U.S. patent application number 16/193331 was filed with the patent office on 2019-03-21 for manganese oxide nanoarchitectures for broad-spectrum removal of toxic gases in air-filtration applications.
This patent application is currently assigned to The Government of the United States of America, as represented by the Secretary of the Navy. The applicant listed for this patent is The Government of the United States of America, as represented by the Secretary of the Navy, The Government of the United States of America, as represented by the Secretary of the Navy. Invention is credited to Jeffrey W. Long, Gregory W. Peterson, Jean M. Wallace.
Application Number | 20190083953 16/193331 |
Document ID | / |
Family ID | 59274676 |
Filed Date | 2019-03-21 |
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United States Patent
Application |
20190083953 |
Kind Code |
A1 |
Long; Jeffrey W. ; et
al. |
March 21, 2019 |
Manganese Oxide Nanoarchitectures for Broad-Spectrum Removal of
Toxic Gases in Air-Filtration Applications
Abstract
A high-surface-area, highly porous manganese oxide (MnOx) in the
form of xerogel or aerogel monoliths or powders comprising a
manganese oxide nanoarchitecture comprising an interior surface
area >200 m.sup.2 g.sup.-1, wherein the MnOx gel has a void
structure comprising pores that are sized from 2-150 nm, and
wherein the manganese oxide nanoarchitecture removes toxic gas from
a toxic gas and air mixture at room temperature via an oxidative
mechanism that converts the toxic gas to an innocuous adsorbed
substance. These high-surface-area, ultraporous manganese oxide
(MnOx) xerogels and aerogels exhibit outstanding filtration
performance for multiple, chemically distinct toxic gases,
including ammonia, sulfur dioxide and hydrogen sulfide. These MnOx
materials use multiple mechanisms for small molecule
capture/catalysis including molecular sieving and oxidative
decomposition, and function in a wide range of humidity
conditions.
Inventors: |
Long; Jeffrey W.;
(Alexandria, VA) ; Wallace; Jean M.; (Bristow,
VA) ; Peterson; Gregory W.; (Belcamp, MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Government of the United States of America, as represented by
the Secretary of the Navy |
Arlington |
VA |
US |
|
|
Assignee: |
The Government of the United States
of America, as represented by the Secretary of the Navy
Arlington
VA
|
Family ID: |
59274676 |
Appl. No.: |
16/193331 |
Filed: |
November 16, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15383938 |
Dec 19, 2016 |
10179319 |
|
|
16193331 |
|
|
|
|
62276348 |
Jan 8, 2016 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 53/02 20130101;
B01D 53/508 20130101; B01D 53/80 20130101; B01D 2257/406 20130101;
B01D 2251/304 20130101; B01D 2253/1124 20130101; B01J 20/3085
20130101; B01J 20/06 20130101; B01D 53/52 20130101; B01D 2257/302
20130101; B01D 53/58 20130101; B01D 53/82 20130101; B01D 2253/306
20130101; B01D 2255/2073 20130101; B01D 53/50 20130101; B01J
20/28083 20130101; B01J 20/041 20130101; B01J 20/28047 20130101;
B01D 2257/304 20130101; B01D 2253/308 20130101; B01J 20/28085
20130101; B01J 20/3071 20130101 |
International
Class: |
B01J 20/04 20060101
B01J020/04; B01D 53/52 20060101 B01D053/52; B01D 53/50 20060101
B01D053/50; B01D 53/58 20060101 B01D053/58; B01J 20/30 20060101
B01J020/30; B01J 20/28 20060101 B01J020/28; B01J 20/06 20060101
B01J020/06 |
Claims
1. A high-surface-area, highly porous manganese oxide (MnOx) in the
form of xerogel or aerogel monoliths or powders comprising: a
manganese oxide nanoarchitecture comprising an interior surface
area >200 m.sup.2 g.sup.-1; wherein the MnOx gel has a void
structure comprising pores that are sized from 2-150 nm; wherein
the manganese oxide nanoarchitecture removes toxic gas from a toxic
gas and air mixture at room temperature via an oxidative mechanism
that converts the toxic gas to an innocuous adsorbed substance; and
wherein the manganese oxide nanoarchitecture removes ammonia from a
contacting gas mixture at sorption capacities >1.0 mol NH.sub.3
kg.sup.-1 MnOx for H--MnOx compositions and >1.5 mol NH.sub.3
kg.sup.-1 MnOx for Na--MnOx compositions under dry conditions.
