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

• Nanocomposites were synthesized via a modified sonochemical-assisted method, ensuring uniform ZIF growth on graphene. • Structural and thermal analyses (SEM, BET, TGA) confirmed high integrity, improved microporosity, thermal stability, and surface area. • Langmuir monolayer CO2 capacities reached 404.16 cm 3 (graphene@ZIF-8) and 353.92 cm 3 (graphene@ZIF-67), exceeding pristine ZIFs. • CO2 adsorption isotherms showed significantly enhanced adsorption capacity while H2 uptake remained low. • Improved CO2/H2 selectivity confirms suitability for gas separation applications like in situ hydrogen production. Selective CO 2 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 CO 2 /H 2 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 m 2 /g (+15%) and ZIF-67 from 1116 to 1413 m 2 /g (+27%), while also increasing total pore volume by ∼10-16%. CO 2 adsorption improved substantially, with Graphene@ZIF-8 (75 cm 3 /g) and Graphene@ZIF-67 (65 cm 3 /g) exhibiting ∼2× higher uptake than their pristine ZIF counterparts (38 and 25 cm 3 /g, respectively). Langmuir monolayer capacities similarly increased from 105 to 404 cm 3 (+285%) for ZIF-8 and from 48 to 354 cm 3 (+637%) for ZIF-67 upon graphene hybridization. In contrast, H 2 uptake remained lower than CO 2 , producing enhanced CO 2 /H 2 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 CO 2 capture from H 2 -rich gas streams in subsurface hydrogen production.