Oxidation Of Salicylic Acid Research Articles

Kinetic Study of the Electrochemical Oxidation of Salicylic Acid and Salicylaldehyde Using UV/vis Spectroscopy and Multivariate Calibration

The electrochemical oxidation of salicylic acid (SA), salicylaldehyde (SH), and their mixtures at Ti/IrO2-SnO2-Sb2O5 electrodes was studied using in situ UV/vis spectroscopy. Plackett-Burman's experimental design was employed to simultaneously investigate the effect of current density, temperature, mass transfer, composition of the electrode materials, initial concentration, and supporting electrolyte on the electrochemical oxidation of SA, revealing that temperature and the applied current density are the two major factors. The kinetics of the electrochemical oxidation of SA and SH was thus investigated at different temperatures and current densities, showing that the electrochemical oxidation of SA and SH is governed by the hydroxyl radical reaction and follows first-order kinetics with the apparent activation energy of 24.8 and 17.2 kJ/mol, respectively. The competitive effects of SA and SH during the electrochemical oxidation of their mixtures were further studied using UV/vis spectroscopy and multivariate calibration.

Photoelectrochemical oxidation of salicylic acid and salicylaldehyde on titanium dioxide nanotube arrays

We report on the kinetics of photoelectrochemical oxidation of salicylic acid (SA) and salicylaldehyde (SH) on titanium dioxide nanotube arrays. The TiO 2 nanotubes were prepared by the electrochemical oxidation of titanium substrates in a nonaqueous electrolyte (DMSO/HF). Scanning electron microscopy (SEM) was employed to examine the morphology of the formed nanotubes. Linear voltammetry was used to study the electrochemical and photoelectrochemical behavior of the synthesized TiO 2 nanotube arrays. The photoelectrochemical oxidation of SA and SH on the TiO 2 nanotubes was monitored by in situ UV–vis spectroscopy, showing that the kinetics of the photoelectrochemical oxidation of SA and SH follows pseudo first-order and that the rate constant of SH oxidation is 1.5 times larger than that of SA degradation. Quantum chemical calculations based on the DFT method were performed on SA and SH to address the large difference in kinetics. The relatively higher E LUMO − E HOMO makes SA more stable and thus more difficult to be oxidized photoelectrochemically. The impact of temperature and initial concentrations on the kinetics of SA and SH photoelectrochemical degradation was also investigated in the present work.

Reconsideration to the deactivation of TiO 2 catalyst during simultaneous photocatalytic reduction of Cr(VI) and oxidation of salicylic acid

Photocatalytic reduction/oxidation and deactivation of TiO 2 photocatalyst was investigated in the systems composed of Cr(VI) and salicylic acid. The selection of analysis method of Cr(IV) was very important to the monitoring of the photocatalytic process. It was found that as previously reported, serious deactivation of TiO 2 catalyst in the simultaneous photo-reduction of Cr(VI) and oxidation of salicylic acid was incorrectly observed if the Cr(VI) level was analyzed by directly monitoring the absorbance at characteristic 348 nm band of Cr(VI), because it seriously suffers from the interferences of the intermediates generated from the degradation of salicylic acid. By using an appropriate method to determine the Cr(VI) concentration, it was observed that all the added Cr(VI) could be reduced, not showing marked deactivation of the photocatalyst. A long time photocatalytic reduction of Cr(VI) under UV illumination induced the deposition of Cr(III) species on the surface of TiO 2 particles, which could cause a mild deactivation of the photocatalyst. However, the accompanied oxidation of salicylic acid was demonstrated to depress the deactivation effect of the deposited Cr(III) species on the photocatalytic activity of the TiO 2 photocatalyst.

Sonocatalytic facilitation of hydroxyl radical generation in the presence of TiO 2

The generation of hydroxyl (OH) radicals was investigated during ultrasonic irradiation and in the presence of TiO 2. The effect of TiO 2 on an ultrasonic system’s oxidation power was evaluated by examining the oxidation of salicylic acid. The generation of the salicylic acid derivatives, 2,3-dihydroxybenzoic acid (DHBA) and 2,5-DHBA, was measured by high-performance liquid chromatography coupled with electrochemical detection under different experimental conditions. The presence of TiO 2 enhanced the generation of DHBA during ultrasonic irradiation, thus indicating a higher oxidation power in the ultrasonic system. Al 2O 3 also increased the generation of DHBA during irradiation; however, the effect of TiO 2 was found to be higher than that of Al 2O 3. The addition of OH radical scavengers such as dimethylsulfoxide (DMSO), methanol and mannitol significantly suppressed the production of DHBA, and DMSO was found to have the highest suppressive effect among all scavengers. The effects of dissolved gases on the generation of OH radicals were further studied, and their power was found to be in the order Xe > Ar > O 2 > N 2. The degassing of the irradiation solution completely suppressed the generation of OH radicals. These results indicate that the presence of TiO 2 accelerates the generation of OH radicals during ultrasonic irradiation, and that the process may be mediated through the induction of cavitation bubbles in irradiating solutions.

