Analysis of CO2 Adsorption in Amine-Functionalized Porous Silicas by

Article pubs.acs.org/EF

Analysis of CO2 Adsorption in Amine-Functionalized Porous Silicas by Molecular Simulations Santiago Builes,*,†,∥ Pedro López-Aranguren,†,‡ Julio Fraile,‡ Lourdes F. Vega,†,§ and Concepción Domingo‡ †

MATGAS Research Center, Campus UAB, 08193 Bellaterra, Barcelona, Spain Institut de Ciència de Materials de Barcelona, Consejo Superior de Investigaciones Científicas. (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Barcelona, Spain § Carburos Metálicos, Air Products Group, C/Aragón 300, 08009 Barcelona, Spain ∥ Grupo de Investigación DDP, Universidad EAFIT, Medellín, Colombia ‡

ABSTRACT: We present the results of a combined experimental−molecular simulations approach concerning the capacity for CO2 adsorption of aminosilica hybrid products synthesized using supercritical fluids. Two porous supports were examined for amine functionalization, an ordered mesoporous silica (MCM-41) and a disordered silica gel (SG40). The textural properties of the studied materials were analyzed by low-temperature N2 adsorption−desorption isotherms and compared to those of molecular simulations using the grand canonical Monte Carlo simulation method. The CO2 adsorption capacity of these materials was evaluated by recording CO2 adsorption isotherms up to 100 kPa. Molecular simulations of the CO2 adsorption behavior of selected samples were performed to gain a fundamental understanding of the effect of functionalization. This study shows that in the functionalized materials, the distance between nitrogen atoms of the grafted amines is a critical factor for the occurrence of CO2 chemisorption, providing some insight into key parameters for designing adsorbent materials for CO2 capture and separation. The relationship between the adsorption results with N2 and CO2 allow us to compare the potential applications of the materials in CO2 adsorption and separation processes. A correlation of the N2 adsorption at a given pressure with the CO2 adsorption at a different pressure allowed the prediction of which materials will perform well for these processes. The hybrid products with high amine density have desirable features required for industrial sorbents, such as an enhanced CO2 adsorption capacity and selectivity. alternative to the use of toluene.16 This loading route has been presented as one of the most effective, simple, and reproducible methods for producing homogeneous, covalently bonded, and high-density alkyl and aminosilane films on the internal surface of porous materials.17−21 For amine-functionalized supports, CO2 uptake is expected to occur mainly by chemisorption at low partial pressure of CO 2 . 22−24 The exact mechanism of CO 2 capture by chemisorption is still the subject of extensive research.22−28 It has been found that under dry conditions the chemisorbed CO2 can be either a carbamate−ammonium ion pair, resulting from the reaction of CO2 with a pair of primary or secondary amine groups or carbamic acid, resulting from the reaction of CO2 with an amine and a hydroxyl group.25−28 Bacsik et al.29 determined from Fourier transform infrared (FTIR) spectroscopy of CO2 adsorption on propylamine-functionalized MCM48 that under dry and moist conditions both carbamates ammonium ion pair and carbamic acid are formed. Previous works have shown that depending on the hybrid adsorbent synthesis route and on the porous support employed for functionalization, different effects on the CO2 capture capabilities of the synthesized products can be attained.5,24,30

1. INTRODUCTION The use of solid porous sorbents modified with aminosilane molecules is being considered as an appropriate technology for the adsorption and separation of CO2 from other light gases.1 Amine-functionalized solid materials open up the possibility for the design of CO2 separation processes2,3 that are less energy intensive than the conventional absorption methods using aqueous alkanolamines. Moreover, the evaporation and degradation of amines during the regeneration step might be reduced when using this technology.4 However, a more complete understanding of the underlying processes governing the adsorption and separation mechanisms of gases on functionalized silica is required for the implementation and design of industrial processes using those materials. In general, mesoporous silica materials at moderate pressures have low CO2 adsorption capacities.5,6 Hence, there is little practical interest in using raw mesoporous silica for gas adsorption processes. Nonetheless, the modification of the surface with amine groups represents an attractive alternative for their application to adsorption processes. Nowadays, the supports most commonly used for grafting amine moieties on porous surfaces are mesoporous silica materials. The amines in mesoporous silica are typically grafted to the surface by admixing aminosilane with the silica support in toluene.7−15 A supercritical CO2 (scCO2) anhydrous method has been developed recently in our group as a cleaner © 2015 American Chemical Society

