Acidic Aqueous Conditions Research Articles

Dehydration of Different Ketoses and Aldoses to 5‐Hydroxymethylfurfural

5-Hydroxymethylfurfural (HMF) is considered an important building block for future bio-based chemicals. Here, we present an experimental study using different ketoses (fructose, sorbose, tagatose) and aldoses (glucose, mannose, galactose) under aqueous acidic conditions (65 g L(-1) substrate, 100-160 °C, 33-300 mM H2 SO4 ) to gain insights into reaction pathways for hexose dehydration to HMF. Both reaction rates and HMF selectivities were significantly higher for ketoses than for aldoses, which is in line with literature. Screening and kinetic experiments showed that the reactivity of the different ketoses is a function of the hydroxyl group orientation at the C3 and C4 positions. These results, in combination with DFT calculations, point to a dehydration mechanism involving cyclic intermediates. For aldoses, no influence of the hydroxyl group orientation was observed, indicating a different rate-determining step. The combination of the knowledge from the literature and the findings in this work indicates that aldoses require an isomerization to ketose prior to dehydration to obtain high HMF yields.

ChemInform Abstract: Cyclometalated Iridium Complexes for Transfer Hydrogenation of Carbonyl Groups in Water.

AbstractA new protocol for the reduction of ketones and aldehydes in acidic aqueous conditions using formic acid as the hydrogen source and cyclometalated iridium complexes as catalysts is described.

Iterative reducible ligation to form homogeneous penicillamine cross-linked polypeptides

The syntheses of homogeneous penicillamine disulfide cross-linked polypeptides are reported. Dodecapeptides containing N-terminal, C-terminal, or N- and C-terminal Pen were serially ligated into 36 amino acid polypeptides linked through Cys-Pen, Pen-Cys or Pen-Pen disulfide bonds. Critical to the syntheses was the incorporation of thiazolidine masked Cys and Pen as the N-terminal residues and selective hydrolysis with silver trifluoromethanesulfonate in acidic aqueous conditions to generate a free thiol for subsequent ligation. This approach allows the synthesis of homogeneous disulfide cross-linked polypeptides that have different reductive stabilities and have application in gene delivery by undergoing a tempered reductive triggered release of DNA.

Electrical and Photoelectrochemical Properties of WO3/Si Tandem Photoelectrodes

Tungsten trioxide (WO3) has been investigated as a photoanode for water oxidation reactions in acidic aqueous conditions. Though WO3 is not capable of performing unassisted solar-driven water splitting, WO3 can in principle be coupled with a low band gap semiconductor, such as Si, to produce a stand-alone, tandem photocathode/photoanode p-Si/n-WO3 system for solar fuels production. Junctions between Si and WO3, with and without intervening ohmic contacts, were therefore prepared and investigated in detail. Thin films of n-WO3 that were prepared directly on p-Si and n-Si substrates exhibited an onset of photocurrent at a potential consistent with expectations based on the band-edge alignment of these two materials predicted by Andersen theory. However, n-WO3 films deposited on Si substrates exhibited much lower anodic photocurrent densities (∼0.02 mA cm–2 at 1.0 V vs SCE) than identically prepared n-WO3 films that were deposited on fluorine-doped tin oxide (FTO) substrates (0.45 mA cm–2 at 1.0 V vs SCE). Deposition of n-WO3 onto a thin layer of tin-doped indium oxide (ITO) that had been deposited on a Si substrate yielded anodic photocurrent densities that were comparable to those observed for n-WO3 films that had been deposited onto FTO-coated glass. An increased photovoltage was observed when an n-Si/ITO Schottky junction was formed in series with the n-WO3 film, relative to when the WO3 was deposited directly onto the Si. Hence, inclusion of the ITO layer allowed for tandem photoelectrochemical devices to be prepared using n-WO3 and n-Si as the light absorbers.

Single-step grafting of aminooxy-peptides to hyaluronan: A simple approach to multifunctional therapeutics for experimental autoimmune encephalomyelitis

The immune response to antigens is directed in part by the presence or absence of costimulatory signals. The ability to coincidently present both antigen and, for example, a peptide that inhibits or activates the costimulatory pathway, would be a valuable tool for tolerization or immunization, respectively. A simple reaction scheme utilizing oxime chemistry was identified as a means to efficiently conjugate different peptide species to hyaluronan. Peptides synthesized with an aminooxy N-terminus reacted directly to hyaluronan under slightly acidic aqueous conditions without the need for a catalyst. The resulting oxime bond was found to rapidly hydrolyze at pH2 releasing peptide, but was stable at higher pH values (5.5 and 7). Two different peptide species, a multiple sclerosis antigen (PLP) and an ICAM-1 ligand (LABL) known to block immune cell stimulation, were functionalized with the aminooxy end group. These peptides showed similar reactivity to hyaluronan and were conjugated in an equimolar ratio. The resulting hyaluronan with grafted PLP and LABL significantly inhibited disease in mice with experimental autoimmune encephalomyelitis, a model of multiple sclerosis. Aminooxy-peptides facilitate simple synthesis of multifunctional hyaluronan graft polymers, thus enabling novel approaches to antigen-specific immune modulation.