Anodic oxidation of salicylic acid at Ta/BDD electrode

Boron-doped diamond (BDD) film electrodes using Ta as substrates were employed for anodic oxidation of salicylic acid (SA). The effects of operational variables including initial concentration, current density, temperature and pH were examined. The results showed that BDD films deposited on the Ta substrates had high electrocatalytic activity for SA degradation. There was little effect of pH on SA degradation. The current efficiency (CE) was found to be dependent mainly on the initial SA concentration, current density and temperature. Chemical oxygen demand (COD) was reduced from 830 mg/L to 42 mg/L under a current density of 200 A/m2 at 30 °C.

Intensification of hydroxyl radical production in sonochemical reactors

The efficacy of sonochemical reactors in chemical processing applications has been well established in the laboratory scale of operation though at a given set of operating parameters and no efforts have been directed in terms of maximizing the free radical production. In the present work, the effect of different operating parameters viz. pH, power dissipation into the system, effect of additives such as air, haloalkanes, titanium dioxide, iron and oxygen on the extent of hydroxyl radical formation in a sonochemical reactor have been investigated using salicylic acid dosimetry. Possible mechanisms for oxidation of salicylic acid in the presence of different additives have also been established. It has been observed that acidic conditions under optimized power dissipation in the presence of iron powder and oxygen result in maximum liberation of hydroxyl radicals as quantified by the kinetic rate constant for production of 2,5- and 2,3-dihydroxybenzoic acid. The study has enabled the optimization of the conditions for maximum efficacy of sonochemical reactors where free radical attack is the controlling mechanism for the chemical processing applications.

Hydroxyl radical generation via photoreduction of a simple pyridine N-oxide by an NADH analogue

Photoreduction of pyridine N-oxide, which has a key structure of antitumor agents for hypoxic solid tumors, by 1-benzyl-1,4-dihydronicotinamide in deaerated aprotic media resulted in generation of hydroxyl radical, leading to the oxidation of salicylic acid to 2,3- and 2,5-dihydroxybenzoic acids, and catechol.

Salicylic acid-induced inactivation of creatine kinase in the presence of lactoperoxidase and H 2O 2

To clarify one mechanism of aspirin-induced gastric mucosal damage, inactivation of creatine kinase (CK) by salicylic acid that is easily produced from aspirin in vivo was examined in the presence of lactoperoxidase (LPO) and H 2O 2 (LPO H 2O 2). Salicylic acid inactivated CK (rabbit muscle) during its interaction with LPO H 2O 2. CK activity in gastric mucosal homogenate decreased dependent on the concentration of salicylic acid in the presence of LPO H 2O 2. Oxygen radical scavengers did not prevent the inactivation of CK. Direct detection of free radicals of salicylic acid by electron spin resonance was unsuccessful. However, glutathionyl radicals were formed during the interaction of salicylic acid with LPO H 2O 2 in the presence of reduced glutathione and 5,5-dimethyl-1-pyrroline oxide as a spin trap agent. Among salicylic acid-related drugs, salsalate, but not aspirin and ethenzamide, inactivated CK, indicating the phenolic hydroxyl group is oxidized by LPO H 2O 2. During oxidation of salicylic acid by LPO H 2O 2, the sulfhydryl group in CK markedly decreased, and salicylic acid bound to CK. These results indicate that CK was inactivated through loss of the sulfhydryl group and binding of salicylic acid.

Voltammetric determination of salicylic acid in pharmaceuticals formulations of acetylsalicylic acid

The electrochemical oxidation of salicylic acid (SA) has been studied on a glassy carbon electrode using cyclic voltammetry and differential pulse voltammetric (DPV) method. SA gives a single irreversible oxidation wave over the wide pH range studied. The irreversibility of the electrode process was verified by different criteria. The mechanism of oxidation is discussed. Using differential pulse voltammetry, SA yielded a well-defined voltammetric response in Britton–Robinson buffer solution, pH 2.37 at 1.088 V (versus Ag/AgCl). The method was linear over the SA concentration range: 1–60 μg ml −1. The method was successfully applied for the analysis of SA as a hydrolysis product, in solid pharmaceutical formulations containing acetylsalicylic acid (ASA).

