José A. Dias

Acidity measurements of zeolite Y by adsorption of several probes

Acid strength distribution measurements of zeolite Y by a combined analysis of calorimetric and adsorption data, Cal-ad, for the reaction of zeolite Y with pyridine, 2,6-lutidine, 2,6-di-tert-butylpyridine, quinoline, tetrahydrothiophene and benzonitrile show the presence of three families of acid sites. The strongest sites (heats above −29 kcal mol−1) contain a significant number of Bronsted sites (n = 0.1 mmol g−1), which are located inside the large cages. The sites of intermediary strength (heats around −17 kcal mol−1) are of Lewis and Bronsted types and the weaker sites (heats below −10 kcal mol−1) contain significant hydrogen bonding. The strength of these sites is probably due to mutual interaction between Lewis and Bronsted sites. The accessibility on zeolite and the size of the probe molecules play fundamental role on the determination of acid site distribution. Application of the electrostatic–covalent model (ECW) to this acid provides quantitative parameters for determination of donor-acceptor contributions based on electrostatic (EA) and covalent (CA) intrinsic characters to each chemical bonding. The results demonstrate that zeolite Y has a stronger covalent character with EA = 5.25, CA = 6.89 (CA/EA = 1.31) consistent with softness properties suggested in the literature.

A escolha da faixa espectral no uso combinado de métodos espectroscópicos e quimiométricos

A method is presented for the choice of spectral regions when absorption measurements are coupled to chemometric tools to perform quantitative analyses. The method is based on the spectral distribution of the relative standard deviation of concentration (sc/c). It has been applied to the development of PLS-FTNIR calibration models for the determination of density and MON of gasoline, and ethanol content and density of ethanol fuel. The new method was also compared with the correlation (R2) method and has proved to generate PLS calibration models that present better accuracy and precision than those based on R2.

Liquid phase calorimetry and adsorption analyses of zeolite beta acidity

Zeolite beta acidity, activated at 550 °C, was characterized by calorimetric and adsorption studies of pyridine in cyclohexane (Cal–Ad). Two different acid sites were found: n1 = 0.1612 mmol g−1 with ΔH1 = −83.6 kJ mol−1; and n2 = 0.3820 mmol g−1 with ΔH2 = −62.8 kJ mol−1. Step calcinations followed by FTIR indicated n1 as Lewis sites and n2 as weak Bronsted sites. When activated at 450 °C, zeolite beta showed three different sites: n1 = 0.0467 mmol g−1 with ΔH1 = −227.3 kJ mol−1; n2 = 0.1163 mmol g−1 with ΔH2 = −93.3 kJ mol−1; and n3 = 0.3322 mmol g−1 with ΔH3 = −80.4 kJ mol−1. Sites n1 and n2 were assigned as Bronsted centers, while n3 as a combination of Bronsted and Lewis sites. These results suggest that the zeolite beta strongest Bronsted sites are converted to Lewis sites at 550 °C. These Lewis sites are not only related to extra-framework Al species, but also to aluminium atoms partially bonded to the zeolite framework, as evidenced by infrared bands at 3777 and 875 cm−1. FTIR and thermogravimetric analyses of gas phase adsorbed pyridine were also used to investigate the materials acidity and a good agreement was found with Cal–Ad data. The results showed that the Cal–Ad method offers a more detailed description of the zeolite surface acidity by providing thermodynamic parameters not easily obtained by other techniques. A linear relationship between enthalpy and entropy values for several zeolites (ZSM-5, TS-1, MOR, Y and BEA) is also reported.

Analytical evaluation of BEA zeolite for the pre-concentration of polycyclic aromatic hydrocarbons and their subsequent chromatographic analysis in water samples.

The analytical performance of BEA - a commercial zeolite - is evaluated for the pre-concentration of fifteen Environmental Protection Agency - polycyclic aromatic hydrocarbons and their subsequent HPLC analysis in tap and lake water samples. The pre-concentration factors obtained with BEA have led to a method with excellent analytical figures of merit. One milliliter aliquots were sufficient to obtain excellent precision of measurements at the parts-per-trillion concentration level with relative standard deviations varying from 4.1% (dibenzo[a,h]anthracene) to 13.4% (pyrene). The limits of detection were excellent as well and varied between 1.1 (anthracene) and 49.9 ng L(-1) (indeno[1,2,3-cd]pyrene). The recovery values of all the studied compounds meet the criterion for regulated polycyclic aromatic hydrocarbons, which mandates relative standard deviations equal or lower than 25%. The small volume of organic solvents (100 μL per sample) and amount of BEA (2 mg per sample) makes sample pre-concentration environmentally friendly and cost effective. The extraction procedure is well suited for numerous samples as the small working volume (1 mL) facilitates the implementation of simultaneous sample extraction. These are attractive features when routine monitoring of numerous samples is contemplated.

Effect of cerium loading on structure and morphology of modified Ce-USY zeolites

Este trabalho descreve detalhadamente o efeito da quantidade de cerio na estrutura e morfologia da zeolita NH4USY. Ce-USY (2-25% m/m de CeO2) foi obtido por impregnacao umida de CeCl3 seguida de calcinacao a 550oC por 8 h. Em quantidades baixas (2-10%), foi observado que as especies de cerio encontram-se nas posicoes de troca ionica na rede, enquanto em maiores teores (15-25%) pequenos agregados formaram-se na superficie da HUSY. Difratometria de raios X (XRD) mostrou apenas reflexoes relacionadas a HUSY, confirmando a alta dispersao das especies de cerio, porem as analises por espectroscopia Raman com transformada de Fourier (FT-Raman) detectaram CeOx para os materiais acima de 10%. A reacao do CeCl3 com NH4USY produziu NH4Cl, o qual se decompoe em HCl, ocasionando a desaluminizacao da rede. Os materiais apresentaram um aumento da razao Lewis/Bronsted com o aumento da quantidade de cerio, devido a interacao do excesso de cerio com os grupos OH da USY e consequente formacao de especies CeOx.

