Molecular sieve abstract
Disclosed are novel crystalline molecular sieves having the framework
of ETS-10 titanium silicate which contain both octahedral and tetrahedral
titanium atoms and are in acid form. The sieves are prepared by
adding a complexing agent such as hydrogen peroxide during synthesis
or by treating a synthesized titanium silicate molecular sieve with
a reagent that replaces tetrahedral silicon with tetrahedral titanium
atoms.
Molecular sieve claims
We claim:
1. An oxidation catalyst comprising a crystalline titanium silicate
molecular sieve having exchangeable cation sites and containing
both octahedrally coordinated titanium atoms and tetrahedrally coordinated
titanium atoms in the framework, wherein the exchangeable cations
are predominantly hydrogen or a cation thermally decomposable to
hydrogen.
2. The catalyst of claim 1 wherein said titanium silicate is of
the ETS-10 type.
3. The catalyst of claim 1 which has a chain-like titanium silicate
framework in which the majority of the titanium atoms are octahedrally
coordinated and a minor amount are tetrahedrally coordinated.
4. The catalyst of claim 3 which has the x-ray pattern of ETS-10
titanium silicate.
5. The catalyst of claim 4 which has an IR peak at 980 cm.sup.-1.
6. The catalyst of claim 5 which has a Si/Ti ratio below 5.
7. The catalyst of claim 6 which has a Si/Ti ratio above 3.
8. The catalyst of claim 6 which has a Si/Ti ratio of about 4.
9. The catalyst of claim 1 wherein more than 65% of the exchangeable
cations are hydrogen.
10. The catalyst of claim 9 wherein the titanium silicate has the
x-ray pattern of ETS-10.
11. The catalyst of claim 1 wherein the titanium silicate is defined
by the formula: (1.0.+-.0.25)M.sub.2a O:TiO.sub.2 :2-5 SiO.sub.2
:2.5-5 H.sub.2 O wherein at least about 65% of M is H, the remainder
being Na, K or combinations thereof, and "a" is the valence
of M.
12. An oxidation catalyst containing both acidic and oxidation
sites which is a porous hydrogen form titanium molecular sieve silicate
based on a framework of a di-charged octahedrally coordinated titanium
atoms and tetrahedrally coordinated silica atoms and contains a
minor amount of tetrahedrally coordinated titanium atoms in the
framework.
13. The catalyst of claim 12 which has the x-ray pattern of ETS-10
and has an IR peak at 980-950 cm.sup.-1.
14. A method for preparing a modified form of the crystalline molecular
sieve ETS-10 which comprises mixing at least one water soluble peroxide
with a reaction mixture capable of producing ETS-10 in the absence
of said peroxide and recovering the resulting modified ETS-10 crystals.
15. The method of claim 14 which comprises mixing an aqueous acidic
solution of a source of titanium ions with an aqueous alkaline solution
of sodium silicate in proportions selected to form ETS-10 crystals,
adding a water soluble peroxide to the mixture either before or
after reaction takes place between the source of titanium ions and
sodium silicate and heating the resulting mixture containing said
peroxide until crystals of modified ETS-10 containing both octahedrally
and tetrahedrally coordinated titanium atoms form, and washing and
thereafter ion-exchanging the crystals with hydrogen or a source
of hydrogen ions to replace alkali metal ion.
16. A method for preparing a titanium silicate oxidation catalyst
containing both discharged octahedrally coordinated titanium atoms
and tetrahedrally coordinated silicon atoms which comprises extracting
silica from a crystalline titanium silicate molecular sieve at an
acidic pH with a reagent that selectively replaces tetrahedral silicon
atoms with tetrahedral titanium atoms.
17. The method of claim 16 wherein said reagent is ammonium titanium
hexafluoride.
18. The method of claim 16 wherein reagent is (NH.sub.4)yXF.sub.6
wherein X is a heteroatom and y is the valence of X-6.
19. A method for oxidizing an organic compound which comprises
treating at least one organic compound in the presence of peroxide
with the catalyst of claim 1.
Molecular sieve description
FIELD OF THE INVENTION
The invention relates to novel crystalline titanium silicate molecular
sieves containing acid sites and both tetrahedrally and octahedrally
coordinated titanium atoms in the molecular sieve framework. These
materials are useful as sorbents and as catalysts for oxidation
reactions. This invention relates also to novel methods for preparing
such catalysts and to the use thereof in oxidation reactions, especially
partial oxidation reactions.
