Reaction of iodine with metal oxides

Article abstract of DOI:10.1139/v11-117

The reaction of iodine with various metal oxides has been studied in both vapour phase (373-873 K) and in solution in cyclohexane, under anhydrous conditions. The course of the reaction has been revised in accordance with literature and experimental data. Apart from adsorbed iodine molecules ([I ₂] < 200 μmol·g^-1), the presence of I ^- ions ([I^-] < 50 μmol·g^-1) has been noted. IO_n^- ions have been observed only when both water and strong basic sites, such as those on MgO or La₂O ₃, are present. It has been found that the strength of interaction of iodine with metal oxide increases with basicity of the oxide.

Full text of DOI:10.1139/v11-117

1370
Reaction of iodine with metal oxides
ꢀ
Marek Glinski and Urszula Ulkowska
Abstract: The reaction of iodine with various metal oxides has been studied in both vapour phase (373–873 K) and in solu-
tion in cyclohexane, under anhydrous conditions. The course of the reaction has been revised in accordance with literature
and experimental data. Apart from adsorbed iodine molecules ([I2] < 200 mmol·g–1), the presence of I– ions
([I–] < 50 mmol·g–1) has been noted. IOn ions have been observed only when both water and strong basic sites, such as
–
those on MgO or La2O3, are present. It has been found that the strength of interaction of iodine with metal oxide increases
with basicity of the oxide.
Key words: iodine, metal oxides, iodides, basicity.
Résumé : On a étudié les réactions de l’iode avec divers oxydes métalliques, tant en phase vapeur (373–873 K) qu’en solu-
tion dans le cyclohexane, dans des conditions anhydres. On a révisé le cours de la réaction afin de le rendre en accord avec
les données expérimentales et celles de la littérature. En plus des molécules d’iode absorbées, ([I2] < 200 mmol·g–1), on a
aussi noté la présence d’ions I– ([I–] < 50 mmol·g–1). On n’a observé la présence d’ions IOn que lorsqu’on est en présence
–
à la fois d’eau et de sites fortement basiques, tels ceux présents sur le MgO ou le La2O3. On a aussi observé que la force de
l’interaction de l’iode avec l’oxyde métallique augmente avec la basicité de cet oxyde.
Mots‐clés : iode, oxydes métalliques, iodures, basicité.
[Traduit par la Rédaction]
The same type of surface reaction has been proposed for the
treatment of alumina with gaseous chlorine, although no
proof for the formation of ClO– ions has been given.3
In a series of articles Klabunde et al. have reported that
MgO-halogen adducts are reactive halogenating agents of or-
ganic molecules and exhibit noticeable biocidal character.4,5
Moreover, magnesium oxide is found to be an active catalyst
in the oxidative dehydrogenation of butane to butadiene in
the presence of oxygen and iodine.6 Based on the results of
Raman spectroscopy, the authors concluded that upon ad-
sorption of iodine on the surface of MgO, the formation of
an Mg–I bond does not take place or it takes place to a neg-
ligible extent.5
Iodine has been used as a molecular probe of electron do-
nor strength of Lewis base sites of various zeolites.7,8 As has
been stated, I2 interacts with the basic framework oxygen and
not the charge-balancing cation. It has also been shown that a
blue shift in the visible spectrum of adsorbed iodine on zeo-
lite increases with increasing basicity of the zeolite.9 This
technique for probing basicity of solids has been extended to
classical supports like alumina, silica, and magnesia.10 The
following decreasing order of the strength of interaction has
been found: MgO (very high) > Al2O3 (significant) > SiO2
(weak).10
Introduction
Although halogens are very potent electron acceptors, little
is known about their reactivity with the surface of metal ox-
ides. What is more, there is some controversy in the literature
about the products of this reaction.
In 1974, Kibblewhite and Tench reported that halogens re-
act at room temperature (RT) with magnesium oxide, and the
effect of the reaction of a given halogen on the oxide surface
strongly depends on the halogen type.1 It has been shown
that MgO treated with iodine contains the appropriate I– ions
and exhibits a lack of IO– and IO3 ions. The authors stated
–
that iodine can react with the surface oxide ions of the lowest
coordination only, e.g., 3-fold coordination, which are the
most reactive.
Also in 1974 a paper on the reduction of iodine in benzene
solution by Al2O3, Al2O3-SiO2, and SiO2 surfaces was pub-
lished by Flockhart et al.2 The authors stated that hydroxyl
groups on the surfaces of metal oxides can act as
one-electron donor sites, and that they are responsible for the
occurrence of the reaction. The authors have proposed the
mechanism of the reduction based on the following equation:
½1ꢀ
I2 þ 2OHꢁ ! IOꢁ þ Iꢁ þ H2O
–
However, they could not explain the absence of IO3 ions,
derived from the transformations of IO– ions, which should
be formed according to:
Very recently we have demonstrated that MgO treated with
a solution of iodine in pentan-2-ol shows a significant in-
crease in the selectivity in the hydrogen transfer reaction to
cyclopentanone.11
3IOꢁ ! IO3 þ 2Iꢁ
ꢁ
½2ꢀ
Received 15 June 2011. Accepted 22 July 2011. Published at www.nrcresearchpress.com/cjc on 18 October 2011.
ꢀ
M. Glinski and U. Ulkowska. Warsaw University of Technology (Politechnika), Faculty of Chemistry, Noakowskiego 3, 00-664 Warsaw,
Poland.
ꢀ
Corresponding author: Marek Glinski (e-mail: marekg@ch.pw.edu.pl.)
Can. J. Chem. 89: 1370–1374 (2011)
doi:10.1139/V11-117
Published by NRC Research Press

