Molecular insights into the effects of lignin on methane hydrate formation in clay nanopores

The formation behavior of CH4 hydrates in marine sediments is inevitably affected by lignin molecules, but the exact molecular mechanism remains unclear. Herein, microsecond molecular dynamics simulations have been conducted to reveal the effects of lignin molecules on CH4 hydrates formation from the salty solutions in clay nanopores. Simulation results show that lignin molecules could kinetically inhibit the formation and growth of CH4 hydrate by promoting the formation of CH4 nanobubble via their functional groups. Specifically, the strong affinity of lignin molecules for CH4 molecules together with the self-aggregation of lignin molecules promotes the formation of CH4 nanobubble, and then inhibits hydrate formation. Meanwhile, most lignin molecules adsorb on the surfaces of spherical CH4 nanobubbles and tightly wrap the nanobubbles, which further stabilizes the nanobubbles and then inhibits hydrate growth. Furthermore, the higher the concentration of lignin molecules in the solution, the slower growth of CH4 hydrate. Interestingly, a few lignin molecules directly build abnormal hydrate cages by replacing the water molecules at the vertices of hydrate cages with their hydroxyl groups, while some lignin molecules interact with hydrate solids by inserting into the cage-like hydrogen bond networks of hydrate solids. Some semi-cage structures form near the lignin molecule, which could encapsulate CH4 molecules or only entrap the methyl groups of lignin. These molecular insights help to understand the formation process of natural gas hydrates in marine sediments and provide inspiration for the development of new hydrate inhibitors.