Received: April 10, 2015 Revised: May 22, 2015 Published: May 26, 2015 3855

DOI: 10.1021/acs.energyfuels.5b00781 Energy Fuels 2015, 29, 3855−3862

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Energy & Fuels Jones and co-workers24 studied the isosteric heats of adsorption of silica-supported amine materials with varying amine densities. They showed that a large increase in the heat of adsorption occurs once the average spacing between the grafted ends of the functionalized chains reaches a value below 1.2 nm. In this calculation they assumed that the chains distribute in a homogeneous way, i.e., assuming a regular distribution of the chains over the surface. However, they argued that the grafted chains form clusters upon grafting and the actual distance between amine groups was likely much lower than this value. Young and Notestein5 showed that the shift on the CO2 capture efficiency observed for materials with high amine density depends on the nature of the support and the functionalizing amine. They attributed this shift to changes in the configuration resulting from each type of support and amine and the resulting distance between neighbor amines. Moreover, it has been observed by different research groups that a large increase in the captured CO2 occurs for amine densities over ∼1 NH 2 /nm 2 in periodic mesoporous silicas.7,8,24,31 This density is similar to the values used by Jones et al.24 to obtain an average amine spacing between 1.1 and 1.2 nm. Nevertheless, for an amorphous silica gel, Aziz et al.27 observed that this value was much higher, around 2 NH2/ nm2. This increase was attributed to the different geometry of the pore and the expected differences in the grafted surface. Although there are numerous works that describe the use of amine-functionalized materials for the separation and capture of CO2, the underlying mechanism of the amine CO2 adsorption processes on amine-functionalized porous materials is not yet fully understood. This incomplete understanding limits the possibilities of designing ad-hoc optimal sorbents for specific applications, highlighting the interest of performing complementary experimental−simulation studies. One of the main advantages of molecular simulations is that they allow studies of certain ideal controlled conditions with a high level of detail. Although models are idealizations of the experimental behavior, they can provide useful information on the behavior of the system under controlled conditions not attainable experimentally, provided that the main assumptions used in developing the models are taken into account when analyzing the results. Moreover, if the models of molecular simulations can accurately predict the behavior of an experimental system, then they can provide valuable information on the interactions at the molecular level responsible for the macroscopic behavior. The present work examines the capacity of the supercritically prepared hybrid products for adsorption of CO2 using a combination of experiments and molecular simulations. This work builds on our previous work as a further step toward developing and understanding specific materials for CO2 separation by adsorption.6,16−21 The two porous supports examined in this work, silica gel (SG40) and MCM-41, were modified using a monoaminosilane (3-aminopropylsilane (AP) or [3-(methylamino) propyl] trimethoxysilane (MAP)). Therefore, computational models for MCM-41 and silica gel functionalized with monoamine molecules were evaluated for N2 and CO2 adsorption. The simulated results are discussed and compared to data obtained experimentally. The goal of the adsorption simulation studies was twofold. First, to assess the ability of using grand canonical Monte Carlo (GCMC) to obtain qualitative and quantitative predictions of the adsorption characteristics of gases on aminosilane supercritically functionalized products; and second, to provide new insights into the CO2 adsorption mechanism in those porous materials.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Aminosilane Postfunctionalized Matrices. MAP (Sigma-Aldrich) was used as the experimental grafting functionality. Ordered mesoporous silica (MCM-41, ACS Materials) and silica gel (SG-40, Fluka) were investigated as supports. Carbon dioxide (CO2 99.999%, Carburos Metálicos S.A., Air Products Group) was used as the solvent for the synthesis of the hybrid materials. The supercritical silanization of silica substrates was performed in scCO2 using a setup and a procedure described elsewhere.17 The optimal functionalization conditions were found at 408 K and 9 MPa with a reaction time of 300 min. 2.2. Characterization. The substrate loading with amine was estimated using thermogravimetric analysis (TGA) (TGA Instrument Q5000 IR) under a N2 flow. The textural properties of raw and silanized substrates were studied by low-temperature N2 adsorption− desorption analysis (ASAP 2000 Micromeritics). The specific surface area (Sa) was determined by the Brunauer−Emmett−Teller (BET) method. The mesopore volume (Pv) was calculated using the Barrett− Joyner−Halenda (BJH) method from the adsorption branch of the isotherm. The pore diameter (Pd) was calculated as 4Pv/Sa. Adsorption isotherms of CO2 up to 100 kPa and at 298 K were obtained using a Micromeritics ASAP 2020 analyzer. For both, N2 and CO2 adsorption, the porous substrates were dried at 393 K for 20 h at reduced pressure prior to the measurements. Amine loadings for the different prepared samples, expressed as millimoles of amine per gram of dry substrate (ρgraft(s)), together with the textural properties, are provided in Table 1.