ꢀ
Glinski and Ulkowska
1371
Here, we describe the reaction of iodine with a series of
metal oxides. The reactions were carried out in a broad range
of conditions: with a solution of I2 in cyclohexane at RT or
with vapours of I2 in the temperature range 373–873 K. The
present study was focused on the strong interaction of iodine
with metal oxides, leading to its reduction to I– ions. We ex-
pected that our work would help to resolve the controversy
found in the literature about the products of the above men-
tioned reaction.
Solvents and reagents
Cyclohexane (99+%, Sigma-Aldrich) was dried by filtra-
tion through a glass column containing anhydrous molecular
sieves, followed by distillation over metallic potassium – ben-
zophenone in nitrogen atmosphere directly to the Schlenk-
type container. Chloroform (pure, BDH) was washed three
times with distilled water to remove ethanol and dried at 273
K under nitrogen over anhydrous CaCl2. A final purification
of CHCl3 was achieved by distillation in a stream of dry ni-
trogen in an all-glass apparatus. The distillate was kept under
nitrogen in a Schlenk-type container, covered with metallic
foil.
Potassium iodate (p.a., Merck) was dried in an oven at 423
K for 4 h and kept in a tightly closed container. Iodine (dou-
ble sublimed, Merck), potassium iodide (p.a., POCh Gliwice,
Poland), and sodium thiosulphate pentahydrate (99.5%,
Sigma-Aldrich) were used as received. A stock solution of
Na2S2O3 was prepared (∼0.1 mol/L) and stabilized by the ad-
dition of CHCl3. Sodium thiosulphate solutions with lower
concentrations (0.001–0.005 mol/L) were prepared by dilu-
tion of the stock solution immediately before use. Their con-
centrations were determined using anhydrous potassium
iodate as a standard in the presence of iodide ions in an
acidic solution. Sulphuric acid (p.a., 98%, Merck) was used
to prepare a 1 mol/L solution in water. Starch (AR, soluble,
BDH) was used as received. Its solution in water was stabi-
lized by addition of small amounts of salicylic acid.
Experimental
Metal oxides Al2O3, SiO2, TiO2, and ZnO
The powders of alumina (type C, Degussa), silica (Aerosil
200, Degussa), titania (P-25, Degussa), and zinc oxide (puriss
p.a., Fluka) were mixed with re-distilled water, and the slur-
ries were dried at 333 and 393 K for 24 h at each tempera-
ture. The lumps of oxide were crushed, sieved, and the grain
fraction 0.4–0.63 mm was collected.
ZrO2 and La2O3
The solutions of ZrOCl2·8H2O and La(NO3)3·6H2O (both
puriss p.a., Fluka) in water were treated with an aqueous am-
monia solution (25%), and the precipitated hydroxides were
successively washed with re-distilled water to remove all
soluble salts. After filtration, metal hydroxides were treated
according to the procedure described above.
Reaction of metal oxides with iodine
MgO
(All operations connected with sample preparation and
handling were done in a dry nitrogen atmosphere.) Two dif-
ferent procedures for the introduction of iodine on the surfa-
ces of metal oxides were used.
Magnesium oxide was prepared by thermal decomposition
of Mg(OH)2, whose preparation has been described in detail
elsewhere.12 The grains of hydroxide with 0.4–0.63 mm di-
ameter were used.
Before characterization studies and treatment with iodine
or its solutions, metal oxides or hydroxides were calcined at
873 K for 1 h in a stream of air and then for 5 h at the same
temperature in a stream of dry nitrogen. They were then
stored under nitrogen in Schlenk-type containers.
In the first procedure, a weighed sample of metal oxide
(250 mg), placed in an Erlenmeyer flask, was suspended in
anhydrous cyclohexane (5 cm3), and a cyclohexane solution
of iodine of known concentration (0.05 mol/L, 5 cm3) was
added. A suspension of the metal oxide in the stoppered
flask was stirred at RT for a certain period of time.
In the second procedure, a stream of anhydrous nitrogen
(100 cm3·min–1) was passed through a glass saturator, filled
with iodine kept at 293 K. The stream of N2, saturated with
I2 vapours, was passed through a metal oxide bed (500 mg),
placed in a tubular quartz reactor, and heated to the appropri-
ate temperature. After a 3 h long contact with I2 vapours, the
bed of metal oxide was purged with nitrogen at the same
temperature for 1 h, and cooled in a stream of N2. It has
been found in a test without a catalyst that the total amount
of iodine reaching the reactor during 3 h is equal to
202 6 mmol.
Characterization of metal oxides
The specific surface areas were measured by nitrogen ad-
sorption at 77 K (BET method) using a Gemini 2360 instru-
ment (Micromeritics).
The strength of the surface acid–base sites of selected
metal oxides was determined by the Hammett method, using
a sequence of indicators in anhydrous toluene as the sol-
vent.13 The following set of indicators has been used (the val-
ues of pKA or pKBH+ are given in parentheses): chalcone
(–5.6), dicinnamylideneacetone (–3.0), crystal violet (0.8),
methyl red (4.8), bromothymol blue (7.2), phenolphtha-
lein (9.3), 2,4-dinitroaniline (15.0), 4-nitroaniline (18.4),
diphenylamine (22.3), 4-chloroaniline (26.5), and triphenyl-
methane (33.0). The concentrations of acidic and basic sites
of metal oxides were determined using solutions (0.01 mol/L)
of triethylamine or benzoic acid in anhydrous toluene, ac-
cording to a procedure described elsewhere.14 The measure-
ments were performed at ambient temperature under dry
nitrogen, after 24 h contact of the oxides with these solu-
tions in grease-less reactors. The results are summarized in
Table 1.
Analytical determinations
These are tests for the presence of I– and IO3 ions. A
–
sample of a metal oxide that had been reacted previously
with an iodine solution or gaseous iodine was extracted with
chloroform (5 times) to remove the free halogen, and finally
the solvent was removed under reduced pressure. The result-
ing solid was triturated with 1 mol/L H2SO4. An acidic solu-
tion was treated either with 30% H2O2 or with a freshly
prepared 0.1 mol/L KI solution in the presence of starch for
Published by NRC Research Press