2. The high-surface-area, highly porous manganese oxide (MnOx) in
the form of xerogel or aerogel monoliths or powders of claim 1
wherein the manganese oxide nanoarchitecture removes sulfur dioxide
from a contacting gas mixture at sorption capacities >2 mol
SO.sub.2 kg.sup.-1 MnOx for H--MnOx compositions and >3 mol
SO.sub.2 kg.sup.-1 MnOx for Na--MnOx compositions under wet
conditions or wherein the humidity is about 80% relative
humidity.
3. The high-surface-area, highly porous manganese oxide (MnOx) in
the form of xerogel or aerogel monoliths or powders of claim 1
wherein the manganese oxide nanoarchitecture removes hydrogen
sulfide from a contacting gas mixture at sorption capacities
>0.3 mol H.sub.2S kg.sup.-1 MnOx for H--MnOx compositions and
>1.5 mol H.sub.2S kg.sup.-1 MnOx for Na--MnOx compositions under
dry conditions or under wet conditions or wherein the humidity is
about 80% relative humidity.
4. The high-surface-area, highly porous manganese oxide (MnOx) in
the form of xerogel or aerogel monoliths or powders of claim 1
wherein the manganese oxide nanoarchitecture removes >35% of HD
mustard agent from a liquid-phase application.
5. The high-surface-area, highly porous manganese oxide (MnOx) in
the form of xerogel or aerogel monoliths or powders of claim 1
wherein the MnOx gel has an average manganese oxidation state
between +3 and +4.
6. A high-surface-area, highly porous manganese oxide (MnOx) in the
form of xerogel or aerogel monoliths or powders for filtering toxic
gases made from the steps of: adding fumaric acid to an aqueous
solution of NaMnO.sub.4 in a 1:3 mole ratio to form a fluid-filled
porous gel of MnOx in which the oxide domains also contain Na.sup.+
and thereby form Na--MnOx; rinsing the Na--MnOx gel with an acid
solution to protonate the oxide and form H--MnOx and remove
Na.sup.+; and rinsing the gel in water to remove residual acid;
drying the fluid-filled porous gel under ambient-pressure
conditions to generate a densified xerogel MnOx material; wherein
the manganese oxide nanoarchitecture has an interior surface area
>200 m.sup.2 g.sup.-1; wherein the MnOx gel has a void structure
comprising pores that are sized from 2-150 nm; exchanging the fluid
in the pores of the fluid-filled porous gel for CO.sub.2; and
removing said CO.sub.2 under supercritical conditions to render a
dry, low-density MnOx aerogel; wherein the low-density MnOx aerogel
with a manganese oxide nanoarchitecture is exposed to a toxic gas
and air mixture; wherein the toxic gas is removed from a toxic gas
and air mixture at room temperature via an oxidative mechanism that
converts the toxic gas to an innocuous adsorbed substance; and
wherein the low-density MnOx aerogel with a manganese oxide
nanoarchitecture removes ammonia from a contacting gas mixture at
sorption capacities >1.0 mol NH.sub.3 kg.sup.-1 MnOx for H--MnOx
compositions and >1.5 mol NH.sub.3 kg.sup.-1 MnOx for Na--MnOx
compositions under dry conditions.
7. The high-surface-area, highly porous manganese oxide (MnOx) in
the form of xerogel or aerogel monoliths or powders for filtering
toxic gases of claim 6 wherein the low-density MnOx aerogel with a
manganese oxide nanoarchitecture removes sulfur dioxide from a
contacting gas mixture at sorption capacities >2 mol SO.sub.2
kg.sup.-1 MnOx for H--MnOx compositions and >3 mol SO.sub.2
kg.sup.-1 MnOx for Na--MnOx compositions under wet conditions or
wherein the humidity is about 80% relative humidity.
8. The high-surface-area, highly porous manganese oxide (MnOx) in
the form of xerogel or aerogel monoliths or powders for filtering
toxic gases of claim 6 wherein the low-density MnOx aerogel with a
manganese oxide nanoarchitecture removes hydrogen sulfide from a
contacting gas mixture at sorption capacities >0.3 mol H.sub.2S
kg.sup.-1 MnOx for H--MnOx compositions and >1.5 mol H.sub.2S
kg.sup.-1 MnOx for Na--MnOx compositions under dry conditions or
under wet conditions or wherein the humidity is about 80% relative
humidity.
9. The high-surface-area, highly porous manganese oxide (MnOx) in
the form of xerogel or aerogel monoliths or powders for filtering
toxic gases of claim 6 wherein the low-density MnOx aerogel with a
manganese oxide nanoarchitecture removes >35% of HD mustard
agent from a liquid-phase application.
