Low-Dose and In-Painting Methods for (Near) Atomic Resolution STEM Imaging of Metal Organic Frameworks (MOFs)

Metal-organic Frameworks (MOFs) are a group of crystalline and highly porous materials consisting of inorganic metal ions/clusters (nodes) that are coordinated by organic linkers. The ability to create a wide range of porous structures, where the pore size can be easily changed in size and shape offers the potential for many applications in gas storage/separation and catalysis. The presence of the organic linkers or “struts” in the sample creates challenges for high resolution microscopy as the sample itself is very sensitive to beam damage. A key challenge for understanding the structures of MOFs and how the applications can be modified by doping the nodes and changing the nature of the organic linkers, is therefore to be able to image the samples on the sub-nm length scale (the nodes are ~1 nm). The study of organics, where large single crystals with long-range order cannot be synthesized, is usually performed by either electron crystallography or direct imaging in the (scanning) transmission electron microscope (S/TEM). In the (S)TEM, large single crystals are not needed as the electron beam can be focused to a very small area (sub-nm if needed). The downside to this ability to see small areas is that because the electron beam has a strong interaction with the sample, it can cause significant levels of electron beam damage. However, the last 40 years of protein crystallography and more recently the use of in-situ liquid stages to study chemical reactions in the (S)TEM, have shown that this beam damage effect can in most cases be mitigated by the use of extremely low-dose imaging (a dose rate of less than 0.1 electrons/angstrom2/s and a cumulative dose of less than 10 electrons/angstrom2). In addition to simply lowering the dose through conventional means (changing the emission current and probe dwell time), more recent use of compressive sensing/in-painting methods for STEM has also been shown to lower the effective dose and dose rate

Enhancing the catalytic activity of hydronium ions through constrained environments

The dehydration of alcohols is involved in many organic conversions but has to overcome high free-energy barriers in water. Here we demonstrate that hydronium ions confined in the nanopores of zeolite HBEA catalyse aqueous phase dehydration of cyclohexanol at a rate significantly higher than hydronium ions in water. This rate enhancement is not related to a shift in mechanism; for both cases, the dehydration of cyclohexanol occurs via an E1 mechanism with the cleavage of Cβ–H bond being rate determining. The higher activity of hydronium ions in zeolites is caused by the enhanced association between the hydronium ion and the alcohol, as well as a higher intrinsic rate constant in the constrained environments compared with water. The higher rate constant is caused by a greater entropy of activation rather than a lower enthalpy of activation. These insights should allow us to understand and predict similar processes in confined spaces

Enhancing the catalytic activity of hydronium ions through constrained environments

The dehydration of alcohols is involved in many organic conversions but has to overcome high free-energy barriers in water. Here we demonstrate that hydronium ions confined in the nanopores of zeolite HBEA catalyse aqueous phase dehydration of cyclohexanol at a rate significantly higher than hydronium ions in water. This rate enhancement is not related to a shift in mechanism; for both cases, the dehydration of cyclohexanol occurs via an E1 mechanism with the cleavage of Cβ–H bond being rate determining. The higher activity of hydronium ions in zeolites is caused by the enhanced association between the hydronium ion and the alcohol, as well as a higher intrinsic rate constant in the constrained environments compared with water. The higher rate constant is caused by a greater entropy of activation rather than a lower enthalpy of activation. These insights should allow us to understand and predict similar processes in confined spaces

Al-27 MAS NMR Studies of HBEA Zeolite at Low to High Magnetic Fields

Al-27 single pulse (SP) MAS NMR :spectra,of RUA zeolites With high. Si/AL ratios of 71 and were obtained at three magnetic field strengths of 7.05, 11.75, and 19.97 T. High field Al-27 MAS NMR spectra acquired at 19.97 T show significantly improved spectral resolution, resulting in at least two well-resolved tetrahedral-Al NMR peaks. Based on, the results obtained from Al-27 MAS and MQMAS NMR, acquired at 19,97 T,four different, quadrupole peaks are used to deconvolute the Al-27 SP MAS spectra acquired at various fields by using the, same set of quadrupole coupling constants, asymmetric parameters and relative integrated peak intensities for the tetrahedral Al peaks. The line shapes of individual peaks change from typical quadrupole line shape at low field to essentially symmetrical line shapes at high field. We demonstrate,thatt for fully, hydrated HBEA zeolites, the effect of second order quadrupole interaction can be ignored, and quantitative spectral analysis can be, erformed by directly fitting the high field, spectra using mixed Gaussian/Lorentzian line shapes. Also, the analytical steps described in our work,allOw direct assignment of spectral intensity to individual Al tetrahedral sites (T-Sites) of zeolite HBEA. Finally, the proposed concept is suggested to be generally applicable to other zeolite framework types, thus allowing a direct probing of Al distributioUs by NMR spectroscopic methods in zeolites with high confidence

Elementary Steps of Faujasite Formation Followed by in Situ Spectroscopy

Ex situ and in situ spectroscopy was used to identify the kinetics of processes during the formation of the faujasite (FAU) zeolite lattice from a hydrous gel. Using solid-state ²⁷Al magic angle spinning (MAS) nuclear magnetic resonance (NMR), the autocatalytic transformation from the amorphous gel into the crystalline material was monitored. Al X-ray absorption near-edge structure shows that most Al already adopts a tetrahedral coordination in the X-ray-amorphous aluminosilicate at the beginning of the induction period, which hardly changes throughout the rest of the synthesis. Using ²³Na NMR spectroscopy, environments in the growing zeolite crystal were identified and used to define the processes in the stepwise formation of the zeolite lattice. The end of the induction period was accompanied by a narrowing of the ²⁷Al and ²³Na MAS NMR peak widths, indicating the increased level of long-range order. The experiments show conclusively that the formation of faujasite occurs via the continuous formation and subsequent condensation of intermediary sodalite-like units that constitute the key building block of the zeolite

Impact of Aqueous Medium on Zeolite Framework Integrity

In this work, Al K-edge extended X-ray absorption fine structure and ²⁷Al MAS NMR spectroscopies in combination with DFT calculations are used to determine both qualitative and quantitative structural changes of two well-characterized samples with the BEA structure. The effects of various properties, including Al concentration, Al distribution, particle size, and structural defects, on zeolite stability are explored. As the samples are degraded by treatment in hot liquid water, the local structure about the Al T-site remains mostly intact, including the Al–O–Si angles and bond distances, while the crystalline structure as measured by XRD and STEM is disrupted. The combined data suggests the crystallinity decreases via selective hydrolysis of the T1- and T2-sites that form the 4-member rings of the zeolite framework. The hydrolysis eventually leads to the dissolution of the T-sites followed by reprecipitation on the particle surface resulting in amorphization of the sample