BACKGROUND OF THE INVENTION
The goal of incorporating tetrahedral titanium(IV) in classical
zeolite structures has been actively pursued for more than a decade.
Such materials include tetrahedral titanium incorporated into ZSM-5
analogs such as TS-1 substitution into ZSM-11 analogs such as TS-2
substitution into zeolite Beta and, more recently, incorporation
into MCM-41. This interest results from the unique catalytic properties
of Ti(IV) sites in molecular sieve configurations.
Commercially, the hydroxylation of phenol to catechol and hydroquinone
is practiced using TS-1. Other reactions which have demonstrated
promise include oximation of cyclohexanone as well as olefin epoxidation,
especially the conversion of propylene to propylene oxide. A potentially
important commercial reaction involving the rearrangement of oxime
to lactam has also been reported.
The known examples of tetrahedral Ti (IV) in zeolite structures
have several elements in common. First, the amount of Ti incorporation
is relatively low, typically 2-3 wt % or less. Second, zeolites
such as TS-1 Ti-Beta, Ti-MCM-41 etc., are silica rich; i.e., there
are only low levels of framework aluminum in these zeolites. This
means that the ion-exchange capacity and the potential acid concentration,
(the number of potential acid sites), in these zeolites is low.
The low level of ion-exchange sites in zeolites with tetrahedral
Ti incorporation means that the ion-exchange properties, the adsorptive
properties and potential catalytic applications are also restricted.
In contrast, the materials of this invention combine relatively
high levels of tetrahedral Ti incorporation with high levels of
ion-exchange capacity. This high ion-exchange capacity arises from
framework charge neutralization associated with octahedrally coordinated
framework titanium atoms. Each octahedral titanium atoms results
in two negative framework charges which must be neutralized with
cations or other appropriate species.
ETS-10 molecular sieve (U.S. Pat. No. 4853202) is a crystalline
structure consisting of silica chains linked to octahedral titania
chains. As such, it contains both tetrahedral and octahedral framework
sites.
Reference is made to the following:
S. M. Kuznicki and K. A. Thrush; U.S. Pat. No. 5244650 and U.S.
Pat. No.
5208006.
M. W. Anderson, A. Philippou, Z. Lin, A. Ferreira and J. Rocha;
Angew. Chem. Int. Engl., 34 1003 (1995).
Rocha, Z. Lin, A. Ferreira and M. W. Anderson; J. Chem. Soc. Chem.
Commun., 867 (1995).
In U.S. Pat. No. 3329481 the synthesis of charge bearing titanium
silicates using a peroxy reagent during synthesis is disclosed.
Distinctions between the resulting "titanium zeolites"
and ETS-10 molecular sieve are set forth in detail in U.S. Pat.
No. 5244650.
U.S. Pat. No. 5208006 commonly assigned, discloses and claims
a host of crystalline titanium molecular sieves of the ETS-10 type
having at least one octahedrally coordinated site comprising titanium
and at least tetrahedrally coordinated silicon. The tetrahedral
sites may include, in addition to silicon, any one of a host of
metals, one of which may be titanium. The terms "octahedral
coordination" and "tetrahedral coordination" are
defined in U.S. Pat. No. 5208006 at col. 19. The teachings of
U.S. Pat. No. 5208006 are incorporated herein in full by cross-reference.
Unlike other atoms, the chemical environment in a conventional
ETS-10 synthesis mixture forces essentially all of the titanium
into octahedral coordination to form titanium silicate chains so
that direct synthesis, especially the controlled direct synthesis
of mixed octahedral/tetrahedral sites is not achieved using conventional
procedures.
Procedures for decreasing the overall Si/Ti ratio of titanium silicate
sieves are not disclosed.
SUMMARY OF THE INVENTION
The invention relates to novel titanium silicate molecular sieve
catalysts having both octahedral and tetrahedral titanium atoms
in the framework. The counterions in the molecular sieve are predominantly
in acid form, i.e., hydrogen or a cation such as ammonium ion that
is thermally convertible to hydrogen ion. Thus, the catalysts contain
both desirable acidic and oxidative sites.
