Selective continuous flow extractive denitrogenation of oil containing S- and N-heteroaromatics using metal-containing ionic liquids supported on monolithic silica with hierarchical porosity

Article abstract of DOI:10.1039/c3ra43585g

The removal of heteroaromatic nitrogen and sulfur impurities from a model oil through extraction with ionic liquids (ILs) containing metal salts was performed in view of the purification of fuel feeds. Chloride and bis(trifluoromethanesulfonyl)imide salts of Cu⁺, Cu²⁺ and Fe³⁺ were used. The systems based on ILs and metal compounds were applied both in batch-like liquid-liquid and continuous flow liquid-solid conditions. In a first phase, liquid-liquid biphasic extraction was used to choose the most adequate IL classes; the suitable systems were immobilized on a solid support to form metal-containing supported ionic liquid phases (SILPs). In a next phase, these SILPs were applied in breakthrough experiments. A selective extraction of N-compounds was achieved with metal-containing ionic liquids, in both liquid-liquid and liquid-solid conditions. The breakthrough experiments using Cu(NTf₂)₂^- and FeCl₄ ^--containing [BMIM][NTf₂] SILPs immobilized on hierarchically structured silica monoliths resulted in an efficient separation of all the nitrogen compounds from the other impurities in the model oil.

Full text of DOI:10.1039/c3ra43585g

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Selective continuous flow extractive
denitrogenation of oil containing S- and N-
heteroaromatics using metal-containing ionic
liquids supported on monolithic silica with
hierarchical porosity†
Cite this: RSC Adv., 2014, 4, 1045
a
b
a
b
a
*
Paulo Forte, Alexander Sachse, Michael Maes, Anne Galarneau and Dirk De Vos
The removal of heteroaromatic nitrogen and sulfur impurities from a model oil through extraction with ionic
liquids (ILs) containing metal salts was performed in view of the purification of fuel feeds. Chloride and
bis(trifluoromethanesulfonyl)imide salts of Cu+, Cu2+ and Fe3+ were used. The systems based on ILs and
metal compounds were applied both in batch-like liquid–liquid and continuous flow liquid–solid
conditions. In a first phase, liquid–liquid biphasic extraction was used to choose the most adequate IL
classes; the suitable systems were immobilized on a solid support to form metal-containing supported
ionic liquid phases (SILPs). In a next phase, these SILPs were applied in breakthrough experiments. A
selective extraction of N-compounds was achieved with metal-containing ionic liquids, in both liquid–
Received 12th July 2013
Accepted 24th October 2013
ꢀ
ꢀ
liquid and liquid–solid conditions. The breakthrough experiments using Cu(NTf2)2 and FeCl4
-
DOI: 10.1039/c3ra43585g
containing [BMIM][NTf2] SILPs immobilized on hierarchically structured silica monoliths resulted in an
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efficient separation of all the nitrogen compounds from the other impurities in the model oil.
benzothiophene (BT) or dibenzothiophene (DBT). These
compounds require higher pressures of hydrogen and overall
harsher reaction conditions.1a,2b,3 The oxidative desulfurization
Introduction
The ever rising consumption rate of fossil fuels implies that
sweet crude sources are depleted at a fast rate and heavier and
dirtier crude sources need to be exploited. This means that the
fuel feedstocks will increasingly contain heavier components
and higher concentrations of pollutant impurities, such as
heteroaromatic nitrogen and sulfur compounds. On the other
hand, recent legislation has steadily set the bar lower regarding
the legal maximum concentration of sulfur impurities in fuel
feeds. European specications, for example, demand a total S
concentration down to as low as 10 ppm.1
The current industrial staples for the deep desulfurization of
fuel feeds are through reductive and/or oxidative processes. The
reductive processes, called hydrodesulfurization (HDS), oen
employ polymetallic clusters of Co, Mo and/or Ni.1a,2 It is
known that HDS is less efficient in the removal of the heavier
and more sterically hindered sulfur heteroaromatics such as
(ODS) methods currently employ such catalysts as organic
acids,4 polyoxometalates,4a,5 titanium-containing zeolites and
silicates,6 FeIII salts,7 etc and produce ultra-low sulfur diesel by
using cheap oxidants such as H2O2 or O2 in mild conditions.8
However, both reductive and oxidative processes are inhibited
by the presence of nitrogen impurities,1a,9 due to adsorption of
these compounds – or their reduction or oxidation products –
on the catalytic sites. Since nitrogen compounds are less reac-
tive than their sulfur counterparts, their removal through
catalytic hydrodenitrogenation is considerably more difficult.
