Clathrate hydrate formation in a (methane + either 3-methyltetrahydropyran or 2-methyltetrahydrofuran + water) system is demonstrated. The first data of the quadruple (water + structure-H hydrate + either 3-methyltetrahydropyran or 2-methyltetrahydrofuran + methane) equilibrium pressure-temperature conditions are measured over temperatures from 273 to 286 K. In the 3-methyltetrahydropyran system, the equilibrium pressures are lower by 1.6-2 MPa than those of the structure-I methane hydrate formed in the methane + water system at given temperatures. In the 2-methyltetrahydrofuran system, equilibrium pressures at temperatures below 278 K are lower than those for the structure-I methane hydrate, and at temperatures above 278 K, they are higher. These phase equilibria suggest the formation of hydrates other than structure-I methane hydrates in the two systems. The crystallographic structures of the hydrates are determined to be structure H by means of X-ray diffraction as expected from considerations of sizes and shapes of the 3-methyltetrahydropyran and 2-methyltetrahydrofuran molecules.
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