Active potassium niobates and titanoniobates as catalysts for organic sulfide remediation

Article abstract of DOI:10.1016/j.catcom.2015.12.023

Pure (KNbO₃) and titanium-substituted (KTi_0.1Nb_0.9O₃ and KTi_0.2Nb_0.8O₃) potassium niobates with perovskite crystalline structures were prepared, characterized and tested as catal

Full text of DOI:10.1016/j.catcom.2015.12.023

Catalysis Communications 76 (2016) 58–61
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Active potassium niobates and titanoniobates as catalysts for organic
sulfide remediation
a,
a
b
b
a
Clara Saux ⁎, Candelaria Leal Marchena , Robinson Dinamarca , Gina Pecchi , Liliana Pierella
a
Centro de Investigación y Tecnología Química (CITeQ), UTN — CONICET, Maestro Marcelo Lopez esq. Cruz Roja Argentina, 5016 Córdoba, Argentina
Department of Physical Chemistry, University of Concepción, Concepción, Chile
b
a r t i c l e i n f o
a b s t r a c t
Article history:
3 3
Pure (KNbO ) and titanium-substituted (KTi0.1Nb0.9O3 and KTi0.2Nb0.8O ) potassium niobates with perovskite
Received 16 November 2015
Received in revised form 21 December 2015
Accepted 22 December 2015
Available online 29 December 2015
crystalline structures were prepared, characterized and tested as catalysts in the selective oxidation of 2-
methylthio)benzothiazole to its corresponding sulfoxide and sulfone. These catalysts, which have not previously
been reported for organic sulfide remediation, are the most active catalysts among those discovered to date for
this type of oxidation reaction. Hydrogen peroxide was employed as the oxidant, and the reaction parameters
were tested in order to find milder reaction conditions.
(
Keywords:
Niobates
©
2015 Elsevier B.V. All rights reserved.
Perovskites
Titanium
Sulfide remediation
Hydrogen peroxide
1
. Introduction
step and reused. In this sense, the employment of mixed oxides appears
as a promising alternative. Perovskite-type oxide have gained attention
as catalysts due to their well-known thermal stabilities in a broad range
of oxygen partial pressures and their resistance to catalytic poisons [9].
In particular, potassium niobate has been widely reported for
photocatalysis [10] and soot oxidation [11]. In the present article, the
The heteroatomic benzothiazole ring is present in diverse and
important organic compounds that are used as pesticides, biocorrosion
inhibitors, chemotherapeutic compounds, etc. [1] with a high produc-
tion volume. However, the largest use of benzothiazoles is as vulcaniza-
tion accelerators in rubber production, as they catalyze the formation of
sulfide linkages between unsaturated elastomeric polymers [2]. Several
benzothiazoles have been detected in surface water, industrial waste-
water and the receiving water from the runoff of impervious urban
surfaces [3]. Considering their acute aquatic toxicities determined in
various test systems [4] and low rates of biodegradability, it is necessary
to find ways to abate them. An alternative for generating more biode-
gradable compounds from benzothizaoles is to oxidize them to their
corresponding sulfoxides and sulfones [5]. Catalytic wet peroxide oxida-
tion (CWPO) is an ecologically attractive catalytic approach aimed at the
deep oxidation of weakly biodegradable toxic organic compounds in
wastewater [6]. Considering that aqueous hydrogen peroxide is inex-
pensive and produces only water as by-product, it has been used as an
attractive and environmental friendly oxidant [7]. Heterogeneous cata-
lysts based on low-valence transition metals have shown good results
and have been widely studied for the CWPO of organic pollutants
because their oxidation efficiencies are relatively high compared with
homogeneous catalysis under the same operating conditions [8]. More-
over, solid catalysts can be recuperated by means of a simple separation
catalytic activities of pure (KNbO
3 3
) and substituted (KTi0.1Nb0.9O and
KTi0.2Nb0.8O
3
) potassium niobate perovskite-type oxides used as
catalysts for an organic sulfide remediation reaction using 2-
(methylthio)benzothiazole as the model substrate are reported for the
first time.
2. Experimental
KNbO
3
, KTi0.1Nb0.9
O
3
and KTi0.2Nb0.8
3
O powders were prepared
adding KNO
3
and Ti(OC H )
4 9 4
to a NbF solution [12]. The viscous gels
5
were dried and then thermally treated in air in an electric furnace at
600 °C for 10 h [13]. Specific areas were calculated from nitrogen ad-
sorption isotherms, on a Micromeritics ASAP 2010 at 77 K. X-ray pow-
der diffraction (XRD) patterns were obtained with nickel-filtered
CuKα
performed in a TPR/TPD 2900 Micromeritics system by passing a 5%
/Ar flow. For the O -TPD experiments, the samples were preheated
in an O flow for 1 h at 700 °C, cooled down to room temperature in
1
radiation using a Rigaku diffractometer. TPR experiments were
H
2
2
2
the same atmosphere, and then switched to a helium flow with the ox-
ygen desorption monitored using a TCD. The catalytic selective oxida-
tion of 2-(methylthio)benzothiazole with H
was carried out in a glass reactor (25 cm ) with magnetic stirring,
2
O
2
as the oxidizing agent
⁎
Corresponding author.
E-mail address: csaux@frc.utn.edu.ar (C. Saux).
3
http://dx.doi.org/10.1016/j.catcom.2015.12.023
566-7367/© 2015 Elsevier B.V. All rights reserved.
1

