NANOPARTICULES D'OXYDE DE CALCIUM STABILISEES PAR DE LA ZIRCONE POUR LA CAPTURE DE CO2 A DES TEMPERATURES ELEVEES

ZIRCONIA-STABILIZED CALCIUM OXIDE NANOPARTICLES FOR CO2 CAPTURE AT HIGH TEMPERATURES

Abrégé français

L'invention fournit des compositions de particules d'oxyde de calcium nanométriques hautement réactives, comprenant des compositions d'oxyde de calcium ayant une surface BET de 200 m²/g. Des sorbants d'oxyde de calcium stabilisés comprenant un matériau qui est constitué d'oxydes métalliques sont également divulgués. Des procédés de synthèse et de fabrication de ces compositions, ainsi que des procédés d'utilisation de ces compositions en tant que sorbants, sont fournis.

Abrégé anglais

The invention provides highly reactive nanosized calcium oxide particle compositions, including calcium oxide compositions with a BET surface area of 200 m²/g. Also disclosed are stabilized calcium oxide sorbents comprising material that consists of metal oxides. Methods for the synthesis and fabrication of these compositions are provided, along with methods for the use of these compositions as sorbents.

Revendications

CLAIMS
1. A method of synthesizing a zirconium-stabilized calcium oxide nanoparticle
sorbent, comprising:
a) forming a calcium oxide nanoparticle by treating calcium compounds in a
calcium compound treatment comprising:
i) dissolving a calcium alkoxide in a mixed aromatic and alcoholic solvent
to form a calcium alkoxide solution;
ii) adding water to the calcium alkoxide solution to form a liquid calcium
hydroxide alcogel;
iii) drying the calcium hydroxide alcogel, to provide a calcium oxide
nanoparticle composition; and,
b) admixing a zirconium compound with one or more of the calcium compounds
in the calcium compound treatment, to form the zirconium-stabilized calcium
oxide
nanoparticle sorbent;
wherein the zirconium-stabilized calcium oxide nanoparticle sorbent has a BET
suiface area of at least 100 m2/g, an average particle size of between 100 and
500
nm, and a carbon dioxide capture capacity of at least 10 mole/kg sorbent.
2. The method of claim 1, wherein the calcium alkoxide has the formula
(R0)3Ca, and
wherein each R is a straight chain alkyl group.
3. The method of claim 2, wherein each R is a methyl or ethyl group.
4. The method of any one of claims 1 to 3, wherein the zirconium compound is a
zirconium alkoxide.
5. The method of claim 4, wherein the zirconium alkoxide is zirconium(IV)
ethoxide
(Zr(ethoxide)4)1 zirconium(IV) propoxide (Zr(propoxide)4) or zirconium(IV)
tertiary
butoxide (Zr(t-Butoxide)4).
6. The method of any one of claims 1 to 3, wherein admixing the zirconium
compound
comprises one or more of (1) mixing a zirconium stabilizer precursor with the
calcium
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alkoxide in step (i) or (ii), so that the liquid calcium hydroxide alcogel
comprises
zirconium; or, (2) incipient wetness impregnation (IWI) of the calcium oxide
nanoparticle composition with a zirconium stabilizer precursor solution; or
(3) shelling
the calcium oxide nanoparticle composition surface, before or after calcining
the
calcium oxide nanoparticle composition, in a core-shell treatment with a core-
shell
surfactant.
7. The method of claim 6, wherein the zirconium stabilizer precursor is a
zirconium
alkoxide.
8. The method of claim 7, wherein the zirconium alkoxide is zirconium(IV)
ethoxide
(Zr(ethoxide)4)1 zirconium(IV) propoxide (Zr(propoxide)4) or zirconium(IV)
tertiary
butoxide (Zr(t-Butoxide)4).
9. The method of any one of claims 6 to 8, wherein mixing the zirconium
stabilizer
precursor with the calcium alkoxide in step (i) or (ii), comprises dissolving
the
zirconium stabilizer precursor in the mixed aromatic and alcoholic solvent.
10. The method of claim 6, wherein shelling comprises adding a mesoporous
zirconia to calcined calcium oxide nanoparticles by the core-shell treatment
using the
core-shell surfactant.
11. The method of claim 6 or 10, wherein the core-shell surfactant is P123 or
TMA.
12. The method of any one of claims 1 to 11, wherein admixing the zirconium
compound comprises adding zirconium tertiary butoxide to calcium methoxide in
the
mixed aromatic and alcoholic solvent.
13. The method of claim 12, wherein zirconium tertiary butoxide and calcium
methoxide are added to the mixed aromatic and alcoholic solvent in a ratio of
0.05-0.1
moles of Zr(t-Butoxide)4 per mole of Ca(CH30)2.
