Cyclodextrins as eco-friendly nucleation promoters for methane hydrate

Abstract

• HPS-ALTA used to rigorously characterise effects of cyclodextrins (CDs) on methane hydrate formation. • CDs can increase the hydrate nucleation rate by up to 6 times at a fixed temperature. • The highest and lowest promotion and H-bonding disruption were observed for α-CD and β-CD, respectively. • No measurable changes to the initial growth rate under stirred conditions caused by CDs. Gas hydrates offer potential methods for the storage and transportation of gases. However, factors such as slow kinetics can hinder storage applications. Here we demonstrate the use of cyclodextrins (CDs) as green chemicals for accelerating methane hydration formation. A second generation high pressure, stirred, automated lag time apparatus (HPS-ALTA) was used to determine the hydrate formation probability as a function of CD concentration, subcooling, and time. Nucleation rates were increased for all three studied (α, β, and γ) CDs, as determined from formation measurements under ramped and fixed temperatures. Analysis of the ramped temperature experiments suggests that the nucleation promotion is related to a CD-induced reduction in the thermodynamic nucleation parameter which provides a measure of nucleation work. Hydrate formation in the presence of CDs was also confirmed to be consistent with the mononuclear nucleation process predicted by the classical nucleation theory. Rates of initial growth were also measured but showed no CD dependence. At the optimum CD loadings found for nucleation, the highest and lowest levels of nucleation promotion were observed for α- and β-CD, respectively. These CDs also generated the highest and lowest levels of H-bonding disruption at the gas-water interface (as measured by sum frequency generation spectroscopy), respectively, suggesting that such disruption may play a role in the hydrate nucleation promotion in CD-containing systems.