The influence of the acid source on the structural and anti-oxidation properties of ordered mesoporous carbons

The synthesis of ordered mesoporous carbons (OMCs) was carried out under acidic aqueous conditions, by cooperative self-assembly of phenol–formaldehyde resol and surfactant F127. The influence of the acid source on the structural and anti-oxidation properties of the OMCs was investigated. The acids (HCl, HNO3, H3PO4 and H2SO4) used in the synthesis are sufficient to induce the self-assembly of resol and F127 to form the mesophase, while the final products carbonized at 600 °C show different structural properties. This was mainly due to the different decomposition behaviors of the surfactant during the carbonization process, which depends on the nature of the anions from the acids. After oxidization in acidic (NH4)2S2O8 solution, the H3PO4 catalyzed sample undergoes a minimum reduction in surface area and pore volume, and exhibits a larger adsorptive capacity for Co2+, compared with those using HCl and HNO3 as catalyst. This was related to the incorporation of the phosphorus atoms in the sample with H3PO4 as catalyst, which is in favour of increasing the carbonization degree of the sample, and they can also act as adsorptive sites for Co2+.

Synthesis and Characterization of Poly (3-hexylthiophene) and Used as Modified Electrode for Determination of Antidepressants in Pharmaceutical Formulations and Urine Samples

Conducting poly (3-hexylthiophene) (P3HT) was synthesized electrochemically in aqueous medium and characterized by electrochemical impedance spectroscopy, Tafel, SEM and TEM. The antidepressants drugs diazepam (DIZ), nitrazepam (NITZ) and imipramine (IMI) were analyzed on nanosize P3HT modified glassy carbon electrode. The voltammetric behavior of the three antidepressants was studied in aqueous acid, neutral, and alkaline conditions. One well-defined reduction peak each for DIZ and NITZ and one oxidation peak for IMI were observed in the cyclic voltammograms. The influence of pH, scan rate on peak current was studied extensively. A systematic study of the experimental parameters that affect the differential pulse stripping voltammetric response was carried out and optimized conditions which yield maximum peak current were arrived. The calibration was made under optimum conditions. The range of study for DIZ, NITZ and IMI was 100–1100, 150–1100 and 200–1300 μg/L and lower limit of determination was 75, 100, and 150 μg/L respectively. The suitability of the method for the determination of the three antidepressants in pharmaceutical preparations and urine samples was also ascertained.

Glycoluril Dimer Isomerization under Aqueous Acidic Conditions Related to Cucurbituril Formation

A water-soluble methylene-bridged glycoluril dimer 2S was isolated. It was shown that 2S is the only kinetic product of the reaction between glycoluril derivative 1 and paraformaldehyde. Compound 2S is subsequently intermolecularly transformed into its diastereomer 2C. The kinetics and thermodynamics of the S- to C-shaped dimer isomerization were investigated under reaction conditions similar to those for cucurbituril synthesis.

Direct Synthesis of 2D-Hexagonal Mesoporous Iron Silicate and its Catalytic Activity for Selective Friedel-Crafts Alkylation

D-hexagonal mesoporous iron silicate (HMFeS) has been synthesized hydrothermally in the presence of a mixture of an amphiphilic triblock copolymer, pluronic F127 and 1,2,4-trivinylcyclohexane (TVCH) as swelling agent under acidic aqueous conditions. The direct incorporation of iron(III) into 2D-hexagonal silicate framework can be monitored in a optimized molar ratio of water and hydrochloric acid. The mesophase of the materials was investigated by using small-angle powder X-ray diffractions (PXRD), transmission electron microscopy (TEM) image analysis and nitrogen adsorption/desorption studies. TEM image and PXRD revealed that the material had 2D-hexagonal mesoporous architecture. The morphology of the material was investigated by using scanning electron microscope (SEM) and framework bonding by utilizing FT IR spectroscopy. The atomic absorption spectrophotometer (AAS) was used to estimate the incorporated iron sites within the silicate framework. BET surface area (780 m 2 g -1 ) and peak pore size of HMFeS (10.07 nm) is much higher than the pure silica SBA-15 (611 m 2 g -1 and peak pore size of 9.09 nm). This mesoporous material (HMFeS) acts as a very good catalyst in the Friedel Craft benzylation and benzoylation reactions of arenes under optimized reaction condition using benzyl chloride and benzoyl chloride as the bezylating and benzoylating agents, respectively.