All these factors make that the selective denitrogenation of fuel
feeds in the presence of sulfur impurities poses a major tech-
nological challenge.
The use of ionic liquids (ILs) for desulfurization of fuel feeds
has been widely researched.1b,10 Their broad liquid ranges and
negligible volatility present obvious engineering advantages
while, from the chemical point of view, their immiscibility with
aCentre for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven, fuel oils and affinity for the target impurities can be systemat-
Kasteelpark Arenberg 23, B-3001 Leuven, Belgium. E-mail: dirk.devos@biw.
ically ne-tuned. Commonly, their use has been as extracting
phase for the biphasic extraction of heteroaromatic impuri-
ties.10,11 ILs have also been found to be excellent solvents for
homogeneous catalysts in biphasic conditions.12 The presence
of metal catalysts dissolved or embedded in the structure of the
IL itself has made IL systems also suitable for use in ODS
kuleuven.be; Fax: +32 16 321998; Tel: +32 16 321639
bInstitut Charles Gerhardt Montpellier, UMR 5253, CNRS-UM2-ENSCM-UM1,
´
ENSCM, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France
† Electronic supplementary information (ESI) available: Detailed descriptions of
the syntheses of the bistriimide metal salts and [BMIM]y[MClx] ILs. See DOI:
10.1039/c3ra43585g
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processes.13 The use of ILs in which ionic metal species are an and they are chemically, mechanically and thermally stable.
intrinsic part of the structure has mostly been investigated for These silica monoliths present a hierarchical structure con-
simple extractive purication, as in the case of the anionic sisting of a highly interconnected, homogeneous network of
halide complexes of aluminum(III),11b,h,14 zinc(II),11d,14,15 cop- macropores of about 2 to 20 mm of diameter with mesopores
per(I),16 copper(II)14 and iron(III).11e,14 The use of this type of ILs from 5 to 15 nm. Our group has already reported the applica-
has, however, limitations: tetrachloroaluminate species are air- tions of monolithic supports in ne chemical synthesis21 and
and moisture-sensitive, and anionic metal halides in general metal extraction from effluents.22 Because of their polar nature
are very corrosive, which limits their use in bulk quantities. The and the presence of the macropore network, they are especially
promising characteristics of ionic liquids combined with the suited to the use with adsorption-immobilized SILPs.23
limitations of the current techniques highlighted above result
The present work aims at the study of the selective extractive
in a rising demand for the development of more benign metal- denitrogenation of fuel feeds containing heavy heteroaromatic
containing IL systems for the selective denitrogenation of fuel N and S impurities by supported ionic liquid phase systems
feeds.
containing bistriimide or halide metal salts immobilized on
Until now, most results on extractive purication of fuels hierarchically structured silica monoliths.
using ionic liquids were obtained with liquid–liquid biphasic,
batch-like processes. This implies regeneration of the IL phase
by either distillation or back-extraction before proceeding to the
next batch. The immobilization of an IL on a solid support to
form a supported ionic liquid phase (SILP) affords many
advantages relative to classical liquid–liquid batch conditions.17
The charged groups allow a very efficient immobilization by
adsorption to polar supports while maintaining the functional
polar or apolar groups that confer the appropriate properties for
the desired selectivity. The well-studied chemistry of biphasic
IL-solvent systems is then improved by the increase in the
surface area available to the substrate, which not only increases
the activity of the IL as catalytic or extracting phase but also
minimizes one of the greatest drawbacks in the use of ILs,
viz. the slow diffusion caused by their high viscosity.18 The
adsorptive immobilization of ILs also presents, over covalent
binding, the advantage of not requiring the chemical modica-
tion of the ILs and/or the metallic species’ structure. The desul-
furization of liquid fuel feeds using SILPs has been reported,
whereby the SILP containing a metal halide/IL mixture is stirred
in contact with the fuel, in a batch-like process.15 Besides the
previously discussed disadvantages of metal halide/IL systems,
these existing studies also do not address the problem of the
selective denitrogenation necessary for the success of other, more
efficient desulfurization processes such as HDS or ODS. More-
over, the choice for a non-continuous process implies that
regeneration steps are necessary.
