Reactions of oxides of some 3d elements with ammonium hydrogen difluoride

Article abstract of DOI:10.1023/B:RJAC.0000031269.89784.b6

Reactions of iron, manganese(II), manganese(IV), copper, and zinc oxides with ammonium hydrogen difluoride were studied by thermogravimetry, X-ray phase analysis, IR spectrometry, and chemical analysis.

Full text of DOI:10.1023/B:RJAC.0000031269.89784.b6

Russian Journal of Applied Chemistry, Vol. 77, No. 3, 2004, pp. 347 352. Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 3,
2004, pp. 353 358.
Original Russian Text Copyright
2004 by Kalinnikov, Nesterov, Makarov, Steshin, Tikhomirova.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Reactions of Oxides of Some 3d Elements
with Ammonium Hydrogen Difluoride
V. T. Kalinnikov, D. P. Nesterov, D. V. Makarov, E. A. Steshin, and E. L. Tikhomirova
Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials, Kola Scientific Center,
Russian Academy of Sciences, Apatity, Murmansk oblast, Russia
Received June 11, 2003
Abstract Reactions of iron, manganese(II), manganese(IV), copper, and zinc oxides with ammonium
hydrogen difluoride were studied by thermogravimetry, X-ray phase analysis, IR spectrometry, and chemical
analysis.
Fluorides of 3d elements find use as active com-
ponents of solid lasers and phosphors, as components
of chemical current sources and fluoride glasses, and
as catalysts in organic synthesis [1]. On the other
hand, recently there has been a considerable progress
in processing of oxide silicate raw materials contain-
ing 3d-metal oxides using ammonium hydrogen di-
fluoride NH HF [2]. Therefore, study of reactions of
hydrolysis followed by potentiometric determination
(using a F -selective electrode). The ammonia content
was determined by Kjeldahl’s method.
The following methods of synthesizing FeF have
3
been described in the literature: reaction of FeCl with
3
anhydrous HF [4] yielding an amorphous product,
which is heated to 1000 C to form a crystalline prod-
uct; calcination of Fe O or hydrated FeF in an HF
4
2
2
3
3
3d-element oxides with NH HF is of scientific and
4
2
flow at 1000 C [5]; and thermal decomposition of
(NH ) FeF at 400 500 C [6], including the reaction
practical importance. It is known that NH HF is
4
2
4 3
6
close to anhydrous hydrogen fluoride in reactivity,
but has a number of advantages: it can be easily dehy-
drated and regenerated from gaseous components, and
fluoridation reactions can be carried out without
sophisticated equipment [3]. In this work we have
studied the fluoridation of manganese(II) and (IV),
iron(III), copper, and zinc oxides with ammonium
hydrogen difluoride by thermogravimetry, chemical
and X-ray phase analyses, and crystal optics. The
chemically pure grade oxides and double-recrystallized
NH HF (analytically pure grade) were used as the
in an F atmosphere [7].
2
These procedures require sophisticated equipment,
and the fluoridation proceeds at high temperatures in
several stages. It was pointed out in [7] that the ab-
sence of iron(II) impurities in the final product
is an advantage of obtaining FeF by decomposing
3
(NH ) FeF in an F atmosphere.
4 3
6
2
When (NH ) FeF is synthesized by the reaction of
4 3
6
natural pyrrhotines Fe S with NH HF [7], iron(II)-
1
x
4
2
containing compounds are inevitably formed, which
4
2
starting substances.
makes necessary the subsequent treatment with F .
2
The preparation of (NH ) FeF by the reaction of
4 3
6
The thermogravimetric study was carried out on
a Q-1500D (Hungary) derivatograph in platinum
crucibles using 200 400-mg samples and calcined
Al O as reference. The experiments were carried out
FeF with NH F in an inert atmosphere [6] or by the
3
4
reaction of FeBr with NH F in methanol [8] was also
3
4
described.
2
3
in dynamic and quasi-isothermal modes at the heating
The derivatogram of a mixture of Fe O with
2
3
1
rate of 2.5 10 deg min .
NH HF in the molar ratio of 1 : 10 is shown in
4
2
Fig. 1. The DTA curve reveals a number of endo-
thermic effects. The first of them at approximately
115 C corresponds to the melting of NH HF [9]. The
The X-ray phase analysis of reaction products was
performed on a DRON-2 diffractometer with the
CuK radiation. The IR absorption spectra were re-
corded on a UR-20 spectrophotometer.
4
2
endothermic effect at 166 C results from the forma-
tion of (NH ) FeF by reaction (1).
4 3
6
The content of iron was determined by the flame-
emission spectrometry, and that of fluoride, by pyro-
Fe2O3 + 6NH4HF2 = 2(NH4)3FeF6 + 3H2O.
(1)
1070-4272/04/7703-0347 2004 MAIK Nauka/Interperiodica

