Photocatalytic Upgrading of Abundant Aromatic Feedstocks on Coated III-V Semiconductors

Para-Xylene is an industrially significant aromatic feedstock, with an annually demand of 50 million tons. One of its oxidized forms, terephthalic acid is a primary component of polyesters, produced at ~7 million tons annually. The industrial synthesis is a high-temperature aerobic oxidation in acetic acid, which occurs in a three stage catalytic process.<br/>Photocatalysis offers a solution via the use of efficient semiconductors for organic synthesis. Through adapting homogeneous reactions that are routinely performed in organic or photoredox synthesis, we can perform these reactions on the surface of photocatalysts. Through synergistic surface C-H bond activation and generation of reactive oxidative species (ROS) the catalytic cycle is compete. III-V Semiconductors coated with ternary TiO₂ based protective layers, coupled with selective co-catalysts can be used to photocatalytically oxidize para-xylene to terephthalic acid (TPA), promising a mild and distributed synthesis method at variable scale. <br/>Oxidation of para-xylene is a model reaction to explore the translation of the principles of homogeneous catalysis to photocatalysis. TiO₂ surfaces have been shown to activate methanol to methoxy species, and photocatalytic partial oxidation of benzyl alcohol to benzaldehyde has been demonstrated, providing support for individual aspects of the work.<br/>UV irradiation is problematic as it can lead to undesired side reactions, reducing selectivity. The addition of photo-inactive protective coatings can stabilize visible light absorbing semiconductors would be otherwise corrode from even trace water. It is necessary to understand the charge transfer process between surfaces and surface bound molecules and the energetics of the respective processes. The energetics of the holes and electrons indirectly control the surface reactivity, production rate, and desorption.<br/>The surface chemistry should affect the energetics at the reductive and oxidative sites in the form of a surface dipole. This has been shown on Si photoanodes where the methoxy that is formed via methanol oxidation shifts the SI band position towards more negative potentials, increasing barrier heights. During operation, electrons and holes would exchanges charges with a variety of surface molecules, rather than one. As a consequence, the valence band positions of high charge-density coatings (&gt;10²⁰) should shift, while low charge density (~10¹⁵)coatings such as amorphous “leaky TiO₂”, would remain fixed.<br/>In this work, we relate product distribution with surface chemistry, molecular binding strength, and hole transfer energetics. Through tuning the Semiconductor band edge positions by varying the Ga, In, and Al content, along with well designed coating composition, the reactivity can be selectively tuned.