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Ultrafiltration membrane synthesis by nanoscale templating of porous carbon
Journal of Membrane Science 198 (2002) 173–186
Ultrafiltration membrane synthesis by nanoscale templating of porous carbon Michael S. Strano a,1 , Andrew L. Zydney a,1 , Howard Barth b , Gilber Wooler b , Hans Agarwal a,1 , Henry C. Foley c,∗ a b
Colburn Laboratory, Department of Chemical Engineering, University of Delaware, Academy Street, Newark, DE 19716, USA DuPont Company, Central Research and Development, Experimental Station, P.O. Box 80228, Wilmington, DE 19880-0228, USA c Department of Chemical Engineering, Center for Catalytic Science and Technology, Pennsylvania State University, University Park, PA 16802, USA Received 22 March 2001; received in revised form 28 June 2001; accepted 9 July 2001
Abstract A novel method for producing carbon membranes for ultrafiltration applications is presented using a spray deposition and pyrolysis of poly(furfuryl alcohol)/poly(ethylene glycol) mixtures on macroporous stainless steel supports. The poly(ethylene glycol) or PEG employed as a carbonization template creates a mesoporosity that leads to pores in the ultrafiltration range. Scanning electron microscopy (SEM) shows that the membranes consisted of 12- to 15-m thick carbon films. Gas permeation and water permeability data were used for the calculation of mean pore sizes, which were found to decrease with decreasing average molecular weight of the PEG template. Ultrafiltration of a polydisperse dextran solution was used to quantify the retention properties of the membranes. Molecular weight cutoffs determined from dextran retention data were shown to vary with template molecular weight: values of 2 × 104 , 3.5 × 104 , and 6 × 104 g mol−1 dextran were measured for respective templates of 2000, 3400, and 8000 g mol−1 PEG. For PEG molecular weights of 2000 or below, the templating effect was ill defined, membrane film cracking became more prominent, and membrane selectivity and reproducibility were adversely affected. © 2002 Published by Elsevier Science B.V. Keywords: Ultrafiltration; Carbon membranes; Membrane formation; Inorganic membranes
1. Introduction Membrane filtration technologies are critical to a variety of industrial process applications including cell harvesting, sterile filtration, protein enrichment, and removal of particulate matter [1]. Ultrafiltration ∗ Corresponding author. Tel.: +1-814-865-2574; fax: +1-814-865-7846. E-mail addresses: [email protected] (M.S. Strano), [email protected] (H.C. Foley). 1 Tel.: +1-302-831-2345; fax: +1-302-831-2085.
in particular, where the membrane retains components with kinetic diameters from 1 to 100 nm, has widespread utility for industrial separation processes. Traditionally, membranes used for ultrafiltration have been polymeric in nature. Asymmetric ultrafiltration membranes are commonly synthesized using phase inversion, where a polymer solution consisting of a base and pore-former in a solvent is induced to form two interdispersed liquid phases. Membranes synthesized in this manner include the bilayer type containing slit shaped fissures or cracks [2] and those that contain plasticizers and are stable while dry [3,4].
0376-7388/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 3 7 6 - 7 3 8 8 ( 0 1 ) 0 0 5 7 4 - 9
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Nomenclature B0 Cb Cp Cw dm J Jv k K0 l M p p rpore R Rc T Greek α β χ δ ε η µ τ
porous structure factor (mol ideal gas m−1 ) concentration of dextran in bulk solution (g l−1 ) concentration of dextran in permeate (g l−1 ) concentration of dextran at membrane surface (g l−1 ) average membrane pore size from gas permeation (m) flux of gas (mol m−2 s−1 ) filtrate flux during ultrafiltration (m s−1 ) solute mass transfer coefficient (m s−1 ) Knudsen permeability (mol m−1 s−1 Pa−1 ) carbon layer thickness (m) molecular weight (g mol−1 ) average trans-membrane pressure difference (Pa) pressure driving force under gas permeation (Pa) cylindrical pore radius (m) ideal gas constant (Pa m3 mol−1 ) rejection coefficient for dextran ultrafiltration temperature of gas permeation (K) symbols geometric constant for porous materials geometric constant for porous materials proportionality constant in mass transfer correlation stagnant film thickness above membrane in stirred-cell (m) membrane porosity gas viscosity (Pa s) viscosity of solvent (Pa s) pore tortuosity
Despite the widespread use of these types of membrane materials, they suffer from several disadvantages. The membranes have limited mechanical integrity which can lead to deformation during operation and adversely affect membrane performance. These materials often have limited temperature and
chemical stability, prohibiting their use in applications involving harsh organic solvents or at elevated temperature [4]. The use of caustic and hypochlorite solutions as cleaning agents can also lead to chemical degradation of the polymeric materials, significantly reducing the life of these membranes. In addition, membranes made from hydrophobic polymers like polysulfone and polyethersulfone must contain a humectant such as glycerol to avoid problems associated with re-wetting, while membranes made from hydrophilic polymers like cellulose must remain wet at all times to avoid collapse of the pore structure. While inorganic ultrafiltration membranes have attracted increasing attention in recent years [5–7], pore size modified carbon membranes have been relatively unexplored. Yet compared to ceramic and metallic ultrafiltration membrane materials, carbon can be considerably less expensive. Nanoporous carbon is also a promising material because it is chemically inert under typical processing conditions and thermally stable at temperatures well above 200 ◦ C [8,9]. Formed from the pyrolysis of carbonizing natural or synthetic polymeric precursors, this material is a disordered solid having a pore size approaching molecular dimensions and has been shown to possess highly shape-selective molecular transport properties [10]. Poly(furfuryl alcohol) (PFA)-derived nanoporous carbons have a mean pore size about 0.5 nm as measured from N2 and methyl chloride adsorption isotherms [11]. Attempts to use this material in the synthesis of defect free, micron scale films on structurally stable macroporous supports have been successful. Nanoporous carbon membranes have been fabricated with high selectivity for some gas phase separations of small molecules [12–14]. While such membranes have a mean pore size on the molecular scale (<1 nm), we [15] have shown that the addition of non-carbonizing polymers such as poly(ethylene glycol) (PEG) to porous carbon precursors forms meso- and macroporosity within the resulting carbon. Polymer mixtures of 25% PEG in PFA were carbonized at a soak temperature of 650 ◦ C. Molecular weights of PEG used were 300, 600, 1000, 2000, 3400, and 8000. The results suggested that lower molecular weight PEG (<2000 g mol−1 ) flashed off before carbonization and had little affect on carbon structure. Alternatively, larger molecular weight PEG created mesopore volumes of 0.45, 0.24
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and 0.28 cm3 g−1 for 2000, 3400, and 8000 g mol−1 templates, respectively. These results have been used in the synthesis of various nanoporous carbon catalysts with greatly enhanced effectiveness factors when compared to catalysts prepared from native nanoporous carbon alone [16,17]. Studies involving the use of porous carbon for ultrafiltration membrane formation in the literature are scarce. Schindler and Maier [4] claimed to have synthesized such a membrane through the carbonization of a pre-existing polymer membrane with identical pore structure. Although pyrolysis of a polymeric microfiltration membrane can reasonably produce a carbon membrane with similar macroporosity, there is little theoretical or empirical evidence to suggest that pores in the entire ultrafiltration range (1–100 nm) would be preserved during the carbonization of a pre-existing film. Additionally, ultrafiltration range pores were undetectable using the bubble-point testing methodology employed by the authors [4]. In this paper, a novel method for producing carbon ultrafiltration membranes is presented. Spray deposition and pyrolysis on macroporous stainless steel of poly(furfuryl alcohol)/poly(ethylene glycol) mixtures is the straightforward fabrication technique employed. The methodology is demonstrated in Fig. 1. As in our earlier work, the PEG is used as a non-carbonizing template molecule to introduce controllable mesoporosity into the carbon film. Permeation of a polydisperse dextran solution was used to quantify the
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retention properties of the membranes as a function of template size. A correlation of partial rejection coefficients for 90% of the dextran mixture with template molecular weight was observed. Membranes were also characterized using scanning electron microscopy (SEM) and gas permeation.
2. Experimental 2.1. Membrane synthesis Carbon based ultrafiltration membranes were synthesized using an adaptation of a spray coating procedure used for creating supported nanoporous carbon membranes [13]. Furfuryl alcohol resin in the form of Durez Resin #16,470 (PFA) from Occidental Chemical Corporation diluted in reagent grade acetone (Aldrich Chemical) was combined with poly(ethylene glycol) (PEG) and spray deposited upon a macroporous stainless steel support. The PFA resin has a specific gravity of 1.21 and a viscosity of 200 cP at 25 ◦ C that decreases to about 5 cP at 80 ◦ C. PEG samples of average molecular weights of 1000, 1500, 2000, 3400, and 8000 g mol−1 were obtained from Aldrich Chemical Company. This PFA/PEG/acetone precursor was prepared by combining PFA resin with a given average molecular weight of PEG in a 50/50 wt.% ratio at 70 ◦ C and subsequent dilution in acetone forming a stable mixture
Fig. 1. Mesoporous carbon membranes via nanoscale templating.