14. The method of any one of claims 1 to 13, wherein adding water comprises
adding 2-5 moles of H20 per mole of calcium alkoxide.
15. The method of any one of claims 1 to 14, further comprising mixing the
calcium
hydroxide alcogel for an alcogel aging period to provide an aged alcogel and
drying
the calcium hydroxide alcogel comprises drying the aged alcogel.
14

16. The method of any one of claims 1 to 15, wherein the calcium alkoxide
solution
comprises zirconium in an opaque slurry.
17. The method of any one of claims 1 to 16, wherein the liquid calcium
hydroxide
alcogel comprises zirconium and is clear and colorless.
18. The method of any one of claims 1 to 17, wherein drying the calcium
hydroxide
alcogel comprises supercritical drying and vacuum dehydration.
19. The method of any one of claims 1 to 17, wherein the calcium hydroxide
alcogel
comprises zirconium and drying the calcium hydroxide alcogel comprises a
thermal
dehydration of zirconium-calcium hydroxide nanoparticles to provide dehydrated
zirconium-calcium hydroxide nanoparticles.
20. The method of claim 19, wherein the thermal dehydration is carried out
at least
partially under a dehydrating vacuum pressure.
21. The method of claim 19 or 20, wherein the thermal dehydration is
carried out
at a dehydration temperature of at least 450 C.
22. The method of any one of claims 19 to 21, further comprising calcining
the
dehydrated zirconium-calcium hydroxide nanoparticles to provide a calcined
mixed
zirconia-calcium oxide sorbent.
23. The method of claim 22, wherein calcining is carried out at a
temperature of at
least 850 C.
24. The method of any one of claims 1 to 23, wherein the zirconium-
stabilized
calcium oxide nanoparticle sorbent has a carbon dioxide capture capacity of at
least
5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or 5 mole/kg sorbent.
25. The method of any one of claims 1 to 24, wherein the zirconium-
stabilized
calcium oxide nanoparticle sorbent has a carbon dioxide capture capacity that
loses
no more than 10, 9, 8, 7, 6, 5, 4, 3 or 2% carbon dioxide capture capacity
after 20
cycles.
26. The method of any one of claims 1 to 25, wherein active Ca0 conversion
by
carbon dioxide capture of the zirconium-stabilized calcium oxide nanoparticle
sorbent
is at least 90, 91, 92, 93, 94, 95, or 96% in a first carbon capture cycle.

27. The method of any one of claims 1 to 26, wherein the zirconium-
stabilized
calcium oxide nanoparticle sorbent has a BET surface area of at least 140
m2/g, 150
m21g, 160 m21g, 170 m2/g, 180 m2/g or 190 m2/g.
28. The method of any one of claims 1 to 27, wherein the zirconium-
stabilized
calcium oxide nanoparticle sorbent has an average particle size of less than
500nm,
400 nm, 350nm, 300nm1 250nm, 200nm, 150nm1 100nm or 90nm.
29. The method of any one of claims 1 to 28, further comprising pelletizing
the
zirconium-stabilized calcium oxide nanoparticle sorbent, to provide a
pelletized
zirconium-stabilized calcium oxide nanoparticle sorbent.
30. The zirconium-stabilized calcium oxide nanoparticle sorbent produced by
the
method of any one of claims 1 to 28, or the pelletized zirconium-stabilized
calcium
oxide nanoparticle sorbent produced by the method of claim 29_
31. Use of the zirconium-stabilized calcium oxide nanoparticle sorbent
produced
by the method of any one of claims 1 to 28 as a carbon dioxide sorbent.
32. Use of the pelletized zirconium-stabilized calcium oxide nanoparticle
sorbent of
claim 29, as a carbon dioxide sorbent in a fixed-bed calcium looping process.
33. Use of a zirconium-stabilized calcium oxide nanoparticle composition as
a
carbon dioxide sorbent, wherein the composition comprises intermixed zirconium
and
calcium oxide nanoparticles in a solid zirconium-calcium oxide sorbent having
a BET
surface area of at least 100 m2/g with average particle size of between 100
and 500
nm.
34. The use according to claim 33, wherein the zirconium-stabilized calcium
oxide
nanoparticle sorbent has a carbon dioxide capture capacity of at least 51 6,
7, 8, 91 10,
11, 12, 13, 14 or 5 mole/kg sorbent
35. The use according to claim 33 or 34, wherein the zirconium-stabilized
calcium
oxide nanoparticle sorbent has a carbon dioxide capture capacity that loses no
more
than 10, 9, 8, 7, 6, 5, 4, 3 or 2% carbon dioxide capture capacity after 20
cycles.
