Boric Acid Post-Modified [Al]ZSM-5 Zeolites: Properties, Acidity, and Ethanol/Ethene Conversion

The effect of boric acid post-modification on [Al]ZSM-5 zeolites is investigated. Boric acid modification reduces surface areas, and high loadings lead to agglomeration of crystals. ¹¹B MAS NMR spectroscopy indicates boron isomorphously substituted into the framework if 1.5 wt% boric acid is applied, while above 5 wt% loading additionally surface bound ^BT¹ species and, increasingly, boric acid deposits ^BT⁰ are found. The Brønsted acid site (BAS) density decreases with boric acid loading. ¹H MAS NMR spectroscopy after acetonitrile-d₃ loading reveals a similar BAS strength as the parent, thus BAS are associated with bridging Si(OH)Al from the parent's structure. A loading with trimethylphosphine oxide (TMPO) is sensitive to weaker Lewis acid sites (LAS). A subsequent hydration changes the nature of water-accessible boric acid surface species. Conversion of methanol, ethanol, or ethene over boric acid-modified MFI was tested. For 1.5 wt% loading, an increased BTEX content and lifetime in ethanol conversion was observed, while other modified catalysts were outperformed by the unmodified parent. Most boron is removed after catalytic application. It is concluded that the introduced weak surface acidity and the introduced LAS density could render the modification method interesting for synthesizing new adsorbents operated at moderate conditions.