Formaldehyde In Aqueous Solution Research Articles

Fabrication of surface‐enhanced Raman scattering‐active ZnO/Ag composite microspheres

AbstractWe show in this paper how zinc oxide (ZnO)/silver (Ag) composite microspheres can be prepared by the reduction of Ag(NH3)2+ with the reducing agent formaldehyde in aqueous solution on the surface of ZnO microspheres. During the preparation, Sn2+ was absorbed on the surface of ZnO microspheres for sensitization and activation, and then Ag(NH3)2+ was reduced to Ag nanoparticles by the reducing agent to obtain ZnO/Ag composite microspheres. SEM and TEM images revealed silver nanoparticles with a diameter ranging from tens to 100 nm. X‐Ray photoelectron spectra (XPS), X‐ray diffraction (XRD) patterns and UV‐vis spectra were used to characterize the structure of the ZnO/Ag composite microspheres. The origin of the surface‐enhanced Raman scattering properties was traced to the surface of the ZnO/Ag composite microspheres. The enhancement factor was estimated in detail, and the enhancement mechanism for the SERS effect was also investigated. Copyright © 2007 John Wiley & Sons, Ltd.

Caracterização das áreas hemófagas da placenta bovina

A região específica da interface materno-fetal corresponde à zona arcada do placentônio ovino e caprino. Em pequenos ruminantes esta área é também caracterizada por sangue materno extravasado (áreas hemófagas). É possível que o ferro seja transferido para o feto por eritrofagocitose trofoblástica nestas áreas. Para investigar as áreas hemófagas na placenta bovina, foram analisados placentônios de 34 vacas zebuínas gestantes (dois a três, quatro a seis, sete a oito, e nove meses de prenhez). O material foi fixado com solução aquosa de formaldeído a 10% e paraformaldeído a 4%, em tampão fosfato, pH 7,4, 0,1M, sendo processado e corado para microscopia de luz e histoquímica. Os hematomas placentários foram observados entre o epitélio uterino e trofoblástico, a partir de três meses de prenhez. A presença de eritrócitos nas células trofoblásticas elucidou a eritrofago-citose. A reação histoquímica de Perl's permitiu provar a existência de ferro férrico no trofoblasto. A reação de PAS foi po-sitiva, marcando substância mucóide nas células epiteliais e, principalmente, nas células binucleadas do epitélio fetal. Baseando-se nas características histológicas e histoquímicas, inferimos que as áreas hemófagas são sítios importantes para a transferência de ferro na placenta bovina.

Fabrication and structure of carbon aerogel spheres prepared by inverse suspension/emulsion polymerization and ambient pressure drying

AbstractCarbon aerogel (CA) spheres were fabricated by inverse suspension polymerization of an emulsified resorcinol–formaldehyde aqueous solution with a basic surfactant used as catalyst, and then drying by gradually heating the resultant sol–gel spheres at ambient pressure. The fabrication conditions, nanostructures of the resultant CA spheres, and their electrochemical properties were investigated. By controlling the time at which stirring started, the stirring speed, and the recipe of the emulsion solution, the diameter of the CA spheres can be controlled from 70 to 1000 μm. The internal carbon nanoparticles of the spheres are of a size ranging from 20 to 40 nm, and are interconnected into a three‐dimensional network. The highest surface area and mesopore volume of the spherical CAs thus prepared reach 626 m2/g and 0.69 cm3/g, respectively. These CAs can be used as an electrode in supercapacitors, and the specific capacitance is as high as 197 F/g. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Development of a new industrial process for trioxane production

