Antonio S. Araujo

Multistep structural transition of hydrogen trititanate nanotubes into TiO2-B nanotubes : a comparison study between nanostructured and bulk materials

H-trititanate nanotubes obtained by alkali hydrothermal treatment of TiO(2) followed by proton exchange were compared to their bulk H(2)Ti(3)O(7) counterpart with respect to their thermally induced structural transformation paths. As-synthesized and heat-treated samples were characterized by XRD, TEM/SAED, DSC and spectroscopy techniques, indicating that H(2)Ti(3)O(7) nanotubes showed the same sequence of structural transformations as their bulk counterpart obtained by conventional solid state reaction. Nanostructured H(2)Ti(3)O(7) converts into TiO(2)(B) via multistep transformation without losing its nanotubular morphology. The transformation occurs between 120 and 400 degrees C through topotactic mechanisms with the intermediate formation of nanostructured H(2)Ti(6)O(13) and H(2)Ti(12)O(25), which are more condensed layered titanates eventually rearranging to TiO(2)(B). Our results suggest that the intermediate tunnel structure H(2)Ti(12)O(25) is the final layered intermediate phase, on which TiO(2)(B) nucleates and grows. The conversion of nanostructured TiO(2)(B) into anatase is completed at a much lower temperature than its bulk counterpart and is accompanied by loss of the nanotubular morphology.

Thermogravimetric investigations during the synthesis of silica-based MCM-41

MCM-41 material was synthesized starting from hydrogel containing colloidal fumed silica, sodium silicate, cetyltetramethylammonium bromide(CTMABr) as surfactant, and distilled water as solvent. These reactants were mixed to obtain a gel with the following composition: 4SiO2:1Na2O:1CTMABr:200H2O. The hydrogel with pH=14 was hydrothermally treated at100°C, for 4 days. Each day, the pH was measured, and then adjusted to 9.5–10 by using 30%acetic acid solution. Thermogravimetry was the main technique, which was used to monitor the participation of the surfactant on the MCM-41 nanophase, being possible to determine the temperature ranges relative to water desorption as well as the surfactant decomposition and silanol condensation.

Synthesis, characterization and catalytic application of cerium-modified MCM-41

The nanostructured CeMCM-41 material was synthesized by the hydrothermal method, using cethyltrimethylammonium bromide as template. The sample was characterized by X-ray diffraction, energy-disperse X-ray, thermogravimetry, nitrogen adsorption isotherm and pore size distribution. The specific surface area of CeMCM-41 was equivalent to 850 m 2 g -1 , with the pore width and mesopore volume of 3.8 nm and 0.8 cm 3 g -1 , respectively. To evaluate the catalytic performance of the CeMCM-41, a mass of ca. 0.1 g was heated from room temperature up to 723 K at heating hate of 10°C min -1 using oxygen flowing at 50 ml min -1 . The CeMCM-41 was tested as catalyst for n-heptane oxidation, in a fixed-bed continuous-flow reactor, at 723 K and F/W ratio (molar flow of reactant per mass of catalyst) of 0.4-0.6 g h mol -1 . The obtained products were analyzed in a gas chromatograph coupled to a mass spectrometer. The GC-MS analysis revealed that the main oxidation products obtained were typically CO and CO 2 , with 12% of hydrocarbon conversion.

Thermogravimetric kinetics of polyethelyne degredation over silicoaluminophosphate

Abstract In the current work, we investigated the Thermogravimetric kinetics of degradation of high-density polyethylene (HDPE) alone, and mixed with silicoaluminophosphate catalyst (SAPO-37/HDPE). To estimate the kinetic parameters of the polymer degradation, the Vyazovkin model-free kinetic method was applied. The activation energy ( E ) was calculated as a function of conversion ( α ) and temperature ( T ), providing an estimate of the time required for the degradation process at a given temperature. The products due to thermal and catalytic cracking of polyethylene were analyzed by gas chromatography/mass spectrometry (GC/MS). The SAPO-37 showed good catalytic activity, decreasing the activation energy for the process.

Kinetic study of HY zeolite regeneration by thermogravimetry

Abstract The optimized regeneration of coked HY zeolite was studied by thermogravimetry using a multiple heating rate integral kinetic method. It was shown that, in order to remove half the coke deposited on zeolite in a period of 1 h, it would be necessary to carry out thermo-oxidation at 742 K. In the same space of time, practically complete regeneration took place at 792 K.

Thermal Analysis Applied to Solid Catalysts Acidity, Activity and Regeneration

The use of catalysts in numerous important processes is widespread throughout the chemical and petroleum-processing industries. Thermal analytical techniques can be used to evaluate important properties and processes associated with solid catalysts. This paper presents examples carried out in our laboratory of the general application of TG and DSC to the acidity, activity and regeneration of solid catalysts.

Evaluation of the relative acid strength of cerium and calcium exchanged Y zeolite by TG and DSC

Abstract The relative acid strength of cerium and calcium Y zeolite was evaluated by thermogravimetry (TG) and differential scanning calorimetry (DSC) methods, using n-butylamine as a molecular probe. The total acidity was calculated from TG curves whereas the relative acid strength given in J per acid site was determined by the DSC curves, using the Borchardt-Daniels kinetic model. The enthalpies of these processes are proportional to the acid-site strength in each specific temperature range.

Catalytic degradation of polyethylene evaluated by TG

HZSM-5 zeolite was screened as catalyst for high density polyethylene degradation at 450‡C, under nitrogen static atmosphere. Two different samples were studied in this condition: HDPE alone and mixed with HZSM-5. The reactor was connected on line to an HP 5890-II gas Chromatograph. Sample degradation was investigated using a Perkin-Elmer Delta 7 Thermobalance, from room temperature to 800‡C, with heating rates of 5.0, 10.0 and 20.0‡C min−1. From TG curves, the activation energies, calculated using an integral kinetic method, decreased 60.6% in the presence of the zeolite.

Hydrothermal synthesis of cerium aluminophosphate

Abstract Cerium(III) aluminophosphate molecular sieve has been synthesized starting from aluminum hydroxide, phosphoric acid, hydrated cerium chloride and water, in the presence of a di-isopropylamine organic template. This material has a tetrahedral framework of aluminum (AlO 2 ) − and phosphorus (PO 2 ) + . The obtained structure was similar to that of ALPO-11. The materials have been characterized by using various physicochemical methods. The incorporation of Ce(III) ions into the aluminophosphate matrix did not change the ALPO-11 structure, but did, however, increase the surface area and total acidity.

Sorption and Diffusion of p-Xylene and o-Xylene in Aluminophosphate Molecular Sieve AlPO4-11

This paper presents experimental results for equilibrium and diffusion of C8 aromatics in laboratory synthesised crystals of AlPO4-11. The samples were prepared by the hydrothermal method, starting from pseudobohemite (CONDEA), 85% phosphoric acid, water and di-isopropilamine as organic template. Adsorption and diffusion data were obtained mainly by gravimetry at temperatures between 60–100°C. Saturation capacities were found in the range of 4 wt%. Equilibrium constants were estimated using virial plots yielding heats of adsorption between 10–12 Kcal/mol at low coverage. Intracrystalline diffusivities at higher temperatures (150–180°C) were also measured, using the Zero-Length-Column (ZLC) method. Diffusivities from both methods (gravimetric and ZLC) agreed reasonably well and followed a typical Arrhenius behaviour, with low activation energy (ca. 7 Kcal/mol).