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Fluorescence characterisation and analysis of 4-carboxybenzaldehyde, a colour precursor to the manufacture of terephthalic acid
Polymer Degradation and Stability 65 (1999) 347±353
Fluorescence characterisation and analysis of 4-carboxybenzaldehyde, a colour precursor to the manufacture of terephthalic acid James Daniels a, Norman S. Allen a,*, Michele Edge a, Stuart Coote b a
Department of Chemistry and Materials, The Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK b DuPont Fibres Ltd., PO BOX 90, Middlesborough, Cleveland TS6 8JE, UK Received 27 November 1998; received in revised form 14 February 1999; accepted 22 February 1999
Abstract The ¯uorescence properties of 4-carboxybenzaldehyde (4-CBA), a main oxidation product in the manufacture of terephthalic acid, have been investigated using dierent polar and acidic solvents. It is shown that these properties are very dependent upon the nature of the solvent, with concentrated sulphuric acid causing the most intense ¯uorescence. An unexpected blue shift was observed in the excitation spectra for solutions of 4-CBA and the models, dimethylterephthalate (DMT) and acetophenone, in sulphuric acid, with decreasing concentration and decreasing acid strength, indicating the existence of a ground state aggregate of the sandwich type. A method for determining the levels of 4-CBA in crude terephthalic acid (CTA) has been developed using the condensation reaction between 4-CBA and o-phenylenediamine (o-phd). # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Fluorescence; Polyester; Luminescence; Terephthalic acid; Degradation
1. Introduction Poly(ethyleneterephthalate) (PET) is used extensively in the ®bre and packaging industries. For most applications it is essential that the base polymer should exhibit only minor variations in physical/chemical properties and the optical properties of luminance, yellowness and ¯uorescence must be tightly controlled for aesthetic reasons. The eect of polymer processing conditions on colour formation is well known [1], consequently, when changes in optical properties occur while operating in standard processing conditions, a raw material quality problem is indicated. Terephthalic acid (TA), one of the two monomers used in the production of PET, is manufactured in a 2stage process. In the ®rst stage p-xylene is oxidised in acetic acid solution using a catalyst consisting of cobalt, manganese and bromide ions [2]. The TA produced at this point is 99.7% pure. Further puri®cation by
* Corresponding author. Tel.: +44-(0)161-247-6520; fax: +44(0)161-247-1438. E-mail address: [email protected] (N.S. Allen)
hydrogenation over a palladium on carbon catalyst can give 99.95% pure TA [3]. The chemical impurities in pure TA (PTA) are therefore potentially trace amounts of the original crude TA impurities together with their reduction products, along with catalyst ®nes arising due to the friable nature of the catalyst. The level of 4-CBA in CTA is used as an indication of the total level of impurities present in the sample during the manufacture process. This monitoring requires a simple method to determine the level of 4-CBA in TA. The results in this paper show that the ¯uorescence analysis is a useful method for the determination of 4CBA. However, the ¯uorescence properties of 4-CBA in solution are dependent on the polar and acidic properties of the solvent used and this appears to be related to the formation of polar associated aggregates. Aromatic ketones, such as acetophenone, were found to exhibit a similar behaviour in acid solution. No evidence for an Aldol condensation was found. The 4-CBA also appears to complex with the terephthalic acid which makes the direct analysis by ¯uorescence dicult. However, it was possible to condense 4-CBA with o-phenylenediamine (o-phd) to produce a highly ¯uorescent product which allows the determination of 4-CBA levels in crude TA samples without interference.
0141-3910/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(99)00023-3
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J. Daniels et al. / Polymer Degradation and Stability 65 (1999) 347±353
2. Experimental 2.1. Materials All compounds and solvents (spectroscopic grade) used in this study were obtained from Aldrich Chemical Company, UK and used as supplied. Unless otherwise stated the sulphuric acid used in this study was 98% v/v concentrated. 2.2. Fluorescence analysis Fluorescence excitation and emission spectra were recorded using a Perkin±Elmer LS-50B luminescence spectrometer. All spectra were recorded in aerobic solutions at room temperature. 2.3. Reactions undertaken 2.3.1. Sodium borohydride reduction of 4-CBA in crude terephthalic acid (CTA) CTA (1.5 g) was added to 2 M NaOH (25 cm3) and left to dissolve. Once dissolution had occurred NaBH4 (0.05 g) was added, this solution was heated gently for 10 min and then allowed to cool to room temperature. Dilute HCl was added drop-wise until precipitation occurred. The precipitate was then ®ltered and washed with distilled water and dried at 110 C. 2.3.2. 4-CBA+amine condensation reactions 4-CBA (0.1 g) and an equimolar amount of the amine were re¯uxed in acetic acid (50 ml) for 1 h. The solution was allowed to cool to room temperature and the acetic acid removed using a rotary evaporator. The resulting solid was recrystallised from methylated spirit. The extent of the reaction was observed by monitoring the reduction of the 4-CBA aldehyde peak at 1684 cmÿ1 using a Madison FT±IR SE80 instrument. The structures of the amines used can be seen in Fig. 1.
