Molecular sieve abstract
A process for producing new crystalline titanium molecular sieve
zeolite compositions having a pore size of about 3-5 Angstrom Units
is disclosed.
Molecular sieve claims
What is claimed is:
1. A process for the preparation of a crystalline titaniumsilicate
molecular sieve zeolite having a pore size of approximately 3-5
Anstrom units which comprises heating a reaction mixture containing
a titanium source, a source of silica, a source of alkalinity, water
and, optionally, an alkali metal fluoride having a composition in
terms of mole ratios falling within the following ranges:
SiO.sub.2 /Ti: 1-10
H.sub.2 O/SiO.sub.2 : 2-100
M.sub.n /SiO.sub.2 : 0.1-10
wherein M indicates the cations of valence n derived from the source
of alkalinity and potassium fluoride to a temperature of from about
100.degree. C. to 300.degree. C. for a period of time ranging from
about 8 hours to 40 days while controlling the pH within the range
of 10.45-11.0.+-.0.1 said crystalline titaniumsilicate having a
composition in terms of mole ratios of oxides as follows:
wherein M is at least one cation having a valence of n, y is from
1.0 to 100 and z is from 0 to 100 and characterized by an X-ray
powder diffraction pattern having the lines and relative intensities
set forth in Table I below:
Molecular sieve description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to methods for preparing new crystalline
titanium molecular sieve zeolite compositions.
2. Background of the Invention and Prior Art
Since the discovery by Milton and coworkers (U.S. Pat. Nos. 2882243
and 2882244) in the late 1950's that aluminosilicate systems could
be induced to form uniformly porous, internally charged crystals,
analogous to molecular sieve zeolites found in nature, the properties
of synthetic aluminosilicate zeolite molecular sieves have formed
the basis of numerous commercially important catalytic, adsorptive
and ion-exchange applications. This high degree of utility is the
result of a unique combination of high surface area and uniform
porosity dictated by the "framework" structure of the
zeolite crystals coupled with the electrostatically charged sites
induced by tetrahedrally coordinated Al.sup.+3. Thus, a large number
of "active" charged sites are readily accessible to molecules
of the proper size and geometry for adsorptive or catalytic interactions.
Further, since charge compensating cations are electrostatically
and not covalently bound to the aluminosilicate framework, they
are generally base exchangeable for other cations with different
inherent properties. This offers wide latitude for modification
of active sites whereby specific adsorbents and catalysts can be
tailormade for a given utility.
In the publication "Zeolite Molecular Sieves", Chapter
2 1974 D. W. Breck hypothesized that perhaps 1000 aluminosilicate
zeolite framework structures are theoretically possible, but to
date only approximately 150 have been identified. While compositional
nuances have been described in publications such as U.S. Pat. Nos.
4524055 4603040 and 4606899 totally new aluminosilicate
framework structures are being discovered at a negligible rate.
With slow progress in the discovery of new aluminosilicate based
molecular sieves, researchers have taken various approaches to replace
aluminum or silicon in zeolite synthesis in the hope of generating
either new zeolite-like framework structures or inducing the formation
of qualitatively different active sites than are available in analogous
aluminosilicate based materials.
It has been believed for a generation that phosphorus could be
incorporated, to varying degrees, in a zeolite type aluminosilicate
framework. In the more recent past (JACS 104 pp. 1146 (1982); Proceedings
of the 7th International Zeolite Conference, pp. 103-112 1986)
E. M. Flanigan and coworkers have demonstrated the preparation of
pure aluminophosphate based molecular sieves of a wide variety of
structures. However, the site inducing Al.sup.+3 is essentially
neutralized by the P.sup.+5 imparting a +1 charge to the framework.
Thus, while a new class of "molecular sieves" was created,
they are not zeolites in the fundamental sense since they lack "active"
charged sites.
