Dilute Aqueous Acid Solutions Research Articles

Chitosan Hydrogels Based on the Diels-Alder Click Reaction: Rheological and Kinetic Study.

The Diels–Alder reaction is recognized to generate highly selective and regiospecific cycloadducts. In this study, we carried out a rheological and kinetic study of N-furfuryl chitosan hydrogels based on the Diels–Alder click reaction with different poly(ethylene)glycol-maleimide derivatives in dilute aqueous acidic solutions. It was possible to prepare clear and transparent hydrogels with excellent mechanical properties. Applying the Winter and Chambon criterion the gel times were estimated at different temperatures, and the activation energy was calculated. The higher the temperature of gelation, the higher the reaction rate. The crosslinking density and the elastic properties seem to be controlled by the diffusion of the polymer segments, rather than by the kinetics of the reaction. An increase in the concentration of any of the two functional groups is accompanied by a higher crosslinking density regardless maleimide:furan molar ratio. The hydrogel showed an improvement in their mechanical properties as the temperature increases up to 70 °C. Above that, there is a drop in G’ values indicating that there is a process opposing to the Diels–Alder reaction, most likely the retro-Diels–Alder.

Dissolution of highly molecular weight cellulose isolated from wheat straw in deep eutectic solvent of Choline/l-Lysine hydrochloride

Green solvents for cellulose dissolution is a key topic for green chemistry, especially natural cellulose with high molecular weight, and there are scarce solvents reported. Deep eutectic solvent (DES) is a typical kind of green solvent that has been attracted much attention recently. Here, high molecular weight natural cellulose (DP > 3000) was first isolated from wheat straw and then be directly dissolved in the choline/l-lysine (Ch/Lys) DES. The solution owns excellent stability, and the solubility reaches ∼5%. Rheological studies revealed that the natural cellulose can be well dispersed in the DES solution and showed gelation at high concentrations. The dissolved cellulose can be regenerated when the dilute acid aqueous solution was added into the solution. It provides an energy conversation and an environmentally friendly route to prepare a cellulose solution, which makes it possible to convert cellulose to valuable chemicals and materials in its homogeneous solution.

Comparing sulphamic acid to acetic and formic acids for the preparation of large mesozoic marine reptile fossils, and a method for monitoring residual acid and salt removal following acid preparation

Chemical preparation is an important technique in palaeontology that uses a dilute aqueous acid solution to remove calcareous matrix from fossils, but which leads to the evolution of salts as a by-product. Acid preparation is usually undertaken using formic and acetic acids, whereas sulphamic acid has only rarely been considered. Sulphamic acid is a strong acid, with many industrial uses, but which has fewer health and safety concerns, and produces fewer irritant fumes, than formic or acetic acids. Three comparative procedures were undertaken to understand the action of sulphamic acid in relation to formic and acetic acids, using calcareous matrix from Colombian (South American) large Mesozoic marine reptiles. The results of these procedures indicate sulphamic acid acts in a comparable manner to formic acid, and more rapidly than acetic acid, in terms of rate of matrix removal. Afourth procedure investigated the removal of acid and salt residues following sulphamic acid preparation of a Colombian large Mesozoic marine reptile fossil, a process essential for the long-term survival of any acid prepared specimen. The fossil was immersed in type 1 deionized water, and increasing electrical conductivity was used as a proxy for ionic leaching. The results imply the preparators 'rule of thumb' of soaking a specimen in water for three times the length of time spent in the acid solution, is inadequate to ensure satisfactory removal of acid and salt residues. Although tested on a specimen prepared using sulphamic acid, the technique for post-preparation ion removal is equally be applicable to all fossils prepared using sulphamic, formic or acetic acids in aqueous solution.

Comparative Study of Different Electrochemical Techniques for the Preparation of Supported Pt-Ru Catalysts

Bimetallic Pt-Ru particles supported on glassy carbon rods were synthesized by simultaneous electrochemical deposition. Pt-Ru alloy particles were deposited from a dilute aqueous acid solution of chloroplatinic acid and ruthenium trichloride by different electrochemical techniques: (i) coulostatic deposition at constant potential; (ii) double potentiostatic steps; and (iii) multiple cycles of potentiostatic pulses. It was found that particle size distribution, and the morphology and composition of the deposits strongly depend on the deposition method. Scanning electron microscopy images showed the presence of agglomerates with diameters in the submicrometre scale composed of nano-sized particles. The catalysts prepared by multiple cycles of potentiostatic pulses exhibited better activity for methanol oxidation and enhanced tolerance to CO poisoning compared with those prepared by the other techniques. This behaviour could be associated with the structure containing a high number of defects of the particles and a higher ruthenium content in the solid solution.

