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. 2023 Dec 7;8(50):48394–48404. doi: 10.1021/acsomega.3c07906

Tuning the Cerium-Based Metal–Organic Framework Formation by Template Effect and Precursor Selection

Dimitry Grebenyuk †,‡, Maria Shaulskaya †, Artem Shevchenko §,†, Mirijam Zobel ∥, Marina Tedeeva †, Alexander Kustov †,⊥, Ilia Sadykov #, Dmitry Tsymbarenko †,*
PMCID: PMC10733954  PMID: 38144061

Abstract

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The novel metal–organic framework [(CH3)2NH2]2[Ce2(bdc)4(DMF)2]·2H2O (Ce-MOF, H2bdc—terephthalic acid, DMF—N,N-dimethylformamide) was synthesized by a simple solvothermal method. Ce-MOF has 3D connectivity of bcu type with a dinuclear fragment connected with eight neighbors, while three types of guest species are residing in its pores: water, DMF, and dimethylammonium cations. Dimethylamine was demonstrated to have a decisive templating effect on the formation of Ce-MOF, as its deliberate addition to the solvothermal reaction allows the reproducible synthesis of the new framework. Otherwise, the previously reported MOF Ce5(bdc)7.5(DMF)4 (Ce5) or its composite with nano-CeO2 (CeO2@Ce5) was obtained. Various Ce carboxylate precursors and synthetic conditions were explored to evidence the major stability of Ce-MOF and Ce5 within the Ce carboxylate-H2bdc-DMF system. The choice of precursor impacts the surface area of Ce-MOF and thus its reactivity in an oxidative atmosphere. The in situ PXRD and TG-DTA-MS study of Ce-MOF in a nonoxidative atmosphere demonstrates that it eliminates H2O and DMF along with (CH3)2NH guest species in two distinct stages at 70 and 250 °C, respectively, yielding [Ce2(bdc)3(H2bdc)]. The H2bdc molecule is removed at 350 °C with the formation of novel modification of Ce2(bdc)3, which is stable at least up to 450 °C. According to the total X-ray scattering study with pair distribution function analysis, the most pronounced local structure transformation occurs upon departure of DMF and (CH3)2NH guest species, which is in line with the in situ PXRD experiment. In an oxidative atmosphere, Ce-MOF undergoes combustion to CeO2 at a temperature as low as 390 °C. MOF-derived CeO2 from Ce-MOF, Ce5, and CeO2@Ce5 exhibits catalytic activity in the CO oxidation reaction.

1. Introduction

Metal–organic frameworks (MOFs) draw substantial attention from many scientific groups nowadays. The main features of MOFs—high surface area and porosity—provide a wide range of potential applications such as gas storage and separation,1−3 sensing,4−7 capture of toxic metals,8,9 drug delivery,10,11 or catalysis12−18 including photo-19,20 and electrocatalysis.21,22 Terephthalic acid (1,4-benzenedicarboxylic acid, H2bdc) is a simple rigid ditopic linker often employed for construction of MOFs with several milestone examples such as Zn4O(bdc)3 (MOF-5),23 Cr(OH)(bdc) (MIL-53),24 or Zr6O4(OH)4(bdc)6 (UiO-66).25

Lanthanide (Ln) MOFs are attracting increasing attention due to their specific geometrical features that provide wide possibilities of topology design and interesting properties related to magnetic and luminescent applications.26−28 Despite the progress in the field and tens of thousands of MOFs reported to date,29 even simple systems such as Ln-H2bdc have yet to be fully explored. Very recent developments on new Ln-based MOFs with terephthalic linkers still continue to emerge.30−32

Cerium MOFs are of particular interest among the Ln-based MOFs due to the unique electronic properties of the Ce ion featuring two stable oxidation states (+3 and +4) and enabling new possible applications as catalysts in various redox processes due to Ce3+/Ce4+ switching.,33−37 e.g., for CO conversion to CO2.33,38,39 Like other lanthanides, Ce can adopt various coordination environments. Therefore, the search for new deliberate and reproducible synthetic methods for Ce MOFs is a challenging task. A high number of Ce4+ MOFs have been synthesized to date.40 The majority of them contain in situ assembled {CeIV6O4(OH)4} cores and are the most extensively studied after the preparation of the Ce-UiO-66 isoreticular analog of the corresponding Zr-based UiO-66 MOF.41 MOFs based on Ce3+ usually contain mono-,42,43 di-,44,48,49 or 1D chain polynuclear30,31,45,47 building blocks connected by organic linkers. A handful of Ce3+ MOFs with nonsubstituted terephthalate linkers have been reported to date (Table 1).

