Microwave-assisted homogeneous precipitation of hydrotalcites by urea hydrolysis

Article abstract of DOI:10.1021/ic7023023

The use of urea as a precipitating agent in the synthesis of Ni-Al and Zn-Al layered double hydroxides having a hydrotalcite-like structure via a microwave-hydrothermal method is reported. For comparison purposes, the samples were also prepared by a conventional hydrothermal method. Ni-Al compounds with the hydrotalcite-like structure were obtained in shorter periods of time by the microwave method than by the conventional method, whereas when zinc cations were involved, no successful synthesis was achieved regardless of the method used. In order to find the best synthesis conditions for the Ni-Al solids, samples were submitted to microwave-hydrothermal treatment at different temperatures for increasing periods of time, and the structural, thermal, and textural properties of the synthesized materials were evaluated. All of the solids were fully characterized by chemical elemental analysis, powder X-ray diffraction (PXRD), FT-IR spectroscopy, and transmission and scanning electron microscopy as well as by N₂ adsorption/desorption at -196°C for assessment of specific surface area and porosity. The PXRD patterns showed that the layered structure appeared after merely 10 min when the synthesis was carried out at 125°C; however, the FT-IR spectra showed the presence of some cyanate groups that were formed during urea hydrolysis and were quite difficult to remove completely. When the conventional hydrothermal treatment was used, longer periods of time were required in order to develop the hydrotalcite-like structure, but increasing the aging time improved the crystallinity of the compounds and yielded large particles.

Full text of DOI:10.1021/ic7023023

Inorg. Chem. 2008, 47, 5453-5463
Microwave-Assisted Homogeneous Precipitation of Hydrotalcites by
Urea Hydrolysis
P. Benito,†,§ M. Herrero,† C. Barriga,‡ F. M. Labajos,† and V. Rives*,†
GIR-QUESCAT, Departamento de Qu´ımica Inorga´nica, UniVersidad de Salamanca,
37008 Salamanca, Spain, and Departamento de Ingenier´ıa Qu´ımica y Qu´ımica Inorga´nica,
UniVersidad de Co´rdoba, 14071 Co´rdoba, Spain
Received November 22, 2007
The use of urea as a precipitating agent in the synthesis of Ni-Al and Zn-Al layered double hydroxides having
a hydrotalcite-like structure via a microwave-hydrothermal method is reported. For comparison purposes, the
samples were also prepared by a conventional hydrothermal method. Ni-Al compounds with the hydrotalcite-like
structure were obtained in shorter periods of time by the microwave method than by the conventional method,
whereas when zinc cations were involved, no successful synthesis was achieved regardless of the method used.
In order to find the best synthesis conditions for the Ni-Al solids, samples were submitted to microwave-hydrothermal
treatment at different temperatures for increasing periods of time, and the structural, thermal, and textural properties
of the synthesized materials were evaluated. All of the solids were fully characterized by chemical elemental analysis,
powder X-ray diffraction (PXRD), FT-IR spectroscopy, and transmission and scanning electron microscopy as well
as by N2 adsorption/desorption at -196 °C for assessment of specific surface area and porosity. The PXRD
patterns showed that the layered structure appeared after merely 10 min when the synthesis was carried out at
125 °C; however, the FT-IR spectra showed the presence of some cyanate groups that were formed during urea
hydrolysis and were quite difficult to remove completely. When the conventional hydrothermal treatment was used,
longer periods of time were required in order to develop the hydrotalcite-like structure, but increasing the aging
time improved the crystallinity of the compounds and yielded large particles.
1. Introduction
easily synthesized by coprecipitation,3 reproducible, fast, and
inexpensive synthesis procedures are still needed in order
to control some of the properties, such as crystallinity,
particle-size distribution, and specific surface area. In this
sense, homogeneous precipitation in the presence of urea
leads to solids of high quality in terms of morphology, size
uniformity, and crystallinity, which are much better than
those of solids prepared by the conventional coprecipitation
method.4 The high crystallinity of these solids has led to
the possibility of determining the cationic order by AFM
microscopy5,6 and performing some Rietveld7,8 and Diffax8
studies.
Layered double hydroxides (LDHs) or hydrotalcite-like
compounds (HTlcs) are layered materials consisting of
positively charged layers having the brucite-like structure,
in which divalent cations have been partially replaced by
trivalent ones; solids with monovalent and tetravalent cations
have been also prepared. In order to balance the excess
positive charge, anions coexisting with water molecules are
located in the interlayer region.1,2 Although these solids are
* To whom correspondence should be addressed. E-mail: vrives@usal.es.
In the homogeneous precipitation process during urea
hydrolysis, massive supersaturation is avoided by slow in
situ generation of the basic medium via thermal decomposi-
Tel: +34 923 29 44 89. Fax: + 34 923 29 45 74.
†
Universidad de Salamanca.
Universidad de Co´rdoba.
Present address: Dip. Chimica Industriale e dei Materiali, University
‡
§
of Bologna, Viale Risorgimento 4, 40136 Bologna, Italy.
(1) Rives, V. In Layered Double Hydroxides: Present and Future; Rives,
V., Ed.; Nova Science Publishers: New York, 2001.
(3) Reichle, W. Solid State Ionics 1986, 22, 135–141.
(4) Adachi-Pagano, M.; Forano, C.; Besse, J.-P. J. Mater. Chem. 2003,
13, 1988–1993.
(2) He, J.; Wei, M.; Li, B.; Kang, Y.; Evans, D. G.; Duan, X. In Layered
Double Hydroxides; Duan, X., Evans, D. G., Eds.; Structure and
Bonding, Vol. 119; Springer-Verlag: Berlin, 2006; p 119.
(5) Cai, H.; Hiller, A. C.; Franklin, K. R.; Nunn, C. C.; Ward, M. D.
Science 1994, 266, 1551–1555.
10.1021/ic7023023 CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 12, 2008 5453
Published on Web 05/22/2008

Benito et al.
tion of urea in aqueous solution.9 Urea hydrolysis proceeds
in two steps: formation of ammonium cyanate (NH4CNO),
which is the rate-determining step, and subsequent hydrolysis
of NH4CNO to ammonium carbonate. Because of the low-
supersaturation conditions, the number of nuclei formed is
small, therefore leading to large, well-crystallized particles.10
The parameters affecting the properties of the materials, such
as phase purity and particle size, are the total concentration
of metal (M) cations, the M2+/(M2+ + M3+) molar fraction,
the urea/(M2+ + M3+) molar fraction in solution, and the
temperature,7,11–13 since altering some of these modifies the
extent of supersaturation as well. Among all of these
parameters, temperature seems to be the most important
one for yielding well-crystallized samples, since urea de-
composition is thermally activated and the decomposition
kinetics can be greatly enhanced by increasing the temperature.
