Thermal degradation of acetate-intercalated hydroxy double and layered hydroxy salts

Article abstract of DOI:10.1021/ic060071k

Two hydroxy double salts (HDSs), zinc copper hydroxy acetate (ZCA) and zinc nickel hydroxy acetate (ZNA), and an analogous layered compound, zinc hydroxy acetate (ZHA), have been prepared by a coprecipitation method. The thermal degradation of these materials was characterized via thermogravimetric analysis (TGA), differential thermal analysis (DTA), and TGA coupled with Fourier transform infrared spectroscopy of gas-phase products, TGA-FTIR. Loss of physisorbed and interlayer H₂O was observed between 50 and 150°C for all compounds. Acetic acid, acetone, water, and CO₂ were released at high temperatures with relative acetone yields found to be dependent on precursor identity, with very little formed from ZCA compared with ZHA and ZNA. Combined FTIR and XRD analysis of solid residues extracted at different points in the heating profile suggests that ketonization occurs via dissociative adsorption of acetic acid on ZnO surfaces. Nanometer-sized ZnO particles were formed from ZHA, showing slight preferential growth in the ZnO (002) lattice direction, while the presence of a second metal, Ni or Cu, served to retard ZnO crystallite growth at temperatures below 600°C and eliminate preferential growth. ZCA leads to the formation of reduced copper species (metallic copper and Cu₂O) when heated to 250°C.

Full text of DOI:10.1021/ic060071k

Inorg. Chem. 2006, 45, 3766−3773
Thermal Degradation of Acetate-Intercalated Hydroxy Double and
Layered Hydroxy Salts
Everson Kandare and Jeanne M. Hossenlopp*
Department of Chemistry, Marquette UniVersity, P. O. Box 1881,
Milwaukee, Wisconsin 53201-1881
Received January 13, 2006
Two hydroxy double salts (HDSs), zinc copper hydroxy acetate (ZCA) and zinc nickel hydroxy acetate (ZNA), and
an analogous layered compound, zinc hydroxy acetate (ZHA), have been prepared by a coprecipitation method.
The thermal degradation of these materials was characterized via thermogravimetric analysis (TGA), differential
thermal analysis (DTA), and TGA coupled with Fourier transform infrared spectroscopy of gas-phase products,
TGA-FTIR. Loss of physisorbed and interlayer H2O was observed between 50 and 150 °C for all compounds.
Acetic acid, acetone, water, and CO2 were released at high temperatures with relative acetone yields found to be
dependent on precursor identity, with very little formed from ZCA compared with ZHA and ZNA. Combined FTIR
and XRD analysis of solid residues extracted at different points in the heating profile suggests that ketonization
occurs via dissociative adsorption of acetic acid on ZnO surfaces. Nanometer-sized ZnO particles were formed
from ZHA, showing slight preferential growth in the ZnO (002) lattice direction, while the presence of a second
metal, Ni or Cu, served to retard ZnO crystallite growth at temperatures below 600
growth. ZCA leads to the formation of reduced copper species (metallic copper and Cu2O) when heated to 250
°
C and eliminate preferential
C.
°
Introduction
single metal. The ability to vary the intralayer metal
composition, as well as the interlayer anion identity, makes
these materials attractive for a broad range of applications
including use as catalysts,1,2 catalyst precursors,3-5 catalyst
support material,6 adsorbents, antacids,7 and ion exchang-
ers.8,9
These materials are also of interest as possible fire
retardant additives for polymers. Previous work in our
laboratory has shown that the addition of copper-containing
HDSs or LHSs to poly(methyl methacrylate)10,11 or poly-
(vinyl ester)12,13 results in enhanced thermal stability. Sig-
nificant reductions were observed in total heat release, as
measured via cone calorimetry, and there was an increase
in the amount of char formed compared with the virgin
polymer. Reduced copper species (Cu and/or Cu2O), which
Layered metal hydroxides with exchangeable interlayer
anions can be utilized to provide nanodimensional structures
with tunable physical and chemical properties.1-9 Examples
of these materials include layered double hydroxides (LDHs)
[M2+xM3+1-x(OH)2]x+(An-x/n)‚mH2O, hydroxy double salts
(HDSs) [(M2+1-xM′2+1+x)(OH)3(1-y)/n]An-(1+3y)/n‚mH2O where
M2+ and M′2+ represent the different divalent metals, and
the analogous layered hydroxy salts (LHSs) containing a
* To whom correspondence should be addressed. E-mail:
jeanne.hossenlopp@mu.edu.
(1) Rives, V. In Layered Double Hydroxides: Present and Future;
Nova: New York, 2001.
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Thermochim. Acta 1996, 276, 65.
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Tech. 2005, in press.
(11) Kandare, E.; Hall, D.; Jiang, D. D.; Hossenlopp, J. M. In Fire and
Polymers IV: Materials and Concepts for Hazard PreVention; Wilkie,
C. A., Nelson, G. L., Eds.; ACS Symposium Series, Oxford University
Press: New York, 2006.
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M. Polym. Degrad. Stab. 2006, in press.
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3766 Inorganic Chemistry, Vol. 45, No. 9, 2006
10.1021/ic060071k CCC: $33.50
© 2006 American Chemical Society
Published on Web 04/04/2006

Thermal Degradation of Acetate-Intercalated Hydroxy Salts
temperature. The resultant suspension was allowed to stand for 24
h. The precipitate was recovered by filtration, washed repeatedly
with water, and dried at room temperature. ZNA and ZCA were
prepared in the same way from 5 mmol equivalents of ZnO and
the corresponding metal acetates in 10 mL of water.
may play a catalytic role in the stabilization process, were
observed in the pyrolysis residues. Detailed characterization
of the thermal degradation pathways of HDSs and LHSs in
the absence of polymers is necessary to fully understand the
role that these materials play in protecting polymer com-
posites.