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Table 1 Summary of synthesis and performance characteristics for membranes used in this worka
M8000-1-0.021 M8000-2-0.018 M8000-3-0.022 M1500-1-0.019 M3400-1-0.019 M3400-2-0.016 M3400-3-0.018 M2000-1-0.019 M2000-2-0.019 M2000-3-0.019 M2000-4-0.019 M1000-1-0.029 M1000-2-0.023 a
Initial weight support (g)
Wet weight support (g)
Net carbon weight (g)
Pressure applied (psig)
Water flux (m s−1 )
10.5107 10.8591 10.4478 10.4659 10.7327 10.286 10.5444 10.6382 10.4353 10.3555 10.6816 10.5297 10.7057
10.6393 10.9768 10.58 10.5728 10.8409 10.3696 10.6481 10.7455 10.534 10.4623 10.7785 10.6695 10.8189
0.021 0.018 0.022 0.019 0.0188 0.0161 0.018 0.0189 0.019 0.019 0.0189 0.0288 0.0227
50.5 54 54 55 75 75 75 50 50 50 50 50 50
1.8 1.1 1.8 5.9 8.3 8.3 8.3 2.0 1.3 8.8 1.0 1.7 5.7
× × × × × × × × × × × × ×
10−6 10−5 10−6 10−7 10−7 10−7 10−7 10−8 10−7 10−8 10−7 10−7 10−8
Water permeance (m s−1 Pa−1 ) 5.1 2.9 4.7 1.5 1.6 1.6 1.6 5.6 3.8 2.5 3.0 5.0 1.6
× × × × × × × × × × × × ×
10−12 10−11 10−12 10−12 10−12 10−12 10−12 10−14 10−13 10−13 10−13 10−13 10−13
Labels are as follows: M(PEG MW)-(sample #)-(carbon mass (in g)).
at room temperature with a viscosity near 5 cP. The resulting solution was subsequently spray deposited onto circular porous stainless steel supports (0.2 m pore size, 11.4 cm2 area, and 1 mm thickness) supplied by Mott Metallurgical Co. Before spray coating the stainless steel supports were cleaned by sonication in acetone. After spray coating approximately 200 mg of the polymer blend solution (wet weight) was deposited on the support. The resulting system was subsequently pyrolyzed in a stream of flowing He (50 sccm) at 5 ◦ C min−1 to 600 ◦ C, held for 2 h at this temperature, and then allowed to cool to room temperature. The final carbon mass on each support was approximately 20 mg. Table 1 presents data on the membranes used in this work. Membrane labels contain the PEG template molecular weight and total coat mass of carbon for each sample. 2.2. Scanning electron microscopy SEM was used to characterize the internal morphology of the membranes. Membrane cross-sections were cut orthogonal to the support surface using a diamond-wafering saw. These sections were mounted in an epoxy resin, polished, and given a coating of Au for imaging with a Hitachi S-4000 field emission scanning electron microscope. Imaging was performed on areas of these cross-sections external to and within the macroporosity of the stainless steel
support and at various radii from the center of the disk-shaped membranes. 2.3. Characterization using gas permeation Gas phase transport of He, Ar, N2 , O2 , SF6 and CO2 was used to characterize the selective porosity and integrity of the carbon membrane. The disk-shaped membranes were sealed using VitonTM gaskets into a stainless steel module setup to measure the transport of a single gas through the membrane. The pure component flux of each gas was measured as a function of pressure. Details concerning the experimental setup and technique can be found in [18]. 2.4. Ultrafiltration of polydisperse dextrans Ultrafiltration of polydisperse dextrans was used to characterize the retention properties of the templated carbon films. Dextrans having average molecular weights of 2 × 106 , 1.7 × 105 , 7.0 × 104 , 3.9 × 104 , 9.9 × 103 g mol−1 (Pharmacia) were obtained from Aldrich Chemical Company. The dextrans were added in equal mass ratios to create a 10 g l−1 solution in a phosphate buffer prepared from monobasic (NaHPO4 ) and dibasic (NaH2 PO4 ) sodium phosphate to achieve a solution pH of 7.4±0.3. When applicable, 1 wt.% of methanol was added to prevent bacterial contamination.
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Retentates and permeates were analyzed using size exclusion chromatography (SEC) with a model 1050 HP chromatograph (Hewlett Packard) and a Waters 410 differential refractometer. SEC analysis was performed using a 7.5-mm i.d. × 30-cm TSK G4000 SW column (Tosoh Corp.), thermostated at 30 ◦ C, with 0.1 M sodium sulfate used as the mobile phase at a flow rate of 0.6 ml min−1 . Samples were injected with an autosampler without additional dilution using a 100-l sample loop. The above mentioned dextrans were injected individually at a concentration of 1 mg ml−1 , and retention volume data were used to construct a molecular weight calibration curve. Data was processed using Waters Millennium 32 HPLC software. Membrane separation was carried out using a dead end filtration setup. A 50 ml volume of dextran solution was loaded above the carbon membrane in a 30 mm diameter Amicon stirred-cell. An attached Ar line supplied gas pressure to the solution above the membrane. The permeate was collected in 30 ml vials, with the total permeate mass weighed continuously by placing the vial directly on a laboratory balance. From this time series data mass flow through the membrane was computed. A series of permeate and retentate samples were collected over time for off-line dextran analysis. 2.5. Batch depletion dextran adsorption Granular carbon membrane samples were prepared using the identical procedure described above for making thin carbon films, except that the solution was poured into a quartz boat and pyrolyzed as a bulk solution. These samples were analyzed for total dextran adsorption capacity using a batch depletion method. Each carbon was ground to 60/140 mesh and suspended in a water solution. After settling, the remaining solution was decanted to remove fine particles. The process was repeated until all fine particles were removed. A mass of 0.5 g of carbon was sealed with 30 ml of the polydisperse dextran solution in a Pyrex vial as described above. Sealed vials were vigorously agitated for 32 h, after which the solutions were passed through a 1 m filter and analyzed using SEC as described above. The average concentration of dextran remaining in the solution above the carbon was integrated over the molecular weight distribu-
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tion, with the results compared to the corresponding integral for the original solution.
3. Results and discussion Table 1 lists the initial support weight, the weight after spray coating (wet), and the weight after carbonization (dry) for each membrane studied in this work. All samples except M8000-3-0.022 were given a single coat and thermal treatment. This particular sample was fabricated in two coating/carbonization cycles. During the carbonization process, the evolution of gases reduced the polymer mass on the support leaving behind the solid carbon film. For poly(furfuryl alcohol) carbonization at 600 ◦ C, the carbon yield produced in this way is approximately 25–30% for similar thin films [14]. The PEG template is expected to produce very little residual carbon as seen in the synthesis of mesoporous catalyst supports (<1%) [19]. Fig. 2 is a plot of carbon yield on the support as a function of template molecular weight. This yield is measured as the ratio of the carbon mass after pyrolysis to the polymer deposition mass and shows a decreasing trend with increasing template PEG molecular weight. Because the template composition was fixed at 50% by weight for this study, it is clear that smaller molecular weight PEG contributes more to the residual carbon layer. Assuming a furfuryl alcohol resin conversion of 30% [16], the carbon yield for the 8000 g mol−1 templated film corresponds to nearly
Fig. 2. Carbon film conversion (net weight carbon deposited/wet polymer weight) as a function of template molecular weight.
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template size although there is considerable variability between samples formed from the same template size. Generally, water permeability decreases with decreasing template size and this may correspond to a decrease in membrane pore size and porosity as discussed below. In Table 1, the values for the hydraulic permeabilities range from 10−14 to 10−11 m s−1 Pa−1 , which are considerably lower than commercial ultrafiltration membranes (typical values are around 10−10 m s−1 Pa−1 . These reduced permeabilities are attributable to both the much greater thickness of the membranes and the decrease in the film porosity that occurs with decreasing template size. These factors are discussed in detail below. Fig. 3. Water permeance through carbon membranes as a function of template molecular weight.
3.1. Scanning electron microscopy
the entire template being emitted as gaseous products. This suggests that volatilization of the template does not cause any significant loss of poly(furfuryl alcohol) prior to carbonization. It is noteworthy that a similar trend is observed for the water permeability (Fig. 3) plotted as a function of
Fig. 4 is a scanning electron micrograph of a cross-section of the external porous carbon layer and the stainless steel support for a membrane synthesized using a 50% mixture of PFA: 8000 g mol−1 PEG at 600 ◦ C. The demarcation between the stainless steel support and external carbon layer is clearly visible,
Fig. 4. Scanning electron micrograph of a porous carbon ultrafiltration membrane (M8000-1-0.021) cross-section showing external carbon layer and stainless steel support.
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and there are also localized along with localized separations or cracks between the two layers. These film separations apparently have a negligible impact on coating integrity and mechanical stability, as there was no observed carbon mass loss during testing or property variations after repeated testing. The combination of the diamond saw cutting and surface polishing of the SEM sample has a smoothing affect on the morphology of the membrane cross-sections. The actual porosity of the steel support is roughly 0.6 as reported by the manufacturer. The carbon layer is 12- to 15-m thick and relatively uniform over the membrane surface, consistent with observations of similarly prepared carbon films for gas separation [14]. The calculated thickness range based on the mass of carbon deposited (Table 1) and assuming a porous carbon density of 1.6 g cm−3 is 9–15 m, which agrees well with this measurement. Fig. 5 is a 500 nm scale micrograph of the microstructure of the PFA/PEG carbon taken from the material external to the support surface. This cross-section was prepared by fracturing the carbon surface while attached to the support using a scalpel.
