Red Mud waste from the Bayer process as a catalyst for the desulfurization of hydrocarbon fuels

Article abstract of DOI:10.1039/c4ra07635d

The management of Red Mud generated as a waste by-product of bauxite processing in the aluminum industry is key to the long-term sustainability of alumina production. At the same time, the desulfurization of fuel oil to low-level sulfur is an ongoing challenge to the petroleum industry. In an attempt to address both these issue in an integrated fashion, Red Mud waste was studied as a catalyst in a desulfurization process using a simulated Diesel containing dibenzothiophene (DBT) as a model heterocyclic organic sulfur compound. The new process combines an oxidative-adsorptive desulfurization in which a combination of H₂O₂//H₃CCOOH and Red Mud was able to catalytically oxidize the sulfur compound to the corresponding sulfoxide (DBTO) and sulfone (DBTO₂) which are then reversibly adsorbed onto the Red Mud. Regenerative tests of Red Mud were performed, maintaining a high activity in recycles suggesting that the Red Mud could be reused after a simple thermal treatment releasing DBTO and DBTO₂. The approach synergistically addresses both problems: removal of sulfur compounds while at the same time offering a potential useful application of Red Mud. Red Mud was characterized by M?ssbauer Spectroscopy, Infrared Spectroscopy, X-ray Diffraction (XRD), Scanning Electron Microscopy with dispersive energy (SEM-EDS), BET Surface area (BET), and release of volatile compounds from Red Mud by Gas chromatography-Mass Spectrometry (GC-MS) using a solvent-free solid injector (SFSI). The kinetics of the desulfurization reactions were monitored by GC-MS. This journal is

Full text of DOI:10.1039/c4ra07635d

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PAPER
Red Mud waste from the Bayer process as a catalyst
for the desulfurization of hydrocarbon fuels†
Cite this: RSC Adv., 2014, 4, 47287
a
a
b
Eliane Cristina de Resende, Iara do Rosario Guimaraes Carvalho, Marcel Schlaf
´
˜
a
and Mario Cesar Guerreiro*
´
´
The management of Red Mud generated as a waste by-product of bauxite processing in the aluminum
industry is key to the long-term sustainability of alumina production. At the same time, the
desulfurization of fuel oil to low-level sulfur is an ongoing challenge to the petroleum industry. In an
attempt to address both these issue in an integrated fashion, Red Mud waste was studied as a catalyst in
a desulfurization process using a simulated Diesel containing dibenzothiophene (DBT) as a model
heterocyclic organic sulfur compound. The new process combines an oxidative-adsorptive
desulfurization in which a combination of H
2
2 3
O //H CCOOH and Red Mud was able to catalytically
oxidize the sulfur compound to the corresponding sulfoxide (DBTO) and sulfone (DBTO
2
) which are then
reversibly adsorbed onto the Red Mud. Regenerative tests of Red Mud were performed, maintaining a
high activity in recycles suggesting that the Red Mud could be reused after a simple thermal treatment
2
releasing DBTO and DBTO . The approach synergistically addresses both problems: removal of sulfur
compounds while at the same time offering a potential useful application of Red Mud. Red Mud was
characterized by M o¨ ssbauer Spectroscopy, Infrared Spectroscopy, X-ray Diffraction (XRD), Scanning
Electron Microscopy with dispersive energy (SEM-EDS), BET Surface area (BET), and release of volatile
compounds from Red Mud by Gas chromatography-Mass Spectrometry (GC-MS) using a solvent-free
solid injector (SFSI). The kinetics of the desulfurization reactions were monitored by GC-MS.
Received 25th July 2014
Accepted 17th September 2014
DOI: 10.1039/c4ra07635d
www.rsc.org/advances
environmentally friendly and energy saving processes that
could be applied to desulfurization are continuously being
Introduction and motivation
8
Red Mud is a solid waste generated by the aluminum industry in explored.
1,2
the processing of bauxite ores following the Bayer process.