Thermal activation of urea decomposition at ambient
pressure requires long reaction times. Several modifications
of this thermal decomposition procedure have been proposed.
For instance, use of hydrothermal conditions leads to faster
precipitation of the compounds than during thermal activation
at ambient pressure and reduces the time required to yield
the pure hydrotalcite phase.14,15 Another modification is the
use of microwave radiation as a heating source. In a previous
paper,16 we reported the synthesis of Mg-Al LDH com-
pounds using microwave-assisted urea hydrolysis, showing
that the formation of the LDH phase depends on the
temperature and irradiation time and that the microwave-
hydrothermal treatment reduces the time required for the
synthesis. Also, Jobba´gy and co-workers17 recently proposed
the synthesis of Ni-Cr samples by a similar method but
using higher temperatures.
materials were evaluated. For comparison purposes, the
conventional hydrothermal treatment was also applied.
2. Experimental Section
2.1. Preparation of the Solids. The solids were prepared by a
method modified from that proposed by Costantino and co-workers7
and similar to that previously reported.16 One liter of a 0.5 M
solution containing 0.333 mol of NiCl2 ·6H2O or ZnCl2 and 0.165
mol of AlCl3 ·6H2O was mixed with 1.65 mol of urea to give a
urea/(M2+ + M3+) ratio of 3.3 (smaller values of this ratio yielded
poorer results and incomplete precipitation), and the mixture was
stirred until the solids were totally dissolved. The solution was
heated at 100, 125, 150, or 175 °C for times ranging from 5 to 300
min, depending on the layer composition, in a Milestone ETHOS
PLUS microwave oven. The times required in order to reach the
desired temperatures were 2.5, 5.0, 7.5, and 10 min for 100, 125,
150, and 175 °C, respectively. The temperature during irradiation
was measured using a thermocouple introduced into the reference
vessel. The software dynamically controlled the temperature profile,
adjusting the delivered power at every moment. The feedback
mechanism optimized the effects of too-high temperatures and
pressures and at the same time prevented thermal runaways.
However, the formation of hot spots within the vessel as a result
of selective microwave absorption by some particles could not be
ruled out. For comparison purposes and in order to study the
exclusive effect of the microwave radiation during the treatment,
the samples were compared to another set prepared using conven-
tional hydrothermal treatment, which was carried out at autogenous
pressure in a Teflon-lined, stainless steel Phaxe 2000 bomb placed
in a static oven at 150 °C for 2, 3, 5, 12, or 24 h. A volume of 50
mL of solution per vessel was used in both the microwave and
conventional procedures. After the sample was cooled to room
temperature, the pH was measured, and then the precipitate was
centrifuged and washed with distilled water until chloride anions
and products of the urea decomposition were completely removed.
Finally, the solids were dried in an oven at 40 °C in air.
The solids prepared under microwave irradiation are named as
XAW-T-t, where X ) N or Z (for Ni or Zn, respectively), T
represents the heating temperature in degrees Celsius, and t refers
to the heating time in minutes. The samples produced under
conventional hydrothermal conditions are named as XAHT-150-t,
where t stands for the heating time in hours.
2.2. Characterization of the Solids. Chemical elemental analysis
for Ni and Al was accomplished via atomic absorption using a Mark
2 ELL-240 apparatus by the Servicio General de Ana´lisis Qu´ımico
Aplicado (University of Salamanca). CHN elemental analyses were
performed using a LECO CHNS-932 elemental analyzer by the
Servicio Interdepartamental de Investigacion (University Autonoma
of Madrid).
In this work, we studied the influence of the cations on
the microwave-assisted homogeneous precipitation using
urea. For this reason, we extended the method to the synthesis
of nickel- and zinc-containing LDHs in order to assess
whether the method can be validated for other systems. The
starting solutions containing the metallic salts and urea were
submitted to microwave-hydrothermal treatment at different
temperatures for increasing periods of time, and the struc-
tural, thermal, and textural properties of the synthesized
(6) Yao, K.; Taniguchi, M.; Nakata, M.; Takahashi, M.; Yamagishi, A.
Langmuir 1998, 14, 2410–2414.
(7) Costantino, U.; Marmottini, F.; Nocchetti, M.; Vivani, R. Eur. J. Inorg.
Chem. 1998, 1439–1446.
(8) Radha, A. V.; Vishnu Kamath, P.; Shivakumara, C. Acta Crystallogr.
2007, B63, 243–250.
Powder X-ray powder diffraction (PXRD) patterns were recorded
on a Siemens D-500 instrument using Cu KR radiation (λ ) 1.54050
Å) and equipped with Diffrac AT software. The crystalline phases
were identified by comparison with JCPDS files.18 Unit cell
parameters were obtained by refining the peak positions of the
PXRD pattern with a least-squares method using the CELREF unit-
cell refinement program,19 assuming a hexagonal unit cell and space
(9) Shaw, W. H. R.; Bordeaux, J. J. J. Am. Chem. Soc. 1955, 77, 4729–
4733.
(10) Gordon, L.; Salutzky, M. L.; Willard H. H. In Precipitation from
Homogeneous Solution; Wiley: London, 1959.
(11) Ogawa, M.; Kaiho, H. Langmuir 2002, 18, 4240–4242.
(12) Oh, J.-M.; Hwang, S.-H.; Choy, J.-H. Solid State Ionics 2002, 151,
285–291.
(13) Okamoto, K.; Iyi, N.; Sasaki, T. Appl. Clay Sci. 2007, 35, 218.
(14) Mohan Rao, M.; Reddy, B. R.; Jayalakshimi, M.; Jaya, V. S.; Sridhar,
B. Mater. Res. Bull. 2005, 40, 347–359.
j
group R3m. Average sizes of crystallites were calculated using the
(15) Kloprogge, J. T.; Hickey, L.; Trujillano, R.; Holgado, M. J.; San
Roman, M. S.; Rives, V.; Martens, W. N.; Frost, R. L. Cryst. Growth
Des. 2006, 6, 1533–1536.
(18) Joint Committee on Powder Diffraction Standards (JCPDS), Interna-
tional Center for Diffraction Data, Swarthmore, PA, 1977.
(19) Laugier, J.; Bochu, B. CELREF: Program for Cell Parameter
Refinement from the Powder Diffraction Diagram, version V3;
Laboratoire des Mate´riaux et du Ge´nie Physique: Grenoble, France,
2003.
(16) Benito, P.; Labajos, F. M.; Rives, V. Cryst. Growth Des. 2006, 6,
1961–1966.
(17) Jobba´gy, M.; Blesa, M. A.; Regazzoni, A. E. J. Colloid Interface Sci.
2007, 309, 72–77.
