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Preparation and characterization of new low MWCO ceramic nanofiltration membranes for organic solvents
Journal of Membrane Science 470 (2014) 421–430
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Preparation and characterization of new low MWCO ceramic nanofiltration membranes for organic solvents Stefanie Zeidler a,b,n, Petra Puhlfürß c, Uwe Kätzel a, Ingolf Voigt c a
Merck KGaA, Frankfurter Str. 250, 64293 Darmstadt, Germany TU Dortmund University, Department of Biochemical and Chemical Engineering, Laboratory of Fluid Separation, Emil-Figge-Strasse 70, 44227 Dortmund, Germany c Fraunhofer Institute for Ceramic Technologies and Systems, Michael-Faraday-Str. 1, 07629 Hermsdorf, Germany b
art ic l e i nf o
a b s t r a c t
Article history: Received 4 April 2014 Received in revised form 16 July 2014 Accepted 17 July 2014 Available online 2 August 2014
Ceramic membranes show advantageous behavior over polymeric membranes under harsh conditions including high pH, high temperature applications or applications involving organic solvents. However, in the range of organic solvent nanofiltration (OSN) the currently available membranes do just offer a molecular weight cut-off (MWCO) of 600 g/mol or above. Here, a new approach is presented to manufacture ceramic membranes with active layers of titanium dioxide/zirconium dioxide with integrated carbon on top of a tubular ceramic multi-layer support. Filtration tests of these membranes with polystyrene mixtures in THF reveal a MWCO of 350 g/mol, thus extending the range of possible nanofiltration applications. The paper also presents a new combination of characterization of the membranes with permporometry and filtration tests that demonstrate the importance of the supporting layers for the creation of a good nanofiltration membrane. & 2014 Elsevier B.V. All rights reserved.
Keywords: Organic solvent nanofiltration Ceramic membranes Inorganic membranes Permporometry
1. Introduction Ceramic membranes are convincing particularly in terms of high mechanical, chemical and thermal stability compared to polymeric membranes enabling their use under harsh conditions as well as the application of chemicals and steam for membrane and module/plant cleaning. Therefore, their use in a wide range of different applications from waste water treatment till clarification and sterilization of beverages are well known. Due to their (in principle) unrestricted resistance against organic solvents, ceramic membranes are of particular interest for organic solvent nanofiltration (OSN). This growing new technology describes the application of nanofiltration (NF) in pure organic solvents or organic solvent mixtures. OSN has just established during the last 15 years as a result of the development of solvent stable polymeric membranes and offers a high potential for applications in pharmaceutical and chemical industries. The first ceramic nanofiltration membrane with a molecular weight cut-off (MWCO) in water below 1000 g/mol was developed by Puhlfürß et al. [1] followed by a successful application in
n Corresponding author at: Merck KGaA, Frankfurter Str. 250, 64293 Darmstadt, Germany. Tel.: þ49 6151 72 94839; fax: þ 49 6151 72 913051. E-mail address: [email protected] (S. Zeidler).
http://dx.doi.org/10.1016/j.memsci.2014.07.051 0376-7388/& 2014 Elsevier B.V. All rights reserved.
process integrated cleaning of waste water from textile finishing [2]. The membrane consisted of amorphous titanium dioxide with a pore size of 0.9 nm and had a water flux of 20 10 5 kg/m2 h Pa and a MWCO of 450 g/mol measured with polyethylene glycols. The application of this membrane in organic solvents resulted in a MWCO above 10,000 g/mol combined with a toluene flux below 2 10 5 kg/m2 h Pa. The low flux and the bad rejection of the membrane despite the low MWCO indicates that the transport of organic solvents occurs only through larger defect pores of the membranes due to the hydrophilic behavior of the titania surface. Similar results were achieved by Guizard et al. [3] and Tsuru et al. [4,5] who prepared mixed oxides (silicon dioxide/titanium dioxide, aluminum oxide/zirconium dioxide and silicon dioxide/zirconium dioxide) with pore sizes below 1 nm. Different post-treatment methods have been suggested to create a hydrophobic membrane surface and to overcome this limitation. First method was the silanisation of mesoporous zirconium dioxide membranes with an initial pore size of 3 nm [6]. Via this way the MWCO in toluene measured with polystyrenes could be reduced down to 600 g/mol [7]. This membrane was successfully used in homogeneous catalyzed reaction removing the product while keeping the catalyst in the reaction [8]. Van Gestel et al. [9] applied the silanisation method to modify mesoporous aluminum oxide/titanium dioxide membranes but did not measure the retention in organic solvents. A second
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method was invented by Buekenhoudt et al. [10] using organometallic compounds for the post-treatment and to transfer the alkyl group to the membrane surface by a Grignard type reaction. Hydrophobisation by a post-treatment in the pore size range of a few nanometers is very sensitive resulting in a limited reproducibility. An alternative could be an in-situ hydrophobization. One approach is the preparation of methylated silicon dioxide membranes by using methylated silicon alkoxides [11] as precursor for the sol–gel process. Unfortunately, this method is limited to silica and resulted in a MWCO of only 1000–2000 g/mol in hexane. In this paper a new method of preparation of hydrophobic ceramic nanofiltration membranes is presented based on the formation of carbon in the amorphous titania or titania/zirconia membranes by sintering the gel layer under inert atmosphere. The amount of carbon was modified by adding complexation agents to the alkoxide precursors.
2. Experimental 2.1. Membrane preparation The method to prepare ceramic NF membranes is based on a polymeric sol–gel technique starting from a mixture of titanium isopropoxide and zirconium n-propoxide. The hydrolysis of the alkoxides is controlled by the amount of added water [1] (Fig. 1). A mixture of titania and zirconia was selected to prevent crystallization during firing the membrane at a temperature of 400 1C resulting in a microporous amorphous mixed oxide. At the same time the titania:zirconia ratio influences the pore size and finally the retention. Smallest pore sizes were achieved with a molar ratio 12:1. This membrane showed excellent NF performance in water with a MWCO measured with polyethylene glycols of 200 g/mol as reported elsewhere [12]. The new approach to generate hydrophobic membranes was the incorporation of carbon into the NF-membrane layer. This was achieved by running the sintering process under inert atmosphere (nitrogen) in order to pyrolyze the remaining alkoxide groups in the gel (type 1). To increase the amount of carbon diethanol amine (DEA) was added to the sol with a molar (titania þzirconia):DEA ratio of 1:2 (type 2). DEA is acting as a complexation agent preventing the complete hydrolysation and supporting the formation of a polymeric sol [13] (Fig. 2). A type 3 membrane was prepared by adding a diluted solution of a phenolic resin (FB8001, FERS, Barcelona, Spain) in isopropyl alcohol instead of DEA. The amount of phenolic resin was 0.16 wt % related to the calculated amount of oxide. In contrast to DEA
the phenolic resin did not act as a complexation agent but resulted in a residual amount of carbon after pyrolysis at 400 1C as well. Membranes were prepared inside of monochannel tubular substrates of α-alumina (A.D.: 10 mm, I.D.: 7 mm, Length: 250 mm) by dip coating under clean room conditions (class 100, 22 1C, humidity o50%). A couple of intermediate layers were used to adapt the pore size and to create a smooth surface. The top layer of this multilayer structure was a zirconium oxide-layer with a thickness of 50 nm and a mean pore size of 3 nm. The NF-membranes were dried for 2 h at room temperature before firing at 400 1C under nitrogen or air. 2.2. Characterization of pore size distribution by permporometry The knowledge of the quality of the active separation layer is of great interest during the development and manufacturing of porous membranes. Conventional analysis methods like mercury-porometry or gas adsorption–desorption measurements require special samples of the active membrane layer. Other analytical methods (e.g. SEM) damage the membrane irreversibly. Permporometry is a measuring process where the active pores of the complete membrane are investigated [14]. Permporometry is based on the effect of capillary condensation in the pores. First, the permeation of an incondensable gas is measured. Then, the incondensable gas is moistened stepwise by a condensable gas. Nitrogen was used as incondensable gas and cyclohexane as condensable gas. The higher the humidity the larger the pores blocked by capillary condensation. Thus, the gas flow through the membrane decreases sequentially. The smaller the pores the lower humidity is sufficient to reach capillary condensation that blocks the pores for gas transport. Capillary condensation is mathematically described by the Kelvin equation. p σ cos ϕ V m ln ¼ ps rp RT With the equation, where p is the partial pressure, ps is the saturation vapor pressure, σ is the interfacial tension, Vm is the molar volume of the condensable gas, R is the gas constant and rp is the pore radius, the pore size distribution of the membrane can be calculated. The contact angle Φ is set to zero as complete wetting is assumed. Prior to the measurements, the membranes were oven treated for 2 h at 120 1C under air. Then, the hot membranes were removed from the heating chamber and were directly implemented into
Fig. 1. Simplified formation of polymeric titania sol by controlled addition of water (type 1).
