MESOPOROUS MATERIAL

Abstract: A mesoporous silica material can be formed by condensing a polysilicic acid from a solution in the presence of a surfactant such as long chain alkylamine, the mesoporous materials have large pore size which enables them to be used in filtration, as catalyst supports and in separations where conventional zeolites have too small a pore size.

Description: Mesoporous Material The present invention relates to porous amorphous structures and methods of making them, in particular it relates to compositions with large pore sizes which can contain metal ions and can be used as adsorbents and catalysts. US Patent 5, 108,725 discloses synthetic compositions of large pore materials and methods of making these compositions. This US Patent gives a detailed description of known and disclosed porous materials and prior art references which are incorporated herein by reference. This US Patent discloses a method of forming porous compounds by reacting certain alumino-silicates with an organic directing agent which is a quaternary ammonium compound under specified conditions to precipitate the compound. It is known from an article by S Gontier and A Tuel in 'Zeolites' 15:601-610, 1995 to form titanium containing mesoporous silicas by reacting a solution of tetraethyl orthosilicate with a solution of tetraisopropyi orthotitanate and adding this reaction mixture to a long chain alkylamine as a templating agent to obtain a Ti-containing mesoporous silica with enlarged pore structure. Silica materials are known which are amorphous in the sense that they have no long range order and are characterised with a pore size distribution over a wide range of sizes and have no X-ray diffraction pattern. Their porosity arises form the voids between dense particles of silica. Paracrystalline materials are known such as the transitional aluminas which have broad X-ray peaks. The microstructure of these materials consists of tiny crystalline regions of condensed alumina phases and the porosity of these materials results from irregular voids between these regions. As there is no controlling long range order, the pore size variability is typically very wide in these materials. Zeolite membranes are known with a narrow defined pore size range and are commonly referred to as molecular sieves, however these have a pore size below 15 Angstroms and are referred to in detail in US Patent 5,108,725 these materials are described as having a microporous structure. However hitherto it has not been possible to obtain a silica material with a narrow pore size distribution which is above the microporous range. We have invented a new silica containing material of enlarged pore size and a method of making it. According to the invention there is provided a silica composition of pore size of above 15 Angstroms and preferably of pore size 15 to 500 Angstroms. The pore size can be measured by using the technique of bubble point pressure as defined in ISO4003 or by nitrogen adsorbtion using the Polimore Head method. The composition should have a regular pore size with a narrow pore size distribution, e.g. the second and third quartile are within the specified range, the pore size distribution may be measured by a Coulter Porometer (Trademark). The structure of the material can be in the form of a chain of molecules linked together in a linear fashion to form what is substantially a chain or it can be in the form of a substantially planar structure of molecules linked together substantially in one plane or it can be in the form of a three dimensional structure with molecules linked together accordingly. In each of the structures the size will depend on the conditions and treatment and each structure will only approach an ideal uniform structure. The materials of the invention preferably have a benzene adsorption capacity of greater than 10 grams benzene/ 100 grams at 50 torr and 25 degrees C as measured in US Patent 5, 108,725. The materials of the present invention essentially comprise a series of polysilicic acid units linked together, each unit comprising a polysilicic acid molecule as described in GB Patent Application 9316350.9 and comprising a plurality of three dimensional species linked together with each species either having silicon atom bridges with an oxygen atom between each silicon atom or hydroxyl groups on the silicon atoms. The linking together of these units forms the structure of the compounds of the invention. The compositions of the present invention can be formed by the condensation of a