New Insights on Gas Hydroquinone Clathrates Using in Situ Raman Spectroscopy: Formation/Dissociation Mechanisms, Kinetics, and Capture Selectivity

Hydroquinone (HQ) is known to form organic clathrates with different gaseous species over a wide range of pressures and temperatures. However, the enclathration reaction involving HQ is not fully understood. This work offers new elements of understanding HQ clathrate formation and dissociation mechanisms. The kinetics and selectivity of the enclathration reaction were also investigated. The focus was placed on HQ clathrates formed with CO2 and CH4 as guest molecules for potential use in practical applications for the separation of a CO2/CH4 gas mixture. The structural transition from the native form (alpha-HQ) to the clathrate form (beta-HQ), as well as the reverse process, were tracked using in situ Raman spectroscopy. The clathrate formation was conducted at 323 K and 3.0 MPa, and the dissociation was conducted at 343 K and 1.0 kPa. The experiments with CH4 confirmed that a small amount of gas can fill the alpha-HQbefore the phase transition from alpha- to beta-HQ begins. The dissociation of the CO2-HQ clathrates highlighted the presence of a clathrate structure with no guest molecules. We can therefore conclude that HQ clathrate formation and dissociation are two-step reactions that pass through two distinct reaction intermediates: guest loaded alpha-HQ and guest-free beta-HQ When an equimolar CO2/CH4 gas mixture is put in contact with either the alpha-HQ or the guest-free beta-HQ the CO2 is preferentially captured. Moreover, the guest-free beta-HQ can retain the CO2 quicker and more selectively.