XRD patterns were obtained by a Rikagu powder diffractometer
operating in parafocusing Bragg-Bretano configuration, with a 1/2°
divergence slit (DS), 1/2° scatter slit (SS), 0.15 mm receiving slit
(RS), and 0.15 mm monochromator receiving slit (MRS) using Cu
KR (λ ) 1.54 Å) radiation source operated at 1 kW. Data acquisition
was performed using 0.036° steps, integrating 20 s/step. Powder
samples were mounted on quartz slides using 10% GE 7031 epoxy
in ethanol after confirming that the epoxy did not perturb the
observed XRD patterns. Peak positions were determined by fitting
XRD data to a pseudo-Voight function using XFIT,31 stripping off
the Cu KR2 wavelength contributions. Basal spacings characterizing
these layered compounds were obtained using Bragg equation, λ
) 2 d sin θ, averaging 00l (l ) 1-3). Average crystallite sizes, τ,
were determined using the Debye-Scherrer equation:
Most literature reports to date have focused on thermal
degradation of LDHs to yield a mixture of metal oxides.14-21
Formation of metal oxides for catalysis has been achieved
through thermal decomposition of metal salts or crystalline
hydrated organic salts.22-25 Both dynamic and static thermal
processes are useful for the preparation of metal oxides for
basic research and technological applications. For instance,
ZnO nanoparticles have been shown to exhibit unique
physical and chemical properties with a wide range of
applications as in cosmetics, surface acoustic wave device
filters, photodetectors, and gas sensors.26 Layered materials
with predetermined metal ratios serve well as precursors for
well-dispersed oxides that can be used as homogeneous
multicomponent catalysts. Variations in the conditions
employed in solvothermal processes27-29 have been used to
prepare nanocrystals of varying shapes and sizes.
τ ) κ λ/(âτ cos θ)
(1.1)
where κ is a constant (0.9 for powders),32 âτ is the full width at
half-maximum height of the target diffraction peak of the material
after correction for Cu KR2 and the instrumental broadening, and
λ is the X-ray wavelength, 1.54 Å for Cu KR1. Assignments of
known compounds were made using the powder diffraction file
(PDF).33
In this work, a detailed analysis of the thermal degradation
pathways is obtained for a set of acetate-containing HDS
and LHS model compounds in order to explore the role of
intralayer metal composition.
Fourier transform infrared (FTIR) spectra of the solid materials
were obtained using the KBr method on a Nicolet Magna-IR 560
spectrometer in the 400-4000 cm-1 region with the spectra
collected at 1 cm-1 resolution, averaging 40 scans. Calcined
inorganic residues were stored in a desiccator prior to making FTIR
measurements. Thermogravimetric analysis (TGA) was performed
to determine thermal stability and degradation pathways using a
Cahn TG-131 device in the temperature range of 50-600 °C at a
heating rate of 20 °C/min in air, flowing at 85 ( 5 mL/min, with
sample sizes of 55.0 ( 5.0 mg contained in quartz cups. The
gaseous products of the decomposition process were analyzed using
a Mattson-FTIR interfaced with the Cahn TG-131 device. Dif-
ferential thermal analysis (DTA) was performed on a SDT 2960
simultaneous DTA-TGA instrument from 50 to 600 °C using 20
( 1 mg samples heated between 50 and 600 °C at 20 °C/min with
air as the purge gas (flow rate, 85 ( 5 mL/min).
Experimental Section
Zinc hydroxy acetate (ZHA), zinc copper acetate (ZCA), and
zinc nickel acetate (ZNA) HDSs were prepared using a literature
synthesis.30 Copper acetate monohydrate, (98.0%) [Cu(CH3COO)2‚
H2O], zinc acetate (99.9%) [Zn(CH3COO)2], nickel acetate tetra-
hydrate, (98.0%) [Ni(CH3COO)2‚4H2O], and zinc oxide (99.9%)
[ZnO] were used as obtained from Aldrich Chemical Co. ZHA was
made from mixing 0.41 g of ZnO (5 mmol) with 0.92 g (5 mmol)
of Zn(CH3COO)2 in 10 mL of water with vigorous stirring at room
(14) Constantino, V. R. L.; Pinnavaia, T. J. Inorg. Chem. 1995, 34, 883.
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11, 624.
(16) Chellam, U.; Xu, Z. P.; Zeng, H. C. Chem. Mater. 2000, 12, 650.
(17) Labajos, F. M.; Rives, V.; Malet, P.; Centeno, M. A.; Ulibarri, M. A.
Inorg. Chem. 1996, 35, 1154.
(18) Prevot, V.; Forano, C.; Besse, J. P. Chem. Mater. 2005, 17, 6695.
(19) Kovanda, F.; Rojka, T.; Dobesova, J.; Machovic, V.; Bezdicka, P.;
Obalova, L.; Jiratova, K.; Grygar, T. J. Solid State Chem. 2006, 179,
812.
(20) Sileo, E. E.; Jobbagy, M.; Paiva-Santos, C. O.; Regazzoni, A. E. J.
Phys. Chem. B. 2005, 109, 10137.
The identities of the combustion gases were determined by
comparing their FTIR spectra to NIST standards.34 Diffuse reflec-
tance spectra (DRS) of the samples were taken on a Shimadzu UV-
2501PC in the reflectance mode at room temperature using BaSO4
as a reference, with the arbitrary absorbance spectra recalculated
using the Kubelka-Munk (KM) equation, defined as follows
(21) Lin, Y.; Adebajo, M. O.; Frost, R. L.; Kloprogge, J. T. J. Therm.
