We report the facile synthesis of aluminum-based metal–organic frameworks (MOFs) with different morphologies and structures by tuning the OH− content in the reaction solution. MIL-96 crystals with dodecahedral and hexagonal column shapes were successfully synthesized by tuning the OH− ion content of the synthesis solution. When the OH− ion content was further increased, MIL-110 with a nanorod morphology appeared as the product. For the first time, we obtained pure MIL-96 and MIL-110 under basic conditions directly by using the 1,3,5-benzenetricarboxylic acid (btc) linker. All samples show significant thermal stability, with a decomposition temperature above 300 °C. We found that the gas adsorption properties of MIL-96 were directly dominated by the crystal morphology. At 0.4 °C and 30 bar, the dodecahedral and hexagonal MIL-96 samples can adsorb 9.3 and 6.5 mmol g−1 CO2, respectively, although they possess similar surface areas and identical crystalline structures. The MIL-110 nanorod shows a CO2 adsorption capacity of up to 16 mmol g−1 under the same conditions.