Molecular sieve abstract
Molecular sieve compositions are prepared by extracting aluminum
and substituting chromium and/or tin for extracted aluminum to give
molecular sieve products containing framework chromium and/or tin
atoms. The process of preparing the chromium and/or tin-containing
molecular sieves invovles contacting a starting molecular sieve
with a solution or slurry of at least one of a fluoro salt of chromium
or a fluoro salt of tin under effective process conditions to provide
for aluminum extraction and substitution of chromium and/or tin.
These compositions are effective as hydrocarbon conversion catalysts
and for separating mixtures of molecular species.
Molecular sieve claims
We claim as our invention:
1. A molecular sieve having a three-dimensional microporous framework
structure which has a unit empirical formula on an anhydrous basis
of:
where w, x and y are the mole fractions of chromium, aluminum and
silicon, respectively, present as the framework tetrahedral oxide
units said mole fractions being such that they are within the triagonal
area defined by points A, B, and C of FIG. 1 which points have
the following values of w, x and y
2. A molecular sieve having a three-dimensional microporous framework
structure which has a unit empirical formula on an anhydrous basis
of
where u is the mole fraction of aluminum and ranges from about
0.01 to about 0.5 v is the mole fraction of silicon and ranges
from about 0.5 to about 0.98 w is the mole fraction of chromium
and ranges from about 0.01 to about 0.49 .quadrature. is framework
defect sites and z is the mole fraction of defect sites in the framework
and ranges from greater than zero to about 0.2 characterized in
that the aluminum, silicon and tin are present as tetrahedral oxide
units in the framework structure.
3. The molecular sieve of claim 1 having the characteristic x-ray
powder diffraction pattern of zeolite Y.
4. The molecular sieve of claim 1 having the characteristic x-ray
powder diffraction pattern of zeolite mordenite.
5. The molecular sieve of claim 1 having the characteristic x-ray
powder diffraction pattern of LZ-202.
6. The molecular sieve of claim 1 having the characteristic x-ray
powder diffraction pattern of L.
Molecular sieve description
FIELD OF THE INVENTION
The instant invention relates to molecular sieve compositions,
the method for their preparation and to processes employing them.
More particularly it relates to molecular sieve compositions topologically
related to prior known molecular sieves but which are characterized
as containing framework atoms of of chromium, and preferably having
a very low content of defect sites in the structure, as hereinafter
disclosed. In general the preparative process involves contacting
a molecular sieve preferably with an aqueous solution of of a fluoro
salt of chromium, preferably a fluoro salt which does not form insoluble
salts with aluminum, under conditions suitable to insert chromium
for aluminum in the framework.
BACKGROUND OF THE INVENTION
The crystal structures of naturally occurring and as-synthesized
zeolitic aluminosilicates are composed of AlO.sub.4.sup.- and SiO.sub.4
tetrahedra which are cross-linked by the sharing of oxygen atoms.
The term AlO.sub.4.sup.-, SiO.sub.4 and the like, are used to depict
the tetrahedral atoms Al, Si and others, in four-fold coordination
with oxygen, within the framework of the zeolite. It is understood
that each of the four oxygen atoms thus depicted is linked to an
additional tetrahedral atom, thus completing the charge requirements
placed on each tetrahedral unit. The electrovalence of each tetrahedron
containing an aluminum atom is balanced by association with a cation.
Most commonly this cation is a metal cation such as Na.sup.+ or
K.sup.+ but organic species such as quaternary ammonium ions are
also employed in zeolite synthesis and in some instances appear
as cations in the synthesized product zeolite. In general the metal
cations are, to a considerable extent at least, replaceable with
other cations including H.sup.+ and NH.sub.4.sup.+. In many instances
the organic cation species are too large to pass through the pore
system of the zeolite and hence cannot be directly replaced by ion
exchange techniques. Thermal treatments can reduce these organic
cations to H.sup.+ or NH.sub.4.sup.+ cations which can be directly
ion-exchanged. Thermal treatment of the H.sup.+ or NH.sub.4.sup.+
cationic forms of the zeolites can result in the substantial removal
of these cations from their normal association with the AlO.sub.4.sup.
- tetrahedra thereby creating an electrovalent imbalance in the
zeolite structure which must be accompanied by structural rearrangements
to restore the electrovalent balance. Commonly when AlO.sub.4.sup.-
tetrahedra constitute about 40% or more of the total framework tetrahedra,
the necessary structural rearrangements cannot be accommodated and
the crystal structure collapses. In more siliceous zeolites, the
structural integrity is substantially maintained but the resulting
"decationized" form has certain significantly different
properties from its fully cationized precursor.
The relative instability of aluminum in zeolites, particularly
in the non-metallic cationic or the decationized form, is well recognized
in the art. For example, in U.S. Pat. No. 3640681 issued to P.
E. Pickert on Feb. 3 1972 there is disclosed a process for extracting
framework aluminum from zeolites which involves dehydroxylating
a partially cation deficient form of the zeolite and then contacting
it with acetylacetone or a metal derivative thereof to chelate and
solubilize aluminum atoms. Ethylenediaminetetraacetic acid has been
proposed as an extractant for extracting aluminum from a zeolite
framework in a process which is in some respects similar to the
Pickert process. It is also known that calcining the H.sup.+ or
NH.sub.4.sup.+ cation forms of zeolites such as zeolite Y in an
environment of water vapor, either extraneous or derived from dehydroxylation
of the zeolite itself, is effective in removing framework aluminum
by hydrolysis. Evidence of this phenomenon is set forth in U.S.
Pat. No. 3506400 issued Apr. 14 1970 to P. E. Eberly, Jr. et
al.; U.S. Pat. No. 3493519 issued Feb. 3 1970 to G. T. Kerr
et al.; and U.S. Pat. No. 3513108 issued May 19 1970 to G. T.
Kerr. In those instances in which the crystal structure of the product
composition is retained after the rigorous hydrothermal treatment
infrared analysis indicated in the presence of substantial hydroxyl
groups exhibiting a stretching frequency in the area of about 3740
3640 and 3550 cm.sup.-1. The infrared analytical data of U.S. Pat.
No. 3506400 is especially instructive in this regard. An explanation
of the mechanism of the creation of these hydroxyl groups is provided
by Kerr et al. in U.S. Pat. No. 3493519 wherein the patentees
state that the aluminum atoms in the lattice framework of hydrogen
zeolites can react with water resulting in the removal of aluminum
from the lattice in accordance with the following equation: ##STR1##
The aluminum removed from its original lattice position is capable
of further reaction with cationic hydrogen, according to Kerr et
al. to yield aluminum-containing i.e., hydroxoaluminum, cations
by the equation: ##STR2##
It has been suggested by Breck, D. W. and Skeels, G. W., "Zeolite
Chemistry II. The Role of Aluminum in the Hydrothermal Treatment
of Ammonium-Exchanged Zeolite Y, Stabilization", Molecular
Sieves - II, A. C. S. Symposium Series 40 pages 271 to 280 (1977),
that stabilization of NH.sub.4 Y occurs through hydrolysis of sufficient
framework aluminum to form stable clusters of these hydroxyaluminum
cations within the sodalite cages, thereby holding the zeolite structure
together while the framework anneals itself through the migration
of some of the framework silicon atoms.
