Preferential cage occupation of CH4 into clathrate hydrate for selective CH4 enrichment from hydrogen-natural gas blends

Abstract

Hydrogen (H2) energy, recognized as a viable alternative source for decarbonization, requires effective transport solutions. Blending hydrogen into natural gas pipelines, forming hydrogen-natural gas blends (HNGB), offers a promising bridging technology toward a low-carbon future. However, high-purity methane (CH4) separation remains crucial for existing natural gas infrastructure. Thus, this study explores gas hydrates as a potential CH4 enrichment media, with employing thermodynamic promoters, tetrahydrofuran (THF), 1,3-dioxolane (DIOX), and 1,3-dioxane (Dioxane), to alleviate thermodynamic formation conditions. Since promoters occupy large cages, promoters inevitably limit CH4 occupancy. Hence, the tuning effect, which can allow CH4 in empty large cages by adjusting promoter concentration, was proposed as a novel approach to enhance CH4 selectivity. In this study, we adjusted promoter concentrations from 5.6 mol% to 4.0 mol% to investigate CH4 selectivity via tuning effect from simulated HNGB, CH4 (80%) + H2 (20%) under conditions of 284.15 K and 7.0 MPa. Thermodynamic stability analysis revealed that THF exhibited the superior promotion effect. Synchrotron XRD confirmed that all promoters at varying concentrations formed sII hydrate. Furthermore, Raman and 13C solid-state NMR analyses provided evidence of enhanced CH4 occupancy in the sII-L, with DIOX (4.0 mol%) achieving a CH4 purity of 99.30 mol% in the hydrate phase. These findings demonstrate that tuning effect can increase CH4 selectivity within the hydrate phase, presenting a promising approach for CH4 enrichment from HNGB. Therefore, we believe that our findings can provide valuable insights into potential hydrate-based separation technology, especially for CH4 enrichment for future transition toward sustainable energy systems.