Investigation of acid-base properties of catalysts obtained from layered double hydroxides

Article abstract of DOI:10.1021/jp002715u

Well-crystallized layered double hydroxides (LDH) containing Mg²⁺ or Ni²⁺ as divalent cations and Al³⁺ as trivalent cations were obtained either by coprecipitation (cp) or with the sol-gel (sg) method. After calcination, m

Full text of DOI:10.1021/jp002715u

J. Phys. Chem. B 2000, 104, 11117-11126
11117
Investigation of Acid-Base Properties of Catalysts Obtained from Layered Double
Hydroxides
Federica Prinetto and Giovanna Ghiotti
Dipartimento di Chimica IFM, UniVersit a` di Torino, Via P. Giuria 7, 10125 Torino, Italy
Robert Durand and Didier Tichit*
UMR 5618 CNRS-ENSCM 8, rue Ecole Normale, 34296 Montpellier Cedex 5, France
Well-crystallized layered double hydroxides (LDH) containing Mg2 or Ni2+ as divalent cations and Al3+ as
trivalent cations were obtained either by coprecipitation (cp) or with the sol-gel (sg) method. After calcination,
mixed oxide structures were achieved, as shown by XRD. Their acid-base properties were investigated through
+
CO
2 3 2 3
and NH TPD experiments and FT-IR spectroscopy, using CO and NH as probe molecules. Basic and
2-
acidic sites of medium-high strength were simultaneously revealed: the basic sites were mainly O atoms
n+
2-
3+
and M -O pairs, while the Lewis acid sites were provided mainly by Al in Mg-containing materials
and by Al3 and Ni in Ni-containing materials. Several parameters, such as the nature of cations and anions
+
2+
2
+
3+
involved in the LDH structure, the M /M ratio, the synthesis method, and the activation conditions,
influenced the acid-base properties of the mixed oxides. In particular, (i) Mg-containing samples showed
higher basicity than Ni-containing samples, (ii) sg materials were more basic than the cp materials due to
different textural and morphological features, (iii) the surface basicity increased with the calcination temperature
up to 723-773 K while it markedly decreased after calcination at 873-923 K due to the surface enrichment
in Al , and (iv) mixed oxides obtained from carbonate-exchanged LDHs showed the highest basicity while
those containing chlorine showed predominantly acidic character. Except for the inactive structures containing
high amounts of chlorine impurities, the OH -intercalated structures of the meixnerite type, obtained by the
3+
-
rehydration of mixed oxides under appropriate conditions, showed high activity and selectivity in the acetone
condensation reaction. The Lewis-type basicity of the mixed oxides correlated well with the Bronsted-type
basicity of the meixnerite-like LDHs, responsible for their remarkable selectivity toward the formation of
diacetone alcohol.
1
. Introduction
a higher basicity than that of MgO.14 Highly specific surface
areas and tailored acido-basicity are generally wished for in
catalytic applications. These result from judicious control of
the cationic nature and the content of the LDHs, but for a given
compound, these qualities are mainly influenced by the prepara-
tion and activation conditions.
Layered double hydroxides (LDHs) are solids with the general
2+
3+
x+ z-
2+
2+
2+
formula [M 1-xM x(OH)2] A x/z‚mH2O (M ) Mg , Ni ,
3
+
3+
3+
z-
2-
-
-
.
..; M ) Al , Fe , ...; A ) CO3 , NO3 , Cl , ...)
belonging to the family of anionic clays, which includes the
naturally occurring Hydrotalcite mineral [Mg6Al2(OH)16](CO3)‚
4
to designate the LDH materials. Their structure consists of
brucite-type layers where the substitution of M cations with
1
The most common route for the preparation of LDHs is
coprecipitation at constant or increasing pH from inorganic salts
H2O. The term “hydrotalcite-type compounds” is often used
2
,4
2
+
in alkaline media. However, in previous work, we explored
a sol-gel method, starting from alkoxides or acetylacetonate
precursors, from which pure and well-crystallized Mg/Al and
Ni/Al LDHs were obtained, leading to mixed oxides with
3
+
M
induces a net positive charge, which is compensated by
2
-4
anions situated in the interlayer space.
LDH materials in the lamellar or mixed-oxide form, which
can be obtained by controlled calcination, have shown very
attractive properties as catalysts in a wide variety of base-
catalyzed reactions, such as self-condensation and cross-aldol
condensation of aldehydes and ketones, double-bond isomer-
ization, selective alkylation of phenol, and epoxidation of
15
peculiar textural and morphological properties. It is reasonable
to expect that these different features induced some modification
in the surface chemistry and, particularly, acid-base properties
of sol-gel LDHs and their mixed oxides in comparison with
the coprecipitated LDHs. Several studies evidenced the influence
5
,6
5
7
2
+
of structural parameters, such as the nature and ratio of M
8
olefins. They have also been largely used as supports or
precursors of supported metal catalysts exhibiting a basic
3
+
and M cations, the nature of interlayer anions, and the
synthesis and activation conditions on the surface basicity and
9
-13
character and a particular metal-support interaction.
Com-
1
4,16,17
acidity of LDHs and their derived mixed oxides.
In this
pared to their parent M2 or M single oxides, the catalysts
obtained from LDHs have their main advantage in the presence
of acid-base pair sites of medium-high strength. In base-
catalyzed reactions, Mg/Al LDH-derived catalysts have shown
+
3+
work, these effects have been carefully investigated for Mg-
and Ni-containing mixed oxides obtained from either sol-gel
or coprecipitated LDHs using CO2 and NH3 as probes, followed
by temperature-programmed adsorption/desorption and micro-
calorimetric experiments and FT-IR spectroscopy.
*
Corresponding author. Phone: + 33 467 144 390. Fax: + 33 467
-
1
44 349. E-mail: tichit@cit.enscm.fr.
LDHs intercalated with OH anions, identifyed as meixnerite-
1
0.1021/jp002715u CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/28/2000

11118 J. Phys. Chem. B, Vol. 104, No. 47, 2000
Prinetto et al.