It is alleged in U.S. Pat. No. 3594331 issued Jul. 20 1971
to C. H. Elliott, that fluoride ions in aqueous media, particularly
under conditions in which the pH is less than about 7 are quite
effective in extracting framework aluminum from zeolite lattices,
and in fact when the fluoride concentration exceeds about 15 grams
active fluoride per 10000 grams of zeolite, destruction of the
crystal lattice by the direct attack on the framework silicon as
well as on the framework aluminum can result. A fluoride treatment
of this type using from 2 to 22 grams of available fluoride per
10000 grams of zeolite (anhydrous) in which the fluorine is provided
by ammonium fluorosilicate is also described therein. The treatment
is carried out for the purpose of improving the thermal stability
of the zeolite. It is theorized by the patentee that the fluoride
in some manner becomes attached to the constructional alkali metal
oxide, thereby reducing the fluxing action of the basic structural
Na.sub.2 O which would otherwise result in the collapse of the crystal
structure. Such treatment within the constraints of the patent disclosure
has no effect on either the overall silicon content of the zeolite
product or the silicon content of a unit cell of the zeolite.
Since stability is quite obviously, in part at least, a function
of the Al.sub.2 O.sub.3 content of the zeolite framework, it would
appear to be advantageous to obtain zeolites having lower proportions
of Al.sub.2 O.sub.3 while avoiding the structural changes inherent
in framework aluminum extraction. Despite considerable effort in
this regard, however, only very modest success has been achieved,
and this has applied to a few individual species only.
A process for increasing the SiO.sub.2 /Al.sub.2 O.sub.3 ratio
in zeolites is disclosed in: commonly assigned U.S. Pat. No. 4503023
issue date Mar. 5 1985; commonly assigned U.S. Pat. No. 4610856
issue date Sep. 9 1986 U.S. Pat. No. 4711770 issue date Dec.
8 1987 (U.S. patent application Ser. No. 880103 filed Jun. 30
1986), and in Skeels, G. W. and Breck, D. W. "Proceedings of
the Sixth International Zeolite Conference", edited by David
Olson and Attilio Bisio, Butterworth & Co. Ltd. pages 87 to
96 (1984). The process disclosed therein comprises inserting silicon
atoms as SiO.sub.4 tetrahedra into the crystal lattice of an aluminosilicate
having a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of at least 3 and
pore diameters of at least 3 Angstroms with a fluoro-silicate salt
in an amount of at least 0.0075 moles per 100 grams of the zeolitic
aluminosilicate on an anhydrous basis, said fluorosilicate salt
being in the form of an aqueous solution having a pH value within
the range of 3 to about 7 and brought into contact with the zeolitic
aluminosilicate at a rate sufficiently slow to preserve at least
60 percent of the crystallinity of the staring zeolitic aluminosilicate.
Commonly assigned European Patent Application Serial No. 85902354.1
describes ammonium fluoride salts of the metal cations iron and/or
titanium which are used to treat the zeolites in an aqueous medium.
Framework aluminum is complexed by the fluoride and removed from
the zeolite. The metal cation is inserted into the framework in
place of the aluminum.
Various attempts have been made to substitute chromium or tin into
a zeolite framework via primary synthesis methods but none have
been truly successful so far. Attempts to synthesize zeolites of
the pentasil family of zeolites (ZSM-5 like) with a number of ions
other than aluminum have been made. In some cases chromium or tin
is found with the zeolite but not in the framework of the zeolite.
The likelihood that either chromium or tin is not a part of the
zeolite framework in primary synthesis products rests on the fact
that such a high pH is required for synthesis that it is probable
that the chromium or tin are present as oxides and/or hydrous oxides.
For example, in U.S. Pat. No. 4405502 (Klotz) discloses the presence
of up to 12.40 weight percent of Cr.sub.2 O.sub.3 with the crystalline
chromosilicate (Example IV), but the Cr.sub.2 O.sub.3 in the product
is present as amorphous or crystalline oxides. The examples teach
that the chromium, initially dissolved in water, is rapidly precipitated
as the hydroxide before ever coming in contact with the silica source.
Further, "these results show that as the chromium factor became
larger, more and more Cr.sub.2 O.sub.3 was detected in the product."
(Column 24 lines 15-17.)
Marosi et al., in German Patent No. 2831630 disclose the presence
of between 0.50 weight percent and 3.00 weight percent of Cr.sub.2
O.sub.3 with a ZSM-5 type structure. The amount of chromium that
would be included in the framework of the ZSM-5 if indeed it were
located therein, would range from 0.4 to 2.5 atoms out of 100 framework
tetrahedral atoms. In the only Example where a product composition
is given (1), the solid product would contain only 0.7 Cr atoms
out of 100 in the framework, a value less than the compositions
of the present invention. In Example 2 in U.K. Patent Application
GB 2024790 (Taramasso et al.), a 6.00 weight percent of Cr.sub.2
O.sub.3 with a ZSM-5 type structure was obtained and which was designated
"TRS-28". While the claims teach that the chromium atoms
either, "entered the crystalline lattice in place of silicon
atoms" or "in the form of salts of bisilicic or polysilicic
acids", the evidence presented in the examples fairly teach
that the chromium is not within the lattice framework of the ZSM-5
product. Surface areas of all of the products of the invention are
given which indicate that there is a substantial reduction relative
to a typical ZSM-5. This is evidence of some amorphous or dense
phase present with the zeolite. Typically ZSM-5 or its more siliceous
analog silicalite will have a surface area, (BET), of greater than
400 square meters per gram. The chromium containing product of the
U.K. patent application GB 2024790 had a surface area (BE) of
380 square meters per gram, a value at least 5% less than what might
be expected of a pure zeolite sample. Additionally, the chromium
containing product of said invention containing 6.0 weight percent
Cr.sub.2 O.sub.3 would be expected to have an ion exchange capacity
of 0.79 meq/gram, providing all of the chromium atoms were to be
positioned in the framework in tetrahedral coordination with four
oxygen atoms. However, only 0.0058 meq/gram of cations were actually
found in the calcined (550.degree. C.) product, a value at least
two orders of magnitude less than what would be necessary to balance
the framework negative charges, if chromium were indeed in the framework.
In order for chromium to be in the framework in tetrahedral coordination
with four oxygen atoms, it is a requirement that there be present
a positively charged species or cation in order to balance the negative
charge caused by the presence of the trivalent chromium ion sharing
the negative charges on four separate oxygen atoms with silicon.
Lacking the cation, it is not possible for the chromium to be tetrahedrally
coordinated with oxygen in this way and hence, the chromium of this
example is not in the framework of the zeolite synthesized in the
example. The converse is not necessarily true, namely, that if a
positively charged cation is found to balance the negative charge
on the chromium to satisfy the requirement of tetrahedral coordination
with oxygen, that the chromium is in the framework. It would be
evident that the chromium is in tetrahedral coordination with oxygen,
but it does not necessarily prove that the chromium is located in
the zeolite framework. It is probable that, like amorphous aluminosilicates,
the amorphous chromosilicates can have tetrahedrally coordinated
chromium atoms and hence ion exchange capacity.
European Patent Application 13630 (Rubin et al.) discloses the
presence of between 0.63 weight percent and 2.90 weight percent
of Cr.sub.2 O.sub.3 with a ZSM-12 type structure. The samples described
in the Tables of the patent application, particularly the products
containing chromium, show a substantial loss of surface area. This
indicates that the purity of the as-synthesized products is questionable
and that they must contain amorphous material. A relative relationship
can also be found in the Tables, namely that as the chromium content
of the synthesis product increases, the reported X-ray crstallinity
decreases.