This paper reports on the development of a new industrial process for the production of trioxane ( C 3 H 6 O 3 ) , the cyclic trimer of formaldehyde ( CH 2 O ) . Trioxane is synthesized from concentrated aqueous formaldehyde solutions, which are inherently reactive complex multicomponent mixtures. Based on the knowledge on the physical chemistry of these complex solutions, reliable modeling of processes containing formaldehyde solutions has recently become possible. This was the key to the process development of the present work. The new trioxane process uses solely distillations for the purification. This is a great advantage compared to the existing process in which an extraction step with tedious solvent recovery is necessary. Conceptual process design was carried out with ∞ / ∞ -analysis using reactive distillation line diagrams. They show the possibility of a pressure swing distillation. Distillation experiments were carried out to validate the results. They prove the feasibility of all distillation cuts and, hence, of the entire process. The resulting new process, including all recycles, was successfully simulated based on a rigorous physico-chemical equilibrium stage model using Chemasim. Moreover, quantitative 1H NMR experiments were carried out, in which the trioxane formation as well as the formation of important side products in highly concentrated formaldehyde solutions containing up to 0.10 g g - 1 sulfuric acid was studied at temperatures up to 338 K to gain reliable information on reaction kinetics, needed for process design.

Asymmetric catalysis in aqueous media

Abstract Lewis acid catalysis has attracted much attention in organic synthesis because of unique reactivity and selectivity attained under mild conditions. Although various kinds of Lewis acids have been developed and applied in industry, these Lewis acids must be generally used under strictly anhydrous conditions. The presence of even a small amount of water handles the reactions owing to preferential reactions of the Lewis acids with water rather than the substrates. In contrast, rare earth and other metal complexes have been found to be water-compatible. Several catalytic asymmetric reactions in aqueous media, including hydroxymethylation of silicon enolates with an aqueous solution of formaldehyde in the presence of Sc(OTf)3-chiral bipyridine ligand or Bi(OTf)3-chiral bipyridine ligand, Sc- or Bi-catalyzed asymmetric meso-epoxide ring-opening reactions with amines, and asymmetric Mannich-type reactions of silicon enolates with N-acylhydrazones in the presence of a chiral Zn catalyst have been developed. Water plays key roles in these asymmetric reactions.

Quantitative Analysis of Formaldehyde Using UV‐VIS Spectrophotometer Pattern Recognition and Artificial Neural Networks

This study is focused on the quantitative analysis of formaldehyde in aqueous solution using a Fluoral‐P reagent and describes the characterization of the reaction, including the effect of reagent concentrations, pH, response time, dynamic range, reproducibility, photostability, and selectivity by using an ultraviolet‐visible (UV‐VIS) spectrophotometer. The relative standard deviation value was 1.79 to 2.12%. The dynamic range of the complex gives a linear stimulation of 0.00 to 3.60 ppm for the concentration of formaldehyde. The reproducibility of this study is high, with 1.79 and 2.12% for 20 and 40 ppm of formaldehyde, respectively. The interference from acetaldehyde (formaldehyde: acetaldehyde=1:100) was lower than 2.10%. In addition, the application of artificial neural networks to quantitative analysis for formaldehyde has also been done in this study to optimize the dynamic range of formaldehyde involved in the formation of Fluoral‐P–formaldehyde complex. A three‐layer feed‐forward network and the back propagation algorithm‐operated training process were used in this study. For quantitative analysis of formaldehyde, artificial neural networks, networking with 23 hidden neurons and 40,000 cycle numbers with 0.001% learning rate, produce the best training results, with sum‐squared error value 0.5847.

Density functional computational studies on ribose and glycine Maillard reaction: Formation of the Amadori rearrangement products in aqueous solution

By following the Hodge-scheme, and considering the formation of the Amadori rearrangement products ( ARPs) as one of the possible intermediates in the early stage, density functional theory calculations have been performed at the standard state on the proposed mechanisms of the Maillard reaction of cyclic ribose ( c-Rib)/open-chain ribose ( Rib) and glycine species under different pH conditions in aqueous solution. The result reveals that both c-Rib and Rib can participate in the reaction. Rib has been found as more reactive than c-Rib in the reaction. The reactions under basic and neutral conditions are supposed to be the most and second most favourable for the formation of ARPs. Production of both of the enol and keto forms of ARP have been found as feasible under basic condition, whereas the neutral condition is only favourable for producing the enol form of ARP. Therefore, the rate of browning under basic condition is assumed higher than that of the neutral condition. Formation of all intermediates in the proposed mechanisms has been found as unfeasible in the reaction under acidic condition and at the isoelectric point of glycine. Therefore, production of ARPs under these conditions is assumed to be stalled under these conditions, resulting into lower browning rate. Formation of Rib through the cleavage of glucose has been assumed less feasible than the formation of glucose from Rib and formaldehyde in aqueous solution.

