Emerging Analytical Methods to Characterize Zeolite-Based Materials

Zeolites and zeolitic materials are, through their use in numerous conventional and sustainable applications, very important to our daily lives, including to foster the necessary transition to a more circular society. The characterization of zeolite-based materials has a tremendous history and a great number of applications and properties of these materials have been discovered in the past decades. This review focuses on recently developed novel as well as more conventional techniques applied with the aim of better understanding zeolite-based materials. Recently explored analytical methods, e.g. atom probe tomography, scanning transmission X-ray microscopy, confocal fluorescence microscopy and photo-induced force microscopy, are discussed on their important contributions to the better understanding of zeolites as they mainly focus on the micro- to nanoscale chemical imaging and the revelation of structure–composition–performance relationships. Some other techniques have a long and established history, e.g. nuclear magnetic resonance, infrared, neutron scattering, electron microscopy and X-ray diffraction techniques, and have gone through increasing developments allowing the techniques to discover new and important features in zeolite-based materials. Additional to the increasing application of these methods, multiple techniques are nowadays used to study zeolites under working conditions (i.e. the in situ/operando mode of analysis) providing new insights in reaction and deactivation mechanisms.

Prospects and challenges for autonomous catalyst discovery viewed from an experimental perspective

Autonomous catalysis research requires elaborate integration of operando experiments into automated workflows. Suitable experimental data for analysis by artificial intelligence can be measured more readily according to standard operating procedures.

Insights on Luminescent Micro- and Nanospheres of Infinite Coordination Polymers

Coordination polymers have been extensively studied in recent years. Some of these materials can exhibit several properties such as permanent porosity, high surface area, thermostability and light emission, as well as open sites for chemical functionalization. Concerning the fact that this kind of compounds are usually solids, the size and morphology of the particles are important parameters when an application is desired. Inside this context, there is a subclass of coordination polymers, named infinite coordination polymers (ICPs), which auto-organize as micro- or nanoparticles with low crystallinity. Specifically, the particles exhibiting spherical shapes and reduced sizes can be better dispersed, enter cells much easier than bulk crystals and be converted to inorganic materials by topotactic transformation. Luminescent ICPs, in particular, can find applications in several areas, such as sensing probes, light-emitting devices and bioimaging. In this review, we present the state-of-the-art of ICP-based spherical particles, including the growth mechanisms, some applications for luminescent ICPs and the challenges to overcome in future commercial usage of these materials.

In situ imaging of two-dimensional surface growth reveals the prevalence and role of defects in zeolite crystallization

Zeolite crystallization predominantly occurs by nonclassical pathways involving the attachment of complex (alumino)silicate precursors to crystal surfaces, yet recurrent images of fully crystalline materials with layered surfaces are evidence of classical growth by molecule attachment. Here we use in situ atomic force microscopy to monitor three distinct mechanisms of two-dimensional (2D) growth of zeolite A where we show that layer nucleation from surface defects is the most common pathway. Direct observation of defects was made possible by the identification of conditions promoting layered growth, which correlates to the use of sodium as an inorganic structure-directing agent, whereas its replacement with an organic results in a nonclassical mode of growth that obscures 2D layers and markedly slows the rate of crystallization. In situ measurements of layered growth reveal that undissolved silica nanoparticles in the synthesis medium can incorporate into advancing steps on crystal surfaces to generate defects (i.e., amorphous silica occlusions) that largely go undetected in literature. Nanoparticle occlusion in natural and synthetic crystals is a topic of wide-ranging interest owing to its relevance in fields spanning from biomineralization to the rational design of functional nanocomposites. In this study, we provide unprecedented insight into zeolite surface growth by molecule addition through time-resolved microscopy that directly captures the occlusion of silica nanoparticles and highlights the prevalent role of defects in zeolite crystallization.

