Molecular sieve abstract
Octahedral molecular sieve sorbents and catalysts are disclosed,
including silver hollandite and cryptomelane. These materials can
be used, for example, to catalyze the oxidation of CO.sub.x (e.g.,
CO), NO.sub.x (e.g., NO), hydrocarbons (e.g., C.sub.3H.sub.6) and/or
sulfur-containing compounds. The disclosed materials also may be
used to catalyze other reactions, such as the reduction of NO.sub.2.
In some cases, the disclosed materials are capable of sorbing certain
products from the reactions they catalyze. Silver hollandite, in
particular, can be used to remove a substantial portion of certain
sulfur-containing compounds from a gas or liquid by catalysis and/or
sorption. The gas or liquid can be, for example, natural gas or
a liquid hydrocarbon.
Molecular sieve claims
1. A method for treating a gas or liquid, comprising exposing the
gas or liquid to silver hollandite such that the silver hollandite
catalyzes oxidation of from about 10% to about 100% of at least
one compound in the gas or liquid.
2. The method according to claim 1 wherein the gas or liquid is
exposed to the silver hollandite at a temperature from about 50.degree.
C. to about 350.degree. C.
3. The method according to claim 1 wherein the gas or liquid is
exposed to the silver hollandite at a temperature from about 100.degree.
C. to about 200.degree. C.
4. The method according to claim 1 wherein the gas or liquid is
a gas, the gas comprises CO, and the silver hollandite catalyzes
oxidation of from about 10% to about 100% of the CO.
5. The method according to claim 1 wherein the gas or liquid is
a gas, the gas comprises NO, and the silver hollandite catalyzes
oxidation of from about 10% to about 100% of the NO.
6. The method according to claim 1 wherein the gas or liquid comprises
a hydrocarbon and the silver hollandite catalyzes oxidation of from
about 10% to about 100% of the hydrocarbon.
7. The method according to claim 1 wherein the gas or liquid is
a gas and the gas comprises a combustion exhaust stream.
8. The method according to claim 1 wherein the gas or liquid comprises
a sulfur-containing compound and the silver hollandite catalyzes
oxidation of from about 10% to about 100% of the sulfur-containing
compound.
9. The method according to claim 8 wherein the sulfur-containing
compound is a sulfur-containing compound having at least one thiol
group, a sulfur-containing compound having at least one sulfide
bond, a sulfur-containing compound having at least one disulfide
bond, H.sub.2S, or a combination thereof.
10. The method according to claim 1 wherein the gas or liquid
comprises a sulfur-containing compound, and the silver hollandite
sorbs from about 10% to about 100% of the sulfur-containing compound.
11. The method according to claim 10 wherein the sulfur-containing
compound is SO.sub.2.
12. A method for treating a gas or liquid, comprising exposing
the gas or liquid to silver hollandite such that the silver hollandite
catalyzes oxidation of a substantial portion of at least one sulfur-containing
compound in the gas or liquid to produce a sulfur-containing product,
and the silver hollandite sorbs a substantial portion of the sulfur-containing
product.
13. The method according to claim 12 wherein the sulfur-containing
product is SO.sub.2.
14. A method for removing a substantial portion of at least one
sulfur-containing compound from a gas or liquid comprising exposing
the gas or liquid to silver hollandite.
15. The method according to claim 14 wherein the gas or liquid
is a gas and the gas comprises natural gas.
16. The method according to claim 15 wherein the natural gas is
exposed to the silver hollandite at a temperature from about ambient
temperature to about 250.degree. C.
17. The method according to claim 14 wherein the gas or liquid
is a liquid and the liquid comprises a liquid hydrocarbon.
18. The method according to claim 17 wherein the liquid hydrocarbon
is exposed to the silver hollandite at a temperature from about
ambient temperature to about 100.degree. C.
19. The method according to claim 17 further comprising rinsing
the liquid hydrocarbon with a polar liquid after exposing the liquid
hydrocarbon to silver hollandite.
20. A method for removing a substantial portion of NO.sub.2 in
a gas, comprising exposing the gas to silver hollandite such that
the silver hollandite catalyzes reduction of a substantial portion
of the NO.sub.2 to form N.sub.2 and O.sub.2.
21. The method according to claim 20 wherein the gas is exposed
to the silver hollandite at a temperature from about 150.degree.
C. to about 350.degree. C.
22. A catalytic device comprising silver hollandite, wherein the
catalytic device is configured to expose a gas or liquid to the
silver hollandite such that the silver hollandite catalyzes oxidation
of a substantial portion of at least one compound in the gas or
liquid.
23. The device according to claim 22 further configured to sorb
a sulfur-containing compound in the gas or liquid.
24. The device according to claim 22 wherein the sulfur-containing
compound is a sulfur-containing product produced by oxidation of
the at least one compound in the gas or liquid.
25. A sulfur-removal device comprising silver hollandite, wherein
the sulfur-removal device is configured to expose a gas or liquid
to the silver hollandite so as to remove a substantial portion of
at least one sulfur-containing compound from the gas or liquid.
Molecular sieve description
RELATED APPLICATION DATA
[0001] This application is a continuation-in-part of currently
pending International Patent Application No. PCT/US/2005/003600
filed Feb. 4 2005 which is a continuation of currently pending
U.S. patent application Ser. No. 10/771866 filed Feb. 4 2004.
This application also claims the benefit of the earlier filing date
of currently pending U.S. Provisional Application No. 60/649656
filed Feb. 3 2005. International Patent Application No. PCT/US/2005/003600
U.S. patent application Ser. No. 10/771866 and U.S. Provisional
Application No. 60/649656 are incorporated herein by reference.
FIELD
[0003] This disclosure is generally related to pollution control
and purification technologies, and more particularly, but not exclusively,
is directed to octahedral molecular sieve sorbents and catalysts
and uses therefore in emissions control and purification.
BACKGROUND
[0004] The combustion waste gases (i.e. the exhaust) of thermal
power plants, factories, on-road vehicles, diesel generators, and
the like typically contain SO.sub.x and NO.sub.x. State and federal
regulations limit the permissible amounts of these emissions because
they create environment problems, such as acid rain. Accordingly,
there is a continual need for improvements in the cost effective
and efficient control of these emissions.
[0005] One mechanism for limiting NO.sub.x and SO.sub.x emissions
is to remove or scrub the pollutants from the exhaust gas using
a sorption bed, trap or similar device. Because many NO.sub.x traps
have been found to be poisoned by the presence of SO.sub.x, it is
important to remove as much SO.sub.x from the exhaust gas as possible.
However, as compared to the large volume of studies on NO.sub.x
reduction, SO.sub.x removal using solid sorbents is an area in need
of scientific advancement. Certain types of materials have been
identified as possible solid sorbents for use in SO.sub.x sorption
beds and traps, such as calcium oxide and alkalized alumina (Na/Al.sub.2O.sub.3
or K/Al.sub.2O.sub.3), copper-based sorbents (e.g., Cu/Al.sub.2O.sub.3),
promoted metal oxides (e.g., TiO.sub.2 Al.sub.2O.sub.3 and ZrO.sub.2),
promoted cerium oxide (La- or Cu-doped CeO.sub.2), and supported
cobalt (Co/Al.sub.2O.sub.3). Unfortunately, over the temperature
range of about 250.degree. C. to 475.degree. C., these materials
typically have a relatively low sorption capacity. For example,
their total sorption capacity of SO.sub.2 is typically less than
about 10 wt % based on the weight of the sorbent, and their breakthrough
sorption capacity can be substantially lower, depending on operating
conditions. As it is combustion in this temperature range that leads
to a significant portion of the total SO.sub.x emissions, a greater
sorption capacity at these temperatures is needed.
[0006] One approach to increasing the sorption capacity of SO.sub.x
sorption beds is to provide an oxidation catalyst upstream or admixed
with the bed so as to convert most of the SO.sub.2 to SO.sub.3
since SO.sub.3 is generally more readily sorbed than SO.sub.2 due
to its ability to form stable surface sulfates. However, the cost
of recovery of the oxidation catalyst (frequently a precious metal)
and the relatively poor conversion efficiency of SO.sub.2 to SO.sub.3
at temperatures below about 300.degree. C. limits the effectiveness
of this approach as well.
SUMMARY
[0007] Disclosed herein are systems and techniques for SO.sub.x
emission control. Some disclosed embodiments concern materials for
sorbing, trapping, or otherwise eliminating sulfur oxides from gases,
such as sulfur oxides present in exhaust gases of internal combustion
engines. These materials can include mixed oxides having a framework
of metal cation or cations (M) each surrounded by 6 oxygen atoms.
The octahedra (MO.sub.6) thus formed can be connected together by
edges and vertices, resulting in a structure with channels in at
least one direction in space. The sides of the channels can be formed
by the linked octahedra. The octahedra can be connected together
by the edges, while the sides of the channels are connected together
by the vertices of the octahedra.
[0008] The width of the channels can vary depending on whether
the sides are composed of 2 3 and/or 4 octahedra, which, in turn,
depends on the mode of preparation. This type of material is known
by its acronym OMS (octahedral molecular sieve). In some disclosed
embodiments, the materials are selected so that they have a structure
that generates channels either with a square cross section composed
of, for example, one octahedra by one octahedra (OMS 1.times.1),
two octahedra by two octahedra (OMS 2.times.2) or three octahedra
by three octahedra (OMS 3.times.3), or with a rectangular cross
section composed of, for example, two octahedra by three octahedra
(OMS 2.times.3). Thus, certain of the materials will have a pyrolusite
(OMS 1.times.1), hollandite (OMS 2.times.2), romanechite (OMS 2.times.3)
or todorokite (OMS 3.times.3) type structure. Other OMS structures,
such as 1.times.3 1.times.4 2.times.4 3.times.4 and 4.times.4
are also contemplated, though the 2.times.2 structure has been found
to be particularly effective in certain applications.
[0009] The disclosed OMS materials are preferably manganese based
(Mn-OMS), which means that, if more than one metal cation (M) is
present, a major portion of the metal cations (M) is manganese (Mn).
A metal cation (M) is in the majority when it satisfies the following
formula: (n.sub.maj/.SIGMA.n.sub.M)>(1/N), where (n.sub.M) is
the number of atoms of each metal cation (M) within the framework,
(N) is the number of different metal cations (M) within the framework,
and (n.sub.maj) is the number of atoms of the metal cation (M) with
the greatest number of atoms of any of the metal cations (M) within
the framework. Most preferably, over about 50% of the metal cations
(M) are manganese for example at least about 75% or at least about
90% of the metal cations (M) by mole. Preferably the manganese has
an oxidation number between +2 and +4. The balance of metal cations
(M) can include one or more elements from groups IIIB to IIIA in
the periodic table such as Zn .sup.2+, Co.sup.2+, Ni.sup.2+, Fe.sup.2+,
Al.sup.3+, Ga.sup.3+, Fe.sup.3+, Ti.sup.3+, In.sup.3+, Cr .sup.3+,
Si.sup.4+, Ge.sup.4+, Ti.sup.4+, Sn.sup.4+, Sb.sup.5+ and combinations
thereof.
[0010] The disclosed materials can have a characteristic structure
with a high surface area and may be capable of oxidizing SO.sub.2
to SO.sub.3 and/or converting SO.sub.3 to a sulfate. In certain
embodiments, another cation, such as H.sup.+, NH .sup.4+, Li.sup.+,
Na.sup.+, Ag.sup.+, K.sup.+, Rb.sup.+, Tl.sup.+, Cs.sup.+, Mg.sup.2+,
Ca.sup.2+, Sr.sup.2+, Ba.sup.2+, Ra.sup.2+, Cu.sup.2+, Pb.sup.2+,
locates in the channels in the OMS structures.
[0011] In embodiments having a sorbing phase of materials with
type OMS 2.times.2 OMS 2.times.3 and/or OMS 3.times.3 the materials
may have a three-dimensional structure that generates channels in
at least one direction in space, is composed of octahedra (MO.sub.6),
and comprises: [0012] at least one metal cation (M) selected from
the group formed by elements from groups IIIB, IVB, VB, VIB, VIIB,
VIII, IB, IIB, IIIA of the periodic table and germanium, each metal
cation (M) being coordinated with 6 oxygen atoms, and being located
at the center of the resulting oxygen octahedra, wherein a major
portion of metal cation or cations (M) is manganese; and [0013]
at least one element (B) selected from the group formed by the alkali
elements IA (such as K.sup.+), the alkaline-earth elements IIA,
the rare earths IIIB, transition metals (such as Ag.sup.+) and elements
from groups IIIA and IVA, element or elements (B) generally being
located in channels in the oxide structure.
[0014] More particularly, metal cation or cations (M) can be selected
from scandium, titanium, zirconium, vanadium, niobium, chromium,
molybdenum, tungsten, manganese, iron, cobalt, nickel, copper, zinc,
aluminum, gallium, and mixtures thereof.