Synthesis of ultrafine carbon cryogel microspheres using a homogenizer

AbstractUltrafine carbon cryogel (UCC) microspheres of diameter <5 μm with desirable mesopores were successfully synthesized using a homogenizer during the inverse emulsion polymerization of a resorcinol–formaldehyde (RF) aqueous solution in an organic solvent as a continuous phase (CP) containing nonionic surfactant, SPAN80 as an emulsifier, followed by solvent exchange, freeze‐drying, and pyrolysis in an Ar atmosphere. The curing time of RF solution before emulsification is a significant factor in controlling the size of the microspheres without changing their porosity. Both the mesoporosity and the size of the microspheres decreased on increasing SPAN80 and RF solution concentration in an inverse emulsion, which showed the possibility of controlling the mesoporosity and the size of the UCC microspheres. Furthermore, hexane could also be used as the continuous phase in the emulsion polymerization. The mean diameter of the UCC microspheres increased and their mesopore volume decreased with increasing pyrolysis temperature. © 2006 American Institute of Chemical Engineers AIChE J, 2007

Combined Multireference Configuration Interaction/ Molecular Dynamics Approach for Calculating Solvatochromic Shifts: Application to thenO→ π* Electronic Transition of Formaldehyde

A combined quantum mechanics/molecular mechanics method is described here for considering the solvatochromic shift of excited states in solution. The quantum mechanical solute is described using high level multireference configuration interaction methods (MRCI), while molecular dynamics is used for obtaining the structure of the solvent around the solute. The electrostatic effect of the solvent is included in the quantum description of the solute in an averaged way. This method is used to study solvent effects on the n(O) --> pi* electronic transition of formaldehyde in aqueous solution. The effects of solute polarization, basis sets, and dynamical correlation on the solvatochromic shift, and on dipole moments, have been investigated.

Quantitative NMR spectroscopy of complex technical mixtures using a virtual reference: chemical equilibria and reaction kinetics of formaldehyde–water–1,3,5-trioxane

Quantitative 1H NMR spectroscopy was used to study chemical equilibria and reaction kinetics of both the formation and decomposition of 1,3,5-trioxane in aqueous formaldehyde solutions. The reaction was homogeneously catalyzed with up to 0.10 g g(-1) sulfuric acid at temperatures between 360 and 383 K so that most of the experiments had to be carried out pressurized. The studied mixtures were complex due to the formation of methylene glycol and poly(oxymethylene) glycols in aqueous formaldehyde and the presence of considerable amounts of ionized species. Most common internal standards are decomposed by the hot sulfuric acid and external standards were not applicable using the flow NMR probe or pressurizable NMR sample tubes. Therefore, for the quantification of the small trioxane signals, a novel procedure was applied, in which electronically generated NMR signals were used as highly stable Virtual References (VR). The NMR decoupler channel with wave-form generator was used as the source of the reference signal, which was irradiated into the probe using the lock coil. Details on the experimental procedure are presented. It is shown that the presented method yields reliable quantitative reaction data for the complex studied mixtures.