Produção de biodiesel via transesterificação etílica com zeólitas básicas

Soybean oil transesterification with ethanol was carried out in a batch reactor using USY zeolites modified with barium and strontium (15 wt.%) as catalysts. A series of three catalytic cycles were performed for each zeolite without any loss of activity. The biodiesel product was analyzed by HPLC and FT-Raman, and the catalysts by pyridine and CO2 adsorption. Ba/USY provided higher conversions (> 97%) than Sr/USY (< 75%). The increased catalytic activity of Ba/USY was attributed to two different effects: a larger number of basic sites; and a lower interaction between barium species and HUSY BrOnsted sites.

Characterization of ZSM-5 modified with niobium pentoxide: the study of thiophene adsorption

ZSM-5 adsorbents impregnated with Nb2O5 were applied in the sulfur removal in the form of thiophene, refractory substance of difficult removal of liquid fuels. For this purpose, a model fuel containing iso-octane contaminated with thiophene in concentrations of 877.5 to 1155 ppmw was prepared. The samples were characterized by X-ray diffractometry (XRD), Fourier transform infrared (FTIR) and Fourier transform-Raman (FT-Raman) spectroscopies for confirmation of the adsorbents, being prioritized the adsorption study with that containing 5 wt.% of niobium pentoxide, because it showed a greater capacity for removal of thiophene. The best results of adsorption were achieved at 353 K, a longer time to reach equilibrium was observed. Under these conditions, the best kinetic fitting was achieved using the equation of pseudo-second order, demonstrating the domain of the phenomenon of chemisorption. While under lower temperatures, the diffusion model presented a better approximation of the experimental results. Also, the increasing of temperature did enhance spontaneous processes.

Acidity and Characterization of 12-Tungstophosphoric Acid Supported on Silica-Alumina

This work deals with preparation and characterization of H3PW12O40 (H3PW) supported on silica-alumina. Impregnation of H3PW (15, 20, 30 and 40 wt.%) on commercial silica-alumina support in acidic aqueous solution is effective for preparing this catalyst keeping its Keggin structure, according to different methods of characterization. The catalysts were tested in a model reaction of acetic acid with ethanol and 30 wt.% H3PW/SiO2-Al2O3 had the highest activity under the conditions: catalyst calcination at 300 oC, temperature of 100 oC, acetic acid:ethanol molar ratio of 2:1 and catalyst:acetic acid mass ratio of 10 wt.%. The reaction yield was 79 and 100% selectivity for ethyl acetate over three reutilizations, for reaction time of 2 h. The calculated total acid site distribution was 0.299 mmol g-1 (97% of the theoretical probed by pyridine), and most of these (0.236 mmol g-1) were Bronsted weak-medium strength (pyridine desorption between 300 and 500 oC).

Síntese e caracterização de mulita utilizando sílica obtida da casca de arroz

Mullite, composed of 1,4Al2O3.SiO2 was synthesized using commercial α-alumina (Alpha Cesar Company) and silica obtained from the calcinations of rice husk. This silica exhibits a high specific surface area, high purity and small average particle diameter. Silica (26.2 wt.%) and α-alumina powders (71.8 wt. %) were mixed and homogenized using an alumina orbital disc mill before and after the calcinations at 1100oC for 4 h in an air atmosphere. Then, the powders were pressed uniaxially at a pressure of 1200 MPa. The green compacts were sintered at 1650oC, for 4 h, in air. The density of compacted sintering samples was determined using the hydrostatic method. The chemical and structural properties were observed using ICP (Inductively Coupled Plasma), XRD (X-ray diffraction) and 29Si MAS (Magic Angle Spinning) NMR (Nuclear Magnetic Resonance) and the microstructural characterization performed using SEM (Scanning Electron Micrography). The results show that it is possible to synthesize mullite ceramic of high-purity and with little glassy phase from the α-alumina and silica obtained from the rice husk that is purified and calcinated.

Effect of Acidity, Structure, and Stability of Supported 12-Tungstophosphoric Acid on Catalytic Reactions

Polyoxometalates are large metal–oxygen nanostructured compounds that can be modified at the molecular level. Catalysts based on 12-tungstophosphoric acid (H3PW12O40) supported on different carries such as silica, silica–alumina, zirconia, and mesoporous MCM-41 have been successfully prepared and characterized by several physicochemical techniques (e.g., FTIR, FT-Raman, 31P MAS-NMR, calorimetry, thermal analysis, TPD of pyridine). The materials were tested in different reactions: transalkylation of benzene with aromatics, esterification of acetic acid and ethanol, esterification of oleic acid with ethanol, and cyclization of (+)-citronellal. The results, coupled with other applications of heteropoly acids in the literature, illustrate the potential for these catalysts to be used in greener processes, i.e., a more effective, selective, economical, and environmentally benign technique. The effects of support interaction, loading of H3PW12O40, calcination temperature and stability, leaching, and acidity were addressed for all studied catalysts. These data were correlated to the conversion and/or selectivity of the main product and were used for recovery of the catalysts. Moreover, it was clear that the strength of interaction of the heteropoly acid with the surface support as well as the acidity define the capacity of utilization of these catalysts. Effective ways to recovery the initial activity is still a challenge to take full advantage of these materials.