Silver metallisation of titania particles: effects on photoactivity for the oxidation of organics

The effects of silver metallisation of titania on the photoactivity were investigated for the oxidations of sucrose, salicylic acid and phenol. The activity of silver/titania particles was dependent on the nature of the organic photooxidised. This dependence was linked to the influence of the parent organic and the intermediate organics on the overall rate of reaction. Silver/titania enhanced the rates of sucrose oxidation by four times but possessed similar activities to bare titania for salicylic acid and phenol oxidation. The enhanced activity of silver/titania for sucrose oxidation was postulated to be due to improved charge separation and to the operation of an alternative reaction mechanism involving both oxidative and reductive pathways. The lack of enhancement in salicylic acid oxidation was postulated to be due to the ability of salicylic acid and the intermediates to function as deep hole traps and mediate efficient charge separation on both bare and modified titania surfaces. Thus, surface metallisation was not required to further enhance charge separation. The absence of rate enhancement in phenol oxidation was explained by a reported oxidation–reduction recombination cycle between the intermediates, hydroquinone and benzoquinone. The recombination of holes and electrons in the cycle was promoted with silver/titania particles. It is postulated that this nullified the beneficial effects of enhanced charge separation.

Concerted effects in the reaction of [rad]OH radicals with aromatics: radiolytic oxidation of salicylic acid

Liquid chromatographic and capillary electrophoretic studies have been used to resolve the products produced in the radiolytic oxidation of salicylic acid in aqueous solution. These studies have shown that, as in the case of phenol, OH radicals preferentially add to the positions ortho and para to the OH substituent. However, in contrast to its reaction with phenol, addition at the ortho position is favored over addition at the para position. Because OH radical is a strong electrophile this difference suggests that the electron population at the ortho position in the salicylate anion is enhanced as a result of the hydrogen bonding in salicylic acid.

Electrogeneration of Hydroxyl Radicals on Boron-Doped Diamond Electrodes

The electrogeneration of hydroxyl radicals was studied at a synthetic boron-doped diamond (BDD) thin film electrode. Spin trapping was used for detection of hydroxyl radicals with 5,5-dimethyl-1-pyrroline-N-oxide and with salicylic acid using electron spin resonance and liquid chromatography measurements, respectively. The production of hydrogen peroxide and competitive oxidation of formic and oxalic acids were also investigated using bulk electrolysis. The results have shown that oxidation of salicylic acid leads to the production of hydroxylated products (2,3- and 2,5-dihydroxybenzoic acids). These results demonstrate that the oxidation process on BDD electrodes involves hydroxyl radicals as electrogenerated intermediates. © 2003 The Electrochemical Society. All rights reserved.

Spectroscopic evidence in support of horseradish peroxidase compound II-catalyzed oxidation of salicylic acid but not of phenylethylamine.

Salicylic acid and phenylethylamine are putative substrates for naturally occurring reactions for generation of reactive oxygen species, which are catalyzed by plant peroxidases. Here, we used commercially available highly purified horseradish peroxidase-C (HRP-C) as a model enzyme for spectroscopic analysis, and obtained data suggesting that the Compound II form of HRP-C does not utilize phenylethylamine as substrate. In contrast, addition of salicylic acid to Compound II resulted in rapid conversion of Compound II to the native form.

A novel fountain photocatalytic reactor: model development and experimental validation

The application of photocatalysis for water treatment and purification on an industrial scale can be accelerated by the development of both new photoreactor designs and mathematical models. A novel, pilot-plant, thin-film, slurry photocatalytic reactor for water treatment and purification is presented in this paper. The reactor is a “fountain” photocatalytic reactor, consisting of a flattened water bell irradiated from above. Such a reactor configuration is particularly suitable for large-scale solar applications of photocatalysis. A dimensionless mathematical model for the fountain photocatalytic reactor is presented in this paper. The model was developed using parameters that can be estimated easily from real systems and model solutions can be obtained with little computational effort. The model was validated with experimental results from the photocatalytic oxidation of salicylic acid in a pilot-scale reactor using titanium dioxide (TiO 2) as the photocatalyst. Experiments were performed under a number of different conditions by varying substrate concentration, intensity of the incident radiation, catalyst loading, flow rate, recycle ratio and water fountain diameter. The model results were found to fit the experimental data well in all cases. The modeling approach can be extended to other thin-film slurry photocatalytic reactors.

Photocatalytic deactivation of commercial TiO 2 samples during simultaneous photoreduction of Cr(VI) and photooxidation of salicylic acid

TiO 2 Degussa P25 and Hombikat UV-100 photocatalysts have been widely characterized. Certain structural and morphological differences have been found. Photocatalytic behaviour has been studied for Cr(VI) reduction and for salicylic acid oxidation, in photoreactions with single and mixed substrates. In the photoreaction with mixed substrates, higher conversion values have been found in comparison to those observed for single substrates. However, clear deactivation process can be noticed. This photocatalytic deactivation is higher for Degussa P25 photocatalyst than for Hombikat UV-100 and can be explained in terms of the surface features exhibited for both TiO 2 during the photoprocesses.