C. Saux et al. / Catalysis Communications 76 (2016) 58–61
59
immersed in a thermostated bath and equipped with a reflux condens-
er. The catalytic tests were performed from room temperature to 60 °C
evaluating different reaction conditions such as the mass of catalyst and
2 2
different molar ratios of sulfide/H O (R). Several solvents with different
dielectric constants were tested. Prior to analysis, the catalyst was sep-
arated out by filtration. Organic compounds were quantitatively ana-
lyzed by gas chromatography (Perkin Elmer Clarus 500) with a HP1
capillary column and a FID as well as qualitatively by GC–MS (Shimadzu
QP 5050 GC-17 A) using a HP-5 (25 m × 0.2 mm i.d) capillary column.
3
3
3
. Results and discussion
.1. Catalyst characterization
.1.1. Specific area
The specific BET areas of the catalysts are summarized in Table 1.
Even though lower surface areas are reported for the perovskite-type
oxides prepared by citrate or sol–gel methods [14], the obtained values
2
−1
in the range 3 to 6 m g are in the range reported for alkaline niobate
perovskites [13,15].
x 3
Fig. 1. X-ray diffraction patterns of KNb1 − xTi O (x = 0.0, 0.1, 0.2) niobates.
3
.1.2. X-ray diffraction (XRD)
The diffraction patterns of the catalysts are shown in Fig. 1. Sharp dif-
600 °C [18]. Increases in both, chemical and lattice oxygen's, in the
peak starting at 500 °C not finished at 700 °C can be seen. As this later
peak is associated with oxygen species occupying the inner vacancies
created by non-stoichiometry's [21], the increase in the oxygen
desorbed as Ti4 content increase can be related to an increases in the
redox properties of the perovskite. Because DTP–MS experiments con-
firm that the evolved gas and the He flow only contain oxygen, the
deconvolution of the oxygen desorption curves using a Lorentzian
shape allows the calculation of the desorbed oxygen in the correspond-
fraction peaks indicative of mixed crystal phases can be observed for the
different titanium contents [16]. A careful analysis of the XRD patterns
in comparison with the K–Nb–F–O components in the database indi-
+
3
cates that for pure potassium niobate powder, the KNbO orthorhombic
structure (32–822) [17] is obtained [13]. Ushikubo reported that niobi-
um oxide can easily react with many other oxides to form mixed oxide
phases with complex structure active [18], therefore it is not surprising
that the large number of additional reflections attributed to segregated
phases. With regard to potassium titanoniobates, the KNbO
phase is also obtained along with new segregated phases identified by
the diffraction peaks as an orthorhombic KTiNbO (71–1747). Some im-
purities that corresponds to KNb F (36–0808) have been previously
reported for SrTi1 − xNb [16]. Moreover, a shift in the reflections of
the KNbO perovskite phase towards higher 2 theta angles is detectable
inset Fig. 1). This behavior indicates a decrease in the interplanar dis-
tance, i.e. a lattice contraction of the crystalline structure [17], due to
3
perovskite
2
ing temperature ranges. The amounts of desorbed O in Table 1 are com-
parable to other substituted perovskites [22]. Lattice oxygen is expected
to be of less importance compared to chemisorbed oxygen in the selec-
tive oxidation of 2-(methylthio)benzothiazole reaction that occurs at
low temperatures.
5
2 5
O
x 3
O
Finally, the characterization results reveal that tetravalent TiIV on
NbV can be successfully incorporated into the octahedral B-site of the
perovskite lattice (XRD), leading to a large extent of oxygen vacancies
3
(
4
+
5+
the smaller radius of Ti (0.062 nm) than Nb (0.064 nm).
2
(O -DTP).
3
.1.3. Temperature programmed reduction (TPR)
3.2. Catalytic activity
The TPR profiles (Appendix 1) do not show any reduction peaks up
to 700 °C, indicative of high thermal stability in reductor atmosphere
and similar to the stoichiometric and non-reducible LaFeO [19]. Al-
3.2.1. Effect of Ti substitution
The catalytic activities in the oxidation of 2-(methylthio)-
benzothiazole (2MTBT) are shown in Table 2. The non-catalyzed
3
though no previous reports of the thermal behavior of niobate
perovskite-type oxides in hydrogen were found, the obtained results
are related to the large stabilities of the oxidation states of the B cations
5
+
4+
(
Nb and Ti ). Moreover, previous reports of decreases in the extent
3
of reduction of SrCo0.8Fe0.2O perovskite upon Nb doping [20] support
this hypothesis.
3
.1.4. Oxygen desorption profiles (O
2
-DTP)
The oxygen desorption profiles shown in Fig. 2 indicate physically
weakly held adsorbed oxygen at ~80 °C, chemically adsorbed oxygen
that desorbs between 200 °C and 500 °C and lattice oxygen or oxygen
species occupying inner vacancies that desorb at temperatures up to
Table 1
x 3
Characterization results of KTi Nb1 − xO (x = 0.0, 0.1, 0.2) niobates.
Catalyst
S
BET, m2 g−1
Detected XRD
phases
Desorbed oxygen
mmol gcat−
1
KNbO
KTi0.1Nb0.9
KTi0.2Nb0.8
3
3
6
6
KNbO
KNbO
KNbO
3
3
3
0.34
0.42
0.50
O
O
3
KNbTiO
KTiNbO
5
3
5
x 3
Fig. 2. Oxygen desorption profiles of KNb1 − xTi O (x = 0.0, 0.1, 0.2) niobates.

