Chemistry of inorganic compounds – Zeolite – Synthesized from naturally occurring product
Reexamination Certificate
1999-12-16
2002-02-26
Sample, David (Department: 1755)
Chemistry of inorganic compounds
Zeolite
Synthesized from naturally occurring product
C423S716000, C423S305000, C423S306000, C423S328200, C423SDIG002, C423SDIG002, C423SDIG002, C423SDIG003
Reexamination Certificate
active
06350429
ABSTRACT:
BRIEF DESCRIPTION OF THE INVENTION
A method that stems from a discovery that the intrinsic porosity characteristics of a nutrient that possesses an amorphous cation oxide-framework can be substantially retained in the final molecular sieve containing product formed by a hydrothermal process by carefully controlling the conditions under which the process is conducted. This invention drives the selection of process conditions to achieve one or more of macro and meso porosity in the final molecular sieve product as a direct product of the hydrothermal reaction producing the molecular sieve. The invention allows the production of a molecular sieve that is incorporated in the framework morphology of a solid cation oxide-framework used in molecular sieve's manufacture.
The invention is directed to a novel solid molecular sieve composition that contains
a) a preformed porous geometric framework where the pores are one or more of macro and meso pores, and
b) interconnected in situ formed crystalline molecular sieve particles that
(i) contain micro pores and
(ii) are structural components of the framework.
BACKGROUND TO THE INVENTION
The definition of molecular sieve, according to Szostak, “Molecular Sieves, Principles Of Synthesis And Identification,” 1989, Van Nostrand Reinhold, New York, N.Y., at page 3, is
“A molecular sieve framework is based on an extensive three-dimensional network of oxygen ions containing generally tetrahedral-type sites. In addition to the Si
+4
and Al
+3
that compositionally define the zeolite molecular sieves, other cations also can occupy these sites. These cations need not be isoelectronic with Si
+4
and A
+3
, but must have the ability to occupy framework sites. Cations presently known to occupy these sites within molecular sieve structures are
(M
+2
O
2
)
−2
where M is Be, Mg, Zn, Co, Fe, Mn
(M
+3
O
2
)
−1
where M is Al, B, Ga, Fe, Cr
(M
+4
O
2
)
0
where M is Si, Ge, Mn, Ti
((M
+5
O
2
)
+1
where M is P
″
The term molecular sieve encompasses the variety of structures within the classification set forth in FIG. 1.1 of Szostak, supra, page 2, which classification is incorporated by reference. Molecular sieves come in two varieties, zeolitic molecular sieves (“ZMS”) and non-zeolitic molecular sieves (“NZMS”). Szostak (page 4) treats aluminosilicates generally to be ZMS provided there is at least one aluminum ion per unit cell based on the bulk composition of the sample. The remaining structures are recognized to be NZMS. According to this characterization, the ZSM-5 structure is considered a ZMS at silica/alumina less than 190 and NZMS above 190. This same convention holds when ZMS's contain trace amounts of other elements in the framework ion positions. (See Szostak, page 5, who considers a crystalline structure a ZMS if the number of other cations in the framework, other than aluminum and silicon, averages less than one per unit cell, all others being a NZMS.)
Zeolitic molecular sieves are typically made from a source of silica that is reacted with a source of aluminum, in the presence of materials that insure significantly alkaline conditions, water and
—
OH. The mix of the reactants may be called the reaction's nutrients. Many of the reactions are conducted in the presence of an organic template or crystal-directing agent, which induces a specific zeolite structure that can not be formed in the absence of the organic template. Most of the organic templates are bases, and many introduce hydroxyl ions to the reaction system. The reaction involves a liquid gel phase (“soup”) in which rearrangements and transitions occurs, such that a redistribution occurs between the alumina and silica nutrients, and structural molecular identities corresponding to specific zeolites or other molecular sieves are formed. It is known that zeolites are not often formed above 350° C., though descriptions of higher temperature formation of certain molecular sieves has been mentioned in the literature (see Szostak, supra, page 52). Lower temperatures than about 100° C. require extensive crystallization time. As Szostak, supra, page 54, points out
Upon mixing of the reagents on the synthesis of zeolite molecular sieves, a gel generally is observed to form, which with time begins to separate into two phases: a solid and a liquid. Visually, as the crystallization progresses, the gel plus the forming crystals increases in density and begins to settle to the bottom of the crystallization vessel, as the forming zeolite crystals have a density greater than that of the initial gel. Thus successful crystallization sometimes can be suspected if a very dense, easily settling solid phase is observed in the crystallization vessel when the crystallization is terminated.
When the desired crystal structure is obtained, the molecular sieve is brought to ambient temperatures and the crystallization process is arrested. The product of the reaction is isolated typically as a loose powder. The crystals that are formed in the powder are so assembled in the structure as to form special micro pores and micro pore openings of a kind that distinguishes the structure. The resulting crystals are an assemblage of individual units the growth of which may be small, medium or large, depending on the conditions employed in the traditional method. The crystals may then, in the usual case, be formed into composite structures that allow their use as, e.g., absorbents and catalysts.
A number of references describe processes for making ZMS by reacting an amorphous precursor in the presence of a small amount of water to form a dense interbonded mass. The amount of water is selected to be less than that which is used in the aforementioned traditional method but large enough to interbond the ZMS particles into dense masses.
For example, Haden et al., U.S. Pat. No. 3,065,064, convert to a ZMS, a dehydrated kaolin clay having a SiO
2
/Al
2
O
3
mol ratio of about 2, in the presence of a “concentrated aqueous solution of NaOH.” The H
2
O to Na
2
O mol ratio in the mixture is within the range of 4.5-11.5 “and being present in an amount such that the Na
2
O/SiO
2
mol ratio in the mixture is about 0.5.” According to Haden et al.:
“. . . the alkali is then reacted with the alumina and/or silica of the dehydrated aluminum silicate until substantially all of the alkali is consumed, such reaction being carried out while controlling the temperature of the mass below that at which water will be evaporated from the mass at the pressure employed and in the absence of an aqueous liquid phase external to and in contact with the mass. The reaction product is a coherent mass of substantially homogeneous amorphous composition and is the precursor of the desired synthetic crystalline zeolite. The amorphous reaction product is then aged without substantial dehydration thereof, preferably at elevated temperature under autogenous pressure or greater, to crystallize the material into the desired substantially homogeneous polycrystalline zeolite of the empirical formula Na
2
O.Al
2
O
3
.2SiO
2
4-5H
2
O in the form of a hard coherent mass of essentially the same volume as the original aluminum silicate-alkali mass.”
According to the patentee, the object of the process is to form the ZMS into a “compact mass or masses” which is defined as a “dense or substantially nonporous mass.” The patentee states,
“only such dense masses react to provide a sufficient number of structural bridges between crystals to form the zeolite in the desired form of hard crystalline aggregates occupying essentially the same volume as the unreacted mass as opposed to the finely divided or pulverulent masses inherently formed in carrying out prior art methods for producing the synthetic crystalline zeolite A.”
Miller, U.S. Pat. No. 5,558,851, patented Sep. 24, 1996, relates to shaped zeolite structures wherein the reactants, in making the structures, are formed into a water-wet thick paste and crystallized after forming into a shaped structure. According to Miller's process, and a
Chang Yun-feng
Murrell Lawrence L.
Overbeek Rudolf A.
van der Puil Nelleke
Yeh Chuen Y.
ABB Lummus Global Inc.
Lillie Raymond J.
Olstein Elliot M.
Sample David
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