Kinetic investigations of the deactivation by coking of a noble metal catalyst in the catalytic hydrogenation of nitrobenzene using a catalytic wall reactor

The vapour phase hydrogenation of nitrobenzene to aniline is a highly exothermic reaction deactivated by coking of the palladium catalyst supported on -alumina carrier. For studying the deactivation of the catalyst a catalytic wall reactor was used in order to ensure isothermal reaction conditions for the kinetic measurements. Furthermore, the catalytic wall reactor allowed the determination of axial coke profiles by total carbon analysis of different wall segments. On the assumption that the main reaction and the deactivation of the catalyst can be assumed separable both the steady state and the unsteady state kinetics were studied. Nitrobenzene was identified as the relevant coke precursor whereas aniline has neither an influence on the main reaction nor on the deactivation. It could be shown that the hydrogenation of nitrobenzene to aniline follows a Langmuir–Hinshelwood mechanism considering the surface reaction of the adsorbed nitrobenzene molecule and one adsorbed hydrogen atom as the rate determining step. The differentiation of coke on the active sites and coke on the support must be taken into account to model the kinetics of coke formation with sufficient accuracy.     

High-Temperature Properties of Mixed α'/β'-SiAlON Materials

The mechanical behavior of mixed α'/β'-SiAlON materials was studied at elevated temperatures. Two different α'/β'-SiAlON compositions along the Si₃N₄-Y₂O₃9AlN composition line in the Si₃N₄-Al₂O₃AlN-YN3AlN plane (α'-SiAlON plane) were prepared using three different raw-material mixtures. Six different materials were obtained with significantly lower values in α'-SiAlON than expected. The high-temperature properties of the materials studied were influenced strongly by the chemical composition (α' content and grain-boundary phase) of the SiAlONs. A high content of α'-SiAlON was beneficial in terms of creep behavior of the materials. The same materials, however, were characterized by a considerably degradated fracture behavior at elevated temperatures in air because of a crack-growth process enhanced by the poor oxidation resistance of these materials. It was concluded that, despite some superior features of the materials studied, a long-term application of mixed α'/β'-SiAlON materials at 1400°C in air is problematical. A combination of all properties required for such applications was not observed. For that reason, it is suggested that the real high-temperature potential of these materials in air should be limited to temperatures <1300°C.     

31P-magnetic resonance spectroscopy (31P-MRS) of human allografts after renal transplantation

BACKGROUND: 31P-Magnetic resonance spectroscopy (31P-MRS) can be used as a non-invasive tool for measuring the relative intracellular concentrations of several phosphorus metabolites in different organs. Various pathological conditions are characterized by different metabolic patterns. We studied the value of 31P-MRS after renal transplantation with both an uneventful and a clinically complicated course. METHODS: We determined the relative concentrations of phosphate-containing metabolites in renal allografts of humans with 31P-MRS (1.5 Tesla) in the first few weeks after transplantation; 18 patients with an uneventful clinical course and 10 patients who required dialysis after transplantation were examined. Six patients with a stable allograft function 2-3 months after transplantation served as controls. RESULTS: In patients with primary allograft function, we found a significant correlation between the phosphomonoester/phosphodiester-ratio (PME/PDE) (r = 0.66, r < 0.01) and the time after transplantation, but no correlation between the nucleoside triphosphate (beta-NTP)-concentration (r = -0.11) and the time course. In the patients with primary or early allograft dysfunction caused by histologically proven rejection (n=5), we found a low beta-NTP compared to patients with an uncomplicated clinical course (0.09+/-0.01 vs 0.15+/-0.03), but no differences in the PME/PDE ratio (0.73+/-0.21 vs 0.80+/-0.21). In contrast, the PME/PDE ratio was lowered in three patients with delayed graft function caused by acute tubular necrosis (0.45+/-0.07 vs 0.80+/-0.21), but the beta-NTP concentration was not reduced (0.15+/-0.003 vs 0.15+/-0.03). The 31P-MR spectrum of two patients with cyclosporin A damage was not altered compared to the controls. CONCLUSIONS: 31P-MRS can be used in patients in the early period after renal transplantation. A significant correlation between the PME/PDE ratio and the time course but no change in the beta-NTP concentration was found in patients with primary allograft function in the first 4 weeks after renal transplantation. Different patterns of 31P-MR spectra were observed depending on the different causes of primary and early transplant dysfunction.     

Biological water and its role in the effects of alcohol

Alcohol and water compete with each other on target membrane molecules, specifically, lipids and proteins near the membrane surface. The basis for this competition is the hydrogen bonding capability of both compounds. But alcohol's amphiphilic properties give it the capability to be attracted simultaneously to both hydrophobic and hydrophilic targets. Thus, alcohol could bind certain targets preferentially and displace water, leading to conformational consequences. This article reviews the clustering and organized character of biological water, which modulates the conformation of membrane surface molecules, particularly receptor protein. Any alcohol-induced displacement of biological water on or inside of membrane proteins creates the opportunity for allosteric change in membrane receptors. This interaction may also prevail in organelles, such as the Golgi apparatus, which have relatively low concentrations of bulk water. Target molecules of particular interest in neuronal membrane are zwitteronic phospholipids, gangliosides, and membrane proteins, including glycoproteins. FTIR and NMR spectroscopic evidence from model membrane systems shows that alcohol has a nonstereospecific binding capability for membrane surface molecules and that such binding occurs at sites that are otherwise occupied by hydrogen-bonded water. The significance of these effects seems to lie in the need to learn more about biological water as an active participant in biochemical actions. Proposed herein is a new working hypothesis that the molecular targets of ethanol action most deserving of study are those where water is trapped and there is little bulk water. Proteins (enzymes and receptors) certainly differ in this regard, as do organelles.     

