Modeling of adsorption and nucleation in infinite cylindrical pores by two-dimensional density functional theory

In this paper, we present an analysis of argon adsorption in cylindrical pores having amorphous silica structure by means of a nonlocal density functional theory (NLDFT). In the modeling, we account for the radial and longitudinal density distributions, which allow us to consider the interface between the liquidlike and vaporlike fluids separated by a hemispherical meniscus in the canonical ensemble. The Helmholtz free energy of the meniscus was determined as a function of pore diameter. The canonical NLDFT simulations show the details of density rearrangement at the vaporlike and liquidlike spinodal points. The limits of stability of the smallest bridge and the smallest bubble were also determined with the canonical NLDFT. The energy of nucleation as a function of the bulk pressure and the pore diameter was determined with the grand canonical NLDFT using an additional external potential field. It was shown that the experimentally observed reversibility of argon adsorption isotherms at its boiling point up to the pore diameter of 4 nm is possible if the potential barrier of 22kT is overcome due to density fluctuations.

Equilibrium adsorption in cylindrical mesopores: A modified Broekhoff and de boer theory versus density functional theory

This paper presents a thermodynamic analysis of capillary condensation phenomena in cylindrical pores. Here, we modified the Broekhoff and de Boer (BdB) model for cylindrical pores accounting for the effect of the pore radius on the potential exerted by the pore walls. The new approach incorporates the recently published standard nitrogen and argon adsorption isotherm on nonporous silica LiChrospher Si-1000. The developed model is tested against the nonlocal density functional theory (NLDFT), and the criterion for this comparison is the condensation/evaporation pressure versus the pore diameter. The quantitative agreement between the NLDFT and the refined version of the BdB theory is ascertained for pores larger than 2 nm. The modified BdB theory was applied to the experimental adsorption branch of adsorption isotherms of a number of MCM-41 samples to determine their pore size distributions (PSDs). It was found that the PSDs determined with the new BdB approach coincide with those determined with the NLDFT (also using the experimental adsorption branch). As opposed to the NLDFT, the modified BdB theory is very simple in its utilization and therefore can be used as a convenient tool to obtain PSDs of all mesoporous solids from the analysis of the adsorption branch of adsorption isotherms of any subcritical fluids.

Modeling of adsorption in finite cylindrical pores by means of density functional theory

Adsorption of argon at its boiling point infinite cylindrical pores is considered by means of the non-local density functional theory (NLDFT) with a reference to MCM-41 silica. The NLDFT was adjusted to amorphous solids, which allowed us to quantitatively describe argon adsorption isotherm on nonporous reference silica in the entire bulk pressure range. In contrast to the conventional NLDFT technique, application of the model to cylindrical pores does not show any layering before the phase transition in conformity with experimental data. The finite pore is modeled as a cylindrical cavity bounded from its mouth by an infinite flat surface perpendicular to the pore axis. The adsorption of argon in pores of 4 and 5 nm diameters is analyzed in canonical and grand canonical ensembles using a two-dimensional version of NLDFT, which accounts for the radial and longitudinal fluid density distributions. The simulation results did not show any unusual features associated with accounting for the outer surface and support the conclusions obtained from the classical analysis of capillary condensation and evaporation. That is, the spontaneous condensation occurs at the vapor-like spinodal point, which is the upper limit of mechanical stability of the liquid-like film wetting the pore wall, while the evaporation occurs via a mechanism of receding of the semispherical meniscus from the pore mouth and the complete evaporation of the core occurs at the equilibrium transition pressure. Visualization of the pore filling and empting in the form of contour lines is presented.

