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Ind. Eng. Chem. Res. 2009, 48, 5794–5801
The Preparation and Characterization of Hydrotalcite Thin Films Tae Wook Kim, Muhammad Sahimi, and Theodore T. Tsotsis* The Mork Family Department of Chemical Engineering and Materials Science, UniVersity of Southern California, Los Angeles, California 90089-1211
Owing to the recent focus on reducing CO2 emissions, the preparation and characterization of CO2-selective inorganic membranes and thin films has received considerable attention. Hydrotalcites (HT) are, potentially, good materials for preparing CO2-selective inorganic membranes and films due to their high CO2 adsorption capacity at elevated temperatures and their good thermal stability. The purpose of this study, therefore, was to prepare CO2-selective HT films and to understand their transport characteristics. A vacuum-suction method was used for the synthesis of the HT films, and their properties were characterized using both single and mixed-gas permeation tests, as well as by nitrogen adsorption and by SEM and XRD. The HT films are microporous, as indicated by both the adsorption data, as well as by the fact that the ideal selectivities of pairs of inert gases (e.g., He, N2, and Ar) through the membrane are higher than the Knudsen values. However, the as-prepared materials contain a substantial fraction of voids and pinholes. Plugging these voids and pinholes using a silicone coating results in CO2-permselective films, which exhibit CO2/N2 and CO2/He ideal separation factors of 34.4 and 12.4, respectively. (II) mx+ M(III) [M1-x x (OH)2] [Ax/m ]·nH2O
1. Introduction Atmospheric concentrations of CO2 have increased significantly in modern times, attributed mostly to increased emissions from the burning of fossil fuels.1 There is concern in recent years about reducing the emissions of CO2. To accomplish this goal, it is important to be able to separate and capture the CO2 from its mixtures with other gases.2 Although there are several methods available, membrane separation processes are a relatively simple and energy-efficient technique.3 Polymeric membranes are the current industry standard finding a variety of applications, including CO2 separation. Inorganic membranes are making recent inroads owing to their increased demand in new applications, such as fuel cells, membrane reactors, and high-temperature separations. One advantage of the inorganic membranes is that they are stable at high temperatures, and resistant to the harsh conditions encountered in industrial environments. In particular, zeolite membranes, carbon molecular-sieve (CMS) membranes, and silica membranes have been widely studied as CO2-selective membranes at higher temperatures.4-7 CO2 separation and capture from power-plants, however, involve exposing the membranes to high-temperature oxidative environments and to gases with high steam concentrations. Some of the conventional inorganic membranes are not capable of functioning in such environments (e.g., CMS membranes in oxidative environments and zeolite and silica membranes in the presence of high-temperature steam). Membranes made of hydrotalcite (HT) materials show promise in this regard, as they are stable in high-temperature steam and in oxidative environments. Hydrotalcites are also known as anionic-layered double hydroxides (LDH) and are being studied today for a variety of scientific and technological uses, such as ion-exchange materials, adsorbents, catalysts and catalytic supports, modified electrodes, and antacids.8 The LDH clays are natural or synthetic lamellarmixed hydroxides with interlayer spaces containing exchangeable anions. The chemical structure of an anionic hydrotalcitetype clay is * To whom correspondence should be addressed. E-mail: tsotsis@ usc.edu. Tel.: 213 740 2069. Fax: 213 740 8053.