[0015] The average charge (oxidation number) carried by the cation
or cations (M) from groups IIIB to IIIA is preferably about +3.5
to +4. Preferably, at least about 50% of the metal cation or cations
(M) in the material is manganese, titanium, chromium, aluminum,
zinc, copper, zirconium, iron, cobalt, and/or nickel. More preferably,
over 50% of the metal cation or cations (M) is manganese, chromium,
copper, iron, titanium, and/or zirconium. In one form, manganese
composes at least about 50% of the metal cation or cations (M) by
mole, for example, at least 75% or 90% of the metal cation or cations
(M).
[0016] Other metal cation or cations (M) from groups IIIB to IIIA,
including the transition metals, can be added in minor quantities
as dopants. Preferably, the element or elements from groups IIIB
to IIIA added in minor quantities is/are selected from scandium,
titanium, zirconium, vanadium, niobium, chromium, molybdenum, tungsten,
manganese, iron, cobalt, nickel, copper, zinc, aluminum, gallium,
and mixtures thereof.
[0017] Element or elements (B) may belong to the group formed by
the alkali elements IA, alkaline-earth elements IIA, rare earth
elements IIIB, transition metals (such as elements from group IB,
e.g. silver) and elements from groups IIIA and IVA. They can be
located in the channels of the material. Preferably, element or
elements (B) is/are selected from potassium, silver, sodium, magnesium,
barium, strontium, iron, copper, zinc, aluminum, rubidium, calcium
and mixtures thereof.
[0018] In a preferred embodiment, the material is of the formula
X.sub.aMn.sub.8O.sub.16 wherein X is an alkali metal, an alkaline
earth metal, or a transition metal, and a is between 0.5 and 2.0.
In still further preferred forms the material is cryptomelane, silver
hollandite, or a combination thereof.
[0019] A number of different methods exist for preparing these
materials (see references 3 and 4 below, for example). They may
be synthesized by mixing and grinding solid inorganic precursors
of metal oxides (metals M and B), followed by calcining. The materials
can also be obtained by heating solutions of precursor salts to
reflux, drying and calcining, by precipitating precursor salts by
the sol-gel method, or by hydrothermal synthesis, which consists
of heating an aqueous solution containing the elements constituting
the final material under autogenous pressure. The materials obtained
from these syntheses can be modified by ion exchange or isomorphous
substitution.
[0020] For example, it has been found that highly crystallized
silver hollandite can be obtained by the ion exchange of cryptomelane
in a silver salt melt. Preferably, this silver salt melt is substantially
pure, i.e. at least about 95% the liquefied silver salt. The ion
exchange typically is carried out at a temperature above the melting
temperature of the silver salt but below the decomposition temperature
of either the cryptomelane or the silver salt. A typical temperature
range for the ion exchange will be between 200.degree. C. and 800.degree.
C. At atmospheric pressure, a typical duration will be at least
about 1 hour. Suitable silver salts include the nitrates, sulfate,
chlorates, bromides, chlorides, fluorides, and iodides of silver
and organic acid silver salts. After ion exchange, the excess salt
is removed by washing with a suitable solvent to produce substantially
pure silver hollandite.
[0021] Previously, silver hollandite has been formed by the thermal
decomposition of AgMnO.sub.4 and Ag.sub.2O in a 1:1 molar ratio
at 970.degree. C. under 5 kbar oxygen over 7 days (see references
11 and 12 below). In contrast, synthesis of silver hollandite via
the disclosed ion exchange process is much simpler and facilitates
the production of large amounts for industrial applications. In
certain embodiments, the silver hollandite can be of high purity
with small crystal size and high surface area. Other metals may
optionally be introduced into the OMS structure using any of the
methods known to the skilled person, such as excess impregnation,
dry impregnation, ion exchange, etc.
[0022] The disclosed material can have a specific surface area
in the range 1 to 300 m.sup.2/g, preferably in the range 2 to 300
m.sup.2/g, and more preferably in the range 30 to 250 m.sup.2/g.
The sorption kinetics typically are improved when the specific surface
area is high, e.g., greater than 10 m.sup.2/g, such as in the range
30 to 250 m.sup.2/g.
[0023] The sorbent phase can be in the form of a powder, beads,
pellets or extrudates and can be deposited or directly prepared
on monolithic supports of ceramic or metal. To increase the dispersion
of the materials and thus to increase their sorption capacity, the
materials can be deposited on porous supports with a high specific
surface area before being formed (extrusion, coating . . . ). These
supports can be generally selected from the group formed by the
following compounds: alumina (alpha, beta, delta, gamma, khi, or
theta alumina), silicas (SiO.sub.2), silica-aluminas, zeolites,
titanium oxide (TiO.sub.2), zirconium oxide (ZrO.sub.2), magnesium
oxide (MgO), divided carbides, for example silicon carbides (SiC),
used alone or as a mixture. Mixed oxides or solid solutions comprising
at least two of the above oxides can be added.
[0024] For many uses, such as in connection with a vehicle exhaust,
it is usually preferable to use rigid supports (monoliths) with
a large open porosity (e.g., more than 70%) to limit pressure drops
that may cause high gas flow rates, and in particular high exhaust
gas space velocities. These pressure drops can be deleterious to
proper functioning of engines and can contribute to reducing the
efficiency of an internal combustion engine (gasoline or diesel).
Further, the exhaust system may be subjected to vibrations and to
substantial mechanical and thermal shocks, so catalysts in the form
of beads, pellets or extrudates run the risk of deterioration due
to wear or fracturing.
[0025] At least two techniques can be used to prepare the disclosed
catalysts on monolithic ceramic or metal supports (or substrates).
The first technique comprises direct deposition on the monolithic
support, such as using a wash coating to coat the sorbing phase.
A wash coating can be prepared, for example, using the operating
procedure described in reference 4 below. The sorbent phase can
be coated just after the co-precipitation step, hydrothermal synthesis
step or heating under reflux step, the final calcining step being
carried out on the phase deposited on the monolith. Alternatively,
the monolith can be coated after the material has been prepared
in its final state, i.e., after the final calcining step.
[0026] The second technique comprises depositing the inorganic
oxide on the monolithic support and then calcining the monolith
between 500.degree. C. and 1100.degree. C. so that the specific
surface area of the oxide is in the range of 20 to 150 m.sup.2/g.
The monolithic substrate covered with the inorganic oxide then can
be covered with the sorbent phase prepared, for example, according
to the steps described in the reference 4 below.
[0027] Monolithic supports that can be used include, but are not
limited to: ceramics (such as alumina, zirconia, cordierite, mullite,
silica, and alumino-silicates), silicon carbide, silicon nitride,
aluminium titanate, metals (such as iron, chromium or aluminium
alloys optionally doped with nickel, cobalt, cerium or yttrium)
or combinations thereof. The structure of a ceramic supports can
be, for example, that of a honeycomb, foam or fibers. Metal supports
can be produced by winding corrugated strips or by stacking corrugated
sheets to constitute a honeycomb structure with straight or zigzag
channels, which may or may not communicate with each other. They
can also be produced from metal fibers or wires which are interlocked,
woven or braided.
[0028] In embodiments in which the supports are made of metal comprising
aluminum, it is recommended that the supports be pre-treated at
high temperature (for example between 700.degree. C. and 1100.degree.
C.) to develop a micro-layer of refractory alumina on the surface.
This superficial micro-layer, with a porosity and specific surface
area which is higher than that of the original metal, encourages
adhesion of the active phase and protects the remainder of the support
against corrosion.
[0029] The quantity of sorbent phase deposited or prepared directly
on a ceramic or metallic support (or substrate) is generally in
the range 20 to 300 g per liter of the support, advantageously in
the range 50 to 200 g per liter.
[0030] Some of the disclosed sorbents can sorb oxides of sulfur
present in the gases, in particular exhaust gases. These materials
typically are capable of sorbing SO.sub.x at a temperature which
is generally in the range 50.degree. C. to 650.degree. C., preferably
in the range 100.degree. C. to 600.degree. C., more preferably in
the range 150.degree. C. to 550.degree. C. For diesel engines in
automobiles, an intended application, the temperature of the exhaust
gas may be in the range 150.degree. C. to 500.degree. C. and rarely
exceeds 600.degree. C.
[0031] The disclosed sorbents may be suitable for sorbing oxides
of sulfur present in the exhaust gases of stationary engines or,
particularly, automotive diesel engines or spark ignition (lean
burn) engines, but also in the gases from gas turbines operating
with gas or liquid fuels. These exhaust gases typically contain
oxides of sulfur in the range of a few tens to a few thousands of
parts per million (ppm) and can contain comparable amounts of reducing
compounds (e.g., CO, H.sub.2 and hydrocarbons) and nitrogen oxides.
These exhaust gases might also contain larger quantities of oxygen
(e.g., 1% to close to 20% by volume) and steam, though the disclosed
sorbents can be effective in oxygen free environments as well. The
disclosed sorbents can be used with HSVs (hourly space velocities),
which correspond to the ratio of the volume of the monolith to the
gas flow rate, for the exhaust gas generally in the range 500 to
150000 hr.sup.-1 for example in the range 5000 to 100000 hr.sup.-1.
[0032] In has been found that sorption of SO.sub.x can lead to
a noticeable color change in the sorbent. Accordingly, in one variation,
a SO.sub.x trap is provided by a quantity of the sorbent contained
in a housing having a window. The color of the sorbent material
can be periodically monitored through the window with the need to
replace or recharge the trap indicated by the color change. In this
or other refinements, a spent SO.sub.x trap can be regenerated by
appropriate reflux synthesis so as to reuse the sorbent support
and the housing.
[0033] The disclosed sorbents can be used anywhere SO.sub.x needs
to be sorbed. It has been found that significant advantages can
be realized in the overall control of emissions from a combustion
exhaust by locating the SO.sub.x sorbent upstream from a particulate
filter or NO.sub.x trap.
[0034] In an exemplary implementation, the disclosed SO.sub.x sorbent
material is used in connection with "regenerable" NO.sub.x
traps without the need to bypass or otherwise protect the SO.sub.x
sorbent during regeneration of the NO.sub.x trap. As discussed more
fully in reference 10 below, regenerable NO.sub.x traps are constructed
to capture NO.sub.x (for example as nitrate) during normal operation
(lean conditions) and then to release the nitrogen (for example
as N.sub.2) during a brief fuel-rich reduction step (rich conditions).
The rich conditions typically last less than about 1 minute, and
the lean conditions are typically at least about 5 times the duration
of the rich conditions. When the NO.sub.x trap and a SO.sub.x sorbent
are arranged in series, the SO.sub.x sorbent also is subject to
this lean/rich cycling, unless measures are employed to limit the
fuel rich reduction conditions to the NO.sub.x trap. These measures,
such as a bypass, can be difficult to implement. Advantageously,
no such measures are needed with some embodiments of the disclosed
sorbents. For example, in certain embodiments, the sorbents are
able to survive lean/rich cycling and still retain substantial sorbent
capacity.
[0035] In some disclosed embodiments, SO.sub.x sorption is provided
by a manganese oxide material which has inherent oxidizing capability,
so that SO.sub.2 can be oxidized and sorbed without use of a separate
and costly oxidation catalyst. The manganese oxide material also
may have a high total sorption capacity, such as greater than about
40% by weight, thereby providing economical and efficient emissions
control.
[0036] As discussed above, silver hollandite can be useful as a
SO.sub.x (e.g., SO.sub.2) sorbent. Silver hollandite also can be
useful as an oxidation catalyst, such as for the catalysis of oxidation
of CO.sub.x (e.g., CO), NO.sub.x (e.g., NO), hydrocarbons (e.g.,
C.sub.3H.sub.6) and certain sulfur-containing compounds, such as
compounds having at least one thiol group, compounds having at least
one sulfide bond, compounds having at least one disulfide bond,
and H.sub.2S. Silver hollandite performs particularly well as an
oxidation catalyst and/or sorbent at low temperatures, such as temperatures
between about 50.degree. C. and about 350.degree. C., especially
temperatures between about 100.degree. C. and about 200.degree.
C. In some implementations, silver hollandite acts as both an oxidation
catalyst and as a sorbent. For example, silver hollandite can be
used to sorb at least a portion of at least one product of an oxidation
process it catalyzes. Such products can include, for example, SO.sub.2
which can be sorbed as sulfate.
[0037] Silver hollandite's properties make it particularly well
suited for use in removing sulfur-containing compounds from a gas
or liquid. For example, silver hollandite can be used to remove
at least a portion of a sulfur-containing compound, such as an organic
sulfur-containing compound, from natural gas or from a liquid hydrocarbon
(e.g., gasoline, diesel or jet fuel). Removing a sulfur-containing
compound from a liquid hydrocarbon may include rinsing the liquid
hydrocarbon with a polar liquid after exposing the liquid hydrocarbon
to silver hollandite. For removing sulfur-containing compounds from
natural gas, the temperature of the natural gas can be, for example,
between about ambient temperature (e.g., about 23.degree. C.) and
about 250.degree. C. For removing sulfur-containing compounds from
a liquid hydrocarbon, the temperature of the liquid hydrocarbon
can be, for example, between about ambient temperature and about
100.degree. C.