The disposal of Red Mud is and has been a considerable chal-
lenge to the aluminium industry and the management of this
waste is considered to be a key sustainability issue for the
Red Mud
The main constituents of Red Mud waste are the oxides Fe O ,
2
3
Al O , SiO , TiO as well as smaller amounts of CaO and Na O
2
2
3
2
2
2
alumina rening industry. The petroleum industry also faces
9
–11
that impart a high alkalinity on the material.
are an estimated 3 billion tons of Red Mud stockpiled around
the world and an additional estimated 120 million tons are
Currently there
ongoing challenges throughout the supply chain, notably
maintaining a consistent quality of the fuel with respect to
3,4
performance and environmental regulations.
Sulfur
12–14
generated by the aluminum industry per year.
The most
compounds represent one of the most undesirable contami-
nants in petroleum products and fuels. They occur in various
forms in the crude oil feed and can been classied into four
main groups: mercaptans, suldes, disuldes and thio-
important barriers to remediation and disposal of bauxite
residue are its high alkalinity (pH > 12) and the large amounts
generated. Several interesting initiatives to design an applica-
tion and alternative use for Red Mud have been explored,
however, despite the numerous conceivable ways to utilise Red
Mud, current applications have not yet shown technical effi-
ciency or economic viability for the full reuse or remediation of
5,6
phenes. The commonly used industrial process for desulfur-
ization is not fully efficient for the removal of compounds such
7
as dibenzothiophene and its derivatives. Therefore, new
9
,12,15–17
this waste.
a
Department of Chemistry, Federal University of Lavras, P.O. Box 3037, Lavras, Minas
In this context, the development of technologies allowing a
full and preferably value-added reuse of Red Mud is of great
interest. Motivated by the recognition that some of the metal
oxides typically present in Red Mud, are known to have catalytic
activity, there have been several initiatives in the scientic
Gerais, 37200-000, Brazil. E-mail: guerreiro.dqi.@ua.br; Fax: + 55 35 3829 1271;
Tel: +55 35 3829 1625
b
Department of Chemistry, The Guelph-Waterloo Centre for Graduate Work in
2
Chemistry (GWC) , University of Guelph, Guelph, Ontario N1G 2W1, Canada
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra07635d
1
,10,18
1
community to use this waste as a catalyst.
Any alternative
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use of Red Mud as a catalyst could contribute to the reduction of DBT oxidation. They highlight that the oxidized molecules with
environmental impact and at the same time add value to this higher polarity migrate to the polar phase aer reaction with the
1,9,19
26
material.
peroxo species on the catalyst. Among different chemical
oxidants, which have been studied in oxidative desulfurization,
the best option is hydrogen peroxide, as it is considered an
environmentally friendly yet powerful oxidant, either by itself or
Desulfurization
6,20
Sulfur, in both, its volatile SO oxide or reduced H S forms, when combined with organic acids.
x
2
constitutes
a
considerable environmental and technical
On the basis of present knowledge, the ODS process is
problem. The sulfur compounds are highly undesirable in therefore in principle able to effect the oxidation of sulfur
rening processes as they can lead to the deactivation of some containing compounds such as benzothiophene compounds,
catalysts used in crude oil processing and cause corrosion into the corresponding sulfoxides and sulfones. The process
problems in pipelines, pumping and rening equipments, consist of the two distinct consecutive steps: (1) oxidation of
6
resulting in substantial nancial losses to the industry. The organosulfur compounds and (2) separation of the oxidation
consequences extend, however, beyond the limit of reneries. products by extraction into a polar solvent or onto an absorbent,
Specically, the presence of elevated levels of sulfur in fuels with no current technology integrating both steps established
21,22
results in the release of compounds, such as carbon disulde on a commercial scale.
While the two-step process is in
(
CS ), dimethyl sulde ((CH S) and sulfur oxides (SO ). These principle promising, it has signicant disadvantages inherent
2
3
)
2
x
compounds affect directly the air quality and the environment, in the use of organic solvents and adsorbents for extracting the
causing several problems: smog above cities, acid rain and oxidation products, representing a technical and economical
respiratory problems for human beings. They also inhibit the barrier to large scale application.
functioning of catalytic converters that are tted to car exhausts
6
further compounding this problem.