5454 Inorganic Chemistry, Vol. 47, No. 12, 2008

MicrowaWe-Assisted Homogeneous Precipitation of Hydrotalcites
Scherrer equation: D ) Kλ/(ꢀ cos θ), where D is the average
crystallite size, K is the shape factor for the average crystallite (the
expected shape factor is 0.9), λ is the X-ray wavelength (1.54056
Å for Cu KR1), θ is the Bragg angle, and ꢀ is the full width at
half-maximum (FWHM) in radians [ꢀ ) (B2 - b2)1/2, where B is
the measured FWHM and b is the instrumental broadening, which
was determined by collecting the diffraction pattern of a NIST-
traceable LaB6 line-width standard (SRM660a)].
FT-IR spectra were recorded on a PerkinElmer FT1730 instru-
ment using KBr pellets; 100 spectra (recorded with a nominal
resolution of 4 cm-1) were averaged in order to improve the signal-
to-noise ratio.
Transmission electron microscopy (TEM) photographs were
taken by the Servicio General de Microscop´ıa Electro´nica (Uni-
versity of Salamanca) using a ZEISS-902 microscope. Each sample
was dispersed in acetone by ultrasound, and a few drops of the
dispersion was deposited on a copper grid previously impregnated
with an amorphous carbon film using a voltaic arc. Scanning
electron microscopy (SEM) images were recorded using a JEOL
6300 instrument at 25 kV at the University of Co´rdoba.
Assessment of specific surface area was carried out using a
Gemini instrument from Micromeritics. Before each measurement,
the sample (80-100 mg) was degassed in flowing nitrogen at 110
°C for 2 h in a Micromeritics FlowPrep 060 apparatus in order to
remove physisorbed water; the data were analyzed using published
software.20 Values of pore volumes were obtained from saturation
adsorption capacities; nitrogen uptake at P/P0 ) 0.95 was converted
into adsorbed volume by assuming that the adsorbate had the normal
liquid density at the operational temperature.
Thermogravimetric analysis and differential thermal analysis
(DTA) were performed using PerkinElmer TG-7 and DTA-7
instruments, respectively, in flowing oxygen and/or nitrogen
(L’Air-Liquide) at a heating rate of 10 °C min-1
.
3. Results
Figure 1. PXRD patterns for Ni-Al solids obtained at 150 °C by (a)
microwave and (b) conventional methods.
3.1. Ni-Al LDHs. 3.1.1. Powder X-ray Diffraction. PXRD
patterns for Ni-Al samples prepared at 150 °C in the
microwave oven and a conventional furnace are displayed
in panels a and b, respectively, of Figure 1. The patterns
show diffraction lines characteristic of the hydrotalcite-like
structure, with nonuniform broadening of the lines that points
to some structural disorder. No secondary crystalline phases
are identified by PXRD, even at low synthesis times. This
is contrary to the behavior showed by Mg-Al-CO3 com-
pounds synthesized using the urea method under both
hydrothermal (conventional and microwave)16 and refluxing4
conditions, where an aluminum hydroxide phase is formed
during the first stages of the synthesis and then incorporation
of the divalent cations takes place, leading finally to the
formation of the LDH structure. However, the absence of
an amorphous secondary phase cannot be completely ruled
out on the basis of PXRD only.
Although an enhancement in the degree of crystallinity
(broadly defined as the sharpness of the diffraction peaks)
with increasing irradiation time is expected, a first exploration
of the patterns in Figure 1a does not show any appreciable
effect of the irradiation time for either short or long treatment
times. It can be concluded that extending the duration of
the microwave treatment does not lead to an appreciable
enhancement of the crystallinity of the Ni-Al LDH com-
pounds once their structures have been formed. Very
recently, Liu et al.21 have also claimed that refluxing Ni-Al
compounds in the presence of urea achieved no improvement
in the crystallinity and that hydrothermal treatment for 2 days
was necessary in order to improve the crystallinity of the
solids. To obtain a more accurate analysis of the evolution
of crystallinity during the synthesis process, FWHM values
for the 003 lines were used in the Scherrer equation to
calculate average crystallite sizes in the stacking direction,
taking into account the instrumental contribution to the
broadness of the lines; the results are included in Table 1. It
should be noted that the line broadening can be also caused
by structural disorder, which must be discounted before
estimating the crystallite size;22 however, for comparison
purposes in the present work, the results give useful
qualitative information concerning crystal growth in the
stacking direction for samples submitted to different treat-
ments. The values of the crystallite sizes are very close to
each other and support the qualitative information obtained
from the analysis of the PXRD patterns, i.e., the sizes do
not change as much as for Mg-Al LDHs.16
(21) Liu, Z.; Ma, R.; Ebina, Y.; Iyi, N.; Takada, K.; Sasaki, T. Langmuir
2007, 23, 861–867.
(20) Rives, V. Adsorpt. Sci. Technol. 1991, 8, 95–104.
(22) Thomas, G. S.; Kamath, P. V. J. Chem. Sci. 2006, 118, 127–133.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5455

Benito et al.
Table 1. Cell Parameters a and c (Å) and Crystallite Sizes D003 (nm)
of Ni-Al-CO3 Compounds Obtained by Microwave-Assisted and
Hydrothermal Precipitation at 150 °C
that the calculated amount of water includes both physisorbed
and interlayer water and that the former could also be
modified during the synthesis steps. In the NAW-150 series,
for instance, a slight increase in the water content takes place
between the samples aged for 10 and 30 min, after which
the water content decreases as the irradiation time is
extended, reaching a value as low as 0.30. Finally, in samples
prepared by the conventional hydrothermal method, the water
content increases with the treatment time. Taking into
account these results, it cannot be stated that the only factor
in determining the value of the parameter c is the water
content; other factors, such as stronger interactions between
layer and interlayer domains and the type and arrangement
of the anions, can determine this value as well. On the other
hand, the variation of the cell parameter a suggests a slight
modification of the layer composition, since this parameter
corresponds to the average closest-cation distance. The a
values for the solids obtained after the shortest synthesis
times are 3.05(3) Å, and these decrease to 3.04(3) Å for the
NAW-150 series and 3.03(1) Å for the NAHT-150 series.
In view of the ionic radii of Ni2+ (0.83 Å) and Al3+ (0.68
Å) in octahedral coordination,29 the decrease in the value of
a suggests the removal of nickel cations at long synthesis
times in both synthesis procedures.
In summary, nickel-aluminum layered double hydroxides
are formed at 150 °C under microwave irradiation at shorter
times than are the corresponding magnesium-aluminum
compounds, but no remarkable improvement in the ordering
of the solid is attained by using microwave radiation as a
heating source. In order to complete the study, the synthesis
was carried out at longer treatment times at 150 °C and at
lower (125 °C) and higher (175 °C) temperatures. The
synthesis at lower temperature was performed in order to
obtain some clues about the precipitation process taking place
in the vessels, whereas prolonging the treatment time and
increasing the temperature were aimed at achieving an
improved degree of crystallinity.
