New hydrotalcite-like anionic clays containing Zr4+ in the layers
S. Velu,a Veda Ramaswamy,a A. Ramani,b Bhanu M. Chandab and S. Sivasanker*a
a Catalysis Division, b Division of Organic Chemistry, Technology, National Chemical Laboratory, Pune 411 008, India
New hydrotalcite-like anionic clays containing Zr4+ in the
brucite-like layers are synthesised by a simple coprecipita-
tion technique; these materials show very interesting proper-
ties as catalysts for liquid-phase hydroxylation of phenol
with H2O2.
isomorphous substitution of Al3+ (Shannon ionic radius 0.53
Å)12 in the octahedral coordination by Zr4+ (Shannon ionic
radius 0.72 Å) in the HT matrix. The increase in c may be due
to the weakening of the interaction between the brucite-like
layer and the interlayer anions or due to the larger number of
anions. Furthermore, since a part of the trivalent cation Al3+ is
being substituted by the tetravalent cation Zr4+, the interlayer
should accommodate more CO322 anions for charge compensa-
tion. This is supported by an increase in carbon content from
2.44% for Zr0.0-HT to 3.55% for Zr0.6-HT (Table 1). Hence,
based on the chemical composition presented in Table 1, the Zr
containing HT-like compounds with Mg/Al + Zr = 3 can be
represented by the following general formula: Mg6Al22xZr-
x(OH)16·[(x + 2)/2]CO3·yH2O, where x can vary from 0 to at
least 1.2.
Hydrotalcite (HT)-like anionic clays are a new family of
interesting materials with applications as catalysts, catalyst
supports, ion exchangers and composite materials.1–3 The
structure of these compounds can be visualised as being made of
brucite [Mg(OH)2]-like octahedral layers in which a part of
Mg2+ is isomorphously substituted by trivalent cations. The
excess positive charge of the layer is compensated by hydrated
anions, such as CO322, present in the interlayer. The materials
are represented generally by the formula: [MII12x,MIIIx,-
(OH)2]x+[(An2
)
x/n·yH2O]x2, where MII = Cu2+, Ni2+, Co2+,
Thermal calcination of these samples at 723 K results in the
formation of an MgO phase whose d(200) value increases from
2.0934 Å for Zr0.0-HT to 2.1174 Å for Zr0.5-HT, indicating the
dissolution of Zr4+ in the MgO lattice. However, a poorly
crystalline ZrO2 phase, in addition to an MgO phase is noticed
in the PXRD of the samples above Zr0.5-HT.
Zn2+, Mn2+; MIII4,5 = Al3+, Fe3+, Cr3+, Ga3+, V3+, and more
recently,6 Ru3+ and Rh3+. A large number of HT-like com-
pounds with a wide variety of cation pairs including a Co–TiIV
layered double hydroxide7 have been reported in recent years.
We now report the synthesis of a Zr4+ containing HT-like
material using a simple coprecipitation technique. The im-
portance of Zr or Ti containing HT-like compounds stems from
the fact that, similar to zeolites containing Zr (Zr-silicalites) or
Ti (Ti-silicalites), they can be used as catalysts for liquid-phase
hydroxylation/oxidation of various organic substrates.8–11 In
the present investigation, we report our preliminary studies on
the synthesis of new Zr-containing anionic clays and their
catalytic performance in the liquid-phase hydroxylation of
phenol.
The dispersion of Zr in the HT framework was studied by
UV–VIS DRS (Fig. 2). All these samples, including that
(a)
(b)
(c)
(d)
Zr-containing hydrotalcites with various Mg:Al:Zr atomic
ratios were synthesized by a coprecipitation method at room
temperature by reacting aqueous solutions containing a mixture
of Mg(NO3)2, Al(NO3)3 and ZrO(NO3)2 and a mixture of NaOH
and Na2CO3 at a constant pH (ca. 10). The resulting precipitate
was filtered, washed with distilled water several times until the
pH of the filtrate was 7 and then dried at 373 K overnight. The
incorporation of Zr in the brucite-like layers was confirmed by
powder X-ray diffraction (PXRD/Rigaku, D-MAX III VC
model, with Ni-filtered Cu-Ka radiation), UV–VIS diffuse
reflectance spectroscopy (UV–VIS DRS; Shimadzu, UV–VIS
Spectrophotometer 2101 PC model) and catalytic hydroxylation
of phenol using hydrogen peroxide (H2O2) as the oxidant.
Fig. 1 shows the PXRD patterns of Mg:Al:Zr-HT with
Mg:Al:Zr atomic ratio ranging from 3:1:0 to 3:0:1. A single
phase corresponding to HT is obtained for all the samples
although the crystallinity of the samples decreases with
increasing Zr content (Table 1). A binary Zr containing HT
without Al forms a poorly crystalline ZrO2 phase [Fig. 1(k)]
rather than a HT phase, indicating that the presence of Al
favours the formation of a pure HT-like phase. The crystallo-
graphic parameters were evaluated employing least-squares
refinement assuming a hexagonal crystal system for samples
Zr0.0-HT–Zr0.6-HT whose PXRD peaks were intense and
sharp enough for accurate determination. The increase in lattice
parameters a and c with a concomitant increase in the unit cell
volume U (Table 1) with increasing Zr content clearly
demonstrates the effective incorporation of Zr4+ in the HT
framework. The increase in a can be attributed to the
(e)
(f )
(g)
(h)
(i )
(j )
(k)
5
20
40
60
2q / °
Fig. 1 PXRD patterns of MgAlZr-HT: (a) Zr0.0-HT, (b) Zr0.1-HT,
(c) Zr0.2-HT, (d) Zr0.3-HT, (e) Zr0.4-HT, (f) Zr0.5-HT, (g) Zr0.6-HT,
(h) Zr0.7-HT, (i) Zr0.8-HT, (j) Zr0.9-HT, (k) Zr1.0-HT
Table 1 Chemical compositions and lattice parameters of MgAlZr-HT
Carbon
contentb
(%)
Lattice parametersc
Mg:Al:Zr
atomic ratioa
Sample
a/Å
c/Å
U/Å3
Zr0.0-HT
Zr0.1-HT
Zr0.2-HT
Zr0.3-HT
Zr0.4-HT
Zr0.5-HT
Zr0.6-HT
3:0.96:0.00
3:0.85:0.07
3:0.78:0.14
3:0.68:0.21
3:0.67:0.33
3:0.57:0.37
3:0.52:0.50
2.44
—
2.71
2.91
3.01
—
3.0584
3.0643
3.0688
3.0687
3.0732
3.0773
3.0780
23.1811
23.4570
23.5305
23.7243
23.7681
23.9383
24.1184
187.8
190.8
191.9
193.5
194.4
196.3
197.9
3.51
a
b
Determined by X-ray fluorescence spectroscopy. Determined by CHN
microanalysis. Refined using least square fitting method for hexagonal
crystal system.
c
Chem. Commun., 1997
2107
Velu
