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WO1996004069A1 - Membrane - Google Patents

Membrane Download PDF

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Publication number
WO1996004069A1
WO1996004069A1 PCT/GB1995/001839 GB9501839W WO9604069A1 WO 1996004069 A1 WO1996004069 A1 WO 1996004069A1 GB 9501839 W GB9501839 W GB 9501839W WO 9604069 A1 WO9604069 A1 WO 9604069A1
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WO
WIPO (PCT)
Prior art keywords
membrane
anatase
suspension
weight
treated
Prior art date
Application number
PCT/GB1995/001839
Other languages
French (fr)
Inventor
David Frank Lewis
Graeme John Baird
David Macdonald Bonnick
Howard Matthew Colquhoun
Original Assignee
North West Water Group Plc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by North West Water Group Plc filed Critical North West Water Group Plc
Priority to AU31836/95A priority Critical patent/AU3183695A/en
Publication of WO1996004069A1 publication Critical patent/WO1996004069A1/en

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/14—Dynamic membranes
    • B01D69/141—Heterogeneous membranes, e.g. containing dispersed material; Mixed matrix membranes
    • B01D69/145—Heterogeneous membranes, e.g. containing dispersed material; Mixed matrix membranes containing embedded catalysts
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0039—Inorganic membrane manufacture
    • B01D67/0041—Inorganic membrane manufacture by agglomeration of particles in the dry state
    • B01D67/00411—Inorganic membrane manufacture by agglomeration of particles in the dry state by sintering
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/14—Dynamic membranes
    • B01D69/141—Heterogeneous membranes, e.g. containing dispersed material; Mixed matrix membranes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/02—Inorganic material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
    • B01J21/06—Silicon, titanium, zirconium or hafnium; Oxides or hydroxides thereof
    • B01J21/063—Titanium; Oxides or hydroxides thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
    • B01J35/39—Photocatalytic properties
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/50—Catalysts, in general, characterised by their form or physical properties characterised by their shape or configuration
    • B01J35/58—Fabrics or filaments
    • B01J35/59—Membranes
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/30—Treatment of water, waste water, or sewage by irradiation
    • C02F1/32—Treatment of water, waste water, or sewage by irradiation with ultraviolet light
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2323/00—Details relating to membrane preparation
    • B01D2323/12—Specific ratios of components used
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/02—Details relating to pores or porosity of the membranes
    • B01D2325/0283—Pore size

