These materials react with CO2 in the temperature range 723–823 K, resembling the actual fuel reforming process, with the advantages of high CO2 capture capacity, innite CO2/N2 or CO2/H
Trang 1Syntheses and structures of lithium zirconates for
Shutao Wang,aChanghua An*aband Qin-Hui Zhang*b The sorption and further application of CO 2 is a highlight in the field of environmental protection and sustainable development Lithium-containing zirconates (LixZryOz) are promising materials for high-temperature chemisorption of CO 2 and have attracted tremendous interest This review presents discussion on the recent status of LixZryOzbased CO 2 sorbents at high temperature Special attention is focused on the solid state chemistry, synthetic strategies, high-temperature CO 2 capture –regeneration properties and possible sorption mechanisms for LixZryOzwith di fferent Li/Zr ratios, including Li 2 ZrO 3 ,
Li 6 Zr 2 O 7 , Li 8 ZrO 6 , and Li 2 ZrO 3 doping with alkali metals.
Climate change and green house gas (GHG) emission
regulation have recently attracted much attention, and were
recognized as critical issues requiring action long before
Studies have shown that increased GHG levels would lead to
global warming Among the various affecting species, carbon
dioxide (CO2) makes up a high proportion with respect to its
amount in the atmosphere, contributing 60 percent of the
global warming effect,1 although methane and
chloro-uorocarbons have much higher warming potentials as per mass of gases According to the prediction of Intergovern-mental Panel on Climate Change (IPCC), by year 2100, the atmosphere may contain up to 570 ppm of CO2, causing a rise
of the mean temperature around 1.9C and an increase in the mean sea level of 3.8 m.2 Accordingly, serious issues such as accompanied species extinction may occur In the IPCC report, approximately three quarters of the increase in atmospheric
CO2 is caused by burning of the fossil fuels Fig 1 shows the change of global-mean CO2 concentration between the years 1850 and 2100.3 It is clearly shown that the global climate change is mainly caused by the increase of CO2 concentration, due to the combustion of fossil fuels to sustain industry and maintain the rapid development rate of economy and technology The conditions will become more severe
Shutao Wang received her PhD
in Chemistry from University of Science and Technology of China in 2006 with Prof Zude Zhang Then she joined the faculty at China University of Petroleum, where she is currently an associate professor
From 2012, she held a post-doctoral position with Prof
Zhang at the State Key Labora-tory of Heavy Oil Processing, China University of Petroleum
Her research involves functional inorganic nanocrystals, with
emphasis on energy-related photocatalysts and hydrogenation
nanocatalysts
Changhua An received his PhD degree from University of Science and Technology of China in 2003 with Prof Yitai Qian, then he worked as post-doctoral research fellow at Seoul National University with Prof Taeghwan Hyeon, Korea from
2004 to 2005 He has been an associate professor at China University of Petroleum from
2005 He worked as a visiting scholar at University of Illinois
at Urbana-Champaign, USA from 2009 to 2010 His research interests focus on the synthesis, characterization, modication, and application of nanomaterials used in the solar-chemical energy transformation
a State Key Laboratory of Heavy Oil Processing, Key Laboratory of New Energy Physics &
Materials Science in Universities of Shandong, College of Science, China University of
Petroleum, Qingdao 266580, P R China E-mail: anchh@upc.edu.cn; Fax:
+86-532-86981787
b State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China
University of Petroleum, Qingdao 266580, P R China E-mail: qhzhang@upc.edu.cn;
Fax: +86-532-86981855
Cite this: J Mater Chem A, 2013, 1,
3540
Received 15th October 2012
Accepted 18th December 2012
DOI: 10.1039/c2ta00700b
www.rsc.org/MaterialsA
Materials Chemistry A
FEATURE ARTICLE
View Article Online
View Journal | View Issue
Trang 2without control of CO2 emissions For both developed
countries like USA and developing countries like China
and India, strategies to reduce CO2 emissions are now
more important than ever Furthermore, the problems associated with CO2 emission may be a primary factor
in restricting economic growth of many countries, e.g., China
Therefore, developing reasonable strategies to reduce CO2 emission and enforcing sequestration are essential to mitigate global warming However, the low concentration of CO2 available in the atmosphere and from the emission sources makes it difficult to full its further applications Selective
CO2 absorption and sequestration could be considered for some scenarios as a viable choice to limit the emission of CO2
into the atmosphere Such a choice is favourable for all the parties as it has minimal interference with the prosperous fossil fuel economy, giving time for the burgeoning alternative energy sources to be optimized for easy implementation and accessibility For example, membrane separation processes requiring a lesser input of energy, only in the range of 0.04– 0.07 kWh per kg CO2, have been used in some systems, like the production of fuel gas from coal However, insufficient downstream purity and low removal rates of CO2 restrict further applications of this technique The capture and storage of CO2 fromue gas is an effective approach for the reduction of CO2 emitted to the atmosphere since the coal burning power plant is one of the largest sources of CO2 emission In light of the fact that the temperature of theue gas between the turbine and the vent is usually in the range of 625–900 K, if CO2is separated from ue gas at high temper-ature and further used as feedstock for the synthesis of fuels through optional techniques, i.e., hydrogenation or photore-duction with the assistance of photocatalysts, the efficiency and economics of the entire process of the power plant might
be improved The availability of new materials with better separation performances to enhance the efficiency of CO2
removal and hence the efficiency of power generating systems
is highly desirable In terms of the in situ removal processes like sorption, the high-temperature stability of the material is
of immense importance as well Lithium based CO2 absor-bents have attracted much attention in thiseld Many good reviews are available for CO2separation/absorption,4 –9among which Yamaguchi et al in 2009 have detailed the performance
of lithium based ceramic materials, particularly on Li2ZrO3
and silicates,5 and related membranes for high temperature
CO2 separation Strategies to enhance the performance of lithium compounds in terms of their stability and the amount and rate of CO2 absorption include improvement of the zirc-onates and investigation of new ceramic systems other than zirconates, which is out of the scope of the current review This review mainly focuses on the lithium-containing zirco-nates (LixZryOz) with different Li/Zr ratios for high-tempera-ture chemisorption of CO2 Firstly, the solid chemistry behavior and syntheses of LixZryOzare briey discussed Then their characteristics of high-temperature CO2 absorption are summarized, which is varied with their preparation condi-tions At the same time, Li2ZrO3 doping with alkali metals is also discussed At the end of the review, future directions of the resourceful use of CO2 and a perspective on the eld are given
Fig 1 E ffect of climate/carbon-cycle feedback on CO 2 increase and global
warming (a) Global-mean CO 2 concentration, and (b) global-mean and
land-mean temperature, versus year Three simulations are shown; the fully coupled
simulation with interactive CO2and dynamic vegetation (red lines), a standard
GCM climate change simulation with prescribed (IS92a) CO2 concentration
and fixed vegetation (dot-dashed lines), and the simulation which neglects
direct CO2-induced climate change (blue lines) The slight warming in the latter
is due to CO2-induced changes in stomatal conductance and vegetation
distri-bution Reproduced from ref 3 with permission Copyright ª 2000, Nature
Publishing Group.
