Method for producing a filter intended to filter nanoparticles, obtained filter and associated method for the collection and quantitative analysis of nanoparticles

Abstract

The present invention relates to a method for impregnating a filter having pores suitable for retaining particles within them that may be present in a flow of air suitable for passing through the filter, according to which the filter made up of a polymer membrane is impregnated with one or more organometallic salts by applying a treatment using supercritical CO.sub.2, the metal M of each salt being chosen from among the group of rare earths, yttrium, scandium, chromium, or a combination thereof. The invention also relates to the obtained filter and an associated method for the collection and quantitative analysis of nanoparticles.

Claims

1. A process for the impregnation of a filter comprising pores which are capable of retaining, within them, particles liable to be present in an air stream intended to pass through the filter, according to which the filter composed of a polymer membrane is impregnated with one or more organometallic salts by applying a treatment with supercritical CO.sub.2, the metal M of each salt being selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, yttrium, scandium, chromium, and a combination of these.

2. The process for the impregnation of a filter as claimed in claim 1, according to which the treatment with supercritical CO.sub.2 is carried out in an autoclave, the organometallic salt(s) being placed inside the autoclave at a distance from the filter comprising pores.

3. The process for the impregnation of a filter as claimed in claim 2, according to which the treatment with supercritical CO.sub.2 consists of the following stages: a/ purging the inside of the autoclave with CO.sub.2 at ambient pressure and temperature, b/ pressurizing the inside of the autoclave to a pressure of between 73.8 and 350 bar, while maintaining the temperature at ambient temperature, c/ raising the temperature of the autoclave to a temperature between 31° C. and 150° C. over a period of time of between 20 minutes and 5 h, d/ maintaining at temperature and at pressure for a period of time of between 20 minutes and 5 hours, e/ naturally cooling the autoclave until ambient temperature is reached, f/ carrying out a calibrated escape of the CO.sub.2 towards the outside of the autoclave until ambient pressure is reached.

4. The process for the impregnation of a filter as claimed in claim 1, according to which a proportion by weight of the organometallic salt(s) with respect to the weight of the filter of less than or equal to 1% is selected.

5. A filter comprising pores which are capable of retaining, within them, nanoparticles liable to be present in an air stream intended to pass through the filter, the filter being composed of a polymer membrane impregnated with one or more organometallic salts, the metal M of each salt being selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, yttrium, scandium, chromium, and a combination of these, the filter being liable to be obtained according to the process as claimed in claim 1.

6. The filter comprising pores as claimed in claim 5, the constituent polymer of the membrane being chosen from saturated polyesters.

7. The filter comprising pores as claimed in claim 6, made of polycarbonate (PC).

8. The filter comprising pores as claimed in claim 5, having a thickness of between 10 and 50 μm.

9. The filter comprising pores as claimed in claim 5, the pores of the filter being holes with a calibrated diameter of between 0.05 and 2 μm.

10. The filter comprising pores as claimed in claim 5, the density of the holes of the filter being between a number of 10.sup.5 and 5×10.sup.8 holes per cm.sup.2.

11. The filter comprising pores as claimed in claim 5, the organometallic salt(s) being a complex of metallocene type, of formula M(Cp).sub.n, in which n is an integer between 1 and 4 and Cp is a cyclopentadiene group.

12. The filter comprising pores as claimed in claim 5, the organometallic salt(s) being a complex in which the metal has been subjected to chelation by groups carrying heteroelements chosen from amines and phosphines.

13. The filter comprising pores as claimed in claim 12, the heteroelements being substituted by alkyl chains with at least four carbons.

14. The filter comprising pores as claimed in claim 5, the organometallic salt(s) being a complex of formula M(PR).sub.n, where n is an integer between 1 and 4 and R is a fluorinated, substituted, or branched aryl group or alkyl group.

15. The filter comprising pores as claimed in claim 5, the organometallic salt(s) being a complex of amide type of formula M(NR.sub.2).sub.n, where n is an integer between 1 and 4 and R is a silyl-aliphatic group or an aromatic group.

16. The filter comprising pores as claimed in claim 5, the organometallic salt(s) being a complex of phenanthroline or bathophenanthroline type of formula M(Phen).sub.n, where n is an integer between 1 and 4 and Phen is phenanthroline.

17. The filter comprising pores as claimed in claim 5, the organometallic salt(s) being a complex of carboxylate type of formula M(OOCR).sub.n, where n is an integer between 1 and 4 and R is an alkyl chain with at least four carbons.

18. The filter comprising pores as claimed in claim 5, the organometallic salt(s) being a complex of β-diketonate type of formula M(RCOCH.sub.2COR).sub.n, where n is an integer between 1 and 4 and R is an alkyl chain with at least four carbons.

