Ind. Eng. Chem. Res. 2008, 47, 201-208
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Polyethyleneimine-Modified MCM-48 Membranes: Effect of Water Vapor and Feed Concentration on N2/CO2 Selectivity Parveen Kumar,† Sangil Kim,†,‡ Junichi Ida,†,§ and Vadim V. Guliants†,* Department of Chemical and Materials Engineering, UniVersity of Cincinnati, Cincinnati, Ohio 45221, Department of Chemical Engineering, Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia 24060, and Department of EnVironmental Engineering for Symbiosis, Faculty of Engineering, Soka UniVersity 1-236 Tngi-cho, Hachioji-shi, Tokyo 192-8577, Japan
Ordered mesoporous MCM-48 membranes with cubic pore structure were prepared by the solution growth method on symmetric R-alumina supports using cetyltrimethylammonium bromide as a surfactant. The surfactant was removed by Soxhlet extraction using an ethanol/HCl mixture to synthesize defect-free MCM48 membranes as confirmed by N2 unsteady-state permeation. MCM-48 membranes and powders were successfully modified with a polymer, polyethyleneimine (PEI) containing amino groups as confirmed by N2 adsorption-desorption data, X-ray diffraction, Thermo gravimetric analysis (TGA), and N2 unsteady-state permeation experiments. The PEI/MCM-48 membranes were permeable to N2 after pore modification, suggesting incomplete filling of MCM-48 pores by PEI, which was confirmed by the N2 adsorption-desorption experiments on the bulk PEI/MCM-48 powder. The PEI-modified MCM-48 membranes were highly selective to N2 permeation from N2/CO2 mixtures at room temperature in the presence of ∼2.6% water vapor. Binarygas N2/CO2 permeation data for the PEI-modified MCM-48 membranes showed much-higher N2 permeance as compared to CO2 permeance in the presence of water vapor. The selectivities increased with CO2 concentration in the feed, and selectivities greater than 80 were observed. The N2/CO2 selectivities diminished significantly at 363 K for the PEI/MCM-48 membranes. 1. Introduction Fossil fuels are likely to remain the main source of energy supply in the 21st century. However, increased CO2 concentration in the atmosphere due to fossil fuel combustion has caused concerns about global warming.1,2 Concentration of CO2 in the atmosphere was estimated to be ∼380 parts per million (ppm) in 2005.3 The Intergovernmental Panel on Climate Change (IPCC) projects that atmospheric CO2 levels could reach 450550 ppm by 2050, possibly resulting in higher temperatures and rising sea levels. Today the United States, the world’s largest economy and consumer of energy, produces about 24% of global CO2 emissions. Therefore, significant reduction of CO2 emissions from the current level is necessary to stabilize atmospheric concentration of CO2. A number of mitigation technologies, including CO2 sequestration (geological, oceanic, and terrestrial) and novel CO2 utilization, are currently under investigation.4-8 Most of the mitigation approaches require CO2 in a concentrated form, whereas the CO2 from power plants is mixed with N2, water vapor, oxygen, and other impurities and is present at a low ∼15% concentration. Therefore, capturing CO2 from flue gas in a concentrated form is a critical step that precedes a variety of proposed sequestration approaches. The current commercial technologies for CO2 capture from flue gas are based on absorption using liquid amines,9-11 most commonly by monoethanolamine and diethanolamine (MEA and DEA). The major drawback of using liquid amines is a large amount of energy required for their regeneration to release concentrated CO2. In commercial MEA applications, a large * To whom correspondence should be addressed. E-mail: Vadim.
[email protected] † University of Cincinnati. ‡ Present address: Polytechnic Institute and State University. § Soka University.