10. The high-surface-area, highly porous manganese oxide (MnOx) in
the form of xerogel or aerogel monoliths or powders for filtering
toxic gases of claim 6 wherein the MnOx gel has an average
manganese oxidation state between +3 and +4.
Description
REFERENCE TO RELATED APPLICATION
[0001] This application is a non-provisional of, and claims
priority to and the benefits of, U.S. patent application Ser. No.
15/383,938 filed on Dec. 19, 2016 and U.S. Provisional Patent
Application 62/276,348 filed on Jan. 8, 2016, the entireties of
each are hereby incorporated by reference.
BACKGROUND
[0002] This disclosure concerns manganese oxide nanoarchitectures
for broad-spectrum removal of toxic gases in air-filtration
applications.
[0003] These sol-gel-derived manganese oxide (MnOx)
nanoarchitectures exhibit broad-spectrum filtration activity at
room temperature for multiple toxic industrial compounds (TICs) and
chemical warfare agents (CWAs).
SUMMARY OF DISCLOSURE
Description
[0004] These sol-gel-derived manganese oxide (MnOx)
nanoarchitectures exhibit broad-spectrum filtration activity at
room temperature for multiple toxic industrial compounds (TICs) and
chemical warfare agents (CWAs).
DESCRIPTION OF THE DRAWINGS
[0005] The following description and drawings set forth certain
illustrative implementations of the disclosure in detail, which are
indicative of several exemplary ways in which the various
principles of the disclosure may be carried out. The illustrated
examples, however, are not exhaustive of the many possible
embodiments of the disclosure. Other objects, advantages and novel
features of the disclosure will be set forth in the following
detailed description when considered in conjunction with the
drawings.
[0006] FIG. 1 is a schematic showing synthesis of MnOx xerogels and
aerogels.
[0007] FIG. 2 illustrates pore-size distribution plots derived from
N.sub.2-sorption isotherms for MnOx xerogels and aerogels.
[0008] FIG. 3 illustrates scanning electron micrographs of (a)
Na--MnOx xerogel and (b) Na--MnOx aerogel; and transmission
electron micrographs of (c) Na--MnOx xerogel and (d) Na--MnOx
aerogel.
[0009] FIG. 4 illustrates powder X-ray diffraction scans for (a)
Na--MnOx xerogel and (b) H-MnOx xerogel.
[0010] FIG. 5 is a schematic of an experimental set-up for
dynamic-challenge microbreakthrough tests.
[0011] FIG. 6 illustrates Breakthrough curves for (a) NH.sub.3; (b)
SO.sub.2; and (c) H.sub.2S challenge, under both dry and 80%
relative humidity conditions and examining Na--MnOx and H--MnOx
xerogel sorbents.
[0012] FIG. 7 illustrates diffuse-reflectance Fourier-Transform
infrared spectra for (a) Na--MnOx and (b) H--MnOx xerogels after
exposure to NH.sub.3 under both dry and 80% RH conditions.
[0013] FIG. 8 is an X-ray photoelectron spectrum in the N is region
for Na--MnOx xerogel exposed to NH.sub.3 under dry conditions.
[0014] FIG. 9 is an X-ray photoelectron spectrum in the S 2p region
for Na--MnOx xerogel exposed to either SO.sub.2 or H.sub.2S under
dry conditions.
[0015] FIG. 10 illustrates diffuse-reflectance Fourier Transform
infrared spectra for (a) Na--MnOx and (b) H--MnOx xerogels after
exposure to H.sub.2S under both dry and 80% RH conditions.
[0016] FIG. 11 illustrates diffuse-reflectance Fourier Transform
infrared spectra for Na--MnOx xerogels powder in its native form
(-) and after exposure to vapor-phase DMMP (---). The position of
the P.dbd.O stretching band (1260 cm.sup.-1) indicates that DMMP is
strongly bound to the MnOx surface.
[0017] FIG. 12 illustrates Breakthrough curves for NH.sub.3
challenge of MnOx aerogel powders. Ammonia-sorption capacity for
MnOx stored for 12 years is comparable to that for a freshly
synthesized and tested MnOx powder (Tests performed in November
2011).
DETAILED DESCRIPTION OF THE INVENTION
[0018] This disclosure concerns manganese oxide nanoarchitectures
as broad-spectrum removal of toxic gases in air-filtration
applications.
[0019] It has been demonstrated that our sol-gel-derived manganese
oxide (MnOx) nanoarchitectures exhibit broad-spectrum filtration
activity at room temperature for multiple toxic industrial
compounds (TICs) and chemical warfare agents (CWAs).
[0020] Manganese oxides are synthesized via the reaction of
NaMnO.sub.4 and fumaric acid to form monolithic gels of disordered
Na--MnOx, which can be further cation-exchanged by acid rinsing to
form more crystalline H--MnOx compositions.
