Efficient Polymerization Inhibition Systems for Acrylic Acid Distillation

Feb 10, 2012 - Because all tubes were filled from the same stock solution of AA and inhibitor, the significant difference observed for the gelation ti...
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Efficient Polymerization Inhibition Systems for Acrylic Acid Distillation: New Liquid-Phase Inhibitors Jaroslav Mosnácě k,†,‡ Renaud Nicolay,̈ †,§ Kishore K. Kar,⊥ Stanley O. Fruchey,⊥ Michael D. Cloeter,⊥ Richard S. Harner,⊥ and Krzysztof Matyjaszewski*,† †

Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States Polymer Institute, Centre of Excellence GLYCOMED, Slovak Academy of Sciences, Dúbravská cesta 9, 845 41 Bratislava, Slovakia § Matière Molle et Chimie, ESPCI-CNRS (UMR 7167), 10 rue Vauquelin, 75005 Paris, France ⊥ The Dow Chemical Company, Midland, Michigan 48674, United States ‡

S Supporting Information *

ABSTRACT: Phenothiazine and other compounds with similar structures, namely, phenoxazine, promazine, promazine hydrochloride, N,N′-dimethylphenazine, carbazole, N-ethylcarbazole, N-benzylphenothiazine, and N-(1-phenylethyl)phenothiazine were tested as liquid-phase inhibitors for acrylic acid. N-Alkylated phenothiazine (PTZ) derivatives, such as Nbenzylphenothiazine and especially N-(1-phenylethyl)phenothiazine, showed improved efficiency for liquid-phase inhibition, in comparison with PTZ. It was also shown that N-alkylated PTZ derivatives could be used in combination with nitroso compounds that can be employed as vapor-phase inhibitors. The combination of N-(1-phenylethyl)phenothiazine and nitrosobenzene showed superior liquid-phase-inhibition efficiency, with time to gelation of >122 h, in comparison with 64 h obtained with PTZ alone. The structure of liquid-phase inhibitors had no influence on the extent of Michael addition reactions during heating of acrylic acid.

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INTRODUCTION An effective polymerization inhibitor is necessary in order to ensure the safe and efficient operation of manufacturing plants and the safe storage of acrylic monomers. Different radical traps have been used as inhibitors. Hydroquinone monomethyl ether (MeHQ), p-benzoquinone, and phenothiazine (PTZ) are examples of commercial inhibitors/stabilizers.1−4 However, there are currently few truly efficient approaches to stabilizing acrylic acid (AA) and preventing its polymerization especially at the higher temperatures applied during distillation. Therefore, the development of effective liquid- and vapor-phase-inhibition systems for ethylenically unsaturated monomers is of significant industrial interest. Indeed, chemical companies are facing some long-term maintenance problems at plants distilling very reactive monomers, such as AA. The aim of this work was to develop an inhibition system that would allow longer operation time. PTZ, which can act in the presence or in the absence of air, is among the most efficient commercially used liquid-phase inhibitors. PTZ acts as a common amine radical trap in aprotic nonpolar media and has an efficiency of two radicals trapped per molecule of PTZ.5 However, in an AA environment, PTZ works via a catalytic process. Monomeric or polymeric carboncentered radicals are reduced by single-electron transfer to form the corresponding carbanion and a PTZ N-radical cation.6 Protonated PTZ is then generated by abstracting a hydrogen atom from another monomeric or polymeric radical, thereby forming a C−C double bond. PTZ is finally regenerated by an acid−base reaction between the protonated PTZ and a monomeric or polymeric carboxylate (Scheme 1). Therefore, © 2012 American Chemical Society

Scheme 1. Inhibition Mechanism of PTZ in an AA Environment

in such a system, the consumption of PTZ is only due to side reactions, such as oxidation (Figure 1).3,6 There are several reports in which the reactivity of PTZ, its derivatives, and other antioxidants toward radicals was studied. However, these studies were primarily based on the bond dissociation energies of N−H or O−H bonds.5,7−9 Only a few reports correlated the radical trapping efficiency, or inhibition efficiency, with inhibitor redox potentials. Lucarini et al. described higher oxidation potentials for N-methylated PTZ Received: Revised: Accepted: Published: 3910

