Alkane metathesis by tandem alkane-dehydrogenation-olefin-metathesis catalysis and related chemistry

Methods for the conversion of both renewable and non-petroleum fossil carbon sources to transportation fuels that are both efficient and economically viable could greatly enhance global security and prosperity. Currently, the major route to convert natural gas and coal to liquids is Fischer-Tropsch catalysis, which is potentially applicable to any source of synthesis gas including biomass and nonconventional fossil carbon sources. The major desired products of Fischer-Tropsch catalysis are n-alkanes that contain 9-19 carbons; they comprise a clean-burning and high combustion quality diesel, jet, and marine fuel. However, Fischer-Tropsch catalysis also results in significant yields of the much less valuable C(3) to C(8)n-alkanes; these are also present in large quantities in oil and gas reserves (natural gas liquids) and can be produced from the direct reduction of carbohydrates. Therefore, methods that could disproportionate medium-weight (C(3)-C(8)) n-alkanes into heavy and light n-alkanes offer great potential value as global demand for fuel increases and petroleum reserves decrease. This Account describes systems that we have developed for alkane metathesis based on the tandem operation of catalysts for alkane dehydrogenation and olefin metathesis. As dehydrogenation catalysts, we used pincer-ligated iridium complexes, and we initially investigated Schrock-type Mo or W alkylidene complexes as olefin metathesis catalysts. The interoperability of the catalysts typically represents a major challenge in tandem catalysis. In our systems, the rate of alkane dehydrogenation generally limits the overall reaction rate, whereas the lifetime of the alkylidene complexes at the relatively high temperatures required to obtain practical dehydrogenation rates (ca. 125 -200 °C) limits the total turnover numbers. Accordingly, we have focused on the development and use of more active dehydrogenation catalysts and more stable olefin-metathesis catalysts. We have used thermally stable solid metal oxides as the olefin-metathesis catalysts. Both the pincer complexes and the alkylidene complexes have been supported on alumina via adsorption through basic para-substituents. This process does not significantly affect catalyst activity, and in some cases it increases both the catalyst lifetime and the compatibility of the co-catalysts. These molecular catalysts are the first systems that effect alkane metathesis with molecular-weight selectivity, particularly for the conversion of C(n)n-alkanes to C(2n-2)n-alkanes plus ethane. This molecular-weight selectivity offers a critical advantage over the few previously reported alkane metathesis systems. We have studied the factors that determine molecular-weight selectivity in depth, including the isomerization of the olefinic intermediates and the regioselectivity of the pincer-iridium catalyst for dehydrogenation at the terminal position of the n-alkane. Our continuing work centers on the development of co-catalysts with improved interoperability, particularly olefin-metathesis catalysts that are more robust at high temperature and dehydrogenation catalysts that are more active at low temperature. We are also designing dehydrogenation catalysts based on metals other than iridium. Our ongoing mechanistic studies are focused on the apparently complex combination of factors that determine molecular-weight selectivity.     

Comments on the paper “Bragg’s law diffraction simulations for electron backscatter diffraction analysis” by Josh Kacher,Colin Landon,Brent L. Adams & David Fullwood

This comment on the paper "Bragg's Law diffraction simulations for electron backscatter diffraction analysis" by Kacher et al. explains the limitations in determining elastic strains using synthetic EBSD patterns.     

Propriétés rédox de fullerènes fonctionnalisés

The C₆₀ physical chemistry properties make it possible to use it an elemental base for the synthesis of new materials. As the functionalization of fullerenes modify these properties, the modifications of their physical chemistry properties, in particular their electrochemical properties have been studied for a series of highly functionalized fullerenes. This article presents the results obtained with the electrochemical studies of fullerenes C₆₀ mono- and polyfunctionalized covalently. Our objective was to analyze the possible correlations between the redox properties of fullerenes and the degree, shape and natue of the functionalization. A series of functionalized fullerenes on positions [6, 6] was synthetised to carry out the investigation. This unique series of mono- and poly-functionalized fullernes provides an effective study of the modifications of the physical chemistry and electrochemical properties vs. the degee, shape and nature of the functionalization. The results obtained have been compared with literature data.     

Microscale synthesis of phosphatidyl-[3H]choline from 1,2-diacylglycerol. Assessment of isomerization by reversed-phase high-performance liquid chromatography

The synthesis ofrac-1-palmitoyl-2-oleoylglycero-3-phospho-[³H]choline of high specific activity was carried out on a microscale by making 7 μmol ofrac-1-palmitoyl-2-oleoylglycerol react first with an equimolar amount of POCl₃ and then of [³H]choline. After purification by thin-layer chromatography and normal-phase high-performance liquid chromatography (HPLC), the yield of the synthesis of [³H]phosphatidylcholine (120 μCi/μmol) was 22%.rac-1-Palmitoyl-2-oleoylglycerol was purified before use by reversed-phase HPLC under conditions which were nonisomerizing and allowed the separation of 1,2- and 1,3-isomers of diacylglycerol. Ethanol, but not benzene, was shown to cause isomerization of long-chain diacylglycerol and, therefore, was not used for drying the substrate before reaction. A rapid and complete separation of 1,2- and 1,3-isomers of long-chain phosphatidylcholine was obtained by reversed-phase HPLC using 20 mM choline chloride in methanol/acetonitrile/water (50∶50∶1, by vol) isocratically as the mobile phase. Under these conditions, analysis of the synthetizedrac-1-palmitoyl-2-oleoylglycero-3-phospho-[³H]choline showed a total absence of 1,3-isomer.     

Preparation of intercalating agent-free epoxy/clay nanocomposites

A simple intercalating agent-free approach to prepare epoxy/montmorillonite (MMT) clay nanocomposite is reported. Through this new approach, no organic modifiers are needed. Thus, the cost for preparing polymer nanocomposites can be significantly reduced. The extent of dispersion and exfoliation of clay in epoxy was characterized by X-ray diffraction and transmission electron microscopy observations. It is found that the MMT clay is well-dispersed in epoxy matrix. The clay platelets in epoxy show a stacked structure with dimensions of about 1–2 μm in length and about 20 nm in thickness. At 4.5 wt% of clay loading level, the flexural modulus of the epoxy nanocomposite is increased by about 35%. No reduction in fracture toughness or glass-transition temperature is observed. The implication of the present finding for commercialization of polymer nanocomposites is discussed. POLYM. ENG. SCI., 47:1708–1714, 2007. © 2007 Society of Plastics Engineers