High Capacity Hydrogen Adsorption in Cu(II) Tetracarboxylate Framework Materials: The Role of Pore Size, Ligand Functionalization, and Exposed Metal Sites

Abstract

A series of isostructural metal-organic framework polymers of composition [Cu2(L)(H2O)2] (L= tetracarboxylate ligands), denoted NOTT-nnn, has been synthesized and characterized. Single crystal X-ray structures confirm the complexes to contain binuclear Cu(II) paddlewheel nodes each bridged by four carboxylate centers to give a NbO-type network of 64.82 topology. These complexes are activated by solvent exchange with acetone coupled to heating cycles under vacuum to afford the desolvated porous materials NOTT-100 to NOTT-109. These incorporate a vacant coordination site at each Cu(II) center and have large pore volumes that contribute to the observed high H2 adsorption. Indeed, NOTT-103 at 77 K and 60 bar shows a very high total H2 adsorption of 77.8 mg g(-)- equivalent to 7.78 wt% [wt% = (weight of adsorbed H2)/(weight of host material)] or 7.22 wt% [wt% = 100(weight of adsorbed H2)/(weight of host material + weight of adsorbed H2)]. Neutron powder diffraction studies on NOTT-101 reveal three adsorption sites for this material: at the exposed Cu(II) coordination site, at the pocket formed by three {Cu2} paddle wheels, and at the cusp of three phenyl rings. Systematic virial analysis of the H2 isotherms suggests that the H2 binding energies at these sites are very similar and the differences are smaller than 1.0 kJ mol-1, although the adsorption enthalpies for H2 at the exposed Cu(II) site are significantly affected by pore metrics. Introducing methyl groups or using kinked ligands to create smaller pores can enhance the isosteric heat of adsorption and improve H2 adsorption. However, although increasing the overlap of potential energy fields of pore walls increases the heat of H2 adsorption at low pressure, it may be detrimental to the overall adsorption capacity by reducing the pore volume.

High gas storage capacities and stepwise adsorption in a UiO type metal–organic framework incorporating Lewis basic bipyridyl sites

“…Single crystal analysis reveals that UiO(bpdc) crystallizes in cubic space group Fm% 3 which has a lower symmetry in contrast to the commonly encountered Fm% 3m space group in UiO series MOFs, 11 because the inclusion of hetero N atoms in bpdc linkers lowers the symmetry of the crystal system. As expected, a six-nuclear 12-connected SBU: Zr 6 O 4 (OH) 4 (CO 2 ) 12 , is formed by the assembly of six zirconium atoms with eight m 3 -O or m 3 -OH bridged oxygen atoms and twelve carboxylate groups (as shown in Fig. 1a).…”

“…At 293 K and 20 bar, the adsorbed amount of CH 4 is 12.2 wt% (7.63 mmol g À1 ), which is far from saturation. The methane isotherms fit well with the Langmuir equation at a higher pressure 12 SBUs. The high capacity and excellent stability make it a very attractive material for methane storage applications.…”

“…The outstanding structural characteristics and excellent stability of UiO type MOFs motivate us to incorporate the pyridyl moieties into the frameworks, with the anticipation that it would further enhance the gas uptake capacities by anchoring the Lewis basic sites onto the pore surface but without sacrificing its original high porosity and exceptional robustness. Herein, we synthesized a UiO MOF, Zr 6 (m 3 -O) 4 (OH) 4 (bpdc) 12 (termed as UiO(bpdc) in this communication), by using a N-heterocyclic carboxylate ligand: 2,2-bipyridine-5,5 0 -dicarboxylate (bpdc), in which the bipyridyl moieties were incorporated as free Lewis basic sites. Upon being activated by Soxhlet-extraction, the adsorption properties of N 2 , H 2 , D 2 , CO 2 and CH 4 were examined.…”

“…Then, each vertex of the 12-connected SBUs is bridged by the bpdc linkers through the carboxylate groups, forming a three-dimensional network with the fcu topology. The formula of UiO(bpdc) can be determined to be Zr 6 (m 3 -O) 4 (OH) 4 (bpdc) 12 on the basis of single crystal X-ray analysis and elemental analysis. Two types of cages, the octahedral cages with a diameter of 1.6 nm and the tetrahedral cages with a diameter of 1.2 nm, are formed by the assembly of the SBUs and linkers (as illustrated in Fig.…”

Studies on Metal–Organic Frameworks of Cu(II) with Isophthalate Linkers for Hydrogen Storage

“…However, when the ligand bridges are lengthened beyond a certain point, this strategy appears to fail due to the onset of interpenetration, which reduces the available pore volume and appears to also lower structural stability. 17 We argued that a more highly-connected network topology might be less likely to form interpenetrated structures. Also, when fused within a network structure, highly-connected metal-organic polyhedra may better maintain their intrinsic porosity on tessellation in 3D space.…”