polysilicic acid from solution in the presence of a surfactant. The polysilicic acid preferably has a weight average molecular weight of 700 to 2000. This acid is preferably dissolved in an alcohol such as ethanol or butanol to form the solution. The surfactant is thought to act to hold the individual units in a suitable orientation and separation to form the mesoporous compounds of the invention when they are joined together. The surfactant is preferably a compound which is at least partially miscible with silicic acid solution and can be in the form of a suspension or solution, e.g. in an alcohol. The surfactant can be a cationic, anionic or non-ionic. Examples of suitable surfactants include amines, quaternary ammonium compounds and siloxanes. Suitable amines include long chain alkyl amines, e.g. containing 6-25 carbon atoms. Suitable quaternary ammonium compounds include tetra-alkyl ammonium compounds. The composition of the present invention can be formed by adding a solution of the silicic acid to a solution or suspension of the surfactant to form the composition. Optionally, other silicon containing compounds can be incorporated in the silicic acid solution to modify the structure of the composition obtained. Suitable compounds include silanes, siloxanes, and functionalised silanes and siloxanes, etc. To form the structures of the present invention, the polysilicic acid solution is mixed with the surfactant solution, preferably with vigorous stirring and the product filtered and dried. The material is preferably calcined, e.g. above 350 degrees C The composition of the present invention can incorporate metals in addition to or in place of the silicon atoms to modify the pore structure of the material. Suitable metals include titanium, zirconium and any metal which can form, e.g. an oxide, hydroxide, alkoxide, acetonate or acetyl acetonate and any other functionality which can undergo a condensation reaction and which can form a solution or gel and which can condense to form a polymeric type structure. This can be carried out by mixing a solution or suspension of a metal oxide or hydroxide with the polysilicic acid solution before mixing with the surfactant. The pore size of the composition formed by the process of the invention will depend on the conditions and the presence of other metals. The compositions of the present invention can be used in filtration, the pore size being larger than in conventional zeolite membranes enables them to be used as filter media for separations which are not possible using zeolite membranes. Their robustness and temperature resistance compared with polymeric membranes enables them to be used in separations which are not possible using polymers. They can also be used as catalyst supports e.g. for preparing polymers such as polyolefins e.g. polyethylene, polypropylene etc. as well as other polymers for example as supports for metallic catalysts such as titanium based catalysts where their pore size enables specific control of the polymer formed to be achieved and in other catalytic processes. The invention will now be described with reference to the following examples - Example 1 Solution A Silicic Acid 31.956grm. of a polysilicic acid weight average molecular weight 800 was dissolved in n-butanol and ethanol to form a solution. Solution B 1 -hexadecylamine 1 -hexadecylamine (0.027 mol.) was added to a solution of 3.6 mol of distilled water and 0.02 mol hydrochloric acid and the resulting mixture vigorously stirred for 30 minutes, and a white creamy mixture formed. Mesoporous Silica Solution A was added slowly to solution B under vigorous stirring conditions for about 15 mins. A white solid was precipitated which was washed several times with distilled water and dried in a fume cupboard for 24 hours. The solid was calcined at 650°C for six hours. The X ray diffraction pattern of the HMS product was taken and was compared with that of HMS fabricated from TEOS as in the S Gontier and A Tuel article referred to above and was found to be identical with a single peak at 3.2°- D- Spacing. Transmission electron micrographs were taken at different magnifications and the results shown in the accompanying micrographs, with fig. 1 being at a magnification of 100 and at 80 KV and fig. 2 being at a magnification of 63 at 80 KV. As can be seen the compounds have a large pore structure. Example 2 Mixture A hexadecylamine (a) A first solution (mixture A) was prepared by adding hexadecylamine (0 027 mol) into a beaker containing distilled water (3 6 mol) and