Table 1. Summary of Ce3+ MOFs with Terephthalate Linkers Reported in Literature with Porosity Analysis Performed with PLATONa46.

formula building unit free volume (%) free volume desolvated (%) CCDC ref code CCDC number ref
Ce6(bdc)9(DMF)9(H2O)3 chain 26.9 49.1 ALUJIB 745286 (30)
Ce5(bdc)7.5(DMF)4 (Ce5) chain 29.6 43.0 GOBXAY 912350 (31)
Ce2(bdc)3(DEF)2 chain 30.3 51.3 QOCSAD 630356 (47)
[(CH3)2NH2]2[Ce2(bdc)4(DMF)2]·2H2O (Ce-MOF) dinuclear 32.3 52.7   2106041 this work
Ce2(bdc)3(DMF)2(DMSO)2 dinuclear 31.9 56.2 BUVHEH 844407 (48)
Ce2(bdc)3(e-urea)2(H2O)2 dinuclear 28.0 45.6 LAGPOA 786945 (49)
Ce2(bdc)2(NMP)4(ac)2 dinuclear 28.2 60.4 UNECEX 868529 (50)
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a

Abbreviations for ligands: H2bdc—terephthalic acid; DMF—N,N-dimethylformamide, DMSO—dimethyl sulfoxide, e-urea—ethylene urea, DEF—N,N-diethylformamide, NMP—N-methyl pyrrolidone, Hac—acetic acid.

Anionic MOFs are of particular interest through the possibility of sensing and separation of charged species, protonic conduction,51−53 dye absorption,54 and capture of toxic metals.55 A vast number of the reported anionic MOFs contain dimethylammonium cations generated in situ via degradation of dimethylformamide usually employed as the solvent for MOF synthesis, e.g.,{[((CH3)2NH2)3(SO4)]2[Zn2(ox)3]}n (ox = oxalate),52 [(CH3)2NH2]2[Cd3(L)]·2H2O (H4L = 2,5′-di(3′,5′-dicarboxylphenyl)pyridine),53 and {[((CH3)2NH2)2][Zn3(L)2]·9H2O} (L = 5,5′-(1,4-phenylenebis(methylene))bis(oxy)diisophthalic acid).55 This potentially decreases synthesis reproducibility with uncontrollable generation of dimethylammonium, as to the best of our knowledge none of the reported synthetic methods employ deliberate addition of dimethylamine to the synthetic mixture.

CO is a toxic gas that is generated by incomplete combustion of fuel from various sources, such as power plants, cement plants, vehicle exhaust emissions, and burning biomass. During CO oxidation, catalysts based on noble metals showed the highest catalytic activity and stability, even at low temperatures.56 However, there are several disadvantages: low abundance, high price, and stringent requirements for the catalyst preparation process. Therefore, the development of highly active and inexpensive CO oxidation catalysts to replace precious metal catalysts is of great importance for large-scale applications. It is known that the excellent catalytic activity of cerium dioxide catalysts in the CO oxidation reaction can be achieved due to oxygen vacancies on the catalyst surface forming upon synthesis of nanoceria. Recently, MOF-derived ceria-based catalysts in the CO oxidation reaction have become a good substitute for precious metal catalysts due to their low price, large reserves, and high catalytic activity.33 Therefore, the exploration of new synthetic pathways to produce ceria for catalytic purposes is an urgent matter.

In this article, we have developed a new approach for the synthesis of anionic MOFs containing dimethylammonium cations. We report on the synthesis of the Ce-based MOF [(CH3)2NH2]2[Ce2(bdc)4(DMF)2]·2H2O (Ce-MOF) and fine-tuning of synthetic conditions leading to this new target compound in the Ln-H2bdc-DMF system. The crystal structure of Ce-MOF was determined by means of single-crystal X-ray diffraction and by PXRD with Rietveld refinement, supported by pair distribution function (PDF) analysis. TGA-DTA-MS, in situ PXRD, and BET add insight on the porosity, specific surface area, and solvent content of the MOF. The effect of the precursor selection on the surface area, reactivity in oxidative conditions, and catalytic activity of MOF-derived CeO2 in CO oxidation reaction is also reported.

2. Experimental Section

Cerium nitrate Ce(NO3)3·6H2O (99%, Sigma-Aldrich), cerium acetate Ce(CH3COO)3·1.5H2O (analytical grade, Reakhim, Russia), terephthalic acid (H2bdc, 98%, Sigma-Aldrich), diethylenetriamine (deta, 99%, Sigma-Aldrich), dimethylamine (33% aq solution, Reakhim, Russia), N,N-dimethylformamide (DMF, 99.9%, Ekos-1, Russia), pivalic acid (Hpiv, 98%, Merck), and NaOH (analytical grade, Reakhim, Russia) were used as received. Ce(III) hydroxo pivalate Ce4(OH)2(piv)10(H2O)2 and Ce(IV) oxo pivalate Ce6O8(piv)8(deta)4 were synthesized according to previously reported methods.57,58