36. The use according to any one of claims 33 to 35, wherein active CaO
conversion by carbon dioxide capture of the zirconium-stabilized calcium oxide
16

nanoparticle sorbent is at least 90, 91, 92, 93, 94, 95, or 96% in a first
carbon capture
cycle.
37. The use according to any one of claims 33 to 36, wherein the zirconium-
stabilized calcium oxide nanoparticle sorbent has a BET surface area of at
least 140
m2/g, 150 m21g, 160 m21g, 170 m2/g, 180 m2/g or 190 m2/g.
38. The use according to any one of claims 33 to 37, wherein the zirconium-
stabilized calcium oxide nanoparticle sorbent has an average particle size of
less than
500nm, 400 nm, 350nm1 300nm, 250nm, 200nm, 150nm1 100nm or 90nm.
39. A method of adsorbing carbon dioxide, comprising exposing a carbon
dioxide
gas to a zirconium-stabilized calcium oxide nanoparticle composition, wherein
the
composition comprises intermixed zirconium and calcium oxide nanoparticles in
a
solid zirconium-calcium oxide sorbent having a BET surface area of at least
100 m2/g
with average particle size of between 100 and 500 nm.
40. The method of claim 39, wherein the zirconium-stabilized calcium oxide
nanoparticle sorbent has a carbon dioxide capture capacity of at least 5, 6,
7, 8, 91 10,
11, 12, 13, 14 or 5 mole/kg sorbent.
41. The method of claim 39 or 40, wherein the zirconium-stabilized calcium
oxide
nanoparticle sorbent has a carbon dioxide capture capacity that loses no more
than
10, 9, 8, 7, 6, 5, 4, 3 or 2% carbon dioxide capture capacity after 20 cycles.
42. The method of any one of claims 39 to 41, wherein active Ca0 conversion
by
carbon dioxide capture of the zirconium-stabilized calcium oxide nanoparticle
sorbent
is at least 90, 91, 92, 93, 94, 95, or 96% in a first carbon capture cycle.
43. The method of any one of claims 39 to 42, wherein the zirconium-
stabilized
calcium oxide nanoparticle sorbent has a BET surface area of at least 140
m21g, 150
m21g, 160 m21g, 170 m2/g, 180 m2/g or 190 m2/g.
44. The method of any one of claims 39 to 43, wherein the zirconium-
stabilized
calcium oxide nanoparticle sorbent has an average particle size of less than
500nm,
400 nm, 350nm, 300nm, 250nm, 200nm, 150nm, 100nm or 90nm.
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45. The method of any one of claims 39 to 44, wherein the sorbent reversibly
adsorbs CO2 at 650-700 C to form CaCO3.
46. The method of claim 45, wherein the sorbent is regenerated from CaCO3
at
850-900 C with the release of CO2.
47. A zirconium-stabilized calcium oxide nanoparticle composition, wherein
the
composition comprises intermixed zirconium and calcium oxide nanoparticles in
a
solid zirconium-calcium oxide sorbent having a BET surface area of at least
100 m2/g
with average particle size of between 100 and 500 nm.
48. The composition of claim 47, wherein the zirconium-stabilized calcium
oxide
nanoparticle sorbent has a carbon dioxide capture capacity of at least 5, 6,
7, 8, 9, 10,
11, 12, 13, 14 or 5 mole/kg sorbent.
49. The composition of claim 47 or 48, wherein the zirconium-stabilized
calcium
oxide nanoparticle sorbent has a carbon dioxide capture capacity that loses no
more
than 10, 9, 8, 7, 6, 5, 4, 3 or 2% carbon dioxide capture capacity after 20
cycles.
50. The composition of any one of claims 47 to 49, wherein active Ca0
conversion
by carbon dioxide capture of the zirconium-stabilized calcium oxide
nanoparticle
sorbent is at least 90, 91, 92, 93, 94, 95, or 96% in a first carbon capture
cycle.
51. The composition of any one of claims 47 to 50, wherein the zirconium-
stabilized
calcium oxide nanoparticle sorbent has a BET surface area of at least 140
m21g, 150
m2/g, 160 m21g, 170 m2/g, 180 m2/g or 190 m2/g.
52. The composition of any one of claims 47 to 51, wherein the zirconium-
stabilized
calcium oxide nanoparticle sorbent has an average particle size of less than
500nm,
400 nm, 350nm, 300nm1 250nm, 200nm, 150nm1 100nm or 90nm.
53. The composition of any one of claims 47 to 52, wherein the sorbent
reversibly
adsorbs CO2 at 650-700 C to form CaCO3.
54. The composition of claim 53, wherein the sorbent is regenerated from
CaCO3
at 850-900 C with the release of CO2.
