Semiclathrate Hydrates in Tri-n-butylphosphine Oxide (TBPO)–Water and TBPO–Water–Methane Systems

Abstract

In the present work, we studied semiclathrate hydrates in the TBPO-HO and TBPO-HO-CH systems. The stoichiometry, temperature, and enthalpy of dissociation of TBPO semiclathrate hydrate crystals formed in the TBPO-HO binary system were found to be TBPO·33.6 ± 0.9HO, 280.0 K, and 253.1 ± 4.7 J g, respectively. The crystal structure determined by single crystal XRD analysis (150 K) was the orthorhombic with space group Pbam and unit cell parameters a = 19.9313(8) Å, b = 23.4660(7) Å, and c = 12.1127(5) Å. The structural stoichiometry is TBPO·34.5HO. The TBPO guest molecules arrangement within the host water framework has been refined for the first time. The discrepancy between the analytically measured and structural stoichiometry is likely to be attributed to the structure defects, which cannot be revealed by the routine single-crystal XRD analysis. The methane capacity of TBPO + CH double hydrate was measured by the thermovolumetric method in the range 14.9-55.8 wt % TBPO aqueous solution at a methane pressure of 8.5 ± 0.5 MPa and temperature of 274 ± 1 and 286 ± 1 K. The maximum included methane volumes of 61.6-74.6 mL/g were observed for the TBPO + CH double hydrates synthesized from ∼26-30 wt % TBPO aqueous solutions. Powder X-ray diffraction measurements of the hydrate samples used in the thermovolumetric experiments revealed that the TBPO + CH double hydrate has the same structural characteristics as the simple TBPO hydrate. The study of the Raman spectra of the TBPO + CH double hydrate and TBPO simple hydrate showed that in the TBPO + CH double hydrate CH molecules selectively occupy the small D cages. The results of the present study did not confirm the earlier suggestion of the formation of several structural types of hydrates in the TBPO-HO system. The obtained results indicate that the TBPO-HO binary system has a potential for application in gas separation and as cold storage and transportation media.

The water molecule arrangement over the side chain of some aliphatic amino acids: A quantum chemical and bottom‐up investigation

“…The presently found water molecule arrangements resemble the semiclathrate structures experimentally characterized, where some water molecules of the cage are replaced by ions …”

Kinetic Measurements on CO2 Hydrate Formation in the Presence of Tetra-n-butyl Ammonium Bromide

“…TBAB·38H 2 O belongs to an orthorhombic structure which contains 6 5 12 -cages, 4 5 12 6 2 -cages, and 4 5 12 6 3 -cages per unit cell, while TBAB·26H 2 O belongs to a tetragonal structure which has 10 5 12 -cages, 16 5 12 6 2 -cages, and 4 5 12 6 3 -cages in a crystal unit . The structural variety should result from the difference in the way of including bromide anions and arranging tetra- n -butyl ammonium cations. , Since the TBAB hydrates only leave 5 12 -cages vacant for gas capture, the selectivity is more evident than that of sI hydrates …”

“…12,13 Since the TBAB hydrates only leave 5 12 -cages vacant for gas capture, the selectivity is more evident than that of sI hydrates. 14 Specifically, the vacant 5 12 -cages, which are squeezed by the cations in the adjacent cages, show good selectivity for CO 2 . 15−17 TBAB hydrates are thus proposed to be applied in ecological CO 2 enrichment systems for greenhouse production.…”