Biochar loading modulates pore accessibility and CO₂ adsorption in dual-phase MIL-100(Al)/MIL-110(Al) hybrids

MIL-100(Al)/MIL-110(Al)-biochar materials were prepared by solvothermal crystallization of aluminum trimesate phases in the presence of activated bamboo-derived biochar. BC-10 and BC-20 denote nominal precursor-basis biochar additions of 10 and 20 wt%, respectively, rather than measured final composite mass fractions. PXRD reflections assigned to MIL-100(Al) and MIL-110(Al) were retained after biochar addition. Conventional TEM revealed coexisting rod-like and faceted/polyhedral particle habits in BC-0 and their spatial association with irregular carbonaceous domains in BC-10. However, peak overlap and the absence of quantitative phase analysis in HRTEM preclude determination of phase fractions or lattice-level interfaces. Among the three investigated formulations, BC-10 gave the highest equilibrium CO₂ uptake at 298 K and a terminal equilibrium pressure of approximately 1 bar (39.59 cm³ STP per g; 1.77 mmol g^-1), compared with 1.16 and 1.37 mmol g^-1 for BC-0 and BC-20, respectively. This trend occurred despite the lower BET area of BC-10 (496.79 m² g^-1) relative to BC-0 (1148.90 m² g^-1), demonstrating that the uptake trend cannot be explained by BET surface area alone. Instrument-integrated CO₂-TPD contributions centred below 500 °C followed the order BC-10 > BC-20 > BC-0; the separate contribution near 560 °C was excluded because thermally evolved carbon-containing species may contribute. Nonlinear Freundlich fitting produced lower RMSE, SSE and AICc values than Langmuir fitting for all samples, supporting an energetically heterogeneous adsorption response without uniquely identifying its molecular origin. Oxygenated biochar groups, Al-containing framework environments, crystallite dispersion and pore accessibility are therefore discussed as plausible contributors rather than demonstrated mechanisms. BC-10 is identified only as the best-performing composition among the formulations tested.