Synthesis strategies in the search for hierarchical zeolites

Abstract

Great interest has arisen in the past years in the development of hierarchical zeolites, having at least two levels of porosities. Hierarchical zeolites show an enhanced accessibility, leading to improved catalytic activity in reactions suffering from steric and/or diffusional limitations. Moreover, the secondary porosity offers an ideal space for the deposition of additional active phases and for functionalization with organic moieties. However, the secondary surface represents a discontinuity of the crystalline framework, with a low connectivity and a high concentration of silanols. Consequently, hierarchical zeolites exhibit a less "zeolitic behaviour" than conventional ones in terms of acidity, hydrophobic/hydrophilic character, confinement effects, shape-selectivity and hydrothermal stability. Nevertheless, this secondary surface is far from being amorphous, which provides hierarchical zeolites with a set of novel features. A wide variety of innovative strategies have been developed for generating a secondary porosity in zeolites. In the present review, the different synthetic routes leading to hierarchical zeolites have been classified into five categories: removal of framework atoms, surfactant-assisted procedures, hard-templating, zeolitization of preformed solids and organosilane-based methods. Significant advances have been achieved recently in several of these alternatives. These include desilication, due to its versatility, dual templating with polyquaternary ammonium surfactants and framework reorganization by treatment with surfactant-containing basic solutions. In the last two cases, the materials so prepared show both mesoscopic ordering and zeolitic lattice planes. Likewise, interesting results have been obtained with the incorporation of different types of organosilanes into the zeolite crystallization gels, taking advantage of their high affinity for silicate and aluminosilicate species. Crystallization of organofunctionalized species favours the formation of organic-inorganic composites that, upon calcination, are transformed into hierarchical zeolites. However, in spite of this impressive progress in novel strategies for the preparation of hierarchical zeolites, significant challenges are still ahead. The overall one is the development of methods that are versatile in terms of zeolite structures and compositions, capable of tuning the secondary porosity properties, and being scaled up in a cost-effective way. Recent works have demonstrated that it is possible to scale-up easily the synthesis of hierarchical zeolites by desilication. Economic aspects may become a significant bottleneck for the commercial application of hierarchical zeolites since most of the synthesis strategies so far developed imply the use of more expensive procedures and reagents compared to conventional zeolites. Nevertheless, the use of hierarchical zeolites as efficient catalysts for the production of high value-added compounds could greatly compensate these increased manufacturing costs.

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

“…8, these strategies can be further subdivided into primary methods, in which all components (including the surfactant) are added at the beginning of a one-step synthesis procedure and secondary methods, which are characterised by a two-steps procedure, where all components except the surfactant are added in the first step, while the surfactant is added in the last step prior to the hydrothermal synthesis. 72,75 In primary methods, surfactants assist the assembly of purely molecular species (framework building units) to form zeolite crystals with additional intra-or intercrystalline porosity. Mostly, the surfactant fulfils two functions.…”

“…The principle behind this method is based on the selective removal or extraction of framework atoms (Al, Si, B or Ti) from crystalline zeolitic materials through acid or alkali treatment, steaming, radiation or a combination of them. 72 Acid treatment at temperatures between 50 and 100 1C results in dealumination. 67 This treatment creates voids, which consequently serve as intracrystalline meso-or macropores.…”

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

“…bottomup and top-down approaches and zeolite recrystallization) have been explored for the formation of hierarchical zeolites, which contain interconnected micro-and mesopores, in prospect of possible industrial applications. [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.…”

One-pot synthesis of nano-crystalline MCM-22

“…A number of reviews categorize and discuss these approaches [30][31][32]. Broadly speaking, two strategies can be followed, namely bottom-up and top-down approaches.…”

“…In top-down approaches, mesopores are introduced after the zeolite has been synthesized, and it is usually achieved by extracting either Al or Si atoms from the zeolite framework. Common methods for demetallation include steaming [33][34][35][36][37][38][39][40], acid-leaching [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46] and base treatment [47,48]. In bottom-up approaches, the mesopores are introduced in the framework during the synthesis.…”