Definitions

  • the present invention relates to a membrane intended particularly, but not exclusively, for water purification.
  • Membranes are used to remove suspended solids from water (e.g. as obtained from a reservoir) before it is supplied to domestic households and other end users.
  • water may also contain dissolved organic compounds (e.g. pesticide residues) which are not removed by the membrane and which either pass untreated to the end user or require a separate treatment for their removal.
  • Photocatalytic oxidation processes have been proposed for the purification of water, particularly potable water, but such processes have so far been hindered by a number of difficulties. If the photocatalyst is used as a suspended powder then clearly it must be removed, for example by filtration or centrifugation. and there is always a danger of catalyst residues remaining in the water. If on the other hand the photocatalyst is immobilised on a continuous solid surface then only a very small fraction of the active area of the catalyst is in contact with the water to be purified, making for an inefficient process.
  • a composite porous membrane comprising an inorganic support having interstices and porous inorganic films of sintered non-metallic particles carried by the support and bridging the interstices thereof wherein exterior and interior surfaces of the membrane are provided with a photocatalyst which upon irradiation with electromagnetic radiation of the appropriate wavelength is effective to oxidatively decompose organic materials in water.
  • a method of purifying water comprising filtering the water with a membrane as defined for the first aspect of the invention and irradiating said membrane with electromagnetic radiation of said appropriate wavelength so as oxidatively to decompose organic materials in the water.
  • Membranes in accordance with the first invention are of high surface area, as provided by the sintered non-metallic particles carried by the support.
  • the catalyst is provided both on external surfaces of the membrane and the internal surfaces of the pores. Therefore the membrane has a high surface area of catalyst.
  • the fact that the support membrane comprises sintered non-metallic particles (e.g. ceramic particles) means that the membrane is oxidatively and hydrolytically stable.
  • the membranes have the dual capability of removing both suspended solids and dissolved organic materials from water to be purified.
  • the membrane is provided with an oxidising agent, e.g. oxygen, ozone, hydrogen peroxide etc., and is irradiated with electromagnetic energy of the appropriate wavelength.
  • an oxidising agent e.g. oxygen, ozone, hydrogen peroxide etc.
  • the upstream side of the membrane may be irradiated with electromagnetic wavelength of the appropriate wavelength. Any paniculate matter accumulating on the surface of the catalyst will also be destroyed by the photocatalytic process so that the membrane is effectively self cleaning.
  • the downstream side of the membrane is irradiated.
  • the membrane acts simultaneously as a microfilter (removing suspended solids on the upstream side of the membrane) and a photocatalyst to catalyse the oxidation of dissolved organic materials on the other side of the membrane.
  • the membrane in accordance with the invention will comprise 0.01% to 30% (more usually 0.2% to 10%) by weight of the photocatalyst based on the total weight of the membrane.
  • the catalyst is preferably one for which irradiation with visible or ultra violet radiation (e.g. near - uv solar radiation) is effective to cause photocatalytic, oxidative decomposition of dissolved organic materials. If near - uv solar radiation is effective to cause the decomposition then there is the possibility of using sunlight as a photon source.
  • visible or ultra violet radiation e.g. near - uv solar radiation
  • the catalyst is preferably the anatase form of titania.
  • Anatase is the less thermodynamically-stable form of titania, the other principal form of which (rutile) is substantially less photocatalytically active.
  • the irradiation of anatase at a wavelength (in the ultraviolet range) for which the photon energy is greater than the semiconductor bandgap results in electron excitation from the valence band to the conduction band.