where M(II) is a divalent metal cation (Mg, Mn, Fe, Co, Ni, Cu, Zn,Ga), and M(III) is a trivalent metal cation (Al, Cr, Mn, Fe, Co, Ni, and La). Am- represents interlayer anions, such as m-valence inorganic (CO32-, OH-, NO3-, SO42-, ClO4-), heteropolyacid (PMo12O403-, PW12O403-), or even organic acid anions.9 In the above formula, the range of x ) M(III) /(M(II) + M(III)) is typically 0.2 e x e 0.33, but significantly higher values, 0.1 e x e 0.5, have also been reported.10 Though hydrotalcites have been widely studied for a variety of applications, we know of only a few published studies about their use for preparing thin films and membranes. A number of different approaches to prepare such thin HT films and coatings have been investigated. Pinnavaia and co-workers11 were the first to prepare thin HT films on glass substrates using alkoxideintercalated Mg-Al LDH colloidal suspensions. Several other studies12-14 reported on the preparation of hybrid films consisting of LDH and various polymeric macromolecules, using a “layer-by-layer” deposition technique, based on the formation of Langmuir-Blodgett films. A different approach to prepare hybrid LDH/polymeric thin films was utilized by Lee et al.15 After coating a Mg-Al-CO3 LDH film on a silicon wafer, they formed a polymeric film on the top by a direct ion-exchange reaction with a PAA binary-solvent solution. In our prior study,16 our group used an electrophoretic deposition (EPD) method for the preparation of hydrotalcite thin films. The permeation properties of these films were investigated by single and mixedgas permeation tests, and the films were shown to be slightly permselective toward CO2.16 In this paper, we report on the preparation of HT films using a vacuum-suction method. The goal of this study is to further validate the use of these materials in the preparation of CO2selective membranes and films, and to understand their transport characteristics through single and mixed-gas permeation tests under various conditions. The advantage of the vacuum-suction over the EPD method is that it is more convenient to utilize for the preparation of larger area films and membranes.
10.1021/ie900371r CCC: $40.75 2009 American Chemical Society Published on Web 05/14/2009
Ind. Eng. Chem. Res., Vol. 48, No. 12, 2009
Figure 1. The PaSD of the Mg70D powder before and after the ball-milling treatment. Adapted from ref 16.
2. Experimental Section 2.1. Materials. The hydrotalcite powder (Mg70D, with a Mg/ Al ratio of 3.0, as measured by ICP-MS analysis), and the silicone coating material (RTV 615A, B) were supplied by the Sasol and GE Silicone Companies, respectively. Alumina powder, from which the membrane film support is made, and heptane (which is used in the preparation of silicone films) were purchased from Accumet Materials and from Aldrich, respectively. In addition, a hydrotalcite (herein after referred to as HTU) was also prepared in our laboratories, through the coprecipitation method, in order to compare its characteristics with those of the commercial HT.17 The Mg/Al ratio for the HTU was 2.89, as measured by ICP-MS analysis. 2.2. Membrane Preparation. Porous R-Al2O3 discs, to be used as support for film preparation, were prepared by pressing 5 g of alumina powder with 1000 kgf/cm2 for 10 min, and then calcining at 1000 °C for 3 h. The thickness of the disks was ∼2 mm, and their porosity was ∼0.34, as measured by the Archimedes method.18 For the preparation of the HT membranes, we utilized the Mg70D powder after its particle size was reduced by ball-milling at the NETZSCH Corporation (the ground powder is referred to as Mg70DS). Figure 1 shows the particle size distribution (PaSD) for the Mg70D powder before and after ball-milling, as measured by dynamic light scattering.16 The average particle diameter for the Mg70D powder is 12.8 µm, while for the Mg70DS powder it is significantly smaller, around 0.17 µm. Prior to film formation, the surface of the support disks was polished with 600 and 2400 grit-sand paper and cleaned several times with deionized water in an ultrasonic bath. Before coating, the cleaned alumina disk support was dried in air at 473 K for about 6 h. For membrane film preparation, colloidal suspensions of 0.76 wt % Mg70DS HT in deionized water were prepared by dispersing the powder with the aid of an ultrasonic bath. The colloidal solution was then coated dropwise, with the aid of a micropipet, on the top of the support, while a mild vacuum of ∼10 Torr (1333.2 Pa) was applied at the bottom of the disk using a mechanical pump, with the goal of enhancing the adhesion of the HT films on the underlying support. The films thus prepared were dried in air at 150 °C for 12 h. Some of the membranes were also coated with a 3.5 wt % silicone solution (GE Silicones, RTV 615A, B) in heptane.19 The goal of this treatment was to plug the intercrystallites voids and the pinhole defects. 