volume of solvent (typically containing 70-88% water) must be heated to release a relatively small amount of CO2 and regenerate the solvent.12,13 CO2 separation employing polymeric membranes is another approach for CO2 separation from lowpurity sources, such as the power plant-flue gas.8,14,15 For a polymeric membrane to be economically attractive, it should provide both high gas permeability and selectivity. Numerous studies have established an inverse relationship between the permeability and selectivity in polymer membranes.16 The CO2/ N2 selectivity and CO2 permeance values for the polymers used for CO2 separation are significantly lower than desired for membrane-separation technology to be cost-effective.17 Koros and co-workers18 suggested the use of polymers confined to a mesoporous inorganic environment as a very attractive system for gas separations. The authors studied a variety of glassy polymers with inherently good diffusivity-based separation characteristics and found that the permselectivity in O2/N2 separation improved by 20-100% when the polymers were confined in the mesoporous silica environment. They proposed that specific and nonspecific energetic interactions between the polymer and silica modify the packing of the polymer to create a better material for sieving. More specifically, the authors hypothesized that within the confined polymer matrix there exist regions of disrupted packing that offer more free volume and regions of enhanced density that provide better selectivity. In this article, we report ordered mesoporous silica membranes modified with amine-containing polymers for CO2 separation from flue gas. Ordered meso- and microporous silica membranes and thin films have a unique advantage that their pore size can be fine-tuned in the 1.6-3.0 nm range,19-22 which can assist in the grafting of large polymeric molecules in the pores. Another unique feature of these membranes is that various amino groups (-NH2) can be attached to their surface silanol groups, which will provide the sites for CO2 adsorption and
10.1021/ie070700d CCC: $40.75 © 2008 American Chemical Society Published on Web 11/29/2007
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facilitate its separation from N2 and O2. In this study, we investigate the use of polyethyleneimine (PEI) confined to the pore channels of ordered mesoporous MCM-48 silica membranes for CO2 separation from N2 in the presence and absence of water vapor. We chose PEI because it contains primary, secondary, and tertiary amino groups, which have high affinity for acidic CO2. The presence of PEI in the pores of mesoporous silica should provide a high CO2/N2 solubility selectivity23,24 because PEI has a high CO2 adsorption capacity and negligible N2 adsorption capacity. MCM-48 possesses a 3D interconnected cubic pore structure, which decreases diffusion limitations and is, therefore, more attractive than MCM-41. The MCM-48 silica membranes contain a large concentration of surface silanol groups inside ordered mesopores, which could be easily functionalized by amine-containing polymers. The functionalization of MCM-48 with PEI is expected to prevent the permeation of weakly interacting gases, such as N2 and H2 and, therefore, lead to selective CO2 permeation. On the other hand, as reports25-27 have suggested that the CO2/N2 selectivities significantly lower than 1 are obtained at very high polymer solubility parameters where the affinity between polymer segments and CO2 reaches a maximum. Freeman et al.26 studied the CO2/H2 system and suggested that any enhancement in CO2/H2 solubility selectivity afforded by PEI molecules was more than offset by the increase in H2/CO2 diffusivity selectivity due to PEI addition. Strong interactions between PEI molecules and CO2 significantly hinder the diffusion of CO2 and hence lead to H2/CO2 diffusivity selectivities significantly greater than 1. Therefore, MCM-48 membranes that are slightly N2 selective, on modification with aminecontaining PEI molecules may also become highly N2 selective because of enhanced N2/CO2 diffusion selectivity owing to strong interaction of CO2 (and hence slow diffusion) with polymeric amine molecules. In this study, we report the successful modification of MCM48 membranes with amine-containing polymer (PEI) to synthesize defect-free PEI/MCM-48 membranes. The PEI-modified membranes were largely free of macroscopic defects that would produce nonselective viscous flow behavior. Furthermore, we observed high N2/CO2 selectivity (>80) for PEI-modified MCM-48 membranes in the presence of water vapor, which we explain by the enhanced affinity of PEI toward CO2 in the presence of water vapor, hindering CO2 diffusion. The current PEI-modified MCM-48 membranes obtained by the solution growth method suffered from support infiltration by the MCM48 particles and, therefore, displayed low N2 permeance, requiring excessively high membrane areas for an economical capture of CO2 from flue gas. The gas permeance of these membranes may be significantly enhanced by reducing the thickness of the MCM-48 layer by support masking and other synthesis approaches to make these membranes an economically viable option for CO2 capture from dilute sources, such as flue gas. 2. Experimental Section The surfactant cetyltrimethylammonium bromide was removed from as-synthesized MCM-48 by Soxhlet extraction with a mixture of EtOH and hydrochloric acid to obtain mesoporous MCM-48 powders and membranes.28,29 PEI (Mn ) ∼10 000, Aldrich) containing 25% primary amines, 50% secondary amines, and 25% tertiary amines was attached
Figure 1. Schematic of PEI attachment on MCM-48 mesoporous silica.
