Thomas Emmler

Solid-state NMR studies of aminocarboxylic salt bridges in L-lysine modified cellulose.

LysCel is a cellulose-based material in which l-lysine molecules are grafted with their amino side chains to the cellulose hydroxyl groups. This modification increases considerably the mechanical strength and resistance of cellulosic structures toward water. It has been attributed to the formation of double salt bridges between lysine aminocarboxyl groups in the zwitterionic state. In order to characterize this unusual structure, we have performed high-resolution solid-state (15)N and (13)C CPMAS NMR experiments on LysCel samples labeled with (15)N in the alpha-position or epsilon-position. Furthermore, (13)C-(15)N REDOR experiments were performed on LysCel where half of the aminocarboxyl groups were labeled in 1-position with 13C and the other half in alpha-position with (15)N. The comparison with the 13C and 15N chemical shifts of l-leucine lyophilized at different pH shows that the aminocarboxyl groups of LysCel are indeed zwitterionic. The REDOR experiments indicate distances of about 3.5 A between the carboxyl carbon and the nitrogen atoms of different aminocarboxyl groups, indicating that the latter are in close contact with each other. However, the data are not compatible with isolated aminocarboxyl dimers but indicate the assembly of zwitterionic aminocarboxyl dimers either in a flat ribbon or as tetramers, exhibiting similar intra- and interdimer (13)C...(15)N distances. This interaction of several aminocarboxyl groups is responsible for the zwitterionic state, in contrast to the gas phase, where amino acid dimers exhibiting two OHN hydrogen bonds are neutral.

A Novel Green Template for the Synthesis of Mesoporous Silica

Mesoporous pure silicas and functionalized silica with a narrow pore size distribution centered at 3.8 nm were prepared by a novel template, amphiphilic dendritic polyglycerol. The resulting silica materials were characterized by electron microscopy; nitrogen adsorption; (1)H, (13)C, and (29)Si solid-state cross-polarization magic-angle spinning NMR spectroscopy. It was shown that the template could be completely removed from the pure and functionalized silica in an environmentally friendly way by means of a simple water extraction procedure. Furthermore, it was shown that these materials could be easily functionalized, for example, by employing aminopropyl groups. Thus, a new environmentally friendly pathway to this fascinating class of silica material has been opened.

11B{15N} REDOR and 11B Spin Echo Studies for structural characterization of Si‐B‐C‐N precursor ceramics

Solid‐state NMR spectroscopy is employed for the structural characterization of precursor‐derived Si‐B‐C‐N ceramics. Particular emphasis is given to the structural composition of the BNCx phase which plays a key role for the unusual high temperature stability of these materials. In the present work 11B{15N} REDOR and 11B spin echo experiments are presented for two 15N enriched precursor systems, made from substituted polysilazanes and polysilylcarbodiimides, which provide interatomic boron‐boron and boron‐nitrogen distances. The obtained results are compatible with the presence of layered structures as reported for hexagonal boron nitride (h‐BN). The derived boron‐nitrogen and boron‐boron distances, however, are larger than in h‐BN, reflecting some layer distortions. The boron‐boron distances are found to decrease with increasing pyrolysis temperature, whereas the boron‐nitrogen distances remain practically unaltered at elevated pyrolysis temperatures. On the basis of the present results it is concluded that intercalated BN and sp2‐carbon layers most likely constitute the BNCx phase. The graphite‐like carbon layers are assumed to create some internal pressure, which in turn is responsible for the observed interatomic distance increase in the BN layers. However, other scenarios, like the direct incorporation of small sp2‐carbon domains into the BN‐sheets, cannot be ruled out completely. Further work along this line appears to be necessary to develop a comprehensive structural model for the BNCx phase in such quarternary ceramic systems.