Qin-Hui Zhang obtained his PhD degree from Tianjin University in
2002 From 2002 to 2004, he held
a postdoctoral position at Tsing-hua University, China From
2004 to 2011, he worked at the State Key Lab of Chemical Engi-neering, College of Chemical Engineering, East China Univer-sity of Science and Technology
Since 2010, he has been a professor of chemical engineering and chemistry In 2011, he joined the State Key Laboratory of Heavy Oil Processing, China University
of Petroleum His research includes energy-related applications of
nanomaterials, in particular carbon capture, photocatalysts,
adsorption/extraction of lithium from brine or seawater
Trang 32 High-temperature CO2 chemisorption on
lithium-containing zirconates
2.1 Structure and synthesis of LixZryOz
Recently, LixZryOz with various stoichiometries have attracted
interest as a class of novel in situ high-temperature CO2
sorbents These materials react with CO2 in the temperature
range 723–823 K, resembling the actual fuel reforming process,
with the advantages of high CO2 capture capacity, innite
CO2/N2 or CO2/H2 selectivity, high stability, and ease of CO2
absorption/desorption reversibility Great effort has been made
to gain insight into LixZryOz compounds with different Li/Zr
molar ratios Investigations of their electronic structures, lattice
dynamics, synthesis, and high-temperature CO2capture
prop-erties have been widely performed And also various synthetic
strategies for LixZryOzcompounds have been developed, aiming
to achieve more reactive phases with improved CO2absorption
kinetics The results have shown that the starting reagents,
calcination temperature and time are all crucial to the nal
stoichiometries and structures of the LixZryOz products In
addition, the preparation history of the samples is also closely
related to the reduced particle size, change of the composition,
and incorporation of appropriate dopants, and hence the CO2
sorption performances of LixZryOz Generally, there are three
kinds of lithium zirconates in the temperature range
investi-gated, including Li2ZrO3, Li6Zr2O7, and Li8ZrO6, progresses on
which will be summarized in the following text
2.1.1 Structure and synthesis of Li2ZrO3 There are two
phases of Li2ZrO3, namely tetragonal t-Li2ZrO3(JCPDS 20-0647,
a¼ b ¼ 9.0 ˚A, c ¼ 3.43 ˚A) and monoclinic m-Li2ZrO3(JCPDS
33-0843, a¼ 5.43 ˚A, b ¼ 9.03 ˚A, c ¼ 5.42 ˚A) t-Li2ZrO3is metastable
and will undergo phase transformation at 1173 K to stable
m-Li2ZrO3 Accordingly, the characteristics associated with
particle size, surface free energy, ionic conductivity, and the
reactivity of the materials are also varied Moreover, Li2ZrO3can
be decomposed rapidly above 1773 K, though stable below
1473 K The Li2ZrO3crystal is usually closely packed with all the
cations octahedrally coordinated (Fig 2).10 Nakagawa and
coworkers rst proposed Li2ZrO3 as a candidate for
high-temperature CO2absorption,11which absorbs and desorbs CO2
cyclically with a theoretical uptake capacity of 0.28 g per g of
acceptor Furthermore, t-Li2ZrO3exhibits better performance as
a CO2 absorbent than its monoclinic counterpart with higher stability, faster uptake rate, and higher absorption capacity Traditional synthesis of Li2ZrO3 via solid state reaction involves mechanical mixing of zirconium oxide (ZrO2) and lithium carbonate (Li2CO3) at high temperatures It is found that as Li2CO3 was substituted by LiOH, both t-Li2ZrO3 and hexa-lithium zirconate (h-Li6Zr2O7) could be synthesized through calcination at 873 and 1073 K, respectively In this process, the initial Li/Zr molar ratios should be a little greater than the chemical stoichiometric amount,12,13since lithium (Li) sublimes easily at high temperature Unfortunately, this method oen needs high energy input and it is difficult to control the sizes and phases of thenal products
Therefore, great efforts have been made in the development of so-chemistry strategies, such as sol–gel or other liquid phase methods at low temperatures using water-soluble precursors to ensure improved mixing of reagents on the molecular/atomic level.14–21For example, Nakagawa et al synthesized Li2ZrO3using a sol–gel procedure and compared its properties with powders obtained by the powder-mixing route and a commercial-grade powder.14They found that the CO2 absorption and membrane separation properties of Li2ZrO3 are closely connected to its particle size, crystal structure and agglomeration state A gelatin assisted biomimetic so solution method has been used to prepare nanoparticles of t-Li2ZrO3 containing monoclinic
m-Li6Zr2O7.19A citrate sol–gel method (or modied Pechini method) via spray drying has been developed to prepare Li2ZrO3 nano-crystals starting from zirconium oxynitrate [ZrO(NO3)2$2H2O] and lithium nitrate (LiNO3) with improved CO2capture efficiency,18at
a temperature (923 K) much lower than that required in solid state reactions In this process, heterogeneous spatial distribution of the Li and Zr elements is oen encountered, leading to incomplete reaction and thus reduced CO2absorption capacity
In addition, porous Li2ZrO3 has been prepared through aqueous reactions using an ultrasound assisted surfactant-template method from feedstocks of ZrO(NO3)2$3xH2O, lithium acetate, and cetyltrimethylammonium bromide (CTAB).22 The prepared Li2ZrO3 presented higher absorption rate, capacity, and cyclic stability than those obtained by the simple surfac-tant-template method (without sonication) and the conven-tional so-chemistry route The crystallite size and surface area