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Description

WO 2021/042212
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ZIRCONIA-STABILIZED CALCIUM OXIDE NANOPARTICLES FOR CO2
CAPTURE AT HIGH TEMPERATURES
FIELD
[0001] The invention is in the field of chemical
and physical processes for the
production and use of stabilized calcium oxide aerogel sorbents for CO2
capture at
high temperature.
BACKGROUND
[0002] Alternative methods have been described for
the preparation and use
of various metal oxide aerogels, such as silica, alumina (Al2O3) aerogels,
magnesia
and calcium oxide nanoparticles (Teichner et at, Inorganic oxide aerogels,
Advances in Colloid and Interface Science (1976), 5(3), 245-73; U54550093;
U54717708). These materials generally have very high surface areas and are
often
excellent sorbents for a range of substances. In general, metal oxide aerogels
and
nanoparticles show much more reactivity than the corresponding metal oxides.
These materials may be prepared as a single metal oxide, or as composites (see
U53963646, U54469816, and U56770584).
[0003] Increasing amounts of CO2 in the
environment have given rise to the
need to find easy and effective ways to capture this gas without producing
toxic by-
products. CO2 has extremely high thermal stability, and can be converted to
value-
added products by known processes or stored in underground fields. For
example,
the capture capacity of calcium oxide has been utilized for CO2 removal at
high
temperatures through carbonate formation. However, the weak structure of
calcium
oxide is prone to sintering, thereby significantly decreasing capacity during
extended
cyclic performance. Many approaches have been proposed to maintain this
capacity
for long periods of time, but problems remain.
[0004] While capturing techniques appear promising
and have favorable
thermodynamics, the cost of these techniques has been considerable, owing in
part
to the fact that, to be effective, the capture reagents must be very finely
divided for
maximum surface area. Moreover, these reactions are non-catalytic and depend
entirely on molecular reaction at the surface of the reagents. There is
accordingly a
real and unsatisfied need for reagents with enhanced capture efficiencies.
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[0005] US2010/0139486A disclosed the design and
development of novel
calcium-oxide-based refractory sorbents synthesized by flame spray pyrolysis
(FSP)
for CO2 capture. FSP, as used herein, refers to a technique for converting
precursor
droplets into solid nanoparticles in flames. FSP allows for the controlled
synthesis of
nanoparticles with high specific surface areas. Ca-naphthenate precursor and
the
calculated refractory dopant precursor are dissolved in xylene and fed by a
syringe
pump through the spray nozzle. The most stable sorbent (40 wt.% ZrO2-60 wt.%
CaO) gives a CO2 capacity of 10.76 mole/kg in an extended cyclic operation of
50
without any activity loss. Carbonation was conducted at 700 C in 30% CO2 for
30
min.
[0006] US2010/0311577 describes a method for
making inert nanoparticle-
doped porous CaO sorbent by physically dry mixing and decomposing calcium
acetate (Ca(CH3C00)2) or calcium oxalate (CaC204) with inert nanoparticles, to
form a nitrate-free mixture, and calcining this mixture to form a stable
porous
microstructure with CO2 sorbent properties. The CO2-capture performance of MgO-
doped CaO (42 wt. % MgO-58 wt_ % CaO) sorbents has been described as having
a capacity of 8.5 mole/kg in a multicyclic operation of 100 without any
activity loss.
Carbonation and decarbonation (calcination) were performed at 758 C in 100%
CO2
for 30 min and at 758 C in 100% He for 30 min respectively. U56087294
describes
the preparation of calcium oxide nanoparticles by the sol-gel method followed
by
supercritical extraction of solvents from the gel. These nanoparticles were
then
coated with reactive elements. U56740141 disclosed that iron-oxide-coated
calcium
oxide nanoparticles (Fe203.Ca0) were excellent adsorbents for a variety of
target
substances, such as CO2 and chlorinated hydrocarbons from gas streams at low
temperatures.
