KR100827248B1 - Multilayer polyelectrolyte membranes and manufacturing method thereof - Google Patents

Abstract

A membrane manufactured by forming a polyelectrolyte layer on a porous supporting film is provided to separate the fluorine ion from the ion mixture, obtain good permeability, and sustain separation performance for a long time even under the long-term operating circumstances by manufacturing a membrane capable of separating a fluorine ion from an ion mixture, and a manufacturing method of the membrane is provided. A multilayer polyelectrolyte membrane comprises: a porous supporting film selected from a porous polymer film, a porous ceramic film, and a porous carbon film; and a polyelectrolyte coating layer formed by alternately laying up an anion polyelectrolyte solution and a cation polyelectrolyte solution to one or more layers on one or both faces of the porous supporting film in such a manner that the anion polyelectrolyte solution and the cation polyelectrolyte solution are laid up to a thickness of 10 to 100 nm, wherein the anion polyelectrolyte is prepared in the form of a solution having a pH of 1 to 12, a supporting salt concentration of 0.01 to 5 M, and a polyelectrolyte concentration of 0.0001 to 1 M by dissolving an anion polyelectrolyte and a supporting salt into a solvent, and the cation polyelectrolyte solution is prepared in the form of a solution having a pH of 1 to 12, a supporting salt concentration of 0.01 to 5 M, and a polyelectrolyte concentration of 0.0001 to 1 M by dissolving a cation polyelectrolyte and a supporting salt into a solvent. The anion polyelectrolyte is at least one selected from poly(styrene sulfonate), polyacrylic acid, poly(ethylene oxide), polyvinyl sulfate, poly(3-sulfopropyl methacrylate), poly(vinyl sulfonate), and poly(acrylamido-2-methyl-propanesulfonate). The cation polyelectrolyte is at least one selected from poly(dialyl dimethyl ammonium chloride), poly(aryl amine chloride), poly(ethlene imine), poly(acryl amide), and poly(vinyl amine). The supporting salt is at least one metal salt selected from NaCl, NaBr, NaF, MnCl2, CaCl2, KCl, NaI, LiCl, RbCl, CsCl, and HCl. Further, the porous ceramic film is one selected from a group consisting of a porous alumina film, a porous zirconia film, a porous silica film and a porous titania film.

Description

Multilayer polyelectrolyte membrane and manufacturing method thereof

The present invention relates to a method for preparing a multilayer polyelectrolyte membrane, and a separator capable of separating fluorine ions from an ion mixture having good permeability prepared therefrom. More specifically, in preparing a separator capable of separating fluorine ions from the ionic mixture, the polymer electrolyte is alternately stacked on a porous support membrane having excellent permeability and mechanical strength, thereby preparing a separator to separate fluorine ions from the ionic mixture as well as permeability. The present invention relates to a separation membrane that exhibits a long lasting permeability even in a long operation condition.

Fluoride is a common element in the earth's crust, and is present in many minerals and rocks in the form of fluorine compounds. However, drinking water containing more than 2 mg / L of fluoride ions is known to cause bone and tooth problems and even alter DNA structure. Nevertheless, in the United States alone, more than 200,000 people drink water containing fluoride ions with concentrations above 4 mg / L. However, since ions such as chlorine ions bind more strongly to fluoride ions than ion exchange resins, it is known that ion exchange resins are not effective in removing fluoride ions in water.

Therefore, fluorine ions in water are generally removed using an adsorbent such as activated alumina. However, alumina has a low fluorine ion binding ability, and many efforts have been made to improve it. Blachman et al. Attempted to improve the adsorption capacity of activated alumina using oxides of alkali or alkaline earth metals such as sodium, potassium, calcium, and magnesium. (US Pat. No. 6,632,3268) As a result, it was possible to improve the adsorption capacity by up to 25% compared to conventional activated alumina. On the other hand, Lebeau et al. Attempted to remove fluoride ions by neutralizing with lime, but did not lower the concentration of residual fluorine ions below 8 mg / L (US Pat. No. 6,436,297). McMullen et al. In order to remove the fluoride ions by the method of forming a magnesium precipitate by using a low concentration of 1 mg / L to less than 0.9 mg / L concentration showed a slight removal ability. (US Pat. No. 6,296,773) Meanwhile, Cargnel et al. Have made efforts to increase the ability to remove fluorine ions using reactors containing two or more aluminum compounds (US Pat. No. 5,824,227). Krulik et al. In order to remove fluorine ions by passing water containing fluorine ions through the reaction and then removing fluorine deposits in the next tank. (U.S. Patent 6,645,385)