Realizing this inherent utility limiting deficiency, for the past
few years the research community has emphasized the synthesis of
mixed aluminosilicate-metal oxide and mixed aluminophosphate-metal
oxide framework systems. While this approach to overcoming the slow
progress in aluminosilicate zeolite synthesis has generated approximately
200 new compositions, all of them suffer either from the site removing
effect of incorporated P.sup.+5 or the site diluting effect of incorporating
effectively neutral tetrahedral +4 metal into an aluminosilicate
framework. As a result, extensive research in the research community
has failed to demonstrate significant utility for any of these materials.
A series of zeolite-like "framework" silicates have been
synthesized, some of which have larger uniform pores than are observed
for aluminosilicate zeolites. (W. M. Meier, Proceedings of the 7th
International Zeolite Conference, pp. 13-22 (1986).) While this
particular synthesis approach produces materials which, by definition,
totally lack active, charged sites, back implementation after synthesis
would not appear out of the question although little work appears
in the open literature on this topic.
Another and most straightforward means of potentially generating
new structures or qualitatively different sites than those induced
by aluminum would be the direct substitution of some charge inducing
species for aluminum in a zeolite-like structure. To date the most
notably successful example of this approach appears to be boron
in the case of ZSM-5 analogs, although iron has also been claimed
in similar materials. (EPA 68796 (1983), Taramasso et al; Proceedings
of the 5th International Zeolite Conference; pp. 40-48 (1980));
J. W. Ball et al; Proceedings of the 7th International Zeolite Conference;
pp. 137-144 (1986); U.S. Pat. No. 4280305 to Kouenhowen et al.
Unfortunately, the low levels of incorporation of the species substituting
for aluminum usually leaves doubt if the species are occluded or
framework incorporated.
In 1967 Young in U.S. Pat. No. 3329481 reported that the synthesis
of charge bearing (exchangeable) titaniumsilicates under conditions
similar to aluminosilicate zeolite formation was possible if the
titanium was present as a "critical reagent" +III peroxo
species. While these materials were called "titanium zeolites"
no evidence was presented beyond some questionable X-ray diffraction
(XRD) patterns and his claim has generally been dismissed by the
zeolite research community. (D. W. Breck, Zeolite Molecular Sieves,
p. 322 (1974); R. M. Barrer, Hydrothermal Chemistry of Zeolites,
p. 293 (1982); G. Perego et al, Proceedings of 7th International
Zeolite Conference, p. 129 (1986).) For all but one end member of
this series of materials (denoted TS materials), the presented XRD
patterns indicate phases too dense to be molecular sieves. In the
case of the one questionable end member (denoted TS-26), the XRD
pattern might possible be interpreted as a small pored zeolite,
although without additional supporting evidence, it appears extremely
questionable.
A naturally occurring alkaline titanosilicate identified as "Zorite"
was discovered in trace quantities on the Siberian Tundra in 1972
(A. N. Mer'kov et al; Zapiski Vses Mineralog. Obshch., pages 54-62
(1973)). The published XRD pattern was challenged and a proposed
structure reported in a later article entitled "The OD Structure
of Zorite", Sandomirskii et al, Sov. Phys. Crystallogr. 24
(6), Nov-Dec 1979 pages 686-693.
No further reports on "titanium zeolites" appeared in
the open literature until 1983 when trace levels of tetrahedral
Ti(IV) were reported in a ZSM-5 analog. (M. Taramasso et al; U.S.
Pat. No. 4410501 (1983); G. Perego et al; Proceedings of the 7th
International Zeolite Conference; p. 129 (1986).) A similar claim
appeared from researchers in mid-1985 (EPA 132550 (1985).) More
recently, the research community reported mixed aluminosilicate-titanium(IV)
(EPA 179876 (1985); EPA 181884 (1985) structures which, along
with TAPO (EPA 121232 (1985) systems, appear to have no possibility
of active titanium sites. As such, their utility is highly questionable.
