The present study reports a controlled synthesis strategy that enhances the CO2 adsorption capacity of MIL-88B-type MOF crystallites by the control of hierarchical pores via the Fe/BDC ratio and synthesis temperature. MIL-88B-type crystallites synthesized at different temperatures with different Fe/BDC ratios are investigated for their CO2 adsorption capacities. The crucial role of narrow micropores (pore sizes less than 1 nm) on CO2 adsorption capacity is shown. The highest CO2 uptake capacit-y of 5.58 wt % (at 1 bar and 298 K) is achieved by the MOF crystallites with a 400.4 m(2)/g BET surface area. This CO2 uptake capacity is greater than those of reported commercially available MOFs (MOF-5 CO2 uptake capacity, 4.5 wt %, and ZIF-8 CO2 uptake capacity, 4.3 wt %). By the formation of hierarchical pores and control of the textural mesoporosity via the Fe/BDC ratio and synthesis temperature, this research paves the way for the synthesis of MOFs with enhanced CO2 adsorption capacity.