EP0630356A1 - Preparation d'alumine stabilisee presentant une resistance accrue a la perte de superficie aux temperatures elevees - Google Patents

Preparation d'alumine stabilisee presentant une resistance accrue a la perte de superficie aux temperatures elevees

Info

Publication number
EP0630356A1
EP0630356A1 EP93907334A EP93907334A EP0630356A1 EP 0630356 A1 EP0630356 A1 EP 0630356A1 EP 93907334 A EP93907334 A EP 93907334A EP 93907334 A EP93907334 A EP 93907334A EP 0630356 A1 EP0630356 A1 EP 0630356A1
Authority
EP
European Patent Office
Prior art keywords
alumina
stabilizer
boehmite alumina
gel
boehmite
Prior art date
Legal status (The legal status 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 status listed.)
Ceased
Application number
EP93907334A
Other languages
German (de)
English (en)
Other versions
EP0630356A4 (fr
Inventor
Lewis Bernard Decker, Jr.
Kevin J. Mclaughlin
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Sasol North America Inc
Original Assignee
Vista Chemical Co
Condea Vista Co
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 Vista Chemical Co, Condea Vista Co filed Critical Vista Chemical Co
Publication of EP0630356A1 publication Critical patent/EP0630356A1/fr
Publication of EP0630356A4 publication Critical patent/EP0630356A4/fr
Ceased legal-status Critical Current

Links

Classifications

    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00—Compounds of aluminium
    • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00—Compounds of aluminium
    • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
    • C01F7/021—After-treatment of oxides or hydroxides
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/12—Surface area
    • C01P2006/13—Surface area thermal stability thereof at high temperatures
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/14—Pore volume
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/16—Pore diameter
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/16—Pore diameter
    • C01P2006/17—Pore diameter distribution