[0021] For both Na--MnOx and H--MnOx forms, controlled pore-fluid
removal yields either densified, yet still mesoporous, xerogels or
low-density aerogels, prepared by supercritical-CO.sub.2 drying.
Using dynamic-challenge microbreakthrough test protocols, we show
that coarse-powder forms of these MnOx nanoarchitectures serve as
highly effective filtration media for three chemically distinct
TICs-NH.sub.3, SO.sub.2, and H.sub.2S-chemicals that are classified
in the "high hazard" category on the "TIC Hazard Index List"
published by the U.S. Occupational Safety and Health
Administration.
[0022] High filtration capacities are observed under both dry and
wet (80% relative humidity, RH) atmosphere conditions.
[0023] These same MnOx materials also exhibit activity for the
removal of sulfur-mustard agents and the CWA simulant,
dimethylmethylphosphonate (DMMP)
[0024] Such manganese oxides offer multiple mechanisms for
filtration/sorption of toxic agents--molecular sieving (for
NH.sub.3); chemisorption (for DMMP); and oxidative decomposition
(for SO.sub.2 and H.sub.2S) to yield such innocuous byproducts as
sulfate.
[0025] The ability to achieve high-capacity sorption and strong
binding for multiple types of TICs and CWAs using a single sorbent
material (MnOx in this case) will ultimately reduce the complexity,
size, and cost of filtration technologies for such critical
applications as respirators.
[0026] Manganese oxides (MnOx) comprise a large family of naturally
occurring and synthetic materials that are of interest for
applications ranging from electrochemical energy storage to
catalysis. Many crystalline polymorphs of MnOx are constructed of
MnO.sub.6 octahedra that assemble into inherently microporous forms
of either tunnel (e.g., hollandite) or layered (e.g., birnessite)
structures, a characteristic that enables their use as molecule-
and ion-sieving sorbents. Hollandite- and birnessite-type MnOx
structures readily incorporate gas-phase NH.sub.3 within their
microporous structures to provide filtration activity for a TIC
that is ordinarily difficult to capture due to its high vapor
pressure.
[0027] In addition to physical capture, many forms of manganese
oxides are catalytically active for oxidation reactions, enabled by
facile interconversion of Mn oxidation state (e.g., between +3 and
+4) and the rich defect chemistry that is available in
nonstoichiometric MnOx compositions. For example, MnOx substrates
effectively promote the oxidation of formaldehyde, another TIC for
which abatement strategies are desired. More recent investigations
have shown that some forms of MnOx adsorb/degrade certain chemical
warfare agents (CWAs) and simulants such as sulfur mustard and
2-chloro-ethyl ethyl sulfide.
[0028] Materials designed for air filtration require not only
reactive or sorptive functionality but also amplified surface areas
that are readily accessible to the gas-phase agents of
interest.
[0029] Here, such properties are achieved with MnOx materials
synthesized via sol-gel chemistry and based on the reaction of
aqueous permanganate with an organic reducing agent (e.g., fumaric
acid) to form monolithic MnOx gels. Removal of the pore-filling
fluid (e.g., H.sub.2O) of the wet gel under ambient-pressure
conditions yields moderately dense MnOx xerogels, while pore-fluid
exchange and supercritical extraction with CO.sub.2 produces
low-density MnOx aerogels. Manganese oxide xerogels and aerogels
possess through-connected networks of mesopores (in the case of
xerogels) and/or small macropores (in the case of aerogels) that
facilitate the long-range transport of gas-phase molecules to
access the extensive interior surface areas (>200 m.sup.2
g.sup.-1) of these nanoarchitecture, even under high humidity
conditions.
[0030] When combined with the sieving and/or catalytic activity of
particular MnOx phases, these structural characteristics enhance
performance in air-filtration applications.
EXAMPLE
[0031] MnOx gels were prepared via established sol-gel chemistry
methods. FIG. 1 illustrates a preparation method.
[0032] A 1:3 mole ratio of fumaric acid was added to a filtered
0.18 M NaMnO.sub.4 aqueous solution with stirring; this mixture was
degassed under vacuum for 8 minutes to remove evolving
CO.sub.2.
[0033] The resulting MnOx sol was poured into polypropylene molds,
and the tops of the molds sealed with Parafilm, followed by aging
overnight.
[0034] The resulting MnOx gels were removed from the molds, and
rinsed for 2 days in several aliquots of water.