TABLE 1: Synthesis Parameters and Chemical Composition of LDH Samples
Chemical composition (g/100 g)
sample
precursors
hydrol. acida
M2+/M3+ solid
C
N
Cl
sg-MgAl4.29
sg-MgAl4.13(e)
sg-MgAl2.53
sg-MgAl2.47(e)
sg-MgAl4.16
sg-MgAl4.10(e)
sg-NiAl3.39
sg-NiAl3.38(e)
cp-MgAl2.69
cp-MgAl2.89
cp-MgAl2.99(e)
cp-NiAl2.98
Mg ethoxide, Al(acac)
Mg ethoxide, Al(acac)
Mg ethoxide, Al(acac)
Mg ethoxide, Al(acac)
Mg ethoxide, Al(acac)
Mg ethoxide, Al(acac)
Ni(acac), Al(acac)
Ni(acac), Al(acac)
Mg, Al nitrates
HCl
HCl
4.29
4.13
2.53
2.47
4.16
4.10
3.39
3.38
2.69
2.89
2.99
2.98
8.12
3.99
8.52
2.71
8.21
3.16
<0.05
<0.05
5.35
<0.05
3.02
8.66
0.15
-
HNO
HNO
HNO
HNO
HCl
HCl
-
3
3
3
3
-
-
-
<0.05
b
b
b
nd
nd
nd
0.27
4.79
0.31
1.51
2.35
0.98
< 0.05
3.30
3.44
-
-
-
0.06
Mg, Al nitrates
Mg, Al chlorides
Mg, Al nitrates
-
-
-
0.41
-
a
Only for sol-gel preparation. b nd ) not determined.
type compounds, exhibit peculiar properties as strong Brønsted
basic catalysts, as exemplified in the self-condensation reaction
of acetone.18 Because of the memory effect of these structures,
they can be obtained by the reconstruction in water of mixed
oxides.1 In this paper, the self-condensation reaction of acetone
and the pH was maintained at about 6 with NH4OH. We
proceeded thereafter with the same procedure used for sg-MgAlx
samples.
2+
2.1.3. cp-MgAlx and cp-NiAlx. A 0.18 mol sample of M
9
2+
2+
3+
(Mg or Ni ) and a 0.06 mol sample of Al nitrates were
-
appeared as a useful means of evaluating the basicity of OH -
coprecipitated at constant pH in the range of 9-10.5 with a 1.5
-
2
intercalated LDHs from coprecipitation and sol-gel chemistry.
Indeed, it must be emphasized that the acid-base properties of
these solids cannot be directly investigated through conventional
physicochemical methods, due on one hand to the high degree
of hydration and on the other hand to the instability of the
lamellar structure upon thermal treatment. The Brønsted basicity
of the meixnerite-type compounds, studied through the model
reaction, was found to be in good agreement with the Lewis
basicity of the mixed oxides.
M NaOH + 2 × 10 M Na CO aqueous solution, as
2
3
18
previously described. The precipitates were then refluxed in
the mother liquor at 353 K for 17 h, repeatedly washed into
deionized water (Na < 100 ppm), and dried at 353 K.
2.1.4. cp-MgAl2.99(e). This sample was prepared following
the same procedure as for the other cp samples but using Mg
and Al chlorides. The precipitate was refluxed in the mother
2+
3
+
-1
liquor at 353 K for 17 h, stirred into a 10 M Na CO solution,
2
3
repeatedly washed into deionized water, and dried at 353 K.
2
+
2+
3+
2
.1.5. Reference Samples. Pure Mg , Ni , and Al oxides
2
. Experimental Section
.1. Sample Preparation. Mg/Al and Ni/Al LDH samples
were obtained by calcination at 773 K of Mg(OH)2, Ni(OH)2,
and Al(OH)3 (Aldrich), respectively.
2
2.2. Characterization Techniques and Procedures. Chemi-
2
+
3+
with different M /Al ratios were prepared through a sol-
gel method (sg) from organic precursors or by the coprecipitation
at constant pH (cp) of inorganic salts (see Table 1. Samples
will be hereafter referred to as sg-Mg(or Ni)Alx and cp-Mg(or
Ni)Alx, where x is the M /M atomic ratio determined in the
solid and sg and cp indicate the preparation method.
cal analyses of the as prepared solids were performed at the
Service Central d’Analyse du CNRS (Solaize, France).
XRD powder patterns were collected on a CGR Theta 60
instrument using monochromatized Cu KR1 radiation (λ ) 1.542
Å, 40 kV, and 50 mA) on samples as prepared or treated as
follows. LDHs were calcined at 723 or 873 K with a N2/O2
2+
3+
-
1
-1
2
.1.1. sg-MgAlx. A 0.15 mol sample of magnesium ethoxide
flow of about 100 mL min . The heating rate was 1 K min ,
and the final temperature was maintained for 6 h.
(Alfa products, 99%) was dissolved by acid hydrolysis with HCl
(Baker, 35% in water) or HNO3 (Carlo Erba, 65%) in ethanol
(Baker, 99.9%); the solution was refluxed at 353 K under
Rehydration of calcined samples was performed following
2
+
18
different procedures, depending on the M involved: (i) on
Mg-samples, rehydration was performed by vigorously stir-
ring the powders for 1 h into decarbonated water at 298 K
constant stirring. A second solution, containing a suitable amount
of aluminum acetylacetonate (Al(acac), Aldrich, 95%) in a 1:1
2+
3+
5
mixture of acetone/ethanol, meant to obtain Mg /Al ratios
in the range of 3-6, was slowly added, and the pH was adjusted
at about 10 with NH4OH (Baker, 33% NH3 in water). Water,
in molar amounts equal to the M2 content, was then slowly
added. The solution was refluxed at 353 K up to 17 h until gel
formation. A portion of the gels was then repeatedly washed
with ethanol and dried overnight at 353 K. The other portion
of the gels was stirred into a 10 M Na2CO3 aqueous solution
to exchange Cl ions coming from the HCl and/or the organic
species provided by the carbonate precursors. The samples were
then repeatedly washed into deionized water (Na < 100 ppm)
and dried overnight at 353 K. These samples will be hereafter
referred to as sg-MgAlx(e), where (e) stands for “exchanged”.
and 1 × 10 Pa, and (ii) on Ni-samples, rehydration occured
5
in an autoclave for 120 h at 433 K and 6 × 10 Pa in NH OH
4
1 wt % decarbonated aqueous solution.
+
BET-specific surface areas (Table 2) were determined by N2
adsorption at 77 K with a Micromeritics ASAP 2100 apparatus
on samples calcined at 723 or 873 K and outgassed at 523 K
-4
(
10 Pa).
-
1
The total basicity of the calcined samples was measured by
-
the adsorption and temperature-programmed desorption (TPD)
of CO2 with a Setaram DSC-111 microcalorimeter. The samples
were previously outgassed at 723 K or, in some cases, 873 K,
cooled to 373 K, and contacted with flowing CO2. Thermal
events in the microcalorimetric cell were recorded. Weakly
adsorbed CO2 was removed under He flow, and then TPD
experiments were performed following weight loss under He
2
.1.2. sg-NiAlx. A 0.06 mol sample of nickel acetylacetonate
Ni(acac), Aldrich, 95%) was dissolved with HCl in ethanol at
53 K. A second solution, containing 0.02 mol of aluminum
acetylacetonate in a 1:1 mixture of acetone/ethanol, was added,
(
3
-1
flow from 373 to 723 K (heating rate, 10 K min ) by mass
coupling.

