164 JOURNAL OF CHEMICAL RESEARCH 2018
Catalyst characterisation
para-selectivity. Similar results were reported by Kostrab and
co-workers,14 who explained that a small excess of tert-butanol
is preferred to suppress the dealkylation reaction (cleavage of
tert-butyl group from the aromatic ring).
XRD patterns of catalysts were obtained on a D/max-2200PC-X-ray
diffractometer using Cu Kα radiation (λ = 0.154 nm) and operated at
40 kV, 30 mA. The scanning rate was 0.05 ° s−1 in the range from 10
to 80°.
The morphology of the HBEA as well as the particle size
distribution for the MxOy/HBEA samples were determined with a
JSM-6360LA SEM.
Infrared spectra of catalysts were recorded on a Bruker FTIR
spectrometer (Tensor 27) in a wavenumber range of 4000–400 cm−1
with 4 cm−1 resolution. Absorbance measurements were carried
out using KBr pellets containing 5% of the powder sample to be
analysed.
Nitrogen adsorption–desorption isotherms of catalysts were
measured at −196 °C using Micromeritics adsorption equipment
type NOVA2000e. The specific surface area of the catalysts was
calculated using the BET method in the partial pressure (P/P0) range
from 0.01 to 0.99. The mesopore size distribution was obtained from
the adsorption branch of the isotherm using the Barret–Joyner–
Halenda (BJH) adsorption model.
Effect of reaction temperature
The dependence of toluene conversion and 4-TBT selectivity
on the reaction temperature was examined. The experiments
using Fe2O3/HBEA catalyst were conducted at 150, 170, 180,
190 and 210 °C. As shown in Fig. 8, there is a sharp increase
in the conversion of toluene when the reaction temperature is
increased from 150 to 180 °C, and the toluene conversion then
decreases on increasing the temperature further. Based on
these results, we suggest that activated toluene molecules and
activated tert-butyl carbenium ions increase as the temperature
increases, which results in increased toluene conversion.
However, a decrease in toluene conversion was evident when
the temperature was higher than 180 °C. Sebastian et al.7 also
reported similar results. They found that butene was formed
through dehydration of tert-butanol on acidic sites, which can
oligomerise to C8 and C12 olefins and then crack to low boiling
species. As these reactions were dominant at higher reaction
temperatures, a decrease in the toluene conversion and 4-TBT
selectivity was expected.
The NH3-TPD procedure was carried out in a Quantachrome
Chembet-3000 Characterization System; 200 mg sample was used
for each measurement. The sample was pretreated in the stream at
550 °C for 60 min in dry helium and cooled to 120 °C. It was then
exposed to 10 v% NH3/He for 1 h. After purging the catalyst with He
for 1 h, a TPD plot was obtained at a heating rate of 10 °C min−1 from
120 to 600 °C.
Conclusions
FTIR spectra of pyridine-adsorbed (Py-IR) samples were obtained
on a Bruker FTIR spectrometer in KBr pellets. Before each
experiment, samples were pressed into thin pellets (10–30 mg) with
diameter of 16 mm and activated in situ over one night under vacuum
(10−5 Pa) at 170 °C. Pyridine was introduced in excess at 150 °C after
the activation period. The concentrations of the B and L sites were
determined from the integrated area bands of the PyH+ and PyL
species using the values of the molar extinction coefficients of both
bands.
Several transition metal oxide-modified HBEA catalysts
were prepared and investigated in the alkylation of toluene
with tert-butanol for the synthesis of 4-TBT. The introduction
of transition metal oxides into the framework structure of
the zeolite led to a loss of crystallinity, but could adjust the
pore structures and decrease the total acidity, especially the
strong acidity of HBEA. On loading with MxOy, the toluene
conversion of MxOy/HBEA catalysts decreased, while
the 4-TBT selectivity increased significantly. A possible
mechanism was proposed to explain the increased selectivity:
(1) the narrowing of the pores of MxOy/HBEA catalysts
promoted the formation of 4-TBT by increasing the shape
selectivity; and (2) the deactivation of strong acid sites caused
by the loading of MxOy increased the selectivity for 4-TBT by
suppressing the further isomerisation of 4-TBT. The effects of
various reaction parameters, such as the amount of catalyst,
the reactant molar ratio and the reaction temperature, were
studied over Fe2O3/HBEA.
Toluene alkylation with tert-butanol
The reactions were performed in a 300 mL laboratory autoclave
reactor. For a typical run, the reaction mixture consisted of tert-
butanol (283 mmol Aladdin, 98%, 28 mL), toluene (94 mmol,
Aladdin, 98%, 10 mL) and cyclohexane (Aladdin, 99%, 60 mL). The
raw material and catalyst were first added into the reactor, then, the
air having been purged with nitrogen, the pressure in the reactor was
adjusted to 1.5 MPa with nitrogen. The reaction temperature was
maintained at 180 °C and the stirring rate was controlled at 300 rpm
to overcome external diffusional limitations. After the experiment,
the pressure was reduced to atmospheric pressure, and the zeolite
catalyst was removed by filtration. Each experiment was repeated
three times, and good reproducibility of the results was found.
The samples were analysed using a GC-14C gas chromatograph
(Shimadzu, Japan), with a SE-30 capillary column (50 m × 0.25
mm × 0.25 μm) and a FID detector. Analyses were carried out with
a temperature programme from 60 to 220 °C (with a slope of 10 °C
min−1) and at 220 °C for 10 min isothermally. The reaction products
were identified by GC/MS analysis.
Experimental
Materials
Na-BEA zeolite powder (Si/Al = 25) was purchased from Zeolyst
Int., China. Ammonium nitrate, manganese(ii) nitrate, iron(iii)
nitrate, cobalt(ii) nitrate, nickel (ii) nitrate, copper(ii) nitrate, toluene,
tert-butanol and cyclohexane (all of analytical grade) were bought
from ACROS Organics.
Catalyst preparation
Received 8 March 2018; accepted 23 March 2018
Paper 1805335
Published online: 29 March 2018
The HBEA zeolite with a Si/Al ratio of 25 used in this study was
prepared according to the reported procedure.15 NaBEA zeolite was
used as a starting material. The HBEA zeolite was prepared by ion
exchange of NaBEA with aqueous NH4NO3. After ion exchange, the
zeolite was filtered, and dried at 110 °C for 12 h. Then the samples
were calcined at 350 °C for 4 h in an air. A series of MxOy/HBEA
catalysts was synthesised by ion exchange of HBEA zeolites with
aqueous transition metal nitrates (0.25 mol L−1) for 24 h at room
temperature.16 The resulting materials were calcined at 350 °C for 4
h in an air.
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