Cryo-EM Structures of Atomic Surfaces and Host-Guest Chemistry in Metal-Organic Frameworks

Abstract

Host-guest interactions govern the chemistry of a broad range of functional materials. However, the weak bonding between host and guest prevents atomic-resolution studies of their structure and chemistry using transmission electron microscopy (TEM). This problem is exacerbated in metalorganic frameworks (MOF), in which the host framework is easily damaged by the electron beam.Here, we use cryogenic-electron microscopy (cryo-EM) to simultaneously address these two challenges and resolve the atomic surface structure of zeolitic imidazolate framework (ZIF-8) andits interaction with guest CO2 molecules. We image atomic step-edge sites on the ZIF-8 surface that provides possible insight to its growth behavior. Furthermore, we observe two distinct binding sites for CO2 within the ZIF-8 pore, which are both predicted by density functional theory (DFT) to be energetically favorable. This CO2 insertion induces an apparent ~3% lattice expansion along the <002> and <011> directions of the ZIF-8 unit cell. The ability to stabilize MOFs and preserve their host-guest chemistry opens a rich materials space for scientific exploration and discovery using cryo-EM. Main Text:Metal-organic frameworks (MOFs) are a large class of highly porous materials 1-3 whose chemistry and crystalline structure can be tuned for potential applications in gas storage 4-6 , separations 7 , and catalysis 8 . Interactions between the host framework and guest molecule are central to such applications but are poorly understood at the single particle level. Although transmission electron microscopy (TEM) has been used to study the empty MOF framework 9-13 , guest insertion within the MOFs could not be imaged. The weak bonding between the guest molecule and framework is easily damaged even under low electron doses 14,15 . Furthermore, guest molecules are likely to desorb from MOF pores under the high vacuum condition (~10 -6 mbar) of the TEM at room temperature (supplementary text). Therefore, current studies rely on ensemble measurements using X-ray/neutron diffraction 16,17 , nuclear magnetic resonance 18 , or theoretical simulations 19 .However, the structural information obtained by these methods is averaged over bulk particles.Direct observations of individual MOF particles and their interaction with guest species at high spatial resolution have not been possible.Recently, cryogenic-electron microscopy (cryo-EM) was shown to be a powerful tool beyond structural biology. For instance, reactive lithium battery materials have been successfully stabilized for imaging and spectroscopy [20][21][22][23][24] . Such cryogenic condition may also reduce radiation damage to the MOF framework (supplementary text). However, cryo-EM procedures developed for biological or battery materials are not necessarily compatible with MOFs. Here, we establish a new cryo-EM protocol to reveal atomic host-guest structures within MOFs, demonstrating that these entities, held together by weak interactions, can be preserved for high-resolution imaging at cryogenic condition (Fig. 1...

High-Resolution Imaging of Unstained Polymer Materials

“…The first direct image of adsorbed gas molecules in MOF crystals was obtained by Li et al (seventh entry in Table ). A ZIF-8 MOF crystal saturated with CO 2 was plunged into liquid ethane to trap the CO 2 molecules inside the crystal. A schematic structure of ZIF-8 MOF crystal with a CO 2 guest molecule is shown in Figure A.1.…”

“…The gray region at the center of the unit cell (indicated by red arrow) corresponds to an adsorbed CO 2 molecule. Reproduced with permission from ref . Copyright 2019 Elsevier Inc. (B) (1) Chemical structure of a polypeptoid diblock copolymer (pNeh- b -pNBrpe) with Br atoms.…”

Revolutionizing the structural design and determination of covalent–organic frameworks: principles, methods, and techniques

“…31 TEM provides a powerful platform that integrates the capabilities of multi-dimensional correlated structural, chemical, and electronic information retrieval at high spatial resolution. [32][33][34][35] Recent advances in the development of high-performance electron detectors enable various low-dose imaging techniques, such as direct detection cameras (DDC)-high resolution transmission electron microscopy (HRTEM), 36 integrated-differential-phase-contrast (iDPC)-STEM and electron ptychography, 37,38 providing unprecedented opportunities in direct imaging of beam-sensitive low-contrast crystalline and non-crystalline organic polymers.…”

Evolution of porous materials from ancient remedies to modern frameworks

“…In recent years, new techniques such as neutron scattering, pair distribution function analysis, highperformance computing, and cryogenic electron microscopy (cryo-EM) have allowed for a better understanding of the structure and dynamics of materials at the molecular level. For example, cryo-EM allowed for the first direct visualization of gas molecules loaded into the pores of a MOF 44 . This proves that even in a field as old as porous materials, there are always new things to learn, and it is never too late to look back at some of the materials studied by previous generations with fresh eyes.…”