Table 1. Loadings and N2 Adsorption Measured Textural Properties of the Experimental Samples sample

ρgraft(s) (mmol/g)

Me 0 Me 1 Me 2

− 1.03 2.08

Se0 Se1

− 0.53

Pd (nm) MCM-41 3.8

Silica Gel 40 4.1

Pv (cm3/g)

Sa (m2/g)

0.92 0.73 0.57

1127 837 681

0.47 0.47

556 532

3. COMPUTATIONAL SECTION 3.1. Structural Models of the Porous Media. The complete modeling methodology developed in our group to reproduce the pristine porous matrixes is described elsewhere for MCM-4130,32 and silica gel.18 Here, we summarize only the details relevant to the present work. For MCM-41, a realistic model of the mesoporous material with hexagonal periodicity (Figure 1a,c) was generated by carving out cylinders with a predefined diameter in an amorphous silica block.33 For the amorphous silica gel, the link between the experimental and the simulated material is merely statistical. The model was built applying the atomistic theory of MacElroy and Raghavan34 using pregenerated vitreous silica spheres. Because of computational constraints, the silica gel model was generated using two interconnected spheres, with 3.5 nm radius, requiring cubic periodic cells of 9.1 nm length (shown in Figure 1b; for visualization purposes, 4 unit cells of the model are used). Although the computational size restrictions, reflected in the low number of spheres that can be used in the model, may limit the validity of some quantitative aspects of the gas adsorption results, these models correctly represent the surface structure of silica gel and are suitable for the analysis of data at the low relative pressures of interest to this work. 3.2. Functionalization with the Aminosilane. For the simulations, AP was used for MCM-41 and SG40 functionaliza3856

DOI: 10.1021/acs.energyfuels.5b00781 Energy Fuels 2015, 29, 3855−3862

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Figure 1. Models of the materials used for the simulations: (a) MCM-41 and (b) SG40; detailed view of the surface of (c) Ms0, (d) Ms1, and (e) Ms2 in Table 1. Color key: Si (support), yellow; bridging O (support), red; nonbridging O (support), green; H, white; N, blue; C, gray; Si (aminosilane), gray; O (aminosilane), gray.

and distributed amine chains, hence mimicking the experimental situation. For the MCM-41 model, amine loading concentrations of 1.0 (Figure 1d) and 2.0 mmol/g (Figure 1e) were studied. For the SG40 model, the support admitted a maximum amine concentration of 0.5 mmol/g (Table 2). These amine concentrations were similar to the values obtained for the experimental materials (Table 1). 3.3. Adsorption Simulations. Adsorption simulations were carried out using the GCMC method, equilibrating the chemical potential of the sorbent and a reservoir of gas by exchanging and moving the adsorbed molecules. Using this methodology, N2 and CO2 adsorption isotherms were generated for the studied substrates at different degrees of functionalization and compared with the obtained experimental data. The chemical potential of the adsorbate was related to the pressure of the bulk fluid by the soft-SAFT equation of state.35 The adsorption isotherms were calculated by simulating the average number of adsorbed gas molecules at constant temperature at different values of bulk pressure. In the hybrid materials, the silica atoms were kept rigid while the organic chains were allowed to move, with their moves limited by the grafted end. For each value of pressure in the isotherms, at least 5.0 × 107 MC steps were used for equilibration and 5.0 × 107 MC steps for data collection. Lennard-Jones 12−6 and Coulombic potentials were used for the calculation of the intermolecular interactions. The TraPPE potential36 was used to model the interactions of both gas molecules, N2 (N−N bond length of 0.110 nm) and CO2 (C−O bond length of 0.116 nm and O−C−O angle of 180°). The semiempirical calculations of Brodka and Zerda37 were taken as a reference for the point charges for the silica framework for MCM-4132 and the amorphous silica.18 The parameters for the Lennard-Jones 12−6 and Coulumbic charges for the aminosilane chains were taken from our previous work, which described the methodology for the calculation of adsorption isotherms of aminofunctionalized silica materials.32 The surface area, pore volume, and pore diameter were calculated by analyzing the N2 isotherms for both the experimental and simulated materials for consistency and ease of comparison.

tion because more literature data is available for this system than for MAP. However, both compounds AP and MAP present almost an equivalent CO2 adsorption capacity and efficiency; thus, the results are comparable.4 In a reciprocal manner to the experimental materials, for silica gel one raw support (Ss0) and a functionalized material (Ss1) were used as models for the simulation. For MCM-41 (Figures 1c−e), the raw support (Ms0) and a low (Ms1) and high (Ms2) amine density material were used as model materials. The adsorption properties of the model materials are provided in Table 2. Table 2. Loadings and N2 Adsorption Properties of the Model Materials Used in the Simulations sample