Molecular sieves of this class are prepared by two general methods.
In one, the sieves are prepared by partitioning titanium in a titanium
silicate synthesis gel into both tetrahedrally and octahedrally
coordinated atoms. The synthesis gel is one that would normally
produce a crystalline molecular sieve with a chain-like structure
in which all titanium is in octahedral coordination. Tetrahedral
titanium takes positions normally occupied by silicon in the unmodified
structure, resulting in a lower Si/Ti ratio. The resulting material
is then ion-exchanged with hydrogen cations or a source thereof.
In another method, the crystalline molecular sieve containing substantially
all titanium in octahedral coordination and tetrahedral silicon
is synthesized, then exchanged with acid and thereafter treated
with a reagent that replaces tetrahedral silicon atoms with tetrahedral
titanium atoms.
While the preferred embodiments involves modification of the molecular
sieve ETS-10 framework, the same principles may be applied to other
molecular sieves including ETS sieves by substituting Ti(IV) for
silicon into other known and possibly not presently known, crystalline
titanium silicate sieves based on a chain-like titanium silicate
framework, as exemplified by: ETS-4 ETAS-10 and ETS-14.
The art indicates that highly siliceous zeolites or molecular sieves
that contain small amounts of tetrahedrally coordinated framework
titanium atoms catalyze the selective oxidation of alkanes, alkenes
and aromatics (e.g. phenol) in the presence of peroxide. For example,
catalysts of this type are the basis for a commercial process for
oxidation of phenol to catechol and hydroquinone.
The art also indicates that framework titanium in tetrahedral coordination
is necessary for these oxidations. Since ETS-10 nominally contains
no tetrahedrally coordinated titanium, it would be expected that
ETS-10 would not catalyze the oxidation of organic compounds by
peroxide. In fact, unmodified ETS-10 has little, if any, oxidation
capability.
An unexpected result is that the products of the oxidation are
different using modified ETS-10. Modified ETS-10 tends to oxidize
carbons adjacent to double bonds or aromatic rings. For example,
oxidation of ethylbenzene gives acetophenone whereas oxidation of
ethylbenzene with peroxide and titanium substituted ZSM-5 (TS-1)
gives ethylphenol.
Still, another unexpected result is that modified ETS-10 is a more
substrate selective, milder oxidizing agent than prior art catalysts.
For example, prior art catalysts oxidize phenol and alpha olefins.
Modified ETS-10 does not. In contrast, prior art titanium based
zeolite catalysts do not oxidize allylic carbons in the presence
of peroxide. Modified ETS-10 catalysts do this oxidation.
Products of the Invention
Detailed Description
The invention relates to oxidation catalysts comprising a crystalline
titanium silicate molecular sieve, comprising both di-charged octahedrally
coordinated titanium atoms and tetrahedrally coordinated titanium
atoms in the framework, wherein exchangeable cations in the molecular
sieve are predominately hydrogen or cations such as ammonium ions
that are thermally convertible to hydrogen cations.
In a presently preferred embodiment, the molecular sieve is based
on ETS-10. Thus, the invention will be described with special emphasis
on modified forms of ETS-10 molecular sieve. These modified forms
have the x-ray diffraction of ETS-10 and a Si/Ti molar ratio (bulk
ratio) in the range of 2 to 5.
In a preferred embodiment, the molar Si/Ti ratio in the modified
ETS-10 type containing both octahedrally and tetrahedrally coordinated
titanium atoms is less than 5 preferably above 3 and most preferably
above 4. Typically, the Si/Ti ratio is in the range of 3.5 to 4.7.
Products of the invention (herein referred to as Ti/ETS-10) contain
the most significant line which are set forth in Table 1.
TABLE 1 ______________________________________ CHARACTERISTIC XRD
d-SPACINGS OF Ti/ETS-10 d-SPACING (ANGS.) I/I.sub.0 ______________________________________
14.7 .+-. 0.35 W-M 7.20 .+-. 0.15 W-M 4.41 .+-. 0.10 W-M 3.60 .+-.