In European Patent Application 14059 (Rubin et al.) between 0.09
weight percent and 1.26 weight percent of Cr.sub.2 O.sub.3 with
a ZSM-11 type structure was obtained. Similar observations can be
made with these products; that the products containing chromium
have reduced X-ray crystallinity, substantially reduced adsorption
capacity for n-hexane and cyclohexane and substantially lower surface
areas when compared to a product which does not contain chromium.
Each observation taken alone would not preclude the incorporation
of chromium in the ZSM-11 framework. However, taken together, these
data are substantive evidence for the precipitation of an amorphous
chromium containing phase with the zeolite, which under the very
basic synthesis conditions employed is the expected result.
Dwyer et al. in U.S. Pat. No. 3941871 disclose the presence of
tin in place of or as part of the organic template in a ZSM-5 type
of a sturcture but not as a part of the ZSM-5 framework structure
itself. In U.S. Pat. No. 4329328 (McAnespic et al.) the synthesis
of a stannosilicate is suggested, but no examples of such synthesis
are give nor are any properties of such materials suggested.
The above-mentioned references, while they may suggest the incorporation
of the chromium or tin metal ions into the frameworks of the respective
zeolites, provide consistent evidence that the metal ions are not
included in the framework, and are merely precipitated with the
zeolite as some other probably amorphous phase during the course
of the synthesis process. Tiele et al. in "Proceedings of the
International Symposium on Zeolite Catalysis", Siofok, Hungary,
May 13 1985 commented on isomorphic substitution in zeolites,
stating that, "Generally speaking these new materials are claimed
based upon their novel chemical composition or XRD spectrum or both.
This novelty does not necessarily mean that the new materials contain
the new element, or at least part of it, substituted in the zeolite
framework. As far as we are aware, only in the case of boron substitution
sound proof is available for its presence in the zeolite lattice."
The reason for this failure is then obvious, since the very synthesis
conditions used to synthesize the zeolite products are such that
a nearly insoluble metal hydroxide precipitates thereby limiting
the ability of the metal oxide to incorporate into the silicate
units during crystal growth. This feature was only recently pointed
out by Szostak et al. in Journal of Chemical Society, Faraday Trans.
I, page 83 (1987). By recognizing the critical nature of the pH
they were able to, for the first time, synthesize the ferrisilicate
analog of ZSM-5.
It should be further pointed out that Tielen at the top of page
9 states "based on this table and the evidence mentioned in
the introduction, elements with ionic radii between 0.020 and 0.061
are potential candidates for incorporation into a framework . .
. ". Using the values given for the Shannon references (radii
in tetrahedral coordination) it is observed that Sn.sup.+4 has an
ionic radius of 0.069 nm or 0.69 .ANG. (note that Shannon reports
this value as the crystal radii) which is outside the range stipulated
by Tielen and thus would not be expected to be in the framework.
Although values for Cr.sup.+3 and Sn.sup.+2 are not provided in
Tielen, they are provided by R. D. Shannon in his original paper
(Acta Cryst. A 32 751-767 (1976) as Cr.sup.+3 =0.755 and Sn.sup.+2
=1.36. It should be noted that the values for Cr.sup.+3 and Sn.sup.+2
are for octahedral coordination and the number for tetrahedral coordination
would be expected to be smaller. Further, the numbers presented
by Tielen are the crystal radii (see Shannon Table 1 page 752).
Accordingly, none of these metal ions would be expected to be in
the framework.
Another reference which has been cited in this art is Canadian
Pat. No. 1127134 to Morrison. This reference discloses an "Alumino-silicate
zeolite" which contains a metal oxide selected from the group
consisting of indium, boron, ruthenium, platinum, chromium, rare
earth, vanadium, palladium, molybdenum, mercury, tellurium, silver
and mixtures thereof. However, there is no mention or hint that
these metals are or could be in the framework. Indeed, on page 6
lines 1-7 the patentees state that it is not known whether the
metal is present as a metal or as a metal compound. Given this uncertainty,
it would be pure speculation to state that Morrison discloses an
aluminosilicate zeolite where some of the aluminum in the framework
has been replaced by chromium or some other metal. Accordingly,
there is nothing in Morrison that suggests a zeolite having chromium
as a framework metal.
The above mentioned references do suggest that it is desirable
to synthesize zeolites or molecular sieves containing chromium or
tin in the framework tetrahedral sites. However the methods employed
in the references leave little doubt that the metal has been deposited
with the zeolite either as an oxide or hydroxide or as an amorphous
metal silicate. The references further demonstrate the difficulty
involved in the incorporation of these metal ions in the zeolite
tetrahedral framework positions. The uniqueness of the method of
the current application which relies on the solubility of the chromium
and tin metal ions in an acidic medium, and the Secondary Synthesis
procedure to incorporate the metal ions into the framework is further
demonstrated. As for the obviousness of the Secondary Synthesis
procedure to incorporate any metal ion into the framework of an
existing zeolite, all attempts to use this process with the ions
of phosphorus or boron have thus far been unsuccessful. Boron is
the only metal ion thus far that has been successfully incorporated
into the pentasil zeolite framework via primary synthesis methods
(Tielen et al.). Only by careful control of the Secondary Synthesis
conditions can one be successful in incorporating iron and/or titanium
(D-13733), or chromium and/or tin into the framework of existing
zeolites or molecular sieves.
Secondary Synthesis as used herein means a process whereby a molecular
sieve product is treated by some method (Secondary Synthesis) to
obtain a molecular sieve product that is either not obtainable by
primary synthesis methods or is prepared with great difficulty or
is not normally found in nature.
The present invention relates to novel zeolite compositions which
contain significant framework tetrahedral atoms, which are not found
to any significant level either in naturally occurring zeolites
or in synthetic zeolites.
In the present invention, zeolite Y, zeolite L, mordenite and zeolite
LZ-202 (an omega type zeolite prepared without the use of a templating
agent as disclosed in U.S. Pat. No. 4840779) are treated with
aqueous ammonium fluoride salts of either or both chromium or tin.
During the treatment aluminum is removed from the molecular sieve
framework and the metal ions is incorporated therein. By means of
this invention, the metal ions of chromium and/or tin can be incorporated
into molecular sieve frameworks where they are not normally found
in nature.
DESCRIPTION OF THE FIGURE
FIG. 1 is a ternary diagram wherein parameters relating to the
instant compositions are set forth as mole fractions.
SUMMARY OF THE INVENTION
A molecular sieve composition having a three-dimensional microporous
framework structure which has an unit empirical formula on an anhydrous
basis of:
where "M" is at least one of chromium or tin; and "w",
"x" and "y" represent the mole fractions of
"M" , aluminum and silicon, respectively, present as framework
tetrahedral oxide units said mole fractions being such that they
are within the triagonal area defined by points, A,B, and C of FIG.
1.
A process for preparing molecular sieve composition containing
at least one of chromium or tin from a starting crystalline microporous
aluminosilicate having a framework structure comprising aluminum
and silicon present as tetrahedral oxides which comprises contacting
said crystalline aluminosilicate having pore diameters of at least
about 3 Angstroms and having a molar SiO.sub.2 Al.sub.2 O.sub.3
ratio of at least 2 with at least one of a fluoro salt of chromium
and a fluoro salt of tin, said fluoro salt being in the form of
a solution or slurry, whereby framework aluminum atoms of the zeolite
are removed and replaced by at least one of chromium or tin.