Fig. 1. Amines used in the condensation reaction with 4-CBA.
were obtained using an excitation wavelength of 325 nm and the intensity of emission was recorded at 395.4 nm. 2.3.4. 4-CBA/o-phd determination Using the same stock solutions and glassware as for the calibration, the determination of 4-CBA in CTA was carried out in triplicate. CTA (0.1 or 0.01 g) was weighed and placed in a 100 cm3 conical ¯ask, o-phd stock solution (20 cm3), DMF (20 cm3) and HCl stock solution (10 cm3) was added and the mixture was heated on a steam bath for 15 min with swirling. After 15 min the 1 ml aliquots were taken and placed directly into 20 cm3 volumetric ¯asks, they were then diluted to the mark and their spectra were obtained using an excitation wavelength of 325 nm and the intensity of emission recorded at 395.4 nm.
2.3.3. 4-CBA/o-phd calibration Stock solutions of 4-CBA, o-phd and HCl in DMF were prepared to the following concentrations,
3. Results and discussion
4-CBA=510ÿ5 M o-phd=2.510ÿ3 M HCl=0.1 cm3 of concentrated HCl in 100 ml of DMF.
The ¯uorescence of 4-CBA was found to be very dependent upon the polarity of the solvent. In methanol, ethanol and propan-2-ol two distinct emissions were observed at 410 and 470 nm respectively. The addition of a small amount of acid (0.1 cm3 2 M sulphuric acid) enhanced the emissions. These two emissions were reported earlier [4] for solutions of 4-CBA in methanol (Fig. 2). They have now been observed for solutions of 4-CBA in ethanol and propan-2-ol (Fig. 3), however, the change for propan-2-ol solutions upon the addition of an acid catalyst are very small. The long wavelength component in 2-propanol is also the dominant
Using the stock solutions above, calibration reaction mixtures were made in triplicate with varying concentrations of 4-CBA and constant concentrations of ophd and HCl. In each case the HCl was added last, the solutions were placed on a steam bath for 15 min with occasional swirling. After 15 min 1 cm3 aliquots were taken and placed directly into 20 ml volumetric ¯asks, these were then diluted to the mark and their spectra
3.1. 4-CBA ¯uorescence: solvent dependence
J. Daniels et al. / Polymer Degradation and Stability 65 (1999) 347±353
peak unlike that observed in methanol. It is well documented using methods such as infra-red spectroscopy [5] and UV/Visible spectroscopy [6] that aldehydes do not exist purely as the free aldehydes in alcoholic media, but as acetals and hemi-acetals. These emissions are, therefore, attributed to the presence of the acetal and hemi-acetal forms of 4-CBA with the corresponding alcohols. The smaller change for propan-2-ol solutions upon acid addition and dominance of the longer wavelength species could be attributed to the secondary nature of the alcohol and its more acidic character compared to the primary alcohols ethanol and methanol. However, the emissions
Fig. 2. Fluorescence emission spectra of a 4-CBA solution in methanol (0.45% w/w) without acid (EX=340 nm) (ÐÐ) (EX=388 nm) (- - - - - -) and with acid (EX=340 nm) (x-x-x-), (EX=388 nm) (- . - . - . -).
Fig. 3. Fluorescence emission spectra of a 4-CBA solution in propan2-ol (0.45% w/w) without acid (EX=340 nm) (ÐÐ) (EX=388 nm) (- - - - - -) and with acid (EX=340 nm) (x-x-x-), (EX=388 nm) (- . - . - . -).