SUMMARY
[0007] Highly reactive nanosized calcium oxide
particle compositions are
provided, including compositions comprising materials that are oxides or
hydroxides
of the elements of groups IIA or the transition metals. The materials may be
intimately
mixed on a nanosized scale. Select compositions showed very small average
particle
sizes and consistently large surface areas. Methods for the synthesis and
fabrication
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of these compositions are provided, along with methods for the use of these
compositions as sorbents. In select embodiments, nanosized zirconia-stabilized
calcium oxide particle sorbents with a BET surface area on the order of 150
m2/g are
disclosed. Methods for preparing calcium hydroxide and oxide compositions are
described herein and in CA3044223 (claiming priority to U.S. provisional
Application
No. 62/675,505, filed May 23, 2018). Calcium oxide nanoparticles are
synthesized
in a high surface area alumina aerogel (2000 m2/9), as follows:
Calcium methoxide + Methanol + co-solvent Hydrolysis Calcium hydroxide
Alcogel
Supercritical
Vacuum Dehydration
Drying
(450'C)
_______________ > Calcium hydroxide Nanoparticles
________________________________ Dehydrated Calcium
[0008] In select embodiments, solid compositions
may be prepared by mixing
solid oxides and/or hydroxides with large surface areas. These compositions
exhibited
excellent performance in the high-temperature carbon capture process. These
compositions may for example include materials selected from the oxides and/or
hydroxides of elements of Groups IIA, IIIA, and the transition metals. The
compositions
used as base sorbents were synthesized by preparing calcium alkoxide slurries,
which
were then hydrolyzed to obtain an alcogel. The alcogel was first
supercritically dried
and then calcined at high temperature to yield calcium oxide. The exemplified
calcium
oxide nanoparticles comprise a uniform dispersion of nanosized particles of
calcium
oxide, characterized as substantially fluffy clusters of particles, having a
BET surface
area in the range of 80 to 150 m2/g, a pore volume in the range of 0.5 to 2.5
cm3/g,
and a bulk density in the range of 0.01 to 0.05 g/cm3.
[0009] Calcined calcium oxide was impregnated
and/or core-shelled with an
inorganic and organic metal precursor to exemplify alternative solid
compositions.
These compositions were used as sorbents for carbon dioxide capture from flue
gases
at high temperature.
[0010] The prepared sorbent of calcium oxide
nanoparticles stabilized with
zirconia exhibited an almost 100% CO2-capture efficiency. The loss of CO2
capacity
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of the zircon ia-stabilized sorbents was shown to be near 10-20% during multi-
cycle
operations.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Figure 1 is a graph showing the particle
size distribution of Sample NCI
(average size, 27.5 nm).
[0012] Figure 2 is a graph showing the particle
size distribution of NCl/10%
ZrO2 (average size, 348.5 nm).
[0013] Figure 3 is a graph showing
thermogravimetric analysis (TGA)
performance of NCI sample at 675 C (CO2 capture).
[0014] Figure 4 is a graph showing thermogravimetric
analysis (TGA)
performance of NCI /10% ZrO2 sample at 675 C (CO2 capture).
[0015] Figure 5 is a graph showing
thermogravimetric analysis (TGA)
performance of NCI /10% ZrO2 -P123 sample at 675 C (CO2 capture).
DETAILED DESCRIPTION
[0016] Methods are provided for preparing a series of calcium
compounds,
including nanoscale oxide and hydroxide particulates with very high surface
areas. In
an initial step, calcium alkoxide solutions are prepared in a suitable
solvent. Calcium
alkoxide may for example have the formula (R0)3Ca, where each R is a C1-C2
straight
chain alkyl group. Exemplary alkoxides comprise methyl and ethyl groups. The
calcium alkoxide solution is then hydrolyzed to yield a calcium hydroxide
alcogel.
Thereafter, the alcogel is dried under supercritical conditions, at a
temperature over
the supercritical point of the solvent, to yield a calcium hydroxide
nanoparticle.
Supercritical drying may for example be carried out for a period of from 1.5-
3.5 hours.
[0017] The calcium hydroxide nanoparticles may in
turn be subject to thermal
dehydration, to provide a dehydrated calcium oxide nanoparticle comprising
calcium
hydroxide. The thermal dehydration may for example be carried out at a
temperature
of 300-500 C, for example for a period of 1-3 hours under vacuum. The
nanosized
calcium hydroxide prepared in this way has a high BET surface area, in some
embodiments of at least 140 m2/g.
[0018] The dried or dehydrated nanoparticle compositions may be
used as
sorbents, for example for chemical adsorption of gases. The dehydrated
nanoparticles
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may be calcined to provide particulate calcium oxide compositions. The
calcination
may for example be performed at a temperature of 750-850 C for a period of 2-
5
hours. The nanosized calcium oxide prepared in this way has a higher BET
surface
area, in some embodiments of at least 20 m2/g compared with a commercial
calcium
oxide with BET surface area less than 1 m2/g.
[0019] Compositions provided by the foregoing
methods may be used as solid
sorbents, for example for the removal of target materials through
physisorption or
chemisorption. Exemplified processes of this kind involve contacting the
selected
compositions with target materials such as CO2 and 802 (exemplary of flue
gases
containing CO2). The following examples also illustrate that nanoparticles-
derived
compositions with high surface areas calcined at high temperatures have
sufficient
surface and stability to provide solid sorbents as part of the calcium-looping
process.
