Enhancing the Acidity\nWindow of Zeolites by Low-Temperature\nTemplate Oxidation with Ozone

Revisiting the impact of the first and often deemed trivial\npostsynthetic\nstep, i.e., a high-temperature oxidative calcination to remove organic\ntemplates, increases our understanding of thermal acid site evolution\nand Al distributions. An unprecedented degree of control over the\nacidity of high-silica zeolites (SSZ-13) was achieved by using a low-temperature\nozonation approach. Fourier transform infrared spectroscopy of adsorbed\nprobe molecules and solid-state NMR spectroscopy reveal the complexity\nof the thermal evolution of acid sites. Low-temperature activated\n(ozonated) zeolites maintain the original Brønsted acidity content\nand high defect content and have virtually no Lewis acidity. They\nalso preserve the “as-made” Al distribution after crystallization\nand show a clear link between synthesis conditions and divalent cation\ncapacity, as measured with aqueous cobalt ion uptake. The synthesis\nprotocol is found to be the main contributor to Al proximity, yielding\nrecord high exchange capacity when ozonated. After conventional calcination\nat 500–600 °C, however, the presence of water leads to\nthe gradual depletion of Brønsted acid sites, in particular,\nin small crystals. This work indicates that low-temperature ozonation\nfollowed by thermal activation at different temperatures can be used\nas a novel tool for tuning the amount and nature of acid sites, providing\ninsights into the activity of zeolites in acid-catalyzed reactions,\nsuch as CO<sub>2</sub> hydrogenation to dimethyl ether, and thereby\nexpanding the possibilities of rational acidity tuning.