The kinetics of photobleaching (by spectrophotometric anal.) and integral photomineralization (by total org. carbon (TOC) anal.) of azobenzene (I) and substituted azobenzenes in aq. soln. were followed in lab.-scale(coating process) runs on photocatalytic membranes immobilizing 30±3 wt. of semiconductor TiO2. Expts. were carried out by the technique described in preceding papers of this series, employing stoichiometric hydrogen peroxide as the oxygen donor. The following azobenzenes were examd.: (4-diethylamino)-phenylazobenzene (II), 4'-(((4-diethylamino)phenyl)azo) benzoic acid (III), 4'-(((2-amino-5-diethylamino)phenyl)azo) benzoic acid (IV), 4'-(((2-acetamido-4-diethylamino)phenyl)azo) benzoic acid (V), 4'-(((4-dimethylamino)phenyl)azo) benzenesulfonic acid, sodium salt (VI) and 4'-(((2-acetamido-4-diethylamino)phenyl)azo) benzenesulfonic acid, sodium salt (VII). From the Langmuir-Hinshelwood treatment of the initial rate data as a function of the initial concn. ((0.10-1.0)×10-3 M), the kinetic parameter k and the pseudo-thermodn. parameter K for photobleaching were obtained. With regard to photobleaching, I and II were certainly the most reactive, followed by IV. The remaining mols. showed a photo-oxidn. rate of one-third to one-quarter of that of I chosen as ref. structure. Consequently, the presence of an amino group in the 4-position (II) does not stabilize the azobenzene structure against photo-oxidn. leading to bleaching, whereas the same group in the 2-position (IV) decreases the photobleaching rate by about 40 when a carboxylic group is also present in the 1'-position. Acetylation of this amino group, such as in V, decreases the photobleaching rate more markedly. With regard to photomineralization, it was obsd. that, when photobleaching was virtually complete, a certain amt. of TOC was already mineralized. The max. amt. of TOC remaining at the end of photobleaching ranged from about 90 to about 30, varying with the dye structure and initial concn. as well as with the power and type of irradn. source. By examg. the TOC concn. profiles as a function of the substituted azobenzene structure, the following hypotheses were proposed: (1) during the photobleaching period, the ring contg. the diethylamino group breaks down (more markedly if further amino or acetamido groups are present in the structure), and photomineralization of the other ring occurs more slowly; (2) both rings break down, within certain limits, during photobleaching; however, the aliph. fragments contg. carboxyl or sulfonic groups are mineralized more slowly. The fact that a small initial plateau in the TOC profile is followed by another more evident plateau at the end of the photobleaching period, for both VI and VII, suggests that hypothesis (1) is more probable in these cases. When the second plateau is reduced to a sigmoidal curve or an inflection point, hypothesis (2) also needs to be considered.

Photobleaching and photomineralization of azobenzene and substituted azobenzenes in aqueous solution by photocatalytic membranes immobilizing titanium dioxide / C. Lagrasta, I.R. Bellobono, M. Bonardi. - In: JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY. A, CHEMISTRY. - ISSN 1010-6030. - 110:2(1997), pp. 201-205.

Photobleaching and photomineralization of azobenzene and substituted azobenzenes in aqueous solution by photocatalytic membranes immobilizing titanium dioxide

C. Lagrasta
Primo
;
I.R. Bellobono
Secondo
;
M. Bonardi
Ultimo
1997

Abstract

The kinetics of photobleaching (by spectrophotometric anal.) and integral photomineralization (by total org. carbon (TOC) anal.) of azobenzene (I) and substituted azobenzenes in aq. soln. were followed in lab.-scale(coating process) runs on photocatalytic membranes immobilizing 30±3 wt. of semiconductor TiO2. Expts. were carried out by the technique described in preceding papers of this series, employing stoichiometric hydrogen peroxide as the oxygen donor. The following azobenzenes were examd.: (4-diethylamino)-phenylazobenzene (II), 4'-(((4-diethylamino)phenyl)azo) benzoic acid (III), 4'-(((2-amino-5-diethylamino)phenyl)azo) benzoic acid (IV), 4'-(((2-acetamido-4-diethylamino)phenyl)azo) benzoic acid (V), 4'-(((4-dimethylamino)phenyl)azo) benzenesulfonic acid, sodium salt (VI) and 4'-(((2-acetamido-4-diethylamino)phenyl)azo) benzenesulfonic acid, sodium salt (VII). From the Langmuir-Hinshelwood treatment of the initial rate data as a function of the initial concn. ((0.10-1.0)×10-3 M), the kinetic parameter k and the pseudo-thermodn. parameter K for photobleaching were obtained. With regard to photobleaching, I and II were certainly the most reactive, followed by IV. The remaining mols. showed a photo-oxidn. rate of one-third to one-quarter of that of I chosen as ref. structure. Consequently, the presence of an amino group in the 4-position (II) does not stabilize the azobenzene structure against photo-oxidn. leading to bleaching, whereas the same group in the 2-position (IV) decreases the photobleaching rate by about 40 when a carboxylic group is also present in the 1'-position. Acetylation of this amino group, such as in V, decreases the photobleaching rate more markedly. With regard to photomineralization, it was obsd. that, when photobleaching was virtually complete, a certain amt. of TOC was already mineralized. The max. amt. of TOC remaining at the end of photobleaching ranged from about 90 to about 30, varying with the dye structure and initial concn. as well as with the power and type of irradn. source. By examg. the TOC concn. profiles as a function of the substituted azobenzene structure, the following hypotheses were proposed: (1) during the photobleaching period, the ring contg. the diethylamino group breaks down (more markedly if further amino or acetamido groups are present in the structure), and photomineralization of the other ring occurs more slowly; (2) both rings break down, within certain limits, during photobleaching; however, the aliph. fragments contg. carboxyl or sulfonic groups are mineralized more slowly. The fact that a small initial plateau in the TOC profile is followed by another more evident plateau at the end of the photobleaching period, for both VI and VII, suggests that hypothesis (1) is more probable in these cases. When the second plateau is reduced to a sigmoidal curve or an inflection point, hypothesis (2) also needs to be considered.
Azo dyes; Photobleaching; Photocatalytic membranes; Photomineralization; Titanium dioxide
Settore CHIM/03 - Chimica Generale e Inorganica
1997
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2434/202128
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