Synthesis of Mesoporous Materials：介孔材料的合成

1、.Julia HoetsSynthesis of Mesoporous MaterialsMesoporous materials are those with pores in the range 20-500Å in diameter.1 www. mesoporous. com, International Mesostructured Material Association.(From Yahoo search engine.) They have huge surface areas, providing a vast number of sites where sorp

2、tion processes can occur. These materials have numerous applications in catalysis, separation and many other fields.2 Davies, S., Goltner, C., Mesoporous Silica from Supramolecular Templating, University of Bristol, Inorganic Practical level II, experiment 8.(From relevant practical handout.) The sy

3、nthesis of these materials is therefore of considerable interest and is constantly being developed to introduce different properties.Originally porosity was introduced to layered systems by pillaring. This process involves the intercalation of a guest species to create a porous network.3 Schöll

4、horn, R., Intercalation Systems as Nanostructured Functional Materials, Chem.Mater.,1996, 8, 1747-1757.(From list of references in reference number 13.) However, the interlayer voids would collapse on removal of the guest species, so pillars were introduced to the framework to maintain the lateral s

5、pacing. Pore size could thus be controlled by the size of the pillar. The spacing in materials synthesised in this fashion was however irregular and the size distribution broad.In 1992 researchers at Mobil demonstrated the synthesis of hexagonally packed mesoporous silicate and aluminosilicate mater

6、ials with uniform pore sizes, evenly distributed throughout the material.4 Kresge, C.T. et al. Ordered Mesoporous Molecular Sieves Synthesised by a Liquid Crystal Template Mechanism, Nature, 1992, 359, 710-712. (Starting reference given.) 5 Beck, J.S. et al. A New Family of Mesoporous Molecular Siev

7、es Prepared with Liquid Crystal Templates, J.Am.Chem.Soc., 1992, 114, 10834-10843.(Starting reference given.)These materials were termed MCM-41. Their synthesis was achieved by the use of surfactant micelles as structure directing agents in a sol-gel process. Amphiphillic surfactants self assemble i

8、nto cylindrical micelles, which are encapsulated by an inorganic material (silicate or aluminosilicate in this case), which balances the charge on the micellular surfaces. Calcination, a thermal processing technique is then used to remove the organic surfactant, leaving a hexagonal arrangement of me

9、sopores.This process employs a liquid crystal templating mechanism, and is used to synthesise pores up to 100Å in diameter with surface areas of up to 700m²/g.Sol-gel technology, first used in 1846 by J.Ebelman6 Lev, O., Sol-gel Materials in Electrochemistry, Chem.Mater., 1997, 9, 2354-337

10、5.(From list of references in reference 15.) , is now commonly used in the synthesis of ceramics. It involves the formation of a solid phase, through the gelation of a colloidal suspension (sol). There are two possible routes for this process7 Pringle, P.G., University of Bristol Level II lecture no

11、tes. (From relevant lecture material.):i)Inorganic route ; this hydrothermal technique involves the formation of a sol from a very fine powder of the silicate starting material. This is then heated and dehydrolysed to form the ceramic via a gel state.ii)Organic route ; here the starting material has

12、 an organic component, for example TEOS (tetraethylorthosilicate) which is hydrolysed to form a gel, thus losing ethanol, and then heated to form the ceramic. This is a more expensive route but forms pure products of optical and electronic quality. The liquid crystal templating mechanism used by Mob

13、il in 1992 involved the dissolution of a surfactant species in the pre-hydrolysed inorganic pre-cursor8 Ogawa, M., Simple Sol-gel Route for Preparation of Silica-Surfactant Mesoporous Materials, Chem.Commun., 1996, 1149-1150.(From computer citation search using Mesoporous Materials as a subject.). T

14、he mechanism is strongly influences by electrostaic and steric interactions between the solvent molecules, the inorganic species and the self assembled organic surfactants9 Vaudry, F., et al. Synthesis of Pure Alumina Mesoporous Materials, Chem.Mater., 1996, 8, 1451-1464. (From list of references in

15、 reference 15.) .A wide variety of surfactants, with different properties; anionic, lipid, zwitterionic even two-tailed species, are used to introduce the mesopores10 Huo, Q., Organisation of Organic Molecules with Inorganic Molecular Species into Nanocomposite Biphase Arrays, Chem.Mater., 1994, 6,

16、1176 1184. (From list of references in reference 14.).Ionic surfactants self assemble into micelles, governed by the matching of charge densities at surfactant inorganic interfaces11 Monnier, A. et al. Cooperative Formation of Inorganic-Organic Interfaces in the Synthesis of Silicate Mesostructures.

