Permeability and Separability of Oxygen Across Perovskite−Alumina

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Ind. Eng. Chem. Res. 2008, 47, 3000-3007

Permeability and Separability of Oxygen Across Perovskite-Alumina Membrane Prepared from a Sol-Gel Procedure M. R. Othman* and J. Kim† School of Chemical Engineering, UniVersiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia

Perovskite membrane precursor was prepared in this work using a sol-gel method. The perovskite precursor sol was a rheopectic fluid and it exhibited Newtonian flow within 2 h of aging. After 24 h of aging, the flow became non-Newtonian. The thickness of the thin layer was approximately 2 µm that was deposited on a macroporous alumina support pretreated with polyvinyl butyral. The precursor was dip-coated on the support to form a perovskite thin layer for use in the oxygen and nitrogen permeability experiment conducted at room conditions. From the permeability result, it appeared virtually impossible to separate oxygen from the oxygen/nitrogen mixture because of the value of the permeability that was nearly identical. From the separability experiment, however, the oxygen separation factor was found to be higher than the theoretical separation factor attributed to the Knudsen diffusion, suggesting the possibility of separation and enrichment of the gas from the binary mixture. The higher separability of oxygen was perceived to be attributable to the affinitive energy by the perovskite in the range between 11-19 kJ/mol. 1. Introduction As energy costs continue to increase, a membrane technology for separating gases is likely to offer an increasingly important option in reducing the environmental impact and costs of industrial processes. Membrane devices for gas separation are, in general, fabricated from either polymeric or inorganic materials. The polymer is typically dense, and its inorganic counterpart can be made porous and dense. Both of them provide unique usefulness and advantages. If they come in a hybrid, the product may be used to fulfill dual purposes, on an unprecedented scale. Cellulose acetate and polyamide are polymeric materials commonly used to prepare polymeric membranes. The former is made up of mainly a polarized hydroxyl group, while the latter is made up of carboxylic and amine groups. Inorganic membranes are conventionally prepared from materials such as carbon, metals, ceramic, or a combination of metal oxides. In recent years, separation of oxygen from air or nitrogen for high-temperature applications has been intensely researched, for there is growing interest to improve catalytic reactions in a process requiring in situ feed of oxygen and the present pressing need for complete combustion to circumvent generation of toxic and noxious gases. Mixed conducting perovskite oxides are among the materials that have been wellreceived since they exhibit long-term stability under hightemperature conditions and their high ion conductivity improves oxygen permeability, such as that demonstrated by the family of SrCo1-xFexO3-δ perovskite.1 The interest in perovskite membrane transcends the boundaries of oxygen permeability and ion conductivity after the physicochemical properties, such as surface exchange kinetics, electronic and oxide ionic conductive, oxygen nonstoichiometry, and structural stability, and its internal transport mechanism have been gaining an equal if not exceptional attention, of late.2-4 The reason for better transport of oxygen into the permeated stream has been found to originate from the fact that the ionconducting perovskite contains many oxygen vacancies that play * To whom correspondence should be addressed. E-mail: chroslee@ eng.usm.my. † College of Environment and Applied Chemistry, Kyung Hee University, Yongin 449-701, Korea.

Figure 1. Schematic of membrane module.

Table 1. Characteristics of the Support

sample

pore size (nm)

BET surface area (m2/g)

surface fractal, D

alumina powder alumina pellet sintered at 1000 °C

56.6 78.4

145.2 83.78

2.481 2.483

a crucial role in providing an oxide ion-conducting path, whose oxygen vacancies is inclined to decrease under high oxygen partial pressure.5 Many researchers in the past and present6-11 reportedly claimed that perovskite membranes were promising materials for oxygen permeation since they exhibited high flux at high temperature and in a constant state of oxygen deficit. Improvement on the oxygen flux was realized after the A site of LaCoO3 composition (regarded as ABO3 formula) was replaced by alkaline earth elements such as Sr and Ba to produce more oxide vacancies in order to provide “highways” for oxygen permeation. Further Zr substitution on the B site of SrCo0.4Fe0.6O3-δ was thought to have stabilized the perovskite structure of the oxygen-vacancy disordered to a reasonably low temperature.12 In another work, the diffusive transport of gas across the membrane was known to be responsive to activity gradient, whose parameters such as pressure, flow rate, and concentration facilitate in accelerating or/and decelerating the mobility of the gas molecule through the membrane path. Temperature played an equally important part as it was reported that permeability

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Figure 4. Viscosity of the perovskite sol as a function of shear rate.