[0020] The following examples describe select
compositions and methods,
illustrating only select aspects of the present innovation. Although various
embodiments of the invention are disclosed herein, many adaptations and
modifications may be made within the scope of the invention in accordance with
the
common general knowledge of those skilled in this art. Such modifications
include the
substitution of known equivalents for any aspect of the invention in order to
achieve the
same result in substantially the same way. Numeric ranges are inclusive of the
numbers defining the range. The word "comprising" is used herein as an open-
ended
term, substantially equivalent to the phrase "including, but not limited to",
and the word
"comprises" has a corresponding meaning. As used herein, the singular forms
"a", "an"
and "the" include plural referents unless the context clearly dictates
otherwise. Thus,
for example, reference to "a thing" includes more than one such thing.
Citation of
references herein is not an admission that such references are prior art to
the present
invention. Any priority document(s) and all publications, including but not
limited to
patents and patent applications, cited in this specification, and all
documents cited in
such documents and publications, are hereby incorporated herein by reference
as if
each individual publication were specifically and individually indicated to be
incorporated by reference herein and as though fully set forth herein. The
invention
includes all embodiments and variations substantially as herein before
described and
with reference to the examples and drawings.
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EXAMPLE 1: Preparation and Characterization of Calcium Oxide
Nanoparticle Compositions
[0021] In this example, calcium oxide nanosized
powder without/with zirconia
stabilizer was synthesized and stored. The synthesis consisted of four main
steps, and
was followed by characterization of the materials:
Synthesis of calcium oxide alcogel without/with zirconia stabilizer
[0022] This step comprises the hydrolysis of a
calcium alkoxide solution
(Ca(R0)3 + alcohol + co-solvent). The chemicals used in the synthesis were
directly
obtained from a commercial source without further purification. Calcium
methoxide
(Aldrich) was added to a 500 ml beaker to prepare 10% wt. calcium methoxide
slurry
in methanol (Aldrich). The Ca(CH30)2 slurry was dispersed in a solution of
toluene co-
solvent (Aldrich) (Vol. of toluene/Vol. of methanol of 5-8) to form a grey
slurry. A
specific amount of DI water (1.5-3 moles of H20 per mole of Ca(CH30)2) was
then
added drop-wise to the slurry to form the calcium hydroxide alcogel. The
reaction
mixture was then stirred at room temperature for 1-3 days for ageing_ During
this time,
the mixture remained an opaque gel, but was dilute enough to maintain a semi-
liquid
state. The step of admixing zirconia stabilizer comprises of adding the
specific
amounts of zirconium precursors (depending on zirconia percentages in the
final
sorbents) such as ethanolic zirconium tetra-butoxide solution to calcium
alkoxide
solution prior to hydrolysis step.
Supercritical drying of the alcogel
[0023] The hydroxide alcogel was transferred to a
100 ml glass liner of a Parr
high-pressure batch reactor. The reactor was first flushed with nitrogen and
then
pressurized to 200-250 Psi with nitrogen. The reactor was slowly heated
without
stirring from room temperature to 250-270 C for a period of 1-3 hours. As the
reactor
was heating, the pressure was increased from 200-250 Psi to 600-800 Psi. After
the
reactor reached the target temperature, it was kept at that temperature for a
while and
then flashed to the atmosphere quickly to remove the solvent vapors.
Afterward, the
heating jacket was removed, and the reactor was flushed with nitrogen for 5
minutes
to remove the remaining solvent vapors. The reactor was then allowed to cool
down
to the room temperature.
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Thermal dehydration of calcium hydroxide
[0024] Data from thermogravimetric analysis (TGA)
confirmed that calcium
hydroxide lost the highest weight at a temperature of 400-450 C to convert to
dehydrated calcium oxide. The fluffy white calcium hydroxide powder was placed
into
a BET tube connected to a degassing vacuum line of the BET instrument. The
tube
was evacuated at room temperature for a while to 10 pHg vacuum. Afterward, the
tube
was slowly heated from room temperature to 400-450 C at a ramp of 10 C/min
under
dynamic vacuum. After the heat treatment was complete, the degassing line was
turned off and the tube cooled down to room temperature under dynamic vacuum.
After this step, the dehydrated calcium oxide had a light white color.
Calcination
[0025] The fresh and heat-treated samples were
calcined to the temperature
up to 850 C to obtain nanoparticle-derived calcium oxide, which exhibited a
higher
surface area than that of the commercial calcium oxide. Specifically, calcium
oxide
nanoparticles were calcined in a muffle furnace without gas flow and at high
temperatures. In an exemplary procedure, first, sample NCI was subjected to a
dynamic vacuum using the BET instrument degassing port at 450 C. The BET
surface
area without vacuum dehydration measured 117 m2/g as shown in Table 1.
