Evidence for the formation of anhydrous zinc acetate and acetic
anhydride during the thermal degradation of zinc hydroxy acetate,
Zn

5

(OH)

8

(CH

3

CO

2

)

2

$

4H

2

O to ZnO

Timothy Biswick

a

,

*

,

1

, William Jones

a

, Aleksandra Pacu1a

b

, Ewa Serwicka

b

, Jerzy Podobinski

b

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

Article history:
Received 5 January 2008
Received in revised form 19 May 2008
Accepted 19 June 2008
Available online 28 June 2008

Keywords:
Hydroxy salts
Anionic clays
Zinc hydroxy acetate
Anhydrous zinc acetate
Thermal decomposition
Zinc oxide
Acetic anhydride

a b s t r a c t

Zinc hydroxy acetate, Zn

5

(OH)

8

(CH

3

CO

2

)

2

$

4H

2

O, has been prepared by the precipitation method. It has

been demonstrated by FTIR analysis that, contrary to previous reports, the interaction of the acetate
anion with the matrix cation is ionic. TG analysis, mass spectral analysis of the evolved gases, and in situ
variable temperature PXRD and FTIR analysis have shown that decomposition of the material to ZnO
involves the formation of Zn

5

(OH)

8

(CH

3

CO

2

), Zn

3

(OH)

4

(CH

3

CO

2

)

2

and anhydrous zinc acetate

(Zn(CH

3

CO

2

)

2

) as some of the acetate-containing intermediate solid products. The acetate anion is finally

lost, at temperatures below 400



C, as acetic anhydride, (CH

3

CO)

2

O.

Ó 2008 Elsevier Masson SAS. All rights reserved.

1. Introduction

Layered metal hydroxide salts containing exchangeable anions

form a family of inorganic solids attracting ever-increasing atten-
tion. The unique structural and physicochemical properties of these
compounds render them excellent substrates for advanced mate-
rials design

[1,2]

. Layered double hydroxides (LDHs) and hydroxy

salts, also known as basic salts, are examples of such materials. The
structures of both families of compounds may be considered as
derived from the layered lattice of brucite, (Mg(OH)

2

). In the case of

LDHs a layer charge develops as a result of partial substitution
of divalent cations by trivalent, thereby necessitating the presence
of charge balancing anions within the interlayer

[3]

. In hydroxy

salts, since the cation composition of the brucite-like layers is
confined to cations of the same valency (usually divalent), the
existence of potentially exchangeable anions may be created by two
mechanisms. In the first, a surplus layer charge is created by the
occurrence of octahedral vacant sites that are capped above and

below by two tetrahedrally coordinated divalent cations. In the
second, whilst there is complete occupancy of octahedral cationic
positions, part of the framework hydroxyl groups are substituted by
structure forming anions such as NO

3



, Cl



etc. The potential

applications and synthesis procedures for these materials have
been described elsewhere

[1]

.

Although the structure of zinc hydroxy acetate, Zn

5

(OH)

8

(CH

3

CO

2

)

2

$

nH

2

O has not been determined, a number of authors

[4–6]

have proposed that it belongs to the hydrozincite structure

[7]

in which zinc occupies both octahedral and tetrahedral coor-

dination sites in the ratio 3:2. The interaction of the anion with the
matrix cation in layered hydroxy salts of the hydrozincite structure
has been observed to vary depending on the anion intercalated as
illustrated by the three structurally related zinc hydroxy salts,
Zn

5

(OH)

8

(NO

3

)

2

$

2H

2

O, Zn

5

(OH)

8

Cl

2

$

H

2

O and Zn

5

(OH)

6

(CO

3

)

2

.

The structure of Zn

5

(OH)

8

(NO

3

)

2

$

2H

2

O has been determined by

single crystal x-ray diffraction method by Stahlin and Oswald

[8]

. It

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
a water molecule occupying the apex. Unbound nitrate groups are
located between the sheets being hydrogen bonded to two water
molecules of one layer and an OH group of the opposite layer

[8]

. In

*

Corresponding author.
E-mail address:

[e-mail]

(T. Biswick).