The application of ow-through methodology has been
increasingly adopted in multiphasic processes which require
the use of solid supports.19 Packed-bed columns with particu-
late supports have been used in industrial processes but tech-
nically, the handling of particulates is regarded more and more
as unwanted by the industry, due to health and safety concerns.
Furthermore, the possible formation of preferential elution
pathways and the broad distribution of residence times may
lead to poor process efficiencies and reproducibility. The use of
hierarchically-structured macroporous/mesoporous monolithic
supports presents a very attractive alternative.20 The homoge-
neity of their pore structure, the high available surface area and
the ease of their synthesis, tted to the reactor shape, eliminate
the drawbacks of packed-bed reactors. Silica-based monoliths
have inherent advantages over polymer- or zeolite-based ones:
Results and discussion
To mimic the typical heavy N and S impurities usually found in
fuel feeds, a model oil was prepared. This was a heptane/toluene
(4 : 1 volume ratio) mixture with a total impurity concentration
of 7.4 ꢁ 10ꢀ3 M, with 4.6 ꢁ 10ꢀ3 M of nitrogen compounds and
2.8 ꢁ 10ꢀ3 M of sulfur compounds. The N-compounds tested
were indole (I), 1,2-dimethylindole (DMI) and N-methyl-
carbazole (NMC) while the S-compounds were benzothiophene
(BT) and dibenzothiophene (DBT) (Scheme 1).
The IL systems suitable for use in continuous setups were
developed in two phases. In a rst phase, a set of preliminary
batch-type experiments were run to choose the most appro-
priate IL system for the selective extraction of nitrogen
compounds from the model oil.
Biphasic liquid–liquid extractions
The initial tests were performed as biphasic liquid–liquid
extraction experiments, subjecting the N- and S-containing
model oil to contact with an ionic liquid phase in a sealed vial
while stirring. Scheme 2 shows the structures of the cations and
anions of the ionic liquids used at this stage.
Ionic liquids and ionic liquid mixtures
The heterocyclic structures have an important effect on the
uptakes, and indole, as a N-heterocycle with an available N–H
bond, is clearly the preferred compound in practically all ionic
their ease of synthesis affords a wide range in macropore size, Scheme 1 Structures of the N and S impurities studied in this work.
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Scheme 2 Molecular structures of the ILs used in the biphasic experiments.
liquids. The preference for indole is especially large in polar
For its generally satisfactory performance, combining high
ionic liquids, like [EMIM][Cl] or [BMIM][OAc], in which the uptake of nitrogen compounds and signicant selectivity,
anion offers ample opportunity for H-bonding, as is well known [BMIM][NTf2] was chosen as a reference liquid phase for the rest
for the chloride and acetate anions.24 These same polar ionic of the N/S selective separations.
liquids show hardly any preferential uptake of larger N-hetero-
cycles like DMI or NMC in comparison with the S-heterocycles,
and similar percentual uptakes are recorded for both
Admixture of metal ions to the ionic liquids
compound classes (entries 1 to 3). These uptake values, typically Aer the initial solvent selection, the same type of batch
below 10%, are particularly poor for the chloride ionic liquids. experiment was repeated but using [BMIM][NTf2] containing
More interesting results are obtained with the bistriimide the bistriimide salt of copper(I).† Copper(I) salts were chosen
(NTf2) ionic liquids. Irrespective of the cation, indole remains in view of the known affinity of Cu+-exchanged zeolites27 and
the preferred compound, but the uptake values for the other, Cu+-containing ionic liquids16 for heterocycles. In such cases,
larger N-heterocycles are 2 or 3 times higher than for the S- direct coordination of the heterocycle on the metal ion is
heterocycles. This indicates that this intrinsic selectivity is to be invoked as one of the mechanisms leading to enhanced uptake
ascribed to an effect of the NTf2 anion, since EMIM, BMIM and or affinity. In order to map the effect of the copper compounds
even the non-aromatic BMPyrr ILs yield highly similar results at various concentration levels, 0.04 M and 1 M solutions of
(entries 4–6). In contrast with the Clꢀ and AcOꢀ anions, the CuNTf2 in [BMIM][NTf2] were prepared. During the biphasic
bistriimide anion does not interact as strongly with the experiments, this corresponds to initial (total heterocycle)/
cation.25 This is reected in the cation’s increased ability to (metal ion) molar ratios of 1.5 and 0.06 respectively; the corre-
interact with the N-alkyl-substituted compounds like DMI and sponding partial ratios (total N-heterocycle)/(metal ion) are 1
NMC, for instance by hydrogen bonding between the hydrogen and 0.04. The results are shown in Fig. 1.
