Superior Mass Transfer Properties of Technical Zeolite Bodies with Hierarchical Porosity

Abstract

Mass transfer in zeolite crystals can be enhanced by the introduction of a hierarchical network of auxiliary mesopores. To fully exploit pore engineering in the design of more efficient industrial catalysts, the benefit needs to be demonstrated over technically relevant forms. Here, the influence of shaping on the adsorption and diffusion properties of hierarchical ZSM‐5 is assessed by studying the gravimetric uptake of 2,2‐dimethylbutane over powders and millimeter‐sized bodies. Formed by extrusion or granulation with clay binders, the latter display a complex trimodal network of micro‐, meso‐, and macropores. The enhanced intracrystalline diffusivity due to the interconnected mesopores is preserved in the macroscopic bodies, independent of the shaping method or binder applied. Furthermore the superior overall diffusivity is retained in the hierarchical bodies compared to their conventional (purely microporous) counterparts, despite the significant extracrystalline resistance to mass transfer. The connective participation of mesopores, leading to a 6 times improved effective diffusivity in hierarchical with respect to conventional zeolite powders, is revealed by the distinct dependence on the adsorbate concentration and the relationship with the mesopore surface area. Analysis of the thermodynamic parameters derived from the adsorption isotherm proves a sensitive method to detect binder‐zeolite interactions induced upon shaping.

Effects of Binders on the Performance of Shaped Hierarchical MFI Zeolites in Methanol-to-Hydrocarbons

“…It is known that the mass transfer within shaped zeolite bodies can be dominated by diffusion in the macropores, representing up to 80% of the global mass transfer, particularly at high temperature. 35,36 In fact, a clear dependence on V macro was observed on comparison of the D global determined at 358 K and 0.1 mbar for the conventional and hierarchical ZSM-5 extrudates prepared with different binders (Figure 3). …”

“…35 This probe molecule is ideal for ZSM-5 zeolites because it exhibits a kinetic diameter (0.61 nm) similar to the diameter of an MFI micropore (0.56 nm), decreasing the rate of uptake. In that study, an almost four times higher global diffusivity (D global ) was evidenced over the hierarchical with respect to the conventional ZSM-5 powder at 358 K and 0.1 mbar, which was correlated with the additional intracrystalline mesoporosity.…”

Heterogeneous Diffusion of Alkanes in the Hierarchical Metal–Organic Framework NU-1000

“…It has been previously suggested that the interconnectedness of mesopores and micropores 19 is essential to increasing diffusion in nanoporous materials 16, 44,45 . Figure 10 illustrates the relationship between the overall mean squared displacements and those in the microchannels and mesochannels at various temperatures.…”

Mesoporous ZSM-5 Zeolites in Acid Catalysis: Top-Down vs. Bottom-Up Approach

“…Indeed, hierarchically porous zeolites offer enhanced accessibility and transport of molecules to and from the active sites [5][6][7][8], therefore enhancing catalyst effectiveness [1][2][3][4]9]. It is of utmost importance to be able to characterise and to guarantee good pore connectivity in order to take benefits from those advantages [10][11][12][13].…”