Molecular sieve abstract
A new class of manganese oxide octahedral molecular sieves possess
a (4.times.4) tunnel structure. The molecular sieves possess the
general composition wherein A is +1 +2 +3 or +4 tunnel cation
or combination thereof, 0.ltoreq.a.ltoreq.8 M is +1 +2 +3 or
+4 framework-substituting metal cation or combination thereof, 0.ltoreq.b.ltoreq.16
and n.gtoreq.0. A method of producing the molecular sieves includes
the steps of dissolving a manganese salt in an organic solvent,
e.g., ethanol, adding a permanganate salt to the resulting solution
to provide a solid intermediate which is recovered and heated to
a temperature which results in producing an octahedral molecular
sieve having a (4.times.4) tunnel structure. The molecular sieves
are useful in such applications as oxidation catalysis, hydrocarbon
conversion, adsorption and electrochemical sensors.
Molecular sieve claims
What is claimed is:
1. A manganese oxide octahedral molecular sieve possessing the
composition:
wherein A is a +1 or +2 tunnel cation or combination thereof, 0<a.ltoreq.8
M is a +1 +2 +3 or +4 framework-substituting metal cation or combination
thereof, 0<b<16 and n.gtoreq.0.
2. The molecular sieve of claim 1 wherein A is an alkali or alkaline
earth metal cation.
3. The molecular sieve of claim 1 wherein A is a metal cation selected
from the group consisting of Li, Na, K, Cs, Mg, Ca and Ba.
4. The molecular sieve of claim 1 wherein A is a metal cation of
K.
5. The molecular sieve of claim 1 wherein M is a transition metal
cation.
6. The molecular sieve of claim 1 wherein M is selected from the
group consisting of Mg, Fe, Co, Ni, Cu, Ti, V, Cd, Mo, W, Cr, Zn,
La, Ir, Rh, Pd and Pt.
7. A method of producing a manganese oxide octahedral molecular
sieve possessing a (4.times.4) tunnel structure which comprises:
a) dissolving a manganese salt in an organic solvent to form a
solution;
b) adding a permanganate salt to the solution to form a solid intermediate;
c) recovering the intermediate; and,
d) heating the intermediate at a temperature effective to produce
an octahedral molecular sieve possessing a (4.times.4) tunnel structure.
8. The method of claim 7 wherein the manganese salt is selected
from the group consisting of MnCl.sub.2 Mn(NO.sub.3).sub.2 MnSO.sub.4
and Mn(CH.sub.3 COO).sub.2.
9. The method of claim 7 wherein the permanganate salt is selected
from the group consisting of Na(MnO.sub.4), KMnO.sub.4 and Mg(MnO.sub.4).sub.2.
10. The method of claim 7 further comprising the step of co-dissolving
a transition metal salt in the organic solvent.
11. The method of claim 7 wherein the solvent corresponds to the
general formula R.brket open-st.OH].sub.n where R is C.sub.1 -C.sub.6
straight-chain alkyl and n is 1 or 2.
12. The method of claim 7 wherein the solvent is ethanol.
13. The method of claim 7 wherein the intermediate is heated from
room temperature to a temperature ranging from about 200.degree.
to about 800.degree. C.
14. The method of claim 7 wherein the intermediate is heated from
room temperature to a temperature ranging from about 500.degree.
to about 700.degree. C.
15. The method of claim 7 wherein heating is carried out for a
period of time ranging from about 0.1 to about 10 hours.
16. The method of claim 7 wherein heating is carried out for a
period of time ranging from about 0.5 to about 3.0 hours.
17. The method of claim 7 wherein prior to heating the intermediate
the intermediate is aged at room temperature for a period of time
ranging from about 1 to about 14 days.
Molecular sieve description
BACKGROUND OF THE INVENTION
This invention relates to a new class of manganese oxide octahedral
molecular sieves (OMS) possessing a (3.times.4) tunnel structure
and to a method for their production.
Manganese oxide octahedral molecular sieves possessing mono-directional
tunnel structures constitute a family of molecular sieves wherein
chains of MnO.sub.6 octahedra share edges to form tunnel structures
of varying sizes. Such materials have been detected in samples of
terrestrial origin and are also found in manganese nodules recovered
from the ocean floor. Manganese nodules have been described as useful
catalysts in the oxidation of carbon monoxide, methane and butane
(U.S. Pat. No. 3214236), the reduction of nitric oxide with ammonia
(Atmospheric Environment, Vol. 6 p. 309 (1972)) and the demetallation
of topped crude in the presence of hydrogen (Ind. Eng. Chem. Proc.
Dev., Vol. 13 p.315 (1974)).