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H NMR (CDCl3): δ 7.15−7.20 (m, 4H, Ar), 6.90−6.95 (m, 4H, Ar), 3.95 (t, 2H, NCH2−), 2.43 (t, 2H, −CH2N(CH3)2), 2.23 (s, 6H, −N(CH3)2), 1.98 (m, 2H, −CH2CH2N(CH3)2). Synthesis of N-Benzylphenothiazine (BPTZ, 7). A solution of PTZ (2 g; 0.01 mol) in 10 mL of dry THF was added at RT under a nitrogen atmosphere to a suspension of sodium hydride (1.0 g; 0.042 mol; obtained from 80% suspension of NaH in mineral oil after washing with n-hexane) in 20 mL of dry THF. The mixture was stirred for 6 h, when no more H2 was being released and the color of mixture had changed to orange. Then 1.8 mL (0.013 mol) of benzyl bromide was added slowly at RT, and the mixture was stirred overnight and finally heated to 60 °C for 1 h. The disappearance of the orange color confirmed that the PTZ base had reacted with the alkyl bromide. The reaction mixture was poured into ice−water acidified with HCl (pH = 1), the crude product was extracted with ethyl acetate and dried over the MgSO4, and the solvent was removed under vacuum at 25− 30 °C. The product was purified by flash-column chromatography on neutral alumina using a mixture of n-hexane/ethyl acetate (50:1) as the eluent. Recrystallization from ethanol yielded white crystals (∼2 g; 60%). The structure and purity of the product were confirmed by 1H NMR. 1 H NMR (CDCl3): δ 7.25−7.40 (m, 5H, ArCH2−), 7.11 (d, 2H, J = 7.5 Hz, Ar-PTZ), 7.00 (t, 2H, J = 8.0 Hz, Ar-PTZ), 6.88 (t, 2H, J = 7.5 Hz, Ar-PTZ), 6.67 (d, 2H, J = 8.0 Hz, ArPTZ), 5.12 (s, 2H, ArCH2N). Synthesis of N-(1-Phenylethyl)phenothiazine (PEPTZ, 8). The synthesis and purification were performed following the procedure described for the preparation of 7. Evaporation of the solvent gave a sticky white solid in 65% yield. The structure and purity of the product were confirmed by 1H NMR. 1 H NMR (CDCl3): δ 7.30−7.45 (m, 5H, ArCH−), 7.14 (d, 2H, J = 7.5 Hz, Ar-PTZ), 6.85−7.05 (m, 4H, Ar-PTZ), 6.74 (d, 2H, J = 7.0 Hz, Ar-PTZ), 5.42 (q, 1H, ArCHCH3), 1.98 (d, 3H, J = 7.0 Hz, CHCH3). Synthesis of N-(Diphenylmethyl)phenothiazine (9). The synthesis was performed in the same manner as that described for 7. Purification by flash-column chromatography on neutral alumina gave three fractions. Precipitation from hexane and/or ethanol was unsuccessful. Dissolving in various solvents led to a change of color to dark red or dark green depending on the type of solvent. 1H NMR spectra measured from all fractions from both column chromatography and crystallization were similar. They were very messy and did not contain a significant amount of the product. Testing Experiments and Analysis. Liquid-phase tests of the single-liquid-phase inhibitors were carried out at 113 °C with an inhibitor concentration of 100 ppm. The inhibitor mixtures tests were conducted with 100 ppm of the liquidphase inhibitor, 100 ppm of nitrosobenzene, and 10 ppm of manganese acetate tetrahydrate. Visual observation of gel formation, supported by 1H NMR analysis, was used to evaluate the inhibition time. The experiments were carried out either in open DOT tubes (glass dilatometer tubes) or in 10 mL tubes sealed with a poly(tetrafluoroethylene) cap, parafilm, and electric tape. Samples required to study changes in the AA composition with the heating time were only taken from the open DOT tubes. Changes in the AA composition were followed by 1H NMR on a 300 MHz Bruker NMR spectrometer using deuterated dimethyl sulfoxide as the solvent. 1

