Solid State Sciences 11 (2009) 330 335
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Solid State Sciences
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Evidence for the formation of anhydrous zinc acetate and acetic
anhydride during the thermal degradation of zinc hydroxy acetate,
Zn5(OH)8(CH3CO2)2$4H2Oto ZnO
a,*,1 a b b b
Timothy Biswick , William Jones , Aleksandra Pacu1a , Ewa Serwicka , Jerzy Podobinski
a
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom
b
Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek 8, 30-329 Krakow, Poland
a r t i c l e i n f o a b s t r a c t
Article history:
Zinc hydroxy acetate, Zn5(OH)8(CH3CO2)2$4H2O, has been prepared by the precipitation method. It has
Received 5 January 2008
been demonstrated by FTIR analysis that, contrary to previous reports, the interaction of the acetate
Received in revised form 19 May 2008
anion with the matrix cation is ionic. TG analysis, mass spectral analysis of the evolved gases, and in situ
Accepted 19 June 2008
variable temperature PXRD and FTIR analysis have shown that decomposition of the material to ZnO
Available online 28 June 2008
involves the formation of Zn5(OH)8(CH3CO2), Zn3(OH)4(CH3CO2)2 and anhydrous zinc acetate
(Zn(CH3CO2)2) as some of the acetate-containing intermediate solid products. The acetate anion is finally
Keywords:
lost, at temperatures below 400 C, as acetic anhydride, (CH3CO)2O.
Hydroxy salts
Ó 2008 Elsevier Masson SAS. All rights reserved.
Anionic clays
Zinc hydroxy acetate
Anhydrous zinc acetate
Thermal decomposition
Zinc oxide
Acetic anhydride
1. Introduction below by two tetrahedrally coordinated divalent cations. In the
second, whilst there is complete occupancy of octahedral cationic
Layered metal hydroxide salts containing exchangeable anions positions, part of the framework hydroxyl groups are substituted by
form a family of inorganic solids attracting ever-increasing atten- structure forming anions such as NO 3 , Cl etc. The potential
tion. The unique structural and physicochemical properties of these applications and synthesis procedures for these materials have
compounds render them excellent substrates for advanced mate- been described elsewhere [1].
rials design [1,2]. Layered double hydroxides (LDHs) and hydroxy Although the structure of zinc hydroxy acetate, Zn5(OH)8
salts, also known as basic salts, are examples of such materials. The (CH3CO2)2$nH2O has not been determined, a number of authors
structures of both families of compounds may be considered as [4 6] have proposed that it belongs to the hydrozincite structure
derived from the layered lattice of brucite, (Mg(OH)2). In the case of [7] in which zinc occupies both octahedral and tetrahedral coor-
LDHs a layer charge develops as a result of partial substitution dination sites in the ratio 3:2. The interaction of the anion with the
of divalent cations by trivalent, thereby necessitating the presence matrix cation in layered hydroxy salts of the hydrozincite structure
of charge balancing anions within the interlayer [3]. In hydroxy has been observed to vary depending on the anion intercalated as
salts, since the cation composition of the brucite-like layers is illustrated by the three structurally related zinc hydroxy salts,
confined to cations of the same valency (usually divalent), the Zn5(OH)8(NO3)2$2H2O, Zn5(OH)8Cl2$H2O and Zn5(OH)6(CO3)2.
existence of potentially exchangeable anions may be created by two The structure of Zn5(OH)8(NO3)2$2H2O has been determined by
mechanisms. In the first, a surplus layer charge is created by the single crystal x-ray diffraction method by Stahlin and Oswald [8]. It
occurrence of octahedral vacant sites that are capped above and consists of infinite brucite-like layers, where one quarter of the
octahedrally coordinated zinc atom sites are vacant and on either
side of the empty octahedra there are zinc atoms tetrahedrally
coordinated by OH groups (forming the base of a tetrahedron), with
* Corresponding author.
a water molecule occupying the apex. Unbound nitrate groups are
E-mail address: [e-mail] (T. Biswick).