Molecular sieves and the process for their preparation are claimed
wherein said molecular sieves have three-dimensional microporous
crystalline framework structures consisting of CrO.sub.4.sup.-,
SnO.sub.4.sup.= or SnO.sub.4 AlO.sub.4.sup.- and SiO.sub.4 tetrahedra
which are cross linked by the sharing of oxygen atoms. These new
molecular sieves expressed as mole fractions of oxides have a unit
empirical formula on an anhydrous basis of :
where "M" is chromium and/or tin; and "w",
"x" and "y" represent one of the mole fractions
of "M", aluminum and silicon, respectively, present as
framework tetrahedral oxide units, said mole fractions being such
that they are within the compositional area defined by points A,
B and C in FIG. 1 where points A, B and C have the following values
for "w", "x" and "y".
______________________________________ Mole Fraction Point w x
y ______________________________________ A 0.49 0.01 0.50 B 0.01
0.49 0.50 C 0.01 0.01 0.98 ______________________________________
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to new molecular sieve compositions
and to the processes for their preparation. The molecular sieves
of the instant invention have three-dimensioned microporous crystal
framework oxide structures of "MO.sub.2 ", AlO.sub.2 and
SiO.sub.2 in tetrahedral units which have a unit empirical formula
on an anhydrous basis of:
wherein "M" represents at least one of chromium or tin;
and "w", "x" and "y" are as defined
above represent the mole fractions of "M", aluminum and
silicon, respectively, present as tetrahedral oxides.
The term "unit empirical formula" is used herein according
to its common meaning to designate the simplest formula where gives
the relative number of moles of chromium and/or tin (M), aluminum
and silicon which form "MO.sub.2 ", AlO.sub.2 and SiO.sub.2
tetrahedral units within the molecular sieve. The unit empirical
formula is given in terms of chromium and/or tin, aluminum and silicon
as shown in Formula (1), above, and does not include other compounds,
cations or anions which may be present as a result of the preparation
or the existence of other impurities or materials in the bulk composition
not containing the aforementioned tetrahedral units.
The instant process generally comprises a method for removing framework
aluminum from zeolites having SiO.sub.2 /Al.sub.2 O.sub.3 mole ratios
of about 2 or greater and substituting therefore one or more elements
selected from the group consisting of chromium and/or tin. The resulting
molecular sieves contain chromium and/or tin and have crystal structures
similar to that of the initial zeolite.
The process of the invention comprises contacting a crystalline
zeolite having pore diameters of at least about 3 Angstroms and
having a molar SiO.sub.2 /Al.sub.2 O.sub.3 ratio of at least 2
with an effective amount of at least one of a fluoro salt of chromium
or a fluoro salt of tin, preferably in an amount of at least 0.001
moles per 100 grams of zeolite starting material, said fluoro salt
being in the form of an aqueous solution or slurry and brought into
contact with the zeolite either incrementally or continuously at
a slow rate (optionally in the presence of a buffer) whereby framework
aluminum atoms of the zeolite are removed and replaced by chromium
and/or tin atoms. It is desirable that the process be carried out
such that at least 60 percent, preferably at least 80 percent, and
more preferably at least 90 percent of the crystal structure of
the starting zeolite is retained and that the Defect Structure Factor
(hereinafter defined) is increased by less than 0.10 and preferably
by less than 0.08.
Crystalline zeolite starting suitable for the practice of the present
invention can be any naturally occurring or synthetically produced
zeolite species which have pores large enough to permit the passage
of water, chromium and/or tin fluoro salts and reaction products
through their internal cavity system. These materials can generally
be represented, in terms of molar ratios of oxides, as
wherein "M" is a cation having the valence "n",
"x" is a value of at least about 2 and preferably about
3and "y" has a value of from zero to about 9. This value
of "y" depends upon the degree of hydration and the capacity
of the particular zeolite to hold adsorbed water. Alternatively,
the framework composition of the naturally occurring or synthetic
zeolite starting material can be expressed in terms of the mole
fraction of framework tetrahedra, TO.sub.2 as:
wherein "a" is the mole fraction of framework tetrahedral
sites occupied by aluminum atoms and "b" is the mole fraction
of framework tetrahedral sites occupied by silicon atoms. Should
the framework of the starting material contain atoms in addition
to silicon and aluminum, these materials may be similarly expressed
in terms of their "TO.sub.2 " formula in terms of their
fractional occupation of the framework of the starting material.
The algebraic sum of all of the subscripts within the brackets is
equal to 1. In the above example, a=b=1.
Representative of the crystalline aluminosilicate zeolite molecular
sieves include, but are not limited to erionite, mordenite, clinoptilolite,
zeolite Y, zeolite L, zeolite LZ-202 (an omega type zeolite prepared
without the use of a templating agent as disclosed U.S. Pat. No.
4840779), zeolite omega, zeolite beta, zeolite TMA offretite,
LZ-105 ZSM5ZSM-34 and ZSM-35. Zeolite Y is disclosed in U.S. Pat.
No. 3130007; zeolite L is disclosed in U.S, Pat. No. 3216789;
LZ-105 is disclosed in U.S. Pat. No. 4257885; zeolite omega is
disclosed in U.S. Pat. No. 4242036; zeolite beta is disclosed
in U.S. Pat. No. 3308069; ZSM-5is disclosed in U.S. Pat. No. 3702886;
ZSM-34 is disclosed in U.S. Pat. No. 4086186; and ZSM-35 is disclosed
in U.S. Pat. No. 3992466. Both naturally occurring and synthetically
prepared zeolite molecular sieves can be used.
For reasons more fully explained hereinafter, the starting zeolite
should be able to withstand the initial loss of framework aluminum
atoms to at least a modest degree without collapse of the crystal
structure unless the process is to be carried out at a very slow
rate, or the process is to be buffered. In general the ability to
withstand aluminum extraction and maintain a high level of crystallinity
is directly proportional to the initial SiO.sub.2 /Al.sub.2 O.sub.3
molar ratio of the zeolite. Accordingly, it is preferred that the
SiO.sub.2 /Al.sub.2 O.sub.3 ratio is preferably at least about 2.0
and more preferably about 3. It is also preferred that at least
about 50 percent, and more preferably at least 95 percent of the
AlO.sub.4.sup.- tetrahedra of the naturally occurring or as-synthesized
zeolite are present in the starting zeolite. Most advantageously
the starting zeolite contains as many as possible of its original
AlO.sub.4.sup.31 tetrahedra, i.e. the starting zeolite has not been
subjected to any post-formation treatment which either extensively
removed aluminum atoms from their original framework sites or converts
them from the normal conditions of 4-fold coordination with oxygen.
The cation population of the starting zeolite is not a critical
factor insofar as substitution of chromium and/or tin for framework
aluminum is concerned, but since the substitution mechanism may
involve the in situ formation of salts of at least some of the zeolitic
cations, it is generally advantageous that these salts be water-soluble
to a substantial degree to facilitate their removal from the molecular
sieve product. It is found that ammonium or hydronium cations form
the most soluble salts in this regard and it is accordingly preferred
that partially or at least 50 percent, most preferably 85 or more
percent, of the zeolite cations be ammonium or hydronium cations.
Sodium, one of the most common cations present in zeolites, is found
to form Na.sub.3 AlF.sub.6 which is only very sparingly soluble
in either hot or cold water. When this compound is formed as precipitates
within the structural cavities of the zeolite it is quite difficult
to remove by water washing. Its removal, moreover, is important
if thermal stability of the molecular sieve product is desired since
substantial amounts of fluoride can cause crystal collapse at temperatures
as low as 500.degree. C.