1372
Can. J. Chem. Vol. 89, 2011
Table 1. Specific surface area and acid–base properties of the studied metal oxides.
Concentration of sites (mmol·g–1)
Strength
MOx
MgO
ZnO
Al2O3
La2O3
SiO2
TiO2
ZrO2
SBET (m2·g–1)
100
3
103
26
253
38
Acidic
9
—
132
–
30
42
—
Basic
1785
—
H0
H–
a
<7.2; 33.0)
—
<7.2; 9.3)
<7.2; 15.0>b
<7.2; 9.3)
<7.2; 9.3)
—
—
408
–
(–5.6; 4.8>
–
358
130
—
(0.8; 4.8>
(0.8; 4.8>
—
32
aH0 > 4.8.
bTaken from Ref. 15.
the confirmation of the presence of I– and IO3 ions, respec-
tively.
–
Table 2. Reaction of I2 with Al2O3 in anhydrous cyclohexane.
Treatment
[I–] (mmol·g–1)/ Time of reaction (h)
I2 on metal oxides
0.25
9
10
18
20
0
22
20
19
0.5
12
12
19
18
—
20
21
18
1
3
10
—
—
23
24
—
—
—
26
24
14
13
29
27
2
24
30
33g
(All operations were performed in a dry nitrogen atmos-
phere). A weighed sample of an oxide (250 mg) was placed
in an Erlenmeyer flask (100 cm3) fitted with a ground-glass
stopper, and the free iodine was extracted with anhydrous
CHCl3 (5 × 5 cm3). The organic extracts were collected and
treated with 10 cm3 0.05 mol/L KI. The resulting two-phase
mixture was titrated with 0.002 mol/L Na2S2O3 in the pres-
ence of starch.
—
—
—
19
19
0
22
—
18
12
13
21
20
1
23
23
20
a
b
a,b
a,b,c
b,d
b,e
b,f
aIn darkness.
I– ions on metal oxides
bWith stirring.
The solid left from the determination of iodine was stirred
with 15 cm3 1 mol/L H2SO4, and 1 cm3 0.005 mol/L NaIO3
was added. The liberated iodine was extracted with chloro-
form (5 × 5 cm3). The extracts were collected, washed with
water, and titrated with 0.002 mol/L Na2S2O3, according to
the procedure described earlier.
c250 mg of quartz.
d125 mg of alumina.
e500 mg of alumina.
f1000 mg of alumina.
g33 2 mmol·g–1 for 5 measurements.
The concentrations given in Tables 2–4 are the averages of
three measurements.
Table 3. Reaction of I2 with metal oxides in anhydrous cyclo-
hexane.
[I–] (mmol·g–1 MOx) / Time of reaction (min)
Results and discussion
MOx
5
2
2
5
14
15
18
21
15
3
5
60
4
We applied the method of Flockhart et al. to introduce io-
dine onto alumina using cyclohexane instead of benzene as
SiO2
the solvent for iodine.2 We have found that no IOn ions
La2O3
MgO
Al2O3
TiO2
ZrO2
ZnO
6a
8a
19
20
25
30
–
were formed. Moreover, the consumption of iodine can be
described by the following equation:
6
18
19
21
23
½3ꢀ
I2 þ 2eꢁ ! 2Iꢁ
Very small amounts of iodides, around 20 mmol·g–1, were
aThe value increases steeply when water is present.
found on the surface of the oxide apart from the unreacted
iodine. Despite the different solvent, the concentration of io-
dine, the type of alumina, and analytical procedure, a quite
good correlation with the results obtained by Flockhart et al.
(max. [I–] = 76 mmol·g–1) has been noted.2 The source of
electrons transferred to iodine is not established. It is said in
the literature that they are derived from surface hydroxyls and
(or) the lowest coordinated oxygen anions acting as one-electron
donor sites.16,17
We have also found that stirring the suspension of alumina
greatly accelerates reaching an equilibrium, and that daylight
has no influence on the rate of the reaction. Stirring of the
suspension for more than an hour increases the I– concentra-
tion because of the progressive grinding of the grains of alu-
mina. The results of preliminary measurements are collected
in Table 2.
Other metal oxides reacted with iodine in cyclohexane in a
similar manner as alumina (Table 3). A partial reduction of
iodine also took place with the formation of iodide ions. No
–
formation of IOn ions was noted in the case of all studied
metal oxides. Very low concentrations of iodide ions, up to
30 mmol·g–1, were reached. It has been found that only for
strong basic oxides such as MgO or La2O3, the presence of
water greatly accelerates the reaction of the oxide with I2 be-
cause of the participation of a well-known disproportionation
reaction of a halogen in basic media:
Published by NRC Research Press