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observed from the solutions of 4-CBA in alcoholic solvents are too weak to be of use in determining 4-CBA concentrations in TA samples. Increased emission was observed from solutions of 4CBA in sulphuric acid, with only one emission band at 460 nm. This ¯uorescence emission is also observed from solutions of CTA in sulphuric acid. Two emission bands are observed for TA samples in sulphuric acid, the shorter wavelength band at 390 nm is associated with the presence of TA and the second band at 460 nm with the presence of 4-CBA. Further evidence to suggest that this longer wavelength emission from CTA solutions is due to the presence of 4-CBA impurities originates from the fact that there is a reduction in this emission for TA samples that have been previously reduced using sodium borohydride (Fig. 4). The species responsible for the emission from solutions of 4-CBA in sulphuric acid was suggested to be the protonated carbonyl group of 4-CBA. Rusakowicz et al. [7] have suggested that aromatic carbonyl compounds can be protonated in strong acids and that these protonated species give rise to the emissions. However, from Fig. 5 it can be seen that there is very little ¯uorescence emission from 4-CBA solutions using other strong acids such as phosphoric acid. There are also inconsistencies in the excitation spectra of the carbonyl compounds. For example, the excitation spectrum of 4-CBA in sulphuric acid shows a blue shift with decreasing concentration (Fig. 6) and also with decreasing sulphuric acid strength (Fig. 7). Campbell and Edward [8] using UV/Visible studies on the ionization of ketones in various concentrations of sulphuric acid observed a blue shift with increasing acid strength. This observation was explained by considering the protonation of a carbonyl group as an acid-base reaction in which the theoretical extreme is complete protonation. The species formed by the interaction between the carbonyl group and the acid will have a hydration shell
Fig. 4. Fluorescence emission spectra (EX=380 nm) of a CTA solution (1% w/v) in sulphuric acid before reduction (ÐÐ) and after sodium borohydride reduction (- - - - - - -).
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associated with it. As the strength of the sulphuric acid is increased the activity of the water in the hydration shell will decrease and there will be a corresponding increase in the strength of the hydrogen bond between the carbonyl group and the proton. This increase in the strength of hydrogen bond causes the blue shift observed. Using a similar argument it would be expected that a blue shift would be observed as the concentration of 4CBA in sulphuric acid is increased. However, as has been stated the observations for 4-CBA are the opposite of those observed by Campbell and Edwards. Thus, the observations for 4-CBA in sulphuric acid need to be
explained by another theory. This is achieved by assuming that there is a dimer formed between two 4CBA molecules in concentrated sulphuric acid that absorbs radiation at longer wavelengths. Possible intermolecular associate structures are shown in Fig. 8. Upon addition of water to the acid or lowering the 4-CBA concentration the formation of these aggregates becomes less favourable and the radiation absorbed shifts to shorter wavelengths. Whilst the intensity of the ¯uorescence is correspondingly reduced the wavelength of radiation emitted does not shift with varying 4-CBA concentration or sulphuric acid strength. A similar blue shift with decreasing concentration has been observed in this study for solutions of acetophenone (Fig. 9) and dimethylterephthalate (DMT) in sulphuric acid. This suggests that the dimer formed must be of the sandwich type as they can not take part in head-tail type dimerisation. Such extended conjugation in the aggregates between the two aromatic rings would eectively give rise to an emissive lowest excited singlet state.
Fig. 5. Fluorescence emission spectra (EX=380 nm) for solutions of 4-CBA (510ÿ3% w/v) in sulphuric acid (ÐÐ) and phosphoric acid (- - - - - -).
Fig. 7. Fluorescence excitation spectra (EM=450 nm) for solutions of 4-CBA (0.05% w/v) in 98% H2SO4 (ÐÐ), 74% H2SO4 (- - - - - - -), 49% H2SO4 (x-x-x-), 25% H2SO4 (- . - . - . -).
Fig. 6. Fluorescence excitation spectra (EM=450 nm) for solutions of 4-CBA in sulphuric acid 0.2% (w/v) (ÐÐ), 0.05% (w/v) (- - - - -), 0.01% (w/v) (x-x-x), 0.002% (w/v) (- . - . - . -).
Fig. 8.
J. Daniels et al. / Polymer Degradation and Stability 65 (1999) 347±353
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3.2. Fluorescence from 4-CBA condensation products The emission from solutions of 4-CBA in organic solvents and sulphuric acid does not lend itself to a determination of 4-CBA concentration in TA due to the complexities of the emission spectra and nature of the associated species. However, the condensation of 4CBA with o-phd produces a Schi Base product that can be determined at sensitivities <5 ppm. The outline of this reaction is shown in Scheme 1. Many amines (Fig. 1) were condensed with 4-CBA but only o-phd produced a product whose emission was not interferred with by the emission from the reactants. The products formed during the condensation reaction have the possibility of being highly ¯uorescent due to the -CHNlinkage allowing conjugation between the two aromatic rings and giving rise to an emissive lowest excited singlet state. The emissions from the Schi bases formed between 4-CBA and the larger amines all overlapped with the emission from the unreacted amines. This would lead to an unnecessary interference in an analytical method. Fig. 10. Structure and ¯uorescence emission spectra (EX=325 nm) of 4-CBA/o-phd complex in DMF 9210ÿ6M.
Fig. 9. Fluorescence excitation spectra (EM=450 nm) for solutions of acetophenone in sulphuric acid 1.610ÿ2M (ÐÐ), 1.610ÿ3M (- - - - - - -), 1.610ÿ4M (x-x-x-x), 1.610ÿ5M (- . - . - . -).