Structural and kinetic investigation of the hydride composite Ca(BH4)2 + MgH2 system doped with NbF5 for solid-state hydrogen storage

Designing safe, compact and high capacity hydrogen storage systems is the key step towards introducing a pollutant free hydrogen technology into a broad field of applications. Due to the chemical bonds of hydrogen-metal atoms, metal hydrides provide high energy density in safe hydrogen storage media. Reactive hydride composites (RHCs) are a promising class of high capacity solid state hydrogen storage systems. Ca(BH4)2 + MgH2 with a hydrogen content of 8.4 wt% is one of the most promising members of the RHCs. However, its relatively high desorption temperature of ∼350 °C is a major drawback to meeting the requirements for practical application. In this work, by using NbF5 as an additive, the dehydrogenation temperature of this RHC was significantly decreased. To elucidate the role of NbF5 in enhancing the desorption properties of the Ca(BH4)2 + MgH2 (Ca-RHC), a comprehensive investigation was carried out via manometric measurements, mass spectrometry, Differential Scanning Calorimetry (DSC), in situ Synchrotron Radiation-Powder X-ray Diffraction (SR-PXD), X-ray Absorption Spectroscopy (XAS), Anomalous Small-Angle X-ray Scattering (ASAXS), Scanning and Transmission Electron Microscopy (SEM, TEM) and Nuclear Magnetic Resonance (NMR) techniques.

Counting of labelled tyrosine molecules in hydrophobic yoctolitre wells filled with water

Time-dependent radioactivity and solid-state 13C-NMR measurements of tyrosine entrapped in water-filled yoctolitre (10(-24) L) wells with hydrophobic walls are reported; the results indicate that such wells induce the formation of quasi solid tyrosine if they are brought in contact with 0.1 M solutions of this edge amphiphile.

Influence of Cation Exchange on the 27Al-NMR Spectra of Zeolites

Abstract The influence of cation exchange on the 27Al-NMR spectra of NaA-zeolites has been studied by 27Al-MAS- and MQ-MAS-Solid State-NMR. From the 27Al-spectra a characterization of the different Al sites in the A zeolites according to their chemical environment and the structural changes on the aluminosilicate network caused by the cation exchange are obtained. It is found that the exchange with cations with smaller ion-radius cause stronger distortions of the 27Al-NMR-spectra than exchange with larger cations like Ba2+. Employing MQ-MAS spectroscopy these distortions are revealed as second order quadrupolar effects for the smaller cations and as a combination of chemical shift and second order quadrupolar interaction for the Ba cation. These changes of the quadrupolar coupling are interpreted numerically via calculations of the lowering of the symmetry of the EFG tensor. Finally it is found that the exchange with divalent cations leads to distortions of the zeolitic framework and the formation of an extra-framework aluminum. To the best of our knowledge this is for the first time that evidence for the production of extra frame work aluminum by pure cation exchange without any thermal treatment has been found in type A zeolites.

Cold gas spraying of semiconductor coatings for the photooxidation of water

This contribution shows the potential of cold gas spraying for the production of photoelectrodes employing photoelectrocatalysts for the water oxidation reaction. Conventional methods of coating usually employ sol-gel methods and calcination to obtain a good binding of the coating to the substrate. In cold gas spraying, particles are accelerated to high velocities by a pressurized gas. Nitrogen is used as process gas, preheated and then expanded in a De Laval type nozzle. On impact with the substrate the particles deform, break up and build an efficient interface to the back contact (as revealed, for example, by scanning electron microscopy). Cold gas spraying is a method for the direct bonding of particles to a substrate and does not require additives that have to be removed e.g. by a calcination step. Thereby it allows the direct fabrication of a working electrode ensemble. In our initial experiments, the state-of-the-art photocatalyst titanium dioxide (TiO2) was explored. The cold-gas-sprayed coatings revealed significantly higher activities for the oxygen evolution reaction (OER), as compared to films derived from wet-chemical processes. Due to the demand for photocatalysts with band gap suitable for visible light absorption, this approach was extended to the promising catalyst material hematite. In correlation with photoelectrochemical measurements, the operating parameters of the cold gas spray process are discussed in terms of their influence on the photocatalytic properties of the semiconductor.