2.3. Membrane Characterization. The HT powders and the resulting membrane films were characterized by a variety of techniques. The FT-IR spectra of the HT were recorded using a Genesis II (Mattson, FT-IR) instrument; the experimental
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operating conditions were a scan-range from 4000 to 500 cm-1, scan numbers 50, and a scan resolution of 2 cm-1. XRD analysis was carried out using a Rigaku X-ray diffractometer, with the Cu KR line used for the X-ray source, with a monochromator positioned in front of the detector. Scans were performed over a 2θ range from 5° to 75° with scan rate of 2°/min and a step rise 0.05. The surface area, the BJH (Barret-Joyner-Halenda), the Horvath-Kawazoe (H-K) pore volumes, and the pore size distribution (PSD) of the HT were calculated from N2 adsorption at 77 K using a Micrometrics ASAP 2010 instrument. The samples for adsorption were pretreated by heating in vacuum overnight. The surface morphology of the HT membrane films was investigated by scanning electron microscopy (SEM), using a Philips/FEI XL-30 field emission scanning electron microscope. Transport properties of the HT membrane films were measured using a Wicke-Kallenbach-type permeation apparatus using a bubble flow-meter to measure the flow, and a mass spectrometer to measure the gas concentration. For the siliconecoated HT films, their permeation characteristics were measured, using the constant volume (or diffusion time-lag) method20 with the apparatus shown in Figure 2. To measure the gas permeance, the pressure of the permeate side was kept at ∼1 × 10-2 Torr (1.33 Pa), while the feed side was maintained at a fixed pressure of either 30 or 40 psi (206 842.7 or 275 790.3 Pa). Gas permeance (Pr [mol/m2 s Pa]) was calculated from eq 1. Permeability (Pe) is calculated by multiplying Pr by the film thickness, if known (Pe ) Pr × L). Pr )
273 × V0 × dp/dt 101325 × 22400 × T × A × ∆p
(1)
where V0 (cm3) is the volume of the downstream side of permeation cell, dp/dt (Pa/s) is the rate of change of pressure in that side with time, A (m2) is the membrane area, ∆p (Pa) is the pressure-drop across the membrane, and T (K) is the temperature. The ideal selectivity (R1/2) or permselectivity of the HT membrane for a pair of gas components 1 and 2 is expressed as the ratio of the two pure gas permeances. R1/2 )
Pr,1 Pr,2
(2)
The separation factor (SF) for the mixed-gas permeation experiments is defined as SF )
J1 / ∆p1 J2 / ∆p2
(3)
where J1 and J2 are the fluxes (mol/(m2 s)) of the individual gases 1 and 2. 3. Results 3.1. Characterization. FTIR spectra of the Mg70D HT and the HTU, reported elsewhere,16 are very similar to each other, and in line with what has been previously published for the same materials.21 The only difference between the two spectra is a single band at 1593.2 cm-1, which results from the lactic acid (CH3-HCOH-COOH) added to the Mg70D HT powder to improve its dispersion characteristics. PXRD spectra for the HTU indicate a basal spacing of 7.72 Å (2θ ) 11.45, λ ) 0.1542 nm) according to Bragg’s equation.16 The PXRD patterns for the Mg70D HT indicate a basal spacing of 7.76 Å (2θ ) 11.4, λ ) 0.1542 nm). These results are consistent with those reported in the literature, indicating a typical basal spacing ∼7.8 Å.22 The XRD spectra of the prepared HT films are very similar to
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Figure 2. The gas-permeation apparatus. Table 1. Surface Area and Pore Size of the HT Powder
Mg70D (uncalcined) Mg70D (calcined @ 550 °C for 4 h in Ar)
pretreatment temp (°C)
surface area (m2/g)
BJH adsorption average pore size (nm)