of Li2ZrO3could be controlled by the sonication time and the CTAB concentration Nanotubes of t-Li2ZrO3with high aspect-ratio can also be prepared using a hydrothermal method in LiOH solution with anodic ZrO2nanotubes as templates.23,24In comparison with bulk and nanoparticles of Li2ZrO3, the nano-tubes of Li2ZrO3 containing a small amount of ZrO2 exhibit enhanced CO2capture properties It is found that the addition
of LiOH can promote ZrO2dissolution and result in the Li2ZrO3 formation on the surfaces of the nanotubes Namely, the alka-linity of the solution plays an important role in guiding the growth of the Li2ZrO3nanotubes.24
2.1.2 Structure and synthesis of Li6Zr2O7 Similar to
Li2ZrO3, two phases of Li6Zr2O7exist, thermodynamically stable monoclinic m-Li6Zr2O7(JCPDS 34-0312, a¼ 10.45 ˚A, b ¼ 5.99 ˚A,
c¼ 10.20 ˚A) and meta-stable triclinic tri-Li6Zr2O7(a¼ 6.0153 ˚A,
Fig 2 Snapshots of m-Li 2 ZrO 3 and Na 2 ZrO 3 structures The spheres represent,
from the brightest to the darkest, the alkaline element (Li or Na), oxygen, and
zirconium atoms, respectively Reproduced from ref 10 with permission
Copy-right ª 2006, American Chemical Society.
Trang 4b¼ 9.1941 ˚A, c ¼ 5.3112 ˚A).25Tri-Li6Zr2O7transforms slowly into
m-Li6Zr2O7and is rapidly sintered at 1223 K.13,26,27Interestingly,
this phase transformation is associated with a
volume-increasing process due to the larger cell parameters of the
monoclinic phase The structures of both Li2ZrO3and Li6Zr2O7
are of the NaCl type,26,28although the distributions of cations
are different in these two phases A more open layer structure
than that of Li2ZrO3has been suggested for Li6Zr2O7(Fig 3),26
where the Zr layer locates along the (111) direction and each Zr
atom is coordinated with six O atoms Consequently, both Li+
and O2 ions have higher mobility, rendering Li6Zr2O7 with
good ionic conductivity
Reactions between Zr(OH)4and LiNO3would produce
phase-pure m-Li6Zr2O7 under appropriate conditions However,
acquiring m-Li6Zr2O7with high purity through solid reactions of
Li2CO3 and ZrO2 is not easy, since decomposition of Li2CO3
occurs under the reaction conditions,12,13which would release
CO2 and further react with Li6Zr2O7 to form Li2ZrO3 On the
other hand, thermal analyses of Li6Zr2O7showed continuous
decomposition behaviour owing to the sublimation of
lithium.29Consequently, the poor thermal stability of Li6Zr2O7
makes it suffer from arduous regeneration.27
Theoretically, phase pure high-lithia zirconates of LixZryOz, i.e
Li6Zr2O7and Li8ZrO6, can be produced if the Li/Zr ratio is high
enough However, few reports concern the structures and
syntheses of these two compounds because of the rigorous
conditions required, such as calcination under ultra-high
vacuum, high temperature, and with prolonged heating time The
substitution of the lithium source of Li2CO3with LiOH$H2O could
produce m-Li6Zr2O7, tri-Li6Zr2O7, and their mixtures with Li2ZrO3
under suitable conditions.13,29Further replacement of ZrO2with
Zr(NO3)4$5H2O leads to the production of pure m-Li6Zr2O7in a
remarkably shortened recrystallization time from 96 h to 24 h.13
However, the zirconium sources of ZrOCl2$8H2O are
accompa-nied with a greater lithium loss through LiCl volatilization It is
noted that Li6Zr2O7exhibits smaller surface area than t-Li2ZrO3
synthesized with similar parameters,27since serious
agglomera-tion happened for Li6Zr2O7when calcined with higher
tempera-ture, longer time, and larger initial Li/Zr molar ratios
2.1.3 Structure and synthesis of Li8ZrO6 Few
investiga-tions have been performed on the detailed structure and CO2
absorption properties of Li8ZrO6because of the difficulty in its preparation, conrmed both experimentally and theoretically through calculations of the standard Gibbs free energies of relevant reactions.30
Traditionally, Li8ZrO6was prepared via solid state reactions between ZrO2 (or Li6Zr2O7) and Li2O (or Li2O2).13,25,30 Zou and Petric calculated the thermodynamic data and proposed the temperature range of 773–1373 K for the synthesis of pure Li8ZrO6
in air.30They prepared Li8ZrO6 from ZrO2 and Li2O2 through complicated heating procedures Wyers and Cordfunke synthe-sized Li8ZrO6 through reactions between ZrO2 and Li2O in a vacuum.25
It is reported that rhombohedral r-Li8ZrO6(JCPDS 26-0867,
a¼ 5.48 ˚A, c ¼ 15.45 ˚A) would decompose slowly into Li6Zr2O7
and Li2O upon prolonged heating above 1073 K.30A mixture of
Li6Zr2O7 and Li8ZrO6 was observed aer the calcination of
Li2CO3and ZrO2,13,29and also of LiOH and Zr(NO3)4$5H2O The authors' group has synthesized r-Li8ZrO6 coexisting with a fraction of Li6Zr2O7via coupling a liquid-phase coprecipitation and calcination at 1223 K for 72 h, using LiOH$H2O, NH3, and Zr(NO3)4$H2O as feedstocks.12 Furthermore, we synthesized pure Li8ZrO6with a liquid mixture of the reagents followed by a simple three step calcination.31 The choice of LiNO3 as the lithium source with the advantages of safety, availability, and low melting point (873 K), is the key to successful production of phase-pure Li8ZrO6 The formation of homogeneous mixture of
Li+and Zr4+is favoured by LiNO3 For Li2CO3, generation of CO2
from its decomposition interfere Li8ZrO6preparation through a carbonate reaction just as the conditions of Li6Zr2O7 Unfortu-nately, the side effects in high-temperature processes also occur for the preparation of Li8ZrO6,12,13,30i.e., lithium loss, serious particle aggregation, large particle size, low surface area, and thus poor CO2absorption properties