Ferric Chloride Complexes in Aqueous Solution: An EXAFS Study

A series of concentrated aqueous solutions of ferric chloride with different chloride:iron(III) ratios has been studied by means of EXAFS to determine the structure around the iron(III) ion of the dominating species in such solutions. The dominating species in dilute acidic aqueous solution of ferric chloride, at less than 1 mmol·dm−3, are the hydrated iron(III) and chloride ions, while in concentrated aqueous solution and in solutions with an excess of chloride ions, up to 1.0 mol·dm−3, it is the trans-[FeCl2(H2O)4]+ complex. Possible higher chloroferrate(III) or dimeric [Fe2Cl6] complexes at room temperature, as proposed in the literature, were not observed in any of the studied solutions in spite of an excess of chloride ions of 1 mol·dm−3.

Fischer–Tropsch acid water processing by Kolbe electrolysis

Fischer–Tropsch synthesis produces roughly equal amounts of water and oil as products. The water product is acidic due to dissolved carboxylic acids. Treating this acid water in small-scale facilities designed for remote locations is a challenge, because conventional biological treatment might be impractical. Kolbe electrolysis was investigated as potential process for acid water treatment in such small-scale facilities. The Kolbe electrolysis of acetic acid in dilute aqueous solution produced methane, ethane and CO2 as expected. However, current efficiencies in the operating range 10–30°C were less than 3% and the energy required for acetic acid conversion was >2GJ/kg acid. Hydrogen was co-produced by water electrolysis and the overall current efficiency of acetic acid and water electrolysis was of the order 20%. Despite being technically viable, it appeared unpromising compared to alternative electrolysis processes for use in small-scale remote Fischer–Tropsch facilities.

High-Yield Production of Levulinic Acid from Pretreated Cow Dung in Dilute Acid Aqueous Solution.

Agricultural waste cow dung was used as feedstock for the production of a high value–added chemical levulinic acid (LA) in dilute acid aqueous solutions. A high LA yield of 338.9 g/kg was obtained from the pretreated cow dung, which was much higher than that obtained from the crude cow dung (135 g/kg), mainly attributed to the breakage of the lignin fraction in the lignocellulose structure of the cow dung by potassium hydroxide (KOH) pretreatment, and thus enhanced the accessibility of cow dung to the acid sites in the catalytic reaction. Meanwhile, another value-added chemical formic acid could be obtained with a yield of ca. 160 g/kg in the process, implying a total production of ca. 500 g/kg yield for LA and formic acid from the pretreated cow dung with the proposed process. The developed process was shown to be tolerant to high initial substrate loading with a satisfied LA yield. This work provides a promising strategy for the value-increment utilization of liglocellulosic agricultural residues.

Effect of temperature on the surface properties of α,ω-amino acids in dilute aqueous solutions

In this work, experimental results for surface tension of aqueous solutions of 3-aminopropanoic acid, 4-aminobutanoic acid, 5-aminopentanoic acid and 6-aminohexanoic acid at T = (293.15, 298.15, 303.15 and 308.15) K at several concentrations are presented. The results were used to evaluate the limiting experimental slopes of surface tension with respect to mole fraction at each temperature. Surface tension measurements were taken using a LAUDA TVT-2 drop volume tensiometer with temperature control better than 0.1 K at T = (293.15, 298.15, 303.15 and 308.15) K The thermodynamic behavior of the aqueous α,ω-amino acid solutions is compared with that reported for α-amino acids in water. The surface behavior is discussed in terms of the effect of the hydrocarbon chain and the position of polar groups on water structure. The temperature dependence of the limiting slopes of surface tension with respect to mole fraction, derived from surface tension measurements, is discussed.

Structure and properties of biodegradable wheat gluten bionanocomposites containing lignin nanoparticles

Lignin nanoparticles (LNPs), prepared through precipitation from an ethylene glycol solution by using diluteacidic aqueous solutions, were obtained. Fourier transform infrared spectroscopy confirmed the removal of hemicelluloses from pristine lignin by acidolysis, while the TGA analysis revealed a retained thermal stability of the nanoparticles with respect of pristine lignin. The lignin nanoparticles presented a uniform size distribution and good stability at various aqueous pH mediums. Wheat gluten (WG) bionanocomposites filled with 0, 1 and 3% wt. of LNPs were produced and thermally and mechanically characterized by means of TGA, modulated DSC and DMTA analysis. UV–Vis spectroscopy and water sensitivity measurements were also performed, in order to fully characterize the produced materials. The results indicated that LNPs could also effectively absorb ultraviolet spectrum and improve the water sensitivity of WG bionanocomposites. Moreover, even if the addition of LNPs reduced the transparency at the two different weight contents, it enhanced the mechanical behavior and thermal stability of resulted bionanocomposites. A strong interaction between LNPs and WG matrix, as well as homogeneous distribution of LNPs within the WG matrix, was confirmed by FESEM investigation and mechanical characterization. The reduction of absorbed water as well as the increase in glass transition suggested that LNPs can be beneficial to enlarge the potential applications for WG based materials and confirmed the great potential of using these novel nanofiller in bio-based matrices.