Figure 2. Schematic of the experimental process flow.

Figure 5. Viscosity of perovskite sol as a function of temperature.

2. Materials and Procedures Figure 3. Viscosity of the perovskite sol as it aged.

and flux of the gas could have been improved at high temperature if pores within the micropore network were allowed to grow.13-15 However, this would certainly undermine the oxygen separability, i.e., greater permeability at the expense of lower separability. Asymmetric perovskite materials that exhibit a high thermal expansion coefficient may induce membrane defects such as cracks and collapsed perovskite structures when operated at extreme conditions. Even though there appears to be intensification of research work in the area of synthesis and modification of perovskite materials in a nanometer scale, rather few reports on the oxygen permeability and separability in mixtures of component gases and in the separation deriving from the compressed air were published because of the difficulties in creating a separator and gas sealing.5 While the information on permeability of pure oxygen gas across perovskite materials has been satisfactorily ample, information on the separability or enrichment factor of the gas from binary gas containing oxygen and other component gas by the membranes has been relatively rare. Thus, separability of oxygen in a binary mixture comprising O2/N2 across a mesoporous perovskite membrane from this work is reported. The present study was carried out using perovskite coated on a macroporous alumina support disc, which was perceived to be practically attainable in order to enhance the surface adsorption transport mechanism in the ceramic membrane, which would lead to improved oxygen separability.

The preparation of a precursor sol to forming a perovskitealumina ((Al2O3)0.5(SrCo0.6Fe0.4O3)0.5) membrane followed a sol-gel-dip-coating approach.16-18 During the sol preparation, aluminum alkoxide was first hydrolyzed to obtain a boehmite sol, in which 1 mol of Al(OC4H9)3 was added to 100 mol of distilled water at 90 °C and stirred for 30 min to achieve complete hydrolysis of the alkoxide. HNO3 with a molar ratio of 0.07:1.00 of H+ to Al3+ was added to peptize the sol. The preparation of the perovskite membrane involved solvation of an appropriate amount of Sr(NO3)2, Co(NO3)2·6H2O, and FeCl3· 7H2O in water followed by dilution of the mixture into the standard boehmite sol prepared earlier. The resulting solution was used as a precursor to the preparation of a perovskite film. The film was developed by immersing a porous alumina support disc (substrate) surface in the precursor for 15 s to allow the coating material to apply itself to the substrate, withdrawing the substrate from the solution, drying at ambient conditions to remove the organic solvent, and sintering so that densification and infusion of the coated material into the substrate was achieved. The sintering temperature was relatively low (400 °C) in order to achieve a pore size in the mesoporosity regime (4.6 nm) and a stable amorphous cubic-perovskite that would inhibit the transition of the oxygen vacancy from disordered to ordered in the asymmetric perovskite membrane. The alumina support disc prepared by the compression-sintering method was in the macroporosity regime with a Brunauer-Emmett-Teller (BET) surface area of 83.8 m2/g. The characteristics of the support are given in Table 1. In the permeation experiment, perovskite coated γ-alumina disc (250 mm diameter and 30 mm thickness) was secured by

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Figure 6. TEM of perovskite gel at ×30 000.

alkoxylation (alcohol condensation). The reaction scheme, where R ) (C4H9), is as follows:

Figure 7. XRD of the sintered perovskite bulk sample.

airtight rubber gaskets (13 mm diameter hole) between the feed and permeate sides prior to being mounted inside a stainless steel cylindrical containment vessel (60 mm diameter and 300 mm length) as illustrated in Figure 1. The flow process of the permeation experiment is illustrated in Figure 2. A pure gas of oxygen and nitrogen was used, respectively, during permeability study, whereas a mixed gas containing 50/50 molar concentrations of each gas was used to perform the separability study. The laboratory-measured permeability was obtained by employing the expression

K)

qptm Am∆P

Al(OR)3 + 2H2O f AlOOH + 3ROH

(3)

H2O + 2AlOOH f Al(OH)3 + H+

(4)

When the metal salts dissolve in water, they formed labile ionic metal species that became attractive toward OH- from water. Water is a polar molecule, which has a relatively strong negative charge in the middle oxygen site and a positive charge at the two ends of its hydrogen sites. The strong negative charge site of the water molecule pulled the metal cations, whereas the positive charge site pushed the cation and attracted any hydrogen deficient species closer to its molecule. Continuous hydrolysis led to solvation of the cations by the water molecules to form hydrolyzed metal species in the following way, where z is the oxidation state of the metal,