[0038] Silver hollandite also can be used to remove at least a
portion of a nitrogen oxide, such as NO.sub.2 from a gas or liquid.
For example, silver hollandite can be used to catalyze the reduction
of NO.sub.2 to form N.sub.2 and O.sub.2. Such a process can be performed,
for example, at a temperature between about 150.degree. C. and about
350.degree. C.
[0039] One or more of the disclosed sorbents, such as cryptomelane
and silver hollandite, can be combined in the same SO.sub.x trap
and/or can be placed in a series configuration in the emissions
stream. For example, a SO.sub.x trap comprising silver hollandite
can be placed upstream of a SO.sub.x trap comprising cryptomelane.
BRIEF DESCRIPTION OF THE FIGURES
[0040] FIG. 1 is a graph showing exemplary plots of the sorption
of SO.sub.2 on 2.times.2 Mn-OMS materials and on MnO.sub.2.
[0041] FIGS. 2a and 2b are exemplary scanning electron microscopy
images of a 2.times.2 Mn-OMS material before and after SO.sub.2
sorption, respectively.
[0042] FIGS. 3a and 3b are exemplary x-ray diffraction patterns
of a 2.times.2 Mn-OMS material before and after SO.sub.2 sorption,
respectively.
[0043] FIG. 4 is a graph showing exemplary plots of the sorption
of SO.sub.2 on a Mn-OMS material at different gas feed temperatures.
[0044] FIG. 5 is a graph showing exemplary plots of the sorption
of SO.sub.2 on a Mn-OMS material at different gas feed rates.
[0045] FIG. 6 is a graph showing exemplary plots of the sorption
of SO.sub.2 on a Mn-OMS material at different concentrations of
SO.sub.2 in the feed gas.
[0046] FIG. 7 is a graph showing exemplary plots of the sorption
of SO.sub.2 on a Mn-OMS material at different feed gas compositions.
[0047] FIG. 8 is a graph showing exemplary plots of the sorption
of SO.sub.2 on 2.times.3 Mn-OMS materials at different gas feed
rates.
[0048] FIG. 9 is a graph showing exemplary plots of the sorption
of SO.sub.2 on a 2.times.4 Mn-OMS material.
[0049] FIG. 10 is a schematic illustration of one embodiment of
an emissions control system implemented on a vehicle producing combustion
exhaust.
[0050] FIG. 11 is a perspective view of one embodiment of a SO.sub.x
filter having a monitoring window and regeneration ports.
[0051] FIG. 12 is an exemplary x-ray diffraction pattern of Ag-hollandite
from synthesis method A.
[0052] FIG. 13 is an exemplary TEM image of Ag-hollandite from
synthesis method A.
[0053] FIG. 14 is an exemplary selected area electron diffraction
pattern of Ag-hollandite from synthesis method A.
[0054] FIG. 15 is an exemplary x-ray diffraction pattern of Ag-hollandite
from synthesis method B.
[0055] FIG. 16 is a first exemplary TEM image of Ag-hollandite
from synthesis method B.
[0056] FIG. 17 is a second exemplary TEM image of Ag-hollandite
from synthesis method B.
[0057] FIG. 18 is an EDS of spot A in FIG. 17.
[0058] FIG. 19 is an EDS of spot B in FIG. 17.
[0059] FIG. 20 is a graph showing exemplary plots of the CO oxidation
properties of Ag-hollandite from synthesis method B and cryptomelane
over a range of temperatures.
[0060] FIG. 21 is a graph showing exemplary plots of the C.sub.3H.sub.6
oxidation properties of Ag-hollandite from synthesis method B and
cryptomelane over a range of temperatures.
[0061] FIG. 22 is a graph showing exemplary plots of the NO oxidation
properties of Ag-hollandite from synthesis method B and cryptomelane
over a range of temperature.
[0062] FIG. 23 is a graph showing exemplary plots of the de-NO.sub.x
properties of Ag-hollandite from synthesis method B and cryptomelane
over a range of temperatures.
[0063] FIGS. 24A-B are gas chromatograph traces of sulfur species
present in a liquid hydrocarbon before (FIG. 24A) and after (FIG.
24B) a sorption process using a Cu(I)Y zeolite.
[0064] FIGS. 25A-C are gas chromatograph traces of sulfur species
present in a liquid hydrocarbon showing how treatment with certain
embodiments of cryptomelane affects the concentration of sulfur
compounds in this liquid hydrocarbon.
[0065] FIG. 26 is a plot illustrating the use of certain embodiments
of cryptomelane to sorb H.sub.2S from nitrogen gas.
[0066] FIG. 27 is a XRD plot showing that MnS is produced by desulfurization
of natural gas using an embodiment of the disclosed process using
cryptomelane.
DETAILED DESCRIPTION
[0067] Reference will now be made to the embodiments illustrated
in the drawings and specific language will be used to describe the
same. It will nevertheless be understood that no limitation of the
scope of the invention is hereby intended. Alterations and further
modifications in the illustrated devices, and such further applications
of the principles of the invention as illustrated herein are contemplated
as would normally occur to one skilled in the art to which the invention
relates.
[0068] The prefix "sorb" includes adsorption and/or absorption,
unless the context indicates otherwise. The separations described
herein can be partial, substantial or complete separations unless
the context indicates otherwise.
[0069] In some disclosed embodiments, a manganese-based octahedral
molecular sieve serves as a high-capacity sulfur oxide solid sorbent.
As compared to other sorbents studied for the removal of SO.sub.2
from waste gases, this material provides surprising high capacity
and efficiency. In a preferred form, this material is referred to
as Mn-OMS 2.times.2.
[0070] The basic structure of the materials employed in some of
the Examples that follow includes MnO.sub.6 octahedra joined at
the edges to form a 2.times.2 hollandite tunnel structure with a
pore size of about 0.46 nm. For cryptomelane, a counter-cation,
K.sup.+, is present within the tunne 1 structure for charge compensation.
Mn can assume an oxidation state of 4+, 3+, or 2+, and the average
Mn oxidation state can be controlled within a certain range during
synthesis. Generally, this material has a high surface area (e.g.,
about 80 m.sup.2/g) and a high redox reaction activity.
[0071] Without intending to be bound by any theory of operation,
the following reaction may be involved in SO.sub.2 sorption using
cryptomelane: SO.sub.2+K.sub.xMn.sub.8O.sub.16.fwdarw.MnSO.sub.4+K.sub.2O
(1) The SO.sub.2 may be oxidized to SO.sub.3 by Mn.sup.4+ and Mn.sup.3+.
The Mn.sup.4+ and Mn.sup.3+ may be simultaneously reduced to Mn.sup.2+
(MnO). The SO.sub.3 produced then may react with the Mn.sup.2+ to
form MnSO.sub.4.
[0072] As explained herein, tunnel structure cryptomelane was found
to be a high-capacity sulfur dioxide sorbent. Its SO.sub.2 capacity
from 250.degree. C. to 475.degree. C. was found to be more than
ten times higher than that of conventional SO.sub.2 sorbents. Its
maximum SO.sub.2 capacity can be as high as about 74 wt %. The dominant
mechanism for SO.sub.2 sorption is believed to be oxidation of SO.sub.2
by Mn.sup.4+ and Mn.sup.3+ to form SO.sub.3 followed by reaction
of the SO.sub.3 with the co-produced Mn.sup.2+ to form MnSO.sub.4.
It has been found that this reaction is primarily controlled by
the mass diffusion of SO.sub.2 through the sorbent, and that it
can surprisingly effectively occur in an oxygen-free environment.
In addition, the visibly significant color change of cryptomelane
from black to yellow after SO.sub.2 sorption can be used as a convenient
indicator for the sorbent replacement.
[0073] Cryptomelane for SO.sub.2 sorption can be synthesized, for
example, either from a mixture of KMnO.sub.4 and MnSO.sub.4 or a
mixture of MnSO.sub.4 and KOH solution. After SO.sub.2 sorption,
MnSO.sub.4 is formed, which can subsequently be dissolved in water
and used as raw material for a subsequent cryptomelane synthesis.
To regenerate the SO.sub.2 sorption trap, therefore, only KOH and
O.sub.2 may be needed because the sorbent support (such as a monolith)
and the MnSO.sub.4 can be re-used.
[0074] The disclosed highly-efficient SO.sub.2 sorbents can be
used for removal of SO.sub.2 generated from thermal power plants,
factories, and on-road vehicles. They can be especially effective
for removal of SO.sub.x that is present in the emissions of diesel
trucks, in order to protect downstream emissions control devices
such as particulate filters and NO.sub.x traps that are poisoned
by SO.sub.x.
[0075] Silver hollandite, in particular, has utility as a low-temperature
oxidation catalyst. For example, silver hollandite can be used as
a catalyst for oxidation of CO.sub.x (e.g., CO), NO.sub.x (e.g.,
NO), hydrocarbons (e.g., hydrocarbons having between one and four
carbon atoms, such as C.sub.3H.sub.6) and sulfur-containing compounds,
such as compounds having at least one thiol group, compounds having
at least one sulfide bond, compounds having at least one disulfide
bond, and H.sub.2S. In general, silver hollandite performs well
when oxidation can be accomplished in the gas phase with a continuous
flow of oxygen to reactivate the material. Oxidation of sulfinur-containing
compounds may produce sulfur oxide products, such as SO.sub.2 which
then can be sorbed by the silver hollandite. Since silver hollandite
can act as both an oxidation catalyst and as a sorbent, it can serve
as a complete solution for the removal of sulfinur-containing compounds.
[0076] Silver hollandite also may be useful for catalyzing the
reaction of other compounds to convert such compounds into forms
that can be sorbed by the silver hollandite. For example, silver
hollandite may catalyze the oxidation of phosphites, phosphonates
and other reduced phosphorus compounds. The products of the catalyzed
oxidation reactions may include phosphates that would likely be
sorbed by the silver hollandite. Similarly, silver hollandite may
be used to catalyze the oxidation of certain metals and metal-containing
compounds into metal oxides that can be sorbed. For example, silver
hollandite may be used to catalyze the oxidation of mercury or alkyl
mercury compounds into mercuric oxide or to catalyze the oxidation
of AsH.sub.3 into As.sub.2O.sub.3. Toxic metal-containing compounds,
such as alkyl mercury compounds and AsH.sub.3 might be present,
for example, in emissions from coal gasification plants.
[0077] In some embodiments, a substantial portion of a sulfinur-containing
compound, such as a sulfur oxide product, is sorbed by silver hollandite.
A substantial portion can be, for example, between about 10% and
about 100% of the sulfur-containing compound, such as between about
50% and about 100% or between about 70% and about 100%. These percentages
and other percentages recited herein in connection with the term
"substantial portion" are weight percentages for a compound
within a defined quantity of a gas or liquid undergoing treatment.
The defined quantity of gas or liquid undergoing treatment typically
is substantially isolated from the surrounding atmosphere. For example,
the defined quantity can be a batch of gas or liquid undergoing
treatment or a stream of gas or liquid flowing past a device containing
silver hollandite for a set amount of time.
[0078] Oxidation using silver hollandite as a catalyst can be performed
continuously or as a batch process. The process typically results
in oxidation of a substantial portion of at least one oxidizable
compound in the gas or liquid being treated, such as between about
10% and about 100% of the oxidizable compound, between about 50%
and about 100%, or between about 70% and about 100%. In some applications,
the gas or liquid to be treated is exposed to silver hollandite
at a temperature between the minimum temperature required to provide
sufficient kinetic energy for the reaction and a temperature at
which the silver hollandite degrades. In some embodiments, the gas
or liquid is treated at a temperature between about 50.degree. C.
and about 350.degree. C., such as between about 100.degree. C. and
about 200.degree. C.
[0079] Due to its utility as a catalyst, silver hollandite may
be used in place of more expensive catalysts, such as platinum-based
catalysts, in many applications. For example, silver hollandite
can be used in place of platinum-based catalysts used for NO.sub.x,
CO.sub.x, hydrocarbon, and carbon soot oxidation (e.g., for emissions
control) and formaldehyde oxidation (e.g., for in-door pollution
abatement).
[0080] In some disclosed embodiments, silver hollandite is used
as a de-NO.sub.x catalyst. For example, silver hollandite can be
exposed to a gas or liquid containing NO.sub.2 and catalyze the
reduction of the NO.sub.2 to form N.sub.2 and O.sub.2 using a reductant,
such as CO or a hydrocarbon. This can be done, for example, at a
temperature between about 50.degree. C. and about 400.degree. C.,
such as between about 150.degree. C. and about 350.degree. C. The
process can result in reduction of a substantial portion of a NO.sub.x
compound in the gas or liquid, such as between about 10% and about
100% of a NO.sub.x compound, between about 50% and about 100% or
between about 70% and about 100%.