1

Present address: Centre for Intelligent Nano-Bio Materials, Department of

Chemistry, Ewha Womans University, Seoul 120-750, Republic of Korea.

Contents lists available at

ScienceDirect

Solid State Sciences

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / s s s c i e

1293-2558/$ – see front matter Ó 2008 Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.solidstatesciences.2008.06.018

Solid State Sciences 11 (2009) 330–335

Zn

5

(OH)

8

Cl

2

$

H

2

O, 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, Zn

5

(OH)

6

(CO

3

)

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

proposed (on the basis of FTIR data) that the acetate anion is
directly coordinated to the matrix cation as a unidentate ligand via
M–OCOCH

3

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
toxicity and a wide range of possible applications, including solar
cell technology, catalysis and photo-catalysis, thin-film gas sensors,
varistors, transparent conductive electrodes and surface acoustic
wave devices

[12–14]

. Detailed characterisation of the solid mate-

rials as well as the gaseous products released at different temper-
atures is, therefore, necessary in order to fully understand the
chemical processes occurring during the thermal decomposition.

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 Zn

5

(OH)

8

(CH

3

CO

2

)

2

$

nH

2

O and also to monitor the phase

changes accompanying the thermal treatment of the material at
different temperatures.

2. Materials and methods

2.1. Materials

Zinc hydroxy acetate with idealised composition Zn

5

(OH)

8

(CH

3

CO

2

)

2

$

nH

2

O was prepared by the titration method. Fifty

milliliters of a 0.75 M NaOH (aq) solution was slowly added to 20 ml
of 3.5 M zinc acetate dihydrate aqueous solution, with constant
stirring at room temperature. The white precipitate was immedi-
ately filtered, washed twice with deionised water and dried
at 60



C.

2.2. Methods

PXRD data was collected on a Philips X’Pert MPD diffractometer

using Cu K

a

radiation (

l

¼ 1.5440 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
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



C to 400



C at increments of 10



C. Only data where there were

observable changes in the PXRD patterns has been presented.

FTIR spectra were recorded on a ThermoNicolet Smart Golden

Gate MKII single reflection ATR spectrometer from 4000 to
500 cm

1

. In situ variable temperature studies were performed in

air using the ATR spectrometer described above with a heating
attachment. The sample was quickly (ca. 10



C/min) heated to the

required temperature and the FTIR spectrum was collected after
equilibration of 60 s. Elemental analysis for C and H was performed
using a CE-440 Exeter Analytical Inc. elemental analyser.

TG profiles were recorded at a heating rate of 5



C min

1

in

flowing nitrogen (50 ml min

1

). MS analysis of the gases evolved

during thermal decomposition was carried out for a 0.05 g sample

heated at the rate of 3



C min in flowing argon (10 ml min) using

RGA200 Stanford Research quadrupole mass spectrometer.

3. Results

3.1. Characterisation of the parent material

The results of chemical elemental analysis of the material show

general agreement between experimental and calculated (based on
the formula Zn

5

(OH)

8

(Ac)

2

$

4H

2

O) values: C – 7.48% obsrvd, 7.35%

calc. and H – 2.79% obsrvd, 3.37% calc. The amount of water was
calculated from the overall mass loss; assuming that the starting
material had the ideal formula Zn

5

(OH)

8

(Ac)

2

$

nH

2

O and that the

final material remaining upon thermal degradation is ZnO. It is
interesting to note that although the observed C content is greater
than the calculated value, the observed H content is significantly
less than the calculated value. This may suggest the presence of
impurity carbonate anions from atmospheric carbon dioxide co-
intercalated with the acetate anions during synthesis and handling
of the sample.

The PXRD pattern of the material, presented in

Fig. 1

, is typical of

a layered material exhibiting sharp and symmetric reflections at

45

5

10

15

20

25

30

35

40

50

55

60

65

10

15

20

25

30

35

40

45

50

55

60

65

2 (degrees)

Relative intensity

2 (degrees)

Fig. 1. PXRD pattern of Zn

5

(OH)

8

(Ac)

2

$

4H

2

O. Insert shows details of the high angle

reflections.

