12 research outputs found
Synthesis and characterization of mesoporous silica thin films as a catalyst support on a titanium substrate
No full textMesoporous silica films with a thickness of 500–900 nm were synthesized on a titanium substrate by the evaporation-induced self-assembly method (with 900–1200 rpm for 90 s) using cetyltrimethylammonium bromide (CTAB) as structure-directing agent and tetraethyl orthosilicate as the silica source. Prior to coating deposition, the titanium substrate was oxidized to increase the surface roughness up to 500 nm and to produce a thin titania layer. Just before the synthesis, the titania layer was made super hydrophilic by an UV treatment for 2 h to provide a better adhesion of the silica film to the substrate. Films with hexagonal and cubic mesostructures with a uniform pore size of 2.8 nm and a surface area of 1080 m2/g were obtained and characterized by different methods. An alternative approach for surfactant removal by gradual heating up to 250 °C in vacuum was applied. Complete removal of CTAB from the as-synthesized silica films was confirmed by infrared spectroscopy
Selective hydrogenation of acetylene alcohols over a Pd/TiO2 coating in a capillary microreactor
No full textUnderstanding the structure-activity/selectivity relationships at the molecular scale is of significant importance in assisting the development of new catalytic processes in microstructured reactors. The selective hydrogenation of 2-methyl-3-butyne-2-ol on a series of well-characterized mesostructured titania supported bimetallic PdZn thin films has been studied to clarify key factors responsible for high selectivity to the semihydrogenated product. The highest selectivity towards the semihydrogenated product (2-methyl-3-butene-2-ol, MBE) over Pd/Ti02 was 83%. The bimetallic PdZn catalyst with a Zn/Pd molar ratio of 3 demonstrated the highest selectivity towards MBE of 90% which was achieved at 60 °C with a H2 partial pressure of 0.7. Addition of a base either on the surface of the catalyst or in the liquid phase further improved selectivity to MBE towards 98%
Selective hydrogenation of acetylene alcohols over a Pd/TiO2 coating in a capillary microreactor
No full textUnderstanding the structure-activity/selectivity relationships at the molecular scale is of significant importance in assisting the development of new catalytic processes in microstructured reactors. The selective hydrogenation of 2-methyl-3-butyne-2-ol on a series of well-characterized mesostructured titania supported bimetallic PdZn thin films has been studied to clarify key factors responsible for high selectivity to the semihydrogenated product. The highest selectivity towards the semihydrogenated product (2-methyl-3-butene-2-ol, MBE) over Pd/Ti02 was 83%. The bimetallic PdZn catalyst with a Zn/Pd molar ratio of 3 demonstrated the highest selectivity towards MBE of 90% which was achieved at 60 °C with a H2 partial pressure of 0.7. Addition of a base either on the surface of the catalyst or in the liquid phase further improved selectivity to MBE towards 98%
Reversible Gelation of Poly(dimethylsiloxane) with Ionic and Hydrogen-Bonding Substituents
No full textPoly(dimethylsiloxane) copolymers containing a small fraction of carboxylic acid or Zn-carboxylate groups were prepared and compared regarding reversible gelation by hydrogen-bonding and ion-pair interaction. The polymers were synthesized by condensation of a t-butylcarboxylate functionalized dichlorosilane with an α,ω-dihydroxy-poly(dimethylsiloxane), followed by thermal cleavage of the ester bond. Neutralization of the resulting carboxylic acid substituents was achieved by addition of Zn (acac)2. Reversible crosslinking was investigated by step stress and oscillating shear experiments. The carboxylic acid containing poly(dimethylsiloxane) became rubberlike upon increasing the temperature and liquified again when it was brought back to room temperature. This observation has been explained tentatively by segregation of the carboxylic acid groups into polar domains at high temperatures [i.e., a behavior like it is observed for systems with a lower critical solution temperature (LCST)]. At ambient temperature, the carboxylic acid groups undergo hydrogen bonding to the Si-O-Si backbone. Clustering of the carboxylic acid groups occurs only as these hydrogen bonds break upon raising temperature. Moisture was found to have a strong influence on the reversal of the crosslinking. Addition of zinc acetylacetonate resulted in the formation of an elastic network already at ambient conditions consistent with the concept of ionomers which undergo reversible gelation by formation of ion-pair multiplets and clusters in the hydrophobic polymer matrix in particularly at low temperatures. At high temperature, both the carboxylic acid and the carboxylate sample exhibited a rather similar viscoelastic behavior consistent with a common structure where transient crosslinks are formed by clusters of the carboxylic acid and the carboxylate groups. © 1999 John Wiley & Sons, Inc
