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2018 | Book

Insights into the Chemistry of Organic Structure-Directing Agents in the Synthesis of Zeolitic Materials

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About this book

This edited volume focuses on the host-guest chemistry of organic molecules and inorganic systems during synthesis (structure-direction). Organic molecules have been used for many years in the synthesis of zeolitic nanoporous frameworks. The addition of these organic molecules to the zeolite synthesis mixtures provokes a particular ordering of the inorganic units around them that directs the crystallization pathway towards a particular framework type; hence they are called structure-directing agents. Their use has allowed the discovery of an extremely large number of new zeolite frameworks and compositions.

This volume covers the main aspects of the use of organic molecules as structure-directing agents for the synthesis of zeolites, including first an introduction of the main concepts, then two chapters covering state-of-the-art techniques currently used to understand the structure-directing phenomenon (location of molecules by XRD and molecular modeling techniques). The most recent trends in the types of organic molecules used as structure-directing agents are also presented, including the use of metal-complexes, the use of non-ammonium-based molecules (mainly phosphorus-based compounds) and the role of supramolecular chemistry in designing new large organic structure-directing agents produced by self-aggregation. In addition the volume explores the latest research attempting to transfer the asymmetric nature of organic chiral molecules used as structure-directing agents to the zeolite lattice to produce chiral enantioselective frameworks, one of the biggest challenges today in materials chemistry.

This volume has interdisciplinary appeal and will engage scholars from the zeolite community with a general interest in microporous materials, which involves not only zeolite scientists, but also researchers working on metal-organic framework materials. The concepts covered will also be of interest for researchers working on the application of materials after encapsulation of molecules of interest in post-synthetic treatments. Further the work explores the main aspects of host-guest chemistry in hybrid organo-inorganic templated materials, which covers all types of materials where organic molecules are used as templates and are confined within framework-structured inorganic materials (intercalation compounds). Therefore the volume is also relevant to the wider materials chemistry community.