The hollandites are naturally occurring hydrous manganese oxides
with tunnel structures (also described as "framework hydrates")
in which Mn can be present as Mn.sup.4+ and other oxidation states,
the tunnels can vary in size and configuration and various mono-
or divalent cations can be present in the tunnels. The hollandite
structure consists of double chains of MnO.sub.6 octahedra which
share edges to form (2.times.2) tunnel structures. The average size
of these tunnels is about 4.6 .ANG. square. Ba, K, Na and Pb ions
are present in the tunnels and coordinated to the oxygens of the
double chains. The identity of the tunnel cations determines the
mineral species. Specific hollandite species include hollandite
(BaMn.sub.8 O.sub.16), cryptomelane (KMn.sub.8 O.sub.16), manjiroite
(NaMn.sub.8 O.sub.16) and coronadite (PbMn.sub.8 O.sub.16).
The hydrothermal method of synthesizing a manganese oxide octahedral
molecular sieve possessing (2.times.2) tunnel structures such as
those possessed by the naturally-occurring hollandites is described
in "Hydrothermal Synthesis of Manganese Oxides with Tunnel
Structures," in Synthesis of Microporous Materials, Vol. II,
333 M. L. Occelli, H. E. Robson Eds. Van Nostrand Reinhold, N.Y.,
1992. Such synthetic octahedral molecular sieves having (2.times.2)
tunnel structures are referred to in the art by the designation
OMS-2. The (2.times.2) tunnel structure of OMS-2 is diagrammatically
depicted in FIG. 1A.
The hydrothermal method of producting OMS-2 involves autoclaving
an aqueous solution of manganese cation and permanganate anion under
acidic conditions, i.e., pH<3 at temperatures ranging from about
80.degree. to about 140.degree. C. in the presence of counter cations
having ionic diameters of between about 2.3 and about 4.6 .ANG..
The counter cations can serve as templates for the formation of
OMS-2 product and be retained in the tunnel structures thereof.
Based on analytical tests, OMS-2 produced via this method is thermally
stable up to about 600.degree. C.
Alternatively, OMS-2 can be produced by the method disclosed in
R-Giovanili and B. Balmer, Chimia, 35 (1981) 53. Thus, when manganese
cation and permanganate anion are reacted under basic conditions,
i.e., pH>12 a layered manganese oxide precursor is produced.
This precursor is ion exchanged and then calcined at high temperatures,
i.e., temperatures generally exceeding about 600.degree. C., to
form OMS-2 product. Analytical tests indicate that OMS-2 produced
via this method is thermally stable up to about 800.degree. C. and
the average oxidation state of manganese ion is lower.
The todorokites are naturally occurring manganese oxides with (3.times.3)
tunnel structures formed by triple chains of MnO.sub.6 edge-sharing
octahedra. Todorokites and related species are described by Turner
et al. in "Todorokites: A New Family of Naturally Occurring
Manganese Oxides", Science, Vol. 212 pp. 1024-1026 (1981).
The authors speculate that since todorokites are often found in
deep-sea manganese nodules containing high concentrations of copper
and nickel, it is probable that such metals substitute for Mn.sup.+2
in the octahedral framework.
Todorokites have attracted particular interest because of their
relatively large tunnel dimension and their cation-exchange behavior
which is similar to that of zeolites (Shen et al., "Manganese
Oxide Octahedral Molecular Sieves: Preparation, Characterization,
and Applications", Science, Vol. 260 pp. 511-515 (1993)).
The naturally occurring todorokites are poorly crystalline, impure
in composition and coexist with other manganese oxide minerals.
Results of high resolution transmission electron microscopy (HRTEM)
show that todorokite contains random intergrowth material of 3.times.2
3.times.3 3.times.4 and 3.times.5 tunnel structure. Because of
their disordered structure, the todorokites exhibit variable and
non-reproducible catalytic activity, a drawback which militates
against their commercial use.
A method of synthesizing a manganese oxide octahedral molecular
sieve possessing (3.times.3) tunnel structures such as those possessed
by the naturally-occurring todorkites is described in U.S. Pat.
No. 5340562. Such synthetic octahedral molecular sieves having
(3.times.3) tunnel structures are referred to in the art by the
designation OMS-1. The (3.times.3) tunnel structure of OMS-1 is
diagrammatically depicted in FIG. 1B.
OMS-1 can be prepared by reacting manganese cation and permanganate
anion under strongly basic conditions to form a layered manganese
oxide precursor, thereafter aging the precursor at room temperature
for at least 8 hours, ion-exchanging the aged precursor and then
autoclaving the ion-exchanged precursor at from about 150.degree.
to about 180.degree. C. for several days. Analytical tests indicate
that OMS-1 produced via this method is thermally stable up to about
500.degree. C.
Methods of substituting the frameworks of OMS-1 and OMS-2 with
a metal other than manganese are described in commonly assigned,
copending U.S. appln. Ser. No. 08/215496.
