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Development and Application of Modeling Tools for Mass Transport and Catalytic Reaction in Nanostructured Membranes

Simon E. Albo, Randall Q. Snurr, and Linda J. Broadbelt. Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road E136, Evanston, IL 60208

The combination of anodic aluminum oxidation and atomic layer deposition offers a pathway to obtain nanostructured membranes with tailored uniform pore geometry and wall composition. Here we focus on developing and applying different modeling tools to study mass transport and selective catalytic oxidation inside these membranes. The main goal is to propose the optimum membrane architecture to maximize a desired product in a reacting system.

The contribution of the possible mass transport mechanisms in pores with diameters between 10 and 150 nm was assessed using a combination of techniques. First, theoretical expressions for convection, molecular diffusion and Knudsen diffusion were employed at the conditions of interest for selective catalytic oxidation, elevated temperature (700 K) and pressure near atmospheric. It was found that diffusion dominates over convection, with Knudsen diffusion being the dominating mechanism. This trend is accentuated in the smaller pores and at lower pressures. Then, molecular dynamics (MD) simulations were performed to determine the role of surface diffusion. The results showed that this mechanism is only present at lower temperatures and negligible at the reaction conditions of interest here. The dominance of Knudsen diffusion was then exploited to develop a model based on its principles: no interaction among particles and diffusive collisions between particles and the pore wall. The simulations were set up to mimic the experimental conditions by having a pressure drop across the pores. This model is less computationally expensive than molecular dynamics and allows simulation of longer pore lengths and timescales than MD, while maintaining a level of detail that provides information regarding the residence times of particles in the membranes and the contact between particles and the pore walls. The residence times along with the number and location of hits on the wall can be obtained for cylindrical and asymmetrical pores for different reactor operation modes, including the sweep gas mode and the pass-through mode. The model is being expanded to include oxidation reactions as stochastic events when the particles are in contact with the catalyst on the pore walls.