Fig. 2. Simplified formation of polymeric titania sol containing DEA as complexation agent (type 2).
S. Zeidler et al. / Journal of Membrane Science 470 (2014) 421–430
permporometry system. After the measurement a dry nitrogen flow was applied until the initial flux was achieved. An example of a determined permporometry curve is given in Fig. 3. The relative permeance related to the pure nitrogen permeance is plotted over the pore diameter calculated by the Kelvin equation. Subsequently, the nominal pore radius was determined by the interpolation of the permeance curve at a relative permeance of 50%.
2.3. Rejection measurements 2.3.1. Test system Rejection measurements were carried out according to a method recommended by See Toh et al. [15]. A styrene oligomer mixture (Fig. 4) containing PSS-ps560, PSS-ps1.8k and PSS-ps5.6k (Polymer Standards Service GmbH, Mainz, Germany) was used. One gram of each standard was dissolved in 1 l of tetrahydrofuran (THF). Furthermore, some experiments were conducted with polystyrene in toluene, n-heptane or ethanol (all Merck KGaA, Darmstadt, Germany). The solutions were prepared in the same way except the ethanol solution. Only 0.5 g of the PSSps560 standard was dissolved in 1 l ethanol due to solubility limitations.
100%
rel. permeance
423
80% 60%
2.3.2. Experimental set-up Nanofiltration experiments and the rejection characterization of the new membranes were carried out in a Multi-Purpose-CrossFlow-Membrane Plant at Merck KGaA, Darmstadt, Germany. A schematic diagram of the part of the plant which was used for the experiments is shown in Fig. 5. The feed is temperature controlled by the double jacketed feed vessel, while the whole tubing is isolated to prevent temperature changes across the test set-up. The feed is circulated across the membrane by a membrane plunger pump (Hydracell G03, Wanner, Minneapolis, USA). The single channel membrane tube is housed in a vertically fixed membrane module (Andreas Junghans, Frankenberg, Germany). The system is pressurized by closing the pressure retention valve behind the membrane module. Both, the retentate and the permeate were recirculated in the feed vessel. Rejection of the membranes were always measured under the same standard process conditions, i.e. a transmembrane pressure of 2 106 Pa, a feed flow of 6 l/min, corresponding to a cross-flow velocity of 3 m/s and a temperature of 20 1C. After equilibration of the process, the membranes were conditioned for 1 h. Subsequently, retentate and permeate samples were taken to determine the rejection of the membrane. Permeate mass flow was determined by a coriolis flow meter in the permeate recirculation. If the permeate flow was below the detection limit, it was determined by measuring the increase of permeate mass per time on an analytical balance. Prior to all experiments, the membranes were dried for at least 12 h in a cabinet desiccator at 80 1C under vacuum to eliminate air humidity or solvent residuals from the pores.
mean pore size
40% 20%
defect pores
0% 0
1
2
3
4
5
6
7
pore diameter [nm] Fig. 3. Determination of the pore size distribution, mean pore size and the amount of defect pores by permporometry measurement.
Fig. 4. Structural formula of polystyrene oligomers.
FI
permeate sample
pressure retention valve
feed vessel
membrane modul retentate sample FIC
membrane plunger pump Fig. 5. Schematic diagram of the membrane test unit.
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2.3.3. Analysis The concentrations of the polystyrene fractions in the retentate and the permeate were determined by gel permeation chromatography (Hitachi Elite LaChrom, Hitachi High Technologies America, Illinois, USA). The used colums Shodex GPC KF-801, KF-802 and KF-806M were supplied by Showa Denko K.K., Tokyo, Japan. The rejection of each molecular weight was then calculated with the concentration in the permeate cP and in the retentate cR by cP 100 R ¼ 1 cR Due to the different molecular weights of the styrene oligomers in the standards it was possible to create a rejection curve from the single data and, thus, the molecular weight cut-off value was determined as the molecular weight at which the interpolated rejection curve reached a value of 90%. Fig. 6 shows an example of such a rejection curve and the respective MWCO.
3. Results and discussion 3.1. Influence of complexation agents First, the influence of sintering under nitrogen atmosphere on the new 200 g/mol membrane was investigated (membrane type 1). The pore size distribution of the membrane can be seen in the
course of the permporometry measurement in Fig. 7 (left). A mean pore size of 1.0 nm with a defect flux of about 7% was detected. The results of the rejection measurements with polystyrene in THF are given in Fig. 7 (right). The rejection curve stagnates at 55% and decreases slightly towards larger molecules. The measured permeate flux was 350 l/m2 h. Apparently, the amount of solvents in the dried gel layer is too marginal to create carbon in the membrane and to change the surface properties from hydrophilic to hydrophobic and to increase the wettability with organic solvents after sintering under inert atmosphere. To enhance the wettability, the amount of carbon should be increased by the use of complexation agents. The first complexation agent which was tested for the preparation of hydrophobic nanofiltration membranes was diethanol amine (DEA) (membrane type 2). Using this agent and sintering the membrane under inert atmosphere results in an OSN membrane with the best MWCO (490 g/mol) to date. The respective rejection curve is given in Fig. 8 (right). Good rejections ( 4 60%) of the small polystyrenes were obtained. Nevertheless, rejection stagnates at 95% for the larger molecules. This can be attributed to a slight portion of defect pores (see Fig. 8 left). The mean pore size of this membrane is around 1.1 nm and hence nearly the same compared to the membrane prepared without complexation agent (Fig. 7). The considerable better rejection can be attributed to the better wettability of the membrane surface. Thus, the smaller pores take part in the separation process and especially larger molecules are retained. Furthermore, the distribution of the pores is very narrow, recognizable by the steep permporometry curve between 0 and 1.5 nm. However, this leads to a loss of flux performance (110 l/m2 h). In Fig. 9 (right) the rejection curve of a membrane prepared with phenolic resin (type 3) in the sol is given. It can be seen, that the membrane has hitherto the best rejection of 98% for the 5600 g/mol polystyrene. Unfortunately, the MWCO ( 1200 Da) is too high and thus does not give a true nanofiltration membrane. A glance at the permporometry diagram (Fig. 9 left) explains the results. The membrane has a very good pore size distribution with a mean pore size of 1.55 nm and no pores bigger than 2.5 nm. Therefore, larger molecules were completely retained. However, the pore size is still too large to achieve significant rejections in the nanofiltration range. Due to the broader pore size distribution, the permeate flux is higher (240 l/m2 h) compared to the type 2 membrane.
3.2. Influence of defect pores
Fig. 6. Rejection curve with MWCO determination.
The rejection measurements show that the mean pore size is not the only factor for a good OSN membrane performance.
Fig. 7. Characterization of a new 200 Da ceramic membrane (N0457) for aqueous applications sintered under nitrogen atmosphere. Permporometry measurement by use of cyclohexane as condensable vapor (left) and rejection curve determined with polystyrenes in THF at 20 bar (right).
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Fig. 8. Characterization of a new membrane (N0265) hydrophobised by DEA in the sol and sintered under nitrogen atmosphere. Permporometry measurement by use of cyclohexane as condensable vapor (left) and rejection curve determined with polystyrenes in THF at 20 bar (right).
Fig. 9. Characterization of a new membrane (N0157) hydrophobised by phenolic resin in the sol and sintered under nitrogen atmosphere. Permporometry measurement by use of cyclohexane as condensable vapor (left) and rejection curve determined with polystyrenes in THF at 20 bar (right).
Fig. 10. Characterization of a new membrane (N0426) for aqueous nanofiltration. Permporometry measurement by use of cyclohexane as condensable vapor (left) and rejection curve determined with polyethylene glycol in water at 10 bar (right).
A closer consideration of the permporometry measurements reveals that defect pores could have a strong negative impact on the performance in organic solvents. Membranes that have a constant relative permeance for larger pores ( 4 5 nm) in the permporometry measurement, show an incomplete rejection for higher weight molecular weight oligomers. The rejection curve often stagnates or even decreases. This implies that the larger molecules are able to pass through some larger pores, which are undesirable and therefore called defect pores.