When heating is performed at 125 °C (see Figure 1 in the
Supporting Information), the single hydrotalcite phase is
formed after a very short time (i.e., 10 min), and the degree
of crystallinity is quite similar to that reported above for
samples prepared at 150 °C. No fast enhancement of the
ordering is observed on prolonging the irradiation time, as
the crystallite sizes range between 8 and 9 nm for samples
NAW-125-10 through NAW-125-120. However, the lattice
parameter a remains almost constant at 3.05(2) Å for all of
the synthesis times, whereas the c values show a slight
increase from 23.68(7) to 23.98(5) Å in the first 20 min and
then a decrease to 23.76(4) Å for sample NAW-125-120.
When the synthesis is carried out at 150 °C, it is necessary
to prolong the synthesis up to 300 min to observe just a small
enhancement in crystallinity. Finally, when the reaction
temperature is set at 175 °C, it is possible to obtain a solid
with a higher degree of crystallinity and smaller c and a
values [22.85(9) and 3.03(3) Å, respectively] than in NAW-
sample
c
a
D003
NAW-150-10
NAW-150-20
NAW-150-30
NAW-150-60
NAW-150-120
23.59(2)
23.37(5)
23.47(2)
23.26(6)
23.28(3)
3.05(3)
3.05(9)
3.05(6)
3.05(2)
3.04(3)
8
7
7
8
9
NAW-175-60
NAW-175-120
22.85(9)
22.76(6)
3.03(3)
3.03(3)
9
9
NAHT-150-2
NAHT-150-3
NAHT-150-5
NAHT-150-12
NAHT-150-24
23.38(9)
23.32(8)
23.24(5)
22.74(3)
22.64(8)
3.05(3)
3.05(7)
3.04(1)
3.03(6)
3.03(1)
9
9
10
13
15
In regard to the conventional hydrothermal procedure,
increasing the treatment time leads to more intense and
symmetric diffraction lines without development of nondes-
ired side phases (Figure 1b). It should be remarked that the
PXRD pattern for NAHT-150-24 also shows well-resolved
(110) and (113) diffraction lines, which are features not
observed for the microwave-irradiated solids. Comparison
of the values obtained for samples synthesized by the
microwave and conventional hydrothermal treatments at the
same temperature (150 °C) for 2, 3, and 5 h reveals that the
microwave treatment leads to particles with similar crystallite
sizes regardless of the irradiation times, while larger crys-
tallites are obtained when the conventional treatment is
prolonged.
Refined values of the lattice parameters a and c are
summarized in Table 1. The values depend on the synthesis
time, and both parameters show a general decrease when
the aging time is prolonged. For example, the c values for
the NAW-150 series range from 23.59(2) to 23.28(3) Å
for the samples aged for 10 and 120 min, respectively. The
basal spacing for sample NAW-150-10 is significantly larger
than the value reported for carbonate-containing LDHs23 but
in agreement with values previously reported for Ni-Al
compounds obtained by the urea method.15,24 Moreover, the
c values for the samples prepared following the conventional
hydrothermal method are smaller and closer to those reported
for LDHs having carbonate in the interlayers. Several authors
have related an increase in basal spacing to the location of
CNO- anions (intermediate species formed in the decom-
position of urea)25,26 in the interlayer; however, such a large
value could be also ascribed to weakness of the electrostatic
interactions between the cationic layers and the interlamellar
anions27 or to the presence of different amounts of interlayer
water.28 The amount of water has been calculated from
thermogravimetric analyses and found to depend on the
synthesis conditions (Table 2). However, it should be stressed
(23) Cavani, F.; Trifiro`, F.; Vaccari, A. Catal. Today 1991, 11, 173–301.
(24) Costantino, U.; Curini, M.; Montanari, F.; Nocchetti, M.; Rosati, O.
J. Mol. Catal. A 2003, 195, 245–252.
(25) Mavis, B.; Akinc, M. J. Power Sources 2004, 143, 308–317.
(26) Shu, X.; Zhang, W.; He, J.; Gao, F.; Zhu, Y. Solid State Sci. 2006, 8,
634–639.
(27) Labajos, F. M.; Rives, V. Inorg. Chem. 1996, 35, 5313–5318.
(28) Iyi, N.; Fujii, K.; Okamoto, K.; Sasaki, T. Appl. Clay Sci. 2007, 35,
218–227.
(29) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry:
Principles of Structure and ReactiVity, 4th ed.; Harper Collins: New
York, 1993.
5456 Inorganic Chemistry, Vol. 47, No. 12, 2008

MicrowaWe-Assisted Homogeneous Precipitation of Hydrotalcites
Figure 2. Evolution of the pH during syntheses in the microwave oven at
125 °C (NAW-125), 150 °C (NAW-150), and 175 °C (NAW-175) and in
the conventional oven under hydrothermal conditions at 150 °C (NAHT-
150).
150-300 in 60 min. Heating the mixture for 120 min (NAW-
175-120) yields only a decrease in the c value [to 22.76(6)
Å].
The differences observed between the synthesis processes
for the Ni-Al-CO3 and Mg-Al-CO3 samples must be
related to their titration curves obtained by titration of
solutions containing divalent and trivalent metal chlorides
with NaOH to yield LDHs.30 The pH of the second plateau
is observed at lower pH values for the nickel-containing
compounds than for the magnesium LDHs. Consequently,
the pH needed to form the LDH structure is more rapidly
attained in the urea hydrolysis. The general shape of the pH
curves shown in Figure 2 (measured after the cooling of the
vessels) is quite similar to that for the Mg-Al-CO3
compounds.16 Only small differences are detected in the
temperatures at which the same pH values are reached. The
pH increases very rapidly at 125 °C: a steady increase of
the basicity takes place during the first 20 min, followed by
a plateau that is maintained until an irradiation time of 60
min and then by another increase, to a pH value close to
6.5. This behavior is similar to that observed during the
synthesis of the Mg-Al-CO3 samples. No important dif-
ferences are observed when the temperature is set to 150
°C, where both the initial pH rise and the posterior plateau
are observed, as well as an increase to a maximum pH
followed by a small decrease as the irradiation time is further
prolonged. The same differences reported between the two
treatments are observed, but the pH rises more slowly here.
Finally, it should be noticed that an increase of the reaction
temperature to 175 °C leads to the most rapid pH increase.
Figure 3. FT-IR spectra of Ni-Al LDHs obtained in the microwave oven
at (A) 150 and (B) 175 °C. In (A), samples were aged for (a) 10, (b) 20,
(c) 30, (d) 60, (e) 180, (f) 240, and (g) 300 min.