Catalysts from Layered Double Hydroxides
J. Phys. Chem. B, Vol. 104, No. 47, 2000 11119
TABLE 2: Acid-Base Characterization of the LDHs Calcined from CO
2
3
and NH TPD Experiments
a
a
calcin. temp
(K)
specif. surface
∆H CO
2
-1
N
b
meq
)
T
NH
max des.
N
a
meq
)
2
-1
(g-1
-1
sample
(m g
)
(kJ mol
)
3
(K)
(g
N
b
/N
a
sg-MgAl4.29
sg-MgAl4.13(e)
723
270
267
873
270
265
265
240
222
190
210
220
212
272
19
81
77
67
67
71
65
34
61
66
69
70
53
0.32
1.42
1.16
2.03
1.99
1.81
0.59
0.30
0.85
1.62
0.49
1.17
0.45
530
488
513
513
508
500
533
533
513
483
529
530
510
1.13
0.13
0.34
0.34
0.22
0.19
0.26
0.25
0.23
0.19
0.33
0.13
0.46
0.3
7
2
23
17
10.9
3.4
6.0
8.9
9.5
2.3
1.2
3.7
8.5
1.5
9.0
1.0
sg-MgAl2.53
sg-MgAl4.16
sg-MgAl4.10(e)
723
723
723
7
8
23
73
sg-NiAl3.38(e)
cp-MgAl2.69
cp-MgAl2.99(e)
cp-NiAl2.98
MgO
723
723
723
823
823
2 3
Al O
a
N
b
) number of basic sites and N ) number of acid sites.
a
The total acidity of the calcined samples was measured by
NH3 adsorption and with TPD experiments using a conductivity
cell for the detection of the effluent gases. The samples were
previously outgassed at 723 K or, in some cases, 873 K, cooled
to 373 K, and contacted with NH3 vapor. After the sample was
purged, the temperature was increased to 723 K (heating rate,
accounted for the affinity of the LDH structure for the different
2
-
-
- 3,20
anions in the order of CO3 . Cl > NO3 . In carbonate-
exchanged sol-gel materials, a fraction of the organic species
from the alkoxide or acetylacetonate precursors remained in the
interlayer region, accounting for their high carbon content (Table
1).
-
1
1
0 K min ), and evolved ammonia was trapped in a HCl
The thermal decomposition of LDH materials at increasing
temperature caused the progressive removal of water and
interlayer anions, as evidenced by thermogravimetric and IR
solution and finally titrated.
For absorption/transmission IR analysis, powdered materials
-
2
15
were pelleted in self-supporting disks of 15-20 mg cm and
placed in an IR cell allowing thermal treatment in vacuo or in
controlled atmospheres. The pellets were activated by heating
in vacuo at increasing temperatures to 923 K and, subsequently,
in dry oxygen at the same temperature and cooled in oxygen.
IR spectra were then recorded before and after interaction at
room temperature (RT), with CO2 or NH3 (Matheson C.P.) used
as probe molecules. The spectra were run at RT on a Perkin-
Elmer FT-IR 1760-X spectrophotometer equipped with a Hg-
Cd-Te cryodetector working in the range of wavenumbers of
measurements. At each calcination temperature, a higher
fraction of carbonates remained in Mg- than in Ni-containing
samples, accounting for the higher basicity of the former. The
calcination at 723-823 K of LDH compounds led to mixed
2
+
3+
oxides, M (M )O, with rock-salt-type structures, as evidenced
15,18
by XRD patterns.
These materials exhibited specific surface
2
-1
areas in the range of 190-270 m g , decreasing by 10-20%
when the calcination temperature was raised from 723 to 873
K (see Table 2). Remarkably, the mixed oxides derived from
sg LDHs exhibited specific surface areas 10-25% higher than
those of cp samples. Furthermore, they showed different N2
adsorption/desorption isotherms and hysteresis loops, charac-
teristic of alcogels for sg materials and clay minerals for cp
materials. These findings have been related to the different
textural and morphological features of sg materials, both as
prepared and after thermal decomposition, well evidenced by
-
1
-1
7
∼
200-580 cm at a resolution of 2 cm (number of scans,
60).
Catalytic tests were performed on samples calcined and
rehydrated as described above. Rehydrated powders were
previously washed four times with acetone to remove remaining
water. Aldol condensation of acetone was performed in an ice
bath at 273 K in a well-agitated 100 mL round-bottom flask
under N2 flow using 1.5 g of solid (catalyst-to-acetone ratio,
1
5
transmission electron microscopies. In particular, TEM and
HRTEM analysis revealed a more heterogeneous morphology
and smaller particle sizes of sg LDHs and their mixed oxides,
inducing a higher number of surface defects. Similarly, sol-
gel-prepared MgO exhibited smaller particle sizes, higher
specific surfaces, and higher defects in comparison with the
4.7 wt-%). Acetone (Aldrich, >99.9%) was previously purified
using a 3X molecular sieve activated at 523 K. The reaction
mixture was analyzed by gas chromatography using an OV1
column. Diacetonealcohol (DAA) was obtained with a selectivity
of >99.5%.
2
1
MgO obtained by conventional methods.
By rehydration at ambient conditions of the Mg-containing
mixed oxides, the reconstruction into lamellar structures inter-
3. Results and Discussion
-
calated with OH anions was easily performed, accounting for
1
9
3
.1. Sample Characterization. The results concerning the
the memory effect of these structures. At variance on Ni-
containing mixed oxides, a reconstruction reaching 70-80%
could be achieved only operating in autoclave during 120 h at
characterization of coprecipitated and sol-gel LDHs as prepared
and upon thermal treatment have been carefully discussed in
our previous works.1
5,18
To elucidate the results in this paper,
433 K and 6 × 10 Pa of NH4OH.
5
18
we will summarize the main characteristics of these samples.
As seen in Table 1, pure sg-MgAlx and sg-NiAlx LDHs were
3.2. CO2 and NH3 TPD Measurements. The results of the
CO2 and NH3 adsorption and TPD experiments are reported in
Table 2. The heat of CO2 adsorption was used as a measure of
the strength of basic sites. This approach is open to criticism,
since the interaction of CO2 with the metal oxides can involve
2+
3+
obtained with M /Al ratios in the ranges of 2.47-4.29 and
.98-3.39, respectively. For sg-NiAlx, the lamellar structure
2
was only obtained using HCl for the hydrolysis of the Ni and
Al acetylacetonate precursors. When HCl was involved in the
hydrolysis step, small amounts of chlorine remained in the
samples, although exchanged with carbonates, while using
HNO3 nitrates were totally exchanged with carbonates. This
n+ 2-
acid-base pairs, M O . Therefore, the energetics of the
system is governed by interactions not only with the basic center
but also with the adjacent cation. Furthermore, contributions
can also arise from surface rearrangements during the formation

11120 J. Phys. Chem. B, Vol. 104, No. 47, 2000
Prinetto et al.
of carbonates.22 The heat of CO2 adsorption is useful for
comparing samples with similar structural features, such as
LDH-derived Mg- and Ni-containing mixed oxides, providing
for each system an average value of the basic strength. The
strength of the acid sites was evaluated from the temperature
of the maximum desorption rate of NH3. The number of basic
and acid sites was determined from the amounts of CO2 and
NH3, respectively, evolved during the TPD experiments.