to the surface of the mesoporous silica by a two-step grafting procedure:30
-Si-OH + C2H5-O-Si-R-Cl f -Si-O-Si-R-Cl + C2H5OH -Si-O-Si-R-Cl + R′-NH f -Si-O-Si-R-NH-R′ + HCl In the first step, the surface silanol groups of MCM-48 silica were reacted with 3-chloropropyltriethoxysilane to form surface chloropropyl-silyl groups. These surface chloropropyl groups served as linkers for the attachment of PEI to the surface of mesopore channels in MCM-48. The MCM-48 membranes and powders were first heated at 120 °C for 4 h in vacuum to remove the physically adsorbed moisture. The MCM-48 silica was then kept in a solution of 3-chloropropyltriethoxysilane (Aldrich) and anhydrous toluene (Aldrich) for 3 h at 110 °C to attach chloropropyl groups to the MCM-48 surface. The chloropropylmodified MCM-48 silica was then functionalized with branched PEI by the nucleophilic substitution of the chlorine atom with the primary, secondary, and tertiary amino groups of PEI in THF for 20 h at 70 °C. A schematic of PEI attachment in the mesopores of MCM-48 is shown in Figure 1. The XRD patterns of the original and PEI-modified MCM48 powders were recorded on a Siemens D-500 diffractometer using Cu KR radiation with a step size of 0.02°/s. The thermogravimetric analysis (TGA) was carried out on PEIattached MCM-48 powders using SDT 2960 simultaneous DSCTGA from TA Instruments. For the TGA analysis, the PEIattached MCM-48 powder was heated at 25-700 °C in flowing air (100 mL/min) at a rate of 10 °C per minute. The N2 adsorption-desorption isotherms for MCM-48 and PEI/MCM48 powders were collected at 77 K using a Micromeritics TriStar 3000 porosimeter. The surface areas were calculated using the BET method, and the pore volumes and diameters were calculated by the BJH method.31 In situ IR studies of CO2 adsorption on the PEI-modified MCM-48 silica were conducted employing a BioRad FTS-60 FTIR spectrometer. For these studies, the PEI-modified MCM-48 silica was pressed into a thin wafer and placed in an in situ IR cell. Prior to CO2 adsorption, these wafers were outgassed in flowing helium (130 mL/min) for 2.5 h at 393 K. The helium flow was then replaced with 1.5% CO2 in helium (in the absence and presence of water vapor) under ambient conditions. After 2 h, the CO2 atmosphere in the in situ cell was displaced with helium, and IR spectra were collected at 4000-13 000 cm-1. The unsteady-state N2 permeation experiments were carried out on MCM-48 and PEI/MCM-48 membranes at room temperature to confirm the absence of macroscopic defects and to study the effect of PEI attachment on gas permeance. A schematic representation of the unsteady-state permeation system was shown in our previous publication.28 Three different pressures (10-30 psig) were used on the feed side. This system is referred to as the unsteady-state permeation system because the pressure on the downstream side changed as a function of
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Figure 2. Schematic representation of the binary-gas permeation system.
time. The gas permeance was calculated from the slope of the downstream pressure versus time plot, which gives a dP′′/dt value according to eq 1. where Vc is the gas tank volume, Am
Permeance,
Vc dP′′ F ) L RTAm(P′-P′′) dt
(1)
is the membrane area, P′ is the upstream pressure, P′′ is the downstream pressure, and dP′′/dt is the rate of a gas pressure increase in the tank. The binary N2/CO2 permeation experiments were carried out both in the absence and in the presence of water vapor at 20 psia feed pressure. A schematic of the binary-gas permeation system is shown in Figure 2. A gas bubbler located on a feed side was used to introduce water vapor into the feed. Furthermore, the pressure drop in the bubbler was kept constant by monitoring the water level. The temperature in the water bubbler and gas lines leading from the bubbler was maintained at 293 ( 1 K. This temperature was chosen to keep the water vapor concentration in the feed constant during permeation experiments at both 293 and 363 K. The water-vapor concentration was estimated to be 2.60 ( 0.16% from psychrometric charts assuming complete saturation of the feed gas at 293 ( 1 K. The water-vapor concentration was confirmed by monitoring weight changes of water in the bubbler as a function of time. The permeate and retentate streams were analyzed using a GC-MS system equipped with fused silica capillary column. The binary N2/CO2 separation factors of the MCM-48 and the PEI-modified MCM-48 membranes were calculated based on the GC-MS analysis data of the permeate gas. The GC-MS system was calibrated for several CO2 and N2 concentrations. The CO2 and N2 concentrations in the permeate stream were obtained by inserting the CO2 and N2 areas in the CO2 and N2 calibration curves. The N2/ CO2 separation factors were calculated using the following formula:
RN2/CO2 )
[XN2/XCO2 ]permeate [XN2 /XCO2 ]feed
where XN2 is the mole fraction of N2 and XCO2 is the mole fraction of CO2 in the gas mixture. A bubble-flow meter was employed to measure the total gasflow rate on permeate and the retentate side. The total gasflow rates were used to calculate the total gas permeance values, which were used in turn to determine the CO2 and N2 permeance values (using the CO2 and N2 concentrations) in the permeate gas. Single-gas CO2 and N2 permeance values were also measured in the absence and in the presence of water vapor.
