Experimental and Molecular Simulation Investigation of Enhanced

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Experimental and Molecular Simulation Investigation of Enhanced CO2 Solubility in Hybrid Adsorbents Ngoc Linh Ho,† Fabien Porcheron,*,† and Roland J.-M. Pellenq‡,§ †

IFP-Lyon, Catalysis and Separation Division, BP 3, Rond-point de l’ echangeur de Solaize, 69360 Solaize, France, ‡Centre Interdisciplinaire des Nanosciences de Marseille, CNRS, Campus de Luminy, 13288, Marseille, Cedex 09 France, and §Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 Received April 21, 2010. Revised Manuscript Received June 24, 2010 Hybrid adsorbents are prepared by confining physical solvents (propylene carbonate, N-methyl-2-pyrrolidone) within the porosity of a solid support (alumina) using both wet and dry impregnation methods. The resulting hybrid solids are analyzed using characterization methods (N2 adsorption isotherm, TGA) to ensure that a proper confinement of the solvent has been achieved. The hybrid adsorbents are then subsequently assessed for CO2 capture by performing solubility measurements. An enhanced CO2 solubility is observed with regard to the ones in the bulk solvent and in the raw solid. In a next step, grand canonical Monte Carlo simulations have been performed on a slit pore model to understand the microscopic mechanisms yielding the apparition of enhanced solubility. The presence of solvent molecules favors the layering of CO2 within the pore, and the resulting local density profile is then markedly increased compared to one found in the raw adsorbent as more carbon dioxide molecules can be accommodated into the pore volume.

Introduction One of the major sources liable for increasing carbon dioxide concentration in the atmosphere comes from factories’ emissions like coal-fired power station. Because of low CO2 partial pressure in these flue gases, nowadays the most common way to selectively remove carbon dioxide is by using amine scrubbing. However, this process usually suffers from high-energy requirement, corrosion, and solvent degradation.1 One prospective alternative to amine scrubbing is the utilization of hybrid adsorbents obtained by confining amine molecules into a porous material. One then expects to combine the advantages of both absorption (high selectivity and capacitive solvent) and adsorption (absence of corrosion, reduced solvent loss) properties to create an efficient generation of hybrid adsorbents with high potential for capturing CO2 from flue gas.2 Generally, there are two common ways to confine the solvent into the solid supports: grafting amine groups on the surface or loading amines in the solid porosity using impregnation. Grafting requires to bound the solvent molecules onto the surface of the solids and is usually achieved by using aminosilane compounds which have the ability to create chemical bonding with the surface through the silane group. Silica-based adsorbents have been widely used for the preparation of this type of hybrid adsorbent. For instance, Knowles et al.3 prepared a series of diethylenetriamine[propyl(silyl)] (DT)-functionalized silicas and compared them with amino[propylsilyl] (AP) and ethylenediamine[propyl(silyl)] (ED)-functionalized silica.4 A greater CO2 adsorption capacity for a given surface tether loading was found *Corresponding author. E-mail: [email protected]. (1) Rochelle, G. T. Science 2009, 325, 1652–1654. (2) Chaffee, A. L.; Knowles, G. P.; Liang, Z.; Zhang, J.; Xiao, P.; Webley, P. A. Int. J. Greenhouse Gas Control 2007, 1, 11–18. (3) Knowles, G. P.; Delaney, S. W.; Chaffee, A. L. Ind. Eng. Chem. 2006, 45, 2626–2633. (4) Knowles, G. P.; Delaney, S. W.; Chaffee, A. L. Surf. Sci. Catal. 2005, 156, 887–896.

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in the DT-functionalized materials as compared to the analogous AP- or ED-functionalized adsorbents. Chang et al.5 reported that aminated-SBA-15 supported CO2 diffusion inside the pore, forcing CO2 adsorption on the surface amine sites in the form of carbonate and bicarbonate. Furthermore, this study reported that amine grafted on SBA-15 exhibited a hydrothermal stability and acceptable repeated use without loss of CO2 adsorption capacity. On the other hand, Filburn et al.6 performed a comparative performance study of aminopropyltriethoxysilane-SBA-15, ethyleneamine-R-IAS, and raw IAS. In all cases, there was a decrease of the CO2 capture capacity upon regeneration, indicating instability of the amines used in these adsorbents or that desorption of the CO2 was incomplete. Possible explanations are deactivation of the adsorbent, redistribution of the amine on the surface of the adsorbent, and/or other factors. In view of these results, it seems that the choice of the amine molecule plays an important role on the hybrid adsorbents performance. Propylamine chain grafted in silica-based support material MCM-48 and silica xerogel were prepared by Huang et al.7 When exposed to a CO2/N2 humid gas, the CO2 adsorption capacity increased and was almost doubled, in comparison to that obtained in the presence of a dry stream. The authors suggested that the behavior of adsorption was reminiscent of the amine-CO2 chemistry in the liquid phase. Sayari et al.8 studied conventional MCM-41 and pore-expanded MCM-41 silica modified with diamines and triamines. They showed a significant increase of the equilibrium adsorption capacity and apparent adsorption rate with pore-expanded MCM-41 due to a larger quantity of aminosilanes accommodation (5) Chang, A. C. C.; Chuang, S. S. C.; Gray, M.; Soong, Y. Energy Fuels 2003, 17, 468–473. (6) Filburn, T. P.; Gray, M. L.; Soong, Y.; Champagne, K. J.; Pennline, H.; Baltrus, J. P.; Stevens, R. W., Jr; Khatri, R.; Chuang, S. S. C. Fuel Process. Technol. 2005, 86, 1449–1455. (7) Huang, H. Y.; Yang, R. T.; Chinn, D.; Munson, C. L. Ind. Eng. Chem. 2003, 42, 2427. (8) Sayari, A.; Harlick, P. J. E. Ind. Eng. Chem. 2006, 45, 3248–3255.