Anal. Cal. 2005, 81, 83.
(22) Arii, Tadashi.; Kishi, A. Thermochim. Acta 2003, 400, 175.
(23) Kawai, Akiko.; Sugahara, Y.; Park, I. Y.; Kuroda, K.; Kato, C. Ceram.
Powder Sci. IV 1991, 22, 75.
F(R∞) ) (1 - R)2/2R
(1-2)
(24) Vithal Ghule, A.; Lo, B.; Tzing, S.; Ghule, K.; Chang, H.; Ling, Y.
C. Chem. Phys. Lett. 2003, 381, 262.
(25) Bera, P.; Rajamathi, M.; Hegde, M. S.; Kamath, P. V. Bull. Mater.
Sci. 2000, 23, 141.
(26) Bae, H. Y.; Choi, G. M. Sens. Actuators, B 1999, 55, 47.
(27) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000,
122, 12700.
(31) Cheary R. W.; Coelho A. A. Programs XFIT and FOURYA, deposited
in CCP14. Powder Diffraction Library, Engineering and Physical
Sciences Research Council, Daresbury Laboratory, Warrington,
England. (http://www.ccp14.ac.uk/tutorial/xfit- 95/xfit.htm), 1996.
(32) Jenkins, R.; Synder, R. L. Introduction to X-ray Powder Diffractom-
etry; Wiley: New York, 1996.
(28) Jiang, X.; Xie, Y.; Lu, J.; Zhu, L.; He, W.; Qian, Y. Chem. Mater.
2001, 13, 1213.
(29) Yu, S.; Wu, Y.; Yang, J.; Han, Z.; Xie, Y.; Qian, Y.; Liu, X. Chem.
Mater. 1998, 10, 2309.
(30) Morioka, H.; Tagaya, H.; Karasu, M.; Kadokawa, J.; Chiba, K. Inorg.
Chem. 1999, 38, 4211.
(33) Powder Diffraction File Alphabetical Indexes. Inorganic Phases.
JCPDS, International Centre for Diffraction Data: Swartmore, PA,
1999.
(34) NIST Spec Data Center, Stein, S. E. dir. IR and Mass Spectra. In
NIST Chemistry Webbook, Acetic acid and acetone: Coblentz Society,
Inc, http://webbook. nist.gov/chemistry/.
Inorganic Chemistry, Vol. 45, No. 9, 2006 3767

Kandare and Hossenlopp
where F(R∞) is analogous to absorbance and R is the reflectance
percent fraction. Elemental analysis was carried out by Huffman
Labs, Colorado, using the atomic emission spectroscopy interfaced
with inductively coupled plasma (AES-ICP) for metals determi-
nation. ZCA: Cu3.6Zn1.4(OH)7.6(CH3CO2)2.4‚5H2O, [Cu (33.6%
calcd, 35.6% exptl), Zn (13.4% calcd, 14.8% exptl), C (8.5% calcd,
9.4% exptl), H (3.7% calcd, 3.2% exptl)], ZHA: Zn5(OH)8-
(CH3CO2)2‚4H2O, [Zn (50.1% calcd, 48.8% exptl), C (7.4% calcd,
7.6% exptl), H (3.4% calcd, 3.25% exptl)] and ZNA: Zn3.2
-
Ni1.5(OH)7.9(CH3CO2)1.5‚1.7H2O [Zn (38.0% calcd, 35.7% exptl),
Ni (16.0% calcd, 14.9% exptl), C (6.5% calcd, 7.6% exptl), H (2.9%
calcd, 3.25% exptl)].
Results and Discussion
ZHA and ZNA are expected to have structures consistent
with brucite-like35,36 layered compounds where the second
metal occupies tetrahedral sites above and below vacant
octahedral positions. Anions bind to the tetrahedral sites.
ZCA has been hypothesized to have a botallackite-like
structure, Cu2(OH)3Cl, (PDF no. 8-88)33 as extensively
described by Masciocchi and co-workers.37 ZHA, ZCA, and
ZNA were synthesized as described above, and some of the
key structural parameters are summarized below. Additional
discussion can be found in our previous study of anion-
exchange kinetics in these model systems.38
Figure 1. (A) TGA and corresponding DTG curves of ZHA from 50 to
600 °C at 20 °C/min in air. (B) DTA curve for ZHA from 50 to 600 °C at
20 °C/min in air.
Basal spacings were found38 to agree well with literature
values. The spacing for ZCA was found to be 9.3 Å,
comparable to the literature value of 9.4 Å.30 Larger basal
spacings of 13.6 and 13.0 Å similar to 13.2 and 12.8 Å30
were observed for ZHA and ZNA, respectively. The crys-
tallite size in the c-axis direction was calculated to be 540
( 30 Å for ZCA, 270 ( 40 Å for ZNA, and 350 ( 45 Å for
ZHA.
The TGA and corresponding derivatized mass, DTG,
curves of ZHA heated in air from 50 to 600 °C are shown
in Figure 1A, and the DTA curve for the corresponding
heating ramp is shown in Figure 1B. The TGA and DTG
curves exhibit two distinct degradation regions: one in the
temperature range of 50-150 °C and the other in the range
of 150-350 °C. A final total mass loss of 38% is observed.
This compares well with the expected mass loss of 37%,
which is based on formation of ZnO as the final product
from the empirical formula of the ZHA precursor. Combining
the TGA and DTA data with TGA-FTIR experiments
provides a detailed view of the decomposition process. FTIR
spectra of the gas-phase products from thermal decomposi-
tion of ZHA in air are presented in Figure 2.