One general method was employed to prepare [(CH3)2NH2]2[Ce2(bdc)4(DMF)2]·2H2O (Ce-MOF), differing only in the precursor compound and its amount. A mixture of H2bdc (136 mg, 0.8 mmol) and (i) Ce4(OH)2(piv)10(H2O)2 (170 mg, 0.1 mmol), (ii) Ce(CH3COO)3·1.5H2O (136 mg, 0.4 mmol), (iii) Ce6O8(piv)8(deta)4 (150 mg, 0.07 mmol), or (iv) Ce(NO3)3·6H2O (178.5 mg, 0.4 mmol) was dispersed in DMF (12 mL). Then, dimethylamine water solution (340 μL, 2 mmol) was added and the mixture was placed in Teflon-lined stainless-steel reactors (23 mL), heated at 120 °C for 48 h, and then cooled to room temperature for 24 h. The resulting powder was filtered off, washed with DMF and ethanol, dried, and stored in air. Yield was determined to be ca. 80%.

Ce5(bdc)7.5(DMF)4 (Ce5) was prepared from H2bdc and (i) Ce4(OH)2(piv)10(H2O)2 or (ii) Ce(CH3COO)3·1.5H2O by the same method as described above in the absence of dimethylamine. Yield was ca. 60%.

The CeO2@Ce5 composite was prepared from H2bdc and (iii) Ce6O8(piv)8(deta)4 by the same method as described above in the absence of dimethylamine. The yield was ca. 50%.

Single crystals of Ce-MOF for X-ray structure analysis were serendipitously obtained during the optimization of the preparation method of Ce5.

3. Results and Discussion

3.1. General Remarks on Synthesis

Recently, we have elaborated the synthetic routes for tetranuclear {Ln4(OH)2}, {Ln4(OH)4} and hexanuclear {Ce6O8}, {Ln6(OH)8} lanthanide carboxylate complexes.57−59 The intent of the current study was to examine the possibility of creating new MOFs by linking the di-, tetra-, and hexanuclear Ce complexes into an extended 3D framework via the terephthalate linker.

The compounds [(CH3)2NH2]2[Ce2(bdc)4(DMF)2]·2H2O (Ce-MOF) and previously reported Ce5(bdc)7.5(DMF)431 (Ce5) were successfully synthesized by solvothermal method from terephthalic acid (H2bdc) and three different Ce carboxylate precursors, (i) Ce4(OH)2(piv)10(H2O)2, or (ii) Ce(CH3COO)3·1.5H2O, or (iii) Ce6O8(piv)8(deta)4, in DMF as outlined in Scheme 1.

Scheme 1. Synthetic Scheme of Ce-MOF and Ce5 Preparation from Various Ce Carboxylate Precursors.

Scheme 1

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SBET values refer to the desolvated Ce-MOF samples obtained from the respective precursors.

The solvothermal reaction that is usually employed for preparation of MOFs is carried out at a relatively high temperature and autogenous pressure of the solvent. Even a slight change in reaction conditions (temperature, pH, presence of modulating reactant) may guide it to a different product.60,61

Apart from the desired reaction, interfering processes could occur, including decomposition of the compounds. Accordingly, in situ generated dimethylammonium cation (CH3)2NH2+ is a frequent guest in many anionic MOF crystal structures due to the solvent (DMF) decomposition in the temperature range of 100–160 °C.62−70

Indeed, in the present work, several serendipitously formed single crystals of the Ce-MOF byproduct were harvested from the reaction product of solvothermal synthesis between Ce6O8(piv)8(deta)4 and H2bdc in DMF, while the main precipitate was the composite CeO2@Ce5. As in previous works, the (CH3)2NH2+ ions have formed due to hydrolysis of DMF in the presence of water traces and acted as a template for the framework assembly (see Section 3.4). However, out-of-control hydrolysis of DMF complicates the synthesis of pure Ce-MOF powder in high yield. We have varied the Ce-containing precursor (i–iii), as well as temperature (120–160 °C), time (2–48 h), and reagent concentration in solvothermal syntheses, but most of the syntheses resulted in formation of Ce5. The only rigorous and easy way to reproducible synthesis of Ce-MOF was discovered to be the deliberate addition of (CH3)2NH excess into the reaction vessel. In our view, this novel approach opens the prospect for reproducible synthesis of a large variety of new and previously reported anionic MOFs on templating (CH3)2NH2+ and related cations.