348
KALINNIKOV et al.
The thermogravigram obtained in the quasi-isother-
(a)
TG
m, %
mal mode contains three plateaus corresponding to the
formation of (NH ) FeF , NH FeF , and FeF , re-
4 3
6
4
4
3
spectively (Fig. 1b). The calculated weight loss in each
stage of the synthesis (10.8, 40.3, and 55.0 wt %, re-
spectively) agrees well with the loss observed experi-
mentally (Fig. 1b). The derivatographic results were
confirmed by X-ray phase analysis. The X-ray pat-
terns of Fe O :NH HF mixtures with the molar ratio
(b)
m, %
DTG
DTA
2
3
4
2
of 1 : 6 at 180, 320, and 430 C are shown in Fig. 2a.
It is seen from Fig. 2a, curve 1 that (NH ) FeF is the
reaction product at 180 C, and at 320 C it decom-
4 3
6
T, C
T
poses to give NH FeF (Fig. 2a, curve 2). Curve 3 in
4
4
Fig. 2a is the X-ray pattern of anhydrous FeF . Its
3
heating in air results in the following pyrohydrolysis
reactions:
Fig. 1. (a) Derivatogram of a mixture Fe O : NH HF =
2
2FeF3 + H2O = Fe2F4O + 2HF,
2FeF3 + 3H2O = Fe2O3 + 6HF.
(4)
(5)
2
3
4
1 : 10 and (b) thermogravigram of a mixture Fe O :
2
3
NH HF = 1 : 6 recorded in the quasi-isothermal mode.
4
2
(T) temperature and ( m) weight loss; the same for Figs. 3,
5, and 6.
The formation of Fe O was confirmed by X-ray
phase analysis. The X-ray patterns of the synthesized
compounds coincide with the data given in [6].
2
3
(a)
(b)
The IR absorption spectra of ammonium fluorofer-
rates(III) and FeF are given in Fig. 2b. The absorp-
3
1
tion bands in the regions of 2800 3400 and 1430 cm
in the spectra of ammonium fluoroferrates(III) are as-
signable to the stretching and bending vibrations of
1
N H bonds. The absorption band at 450 550 cm ,
which is observed in all the IR spectra under con-
sideration, is characteristic of the Fe F stretching
vibration. A weak absorption band in the region of
bending vibrations of water molecules is present in
the spectrum of ammonium tetrafluoroferrate(III) at
1630 cm . This is apparently hygroscopic water ap-
pearing owing to fine dispersity of the substance.
1
2 , deg
, cm
1
Fig. 2. (a) X-ray patterns of cakes of Fe O : NH HF =
2
3
4
2
1 : 6 mixtures and (b) IR absorption spectra of the same
samples. (2 ) Bragg’s angle and ( ) wave number. T, C:
(1) 180, (2) 320, and (3) 430.
Thus, we found the conditions for obtaining anhy-
drous FeF by the reaction of Fe O with NH HF .
3
2
3
4
2
It should be taken into account that a 1.5 2-fold ex-
cess of NH HF relative to the stoichiometry is
On further heating, (NH ) FeF decomposes by the
4 3
6
4
2
scheme
required to obtain FeF as a final product free from
3
iron(III) oxide formed by pyrohydrolysis. Depending
(NH4)3FeF6 = NH4FeF4 + 2HF + 2NH3.
(2)
on the requirements to FeF purity, it is appropriate
3
to synthesize it either in inert or in fluorinating
atmosphere, the repeated treatment with a fluorinating
agent being possible.
The decomposition is completed at 314 C, as
proved by a series of the corresponding endothermic
effects. Boiling of excess NH HF at approximately
4
2
231 C [3, 9] is accompanied by the corresponding
endothermic effect. The final stage (3) of the synthesis
is completed at approximately 430 C by the formation
of anhydrous iron(III) fluoride.
The main method of obtaining MnF is the reaction
2
of MnCO with HF [4].
3
The derivatogram of an MnO : NH HF mixture
4
2
with the molar ratio of 1 : 2 is shown in Fig. 3a. A
number of peaks corresponding to endothermic effects
NH4FeF4 = FeF3 + HF + NH3.
(3)
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 3 2004