In aqueous applications this effect seems to be negligible. Rejection measurements of aqueous NF membranes (type 1 sintered under air) with a mixture of polyethylene glycols in water show excellent rejections despite a considerable portion of defect pores. Fig. 10 illustrates this clearly. There are different explanations for the observed behavior. On the one hand the water molecules are smaller than the solvent molecules and additionally show a better wettability of the membrane surface. Thus, they can pass more easily through the very small pores and the relative transport through the defect
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Fig. 11. Permporometry measurements of the support membranes and the respective NF membrane after coating the layer with phenolic resin (membrane type 3) as complexation agent.
pores becomes marginal. On the other hand, in water there are also electrostatic repulsion forces acting between the membrane surface and charged solutes (not the case for PEG) so that larger pores may also be masked by such an effect, which is not existent in organic solvents. However, the results clearly show that if good OSN performance of ceramic membranes is sought, the reduction of the number of defect pores in the membranes active layer is essential. The mesoporous supports of the membrane (3 nm ZrO2) before coating the NF layer were investigated to evaluate the root cause for the formation of the defect pores. Permporometry measurements show that the defect pores already exist in the membrane support. Fig. 11 shows the results for the support layer and the finished NF membrane of two membranes containing the complexation agent phenolic resin. In Fig. 11A the support possesses a relative permeance of 10% at pores sizes 44 nm. These defect pores still exist in the finished membrane showing that an additional coating does not close the defect pores. Only if the permporometry of the support showed no defect pores (Fig. 11B), membranes without defect pores and hence membranes with complete rejection for higher molecular weight compounds could be produced. This also proves the usefulness of permporometry measurements as a quality control of ultrafiltration support membranes for OSN membranes manufacturing.
layer. Thus, the positive properties of the two materials, the ability to build tight pores and the hydrophobicity, could be combined. In addition, special attention was paid to the quality and the permporometry results of the support layer to reduce the amount of defect pores in the membrane. First, the membrane was produced and sintered under air conditions. The rejection curve and the permporometry diagram are given in Fig. 12. The diagrams indicate that the optimization procedure was successful concerning the reduction of defect pores. The permporometry curve approaches a relative permeance of 3% for larger pores and the 5600 g/mol polystyrene is completely retained. The measured perameate flux was 235 l/m2 h. However, the measured MWCO is still too high. Those results change dramatically, if the membrane is sintered under inert conditions. The permporometry result of the membrane produced in this way is given in Fig. 14 (left). The diagram indicates the success of the optimization. The low number of pores 42 nm in the support (see Fig. 13) could be closed and the mean pore size reduced ( 0.6 nm). The rejection curve measured with polystyrene in THF is given in Fig. 14 (right) and proves that a ceramic membrane with a cut-off of 350 g/mol could be developed. Molecules larger than 1800 g/mol are completely retained. The permeate flux at 20 bar transmembrane pressure was 70 l/m2 h.
3.3. Molecular weight cut-off optimization
3.4. Reproducibility
Further investigations were focused on DEA as complexation agent since this preparation route revealed the lowest MWCO so far (cf. Fig. 8). On the one hand optimization concentrated on the reduction of the mean pore size and on a method that closes the defect pores on the other hand. To achieve the former, the sintering conditions were changed and the active layer was coated twice. An aqueous NF layer (type 1) [12] was coated below the DEA
In order to check the reproducibility of the new membrane synthesis route, the optimized membrane was reproduced four times under the same conditions. The results of the rejection measurements with polystyrene in THF are shown in Fig. 15. Regrettably, the rejections are a little bit lower (MWCO4420 g/mol) than the original membrane (MWCO¼ 350 g/mol) and the four membranes differ in their MWCOs.
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Fig. 12. Characterization of a new membrane (N0341) hydrophobised by DEA in the sol and sintered under air. Permporometry measurement by use of cyclohexane as condensable vapor (left) and rejection curve determined with polystyrenes in THF at 20 bar (right).
rel. permeance
100% 80% 60% 40% 20% 0% 0
1
2
3
4
5
6
7
pore diameter [nm] Fig. 13. Permporometry measurement of the mesoporous support of the optimized membrane (N0328).
Nevertheless, this synthesis route with the chosen additive provides the best results and the lowest MWCO reported so far. The reproducibility is limited by a couple of parameters based on the membrane preparation, pore size distribution, support quality as well as the filtration tests and GPC measurements. Therefore the variation between 350 g/mol and 420 g/mol is quite good. In particular, the high rejections of the larger molecules are important to note. All membranes show a rejection 498.5% for the 6000 g/mol oligomers, which indicates the marginal amount of defect pores and the quality of the support layer. 3.5. Applicability in other solvents It is known from polymeric OSN membranes that considerable deviations in fluxes and rejections occur depending on different solvents used for separations [16–20]. All known transport models for solvent permeation and solute transport through ceramic OSN membranes [21–24] are more or less based on the pore-flow theory. Some authors extended the models by further influencing factors as the surface tension, dielectric constant, dipole moment, effective molecular diameter or a solubility parameter. However, with all the models and according to Darcy's law, only the viscosity, the applied pressure and the molecular size should cause differences in permeations of the solvent. Assuming the validity of the models also for the new nanofiltration membranes, MWCO should not change much when switching from THF to other solvents. In an experimental assessment the single layer DEA membrane (MWCO: 490 g/mol) was also characterized with polystyrenes in n-heptane and ethanol. The results are given in Fig. 16. For the purpose of comparison, the rejections measured in THF, which were presented Fig. 8, are also
incorporated. The experiments were conducted in the sequence THF, ethanol, n-heptane, n-heptane and ethanol again. As the results of the second measurements were comparable, only the results of the first is given in the figure. The rejection curves clearly illustrate that, similarly to the investigations with polymeric membranes, substantial differences exist between the rejections in the different solvents. THF provides the highest rejections. The rejections in ethanol are about 40% lower and polystyrene in n-heptane is not retained at all up to a molecule size of 2000 g/mol. Those results contradict the expectations based on the pore flow theory. If rejections are solely dependent on the different viscosities of the solvents and the effective molecular solute size in the solvent, rejection curves should only be shifted along the xaxis. Even the polarity of the solvents cannot solely explain those extreme differences in rejections. The polarity as well as the viscosity increases from n-heptaneo THF oethanol, whereas rejection increases from n-heptane to ethanol and THF. It has been proposed for solvent filtration, that the performance of a membrane is dependent on the first medium permeating the membrane [24]. As the rejections in THF (cyclic solvent) were always better than the rejections in other solvents, a similar effect based on the cyclohexane used in permporometry could be assumed here. In order to check the influence of the precharacterization by permporometry on the performance of the membranes, nine membranes were produced via the same synthesis route. Three of them were characterized by permporometry to ensure the quality of the membranes. Subsequently, the rejection curves of one precharacterized membrane and two membranes without a pretreatment were determined in THF, ethanol and n-heptane, respectively. The results are shown in Figs. 17–19. Rejections in THF (Fig. 17) seem to be better after preconditioning with cyclohexane by permporometry measurements. This may be due to the structural similarity between THF and cyclohexane which are both cyclic solvents. Cyclohexane could potentially adsorb at the pore walls whereby the THF permeate could pass easier through the pores by reason of a higher affinity. However, as the quality of the supports and the separation layer could not be verified beforehand, a deteriorated performance based on defect pores cannot be excluded. If a permanent solvent layer is really formed at the first use of the membrane, the rejections of a characterized membrane in ethanol should be lower than the rejections of an untreated membrane as the affinity of the nonpolar cyclohexane to the small polar ethanol molecule is very small. The permeate fluxes of the measurements are given in Table 1.
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100
100
80
80
rejection [%]
rejection [%]
Fig. 14. Characterization of the optimized membrane (N0328) hydrophobised by DEA in the sol and sintered under nitrogen atmosphere. Permporometry measurement by use of cyclohexane as condensable vapor (left) and rejection curve determined with polystyrenes in THF at 20 bar (right).
60
40
N0328 N0327 N0337 N0338
20
0 0
500
1000
1500
60
40
after permporometry
20
without precharacterization 1 without precharacterization 2 0 2000
0
MW [g/mol] Fig. 15. Reproducibility of the new membrane for organic solvent nanofiltration.
100
rejection [%]
n-Heptane (10,5l/m²h)
After permporometry Without pretreatment 1 Without pretreatment 2
20
0 2000
4000
8000
Permeate flux [l/m2 h]
THF (110 l/m²h)
0
6000
Fig. 17. Rejection measurements of membranes containing DEA in THF. One membrane was characterized with permporometry prior to the rejection measurements, the other two membranes were measured untreated.
Ethanol (20 l/m²h)
40
4000
Table 1 Permeate fluxes of THF, ethanol and n-heptane in l/m2 h of type 2 membranes with and without precharacterization.
80
60
2000
molecular weight [g/mol]
6000
8000
-20
molecular weight [g/mol] Fig. 16. Rejection curves and permeate fluxes of the DEA membrane (N0265, MWCO: 490 Da) determined with polystyrene in n-heptane, ethanol and THF (adapted from Fig. 8).