For purposes of simplifying the description of the products of
the above process, as above defined the framework composition of
the zeolite starting material and the products of the instant process
are expressed in terms of mole fractions of framework tetrahedra,
i.e., the "TO.sub.2 ", where T represents the substituting
tetrahedral atom in the framework. The starting zeolite may be expressed
as:
whereas "a" is the mole fraction of aluminum tetrahedra
in the framework; "b" is the mole fraction of silicon
tetrahedra in the framework; "[ ]" denotes defect sites
in the framework; and "z" is the mole fraction of defect
sites in the zeolite framework. In many cases the "z"
value for the starting zeolite is zero and the defect sites are
simply eliminated from the expression. Numerically the sum of the
values a+b+z=1.
If defect sites are present, the molecular sieves of this invention
can have a unit empirical formula expressed in terms of the mole
fractions of framework tetrahedra (TO.sub.2) of :
where u is the mole fraction of aluminum and ranges from about
0.01 to about 0.5 v is the mole fraction of silicon and ranges
from about 0.5 to about 0.98 M is chromium, tin or mixtures thereof,
w is the mole fraction of M and ranges from about 0.01 to about
0.49 [ ] is framework defect sites and z is the mole fraction of
defect sites in the framework and ranges from greater than zero
to about 0.2.
When the molecular sieves of this invention are prepared by the
specific process of this invention, then the molecular sieve can
have a unit empirical formula expressed in terms of the mole fractions
of tetrahedra (TO.sub.2) of:
wherein: "N" is defined as the mole fraction of aluminum
tetrahedra removed from the framework during the treatment; "a"
is the mole fraction of aluminum tetrahedra present in the framework
of the starting zeolite; "b" is the mole fraction of silicon
tetrahedra present in the framework of the zeolite; "[ ]"
is the framework defect sites; "z" is the mole fraction
of defect sites in the framework and varies from greater than zero
to about 0.2; "M" denotes chromium and/or tin; and "c"
is the mole fraction of chromium and/or tin tetrahedra resulting
from the fluoro salt treatment of the instant process and varies
from about 0.01 to 0.5. Numerically, the sum of the values:
The term "Defect Structure Factor" for any given zeolite
is equivalent to the "z" value of that particular zeolite.
The net change in Defect Structure Factors between the starting
zeolite and the product zeolite is equivalent "Z".
Theoretically, there should be no change in the silicon content
and therefore "c" should equal (N-z) where "z"
is the net change in the mole fraction of defect sites in the zeolite
framework resulting from the treatment. However, in reality fluoride
does sometimes react with silicon of the molecular sieve particularly
on the surface of the crystals of the more siliceous molecular sieves
causing etching and transport of silicon atoms to other defect sites
of the crystal. Hence "c" will not always be actually
equal to (N-z).
The chromium and/or tin-containing molecular sieve compositions
prepared by the instant processes have framework aluminum removed
from the starting zeolite with substitution of chromium and/or tin.
The process generally comprises:
(a) contacting at effective processes conditions a zeolite with
an effective amount of at least one of a fluoro salt of chromium
or a fluoro salt of tin; and
(b) isolating the chromium and/or tin-containing molecular sieve
product from the reaction mixture.
The instant process generally comprises contacting a crystalline
zeolite having a pore diameter of at least about 3 Angstroms and
having a molar SiO.sub.2 /Al.sub.2 O.sub.3 ratio of at least 2
with at least 0.0075 moles of a fluoro salt of chromium or a fluoro
salt of tin, per 100 grams of zeolite starting material, said fluoro
salt being in the form of a solution or slurry. The fluoro salt
is preferably provided as an aqueous solution or slurry but is believed
that solutions or slurries employing alcohols and other organic
solvents may be employed.
It is necessary that the solution or slurry be maintained at an
effective pH. The "effective pH" is a pH such that under
effective process conditions; a) a monomeric speices of the chromium
and/or tin is present in the reaction solution; and b) the pH is
high enough to avoid undue destructive acidic attack on the particular
zeolite structure, apart from the intended reaction with an effective
amount of the fluoro salt. The effective amount of fluoro salt is
that amount which provides sufficient fluoride and chromium and/or
tin for the process and the desired amount of chromium and/or tin
in the final molecular sieve product. The effective pH value for
this invention is generally greater than one (1), more preferably
greater than 3 (three) and most preferably in the range of about
3 to about 7 (seven).
A pH of about 3 or more usually assures that no acid degradation
of the zeolite occurs but it may not necessarily be the optimum
pH for the formation of monomeric species of either chromium and/or
tin in the solution. At pH values below about 3 crystal degradation
of many zeolites is found to be unduly severe. Whereas at pH values
higher than 7 insertion of the chromium and/or tin may be slow
from a practical standpoint as a result of the solubility of chromium
and/or tin at these pHs and as a result of certain polymerization
reactions. A pH of 7 and above typically results in no monomeric
species of either chromium and/or tin being present in the solution
so that very little substitution of these metal atoms in the framework
would occur. Frequently the polymeric species of chromium and/or
tin will precipitate as solid oxides or hydrous oxides at pH 7 or
above.
The fluoro salt solution or slurry is brought into contact with
the zeolite either incrementally or continuously at a slow rate
whereby framework aluminum atoms of the zeolite are removed and
replaced by chromium and/or tin atoms from the fluoro salt.
The solution or slurry of the fluoro salt, preferably aqueous,
is brought into contact with the zeolite either incrementally or
continuously at an effective rate such that a portion of the framework
aluminum atoms are removed and replaced by chromium and/or tin atoms
at a rate which preferably retains at least 80 percent and more
preferably at least 90 percent of the crystal structure of the starting
zeolite.
The fluoro salt used as the aluminum extractant and also as the
source of chromium and/or tin, which is inserted into the zeolite
structure in place of the extracted aluminum, can be any of the
fluoro salts having the general formula:
where "M" is tin and "A" is a metallic or non-metallic
cation, having the valence "b". Cations represented by
"A" include alkylammonium, H.sup.+, NH.sub.4.sup.+, Mg.sup.++,
Li.sup.+, Na.sup.+, K.sup.+, Ba.sup.++, Cd.sup.++, Cu.sup.+, Ca.sup.++,
Cs.sup.+, Fe.sup.++, Co.sup.++, Pb.sup.++, Mn.sup.++, Rb.sup.+,
Ag.sup.+, Sr.sup.++, Tl.sup.+ and Zn.sup.++, or the formula A.sub.2/b
MF.sub.5 where "M" is chromium. The ammonium and hydronium
cation forms of the fluoro salts are generally preferred because
of their solubility in water and also because these cations form
water soluble by-product salts upon reaction with the zeolite, e.g.,
(NH.sub.4).sub.3 AlF.sub.6 and/or (NH.sub.4).sub.2 AlF.sub.5. Other
salts which may be used include a combination of salts of MF.sub.3
and 3/2(NH.sub.4 HF.sub.2) or MF.sub.4 and NH.sub.4 HF.sub.2 where
M is chromium or tin. Preferred fluoro salts are (NH.sub.4).sub.3
CrF.sub.5 ; CrF.sub.3.3/2((NH.sub.4)HF.sub.2); NH.sub.4 SnF.sub.
3 ; SnF.sub.2 .3/2(NH.sub.4 HF.sub.2) and SnF.sub.4.NH.sub.4 HF.sub.2.
The manner in which the fluoro salt of chromium or the fluoro salt
of tin and the starting zeolite are brought into contact and the
overall process of substituting chromium and/or tin for aluminum
in the zeolite framework is believed to be a two step process in
which the aluminum extraction step tends to, unless controlled,
proceed very rapidly while the insertion of chromium and/or tin
is generally relatively slow. If dealumination becomes too extensive
without the substitution of chromium and/or tin the crystal structure
become seriously degraded and ultimately collapses. While not wishing
to be bound by any particular theory, it appears that fluoride ion
acts as the agent for extraction of framework aluminum in accordance
with the equation: ##STR3##
It is important, therefore, that the initial dealumination step
be inhibited and the step involving insertion of chromium and/or
tin be promoted to achieve the desired molecular sieve product.
