Boosting molecular diffusion following the generalized Murray's Law by constructing hierarchical zeolites for maximized catalytic activity

Abstract

Diffusion is an extremely critical step in zeolite catalysis that determines the catalytic performance, in particular for the conversion of bulky molecules. Introducing interconnected mesopores and macropores into a single microporous zeolite with the rationalized pore size at each level is an effective strategy to suppress the diffusion limitations, but remains highly challenging due to the lack of rational design principles. Herein, we demonstrate the first example of boosting molecular diffusion by constructing hierarchical Murray zeolites with a highly ordered and fully interconnected macro-meso-microporous structure on the basis of the generalized Murray's Law. Such a hierarchical Murray zeolite with a refined quantitative relationship between the pore size at each length scale exhibited 9 and 5 times higher effective diffusion rates, leading to 2.5 and 1.5 times higher catalytic performance in the bulky 1,3,5-triisopropylbenzene cracking reaction than those of microporous ZSM-5 and ZSM-5 nanocrystals, respectively. The concept of hierarchical Murray zeolites with optimized structural features and their design principles could be applied to other catalytic reactions for maximized performance.

Keywords: catalytic cracking; generalized Murray's Law; hierarchical Murray structure; ordered porous hierarchy; zeolites.

Figure 1.

(a) Schematic of the synthesis of hierarchically ordered macro–meso–microporous zeolite ZSM-5 (OMMM–ZSM-5) assembled by zeolite nanocrystals. Steps: (1) self-assembly, (2) carbonization, (3) hydrothermal crystallization and (4) template removal by calcination. (i) Monodispersed polystyrene (PS) nanospheres and SiO₂ nanospheres in sucrose solution, (ii) three-dimensionally periodic close-packed PS/SiO₂ spheres in sucrose solution, (iii) PS/SiO₂/carbon matrix, (iv) PS/ZSM-5 zeolite nanocrystals/carbon matrix and (v) OMMM–ZSM-5 by template removal via calcination. (b–j) Characterizations of OMMM–ZSM-5(400), as a representative sample. (b–d) SEM images. (e) TEM image and ED pattern (inset). (f) TEM image of enlarged area outlined in (e) by the green box. (g) HRTEM image of enlarged area outlined in (f). (h) SAXS and (i) WAXS pattern. (j) N₂ adsorption–desorption isotherms and micropore-size, mesopore-size distribution (inset).

Figure 2.

Crystallization process of OMMM–ZSM-5(400). TEM images of OMMM–ZSM-5(400) obtained at (a) 0 h, (b) 8 h, (c) 16 h and (d) 24 h. (e) SAXS data, (f) WAXS data, (g) ²⁹Si NMR spectra (the black plots are the fitted data), (h) ²⁷Al NMR spectra and (i) nitrogen adsorption isotherms, (j) micropore-size distributions and (k) mesopore-size distributions.

Figure 3.

(a) Laser-hyperpolarized ¹²⁹Xe NMR spectra with temperature varied from 273 to 173 K of OMMM–ZSM-5(400). (b and c) The macroscopic diffusion measurement. Adsorption and diffusion performance of 1,3,5-TMB within ZSM-5 samples. (b) The isothermal adsorption curve. (c) Normalized uptake (Q_t/Q₀) profiles of 1,3,5-trimethylbenzene over different catalysts. (d)–(h) The microscopic diffusion measurement. (d)–(f) ¹H PFG NMR attenuation curves for methane with loading of (d) 2.31, (e) 3.7 and (f) 5.13 mmol·g⁻¹ in OMMM–ZSM-5(400) for different observation/diffusion time t measures at 298 K. (g) Intracrystalline diffusivities D_f-intra, (h) the mean squared value of the displacements 〈 r² 〉 ^1/2 and (i) the relative number of methane molecules still within OMMM–ZSM-5(400) p_intra(t) of methane with different loadings in OMMM–ZSM-5(400) determined from PFG NMR attenuation curves in (d–f).

Figure 4.

Catalytic performance of various zeolite ZSM-5 catalysts in the cracking reaction of 1,3,5-TIPB (a) catalytic activities (1,3,5-TIPB conversions) at different reaction times. (b–e) The product distributions in the 1,3,5-TIPB cracking using (b) C–ZSM-5, (c) Nano–ZSM-5, (d) OMMM–ZSM-5(200), (e) OMMM–ZSM-5(400) and (f) OMMM–ZSM-5(600).