atom at C2 in imidazolium rings and the substituted N in the
Upon increasing the concentration of the copper(I)
heteroaromatic compound.26 The generally lower uptakes of compound in the IL from 0 to 1 M, there was some improve-
heteroaromatics in the C2-substituted imidazolium IL [BDMIM] ment in the overall sorption capacity and a somewhat increased
[NTf2] (entry 8) when compared with [BMIM][NTf2] prove this affinity was noted for NMC and the sulfur compounds, when
hypothesis and show that polarity and/or the labile hydrogen on compared with previous experiments. However, the extent of
the imidazolium structure favour the interaction with the the increase in copper concentration from 0.04 M to 1 M was,
solutes. Increasing the cation polarity, e.g. by inserting a nitrile disappointingly, not reected on the total heterocycle uptake,
group in the side chain, generally raises the affinity for all which only increased from 45% to 54%. This can be explained
heteroaromatic compounds, while the selectivity stays similar by the effect that the increase of the copper salt concentration
(entry 7) without indole but lower when taking into account this has on the polarity of the IL phase, the increased polarity
compound.
affecting the heterocyclic compounds’ solubility in such a way
In order to ne-tune the properties of the bistriimide ionic that the specic interactions of the metal species with the
liquids, mixtures with ionic liquids containing other cations heterocycles are not enough to stabilize the impurities in the IL
were employed. Generally, cations like choline or betainium phase. Surprisingly, when the mixture with the IL that con-
seem to suppress thiophenes uptake in comparison with tained 0.04 M copper(I) was le for a longer time (48 h), a trend
[BMIM][NTf2]; however this occurs slightly at the expense of the towards a more selective N vs. S heterocycle uptake was
NMC and DMI uptake (entries 9 and 10). The more apolar observed. Although there was no change in the IL + Cu system’s
methyltrioctylammonium increased again the affinity for the S- colour, this was attributed to the oxidation of the metal species
compounds (entry 11).
to copper(II), in the presence of the model oil. Apparently, in the
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bistriimide or chloride anions.† All bistriimide metal salts
are in 0.04 M concentration for direct comparison with the
ꢀ
previously investigated NTf2 salts of copper(I) and copper(II).
Results are shown in Fig. 2. Chloride salts were prepared by
mixing [BMIM][Cl] with the respective metal chlorides in a 1 : 1
or 1 : 2 metal/IL ratio required to form the respective [BMI-
M]y[MClx] ionic liquids, as shown in Fig. 3.29†
The bistriimide salts offered, in general, a very good uptake
of both I and DMI. In this group of ILs, CuII(NTf2)2 offered the
best results in the uptake of all N-impurities in the model oil as
well the best total heterocycle uptake. FeII(NTf2)2 afforded a
similar total uptake but appeared to have signicantly less
affinity for the more apolar NMC. When comparing with the
pure [BMIM][NTf2] IL, the advantage of theꢀpresence of the
metal salt can clearly be seen: all the NTf2 metal salt-con-
taining IL performed better than the metal-free [BMIM][NTf2],
regarding both total uptake and selectivity.
Fig. 1 Comparison of the uptakes in the biphasic experiments using
Cu-containing ionic liquid mixtures at 0.04 M and 1 M concentrations
in [BMIM][NTf2]. The histograms show the percentual molar uptake of
each impurity for each ionic liquid phase. The full lines indicates the N/
S selectivities. Conditions as in Table 1.
