Novel Modification of a Macroporous Stainless Steel Tube by

Article pubs.acs.org/IECR

Novel Modification of a Macroporous Stainless Steel Tube by Electroless Ni plating for Use as a Substrate for Preparation of Nanoporous Carbon Membranes B. S. Liu,* Y. H. Guo, and F. Yuan Department of Chemistry, School of Science, Tianjin University, Tianjin 30072, P.R. China ABSTRACT: Nanoporous carbon (NPC)/Ni−P composite membranes were prepared on electroless Ni-plating-modified porous stainless steel tubes (PSSTs) by carbonizing polyimide or cellulose acetate precursors at 700 °C in a nitrogen atmosphere. The performance of the membranes was evaluated by single-gas permeation and binary-gas mixture separation. The effects of the substrate with or without modification on gas permeation were studied, and the diffusion mechanism of the gases through the membranes was analyzed. The real separation factors of gas mixtures through the NPC/Ni−P composite membranes were much higher than those through the nanoporous carbon membranes (NPCMs) prepared on the unmodified substrate, especially those for H2/CH4 and H2/CO2. The weight loss rate of polymer precursors was investigated by TG/DTG. The specific surface areas of the membranes and the crystallization of the Ni−P alloy layer were characterized by means of the BET and DSC techniques, respectively, indicating that localized electroless Ni plating can reduce the pore diameter of PSST effectively and regulate the pore size distribution.

1. INTRODUCTION Although polymeric membranes are widely used in different gas separation processes, an inorganic membrane technology is rapidly receiving global attention because it offers a better alternative for superior separation properties. Among the various inorganic membranes known, such as silica, zeolites, carbon,1,2 and silica−carbon,3,4 carbon is one of the most promising membrane materials becase of its high mechanical, chemical,5 and thermal6,7 stabilities, along with good permeability and selectivity. In 1983, Koresh and Sofer8 prepared carbon molecular sieving membranes with high permeability by the carbonization of cellulose. Since then, there have been many reports about preparation techniques for nanoporous carbon membranes (NPCMs), including the selection and pretreatment of precursors, the optimization of carbonization conditions, and the post-treatment of membranes. 9−12 Although carbon nanotube (CNT)−polymer membranes offer the flexibility of the polymer as well as the selectivity and thermal stability of the CNT,13,14 the greatest challenge for NPCMs that rely on differences in molecular size to achieve separation at present is to find new approaches to reduce their costs and improve their properties. Among NPCMs, there are two main configurations: unsupported and supported on a macroporous substrate.15 The brittleness of the former makes it difficult for practical applications. Moreover, NPCMs supported on a macroporous substrate show better mechanical strength than CNT−polymer membranes, whose mechanical strength is governed by the interactions between the nanotubes and the polymer. Therefore, many researchers have concentrated on studying the preparation of NPCMs on strong porous supports, such as αAl2O 3,16,17 γ-Al2O3,18 and porous stainless steel tubes (PSSTs).19 In particular, PSSTs are gradually coming into view because the problems with stabilizing seals encountered when using Al2O3 ceramic tubes can be solved by simple © 2012 American Chemical Society

welding with PSSTs. However, the composition of PSSTs is very complex, and the pore structure is irregular. Therefore, before intact carbon membranes are formed, the irregular pore structure of the PSSTs must be modified to lessen the defects as much as possible and obtain a support with uniform pores. Merritt et al.20,21 filled the pores of PSSTs with silica nanoparticles, but they still needed strenuous processing involving several sequential coating and pyrolysis steps to obtain a selective membrane. In the present work, NPCMs were prepared by one-time coating and then carbonization of polyimide or cellulose acetate precursors on PSSTs modified by a novel localized electroless nickel-plating technique22 at 700 °C in a nitrogen atmosphere. The separation performance of the prepared membranes was found to meet the requirements for practical applications. That is, this modification process can save time and cost during the preparation of supported NPCMs. It is a potential and promising pretreatment for macro-PSSTs.

2. EXPERIMENTAL METHOD 2.1. Modification of PSSTs. PdCl2 (0.05 g) was dissolved in deionized water and then mixed with 50 mL of aluminum sol23 by stirring. The outer surface of PSSTs was brushed with a layer of the obtained Pd sol and then dried at room temperature. The dried PSSTs were activated at 580 °C in H2 atmosphere (30 mL/min) for 10 min. During the activation process, PdCl2 on the top layer of the PSSTs decomposed into fine metallic palladium particles. Then, both ends of the PSSTs were sealed up by Teflon tape. After pre-electroless nickel plating in a bath containing nickel acetate (1.57 g/L) and Received: Revised: Accepted: Published: 9007

December 16, 2011 June 10, 2012 June 14, 2012 June 14, 2012 dx.doi.org/10.1021/ie202953n | Ind. Eng. Chem. Res. 2012, 51, 9007−9015

Industrial & Engineering Chemistry Research

Article

sodium hypophosphite (30 g/L) (40−50 °C, pH 6) for 10 min, the PSSTs were transferred to an electroless nickel-plating bath (30−60 °C, pH = 9−10) (Table 1) and kept for 20 min to

3%. For each pair of gases the ideal separation factor29 was calculated as the permeation ratio of the two gases P=

