Systematic Investigations of the Transition between Framework Topologies in Ce/Zr-MOFs.

Abstract

1- H-Pyrazole-3,5-dicarboxylic acid (H2PZDC), a small, strongly bent linker molecule with an angle of 147.4° between the carboxylate groups, was used in the synthesis of metal organic frameworks (MOFs) with fcu, bcu and reo topology. In systematic studies of the chemical system Ce4+/Zr4+/H2PZDC/HCOOH, their fields of formations were established. The decisive factors for the product formation and hence the transition between the framework topologies are the HCOOH/metal ratio and the molar ratio of Ce4+/Zr4+ employed in the synthesis. All title compounds crystallize with the well-known hexanuclear cluster {M6(μ3-O)4(μ3-OH)4(-CO2) n}, with n = 8 or 12 and M = Ce4+ and Zr4+, as the inorganic building unit (IBU). Connection through 12 or eight linker molecules leads to three framework topologies: fcu, bcu, and reo, respectively. The dominant phase observed in this system crystallizes with reo topology and is known as DUT-67. The pure Zr-MOF of composition [Zr6(μ3-O)4(μ3-OH)4(PZDC)4(OH)2(H2O)2] (Zr-DUT-67-PZDC) as well as the mixed-metal compounds Ce/Zr-DUT-67-PZDC are accessible and the molar ratio Ce4+/Zr4+ can be adjusted between 0 and 1. At low HCOOH/metal ratios, surprisingly, the UiO-66 type structure with fcu topology is formed despite the nonlinear geometry of the linker. Thus, using exclusively Zr4+ ions in the starting mixture the pure Zr-MOF with ideal composition [Zr6(μ3-O)4(μ3-OH)4(PZDC)6] (Zr-UiO-66-PZDC) was obtained. Variation of the Ce/Zr molar ratio leads to a continuous increase in linker defects with increasing Ce content in the MOF. At a Ce/Zr value of ∼ 1:1 a transition from the fcu to the reo framework topology takes place. Using high HCOOH/metal ratios, a transition from the reo to the bcu topology is observed when a molar ratio of Ce/Zr ≥ 1:5 is employed. Irrespective of the molar ratio used in the reaction mixture, the mixed-metal MOF of composition [CeZr5(μ3-O)4(μ3-OH)4(PZDC)4(OH)2(H2O)2] (Ce/Zr-CAU-38-PZDC) is always formed as confirmed by comprehensive EDX analyses. Rietveld refinement strongly indicates the presence of exclusively hexanuclear {CeZr5(μ3-O)4(μ3-OH)4} clusters and thus CAU-38 is the first Ce/Zr-MOF which solely occurs at a specific metal stoichiometry. In addition to the detailed synthetic study, the compounds were thoroughly characterized regarding their composition, lattice parameters, and porosity, as well as thermal and chemical stability.