Structures and H₂ Adsorption Properties of Porous Scandium Metal–Organic Frameworks

Abstract

Two new three-dimensional Sc(III) metal-organic frameworks {[Sc(3)O(L(1))(3)(H(2)O)(3)]·Cl(0.5)(OH)(0.5)(DMF)(4)(H(2)O)(3)}(∞) (1) (H(2)L(1)=1,4-benzene-dicarboxylic acid) and {[Sc(3)O(L(2))(2)(H(2)O)(3)](OH)(H(2)O)(5)(DMF)}(∞) (2) (H(3)L(2)=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)(O(2)CR)(6)] building block featuring three Sc(III) centres joined by a central μ(3)-O(2-) 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 μ(3)-O(2-) donor completes the six coordination at the metal centre. Frameworks 1 and 2 show high thermal stability with retention of crystallinity up to 350 °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).

Structural aspects of metal-organic framework-based energy materials research at Diamond

“…We anticipate that structural results from Beamline I19 will continue to feature prominently in our high-profile publications, as they have done in the recent past [6][7][8]11,15,[20][21][22][23][24][25][26][27][28][29].…”

Solid‐State NMR Spectroscopy Proves the Presence of Penta‐coordinated Sc Sites in MIL‐100(Sc)

“…The ligandsd iffer between Sc 3 BTB 2 and MIL-100(Sc). [16] X-ray thermodiffraction analysisa llows the probing of modificationso ft he long-range positional order versus temperature for Sc 3 BTB 2 and MIL-100(Sc) (FigureS4a nd S5 in Supporting Information,respectively). The crystal structures of Sc 3 BTB 2 and MIL-100(Sc) also differ.The Sc 3 BTB 2 crystal-lizes in the hexagonal space group P6 2c forming hexagonal channels with ad iameter of approximately 23 (Figure 1b).…”

“…This shift is tentatively ascribed to the evacuation of the pores and the flippingo ft he ligand phenylr ings around the singleb onds between sp 2 hybridized carbona toms (Figure 1e). [16] To the best of our knowledge, the NMR observation of the ScO 5 site in MOFs has not been reported so far.F urthermore, the fraction of 34 %p enta-coordinated scandium is in agreement with the structure ( Figure 3) and with results already published fora nalogousM OFs. [28] Moreover,t he lineshape of the ScO 6 signal at 170 8Ci sb roader than for the www.chemeurj.org as-synthesized material.…”

“…[8] These MOFs have been widely investigatedo wing to their numerous interesting properties including:1 )high sur-face areas, 2) Lewis-acidity, and 3) remarkable thermal stability. [16] Nevertheless, the Lewis acidityo ft his MOF has not been investigated and hence it has not been possible to relate the Lewis acidity of Sc-based MOFs to the formation of ScO 5 sites. [14,15] IR spectroscopya nd H 2 adsorptionstudies have recently demonstrated the formationo ff ive-fold coordinated scandium atoms, ScO 5 ,u pon the thermala ctivation of another MOF:[ Sc 3 O(BTB) 2 (H 2 O) 3 ](OH)(H 2 O) 5 (DMF),w ith H 3 BTB = 1,3,5tris(4-carboxyphenyl)benzene, called Sc 3 BTB 2 hereafter.…”

“…[6,17,18] Furthermore, 45 Sc chemicals hifts are knownt ob every sensitive to the coordination number of the scandium [17,19,20] and their assignment can be supported by density functional theory (DFT) calculations, [19,21,22] notably in Sc-based MOFs. [16] However,i nM OFs, only hexa-coordinated Sc sites have been seen by solid-stateN MR spectroscopy so far. [17,22] Nevertheless, the 45 Sc isotropic chemical shifti sa lso quite sensitive to the others tructural differences, such as bond angles and bond distances [19,20] and hence the shift regionso ft he different Sc coordinations in different families of materials overlap heavily.F urthermore, this spin-7/2 isotopei ss ubjectt ol arge quadrupolari nteractions [19,20] and hence the second-order quadrupolarb roadening can obscure the chemical shift information.S uch quadrupolar interaction is especially large for asymmetricalS ce nvironments,s uch as ScO 5 .A na dditional difficulty for the NMR detection of 45 ScO 5 sites is the low concentration of those sites, since most materials contain mainly six, seven, or eight-coordinated scandiume nvironments.…”