Table 1. Composition of Electroless Nickel-Plating Bath chemicals nickel acetate (AR) sodium citrate (AR) triethanolamine (AR) ammonium chloride (AR) sodium hypophosphite (AR)

concentration

P0V /RT ΔpSΔt

(1)

where P is permeation (mol·m−2·s−1·Pa−1), P0 is the pressure of measured gas (Pa) at the operating temperature T (K), V is the volume (m3) of permeated gas at time Δt (s), R is the ideal gas constant (J·mol−1·K−1), S is the outer surface area (m2) of the membrane, and Δp is the pressure difference (Pa) between the outer (feed) and inner (permeate) tubes. 2.3.2. Measurement of Mixed Gas through the Membrane. A similar membrane module26 was used for the separation of mixed gases. A binary gas mixture (with a 1:1 molar ratio of gas A to gas B) was introduced into the outer tube of the membrane module. The gas permeated into the inner tube was carried away by helium or nitrogen as sweep gas. The pressure difference (Δp) between the two tubes was ca. 40 kPa, and the pressure on the permeate side of the membrane was atmospheric at all times. The time required from the introduction of the sweep gas and the binary gas mixture to reach permeation equilibrium was about 3 h. That is, after 3 h, the gas compositions on both the permeate and feed sides remained constant and were analyzed using an online gas chromatograph (102 G) with a TDX-01 packed column. The apparatus for binary-gas mixture separation is shown in Figure 1. The real separation factor was calculated as

20−42 g/L 40−100 g/L 70−90 mL/L 30−45 g/L 5−30 g/L

form nickel−phosphorus (Ni−P) amorphous membranes. Finally, the nickel-plated PSSTs were washed with tap water several times, then dehydrated with anhydrous ethanol, and dried at 80 °C for 1 h. 2.2. Preparation of NPCMs and NPC/Ni−P Composite Membranes. PSSTs with or without modification by electroless nickel plating were used as supports. Precursors of 20 wt % polyimide and 7.23 wt % cellulose acetate were prepared by separately dissolving polyimide and cellulose acetate in Nmethylpyrrolidone (NMP) at 60 °C with continuous stirring to form transparent sols. A layer of polymeric precursor was coated on the outer surface of a modified PSST by dipping a sealed end of the PSST vertically into the prepared sol for 3 s. Then, the PSST was spun and dried under infrared light. Polymeric precursors coated on PSSTs were carbonized by subsequent pyrolysis in N2 atmosphere (45 mL/min). First, the sample was heated from room temperature to 250 °C at a rate of 10 °C/min and then from 250 to 550 °C at a rate of 4.0 °C/ min. Second, the heating rate was lowered to 2.5 °C/min until the temperature reached 650 °C. The variation in heating rate was due to the fact that the evolution rate of volatile components during pyrolysis can control and affect the initial pore formation of NPCMs.24,25 Third, the temperature was raised to 700 °C at a rate of 0.2 °C/min to avoid uncontrollable shrinkage of the membranes, and this temperature was held for 2 h to form the NPCMs. Finally, the temperature was decreased to 100 °C at a rate of 3 °C/min, and the sample was cooled to room temperature. 2.3. Evaluation of the Membrane Performance. The performances of a commercial PSST, PSST-supported Ni−P amorphous alloy membranes, NPCMs, and NPC/Ni−P alloy composite membranes were investigated by single-gas permeation and binary-gas mixture separation. 2.3.1. Investigation of Single-Gas Permeation. A single-gas permeation measurement was performed in a two-tube membrane apparatus with a quartz tube and a membrane tube as the outer and inner tubes, respectively. A schematic was provided elsewhere.26 Prior to testing, the membranes were treated in an atmosphere of N2 (20 mL/min) for 12 h at 150 °C to remove physically adsorbed water.27,28 Single-gas samples, namely, N2, CH4, CO2, and H2, were introduced into the outer tube and allowed to permeate through the membranes into the inner tube with different pressure differences between the two tubes. The pressure differences were measured with a precision pressure gauge. The pressure on the permeate side of the membrane was atmospheric at all times. The amount of permeated gas was measured with a soapfilm flow meter. All related experiments were repeated three times (average), and the relative derivatives remained within

Figure 1. Apparatus for the evaluation of binary-gas mixture separation: (1) helium or nitrogen gas cylinder (both sweep gas and GC carrier gas; when H2 is separated, nitrogen is used as the carrier gas, whereas for other gases, helium is used as the carrier gas), (2) gas cylinder (CH4/H2 or CO2/N2), (3) flow meter, (4) pressure gauge, (5) temperature controller, (6) power supply, (7) furnace, (8) thermocouple, (9) membrane separation module, (10) four-way valve, (11) six-way valve, (12) gas chromatograph.