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The microstructure suggests pores with diameters in the nanometer range. As expected from the inferred hypothesis for pore formation, these pores are in the form of fissures and voids between purely nanoporous carbon domains. Porous carbon was also observed to be present on the opposite side of the support (the side opposite from the initial deposition) and may fill some portion of the macroporosity of the support. During pyrolysis the polymer blend drops in viscosity prior to carbonization, making deeper pore penetration more facile. 3.2. Porosity characterization by gas permeation Fig. 6(a) is a plot of the flux versus pressure for several gases through an uncoated stainless steel 0.2 m support. The data show both a high rate of permeation and low gas selectivity. A nanoporous carbon film pyrolized in the absence of polymer template shows a linear relationship between the flux and applied pressure for He, O2 , N2 and Ar (Fig. 6(b)). This permeation behavior is typical of PFA derived carbon membranes [18]. Fig. 7 is characteristic of gas
Fig. 5. Scanning electron micrograph of the pore structure of the carbon ultrafiltration membrane showing grain boundaries between nanoporous carbon domains (M8000-1-0.021).
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Fig. 6. Flux as a function of driving force pressure for: (a) uncoated stainless steel support; (b) 0% PEG/PFA carbon membrane carbonized at 600 ◦ C; (c) 50% 8000 amu PEG/PFA templated membrane carbonized at 600 ◦ C.
permeation through a templated carbon membrane (50% PFA/8000 g mol−1 PEG) showing a quadratic dependence of the flux versus pressure and Knudsen gas selectivity at low pressure. This behavior is
observed on all PEG/PFA carbon membranes regardless of coating deposition mass or template size employed. Hence, the permeation results cannot necessarily be attributed to partially coated or partially formed nanoporous films such as that shown in Fig. 6(b). For a membrane with a mean pore size between 100 and 1 nm, the pore dimension can be estimated by examining data for the membrane gas permeability versus trans-membrane pressure. The linear relationship yields an intercept equal to the Knudsen permeability with slope inversely proportional to the gas viscosity (Fig. 7). The gas flux through the membrane is written as B0 p J = K0 + (1) p¯ l η The structure factor B0 is proportional to the square of the mean pore size B0 =
Fig. 7. Ultrafiltration membrane (M3400-1-0.019) gas permeability as a function of trans-membrane pressure for mean pore size characterization.
ε 2 d 2 τ α m
(2)
with α = 2.5 for consolidated media [20]. The Knudsen permeability K0 can be written as
M.S. Strano et al. / Journal of Membrane Science 198 (2002) 173–186 Table 2 Mean pore sizes of membranes measured by gas permeation
Table 3 Parameters used for stagnant film/concentration polarization correction
dm (m) PEG1500 × × × × × ×
10−9 10−8 10−8 10−9 10−8 10−8
He N2 Ar O2 CO2 SF6
9.92 1.03 1.37 9.15 1.43 1.13
Mean S.D.
1.15 × 10−8 2.10 × 10−9
4βε K0 = 2 dm 3τ α
PEG3400 1.19 1.28 6.81 2.39 1.22 1.10
× × × × × ×
10−8 10−8 10−9 10−8 10−8 10−8
1.31 × 10−8 5.69 × 10−9
PEG8000 2.83 2.59 2.21 2.87
10−8
× × 10−8 × 10−8 × 10−8
2.63 × 10−8 3.05 × 10−9
8RT M
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Water kinematic viscosity, µ (m2 s−1 ) Cell radius, b (m) Stir speed, ω (rad s−1 ) Coefficient for Amicon 30 mm cella , χ Free solution diffusivityb,c , D∞ (m2 s−1 ) Reynolds numberc , Re Schmidt numberc , Sc Sherwood numberc , Sh Mass transfer coefficientc , k (m s−1 )
8.50 × 10−7 0.0125 2π × 5 0.23 a = 7.67 × 10−9 ; b = −0.478 a a a a
= 5.77 × 103 ; b = 0.000 = 1.11 × 102 ; b = 0.478 = 148.0; b = 0.158 = 9.1 × 10−5 ; b = −0.320
a
(3)
where β is 0.8 for consolidated media [20]. The gas permeation data can be used to evaluate both B0 and K0 , with the ratio of these two parameters used to evaluate the mean pore size of the membrane, dm without any assumptions regarding the membrane porosity, ε or tortuosity factor, τ . Here, we do assume that the pore size of the carbon film is significantly less than that of the support (i.e. <200 nm.) The results in Table 2 show that the mean pore sizes decrease with decreasing PEG template molecular diameter. It should be noted, however, that this simple analysis cannot account for complexities in pore structure, including multimodal pore size distributions. 3.3. Ultrafiltration of polydisperse dextrans The trends observed above for membrane pore size as a function of template molecular weight can be clarified by measuring dextran rejection data [21]. Under steady-state filtration conditions, concentration polarization can magnify the driving forces of retained solutes and yield fictitiously lower partial rejection coefficients for a given membrane. Several researchers have corrected the observed partial rejection coefficients obtained under these conditions to yield actual rejection coefficients — those applicable in the limit of zero polarization — through the use of a stagnant film and solute mass transfer model [1]. The analysis requires knowledge of dextran-free solution diffusion coefficients, stirred-cell operating conditions and geometry, as well as overall mem-
[23]. [22]. c a × (dextran molecular weight)b . b
brane flux. A correlation of free solution diffusion coefficient versus dextran molecular weight has been produced by Granath [22]. Table 3 summarizes the relevant parameters used in the analysis. Under dead end filtration conditions in a stirred-cell, a boundary layer of thickness, δ develops on the surface of the membrane with the concentration of solute at the membrane surface, Cw greater than that in the bulk, Cb due to polarization effects. The solute flux through the membrane is related to the effective concentration driving force as [1] Cw − Cp Jv = k ln (4) Cb − C p where k is the mass transfer coefficient of the dextran, which is a function of the dextran molecular weight, and Cp the permeate solute concentration. Measuring Jv and Cp at a particular Cb , one can calculate Cw using Eq. (4). The value of k can be obtained from the correlation Sh = χ Re0.567 Sc0.33
(5)
where Sh, Re and Sc are the Sherwood, Reynolds and Schmidt numbers for the stirred-cell filtration process, respectively. Values for these parameter groupings as a function of molecular weight appear in Table 3. Zeman and Zydney provide a more detailed discussion of the analysis and the assumptions involved [1]. Fig. 8(a) through e show the observed (1 − C p /Cb ) and actual (1 − C p /Cw ) partial rejection coefficients
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Fig. 8. Observed (dotted line) and actual (solid line) partial rejection coefficients vs. dextran molecular weight for carbon membranes synthesized from poly(ethylene glycol) template: (a) 1000 MW PEG; (b) 1500 MW PEG; (c) 2000 MW PEG; (d) 3400 MW PEG; (e) 8000 MW PEG.
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as a function of dextran molecular weight for membranes synthesized using the five different template PEG sizes. The results for 1000 MW PEG shown in Fig. 8(a) demonstrate a lack of dextran retention with respect to molecular weight indicating that large, non-selective pores exist across the film. The polarization correction reveals that the broad, non-selective behavior of the membrane cannot be attributed to polarization, but rather that the membrane is defective. As the template size tends toward zero, the carbon membrane behaves as an untemplated film with a significant tendency towards cracking. It has been well established that furfuryl alcohol resin-based membranes require several coatings before a crack-free layer can be constructed [12,14]. The curves for 1500 (Fig. 8(b)) and 2000 PEG templates (Fig. 8(c)) show an increase in membrane selectivity. However, a tendency towards film cracking is retained and membrane reproducibility is affected. Fig. 8(c) in particular shows a lack of reproducibility for four membranes synthesized at the same carbon deposition mass. Membrane M2000-4-0.019 (Fig. 8(c)) shows a significantly enhanced retention compared to those in Fig. 8(a), which may represent the properties of the film in the absence of any significant cracking. Such retention could not be attained using a 1000 MW precursor. The templating of nanoporous carbon using PEG has been observed to yield accessible mesoporosity only at molecular weights of 2000 g mol−1 and higher for granular materials [15], although this study represents the most comprehensive work undertaken to determine the exact threshold. Problems of film cracking and reproducibility diminish for higher molecular weight templates as transport through these films becomes dominated by that which takes place through the porosity created via the templates rather than through adventitious cracks and fissures. Fig. 8(d) and (e) show reproducible behavior for 3400 and 8000 MW PEG templated membranes, respectively. Retention properties tend to increase from those seen in Fig. 8(b) and (c) and are apparently independent of carbon thickness and water permeance as expected. Membrane M8000-3-0.022 is the result of two successive coating and carbonization cycles and shows an increase in retention behavior likely from a reduction in pore size after the initial coat. Here, the pore structure of the first coat may become impregnated with a second coating of precursor
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Fig. 9. Dextran R90 value and calculated pore size as a function of template molecular weight of carbon membrane.