Fig. 18 illustrates that the measured results substantiate this hypothesis. The rejections after the permporometry measurement are negative meaning that the solutes preferentially permeate instead of the solvent. As the solute (polystyrene) is similar to the
THF
Ethanol
n-Heptane
200 350 200
270 400 200
170 230 400
precharaterization solvent (cyclohexane), the affinity is higher than to ethanol and therefore permeation could be preferred. In particular the rejection of the smallest molecules is significantly worse than that observed with the untreated membrane. For the larger molecules this effect might be extenuated due to sterical hindrance. The rejection curves in n-heptane are given in Fig. 19. Here, no differences between the membranes could be identified. None of the molecules was retained by any of the membranes. Concluding, some differences of the pretreatment of the membranes with cyclohexane in permporometry measurements could be identified. These, however, cannot solely explain the large change in rejections for different solvents in ceramic OSN membranes. For instance, the membranes show no rejection in n-heptane independent of the pretreatment.
S. Zeidler et al. / Journal of Membrane Science 470 (2014) 421–430
Thus, several substantial requirements of a ceramic membrane for organic solvent nanofiltration were identified:
100
after permporometry without precharacterization 1 without precharacterization 2
rejection [%]
50
– The uniformity of the support layer as defects in the support cannot be closed by the final active separation layer. – Pores have to be small enough to retain the molecules, but big enough to enable solvent permeation (mean pore size 40.5 nm in permporometry). – The elimination of defect pores is besides the reduction of the pore size the most important target for the development of tighter ceramic OSN membranes.
0 -50 -100 -150 -200 0
100
200
300
400
429
500
Contrary to widespread expectations, even the rejection of ceramic membranes depends substantially on the specific solvent. This dependency and the influencing factors should be further investigated in the future in order to enable industrial application for ceramic OSN membranes.
molecular weight [g/mol] Fig. 18. Rejection measurements of membranes containing DEA in ethanol. One membrane was characterized with permporometry prior to the rejection measurements, the other two membranes were measured untreated.
The authors would like to thank the German Federal Ministry of Education and Research for funding of the project “nanomembrane” (03 0080 A-K).
100
after permporometry without precharacterization 1 without precharacterization 2
80
rejection [%]
Acknowledgments
References
60
40
20
0
-20
0
2000
4000
6000
molecular weight [g/mol] Fig. 19. Rejection measurements of membranes containing DEA in n-heptane. One membrane was characterized with permporometry prior to the rejection measurements, the other two membranes were measured untreated.
This discrepancy should be topic of further research and is believed essential for a successful implementation of ceramic OSN membranes in industry.
4. Conclusion A new method for the preparation of ceramic membranes for organic solvent nanofiltration with a molecular weight-cut-off of 350 g/mol was developed. We have circumstantiated, that complexation agents significantly enhance the separation performance. Addition of complexation agents into the sol and the subsequent sintering of the gel layer under inert atmosphere integrates hydrophobic patterns containing carbon in the amorphous titania/zirconia membranes. In this way, the hydrophobicity of the active membrane layer can be increased. Permporometry was identified as a suitable and helpful tool for quality control of the support layers as well as the finished membrane.
[1] P. Puhlfürß, A. Voigt, R. Weber, M. Morbé, Microporous TiO2 membranes with a cut off o500 Da, J. Membr. Sci. 174 (2000) 123–133. [2] I. Voigt, M. Stahn, S. Wöhner, A. Junghans, J. Rost, W. Voigt, Integrated cleaning of coloured waste water by ceramic NF membranes, Sep. Purif. Technol. 25 (2001) 509–512. [3] C. Guizard, A. Ayral, A. Julbe, Potentiality of organic solvents filtration with ceramic membranes. A comparison with polymer membranes, Desalination 147 (2002) 275–280. [4] T. Tsuru, T. Sudou, S.I. Kawahara, T. Yoshioka, M. Asaeda, Permeation of liquids through inorganic nanofiltration membranes, J. Colloid Interface Sci. 228 (2000) 292–296. [5] T. Tsuru, T. Sudoh, T. Yoshioka, M. Asaeda, Nanofiltration in non-aqueous solutions by porous silica–zirconia membranes, J. Membr. Sci. 185 (2001) 253–261. [6] G. Dudziak, T. Hoyer, A. Nickel, P. Puhlfuerss, I. Voigt, Ceramic Nanofiltration Membrane for Use in Organic Solvents and Method for The Production Thereof, EP Patent 1603663, 2010. [7] I. Voigt, P. Puhlfürß, T. Holborn, G. Dudziak, M. Mutter, A. Nickel, Ceramic nanofiltration membranes for applications in organic solvents, in: Proceedings of 9th Aachener Membrankolloquium, Aachen, 2003. [8] I. Voigt, P. Puhlfurb, G. Dudziak, M. Mutter, Ceramic nanofiltration membranes for retention of homogeneous catalysts, in: Proceedings of 8th International Congress on Inorganic Membranes, Cincinnati, USA, 2004. [9] T. Van Gestel, B. Van der Bruggen, A. Buekenhoudt, C. Dotremont, J. Luyten, C. Vandecasteele, G. Maes, Surface modification of γ-Al2O3/TiO2 multilayer membranes for applications in non-polar organic solvents, J. Membr. Sci. 224 (2003) 3–10. [10] A. Buekenhoudt, K. Wyns, V. Meynen, B. Maes, P. Cool, Surface-Modified Inorganic Matrix and Method for Preparation Thereof, WO Patent 2,010,106,167, 2010. [11] T. Tsuru, T. Nakasuji, M. Oka, M. Kanezashi, T. Yoshioka, Preparation of hydrophobic nanoporous methylated SiO2 membranes and application to nanofiltration of hexane solutions, J. Membr. Sci. 384 (2011) 149–156. [12] I. Voigt, P. Puhlfuerss, H. Richter, A. Endter, S. Duscher, K. Herrmann, Ceramic NF-membranes with a cut-off of 200 Da in: ICIM, Enschede, 2012. [13] H. Richter, A. Piorra, G. Tomandl, Ceramic membranes for nanofilters, Ceram. Trans. 83 (1997) 367–373. [14] F.P. Cuperus, D. Bargeman, C.A. Smolders, Permporometry: the determination of the size distribution of active pores in UF membranes, J. Membr. Sci. 71 (1992) 57–67. [15] Y.H. See Toh, X.X. Loh, K. Li, A. Bismarck, A.G. Livingston, In search of a standard method for the characterisation of organic solvent nanofiltration membranes, J. Membr. Sci. 291 (2007) 120–125. [16] X.J. Yang, A.G. Livingston, L. Freitas Dos Santos, Experimental observations of nanofiltration with organic solvents, J. Membr. Sci. 190 (2001) 45–55. [17] S. Zeidler, U. Kätzel, P. Kreis, Systematic investigation on the influence of solutes on the separation behavior of a PDMS membrane in organic solvent nanofiltration, J. Membr. Sci. 429 (2013) 295–303. [18] P. Schmidt, T. Köse, P. Lutze, Characterisation of organic solvent nanofiltration membranes in multi-component mixtures: membrane rejection maps and
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membrane selectivity maps for conceptual process design, J. Membr. Sci. 429 (2013) 103–120. [19] S. Postel, G. Spalding, M. Chirnside, M. Wessling, On negative retentions in organic solvent nanofiltration, J. Membr. Sci. 447 (2013) 57–65. [20] J. Geens, K. Boussu, C. Vandecasteele, B. Van der Bruggen, Modelling of solute transport in non-aqueous nanofiltration, J. Membr. Sci. 281 (2006) 139–148. [21] S. Darvishmanesh, A. Buekenhoudt, J. Degrève, B. Van der Bruggen, Coupled series–parallel resistance model for transport of solvent through inorganic nanofiltration membranes, Sep. Purif. Technol. 70 (2009) 46–52.
[22] S. Darvishmanesh, A. Buekenhoudt, J. Degrève, B. Van der Bruggen, General model for prediction of solvent permeation through organic and inorganic solvent resistant nanofiltration membranes, J. Membr. Sci. 334 (2009) 43–49. [23] P. Marchetti, A. Butté, A.G. Livingston, An improved phenomenological model for prediction of solvent permeation through ceramic NF and UF membranes, J. Membr. Sci. 415–416 (2012) 444–458. [24] A. Buekenhoudt, F. Bisignano, G. De Luca, P. Vandezande, M. Wouters, K. Verhulst, Unravelling the solvent flux behaviour of ceramic nanofiltration and ultrafiltration membranes, J. Membr. Sci. 439 (2013) 36–47.