Definitions

  • the present invention relates to a process for producing alumina which can be converted to catalyst supports exhibiting enhanced resistance to loss of surface area when subjected to high temperatures.
  • a particular problem with autocatalyst supports involves the high temperatures to which the supports are subjected. High temperatures deleteriously effect the structural integrity of the catalyst support resulting in a loss of surface area.
  • stabilizers such as oxides of barium and the lanthanide series of elements can stabilize autocatalysts in the sense that the loss of structural integrity of the support is retarded.
  • oxides of barium, lanthanum or other lanthanide elements have been used in alumina based autocatalyst supports as heat
  • a stabilized alumina of enhanced resistance to high temperature surface area loss is prepared by forming a gel of a boehmite alumina, the boehmite alumina being obtained by hydrothermally treating an aqueous mixture of a precursor boehmite alumina having a pH of from about 5 to about 9 for a period of time sufficient to convert the greater portion of the precursor boehmite alumina to a colloidal sol.
  • the gel is subjected to working, i.e. by using a sufficient shearing force for a sufficient period of time to produce a worked boehmite alumina and increase the pore volume by at least 30 percent and the median pore radius by at least 20 percent.
  • a stabilizer is added to the boehmite alumina, the stabilizer being an oxide of a metal such as barium or a metal included in the lanthanide series of metals or a compound of such metals which converts to an oxide at elevated temperatures. Mixtures of such stabilizers can be employed if desired, the amount of the stabilizer used being sufficient to decrease loss of porosity of a calcined alumina produced from the worked alumina.
  • the stabilizer can be added to a calcined product obtained by calcining the worked (sheared) boehmite alumina.
  • the aluminas which can be treated according to the process of the present invention are boehmite aluminas which have been hydrothermally treated under conditions to convert the greater portion of the boehmite alumina to a colloidal sol, the thus hydrothermally treated aluminas forming the starting material boehmite alumina for use in the process of the present invention.
  • the beohmite alumina which is hydrothermally treated, hereinafter referred to as precursor boehmite alumina is preferably, although not necessarily, obtained by the hydrolysis of an aluminum alkoxide in the well known fashion.
  • the aluminum alkoxide can be produced, in the well known manner, by reacting a low molecular weight alcohol, a linear or branched chain, with an aluminum-bearing material.
  • Such aluminum- bearing materials include pure aluminum and mixed alloy scrap. Typical methods for preparing such aluminum alkoxides are shown, for example, in U.S. Patent No. 4,242,271 , incorporated herein by reference for all purposes.
  • the aluminum alkoxide can be hydrolyzed in the well known manner, such as by the process taught in U.S. Patent No. 4,202,870, incorporated herein by reference for all purposes.
  • aluminas obtained from the hydrolysis of aluminum alkoxides derived from Ziegler Chemistry in the well known manner.
  • alumina slurry particularly a slurry produced by the hydrolysis of aluminum alkoxides
  • aluminas from other sources can be formed into slurries and hydrothermally treated to produce the precursor alumina.
  • the starting material boehmite alumina used in the process of the present invention can be obtained according to the process disclosed and claimed in U.S. Patent No. 4,676,928, incorporated herein by reference for all purposes.
  • the process disclosed in U.S. Patent No. 4,676,928 involves taking a precursor boehmite alumina, forming the precursor alumina into an aqueous slurry or mixture, the pH being in the range of from about 5 to about 9, and then heating the aqueous slurry of the precursor alumina at elevated temperatures, generally about 70 °C or greater, for a sufficient period of time to convert the greater portion of the precursor boehmite alumina to a colloidal sol.
  • a colloidal sol can be employed.
  • a colloidal sol which has been dried to form a dried powder can be formed into an aqueous dispersion and used.
  • the alumina content will range from about 15 to about 55 percent-by- weight calculated as Al 2 O 3 , depending on whether or not a gelling agent is employed.
  • the gel will normally contain from about 15 to about 25 percent- by-weight Al 2 O 3 .
  • the gel will generally contain from about 35 to about 55 percent-by-weight Al 2 O 3 .
  • the process is conducted by forming an aqueous slurry or dispersion, either as the sol as described above, or by dispersing a dried sol in an aqueous medium.
  • a slurry of the starting material boehmite alumina Once the slurry of the starting material boehmite alumina has been formed, it must be gelled or thickened to increase the viscosity prior to being worked.
  • gel refers to a suspension, colloidal in nature, in which shearing stresses below a certain finite value fail to produce permanent deformation. Gelling of the alumina slurry can be carried out simply by concentrating the slurry by the removal of water to form a viscous gel of increased alumina content.
  • the gelling of the dispersion can be carried out by the addition of gelling agents.
  • gelling agents are generally water-soluble compounds which are well known by those skilled in the art to be compounds which will de- stabilize aqueous colloidal systems.
  • Non-limiting examples of such gelling agents include mineral acids such as nitric acid, hydrochloric acid, etc., organic acids such as formic acid, acetic acid, etc., polyvalent metal salts, etc.
  • water- soluble salts of certain polyvalent metals such as the nitrates, chlorides, acetates, sulfates, etc., of metals such as aluminum, iron, magnesium, manganese, etc. can be used.