19. The filter comprising pores as claimed in claim 5, the organometallic salt(s) being a complex of alkoxide type of formula M(OR).sub.n, where n is an integer between 1 and 4 and R is an alkyl chain with at least four carbons.

20. The filter comprising pores as claimed in claim 5, the organometallic salt(s) being a complex of β-ketocarboxylate type of formula M(OOCCH.sub.2COR).sub.n, where n is an integer between 1 and 4 and R is an alkyl chain with at least four carbons.

21. The filter comprising pores as claimed in claim 5, the organometallic salt(s) being a complex of phosphonate type of formula M(O.sub.2(O)PR).sub.n, where n is an integer between 1 and 4 and R is an alkyl chain with at least four carbons.

22. The filter comprising pores as claimed in claim 5, the organometallic salt(s) being a complex of sulfonate type of formula M(O.sub.3SR).sub.n, where n is an integer between 1 and 4 and R is an alkyl chain with at least four carbons.

23. The filter comprising pores as claimed in claim 17, the alkyl chain being fluorinated.

24. A process for the collection and analysis of nanoparticles according to which the following stages are carried out: suction of an air stream liable to be laden with nanoparticles through the filter as claimed in claim 5 with an organometallic salt impregnated therein; introduction of the filter into an X-ray fluorescence spectrometer; quantitative analysis by X-ray fluorescence of the nanoparticles retained by the filter by division between the fluorescence signal emitted by a chemical element of the nanoparticles and the fluorescence signal emitted by the organometallic salt.

Description

DETAILED DESCRIPTION

(1) Other advantages and characteristics will emerge more clearly on reading the detailed description, made by way of illustration and without implied limitation, with reference to the following figures, among which:

(2) FIG. 1 is a diagrammatic view of an installation for the implementation of a process for the impregnation of a filter according to the invention;

(3) FIGS. 2A and 2B are photographic reproductions of the inside of an autoclave used in an installation according to FIG. 1, these reproductions showing devices for positioning, in the autoclave, filters and organometallic salts in accordance with the invention;

(4) FIGS. 3A and 3B represent an X-ray fluorescence spectrum of a polycarbonate filter, respectively before and after impregnation according to the invention with a chromium salt;

(5) FIGS. 4A and 4B represent a measurement signal, measured by X-ray fluorescence spectrometry, of a polycarbonate filter in accordance with the invention as a function of the amounts of zinc oxide (ZnO) nanoparticles charged beforehand to the filter, according to a measurement respectively without correction and with correction by the signal of the chromium salt impregnated into the filter;

(6) FIGS. 5A and 5B are images by scanning microscopy at the surface of a polycarbonate filter, respectively before and after impregnation according to the invention with a chromium salt.

(7) An example of a filter used to collect the nanoparticles according to the invention is a microporous membrane, made of polycarbonate, with a thickness of a few tens of microns and pierced by a multitude of holes of controlled diameter. By way of example, the holes of controlled diameter have a diameter of 0.4 μm and exhibit a density of holes of 10.sup.5 perforations per cm.sup.2. Such a microporous filter makes it possible to achieve retention efficiencies of greater than 99.5% of the nanoparticles in suspension in the air, with a diameter of between 10 and 300 nm, with a collection flow rate of between 0.1 and 10 l.Math.min.sup.−1.

(8) In order to make it possible to pull such a filter flat and to keep it flat under mechanical tension, it is possible advantageously to produce a filtration assembly with a filter support in accordance with the patent application FR 12 55785.

(9) Although not represented, such a filtration assembly is intended to be fitted into a suitable sampling cartridge in order to allow the sunctioned air stream to pass through the filter in order to carry out the collecting proper of the nanoparticles.

(10) According to the invention, the filter is impregnated with one or more organometallic salts by applying a treatment with supercritical CO.sub.2, the metal M of each salt being chosen from the group of the rare earth metals, yttrium, scandium, chromium or a combination of these.

(11) In order to do this, the installation 1 shown diagrammatically in FIG. 1 is used: it comprises an autoclave 2, the inlet of which is connected to a CO.sub.2 feed line and an outlet of which is connected to a CO.sub.2 extraction line. In order to open/close, the inlet and outlet of the autoclave 2 are provided with valves 3. Finally, a manometer 4 makes it possible to monitor the pressure prevailing inside the autoclave 2.

(12) By way of example, the autoclave has a reactor part in the form of a cylinder with an internal diameter of 50 mm and a height of 100 mm, made of stainless steel 316: it is capable of withstanding temperatures up to 150° C. and pressures ranging up to approximately 210 bar.

(13) The valves 3 may be valves of needle gate type.