17、 Science, 1993, 261, 1299 - 1303.(From list of references in reference 14.) .Neutral templating, involving non-charged species (often primary amines) is based upon hydrogen bonding.Larger pores, up to 140Å, can be synthesised by using amphiphilic block copolymers12 Yang, P., et al. Generalised

18、Synthesis of Large Pore Mesoporous Metal Oxides with Semi-Crystalline Frameworks, Nature, 1998, 396, 152 - 155. (From computer citation search using Yang, P. as an author.) with a wide variety of characteristics13 Yang, P., et al. Triblock Copolymer Directed Synthesis of Large Pore Mesoporous Silica

19、 Fibres. Chem.Mater. 1998, 10, 2033 2036.(From list of references in reference 15.). Materials formed by this method are hydrothermally stable due to their excellent interfacial stabalisation properties14 Zhao, D., et al. Non-ionic Triblock and Star Diblock Copolymer and Oligomeric Surfactant Synthe

20、sis of Highly Ordered, Hydrothermically Stable, Mesoporous Silica Structures, J.Am.Chem.Soc., 1999, 12, 6024 6036.(From computer citation search using mesoporous as a subject.). They also have long ordering lengths, thick walls and large pores15 Yang, P., et al. Block Copolymer Templating Synthesis

21、of Mesoporous Metal Oxides with Large Ordering Lengths and Semicrystalline Frameworks, Chem.Mater. 1999, 11, 2813 2826.(From computer citation search using Yang, P. as an author.).Surfactants can even be covalently bonded to the inorganic precursor as a ligand16 Yang, P., Synthesis and Characterisat

22、ion of Hexagonally Packed Mesoporous Tantalum Oxide Molecular Sieves. Chem.Mater. 1996, 8, 874 881.(From computer citation search using Yang, P. as an author.).The removal of the surfactant also has many options.The traditional method is calcinations. The inorganic surfactant complex is heated to hi

23、gh temperatures for extended periods under a flow of air, oxygen or nitrogen, to “burn off” the surfactant moleculesp. Forceful conditions are required to break the strong electrostatic bonds between the micelles and the inorganic species.The non-ionic templating approach allows the removal of the s

24、urfactant by solvent extraction, as the hydrogen bonding involves is more easily broken. The surfactant is recovered and therefore can be recycled17 Tanev, P.T., et al. A Neutral Templating Route to Mesoporous Molecular Sieves, Science, 267, 865 867.(From list of references in reference 15.).Other m

25、ethods include plasma and super-critical fluid extraction.A recent journal (feb 2001) published a paper detailing surfactant removal by ozone18 Büchel, G., In situ Surfactant Removal of MCM-type Mesostructures by Ozone Treatment, J.Mater.Chem. 2001, 11 (2), 589 595. (From reading new editions o

26、f the journal of materials chemistry.). This involves a highly exothermic process, which cannot currently be controlled. Treatment with ozone forms larger pores, with a decreased size distribution and increased order.Pore size can be controlled by the choice of surfactant. The diameter can be increa

27、sed by lengthening the alkyl chain, addition of an auxiliary hydrocarbon such as an alkylated benzene, or solution of an organic species in the hydrophobic micellular regions.Aqueous emulsions of amines can be used as swelling agents, inducing up to a 3-fold enlargement of pores, at the cost of a de

28、crease in ordering19 Sayari, A., Yang, Y., Expanding the Pore Size of MCM-41 Silicas : Use of Amines as Expanders in Direct Synthesis and Postsynthesis Procedures. J.Phys.Chem. B, 1999, 103 (18), 3651 3658.(From computer citation search using MCM-41 as a subject.). Manipulation of temperature and ac

29、idity can also affect the size and even the shape of the pores20 Yang, H., et al. Synthesis of Mesoporous Silica Spheres under Quiescent Aqueous Acidic Conditions. J.Mater.Chem., 1998, 8 (3), 743 750.(From list of references in reference 18.) creating gyroid rather than spherical mesopores.Branched,

30、 3D networks of pores can be synthesised by polymerisation of the silicate anions around micelles with organic salts at high temperatures21 Ryoo, R., Disordered Molecular Sieve with Branched Mesoporous Channel Network, J.Phys.Chem., 1996, 100, 17718 17721.(From computer citation search using mesopor

31、ous as a subject.) this allows the porous system to become more resistant to blocking, leading to faster diffusion through the system.Mesopores can be further modified by the functionalisation of the pore surface. This can alter surface properties such as acidity, hydrophobicity, or specific affinit

32、y. The functional group is added to the inorganic precursor by copolycondensation where it will replace an alkoxy group.Transition metal oxide structural equivalents of mesoporous materials are highly sought after, due to the combination of a high surface area and strong catalytic properties. They a

33、lso have useful applications in electrochemistry22 Huo, Q., et al. Generalised Synthesis of Periodic Surfactant / Inorganic Composite Materials, Nature, 1994, 368, 317 - 321.(From list of references in reference 6.). The first synthesised was a TiO2 based hexagonally packed material, synthesised by

34、a similar process as the silicate but under acidic conditions23 Antonelli, D., et al. Synthesis of Hexagonally Packed Mesoporous TiO2 by a Modified Sol-gel Method, Agnew.Chem.Int.Ed.Eng., 1995, 34, 2014 2017.(From computer citation search using mesoporous as a subject.). Mesoporous materials of Sb and Pb oxides have since been synthesised using poly