Comparative Analysis of Structural and Morphological Properties of Large-Pore Periodic Mesoporous Organosilicas and Pure Silicas

A systematic study on the structural properties and external morphologies of large-pore mesoporous organosilicas synthesized using triblock copolymer EO20PO70EO20 as a template under low-acid conditions was carried out. By employing the characterization techniques of SAXS, FE-SEM, and physical adsorption of N-2 in combination with alpha(s)-plot method, the structural properties and external morphologies of large-pore mesoporous organosilicas were critically examined and compared with that of their pure-silica counterparts synthesized under similar conditions. It has been observed that unlike mesoporous pure silicas, the structural and morphological properties of mesoporous organosilicas are highly acid-sensitive. High-quality mesoporous organosilicas can only be obtained from synthesis gels with the molar ratios of HCl/H2O between 7.08 x 10(-4) and 6.33 x 10(-3), whereas mesoporous pure silicas with well-ordered structure can be obtained in a wider range of acid concentration. Simply by adjusting the HCl/H2O molar ratios, the micro-, meso-, and macroporosities of the organosilica materials can be finely tuned without obvious effect on their structural order. Such a behavior is closely related to their acid-controlled morphological evolution: from necklacelike fibers to cobweb-supported pearl-like particles and to nanosized particulates.

Application of a generalized thermodynamic approach to characterize mesoporous materials

Equilibrium adsorption and desorption in mesoporous adsorbents is considered on the basis of rigorous thermodynamic analysis, in which the curvature-dependent solid-fluid potential and the compressibility of the adsorbed phase are accounted for. The compressibility of the adsorbed phase is considered for the first time in the literature in the framework of a rigorous thermodynamic approach. Our model is a further development of continuum thermodynamic approaches proposed by Derjaguin and Broekhoff and de Boer, and it is based on a reference isotherm of a non-porous material having the same chemical structure as that of the pore wall. In this improved thermodynamic model, we incorporated a prescription for transforming the solid-fluid potential exerted by the flat reference surface to the potential inside cylindrical and spherical pores. We relax the assumption that the adsorbed film density is constant and equal to that of the saturated liquid. Instead, the density of the adsorbed fluid is allowed to vary over the adsorbed film thickness and is calculated by an equation of state. As a result, the model is capable to describe the adsorption-desorption reversibility in cylindrical pores having diameter less than 2 nm. The generalized thermodynamic model may be applied to the pore size characterization of mesoporous materials instead of much more time-consuming molecular approaches. (c) 2005 Elsevier B.V. All rights reserved.

Effect of the preparation technique on the catalytic properties of mesoporous V-HMS for the oxidation of toluene

Various mesoporous catalysts with vanadium loadings between 0.5 and 6 V wt.% and surface areas around 1300 m(2)/g were synthesized using the isomorphous substitution (IS) and molecular designed dispersion (MDD) techniques. Their catalytic properties were tested using toluene as a model VOC in a fixed bed reactor at temperatures between 300 and 550 degrees C. It was found that during the oxidation of toluene, over V-HMS synthesized via IS, conversion of toluene mainly results in carbon oxides, benzene, benzaldehyde and water. Total conversion is greatly improved when the vanadium content is increased from around 1.5 to 3.0 wt.%, but an increase in the textural porosity (V-TEX/V-MESO) from 0.3 to 0.6 had no discernable effect on the conversion. This can be explained by the fact that a V-TEX/V-MESO as low as 0.3 is sufficient to facilitate the access of toluene into the framework confined mesopores without any molecular transport limitations. However, when using V-HMS synthesized by MDD, conversion of toluene is greatly improved when the V-TEX/ V-MESO ratio is increased from 0.1 to 0.6. This is because the diffusion limitations are minimized by this increase. V-HMS synthesized via MDD does not exhibit selectivity to benzaldehyde, favoring total oxidation to CO and CO2. This different oxidation mechanism can be explained in terms of location, accessibility and number of active species on the surface of the HMS support. (c) 2005 Elsevier Inc. All rights reserved.

Interdependent lateral interactions, hydrophobicity and acid strength and their influence on the catalytic activity of nanoporous sulfonic acid silicas