2.2 High-temperature CO2chemisorption on LixZryOz The high-temperature stability and CO2 capture–regeneration (absorption–desorption) kinetics of LixZryOz compounds with diverse stoichiometries have attracted much attention, partic-ularly for Li2ZrO3, Li6Zr2O7, and Li8ZrO6 The CO2 absorption originates from the reaction between CO2 molecules and Li+ ions derived from LixZryOz Hence, the reactivity and absorption capacity of LixZryOztowards CO2are greatly dependent on the diffusion and amount of Li+ions Theoretically, Li6Zr2O7and
Li8ZrO6 might absorb more CO2 than Li2ZrO3 under similar conditions, since the Li/Zr ratio of Li6Zr2O7and Li8ZrO6is 1.5 and 4.0 times higher than that of Li2ZrO3, respectively This conclusion has been proved through simulation of CO2
absorption kinetics under various CO2 partial pressures following a double-exponential model as shown in eqn (1),22,27,32–34where y is the weight percentage of CO2absorbed, t
is the absorption time, kCO2is the rate constant for CO2 diffu-sion on the surfaces of the particles, kLiis the rate constant of
Li+diffusion from the core to the reaction interface, and A, B, and C are pre-exponential factors
y ¼ Aexpk CO2 t+ Bexpk Li t+ C (1)
Fig 3 The crystal structures of Li 6 ZrO 7 The biggest ball represents Zr, the
smallest O c axis is vertical Reproduced from ref 26 with permission Copyright ª
2011, American Institute of Physics.
Trang 5It is desired that the CO2absorbents at high temperature
have high selectivity and absorption capacity, as well as good
absorption–regeneration kinetics Theoretically, the absorption
performance of the absorbents is determined mainly by their
internal structures Furthermore, the sorbents should possess
high stability during recycling That is, the absorbents should
not only easily absorb CO2 in therst half cycle, but also to
release CO2 in the second half cycle Usually, the absorbents
with accelerated rate of gas diffusion and CO2chemisorption
have the characteristics of decreased particle sizes and enlarged
surface areas, facilitating the access of CO2molecules into the
internal layers of active sites On the other hand, the CO2uptake
capacity and kinetics of LixZryOz depend strongly on the
temperature and CO2partial pressure or concentration of the
atmosphere, which could be judged by Le Chatelier's principle
of equilibrium
2.2.1 High-temperature CO2 chemisorption on Li2ZrO3
The CO2absorption–desorption process for Li2ZrO3is ascribed
to a reaction model whereby lithium oxide in the Li2ZrO3
structure reacts reversibly with CO2 As demonstrated by eqn
(2), the stoichiometric capacity of CO2absorption for Li2ZrO3is
up to 28.7 wt%, since 1 mol Li2ZrO3compensates 1 mol CO2
Apart from the selective absorption capability irrespective of the
gas species other than CO2, Li2ZrO3has the advantages of
high-temperature stability over other known CO2absorbers, coupled
with a low morphologic/volumetric change no more than
134%.11,14Moreover, Ochoa-Fern´andez et al demonstrated that
t-Li2ZrO3 has superior CO2 capture–regeneration performance
to that of m-Li2ZrO3,35i.e., a faster uptake rate and a higher
absorption capacity around 26 wt%, which is about 90% of its
stoichiometric capacity (28.7 wt%)
Li2ZrO3(s) + CO2(g) ! ZrO2(s) + Li2CO3(s) (2)
Selective removal of CO2from gas mixtures according to eqn
(2) oen works at a specic temperature, because the absorption
process takes place normally in the temperature range of 723–
923 K,5,11,16,34,36 meanwhile, the regeneration reaction initiated
above 923 K Ida et al proposed a double shell model to describe
the mechanism of the CO2absorption/desorption process on
both Li2ZrO3 and potassium (K) modied Li2ZrO3.4,34,36 The
forward direction of eqn (2) would be accelerated through a
carbonation mechanism,11–14,22,32,34,36–38with the formation of a
solid Li2CO3–ZrO2 layer surrounding an unreacted core of
Li2ZrO3(Fig 4) Then the formation and growth of the external
shell limits the diffusion of gases and ions (Li+and O2), leading
to a decreased rate of CO2 sorption Desorption of CO2takes
place accompanied by the regeneration of Li2ZrO3through the
reverse process of eqn (2),26,39,40favoured by the generation of
Li2O through Li2CO3decomposition at the operating
tempera-ture Fig 5 gives a general scheme for the possible external
Li2CO3shells with different compositions (phase pure Li2CO3,
coexist with metal oxide or other lithium secondary phases of
different lithium diffusion capacities) However, this model is
applied only for the case where Li2CO3is in a solid state.41
It has been reported that small particle sizes are preferred for
a faster rate of CO2 uptake on Li2ZrO3 particles,42while large
particle sizes and serious aggregations restrict the migration of gases and ions The smaller particle size always means a thinner
Li2CO3–ZrO2 shell/layer and a shorter diffusion distance, showing that the CO2absorption is diffusion controlled.36,37For example, porous Li2ZrO3with signicantly reduced aggregation exhibited the maximum capacity of 22 wt% at 100% CO2
compared to 15.2 wt% for conventional aggregated samples.22
The temperature effect is observed with a faster absorption rate at a lower temperature for solid CO2 absorbents This effect is closely related to the size effect mentioned above,14,22as high-temperature oen leads to aggregation and sintering of
Fig 4 Schematic illustration of carbonation mechanism on (A and B) pure and (C and D) potassium modi fied Li 2 ZrO3 Reproduced from ref 36 with permission Copyright ª 2003, American Chemical Society.
