Simultaneous and cooperative gas storage and gas production using bifunctional zeolites

Article abstract of DOI:10.1039/b813976h

Nitric oxide can be stored in and produced from zeolites in a simultaneous and cooperative process The Royal Society of Chemistry.

Full text of DOI:10.1039/b813976h

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Simultaneous and cooperative gas storage and gas production using
bifunctional zeolitesw
a
a
a
b
a
Astrid-Karla Boes, Paul S. Wheatley, Bo Xiao, Ian L. Megson and Russell E. Morris
Received (in Cambridge, UK) 12th August 2008, Accepted 22nd September 2008
First published as an Advance Article on the web 14th October 2008
DOI: 10.1039/b813976h
2À
Nitric oxide can be stored in and produced from zeolites in a
simultaneous and cooperative process
the transformation of NO to NO. This occurs in bacterial
1
nitrite reductase enzymes, and in solution state
6
17,18
and
polymer-bound models for this reaction. However, the
1
5
À
The interaction of nitrogen oxides with zeolites is of great
interest in catalytic and adsorption technologies for environ-
NO
2
to NO reaction has never been demonstrated in zeolites.
Extra-framework copper(II) ions are easily incorporated into
zeolites, such as those with the zeolite-X (FAU) or ZSM-5
(MFI) framework types (Fig. 1), by standard ion-exchange
procedures (See ESIw). The as-synthesised Cu(II)-containing
1
–3
mental remediation
4
and for medical gas-storage applica-
There is a wide range of knowledge about this
,5
tions.
interaction from thousands of publications. However, the
research has almost exclusively concentrated on the destruc-
tion of nitric oxide (NO) using zeolites, particularly in relation
materials (i.e. directly after ion exchange) are not active for the
À
production of NO from NO
2
. This control experiment proves
6
–8
to selective catalytic reduction deNO processes. There are
x
that the environment (e.g. the buffer) does not reduce the
copper ions in the zeolite. In addition, the addition of nitrites
to a solution containing only aqueous cupric cations did not
induce any NO release, which illustrates the critical role of
cuprous cations. However, thermal activation of copper-
containing zeolites leads to the formation of Cu(I) species in
no examples of zeolites being used to produce NO. Here we
demonstrate that copper-containing zeolites with the X (FAU)
and ZSM-5 (MFI) framework structures can be used both to
À
store NO and to produce NO from nitrite ions (NO
2
),
facilitating the delivery of the gas for extended periods of time
that are significantly longer than possible with gas storage
alone and increasing the deliverable capacity by an order of
magnitude. That zeolites can be used to produce NO goes
1
9
the zeolites—the so called ‘self reduction’ reaction (see ESIw).
Self-reduction involves several different steps as the zeolites are
heated, including a complex reorganization of the copper over
the cation sites as the water of hydration is eliminated, followed
by reduction from Cu(II) to Cu(I) at higher temperatures and
completely against their traditional uses as deNO catalysts.
x
Furthermore, we also show that these two mechanisms, the
release of stored gas and the chemical production of gas, occur
simultaneously and, most surprisingly, that there is a coopera-
1
9
structural rearrangement to maintain neutrality.
À
Addition of NO
2
solution to activated (dehydrated) copper-
tive effect in that the amount and lifetime of gas produced
À
exchanged zeolite-X (framework Si/Al ratio = 1, copper
exchange = 43%, 10.3 wt% copper determined by ICP) or
to copper-exchanged zeolite-X (framework Si/Al ratio = 60,
copper exchange = 92%, 3.2 wt% of copper) leads to a
significant production of NO (Fig. 2a). The amount of NO
from NO
2
is considerably enhanced if the zeolite is pre-
adsorbed with stored NO. Such cooperative effects between
gas storage and gas production are unprecedented.
9
Gas-delivery technologies are of increasing importance in
many areas of science, with emerging applications that include
1
0
11
the storage of hydrogen and methane as energy carriers
1
2
and the delivery of nitric oxide (NO) for medical therapies.
common method of achieving such delivery is to store the gas
A
9
,13
inside a storage material.
