Propylene glycol oxidation with hydrogen peroxide over Zr-containing metal-organic framework UiO-66

Valorization of Glycerol through Plasma-Induced Transformation into Formic Acid

To cope with climate change issues, a significant shift is required in worldwide energy sources. Hydrogen and bioenergy are being considered as alternatives toward a carbon neutral society, making formic acid - a hydrogen carrying product of glycerol - of interest for the valorization of glycerol. Here we investigate the plasma-induced transformation of glycerol in an aqueous nanosecond repetitively pulsed discharge reactor. We found that the water content in the aqueous mixture fulfilled a crucial role in both the gas phase (as a source of OH radicals) and the liquid phase (as a promotor of the dissolved OH radical's mobility and reactivity). The formic acid produced was linearly proportional to the specific input energy, and the most cost-effective production of formic acid was found with 10 % v/v glycerol in the aqueous mixture. A plausible reaction pathway was proposed, consisting of the OH radical-driven dehydrogenation and dehydration of glycerol. The results provide a fundamental understanding of plasma-induced transformation of glycerol to formic acid and insights for future practical applications.

Butane isomers mobility and framework dynamics in UiO-66 (Zr) MOF: Impact of the hydroxyl groups in zirconia cluster

UiO-66 (Zr) is a metal-organic framework (MOF) known for its thermal and chemical stability and wide range of adsorption-based applications. This MOF exhibits high separation selectivity for butane isomers. It has been earlier inferred that the separation performance of the material depends on the hydroxylation state of the zirconia cluster. In this contribution, we apply ²H solid-state NMR to characterize the dynamics of both the MOF organic framework itself and butane isomers in hydroxylated and dehydroxylated forms of UiO-66. It is established that the rate of π-flipping and the amplitude of the phenylene ring plane librations in the framework are higher for the dehydroxylated form. Self-diffusion coefficients of butane isomers have been estimated for both forms of UiO-66. The diffusivity is higher for n-butane in the dehydroxylated form, whereas the diffusion of isobutane is not affected by the presence of OH groups in the zirconia cluster of the MOF. Higher diffusivity of n-butane in dehydroxylated form is accounted for by the larger effective diameter of the window between the adjacent cages in this form, which arises from faster rotation and larger amplitude of framework linker libration. This rationalizes the higher efficiency of the dehydroxylated form of UiO-66(Zr) material for butane isomers separation.

Catalytic Performance of Zr-Based Metal-Organic Frameworks Zr-abtc and MIP-200 in Selective Oxidations with H2 O2

The catalytic performance of Zr-abtc and MIP-200 metal-organic frameworks consisting of 8-connected Zr₆ clusters and tetratopic linkers was investigated in H₂ O₂ -based selective oxidations and compared with that of 12-coordinated UiO-66 and UiO-67. Zr-abtc demonstrated advantages in both substrate conversion and product selectivity for epoxidation of electron-deficient C=C bonds in α,β-unsaturated ketones. The significant predominance of 1,2-epoxide in carvone epoxidation, coupled with high sulfone selectivity in thioether oxidation, points to a nucleophilic oxidation mechanism over Zr-abtc. The superior catalytic performance in the epoxidation of unsaturated ketones correlates with a larger amount of weak basic sites in Zr-abtc. Electrophilic activation of H₂ O₂ can also be realized, as evidenced by the high activity of Zr-abtc in epoxidation of the electron-rich C=C bond in caryophyllene. XRD and FTIR studies confirmed the retention of the Zr-abtc structure after the catalysis. The low activity of MIP-200 in H₂ O₂ -based oxidations is most likely related to its specific hydrophilicity, which disfavors adsorption of organic substrates and H₂ O₂ .