Figure 4. FT-IR spectra of Ni-Al LDHs obtained by the conventional
hydrothermal method using a temperature of 150 °C and reaction times of
5, 12, and 24 h.
3.1.2. FT-IR Spectra. FT-IR spectra of the samples
prepared in the microwave oven at 150 and 175 °C and by
the conventional hydrothermal treatment are displayed in
Figures 3 and 4, respectively.
The characteristic bands related to the hydrotalcite struc-
ture are observed in all cases.31 The broad and intense band
at 3420 cm-1 is ascribed to the stretching vibrations of OH
groups in the layers and in water molecules. The weak band
at 1630 cm-1 is due to the water deformation mode and that
close to 1500-1360 cm-1 to the ν3 mode of carbonate anions
released during urea hydrolysis. The sharp but not very
intense band observed at 2180 cm-1 is related to the presence
(31) Kloprogge, J. T.; Frost, R. L. In Layered Double Hydroxides: Present
and Future; Rives, V., Ed.; Nova Science Publishers: New York, 2001;
pp 139-192.
(30) Boclair, J.; Braterman, P. Chem. Mater. 1999, 11, 298–302.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5457

Benito et al.
Table 2. Chemical Analyses and Chemical Formulas for Selected Samples
a
a
b
sample
Ni2+
Al3+
Ni2+/Al3+
Na
Ca
N/Cb
formula
NAW-150-10
NAW-150-30
NAW-150-60
NAW-150-180
NAW-150-300
35.40
36.41
36.10
35.47
34.94
8.40
8.28
8.28
8.28
8.52
1.94
2.00
2.00
1.97
1.88
2.11
1.12
0.30
0.18
0.16
1.64
1.75
1.80
1.78
1.82
1.09
0.53
0.13
0.09
0.07
[Ni0.660Al0.340(OH)2](OCN)0.164(CO3)0.000 ·0.45H2O
[Ni0.669Al0.331(OH)2](OCN)0.084(CO3)0.073 ·0.51H2O
[Ni0.667Al0.333(OH)2](OCN)0.022(CO3)0.141 ·0.39H2O
[Ni0.663Al0.337(OH)2](OCN)0.014(CO3)0.148 ·0.35H2O
[Ni0.653Al0.347(OH)2](OCN)0.012(CO3)0.155 ·0.30H2O
NAW-175-60
NAW-175-120
35.21
33.99
8.55
8.78
1.77
1.78
0.14
0.10
1.76
1.83
0.07
0.05
[Ni0.654Al0.346(OH)2](OCN)0.011(CO3)0.155 ·0.32H2O
[Ni0.640Al0.360(OH)2](OCN)0.008(CO3)0.162 ·0.34 H2O
NAHT-150-5
NAHT-150-12
NAHT-150-24
34.71
33.58
34.06
8.63
8.72
8.71
1.85
1.77
1.80
0.10
0.06
0.04
1.73
1.80
1.83
0.05
0.03
0.01
[Ni0.649Al0.351(OH)2](OCN)0.008(CO3)0.150 ·0.34H2O
[Ni0.639Al0.361(OH)2](OCN)0.004(CO3)0.163 ·0.37H2O
[Ni0.642Al0.358(OH)2](OCN)0.002(CO3)0.167 ·0.54H2O
a Weight percent. b Molar ratio.
of products formed during urea decomposition;25,26,32,33 for
example, it may be the C-N stretching mode of cyanate
anions in the interlayer region25,26,33 or adsorbed on the
surface.33 On the basis of Raman and FT-IR spectra,
Klopproge and co-workers15 have recently ascribed this band
to the presence of intercalated (NH2)COO- species formed
during urea decomposition. It is worth noting that this band
is not observed in the FT-IR spectra of Mg-Al LDHs.
The intensity of the ν3(CO32-) band increases as the
irradiation time at 150 °C increases, while the band at 2180
cm-1 becomes less intense (Figure 3A). These features
suggest a steady intercalation of CO32- with a simultaneous
release of the urea-derived anions, although some species
still persist after treatment for 300 min at 150 °C. Increasing
the temperature to 175 °C does not completely remove the
band at 2180 cm-1 (Figure 3B). However, conventional
hydrothermal treatment for 12 and 24 h is enough to almost
remove this species (Figure 4).
In regard to the low-wavenumber region (where bands due
to M-O modes and other bands of the interlayer anions
usually appear), a broad band with a minimum at 620 cm-1
is observed for the solids prepared at 150 °C for short periods
of time; this feature is attributed to the nitrogen-bonded
cyanate deformation of OCN- species [δ(OCN-)].33 As the
exposure time increases, this band steadily disappears and
the bands corresponding to LDHs start to appear, but no
resolution of this region is achieved even when the exposure
time is prolonged to 300 min. However, a well-resolved
spectrum containing merely the vibrational modes due to the
brucite-like layers and the carbonate species is observed for
the hydrothermally synthesized samples, from which the
cyanate species are almost completely removed.
presence of [Ni(NH3)6]2+, the UV-vis spectrum of the
supernatant liquid of the NAHT-150-24 sample was recorded
(see Figure 2 in the Supporting Information). The spectrum
shows two bands centered at 368 and 598 nm (27 170 and
3
16 720 cm-1, respectively), which are ascribed to the T2g
3
3
3
r Ag (ν3) and T1g r Ag (ν2) transitions, respectively, of
Ni2+ cations in an octahedral environment.34 The values
calculated for the parameters Dq and B using the Dou
equations35 are 1045 and 836 cm-1, respectively. The Dq
value agrees with that reported for [Ni(NH3)6]2+, so it can
be concluded that upon hydrothermal treatment, nitrogen-
containing species are removed as an ammonia complex.
3.1.3. Chemical Elemental Analyses. Elemental analysis
data for metal cations in selected samples are summarized
in Table 2, together with the chemical formulas for the
compounds synthesized. In regard to the NAW-150 samples,
the one synthesized in 10 min shows a slightly lower Ni2+/
Al3+ ratio than expected, probably because nickel precipita-
tion was not completed in such a short period of time;
nevertheless, the Ni2+/Al3+ molar ratios are very close to
that in the starting solutions for the solids obtained in short
periods of time (30, 60, and 180 min), and a small release
of Ni2+ cations is observed with increasing irradiation time
(sample NAW-150-300). Increasing the temperature to 175
°C gives slightly smaller Ni2+/Al3+ molar ratio values,
suggesting decreased incorporation of Ni2+ cations into the
lattice. For the conventional hydrothermally treated samples,
a solid with a ratio of 1.85 is obtained in 5 h, and increasing
the treatment time leads to a decrease in this value as well.
The decrease observed in the Ni2+/Al3+ molar ratio is in
agreement with the smaller value of lattice parameter a.