Globally multiple avenues are being pursued to reduce the Red Mud as a desulfurization catalyst
negative impacts to the environment caused by the emission of
A possible and desirable strategy for the realization of an
economically and ecologically viable ODS process would be to
use a material able to selectively remove sulfur compounds
from fuels by a combined process of oxidation and adsorption
of the oxidized products formed, thus eliminating the need for
solvents.
Given the large amount of Red Mud available, its low or
effectively negative value and the known potential catalytic
activity of the metal oxides present, we here report the use of
this material as a catalyst for the oxidative desulfurization of a
simulated diesel (representing heavy oil fractions) and at the
same time as an adsorbent for the resulting sulfoxide/sulfone
reaction products.
20–23
sulfur compounds and other pollutants.
Among these is
hydrodesulphurization (HDS), the conventional method for
removal of sulfur from fuels, which employs a high temperature
and high pressure reductive catalytic process. HDS is an effi-
cient method, however, it is not or only partially effective for
removing heterocyclic sulfur compounds such as dibenzothio-
phene (DBT) and its derivatives. The development of new
technologies and materials applied in a complementary
manner to the existing conventional sulfur removing industrial
process has been highlighted as a possible approach to meet
4,21,22
government regulations worldwide
and academia as well as
renery operators are expending substantial research efforts to

nd a viable and feasible solution to achieve deep desulfuriza-
6,20,24
tion of fuels.
Recently studies have therefore focused on the oxidative
desulfurization (ODS) as one of the new strategies with which it
may be economically feasible to achieve the low levels of
Results and discussion
Characterization of Red Mud waste
contaminants required by present and future environmental The powder X-ray diffraction pattern of the actual Red Mud
5,20,21
regulations, in particular for Diesel oil.
Several ODS used in this study is given in Fig. 1. The XRD analysis of Red
systems have been studied, such as tert-butyl hydroperoxide or Mud showed a typical pattern with the presence of poorly
3
H
2
O
2
/V
2
O
5
catalysts supported on different metallic oxides,
crystalline phases, with low intensity broad diffraction lines. As
expected, a multi-component pattern was produced. With such
4
cyclohexanone peroxide/tungsten supported on resin D152,
22
H O and H CCOOH/sol–gel Fe–TiO , H O and H CCOOH/ complex patterns, it is possible to t a number of different
2
2
3
2
2
2
3
23
ionic liquids,
tic acid, and others.
H
2
O
2
/nano-magnetic silica with phosphotungs- combinations of phases. Accordingly, we have associated these
25
21
peaks to phases which are known to be major components of
Souza et al. have applied the limonite/H
as a oxidant. Their results showed an almost complete removal quartz (Qz ¼ SiO
of sulfur compounds, with approximately 96% of the dibenzo- FeOOH) and hematite (Hm ¼ Fe
thiophene being oxidized aer 240 min. Sulfone was the prin- giving the highest intensity. The gibbsite phase represents the
2
O
2
//HCOOH system Red Mud. The pattern shows peaks assignable to the phases
), gibbsite (Gb ¼ g-Al(OH) ), goethite (Gt ¼ a-
), with the iron oxides peaks
2
3
2 3
O
2
4
cipal product formed and was identied by GC/GC-MS.
fraction of aluminum hydroxides, which were not completely
27
Oliveira and co-workers developed an amphiphilic niobium extracted in the Bayer process. According to the literature, Red
oxyhydroxide which was applied as a hybrid catalyst in oxidative Mud typically also contains other amorphous phases such as
desulphurization, specically with this material being active for AlO(OH), TiO , CaCO , not identiable by XRD.
2 3
11,27,28
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The M ¨o ssbauer spectrum of the Red Mud (acquired at
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ambient temperature, ꢀ298 K) is shown in Fig. 2.
The spectrum shows two sextets related to hematite (d 0.36
ꢁ
1
ꢁ1
mm s , D ꢁ0.22 mm s , Bhf 50.8 T, relative area 58%) and
ꢁ
1
ꢁ1
goethite (d 0.74 mm s , D ꢁ0.031 mm s , Bhf 48.6 T, relative
area 2%). Two central accentuated doublets were also observed
ꢁ1
ꢁ1
(
d 0.36 mm s , D 0.58 mm s , relative area 35%) and (d ꢁ0.38
ꢁ1 ꢁ1
mm s , D 4.11 mm s , relative area 5%). They are typical of
iron oxides with small particle sizes, congruent with the
previous analysis performed by Rio Tinto ALCAN, which showed
that the majority of particles (>70%) are less than 10 mm. The
M ¨o ssbauer spectrum is consistent with the XRD results as the
goethite and hematite phases were also identied. Small vari-
ations of the value of Bhf parameter of M ¨o ssbauer analysis have
been observed for materials with low crystallinity or when they
29–31
Fig. 3 PCZ of Red Mud.
have isomorphic substitution (Si, Al).
