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).