SUMMARY OF THE INVENTION
In accordance with the present invention an octahedral molecular
sieve possessing a (4.times.4) tunnel structure is provided. The
molecular sieve possesses the general formula:
wherein A is a +1 +2 +3 or +4 tunnel cation or combination thereof,
0<a.ltoreq.8 M is a +1 +2 +3 or +4 framework-substituting
metal cation or combination thereof, 0<b<16 and and n.gtoreq.0.
The octahedral molecular sieve herein possesses an average pore
diameter of about 9.2 .ANG.. The manganese oxide octahedral molecular
sieve of this invention possesses a highly uniform and homogeneous
structure, i.e., one made up substantially entirely of (4.times.4)
tunnel structure species without admixture of any significant amount
of other tunnel structure species.
The novel manganese oxide octahedral molecular sieve of this invention,
which shall be referred to throughout the specification by the designation
OMS-3 can be prepared by the method which comprises:
a) dissolving a manganese salt in an organic solvent to form a
solution;
b) adding a permanganate salt to the solution to form a solid intermediate;
c) recovering the intermediate; and,
d) heating the intermediate at temperatures effective to produce
the desired OMS-3 product.
OMS-3 can be effectively utilized in a wide variety of applications
such as oxidation catalysis, hydrocarbon conversion, adsorption
and electrochemical sensors.
BRIEF DESCRIPTION OF THE DRAWINGS
In the attached figures of drawing:
FIGS. 1A and 1B are diagrammatic representations of OMS-2 and OMS-1
respectively;
FIG. 2 is a diagrammatic representation of OMS-3; and,
FIG. 3 presents the x-ray powder diffraction pattern of OMS-3 produced
in accordance with the method of this invention.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, a manganese oxide octahedral
molecular sieve is provided which possesses unique (4.times.4) tunnel
structures. The molecular sieve possesses the general formula:
wherein A is a +1 +2 +3 or +4 tunnel cation or combination thereof,
0<a.ltoreq.8 M is a +1 +2 +3 or +4 framework-substituting
metal cation or combination thereof, 0<b<16 and n.gtoreq.0.
OMS-3 is characterized by the (4.times.4) tunnel structure which
is diagrammatically depicted in FIG. 2. In FIG. 3 the x-ray powder
diffraction pattern of OMS-3 is presented, thus confirming its structure
as having (4.times.4) tunnel structures. Preferably, the manganese
oxide octahedral molecular sieve is provided with tunnel-substituting
and/or framework-substituting metal cations as more fully described
hereinbelow. It is believed that the presence of these additional
cations will enhance the crystallinity, stability and catalytic
effectiveness of the resulting OMS-3 product.
The octahedral molecular sieve of this invention is produced by
the method comprising:
a) dissolving a manganese salt in an organic solvent to form a
solution;
b) adding a permanganate salt to the solution to form a solid intermediate;
c) recovering the intermediate; and,
d) heating the intermediate at a temperature effective to produce
the desired OMS-3 product.
In general, any organic solvent-soluble manganese salt, whether
inorganic or organic, can be employed herein so long as it is soluble
in the organic solvent. Suitable salts include, for example, the
sulfate, nitrate and perchlorate salts and salts of organic acids
such as acetates.
The permanganate salt is likewise not limited so long as it is
soluble in the organic solvent. In general, the permanganate salt
can be an alkali or alkaline earth metal permanganate such as the
permanganates of sodium, potassium, cesium, magnesium, calcium and
barium. Ammonium or tetraalkylammonium permanganates can also be
employed. The counter ions of the aforementioned permanganates,
i.e., alkali metal cations, alkaline earth metal cations, ammonium
cations and tetraalkylammonium cations, often enhance dissolution
of the permanganate anion in the organic solvent. In some cases,
the counter ions, especially in the case of the larger counter ions
such as potassium and barium, serve as templates for crystallization
of OMS-3 product and will remain in the tunnel structures of OMS-3
as tunnel cations. Therefore, the particular permanganate salt employed
in the practice of this invention can be selected for its ability
to facilitate the formation and stabilization of the desired OMS-3
product. Where a smaller counter ion, for example, sodium cation
and/or magnesium cation, is utilized, the counter ion can have the
desirable effect of allowing template materials other than the counter
ion to affect the formation of OMS-3. The ionic diameters of some
alkali and alkaline earth metal cations which can be employed are
listed below:
______________________________________ Cation Li.sup.+ Na.sup.+
K.sup.+ Cs.sup.+ Mg.sup.2+ Ca.sup.2+ Ba.sup.2+ ______________________________________
r(.ANG.) 1.36 1.96 2.66 3.78 1.30 1.98 2.70 ______________________________________
Template materials which can be employed in producing OMS-3 include
the tetraakylammonium salts in which the alkyl groups can contain
from 1 to about 5 carbon atoms, can be the same or different and
can be normal or branched in structure. Methyl, ethyl and propyl
groups are representative alkyl groups which can advantageously
be employed herein. The anion of the aforementioned salts can be
any suitable inorganic or organic ion which will dissolve and remain
in solution without interfering with the reaction or, optionally,
form a precipitate with the counter ion of the permanganate salt
employed in the method herein. Examples of such anions include the
halides, hydroxides, bisulfates, sulfates, perchlorates, acetates
and the like.