Preparation of highly regioselective amphiprotic chitosan derivative via “click chemistry”

Synthesis of a highly regioselective amphiprotic chitosan derivative was achieved by click chemistry. The Huisgen cycloaddition between 6-azido-6-deoxy-N-pthaloyl-chitosan and methyl propiolate was successfully carried out in the presence of Cu(I) catalyst. After the reaction, both the phthaloyl protecting group and methyl group were completely removed by hydrazine. FT-IR and NMR spectroscopy as well as elemental analysis strongly support the structural uniformity of the desired amphiprotic chitosan derivative, which has both a carboxylic group with a 1,4-triazole linker at the C-6 position and an amino group at the C-2 position per repeating unit. The amphiprotic chitosan derivative was soluble under both acidic and basic aqueous conditions. In contrast, it formed nanoparticle under neutral condition due to the interaction between the positive (NH3+) and negative (COO−) ions on the chitosan derivative.

ChemInform Abstract: Unique Salt Effect on Highly Selective Synthesis of Acid‐Labile Terpene and Styrene Oxides with a Tungsten/H2O2 Catalytic System under Acidic Aqueous Conditions.

AbstractAddition of a saturated amount of anhydrous sodium sulfate significantly improves the tungsten‐catalyzed H2O2 epoxidation to give acid‐labile epoxides in high yields.

A Cationic Antimonite Chain Templated by Sulfate: [Sb6O74+][(SO42–)2

An extended metal oxide possessing a cationic charge on the host has been synthesized by hydrothermal methods. The structure consists of 1D antimony oxide [Sb(6)O(7)](4+) chains with a new structural motif of four Sb atoms wide and unprotonated sulfate anions between the chains. The material was characterized by powder and single-crystal X-ray diffraction. Thermal behavior and chemical resistance in aqueous acidic conditions (pH ~2) indicate a highly stable cationic material. The stability is attributed to the entirely inorganic composition of the structure, where 1D covalently extended chains are electrostatically bound to divalent anions.

Decolourization of the azo dye Orange G in aqueous solution via a heterogeneous Fenton-like reaction catalysed by goethite

Decolourization of the azo dye Orange G (OG) was investigated by using goethite/H2O2 as a heterogeneous Fenton-like reagent. Five principle operational parameters, namely pH, ion strength, concentrations of goethite (α -FeOOH) and hydrogen peroxide (H2O2), and reaction temperature, were taken into account to investigate how these controlling factors mediated OG decolourization. Goethite surfaces catalysed a Fenton-like reaction responsible for decolourizing OG following pseudo-first-order kinetics (R 2>0.964). This process was effective but seriously impacted by the medium pH and the dosages of both α -FeOOH and H2O2. The decolourization efficiencies of OG increased with the decrease of solution pH and NaCl (chloride ion) concentration and/or the increase of H2O2. The acidic aqueous medium conditions were likely favourable due to the surface adsorption of the negatively charged OG leading to the promotion of decolourizing OG. The apparent activation energy (E) for this reaction was 42.18 kJ mol−1, a relatively low value. This is consistent with the OG decolourization being enhanced with the reaction temperature increase.

Nanoscale titania ceramic composite supports for PEM fuel cells

Abstract

Unique Salt Effect on Highly Selective Synthesis of Acid-Labile Terpene and Styrene Oxides with a Tungsten/H2O2 Catalytic System under Acidic Aqueous Conditions

Acid-labile epoxides such as terpene and styrene oxides are effectively synthesized in high yields with good selectivities using tungsten-catalyzed hydrogen peroxide epoxidation in the presence of Na2SO4. The salt effect is thought to originate with the addition of a saturated amount of Na2SO4 to aqueous H2O2; this addition strongly inhibited the undesired hydrolysis of the acid-labile epoxy products, despite the biphasic conditions of substrate as oil phase and H2O2 as acidic aqueous phase.