Figure 3. XRD patterns of (A) MCM-48 and (B) PEI-modified MCM-48 powders.
Figure 4. DTG curve for PEI/MCM-48 powder sample.
The steady-state permeances and separation factors were attained after 3-4 days on stream. However, in some cases permeation experiments were conducted for up to 2 weeks to confirm the steady state. 3. Results and Discussion The powder XRD patterns of the MCM-48 phase before and after pore modification with PEI are shown in Figure 3. The XRD pattern of the bulk PEI/MCM-48 sample was similar to that of the original MCM-48 silica, suggesting that the MCM-48 structure was preserved after pore modification with PEI. A significant decrease in the intensity of MCM-48 peaks was observed after PEI attachment. The decrease in intensity of the peaks was observed because the peak intensity is a function of scattering contrast between the silica walls and the pore channels and decreases with decreasing scattering contrast after filling of the MCM-48 pores with PEI. The derivative TGA weight loss (DTG) curve for the representative PEI/MCM-48 powder is shown in Figure 4. The sample showed weight loss below 100 °C associated with physically adsorbed water. Above 100 °C, the sample exhibited a small weight loss associated with condensation of surface silanol groups. Above 200 °C, a large weight loss associated with decomposition of surface-attached polymeric amines was
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Ind. Eng. Chem. Res., Vol. 47, No. 1, 2008 Table 2. The N2/CO2 Separation Factors for the PEI/MCM-48 Membrane at 293 K in the Absence of Water Vapor
pressure 20 psi (138 KPa)
feed composition (%)
permeate composition (%)
separation factor (R)
N2
CO2
N2
CO2
N2/CO2
80
20
83.8
16.2
1.31
Table 3. The N2/CO2 Separation Factors for the PEI/MCM-48 Membrane at 293 K in the Presence of 2.6 vol % Water Vapor
pressure Figure 5. N2 permeance of MCM-48 and PEI-modified MCM-48 membranes at 293 K plotted against pressure difference (∆P) across the membranes. Table 1. Structural Properties of MCM-48 and PEI/MCM-48 Silicas
sample
BET surface area (m2/g)
BJH ads. pore volume (cm3/g)
BJH ads. pore diameter (nm)
MCM-48 PEI/MCM-48
1027 80
1.14 0.08
2.6 -
observed. TGA analysis confirmed that MCM-48 silica was successfully modified with polymeric amine, PEI. The BET surface areas, pore volumes, and diameters for the original MCM-48 and PEI/MCM-48 powders are summarized in Table 1. A significant decrease in the surface areas and pore volumes was observed after MCM-48 pore modification with PEI. As shown in Table 1, the PEI/MCM-48 sample possessed 0.08 cm3/g pore volume and 80 m2/g surface area compared to 1.14 cm3/g pore volume and 1027 m2/g surface area for extracted MCM-48 powder. This suggested incomplete filling of MCM48 pores with PEI. The pore size for the PEI/MCM-48 sample could not be measured using the BJH method. The crosssectional SEM/EDS data for the MCM-48 membranes reported in our previous publication28 suggested the effective MCM-48 layer thickness to be ∼0.6 mm. Nearly 70% of the MCM-48 mesopore space was functionalized with PEI, suggesting the thickness of the PEI modification layer to be ∼0.4 mm. The unsteady-state N2 permeation studies were first carried out on extracted MCM-48 membranes at three different feed pressures to confirm the absence of macroscopic defects. The synthesis procedures for MCM-48 membranes have been discussed elsewhere.28,29 MCM-48 membranes after pore modification with PEI (denoted as PEI/MCM-48 membranes) were characterized by N2 unsteady state permeation to study the effect of PEI attachment on gas permeation and to confirm the synthesis of defect-free PEI/MCM-48 membranes. The unsteady state N2 permeation data for MCM-48 and PEI/MCM-48 membranes are shown in Figure 5. The N2 permeance values for MCM-48 and PEI-attached MCM-48 membranes were independent of feed pressure, confirming the synthesis of defectfree MCM-48 and PEI/MCM-48 membranes. A large drop (∼40 fold) in the N2 permeance values was observed after PEI attachment, suggesting decreased pore sizes and pore volumes for the PEI-modified MCM-48 membranes as compared to the original MCM-48 membranes. The PEI-modified MCM-48 membranes displayed appreciable N2 permeance after pore modification (Figure 5), suggesting incomplete filling of MCM48 pores with the PEI. The N2/CO2 mixtures of different compositions were used to calibrate the GC-MS system for the calculation of N2/CO2 separation factors for the PEI/MCM-48 membranes. On the basis