Fig 5 Scheme of the lithium di ffusion processes controlled by different possible external shell compositions (A) Lithium di ffusion controlled exclusively by Li 2 CO 3
in solid state; (B) lithium di ffusion controlled by Li 2 CO 3 , but limited by the metal oxide presence; (C) lithium di ffusion controlled by Li 2 CO 3 , which is reduce by the presence of other lithium secondary phase with a smaller lithium di ffusion capacity; (D) lithium di ffusion controlled by Li 2 CO 3 , which is enhanced, at a determined temperature, by the presence of the other lithium secondary phase with a larger lithium di ffusion capacity Reproduced from ref 41 with permission Copyright ª 2011, Akad´emiai Kiad´o, Budapest, Hungary.
Trang 6adjacent particles Similarly, the synthetic temperature also
affects the structure and the morphology of Li2ZrO3,43and thus
its CO2absorption ability In addition, sublimation of lithium
as Li2O occurs through decomposition of Li2CO3 when the
temperature increased to 1173 K as eqn (3),39resulting in an
incomplete precursor reaction and loss of absorption capacity
aer cyclic operations On the other hand, Li2O migrate to the
particle surfaces and absorb CO2 efficiently through reverse
reaction of eqn (3),39,40,44since this process needs very low CO2
pressure and/or very high temperature For ZrO2, it might act as
a dispersant and introduce more reactive boundaries, the
presence of which in the lithium outer shell will decrease the
rate of lithium diffusion to the surface which controls the rate of
absorption.41Therefore, the ratio of Li2O to ZrO2considerably
inuences not only the CO2 capture rate, but also the CO2
capture capacity of Li2ZrO3.21 Moreover, it is found that the
absorption capacity of Li2ZrO3prepared from tetragonal ZrO2
(t-ZrO2, metastable) (25 0.6 wt%) is higher than that produced
from monoclinic ZrO2(m-ZrO2, stable) (only 9 0.6 wt%) under
the same sorption conditions (at 773 K for 3 h, atmosphere of
20% CO2and 80% air).45
Li2CO3(s) ! Li2O (s) + CO2(g) (3) The main obstacle for practical application of Li2ZrO3as a
CO2absorbent is the kinetic limitation.4Many efforts have been
made to improve its performance for CO2capture/release Duan
and coworkers studied the CO2capture capabilities of various
alkali metal zirconates by calculating the chemical potential
changeDm (T, P) for the capture reactions under various CO2
pressures and temperatures.26,39,46 A kinetic equation for the
sorption on Li2ZrO3as a function of CO2partial pressure and
temperature has been acquired.47As shown in Fig 6, the curve of
Dm ¼ 0 means the equilibrium between the absorption and
desorption process at given temperatures and pressures Above
the curve (Dm < 0), the forward reactions are favourable for both
Li2ZrO3and Li6Zr2O7to absorb CO2and form Li2CO3 Below the
curve (Dm > 0), the reversed reaction to release CO2is favourable,
indicating decomposition of Li2CO3to release CO2and
regen-eration of the sorbents Consequently, Li2ZrO3can absorb CO2
over a wide range of CO2pressures (1025to 102atm) below 700 K
The reverse reaction to release CO2happens by increasing the
temperature over a CO2pressure range of 1025to 101atm.14
Practically, the CO2absorption rate and capacity of Li2ZrO3
are also conrmed to be strongly dependent on the working
temperature and CO2concentrations.17,27A higher percentage of
CO2in the atmosphere is benecial to the CO2absorption by
Li2ZrO3,43while low CO2partial pressure (<0.1 bar) close to its
equilibrium partial pressure limits the external gas and ion
diffusion to the surfaces of the absorbent, leading to a
decreased absorption rate.22
Additionally, the LixZryOzcompounds tend to be hydrolysed
or attacked by water vapour in wet air conditions.30
Ochoa-Fern´andez et al explored the effects of steam addition on the
performance of the CO2acceptors.35They found that the
pres-ence of water could enhance the CO2capture–regeneration rate
because of the high mobility of the alkaline ions However, there
is a large decay of the capacity of the absorbents compared to dry conditions, due to sintering of the particles, vaporization of alkali metals, and phase segregation
2.2.2 High-temperature CO2 chemisorption on Li6Zr2O7 Theoretically, the LixZryOz compounds with richer lithium content would exhibit a larger capacity for CO2 absorption than Li2ZrO3.12,29In fact, both tri/m-Li6Zr2O7show smaller CO2
absorption capacity compared with t-Li2ZrO3.12,26Results from many researchers indicated that Li6Zr2O7 can be fully con-verted into ZrO2 and Li2CO3 only in the rst cycle of CO2
capture through eqn (4),12,26,29 with the maximum theoretical capacity of 39.28 wt% Aer CO2 desorption, Li2ZrO3 rather than Li6Zr2O7 would be regenerated as eqn (5) shows, when the temperature is not high enough to regenerate Li6Zr2O7 In the following cycles, Li2ZrO3 absorbs/desorbs CO2 following eqn (2) Consequently, the CO2absorption capacity of Li6Zr2O7 through eqn (5) gains only 13 wt%.29 Although the capacity reduced gradually, multi-cycle tests demonstrate that
m-Li6Zr2O7in low CO2 concentration stream (10% CO2 stream) exhibits fast CO2 uptake and release rates.27 There are also reports that the absorbent is composed of tri-Li6Zr2O7instead
of m-Li6Zr2O7 aer the rst cycle,27 whereas the desorption temperature is too low for the phase transformation of
Li6Zr2O7from triclinic to monoclinic For tri-Li6Zr2O7, most of the absorbed CO2 could be released, while no obvious desorption was observed for m-Li6Zr2O7, even at high temper-ature The regeneration of Li6Zr2O7could be realized through eqn (6) between Li2O and Li2ZrO3.12,29
In other words, the capture behaviour of both Li6Zr2O7and
Li2ZrO3 is similar aer the rst cycle of the CO2 sorption/ desorption process The weak cycle stability and gradually reduced CO2 capacity of Li6Zr2O7 thus become the disadvan-tages to use Li6Zr2O7 over Li2ZrO3 as CO2 sorbents.12 These conclusions are in agreement with Pfeiffer's work that Li6Zr2O7
absorbed four times the amount of CO2with faster sorption rate
Fig 6 The contour plotting of calculated chemical potentials vs CO 2 pressures and temperatures of the reactions Y-axis plotted in logarithm scale Only Dm ¼
0 curve is shown explicitly For each reaction, above its Dm ¼ 0 curve, their Dm < 0, which means the lithium zirconates absorb CO2and the reaction goes forward, whereas below the Dm ¼ 0 curve, their Dm > 0, which means the CO 2 starts to release and the reaction goes backward to regenerate the sorbents Reproduced from ref 26 with permission Copyright ª 2011, American Institute of Physics.