1
located between the sheets being hydrogen bonded to two water
Present address: Centre for Intelligent Nano-Bio Materials, Department of
Chemistry, Ewha Womans University, Seoul 120-750, Republic of Korea. molecules of one layer and an OH group of the opposite layer [8]. In
1293-2558/$  see front matter Ó 2008 Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.solidstatesciences.2008.06.018
T. Biswick et al. / Solid State Sciences 11 (2009) 330 335 331
Zn5(OH)8Cl2$H2O, the fourth bond from the tetrahedrally coordi-
nated zinc is to a chloride anion and the water molecule is located
between the layers [9] whereas in hydrozincite, Zn5(OH)6(CO3)2,
the carbonate group is bonded to a tetrahedral zinc of one layer and
also to an octahedral zinc of the opposite layer [7] with the result
that this is a rigid three-dimensional structure. The mode of
interaction of the acetate anions with the matrix cation in zinc
hydroxy acetate is, however, not clear. Poul et al. [10] and Hosono
et al. [4] proposed that the acetate anion occurs as a free species
while in a recent report, Kandare and Hossenlopp [11] have
10 15 20 25 30 35 40 45 50 55 60 65
proposed (on the basis of FTIR data) that the acetate anion is
2 (degrees)
directly coordinated to the matrix cation as a unidentate ligand via
M OCOCH3 bonds.
Most studies on zinc hydroxy acetate have mainly focused on
the physical and chemical properties of ZnO produced upon its
thermal degradation. ZnO is of particular interest in view of its low
5 10 15 20 25 30 35 40 45 50 55 60 65
toxicity and a wide range of possible applications, including solar
2 (degrees)
cell technology, catalysis and photo-catalysis, thin-film gas sensors,
varistors, transparent conductive electrodes and surface acoustic
Fig. 1. PXRD pattern of Zn5(OH)8(Ac)2$4H2O. Insert shows details of the high angle
wave devices [12 14]. Detailed characterisation of the solid mate- reflections.
rials as well as the gaseous products released at different temper-
atures is, therefore, necessary in order to fully understand the
heated at the rate of 3 C min in flowing argon (10 ml min) using
chemical processes occurring during the thermal decomposition.
RGA200 Stanford Research quadrupole mass spectrometer.
The objectives of the work in the present report are to investi-
gate the type of interaction of the acetate anions with the matrix
cation in Zn5(OH)8(CH3CO2)2$nH2O and also to monitor the phase
3. Results
changes accompanying the thermal treatment of the material at
different temperatures.
3.1. Characterisation of the parent material
2. Materials and methods
The results of chemical elemental analysis of the material show
general agreement between experimental and calculated (based on
2.1. Materials
the formula Zn5(OH)8(Ac)2$4H2O) values: C  7.48% obsrvd, 7.35%
calc. and H  2.79% obsrvd, 3.37% calc. The amount of water was
Zinc hydroxy acetate with idealised composition Zn5(OH)8 calculated from the overall mass loss; assuming that the starting
(CH3CO2)2$nH2O was prepared by the titration method. Fifty
material had the ideal formula Zn5(OH)8(Ac)2$nH2O and that the
milliliters of a 0.75 M NaOH (aq) solution was slowly added to 20 ml
final material remaining upon thermal degradation is ZnO. It is
of 3.5 M zinc acetate dihydrate aqueous solution, with constant
interesting to note that although the observed C content is greater
stirring at room temperature. The white precipitate was immedi-
than the calculated value, the observed H content is significantly
ately filtered, washed twice with deionised water and dried
less than the calculated value. This may suggest the presence of
at 60 C.
impurity carbonate anions from atmospheric carbon dioxide co-
intercalated with the acetate anions during synthesis and handling
2.2. Methods
of the sample.
The PXRD pattern of the material, presented in Fig. 1, is typical of
PXRD data was collected on a Philips X Pert MPD diffractometer
a layered material exhibiting sharp and symmetric reflections at
using Cu Ka radiation (lź1.5440 Å) operating at 40 kv and 40 mA.
