Article pubs.acs.org/crystal
Crystal Growth of Hydrogen/Tetra‑n‑butylammonium Bromide Semiclathrates Based on Morphology Study Hari Prakash Veluswamy, Ting Yang, and Praveen Linga* Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore, Singapore 117 576 S Supporting Information *
ABSTRACT: Morphology studies were conducted for the first time on the mixed hydrogen/tetrabutylammonium bromide (TBAB) semiclathrates. Morphology study dealt with the observation of nucleation of the first hydrate crystal, the growth of hydrates and the characteristic appearance of crystals through the microscope. Morphology changes occurring during the formation of mixed hydrogen hydrates using TBAB as a promoter were observed through the microscope. Concentration of TBAB was varied and the influence of concentration on the hydrate crystal morphology was studied. At lower TBAB concentration (1 and 2 mol %) needle like and equiaxial crystals were formed initially which later developed into columnar crystals having high transmittance and retained their individual structure. However, at higher TBAB concentration (2.5 mol % and above), cylinder like crystals were observed which developed into dense irregular shaped crystals with lower transmittance. The individual crystals cannot be distinguished, they become mushy and grow as layers. Characteristic wave formations were observed during crystal formation in all the cases. With the decrease in subcooling or with the increase in experimental pressure (driving force), the dimensions of the formed crystals was initially same but later increased to around 2−4 times the original size.
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INTRODUCTION Gas hydrates are emerging to be prominent materials for energy storage having significant advantages like high gravimetric/ volumetric storage density, environmentally benign, and offer safe mode for storing energy compared to conventional energy storage methods. Methane is the predominant gas that is reported to be stored and transported in the form of hydrates and it has been shown to be economically and technologically feasible including process scale-up.1−3 Hydrogen gas has also been reported to form hydrates and the prospects of using hydrates as hydrogen storage media has been explored in the past two decades.4−8 Addition of small concentration of “promoter” thermodynamically favors hydrogen hydrate formation at moderate conditions. Recent research efforts have been put forth to study the kinetics of mixed hydrogen hydrates on a macroscopic scale.9−12 Apart from energy storage, gas hydrates also find their application in gas separation, desalination, cold storage, and CO2 capture.13−19 Gas hydrates are nonstoichiometric, crystalline, and caged compounds having specific structures.20,21 In hydrates, the guest gas molecules are trapped inside cages formed by host water molecules. The reported structures of hydrates include sI, sII, and sH. Each of these structures has specific number of small/medium/large cavities formed by water molecules.21,22 The hydrate structure formed depends on the nature and size of guest molecules involved in forming hydrates. Small guest molecules having diameter in the size range of approximately 4−6 Å form sI structure (made of two 512 cages and six 51262 © 2014 American Chemical Society
cages formed by 46 molecules of water), molecules having diameter of 6−7 Å typically form sII structure (made of sixteen 512 cages and eight 51264 cages formed by 136 water molecules) and the molecules larger than 7 Å form sH hydrates (made of three 512 cages, two 435663 cages, and one 51268 cage requiring 34 water molecules).21 However, there are few exceptions to the above listed molecule diameter as a criterion for specific hydrate structure formation. Hydrogen and argon having diameter