The separation membrane can be considered as a separation process that can replace the existing activated alumina adsorption method, which does not have a large adsorption amount. Membrane technology has made significant progress in the past few decades in the field of separation of suspended solids in water, desalination of seawater, removal of dyes and separation of amino acids. Nanofiltration membranes remove most of divalent ions and pass most monovalent ions among various membranes depending on the pore size of the membrane. On the other hand, reverse osmosis membranes, which have smaller pore sizes, allow only water to pass through as well as divalent ions. Therefore, the use of reverse osmosis membrane can effectively remove fluorine ions. However, the reverse osmosis membrane has a problem that the amount of permeation is very small compared to the nanofiltration membrane and the minerals need to be replenished because not only high pressure is required but also all ions are removed.

Therefore, there is an urgent need for the development of a separation membrane that can effectively separate fluorine ions from the ion mixture but does not require high pressure during operation.

As described above, a general reverse osmosis membrane does not effectively separate ionic mixtures having similar ion sizes and charges. Therefore, it is necessary to use a nanofiltration membrane that selectively separates only fluorine ions but does not require high pressure during operation. However, conventional nanofiltration membranes can separate divalent ions, but pass monovalent ions as it is, and thus, a new concept of separation membrane is required.

In order to solve the above problems, the present invention provides a composite membrane by alternately stacking a polymer electrolyte on a porous support membrane to apply fluorine ions from an ionic mixture, thereby increasing the selectivity to fluorine ions and solution flux. In addition, the present invention aims to provide a separator that exhibits excellent characteristics such as sustaining permeation performance even in long-term operation.

That is, in the present invention, the polymer electrolyte is alternately stacked on the porous support membrane to prepare a separator having good permeability and high pressure during operation while separating fluorine ions from the ion mixture.

In addition, in the multilayer polymer electrolyte membrane prepared according to the present invention, the pore size of the membrane is appropriately adjusted by appropriately changing the type of polymer electrolyte used, the lamination conditions, and the number of laminations, so that the fluorine ions can be selectively separated from the ion mixture. To provide.

The present invention relates to a method for preparing a multilayer polyelectrolyte membrane, and a separator capable of separating fluorine ions from an ion mixture having good permeability prepared therefrom, and more specifically, a porous support membrane; And a polymer electrolyte layer in which one or more anionic polymer electrolyte solutions and a cationic polymer electrolyte solution are alternately stacked on one or both surfaces of the porous support membrane, to selectively separate fluorine ions.

Hereinafter, the present invention will be described in detail.

The separator prepared in the present invention is composed of a polymer electrolyte layer exhibiting selective permeability to monovalent anions other than fluorine and a porous support membrane supporting the separator. The support membrane can be used as long as it has good permeability and can maintain sufficient mechanical strength. For example, a general porous polymer membrane or a ceramic membrane can be used, and the shape of the support membrane can also be used in the form of a flat plate, a tube, or a tubular tube.

More specifically, high density polyethylene, low density polyethylene, linear low density polyethylene, polypropylene, high crystalline polypropylene, polyethylene-propylene air Polyethylene-propylene copolymer, Polyethylene-butylene copolymer, Polyethylene-hexene copolymer, Polyethylene-octene copolymer, Polystyrene-butylene- Styrene copolymer (polystyrene-butylene-styrene copolymer), polystyrene-ethylene-butylene-styrene copolymer (polystyrene), polystyrene, polyphenylene oxide, polysulfone ( polysulfone, polycarbonate, polyester, polyamide, poly Polyurethane, polyacrylate, polyvinylidene chloride, polyvinylidene fluoride, polysiloxane, polyolefin, ionomer, polymethylpentene (polymethyl pentene), hydrogenated oligocyclopentadiene (HOCP), at least one selected from the group consisting of one or more copolymers or derivatives thereof, porous polymer membrane or porous alumina membrane, porous zirconia membrane, One or more porous ceramic membranes or porous carbon membranes selected from porous silica membranes, porous titania membranes, and the like may be used, but is not limited thereto.

