Nano-Scale Confinement for Efficient Multi-Step Reaction Cascades

Abstract

Strategic operation of multi-step reaction cascades for energy conversion/storage applications such as biofuel cells is a necessary step to increase process efficiency [1]. Beyond maximizing catalytic activity, efficient transport of reaction intermediates between active sites with minimal diffusion loss into the bulk is critical. Therefore, implementation of intermediate channeling mechanisms, many of which exist in nature, to multi-step energy cascades is a critical aspect of an overall design strategy. Molecular tunneling is one mechanism of substrate channeling that minimizes intermediate diffusional losses into the bulk by confining intermediate pathway [2].In this work, we studied intermediate transport between two active sites confined by single-walled carbon nanotube (SWCNT) using continuum and molecular dynamics (MD) approaches [3]. MD was performed using model intermediates (oxalate and ethanol) inside SWCNTs with diameter 1 nm to 4 nm. Due to water molecule orientation inside the SWCNT, the intermediates experienced a constrained environment that resulted in Knudsen diffusion and decreased diffusion coefficient [4]. In order to increase intermediate residence time inside the nanotube, SWCNT termini were modified with varying numbers of negatively-charged carboxylate groups. Due to attractive-repulsive electrostatic interactions, ethanol, which is uncharged but polar, was retained for a longer time inside the SWCNT. Like-charged intermediates such as oxalate were less affected. This approach represents one possible technique to enhance confinement and therefore increase reaction efficiency.