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Preparation and characterization of new low MWCO ceramic nanofiltration membranes for organic solvents Stefanie Zeidler a,b,n, Petra Puhlfürß c, Uwe Kätzel a, Ingolf Voigt c a
Merck KGaA, Frankfurter Str. 250, 64293 Darmstadt, Germany TU Dortmund University, Department of Biochemical and Chemical Engineering, Laboratory of Fluid Separation, Emil-Figge-Strasse 70, 44227 Dortmund, Germany c Fraunhofer Institute for Ceramic Technologies and Systems, Michael-Faraday-Str. 1, 07629 Hermsdorf, Germany b
art ic l e i nf o
a b s t r a c t
Article history: Received 4 April 2014 Received in revised form 16 July 2014 Accepted 17 July 2014 Available online 2 August 2014
Ceramic membranes show advantageous behavior over polymeric membranes under harsh conditions including high pH, high temperature applications or applications involving organic solvents. However, in the range of organic solvent nanofiltration (OSN) the currently available membranes do just offer a molecular weight cut-off (MWCO) of 600 g/mol or above. Here, a new approach is presented to manufacture ceramic membranes with active layers of titanium dioxide/zirconium dioxide with integrated carbon on top of a tubular ceramic multi-layer support. Filtration tests of these membranes with polystyrene mixtures in THF reveal a MWCO of 350 g/mol, thus extending the range of possible nanofiltration applications. The paper also presents a new combination of characterization of the membranes with permporometry and filtration tests that demonstrate the importance of the supporting layers for the creation of a good nanofiltration membrane. & 2014 Elsevier B.V. All rights reserved.
Keywords: Organic solvent nanofiltration Ceramic membranes Inorganic membranes Permporometry
1. Introduction Ceramic membranes are convincing particularly in terms of high mechanical, chemical and thermal stability compared to polymeric membranes enabling their use under harsh conditions as well as the application of chemicals and steam for membrane and module/plant cleaning. Therefore, their use in a wide range of different applications from waste water treatment till clarification and sterilization of beverages are well known. Due to their (in principle) unrestricted resistance against organic solvents, ceramic membranes are of particular interest for organic solvent nanofiltration (OSN). This growing new technology describes the application of nanofiltration (NF) in pure organic solvents or organic solvent mixtures. OSN has just established during the last 15 years as a result of the development of solvent stable polymeric membranes and offers a high potential for applications in pharmaceutical and chemical industries. The first ceramic nanofiltration membrane with a molecular weight cut-off (MWCO) in water below 1000 g/mol was developed by Puhlfürß et al. [1] followed by a successful application in
n Corresponding author at: Merck KGaA, Frankfurter Str. 250, 64293 Darmstadt, Germany. Tel.: þ49 6151 72 94839; fax: þ 49 6151 72 913051. E-mail address: [email protected] (S. Zeidler).
http://dx.doi.org/10.1016/j.memsci.2014.07.051 0376-7388/& 2014 Elsevier B.V. All rights reserved.
process integrated cleaning of waste water from textile finishing [2]. The membrane consisted of amorphous titanium dioxide with a pore size of 0.9 nm and had a water flux of 20 10 5 kg/m2 h Pa and a MWCO of 450 g/mol measured with polyethylene glycols. The application of this membrane in organic solvents resulted in a MWCO above 10,000 g/mol combined with a toluene flux below 2 10 5 kg/m2 h Pa. The low flux and the bad rejection of the membrane despite the low MWCO indicates that the transport of organic solvents occurs only through larger defect pores of the membranes due to the hydrophilic behavior of the titania surface. Similar results were achieved by Guizard et al. [3] and Tsuru et al. [4,5] who prepared mixed oxides (silicon dioxide/titanium dioxide, aluminum oxide/zirconium dioxide and silicon dioxide/zirconium dioxide) with pore sizes below 1 nm. Different post-treatment methods have been suggested to create a hydrophobic membrane surface and to overcome this limitation. First method was the silanisation of mesoporous zirconium dioxide membranes with an initial pore size of 3 nm [6]. Via this way the MWCO in toluene measured with polystyrenes could be reduced down to 600 g/mol [7]. This membrane was successfully used in homogeneous catalyzed reaction removing the product while keeping the catalyst in the reaction [8]. Van Gestel et al. [9] applied the silanisation method to modify mesoporous aluminum oxide/titanium dioxide membranes but did not measure the retention in organic solvents. A second
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method was invented by Buekenhoudt et al. [10] using organometallic compounds for the post-treatment and to transfer the alkyl group to the membrane surface by a Grignard type reaction. Hydrophobisation by a post-treatment in the pore size range of a few nanometers is very sensitive resulting in a limited reproducibility. An alternative could be an in-situ hydrophobization. One approach is the preparation of methylated silicon dioxide membranes by using methylated silicon alkoxides [11] as precursor for the sol–gel process. Unfortunately, this method is limited to silica and resulted in a MWCO of only 1000–2000 g/mol in hexane. In this paper a new method of preparation of hydrophobic ceramic nanofiltration membranes is presented based on the formation of carbon in the amorphous titania or titania/zirconia membranes by sintering the gel layer under inert atmosphere. The amount of carbon was modified by adding complexation agents to the alkoxide precursors.
2. Experimental 2.1. Membrane preparation The method to prepare ceramic NF membranes is based on a polymeric sol–gel technique starting from a mixture of titanium isopropoxide and zirconium n-propoxide. The hydrolysis of the alkoxides is controlled by the amount of added water [1] (Fig. 1). A mixture of titania and zirconia was selected to prevent crystallization during firing the membrane at a temperature of 400 1C resulting in a microporous amorphous mixed oxide. At the same time the titania:zirconia ratio influences the pore size and finally the retention. Smallest pore sizes were achieved with a molar ratio 12:1. This membrane showed excellent NF performance in water with a MWCO measured with polyethylene glycols of 200 g/mol as reported elsewhere [12]. The new approach to generate hydrophobic membranes was the incorporation of carbon into the NF-membrane layer. This was achieved by running the sintering process under inert atmosphere (nitrogen) in order to pyrolyze the remaining alkoxide groups in the gel (type 1). To increase the amount of carbon diethanol amine (DEA) was added to the sol with a molar (titania þzirconia):DEA ratio of 1:2 (type 2). DEA is acting as a complexation agent preventing the complete hydrolysation and supporting the formation of a polymeric sol [13] (Fig. 2). A type 3 membrane was prepared by adding a diluted solution of a phenolic resin (FB8001, FERS, Barcelona, Spain) in isopropyl alcohol instead of DEA. The amount of phenolic resin was 0.16 wt % related to the calculated amount of oxide. In contrast to DEA
the phenolic resin did not act as a complexation agent but resulted in a residual amount of carbon after pyrolysis at 400 1C as well. Membranes were prepared inside of monochannel tubular substrates of α-alumina (A.D.: 10 mm, I.D.: 7 mm, Length: 250 mm) by dip coating under clean room conditions (class 100, 22 1C, humidity o50%). A couple of intermediate layers were used to adapt the pore size and to create a smooth surface. The top layer of this multilayer structure was a zirconium oxide-layer with a thickness of 50 nm and a mean pore size of 3 nm. The NF-membranes were dried for 2 h at room temperature before firing at 400 1C under nitrogen or air. 2.2. Characterization of pore size distribution by permporometry The knowledge of the quality of the active separation layer is of great interest during the development and manufacturing of porous membranes. Conventional analysis methods like mercury-porometry or gas adsorption–desorption measurements require special samples of the active membrane layer. Other analytical methods (e.g. SEM) damage the membrane irreversibly. Permporometry is a measuring process where the active pores of the complete membrane are investigated [14]. Permporometry is based on the effect of capillary condensation in the pores. First, the permeation of an incondensable gas is measured. Then, the incondensable gas is moistened stepwise by a condensable gas. Nitrogen was used as incondensable gas and cyclohexane as condensable gas. The higher the humidity the larger the pores blocked by capillary condensation. Thus, the gas flow through the membrane decreases sequentially. The smaller the pores the lower humidity is sufficient to reach capillary condensation that blocks the pores for gas transport. Capillary condensation is mathematically described by the Kelvin equation. p σ cos ϕ V m ln ¼ ps rp RT With the equation, where p is the partial pressure, ps is the saturation vapor pressure, σ is the interfacial tension, Vm is the molar volume of the condensable gas, R is the gas constant and rp is the pore radius, the pore size distribution of the membrane can be calculated. The contact angle Φ is set to zero as complete wetting is assumed. Prior to the measurements, the membranes were oven treated for 2 h at 120 1C under air. Then, the hot membranes were removed from the heating chamber and were directly implemented into
Fig. 1. Simplified formation of polymeric titania sol by controlled addition of water (type 1).