60
C. Saux et al. / Catalysis Communications 76 (2016) 58–61
Table 2
Catalytic activities and selectivities of KTi
oxidation.
x
Nb
1
−
x
O
3
(x = 0.0, 0.1, 0.2) on 2MTBT
⁎
⁎
Entry
Catalyst
Conversion
Selectivity
Sulfoxide
Scheme 1. Proposed 2MTBT oxidation pathway.
Sulfone
1
2
3
4
KNbO
3
99.7
99.5
99.9
5.1
29.7
5.4
6.5
70.2
94.6
93.5
18.7
Moreover, the relationship between the chemical bonding nature of the
Ti-substituted niobates with the oxygen vacancies should be an efficient
tool for designing and developing new efficient and active catalysts to
effectively decompose some organic molecules.
KTi0.1Nb0.9
KTi0.2Nb0.8
None
O
O
3
3
81.3
Reaction conditions: 40 °C, acetonitrile, R = 0.1, 0.1 g of catalyst, after 1 h.
Mol.%.
⁎
3.2.2. Effect of the nature of the solvent
reaction (entry 4, Table 2) results are also included. The large catalytic
activities and selectivities of these potassium niobates in the 2MTBT
conversion are noticeable, as the corresponding sulfoxide and sulfone
are the only reaction products. Additionally, niobium oxides have
been proposed to be catalysts for pollution abatement due to their
large selective to hydrocarbons for oxidation reactions [18].
Considering that the 2MTBT oxidation is a reaction that takes place
in two liquid phases (aqueous and organic) and that the catalyst adds
another phase (solid), the minimization of mass transfer problems is
greatly necessary. A study on the effects of the nature of the solvent,
using species with different polarities and proticities to optimize the
so-called triple contact point where the catalyst, the oxidant and the re-
actant meet together, was proposed. The conversion levels and selectiv-
ities to sulfoxide and sulfone using acetone, acetonitrile, ethanol,
methanol and 2-propanol were studied. When water and hexane
were used as solvents, it was not possible to obtain a homogeneous liq-
uid phase, and thus, 2MTBT conversion was extremely low. No changes
in the conversion level (approx. 99.5 mol.% after the first reaction hour)
using acetonitrile, ethanol, methanol and 2-propanol as the solvent
were observed. In the case of acetone, a 63 mol.% conversion was ob-
tained after 1 h of reaction time. Even when conversion values are not
negligible, this behavior could be assigned to a more difficult contact be-
tween the hydrophilic catalyst surface and this aprotic solvent, which
has the lowest dielectric constant (20.7 D). With regard to sulfone and
sulfoxide selectivities, no significant differences were obtained,
reaching approx. 40 and 60 mol.%, respectively. Moreover, higher initial
reaction rates were obtained using 2-propanol, ethanol and methanol,
indicating that protic solvents favored the triple contact due to the hy-
drophilic character of these alkaline niobates.
It should be noted that the conversion levels (≥99.5%), are higher
than those reported by many authors for this kind of reaction [23,24].
In general, in ABO
ly related to the nature and redox behavior of the B-site. Due to no