αA/B =

(YA /YB)permeate (XA /XB)feed

(2)

where αA/B is the real separation factor between gases A and B and XA, XB, YA, and YB represent the mole fractions of species A and B on the feed and permeate sides, respectively. 9008

dx.doi.org/10.1021/ie202953n | Ind. Eng. Chem. Res. 2012, 51, 9007−9015


Article

2.4. Characterization of the Membranes. Thermogravimetric (TG) and differential scanning calorimetry (DSC) analyses were performed using a NETZSCH STA 409 PC/PG instrument. The polymer precursors or NPCMs were heated in a N2 atmosphere (30 mL/min) from 30 to 800 °C at a rate of 10 °C/min. N2 adsorption isotherms and pore size distributions were obtained at 77 K on a domestic adsorption analyzer.30

Table 2. Ideal Separation Factors of Different Gases through Membranes M1−M3 and Theoretical Value of Knudsen Diffusion at 25 °C for a Pressure Difference of 0.1 MPa separation factors

3. RESULTS AND DISCUSSION 3.1. Modification Action of Supports. Commercial PSSTs with and without modification by localized electroless nickel plating are referred to as M1 and M0, respectively. To probe the influence of modification on the properties of the support, the permeations of different gases through M0 and M1 at 25 °C were measured, as shown in Figure 2. The

item

H2/ N2

H2/ CH4

H2/ CO2

N2/ CH4

CO2/ CH4

CO2/ N2

M1 M2 M3 Knudsen diffusion

2.45 2.54 8.46 3.74

1.55 1.73 12.49 2.83

3.05 2.71 5.15 4.69

0.63 0.68 1.48 0.76

0.51 0.64 2.43 0.60

0.80 0.94 1.64 0.80

summary, the electroless nickel-plating modification promoted the adsorption of H2 and reduced the pore diameter of the support. This phenomenon was also observed in our previous work.34 The permeability of H2 through Ni−P amorphous alloy/ceramic composite membranes increased with increasing pressure difference. According to the report of Hosseinzadeh Hejazi et al.,35 permeation related to Langmuir adsorption slightly increases with increasing pressure in the region of low pressure. 3.2. Pyrolysis Analysis of Precursor and Preparation of NPCMs. Figure 3 shows thermogravimetric/differential

Figure 2. Permeation of gases through M0 (open symbols) and M1 (solid symbols) as a function of the pressure difference between the outer and inner tubes (25 °C).

permeations of gases such as H2, N2, and CO2 through M0 were extremely high [ca. (2.5−4.4) × 10−5 mol·m−2·s−1·Pa−1], indicating that the pore diameter of the commercial PSSTs was very large (ca. 2.2 μm31). Therefore, the diffusion of gases through M0 was mainly controlled by viscous flow (i.e., the flow of gas molecules in the pore channel depended on the collisions among the molecules). According to the Hagen−Poiseuille law, gases existing in a viscous state cannot be separated. After modification of the PSSTs, the permeations of gases through M1 decreased by more than 2 orders of magnitude (Figure 2) and changed slightly with increasing pressure difference, except for a significant increase for hydrogen. The ideal separation factors (Table 2) of gases through M1 at 0.1 MPa were close to the Knudsen diffusion coefficients, meaning that Knudsen diffusion played an important role in the permeation of gas through M1. Based on H2 temperatureprogrammed desorption (TPD) spectra of Ni−P membranes reported by Liu et al.,32 H2 desorption peaks in the vicinity of 103 and 400−600 °C represent active sites of hydrogen adsorption. Therefore, the permeation of hydrogen through M1 was controlled partially by surface diffusion in addition to Knudsen diffusion. According to the scanning electron microscopy images reported by Liu et al.,33 the configuration of Ni−P films consists mainly of Ni−P amorphous alloy clusters of 3−4 μm and interparticle voids in the Ni−P alloy providing space for the flow of H2 through the membrane. In

Figure 3. TG/DTG curves for (a) polyimide and (b) cellulose acetate precursors.

thermogravimetric (TG/DTG) curves of the two polymer precursors. For the polyimide precursor (Figure 3a), there were three variation processes: The 36% weight loss from room temperature to 270 °C corresponds to solvent NMP evaporation (boiling point 203 °C), with one peak at 240 °C in the DTG curve. The 47% weight loss from 270 to 625 °C indicates that polyimide started to polymerize with NMP and decompose gradually with the removal of small gas molecules. At temperatures higher than 625 °C, the mass loss was only 10%, indicating that the decomposition reaction was almost finished, along with the formation of the pore structure and the porosity of the NPCMs. For the cellulose acetate precursor, it can be seen that there were two peaks of weight loss at 135 and 355 °C in the DTG curve, corresponding to the evaporation of solvent NMP (34% mass loss) and cellulose acetate decomposition (55% mass loss) with the removal of small gas molecules, respectively. This shows that the cellulose acetate precursor had a lower decomposition temperature than the polyimide precursor, which affected the pore structure of the NPCMs. 9009



Article

Table 3. Preparation Conditions and Propertiesa of Different Membranes Ni−P alloy membranes

NPCMs

membraneb

numberc

precursord

S (cm2)

Δg (mg)

δ (μm)

S (cm2)

Δg (mg)

δ (μm)

M1 M2 M3 M4

1 0 1 1

PI PI CA

9.43  9.43 9.43

54.7  55.4 53.1

9.3  9.4 9.0

 15.7 17.3 17.3

 35.0 40.9 16.6

 13.9 14.8 6.0

a S, area (cm2); Δg, quality (mg); and δ, thickness (μm). bM1, commercial PSST modified by electroless nickel plating; M2, NPCM on commercial PSST obtained by carbonizing PI precursor; M3, NPC/Ni−P composite membrane on commercial PSST obtained by carbonizing PI; M4, NPC/Ni− P composite membrane on commercial PSST obtained by carbonizing CA. cNumber of times of localized electroless nickel plating. dPI, polyimide; CA, cellulose acetate.

