Boron Nitride: A Support for Highly Active Heteropolyacids in the Mehtanol-to-DME Reaction

Due to its superacidity and ability of its bulk to react following a pseudoliquid mechanism, the Keggin H3PW12O40 heteropolyacid attracts more and more attention as a catalyst for the gas phase methanol-to-DME reaction. However, in its pure state, H3PW12O40 has a very low surface area (typically 5−10 m2/g), which limits the accessibility of its inner protons due to diffusional constraints and explains why teams investigate H3PW12O40in its supported form. In the present work, we highlight the interest of using hexagonal boron nitride (BN) as a support. We show that, in contrast to commonly used supports such as TiO2, BN is able to increase the accessibility of H3PW12O40’s acid sites (i.e., stabilizing small enough crystallites) while preserving their strong acidity (i.e., not interacting too much with the Keggin units). At low loadings(typically around 16% of one ideal Keggin monolayer), BN leads H3PW12O40 to reach an almost 2 times higher methanol conversion than obtained with the adequately activated pure bulk sample, and an almost 10 times higher conversion than an optimized TiO2-supported H3PW12O40catalyst. At higher H3PW12O40 loadings, the BN-supported catalysts are still much more active than the optimized TiO2-supported one, but less active than the pure bulk-activated H3PW12O40,which we attribute to the partial intercalation of H3PW12O40 within the interlayers of BN.