material that modifies the retention characteristics as shown. This procedure, which is analogous to a slip casting design, may in fact provide a route to the synthesis of ultra-thin nanofiltration membranes. With successive layer deposition, the retentive properties decrease with only a marginal increase in film thickness and deposition mass. In Fig. 9, the molecular weight of dextran corresponding to 90% rejection is plotted as a function of the molecular weight of the PEG template. This R90 value is obtained by interpolation or extrapolation of the data in Fig. 8(a)–(e) as needed. The extrapolated value for the 1000 g mol−1 template determined from Fig. 8(a) is actually an underestimate of this molecular weight. As observed above, the analysis suggests that these films possess large cracks and defects that are non-retentive. The trend shows an increase in dextran R90 with increasing PEG molecular weight starting from the 2000 MW PEG template size. Below this cutoff, the selectivity decreases sharply with decreasing template size suggesting the formation of cracks and defects. Fig. 9 also plots the calculated pore size based upon a simple partitioning model assuming a cylindrical pore structure and using the Stokes radius of the dextran molecule for the corresponding R90 molecular weight [1] rdextran 2 Rc = 1 − 1 − rpore
(6)
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Pore sizes estimated in this way for the membranes formed from the larger PEG templates are generally smaller than those calculated from gas permeability data. For example, the values of 15 and 12.4 nm corresponding to 8000 and 3400 molecular weight PEG compare to 26.3 ± 6.1 and 13.1 ± 11.4 nm calculated from gas permeation, respectively. In contrast, the pore size calculated from the dextran data for the 1500 PEG membrane was 28.2 nm, which is significantly greater than the 11.5 ± 4.2 nm determined from gas permeation. This large discrepancy is due to the greater effect of the pore defects on dextran transport than on the gas permeation results. The water permeability can also be used to provide an estimate of the average pore size or porosity using the Hagen–Poiseuille equation. Here, the membrane porosity/tortuosity ratio, ε/τ , can be expressed as a function of the pore size and water permeability Lp assuming a cylindrical pore structure [1] 8lµLp ε = τ (rpore )2
(7)
Using pore sizes from dextran rejection data (Eq. (6)), application of Eq. (7) predicts that the ratio of porosity to tortuosity decreases with decreasing template size. Average values are 0.024, 0.005, 0.001 and 0.001 for 8000, 3400, 2000, and 1000 g mol−1 template. This decrease in ε/τ is consistent with the greater carbon yield, and thus, lower porosity, of the membranes formed using the smaller PEG templates. Results of the batch depletion of dextran over granular templated carbons yield relative values of the accessible pore volumes of the various carbons. This allows for a comparison between properties resulting from the templating process and those resulting from pores developed as a result of film cracking. Fig. 10 is a plot of relative dextran amount adsorbed versus PEG molecular weight used during carbon synthesis. The results show negligible uptake at values of 2000 MW PEG and below with substantial increases in accessible area at larger template molecular weights (sizes). The data suggest a maximum in adsorption uptake near 8000 MW PEG. At 18,500 MW PEG, the extent of adsorption falls to values below that of the 3400 MW PEG. The initial portion of the curve agrees well with previous gas adsorption characterization, where it has also been noted that accessibility of the mesopore distribution becomes hampered for lower
Fig. 10. Dextran uptake from batch depletion on granular carbons synthesized using templates of 300–18,500 g mol−1 of PEG.
template molecular weights [15]. It is this lack of accessibility that may be responsible for the “trapping” of template material within the carbonizing system and hence the increase in carbon yield at low template molecular weights (Fig. 2). This retention of material would explain the observed decrease in membrane porosity. In Fig. 10, the subsequent decrease in capacity with 18,500 PEG may be the effect of a decrease in the rate of pyrolysis of this larger polymeric template. The subsequent decrease in the rate of evolution of the pyrolysis gases could also lead to a retention of template carbon within the forming solid, as in the case of the diffusion limitations associated with smaller template sizes. The internal pore volume of membranes made from the smaller templates is largely inaccessible, leading to a small amount of dextran uptake. As the template size increases, the accessibility of the pore volume increases, but at the expense of an increasing average pore radius, which ultimately reduces the internal surface area. The emerging picture of the actual templating process from the above observations is one that is kinetically driven by the diffusion of template decomposition products from the forming solid. At small template pore sizes (<2000 g mol−1 for PEG) the templated pores are largely unconnected. This may force the evolving decomposition products to diffuse through nanoporous domains, where mass transfer
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resistance is exceedingly high. These diffusion limitations increase carbon yield and decrease carbon porosity as observed. The lack of mesopore interconnection also means that the membrane behaves as a purely nanoporous thin film, and hence is subject to cracking and the formation of defects after a single coating. As the template size increases, this diffusion length scale increases and membrane properties become a function of the templated carbon itself with a subsequent improvement in retention properties and reproducibility. The results presented here underscore several potential improvements that can be made through modification of the synthesis procedure. The most notable of these is improvement in water permeability by decreasing carbon film thickness. The spray deposition on macroporous support was readily adapted from an analogous method of gas-separation membrane fabrication [15], although this may not be the optimal system for liquid separation applications. The results of microscopy and water permeation measurements suggest that decreasing the carbon deposition mass while maintaining coating uniformity can accomplish this goal. Additionally, this work presents indirect evidence that lower molecular weight templates have a discernable affect on pore structure and may not be entirely flashed off during carbonization as originally proposed [17]. This warrants a more complete investigation of the templating process with a particular emphasis on the composition and molecular weight dependence of the template as well as the pyrolysis ramp rate during carbonization.
4. Conclusions A novel method for producing ultra and nanofiltration carbon membranes is presented using spray deposition on macroporous stainless steel and pyrolysis of poly(furfuryl alcohol)/poly(ethylene glycol) mixtures. Membranes characterized by SEM show 12- to 15-m thick carbon films. Pores in the ultrafiltration range clearly vary with the average molecular weight of the template for PEG with molecular weight of 2000 g mol−1 and above. Below this threshold, the film behaves similar to an untemplated carbon membrane with crack formation a significant factor for the single coated membranes. Transport of a polydisperse
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dextran solution was used to quantify the retention properties of the membranes. Partial rejection coefficients for 90% of the dextran mixture were shown to vary with template molecular weight: 90% cutoffs of 2 × 104 , 3.5 × 104 , 6 × 104 g mol−1 of dextran were measured for 2000, 3400, and 8000 g mol−1 of template employed, respectively. Although generally of less accuracy, the mean pore sizes as measured from gas permeation data also demonstrate a similar decrease in value with decreasing template size.
Acknowledgements Support for this research was provided by the Department of Energy Office of Basic Energy Science and the DuPont Co. Michael Strano is grateful for financial support in the form of a Presidential Graduate Fellowship from the University of Delaware. References [1] L.J. Zeman, A.L. Zydney, Microfiltration and Ultrafiltration: Principles and Applications, Marcel Dekker, New York, 1996. [2] A.S. Michaels, High flow membrane, US Patent #3615024 (1971). [3] T.A. Tweddle, W.L. Thayer, O. Kutowy, S. Sourirajan, Method of gelling cast, polysulfone memrbrane, US Patent #4451424 (1984). [4] E. Schindler, F. Maier, Manufacture of porous carbon membranes, US Patent #4919860 (1990). [5] W.M. Clark, A. Bansal, M. Sontakke, Y.H. Ma, Protein adsorption and fouling in ceramic ultrafiltration membranes, J. Membr. Sci. 55 (1991) 21–38. [6] H.P. Hsieh, R.R. Bhave, H.L. Fleming, Microporous alumina membranes, J. Membr. Sci. 39 (1988) 221–241. [7] J.C.S. Wu, L.C. Cheng, An improved synthesis of ultrafiltration zirconia membranes via the sol–gel route using alkoxide precursor, J. Membr. Sci. 167 (2) (2000) 253–261. [8] H.C. Foley, Carbogenic molecular-sieves — synthesis, properties and applications, Micropor. Mater. 4 (6) (1995) 407–433. [9] H.C. Foley, Nanoporous carbons and related materials for small molecule separations, Abstr. Papers Am. Chem. Soc. 211 (1996) 2. [10] M. Acharya, M.S. Strano, J. Mathews, S.J.L. Billinge, et al., Simulation of nanoporous carbons: a chemically constrained structure, Philos. Mag. B 79 (10) (1999) 1499–1518. [11] R.K. Mariwala, H.C. Foley, Calculation of micropore sizes in carbogenic materials from the methyl-chloride adsorptionisotherm, Ind. Eng. Chem. Res. 33 (10) (1994) 2314–2321. [12] M. Acharya, B.A. Raich, H.C. Foley, M.P. Harold, J.J. Lerou, Metal-supported carbogenic molecular sieve membranes:
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M.S. Strano et al. / Journal of Membrane Science 198 (2002) 173–186 synthesis and applications, Ind. Eng. Chem. Res. 36 (8) (1997) 2924–2930. M. Acharya, H.C. Foley, Spray-coating of nanoporous carbon membranes for air separation, J. Membr. Sci. 161 (1999) 1–5. M.B. Shiflett, H.C. Foley, Ultrasonic deposition of high selectivity nanoporous carbon membranes, Science 285 (17) (1999) 1902–1905. D.S. Lafyatis, J. Tung, H.C. Foley, Poly(furfuryl alcohol)derived carbon molecular-sieves — dependence of adsorptive properties on carbonization temperature, time and poly(ethylene glycol) additives, Ind. Eng. Chem. Res. 30 (5) (1991) 865–873. M.S. Kane, L.C. Kao, R.K. Mariwala, D.F. Hilscher, H.C. Foley, Effect of porosity of carbogenic molecular sieve catalysts on ethylbenzene oxidative dehydrogenation, Ind. Eng. Chem. Res. 35 (10) (1996) 3319–3331. M.G. Stevens, H.C. Foley, Alkali metals on nanoporous carbon: new solid-base catalysts, Chem. Commun. (6) (1997) 519–520.