The patterns were recorded from 5 to 80 with a scan step of 19.7 s
and a step size of 0.02 . Variable temperature PXRD was collected
on the Philips diffractometer described above using an Anton Paar
b
TTK450 Low-Temperature attachment in flowing nitrogen. The
procedure involved heating the sample to a specified temperature
(at a heating rate of 5 C/min) and holding the temperature at that
value for 2 h.. PXRD patterns of the sample were continuously
measured during this time. Measurements were performed from
60 Cto400 C at increments of 10 C. Only data where there were
observable changes in the PXRD patterns has been presented. a
FTIR spectra were recorded on a ThermoNicolet Smart Golden
f
Gate MKII single reflection ATR spectrometer from 4000 to
500 cm 1. In situ variable temperature studies were performed in
e
air using the ATR spectrometer described above with a heating
attachment. The sample was quickly (ca. 10 C/min) heated to the
c
required temperature and the FTIR spectrum was collected after
d
equilibration of 60 s. Elemental analysis for C and H was performed
using a CE-440 Exeter Analytical Inc. elemental analyser.
3500 3000 2000 1800 1600 1400 1200 1000 800
TG profiles were recorded at a heating rate of 5 C min 1 in
Wavenumber (cm-1)
flowing nitrogen (50 ml min 1). MS analysis of the gases evolved
during thermal decomposition was carried out for a 0.05 g sample Fig. 2. FTIR spectrum of Zn5(OH)8(Ac)2$4H2O.
Relative intensity
Reflectance
332 T. Biswick et al. / Solid State Sciences 11 (2009) 330 335
low angle and weak asymmetric reflections at high degree angle. temperatures [4,10,17]. In this section we report a detailed analysis
The pattern may be indexed on the basis of hexagonal cell of the thermal decomposition of Zn5(OH)8(Ac)2$4H2O using
parameters aź3.12 Å, cź13.4 Å. The values observed in this study a number of complimentary characterisation techniques in order to
are comparable to those reported in earlier studies [10]. Since the fully understand the decomposition pathway of the material.
length of the acetate anion may be estimated as ca. 3.6 Å and the The TG and DTG profiles for Zn5(OH)8(Ac)2$4H2O are presented
hydroxide layer thickness is approximately 5.2 Å, the observed in Fig. 3, showing that the mass of the sample reaches a constant
layer expansion (8.2 Å), which is more than twice the size of the value at ca. 370 C before which three major steps of mass loss, I, II
acetate anion, may suggest that the anions are assembled in bila- and III are observed. The solid residue collected at the end of step III
yers in the gallery. was identified, by PXRD analysis, as ZnO. The expected mass loss for
The FTIR spectrum and spectral assignments of the material are the total decomposition of the material to ZnO is 37.8% while the
presented in Fig. 2 and Table 1, respectively. The OH stretching observed value is 38.2%.
vibration bands are observed from 2900 to 3500 cm 1. The sharp Mass spectral analysis of the evolved gases (Fig. 4), however,
band observed at ca. 3570 cm 1 may be assigned to stretching shows two major temperature regions (50 200 C, A and 220
vibrations of OH groups not involved in hydrogen bonding, whereas 370 C, B) in which gaseous products are released. It is interesting
the broad bands at 3473 and 3367 cm 1 may be assigned to to note that the signal for mass 18, corresponding to the species
vibrations of OH groups involved in significant hydrogen bonding. H2Oþ, has maxima only in the first temperature region. This may
CH symmetric and asymmetric stretching vibration bands of the suggest that the second region of the decomposition process (B)
interlayer acetate anion are observed at ca. 2900 cm 1. The strong involves an anhydrous zinc salt and that the acetate anion does not
absorption bands at 1542 and 1390 cm 1 may be assigned to the decompose to CO2 and H2O, as proposed by earlier studies [10], but
asymmetric and symmetric stretching vibrations of the carboxylate is probably released intact.
