Theoretical and experimental investigations have been performed to obtain carbonic anhydrase (CA) mimic catalysts that are effective for CO2 absorbents containing highly concentrated amines. One of the most serious problems associated with such absorbents is the inhibition of catalysts due to strong binding of the reaction products-bicarbonates and alkylcarbonates-as well as amine compounds at the active site of the catalysts. The active form of the CA mimic catalyst, M-OH, must be formed prior to the binding of such inhibitors. Taking into account the fact that OH- is a Lewis base that is harder than the inhibitors, we focus on hard Lewis acids, in particular, boron compounds, on the basis of the Hard and Soft Acids and Bases principle. With the aid of density functional theory (DFT) calculations, we experimentally confirm that triphenylborane and boric acid derivatives are efficient catalysts for CO2 absorption in an aqueous solution of 30 wt % 2-(isopropylamino)ethanol. The DFT calculations predict that the intrinsic reactivity toward CO2 is improved, while the resistance to inhibitions is reduced, as the hardness of the boron center is decreased.