According to literature reports, the point of zero charge (PZC)
of Red Mud is strongly dependent of the origin and composition
of the bauxite, variability in the Bayer process parameters, the found for this particular Red Mud was 6.10, very similar to the
addition of different types of occulants and the composition of values reported in other studies with ranges of 8.3 to 5.3 having
11
11,32
the background electrolyte. Fig. 3 shows the curve of electro- been observed previously.
phoretic mobility of material studied. The electrokinetic
As illustrated by the SEM-EDX images shown in Fig. 4, on the
potential was determined by plotting the zeta potential vs. pH, microscopic scale (#20 mm) the morphology of the catalyst
which allows the determination of the pH at which the surface
charges are in equilibrium (point of zero charge, PZC). The PZC
Fig. 1 Powder XRD pattern of Red Mud.
Fig. 4 Scanning electron microscopy (SEM) coupled with energy
Fig. 2 M o¨ ssbauer spectrum of Red Mud.
dispersive spectroscopy (EDS) of Red Mud.
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Table 1 Semi-quantitative elemental analysis by EDX
Element
% Mass
% Atomic
Oxygen
Sodium
Aluminum
Silicon
37
9
17
6
55
10
16
5
Calcium
Titanium
Iron
2
5
24
1
2
11
Fig. 7 Comparison of DBT removal from adsorptive and oxidative
tests.
shapes may reect the diverse composition of the material,
formed by the junction of different minerals.
The EDX analyses provides a semi-quantitative elemental
33
analysis of the Red Mud surface as listed in Table 1.
Fig. 5 Oxidation of DBT as a function of time followed by quantitative
ꢁ
1
GC-MS. Reaction conditions: [DBT] ¼ 50 mgL ; [H
2
2 3
O /H CCOOH] ¼
ꢁ1
ꢃ
3
.5 mmol L ; mcatalyst ¼ 10 mg; T ¼ 25 ꢂ 1 C.
shows as an agglomerate of particles with rounded shapes and
irregular size distribution, i.e., the aggregate particles are of Fig. 8 (a) SEM image and (b) EDS mapping of sulfur on Red Mud after
33
use as a catalyst and adsorbent.
poorly-crystallized/amorphous forms. This variety of sizes and
Fig. 6 Integrated peak area and approximate reaction order.
Fig. 9 Release profile of volatile compounds monitored by GC-MS.
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The Red Mud was also characterized by adsorption and medium) and 0.1 mL of an equimolar mixture of [H
2
O
2
//
ꢁ1
desorption of nitrogen. The specic surface area (SSA) found to H CCOOH] ¼ 3.5 mmol L as the oxidant. The acetic acid was
3
2
ꢁ1
this material was SBET ¼ 14 m g , consistent with the values applied as an activity promoter, as in the presence of hydrogen
11,15,34,35
reported for Red Mud by literature and previous analysis.
peroxide under acidic condition, the more active peroxyacetic
22
acid is formed.
The oxidation of DBT was monitored by the variation in
initial concentration by gas chromatography with mass spec-
trometric detection (GC-MS). The total ion count chromato-
grams are shown in Fig. 5.
Catalytic removal of DBT
Red Mud (10 mg) was tested for its principal effectiveness for
the oxidation of dibenzothiophene (DBT) using 10 mL of a
ꢁ
1
solution of DBT (50 mg L in toluene, used as the hydrocarbon
ꢃ
ꢃ
Fig. 10 Mass spectra of ions volatilized from Red Mud after catalytic DBT oxidation and virgin Red Mud obtained at 225 C and 280 C.