When compared with their bistriimide counterparts, the
ILs containing the chloride salts of copper(I), copper(II) or
iron(II) presented a low uptake of the more substituted N-
compounds, DMI and NMC (Fig. 3). This resulted in a very poor
N/S selectivity, especially when considering the values for DMI +
NMC versus BT + DBT (“Selectivity w/o I”, in Fig. 3). This is in
line with what was observed for the pure [BMIM][Cl] IL.
However, the uptake of DMI and NMC was still larger than the
one observed for the pure IL. The same trend for augmented
uptake capacity in the presence of the metal species was also
observed for the experiments with NTf2ꢀ compounds, as shown
in Fig. 2. The [FeIIICl4]ꢀ-containing IL presented a total uptake
capacity for the more apolar nitrogen compounds and even for
the sulfur compounds which was larger than that of any of the
other ILs described in this set and the previous, contrasting
with the relatively low uptakes from the other IL + [MClx]
systems. Noteworthy is the fact that, apart from [BMIM]
[FeIIICl4], all bistriimide salt containing ILs offered a better
total uptake and selectivity than their chloride salt counterparts
even though the metal concentration was much lower.
presence of the copper(II) compound, the ionic liquid phase
increases its overall affinity for the N compounds, indole and
dimethylindole in particular, while the thiophenes interact
more poorly. Considering Pearson’s HSAB concept of Lewis
acidity/basicity, it must be noted that Cu2+ is a considerably
harder Lewis acid than the very so Cu+ ion, and therefore has
less affinity for the soer Lewis bases in solution: the S
heterocycles.28 This assumption was reinforced by the uptake
and selectivity values obtained when performing the same
experiment with an IL containing 0.04 M CuII(NTf2)2 (Fig. 1).†
This IL phase presented a complete uptake of both I and DMI
and an increased uptake of NMC, when compared with the
copper(I)-containing IL phases, which is reected in an
increased N/S selectivity.
To assess in more detail the role of the nature of the metal
ion, the same type of liquid–liquid extraction experiments was
performed using ILs containing various metal salts, namely of
copper(I), copper(II), zinc(II), iron(II) and iron(III) with
Table 1 Uptake of N- and S-compounds by metal-free ILs in batch-like liquid–liquid extraction experimentsa
Uptake (molar%)c
N/S selectivityd
Ionic liquidb
I
DMI
NMC
BT
DBT
Total
with I
without I
1
2
3
4
5
6
7
8
[EMIM][Cl]
[BMIM][Cl
[BMIM][AcO]
[EMIM][NTf2]
96
96
3
6
4
7
5
6
2
4
28
29
39
41
45
45
49
30
34
39
44
11.5
8.0
2.7
3.6
3.4
3.1
2.6
8.7
8.9
9.6
2.8
1.2
1.3
1.0
2.3
2.1
2.1
2.0
4.3
4.3
6.0
1.9
100
90
91
91
93
81
86
93
87
18
39
45
44
47
28
31
43
40
22
34
40
39
49
12
21
19
39
20
17
21
21
24
7
10
7
19
18
14
19
18
25
2
4
3
21
[BMIM][NTf2]
[BMPyrr][NTf2]
[C2NC1Pyrr][NTf2]e
[BDMIM][NTf2]e
9
10
11
[BMIM][NTf2] + [Choline][NTf2]
[BMIM][NTf2] + [HBetaine][NTf2]
[BMIM][NTf2] + [N8881][NTf2]
a
Batch experiment using 0.2 ml of ionic liquid and 1.7 ml of heptane/toluene model oil contaminated with 1.9 mM I, 1.5 mM DMI, 1.2 mM NMC,
1.6 mM BT and 1.2 mM DBT; equilibration for 0.75 h at RT. b All IL mixtures are 1 : 1 molar ratios. c Molar% of the compounds transferred to the IL
d
phase; repetitions resulted in a uctuation of less than 2%. N/S selectivity ¼ (moles of N-heterocyclic compounds in the IL)/(moles of S-
e
heterocyclic compounds in the IL). Values were calculated including indole or not. [C2NC1Pyrr] and [BDMIM] refer to the 1-(1-cyanomethyl)-1-
methylpyrrolidinium and 1-butyl-2,3-dimethylimidazolium cations, respectively.