3.2. Synthesis and Characterization of Ce5 and CeO2@Ce5

We found that the reaction between terephthalic acid (H2bdc) and either (i) Ce4(OH)2(piv)10(H2O)2,57 (ii) Ce2(ac)6(H2O)3, or (iii) Ce6O8(piv)8(deta)458 in DMF under a broad range of solvothermal conditions leads to formation of a previously reported stable Ce3+-based MOF Ce5(bdc)7.5(DMF)4 (Ce5) (Scheme 1 and Figure S1).31,71

Notably, if Ce6O8(piv)8(deta)4 is used as a precursor, Ce4+ is only partially reduced to Ce3+ and the reaction leads to the formation of a CeO2@Ce5 composite, according to PXRD data (Figure 1). The possible mechanism includes reduction of Ce4+ to Ce3+ by deta, which can be readily oxidized to form polyamines (Figure S2).72,73 Such types of CeO2/MOF-based composites are currently studied as catalysts having a prospect to enhance catalytic activity and selectivity.74,75

Figure 1.

Figure 1

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Experimental PXRD pattern (λ = 1.5419 Å) of CeO2@Ce5 (blue) and calculated patterns of Ce5 (red), nano-CeO2 (yellow), and their composition Ce5+nano-CeO2 (green). Peak broadening of the nano-CeO2 pattern was calculated for a grain size of 4.5 nm according to the Scherrer equation. Vertical orange bars show the positions of the Bragg peaks of CeO2.

The broad peaks on the experimental PXRD pattern corresponding to CeO2 reflect the nanoscale state of the particles in the composite. This is supported by the results of scanning electron microscopy (Figure S3), indicating that nano-CeO2 is distributed on the surface of Ce5 crystals. Total X-ray scattering with PDF analysis demonstrated that the experimental PDF is in good agreement with the theoretical one of CeO2 nanoparticles (Figure 2).

Figure 2.

Figure 2

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PDF refinement (red) of measured data of CeO2@Ce5 (blue) with the CeO2 spherical particle model for the distance range 1–50 Å. Difference curve (green) is offset for clarity. Weighted agreement factor Rw and Pearson correlation coefficient r between the experimental and fitted curve are shown below the graphs. Insets show the TEM image of the composite (left) and selected-area diffraction pattern with clearly visible diffraction rings corresponding to CeO2 (right).

The average particle size of the nanomaterial can be deduced from the PDF being a histogram of interatomic distances as the distance at which the curve flattens out since the particle diameter is reached, and no interatomic distances exist any further. The CeO2 particle size value of 4.5 nm was obtained by the PDF fit using a spherical shape function and is in line with the results of transmission electron microscopy (Figure 2).

It should be noted that the CeO2 signal dominates the PDF despite the comparable amounts of individual phases in CeO2@Ce5 (Synthesis section in the SI). The reason for this lies in the higher frequency of Ce–O and Ce–Ce distances in CeO2 involving the strong scatterer Ce compared to many light scatterers (H, C, O, N) in Ce5 making the Ce5 signal hardly visible on the PDF of the composite. Yet, the structural remnants can be seen in the difference curve (Figure S4).

XANES spectra of Ce3+ near its L3 edge (Figure 3) feature a single strong peak at 5726 eV, whereas those of Ce4+ have two maxima of lower intensity at 5730 and 5739 eV. The experimental spectrum of CeO2@Ce5 can be represented as a sum of the two components, confirming the presence of Ce in both oxidation states. The fitting of XANES spectra of model compounds to the spectrum of CeO2@Ce5 estimates the Ce3+:Ce4+ molar ratio as 0.47 ± 0.01:0.53 ± 0.01.

Figure 3.

Figure 3

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X-ray absorption near edge structure (XANES) spectra of CeO2@Ce5 recorded near the Ce L3 absorption edge fitted with experimental spectra of the model compounds Ce2(bdc)3(H2O)4 and CeO2.

3.3. Synthesis of Ce-MOF

Inspired by a serendipitous formation of several Ce-MOF crystals as a byproduct due to hydrolysis of DMF, we decided to deliberately add the templating (CH3)2NH to the reaction mixture. This led to the formation of pure Ce-MOF for all three investigated carboxylate precursors—Ce6O8(piv)8(deta)4, Ce2(ac)6(H2O)3, and Ce4(OH)2(piv)10(H2O)2 (Figure 4). Notably, synthesis from Ce6O8(piv)8(deta)4 is accompanied by full reduction of Ce4+ to Ce3+ due to amine excess.

Figure 4.

Figure 4

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PXRD patterns (λ = 1.5419 Å) of Ce-MOF samples prepared from various Ce precursors.

Variation of (CH3)2NH content demonstrated that Ce-MOF could be obtained in pure form within a wide range of (CH3)2NH content (1 to 10 equiv with respect to Ce), indicating the stability of the framework. Notably, if a simple and commercially available Ce source Ce(NO3)3(H2O)6 was used as a precursor, it was not possible to obtain pure Ce-MOF (Figure S5), highlighting the importance of preorganized Ce atom arrangement in the carboxylate precursors for the synthesis. Despite the identical composition of the Ce-MOF powders derived from three different precursors, BET surface area measurements clearly indicate a substantial difference in specific surface area of the sample prepared from Ce4(OH)2(piv)10(H2O)2 having SBET (222 m2/g) twice as large as that of the ones prepared from Ce6O8(piv)8(deta)4 (110 m2/g) and Ce2(ac)6(H2O)3 (112 m2/g) (Scheme 1 and Figures S6 and S7).

