Clustering of Ti on a C60 surface and its effect on hydrogen storage

Recent efforts in finding materials suitable for storing hydrogen with large gravimetric density have focused attention on carbon-based nanostructures. Unfortunately, pure carbon nanotubes and fullerenes are unsuitable as hydrogen storage materials because of the weak bonding of the hydrogen molecules to the carbon frame. It has been shown very recently that coating of carbon nanostructures with isolated transition metal atoms such as Sc and Ti can increase the binding energy of hydrogen and lead to high storage capacity (up to 8 wt % hydrogen, which is 1.6 times the U.S. Department of Energy target set for 2005). This prediction has led to a great deal of excitement in the fuel cell community [see The Fuel Cell Review, http://fcr.iop.org/articles/features/2/7/4]. However, this prediction depends on the assumption that the metal atoms coated on the fullerene surface will remain isolated. Using first-principles calculations based on density functional theory, we show that Ti atoms would prefer to cluster on the C60 surface, which can significantly alter the nature of hydrogen bonding, thus affecting not only the amount of stored hydrogen but also their thermodynamics and kinetics.     

Synthesis of aromatic polyamide-imides from N,N′-bis(trimethylsilyl)-substituted aromatic diamines and 4-chloroformylphthalic anhydride

The polymerization of N,N′-bis(trimethylsilyl)-substituted aromatic diamines with 4-chloroformylphthalic anhydride in various solvents at a temperature range between 10 and 70°C afforded polyamide-amic acid trimethylsilyl esters having inherent viscosities of 0.8–1.4 dL/g. Transparent and flexible films of the silylated precursor polymers were obtained by casting directly from the polymer solutions. Desilylation of the silylated polymers with methanol resulted in the formation of the corresponding polyamide-amic acids. Subsequent thermal imidization of the silylated precursor polymers with the elimination of trimethylsilanol afforded yellow, transparent, and tough films of the aromatic polyamide-imides. The thermal conversion of the silylated precursor polymer to polyamide-imide proceeded almost as rapidly as that of the corresponding polyamide-amic acid prepared by a conventional method from the parent aromatic diamine and 4-chloroformylphthalic anhydride.     

Comparative study on pore structures of mesoporous ZSM-5 from resorcinol-formaldehyde aerogel and carbon aerogel templating

Resorcinol-formaldehyde aerogels and carbon aerogels of different mesoporosities have been used as templates for preparing bimodal zeolites of mesopores. Samples were thoroughly characterized with X-ray diffraction, field emission scanning electron microscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, N(2) adsorption at 77 K, as well as FT-IR spectroscopy and (29)Si nuclear magnetic resonance spectroscopy. The mesoporous ZSM-5 zeolites have additional mesopores of 9-25 nm in widths and 0.07-0.2 cm(3)/g in volumes, besides their perfect inherent micropores. Experimental results show the mesoporous systems of the finally obtained zeolites can be influenced by proper preparation of resorcinol-formaldehyde aerogels and carbon aerogels through solution chemistry. Consequently, zeolites of tunable mesoporosities can be prepared with this unique methodology.     

Steady laminar flow of a power-law fluid in a curved pipe of circular cross-section with varying curvature

The steady laminar flow of power-law fluid through pipes of circular cross-section, whose center-line curvature varies locally, is analyzed theoretically. The flows, in three kinds of pipes whose center-lines are specified by

as examples of once, twice, and periodically-curved pipes, respectively, are discussed in comparison with Newtonian flow. The analysis is valid for any other two-dimensionally curved pipes, when the center-line curvature is small.     

Oxidative formation of thiolesters in a model system of the pyruvate dehydrogenase complex

In the presence of a catalytic amount of 3-butyl-4-methylthiazolium bromide, the reaction of benzaldehydes with azobenzene in dichloromethane containing octanethiol and Et(3)N gave the corresponding S-octyl thiobenzoates in good yields. The thiolesters were produced by trapping of the 2-benzoylthiazolium salts with the thiol, which were generated through the azobenzene oxidation of the active aldehydes. This is the first example for the thiolester formation mimicking the function of the pyruvate dehydrogenase complex. An electron-withdrawing substituent at the 4-position of benzaldehyde enhanced the reaction rate. The effect of benzaldehyde substituents on the reaction rate was examined quantitatively on the basis of kinetic measurements, leading to a nonlinear correlation of log(k(obs)) with Hammett's substituent constants (sigma). The origin of the nonlinear Hammett plot was interpreted in terms of a shift in the rate-determining step of the multistep reaction with change of the electronic nature of substituent. Further support for this assumption was given by the observation that the reaction constant (rho) of the Hammett plot for the azobenzene substituent effect on the oxidation rate of 4-bromobenzaldehyde was much smaller than that of 4-cyanobenzaldehyde.     

A three-component approach to isoquinoline derivatives by cycloaddition/Heck reaction sequence

Here, we report a three-component coupling reaction approach between an aldehyde, an allyloxyamine, and a maleimide toward isoquinoline derivatives. At first, an oxime O-allylic ether, prepared by dehydrative condensation of the aldehyde and the allyloxyamine, was reacted with the maleimide in the presence of a Pd²⁺ species. The cycloadduct obtained was then subjected to the Heck cyclization employing a Pd⁰ species to give thermodynamically stable diastereomer of isoquinoline derivatives selectively in 25-78% yields.     

Novel approaches to synthesize self-supported ultrathin carbon nanowire arrays templated by MCM-41

Ordered self-supported ultrathin carbon nanowire arrays with small mesopore sizes have been fabricated for the first time employing mesoporous silica MCM-41 as the templates, and two possible related mechanisms are proposed.