A series of propylsulfonic (MCM-SOH) and octyl co-functionalised propylsulfonic (MCM-Oc-SOH) catalysts have been prepared by post modification of MCM-41 with mercaptopropyltrimethoxysilane (MPTS) to achieve SOH surface coverages spanning the range 0.12-1 monolayer. Within the MCM-Oc-SOH series, samples with submonolayer MPTS coverages were further grafted with octyltrimethoxysilane to cap bare hydroxyl sites and tune the hydrophobicity of the support. For the MCM-SO H series NH calorimetry revealed acid strength increases as a function of sulfonic acid loading, with -ΔH(NH ) increasing from 87 to 118 kJ mol. In contrast, MCM-Oc-SOH exhibits a dramatic enhancement of acid strength for submonolayer SOH coverages, with -ΔH(NH ) found to increase to 103 kJ mol. In line with these acid strength measurements the per-site activity of the MCM-SOH series in the esterification of butanol with acetic acid was found to increase with SOH content. Incorporation of octyl groups further promotes esterification activity of all the samples within the MCM-Oc-SOH series, such that the turn over frequency of the sample with the lowest loading of SOH more than doubles. Molecular dynamic simulations indicate that the interaction of isolated sulfonic acid groups with the pore walls is the primary cause of the decrease in acid strength and activity of submonolayer samples within the MCM-SOH series. Incorporation of octyl groups results in a combination of increased hydrophobicity and lateral interactions between adjacent sulfonic acid head groups, resulting in a striking enhancement of acid strength and esterification activity. © 2010 The Royal Society of Chemistry.

Green preparation of tuneable carbon-silica composite materials from wastes

Bio-oil has successfully been utilized to prepare carbon-silica composites (CSCs) from mesoporous silicas, such as SBA-15, MCM-41, KIT-6 and MMSBA frameworks. These CSCs comprise a thin film of carbon dispersed over the silica matrix and exhibit porosity similar to the parent silica. The surface properties of the resulting materials can be simply tuned by the variation of preparation temperatures leading to a continuum of functionalities ranging from polar hydroxyl rich surfaces to carbonaceous aromatic surfaces, as reflected in solid state NMR, XPS and DRIFT analysis. N2 porosimetry, TEM and SEM images demonstrate that the composites still possess similar ordered mesostructures to the parent silica sample. The modification mechanism is also proposed: silica samples are impregnated with bio-oils (generated from the pyrolysis of waste paper) until the pores are filled, followed by the carbonization at a series of temperatures. Increasing temperature leads to the formation of a carbonaceous layer over the silica surface. The complex mixture of compounds within the bio-oil (including those molecules containing alcohols, aliphatics, carbonyls and aromatics) gives rise to the functionality of the CSCs.

High pressure and low temperature study of ammonia borane and lithium amidoborane

Hydrogen has been considered as a potentially efficient and environmentally friendly alternative energy solution. However, one of the most important scientific and technical challenges that the "hydrogen economy" faces is the development of safe and economically viable on-board hydrogen storage for fuel cell applications, especially to the transportation sector. Ammonia borane (BH3NH 3), a solid state hydrogen storage material, possesses exceptionally high hydrogen content (19.6 wt%).However, a fairly high temperature is required to release all the hydrogen atoms, along with the emission of toxic borazine. Recently research interests are focusing on the improvement of H2 discharge from ammonia borane (AB) including lowering the dehydrogenation temperature and enhancing hydrogen release rate using different techniques. Till now the detailed information about the bonding characteristics of AB is not sufficient to understand details about its phases and structures. ^ Elemental substitution of ammonia borane produces metal amidoboranes. Introduction of metal atoms to the ammonia borane structure may alter the bonding characteristics. Lithium amidoborane is synthesized by ball milling of ammonia borane and lithium hydride. High pressure study of molecular crystal provides unique insight into the intermolecular bonding forces and phase stability. During this dissertation, Raman spectroscopic study of lithium amidoborane has been carried out at high pressure in a diamond anvil cell. It has been identified that there is no dihydrogen bond in the lithium amidoborane structure, whereas dihydrogen bond is the characteristic bond of the parent compound ammonia borane. It has also been identified that the B-H bond becomes weaker, whereas B-N and N-H bonds become stronger than those in the parent compound ammonia borane. At high pressure up to 15 GPa, Raman spectroscopic study indicates two phase transformations of lithium amidoborane, whereas synchrotron X-ray diffraction data indicates only one phase transformation of this material. ^ Pressure and temperature has a significant effect on the structural stability of ammonia borane. This dissertation explored the phase transformation behavior of ammonia borane at high pressure and low temperature using in situ Raman spectroscopy. The P-T phase boundary between the tetragonal (I4mm) and orthorhombic (Pmn21) phases of ammonia borane has been determined. The transition has a positive Clapeyron slope which indicates the transition is of exothermic in nature. Influence of nanoconfinemment on the I4mm to Pmn2 1 phase transition of ammonia borane was also investigated. Mesoporus silica scaffolds SBA-15 with pore size of ~8 nm and MCM-41 with pore size of 2.1-2.7 nm, were used to nanoconfine ammonia borane. During cooling down, the I4mm to Pmn21 phase transition was not observed in MCM-41 nanoconfined ammonia borane, whereas the SBA-15 nanocondfined ammonia borane shows the phase transition at ~195 K. Four new phases of ammonia borane were also identified at high pressure up to 15 GPa and low temperature down to 90 K.^

