In this study, clathrate-based CO2 capture was investigated in the presence of thermodynamic promoters such as tetrahydrofuran (THF) and tetra-n-butyl ammonium chloride (TBAC) for upgrading CO2-rich natural gas and biogas. The phase equilibria, gas uptakes, gas composition measurements, and spectroscopic analyses of CH4 (50%), CO2 (50%), and promoter clathrates were examined with a primary focus on the effects of thermodynamic promoters on clathrate stability and cage filling behavior. The addition of THF and TBAC significantly enhanced the thermodynamic stability of CH4 (50%) and CO2 (50%) clathrates. 13C NMR and Raman spectroscopy clearly revealed that CO2 and CH4 are enclathrated in the clathrate cages. THF solutions demonstrated a faster growth rate of clathrates, but CO2 was less selective than CH4 in the THF clathrate phase due to the lower thermodynamic stability of the CO2 and THF clathrate compared to the CH4 and THF clathrate. TBAC solutions produced higher CO2 selectivity in the semiclathrate phase due to the presence of distorted small cages, which have a strong preference for CO2 molecules. The experimental results demonstrated that CO2 selectivity in the clathrate phase can be influenced by the thermodynamic stability, cage shape and dimension, and cage filling behavior in the presence of thermodynamic promoters, and thus, a suitable promoter and their optimum concentration should be carefully determined in designing and operating clathrate-based CO2 capture from natural gas or biogas.

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