In the present invention, the polymer electrolyte layer is composed of an anionic polymer electrolyte and a cationic polymer electrolyte. However, anionic polymer electrolyte and cationic polymer electrolyte are alternately stacked on the polymer electrolyte layer. In the separation of anions such as fluorine ions, it is preferable to stack an anionic polymer electrolyte on the last layer.

Preferably, the anionic polymer electrolyte solution is prepared by dissolving an anionic polymer electrolyte and an auxiliary salt in a solvent with a pH of 1 to 12, an auxiliary salt of 0.01 to 5M, and a polymer electrolyte of 0.0001M to 1M. The cationic polymer electrolyte solution is prepared by dissolving a cationic polymer electrolyte and an auxiliary salt in a solvent, wherein the pH is 1 to 12, the concentration of the auxiliary salt is 0.01 to 5M, and the concentration of the polymer electrolyte is 0.0001M to 1M. The anionic polymer electrolyte is poly (styrene sulfonate), poly (acrylic acid), poly (ethylene oxide), poly (vinyl sulfate), poly (3-sulfopropyl methacrylate), poly (vinyl sulfonate), poly (acrylic) Amido-2-methyl-propanesulfonate), and the cationic polymer electrolyte is poly (diallyldimethylammonium chloride), poly (aryl Min chloride), poly (ethyleneimine), poly (acrylamide), it is preferable to use at least one selected from poly (vinylamine). However, the present invention is not limited thereto, and as the polymer electrolyte, any kind of polymer electrolyte that can be laminated alternately may be used. More preferably, it is preferable to use poly (styrene sulfonate) as the anionic polymer electrolyte, and it is preferable to use poly (diallyldimethylammonium chloride) as the cationic polymer electrolyte.

Auxiliary salts that can be used in the manufacture of the multilayer polymer electrolyte membrane include NaCl, NaBr, NaF, MnCl ₂ , CaCl ₂ , KCl, NaI, LiCl, RbCl, CsCl, HCl and the like. However, in addition to the salts exemplified herein, there are many types of salts and auxiliary salts consistent with the object of the present invention are not limited to those exemplified herein. In addition, the separation membrane of the present invention can be used not only a single salt but also a physical mixture of one or more salts.

Substantially affecting the selective separation of fluorine ions in the separator prepared in the present invention is the charge and pore size of the separator and the thickness of the polymer electrolyte layer affects the permeation amount. Factors affecting the pore size, charge, and permeation of the membrane are the type of polymer electrolyte, the pH of the stacking solution, the concentration of the polymer electrolyte, the concentration of the auxiliary salt in the polymer electrolyte solution, the stacking time, and the number of stacks. These variables determine the characteristic of selectively permeating fluorine ions in the ionic mixture. That is, the type of the polymer electrolyte, the pH of the lamination solution, the concentration of the salt in the lamination solution, the lamination time (lamination thickness), and the number of laminations must be carefully selected to obtain a separator having high selectivity and permeability.

In the present invention, it has been found that the best fluorine ion rejection rate can be achieved when the anionic polymer electrolyte and the cationic polymer electrolyte are alternately stacked one or more layers at the same time and the thickness of the stacked electrolyte layers is 10 to 100 nm. In case of less than 10 nm, it is difficult to cover all the pores of the porous alumina membrane, and if it exceeds 100 nm, the permeate flux becomes too low, which is not preferable. In addition, the pH of the electrolyte polymer solution is prepared to 1 to 12, the concentration of the auxiliary salt is preferably prepared to 0.01 to 5M, the concentration of the polymer electrolyte is preferably prepared to the solution of 0.0001 ~ 1M, satisfies the above range In this case, it was confirmed that the exclusion rate of fluorine ions was high.

The multilayer polymer electrolyte membrane according to the present invention has a fluorine ion exclusion rate of at least 60%, a selectivity of at least 2.5, and a flux of solution of at least 2 (m 3 / m 2 -day) with respect to the ion mixture to be separated. Selective separation has the effect of improving.

The ion mixture includes one or more monovalent anions and fluorine ions, the monovalent anions preferably include chlorine ions, iodine ions, nitrate ions, nitrite ions, bromine ions, and the ionic mixtures are monovalent anions. It is also possible to include a divalent anion.