  • the electron-hole pairs so created have considerable potential for redox chemistry, especially for generation of the powerfully oxidising hydroxyl radical which is capable of attacking and degrading most organic compounds.
  • Photo-excitation of anatase powder in aqueous suspension is known to result in the complete, catalytic, oxidative degradation ("mineralisation") of organic contaminants including herbicides and pesticides which may be present in the water. Typical end products are carbon dioxide, and nitrate, sulphate, and halide ions.
  • the reduction potential of irradiated titania can result in soluble metal ions in high oxidation states being reduced to insoluble, less hazardous forms.
  • Membranes in accordance with the invention preferably have a pore size in the range 0.01 - lO ⁇ m.
  • the membranes in accordance with the invention may be produced from a substrate membrane (to which the photocatalyst is applied, e.g. using the techniques described below) comprising said inorganic support and said porous inorganic films of sintered non-metallic particles bridging interstices of the support.
  • a substrate membrane to which the photocatalyst is applied, e.g. using the techniques described below
  • Such substrate membranes may be as disclosed in EP-A-0 348 041.
  • a particularh suitable substrate membrane comprises a chemically-resistant metal mesh or an alloy such as inconel, supporting a porous film of zirconia (ZrO 2 ) having a pore-size in the range of 0.01 - lO ⁇ m.
  • the substrate membrane has an effective pore size in the range 0.01 to 10 microns, more preferably 0.05 to 0.5 ⁇ m.
  • the membrane in accordance with the invention may be produced in a number of ways.
  • the substrate membrane may be treated (e.g. by immersion) with a suspension of the photocatalyst which is to be incorporated in the membrane of the invention or with a solution or suspension of a precursor thereof which is decomposable by heat to give the final form of the catalyst.
  • the treated membrane is then heated at a temperature which is sufficiently high to effect any necessary conversion of catalyst precursor and to provide sintered catalyst on exterior and interior surfaces of the membrane. The temperature should not be such that catalytic activity is lost.
  • the preferred catalyst is anatase.
  • the substrate membrane may initially be immersed in a suspension of anatase particles.
  • the dispersed particles preferably have a size of about 10 to 100 nm, more preferably 10 to 40 nm, most preferably about 20 nm.
  • a suitable anatase may be obtained in Degussa P25. If the anatase as supplied contains larger aggregates of such particles then such aggregates should be 'broken down' so as to provide the primary particles in dispersion, e.g. by using ultrasonics.
  • the dispersion preferably comprises 1% to 10% by weight of the anatase (more preferably 4 to 6%) and most preferably about 5% on the same weight basis.
  • the dispersion preferably also incorporates a binder in an amount less than that of the anatase.
  • the binder serves to increase viscosity and hold the particles in suspension. Typically the binder will be used in an amount of 2-5% by weight.
  • suitable binders include methylcellulose and xanthan gum.
  • the dispersion also includes a trace amount (e.g. less than 0.1% by weight) of a surface active agent, e.g. Miravon B79.
  • the substrate membrane will usually be dipped in the abovedescribed dispersion for a period of 15 to 25 seconds and then allowed to dry to the extent that visually the membrane appears dry. A drying period of about 1 hour at ambient temperature will generally be suitable.
  • the treated membrane is heated in a furnace to effect sintering of the anatase without conversion to rutile.
  • a peak furnace temperature of 300-600°C will generally be used.
  • the temperature of the treated membrane will be increased to this peak temperature over a period of 2 to 3 hours and the membrane will then be allowed to dwell at the peak temperature for about 1 hour before being cooled in the furnace.
  • the overall firing procedure will generally take about 8-9 hours.
  • anatase dispersion it is possible to treat the substrate membrane with a catalyst precurosr, e.g. with Titanium (IV) bis/ammonium lactato)dihydroxide of formula (CH 3 CH(O)CO 2 NH 4 ) 2 Ti(OH) 2 which may be used as an aqueous solution, preferably at a concentration of 40-60% by weight, ideally about 50% by weight.
