Theoretical studies on microstructures, stabilities and formation conditions of some sour gas in the type I, II, and H clathrate hydrates

Clathrate hydrates are well known for the water cage structures and the capability of encapsulating natural gas, generally considered as sour gas if containing appreciable amount of hydrogen sulfide. Using the ab initio calculations at the wB97X-D/6-311++G(2d,2p) level, we have investigated systematically the microstructures of five standard water cavities (ID, D, T, H and I) with single and multiple hydrogen sulfide inside. The interaction energies and deformation energies are predicted to ensure the stabilities and maximum occupancies of cages. In addition, the Gibbs free energies forming various water cavities enclosing CH4 and H2S molecules at temperature range 253 K-283 K are also calculated to explore the selectivity on hydrate types and corresponding formation conditions. The results from this work may provide new insight into the theory for the replacement scheme in the exploitation of natural gas hydrate.