  • such gelling agents When employed, such gelling agents will be added in an amount sufficient to increase the viscosity to the desired degree, i.e. until a gel is formed, amounts of from about 0.1 to about 50 percent-by-weight based on the weight of alumina in the gel being generally used.
  • alumina dispersion it is generally necessary, when viscosifying the alumina dispersion, whether such be accomplished by concentrating the dispersion and/or the addition of gelling agents, to add sufficient acid to maintain the gelled alumina in a flowable condition.
  • monobasic acids such as nitric acid, hydrochloric acid, formic acid, acetic acid, and so forth can be employed.
  • the amount of acid added should be kept to a minimum, consistent with achieving desired gelling, as increased acid decreases porosity.
  • Working or shearing of the gel to the desired extent can be accomplished in a variety of equipment and under widely varying conditions.
  • any apparatus which is capable of imparting high shear to viscous systems can be employed.
  • apparatus which can be used to carry out the working or shearing step include plastic melt viscometers, mullers commonly used for mixing paste-like materials, driers for preparing high viscosity pastes and gels and the like. Parameters such as shear rate, shear time, temperature, etc.
  • the concentration of alumina in the gel will vary depending upon the concentration of alumina in the gel, the type of gelling agent employed, the type of precursor boehmite employed and the type of hydrothermal treatment applied to the precursor alumina to obtain the staring material boehmite used in the process of the present invention.
  • conditions of high shearing, high concentration of alumina in the gel and minimum acid concentration are preferred.
  • Temperature can vary widely as from ambient to about 100°C.
  • the gel will be subjected to a sufficient shearing force, for a sufficient period of time to increase the pore volume by at least 30% and the median pore radius by at least 20% over that of the alumina in the unworked gel.
  • Such increase in porosity parameters can be determined by techniques well known to those skilled in the art.
  • 4,676,928 exists in the form of extensive aggregates of individual crystallites of relatively small size, i.e. less than about 5 ⁇ A in thickness (020 plane). Such aluminas exhibit extensive aggregation of the crystallites, i.e. microgels. Aluminas which have been preparing according to the process of U.S. Patent No. 4,676,928, as seen by TEM, also exist as aggregates but unlike ordinary boehmite the microgels are made up of stacks of plate-like crystallites which are generally highly oriented.
  • Such staring material aluminas can be characterized as being comprised of microgels which are comprised of numerous, associated stacked crystallites on the order of from about 50 to about 150 nm in diameter, the individual crystallite size being on the order of from about 50 to about 15 ⁇ A in thickness (020 plane).
  • the process of the present invention includes the addition of a stabilizer to a boehmite alumina which has been worked, i.e. sheared, as described above.
  • stabilizer refers to a compound or process which acts to decrease or retard loss of surface area when the alumina, calcined to Al 2 O 3 , is subjected to elevated temperatures, i.e. 1000°C or greater, generally 1200°C or greater.
  • the stabilizer can be an oxide of barium, an oxide of a lanthanide metal such as lanthanum, cerium, etc., a compound of barium which is converted to an oxide upon heating at an elevated temperature or a compound of a lanthanide metal which is converted to an oxide at an elevated temperature.
  • Especially preferred stabilizers are oxides or barium or lanthanum, or a compound of barium or lanthanum which is converted to an oxide upon heating at an elevated temperature.
  • a compound of barium or a lanthanide metal which can be converted to the oxide is used rather than the oxide thereof. This permits the stabilizer to be incorporated in the form of an aqueous solution or dispersion ensuring more uniform distribution of the stabilizer throughout the alumina.
  • the stabilizer may be added at various points in the process.
  • the stabilizer can be added to the boehmite alumina prior to gelling, during the gelling or after the boehmite alumina is sheared.
  • the stabilizer can be added to the boehmite alumina prior to the boehmite alumina being worked or after the boehmite alumina is worked.
  • the worked boehmite alumina can be dried and the stabilizer added to the dried, worked beohmite alumina.
  • the worked boehmite alumina can be dried and calcined to produce a calcined product, i.e.
  • the stabilizer will be added in an amount sufficient to decrease loss of porosity of a calcined alumina which is subjected to elevated temperatures.
  • the amount of the stabilizer added will be such as to provide a stabilizer content of from about 0.5 to about 20 weight percent based on Al 2 O 3 whether in the boehmite alumina or in the calcined product.
  • the unexpected stability of alumina prepared according to the process of the present invention results from the fact that the starting material boehmite is comprised of aggregations of individual pseudoboehmite crystallites, the crystallites being of a generally larger size, i.e. from about 50 to about 150 A in thickness (020 plan), than the conventional boehmite aluminas wherein the individual crystallites are generally about 50 A and smaller in thickness (020 plan).
  • the individual crystallites are plate-like structures which are generally arranged in an ordered, stacked configuration as can be seen by transmission electron microscopy (TEM).
  • the individual crystallites become more randomly distributed, i.e. the stacks of crystallites are disoriented leaving voids or pores, i.e. greater porosity and higher surface area.
  • This porosity provides for a reactive, accessible surface yielding higher catalytic activity.
