Process for synthesising aluminas in a basic medium

Details Process for synthesising aluminas in a basic medium Process for synthesising aluminas in a basic medium

1999-07-21

2001-04-10

Bos, Steven (Department: 1754)

Chemistry of inorganic compounds

Oxygen or compound thereof

Metal containing

C423S626000, C423S630000, C423S631000

Reexamination Certificate

active

06214312

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to the field of aluminas with a controlled porosity and to a process for synthesising aluminas with a controlled porosity in a basic medium.
BACKGROUND OF THE INVENTION
Such materials are characterized by their pore volume, their specific surface area and their X ray diffraction spectrum. These materials have a high adsorbent capacity, and are usually used as catalyst supports, adsorbents, or separation means.
The preparation of oxides with a controlled porosity is based on the fact that organic molecules which are classified as surfactants can associate together, in particular when they are placed under controlled porosity oxide synthesis conditions. They then form micelles which have surface electrical charges and a variety of geometries depending on the conditions of the synthesis medium.
Such micelles can then structure polymeric species of mineral oxides such as silica, alumina or other metals such as titanium about them, and thus generate a solid mesoporous network.
International patent WO 96/39357 describes a complementary route using non-ionic surfactants such as polyethylene oxides and neutral precursors of inorganic oxides as sources of the reactants. Such a synthesis is carried out in an organic solvent. This method enables the surfactant to be eliminated readily. Porous inorganic oxides are prepared from non-ionic surfactants using that method.
Studies primarily aimed at controlled hydrolysis of organic Al(OR′)
3
type aluminium precursors, where R′ is a linear or branched alkyl group containing 1 to 5 carbon atoms, highlighted the case where the structuring agent is a quaternary ammonium type, where the medium is a mixed water-alcohol type, and the case where the structuring agent is a micellar type, the reaction then being carried out in an organic solvent. Vaudry and Davis in Chem. Mater., 1996, p. 1451 used fatty acid type surfactants. S. Bagshaw, T. J. Pinnavaia, in Mesoporous alumina molecular sieves, Angew. Chem. Intl. Ed. Engl. 1996, 35; 10, p. 1102-1105 described the production of lamellar phases.
When the structuring agent is a quaternary ammonium type R
4
N
+
, the X ray diffraction diagram generally exhibits a single diffuse diffraction below 2&thgr;=5° (CuK&agr;), indicating the presence of a mesostructure with a periodic short range order. With micellar surfactants, the X ray diffraction diagram generally exhibits a peak corresponding to an interplanar distance which is strictly over 3 nanometers indicating a periodic arrangement with a longer range order.
A mesoporous alumina with a hexagonal structure has been synthesised from Al
3+
species in the presence of sodium dodecyl sulphate; that synthesis has been described by M. Yada, M. Machida, T. Kijima in: “Synthesis and deorganization of an aluminium-based dodecyl sulphate mesophase with a hexagonal structure”, Chem. Commun., 1996, 769-770. That method requires the use of large quantities of urea the decomposition of which leads to a slow increase in the pH during prolonged heating. Thus the Al
3+
cations are hydrolysed, which enables them to polycondense to a lamellar phase then to a hexagonal phase when the pH is above 7. That structure is preserved after calcining the unwashed product but no mention was made of the existence of controlled porosity.
A. S. Stein, B. Holland, in “Aluminium-containing mesostructural materials”, J. Porous Materials, 1996, 3, 83-92 synthesised an alumina from Keggin type polycations based on Al
13
aluminium in the presence of dodecyl sulphate. However, after heat treatment to eliminate the surfactant, the solid obtained had a very low specific surface area of 18 m
2
/g, obtained using the BET method.
In the present description, the abbreviation “nm” will be used for “nanometer”, i.e., 10
−9
meters.
SUMMARY OF THE INVENTION
The invention relates to a process for synthesising aluminas with a controlled porosity such that the pore diameter is in the range 0.6 nm to 80 nm.
This process is carried out in a plurality of steps including at least one step a) in which an alumina precursor is prepared by hydrolysis of at least one anionic inorganic source of aluminium in the presence of at least one surfactant. Step a) of the process is carried out in an essentially aqueous medium the pH of which is generally above the isoelectric point of the alumina. The process of the invention also comprises at least one step b) in which the precipitate obtained is dried in air at a temperature of about 40° C. to about 110° C. for a period of about 2 to 30 hours, and at least one step c) in which the dried precipitate is calcined at a temperature which is sufficient to eliminate the molecules of surfactant present in the precursor. Hydrolysis of the inorganic aluminium source is generally carried out with a compound having an acid function.
The present invention also relates to aluminas with a controlled porosity with a pore diameter in the range 0.6 nm to 80 nm; this invention also relates to the hydrated (as-synthesised) compound obtained before calcining.
The advantages of the process of the invention include: carrying out the process with inorganic sources of aluminium which are cheaper and easy to use, in a low pollution and low cost medium where the main and only solvent is water.


REFERENCES:
patent: 4579729 (1986-04-01), Schoenthal et al.
patent: 5695735 (1997-12-01), Benazzi et al.
patent: 5800797 (1998-09-01), Matsumoto et al.
patent: 6027706 (2000-02-01), Pinnavaia et al.
patent: 0 670 286 (1995-09-01), None
patent: 670286 (1995-09-01), None
patent: 0 714 851 (1996-06-01), None
patent: 0 775 520 (1997-05-01), None
patent: 96/39357 (1996-12-01), None
Vaudry et al., “Synthesis of Pure Alumina Mesoporous Materials,”Chem. Mater. 1996, vol. 8, 1451-1464.
Yada et al., “Synthesis and deorganization of an aluminium-based dodecyl sulfate mesophase with a hexagonal structure,”Chem. Commun., 1996, XP-002075632, pp. 769-770.
Stein et al., “Aluminum-Containing Mesostructural Materials,”Journal of Porous Materials, vol. 3, 83-92 (1996).
Huo et al., “Organization of Organic Molecules with Inorganic Molecular Species Into Nanocomposite Biphase Arrays,”Chem. Mater., 1994, 6, No. 8, 1176-1191.
Tanev et al., “A Neutral Templating Route to Mesoporous Molecular Sieves”Science,vol. 267, No. 5199, Feb. 10, 1995, pp. 865-867.
English Abstract of EP 0 670 286 no date.

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