Influence of SO3H groups incorporated as Brønsted acidic parts by tandem post-synthetic functionalization on the catalytic behavior of MIL-101(Cr) MOF for methanolysis of styrene oxide

Extending the visible light absorption of NH2-UiO-66 through diazotization reaction for photocatalytic chromium (VI) reduction

The optical properties of NH₂-UiO-66 as a visible light-active metal organic framework was further enhanced through the diazotization reaction with π-conjugated 1-naphthol reagent. Diffuse reflectance UV-Vis spectrum of diazotized MOF, named as Azo-UiO-66, exhibited a significant red shift compared to unfunctionalized NH₂-UiO-66 due to the formation of diazo compound. Also, Tauc calculations indicated considerable decrease in band gap energy from 2.68 to 1.7 eV, resulting in improvement of visible light harvesting. Furthermore, other physicochemical techniques, e.g., X-ray diffraction (XRD), N₂ adsorption-desorption analysis, thermogravimetric analysis (TGA), energ-dispersive X-ray (EDX), and CHN elemental analyses demonstrated the successful MOF diazotization with 1-naphthol and preservation of NH₂-UiO-66 framework upon post-modification process. The reduction of hexavalent chromium, Cr(VI), as a serious contaminant in wastewater to less toxic Cr(III) was performed over prepared photocatalyst, which demonstrated the positive role of ligand functionalization and enhancement of visible light absorption on overall photocatalytic performance of Azo-UiO-66.

Sulfonic Acid-Grafted Hybrid Porous Polymer Based on Double-Decker Silsesquioxane as Highly Efficient Acidic Heterogeneous Catalysts for the Alcoholysis of Styrene Oxide

β-Alkoxyalcohols generated from epoxide ring-opening reactions are significant due to their enormous value as pharmaceutical intermediates and fine chemicals. Using a phenyl-substituted double-decker-type silsesquioxane as the precursor, a hybrid porous material (PCS-DDSQ) was synthesized through a Scholl coupling reaction with an AlCl₃ catalyst. Then, novel excellent Brønsted acid-derived silsesquioxane solid catalysts (PCS-DDSQ-SO₃H-x) were successfully obtained through an electrophilic aromatic substitution reaction of chlorosulfonic acid on phenyl rings of PCS-DDSQ, fully characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, powder X-ray diffraction, temperature-programmed desorption, water contact angle, Brunauer-Emmett-Teller model, thermogravimetric analysis, and solid-state ¹³C and ²⁹Si nuclear magnetic resonance techniques. The catalytic behavior of the PCS-DDSQ-SO₃H-x with different SO₃H loadings for the methanolysis of styrene oxide was compared and evaluated. The presence of SO₃H groups endows them with excellent catalytic abilities, achieving the highest values from PCS-DDSQ-SO₃H-1 (the acid site of its catalyst is 1.84 mmol/g) as 99% conversion and 100% selectivity for the methanolysis of styrene oxide in 30 min, which shows superior catalytic properties of low dosage and high efficiency. Furthermore, the PCS-DDSQ-SO₃H-1 catalyst can maintain high activity and selectivity after three cycles. This study provides a feasible method for the preparation of Brønsted solid acid catalysts with different acid loadings by introducing the sulfonic group into PCS-DDSQ.

Advances in Metal-Organic Frameworks MIL-101(Cr)

MIL-101(Cr) is one of the most well-studied chromium-based metal-organic frameworks, which consists of metal chromium ion and terephthalic acid ligand. It has an ultra-high specific surface area, large pore size, good thermal/chemical/water stability, and contains unsaturated Lewis acid sites in its structure. Due to the physicochemical properties and structural characteristics, MIL-101(Cr) has a wide range of applications in aqueous phase adsorption, gas storage and separation, and catalysis. In this review, the latest synthesis of MIL-101(Cr) and its research progress in adsorption and catalysis are reviewed.

Cooperative Interplay of Brønsted Acid and Lewis Acid Sites in MIL-101(Cr) for Cross-Dehydrogenative Coupling of C-H Bonds

Cross-dehydrogenative coupling (CDC) is an effective tool for carbon-carbon bond formation in chemical synthesis. Herein, we report a metal-organic framework (MOF) possessing dual Lewis acidic Cr sites and sulfonic acid sites (MIL-101(Cr)-SO₃H) as an efficient catalytic material for direct cross-coupling of xanthene and different nucleophiles using O₂ as the oxidant. The highly porous structure of MIL-101(Cr)-SO₃H enables the free access of reactants to the catalytic active sites inside MOF pores. Kinetic studies indicated that the Cr sites of MOF accelerate the rate-limiting autoxidation reaction of xanthene, which synergistically work with the sulfonic acid group on MOF ligands in promoting the CDC reactions. Besides, the catalytic system shows excellent functional group compatibility, and a variety of valuable xanthene derivatives were synthesized with satisfactory yields. Furthermore, MIL-101(Cr)-SO₃H can be reused and its catalytic activity and crystal structure remain after six consecutive runs.