Published on Web 07/20/2010

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in the pore-expanded silica. However, the disadvantage of grafting technique is the limited amounts of amine maintained on the support, in contrast with impregnation method, in which a larger quantity of solvent can be introduced. Preparation of impregnated hybrid adsorbents usually involves filling the pore structure of the solid support with a solution of amine and then removing the excess solution by evaporation of the diluted solvent. Monoethanolamine (MEA)-modified zeolite 13X adsorbents have been prepared and evaluated for CO2 adsorption using impregnation by Rayalu et al.9 Small MEA loadings hardly changed the CO2 adsorption capacity compared to the raw zeolite, but 10 wt % of MEA impregnating the porosity results in an improvement of CO2 adsorption capacity over the unmodified zeolite by a factor of ∼1.6 at T = 30 C and ∼3.5 at T = 120 C. On the other hand, with very much higher loadings (i.e., 50 wt % loading), the adsorption capacity decreased due to reduced surface area and pore volume leading to a restricted access to adsorption sites for CO2. A novel CO2 “molecular basket” adsorbent was investigated by Song and co-workers.10-17 The hybrid adsorbent was based on a mesoporous molecular sieve of MCM-41 impregnated with polyethylenimine (PEI). They found that CO2 adsorption capacity of MCM-41-PEI hybrid adsorbent was much higher than that of the MCM-41 alone but also twice that of the pure PEI. When the loading was higher than 30 wt %, the hybrid adsorbent showed a synergetic effect on the CO2 adsorption by the guest PEI solvent, and for 50 wt % of PEI, the highest CO2 adsorption capacity was obtained. The authors suggested that by adding functional amine groups to the surface of the solid an increased gas-adsorbent interaction could be achieved, but no further explanation was given. Surprisingly, despite the exothermic nature of CO2 adsorption in PEI or MCM-41, the experimental results showed that CO2 adsorption capacity in these hybrid adsorbents increased with the temperature. The authors claimed that at low temperature the PEI molecules confined in the channels of MCM-41 are like nanosized particles; only the nitrogen sites on the surface of these particles can eagerly react with the CO2. In this case, the diffusion of carbon dioxide molecules toward the affinity sites inside the PEI particles was subject to limitation. CO2 molecules may reach additional affinity sites if the diffusion time is sufficiently long. When the temperature was increased, the nanosized particles expanded and the totality of the nitrogen sites was now accessible to CO2. In conclusion, there are numerous works dedicated to the preparation and evaluation of hybrid adsorbents for CO2 capture,18 and we only provide here a short review of the existing literature. However, most of these works deal with chemical solvent as amine molecules are loaded within the pores of the solids and physical solvents which usually display low CO2 solubility received little attention. Besides, due the complexity (9) Rayalu, S. S.; Jadhav, P. D.; Chatti, R. V.; Biniwale, R. B.; Labhsetwar, N. K.; Devotta, S. Energy Fuels 2007, 21, 3555–3559. (10) Song, C. S.; Xu, X. C.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Surf. Sci. Catal. 2004, 153, 411–416. (11) Song, C. S. Catal. Today 2006, 115, 2–32. (12) Xu, X. C.; Song, C. S.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Energy Fuels 2002, 16, 1463–1469. (13) Xu, X. C.; Song, C. S.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Microporous Mesoporous Mater. 2003, 62, 29–45. (14) Xu, X. C.; Song, C. S.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Environ. Technol. Manage. 2004, 4, 32–52. (15) Xu, X. C.; Graeffe, B.; Song, C. S. Fuel Chemistry 2004, 49, 259–260. (16) Xu, X. C.; Song, C. S.; Miller, B. G.; Scaroni, A. W. Fuel Chemistry 2004, 49, 300–301. (17) Xu, X. C.; Song, C. S.; Miller, B. G.; Scaroni, A. W. Ind. Eng. Chem. 2005, 44, 8113–8119. (18) Choi, S.; Drese, J. H.; Jones, Ch.W. Chem. Sus. Chem. 2009, 2, 769–854.

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of the systems, there is a lack of work dedicated to understanding the microscopic mechanisms of adsorption occurring in such systems. Recently, Miachon et al.19,20 found that a remarkable enhancement of hydrogen and light hydrocarbon solubility can be achieved when solvents are confined in mesoporous solids. For instance, by confining hexane in low-density mesoporous materials (i.e., silica aerosol), the authors observed an “oversolubility” of H2 by a factor of 4 compared to the bulk hexane system. They suggested that by reducing the volume of the solvent down to the nanometric scale a dramatic increase of gas solubility can be realized. Indeed, confining a fluid into a porous solid at such scale commonly induces significant alteration of the fluid structure as well as dynamic and physical properties as compared to the bulk ones.21-23 Once again, no detailed explanation of the microscopic mechanisms yielding the apparition of this “oversolubility” was given. The aim of this work is to investigate the possible existence of enhanced solubility in hybrid adsorbents in the framework of CO2 capture. We choose to focus on physical solvents as the most dramatic effects are expected for this kind of solvent that usually exhibit low CO2 capacities at low partial pressures. Impregnation is chosen over grafting as this technique more easily permits to screen a wide range of solids and solvents. We also perform Monte Carlo molecular simulations in the grand canonical ensemble to understand the microscopic mechanisms yielding the existence of enhanced solubility in hybrid adsorbents.

