Integrated desulfurization of diesel by combination of metal-free oxidation and product removal by molecularly imprinted polymers

Article abstract of DOI:10.1039/c4ra11666f

The production of ultra-low-sulfur diesel is an important worldwide demand. In this work a novel integrated method for desulfurization of diesel is proposed based on the combination of Bronsted acid catalyzed oxidation and the selective removal of the oxidized products using a molecularly imprinted polymer (MIP) produced in supercritical carbon dioxide (scCO₂). The biphasic oxidation reaction of dibenzothiophene sulfone (DBT), as model substrate, and H₂O₂ as oxidant, was optimized by testing different acid catalysts, and also different phase transfer catalysts (PTC), including two different ionic liquids (ILs) trihexyl(tetradecyl)phosphoniumchloride [P_6,6,6,14]Cl and Aliquat. The products of the efficient oxidation of DBT, dibenzothiophene sulfoxide (DBTSO) and dibenzothiophene sulfone (DBTSO₂), were then selectively removed from real diesel using the MIP.

Full text of DOI:10.1039/c4ra11666f

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Integrated desulfurization of diesel by combination
of metal-free oxidation and product removal by
molecularly imprinted polymers†
Cite this: RSC Adv., 2014, 4, 54948
Joana P. Ferreira,a Raquel Viveiros,b Anita Lourenço,b Mara Soares da Silva,b
Received 16th August 2014
Accepted 9th October 2014
a
b
a
*
*
Andreia Rosatella, Teresa Casimiro and Carlos A. M. Afonso
DOI: 10.1039/c4ra11666f
www.rsc.org/advances
The production of ultra-low-sulfur diesel is an important worldwide benzothiophenes and dibenzothiophenes from heavier frac-
demand. In this work a novel integrated method for desulfurization of tions of petroleum.2
diesel is proposed based on the combination of Brønsted acid cata-
Other desulfurization processes have been reported, such as
lyzed oxidation and the selective removal of the oxidized products using biological systems to oxidize sulfur compounds (bio-
using a molecularly imprinted polymer (MIP) produced in supercritical desulfurization),1–5 or using a reactive or non-destructive
carbon dioxide (scCO2). The biphasic oxidation reaction of dibenzo- adsorption system.1 Oxidative desulfurization is currently the
thiophene sulfone (DBT), as model substrate, and H2O2 as oxidant, was most studied process to remove sulfur compounds from fuel.
optimized by testing different acid catalysts, and also different phase This method involves the modication of physical and chemical
transfer catalysts (PTC), including two different ionic liquids (ILs) tri- properties (e.g., change in solubility), improving the last step of
hexyl(tetradecyl)phosphoniumchloride [P6,6,6,14]Cl and Aliquat®. The separation of oxidized compounds. Initially, the reported
products of the efficient oxidation of DBT, dibenzothiophene sulfoxide oxidative processes involved the use of nitric oxides as oxidizing
(DBTSO) and dibenzothiophene sulfone (DBTSO2), were then selec- agents.6 However, with the growing environmental awareness,
tively removed from real diesel using the MIP.
emerging studies started to be more sustainable, using organic
or hydro-peroxides as oxidants.7–14 A more recent methodology
to remove sulfur compounds from fuels is based on the use of
selective two-phase extraction, such as ionic liquids (ILs) as
phase-transfer promoters and receiving phase, i.e., triggering
the molecules to move from diesel phase to the IL, where the
oxidation reaction takes place.15 In the oxidative approach the
second important step is the efficient removal of oxidized
products from the diesel, namely by precipitation and/or
extraction. Yet this approach has some drawbacks such the
nal fuel contamination by IL and the difficult and incomplete
removal of the oxidation products from diesel.
Molecular imprinted polymers (MIPs) are cross-linked poly-
mers formed in the presence of a template, molecule for which
the affinity is wanted. During the process the template forms a
stable complex with the functional monomer(s), in the presence
of a porogen and a cross-linker that freezes the complex within a
rigid porous polymer matrix.16 At the end, the template is
removed leaving within the three-dimensional network, specic
sites complementary to the template in size and functionality.
