Temperature-Dependent Release of Guest Molecules and Structural Transformation of Hydroquinone Clathrates

Abstract

The CO2-, CH4-, and CO2/CH4-loaded β-form hydroquinone (HQ) clathrates were synthesized by the gas-phase reaction between α-form HQ and high pressure gases. Temperature-dependent Raman spectra of guest-free, CO2-loaded, CH4-loaded, and CO2/CH4-loaded β-form HQ clathrates were measured in the temperature range 300–385 K at increments of 5 K. The CH4 molecules rapidly escaped from the β-form HQ clathrate in the temperature range 360–380 K, whereas the CO2 molecules were gradually released from the β-form HQ clathrate framework in the wide temperature range 300–380 K. It was also found that both CO2 and CH4 molecules were rapidly released from the CO2/CH4-loaded β-form HQ clathrate framework in the temperature range 360–380 K. However, all of the guest-free and guest-loaded β-form HQ clathrates were fully converted to the α-form HQ at the same temperature of 385 K. These results demonstrate the strong effect of temperature on both guest–host interactions and the stability of the framework structure.

Characterization Study of CO2, CH4, and CO2/CH4 Hydroquinone Clathrates Formed by Gas–Solid Reaction

“…This difference in gas release temperatures for the CO 2 -HQ, CO 2 / CH 4 -HQ, and CH 4 -HQ clathrates confirms (i) the greater thermal stability of CH 4 -HQ clathrates and (ii) the role played by CH 4 molecules in the thermal stabilization of the mixed CO 2 /CH 4 -HQ clathrates, as already pointed out in the literature. 42 In the case of CO 2 /CH 4 -HQ and CH 4 -HQ clathrates, the start-of-release temperatures were, respectively, equal to and higher than their measured structural transition temperatures (i.e., 383 ± 6 K for CO 2 /CH 4 -HQ and 378 ± 5 K for CH 4 -HQ clathrates) above which no more gas is theoretically retained in the structure. Indeed, by applying a heating rate of 5 K/min during the TGA, the HQ clathrates dissociate at a higher temperature than at the rate of 1 K/min set for the in situ Raman experiments.…”

Extremely Slow Diffusion of Argon Atoms in Clathrate Cages: Implications for Gas Storage in Solid Materials

“…The characteristic Raman features between α-HQ and β-HQ have been well established. 10,14,18 In the spectral region by the C−H in-plane bending mode around 1160 cm −1 , β-HQ shows a single peak whereas α-HQ shows two split peaks. In addition, the triplet-shaped band at around 1600 cm −1 and one strong peak at around 1250 cm −1 respectively corresponding to the C−O and C−C stretching mode are characteristic features of β-HQ.…”

“…It should be noted that this temperature is very close to the dissociation temperature of HQ clathrates including other guests such as CH 4 , CO 2 , and methanol. 11,18,29 Since there is no penetration pathway along directions perpendicular to the c-axis, the diffusion path of argon atoms is essentially along the c-axis in the direction parallel to the long axis of the cage, 30 leading to a reasonable approximation of 1D diffusion. The argon atoms migrate via the 1D cage nanochannels connected by the hydrogen-bonded hexagon between the ∼4 Å-sized cages composed of host HQ molecules (Figure 3d).…”

Selective Recovery of H2S from Gas Mixtures Using a Hydroquinone Clathrate

“…All of the spectra showed patterns similar to those in the previous results. ,− Considering the C–O and C–C stretching modes at around 1200 cm –1 and the strong C–H mode at around 3000 cm –1 , the samples were found to be mainly HQ clathrates (β form) with some residual pure HQ (α form). , Clathration-induced changes in the Raman spectra were clearly shown in the frequency ranges of 1100–1200 and 1500–1700 cm –1 (Figure ). The two split bands corresponding to the C–H bending mode at 1163 and 1169 cm –1 in the spectra of pure HQ appeared as a single band at 1162 cm –1 in the spectra of HQ clathrates. , Changes in the shape and intensity of unresolved triplet bands at 1601, 1611, and 1625 cm –1 , attributed to the C–C stretching vibration mode, are also shown in Figure and compared to the α-form pure HQ. Clearly, the Raman spectra of the HQ clathrate with the gas mixture at 2.0 MPa revealed a small amount of pure HQ, indicating incomplete conversion of the HQ clathrate.…”

“…The two split bands corresponding to the C−H bending mode at 1163 and 1169 cm −1 in the spectra of pure HQ appeared as a single band at 1162 cm −1 in the spectra of HQ clathrates. 24,25 Changes in the shape and intensity of unresolved triplet bands at 1601, 1611, and 1625 cm −1 , attributed to the C−C stretching vibration mode, are also shown in Figure 4 and compared to the α-form pure HQ. Clearly, the Raman spectra of the HQ clathrate with the gas mixture at 2.0 MPa revealed a small amount of pure HQ, indicating incomplete conversion of the HQ clathrate.…”