Figure 1. PTZ oxidative byproduct.

derivatives in comparison with nonalkylated PTZ derivatives.5 Gutierrez-Correa studied the antioxidant efficiency of various PTZ derivatives for Trypanosoma cruzi LADH (T. cruzi LADH) inactivation.10 The inactivation of T. cruzi LADH depends on the rate of production of PTZ radical cations. The production of PTZ radical cations was correlated with the electron donor ability of the substrates and of the alkylated PTZ. Promazine (6), an alkylated derivative of PTZ, was found to be the most effective inactivating agent. Because the amine moiety present in PTZ plays a central role in the inhibition mechanism, several compounds with structures similar to PTZ were investigated as liquid-phase inhibitors for AA at high temperature.

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EXPERIMENTAL PART

Materials. Phenothiazine (PTZ), phenoxazine, promazine hydrochloride, N,N′-dimethylphenazine, carbazole, N-ethylcarbazole, and all reagents were purchased from Aldrich and used as received. Tetrahydrofuran (THF) was purchased from Aldrich and was dried prior to use by refluxing over sodium/ benzophenone until a dark blue color appeared. Acrylic acid (AA), purchased from Aldrich, was purified in order to remove the inhibitor. AA Purification. The inhibitor was removed from commercial AA by conducting two distillations as follows: 1 L of AA with 500 ppm of PTZ in a 2 L flask was heated to 53−55 °C. Distilation was performed under a pressure of 5−7 Torr using a condenser cooled by water at 6−10 °C. About 600 mL of distilled AA was collected. The second distillation was carried out under the same conditions, collecting about 750 mL from 1.2 L of AA. Two successive distillations were performed to reduce the concentration of the inhibitor present in the commercially available AA below 1 ppm to eliminate a potential influence on the inhibition efficiency of studied inhibitors. The purified AA was subsequently stored at −18 °C in 20 mL vials. Synthesis of 6. Promazine hydrochloride (5; 2 g, 6.2 × 10−3 mol) in 100 mL of diethyl ether was added to a 5 wt % solution of sodium bicarbonate. The mixture was stirred for 10 min at room temperature (RT), and then the product was extracted with 100 mL of diethyl ether. The organic layer was washed with water and dried over anhydrous sodium sulfate. After evaporation of the solvent, 1.5 g of slightly yellowish oil was obtained as a pure product. The purity and structure of the product were confirmed by 1H NMR analysis. 3911

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Article

RESULTS AND DISCUSSION Evaluation of Various PTZ Analogues. PTZ, a commercially used radical stabilizer, was included in the series of tests as a reference. Because of the limited solubility of PTZ in AA at RT, the compound was dissolved in the reaction medium at 50 °C. Preliminary experiments, using PTZ as the inhibitor, were performed simultaneously at 113 °C in open DOT tubes and in sealed tubes. The open DOT tubes were used for continuous analysis of changes in the composition of AA by 1H NMR. These experiments showed that when polymerization of AA started, the gel formed immediately in both the open and sealed systems. This observation allowed us to perform the inhibition studies only in sealed tubes by visual evaluation, i.e., determination of the time for gel formation. The standard tests for all inhibitors were, therefore, performed at 113 °C in 10 mL sealed tubes containing 5 mL of AA and 100 ppm of inhibitor. During the test, the color of the solutions containing PTZ slowly changed from colorless to pink and finally dark red (see Figure S1 in the Supporting Information). This change in color reflects the slow oxidation of PTZ. In two tubes, gelation occurred after only 48 and 58 h, while in the remaining three tubes, gel was formed after 69, 72, and 73 h of heating at 113 °C, giving an average time to gel formation of 64 ± 10 h (Table 1). Because all tubes were filled from the

