Zeolitic imidazolate framework/graphene hybrid nanocomposites for downhole CO2/H2 separation during in-situ hydrogen production from hydrocarbon reservoirs

Selective CO₂ separation from H₂-rich streams is crucial for advancing in-situ hydrogen production from depleted natural gas reservoirs. This study investigates pristine-graphene-based nanocomposites of ZIF-8 and ZIF-67 as solid sorbents for high-pressure CO₂/H₂ separation. Graphene@ZIF-8 and Graphene@ZIF-67 were synthesized via a modified sonochemical route that enabled uniform ZIF crystal growth on graphene. XRD confirmed complete retention of the sodalite topology with no impurity phases. SEM and BET analyses showed that incorporating graphene increased the BET surface area of ZIF-8 from 1284 to 1471 m²/g (+15%) and ZIF-67 from 1116 to 1413 m²/g (+27%), while also increasing total pore volume by ∼10-16%. CO₂ adsorption improved substantially, with Graphene@ZIF-8 (75 cm³/g) and Graphene@ZIF-67 (65 cm³/g) exhibiting ∼2× higher uptake than their pristine ZIF counterparts (38 and 25 cm³/g, respectively). Langmuir monolayer capacities similarly increased from 105 to 404 cm³ (+285%) for ZIF-8 and from 48 to 354 cm³ (+637%) for ZIF-67 upon graphene hybridization. In contrast, H₂ uptake remained lower than CO₂, producing enhanced CO₂/H₂ selectivity; Graphene@ZIF-8 achieved a maximum selectivity of ∼11, compared to ∼10 for ZIF-8 and <4 for ZIF-67. These improvements demonstrate that pristine-graphene-supported ZIF nanocomposites provide significantly enhanced adsorption capacity and selectivity, making them strong candidates for CO₂ capture from H₂-rich gas streams in subsurface hydrogen production.