This can originate from a rather loose arrangement of 1D polymeric structural entities in Ce4(OH)2(piv)10(H2O)2 due to the bulky pivalate ligands or from a different reactivity of the precursors but requires further investigation. Thus, to the best of our knowledge, Ce-MOF having SBET of 222 m2/g shares the second highest reported specific surface area value among Ce3+ MOFs with dicarboxylic linkers.60,71

3.4. Crystal Structures of Ce-MOF and Ce-MOF–H2O

Ce-MOF crystallizes in the P21/n space group, the unit cell contains one symmetrically independent Ce1 atom, and the crystal structure is composed of the centrosymmetric dinuclear [Ce2(bdc)4]2– species (Ce1–Ce1i separation equals 4.185 Å) linked by two bridging and two chelate bridging carboxylic groups of four terephthalate anions (bdc2–) (Figure 5 and Figure S8). Each Ce1 is coordinated by O1 and O2 atoms of a chelating bdc2– ion, O3 and O4 atoms of a chelating bridging bdc2–, O5 and O6 atoms of a bridging bdc2–, O3 from another chelating bridging bdc2–, O7 atom from a terminal bdc2–, and O1S from the coordinated DMF molecule. The ninefold coordination environment of Ce1 is best described as a muffin according to CShM (continuous shape measures) analysis (Figure S9 and Table S4).76 It is worth noting that the geometry of dinuclear [Ce2(bdc)4]2– species is typical for cerium carboxylates and, for instance, occurs in Ce2(bdc)3(DMF)2(DMSO)248 and Ce2(bdc)2(NMP)4(ac)250 MOF structures. However, the presence of a terminal COO group is quite unusual and is due to the formation of ion pairs with [(CH3)2NH2]+ cations. The dinuclear [Ce2(bdc)4]2– fragments are joined together into a three-dimensional anionic framework [Ce2(bdc)4]2–∞ by the dicarboxylate terephthalate anions of two types, which are considered as μ2 and μ4 connectors (Figure 6).

Figure 5.

Figure 5

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Crystal structure of Ce-MOF. The dinuclear building unit on the left is presented as a ball-and-stick model for clarity; see the SI for the thermal ellipsoid plot of the building unit. Dashed lines show H-bonds. Symmetry codes: (i) 1–x, 1–y, 1–z; (ii) 1.5–x, 0.5+y, 1.5–z; (iii) −0.5+x, 0.5–y, −0.5+z. Guest molecules on the right are shown as semitransparent for clarity. The right panel shows the view along the channels in the [1̅1̅1] direction. Semitransparent green lines show unit cell edges.

Figure 6.

Figure 6

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Dinuclear building unit of Ce-MOF surrounded by eight neighbor units as a node of an 8-c topological bcu net.

The framework features a complex intertwined channel system along the [11̅1̅] and [1̅1̅1] directions with the largest rhombic aperture of 9.47 Å × 17.58 Å along the diagonals, as calculated from the distances between Ce ions (Figure 5 and Figure S10). Topology analysis of the Ce-MOF crystal structure with ToposPro software77 indicates that the structure can be represented as an 8-c net (point symbol {4.2464}) of a bcu-type topology with Ce2 building units as nodes (Figure 6 and Figure S11). Porosity analysis of the crystal structure demonstrates the highest porosity of Ce-MOF among the reported Ce3+-based MOFs with a terephthalic linker and the third highest porosity after virtual solvent exclusion (Table 1).

Structural analysis of dehydrated Ce-MOF–H2O demonstrates that removal of water molecules does not affect the crystal structure of the framework substantially (Tables S1 and S2) due to its rigidity and only leads to disorder of dimethylammonium cations and dimethylformamide molecules. Porosity analysis of the crystal structure demonstrates the highest porosity of Ce-MOF among the reported Ce3+-based MOFs with the terephthalic linker and the third highest porosity after virtual solvent exclusion (Table 1).

Crystal structures of Ce-MOF and Ce-MOF–H2O were also determined at room temperature and were demonstrated to have no noticeable differences from the ones at 100 K (Tables S1 and S2). Figure S12 shows the Rietveld refined powder XRD profile of Ce-MOF.

