1,721,033 research outputs found
An exploration of Silsesquioxanes and Zeolites using High-Speed experimentation
No full textCombinatorial Chemistry and High-Speed Experimentation techniques allow the rapid preparation and testing of large numbers of samples by using automated workstations. These techniques are increasingly applied to various fields of chemical research and particularly to catalysis. In this project, High-Speed Experimentation techniques were used to study two families of compounds with a silicon-oxygen framework: silsesquioxanes and zeolites. Silsesquioxanes are inorganic-organic hybrid materials with broad applications as model compounds for silica surfaces and as ligands in coordination chemistry and catalysis. Here, the synthesis of incompletely condensed silsesquioxanes as precursors for titanium catalysts active in the epoxidation of alkenes was optimised by means of High-Speed Experimentation techniques. This thorough study led to the identification of a number of trends, to new and more efficient methods to synthesise known silsesquioxanes and to the discovery of new silsesquioxane precursors for active catalysts. The most interesting results were reproduced at a conventional laboratory scale and the silsesquioxane products were fully characterised. One of these silsesquioxane structures was used to prepare an osmium complex that proved to be a useful model compound for a known heterogeneous catalyst and an active and safe homogenous catalyst for the dihydroxylation of alkenes. Zeolites are microporous crystalline materials with applications as heterogeneous catalysts, ion-exchangers and molecular sieves. The synthesis of aluminium-rich zeolite beta was investigated by means of High-Speed Experimentmation techniques in order to identify the lowest Si/Al ratio to obtain pure zeolite beta with hydrothermal methods.Applied Science
Chiral dirhodium catalysts immobilised in porous hosts: Synthesis and performance
No full textThe main objective of this research is to modify the catalytic properties of homogeneous chiral dirhodium catalysts upon immobilisation on the porous support materials silica, MCM-41 and TUD-1. The catalysts were immobilised via ligand exchange of one chiral ligand with a carboxylate tether group. This influences the enantio- and trans/cis selectivity of the catalysts in the cyclopropanation reaction of styrene with ethyl or tert-butyl diazoacetate and the Si-H insertion reaction of dimethylphenylsilane with methyl phenyldiazoacetate. The results show that it is possible to influence the selectivity of these catalysts by immobilising them. Leaching of the catalyst from the carrier materials remains a problem. Full conversions are still possible upon recycling, but analysis shows that a significant amount of rhodium leaches. In some cases the active catalyst leaches, in others the filtrate is not active, so non-active rhodium leaches. Future work needs to be addressed to the optimisation of the constraint induced by the pore walls as well as to the problem of leaching. The former would be greatly facilitated by computational modelling studies and the latter by ligands that are giving a more stable complex (e.g. four- or six-dentate ones).Applied Science
Probing the titanium sites in Ti-MCM41 by diffuse reflectance and photoluminescence UV-vis spectroscopies
No full textGoing Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The identity of titanium centres in microporous aluminophosphates compared with Ti-MCM-41 mesoporous catalyst and titanosilsesquioxane dimer molecular complex: a spectroscopy study
No full textOne-pot Incorporation of Titanium Catalytic Sites into Mesoporous True Liquid Crystal Templated (TLCT) Silica
No full textL.Marchese, E. Gianotti, T. Maschmeyer, G. Martra, S. Coluccia, J.M. Thomas Spectroscopic Tools for Probing the Isolated Titanium Centres in MCM41 Mesoporous Catalysts
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