hydrochloric acid (0 002 mol) After the mixture was stirred vigorously for 30 minutes at room temperature, a thick creamy white acidic surfactant mixture was formed Mixture B Silicic Acid (b) A second solution mixture B was prepared by adding silicic acid/n-butanol solution (containing 0 1 mol Si) to absolute ethanol (0 65 mol) The silicic acid/n-butanol solution was prepared by adding 2 gram of sodium silicate powder into 8 35 gram of distilled water with constant stirring for 15 minutes The sodium silicate solution was added slowly into 100ml of cold 3M hydrochloric acid with constant stirring The mixture was stirred vigorously for 2 hours and the silicic acid extracted with n-butanol to form the silicic acid/n-butanol solution Mesoporous Silica (c) Mixture B was added slowly to mixture A under vigorous stirring The stirring was maintained for approximately 15 minutes White solids were formed instantaneously on mixing the two mixtures The product was recovered by filtration, washed with an excess amount of distilled water, and allowed to dry at room temperature The organic materials were removed by calcination of the as-synthesised solids in air at 650°C for 6 hours. The as-synthesised and calcined product consisted of a very fine white powder The adsorption isotherm for this material is shown in figure 3 The inflection is at p/p 0 ~0 35, the pore diameter was 30 Angstroms and the surface area was 1161m /g. Cetyltrimethylammoniumbromide (CTMABr) Silicic Acid (a) A silicic acid solution was prepared as in Example 2 except that the polymeric silicic acid solution formed was not extracted with butanol but was used immediately CTMABr (b) The surfactant mixture was prepared by dissolving 1 gram CTMABr in 10 3 gram distilled water Mesoporous Silica (c) The silicic acid solution of (a) was added to the surfactant mixture The resulting mixture was transferred into a sealed plastic bottle and placed in an oven at 80°C The resulting mixture was left for 24 hours The product was recovered by filtration, washed with an excess amount of distilled water and allowed to dry at room temperature The organic materials were removed by calcination of the as-synthesised solids in air at 650°C for 6 hours The as-synthesised and calcined product consisted of a very fine white powder This was shown to be to be hexagonal mesoporous silica, the diffraction pattern is shown in figure 4 The pore size was 30 Angstroms

Claims: Claims 1 A mesoporous silica composition of pore size of above 15 Angstroms and which comprises a plurality of polysilicic acid molecules linked together 2 A mesoporous silica composition as claimed in claim 1 in which the polysilicic acid molecules are linked together to form a three dimensional structure 3 A mesoporous silica composition as claimed in claim 1 or 2 in which the polysilicic acid molecules have an average molecular weight of from 700 to 2000 4 A composition as claimed in any one of claims 1 to 3 which has an average pore size of from 15 to 500 Angstroms. 5 A composition as claimed in any one of claims 1 to 4 which has a benzene adsorption capacity of greater than 10 grams benzene/100 grams at 50 torr and 25°C 6 A composition as claimed in any one of claims 1 to 5 which incorporates a metal 7 A composition as claimed in claim 6 in which the metal is titanium or zirconium 8 A method of forming a mesoporous silica composition which comprises condensing a polysilicic acid from a solution in the presence of a surfactant 9 A method as claimed in claim 8 in which the polysilicic acid has an average molecular weight of from 700 to 2000 10 A method as claimed in claim 9 in which the polysilicic acid is dissolved in an alcohol 1 1 A method as claimed in any one of claims 8 to 10 in which the surfactant is an amine, quaternary ammonium compound or a siloxane 12 A method as claimed in claim 1 1 in which the amine is a long chain alkyl amine containing from 6 to 25 carbon atoms 13 A method as claimed in any one of claims 8 to 12 in which a solution of the silicic acid is added to a solution or suspension of the surfactant 14 A mesoporous silica composition formed by the method of any one of claims 8 to 13

Inventors: BRATTON GRAHAM JOHN, BUCK KARON DOREEN, NAYLOR TIMOTHY DE VILLIERS, BRATTON, GRAHAM, JOHN, BUCK, KARON, DOREEN, NAYLOR, TIMOTHY, DE VILLIERS
Applicants: BRATTON GRAHAM JOHN, BUCK KARON DOREEN, NAYLOR TIMOTHY DE VILLIERS, BRATTON, GRAHAM, JOHN, BUCK, KARON, DOREEN, NAYLOR, TIMOTHY, DE VILLIERS
Application year: 1997
Main classification: C01B37
IPC codes: B01D71, B01J20, C01B37