PH-Dependent Switchable Permeability from Core–Shell Microcapsules

We recently reported cationic cyclopolymerization of o-vinylbenzaldehydes initiated by boron trifluoride to generate acid-sensitive poly(o-(α-alkyl)vinylbenzaldehyde). Herein we report preparation of core-shell microcapsules (μCs) using flow-focusing microfluidic techniques with shells composed of poly(o-(α-methyl)vinylbenzaldehyde) (PMVB) that release their payload in response to dilute aqueous acid solution. Release profiles of encapsulated fluorescein isothiocyanate-labeled dextran from μCs are controlled by varying the proton concentration and shell-wall thickness. SEM studies indicate that the system's unique reversible release mechanism involves porosity changes in the shell wall due to microcrack formation.

High grafting density of cyclodextrin polymer for fast removal of aromatic compounds from water

The synthesized β-cyclodextrin grafted poly(glycidyl methacrylate) shows great potential in water treatment as it can complex with organic compounds such as 1-naphthylamine and 4′-aminoazobenzene-4-sulfonic acid in dilute aqueous solution.

Preparation and characterization of silver-modified La0.8Sr0.2MnO3 cathode powders for solid oxide fuel cells by chemical reduction method

Herein a chemical reduction method is proposed in order to modify the solid oxide fuel cells (SOFC) traditional cathode material La0.8Sr0.2MnO3−x (LSM). Silver nanoparticles were prepared by the reduction of ammoniacal silver nitrate with ascorbic acid in dilute aqueous solutions containing PVP. The obtained LSM–Ag composite powders were characterized by XRD, SEM, EDX, and STEM. The results showed that the LSM–Ag composite powder possess an elaborated fine structure with a homogeneous distribution of Ag and LSM, which effectively shortens the diffusion pathway for electrons and adsorbed oxygen. The electrochemical performance of the LSM–Ag cathode with different Ag loadings was investigated. A cathode loading with 1 wt.% Ag exhibited an area specific resistance as low as 0.45 Ω cm2 at 750 °C, compared to around 1.1 Ω cm2 for a pure LSM electrode. Similarly an anode-supported SOFC with 1 wt.% Ag in the cathode shows a peak power density of 1199 mW cm−1, higher than the value of 717 mW cm−1 achieved for a similar cell with a LSM cathode. Increasing the Ag loading is shown to have an insignificant effect on improving electrocatalytic performance at 750 °C, however it can increase output power at 650 °C.

Furfural formation from d-xylose: the use of different halides in dilute aqueous acidic solutions allows for exceptionally high yields

Starting from the results achieved in a previous work on the effects of Cl − ions on furfural formation in aqueous acid solution [Marcotullio, G. et al., Green Chem. 2010, 12, 1739], the general effect of different halides is addressed. Experimental results show the halides to influence at least two distinct steps in the reaction leading from d-xylose to furfural under acidic conditions, via different mechanisms. The nucleophilicity of the halides appears to be critical for the dehydration, but not for the initial enolization reaction. By combining different halides synergic effects become evident resulting in very high selectivities and furfural yields.

Hydration of Simple Carboxylic Acids from Infrared Spectra of HDO and Theoretical Calculations

The hydration of carboxylic acids in dilute aqueous solutions is important for our understanding of their functioning in the biochemical context. Here we apply vibrational spectra of HDO isotopically diluted in H(2)O to study this phenomenon, using the difference spectra method for analysis and interpretation of the results. The spectra of HDO affected by formic, acetic, and propionic acid display characteristic component bands, significantly red-shifted from the bulk HDO band position. The appearance of these component bands is linked with isotopic substitution on the carboxylic acid molecule, which forms a short and strong hydrogen bond with a water molecule. Additionally, a charge separation due to the proton transfer in the neutral form of the complex leading to a contact ion pair formation may be inferred from the affected HDO spectra. Apart from the contraction of the principal acid-water hydrogen bond, it results in other major structural changes in the hydration shell, as revealed by density functional theory (DFT) calculations of optimal geometries of aqueous clusters of the studied acids.