Determination of toluene hydrogenation kinetics with neutron diffraction

Total neutron scattering has been used to follow the hydrogenation of toluene-d₈ to methylcyclohexane-d₁₄ over 3 wt% platinum supported on highly ordered mesoporous silica (MCM-41) at 298 K and under 150 mbar D₂ pressure. The detailed kinetic information so revealed indicates that liquid reorganisation inside pores is the slowest step of the whole process. Additionally, the results were compared with the reaction performed under 250 mbar D₂ pressure as well as with toluene-h₈ hydrogenation using D₂ at 150 mbar.

Matériaux mésoporeux fonctionnalisés pour l'extraction sélective des terres rares

Les matériaux mésoporeux à base de silice sont des plateformes polyvalentes qui offrent une réponse aux besoins de domaines variés comme l’environnement, la santé et les énergies. La fonctionnalisation avec des groupements organiques en fait des matériaux hybrides qu’il est aisé d’orienter vers une application spécifique. Ainsi, afin de fournir une alternative aux procédés industriels, dommageables pour l’environnement actuellement utilisés pour l’extraction et la purification des terres rares, à savoir l’extraction liquide-liquide (ELL) majoritairement, les silices mésoporeuses ont été sollicitées à titre d’adsorbant dans l’extraction sur phase solide. Cette dernière, en opposition à l’ELL, présente de nombreux avantages dont, la suppression des solvants organiques, le contrôle de la sélectivité envers et parmi le groupe des éléments de terres rares (ÉTR) à travers l’ancrage du ligand sur un support solide et la possibilité de réutiliser plusieurs fois l’adsorbant. Les ÉTR sont des métaux qui participent à la transition vers des technologies moins coûteuses en énergie, il est donc primordial de rendre leurs procédés d’extraction plus verts. Dans le cadre de ce travail, différents types de silices ordonnées mésoporeuses, MCM-41, SBA-15 et SBA-16, ont été synthétisées, fonctionnalisées avec un ligand approprié, et leurs comportements vis à vis de ces éléments, comparés. Ces matériaux ont de nombreux points communs mais certaines caractéristiques les différencient néanmoins : la taille et la géométrie des pores, la connexion entre les pores, l’épaisseur des parois, l’accessibilité aux pores ou encore la diffusion des liquides ou gaz dans la matrice. C’est pourquoi, le but de cette étude est d’élucider l’impact de ces diverses propriétés sur l’adsorption sélective des ÉTR en condition statique et dynamique.

Mesoestruturas porosas a partir de materiais naturais

The MCM-41 mesoporous synthesis was done using rice hulls ash and chrysotile as natural alternative silica sources. For the using of these sources, chemical and thermic treatments were done in both materials. After chemical and thermic treatments, these materials were employed on the MCM-41 mesoctructures synthesis. The natural materials treated and employed in the synthesis were characterized by several techniques such as X-ray diffraction, N2 adsorption and desorption, scanning electronic microscopy and thermogravimetric analysis. MCM-41 standart samples synthetized with aerosil 200 commercial sílica were used to evaluation. The formed material from rice hulls ash showed values from BET specific area about 468 m².g-1, N2 adsorption and desorption isotherms and loss mass similar to reference materials. The silica from chrysotile calcined and leached was employed to mesoporous materials synthesis. The BET specific area showed values about 700 m².g-1, N2 adsorption and desorption isotherms type IV and loss mass similar to mesoporous materials. The formed material from calcined and leached chrysotile, without calcination, applied to phenol remotion carried high performance liquid chromatography and evaluated with organophilic clays with different treatments. By the characterization techniques were proved that mesoporous materials with lesser order that reference samples. The material formed from rice hulls ash without the calcination step achieved better adsorption results than organophilic clays

