Molecular sieve abstract
Manganese oxide octahedral molecular sieve (OMS) is produced by
the method comprising: a) forming an aqueous reaction medium containing
manganese cation and permanganate anion, the reaction medium being
maintained at a pH of not greater than about 4.5; b) refluxing the
aqueous reaction medium under conditions which are effective to
produce solid crystalline manganese oxide octahedral molecular sieve
product; and, c) recovering the solid crystalline product. The method
of this invention is carried out in an open system, i.e., a reflux
condenser, and results in the formation of OMS which is thermally
stable up to about 600.degree. C.
Molecular sieve claims
What is claimed is:
1. A method for producing a manganese oxide octahedral molecular
sieve which comprises:
a) forming an aqueous reaction medium containing manganese cation
and permanganate anion and having a pH of not greater than about
4.5;
b) refluxing the aqueous reaction medium to produce solid crystalline
manganese oxide octahedral molecular sieve product; and,
c) recovering the solid crystalline product.
2. The method of claim 1 wherein the solid crystalline product
is a manganese oxide octahedral molecular sieve having a 2.times.2
tunnel structure.
3. The method of claim 2 wherein the tunnel structure of the solid
crystalline product contains counter ions.
4. The method of claim 1 wherein the aqueous reaction medium is
formed by:
a) dissolving a manganese salt in aqueous medium to provide a first
solution;
b) dissolving a permanganate salt in aqueous medium to provide
a second solution; and,
c) combining the first solution and second solution to provide
the aqueous reaction medium.
5. The method of claim 4 wherein the manganese salt is selected
from the group consisting of MnSO.sub.4 MnNO.sub.3 MnClO.sub.4
and Mn(CH.sub.3 COO).sub.2.
6. The method of claim 4 wherein the permanganate salt is an alkali
metal permanganate, alkaline earth metal permanganate, ammonium
permanganate or tetraalkylammonium permanganate.
7. The method of claim 6 wherein the permanganate salt is selected
from the group consisting of LiMnO.sub.4 NaMnO.sub.4 KMnO.sub.4
CsMnO.sub.4 Mg(MnO.sub.4).sub.2 Ca(MnO.sub.4).sub.2 and Ba(MnO.sub.4).sub.2.
8. The method of claim 1 wherein the pH of the reaction medium
ranges from about 0 to about 4.
9. The method of claim 1 wherein the pH of the reaction medium
ranges from about 1 to about 3.
10. The method of claim 1 wherein the aqueous reaction medium is
refluxed in the presence of a template material.
11. The method of claim 1 wherein the molar ratio of manganese
cation to permanganate anion is from about 0.05 to about 3.
12. The method of claim 1 wherein the molar ratio of manganese
cation to permanganate anion is from about 0.1 to about 1.5.
13. The method of claim 1 wherein the temperature at which the
reaction medium is refluxed ranges from about 40.degree. to about
255.degree. C.
14. The method of claim 1 wherein the temperature at which the
reaction medium is refluxed ranges from about 70.degree. to about
155.degree. C.
15. The method of claim 1 wherein the reaction medium is refluxed
for a period of time ranging from about 2 to about 48 hours.
16. The method of claim 1 wherein the reaction medium is refluxed
for a period of time ranging from about 12 to about 36 hours.
17. The method of claim 1 wherein the product is recovered by the
successive steps of filtering, washing and drying.
Molecular sieve description
BACKGROUND OF THE INVENTION
This invention relates to a method for producing a manganese oxide
octahedral molecular sieve (OMS). More particularly, this invention
relates to a method for producing a manganese oxide octahedral molecular
sieve which is carried out in an open system, e.g., under refluxing
conditions.
Manganese oxide octahedral molecular sieves (OMS) 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)).
Pyrolusite, .beta.-MnO.sub.2 is a naturally occurring manganese
oxide characterized by single chains of MnO.sub.6 octahedra which
share edges to form (1.times.1) tunnel structures which are about
2.3 .ANG. square. Ramsdellite, MnO.sub.2 is a naturally-occurring
manganese oxide characterized by single and double chains of MnO.sub.6
octahedra which share edges to form (2.times.1) tunnel structures
which are about 4.6 .ANG. by about 2.3 .ANG. square. Nsutite, .gamma.-MnO.sub.2
is a naturally-occurring manganese oxide characterized by an intergrowth
of pyrolusite-like and ramsdellite-like tunnel structures. Pyrolusite,
ramsdellite and nsutite do not possess cations in their tunnel structures.
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 and R. Giovanili and B. Balmer, Chimia, 35 (1981) 53. 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. 1.
The hydrothermal method of synthesizing 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.
SUMMARY OF THE INVENTION
In accordance with the present invention a manganese oxide octahedral
molecular sieve is produced by the method which comprising:
a) forming an aqueous reaction medium containing manganese cation
and permanganate anion, the reaction medium being maintained at
a pH of not greater than about 4.5;
b) refluxing the reaction medium under conditions effective to
produce solid crystalline manganese oxide octahedral molecular sieve
product; and,
c) recovering the solid crystalline product.
