Browsing by Author "Sampaio, Maria"
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- Activated carbons functionalized with thiol and sulfonic acid groups for catalytic wet peroxide oxidationPublication . Gomes, Helder; Miranda, Sandra; Sampaio, Maria; Silva, Adrián; Faria, JoaquimActivated carbons are known catalysts for hydrogen peroxide decomposition through a pathway involving the formation of hydroxyl radicals. Thus, it seems logical to take advantage of the strong oxidizing properties of these radicals for the degradation of organic pollutants adsorbed and concentrated on the surface of activated carbons. In a recent work, we established the unprecedented performance of acidic activated carbons functionalized with sulphuric acid in the catalytic wet peroxide oxidation of Chromotrope 2R, an anionic azo dye. In the present work, we explore the influence of the amounts and types of surface groups with sulphur on the catalytic efficiency of the activated carbons.
- Activated carbons treated with sulphuric acid: catalysts for catalytic wet peroxide oxidationPublication . Gomes, Helder; Miranda, Sandra; Sampaio, Maria; Silva, Adrián; Faria, JoaquimDifferent liquid phase thermal treatments were applied to a commercial activated carbon (Norit ROX 0.8) in order to produce modified activated carbons with varying surface chemistry and increased acidic character. Chemical characterization of the prepared materials includes determination of the point of zero charge and evaluation of the concentration and nature of acidic and basic surface functionalities by acid/base titrations and temperature programmed desorption. The prepared materials were used as catalysts in the catalytic wet peroxide oxidation of the acid dye Chromotrope 2R in order to assess their removal efficiency. The relationship between the surface chemistry and efficiency for dye removal is discussed. As expected, decreasing acidity of the catalysts surface will correlate with increasing dye conversion. Unexpectedly, treatment with sulphuric acid leads to a very high yield of dye removal which falls out of the previous correlation. This was explained in terms of the introduction of sulphur containing groups on the carbon surface, which promotes the surface interaction between the pollutant and hydrogen peroxide: higher production of hydroxyl radicals close to the pollutant leads to improved dye removal. In addition, reutilization studies show that the catalyst prepared by sulphuric acid treatment is able to keep its performance in successive runs.
- Activated carbons treated with sulphuric acid: catalysts for catalytic wet peroxide oxidationPublication . Gomes, Helder; Miranda, Sandra; Sampaio, Maria; Silva, Adrián; Faria, JoaquimThe commercial activated carbon Norit ROX 0.8 was subjected to different liquid phase treatments in order to produce activated carbons with varying surface chemistry and acid/base character. The prepared materials were used as catalysts in the catalytic wet peroxide oxidation of the acid dye Chromotrope 2R in order to assess their removal activity. Chemical characterization of the prepared materials includes determination of the point of zero charge and evaluation of the concentration and nature of acidic and basic surface functionalities by acid/base titrations and temperature programmed desorption. The relationship between the surface chemistry and efficiency for dye removal is discussed. In general, dye removal is increased when using catalysts with higher basic character, due to enhanced electrostatic attraction between the acid dye and the catalyst surface when compared with catalysts presenting acidic character. In addition, treatment with sulphuric acid introduces on the catalysts surface sulphur containing groups, which promotes the surface interaction between the pollutant and hydrogen peroxide. This leads to improved dye removal resulting from higher production of hydroxyl radicals close to the pollutant. Reutilization studies show that the catalyst prepared by sulphuric acid treatment is able to keep its performance in successive runs.
