Diffusion and catalyst efficiency in hierarchical zeolite catalysts

Abstract

The preparation of hierarchical zeolites with reduced diffusion limitation and enhanced catalyst efficiency has become a vital focus in the field of zeolites and porous materials chemistry within the past decades. This review will focus on the diffusion and catalyst efficiency of hierarchical zeolites and industrial catalysts. The benefits of diffusion and catalyst efficiency at two levels of hierarchies (zeolitic component level and industrial catalyst level) from a chemical reaction engineering point of view will be analysed. At zeolitic component level, three types of mesopores based on the strategies applied toward enhancing the catalyst effectiveness factor are presented: (i) 'functional mesopores' (raising effective diffusivity); (ii) 'auxiliary mesopores' (decreasing diffusion length); and (iii) 'integrated mesopores' (a combination thereof). At industrial catalyst level, location and interconnectivity among the constitutive components are revealed. The hierarchical pore interconnectivity in multi-component zeolite based industrial catalysts is exemplified by fluid catalytic cracking and bi-functional hydroisomerization catalysts. The rational design of industrial zeolite catalysts at both hierarchical zeolitic component and catalyst body levels can be fully comprehended using the advanced in situ and/or operando spectroscopic, microscopic and diffraction techniques.

Keywords: advanced characterization; diffusion; effectiveness factor; hierarchical zeolite; industrial catalyst; pore connectivity.

Figure 1.

Pore size distribution of mesoporous (solid circle and hollow diamond) and conventional ZSM-5 zeolite (solid line) contained industrial catalyst bodies. Re-drawn and adapted with permission from [19].

Figure 2.

Overview of the synthesis routes toward zeolite-based hierarchical materials.

Figure 3.

Representative functional mesopores. (a) Zeolitized ordered mesoporous material. Adapted with permission from [27]. (b) Framework of ITQ-43. Adapted with permission from [33]. (c) Framework of ITQ-37. Adapted with permission from [34]. (d) Crystallization of hierarchical single-unit-cell nanosheets of zeolite MFI. Adapted with permission from [37]. (e) Single quaternary ammoniums in the template molecules are located in the straight channel and serve as a template to direct the formation of hierarchical single-crystalline mesostructured zeolite nanosheets. Adapted with permission from [44].

Figure 4.

Decreased diffusion length (shown as double-headed arrows in the figure) in the micropores within zeolite crystals via auxiliary mesopores: (a) without mesopores and (b) with auxiliary mesopore inside the zeolite crystal. Re-drawn and adapted with permission from [45].

Figure 5.

Representative auxiliary mesopores formed via post-treatment strategies. (a) Mechanism of formation of secondary mesopores via dealumination. Adapted with permission from [46]. (b) Properties of desilication method. Adapted with permission from [48]. (c) Opening the cages of FAU zeolite via fluoride-mediated post-treatment. Adapted with permission from [55].

Figure 6.

Representative auxiliary mesopores formed via in situ strategies. (a) Mesopores formed in ZSM-5 zeolite via carbon template. Adapted with permission from [57]. (b) Formation of mesopores via organosilane surfactant template. Adapted with permission from [43]. (c) Nanosized zeolite crystals with a diverse morphology and size, synthesized from colloidal suspensions, self-supported shapes, porous membranes and optical quality films. Adapted with permission from [63].

Figure 7.

Schematic representation of synthesis procedure leading to different types of integrated mesopores. Adapted with permission from [78].

Figure 8.

Examples demonstrating the influence of hierarchy at zeolitic level on catalyst effectiveness. (a) Experimental and simulated X-ray diffraction patterns for zeolite framework structures and comparison of benzyl alcohol alkylation performance of self-pillared pentasil nanosheets with pillared, 3DOm-i, commercial and conventional (0.2, 1.4 and 17 μm) MFI zeolites. Adapted with permission from [40]. (b) Method to differentiate the origin of enhanced catalyst efficiency by ion-exchange with bulky (i.e. tetra propyl ammonium TPA⁺) and/or small cations (i.e. ammonium NH₄⁺). Adapted with permission from [81]. (c) Schematic representation of various materials and their corresponding characteristic length. Adapted with permission from [82]. (d) Mass transfer advantage of hierarchical zeolites promotes methanol converting into para-methyl group in toluene methylation. Adapted with permission from [83].

Figure 9.

Schematic presentation of positron annihilation lifetime spectroscopy (PALS). Adapted with permission from [105].

Figure 10.

Scheme of hydrocracking reactions that use a bifunctional catalyst. Re-drawn and adapted with permission from [116].

Figure 11.

The non-ideal matching of the hierarchical pore structure between the zeolite and non-zeolite components leads to a decrease in the catalyst effectiveness factor. Adapted with permission from [120].

Figure 12.

Schematic of the research approach. (a) Confocal fluorescence microscopy is used to visualize distinct components of fluid catalytic cracking (FCC) catalyst particles after staining with thiophene (green) and Nile Blue A (red); confocal fluorescence microscopy image of the stained FCC industrial catalyst bodies with (b and c) and without (d and e) zeolite Y. Adapted with permission from [127].

Figure 13.

Integrated approaches for visualization of an industrial zeolite catalyst from macro- to nano-length scales. (a) Optical microscopy: macroscopic structure of a hierarchical zeolite catalyst granule; (b) X-ray micro-tomography (micro-CT): the internal structure; (c) profilometry and (d) confocal laser scanning microscopy (CLSM): external surface; (e) synchrotron radiation X-ray tomographic microscopy (SRXTM) and (f) focused ion beam scanning electron microscopy (FIB-SEM): homogeneous internal distribution of zeolite and binder phases; (g) visualized and calculated macro- and mesopore structures based on SRXTM and FIB-SEM; (h) SEM: arrangement of binder particles at the external surface of zeolite particles; (i) energy dispersive X-ray spectroscopy (EDX): elemental maps of silicon (green) and aluminium (red); (j) high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM): uniform distribution of intracrystalline mesopores within individual zeolite aggregates; (k) transmission electron microscopy (TEM) and (l) high resolution transmission electron microscopy (HRTEM): nanostructural insights of microtome cross-sections. Adapted with permission from [19].