Experimental Section Hybrid Adsorbents Preparation. Commercial mesoporous alumina (Axens/Procatalyse) is chosen as a raw support in our study. Two physical solvents, propylene carbonate (PC, Merck) and N-methyl-2-pyrrolidone (NMP, Merck), are used as the impregnated agents. These solvents are commonly used for gas treating processes, especially in coal gasification applications (PC and NMP are used in Fluor Solvent and Purisol processes, respectively24). There are two ways to impregnate the solvent into the solid support. If the volume of the solution is either equal to or less than the pore volume of the support, the technique is referred to as “incipient wetness” or dry impregnation. The other method is wet impregnation, in which an excessive amount of solution is used. For solid particles, the wet impregnation method is commonly used, whereas due to very strong capillary forces existing in powdered solid, dry impregnation proves to be more efficient for this kind of system.25 Details on the two preparation methods are given below. Wet Impregnation. During wet impregnation, 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 is evacuated to the vapor pressure. Finally, the obtained (19) Miachon, S.; Syakaev, V. V.; Rakhmatullin, A.; Pera-Titus, M.; Caldarelli, S.; Dalmon, J.-A. ChemPhysChem 2008, 9, 78–82. (20) Pera-Titus, M.; El-Chahal, R.; Rakotovao, V.; Daniel, C.; Miachon, S.; Dalmon, J.-A. ChemPhysChem 2009, 10, 2082–2089. (21) Alba-Simionesco, C.; Coasne, B.; Dosseh, G.; Dudziak, G.; Gubbins, K. E.; Radhakrishnan, R.; Sliwinska-Bartkowiak, M. J. Phys.: Condens. Matter 2006, 18, R15–R68. (22) Porcheron, F.; Rousseau, B.; Fuchs, A. H. Mol. Phys. 2002, 100, 2109– 2119. (23) Porcheron, F.; Schoen, M. Phys. Rev. E 2002, 66, 041205. (24) Bucklin, R. W.; Schendel, R. L. In Acid and Sour Gas Treating Processes; Newman, S. S., Ed.; Gulf Publishing Co.: Houston, TX, 1985. (25) Hogendoorn, J. A.; van Swaaij, W. P. M.; Versteeg, G. F. Chem. Eng. Sci. 1994, 49, 3421–3438.

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Article closing valve A between the cell and the reservoir; (ii) introducing additional CO2 in the reservoir and measuring the initial pressure, PI; (iii) opening valve A and waiting until the system has reached equilibrium; (iv) measuring the final pressure of the system, PF. Since we are performing experiments in a reduced pressure range (PCO2 ∈ 0-3 bar), the CO2 gas phase is obeying the ideal gas law. Therefore, at the end of a cycle (i - 1), the total number of CO2 moles in the whole system (NTF), i.e., reservoir (NR F ) and cell (NC F ), is given by NFT ½i - 1 ¼ NFR ½i - 1 þ NFC ½i - 1 ¼

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

hybrid adsorbent is dried in air overnight to remove excess solution and entrapped gas. Dry Impregnation. In the dry impregnation procedure, the volume of the solution which equals the pore volume of the support is prepared by diluting the desired physical solvent in ethanol. Afterward, the solution is gradually added to the stirred powder until the powder becomes clotty. The hybrid adsorbent is evacuated to the vapor pressure and then exposed to dry air overnight for the removal of remaining entrapped gases. The resulting hybrid adsorbents are constituted of a raw solid support where a fraction of the pore volume is filled with a liquid absorbent. In order to distinguish the different hybrid adsorbents studied in this work, we referred to X%-liquid-solid hybrid adsorbents. For instance, 70%-NMP-Al will refer to a support of alumina which 70% of the pore volume is filled with liquid NMP. Characterization Methods. N2 adsorption/desorption experiments are performed on various solid samples to evaluate adsorbents’ physical properties. Surface areas are obtained by using Brunauer-Emmet-Taylor (BET) equation while the pore size distribution is calculated with the Barrett-Joyner-Halenda (BJH) method. To eliminate remaining adsorbed volatile compounds, adsorbents are outgassed at 77 K under vacuum for only 6 min, prior to their analysis. In order to evaluate the impregnation method used for the preparation of hybrid adsorbents, the amount of solvent confined in the mesoporous adsorbent is measured by thermogravimetric analysis (TGA). The sample is heated at 10 C min-1 to 580 C in He flow (25 mL min-1). Mass loss evolution of the sample is followed during that time. Solubility Measurements. The experimental apparatus used for measuring CO2 solubility in hybrid adsorbents is schematically represented in Figure 1. It consists of a reactor connected upstream to a gas reservoir. The whole system 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, the reactor cell is loaded with about 5 g of adsorbent. Prior to starting the experiments, the cell and the gas reservoir are vacuumed, and the system is set to reach a steady temperature at which the isotherm will be measured. CO2 gas is then introduced within the reservoir. The valve between the reactor and the reservoir is opened and remained that way until the system reaches equilibrium. The valve is then closed, and a small amount of CO2 is introduced into the reservoir to proceed to the next step of the isotherm. To determine the complete CO2 adsorption isotherm, the procedure is repeated in a pressure domain ranging from 0 to 3 bar. Pressures and temperatures within the cell and the reservoir are recorded every 1 s. We now detail the procedure used to calculate the adsorption isotherm. First, the volumes of the reservoir (VR) and the cell (VC) are estimated using a specific volumetric device. The volume available to the gas phase in the cell (VG) is then calculated by subtracting the adsorbent volume to the cell volume. Along the whole adsorption isotherm, we proceed to N injections (cycles). Each cycle of the experiment consists of the following steps: (i) Langmuir 2010, 26(16), 13287–13296

PF ½i - 1ðVR þ VG Þ ð1Þ RT

with R the ideal gas constant and PF the equilibrium gas phase pressure. When the valve A is closed, the next cycle i is initiated and an additional amount of carbon dioxide is injected within the reservoir, yielding PI ½iVR NIR ½i ¼ ð2 e i e NÞ ð2Þ RT with NR I the number of CO2 moles in the reservoir. At this step, the total number of carbon dioxide moles in the overall gas phase (NTI ) is given by NIT ½i ¼ NIR ½i þ NFC ½i - 1 ð2 e i e NÞ

ð3Þ

Finally, when the valve A is opened, CO2 is introduced within the cell and adsorbed by the solid samples. Once equilibrium is reached, we obtain NFT ½i ¼

PF ½iðVR þ VG Þ ð1 e i e NÞ RT

ð4Þ

The number of moles of CO2 adsorbed by the solid (NA) in step i is NA ½i ¼ NFT ½i - NIT ½i ð1 e i e NÞ