  • the incorporation of a stabilizer enhances the structural integrity of the alumina in the sense that when subjected to high temperature, the surface area remains, i.e. the alumina does not collapse upon itself.
  • a boehmite alumina which has been prepared in accordance with the process of U.S. Patent No. 4,676,928 or an equivalent wherein the alumina exists essentially as microgels comprising stacks of plate-like crystallites.
  • Such starting material aluminas can be characterized as being comprised of microgels which are comprised of numerous, associated stacked crystallites on the order of from about 50 to about 150 nm in diameter, the individual crystallite size being, as noted, on the order of from about 50 to about 15 ⁇ A in thickness (020 plan).
  • the process of the present invention can be used to make catalyst supports which retain a high surface area, i.e. about 50 m 2 /g or greater upon calcination at 1200 °C for three hours.
  • DISPAL ® aluminas used in the following examples are boehmite aluminas marketed by Vista Chemical Company and made in accordance with the teachings of U.S. Patent No. 4,676,928. In all cases surface area was obtained by the multi-point BET method.
  • a series of samples were prepared by adding a predetermined amount of a 62.8 percent-by-weight lanthanum nitrate hexahydrate solution to a predetermined amount of DISPAL ® 120 alumina sol or DISPAL ® 180 alumina powder.
  • the addition of the lanthanum solution resulted in gelation of the alumina sol.
  • the alumina/lanthanum mixture was then worked on a Haake Torque Rheometer. The material was then removed from the rheometer/mixer, dried over night at 70°C, and then fired at 1200°C for three hours. The firing temperature and time were selected, to mimic the conditions that cause loss of surface area and porosity collapse, i.e.
  • samples prepared in accordance with the process of the present invention wherein the alumina is worked, i.e. sheared, and contains a stabilizer exhibit high surface area retention, i.e. generally greater than about 50 ⁇ r/g even after being subjected to a temperature of 1200°C for three hours.
  • Sample 1 in which an unworked alumina containing stabilizer showed a surface area markedly less than 50 m 2 /g after being heated to 1200°C for three hours.
  • Examples 2-4 demonstrate that retention of high surface area of calcined products is not achieved with conventional boehmite aluminas.
  • the CATAPAL ® aluminas used are conventional aluminas marketed by
  • Example 2 100 g of CATAPAL A ® alumina and 452 g deionized water were placed in a Baker-Perkins Muller and sheared for 20 minutes. The resulting material was dried at 66°C and calcined three hours as 1200°C. The surface area on the calcined product was determined to be 5.8 m 2 /g.
  • a sample of DISPAL ® 18N4-80 alumina powder was calcined three hours at 1200°C and found to have a surface area of 4.7 m 2 /g.
  • Example 8 700 g DISPAL ® 18N4-80 alumina, 452 g deionized water and 75.24 lanthanum nitrate solution (61.1 wt. % lanthanum nitrate) were placed in a Baker- Perkin Muller and sheared for 20 minutes. The resulting material was dried at 66°C and calcined three hours at 1200 °C. The resulting calcined material was found to have a surface area of 52.9 n /g.
  • Example 8 the combination of working and stabilizing (Example 8) DISPAL ® alumina, i.e. aluminas prepared in accordance with the teaching of U.S. Patent No. 4,676,928, results in a dramatic increase in retained surface area of the final, calcined product, i.e. a surface area of greater than 50 m 2 /g is obtained even after the material has been subjected to a temperature of 1200°C for three hours.
  • DISPAL ® alumina i.e. aluminas prepared in accordance with the teaching of U.S. Patent No. 4,676,928, results in a dramatic increase in retained surface area of the final, calcined product, i.e. a surface area of greater than 50 m 2 /g is obtained even after the material has been subjected to a temperature of 1200°C for three hours.
  • DISPAL ® 18N4-20 alumina and 4.54 g barium acetate powder were mixed for 10 minutes and dried at 66°C.
  • the resulting powder was calcined three hours at 1200°C.
  • the calcined material was found to have a surface area of 63 m 2 /g.
  • Example 10 with the surface area of the calcined materials in Examples 5-7).
  • Example 9 the presence of barium stabilization alone gives a surface area of greater than 50 Mm 2 /g, barium presents certain toxicity problems not presented by the use of lanthanum.
  • Example 10 the use of both barium stabilization and working gives sharply increased retained surface area
  • Example 14 (50 wt. % aluminum nitrate, 50 wt. % deionized water) were mixed to form an alumina gel.
  • the gel was sheared on a Haake Torque Rheometer for 10 minutes at 60°C, 110 rpm.
  • the Al 2 O 3 content of the sheared gel was 26.8 percent.
  • 53.0 g of the sheared gel, 2.6 g barium acetate powder, and 80.0 g deionized water were mixed for 10 minutes and dried at 66°C.
  • the resulting powder was calcined three hours at 1200°C.
  • the calcined material was found to have a surface area of 67.8 m 2 /g.
  • Example 14 Example 14
  • Example 13 55.75 g of the sheared gel of Example 13 were dried at 66°C.
  • the resulting dried gel (18 g), 1.94 g lanthanum nitrate solution, and 80.0 g deionized water were mixed for 10 minutes and dried at 66°C.
  • the resulting powder was calcined three hours at 1200°C.
  • the calcined material was found to have a surface area of 47.7 m 2 /g.
  • Example 18 52.24 g of the sheared gel of Example 13 were dried at 66°C.
  • the resulting dried gel was calcined two hours at 250°C, followed by 24 hours at 600°C.
  • the resulting material was mixed for 10 minutes with 1.82 g lanthanum nitrate solution (61.1 wt.% lanthanum nitrate) and 20.0 g deionized water.
  • the slurry was dried at 66°C and the resulting powder calcined three hours at 1200°C.
  • the calcined material was found to have a surface area of 52.2 nr/g.