3.5. Thermal Behavior

Since MOFs synthesized under solvothermal conditions usually contain guest solvent molecules in the voids, their desolvation upon heating at moderate temperatures is required to activate the porosity. The thermal behavior of the new Ce-MOF was investigated by means of in situ PXRD, TG-DTA with mass spectrometric evolved gas analysis, and total X-ray scattering with PDF analysis. In the process of thermal decomposition of Ce-MOF, several stages can be distinguished. The decomposition begins from the elimination of the weakly bound water molecules (m/z = 18, 17) from the pores at 50–100 °C, the experimental weight loss is 3.1%, which is consistent with the theoretically calculated weight loss of 3.0% (Figure 7 and Figure S13). Judging by the data obtained in the in situ PXRD experiment (Figure 7 and Figure S14), only slight structural changes occur (at Q of ca. 1.4 and 1.8 Å–1) with most of the peaks retaining their positions and intensity. This is in line with the SC XRD crystal structure of a Ce-MOF–H2O.

Figure 7.

Figure 7

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In situ PXRD pattern (λ = 0.5594 Å) of Ce-MOF upon heating in a capillary sealed in vacuum (left). Semitransparent dashed lines divide the temperature range into several stages of the sample evolution. TG curve of Ce-MOF in argon with scaled ionic currents of ions in evolved gas with the respective m/z (right).

Upon further heating, DMF and dimethylamine molecules leave the pores on the next stage of weight loss at 250–275 °C (exp. 22.8%, calcd 22.4%) and appearance of peaks in the mass spectrum with m/z = 73 (DMF+), 44 ((CH3)2N+), 18 (NH4+), 17 (NH3+). This corresponds to the major change in PXRD pattern (see detailed view in Figures S15 and S16) manifested in intensity drop and disappearance of several diffraction peaks (e.g., at 0.8–1 Å–1). Lighter dimethylamine molecules residing in the pores of the framework are eliminated simultaneously with the heavier coordinated dimethylformamide molecules due to the firm retention in the form of (CH3)2NH2+ cations by the anionic framework [Ce2(bdc)4]2– and a system of H-bonds. The proton that should remain upon elimination of dimethylamine for charge compensation is supposedly bound to the O8 atom of the terminal bdc2– ligand that acted as the H-bond acceptor from (CH3)2NH2+ in the as-obtained Ce-MOF.

At ca. 350 °C, one H2bdc molecule per formula unit is eliminated, leaving the Ce2(bdc)3 framework (exp. and calcd weight losses of 36.0%). According to TG-DTA-MS data, the framework exhibits thermal stability up to at least 450 °C in argon while the peaks of m/z 44, 73, and 166 at higher temperatures originate from terephthalic linkers indicating MOF destruction. The argon atmosphere impedes oxidation, and the formation of CeO2 is only achieved by 1000 °C (exp. and calcd weight losses of 72.0 and 71.6%, respectively). The thermal behavior of Ce-MOF in air demonstrates the same three elimination stages at temperatures of 100–150, 200–250, and 300–350 °C while combustion of the framework occurs in the temperature range of 390–500 °C with the formation of CeO2 (Figure S17).

To get more insight into the structural changes in Ce-MOF upon heating, we performed total scattering experiments with PDF analysis of as-synthesized Ce-MOF and of the products of its heating at 100 (Ce-MOF–H2O), 250 ([Ce2(bdc)3(H2bdc)]), and 400 °C ([Ce2(bdc)3]) in a vacuum (Figure 8). The data were collected according to our previously developed procedure.78 See the Supporting Information for the details on data collection and choice of structure models for the refinements.

Figure 8.

Figure 8

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Measured PDF data (shades of blue) of Ce-MOF samples (as is and heated to different temperatures) and refinements (red) with the corresponding periodic models. Difference curves (gray) are offset for clarity. Weighted residual factor Rw and Pearson correlation coefficient r between experimental and fitted curves are shown below the graphs.

The PDFs of Ce-MOF and Ce-MOF–H2O closely resemble each other, which is reflected in their Pearson correlation coefficient r value of 0.98 (Figure S18) and are well described by the corresponding structure models of Ce-MOF and Ce-MOF–H2O within a distance range of up to 40 Å, which indicates that the local structure remains unchanged upon elimination of water molecules.

Heating of Ce-MOF up to 250 °C is accompanied by departure of DMF and dimethylamine molecules and leads to formation of [Ce2(bdc)3(H2bdc)], and a substantial change of the structure is expressed in both in situ PXRD and PDF data (Figures 7 and 8, respectively). The local structure changes and the peaks corresponding to Ce–Ce distances (e.g., at 11.4 and 28.4 Å) become less pronounced in the PDF of [Ce2(bdc)3(H2bdc)] indicating partial loss of the long-range order. PDF of [Ce2(bdc)3] is similar to the one of [Ce2(bdc)3(H2bdc)] with an r value of 0.91.

