Mesopore quality determines the lifetime of hierarchically structured zeolite catalysts

Abstract

Deactivation due to coking limits the lifetime of zeolite catalysts in the production of chemicals and fuels. Superior performance can be achieved through hierarchically structuring the zeolite porosity, yet no relation has been established between the mesopore architecture and the catalyst lifetime. Here we introduce a top-down demetallation strategy to locate mesopores in different regions of MFI-type crystals with identical bulk porous and acidic properties. In contrast, well-established bottom-up strategies as carbon templating and seed silanization fail to yield materials with matching characteristics. Advanced characterization tools capable of accurately discriminating the mesopore size, distribution and connectivity are applied to corroborate the concept of mesopore quality. Positron annihilation lifetime spectroscopy proves powerful to quantify the global connectivity of the intracrystalline pore network, which, as demonstrated in the conversions of methanol or of propanal to hydrocarbons, is closely linked to the lifetime of zeolite catalysts. The findings emphasize the need to aptly tailor hierarchical materials for maximal catalytic advantage.

Structure–performance descriptors and the role of Lewis acidity in the methanol-to-propylene process

“…Preparation of M4 involved extraction of framework Al by steaming, leading to the formation of "inefficient" occluded mesopores barely impacting intracrystalline diffusion. 31,32,33 Milina et al 6 have…”

“…3 Zeolite topology is an important 4 yet not the sole parameter in defining catalyst performance in MTO. 5 Accessibility, 6 strength, distribution, 7 amount 8 and nature 9 of acid sites are also of high importance.…”

Hierarchy concepts: classification and preparation strategies for zeolite containing materials with hierarchical porosity

“…Very often it is still difficult to characterise the interconnectivity of the existing -at least two different -pore systems with the different length scales precisely. Few recent papers have shown the progress in this respect in specified methods such as 3D reconstruction from data by STEM-tomography 55,196 or by face-scanning techniques, 197 spectroscopic techniques such as positron annihilation lifetime spectroscopy 198 or continuously improved conventional methods like physical sorption. 199 Alternatively, molecular uptake and release studies have been performed to demonstrate the role of pore interconnectivity and the enhancement of mass transport properties of zeolites with hierarchical porosity.…”

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].…”