TG analysis can help to judge the weight loss rate of the precursor in different temperature ranges, which has a marked influence on the carbonization rate. The residual carbon quantity during the carbonization of the polymer precursors can be estimated from the TG curves. Based on the weight loss rate of the polymer precursors in different temperature ranges (Figure 3), we regulated the heating rate properly (section 2.2) to avoid the instantaneous formation of small gas molecules that could result in the deterioration of the carbon nanopores during the pyrolysis of the polymer precursor. After the intact NPCMs were formed, the temperature was decreased slowly (3 °C/min) to 100 °C. Finally, the thickness of the NPCMs was estimated by the weight of the commercial PSSTs before and after preparation of the membranes (the density of carbon is 1.6 g/cm3). The preparation conditions and properties of different membranes are listed in Table 3. For comparison, the thickness of the electroless nickel-plating membranes (the density of the Ni−P membranes was 6.24 g/cm3) is also listed in Table 3. It can be seen that the thicknesses of the Ni−P membranes were identical, whereas the NPC/Ni−P composite membrane (M3) obtained by carbonizing polyimide was much thicker than that (M4) obtained by carbonizing cellulose acetate, because the mass fraction (20%) of the polyimide precursor was larger than that (7.23%) of the cellulose acetate precursor. In addition, the carbon residue of the polyimide was also larger than that of the cellulose acetate during the preparation of NPCMs. 3.3. Brunauer−Emmett−Teller (BET) Characterization of Pure NPCMs and NPC/Ni−P Composite Membranes. N2 adsorption isotherms of pure NPCMs and NPC/Ni−P composite membranes prepared by the carbonization of polyimide precursor are shown in Figure 4. When the relative pressure, p/p0, approached 0.07, single-layer adsorption on both membranes was saturated, similar to Langmuir adsorption isotherms according to the IUPAC classification. This suggests the presence of a large amount of micropores in the membranes. When p/p0 was close to 1, the pores in the NPCMs were entirely fulled. The specific surface area and pore volume (Table 4) of the pure NPCMs were slightly larger than those of the NPC/Ni−P composite membranes because of the low pore structure of Ni−P alloys. The average pore diameter of both membranes was ca. 0.9 nm. The pore size distribution curves calculated by a simplified local density are shown in the inset of Figure 4. It can be seen that both pure NPCMs and NPC/Ni−P composite membranes presented very narrow pore size distributions between 0.7 and 1.0 nm. 3.4. DSC Analysis of Different Membranes. DSC curves of membranes obtained in a N2 atmosphere are shown in Figure 5. There was an exothermic peak at 498 °C for all samples, originating from the crystallization of Ni−P

Figure 4. N2 adsorption isotherms of (a) NPCMs and (b) NPC/Ni−P composite membranes at 77 K. Inset: Pore size distributions of the two membranes.

Table 4. Specific Surface Areas (SBET), Total Pore Volumes (VT), Micropore Volumes (Vmic), and Average Pore Diameters (Da) of NPCM and NPC/Ni−P Composite Membranes sample

SBET (m2/g)

VT (cm3/g)

Vmic(cm3/g)

Da (nm)

NPCM NPC/Ni−P

401 369

0.127 0.103

0.102 0.070

0.9 0.9

Figure 5. DSC curves of different precursor/Ni−P membranes.

amorphous alloy. The crystallization temperature was slightly higher than that (344−381 °C) reported by Liu et al.,36 because the polymer precursor entered the voids between the Ni−P amorphous alloy particles and further prohibited the crystal9010



Article

lization of Ni−P alloy. In addition, there is one exothermic peak at 600 °C in curve b, corresponding to the linkage and cyclization between polyimide and NMP, as well as the formation of NPCMs accompanied by the removal of small gas molecules. These results are consistent with the TG/DTG analysis of the precursor (Figure 3). 3.5. Influence of Supports with and without Modification on Gas Permeation. To probe the effects of the support with and without modification on the separation performance of NPCMs, the relationships between the permeation of gas through membranes M2 and M3 and the pressure difference, gas molecular kinetic diameter, and temperature were investigated. 3.5.1. Influence of Pressure Difference on Permeation. The permeations of different gases through membranes M2 and M3 at 25 °C as functions of pressure difference are shown in Figure 6. The permeations of N2, CH4, and CO2 through M2 were

Figure 7. Permeation of gases through membranes M2 and M3 as a function of the kinetic diameter of the gas molecules at 25 °C at a pressure difference of 0.1 MPa. (Values in parentheses are the square roots of the gas molecular weights.)