Conducting Metallopolymers as Precursors to Fabricate Palladium Nanoparticle/Polymer Hybrids for Oxygen Reduction

The novel conducting metallopolymer [N,N'-((2,2'-dimethyl)propyl)bis(5-(2,2'-bithiophene-5-yl)salcylideniminato-palladium(II)](n) prepared by electropolymerization provides a polymer matrix in which the palladium metal centers are evenly distributed. The metal centers embedded directly in the conducting metallopolymer backbone serve as seed points for size-controlled palladium nanoparticle (NP) growth of 3.8 to 4.9 nm NPs within the conducting metallopolymer film. The palladium NP/conducting metallopolymer hybrid material has demonstrated electrocatalytic behavior toward oxygen reduction with peak current densities around 400 μA/cm(2) in acidic aqueous conditions. These results demonstrate a promising new method for the production of electrocatalytically active hybrid materials.

Efficient synthesis and sulfonation of ordered mesoporous carbon materials

Ordered mesoporous carbons (OMCs) with hexagonal structure were efficiently synthesized via cooperative self-assembly of phenol/formaldehyde resol and surfactant F127 under acidic aqueous conditions. Induced by HCl, a gel phase mainly containing phenol/formaldehyde resol and F127 was obtained within several hours. X-ray diffraction (XRD), transmission electron microscope (TEM) and nitrogen adsorption isotherms indicated that the synthesized samples possess 2-D hexagonal mesostructure. The influence of the synthesis conditions, including acid concentration and mass ratio of resol to F127, was investigated. When the acid concentration was fixed in the range of 0.6–2.0M and the mass ratio of resol to F127 in the range of 3.5–4.0, highly ordered mesoporous carbon could be synthesized. The synthesized OMCs could be easily sulfonated in concentrated sulfuric acid at elevated temperature. The results indicate that the mesostructural stability and the content of the surface sulfonic acid (SO3H) groups depend mainly on the pyrolysis temperature of the OMCs and the sulfonation temperature, suggesting that the combination of pyrolysis and sulfonation temperature is essential for developing OMCs with high densities of SO3H groups.

ChemInform Abstract: Unique Salt Effect on the High Yield Synthesis of Acid‐Labile Terpene Oxides Using Hydrogen Peroxide under Acidic Aqueous Conditions.

AbstractThe use of sodium sulfate as an additive in the oxidation of various terpenes with hydrogen peroxide allows the synthesis of acid‐labile epoxides in high yields.

Gold/Palladium Bimetallic Alloy Nanoclusters Stabilized by Chitosan as Highly Efficient and Selective Catalysts for Homocoupling of Arylboronic Acid

Aerobic oxidative homocoupling of arylboronic acid under acidic aqueous conditions (pH 4.0) using bimetallic Au/Pd alloy nanoclusters stabilized by chitosan has been investigated. It was found that a Au0.81Pd0.19 catalyst (3.1 ± 0.8 nm) exhibited superior catalytic activities as compared to monometallic Au (2.3 ± 0.3 nm) and other series of bimetallic nanoclusters, giving the corresponding biaryls in nearly quantitative yield.

Photo-excitation energy transfer between a titanium(iv)–porphyrin complex and oxygen molecule

The Ti-TPyP reagent, i.e. an acidic aqueous solution of oxo[5,10,15,20-tetra(4-pyridyl)-porphyrinato]titanium(IV) complex, denoted as TiO(tpyp), was developed as a highly sensitive and specific reagent for detecting hydrogen peroxide. In our previous paper, the photosensitizing effect of the TiO(tpyp) complex on the detection of hydrogen peroxide using this reagent was explored based on the experimental findings. In the present paper, computational analysis was carried out for the molecular level elucidation of the energy transfer mechanism between the photo-excited TiO(tpyp) complex and triplet oxygen to form singlet oxygen under aqueous acidic conditions. The protonated form of the TiO(tpyp) complex, i.e. [TiO(tpypH4)]4+ complex, was found to be the entity responsible for the energy transfer to triplet oxygen. The interaction sites between the [TiO(tpypH4)]4+ complex with triplet oxygen were then discussed based on their molecular orbital characteristics and the potential energy changes accompanying the approach of triplet oxygen molecule Σ3O2 to the [TiO(tpypH4)]4+ complex. The obtained results show that the C–N portion of the porphyrin ring plays a predominant role in the energy transfer from the photo-excited porphyrin to Σ3O2. Σ3O2 is then converted to Δ1O2 through Σ1O2. The Δ1O2 molecule reaches its stable state at the distance of 3.21 Å from the C–N portion of the porphyrin ring with its stabilization energy of 0.99 kcal mol−1. The stabilization energy for the Δ1O2 molecule thus formed is small, implying no stable bond formation between the Δ1O2 molecule and the [TiO(tpypH4)]4+ complex and that it moves away from the C–N portion immediately after the energy transfer from the photo-excited porphyrin.