Trang 7than Li2ZrO3in short times, but they became similar aer long
reaction times.29
Li6Zr2O7(s) + 3CO2(g) ! 3Li2CO3(s) + 2ZrO2(s) (4)
Li6Zr2O7(s) + CO2(g) ! Li2CO3(s) + 2Li2ZrO3(s) (5)
Li2O (s) + 2Li2ZrO3(s) ! Li6Zr2O7(s) (6)
Li6Zr2O7(s) + 2CO2(g) ! 2Li2CO3(s) + ZrO2+ Li2ZrO3(s)(7)
The possible pathway for CO2absorption–desorption on tri/
m-Li6Zr2O7at high temperature has been suggested as a series
of reactions as shown by eqn (4)–(7),12,26,27,29 with the nal
products of ZrO2, Li2ZrO3, and Li2CO3 There are two different
sections during the multi-cycle processes on tri/m-Li6Zr2O7 at
the operating temperatures, including the absorption process
and desorption process with similar diffusion behaviours of
CO2 and ions (Li+ and O2) (Fig 7),27 corresponding to a
carbonation and decarbonation mechanism similar to that of
Li2ZrO3(Fig 8).29The absorption of CO2on Li6Zr2O7happens
when lithium from Li6Zr2O7 structures reacts with CO2 to
produce a Li2CO3shell,rstly on the surfaces of the particles, as
shown in eqn (4) Then Li2CO3further decomposes into CO2
and Li2O through eqn (3) when the temperature is higher than
973 K, reacting continuously with ZrO2 to form an internal
Li2ZrO3 core, and the other parts sublimate as Li2O (g) The
desorption process to release CO2happens with the
regenera-tion of tri-Li6Zr2O7 through diffusion of lithium and CO2 in
opposite ways Moreover, the activation energies for the
diffu-sion of CO2and Li+in m-Li6Zr2O7was estimated to be 22.684
and 56.084 kJ mol1under 10% CO2atmosphere,27respectively
These data prove that the diffusion of lithium is a dominating
step in the process of whole CO2absorption.41
The authors' group also studied the effect of temperatures and
CO2 partial pressures on CO2 absorption for m-Li6Zr2O7.27The results indicated that about 86.7% of the capacity is preserved for m-Li6Zr2O7at 1023 K as the CO2partial pressure decreases from 1.0 to 0.1 bar Calculations of the chemical potentials versus CO2 pressures and temperatures for the CO2 capture reaction on
Li6Zr2O7 are shown in Fig 6.26 It is proved that the optimal temperature for CO2absorption on Li6Zr2O7is around 823 K.29
Higher temperature would cause loss of lithium and reduced capacity of CO2absorption during the multi-cycle processes.12,30
Kinetic studies ensure that there are no mass transfer limi-tations for CO2absorption So control over the gasow rate with
a minimum value is essential The CO2absorption rate for
m-Li6Zr2O7at 1023 K increases obviously with the gasow rate switched from 50 to 100 ml min1.27But upon further switching the ow rate up to 150 ml min1, no more increase can be observed
2.2.3 High-temperature CO2 chemisorption on Li8ZrO6 Higher capacity of CO2absorption is expected for rhombohe-dral r-Li8ZrO6 because of its higher lithium content than
Li2ZrO3and Li6Zr2O7 However, reports about the CO2 absorp-tion properties of Li8ZrO6are few because of the difficulty in obtaining pure Li8ZrO6 The CO2absorption capacity of Li8ZrO6
could be well maintained within a wide range of CO2partial pressures,31which is very different from that of Li2ZrO3 The temperature effect on CO2sorption is dependent on both kinetic and thermodynamic factors.34 Investigations of the temperature effect indicated that both Li6Zr2O7 and Li8ZrO6
showed slow CO2uptake rates below 973 K.12,31The limitation of ion (e.g Li+and O2) migration and CO2diffusion at low temper-atures are ascribed to the formation of the same solid carbonate shell and aggregation of the particles as described for Li6Zr2O7. Similar to Li6Zr2O7, as the operating temperature was enhanced above the melting point of Li2CO3(983 K),31,36the CO2uptake rates would be dramatically increased for Li8ZrO6 with a capacity of
52 wt%, resulting from the facile diffusion of CO2and ions Thermal stability tests demonstrated that Li8ZrO6 exhibited gradually reduced uptake capacity during the multi-cycle processes of CO2 absorption–desorption,12 due to the
Fig 7 Schematic illustration of CO 2 absorption ( #1073 K) and desorption
( $1123 K) on Li 6 Zr 2 O 7 Section (A) is the first cycle processes including the
absorption process from (a) to (c) in CO 2 flow, and desorption process from (d) to
(f) in N 2 flow, for monoclinic phase Li 6 Zr 2 O 7 ; section (B) is the following
multi-cycle process for triclinic phase Li 6 Zr 2 O 7 with the absorption process from (g) to (i)
in CO 2 flow and the regeneration process from (d) to (f) in N 2 flow Reproduced
from ref 27 with permission Copyright ª 2010, American Chemical Society.
