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

Abstract

'Hierarchy' is a property which can be attributed to a manifold of different immaterial systems, such as ideas, items and organisations or material ones like biological systems within living organisms or artificial, man-made constructions. The property 'hierarchy' is mainly characterised by a certain ordering of individual elements relative to each other, often in combination with a certain degree of branching.Especially mass-flow related systems in the natural environment feature special hierarchically branched patterns. This review is a survey into the world of hierarchical systems with special focus on hierarchically porous zeolite materials. A classification of hierarchical porosity is proposed based on the flow distribution pattern within the respective pore systems. In addition, this review might serve as a toolbox providing several synthetic and post-synthetic strategies to prepare zeolitic or zeolite containing material with tailored hierarchical porosity. Very often, such strategies with their underlying principles were developed for improving the performance of the final materials in different technical applications like adsorptive or catalytic processes. In the present review, besides on the hierarchically porous allzeolite material, special focus is laid on the preparation of zeolitic composite materials with hierarchical porosity capable to face the demands of industrial application.

Sol‐Gel and Porous Glass‐Based Silica Monoliths with Hierarchical Pore Structure for Solid‐Liquid Catalysis

“…By careful control of chemical composition, pore and surface structure, as well as physical properties down to the nanometer scale, it is possible to produce materials with tailored optical, mechanical, electrical, or hydrodynamic characteristics and increase selectivity in adsorption, separation, and catalysis. The introduction of hierarchical design aspects in the interconnected pore network as well as the independent manipulation of the contributing sets of pores [17] allows the preparation of highly permeable media and the optimization of mass transport to and from the active surface sites. Monoliths exhibit more degrees of freedom in their design than conventional particulate fixed-bed adsorbers, separators, and reactors from the local pore scale up to the bulk material scale, allowing them to combine adsorption/reaction selectivity, mobile phase velocity (analysis speed, contact time), heat and mass transfer (efficiency, residence time distribution), as well as specific surface area (loading capacity) in a unique manner.…”

Catalytic test reactions for the evaluation of hierarchical zeolites

“…Thus, hierarchical zeolites can have micropores and mesopores (also known as micro/ mesoporous zeolites or mesoporous zeolites) or micropores and macropores (also known as micro/macroporous zeolites or macroporous zeolites) or micropores, mesopores and macropores. However, in hierarchical catalysts these different pore systems must be interconnected in order to classify the overall pore system as hierarchical and gain the full benefit in catalysis (for a good definition of hierarchy see the review of Schwieger et al 3 or Perez-Ramirez et al 6 ).…”

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.…”