The heats of CO2 adsorption in the range of 61-81 kJ mol-
were found for pure MgO and MgAlx samples. Interestingly,
the highest values corresponded to the sg-MgAlx samples. An
exception was the samples containing high amounts of chlorine
1
-
1
impurities, which showed values beyond 20 kJ mol . For NiO
and NiAlx samples, slightly lower heats of CO2 adsorption, in
the range of 58-69 kJ mol , were found. A further decrease
was observed for pure Al2O3 (53 kJ mol ). The number of
basic sites per mass unit, Nb, increased up to three times from
Ni- to Mg-containing mixed oxides. Furthermore, it markedly
increased from cp to sg materials. Noticeably, MgAlx samples
exhibited Nb values higher than those of MgO, in agreement
with previous results.14
-
1
-
1
Figure 1. Interaction with CO
2
at RT on LDHs and reference samples
2
activated at 773 K. FT-IR spectra after admission of 200 Pa of CO at
RT on sg-NiAl3.39 (curve 1), sg-MgAl4.29 (curve 2), NiO (curve 3),
sg-NiAl3.38(e) (curve 4), MgO (curve 5), cp-MgAl2.69 (curve 6), sg-
The NH3 TPD profiles extended in a wide temperature range
and showed several overlapped components. The temperature
of the maximum desorption rate of NH3 was up to 50 K higher
MgAl2.53 (curve 7), and Al
as the difference from the spectrum before CO
2
O
3
(curve 8). Each spectrum is reported
admission and translated
2
along the Y axis for clarity. The dotted portions of curves 3 and 8 are
artifacts.
2
+
3+
in Ni- than in Mg-containing samples with similar M /M
ratios. On pure MgO, only the highest temperature component,
with a maximum around 530 K, was observed and ascribed to
the NH2 species, which showed a thermal stability higher than
an increase in the LDH precalcination temperature from 723 to
-
873 K. We found a sharp decrease of the N /N ratio for both
b
a
that for coordinated NH3 (vide infra).
Mg- and Ni-containing samples due to structural changes
causing an enrichment in the Al on the surface, as evidenced
by IR results (see below).
3+
18
The number of acid sites, Na, showed an opposite trend with
respect to Nb: indeed, Na decreased from NiAlx samples to MgO
or MgAlx samples. Furthermore, it decreased from cp- to sg-
MgAlx samples, while for MgAlx samples, it increased with
the Al3 content. The highest Na values were found on pure
alumina and highly chlorinated mixed oxides.
3.3. FT-IR characterization. 3.3.1. CO Interaction. The IR
2
study of CO adsorption at RT on the mixed oxides obtained
2
+
by thermal decomposition at increasing temperature of the LDHs
provided useful information on the nature, strength, and relative
amount of acid and basic sites.
Significant information was given by the ratios of the numbers
of basic and acid sites, Nb/Na (Table 2). First, they clearly
evidenced the influence of the synthesis method on the basicity
of the mixed oxides: both Mg- and Ni-containing mixed oxides
obtained from sg LDHs showed Nb/Na ratios higher than those
of the cp counterparts. Second, they revealed the influence of
On both Mg- and Ni-containing samples evacuated below
473 K, no surface species were formed upon CO admission
2
due to the high amounts of water and interlayer anions still
present in the samples. Conversely, surface carbonates were
formed in increasing amounts when the evacuation temperature
was raised from 473 to 773 K. The great number of components
the interlayer anions which were contained in the LDHs before
calcination, in agreement with previous reports.5
,16,17
Carbonated
-1
in the 1700-900 cm region, where carbonate modes fall,
samples were generally found to be more basic and catalytically
active than those containing halides, sulfates, or chromates,
anions whose stabilities were improved by grafting to the mixed
oxide matrix. In the present case, carbonate-exchanged cp LDHs
showed Nb/Na ratios markedly higher in comparison to those
of nitrate-containing samples, accounting for the slightly higher
pointed to the heterogeneity of the surface acid-basic sites, as
evidenced in Figure 1 for some representative LDHs and
reference samples activated at 773 K. The presence of sites with
different basic strengths can depend on the coordination degree
22,24
of the surface oxygen atoms, as evidenced for MgO
and
2
+
2+
3+
with the nature of the vicinal cations, Mg , Ni , and Al .
1
5
relative thermal stability of nitrate-containing samples. Fur-
thermore, the remaining carbonates, if any, were more basic
than the other anions. Accordingly, for sg materials, the
exchange of intercalated organic species with carbonates induced
a slight increase in the Nb/Na ratios, while the lowest values
were found for the mixed oxides containing high amounts of
chlorine. A similar behavior has been observed for Cl-containing
LDHs.1
The main species formed upon CO2 adsorption, in relative
amounts depending on the chemical composition and activation
temperature, were (i) chelating or bridging bidentate carbonates
-1
-1
(
1680-1570 cm , ν(CdO); 1350-1250 and 1060-1040 cm ,
νasym and νsym(O-C-O), respectively), (ii) monodentate carbon-
-1
ates (1550-1500 and 1450-1400 cm , νasym and νsym(O-C-
-1
O), respectively; 860-920 cm , ν(C-O)), (iii) hydrogen
4,23
-1
-1
carbonates (3616 cm , ν(OH); 1670 and 1470 cm , νasym and
2
+
Accounting for the above results, the influence of the M /
M3 ratio should be examined on samples obtained from the
same synthesis method and intercalated with the same species.
-1
νsym(O-C-O-H), respectively; 1230 cm , δ(C-O-H)), (iv)
+
n+
-1
CO2 linearly coordinated on M sites (2338-2357 cm , νasym-
-1
13
(
OdCdO); 2297 cm , νasym(OdCdO) of CO2 with natural
2
+
We noted a decrease in the Nb/Na values with decreasing M /
abundance), and (v) on Al2O3 “strongly perturbed CO2 mol-
3
+
M
ratios, as expected from the increasing amount of Lewis
25
-1
ecules” (1900-1750 and 1250-1150 cm , νasym and νsym-
acid sites.
(
O-C-O), respectively). These bands have been alternatively
assigned to “organic-like” carbonates.