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Fig. 2 Comparison of the uptakes in biphasic experiments using ILs containing different metal NTf2ꢀ salts. Conditions as in Table 1.
support was also tested: all impurities eluted at the dead time of
the column, implying no uptake occurred. Next, [BMIM][NTf2]
was immobilized on the surface of the mesoporous silica in
such an amount as to ll half of the solid’s total pore volume
(0.5 ml per g of mesoporous silica). The rst run performed with
this SILP (Fig. 4a) shows that the SILP causes a delay in the
elution of indole but, unlike in the biphasic liquid–liquid
experiments, none of the other N-compounds was signicantly
retained. Noticeably, the breakthrough prole of indole displays
a broadened shape, which is indicative for too slow diffusion of
the compound into the adsorbent phase.30 The column was
regenerated with pure solvent mixture and used in a second run
(Fig. 4b), this time using a model oil containing no indole, in an
effort to determine whether the uptake of this compound
impeded the uptake of the other compounds. Fig. 4b shows that
the selectivity of the SILP for indole does not affect the affinity
for the other N-compounds: again, neither DMI nor NMC were
signicantly retained.
Breakthrough experiments
Aer determining the most adequate metal/IL systems for
selective denitrogenation, the next natural step, in view of a
viable industrial application of such systems, is their immobi-
lization on the pore surface of solid supports. The resulting
supported ionic liquid phase (SILP) allows for application in
breakthrough experiments. In these experiments, the ionic
liquid was non-covalently adsorbed on the pore surface of a
silica support which was inserted in a column. The model oil
containing the N and S impurities was ushed through the
column and the eluent was collected and analysed.
Following the trends demonstrated in the liquid–liquid
extraction experiments, breakthrough experiments were per-
formed with SILP columns of [BMIM][NTf2] – either pure or
loaded with 0.04 M CuII(NTf2)2 – and of [BMIM][Cl]/FeCl3.
Packed-bed columns
The packed-bed columns contained powdered mesoporous
In view of the good result obtained with the ionic liquid
silica loaded with a SILP, and were used as stainless steel containing copper(II) in the liquid–liquid experiments (Fig. 1),
columns. The blank column containing only the solid silica two consecutive breakthrough experiments were run with a
Fig. 3 Comparison of the uptakes in biphasic experiments using ILs containing different metal chloride salts. Conditions as in Table 1.
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Fig. 4 Breakthrough profiles, in 2 consecutive runs, for the pure [BMIM][NTf2] SILP packed-bed column at 1 ml minꢀ1 flow rate: (a) experiment
including indole; (b) experiment without indole. The column was abundantly rinsed with H/T 4 : 1 mixture in between the runs. A GC analysis of
the eluent after the column regeneration showed no N or S impurity.
packed-bed column containing [BMIM][NTf2] with 4 ꢁ 10ꢀ2
M
bed column presents a slightly better uptake of both DMI and
CuII(NTf2)2. Both runs presented near–identical proles NMC than at 1 ml minꢀ1, but still the proles did not match the
(Fig. 5a). Comparing these with the metal-free SILP prole expectations from the batch experiments.
(Fig. 4a), the copper(II)-containing SILP presents a similar
prole but a sharper uptake of indole. This column displayed
only a slight uptake of DMI, as seen in the rst breakthrough
Monolithic silica columns
prole of Fig. 5a. Given the discrepancy between the liquid– Next, similar experiments were performed, now using hierar-
liquid and the breakthrough experiments in the uptake of the chically structured silica monoliths as solid support. Aer a
more apolar N-compounds (DMI and NMC), it was suspected blank run, in which it was established that the monolith itself
that the contact times between the mobile phase and the SILP does not have a separation capacity, pure, metal-free [BMIM]
phase were not high enough for an efficient uptake. Hence, the [NTf2] was immobilized onto the pore surface to create a SILP
copper(II)-SILP was also tested at a lower ow rate of 0.2 ml monolith column. Running the mobile phase at a ow rate of
minꢀ1 (Fig. 5b). In these conditions, the copper(II)-SILP packed- 0.2 ml minꢀ1 afforded the prole on Fig. 6a.
Fig. 5 Breakthrough profiles with the CuII(NTf2)2 containing [BMIM][NTf2] SILP packed-bed column: (a) two consecutive runs at 1 ml minꢀ1 flow
rate, (b) run at 0.2 ml minꢀ1 eluent flow rate.