Hydroxymethylation and polycondensation reactions in urea–formaldehyde resin synthesis

AbstractFormaldehyde–urea (F/U) reaction products with molar ratios of 1.8, 2.1, and 2.4 were synthesized at pH 8.3, and the last one also at pH 4.5 using 45% formaldehyde aqueous solution. For obtaining the resin, the synthesis of F/U 2.1 was continued by acid‐catalyzed condensation at pH 4.5 and posttreatment with second part of U (F/U 1.05/1) at 70°C and pH 8.3. The products were analyzed using 13C‐NMR spectrometry. Higher excess of F increases the dihydroxymethyl content on account of smaller dimethylene ether content. Certain 13C chemical shifts in carbonyl and methylene region of spectra were assigned to trishydroxymethylurea, being the main trisubstituted urea compound in hydroxymethylated product. Acid catalyst promotes the formation of methylene groups by polycondensation of hydroxymethyl groups, against the background of similar content of dimethylene ethers in both catalytic conditions. The ratio of linear/branched chains is emphasized in characterizing the resin structure. Higher hydroxymethyl content in acid‐catalyzed polycondensation is an advantage of three‐step synthesis technology. The amount of binding methylene and dimethylene ether groups linked only to secondary amino groups can be increased by transhydroxymethylation with subsequent polycondensation in posttreatment with U in suitable reaction conditions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1673–1680, 2006

π* → n Fluorescence Transition in Formaldehyde in Aqueous Solution: A Combined Quantum Chemical Statistical Mechanical Study

The solvent shift to the fluorescence transition pi --> n in formaldehyde in aqueous solution is theoretically analyzed. The solvent model has explicit representation of the solvent and uses the complete active space state interaction (CASSI) method to obtain a description of the wave function of the solute similar to what the complete active space self-consistent-field (CASSCF) method would give. In the description of the solute-solvent interaction the discrete set of solvent molecules perturb the solute not only through an electrostatic perturbation but also through a nonelectrostatic operator. The latter describes in a way analogous to pseudopotential theory the effect the Pauli principle has on the solute embedded in the solvent. This way the exchange repulsion between solute and solvent is accounted for which therefore can be anisotropic. The best estimate of the average shift is a blue shift of 0.003 eV, and for the current transition the nonelectrostatic perturbation broadens the distribution but has no significant effect on the average shift.

Study on Reactive Extraction Process for Trioxane Synthesis with Supercritical Fluid

Trioxane extraction with supercritical fluids was directly applied to an actual trioxane synthesis system. The reaction/extraction was conducted for an aqueous solution of formaldehyde with an acid catalyst in the presence of supercritical fluid. It was found that carbon dioxide is the best extractant and a trioxane solution at a concentration of more than 40% was obtained by one pot with supercritical fluid extraction from aqueous formaldehyde and a catalyst.Study on the reactive extraction by supercritical carbon dioxide fed continuously revealed that the larger vapor/liquid phase ratio is preferable, and the optimized reaction temperature is 373 K.

Bismuth Triflate−Chiral Bipyridine Complexes as Water-Compatible Chiral Lewis Acids

[reaction: see text] Catalytic asymmetric hydroxymethylation of silicon enolates with an aqueous formaldehyde solution has been developed using a chiral bismuth complex. This is the first example of highly enantioselective reactions using a chiral bismuth catalyst in aqueous media. In this paper, we have added Bi(OTf)(3)-1 complex as a "water-compatible Lewis acid". Bi(OTf)3 is unstable in the presence of water but is stabilized by the basic ligand.

On-line collection/concentration of trace amounts of formaldehyde in air with chromatomembrane cell and its sensitive determination by flow injection technique coupled with spectrophotometric and fluorometric detection