[18] M.S. Strano, H.C. Foley, Deconvolution of permeance through supported nanoporous membranes, AIChE J. 46 (3) (2000) 651–658. [19] M. Stevens, Cesium/nanoporous carbon composite materials: synthesis, characterization, and base catalysis, Ph.D. Thesis, University of Delaware, Newark, 1999 (Chapter 2). [20] P.C. Carman, Flow of Gases through Porous Media, Academic Press, London, 1956. [21] R. Nobrega, H. Balmann, P. Aimar, V. Sanchez, Transfer of dextran through ultrafiltration membranes: a study of rejection data analysed by gel permeation chromatography, J. Membr. Sci. 45 (1989) 17–36. [22] K.A. Granath, Solution properties of branched dextrans, J. Colloid Interf. Sci. 13 (1958) 308. [23] K.A. Smith, C.K. Colton, E.W. Merrill, L.B. Evans, Convective transport in a batch dialyzer: determination of the true membrane permeability from a single measurement, AIChE Symp. Ser. 64 (1968) 45.
Ultrafiltration membrane synthesis by nanoscale templating of porous carbon Michael S. Strano a,1 , Andrew L. Zydney a,1 , Howard Barth b , Gilber Wooler b , Hans Agarwal a,1 , Henry C. Foley c,∗ a b
Colburn Laboratory, Department of Chemical Engineering, University of Delaware, Academy Street, Newark, DE 19716, USA DuPont Company, Central Research and Development, Experimental Station, P.O. Box 80228, Wilmington, DE 19880-0228, USA c Department of Chemical Engineering, Center for Catalytic Science and Technology, Pennsylvania State University, University Park, PA 16802, USA Received 22 March 2001; received in revised form 28 June 2001; accepted 9 July 2001
Abstract A novel method for producing carbon membranes for ultrafiltration applications is presented using a spray deposition and pyrolysis of poly(furfuryl alcohol)/poly(ethylene glycol) mixtures on macroporous stainless steel supports. The poly(ethylene glycol) or PEG employed as a carbonization template creates a mesoporosity that leads to pores in the ultrafiltration range. Scanning electron microscopy (SEM) shows that the membranes consisted of 12- to 15-m thick carbon films. Gas permeation and water permeability data were used for the calculation of mean pore sizes, which were found to decrease with decreasing average molecular weight of the PEG template. Ultrafiltration of a polydisperse dextran solution was used to quantify the retention properties of the membranes. Molecular weight cutoffs determined from dextran retention data were shown to vary with template molecular weight: values of 2 × 104 , 3.5 × 104 , and 6 × 104 g mol−1 dextran were measured for respective templates of 2000, 3400, and 8000 g mol−1 PEG. For PEG molecular weights of 2000 or below, the templating effect was ill defined, membrane film cracking became more prominent, and membrane selectivity and reproducibility were adversely affected. © 2002 Published by Elsevier Science B.V. Keywords: Ultrafiltration; Carbon membranes; Membrane formation; Inorganic membranes
1. Introduction Membrane filtration technologies are critical to a variety of industrial process applications including cell harvesting, sterile filtration, protein enrichment, and removal of particulate matter [1]. Ultrafiltration ∗ Corresponding author. Tel.: +1-814-865-2574; fax: +1-814-865-7846. E-mail addresses: [email protected] (M.S. Strano), [email protected] (H.C. Foley). 1 Tel.: +1-302-831-2345; fax: +1-302-831-2085.
in particular, where the membrane retains components with kinetic diameters from 1 to 100 nm, has widespread utility for industrial separation processes. Traditionally, membranes used for ultrafiltration have been polymeric in nature. Asymmetric ultrafiltration membranes are commonly synthesized using phase inversion, where a polymer solution consisting of a base and pore-former in a solvent is induced to form two interdispersed liquid phases. Membranes synthesized in this manner include the bilayer type containing slit shaped fissures or cracks [2] and those that contain plasticizers and are stable while dry [3,4].
0376-7388/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 3 7 6 - 7 3 8 8 ( 0 1 ) 0 0 5 7 4 - 9
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Nomenclature B0 Cb Cp Cw dm J Jv k K0 l M p p rpore R Rc T Greek α β χ δ ε η µ τ
porous structure factor (mol ideal gas m−1 ) concentration of dextran in bulk solution (g l−1 ) concentration of dextran in permeate (g l−1 ) concentration of dextran at membrane surface (g l−1 ) average membrane pore size from gas permeation (m) flux of gas (mol m−2 s−1 ) filtrate flux during ultrafiltration (m s−1 ) solute mass transfer coefficient (m s−1 ) Knudsen permeability (mol m−1 s−1 Pa−1 ) carbon layer thickness (m) molecular weight (g mol−1 ) average trans-membrane pressure difference (Pa) pressure driving force under gas permeation (Pa) cylindrical pore radius (m) ideal gas constant (Pa m3 mol−1 ) rejection coefficient for dextran ultrafiltration temperature of gas permeation (K) symbols geometric constant for porous materials geometric constant for porous materials proportionality constant in mass transfer correlation stagnant film thickness above membrane in stirred-cell (m) membrane porosity gas viscosity (Pa s) viscosity of solvent (Pa s) pore tortuosity
Despite the widespread use of these types of membrane materials, they suffer from several disadvantages. The membranes have limited mechanical integrity which can lead to deformation during operation and adversely affect membrane performance. These materials often have limited temperature and
chemical stability, prohibiting their use in applications involving harsh organic solvents or at elevated temperature [4]. The use of caustic and hypochlorite solutions as cleaning agents can also lead to chemical degradation of the polymeric materials, significantly reducing the life of these membranes. In addition, membranes made from hydrophobic polymers like polysulfone and polyethersulfone must contain a humectant such as glycerol to avoid problems associated with re-wetting, while membranes made from hydrophilic polymers like cellulose must remain wet at all times to avoid collapse of the pore structure. While inorganic ultrafiltration membranes have attracted increasing attention in recent years [5–7], pore size modified carbon membranes have been relatively unexplored. Yet compared to ceramic and metallic ultrafiltration membrane materials, carbon can be considerably less expensive. Nanoporous carbon is also a promising material because it is chemically inert under typical processing conditions and thermally stable at temperatures well above 200 ◦ C [8,9]. Formed from the pyrolysis of carbonizing natural or synthetic polymeric precursors, this material is a disordered solid having a pore size approaching molecular dimensions and has been shown to possess highly shape-selective molecular transport properties [10]. Poly(furfuryl alcohol) (PFA)-derived nanoporous carbons have a mean pore size about 0.5 nm as measured from N2 and methyl chloride adsorption isotherms [11]. Attempts to use this material in the synthesis of defect free, micron scale films on structurally stable macroporous supports have been successful. Nanoporous carbon membranes have been fabricated with high selectivity for some gas phase separations of small molecules [12–14]. While such membranes have a mean pore size on the molecular scale (<1 nm), we [15] have shown that the addition of non-carbonizing polymers such as poly(ethylene glycol) (PEG) to porous carbon precursors forms meso- and macroporosity within the resulting carbon. Polymer mixtures of 25% PEG in PFA were carbonized at a soak temperature of 650 ◦ C. Molecular weights of PEG used were 300, 600, 1000, 2000, 3400, and 8000. The results suggested that lower molecular weight PEG (<2000 g mol−1 ) flashed off before carbonization and had little affect on carbon structure. Alternatively, larger molecular weight PEG created mesopore volumes of 0.45, 0.24
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and 0.28 cm3 g−1 for 2000, 3400, and 8000 g mol−1 templates, respectively. These results have been used in the synthesis of various nanoporous carbon catalysts with greatly enhanced effectiveness factors when compared to catalysts prepared from native nanoporous carbon alone [16,17]. Studies involving the use of porous carbon for ultrafiltration membrane formation in the literature are scarce. Schindler and Maier [4] claimed to have synthesized such a membrane through the carbonization of a pre-existing polymer membrane with identical pore structure. Although pyrolysis of a polymeric microfiltration membrane can reasonably produce a carbon membrane with similar macroporosity, there is little theoretical or empirical evidence to suggest that pores in the entire ultrafiltration range (1–100 nm) would be preserved during the carbonization of a pre-existing film. Additionally, ultrafiltration range pores were undetectable using the bubble-point testing methodology employed by the authors [4]. In this paper, a novel method for producing carbon ultrafiltration membranes is presented. Spray deposition and pyrolysis on macroporous stainless steel of poly(furfuryl alcohol)/poly(ethylene glycol) mixtures is the straightforward fabrication technique employed. The methodology is demonstrated in Fig. 1. As in our earlier work, the PEG is used as a non-carbonizing template molecule to introduce controllable mesoporosity into the carbon film. Permeation of a polydisperse dextran solution was used to quantify the
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retention properties of the membranes as a function of template size. A correlation of partial rejection coefficients for 90% of the dextran mixture with template molecular weight was observed. Membranes were also characterized using scanning electron microscopy (SEM) and gas permeation.
