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be responsible for its basicity. According to the proposal of Coluc-
cia and Tench, several MgO ion pairs of other coordination numbers
exist on the surface of this MgO, such as the corners, edges and high
Miller index surfaces of the magnesia crystal, and the coordination
numbers of the oxygen atom would be 3, 4 and 5 at these sites [24].
The crystal defects on the magnesia surface result in the imbal-
ance of electronic charge of O2− anion, leading to the formation of
strongly basic sites.
The hydroxyl groups of the catalysts were identified by FT-IR
using a Nicolet 750 spectrometer (the resolution of spectrome-
ter is 0.125 cm−1) in the range of 4000–3000 cm−1 at a resolution
of 4.0 cm−1 using the KBr pellet technique. The structure of CO2
chemisorbed on samples was also determined by FT-IR, the method
of catalysts pretreatment corresponded to previous literatures
[26,27].
Gillespie et al. developed the solid solutions of metal oxides
MgO/Al2O3 and NiO/MgO/Al2O3 for use as solid bases to catalyze
the oxidation of mercaptans to disulfides successfully [25].
A novel catalyst which has adequate basicity and defects may
solve the problems associated with desulfurization and octane
value loss during the traditional gasoline hydro-desulfurization
process. It is possible to completely eliminated aqueous sodium
hydroxide by incorporating solid basic materials into the cata-
lyst formation. Furthermore, the commonly used Merox process of
sweetening is rather expensive since it involves expensive catalyst,
such as CoPc, which is costly and cannot convert iso-mercaptans
efficiently. Thus, a CoPc-free sweetening process could result in
significant savings over the use of Merox process of sweetening.
The main objective of the present study was to find a lower-cost
and more effective catalyst which has better stability capable of
solving over proof mercaptans in hydrogenated gasoline, a series
of solid-base catalysts with magnesia matrix were synthesized and
tested in gas-liquid-solid heterogeneous base-catalyzed oxidation
of mercaptans. Most of all, the critical factor that affects the con-
version of tert-butyl thiol, such as basic centers and active oxygen
species of solid-base catalysts were studied.
X-ray diffraction (XRD) technique was used to characterize the
crystal structure. A D8 ADVANCE X-ray diffractometer equipped
with Ni-filtrated Cu-K␣ radiation source (40 kV, 40 mA) was used.
Samples were analyzed with the continuous scan mode at 2 ◦/min
over a 2ꢀ range of 5–80◦.
Temperature programmed desorption (TPD) of oxygen, CO2 and
H2 was carried out on a FINESORB-3010 apparatus; the samples
were treated at 400 ◦C for an hour in oxygen (or CO2 or H2) and
cooled to room temperature in the same atmosphere then swept
with helium at a rate of 30 mL/min until the chromatogram has
steadied. Finally, the sample was heated at a rate of 20 ◦C/min in
helium for recording the TPD spectra.
The electron paramagnetic resonance (EPR) spectra were
obtained at room temperature by a JEOL JES FA200 machine. The
concentration of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) radical
trap was 1 mol/L.
2.3. Catalytic test for oxidation of tert-butyl thiol
Tert-butyl thiol was considered as the model compound repre-
sentative of iso-mercaptans in hydrogenated gasoline [28]. It was
dissolved in n-hexane, air was used as the oxidizing agent.
The catalytic performance of the solid-base samples were com-
pared to a commercial CoPc catalyst. A conventional fixed bed flow
reactor (1.2 in. i.d.) was used for the catalysts performance tests.
6 mL of catalyst was crushed to particles of 40–60 mesh and packed
in the reactor. Liquid stream and the catalyst were in neat contact
with each other as the liquid stream flowed upwardly through the
catalyst. Contact time was equivalent to a liquid hourly space veloc-
ity (LHSV) of 3.0 h−1 or 1.0 h−1. Air, at flows of 6 mL/min, passed
through the reactor at 40 ◦C. The apparatus was consisted of ther-
mostat water bath maintained by recycling water with a pump. A
static mixer was placed in front of the reactor to extensively mix
acid hydrocarbon with air.
The feedstock was tert-butyl thiol dissolved in n-hexane with
the initial mercaptan-type sulfur content of 75 weight ppm; when
the treating time reached 6 h, the mercaptan-type sulfur content
was 1513 mass ppm. When the conversion rate of tert-butyl thiol
decreased to a level of 80% or below, the catalysts were thought to
be inactive and the evaluation tests were ceased.
The contents of mercaptan-type sulfur were measured by poten-
tiometric titration according to ASTM D3227-83. Products were
identified with GC–MS.
2. Experimental
2.1. Preparation of solid-base catalysts
The solid-base catalysts were prepared at ambient pressure
and temperature. Light magnesia, NaOH, Cu(NO3)2·3H2O and
Ni(NO3)2·6H2O purchased from Sinopharm Chemical Reagent Co.
Ltd were of analytical grade. A series of solid-base catalysts used
in this study were prepared via the protocol below: (1) prepara-
tion of sample A: the powders of 190 g light magnesia and 10 g
palygorskite were mixed in a 500 mL beaker until the mixture
was homogenous. 80 mL NaOH aq. solution was then added into
the above mixture under constant stirring until the mixture was
uniform. The concentration of NaOH solution of A is 3.3%. The mix-
ture was then extruded and the extrudate was dried at 120 ◦C for
3 h and calcinated at 500 ◦C for 4 h. (2) Preparation of sample B
and C: the powders of 190 g light magnesia and 10 g palygorskite
were mixed in a 500 mL beaker until the mixture was homogenous.
80 mL aq. solution of NaOH and Cu(NO3)2·3H2O or Ni(NO3)2·6H2O
was added into the above admixture under constant stirring until
the mixture was uniform, and the concentration of solution of
NaOH, Cu(NO3)2·3H2O and Ni(NO3)2·6H2O is 3.5%, 40.4% and 51.8%,
respectively; then the mixture was extruded and the extrudate
was dried at 120 ◦C for 3 h and calcinated at 500 ◦C for 4 h. (3)
Preparation of sample D: the powders of 190 g light magnesia and
10 g palygorskite were mixed in a 500 mL beaker until the mix-
ture was homogenous. 80 mL solution of NaOH, Cu(NO3)2·3H2O and
Ni(NO3)2·6H2O was added into the above admixture under con-
stant stirring until the mixture was uniform, and the concentration
of solution of NaOH, Cu(NO3)2·3H2O and Ni(NO3)2·6H2O is 3.7%,
the extrudate was dried at 120 ◦C for 3 h and calcinated at 500 ◦C
for 4 h.
3. Results
3.1. Measurement of sweetening reactivity
In order to clarify the property of the catalyst specific to cat-
alyze the oxidation of tert-butyl thiol, the catalytic performances
of synthesized solid-base catalysts were compared with that of
a commercial CoPc catalyst. The conversions are shown in Fig. 1.
According to the analysis result of GC–MS, the oxidation product
of the tert-butyl thiol is disulfide whose figure of mass spectrum
is as shown in Fig. 2, which is consistent with the base-catalyzed
oxidation reaction mechanism of tert-butyl thiol mentioned above.
The composition and physical properties of samples are shown
in Table 1.