Hydroquinone adsorption

Ethers contain additives to stabilise them against peroxide formation. For instance, tetrahydrofuran is commonly stabilised by the addition of small amounts of hydroquinone. This absorbs uv radiation strongly and so interferes with uv absorbance detection. It can be removed by distilling the solvent from KOH pellets. If you use inhibitor-free tetrahydrofuran, it should be stored in a dark bottle and flushed with nitrogen after each use. Any peroxides that form should be periodically removed by adsorption onto alumina. 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

The total conversions of catechol, 3-methylcatechol, hydroquinone, 2-methyl-hydroquinone, and 2,3-dimethyldhydroquinone were compared in the presence and absence of the nanoparticle iron oxide in quartz chips beds. It is evident that the presence of nanoparticle iron oxide lowered the temperature for a given conversion by about 180°C for all starting materials. In Fig. 12.4, representative results of the comparison are shown for catechol and hydroquinone over nanoparticle iron oxide/quartz mixture and quartz only, as a function of temperature. Each data point in Fig. 12.4 represents the averaged result of more than two experiments under the same conditions and a fresh catalyst was used for each new experiment. Catechol showed lower reactivity (50% conversion) than hydroquinone (100% conversion) at 260°C in the presence of the catalyst. This could be attributed to two phenomena. When the dihydroxybenzenes approach the catalyst surface in a co-planar fashion, intermolecular hydrogen bonding will lower the adsorption of catechol onto the catalyst surface and its interaction... 

To elucidate some enzymatic characteristics of the isolated laccases I, II, and III, substrate specificities for several simple phenols, electrophoresis patterns, ultraviolet spectra, electron spin resonance spectra, copper content, and immunological similarities were investigated. Tyrosine, tannic acid, g c acid, hydroquinone, catechol, pyrogallol, p-cresol, homocatechol, a-naphthol, -naphthol, p-phenylenediamine, and p-benzoquinone as substrates. No differences in the specificities of these substrates was found. The UV spectra for the laccases under stucfy are shown in Figure 4. Laccase III displays three adsorption bands (280, 405, and 600nm), laccase II shows one band 280nm), and laccase I shows two bands (280 and 405 nm). These data appear to indicate differences in chemical structure. The results of the copper content analysis (10) and two-dimensional electrophoresis also indicate that these fractions are completely different proteins (10), Therefore, we may expect differences in substrate specificities between the three laccase fractions for more lignin-like substrates, yet no difference for some simple phenolic substrates. 

Thus, the available evidence indicates that little or no adsorption of hydroquinone by silver occurs. Rabinovich s data are unacceptable because of the large experimental errors involved. The possible amount of adsorption indicated by the data of Perry, Ballard, and Sheppard does not exceed the limits of error in their analytical determination of hydroquinone and could not under any circumstances cover more than a small fraction of the silver surface. The kinetics of the reaction between hydroquinone and silver ions do not indicate adsorption of the reducing agent, although the first-order dependence of rate on concentration is not incompatible with weak adsorption. It seems unlikely, accordingly, that adsorption of hydroquinone by silver plays a role of any consequence in the silver catalysis of the reaction between hydroquinone and silver ion. 

Thus, an equation in agreement with the experimental data for the hydroquinone-silver ion reaction can be derived either on the basis of the assumption that adsorption of silver ions by the silver is a prelude to the reaction, or on the basis of the assumption that the rate-controlling step in an electrode process is the rate of transfer of electrons to the silver electrode. The first mechanism carries with it the assumption that a silver ion adsorbed by silver is more easily reduced than an ion in solu-... 

Rabinovich and his coworkers (Rabinovich et ah, 13) have made measurements on the adsorption of hydroquinone on silver bromide from alkaline solution. They report that the measured adsorption is proportional to the area of the silver bromide surface and give a value of... 

In view of its importance, reductive dissolution of Fe oxides has been widely studied. Reductants investigated include dithionite, thioglycolic acid, thiocyanate, hydrazine, ascorbic acid, hydroquinone, H2S, H2, Fe ", tris (picolinato) V", fulvic acid, fructose, sucrose and biomass/bacteria (Tab. 12.3). Under the appropriate conditions, reductive dissolution may also be effected photochemically. As with protonation, the extent of reduction may be strongly influenced by ligand and proton adsorption on the oxide surface. 