group from the acetate anion. The bands at 1336 and 1018 cm 1 The first two decomposition steps in the TG/DTG profiles (I and
may be assigned to CH3 asymmetric deformation and OH bending II) are observed from 50 to 160 C and 160 to 230 C, respectively,
vibrations, respectively. The water bending vibration mode, and correspond to dehydration of the material followed by dehy-
expected to appear at ca. 1600 cm 1, is probably obscured by the droxylation of the hydroxide layers according to reaction (1). The
strong and broad band due to CO 2 asymmetric stretch (labelled c in expected mass loss for these two processes is 22.1% while
Fig. 2). the observed is 22.2%, showing close agreement. Evidence for
The nature and extent of interaction of the carboxylate group the formation of anhydrous zinc acetate will be presented and
with the matrix cation may be inferred from the differences in discussed later.
energy (Dn) between the asymmetric and symmetric stretching
Zn5ðOHÞ8ðCH3CO2Þ2$4H2O / ZCH3CO2Þ2 D 4ZnO D 8H2O (1)
vibrations in comparison with Dn values for ionic carboxylate
(usually taken as that of sodium or potassium salts of the anion)
The first decomposition step in the TG/DTG profiles, step I (50
[15,16]. In hydroxy salts, the carboxylate group may coordinate to
160 C), shows a shoulder (on the higher temperature side) which
the matrix cation as a unidentate ligand or occur as a free species.
may suggest that this step involves two separable thermal
Unidentate coordination increases the energy of the asymmetric
processes which are highly overlapped under the present experi-
stretch and reduces the energy of the symmetric stretch with
mental conditions. It should be noted that when the TG profile was
respect to ionic carboxylate, thereby increasing the value of Dn.
collected at a lower heating rate (1 C/min), the two processes were
Unidentate coordination usually exhibits Dn values greater than
resolved. We suggest that the first (and major) of these processes
200 cm 1 while ionic carboxylate interaction gives Dn values
involves dehydration of the material and grafting of the anion on
similar to those for the sodium salts of the anion. The Dn value
the matrix cation according to reaction (1a).
observed in the present study (152 cm 1) is similar to the Dn value
observed for sodium acetate (Aldrich, 98% purity), 155 cm 1. This
Zn5ðOHÞ8ðCH3CO2Þ2$4H2O / Zn5ðOHÞ8ðCH3CO2Þ2 D 4H2O (1a)
suggests that the interaction of the acetate anion with the cation
Subsequently, the anhydrous zinc hydroxy acetate, Zn5(OH)8
matrix in the zinc salt is purely ionic i.e. contrary to previous
(Ac)2 undergoes partial dehydroxylation to give a phase analogous
reports, the acetate anion is not directly attached to the matrix
to Zn3(OH)4(NO3)2, which has been observed in the thermal
cation in Zn5(OH)8(Ac)2$4H2O but interacts with it via hydrogen
bonding, similar to the case for nitrate anions in Zn5(OH)8 decomposition of Zn5(OH)8(NO3)2$2H2O [18], as outlined in reac-
tion (1b).
(NO3)2$2H2O [8].
3.2. Thermal analysis
0.01
II III
100 I (b)
Although a number of authors have investigated the thermal
0.00
decomposition of zinc hydroxy acetate, there are differences in the
-0.01
interpretation of the decomposition profiles and identification of
90
the solid phases and gaseous products released at different
-0.02
80
-0.03
Table 1
FTIR spectral assignments for Zn5(OH)8(Ac)2$4H2O
-0.04
Peak label Wavenumbers (cm 1) Assignment
70
a 3570, 3473, 3367 OH stretching vibrations
(a) -0.05
b w3000 CH stretch
c 1542  CO2 asymmetric stretch
60 -0.06
d 1390  CO2 symmetric stretch
50 100 150 200 250 300 350 400 450 500
e 1336 CH3 symmetric deformation
Temperature (°C)
f 1018 CH3 rock
Dn 152 c d
Fig. 3. (a) TG and (b) DTG profiles for Zn5(OH)8(Ac)2$4H2O. Heating rateź5 C/min.