Table of Contents

Frontmatter
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 structure-directing 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 drive 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 (H2O/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 imidazolium-based organic cations as SDAs because of their current relevance in the synthesis of new zeolite materials.
Luis Gómez-Hortigüela, Miguel Á. Camblor
Location of Organic Structure-Directing Agents in Zeolites Using Diffraction Techniques
Abstract
In this chapter, we delve into the X-ray diffraction techniques that can be used to address the question as to where the organic structure-directing agents (OSDAs) are located in the pores of a zeolite framework structure and give an overview of some of the practical issues involved. By examining the results of such investigations, we attempt to establish whether the OSDAs are really disordered, as is often claimed, or if it is the methods we use that give this impression. In fact, the non-framework species in the channels of a zeolite appear to be arranged quite logically in a chemically sensible manner. In most cases, the OSDA within the pores can be described well as a superposition of just a few discrete, symmetry-related positions, provided the discrepancies between the OSDA and framework symmetries can be resolved. On the basis of some selected examples, we show that their arrangements can be extracted from experimental data using a systematic strategy and sometimes supplementary information.
Stef Smeets, Lynne B. McCusker
Molecular Modelling of Structure Direction Phenomena
Abstract
Organic structure-directing agents (OSDAs) are widely used in the synthesis of zeolitic materials. Molecular modelling methods are playing a key part in helping to establish the role of the OSDA in the synthesis process. Moreover, modelling is increasingly being used to design and screen new OSDAs for specific targets. This review aims to provide an overview of the methods used to investigate the relationship between OSDAs and their zeolitic products and to provide a series of examples to highlight the important contribution that modelling is making in this field.
Alessandro Turrina, Paul A. Cox
Beyond Nitrogen OSDAs
Abstract
The use of organic structure-directing agents (OSDAs) is perhaps the most important factor to be considered for the synthesis of zeolitic materials. Several OSDAs had been used along the last 70 years, especially ammonium organic cations, letting the synthesis of a large number of materials. But besides ammonium cations, organic cations with different chemical natures had also been used, which resulted in the synthesis of very interesting zeolitic materials. This review includes most of the non-ammonium cations used up to date, namely, phosphorous cations, sulfonium cations, crown macrocycles and metal complexes, but also when N-containing OSDAs play a different role than in conventional zeolite syntheses, such as proton sponges, self-assembled compounds or ionic liquids.
Fernando Rey, Jorge Simancas
Role of Supramolecular Chemistry During Templating Phenomenon in Zeolite Synthesis
Abstract
In the last years, there is an increasing interest in the use of organic molecules with the appropriate functionalities to interact with other organic molecules and/or inorganic cations through non-covalent supramolecular interactions, as very specific organic structure-directing agents (OSDAs) for zeolite synthesis. These assembled molecular subunits allow directing the crystallization of zeolite structures with particular physico-chemical properties, such as novel framework topologies, crystal size, chemical compositions, acid-base properties, or metal incorporation, which otherwise would not be achieved using “classical” amine or ammonium-based OSDA molecules. Along the present chapter, different zeolite synthesis strategies employing assembled molecular subunits will be presented, including the use of crown ether-based supramolecular templates, metal-organic complexes, aromatic molecules able to interact through π–π interactions, or supramolecular assembled amphiphilic molecules, among others. The most relevant results described in the literature using these supramolecular-based templating routes will be discussed, together with the current challenges and perspectives.
Cecilia Paris, Manuel Moliner
Metal Complexes as Structure-Directing Agents for Zeolites and Related Microporous Materials
Abstract
Metal complexes can act as structure-directing agents (SDAs) for zeolites and zeotypes, either alone or together with additional SDAs in dual-templating approaches. Such complexes include organometallic cobaltocenium ions, alkali metal crown ether complexes, first-row transition-metal (Fe, Co, Ni, Cu) polyamines and thiol-complexed second- and third-row transition metals (Pd, Pt). Their inclusion has been demonstrated in some cases by crystallographic methods but more commonly by spectroscopy (UV-visible, X-ray absorption, Mössbauer). The unique feature of this class of template is that they can not only direct crystallisation but also give solids with homogeneously distributed metal cations or metal oxide species upon calcination, precluding the need for an additional post-synthesis modification step. Materials prepared via this ‘one-pot’ synthetic route have been shown to give shape-selective catalysts for reactions such as the selective catalytic reduction of NO x with ammonia and the hydrogenation, dehydration and oxidative dehydrogenation of small hydrocarbons and oxygenates.
Abigail E. Watts, Alessandro Turrina, Paul A. Wright
Chiral Organic Structure-Directing Agents
Abstract
Chirality is crucial for life. The preparation of enantiopure chiral compounds is highly desirable in the chemical industry, especially in the pharmaceutical sector. In this context, the design of chiral solids able to discriminate between enantiomers of chiral compounds, either during adsorption or asymmetric catalytic processes, is one of the greatest challenges nowadays in chemical research. Zeolite-type materials represent ideal candidates to achieve enantioselective chiral solids since they could combine their high stability, surface area, and shape-selectivity with a potential enantioselectivity that could be enhanced by the confinement effect. Despite the occurrence of chiral zeolite frameworks and the strong interest in preparing these chiral solids, very little success has been met in preparing these in homochiral form. The main strategy to induce chirality in zeolite materials has been the use of chiral structure-directing agents, in an attempt to transfer their chiral feature into the nascent zeolite structure. However, although many chiral organic species have directed the crystallization of zeolite frameworks, some of them even being chiral, there is only one unique very recent example of success in transferring the chirality from the organic structure-directing agent into an enantioenriched chiral zeolite material. Chiral coordination compounds have been very successful in transferring their chirality onto inorganic frameworks through the development of extensive H-bond host–guest interactions, but these chiral materials usually collapse upon removal of the guest species. In this chapter we report the different types of chiral molecules, both organic and organometallic compounds, used so far as structure-directing agents in an attempt to promote the crystallization of homochiral zeolites; we analyze in detail the possible reasons for the general failure in transferring their chirality, and we propose approaches to prepare known chiral zeolite frameworks in homochiral form. Furthermore, we also review a different approach we have followed in our group in order to induce chirality in zeolite materials, consisting in the development of chiral spatial distributions of dopants embedded in otherwise achiral zeolite frameworks.
Luis Gómez-Hortigüela, Beatriz Bernardo-Maestro
Backmatter
Metadata
Title
Insights into the Chemistry of Organic Structure-Directing Agents in the Synthesis of Zeolitic Materials
Editor
Dr. Luis Gómez-Hortigüela
Copyright Year
2018
Electronic ISBN
978-3-319-74289-2
Print ISBN
978-3-319-74288-5
DOI
https://doi.org/10.1007/978-3-319-74289-2

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