3D-3D topotactic transformation in aluminophosphate molecular sieves and its implication in new zeolite structure generation

Structure determination of a low-crystallinity covalent organic framework by three-dimensional electron diffraction

Covalent organic frameworks (COFs) have been attracting intense research due to their permanent porosity, designable architecture, and high stability. However, COFs are challenging to crystallize and their synthesis often results in tiny crystal sizes and low crystallinities, which hinders an unambiguous structure determination. Herein, we demonstrate that the structure of low-crystallinity COF Py-1P nanocrystals can be solved by coupling three-dimensional electron diffraction (3DED) with simulated annealing (SA). The resulting model is comparable to that obtained from high-crystallinity samples by dual-space method. Moreover, for low-resolution 3DED data, the model obtained by SA shows a better framework than those provided by classic direct method, dual-space method, and charge flipping. We further simulate data with different resolutions to understand the reliability of SA under different crystal quality conditions. The successful determination of Py-1P structure by SA compared to other methods provides new knowledge for using 3DED to analyze low-crystallinity and nanosized materials.

Single-crystal structure determination of nanosized metal-organic frameworks by three-dimensional electron diffraction

Metal-organic frameworks (MOFs) have attracted considerable interest due to their well-defined pore architecture and structural tunability on molecular dimensions. While single-crystal X-ray diffraction (SCXRD) has been widely used to elucidate the structures of MOFs at the atomic scale, the formation of large and well-ordered crystals is still a crucial bottleneck for structure determination. To alleviate this challenge, three-dimensional electron diffraction (3D ED) has been developed for structure determination of nano- (submicron-)sized crystals. Such 3D ED data are collected from each single crystal using transmission electron microscopy. In this protocol, we introduce the entire workflow for structural analysis of MOFs by 3D ED, from sample preparation, data acquisition and data processing to structure determination. We describe methods for crystal screening and handling of crystal agglomerates, which are crucial steps in sample preparation for single-crystal 3D ED data collection. We further present how to set up a transmission electron microscope for 3D ED data acquisition and, more importantly, offer suggestions for the optimization of data acquisition conditions. For data processing, including unit cell and space group determination, and intensity integration, we provide guidelines on how to use electron and X-ray crystallography software to process 3D ED data. Finally, we present structure determination from 3D ED data and discuss the important features associated with 3D ED data that need to be considered. We believe that this protocol provides critical details for implementing and utilizing 3D ED as a structure determination platform for nano- (submicron-)sized MOFs as well as other crystalline materials.

Structural Transformation-Involved Synthesis of Nanosized ERI-Type Zeolite and Its Catalytic Property in the MTO Reaction

Nanosized ERI-type aluminophosphate was prepared by the calcination of a precursor material (denoted as ECNU-38P) synthesized using 1,1,6,6-tetramethyl-1,6-diazacyclododecane-1,6-diium hydroxide (TDDH) as a structure-directing agent. The structure of ECNU-38P is related to ERI topology but exhibits a highly disordered manner and contains both four- and six-coordinated Al atoms. In situ XRD patterns revealed a rarely reported temperature-induced three-dimensional (3D)-to-3D structural transformation from ECNU-38P to the ordered ERI-type ECNU-38 zeolite at 573-623 K. Nanosized ERI-type silicoaluminophosphate Si-ECNU-38 was also obtained by introducing Si atoms into the synthetic system of ECNU-38P. The catalytic performance of ERI-type silicoaluminophosphates in the methanol-to-olefin (MTO) reaction was revealed to be highly related to the crystal sizes.

Revisiting the Structure of Calcined and Hydrated AlPO-11 with DFT-Based Molecular Dynamics Simulations*

Published crystal structures of the AEL-type aluminophosphate AlPO-11 in its calcined form (space group I m a 2 ) show some peculiar features, such as unusually short Al-O and P-O bonds and near-linear Al-O-P angles. Although experimental evidence for the presence of dynamic disorder was presented, the nature of the associated distortions remained unresolved. In this study, ab initio molecular dynamics (AIMD) calculations in the framework of density functional theory (DFT) were employed to study the dynamic behaviour of this zeotype. At 100 K, static local distortions that break the I m a 2 symmetry are present in the time-averaged structures computed from the AIMD trajectories. At 300 and 500 K, the time-averaged structures approach I m a 2 symmetry. Although shortened Al-O and P-O bonds and near-linear Al-O-P angles were found in the average structures, an analysis of radial and angular distribution functions confirmed their absence in the instantaneous structures. This deviation is due to a precession-like motion of some oxygen atoms around the Al-P connection line, which moves their time-averaged positions closer to this line. In hydrated AlPO-11, some of the water molecules are coordinated to framework Al atoms, leading to an octahedral coordination of 1/5 of the Al sites. DFT optimisations and AIMD simulations on partially hydrated models delivered evidence for a preferential adsorption at the Al1 site. No dynamic disorder was observed for the hydrated form.

On the key role of aluminium and other heteroatoms during interzeolite conversion synthesis

Interzeolite conversion, a synthesis technique for several zeolite frameworks, has recently yielded a large amount of high-performing catalytic zeolites. Yet, the mechanisms behind the success of interzeolite conversion remain unknown. Conventionally, small oligomers with structural similarity between the parent and daughter zeolites have been proposed, despite the fact these have never been observed experimentally. Moreover, recent synthesis examples contradict the theory that structural similarity between the parent and daughter zeolites enhances interzeolite conversion. In this perspective it is proposed that heteroatoms, such as aluminium, are key players in the processes that determine the successful conversion of the parent zeolite. The role of Al during parent dissolution, and all consecutive stages of crystallization, are discussed by revising a vast body of literature. By better understanding the role of Al during interzeolite conversions, it is possible to elucidate some generic features and to propose some synthetic guidelines for making advantageous catalytic zeolites. The latter analysis was also expanded to the interconversion of zeotype materials where heteroatoms such as tin are present.

