Characterization and Testing of Porous Boron Nitride Towards Application in Adsorption-Based Processes

Industrial separation processes account for 10-15% of the global energy consumption.¹ The energy cost of large-scale gas and liquid separations like distillation can be significantly reduced by moving towards adsorption processes. An example of an inorganic adsorbent is porous boron nitride (BN): this material exhibits high surface area and porosity, and benefits from greater oxidative and thermal stability than common carbonaceous adsorbents.² Our group previously developed a new method to produce porous BN with enhanced surface area, tunable porosity³ and promising liquid and gas separations performance. In more recent work, we explored the potential of porous BN as an efficient and moisture-resistant adsorbent for molecular separations at industrial scale. To do so and foster scaling-up, two questions have been addressed: 1) “How does porous BN form and what are the controlling synthesis parameters?” and 2) “How can one enhance the water stability of this adsorbent?”.<br/>To address point 1, we investigated the formation mechanism of porous BN using a range of analytical and spectroscopic techniques including, among others, solid-state NMR and synchrotron-based X-ray absorption spectroscopy.⁴ This in-depth characterization exercise of porous BN and its intermediates obtained at different temperatures allowed us to propose a detailed formation mechanism that involves carbon nitride as intermediate. We showed how the porosity develops in the material, what synthesis factors can influence it and what gases are responsible for its generation. Overall, our study allowed to support with more certainty the hypotheses formulated in previous reports in the field.<br/>To address point 2, we developed two functionalization routes to modify the surface of porous BN and enhance its hydrophobicity and resistance to moisture.⁵ The first route involved direct silylation of porous BN powder followed by pelletization, whereas the second route used chemical vapor deposition on porous BN pellets. In parallel, we developed a method relying on saturated salt solutions⁶ to mimic two different levels of humidity exposure relevant to storage and sorption testing conditions. Using FTIR, XRD, XPS, μXRF, TGA, N₂ sorption and mercury intrusion porosimetry, we analyzed the samples before and after functionalization, as well as before and after exposure to humidity. Our results pointed to the efficiency of the functionalization approach to produce moisture-resistant BN-based adsorbents. Therefore, thanks to the extensive use of characterization techniques, this research paved the way for scaling up the synthesis of porous BN towards lower carbon emissions in industrial applications.⁷<br/><br/>References:<br/>1. D. S. Sholl and R. P. Lively, <i>Nature</i> <b>532</b> (2016) 435-437<br/>2. X.-F. Jiang et al., J. Mater. <i>Sci. Technol.</i> <b>31</b> (2015) 589-598<br/>3. S. Marchesini et al., <i>ACS Nano</i> <b>11</b> (2017) 10003-10011<br/>4. A. L’Hermitte et al., <i>J. Phys. Chem. C</i> <b>125</b> (2021) 27429–27439<br/>5. A. L'Hermitte et al., <i>Microporous Mesoporous Mater.,</i> <b>352</b> (2023) 112478<br/>6. L. Greenspan, <i>J. Res. Natl. Bur. Stand. A Phys. </i><i>Chem</i>. (1977) 89-96<br/>7. I. Itskou et al., <i>Acc. Mater. Res.</i> <b>4</b> (2023) 143-155