2. Experimental 2.1. Membrane synthesis Carbon based ultrafiltration membranes were synthesized using an adaptation of a spray coating procedure used for creating supported nanoporous carbon membranes [13]. Furfuryl alcohol resin in the form of Durez Resin #16,470 (PFA) from Occidental Chemical Corporation diluted in reagent grade acetone (Aldrich Chemical) was combined with poly(ethylene glycol) (PEG) and spray deposited upon a macroporous stainless steel support. The PFA resin has a specific gravity of 1.21 and a viscosity of 200 cP at 25 ◦ C that decreases to about 5 cP at 80 ◦ C. PEG samples of average molecular weights of 1000, 1500, 2000, 3400, and 8000 g mol−1 were obtained from Aldrich Chemical Company. This PFA/PEG/acetone precursor was prepared by combining PFA resin with a given average molecular weight of PEG in a 50/50 wt.% ratio at 70 ◦ C and subsequent dilution in acetone forming a stable mixture
Fig. 1. Mesoporous carbon membranes via nanoscale templating.
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Table 1 Summary of synthesis and performance characteristics for membranes used in this worka
M8000-1-0.021 M8000-2-0.018 M8000-3-0.022 M1500-1-0.019 M3400-1-0.019 M3400-2-0.016 M3400-3-0.018 M2000-1-0.019 M2000-2-0.019 M2000-3-0.019 M2000-4-0.019 M1000-1-0.029 M1000-2-0.023 a
Initial weight support (g)
Wet weight support (g)
Net carbon weight (g)
Pressure applied (psig)
Water flux (m s−1 )
10.5107 10.8591 10.4478 10.4659 10.7327 10.286 10.5444 10.6382 10.4353 10.3555 10.6816 10.5297 10.7057
10.6393 10.9768 10.58 10.5728 10.8409 10.3696 10.6481 10.7455 10.534 10.4623 10.7785 10.6695 10.8189
0.021 0.018 0.022 0.019 0.0188 0.0161 0.018 0.0189 0.019 0.019 0.0189 0.0288 0.0227
50.5 54 54 55 75 75 75 50 50 50 50 50 50
1.8 1.1 1.8 5.9 8.3 8.3 8.3 2.0 1.3 8.8 1.0 1.7 5.7
× × × × × × × × × × × × ×
10−6 10−5 10−6 10−7 10−7 10−7 10−7 10−8 10−7 10−8 10−7 10−7 10−8
Water permeance (m s−1 Pa−1 ) 5.1 2.9 4.7 1.5 1.6 1.6 1.6 5.6 3.8 2.5 3.0 5.0 1.6
× × × × × × × × × × × × ×
10−12 10−11 10−12 10−12 10−12 10−12 10−12 10−14 10−13 10−13 10−13 10−13 10−13
Labels are as follows: M(PEG MW)-(sample #)-(carbon mass (in g)).
at room temperature with a viscosity near 5 cP. The resulting solution was subsequently spray deposited onto circular porous stainless steel supports (0.2 m pore size, 11.4 cm2 area, and 1 mm thickness) supplied by Mott Metallurgical Co. Before spray coating the stainless steel supports were cleaned by sonication in acetone. After spray coating approximately 200 mg of the polymer blend solution (wet weight) was deposited on the support. The resulting system was subsequently pyrolyzed in a stream of flowing He (50 sccm) at 5 ◦ C min−1 to 600 ◦ C, held for 2 h at this temperature, and then allowed to cool to room temperature. The final carbon mass on each support was approximately 20 mg. Table 1 presents data on the membranes used in this work. Membrane labels contain the PEG template molecular weight and total coat mass of carbon for each sample. 2.2. Scanning electron microscopy SEM was used to characterize the internal morphology of the membranes. Membrane cross-sections were cut orthogonal to the support surface using a diamond-wafering saw. These sections were mounted in an epoxy resin, polished, and given a coating of Au for imaging with a Hitachi S-4000 field emission scanning electron microscope. Imaging was performed on areas of these cross-sections external to and within the macroporosity of the stainless steel
support and at various radii from the center of the disk-shaped membranes. 2.3. Characterization using gas permeation Gas phase transport of He, Ar, N2 , O2 , SF6 and CO2 was used to characterize the selective porosity and integrity of the carbon membrane. The disk-shaped membranes were sealed using VitonTM gaskets into a stainless steel module setup to measure the transport of a single gas through the membrane. The pure component flux of each gas was measured as a function of pressure. Details concerning the experimental setup and technique can be found in [18]. 2.4. Ultrafiltration of polydisperse dextrans Ultrafiltration of polydisperse dextrans was used to characterize the retention properties of the templated carbon films. Dextrans having average molecular weights of 2 × 106 , 1.7 × 105 , 7.0 × 104 , 3.9 × 104 , 9.9 × 103 g mol−1 (Pharmacia) were obtained from Aldrich Chemical Company. The dextrans were added in equal mass ratios to create a 10 g l−1 solution in a phosphate buffer prepared from monobasic (NaHPO4 ) and dibasic (NaH2 PO4 ) sodium phosphate to achieve a solution pH of 7.4±0.3. When applicable, 1 wt.% of methanol was added to prevent bacterial contamination.
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Retentates and permeates were analyzed using size exclusion chromatography (SEC) with a model 1050 HP chromatograph (Hewlett Packard) and a Waters 410 differential refractometer. SEC analysis was performed using a 7.5-mm i.d. × 30-cm TSK G4000 SW column (Tosoh Corp.), thermostated at 30 ◦ C, with 0.1 M sodium sulfate used as the mobile phase at a flow rate of 0.6 ml min−1 . Samples were injected with an autosampler without additional dilution using a 100-l sample loop. The above mentioned dextrans were injected individually at a concentration of 1 mg ml−1 , and retention volume data were used to construct a molecular weight calibration curve. Data was processed using Waters Millennium 32 HPLC software. Membrane separation was carried out using a dead end filtration setup. A 50 ml volume of dextran solution was loaded above the carbon membrane in a 30 mm diameter Amicon stirred-cell. An attached Ar line supplied gas pressure to the solution above the membrane. The permeate was collected in 30 ml vials, with the total permeate mass weighed continuously by placing the vial directly on a laboratory balance. From this time series data mass flow through the membrane was computed. A series of permeate and retentate samples were collected over time for off-line dextran analysis. 2.5. Batch depletion dextran adsorption Granular carbon membrane samples were prepared using the identical procedure described above for making thin carbon films, except that the solution was poured into a quartz boat and pyrolyzed as a bulk solution. These samples were analyzed for total dextran adsorption capacity using a batch depletion method. Each carbon was ground to 60/140 mesh and suspended in a water solution. After settling, the remaining solution was decanted to remove fine particles. The process was repeated until all fine particles were removed. A mass of 0.5 g of carbon was sealed with 30 ml of the polydisperse dextran solution in a Pyrex vial as described above. Sealed vials were vigorously agitated for 32 h, after which the solutions were passed through a 1 m filter and analyzed using SEC as described above. The average concentration of dextran remaining in the solution above the carbon was integrated over the molecular weight distribu-
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tion, with the results compared to the corresponding integral for the original solution.
3. Results and discussion Table 1 lists the initial support weight, the weight after spray coating (wet), and the weight after carbonization (dry) for each membrane studied in this work. All samples except M8000-3-0.022 were given a single coat and thermal treatment. This particular sample was fabricated in two coating/carbonization cycles. During the carbonization process, the evolution of gases reduced the polymer mass on the support leaving behind the solid carbon film. For poly(furfuryl alcohol) carbonization at 600 ◦ C, the carbon yield produced in this way is approximately 25–30% for similar thin films [14]. The PEG template is expected to produce very little residual carbon as seen in the synthesis of mesoporous catalyst supports (<1%) [19]. Fig. 2 is a plot of carbon yield on the support as a function of template molecular weight. This yield is measured as the ratio of the carbon mass after pyrolysis to the polymer deposition mass and shows a decreasing trend with increasing template PEG molecular weight. Because the template composition was fixed at 50% by weight for this study, it is clear that smaller molecular weight PEG contributes more to the residual carbon layer. Assuming a furfuryl alcohol resin conversion of 30% [16], the carbon yield for the 8000 g mol−1 templated film corresponds to nearly
Fig. 2. Carbon film conversion (net weight carbon deposited/wet polymer weight) as a function of template molecular weight.
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template size although there is considerable variability between samples formed from the same template size. Generally, water permeability decreases with decreasing template size and this may correspond to a decrease in membrane pore size and porosity as discussed below. In Table 1, the values for the hydraulic permeabilities range from 10−14 to 10−11 m s−1 Pa−1 , which are considerably lower than commercial ultrafiltration membranes (typical values are around 10−10 m s−1 Pa−1 . These reduced permeabilities are attributable to both the much greater thickness of the membranes and the decrease in the film porosity that occurs with decreasing template size. These factors are discussed in detail below. Fig. 3. Water permeance through carbon membranes as a function of template molecular weight.
3.1. Scanning electron microscopy
the entire template being emitted as gaseous products. This suggests that volatilization of the template does not cause any significant loss of poly(furfuryl alcohol) prior to carbonization. It is noteworthy that a similar trend is observed for the water permeability (Fig. 3) plotted as a function of
Fig. 4 is a scanning electron micrograph of a cross-section of the external porous carbon layer and the stainless steel support for a membrane synthesized using a 50% mixture of PFA: 8000 g mol−1 PEG at 600 ◦ C. The demarcation between the stainless steel support and external carbon layer is clearly visible,
Fig. 4. Scanning electron micrograph of a porous carbon ultrafiltration membrane (M8000-1-0.021) cross-section showing external carbon layer and stainless steel support.