Scheme 1. Schematic of the schi base reaction between 4CBA and Aniline
Fig. 11. Calibration graph of ¯uorescence intensity at 400 nm of the condensation product of 4-CBA with o-phenylenediamine.
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Table 1 Results for determination of 4-CBA samples of known 4-CBA content CTA (0.14% 4CBA)a MASS CTA (g)
Intensity
[4-CBA], react (M)
[CTA,M]
CTA (%)
0.011 0.010 0.011 0.1 0.099 0.104
34.6 31.0 32.8 279.9 274.3 295.3
1.6610ÿ6 1.4810ÿ6 1.56710ÿ6 1.910ÿ5 1.3610ÿ5 1.4710ÿ5
1.3010ÿ3 1.2310ÿ3 1.3010ÿ3 1.2010ÿ2 1.1910ÿ2 1.2510ÿ2
0.13 0.12 0.12 0.12 0.12 0.12
64.5 59.7 53.8 392.5 392.9 388.5
3.1510ÿ6 2.9110ÿ6 2.6110ÿ6 1.9610ÿ5 1.9610ÿ5 1.9210ÿ5
1.3310ÿ3 1.2910ÿ3 1.15710ÿ3 1.2010ÿ2 1.2210ÿ2 1.2010ÿ2
0.24 0.23 0.23 0.16 0.16 0.16
CTA (0.24% 4CBA)b 0.011 0.011 0.01 0.100 0.102 0.100 a b
0.01 g; s=4.010ÿ3, CV=3.3%; 0.1 g; s=1.010ÿ3, CV=0.86%. 0.01 g; s=6.910ÿ3, CV=3.0%; 0.1 g; s=16010ÿ3, CV=0.95%.
Of the phenylenediamines used, only the ortho compound resulted in the formation of a ¯uorescent product. The structure of the product between 4-CBA and o-phd is shown in Fig. 10. This shows the possibility of hydrogen bonding between the unreacted amine group and the nitrogen of the -CHN- linkage. This hydrogen bond imparts rigidity to the product that is not possible in the non-¯uorescent products formed between 4-CBA and p-phd and m-phd. This rigidity also increases the probability of ¯uorescence over other non-radiative deactivation processes. The emission from the 4-CBA/o-phd product in N,Ndimethylformamide (DMF) is at a long enough wavelength, 395 nm (Fig. 10), not to be re-absorbed by any TA, lmax 261 nm, that would be present in the matrix in which the 4-CBA is being determined. The 4-CBA/o-phd product emits only very weakly in acidic solutions, however, the reaction requires an acid catalyst. To overcome this problem the 1 cm3 aliquot was taken and diluted to 20 cm3, this had the eect of diluting the acid and of reducing the temperature of the solution. The calibration graph used to determine 4-CBA concentrations in CTA samples is shown in Fig. 11. Linear regression of the data points plotted gives a linear equation of y 4 108 x 1:4721, with an R2 value of 0.995. Values of 4-CBA concentration were determined using the method described above for CTA samples with 4-CBA concentrations previously determined using a HPLC method. These values are shown in Table 1. It should be noted that the values for 4-CBA concentrations
have been corrected for the 1 cm3 aliquot diluted to 20 cm3 after the reaction. The values obtained when only 0.01 g TA was used in the reaction are closer to the values accepted as being present in the CTA samples. This is to be expected as the high levels of TA present in the reaction mixture will slow the rate of reaction between the 4-CBA and the ophd during the 15 min allowed for the reaction. However, the values obtained for the larger, 0.1 g, samples have much smaller values for the coecient of variation (CV). Thus, a compromise needs to be made between the size of sample used and the precision of the results. The emission obtained from poly(ethylene terephthalate) (EX=395 nm) was found to match very well the emission from a 0.1% solution of 4-CBA in TFA, although the emission from 4-CBA in TFA is very weak. The implications of this contamination will be the subject of further studies on photooxidation. 4. Conclusions It has been demonstrated that the emission from 4-CBA is very dependent upon the nature of the solvent and that in sulphuric acid ground state dimers and aggregates are formed which give rise to a blue shift with decreasing 4CBA content and decreasing acid strength. It is suggested that the structure of this dimer is predominantly of the sandwich type. A method involving the condensation of 4CBA with o-phd allows the determination of 4-CBA in CTA samples. At low concentrations sensitivities <5 ppm are possible by ¯uorescence analysis.
J. Daniels et al. / Polymer Degradation and Stability 65 (1999) 347±353
References [1] Buxbaum LH. Angew Chem Int Ed 1968;7:102. [2] Landau R, Staer A. Chem Eng Prog 1968;64:20. [3] Meyer, DH. (Standard oil) US Pat. 2,584,039, 1971.
