Zr and Hf-metal-organic frameworks: Efficient and recyclable heterogeneous catalysts for the synthesis of 2-arylbenzoxazole via ring open pathway acylation reaction

The use of metal-organic frameworks as heterogeneous catalysts

This article presents an overview of some of the available research studies of MOFs as catalysts. Catalytic studies of magnetic iron oxide nanoparticles with modified surfaces, MOFs with precious metals such as palladium, platinum, and silver, with zirconium, hafnium, copper, alkaline earth metals, lanthanides are generalized. The studies of the catalytic activity of micro- and mesoporous MOF structures are described.

Design, synthesis and in vitro antiproliferation activity of some 2-aryl and -heteroaryl benzoxazole derivatives

Background: Phortress produces reactive electrophilic metabolites that form DNA adducts only in sensitive tumor cells. The authors converted the 2-phenylbenzothiazole nucleus in phortress to 2-aryl and -heteroaryl benzoxazole derivatives (11 new and 14 resynthesized). All synthesized compounds were studied for antitumor activity in various cancer cells. Materials & methods: Cytotoxicity, cell morphology, flow cytometry and cell-cycle analyses of compounds were performed and more active derivatives were tested in the MCF-7 cell line. Conclusion: Methyl 2-(thiophen-2-yl)benzo[d]oxazole-6-carboxylate (BK89) has a higher effect than fluorouracil to induce apoptotic cell death (apoptosis value of 49.44%). Cell-cycle analysis shows that the compounds BK89 and methyl 2-(furan-2-yl)benzo[d]oxazole-6-carboxylate (BK82) can be used as potential cell-cycle blockers by arresting MCF-7 cells in G0/G1 phase at rates of 63% and 85%, respectively.

Recyclable Zr/Hf-Containing Acid-Base Bifunctional Catalysts for Hydrogen Transfer Upgrading of Biofuranics: A Review

The massive burning of a large amount of fossil energy has caused a lot of serious environmental issues (e.g., air pollution and climate change), urging people to efficiently explore and valorize sustainable alternatives. Biomass is being deemed as the only organic carbon-containing renewable resource for the production of net-zero carbon emission fuels and fine chemicals. Regarding this, the selective transformation of high-oxygen biomass feedstocks by catalytic transfer hydrogenation (CTH) is a very promising strategy to realize the carbon cycle. Among them, the important Meerwein-Ponndorf-Verley (MPV) reaction is believed to be capable of replacing the traditional hydrogenation strategy which generally requires high-pressure H2 and precious metals, aiming to upgrade biomass into downstream biochemical products and fuels. Employing bifunctional heterogeneous catalysts with both acidic and basic sites is needed to catalyze the MPV reaction, which is the key point for domino/cascade reaction in one pot that can eliminate the relevant complicated separation/purification step. Zirconium (Zr) and hafnium (Hf), belonging to transition metals, rich in reserves, can demonstrate similar catalytic efficiency for MPV reaction as that of precious metals. This review introduced the application of recyclable heterogeneous non-noble Zr/Hf-containing catalysts with acid-base bifunctionality for CTH reaction using the safe liquid hydrogen donor. The corresponding catalysts were classified into different types including Zr/Hf-containing metal oxides, supported materials, zeolites, metal-organic frameworks, metal-organic hybrids, and their respective pros and cons were compared and discussed comprehensively. Emphasis was placed on evaluating the bifunctionality of catalytic material and the key role of the active site corresponding to the structure of the catalyst in the MPV reaction. Finally, a concise summary and prospect were also provided centering on the development and suggestion of Zr/Hf-containing acid-base bifunctional catalysts for CTH.

Study of the Cycloaddition of CO2 with Styrene Oxide Over Six-Connected spn Topology MOFs (Zr, Hf) at Room Temperature

A series of MOFs with a 6-connected spn topology were synthesized (MOF-808-(Zr, Hf), PCN-777-(Zr, Hf), MOF-818-(Zr, Hf)). Through the in situ DRIFTS of NH₃ adsorption-desorption, we found that the activated catalyst mainly contains Lewis acid sites. The effects of different organic ligands on the Lewis acid of the Zr₆ cluster were analyzed by XPS and NH₃ -TPD, and the relative Lewis acidity of the same metal was obtained: PCN-777>MOF-808>MOF-818. In the Py-FTIR results, we confirmed that MOF-818 has a higher acid site density. In the activity test, MOFs with mesoporous structure showed better catalytic activity under normal temperature and pressure. Among them, MOF-818 can still maintain a high degree of crystallinity after catalysis. Finally, we use density functional theory to propose the mechanism of the cycloaddition reaction of carbon dioxide and styrene oxide. The results show that the metal is coordinated with styrene oxide and halogens attack the β carbon of the epoxide.

Heterogeneous nanozymatic activity of Hf oxo-clusters embedded in a metal-organic framework towards peptide bond hydrolysis

Materials with enzyme-like activities and proteolytic potential are emerging as a robust and effective alternative to natural enzymes. Herein, a Hf₆O₈-based NU-1000 metal organic framework (Hf-MOF) is shown to act as a heterogeneous catalyst for the hydrolysis of peptide bonds under mild conditions. In the presence of Hf-MOF, a glycylglycine model dipeptide was hydrolysed with a rate constant of k_obs = 8.33 × 10^-7 s^-1 (half-life (t_1/2) of 231 h) at 60 °C and pD 7.4, which is significantly faster than the uncatalyzed reaction. Other Gly-X peptides (X = Ser, Asp, Ile, Ala, and His) were also smoothly hydrolysed under the same conditions with similar rates, except for the faster reactions observed for Gly-His and Gly-Ser. Moreover, the Hf₆O₈-based NU-1000 MOF also exhibits a high selectivity in the cleavage of a protein substrate, hen egg white lysozyme (HEWL). Our results suggest that embedding Hf₆O₈ oxo-clusters is an efficient strategy to conserve the hydrolytic activity while smoothing the strong substrate adsorption previously observed for a discrete Hf oxo-cluster that hindered further development of its proteolytic potential. Furthermore, comparison with isostructural Zr-NU-1000 shows that although the Hf variant afforded the same cleavage pattern towards HEWL but slightly slower reaction rates, it exhibited a larger stability window and a better recyclability profile. The results suggest that these differences originate from the intrinsic differences between Hf^IV and Zr^IV centers, and from the lower surface area of Hf-NU-1000 in comparison to Zr-NU-1000.