  • a catalyst precurosr e.g. with Titanium (IV) bis/ammonium lactato)dihydroxide of formula (CH 3 CH(O)CO 2 NH 4 ) 2 Ti(OH) 2 which may be used as an aqueous solution, preferably at a concentration of 40-60% by weight, ideally about 50% by weight.
  • the treated substrate membrane is allowed to dry and then heated in a furnace for similar temperature and time conditions to those disclosed above.
  • a further method of applying anatase to the substrate membrane is to treat the latter with a titanium alkoxide dissolved in a volatile organic solvent, and then allowing the membrane to dry, the procedure normally being effected under nitrogen.
  • the heating step to decompose the alkoxide may be effected in air at a temperature which is sufficient to cause hydrothermal conversion of the precursor to the anatase form of titania but not so high as to convert the titania to the rutile form.
  • the temperature will be in the range 200-600°C and maintained for 1 - 10 hours.
  • This process of treatment with titanium alkoxide and subsequent heat decomposition may be repeated until an optimum thickness of the catalyst is achieved.
  • photocatalytic activity may not be observed after a single coating, but after several coatings an active catalyst surface will be produced.
  • the reduction in permeability after several coating cycles is normally additive, so that for example after 5 cycles the permeability of the substrate membrane would be reduced by some 15-25%).
  • the alkoxide group may, for example, contain from 1 to 10 carbon atoms. Most preferably the alkoxide is the iso-propoxide.
  • the conversion of titanium tetra- isopropoxide to titania is represented by the following equation:
  • Ceramic flat-sheet/metal hybrid membranes as described in EP 348041 were then dipped in the dispersion.
  • One membrane had a pore size of 0.1 ⁇ m and the other had a pore size of 0.2 ⁇ m. Dipping was effected by immersing the membranes into the dispersion at an immersion speed of 1 cm/sec, allowing 20 seconds immersion, and then withdrawing the membrane at a speed of 2.5 cm/min.
  • the treated membranes were then allowed to dry (as assessed visually) before firing in a furnace.
  • the membranes were heated in the furnace at a rate of 5°C/min until a temperature of 600°C was reached which was then maintained for 1 hour prior to cooling at a rate of 5°C/min to room temperature.
  • the weight gain of the 0.2 ⁇ m membrane was found to be 1.9%.
  • Pore size This was measured using a Porometer (Coulter Electronics). A 25 mm membrane disc was wetted out using "Porofill” and then exposed to incrementally increasing air pressure. The pore size was determined using the "bubble point” technique.
  • the membranes were also tested for photocatalytic activity by means of a qualitative colour test using an aqueous solution containing paraquat ( 10 ' M) and methanol (4% v/v).
  • a small (10 x 2 cm) strip of the membrane was placed in a vessel containing the paraquat solution under nitrogen.
  • the vessel was exposed (at a distance of 10 cm) to a 365 nm UV source for 30 minutes.
  • Photocatalytic activity in the membrane produces a blue colour. This is caused by photoexcited electrons captured by the bipyridinium dication, giving an intensely blue radical monocation. and the holes formed by photo-excitation oxidise the methanol.
  • the overall photocatalytic process is thus:
  • Example 2 The results are shown in Table 1 which also includes results obtained using the untreated substrate membrane.
  • Example 2 The results are shown in Table 1 which also includes results obtained using the untreated substrate membrane.
  • Example 1 was repeated for the ceramic flat-sheet/metal hybrid membrane having a pore size of 0.1 ⁇ m save that the anatase dispersion was replaced by a 50% solution of (CH 3 CH(O)CO 2 NH 4 ) 2 Ti(OH) 2 in water.
  • the solution was obtained from Aldrich and was used as received. Furthermore the peak heating temperature in the furnace was 500°C as compared to 600°C for Example 1.
  • the coating process was therefore repeated, five times in total, to produce an overall loading of about 0.7 wt% titania.
  • the modified membrane demonstrated photocatalytic activity.
  • the permeability of the modified membrane to water was about 85%o of its original value, the coating was adherent, and the mechanical properties and filtration capability of the membrane were unaffected.