[4] [5] [6] [7] [8]
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Allen NS, Daniels J, Edge M, Royall D. Polym Deg Stab 1998;62:373. Ashdown A, Kletz TA, J Chem Soc Trans 2 1948:1454. Bell J, Kubler DJ, Zepp RG. J Org Chem 1965;30:4284. Rusakowicz R, Byers GW, Leermakers PA. JACS 1971;93: 3263. Campbell HJ, Edward JT. Can J Chem 1960;38:2109.
Fluorescence characterisation and analysis of 4-carboxybenzaldehyde, a colour precursor to the manufacture of terephthalic acid James Daniels a, Norman S. Allen a,*, Michele Edge a, Stuart Coote b a
Department of Chemistry and Materials, The Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK b DuPont Fibres Ltd., PO BOX 90, Middlesborough, Cleveland TS6 8JE, UK Received 27 November 1998; received in revised form 14 February 1999; accepted 22 February 1999
Abstract The ¯uorescence properties of 4-carboxybenzaldehyde (4-CBA), a main oxidation product in the manufacture of terephthalic acid, have been investigated using dierent polar and acidic solvents. It is shown that these properties are very dependent upon the nature of the solvent, with concentrated sulphuric acid causing the most intense ¯uorescence. An unexpected blue shift was observed in the excitation spectra for solutions of 4-CBA and the models, dimethylterephthalate (DMT) and acetophenone, in sulphuric acid, with decreasing concentration and decreasing acid strength, indicating the existence of a ground state aggregate of the sandwich type. A method for determining the levels of 4-CBA in crude terephthalic acid (CTA) has been developed using the condensation reaction between 4-CBA and o-phenylenediamine (o-phd). # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Fluorescence; Polyester; Luminescence; Terephthalic acid; Degradation
1. Introduction Poly(ethyleneterephthalate) (PET) is used extensively in the ®bre and packaging industries. For most applications it is essential that the base polymer should exhibit only minor variations in physical/chemical properties and the optical properties of luminance, yellowness and ¯uorescence must be tightly controlled for aesthetic reasons. The eect of polymer processing conditions on colour formation is well known [1], consequently, when changes in optical properties occur while operating in standard processing conditions, a raw material quality problem is indicated. Terephthalic acid (TA), one of the two monomers used in the production of PET, is manufactured in a 2stage process. In the ®rst stage p-xylene is oxidised in acetic acid solution using a catalyst consisting of cobalt, manganese and bromide ions [2]. The TA produced at this point is 99.7% pure. Further puri®cation by
* Corresponding author. Tel.: +44-(0)161-247-6520; fax: +44(0)161-247-1438. E-mail address: [email protected] (N.S. Allen)
hydrogenation over a palladium on carbon catalyst can give 99.95% pure TA [3]. The chemical impurities in pure TA (PTA) are therefore potentially trace amounts of the original crude TA impurities together with their reduction products, along with catalyst ®nes arising due to the friable nature of the catalyst. The level of 4-CBA in CTA is used as an indication of the total level of impurities present in the sample during the manufacture process. This monitoring requires a simple method to determine the level of 4-CBA in TA. The results in this paper show that the ¯uorescence analysis is a useful method for the determination of 4CBA. However, the ¯uorescence properties of 4-CBA in solution are dependent on the polar and acidic properties of the solvent used and this appears to be related to the formation of polar associated aggregates. Aromatic ketones, such as acetophenone, were found to exhibit a similar behaviour in acid solution. No evidence for an Aldol condensation was found. The 4-CBA also appears to complex with the terephthalic acid which makes the direct analysis by ¯uorescence dicult. However, it was possible to condense 4-CBA with o-phenylenediamine (o-phd) to produce a highly ¯uorescent product which allows the determination of 4-CBA levels in crude TA samples without interference.
0141-3910/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(99)00023-3
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J. Daniels et al. / Polymer Degradation and Stability 65 (1999) 347±353
2. Experimental 2.1. Materials All compounds and solvents (spectroscopic grade) used in this study were obtained from Aldrich Chemical Company, UK and used as supplied. Unless otherwise stated the sulphuric acid used in this study was 98% v/v concentrated. 2.2. Fluorescence analysis Fluorescence excitation and emission spectra were recorded using a Perkin±Elmer LS-50B luminescence spectrometer. All spectra were recorded in aerobic solutions at room temperature. 2.3. Reactions undertaken 2.3.1. Sodium borohydride reduction of 4-CBA in crude terephthalic acid (CTA) CTA (1.5 g) was added to 2 M NaOH (25 cm3) and left to dissolve. Once dissolution had occurred NaBH4 (0.05 g) was added, this solution was heated gently for 10 min and then allowed to cool to room temperature. Dilute HCl was added drop-wise until precipitation occurred. The precipitate was then ®ltered and washed with distilled water and dried at 110 C. 2.3.2. 4-CBA+amine condensation reactions 4-CBA (0.1 g) and an equimolar amount of the amine were re¯uxed in acetic acid (50 ml) for 1 h. The solution was allowed to cool to room temperature and the acetic acid removed using a rotary evaporator. The resulting solid was recrystallised from methylated spirit. The extent of the reaction was observed by monitoring the reduction of the 4-CBA aldehyde peak at 1684 cmÿ1 using a Madison FT±IR SE80 instrument. The structures of the amines used can be seen in Fig. 1.