Hereinafter, a method of manufacturing a multilayer polymer electrolyte separator according to the present invention will be described.

a) dissolving an anionic polyelectrolyte or cationic polyelectrolyte together with an auxiliary salt in a solvent to prepare a solution having a pH of 1 to 12, an auxiliary salt of 0.01 to 5M, and a polymer electrolyte of 0.0001M to 1M;

b) stacking the prepared anion polymer electrolyte solution on the porous support membrane to form an anion polymer electrolyte layer;

c) forming a cationic polymer electrolyte layer by laminating a cationic polymer electrolyte solution on the anion polymer electrolyte layer;

d) stacking an anionic polymer electrolyte solution on top of the cationic polymer electrolyte layer to form an anion polymer electrolyte layer at the outermost part;

It may include a multi-layer polymer electrolyte separator to selectively separate the fluorine ions, including.

More preferably, the steps b) and c) are repeated to prepare the polymer electrolyte layer so that the thickness of the polymer electrolyte layer is 10 to 100 nm, thereby improving the resolution of fluorine ions.

Specifically, the membrane of the present invention is prepared by first dissolving an anionic polymer electrolyte and a cationic polymer electrolyte in a liquid solvent containing an auxiliary salt, and then stacking the solutions alternately on a porous support membrane. At this time, the excess polymer electrolyte that is not laminated is removed using water between the stacks of the respective electrolyte layers. The liquid solvent used in the lamination process should be capable of dissolving the polymer electrolyte and may be used as long as it does not damage the support membrane. If the polymer constituting the polymer electrolyte is water soluble, water may be used as a solvent. The concentration of the auxiliary salt in the polymer electrolyte solution, the pH of the polymer electrolyte solution, the concentration of the polymer electrolyte solution, the lamination time, and the number of laminations are determined in consideration of the thickness and charge of the polymer electrolyte layer. There are various ways to apply the electrolyte coating solution on the support membrane, as is well known, and simply, a method such as a spin coating method or a dip coating method may be used. It is preferable to make the thickness of the polymer electrolyte layer formed on the support membrane as small as possible in order to increase permeability. However, if the thickness of the polymer electrolyte layer is too small, the pores of the porous support membrane may not be blocked, or a hole may be generated due to a pressure difference during operation, which may reduce selectivity. Another characteristic of the membrane thus prepared is the high rejection rate and selectivity of fluorine ions. Here, the exclusion rate and selectivity are defined as in Equations 1 and 2.

Exclusion rate = 1-[concentration / feed concentration] x 100 (1)

Selectivity = [100-rejection rate of other anions] / [100-rejection rate of fluoride ions] (2)

The exclusion rate of fluorine ions is a value indicating how well fluorine ions can be removed, and the selectivity is a value indicating how selectively fluorine ions can be separated from other anions and measured using a 1 mM mixed solution. Therefore, the greater the rejection rate and selectivity of fluorine ions, the better the separation performance and the better the practical application. Separation membrane prepared in the present invention has a high fluorine ion rejection rate of 60% or more, more preferably 60 to 90%, selectivity of 2.5 or more, more preferably 2.5 to 5. In addition to selectivity in membranes, an important value is the flux of the solution. The separator prepared in the present invention has a high flux of the solution of 2 (m 3 / m 2 -day) or more, more preferably 2 to 5 (m 3 / m 2 -day).

The following examples are intended to illustrate the invention in more detail, but the invention is not limited thereto.

Example 1

A porous alumina membrane [0.02 μm Whatman Anodisk filter] was mounted in the holder, followed by pouring 0.02 M poly (styrene sulfonate) (molecular weight = 70,000, Aldrich) (PSS) solution (pH = 5.5) containing 0.5 M NaCl 3 After holding for 1 minute, the mixture was washed with water for 1 minute. Thereafter, 0.02M poly (diallyldimethyl ammonium chloride) containing 0.5M NaCl (molecular weight = 150,000, 20 wt% aqueous solution, Aldrich) (PDADMAC) solution (pH = 5.2) was poured and held for 3 minutes, and then water was Washed for 1 min. Again pour the PSS solution was maintained for 3 minutes, washed with water for 1 minute. The lamination process was performed five times for the PSS solution, four times for the PDADMAC solution, and a final solution for the PSS solution. The separator thus prepared contained a polymer electrolyte coating layer having a thickness of about 30 nm on one surface thereof, and the membrane area was 1.5 cm 2.