One of the drawbacks of this
approach is that the reservoir of stored gas in a material is
finite and is inevitably depleted as it is released for use. An
alternative method of delivering an active gas is to produce the
1
4,15
gas chemically from a suitable substrate.
Combining these
two methods has the potential to significantly increase the
capacity and lifetime of a gas-delivery material beyond what is
possible with gas storage alone.
Copper-containing zeolites make some of the most active
deNOx catalysts known, and have been extensively studied
6–8
over many years. However, Cu(I) species are also active for
a
EaStChem School of Chemistry, University of St Andrews, Purdie
Building, St Andrews, KY16 ST, UK. E-mail: rem1@st-and.ac.uk
Free Radical Research Facility, UHI Millennium Institute, Inverness,
b
UK IV2 3 BL. E-mail: Ian.Megson@uhi.ac.uk
w Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b813976h
Fig. 1 The adsorption/desorption isotherms for NO on the Cu-
zeolite-X (top) and Cu-ZSM-5 (bottom) samples used in this study.
6
146 | Chem. Commun., 2008, 6146–6148
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produced scales well with the amount of copper in the samples
À5
of the gas pressure). Cu-ZSM-5 only irreversibly adsorbs
B0.6 mmol NO per g of zeolite because it has a much higher
framework Si/Al ratio (60) and therefore considerably fewer
copper ions are needed to balance the framework charge.
Adsorption/desorption isotherms for NO on both copper-
exchanged zeolites are shown in Fig. 1.
À1
(
4.71 Â 10 mol NO g in 300 min for Cu-X versus 1.28 Â
À5
À1
1
0
mol NO g
in 300 min for Cu-ZSM-5). That the
dehydrated zeolites are active is further evidence that the
mechanism requires Cu(I). However, the NO production from
thermally activated samples decreases quickly so that the flux
of NO is reduced to zero after only a few hours (Fig. 2a).
There is no added reductant in the reaction and so no
way of re-reducing Cu(II) to Cu(I), therefore this reaction is
stoichiometric.
4
Release of the stored gas is triggered by contact with water
(Fig. 2) with a relatively short-lived burst, followed by a
steadily decreasing release of a small amount of NO that lasts
for about 100 minutes. For both Cu-ZSM-5 and Cu-X only a
small proportion of the stored NO is releasable on contact
with water at ambient temperature meaning that there is
significant gas inside the zeolites that is irreversibly adsorbed
even in the presence of water. A similar situation is also
seen on NO adsorption on some metal organic framework
Water is known to be important in the deactivation of
x
copper zeolite-based deNO catalysts by favouring the re-
1
9
oxidation of Cu(I) species to Cu(II) and explains why the
activity of the dehydrated sample is reduced so quickly.
Activated copper zeolites are therefore of limited value for
the production of NO in the presence of water.
2
1,22
materials.
Addition of NO2 to the solution in contact with the NO-
À
In some ways, the fact that zeolites can be used to produce
NO at all is a surprise, given their extensive deNO
x
chemistry.
loaded zeolites immediately produces a large amount of NO.
À
However, even more surprising is the effect of pre-adsorbing
and storing NO on the gas production reaction. Zeolites,
including copper-exchanged ones, can also be used as NO
storing materials that release NO on exposure to moisture.
Such materials have been used to remove NO from gas streams
For example, adding 25 mL of 0.05 M NO
physiological solution (total volume 2.6 mL, pH 7.4, [NO
2
to a simulated
À
2
] =
470 mM) in contact with NO-loaded Cu-X leads to an average
NO flux during the experiment time scale of B0.2 nmol
per min per mg of zeolite that lasts ten times longer than the
release of any stored NO. Importantly, unlike the activated
samples containing no stored gas, the NO production does not
decrease quickly but is relatively constant for about 10 hours,
lasting well beyond the time when the production from
NO-free zeolites has reduced to zero. In our batch experi-
ments, after 10 hours the NO production slowly decreases
until it reaches near zero levels after approximately 20 hours.