High Pressure and Low Temperature Study of Ammonia Borane and Lithium Amidoborane

Hydrogen has been considered as a potentially efficient and environmentally friendly alternative energy solution. However, one of the most important scientific and technical challenges that the “hydrogen economy” faces is the development of safe and economically viable on-board hydrogen storage for fuel cell applications, especially to the transportation sector. Ammonia borane (BH3NH3), a solid state hydrogen storage material, possesses exceptionally high hydrogen content (19.6 wt%).However, a fairly high temperature is required to release all the hydrogen atoms, along with the emission of toxic borazine. Recently research interests are focusing on the improvement of H2 discharge from ammonia borane (AB) including lowering the dehydrogenation temperature and enhancing hydrogen release rate using different techniques. Till now the detailed information about the bonding characteristics of AB is not sufficient to understand details about its phases and structures. Elemental substitution of ammonia borane produces metal amidoboranes. Introduction of metal atoms to the ammonia borane structure may alter the bonding characteristics. Lithium amidoborane is synthesized by ball milling of ammonia borane and lithium hydride. High pressure study of molecular crystal provides unique insight into the intermolecular bonding forces and phase stability. During this dissertation, Raman spectroscopic study of lithium amidoborane has been carried out at high pressure in a diamond anvil cell. It has been identified that there is no dihydrogen bond in the lithium amidoborane structure, whereas dihydrogen bond is the characteristic bond of the parent compound ammonia borane. It has also been identified that the B-H bond becomes weaker, whereas B-N and N-H bonds become stronger than those in the parent compound ammonia borane. At high pressure up to 15 GPa, Raman spectroscopic study indicates two phase transformations of lithium amidoborane, whereas synchrotron X-ray diffraction data indicates only one phase transformation of this material. Pressure and temperature has a significant effect on the structural stability of ammonia borane. This dissertation explored the phase transformation behavior of ammonia borane at high pressure and low temperature using in situ Raman spectroscopy. The P-T phase boundary between the tetragonal (I4mm) and orthorhombic (Pmn21) phases of ammonia borane has been determined. The transition has a positive Clapeyron slope which indicates the transition is of exothermic in nature. Influence of nanoconfinemment on the I4mm to Pmn21 phase transition of ammonia borane was also investigated. Mesoporus silica scaffolds SBA-15 with pore size of ~8 nm and MCM-41 with pore size of 2.1-2.7 nm, were used to nanoconfine ammonia borane. During cooling down, the I4mm to Pmn21 phase transition was not observed in MCM-41 nanoconfined ammonia borane, whereas the SBA-15 nanocondfined ammonia borane shows the phase transition at ~195 K. Four new phases of ammonia borane were also identified at high pressure up to 15 GPa and low temperature down to 90 K.

Preparação, caracterização e modificação de adsorventes para remoção de pesticidas de águas