These affinity materials have been successfully applied in
several areas, as recently reviewed in literature.17 MIPs have
high potential because they mimic the affinity of natural
materials, but they are very robust to extreme pH, temperature
and pressure conditions and to organic solvents. Some draw-
backs for their broad use are the intensive use of organic
Introduction
Petroleum is a fossil fuel with numerous organic compounds
with small amounts of nitrogen, oxygen, sulfur and metal ions.
Some of these compounds, especially sulfur containing ones,
are not favourable since their combustion leads to the release of
SOx and other oxides, increasing atmospheric pollution.
Therefore, good desulfurization processes are of high impor-
tance in petroleum and fuels processing, and a range of
approaches have been reported concerning this issue. The main
method to remove sulfur compounds from fuels is the hydro-
desulfurization, that subjects the oil to high pressure and
temperature in the presence of hydrogen and usually a metal
catalyst,1 resulting hydrogen sulphide as by-product. Hydro-
desulfurization is effective in removing less hindered sulfur
compounds however, it is inefficient in the removal of
a
˜
Instituto de Investigaçao do Medicamento (iMed.ULisboa), Faculty of Pharmacy,
University of Lisbon, Av. Prof. Gama Pinto, 1649-003, Lisboa, Portugal. E-mail:
carlosafonso@ff.ulisboa.pt
bREQUIMTE, Departamento de Quımica, Faculdade de Ciencias
e Tecnologia,
´
ˆ
Universidade Nova de Lisboa, Campus de Caparica, 2829-516 Caparica, Portugal.
E-mail: teresa.casimiro@fct.unl.pt
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra11666f
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solvents, multi-step preparation with typical grinding and These prompted us to achieve an integrated, more efficient and
sieving of the synthesized polymers. Recently the development mild process for the removal of more demanding
of MIPs using supercritical uid technology have been benzothiophene-based compounds from diesel, by non-metal
proposed18 and since then several applications have been sulfur oxidation using hydrogen peroxide, followed by a MIP-
reported.19–22 The use of supercritical carbon dioxide (scCO2) selective removal of the products from diesel.
has shown to be an excellent alternative since MIPs are obtained
ready-to-use as dry powders, with no need of further drying or
mechanical steps. The powder is very homogeneous and easily
Results and discussion
packed into columns.23 The high diffusivity and low viscosity of The two-phase oxidation of dibenzothiophene (DBT), as model
scCO2 decreases the mass transfer limitations found in both of related benzothiophenes and dibenzothiophenes, by
conventional synthesis and template desorption.24 In addition aqueous H2O2 and using p-TsOH as catalyst, was explored in
CO2 is cheap, non-toxic, non-ammable, can be removed by hexane, dicholoroethene and diesel. Under different conditions
simple depressurization and re-used thus replacing the use of such as temperature (RT to 80 ꢀC) and catalyst loading (10 to 50
organic solvents and leaving no residues in the MIP.
mol%) only modest oxidation conversion was observed by GLC
MIPs have been applied to the removal of benzothiophene (see ESI†) and the maximum conversion of 39% was obtained at
ꢀ
(DBT)-based compounds from diesel and/or model oil (e.g. 65 C using 30 mol% of p-TsOH (Table 1, entry 1).
hexane, n-octane or acetonitrile). For example dibenzothio-
The use of ionic liquids (ILs) as co-solvents has been
phene sulfone (DBTSO2)-imprinted chitosan hydrogel proved to demonstrated to increase the efficiency of different catalytic
be selective in the absorption of DBTSO2 from an acetonitrile systems and oxidations. These compounds are also reported as
solution when compared to other organosulfur compounds efficient extractors of sulfur compounds from diesel.1,2 Thus,
found in diesel.25 A similar test was performed more recently, two different ILs (trihexyl(tetradecyl)phosphoniumchloride
also using molecularly imprinted chitosan, showing that the [P6,6,6,14]Cl and Aliquat®) where tested as co-solvent, also acting
MIP could be used to selectively adsorb organosulfurs from as phase-transfer catalyst (PTC). Table 1 shows data for the
gasoline.26 Another approach to molecular imprinting, was the oxidation of DBT to DBTSO and DBTSO2 in model oil and diesel
preparation of surface-imprinted polymers using titanium using ILs. The addition of ILs provided conversions above 94%
dioxide27 or potassium tetratitanate whisker (K2Ti4O9)28 as either in hexane (entries 2–4), dichloroethane (DCE) (entries 5
support, that presented some imprinting effects. However the and 6) or diesel (entry 7). Interestingly, in diesel there was an
adsorption tests in both studies were performed in model oil increase on the DBTSO2/DBTSO ratio (entries 2–6 vs. entry 7).