Therefore, a release of Ni2+ cations takes place under
hydrothermal conditions, in agreement with the observed
formation of [Ni(NH3)6]2+ explained above.
It should also be mentioned that the supernatant liquids
of the samples whose FTIR spectra do not display the 2180
cm-1 band are blue, typical of [Ni(NH3)6]2+ species. Klop-
proge and co-workers15 pointed out that upon hydrothermal
treatment, the (NH2)COO- species decomposes through
reaction with interlayer water, resulting in the formation of
The chemical analyses also show that the nitrogen content
decreases with increasing aging time (microwave and hy-
drothermal) and temperature, becoming almost negligible for
the NAHT-150-12 and NAHT-150-24 samples, as suggested
by FT-IR spectroscopy.
+
CO32- and NH4 ; the strongly basic medium also gives rise
to liberation of ammonia, which coordinates unprecipitated
Ni2+ cations to form [Ni(NH3)6]2+. In order to confirm that
the blue color of the supernatant liquids was due to the
3.1.4. Thermal Analyses. DTA curves for representative
solids prepared by the microwave-assisted method at 150
and 175 °C and by the conventional hydrothermal treatment
(32) Zhao, Y. L.; Wang, J. M.; Chen, H.; Pan, T.; Zhang, J. Q.; Cao, C. N.
Int. J. Hydrogen Energy 2004, 29, 889–896.
(34) Sutton, D. Espectros Electro´nicos de los Complejos de los Metales
de Transicio´n; Editorial Reverte´: Barcelona, 1975.
(33) Mavis, B.; Akinc, M. Chem. Mater. 2006, 18, 5317–5325.
5458 Inorganic Chemistry, Vol. 47, No. 12, 2008

MicrowaWe-Assisted Homogeneous Precipitation of Hydrotalcites
Figure 6. DTA curves for Ni-Al samples conventionally heated at
150 °C.
shifts toward higher temperatures. The curves recorded for
the samples submitted to conventional hydrothermal treat-
ment (Figure 6) follow the same evolution as those for the
microwave-assisted solids, but it should be noticed that
sample NAHT-150-24 has a rather sharp I2 peak with the I1
peak appearing as a weak shoulder. The second endothermic
effect (I3) is due to structural collapse with simultaneous
elimination of the interlamellar anions.36 This feature is sharp
and intense with a minimum at 365 °C for all of the solids,
regardless of the treatment to which the samples are
submitted, with the exception of compound NAW-150-10,
where the minimum occurs at a lower temperature, and
compounds NAHT-150-12 and NAHT-150-24, where it
occurs at higher temperatures.
In the present work, the strength with which surface-
adsorbed and interlayer water molecules (whose removal
accounts for the first two endothermic effects) are held seems
to depend on the synthesis time, while the lamellar stability
is only slightly improved when the most drastic synthesis
conditions [i.e., 175 °C for 2 h (NAW-150-120) or 150 °C
for 12 h (NAHT-150-12)] are applied. However, the shift
of the second endothermic peak toward higher temperatures
observed for samples submitted to conventional treatment
could be also attributed to the slight modification of the Ni/
Al ratio.
Figure 5. DTA curves for microwave-irradiated Ni-Al samples at (a) 150
and (b) 175 °C.
at 150 °C are shown in Figures 5a,b and 6, respectively.
Changes in the shapes and positions of the effects are
observed as both the time and the temperature of the
treatment are increased.
In regard to the first endothermic effect, which is related
to water release,36 a broad peak I1 displaying a low-
temperature shoulder is observed for the samples aged for
short periods of time at 150 °C (i.e., samples NAW-150-10,
NAW-150-20, and NAW-150-30). As the microwave-hy-
drothermal treatment is prolonged, the shoulder disappears
and the peak becomes broader and splits into two overlapped
peaks (I1 and I2). No differences are found in the DTA
profiles upon further exposure to the microwave field.
Finally, in the sample treated for 120 min at 175 °C (NAW-
175-120), the second peak (I2) becomes more intense and
3.1.5. N2 Adsorption/Desorption at -196 °C. Regardless
of the microwave irradiation time or temperature conditions
used, the adsorption/desorption isotherms were type-IIb
isotherms,37 having nearly parallel adsorption and desorption
branches in spite of the relatively low crystallinities of some
of the samples. The hysteresis shape is related to the pore
structure: the mesoporosity of LDHs arises from interparti-
cle pores, since the nitrogen molecules are unable to enter
(35) Dou, Y. J. Chem. Educ. 1990, 67, 134.
(37) Rouquerol, F.; Rouquerol, J.; Sing, K. In Adsorption by Powders and
Porous Solids: Principles, Methodology and Applications; Academic
Press: London, 1999; p 441.
(36) Rives, V. In Layered Double Hydroxides: Present and Future; Rives,
V., Ed.; Nova Science Publishers: New York, 2001; Chapter 4, p 115.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5459

Benito et al.
Table 3. Values of Specific Surface Areas (SBET) and Pore Volumes
(Vp) for the Microwave-Assisted and Hydrothermal Samples
sample
SBET (m2 g-1
)
Vp (mm3 g-1
)
NAW-150-10
NAW-150-20
NAW-150-30
NAW-150-60
NAW-150-120
NAW-150-180
NAW-150-240
NAW-150-300
54
85
74
87
80
85
85
85
135
199
163
196
186
241
237
220
NAW-175-60
NAW-175-120
81
80
216
247
NAHT-150-2
NAHT-150-3
NAHT-150-5
NAHT-150-12
NAHT-150-24
34
69
73
64
56
84
152
183
168
84
Figure 7. SEM micrograph of sample NAW-150-120.
the interlayer space. As the urea hydrolysis is supposed to
give rise to uniformly sized particles, the particle-particle
interactions, which in the end determine the pore structure,
might be better defined, giving rise to a narrow hysteresis
loop. The only differences are found for the samples obtained
by conventional heating, for which broader hysteresis loops
were recorded.
The values of the specific surface area (SBET) are given in
Table 3. An increase in SBET is observed in going from
sample NAW-150-10 to sample NAW-150-20.38 However,
in contrast to what would be expected, extending the
irradiation time further does not modify the SBET values
during the treatment: a value of 85 m2 g-1 is always observed.
The almost constant SBET values agree with the slight increase
in the degree of crystallinity concluded from the PXRD
patterns. Changes in the pore volumes (Vp) run parallel with
changes in the specific surface areas. The samples prepared
by the conventional hydrothermal method show similar
textural properties; the initial surface-area development is
observed on increasing the time from 2 to 3 h; however, a
cancellation of the surface area is observed under conven-
tional hydrothermal conditions as the treatment is prolonged.
place. These results agree with those recently reported by
Okamoto et al.13 for the formation of LDHs by the urea
method; these authors observed imperfect particles beside
the perfect hexagonal ones. They explained this behavior by
assuming that formation of LDHs took place in circles around
aluminum hydroxide seeds: if a circle became closed,
hexagonal particles were observed, but if the growth was
not complete, imperfect particles shaped as semihexagons
or hexagonal platelets with a hole in the center were formed.