0.05 VS 3.28 .+-. 0.05 W-M ______________________________________
Wherein VS = 50-100 W-M = 15-50
The d-spacings reported in Table 1 are the significant lines for
ETS-10. However, products of the invention can be readily distinguished
from known forms of ETS-10. The first X-ray peak at 14.7.+-.0.35
and the strongest peak at 3.60.+-.0.05 are slightly higher than
classical ETS-10 and are consistent with the substitution of slightly
larger Ti(IV) for a portion of the tetrahedral Si(IV) units. Also,
the IR (infra-red) spectrum of certain products of the invention
distinguish from ETS-10.
Spectra of both contain silicon stretching bands at a 1030 cm.sup.-1
wave number as the strongest feature in the spectrum. In ETS-10
there is essentially no peak in the region of 1000-900 wave numbers
and in the acid exchanged material there is a very small peak in
the 960-980 region. In contrast, for as synthesized Ti/ETS-10 wherein
tetrahedral titanium is introduced during synthesis, there is a
very large peak at approximately 980 cm.sup.-1 where there is no
peak in as synthesized ETS-10. In addition, the infra-red spectrum
of the acid form of Ti/ETS-10 has a very large peak at approximately
975 cm.sup.-1 whereas there is a small peak in acid exchanged ETS-10.
Furthermore, catalytic data shown in the accompanying examples confirm
differences from ETS-10. In the scientific literature, the exact
assignment of the IR peak at the 980 cm.sup.-1 wave number is a
matter of some controversy. However, it is more or less universally
regarded as an indication of the modification of a molecular sieve
high silicon framework by inclusion of heteroatoms such as titanium.
For example, this peak is diagnostic for inclusion of Ti into the
framework of ZSM-5 to give the sieve TS-1. Thus, the presence of
this infra-red peak along with the catalytic activity of Ti/ETS-10
is very strong evidence that the framework of ETS-10 has been changed
by the inclusion of Ti into silicon sites and the two materials
are substantively different.
While all scientific evidence to date is that the nonoctahedral
titanium position of products of the invention is tetrahedral, it
is possible that during use in reactions such as oxidative catalysis,
the titanium may assume other configurations such as penta or hexa
coordination or combination.
Preparation of Catalyst by Direct Gel Synthesis
Detailed Description
The novel titanium silicate molecular sieve catalysts of this invention
having a Si/Ti ratio less than 5 can be prepared by modification
of a conventional gel synthesis mixture for preparing ETS-10 molecular
sieve by adding a chelating agent for octahedral titanium atoms,
preferably a peroxide, most preferably hydrogen peroxide, to such
mixture prior to gelation and crystallization. Other peroxides include
water soluble peroxides such as sodium peroxide, organic peroxide
such as t-butyl peroxide, cyclohexyl peroxide, sodium percarbonates,
peroxidisulfates, and peroxy compounds such as dialkyl peroxides
and alkyl peroxy esters mixtures can be used. The chelating agent
must be capable of complexing octahedral titanium atoms to form
titanium atoms having tetrahedral coordination and keep them in
such coordination state prior to crystallization so that a significant
fraction of silicon atoms are replaced by tetrahedral Ti atoms in
the tetrahedral silicon containing chains that make-up ETS-10. In
effect, the titanium reservoir in the ETS-10 synthesis mixture is
partitioned into tetrahedrally and octahedrally coordinated atoms
which are incorporated into their respective chains. The net effect
is that some Ti atoms replace Si atoms in the tetrahedral chain
and the resulting structure is ETS-10-like with a lower Si/Ti ratio.
Typical pore size is about 8 Angstrom units.
The mole ratio of chelating agent to TiO.sub.2 is generally from
0.1 to 50 preferably from 0.5 to 10 and most preferably about
0.5. Preferably the atomic Si/Ti ratio (total) is from 2 to 20
most preferably 2 to 7. If too little peroxide is added, insufficient
tetrahedral titanium is present in the crystallized product.
As mentioned, catalysts of the invention can be prepared by modification
of a conventional gel synthesis mixture for preparing ETS-10 molecular
sieve. Such conventional synthesis utilizes 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 mineralizer having a composition
in terms of mole ratios falling within the range set forth in Table
2.