Surface-adsorbed and intercrystalline water molecules are
lost in the first degradation stage. A shoulder at about 110
°C in the DTA curve, shown in Figure 1B, is assigned
primarily to the loss of the external surface water molecules,
while a stronger endothermic feature centered at 140 °C is
attributed to the loss of the intergallery hydrogen-bonded
water molecules. The FTIR spectrum at 121 °C confirms
Figure 2. FTIR spectra of evolved gaseous products collected at the
temperature (°C) indicated on the right of each spectrum during thermal
combustion of ZHA in air (20 °C/min). FTIR spectra were scaled by dividing
by 10 except for those at 121 and 198 °C, which were not scaled. Expected
positions of absorption bands for water stretching modes (a), bending mode
(b), CO2 antisymmetric stretch (c), overlapping acetic acid and acetone
CdO stretch (d), CH3 deformation (e), and C-C bending for acetone and
C-O stretch for acetic acid (f) are shown. See Figure 3 for further detail
on acetic acid and acetone spectra.
the departure of water is the primary channel during the first
degradation step. The expected weight loss due to simple
dehydration, based on having four water molecules per
formula unit, is 11%. However, the weight loss in the first
stage is significantly higher, approximately 18%. This
suggests that there may be partial dehydroxylation in this
low-temperature regime. The second degradation stage is
attributed to the collapse of the intralayer structure releasing
a variety of products. Stable products may include acetic
acid produced via deanation; acetone generated via keton-
ization of acetic acid; water which may be generated via
dehydroxylation of Zn-OH linkages, thermo-oxidation of
organic species, and ketonization of acetic acid; and carbon
dioxide resulting from thermo-oxidation of organic species
and ketonization of acetic acid. The evolution of CH3CO2H
(35) Rajamathi, M.; Thomas, G. S.; Kamath, P. V. Proc. Indian Acad. Sci.
(Chem Sci.) 2001, 113, 671.
(36) Stahlin, W.; Oswald, H. R. Acta Crystallogr. B, 1970, 26, 860.
(37) Masciocchi, N.; Corradi, E.; Sironi, A.; Moretti, G.; Minelli, G.; Porta,
P. J. Solid State Chem. 1997, 131, 252.
(38) Kandare, E.; Hossenlopp, J. M. J. Phys. Chem. B. 2005, 109, 8469.
3768 Inorganic Chemistry, Vol. 45, No. 9, 2006

Thermal Degradation of Acetate-Intercalated Hydroxy Salts
Figure 3. FTIR spectra of evolved gaseous products collected at 328 °C
during thermal combustion of ZHA in air, acetic acid, and acetone.
Literature34 acetic acid and acetone spectra were scaled to fit on the same
plot.
Figure 4. XRD analysis of residual products collected at various times
during thermal combustion of ZHA in air (50-600 °C). Samples were heated
at 20 °C/min to the final temperature shown on the right. ZnO expected
peak positions are shown with sticks on the bottom of the figure; Miller
indices are labeled on the top.
begins at about 150 °C and peaks at around 305 °C,
suggesting that the second stage of decomposition is mainly
due to the loss of the acetate groups. In addition to formation
of acetic acid, acetone, water, and CO2 are formed at
temperatures above 300 °C. The FTIR spectrum of gaseous
products evolved at 328 °C along with reference spectra34
of acetic acid and acetone are shown in Figure 3; additional
spectra obtained between 300 and 400 °C are found in the
Supporting Information.
Table 1. ZHA Thermal Residue: Relative Intensities of ZnO X-ray
Diffraction Peaks
relative intensity
ZHA
polycrystalline
ZnOb
lit. ZHA heated
2θ (deg)
hkl
(600 °C)a
at 600 °C c
31.9
34.5
36.4
47.7
56.8
63.1
100
002
101
102
110
103
0.55
0.67
1.00
0.27
0.33
0.36
0.57
0.44
1.00
0.23
0.32
0.29
0.1
1.0
0.3
0.2
Aliphatic carboxylic acids having R-hydrogens undergo
bimolecular decarboxylative coupling (ketonization) yielding
ketones.39,40 Two mechanisms for the formation of acetone
from acetic acid over metal oxide catalysts have been
proposed. In the first mechanism, proposed for oxides of low
lattice energy, bulk acetates are formed first and then
subsequently decompose pyrolytically into acetone and
CO2.41-43 This is promoted by strong basicity (ionicity) of
these oxides. In the second mechanism, for oxides of high
lattice energy, the initial step of ketonization involves
adsorptive interaction of acetic acid molecules with the metal
oxide surface generating surface acetate species. Abstraction
of an R-hydrogen atom from the acetate anion oriented
parallel to the surface of the catalyst leads to the formation
of an alkylidene group which in turn reacts with a neighbor-
ing carboxylate and hydrogen to form acetone, water, and
CO2.39,40 To evaluate these potential mechanisms, solid
residues were extracted at different points along the heating
ramp and analyzed via XRD and FTIR spectroscopy.
XRD patterns of the inorganic residue collected after
heating ZHA in air at a ramp rate of 20 °C/min up to the
indicated temperatures are shown in Figure 4. Calcination
to 150 °C yields a mixture of ZHA and wurtzite ZnO
(P 6(3) mc; PDF no. 36-1451)33 marked with open
diamonds. The observation of ZnO reflections after heating
to this temperature is consistent with the previously noted
extent of water loss beyond that which is expected from the
a This work; sample heated at 20 °C/min in air to final temperature
indicated. b PDF no. 36-1451, ref 33. c Estimated from data presented in
ref 30; sample heated for 60 min.
number of water molecules per precursor formula unit. Some
dehydroxylation of ZHA to form ZnO and water may be
occurring in this low-temperature regime. However, the
existence of (00l) reflections of ZHA to the third order at
temperatures as high as 250 °C implies that much of the
ZHA phase is thermally stable throughout the first stage of
mass loss. Surprisingly, the interlayer spacing increased by
1.7 Å with temperature, probably a result of the rearrange-
ment of the acetate anions in the gallery space following
the loss of intercrystalline water molecules.
