1 Introduction
ZSM-5 zeolites with excellent catalytic performance have been widely
applied in the petrochemical industry as shape-selective catalysts
[1-4], such as methanol-to-aromatics reaction [5],
methanol-to-olefins reaction [6], fluid catalytic cracking reaction
[7], deep desulfurization [8] and so on. While
their morphological, textural and
acidic properties often affect catalytic performance, it remains a
research focus to control the physical and chemical properties of
zeolites.
In an attempt to improve the performance of zeolites, two routes have
been proposed: (1) Tailoring acid sites in zeolites [9]. Various
approaches have been developed to improve acid sites of zeolites
including dealumiumazation [10,11], SiO2deposition
[12], the load of superacid [13], and so on. Dealuminization of
zeolites prompts the aluminum species in the lattice to migrate out with
high-temperature treatment, resulting in a transformation from framework
aluminum to extra-framework aluminum. The formation of extra-framework
aluminum is beneficial to increasing acid amount of Lewis acidity, and
consequently, enhancing the synergistic effect of Brønsted and Lewis
acidity [11]. The vapor
deposition
of SiO2 could facilitate the formation of framework
aluminum in zeolites, resulting in an increase of the Brønsted acidity
[12]. The load of superacid enhances both Brønsted acidity and Lewis
acidity of zeolites [13]. However, the secondary high-temperature
and acid treatment in the above processes often destroyed the pore
structure and hydrothermal stability of zeolites [14]. (2)
Decreasing particle sizes and donating mesopores or macropores to
zeolites by adding templates in the reaction system or enhancing
micromixing during the hydrothermal synthesis process [15]. Kim et
al. [16] have prepared nanosized ZSM-5 zeolites with particle sizes
in the range of 12-100 nm via confined-space synthesis using
colloid-imprinted carbons as templates. Narayanan et al. [17] have
synthesized microporous ZSM-5 zeolites with an average size of 340 nm by
using nonionic surfactants (Triton X-100), and Dong et al. [18] have
prepared large macropores (average pore-diameter: 1.5 μm) zeolite
monoliths by the transformation of mesoporous silica spheres. On the
other hand, enhancing micromixing in the nucleation/crystal growth
period was reported to be a useful method for the fast synthesis of
hierarchical zeolites [19]. During the nucleation process of
zeolites, the uniformity of various reaction species affects the
assembly of structural units, resulting in the differences of the size,
amount, and composition of the crystal nucleus. According to the Ostwald
ripening growth mechanism, a large crystal nucleus will grow up based on
the disappearance of the small crystal nucleus. Thus, the morphological,
textural, and acidic properties of the final crystal are closely related
to the uniformity of the initial crystal nucleus.
In the traditional stirring tank reactor (STR), it’s difficult to obtain
a homogeneous nucleation
environment and uniformity of initial crystal nucleus [20],
particularly in high viscosities and large solution-volume systems
[21, 22]. A rotating packed bed (RPB) as an efficient process
intensification device can provide a high-gravity environment with
considerable shear force, leading to the excellent micromixing effect
[23-26]. During the ZSM-5 synthesis process, RPB can create reaction
precursors with uniform concentration and supersaturation, resulting in
uniform nucleation and consequently the acceleration of crystallization
of zeolites [19]. Also, enhancing micromixing to improve acid
strength and acid quantity by controlling Al contents during the
nucleation/growth process of zeolite has not been extensively studied
[27].
In this study, four operating processes, including RPB premix - dynamic
crystallization, RPB premix - static crystallization, STR premix -
dynamic crystallization, and STR premix - static crystallization, were
carried out to investigate the effects of enhancing micromixing in the
zeolite synthetic pathway on nucleation/crystal growth process and
material properties of ZSM-5 zeolites. The nucleation/crystal growth
mechanism of ZSM-5 zeolites was studied based on experimental data
characterized by Fourier transform infrared (FTIR) spectrophotometry and
powder X-ray diffraction (XRD). And the catalytic cracking reaction of
C4 olefins was adopted for evaluating the catalytic performance of the
prepared ZSM-5 zeolites obtained by different operating processes.