[0035] For the purposes of this study, MnOx nanoarchitectures were
prepared in four variations that include xerogel and aerogel forms
of MnOx gels either processed with only H.sub.2O rinsing (a
composition designated hereafter as "Na--MnOx") or rinsed in 1 M
H.sub.2SO.sub.4 after initial synthesis to exchange Na.sup.+ for
H.sup.+ (compositions noted as "H--MnOx").
[0036] Following the rinsing steps, dry xerogels are prepared by
ambient-pressure evaporation of H.sub.2O from the wet gel;
capillary forces that arise during drying result in significant
densification (factor of .about.8-10) to produce durable monolithic
pellets (Na--MnOx and H--MnOx xerogels are visibly
indistinguishable at this point).
[0037] Aerogels are prepared by rinsing H.sub.2O-filled gels with
acetone for 2 days followed by CO.sub.2 supercritical extraction
(Polaron E3000 Series Critical Point Drying Apparatus). The
resulting aerogels retain the approximate dimensions of the wet
gel; as a consequence of minimal densification, MnOx aerogels,
although monolithic, are relatively fragile when handled.
[0038] As demonstrated herein, the low-density aerogel architecture
provides higher mass-normalized TIC-filtration capacities in some
cases, but when normalized to volume occupied by the powdered
sorbent, the densified xerogel forms will provide better and
superior performance. The ambient-pressure drying process for
xerogels is also more economically attractive for large-scale
production.
[0039] These MnOx nanoarchitectures are distinctive in that
specific surface area for xerogels is comparable to those for the
supercritically dried aerogels, whereas with other sol-gel-derived
metal oxides, the capillary forces that arise during
ambient-pressure drying to form xerogels results in coalescence of
the networked oxide particles and loss of surface area.
[0040] For example, MnOx xerogels and aerogels studied herein have
comparable specific surface areas in the 240-290 m.sup.2 g.sup.-1
range, as determined by N.sub.2-sorption analysis. Pore-size
distribution plots derived from N.sub.2-sorption isotherms show the
distinctions in pore--solid architecture--xerogels have their void
volume expressed within a narrow 2-15 nm size range, whereas the
pores in the aerogels span 10-80 nm in size (FIG. 2).
[0041] Cumulative pore volumes for aerogels are .about.4-5 times
greater than for xerogels (Table 1), evidence of the degree of pore
collapse that occurs during ambient-pressure drying to form the
xerogel. Despite this densification, the Na--MnOx and H--MnOx
xerogels have pore volumes of 0.34 and 0.50 cm.sup.3 g.sup.-1,
respectively; such values are comparable to those of activated
carbon-based sorbents.
TABLE-US-00001 TABLE 1 Summary of results from
N.sub.2-physisorption measurements. Cumulative Specific surface
pore volume Mean pore area (m.sup.2 g.sup.-1) (cm.sup.3 g.sup.-1)
size (nm) Na--MnOx xerogel 263 0.34 5.3 H--MnOx xerogel 289 0.50
6.1 Na--MnOx aerogel 264 1.9 28 H--MnOx aerogel 246 2.4 40
[0042] Scanning electron microscopy confirms the 3D-porous nature
of these MnOx materials. While the aerogels exhibit the most open
architectures (FIG. 3b), even the xerogels, which experience
significant densification and pore collapse during drying, retain a
sponge-like structure with void sizes up to .about.20 nm (FIG.
3a).
[0043] The solid MnOx domains comprise filament- or needle-like
particle morphologies, as commonly observed with sol-gel derived
MnOx. The networked nanoscale morphology of MnOx xerogels and
aerogels is best shown by TEM (FIGS. 3c and 3d). The Na--MnOx
xerogels are relatively ill-defined, whereas lattice fringes are
more clearly observed in micrographs for the H--MnOx form, and the
Na--MnOx and H--MnOx aerogels. The TEM images show oblong particles
that are approximately 3-5 nm by 25-40 nm. It is suggested that the
needle-like morphology of the MnOx domains, when expressed in 3-D
networked architectures, prevents complete collapse of the initial
pore structure of the wet MnOx gel that might otherwise occur due
to the strong capillary forces that develop during ambient-pressure
drying to form the xerogel.
[0044] X-ray diffraction (FIG. 4) confirms that the Na--MnOx forms
are poorly crystalline, with only two broad peaks at 38 and
66.degree. 2-theta; the lack of crystallinity is not unexpected for
sol-gel-derived materials that have not been thermally treated.