although new tubes were used for each experiment. For comparison, in the preliminary experiments, gel formation in a sealed tube was observed after 61 h. Eight compounds presenting structures similar to that of PTZ were tested as liquid-phase inhibitors for AA during heating. The structures of the molecules are shown in Figure 2. Phenoxazine (1), which has a structure similar to that of PTZ with an atom of oxygen instead of the atom of sulfur, was tested in 10 mL sealed tubes under the same conditions as those used for PTZ. Because the solubility of 1 is higher than that of PTZ, 1 could be readily dissolved at RT. During the test, the color changed from colorless to yellow-brown and finally dark redbrown, once again reflecting the gradual oxidation of the inhibitor 1. The inhibition efficiency of 1 was only half that of PTZ, with an average time to gelation of 32.8 ± 1.3 h (Table 1). N,N′-Dimethylphenazine (2) has two nitrogen atoms linked to two aromatic rings and is known to exhibit two successive reversible, one-electron oxidation−reduction steps. Therefore, improved inhibition efficiency could be expected when compared to PTZ. The solubility of 2 is slightly lower than that of 1. However, it completely dissolved in AA after a couple of minutes at 30 °C, yielding a yellow solution that became green after a short time at RT. The color of the solution continued to change during the test to dark red-brown. Although improved inhibition efficiency could be expected for 2 because of the presence of two nitrogen atoms, a gel was formed in all five sealed tubes after 20 h of heating at 113 °C (Table 1). Both carbazole (3) and its alkylated derivative N-ethylcarbazole (4) were found to have no inhibition efficiency, and solid polymer was observed in all sealed vials after 1.5 h (Table 1). The use of N-alkylated PTZ derivatives as liquid-phase inhibitors could bring some improvement compared to PTZ because it might, partially, prevent consumption of PTZ by side reactions, such as oxidation (Figure 1).6 5 was found to be highly soluble in AA and dissolved instantaneously in AA at RT. During the test, the color of the solution changed from colorless to pink and finally red-brown. Because an increase in the oxidation potential for N-alkylated PTZ derivatives compared to PTZ is described in the literature, it was expected that the alkylation should not negatively affect the single-electrontransfer inhibition mechanism of PTZ. Nevertheless, an average time to gelation of 36.6 ± 2.7 h was obtained (Table 1). To assess if the presence of hydrochloride could affect the inhibition efficiency of 5, its dehydrochloration was carried out. The dehydrochloration was performed by mixing 5 with sodium bicarbonate in a water/diethyl ether mixture (Scheme 2). 6 was

Table 1. Inhibition of AA with 100 ppm of Inhibitor at 113 °C time to gelation (h) for vial no. inhibitor

1

2

3

4

5

average time to gelation (h)

PTZ 1 2 3 4 5 6 7 8

48 31 20 1.5 1.5 33 37 81 94

58 32 20 1.5 1.5 35 40 81 98

69 33 20 1.5 1.5 37 40 81 98

72 34 20 1.5 1.5 38 43 88 99

73 34 20 1.5 1.5 40 43 97 >94a

64 ± 10.7 32.8 ± 1.3 20 1.5 1.5 36.6 ± 2.7 40.6 ± 2.5 85.6 ± 7.1 96.6 ± 2.4

a

the vial was opened after 94 h, before gelation occurred, in order to analyze the composition of the solution by 1H NMR.

same stock solution of AA and inhibitor, the significant difference observed for the gelation time of two samples might be explained by the presence of impurities in the tubes,