ꢀ
Glinski and Ulkowska
1373
Table 4. Reaction of I2 with metal oxides heated to TR for 3 h.
Concentration
Metal oxide
TR (K)
(mmol·g–1 MOx)
Density (mmol·m–2 MOx)
I2
0
I–
0
I2
I–
a
—
473
0.0
0.0
0.0
0.0
0.7
0.0
0.1
0.0e
0.0
4.7
1.9
0.2
0.0
1.9
0.6
0.1
0.0
0.0
0.0
0.0
0.0c
1.0
1.0
0.0d
0.0d
0.0d
0.1
0.2
0.2
0.3
0.1
0.1
0.2
0.2
SiO2
ZrO2
TiO2
ZnO
0b
0b
0b
2
0b
1
1
3
3
2
2
2
3
5
6
8
10
12
15
22
673
373
473
573
373
473
673
873
373
473
673
873
0
12
4
Al2O3
La2O3
0
121
49
4
0b
190
58
14
0b
MgO
aQuartz.
bLess than 0.2 mmol·g–1.
c0.03 mmol·m–2.
d0.02 mmol·m–2.
e0.04 mmol·m–2.
The results correlate well with those obtained in UV–vis
spectroscopy studies.10
½4ꢀ
6 MgðOHÞ2 þ 6 I2 ! MgðIO3Þ2 þ 5 MgI2 þ 6 H2O
Conclusion
However, such a reaction does not occur under the rigorous
anhydrous conditions adopted in this work.
Iodine, both in vapour phase or in solution in anhydrous
cyclohexane, reacts with the surfaces of various metal oxides.
Apart from adsorbed iodine molecules ([I2] < 200 mmol·g–1),
the presence of I– ions ([I–] < 50 mmol·g–1) has been noted.
Based on the results presented above we decided to further
investigate the method of introduction of iodine onto the sur-
face of metal oxides. To simplify the reaction conditions be-
tween metal oxide and iodine, we applied a reaction method
of heating the oxide to different temperatures with a stream
of iodine vapour diluted with anhydrous nitrogen. In this
solvent-free method, the absence of any complications con-
nected with the use of a solvent is ensured. The results are
summarized in Table 4.
–
The formation of IOn ions occurs only when both water and
strong basic sites, such as those on MgO or La2O3, are
present. The strength of interaction of iodine with metal ox-
ide increases with basicity of the oxide.
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