Thermodynamic and Spectroscopic Identification of Methane Inclusion in the Binary Heterocyclic Compound Hydrates

Abstract

Two kinds of heterocyclic organic compounds, a five-membered ring with the chemical formula of C4H4NH (pyrrole, PRL) and a six-membered ring with the chemical formula of C5H5N (pyridine, PRD), are introduced into clathrate hydrate structures as a coguest with methane (CH4) gas. The results from powder X-ray diffraction of the binary (pyrrole + CH4) and (pyridine + CH4) clathrate hydrates showed that the lattice size of the former is smaller than that of the latter but the crystal structure of both binary hydrates is identified as a cubic Fd3m structure II hydrate. Raman spectroscopy also provided the clear evidence of CH4 and each aromatic ring compound occupying in the small and large cavities of structure II hydrates. The thermodynamic behaviors of the two binary systems were compared with those of the pure methane system to identify the role of the two aromatic compounds in the clathrate hydrate system by using high-pressure micro-differential scanning microcalorimetry. It is striking that the two heterocyclic compounds containing a part of hydrate inhibiting functional groups have a promoting effect on the hydrate formation of the methane-water system.