H-K median pore size (Å)
150 300 150
1.5 23.9 219.9
18.0 12.3 3.1
10.7 8 8.4
Table 2. Pore Volumes and Surface area for the Supported HT Membrane as Well as the Support Disk
HT membrane support
HK pore volume (cm3/g) [ < 1.7 nm]
BJH average pore volume (cm3/g) [1.7-300 nm]
BET surface area (m2/g)
0.001258 0.000560
0.01271 0.00791
4.47 1.91
those of the powders (other than the bands characteristic of the support), indicating that the films retain their crystallinity during deposition. N2 adsorption is utilized to study the porous structure of the HT materials. Table 1, for example, shows the structural characteristics of the Mg70D powder (the microporosity in Table 1 is evaluated using the HK equation, while the mesoporosity is evaluated using the BJH model). Prior to the adsorption, the powder was evacuated overnight at an elevated temperature, as indicated in the second column in Table 1. As reported in greater detail elsewhere,23 the structural characteristics of these materials remain fairly unchanged for temperatures below 280 °C. However, above this temperature the surface area increases (while still remaining fairly low), and the average mesopore and micropore sizes decrease. Treating the powder at much higher temperatures (see row 3 in Table 1 - similar observations have been made by other groups24) results in very high surface areas and in very significant reductions in the average mesopore size (while the average micropore size remains fairly invariant); this is consistent with the collapse of long-range structural order in these materials.25 Table 2 shows the BJH and HK pore volumes and the surface area of one the HT supported membranes measured by BET analysis, as well as the corresponding values for the alumina support disk. Despite the fact that the HT layer is a small fraction of the total weight in the composite structure, the surface area of the supported membrane is more than twice that of the alumina disk. Figure 3 shows the morphology of the top surface of a composite membrane (consisting of a thin HT layer deposited on the top of an alumina support) prepared by the vacuumsuction method. The surface is smooth, as a result of using for coating the suspension of the finely gound Mg70DS HT powder.
Figure 4a shows the cross-section of one of the HT films. The film thickness is ∼6 to 7 µm, and the layer appears to consist of well-intergrown HT crystallites. Figure 4b shows the crosssection of one of the silicone-coated HT membranes; clearly
Figure 3. SEM picture of the surface (top view) of a vacuum-suction membrane: (a) magnification ×10 K and (b) magnification ×40 K.
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Figure 4. The cross-section of (a) the HT membrane and (b) the siliconecoated HT membrane (magnification ×5 K).
shown on this figure are the HT layer and the silicone layers sitting on its top. 3.2. Permeation Results. Table 3 shows the permeance and ideal permselectivity for single gases, such as He, N2, Ar, and CO2, measured at two different transmembrane pressure drops of 30 or 40 psi (206 842.7 or 275 790.3 Pa), at room temperature, for two different supported HT membranes prepared by the vacuum-suction technique. The ideal permselecitivities of the HT membranes are generally higher than the ideal Knudsen values, with the He permeance being of the order of ∼10-8 mol/m2 s Pa. By comparison, the permeance of the underlying alumina support is 2 orders of magnitude higher than that of the HT composite membranes, with ideal permselectivities lower than the Knudsen values. This is indicative of the presence of substantial microporosity in the HT films. A simple model was used to analyze the permeation characteristcs of these composite HT membranes (see the Appendix). The model assumes that transport in the support layer is a combination of convective flow and Knudsen transport, whereas in the HT layer it is due to the combined surface and Knudsen diffusion. This model indicates that for the supported HT membranes more than 99% of the resistance to transport resides in the HT layer, indicative that the transport characteristics measured reflect the properties of the HT film itself. The effect of temperature on the permeation characteristics of the HT membranes is shown in Table 4. The permeance of all the gases decreases as the temperature increases (from 298 to 503 K), with the exception of CO2 for which the permeance first slightly increases as the temperature increases from 298 to 373, and then decreases. Notice, that the ideal separation factors first decrease with temperature and then increase. These results