3500

3000

2000

1800

1600

1400

1200

1000

800

f

e

d

c

b

Reflectance

Wavenumber (cm

-1

)

a

Fig. 2. FTIR spectrum of Zn

5

(OH)

8

(Ac)

2

$

4H

2

O.

T. Biswick et al. / Solid State Sciences 11 (2009) 330–335

331

low angle and weak asymmetric reflections at high degree angle.
The pattern may be indexed on the basis of hexagonal cell
parameters a ¼ 3.12 Å, c ¼ 13.4 Å. The values observed in this study
are comparable to those reported in earlier studies

[10]

. Since the

length of the acetate anion may be estimated as ca. 3.6 Å and the
hydroxide layer thickness is approximately 5.2 Å, the observed
layer expansion (8.2 Å), which is more than twice the size of the
acetate anion, may suggest that the anions are assembled in bila-
yers in the gallery.

The FTIR spectrum and spectral assignments of the material are

presented in

Fig. 2

and

Table 1

, respectively. The OH stretching

vibration bands are observed from 2900 to 3500 cm

1

. The sharp

band observed at ca. 3570 cm

1

may be assigned to stretching

vibrations of OH groups not involved in hydrogen bonding, whereas
the broad bands at 3473 and 3367 cm

1

may be assigned to

vibrations of OH groups involved in significant hydrogen bonding.

CH symmetric and asymmetric stretching vibration bands of the

interlayer acetate anion are observed at ca. 2900 cm

1

. The strong

absorption bands at 1542 and 1390 cm

1

may be assigned to the

asymmetric and symmetric stretching vibrations of the carboxylate
group from the acetate anion. The bands at 1336 and 1018 cm

1

may be assigned to CH

3

asymmetric deformation and OH bending

vibrations, respectively. The water bending vibration mode,
expected to appear at ca. 1600 cm

1

, is probably obscured by the

strong and broad band due to CO

2



asymmetric stretch (labelled c in

Fig. 2

).

The nature and extent of interaction of the carboxylate group

with the matrix cation may be inferred from the differences in
energy (

Dn

) between the asymmetric and symmetric stretching

vibrations in comparison with

Dn

values for ionic carboxylate

(usually taken as that of sodium or potassium salts of the anion)

[15,16]

. In hydroxy salts, the carboxylate group may coordinate to

the matrix cation as a unidentate ligand or occur as a free species.
Unidentate coordination increases the energy of the asymmetric
stretch and reduces the energy of the symmetric stretch with
respect to ionic carboxylate, thereby increasing the value of

Dn

.

Unidentate coordination usually exhibits

Dn

values greater than

200 cm

1

while ionic carboxylate interaction gives

Dn

values

similar to those for the sodium salts of the anion. The

Dn

value

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

suggests that the interaction of the acetate anion with the cation
matrix in the zinc salt is purely ionic i.e. contrary to previous
reports, the acetate anion is not directly attached to the matrix
cation in Zn

5

(OH)

8

(Ac)

2

$

4H

2

O but interacts with it via hydrogen

bonding, similar to the case for nitrate anions in Zn

5

(OH)

8

(NO

3

)

2

$

2H

2

O

[8]

.

3.2. Thermal analysis

Although a number of authors have investigated the thermal

decomposition of zinc hydroxy acetate, there are differences in the
interpretation of the decomposition profiles and identification of
the solid phases and gaseous products released at different

temperatures

[4,10,17]

. In this section we report a detailed analysis

of the thermal decomposition of Zn

5

(OH)

8

(Ac)

2

$

4H

2

O using

a number of complimentary characterisation techniques in order to
fully understand the decomposition pathway of the material.

The TG and DTG profiles for Zn

5

(OH)

8

(Ac)

2

$

4H

2

O are presented

in

Fig. 3

, showing that the mass of the sample reaches a constant

value at ca. 370



C before which three major steps of mass loss, I, II

and III are observed. The solid residue collected at the end of step III
was identified, by PXRD analysis, as ZnO. The expected mass loss for
the total decomposition of the material to ZnO is 37.8% while the
observed value is 38.2%.