Exploring the origins of crystallisation kinetics in hierarchical materials using in situ X-ray diffraction and pair distribution function analysis

The discovery of novel catalytic materials is predicated on understanding contemporary synthetic processes. With this fundamental knowledge in place it becomes possible to modify the final material with subtle changes to the synthesis process. In this vein, hierarchical materials, formed by the addition of a mesoporogen within the hydrothermal synthesis, have attracted a significant amount of attention due to their catalytic benefits over analogous microporous species. In this work we monitor the hydrothermal synthesis in situ of a hierarchical and a microporous aluminophosphate, for the first time, combining total scattering and pairwise distribution function data. In doing so we observe the local formation of the species, and the longer range crystallisation processes concurrently.

Multi-Spectroscopic Interrogation of the Spatial Linker Distribution in Defect-Engineered Metal-Organic Framework Crystals: The [Cu3 (btc)2-x (cydc)x ] Showcase

In the past few years, defect-engineered metal-organic frameworks (DEMOFs) have been studied due to the plethora of textural, catalytic, or magnetic properties that can be enhanced by carefully introducing defect sites into the crystal lattices of MOFs. In this work, the spatial distribution of two different non-defective and defective linkers, namely 1,3,5-benzenetricarboxylate (BTC) and 5-cyano-1,3-benzenedicarboxylate (CYDC), respectively, has been studied in different DEMOF crystals of the HKUST-1 topology. Raman micro-spectroscopy revealed a nonhomogeneous distribution of defect sites within the [Cu3 (btc)2-x (cydc)x ] crystals, with the CYDC linker incorporated into defect-rich or defect-free areas of selected crystals. Additionally, advanced bulk techniques have shed light on the nature of the copper species, which is highly dynamic and directly affects the reactivity of the copper sites, as shown by probe molecule FTIR spectroscopy. Furthermore, electron microscopy revealed the effect of co-crystallizing CYDC and BTC on the crystal size and the formation of mesopores, further corroborated by X-ray scattering analysis. In this way we have demonstrated the necessity of utilizing micro-spectroscopy along with a whole array of bulk spectroscopic techniques to fully describe multicomponent metal-organic frameworks.

Spectroscopy, microscopy, diffraction and scattering of archetypal MOFs: formation, metal sites in catalysis and thin films

A comprehensive overview of characterization tools for the analysis of well-known metal–organic frameworks and physico-chemical phenomena associated to their applications.

Nuclear Magnetic Resonance Spectroscopy for In Situ Monitoring of Porous Materials Formation under Hydrothermal Conditions

The employment of nuclear magnetic resonance (NMR) spectroscopy for studying crystalline porous materials formation is reviewed in the context of the development of in situ methodologies for the observation of the real synthesis medium, with the aim of unraveling the nucleation and growth processes mechanism. Both liquid and solid state NMR techniques are considered to probe the local environment at molecular level of the precursor species either soluble in the liquid phase or present in the reactive gel. Because the mass transport between the liquid and solid components of the heterogeneous system plays a key role in the synthesis course, the two methods provide unique insights and are complementary. Recent technological advances for hydrothermal conditions NMR are detailed and their applications to zeolite and related materials crystallization are illustrated. Achievements in the field are exemplified with some representative studies of relevance to zeolites, aluminophosphate zeotypes, and metal-organic frameworks.

POMzites: A Family of Zeolitic Polyoxometalate Frameworks from a Minimal Building Block Library