20 psi (138 KPa) 20 psi (138 KPa) 20 psi (138 KPa)
feed composition (%)
permeate composition (%)
separation factor (R)
N2
CO2
N2
CO2
N2/CO2
80 50 20
20 50 80
98.6 98 95.3
1.4 2 4.7
17.6 49 81
Table 4. N2/CO2 Separation Factors for the PEI/MCM-48 Membrane at 363 K in the Presence of 2.6 vol % Water Vapor
pressure 20 psi (138 KPa) 20 psi (138 KPa) 20 psi (138 KPa)
feed composition (%)
permeate composition (%)
separation factor (R)
N2
CO2
N2
CO2
N2/CO2
80 50 20
20 50 80
85.2 61 46.2
15.8 39 53.8
1.35 1.56 3.4
of the calibration, the composition of the permeate stream was obtained. The separation factors were calculated using the permeate and feed compositions. The N2/CO2 binary-gas permeation data for the PEI/MCM-48 membrane in the absence of water vapor, shown in Table 2, are indicative of the N2/CO2 separation factors comparable to those in the Knudsen regime, that is, 1.25, indicating that this membrane was slightly N2 selective. The binary-gas N2/CO2 permeation results for the PEI/MCM48 membrane in the presence of water vapor (∼2.6%) at 20 psia feed pressure shown in Table 3 are indicative of the N2/ CO2 separation factors at room temperature and are dramatically higher than those expected in the Knudsen regime, for example, greater than 80 for the highest CO2 feed concentration investigated, indicating that these membranes were highly N2 selective. Contrary to the expectations of high CO2 permselectivity owing to high CO2/N2 solubility selectivity expected for PEI, very high N2 selectivity was observed for the PEI-modified MCM-48 membranes. Increased N2/CO2 separation factors were observed with an increase in CO2 concentration in the feed mixture (Table 3). The high N2/CO2 selectivity for the PEI/MCM-48 membranes was observed at room temperature. Binary (N2/CO2) gas permeation experiments were then performed at higher temperatures in the presence of water vapor (Table 4). Diminished N2/CO2 selectivities were observed at 363 K. Very low N2/ CO2 separation factors (∼3.5) were observed even at the highest CO2 concentration (80%) in the feed at this temperature. To explain the N2/CO2 separation behavior of the PEI/MCM48 membranes at room temperature, single-gas CO2 and N2 permeances were measured in the presence and absence of water vapor under steady-state conditions. Single-gas CO2 and N2 permeance values for PEI/MCM-48 membranes are summarized in Table 5. The single-gas CO2 and N2 permeances for the PEI/ MCM-48 membranes in the absence of water vapor were comparable, suggesting negligible separation selectivity for these
Ind. Eng. Chem. Res., Vol. 47, No. 1, 2008 205 Table 5. Single Gas Steady-State Permeance Values for PEI-modified MCM-48 Membranes at 293 K P (psia) temperature (K) water vapor 20 20 20 20
293 293 363 363
no 2.6% no 2.6%
CO2 permeance N2 permeance (mol/m2‚s‚Pa) (mol/m2‚s‚Pa) 24 × 10-9 3 × 10-9 25 × 10-9 27 × 10-9
29 × 10-9 9 × 10-9 33 × 10-9 34 × 10-9
membranes, which was also observed in binary-gas permeation experiments. In the presence of water vapor, the N2 permeance was much higher than the CO2 permeance (Table 5). The ratio of N2 and CO2 permeances in single-gas permeation experiments was much lower than the N2/CO2 selectivities observed for the PEImodified MCM-48 membranes in binary-gas permeation experiments. We explain these observations by a competitive effect between N2 and CO2 permeation during binary-gas permeation, which results in a significantly different separation behavior than that expected from single-gas permeation experiments. Nonetheless, even for single-gas permeation experiments, these membranes showed a significant permeation preference for N2 as shown in Table 5. Moreover, it was observed that the gas permeances were significantly lower in the presence of water vapor. The decrease in gas permeance values is a result of watervapor adsorption26 in PEI/MCM-48 membranes, expected to lower the free volume available for permeation. The CO2 permeance in the presence of water vapor in binarygas permeation experiments (Table 6) was