diffusion of gases through M2 was dominated by the Knudsen diffusion mechanism and that the pore size distribution of NPCMs prepared on the commercial PSSTs was relatively large. CO2 has a high molecular weight; hence, its permeation was low even though the kinetic diameter of CO2 is smaller than those of N2 and CH4. The ideal separation factors for H2/ N2 and H2/CH4 were 2.54 and 1.73, respectively, which are lower than the value (2.71) for H2/CO2 (Table 2). As expected, the permeations of gases through M3 decreased based on the increase of the corresponding molecular kinetic diameter [H2 (2.89 Å) < CO2 (3.3 Å) < N2 (3.64 Å) < CH4 (3.8 Å)] (Figure 7). That is, when the pore diameter of the NPCMs approached the molecular dimensions of the gases, the permselectivity of these gases through the membranes depended on the size and shape of the molecules. For example, the permeation of CO2 through M3 was higher than those of N2 and CH4 because of the small kinetic diameter of CO2, similar to reports by Centeno and Fuertes37and Kim et al.38 The BET characterization also verified that the pore size of as-prepared NPCMs was approximately 0.9 nm (inset of Figure 4). In addition, the ideal separation factors for H2/N2 and H2/CH4 were 8.46 and 12.49, respectively. These are significantly higher than the theoretical values (3.74, 2.83) of the Knudsen diffusion (Table 2), indicating that the mechanism of molecular sieving played an important role in the process of gas diffusion through M3. This confirms that the modification of the commercial macro-PSSTs by electroless nickel plating regulated the pore diameter distribution of the support and simplified the preparation process of the NPCMs. (Specifically, the NPCMs were prepared by carbonizing polyimide or cellulose acetate precursor only once.) Hence, we resolved the scientific issue in the chemical modification of PSSTs for a long period of time and greatly improved the separation performance of the membranes. 3.5.3. Effect of Temperature on Gas Permeation. The relationship between the permeations of the gases through the membranes (M2, M3) and temperature is shown in Figure 8. It can be seen that the permeations of gases through M2 decreased slightly with increasing temperature except for H2, which followed the characteristics of Knudsen diffusion according to the relationship between permeation and temperature described in the equation39

Figure 6. Permeation of gases through M2 (solid symbols) and M3 (open symbols) as a function of the pressure difference between the outer and inner tubes (25 °C).

about 10−7 mol·m−2·s−1·Pa−1 and increased slightly with increasing pressure difference. The ideal separation factors of gases through M2 were found to be similar to those for M1 (Table 2). Moreover, at the same pressure difference, the permeations of different gases were mostly in the order H2 > CH4 > N2 > CO2, following the same trend as the gases permeating through M1, indicating that transport of these gases entered the range of Knudsen diffusion. However, the permeation of H2 through membrane M2 increased gradually with increasing pressure difference possibly because of adsorption and surface diffusion action. For the NPCM (M3) formed on the PSST modified by electroless nickel plating, the permeation of N2, CH4, and CO2 through membrane M3 was about 10−9 mol·m−2·s−1·Pa−1 and remained almost constant with increasing pressure difference except that there was a slight variation for H2. However, at the same pressure difference, the permeations of different gases were in the order H2 > CO2 > N2 > CH4, which was different from that of gases through membranes M1 and M2. This indicates that the diffusion of gases through NPC/Ni−P alloy composite membranes entirely surpassed the range of Knudsen diffusion and was controlled mainly by molecular sieving. 3.5.2. Influence of Molecular Kinetic Diameter on Gas Permeation. As shown in Figure 7, the permeations of gases through M2 decreased with the rise of the square root of the corresponding molecular weight. This indicates that the

P ∝ (MRT )−0.5 9011

(3)



Article

where P is the gas permeation (mol·m−2·s−1·Pa−1), A is a preexponential factor, and Ep is the apparent activation energy (kJ/ mol). It can be seen from Figure 9 that the apparent activation energy of the gas increased with increasing kinetic diameter of the gas molecule. The apparent activation energy of H2 was 2.9 kJ/mol; because the kinetic diameter of H2 was small (2.89 Å), there was almost no resistance to H2 permeation. The kinetic diameters of N2 (3.64 Å) and CH4 (3.8 Å) were relatively large, so large resistances were generated. The apparent activation energies for N2 and CH4 were 9.7 and 14.3 kJ/mol, respectively, similar to the results (9.8 and 14.5 kJ/mol) reported by Fuertes and Centeno.15 For CO2, the apparent activation energy was slightly higher than the result (0 kJ/mol) obtained for permeation through NPCM/carbon15 because the transport occurred mainly by diffusion in the gas phase (molecular sieving) without surface diffusion at high temperatures. Permeation of the large gas molecules through membrane M3 required generally high activation energies because of the molecular sieving effect. This result also explains the decrease of gas permeation mentioned in section 3.5.2. The ideal separation factors for binary gas mixtures through M3 decreased with increasing temperature (Figure 10). This

Figure 8. Permeation of gases through M2 (solid symbols) and M3 (open symbols) at different temperatures for a pressure difference between the inner and outer tubes of 0.04 MPa.

where P is the gas permeation (mol·m−2·s−1·Pa−1) at temperature T (K), M is the gas molecular weight, and R is the ideal gas constant (J·mol−1·K−1). However, the permeation of H2 through M2 increased slightly with increasing temperature because of the adsorption of H2 on the NPCMs and the increasing tendency of H2 to be adsorbed at higher temperatures. We also observed H2 adsorption in the molecular or atomic state on NPCMs in H2 TPD studies.40 The permeations of gases through M3 increased gradually with increasing temperature, in accordance with the activated diffusion mechanism, indicating that the diffusion of gases through M3 exceeded the range of Knudsen diffusion. According to the surface diffusion or molecular sieving mechanism, the permeation of gases through membranes depends strongly on the apparent activation energy or the kinetic diameter of the gas. The permeations of gases through M3 as function of temperature are shown in an Arrhenius plot in Figure 9. The apparent activation energies (Ep) of the gases through the membranes were calculated according to the equation41 log P = −

Ep 2.303RT

+ log A

Figure 10. Ideal separation factor of gases through M3 as a function of temperature for a pressure difference between the inner and outer tubes of 0.04 MPa.

