. The amount adsorbed is plotted in (a) volumetric units and (b) gravimetric units. Symbols: (O) unmodified, 42 Å; (b) unmodified, 19 Å; (∆) bromophenyl-modified, 42 Å; (2) bromophenylmodified, 19 Å; (0) diaminophenyl-modified, 42 Å; (9) diaminophenylmodified, 19 Å.

explicitly takes into account the larger molecular weight of the functionalized materials. Here, the adsorption of CO2 still increases in the order unmodified MCM-41 < bromophenyl < diaminophenyl, but the differences between the two different pore sizes are not as pronounced. The larger density of the 19-Å diaminophenyl-functionalized adsorbent results in a lower CO2 capacity than for the corresponding 42-Å pore material. Our simulations so far have demonstrated that rigid, functionalized surface groups, in particular the diaminophenyl surface group, enhance the uptake and selectivity of the material for CO2 and that this effect is largely due to the presence of the amino moiety projected toward the pore center. The next step toward further improving these beneficial properties is to design surface groups that incorporate larger numbers of polar moieties and do not bend back. As the amino-functionalized phenyl group was shown to give better CO2 uptakes and selectivities than any of the halo-functionalized materials simulated, the design of new surface groups was based on this moiety. The new surface groups consist of polyphenyl chains that act as a stiff scaffold for two amino moieties per phenyl ring (see Figure 2). As an example, Figure 8a,b shows snapshots of MCM-41 functionalized with hexaaminoterphenyl. For a pore that is functionalized with relatively bulky surface groups, such as the one shown in Figure 8, a number of factors have to be taken into consideration: Although a surface group might increase the selectivity toward one species over another, a high degree of functionalization will reduce the available pore space for adsorption and reduce the overall adsorption capacity of a material. This results in pore filling occurring at lower pressures than for the unfunctionalized material. Volumetric adsorption isotherms of CO2 from flue gases in 42-Å-diameter MCM-41 pores functionalized with different quantities of diaminophenyl, tetraaminobiphenyl, and hexaaminoterphenyl surface groups are shown in Figure 9a-c. The same

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Figure 10. Snapshot of the 50%-hexaaminoterphenyl-functionalized material (pore diameter ) 42 Å) containing flue gases at 25 bar and 298 K. Key: gray, Si; silver, O; white, H; red, CO2; yellow, N2.

Figure 8. MCM-41 functionalized with 50% hexaaminoterphenyl (pore diameter ) 42 Å). (a) Surface groups in skeletal form and (b) surface groups represented as van der Waals spheres. Key: yellow, Si; red, O; white, H; dark blue, N; light blue, C. Figure 11. Selectivities of the 50%-functionalized materials (pore diameter ) 42 Å) as a function of pressure at 298 K: (2) diaminophenyl, (O) tetraaminobiphenyl, (9) hexaaminoterphenyl. Unless otherwise shown the error bars are within the symbol size.

Figure 9. Isotherms for the adsorption of CO2 from a flue-gas mixture in (a) diaminophenyl-, (b) tetraaminobiphenyl-, and (c) hexaaminoterphenyl-substituted materials at 298 K plotted on a volumetric scale. (d) Hexaaminoterphenyl-substituted materials at 298 K plotted on a gravimetric scale. All pores are 42 Å in diameter. Symbols: (9) unmodified and (O) 25%-, (0) 50%-, (2) 75%-, and (]) 100%functionalized.

scale has been used on each axis so that the volumetric isotherms can be directly compared. Figure 9d shows the data for hexaaminoterphenyl-functionalized MCM-41 where the amount adsorbed is plotted on a gravimetric scale. It can be seen that, for diaminophenyl and tetraaminobiphenyl, a 75% surface group functionalization results in the highest CO2 adsorption over the entire pressure range. For the hexaaminoterphenyl-substituted materials, where the surface group is significantly bulkier, the highest CO2 adsorption was observed for 50% functionalization. Higher degrees of functionalization resulted in crowding of the pore space and a decrease in the CO2 adsorption capacity of the material. The same trend was observed for both volumetric (Figure 9c) and gravimetric (Figure 9d) uptake of the hexaaminoterphenyl material. In the case of the 100%-functionalized hexaaminoterphenyl material, the isotherm leveled off rapidly, and the amount of CO2 adsorbed