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Fig. 6 Breakthrough profiles with SILP containing monoliths: (a) SILP based on pure [BMIM][NTf2]; (b) and (c) two consecutive runs with a [BMIM]
[NTf2] SILP containing 4 ꢁ 10ꢀ2 M CuII(NTf2)2. Flow rate: 0.2 ml minꢀ1. After each run, the column was regenerated by flushing with the pure
solvent mixture.
Comparison with the prole for the packed-bed columns at no signicant leaching of the ionic liquid phase nor any
the same ow rate (Fig. 5b) shows that the SILP monolith permanent chemical modication of the metal-containing
presents the same adsorption prole, with the preferential SILP during the process takes place.
uptake of indole. The column regeneration is also efficient, with
the expected delay in the release of indole. A slight selectivity for the very good result obtained with the iron(III) tetrachloride IL in
the uptake of the apolar DMI and NMC is observed. the liquid–liquid experiments (Fig. 3) motivated the application
The good performance of the monolithic silica supports, and
A new monolithic SILP column was then prepared with of this IL in breakthrough runs using a monolith column. As
[BMIM][NTf2] containing 4 ꢁ 10ꢀ2 M CuII(NTf2)2 and the same seen in Fig. 7, indole is taken up until about 20 ml into the
experiment was run once again at 0.2 ml minꢀ1 (Fig. 6b). The experiment, in the same manner as on the metal-free SILP
monolith column was regenerated and a second run was per-
formed (Fig. 6c).
Comparing this result with the corresponding copper(II)-
SILP packed-bed column at the same ow rate (Fig. 5b), it can
be concluded that the monolith plays a decisive role in the
uptake process. The hierarchical structure of the monolith,
with macro- and mesopores, aids the diffusion of the solutes
and increases the SILP’s contact with the bulky N- and S-
compounds. As a result, the preferences of the SILP mono-
lithic system perfectly mirror those of the same ionic liquid in
batch mode (Fig. 2), with a decreasing preference in the order:
I > DMI, NMC > BT, DBT. Importantly, the nitrogen contami-
nants are well separated from the sulfur contaminants. The
similar breakthrough proles obtained for both runs show
that this system can be successfully regenerated and used in
Fig. 7 Breakthrough profile at 0.2 ml minꢀ1 of the Fe(III)Cl4-SILP
multiple cycles. Moreover, the identical proles indicate that
monolith column.
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monolith column (Fig. 6a). However, in this case, 1,2-dimethy-
lindole is also taken up in the same extent, resulting in almost
20 ml of model oil containing only 33% of its normal nitrogen
content. Comparing with the corresponding liquid–liquid
experiment performed with the same IL (Fig. 3), there is now a
more marked selectivity towards the nitrogen compounds.
While NMC was also efficiently retained in the Cu-doped
[BMIM][NTf2] monolith, this compound is less efficiently with-
held on the FeIIICl4ꢀ-containing ionic liquid. This is somehow
at contrast with the liquid–liquid equilibrium data of Fig. 3, and
suggests that, in the breakthrough test with the FeIIICl4ꢀ-con-
taining ionic liquid, adsorption equilibrium was not reached for
NMC due to the slow diffusion of this large compound.
General procedure for SILP monolith preparation
This type of column consists of a cylindrical silica monolith
encased between two glass tubes and in a Teon sheath. The
monoliths used have a macropore diameter of 5–6 mm, meso-
pore diameter of 11 nm, a specic surface area of 550 m2 gꢀ1
,
mesopore volume 1.1 ml gꢀ1 and macropore volume of 1.7 ml
gꢀ1. They have a diameter of 6 mm and 4 cm in length, corre-
sponding to a cylindrical volume of 1.1 ml. The synthesis of the
monoliths is described in the literature.21b The dry ionic liquid
was dissolved in an amount of dry dichloromethane such as
that the resulting solution’s volume was the same as the
monolith’s total pore volume. The SILP was prepared by lling
the entire pore volume of the monolith with the solution. The
amount of IL was such as to ll half the monolith’s total pore
volume, as in the preparation of the powder SILPs. The solvent
was removed and the SILP-containing monolith dried overnight
under vacuum. This SILP column was tted to the HPLC pump
directly by the glass ends.