(n-octane) and not in real gasoline or diesel. Some recent
Despite all the known advantages, the use of ILs shows
examples also explored the use of other solid support surfaces disadvantages for industrial applications derived from the high
namely carbon microspheres,29 silica nanoparticles,30 magnetic cost of the IL, potential nal IL contamination of the diesel and
particles31 and double-template MIP on the surface of carbon the need of further removal of the oxidized products from the IL
microspheres32 and graphene oxide hybrids for33 selective phase in order to enable the desirable IL reuse. With the
absorption of DBT dissolved in fuel and/or in n-alkane. Cross- purpose of circumvent these limitations we explored the use of
linked chitosan microspheres and electrospun chitosan nano- the acid catalyst/aqueous H2O2 system in diesel without any co-
bers were recently applied for DBTO2 removal from diesel.34
recent review by Liu et al. provides an overview on the use of MIP Table 2, were performed using real diesel by monitoring the
for deep desulfurization of fuel oils.35 DBT, DBTSO and DBTSO2 present in diesel (5 mg mLÀ1) at
A
solvent or at least using a PTC. The experiments, summarized in
Herein we propose an integrated method for desulfurization optimized conditions. The observed yields were determined by
of diesel comprising two steps: (1) free-metal oxidation of DBT previous removal of the main diesel constituents by absorbing
into oxidized products (Fig. 1), and (2) selective removal of the DBT, DBTSO and DBTSO2 in silica followed by HPLC quanti-
oxidized species using a MIP prepared using scCO2.
cation (see ESI†).
We take advantage of the fact that p-toluenesulfonic acid
In the absence of any PTC different acids were tested, such as
(p-TsOH) is an efficient catalyst for the dihydroxylation of p-TsOH, camphorsulfonic acid, dodecyl benzene sulfonic acid,
olens using hydrogen peroxide36 in combination with a clean formic acid, phosphoric acid and Amberlyst®. Apart of p-TsOH
method for the development of an easy-to-make and ready-to- (entry 1 of Table 2, 21.1%), all the others provided lower yields
use MIP with molecular recognition for the oxidized products. (7–17%, entries 2–6) and in the absence of catalyst an even lower
yield of oxidized product was observed (1.8%, entry 7). Several
potential PTCs were also tested namely Aliquat® (entry 8),
CHAPS (entry 10) and sodium dodecylsulfonate (SDS, entry 11).
Only SDS provided slightly better yields (29.1%, entry 11) than
in the absence of PTC (21.1%, entry 1), and in general the
sulfone (DBTSO2) is the predominant product. The use of an
higher excess of H2O2 (8 eq.) which was either added at once in
one experiment or when H2O2 (4 eq.) were added followed by
another 4 eq. 24 hours aer and p-TsOH (30 mol%) provided
Fig. 1 Oxidation of dibenzothiophene (DBT) to dibenzothiophene
sulfoxide (DBTSO) and dibenzothiophene sulfone (DBTSO2).