ρgraft(s) (mmol/g)

Ms 0 Ms 1 Ms 2

− 1.0 2.0

Ss0 Ss1

− 0.5

Pd (nm) MCM-41 2.9

Silica Gel 40 3.4

Pv (cm3/g)

Sa (m2/g)

0.81 0.67 0.47

1134 961 573

0.40 0.37

479 395

We have developed an efficient methodology to build the models of alkyl and aminosilane functionalities postgrafted on the silanol groups of the internal surface of the porous supports.18,30 The option of grafting the complete hydrolyzed molecule (NH2(CH2)3Si(OH)2O−)32 was preferred against the more common choice of adding only the organic part (NH2(CH2)3−).18 The surface chemistry in the model is simplified by considering that all the functionalized chains are covalently tethered to the surface in a monodentate manner and that siloxane bridges are not formed between neighbor chains. The surface AP groups were introduced to the silica models by randomly replacing a fixed amount of silanols by the first oxygen of the AP molecules and then growing the rest of the AP molecule using a configurational bias algorithm.32 This model ensures that the grafted chains can form clusters depending on their enthalpic and entropic interactions and that the functionalized surface does not consist of perfectly ordered 3857

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Energy & Fuels Simulated CO2 adsorption isotherms were generated considering chemisorption and physisorption at low CO2 pressure and physisorption at all pressures.6 The reader is referred to the original work for full details of the technique for considering chemisorption in GCMC simulations.32 Chemisorption was included in the molecular simulation calculations by adding during functionalization a predefined number of carbamate and protonated amine chains in addition to the nonreacted amine chains, instead of including only the aminosilane chains.32 This method considers that the only reaction of CO2 with the amines is the formation of carbamate−ammonium ion pairs and that this reaction occurs at low partial pressures of CO2. Although it has been shown that dry CO2 can also form carbamic acid, the formation of carbamate−ammonium ion pairs is the leading mechanism for the chemisorption of CO2.27 It has been shown by FTIR that most of the chemisorbed CO2 occurs at low CO2 partial pressures.22−24,27 For instance, Aziz et al.27 estimated the amount of CO2 physisorbed and chemisorbed on silica gel by extrapolating, using Toth’s isotherm equation, the physisorbed amount at low pressure determined from the adsorption isotherms and FTIR spectroscopy. They observed that, except for very large amine densities, after ∼10 kPa there are no significant contributions of chemisorption to the CO2 captured by functionalized silica gel materials. Thus, the inclusion of the carbamates at zero coverage emulates the low CO2 pressure conditions at which the CO2-amine reaction mainly occurs. In this work, the amount of CO2 chemisorbed in the system was fixed at the carbamate−ammonium ion pair formation stoichiometric ratio of 0.5 mol of CO2 per mol of functionalized amine. That is, half of the amine molecules were replaced by carbamates and the other half by protonated amines. This simple model for considering the chemisorption allows taking into account the effect of the chemisorption and physisorption on the CO2 capture capabilities of functionalized materials by employing only physical interaction potentials (Lennard-Jones and Coulombic interactions) instead of more complex and more computing-intensive procedures.

Figure 2. N2 adsorption isotherms for synthesized and simulated MCM-41 samples up to (a) 100 kPa and (b) 50 kPa. Experimental results are represented by filled symbols, while predictions from the simulations are denoted by open symbols; lines are provided as guides to the eye. N2 isotherms for Me0 (squares), Me1 (diamonds), Me2 (circles), Ms0 (squares), Ms1 (diamonds), and Ms2 (circles).

