On the nature and accessibility of the Brønsted-base sites in activated hydrotalcite catalysts

Base catalysis is of importance for organic synthesis in general and fine chemicals manufacture in particular. Activated hydrotalcites have recently received a great deal of attention as solid base catalysts; however, no systematic work on the nature of their active sites has been published up till now. In this work two different methods have been applied to activate Mg-Al hydrotalcites to obtain Br?nsted-base catalysts for liquid-phase condensation reactions. Activation via thermal treatment followed by rehydration (HT-reh) resulted in irregularly stacked platelets ( approximately 60 nm), whereas the sample activated via aqueous ion-exchange (HT-exc) preserved its original hexagonal hydrotalcite platelets ( approximately 100 nm). The specific activity for the self-condensation of acetone of HT-reh was over 10 times that of HT-exc. The enthalpy of CO2 adsorption on the activated hydrotalcites determined with calorimetry to gain insight into the strength of the basic sites showed very similar values. IR spectra of adsorbed CDCl3 as probe molecule on the differently activated samples revealed large differences in adsorbed amounts, but again the strength of the basic sites appeared to be the same. These results point to steric hindrance for the substrate molecules as the main factor determining differences in catalytic activity. The high accessibility of Br?nsted-base sites in HT-reh is proposed to involve a distorted edge structure of the platelets. The edge structure of exchanged samples could be distorted too, either by exchange under reflux conditions or under ultrasonic treatment. In line with the proposed model, the distorted exchanged samples displayed a much higher catalytic activity than HT-exc.     

Kinetic,isotherm, and mechanism investigations of the removal of nitrate and nitrite from water by the synthesized hydrotalcite Mg–Al

Research on Chemical Intermediates - The objective of this work is to test the effectiveness of hydrotalcite as an adsorbent for the retention of nitrate and nitrite ions and to study the influence...     

Co–Mg–Al oxides issued of hydrotalcite precursors for total oxidation of volatile organic compounds. Identification and toxicological impact of the by-products

Catalysts based on Co-Mg-Al, which were used for the total oxidation of toluene, were synthesized by using the hydrotalcite pathway. The calcination allowed us to obtain various mixed oxide types (i.e. Co₃O₄, Co₂AlO₄ or CoAl₂O₄), presenting mesopores of about 8 nm and high specific surface areas. The solids were tested for the total oxidation of toluene and showed a total selectivity in CO₂ and H₂O for 100% of toluene conversion. However, studies using diffuse reflectance infrared “operando” and GC-MS allowed us to identify intermediary by-products stemming from the catalytic oxidation of toluene: benzene and small quantities of benzaldehyde, styrene and acetophenone. In order to contribute to the improvement of the current scientific knowledge on volatile organic compound (VOC) toxicity in humans, the lung toxicity of toluene, benzene or their association was determined by using a human epithelial lung cell model (i.e. L132 cell line). VOC cytotoxicity was studied with three complementary methods: the enzymatic activity of extracellular lactate dehydrogenase (LDH), the enzymatic activity of mitochondrial dehydrogenase (mDH), and the incorporation of 5-Bromodesoxyuridine (5-BrdU). Taken together, these results showed the occurrence of adverse effects, notably reported by significant increases in LDH activity in cell culture supernatants, 24 hours after L132 cell exposure not only to toluene alone or benzene alone, but also to their association. This original approach allowed us to integrate some toxicological parameters to help the choice of new-dedicated catalysts for the oxidative conversion of VOC.     

Preparation of Mg–Al hydrotalcite from the effluent of Friedel–Crafts reaction and its application as a catalyst in Knoevenagel reaction

Abstract

Hydrotalcite containing magnesium and aluminium (Mg–Al HT) with a molar ratio of Mg(II)/Al(III) = 2.5 has been prepared by a co-precipitation method using the effluent of a Friedel–Crafts acylation reaction. The HT was calcined at 500°C and reconstructed with deionized water. The synthesized HT was characterized by XRD and FT-IR spectroscopy and was successfully used as a catalyst in the Knoevenagel reaction of aldehydes and active methylene compounds. The catalyst was found to be reusable.     

Tuning the base properties of Mg–Al hydrotalcite catalysts using their memory effect

Herein,the relationship between the structure and base properties of Mg–Al hydrotalcite catalysts was comprehensively investigated in relation to heat treatment and rehydration processes,which are well known as memory effects of hydrotalcite.The structure of Mg–Al hydrotalcites changed from layered double hydroxide(LDH)to mixed metal oxide and subsequently to a spinel structure during heat treatment,and it was returned from mixed metal oxide to a LDH structure by rehydration.Based on various characterizations,we successfully proposed a detailed mechanism of memory effect.We also confirmed that the Mg–Al hydrotalcites had weak or strong base sites and that their base properties could be systematically tuned by heat treatment and rehydration.The prepared Mg–Al hydrotalcites were applied to a model reaction,isomerization of glucose to fructose,as base catalysts.Among the catalysts tested,the rehydrated Mg–Al hydrotalcite efficiently produced fructose due to its appropriate base and structure properties.We finally concluded that the base sites of Mg–Al hydrotalcites can be designed as desired by heat treatment and rehydration.Moreover,through systematic design of the base sites of Mg–Al hydrotalcites,these can be promising catalysts for various heterogeneous reactions over base catalysts,giving excellent catalytic performances.     