Wt
1st derivative
T. Biswick et al. / Solid State Sciences 11 (2009) 330 335 333
Fig. 4. Mass spectral profile for evolved gases during the thermal decomposition of Zn5(OH)8(Ac)2$4H2O. Heating rateź3 C/min.
anion may be lost as CO2 and H2O through thermal decomposition
or it may be released as molecular acetic acid or acetone as has been
Zn5ðOHÞ8ðCH3CO2Þ2 /   Zn3ðOHÞ4ðCH3CO2Þ00D 2H2O D 2ZnO
2
previously proposed for zinc hydroxy acetate and other acetate-
(1b)
containing hydroxy salts [21 23]. As stated earlier, the species with
Finally, in step II,   Zn3(OH)4(CH3CO2)2  undergoes dehydrox- mass 18 (H2Oþ) does not have a maximum in the final stage of
decomposition of the material and this observation rules out the
ylation to give anhydrous zinc acetate, Zn(CH3CO2)2.
possibility of the anion decomposing to CO2 and H2O. In order to
  Zn3ðOHÞ4ðCH3CO2Þ00/ ZCH3CO2Þ2 D 2H2O D 2ZnO (1c)
check if the acetate anion is lost as acetic acid, it is important to
2
compare the mass spectrum of the evolved gases in the second
The three steps described above all fall within step A of the mass
temperature range with the mass spectrum of pure acetic acid. The
spectral profile and involve the loss of water only.
reported mass spectrum for pure acetic acid [24] contains six major
In situ variable temperature PXRD analysis of the material
peaks with mass 60 (the molecular ion, CH3CO2Hþ), 59 (CH3COþ),
2
(Fig. 5) has revealed the formation, at ca. 100 C, of a phase with
45 (CO2Hþ), 43 (CH3COþ), 17 (OHþ) and 15 (CHþ). The peak with
3
a first reflection at ca. 9.35 (9.45 Å). The position of the first
mass 43 is the base peak and the other high intensity peaks are
reflection is similar to that observed for the hydroxy acetates of
those with mass 45, 60, 15 and 17 in the order of decreasing
copper (9.34 Å) and nickel (9.08 Å) [19], which are expected to be
intensity. The absence of the molecular ion peak (mass 60), the
structurally similar to Zn3(OH)4(Ac)2  compare with the hydroxy
presence of the peak with mass 44 (COþ) and the relative intensi-
2
nitrates, Cu2(OH)3NO3, Ni2(OH)3NO3 and Zn3(OH)4(NO3)2 which
ties of the observed peaks (compared to observed intensities for
have similar interlayer spacings, 6.9 Å [20]. We, therefore, propose
acetic acid) may suggest that the acetate anion is not lost as acetic
that the observed reflections are from a  Zn3(OH)4(Ac)2 phase
released from the partial dehydroxylation of Zn5(OH)8(Ac)2.
Reflections associated with a ZnO phase are also observed in the
PXRD pattern of the sample heated at this temperature (Fig. 5). ZnO
is one of the solid phases expected from the partial dehydroxylation
of Zn5(OH)8(Ac)2 (reaction (1b)). It is interesting to note that upon
exposure to high relative humidity (80% RH), this phase (Zn3
(a) RT
(OH)4(Ac)2) readily converts to Zn5(OH)8(Ac)2$nH2O(Fig. 5).
Additionally, reflections that may be assigned to anhydrous zinc
acetate have been observed in the PXRD pattern of the sample *
*
* (b) 100 °C, 45 min
heated at temperatures above 150 C (Fig. 6) i.e. anhydrous zinc
acetate produced from the dehydroxylation of Zn3(OH)4(Ac)2
according to reaction (1c).
(c) 100 °C, 70 min
Although the three reactions outlined above (1a) (1c) involve
loss of water only, the mass spectral profile of the gaseous products
evolved in the temperature region corresponding to steps I and II of
(d) 80 RH at RT
the TG/DTG profile (i.e. step A of mass spectral profile) also shows
maxima for fragments with mass 45, 44, and 16 which may origi-
5 10 15 20 25 30 35 40 45 50 55 60
nate from the decomposition of impurity carbonate anions or other
2 (degrees)
carbon-containing species.
