Structures and H-2 Adsorption Properties of Porous Scandium Metal-Organic Frameworks

Two new three-dimensional Sc-III metal-organic frameworks {[Sc3O(L-1)(3)(H2O)(3)]center dot Cl-0.5(OH)(0.5)(DMF)(4)-(H2O)(3)}(infinity) (1) (H2L1 = 1,4-benzene-dicarboxylic acid) and {[Sc3O(L-2)(2)-(H2O)(3)](OH)(H2O)(5)(DMF)}(infinity) (2) (H3L2 = 1,3,5-tris(4-carboxyphenyl)benzene) have been synthesised and characterised. The structures of both 1 and 2 incorporate the trinuclear trigonal planar [Sc-3(O)(O2CR)(6)] building block featuring three Sc-III centres joined by a central mu(3)-O2- donor. Each Sc-III centre is further bound by four oxygen donors from four different bridging carboxylate anions, and a molecule of water located trans to the mu(3)-O2- donor completes the six coordination at the metal centre. Frameworks 1 and 2 show high thermal stability with retention of crystallinity up to 350 degrees C. The desolvated materials 1a and 2a, in which the solvent has been removed from the pores but with water or hydroxide remaining coordinated to Sc-III, show BET surface areas based upon N-2 uptake of 634 and 1233 m(2) g(-1), respectively, and pore volumes calculated from the maximum N-2 adsorption of 0.25 cm(3) g(-1) and 0.62 cm(3) g(-1), respectively. At 20 bar and 78 K, the H-2 isotherms for desolvated 1a and 2a confirm 2.48 and 1.99 wt% total H-2 uptake, respectively. The isosteric heats of adsorption were estimated to be 5.25 and 2.59 kJ mol(-1) at zero surface coverage for 1a and 2a, respectively. Treatment of 2 with acetone followed by thermal desolvation in vacuo generated free metal coordination sites in a new material 2b. Framework 2b shows an enhanced BET surface area of 1511 m(2) g(-1) and a pore volume of 0.76 cm(3) g(-1), with improved H-2 uptake capacity and a higher heat of H-2 adsorption. At 20 bar, H-2 capacity increases from 1.99 wt% in 2a to 2.64 wt% for 2b, and the H-2 adsorption enthalpy rises markedly from 2.59 to 6.90 kJ mol(-1).