It is found that the various zeolites have varying degrees of resistance
toward degradation as a consequence of framework aluminum extraction
without substitution of chromium and/or tin into the framework.
Accordingly, for the reasons stated above the pH is preferably within
the range of 3 to 7. The higher pH inhibits the rate and amount
of dealumination. Also, increasing the reaction temperature tends
to increase the rate of substitution of chromium and/or tin. Increasing
the reaction temperature has been found to have less of an effect
on dealumination than the pH of the solution. Therefore, the pH
may be considered a means of controlling dealumination while temperature
may be considered as a means of controlling the substitution rate.
Whether it is necessary or desirable to buffer the reaction system
or select a particular fluoro salt concentration to control the
pH is readily determined for each zeolite species by routine observation
and evaluation. The question of whether the reaction system may
advantageously be buffered will in large part depend on the selection
of the particular starting zeolite, since zeolites have varying
tolerances to acid and base media. For example, some zeolites such
as mordenite and clinoptilolite can withstand very low pH conditions
and a high level of dealumination without collapse of the crystal
structure. When it is advantageous to buffer the reaction mixture
in a particular pH range the reaction mixture may be buffered in
a manner as generally heretofore employed in the art. The use of
buffering salts, such as ammonium acetate, or use of an inert solid
to react with excess acid or base, e.g. clays or aluminas, may be
suitable to buffer the pH of the reaction mixture.
Theoretically, there is no lower limit for the concentration of
fluoro salt of chromium and/or tin in the aqueous solution or slurry
employed. A slow rate of addition of the fluoro salt generally provides
adequate time for the insertion of chromium and/or tin as a framework
substitute for extracted aluminum before excessive aluminum extraction
occurs with consequent collapse of the crystal structure. Practical
commercial considerations, however, may require that the reaction
proceed as rapidly as possible, and accordingly the conditions of
reaction temperature and reagent concentrations will necessarily
be optimized with respect to each zeolite starting material and
with respect to commercial operation. In general it is believed
that the more highly siliceous the zeolite, the higher the permissible
reaction temperature and the lower the pH conditions which may be
employed in the instant process. In general the preferred effective
reaction temperature is within the range between about 10.degree.
C. and about 99.degree. C., preferably between about 20.degree.
C. and 95.degree. C., but temperatures of 125.degree. C. or higher
and as low as 0.degree. C. are believed employable in some instances
with some zeolite starting materials and with fluoro salts in a
form other than aqueous solutions or slurries. The maximum concentration
of fluoro salt in the aqueous solution employed is, of course, interrelated
to the temperature and pH factors and also with the time of contact
between the zeolite and the solution and the relative proportions
of zeolite and fluoro salt. Solutions having fluoro salt concentrations
of between about 10.sup.-3 moles per liter of solution and up to
saturation of the solution can be employed, but it is preferred
that concentrations in the range of between about 0.5 and about
1.0 moles per liter of solution be used. In addition, as hereinbefore
discussed, slurries of the fluoro salts of chromium and/or tin may
be employed. The aforementioned concentration values are with respect
to true solutions, and are not intended to apply to the total fluoro
salts in slurries of the salts in water. Even very slightly soluble
fluoro salts can be slurries in water and used as a reagent, the
undissolved solids being readily available to replace dissolved
molecular species consumed in reaction with the zeolite. As stated
hereinabove, the amount of dissolved fluoro salts employed with
respect to the particular zeolite being treated will depend to some
extent upon the physical and chemical properties of the individual
zeolites and other effective process conditions. However, the minimum
value for the amount of fluoro salt to be added is preferably at
least equivalent to the minimum mole fraction of aluminum to be
removed from the zeolite.
In specifying the proportions of the zeolite starting material
or adsorption properties of the zeolite product and the like herein,
the "anhydrous state" of the zeolite will be intended
unless otherwise stated. The term "anhydrous state" is
employed herein to refer to a material substantially devoid of both
physically adsorbed and chemically adsorbed water. In general a
zeolite may be prepared in the anhydrous state by heating the zeolite
in dry air at about 450.degree. C. for about 4 hours.
It is apparent from the foregoing that, with respect to effective
process conditions, it is desirable that the integrity of the zeolite
crystal structure be substantially maintained throughout the process,
and that, in addition to having chromium and/or tin atoms inserted
into the lattice, the zeolite retains at least 60 percent, preferably
at least 80 and more preferably at least 90 percent of its original
crystallinity. A convenient technique for assessing the crystallinity
of the products relative to the crystallinity of the starting material
is the comparison of the relative intensities of the d-spacings
of their respective X-ray powder diffraction patterns. The sum of
the peak heights, in terms of arbitrary units above background,
of the starting material is used as the standard and is compared
with the corresponding peak heights of the products. When for example,
the numerical sum of the peak heights of the molecular sieve product
is 85 percent of the value of the sum of the peak heights of the
starting zeolite, then 85 percent of the crystallinity has been
retained. In practice it is common to utilize only a portion of
the d-spacing peaks for this purpose, as for example, five of the
six strongest d-spacings. In zeolite Y these d-spacings correspond
to the Miller Indices 331 440 533 642 and 555. Products of the
instant invention will have a certain fraction of the framework
tetrahedra replaced by tin and/or chromium atoms. Because atoms
of these heavier elements are incorporated there may be a decrease
in the X-ray crystallinity values due to scatter because of the
heavier elements. In this case, more reliable indicia of the crystallinity
retained by the zeolite product are the degree of retention of surface
area or the degree of retention of the adsorption capacity. Surface
areas can be determined by the well-known Brunauer-Emmett-Teller
method (B-E-T). See for example, Journal of American Chemical Society,
Volume 60 page 309 (1938) using nitrogen as the adsorbate. In determining
the adsorption capacity, the capacity for oxygen at -183.degree.
C. (90.degree. K.) at 100 Torr is preferred.
Analysis of the Substitution Mechanism
The following is a hypothetical description of the mechanism involved
and may not be the actual mechanism that is taking place. This description
is based upon the present available data and analysis of the substitution
products of this invention. This hypothetical description seems
to be consistent with that data and may help to explain this unique
process.
All available evidence, to date, indicates that the above described
process of this invention is unique is being able to produce zeolites
essentially free of defect structure and having chromium and/or
tin inserted into the framework by a secondary synthesis process.
In untreated, i.e. naturally occurring or as-synthesized zeolites
the original tetrahedral structure is conventionally represented
as ##STR4##
After treatment with a complexing agent such as ethylene-diamine-tetraacetic
acid (H.sub.4 EDTA) in which a stoichiometric reaction occurs whereby
framework aluminum atoms along with an associated cation such as
sodium is removed as NaAlEDTA, it is postulated that the tetrahedral
aluminum is replaced by four protons which form a hydroxyl "nest",
as follows: ##STR5##
In the practice of this invention, a two-step process is envisioned.
In the first step of the treatment, tetrahedral aluminum atoms are
first hydrolyzed and removed from the zeolite framework, whereupon
they immediately react to form a more stable aluminum species or
compound (i.e. aluminum fluoride species).
In the second step, ions of suitable size and coordination number
are inserted into the vacant tetrahedral sites created by the dealumination.