did not increase significantly above 25 bar, at which point the pore reached its maximum loading. Further evidence of this trend is given in Figure 10, which shows a snapshot of the pore of the 50%-functionalized hexaaminoterphenyl material at 25 bar containing adsorbed gases. At this pressure, much of the available pore space has been occupied by the surface groups and flue-gas species, leaving little room for further adsorption. We next investigated the selectivity of this material. For each of the surface groups investigated, GCMC simulations were carried out at 298 K using the flue-gas model described previously. Figure 11 shows the CO2/N2 selectivities of diaminophenyl, tetraaminobiphenyl, and hexaaminoterphenyl. In each case, the degree of surface group functionalization was 50%. Figure 11 shows that the selectivity increases with increasing complexity of the surface group, with the larger hexaaminoterphenyl surface group giving CO2/N2 selectivities in the range of 28-35 over the simulated pressure range. The data show a general trend of slightly decreasing CO2/N2 selectivity with pressure. This reflects the fact that, at higher pressures, adsorption is not occurring directly on the pore wall but rather on a monolayer of adsorbed molecules. At low pressures, CO2 adsorbs selectively for two reasons: CO2 has a stronger quadrupole than N2 and is also smaller along its axis. The latter fact allows CO2 molecules to access areas of microporosity created by the amorphous pore walls and surface groups that N2 molecules cannot. Such areas of microporosity allow molecules to maximize their interactions with the adsorbent, and occupation of these areas by adsorbate molecules is energetically favorable. At higher pressures, these sites of microporosity have been filled, and the molecules start to adsorb on a monolayer of existing adsorbed molecules. Here, the advantages of a smaller cross-sectional diameter are diminished, resulting in a decrease in the selective adsorption of CO2. It has been shown50 that, in the medium coverage range, the amount adsorbed is proportional to the accessible surface area, which can be determined by rolling a probe molecule with a diameter similar to that of the adsorbate along the pore surface.51

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Figure 13. Isotherms for the adsorption of CO2 from flue gas in MCM41 (pore diameter ) 42 Å) at 298 K: (O) unmodified and (0) terphenyland (2) hexaaminoterphenyl-modified.

Figure 12. (a) Surface areas available to a probe molecule of N2 for each of the 42-Å-diameter functionalized pores. (b) Selectivities at 10 bar of the diaminophenyl-, tetraaminobiphenyl-, and hexaaminoterphenyl-functionalized materials at different concentrations at 298 K: (2) diaminophenyl, (O) tetraaminoterphenyl, (9) hexaaminoterphenyl.

To investigate what role the increased surface area plays, we calculated the accessible surface area using a probe diameter of 3.3 Å, this being equal to the Lennard-Jones parameter σ of the nitrogen atom of the N2 atom.49 A plot of the surface area accessible to a CO2 probe molecule is similar to that of N2 and is included in the Supporting Information (Figure S3). Figure 12a shows the surface areas accessible to N2 for structures having different degrees of surface group functionalization. The general trend is that the available surface area increases with increasing number of phenyl rings, with the larger hexaaminoterphenyl structure providing a greater surface area than either of the two smaller surface groups. The exception here is at very high pore loadings (100% corresponding to 0.3907 g of surface groups per gram of adsorbent) where the available surface area of the hexaaminoterphenyl is reduced to less than that of the tetraaminobiphenyl- or diaminophenylfunctionalized materials. A snapshot of the hexaaminoterphenyl material in space-filling representation (Figure 8b) shows that, at this degree of functionalization, the individual hexaaminoterphenyl surface groups come into close contact and, in these regions, the available surface area for adsorption is reduced. The results of these simulations indicate that the optimal surface group coverage for maximizing the accessible surface area is 50% for the tetraaminobiphenyl and hexaaminoterphenyl materials and 75% for the diaminophenyl material. These results correlate well with the trends for CO2 uptake in Figure 9, which show that the CO2 uptake is largely dependent on the number of available adsorption sites. To investigate how the selectivity changes with degree of surface group functionalization, S(CO2/N2) was calculated at 10 bar for each of the three materials using different degrees of functionalization in the 42-Å-diameter pore. The results are presented in Figure 12b and show that, for the diaminophenyland tetraaminobiphenyl-functionalized materials, the highest selectivity is achieved with 75% functionalization. In the case of the larger hexaaminoterphenyl-functionalized material, a lower functionalization of 50% gives the maximum CO2 selectivity. Thus, selectivity follows the same pattern as the surface area shown in Figure 12a and the CO2 adsorption