- Mechanistic insights into catalytic wet peroxide oxidation over activated carbons treated with sulfuric acidPublication . Gomes, Helder; Miranda, Sandra; Sampaio, Maria; Figueiredo, José; Silva, Adrián; Faria, Joaquim
- Produção de biodiesel por catálise heterogéneaPublication . Sampaio, Maria; Gomes, HelderDevido à limitada disponibilidade de novas reservas petrolíferas e ao consequente aumento dos preços do petróleo na última década, a procura de biocombustíveis tem vindo a ser alvo de intensa investigação. O biodiesel parece ser o melhor candidato a substituto do diesel convencional, uma vez que é ambientalmente sustentável e facilmente produzido por transesterificação de triglicéridos com metanol, usando catalisadores homogéneos alcalinos ou ácidos. Para ultrapassar as desvantagens inerentes à utilização da catálise homogénea, nomeadamente, a neutralização do catalisador e sua separação do produto final, nos últimos anos, tem vindo a ser propostos sistemas catalíticos heterogéneos. Os catalisadores heterogéneos surgem como uma alternativa desejável para optimizar a síntese de biodiesel. Neste contexto, este trabalho teve como objectivo o estudo da performance de catalisadores sólidos com características ácidas. Foram testados diferentes materiais na reacção de transesterificação do triacetato de glicerol (triglicérido modelo), nomeadamente, resinas contendo grupos sulfónicos (Amberlyst-15 e Nafion SAC-13) e carvões activados com diferentes químicas superficiais. A reacção realizou-se num reactor batch, definindo-se como condições padrão, uma temperatura de 323 K, uma razão molar triacetato de glicerol:metanol de 1:6 e uma concentração de catalisador sólido de 20 g/L. Os catalisadores Amberlyst-15 e Nafion SAC-13 apresentaram uma elevada actividade na reacção de transesterificação, similar à obtida usando H2SO4. No entanto, comparado com o NaOH, a sua actividade é bastante inferior. Os carvões activados, sem tratamento específico para aumentar a sua acidez superficial, revelaram-se completamente inactivos nas condições padrão empregues. As variáveis pertinentes do processo foram estudadas utilizando a resina Amberlyst-15 como catalisador. A presença de grupos ácidos -SO3H, o baixo valor de PZC (ponto de carga zero) e a elevada concentração de centros activos ácidos parece ser determinante na eficiência catalítica da Amberlyst-15. Com o objectivo de introduzir grupos ácidos na superfície dos materiais de carvão testados, procedeu-se ao seu tratamento com H2SO4 e HNO3. Após o tratamento, verificou-se que os carvões se tornam activos para a produção de biodiesel, contudo com baixa actividade, quando comparada com a actividade apresentada pela Amberlyst-15. Because of the restricted availability of new petroleum reserves and the consequent crude´s rise in price in the last decade the search of green fuels has been subject of intense research. Currently, biodiesel seems to be the best candidate to substitute candidate of conventional diesel, since it is environmentally sustainable and easily produced by transesterification of triglycerides with methanol, using homogeneous alkali or acid catalysts. In order to overcome the problems presented by homogeneous catalytic systems, namely the need for catalyst neutralization and their separation from the final product, in the last years heterogeneous catalysts have been suggested. Heterogeneous catalysts are desired alternatives to optimize the biodiesel synthesis. In this context, the purpose of this work was to investigate the performance of solid acid catalysts. Different materials were tested in the transesterification of glycerol triacetate (triglycerides model) namely resins containing sulfonic acid groups (Amberlyst-15 and Nafion SAC-13) and activated carbon with different superficial chemistry. The screening reaction runs were carried out at 323 K in a batch reactor, with a glycerol triacetate molar:methanol ratio of 1:6 and a solid catalysts concentration of 2 g/L. The Amberlyst-15 and Nafion SAC-13 catalysts showed a high activity in the transesterification reaction, similar to the H2SO4 one. Never the less, when compared with NaOH, its activity is low. The activated carbons without specific treatment in order to rise its superficial acidity, showed to be completely inactive in the applied conditions. The process conditions have been studied using the Amberlyst-15 resin as catalyst. The presence of acid groups -SO3H, a low PZC (point of zero charge) and the high concentration of active acid sites seems to be determinate to the observed catalytic efficiency of Amberlyst-15. Accordingly, in order to introduce acid groups at the carbon materials surface, they were treated with H2SO4 and HNO3. After the treatment, it was verified that the activated carbons present activity, although low, when compared with of Amberlyst-15.
- Production of biodiesel using heterogeneous acid catalystsPublication . Sampaio, Maria; Gomes, Helder
- The role of activated carbons functionalized with thiol and sulfonic acid groups in catalytic wet peroxide oxidationPublication . Gomes, Helder; Miranda, Sandra; Sampaio, Maria; Figueiredo, José; Silva, Adrián; Faria, JoaquimNorit ROX 0.8 commercial activated carbon was subjected to liquid phase treatments with sulfuric acid at different concentrations (5-18 M) and temperatures (353 and 423 K), to generate different amounts (760-1000 mu mol g(-1)) of sulfur groups (thiol and sulfonic acid), with varying surface acidity. The prepared materials were chemically characterized by the determination of the point of zero charge (2.0-5.8) and by mass titration, temperature programmed desorption and X-ray photoelectron spectroscopy, to evaluate the nature and concentration of acidic surface functionalities. The materials were tested as catalysts in the catalytic wet peroxide oxidation of aqueous solutions of the acid dye Chromotrope 2R. A correlation between the surface acidic strength and the efficiency for dye removal from solution was found, which was ascribed both to adsorption and to reaction. In general, dye removal by adsorption is increased when using adsorbents with lower acidic character, due to enhanced electrostatic attraction between the acid dye and the catalyst surface. In addition, the introduction of sulfur containing groups promotes the decomposition of hydrogen peroxide, leading to increased production of hydroxyl radicals close to the pollutant and subsequent improved dye degradation.