Mass spectral analysis of the evolved gases (

Fig. 4

), however,

shows two major temperature regions (50–200



C, A and 220–

370



C, B) in which gaseous products are released. It is interesting

to note that the signal for mass 18, corresponding to the species
H

2

O

þ

, has maxima only in the first temperature region. This may

suggest that the second region of the decomposition process (B)
involves an anhydrous zinc salt and that the acetate anion does not
decompose to CO

2

and H

2

O, as proposed by earlier studies

[10]

, but

is probably released intact.

The first two decomposition steps in the TG/DTG profiles (I and

II) are observed from 50 to 160



C and 160 to 230



C, respectively,

and correspond to dehydration of the material followed by dehy-
droxylation of the hydroxide layers according to reaction

(1)

. The

expected mass loss for these two processes is 22.1% while
the observed is 22.2%, showing close agreement. Evidence for
the formation of anhydrous zinc acetate will be presented and
discussed later.

Zn

5

ðOHÞ

8

ðCH

3

CO

2

Þ

2

$

4H

2

O / ZnðCH

3

CO

2

Þ

2

D

4ZnO D 8H

2

O

(1)

The first decomposition step in the TG/DTG profiles, step I (50–

160



C), shows a shoulder (on the higher temperature side) which

may suggest that this step involves two separable thermal
processes which are highly overlapped under the present experi-
mental conditions. It should be noted that when the TG profile was
collected at a lower heating rate (1



C/min), the two processes were

resolved. We suggest that the first (and major) of these processes
involves dehydration of the material and grafting of the anion on
the matrix cation according to reaction

(1a)

.

Zn

5

ðOHÞ

8

ðCH

3

CO

2

Þ

2

$

4H

2

O / Zn

5

ðOHÞ

8

ðCH

3

CO

2

Þ

2

D

4H

2

O

(1a)

Subsequently, the anhydrous zinc hydroxy acetate, Zn

5

(OH)

8

(Ac)

2

undergoes partial dehydroxylation to give a phase analogous

to Zn

3

(OH)

4

(NO

3

)

2

, which has been observed in the thermal

decomposition of Zn

5

(OH)

8

(NO

3

)

2

$

2H

2

O

[18]

, as outlined in reac-

tion

(1b)

.

Table 1
FTIR spectral assignments for Zn

5

(OH)

8

(Ac)

2

$

4H

2

O

Peak label

Wavenumbers (cm

1

)

Assignment

a

3570, 3473, 3367

OH stretching vibrations

b

w3000

CH stretch

c

1542

–CO

2

– asymmetric stretch

d

1390

–CO

2

– symmetric stretch

e

1336

CH

3

symmetric deformation

f

1018

CH

3

rock

Dn

152

c–d

Temperature (°C)

50

100

150

200

250

300

350

400

450

500

Wt

60

70

80

90

100

1st derivative

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0.01

I

II

III

(a)

(b)

Fig. 3. (a) TG and (b) DTG profiles for Zn

5

(OH)

8

(Ac)

2

$

4H

2

O. Heating rate ¼ 5



C/min.

T. Biswick et al. / Solid State Sciences 11 (2009) 330–335

332

Zn

5

ðOHÞ

8

ðCH

3

CO

2

Þ

2

/

‘‘

Zn

3

ðOHÞ

4

ðCH

3

CO

2

Þ

00

2

D

2H

2

O D 2ZnO

(1b)

Finally, in step II, ‘‘Zn

3

(OH)

4

(CH

3

CO

2

)

2

’’ undergoes dehydrox-

ylation to give anhydrous zinc acetate, Zn(CH

3

CO

2

)

2

.

‘‘

Zn

3

ðOHÞ

4

ðCH

3

CO

2

Þ

00

2

/

ZnðCH

3

CO

2

Þ

2

D

2H

2

O D 2ZnO

(1c)

The three steps described above all fall within step A of the mass

spectral profile and involve the loss of water only.