changes in the redox properties of KNbO upon Ti substitution, the cat-
3
perovskite-type oxides, the catalytic activity is close-
3
alytic activity behavior should be related to the formation of oxygen va-
cancies. For K-substituted lanthanum perovskites, the catalytic activity
has been related to the formation of surface oxygen-containing com-
plexes in nano-sized particles [25]. Moreover, for lanthanoid perov-
skites, Li et al. [26] reported that surface oxygen species are also active
species, which migrate from the perovskite structure.
Fig. 3 shows the reaction evolution as function of reaction time. It is
possible to observe that sulfoxide selectivity decreases while sulfone se-
lectivity increases with time. This observation allows us to propose a
two-step mechanism (Scheme 1).
In the first step, the sulfide is converted into sulfoxide and then the
sulfoxide is oxidized into sulfone. A large increase in the selectivity to
sulfone (entries 2 and 3, Table 2) upon titanium substitution into the
KNbO
3
perovskite structure is detected. According to the obtained re-
3.2.3. Effect of the oxidant concentration
sults, it is proposed that the substitution of such electronegativity cat-
ions gives rise to the enhancement of the activity to effectively
decompose 2MTBT. These observations could be understood in terms
of the weakening of transition metal–oxygen bond upon the Ti substitu-
tion responsible for a decrease in the activation energy of the second
step of the oxidation pathway thus increasing the production of sulfone.
The effectiveness of H
2 2
O was probed because no conversion was
achieved in its absence. The effect of the initial oxidant concentration
was studied by changing the volume added while maintaining the
other initial concentrations and reaction conditions constant. The ob-
tained results shown in Fig. 4 indicate a progressive increase in the
2MTBT conversion level up to 2.0 ml of hydrogen peroxide. Further in-
creases in the concentration of hydrogen peroxide do not modify the
catalytic behavior. The increase by 5 mol.% of sulfone selectivity with
2 2
an increase in the H O volume from 2.0 to 3.0 ml is the expected behav-
ior considering that in the presence of a larger amount of oxidant
Fig. 4. 2MTBT conversion and sulfoxide and sulfone selectivities as function of H
O
2 2
vol-
Fig. 3. Effect of reaction time on 2MTBT oxidation. Reaction conditions: room temperature;
3
ume. Reaction conditions: room temperature; 0.05 g of KNbO ; acetonitrile as solvent;
after 1 h.
3
0.1 g KNbO ; acetonitrile; R = 0.1.

C. Saux et al. / Catalysis Communications 76 (2016) 58–61
61
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Acknowledgments
[
This project was partially supported by UTN-PID 1275 and 25/E172.
Pierella, Saux and Leal Marchena thank CONICET-Argentina. Pecchi and
Dinamarca thank CONICYT — Chile Fondecyt 1130005 and Red Doctoral
REDOC, MINEDUC project UCO1202.
[
[
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2015.12.023.