Also useful as organic templates are polymer chains containing
synthetic polymers such as those described as cationic polymers,
quaternary ammonium polymers and ionene polymers by Daniels et al.
in "Cationic Polymers as Templates in Zeolite Crystallization,
"J. Am. Chem. Soc. 100 pp. 3097-3100 (1978) and Davis et al.
in "Synthesis of Gmelinite and ASM-12 Zeolites with a Polymer
Template," J. Chem. Soc., Chem. Commun. 1988 pp. 920-921.
In another embodiment of this invention, the framework of OMS-3
is substituted with transition metal cation(s). The transition metal
cation is incorporated into the framework of OMS-3 by co-dissolving
a transition metal salt in the organic solvent used to dissolve
the manganese salt. The transition metal cation(s), which can be
designated as M.sup.+n (where M indicates the transition metal and
n indicates an oxidation state which is stable in the organic solvent
solution), can be any metal selected from Groups IIIA, IVA, VA,
VIA, VIIA, VIIIA, IB and IIB of the Periodic Table of the Elements.
Preferably, the transition metal is a metal selected from Groups
1B, IIB and VIIIA of the Periodic Table of the Elements. Examples
of useful framework-substituting transition metals include Mg, Fe,
Co, Ni, Cu, Ti, V, Cd, Mo, W, Cr, Zn, La, Ir, Rh, Pd and Pt. Preferred
metals include Co, Cu, Ni, Zn, La and Pd. Transition metal cation(s)
M.sup.+n should be present in the organic solvent in a concentration
effective to introduce the desired proportions of the metal(s) into
the framework of OMS-3 structure during the course of the reaction.
Therefore, any suitable salt (inorganic or organic) of the selected
metal(s) can be used which is sufficiently soluble provided, of
course, that the anion does not interfere with the other reactants
or the course of the reaction. For example, the nitrates, sulfates,
perchlorates, alkoxides, acetates, and the like, can be used with
generally good results.
The preferred synthesis of OMS-3 initially involves dissolving
a manganese salt in a lower alkanol as solvent. Examples of manganese
salts which can be employed include MnCl.sub.2 Mn(NO.sub.3).sub.2
MnSO.sub.4 Mn(CH.sub.3 COO).sub.2 and the like, with Mn(CH.sub.3
COO).sub.2 being preferred. Suitable lower alkanols include those
of the general formula R.brket open-st.OH].sub.n wherein R is straight-chain
C.sub.1 -C.sub.6 alkyl and n is 1 or 2. Ethanol is preferably employed.
Thereafter, a permanganate salt is added to the solution, resulting
in the formation of a solid intermediate. Suitable permanganate
salts include Na(MnO.sub.4), KMnO.sub.4 Mg(MnO.sub.4).sub.2 etc.,
with KMnO.sub.4 providing particularly good results. The solution
is preferably heated during the reaction under stirring. After the
formation of a solid intermediate, the heat is removed and the remaining
solvent is allowed to evaporate at room temperature. The solid intermediate
is thereby recovered and preferably allowed to age at room temperature
for a period of time ranging from about 1 to about 14 days, and
preferably from about 6 to about 8 days. The aged solid intermediate
is then gradually heated from room temperature to a temperature
generally ranging from about 200.degree. to about 800.degree. C.,
with temperatures ranging from about 500.degree. C. to about 700.degree.
C. being preferred, to provide the desired OMS-3 product. The heating
step will generally take from about 0.1 to about 10 preferably
from about 0.5 to about 3 hours.
As the example which follows demonstrates, x-ray powder diffraction
(XRD) patterns of products resulting from the method disclosed herein
verify the presence (4.times.4) tunnel structures therein.
The octahedral molecular sieve of this invention possesses acid
sites, including Lewis and Bronsted sites. Applications include
catalyzed reactions, e.g., isomerization and polymerization, and
adsorption. Specific examples of catalysis and adsorption applications
of OMS-3 include the decomposition of alcohol, oxidation of CO,
dehydrogenation of hydrocarbons, reduction of NO, hydrogenation
of olefins, demetallation of petroleum residua, decomposition of
organic sulfur compounds, decomposition of organic nitrogen compounds,
decomposition of asphalt, adsorption of noxious gases and adsorption
of heavy metal ions.
The following examples are presented to illustrate specific embodiments
of the practice of this invention and are not to be interpreted
as limitations upon the scope of the invention. |