Permeation experiment was performed using the prepared membrane. The feed contained 1 mM NaCl and 1 mM NaF, permeation experiments were carried out at room temperature with feed pressure of 70 psig and permeate pressure of 0 psig, with a feed flow rate of 18 mL / min. The volume flow of the permeate was measured using a graduated cylinder to calculate the solution flux and the concentration of the permeate and feed was measured using ion chromatography (Dionex 600). Table 1 shows the solution flux and selectivity of the separator.

TABLE 1

Example 2

Using the method described in Example 1, a composite film of PSS and PDADMAC was prepared. However, the feed contains 1 mM NaBr and 1 mM NaF. Table 2 shows the solution flux, rejection rate, and selectivity.

TABLE 2

Example 3

Using the method described in Example 1, a composite film of PSS and PDADMAC was prepared. However, 0.02M poly (styrene sulfonate) (PSS) solution containing 1.0M NaCl was used for lamination of the last layer. Table 3 shows the solution flux, rejection rate, and selectivity.

TABLE 3

[Examples 4 to 6]

In Examples 4 to 6, permeation experiments were carried out while varying the feed pressure of the nano-membrane of the PSS and PDADMAC prepared in Example 1. Table 4 shows the solution flux, rejection rate, and selectivity.

TABLE 4

[Examples 7 to 8]

In Examples 7 to 8, permeation experiments were carried out while varying the flow rate of the feeds for the nano separators of the PSS and PDADMAC prepared in Example 1. Table 5 shows the solution flux, rejection rate, and selectivity.

TABLE 5

As can be seen in the above embodiment, the separation membrane according to the present invention has a fluorine ion rejection rate of 60% or more, selectivity of 2.5 or more, and the flux of the solution is 2 (m 3 / m 2 -day) or more with respect to the ion mixture Could know.

In the present invention, a membrane was prepared by alternately stacking a polymer electrolyte solution on a porous support membrane. The prepared membrane has an appropriate charge and pore size, which not only has high exclusion rate and selectivity of fluorine ions, but also has high flux of solution, which is suitable for real fluorine ion separation process.

Claims (13)

