Metal-organic frameworks (MOFs) have gained widespread attention due to their modular construction that allows the tuning of their properties. Within this vast class of compounds, metal carboxylates containing tri- and...
Bimetallic Ce/Zr-UiO-66
metal–organic frameworks (MOFs)
proved to be promising materials for various catalytic redox applications,
representing, together with other bimetallic MOFs, a new generation
of porous structures. However, no direct proof for the presence of
both metals in a single cornerstone of UiO-type MOFs was reported
so far. Employing element-selective X-ray absorption spectroscopy
techniques herein, we demonstrate, for the first time, that our synthesis
route allows obtaining Ce/Zr-UiO-66 MOFs with desired Ce content and
bimetallic CeZr5 cornerstones. Performing multiple-edge
extended X-ray absorption fine structure analysis, we determine the
exact stoichiometry of the cornerstones, which explains the dependence
of thermal and chemical stability of the materials on Ce content.
We report on the results of an in situ synchrotron powder X‐ray diffraction study of the crystallisation in aqueous medium of two recently discovered perfluorinated CeIV‐based metal–organic frameworks (MOFs), analogues of the already well investigated ZrIV‐based UiO‐66 and MIL‐140A, namely, F4_UiO‐66(Ce) and F4_MIL‐140A(Ce). The two MOFs were originally obtained in pure form in similar conditions, using ammonium cerium nitrate and tetrafluoroterephthalic acid as reagents, and small variations of the reaction parameters were found to yield mixed phases. Here, we investigate the crystallisation of these compounds, varying parameters such as temperature, amount of the protonation modulator nitric acid and amount of the coordination modulator acetic acid. When only HNO3 is present in the reaction environment, only F4_MIL‐140A(Ce) is obtained. Heating preferentially accelerates nucleation, which becomes rate determining below 57 °C. Upon addition of AcOH to the system, alongside HNO3, mixed‐phased products are obtained. F4_UiO‐66(Ce) is always formed faster, and no interconversion between the two phases occurs. In the case of F4_UiO‐66(Ce), crystal growth is always the rate‐determining step. A higher amount of HNO3 favours the formation of F4_MIL‐140A(Ce), whereas increasing the amount of AcOH favours the formation of F4_UiO‐66(Ce). Based on the in situ results, a new optimised route to achieving a pure, high‐quality F4_MIL‐140A(Ce) phase in mild conditions (60 °C, 1 h) is also identified.
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.
Herein is reported the utilization of acetonitrile as a new solvent for the synthesis of the three significantly different benchmark metal–organic frameworks (MOFs) CAU‐10, Ce‐UiO‐66, and Al‐MIL‐53 of idealized composition [Al(OH)(ISO)], [Ce6O4(OH)4(BDC)6], and [Al(OH)(BDC)], respectively (ISO2−: isophthalate, BDC2−: terephthalate). Its use allowed the synthesis of Ce‐UiO‐66 on a gram scale. While CAU‐10 and Ce‐UiO‐66 exhibit properties similar to those reported elsewhere for these two materials, the obtained Al‐MIL‐53 shows no structural flexibility upon adsorption of hydrophilic or hydrophobic guest molecules such as water and xenon and is stabilized in its large‐pore form over a broad temperature range (130–450 K). The stabilization of the large‐pore form of Al‐MIL‐53 was attributed to a high percentage of noncoordinating −COOH groups as determined by solid‐state NMR spectroscopy. The defective material shows an unusually high water uptake of 310 mg g−1 within the range of 0.45 to 0.65 p/p°. In spite of showing no breathing effect upon water adsorption it exhibits distinct mechanical properties. Thus, mercury intrusion porosimetry studies revealed that the solid can be reversibly forced to breathe by applying moderate pressures (≈60 MPa).
In theory, bimetallic UiO-66(Zr:Ce) and UiO-66(Zr:Hf) metal-organic frameworks (MOFs) are extremely versatile and attractive nanoporous materials as they combine the high catalytic activity of UiO-66(Ce) or UiO-66(Hf) with the outstanding stability of . Using in situ highpressure powder X-ray diffraction, however, we observe that this expected mechanical stability is not achieved when incorporating cerium or hafnium in . This observation is akin to the earlier observed reduced thermal stability of UiO-66(Zr:Ce) compounds. To elucidate the atomic origin of this phenomenon, we chart the loss-of-crystallinity pressures of 22 monometallic and bimetallic UiO-66 materials and systematically isolate their intrinsic mechanical stability from their defect-induced weakening. This complementary experimental/computational approach reveals that the intrinsic mechanical stability of these bimetallic MOFs decreases nonlinearly upon cerium incorporation but remains unaffected by the zirconium: hafnium ratio. Additionally, all experimental samples suffer from defect-induced weakening, a synthesis-controlled effect that is observed to be independent of their intrinsic stability.
A microwave-assisted synthesis method for Ce(IV)-based MOFs crystallizing in the MIL-140 structure has been developed. Three different linker molecules, i.e. terephthalic acid (H2BDC), 2-chloroterephthalic acid (H2BDC-Cl) and 2,6-naphtalenedicarboxylic acid (H2NDC)...
Six different chiral and achiral alkane dicarboxylic C4-acids resulted in the formation of Ce(iv)-MOFs crystallizing in three different framework topologies.
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