Estudio de las propiedades de transporte en materiales porosos mediante espectroscopía de infrarrojo con transformada de fourier (FTIR)

Abstract

La preparación de sílices mesoporosas ordenadas y su utilización en procesos que involucran transporte difusivo de masa es un tema de interés. La sílice SBA-16 es un material que presenta un arreglo cúbico 3D de mesoporos interconectados el cual se puede indexar en un grupo espacial Im3m. Los poros tipo caja (entre 5 - 15 nm de diámetro (Gobin, 2006)) interconectados con poros cilíndricos (con diámetros entre 1 - 6 nm (Kim et al., 2004)) proporcionan condiciones favorables para el transporte de masa. En la preparación de estas sílices se busca obtener estructuras porosas que permitan un adecuado transporte de las sustancias para maximizar la eficiencia de los procesos. Por lo tanto, es importante cuantificar la propiedad de transporte en estos materiales. Esta cuantificación se realiza mediante el coeficiente de difusión efectiva o difusividad efectiva (De). Para la determinación de la difusividad efectiva se han utilizado diferentes técnicas experimentales y de simulación computacional. En este trabajo se emplea la técnica experimental espectroscopía de infrarrojo con transformada de Fourier (FTIR) para la determinación de los coeficientes de difusión efectiva del hidrocarburo ciclohexano en la sílice SBA-16 a diferentes temperaturas y velocidades de flujo. Los coeficientes obtenidos se encuentran en el intervalo comprendido entre 5,5x10-14 y 8,4x10-14 m2/s. Por otro lado, y como estudio preliminar al transporte de gases en materiales porosos desordenados, se utilizaron la teoría de la percolación y la técnica de Monte Carlo Cinético para representar la estructura porosa y simular el proceso de difusión de hidrógeno en este tipo de materiales. Se corroboró la aleatoriedad de los poros en el medio, inscrito dentro de un retículo cúbico simple, calculando el umbral de percolación a partir del parámetro de orden. En cuanto al transporte de masa, los resultados sugieren que el tamaño del poro, variado entre 1 y 5 nm, sólo afecta a la difusividad efectiva si los valores de porosidad se encuentran cerca al umbral de percolación, y que dicha difusividad es prácticamente independiente de la concentración de las moléculas de H2. La simulación tridimensional permitió caracterizar el comportamiento difusional anómalo en función de la porosidad, siendo este consecuente con el reportado previamente para retículos bidimensionales./Abstract. The preparation of ordered mesoporous silicas and their use in process involving diffusive transport of mass is a topic of interest. SBA-16 silica is a material which has a cubic arrangement of mesopores interconnected 3D which can be indexed in space group Im3m. The cage-like pores (between 5 to 15 nm (Gobin, 2006)) interconnected with cylindrical pores (with diameters ranging from 1 to 6 nm (Kim et al., 2006)) Provide favorable conditions for mass transport. In preparing these silicas is sought porous structures that allow adequate transport of substances to maximize the efficiency of processes. Therefore, it is necessary to quantify the transport in these materials. This quantification is done using the effective diffusion coefficient (De). To determine effective diffusion coefficient have been used experimental and computer simulation techniques. In this study fourier transform infrared spectroscopy was applied to determine of effective diffusion coefficient of ciclohexane in SBA-16 silica at different temperatures and flow rates. The results of the coefficients obtained are in the range between 5,5x10-14 y 8,4x10-14 m2/s. Besides, as preliminary study the gas transport in disordered porous materials were used Percolation Theory and Kinetic Monte Carlo technique to represent porous structure and simulate the diffusion of hydrogen in these materials. It confirmed the randomness of the pores in the structure, inscribed in a simple cubic lattice, calculating the percolation threshold from order parameter. With regard to mass transport, the results suggest that the pore size, varied between 1 and 5 nm, only affects the effective diffusivity if the porosity values are close to the percolation threshold, and that the diffusivity is almost independent of the concentration of H2 molecules. The simulation allowed to characterize the anomalous diffusional behavior as a function of porosity, this being consistent with that reported previously for two-dimensional lattices.

Ambstracts: The preparation of ordered mesoporous silicas and their use in process involving diffusive transport of mass is a topic of interest. SBA-16 silica is a material which has a cubic arrangement of mesopores interconnected 3D which can be indexed in space group Im3m. The cage-like pores (between 5 to 15 nm (Gobin, 2006)) interconnected with cylindrical pores (with diameters ranging from 1 to 6 nm (Kim et al., 2006)) Provide favorable conditions for mass transport.In preparing these silicas is sought porous structures that allow adequate transport of substances to maximize the efficiency of processes. Therefore, it is necessary to quantify the transport in these materials. This quantification is done using the effective diffusion coefficient (De). To determine effective diffusion coefficient have been used experimental and computer simulation techniques. In this study fourier transform infrared spectroscopy was applied to determine of effective diffusion coefficient of ciclohexane in SBA-16 silica at different temperatures and flow rates. The results of the coefficientsobtained arein the range between 5,5x10-14y 8,4x10-14m2/s.Besides, as preliminary study the gas transport in disordered porous materials were used Percolation Theory and Kinetic Monte Carlo technique to represent porous structure and simulate the diffusion of hydrogen in these materials. It confirmed the randomness of the pores in the structure, inscribed in a simple cubic lattice, calculating the percolation threshold from order parameter. With regard to mass transport, the results suggest that the pore size, varied between 1 and 5 nm, only affects the effective diffusivity if the porosity values are close to the percolation threshold, and that the diffusivity is almost independent of the concentration of H2molecules. The simulation allowed to characterize the anomalous diffusional behavior as a function of porosity, this being consistent with that reported previously for two-dimensional lattices.