We describe why the cyclic heteropolyanion [P8W48O184]40- (abbreviated as {P8W48}) is an ideal building block for the construction of intrinsically porous framework materials by classifying and analyzing >30 coordination polymers incorporating this polyoxometalate (POM) ligand. This analysis shows that the exocyclic coordination of first-row transition metals (TMs) to {P8W48} typically yields frameworks which extend through {W-O-TM-O-W} bridges in one, two, or three dimensions. However, despite the rich structural diversity of such compounds, the coordination of TMs to the {P8W48} ring is poorly understood, and therefore largely unpredictable, and had not until now been present with any structural classification that could allow rational design. Herein, not only do we present a new approach to understand and classify this new class of materials, we also present three {P8W48}-based frameworks which complement those frameworks which have previously been described. These new compounds help us postulate a new taxonomy of these materials. This is possible because the TM coordination sites of the {P8W48} ring are found, once fully mapped, to lead to well-defined classes of connectivity. Together, analysis provides insight into the nature of the building block connectivity within each framework, to facilitate comparisons between related structures, and to fundamentally unite this family of compounds. Hence we have tentatively named these compounds as "POMzites" to reflect the POM-based composition and zeolitic nature of each family member, although crucially, POMzites differ from zeolites in the modular manner of their preparation. As the synthesis of further POMzites is anticipated, the classification system and terminology introduced here will allow new compounds to be categorized and understood in the context of the established materials. A better understanding of TM coordination to the {P8W48} ring may allow the targeted synthesis of new frameworks rather than the reliance on serendipity apparent in current methods.

Exchange of Coordinated Solvent During Crystallization of a Metal-Organic Framework Observed by In Situ High-Energy X-ray Diffraction

Using time-resolved monochromatic high energy X-ray diffraction, we present an in situ study of the solvothermal crystallisation of a new MOF [Yb2(BDC)3(DMF)2]⋅H2O (BDC=benzene-1,4-dicarboxylate and DMF=N,N-dimethylformamide) under solvothermal conditions, from mixed water/DMF solvent. Analysis of high resolution powder patterns obtained reveals an evolution of lattice parameters and electron density during the crystallisation process and Rietveld analysis shows that this is due to a gradual topochemical replacement of coordinated solvent molecules. The water initially coordinated to Yb(3+) is replaced by DMF as the reaction progresses.

Chiral chemistry of metal-camphorate frameworks

This critical review presents the various synthetic approaches and chiral chemistry of metal-camphorate frameworks (MCamFs), which are homochiral metal-organic frameworks (MOFs) constructed from a camphorate ligand. The interest in this unique subset of homochiral MOFs is derived from the many interesting chiral features for both materials and life sciences, such as asymmetrical synthesis or crystallization, homochiral structural design, chiral induction, absolute helical control and ligand handedness. Additionally, we discuss the potential applications of homochiral MCamFs. This review will be of interest to researchers attempting to design other homochiral MOFs and those engaged in the extension of MOFs for applications such as chiral recognition, enantiomer separation, asymmetric catalysis, nonlinear sensors and devices.

Probing zeolites by vibrational spectroscopies

This review addresses the most relevant aspects of vibrational spectroscopies (IR, Raman and INS) applied to zeolites and zeotype materials. Surface Brønsted and Lewis acidity and surface basicity are treated in detail. The role of probe molecules and the relevance of tuning both the proton affinity and the steric hindrance of the probe to fully understand and map the complex site population present inside microporous materials are critically discussed. A detailed description of the methods needed to precisely determine the IR absorption coefficients is given, making IR a quantitative technique. The thermodynamic parameters of the adsorption process that can be extracted from a variable-temperature IR study are described. Finally, cutting-edge space- and time-resolved experiments are reviewed. All aspects are discussed by reporting relevant examples. When available, the theoretical literature related to the reviewed experimental results is reported to support the interpretation of the vibrational spectra on an atomic level.

Crystal growth of nanoporous metal organic frameworks

Nanoporous metal organic frameworks (MOFs) form one of the newest families of crystalline nanoporous material that is receiving worldwide attention. Successful use of MOFs for application requires not only development of new materials but also a need to control their crystal properties such as size, morphology, and defect concentration. An understanding of the crystal growth processes is necessary in order to aid development of routes to control such properties of the crystallites. In this Perspective article we aim to provide a short overview of the current work and understanding concerning the nucleation and growth processes of nanoporous MOFs and how this work may be expanded upon to further our comprehension of this subject. We also focus heavily on in situ studies that provide real time information on the developing materials and generally provide the most conclusive findings on the processes under investigation.