ObjectiveThe main objective of this study is to unravel the microscopic mechanisms underlying the adsorption-desorption processes of volatile organic compounds (VOCs), particularly ethanol, using graphene-based materials and metal-organic frameworks (MOFs) as adsorbents. Adsorption technologies have been widely adopted in view of their high efficiency and cost-effectiveness in the treatment of VOCs, but the underlying mechanisms surrounding these processes are not well understood, which poses a major obstacle to further improvement of adsorption rates. To address this challenge, we aim to utilize the unique capabilities of terahertz time-domain spectroscopy (THz-TDS) as an emerging tool to gain insight into the dynamic interactions between adsorbents and adsorbates.
MethodsIn this study, THz-TDS was employed to monitor and track the real-time adsorption-desorption dynamics of ethanol on four different graphene-based materials (rGO, FG, GO, and GO-COOH) and three MOFs (ZIF-8, ZIF-67, and MOF-177). By adjusting the experimental temperature, the variation of spectral peaks (
EP) in the THz-TDS signal was systematically recorded as an indicator of adsorption-desorption behavior. In this way, we analyzed the effect of temperature on the equilibrium state of the adsorption process and deepened our understanding of the mechanism of warming-triggered desorption. In addition,the effect of temperature on the equilibrium state of the adsorption process was analyzed and the understanding of the mechanism of warming-triggered desorption was deepened. In addition, the adsorption capacity of each material was quantitatively characterized using THz-TDS and the correctness of the characterization results was explained by the physicochemical properties of the adsorbents.
Results and DiscussionsThe adsorption of graphene-based materials was in the order of Q
GO-COOH>Q
FG>Q
GO>Q
rGO, while the adsorption of MOFs was in the order of Q
ZIF-8>Q
ZIF-67>Q
MOF-177. In addition, the analysis of THz-TDS signals at different temperatures (Fig.5) confirmed the existence of a warming-triggered desorption mechanism, thus revealing the dynamic equilibrium of the adsorption process. The quantitative characterization of adsorption achieved by THz-TDS provides a solid foundation for comparing the performance of different adsorbents and exploring the underlying microscopic mechanism of their interaction with ethanol.
ConclusionsIn this study, the effect of temperature on the adsorption-desorption process of ethanol by four graphene-based materials and three MOFs was analyzed using THz-TDS. During the adsorption process, the
EP values of all seven adsorbent materials increased with the increase of temperature, which was attributed to the transformation of the ethanol molecules from the adsorbed state to the gaseous state, with the consequent breakage of the hydrogen bonds and the corresponding weakening of the absorption of terahertz waves. Meanwhile, the adsorption (26 ℃) and desorption processes (46 ℃, 66 ℃, and 76 ℃) of four graphene-based materials and three MOFs were tracked using the THz-TDS technique, and the parameter of adsorption capacity was indirectly assessed by calculating the difference in EP between 26 ℃ and 76 ℃. The correlation between adsorption capacity and adsorption capacity is shown in the following table. The correlation between the adsorbed amounts was Q
GO-COOH>Q
FG>Q
GO>Q
rGO and Q
ZIF-8>Q
ZIF-67>Q
MOF-177, respectively, which was verified by the information of polarity, SSA, pore size distribution, and surface functional groups. This study confirms the feasibility of using THz-TDS to track the adsorption-desorption process and to indirectly characterize the adsorption amount by monitoring the hydrogen bond breakage, which provides a valuable reference to achieve efficient treatment of VOCs.