[0081] Many of the materials disclosed as sorbents and/or catalysts,
including cryptomelane and silver hollandite, can be used as oxidation
catalysts and/or as sorbents in desulfurization applications, such
as the desulfurization of natural gas and liquid hydrocarbons. For
example, cryptomelane and silver hollandite can be used to remove
organic sulfur-containing compounds from liquid hydrocarbons. Examples
of liquid hydrocarbons include gasoline, diesel and jet fuel.
[0082] Desulfurization of natural gas has particularly broad utility.
Sulfur-containing compounds naturally occur as contaminants in natural
gas and often are added to give natural gas a distinctive smell.
For many applications, such as analytical applications and fuel
cell applications, substantially all sulfur-containing compounds
must be removed before natural gas is burned. To accomplish this,
natural gas can be exposed to silver hollandite or cryptomelane
before use. This can be done, for example at a temperature between
about 0.degree. C. and about 350.degree. C., such as between about
ambient temperature and about 250.degree. C.
[0083] Desulfurization of a gas or liquid using silver hollandite
or cryptomelane can be performed continuously or as a batch process.
The process typically results in removal of a substantial portion
of at least one sulfur-containing compound in the gas or liquid
being treated, such as between about 10% and about 100% of the compound
to be removed, between about 50% and about 100%, or between about
70% and about 100%. In some applications, the gas or liquid to be
treated is exposed to silver hollandite or cryptomelane at a temperature
between about 0.degree. C. and about 350.degree. C., such as between
about ambient temperature (e.g., about 23.degree. C.) and about
100.degree. C.
[0084] Removing a sulfur-containing compound from a liquid hydrocarbon
may include rinsing the liquid hydrocarbon with a polar liquid after
exposing the liquid hydrocarbon to silver hollandite. This may be
useful to remove oxidation products, such as sulfones, that are
somewhat polar. Suitable solvents include, for example, solvents
with dielectric constants greater than 10 such as water, ethanol,
isopropanol, formic acid, acetone, and acetonitrile.
[0085] The disclosed molecular sieve materials, such as silver
hollandite, also may have utility for removing compounds that cause
odors, such as volatile organic compounds released by consumer products,
including carpet, paint, etc. Formaldehyde is one such compound.
Some of the disclosed materials may be used to catalyze the oxidation
of formaldehyde into H.sub.2O and CO.sub.2. In comparison to many
conventional catalytic materials for removing odors, such as titania-based
materials, the disclosed molecular sieve materials typically do
not need to be activated by light energy. In some embodiments, however,
the disclosed materials are heated to improve their catalytic activity.
[0086] In sorption and/or catalysis applications, the disclosed
materials may be present in an amount and form that allow sorption
and/or catalysis to occur. In catalysis applications, the amount
can be, for example, an amount that provides a substantially greater
number of catalysis sites at any one time than the amount of potentially
oxidizable compound in the feed. For example, in sulfur catalysis
and/or sorption applications, the amount of catalyst and/or sorbent
can be greater than about 0.1 grams per gram of sulfur in the gas
or liquid being treated, such as between about 0.1 and about 100
grams catalyst and/or sorbent per gram of sulfur or between about
0.5 and about 10 grams catalyst and/or sorbent per gram of sulfur.
The form can be a form that maximizes surface area, such as a powder
form. As discussed above, the disclosed catalytic materials also
can be affixed to supports, such as monoliths.
[0087] Turning now to FIG. 10 a vehicle 20 implementing a simplified
embodiment of the disclosed emissions control system is depicted.
Vehicle 20 has an engine 22 fluidly connected to upstream and downstream
emissions control devices 24 and 26 respectively. Devices 24 and
26 perform different emissions control functions, and while they
could be combined into a single device, as described more fully
below, certain problems are avoided by the provision of separate
devices.
[0088] The exhaust 21 from the engine 22 is first fed to the upstream
device 24. The transfer of the exhaust 21 and all other fluid transfer
operations, can be in any conventional fashion, such as the exhaust
piping of a conventional automobile, and may include intermediate
fluid processing operations, such as catalytic conversion, mixing
with other gases, or recycling of exhaust to the engine.
[0089] The upstream device 24 is a SO.sub.x scrubber functioning
to remove any sulfur oxides from the exhaust gas 21 and to prevent
their passage via channel 25 to the downstream device 26 and eventually
to the atmosphere via the exhaust 28. The SO.sub.x scrubber functions
to remove most if not all of any sulfur oxides in the gaseous exhaust
21. The SO.sub.x scrubber can contain a solid SO.sub.x sorbent,
as described herein, preferably one supported on a monolith or similar
support. The sorbent can function to remove the SO.sub.x from the
passing gas stream, such as by permanent or reversible sorption,
trapping, filtering, or chemical reaction therewith, and thereby
can be used to prevent SO.sub.x from entering the downstream device
26.
[0090] The downstream device 26 provides a different emissions
control function than the upstream device 24. As illustrated, the
downstream device is a NO.sub.x scrubber or particulate filter,
although any conventional scrubber or filter can be employed. As
many conventional NO.sub.x traps and/or particulate filters are
fouled or poisoned by the presence of SO.sub.x, the provision of
upstream device 24 inventively reduces or eliminates this possibility
by providing an inlet stream to device 26 that is substantially
SO.sub.x free. For example, it is contemplated that device 24 will
function to cause fluid at channel 25 to have less than 1% of the
SO.sub.x concentration in the exhaust 21 more preferably less than
about 0.1%.
[0091] Turning now to FIG. 11 an exemplary SO.sub.x scrubber 30
which can be employed as device 24 in the FIG. 10 system, is depicted.
Scrubber 30 includes a housing 32 having a fluid inlet 34 a fluid
outlet 36 and a fluid flow path therebetween. The housing 32 contains
a SO.sub.x sorbent in the flow path so as to facilitate the removal
of SO.sub.x from the fluid as it passes through the SO.sub.x scrubber.
[0092] Housing 32 also contains a window 38 providing visual access
to the sorbent contained therein. As the sorbent contained in the
housing 32 sorbs the SO.sub.x, it will undergo a noticeable color
change, with the sorbent nearer the inlet 34 becoming saturated
(and thus changing color) sooner than the sorbent near the outlet
36. The resulting transition between different colored portions
of the sorbent provides an indication on the extent that the sorbent
packing has become spent. Accordingly, a series of indicator marks
39 are provided on the window 38 or on the housing 32 adjacent the
window 38 for measuring the remaining sorption capacity of the scrubber
30. For example, during routine maintenance of a machine on which
scrubber 30 is implemented, such as vehicle 20 the window 38 can
be checked to determine whether replacement of the scrubber 30 is
necessary.
[0093] When replacement is needed, i.e. when the sorbent is saturated
and entirely changed in color, the scrubber 30 can simply be removed
from the exhaust stream and replaced. In another embodiment, once
removed, the spent sorbent and/or the scrubber 30 can be reused.
For example when the spent sorbent is converted to MnSO.sub.4 this
MnSO.sub.4 can be used as a starting material to reform the Mn-OMS
material on the support. This reforming can be accomplished, for
example, by removing the spent sorbent and its support (such as
a monolith) from the housing 32. After processing and appropriate
calcinations, the spent sorbent is returned to its OMS structure
and is ready to sorb additional SO.sub.x.
[0094] Alternatively, the necessary reagents for reforming the
spent sorbent, for example KOH and O.sub.2 can be circulated through
the housing 32 without removing the spent sorbent. The inlet and
outlet ports 34 and 36 can be used as the reagent inlet and outlet
ports to recharge the sorbent in this fashion when the scrubber
30 is removed from the exhaust stream. However, as illustrated,
scrubber 30 can include optional dedicated inlet and outlet ports
44 and 42 for this purpose. Ports 42 44 permit recharging without
removal from the exhaust stream, or they may be used in conjunction
with offline recharging via inlet and outlet 34 36.
[0095] Reference will now be made to examples illustrating specific
features of inventive embodiments. It is to be understood, however,
that these examples are provided for illustration and that no limitation
to the scope of the invention is intended thereby. Further, certain
observations, hypotheses, and theories of operation are presented
in light of these examples in order to further understanding, but
these are likewise not intended to limit the scope of the invention.
EXAMPLES
[0096] The following examples are provided to illustrate certain
particular embodiments of the disclosure. Additional embodiments
not limited to the particular features described are consistent
with the following examples.
Example 1
Sample Preparations and Test Conditions OMS 2.times.2
[0097] 2.times.2 manganese based octahedral molecular sieve (tunnel
structure cryptomelane) was prepared using the methods described
in reference 3 below. A typical synthesis was carried out as follows:
11.78 g KMnO.sub.4 in 200 ml of water was added to a solution of
23.2 g MnSO.sub.44H.sub.2O in 60 ml of water and 6 ml of concentrated
HNO.sub.3. The solution was refluxed at 100.degree. C. for 24 hours,
and the product was washed and dried at 120.degree. C. Hydrothermal
reaction in Teflon bottles at 90.degree. C., instead of the reflux
method, was also used for the synthesis. In one example, this approach
yielded cryptomelane with a density of 0.66 g/cm.sup.3 and a surface
are of 74.1 m.sup.2/g.
[0098] An alternative synthesis method for cryptomelane included
purging O.sub.2 through a mixture of MnSO.sub.4 and KOH solution,
followed by calcination at 600.degree. C. A typical preparation
was: a solution of 15.7 g KOH in 100 ml of cold water was added
to a solution of 14.9 g of MnSO.sub.4H.sub.2O in 100 ml of water.
Oxygen gas was bubbled (about 10 L/min) through the solution for
4 hours. The product was washed with water and calcined in air for
20 hours. In one example, this approach yielded cryptomelane with
a density of 0.99 g/cm.sup.3 and a surface are of 32.0 m.sup.2/g.
[0099] The dried materials were sieved to provide 40-80 mesh particles
for the SO.sub.2 sorption tests, which was carried out in a temperature
controlled reactor with a Sulfur Chemiluminescent Detector (SCD)
analysis system. Unless otherwise stated, the sorption testing conditions
were 0.5 gram 40-80 mesh sorbent particles, 100 standard cubic centimeters
per minute (sccm) exhaust air flow with 250 parts per million (ppm)
SO.sub.2 75% N.sub.2 12% O.sub.2 and 13% CO.sub.2.
[0100] The SO.sub.2 sorption performance of cryptomelane material
synthesized by refluxing mixture of KMnO.sub.4 and MnSO.sub.4 solutions
was also systemically tested under different temperature, gas hour
space velocity (GHSV), SO.sub.2 concentrations, and feed gas compositions.
The results are summarized in Table 1.
[0101] Before each SO.sub.2 sorption measurement, the material
was heated at 500.degree. C. for 2 hours in flowing air. To characterize
the property changes before and after SO.sub.2 sorption, powder
X-ray diffraction pattern (XRD), particle surface area (SA), and
scanning electron microscopy (SEM) images were collected on some
of the tested materials. TABLE-US-00001 TABLE 1 SO.sub.2 Sorption
Test Conditions for Cryptomelane Material Variable conditions Other
conditions SO.sub.2 sorption temperature 0.5 g 40-80 mesh sorbent,
250.degree. C., 325.degree. C., feed gas: 250 ppm SO.sub.2 82%
and 475.degree. C. N.sub.2 18% O.sub.2 .about.8000 hr.sup.-1
GHSV Gas Hour Space Velocity, GHSV, hr.sup.-1 0.5 g 40-80 mesh sorbent
for 8000 8K and 30K hr.sup.-1 GHSV test, 30000 and 0.25 g for
60K test, and 60000 325.degree. C., feed gas: 250 ppm SO.sub.2
82% N.sub.2 18% O.sub.2 SO.sub.2 concentration in feed gas 0.5
g 40-80 mesh sorbent for 50 ppm 250 ppm SO.sub.2 test, and 0.25
g and 250 ppm for 50 ppm test 325.degree. C., feed gas: 82% N.sub.2
18% O.sub.2 .about.30000 hr.sup.-1 GHSV, Feed gas composition
For CO--NO--H.sub.2O test Air 0.25 g 40-80 mesh sorbent, (250 ppm
SO.sub.2 82% N.sub.2 and 18% O.sub.2) 325.degree. C., 17K hr.sup.-1
GHSV CO.sub.2 effect For others (250 ppm SO.sub.2 75% N.sub.2
12% O.sub.2 and 13% CO.sub.2) 0.5 g 40-80 mesh sorbent, NO effect
325.degree. C., .about.8K hr.sup.-1 GHSV (178 ppm SO.sub.2 178
ppm NO, 9% N.sub.2 20% O.sub.2 71% He) CO effect (250 ppm SO.sub.2
250 ppm CO, 87% N.sub.2 13% O.sub.2) CO--NO--H.sub.2O effect (125
ppm SO.sub.2 125 ppm CO, 125 ppm NO, 11% H.sub.2O*, 19% N.sub.2
20% O.sub.2 50% He) O.sub.2-free effect (250 ppm SO.sub.2 12.5%
N.sub.2 and 87.5% He) *Steam was introduced by purging O.sub.2
through a flask containing temperature-controlled de-ionized water.
