Parallel Microwave-Assisted Synthesis of Ionic Liquids
Combinatorial Chemistry & High Throughput Screening, 2012, Vol. 15, No. 5 431
Effect of the substituent R2 at C2 in imidazolium type
ionic liquids: Ionic liquids with R2 = H are more efficient
extracting agents than those with R2 = Me; perhaps the
methyl substituent at R2 promoted a steric effect that made
the interaction of nitrogenated compounds diffcult in ionic
liquids (1 vs 29; 2 vs 30; 17 vs 32 and 18 vs 31).
containing Fe or Al as metal and n = 3 (see entries: 8-13, 17-
19, 31 and 32, 40-43 and 51, 54 and 56). Being
fundamentally basic nitrogenated compounds, the results
obtained with Lewis acid ionic liquid could be expected, as
the consequence of the low viscosity obtained for ionic
liquids containing IL:MeCln = 1:1.5. Lewis acid ionic liquids
containing only one halogen in the structure (i.e. ClFeCl3) of
the anion are more efficient extractanting agents than the
analogues containing Br and Cl (i.e. BrFeCl3, see entry 10 vs
Effect of the halogen type (Cl vs Br) for the “first
generation” ionic liquids: In general, halogenated ionic
liquids are efficient extractants of nitrogenated compounds
and Cl containing anions are slightly more efficient than Br
for imidazolium type cation, while Br are more efficient for
pyridinium type cations. However, ionic liquids containing
Br are obtained quickly and with higher yield. These first
generation ionic liquids are cheaper than ionic liquids
containing other anions because additional reagents are
required in their synthesis and the yield is lower. Halogen-
containing ionic liquids are also moisture stables and can be
recuperated and reused in several reaction cycles (see entries
1 vs 2, 14 vs 15, 33 vs 34 and 49 vs 50).
13 and 8 vs 11). For these kinds of ionic liquids, the
imidazolium cation showed better performance than the
pyridinium analog (see entries 32 vs 56). The effect of the
type of metal was discussed by us before in relation with the
desulfurization process, and the same analysis can be applied
for denitrogenation [13].
Lewis acid ionic liquids remove the basic nitrogenated
compounds presumably by means of chemisorptions by the
interaction of N…Metal through Dewar-Chatt-Duncanson-
like mechanism of electron donation-backdonation among
electron rich nitrogenated compounds and transition metal
centers of Me2Cl7¯ anions in the ionic liquid [14].
Effect of the cation type: As can be observed in Table 2,
the cation effect is less significant than that of anion,
however, a slightly higher extraction is observed for
pyridinium type (II) cations with respect to imidazolium type
cations containing the same anion (see entries 1 vs 33, 2 vs
34, 4 vs 39, 13 vs 54, 14 vs 49 and 15 vs 50). This results can
be attributed to a major structural similarity between
pyridonium cation (five member one N atom heterocycles)
and nitrogenated compounds present in the Diesel feed.
However, these kinds of liquids suffer from being
moisture sensitive and decomposing after being used, while
the ionic liquids containing Br and Cl as anion are
chemically stables and cheaper. The latest can be recuperated
and reused in several cycles of extractions [9]. On the other
hand, it is well known that Lewis acid ionic liquids can be an
alternative for removing the sulfurated compounds [4].
Effect of the anion type: The anion plays a very
important role in the extraction ability of nitrogenated
compounds. Anions like halogens (Cl and Br), XFeCl3 and
XAlCl3 (X = Cl and Br) show a good ability to remove
nitrogenated compounds, while other than CH3COO,
PhCOO, BF4, PF6, and Tf showed relatively low efficiency
of extraction (see entries 2-7, 45 and 47 in Table 2);
however, the anion effect is more significant for the
extraction of sulfurated compounds [13].
In summary, we found that ionic liquids are good
alternatives for denitrogenation of straight-run Diesel feed.
Several types of ionic liquids showed a good performance
for the selective extraction of nitrogenated compounds by
liquid-liquid extraction. In particular, the halogenated ionic
liquids are excellent alternatives due to these compounds are
relatively inexpensive, presenting a high selectivity for the
extraction of nitrogen-containing compounds and can be
regenerated and recycled. All ionic liquids probed were
synthesized in parallel under solvent-free, one-pot
microwave-assisted synthesis.
Substitutent effect in pyridinium type ionic liquids:
Pyridinium cation (II with R1 and R2 = H) was the most
appropriate for the extraction of nitrogenated compounds
and their performances decrease when Me substituents are
included in their structure as in the case of 2-
methylpyridinium and 2.6-dimethylpyridinium (lutidinium)
as shown in entries 44 and 45.
ACKNOWLEDGEMENT
We would like to thank to Instituto Mexicano del
Petróleo (IMP) for both granting permission to publish
results and for their financial support (IMP project D.00412).
Selective extraction of nitrogenated compounds vs
sulfur compounds: Selective extraction of nitrogenated
compounds is very important, especially in this kind of
predesulfurized feed containing a very high content of
sulfur-containing compounds, because these compounds can
be transformed during HDS in valuable hydrocarbon feed
and significant separation of these compounds from the feed
can be translated in a considerable volumetric loss of
hydrocarbons. In our case, halogenated ionic liquids showed
a high selectivity for nitrogen-containing compounds, the
desulfurization degree with [BuPy]Br, [OMOM]Cl and
[OctPy]Br was less than 2%.
CONFLICT OF INTEREST
Declared none.
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Effect of the ratio halogenated IL/MeCln for ionic
liquid obtained by metathesis: As can be observed in Table
1, the best results were obtained for Lewis acidic ionic
liquids (ratio IL:MeCln = 1:1.5), especially from those
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