2
6
Finally, for some sg samples, we examined the influence of

Catalysts from Layered Double Hydroxides
J. Phys. Chem. B, Vol. 104, No. 47, 2000 11121
On the other hand, the spectra of MgAlx samples markedly
differed from those of pure Al2O3, where mainly hydrogen
carbonates (bands at 3618, 1660-1645, 1490-1470, and 1230
-
1
cm ) and “strongly perturbed CO2 molecules” were detected
Figure 1, curve 8). On alumina, the peak due to linearly bound
(
-
1
CO2 showed a maximum at 2364 cm and a shoulder at 2345
-
1
3+
cm , assigned to Al sites with tetrahedral and octahedral
configurations, AlIVcus and AlVIcus, respectively.
24,29
The spectra of CO2 adsorbed on cp- and sg-Ni-containing
mixed oxides, except for those of highly chlorinated sample
and NiO, showed similar features. Smaller amounts of carbonate
species were formed in comparison to those of Mg-containing
samples, evidencing fewer basic sites (Figure 1, curves 3 and
4
). Furthermore, the lower monodentate/bidentate carbonate ratio
2
-
accounted for a small amount of strong basic O sites relative
to the amount of M -O pairs. Small amounts of “strongly
perturbed CO2 molecules” (1900-1750 and 1250-1150 cm )
similar to those observed on pure Al2O3 were also detected,
evidencing the presence of alumina islands. For both NiO and
NiAlx samples, the peaks due to linear CO2 exhibited lower
n+
2-
-1
-
1
frequencies (2338-2340 cm ) than those for Mg-containing
2
+
3+
samples. Both Ni and Al concurred as Lewis acid sites to
CO2 coordination so that their single contributions were hardly
distinguishable. In any case, the low frequency of the absorption
evidenced the influence of the NiO-like matrix on the Al3
cations.
+
As previously stated, sg-MgAlx and sg-NiAlx samples
containing high amounts of Cl showed a particular behavior:
Figure 2. Interaction with CO
temperature. FT-IR spectra after admission of 200 Pa of CO
sg-MgAl4.10(e) (section a) and sg-NiAl3.38(e) (section b) evacuated
at 773 K (curves 1) and 923 K (curves 2). Each spectrum is reported
as the difference from the spectrum before CO
2
at RT on LDHs activated at increasing
at RT on
-
2
upon CO2 admission, only linearly coordinated CO2 was
detected, exhibiting a peak slightly shifted toward frequencies
-
1
2
admission and translated
(3-5 cm ) higher that those of the Cl-free materials, while
along the Y axis for clarity.
negligible amounts of carbonates were formed (Figure 1, curves
1
and 2). These findings indicated that due to the grafting of
The modes of carbonate species have been assigned on the
-
Cl ions to the surface, very few surface oxygens with basic
character were available for the formation of carbonates, while
the acidic strength of Lewis acid sites was slightly in-
basis of the splitting, ∆ν3, of the ν3(E) mode, νasym (CO), which
-
1
is at 1415 cm in the free carbonate ion. For ∆ν3 values of
-1
about 100, 300, and 400 cm , monodentate, chelating bidentate,
and bridging bidentate structures, respectively, are generally
1
6,23,30
creased.
2
5,27,28
The effect of the activation temperature between 773 and 923
K was examined next. On both MgAlx and NiAlx samples, we
noted that (i) the integrated intensity of the overall absorption
in the 1700-900 cm region markedly decreased, (ii) the
monodentate/bidentate carbonate ratio decreased, and (iii)
hydrogen-carbonates were no longer formed. These findings
indicated that at high calcination temperature (923 K), the
proposed.
Monodentate species are formed on the strongest basic oxygen
ions and chelating and bridging bidentate carbonates on the sites
of decreasing basic strength with the participation of an adjacent
cationic site. Finally, hydrogen carbonates reveal the presence
of basic hydroxyls.
-
1
Except for those containing high amounts of chlorine, the
cp- and sg-MgAlx samples (Figure 1 curves 6, 7) exhibited
similar features. On these samples, mainly bidentate and, in
minor amounts, monodentate and hydrogen carbonates were
formed upon CO2 adsorption. Furthermore, linearly coordinated
2-
amount of basic sites and, particularly, the strongly basic O
n+
2-
sites relative to the amount of M -O pairs markedly
decreased, while basic hydroxyls were no longer present.
Furthermore, weak amounts of “strongly perturbed CO2 mol-
ecules” were also detected on MgAlx samples, and the peak of
the linearly coordinated CO exhibited two distinct maxima at
2
2369 and 2350 cm , ascribed to Al
-
1
CO2 was detected (2352-2355 cm ), mainly bound to coor-
3
+
dinatively unsaturated Al Lewis acid sites with octahedral
-
1
IV
VI
VI
24,29
cus
and Al cus, sites,
configuration, Al cus.
respectively. These findings could not be attributed merely to
the diminution of the specific surface area in this temperature
range but also to structural changes and, particularly, the
segregation of Al2O3 and/or the formation of inverse spinel
Noticeably, the normalized integrated intensity of the enve-
lope of carbonate bands at 1700-900 cm-1 was higher in sg
than in cp samples (compare curves 6 and 7 in Figure 1),
indicating a larger amount of basic sites in the sg samples. At
variance, no appreciable differences were observed when
11,31
phases.
However, as no bulk MgAl2O4 is observed by XRD,
2
+
3+
we must suppose that the spinel domains were small or located
at the surface of the Mg(Al)O particles. On NiAlx samples in
comparing the spectra of samples with different Mg /Al
ratios.
IV
the examined range of calcination temperature, Al cus sites were
On one hand, the spectra of MgAlx samples showed some
significant differences in comparison to those of pure MgO
Figure 1 curve 5): (i) a higher monodentate/bidentate carbonate
ratio, (ii) the presence of hydrogen carbonates not formed on
MgO, though presenting similar amounts of hydroxyl species,
and (iii) a higher amount of linearly coordinated CO2.
not detected, indicating that no inverse spinel formation oc-
curred. However, as previously stated, in this case the contribu-
(
2
+
3+
tions of Ni and Al are hardly distinguished.
The thermal stability of the different species formed upon
CO2 adsorption was investigated by examining their resistance

11122 J. Phys. Chem. B, Vol. 104, No. 47, 2000
Prinetto et al.
Figure 3. Stability of species formed upon CO
sg samples activated at 773 K. FT-IR spectra of sg-MgAl4.13(e) (section
a) and sg-NiAl3.38(e) (section b) after admission of 200 Pa of CO at
RT (curves 1) and subsequent evacuation (15′) at 298 K (curves 2),
73 K (curves 3), 473 K (curve 4), and 573 K (curve 5). Each spectrum
is reported as the difference from the spectrum before CO admission.