(1)

metal salts + H2O f Mz+

(5)

where K is the permeability of the membrane, qp is the gas flow rate from the permeate stream, tm is the membrane thickness, Am is the membrane’s effective surface area, and ∆P is the pressure difference between the feed and permeate streams. The separability factor, R, was obtained from gas chromatography by observing the mole fraction of oxygen in the permeate (y) and retentate (x) streams, respectively, and the calculation was made using the following expression:

Mz+ + :OH2 f M(OH2)z+

(6)

R)

(1 -y y)(1 -x x)

(2)

3. Results and Discussion Synthesis of the beohmite (AlOOH) sol from the alkoxide yielded a byproduct butanol during hydrolysis and a prelude to

Addition of the solution into the boehmite produced highly complex ligands, rendering the system unstable, initially, due to the formation of aquo, hydroxo, or/and oxo ligands. The resulting sol might also contain intermediaries of complex species such as aquo-hydroxo complex and hydroxo-oxo complex at this stage prior to equilibrium. According to a partial charge distribution model, two possible reaction mechanisms called nucleophilic substitution and nucleophilic addition might occur during the processes of hydrolysis and condensation above. The former reaction mechanism was perceived to prevail when the substituent with the largest partial positive charge became the nucleofugal or leaving group after the preferred coordination number was satisfied. The latter reaction

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Figure 8. SEM of a sintered perovskite layer at ×2 000.

Figure 9. TEM of perovskite layer at ×30 000.

mechanism occurred when the possible substituent with the largest partial negative charge became the nucleophile and the preferred coordination number was not satisfied during the reaction. The viscosity of the precursor sol prepared from this work increased with time as it was allowed to age to enable continued hydroxylation, alkoxylation, or condensation to occur. Figure 3 shows that an immediate increase in viscosity was observed at 2 h of aging, indicating that the gelation began (gelation time

was ∼1.5 h). The increase in viscosity with time demonstrates that the sol exhibited rheopectic flow behavior. The viscosity decreased with the increase in shear rate, as shown in Figure 4 to further confirm that the sol exhibited a non-Newtonian, pseudo-plastic flow behavior. The viscosity decrease as the sol was exposed to higher temperature, as shown in Figure 5, was due to the fact that the heat energy weakened the interparticle interaction (bonding) as the particles received the excess energy from the heat.

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Figure 10. SEM of mesoporous alumina support (×5 000 magnification).

Figure 11. SEM of alumina support coated with PVB (×5 000 magnification).

Figure 6 shows the transmission electron microscopy (TEM) image of the gel sample after 24 h of aging. Apparent agglomeration and fiberlike structures (goethite needle) were observed, especially plausible when there was a structural evolution from the primary spherical perovskite particles that evolved and grouped into polymeric and particulate gel networks during the course of the colloid growth. Coarsening was primarily due to the dissolution and reprecipitation of a primary unit particle as a consequence of the Ostwald ripening effect, combined with the agglomeration of the grown primary unit particle around the “needle” or crystalloid structure into what is known as Cayley tree or Bethe lattice.

Figure 7 shows the X-ray diffraction (XRD) that confirmed the presence of a highly amorphous structure within the perovskite network due to the low intensity from the diffraction curve. The sample exhibited extremely narrow pore size distribution with a nominal pore size of 3.88 nm and a surface area of 341 m2/g.18 The other relevant parameters are reported in the previous work.16,17 The scanning electron microscopy (SEM) image from Figure 8 shows the appearance of microcracks formed into fractal networks that are barely visible on the surface of the sintered perovskite layer. Since cracks and pinholes are considered as defects, their development was to be avoided. Figure 9 is a TEM image after multiple coatings

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Figure 12. SEM perovskite cross section at ×699.18

Figure 13. Permeability of the gases as a function of inlet pressure at 200 mL/min.

