Chemistry of inorganic compounds – Zeolite – Isomorphic metal substitution
Reexamination Certificate
2002-10-18
2004-09-14
Sample, David R (Department: 1755)
Chemistry of inorganic compounds
Zeolite
Isomorphic metal substitution
C423S714000, C423S715000, C502S085000, C502S061000, C502S073000, C502S074000
Reexamination Certificate
active
06790433
ABSTRACT:
I. FIELD OF THE INVENTION
This invention relates to new methods for improving the lattice substitution of heteroatoms in large and extra-large pore borosilicate zeolites. In particular, the aforesaid methods include (1) the deboronation of essentially aluminum free borosilicate zeolites under acid conditions and (2) the reinsertion of heteroatoms in the lattices of deboronated zeolites using aqueous solutions of salts of the corresponding heteroatoms.
II. BACKGROUND OF THE INVENTION
Natural and synthetic microporous crystalline molecular sieves including metallosilicates have found widespread industrial applications as catalysts, adsorbents and ion exchangers. These molecular sieves have distinct crystal structures with ordered pore structures which are demonstrated by distinct X-ray diffraction patterns. The crystal structure defines cavities and pores which are characteristic of the different species and are similar in size to small organic molecules (roughly 3-15 Å). The adsorptive, catalytic and/or ion exchange properties of each molecular sieve depend largely on its large internal surface area and highly distributed active sites, both of which are accessible through uniform molecularly sized channels and cavities.
According to the Structure Commission of the International Zeolite Association, there are above 120 different microporous crystalline molecular sieve structures. The cage or pore size of these materials is denoted by the number of oxygen atoms (likewise the number of tetrahedral atoms) circumscribing the pore or cavity, e.g., a pore circumscribed by n oxygen-atoms is referred to as an n membered-ring pore, or more simply, n-MR. Molecular sieves containing pores and/or cages with molecular-sized windows (containing 8-MR or larger) can have industrial utility for separations, ion exchange and catalysis. Depending on the largest pore openings that they possess, they are usually categorized into small (8-MR), medium (10-MR), large (12-MR) and extra-large (≧14-MR) pore molecular sieves.
The metallosilicates are molecular sieves with a silicate lattice wherein a metal (referred here to as “heteroelement”) can be substituted into the tetrahedral positions of the silicate framework. Examples of these metals are boron, aluminum, gallium, iron and mixtures thereof. The substitution of boron, aluminum, gallium and iron for silicon results in an imbalance in charge between the silicon and the corresponding trivalent ions in the framework. In turn, such a change in the framework charge alters the ion exchange capacity of a material as well as the adsorptive and catalytic behavior because of the distinct physicochemical properties of these heteroelements. Thus, the utility of a particular molecular sieve in a particular adsorptive, catalytic or ion exchange application depends largely not only on its crystal structure but also on its properties related to the framework compositions. For example, stronger acid strength in zeolite catalysts is required for iso-butane/butene alkylation at lower reaction temperatures to simultaneously achieve higher activity and lower deactivation rate of the catalyst. By contrast, as demonstrated by S. Namba et al. (
Zeolites
11, 1991, p.59) in studies on the alkylation of ethylbenzene with ethanol over a series of metallosilicates with MFI (ZSM-5) zeolite structure, namely, B-ZSM-5, Sb-ZSM-5, Al-ZSM-5, Ga-ZSM-5 and Fe-ZSM-5, the para-selectivity to para-diethylbenzene is largely related to the acid strength of the catalysts and the weaker acid sites provide a higher para-selectivity.
In nature, molecular sieves commonly form as geothermally heated ground water passes through silicate volcanic ash. Early attempts to synthesize zeolites centered around recreating the high-pressure, high-temperature conditions found in nature. Barrer (
J. Chem. Soc.,
1948, p.127) demonstrated the first successful zeolite synthesis (mordenite) while Milton (U.S. Pat. No. 2,882,243 (1959)) developed the large-scale zeolite synthesis at low temperatures and pressures that allowed zeolites to gain industrial importance. These zeolite syntheses relied on the presence of alkali metal cations in the synthesis mixture to serve as a mineralizing agent. The alkali metal cations also play a role in the structure direction of the particular zeolite that forms.
Building on the concept of cationic structure direction, the range of cations was subsequently expanded later on from the inorganic metal cations to organic cations such as quaternized amines. In the recent years, the use of organic molecules to direct the formation of zeolites and other molecular sieves has become commonplace and given rise to an increasing number of novel molecular sieves, leading to breakthroughs in molecular sieve science and providing an impetus in developing new process chemistry.
As mentioned before, today over 120 molecular sieve structures have been discovered. Some of them counterparts to the naturally occurring molecular sieves, whereas others have no natural analog. Theoretical studies of molecular sieve structures and structure types indicate that only a small fraction of the configurations possible for microporous, crystalline molecular sieves have been discovered. Apparently, the major roadblock in tailoring and utilizing molecular sieve materials for specific applications in catalysis, adsorption and ion exchange is the development of synthesis methods to produce the desirable structure with the desirable framework composition.
In the principle, there are two routes leading to the formation of a particular molecular sieve structure with a particular framework composition, e.g., a particular metallosilicate such as aluminosilicate, gallosilicate, ferrosilicate or borosilicate of the same crystal structure: (1) direct synthesis and (2) post-synthetic treatment (secondary synthesis). These two routes will be discussed next.
The direct synthesis is the primary route of the synthesis of molecular sieves. The major variables that have a predominant influence on the molecular sieve structure crystallized include: the gross composition of the synthesis mixture, temperature and time. Even though each variable contributes to a specific aspect of the nucleation and crystallization of the molecular sieves, there is substantial interplay between these elements during the formation of molecular sieves. In the presence of heteroelement X (X=Al, Ga, Fe or B, for example, or X=none for pure-silica molecular sieves), the Si/X ratio will determine the elemental framework composition of the crystalline product; but the amount of the heteroelement in the synthesis mixture also can determine which structure, if any, crystallizes. In addition to the Si/X ratio, various other factors related to the gross composition of the synthesis mixture also play an important role. These factors include: OH
−
(or F
−
) concentration, cations (both organic and inorganic), anions other than OH
−
(or F
−
), and water concentration. There are also history-dependent factors such as digestion or aging period, stirring, nature (either physical or chemical) of the synthesis mixture, and order of mixing. In short, depending on the nature of the molecular sieves and the chemistry of their formation, some of these molecular sieve structures can be synthesized in a broad spectrum of framework compositions such as ZSM-5 containing none heteroatoms (Si-ZSM-5 or silicalite-1), Al (Al-ZSM-5), B (B-ZSM-5), Fe (Fe-ZSM-5) and Ga (Ga-ZSM-5), whereas the synthesis of other structures succeeds only if certain heteroatom is present in the synthesis mixture and, in turn, incorporated into the framework, or some structures containing specific heteroatom(s) can be synthesized only in a limited range of Si/X ratio, or some structures containing specific heteroatom(s) can be synthesized only if certain specific, usually more expensive, structure-directing agents are employed. These complicated relationships between zeolite structures, framework compositions and structure-directing agents have been di
Chen Cong-Yan
Zones Stacey I.
Chevron U.S.A. Inc.
Sample David R
Tuck D. M.
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