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Engineering & Computer Science (AREA)
  • Organic Chemistry (AREA)
  • Inorganic Chemistry (AREA)
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  • Dispersion Chemistry (AREA)
  • Manufacturing & Machinery (AREA)
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  • Environmental & Geological Engineering (AREA)
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Abstract

A composite porous membrane for use in water treatment comprises an inorganic support having interstices and porous inorganic films of sintered non-metallic particles carried by the support and bridging the interstices thereof. Exterior and interior surfaces of the membrane are provided with a photocatalyst which upon irradiation with electromagnetic radiation of the appropriate wavelength is effective oxidatively to decompose organic materials in water. The preferred photocatalyst is the anatase form of titania.

Description

MEMBRANE
The present invention relates to a membrane intended particularly, but not exclusively, for water purification.
Membranes are used to remove suspended solids from water (e.g. as obtained from a reservoir) before it is supplied to domestic households and other end users. However the water may also contain dissolved organic compounds (e.g. pesticide residues) which are not removed by the membrane and which either pass untreated to the end user or require a separate treatment for their removal.
Photocatalytic oxidation processes have been proposed for the purification of water, particularly potable water, but such processes have so far been hindered by a number of difficulties. If the photocatalyst is used as a suspended powder then clearly it must be removed, for example by filtration or centrifugation. and there is always a danger of catalyst residues remaining in the water. If on the other hand the photocatalyst is immobilised on a continuous solid surface then only a very small fraction of the active area of the catalyst is in contact with the water to be purified, making for an inefficient process.
It has been proposed to incorporate a photocatalyst in the bulk phase of an organic, polymeric membrane (which is used for removing suspended solids from water). However the majority of the catalyst is inaccessible, and the membrane itself becomes susceptible to photo-oxidative degradation.
It is an object of the present invention to obviate or mitigate the abovementioned disadvantages
According to a first aspect of the present invention there is provided a composite porous membrane comprising an inorganic support having interstices and porous inorganic films of sintered non-metallic particles carried by the support and bridging the interstices thereof wherein exterior and interior surfaces of the membrane are provided with a photocatalyst which upon irradiation with electromagnetic radiation of the appropriate wavelength is effective to oxidatively decompose organic materials in water.
According to a further aspect of the invention there is provided a method of purifying water comprising filtering the water with a membrane as defined for the first aspect of the invention and irradiating said membrane with electromagnetic radiation of said appropriate wavelength so as oxidatively to decompose organic materials in the water.
Membranes in accordance with the first invention are of high surface area, as provided by the sintered non-metallic particles carried by the support. The catalyst is provided both on external surfaces of the membrane and the internal surfaces of the pores. Therefore the membrane has a high surface area of catalyst. The fact that the support membrane comprises sintered non-metallic particles (e.g. ceramic particles) means that the membrane is oxidatively and hydrolytically stable.
The membranes have the dual capability of removing both suspended solids and dissolved organic materials from water to be purified. For the purpose of removing dissolved organic materials, the membrane is provided with an oxidising agent, e.g. oxygen, ozone, hydrogen peroxide etc., and is irradiated with electromagnetic energy of the appropriate wavelength. As a result, the photocatalyst causes oxidative decomposition of the dissolved organic materials.
In one embodiment of the invention for purifying water, the upstream side of the membrane may be irradiated with electromagnetic wavelength of the appropriate wavelength. Any paniculate matter accumulating on the surface of the catalyst will also be destroyed by the photocatalytic process so that the membrane is effectively self cleaning.
In a further embodiment of the invention for purifying water, the downstream side of the membrane is irradiated. In this case, the membrane acts simultaneously as a microfilter (removing suspended solids on the upstream side of the membrane) and a photocatalyst to catalyse the oxidation of dissolved organic materials on the other side of the membrane.
Typically the membrane in accordance with the invention will comprise 0.01% to 30% (more usually 0.2% to 10%) by weight of the photocatalyst based on the total weight of the membrane.
The catalyst is preferably one for which irradiation with visible or ultra violet radiation (e.g. near - uv solar radiation) is effective to cause photocatalytic, oxidative decomposition of dissolved organic materials. If near - uv solar radiation is effective to cause the decomposition then there is the possibility of using sunlight as a photon source.
The catalyst is preferably the anatase form of titania. Anatase is the less thermodynamically-stable form of titania, the other principal form of which (rutile) is substantially less photocatalytically active.