ð5Þ

As a result, the total number of moles of CO2 adsorbed at each pressure point (NAc) is given by the sum of the incremental contribution calculated from eq 5: i X NAc ½i ¼ NA ½i ð1 e i e NÞ ð6Þ j¼1

Molecular Simulation Molecular Models. The system studied in this work contains two different adsorbates (CO2 and solvent) confined in a porous solid material. We choose to consider a model that is simple enough to reduce the computational burden but also realistic enough to capture the main mechanisms of adsorption occurring in hybrid adsorbents. Work is in progress to consider more realistic molecular models. The solvent and CO2 molecules are modeled as spherical and nonpolar molecules interacting via a Lennard-Jones (LJ) potential. Solvent interaction parameters are chosen to model the behavior of OMCTS which is consider to behave like a model LJ fluid.26 CO2 is by no means a Lennard-Jones fluid, but it is possible to describe the bulk vapor-liquid equilibrium using an effective LJ potential.27 We use a cutoff distance beyond which the interaction potential is set to zero. The resulting expression is then 8 "   6 # > εij 12 εij < LJ r < rc U ðrÞ ¼ 4εij ð7Þ r r > : LJ U ðrÞ ¼ 0 r > rc (26) Ayappa, K. G.; Mishra, R. K. J. Phys. Chem. B 2007, 111, 14299–14310. (27) Vishnyakov, A.; Ravikovitch, P. I.; Neimark, A. V. Langmuir 1999, 15, 8736–8742.

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Ho et al. Table 1. Fluid and Solid Interaction Parameters Used for the GCMC Molecular Simulations

Figure 2. Molecular model used to simulate CO2 capture in hybrid adsorbents.

where rc is the cutoff radius of the potential, r is the distance between any pair of molecules, ε is the depth of the potential well, and σ is the particle diameter. The subscripts i and j refer to the type of molecule interacting, i.e., solvent (A) or CO2 (B), respectively. The cross-interaction parameters (σij, εij) are calculated using the following combination rules: ( σii þ σ jj σ ij ¼ ð8Þ 2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi p εij σ ij 6 ¼ εii σii 6 εjj σjj 6 where (σii, εii) and (σjj, εjj) are the like interaction parameters. For εij, due to the large size difference between CO2 and solvent molecules, the Kong combination rule is preferred to the Lorentz-Berthelot rule which is commonly used for these kinds of simulations.28 A slit-pore geometry in which two substrates are separated by a distance sz was chosen to model the confinement imposed by the solid. The resulting geometry is represented in Figure 2. In this model, the surface is modeled as smooth, flat, and homogeneous (i.e., structureless). The interactions between the solid wall and the fluid molecules are represented via the 10-4 Steele potential: 2 0 110 0 14 3 62B B B σ SF C C B σSF C C7 7 USF ðzÞ ¼ 2πFS εSF σ SF 2 6 45@ Sz A - @ Sz A 5 z( z( 2 2

ð9Þ

where FS is the number density of the solid support and z is the distance between an adsorbate molecule and the surface. The solid-adsorbate interaction parameters (σSF, εSF) are also calculated using the same combination rules, and the number density is set to mimic a faced centered cubic surface (i.e., FSσS3 = 21/2). Potential parameters of the solid substrate are chosen to model a strongly attractive surface. Interaction parameters are listed in Table 1. Simulation Methodology. We perform grand canonical Monte Carlo (GCMC) simulation to compute the adsorption of carbon dioxide molecules in the slit pore model of hybrid solid. In GCMC, the temperature T, the volume of the simulation cell V, and the chemical potential μ are fixed. For comparison with the experimental result, the input value of the chemical potential is converted into the corresponding gas pressure. The number of molecules in the simulation box is allowed to vary through particles creation and destruction. The following methodology is used to simulate CO2 adsorption in hybrid adsorbents. First, a slit pore geometry is created with standard periodic boundary conditions in the directions parallel (i.e., x and y) to the (28) Delhommelle, J.; Millie, P. Mol. Phys. 2001, 99, 619–625.

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system

σ (A˚)

ε (K)

CO2 OMCTS slit pore

3.64 7.70 3.00

246 351 800

pore walls. The lateral dimensions (sx, sy) are set to 100 A˚, while sz is set to 40 A˚, to represent typical mesoporous pore size. The model hybrid adsorbent is then prepared by filling the pore volume with the solvent molecules. A first GCMC simulation is performed with a pressure of 1010 Pa until equilibration to simulate the solvent loading process. In a subsequent step, pressure is slowly reduced to 105 Pa (= 1 bar) as in experimental conditions in order to relax the system. The resulting configuration is used as an initial configuration prior to simulating the CO2 adsorption process. After preparing the model hybrid adsorbent, we perform GCMC simulations of CO2 adsorption in the slit pore. Conformations of the systems are sampled using translation, creation/ deletion, and particle exchange between CO2 and solvent molecules. The final configuration of the previous equilibrium state of the isotherm is used as an initial configuration for the following simulation at higher pressure. Typical runs consist of 103 MC steps per each pressure point to guarantee equilibration, followed by 104 configurations to perform averages on the thermodynamic properties of interest. All simulations are performed at a temperature of T = 40 C with 10 points of pressure in the range PCO2 = 5-1000 kPa. The cutoff radius is set to rc = 35 A˚. The theoretical adsorption isotherm is constructed by the computation of the average CO2 density inside the slit pore, FCO2, as a function of the pressure of the gas phase in equilibrium with the adsorbed CO2 phase, yielding FCO2 ¼

ÆNCO2 æ V

ð10Þ

where ÆNCO2æ is the average number of CO2 molecules in the slit pore. In order to understand the microscopic mechanisms of CO2 adsorption occurring in the model hybrid adsorbent, the local density profile, FCO2(z), is also computed using FCO2 ðzÞ ¼

ÆNCO2 ðzÞæ ÆNCO2 ðzÞæ ¼ ΔV sx sy Δz

ð11Þ

where ÆNCO2(z)æ is the ensemble average of the number of CO2 molecule in an elementary slit pore volume ΔV =sxsyΔz.