2
adsorption at RT on
2
3
Figure 4. Interaction with NH
3
at RT on sg-MgAl4.13(e) activated at
2
7
73 K and the stability of the species formed. FT-IR spectra in the NH
stretching-mode (section a) and bending-mode (section b) regions upon
upon heating under vacuum at increasing temperature (Figure
). Linearly coordinated CO2 was readily removed by evacuation
at RT, while hydrogen carbonates desorbed at 373 K. On both
cp- and sg-MgAlx samples carbonate species were progressively
removed by evacuation at increasing temperature: at 573 K,
admission of 200 Pa of NH at RT (curve 1) and subsequent evacuation
3
3
(15′) at RT (curve 2), 373 K (curve 3), and 473 K (curve 4). Each
spectrum is reported as the difference from the spectrum before NH
admission (curve 0 in section a).
3
resonance with the first overtone of the δasym(NH3) mode.
Similarly, the free hydroxyl stretching modes decreased, and a
large absorption with maximum approximatively centered at
∼
15% of the overall carbonates was still present. However,
monodentate carbonates showed higher stability than bidentate
carbonates: the latter were reduced to 5-7% of their initial
intensity after evacuation at 573 K, while a large fraction of
monodentate species was still present. On Ni-containing samples,
carbonates showed much lower stability. Indeed, after evacuation
at 373 K, >70% of carbonates were desorbed, while mono-
dentate species still maintained the higher resistance. The higher
thermal stability of monodentate carbonates compared to that
of bidentate species confirmed that they were formed on the
-
1
3
300-3200 cm appeared (Figure 4a). The erosion of the free
hydroxyls was only partial (about 30%), as seen when compar-
ing the integrated intensity of the negative bands at 3800-3700
-
1
cm with that of free hydroxyls on the surface before the NH3
admission (Figure 4a, curve 0). The bands at 1600 and 1130
-
1
cm were assigned to asymmetric and symmetric δ(NH3)
modes, respectively, of coordinated ammonia (Figure 4b). A
-1
strongest basic oxygen ions.2
5,28
further component of minor intensity at 1555 cm was ascribed
-
to the NH2 species formed through the heterolytic dissociation
3
.3.2. NH3 Interaction. The results obtained from NH3 TPD
of ammonia on acid-base pairs involving a particularly basic
experiments make it useful in complementing careful investiga-
tion of the nature of acidic sites present at the surface of mixed
oxides. Ammonia was used as a probe, although the spectral
3
4
oxygen atom.
-
1
The deep negative band centered at ∼1000 cm in the
difference spectra of Figure 4b was due to an erosion of the
high-frequency metal-oxygen vibration modes. This effect was
due to the relaxation of some surface-localized M-O vibrations,
caused by the ammonia addition to the coordination sphere of
analysis is relatively complex due to the possibility of several
adsorbed forms.32 NH3 molecule can also provide information
on the nature of basic oxygens, as reported for CoO, NiO, and
MgO.33
2
9
surface ions.
The spectra of cp and sg Mg-containing mixed oxides upon
NH3 adsorption at RT showed similar features, with the
exception of the highly chlorinated samples that will be
In Scheme 1, the possible structures which ammonia can
possess when coordinated onto the surface sites of an oxide are
shown.
-
1
discussed separately. Bands at 3400 and 3360 cm , due to the
νasym(NH) modes of coordinated ammonia, and three overlapped
components at 3250-3150 cm were formed (see Figure 4a,
curve 1 for sg-MgAl4.13(e)). The latter were assignable to νsym-
From the integrated intensities of the bands in the NH
stretching or bending regions, it was calculated that about 30%
of adsorbed ammonia was removed by evacuation at RT (curves
1 and 2 in Figure 4). This reversible fraction could be ascribed
-
1
(NH) modes of coordinated ammonia splitted for the Fermi

Catalysts from Layered Double Hydroxides
J. Phys. Chem. B, Vol. 104, No. 47, 2000 11123
SCHEME 1
to ammonia hydrogen-bonded either to basic surface oxygens
or to hydroxyls (structures c and e in Scheme 1). The presence
of the latter species was confirmed by the partial restoration
(about 40%) of free hydroxyls upon outgassing at RT.
The major fraction of ammonia, stable under vacuum up to
4
73 K, was ascribed to NH3 molecules coordinated through
the nitrogen lone pair onto Lewis acid sites. Strong evidence
suggested that structures b and d in Scheme 1, where coordinated
ammonia interacts via H bonding with an adjacent oxygen atom,
rather than structure a should be proposed, i.e., (i) the presence
of two bands from the splitting of the triply degenerate νasym-
(NH3) mode as a consequence of the decreased molecular
symmetry and (ii) the persistence after evacuation at RT of the
-
1
large absorption centered at 3300-3200 cm , which was
assignable to N-H‚‚‚O vibrations. The presence of structure a
of Scheme 1 could not be ruled out, but its spectral features
could be hardly distinguished. In fact, on several oxides, the
triply degenerate νasym(NH3) modes of species a have been found
to be eavily superposed onto the ν(NH2) modes of species such
as b.33
The Lewis sites which coordinated ammonia exhibited
Figure 5. Interaction with NH
activated at 773 K. FT-IR spectra after admission of 200 Pa of NH
RT on (section a) MgO (curve 1), sg-MgAl4.13(e) (curve 2), and Al
curve 3) and (section b) sg-NiAl3.38(e) (curve 1) and NiO (curve 2).
Each spectrum is reported as the difference from the spectrum before
NH admission. The dotted portion is an artifact.
3
at RT on LDHs and reference samples
3
at
medium-high acidic strength, as indicated by the intensity and
large blue shift of the δsym (NH3) mode, occurring at 950 cm-1
2 3
O ,
(
in gaseous ammonia. An indication of the acidic strength also
came from the thermal stability under vacuum of coordinated
ammonia, which was completely removed only by evacuation
3
-
region at 3360 cm-1 was observed, while the broad absorption
at 523 K. NH2 species showed higher resistance, being stable
-
1
up to 573 K (Figure 4 curves 2-4).
centered at 3300-3200 cm was almost lacking (Figure 6a).
These findings indicated that the major fraction of ammonia
coordinated onto Lewis acid sites could not interact via H
In Figure 5a, the spectra of MgAlx and reference samples
are compared in the δ(NH3) mode region. Going from MgO to
MgAlx and Al2O3, the integrated intensity of the bands due to
coordinated ammonia markedly increased, and the δsym(NH3)
mode progressively shifted toward the higher wavenumbers,
evidencing an increase in the amount and strength of Lewis
acid sites. The δsym(NH3) mode showed a maximum at 1080
-
bonding with the surface oxygens or hydroxyls replaced by Cl
ions, thus leading to the type a structures of Scheme 1. In
agreement, the intensity of the νasym(NH3) mode was higher than
that on the exchanged sample, accounting for the contribution
of three degenerate vibration modes.