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are qualitatively consistent with the changes in the structural characteristics and the average pore size measurements made with HT powders, as shown in Table 1. Table 5 shows the effect of pressure-drop on the single gas permeances, and the ideal separation factor for three different membrane samples. The He:N2 ideal separation factor remains fairly invariant with respect to the pressure-drop (particularly for membranes HT-4 and HT-5); the He:CO2 and to a lesser extent the N2:CO2 separation factors, on the other hand, decrease as the pressure-drop increases. It is interesting to note that the CO2 permeance increases with pressure-drop, reflecting the fact that CO2 has a significant affinity for the HT surface; in fact, the HT materials are known to be excellent high-temparature CO2 adsorbents.26 As noted in Tables 3 and 5, these HT films are microporous with ideal permselectivities that exceed the Knudsen values. On the other hand, these membrane films show no preferential transport toward CO2, as one would have expected, if transport through these films were dominated by surface diffusion alone, since CO2 is known to adsorb preferentially in these materials. The observed behavior is, in our opinion, the outcome of the complex three-dimensional porous structure of these materials, which consist of completely interwoven mesoporous and microporous regions.27 In principle, if one were able to completely “plug” the mesoporous regions, so that transport through these HT films was only through the interlayer HT space in the individual crystallites (assuming that they are wellintergrown to form a percolating region of such interlayer spacings throughout the film), one would observe significantly higher permeation rates for CO2, when compared to such inert gases such as He, Ar, and N2. To test this hypothesis, we utilized a silicone material (RTV 615A, B supplied by GE Silicone Companies) which is composed of vinyl-polydimethylsiloxane (VPDMS) and modified silica. This compound has good thermal resistance, with the supplier’s data indicating the operating temperature to be as high as 204 °C. Silicone materials are known to be CO2permselective, because of their high CO2 solubility, when compared to that of fixed gases such as N2, He, H2.28,29 The supported HT membranes were coated utilizing this silicone material using the procedure described in the Experimental Section. To study the effect of coating, we first measured the properties of the membrane without the coating, and then, after the membrane was coated, measured its transport properties again. As expected, after the silicone coating, the HT membrane permeances significantly decreased, as shown in Table 6, which also shows the permeances and separation factors of an alumina disk also coated with the silicone layer. We calculated the permeability and ideal separation factors for the silicone layer alone, based on the experimental data with the silicone-coated alumina membranes, using the resistancein-series model presented in the Appendix, and the thickness measured from the SEM pictures. The values are shown in Table 7, where they are compared with the corresponding values for PDMS from the literature (we have been unable, unfortunately, to locate permeation measurements in the literature with the VPDMS material). Using the calculated permeabilities for the silicone layer, its measured thickness (SEM) on the supported HT membrane, and the experimentally measured permeances of the silicone-coated HT membrane, we calculated the permeances and ideal selectivities of the HT membrane itself using the “resistance-inseries” model (see the Appendix). The calculated values (based on the experimentally measured permeances of the silicone-
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Table 3. The Permeation Characteristics of HT Membranes Prepared by the Vacuum-Suction Method (Temperature, 25 °C) permselectivity permeance × 108 (mol/(m2 s Pa)) membrane HT-1