Generally,
at temperature dehydration of 450 C, the surface area was highest (140 m2/g).
The
significant decrease in surface areas at temperatures above 500 C can be
explained
by sintering. Calcination of NC1 at 850 C, after vacuum dehydration reduced
its
surface area to 28 m2/g. The additional five samples vacuum dehydrated at 450
C
showed different surface areas. The highest surface area belonged to sample
NC1/10% ZrO2, as shown in Table 1. Two samples of NC2 and NC3 calcined at the
higher temperature of 850 C showed low surface areas of less than 20 m2/g
Characterization
[0026] The Brunauer-Emmett-Teller (BET) surface
areas and pore size
distributions were measured using nitrogen adsorption and desorption isotherms
at
-196 C on a Micromeritics 2020 volumetric adsorption analyzer, using pressure
values ranging from 1 to 760 mmHg. The samples were degassed at 150-450 C for
at least 2-5 hours. The pore size distribution was calculated using the
Barrett-Joyner-
Halenda (BJH) pore size and volume analysis method.
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[0027] Thermogravimetric Analysis (TGA) was used
to determine the cyclic
performance of Ca(OH)2 and the ZrO2-stabilized Ca(OH)2 during CO2 capture.
These
studies were conducted under nitrogen and CO2 flows. To measure weight loss
and
gain, the samples were placed in a crucible and heated at a rate of 40 /min
from room
temperature to 850 C. The instrument used was a thermogravimetric analyzer,
the
TGA-STA-6000 from the PerkinElmer Company.
[0028] The Malvern zetasizer (nano-series, Nano-
ZS) dynamic light scattering
(DLS) instrument was used to measure the size of alumina aerogel particles.
This
instrument uses a 633 nm wavelength laser through which the sample particles
scattered light in all directions, including towards a detector. The change in
the
movement of the particle and a correlation function were used in the software
(version
7.12) to draw size distribution graphs.
[0029] In accordance with the foregoing methods,
several embodiments were
prepared using calcium methoxide as a starting material, methane solvent and
toluene
co-solvent, and by varying the concentration of the solutions, time of ageing
and
duration of supercritical drying. All these parameters influence the surface
area and
particle sizes of the resulting samples. Results are shown in Table 1.
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Table 1. Result of BET surface areas of the prepared samples
Calcium BET Ads BJH Des BJH
Pore Pore Mean
oxide (m2/g) i 1m2/9) (m2/g) volume Wide Particle
Nanoparticles
(cms/g) (nm) size (rim)
NC1 116.4 149 167
0.38 9.2 27.5
NC1-450 140.3 221.8 138
0.4 10.3 82.4
NC2 93.6 93.7 102.6
0.4 15.3 217.7
NC2-450 101.7 123.8 138.3
0.27 7.84 -
NC3 85 92 95.2
0.45 18.7
NC1/10% 191.7 230 238
0.62 10.24 348.5
ZrO2
NC2/10% 106-5 126 139
0.5 14.3 -
ZrO2
Cadom in 18.1 - -
0.21 - -
[0030] Samples NCI and NCl/10% ZrO2, in which
calcium tri-methoxide and
zirconium tetra-butoxide were used as the starting material, exhibited the
highest
surface areas. In order to obtain samples with the highest surface areas,
sufficient co-
solvent/solvent ratios are required to be provided for the solutions. The
ratios of
alcoholic solvent (e.g. methanol) to aromatic co-solvent (e.g. toluene) were
varied and
found to have an effect on the surface area. This ratio was over 5 for samples
samples
NCI and NCl/10% ZrO2, and below 3 for samples NC2 and NC2/10% ZrO2 which
resulted in lower surface areas. It was found that decreasing the ratio
resulted in a
significant decrease in the surface area, changing from 200 to 80 m2/g as for
sample
NC3. This sample had a methanol/toluene ratio of less than 2. The amount of
calcium
alkoxide used ranged from 0.5 to 1.3 g and solution mixture concentration
varied
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between 10 and 26 g alkoxide/L, corresponding to 0.3-0_7g CaO/L. The lowest
concentrations resulted in the highest surface areas as for all samples in
Table 1.
[0031] Solution concentration plays a significant
role in increasing surface area.
The slurry of calcium methoxide in the mixture solution was much clearer than
in the
individual alcohol solution. Using different alcohols resulted in the same
surface areas.