Calcination of ZHA at higher temperatures leads to
formation of polycrystalline ZnO; peak positions and relative
intensities for the sample heated to 600 °C are found in Table
1. Heating between 300 and 600 °C yields wurtzite ZnO as
the only polycrystalline phase. Average ZnO crystallite sizes,
as determined from the width of the (100) reflection, are
listed in Table 2. The crystallite size increases with temper-
ature from 322 ( 22 Å after annealing to 250 °C to 585
(56 Å upon heating to 600 °C. Relative intensities of the
ZnO reflections exhibit a slight propensity for growth in the
(002) lattice dimension. The (002) peak is more intense than
the (100) peak, a reversal of what is expected for polycrys-
talline ZnO.33 Morioka and co-workers reported a more
pronounced preference for crystalline growth along the (002)
lattice direction when annealing ZHA at 600 °C for 60 min;
their data are also summarized in Table 1.30 ZnO micro-
structure derived from ZHA is thus sensitive to the thermal
degradation protocol employed.
(39) Pestman, R.; Koster, R. M.; van Duijne, A.; Pieterse, J. A. Z.; Ponec,
V. J. Catal. 1997, 168, 265.
(40) Mekhemer, G. A. H.; Halawy, S. A.; Mohamed, M. A.; Zaki, M. I. J.
Catal. 2005, 230, 109.
(41) Davis, R.; Schultz, H. P. J. Org. Chem. 1962, 27, 854.
(42) Gonzalez, F.; Munuera, G.; Prieto, J. A. J. Chem. Soc, Faraday Trans.
1 1978, 74, 1517.
(43) Nakai, R.; Sugh, M.; Nakao, H. J. Am. Chem. Soc. 1959, 81, 1003.
Inorganic Chemistry, Vol. 45, No. 9, 2006 3769

Kandare and Hossenlopp
Table 2. Average Crystallite Sizes of Metal Oxide Phases
crystallite sizes (Å)
ZnO
CuO
NiO
precursor
250 °C
300 °C
400 °C
600 °C
600 °C
600 °C
ZHA
ZNA
ZCA
322 ( 24
159 ( 11
338 ( 68
400 ( 36
145 ( 13
361 ( 45
419 ( 39
188 ( 17
293 ( 54
585 ( 56
482 ( 48
496 ( 62
-
-
-
205 ( 50
-
378 ( 28
FTIR spectra of the residue obtained after calcinations of
the ZHA to specified temperatures and that of bulk ZnO
exposed to acetic acid vapor at room temperature for 2 h,
identified as ZnO-Ac, are shown in Figure 5. The FTIR
spectrum obtained after heating ZHA to 250 °C can be
assigned in analogy to acetate adsorbates on ZnO.44 A weak
but sharp M-O absorption band is observed at 620 cm-1,
weak and broad M-O and δ(CH3) absorption bands are
found at 700 and 1030 cm-1, respectively, and broadened
symmetric and antisymmetric C-O stretches occur at around
1420 and 1560 cm-1, respectively. The shift in position of
the C-O stretches, in comparison to the ZHA precursor, is
consistent with the increase in interlayer spacing observed
in the XRD data in Figure 4, ascribed to reorganization of
the anions in the galleries. The most intense bands, assigned
to the C-O stretching modes, persisted in samples heated
to 350 °C, beyond the temperature where evidence for ZHA
layered structures were observed in XRD analysis of residues.
The combined FTIR spectra and XRD data for samples
heated to 300 and 350 °C suggest acetate in contact with
ZnO. A shoulder at 1460 cm-1 in the FTIR trace at 250 °C
is also observed in the ZnO-Ac spectrum, and this is
consistent with adsorption of acetate groups on the ZnO
surface during thermal decomposition. Peak assignments for
the acetate anions bound to the ZnO surface are as follows.
1554 cm-1 (antisymmetric O-C-O stretching), 1460 cm-1
(δ(CH3)), 1428 cm-1 (symmetric O-C-O stretching), and
1037 cm-1 (δ(CH3)).
during the thermal decomposition process of ZHA in this
temperature regime.
TGA and corresponding DTG curves of ZNA in air are
shown in Figure 6A, while the DTA curve is shown in Figure
6B. ZNA is structurally similar to ZHA; thus, it would be
expected to show a similar thermal degradation pathway. One
difference between these two precursors is the more evident
third degradation step observed in the ZNA TGA data. For
ZNA in air, a total mass loss of ∼ 8% (calcd 6% based on
elemental analysis of precursor) is observed between 50 and
150 °C and is attributed to the loss of physisorbed and
intergallery water molecules. This is consistent with a DTA
Figure 5. FTIR spectra of residual products collected after heating ZHA
at 20 °C/min in air to the indicated temperature (°C). Reference spectra for
ZHA precursor (bottom trace) and for ZnO exposed to acetic acid vapor
(top trace) are also provided. Dotted vertical lines indicate absorption band
positions for O-C-O antisymmetric (a) and symmetric stretch (b),
respectively, deformation mode (δ(CH3)) (c), and Zn-O stretch (d) in the
precursor ZHA.