In situ variable temperature PXRD analysis of the material

(

Fig. 5

) has revealed the formation, at ca. 100



C, of a phase with

a first reflection at ca. 9.35



(9.45 Å). The position of the first

reflection is similar to that observed for the hydroxy acetates of
copper (9.34 Å) and nickel (9.08 Å)

[19]

, which are expected to be

structurally similar to Zn

3

(OH)

4

(Ac)

2

– compare with the hydroxy

nitrates, Cu

2

(OH)

3

NO

3

, Ni

2

(OH)

3

NO

3

and Zn

3

(OH)

4

(NO

3

)

2

which

have similar interlayer spacings, 6.9 Å

[20]

. We, therefore, propose

that the observed reflections are from a ‘Zn

3

(OH)

4

(Ac)

2

’ phase

released from the partial dehydroxylation of Zn

5

(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 Zn

5

(OH)

8

(Ac)

2

(reaction

(1b)

). It is interesting to note that upon

exposure to high relative humidity (80% RH), this phase (Zn

3

(OH)

4

(Ac)

2

) readily converts to Zn

5

(OH)

8

(Ac)

2

$

nH

2

O (

Fig. 5

).

Additionally, reflections that may be assigned to anhydrous zinc

acetate have been observed in the PXRD pattern of the sample
heated at temperatures above 150



C (

Fig. 6

) i.e. anhydrous zinc

acetate produced from the dehydroxylation of Zn

3

(OH)

4

(Ac)

2

according to reaction

(1c)

.

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
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-
nate from the decomposition of impurity carbonate anions or other
carbon-containing species.

The final major step of decomposition in the TG/DTG profile,

step III, is observed from 230 to 375



C and corresponds to the

decomposition of the anhydrous zinc acetate to ZnO. The acetate

anion may be lost as CO

2

and H

2

O through thermal decomposition

or it may be released as molecular acetic acid or acetone as has been
previously proposed for zinc hydroxy acetate and other acetate-
containing hydroxy salts

[21–23]

. As stated earlier, the species with

mass 18 (H

2

O

þ

) does not have a maximum in the final stage of

decomposition of the material and this observation rules out the
possibility of the anion decomposing to CO

2

and H

2

O. In order to

check if the acetate anion is lost as acetic acid, it is important to
compare the mass spectrum of the evolved gases in the second
temperature range with the mass spectrum of pure acetic acid. The
reported mass spectrum for pure acetic acid

[24]

contains six major

peaks with mass 60 (the molecular ion, CH

3

CO

2

H

þ

), 59 (CH

3

CO

2

þ

),

45 (CO

2

H

þ

), 43 (CH

3

CO

þ

), 17 (OH

þ

) and 15 (CH

3

þ

). The peak with

mass 43 is the base peak and the other high intensity peaks are
those with mass 45, 60, 15 and 17 in the order of decreasing
intensity. The absence of the molecular ion peak (mass 60), the
presence of the peak with mass 44 (CO

2

þ

) and the relative intensi-

ties of the observed peaks (compared to observed intensities for
acetic acid) may suggest that the acetate anion is not lost as acetic

Fig. 4. Mass spectral profile for evolved gases during the thermal decomposition of Zn

5

(OH)

8

(Ac)

2

$

4H

2

O. Heating rate ¼ 3



C/min.

5

10

15

20

25

30

35

40

45

50

55

60

*

*

Intensity (a.u)

2 (degrees)

(a) RT

(b) 100 °C, 45 min

(c) 100 °C, 70 min

(d) 80

RH at RT

*

Fig. 5. Variation of the PXRD pattern of Zn

5

(OH)

8

(Ac)

2

$

4H

2

O at 100



C with time

showing the formation of ‘Zn

3

(OH)

4

(Ac)

2

’ and its hydrolysis to Zn

5

(OH)

8

(Ac)

2

$

4H

2

O

under high humidity. The studies were conducted in flowing nitrogen. * Denotes ZnO.

T. Biswick et al. / Solid State Sciences 11 (2009) 330–335

333

acid. Similarly, the reported mass spectrum of pure acetone
contains three major peaks with mass 58 (molecular ion,
CH

3

COCH

3

þ

), 43 (base peak, CH

3

CO

þ

) and 15 (corresponding to the

species CH

3

þ

)

[24]

. Although there are peaks with mass 43 and 15,

the absence of a peak with mass 58 may suggest that the acetate
anion is not released as acetone either.

We therefore propose that in the final step, the anhydrous zinc

acetate decomposes according to reaction

(2)

such that the acetate

anion is released as acetic anhydride.