Fig. 1. Amines used in the condensation reaction with 4-CBA.
were obtained using an excitation wavelength of 325 nm and the intensity of emission was recorded at 395.4 nm. 2.3.4. 4-CBA/o-phd determination Using the same stock solutions and glassware as for the calibration, the determination of 4-CBA in CTA was carried out in triplicate. CTA (0.1 or 0.01 g) was weighed and placed in a 100 cm3 conical ¯ask, o-phd stock solution (20 cm3), DMF (20 cm3) and HCl stock solution (10 cm3) was added and the mixture was heated on a steam bath for 15 min with swirling. After 15 min the 1 ml aliquots were taken and placed directly into 20 cm3 volumetric ¯asks, they were then diluted to the mark and their spectra were obtained using an excitation wavelength of 325 nm and the intensity of emission recorded at 395.4 nm.
2.3.3. 4-CBA/o-phd calibration Stock solutions of 4-CBA, o-phd and HCl in DMF were prepared to the following concentrations,
3. Results and discussion
4-CBA=510ÿ5 M o-phd=2.510ÿ3 M HCl=0.1 cm3 of concentrated HCl in 100 ml of DMF.
The ¯uorescence of 4-CBA was found to be very dependent upon the polarity of the solvent. In methanol, ethanol and propan-2-ol two distinct emissions were observed at 410 and 470 nm respectively. The addition of a small amount of acid (0.1 cm3 2 M sulphuric acid) enhanced the emissions. These two emissions were reported earlier [4] for solutions of 4-CBA in methanol (Fig. 2). They have now been observed for solutions of 4-CBA in ethanol and propan-2-ol (Fig. 3), however, the change for propan-2-ol solutions upon the addition of an acid catalyst are very small. The long wavelength component in 2-propanol is also the dominant
Using the stock solutions above, calibration reaction mixtures were made in triplicate with varying concentrations of 4-CBA and constant concentrations of ophd and HCl. In each case the HCl was added last, the solutions were placed on a steam bath for 15 min with occasional swirling. After 15 min 1 cm3 aliquots were taken and placed directly into 20 ml volumetric ¯asks, these were then diluted to the mark and their spectra
3.1. 4-CBA ¯uorescence: solvent dependence
J. Daniels et al. / Polymer Degradation and Stability 65 (1999) 347±353
peak unlike that observed in methanol. It is well documented using methods such as infra-red spectroscopy [5] and UV/Visible spectroscopy [6] that aldehydes do not exist purely as the free aldehydes in alcoholic media, but as acetals and hemi-acetals. These emissions are, therefore, attributed to the presence of the acetal and hemi-acetal forms of 4-CBA with the corresponding alcohols. The smaller change for propan-2-ol solutions upon acid addition and dominance of the longer wavelength species could be attributed to the secondary nature of the alcohol and its more acidic character compared to the primary alcohols ethanol and methanol. However, the emissions
Fig. 2. Fluorescence emission spectra of a 4-CBA solution in methanol (0.45% w/w) without acid (EX=340 nm) (ÐÐ) (EX=388 nm) (- - - - - -) and with acid (EX=340 nm) (x-x-x-), (EX=388 nm) (- . - . - . -).
Fig. 3. Fluorescence emission spectra of a 4-CBA solution in propan2-ol (0.45% w/w) without acid (EX=340 nm) (ÐÐ) (EX=388 nm) (- - - - - -) and with acid (EX=340 nm) (x-x-x-), (EX=388 nm) (- . - . - . -).