REACTIONS OF OXIDES OF SOME 3d ELEMENTS
349
(a)
TG
m, %
m, %
(b)
DTA
T
T, C
(c)
m, %
(d)
m, %
TG
DTA
T
T, C
Fig. 3. (a) Derivatogram of an MnO : NH HF = 1 : 2 mixture; (b) thermogravigram of an MnO : NH HF = 1 : 1.5 mixture
4
2
4
2
recorded in the quasi-isothermal mode; (c) derivatogram of an MnO : NH HF = 1 : 4 mixture; and (d) thermogravigram of
4
2
an MnO : NH HF = 1 : 2.5 mixture recorded in the quasi-isothermal mode.
2
4
2
are observed in the DTA curve. The endothermic
effect at 195 C corresponds to the formation of the
complex NH MnF by the reaction
The reaction of MnO with NH HF was studied
2 4 2
by Rakov and co-workers [11]. They found that the
reaction proceeded stepwise to give (NH ) MnF ,
4
3
4 2
5
NH MnF , and MnF at 175, 230, and 300 C, respec-
4
4
2
MnO + 2 NH4HF2 = NH4MnF3 + H2O.
(6)
tively. Another reaction scheme was given in [12],
where the reaction of iron-manganese concretions with
NH HF was studied. The product of the exothermic
As the temperature is increased to 350 C, NH MnF
4
3
4
2
decomposes to give MnF :
2
reaction of MnO and NH HF is a compound iso-
2
4
2
structural to (NH ) MnF but having a 10% defi-
NH4MnF3 = MnF2 + HF + NH3.
(7)
4 3
6
ciency of fluorine as compared to this stoichiometry
owing to the isomorphous replacement of fluorine by
hydroxide ions. The final stage of the process at
The scheme of the MnO fluoridation is confirmed
by the thermogravigram taken in the quasi-iso-
thermal mode at the molar ratio of components
MnO : NH HF = 1 : 1.5. The experimental weight
320 C is the formation of MnF .
2
4
2
The thermogram of a mixture of MnO with
losses of 17.6 and 42%, respectively, agree well the
calculated losses of 17.4 and 41% (Fig. 3b). Com-
pounds identified as NH MnF and MnF by X-ray
2
NH HF in the molar ratio of 1:4 is shown in Fig. 3c.
4
2
The reaction proceeds in several stages (endothermic
effects at 185 and 224 C). The process can be de-
scribed by the equation
4
3
2
phase and chemical analyses [10] were synthesized at
200 and 325 C, respectively (Figs. 4a and 4b).
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 3 2004

350
KALINNIKOV et al.
(a)
(c)
(d)
(b)
2 , deg
2 , deg
Fig. 4. X-ray patterns of MnO : NH HF and MnO : NH HF mixtures at various temperatures: (a) NH MnF (200 C),
4
2
2
4
4
2
4
3
(b) MnF (325 C), (c) (NH ) MnF (200 C), and (d) NH MnF (300 C).
2
4 2
5
4
6MnO2 + 24NH4HF2 = 6(NH4)2MnF5 + 18HF + 10NH3
Preparation of anhydrous copper(II) fluorides is
complicated by the reaction with materials of cru-
cibles and by the pyrohydrolysis [14]. The following
+ N2 + 12H2O.
(8)
methods for preparing CuF have been described in
2
A series of endothermic effects observed on further
heating (Fig. 3c) correspond to the reactions
the literature: fluoridation of dehydrated CuCl crystal
2
hydrates using F or ClF at 400 C [4], dissolution
2
3
of CuO or (CuOH) CO in excess 40% hydrofluoric
2
3
(NH4)2MnF5 = NH4MnF4 + HF + NH3,
NH4MnF4 = MnF2 + HF + 1/2F2 + NH3.
(9)
acid with the subsequent dehydration of crystal hy-
drates of copper(II) compounds (CuF 2H O or
2
2
(10)
CuF 5H O 5HF according to the data of various
2
2
authors) in a dry HF flow [4, 14], and the reaction of
The thermogravigram obtained in the quasi-iso-
thermal mode is shown in Fig. 3d. It contains three
CuS with SF in an autoclave at 190 C followed by
4
treatment of the resulting products with anhydrous
carbon disulfide to remove sulfur [15]. The dehydra-
tion of CuF 2H O crystal hydrate both in air and in
plateaus corresponding to the formation of (NH )
4 2
MnF , NH MnF , and MnF , respectively. The ther-
5
4
4
2
2
2
mogravimetric data obtained are confirmed by the
results of the X-ray phase analysis [13] (Figs. 4c and
4d). The compounds (NH ) MnF , NH MnF , and
an inert atmosphere with heating results in the forma-
tion of mixtures of CuOHF, CuO, and CuF [16, 17].
2
4 2
5
4
4
Typical thermogravimetric curves of a mixture of
CuO with NH HF in the molar ratio of 1 : 2 are
MnF were isolated at 200, 300, and 320 C.
2
4
2
Thus, the established reaction scheme of the fluori-
dation of manganese(IV) oxide coincides with the
results of [11].
shown in Fig. 5a. The DTA curve contains a number
of endothermic effects. The first effect at 125 C corre-
sponds to the melting of NH HF [3, 9]. The endo-
4
2
The nature of exothermic effects (Figs. 3a, 3c)
remains to be elucidated. They may be due to the
crystallization of the forming amorphous manganese
fluoride.
thermic effect at 200 C corresponds to the formation
of the complex (NH ) CuF by reaction (11):
4 2
4
CuO + 2NH4HF2 = (NH4)2CuF4 + H2O.
(11)
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 3 2004