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  • Chemical & Material Sciences (AREA)
  • Organic Chemistry (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Geology (AREA)
  • Inorganic Chemistry (AREA)
  • Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)
  • Catalysts (AREA)

Abstract

Procédé de préparation d'alumine stabilisée présentant une plus grande aptitude à conserver sa superficie aux températures élevées. Selon le procédé, on prend un gel d'alumine de boehmite obtenu par le traitement hydrothermique d'un mélange aqueux d'une alumine de boehmite précurseur dont le pH est compris entre environ 5 et environ 9, pendant une durée suffisamment longue pour transformer en sol colloïdal une majeur partie de l'alumine de boehmite précurseur et on le soumet, par exemple, à un effort de cisaillement suffisant pour obtenir, après une durée suffisante, une alumine de boehmite traitée dont le volume des pores est supérieur d'au moins environ 30 %, et dont le rayon moyen des pores est supérieur d'au moins environ 20 %. On ajoute à l'alumine de boehmite un stabilisant constitué de l'oxyde d'un métal tel que le baryum ou un métal figurant parmi les lanthanides, ou bien d'un composé de ces métaux qui se transforme en oxyde à des températures élevées.
EP93907334A 1992-03-12 1993-03-10 Preparation d'alumine stabilisee presentant une resistance accrue a la perte de superficie aux temperatures elevees. Ceased EP0630356A4 (fr)

Applications Claiming Priority (5)

Application Number Priority Date Filing Date Title
US84988292A 1992-03-12 1992-03-12
US84974792A 1992-03-12 1992-03-12
US849882 1992-03-12
US849747 1992-03-12
PCT/US1993/002104 WO1993017968A1 (fr) 1992-03-12 1993-03-10 Preparation d'alumine stabilisee presentant une resistance accrue a la perte de superficie aux temperatures elevees

Publications (2)

Publication Number Publication Date
EP0630356A1 true EP0630356A1 (fr) 1994-12-28
EP0630356A4 EP0630356A4 (fr) 1995-06-14

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Family Applications (1)

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EP93907334A Ceased EP0630356A4 (fr) 1992-03-12 1993-03-10 Preparation d'alumine stabilisee presentant une resistance accrue a la perte de superficie aux temperatures elevees.

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Country Link
EP (1) EP0630356A4 (fr)
CA (1) CA2131795C (fr)
WO (1) WO1993017968A1 (fr)

Families Citing this family (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5389589A (en) * 1992-12-22 1995-02-14 Allied-Signal Inc. Barium-containing alumina
DE19634855C2 (de) * 1996-08-28 1998-07-02 Haldenwanger Tech Keramik Gmbh Verfahren zur Herstellung eines Oxidationsschutzes für poröse Keramiken auf der Basis von SiC und Si¶3¶N¶4¶
US6129904A (en) * 1996-09-19 2000-10-10 Sud-Chemie A.G. Aluminum oxide masses with very narrow pore radius distribution
CN102448606B (zh) * 2009-05-27 2014-03-26 株式会社科特拉 废气净化用催化剂
GB201220912D0 (en) 2012-11-21 2013-01-02 Johnson Matthey Plc Oxidation catalyst for treating the exhaust gas of a compression ignition engine
EP3915675A1 (fr) * 2020-05-26 2021-12-01 SASOL Germany GmbH Supports d'alumine de forme stable et leur procédé de production

Citations (1)

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Publication number Priority date Publication date Assignee Title
FR2550099A1 (fr) * 1983-08-04 1985-02-08 Nikki Universal Co Ltd Procede de preparation d'alumine destinee a etre utilisee comme support de catalyseur

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Publication number Priority date Publication date Assignee Title
US4220559A (en) * 1978-02-14 1980-09-02 Engelhard Minerals & Chemicals Corporation High temperature-stable catalyst composition
JPS5527830A (en) * 1978-08-15 1980-02-28 Chiyoda Chem Eng & Constr Co Ltd Production of alumina carrier
US4861410A (en) * 1985-02-25 1989-08-29 University Of Florida Method of joining metal oxide containing ceramic bodies
JPS61245844A (ja) * 1985-04-25 1986-11-01 Babcock Hitachi Kk 燃焼用触媒担体およびその製法
US4676928A (en) * 1986-01-30 1987-06-30 Vista Chemical Company Process for producing water dispersible alumina
JPS62180751A (ja) * 1986-02-03 1987-08-08 Toyota Central Res & Dev Lab Inc 耐熱性アルミナ担体
DE3823895C1 (fr) * 1988-07-14 1989-12-21 Condea Chemie Gmbh, 2212 Brunsbuettel, De

Patent Citations (1)

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Publication number Priority date Publication date Assignee Title
FR2550099A1 (fr) * 1983-08-04 1985-02-08 Nikki Universal Co Ltd Procede de preparation d'alumine destinee a etre utilisee comme support de catalyseur

Non-Patent Citations (1)

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Title
See also references of WO9317968A1 *

Also Published As

Publication number Publication date
EP0630356A4 (fr) 1995-06-14
WO1993017968A1 (fr) 1993-09-16
CA2131795C (fr) 2004-02-03
CA2131795A1 (fr) 1993-09-16

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