Figure 14. O2 separability as a function of inlet flow rate (50/50 molar concentration of O2/N2) at 1 bar.

were applied to cover the defects. Subsequent thin films were observed to develop, commencing from the uncovered pinhole (bright side) toward the most upper layer (darkest sheets) of

Figure 15. O2 thermal swing adsorption and desorption of perovskite.

the coated film. The image also indicates that the sample displayed characteristics of an intercalated material. Figure 10 shows the SEM image of the alumina disc that was used as a support in the development of the perovskite film. The support was rough with gritty surfaces comprising open pores and wide gaps. The degree of irregularity as quantified by the surface fractals (Table 1) was confirmed by this image. An attempt to obtain the perovskite film was futile, initially, due to the solution that seeped, percolated, and meandered through these gaps. The membrane was formed only after the support was first dip-coated with polyvinyl butyral (PVB), even with open pores that still existed. PVB was employed (as a surfactant) to create high surface tension on the support surface so that the precursor sol would advance and spread on to the surface, facilitated by the Marangoni effect/force. The Marangoni force defines that, if the surface tension is greater near the edge of a liquid, the liquid is forced to move in a direction from a low to a high surface tension. The liquid spontaneously moves over the solid surface without the application of external force. The SEM image of the alumina support pretreated with

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Figure 16. Internal energies of O2 and N2 as a result of surface and saturation temperature difference.

PVB is displayed in Figure 11. Figure 12 shows the first appearance of perovskite membrane on the porous alumina support with thickness of 2 µm.18 Figure 13 shows the permeability of pure O2 and N2 across the perovskite membrane deposited on the alumina porous support. The permeability values of the gases were too close to ensure effective separation. Both gases, however, were virtually independent of pressure, as expected when the pore size is in the micro- and mesoporosity regimes, which is, in effect, a manifestation of the Knudsen flow dominance.19 Since the pressure increase did not effect any improvement in the gas permeability, a decision was made to limit the subsequent separability experiment at a pressure of 1 bar. Figure 14 shows the separability of oxygen across the microporous adsorptive perovskite thin layer. The separability was slightly higher than the theoretical Knudsen separation, which was defined as the square root of the ratio of the heavier molecular weight gas (O2) to the lighter molecular weight gas (N2). The Knudsen separation factor of 1.07 indicates that further enrichment and separation of this gas from the binary mixture was possible. The adsorption analysis of the perovskite bulk sample (Figure 15) reveals that, at the peak of the first adsorption cycle, 36 kJ/mol of energy was liberated to adsorb O2 (at a surface temperature of 164 °C), followed by 28 kJ/mol of energy at the apex of the second adsorption cycle (at a lower surface temperature of 139 °C). From the application of the first law of thermodynamics, assuming an isobaric, closed system at rest in the absence of magnetic field and that the total energy equals the internal energy U, the calculated change of internal energy as a result of temperature difference between the surface and the gassaturation temperature is plotted in Figure 16. The result suggests that 17 kJ/mol was due to the change of internal energy of O2 following the temperature drop from 164 to 139 °C. Therefore, 19 kJ/mol (36-17 kJ/mol) might actually be due to the affinitive force by perovskite in the first cycle of adsorption, whereas 11 kJ/mol was possibly due to the affinitive force by perovskite in the second cycle of adsorption. 8 kJ/mol of perovskite’s affinitive energy was lost possibly as a result of the surface temperature drop, attributable to the weaker latticed network to facilitate ionization of oxygen. Further improvement in the separability would be possible if the perovskite material were to be made dense, which would allow only the oxygen ion to permeate through the membrane at elevated temperature.

4. Conclusion The perovskite material prepared from the sol-gel technique in this work demonstrated ordered, intercalated structures as observed from TEM, which would provide anion exchange or adsorptive capacity for the intended application. From the permeation test employing pure gas in a single system, the permeability of O2 and N2 across the perovskite membrane was observed to be independent of pressure. The permeability values were nearly identical. Separability of oxygen in a binary gas mixture using perovskite membrane, nevertheless, was observed to be higher than the theoretical separation factor attributed to the Knudsen diffusion. The higher separability of oxygen was possibly due to the competitive adsorption by the perovskite in the range between 11-19 kJ/mol. Acknowledgment The work has been supported by the Ministry of Science, Technology and Innovation of Malaysia (MOSTI) and the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2007-313D00137). The authors also acknowledge the contribution made by N. F. Idrus, T. A. Guek, and H. Abdullah. Literature Cited (1) Zeng, Y.; Lin, Y. S.; Swartz, S. L. Perovskite-type ceramic membrane: synthesis, oxygen permeation and membrane reactor performance for oxidative coupling of methane. J. Membr. Sci. 1998, 150, 87. (2) Qiu, L.; Lee, T. H.; Liu, L. M.; Yang, Y. L.; Jacobson, A. J. Oxygen permeation studies of SrCo0.8Fe0.2O3-δ. Solid State Ionics 1995, 76, 321. (3) Kim, S.; Yang, Y. L.; Jacobson, A. J.; Abeles, B. Diffusion and surface exchange coefficients in mixed ionic electronic conducting oxides from the pressure dependence of oxygen permeation. Solid State Ionics 1998, 106, 189. (4) Kim, S.; Yang, Y. L.; Jacobson, A. J.; Abeles, B. Oxygen permeation, electrical conductivity and the stability of the perovskite oxide La0.2Sr0.8Cu0.4Co0.6O3-x. Solid State Ionics 1997, 104, 57. (5) Ito, W.; Nagai, T.; Sakon, T. Oxygen separation from compressed air using a mixed conducting perovskite-type oxide membrane. Solid State Ionics 2007, 178, 809. (6) Fan, C. G.; Zuo, Y. B.; Li, J. T.; Lu, J. Q.; Chen, C. S.; Bae, D. S. Highly permeable La0.2Ba0.8Co0.8Fe0.2-xZrxO3-δ membranes for oxygen separation. Sep. Purif. Technol. 2007, 55, 35. (7) Teraoka, Y.; Nobunaga, T.; Okamoto, K.; Miura, N.; Yamazoe, N. Influence of constituent metal cations in substituted LaCoO3 on mixed conductivity and oxygen permeability. Solid State Ionics 1991, 48, 207.

