Mesoporous materials and methods of making the same

Details Mesoporous materials and methods of making the same Mesoporous materials and methods of making the same

2000-06-21

2004-02-24

Le, Long V. (Department: 1641)

Chemistry: molecular biology and microbiology

Measuring or testing process involving enzymes or...

Involving antigen-antibody binding, specific binding protein...

C435S004000, C435S007100, C435S014000, C436S518000, C436S523000, C436S524000, C436S528000, C423S702000, C424S484000, C424S489000, C424S499000

Reexamination Certificate

active

06696258

ABSTRACT:

BACKGROUND OF THE INVENTION
The synthesis of mesoporous molecular materials using pore forming materials or “templates” has emerged as an important area of research. However, currently the most successful of such templates for directing the mesophase formation are surfactants. The choice of such templates is relatively limited and many surfactants, e.g. alkylammonium ions and aikylamines, are costly and toxic, particularly with respect to biologically active agents.
The synthesis of mesoporous materials using surfactants as templates has been studied extensively, and the materials were intended for applications in catalysis, biochemical separation technology and molecular engineering. Surfactants, ionic and neutral have been the most commonly used pore forming materials for directing the formation of mesoporosity. The ionic pathways are based on charge matching between the ionic surfactants and ionic inorganic precursors through electrostatic interaction. Neutral surfactants are theorized to use hydrogen bonding between the surfactants and the precursors to direct formation of mesostructures.
The discovery of the M41S family of mesoporous silicate and aluminosilicate molecular mesoporous materials, or sieves, using surfactant templated hydrothermal sol-gel processes was reported in 1992 by C. T. Kresge et al.,
Nature
, “Ordered Mesoporous Molecular Sieves Synthesized By A Liquid Crystal Template Mechanism,” vol. 359, page 710 (1992). This discovery drew great interest because of the potential applications of such mesoporous materials as catalysts, catalyst supports, separation media, and host material for inclusion compounds. Numerous mesoporous or nanoporous materials have been synthesized and the pore diameter extended from less than 13 Å for conventional zeolites to about 100 Å and lately up to 300 Å. Many synthetic routes and strategies have been developed to yield a wide diversity of materials of various framework chemical compositions and pore structures. The mostly commonly used templates, however, for directing the mesophase formation have been ionic or non-ionic surfactants. A number of mechanistic pathways have been proposed to account for the formation of nanophase structures, based on the electrostatic interactions and charge-matching for the systems with ionic surfactant templates and the hydrogen bonding interactions for the systems with neutral surfactant templates such as amines and polyethylene oxide (PEO) copolymers. A covalent bonding pathway has also been proposed. In addition, mesoporous materials may also be prepared from interlayer crosslinking of a layered silicate through ion exchange reactions with organic cations.
MCM-41 mesoporous materials have an array of hexagonal arrangements of uniform mesopores of 15 to 100 Å in diameter, which could be controlled by the hydrophobic alkylchain length of ionic surfactants or with the aid of auxiliary organic compounds as spacers. The ionic templates are usually removed by high temperature calcination or ion exchange. Strong electrostatic interactions among the ionic surfactants and the silicate intermediates result in matrices with limited pore wall thicknesses of 0.8-1.3 nm that are influenced little by pH in the synthesis. As a consequence, the materials often have limited thermal stability as evidenced by significant pore contraction or even structure collapse during calcination. Nonetheless, MCM-41 and their analogues have been explored for many applications. They may serve as model adsorbents for the study of gas sorption in mesoporous solids, as catalysts especially when transition metal elements or organic functional groups are incorporated into the framework structure, and as host materials for inclusion of other molecules.
With neutral, primary-amine surfactants as the template, a family of hexagonal mesoporous silicas have been prepared. The pore size may be adjusted by changing the hydrophobic tail length of the amines. The template can be removed by solvent extraction. The mesoporous materials have greater wall thicknesses (1.7-3.0 nm) due to the absence of electrostatic or charge-matching effects, and thus higher thermal stability than M41S materials. However, the materials exhibit both complementary textural and framework-confined mesoporosity. The toxicity of amines also remains a concern if a large scale production is intended.
The use of neutral, polymeric PEO surfactants as pore forming materials has been demonstrated as advantageous in solving the problems of ionic surfactant charge-matching and organic amine toxicity, since the PEO surfactants are neutral, non-toxic and biodegradable. In addition, the pore size can be controlled by varying the size and structure of the PEO surfactant molecules though the channels are largely disordered. Recently, highly ordered porous silicas (20-300 Å) with large wall thickness values of 3.1-6.4 rum and pore volumes up to 2.5 cm
3
/g were synthesized by using alkyl PEO oligomeric surfactants and poly(alkylene oxide) block copolymers as templates in strongly acidic media, however, such acidic media are not biocompatible and would present problems with respect to applications involving biologically active agents. In addition, it is difficult to remove such templates using solvent extraction due to their high molecular weight. Further, such attempts do not generally form transparent, monolithic materials which are important for specific applications.
One area of application of microporous and mesoporous materials is for the immobilization of biologically active agents within these materials. Immobilization of enzymes, in particular, has been a subject of extensive research efforts because of its immense technological potentials. Among the popular methods of immobilization is formation of chemical bonding between enzymes and a solid support, which often alters the enzymatic activity. A variety of enzymes and other bioactive substances have been entrapped in inorganic oxides such as silica for biocatalysis and biosensor applications through conventional sol-gel processes. However, because of the microporous nature of the conventional silica matrices (i.e. typical pore diameter<15 Å and pore volume<0.25 cm
3
/g), the catalytic activities of enzymes are hindered by low diffusion rates of substrate molecules and poor accessibility of enzymes inside the materials.
Mesoporous materials are valuable to the life sciences because the larger pore size in comparison with microporous materials allows for a more suitable environment and better mass transfer for biologically active agents. Much of the prior art involving the immobilization of biologically active agents in porous materials involves use of microporous materials, not mesoporous materials. Biologically active agents previously have been bound to microporous materials, but the pore diameters result in steric hindrance and mass transfer limitations on the use of such materials in biological reactions.
Recent advances in the development of mesoporous materials have enabled the immobilization of biologically active agents, but these techniques primarily involve the use of surfactants as templating agents. The syntheses are either detrimental to the activity of biologically active agents, or employ extreme synthesis conditions (such as high temperature, or low pH). Further, such procedures generally do not provide transparent, monolithic mesostructured materials having immobilized enzymes have been achieved using such methods.
As such, there is a need in the art for an easy-to-synthesize mesoporous material. There is further a need for a process for making such materials, and in which the materials can be biocompatible. There is also a need for a method for immobilizing biologically active agents which enables biologically active agents to fit in the mesopores while also preserving biological activity. Those prior art methods using templating agents have been generally unsuccessful in that the templating agents used in forming the mesoporous materials were toxic or denaturi

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