the experimental material, data was compared until a relative pressure of 0.5 (Figure 2b). Figure 2 shows that after functionalization with ca. 1 mmol/g of aminosilane the reduction in the N2 adsorption capacity for the model and the experimental materials was similar. At 50 kPa, the N2 adsorption for both Me1 and Ms1, functionalized with ca. 1 mmol/g, was reduced in ca. 25%. At the same pressure, a decrease of ca. 55% and 65% was estimated for samples Ms2 and Me2, respectively. This 10% difference in the N2 uptake can be explained as an artifact resulting from the low temperature of the measurements. At 77 K, the grafted chains are in a solid state; therefore, experimental blockage and diffusion problems are expected to occur. These experimental issues are highlighted in the materials with the highest amine loading, which considerably reduce the N2 uptake. Conversely, in Monte Carlo simulations, the molecules are adsorbed in the available pore space without considering the trajectory; thus, pore blocking and diffusion problems are not present in this kind of simulation. As a result, the estimated pore volume was higher for sample Ms2 than for Me2. This effect is expected to be reduced at the higher temperature employed for the CO2 adsorption (298 K) compared to the N2 adsorption (77 K). Nevertheless, the MCM-41 model employed in this work accurately reproduces the adsorption behavior of the experimental materials and can be used as an aid in the characterization of these mesoporous hybrid supports for CO2 capture. 4.1.2. CO2 Adsorption. Experimental CO2 adsorption isotherms for the functionalized MCM-41 materials were recorded up to pressures of 100 kPa at 298 K to account for specific chemisorption information, and they were compared to the corresponding models in Figure 3. Pure silica surfaces do not interact very strongly with carbon dioxide because the residual hydroxyl groups are not able to induce strong interactions. Hence, for raw mesoporous materials, the adsorption can be denoted by a linear equation representative of physisorption in a mesopore.38 As expected from the N2 results, the model Ms0 accurately predicts the

4. RESULTS AND DISCUSSION 4.1. Adsorption Isotherms of MCM-41. 4.1.1. N2 Adsorption. Figure 2 depicts a comparison of the N2 adsorption isotherms in MCM-41 obtained by GCMC simulations versus those obtained experimentally for the same material. As shown in Figure 2, the shape of the isotherm for the raw MCM-41 support is the same in the experimental and simulated materials, indicating mesoporosity in all the studied samples. Measured N2 uptakes for the functionalized substrates were lower than those for the raw materials Me0 and Ms0. Small discrepancies between data for experimental and simulated raw materials were found regarding the total accessible pore volume, which was larger for Me0 than for Ms0. This underestimation of the pore volume in the simulated product, reflected in the filling point, was caused by a slightly smaller pore size in the MCM-41 model (Table 2) than in the experimental material (Table 1). The filling point was found at a lower relative pressure for the models than for the experimental materials. Note that this is a direct comparison between the results obtained by simulations with the experimental results, without any refinement of the model. To minimize the influence of the extra pore volume present in 3858

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atoms in the amine moieties was calculated as an average of the shortest distance of each N atom to their closest neighbor N atom during the production steps of low-pressure adsorption simulations. For those calculations, the models used consisted of only amine chains, i.e., no carbamate−ammonium ion pairs were included. The standard deviation corresponds to the variance of this average distance due to the different atoms present in the model and their displacement during the simulation. This distance is used as an indicator of the average separation between amine groups inside the porous support. The results indicate that this distance was of 0.57 nm (standard deviation of 0.37 nm) for Ms1 (with an amine density of 0.5 NH2/nm2) and of 0.26 nm (standard deviation of 0.13 nm) for Ms2 (with an amine density of 1.1 NH2/nm2). Thus, the latter material is more likely to find a pair of amine groups to react forming carbamate−ammonium ion pairs. Hence, the size of the CO2 molecules (0.23 nm) is only comparable to the average shortest distance between N atoms in Ms2. Therefore, it is consequent with the behavior of the latter material, which, unlike Ms1, exhibits a significant CO2 chemisorption. This result is complementary to previous observations that determined that around 1 NH2/nm2 in periodic mesoporous silica a shift occurs from the amine groups in the grafted chains being far apart to being at close distance to allow the formation of carbamate−ammonium ion pairs. As suggested by Jones and co-workers,24 the average spacing between grafted amines calculated assuming that the amines distribute homogeneously on the surface (∼1.2 nm for a density of 1 NH2/nm2) corresponds to an overestimation due to the clustering of the funcionalized chains. To the best of our knowledge, this is the first report of the average amine spacing (calculated as the distance between the N atoms) in amine-functionalized MCM41 obtained from a realistic model of the system. 4.2. Adsorption Isotherms of Silica Gel. The effect of the geometry of the substrate on the adsorption can be quantified by comparing the adsorption of N2 and CO2 in functionalized silica gels versus the mesoporous ordered MCM-41. We have proceeded with the experimental and modeling work in a similar manner as we did with MCM-41, and results are presented next. 4.2.1. N2 Adsorption. For the silica gel materials, the smaller pore size of bare silica gel in the model Ss0 with respect to the experimental material Se0 is reflected in the textural data (Tables 1 and 2). The N2 adsorption isotherms for the model and experimental silica gel material are shown in Figure 4. Thus, the model of the pristine Ss0 represents only the microporous section and the small size mesopores segment of the experimental adsorbent Se0 and does not represent the large size mesopores of the pristine silica gel material. Qualitative information can be extracted from the Ss0 model by comparing with the experimental adsorption data of the silica gel materials at low pressure (