The similarity between FTIR spectra of ZnO-Ac and bulk
zinc acetate precludes a definitive assignment to either
species. However, the absence of polycrystalline zinc acetate
phases in the XRD patterns of the inorganic residues in this
temperature regime suggests that ketonization occurs via
surface adsorption of acetic acid over ZnO. The basic nature
of the ZnO surface promotes surface adsorption of acidic
molecules providing catalytic sites over which ketonization
progresses.
Loss of surface adsorbed and intercrystalline water mol-
ecules occur primarily in the temperature range of 50-150
°C together with possible partial dehydroxylation of the
precursor ZHA. In the temperature region of 150-250 °C,
simultaneous dehydroxylation and deanation results in the
formation of ZnO, ZnO surface-adsorbed acetates (ZnO-
Ac), acetic acid, and water. At temperatures higher than 250
°C, in addition to products mentioned above, evolution of
CO2 and acetone is observed. This is attributed to the thermal
decomposition of organic species and possible ketonization
Figure 6. (A) TGA and corresponding DTG curves of ZNA from 50 to
600 °C at 20 °C/min in air. (B) DTA curve for ZNA from 50 to 600 °C at
20 °C/min in air.
(44) Spitz, R. N.; Barton, J. E.; Barteau, M. A.; Staley, R. H.; Sleight, A.
W. J. Phys. Chem. 1986, 90, 4067.
3770 Inorganic Chemistry, Vol. 45, No. 9, 2006

Thermal Degradation of Acetate-Intercalated Hydroxy Salts
Figure 9. FTIR spectra of residual products collected after heating ZNA
at 20 °C/min in air to the indicated temperature (°C). Reference spectra for
ZNA precursor (bottom trace) and for ZnO exposed to acetic acid vapor
(top trace) are also provided. Dotted vertical lines indicate absorption band
positions for O-C-O anti-symmetric (a) and symmetric stretch (b),
respectively, deformation mode (δ(CH3)) (c), and Zn-O stretch (d) in the
precursor ZNA.
Figure 7. FTIR spectra of evolved gaseous products collected at the
temperature (°C) indicated on the right of each spectrum during thermal
combustion of ZNA in air (20 °C/min). FTIR spectra were scaled by dividing
by 10 except for those at 120 and 197 °C which were not scaled. Expected
positions of absorption bands for water stretching modes (a), bending mode
(b), CO2 antisymmetric stretch (c), overlapping acetic acid and acetone
CdO stretch (d), CH3 deformation (e), and C-C bending for acetone and
C-O stretch for acetic acid (f) are shown.
summarized in Table 2. The crystallite size of ZnO was found
to be 145 ( 13 Å after heating to 300 °C and increased to
482 ( 48 Å after heating to 600 °C. The crystallite size of
NiO after heating to 600 °C was calculated to be 205 ( 50
Å using the nonoverlapping 200 reflection of NiO at 43.2 °.
The presence of Ni2+ in the intralayer of ZNA retards growth
of the ZnO phase, with crystallite sizes observed to be one-
half or less of the average sizes obtained from ZHA at
temperatures up to 400 °C.
Careful inspection of FTIR spectra of the gas-phase
products evolved in the temperature region of 300-400 °C
(Supporting Information) reveals some differences in the
profile for evolution of acetic acid, acetone, CO2, and water
from ZNA when compared with ZHA. Evolution of acetic
acid from ZNA begins at temperatures <250 °C. There is a
temperature shift in terms of the relative contributions of
deanation versus ketonization channels for the ZNA relative
to ZHA. Examination of the relative intensities of the
carbonyl stretching bands centered at 1788 and 1730 cm-1
for acetic acid and acetone, respectively, suggests that, for
temperatures below 340 °C, the production of acetone relative
to acetic acid from ZNA is lower than that observed at the
same temperature from ZHA. Further work is necessary to
quantify yields. However, when combined with the XRD
analysis of solid residues, which exhibit smaller ZnO average
crystallite size for ZNA, the pattern of gas-phase product
evolution suggests that the presence of ZnO catalytic surface
is responsible for promoting ketonization.
FTIR spectra of solid residues of ZNA obtained at different
points in the heating ramp and ZnO-Ac are shown in Figure
9. The FTIR trace for the sample heated to 300 °C suggests
adsorption of acetic acid moieties on the ZnO surface leading
to its ketonization. CO2 produced during ketonization may
then bind to ZnO, forming a metal carbonate species, as
evident from a very broad absorption band (1250-1800
cm-1) in the FTIR trace at 350 °C.40 The continuous release
of CO2 and water at temperatures above 400 °C may be
attributed to the thermo-oxidative degradation of organic
species such as acetone.
Figure 8. XRD analysis of the inorganic residue from heating ZNA in
air to temperatures indicated; trace for the residue at 600 °C shows ZnO
reflections (]) and NiO reflections (0).
endothermic feature at around 140 °C, suggesting absorption
of heat necessary to evaporate water molecules. A significant
mass loss (∼10%) in the 150-250 °C region is attributed
to the loss of water and acetic acid.
Multiple weight losses totaling 17% are observed in the
region of 250-350 °C with simultaneous loss of water, acetic
acid, and acetone in the earlier stages followed by the
evolution of CO2 at temperatures above 300 °C, as shown
in the FTIR spectra of TGA evolved gases shown in Figure
7. A weak DTA endothermic feature at 230 °C and a
vigorous exothermic peak at 340 °C in Figure 6A correspond
to the complete thermal decomposition of ZNA resulting
from the loss of OH ions bound to Ni2+ and the decomposi-
tion of the acetate anions, respectively.45
Data obtained from XRD analysis of solid residues
obtained at various points in the heating ramp for ZNA are
shown in Figure 8. At 150 and 200 °C, the XRD patterns
indicate the presence of the polycrystalline layered ZNA
phase. After heating to 250 °C, ZNA and ZnO (marked in
empty diamonds) are observed. Complete collapse of layered
structure occurs at 300 °C, yielding polycrystalline ZnO and
NiO (marked in empty squares; PDF no. 47-1049).33 Further
heating in the range of 300-600 °C yields ZnO and NiO.