better results, respectively 25.5% and 29.7% (Table 2, entries 13
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Table 1 Selection of experiments for the oxidation of DBT to DBTSO and DBTSO2 in model oil and diesela
Conv. DBTSOb (%)
Conv. DBTSO2 (%)
Total conv.b (%)
b
IL (g)
Solvent
T (ꢀC)
1
2
3
4
5
6
7
—
Hexane
Hexane
Hexane
Hexane
DCE
60
50
50
65
65
65
50
0
39.0
8.8
8.1
11.0
19.3
12.8
35.8
39.0
97.4
94.4
98.8
99.0
96.6
98.9
Aliquat® (1.1)
[P6,6,6,14]Cl (1.1)
Aliquat® (1.1)
Aliquat® (0.27)
[P6,6,6,14]Cl (0.26)
Aliquat® (0.26)
88.6
86.3
87.8
79.7
83.8
63.1
DCE
Diesel
a
General procedure: DBT (entry 1: 1.35 mmol; entries 2–7: 2.71 mmol) was dissolved in the solvent (entry 1: 6 mL; entries 2–7: 3 mL) and added to a
solution of H2O2 (4 equivalents) and p-TsOH (30 mol%), followed by the addition of the IL, for 24 hours (except entry 4, 48 h), in a closed reactor.
b
Observed conversion by GLC to dibenzothiophene sulfoxide (DBTSO), dibenzothiophene sulfone (DBTSO2) and total conversion (DBTSO +
DBTSO2). A complete list of experiments is provided at ESI.
and 12). Those results suggest that the occurrence of competi- In addition, the observed purity of retained DBTSO2 is higher
tive H2O2 decomposition plays an important role in the product for MIP (see ESI†). When lower amount of DBTSO2 was passed
formation.
Aer this study on the acid catalyzed oxidation of DBT in
through the MIP (although with the same concentration),
diesel, the selective removal of the oxidized products from the
diesel was explored using a MIP synthesized in scCO2 using
DBTSO2 as template. Three different MIPs were synthesized
based on MAA (methacrylic acid), 2-vinylpyridine (2Vpy) and 2-
(dimethylamino)ethyl methacrylate (DMAEMA) as functional
monomers, and EGDMA (ethylene glycol dimethacrylate) (see
ESI†). Preliminary binding studies showed that MAA based MIP
presented the best recognition ability for DBTSO2.
The MIP and the corresponding non-imprinted polymer
(NIP) were loaded into a glass column (ID 1 Â L20 cm). The
following procedure was followed: (1) diesel was passed through
the column; (2) the polymer was washed with hexane to remove
the non-retained DBTSO2; (3) acetonitrile was passed to remove
the retained DBTSO2. Samples were analysed by HPLC and the
results are shown in Table 3.
Table 3 Observed binding of DBTSO2 in diesel by MIP and NIPa
DBTSO2
DBTSO2/pol
DBTSO2 (mg)
Diesel (mL)
retainedb (%)
(mg gÀ1
)
1
2
3
4
75.4
75.6
50.0
30.0
75.4
75.0
75.1
15
15
10
6
15
15
15
47.2
36.4
56.7
69.9
41.8
43.7
47.5
177.0
136.5
141.5
105.0
156.5
164.0
178.0
5
6c
7c
a
Diesel containing DBTSO2 (23.1 mM) was passed through a glass
column loaded with polymer (200 mg), then washed successively with
hexane and acetonitrile. In all entries MIP was used except entry 2
b
where NIP was used. DBTSO2 retained by the polymer that was
present in the acetonitrile fraction as determined by HPLC (duplicate
c
analysis). Observed results of reused MIP from the previously
MIP revealed higher affinity to DBTSO2 than NIP (177 mg per
g MIP; 47.2% vs. 136.5 mg per g NIP; 36.4%, entry 1 vs. entry 2).
experiment.