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and there are also localized along with localized separations or cracks between the two layers. These film separations apparently have a negligible impact on coating integrity and mechanical stability, as there was no observed carbon mass loss during testing or property variations after repeated testing. The combination of the diamond saw cutting and surface polishing of the SEM sample has a smoothing affect on the morphology of the membrane cross-sections. The actual porosity of the steel support is roughly 0.6 as reported by the manufacturer. The carbon layer is 12- to 15-m thick and relatively uniform over the membrane surface, consistent with observations of similarly prepared carbon films for gas separation [14]. The calculated thickness range based on the mass of carbon deposited (Table 1) and assuming a porous carbon density of 1.6 g cm−3 is 9–15 m, which agrees well with this measurement. Fig. 5 is a 500 nm scale micrograph of the microstructure of the PFA/PEG carbon taken from the material external to the support surface. This cross-section was prepared by fracturing the carbon surface while attached to the support using a scalpel.
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The microstructure suggests pores with diameters in the nanometer range. As expected from the inferred hypothesis for pore formation, these pores are in the form of fissures and voids between purely nanoporous carbon domains. Porous carbon was also observed to be present on the opposite side of the support (the side opposite from the initial deposition) and may fill some portion of the macroporosity of the support. During pyrolysis the polymer blend drops in viscosity prior to carbonization, making deeper pore penetration more facile. 3.2. Porosity characterization by gas permeation Fig. 6(a) is a plot of the flux versus pressure for several gases through an uncoated stainless steel 0.2 m support. The data show both a high rate of permeation and low gas selectivity. A nanoporous carbon film pyrolized in the absence of polymer template shows a linear relationship between the flux and applied pressure for He, O2 , N2 and Ar (Fig. 6(b)). This permeation behavior is typical of PFA derived carbon membranes [18]. Fig. 7 is characteristic of gas
Fig. 5. Scanning electron micrograph of the pore structure of the carbon ultrafiltration membrane showing grain boundaries between nanoporous carbon domains (M8000-1-0.021).
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Fig. 6. Flux as a function of driving force pressure for: (a) uncoated stainless steel support; (b) 0% PEG/PFA carbon membrane carbonized at 600 ◦ C; (c) 50% 8000 amu PEG/PFA templated membrane carbonized at 600 ◦ C.
permeation through a templated carbon membrane (50% PFA/8000 g mol−1 PEG) showing a quadratic dependence of the flux versus pressure and Knudsen gas selectivity at low pressure. This behavior is
observed on all PEG/PFA carbon membranes regardless of coating deposition mass or template size employed. Hence, the permeation results cannot necessarily be attributed to partially coated or partially formed nanoporous films such as that shown in Fig. 6(b). For a membrane with a mean pore size between 100 and 1 nm, the pore dimension can be estimated by examining data for the membrane gas permeability versus trans-membrane pressure. The linear relationship yields an intercept equal to the Knudsen permeability with slope inversely proportional to the gas viscosity (Fig. 7). The gas flux through the membrane is written as B0 p J = K0 + (1) p¯ l η The structure factor B0 is proportional to the square of the mean pore size B0 =
Fig. 7. Ultrafiltration membrane (M3400-1-0.019) gas permeability as a function of trans-membrane pressure for mean pore size characterization.
ε 2 d 2 τ α m
(2)
with α = 2.5 for consolidated media [20]. The Knudsen permeability K0 can be written as
M.S. Strano et al. / Journal of Membrane Science 198 (2002) 173–186 Table 2 Mean pore sizes of membranes measured by gas permeation
Table 3 Parameters used for stagnant film/concentration polarization correction
dm (m) PEG1500 × × × × × ×
10−9 10−8 10−8 10−9 10−8 10−8
He N2 Ar O2 CO2 SF6
9.92 1.03 1.37 9.15 1.43 1.13
Mean S.D.
1.15 × 10−8 2.10 × 10−9
4βε K0 = 2 dm 3τ α
PEG3400 1.19 1.28 6.81 2.39 1.22 1.10
× × × × × ×
10−8 10−8 10−9 10−8 10−8 10−8
1.31 × 10−8 5.69 × 10−9
PEG8000 2.83 2.59 2.21 2.87
10−8
× × 10−8 × 10−8 × 10−8
2.63 × 10−8 3.05 × 10−9
8RT M
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Water kinematic viscosity, µ (m2 s−1 ) Cell radius, b (m) Stir speed, ω (rad s−1 ) Coefficient for Amicon 30 mm cella , χ Free solution diffusivityb,c , D∞ (m2 s−1 ) Reynolds numberc , Re Schmidt numberc , Sc Sherwood numberc , Sh Mass transfer coefficientc , k (m s−1 )
8.50 × 10−7 0.0125 2π × 5 0.23 a = 7.67 × 10−9 ; b = −0.478 a a a a
= 5.77 × 103 ; b = 0.000 = 1.11 × 102 ; b = 0.478 = 148.0; b = 0.158 = 9.1 × 10−5 ; b = −0.320
a
(3)
where β is 0.8 for consolidated media [20]. The gas permeation data can be used to evaluate both B0 and K0 , with the ratio of these two parameters used to evaluate the mean pore size of the membrane, dm without any assumptions regarding the membrane porosity, ε or tortuosity factor, τ . Here, we do assume that the pore size of the carbon film is significantly less than that of the support (i.e. <200 nm.) The results in Table 2 show that the mean pore sizes decrease with decreasing PEG template molecular diameter. It should be noted, however, that this simple analysis cannot account for complexities in pore structure, including multimodal pore size distributions. 3.3. Ultrafiltration of polydisperse dextrans The trends observed above for membrane pore size as a function of template molecular weight can be clarified by measuring dextran rejection data [21]. Under steady-state filtration conditions, concentration polarization can magnify the driving forces of retained solutes and yield fictitiously lower partial rejection coefficients for a given membrane. Several researchers have corrected the observed partial rejection coefficients obtained under these conditions to yield actual rejection coefficients — those applicable in the limit of zero polarization — through the use of a stagnant film and solute mass transfer model [1]. The analysis requires knowledge of dextran-free solution diffusion coefficients, stirred-cell operating conditions and geometry, as well as overall mem-
[23]. [22]. c a × (dextran molecular weight)b . b
brane flux. A correlation of free solution diffusion coefficient versus dextran molecular weight has been produced by Granath [22]. Table 3 summarizes the relevant parameters used in the analysis. Under dead end filtration conditions in a stirred-cell, a boundary layer of thickness, δ develops on the surface of the membrane with the concentration of solute at the membrane surface, Cw greater than that in the bulk, Cb due to polarization effects. The solute flux through the membrane is related to the effective concentration driving force as [1] Cw − Cp Jv = k ln (4) Cb − C p where k is the mass transfer coefficient of the dextran, which is a function of the dextran molecular weight, and Cp the permeate solute concentration. Measuring Jv and Cp at a particular Cb , one can calculate Cw using Eq. (4). The value of k can be obtained from the correlation Sh = χ Re0.567 Sc0.33
(5)
where Sh, Re and Sc are the Sherwood, Reynolds and Schmidt numbers for the stirred-cell filtration process, respectively. Values for these parameter groupings as a function of molecular weight appear in Table 3. Zeman and Zydney provide a more detailed discussion of the analysis and the assumptions involved [1]. Fig. 8(a) through e show the observed (1 − C p /Cb ) and actual (1 − C p /Cw ) partial rejection coefficients
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Fig. 8. Observed (dotted line) and actual (solid line) partial rejection coefficients vs. dextran molecular weight for carbon membranes synthesized from poly(ethylene glycol) template: (a) 1000 MW PEG; (b) 1500 MW PEG; (c) 2000 MW PEG; (d) 3400 MW PEG; (e) 8000 MW PEG.
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as a function of dextran molecular weight for membranes synthesized using the five different template PEG sizes. The results for 1000 MW PEG shown in Fig. 8(a) demonstrate a lack of dextran retention with respect to molecular weight indicating that large, non-selective pores exist across the film. The polarization correction reveals that the broad, non-selective behavior of the membrane cannot be attributed to polarization, but rather that the membrane is defective. As the template size tends toward zero, the carbon membrane behaves as an untemplated film with a significant tendency towards cracking. It has been well established that furfuryl alcohol resin-based membranes require several coatings before a crack-free layer can be constructed [12,14]. The curves for 1500 (Fig. 8(b)) and 2000 PEG templates (Fig. 8(c)) show an increase in membrane selectivity. However, a tendency towards film cracking is retained and membrane reproducibility is affected. Fig. 8(c) in particular shows a lack of reproducibility for four membranes synthesized at the same carbon deposition mass. Membrane M2000-4-0.019 (Fig. 8(c)) shows a significantly enhanced retention compared to those in Fig. 8(a), which may represent the properties of the film in the absence of any significant cracking. Such retention could not be attained using a 1000 MW precursor. The templating of nanoporous carbon using PEG has been observed to yield accessible mesoporosity only at molecular weights of 2000 g mol−1 and higher for granular materials [15], although this study represents the most comprehensive work undertaken to determine the exact threshold. Problems of film cracking and reproducibility diminish for higher molecular weight templates as transport through these films becomes dominated by that which takes place through the porosity created via the templates rather than through adventitious cracks and fissures. Fig. 8(d) and (e) show reproducible behavior for 3400 and 8000 MW PEG templated membranes, respectively. Retention properties tend to increase from those seen in Fig. 8(b) and (c) and are apparently independent of carbon thickness and water permeance as expected. Membrane M8000-3-0.022 is the result of two successive coating and carbonization cycles and shows an increase in retention behavior likely from a reduction in pore size after the initial coat. Here, the pore structure of the first coat may become impregnated with a second coating of precursor
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Fig. 9. Dextran R90 value and calculated pore size as a function of template molecular weight of carbon membrane.