Unlike the hydrothermal method of producing OMS-2 which involves
the use of a closed-system reactor, i.e., an autoclave, and the
application of autogenous pressure, the method of this invention
is carried out in an open system, i.e., in a reflux condenser, which
does not involve the application of pressure. OMS-2 produced by
the refluxing method herein is thermally stable up to about 600.degree.
C.
BRIEF DESCRIPTION OF THE DRAWINGS
In the attached figures of drawing:
FIG. 1 is a diagrammatic representation of the three dimensional
tunnel structure of OMS-2.
DETAILED DESCRIPTION OF THE INVENTION
The aqueous reaction medium containing manganese cation and permanganate
anion is preferably formed by first dissolving a manganese salt
in aqueous medium, e.g., distilled deionized water, which is maintained
at an initial pH of not greater than about 4.5 to provide a first
solution. The concentration of manganese cation in the first solution
is not narrowly critical and can range from about 0.5 to about 1M,
preferably from about 0.1 to about 0.5M. Preferably, the pH of the
first solution ranges from about 0 to about 4.0 and more preferably
from about 1.0 to about 3.0. Suitable acids for adjusting the pH
of the solution include the mineral acids, e.g., HCl, H.sub.2 SO.sub.4
HNO.sub.3 and strong organic acids such as toluene sulfonic acid
and trifluoroacetic acid. A permanganate-salt is then dissolved
in a separate aqueous medium, e.g., distilled deionized water, to
provide a second solution. The concentration of permanganate anion
in the second solution is likewise not narrowly critical and can
range from about 0.05 to about 1M preferably, from about 0.1 to
about 0.5M. Thereafter, the first solution and second solution are
combined to form the aqueous reaction medium containing manganese
cation and permangante anion. In another embodiment, the permanganate
salt can be co-dissolved with the manganese salt in aqueous medium
to provide the aqueous reaction medium containing manganese cation
and permanganate anion. After formation of the reaction medium,
the pH of the reaction medium can be adjusted to its initial level,
if necessary, by the addition of an appropriate amount of a suitable
acid such as one or more of the aforementioned pH-adjusting acids.
In general, any manganese salt, whether inorganic or organic, can
be employed herein so long as it is soluble in aqueous medium. 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 remains
soluble in the aqueous reaction medium. In general, the permanganate
salt can be an alkali or alkaline earth metal permanganate such
as a permanganate 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 solubility
of the permanganate anion in the aqueous reaction medium. 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 product and can remain in the tunnel structures of OMS as
tunnel cations. Counter cations having ionic diameters of less than
about 2.3 .ANG. produce a nsutite structure, while those having
ionic diameters ranging from about 2.3 to about 4.6 .ANG. produce
a (2.times.2) tunnel structure, i.e., OMS-2. 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 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. 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 include
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 of those alkyl groups which can advantageously
be employed herein. The counter ion of the tetraalkylammonium salt
can be any suitable inorganic or organic anion 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.
The molar ratio of manganese cation to permanganate anion, [Mn.sup.+2
]/[MnO.sub.4.sup.- ], which can be expressed as [Mn.sup.2+ ]/[Mn.sup.7+
] for convenience, is one of the critical factors or parameters
in determining the nature of the product obtained via the method
of this invention. The [Mn.sup.2+ ]/[Mn.sup.7+ ] ratio will generally
be about 0.05 to about 3 preferably about 0.1 to about 2. When
a ratio of about 0.1 to about 1.5 is employed, OMS-2 is formed.
When a ratio of greater than about 2.5 is employed, OMS corresponding
to the nsutites are formed.
The temperatures at which the reaction medium is refluxed can range
broadly from about 40.degree. C. to about 255.degree. C. with the
lower end of this temperature range tending to produce slower reactions.
Temperatures in the range of from about 40.degree. to about 70.degree.
C. will tend to produce the nsutite structures which have generally
low crystallinities but contain structures characterized by tunnels
of dimension l.times.n where the basic unit dimension is a manganese
oxide octahedron and can be an integer of 1 or 2. Given an appropriate
pH, the process of the invention can be carried out to produce materials
of the OMS-2 structure at temperatures ranging from about 70.degree.
C. to about 155.degree. C., preferably from about 80.degree. to
about 120.degree. C. and more preferably from about 90.degree. to
about 110.degree. C. For the production of pyrolusite (1.times.1)
structures, the temperature preferably ranges from about 155.degree.
C. to about 255.degree. C.
Generally, the reaction medium is refluxed in an open system, e.g.,
a condenser, for a period of time ranging from about 2 to about
48 preferably from about 12 to about 36 hours. The refluxing operation
will result in the formation of a crystalline product characterized
by three dimensional mono-directional tunnel structures formed by
chains of edge-sharing MnO.sub.6 octahedra. Following the refluxing
step, the crystalline product can be recovered from the reaction
medium by any suitable technique. In general, the product will be
filtered, e.g., in a filter funnel under vacuum, washed with purified
water and dried, preferably in an oven at about 120.degree. C. for
about 12 hours.
The octahedral molecular sieve produced by the method 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 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 example is presented to illustrate specific embodiments
of the practice of this invention and is not intended to be a limitation
upon the scope of this invention. |