Fig. 1. Schematic diagram of four operating processes for
synthesizing ZSM-5 zeolites: a) RPB premix - static crystallization, b)
RPB premix - dynamic crystallization, c) STR premix - static
crystallization and d) STR premix - dynamic crystallization
To start a synthesis, a certain amount of chemicals (silica sol (40 wt%
aqueous solution),
Al2(SO4)3.18H2O,
TPAOH (25 wt% aqueous solution), NaCl, H2O) were
measured following a molar ratio of 200SiO2:
Al2O3: 80TPAOH: 30NaCl:
4500H2O. Two sources, including (A) silica source
prepared by silica sol, and (B)
aluminum source prepared by
Al2(SO4)3.18H2O,
NaCl, water and TPAOH, were prepared respectively at room temperature
under vigorous stirring for 0.5 h to obtain a homogeneous solution. In
an RPB premixing process, the above two sources were pumped into RPB via
two flow meters, respectively. And then the initial gel was cycled by a
peristaltic pump for 30 min, operating with a circulation rate of 540
mL·min-1 and a rotation speed of 1000 rpm. While in an
STR premixing process, the silica source was slowly added dropwise into
the aluminum source at a rotation speed of 1000 rpm for 1 h. The
as-obtained gel was then poured into 100 mL Teflon-lined autoclave for
traditional static crystallization and 500 mL STR for dynamic
crystallization, respectively. In the hydrothermal crystallization
process, static crystallization was carried out at 130 ℃ for 24 h, and
dynamic crystallization was at 130 ℃ for 16 h with a stirring speed of
1000 rpm in the STR. After these processes, as-obtained samples were
washed with deionized water and centrifuged at 6000 rpm for 5 min with
repeating four times, followed by drying at 110 °C for 12 h and
calcining at 550 °C for 3.5 h to remove the template. Finally, samples
were converted to proton form by refluxing twice in 1.0 M
NH4NO3 (5.0 g solid per 200 mL solution)
for 4 h, followed by drying at 110 °C for 12 h and calcining at 550 °C
for 3.5 h. The resulting samples are denoted as ZSM-5 samples.
2.3 Nucleation/crystal growth mechanism of zeolite ZSM-5
studies
During the premix and crystallization processes, small aliquots of the
reaction solution were taken in different time for analysis to explore
the effects of enhancing micromixing on the nucleation/crystal growth
process of ZSM-5 zeolites. The aliquot was washed with deionized water
and centrifuged at 6000 rpm for 5 min until the pH was below 9, followed
by drying at room temperature. After drying, the aliquots were analyzed
by FTIR and XRD to study the nucleation/crystal growth mechanism of
ZSM-5 zeolites. These measurements were repeated three times.
2.4 Characterization
XRD patterns of the ZSM-5 zeolites and its reaction intermediates were
recorded on an XRD-6000 diffractometer (Shimadzu Inc.) using CuK a radiation (40 kV, 40 mA), in the 2θ range from
5°
to 90°, with a step size of 0.02° and a counting time of 10 s per step.
The elemental composition (Si/Al ratio) of the sample was determined by
XRF spectrometer (Rigaku). Morphology and particle size of ZSM-5
zeolites were observed by scanning electron microscopy (SEM, Tescan).
FTIR spectroscopic analyses were carried out with pressing potassium
bromide troche (in a mass ratio of 1/100) on a Nicolet 6700 spectrometer
(Nicolet Instrument Co., USA) in the wavenumber range of
4000–400 cm−1. The textural properties of the sample
were characterized on a Quantachrome surface area analyzer. The pore
size distributions were calculated by Barrett-Joyner-Halenda method from
the desorption branch of the isotherm. The surface elemental composition
(Si/Al ratio) was tested using X-ray photoelectron spectroscopy (XPS;
Kratos AXIS SUPRA, Shimadzu) equipped with an Al K α excitation
source. NH3 temperature-programmed desorption (TPD)
experiments were conducted on a chemisorption AutoChem II 2920 analyzer
with a thermal conductivity detector (TCD) for analyzing surface acid
properties. 100 mg of the sample was pretreated under Helium at 550°C
for 1 h and then saturated with a flow mixed with 10% ammonia and 90%
He at 100 °C for 80 min. Afterward, the sample was kept in He flow for
30 min to remove physically adsorbed ammonia, followed by heating to 600
°C at a rate of 10 °C /min in He (30 mL/min). The amount of Brønsted and
Lewis acid of the sample was measured by Pyridine-adsorbed FT-IR spectra
using Nicolet Model 710 instrument. The sample was treated under vacuum
at 350 °C for 1 h and then cooled to 150 °C. After that, the sample was
maintained in the pyridine vapor at 150 °C for 2 h. Before analysis by
FTIR, the test system was evacuated for another 1 h to remove physically
adsorbed pyridine. The FT-IR spectra of the saturated sample were
recorded from 40 to 350 °C under vacuum for 1 h, respectively.
2.5 Catalytic test
The catalyzing C4-olefin cracking reaction was carried out to test the
catalytic activities of ZSM-5 samples in a stainless catalytic reactor.
C4-olefin was used as the reactant without further purification. The
cracking of the C4-olefin test was carried out in a continuous flow
fixed-bed system with a stainless steel tubular reactor (Φ10 mm × 530
mm) equipped with a laboratory-scale piston pump for C4-olefin input.
The reaction was conducted at 823 K under a total pressure of 0.1 MPa.
Before the experiments, the catalyst was in situ heated at a ramp rate
of 5 K min−1 to the reaction temperature in
N2 flow (100 ml min−1) and maintained
at this temperature for 2 h. The output products were analyzed online on
an HP 6890 gas chromatograph equipped with a flame ionization detector
(FID).