Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
1999-11-09
2003-03-04
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
Reexamination Certificate
active
06528034
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to novel mesoporous, lamellar silica compositions and to a method for the preparation thereof. In particular the present invention relates to the use of novel gemini amine surfactants as templating or structure directing agents.
DESCRIPTION OF RELATED ART
Porous materials created by nature or by synthetic design have found great utility in all aspects of human activity. The pore structure of the solids is usually formed in the stages of crystallization or subsequent treatment. Depending on their predominant pore size, the solid materials are classified as: (i) microporous, having pore sizes <20 Å; (ii) macroporous, with pore sizes exceeding 500 Å; and (iii) mesoporous, with intermediate pore sizes between 20 and 500 Å. The use of macroporous solids as adsorbents and catalysts is relatively limited due to their low surface area and large non-uniform pores. Microporous and mesoporous solids, however are widely used in adsorption, separation technology and catalysis.
Owing to the need for higher accessible surface area and pore volume for efficient chemical processes, there is a growing demand for new, highly stable mesoporous materials. Porous materials can be structurally amorphous, paracrystalline, or crystalline. Amorphous materials, such as silica gel or alumina gel, do not possess long range order, whereas paracrystalline solids, such as &ggr;- or &eegr;-Al
2
O
3
are quasiordered as evidenced by the broad peaks on the x-ray diffraction patterns. Both classes of materials exhibit a broad distribution of pores predominantly in the mesoporous range. This wide pore size distribution limits the shape selectivity and the effectiveness of the adsorbents, ion-exchanges and catalysts prepared from amorphous and paracrystalline solids.
Hereafter, in order to clarify one of the objects of the present invention, the terms framework-confined uniform porosity and textural porosity are defined and differentiated. Framework-confined uniform pores are pores formed by nucleation and crystallization of the framework elementary particles. These pores typically are cavities and channels confined by the solid framework. The size of the cavities and channels, i.e. the size of the framework-confined uniform pores, in molecular sieve materials is highly regular and predetermined by the thermodynamically favored assembly routes. The framework-confined pores of freshly crystallized product are usually occupied by the template cations and water molecules. While water molecules are easily removed by heating and evacuation the ionic templating materials, such as quaternary ammonium cations, due to their high charge density, are strongly bonded or confined to the pore cavities and channels of the negatively charged framework. The same concepts are expected to apply for the charge reversed situation where an anionic template is confined in the pores of a positively-charged framework. Therefore, a cation or anion donor or ion pairs are necessary in order to remove the charged template from the framework of the prior art molecular sieves.
Textural porosity is the porosity that can be attributed to voids and channels between elementary particles or aggregates of such particles (grains). Each of these elementary particles in the case of molecular sieves is composed of certain number of framework unit cells or framework-confined uniform pores. The textural porosity is usually formed in the stages of crystal growth and segregation or subsequent thermal treatment or by acid leaching. The size of the textural pores is determined by the size, shape and the number of interfacial contacts of these particles or aggregates. Thus, the size of the textural pores is usually at least one or two orders of magnitude larger than that of the framework-confined pores. For example, the smaller the particle size, the larger the number of particle contacts, the smaller the textural pore size and vice versa. One skilled in the art of transmission electron spectroscopy (TEM) could determine the existence of framework-confined micropores from High Resolution TEM (HRTEM) images or that of framework-confined mesopores from TEM images obtained by observing microtomed thin sections of the material as taught in U.S. Pat. No. 5,102,643.
One skilled in the art of adsorption could easily distinguish and evaluate framework-confined uniform micropores by their specific adsorption behavior. Such materials usually give a Langmuir type (Type I) adsorption isotherm without a hysteresis loop (Sing et al., Pure Appl. Chem. vol. 57, 603-619 (1985)). The existence of textural mesoporosity can easily be determined by one skilled in the art of SEM, TEM and adsorption. The particle shape and size can readily be established by SEM and TEM and preliminary information concerning textural porosity can also be derived. The most convenient way to detect and assess textural mesoporosity is to analyze the N
2
or Ar adsorption-desorption isotherm of the solid material. Thus, the existence of textural mesoporosity is usually evidenced by the presence of a Type IV adsorption-desorption isotherm exhibiting well defined hysteresis loop in the region of relative pressures Pi/Po >0.4 (Sing et al., Pure Appl. Chem. 57 603-619 (1985)). This type of adsorption behavior is quite common for a large variety of paracrystalline materials and pillared layered solids.
The only class of porous materials possessing rigorously uniform pore sizes is that of zeolites and related molecular sieves. Zeolites are microporous highly crystalline aluminosilicates. Their lattice is composed by TO
4
tetrahedra (T=Al and Si) linked by sharing the apical oxygen atoms. Oriented TO
4
tetrahedra, consists of cavities and connecting windows of uniform size (Breck, D. W., Zeolite Molecular Sieves: Structure, Chemistry and Use; Wiley and Sons; London (1974)). Because of their aluminosilicate composition and ability to discriminate small molecules, zeolites are considered as a subclass of molecular sieves. Molecular sieves are crystalline non-aluminosilicate framework materials in which Si and/or Al tetrahedral atoms of a zeolite lattice are substituted by other T atoms such as B, Ga, Ge, Ti, V, Fe, or P.
Zeolite frameworks are usually negatively charged due to the replacement of Si
4+
by Al
3+
. In natural zeolites this charge is compensated by alkali or alkali earth cations such as Na
+
, K
+
or Ca
2+
. In synthetic zeolites the charge can also be balanced by ammonium cations or protons. Synthetic zeolite and molecular sieves are prepared usually under hydrothermal conditions from aluminosilicate or phosphate gels. Their crystallization, according to the hereafter discussed prior art, is accomplished through prolonged reaction in an autoclave for 1-50 days and, often times, in the presence of structure directing agents (templates). The proper selection of template is of extreme importance for the preparation of a particular framework and pore network. A large variety of organic molecules or assemblies of organic molecules with one or more functional groups are known in the prior art to give more than 85 different molecular sieve framework structures (Meier et al., Atlas of Zeolite Structure Types, Butterworth, London (1992)). An excellent review of the use of various organic templates and their corresponding structures, as well as the mechanism of structure directing is given for example in Gies et al., Zeolites, vol. 12, 42-49 (1992). Due to their uniform pore size, unique crystalline framework structure and ability for isomorphous substitution synthetic zeolites and molecular sieves are extremely suitable for a number of adsorption, separation and catalytic processes involving organic molecules. Recently, it has been discovered that synthetic zeolites and molecular sieves can be functionalized by partially substituting the framework T-atoms with such metal atoms capable of performing different chemical (mostly catalytic) tasks. As a result, a large variety of highly selective cataly
Kim Seong-Su
Pinnavaia Thomas J.
Zhang Wenzhong
Board of Trustees of Michigan State University
Hendrickson Stuart L.
McLeod Ian C.
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