2,463 research outputs found
Conversion of biorenewable feedstocks: New challenges in heterogeneous catalysis
For biorenewable feedstocks to serve as a significant source of chemicals and/or fuels, the development of new chemical processes as well as biological processes will be required. However, the conversion of biorenewable feedstocks with heterogeneous catalyst-based processes provides new challenges in inorganic catalyst research and development relative to historical work with petrochemical feedstocks. These catalyst and process challenges include the need to convert highly functionalized molecules with high selectivity, to develop stable catalytic liquid-solid interfaces in which the liquid phase is commonly aqueous, to control solvent phase effects and to develop novel reaction systems. While some of these challenges will be addressed using novel catalytic materials, others will need to be overcome through design of new catalytic reaction systems. Examples of emerging research results demonstrating unique approaches that have been taken to begin to address the efficient conversion of biorenewable feedstocks to chemicals and fuels are discussed
A comparative study of macroporous metal oxides synthesized via a unified approach
A variety of macroporous metal oxides was synthesized via a spontaneous self-assembly process in aqueous solution starting from organic alkoxide precursors under a common set of conditions to present a consistent set of data for evaluation of macropore formation in these materials that are known to yield such structural patterns. Sol-gel-type chemistry appears to be governing the structure formation in these materials, so the influence of sol-gel parameters such as the alkyl group in the alkoxide, the central metal atom, and the pH of the reaction mixture were investigated in order to gain insight on the denning mechanism dictating the formation of these structures. The results revealed that depending upon the central metal atom or the alkyl group, the alkoxide precursors had characteristic hydrolysis and condensation rates, which, when balanced appropriately, resulted in structured macroporous pattern formation in the final materials. Powders obtained in the spontaneous self-assembly processes were found to have varying macropore sizes as well as extents of macroporosity upon adjusting standard sol-gel synthesis parameters, which control the relative rates of the hydrolysis and condensation reactions
Positional Coincidence between the High-latitude Steady Unidentified Gamma-ray Sources and Possibly Merging Clusters of Galaxies
We report an evidence for the first time that merging clusters of galaxies
are a promising candidate for the origin of high galactic-latitude, steady
unidentified EGRET gamma-ray sources. Instead of using past optical catalogs of
eye-selected clusters, we made a matched-filter survey of galaxy clusters over
4\arcdeg \times 4\arcdeg areas around seven steady unidentified EGRET sources
at |b|>45\arcdeg together with a 100 \sq \arcdeg area near the South
Galactic Pole as a control field. In total, 154 Abell-like cluster candidates
and 18 close pairs/groups of these clusters, expected to be possibly merging
clusters, were identified within estimated redshift . Five
among the seven EGRET sources have one or two cluster pairs/groups (CPGs)
within 1\arcdeg from them. We assess the statistical significance of this
result by several methods, and the confidence level of the real excess is
maximally 99.8% and 97.8% in a conservative method. In contrast, we found no
significant correlation with single clusters. In addition to the spatial
correlation, we also found that the richness of CPGs associated with EGRET
sources is considerably larger than those of CPGs in the control field. These
results imply that a part of the steady unidentified EGRET sources at
high-latitude are physically associated with close CPGs, not with single
clusters. We also discuss possible interpretations of these results. We argue
that, if these associations are real, they are difficult to explain by hadronic
processes, but best explained by the inverse-Compton scattering by high energy
electrons accelerated in shocks of cluster formation, as recently proposed.Comment: 9 pages, 2 PostScript figures, uses emulateapj5.sty, added new
analysis and discussion, ApJ accepte
Synthesis of hierarchically structured aluminas under controlled hydrodynamic conditions
Aluminas having hierarchical bimodal pore structures with regular arrayed macropores interconnected with mesopores were synthesized in a cone/plate apparatus that provided well-defined hydrodynamic I conditions. A parametric synthesis study was performed to better understand the synthesis conditions required to form the hierarchical structures. The synthesis experiments demonstrated that the hierarchnical structure could only be formed under limited mixing conditions. The mesoporous structure, which was created by interstitial porosity between boehmite nanoparticles, was formed independently of the macropores and was not significantly impacted by the use of a surfactant, whereas the formation of the macropores required the presence of an alkoxide droplet within the synthesis mixture and the surfactant played no role other than to influence the hydrodynamic conditions during synthesis
Kinetic Analysis of the Hydrogenolysis of Lower Polyhydric Alcohols: Glycerol to Glycols
The production of ethylene glycol and propylene glycol from higher polyhydric alcohols has been parametrically examined numerous times. However, efforts to develop improved catalyst systems require a better understanding of the reaction mechanism. Glycerol conversion to the glycols represents an initial system for developing an improved mechanistic understanding of the conversion of the more complex higher polyhydric alcohols. Batch reactor studies with ruthenium on carbon catalysts were performed at two pH levels to obtain kinetic data. Langmuir-Hinshelwood-type models were developed from the experimental data to describe the hydrogenolysis of glycerol into ethylene glycol and propylene glycol as well as further degradation of the glycols. Detailed information on the competitive adsorption coefficients for the reaction species was determined, which led to conclusions about the limitations of previous parametric analysis
Reduction behavior of potassium-promoted iron oxide under mixed steam/hydrogen atmospheres
Potassium-promoted iron oxide catalysts are used in large volume for the commercial ethylbenzene dehydrogenation to styrene process. Short-term deactivation of these catalysts, which is addressed by operating in excess steam, is thought to be caused due to reactive site loss through coking and/or reduction. However, the relative importance of the two mechanisms is not known. Presented are results concerning the reduction behavior of potassium-promoted iron oxide materials in the absence of carbon. Thermogravimetric experiments and X-ray diffraction analysis were used to examine the reduction behavior of potassium-promoted iron oxide materials. The reduction behavior was then compared with results from isothermal ethylbenzene dehydrogenation reactor studies under low steam-to-ethylbenzene operation. Potassium incorporation was found to stabilize the iron oxide against reduction apparently through the formation of KFeO 2. Chromium addition improved the reduction resistance, which gave good qualitative agreement with the dehydrogenation reaction studies. In contrast, vanadium incorporation led to more significant reduction as well as poor stability in the dehydrogenation reaction
Catalyst and sorbent material for the production of hydrogen
A catalyst and sorbent is disclosed which comprises pellets with an absorbent core and a protective shell with a catalyst in the shell. Such material is especially well suited for steam reforming of hydrocarbons to produce hydrogen since a reforming catalyst can be incorporated in the shell and a sorbent for the by-product carbon dioxide can be used for the core. It is also well suited for producing hydrogen from carbon monoxide by means of the water gas shift reaction. The shell can be made sufficiently strong and durable for moving bed applications as well as fixed bed applications
Baby-Step Giant-Step Algorithms for the Symmetric Group
We study discrete logarithms in the setting of group actions. Suppose that
is a group that acts on a set . When , a solution
to can be thought of as a kind of logarithm. In this paper, we study
the case where , and develop analogs to the Shanks baby-step /
giant-step procedure for ordinary discrete logarithms. Specifically, we compute
two sets such that every permutation of can be
written as a product of elements and . Our
deterministic procedure is optimal up to constant factors, in the sense that
and can be computed in optimal asymptotic complexity, and and
are a small constant from in size. We also analyze randomized
"collision" algorithms for the same problem
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