TABLE 2 ______________________________________ CONVENTIONAL COMPOSITION
OF REACTANTS TO PREPARE ETS-10 BY GEL SYNTHESIS Broad Preferred
Most Preferred ______________________________________ SiO.sub.2
/Ti 2-20 2-10 2-7 H.sub.2 O/SiO.sub.2 2-100 5-50 10-25 M.sub.a /SiO.sub.2
0.1-20 0.5-5 1-3 ______________________________________
wherein "M" indicates the cations derived from the alkali
metal hydroxide and fluoride and/or alkali metal salts used for
preparing the titanium silicate according to the invention and "a"
is the valence of M. To the reaction mixture the chelating agent,
preferably hydrogen peroxide, is added in order to prevent all titanium
from entering octahedral coordination. The reaction mixture is heated
to a temperature of from about 100.degree. C. to 250.degree. C.
for a period of time ranging from about 2 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 150.degree.
C. to 225.degree. C. for a period of time ranging from 4 hours to
7 days. Crystallization is performed in a continuous or batchwise
manner under autogenous pressure in an autoclave or static bomb
reactor. Following the water washing step, the crystalline ETS-10
is dried at temperature of 100.degree. to 600.degree. F. for periods
up to 30 hours.
Prior to crystallization, the gel resulting from the reaction mixture
can be subjected to one or more thermal treatments at temperature
of from about 150.degree. C. to 800.degree. C. for 1-48 hours. The
thermally treated gel is mixed with water and crystallized.
Quite obviously, it is possible to use less caustic, or other reactants
in the gel than set forth in Table 2 and supply these during the
crystallization step after the gel has been thermally treated.
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 can be a trivalent or tetravalent and
compound. Examples include titanium trichloride, titanium sulfate
or oxysulfate, TiCl.sub.3 titanium tetrachloride, TiCl.sub.4 and
titanium oxychloride, TiOCl.sub.2.
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 .about.10.0 to 12.0 using the technique elaborated upon in U.S.
Pat. No. 4853202. The alkali metal hydroxide serves as a source
of sodium oxide which can also be supplied by an aqueous solution
of sodium silicate.
The exchangeable cations in the crystallized product are usually
Na and K. To produce acidic catalysts, it is essential to extensively
replace these alkali metal cations with hydrogen ion or precursors
thereof such as ammonium ion followed by calcination in known manner.
Preferably, the maximum amount of total alkali metal cations in
the exchanged product is less than 55% and is most preferably minimal,
for example (Na.sub.2 O+K.sub.2 O)<2.5 wt. % based on the volatile
free weight. However, unlike other oxidation catalysts based on
molecular sieves or zeolites, the products of this invention can
tolerate moderate quantities of alkali metals.
Because the products of the invention contain a unique combination
of acid sites as well as tetrahedrally coordinated titanium sites,
they are uniquely adapted for catalytic applications in which both
type of sites are desirable, for example converting olefins to diols.
Typically, well-washed prior art forms of ETS-10 have a Si/Ti ratio
in the range of 4.8 to 5.2/1 (U.S. Pat. No. 4853202). Typical
products of the invention obtained by modification of gel synthesis
have a Si/Ti ratio of 4.7 or less .
Thus, the formula for a typical hydrogen form of compositions of
the invention expressed as oxides is typically as follows:
wherein at least 65% of M is H.sup.+ and the balance of M is sodium,
potassium or mixture thereof, and "a" is the valence
of M.
XRD analysis of products of this invention contain at least the
significant line set forth in Table 1 (supra).
As mentioned, products of the invention can be readily distinguished
from known forms of ETS-10. The first x-ray peak at 14.7.+-.0.35
and the strongest peak at 3.60.+-.0.05 are slightly higher than
classical ETS-10 and are consistent with the substitution of slightly
larger (Ti(IV) for a portion of the tetrahedral Si(IV) units. Also,
the IR spectrum of products of the invention obtained during direct
synthesis distinguish it from ETS-10. Spectra of both contain silicon
stretching bands of 1030 wave number as the strongest feature in
the spectrum. However, the peak at 980 wavelength, indicative of
tetrahedral titanium, is completely absent from the spectrum for
conventional ETS-10 but is readily apparent in the spectrum for
products of the invention. Furthermore, catalytic data shown in
the accompanying examples confirm differences from ETS-10.
Preparation of Catalyst by Post Synthesis Chemical Modification
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