349
observed from the solutions of 4-CBA in alcoholic solvents are too weak to be of use in determining 4-CBA concentrations in TA samples. Increased emission was observed from solutions of 4CBA in sulphuric acid, with only one emission band at 460 nm. This ¯uorescence emission is also observed from solutions of CTA in sulphuric acid. Two emission bands are observed for TA samples in sulphuric acid, the shorter wavelength band at 390 nm is associated with the presence of TA and the second band at 460 nm with the presence of 4-CBA. Further evidence to suggest that this longer wavelength emission from CTA solutions is due to the presence of 4-CBA impurities originates from the fact that there is a reduction in this emission for TA samples that have been previously reduced using sodium borohydride (Fig. 4). The species responsible for the emission from solutions of 4-CBA in sulphuric acid was suggested to be the protonated carbonyl group of 4-CBA. Rusakowicz et al. [7] have suggested that aromatic carbonyl compounds can be protonated in strong acids and that these protonated species give rise to the emissions. However, from Fig. 5 it can be seen that there is very little ¯uorescence emission from 4-CBA solutions using other strong acids such as phosphoric acid. There are also inconsistencies in the excitation spectra of the carbonyl compounds. For example, the excitation spectrum of 4-CBA in sulphuric acid shows a blue shift with decreasing concentration (Fig. 6) and also with decreasing sulphuric acid strength (Fig. 7). Campbell and Edward [8] using UV/Visible studies on the ionization of ketones in various concentrations of sulphuric acid observed a blue shift with increasing acid strength. This observation was explained by considering the protonation of a carbonyl group as an acid-base reaction in which the theoretical extreme is complete protonation. The species formed by the interaction between the carbonyl group and the acid will have a hydration shell
Fig. 4. Fluorescence emission spectra (EX=380 nm) of a CTA solution (1% w/v) in sulphuric acid before reduction (ÐÐ) and after sodium borohydride reduction (- - - - - - -).
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associated with it. As the strength of the sulphuric acid is increased the activity of the water in the hydration shell will decrease and there will be a corresponding increase in the strength of the hydrogen bond between the carbonyl group and the proton. This increase in the strength of hydrogen bond causes the blue shift observed. Using a similar argument it would be expected that a blue shift would be observed as the concentration of 4CBA in sulphuric acid is increased. However, as has been stated the observations for 4-CBA are the opposite of those observed by Campbell and Edwards. Thus, the observations for 4-CBA in sulphuric acid need to be
explained by another theory. This is achieved by assuming that there is a dimer formed between two 4CBA molecules in concentrated sulphuric acid that absorbs radiation at longer wavelengths. Possible intermolecular associate structures are shown in Fig. 8. Upon addition of water to the acid or lowering the 4-CBA concentration the formation of these aggregates becomes less favourable and the radiation absorbed shifts to shorter wavelengths. Whilst the intensity of the ¯uorescence is correspondingly reduced the wavelength of radiation emitted does not shift with varying 4-CBA concentration or sulphuric acid strength. A similar blue shift with decreasing concentration has been observed in this study for solutions of acetophenone (Fig. 9) and dimethylterephthalate (DMT) in sulphuric acid. This suggests that the dimer formed must be of the sandwich type as they can not take part in head-tail type dimerisation. Such extended conjugation in the aggregates between the two aromatic rings would eectively give rise to an emissive lowest excited singlet state.
Fig. 5. Fluorescence emission spectra (EX=380 nm) for solutions of 4-CBA (510ÿ3% w/v) in sulphuric acid (ÐÐ) and phosphoric acid (- - - - - -).
Fig. 7. Fluorescence excitation spectra (EM=450 nm) for solutions of 4-CBA (0.05% w/v) in 98% H2SO4 (ÐÐ), 74% H2SO4 (- - - - - - -), 49% H2SO4 (x-x-x-), 25% H2SO4 (- . - . - . -).
Fig. 6. Fluorescence excitation spectra (EM=450 nm) for solutions of 4-CBA in sulphuric acid 0.2% (w/v) (ÐÐ), 0.05% (w/v) (- - - - -), 0.01% (w/v) (x-x-x), 0.002% (w/v) (- . - . - . -).
Fig. 8.
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3.2. Fluorescence from 4-CBA condensation products The emission from solutions of 4-CBA in organic solvents and sulphuric acid does not lend itself to a determination of 4-CBA concentration in TA due to the complexities of the emission spectra and nature of the associated species. However, the condensation of 4CBA with o-phd produces a Schi Base product that can be determined at sensitivities <5 ppm. The outline of this reaction is shown in Scheme 1. Many amines (Fig. 1) were condensed with 4-CBA but only o-phd produced a product whose emission was not interferred with by the emission from the reactants. The products formed during the condensation reaction have the possibility of being highly ¯uorescent due to the -CHNlinkage allowing conjugation between the two aromatic rings and giving rise to an emissive lowest excited singlet state. The emissions from the Schi bases formed between 4-CBA and the larger amines all overlapped with the emission from the unreacted amines. This would lead to an unnecessary interference in an analytical method. Fig. 10. Structure and ¯uorescence emission spectra (EX=325 nm) of 4-CBA/o-phd complex in DMF 9210ÿ6M.
