Compounds, Volume 4, Issue 2 (June 2024) – 9 articles

The basic hydrolysis of Malachite Green (MG) in the presence of β-Cyclodextrin (β-CD) has been studied using UV-Vis spectroscopic techniques and at 20 °C. β-CD was found to catalyze the basic hydrolysis. Indeed, this basic hydrolysis is catalyzed by the interaction cyclodextrin hydroxyl group, in its deprotonated form with the carbocation in the host-guest complex. The proposed model has been successfully applied to a reaction catalyzed by CD. It considers two simultaneous pathways in the aqueous medium involving free hydroxyl ions and the substrate-CD complex. The model allows us to obtain the kinetic parameters including the bimolecular rate constant between MG and HO⁻ in bulk water (k_w = 1.47 ± 0.01 mol⁻¹s⁻¹), the rate constant between MG and the deprotonated hydroxyl group of β-CD inside the host-guest complex (k_CD = 0.25 ± 0.03 s⁻¹) and the binding constant of MG inside the β-CD (K_S = 2500 ± 50). This behavior is like the hydrolysis of Cristal Violet (CV) in the same reaction media. Full article

The reaction of ZrCl₄ or HfCl₄ with excess 2-methylpropanoic acid when boiling under reflux has been studied. The formation of polynuclear Zr and Hf complexes of the composition M₂O(i-C₃H₇CO₂)₆ during prolonged reflux of the reaction mixtures was found. The complexes are very sensitive to hydrolysis, forming hexanuclear [M₆(O)₄(OH)₄(i-C₃H₇CO₂)₁₂]. The reactions have a general character for aliphatic acids and can be used as an alternative to the known methods for the synthesis of polynuclear carboxylate clusters of Group 4 metals. The crystal and molecular structures of previously undescribed {[Hf₆(μ₃-O)₄(μ₃-OH)₄(i-C₃H₇CO₂)₁₂(H₂O)]·3i-C₃H₇COOH} have been determined. The molecular structure is a completely asymmetric hexanuclear cluster containing six Hf(IV) atoms united by a 4:4 μ₃-O/OH system of bridges, and stabilized by twelve 2-methylpropanoate ligands, eight of which are bidentate bridging, three are chelating, and one is monodentate. The crystal structure of the complex includes three independent solvating 2-methylpropanoic acid molecules. The obtained IR spectroscopy data make it possible to determine the type of complexes in the reaction mixture. The results of the study may be useful for improving the catalytic systems for ethylene oligomerization. Full article

Metal–organic frameworks (MOFs) have been the subject of extensive scientific investigation in the last three decades and, currently, they make up one of the types of compounds most studied for their potential application in a wide range of distinct catalytic processes. Pristine MOF compounds provide several intriguing benefits for catalytic applications, including large interior surface areas and high densities of active sites; high catalytic reaction rates per volume; post-synthesis modifications with complementary catalytic groups; and the ability for multiple functional groups to catalyze the reaction. For most large-scale catalytic applications, including those in fuel processing, gas emission reduction, and chemical synthesis, pristine MOFs often show limited stabilities and opportunities for regeneration at high temperatures. As a result, the real applications of MOFs in these technologies are likely to be constrained, and a controlled thermal modification to prepare MOF-derivative compounds has been applied to induce crystalline structural changes and increase the structural stability of the MOFs, enhancing their potential applicability in more severe catalytic processes. Recent advances concerning the use of this strategy to boost the catalytic potential of MOF-derivative compounds, particularly for stable Zr-based MOFs, are outlined in this short review article. Full article

The citrus processing industry is responsible for the generation of large volumes of waste side streams, represented principally by fruit peels. These tissues are exceptionally rich in polyphenolic bioactive phytochemicals, and there has been a great industrial interest for their valorization. The examination presented herein targeted at developing a fast and straight-forward aqueous extraction process, based on ultrasonication, for the efficacious recovery of polyphenolic compounds from waste orange peels. After an initial single-factor examination, the response surface optimization showed that a maximum total polyphenol yield of 12.81 mg chlorogenic acid equivalents (GAE) per g⁻¹ dry mass could be achieved by setting sonicator amplitude at 80%, for 15 min, using a duty cycle of 2/2 (2 s on/2 s off). Comparison of this methodology with a stirred-tank extraction demonstrated that the ultrasonication technique was equally effective, requiring ambient temperature and considerably shorter resident time. The combination of both techniques using the ultrasonication process as a pretreatment step did not boost extraction yield, and the extracts produced had similar polyphenolic composition and antioxidant activity. However, a slight enhancement of the recovery of individual constituents was noted. It is proposed that efficient extraction of polyphenolic substances from waste orange peels may be accomplished using the present methodology, which is a low-cost (ambient temperature, short time) and sustainable (water as solvent) process. Full article

