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.