[0045] Acid-rinsing of the wet MnOx gel to form H--MnOx ultimately
generates a modestly more crystalline material upon drying. The
H--MnOx form exhibits multiple broad diffraction peaks in the
diffraction scan that index most closely to vernadite
(.delta.-MnO.sub.2), a turbostratic relative of the layered
birnessite-MnOx structure. X-ray photoelectron spectroscopy
confirms that these oxides exist in mixed-valent Mn.sup.3+/4+
forms, with an average oxidation state of 3.4 for Na--MnOx and 3.5
for H--MnOx. This characteristic is critical for promoting
oxidative-decomposition mechanisms.
[0046] Breakthrough testing was conducted on MnOx aerogel and
xerogel powders using a microbreakthrough setup that has been
described previously (FIG. 5). Briefly, each chemical (TIC) was
sampled via syringe from a neat cylinder, and delivered to a steel
ballast, which was then pressurized to approximately 15 psig. A
stream from this ballast was delivered via mass flow controller and
mixed with a humidity-controlled stream at rates necessary to
achieve the appropriate challenge concentration (2,000 mg/m.sup.3
for NH.sub.3, 1,000 mg/m.sup.3 for H.sub.2S and SO.sub.2). The
mixed stream was delivered at a total flow rate of 20 ml/min to a
glass-fritted tube submerged in a temperature controlled bath at
20.degree. C. Within the 4 mm ID tube, xerogels and aerogels were
packed to a bed depth of approximately 4 mm, resulting in a
residence time of approximately 0.15 s. Breakthrough was measured
on the effluent side of the bed using HP5890 Series II gas
chromatographs, one equipped with a photoionization detector for
NH.sub.3, and the other a flame photometric detector for H.sub.2S
and SO.sub.2. Sorption capacities for NH.sub.3, SO.sub.2, and
H.sub.2S under dry and humid conditions, and for each of the four
MnOx materials examined is are summarized in Table 2. Corresponding
breakthrough curves are shown in FIG. 6.
TABLE-US-00002 TABLE 2 Summary of TIC sorption capacities from
microbreakthrough tests. NH.sub.3 capacity SO.sub.2 capacity
H.sub.2S capacity Dry 80% RH Dry 80% RH Dry 80% RH Na--MnOx 1.9 mol
kg.sup.-1 1.0 mol kg.sup.-1 1.0 mol kg.sup.-1 3.5 mol kg.sup.-1 1.7
mol kg.sup.-1 9.9 mol kg.sup.-1 xerogel (32 mg g.sup.-1) (17 mg
g.sup.-1) (64 mg g.sup.-1) (220 mg g.sup.-1) (58 mg g.sup.-1) (340
mg g.sup.-1) H--MnOx 2.0 mol kg.sup.-1 2.0 mol kg.sup.-1 0.6 mol
kg.sup.-1 2.4 mol kg.sup.-1 0.3 mol kg.sup.-1 0.5 mol kg.sup.-1
xerogel (34 mg g.sup.-1) (34 mg g.sup.-1) (38 mg g.sup.-1) (150 mg
g.sup.-1) (10 mg g.sup.-1) (17 mg g.sup.-1) Na--MnOx 4.8 mol
kg.sup.-1 2.3 mol kg.sup.-1 0.9 mol kg.sup.-1 3.1 mol kg.sup.-1 2.7
mol kg.sup.-1 20 mol kg.sup.-1 aerogel (82 mg g.sup.-1) (39 mg
g.sup.-1) (58 mg g.sup.-1) (200 mg g.sup.-1) (92 mg g.sup.-1) (680
mg g.sup.-1) H--MnOx 2.6 mol kg.sup.-1 1.2 mol kg.sup.-1 0.6 mol
kg.sup.-1 2.7 mol kg.sup.-1 0.4 mol kg.sup.-1 0.5 mol kg.sup.-1
aerogel (44 mg g.sup.-1) (20 mg g.sup.-1) (38 mg g.sup.-1) (170 mg
g.sup.-1) (14 mg g.sup.-1) (17 mg g.sup.-1)
[0047] Using room-temperature dynamic-challenge test conditions,
these MnOx xerogels and aerogels exhibit NH.sub.3-sorption
capacities ranging from 1.0-4.8 mol kg.sup.-1, achieved via a
combination of physisorption and chemisorption/sieving mechanisms.
Ammonia sorption under 80% relative humidity (RH) was generally
lower, potentially due to competition between NH.sub.3 and H.sub.2O
for sorption sites, but capacity is still competitive even under
these wet conditions. For comparison, Wang et al. used equilibrium
adsorption measurements at 298 K (nominally dry conditions) to
measure "static irreversible" NH.sub.3 capacities of 1.27 and 0.59
mol kg.sup.-1 for H-hollandite and K-hollandite MnOx, respectively.