material that modifies the retention characteristics as shown. This procedure, which is analogous to a slip casting design, may in fact provide a route to the synthesis of ultra-thin nanofiltration membranes. With successive layer deposition, the retentive properties decrease with only a marginal increase in film thickness and deposition mass. In Fig. 9, the molecular weight of dextran corresponding to 90% rejection is plotted as a function of the molecular weight of the PEG template. This R90 value is obtained by interpolation or extrapolation of the data in Fig. 8(a)–(e) as needed. The extrapolated value for the 1000 g mol−1 template determined from Fig. 8(a) is actually an underestimate of this molecular weight. As observed above, the analysis suggests that these films possess large cracks and defects that are non-retentive. The trend shows an increase in dextran R90 with increasing PEG molecular weight starting from the 2000 MW PEG template size. Below this cutoff, the selectivity decreases sharply with decreasing template size suggesting the formation of cracks and defects. Fig. 9 also plots the calculated pore size based upon a simple partitioning model assuming a cylindrical pore structure and using the Stokes radius of the dextran molecule for the corresponding R90 molecular weight [1] rdextran 2 Rc = 1 − 1 − rpore
(6)
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Pore sizes estimated in this way for the membranes formed from the larger PEG templates are generally smaller than those calculated from gas permeability data. For example, the values of 15 and 12.4 nm corresponding to 8000 and 3400 molecular weight PEG compare to 26.3 ± 6.1 and 13.1 ± 11.4 nm calculated from gas permeation, respectively. In contrast, the pore size calculated from the dextran data for the 1500 PEG membrane was 28.2 nm, which is significantly greater than the 11.5 ± 4.2 nm determined from gas permeation. This large discrepancy is due to the greater effect of the pore defects on dextran transport than on the gas permeation results. The water permeability can also be used to provide an estimate of the average pore size or porosity using the Hagen–Poiseuille equation. Here, the membrane porosity/tortuosity ratio, ε/τ , can be expressed as a function of the pore size and water permeability Lp assuming a cylindrical pore structure [1] 8lµLp ε = τ (rpore )2
(7)
Using pore sizes from dextran rejection data (Eq. (6)), application of Eq. (7) predicts that the ratio of porosity to tortuosity decreases with decreasing template size. Average values are 0.024, 0.005, 0.001 and 0.001 for 8000, 3400, 2000, and 1000 g mol−1 template. This decrease in ε/τ is consistent with the greater carbon yield, and thus, lower porosity, of the membranes formed using the smaller PEG templates. Results of the batch depletion of dextran over granular templated carbons yield relative values of the accessible pore volumes of the various carbons. This allows for a comparison between properties resulting from the templating process and those resulting from pores developed as a result of film cracking. Fig. 10 is a plot of relative dextran amount adsorbed versus PEG molecular weight used during carbon synthesis. The results show negligible uptake at values of 2000 MW PEG and below with substantial increases in accessible area at larger template molecular weights (sizes). The data suggest a maximum in adsorption uptake near 8000 MW PEG. At 18,500 MW PEG, the extent of adsorption falls to values below that of the 3400 MW PEG. The initial portion of the curve agrees well with previous gas adsorption characterization, where it has also been noted that accessibility of the mesopore distribution becomes hampered for lower
Fig. 10. Dextran uptake from batch depletion on granular carbons synthesized using templates of 300–18,500 g mol−1 of PEG.
template molecular weights [15]. It is this lack of accessibility that may be responsible for the “trapping” of template material within the carbonizing system and hence the increase in carbon yield at low template molecular weights (Fig. 2). This retention of material would explain the observed decrease in membrane porosity. In Fig. 10, the subsequent decrease in capacity with 18,500 PEG may be the effect of a decrease in the rate of pyrolysis of this larger polymeric template. The subsequent decrease in the rate of evolution of the pyrolysis gases could also lead to a retention of template carbon within the forming solid, as in the case of the diffusion limitations associated with smaller template sizes. The internal pore volume of membranes made from the smaller templates is largely inaccessible, leading to a small amount of dextran uptake. As the template size increases, the accessibility of the pore volume increases, but at the expense of an increasing average pore radius, which ultimately reduces the internal surface area. The emerging picture of the actual templating process from the above observations is one that is kinetically driven by the diffusion of template decomposition products from the forming solid. At small template pore sizes (<2000 g mol−1 for PEG) the templated pores are largely unconnected. This may force the evolving decomposition products to diffuse through nanoporous domains, where mass transfer
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resistance is exceedingly high. These diffusion limitations increase carbon yield and decrease carbon porosity as observed. The lack of mesopore interconnection also means that the membrane behaves as a purely nanoporous thin film, and hence is subject to cracking and the formation of defects after a single coating. As the template size increases, this diffusion length scale increases and membrane properties become a function of the templated carbon itself with a subsequent improvement in retention properties and reproducibility. The results presented here underscore several potential improvements that can be made through modification of the synthesis procedure. The most notable of these is improvement in water permeability by decreasing carbon film thickness. The spray deposition on macroporous support was readily adapted from an analogous method of gas-separation membrane fabrication [15], although this may not be the optimal system for liquid separation applications. The results of microscopy and water permeation measurements suggest that decreasing the carbon deposition mass while maintaining coating uniformity can accomplish this goal. Additionally, this work presents indirect evidence that lower molecular weight templates have a discernable affect on pore structure and may not be entirely flashed off during carbonization as originally proposed [17]. This warrants a more complete investigation of the templating process with a particular emphasis on the composition and molecular weight dependence of the template as well as the pyrolysis ramp rate during carbonization.
4. Conclusions A novel method for producing ultra and nanofiltration carbon membranes is presented using spray deposition on macroporous stainless steel and pyrolysis of poly(furfuryl alcohol)/poly(ethylene glycol) mixtures. Membranes characterized by SEM show 12- to 15-m thick carbon films. Pores in the ultrafiltration range clearly vary with the average molecular weight of the template for PEG with molecular weight of 2000 g mol−1 and above. Below this threshold, the film behaves similar to an untemplated carbon membrane with crack formation a significant factor for the single coated membranes. Transport of a polydisperse
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dextran solution was used to quantify the retention properties of the membranes. Partial rejection coefficients for 90% of the dextran mixture were shown to vary with template molecular weight: 90% cutoffs of 2 × 104 , 3.5 × 104 , 6 × 104 g mol−1 of dextran were measured for 2000, 3400, and 8000 g mol−1 of template employed, respectively. Although generally of less accuracy, the mean pore sizes as measured from gas permeation data also demonstrate a similar decrease in value with decreasing template size.
Acknowledgements Support for this research was provided by the Department of Energy Office of Basic Energy Science and the DuPont Co. Michael Strano is grateful for financial support in the form of a Presidential Graduate Fellowship from the University of Delaware. References [1] L.J. Zeman, A.L. Zydney, Microfiltration and Ultrafiltration: Principles and Applications, Marcel Dekker, New York, 1996. [2] A.S. Michaels, High flow membrane, US Patent #3615024 (1971). [3] T.A. Tweddle, W.L. Thayer, O. Kutowy, S. Sourirajan, Method of gelling cast, polysulfone memrbrane, US Patent #4451424 (1984). [4] E. Schindler, F. Maier, Manufacture of porous carbon membranes, US Patent #4919860 (1990). [5] W.M. Clark, A. Bansal, M. Sontakke, Y.H. Ma, Protein adsorption and fouling in ceramic ultrafiltration membranes, J. Membr. Sci. 55 (1991) 21–38. [6] H.P. Hsieh, R.R. Bhave, H.L. Fleming, Microporous alumina membranes, J. Membr. Sci. 39 (1988) 221–241. [7] J.C.S. Wu, L.C. Cheng, An improved synthesis of ultrafiltration zirconia membranes via the sol–gel route using alkoxide precursor, J. Membr. Sci. 167 (2) (2000) 253–261. [8] H.C. Foley, Carbogenic molecular-sieves — synthesis, properties and applications, Micropor. Mater. 4 (6) (1995) 407–433. [9] H.C. Foley, Nanoporous carbons and related materials for small molecule separations, Abstr. Papers Am. Chem. Soc. 211 (1996) 2. [10] M. Acharya, M.S. Strano, J. Mathews, S.J.L. Billinge, et al., Simulation of nanoporous carbons: a chemically constrained structure, Philos. Mag. B 79 (10) (1999) 1499–1518. [11] R.K. Mariwala, H.C. Foley, Calculation of micropore sizes in carbogenic materials from the methyl-chloride adsorptionisotherm, Ind. Eng. Chem. Res. 33 (10) (1994) 2314–2321. [12] M. Acharya, B.A. Raich, H.C. Foley, M.P. Harold, J.J. Lerou, Metal-supported carbogenic molecular sieve membranes:
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