ZnðCH

3

CO

2

Þ

2

/

ZnO D ðCH

3

COÞ

2

O

(2)

The mass spectrum of the gases released in the temperature

region corresponding to step III of the TG/DTG profile shows frag-
ments with mass 45, 44, 43, 16 and 15 which correspond to the
species, CO

2

H

þ

, CO

2

þ

, CH

3

CO

þ

, O

þ

and CH

3

þ

, respectively. This is in

agreement with literature data for the mass spectrum of pure acetic
anhydride, which shows a similar fragmentation pattern

[24]

.

The decomposition trends observed by in situ variable temper-

ature PXRD analysis are in agreement with the trends observed
from in situ variable temperature FTIR analysis (

Fig. 7

,

Table 2

).

Thus, when the material is heated to 80



C, broadening of both the

asymmetric and symmetric carboxylate vibration bands is
observed. The apparent broadening of these bands is probably due
to the presence of a number of overlapping absorption bands. This
temperature coincides with the start of step I of the TG/DTG profile
(See

Fig. 3

) i.e. the start of the dehydration process. At this

temperature, Zn

5

(OH)

8

(Ac)

2

$

4H

2

O and Zn

5

(OH)

8

(Ac)

2

are the two

acetate-containing materials expected.

As the temperature is increased further to 105–110



C, the

asymmetric stretching vibration band resolves into three separate
peaks at 1607, 1576 and 1545 cm

1

. As discussed in the previous

section, in the temperature region between 50 and 150



C,

a number of thermal processes are occurring including dehydration
of the parent material (to form Zn

5

(OH)

8

(Ac)

2

) and partial dehy-

droxylation of the hydroxide layers (to form Zn

3

(OH)

4

(Ac)

2

phase).

The band at 1545 cm

1

which is also present at lower temperatures

may, therefore, be assigned to vibrations of acetate anions from
remnants of the starting material (Zn

5

(OH)

8

(Ac)

2

$

4H

2

O). The bands

at 1576 and 1607 cm

1

are probably due to asymmetric stretching

vibrations of acetate anions from anhydrous zinc hydroxy acetate,
Zn

5

(OH)

8

(Ac)

2

and Zn

3

(OH)

4

(Ac)

2

. The corresponding symmetric

stretching vibration modes are perhaps part of the broad band at ca.
1386 cm

1

.

Between 120 and 150



C, the three antisymmetric vibration

bands merge again, centred at ca. 1544 cm

1

while the symmetric

stretching vibration band shifts from 1387 to 1417 cm

1

. The

difference in energy between the asymmetric and symmetric
vibrations at 150



C, 127 cm

1

, is characteristic of bridging biden-

tate carboxylate coordination

[15]

, probably confirming the

formation of anhydrous zinc acetate, Zn(Ac)

2

. In anhydrous zinc

acetate the zinc atoms are tetrahedrally coordinated by four
oxygens of four bridging bidentate carboxylate groups

[25]

.

The trends in the mechanism of the thermal decomposition of

zinc hydroxy acetate observed in the present study are, in general,
similar to those observed by other authors in terms of the number
of thermal processes and the temperatures at which they occur –
taking into account different heating rates. However, there are
differences in the identification of the solid phases observed at
different temperatures and the form in which the acetate anion is
released. Poul et al.

[10]

, for example, also observed a phase with

a first reflection at around 9.30



2

q

upon heating the material to

120



C, which they erroneously designated as ZnO. This is the phase

we have identified as Zn

3

(OH)

4

(CH

3

CO

2

)

2

. Additionally the authors

were not able to observe an anhydrous zinc acetate phase as we
have in the present study. Since the conditions under which they
performed the temperature resolved X-ray diffraction studies were
not indicated, it is difficult to surmise the reasons for the differ-
ences. Furthermore, the authors suggested that the acetate anion
was lost as carbon dioxide, apparently on the basis of mass spectral
data of the evolved gases, which unfortunately was not presented
in the paper. In a later report, Kandare and Hossenlopp