significantly lower than the single-gas CO2 permeance in the presence of water vapor. Paul et al.32 suggested that in polymeric membranes, the clustering of water molecules with interacting gases and with one another increases the effective size of the diffusing units beyond that of individual molecules. In our case, H2O may hydrogen bond with other H2O molecules and further interact with CO2 adsorbed in the PEI component, thereby forming larger clusters that are characterized by decreased diffusivity at room temperature. On the other hand, H2O interacts only weakly with N2 and, therefore, the diffusion of N2 is not expected to be impeded to the same extent as that of CO2. This phenomenon possibly leads to the observed N2/CO2 selectivity in presence of water vapor at room temperature. Similar observations were made by Chenar et al.33 who studied the CO2/CH4 separation in the absence and presence of water vapor for poly(2,6dimethyl-1,4-phenylene oxide) membranes. The CO2 permeability decreased significantly in the presence of water vapor, whereas the CH4 permeability showed only a moderate decrease. They explained the observed behavior by competitive sorption theory advanced by Koros et al.,34 according to which, the presence of water vapor affects the permeation of high-affinity components to a greater extent as compared to the permeation of low-affinity ones. This theory was supported by the results of their study where the permeation of CO2 displaying higher affinity toward the membrane was affected more appreciably as compared to that of the low-affinity CH4. As a result, the CO2/CH4 selectivity in the presence of water vapor was lower than in its absence. On the basis of the results reported in the present and past studies,25-27,32-34 we explained the observed N2 selectivity of PEI/MCM-48 membranes by enhanced CO2-PEI interactions in the presence of water vapor. The N2/CO2 selectivity for PEI/ MCM-48 membranes in the presence of water vapor arises from the enhanced CO2-PEI interactions as discussed earlier.29-31
Previously, we reported a CO2 adsorption capacity of ∼0.4 mmol/g at 1 atm pressure and room temperature for PEImodified MCM-48 powders.30 The CO2 adsorption capacity increased slightly to ∼0.5 mmol/g at higher temperature (363 K). The CO2 adsorption in the presence of water vapor was not measured because of equipment limitations. Xu et al.35 reported an increase in the CO2 adsorption capacity for PEImodified MCM-41 powders in the presence of water vapor. The CO2 adsorption capacities for simulated flue gas in the absence and presence of moisture were measured at atmospheric pressure. The CO2 adsorption capacity increased from ∼90 mL (STP)/g-PEI in the absence of moisture to ∼100 mL (STP)/gPEI in presence of 3% moisture. CO2 adsorption capacities of ∼140 mL (STP)/g-PEI were reported at a moisture concentration of ∼14%. These studies clearly suggest that the CO2 adsorption capacity for PEI-modified MCM-48 powders increases in the presence of water vapor. High N2/CO2 selectivities for the PEI-modified MCM-48 membranes were observed only in the presence of water vapor. The PEI polymer containing primary, secondary, and tertiary amino groups occupies the pores of MCM-48. The N2 adsorption-desorption measurements and unsteady state gas permeation experiments showed that the complete filling of MCM48 pores with the PEI polymer does not occur. In the presence of water vapor, the CO2 adsorption capacity of PEI/MCM-48 increases considerably, leading to higher CO2 solubility.35 The nature of chemical interactions between PEI and CO2 changes in the presence of moisture. In the absence of water vapor, the main reaction is the formation of carbamate as shown in the following reaction equations:
CO2 + 2RNH2 f NH4+ + R2NCOO-
(a)
CO2 + 2R2NH f R2NH2+ + R2NCOO-
(b)
CO2 + 2R3N f R4N+ + R2NCOO-
(c)
This limits the adsorption capacity to 1 mol of CO2 for 2 mol of amino groups. In the presence of moisture, the carbamate formed in reactions (a)-(c) will further react with CO2 and H2O to form bicarbonate according to reaction (d). The amino group itself can directly react with CO2 and H2O to form bicarbonate in reactions (e)-(g). In the presence of water vapor, one amine molecule can adsorb one molecule of CO2.