-
1
-1
cm on MgO, 1130-1140 cm on MgAlx samples, and 1230
The spectra of NiAlx samples upon ammonia adsorption
showed minor differences in comparison to those of MgAlx
samples, although in the NH stretching mode region (not
reported), the IR transparency became critical for the spectral
analysis. The band due to δsym(NH3) mode was broader than
-
1
-1
cm (with a shoulder at 1180 cm ) on Al2O3. The parallel
shift of the δasym mode was much more limited, the position of
-
1
-1
its maximum ranging from 1600 cm on MgO to 1620 cm
on Al2O3. On MgO, NH2- species were formed in considerable
amounts compared to coordinated ammonia. On alumina, weak
amounts of NH4 species (1470 and 1680 cm , δasym and δsym-
-
that in MgAlx samples, while NH2 species were not formed
+
-1
(Figure 5b, curve 1). Unfortunately, the comparison with pure
NiO could be made only in a limited frequency region. Indeed,
on one hand this sample did not show enough IR transparency
+
35
(
NH4 ), respectively), formed by proton transfer from Brønsted
acid sites to the ammonia molecule were also observed. The
position of the δsym(NH3) mode provided information concerning
the influence of the synthesis method and the chemical
composition on the surface acidity. First, a small but significant
-
1
at νj > 2500 cm , and on the other hand, artifacts arising in
the difference spectra from the perturbation of NiO mul-
tiphononic absorptions upon ammonia coordination did not
-1
-1
blue shift (7-8 cm ) from sg to cp MgAlx samples with rather
allow the spectral analysis at νj < 1400 cm . The δasym(NH3)
2+
3+
-1
similar Mg /Al ratios was observed, evidencing a slightly
higher acidity in the cp samples. Second, for sg samples, the
mode could be detected at 1600 cm and showed features
comparable to those on NiAlx samples (Figure 5b, curve 2).
2+
3+
3+
influence of the Mg /Al ratio, which ranged from 2.47 to
.29, was evidenced: the δsym(NH3) mode shifted toward higher
These findings suggest that in NiAlx samples, both Al and
2+
4
Ni contributed as Lewis acid sites to ammonia chemisorption,
which was different from the observations for MgAlx samples,
-1
3+
frequencies (5 cm ), increasing the Al content.
2
+
The presence of chlorine had a marked influence on the
surface acidity (Figure 6). Indeed, a large blue shift of the δsym-
where the contribution of Mg was negligible.
Considerations similar to those reported for MgAlx samples
could be made for the influence of the preparation method and
the presence of chlorine.
Ammonia coordinated on Ni-containing mixed oxides showed
thermal stabilities similar to those on Mg-containing samples.
-1
(
NH3) mode (50 cm ) was observed for the highly chlorinated
sg samples, revealing their high acidity. Furthermore, their
spectra in the NH stretching region exhibited a different shape:
a unique slightly asymmetric band in the νasym(NH3) mode

11124 J. Phys. Chem. B, Vol. 104, No. 47, 2000
Prinetto et al.
Figure 8. Selectivity toward DAA (mol of DAA/2mol of acetone ×%)
vs reaction time for cp-MgAl2.69 ([) and sg-MgAl4.13(e) (O).
selectivity toward DAA, markedly higher than those exhibited
5
,36
by Mg/Al mixed oxides.
Remarkably, Mg-containing cata-
Figure 6. Interaction with NH
the effect of chlorine. FT-IR spectra in the NH stretching-mode (section
a) and bending-mode (section b) regions after admission of 200 Pa of
3
at RT on LDHs activated at 773 K:
lysts obtained from sg LDHs showed conversion levels and
initial reaction rates higher than those from cp LDHs (Figure
-
7
a), accounting for more OH sites and possibly stronger
NH
). Each spectrum is reported as the difference from the spectrum before
NH admission.
3
at RT on sg-MgAl4.29 (curves 1) and sg-MgAl4.13(e) (curves
basicity. The conversion levels were close to those at the
2
38
thermodynamic equilibrium, i.e., 23% at 273 K. The selectivity
toward DAA, exceptionally high (>99.5%) for all meixnerite-
like MgAlx samples, was slightly higher for the sg LDHs (Figure
3
8
). The high activity of the sg-MgAl4.13(e) catalyst was
particularly remarkable considering its residual Cl content (0.15
wt-%), which seems to have a less pronounced poisoning effect
on sg than on cp samples.
We have previously shown for meixnerite-like catalysts
2+
3+
obtained from cp LDHs that an increase in the Mg /Al ratio,
inducing a lower positive charge of the brucite-like layers and
-
therefore fewer OH compensating anions, caused a decrease
1
8
2+
in the catalytic activity. At variance, the influence of the Mg /
3
+
Al ratio was not obvious for sg catalysts. In particular, the
carbonate-exchanged sg catalysts with different Mg /Al ratios
exhibited similar activities and the highest initial reaction rates
among all the samples tested. Therefore, the Mg /Al ratio
appears as a secondary parameter compared to both the textural
modifications induced by the sg method and the presence of
carbonates as compensating anions.
Figure 7. Catalytic activity in the aldol condensation of acetone of cp
and sg meixnerite-like LDHs. DAA conversion (%) vs reaction time.
Section a: sg-MgAl4.29 (curve 1), cp-MgAl2.69 (curve 2), and sg-
MgAl4.13(e) (curve 3); each sample was calcined at 723 K and
rehydrated at RT. Section b: sg-NiAl3.39 (curve 1), cp-NiAl2.98 (curve
2+
3+
2
+
3+
2
), and sg-NiAl3.38(e) (curve 3); each sample was calcined at 673 K
5
and rehydrated at 433 K and 6 × 10 Pa in NH
4
OH solutions.
Also in the case of Ni-containing samples, the abundance of
Cl completely abated the catalytic activity (Figure 7b, curve
). The other NiAlx catalysts in the meixnerite-type form were
3
.4. Catalytic Activity. The sg and cp LDHs were tested in
-
the acetone condensation reaction, chosen as a model. It has
been extensively shown that DAA and mesityl oxide are formed
with mixed-oxide catalysts, where acid-base pairs are the major
1
active, but the conversion levels attained were lower than those
of the MgAlx samples. Different from the observations for Mg-
containing catalysts, the conversion level decreased from the
cp to the sg catalyst (Figure 7b) due to the high amounts of
-
sites, while DAA alone is obtained with OH -exchanged
LDHs.3
6,37
All the sg and cp as-prepared materials were catalytically
inactive due to water and compensating anions in the interlayer
space blocking the catalytic sites.
The meixnerite-type compounds obtained by calcination of
the LDHs and subsequent rehydration under appropriate condi-
tions (see Experimental Section) were thereafter tested.