Fig. 2. Simplified formation of polymeric titania sol containing DEA as complexation agent (type 2).
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permporometry system. After the measurement a dry nitrogen flow was applied until the initial flux was achieved. An example of a determined permporometry curve is given in Fig. 3. The relative permeance related to the pure nitrogen permeance is plotted over the pore diameter calculated by the Kelvin equation. Subsequently, the nominal pore radius was determined by the interpolation of the permeance curve at a relative permeance of 50%.
2.3. Rejection measurements 2.3.1. Test system Rejection measurements were carried out according to a method recommended by See Toh et al. [15]. A styrene oligomer mixture (Fig. 4) containing PSS-ps560, PSS-ps1.8k and PSS-ps5.6k (Polymer Standards Service GmbH, Mainz, Germany) was used. One gram of each standard was dissolved in 1 l of tetrahydrofuran (THF). Furthermore, some experiments were conducted with polystyrene in toluene, n-heptane or ethanol (all Merck KGaA, Darmstadt, Germany). The solutions were prepared in the same way except the ethanol solution. Only 0.5 g of the PSSps560 standard was dissolved in 1 l ethanol due to solubility limitations.
100%
rel. permeance
423
80% 60%
2.3.2. Experimental set-up Nanofiltration experiments and the rejection characterization of the new membranes were carried out in a Multi-Purpose-CrossFlow-Membrane Plant at Merck KGaA, Darmstadt, Germany. A schematic diagram of the part of the plant which was used for the experiments is shown in Fig. 5. The feed is temperature controlled by the double jacketed feed vessel, while the whole tubing is isolated to prevent temperature changes across the test set-up. The feed is circulated across the membrane by a membrane plunger pump (Hydracell G03, Wanner, Minneapolis, USA). The single channel membrane tube is housed in a vertically fixed membrane module (Andreas Junghans, Frankenberg, Germany). The system is pressurized by closing the pressure retention valve behind the membrane module. Both, the retentate and the permeate were recirculated in the feed vessel. Rejection of the membranes were always measured under the same standard process conditions, i.e. a transmembrane pressure of 2 106 Pa, a feed flow of 6 l/min, corresponding to a cross-flow velocity of 3 m/s and a temperature of 20 1C. After equilibration of the process, the membranes were conditioned for 1 h. Subsequently, retentate and permeate samples were taken to determine the rejection of the membrane. Permeate mass flow was determined by a coriolis flow meter in the permeate recirculation. If the permeate flow was below the detection limit, it was determined by measuring the increase of permeate mass per time on an analytical balance. Prior to all experiments, the membranes were dried for at least 12 h in a cabinet desiccator at 80 1C under vacuum to eliminate air humidity or solvent residuals from the pores.
mean pore size
40% 20%
defect pores
0% 0
1
2
3
4
5
6
7
pore diameter [nm] Fig. 3. Determination of the pore size distribution, mean pore size and the amount of defect pores by permporometry measurement.
Fig. 4. Structural formula of polystyrene oligomers.
FI
permeate sample
pressure retention valve
feed vessel
membrane modul retentate sample FIC
membrane plunger pump Fig. 5. Schematic diagram of the membrane test unit.
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2.3.3. Analysis The concentrations of the polystyrene fractions in the retentate and the permeate were determined by gel permeation chromatography (Hitachi Elite LaChrom, Hitachi High Technologies America, Illinois, USA). The used colums Shodex GPC KF-801, KF-802 and KF-806M were supplied by Showa Denko K.K., Tokyo, Japan. The rejection of each molecular weight was then calculated with the concentration in the permeate cP and in the retentate cR by cP 100 R ¼ 1 cR Due to the different molecular weights of the styrene oligomers in the standards it was possible to create a rejection curve from the single data and, thus, the molecular weight cut-off value was determined as the molecular weight at which the interpolated rejection curve reached a value of 90%. Fig. 6 shows an example of such a rejection curve and the respective MWCO.
3. Results and discussion 3.1. Influence of complexation agents First, the influence of sintering under nitrogen atmosphere on the new 200 g/mol membrane was investigated (membrane type 1). The pore size distribution of the membrane can be seen in the
course of the permporometry measurement in Fig. 7 (left). A mean pore size of 1.0 nm with a defect flux of about 7% was detected. The results of the rejection measurements with polystyrene in THF are given in Fig. 7 (right). The rejection curve stagnates at 55% and decreases slightly towards larger molecules. The measured permeate flux was 350 l/m2 h. Apparently, the amount of solvents in the dried gel layer is too marginal to create carbon in the membrane and to change the surface properties from hydrophilic to hydrophobic and to increase the wettability with organic solvents after sintering under inert atmosphere. To enhance the wettability, the amount of carbon should be increased by the use of complexation agents. The first complexation agent which was tested for the preparation of hydrophobic nanofiltration membranes was diethanol amine (DEA) (membrane type 2). Using this agent and sintering the membrane under inert atmosphere results in an OSN membrane with the best MWCO (490 g/mol) to date. The respective rejection curve is given in Fig. 8 (right). Good rejections ( 4 60%) of the small polystyrenes were obtained. Nevertheless, rejection stagnates at 95% for the larger molecules. This can be attributed to a slight portion of defect pores (see Fig. 8 left). The mean pore size of this membrane is around 1.1 nm and hence nearly the same compared to the membrane prepared without complexation agent (Fig. 7). The considerable better rejection can be attributed to the better wettability of the membrane surface. Thus, the smaller pores take part in the separation process and especially larger molecules are retained. Furthermore, the distribution of the pores is very narrow, recognizable by the steep permporometry curve between 0 and 1.5 nm. However, this leads to a loss of flux performance (110 l/m2 h). In Fig. 9 (right) the rejection curve of a membrane prepared with phenolic resin (type 3) in the sol is given. It can be seen, that the membrane has hitherto the best rejection of 98% for the 5600 g/mol polystyrene. Unfortunately, the MWCO ( 1200 Da) is too high and thus does not give a true nanofiltration membrane. A glance at the permporometry diagram (Fig. 9 left) explains the results. The membrane has a very good pore size distribution with a mean pore size of 1.55 nm and no pores bigger than 2.5 nm. Therefore, larger molecules were completely retained. However, the pore size is still too large to achieve significant rejections in the nanofiltration range. Due to the broader pore size distribution, the permeate flux is higher (240 l/m2 h) compared to the type 2 membrane.
3.2. Influence of defect pores
Fig. 6. Rejection curve with MWCO determination.
The rejection measurements show that the mean pore size is not the only factor for a good OSN membrane performance.
Fig. 7. Characterization of a new 200 Da ceramic membrane (N0457) for aqueous applications sintered under nitrogen atmosphere. Permporometry measurement by use of cyclohexane as condensable vapor (left) and rejection curve determined with polystyrenes in THF at 20 bar (right).
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Fig. 8. Characterization of a new membrane (N0265) hydrophobised by DEA in the sol and sintered under nitrogen atmosphere. Permporometry measurement by use of cyclohexane as condensable vapor (left) and rejection curve determined with polystyrenes in THF at 20 bar (right).
Fig. 9. Characterization of a new membrane (N0157) hydrophobised by phenolic resin in the sol and sintered under nitrogen atmosphere. Permporometry measurement by use of cyclohexane as condensable vapor (left) and rejection curve determined with polystyrenes in THF at 20 bar (right).
Fig. 10. Characterization of a new membrane (N0426) for aqueous nanofiltration. Permporometry measurement by use of cyclohexane as condensable vapor (left) and rejection curve determined with polyethylene glycol in water at 10 bar (right).
A closer consideration of the permporometry measurements reveals that defect pores could have a strong negative impact on the performance in organic solvents. Membranes that have a constant relative permeance for larger pores ( 4 5 nm) in the permporometry measurement, show an incomplete rejection for higher weight molecular weight oligomers. The rejection curve often stagnates or even decreases. This implies that the larger molecules are able to pass through some larger pores, which are undesirable and therefore called defect pores.
