Introduction to the Zeolite Structure-Directing Phenomenon by Organic Species: General Aspects

Abstract

During the last years, a tremendous progress has been achieved in the application of new zeolite materials in many different sectors through different pioneering innovations in the field of zeolite synthesis. At the very core of the production of these new zeolite materials lies the use of organic species as structuredirecting agents (SDA), which has been recognized as the most important factor to determine the zeolite product rendered after the crystallization process. These organic species organize the inorganic zeolitic units and drives the crystallization pathway towards the production of particular zeolite framework types. This structure-direction phenomenon frequently works in combination with several other factors related to the chemical composition of the synthesis gels, mainly use of fluoride, concentration (H 2 O/T ratio) and presence of different heteroatoms, which are also relevant for the crystallization of particular zeolite materials. Several properties determine the structure-directing effect of these organic species, especially their molecular size and shape, hydrophobicity, rigidity vs flexibility, and hydrothermal stability. The properties of the zeolitic materials synthesized can be tuned up to a certain point through the use of rationally-selected organic species with particular physico-chemical features as SDA. In this introductory chapter, we briefly review the history of the use of organic cations as SDAs, and give the fundaments of the different aspects related to this structure-direction phenomenon and factors affecting it, explaining the main properties of SDAs, providing some examples of recent uses and trends of organic SDAs, as well as the host-guest chemistry involved. In addition, we pay particular attention to the use of imidazoliumbased organic cations as SDAs because of their current relevance in the synthesis of new zeolite materials.

Synthesis of a germanosilicate zeolite HPM-12 using a short imidazolium-based dication: structure-direction by charge-to-charge distance matching

“…Since the last two decades we have been exploring the structure-direction ability of non-quaternary imidazolium cations in fluoride media. 9 In the case of imidazolium-based dications we have found consistent evidence of length-dependent structure-direction effects. For instance, dications based on 1-methylimidazolium tend to give zeolite TON (10MR) as the default phase, and only for n = 4, 8 and 9, MFI appears under highly concentrated conditions.…”

Organic Mesopore Generating Agents (OMeGAs) for Hierarchical Zeolites: Combining Functions on Multiple Scales

“…Such interactions occur early during the formation of the zeolite nuclei. The success of OSDA for particular zeolite framework depends on several factors including their size, shape, hydrophobicity/hydrophilicity, hydrothermal stability, and their rigidity/flexibility …”

Conformational sieving effect of organic structure-directing agents during the synthesis of zeolitic materials

“…In particular, we have found that the presence of such H-bond donor and acceptor groups within the molecule provides a very rich conformational space, which in turn alters the resulting supramolecular chemistry and the structure-directing behavior of these chiral SDAs [35,37,[39][40][41][42]. Moreover, we have shown that 13 C NMR spectroscopy can be very useful for monitoring the conformational space of these organic species upon confinement in nanoporous spaces [35], revealing stabilization of particular conformations under confinement that are unstable in open spaces (in solution). On the other hand, the occurrence of diastereoisomerism can also notably alter the outcome of a crystallization route [43][44][45].…”

“…Hence, in order to gain advantage of confinement effects in diverse applications it is crucial to adequately control the zeolite porous architecture that is to be used. In this context, the main control that zeolite scientists have gained over the outcome (in terms of topological framework) of a zeolite hydrothermal synthesis is through the use of the so-called organic structure-directing agents (SDAs) [9][10][11][12][13]. These are typically ammonium-based organic cations that are added to the synthesis gels, and provide a control on the crystallizing zeolite framework by host-guest size-and shape-relationships.…”