Effect of Temperature on the Volumetric Properties of α,ω-Amino Acids in Dilute Aqueous Solutions

The partial molar volume at infinite dilution of aqueous solutions of 3-aminopropanoic acid, 4-aminobutanoic acid, 5-aminopentanoic acid and 6-aminohexanoic acid were determined at T=(293.15,298.15,303.15 and 308.15 K) from density measurements. The thermodynamic behavior of the aqueous α,ω-amino acid solutions is compared with that reported for α-amino acids in water. The interaction volume is calculated and the influence of charged and uncharged groups of the amino acids is discussed in terms of solute–solvent interactions.

Homogeneous photocatalytic mineralization of acetic acid in an aqueous solution of iron ion

Homogeneous photocatalytic mineralization (decomposition to carbon dioxide (CO2)) of acetic acid in dilute aqueous solutions of various metal ions was examined at room temperature under irradiation of UV light in the presence of oxygen (O2). Iron ion exhibited a much higher level of activity among the metal ions studied for mineralization of acetic acid. The rate of CO2 formation was proportional to first-order partial pressure of O2 and concentrations of iron ion and acetic acid, whereas the rate was in inverse proportional to first-order proton concentration. The rate equation, r=kPO2 [[FeIII(H2O)6]3+] [AcOH] [H+]−1, in overall photocatalytic mineralization of acetic acid was derived from these results. The mechanism of the photocatalytic mineralization of acetic acid in an aqueous solution of ion was proposed.

Water Hydrogen-Bond Dynamics around Amino Acids: The Key Role of Hydrophilic Hydrogen-Bond Acceptor Groups

Water hydrogen-bond (HB) dynamics around amino acids in dilute aqueous solution is investigated through molecular dynamics simulations and analytic modeling. We especially highlight the critical role played by hydrophilic HB acceptors: the strength of the HB formed with water has a pronounced effect on the HB dynamics, in accord with several experimental observations. In contrast, we evidence that hydrophilic HB donors induce a moderate slowdown in the water HB exchange dynamics due to an excluded volume effect, similar to that of hydrophobic groups. We present an analytic model which rationalizes the effect of all examined amino acid sites on the HB dynamics and whose predictions are in excellent agreement with the numerical simulations. This model provides the acceleration or retardation in the HB exchange time with respect to the bulk through the combination of the solute excluded volume factor with the solute-water HB strength factor, both referring to the HB exchange transition state.

Chloride ions enhance furfural formation from d-xylose in dilute aqueous acidic solutions

Furfural production through traditional processes is accompanied by acidic waste stream production and high energy consumption. Modern furfural production process concepts will have to consider environmental concerns and energy requirements besides economics, moreover will have to be integrated within widened biorefinery concepts. In this paper, some particular aspects of the chemistry of D-xylose reaction to furfural are addressed, with the aim to clarify the reaction mechanism leading to furfural and to define new green catalytic pathways for its production. Specifically, reducing the use of mineral acids is addressed by the introduction of alternative catalysts. In this sense, chloride salts were tested in dilute acidic solutions at temperatures between 170 and 200 °C. Results indicate that the Cl− ions promote the formation of the 1,2-enediol from the acyclic form of xylose, and thus the subsequent acid catalyzed dehydration to furfural. For this reason the presence of Cl− ions led to significant improvements with respect to the H2SO4 case. The addition of NaCl to a 50 mM HCl aqueous solution gave 90% selectivity to furfural. Among the salts tested FeCl3 showed very interesting preliminary results, producing exceptionally high xylose reaction rates.

Unusual fluorescein prototropism within aqueous acidic 1-butyl-3-methylimidazolium tetrafluoroborate solution

Fluorescence emission of the cationic form of fluorescein is observed from dilute aqueous acidic solutions in the presence of ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate which is otherwise observed only from aqueous solutions of very high acidity.

Photocatalytic oxidation of oxalic acid in dilute aqueous solution, in a fully illuminated fluidized bed reactor

Abstract A comprehensive kinetic model for the photooxidation of oxalic acid (C 2 O 4 H 4 ) in a fluidized bed (FB) of a TiO 2 /quartz sand composite, in a UV-fully illuminated photoreactor, was developed and correlated with experimental results. A proper solution for the mass balance of the reacting system was achieved by combining the complete radiation field profile and the most recent and updated pathway for the adsorption and photodecomposition of oxalic acid onto TiO 2 ( Mendive et al., 2007 ), which was employed to determine the kinetic expression and model parameters.