Considerable attention has been paid to (1E)-1,2-diaryldiazenes (azo dyes) possessing liquid–crystalline (LC) and optical properties because they can switch color through thermal phase transitions and photoisomerizations. Although multifunctional molecules with both LC and fluorescent properties based on a donor–π-acceptor (D-π-A)-type tolane skeleton have been developed, functional molecules possessing LC and dye properties have not yet been developed. Therefore, this study proposes to develop LC dyes consisting of (1E)-1,2-diaryldiazenes with a D–π-A-type tolane skeleton as the aryl moiety. The (1E)-1,2-diaryldiazene derivatives exhibited a smectic phase, regardless of the flexible-chain structure, whereas the melting temperature was significantly increased by introducing fluoroalkyl moieties into the flexible chain. Evaluation of the optical properties revealed that compounds with decyloxy chains exhibited an orange color, whereas compounds with semifluoroalkoxy chains absorbed at a slightly blue-shifted wavelength, which resulted in a pale orange color. The thermal phase transition caused a slight color change accompanied by a change in the absorption properties, photoisomerization-induced shrinkage, and partial disappearance of the LC domain. These results indicate that (1E)-1,2-diaryldiazenes with a D–π-A-type tolane skeleton can function as thermo- or photoresponsive dyes and are applicable to smart windows and in photolithography. Full article

Here, we successfully synthesized Sr-doped perovskite-type oxides of La_1−xSr_xCo_1−λO_3−δ, “LSX” (x = 0, 0.1, 0.3, 0.5, 0.7), using the glycine-assisted solution combustion method. The effect of strontium doping on the catalyst structure, NO to NO₂ conversion, NO_x adsorption and storage, and NO_x reduction performance were investigated. The physicochemical properties of the catalysts were studied by XRD, SEM-EDS, N₂ adsorption–desorption, FTIR, H₂-TPR, O₂-TPD, and XPS techniques. The NSR performance of LaCoO₃ perovskite was improved after Sr doping. Specifically, the perovskite with 50% of Sr doping (LS5 sample) exhibited excellent NO_x storage capacity within a wide temperature range (200–400 °C), and excellent stability after hydrothermal and sulfur poisoning. It also displayed the highest NO_x adsorption–storage capacity (NAC: 1889 μmol/g; NSC: 1048 μmol/g) at 300 °C. This superior performance of the LS5 catalyst can be attributed to its superior reducibility, better NO oxidation capacity, increased surface Co²⁺ concentration, and, in particular, its generation of more oxygen vacancies. FTIR results further revealed that the LSX catalysts primarily store NO_x through the “nitrate route”. During the lean–rich cycle tests, we observed an average NO_x conversion rate of over 50% in the temperature range of 200–300 °C, with a maximum conversion rate of 61% achieved at 250 °C. Full article

Bipolar Plates (BPPs) play a critical role in Polymer Electrolyte Membrane Water Electrolysers (PEMWEs) for effective hydrogen generation. The performance and longevity of the system can be considerably impacted by choosing the suitable material for these components. Polymer electrolyte membrane water electrolysis technology relies on cost-effective and corrosion-resistant BPPs. Tantalum, niobium, and titanium are low-cost, easy-to-machine materials that have good electrical and thermal conductivity; however, they exhibit low corrosion resistance. Noble metal and metal nitride coatings are usually investigated to minimize corrosion and interfacial contact resistance. Because of its performance-to-cost ratio, Stainless Steel (SS) based materials are among the most popular materials for BPP development. This study recommends material and operating parameters to improve PEMWE systems for sustainable hydrogen production’s efficiency, durability, and economic viability. Full article

Aluminum hydride (AlH₃) has attracted wide attention due to its high gravimetric and volumetric hydrogen capacity. AlH₃ can easily release hydrogen when heated at relatively low temperature. Such high hydrogen density and low dehydrogenation temperature make it one of the most promising high-energy fuels for solid propellants. In particular, AlH₃ as a component of solid propellants may greatly increase the specific impulse of rocket engines. However, AlH₃ exhibits low chemical and thermal stability in an ambient atmosphere. In this paper, the research progress about the synthesis, dehydrogenation thermodynamics, and kinetics, the stabilization of AlH₃ over the past decades are reviewed, with the aim of exploring more a economical synthesis and suitable stabilization methods for large-scale use in solid propellants. Finally, some suggestions regarding future research directions in this filed are proposed. Full article

Distributed throughout the environment are various microorganisms such as bacteria, fungi, parasites, and viruses. Although many are part of the human microbiome, many are pathogenic and cause infections ranging from mild to severe. In recent years, the identification of multidrug-resistant microorganisms has become a serious public health problem. The resulting infections call into question the therapeutic capacity of health systems and lead to approximately 70,000 deaths annually worldwide. The progressive resistance to antibiotics and antifungals has been a major challenge for the medical and pharmaceutical community, requiring the search for new compounds with antimicrobial properties. Several studies have demonstrated the potential of natural and synthesized flavonoids, especially the dimers of these molecules. In this review are presented many examples of dimeric flavonoids that have demonstrated antimicrobial activity against viruses, like influenza and Human Immunodeficiency Virus (HIV), protozoal infections, such as Leishmaniasis and Malaria, fungal infections by Candida albicans and Cryptococcus neoformans, and bacterial infections caused, for example, by Staphylococcus aureus and Escherichia coli. In the pursuit to find potential safe agents for therapy in microbial infections, natural dimeric flavonoids are an option not only for the antimicrobial activity, but also for the low toxicity usually associated with these compounds when compared to classic antimicrobials. Full article