As expected, MgAlx samples still containing high amounts
of chlorine were inactive (Figure 7a, curve 1). The other Mg-
containing meixnerite catalysts showed high activity and
-
remaining Cl ions and organic species in the latter. Indeed,
calcination of the LDH at temperature higher than 673 K should
be required but could not be adopted, therefore hindering the
reconstruction of the mixed oxide into the meixnerite-like
1
8
lamellar form. Accordingly, the catalytic activity of Ni-
containing catalysts precalcined at 773-823 K dramatically
decreased due to the segregation of single oxide phases and
therefore to the poor reconstruction of the lamellar structure
during the rehydration treatment.

Catalysts from Layered Double Hydroxides
J. Phys. Chem. B, Vol. 104, No. 47, 2000 11125
4
. Conclusions
mixed oxides. In the case of Mg/Al oxides, a diminution of the
activity in the aldol condensation of acetone was observed,
TPD and IR results reported above clearly evidence the
2+
3+
37
decreasing the Mg /Al ratio from 3.35 to 2.33 , while Corma
peculiar properties of the mixed oxides obtained by the thermal
decomposition of Mg/Al and Ni/Al LDHs compared to that of
46
et al. found a maximum of activity in Knoevenagel condensa-
2
+
3+
tion reactions at 3:1 Mg /Al , but these reactions require
different basic strengths. In contrast, for the meixnerite-like
catalysts obtained from cp LDHs, the catalytic activity increased
3+
pure MgO and NiO. Indeed, due to the presence of Al cations
in the lattice, these solids show dual character, simultaneously
possessing basic and acid sites of medium-high strength.
Interestingly, CO2 adsorption followed by IR spectroscopy
evidenced a higher number of strongly basic O2 sites on MgAlx
mixed oxides than on pure MgO. The relative amounts of basic
and acidic sites strongly depend on the nature of the divalent
2
+
3+
18
at the reverse of the Mg /Al ratio, while for the sg
meixnerite-like catalysts studied, the influence of this parameter
-
3
+
is not revealed. On one hand, the increase in Al content is
expected to increase the number of surface defects in the mixed-
oxide lattice and, therefore, of catalytically active O2 basic
Lewis sites. In the meixnerite-like LDHs, it induces an increase
in the layer positive charge and, therefore, in the number of
-
cation, Mg2 or Ni , involved in the LDH structure. TPD and
IR data are well in agreement on the higher amount and strength
of basic sites in Mg-containing mixed oxides.
Besides the nature of the divalent cations, the synthesis
method, the calcination temperature, and the nature of the
interlayer species have shown a determinant influence on the
surface properties of the LDH-derived mixed oxides, in agree-
+
2+
-
OH compensating anions. On the other hand the number of
acidic sites and the possibility of formation of alumina islands
3+
increase with the Al content, so the maximum catalytic activity
3+
is actually reached for Al contents, thereby ensuring a good
compromise between these two aspects.
14,16,39-41
ment with previous results.
The results clearly show a strict correlation between the
basicity of the mixed oxides, which is mainly of the Lewis type,
and the Brønsted-type basicity of the meixnerite-like materials,
which was studied through the acetone self-condensation model
reaction. This correlation is particularly interesting for obtaining
information on the basicity of the meixnerite-like LDHs, which
cannot be directly investigated through the spectroscopic
methods due to their high hydration degree and unstability upon
thermal treatments.
The sol-gel route leads to mixed oxides with increased
number and strength of basic sites. This finding can be ascribed
to the different textural and morphological features shown by
sg materials and, particularly, to their greater number of defects
in comparison with cp materials.15 The higher basicity of mixed
oxides obtained from sg LDHs turns into a higher catalytic
activity of the corresponding meixnerite-type catalysts, i.e., the
OH -intercalated LDHs, in comparison to those obtained from
cp materials.
An increase in the calcination temperature from 723 to 923
K induces a marked decrease in the surface basicity of the mixed
oxides, accounting for structural changes such as the segregation
of alumina phases and/or the formation of inverse spinel
-
The results concerning the high basicity and catalytic activity
of mixed oxides and meixnerite-like materials obtained from
sg LDHs, together with results previously reported evidencing
15
their peculiar textural and morphological features, are encour-
aging the synthesis and the catalytic application of sg LDHs.
1
1,31
42
domains.
In line with this behavior, Di Cosimo et al.
demonstrated a decrease in the density of surface basic sites
due to a surface aluminum enrichment of the mixed oxides
already increasing the calcination temperature from 473 to 573
K. Del Arco et al. evidenced by pyridine adsorption an increase
in the surface acidity of the mixed oxides from the calcination
at 1023 K of Mg/Al LDHs compared to MgO.
References and Notes
(
(
(
1) Vaccari, A. Appl. Clay Sci. 1999, 14, 161.
2) Cavani, F.; Trifiro’, F.; Vaccari, A. Catal. Today 1991, 11 (2).
3) Reichle, W. T. CHEMTECH 1986, 16, 58.
43
(4) de Roy, A.; Forano, C.; El Malki, K.; Besse, J. P. In Synthesis of
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Reinhold: New York, 1992; p 108.
The nature of the interlayer anions which decompose during
the thermal treatments plays an important role on the acid-
base properties of the LDH-derived mixed oxides. TPD results
clearly evidenced the higher basicity of mixed oxides obtained
by the thermal decomposition of carbonate-exchanged LDHs.
Chlorine, able to substitute oxygens in the mixed oxide lattice,
has a detrimental effect both on the basicity of the mixed oxides,
which showed a preminent acidic character, and on the catalytic
(
5) Reichle, W. T. J. Catal. 1985, 94, 547.
(6) Dumitriu, E.; Hulea, V.; Chelaru, C.; Catrinescu, C.; Tichit, D.;
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Nielsen, P. In New Frontiers in Catalysis; Guczi, L., Solymosi, F., T e´ t e´ nyi,
P., Eds.; Elsevier: Amsterdam, 1993; p 1031.
12) Rives, V.; Labajos, F. M.; Trujillano, R.; Romero, E.; Royo, C.;
Monzon, A. Appl. Clay Sc. 1998, 13, 363,
13) Monzon, A.; Romero, E.; Royo, C.; Trujillano, R.; Labajos, F. M.;
Rives, V. Appl. Catal., A 1999, 185, 53.
(14) Sanchez Valente, J.; Figueras, F.; Gravelle, M.; Kumbhar, P.; Lopez,
(
(
16,40,44,45
activity of the meixnerite-type LDHs.
Therefore, an
-
accurate removal of the Cl species is strictly required, although
small amounts of remaining chlorine in the carbonate-exchanged
sg catalysts have shown to be more endurable than in cp
materials. However, Constantino and Pinnavaia have found
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still in the lamellar form, behaved as basic catalysts causing
the disproportionation of 2-methyl-3-butyn-2-ol to acetone and
acetylene. Such discrepancies, resulting from the different
hydration degree of the catalyst samples, evidence the versatile
character of LDH-derived materials.
(
(
23
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