(4)

was because the temperature of the separation system was higher than the “isoconcentration temperature” defined by Shelekhin et al.,42 so that gas-phase diffusion (molecular sieving) dominated the transport instead of surface diffusion. However, in the limit of high temperature, the gas diffusion coefficient approaches the conventional Knudsen diffusion coefficient. In addition, there is little understanding about the transition temperature. Gilron and Soffer39 pointed out that, at high temperatures, even if the average pore size of a membrane is within the range of molecular sieving, gas permeation approaches Knudsen diffusion value and the separation factor decreases with increasing temperature. 3.6. Separation Performance of Mixed Gas through Membranes. When the binary gas mixtures H2/CH4 and CO2/N2 were introduced into the membrane module, the relative contents of different gases on the two sides of membranes M2−M4 were measured as functions of time, as shown in Figure 11. The resistances to H2 and CO2 diffusion were much smaller than those to CH4 and N2 diffusion, so the contents of H2 and CO2 increased gradually with time on the permeate sides of membranes M2−M4 but decreased on the

Figure 9. Permeation of gases through M3 with temperature in an Arrhenius plot for a pressure difference between the inner and outer tubes of 0.04 MPa. 9012



Article

Figure 12. Real separation factors of gases through (A) M3 and (B) M4 as functions of time.

membranes had reached equilibrium. The real separation factors of binary gas mixture through the membranes are listed in Table 5. It can be seen that the real separation factors of Table 5. Separation Factors of Binary Gas Mixtures through Membranes M1−M4a)

M1(real) M2(real) M3(real) M3 (ideal) M4(real) Knudsen diffusion a

H2/ N2

H2/ CH4

H2/ CO2

N2/ CH4

CO2/ CH4

CO2/ N2

   9.06  3.74

1.11 4.72 81.42 13.27 131.7 2.83

1.01 4.68 13.49 5.22 12.40 4.69

1.04 1.06 2.61 1.47 2.53 0.76

1.00 0.89 2.46 2.54 2.26 0.60

1.01 0.81 1.80 1.74 1.88 0.80

Temperature of 25 °C, pressure difference of 0.04 MPa.

binary gas mixtures (1:1) through membrane M1 were small, indicating that Ni−P alloy membranes deposited on commercial PSSTs cannot be used for the separation of mixed gases. The real separation factors of different gas mixtures through membranes M3 and M4 were larger than those for M2, meaning that the NPC/Ni−P composite membranes presented more uniform pore distributions. In addition, as listed in Table 5, the real separation factors for H2/ CH4 through membranes M3 and M4 were much higher than the ideal values because, when CH4 and H2 diffused simultaneously through the membranes, the adsorption of CH4 molecules on the pore walls effectively reduced the pore size of the NPC/Ni−P composite membranes, further prohibiting the diffusion of gas molecules with large kinetic diameter through the membranes and promoting the separation of H2/CH4.43,44 In addition, even though the ideal separation factor for H2/CO2 (5.22) was slightly higher than that (5.04) reported by Foley et al.,21 the single-gas permeations for H2 and CO2 at 0.04 MPa were 3 times greater than those through silica−carbon membranes prepared by four cycles of coating and carbonization, as shown in Figure 6. Furthermore, we observed that the real separation factor (131.7) for H2/CH4 through M4 was much larger than that (81.42) through M3, possibly because the thickness of M3 (14.8 μm) was greater than that of M4 (6.0 μm) (Table 3). The thicker the NPC/Ni− P composite membrane prepared by single carbonization, the greater the likelihood that it contained cracks or/and pinholes, hence reducing the permselectivity of gases through the

Figure 11. Relative contents of different gases on the two sides [permeate (open symbols), retentate (solid symbols)] of membranes (A,D) M2, (B,E) M3, and (C,F) M4 as functions of time at 25 °C and a pressure difference of 0.04 MPa: (■) H2, (●) N2, (▲) CH4, (▼) CO2.

retentate side. In addition, it was interesting to note that the permeation of gases through M3 was entirely different from that of gases through M4, because of differences in the microstructures of the NPC/Ni−P composite membranes obtained from the two precursors, polyimide and cellulose acetate. As shown in Figure 11E, the content of CO2 on the retentate side decreased markedly with time, because of the strong adsorption properties between CO2 and −NH groups in the NPC/Ni−P composite membranes generated from polyimide. In addition, the content of N2 on the retentate side (M3) increased quickly with time, plausibly due to the repulsive action between N2 and −NH groups. Moreover, it can be seen that the diffusion of gases through membrane M4 was more moderate and the concentrations of all gases became stable within 3 h. The relationship between the real separation factor and the time of gas diffusion through membranes M3 and M4 is shown in Figure 12. The real separation factors of binary gas mixtures through membranes M3 and M4 increased with time, and after 3 h, the real separation factor remained almost constant, indicating that the permeation of the gases through the 9013



Article

membrane. As reported by Foley et al.,21,45 a large amount of cracks in NPCMs formed when the thickness of the membranes was greater than 20 μm.