Figure 2. Structures of compounds tested as liquid-phase inhibitors for AA. 3912

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Hence, we did not pursue our effort to synthesize and test this compound as a liquid-phase inhibitor for AA distillation. 7 and 8 were subsequently tested as a liquid-phase inhibitor for AA distillation, using the same experimental procedure as that described for the previous compounds. When 7, was used as a radical polymerization inhibitor, gelation was observed after 81 h in three sealed tubes, 88 h in the fourth, and 97 h in the last tube (Table 1). Thus, the average time to gelation was 85.6 h with 100 ppm of 7 as the liquid-phase inhibitor. When 8 was used as a radical polymerization inhibitor, gelation occurred after 94−100 h of heating at 113 °C. The average time to gelation with PEPTZ was 96.3 h (Table 1). Therefore, both 7 and 8 showed significantly higher inhibition efficiency than the commercially used PTZ. This corresponds to an improvement of 33.8 and 50.5%, respectively, compared to the average time to gelation of 64 h observed with PTZ. These results are in accordance with the higher oxidation potentials reported for N-alkylated PTZ derivatives, as mentioned in the introduction.5 Moreover, both N-alkylated PTZ derivatives were soluble in AA at RT; this is in contrast to the commercially available PTZ, which requires an elevation of the temperature to be completely and rapidly soluble in AA. Michael Addition. It is well-known that AA forms dimers through Michael addition reactions. In the preliminary experiments with PTZ, we observed, by 1H NMR, the gradual formation of dimers and even trimers of AA (Figure 3). Michael addition can be catalyzed by both acidic and basic catalysts. Therefore, we were interested in evaluating the potential effect of inhibitors with various basic properties on dimer and trimer formation. As seen from Figure 4, the type of inhibitor had no significant influence on the monomer, dimer, and trimer composition, as observed by 1H NMR analysis. The amount of Michael addition products increased with time at a similar rate in the presence of all tested inhibitors. Thus, Michael addition seems to be mainly dependent on the time of heating, and it can be concluded that in our systems it is predominately thermodynamically controlled. Evaluation of the Combination of Inhibitors. Industrialscale equipments, such as distillation towers for vinyl monomers in chemical plants, require inhibition systems that are effective not only in the liquid phase but throughout the plant. A combination of liquid-phase and volatile inhibitors, with the latter being able to migrate with the vapors of the distilled monomer to inhibit polymerization in the column trays, is required for a significant improvement in inhibition packages. However, such a combination of inhibitors can have an antagonistic inhibition effect, for example, because of side reactions between the inhibitors. Recently, we found nitrosobenzene to be a very effective inhibitor in the vapor phase during AA distillation.11 On the other hand, it was also shown that the inhibition efficiency in both the liquid and vapor phases was poor when nitrosobenzene was used in combination with some standard liquid inhibitors, such as PTZ. This decrease in the inhibition efficiency was caused by side reactions between PTZ and nitroxyl radicals, formed in situ from nitrosobenzene, leading to the formation of hydroxylamine and green oxidation products from the reaction with PTZ.11 N-Alkylation of PTZ could prevent this side reaction. Therefore, the new N-alkylated PTZ derivatives were also studied in combination with nitrosobenzene in order to test the liquid-phase-inhibition efficiency of such mixtures. A mixture of 100 ppm of 8 with 100 ppm of nitrosobenzene and 10 ppm of manganese acetate tetrahydrate was tested under the same conditions as those used for the

Scheme 2. Dehydrochloration of 5

obtained in quantitative yield as a slightly yellowish oil (melting point of 6 < 25 °C). Since 1H NMR analysis of the prepared 6 confirmed its purity, it was subsequently used for inhibition tests. The inhibition efficiency of 6, with average times to gelation of 40.6 h (Table 1), was quite similar to that of 5. Therefore, the presence of hydrochloride in structure 5 has no or minimal effect on its inhibition efficiency. One can assume that protonation of 6 in acidic media, such as AA, will happen anyway. We further synthesized a series of another N-alkylated PTZ derivatives, 7−9, and evaluated them as liquid-phase inhibitors for AA. 7−9 were synthesized in order to assess the inhibition efficiency of other N-alkylated PTZ derivatives (Scheme 3 and Scheme 3. Synthesis of N-Alkylated PTZ Derivatives 7−9

Figure 2). All three compounds were prepared following a similar procedure. The reactions were carried out in dry THF using NaH as a strong base to abstract the hydrogen atom from the amino group of PTZ. The so-formed nitrogen-centered anion can subsequently attack the carbon bearing the bromine atom of benzyl bromide, 1-phenylethyl bromide, or diphenylbromomethane, to yield compounds 7−9, respectively. 7 was purified by column chromatography on neutral alumina followed by recrystallization from ethanol to give white crystals with a yield of 60%. 8 was purified following a similar procedure but could not be recrystallized from ethanol. Only an oily solid was formed during crystallization. The failure to obtain crystals might be caused by the presence of a mixture of two enantiomers, each of which acts as an impurity for the other. No reference describing the synthesis or the properties of 8 was found in the literature. Nevertheless, the structure and purity of the product were confirmed by 1H NMR. The purification of 9 was challenging. A change of color to green or dark red-violet was observed depending on the solvent used for purification. After purification either by flash chromatography on an alumina column with subsequent solvent evaporation or by precipitation from ethanol, white to red solids or oils were obtained. However, 1H NMR of all fractions appeared to be similar. They were very poorly resolved, and no clear signal of the CH group of diphenylmethyl linked to the nitrogen of PTZ could be observed. We presume that 9 is significantly less stable than the other prepared PTZ derivatives. 3913