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ꢃ
According to the chromatogram, the combination of Red at 307 C. Mass spectra obtained at various temperatures
Mud with the oxidants results in an efficient removal of DBT covered by the large peak in Fig. 9 are shown in Fig. 10 and
from the solution: aer 10 min of reaction time a signicant compared to controls using virgin Red Mud.
ꢃ
decrease, about 35%, in the intensity of the signal related to
The mass spectrum at 225 C of the Red Mud aer its use as a
sulfur compound is observed and aer 60 min. the remaining catalyst shows a peak at m/z ¼ 216, assignable to the sulfone
ꢃ
signal approaches the detection limit. No other peaks for species. At 280 C, an additional signal at m/z ¼ 200, associated
oxidized products were detected by GC-MS analysis suggesting with the release of the intermediate sulfoxide is seen. The peak
the existence of a removal mechanism by an adsorption at m/z ¼ 184 is assigned to a fragment in dibenzothiophene
2
process, i.e., the reagent combination Red Mud/active oxygen is sulfone DBTO spectrum, which also can be seen at MS spectra
able to oxidize DBT and at the same time adsorb the oxidation of authentic DBTO shown in Fig. 10. These fragments were not
2
products.
observed in the mass spectra of the virgin Red Mud. The
A plot of the integrated peak areas from the total ion count differences between the MS spectra are further direct evidence
chromatograms as a function of reaction time is shown in for the removal of DBT from the hydrocarbon medium.
Fig. 6.
The composition of the Red Mud before and aer the
Using the limited number of data points obtained we oxidation reaction was also studied by infrared spectroscopy.
attempted an approximate kinetic analysis. Both a direct plot of Fig. 11 compares the two spectra, which show the typical band
ln(CA0/CA) vs. time and a van't Hoff analysis expressing k as a of the oxides constituting Red Mud along with peaks for bound
function of time and reaction order gave the best t for a rst water present in the material (ꢀ10% w/w). The spectra inter-
order process, for which the rate constant for the apparent pretation is complicated because of the complex composition of
ꢁ
1
36
consumption of DBT was calculated to be k z 0.08375 min
.
Red Mud due to the presence of several mineralogical phases
The pH of the solution was monitored throughout the reaction containing numerous adsorption and oxidative sites. The
using an indicator paper and remained constant at pH ꢀ6. A positions of the absorption bands are similar in both spectra.
ꢁ
1
test of active phase leaching was performed, by leaving the Red The bands 535 and 465 cm are attributed to the Fe–O bond in
Mud in contact with DBT solution for 60 min. Aer that the Red goethite and hematite, however, the bending vibration of Si–O
ꢁ
1
27,33
Mud was removed by ltration and the oxidants (H
2
O
2
// and Al–O, are also in the 465 cm region.
The overlapping
ꢁ
1
H CCOOH) added without any activity being observed con- bands in the 1000 cm region are assigned to angular defor-
3
37

rming the catalyst is truly heterogeneous.
mation of Al–O and Si–O. The Red Mud aer DBT shows
The hypothesis that Red Mud acts as an oxidation catalyst additional bands at 1418 and 1465 cm , attributed to C]C
and adsorbent at the same time is supported by the following bonds from aromatic compounds and at 1160 cm there is a
ꢁ
1
ꢁ
1
control reactions:
so shoulder peak characteristic of angular deformation to C–H
-
In the absence of Red Mud, H
2
O
2
by itself as well as the bond from aromatic compounds. These bands are present only
2 2 3
combination of H O //H CCOOH by itself are inactive as an in the spectrum of Red Mud aer the oxidation reaction. The
oxidant and the concentration of DBT remained constant in S]O bands from sulfone also occur in this region, but are
2
2
31
both experiments. This is congruent with earlier observations.
obscured by the overall broad peak shape. Also present in both
ꢁ1
-
Vice versa in the absence of oxidants but presence of Red spectra are stretching vibrations between 3100–3500 cm and
Mud no decrease of the DBT concentration was observed, i.e., 1644 cm from O–H and H–O–H bonds from the surface of
ꢁ1
the Red Mud does not act as an adsorbent for DBT by itself.
The GC-MS traces of the oxidation and adsorption control
goethite and hematite, as well as from bound water present in
-
reaction solution tests aer 60 minutes are shown in Fig. 7.
