Natural gas and oil typically require purification before processing because these feedstocks contain a wide range of sulfur compounds that can damage equipment and catalysts and that are hazardous to humans and the environment. While metal oxides are widely used to remove trace amounts of these contaminants via chemical reaction, these materials suffer from incomplete utilization that can often lead to unexpected process shutdowns or fugitive emissions. Indeed, more research is needed to understand these reactions at the molecular level so that complete active phase utilization can be achieved to mitigate premature contaminant breakthrough. Therefore, we combine information from reaction and characterization experiments across multiple length scales with computational chemistry studies to elucidate the fundamental structure-function relationships for reactions of sulfur compounds with copper oxide. Fixed bed breakthrough studies showed that removal capacities are less sensitive to macroscopic size and intra-particle void space than to atomistic features such as CuO crystallite size and the presence of foreign atoms (i.e., dopants and surface carbon). In-situ X-ray absorption spectroscopy studies revealed that average capacities and kinetics measured from fixed beds are representative of those measured at individual particle scales and that introducing a foreign atom into the CuO lattice can influence the reactivity of CuO towards sulfur by disrupting the lattice structure (manifested as perturbed Cu-O distances). The presence of foreign atoms could also alter the electronic structure by changing the density of states at the fermi level which affects oxygen and cation reactivity. DFT calculations supported these conclusions and indicated that decreasing crystallite size exposes a greater extent of reactive surface facets in CuO and increases O-vacancies. These open facets facilitate H-S bond cleavage which is necessary to produce S atoms that react with CuO moieties. Open facets may also enhance the diffusion of sulfur atoms to fresh substrate layers allowing for a greater extent of propagation of the reaction front. This knowledge of the influence of crystallite size and composition on CuO capacity for H2S can serve as a guide for rational design or selection of copper oxide sources for sorbent materials.

Fundamental insights into reactions for trace contaminant removal

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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