Average crystallite sizes of the metal oxide phases are
TGA and corresponding DTG curves of ZCA in air are
shown in Figure 10A, with corresponding DTA data provided
in Figure 10B. A 7% mass loss in the temperature range of
(45) Nishizawa H.; Yuasa, K. J. Solid State Chem. 1998, 141, 229.
Inorganic Chemistry, Vol. 45, No. 9, 2006 3771

Kandare and Hossenlopp
Figure 12. XRD analysis of the residue from thermal combustion of ZCA
in air; the temperatures to which ZCA was heated are indicated. Only ZnO
(]) and CuO (b) were observed after heating to 600 °C. Peaks assigned to
metallic copper (*) and Cu2O (4) were observed in samples heated to 250
and 300 °C.
feature in the DTA curve. FTIR spectra shown in Figure 11
(with additional spectra provided as Supporting Information)
show that acetic acid, water, and CO2 are the primary
products in the temperature range of 250-400 °C with very
little formation of acetone.
Figure 10. (A) TGA and corresponding DTG curves of ZCA from 50 to
600 °C at 20 °C/min in air. (B) DTA curve for ZCA from 50 to 600 °C at
20 °C/min in air.
XRD patterns of the inorganic residue in air after heating
to various temperatures are shown in Figure 12. The XRD
data for the residue obtained from heating ZCA to 150 °C
reveal the presence of ZCA as the only polycrystalline
product. Heating to 200 °C yields an amorphous product
showing a broad feature at 6.6° corresponding to an
unidentified phase. Note that the ZCA layered structure
disappears at an earlier temperature than it does for ZHA or
ZNA. Weak, broad ZnO features are observed after heating
to 250 °C, along with metallic copper (labeled with asterisks
in Figure 12; PDF no. 4-836)33 and Cu2O (open triangles
triangles; PDF no. 35-1091).33 Simultaneous presence of
metallic copper, Cu2O, and ZnO suggests macroscopic
heterogeneity. After heating to 600 °C, ZnO and CuO are
the only polycrystalline phases formed.
Results reported herein are similar to those observed by
Guo and co-workers from the calcination of Zn[Cu(CN)3].5
Cu is oxidized to Cu2O, which in turn is oxidized to CuO as
a result of direct contact with oxygen. The approximate
crystallite sizes of ZnO and CuO particles obtained from
ZCA after heating to 600 °C in air are 496 ( 62 and 378 (
28 Å, respectively. When fitting the XRD data to extract
peak widths for determination of average crystallite size of
ZnO, the overlapping ZnO (100) and CuO (110) reflections
were fit simultaneously. However, the overlapping peaks and
relatively weak ZnO reflections lead to larger uncertainty in
the extracted crystallite sizes indicated in Table 2. Within
the uncertainty limits, there appears to be little ZnO crystallite
growth until 600 °C, as was also observed with ZNA,
possibly due to phase separation effects.
Figure 11. FTIR spectra of evolved gaseous products collected at the
temperature (°C) indicated on the right of each spectrum during thermal
combustion of ZCA in air (20 °C/min). FTIR spectra were scaled by dividing
by 10 except for those at 121 and 192 °C which were not scaled. Expected
positions of absorption bands for water stretching modes (a), bending mode
(b), CO2 antisymmetric stretch (c), overlapping acetic acid and acetone
CdO stretch (d), CH3 deformation (e), and C-C bending for acetone and
C-O stretch for acetic acid (f) are shown.
50-150 °C in air, Figure 10A, is attributed to the dehydration
process resulting from the loss of surface and interlayer water
molecules consistent with a DTA endothermic feature at
about 140 °C shown in Figure 10B. The weight increase
observed at temperatures above 400 °C is due to oxidation
of Cu and Cu2O to CuO.
FTIR spectra of gas-phase products formed at 121 and
192 °C, shown in Figure 11, reveal that water is the primary
product in this temperature range. A second endothermic
degradation step that begins at around 200 °C is consistent
with both deanation and dehydroxylation, with water and
acetic acid observed in the FTIR spectra in this temperature
range. The DTA curve for ZCA in air shows two overlapping
exothermic peaks between 230 and 320 °C. The final step
just above 230 °C is consistent with the loss of organic anions
through deanation and/ or thermo-oxidation, leading to the
formation of CO2 and H2O, as evident from a sharp weight
loss occurring at 240 °C accompanied by an exothermic
FTIR spectra of the residues obtained when heating ZCA
are shown in Figure 13. The inorganic residues at temper-
atures above 250 °C are acetate-free. There is no evidence
of either metal acetate formation or adsorption of the acetate
anions on metal oxides, consistent with the minimal acetone
production observed from this precursor. Instead, carbonate-
containing species are seen at temperatures of 300 °C and
beyond probably as a result of combination of CO2 with
3772 Inorganic Chemistry, Vol. 45, No. 9, 2006

Thermal Degradation of Acetate-Intercalated Hydroxy Salts
formation of reduced copper is thus important for enhancing
selected chemical pathways.