As well as increasing the lifetime by an order of magnitude, the
NO delivery capacity of the material is also increased approxi-
mately ten-fold. Cu-ZSM-5 shows similar properties. The
effect is not due to copper leaching from the zeolites, as
removal of the solid by centrifugation completely inhibits
the NO production, even when a reducing agent is added to
the solution to reform Cu(I) from any copper that has been
oxidised to Cu(II).
2
0
in pressure swing adsorption applications and as a source of
5
NO in biologically relevant amounts. Exposure of activated
copper-exchanged zeolite-X to gaseous NO leads to a material
that ‘irreversibly’ adsorbs B3.6 mmol of NO per g of zeolite
(
the term irreversibly stems from pressure swing adsorption
terminology and means that NO is not released on reduction
Fig. 3 shows that the release of stored NO and the produc-
tion of NO occur simultaneously. Exposure of the NO loaded
Cu zeolite-X to a buffered solution leads to delivery of NO
À
from the zeolite as indicated. Addition of an aliquot of NO
À4
2
(
100 ml, 2.5 Â 10 M; final concentration, 9.6 mM) to the
solution in contact with the zeolite produces measurable NO
over and above that already being delivered from the stored
reservoir of NO. Subsequent addition of similar quantities of
the same nitrite solution produces broadly the same increases
in NO flux at each addition, taking into account the change in
overall concentration caused by adding new liquid to the
buffer. Further additions of more concentrated nitrite solution
Fig. 2 The nitrite to NO reaction over zeolites. The top panel shows
the NO production, on addition of nitrite (total volume 2.6 mL,
2À
À4
À
(
100 ml, 5 Â 10 M; final NO
2
concentration, 19.2 mM)
À3
pH 7.4, [NO ] = 470 mM, 1.60 Â 10
g of Cu FAU X i.e.
induce approximately twice the response in the NO analyzer,
indicating that the concentration-response is broadly propor-
tional at these concentrations of substrate. The first
aliquot addition, highlighted in Fig. 3, shows that the
stored NO is being delivered at the same time as it is being
produced from nitrite. Such simultaneous stored-gas release
and gas production is previously unknown as far as we
are aware.
À4
1
.65 Â 10 g of Cu), to an activated sample of Cu-exchanged zeolite
X (no pre-loaded NO). The bottom panel illustrates the effect of pre-
adsorbing the same zeolite with NO. On exposure of the NO-loaded
zeolite to water some of the NO is released while on addition of nitrite
2À
À3
solution (total volume 2.6 mL, pH 7.4, [NO ] = 470 mM, 2 Â 10
g
À4
of Cu FAU X i.e. 2.06 Â 10 g of Cu) the lifetime of the NO
production is extended significantly when compared to the experiment
depicted in the top panel.
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to recharge NO-releasing materials so that the anti-bacterial,
anti-thrombosis and anti-mitogenic properties of NO are
extended at least an order of magnitude beyond those possible
with gas-storage materials alone. The very slow leaching of
copper from zeolites means that, at least for applications that
only require the device to be in the body for several hours the
toxicology of copper may not be disadvantageous, as is likely
for the bare metal. There are also potential in vitro applica-
tions (e.g. anti-bacterial materials) where any potential toxicity
of copper is much less of an issue.
We thank the EPSRC and the GEMI fund for financial
assistance.
Fig. 3 The dose–response curve for addition of nitrite to NO-loaded
À3
À4
Cu zeolite-X (2.2 Â 10 g of Cu FAU X i.e. 2.27 Â 10 g of Cu).
À4
Addition of an aliquot of nitrite (100 ml, 2.5 Â 10 M) to the 2.6 mL
Notes and references
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This journal is ꢀc The Royal Society of Chemistry 2008