(14) Before carrying out the impregnation proper, organometallic salts according to the invention, which have been weighed beforehand, are placed in a porcelain crucible 5 inserted at the bottom of the autoclave, as seen in FIG. 2A. By way of example, the weight of Cr salts placed in the crucible may be between 5 and 10 mg.

(15) A pan 6 is then installed above the crucible 5, as seen in FIG. 2B.

(16) A plurality of commercial PC filters, free from any other material, are stacked with their conditioning separators, this being done in order to avoid any direct contact between them. By way of example, the commercial PC filters may be those sold under the Isopore commercial reference by Millipore.

(17) Thus, each filter is placed at a distance from another and also at a distance from the organometallic salts.

(18) In other words, the organometallic salts are weighed and introduced into the crucible 5 itself at the bottom of the autoclave 2. The pan-support 6, comprising the PC filters separated from one another by conditioning separator membranes, is placed over the top, without direct contact with the salts.

(19) The following stages are then carried out: purging the autoclave 2 with CO.sub.2, closing the outlet valve 3, pressurizing the autoclave 2 to approximately 60 bar at ambient temperature, placing the autoclave 2 under pressure in an oven, not represented, at 70° C. for a period of time of one hour: the pressure then rises to approximately 150 bar over the course of one hour, maintaining at this temperature of 70° C. and pressure of 150 bar for half an hour, removing the autoclave from the oven, naturally cooling to ambient temperature, applying a controlled escape for the decrease in pressure.

(20) The PC filters according to the invention are then impregnated throughout their bulk with the organometallic salts.

(21) FIGS. 3A and 3B represent the X-ray fluorescence spectra obtained before and after doping, with chromium Cr, a polycarbonate PC filter, free from any other material. A peak corresponding to the Ka re-emission line of chromium at 5.4 keV may be observed after doping.

(22) FIGS. 4A and 4B a measurement signal, measured by X-ray fluorescence spectrometry, under grazing incidence, of a polycarbonate (PC) filter in accordance with the invention as a function of the amounts of zinc oxide (ZnO) nanoparticles charged beforehand to the filter by aerosol sampling, according to a measurement respectively without correction and with correction by the signal of the chromium salt impregnated in the filter.

(23) Thus, the raw measurement signal of the chemical element retained in the filter, zinc Zn (FIG. 4A), is divided by that of the chromium salt impregnated in the PC filter. The corrected signal is thus linearized as a function of the concentration of the zinc (FIG. 4B). Using this straight calibrating line, it is thus possible to determine, from the signal measured, the precise amount of a sample of unknown concentration of Zn.

(24) By way of example, if a corrected signal of 12 counts per second is measured, it is possible to quantify the prior charging density of the filter with ZnO nanoparticles equal to approximately 4.3 μg/cm.sup.2.

(25) It is possible to proceed thus with different chemical elements liable to occur in the form of nanoparticles in suspension in an aerosol.

(26) Thus, the process for the impregnation of the filter with supercritical CO.sub.2 according to the invention makes it possible to incorporate organometallic salts in the body of the filter in order to use their response in XRF fluorescence to calibrate the raw spectra of the chemical elements present in the particles to be collected by the filter and consequently to be able to quantify them.

(27) In order to confirm the impact of the treatment with supercritical CO.sub.2 according to the invention undergone on the polycarbonate constituting the filter, the surface condition of the latter was displayed by scanning electron microscopy (SEM), respectively before and after said treatment.

(28) The aim of this confirmation is to ensure that the treatment with supercritical CO.sub.2 according to the invention does not modify the mechanical properties of a PC filter and in particular that the size of the pores is well preserved.

(29) FIGS. 5A and 5B respectively present the images of a fresh PC filter and of the same filter after treatment with supercritical CO.sub.2 according to the invention.

(30) A modification to the structure of the surface of the filter with a slight deterioration in flatness is observed, which is not disadvantageous to the analysis by XRF.

(31) The size of the pores does not appear to be affected by the treatment.

(32) This is corroborated by measurement of the pressure drop generated by the filter. Thus, when an air stream of 1 l.Math.min.sup.−1 is passed through a PC filter, the pressure drop does not vary after treatment with supercritical CO.sub.2 and remains substantially constant at approximately 50 mbar.

(33) The invention is not limited to the examples which have just been described; it is possible in particular to combine, with one another, characteristics of the examples illustrated within alternative forms not illustrated.

REFERENCES CITED

(34) [1]: Ying Zhang and Can Erkey, “Preparation of supported metallic nanoparticles using supercritical fluids: a review”, J. of Supercritical Fluids, 38 (2006), 252-267. [2]: D. Kim, J. Sauk, J. Byun, K. S. Lee and H. Kim, “Palladium composite membranes using supercritical CO.sub.2 impregnation method for direct methanol fuel cells”, Solid State Ionics, 178 (2007), 865-870.