The chemistry and applications of hafnium and cerium(iv) metal-organic frameworks

The coordination connection of organic linkers to the metal clusters leads to the formation of metal-organic frameworks (MOFs), where the metal clusters and ligands are spatially entangled in a periodic manner. The immense availability of tuneable ligands of different length and functionalities gives rise to robust molecular porosity ranging from several angstroms to nanometres. Among the large family of MOFs, hafnium (Hf) based MOFs have been demonstrated to be highly promising for practical applications due to their unique and outstanding characteristics such as chemical, thermal, and mechanical stability, and acidic nature. Since the report of UiO-66(Hf) and DUT-51(Hf) in 2012, less than 200 Hf-MOFs (ca. 50 types of structures) have been reported. Besides, tetravalent cerium [Ce(iv)] has been proven to be capable of forming similar topological MOF structures to Zr and Hf since its first discovery in 2015. So far, ca. 40 Ce(iv) MOFs with 60% having UiO-66-type structure have been reported. This review will offer a holistic summary of the chemistry, uniqueness, synthesis, and applications of Hf/Ce(iv)-MOFs with a focus on presenting the development in the Hf/Ce(iv)-clusters, topologies, ligand structures, synthetic strategies, and practical applications of Hf/Ce(iv)-MOFs. In the end, we will present the research outlook for the development of Hf/Ce(iv)-MOFs in the future, including fundamental design of Hf/Ce(iv)-clusters, defect engineering, and various applications including membrane development, diversified types of catalytic reactions, irradiation absorption in nuclear waste treatment, water production and wastewater treatment, etc. We will also present the emerging computational approaches coupled with machine-learning algorithms that can be applied in screening Hf and Ce(iv) based MOF structures and identifying the best-performing MOFs for tailor-made applications in future practice.

Functionalization of zirconium-based metal-organic frameworks for gas sensing applications

The functionalization and incorporation of noble metals in metal-organic frameworks have been widely used as efficient methods to enhance their applicability. Herein, a sulfone-functionalized Zr-MOF framework labeled Zr-BPDC-SO₂ (BPDC-SO₂ =dibenzo[b,d]-thiophene-3,7-dicarboxylate 5,5-dioxide) and its Pd-embedded composite were efficiently synthesized by adjusting their functional groups. The obtained compounds were characterized to assess their potential for gas sensing applications. X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, specific surface area measurements, and thermogravimetric analysis were employed to characterize the new sensor materials. The gas sensing properties of the novel functionalized sensor materials were systematically investigated under various temperature, concentration, and gas type conditions. Owing to the strong hydrogen bonds of the sulfonyl groups and Zr₆ clusters in the framework with the hydroxyl groups of ethanol, Zr-BPDC-SO₂ emerged as an effective sensor for ethanol detection. In addition, Pd@Zr-BPDC-SO₂ exhibited efficient hydrogen sensing performance, in terms of sensor dynamics and response. More importantly, the material showed a higher sensing response to hydrogen than to other gases, highlighting the important role of Pd in the Zr-MOF-based hydrogen sensor. The results of the sensing tests carried out in this study highlight the promising potential of the present materials for practical gas monitoring applications.

Photocatalytic green synthesis of benzazoles from alcohol oxidation/toluene sp3 C–H activation over metal-free BCN: effect of crystallinity and N–B pair exposure

Porous borocarbonitride, with characteristics of enhanced crystallinity and improved N–B pairs exposure, was employed for the heterogeneous photocatalytic tandem synthesis of benzazoles from alcohol oxidation/toluene sp3 C–H activation.

Synthesis of 5-hydroxymethylfurfural from monosaccharides catalyzed by superacid VNU-11-SO4 in 1-ethyl-3-methylimidazolium chloride ionic liquid

Superacid VNU-11–SO₄, a modified metal–organic framework by post-synthetic treatment with a sulfuric acid solution, has been considered as a promising heterogeneous catalyst in the isomerization of glucose to fructose and further dehydration to form 5-hydroxymethylfurfural (HMF) due to its possession of both Lewis and Brønsted acid sites. In this work, we focused on using VNU-11–SO₄ for the optimization of the conversion of fructose and glucose into HMF using an ionic liquid as a green solvent. The highest yields of HMF from glucose and fructose could be obtained in 28% (140 °C, 8 h) and 86% (110 °C, 3 h), respectively, with the use of VNU-11–SO₄ catalyst in 1-ethyl-3-methylimidazolium chloride ionic liquid. Recycling examination of the catalyst showed only a slight decrease in the HMF yield, implying its potential industrial application in biomass transformation.

Superacid VNU-11–SO₄ has been considered as a promising heterogeneous catalyst in the conversion of glucose and fructose to 5-hydroxymethylfurfural (HMF) in ionic liquid.

Effect of Fe(iii)-based MOFs on the catalytic efficiency of the tandem cyclooxidative reaction between 2-aminobenzamide and alcohols

Fe-MOFs were used as efficient heterogeneous catalysts in the tandem cyclooxidative reaction under microwave irradiation.