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- ItemApplication of sulfonated nanoporous carbons as acid catalysts for Fischer esterification reactions(Amsterdam : Elsevier, 2015) Tamborini, L.H.; Militello, M.P.; Balach, J.; Moyano, J.M.; Barbero, C.A.; Acevedo, D.F.Heterogeneous acid catalysts were prepared by sulfonation of nanoporous carbons (NPCs). The NPCs were produced by pyrolysis of resorcinol–formaldehyde nanoporous resins (NPRs). The NPRs were synthesized as wet gels by condensation of resorcinol and formaldehyde in a sol–gel polycondensation using Na2CO3 as catalyst. A cationic polyelectrolyte (poly(diallyl dimethyl ammonium chloride)) was used as pore stabilizer, allowing to dry the gels in air without any special procedures. Five NPRs with different properties were synthesized by varying the monomer to catalyst ratio (Resorcinol/Na2CO3, R/C). The morphological and textural characterizations of the NPCs were performed by scanning electron microscopy and nitrogen adsorption–desorption isotherms. The results indicate that using a molar ratio of R/C = 200, a nanoporous carbon NPC with large surface area (695 m2/g) is obtained. This NPC was sulfonated by reaction with three different sulfonating agents: (i) concentrated sulfuric acid; (ii) fuming sulfuric acid and (iii) chlorosulfonic acid in order to obtain a novel Fischer esterification catalyst. The amount of acid groups attached to the NPC surface was determined by titration using a modified Boehm method. The catalytic activity, for Fischer esterification reaction of different sulfonated NPCs, was compared with sulfonated NPRs, sulfonated commercial porous carbons and polymeric acid catalyst (cationic ion exchange resins, sulfonated fluoropolymers). The modification of NPCs with concentrated sulfuric acid seems to render the materials with more catalytic activity. The best sulfonated material NPC (PC200-H2SO4) shows a high catalytic activity for the esterification of acetic acid (90.8%) and oleic acid (60.6%) with ethanol. The conversion and conversion rate values are better than commercial acid catalysts. The results suggest that sulfonated NPC catalysts are promising materials for the synthesis of biodiesel and related reactions.
- ItemNew electronic device for driving surface acoustic wave actuators(Amsterdam : Elsevier, 2009) Brünig, R.; Mensel, K.; Kunze, R.; Schmidt, H.Surface acoustic wave (SAW) actuators are driven by a high frequency signal. The frequency range for an ideal SAW-generation is usually very narrow banded and may shift depending on various environmental conditions. We present a new electronic device which self-aligns to the optimal excitation frequency within a wide range. Any kind of SAW-actuator can be used. The device continuously scans a certain frequency range and characterizes the SAW-component. The ideal excitation frequency is then determined and used to drive the SAW-device. In case of changes like loading conditions or temperature variations the device automatically readjusts to the optimal frequency and prevents possible damage of the device or actuator in case of an error. © 2009.
- ItemComplex loading and simulation of acoustic thickness shear mode resonator(Amsterdam : Elsevier, 2010) Brünig, R.; Weihnacht, M.; Guhr, G.; Schmidt, H.During the last decades thickness shear mode resonators (TSM, QCM) have been object of comprehensive research. Many approaches were made to describe the behavior and physical effects when loaded. We present a physical model that describes the TSM in the full frequency range, including overtones for a large variety of loadings (e.g. gases, liquids or solid materials). By using an automated curve fit algorithm, absolute values for the loaded material (e.g. thickness, viscosity) can be extracted. The model has been validated with a large number of experiments including liquids with complex viscosities, biomolecule interactions, electrochemisty or vacuum deposition techniques. Additionally, the appearance of layer resonances have been predicted and verified. Layer resonances are remarkable because they appear at even-numbered overtones, which have been considered to be impossible.
- ItemElectrodeposition of Fe70Pd30 nanowires from a complexed ammonium-sulfosalicylic electrolyte with high stability(Amsterdam : Elsevier, 2010) Haehnel, V.; Fähler, S.; Schultz, L.; Schlörb, H.A highly stable plating bath for the electrodeposition of Fe-Pd nanowires into nanoporous alumina templates has been developed. Complexing of both metal ions and exchanging Fe2+ by Fe3+ avoid chemical reduction of Pd ions and, therefore, undesirable deposition. By using a pulse potential mode and appropriate adjustment of deposition potentials homogeneously filled templates without surface deposits and nanowires close to the desired composition of Fe70Pd30 have been achieved. These alloy nanowires represent a key step towards nanoactuators based on magnetic shape memory alloys. © 2010 Elsevier B.V. All rights reserved.