Preparation of acid–base bi-functional hydrotalcite based catalyst for converting Vietnamese coconut oil to green hydrocarbons

Acid–base bi-functional hydrotalcite like compounds based on partial incorporation of Al³⁺ into brucite structure of Mg(OH)₂ with various molar ratios were prepared through co-precipitation method. The co-precipitation of the precursors produced precipitations followed by drying at 120 °C for 12 h and calcination in air flow at 500 °C for 6 h to obtain the catalysts (Mg–Al HLCs). Many techniques including XRD, TG–DTA, EDX, NH₃-TPD, CO₂-TPD, GC–MS and XANES were used to characterize and optimize Mg/Al molar ratio based on the thermal stability of the Mg–Al HLCs and their activities in decarboxylation process of coconut oil. The results showed that the best molar ratio of Mg/Al was 3/1 providing a stable hydrotalcite like structure, and the catalyst possessed both acid and base sites on its surface enhancing its activity and selectivity in the decarboxylation process. The catalysts revealed high performance in the decarboxylation process of coconut oil established at 400 °C for 4 h for green hydrocarbons belonging to kerosene fractions.     

Hierarchical Ni−Al hydrotalcite supported Pt catalyst for efficient catalytic oxidation of formaldehyde at room temperature

Catalytic oxidation at room temperature is recognized as the most promising method for formaldehyde (HCHO) removal. Pt-based catalysts are the optimal catalyst for HCHO decomposition at room temperature. Herein, flower-like hierarchical Pt/NiAl-LDHs catalysts with different [Ni²⁺]/[Al³⁺] molar ratios were synthesized via hydrothermal method followed by NaBH₄ reduction of Pt precursor at room temperature. The flower-like hierarchical Pt/NiAl-LDHs were composed of interlaced nanoplates and metallic Pt nanoparticles (NPs) approximately 3–4 nm in diameter were loaded on the surface of the Pt/NiAl-LDHs with high dispersion. The as-prepared Pt/NiAl21 nanocomposite was highly efficient in catalyzing oxidation of HCHO into CO₂ at room temperature. The high activity of the hierarchical Pt/NiAl21 nanocomposite was maintained after seven recycle tests, suggesting the high stability of the catalyst. Based on in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies, a reaction mechanism was put forward about HCHO decomposition at room temperature. This work provides new insights into designing and fabricating high-performance catalysts for efficient indoor air purification.     

Interactions and influence of α-cyclodextrin on the aggregation and interfacial properties of mixtures of nonionic and zwitterionic surfactants

The interactions of α-cyclodextrin (α-CD) with the nonionic surfactant decanoyl-N-methyl-glucamide (Mega-10) and the zwitterionic surfactant dimethyldodecylammoniopropanesulfonate (DPS) in their mixed system have been studied using interfacial tension, fluorescence, and nuclear magnetic resonance measurements. From the plots of interfacial tension vs. log of total surfactant concentration, we have obtained values of the surface excess of surfactant, the critical micellar concentration (cmc), the standard free energy of micelle formation, and association constant of surfactant/α-CD inclusion complexes (assuming a 1:1 stoichiometry). A comparison of the K _a values obtained for the interaction between α-CD and DPS and Mega-10, respectively, shows that DPS interacts stronger with α-CD than Mega-10. The experimental mixed cmc was analyzed by the pseudophase separation model and regular solution theory for the evaluation of ideality or nonideality of the mixed micelle formation. The interaction parameters in the mixed micelle and the micelle composition at different mole fractions of DPS were also computed. The fluorescence anisotropy (r) values of rhodamine B decreases with the increase of α-CD concentrations.     

Synthesis and Spectroscopic Characteriration of Transition MetalComplexes of Maleionitriledithiolene and 1,10-Phenanthroline

Thedithiolenesanddiiminesandtheirmetalcomplexesareanimportantresearchfieldintheorganicandcoordinationchemistry'-'.Metalcomplexesofadithioleneandadiiminehaveexcellentelectronicfullctionsduetotheintramolecularchargetransferfromaligandtootherligand(LL'C...     

Synthesis and study of the cyclization pathways and reactivities of α-diazoimidolates

The reaction of unsymmetrical N,N-disubstituted malonamides with benzenesulfonyl azide in the presence of sodium ethoxide gives individual sodium 1,2,3-triazol-5-olates or mixtures of their isomers, from the relative amounts of which the effect of substituents in the amido groups on the cyclization pathways and reactivities of -diazoimidolates was ascertained.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1521–1527, November, 1991.     

