Microcoil High-Resolution Magic Angle Spinning NMR Spectroscopy

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Uncovering the Local Magnesium Environment in the Metal-Organic Framework Mg-2(dobpdc) Using Mg-25 NMR Spectroscopy

The incorporation of N,N'-dimethylethylenediamine into an expanded MOF-74 framework has yielded a material (mmen-Mg-2(dobpdc)) exhibiting "step-shaped" CO, adsorption isotherms. The coordination of mmen at the Mg open metal center is essential for the unique cooperative adsorption mechanism elucidated for this material. Despite its importance for carbon capture, there is as yet no experimental structure determination available for the underlying metal organic framework Mg-2(dobpdc). Our Mg-25 solid-state NMR data unravel the local Mg environments in several Mg2(dobpdc) samples, unambiguously confirming the formation of five coordinate Mg centers in the activated material and six-coordinate Mg centers in the solvent- or diamine-loaded samples, such as mmen-Mg2(dobpdc). A fraction of the Mg centers exhibit local disorder due to the framework deformation accompanied by the guest distributions and dynamics

A structural investigation relating to the pozzolanic activity of rice husk ashes

Various factors determine the applicability of rice husk ash (RHA) as a pozzolanic material. The amount and accessibility of reactive sites is thought to be a key factor. A structural study of RHA samples in relation to their reactivity has been performed; Silica in RHA formed by burning rice husk in a laboratory furnace under continuous supply of air have been characterized as a function of incineration temperature, time and cooling regime. The characterization methods included chemical analyses, conductivity measurements, microscopic analysis, X-ray diffraction (XRD) and 29Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR). In line with earlier observations, the analyses show that the highest amounts of amorphous silica occur in samples burnt in the range of 500 °C–700 °C. The 29Si NMR data allow direct identification of the reactive silanol sites in the RHA samples. De-convolution of the NMR spectra clearly shows that the quickly cooled RHA resulting from burning rice husk for 12 h at 500 °C has the highest amount of silanol groups. This sample also induced the largest drop in conductivity when added to a saturated calcium hydroxide solution giving an indication of its reactivity towards lime. Therefore, this RHA is the favorable sample to be used as pozzolanic cement additiveCement and Concrete Research 38 (2008) 861–86

Solid-State NMR Investigations of MgCl2 Catalyst Support

MgCl2 is a vital component of Ziegler-Natta catalysts for olefin polymerization. Here we:synthesized anhydrous MgCl2 using different drying protocols and exploited H-1 NMR to quantify the proton content. We report on our study of neat and ball-milled MgCl2 samples by means of Mg-25 and Cl-35 solid-state NMR. DFT calculations of the quadrupole tensor aid in analysis of the spectra. The results show that, due to the morphology of the neat particles, a preferred Orientation is induced which manifests itself in unusual powder line shapes. Ball Milling reduces particle size, which subsequently leads to a small distribution of quadrupole parameters for the bulk. Surface sites, highly relevant for catalysis, are not directly observed, due to their broad lines of low intensity

Structural Characterization of Electron Donors in Ziegler–Natta Catalysts

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Effect of H-bonding on network junction and macroscopic elastomer properties in photocured polyacrylate films

Two series of polyacrylate films with different H-bonding capable monoacrylate copolymers were synthesized by UV-initiated photo-polymerization. Detailed IR analyses on cured samples show that networks made from structurally distinguishable mono-acrylate have significantly different extent of H-bonded monoacrylate present. We found that this is induced by differences in the relative reaction rate between homo-polymerization and copolymerization of the relevant monoacrylates and crosslinker (PEGDA). Although ‘pre-organization’ of H-bond capable monoacrylates was observed for those tested formulations prior to crosslinking, which should significantly increase the homo-polymerization rate, it was found that the co-polymerization rate was also significantly increased. Furthermore, DQ NMR analyses on these networks pinpoints the presence of H-bonding clusters, which decreases mobility of chain segments near the H-bonding functional groups in the monoacrylate sidechains (dangling chain segments of monoacrylate). However, the overall network topology does not alter significantly upon forming H-bonding clusters in the monoacrylate sidechains. Finally it was also found that larger H-bonding clusters increase the apparent network junction functionality. These findings are discussed in the context of utilizing the H-bonding capable monoacrylates to tailor the microscopic topological properties as well as macroscopic physical properties of photocured polyacrylate polymer films

Probing Interactions between Electron Donors and the Support in MgCl₂‑Supported Ziegler–Natta Catalysts

Olefin polymerization using Ziegler–Natta catalysts (ZNCs) is an important industrial process. Despite this, fundamental insight into the inner working mechanisms of these catalysts remains scarce. Here, we focus on the low-γ nuclei ²⁵Mg and ³⁵Cl for an in-depth solid-state NMR and density functional theory (DFT) study of the catalyst’s MgCl₂ support in binary adducts prepared by ball-milling. Besides the bare MgCl₂ support and a MgCl₂–TiCl₄ adduct, samples containing donors that are part of the families of 2,2-dialkyl-1,3-dimethoxypropanes and phthalates used in fourth- and fifth-generation ZNCs are studied. DFT calculations indicate that the quadrupolar coupling parameters of the chlorines differ significantly between bulk and surface sites. As a result, the NMR visibility of the chlorine sites correlates with the particle size except for the adduct with 2,2-dimethyl-1,3-dimethoxypropane donor. The DFT calculations furthermore show that the surface sites are fairly insensitive to binding of different donor molecules, making it difficult to identify specific binding motives. The surface sites with large ³⁵Cl NMR line widths can be observed using high radio frequency field strengths. For the 2,2-dimethyl-1,3-dimethoxypropane donor, we observe additional surface sites with intermediately high quadrupolar couplings, suggesting a different surface structure for this particular adduct compared to the other systems. For ²⁵Mg, pronounced effects of donor binding on the quadrupole interaction parameters are observed, both computationally and experimentally. Again the adduct with the 2,2-dimethyl-1,3-dimethoxypropane donor shows a different behavior of the surface sites compared to the other adducts, which display more asymmetric coordinations of the surface Mg sites. Identifying specific binding motives by comparing ²⁵Mg NMR results to DFT calculations also proves to be difficult, however. This is attributed to the existence of many defect structures caused by the ball-milling process. The existence of such defect structures both at the surface and in the interior of the MgCl₂ particles is corroborated by NMR relaxation studies. Finally, we performed heteronuclear correlation experiments, which reveal interactions between the support and Mg–OH surface groups, but do not provide indications for donor–surface interactions