Ind. Eng. Chem. Res., Vol. 47, No. 9, 2008 3007 (8) Kharton, V. V.; Viskup, A. P.; Marozau, I. P.; Naumovich, E. N. Oxygen permeability of perovskite-type Sr0.7Ce0.3MnO3-δ. Mater. Lett. 2003, 57, 3017. (9) Ikeguchi, M.; Yoshino, Y.; Kanie, K.; Nomura, M.; Kikuchi, E.; Matsukata, M. Effects of preparation method on oxygen permeation properties of SrFeCo0.5Ox membrane. Sep. Purif. Technol. 2003, 32, 313. (10) Deng, Z. Q.; Liu, W.; Peng, D. K.; Chen, C. S.; Yang, W. S. Combustion synthesis, annealing, and oxygen permeation properties of SrFeCo0.5Oy membranes. Mater. Res. Bull. 2004, 39, 963. (11) Kruidhof, H.; Bouwmeester, H. J. M.; Doorn, R. H. E. v.; Burggraaf, A. J. Influence of order-disorder transitions on oxygen permeability through selected nonstoichiometric perovskite-type oxides. Solid State Ionics 1993, 63, 816. (12) Yang, L.; Gu, X. H.; Tian, L.; Jin, W. Q.; Zhang, L. X.; Xu, N. P. Oxygen Transport Properties and Stability of Mixed-Conducting ZrO2Promoted SrCo0.4Fe0.6O3-δ Oxides. Ind. Eng. Chem. Res. 2002, 41, 4273. (13) Wu, C. S. J.; Flowers, D. F.; Liu, P. K. T. High-temperature separation of binary gas mixtures using microporous ceramic membranes. J. Membr. Sci. 1993, 77, 85. (14) Ahmad, A. L.; Othman, M. R.; Mukhtar, H. H2 separation from binary gas mixture using coated alumina-titania membrane by sol-gel

technique at high-temperature region. J. Hydrogen Energy 2003, 29, 817. (15) Othman, M. R.; Mukhtar, H.; Ahmad, A. L. Permeability of Pure Gases in Porous Inorganic Membrane. J. Phys. Sci. 2000, 11, 37. (16) Othman, M. R.; Ahmad A. L.; Idrus, N. F. Characterization of Perovskite Ceramic Membrane for Oxygen Separation. Int. Conf. AdV. Str. Tech. Proc. 2003, 383-386. (17) Othman, M. R.; Kim, J. Al2O3-SrCo0.6Fe0.4O3 membrane coated on a meso-porous Al2O3 support. Int. Conf. AdV. Mater., Nanotech 2007. (18) Ahmad, A. L.; Idrus, N. F.; Othman, M. R. Preparation of perovskite alumina ceramic membrane using sol-gel method. J. Membr. Sci. 2005, 262, 129. (19) Othman, M. R.; Mukhtar, H.; Ahmad, A. L. Permeabilities of Gases in Porous Inorganic Membrane. 2nd Int. Conf. AdV. Strat. Tech. 2000.

ReceiVed for reView November 28, 2007 ReVised manuscript receiVed February 9, 2008 Accepted February 14, 2008 IE0716215