Due to the change of structure between 100 and 250 °C, the last two PDFs differ significantly from the first two, which is quantitatively expressed in the Pearson correlation coefficient values of ca. 0.7 (Figure S17). At the same time, the two PDFs are fitted rather well with the Ce5(bdc)7.5 model constructed from the Ce5(bdc)7.5(DMF)4 crystal structure31 by exclusion of DMF within the short distance range up to 15 Å, which points to substantial structure change upon elimination of DMF and dimethylamine and only a minor change between 250 and 400 °C.

PXRD patterns of Ce-MOF above 350 °C (Figure 7), a previously reported product of Ce5 heating above 275 °C in the N2 atmosphere,71 and that of CeO2@Ce5 above 275 °C (Figure S19) are similar to each other, and these three substances contain the same modification of Ce2(bdc)3. It should be noted that poor crystallinity of that phase hinders the structural study by means of Rietveld refinement; therefore, its crystal structure remains to be determined.

In an oxidizing atmosphere, Ce2(bdc)3 is readily oxidized to form nano ceria (Figures S15). The oxidation process depends on the surface area of the initial MOF and thus on the precursor employed. The largest surface area of Ce-MOF is obtained in the case of the Ce4(OH)2(piv)10(H2O)2 precursor (222 m2/g) and the corresponding Ce-MOF is fully oxidized to CeO2 at 390 °C, while the Ce-MOF sample synthesized from Ce2(ac)6(H2O)3 only forms traces of CeO2 at that temperature and full combustion occurs after heating at a constant temperature of 400 °C for 10 min (Figure S20).

Here, we have also studied the CeO2@Ce5 composite behavior upon heating in a vacuum (Figure S19). It has been found that nanoceria does not influence thermal decomposition of the Ce5 framework in composite, which loses DMF at 250 °C and undergoes pyrolysis after 475°C similarly to pure Ce5, while nanoceria gradually crystallizes upon heating.

3.6. Catalytic Performance of MOF-Derived CeO2

MOFs are widely explored as catalysts for various industrially important processes with the Ce-based ones and their derivatives, e.g., MOF-derived ceria being the most prominent in redox reactions due to the two stable oxidation states of Ce.33−36 Many reports point to the high catalytic activity of CeO2 and its composites in the CO oxidation reaction:38,39,79,80

3.6.

Less attention was devoted to Ce-based MOFs with only several reported examples indicating high efficiency of these MOFs and corresponding composites.81,82

Preliminary catalytic experiments demonstrated that no activity is exhibited by Ce-MOF and Ce5 per se at lower temperatures and the activation to form the MOF-derived CeO2 is essential. The Ce-MOF sample with the lowest surface area (112 m2/g) prepared from the Ce2(ac)6(H2O)3 precursor did not oxidize to CeO2 under the conditions of the catalytic reaction even at 400°C but transformed to [Ce2(bdc)3] and thus did not exhibit catalytic activity (Figure 9, Figure S21, and Table S5). At the same time, all other MOFs that transformed to CeO2in situ demonstrate comparable activity with the conversion of CO at the level of ca. 30% corresponding to the 16 mmol h–1 g–1 reaction rate, which is significantly higher than that of bulk CeO2 (Figure 9). A minor decrease of activity of derived CeO2 from Ce5, CeO2@Ce5, and Ce-MOF is in perfect correlation with the decrease of the CeO2 BET surface area: 224, 188, and 182 m2/g, respectively (Table S5). The only reaction product is CO2; accordingly, the selectivity for CO2 is 100%. The material balance of the reaction for carbon calculated from gas chromatography data is 95–98% (Figure S22 and Table S6).

Figure 9.

Figure 9

Open in a new tab

Reaction rate of CO oxidation on catalysts in situ derived from Ce-MOF, Ce5, and CeO2@Ce5 per gram of loading. Ce2(bdc)3 is formed from Ce-MOF (SBET = 112 m2/g) synthesized from the Ce2(ac)6(H2O)3 precursor, while Ce-MOF (SBET = 222 m2/g) synthesized from the Ce4(OH)2(piv)10(H2O)2 precursor forms CeO2. The reaction rate on bulk crystalline CeO2 is provided for reference. CO:air ratio is 1:4; gas flow is 20 mL/min. Loading mass 0.2 g of sample +0.3 g of quartz.

Further studies of the activity and life cycle of the catalyst were performed for Ce-MOF. Since only around 30% conversion of CO was achieved during the preliminary experiments, the loading mass of the catalyst was increased, keeping all other reaction conditions unchanged. The efficiency of the catalyst increases significantly upon in situ transformation to CeO2 at ca. 350°C on the first cycle (Figure 10), and high CO conversion (80%) is already observed at a temperature of 270 °C on the second cycle. The lifetime of the catalyst was examined, and it demonstrated to maintain its high efficiency for more than 14 h at 300 °C (Figure S23). The catalytic activity of the Ce-MOF-derived CeO2 is comparable to the reported catalysts in CO oxidation (Table S7).38,39,79,81,82

Figure 10.