Results and Discussion In this part we detail the results obtained on our hybrid adsorbents for CO2 capture. In a first step we prepare hybrid adsorbents by confining PC in the alumina support. Characterization methods are subsequently applied to ensure that the solvent is properly confined in the porosity of the solid. We then assess the potential of hybrid adsorbents for CO2 capture and investigate the existence of enhanced solubility by confining NMP in the same alumina support. Finally, molecular simulations are performed to investigate the microscopic mechanisms yielding the apparition of enhanced solubility. Characterization of Hybrid Adsorbents. Hybrid adsorbents are prepared using dry and wet impregnation to load liquid propylene carbonate within the pore of the solid. We first focus Langmuir 2010, 26(16), 13287–13296

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Article Table 2. Textural Properties of Raw Alumina Support and PC-Al Hybrid Adsorbents impregnation method wet

dry

adsorbent

surface area BET (m2/g)

pore volume (cm3/g)

Al 10%-PC-Al 30%-PC-Al 50%-PC-Al 70%-PC-Al 100%-PC-Al 10%-PC-Al 30%-PC-Al 50%-PC-Al 70%-PC-Al 100%-PC-Al

278 185 104 60 31 2 215 111 64 32 1

0.520 0.568 0.385 0.262 0.154 0.024 0.605 0.404 0.277 0.157 0.003

Figure 3. N2 adsorption isotherms on raw alumina (solid line) and 10%-, 30%-, 50%-, 70%-, and 100%-PC-Al hybrid adsorbents (dashed lines) prepared by wet impregnation. T = 77 K.

on the efficiency of these methods to impregnate the liquid without disrupting the initial solid properties and to the resulting location of the liquid in the solid. We perform N2 porosimetry on the hybrid adsorbents. Results are reported on Figure 3 for the raw alumina support and the hybrid adsorbents prepared by wet impregnation. The isotherm of the alumina support is of type IV and displays an hysteresis loop starting around P=0.5 bar. The same behavior is observed for hybrid samples after impregnation, which proves that the original mesoporous structure is retained. However, the adsorbed nitrogen quantity markedly drops when the PC loading increases, reaching almost zero when pores are completely filled with the solvent. The resulting hysteresis is also shifted toward higher pressures. Table 2 summarizes the textural properties of all the prepared samples. In comparison with alumina, the BET surface area and pore volume of the hybrid adsorbents are considerably smaller. Alumina displays a surface area of 278 m2/g, which then decreases down to ∼2 m2/g for the 100%-PCAl hybrid adsorbent regardless of the impregnation technique. The effect of solvent loading can be explained by the increasing packing of solvent molecules in the porous support leaving a decreasing volume available for nitrogen molecules, hence supporting the fact that solvent has been loaded successfully within the porous volume of the solid. The corresponding pore size distribution of the hybrid adsorbents is reported in Figure 4. The calculated mean pore size of the hybrid materials is shifted to the higher values as the solvent loading increases. This can be explained by the heterogeneity of the solid sample and by the fact that most of the micro- and mesochannels of alumina are occupied by solvent molecules after the loading process, leaving larger pores unoccupied. N2 molecules can then only access pores with larger size, and the average pore size of the hybrid materials increases consequently. To further support this conclusion, we measure the amount of solvent loaded into the mesoporous solid support using thermogravimetry analysis. The results from TGA experiments are reported in Figure 5 for the bulk solvent and the hybrid adsorbents prepared by wet impregnation. TGA experiments were also performed for the raw alumina support, but for sake of clarity the result is not reported in the figure. The mass loss curve of the original alumina remains remarkably stable up to T = 600 C while hybrid adsorbents on the other side display a different kind of behavior. For these solids, a slight loss in mass which occurs in the temperature range of Langmuir 2010, 26(16), 13287–13296

Figure 4. Pore size distribution of raw alumina (solid line) and 30%-, 50%-, and 70%-PC-Al hybrid adsorbents (dashed lines) prepared by wet impregnation.

T = 100-200 C can be attributed to the evaporation of preadsorbed water and other trapped gases. In addition, a second sharp slope is observed between T=200 and 230 C, which is close to the boiling point of PC (T=240 C). Above T = 250 C, the rate of mass loss starts to increase. A further increase in temperature up to roughly T=600 C results in a complete decomposition of PC. Undoubtedly, a decomposition retarding effect of the solvent is attained when confining the solvent down to mesometer scale, which was also reported by Xu et al.12 and Son et al.29 In addition, the observation of uniform mass loss peaks for the hybrid adsorbents with different amount of PC loading sustains the fact that the pores of alumina are large enough for the molecules of solvent to move in and out with little hindrance. In view of these results, it seems that a successful confinement of the solvent into the porosity of alumina is achieved. Furthermore, solvent weight losses measured from TGA experiments are compared to the initial mass of solvent used in the adsorbents preparation (Table 3). The good agreement between the values indicates that the PC loss is negligible during the preparation process. TGA values are slightly higher than the initial mass of solvent used in the preparation step. These minor discrepancies could reasonably be attributed to the presence of adsorbed moisture in the samples. For example, the total weight loss for 100%-PC-alumina was 47.7 wt % at 580 C, and the adsorbed moisture excluded from the total weight is ∼3 wt %. The resulting PC loading is then about 44.7 wt %, which is in good (29) Son, W. J.; Choi, J. S.; Ahn, W. S. Microporous Mesoporous Mater. 2008, 113, 31–40.