Fig. 9. Fluorescence excitation spectra (EM=450 nm) for solutions of acetophenone in sulphuric acid 1.610ÿ2M (ÐÐ), 1.610ÿ3M (- - - - - - -), 1.610ÿ4M (x-x-x-x), 1.610ÿ5M (- . - . - . -).
Scheme 1. Schematic of the schi base reaction between 4CBA and Aniline
Fig. 11. Calibration graph of ¯uorescence intensity at 400 nm of the condensation product of 4-CBA with o-phenylenediamine.
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Table 1 Results for determination of 4-CBA samples of known 4-CBA content CTA (0.14% 4CBA)a MASS CTA (g)
Intensity
[4-CBA], react (M)
[CTA,M]
CTA (%)
0.011 0.010 0.011 0.1 0.099 0.104
34.6 31.0 32.8 279.9 274.3 295.3
1.6610ÿ6 1.4810ÿ6 1.56710ÿ6 1.910ÿ5 1.3610ÿ5 1.4710ÿ5
1.3010ÿ3 1.2310ÿ3 1.3010ÿ3 1.2010ÿ2 1.1910ÿ2 1.2510ÿ2
0.13 0.12 0.12 0.12 0.12 0.12
64.5 59.7 53.8 392.5 392.9 388.5
3.1510ÿ6 2.9110ÿ6 2.6110ÿ6 1.9610ÿ5 1.9610ÿ5 1.9210ÿ5
1.3310ÿ3 1.2910ÿ3 1.15710ÿ3 1.2010ÿ2 1.2210ÿ2 1.2010ÿ2
0.24 0.23 0.23 0.16 0.16 0.16
CTA (0.24% 4CBA)b 0.011 0.011 0.01 0.100 0.102 0.100 a b
0.01 g; s=4.010ÿ3, CV=3.3%; 0.1 g; s=1.010ÿ3, CV=0.86%. 0.01 g; s=6.910ÿ3, CV=3.0%; 0.1 g; s=16010ÿ3, CV=0.95%.
Of the phenylenediamines used, only the ortho compound resulted in the formation of a ¯uorescent product. The structure of the product between 4-CBA and o-phd is shown in Fig. 10. This shows the possibility of hydrogen bonding between the unreacted amine group and the nitrogen of the -CHN- linkage. This hydrogen bond imparts rigidity to the product that is not possible in the non-¯uorescent products formed between 4-CBA and p-phd and m-phd. This rigidity also increases the probability of ¯uorescence over other non-radiative deactivation processes. The emission from the 4-CBA/o-phd product in N,Ndimethylformamide (DMF) is at a long enough wavelength, 395 nm (Fig. 10), not to be re-absorbed by any TA, lmax 261 nm, that would be present in the matrix in which the 4-CBA is being determined. The 4-CBA/o-phd product emits only very weakly in acidic solutions, however, the reaction requires an acid catalyst. To overcome this problem the 1 cm3 aliquot was taken and diluted to 20 cm3, this had the eect of diluting the acid and of reducing the temperature of the solution. The calibration graph used to determine 4-CBA concentrations in CTA samples is shown in Fig. 11. Linear regression of the data points plotted gives a linear equation of y 4 108 x 1:4721, with an R2 value of 0.995. Values of 4-CBA concentration were determined using the method described above for CTA samples with 4-CBA concentrations previously determined using a HPLC method. These values are shown in Table 1. It should be noted that the values for 4-CBA concentrations
have been corrected for the 1 cm3 aliquot diluted to 20 cm3 after the reaction. The values obtained when only 0.01 g TA was used in the reaction are closer to the values accepted as being present in the CTA samples. This is to be expected as the high levels of TA present in the reaction mixture will slow the rate of reaction between the 4-CBA and the ophd during the 15 min allowed for the reaction. However, the values obtained for the larger, 0.1 g, samples have much smaller values for the coecient of variation (CV). Thus, a compromise needs to be made between the size of sample used and the precision of the results. The emission obtained from poly(ethylene terephthalate) (EX=395 nm) was found to match very well the emission from a 0.1% solution of 4-CBA in TFA, although the emission from 4-CBA in TFA is very weak. The implications of this contamination will be the subject of further studies on photooxidation. 4. Conclusions It has been demonstrated that the emission from 4-CBA is very dependent upon the nature of the solvent and that in sulphuric acid ground state dimers and aggregates are formed which give rise to a blue shift with decreasing 4CBA content and decreasing acid strength. It is suggested that the structure of this dimer is predominantly of the sandwich type. A method involving the condensation of 4CBA with o-phd allows the determination of 4-CBA in CTA samples. At low concentrations sensitivities <5 ppm are possible by ¯uorescence analysis.
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