RESULTADOS DEL NIVEL DE LA GESTION MUNICIPAL

Currently the commercial applications of microporous materials are much more prevalent than uses of mesoporous materials which leaves large scope and potential for mesoporous silica. Recent advances have led to interest in mesoporous silica for the purpose of catalysis [103], sensing and drug delivery[104]. These mesoporous molecular sieves have application potential in adsorption, molecular sieving, membrane separation and chemical sensing. If a faster mass and transfer is required then it is possible to use colloidal particles of mesoporous molecular sieves.

1.11.1 Adsorption

As mesoporous materials were developed many potential applications emerged including utilising the pores for their ability to adsorb a variety of molecules. One of the first studies involving mesoporous solids as adsorbates was investigating MCM-41 by introducing a variety of gases into the pores, including nitrogen, oxygen and argon. It was observed that oxygen and argon isotherms exhibited hysteresis loops whereas the nitrogen isotherm was seen to be reversible, attributed to capillary condensation within the narrow range or pores.

Various studies have been carried out investigating adsorption of argon, nitrogen, oxygen, water, and hydrocarbons such as cyclopentane and benzene[105,106].

1.11.2 Catalysis

With the synthesis of the first mesoporous silicas, possibilities were apparent in the area of catalysis. Zeolites and variations of these are typically used as catalysts [2, 107] due to their open structure and active sites but there were problems due to their limiting pore diameter, generally of approx 10 Å. Progress was limited to conversion of molecules which could fit within these pores but this changed with the development of larger pore materials, particularly silica frameworks. Silica has many attributes of an ideal catalyst support including well defined pore system, high surface area, stable in solvents at varying temperatures and pressures and high surface concentration of silanol groups.

Mesoporous solids can also be readily functionalised in such a way as to incorporate active sites within the walls of the silica or by coating the inner surface with a catalytically active species.

It has been reported that by incorporation of aluminium into MCM-41 it is possible to use this material for acid catalysis [108]. Alkylation of 2, 4-di-tert-butylphenol with cinnamyl alcohol is possible within the pores of aluminosilicate MCM-41[109,110], unlike in zeolite Y where pore size restricts the formation of the primary alkylation product 6,8-di-tert-2- phenyl-2-3-dihydro[4H]benzopyran. Friedel Crafts alkylation and acylation are possible using aluminosilicate MCM-41 [110-116]. In addition to alkylation it is possible to incorporate metals such as titanium [117], vanadium [118,119], and zirconia [120] into mesoporous solids and investigate their effect on catalysis.

As hydrothermal stability is important in the application of mesoporous materials in catalysis, several methods have been investigated to improve these properties. They include decreasing the silanol group content of the framework which makes the surface more hydrophobic and stable in water, thickening the walls of MCM-41 to improve the hydrothermal stability and generating microporous zeolite-mesostructure composite mixtures.

Thiol-functionalised materials have been used as solid acid catalysts once the thiols have been oxidised to sulfonic acid groups, as ion exchangers and as heavy metal adsorbers.

These have advantages over previous catalytic systems as they avoid problems such as corrosion, toxic waste and hazards during handling.

1.11.3 Adsorption of proteins; Immobilisation of enzymes

The wide range of mesoporous materials now available with differing pore sizes and morphology has opened up opportunities for the immobilisation of homogeneous and enzymatic catalysts [121-124]. Enzymes have been shown to give high turnovers at mild conditions in comparison to transition metal catalysts[125].

Functionalisation can enhance the properties of mesoporous silica as a support for enzymes

[126,127]. By functionalising the surface of mesoporous molecular sieves with enzymes this allows highly selective catalysis to be carried out using materials that are chemically and mechanically tough and which can be easily separated from reaction mixtures. Mesoporous molecular sieves such as MCM-48, MCM-41 and SBA-15 have been used in enzyme immobilisation [128-131]. Yiu et al. [59] showed that larger pore mesoporous molecular sieves such as SBA-15 have a greater potential in immobilising enzymes than the more traditional MCM-41. Methods of enzyme immobilisation and the use of mesoporous silica as a support are described in more detail in chapter 5.

Until recently the main focus of protein adsorption has been on controlled pore glass and sol-gel type materials. Controlled pore glass (CPG) can be synthesised with pores ranging from 2 – 20 nm and extensive work has been carried out using it as a support for biological molecules. Problems arise with this material as it is relatively expensive and the surface area decreases strongly as the pore size is increased. Sol-gels have also been investigated but their pore size distribution is usually broad and their stability in aqueous media can be poor. Soon after the publication of the first mesoporous materials [4,5,7]researchers began looking at these new materials and their possible applications. Due to their thick walls, high stability and well defined pore systems it was not long before groups started working on using these materials as supports for proteins and this is covered in detail in chapters 4 and 5.

1.11.4 Drug Delivery

The development of drug delivery systems (DDS) utilising mesoporous materials has expanded rapidly in the last few years since MCM-41 was first used as a delivery system

[132]. Since then a variety of materials have been employed as DDS, including MCM-41, SBA-15 and MCM-48 [133, 134]. These mesoporous materials can be described as potential drug carriers due to several key features; they possess homogenous, ordered pore structures which allow the control of drug loading and release kinetics. High pore volumes allow sufficient loading of the drug required and a high surface area gives a large potential for drug adsorption. The surface of the material usually possesses silanol groups which provide opportunities for surface functionalisation which can in some cases allow the modification and control of loading and release of drug molecules. This is discussed further in chapter 6.