The irradiation of anatase at a wavelength (in the ultraviolet range) for which the photon energy is greater than the semiconductor bandgap results in electron excitation from the valence band to the conduction band. The electron-hole pairs so created have considerable potential for redox chemistry, especially for generation of the powerfully oxidising hydroxyl radical which is capable of attacking and degrading most organic compounds. Photo-excitation of anatase powder in aqueous suspension is known to result in the complete, catalytic, oxidative degradation ("mineralisation") of organic contaminants including herbicides and pesticides which may be present in the water. Typical end products are carbon dioxide, and nitrate, sulphate, and halide ions. At the same time, the reduction potential of irradiated titania can result in soluble metal ions in high oxidation states being reduced to insoluble, less hazardous forms.
Membranes in accordance with the invention preferably have a pore size in the range 0.01 - lOμm.
The membranes in accordance with the invention may be produced from a substrate membrane (to which the photocatalyst is applied, e.g. using the techniques described below) comprising said inorganic support and said porous inorganic films of sintered non-metallic particles bridging interstices of the support. Such substrate membranes may be as disclosed in EP-A-0 348 041. A particularh suitable substrate membrane comprises a chemically-resistant metal mesh or an alloy such as inconel, supporting a porous film of zirconia (ZrO2) having a pore-size in the range of 0.01 - lOμm.
Preferably the substrate membrane has an effective pore size in the range 0.01 to 10 microns, more preferably 0.05 to 0.5 μm. The membrane in accordance with the invention may be produced in a number of ways. Thus the substrate membrane may be treated (e.g. by immersion) with a suspension of the photocatalyst which is to be incorporated in the membrane of the invention or with a solution or suspension of a precursor thereof which is decomposable by heat to give the final form of the catalyst. The treated membrane is then heated at a temperature which is sufficiently high to effect any necessary conversion of catalyst precursor and to provide sintered catalyst on exterior and interior surfaces of the membrane. The temperature should not be such that catalytic activity is lost.
As indicated, the preferred catalyst is anatase. In order to produce a membrane in accordance with the invention incorporating anatase as the catalyst, the substrate membrane may initially be immersed in a suspension of anatase particles. The dispersed particles preferably have a size of about 10 to 100 nm, more preferably 10 to 40 nm, most preferably about 20 nm. A suitable anatase may be obtained in Degussa P25. If the anatase as supplied contains larger aggregates of such particles then such aggregates should be 'broken down' so as to provide the primary particles in dispersion, e.g. by using ultrasonics. The dispersion preferably comprises 1% to 10% by weight of the anatase (more preferably 4 to 6%) and most preferably about 5% on the same weight basis. The dispersion preferably also incorporates a binder in an amount less than that of the anatase. The binder serves to increase viscosity and hold the particles in suspension. Typically the binder will be used in an amount of 2-5% by weight. Examples of suitable binders include methylcellulose and xanthan gum. Preferably the dispersion also includes a trace amount (e.g. less than 0.1% by weight) of a surface active agent, e.g. Miravon B79.
The substrate membrane will usually be dipped in the abovedescribed dispersion for a period of 15 to 25 seconds and then allowed to dry to the extent that visually the membrane appears dry. A drying period of about 1 hour at ambient temperature will generally be suitable.
In the subsequent production step, the treated membrane is heated in a furnace to effect sintering of the anatase without conversion to rutile. A peak furnace temperature of 300-600°C will generally be used. The temperature of the treated membrane will be increased to this peak temperature over a period of 2 to 3 hours and the membrane will then be allowed to dwell at the peak temperature for about 1 hour before being cooled in the furnace. The overall firing procedure will generally take about 8-9 hours.
As an alternative to using an anatase dispersion, it is possible to treat the substrate membrane with a catalyst precurosr, e.g. with Titanium (IV) bis/ammonium lactato)dihydroxide of formula (CH3CH(O)CO2NH4)2Ti(OH)2 which may be used as an aqueous solution, preferably at a concentration of 40-60% by weight, ideally about 50% by weight. The treated substrate membrane is allowed to dry and then heated in a furnace for similar temperature and time conditions to those disclosed above.
A further method of applying anatase to the substrate membrane is to treat the latter with a titanium alkoxide dissolved in a volatile organic solvent, and then allowing the membrane to dry, the procedure normally being effected under nitrogen. The heating step to decompose the alkoxide may be effected in air at a temperature which is sufficient to cause hydrothermal conversion of the precursor to the anatase form of titania but not so high as to convert the titania to the rutile form. Typically the temperature will be in the range 200-600°C and maintained for 1 - 10 hours.