Kinetics based determination of trace amounts of silver at the excess of copper ions

The determination of silver traces, based on the catalytic action of Ag + ions in the oxidation of salicylic acid by persulfate in the presence of 2,2′-dipyridyl as an activator, has been elaborated. The influence of several parameters, such as temperature, pH, salicylic acid concentration, persulfate concentration and 2,2′-dipyridyl concentration on the course of reaction has been investigated. The activation energies of the catalyzed (30.79 kJ mol −1) and non-catalyzed (44.77 kJ mol −1) processes were determined. Optimum conditions for the silver determination in the range of 4.3–38.8 ng cm −3 were established. The proposed method is more sensitive than atomic absorption spectrometry (AAS). The detection limit was 0.38 ng cm −3, so it was at least three orders of magnitude lower than that for the AAS method. The described method was used in the determination of the silver content in samples containing excess of copper and/or sulfate ions at concentrations of 8×10 −6–2.4×10 −2 M. The sulfate ions do not disturb the course of the reaction. The results of the determination of silver in a copper sheet by the proposed method were compared with the results of the analysis performed by AAS.

A laminar falling film slurry photocatalytic reactor. Part I—model development

The application of photocatalysis for wastewater treatment on an industrial scale has been partially impeded by the lack of simple mathematical models and optimisation studies. In this paper, a mathematical model (LSSE-LSPP model) for a laminar falling film slurry (LFFS) photocatalytic reactor is presented. The model is expressed in terms of five dimensionless parameters and can be solved with a minimal number of simple integrations. Model predictions are presented for different reaction kinetics and model parameters. As the parameters used in the model can be readily estimated from real systems, the model may be used for scale-up. A study of the sensitivity of the model to variations of the model parameters and validation of the model with experimental results of the oxidation of salicylic acid in a pilot-scale LFFS photocatalytic reactor are presented in Part II of this paper.

A laminar falling film slurry photocatalytic reactor. Part II—experimental validation of the model

The dimensionless mathematical model (LSSE–LSPP model) developed in Part I of this paper for a laminar falling film slurry (LFFS) photocatalytic reactor provides a method for scale-up and design of this type of reactor. Part II of the paper reports the experimental validation of the LSSE–LSPP model and a study of the sensitivity of the model with variations in the model parameters. The LSSE–LSPP model was used to calculate the rate of photocatalytic oxidation of salicylic acid in a pilot-scale LFFS photocatalytic reactor over a wide range of experimental conditions. The model results were found to fit the experimental data well, under conditions with varying substrate concentration, intensity of the incident radiation, catalyst loading, flow rates, recycle ratio and reactor geometry. The sensitivity analysis of the model parameters indicated that the reaction kinetics parameters are the major source of uncertainty in the modelling of photocatalytic reactors. The model revealed that, in the range of radiation wavelengths of 310–380 nm, light scattering in water suspensions of TiO 2 (Degussa P25) is negligible at high optical thicknesses but cannot be neglected at low optical thicknesses.

Concerning the role of oxygen in photocatalytic decomposition of salicylic acid in water

The photocatalyzed oxidation of salicylic acid on pure TiO 2 and ZnO films as well as on TiO 2 films loaded with various amounts of ZnO in N 2O-saturated aqueous solutions has been investigated. It was found that the photoassisted decomposition of salicylic acid did not take place on the irradiated surface of pure TiO 2 and took place on ZnO when N 2O was bubbled through the solution. The deposition of a small amount of ZnO onto the TiO 2 surface was responsible for the appearance of marked photocatalytic activity of material. The information about the electron-scavenger ability of N 2O in interaction with TiO 2 has been obtained by conductivity and photoconductivity measurements. The main conclusion is that the major role of oxygen in photocatalytic oxidation of organic compounds is to be a scavenger for continuous electron removal from photocatalysts.

Is the oxidation of salicylic acid to 2,5-dihydroxybenzoic acid a specific reaction of singlet oxygen?

Exclusive formation of 2,5-dihydroxybenzoic acid from salicylic acid is not a proof 1O2 intermediacy in photochemical systems as was previously proposed. In the photocatalytic transformation of salicylic acid in the presence of an OH. scavenger, the formation of 2,5-dihydroxybenzoic acid as the hydroxylated product is observed and attributed to an oxidation by positive holes.