The main difference among the samples prepared by the same alcohol was the
ageing
time and the toluene/alcohol ratio. It was found that changing the solvent
resulted in
reducing the surface area by half. The other important factor that affected
the
performance of the samples was storage time. A long storage time resulted in
the
degradation of the fresh sample due to the very reactive surface triggering
the particle
growth and strong adsorption of gases. The surface area of sample NC1
decreased
from 116 to 75 m2/g after one week of storage. Therefore, to avoid the effects
of this
instability, a fresh sample obtained immediately after the supercritical
drying process
may beneficially be thermally converted to more stable phases_ Particle sizes
of three
samples NC1, NC2 and NC1 /10% ZrO2 were measured by the zetasizer particle
analyzer in a range of 20400 nm (Figures 1,2 and 3). The particles of the
calcined
CaO nanoparticles grew to larger values (range of 500-800 nm) due to
sintering.
EXAMPLE 2: High temperature carbon capture
[0032] In this example, the prepared calcium oxide aerogel was used
for high-
temperature CO2 capture from flue gases, and embodiments were exemplified that
used zirconia (ZrO2) stabilized calcium oxide CaO for high-temperature CO2
capture.
This sorbent adsorbs CO2 at 650-700 C by the following reaction to form
CaCO3: CaO
+ CO2 4- CaCO3 (1).
[0033] Regeneration of CaCO3 occurs at 850-900 C with the release of
CO2.
Based on reaction (1), the theoretical amount of CO2 adsorbed per kilogram of
calcium oxide is calculated at 17.857 mole or 785.7 gram. Pure CaO is a weak
substance that must be modified to achieve high CO2-capture efficiency for
long cyclic
operations. CaO stabilized by zirconia ceramic was found to be an efficient
sorbent
due to the zirconia's high thermal barrier coating, high physical strength and
significant resistance to sintering. The CO2-capture efficiency of 10%-20% CaO-
stabilized zirconia was illustrated using various preparation methods such as:
(1)
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mixing stabilizer precursor during alcogel preparation, (2) incipient wetness
impregnation (IWI, also called capillary impregnation or dry impregnation) of
nanoparticles with stabilizer precursor solution, comprises of zirconium
oxynitrate
dissolved in sufficient amount of water which only covers the particles pores
and
surface& Next, the partially wet particles are dried and (3) shelling the
nanoparticles
or calcined nanoparticles surface by the core-shell method using two types of
surfactants, P123 and TMA, comprises of an ethanolic zirconium precursor
solution
containing zirconium tetra-butoxide, acetylacetone, surfactant and water is
used for
shelling. Nanoparticles are dipped in this solution several times, between
each dip,
nanoparticles are dried.
[0034] All sorbents were tested at a carbonation
temperature of 675 C and
decarbonation temperature of 850 C. The exemplary CaO sorbent showed high
reactivity compared to other calcium-based sorbents tested at the higher
calcination
temperature of 850 C. The sorbent fabricated by mixing zirconia precursor
(zirconium
n-butoxide) with calcium hydroxide alcogel solution with a surface area of 190
m2/g
showed the highest CO2-capture efficiency and stability compared with the
other
fabricated sorbents by different preparation methods. The pure CaO sorbent
prepared
by the same sol-gel method with a surface area of 140 m2/g showed the lowest
CO2-
capture stability due to having a high sintering characteristic. Higher
sintering of the
sorbent resulted in converting the particulate-active CaO precursor to an
agglomerated inactive precursor of CaCO3 that cannot be completely regenerated
at
the selected calcination conditions.
[0035] The zirconia-stabilized calcium oxide
sorbent (sample NC1) provided
higher surface area to disperse the active component (zirconium) on the
surface more
effectively. These results explain that the highest CO2-capture efficiency and
stability
(less activity loss) was displayed by the 10% ZrO2 stabilized CaO. Results are
shown
in Table 2. The pelleting of samples at a size of 1 mil using the Parr pellet
press (up
to 1000kg. total force can be exerted on the punch) decreases the activity to
10-15%
due to the gas diffusion barrier. Figures 4,5 and 6 show the cyclic operations
of three
sorbents NC1, NC1/10% ZrO2 and NC1/10% ZrO2 (P123). As can be seen in Figure
5, the stabilized sorbent remained stable after 20 carbonation/calcination
cycles.
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Table 2. Result of CO2 capture performance of the prepared sorbents
Sorbent CO2 uptake at 151 cycle
CO2 uptake at 21st Activity loss %
(Mole CO2/kg of sorbent) cycle (Mole CO2/kg
of sorbent)
NCI 17.2
6.9 60
NC2 15.3
5.78 62
NCl/10% 15.5
13.95 10
ZrO2
NC2/10% 11.7
9.5 18.8
ZrO2
Cadomin 13.54
4.2 68.98
NCl/10% 10.64
10.43 2
ZrO2 (P123)
NCl/10% 12.63
10.04 20.5
Zr02(TMA)
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