(11) Song, C. W.; Wang, T. H.; Qiu, Y. H.; Qiu, J. S.; Cheng, H. M. Effect of carbonization atmosphere on the structure changes of PAN carbon membranes. J. Porous Mater. 2009, 16, 197−203. (12) Lee, H. J.; Suda, H.; Haraya, K. Characterization of the postoxidized carbon membranes derived from poly(2,4-dimethyl-1,4phenylene oxide) and their gas permeation properties. Sep. Purif. Technol. 2008, 59, 190−196. (13) Sholl, D. S.; Johnson, J. K. Making high-flux membranes with carbon nanotubes. Science 2006, 312, 1003−1004. (14) Cong, H. L.; Radosz, M.; Towler, B. F.; Shen, Y. Q. Polymer− inorganic nanocomposite membranes for gas separation. Sep. Purif. Technol. 2007, 55, 281−291. (15) Fuertes, A. B.; Centeno, T. A. Preparation of supported asymmetric carbon molecular sieve membranes. J. Membr. Sci. 1998, 144, 105−111. (16) Hayashi, J.; Yamamoto, M.; Kusakabe, K.; Morooka, S. Simultaneous improvement of permeance and permselectivity of 3,3′,4,4′-biphenyltetracarboxylic dianhydride-4,4′-oxydianiline polyimide membrane by carbonization. Ind. Eng. Chem. Res. 1995, 34, 4364−4370. (17) Sedigh, M. G.; Onstot, W. J.; Xu, L. F.; Peng, W. L.; Tsotsis, T. T.; Sahimi, M. Experiments and simulation of transport and separation of gas mixtures in carbon molecular sieve membranes. J. Phys. Chem. A 1998, 102, 8580−8589. (18) Carretero, J.; Benito, J. M.; Guerrero-Ruiz, A.; RodriguezRamos, I.; Rodriguez, M. A. Infiltrated glassy carbon membranes in γAl2O3 supports. J. Membr. Sci. 2006, 281, 500−507. (19) Acharya, M.; Foley, H. C. Spray-coating of nanoporous carbon membranes for air separation. J. Membr. Sci. 1999, 161, 1−5. (20) Merritt, A.; Rajagopalan, R.; Foley, H. C. High performance nanoporous carbon membranes for air separation. Carbon 2007, 45, 1267−1278. (21) Rajagopalan, R.; Merritt, A.; Tseytlin, A.; Foley, H. C. Modification of macroporous stainless steel supports with silica nanoparticles for size selective carbon membranes with improved flux. Carbon 2006, 44, 2051−2058. (22) Liu, B. S.; Zhang, F. M.; Yang, X. Y. Formation and performance of ohmic contact electrodes on BaTiO3-based thermistors by localized electroless Cu-plating. Thin Solid Films 2010, 519, 373−377. (23) Liu, B. S.; Cao, Y.; Deng, J. F. Catalytic dehydrogenation of ethanol in Ru-modified alumina membrane reactor. Sep. Sci. Technol. 1997, 32, 1683−1697. (24) Saufi, S. M.; Ismail, A. F. Fabrication of carbon membranes for gas separationA review. Carbon 2004, 42, 241−259. (25) Centeno, T. A.; Vilas, J. L.; Fuertes, A. B. Effects of phenolic resin pyrolysis conditions on carbon membrane performance for gas separation. J. Membr. Sci. 2004, 228, 45−54. (26) Liu, B. S.; Au, C. T. A La2NiO4-zeolite membrane reactor for CO2 reforming of methane to syngas. Catal. Lett. 2001, 77, 67−74. (27) Ismail, A. F.; David, L. I. B. A review on the latest development of carbon membranes for gas separation. J. Membr. Sci. 2001, 193, 1− 18. (28) Menendez, I.; Fuertes, A. B. Aging of carbon membranes under different environments. Carbon 2001, 39, 733−740. (29) Strano, M. S.; Foley, H. C. Modeling ideal selectivity variation in nanoporous membranes. Chem. Eng. Sci. 2003, 58, 2745−2758. (30) Liu, B. S.; Wei, X. N.; Zhan, Y. P.; Chang, R. Z.; Subhan, F.; Au, C. T. Preparation and desulfurization performance of LaMeOx/SBA15 for hot coal gas. Appl. Catal. B 2011, 102, 27−36. (31) Ames, R. L.; Bluhm, E. A.; Way, J. D.; Bunge, A. L.; Abney, K. D.; Schreiber, S. B. Physical characterization of 0.5 μm cut-off sintered stainless steel membranes. J. Membr. Sci. 2003, 213, 13−23. (32) Liu, B. S.; Lian, P. Y.; Zhao, X. H. A novel NiP−Cu composite membrane reactor for catalyic dehydrogenation of ethanol. Sep. Purif. Technol. 2003, 32, 281−287. (33) Liu, B. S.; Dai, W. L.; Wu, G. H.; Deng, J. F. Amorphous alloy/ ceramic composite membrane: Preparation, characterization and reaction studies. Catal. Lett. 1997, 49, 181−188.

