Focused ion beam milling and MicroED structure determination of metal-organic framework crystals

We report new advancements in the determination and high-resolution structural analysis of beam-sensitive metal organic frameworks (MOFs) using microcrystal electron diffraction (MicroED) coupled with focused ion beam milling at cryogenic temperatures (cryo-FIB). A microcrystal of the beam-sensitive MOF, ZIF-8, was ion-beam milled in a thin lamella approximately 150 nm thick. MicroED data were collected from this thin lamella using an energy filter and a direct electron detector operating in counting mode. Using this approach, we achieved a greatly improved resolution of 0.59 Å with a minimal total exposure of only 0.64 e-/A2. These innovations not only improve model statistics but also further demonstrate that ion-beam milling is compatible with beam-sensitive materials, augmenting the capabilities of electron diffraction in MOF research.

Beam-sensitive metal-organic framework structure determination by microcrystal electron diffraction

Analysis of metal-organic framework (MOF) structure by electron microscopy and electron diffraction offers an alternative to growing large single crystals for high-resolution X-ray diffraction. However, many MOFs are electron beam-sensitive, which can make structural analysis using high-resolution electron microscopy difficult. In this work we use the microcrystal electron diffraction (MicroED) method to collect high-resolution electron diffraction data from a model beam-sensitive MOF, ZIF-8. The diffraction data could be used to determine the structure of ZIF-8 to 0.87 Å from a single ZIF-8 nanocrystal, and this refined structure compares well with previously published structures of ZIF-8 determined by X-ray crystallography. This demonstrates that MicroED can be a valuable tool for the analysis of beam-sensitive MOF structures directly from nano and microcrystalline material.

Postsynthetic Modification of ZIF-8 Membranes via Membrane Surface Ligand Exchange for Light Hydrocarbon Gas Separation Enhancement

The ability to tailor the pore structure of metal-organic framework (MOF) membranes enables synthesis of new or modified MOF membranes with enhanced separation characteristics. This work employs a modified version of solvent-assisted ligand exchange, termed membrane surface ligand exchange (MSLE), to modify the pore structure of zeolitic imidazolate framework-8 (ZIF-8) membranes. This paper is the first to perform a time-based, ex situ characterization and gas permeation study of ZIF-8 MSLE with 5,6-DBIM (DBIM, dimethylbenzimidazole) to effectively narrow the ZIF-8 pores, enhance light hydrocarbon gas-phase separations, and give insight into the exchange mechanism with respect to time and temperature. The results show that relatively fast exchange kinetics occur mainly at the outer surface of the ZIF-8 membrane during the initial 30 min of exchange and enables significant (40-70%) increases in propylene/propane selectivity with minimal (10-20%) propylene permeance losses for the modified ZIF-8 membranes. We postulate as the reaction time proceeds, the ligand-exchange rate slows as the DBIM linker diffuses into the ZIF-8 membrane beyond the external surface, exchanges with the original linker, disrupts the original framework's crystallinity, and then increases long-range order/crystallinity as the reaction proceeds. The H₂/C₂ separation factor increases with increased 5,6-DBIM content in the ZIF-8 framework which is facilitated by increased MSLE time and reaction temperature.

Catalytic mixed conducting ceramic membrane reactors for methane conversion

Schematic of catalytic mixed conducting ceramic membrane reactors for various reactions: (a) O₂ permeable ceramic membrane reactor; (b) H₂ permeable ceramic membrane reactor; (c) CO₂ permeable ceramic membrane reactor.

Strategy for stabilizing noble metal nanoparticles without sacrificing active sites

We report a facile surface fluorination strategy for restricting Pt nanoparticle sintering through providing anchoring sites on the TiO₂ support and enhancing metal–support interaction via improved electronic interaction without sacrificing the active sites.

A highly permeable and selective amino-functionalized MOF CAU-1 membrane for CO2–N2 separation

A CAU-1 membrane with high separation performance for CO₂–N₂ mixtures demonstrates for the first time the potential of amino-functionalized MOF membranes for CO₂ capture.

Direct evidence of the evolutionary mechanism of zeolite monolayers on the substrate surface in a hydrothermal reaction

The microstructure control and optimization of zeolite films and membranes is an indispensable challenge for various innovative applications. It can be steered by understanding the formation process. Here we design an unprecedented strategy to uncover direct evidence via the hydrothermal synthesis of chitosan-supported zeolite monolayers. The chitosan-supported layer involved in the hydrothermal reaction is observed using SEM, AFM, EPMA, and HRTEM while nucleation and crystal growth in the bulk synthesis solution are pursued with HRTEM, DLS, and SEM. The direct HRTEM observation is achieved on the chitosan-supported layer by peeling chitosan from its support. It reveals that a gel layer is initially formed on the chitosan layer where the subsequent crystal growth is fatally restrained. Our own experimental evidence and the literature reports clearly demonstrate that the formation mechanism is homogeneous for severely reduced crystal growth on the substrate but is heterogeneous when crystal growth on the substrate is significantly enhanced.

Gas Diffusion and Microstructural Properties of Ordered Mesoporous Silica Fibers

Pore and surface diffusion of carbon dioxide (CO(2)) and ethylene (C(2)H(4)) in the nanopores of ordered mesoporous silica fibers about 200 microm in length was measured by the transient gravimetric method. The experimentally determined pore diffusivity data, coupled with the porosity, pore size, and fiber length, are used to obtain the actual length of the nanopores in silica fibers. These measurements reveal a structure of the ordered nanopores whirling helically around the fiber axis with a spiral diameter of about 15 microm and a pitch value of 1.6 microm. At room temperature the surface diffusion contributes about 10% to the total diffusional flux for these two gases in the nanopores of the ordered mesoporous silica fibers. The surface diffusion coefficients for the ordered mesoporous silica fibers are about 1 order of magnitude larger than the non-ordered mesoporous alumina or silica with similar pore size.

Tailoring Pore Properties of MCM-48 Silica for Selective Adsorption of CO2

Four different types of amine-attached MCM-48 silicas were prepared and investigated for CO(2) separation from N(2). Monomeric and polymeric hindered and unhindered amines were attached to the pore surface of the MCM-48 silica and characterized with respect to their CO(2) sorption properties. The pore structures and amino group content in these modified silicas were investigated by XRD, FT-IR, TGA, N(2) adsorption/desorption at 77 K and CHN/Si analysis, which confirmed that in all cases the amino groups were attached to the pore surface of MCM-48 at 1.5-5.2 mmol/g. The N(2) adsorption/desorption analysis showed a considerable decrease of the pore volume and surface area for the MCM-48 silica containing a polymeric amine (e.g., polyethyleneimine). The CO(2) adsorption rates and capacities of the amine-attached MCM-48 samples were studied employing a sorption microbalance. The results obtained indicated that in addition to the concentration of surface-attached amino groups, specific interactions between CO(2) and the surface amino groups, and the resultant pore structure after amine group attachment have a significant impact on CO(2) adsorption properties of these promising adsorbent materials.