3.2. Zn-Al LDHs. It is well-known that the coprecipi-
tation method usually followed to prepare various LDHs fails
when Zn-containing solids are pursued if the precipitation
parameters are not carefully controlled;40 actually, the best
method for preparing Zn-containing hydrotalcites is the so-
called oxide-salt method.41 Here we report several attempts
to prepare Zn-containing hydrotalcites via the urea hydrolysis
method using both conventional hydrothermal conditions and
microwave irradiation.
PXRD patterns of the solids prepared at 150 °C by both
microwave and conventional hydrothermal treatments (Figure
9) indicate that a pure hydrotalcite-like phase was not ever
formed. Precise identification of the phases is very difficult
because of the large number of diffraction lines recorded,
but at least the following phases can be identified for sample
ZAW-150-10 (i.e., the one formed after only 10 min of
microwave irradiation): (i) hydrozincite and hydrozincite-
like compounds [Zn5(CO3)2(OH)6 and Zn5Cl2(OH)6, respec-
tively], since both carbonate and chloride anions exist in the
reaction mixture, the former as a product of urea hydrolysis
and the latter as a constituent of the starting metallic salts;
(ii) ZnO; (iii) several aluminum oxide phases, such as Al2O3,
although the presence of an amorphous alumina phase not
detectable by X-ray diffraction cannot be ruled out; and (iv)
a Zn-Al hydrotalcite-type phase that produces a weak
diffraction line. Increasing the irradiation time enhances all
of the diffraction lines due to the hydrotalcite-like structure,
3.1.6. SEM and TEM Images. Representative SEM and
TEM micrographs are shown in Figures 7 and 8, respectively.
The SEM micrograph, Figure 7, shows very small primary
particles with disklike shapes and rather uniform sizes that
adopt a sand-rose morphology upon aggregation.39 Increasing
the irradiation time and the temperature causes larger
particles to form.
The TEM images in Figure 8 show formation of very thin
particles in some cases (e.g., NAW-150-180), and even at
long irradiation times, small particles without a well-defined
shape are obtained (not shown). However, the samples
prepared by conventional hydrothermal treatment (e.g.,
NAHT-150-12) form larger particles. It should be noticed
that the darkness of these hexagonal particles is not
homogeneous, suggesting that some agglomeration takes
(40) Kloprogge, J. T.; Hickey, L.; Frost, R. L. J. Solid State Chem. 2004,
177, 4047–4057.
(38) Benito, P.; Labajos, F. M.; Rocha, J.; Rives, V. Microporous
Mesoporous Mater. 2006, 94, 148–158.
(41) de Roy, A.; Forano, C.; Besse, J. P. In Layered Double Hydroxides:
Present and Future; Rives, V., Ed.; Nova Science Publishers: New
York, 2001; Chapter 1, pp 1-37.
(39) Vial, S.; Prevot, V.; Forano, C. J. Phys. Chem. Solids 2006, 67, 1048–
1053.
5460 Inorganic Chemistry, Vol. 47, No. 12, 2008

MicrowaWe-Assisted Homogeneous Precipitation of Hydrotalcites
Figure 9. PXRD patterns of Zn-Al compounds prepared at 150 °C (a) in
the microwave oven and (b) by the conventional process: (9) hydrotalcite,
(*) hydrozincite, (+) ZnCO3, (o) ZnO, (&) Al(OH)3, ($) Zn(OH)2.
Figure 8. TEM micrographs of samples (A) NAW-150-180 and (B) NAHT-
150-12 [with the same bar scale as in (A)].
performed under refluxing conditions.7,21 This behavior must
be related to the different sort of heating that initiates urea
decomposition using the microwave-assited method rather
than the conventional hydrothermal method and to the severe
temperature and pressure conditions operating in hydrother-
mal treatments in comparison with the mild conditions under
reflux. Very short treatment times in the conventional
hydrothermal treatment are probably not enough to achieve
the set temperature within the reactor, and temperature
gradients are generated inside the autoclave bomb. The
microwave radiation, which is directly transmitted to
the solution inside the reactor (rather than to the container)
and converted into heat, enables the established temperature
to be achieved rapidly. It is well-known that the size of the
particles prepared by the urea method depends on several
factors, such as temperature, the concentrations of the metal
salts, and the urea concentration. All of these factors affect
the extent of supersaturation of the solution, which is
responsible for the formation of nuclei that serve as precur-
sors of the final particles. In our case, it can be assumed
that microwave radiation increases the supersaturation within
the whole volume of solution in the reaction vessel because
of a rapid and homogeneous decomposition of the urea. This
should lead to a decrease in the nucleation rate, yielding a
higher number of nuclei and thus smaller particles. Conse-
but the side products are not completely removed. In an
attempt to prepare pure Zn-Al compounds, the synthesis
temperature in the microwave oven was decreased to 100
or 125 °C without success (see Figure 3 in the Supporting
Information).
The same phases as those recorded for the microwave-
assisted solids are observed when using the conventional
hydrothermal synthesis (Figure 9b) for the compounds
obtained after treatment for 2 and 5 h; however, when the
synthesis time was prolonged to 12 h, the only intense
diffraction line recorded was that corresponding to d(003)
planes of the hydrotalcite phase. After 24 h of treatment,
the structure is destroyed and only broad, poorly resolved
diffraction peaks are observed. The same negative results
were obtained when the microwave treatment was carried
out at 100 or 125 °C.
4. Discussion
On the basis of the results reported here for the synthesis
of Ni-Al LDHs, it can be stated that urea decomposition is
greatly enhanced using microwave irradiation, under which
the coprecipitation conditions for LDH formation are achieved
in times that are similar to those for Mg-Al compounds
and shorter than those for the conventional hydrothermal
process and the ones reported previously for the syntheses
Inorganic Chemistry, Vol. 47, No. 12, 2008 5461

Benito et al.
quently, increasing the irradiation time should lead to
formation of larger particles with a lower free energy in the
growing step.