This process of treatment with titanium alkoxide and subsequent heat decomposition may be repeated until an optimum thickness of the catalyst is achieved. Thus, for example, photocatalytic activity may not be observed after a single coating, but after several coatings an active catalyst surface will be produced. The reduction in permeability after several coating cycles is normally additive, so that for example after 5 cycles the permeability of the substrate membrane would be reduced by some 15-25%).
The alkoxide group may, for example, contain from 1 to 10 carbon atoms. Most preferably the alkoxide is the iso-propoxide. The conversion of titanium tetra- isopropoxide to titania is represented by the following equation:
TI(OPr')4 + 2H20 TiO, + 4Pr'OH The invention is illustrated by the following non-limiting Examples. In the Examples.
Example 1
A dispersion of the following composition
He Quantity (% bv weight
Degussa P25 (anatase) 5
Methyl cellulose 3
Miravon B79 <0.
Deionised Water 92
was prepared by drying mixing P25 powder and methyl cellulose in a ball mill for 30 minutes. The water and Miravon were added to the P25/methyl cellulose mixture and ball milling continued for 5 hours. The resultant mixture was treated with ultrasonics for 15 minutes to complete dispersion of the P25.
Ceramic flat-sheet/metal hybrid membranes as described in EP 348041 (zirconia on inconel) were then dipped in the dispersion. One membrane had a pore size of 0.1 μm and the other had a pore size of 0.2μm. Dipping was effected by immersing the membranes into the dispersion at an immersion speed of 1 cm/sec, allowing 20 seconds immersion, and then withdrawing the membrane at a speed of 2.5 cm/min.
The treated membranes were then allowed to dry (as assessed visually) before firing in a furnace. The membranes were heated in the furnace at a rate of 5°C/min until a temperature of 600°C was reached which was then maintained for 1 hour prior to cooling at a rate of 5°C/min to room temperature. The weight gain of the 0.2μm membrane was found to be 1.9%.
The fired membranes were then tested to establish water flux and pore size characteristics using the following procedures: (I) Water Flux:- A 25 mm membrane disc was placed in a Swinnex type holder and the flux of filtered (0.2μm) de-ionised water at 25°C and 1 bar pressure through the membrane measured.
(ii) Pore size:- This was measured using a Porometer (Coulter Electronics). A 25 mm membrane disc was wetted out using "Porofill" and then exposed to incrementally increasing air pressure. The pore size was determined using the "bubble point" technique.
The membranes were also tested for photocatalytic activity by means of a qualitative colour test using an aqueous solution containing paraquat ( 10' M) and methanol (4% v/v). A small (10 x 2 cm) strip of the membrane was placed in a vessel containing the paraquat solution under nitrogen. The vessel was exposed (at a distance of 10 cm) to a 365 nm UV source for 30 minutes. Photocatalytic activity in the membrane produces a blue colour. This is caused by photoexcited electrons captured by the bipyridinium dication, giving an intensely blue radical monocation. and the holes formed by photo-excitation oxidise the methanol. The overall photocatalytic process is thus:
Q1 Λ ♦ 211 *
Figure imgf000009_0001
colourless eeo D i je
(The photoactivity colour-test was validated using suspended anatase powder, which gave a strong response after 10 minutes illumination with a 300 W Phillips MLU- UV source).
The results are shown in Table 1 which also includes results obtained using the untreated substrate membrane. Example 2
Example 1 was repeated for the ceramic flat-sheet/metal hybrid membrane having a pore size of 0.1 μm save that the anatase dispersion was replaced by a 50% solution of (CH3CH(O)CO2NH4)2Ti(OH)2 in water. The solution was obtained from Aldrich and was used as received. Furthermore the peak heating temperature in the furnace was 500°C as compared to 600°C for Example 1.
The results are included in table 1.
Table
Flux Mean Photocat
Membrane 0/m2/hr/bar) Pore Size ( μm) Activity
0.1 μm substrate 555 0.10 -
0.2μm substrate 1653 0.19 -
0.1μm / 5% P25 (Ex. 1) 347 < 0.08 Distinct
0.2μm / 5% P25 (Ex. 1) 805 0.10 Distinct
0.2μm / (Ex.2) 740 0.17 Distinct
(CH3CH(0)C02NH4),Ti(OH)2
Example 3
A weighed sample of a flat-sheet ceramic/metal hybrid membrane, as described in EP 348041 (0.1 mm pore-size; zirconia on inconel). was dipped in a 5% (w/v) solution of titanium tetra-isopropoxide in anhydrous ethanol, drained, and allowed to dry in a nitrogen atmosphere. It was transferred to a furnace and heated in air, the temperature being raised from room temperature to 400°C over some 2 hours. After a further 2 hours at 400°C, the samples were cooled and evaluated for weight-increase, mechanical cohesion, permeability to water at 1 bar. and photo-activity. After a single dipping/drying/firing cycle, the above process gave a titania loading of some 0.15 wt%. The membrane was still cohesive, flexible, and hydrophilic, and its permeability to water had fallen by some 2 - 3%. However, it displayed no evidence of photo-activity.
The coating process was therefore repeated, five times in total, to produce an overall loading of about 0.7 wt% titania. The modified membrane demonstrated photocatalytic activity. The permeability of the modified membrane to water was about 85%o of its original value, the coating was adherent, and the mechanical properties and filtration capability of the membrane were unaffected.
A control experiment after removing the modified membrane from the reactor showed that the result was not due to paniculate or colloidal titania being lost from the surface, and a second control using the unmodified zirconia membrane confirmed that photo-activity originated in the applied TiO2 coating. Finally, the active membrane was rinsed, dried, and re-tested. No loss of photocatalytic-activity had occurred.