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Ho et al. Table 3. Comparison between TGA Mass Loss Measurements and Initial Mass of Solvent Used for the Preparation of Hybrid Adsorbents impregnation method wet

dry

adsorbent

mass % of solvent used for impregnation

mass % loss measured by TGA

100%-PC-Al 70%-PC-Al 50%-PC-Al 30%-PC-Al 10%-PC-Al 100%-PC-Al 70%-PC-Al 50%-PC-Al 30%-PC-Al 10%-PC-Al

43.8 35.1 29.0 19.0 7.5 43.6 35.1 28.3 18.8 7.2

47.7 41.5 36.3 22.4 11.6 46.9 41.4 34.9 19.2 10.9

Figure 5. TGA mass loss profiles of bulk PC (solid line) and 30%-, 50%-, 70%-, and 100%-PC-Al hybrid adsorbents (dashed lines) prepared by wet impregnation.

agreement with the value of mass loss of solvent measured from TGA, and well-matched with the targeted solvent loading in the preparation step. In conclusion, the solvent can be confined in the porous volume without modifying the original properties of the solid. Pore loading can also be controlled efficiently, and the solvent can occupy a part or the totality of the porous volume. In the following section we will evaluate the thermodynamic properties of these hybrid adsorbents with regard to CO2 capture. Evidence of Enhanced CO2 Solubility in Hybrid Adsorbents. First, we need to set a common basis to compare all the systems with each other. Indeed, as we are comparing different phenomena, i.e., absorption and adsorption, the usual expression of solubility within the system may differ. In adsorption, it is often common to express solubility in term of gram of gas adsorbed per gram of solid. For absorption, however, one prefers to consider the number of moles of gas absorbed per mole of solvent (also known as loading R). To combine these two aspects, we choose to consider a volumetric approach as we consider that empty pore volume of the raw adsorbent is replaced by a liquid volume when dealing with hybrid adsorbents. We therefore calculate the number of CO2 moles adsorbed in the hybrid adsorbent per cm3 of pore volume. The corresponding quantity is obtained for the bulk solvent by calculating the number of CO2 moles absorbed per cm3 of liquid. CO2 adsorption isotherm in 100%-NMP-Al hybrid adsorbent prepared by wet impregnation at T = 40 C is presented in Figure 6, together with the corresponding raw solid and bulk solvent isotherms. We clearly observe the apparition of an enhanced CO2 solubility in the hybrid adsorbent over the whole range of pressures studied in this work. Roughly, the hybrid adsorbent capacity is 5 times larger than that of the bulk fluid and about 20% larger than the raw adsorbent. The shape of the adsorption isotherm in the hybrid adsorbent resembles the one in the raw adsorbent, suggesting that adsorption mechanism is still predominant over absorption in these systems. This demonstrates that a specific effect can occur when a solvent is confined within a porous material. Hybrid adsorbents prepared via wet or dry impregnation exhibit the same performance of CO2 adsorption. NMP is a relatively nonviscous solvent which may favor the diffusion of molecules within the solid regardless of the impregnation method used. Thus, for low viscous solvents, the chosen impregnation technique does not considerably influence the resulting CO2 adsorption performance of the hybrid adsorbent. 13292 DOI: 10.1021/la1015934

Figure 6. CO2 adsorption isotherms at T = 40 C for the raw alumina solid (filled triangles), bulk PC (filled squares), and 100%NMP-Al hybrid adsorbent (filled circles).

Thermodynamic Modeling of Enhanced CO2 Solubility in Hybrid Adsorbents. We now consider hybrid adsorbents displaying a set of different loadings ranging from 30% to 100% of the total pore volume of the adsorbent. Results are presented in Figure 7. The bulk solvent exhibits a relatively weak absorption capacity throughout the entire pressure range investigated in this work. For instance, the CO2 absorption capacity is Q=5.4 mg/cm3 at PCO2=1 bar. Comparatively, the CO2 adsorption capacity of the raw adsorbent is about 3 times larger at this pressure (Q = 18.6 mg/cm3). When 30% of the pore volume is loaded with NMP, the adsorption isotherm of the hybrid adsorbent appears relatively close to the one of nonimpregnated solid (Q = 17.4 mg/cm3 at PCO2=1 bar). However, when increasing the concentration of solvent in the pores of the solid, the experimental adsorption capacity shows a steady increase in the whole range of pressure. At PCO2=1 bar, CO2 adsorption capacities of the 50, 70, and 100% hybrid adsorbents are about Q=20, 21, and 24 mg/cm3, respectively. Since physical interactions are predominant between CO2 and both the solid support and the solvent, hybrid adsorbent properties could theoretically be modeled as a simple linear combination of the two constitutive systems (i.e., solid support and bulk solvent) properties, using QT ¼ %VSP Qs þ %VLP Ql

ð12Þ

where QT is the theoretical adsorption capacity, Qs and Ql are the adsorption capacity of the nonimpregnated solid and the solvent Langmuir 2010, 26(16), 13287–13296

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Figure 7. CO2 adsorption isotherm at T = 40 C for the raw alumina solid (filled triangles), bulk NMP (filled squares), and 30%- (open circles), 50%- (open triangles), 70%- (open diamond), and 100%-NMP-Al (filled circles) hybrid adsorbents.

bulk, %VPS, and %VPL are the pore volume percentage occupied by the pure solid and the liquid solvent in the hybrid adsorbent, respectively. Any deviation from this ideal behavior may be quantified using SF ¼ QE - QT