That charge bearing, exchangeable titanium silicates are possible
is inferred not only from the existence of exchangeable alkali titanates
and the early work disclosed in U.S. Pat. No. 3329481 on ill defined
titaniumsilicates but also from the observation (S. M. Kuznicki
et al; J. Phys. Chem.; 84; pp. 535-537 (1980)) of TiO.sub.4 - units
in some modified zeolites.
SUMMARY OF THE INVENTION
The present invention relates to a method of preparation of a new
family of stable, small-pore crystalline titaniumsilicate molecular
sieve zeolites, hereinafter designated ETS-4. ETS-4 can be used
as adsorbents and catalysts for the conversion of compounds of small
molecular diameter. Additionally, these materials can be used as
scavengers for ions of certain divalent metals such as mercury and
lead.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a new family of stable crystalline
titaniumsilicate molecular sieve zeolites which have a pore size
of approximately 3-4 Angstrom units and a titania/silica mole ratio
in the range of from 1.0 to 10. These titanium silicates have a
definite X-ray diffraction pattern unlike other molecular sieve
zeolites and can be identified in terms of mole ratios of oxides
as follows:
wherein M is at least one cation having a valence of n, y is from
1.0 to 10.0 and z is from 0 to 100. In a preferred embodiment,
M is a mixture of alkali metal cations, particularly sodium and
potassium, and y is at least 2.5 and ranges up to about 5.
The original cations M can be replaced at least in part with other
cations by well known exchange techniques. Preferred replacing cations
include hydrogen, ammonium rare earth, and mixtures thereof. Members
of the family of molecular sieve zeolites designated ETS-4 in the
rare earth-exchanged form have a high degree of thermal stability
of at least 450.degree. C. or higher depending on cationic form,
thus rendering them effective for use in high temperature catalytic
processes. ETS zeolites are highly adsorptive toward molecules up
to approximately 3-5 Angstroms in critical diameter, e.g., water,
ammonia, hydrogen sulfide, SO.sub.2 and n-hexane and are essentially
non-adsorptive toward molecules which are larger than 5 Angstroms
in critical diameter.
Members of the ETS molecular sieve zeolites have an ordered crystalline
structure and an X-ray powder diffraction pattern having the following
significant lines:
TABLE 1 ______________________________________ XRD POWDER PATTERN
OF ETS-4 (0-40.degree. 2 theta) SIGNIFICANT d-SPACING (ANGS.) I/I.sub.o
______________________________________ 11.65 .+-. 0.25 S-VS 6.95
.+-. 0.25 S-VS 5.28 .+-. .15 M-S 4.45 .+-. .15 W-M 2.98 .+-. .05
VS ______________________________________
In the above table,
VS=50-100
S=30-70
M=15-50
W=5-30
The above values and values later mentioned were collected using
standard techniques on a Phillips APD3720 diffractometer equipped
with a theta compensator. The theta compensator maintains a constant
area of illumination on the sample, so X-ray intensities obtained
from a theta compensated unit are not directly comparable to those
of a non-compensated unit. Thus, all values mentioned in the specification
and claims with regard to ETS-4 were determined by said theta compensated
X-ray equipment. The radiation was the K-alpha doublet of copper,
and a scintillation counter spectrometer was used. The peak heights,
I, and the positions as a function of 2 times theta, where theta
is the Bragg angle, were read from the spectrometer chart. From
these, the relative intensities, 100 I/I.sub.o, where I.sub.o is
the intensity of the strongest line or peak, and d (obs.), the interplanar
spacing in A, corresponding to the recorded lines, were calculated.
It should be understood that this X-ray diffraction pattern is characteristic
of all the species of ETS-4 compositions. Ion exchange of the sodium
ion and potassium ions with cations reveals substantially the same
pattern with some minor shifts in interplanar spacing and variation
in relative intensity. Other minor variations can occur depending
on the silicon to titanium ratio of the particular sample, as well
as if it had been subjected to thermal treatment. Various cation
exchanged forms of ETS-4 have been prepared and their X-ray powder
diffraction patterns contain the most significant lines set forth
in Table 1.