In aqueous applications this effect seems to be negligible. Rejection measurements of aqueous NF membranes (type 1 sintered under air) with a mixture of polyethylene glycols in water show excellent rejections despite a considerable portion of defect pores. Fig. 10 illustrates this clearly. There are different explanations for the observed behavior. On the one hand the water molecules are smaller than the solvent molecules and additionally show a better wettability of the membrane surface. Thus, they can pass more easily through the very small pores and the relative transport through the defect
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Fig. 11. Permporometry measurements of the support membranes and the respective NF membrane after coating the layer with phenolic resin (membrane type 3) as complexation agent.
pores becomes marginal. On the other hand, in water there are also electrostatic repulsion forces acting between the membrane surface and charged solutes (not the case for PEG) so that larger pores may also be masked by such an effect, which is not existent in organic solvents. However, the results clearly show that if good OSN performance of ceramic membranes is sought, the reduction of the number of defect pores in the membranes active layer is essential. The mesoporous supports of the membrane (3 nm ZrO2) before coating the NF layer were investigated to evaluate the root cause for the formation of the defect pores. Permporometry measurements show that the defect pores already exist in the membrane support. Fig. 11 shows the results for the support layer and the finished NF membrane of two membranes containing the complexation agent phenolic resin. In Fig. 11A the support possesses a relative permeance of 10% at pores sizes 44 nm. These defect pores still exist in the finished membrane showing that an additional coating does not close the defect pores. Only if the permporometry of the support showed no defect pores (Fig. 11B), membranes without defect pores and hence membranes with complete rejection for higher molecular weight compounds could be produced. This also proves the usefulness of permporometry measurements as a quality control of ultrafiltration support membranes for OSN membranes manufacturing.
layer. Thus, the positive properties of the two materials, the ability to build tight pores and the hydrophobicity, could be combined. In addition, special attention was paid to the quality and the permporometry results of the support layer to reduce the amount of defect pores in the membrane. First, the membrane was produced and sintered under air conditions. The rejection curve and the permporometry diagram are given in Fig. 12. The diagrams indicate that the optimization procedure was successful concerning the reduction of defect pores. The permporometry curve approaches a relative permeance of 3% for larger pores and the 5600 g/mol polystyrene is completely retained. The measured perameate flux was 235 l/m2 h. However, the measured MWCO is still too high. Those results change dramatically, if the membrane is sintered under inert conditions. The permporometry result of the membrane produced in this way is given in Fig. 14 (left). The diagram indicates the success of the optimization. The low number of pores 42 nm in the support (see Fig. 13) could be closed and the mean pore size reduced ( 0.6 nm). The rejection curve measured with polystyrene in THF is given in Fig. 14 (right) and proves that a ceramic membrane with a cut-off of 350 g/mol could be developed. Molecules larger than 1800 g/mol are completely retained. The permeate flux at 20 bar transmembrane pressure was 70 l/m2 h.
3.3. Molecular weight cut-off optimization
3.4. Reproducibility
Further investigations were focused on DEA as complexation agent since this preparation route revealed the lowest MWCO so far (cf. Fig. 8). On the one hand optimization concentrated on the reduction of the mean pore size and on a method that closes the defect pores on the other hand. To achieve the former, the sintering conditions were changed and the active layer was coated twice. An aqueous NF layer (type 1) [12] was coated below the DEA
In order to check the reproducibility of the new membrane synthesis route, the optimized membrane was reproduced four times under the same conditions. The results of the rejection measurements with polystyrene in THF are shown in Fig. 15. Regrettably, the rejections are a little bit lower (MWCO4420 g/mol) than the original membrane (MWCO¼ 350 g/mol) and the four membranes differ in their MWCOs.
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Fig. 12. Characterization of a new membrane (N0341) hydrophobised by DEA in the sol and sintered under air. Permporometry measurement by use of cyclohexane as condensable vapor (left) and rejection curve determined with polystyrenes in THF at 20 bar (right).
rel. permeance
100% 80% 60% 40% 20% 0% 0
1
2
3
4
5
6
7
pore diameter [nm] Fig. 13. Permporometry measurement of the mesoporous support of the optimized membrane (N0328).
Nevertheless, this synthesis route with the chosen additive provides the best results and the lowest MWCO reported so far. The reproducibility is limited by a couple of parameters based on the membrane preparation, pore size distribution, support quality as well as the filtration tests and GPC measurements. Therefore the variation between 350 g/mol and 420 g/mol is quite good. In particular, the high rejections of the larger molecules are important to note. All membranes show a rejection 498.5% for the 6000 g/mol oligomers, which indicates the marginal amount of defect pores and the quality of the support layer. 3.5. Applicability in other solvents It is known from polymeric OSN membranes that considerable deviations in fluxes and rejections occur depending on different solvents used for separations [16–20]. All known transport models for solvent permeation and solute transport through ceramic OSN membranes [21–24] are more or less based on the pore-flow theory. Some authors extended the models by further influencing factors as the surface tension, dielectric constant, dipole moment, effective molecular diameter or a solubility parameter. However, with all the models and according to Darcy's law, only the viscosity, the applied pressure and the molecular size should cause differences in permeations of the solvent. Assuming the validity of the models also for the new nanofiltration membranes, MWCO should not change much when switching from THF to other solvents. In an experimental assessment the single layer DEA membrane (MWCO: 490 g/mol) was also characterized with polystyrenes in n-heptane and ethanol. The results are given in Fig. 16. For the purpose of comparison, the rejections measured in THF, which were presented Fig. 8, are also
incorporated. The experiments were conducted in the sequence THF, ethanol, n-heptane, n-heptane and ethanol again. As the results of the second measurements were comparable, only the results of the first is given in the figure. The rejection curves clearly illustrate that, similarly to the investigations with polymeric membranes, substantial differences exist between the rejections in the different solvents. THF provides the highest rejections. The rejections in ethanol are about 40% lower and polystyrene in n-heptane is not retained at all up to a molecule size of 2000 g/mol. Those results contradict the expectations based on the pore flow theory. If rejections are solely dependent on the different viscosities of the solvents and the effective molecular solute size in the solvent, rejection curves should only be shifted along the xaxis. Even the polarity of the solvents cannot solely explain those extreme differences in rejections. The polarity as well as the viscosity increases from n-heptaneo THF oethanol, whereas rejection increases from n-heptane to ethanol and THF. It has been proposed for solvent filtration, that the performance of a membrane is dependent on the first medium permeating the membrane [24]. As the rejections in THF (cyclic solvent) were always better than the rejections in other solvents, a similar effect based on the cyclohexane used in permporometry could be assumed here. In order to check the influence of the precharacterization by permporometry on the performance of the membranes, nine membranes were produced via the same synthesis route. Three of them were characterized by permporometry to ensure the quality of the membranes. Subsequently, the rejection curves of one precharacterized membrane and two membranes without a pretreatment were determined in THF, ethanol and n-heptane, respectively. The results are shown in Figs. 17–19. Rejections in THF (Fig. 17) seem to be better after preconditioning with cyclohexane by permporometry measurements. This may be due to the structural similarity between THF and cyclohexane which are both cyclic solvents. Cyclohexane could potentially adsorb at the pore walls whereby the THF permeate could pass easier through the pores by reason of a higher affinity. However, as the quality of the supports and the separation layer could not be verified beforehand, a deteriorated performance based on defect pores cannot be excluded. If a permanent solvent layer is really formed at the first use of the membrane, the rejections of a characterized membrane in ethanol should be lower than the rejections of an untreated membrane as the affinity of the nonpolar cyclohexane to the small polar ethanol molecule is very small. The permeate fluxes of the measurements are given in Table 1.
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100
100
80
80
rejection [%]
rejection [%]
Fig. 14. Characterization of the optimized membrane (N0328) hydrophobised by DEA in the sol and sintered under nitrogen atmosphere. Permporometry measurement by use of cyclohexane as condensable vapor (left) and rejection curve determined with polystyrenes in THF at 20 bar (right).
60
40
N0328 N0327 N0337 N0338
20
0 0
500
1000
1500
60
40
after permporometry
20
without precharacterization 1 without precharacterization 2 0 2000
0
MW [g/mol] Fig. 15. Reproducibility of the new membrane for organic solvent nanofiltration.
100
rejection [%]
n-Heptane (10,5l/m²h)
After permporometry Without pretreatment 1 Without pretreatment 2
20
0 2000
4000
8000
Permeate flux [l/m2 h]
THF (110 l/m²h)
0
6000
Fig. 17. Rejection measurements of membranes containing DEA in THF. One membrane was characterized with permporometry prior to the rejection measurements, the other two membranes were measured untreated.
Ethanol (20 l/m²h)
40
4000
Table 1 Permeate fluxes of THF, ethanol and n-heptane in l/m2 h of type 2 membranes with and without precharacterization.
80
60
2000
molecular weight [g/mol]
6000
8000
-20
molecular weight [g/mol] Fig. 16. Rejection curves and permeate fluxes of the DEA membrane (N0265, MWCO: 490 Da) determined with polystyrene in n-heptane, ethanol and THF (adapted from Fig. 8).
