Gustavo Larsen

Preparation and evaluation of the electrospun chitosan/PEO fibers for potential applications in cartilage tissue engineering

Fibrous materials have morphological similarities to natural cartilage extracellular matrix and have been considered as candidate for bone tissue engineering scaffolds. In this study, we have evaluated a novel electrospun chitosan mat composed of oriented sub-micron fibers for its tensile property and biocompatibility with chondrocytes (cell attachment, proliferation and viability). Scanning electronic microscope images showed the fibers in the electrospun chitosan mats were indeed aligned and there was a slight cross-linking between the parent fibers. The electrospun mats have significantly higher elastic modulus (2.25 MPa) than the cast films (1.19 MPa). Viability of cells on the electrospun mat was 69% of the cells on tissue-culture polystyrene (TCP control) after three days in culture, which was slightly higher than that on the cast films (63% of the TCP control). Cells on the electrospun mat grew slowly the first week but the growth rate increased after that. By day 10, cell number on the electrospun mat was almost 82% that of TCP control, which was higher than that of cast films (56% of TCP). The electrospun chitosan mats have a higher Youngs modulus (P <0.01) than cast films and provide good chondrocyte biocompatibility. The electrospun chitosan mats, thus, have the potential to be further processed into three-dimensional scaffolds for cartilage tissue repair.

Metal-support effects in Pt/L-zeolite catalysts

A series of Pt/L-zeolites with different cations (varying acidity) were prepared and characterized by H2 and CO chemisorption and competitive toluene/benzene hydrogenation. The ratio of adsorption equilibrium constants for toluene/benzene was extracted from rates of hydrogenation. The variation of this ratio across a series of alkaline-earth exchanged Pt/L-zeolites indicates that there is a varying support interaction with Pt clusters in the zeolite and suggests that the clusters interact electronically with the support.

Characterization of Pt/L-zeolite catalysts by chemisorption, EXAFS and reaction of neopentane with H2

Abstract We have prepared four series of Pt/L-zeolite Catalysts that were calcined at six different temperatures and subsequently reduced at the same temperature (773K). Two series were prepared by ion exchange and two by impregnation. In all cases the dispersion (H/Pt or CO/Pt) was a maximum after calcination at 543–643 K. The Catalysts were further characterized by EXAFS and reaction of neopentane with H2. There is a good correlation between the particle size determined by chemisorption and the Pt first shell coordination number determined by EXAFS. The neopentane isomerization selectivity ratio was observed to be a linear function of particle size measured by chemisorption or EXAFS coordination number. In addition, enhanced isomerization and hydrogenolysis activities are found to correlate with an increase in the support acidity.

Facile sol-gel synthesis of porous silicas using poly(propylene)imine dendrimers as templates

Commercially available poly(propylene)imine (DAB-Am-32 and DAB-Am-64) dendrimers were used as single-molecule templates to tailor the porosity of silicas via a nonacidic sol-gel method. X-ray diffraction on both the as-prepared (oven-dried at 373 K) and the calcined (833 K) materials revealed that modest contraction took place on template removal and that the cavities created did not achieve three-dimensional ordering under the current synthesis conditions. Transmission electron microscopy of Pt-stained samples supported this picture. A modified Horvath-Kawazoe analysis of the argon adsorption isotherms indicated that DAB-Am-64 is a much more effective template than DAB-Am-32. Pyrolysis and oxidation protocols for template removal are also presented. (c) 2000 Materials Research Society.

On the deactivation of Pt/L-zeolite catalysts

Pt/L-zeolite catalysts have a unique activity forn-hexane aromatization to benzene. There have been proposals which attribute this to electronic and to geometric origins of the L-zeolite. Recently, the uniqueness of the L-zeolite support has been understood to derive from the ability to stabilize very small particles in a non-acidic environment and it has been proposed that a further stabilization against deactivation (by geometric constraint of bimolecular coke precursor reactions) is what distinguishes these catalysts relative to SiO2 supported small Pt particles. We have investigated the initial deactivation rate of four Pt/L-zeolite catalysts and a Pt/SiO2 reference during reaction ofn-hexane, neopentane and 2-methyl-2-pentene. In all cases, the relative rates of deactivation correlate with the apparent acidity (as determined by competitive benzene/toluene hydrogenation) suggesting that the deactivation stabilization may have an electronic component.

Platinum-Nickel/L-zeolite Bimetallic Catalysts: Effect of Sulfur Exposure on Metal Particle Size and n-Hexane Aromatization Activity and Selectivity

Abstract Experimental evidence that Ni can be used to stablizie Pt particles supported in L-zeolite against deactivation by sulfur is presented. A series of Pt-Ni/L-zeolite bimetallic catalysts prepared by co-impregnation were tested for n-hexane aromatization and characterized by Extended X-ray Absorption Fine Structure (EXAFS), hydrogen chemisorption and Transmission Electron Microscopy (TEM) before and after sulfiding. It was found that bimetallic particles are significantly less subject of sulfur-induced particle growth than their monometallic (pt) counterpart. This mechanism of catalyst fouling is known to cause deterioration of Pt/L aromatization catalysts.

The effect of Preparation Method on Metal-Support Interaction in Pd/L-Zeolite Catalysts

Abstract The method of preparation, ion exchange (IE) versus impregnation (IP), effects the final dispersion, acidity and sintering stability of L-zeolite supported catalysts. The origin of the dispersion effect appears to be in the shape and size of the PdO precursor formed upon calcination which is more elongated with respect to the zeolite channel and has a lower nearest neighbor oxide coordination number when IP is used. While acidity introduced into the L-zeolite before preparation does not appear to effect the initial dispersion, acidity introduced after reduction of the IE preparation stabilizes the Pd particles toward sintering in H 2 .

Methoxy formation/spillover on Pd/Al2O3 studied by 13C, 1H NMR

Abstract The reaction of H 2 with 13 CO adsorbed on Pd/Al 2 O 3 has been investigated by solid state 13 C NMR. As the reaction proceeds, the peak for adsorbed CO on the Pd particles decreases while the intensity of a new peak near 60 ppm increases. This new peak is tentatively assigned to methoxy groups. When the sample was annealed at temperatures higher than 180°C, gas phase CH 4 was observed as a sharp peak near -10 ppm. The metallic character of the Pd particles was studied by observing the first moment and spin lattice relaxation time of the 13 CO on Pd which changed as the reaction progressed. Proton NMR was also employed to confirm the assignment of 13 C peaks as well as the presence of palladium hydride.

CHARACTERIZATION OF Pt-Ni/KL-ZEOLITE BIMETALLIC CATALYSTS BY CHEMISORPTION, CATALYSIS AND X-RAY ABSORPTION

A series of Pt-Ni bimetallic catalysts with varying Ni/Pt ratio were characterized by H2 chemisorption, cyclopropane hydrogenation/hydrogenolysis, and X-ray absorption (XANES and EXAFS). All techniques indicate that there is metal-metal interaction. Of particular interest is the Pt-Ni electronic interaction as determined by XANES in the presence of H2 chemisorption.