Table 2 Effect of the catalyst and PTCs for the oxidation of DBT in diesela
DBTSOb (%)
hb (%)
b
Catalyst (30 mol%)
PTC (30 mol%)
DBTSO2 (%)
1
2
3
4
5
6
7
p-TsOH
Camphor-sulfonic acid
Dodecyl benzene sulfonic acid
Formic acid
Phosphoric acid
Amberlyst
—
—
—
—
—
—
—
—
16.1
9.3
8.7
0.8
5.6
5.0
5.6
8.1
6.2
4.4
3.8
0
21.1
14.9
16.8
7.0
10.0
16.6
1.8
12.8
1.8
8
9
10
11
12c
13d
p-TsOH
p-TsOH
p-TsOH
p-TsOH
p-TsOH
p-TsOH
Aliquat®
[P6,6,6,14]Cl
CHAPS
SDS
—
—
13.1
11.0
18.8
24.9
24.9
16.7
8.1
10.4
1.6
4.2
4.8
8.8
21.2
21.4
20.4
29.1
29.7
25.5
a
General procedure: DBT (50.0 mg, 0.27 mmol) was dissolved in diesel (10 mL), and added to a solution of H2O2 (4 equivalents), 30 mol% of catalyst,
and 30 mol% of phase-transfer catalyst (PTC), then stirred for 24 hours at 60 ꢀC, in a closed reactor. b Observed yield by HPLC of DBTSO2, DBTSO
and total (DBTSO2 + DBTSO), average of two independent analysis. c Aer 24 h, more H2O2 (4 eq.), p-TsOH (30 mol%) were added and the reaction
d
was continued for more 24 h (total time of 48 h). H2O2 was used (8 equivalents).
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higher percentage of DBTSO2 was retained (69.9% vs. 47.2%, monomer, EGDMA as cross-linker, AIBN as initiator and
entry 4 vs. entry 1, Table 3) although with lower loading (105 mg DBTSO2 as template, with a molar ratio of monomer/template/
per g MIP, entry 4). In addition, the reusability of the MIP was cross-linker of 1 : 5 : 25. The MIP was obtained as a dry, white,
tested by performing three consecutive runs using fresh diesel free owing powder, ready to be packed in the glass column.
solutions containing DBTSO2. Interestingly the retained capa-
bility increased with the reuse (1st run: 41.8%, 2nd run: 43.7%,
3rd run: 47.5%, entries 5–7, Table 3) but the nal amount that
General procedure for the integrated oxidation of
dibenzothiophene (DBT) followed by DBTSO2 removal using
MIP
the structure can recognize decreases which is probably due to
the combination of the retained performance of the polymer
and the increase of the polymer compaction inside the column,
as it was observed an increase of the column head pressure. We
should also mention that the MIP loading capability of 177 mg
(DBTSO2 per g MIP) is considerable higher than the one
reported recently for DBTO2 (4.2 mg DBTO2 per g MIP,34) and for
DBT namely 20 mg DBT per g MIP,30 22.7 mg DBT per g MIP,37
88.8 mg DBT per g MIP,29 67.2 mg DBT per g MIP,32 and in the
same range as using graphene oxide (181.9 mg DBT per g
MIP).33
To a solution of 50 mg of dibenzothiophene (DBT) in 10 mL of
diesel was added a solution of 30 mol% of p-TsOH in 4 equiv-
alents of H2O2. Aer 24 hours at 60 ꢀC, the reaction mixture was
passed through the glass column containing the MIP (200 mg),
and dragged with hexane followed by acetonitrile to remove
binding compounds from the polymer. The acetonitrile fraction
was analysed by HPLC providing 12.7 mg of DBTSO2 (10.8%).
Finally, the integrated process of the organocatalyzed DBT
oxidation and removal of the oxidized product in diesel by the
synthesised MIP was tested. DBT (50 mg) was dissolved in diesel
(10 mL), the oxidation step was performed (H2O2 (4 eq.), p-TsOH
(30 mol%), 60 ꢀC, 24 h), the diesel phase was decanted and
passed through a MIP packed column (200 mg), allowing the
removal of 10.8% of oxidized DBTSO2. This result is comparable
to the expected overall yield of 12% derived from the combi-
nation of the oxidation step (21.1%, Table 2, entry 1) and
binding step (56.7%, Table 3, entry 3).
Acknowledgements
˜
ˆ
The authors are grateful to Fundaçao para a Ciencia e Tecno-
logia (FCT-Lisbon), FSE and FEDER for nancial support
through projects PTDC/QUI-QUI/102460/2008, PEst-OE/SAU/
UI4013/2011, PTDC/QEQ-PRS/2757/2012 and PEst-C/EQB/
LA0006/2013. RV acknowledges FCT-Lisbon and Hovione for
BDE doctoral grant SFRH/BDE/51907/2012.
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