R2NCOO- + CO2 + 2H2O f R2NH2++ 2HCO3- (d) CO2 + RNH2 + H2O f RNH3+ + HCO3-
(e)
CO2 + R2NH + H2O f R2NH2+ + HCO3-
(f)
CO2 + R3N + H2O f R3NH+ + HCO3-
(g)
As shown in the reactions above, bicarbonate formation takes place in the presence of water vapor, whereas carbamate formation occurs in the absence of water vapor. The in situ IR spectroscopy data for the CO2 adsorption on PEI-modified MCM-48 silica are shown in Figure 6. The IR data confirmed that the bicarbonate formation occurs in the presence of water vapor whereas carbamate is formed in the absence of water vapor. Therefore, the adsorption of CO2 on PEI-modified MCM48 silica is enhanced in the presence of water vapor. In the presence of moisture, CO2 adsorption is significantly enhanced and CO2 has a very high affinity for PEI. Therefore, in the
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Figure 6. In situ IR spectra of PEI-modified MCM-48 samples. Table 6. Binary Gas Steady-State CO2 Permeances of PEI-modified Membranes at 293 K in Presence of ∼2.6 vol % Water Vapor feed composition (%)
pressure 20 psi (138 KPa) 20 psi (138 KPa) 20 psi (138 KPa)
permeate composition (%)
N2
CO2
N2
CO2
80 50 20
20 50 80
98.6 98 95.3
1.4 2 4.7
presence of water vapor, the CO2 diffusivity is very low because of strong CO2-PEI interactions. Decreased single-gas N2 permeance was also observed for PEI/MCM-48 membranes in the presence of water vapor (Table 5). The decrease in the CO2 permeance in the presence of water vapor was more pronounced as compared to the N2 permeance. The decrease in N2 permeance results from the adsorption of water vapor in the PEI/MCM-48 membranes. The N2/CO2 diffusivity selectivity is significantly enhanced in the presence of water vapor because of enhanced specific interactions between CO2 and amino groups of PEI. As discussed above, the solubility of CO2 in PEI-modified MCM-48 increased in the presence of water vapor. Furthermore, we demonstrated that the CO2 permeance decreases considerably in the presence of water vapor (Table 5), suggesting that the CO2 diffusivity is decreased considerably in the presence of water vapor. The emergence of high N2/CO2 selectivity in the presence of water vapor at 293 K suggested that the enhanced CO2-PEI interactions in the presence of water vapor may lead to high N2/CO2 diffusivity selectivity, which is primarily responsible for the observed N2/CO2 selectivity of PEI-modified MCM-48 membranes. Ghosal et al.25 and Davies et al.27 showed that the CO2/ N2 selectivities significantly lower than 1 are obtained at very high polymer solubility parameters where the affinity between polymer segments and CO2 reaches a maximum. In the presence of water vapor, when the interactions between PEI and CO2 are very strong, N2/CO2 diffusivity selectivities significantly greater than 1 may be observed. This leads to high N2/CO2 permselectivities of PEI/MCM-48 in presence of moisture. Freeman et al.26 studied the CO2/H2 system and suggested that any enhancement in CO2/H2 solubility selectivity afforded by the PEI domains is more than offset by an increase in the H2/ CO2 diffusivity selectivity on PEI addition. They suggested that PEI itself does not have high permeability for CO2 despite the high CO2 sorption capacity reported for this polyamine in literature. Freeman et al. also suggested that CO2 is held by PEI so strongly that it cannot easily desorb at low pressures and room temperature, the conditions at which our membranes showed high N2/CO2 selectivities. The PEI/MCM-48 membranes did not exhibit significant N2/ CO2 selectivities at 363 K in the presence of water vapor. The
CO2 permeance (× 10-9 mol/m2‚s‚Pa) 0.112 0.13 0.188
diffusion of gases through the polymers is a thermally activated process and the diffusion coefficient is expressed as36
D ) D0 exp(-Ed/RT) where Ed is the activation energy of diffusion and D0 is a constant. Gas diffusion coefficients typically increase appreciably with temperature. Increasing temperature can elevate the diffusivity of the less-permeable component more than the diffusivity of more-permeable components, thereby decreasing diffusivity selectivity.37 For small gas molecules, diffusivity is typically much more temperature sensitive than solubility.36 The decrease in diffusivity selectivity at higher temperatures often leads to decreases in the overall permselectivity. Because the CO2 solubility in PEI decreases with increasing temperature,30 we speculate that the observed loss of the N2/CO2 permselectivity for the PEI-modified MCM-48 membranes at 363 K is probably due to a loss in the N2/CO2 diffusivity selectivity. Concentration polarization may have a pronounced effect on the gas-separation behavior of inorganic membranes. However, the impact of this phenomenon is typically small for membranes displaying low fluxes38 and may be significant