The final major step of decomposition in the TG/DTG profile,
Fig. 5. Variation of the PXRD pattern of Zn5(OH)8(Ac)2$4H2O at 100 C with time
step III, is observed from 230 to 375 C and corresponds to the
showing the formation of  Zn3(OH)4(Ac)2 and its hydrolysis to Zn5(OH)8(Ac)2$4H2O
decomposition of the anhydrous zinc acetate to ZnO. The acetate under high humidity. The studies were conducted in flowing nitrogen. * Denotes ZnO.
Intensity (a.u)
334 T. Biswick et al. / Solid State Sciences 11 (2009) 330 335
OH stretch
b
60 °C a
60 °C
Zn3(OH)4(Ac)2
80 °C
100 °C
1576 cm-1
105 °C
Zn(Ac)2 1607 cm-1
110 °C
110 °C
1545 cm-1
140 °C
120 °C
180 °C
130 °C
10 20 30 40 50 60 140 °C
2 (degrees)
Fig. 6. Variation of the PXRD pattern of Zn5(OH)8(Ac)2$4H2O with temperature
3600 3200 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000
showing the formation of anhydrous zinc acetate. The studies were conducted in
flowing nitrogen. ; Denotes ZnO.
Wavenumber (cm-1)
Fig. 7. The evolution of the FTIR spectrum of Zn5(OH)8(Ac)2$4H2O with temperature.
a and b denote CO 2 asymmetric and symmetric stretch, respectively.
acid. Similarly, the reported mass spectrum of pure acetone
contains three major peaks with mass 58 (molecular ion,
CH3COCHþ), 43 (base peak, CH3COþ) and 15 (corresponding to the
3
stretching vibration band shifts from 1387 to 1417 cm 1. The
species CHþ) [24]. Although there are peaks with mass 43 and 15,
3
difference in energy between the asymmetric and symmetric
the absence of a peak with mass 58 may suggest that the acetate
vibrations at 150 C, 127 cm 1, is characteristic of bridging biden-
anion is not released as acetone either.
tate carboxylate coordination [15], probably confirming the
We therefore propose that in the final step, the anhydrous zinc
formation of anhydrous zinc acetate, Zn(Ac)2. In anhydrous zinc
acetate decomposes according to reaction (2) such that the acetate
acetate the zinc atoms are tetrahedrally coordinated by four
anion is released as acetic anhydride.
oxygens of four bridging bidentate carboxylate groups [25].
The trends in the mechanism of the thermal decomposition of
ZCH3CO2Þ2 / ZnO DðCH3COÞ2O (2)
zinc hydroxy acetate observed in the present study are, in general,
similar to those observed by other authors in terms of the number
The mass spectrum of the gases released in the temperature
region corresponding to step III of the TG/DTG profile shows frag- of thermal processes and the temperatures at which they occur 
taking into account different heating rates. However, there are
ments with mass 45, 44, 43, 16 and 15 which correspond to the
differences in the identification of the solid phases observed at
species, CO2Hþ, COþ, CH3COþ, Oþand CHþ, respectively. This is in
2 3
different temperatures and the form in which the acetate anion is
agreement with literature data for the mass spectrum of pure acetic
released. Poul et al. [10], for example, also observed a phase with
anhydride, which shows a similar fragmentation pattern [24].