O objectivo principal deste trabalho de mestrado consistiu em avaliar a potencial utilização de materiais adsorventes, nomeadamente, de carvões activados (AC) preparados por activação química com KOH, a partir de PET reciclado, e de materiais com estrutura mesoporosa ordenada, do tipo MCM-41 e SBA-15, na remoção de ácido 4 - cloro - 2 - metilfenoxiacético (MCPA) e de azul de metileno (MB), presentes nas águas. Nesta tese apresentam-se estudos de preparação e caracterização de materiais micro e mesoporosos e também estudos de avaliação da capacidade adsortiva em fase líquida. Prepararam-se três materiais microporosos, nomeadamente, PET-2-700, PET-2-700ox (AC oxidado) e PET-2-700red (AC reduzido), dois materiais constituídos exclusivamente por mesoporos, Si-MCM-41 e Ti-MCM-41-50 e dois materiais contendo maioritariamente mesoporos, mas que também possuem alguma microporosidade, tais corno, Si-SBA-15 e Ti-SBA-15-50. A caracterização textural dos adsorventes foi inferida por adsorção de azoto a 77K e por de difracção de raios X. Recorreu-se a três métodos de análise das isotérmicas, nomeadamente, Dubinin-Radushkevich, Brunauer-Ernrnett-Teller e alfa-s (as). A caracterização química dos AC foi realizada recorrendo-se a técnicas de análise elementar (AE) e espectroscopia de infravermelho com transformadas de Fourier (FTIR) e à determinação do ponto de carga zero. Os três carvões activados possuem valores de área superficial externa idênticos, o PET-2-700 possui o maior volume microporoso e o PET-2-700ox exibe o maior diâmetro de poros. Por outro lado, o PET-2-700ox possui um carácter fortemente ácido, o PET-2-700 exibe carácter ligeiramente ácido e o PET-2-700red apresenta propriedades ligeiramente alcalinas. Com base na AE, todas as amostras possuem percentagens de carbono elevadas, sendo que o PET-2-700red apresenta o valor mais elevado. Os resultados obtidos para a caracterização estrutural dos revelaram a obtenção de materiais mesoporosos de alta qualidade, definida pela elevada regularidade e uniformidade da estrutura porosa. A análise dos parâmetros de caracterização textura! permitiu inferir que os quatro materiais mesoporosos possuem valores de área superficial elevados, e que os materiais SBA-15 apresentam valores de volume poroso total e de tamanho de poros superiores aos manifestados pelos MCM-41. A incorporação de titânio não conduziu a uma perda significativa de qualidade dos materiais substituídos em relação às correspondentes amostras de sílica. Efectuaram-se estudos de adsorção em fase líquida de forma a avaliar a possível aplicação dos vários adsorventes na remoção de MCPA e de MB de efluentes líquidos. concluiu-se que o tempo de equilíbrio de 72 horas seria adequado e que a capacidade de adsorção dos vários AC era superior em meio ácido. Com base nas isotérmicas de adsorção do MCPA e do MB e na aplicação da representação de Langmuir e de Freundlich, foi possível concluir que o PET-2-700 possui a maior capacidade de adsorção do MCPA, 1.42 mmol/g, enquanto que o PET-2-700ox revelou a maior capacidade de adsorção do MB, 1.43 mmol/g. Na realidade, os materiais microporosos estudados apresentaram percentagens de remoção elevadas, tanto do MCPA como do MB. Relativamente aos materiais mesoporosos ordenados preparados neste trabalho, a percentagem de remoção de para os poluentes em estudo foi relativamente baixa, constatando-se que nesta fase dos estudos não constituem uma alternativa viável à utilização dos AC. No entanto, uma funcionalização criteriosa dos mesmos pode eventualmente proporcionar um aumento da capacidade adsortiva. ABSTRACT: The work presented in this master thesis, consisted of evaluating the potential use of different adsorbents materials, like activated carbon (AC) prepared by chemical activation with KOH, from recycled poly (ethylene terephthalate) (PET) and materials with ordered mesoporous structure such as MCM-41 and SBA-15, for removing acid 4-chlorine-2-metilfenoxiacétic and methylene blue from aqueous phase. We had prepared three microporous materials, PET-200-700, PET-2-700ox (AC oxidized) and PET-2-700red (reduced AC), two materials consisting exclusively of mesopores, Si-MCM-41 and Ti-MCM-41-50 and two materials containing mainly mesopores, but also having some microporosity, such as Si-SBA-15 and Ti-SBA-15-50. The textural characterization of the adsorbents was inferred by nitrogen adsorption at 77K and X-ray diffraction. Three methods were used to analyse the isotherms, namely, Dubinin-Radushkevich, Brunauer-Emmett-Teller and alpha-s (as). The chemical characterization of AC was performed using the elementary analysis, Fourier transform infrared spectroscopy (FTIR) and determination of the point of zero charge. Concerning the AC, the three present almost the same externa! surface area, PET-2-700 has a high micropore volume and PET-2-700ox shows the largest pore size diameter. On the other hand, PET-2-700ox had a strong acid character, PET-2-700 exhibits just a slightly acid character and PET-2-700red presents alkaline properties. The AE analysis allows confirming the high carbon content of theses AC, with PET-2-700red exhibiting the highest carbon proportion. The results from the structural characterization of the mesoporous materials, had disclosed the attainment of materials with high quality, defined by the raised regularity and uniformity of the porous structure. The analysis of the textural parameters allowed inferring that the four studied mesoporous materials possess high superficial area. The SBA-15 type materials present higher values of total porous volume and pores size diameter as the MCM-41. Also, the titanium incorporation did not lead to a significant loss of quality of the materials substituted in relation to the corresponding silica samples. The adsorption studies in liquid phase allow evaluating the possibility of using the different adsorbents for the MCPA and the MB removal. The kinetic studies had allowed to state the equilibrium time as 72 hours and a higher adsorption capacity was achieved in an acid medium. The influence of the pH of the medium, on the MCPA adsorption was evaluated. The MCPA and MB isotherms were analysed based on the Langmuir and Freundlich equation, the representations presented an excellent linearity, indicating the applicability of these equations to these systems. Also, it allows concluded that PET-2-700 had a higher adsorption capacity for MCPA, 1.42 mmol/g, and PET-2-700ox had a higher adsorption capacity for MB, 1.43 mmol/g. The AC used presented high removal percentages for MCPA and MB. Concerning the mesoporous materials prepared in this work, the percentage removal for the pollutants in study was relatively low, and evidencing that at the moment these mesoporous materials do not constitute a viable alternative to the AC. However, an astute funcionalisation of the same ones can, eventually provide an increase of the adsorption capacity.

