Reply to the ‘Comment on Cage occupancy of methane clathrate hydrates in the ternary H₂O–NH₃–CH₄ system’ by S. Alavi and J. Ripmeester, Chem. Commun., 2022, 58, DOI: 10.1039/D1CC06526B

Abstract

Our recent Communication suggested that ammonia in aqueous solution may preferentially destabilize large cages in methane clathrate hydrates.

Phase Behavior of the Ternary H2O–THF–NH3 System under Cryogenic Conditions: Implications for the Destabilization of Clathrate Hydrates on Titan

“…Recently, the incorporation of ammonia in the clathrate structure, either within the water cavities or as part of the host lattice, has been reported for hydrogen-bonding guests, such as THF. − This incorporation could actually be facilitated by host–guest hydrogen bonds, which disrupt the intrinsic water–water hydrogen bonds in the host lattice and create Bjerrum defects . On the other hand, stable NH 3 incorporation in methane clathrates is still debated ,,, and appears to be strongly dependent upon formation conditions. The incorporation of ammonia in clathrates has been supported in CH 4 clathrates synthesized from cryogenic vapor deposition methods at relatively low NH 3 concentrations (5 wt % or less), while it was not required to explain results obtained on CH 4 clathrates formed from freezing of aqueous solutions. , This emphasizes that the chemical system and formation method must be considered when discussing stable NH 3 incorporation in clathrate hydrates …”

“…On the other hand, stable NH 3 incorporation in methane clathrates is still debated ,,, and appears to be strongly dependent upon formation conditions. The incorporation of ammonia in clathrates has been supported in CH 4 clathrates synthesized from cryogenic vapor deposition methods at relatively low NH 3 concentrations (5 wt % or less), while it was not required to explain results obtained on CH 4 clathrates formed from freezing of aqueous solutions. , This emphasizes that the chemical system and formation method must be considered when discussing stable NH 3 incorporation in clathrate hydrates …”

“…The incorporation of ammonia in clathrates has been supported in CH 4 clathrates synthesized from cryogenic vapor deposition methods at relatively low NH 3 concentrations (5 wt % or less), 27 while it was not required to explain results obtained on CH 4 clathrates formed from freezing of aqueous solutions. 13,32 This emphasizes that the chemical system and formation method must be considered when discussing stable NH 3 incorporation in clathrate hydrates. 32 In the ternary H 2 O−THF−NH 3 system under superstoichiometric THF conditions as presented in this study, the data collected using XRD and Raman spectroscopy show no clear evidence of NH 3 incorporation in the first generation of THF clathrates, THF-c1, that form upon freezing of the aqueous solution.…”

“…13,32 This emphasizes that the chemical system and formation method must be considered when discussing stable NH 3 incorporation in clathrate hydrates. 32 In the ternary H 2 O−THF−NH 3 system under superstoichiometric THF conditions as presented in this study, the data collected using XRD and Raman spectroscopy show no clear evidence of NH 3 incorporation in the first generation of THF clathrates, THF-c1, that form upon freezing of the aqueous solution. The second generation of THF clathrates, THF-c2, is not discernible from THF-c1 in our Raman spectra, in agreement with previous studies.…”

“…27−29 This incorporation could actually be facilitated by host−guest hydrogen bonds, which disrupt the intrinsic water−water hydrogen bonds in the host lattice and create Bjerrum defects. 30 On the other hand, stable NH 3 incorporation in methane clathrates is still debated 13,27,31,32 and appears to be strongly dependent upon formation conditions. The incorporation of ammonia in clathrates has been supported in CH 4 clathrates synthesized from cryogenic vapor deposition methods at relatively low NH 3 concentrations (5 wt % or less), 27 while it was not required to explain results obtained on CH 4 clathrates formed from freezing of aqueous solutions.…”