Influence of the chemical environment on CO2-sequestration by clathrate hydrates: a Raman study

Clathrate hydrates (CHs) are nonstoichiometric solid compounds formed by cage-like crystalline structure of water molecules where a guest molecule is entrapped under proper conditions of temperature and pressure. CHs offers a solution for the sequestration of Carbon Dioxide (CO2) in its structure as trapped molecule both in seawater and under the seafloor.1 The formation conditions and a deeply investigation about the chemical properties of seawater, sediments and the environmental parameters are fundamental to understand the process and help the know-how for a technological solution for the CO2 sequestration as CO2-CHs. Whitin the PRIN2022 project CO2-RESTO, the water hydrates features were deeply investigated by Raman spectroscopy, since important information can be obtained from the observation of the -OH stretching bands of water that are present in CO2-CHs. The Raman study has been performed by using lab-reproduction CHs with and without CO2, in presence or in absence of salts and sediments. The in-depth analysis of Raman shifts showed that the presence of salt, as NaCl, influences the water structure decreasing the hydrogen bonds, but in the same conditions, in presence of sand, the latter limiting the inhibition effect of salts. Moving towards the CO2-CHs test, using Raman spectroscopy the characteristic two peaks of CO2, in the region 1200-1400 cm- 1, can be easily detected. Also in this case, the Raman bands of the water -OH stretching in the CO2-CHs, showed useful information. At different temperature, the CO2 hydrates showed a less ordered structure in the presence of sand but however, thanks to the interaction between water and silanol groups of sand particles, their stability was maintained with the variation of temperature, favoring the expansion of the cages that was highlighted by the Raman shifts of the CO2 Fermi diad.2 All the obtained experimental results will be used to the devolpment of a theoretical model applicable to the CO2-CHs formation for their reproduction in lab-scale and for the technological solution as the CO2-injection.