Aniline is a compound used in the synthesis of insecticides, chemical brighteners, and dyes and is a by-product of the petroleum, paper, and coal industries. The photocatalytic oxidation of aniline was studied by Sanchez et al. (1997). The reaction was found to follow Langmuir-Hinshelwood kinetics. The adsorption rate constant and the reaction rate constants were also reported. Higher yields were reported for acidic conditions and values near the pH at the point of zero charge (pHpZC) of Ti02. The rate of photocatalytic oxidation was also found to increase with the addition of small amounts of Fe. Hydroquinone is the main intermediate formed from the reaction. Photocatalytic reactions were carried out in a 130-cm3 cylindrical Pyrex cell. Medium-pressure mercury lamps provided UV light. Initial concentrations of 1.0 x 104 and 2 g/L were prepared. The pH of the solutions was adjusted and reactions were carried out for 15 min. Concentrations of aniline and byproducts formed were determined by HPLC. 

Waugh et al.131 discussed the selective oxidation of benzene to maleic anhydride on the basis of a detailed study of maleic anhydride and benzene adsorption on a V-Mo oxide catalyst supported on a-Al203. Hydroquinone is found to be an intermediate in this reaction and p-benzoquinone, formed from the hydroquinone, is the main intermediate in the non-selective pathway. The maleic anhydride is observed to be immobile adsorbed and the surface oxidation reaction has a relatively low activation energy. From this the authors conclude that it is not lattice oxygen but weakly bound molecular 02 which is responsible for the selective oxidation and a detailed mechanism, in which use is made of orbital symmetry arguments, is presented. 

Figure 4.6 Dependence of the surface coverage on the bulk concentration of the quinone (where and A denote the areas under the anodic and cathodic peaks, respectively) and hydroquinone (where denotes both anodic and cathodic data) forms of 1,2,4-AQASH. The supporting electrolyte is 1.0 M HCIO4. The dashed lines represent the best fits to the Frumkin adsorption isotherm where error bars are not shown, the errors determined from at least three independently formed monolayers are comparable to the sizes of the symbols. Reprinted with permission from R.J. Foster, T.E. Keyes, M. Farrell and D. O Hanlon, Langmuir, 16, 9871 (2000). Copyright (2000) American Chemical Society...

In addition to quinone reduction and hydroquinone oxidation, electrode reactions of many organic compounds are also inner-sphere. In these charge transfer is accompanied by profound transformation of the organic molecules. Some reactions are complicated by reactant and/or product adsorption. Anodic oxidation of chlorpro-mazine [54], ascorbic acid [127], anthraquinone-2,6-disulfonate [128], amines [129], phenol, and isopropanol [130] have been investigated. The latter reaction can be used for purification of wastewater. The cyclic voltammogram for cathodic reduction of fullerene Cm in acetonitrile solution exhibits 5 current peaks corresponding to different redox steps [131]. 

Numerous compounds related to hydroquinone have been studied with regard to their adsorption and adsorbed state reactivity at polycrystalline Pt TLE. These compounds were chosen because their electrochemical reactivity makes them an inviting target and because this large family of compounds offers a wide range of properties for investigation. 

It is also noteworthy that, as implied by eqn. (29), adsorption from hydroquinone (HQ) solutions leads to double dehydrogenation of the adsorbate in either final orientation. 

The electroactivity of the adsorbed states of this group of compounds is evidently associated with the presence on the adsorbed layer of a pendant functionality which is electroactive in the unadsorbed molecule and is attached to the surface in such a way that the pendant is virtually unperturbed structurally or electronically by the surface. In contrast, a molecule such as hydroquinone which is electroactive prior to adsorption but is strongly perturbed by adsorption (that is, by direct covalent attachment to the Pt surface) is not reversibly electroactive in the adsorbed state (Fig. 26). Accordingly, adsorbed DMBM is reversibly electroactive under almost all conditions, while adsorbed HQ is not reversibly electroactive under any conditions thus far studied. In between these two extremes is THBP, which is reversibly electroactive in one of its adsorbed states but not in the other two. 

Adsorption and electrochemical oxidation of a series of compounds related to hydroquinone (HQ) at Pt(lll) in aqueous solutions has been studied [81, 82], including benzoquinone (BQ), phenol (PL), perdeuterophenol (PDPL), tetrafluorohydroquinone (TFHQ), 2,2, 5,5 -tetrahydroxygiphenyl (THBP), and 2,2, 5,5 -tetraketodicyclohexadiene (TKCD). 

Recently, the use of sulfolane solvent allowed better kinetic control of the oxidation chain, with an increase of the selectivity to 80% or greater, at ca 8% benzene conversion. The by-products were catechol (7%), hydroquinone (4%), 1,4-benzo-quinone (1%) and tar (5%) [53, 54]. According to these authors, a rather stable complex, formed by hydrogen bonding with sulfolane, promoted desorption and hindered the re-adsorption of phenol, protecting it from consecutive oxidation (Equation 18.7). Actually, the rate of oxidation of phenol in the presence of sulfolane was only 1.6 times that of benzene, while it was 10 times higher in the presence of acetone. 

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