Study
on alternative methods to hydrodesulfurization is always
an interesting area, because of the ineffectiveness for hydrodesulfurization
to remove some cyclic sulfur compounds (S-compounds) and the high
cost from harsh operation conditions and expensive catalyst, among
which oxidative desulfurization (ODS) using ionic liquids (ILs) as
solvent is intensively studied recently. Here, we synthesize a series
of Brønsted–Lewis acidic ILs of N-methylpyrrolidonium
zinc chloride ([Hnmp]Cl
x
/(ZnCl2)
y
, x:y from 2:1 to 1:2) and investigate the ODS of both model diesel fuel
composed of n-octane and dibenzothiophene and real
FCC feedstock where such ILs are used as extractant and catalyst and
30 wt % H2O2 is used as oxidant, involving the
factors such as IL composition (or x:y), temperature, dose of oxidant (or molar ratio of O/S), dose of
IL (or mass ratio of IL/oil), recycling of IL and multistage desulfurization.
IL composition has an important effect on sulfur removal (S-removal)
efficiency, and [Hnmp]Cl/ZnCl2 (x:y = 1, the structure nature was characterized with ESI-MS
and FT-IR) shows the highest desulfurization capability with good
recyclability. With [Hnmp]Cl/ZnCl2, the S-content in model
diesel fuel can be reduced to <1 ppm from 500 ppm with 99.9% S-removal
at 75 °C, IL/oil = 1/3 and O/S = 8 after only one stage, while
the sulfur removal for real FCC diesel fuel is less than 38% in one
stage and can reach 83% after five stages, which might be ascribed
to more-complex S-species in real fluidized catalytic cracking (FCC)
diesel fuel as indicated by gas chromatography–sulfur chemiluminescence
detection (GC-SCD) chromatogram analyses. The sulfur content (S-content)
in FCC diesel fuel, however, can be reduced to 5.3 ppm with a total
S-removal of 97.6% after five-stage ODS with one more extractive desulfurization
with furfural as the extractant. This work shows that such Brønsted–Lewis
acidic ILs are potential solvents used in ODS to produce clean fuel
oils.
To screen and use ionic liquids (ILs) as environmental-friendly extractive solvents in removing aromatic sulfur compounds (S-compounds) from fuel oils, the knowledge of their capacity for S-compounds (or solubility of S-compounds in ILs) is very important. In this work, the capacities of 1860 potential ILs (30 anions, 62 cations) for two representative S-compounds of thiophene (TS) and dibenzothiophene (DBT) are calculated using conductor-like screening model for real solvents (COSMO-RS). The influences of cation family, cation alkyl chain length, cation symmetry, anion nature, anion alkyl chain length, and functional group on the capacity are extensively discussed and are understood from microlevel view with σ-profile, σ-moments, and COSMO-RS energies. It is observed that the capacity is very dependent on cation and anion structure characteristics and is in a very wide range (e.g., 10 −3 ∼10 1 for TS, 10 −3 ∼10 2 for DBT); the van der Waals (vdW) and hydrogenbonding (HB) energies have significant effects on the capacity. Increasing the nonpolarity and vdW energies of cation or alkyl chain on anion, or the polarity and HB energies of anion, can favor the capacity. This work is valuable to rationally select or design the ILs for desulfurization of fuel oils.
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