After passing through the sorbent, the steam was removed before
analysis with the SCD detector. The steam was removed using a MD
Gas Dryer (from Perma Pure Inc.), which can selectively separate
H.sub.2O from other gases in the mixture.
Example 2 (Comparative)
Comparative Breakthrough Sorption Capacities
[0102] For purposes of comparison, the SO.sub.2 sorption capabilities
of several commercially available materials were tested at a temperature
range from 250.degree. C. to 475.degree. C. under the testing conditions
indicated above (0.5 gram 40-80 mesh sorbent particles, 100 sccm
exhaust air flow with 250 ppm SO.sub.2 75% N.sub.2 12% O.sub.2
and 13% CO.sub.x). The tested materials included La.sub.2O.sub.3
or BaO doped ZrO.sub.2--CeO.sub.2 mixtures (from Daiichi Kigenso
Kagaku Kogyo Co., Ltd.), ZrO.sub.2 (from RC100 Inc.), A1203 (from
Engelhard, acidic), CaO (from Alfa Aesar, Inc.), and MnO.sub.2 (from
Erachem Comilog, Inc.). These materials were obtained from their
respective commercial sources.
[0103] Table 2 presents a summary of the SO.sub.2 sorption capacities
for some of these SO.sub.2 sorbents. The SO.sub.2 sorption capacity
was calculated based on weight of SO.sub.2 sorbed per gram of sorbent
when 1% of the initial SO.sub.2 concentration was observed eluting
from the sorbent bed. This is defined as the breakthrough sorption
capacity. As seen in Table 2 the SO.sub.x breakthrough sorption
capacities for these materials are generally less than 5 wt %. Of
the other materials tested, the sorption capacities for the MnO.sub.2
was selected for more direct comparison to the materials of Example
1 and are presented in the Examples below. TABLE-US-00002 TABLE
2 SO.sub.2 Breakthrough Sorption Capacity of Conventional SO.sub.2
Sorbents Materials tested 200.degree. C. 325.degree. C. 400.degree.
C. 475.degree. C. 73.8% ZrO.sub.2--26.2% CeO.sub.2 mixed oxide 2.2
wt % 2.2 wt % 2.2 wt % SA 53.5 m.sup.2/g, 10000 hr.sup.-1 GHSV
73.2% ZrO.sub.2 1.75% La.sub.2O.sub.3 5.22% Nd.sub.2O.sub.3 2.4
wt % 3.1 wt % 3.6 wt % and 19.9% CeO.sub.2 mixed oxide SA 60.3 m.sup.2/g,
7236 hr.sup.-1 GHSV 61.8% ZrO.sub.2 29.4% CeO.sub.2 and 8.9% 2.0
wt % 3.5 wt % 5.0 wt % 5.3 wt % La.sub.2O.sub.3 mixed oxide SA 69.1
m.sup.2/g, 11400 hr.sup.-1 GHSV 70.3% ZrO.sub.2 4.0% BaO, and
25.8% 1.7 wt % 1.7 wt % 2.5 wt % CeO.sub.2 mixed oxide SA 29.2 m.sup.2/g,
.about.10000 hr.sup.-1 GHSV ZrO.sub.2 SA 95.7 m.sup.2/g, 2.2 wt
% 10000 hr.sup.-1 GHSV Al.sub.2O.sub.3 Engelhard Corp. SA 150
m.sup.2/g, 1.0 wt % Acidic 7281 hr.sup.-1 GHSV CaO, SA 2.7 m.sup.2/g,
.about.10000 hr.sup.-1 GHSV <0.2 wt % <0.2 wt % <0.2 wt
% 1. Other test conditions: 0.5 g 40-80 mesh sorbent, feed gas:
250 ppm SO.sub.2 75% N.sub.2 12% O.sub.2 and 13% CO.sub.2 2.
SO.sub.2 capacity based on gram of SO.sub.2 sorbed per gram of catalyst
at 1% SO.sub.2 breakthrough point
Example 3
Breakthrough Sorption Capacities
[0104] Table 3 gives the SO.sub.2 breakthrough and total sorption
capacities of the materials synthesized according to Example 1
K.sub.xMn.sub.8O.sub.16 A (reflux synthesis, with projected final
average Mn oxidation state 3.5.sup.+), K.sub.xMn.sub.8O.sub.16 B
(reflux synthesis, with projected final average Mn oxidation state
4.sup.+), Cu-doped K.sub.xMn.sub.8O.sub.16 B (hydrothermal synthesis,
with projected final average Mn oxidation state 4.sup.+), and K.sub.xMn.sub.8O.sub.16
C (synthesized from MnSO.sub.4 and KOH). The projected final Mn
oxidation state (PAOS) is calculated based on the relative amount
of KMnO.sub.4 and MnSO.sub.4 in the starting solution, i.e. PAOS=(moles
of KMnO.sub.4*7+moles of MnSO.sub.4*2)/(moles of KMnO.sub.4+moles
of MnSO.sub.4). Breakthrough capacities were measured at 1% breakthrough
as described in Example 2 and total SO.sub.2 sorption capacity
was also measured with the values given in parentheses in Table
3. For example, the breakthrough and maximum SO.sub.2 sorption capacities
for K.sub.xMn.sub.8O.sub.16 B are 58 wt % and 68 wt %, respectively.
Under similar reaction conditions, these materials have significantly
higher breakthrough SO.sub.2 sorption capacity than the conventional
SO.sub.2 sorbents given in Table 2. To facilitate comparison, the
results for the commercially obtained electrolytic MnO.sub.2 (EMD,
Erachem Comilog, Inc.) discussed above are presented in Table 3.
TABLE-US-00003 TABLE 3 SO.sub.2 Sorption Capacity of Cryptomelane
Materials Synthesized SO.sub.2 breakthrough capacity Materials tested
SA, m.sup.2/g (total capacity) K.sub.xMn.sub.8O.sub.16 A, 14000
hr.sup.-1 GHSV 51 28 (45).sup.b wt % Cu-doped K.sub.xMn.sub.8O.sub.16
B.sup.a 9637 hr.sup.-1GHSV 88 57.5 (67) wt % K.sub.xMn.sub.8O.sub.16B,
.about.7500 hr.sup.-1 GHSV 74 58 (68) wt % K.sub.xMn.sub.8O.sub.16
C, .about.8000 hr.sup.-1GHSV 32 48 (60) wt % EMD MnO.sub.2 12000
hr.sup.-1 GHSV 30 3.5 (9) wt % *Other test conditions: 0.5 g 40-80
mesh sorbent, 325.degree. C., feed gas: 250 ppm SO.sub.2 82% N.sub.2
and 18% O.sub.2 for K.sub.xMn.sub.8C.sub.16 C, for others: 75% N.sub.2
12% O.sub.2 and 13% CO.sub.2 .sup.aCuSO.sub.4 was added in MnSO.sub.4
solution .sup.bData in parentheses are maximum SO.sub.2 sorption
capacities
Example 4
SO.sub.2 Sorption at 325.degree. C.
[0105] FIG. 1 is a graph showing plots of SO.sub.2 sorption on
K.sub.xMn.sub.8O.sub.16 A, B, Cu-doped K.sub.xMn.sub.8O.sub.16 B,
and EMD MnO.sub.2 as a function of the weight percentage of SO.sub.2
fed at 325.degree. C. The left axis is the percentage of SO.sub.2
not sorbed (i.e. that passed through the bed) and corresponds to
the S-shaped curves. The right axis is the wt % SO.sub.2 sorbed
and corresponds to the curves whose slope is initially 1 at low
feed amounts and then tends towards slope of zero at high feed amounts.
All weight percentages are relative weight of sorbent.
Example 5
Changes after Sorption
[0106] FIGS. 2 and 3 are before and after scanning electron microscopy
(SEM) images and x-ray diffraction patterns (XRD), respectively,
for the SO.sub.2 sorption by K.sub.xMn.sub.8O.sub.16 B at 325.degree.
C. As shown in FIG. 2 the morphology and the crystal structure
of the K.sub.xMn.sub.8O.sub.16 material significantly changes after
SO.sub.2 sorption.
[0107] The surface area of this material also decreased sharply
from 74 m.sup.2/g to 4.6 m.sup.2/g, and the XRD patterns indicate
that the OMS structure had converted to a mixture of MnSO.sub.4
and manganolangbeinite K.sub.2Mn.sub.2(SO.sub.4).sub.3. A visible
color change in the sorbent was also evident. It was initially black
and changed to yellow after the SO.sub.2 sorption.
Example 6
Temperature Dependence
[0108] FIG. 5 is a plot of the wt % of SO.sub.2 sorption on K.sub.xMn.sub.8O.sub.16
B at 250.degree. C., 325.degree. C. and 475.degree. C. under the
other test conditions as indicated in Table 1 (0.5 g 40-80 mesh
sorbent, feed gas: 250 ppm SO.sub.2 82% N.sub.2 18% O.sub.2 about
8000 hr.sup.-1 GHSV). Even at as low a temperature as 250.degree.
C., this material could sorb more than 66 wt % SO.sub.2 although
sorption is not 100% and some SO.sub.2 breakthrough was observed
even initially.
Example 7
Feed Gas Flow Rate Dependence
[0109] FIG. 6 shows the feed gas GHSV effect on the SO.sub.2 sorption
with K.sub.xMn.sub.8O.sub.16 B at 325.degree. C. The breakthrough
SO.sub.2 capacity decreased from 61 to 44 and 33 wt % as the feed
GHSV increased from 8K, to 30K and 60K hr.sup.-1. As the feed GHSV
increased, the total SO.sub.2 sorption capacity decreased, but not
significantly, from 74 to 64 and 63 wt %.
Example 8
Feed Gas Composition Dependence
[0110] FIG. 7 shows that increasing the SO.sub.2 concentration
in the feed gas from 50 ppm to 250 ppm had almost no effect on the
SO.sub.2 sorption with K.sub.xMn.sub.8O.sub.16 B.
[0111] FIG. 8 shows the effect of the feed gas composition on the
sorption of SO.sub.2 with K.sub.xMn.sub.8O.sub.16 B at 325.degree.
C. CO and NO, which can be present in combustion waste gases, did
not have any effect on the SO.sub.2 sorption with K.sub.xMn.sub.8O.sub.16.
CO.sub.2 at about 13%, slightly deceased the SO.sub.2 breakthrough
capacity (from 61 wt % to 59 wt %) and the total SO.sub.2 capacity
(from 74 wt % to 68 wt %).
[0112] Surprisingly, the OMS material K.sub.xMn.sub.8O.sub.16 B
showed a higher SO.sub.2 breakthrough capacity of 74 wt % and total
SO.sub.2 capacity of about 80 wt % in the CO--NO--SO.sub.2--H.sub.2O
mixture feed gas even through the GHSV was 17K hr.sup.-1. When steam
was introduced into the system, the SCD signals were not very stable,
which may have generated some error in the measurements. In the
absence of O.sub.2 cryptomelane material K.sub.xMn.sub.8O.sub.16
B still had a SO.sub.2 breakthrough capacity of 41 wt % and a total
SO.sub.2 capacity of 58 wt %.
[0113] After the SO.sub.2 sorption test, the weight gain of the
sorbent was measured, though it was not possible to collect all
the sorbent particles. Table 4 gives the weight gain data of the
feed gas composition trials. For comparison, the total SO.sub.2
sorption calculated from the SCD concentration change for each test
also is listed. TABLE-US-00004 TABLE 4 Sorbent Weight Gain after
SO.sub.2 Sorption Test in Different Feed Gas Compositions at 325.degree.
C. CO.sub.2 NO O.sub.2-free CO--NO--H.sub.2O Feed Gas Air effect
CO effect effect effect effect Weight 65.7 65.6 66.2 64 50 66.8
Gain, % Total SO.sub.2 74.6 68.1 74.1 70.7 58 80.6 sorbed, % *See
Table 1 for detailed test conditions
Discussion of Examples 1-8
[0114] Based on reaction (1), the maximum SO.sub.2 capacity is
believed to be controlled by the oxidation state of Mn in the sorbent.