A porous support membrane selected from a porous polymer membrane, a porous ceramic membrane or a porous carbon membrane; and Anionic polymer electrolyte and an anionic salt were dissolved in a solvent on one or both sides of the porous support membrane, and the anionic polymer electrolyte prepared as a solution having a pH of 1 to 12, an auxiliary salt of 0.01 to 5M, and a polymer electrolyte of 0.0001M to 1M. A solution, a cationic polymer electrolyte, and an auxiliary salt were dissolved in a solvent to alternately form a cationic polymer electrolyte solution prepared from a solution having a pH of 1 to 12, a concentration of the auxiliary salt of 0.01 to 5M, and a concentration of the polymer electrolyte of 0.0001M to 1M. A polymer electrolyte coating layer having a thickness of 10 to 100 nm by laminating above; Including; The anionic polymer electrolyte is poly (styrene sulfonate), poly (acrylic acid), poly (ethylene oxide), poly (vinyl sulfate), poly (3-sulfopropyl methacrylate), poly (vinyl sulfonate), poly (acrylic) Amido-2-methyl-propanesulfonate), and the cationic polymer electrolyte is poly (diallyldimethylammonium chloride), poly (aryl amine chloride), poly (ethylene imine), poly (Acryl amide), any one or more selected from poly (vinyl amine) is used, The auxiliary salt is a multi-layered polymer that selectively separates fluorine ions, characterized by using any one or more metal salts selected from NaCl, NaBr, NaF, MnCl ₂ , CaCl ₂ , KCl, NaI, LiCl, RbCl, CsCl, HCl Electrolyte separator. delete delete delete The method of claim 1, The porous polymer membrane is a high density polyethylene, low density polyethylene, linear low density polyethylene, polypropylene, high crystalline polypropylene, polyethylene-propylene air Polyethylene-propylene copolymer, Polyethylene-butylene copolymer, Polyethylene-hexene copolymer, Polyethylene-octene copolymer, Polystyrene-butylene- Styrene copolymer (polystyrene-butylene-styrene copolymer), polystyrene-ethylene-butylene-styrene copolymer (polystyrene), polystyrene, polyphenylene oxide, polysulfone ( polysulfone, polycarbonate, polyester, polyamide, Polyurethane, polyacrylate, polyvinylidene chloride, polyvinylidene fluoride, polysiloxane, polyolefin, ionomer, polymethyl Any one selected from pentene (polymethyl pentene), hydrogenated oligocyclopentadiene (HOCP), copolymers or derivatives thereof, The porous ceramic membrane is a multilayer polymer electrolyte membrane, characterized in that any one selected from porous alumina membrane, porous zirconia membrane, porous silica membrane, porous titania membrane. The method according to any one of claims 1 to 5, The outermost layer of the polymer electrolyte layer is a multilayer polymer electrolyte membrane, characterized in that the anionic polymer electrolyte solution is laminated. The method of claim 1, The multilayer polymer electrolyte separator has a fluorine ion exclusion rate of 60% or more, a selectivity of 2.5 or more, and a flux of a solution of 2 (m 3 / m 2 -day) or more with respect to the ion mixture to be separated. . The method of claim 7, wherein The ion mixture is a multilayer polymer electrolyte membrane, characterized in that it comprises one or more monovalent anions and fluorine ions. The method of claim 8, The monovalent anion is a multilayer polymer electrolyte membrane, characterized in that it comprises a chlorine ion, iodine ion, nitrate ion, nitrite ion, bromine ion. The method of claim 9, The ion mixture is a multilayer polymer electrolyte membrane, characterized in that it comprises a divalent anion in addition to the monovalent anion. a) at least one auxiliary salt selected from NaCl, NaBr, NaF, MnCl ₂ , CaCl ₂ , KCl, NaI, LiCl, RbCl, CsCl, HCl and poly (styrene sulfonate), poly (acrylic acid), poly ( At least one anion selected from ethylene oxide), poly (vinyl sulfate), poly (3-sulfopropyl methacrylate), poly (vinyl sulfonate), poly (acrylamido-2-methyl-propanesulfonate) Dissolving a polymer electrolyte in a solvent to prepare an anionic polymer electrolyte solution having a pH of 1 to 12, an auxiliary salt of 0.01 to 5M, and a polymer electrolyte of 0.0001M to 1M; b) at least one auxiliary salt selected from NaCl, NaBr, NaF, MnCl ₂ , CaCl ₂ , KCl, NaI, LiCl, RbCl, CsCl, HCl and poly (diallyldimethylammonium chloride), poly (aryl amine chloride), One or more cationic polymer electrolytes selected from poly (ethylene imine), poly (acrylamide), and poly (vinyl amine) are dissolved in a solvent to have a pH of 1 to 12, an auxiliary salt of 0.01 to 5M, and a concentration of the polymer electrolyte. Preparing a cationic polymer electrolyte solution having 0.0001M to 1M; c) The anionic polymer electrolyte solution and the cationic polymer electrolyte solution are alternately laminated on the porous support membrane selected from the porous polymer membrane, the porous ceramic membrane or the porous carbon membrane, and the outermost layer is laminated so that the anionic polymer electrolyte solution is laminated. Forming a polymer electrolyte coating layer having a thickness of 10 to 100 nm; Method for producing a multilayer polymer electrolyte membrane for selectively separating fluorine ions, including. The method of claim 11, The porous polymer membrane is a high density polyethylene, low density polyethylene, linear low density polyethylene, polypropylene, high crystalline polypropylene, polyethylene-propylene air Polyethylene-propylene copolymer, Polyethylene-butylene copolymer, Polyethylene-hexene copolymer, Polyethylene-octene copolymer, Polystyrene-butylene- Styrene copolymer (polystyrene-butylene-styrene copolymer), polystyrene-ethylene-butylene-styrene copolymer (polystyrene), polystyrene, polyphenylene oxide, polysulfone ( polysulfone, polycarbonate, polyester, polyamide, Polyurethane, polyacrylate, polyvinylidene chloride, polyvinylidene fluoride, polysiloxane, polyolefin, ionomer, polymethyl Any one selected from pentene (polymethyl pentene), hydrogenated oligocyclopentadiene (HOCP), copolymers or derivatives thereof, The porous ceramic membrane is a method for producing a multilayer polymer electrolyte membrane, characterized in that any one selected from porous alumina membrane, porous zirconia membrane, porous silica membrane, porous titania membrane. delete