Observed here is some NH.sub.3 desorption after feed termination
for the MnOx sorbents, indicating that at least a portion the
NH.sub.3 is weakly physisorbed. Yet, post-breakthrough
characterization of NH.sub.3-exposed MnOx using diffuse-reflectance
infrared Fourier transform spectroscopy (DRIFTS) and X-ray
photoelectron spectroscopy (XPS) indicates that a significant
fraction of NH.sub.3, and its protonated form, NH.sub.4.sup.+, are
strongly retained within the MnOx structure (FIG. 7,8).
[0048] The MnOx xerogel and aerogel nanoarchitectures also show
significant activity for SO.sub.2 removal at room temperature via
oxidative mechanisms that convert this TIC to innocuous adsorbed
sulfate, SO.sub.4.sup.2- (see XPS in FIG. 9). Sorption capacities
range from 0.6 mol kg.sup.-1 for the H--MnOx gels under dry
conditions to a high of 3.5 mol kg.sup.-1 for the Na--MnOx xerogel
under humid conditions (FIG. 6b and Table 2), and after feed
termination no elution of SO.sub.2 is observed, indicating strong
retention/conversion. In all cases, Na--MnOx compositions exhibit
higher SO.sub.2 capacities, a trend that may be due to the lower
average Mn oxidation state of that form.
[0049] Capacities obtained at 80% RH are significantly greater than
for dry conditions, in agreement with previous reports. For
comparison, Yi-Fan et al. reported SO.sub.2-sorption capacities as
high as 1.5 mol kg.sup.-1 for MnOx-containing activated carbons
when studied by dynamic breakthrough protocols under humid
conditions and with the sorbent bed maintained at 80.degree. C.
Manganese oxides have long been used to remove SO.sub.2 from flue
gas streams, but in such cases the MnOx sorbent/catalyst bed is
typically operated at high temperatures (>200.degree. C.). Our
work demonstrates that nanostructured, mixed-valent MnOx are active
for SO.sub.2 oxidation/decomposition at room temperature.
Post-exposure analysis shows that SO.sub.2 is converted to adsorbed
SO.sub.4.sup.2- concomitant with an increase in the average Mn
oxidation state.
[0050] Typical breakthrough curves for dynamic H.sub.2S challenge
are shown in FIG. 6c. Similar to our results for SO.sub.2, effluent
concentration immediately drops to the baseline after H.sub.2S feed
termination, indicating strong retention or conversion of H.sub.2S
to a nonvolatile product. Filtration of H.sub.2S is more sensitive
to the particular form of MnOx sorbent. Whereas the H--MnOx
compositions show very little H.sub.2S removal, Na--MnOx forms of
both the aerogel and xerogel show outstanding reactivity,
especially under humid conditions. The Na--MnOx xerogel exhibits
capacities of 1.7 and 9.9 mol kg.sup.-1 (58 and 340 mg g.sup.-1)
under dry and humid conditions, respectively, while the aerogel
approximately doubles the removal capacity, with loadings of 2.7
and 20 mol kg.sup.-1 (92 and 680 mg g.sup.-1, respectively) under
the same conditions.
[0051] Analysis of H.sub.2S-exposed MnOx confirms that Hydrogen
sulfide undergoes similar oxidative-decomposition mechanisms at
MnOx to that observed with SO.sub.2, yielding both sulfate and
adsorbed sulfur/polysulfide byproducts, as determined by XPS (FIG.
9) and DRIFTS (FIG. 10). As part of the H.sub.2S-decomposition
process some of the mixed valent MnOx is converted to MnOOH (see
FIG. 10), concomitant with a decrease in average Mn oxidation state
to 3.1. The through-connected pore structure and high total pore
volume of these MnOx xerogels and aerogels readily accommodate the
formation of solid byproducts of H.sub.2S decomposition, leading to
high capacities. A general reaction scheme for the reactivity of
Mn.sup.IVO.sub.2 with H.sub.2S is shown below.
2Mn.sup.IVO.sub.2(s)+H.sub.2S(g).fwdarw.2Mn.sup.IIIOOH(s)+S(s)
[0052] Prior formulations of manganese oxides have been
successfully used for H.sub.2S removal at high temperatures, but
were generally less effective at room temperature and with loadings
that do not approach those seen with the MnOx xerogels and
aerogels. Recently, Peterson and co-workers examined broad-spectrum
carbon, an excellent material for acidic, basic, and oxidizable
gases, tested with the same microbreakthrough apparatus used herein
reporting breakthrough capacity of approximately 8 and 3 mol
kg.sup.-1 for H.sub.2S under dry and humid conditions,
respectively, a dry-vs.-wet trend that is opposite of that seen
with the MnOx materials, even though both materials remove H.sub.2S
via oxidation. Whereas water absorption under wet conditions may
block active sites on the carbon sorbent, water actually acts
synergistically to enhance the H.sub.2S removal mechanism within
the MnOx xerogels and aerogel materials. Further, the
mesoporous/macroporous structure of these xerogels and aerogel
architectures should minimize pore occlusion by capillary
condensation of H.sub.2O, which is often an issue with microporous
sorbents.