A simple, rapid and highly sensitive method for the determination of trace amounts of formaldehyde in air by using flow injection analysis (FIA) system coupled with a three-hole chromatomembrane cell (CMC) was investigated by using a spectrophotometer and a fluorometer. The CMC was applied to on-line collection/concentration of trace amounts of formaldehyde in air into water as an absorbing solution; formaldehyde in the air was found to be quantitatively transferred into the absorbing solution in CMC. The solution, containing absorbed formaldehyde, was introduced into the carrier stream of the FIA system. The amount of formaldehyde in an absorbing solution was measured spectrophotometrically and fluorometrically after the reaction with a mixed reagent of acetylacetone and ammonium acetate at pH 5.6–5.8. The amount of formaldehyde in the absorbing solution, measured by the proposed system, could be converted to the concentration of formaldehyde in the air sample. A calibration graph prepared by a series of standard formaldehyde aqueous solutions was adopted. The formaldehyde in indoor air, determined as exampled by the proposed spectrophotometric FIA, was found to be 5.14 ± 0.08 ppbv for 20 ml of the air sample at the air flow rate of 6 ml min −1, and the relative standard deviation (R.S.D.) was 1.56%. The limit of detections (LODs) of HCHO in an absorbing solution was 2 × 10 −8 M (0.6 ppb) and 8 × 10 −9 M (0.2 ppb), respectively, by the spectrophotometric and the fluorometric FIA, and the LODs of HCHO in air sample of 40 ml were 0.05 and 0.03 ppbv, respectively. The interferences from foreign species were examined; tolerable concentrations of other aldehydes were more than 50-fold of formaldehyde (1 × 10 −6 M).

Catalytic Asymmetric Hydroxymethylation of Silicon Enolates Using an Aqueous Solution of Formaldehyde with a Chiral Scandium Complex.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Putative mechanism of the sugar formation on prebiotic Earth initiated by UV-radiation

UV-irradiation of neutral aqueous formaldehyde solutions leads to the formation of a number of products. CO, CH4, CO2 and H2 are the main products detected in the gas phase over the solutions. In the liquid phase, glycolaldehyde and glyceraldehyde are among the products with the maximum yields of 4.2% and 0.18%, respectively. These aldehydes are shown to be the most active initiators of the formose reaction in the dark to synthesize monosaccharides from formaldehyde. The amounts of the photolytically formed initiators are sufficient for the initiating the formose reaction.

Solvent effects on the n→π* electronic transition in formaldehyde: A combined coupled cluster/molecular dynamics study

We present a study of the blueshift of the n-->pi* electronic transition in formaldehyde in aqueous solution using a combined coupled cluster/molecular mechanics model including mutual polarization effects in the Hamiltonian. In addition, we report ground and excited state dipole moments. Configurations are generated from molecular dynamics simulations with two different force fields, one with and one without an explicit polarization contribution. A statistical analysis using 1200 configurations is presented. Effects of explicit polarization contributions are found to be significant. It is found that the main difference in the effects on the excitation energies arises from the fact that the two force fields result in different liquid structures, and thus a different set of configurations is generated for the coupled cluster/molecular mechanics calculations.

Kinetics of oligomerization reactions in formaldehyde solutions: NMR experiments up to 373 K and thermodynamically consistent model

Aqueous and methanolic formaldehyde solutions are industrially important complex reacting mixtures in which formaldehyde is bound in a variety of oligomerization products. Reaction kinetics in these systems have to be taken into account in the design of reactors and separation equipment. The present paper reports on studies on reaction kinetics of aqueous and methanolic formaldehyde solutions by quantitative high-resolution on-line 1H NMR spectroscopy. The experiments were carried out at temperatures up to 373 K partially with a pressurized NMR probe and extend the temperature range in which data were available up to now. The NMR data are used to develop a reaction kinetic model, which is consistent with a physico-chemical thermodynamic equilibrium model, for formaldehyde containing mixtures. This is an important step towards developing thermodynamically consistent process models for reactions and separations with formaldehyde containing mixtures, which take reaction kinetics explicitly into account.

Catalytic asymmetric hydroxymethylation of silicon enolates using an aqueous solution of formaldehyde with a chiral scandium complex.

Catalytic asymmetric hydroxymethylation of silicon enolates has been achieved. In this reaction, an aqueous solution of formaldehyde can be used to realize an easy and safe procedure, and high enantioselectivities have been obtained. This is the first example of catalytic asymmetric reactions in aqueous media with a chiral scandium complex.