REACTIONS OF OXIDES OF SOME 3d ELEMENTS
Further heating causes thermal decomposition of
351
(a)
m, %
(NH ) CuF according to scheme (12), completed
TG
4 2
4
at 230 C.
DTG
m, %
(b)
(NH4)2CuF4
=
NH4CuF3
+
HF
+
NH3. (12)
DTA
The final stage (13) of the synthesis is completed
at approximately 290 C by the formation of anhy-
drous CuF .
2
NH4CuF3 = CuF2 + HF + NH3.
(13)
T
T, C
At 200, 230, and 290 C, we have synthesized the
compounds identified by X-ray phase and chemical
analyses as (NH ) CuF , NH CuF , and CuF [17].
Fig. 5. (a) Derivatogram of a mixture CuO:NH HF = 1:2
4
2
4
and (b) thermogravigram of a mixture CuO : NH HF
=
2
4 2
4
4
3
2
1:2 recorded in the quasi-isothermal mode.
(a)
Three steps corresponding to the formation of (NH )
4 2
CuF , NH CuF , and CuF (Fig. 5b) are detected in
4
4
3
2
m, %
TG
the TG curve obtained in the quasi-isothermal mode.
The weight loss for process (11) in the established
temperature range takes place even under conditions
of the quasi-isothermal measurement. An increase in
m, %
(b)
DTA
temperature results in the pyrohydrolysis of CuF and
2
in the contamination of the product with copper oxide.
In the copper(II) difluoride obtained, the atomic ratio
F : Cu 2. According to the data from [14], the for-
mula of copper(II) fluoride is CuF
(0
x
0.33);
the stoichiometric samples were synthesized by treat-
ment of CuF with xenon difluoride.
2
x
T
T, C
Fig. 6. (a) Derivatogram of a mixture ZnO:NH HF = 1:2
and (b) thermogravigram of a mixture ZnO:NH HF = 1:2
recorded in the quasi-isothermal mode.
4
4
2
2
2
x
The thermal decomposition of NH CuF to an-
4
3
hydrous CuF occurs at a lower temperature than the
2
corresponding processes for NH CoF and NH NiF
to the thermal decomposition of (NH ) ZnF :
4
3
4
3
4 2
4
(340 and 370 C) [18].
(NH4)2ZnF4 = NH4ZnF3 + HF + NH3.
(15)
The complete fluoridation is reached with a certain
excess of NH HF as compared to the stoichiometry
4
2
It is completed at 240 C, which is confirmed by a
series of the corresponding endothermic effects.
The final stage (16) of the synthesis proceeds up to
approximately 337 C.
of Eq. (11). This is associated with possible partial
decomposition of NH HF [9]. To obtain the pure
4
2
product from copper oxide, it is better to perform the
synthesis in an inert atmosphere.
Anhydrous ZnF is obtained by the reaction of
2
NH4ZnF3 = ZnF2 + HF + NH3.
(16)
ZnCO with excess hot hydrofluoric acid, followed by
3
heating of the precipitate at 800 C without air access
[4]. According to the review [3], zinc(II) forms the
complex compounds (NH ) ZnF H O and NH ZnF .
Three steps corresponding to the formation of
(NH ) ZnF , NH ZnF , and ZnF (Fig. 6b) are ob-
served in the TG curve obtained in the quasi-iso-
thermal mode. These compounds were synthesized
and isolated at the corresponding temperatures and
identified by X-ray phase and chemical analyses.
4 2
4
4
3
2
4 2
4
2
4
3
The temperature of NH ZnF decomposition is 250 C.
4
3
It is seen from Fig. 6a that the reaction in the mix-
ture of ZnO with NH HF (molar ratio 1 : 2) starts
4
2
even before NH HF melting and is accompanied by a
4
2
weight loss. It results in the formation of (NH ) ZnF :
CONCLUSION
4 2
4
ZnO + 2NH4HF2 = (NH4)2ZnF4 + H2O.
(14)
The reaction of ZnO with NH HF proceeds simi-
4
2
larly to the reactions of NH HF with nickel, cobalt,
4
2
Further increase in the temperature above 178 C leads
and copper oxides considered in the literature [18].
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KALINNIKOV et al.
Novosibirsk: Sib. Otd. Ross. Akad. Nauk, 1981,
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