Metal-Organic Frameworks in Oxidation Catalysis with Hydrogen Peroxide

In recent years, metal–organic frameworks (MOFs) have received increasing attention as selective oxidation catalysts and supports for their construction. In this short review paper, we survey recent findings concerning use of MOFs in heterogeneous liquid-phase selective oxidation catalysis with the green oxidant–aqueous hydrogen peroxide. MOFs having outstanding thermal and chemical stability, such as Cr(III)-based MIL-101, Ti(IV)-based MIL-125, Zr(IV)-based UiO-66(67), Zn(II)-based ZIF-8, and some others, will be in the main focus of this work. The effects of the metal nature and MOF structure on catalytic activity and oxidation selectivity are analyzed and the mechanisms of hydrogen peroxide activation are discussed. In some cases, we also make an attempt to analyze relationships between liquid-phase adsorption properties of MOFs and peculiarities of their catalytic performance. Attempts of using MOFs as supports for construction of single-site catalysts through their modification with heterometals will be also addressed in relation to the use of such catalysts for activation of H2O2. Special attention is given to the critical issues of catalyst stability and reusability. The scope and limitations of MOF catalysts in H2O2-based selective oxidation are discussed.

Selective oxidation of bio-based platform molecules and their conversion products over metal nanoparticle catalysts: a review

The conversion of bio-renewable raw materials into valuable products (biofuels, bifunctional carbonyls/carboxyls) that serve as the basis for biopolymers, has become one of the most important areas in the development of novel hybrid catalysts.

Bimetallic AgPd/UiO-66 Hybrid Catalysts for Propylene Glycol Oxidation into Lactic Acid

Different methods (the wetness impregnation of Ag and Pd precursors dissolved in water or acetonitrile solution, and the double solvent impregnation technique) were employed to immobilize Ag–Pd nanoparticles (NPs) into the pores of the microporous zirconium-based metal-organic framework known as UiO-66. The obtained materials were characterized by using nitrogen adsorption-desorption at −196 °C, powder X-ray diffraction, UV-Vis diffusion reflectance spectroscopy, and transition electron microscopy measurements. Special attention was paid to the acid and redox properties of the obtained materials, which were studied by using temperature-programmed desorption of ammonia (TPD-NH₃) and temperature-programmed reduction (TPR-H₂) methods. The use of a drying procedure prior to reduction was found to result in metallic NPs which, most likely, formed on the external surface and were larger than corresponding voids of the metal-organic framework. The formation of Ag–Pd alloy or monometallic Ag and Pd depended on the nature of both metal precursors and the impregnation solvent used. Catalytic activity of the AgPd/UiO-66 materials in propylene glycol oxidation was found to be a result of synergistic interaction between the components in AgPd alloyed NPs immobilized in the pore space and on the external surface of UiO-66. The key factor for consistent transformation of propylene glycol into lactic acid was the proximity between redox and acid-base species.

Effect of solvent nature on propylene glycol oxidation with tert-butyl hydroperoxide over metal-organic framework Cr-MIL-101

The effect of solvent nature on conversion, product yields and reaction kinetics of selective propylene glycol oxidation with tert-butyl hydroperoxide over porous chromium terephthalate Cr-MIL-101 used as a heterogeneous catalyst is considered. Differences in hydrogen bonding of propylene glycol molecules in different solvents and adsorption of components of the reaction mixture on the active sites of the catalyst are studied by Fourier-transformed infrared spectroscopy. The characteristics of the solvent are shown to play a key role in the process under consideration. In the case of aprotic solvents, the oxidant utilization efficiency, the propylene glycol conversion and the product yields are significantly higher in comparison with those in protic solvents. The protic solvents can adsorb on the active sites of the catalyst which leads to a decrease of their accessibility for the reagents. The initial rate of propylene glycol oxidation decreases linearly with the increasing of the diameter of molecules of the protic solvents. DFT calculations support the competitive adsorption of the molecules of protic solvents on Cr active sites of MIL-101. In the aprotic solvents, the reactivity and distribution of propylene glycol molecules in the solution are determined by the involvement of hydroxyl groups of the substrate into the intermolecular interactions. The ability of the aprotic solvents to break the hydrogen bond network in the associates of propylene glycol is responsible for the concentration of the substrate molecules in the pores of Cr-MIL-101. The highest selectivities towards hydroxyacetone for Cr-MIL-101 catalyst are obtained in solvents, where the initial rates of propylene glycol oxidation are the lowest ones.