Fig 8 Schematic illustration of the carbonation and decarbonation mechanisms
of Li 6 Zr 2 O 7 at high temperatures Reproduced from ref 29 with permission Copyright ª 2005, American Chemical Society.
Trang 8volatilization of Li2O at high temperature Fig 9 illustrates the
schematic processes of CO2 absorption on Li8ZrO6 at 1023 K
and desorption at 1173 K.12,31 Aer the rst cycle of CO2
absorption, Li8ZrO6decomposes as proposed in equations (8)
and (9), producing more Li2CO3 than Li6Zr2O7 and Li2ZrO3
Then the CO2absorption proceeds following the same pathway
as that for Li6Zr2O7 and Li2ZrO3, with a theoretical CO2
absorption capacity of 54.4 wt%
2Li8ZrO6+ 5CO2! 5Li2CO3+ Li6Zr2O7 (8)
Li8ZrO6+ 3CO2! 3Li2CO3+ Li2ZrO3 (9)
2.2.4 High-temperature CO2 chemisorption on Li2ZrO3
doped with other alkali metals Recently, Li2ZrO3 sorbents
doped with other alkali metals (Na and/or K) have attracted
much interest, with a noticeably improved CO2uptake rate and
sorption capacity than the unmodied counterparts As far as
we know, there are no reports about doped Li6Zr2O7and Li8ZrO6
involved in CO2chemisorption
Na2ZrO3and Na promoted absorbents are good candidates
for increased CO2capture Duan reported that the structure of
Na2ZrO3is isotypic to that of Li2TiO3,46and both of them are
different from that of K2ZrO3 The CO2capture performance of
Na2ZrO3has been proved to be greater than that of Li2ZrO3,48–50
though they have a similar enthalpy of reaction.46Furthermore,
the introduction or doping of alkaline elements into Li2ZrO3
will change the melting points of the system apparently and
produce a liquid eutectic mixed-salt molten shell (Fig 4 and 10)
on the outer surfaces of Li2ZrO3,14,16,21,36,51 hence signicantly
improving the diffusion rate of CO2on the samples The molten
carbonate shell allows diffusion and sorption of CO2, which
changes the viscoelastic properties of the sorbent and
deter-mines the effectiveness of the sorbent for CO2uptake
Never-theless, particle coarsening leads to low capacities and poor
stability of the absorbent Nakagawa et al pointed out that the
amount of molten alkali carbonates may improve the CO2
diffusion rate in the carbonate shell and enhance the CO2
uptake rate compared with that happened in solid carbonate
shell.14,52 The rheological properties of pure Li2ZrO3 and K-doped Li2ZrO3under CO2 atmosphere have been studied to investigate the inuence of the molten carbonate in the sorbent mixture on the CO2sorption.51,52Pfeiffer et al found that the
Li2xNaxZrO3solid solutionsrstly chemisorbed CO2through the formation of a carbonate shell on the surface of the particles
at low temperatures (473–573 K).53Then lithium and/or sodium atoms diffuse from the core to the surfaces of the particles through the external carbonate shell when the temperature reached 673 K or higher
A compromise between the kinetic enhancement and thermal stability should be considered to develop efficient CO2
absorbents promoted with elements of Na and/or K Therefore,
a number of binary and ternary eutectic salt-modied Li2ZrO3
sorbents through addition of NaF in combination with K2CO3
and Na2CO3have been identied and evaluated as CO2 absor-bents at temperatures between 723 and 973 K,10,14,45,52–54with enhanced chemisorption capacity and diffusion kinetics Theoretically, Duan calculated the reaction heats and the rela-tionships ofDm (T, P) versus temperatures and CO2pressures of the M2ZrO3(M¼ K, Na, Li) system as shown in Fig 11 and eqn (10).46Above the line ofDm < 0, M2ZrO3(M¼ K, Na, Li) tends to absorb CO2 to form M2CO3, while below the line of Dm > 0,
M2CO3 is easy to decompose to release CO2 and regenerate
M2ZrO3 again The molar ratios of Li to alkali metals (Na and/or K) are vital in determining the absorption/regeneration features of CO2at high temperature
M2ZrO3(M ¼ K, Na, Li) + CO2! ZrO2+ M2CO3 (10) The atomic radii of lithium and sodium are 2.05 and 2.23 ˚A, respectively The solubility limits of sodium into Li2ZrO3and lithium into Na2ZrO3 are different,10with smaller and lighter lithium atoms diffusing more easily into the Na2ZrO3 lattice than the sodium atoms into the Li2ZrO3network (Fig 2) As a consequence, the maximum amount of sodium in Li2xNaxZrO3
is 0.2, and the amount of lithium in the lattice of Na2xLixZrO3
is 0.6 Li2xNaxZrO3(0# x # 2) enclosed Na2xLixZrO3as in a cherry model has been prepared by a precipitation method.10It was found that Li2(1x)Na2xZrO3with x¼ 0.02 possesses the best performance with a CO2absorption capacity of 25 0.4 wt% in
Fig 9 Schematic illustration of CO 2 absorption (at 1023 K) and desorption
(at 1173 K) on Li 8 ZrO 6 Steps (A) –(C) correspond to the first cycle of CO 2
absorption –desorption, and steps (D)–(F) correspond to the second cycle of CO 2
absorption –desorption Reproduced from ref 31 with permission Copyright ª
2011, American Chemical Society.
Fig 10 Phase diagram of the Li 2 CO 3 –K 2 CO 3 binary systems Reproduced from ref 21 with permission Copyright ª 2008, American Chemical Society.