HT-2 R-alumina support
experimental results
gas (MW)
∆p ) 30 psi
∆p ) 40 psi
ideal Knudsen value
He/gas ) 30 psi
He/gas ) 40 psi
He (4) N2 (28) Ar (40) CO2 (44) He (4) N2 (28) CO2 (44) He (4) N2 (28) Ar (40) CO2 (44)
5.292 1.18 0.873 0.611 2.02 0.869 0.261 647.9 326.5 279.9 308.4
4.93 1.24 0.883 0.604 1.90 0.881 0.278 686.2 354.8 292.1 337.5
1.0 2.65 3.16 3.32 1.0 2.65 3.32 1.0 2.65 3.16 3.32
1.0 4.48 6.06 8.66 1.0 2.32 7.74 1.0 1.98 2.32 2.10
1.0 3.98 5.58 8.16 1.0 2.16 6.83 1.0 1.93 2.35 2.03
Table 4. The Temperature Effect on the Transport Properties of the HT Membranes (HT-3, ∆p ) 20 psi (137 895.1 Pa)) permeance × 108 (mol/m2 s Pa)
permselectivity
temp (K)
He
N2
Ar
CO2
N2/CO2
He/CO2
He/N2
He/Ar
298 373 423 473 503
2.04 1.51 1.10 0.745 0.701
0.543 0.455 0.319 0.204 0.192
0.42 0.315 0.241 0.13 0.134
0.336 0.341 0.232 0.138 0.111
1.62 1.33 1.38 1.48 1.73
6.07 4.43 4.74 5.40 6.32
3.76 3.32 3.45 3.65 3.65
4.86 4.79 4.56 5.73 5.23
Table 5. The Effect of Pressure-Drop on the Transport Properties of the HT Membranes (Temperature, 25°C) permeance × 108 (mol/(m2 s Pa))
permselectivity
membrane
∆p (psi)
He
N2
CO2
He/CO2
He/N2
N2/CO2
HT-2
20 30 40 20 30 40 20 30 40
2.19 2.02 1.90 3.35 3.15 3.27 2.94 2.85 2.97
0.881 0.869 0.881 0.691 0.659 0.688 0.619 0.600 0.619
0.240 0.261 0.278 0.249 0.331 0.394 0.303 0.319 0.358
9.13 7.74 6.83 13.45 9.52 8.30 9.70 8.93 8.30
2.49 2.32 2.16 4.85 4.78 4.75 4.75 4.75 4.80
3.67 3.33 3.17 2.78 1.99 1.75 2.04 1.88 1.73
HT-4 HT-5
Table 6. Permeation Properties before and after the Silicone-Coating (Sil-HT-1) Permeance Units: ×109 (mol/m2 s Pa), Temperature 25 °C HT membrane before coating
HT membrane after coating
∆p (psi)
gas
permeance
CO2/N2
permeance
CO2/ N2
silicone-coated alumina membrane permeance
CO2/N2
30
N2 CO2
26.5 18.1
0.68
0.0159 ( 0.0014 0.494 ( 0.002
31.1 ( 2.5
0.204 1.97
9.7
40
N2 CO2
24.9 17.5
0.70
0.0165 ( 0.0013 0.450 ( 0.008
27.3 ( 1.8
0.204 2.04
10.0
Table 7. The Permeabilities and Ideal Selectivities of the Silicone Coating (Temperature, 25 °C) permeability (barrer) [permeance × 109(mol/(m2 s Pa))] exptl ∆p (psi) 15 30 40 58 a
ref (PDMS)
CO2 19.44 [1.97] 20.13[2.04]
N2 2.01 [0.204] 2.01 [0.204]
ideal SF for CO2/N2
CO2
N2
2645 [25.3] 1300 [4.35]
251.9 [2.41] 299 [1.00] [1758- 8040]
exptl 9.7 10.0
ref
ref
10.5 4.35
28a 29b 30
Thickness of the silicone film ) 100 µm. Thickness of the silicone filem ) 35 µm. b
coated HT membranes after accounting for the effect of silicone coating itself) and the experimental values (measured with the HT membranes prior to being coated with the silicone layer) are compared in Table 8. The experimental values are significantly higher than the calculated values, which indicates that the silicone layer penetrates deep into the underlying HT film’s structure (since, when accounting for the effect of the silicone layer, we used the thickness of the silicone layer on the top of the HT membrane, calculated from the SEM images).
Table 9 shows the permeation characteristics of a different silicone-coated HT membrane for CO2 as well as other molecules like He, N2, and H2. In the same table we also present the values of the silicone layer itself, measured in composite structures which are utilizing the alumina disks as supports. Again, the silicone-coated HT membrane film exhibits enhanced CO2 permeation. Furthermore, the measured ideal separation factors of the silicone-coated HT membrane significantly exceed those of the silicone layer itself, likely pointing out that the
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Table 8. Comparison between the Calculated and Experimental Values for the HT Membrane Films (Temperature, 25°C, Sil-HT-1) permeance × 109 (mol/m2 s Pa) calcd (after coating with silicone)
exptl (before coating with silicone)
permselectivity CO2/N2
∆p (psi)
CO2
N2
CO2
N2
calcd
exptl
30 40
0.659 0.577
0.0172 0.0180
18.1 17.5
26.5 24.9
38.31 32.09
0.68 0.70
Table 9. The Permeation Characteristics of a Silicone-Coated HT Membrane (Sil-HT-2), and of a Silicone Film permeance × 109 (mol/(m2 · sec · Pa))
permselectivity
membrane
∆p (psi)
CO2
N2
He
H2
CO2/N2
CO2/H2
CO2/He
Sil-HT-2
30 40 30 40
0.546 ( 0.003 0.489 ( 0.009 1.97 2.04
0.0159 ( 0.0014 0.0165 ( 0.0013 0.204 0.204