does not influence the nature of the final products. An
important point is that in all these reports, urea was
decomposed under refluxing, i.e., under milder conditions
than in our case. However, preparation of well-crystallized
solids was achieved under different conditions: Costantino
et al. reported that a very good crystallized material was
obtained after 48 h at 90 °C but only good crystallinity was
observed when the synthesis conditions were 100 °C for 36 h,
suggesting that the crystallinity decreased when the temper-
ature increased; Liu et al. claimed that highly crystalline
Zn-Al LDHs required shorter reaction times (1 instead of
2 days) in order to avoid formation of a layered hydroxy-
carbonate impurity through dissolution of initially formed
LDH and reprecipitation of Zn2+ into another phase. Con-
sequently, our lack of success should be mainly attributed
to the severe hydrothermal conditions used, which might have
affected several aspects of the preparation procedure. If the
LDH structure was ever formed, it could have been destroyed
during the hydrothermal treatment, with Zn2+ cations leading
to ZnO segregation, which is easily produced under hydro-
thermal conditions.44 On the other hand, crystallization of
pseudoboehmite also could have taken place under such
conditions. Titration curves of Zn-Al mixtures with NaOH
in an aqueous medium reveal that the plateau corresponding
to precipitation of a Zn/Al phase is poorly defined and close
to those for precipitation of Zn(OH)2 and Al(OH)3.30,45 When
studying the synthesis of sol-gel Zn-Al LDHs, Tichit et
al.46 concluded that Zn2+ and Al3+ cations are more likely
to be involved in the formation of pure hydroxides or oxides
rather than a coprecipitated Zn-Al LDH structure when the
synthesis is performed at high temperatures. Under these
conditions, the kinetically stable product is formed. During
synthesis by the sol-gel method, the pure hydroxides are
preferentially formed, leading to a mixture of ZnO [formed
by dehydration of Zn(OH)2] and alumina. In the present
work, the kinetics of urea decomposition is enhanced either
by the microwave-assisted or the conventional hydrothermal
synthesis, a fact that could account for the rapid formation
of the kinetically favored products. Actually, when the
treatment time is increased, the thermodynamically favored
product, i.e., the Zn-Al layered double hydroxide, is formed
in a small amount, but the nondesired side products remain
in the final solid.
However, despite the decrease in the precipitation time,
no enhancement of the degree of crystallinity was achieved.
Once the hydrotalcite phase appears, crystallization of Ni-Al
compounds seems to be unaffected by the irradiation time,
as no enhancement of the crystallization degree was achieved
and no decrease in the SBET values observed. However,
hydrothermal treatment for longer periods of time (24 h)
yielded a better-crystallized compound. Mavis and Akinc42
observed that although the particles grow up to several
micrometers, primary crystallites remain nanosized; they
related this behavior to an agglomerative growth mechanism.
Liu et al.21 reported that hydrothermal treatment at 190 °C
for 2 days was necessary to improve cystallinity, but these
authors did not report an explanation for this behavior. In
order to explain all of these features, the presence of some
nitrogen-containing species (i.e., cyanate), as confirmed by
elemental analyses and FT-IR spectroscopy, should be noted.
Cyanate is an intermediate product of the decomposition of
urea, and its concentration reaches a maximum during the
first hours of heating in conventional urea hydrolysis.42
However, in the synthesis reported here, this maximum can
be reached in shorter times, so for the samples prepared at
short periods of time and lower temperature (150 °C), cyanate
ions together with some carbonate anions and water mol-
ecules are incorporated into the interlayer region to balance
the excess positive charge. Thus, an interstratified compound
is formed, with cyanate in a given interlayer and carbonate
in the next one (although a precise model cannot be
proposed). This interstratification destroys the periodicity
along the c axis, so the crystallinity cannot be improved until
cyanate anions are removed and a carbonate-containing phase
is formed. Increasing the synthesis time (microwave or
hydrothermal) and temperature causes an Ostwald ripening
mechanism (dissolution/reprecipitation) to occur. During this
process, release of cyanate species, which are replaced by
carbonates because of their high affinities for LDHs, can take
place. Cyanate species are then further hydrolyzed to NH3
and CO2, the former of which binds to Ni2+ cations from
the layers to form [Ni(NH3)6]2+ complexes, which are
removed in the supernatant liquid as observed using UV-vis
spectroscopy. Removal of these species is accompanied by
an improvement in the crystallinity observed when the
sample is aged under conventional hydrothermal treatment
for 24 h, which leads to smaller, better-defined particles
having lower specific surface areas.
5. Conclusions
Ni-Al hydrotalcite-like compounds can be synthesized
quickly using microwave-assisted urea decomposition. A
pure hydrotalcite-type phase is obtained in very short periods
of time; however, some CNO- species were detected, and it
was necessary to increase both the temperature and the
irradiation time in order to remove them under these
conditions. By this method, small particles having a fila-
mentous shape are obtained in the first stages, and the shape
On the other hand, concerning Zn-Al samples, Costantino
et al.,7 Yang et al.,43 and Liu et al.21 have reported the
preparation of well-crystallized Zn-Al-CO3 hydrotalcite by
urea hydrolysis. Different sources of zinc cations were used:
while Costantino et al. used ZnO dissolved in HCl, Yang et
al. and Liu et al. employed nitrates and chlorides, respec-
tively. Therefore, it would seem that the source of Zn(II)
(44) Benito, P.; Guinea, I.; Labajos, F. M.; Rocha, J.; Rives, V. Microporous
Mesoporous Mater. 2008, 110, 292–302.
(42) Mavis, B.; Akinc, M. J. Am. Ceram. Soc. 2006, 89, 471–477.
(43) Yang, P. P.; Yu, J. F.; Wu, T. H.; Liu, G. Z.; Chang, T. S.; Lee,
D. K.; Cho, D. H. Chin. Chem. Lett. 2004, 15, 90–92.
(45) Radha, A. V.; Kamath, P. V. Bull. Mater. Sci. 2003, 26, 661–666.
(46) Tichit, D.; Lorret, O.; Coq, B.; Prinetto, F.; Ghiotti, G. Microporous
Mesoporous Mater. 2005, 80, 213–220.
5462 Inorganic Chemistry, Vol. 47, No. 12, 2008

MicrowaWe-Assisted Homogeneous Precipitation of Hydrotalcites
becomes hexagonal in samples irradiated for long times;
specific surface area values of 80 m2 g-1 are obtained. On
the other hand, when the urea decomposition was carried
out under conventional hydrothermal conditions, the hydro-
talcite-type structure is formed after longer reaction times,
and larger particles with smaller specific surface areas are
formed. Finally, no successful results were obtained from
attempts to synthesize Zn-Al compounds, regardless of the
hydrothermal treatment method (conventional or microwave-
assisted).
Acknowledgment. The authors thank MCyT (Grant
MAT2003-06605-C02), MEC (Grant MAT2006-10800-C02-
01), Fundacio´n Samuel Solorzano, JCyL (Grant SA030/03),
and ERDF for financial support. A grant from JCyL to P.B.
is acknowledged.
Supporting Information Available: PXRD patterns and UV-vis
spectra for various samples. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC7023023
Inorganic Chemistry, Vol. 47, No. 12, 2008 5463