for high gasflux membranes used in industrial vapor-gas-separation processes and in high-pressure applications.39 For instance, Gaohong et al.40 studied in detail the effect of concentration polarization on gas-separation factors as a function of permeation rate. The permeation rate was found to be a dominating factor in affecting concentration polarization. They found that the separation factors dropped ∼10% when the permeation rate of the more-permeable gas was more than 100 GPU (1 GPU ) 10-6 cm3 (STP) cm-2 s-1 cmHg-1). The concentration polarization effects were not detected at lower permeation rates. However, the N2 (the more-permeable component) permeances calculated for the PEI-modified membranes in the presence of water vapor were 23.5, 19.0, and 11.4 GPU, respectively, for the CO2 feed concentrations of 20, 50, and 80%, suggesting that the concentration polarization was insignificant for the PEImodified MCM-48 membranes. Furthermore, Pinnau et al.41 studied the effect of concentration polarization on methane/hydrogen separation, employing polydimethylsiloxane membranes. They observed concentration
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polarization for thin membranes (∼10 µm) and found that it was practically absent for thicker membranes (>100 µm). Muchgreater effective thickness of our PEI-modified MCM-48 membranes (∼ 0.4 mm) leading to low gas permeances further suggested that concentration polarization effects are not significant for our membranes. 4. Conclusions The polymeric amine, polyethyleneimine (PEI), was successfully introduced into the mesopore channels of MCM-48 membranes, as indicated by the N2 adsorption-desorption data, TGA, and XRD experiments. The PEI/MCM-48 membranes were permeable to N2 after pore modification, suggesting incomplete filling of MCM-48 pores by PEI, which was confirmed by the N2 adsorption-desorption experiments on the bulk PEI/MCM-48 powder. The PEI-modified MCM-48 membranes were highly selective to N2 permeation from N2/CO2 mixtures at room temperature in the presence of ∼2.6% water vapor. The selectivities increased with CO2 concentration in the feed, and selectivities greater than 80 were observed. The N2/ CO2 selectivities diminished significantly at 363 K for the PEI/ MCM-48 membranes. Acknowledgment This work was supported by Ohio Coal Development Office (OCDO), Project C2-4.14 and Ohio Coal Consortium. We thank Dr. Hatem Alsyouri for his valuable suggestions. Literature Cited (1) Azar, C.; Rodhe, H. Targets for Stabilization of Atmospheric CO2. Science 1997, 276, 1818. (2) Siriwardne, R. V.; Shen, M. S.; Fisher, E. P.; Poston, J. A. Adsorption of CO2 on molecular sieves and activated carbon. Energy Fuels 2001, 15, 279. (3) Cochelin, A. S. B.; Mysak, L. A.; Wang, Z.; Simulation of LongTerm Future Climate Changes with the Green McGill Paleoclimate Model: The Next Glacial Inception. Climate Change 2006, 79, 381. (4) Herzog, H. J. What Future for Carbon Capture and Sequestration? EnViron. Sci. Technol. 2001, 35, 148A. (5) Yegulalp, T. M.; Lackner, K. S.; Ziock, H. J. A Review of Emerging Technologies for Sustainable Use of Coal for Power Generation. Int. J. Surf. Min., Reclam. EnViron. 2001, 15, 52. (6) Lackner, K. S.; Ziock, H. J. From low to no emissions. Mod. Power Sys. 2000, 20, 31. (7) Elliott, S.; Lackner, K. S.; Ziock, H. J.; Dubey, M. K.; Hanson, H. P.; Barr, S.; Ciszkowski, N. A.; Blake, D. R. Compensation of Atmospheric CO2 Buildup through engineered chemical sinkage. Geophys. Res. Lett. 2001, 28, 1235. (8) Plasynski, S. I.; Chen, Z.-Y. Review of CO2 Capture Technologies and Some Improvement Opportunities. Abs. of Papers of the American Chemical Society 2000, 220, U391. (9) Bounaceur, R.; Lape, N.; Roizard, D.; Vallieres, C.; Favre, E. Membrane Processes for Post-Combustion Carbon Dioxide Capture: A Parametric Study. Energy 2006, 31, 2220. (10) Zhang, X.; Zhang, C. F.; Qin, S. J.; Zheng, Z. S. A Kinetics Study on the Absorption of Carbon Dioxide into a Mixed Aqueous Solution of Methyldiethanolamine and Piperazine. Ind. Eng. Chem. Res. 2001, 40, 3785. (11) Rinker, E. B.; Ashour, S. S.; Sandall, O. C. Absorption of Carbon Dioxide into Aqueous Blends of Diethanolamine and Methyldiethanolamine. Ind. Eng. Chem. Res. 2000, 39, 4346. (12) Mimura, T.; Simayoshi, H.; Suda, T.; Iijima, M.; Mituoka, S. Development of Energy Saving Technology for Flue Gas Carbon Dioxide Recovery in Power Plant by Chemical Absorption Method and Steam System. Energy ConVers. Manage. Suppl. 1997, 38, S57. (13) Leci, C. L. Development Requirement for Absorption Processes for Effective CO2 Capture from Power Plants. Energy ConVers. Manage. Suppl. 1997, 38, S45.
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ReceiVed for reView May 16, 2007 ReVised manuscript receiVed September 20, 2007 Accepted October 1, 2007 IE070700D