capacity shown in Figure 9. This simplifies the choice of an optimal material, as there is no tradeoff between CO2 selectivity and CO2 capacity for these functionalized materials. At 10 bar, the S(CO2/N2) values of 50%-substituted diaminophenyl-, tetraaminobiphenyl-, and hexaaminoterphenyl-functionalized materials are 14, 20, and 32, respectively. These selectivities are significantly higher than those of any of the surface groups reported in Table 1 and, particularly for the hexaaminoterphenyl material, compare favorably with the selectivities of commercially used adsorbents for flue-gas separations.52 Having demonstrated the correlation between surface area and CO2 uptake and selectivity, we now consider other structural features of the adsorbent that might influence its adsorption properties. As well as a larger surface area, the hexaamino material presents a larger number of polar amino moieties than either the diaminophenyl or the tetraaminobiphenyl material (Figure 2). To establish the relative contributions of each of these factors, we performed simulations that investigated the effect of chain length independently of increasing the amino moiety concentration. Figure 13 shows isotherms for the adsorption of CO2 from flue gas for the unmodified material; 50%-functionalized hexaaminoterphenyl MCM-41; and 50%functionalized terphenyl MCM-41, which corresponds to the hexaaminoterphenyl material without the amino moiety. Here, it can be seen that, even in the absence of polar amino moieties, the adsorption capacity for CO2 is increased over that of the unmodified material. This is due to the presence of a long aromatic chain. The benzene rings contain delocalized π electrons that result in regions of high electron density that are attractive to the carbon of CO2. However, when amino moieties are present on the polyphenyl surface group, the CO2 uptake is further increased. A plot of the surface area available to a probe molecule of CO2 for the polyphenyl materials with and without amino moieties (Figure S4 in the Supporting Information) shows that the additional amino moieties present do not increase the surface area significantly. This means that it is predominantly the attractive forces between the polar amino moieties and the quadrupolar surface groups, in particular CO2, that causes the increased CO2 uptake and selectivity in materials containing amino-functionalized surface groups. Conclusions We have used molecular simulations to investigate the performance of functionalized MCM-41 materials for the removal of CO2 from flue gases. Variations in the pore diameter and surface group led to materials exhibiting wide ranges of CO2 capacities and CO2/N2 selectivities. The GCMC simulations allowed for detailed examination of molecular-level phenomena and an atomistic interpretation for

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the lower-than-expected CO2 adsorption in alkylamine-functionalized MCM-41. Alkylamine chains were found to bend backward such that the terminal amino moiety was oriented toward the pore wall rather than directed into the pore space. In such an orientation, the polar amino moiety was unable to interact with adsorbed species, and sections of the apolar chain also shielded parts of the polar silanol wall. Knowledge of how configurational flexibility affected the adsorption properties of materials allowed us to tailor-make a series of surface groups encompassing favorable structural characteristics. These surface groups containing rigid phenyl rings increased CO2 adsorption and CO2/N2 selectivities by providing additional adsorption sites and by acting as a skeleton for projection of polar halo and amino groups directly into the pore space. It was determined that the polar amino moieties were largely responsible for the increased CO2 uptakes and selectivities over N2, as these groups undergo strong Coulombic interactions with the quadrupolar CO2 molecules, thus providing interaction sites where adsorption is highly favored. Finally, the selectivities of the MCM-41 material were further improved by functionalization with a series of long polyphenyl chains, which acted as scaffolds for a high concentration of amino moieties. For these polyphenyl chain surface groups, simulations were used to determine to what extent the different structural features of the functional group influenced the adsorption and selectivities of the material. It was determined that at 50% loading, the hexaaminoterphenyl group showed optimal CO2 uptake and selectivity due to the combined effects of increased surface area and the presence of six polar amino moieties per surface group. The selectivity of hexaaminoterphenyl modified MCM-41, which was calculated to be approximately 32, compares well with those of other typical adsorbents such as alumina [S(CO2/ N2) ) 6],53 zeolite Y [S(CO2/N2) ) 8],54 SAPO-34 [S(CO2/N2) ) 16],55 and carbon membranes [S(CO2/N2) ) 22].56 The isotherm presented by hexaaminoterphenyl-modified MCM-41, Figure 9c, is, in fact, the ideal type of isotherm for use in a pressure-swing adsorption (PSA) application: The pores are nearly full at 10 bar, a moderate pressure in PSA terms, and the low initial slope means that the capacity of the adsorbent can be substantially recovered in the regeneration step. The results of our simulations clearly demonstrate that functionalization with organic surface groups allows a large degree of control over the adsorption properties of MCM-41. This, along with the large surface area of the material, the uniform mesopores, and the ability to tune the pore size, makes functionalized MCM-41 materials of the type investigated here ideal materials for CO2/N2 separation applications. Acknowledgment. Financial support from the EU FP6 framework program (“Fusion”) and the EPSRC (EP/F034520/ 1) is gratefully acknowledged. This work made use of the resources provided by the Edinburgh Compute and Data Facility (ECDF; http://www.ecdf.ed.ac.uk/). The ECDF is partially supported by the eDIKT initiative (http://www.edikt.org.uk). Supporting Information Available: Full details of the parameters used in the simulations, lattice cell parameters, and figures mentioned in the main text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Veawab, A.; Tontiwachwuthikul, P.; Chakma, A. Ind. Eng. Chem. Res. 1999, 38, 3917.

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