0.0440 ( 0.0005 0.0504 ( 0.0016 0.291 0.296
0.108 ( 0.001 0.102 ( 0.001 0.504 0.516
34.3 ( 3.1 29.6 ( 1.8 9.7 10.0
5.1 ( 0.1 4.8 ( 0.1 3.9 4.0
12.4 ( 0.1 9.64 ( 0.2 6.8 6.9
silicone
Table 10. Mixed-Gas Experimental Data for a Silicone-Coated HT Membrane, Temperature, 25 °C, ∆p ) 30 psi (206,842.7 Pa) permeance (mol/(m2 s Pa))
mole fraction membrane Sil-HT-2 Sil-HT-3
CO2 0.5 0.5
He 0.5 0.5
CO2
He -10
5.65 ( 0.035 × 10 4.94 ( 0.03 × 10-10
-11
4.83 ( 0.37 × 10 5.15 ( 0.3 × 10-11
SF CO2/He
single-gas CO2/He
11.70 ( 0.9 9.60 ( 0.7
12.4 ( 0.1 11.2 ( 0.2
Table 11. The Permeation Characteristics of a Silicone-Coated HT Membrane (Sil-HT-4) at Elevated Temperatures, ∆p ) 30 psi (206 842.7 Pa) permeance × 109 (mol/(m2 · sec · Pa))
permselectivity
membrane
temp (°C)
CO2
N2
He
CO2/He
CO2/N2
He/N2
Sil-HT-4
25 50 80 120 150
1.42 0.965 0.945 0.921 0.885
0.0851
0.194 0.230 0.328 0.557 0.631
7.32 4.20 2.88 1.65 1.40
16.65
2.28
7.21
2.50
3.48
2.48
0.131 0.254
separation characteristics of the silicone-coated HT membrane reflect the intrinsic properties of the HT material itself. They likely, furthermore, reflect the strong affinity of the CO2 molecules for the intercrystalline space of the hydrotalcite material itself. Table 10 shows the results of mixed-gas permeation tests with two silicone-coated HT membranes. These experiments were carried out using the permeation apparatus of Figure 2 connected with a mass spectrometer to measure the composition of the permeated gas mixture. The CO2/He mixed-gas separation factors, as defined by eq 3, are somewhat lower, but still fairly close to the measured single-gas values. The slight differences between the mixed-gas and single-gas separation factors are not unsual to observe for microporous membranes and are the result of the complicated three-dimesional porous structure of these materials and the complex phenomena (adsorption/surface and Knudsen transport) that take place during mixture transport. Finally, Table 11 shows gas permeation data for another silicone-coated HT membrane for a range of temperatures. As shown in the Table, the CO2/He and CO2/N2 ideal gas selectivities decrease with increasing temperature, while the He/ N2 permselectivities remain mostly unaffected. The permeance of CO2 decreases as a function of temperature, which is very much in line with the behavior of the uncoated films (e.g., Table 4) and unlike the behavior of the PDMS film for which the CO2 permeance increases as function of temperature.34 4. Conclusions We have presented the results of our ongoing research, the goal of which is the preparation and study of CO2-selective hydrotalcite films and membranes. HT films are prepared by a vacuum-suction method using alumina macroporous disks as supports. The membrane films were tested for their transport characteristics using single and mixed-gas permeation tests, as
well as by a variety of other characterization techniques, including SEM, FTIR, XRD, and BET. Microporous membrane films were prepared, which show significantly higher permeation rates for gases with smaller kinetic diameters, such as He, as compared with gases with larger kinetic diameters, such as Ar. On the other hand, these membranes show no preferential transport toward CO2, even though the HT films are expected to be CO2-selective. Coating the HT membranes with a silicone layer changes their separation characteristics, making them significantly more permeable toward CO2. We attribute this to the plugging, by the silicone layer, of the mesoporous regions of these HT films, which then forces the molecules to transport mostly through the microporous regions, where one expects surface transport by the CO2 molecules to dominate because of their strong affinity toward the HT surface. Acknowledgment The support of the U.S. Department of Energy is gratefully acknowledged. Appendix The reciprocal of the permeance of a multilayer membrane can be thought of as its resistance for mass transport,31 and can be expressed as the sum of the transport resistances of the individual membrane layers according to 1 1 1 1 + + + ··· ) Pr,tot Pr(support) Pr(layer1) Pr(layer2) (A1) For a two-layer membrane at steady-state, the flux through the membrane Jtot is equal to the fluxes through the support JS, and through the top layer JSL.