Figure 10

Open in a new tab

Activity of the catalyst in the CO oxidation reaction. The CO:air ratio is 1:4; the gas flow is 20 mL/min. Loading mass of Ce-MOF 0.4 g. 1st cycle (blue line) corresponds to in situ combustion of the loaded Ce-MOF to CeO2 under the conditions of the catalytic reaction; 2nd cycle (orange line) corresponds to the activity of CeO2 derived from Ce-MOF for the whole examined temperature range.

4. Conclusions

To conclude, the novel Ce3+-based MOF [(CH3)2NH2]2[Ce2(bdc)4(DMF)2]·2H2O (Ce-MOF) has been prepared by solvothermal reaction in DMF from various Ce carboxylates with terephthalic acid (H2bdc) and dimethylamine. The choice of carboxylate precursor did not impact the phase composition; however, it affected the specific surface area of the obtained material. In the absence of added dimethylamine, the solvothermal reaction leads to the formation of Ce5(bdc)7.5(DMF)4 (Ce5) or its composite CeO2@Ce5. It is worth noting that while nano-CeO2 is hardly visible in PXRD patterns of the composite, its signal dominates the PDF. This highlights the value of total scattering for characterization of the MOFs that could contain amorphous or nanocrystalline admixtures.

Ce-MOF has 3D connectivity of the bcu type with a dinuclear fragment connected with eight neighbors and three types of solvate species residing in its pores: water, DMF, and dimethylammonium cation.

The framework retains its integrity upon elimination of water molecules, while departure of DMF and dimethylamine leads to substantial structural transformation, which was evidenced through in situ XRD, thermogravimetric analysis, and total scattering experiments with PDF analysis.

Further heating leads to formation of Ce2(bdc)3, which is stable up to at least 450 °C in nonoxidative conditions and undergoes combustion after 390 °C to form nano-CeO2. MOF-derived CeO2 has a mean crystallite size of ca. 5–6 nm and exhibits catalytic activity in the CO oxidation reaction.

Finally, we developed a synthetic strategy for a new anionic Ce-based MOF formed due to the templating effect of deliberately added dimethylamine. We believe that the proposed approach could pave the way for the synthesis of new anionic MOFs.

Acknowledgments

D.T. and D.G. acknowledge the Elettra synchrotron facility (XRD1 and MCX beamlines, Trieste, Italy) and the Kurchatov synchrotron radiation source (Belok/XSA beamline, Moscow, Russia) for the awarded beamtime. The valuable support from Dr. Maurizio Polentarutti, Dr. Giorgio Bais, Dr. Jasper Plaisier, and Dr. Roman Svetogorov during powder and single-crystal XRD experiments is gratefully acknowledged. A.S. is very thankful to Dr. Sebastian Bette (Institute for Solid State Research, Stuttgart, Germany) for his help in PXRD experiments. M.T. and A.K. thank Dr. Gennady Kapustin from the Institute of Organic Chemistry for surface area measurements. D.T. acknowledges the support from the M.V. Lomonosov Moscow State University Program of Development.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c07906.

  • Crystal structure and refinement data (ZIP)

  • Experimental section; synthesis; X-ray crystallography; thermal behavior; and catalytic performance (PDF)

Accession Codes

CCDC 2106041–2106042, 2238303–2238304 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033

Author Present Address

∇ Max Planck Institute for Solid State Research, Stuttgart 70569, Germany

Author Contributions

D.T. conceived the idea, secured funding, and supervised the project. D.G. and M.S. synthesized and characterized the compounds. M.Z. enabled access to lab diffractometer for PDF data collection, provided expertise in the field, and contributed to the data analysis and discussion. D.T. and D.G. collected single-crystal and powder diffraction data and solved and refined the crystal structures. A.S., D.G., and D.T. performed the in situ PXRD experiments. A.K. and M.T. measured the nitrogen adsorption isotherms and performed catalytic activity studies. I.S. measured and interpreted XANES spectra. D.G. took the lead in writing of the manuscript with contributions from D.T., M.Z., and A.K. All authors have given approval to the final version of the manuscript.

D.G., M.S., M.T., and D.T. acknowledge the support by RSF (project no. 22-73-10089). Diffraction data collection at Elettra synchrotron was partially funded by the CALIPSOplus project from the EU Framework Programme for Research and Innovation HORIZON 2020 (Grant Agreement 730872). A.S. acknowledges the DFG-Grant BE 7407/1-1 for partial funding of in situ PXRD data collection in an inert atmosphere.

The authors declare no competing financial interest.

Supplementary Material

ao3c07906_si_001.zip (1.4MB, zip)
ao3c07906_si_002.pdf (3.2MB, pdf)

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