For the OMS materials K.sub.xMn.sub.8O.sub.16 B and Cu-doped K.sub.xMn.sub.8O.sub.16
B, the average projected oxidation state of Mn is 4; there are very
few K.sup.+ cations present in the structure. If the small amount
of K.sup.+ is ignored, the maximum SO.sub.2 capacity should be about
73.5 wt %. The measured maximum SO.sub.2 capacities (see Tables
3 and 4) are about 70 wt % for these two materials, which is very
close to 73.5 wt % and reflects the contribution of the K.sub.2O
byproduct. This result supports the hypothesis that reaction (1)
dominates SO.sub.2 sorption. Direct evidence is also seen in the
XRD patterns before and after the SO.sub.2 sorption. After SO.sub.2
sorption, the OMS structure completely changed to MnSO.sub.4 and
manganolangbeinite K.sub.2Mn.sub.2(SO.sub.4).sub.3. The formation
of K.sub.2Mn.sub.2(SO.sub.4).sub.3 means more SO.sub.4.sup.2- than
Mn.sup.2+ is formed and that reaction (1) needs to be modified slightly.
A postulate is that the cryptomelane material itself or the SO.sub.2
sorbed material (mostly MnSO.sub.4) acts as a catalyst for the following
reaction and formation of K.sub.2Mn.sub.2(SO.sub.4).sub.3. SO.sub.2+O.sub.2.fwdarw.SO.sub.3
(2) Because the amount of K.sup.+ is small, the whole SO.sub.2 sorption
process is mostly dominated by manganese oxidation. It should be
noted that, while the presence of O.sub.2 in the feed gas does help
increase sorption, it is not necessary, as demonstrated by the satisfactory
sorption performance in the oxygen-free feed gas test (feed gas
composition: 250 ppm SO.sub.2 12.5% N.sub.2 and 87.5% He). After
SO.sub.2 sorption in an O.sub.2-free environment, both MnSO.sub.4
and K.sub.2Mn.sub.2(SO.sub.4).sub.3 were formed, suggesting that
other reactions involving oxygen transfer and formation of some
amorphous phases also happened in an O.sub.2-free environment.
[0115] The synthesized K.sub.xMn.sub.8O.sub.16 A had an average
projected Mn oxidation state of +3.5. The ideal formula for this
material should be K.sub.4Mn.sub.8O.sub.16 (after water removal
at 500.degree. C. and assuming all the counter-cations are K.sup.+).
Then according to reaction (1), the maximum capacity is 45 wt %,
which exactly matches the measurement (see Table 3). If reaction
(2) also exists with this material, K.sub.2Mn.sub.2(SO.sub.4).sub.3
should form, which was not seen in an XRD pattern (not shown), and
the maximum SO.sub.2 capacity should be 60 wt %. This suggests that
reaction (1) predominates, and that reaction (2) occurs only to
a very small extent or not at all. Based on the results from K.sub.xMn.sub.8O.sub.16
A (hydrothermal synthesis, with projected final average Mn oxidation
state 3.5), K.sub.xMn.sub.8O.sub.16 B (reflux synthesis, with projected
final average Mn oxidation state 4) and Cu-doped K.sub.xMn.sub.8O.sub.16
B (hydrothermal synthesis, with projected final average Mn oxidation
state 4), neither the choice between reflux or hydrothermal synthesis,
nor doping with Cu significantly changes SO.sub.2 break through
sorption capacity or maximum sorption capacity. This suggests that
there is no kinetic or thermodynamic effect attributable to doping
or the synthesis mechanism.
[0116] The alternative synthesis method of purging O.sub.2 through
a mixture of MnSO.sub.4 and KOH solution followed by calcination
at 600.degree. C. did show some effect. K.sub.xMn.sub.8O.sub.16
synthesized using this method, gives .about.50 wt % SO.sub.2 breakthrough
capacity and .about.60 wt % SO.sub.2 total capacity, which are lower
than those for K.sub.xMn.sub.8O.sub.16 B. Potential reasons for
this disparity include 1) possible incomplete oxidation of Mn.sup.2+
to Mn.sup.4+, 2) the relative low surface area (32 m.sup.2/g vs.
75 m.sub.2/g) and high density (.about.1 g/cm.sup.3 vs. 0.67 g/cm.sup.3),
and 3) the product is not pure OMS as a small amount of K.sub.2SO.sub.4
was found to exists (see FIG. 9 for its XRD pattern). While the
synthesis conditions could still be optimized to improve sorption
performance, the sorption performance is adequate to be effective
and consideration of other factors renders this a desirable approach.
For example, this synthesis method can significantly decrease the
overall cost of the sorbent production. After SO.sub.2 sorption,
almost pure MnSO.sub.4 is formed, which, being one of the starting
materials, can be recaptured and reused. Also the density of K.sub.xMn.sub.8O.sub.16
C is about 50% higher than that of K.sub.xMn.sub.8O.sub.16 B, which
indicates more sorbents can be loaded and a higher SO.sub.2 capacity
in a given volume can be achieved.
[0117] Although electrolytic manganese dioxide (EMD) from Erachem
Comilog, Inc. has Mn.sup.4+ and a surface area of about 30 m.sup.2/g,
the SO.sub.2 capacity for this material does not approach that of
the OMS materials. This indicates that the OMS structure may be
important for high SO.sub.2 sorption.
[0118] K.sub.xMn.sub.8O.sub.16 B was tested at a temperature of
250.degree. C., 325.degree. C., and 475.degree. C. At 250.degree.
C., a lower SO.sub.2 sorption rate was observed, but the maximum
SO.sub.2 capacity was almost the same as that measured at 325.degree.
C. and 475.degree. C. In comparing the results at 325.degree. C.
and 475 CC, a minimal difference was observed, except that at 475.degree.
C., after the breakthrough capacity was reached, the SCD detector
background slightly increased, indicating that some SO.sub.3 was
being released even though total sorption amount was still increasing.
[0119] Substantial variation in the feed gas flow rate affected
the SO.sub.2 sorption performance of K.sub.xMn.sub.8O.sub.16 B sorbent,
although in practice this affect can be mitigated with appropriate
sizing and design of the SO.sub.x trap. For example, the SO.sub.2
breakthrough capacity decreased about 50% when the feed GHSV increased
from 8K to 60K hr.sup.-1. This indicates the SO.sub.2 sorption reaction
is mostly controlled by SO.sub.2 mass diffusion through the sorbent.
Since the SO.sub.2 concentration in the feed gas had little or no
effect on its sorption suggests that reaction (1) is 0.sup.th order
for SO.sub.2 and it is mostly controlled by the available active
sites on the sorbent.
[0120] Most components in the simulated exhaust combustion gases
tested, CO, NO, CO.sub.2 and H.sub.2O, did not have a significant
effect on the SO.sub.2 sorption capacity of the K.sub.xMn.sub.8O.sub.16
B sorbent, and the sorption capacity in an oxygen-free environment,
while lower, was still acceptable. Therefore, it is expected that
the Mn-OMS materials should be useful to remove SO.sub.2 from gas
streams in a wide variety of applications.
Example 9
Other OMS Structures
[0121] SO.sub.2 sorption capacities of other manganese oxides with
tunnel structures, including Todorokite-type magnesium manganese
oxide with channels of 3.times.3 MnO.sub.6 units, sodium manganese
oxide with channels of 2.times.4 MnO.sub.6 units, sodium manganese
oxide with channels of 2.times.3 MnO.sub.6 units, and pyrolusite
manganese oxide with channels of 1.times.1 MnO.sub.6 units, were
studied.
[0122] Pyrolusite, MnO.sub.2 1.times.1 was obtained from Stream
Chemicals. The as-received chemical was ball-milled for 1 hr to
get about 1 .mu.m particles before the SO.sub.2 sorption test. One
todorokite material, OMS-1 was provided by Engelhard Corporation.
Other tunnel-structured manganese oxides were prepared in the lab
using the methods described in the published literature.
[0123] Birnessite was used as a precursor for the synthesis of
channel-structured manganese oxides. Birnessite-type layered manganese
oxides were prepared using the methods described in references 5
and 6 below. A typical synthesis included mixing 250 ml 6.4M NaOH
solution with 200 ml 0.5M MnSO.sub.4 at room temperature. Oxygen
was immediately bubbled through a glass frit at a rate of 4 L/min.
After 4.5 hours the oxygenation was stopped and the precipitate
was filtered out and washed with deionized water 4 times, and then
dried in air at 100.degree. C. About 13 g of grey-colored birnessite
product was obtained.
[0124] Two sodium manganese oxides with channels of 2.times.3 MnO.sub.6
units (Na 2.times.3 A & B) were prepared by directly calcination
of birnessite in air for 12 hours at 500.degree. C., and 650.degree.
C., respectively as described in reference 7 below.
[0125] Todorokite (magnesium manganese oxide with channels of 3.times.3
MnO.sub.6 units) was prepared as described in references 5 and 8
below. About 3 g birnessite was added to 100 ml 1M MgCl.sub.2 solution
and the mixture was shaken overnight at room temperature to cause
Mg.sup.2+ ion exchange for Na.sup.+. The slurry was washed four
times with deionized water. Then, Mg.sup.2+-birnessite, together
with 25 ml H.sub.2O, was autoclaved at 150.degree. C. for 48 hours.
After washing with DI water 3 times, the product was dried in air
at 100.degree. C. About 2.0 g todorokite-type tunnel structure manganese
oxide (Mg 3.times.3) was obtained.
[0126] Sodium manganese oxide with channels of 2.times.4 MnO.sub.6
units was prepared using method described in reference 9 below.
About 5 g birnessite, together with 25 ml 2.5M NaCl solution, was
autoclaved at 210.degree. C. for 48 hours. After washing with DI
water 3 times, the product was dried in air at 100.degree. C. About
4.4 g of black-colored product (Na 2.times.4) was obtained.
[0127] The dried materials were sieved to provide 40-80 mesh particles
for the SO.sub.2 sorption test, which was carried out in a temperature
controlled reactor with an SCD analytical system. All of the materials
were tested under the same conditions (0.5 gram 40-80 mesh sorbent
particles, 100 sccm feed flow of 250 ppm SO.sub.2 in 82% N.sub.2
18% O.sub.2) at a temperature of 325.degree. C. Additional details
are shown in Table 5. Before each SO.sub.2 sorption measurement,
the sorbent material was heated at 500.degree. C. for 2 hours in
flowing air to remove residual moisture. To characterize the structure
change before and after SO.sub.2 sorption, powder X-ray diffraction
pattern (XRD) was collected on some of the tested materials. TABLE-US-00005
TABLE 5 SO.sub.2 Sorption Tests of Other OMS Materials Sorbent Test
Conditions* MnO.sub.2 1 .times. 1 0.5 g 40-80 mesh sorbent, 325.degree.
C., 100 sccm pyrolusite from Strem Chemicals, flow of 250 ppm SO.sub.2
82% N.sub.2 18% O.sub.2 with 1 .times. 1 tunnels GHSV = 18K hr.sup.-1
Na 2 .times. 3 A 0.5 g 40-80 mesh sorbent, 325.degree. C., 100 sccm
Na.sub.2Mn.sub.5O.sub.10 with 2 .times. 3 tunnels, flow of 250 ppm
SO.sub.2 82% N.sub.2 18% O.sub.2 calcined at 500.degree. C. for
12 h GHSV = 3.4K hr.sup.-1 Na 2 .times. 3 B 0.5 g 40-80 mesh sorbent,
325 C., 100 sccm Na.sub.2Mn.sub.5O.sub.10 with 2 .times. 3 tunnels,
flow of 250 ppm SO.sub.2 82% N.sub.2 18% O.sub.2 calcined at
650.degree. C. for 12 h GHSV = 5.1K hr.sup.-1 Na 2 .times. 3 A 0.5
g 40-80 mesh sorbent, 325.degree. C., 100 sccm Na.sub.2Mn.sub.5O.sub.10
with 2 .times. 3 tunnels, flow of 250 ppm SO.sub.2 82% N.sub.2
18% O.sub.2 calcined at 500.degree. C. for 12 h GHSV = 11K hr.sup.-1
Na 2 .times. 4 0.5 g 40-80 mesh sorbent, 325.degree. C., 100 sccm
sodium manganese oxide with 2 .times. 4 flow of 250 ppm SO.sub.2
82% N.sub.2 18% O.sub.2 tunnels GHSV = 11K hr.sup.-1 OMS-1 0.5
g 40-80 mesh sorbent, 325.degree. C., 100 sccm todorokite, with
3 .times. 3 tunnels, flow of 250 ppm SO.sub.2 82% N.sub.2 18%
O.sub.2 provided by Engelhard Corporation GHSV = 11K hr.sup.-1
Mg 3 .times. 3 0.5 g 40-80 mesh sorbent, 325.degree. C., 100 sccm
Todorokite, with 3 .times. 3 tunnels flow of 250 ppm SO.sub.2 82%
N.sub.2 18% O.sub.2 Synthesized GHSV = 2.7 hr.sup.-1 *Before each
SO.sub.2 sorption measurement, the sorbent material was heated at
500.degree. C. for 2 hours in 100 sccm air.