Trang 9an atmosphere of CO2(20 wt%) and air (80 wt%) at 773 K within
3 h,55losing only 0.9% of its CO2-absorption capacity aer 10
cycles of absorption–desorption
Although it has a similar structure, the CO2capture
perfor-mance of K2ZrO3is inferior to that of Na2ZrO3and Li2ZrO3,46
because a high regeneration temperature is required However,
resembling the cases of Na-doped Li2ZrO3, Xiong and Ida et al
reported the synthesis of K-doped Li2ZrO3 with remarkably
enhanced CO2 sorption kinetics compared to Li2ZrO3
alone.34,36,37,56Solid solutions of Li2xKxZrO3(0# x # 2) have
been prepared by a coprecipitation method and tested as CO2
captors,54with the solubility limit of potassium in Li2ZrO3about
x¼ 0.2 Li2xKxZrO3hasve times the CO2absorption rate of
Li2ZrO3 alone in short times.54 On the other hand, the CO2
absorption rate for Li2ZrO3synthesized from m-ZrO2is evidently
slower and impacted more obviously by K doping than that
from t-ZrO2.45
Size effects also take place here, since the CO2sorption rate
on K-doped Li2ZrO3increases with decreasing sizes of the CO2
sorbents.34The K-doped Li2ZrO3synthesized via a citrate route
have been investigated at different temperatures and CO2
partial pressures,56 which possess better CO2 capture
perfor-mance especially at low CO2partial pressures However, kinetic
analyses demonstrated that the CO2absorption of Li2xKxZrO3
was similar to that of Li2ZrO3 aer long times,46 due to the
diffusion of lithium and potassium through the external
carbonate shell of Li2CO3and K2CO3
It was also found that increase of the amount of doping
elements can enhance the CO2 uptake rate, associating with
decreased CO2 absorption capacity of Li2ZrO3 For example,
higher potassium concentrations lead to the formation of a new
phase of Li2.27K1.19Zr2.16O6.05,54or even different phases of ZrO2
In the structure of Li2.27K1.19Zr2.16O6.05, the lithium atoms are
located on different positions, caused by the coulombic
repul-sion energies associated with the proximity of lithium and
potassium atoms While some lithium atoms are
hexa-coordinated with the LiO6polyhedra distorted along the c-axis, other lithium atoms are lonely pentacoordinated without distortion to the LiO5polyhedra
Design and synthesis of novel CO2 absorbents represents a rapidly expanding research area with respect to environmental protection and resourceful application of CO2 This review briey summarized recent developments, with special attention focused on the synthesis and modication of lithium contain-ing zirconates LixZryOzand their applications for CO2sorption Currently, the main challenges of most of the absorbents, including LixZryOz, are their insufficient absorption capacity, kinetics, and stability The development of in situ high temperature precise measurement of weight variation of absorbents before and aer absorption of CO2and the deter-mination of internal correlation between the absorbent struc-tures and their performance will provide a rational strategy to explore efficient absorbents LixZryOz reacts with CO2 and releases CO2following reverse reactions Analyses conrm that high lithium containing LixZryOz does not regenerate easily aer repeated CO2capture–regeneration cycles, since it suffers severely from textural degradation and gradually reduced uptake capacity because of the volatilization of Li2O Therefore, the long-term stability of the sorbents during the multi-cycle processes is a limiting issue affecting continuous CO2 separa-tion and practical applicasepara-tions
The CO2capture performance of LixZryOzcould be tuned via controlling the synthetic parameters or introduction of external elements with the dual effects of phase stabilization and increase in conductivity Meanwhile, partial substitution of Li
by doping elements within the LixZryOzstructures would form new structures to enhance the reactivity of Li+ with CO2, and consequently producing faster CO2 absorption kinetics with respect to non-doped LixZryOz More detailed studies on the synthesis of new phases as well as the determination of the doping effects on the structures and CO2capture properties of these materials are necessary in the future
Furthermore, possible sorption pathways and mechanisms would provide guidelines for further research in the search for more efficient high-temperature CO2absorbents The effect of temperature and pressure on the sorption performances is complicated, depending on both thermodynamic and kinetic factors It is desired to reach an optimized balance between the absorption thermodynamics and regeneration kinetics Conse-quently, special attention should be paid to the structures of the active sites and the structural evolutions of these materials during the CO2sorption and desorption, which are not yet clear and signicant for the process
However, to date, most of the research has been performed
at laboratory scale For realistic CO2capture operations, there are a lot of complexities For example, both steam and other gases such as SO2will be present in theue gas.5,57On the other hand, although kinetics would be enhanced, smaller particle size could always provoke important problems in industrial scale operations, such as pressure drop due to particle
Fig 11 The contour plotting of calculated chemical potentials versus CO 2
pressures and temperatures of the reactions of M2ZrO3(M ¼ K, Na, Li) capturing
CO2 Y-axis plotted in logarithm scale Only Dm ¼ 0 curve (van't Hoff relation) is
shown explicitly For each reaction, above its Dm ¼ 0 curve, their Dm < 0, which
means the alkali metal zirconates absorb CO2and the reaction goes forward,
whereas below the Dm ¼ 0 curve, their Dm > 0, which means the CO 2 starts to
release and the reaction goes backward to regenerate the sorbents Reproduced
from ref 46 with permission Copyright ª 2012, American Institute of Physics.
Trang 10agglomerations So, the complexity of real system is of
signi-cance and in the next few years, scalable and controllable
sample preparation should be paid more attentions Careful
investigations should be made to explore the effect of real
conditions with as many of the factors present as possible on
CO2capture efficiency in both laboratory and batch scale, such
as the coexisting species of water vapor, etc where caution
should be exercised to evaluate the sorbent performance before
drawing conclusions
In summary, the exciting applications of CO2 sorbent
materials with high capacity and good recycling stabilities are
strongly desired To make them available in the near future to
address the pressing need for CO2xation, particularly at low
CO2partial pressures, more systematic efforts should be made
both fundamentally and practically
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