Gas sorption in porous ionic packings

Details

Supervisors

Filinchuk, Yaroslav ; Steenhaut, Timothy

Faculty

Faculté des sciences

Degree label

Master [120] en sciences chimiques, à finalité approfondie

Abstract

This project focused on the synthesis, characterization, and sorption performance of porous ionic packings (PIPs), a new class of compounds made of large cations and/or anions. The thesis is divided into three experimental parts covering respectively synthesis of (NMe4)2B12H12 (TMAD), extension of the PIP concept through the synthesis of other PIPs, and investigation of TMAD gas sorption properties using volumetric and diffraction methods. In the first part, the successful synthesis of face-centered cubic (fcc) porous TMAD was confirmed, utilizing the ionic exchange in water between Na2B12H12·4diglyme and NMe4Br. The synthesis conditions, particularly water temperature, were found to significantly affect the relative intensities and lattice expansion of the TMAD. Synthesis in hot water revealed an additional unidentified phase, while synthesis in cold water optimized the yield of the fcc TMAD. The second part demonstrated that the PIP concept can be extended by synthesis of another PIP, (PMe4)2B12H12 (TMPD), the latter exhibited an fcc porous phase and a monoclinic non-porous phase. The fcc phase was obtained in pure form by synthesis in cold water. TMPD displayed thermal stability up to 420 °C, surpassing TMAD. Attempts to synthesize a dodecaborate with a lower symmetry cation, (t-BuNH3)2B12H12, resulted in non-porous (t BuNH3)2B12H12·H2O and (t BuNH3)3B12H12Br. The presence of water in (t BuNH3)2B12H12·H2O is owing to the multiple possibilities to form both hydrogen and dihydrogen bonds. In the third part, gas sorption properties of PIPs were investigated. Crystal structures of TMAD and Cs2B12H12 reveal empty non-connected pores of similar size. TMAD demonstrated CO2 and H2 sorption capacities of 5.92 and 0.21 wt% respectively, at room temperature. The adsorption kinetics is very slow, we attribute this to slow diffusion between the non-connected pores. On the other hand, Cs2B12H12 showed no gas sorption, suggesting the role of the rotational dynamics of cations for gas diffusion and ultimately the gas adsorption. Structural disorder hindered precise localization of CO2 inside the pores. Exposure to air showed TMAD's ability to sorb water, as confirmed by dynamic vapor sorption (DVS) measurements. Both TMAD and TMPD exhibited phase transitions under high CO2 or Xe pressure, indicating the high stability of the porous phase. Sequential Rietveld refinement on Xe in TMPD revealed 25 % pore filling at 400 K. This work shows that large ions can be assembled into close-packed structures containing relatively large, isolated pores. They are not easily accessible, since they are not connected, but they demonstrate the ability to adsorb guests, showing slow gas adsorption kinetics. The latter is likely dominated by the rotational dynamics of cations and/or anions. PIPs with these unique properties may find applications in gas storage and separation, allowing to exploit advantages of the large differences with classical porous solids.