[0053] Having demonstrated high filtration activity for multiple
TICs, we also investigated the sorption/capture of two CWA
agents/simulants using these same MnOx nanoarchitectures.
Dimethylmethylphosphonate (DMMP) is a common simulant for sarin and
related nerve gases. We exposed Na--MnOx xerogels to vapor-phase
DMMP at room temperature, followed by venting the DMMP-exposed
Na--MnOx to remove any weakly bound DMMP. Analysis of the resulting
Na--MnOx xerogels by DRIFTS confirms that DMMP is present on the
MnOx sorbent at significant quantities. Although DMMP does not
undergo chemical decomposition, the position of the P.dbd.O
vibrational band indicates that DMMP is strongly adsorbed to the
MnOx surface (FIG. 11). In another series of experiments, sulfur
mustard (HD) was applied to MnOx sorbents at 5 wt. % agent loading,
followed by extraction after 60 min. The Na--MnOx and H--MnOx
xerogels showed 36% and 47% HD removal, respectively, values that
surpass those of many other common filtration materials.
[0054] The MnOx xerogels and aerogels described herein provide
highly effective filtration performance against a broad range of
TICs and CWAs, all in a single sorbent material.
[0055] Achieving such broad-spectrum protection often requires the
use of multiple distinct sorbents, or chemically complex
composites, such as metal- and organic-doped carbons.
[0056] The mesoporous/macroporous structures inherent to MnOx
xerogels and aerogels support rapid flux of gas/vapor-phase agents
to active sites within the materials, while also minimizing
flooding/occlusion under high-humidity conditions. In addition to
capture by sieving (for NH.sub.3) and chemisorption (for DMMP), the
ability of mixed-valent Mn.sup.3+4+-oxide compositions to readily
undergo Mn oxidation state changes, either positively or
negatively, facilitates redox reactions at the MnOx surface that
promote oxidative decomposition (e.g., for SO.sub.2 and H.sub.2S).
The nanoscale nature of these MnOx materials enables reactivity at
room temperature, whereas prior examples of MnOx sorbents/catalysts
were operated at elevated temperatures. We have also demonstrated
that MnOx nanoarchitectures exhibit excellent shelf-life, with
filtration performance unaffected even after more than a decade of
storage (see FIG. 12). Xerogel forms of MnOx offer the added
benefits of mechanically rugged structures, as well as superior
volume-normalized performance when used in a powder-bed filter
configuration.
[0057] Various other porous materials are used for the removal and
reaction of toxic chemicals. Activated carbons are typically
impregnated with metal salts to react with acidic and basic gases;
however, deleterious effects are often seen due to neutralization
over time. Metal-organic frameworks (MOFs) have been developed to
remove compounds such as ammonia and sulfur dioxide, but only a
very small subset, mainly M-MOF-74 analogs, exhibit broad spectrum
removal capabilities. The MOF HKUST-1 is able to remove ammonia and
hydrogen sulfide, but not sulfur dioxide. Like activated carbon,
however, these examples also have limitations due to stability to
ambient environments. Metal oxides other than oxides of manganese,
such as zirconium hydroxide, zinc oxide, and alumina, have been
developed in the past to remove toxic chemicals, yet these examples
typically target specific groups of chemicals (e.g., acidic
compounds), and not a broad range of chemistries. Similarly, metal
oxides such as zirconium hydroxide, titania, alumina, and others
have been investigated for reaction with chemical warfare agents;
yet none of these compounds has shown the same ability to react
with mustard.
[0058] The above examples are merely illustrative of several
possible embodiments of various aspects of the present disclosure,
wherein equivalent alterations and/or modifications will occur to
others skilled in the art upon reading and understanding this
specification and the annexed drawings. In addition, although a
particular feature of the disclosure may have been illustrated
and/or described with respect to only one of several
implementations, such feature may be combined with one or more
other features of the other implementations as may be desired and
advantageous for any given or particular application. Also, to the
extent that the terms "including", "includes", "having", "has",
"with", or variants thereof are used in the detailed description
and/or in the claims, such terms are intended to be inclusive in a
manner similar to the term "comprising".
* * * * *