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Jtot ) JS ) JSL
(A2)
The transport of a single gas through the support layer is by combined Knudsen diffusion and by viscous flow.32
(
)
BS,0p dp 1 eff (A3) DS,K + RT η dx Integration of eq A3 for the support disk alone (1-layer membrane) results in the equation below for the membrane permeance: JS ) -
Js BS,0 1 ) Deff + p¯ ∆p RTLs S,K RTηLs
(A4)
with
p¯)
(p1 + p2)
2 where p2 is the pressure on the feed-side, and p1 is the pressure on the permeate-side of the membrane By plotting the permeance measured with the support disks eff as a function of pj, the average pressure, one calculates DS,K (effective Knudsen diffusion coefficient, m2/s) as the intercept, and BS,o(viscous-flow membrane parameter, m2) as the slope of the plot. If one is to assume further that the support pore structure consists of straight, parallel, nonintersecting pores, then 2
BS,o
ε dp , ) τ 32
eff DS,K
ε 0 ε dp ) DS,K ) τ τ3
8RT πMi
(A5)
where ε is the membrane porosity, dp [m] is the membrane pore diameter, and τ is the membrane tortusity. One can then also calculate dp and τ for the support layer. For a two-layer membrane the flux JS through the support layer (equal to the experimental membrane flux Jtot, see eq A2) can be expressed as33 JS )
(
)
BS,0 1 eff (p 2 - p12) DS,K (p12 - p1) + RTLs 2η 12
(A6)
where P12 is the pressure at the interface between the support eff and the top layer. From eq A6, using the known values of DS,K , and BS,o, one can estimate P12. One then calculates the permeance of the support layer (equal to Js/∆p, where ∆p ) p12 - p1), and from eq A1, the permeance of the top layer. As already noted in the paper, for the supported HT membranes the permeance of the support layer is typically 2 orders of magnitude higher that the permeance of the top layer, and so the transport characteristcs of the composite membrane reflect those of the HT film. The permeance of the silicone layer is estimated from the measured permeance of the silicone-coated alumina disks, after one subtracts the contribution of the alumina support. Once the silicone film’s permeance is estimated, from the known thickness of the film (based on the SEM images), its permeability can be calculated (see Table 7). From the estimated permeability for the silicone coating, its measured thickness on the HT membranes, and the measured permeance of the silicone-coated HT membrane, one calculates, using eq A1, the permeance of the HT membrane alone (equivalent to the permeance of the HT layer, since the alumina support offers no resistance to transport, as noted previously), see Table 8. Nomenclature A ) membrane area (m2) BS,O ) viscous-flow membrane parameter (m2) C ) gas concentration in the pore (mol/m3)
dp ) nominal membrane pore diameter (m) D ) diffusion coefficient (m2/s) 0 DS,K ) Knudsen diffusion coefficient (m2/s) eff DS,K ) Effective Knudsen diffusion coefficient (m2/s) J ) gas flux (mol/m2 s) L ) nominal membrane thickness (m) Mi ) molecular weight of component i (g/mol) p ) pressure (Pa, psi) Pe ) membrane permeability (1 barrer ) 1 × 10-10(cm3(STP) · cm)/ (cm2 · s · cmHg)) Pr ) gas permeance through the membrane (mol/m2 s Pa) p1 ) pressure on the permeate side (Pa, psi) p12 ) interlayer pressure (Pa, psi) p2 ) pressure on feed side (Pa, psi) R ) universal gas constant (8.314 J/mol K) SF ) separation factor for mixed gases V0 ) volume of downstream side of permeation cell (cm3) Greek Letters R ) ideal separation factor or permselectivity ε ) membrane porosity η ) viscosity (Pa-s) τ ) tortusity Subscripts K ) Knudsen flow S ) support SL ) selective layer (HT or silicone layer in this study) tot ) total
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ReceiVed for reView June 26, 2008 ReVised manuscript receiVed April 20, 2009 Accepted April 27, 2009 IE900371R