Fig. 18 illustrates that the measured results substantiate this hypothesis. The rejections after the permporometry measurement are negative meaning that the solutes preferentially permeate instead of the solvent. As the solute (polystyrene) is similar to the
THF
Ethanol
n-Heptane
200 350 200
270 400 200
170 230 400
precharaterization solvent (cyclohexane), the affinity is higher than to ethanol and therefore permeation could be preferred. In particular the rejection of the smallest molecules is significantly worse than that observed with the untreated membrane. For the larger molecules this effect might be extenuated due to sterical hindrance. The rejection curves in n-heptane are given in Fig. 19. Here, no differences between the membranes could be identified. None of the molecules was retained by any of the membranes. Concluding, some differences of the pretreatment of the membranes with cyclohexane in permporometry measurements could be identified. These, however, cannot solely explain the large change in rejections for different solvents in ceramic OSN membranes. For instance, the membranes show no rejection in n-heptane independent of the pretreatment.
S. Zeidler et al. / Journal of Membrane Science 470 (2014) 421–430
Thus, several substantial requirements of a ceramic membrane for organic solvent nanofiltration were identified:
100
after permporometry without precharacterization 1 without precharacterization 2
rejection [%]
50
– The uniformity of the support layer as defects in the support cannot be closed by the final active separation layer. – Pores have to be small enough to retain the molecules, but big enough to enable solvent permeation (mean pore size 40.5 nm in permporometry). – The elimination of defect pores is besides the reduction of the pore size the most important target for the development of tighter ceramic OSN membranes.
0 -50 -100 -150 -200 0
100
200
300
400
429
500
Contrary to widespread expectations, even the rejection of ceramic membranes depends substantially on the specific solvent. This dependency and the influencing factors should be further investigated in the future in order to enable industrial application for ceramic OSN membranes.
molecular weight [g/mol] Fig. 18. Rejection measurements of membranes containing DEA in ethanol. One membrane was characterized with permporometry prior to the rejection measurements, the other two membranes were measured untreated.
The authors would like to thank the German Federal Ministry of Education and Research for funding of the project “nanomembrane” (03 0080 A-K).
100
after permporometry without precharacterization 1 without precharacterization 2
80
rejection [%]
Acknowledgments
References
60
40
20
0
-20
0
2000
4000
6000
molecular weight [g/mol] Fig. 19. Rejection measurements of membranes containing DEA in n-heptane. One membrane was characterized with permporometry prior to the rejection measurements, the other two membranes were measured untreated.
This discrepancy should be topic of further research and is believed essential for a successful implementation of ceramic OSN membranes in industry.
4. Conclusion A new method for the preparation of ceramic membranes for organic solvent nanofiltration with a molecular weight-cut-off of 350 g/mol was developed. We have circumstantiated, that complexation agents significantly enhance the separation performance. Addition of complexation agents into the sol and the subsequent sintering of the gel layer under inert atmosphere integrates hydrophobic patterns containing carbon in the amorphous titania/zirconia membranes. In this way, the hydrophobicity of the active membrane layer can be increased. Permporometry was identified as a suitable and helpful tool for quality control of the support layers as well as the finished membrane.
[1] P. Puhlfürß, A. Voigt, R. Weber, M. Morbé, Microporous TiO2 membranes with a cut off o500 Da, J. Membr. Sci. 174 (2000) 123–133. [2] I. Voigt, M. Stahn, S. Wöhner, A. Junghans, J. Rost, W. Voigt, Integrated cleaning of coloured waste water by ceramic NF membranes, Sep. Purif. Technol. 25 (2001) 509–512. [3] C. Guizard, A. Ayral, A. Julbe, Potentiality of organic solvents filtration with ceramic membranes. A comparison with polymer membranes, Desalination 147 (2002) 275–280. [4] T. Tsuru, T. Sudou, S.I. Kawahara, T. Yoshioka, M. Asaeda, Permeation of liquids through inorganic nanofiltration membranes, J. Colloid Interface Sci. 228 (2000) 292–296. [5] T. Tsuru, T. Sudoh, T. Yoshioka, M. Asaeda, Nanofiltration in non-aqueous solutions by porous silica–zirconia membranes, J. Membr. Sci. 185 (2001) 253–261. [6] G. Dudziak, T. Hoyer, A. Nickel, P. Puhlfuerss, I. Voigt, Ceramic Nanofiltration Membrane for Use in Organic Solvents and Method for The Production Thereof, EP Patent 1603663, 2010. [7] I. Voigt, P. Puhlfürß, T. Holborn, G. Dudziak, M. Mutter, A. Nickel, Ceramic nanofiltration membranes for applications in organic solvents, in: Proceedings of 9th Aachener Membrankolloquium, Aachen, 2003. [8] I. Voigt, P. Puhlfurb, G. Dudziak, M. Mutter, Ceramic nanofiltration membranes for retention of homogeneous catalysts, in: Proceedings of 8th International Congress on Inorganic Membranes, Cincinnati, USA, 2004. [9] T. Van Gestel, B. Van der Bruggen, A. Buekenhoudt, C. Dotremont, J. Luyten, C. Vandecasteele, G. Maes, Surface modification of γ-Al2O3/TiO2 multilayer membranes for applications in non-polar organic solvents, J. Membr. Sci. 224 (2003) 3–10. [10] A. Buekenhoudt, K. Wyns, V. Meynen, B. Maes, P. Cool, Surface-Modified Inorganic Matrix and Method for Preparation Thereof, WO Patent 2,010,106,167, 2010. [11] T. Tsuru, T. Nakasuji, M. Oka, M. Kanezashi, T. Yoshioka, Preparation of hydrophobic nanoporous methylated SiO2 membranes and application to nanofiltration of hexane solutions, J. Membr. Sci. 384 (2011) 149–156. [12] I. Voigt, P. Puhlfuerss, H. Richter, A. Endter, S. Duscher, K. Herrmann, Ceramic NF-membranes with a cut-off of 200 Da in: ICIM, Enschede, 2012. [13] H. Richter, A. Piorra, G. Tomandl, Ceramic membranes for nanofilters, Ceram. Trans. 83 (1997) 367–373. [14] F.P. Cuperus, D. Bargeman, C.A. Smolders, Permporometry: the determination of the size distribution of active pores in UF membranes, J. Membr. Sci. 71 (1992) 57–67. [15] Y.H. See Toh, X.X. Loh, K. Li, A. Bismarck, A.G. Livingston, In search of a standard method for the characterisation of organic solvent nanofiltration membranes, J. Membr. Sci. 291 (2007) 120–125. [16] X.J. Yang, A.G. Livingston, L. Freitas Dos Santos, Experimental observations of nanofiltration with organic solvents, J. Membr. Sci. 190 (2001) 45–55. [17] S. Zeidler, U. Kätzel, P. Kreis, Systematic investigation on the influence of solutes on the separation behavior of a PDMS membrane in organic solvent nanofiltration, J. Membr. Sci. 429 (2013) 295–303. [18] P. Schmidt, T. Köse, P. Lutze, Characterisation of organic solvent nanofiltration membranes in multi-component mixtures: membrane rejection maps and
430
S. Zeidler et al. / Journal of Membrane Science 470 (2014) 421–430
membrane selectivity maps for conceptual process design, J. Membr. Sci. 429 (2013) 103–120. [19] S. Postel, G. Spalding, M. Chirnside, M. Wessling, On negative retentions in organic solvent nanofiltration, J. Membr. Sci. 447 (2013) 57–65. [20] J. Geens, K. Boussu, C. Vandecasteele, B. Van der Bruggen, Modelling of solute transport in non-aqueous nanofiltration, J. Membr. Sci. 281 (2006) 139–148. [21] S. Darvishmanesh, A. Buekenhoudt, J. Degrève, B. Van der Bruggen, Coupled series–parallel resistance model for transport of solvent through inorganic nanofiltration membranes, Sep. Purif. Technol. 70 (2009) 46–52.
[22] S. Darvishmanesh, A. Buekenhoudt, J. Degrève, B. Van der Bruggen, General model for prediction of solvent permeation through organic and inorganic solvent resistant nanofiltration membranes, J. Membr. Sci. 334 (2009) 43–49. [23] P. Marchetti, A. Butté, A.G. Livingston, An improved phenomenological model for prediction of solvent permeation through ceramic NF and UF membranes, J. Membr. Sci. 415–416 (2012) 444–458. [24] A. Buekenhoudt, F. Bisignano, G. De Luca, P. Vandezande, M. Wouters, K. Verhulst, Unravelling the solvent flux behaviour of ceramic nanofiltration and ultrafiltration membranes, J. Membr. Sci. 439 (2013) 36–47.