ð13Þ

where QE is the hybrid adsorbent capacity and SF is defined as the synergetic factor in mg of CO2 per cm3 of pore volume. For example, the theoretical adsorption capacities of 30-, 50-, 70-, and 100%-NMP-Al at PCO2 =1 bar are estimated to QT = 14.6 mg/cm3 (18.6  0.7 þ 5.4  0.3), 12.0 mg/cm3 (18.6  0.5 þ 5.4  0.5), 9.4 mg/cm3 (18.6  0.3 þ 5.4  0.7), and 5.4 mg/cm3 (18.6  0.0 þ 5.4  1.0). All these values are lower than the ones obtained from the experiment. This reflects a positive synergetic enhancement in the adsorption of CO2, as illustrated in Figure 8. We clearly observe a increasing trend of the synergetic effect when the pore loading increases. Therefore, the sorption in hybrid adsorbents cannot be modeled as a simple linear combination of adsorption in raw support and absorption in bulk solvent and is somehow a more complex phenomenon. In fact, when the pores are completely loaded with solvent, the adsorption capacity roughly equals the sum of the two constitutive systems. Therefore, there is clearly a synergetic effect which is maximized when the whole pore is totally filled with liquid. However, between the two sorption mechanisms, there might be one phenomenon that prevails, but that is different from the one encountered in the constitutive systems of the hybrid adsorbent. Looking at the isotherm shape, it seems that adsorption of CO2 molecules at the solid surface is still predominant. Further investigations on the molecular mechanisms will be pursued in the next section by performing molecular simulations of CO2 adsorption in hybrid adsorbents. In order to extract thermodynamic properties of interest, the experimental adsorption data of all the prepared samples are then fitted using a Langmuir-Freundlich isotherm modeled by the Sips equation, which is a combined form of Langmuir and Freundlich models. The Sips equation is expressed as Qs ¼

Qsmax ðbPÞ1=n 1 þ ðbPÞ1=n

ð14Þ

where Qs and Qsmax are the CO2 uptake and the maximum adsorption capacity, respectively; P is the equilibrium pressure of CO2, b is an affinity constant of CO2 adsorption, and n is a Langmuir 2010, 26(16), 13287–13296

Figure 8. Synergetic adsorption effect for NMP-Al hybrid adsorbents with different solvent loadings at PCO2=1 bar. The line is intended to guide the eye.

parameter of the Sips equation reflecting the heterogeneity of the sample. CO2 adsorption isotherms on raw alumina and hybrid adsorbents are measured at T = 40, 60, and 80 C to evaluate the evolution of hybrid adsorbents capacities with regard to temperature and fitted with the Langmuir-Freundlich model (Figure 9). The Sips parameters obtained for these adsorbents are presented in Table 4. An increase of temperature consistently results in a significant decrease of the CO2 adsorption capacity, as gas adsorption in porous solids and CO2 absorption in physical solvents are both exothermic. Besides, the isosteric heat of adsorption could be obtained from the series of adsorption isotherms at different adsorption temperatures, helping to estimate the interaction intensity between CO2 and the adsorbent, which is accounted for in the van’t Hoff equation:   2 D ln P - ΔHst ¼ RT ð15Þ DT θ where -ΔHst is the isosteric heat of adsorption, R is the universal gas constant (R = 8.3145 J mol-1 K-1), and θ is the fractional loading of the adsorbate. Figure 10 displays the evolution of the isosteric heat of adsorption with CO2 loading in the raw alumina support, the bulk solvent, and the 100%-NMP-Al hybrid adsorbent. The isosteric heat of CO2 adsorption in the hybrid adsorbent is not only larger than in the bulk solvent but also than in the original mesoporous alumina. At low CO2 coverage, Qst=25.2 kJ mol-1 for the hybrid adsorbent compared to Qst = 13.3 kJ mol-1 and Qst = 12.4 kJ mol-1 in the raw alumina and NMP bulk systems, respectively. Therefore, the energy required to remove CO2 molecules from the solid increases in the hybrid adsorbent. A closer look at the data reveals that, at low coverage, the isosteric heat of adsorption in the hybrid adsorbent is roughly the sum of the ones in the raw support and in the bulk solvent. We may then suppose that, at low pressures, the CO2 molecules are still adsorbed onto the surface of alumina but are experiencing an additional potential field created by the solvent molecules in the pore, thereby not only enhancing the adsorption capacity but also increasing the total interaction energy of CO2 with its surrounding. The isosteric heat of adsorption of the hybrid adsorbent then decreases rapidly with increasing CO2 coverage, suggesting that additional CO2 molecules do not experience the same energetic DOI: 10.1021/la1015934

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Figure 10. Isosteric heat of CO2 adsorption for raw alumina (filled triangles), bulk NMP (filled squares), and 100%-NMP-Al hybrid adsorbent (filled circles).

Figure 9. Langmuir-Freundlich modeling of CO2 adsorption in raw alumina (top) and 100%-NMP-Al hybrid adsorbent (bottom) at T1 = 40 C (solid line), T2 = 60 C (dashed lines), and T3 = 80 C (dotted lines). Table 4. Sips Parameters Used To Model CO2 Adsorption Isotherms in Raw Alumina and 100%-NMP-Al Hybrid Adsorbent adsorbent Al 100%-NMP-Al

T (K)

Qmax (mmol/g pure solid)

b (kPa-1)

n

313 333 353 313 333 353

2.9750 2.9746 2.9746 3.9670 3.9652 3.9629

0.000 429 0.000 317 0.000 243 0.000 184 0.000 121 0.000 084

1.4766 1.4763 1.4761 1.8614 1.8424 1.8259

environment. Possibly, the molecules are located further away from the solid surface and are thus more surrounded by solvent molecules. In fact, the mechanism of CO2 adsorption in hybrid adsorbents is a very complex phenomenon that depends on many parameters such as thermodynamic conditions, solid surface structure, and the interaction between the adsorbed molecules in the pores (solvent-solvent, solvent-CO2) as well as between the surface and these adsorbates (surface-solvent, surface-CO2). Therefore, in order to gain a more in-depth view on the microscopic mechanisms occurring in the investigated hybrid adsorbents, in the last part of this study, we have used grand canonical Monte Carlo simulations to interpreting CO2 solubility behavior in a model system of hybrid adsorbent. Microscopic Mechanisms of Enhanced CO2 Solubility in Hybrid Adsorbents. Before performing simulation of CO2 adsorption within porous adsorbents, we first check the consistency of the molecular models used in this work. The absorption 13294 DOI: 10.1021/la1015934