The decomposition trends observed by in situ variable temper- a first reflection at around 9.30 2q upon heating the material to
120 C, which they erroneously designated as ZnO. This is the phase
ature PXRD analysis are in agreement with the trends observed
we have identified as Zn3(OH)4(CH3CO2)2. Additionally the authors
from in situ variable temperature FTIR analysis (Fig. 7, Table 2).
were not able to observe an anhydrous zinc acetate phase as we
Thus, when the material is heated to 80 C, broadening of both the
have in the present study. Since the conditions under which they
asymmetric and symmetric carboxylate vibration bands is
performed the temperature resolved X-ray diffraction studies were
observed. The apparent broadening of these bands is probably due
not indicated, it is difficult to surmise the reasons for the differ-
to the presence of a number of overlapping absorption bands. This
ences. Furthermore, the authors suggested that the acetate anion
temperature coincides with the start of step I of the TG/DTG profile
was lost as carbon dioxide, apparently on the basis of mass spectral
(See Fig. 3) i.e. the start of the dehydration process. At this
data of the evolved gases, which unfortunately was not presented
temperature, Zn5(OH)8(Ac)2$4H2O and Zn5(OH)8(Ac)2 are the two
in the paper. In a later report, Kandare and Hossenlopp [17] sug-
acetate-containing materials expected.
gested that the acetate anion is lost as acetone, acetic acid and
As the temperature is increased further to 105 110 C, the
carbon dioxide. This they concluded from FTIR monitoring of the
asymmetric stretching vibration band resolves into three separate
gaseous materials released from TG analysis under air atmosphere.
peaks at 1607, 1576 and 1545 cm 1. As discussed in the previous
Interestingly, the absorption bands that the authors used to identify
section, in the temperature region between 50 and 150 C,
acetone and acetic acid as the gaseous products released may be
a number of thermal processes are occurring including dehydration
of the parent material (to form Zn5(OH)8(Ac)2) and partial dehy-
droxylation of the hydroxide layers (to form Zn3(OH)4(Ac)2 phase).
Table 2
The band at 1545 cm 1 which is also present at lower temperatures
FTIR spectral assignments for Zn5(OH)8(Ac)2$4H2O heated at different temperatures
may, therefore, be assigned to vibrations of acetate anions from
in situ
remnants of the starting material (Zn5(OH)8(Ac)2$4H2O). The bands
Peak label Temperature/ C (bold) and peak position/cm 1
at 1576 and 1607 cm 1 are probably due to asymmetric stretching
60 80 105 110 120 130 150
vibrations of acetate anions from anhydrous zinc hydroxy acetate,
a 1542 1549 1607 1607 1544 1544 1544
Zn5(OH)8(Ac)2 and Zn3(OH)4(Ac)2. The corresponding symmetric
1576 1576
stretching vibration modes are perhaps part of the broad band at ca.
1545 1544
1386 cm 1.
b 1390 1388 1386 1386 1417 1417 1417
Between 120 and 150 C, the three antisymmetric vibration
1387
bands merge again, centred at ca. 1544 cm 1 while the symmetric
Intensity (a.u)
Reflectance
T. Biswick et al. / Solid State Sciences 11 (2009) 330 335 335
easily accounted for by acetic anhydride alone [26]. This observa- Acknowledgments
tion coupled with our mass spectral data  which has shown that
acetone or acetic acid are not likely candidates  is strong evidence We acknowledge the Cambridge Commonwealth Trust and the
that the acetate anion is lost as acetic anhydride. Royal Society of London for financial support. We are also grateful
to Ms. Mi-Mi Hong for assistance in conducting supplementary TG
experiments.
4. Discussion
References
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zinc sulphate decomposing to ZnO and SO3. Additionally, we have
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seems to be a common feature for these materials.
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5. Conclusions
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This study has demonstrated that contrary to previous reports,
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the interaction of the acetate anion with the matrix cation in zinc
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hydroxy acetate, Zn5(OH)8(Ac)2$4H2O is purely electrostatic,
Benjamin, New York, 1966.
similar to what has been observed for zinc hydroxy nitrate
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(Zn5(OH)8(NO3)2$2H2O). Additionally, we have shown that the
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layered zinc hydroxy salts, forming anhydrous zinc acetate towards
[28] O.K. Srivastava, E.A. Secco, Can. J. Chem. 45 (1967) 579.
the end of the decomposition process. The acetate anion is finally
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lost as acetic anhydride. (1981) 888.