The crucial roles of aluminium in driving and controlling interzeolite conversion, a useful catalyst synthesis protocol, are put under scrutiny.

On the completeness of three-dimensional electron diffraction data for structural analysis of metal-organic frameworks

Three-dimensional electron diffraction (3DED) has been proven as an effective and accurate method for structure determination of nano-sized crystals. In the past decade, the crystal structures of various new complex metal-organic frameworks (MOFs) have been revealed by 3DED, which has been the key to understand their properties. However, due to the design of transmission electron microscopes (TEMs), one drawback of 3DED experiments is the limited tilt range of goniometers, which often leads to incomplete 3DED data, particularly when the crystal symmetry is low. This drawback can be overcome by high throughput data collection using continuous rotation electron diffraction (cRED), where data from a large number of crystals can be collected and merged. Here, we investigate the effects of improving completeness on structural analysis of MOFs. We use ZIF-EC1, a zeolitic imidazolate framework (ZIF), as an example. ZIF-EC1 crystallizes in a monoclinic system with a plate-like morphology. cRED data of ZIF-EC1 with different completeness and resolution were analyzed. The data completeness increased to 92.0% by merging ten datasets. Although the structures could be solved from individual datasets with a completeness as low as 44.5% and refined to a high precision (better than 0.04 Å), we demonstrate that a high data completeness could improve the structural model, especially on the electrostatic potential map. We further discuss the strategy adopted during data merging. We also show that ZIF-EC1 doped with cobalt can act as an efficient electrocatalyst for oxygen reduction reactions.

Zeolite Composite Nanofiber Mesh for Indoxyl Sulfate Adsorption toward Wearable Blood Purification Devices

A nanofiber mesh was prepared for the adsorption of indoxyl sulfate (IS), a toxin associated with chronic kidney disease. Removing IS is highly demanded for efficient blood purification. The objective of this study is to develop a zeolite composite nanofiber mesh to remove IS efficiently. Eight zeolites with different properties were used for IS adsorption, where a zeolite with a pore size of 7 Å, H+ cations, and a silica to aluminum ratio of 240 mol/mol exhibited the highest adsorption capacity. This was primarily attributed to its suitable silica to aluminum ratio. The zeolites were incorporated in biocompatible poly (ethylene-co-vinyl alcohol) (EVOH) nanofibers, and a zeolite composite nanofiber mesh was successfully fabricated via electrospinning. The nanofiber mesh exhibited an IS adsorption capacity of 107 μg/g, while the adsorption capacity by zeolite increased from 208 μg/g in powder form to 386 μg/g when dispersed in the mesh. This also led to an increase in cell viability from 86% to 96%. These results demonstrated that this zeolite composite nanofiber mesh can be safely and effectively applied in wearable blood purification devices.

Three-Dimensional Electron Diffraction for Structural Analysis of Beam-Sensitive Metal-Organic Frameworks

Electrons interact strongly with matter, which makes it possible to obtain high-resolution electron diffraction data from nano- and submicron-sized crystals. Using electron beam as a radiation source in a transmission electron microscope (TEM), ab initio structure determination can be conducted from crystals that are 6–7 orders of magnitude smaller than using X-rays. The rapid development of three-dimensional electron diffraction (3DED) techniques has attracted increasing interests in the field of metal-organic frameworks (MOFs), where it is often difficult to obtain large and high-quality crystals for single-crystal X-ray diffraction. Nowadays, a 3DED dataset can be acquired in 15–250 s by applying continuous crystal rotation, and the required electron dose rate can be very low (<0.1 e s−1 Å−2). In this review, we describe the evolution of 3DED data collection techniques and how the recent development of continuous rotation electron diffraction techniques improves data quality. We further describe the structure elucidation of MOFs using 3DED techniques, showing examples of using both low- and high-resolution 3DED data. With an improved data quality, 3DED can achieve a high accuracy, and reveal more structural details of MOFs. Because the physical and chemical properties of MOFs are closely associated with their crystal structures, we believe 3DED will only increase its importance in developing MOF materials.

Three-dimensional electron diffraction for porous crystalline materials: structural determination and beyond

Porous crystalline materials such as zeolites, metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have attracted great interest due to their well-defined pore structures in molecular dimensions. Knowing the atomic structures of porous materials is crucial for understanding their properties and exploring their applications. Many porous materials are synthesized as polycrystalline powders, which are too small for structure determination by X-ray diffraction. Three-dimensional electron diffraction (3DED) has been developed for studying such materials. In this Minireview, we summarize the recent developments of 3DED methods and demonstrate how 3DED revolutionized structural analysis of zeolites, MOFs, and COFs. Zeolites and MOFs whose structures remained unknown for decades could be solved. New approaches for design and targeted synthesis of novel zeolites could be developed. Moreover, we discuss the advances of structural analysis by 3DED in revealing the unique structural features and properties, such as heteroatom distributions, mixed-metal frameworks, structural flexibility, guest-host interactions, and structure transformation.