Design of hierarchical zeolite catalysts by desilication

Abstract

Hierarchical (or mesoporous) zeolites have received an ever-increasing attention due their improved performance in catalysed reactions with respect to conventional (purely microporous) zeolites. Desilication in alkaline media has become a widely applied preparation method to tailor these modified zeolites, due to an optimal combination of efficiency and simplicity. This Minireview presents recent developments that have expanded its general understanding and turned this top-down treatment highly versatile, controllable, and scalable. Design aspects of mesoporous zeolites for catalytic applications are emphasised, encircling the establishment of synthesis-property-function relationships. Alkaline treatment is a key step in strategic combinations with other post-synthesis modifications towards superior zeolite catalysts. The outlook of the field, pinpointing present needs and short-term priorities, is discussed. 65 increased zeolite performance in adsorption and catalysis was first patented in the 1960s. 22 It was claimed that alkaline-treated mordenite displayed preserved crystallinity and a significantly increased benzene adsorption capacity. Moreover, catalytic evaluation in gas-oil hydrocracking revealed a 3 times higher 70 conversion for an alkaline-treated Pd/mordenite than for the untreated catalyst. Already then, Young speculated that the improved performance of the modified material could be due to better access to the micropores. In the 70s, other patents claimed superior properties of alkaline-treated zeolites as olefin 75 adsorbents 23 and molecular sieves, 24 but the obtained benefits remained poorly understood from a scientific ground. Remarkably, open literature concerning zeolite modification in 65 65

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

“…It involves the selective extraction of silicon from the zeolite framework via treatment with diluted alkaline solutions at temperatures between 50 and 80 1C. 90,91 This method can be used to introduce intracrystalline mesopores with high degree of interconnectivity in zeolite crystals of different framework types. 36,[92][93][94][95][96] In contrast to dealumination, desilication does not significantly affect the acidity and the crystalline structure of zeolites.…”

“…The demetalation of pristine zeolite crystals has already been discussed in Section 3.4 and was found to be an efficient post-synthetic route for the preparation of hierarchical all-zeolite materials by removing parts of the zeolitic network. Moreover, Perez-Ramirez and co-workers 90,119 recently reported that this route can even be used as a 'postcompaction' treatment for the introduction of an additional pore system into already shaped zeolitic bodies (granules). Technically, the demetalation of already shaped zeolitic material has some advantages as compared to the demetalation of the initial fine zeolite powder, such as the easier handling of granular systems.…”

Creating Accessible Active Sites in Hierarchical MFI Zeolites for Low‐Temperature Acid Catalysis

“…[17] The hierarchical zeolites have shown enhanced catalytic performances with respect to the microporous analogues in diffusion-constrained reactions or those carried out in the liquid-phase at low temperatures. [11,18] In the present study, a post-synthetic desilication strategy in alkali media was followed to achieve hierarchical H-ZSM-5 ( Fig.1), starting from parent MFI zeolites with different Si/Al ratio. H-ZSM-5 is a wellknown microporous zeolite with a cross-section of straight channels (5.1 Å x 5.5 Å) intersected by zig-zag channels (5.3 Å x 5.6 Å) and it is widely used as a heterogeneous acid catalyst in a large number of reactions.…”

“…[6][7][8][9][10] Among the different synthetic procedures that can be followed to produce hierarchical zeolites, the top-down routes based on post-synthetic treatments in acid or alkali solutions are widely used, owing to favourable HSE (health-safety-environment) considerations and low production costs. [6,11,12] This method partially extracts the framework constituents (Al or Si) and depending on type of treatment, it can be classified into dealumination or desilication. In particular, under optimized conditions (acid or alkali sources, concentration, temperature and contact time), desilication in alkali media can be extensively used to create mesopores in zeolites, whilst preserving the crystalline structure almost entirely with a decreasing of the Si/Al ratio.…”

Active Site Elucidation in Heterogeneous Catalysis via In Situ X-Ray Spectroscopies

“…[12] The absence of a unified theory of heterogeneous catalysis has largely reflected difficulties in (i) synthesizing well defined, high surface area materials possessing a narrow distribution of coordination environments and oxidation states, and (ii) identifying the active surface species participating in the catalytic cycle. While advances in top-down [13][14][15][16] and bottom-up [17][18][19][20][21][22][23] engineering of nanocrystals and nanoporous solids are helping to address the former, a limited ability to directly observe surface reactions, coupled with reaction-induced restructuring, [24] has hampered past efforts to fingerprint and quantify structure-function relationships even in simple gas phase heterogeneously catalyzed processes such as CO [25,26] /alcohol oxidation [27] and alkene reduction. [28] This highlight paper illustrates how recent technical breakthroughs in X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES), are enabling researchers to visualize catalytic processes under in situ or operando (true working) conditions, and use the resulting insight to nanoengineer improved heterogeneous catalysts.…”