Untersuchungen zur thioladdition an polybutadiene

The free-radical addition of 1-octanediol (RSH) on polybutadienes with different molecular weights and configuration of the C? C double bonds occurs selectively to the 1,2-vinyl and 1,4-cis double bond, while the 1,4-trans double bond remains unchanged. The reactivity order 1,2- > cis > trans is confirmed by the model experiments with the isomers of pentene. The addition is favoured by the formation of an olefin/RSH charge-transfer complex as preinitiation step.     

Two cold legs

Bilateral acute lower limb ischaemia is rare. Usually the diagnosis is based on clinical findings. In four patients, three women aged 51, 48, and 72 and a man aged 64 years, bilateral acute ischaemia of the lower limbs was diagnosed, due to different causes: arterial cardiac myxoma embolism, arterial thrombosis probably due to paraneoplastic coagulopathy, aortic dissection, and arterial thrombosis due to cardiac insufficiency, respectively. The management of these conditions includes restoring the circulation as soon as possible. Reperfusion can be achieved by thromboembolectomy or thrombolysis. In patients with underlying atherosclerosis angiography is useful, but time loss must be avoided. The outcome in patients with bilateral ischaemia of the lower limbs depends on the preoperative ischaemia time and the cardiac situation. The mortality varies between 20 and 50%.     

Theory of the Phase-Sensitive Bi2Sr2CaCu2O8+δc-Axis Twist Josephson Junction Experiments

We studied theoretically the phase-sensitive c-axis Bi₂Sr₂CaCu₂O₈₊ (BSCCO) bicrystal twist Josephson junction experiments of Li et al. (Phys. Rev. Lett. 83, 4160 (1999)), calculating the critical current as a function of the twist angle in a wide variety of models. The data provide strong evidence that the c-axis tunneling in BSCCO is incoherent, and that the order parameter is s-wave for T T _c.     

The Pseudogap and the Superconducting Order Parameter in Inhomogeneous Bi2Sr2CaCu2O8+δ

Recent experiments on Bi₂Sr₂CaCu₂O₈₊ (Bi2212) have provided compelling evidence that the samples are inhomogeneous on a nanoscale, with spatially separated superconducting gap (SG) and nonsuperconducting, pseudogap (PG) regions that arise from hole–hole and particle–hole pairings, respectively. We compare and contrast the nonphase sensitive experiments that cannot distinguish these orderings from the phase-sensitive experiments that can. Although the wave vector dependence of the pseudogap is highly anisotropic, we conclude that in Bi2212, the c-axis quasiparticle tunneling is incoherent and that the c-axis supercurrent arises solely from a rather isotropic s-wave superconducting order parameter (OP) for T < T_c.     

The Synthesis of Cresol from Toluene and N2O on H[Al]ZSM-5: Minimizing the Product Diffusion Limitation by the Use of Small Crystals

The direct hydroxylation of toluene with nitrous oxide to cresol has been studied on two different H[Al]ZSM-5 zeolites with an Si/Al ratio of around 25 and different crystal sizes (30-70 nm and 1-3 m). The samples were activated by calcination and characterized by X-ray diffraction, temperature programmed desorption of ammonia, adsorption of nitrogen and transmission electron microscopy. For the two different crystal sizes, different macroscopic cresol yields and time on stream behaviours are observed. The sample having larger crystals shows a decrease in toluene conversion with increasing reaction temperature. For the smaller crystals an increase in toluene conversion, selectivity to cresol and amount of para-cresol in the cresol fraction with increasing reaction temperature is observed. The para-cresol selectivity is lower on the sample with the longer diffusion path. The findings are explained by product diffusion limitation caused by high reactivity and strong adsorption of the polar product cresol on H[Al]ZSM-5, resulting in a rapid deactivation of the larger crystals.     

A microelectrochemical scanning flow cell with downstream analytics

The combination of a capillary based microelectrochemical flow cell system and downstream UV–vis analytics allows obtaining synchronized electrochemical and spectroscopic data in a fully automated mode. This method combination can be generally applied to microelectrochemical studies in which an electrochemical species is released or consumed during the electrochemical reaction. For the example of pure zinc surfaces, the characterization of the integrated spectroscopic system is presented with a Zn²⁺ detection limit below 0.1 μmol l⁻¹ using Zincon as complexing agent. A parameter screening of the effect of pH in the range of 6.6–9.0 in borate buffer reveals a linear increase in zinc dissolution with proton concentration but a distinct step in the open circuit potential from the active state (around −700 mV SHE, pH 6.6–7.1) to the passive state (around −300 mV SHE, pH 7.4–9.0) indicating the formation of a closed passive layer. This mechanism is strongly influenced by sulfate anions which increase the dissolution rate of the passive film and promote the active state as monitored by the dissolution profile and OCP (open circuit potential) values. Within the scope of this parameter variation, the congruency between OCP transients, potentiodynamic sweeps and time resolved dissolution profiles is discussed.