ETS-4 molecular sieve zeolites can be prepared from a reaction
mixture containing a titanium source such as titanium trichloride,
a source of silica, a source of alkalinity such as an alkali metal
hydroxide, water and, optionally, an alkali metal fluoride having
a composition in terms of mole ratios falling within the following
ranges.
wherein M indicates the cations of valence n derived from the alkali
metal hydroxide and potassium fluoride and/or alkali metal salts
used for preparing the titanium silicate according to the invention.
The reaction mixture is heated to a temperature of from about 100.degree.
C. to 300.degree. C. for a period of time ranging from about 8 hours
to 40 days, or more. The hydrothermal reaction is carried out until
crystals are formed and the resulting crystalline product is thereafter
separated from the reaction mixture, cooled to room temperature,
filtered and water washed. The reaction mixture can be stirred although
it is not necessary. It has been found that when using gels, stirring
is unnecessary but can be employed. When using sources of titanium
which are solids, stirring is beneficial. The preferred temperature
range is 100.degree. C. to 175.degree. C. for a period of time ranging
from 12 hours to 15 days. Crystallization is performed in a continuous
or batchwise manner under autogeneous pressure in an autoclave or
static bomb reactor. Following the water washing step, the crystalline
ETS- 4 is dried at temperatures of 100 to 400.degree. F. for periods
ranging up to 30 hours.
The method for preparing ETS-4 compositions comprises the preparation
of a reaction mixture constituted by sources of silica, sources
of titanium, sources of alkalinity such as sodium and/or potassium
oxide and water having a reagent molar ratio composition as set
forth in Table 2. Optionally, sources of fluoride such as potassium
fluoride can be used, particularly to assist in solubilizing a solid
titanium source such as Ti.sub.2 O.sub.3. However, when titanium
silicates are prepared from gels, its value is greatly diminished.
The silica source includes most any reactive source of silicon
such as silica, silica hydrosol, silica gel, silicic acid, alkoxides
of silicon, alkali metal silicates, preferably sodium or potassium,
or mixtures of the foregoing.
The titanium oxide source is a trivalent titanium compound such
as titanium trichloride, TiCl.sub.3.
The source of alkalinity is preferably an aqueous solution of an
alkali metal hydroxide, such as sodium hydroxide, which provides
a source of alkali metal ions for maintaining electrovalent neutrality
and controlling the pH of the reaction mixture within the range
of 10.45 to 11.0.+-..1. As shown in the examples hereinafter, pH
is critical for the production of ETS-4. The alkali metal hydroxide
serves as a source of sodium oxide which can also be supplied by
an aqueous solution of sodium silicate.
It is to be noted that at the lower end of the pH range, a mixture
of titanium zeolites tends to form while at the upper end of the
pH range, quartz appears as an impurity.
The titanium silicate molecular sieve zeolites prepared according
to the invention contain no deliberately added alumina, and may
contain very minor amounts of Al.sub.2 O.sub.3 due to the presence
of impurity levels in the reagents employed, e.g., sodium silicate,
and in the reaction equipment. The molar ratio of SiO.sub.2 /Al.sub.2
O.sub.3 will be 0 or higher than 5000 or more.
The crystalline titanium silicate as synthesized can have the original
components thereof replaced by a wide variety of others according
to techniques well known in the art. Typical replacing components
would include hydrogen, ammonium, alkyl ammonium and aryl ammonium
and metals, including mixtures of the same. The hydrogen form may
be prepared, for example, by substitution of original sodium with
ammonium. The composition is then calcined at a temperature of,
say, 1000.degree. F. causing evolution of ammonia and retention
of hydrogen in the composition, i.e., hydrogen and/or decationized
form. Of the replacing metals, preference is accorded to metals
of Groups II, IV and VIII of the Periodic Table, preferably the
rare earth metals.