Some of the key differences in structure between ZHA,
ZNA, and ZCA were discussed in our previous report.38 FTIR
spectra suggest that the bonding of the acetate to the metal
centers in the gallery spacing is pseudo-bridging in ZCA with
one acetate oxygen bound to Cu2+ and the other acetate
oxygen forming a hydrogen bond with an adjacent OH group
from the metal hydroxide layer, similar to that reported37
for Cu2(OH)3(CH3CO2)‚H2O. ZHA and ZNA spectra were
more consistent with unidentate binding;38 however, the
possibility of hydrogen-bonding interaction of the “free”
carbonyl oxygen cannot be precluded on the basis of the
FTIR data. DTA curves from thermal degradation of ZCA
in air reported here exhibit a pronounced endothermic event
around 210 °C, which was not observed for ZHA and barely
observed for the ZNA thermal degradation in air. This is
consistent with a stronger anion-cation interaction for the
ZCA HDS as compared to ZHA and ZNA. The departure
of the acetate anion without decomposition requires more
energy if it were tightly bound to the metal center as assumed
in ZCA, while less energy will be required for the same
processes when the interaction between the metal center and
the anion is weaker, as predicted for ZHA and ZNA.
Figure 13. FTIR spectra of residual products collected after heating ZCA
at 20 °C/min in air to the indicated temperature (°C). Reference spectrum
for ZCA precursor (bottom trace) is also provided. Dotted vertical lines
indicate absorption band positions for O-C-O antisymmetric (a) and
symmetric stretch (b), respectively, deformation mode (δ(CH3)) (c), and
Zn-O stretch (d) in the precursor ZCA.
metal oxides.40 This might explain the relative lack of
ketonization observed for this system. Metallic copper and
CuO have been reported to have no catalytic effect on the
ketonization of acetic acid to acetone.46 Instead, Pestman and
co-workers46 observed that CuO promotes formation of
acetaldehyde from acetic acid. In the same work, copper in
its partially reduced form, Cu2O, has been shown to promote
ketonization. Even though Cu2O is present in the temperature
region over which acetic acid is released, its concomitant
presence with metallic copper and/or CuO or the existence
of bound carbonate species may render it inactive. In
addition, ZCA has the smallest mole fraction of zinc of the
three model compounds investigated, limiting the ZnO
surface area available for catalyzing the ketonization channel.
Surface-adsorbed and intercrystalline water molecules are
largely lost in the temperature range of 50-150 °C.
Decomposition of ZCA via dehydroxylation and deanation
in the 150-400 °C temperature region results in the
formation of ZnO, CuO, and reduced copper-containing
species, Cu, Cu2O. Acetic acid, water, and CO2 are evolved
during the thermal decomposition at elevated temperatures.
Very little acetone is observed, suggesting that calcination
of ZCA does not promote ketonization. Polycrystalline ZnO
and CuO are the final solid residues remaining after heating
to 600 °C or higher.
Conclusions
The thermal degradation of zinc-containing layered hy-
droxy salt or hydroxy double salts with interlayer acetate
depends on the structure/composition of the metal hydroxide
layer. The initial thermal degradation step for ZHA, ZNA,
and ZCA is loss of intercalated water in the temperature
range of 50-150 °C. Other gas-phase products observed at
higher temperatures include acetic acid, acetone, CO2, and
H2O, and the solid residues contain a mixture of metal oxide
phases. Catalytic ketonization of acetic acid to form acetone
is observed for ZHA and ZNA, with slightly higher tem-
peratures required for the ketonization channel in ZNA. Little
acetone was observed in the thermal decomposition of ZCA.
Combined FTIR and XRD analysis of solid residues ex-
tracted at different points in the heating profile suggests that
ketonization occurs via dissociative adsorption of acetic acid
on ZnO surfaces.
ZnO from the thermal degradation of ZHA revealed a
slight preferential growth in (002) lattice dimension. Inclusion
of the other metals (Ni and Cu) in the HDS structure
containing Zn eliminated preferential crystal growth in any
lattice directions for ZnO and also served to retard the ZnO
crystallite growth. In the case of ZCA, reduced copper
species, Cu2O and metallic copper, were observed in the
thermal degradation of ZCA at 250-500 °C in an air
atmosphere with oxidation to CuO completed by 600 °C.
While heating ZCA did not promote ketonization, we note
the reduced metal species observed in the thermal degrada-
tion process. The presence of reduced metallic species has
been shown to play an important role in thermal stabilization
of polymeric composites.10-13,47,48 Klenov and co-workers49
reported the appearance of Cu(I) and metallic copper on the
surface of the Cu2+-containing ZnO after catalyst reduction
and they believe these species play an important role in the
catalytic synthesis of methanol from H2 and CO. The
(46) Pestman, R.; Koster, R. M.; Pieterse, J. A. Z.; Ponec, V. J. Catal.
1997, 168, 255.
(47) Starnes, W. H.; Pike, R. D.; Cole, J. R.; Doyal, A. S.; Kimlin, E. J.;
Lee, J. T.; Murray, P. J.; Quinlan, R. A.; Zhang, J. Polym. Degrad.
Stab. 2003, 82, 15.
(48) Pike, R. D.; Starnes, W. H.; Jeng, J. P.; Bryant, W. S.; Kourtesis, P.;
Adams, C. W.; Bunge, S. D.; Kang, Y. M.; Kim, A. S.; Kim, J. H.;
Macko, J. A.; O’Brien, C. P. Macromolecules 1997, 30, 6957.
(49) Klenov, D. O.; Kryukova, G. N.; Plyasova, L. M. J. Mater. Chem.
1998, 8, 1665.
Acknowledgment. We thank Marquette University Com-
mittee on Research for partial support of this work and J.
Collins for support of the X-ray diffractometer.
Supporting Information Available: TGA-FTIR profiles.
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC060071K
Inorganic Chemistry, Vol. 45, No. 9, 2006 3773