[17]

sug-

gested that the acetate anion is lost as acetone, acetic acid and
carbon dioxide. This they concluded from FTIR monitoring of the
gaseous materials released from TG analysis under air atmosphere.
Interestingly, the absorption bands that the authors used to identify
acetone and acetic acid as the gaseous products released may be

10

20

30

40

50

60

Intensity (a.u)

60 °C

100 °C

110 °C

140 °C

180 °C

Zn(Ac)

2

Zn

3

(OH)

4

(Ac)

2

2 (degrees)

Fig. 6. Variation of the PXRD pattern of Zn

5

(OH)

8

(Ac)

2

$

4H

2

O with temperature

showing the formation of anhydrous zinc acetate. The studies were conducted in
flowing nitrogen. ; Denotes ZnO.

3600 3200 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000

1545 cm

-1

1576 cm

-1

1607 cm

-1

b

a

80 °C

105 °C

110 °C

120 °C

130 °C

140 °C

Reflectance

Wavenumber (cm

-1

)

60 °C

OH stretch

Fig. 7. The evolution of the FTIR spectrum of Zn

5

(OH)

8

(Ac)

2

$

4H

2

O with temperature.

a and b denote CO

2



asymmetric and symmetric stretch, respectively.

Table 2
FTIR spectral assignments for Zn

5

(OH)

8

(Ac)

2

$

4H

2

O heated at different temperatures

in situ

Peak label

Temperature/



C (bold) and peak position/cm

1

60

80

105

110

120

130

150

a

1542

1549

1607

1607

1544

1544

1544

1576

1576

1545

1544

b

1390

1388

1386

1386

1417

1417

1417

1387

T. Biswick et al. / Solid State Sciences 11 (2009) 330–335

334

easily accounted for by acetic anhydride alone

[26]

. This observa-

tion coupled with our mass spectral data – which has shown that
acetone or acetic acid are not likely candidates – is strong evidence
that the acetate anion is lost as acetic anhydride.

4. Discussion

The formation of anhydrous metal salts during the decomposi-

tion of layered zinc hydroxy salts has been observed before for zinc
hydroxy chloride, Zn

5

(OH)

8

Cl

2

$

H

2

O and zinc hydroxy sulphate,

Zn

4

(OH)

6

SO

4

$

5H

2

O. Garcia-Martinez et al.

[27]

and Srivistava and

Secco

[28]

investigated

the

thermal

decomposition

of

Zn

5

(OH)

8

Cl

2

$

H

2

O and observed that the material decomposes to

ZnO in three stages with the successive formation of Zn

5

(OH)

8

Cl

2

,

a mixture of ZnOHCl and ZnO, ZnO and anhydrous zinc chloride
(ZnCl

2

). Similarly, in their investigations on the thermal properties

of Zn

4

(OH)

6

SO

4

$

5H

2

O, Ben’yash et al.

[29]

observed that the

material decomposes to ZnO in four major stages with the
successive formation of Zn

4

(OH)

6

SO

4

, Zn

2

(OH)

2

SO

4

and ZnO, ZnO

and anhydrous zinc sulphate (ZnSO

4

) and finally the anhydrous

zinc sulphate decomposing to ZnO and SO

3

. Additionally, we have

recently reported the formation of anhydrous zinc nitrate during
the decomposition of zinc hydroxy nitrates

[18]

. The formation

anhydrous zinc salts during the decomposition of zinc hydroxy salts
seems to be a common feature for these materials.

5. Conclusions

This study has demonstrated that contrary to previous reports,

the interaction of the acetate anion with the matrix cation in zinc
hydroxy acetate, Zn

5

(OH)

8

(Ac)

2

$

4H

2

O is purely electrostatic,

similar to what has been observed for zinc hydroxy nitrate
(Zn

5

(OH)

8

(NO

3

)

2

$

2H

2

O). Additionally, we have shown that the

material decomposes to ZnO via similar stages as observed for other
layered zinc hydroxy salts, forming anhydrous zinc acetate towards
the end of the decomposition process. The acetate anion is finally
lost as acetic anhydride.

Acknowledgments

We acknowledge the Cambridge Commonwealth Trust and the

Royal Society of London for financial support. We are also grateful
to Ms. Mi-Mi Hong for assistance in conducting supplementary TG
experiments.

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