■

CONCLUSIONS The separation performance of gases through NPC/Ni−P composite membranes was found to be significantly higher than that through NPCMs on commercial PSSTs, indicating that the modification of electroless nickel plating for macro-PSSTs played an important role. The results revealed that the diffusion of gases through NPC/Ni−P composite membranes is controlled by molecule sieving, which correlates with the activation energy, the temperature, and the kinetic diameter of the gas molecules. TG analysis of the precursors provided reasonable conditions for the preparation of NPCMs with high performance, and DSC results verified that the polymer precursors coated on the outer surface might prohibit the deep crystallization of Ni−P alloy membranes. The specific surface areas and pore volumes of pure NPCMs were found to be higher than those of NPC/Ni−P composite membranes because of the low pore structure of the Ni−P alloys.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 86-22-27892471. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We are grateful for the financial support of the National Natural Science Foundation of China and the BAOSTEEL Group Corporation (Grant 50876122), as well as the Scientific Research Foundation (SRF) for Returned Overseas Chinese Scholars (ROCS), State Education Ministry (SEM), People’s Republic of China (2002-247).

■

REFERENCES

(1) Rao, M. B.; Sircar, S. Performance and pore characterization of nanoporous carbon membranes for gas separation. J. Membr. Sci. 1996, 110, 109−118. (2) Shiflett, M. B.; Foley, H. C. Reproducible production of nanoporous carbon membranes. Carbon 2001, 39, 1421−1446. (3) Park, H. B.; Lee, Y. M. Pyrolytic carbon−silica membrane: A promising membrane material for improved gas separation. J. Membr. Sci. 2003, 213, 263−272. (4) Park, H. B.; Suh, I. Y.; Lee, Y. M. Novel pyrolytic carbon membranes containing silica: Preparation and characterization. Chem. Mater. 2002, 14, 3034−3046. (5) Liang, C. H.; Sha, G. Y.; Guo, S. C. Carbon membrane for gas separation derived from coal tar pitch. Carbon 1999, 37, 1391−1397. (6) Petersen, J.; Matsuda, M.; Haraya, K. Capillary carbon molecular sieve membranes derived from kapton for high temperature gas separation. J. Membr. Sci. 1997, 131, 85−94. (7) Ismail, A. F.; David, L. I. B. Future direction of R&D in carbon membranes for gas separation. Membr. Technol. 2003, 2003, 4−8. (8) Koresh, J. E.; Sofer, A. Molecular sieve carbon permselective membrane. Part I. Presentation of a new device for gas mixture separation. Sep. Sci. Technol. 1983, 18, 723−734. (9) Suda, H.; Haraya, K. Gas permeation through micropores of carbon molecular sieve membranes derived from kapton polyimide. J. Phys. Chem. B 1997, 101, 3988−3994. (10) Tin, P. S.; Chung, T. S.; Hill, A. J. Advanced fabrication of carbon molecular sieve membranes by nonsolvent pretreatment of precursor polymers. Ind. Eng. Chem. Res. 2004, 43, 6476−6483. 9014



Article

(34) Liu, B. S.; Li, H. X.; Cao, Y.; Deng, J. F.; Sheng, C.; Zhou, S. Y. Preparation and characterization of Ni−P amorphous alloy/ceramic composite membrane. J. Membr. Sci. 1997, 135, 33−39. (35) Hosseinzadeh Hejazi, S. A.; Avila, A. M.; Kuznicki, T. M.; Weizhu, A.; Kuznicki, S. M. Characterization of natural zeolite membranes for H2/CO2 separation by single gas permeation. Ind. Eng. Chem. Res. 2011, 50, 12717−12726. (36) Liu, B. S.; Zhang, W. D.; Dai, W. L.; Deng, J. F. Thermal stability, hydrogen adsorption and separation performance of Ni-based amorphous alloy membranes. J. Membr. Sci. 2004, 244, 243−249. (37) Centeno, T. A.; Fuertes, A. B. Supported carbon molecular sieve membrane based on a phenolic resin. J. Membr. Sci. 1999, 160, 201− 211. (38) Kim, Y. K.; Lee, J. M.; Park, H. B.; Lee, Y. M. The gas separation properties of carbon molecular sieve membranes derived from polyimides having carboxylic acid groups. J. Membr. Sci. 2004, 235, 139−146. (39) Gilron, J.; Soffer, A. Knudsen diffusion in microporous carbon membranes with molecular sieving character. J. Membr. Sci. 2002, 209, 339−352. (40) Liu, B. S.; Wang, N.; He, F.; Chu, J. X. Separation performance of nanoporous carbon membranes fabricated by catalytic decomposition of CH4 using Ni/polyamideimide templates. Ind. Eng. Chem. Res. 2008, 47, 1896−1902. (41) Yamamoto, M.; Kusakabe, K.; Hayashi, J.; Morooka, S. Carbon molecular sieve membrane formed by oxidative carbonization of a copolyimide film coated on a porous support tube. J. Membr. Sci. 1997, 133, 195−205. (42) Shelekhin, A. B.; Dixon, A. G.; Ma, Y. H. Theory of gas diffusion and permeation in inorganic molecular-sieve membranes. AIChE J. 1995, 41, 58−67. (43) Rao, M. B.; Sircar, S. Performance and pore characterization of nanoporous carbon membranes for gas separation. J. Membr. Sci. 1996, 110, 109−118. (44) Sznejer, G. A.; Efremenko, I.; Sheintuch, M. Carbon membranes for high temperature gas separation: Experiment and theory. AIChE J. 2004, 50, 596−610. (45) Shiflett, M. B.; Pedrick, J. F.; Mclean, S. R.; Subramoney, S.; Foley, H. C. Characterization of supported nanoporous carbon membranes. Adv. Mater. 2000, 12, 21−25.

9015


Novel Modification of a Macroporous Stainless Steel Tube by

Recommend Documents