Hexagonal boron nitride (BN) is an excellent support for H3PW12O40 Keggin unit

In the frame of her PhD thesis, Josefine Schnee (IMCN Institute / pole MOST), team of Eric Gaigneaux, and teaching assistant at the faculty of bioscience engineering) demonstrated for the first time that hexagonal boron nitride (BN) is an excellent support for H3PW12O40 Keggin units (KU) used as catalysts for the gas phase 100% selective methanol-to-dimethylether (DME) reaction at 150°C. Only mildly interacting with the KU, BN preserves their strong acidity, and leads to catalysts outcompeting by a factor 10 the KU supported on the most commonly used TiO2. KU on BN perform even better than adequately activated pure bulk KU crystals which were the most efficient catalysts to date thanks to their pseudo-liquid behavior. This effect is due to the ability of BN to stabilize still small enough KU crystallites. The dehydration of methanol to DME currently attracts much attention as the latter is indeed one of the most promising alternative fuels for the future. Indeed DME has a particularly low climate impact, being biodegradable, noncorrosive, and nontoxic and burning without emission of particulates or nitrous oxides.
Reprinted with permission from ACS Catalysis, 2017, 7, pp 4011–4017. DOI: 10.1021/acscatal.7b00808. Copyright (2017) American Chemical Society

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 H3PW12O40 in its supported form. In this work, it is highlighted the interest of using hexagonal boron nitride (BN) as a support. It is shown 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 H3PW12O40 catalyst. 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 was attributed to the partial intercalation of H3PW12O40 within the interlayers of BN.
Reproduced with permission from ACS Catalysis, 2017, 7, pp 4011–4017. DOI: 10.1021/acscatal.7b00808. Copyright (2017) American Chemical Society
More on the work of Josefine Schnee and Eric Gaigneaux on catalytic behavior of Keggin H3PW12O40 heteropolyacid in the methanol-to-DME dehydration, via in situ and operando spectroscopy, can be found in :
Applied Catalysis A, 538 (2017) 174-180; Catalysis Science & Technology, 7 (2017) 817-830; Journal of Physical Chemistry C, 121 (2017) 556–566; Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 173 (2017) 151-159.

Published on August 03, 2017