Estudo do uso de hidrotalcites de Mg-Al como catalisadores heterogéneos para produção de biodiesel

A transesterificação de óleos vegetais ou gorduras animais com um álcool de baixo peso molecular é o principal processo utilizado na produção de biodiesel. Actualmente os processos industriais utilizam catalisadores homogéneos para acelerar a reacção. No entanto a utilização de catalisadores heterogéneos, no processo de transesterificação, tem sido sugerido por vários investigadores pois, são amigos do ambiente e podem ser regenerados e reutilizados portanto possibilitam a utilização de processos contínuos. Neste contexto, a utilização de hidrotalcites Mg-Al, como catalisadores heterogéneos para produção de biodiesel foi investigada neste trabalho experimental. As hidrotalcites com diferentes razões molares Mg/Al (Mg/Al=1, 2, 3 e 4) foram preparadas pelo método de co-precipitação. As diversas matrizes catalíticas obtidas, calcinadas a diferentes temperaturas, foram caracterizadas por difracção de raios X (DRX), análise térmica (TG-DSC), espectroscopia de infravermelhos (MIR), microscopia electrónica de varrimento (SEM) e isotérmicas de adsorção com azoto (BET). Estes catalisadores foram testados na metanólise de óleos vegetais para produzir biodiesel. As hidrotalcites Mg/Al=2, HT2A e HT2B (preparada com metade da quantidade de NaOH) calcinadas a 507 ºC e 700 ºC, respectivamente, foram as que apresentaram melhores resultados ao catalisar a reacção com um rendimento em éster superior a 97%, utilizando 2.5% da massa de catalisador, em relação à massa do óleo, razão molar metanol/óleo igual a 12, temperatura reaccional de 65 ºC durante 4h. Foi também investigada a reutilização do catalisador e o efeito da temperatura de calcinação. Constatou-se que o catalisador hidrotalcite HT2B apresentou melhor comportamento catalítico pois permitiu catalisar a reacção de transesterificação até três ciclos reaccionais, convertendo em ésteres 97%, 92% e 34% no primeiro, segundo e terceiro ciclos reaccionais, respectivamente. A análise de, algumas propriedades do biodiesel obtido como, o índice de acidez, a viscosidade e o índice de iodo mostraram que os resultados obtidos estão dentro dos valores limite recomendados pela norma EN 14214. Em anexo apresenta-se uma comunicação à First International Conference on Materials for Energy, Karlsruhe, 2010.