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Figure 3. 1H NMR spectrum of AA after 31 h of heating at 113 °C in the presence of 100 ppm of PTZ.

Table 2. Liquid-Phase Inhibition of AA with a Combination of Liquid- and Vapor-Phase Inhibitors at 113 °C inhibition system 100 ppm of 8 + 100 ppm of nitrosobenzene +10 ppm of MnIIAc2·4H2O

100 ppm of TEMPOL + 100 ppm of nitrosobenzene + 10 ppm of MnIIAc2·4H2O

vial no.

time to gelation (h)

1 2 3 4 5 1 2 3 4 5

122a 180a 245a 640 >400b 105 109 111 125 131

average time to gelation (h) >122

116 ± 11

a

Poly(acrylic acid) was formed on the walls above the solution before gelation was observed. bThe vial was opened before gelation was observed to check the composition by 1H NMR.

Figure 4. Change in the content of AA, AA dimer, and AA trimer with the time of heating at 113 °C because of Michael addition for various inhibitors: PTZ, 1, 2, 5, and 8.

As seen in Table 2, the time to gelation for this combination of inhibitors is extremely high compared to times previously observed. It is worth mentioning that during these tests some poly(acrylic acid) was formed on the walls above the solution for tubes 1−3, when heated over 100 h, even though no gel was observed yet in the liquid phase. The observed polymerization

other liquid-phase tests (Table 2). Manganese(II) compounds can be used in combination with nitroxides as liquid-phaseinhibition systems. Their role in such systems is to regenerate nitroxides from hydroxylamines.12 3914

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in the condensed vapor phase could explain the significant discrepancies between the tested tubes. In the remaining two tubes, where no polymerization was observed above the solution, no gel was formed even after 400 h of heating. Although no gel had formed, one of these tubes was opened, and the composition was checked by 1H NMR. The results confirmed the absence of polymer in the solution. The time to gelation at 113 °C was greater than 122 h. This is significantly higher than the previous largest value of 96.3 h obtained with 8 alone. These results indicate that no side reactions affecting the inhibition efficiency occurred between 8 and nitrosobenzene or the in situ formed nitroxides. Another commercial liquid-phase stabilizer, namely, 4-hydroxy2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL), was also used instead of PTZ derivative 8. A mixture of 100 ppm of TEMPOL, 100 ppm of nitrosobenzene, and 10 ppm of manganese acetate tetrahydrate was tested in the liquid phase at 113 °C under the same conditions as those used for the previous tests (Table 2). It is worth mentioning that this combination was found to be an efficient vapor-phase-inhibition system for AA distillation.11 As shown in Table 2, the average time to gelation for this combination of inhibitors was 116 h in the liquid-phase tests. This is lower than the 122 h obtained with 8. Again, some poly(acrylic acid) was observed on the walls above the solution for tubes 1−3 shortly after 100 h of heating, although no gel was observed in the liquid phase.