Claims

1. A composite porous membrane comprising an inorganic support having interstices and porous inorganic films of sintered non-metallic particles carried by the support and bridging the interstices thereof wherein exterior and interior surfaces of the membrane are provided with a photocatalyst which upon irradiation with electromagnetic radiation of the appropriate wavelength is effective to oxidatively decompose organic materials in water.
2. Membrane as claimed in claim 1 wherein the catalyst is one for which irradiation with visible or ultra violet radiation is effected to cause decomposition of dissolved organic materials.
3. Membrane as claimed in claim 1 or 2 comprising 0.01% - 30%, preferably 0.2 to 10% by weight of the catalyst based on the total weight of the membrane.
4. Membrane as claimed in any one of claims 1 to 3 having a pore size of 0.01 - lOμm.
5. Membrane as claimed in any one of claims 1 to 4 wherein the catalyst is the anatase form of titania.
6. A method of producing a membrane wherein a support membrane comprised of an inorganic support and porous inorganic films of sintered non-metallic particles bridging interstices of the support is treated with a suspension of the photocatalyst or with a solution or suspension of a precursor thereof which is decomposable by heat to give the final form of the catalyst, and heating the treated substrated membrane to effect any necessary conversion of catalyst precursor and to provide sintered photocatalyst on exterior and interior surfaces of the membrane.
7. A membrane as claimed in claim 6 wherein the substrate membrane is treated with a suspension of anatase.
8. A method as claimed in claim 7 wherein the suspension comprises 1% to 10% by weight of anatase.
9. A method as claimed in claim 8 wherein the suspension comprises 4% to 6% by weight of anatase.
10. A method as claimed in any one of claims 7 to 9 wherein the anatase has a particle size of about 10 to 100 nanometres.
1 1. A method as claimed in any one of claims 7 to 10 wherein the suspension incorporates a binder.
12. A method as claimed in claim 11 wherein the binder is present in an amount of 2 to 5% by weight of the suspension.
1 . A method as claimed in claim 11 or 12 wherein the binder is methylcellulose or xanthan gum.
14. A method as claimed in any one of claims 7 to 13 wherein the suspension incorporates a surface active agent.
15. A method as claimed in claim 14 wherein the surface active agent is used in an amount of less than 0.1 % by weight.
16. A method as claimed in claim 6 wherein the substrate membrane is treated with a solution of (CH3CH(0)C02NH4)2Ti(OH)2, and said heating is effective to produce the anatase form of titania.
17. A method as claimed in claim 16 wherein the solution contains 40 to 60% by weight of (CH3CH(O)CO2NH4)2Ti(OH)2.
18. A method as claimed in any one of claims 7 to 17 wherein the treated substrate membrane is heated to a peak temperature in the range 300 to 600°C.
19. Method as claimed in claim 6 wherein the substrate membrane is treated with a solution of a titanium alkoxide dissolved in a volatile organic solvent, said heating being effective to produce the anatase form of titania.
20. Method as claimed in claim 19 wherein the titanium alkoxide is titanium tetra- isopropoxide.
21. Method as claimed in claim 19 or 20 wherein the solvent is ethanol.
22. Method as claimed in any one of claims 19 to 21 wherein said heating is at a temperature of 200-600°C.
23. Method as claimed in any one of claims 7 to 22 comprising a plurality of successive cycles of treatment with the solution of photocatalyst precursor and heating to effect decomposition thereof.
24. A method of purifying water comprising filtering the water with a membrane as claimed in any one of claims 1 to 5 and irradiatmg said membrane with electromagnetic radiation of said appropriate wavelength so as oxidatively to decompose organic materials in the water.
25. A method as claimed in claim 24 wherein the upstream side of the membrane is irradiated.
26. A method as claimed in claim 24 wherein the downstream side of the membrane is irradiated.
PCT/GB1995/001839 1994-08-02 1995-08-02 Membrane WO1996004069A1 (en)

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EP0950025A4 (en) * 1996-12-31 2000-05-17 Uv Technologies Inc Water treatment and purification
RU2171708C1 (en) * 2000-10-23 2001-08-10 Махмутов Фаниль Ахатович Composite inorganic porous membrane
EP2409954A1 (en) * 2010-07-20 2012-01-25 National Center for Scientific Research Demokritos Photocatalytic purification device
CN104874298A (en) * 2015-05-25 2015-09-02 天津理工大学 Method for preparing nanometer ZnS/cellulose complex film with photocatalytic activity
CN111359450A (en) * 2020-03-19 2020-07-03 海加尔(厦门)科技有限公司 Ceramic ultrafiltration membrane with efficient photocatalytic function and preparation method thereof

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Cited By (7)

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EP0950025A4 (en) * 1996-12-31 2000-05-17 Uv Technologies Inc Water treatment and purification
RU2171708C1 (en) * 2000-10-23 2001-08-10 Махмутов Фаниль Ахатович Composite inorganic porous membrane
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WO2012010645A1 (en) * 2010-07-20 2012-01-26 National Center For Scientific Research Demokritos Photocatalytic purification device
CN104874298A (en) * 2015-05-25 2015-09-02 天津理工大学 Method for preparing nanometer ZnS/cellulose complex film with photocatalytic activity
CN111359450A (en) * 2020-03-19 2020-07-03 海加尔(厦门)科技有限公司 Ceramic ultrafiltration membrane with efficient photocatalytic function and preparation method thereof
CN111359450B (en) * 2020-03-19 2022-02-22 海加尔(厦门)科技有限公司 Ceramic ultrafiltration membrane with efficient photocatalytic function and preparation method thereof

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