85902354.1 (U.S. patent application Ser. No. 604179), involves
replacement of framework aluminum by either iron and/or titanium.
The present work involves replacement by either tin atoms or chromium
atoms or both into vacant framework sites created by the dealumination.
The individual steps of the process can be accomplished in separate
operations. However, it is more desirable to perform both steps
in a single efficient operation. A particularly efficient class
of compounds which can effect the dealumination and framework substitution
steps in a single operation can be designated by J.sub.x TF.sub.y,
where T represents the substituting tetrahedral atom. The substituting
tetrahedral atom (T) when hydrolyzed in solution forms a hydroxylated
species and an acid. The acid subsequently attacks the Al in the
framework to cause the dealumination. The fluoride (F) serves to
complex with the removed aluminum atoms, and J is the charge-balancing
cation or cations. While the process is carried out in an aqueous
system, it is not necessary that the J.sub.x TF.sub.y compound be
dissolved in the solution. It is only necessary that it be sufficiently
soluble to initiate the reaction with the zeolite. It is important
that the reaction byproduct (the aluminum fluoride) be in a form
that is readily removed from the zeolite by a washing step, subsequent
to the substitution reaction. The presence of fluoride in the zeolite
product in concentrations as low as 1 weight percent (or even lower),
results in decreased thermal stability of the zeolite crystals.
The residual fluoride can react with Si in the zeolite at elevated
temperatures to cause the zeolite crystal to collapse.
Salts of the class of compounds J.sub.x TF.sub.y which have been
used in the practice of this invention are:
NH.sub.4 SnF.sub.3 ; 3(NH.sub.4 F).CrF.sub.3 ; 3(NH.sub.4 HF.sub.2).CrF.sub.3
SnF.sub.2.NH.sub.4 HF.sub.2 ; SnF.sub.4.NH.sub.4 HF.sub.2 ; and
CrF.sub.3.NH.sub.4 HF.sub.2.
Among the list of zeolites known to react with one or more of the
above listed compounds to effect framework substitution are: the
synthetic zeolite Y, mordenite, zeolite L and zeolite LZ-202 (an
omega type zeolite prepared without the use of a templating agent
as disclosed in (U.S. Pat. No. 3840779). With all of these zeolites,
the reaction to dealuminate the starting zeolite and replace the
removed aluminum atoms with a different tetrahedral atom did take
place, at least to some extent, although the resulting zeolite may
not have been the optimum product.
The chemistry of the process can be envisioned approximately in
the following way. In the first step an aqueous slurry of the zeolite
is contacted with a solution of J.sub.x TF.sub.y salt. In some cases,
because of the limited solubility of J.sub.x TF.sub.y, the zeolite
and the salt can be slurried together. The salt hydrolyzes in aqueous
solution to form acid, H.sub.3 O.sup.+ and free fluoride. One example
of this hydrolysis where T=Sn and J=NH.sub.4 can be depicted as
follows:
The acid thus formed (H.sub.3 O).sup.+, reacts rapidly to dealuminate
the zeolite. The removed aluminum rapidly reacts with the free fluoride
to form aluminum fluoride salts such as AlF.sub.3 (NH.sub.4).sub.2
AlF.sub.5 and (NH.sub.4).sub.3 AlF.sub.6.
This reaction is the most crucial part of the process, since the
dealumination step is very rapid. If too much dealumination occurs
(without substitution into the vacant tetrahedral sites), the zeolite
quickly loses its crystal structure. The use of a buffer such as
ammonium acetate, thereby keeping the pH greater than about 6.0
can be used to slow down the hydrolysis so that the slower substitution
step can take place. Another method of controlling the dealumination
step is to add the J.sub.x TF.sub.y solution very slowly to the
zeolite slurry. In this manner, some substitution can occur before
the zeolite framework is excessively dealuminated to the point of
causing crystal collapse. With the slow addition of the J.sub.x
TF.sub.y solution, the zeolite itself acts as a "buffer"
in the system.
The second step is the substitution of a new tetrahedral atom into
the zeolite structure in place of the removed aluminum atom. This
step has been found to be the overall rate-limiting or slow step.
Increasing the temperature of the system increases the rate of substitution,
but it may also speed up the rate of other undesirable side reactions
such as the dealumination or the continued hydrolysis of T to form
a polymeric species which will no longer be able to substitute in
the framework defect sites. The exact chemistry of the substitution
step is not known in detail. It can be suggested that dealumination
of the zeolite leaves a hydroxyl nest in the vacant site, which
in turn reacts with the hydrolyzed form of the substituting tetrahedral
atom.
The stepwise reaction can be depicted as follows: ##STR6##
THE EXPERIMENTAL CONDITIONS
The infrared spectrum of the aluminum depleted zeolite will show
a broad nondescript absorption band beginning at about 3750 cm.sup.-1
and extending to about 3000 cm.sup.-1. The size of this absorption
band or envelope increases with increasing aluminum depletion of
the zeolite. The reason that the absorption band is so broad and
without any specific absorption frequency is that the hydroxyl groups
in the vacant sites in the framework are coordinated in such a way
that they interact with each other (hydrogen bonding). The hydroxyl
groups of adsorbed water molecules are also hydrogen-bonded and
produce a similar broad adsorption band as do the "nest"
hydroxyls. Also, certain other zeolitic hydroxyl groups, exhibiting
specific characteristic absorption frequencies within the range
of interest, will if present, cause infrared absorption bands in
these regions which are superimposed on the band attributable to
the "nest" hydroxyl groups. These specific hydroxyls are
created by the decomposition of ammonium cations or organic cations
present in the zeolite.
It is, however, possible to treat zeolites, prior to subjecting
them to infrared analysis, to avoid the presence of the interfering
hydroxyl groups and thus be able to observe the absorption attributable
to the "nest" hydroxyls only. The hydroxyls belonging
to adsorbed water are avoided by subjecting the hydrated zeolite
sample to vacuum activation at a moderate temperature of about 200.degree.
C. for about 1 hour. This treatment permits desorption and substantially
complete removal of the adsorbed water. Complete removal of adsorbed
water can be ascertained by noting when the infrared absorption
band at about 1640 cm.sup.-1 the bending frequency of water molecules,
has been removed from the spectrum.
The decomposable ammonium cations can be removed, at least in large
part, by ion-exchange and replaced with metal cations, preferably
by subjecting the ammonium form of the zeolite to a mild ion exchange
treatment with an aqueous NaCl solution. The OH absorption bands
produced by the thermal decomposition of ammonium cations are thereby
avoided. Accordingly the adsorption band over the range of 3745
cm.sup.-1 to about 3000 cm.sup.-1 for a zeolite so treated is almost
entirely attributable to hydroxyl groups associated with defect
structure and the absolute absorbance of this band can be a measure
of the degree of aluminum depletion.
It is found, that the ion-exchange treatment, which must necessarily
be exhaustive even though mild, required considerable time. Also
the combination of the ion-exchange and the vacuum calcination to
remove adsorbed water does not remove every possible hydroxyl other
than defect hydroxyls which can exhibit absorption in the 3745 cm.sup.-1
to 3000 cm.sup.-1 range. For instance, a rather sharp band at 3745
cm.sup.-1 has been attributed to the Si--OH groups situated in the
terminal lattice positions of the zeolite crystals and to amorphous
(non-zeolitic) silica from which physically adsorbed water has been
removed. For these reasons it is preferred to use a somewhat different
criterion to measure the degree of defect structure in the zeolite
products of this invention.