Example 10
Other OMS Structures Results and Comparison
[0128] The measured breakthrough capacities at selected gas flow
rates for the materials prepared in Example 9 are given in Table
6 along with exemplary capacities for the 2.times.2 structure of
Example 1. TABLE-US-00006 TABLE 6 SO.sub.2 Sorption Capacity of
OMS Sorbents Break through capacity, Material tested GHSV, hr.sup.-1
wt % MnO.sub.2 from Strem Chemicals, 1 .times. 1 18K <0.1 tunnels
of MnO.sub.6 units Na.sub.2Mn.sub.5O.sub.10 2 .times. 3 tunnels
of MnO.sub.6 units, 3.4K 57.5 calcined at 500.degree. C. for 12
h (A) Na.sub.2Mn.sub.5O.sub.10 2 .times. 3 tunnels of MnO.sub.6
units, 5.1K 31 calcined at 650.degree. C. for 12 h (B) Na.sub.2Mn.sub.5O.sub.10
2 .times. 3 tunnels of MnO.sub.6 units, 11K 12 calcined at 500.degree.
C. for 12 h (A) Sodium Manganese Oxide, 2 .times. 4 tunnels of 11K
33 MnO.sub.6 units MgMn.sub.2O.sub.4 from todorokite (3 .times.
3) provided 11K 1.5 by Engelhard Corporation MgMn.sub.2O.sub.4 from
the synthesized todorokite 2.7K 53 (3 .times. 3) Cryptomelane, 2
.times. 2 tunnels of MnO.sub.6 units 8K 62 Cryptomelane, 2 .times.
2 tunnels of MnO.sub.6 units 30K 42
[0129] Except for two of the materials, the SO.sub.2 breakthrough
sorption capacities for these materials were generally much higher
than those of conventional SO.sub.x sorbents, which typically have
SO.sub.2 breakthrough sorption capacities less than 5 wt %. This
establishes the usefulness of these materials as SO.sub.x sorbents.
[0130] The 1.times.1 MnO.sub.2 from Strem Chemicals was confirmed
by XRD to be well-crystallized pyrolusite, comprising 1.times.1
MnO.sub.6 tunnels. After calcination at 500.degree. C. for 2 hours
in air, the structure remained stable. However, the material exhibited
poor sorption capacity, with a breakthrough capacity of about 0.1%
and a maximum sorption capacity less than 3%.
[0131] The synthesized birnessite was calcined in air either at
500.degree. C. for 12 hours (A) or at 650.degree. C. for 12 hours
(B). In both cases, sodium manganese oxide (Na.sub.2Mn.sub.5O.sub.10)
formed, the basic structure consisting of MnO.sub.6 octahedra joined
at the edges to form a 2.times.3 tunnel structure. FIG. 8 shows
the SO.sub.2 sorption performance of both the A and B formulations
of this microporous manganese oxide under different GHSV. Similar
to the 2.times.2 cryptomelane materials, this microporous manganese
oxide also was found to have a very high SO.sub.2 sorption capacity.
At 3444 hr.sup.-1 GHSV, the total SO.sub.2 sorbed was about 70 wt
%. At higher GHSV, the SO.sub.x sorption performance decreased,
indicating the reaction is controlled by the mass diffusion of SO.sub.2
through the sorbent. After SO.sub.2 sorption, MnSO.sub.4 formed.
[0132] The XRD pattern of the sodium manganese oxide synthesized
by hydrothermal reaction matched very well with the XRD pattern
of the sodium manganese oxide material with 2.times.4 tunnel structure
described in reference 10 below. Likewise after calcination at 500.degree.
C. for 12 hours in air, the 2.times.4 tunnel structure remained
unchanged. FIG. 9 gives the SO.sub.2 sorption test result at 10755
hr.sup.-1 GHSV. This material also was found to have a high SO.sub.2
sorption capacity.
[0133] While the todorokite magnesium manganese oxide prepared
by hydrothermal reaction was not well crystallized, the todorokite
structure could still be identified in the XRD. However, after calcination
at 500.degree. C. for 2 hours in air, the 3.times.3 tunnel structure
changed mostly to MgMn.sub.2O.sub.4. This structure change was evident
in the MgMn.sub.2O.sub.4 provided by Engelhard Corporation as well.
While the todorokite materials both exhibited relative high sorption
capacities, this instability at moderately high temperatures is
a relative disadvantage for most applications. While they could
be used as sorbents, most conventional implementations require stability
above temperatures of 500.degree. C. or sometimes higher.
Introduction to Examples 10 through 13
[0134] The stability of cryptomelane was studied under oxidizing
conditions, reducing conditions, and certain lean-rich cycling conditions.
Cryptomelane was confirmed to be stable during oxidizing conditions
and under lean-rich cycling, but its stability under certain reducing
conditions was found to be temperature and condition dependent.
[0135] The studies described in Examples 10 through 13 were mostly
performed using a Netzsch STA 409 TGA/DSC/MS, where TGA is Thermogravimetric
Analysis, DSC is Differential Scanning Calorimetry, and MS is Mass
Spectroscopy. Different gases, including air, 2% H.sub.2 in Ar and
2% C.sub.3H.sub.6 (propylene) in Ar, were used for TGA-DSC analysis.
To get a large amount of samples, cryptomelane materials were also
treated in a tube furnace with flowing air, 2% C.sub.3H.sub.6 in
Ar, a simulated rich condition exhaust from a diesel engine, a simulated
lean condition exhaust, and a simulated lean-rich cycling exhaust
from a diesel engine.
[0136] The compositions of the simulated exhausts are displayed
in Table 7. Unless otherwise specified, the sorption tests were
performed at 325.degree. C., using feed gas of 250 ppm SO.sub.2
in air at 8000 hr.sup.-1 GHSV. Before each sorption test, the sorbent
was pretreated in air at 500.degree. C. for 2 hours.
[0137] Additional details of these studies can be found in U.S.
Provisional Application Ser. No. 60/649656 filed Feb. 3 2005
the disclosure of which is hereby incorporated by reference to the
extent not inconsistent with the present disclosure. TABLE-US-00007
TABLE 7 Composition of Simulated Diesel Engine Exhausts Used in
Examples 10 through 13 Carbon Simulated Exhaust Soot CO CO.sub.2
C.sub.3H.sub.6 H.sub.2 H.sub.2O O.sub.2 NO N.sub.2 Rich Condition
2 .times. 10.sup.-5 g/L 2000 ppm 10% 1000 ppm -- 10% 0.about.1%
500 ppm Balance Lean Condition 5 .times. 10.sup.-5 g/L 3250 ppm
7.11% 360 ppm -- -- 10.2% 230 ppm Balance Lean-Rich Lean -- -- 10%
-- -- 10% 12% 500 ppm Balance Cycling Cycle Rich -- 4% 10% 4000
ppm 1.3% 10% 1.5% 500 ppm Balance Cycle
Example 10
Stability of Cryptomelane Under Oxidizing Conditions
[0138] The stability of cryptomelane after two different oxidation
pretreatments was tested. The first pretreatment was air containing
10% H.sub.2O for three hours at 600.degree. C. and 30000 hr.sup.-1
GHSV. The second pretreatment was the simulated lean exhaust gas
(composition listed in Table 7) for one hour at 500.degree. C. Subsequent
sorption tests on 250 ppm SO.sub.x in air at 325.degree. C. and
80000 hr.sup.-1 GHSV confirmed that the cryptomelane retained its
high sorption capacity, and XRD analysis of the cryptomelane after
exposure to each oxidizing stream confirmed that the cryptomelane
maintained its structural configuration.
Example 11
Stability of Cryptomelane Under Reducing Conditions
[0139] As a highly active, high valance manganese oxide, cryptomelane
was expected to be somewhat unstable under reducing conditions.
DSC analysis of cryptomelane exposed to a reducing stream of 2%
C.sub.3H.sub.6 in Ar flowing at 40 ml/min while subjected to a heating
rate of 10K/min showed that the cryptomelane was reduced at a temperature
as low as 300.degree. C. As the cryptomelane was reduced, it transitioned,
at least in part, into Mn.sub.3O.sub.4 and MnO. When the cryptomelane
was reduced at temperatures higher than 550.degree. C., the Mn.sub.3O.sub.4
was further reduced to MnO. Similar results were obtained when the
reducing gas comprised 2% H.sub.2 in Ar. The doping of Cu and Cr
into cryptomelane was not seen to increase its stability under these
reducing conditions.
[0140] The MnO from reduced cryptomelane was found to be easily
re-oxidized when heated in air, but the properties of the re-oxidized
species showed some variation based on the particular reduction-oxidation
cycle. For the MnO formed from reduction in 2% C.sub.3H.sub.6 in
He at 550.degree. C., DSC analysis in 40 ml/min flowing air with
a heating rate of 10K/min showed that the MnO was re-oxidized at
a temperature as low as 250.degree. C. After oxidation for one hour
at 500.degree. C., XRD analysis revealed that most of the cryptomelane
crystal structure was recovered.
[0141] However, the BET surface area of the cryptomelane decreased
from 74 m.sup.2/g to 4.9 m.sup.2/g, and SEM images demonstrated
that the needle-like crystals no longer exited. Subsequent SO.sub.2
sorption tests at 325.degree. C. of 250 ppm SO.sub.2 in air at 80000
hr.sup.-1 GHSV revealed a loss of sorption capacity due to the reduction-oxidation
cycle. Post sorption test XRD patterns showed that a large portion
of the cryptomelane that had been subjected to this reduction-oxidation
cycle remained unused after the SO.sub.2 sorption.
[0142] As discussed above, at least a portion of the reduced cryptomelane
is able to recover its original crystalline structure after it is
subjected to re-oxidation. Additional tests were performed to determine
phase compositions that were formed after re-oxidation occurred.
These tests were performed using the following reducing gases: 2%
H.sub.2 in He and simulated rich exhaust gas (see Table 7). Table
8 summarizes the phases found after re-oxidation was performed.
For all these tests, the most abundant phase formed after re-oxidation
was either Mn.sub.3O.sub.4 or Mn.sub.2O.sub.3 not cryptomelane.
TABLE-US-00008 TABLE 8 Phases Formed After Reduction-Oxidation Tests
of Cryptomelane Phases after 500.degree. C. Phases after 600.degree.
C. Reduction condition Phases after reduction* 1 hr in air 1 hr
in air 2% H.sub.2 in He, KMn.sub.8O.sub.16 KMn.sub.8O.sub.16 KMn.sub.8O.sub.16
250.degree. C., 1 hr 2% H.sub.2 in He, Mn.sub.3O.sub.4 MnO, Mn.sub.2O.sub.3
KMn.sub.8O.sub.16 Mn.sub.2O.sub.3 KMn.sub.8O.sub.16 300.degree.
C., 1 hr KMn.sub.8O.sub.16 (Mn.sub.3O.sub.4) (Mn.sub.3O.sub.4) 2%
H.sub.2 in He, MnO, (Mn.sub.3O.sub.4) Mn.sub.3O.sub.4 (KMn.sub.8O.sub.16
Mn.sub.2O.sub.3 KMn.sub.8O.sub.16 350.degree. C., 1 hr MnO) (Mn.sub.3O.sub.4)
2% H.sub.2 in He, MnO Mn.sub.3O.sub.4 (MnO Mn.sub.2O.sub.3 KMn.sub.8O.sub.16
550.degree. C., 1 hr KMn.sub.8O.sub.16) (Mn.sub.3O.sub.4) Rich exhaust
(table Mn.sub.3O.sub.4 K.sub.2Mn.sub.4O.sub.8 Mn.sub.3O.sub.4 KMn.sub.8O.sub.16
Mn.sub.2O.sub.3 KMn.sub.8O.sub.16 7), 550.degree. C. 1 hr (MnO)
(MnO) (Mn.sub.3O.sub.4) *Major phase listed first, minor phase listed
in parentheses. # Rich exhaust composition given in Table 7.
The properties of the reduction-oxidation treated cryptomelane
were seen to change significantly. The rich-exhaust treated cryptomelane
(550.degree. C. for one hour) was found to have a significantly
reduced surface area (5.1 m.sup.2/g) and SO.sub.2 sorption capacity.
Example 12
Stability of Cryptomelane Under Lean-Rich Cycling
[0143] An analysis of the results discussed in Examples 10 and
11 show that whether or not cryptomelane can recover its original
crystal structure and its original SO.sub.2 sorption performance
after a reduction-oxidation cycle largely depends on the composition
of the reductant and the cryptomelane treatment history. In an effort
to study the effect of lean-rich cycling (NO.sub.x trap approach)
on cryptomelane, cryptomelane was subjected to lean-rich cycles
at 475.degree. C. for 6.5 hours (cycling at 360 seconds lean and
30 seconds rich). The exhaust gas flow was 26000 hr.sup.-1 GHSV
and its composition was the same as is listed in Table 7.
