Low-valent homobimetallic Rh complexes: influence of ligands on the structure and the intramolecular reactivity of Rh–H intermediates

Supporting two metal binding sites by a tailored polydentate trop-based (trop - 5H-dibenzo[a,d] cyclohepten-5-yl) ligand yields highly unsymmetric homobimetallic rhodium(I) complexes. Their reaction with hydrogen rapidly forms Rh hydrides that undergo an intramolecular semihydrogenation of two C≡C bonds of the trop ligand. This reaction is chemoselective and converts C≡C bonds to a bridging carbene and an olefinic ligand in the first and the second semihydrogenation steps, respectively. Stabilization by a bridging diphosphine ligand allows characterization of a Rh hydride species by advanced NMR techniques and may provide insight into possible elementary steps of H₂ activation by interfacial sites of heterogeneous Rh/C catalysts

Thermal restructuring of silica-grafted -CrO2Cl and -VOCl2 species

The volatile molecular precursors CrO2Cl2 and VOCl3 were grafted to thermally dehydrated silica in order to obtain site-isolated, monopodal [triple bond, length as m-dash]SiO–MOxCly−1 species (M = V, Cr). Thermal restructuring under dynamic vacuum was investigated up to 450 °C with different spectroscopic techniques (viz., NMR, UV-Vis, IR, Raman and XPS). During this thermal restructuring, VOCl3 or CrO2Cl2 is partially eliminated from the surface, whilst the remaining surface species become multiply bound to the silica surface. This restructuring increases both the chemical and thermal stability of these materials, and has significant consequences for their performance as heterogeneous catalysts

Thermal restructuring of silica-grafted TiClx species and consequences for epoxidation catalysis

Anchoring titanium: Grafting of TiCl4 to site-isolated silanol groups on silica initially leads to freely rotating species. Upon heating, a thermal restructuring takes place, causing the elimination of TiCl4 and the formation of more strongly anchored titanium species, which were found to be highly active and stable epoxidation catalysts

Direct evidence of the effect of synthesis conditions on aluminum siting in zeolite ferrierite: A 27Al MQ MAS NMR study

The tailoring of the aluminum distribution over the various T sites in zeolites remains a major challenge. By using differing combinations of templating molecules, we managed to control the aluminum siting over the T sites as identified by 27Al MQ MAS NMR in samples of the same Si/Al ratio. This is unique evidence of a modified aluminum distribution in samples prepared following a strategy to tailor the location of aluminum in the zeolite ferrierite framework.A.B.P. is thankful for the support of the European Community under a Marie Curie Intra-European Fellowship for Career Development. Funding from the Spanish Ministry of Economy (project MAT2012-31127) is greatly acknowledged.Peer Reviewe

Correlating the Structural Evolution of ZnO/Al2O3 to Spinel Zinc Aluminate with its Catalytic Performance in Propane Dehydrogenation

Zn-based Al2O3-suported materials have been proposed as inexpensive and environmentally friendly catalysts for the direct dehydrogenation of propane (PDH), however, our understanding of these catalysts’ structure and deactivation routes is still limited. Here, we correlate the catalytic activity for PDH of a series of Zn-based Al2O3 catalysts with their structure and structural evolution. To this end, three model catalysts are investigated. (i) ZnO/Al2O3 prepared by atomic layer deposition (ALD) of ZnO onto γ-Al2O3 followed by calcination at 700 °C, which yields a core-shell spinel zinc aluminate/γ-Al2O3 structure. (ii) Zinc aluminate spinel nanoparticles (ZnxAlyO4 NPs) prepared via a hydrothermal method. (iii) A reference core-shell ZnO/SiO2 catalyst prepared by ALD of ZnO on SiO2. The catalysts are characterized in detail by synchrotron X-ray powder diffraction (XRD), Zn K-edge X-ray absorption spectroscopy (XAS), and 27Al solid state nuclear magnetic resonance (ssNMR). These experiments allowed us to identify tetrahedral Zn sites in close proximity to Al sites of a zinc aluminate spinel phase (ZnIV–O–AlIV/VI linkages) as notably more active and selective in PDH relative to the supported ZnO wurtzite phase (ZnIV–O– ZnIV linkages) in ZnO/SiO2. The best performing catalyst, 50ZnO/Al2O3 gives 77% selectivity to propene (gaseous products based) at 9 mmol C3H6 gcat−1 h−1 space time yield (STY) after 3 min of reaction at 600 °C. On the other hand, the core-shell ZnO/Al2O3 catalyst shows an irreversible loss of activity over repeated PDH and air-regeneration cycles, explained by Zn depletion on the surface due to its diffusion into subsurface layers or the bulk. ZnxAlyO4 NPs gave a comparable initial selectivity and catalytic activity as 50ZnO/Al2O3. With time on stream, ZnxAlyO4 NPs deactivate due to the formation of coke at the catalyst surface, yet the extend of coke deposition is lower than for the ZnO/Al2O3 catalysts, and the activity of ZnxAlyO4 NPs can be regenerated almost fully using calcination in air.<br /

Characterization of different water pools in solid-state NMR protein samples

We observed and characterized two distinct signals originating from different pools of water protons in solid-state NMR protein samples, namely from crystal water which exchanges polarization with the protein (on the NMR timescale) and is located in the protein-rich fraction at the periphery of the magic-angle spinning (MAS) sample container, and supernatant water located close to the axis of the sample container. The polarization transfer between the water and the protein can be probed by two-dimensional exchange spectroscopy, and we show that the supernatant water does not interact with protein on the timescale of the experiments. The two water pools have different spectroscopic properties, including resonance frequency, longitudinal, transverse and rotating frame relaxation times. The supernatant water can be removed almost completely physically or can be frozen selectively. Both measures lead to an enhancement of the quality factor of the probe circuit, accompanied by an improvement of the experimental signal/noise, and greatly simplify solvent-suppression by substantially reducing the water signal. We also present a tool, which allows filling solid-state NMR sample containers in a more efficient manner, greatly reducing the amount of supernatant water and maximizing signal/noise

The fold of alpha-synuclein fibrils

The aggregation of proteins into amyloid fibrils is associated with several neurodegenerative diseases. In Parkinson's disease it is believed that the aggregation of alpha-synuclein (alpha-syn) from monomers by intermediates into amyloid fibrils is the toxic disease-causative mechanism. Here, we studied the structure of alpha-syn in its amyloid state by using various biophysical approaches. Quenched hydrogen/deuterium exchange NMR spectroscopy identified five beta-strands within the fibril core comprising residues 35-96 and solid-state NMR data from amyloid fibrils comprising the fibril core residues 30-110 confirmed the presence of beta-sheet secondary structure. The data suggest that beta1-strand interacts with beta2, beta2 with beta3, beta3 with beta4, and beta4 with beta5. High-resolution cryoelectron microscopy revealed the protofilament boundaries of approximately 2 x 3.5 nm. Based on the combination of these data and published structural studies, a fold of alpha-syn in the fibrils is proposed and discussed

Low-Temperature Preparation of Tailored Carbon Nanostructures in Water

The development of low-temperature carbonization procedures promises to provide novel nanostructured carbon materials that are of high current interest in materials science and technology. Here, we report a "wet-chemical" carbonization method that utilizes hexayne amphiphiles as metastable carbon precursors. Nearly perfect control of the nanoscopic morphology was achieved by self-assembly of the precursors into colloidal aggregates with tailored diameter in water. Subsequent carbonization furnished carbon nanocapsules with a carbon microstructure resembling graphite-like amorphous carbon materials