Fig. 8 shows the EDS-SEM mapping of Red Mud aer its use
as an oxidation catalyst and adsorbents.
The mapping EDS shows elevated dispersion of the sulfur on
the Red Mud surface aer the reaction. The pink points are
associated with the presence of oxidized DBT on active sites on
surface of the material with a fairly homogeneous distribution
being observed. (EDS mapping of Red Mud before the reaction
did not show the presence of any sulfur).
The presence of oxidized DBT on the surface of the Red Mud
was also probed by gas chromatography-mass spectrometry
(GC-MS) using a solvent-free solid injector (SFSI). The difference
of the GC-MS trace of gas samples volatilized from virgin and
Red Mud aer a DBT oxidation reaction aer 60 min are shown
in Fig. 9.
The release prole of volatile compounds from Red Mud
ꢃ
shows a substantially higher intensity peaking at T z 325 C,
which is close to the boiling points of dibenzothiophene sulf-
Fig. 11 Infrared spectra of Virgin Red Mud and Red Mud after use as a
catalyst.
ꢃ
oxide (DBTO) at 285 C and dibenzothiophene sulfone (DBTO
2
)
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some of the mineral phases of Red Mud. The distinct band at could be removed allowing regeneration and subsequent recy-
ꢁ
1
3
100 cm is due to the bulk hydroxyl stretch and two less cling of the oxidation catalyst, an important feature for a
ꢁ1
intense bands at 3300 and 3660 cm can be attributed to the potential industrial application. Several tests were conducted to
surface hydroxyl groups in goethite.
31
evaluate the regeneration of the Red Mud aer the DBT oxida-
tion reaction by a thermal desorption process. The catalyst was
ꢃ
ꢁ1
ꢃ
ꢃ
subjected to a heat treatment (10 C min from 25 C to 300 C,
Recycling and regeneration of the catalyst
kept at the nal temperature for one hour under an oxidizing
atmosphere of air at a 10 mL min ow). The material recov-
The deactivation of catalysts is generally a consequence of
processes that result in the loss of the active phase or by
poisoning or occupation and blocking of the active sites by
adsorption. In this work deactivation would logically occur by
adsorption of the oxidized sulfur compounds. Since chemi-
sorption was identied as reversible process in our work and on
the basis of the GC-MS results discussed above (Fig. 9 and 10),
we postulated that any surface adsorbed species in Red Mud
ꢁ1
ered from this regenerative treatment was highly active in a new
oxidation cycle giving ꢀ97% removal of DBT under the same
reaction conditions specied earlier, i.e., within the estimated
analytical error range the material can be fully regenerated and
reused for sulfur compound removal.
] ¼ 50 mg Lꢁ ; madsorbent ¼ 10 mg; T ¼ 25 ꢂ 1 C. (b)
1
ꢃ
2 2
Fig. 12 (a) Adsorption of DBTO standard after 60 min. Experimental condition: [DBTO
Release profile of dibenzothiophene sulfone monitored by GC-MS.
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Adsorption of authentic dibenzothiophene sulfone standards
In order to probe the mechanism of DBT oxidation, the
adsorption of an authentic dibenzothiophene sulfone standard
ꢁ
1
([DBTO
2
] ¼ 50 mg L ) onto Red Mud (10 mg) was evaluated. As
with the test with DBT (see above), the adsorption over time was
monitored by GC-MS and the desorption from the Red Mud
aer impregnation with DBTO
SFSI). The results of both tests are shown in Fig. 12.
Fig. 12a shows that aer 60 minutes the Red Mud had
2
using solvent-free solid injector
(
adsorbed ꢀ50% of DBTO present, while the reversibility of the
Fig. 13 Proposal adsorption modes of DBTO
2
on the Red Mud surface
2
(
gray spheres are Fe, Al, Si or Ti; black spheres are O).
process was conrmed by the corresponding release prole,
which identied two distinct sites and stages of ad- and
2
desorption. Since the release of the DBTO occurred at two
distinctly different temperatures, this suggests the presence of
two different interactions between the surface and the
substrate, as proposed in Fig. 13.