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CONCLUSIONS

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ASSOCIATED CONTENT

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REFERENCES

(1) Cutie, S. S.; Henton, D. E.; Powell, C.; Reim, R. E.; Smith, P. B.; Staples, T. L. The effects of MEHQ on the polymerization of acrylic acid in the preparation of superabsorbent gels. J. Appl. Polym. Sci. 1997, 64 (3), 577−589. (2) Becker, H.; Vogel, H. The role of hydroquinone monomethyl ether in the stabilization of acrylic acid. Chem. Eng. Technol. 2006, 29 (10), 1227−1231. (3) Becker, H.; Vogel, H. Phenothiazine as stabilizer for acrylic acid. Chem. Eng. Technol. 2006, 29 (8), 931−936. (4) Li, R. J.; Schork, F. J. Modeling of the inhibition mechanism of acrylic acid polymerization. Ind. Eng. Chem. Res. 2006, 45 (9), 3001− 3008. (5) Lucarini, M.; Pedrielli, P.; Pedulli, G. F.; Valgimigli, L.; Gigmes, D.; Tordo, P. Bond Dissociation Energies of the N−H Bond and Rate Constants for the Reaction with Alkyl, Alkoxyl, and Peroxyl Radicals of Phenothiazines and Related Compounds. J. Am. Chem. Soc. 1999, 121 (49), 11546−11553. (6) Levy, L. B. Inhibition of acrylic acid polymerization by phenothiazine and p-methoxyphenol. II. Catalytic inhibition by phenothiazine. J. Polym. Sci., Part A: Polym. Chem. 1992, 30 (4), 569−576. (7) Amorati, R.; Lucarini, M.; Mugnaini, V.; Pedulli, G. F.; Minisci, F.; Recupero, F.; Fontana, F.; Astolfi, P.; Greci, L. Hydroxylamines as Oxidation Catalysts: Thermochemical and Kinetic Studies. J. Org. Chem. 2003, 68 (5), 1747−1754. (8) Kumar, S.; Engman, L.; Valgimigli, L.; Amorati, R.; Fumo, M. G.; Pedulli, G. F. Antioxidant Profile of Ethoxyquin and Some of Its S, Se, and Te Analogues. J. Org. Chem. 2007, 72 (16), 6046−6055. (9) Li, M.-J.; Liu, L.; Fu, Y.; Guo, Q.-X. Accurate bond dissociation enthalpies of popular antioxidants predicted by the ONIOM-G3B3 method. THEOCHEM 2007, 815 (1−3), 1−9. (10) Gutierrez-Correa, J. Trypanosoma cruzi dihydrolipoamide dehydrogenase as target for phenothiazine cationic radicals. Effect of antioxidants. Curr. Drug Targets 2006, 7 (9), 1155−1179. (11) Nicolay, R.; Mosnácě k, J.; Kar, K. K.; Fruchey, S. O.; Cloeter, M. D.; Harner, R. S.; Matyjaszewski, K. Efficient Polymerization Inhibition Systems for Acrylic Acid Distillation: Vapor Phase Inhibitors. Ind. Eng. Chem. Res., submitted for publication. (12) Minisci, F.; Recupero, F.; Cecchetto, A.; Gambarotti, C.; Punta, C.; Faletti, R.; Paganelli, R.; Pedulli, G. F. Mechanisms of the aerobic oxidation of alcohols to aldehydes and ketones, catalysed under mild conditions by persistent and non-persistent nitroxyl radicals and transition metal saltsPolar, enthalpic, and captodative effects. Eur. J. Org. Chem. 2004, 1, 109−119.

New efficient liquid-phase inhibitors for AA were developed using simple N-alkylation of the commercially used PTZ. The most efficient inhibitor, 8, stabilized AA during heating at 113 °C approximately 50% longer than PTZ. The average time to gel formation with 8 was about 96 h in comparison to 64 h observed for PTZ in the tests. Another advantage of 8 over PTZ is that this compound is readily soluble in AA at RT, unlike standard PTZ. An additional significant advantage of N-alkylated PTZ derivatives is the fact that they can be combined with vaporphase inhibitors based on nitroso compounds because they do not undergo side reactions with nitroxides, formed in situ from the nitroso compounds. A combination of 100 ppm of 8 with 100 ppm of nitrosobenzene and 10 ppm of manganese acetate tetrahydrate yielded a liquid-phase-inhibition system with superior inhibition efficiency and an average time to gelation of more than 122 h.

S Supporting Information *

Images of AA before the start of the test and after 58 h of heating at 113 °C in the presence of 100 ppm of PTZ. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +1 412 268 3209. Fax: +1 412 268 6897. Notes

The authors declare no competing financial interest. 3915

dx.doi.org/10.1021/ie201708n | Ind. Eng. Chem. Res. 2012, 51, 3910−3915