In the absence of hydrogen-bonded hydroxyl groups contributed by
physically adsorbed water, the absorption frequency least affected
by absorption due to hydroxyl groups other than those associated
with framework vacancies or defect sites is at 3710.+-.5 cm.sup.-1.
Thus the relative number of defect sites remaining in a zeolite
product of this invention can be gauged by first removing any adsorbed
water from the zeolite, determining the value of the absolute absorbance
in its infrared spectrum at a frequency of 3710 cm.sup.-1 and comparing
that value with the corresponding value obtained from the spectrum
of a zeolite having a known quantity of defect structure. The following
specific procedure has been arbitrarily selected and used to measure
the amount of defect structure in the products prepared in the Examples
appearing hereinafter. Using the data obtained from this procedure
it is possible, using simple mathematical calculation, to obtain
a single and reproducible value hereinafter referred to as the "Defect
Structure Factor", denoted hereinafter by the symbol "z",
which can be used in comparing and distinguishing the present novel
zeolite compositions from their non-chromium and/or tin containing
counter-parts.
DEFECT STRUCTURE FACTOR "Z"
(A) Defect Structure Zeolite Standard
Standards with known amounts of defect structure can be prepared
by treating a crystalline zeolite of the same species as the product
sample with ethylenediaminetetraacetic acid by the standard procedure
of Kerr as described in U.S. Pat. No. 3442795. In order to prepare
the standard it is important that the starting zeolite be well crystallized,
substantially pure and free from defect structure. The first two
of these properties are readily determined by conventional X-ray
analysis and the third by infrared analysis using the procedure
set forth in part (B) hereof. The product of the aluminum extraction
should also be well crystallized and substantially free from impurities.
The amount of aluminum depletion, i.e., the mole fraction of tetrahedral
defect structure of the standard samples can be ascertained by conventional
chemical analytical procedure. The molar SiO.sub.2 /Al.sub.2 O.sub.3
ratio of the starting zeolite used to prepare the standard sample
in any given case is not narrowly critical, but is preferably within
about 10% of the molar SiO.sub.2 /Al.sub.2 O.sub.3 ratio of the
same zeolite species used as the starting material in the practice
of the process of the present invention.
(B) Infrared Spectrum of Product Samples and Defect Structure Zeolite
Standard
Fifteen milligrams of the hydrated zeolite to be analyzed are pressed
into a 13 mm. diameter self-supporting wafer in a KBr die under
5000 lbs. pressure. The wafer is then heated at 200.degree. C. for
1 hour at a pressure of not greater than 1.times.10.sup.-4 mm Hg
to remove all observable traces of physically adsorbed water from
the zeolite. This condition of the zeolite is evidenced by the total
absence of an infrared absorption band at about 1640 cm.sup.-1.
Thereafter, and without contact with adsorbable substances, particularly
water vapor, the infrared spectrum of the wafer is obtained on an
interferometer system at 4 cm.sup.-1 resolution over the frequency
range of at least 3745 to 3000 cm.sup.-1. Both the product sample
and the standard sample are analyzed using the same interferometer
system to avoid discrepancies in the analysis due to different apparatus.
The spectrum, normally obtained in the transmission mode of operation
is mathematically converted to and plotted as wave number vs. absorbance.
(C) Determination of the Defect Structure Factor
The defect structure factor (z) is calculated by substituting the
appropriate data into the following formula: ##EQU1## wherein AA
.sub.(ps) is the infrared absolute absorbance measured above the
estimated background of the product sample at 3710 cm.sup.-1 ; AA
.sub.(std) is the absolute absorbance measured above the background
of the standard at 3710 cm.sup.-1 and the mole fraction of defects
in the standard are determined in accordance with part (A) above.
Once the defect structure factor, z, is known, it is possible to
determine from the wet chemical analysis of the product sample for
SiO.sub.2 AlO.sub.3 chromium and/or tin and the cation content
as M.sub.2/n O whether chromium and/or tin has been substituted
for aluminum in the zeolite as a result of the treatment and also
the efficiency of the substitution of chromium and/or tin.
The essential X-ray powder diffraction patterns appearing in this
specification and referred to in the appended claims are obtained
using either: 1) standard X-ray powder diffraction techniques; or
2) computer based techniques using copper K-alpha radiation and
using Siemens D-500 X-ray powder diffractometers with Siemens Type
K-805 X-ray sources, available from Siemens Corporation, Cherry
Hill, N.J., with appropriate computer interface. When employing
the standard X-ray technique the radiation source is a high-intensity,
copper target, x-ray tube operated at 50 Kv and 40 ma. The diffraction
pattern from the copper K alpha radiation and graphite monochromator
is suitably recorded by an X-ray spectrometer scintillation counter,
pulse-height analyzer and strip-chart recorder. Flat compressed
powder samples are scanned at 2.degree. (2 theta) per minute, using
a 2 second time constant. Interplanar spacings (d) are obtained
from the position of the diffraction peaks expressed as 2 theta,
where 2 theta is the Bragg angle as observed on the strip chart.
Intensities are determined from the heights of diffraction peaks
after subtracting background.
All of the zeolite samples were evaluated according to standard
analytical procedures. The x-ray crystallinity of most samples was
measured using the Siemens D-500 where peak areas as well as peak
intensities of all major reflections were measured and compared
against untreated samples of the starting materials. It was expected
that the product of a successful experiment would maintain a major
fraction of its x-ray crystallinity. Unit cell values were measured
on materials possessing cubic unit cells (a.sub.o).
Framework infrared spectra of the treated zeolites were compared
to the framework spectra of the respective starting materials. A
general overall shift of the framework absorption frequencies to
higher wave numbers is a good indication of a higher silicon content
in the framework. Shift of the asymmetric stretch band at about
950-1250 cm.sup.-1 accompanies dealumination. The symmetric stretch
band, 750-835 cm.sup.-1 is more sensitive to the actual silicon
content in the framework, shifting to higher wave numbers as the
silicon content increases. Very little is known about the effect
of substitution of atoms other than silicon into the zeolite framework
on the position of these bands. Very little effect on the position
of the symmetric stretch band has been observed as a result of simple
dealumination. However, because there are not studies of the effect
of dealumination on the positions or shifts of the framework infrared
bands with zeolites other than Y and perhaps mordenite; the lack
of a substantial shift of the symmetric stretch band was not used
as the sole criterion to judge the degree of metal atom substitution.
More specifically, there are no studies of the effect of substituting
either chromium or tin or both for aluminum in the zeolite framework
on shifts of framework infrared bands. A general assumption would
be that ions larger than Al would increase the unit cell size causing
a decrease in framework infrared absorption band positions. Conversely,
substitution of ions smaller than Al into the zeolite framework
would cause a decrease in unit cell size and an increase in framework
infrared absorption band positions.
The hydroxyl region infrared spectrum was used to evaluate the
relative amount of framework defect sites in the zeolite product
of this invention. For a more thorough description of this method
of evaluation see U.S. Pat. No. 4503023. Briefly, using standard
procedures, the absolute absorbance (above background) at 3710 cm.sup.-1
was measured and compared to a standard sample of aluminum-depleted
NaY which contained a known number of defects. The defect structure
factor (z) of the reference standard was 0.140 and gave rise to
an absolute absorbance value of 0.330 at 3710 cm.sup.-1 of the infrared
spectrum. The reference value of z in this case is the mole fraction
of vacant tetrahedral sites in the zeolite framework of the aluminum-depleted
NaY. Fourteen percent of all of the tetrahedral sites do not contain
a tetrahedral atom (Si or Al), but rather, some form of hydrogen-bonded
OH groups. |