Figure 11. Experimental (squares) and simulation (solid line) absorption isotherms of CO2 in OMCTS bulk at T = 40 C.

isotherm of CO2 in the OMCTS bulk solvent obtained from our GCMC simulation model is plotted together with the experimental data (Figure 11). The simulation results exhibit an excellent match with the experimental isotherm at the thermodynamic conditions probed in this work. This good correlation proves that, although simple enough, the CO2 and solvent models are sufficiently accurate to correctly describe CO2 absorption in the bulk solvent. Figure 12 reports simulation results of CO2 adsorption at T= 40 C in three different models: raw slit pore, solvent bulk, and hybrid solid. We first notice that the shapes of the adsorption isotherms are similar for each models, in that the CO2 concentration increases steadily with the pressure. The average density of CO2 increases following hybrid adsorbent > solid support > solvent, in good qualitative agreement with the experimental results on systems displaying enhanced CO2 solubility in hybrid materials. In order to get a more in-depth view on the microscopic structure of CO2 confined in the hybrid solid model, CO2 local density profiles are plotted in Figure 13 at a pressure PCO2=1 bar for the three kinds of systems. The distribution probability is plotted as a function of the distance from the slit walls located at sz = (0.5. Density profiles in the solids display an oscillatory damped behavior reflecting the organization of CO2 molecules in discrete layers parallel to the Langmuir 2010, 26(16), 13287–13296

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Figure 12. CO2 adsorption isotherms in the models of bulk solvent (filled squares), raw slit pore (filled triangles), and hybrid adsorbent (filled circles) at T = 40 C.

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Figure 14. Local CO2 density profiles in the models of raw slit pore (filled triangles) and hybrid adsorbent (open squares) at PCO2 = 1 bar (raw slit pore) and PCO2 = 10, 50, and 100 kPa (hybrid adsorbent); T = 40 C.

(Figure 14). A homogeneous decrease of the CO2 density is observed when PCO2 decreases. However, the density profile never matches the one in the raw solid at PCO2=1 bar, thereby proving that the two adsorption mechanisms are totally different.

Conclusions

Figure 13. Local CO2 density profiles in the models of bulk solvent (solid line), raw slit pore (filled triangles), and hybrid adsorbent (filled squares) at PCO2=100 kPa and T = 40 C.

solid surface. One single peak near the substrate is observed for the raw solid while two other peaks appear further away from the wall in the hybrid adsorbent. The density of the contact layer at the solid interface is increased in the hybrid adsorbent with regards to the one formed in the raw solid. In the raw solid, prior to confining the solvent, the CO2-wall interactions prevail, resulting in the formation of a contact layer. However, after pores are filled up with solvent, the solvent molecules create an additional potential field, thereby enabling more CO2 to be adsorbed at the solid interface. In other words, the solvent acts as a “structuring promoter” for the CO2 molecules. This promoting effect is further identify as a second (and possibly a third) layer of CO2 molecules starts to form in the pore. In this region, the CO2wall interactions are weaker; hence, CO2 molecules experience mostly a potential field created by the solvent molecules. Since these ones are structured as discrete layers, the layering propagates throughout the pore volume. It should also be noted that the CO2 density in the pore is always greater than the one of the bulk solvent, sometimes surpassing this value by a factor of 10. In order to further investigate the microscopic mechanisms of adsorption in these systems, we report CO2 density profile in hybrid adsorbents at different pressures along the isotherm Langmuir 2010, 26(16), 13287–13296

In this study, hybrid adsorbents composed of physical solvents confined within the porosity of a solid are prepared and evaluated for CO2 capture. Impregnation methods are employed to confine the liquid solvent into the pore of a solid support, and the analysis of the resulting hybrid solid samples conclusively demonstrates that an efficient confinement of the liquid in the porous volume is reached. CO2 adsorption experiments are carried out on hybrid adsorbents, raw solid, and bulk solvent. Hybrid adsorbents can display an enhanced CO2 solubility with regard to the raw solid and the bulk solvent. Moreover, examination on the effect of solvent loadings in the solid support clearly shows an increase of CO2 adsorption capacity with increasing the concentration of the solvent. The highest adsorption capacity is achieved when pore channels of the support medium are completely filled up with solvent. The effect of temperature on CO2 adsorption is also investigated, and isotherms are fitted using the Langmuir-Freundlich equation which enables the calculation of the isosteric heat of CO2 adsorption. The adsorption mechanism seems to be still predominant but is somewhat modified as compared to the one encountered in the raw solid. Furthermore, to gain deeper insights on the microscopic mechanisms occurring in the investigated hybrid adsorbents, GCMC simulations are performed using simplified models capturing the main physics of CO2 adsorption in hybrid adsorbents. A structureless slit pore is used as the solid support and adsorbate molecules are modeled as Lennard-Jones fluids. A good qualitative agreement is found between the simulated and experimental data regarding the enhancement of CO2 solubility in the hybrid material. Inspection of the CO2 density profile reveals that the solvent molecules create an additional “structuring” field allowing closer-packing of CO2 molecules at the solid interface and in the pore volume. This adsorption mechanism is different from the one found in the raw solid. However, as a predictable consequence of using a rather simple model to perform GCMC simulation, the overall adsorption DOI: 10.1021/la1015934

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behavior reasonably matches the experimental data, but the adsorption isotherms shape hardly resemble the ones measured experimentally. It is therefore necessary to consider more realistic molecular models as for instance the electrostatic nature of CO2 has to be taken into account, and we also plan (30) Coasne, B.; Galarneau, A.; Di Renzo, F.; Pellenq, R. J.-M. Langmuir 2006, 22, 11097–11105.

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to consider more realistic porous materials like the recently introduced MCM-41 atomic scale pore model.30 Besides, a real gas composition is much more complex and contains other species (e.g., N2) for which we might also observe the apparition of enhanced solubility. In a future work, we will develop this aspect and study the selective removal of CO2 from the flue gas.

Langmuir 2010, 26(16), 13287–13296