Synthesis and Characterization of ABBA Block Copolymer of Glycolide and ε-Caprolactone

A biodegradable ABBA block copolymer was synthesized via the ring-opening co-polymerization of ε-caprolactone(CL, B) and glycolide(A) by means of step polymerization in the presence of ethylene glycol as an initiator and stannous octanoate as a catalyst at 110 ℃ for 48 h. The molecular length of the PCL pre-polymer(BB) could be adjusted by controlling the molar ratio of the ethylene glycol initiator to ε-caprolactone monomer. The structure and the composition of the block copolymer were determined by the weight ratio of the monomer glycolide(A) to PCL pre-polymer(BB). The block copolymers were characterized by 1H NMR, GPC, DSC and X-ray. The results confirm the successful synthesis of an ABBA block copolymer.     

Spectral and Electrochemical Investigation of Intercalations of Adrenaline and CT－DNA

A strong interaction between double stranded calf-thymus DNA (ds-DNA) and adrenaline in solution, but no interaction between single stranded calf-thymus DNA (ss-DNA) and adrenaline was observed by the use of UV-visible spectroscopy and voltammetric techniques. It is suggested that the interaction leads to an intercalation of adrenaline molecules into the groove of ds-DNA and the formation of ds-DNA (adrenallne)n complex. The binding site size of the interaction of adrenaline with CT-DNA in nucleotide phosphate [ NP] has been determined as 25. The interaction of different concentration adrenaline with DNA modified GCE shows that the DNA modified GCE can be a good tool to detect lower concentration adrenaline.     

Fluorometric Investigation of the Acid-Base and Complexation Behaviour of Tetracycline and Oxytetracycline

The widely used antibiotics tetracyclines have been effectively used for ailing heart attack, ulcer cure and gene therapy. The actual mechanism of their activity has been proposed to link with the complexes with many metal ions.However, the sites at which complex formation takes place are not well established. In the present work, the deprotonation sequence of tetracycline (TC) and oxytetracycline (OTC), and their specific group used to bind europium ion were investigated by examining the character of fluorescence of TC and OTC as well as that of their complexes.It was concluded that the site of complexation is coordinated with the deprotonation sequence changing with the acidity/basicity of the solution. And it was inferred that five hydrogens in TC and OTC could be dissociated. The deprotonation sequence is as follows: C(3) hydroxy, C(10) phenol, C(4) dimethylamine, C(12) hydroxy and C(12a) hydroxy. The corresponding complexation site changed with pH increase in solution as follows: C(2) acylamino and C(3) hydroxy moiety, C(10)-C(11) ketophenol moiety, C(4) dimethylamine and C(3) hydroxy moiety,C(11)-C(12)β-diketone moiety, C(12) hydroxy and C(12a) hydroxy moiety, and C(12) hydroxy and C(1) ketonemoiety respectively.     

Synthesis and Characterization of ABBA Block Copolymer of Glycolide and ε-Caprolactone

A biodegradable ABBA block copolymer was synthesized via the ring-opening co-polymerization of ε-Scaprolactone (CL, B) and glycolide (A) by means of step polymerization in the presence of ethylene glycol as an initiator and stannous octanoate as a catalyst at 110 ℃ for 48 h. The molecular length of the PCL prepolymer(BB) could be adjusted by controlling the molar ratio of the ethylene glycol initiator to ε-caprolactone monomer. The structure and the composition of the block copolymer were determined by the weight ratio of the monomer glycolide (A) to PCL pre-polymer(BB). The block copolymers were characterized by ^1H NMR, GPC, DSC and X-ray. The results confirm the successful synthesis of an ABBA block copolymer.     

Chromatographic and Computational Studies of the Physicochemical Properties of Cholinesterase Inhibitors — Alkyl- and Arylcarbamate Derivatives of N-Benzylpiperidine and N-Benzylpiperazine

JPC – Journal of Planar Chromatography – Modern TLC - This paper reports an investigation of physicochemical properties of four series of carbamates of 3-hydroxy and...     

Synthesis Characterization and Viscosity Studies of Homopolymer of Methyl Methacrylate and Copolymer of Methyl Methacrylate and Styrene

A homopolymer of methyl methacrylate (MMA) and its copolymer with styrene at different compositions were synthesized and characterized. Viscosity measurements of the synthesized homopolymer and the copolymers in toluene solutions were performed at 313 K. Different equations were used to calculate the intrinsic viscosity, viscometric constants values, and molecular weight of the synthesized polymers. The values of intrinsic viscosity and viscosity average molecular weight obtained by the two methods (single point determination and graphical extrapolation) were compared in order to verify the validity of the single point determination for the polymers. Viscometric properties derived included the specific viscosity (it determines the contribution of the solute to the viscosity of the solution), the reduced viscosity (that provides a measure of the polymer capacity for increasing the solution viscosity), and the intrinsic viscosity.     

Study of Preparation and Characteristics of Pyridoxalated Polyhemoglobin

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