At site (A) the sulfone interacts with a hydroxyl group on the
surface through a single hydrogen bond, which is would explain
The Red Mud obtained from the thermal regeneration
process, was also analyzed by solvent-free solid injector (SFSI)
coupled with gas chromatography-mass spectrometry (GC-MS).
The GC-MS traces are part of Fig. 9 and verify a complete
removal of adsorbed compounds from the material aer its use
in the DBT oxidation reaction. This result asserts that the
material was regenerated and can be reused in successive
reaction cycles.
the release of DBTO
2
at the lower temperatures (beginning at
ꢃ
ꢀ
130 C). On the other hand, for site (B) we propose a stronger
interaction, assuming that the oxygen of DBTO
2
molecule can

ll vacancies on the Red Mud surface. In this case, the release of
The regenerative system described here is promising. Since
during the heating of Red Mud, the oxidized sulfur compounds
DBTO
2
molecule occurs at a higher temperature, starting near
ꢃ
270 C. Evidence for the presence of vacancies on the Red Mud
(
DBTO and DBTO) are released in sequence, these compounds
2
surface was provided by a H O decomposition test, in which
2 2
could be recovered in a small volume with high concentrations
possibly allowing a specic separate treatment and trans-
formation to value-added products, which allows the treatment
of the effluent be made by HDS process, already established in
the industry, thus leading to a complete use of the carbon value
chain.
the O liberation in presence of organic compounds and water
is similar.
2
38
On the basis of these results, we propose that the DBT
removal is in fact a process that occurs on the surface of Red
Mud. That hypothesis is further supported by the comparison of
the proles of the release compounds from Red Mud aer the
DBT oxidation test (Fig. 9) and from the Red Mud aer DBTO2
Fig. 14 Possible mechanism of an oxidative adsorptive process of DBT on the surface of Red Mud (gray spheres are Fe, Al, Si or Ti; black spheres
are O).
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adsorption (Fig. 12b), both of which show a different tempera- Catalytic reactions
ture region for release of compounds by heating the solids. A
possible pathway and mechanism of DBT oxidation on the Red
Mud surface is presented in Fig. 14.
The catalytic reactions were carried out with a total volume of 10
mL (9.9 mL of the DBT solution at 50 mg L concentration and
ꢁ1
0
.1 mL of the H
2
O
2
//H
3
COOH) also 10 mg of Red Mud as a
(50% v/v) and H COOH
We propose that the rst step is the formation of peroxo-
catalyst. The oxidant mixture of H
2
O
2
3
21,22
acetic from the reaction H
2
O
2
with H
3
CCOOH.
The peroxo-
was prepared 2 hours before mixing with the DBT. The
adsorption of DBT was also performed using 10 mL of DBT
solution and 10 mg of the Red Mud. The DBT removal was
monitored by gas chromatography coupled to mass spectros-
copy (GC-MS) using a Shimadzu, model 2010. The analyses were
made using a RTX-5MS column (30 m ꢄ 0.25 mm ꢄ 0.25 mm).
acetic formed is able to insert an oxygen into a vacancy present
on the Red Mud surface. The oxidation of DBT then occurs by
interaction with this oxygen, resulting in a formation of
dibenzothiophene sulfoxide and dibenzothiophene sulfone.
Two types of interactions could occur. In the rst the DBT would
interact with two neighboring oxygens, resulting in the forma-
ꢃ
ꢃ
The heating ramp was 60 C for 1 min, heating rate of 15 C
tion of DBTO
2
. This is substantiated by the mass spectrum from
ꢁ1
ꢃ
min to 270 C and remaining there for 2 min.
solvent-free solid injector (SFSI) (Fig. 10), in which the release of
ꢃ
this compound is seen at 225 C. In the second the DBT inter-
acts with just with one oxygen, resulting in formation DBTO. Conclusion
ꢃ
This is substantiated by the release of this compound at 280 C
from mass spectrum of solvent-free solid injector (SFSI).
Red Mud is an effective catalyst for the oxidation of dibenzo-
thiophene (DBT) to dibenzothiophene sulfoxide (DBTO) and
dibenzothiophene sulfone (DBTO ). The process is a promising
2
Experimental
Source of sample
alternative with potential for application on an industrial scale,
at it is in principle easily implemented, efficient and low cost, as
it employs a waste material of negative value and can be carried
out at room temperature and atmospheric pressure.