The crystalline titanium silicates are then preferably washed with
water and dried at a temperature ranging from 150.degree. F. to
about 600.degree. F. and thereafter calcined in air or other inert
gas at temperatures ranging from 500.degree. F. to 1500.degree.
F. for periods of time ranging from 1 to 48 hours or more.
Regardless of the synthesized form of the titanium silicate the
spatial arrangement of atoms which form the basic crystal lattices
remain essentially unchanged by the replacement or sodium or other
alkali metal or by the presence in the initial reaction mixture
of metals in addition to sodium, as determined by an X-ray powder
diffraction pattern of the resulting titanium silicate. The X-ray
diffraction patterns of such products are essentially the same as
those set forth in Table I above.
The crystalline titanium silicates prepared in accordance with
the invention are formed in a wide variety of particular sizes.
Generally, the particles can be in the form of powder, a granule,
or a molded product such as an extrudate having a particle size
sufficient to pass through a 2 mesh (Tyler) screen and be maintained
on a 400 mesh (Tyler) screen in cases where the catalyst is molded
such as by extrusion. The titanium silicate can be extruded before
drying or dried or partially dried and then extruded.
When used as a catalyst, it is desired to incorporate the new crystalline
titanium silicate with another material resistant to the temperatures
and other conditions employed in organic processes. Such materials
include active and inactive materials and synthetic and naturally
occurring zeolites as well as inorganic materials such as clays,
silica and/or metal oxides. The latter may be either naturally occurring
or in the form of gelatinous precipitates or gels including mixtures
of silica and metal oxides. Use of a material in conjunction with
the new crystalline titanium silicate, i.e., combined therewith
which is active, tends to improve the conversion and/or selectivity
of the catalyst in certain organic conversion processes such as
the cracking of n-hexane. Inactive materials suitably serve as diluents
to control the amount of conversion in a given process so that products
can be obtained economically and in an orderly manner without employing
other means for controlling the rate of reaction. Normally, crystalline
materials have been incorporated into naturally occurring clays,
e.g., bentonite and kaolin to improve the crush strength of the
catalyst under commercial operating conditions. These materials,
i.e., clays, oxides, etc., function as binders for the catalyst.
It is desirable to provide a catalyst having good crush strength
because in a petroleum refinery the catalyst is often subjected
to rough handling which tends to break the catalyst down into powder-like
materials which cause problems in processing. These clay binders
have been employed for the purpose of improving the crush strength
of the catalyst.
Naturally occurring clays that can be composited with the crystalline
titanium silicate described herein include the montmorillonite and
kaolin family, which families include the sub-bentonites and the
kaolins known commonly as Dixie, McNamee, Georgia and Florida or
others in which the main constituent is halloysite, kaolinite, dickite,
nacrite or anauxite. Such clays can be used in the raw state as
originally mined or initially subjected to calcination, acid treatment
or chemical modification.
In addition to the foregoing materials, the crystalline titanium
silicate may be composited with a porous matrix material such as
silica-alumina, silica-magnesia, silica-zirconia, silica-thoria,
silica-berylia, silica-titania as well as ternary compositions such
as silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia
and silica-magnesia-zirconia. The matrix can be in the form of a
cogel. The relative proportions of finally divided crystalline metal
organosilicate and inorganic oxide gel matrix can vary widely with
the crystalline organosilicate content ranging from about 1 to 90
percent by weight and more usually in the range of about 2 to about
50 percent by weight of the composite.
As is known in the art, it is desirable to limit the alkali metal
content of materials used for acid catalyzed reactions. This is
usually accomplished by ion exchange with hydrogen ions or precursors
thereof such as ammonium and/or metal cations such as rare earth.
In order to more fully illustrate the nature of the invention and
a manner of practicing the same, the following examples illustrate
the best mode now contemplated. |