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

Abstract

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 CO and CH as guest molecules for potential use in practical applications for the separation of a CO/CH gas mixture. The structural transition from the native form (α-HQ) to the clathrate form (β-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 CH confirmed that a small amount of gas can fill the α-HQ before the phase transition from α- to β-HQ begins. The dissociation of the CO-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 α-HQ and guest-free β-HQ. When an equimolar CO/CH gas mixture is put in contact with either the α-HQ or the guest-free β-HQ, the CO is preferentially captured. Moreover, the guest-free β-HQ can retain the CO quicker and more selectively.

Hydroquinone clathrate based gas separation (HCBGS): Application to the CO2/CH4 gas mixture

“…5 d), we can infer that the presence of CH 4 in the mixture significantly impedes the cap ture kinetics. These observations confirm that the gas capture kinetics is enhanced for high CO 2 mole fractions, as already reported in literature [32].…”

“…However, as discussed previously in the section on "Mate rials", we believe that after the dissociation step, most of the HQ crystals have a core shell structure composed of a core of preformed α HQ surrounded by a shell of guest free β HQ. We have already de monstrated in a previous study that the HQ guest free structure formed with CO 2 is much more selective to CO 2 than preformed α HQ [32]. Accordingly, the highest selectivity values measured with the compo site just after contacting the gas mixture to HQ could be explained by the presence of the guest free β HQ structure.…”

“…crystallites sized 100 nm at most) is much higher than that of native HQ, it would be expected to contain a greater pro portion of guest free β HQ and thus have a greater effect on selectivity in the HQ based composite. We believe that the same gas capture mechanisms by enclathration through HQ [32] occur both with the native crystals and with the composite. Nevertheless, as the proportion of guest free structures is supposed to be much higher in the composite than in the native HQ crystals (proportional to the surface to volume ratio of the HQ in the two reactive media), the effects on selectivity are much more visible when the composite is used.…”

“…Native HQ was put in contact with pure CO 2 gas at 3.0 MPa and ambient temperature for 1 month to form CO 2 HQ clathrates, followed by a dissociation step at 1 kPa and 343 K until all the stored gas was released. This process generates a preformed form of HQ with a highly reduced induction time [32]. The structure of the preformed HQ was investigated by Raman spectroscopy just before contacting the CO 2 / CH 4 gas mixture (by taking a sample inside the reactor).…”

“…The system is then put under vacuum: valves NV 3 and V 10 are closed, valves V 12 and NV 5 are opened, and the vacuum pump is turned on. Once the primary vacuum is reached, valves V 12 and NV 5 are closed, the vacuum pump is turned off, and the thermostatic bath temperature is set to 343 K. The operating conditions of the release step are set to insure the dissociation of the HQ clathrates in agreement with literature data [32]. At this point, the pressure of the system slowly increases due to the gas re leased by the reactive medium until it reaches a constant value (Fig.…”