Red Mud samples (ꢀ41% of Fe) were obtained from Rio Tinto
Alcan's Jonqui ´e re, Quebec operation, Canada. Before use the
material was dried to constant weight at 60 C (ꢂ1 ꢃC) and
sieved in 35 mesh sieve, to <0.5 mm grain size. Further details of
the Red Mud composition as previously determined by the
The removal DBT occurs by combined oxidative adsorption
processes on the Red Mud surface which presents two different
types of interaction resulting in the formation of dibenzothio-
phene sulfoxide and dibenzothiophene sulfone. Additional
experiments not reported here in detail suggest that a variation
in chemical constitution of Red Mud does not signicantly
ꢃ
39
supplier are given previous publications.
40
Catalyst characterization
affect its activity. In addition, the Red Mud could be regen-
erated by a thermal treatment in air resulting in a recovered
catalyst activity of 97% compared to virgin, i.e., unused Red
Mud.
The transmission M ¨o ssbauer spectroscopy experiments were
5
7
carried out in a CMTE model MA250 spectrometer with a Co/
Rh source at room temperature, using a-Fe as a reference. The
zeta potential determination was performed by measuring
electrophoretic mobility using an apparatus Zeta-Meter System
What differentiates the system described here is the ability to
effect DBT removal in one-step, using a solvent free system, in
which the polar products (sulfoxides and sulfones) do not have
to be extracted by solvents or adsorbed in a separate step. In this
process, both an environmentally friendly oxidation system
3
5
.0. The average of 10 readings electrophoretic mobility in pH 3,
, 6, 7, 9 and 11 were taken to determine the PZC. Fourier
transform infrared spectroscopy (FTIR) spectra were recorded
on a Digilab Excalibur spectrometer with a spectral range from
(
H O and CH COOH) and catalyst (Red Mud waste) are
2 2 3
ꢁ
1
employed.
4
00 to 4000 cm . Samples were ground to a powder and
The possibility to recover the sulfur compounds during the
regeneration process from the Red Mud also shows potential
prospect for industrial applications, since the Sulfoxides and
Sulfone at higher concentration could be amenable to conven-
tional HDS processes potentially yielding comparatively valu-
able biphenyls. The application of Red Mud as a catalyst for
sulfur compound removal from heavy fuel oils may thus make a
contribution to solve two pressing problems in the aluminum
and petrochemical industries in Brazil and elsewhere.
pressed into KBr pellets. The FT-IR spectra were collected with a
resolution of 4 cm over an average of 32 scans. The powder
XRD data were obtained with a Phillips model PW1830/40
ꢁ1
˚
equipment using CuKa radiation (l ¼ 1,54 A), current of 30 mA,
ꢃ
voltage of 30 kV and scanning from 10 to 80 (2q) at a scan rate
ꢃ
ꢁ1
of 1 min . The morphology were analysed by scanning elec-
tron microscopy (SEM) coupled with energy dispersive spec-
troscopy (EDS), using a FEI Inspect 50S apparatus with a 15 kV
of tension. The release proles of volatile compounds moni-
tored before and aer the DBT oxidation were analyzed by Gas
chromatography-mass spectrometry (GC-MS) using a solvent-
free solid injector (SFSI), 1 mg of Red Mud was inserted into a
Acknowledgements
glass capillary vial (1 mm i.d. ꢄ 5 mm in length). The vial was The authors acknowledge the support from CAPES program of
placed in a solvent-free solid injector, which are connected to the Federal Brazilian Government and Rio Tinto ALCAN (Jon-
ꢃ
ꢁ1
ꢃ
the GC-MS, the rate heating used was 10 C min from 25 C to quiere, QC, Canada) also ALCOA (Poços de Caldas, Minas Ger-
ꢃ
350 C.
ais, Brazil) for supplying Red Mud samples.
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2
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ALCOA (Pocos de Caldas, Minas Gerais, Brazil), with 28% of
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following the same experimental condition reported here, 2012.
47296 | RSC Adv., 2014, 4, 47287–47296
This journal is © The Royal Society of Chemistry 2014