[0144] XRD patterns of cryptomelane taken both before and after
the treatment cycle were almost identical. Some morphology changes
were observed under SEM, and cryptomelane's BET surface area decreased
from 74 m.sup.2/g to 20 m.sup.2/g after treatment. However, the
SO.sub.2 sorption performance of the lean-rich treated cryptomelane
was very close to that of fresh cryptomelane. Additionally, as revealed
from the XRD pattern of lean-rich cycled cryptomelane loaded with
SO.sub.2 a substantial amount of the cryptomelane was used for
SO.sub.2 sorption.
[0145] The DSC analysis above indicated that cryptomelane can be
re-oxidized at a lower temperature (e.g., 250.degree. C.) than the
temperature at which it can be reduced (e.g., 300.degree. C.).
[0146] Since the lean-rich cycling was performed at a constant
temperature of 475.degree. C., any cryptomelate that was reduced
during the rich conditions was re-oxidized during the lean condition
with the end result being that the cryptomelane structure remained
unchanged. Moreover, the fact that the re-oxidation temperature
is below the reduction temperature of cryptomelane provides ample
flexibility in selecting the operating temperature (or temperature
range) for lean-rich cycling such that any cryptomelate that is
reduced during the rich conditions can be re-oxidized during the
lean condition.
Example 13
Stability of SO.sub.2-Loaded Cryptomelane
[0147] After SO.sub.2 sorption occurs on cryptomelane, MnSO.sub.4
and K.sub.2Mn.sub.2(SO.sub.4).sub.3 form. As determined by TGA,
these compounds are very stable in an oxidizing atmosphere. The
stability of the SO.sub.2-sorbed cryptomelane was studied using
TGA-MS analysis on SO.sub.2-loaded cryptomelane. From a sulfur stabilization
point of view, the used cryptomelane was stable at temperature up
to approximately 600.degree. C. in 2% H.sub.2 in He. At temperatures
above 600.degree. C. SO.sub.2 and H.sub.2S were released from the
cryptomelane. At approximately 200.degree. C., some H.sub.2O was
released from the sorbent. Similar results were also observed when
Cu-doped and Cr-doped SO.sub.2-sorbed cryptomelane was studied.
Doping Cu or Cr into cryptomelane did not affect the stability of
the SO.sub.2-loaded sorbent.
[0148] Stability of the used sorbent under a rich stimulant was
studied by continuously passing 2% CO and 2000 ppm C.sub.3H.sub.6
(balanced in Ar) through the used sorbent at 70000 hr.sup.-1 GHSV.
Table 9 shows the sulfur concentration in the off gas at different
temperatures. At temperatures below about 450.degree. C., the used
sorbent was stable in rich exhaust gas, as only a small amount of
sulfur was released from the sorbent. At approximately 500.degree.
C., the used sorbent was relatively unstable and a larger amount
of sulfur (100 ppm) was released from the sorbent. After 20 hours
of reduction, it was found, using XRD analysis, that MnSO.sub.4
had been reduced to MnO and MnS. K.sub.2Mn.sub.2(SO.sub.4).sub.3
appeared to remain stable under this extreme condition. When exposed
again to air at 500.degree. C., the MnO and MnS were oxidized back
to Mn.sub.3O.sub.4 and MnSO.sub.4. At the same time, some SO.sub.x
was released from the sorbent. TABLE-US-00009 TABLE 9 Sulfur Concentration
in Off Gas from Used KMn.sub.8O.sub.16 (MnSO.sub.4) Reduced in 2%
CO/2000 ppm C.sub.3H.sub.6 at 70 K.sup.-1 GHSV Temperature, .degree.
C. 20 150 200 250 300 [S] out, ppm <100 ppb <100 ppb <100
ppb <100 ppb <100 ppb Temperature, .degree. C. 350 400 450
500 [S] out, ppm 300 ppb 1.8 ppm 10 ppm .about.100 ppm
Discussion of Examples 10 through 13
[0149] Cryptomelane, a high capacity SO.sub.x sorbent, was shown
to be stable under oxidizing conditions and, in general, to be easily
re-oxidized after reduction. However, when subjected to reducing
conditions for extended periods, the SO.sub.2 capacity upon re-oxidation
can be significantly less than that for fresh cryptomelane. However,
for the type of lean-rich cycling proposed for NO.sub.x trap regeneration,
cryptomelane showed no noticeable change in its crystal structure
or in its SO.sub.2 sorption capacity. Further, after SO.sub.2 sorption,
the used cryptomelane was shown to be stable and avoided releasing
any significant amount of sulfur up to moderate temperatures.
Example 14
Silver Hollandite Synthesis and Characterization
[0150] Ag-hollandite was synthesized using post-ion exchange treatment
of cryptomelane in AgNO.sub.3 solution, followed by calcination
in air. The cryptomelane was prepared using the method described
in reference 3 below. A typical post-ion exchange treatment of cryptomelane
(denoted as method A) included adding 1.0 g of cryptomelane to 100
mL 1M AgNO.sub.3 solution, putting the mixture into a temperature-controlled
shaker and heating to 55.degree. C. under continuous shaking. The
total ion exchange duration was 24 hrs. The liquid was decanted,
and the solid was dried in air at 120.degree. C. overnight. The
dried powder was calcined in air at 500.degree. C. for 2 hours.
The yield was 1.3 g.
[0151] A second method (denoted as method B) to synthesize silver
hollandite from cryptomelane involved performing the ion exchange
in a melt of the silver salt, rather than in solution. More specifically
2.0 g of cryptomelane were added to 20 mL of 1M AgNO.sub.3 solution,
the mixture was put into a temperature-controlled furnace and heated
to 150.degree. C. for 3 hours, and then to 250.degree. C. for 12
hours. After the ion exchange treatment, the extra AgNO.sub.3 in
the mixture was washed off using deionized water. The yield of dried
powder was 2.54 g. Table 10 displays some sample compositions that
were synthesized in this study. TABLE-US-00010 TABLE 10 Cation Molar
Composition of Cryptomelane and Ag-Hollandite from ICP Analysis
Sample K % Ag % Mn % Cryptomelane 7.25 0 92.75 Ag-hollandite A 3.95
21.38 74.67 Water-washed Ag-hollandite A 3.72 19.01 77.26 Acid-washed
Ag-hollandite A 3.03 8.07 88.89 Ag-hollandite B 1.52 21.22 77.26
[0152] Powder X-ray diffraction (XRD) patterns were collected with
a Sintag IV diffractimeter using Cu K.alpha. radiation. Transmission
electron microscope (TEM) with electron dispersion spectrum (EDS)
pattern, BET surface area, ICP elemental analysis, TPR analysis,
TGA-DSC, and XPS spectra were also collected on some of the tested
samples. For ICP elemental analysis, about 20 mg of solid powder
was dissolved in a solution of 12 ml of 2% HNO.sub.3-30% H.sub.2O.sub.2
in water. The mixture was then diluted 1000 times with 2% HNO.sub.3
before ICP chemical analysis. XPS measurements were performed using
a Physical Electronics Quantum 2000 Scanning ESCA Microprobe.
SO.sub.2 Uptake and CO, Hydrocarbon (as C.sub.3H.sub.6), and NO
Oxidation Measurements
[0153] The test setup included a small, fixed bed quartz tube reactor,
which was heated by a small clam-shell furnace. Reactant gases were
metered using mass flow controllers. In different runs, different
feed gases were utilized. The SO.sub.2 analytical system comprised
a HP6890 gas chromatograph equipped with a Sulfur Chemiluminescent
Detector (SCD). The analytical system used was the same as that
described above. The concentration of CO, C.sub.3H.sub.6 and CO.sub.2
were measured using an Agilent Quad Series Micro GC. The NO, NO.sub.2
and total NO.sub.x were measured using a 600-HCLD Digital NO.sub.x
Meter (California Analytical Instruments, Inc.). During the experimental
runs, the analytical system was operated continuously, sampling
the effluent every three minutes. The maximum sensitivity of the
system to SO.sub.2 (with SO.sub.2 feed levels at 10 ppm) was approximately
50 ppb, and to CO, CO.sub.2 C.sub.3H.sub.6 NO, NO.sub.2 was approximately
5 ppm. Typical measurements employed a 0.2 g sample, pressed and
sieved to 40-80 mesh. Each sample was pretreated in flowing air
(100 sccm) at 500.degree. C. for two hours prior to measuring SO.sub.2
uptake and oxidation performance.
Results and Discussion
[0154] The structure of silver hollandite is different than that
of traditional hollandite in that the silver cations do not occupy
the centers of the cubic cages formed by MnO.sub.6 octahedra, but
rather the common faces of the cubes, coordinated with four oxygen
anions at about 0.24 nm. This special structure, as well as the
special property of Ag cations and Ag--Mn mixtures, may contribute
to some unique catalytic and ion conductive properties.
[0155] The Ag.sup.+ cation can be doped into the tunnel structure
of cryptomelane by post ion-exchange. Actually, cryptomelane can
be effective for selective sorption of Ag.sup.+ at a low pH range,
even in the presence of large amount of other cations. After the
Ag.sup.+ ion exchange, the original cryptomelane crystal structure
disappears, and a crystallized Ag-hollandite forms.
[0156] Highly crystallized Ag-hollandite was obtained using synthesis
method A (Ag-hollandite A) as described above, i.e., by ion exchanging
at 55.degree. C. and calcining without intermediate wash of the
product with DI water. FIGS. 13-15 show the resulting XRD pattern,
TEM image and selected area electron dispersion spectrum, respectively.
The cation composition is given in Table 10.
[0157] The XRD pattern (FIG. 12) matched well with that of Ag-hollandite,
Ag.sub.1.8Mn.sub.8O.sub.16. Certain amounts of metallic silver also
existed in the final product. Because there was no product washing
before final calcination, it is believed that this metallic silver
came from the decomposition of the extra AgNO.sub.3. These silver
particles were so big that they were visible to the naked eye. The
crystal size of Ag-hollandite A was small. The crystals were needle-like,
being approximately 30 nm in width and 100-200 nm in length. This
morphology is quite similar to that of the cryptomelane, except
that the Ag-hollandite crystals were relatively shorter.
[0158] The EDS pattern and ICP analysis both show evidence of a
small amount of K.sup.+ in the crystal structure, which indicated
that the ion exchange process was not substantially complete. This
also suggests that the Ag-hollandite crystal structure was able
to accommodate a small amount of K.sup.+.
[0159] Chemical analysis of a water-washed sample of Ag-hollandite
A with large silver particles manually picked out indicated that
the K+ ions were potentially located inside the hollandite structure.
The BET surface area of the as-synthesized material (Ag-Hollandite
A) was 35 m.sup.2/g, which can be mostly attributed to the small
particle size.
[0160] In an attempt to remove the extra metallic silver, the Ag-hollandite
A final product was treated with a 2M HNO.sub.3 solution. It was
found, using XRD pattern analysis, that the silver phase could be
removed, but that extra hollandite structure also resulted. Additionally,
ICP results showed that the amount of silver in the sample was less
than that in Ag-hollandite A. This indicated that the Ag.sup.+ cations
inside the tunnel structure of Ag-hollandite could be easily removed
and replaced by H+ cations, and that the use of an acid wash to
purify the Ag-Hollandite A was not advantageous.
[0161] In another attempt to remove the extra silver phase, the
ion-exchanged cryptomelane was washed with DI water before the final
calcinations occurred. Although the silver phase was removed in
the final product, excess hollandite structure showed up again.
The XRD pattern of this sample was similar to that of the intermediate
product for Ag-hollandite A, i.e., cryptomelane ion exchanged in
1M AgNO.sub.3 at 55.degree. C., washed off extra AgNO.sub.3 with
DI water, and dried at 120.degree. C. in air with no calcination
at 500.degree. C.
[0162] Because no other cations were available for ion exchange
with Ag.sup.+ during washing with DI water, those Ag.sup.+ cations
already exchanged with the K.sup.+ cations likely remained in their
original structure. This also suggests that the final calcination
step did not change the structure when no additional AgNO.sub.3
was available. In other words, the difference in the final product
structure and cation composition between the intermediated washed
sample and the non-washed sample likely came from changes during
the calcination step. Thus, to get highly crystallized Ag-hollandite
in the Ag-Hollandite A synthesis method, the calcination would appear
to benefit from being carried out without washing off the extra
AgNO.sub.3.
[0163] In the Ag-Hollandite A synthesis method, the final calcination
step involved heating in air from 25 to 500.degree. C. at a rate
of 10.degree. C./min, and then maintaining at 500.degree. C. for
2 hours. Under this condition, the extra AgNO.sub.3 melted first
(at approximately 212.degree. C.) and then decomposed (at approximately
444.degree. C.). It was postulated that most of the ion exchange
reaction between Ag.sup.+ and K.sup.+ occurred between 212.degree. |