Experimental section
Bistriimide ionic liquids and [BMIM][Cl] were obtained
commercially with a purity of $95%. The ILs with anions of the
type [MClx] were prepared by mixing [BMIM][Cl] and the metal
chloride in the appropriate proportions at room temperature.29
All syntheses of metal bistriimide salts and ILs of the type
[BMIM]y[MClx] are described in the ESI.† Residual solvent and
water were removed under vacuum aer the formation of the IL.
All ILs were dried at <0.1 mbar vacuum at 90 ꢂC overnight prior
to use.
The model oil was prepared by dissolving 55 mg of each
compound (indole, 1,2-dimethylindole, N-methylcarbazole,
benzothiophene and dibenzothiophene) in 250 ml of a solvent
mixture composed of heptane and toluene in a 4 : 1 volume
ratio.
General procedure for the breakthrough experiments
A typical breakthrough experiment was performed by attaching
a stainless steel packed-bed column or a glass-ended, Teon-
sheathed monolith column to a HPLC pump. The column was
then conditioned with at least 20 ml of pure solvent mixture at
1 ml minꢀ1 (0.5 ml minꢀ1 for the monoliths). The model oil was
eluted at the chosen ow rate, at which point t ¼ 0. Each mil-
lilitre of the eluent was collected and analysed with GC. The
dead volume for a normal system, was approximately 4 ml.
Blank experiments were performed by using a column con-
taining only the respective solid support without any IL phase
and eluting the model oil. The blank breakthrough proles
show no preferential uptake of any compound.
General procedure for the biphasic liquid–liquid extraction
experiments
The experiments were performed, in triplicate when possible,
in GC vials with 0.2 ml of IL phase and the rest of the vials’
volume (1.7 ml) containing the model oil with the N- and S-
compounds, leaving no headspace. The mixtures were stirred
for 45 minutes at room temperature, aer which period the
model oil phase was analysed with GC. Each liquid–liquid
experiment was accompanied by a blank in which the vial was
lled only with the model oil and no IL phase. No solvent
uptake was observed.
Conclusions
The results shown allow the conclusion that ionic liquids con-
taining the bistriimide and chloride salts of copper(II) and
iron(III), respectively, show a promising selectivity for the uptake
of heteroaromatic nitrogen compounds when in the presence of
other impurities usually found in fuel feeds, namely structurally
similar heteroaromatic sulfur compounds. The ionic liquid
[BMIM][NTf2] containing CuII(NTf2)2 was found to perform
better than its copper(I) counterpart, affording a higher uptake
and better selectivity in the uptake of N compounds over S. This
General procedure for SILP powder preparation
Silica gel, large pore from Alfa Aesar (58 mm particle size, species’ harder Lewis acid character accounts for lower affinity
300 m2 gꢀ1 surface area, 1.6 ml gꢀ1 pore volume, 15 nm pore for the soer sulfur compounds. Ionic liquids containing metal
diameter) was used. The appropriate volume of dry ionic liquid bistriimide salts in a relatively low concentration were shown
to prepare a SILP with half of the solid’s initial pore volume to outperform the current literature staple for metal-containing,
lled was dissolved in 50 ml of anhydrous dichloromethane. IL-based extractive denitrogenation, [BMIM]y[MXx] ionic liq-
This solution was added to the silica, previously activated for 3 h uids,11d,e,h,14,16 in which the metal halide (MXxꢀn) is present in a
at 500 ꢂC and cooled under vacuum, and the mixture was stirred very high 1 : 1 or 1 : 2 proportion. Metal bistriimide IL systems
for 2 h under N2 atmosphere. The solvent was removed under present, therefore, a more benign alternative to the use of
vacuum, resulting in a white, free-owing powder. The SILP corrosive metal halides for the purication of fuels feeds.
powder was packed into the 10 cm long, 4 mm inner diameter
The best performing IL systems were non-covalently immo-
stainless steel column (1.3 ml inner cylindrical volume) under bilized on the pore surface of hierarchically structured silica
N2 atmosphere.
monoliths to form SILP columns. This allowed the run of
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breakthrough experiments using these columns. The break-
through proles of the impurities showed that these monolithic
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