Changes in the Structure of Hydrocarbon Medium

Petroleum Chemistry, Vol. 41, No. 1, 2001, pp. 37–42. Translated from Neftekhimiya, Vol. 41, No. 1, 2001, pp. 41–46. Original Russian Text Copyright © 2001 by Bakunin, Popova, Ovanesova, Kuz’mina, Kharitonov, Parenago. English Translation Copyright © 2001 by MAIK “Nauka /Interperiodica” (Russia).

Changes in the Structure of Hydrocarbon Medium during Liquid-Phase Oxidation V. N. Bakunin*, Z. V. Popova*, E. Yu. Ovanesova*, G. N. Kuz’mina*, V. V. Kharitonov**, and O. P. Parenago* * Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninskii pr. 29, Moscow, 117912 Russia ** Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia Received July 7, 2000

Abstract—The product buildup kinetics for hexadecane oxidation at 170°C with oxygen were investigated, and the influence of the products on variations in the phase composition of the medium subjected to oxidation was studied. Experimental evidence for the formation of entities of the inverse micelle type in as early as the initial steps (20 min) of hexadecane oxidation was first obtained. Polar oxidation products responsible for micellization were isolated from the samples of oxidized hexadecane.

Investigation of the features of high-temperature oxidation of hydrocarbons is closely related to searching for new ways of protecting hydrocarbonaceous materials against oxidative degradation. Jensen et al. [1, 2] who studied the kinetics and the set of products of hexadecane oxidation at 120–180°ë found that not only monofunctional but also bi- and trifunctional compounds including di and trihydroperoxides, as well as compounds bearing different functional groups, for example, ketohydroperoxides, appeared to form in the oxidized hydrocarbon as early as 2–4 min after the beginning of the reaction. It is essential that the detected mono-, bi-, and trifunctional oxygen compounds are sparingly soluble in nonpolar hydrocarbons. Thus, the oxidation of hydrocarbons at high temperatures results in rapid accumulation of polar compounds having a limited solubility in the medium subjected to oxidation.

tal evidence for the formation of a micellar phase upon oxidation of hydrocarbons. EXPERIMENTAL Hexadecane used in the work was purified by the conventional procedure [7]. The oxidation of hexadecane was carried out with oxygen at 170°ë in a bubble reactor equipped with a reflux condenser, which provided return of volatile oxidation products (and, partially, hexadecane) to the reaction volume, and a cold trap for condensation of volatile products at –20°ë. Absorption spectra in the visible and ultraviolet regions were recorded with a UV–VIS Specord M-40 instrument. Hydroperoxides and acids were determined by iodometry and alkalimetry, respectively, and the amount of enols was determined by absorption of a enol coordination compound with iron(III) chloride in the visible part of the spectrum at 460 nm [8]; spectral changes in oxidized hexadecane were monitored at 360 nm. A change in the phase composition of the medium was studied by the solubilization technique using the methyl orange dye with a dye sample of 0.001 g (3 × 10–6 mol), an oxidized hexadecane sample size of 5 ml, and a stirring time with the dye of 3.5 h. The absorption was measured at 420 nm in the visible region. In calculating the dye concentration, the molar absorption coefficient ε = 35300 was used as obtained for a methanolic solution of the dye. Polar oxidation products were isolated from oxidized hexadecane samples with methanol. First, the samples were mixed with methanol in the 1 : 1 ratio (by volume) and then allowed to settle for 24 h; two fractions were isolated, the one soluble in methanol (MS)

At the same time, polar compounds that can undergo further chemical transformations must exhibit surface activity and, hence, the ability to form micelles. In addition, they may take part in the formation of highmolecular-weight products similar to those produced by the oxidation of pentaerythritol esters and mineral oils [3, 4]. Previously, it has been assumed that the micellization mechanism includes the association of oxidation products by hydrogen bonding and subsequent formation of inverse micelles and it is the inverse micelle formation which has to exert a substantial influence on the process of further oxidation of hydrocarbons [5, 6]. The objective of this work was to study the kinetics of buildup of oxidation products and their influence on the change in the phase composition of the medium subjected to oxidation, as well as to obtain experimen37

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and the other insoluble in methanol but soluble in hydrocarbon (HS); the obtained fractions were freed of methanol, and their ability to solubilize the dye was examined. RESULTS AND DISCUSSION The buildup kinetics of acids, hydroperoxides, enols, and total oxidation products (UV absorption at λ = 360 nm) represented in Fig. 1 have a common feature. There is the alternation of a drastic rise and a subsequent fall in the concentration of oxidation products, which reflects the general tendency of reactions to occur in the oscillating mode in the hexadecane oxidation. For example, the first peak is observed in 20–30 min, then follows a short period of self-deceleration (10–20 min), and, after that, the process continues, but at a lower rate. It may be assumed that the first deceleration period is due to the formation of inverse micelle aggregates [5, 6]. Indeed, it is quite likely that the produced polar oxidation products having a surface activity first form associates by hydrogen bonding and then take part in Concentration, mol/l 0.4 (a)

Concentration, mol/l 0.7 0.6

0.3

0.5 0.4

0.2 0.3 0.2

H

0.1

ROOH

0

0

Absorbance 0.7 0.6

Absorbance 4

(b)

3

0.5 0.4

2 Oxidate absorption Diols

0.3 0.2 0.1

0.1

0

20

40 Time, min

60

1 0 80

Fig. 1. (a) The amount of carboxylic acids and (b) the absorption intensity of the oxidate (360 nm) and the FeCl3 complex (460 nm, amount of enols) depending on the hexadecane oxidation time (170°ë, oxygen).

the formation of a microheterogeneous phase of the inverse micelle type. The micelle core consists of polar compounds including short-chain substances sparingly soluble in a nonpolar hydrocarbon medium. The micelle shell can be formed either by compounds having a polar group and a long hydrocarbon radical in one case or by condensed “oligomeric” molecules produced via condensation reactions from hydroperoxides, ketohydroperoxides, alcohols, and enols in the other case. The presence of the microheterogeneous phase was determined by a dye solubilization technique, the method used in colloid chemistry. This method is widely applied, for example, to prove the ability of surface active detergents to solubilize additives that are insoluble in hydrocarbons [9]. The methyl orange dye, as other polar water-soluble dyes, is frequently used as an indicator of solubilization by inverse micelles which are formed in nonpolar hydrocarbon media in the presence of some surfactants [10, 11]. In this case, the methyl orange concentration is easy to measure by UV spectroscopy. A rate curve showing a change in the concentration of the dye solubilized by oxidized hexadecane samples is depicted in Fig. 2. From Fig. 2, we can see that, like other properties, the amount of the solubilized dye increases in a stepwise manner: the dye almost does not dissolve during the first minutes, but, starting from 15 min, its concentration in the oxidized product increases with an increasing oxidation time. As shown by the experimental data (Fig. 1), products that are capable to form micelles appear and accumulate in the time span 20–30 min. The most popular method for proving the formation of micelles (both normal and inverse) is determining the critical micelle concentration using the dependence of some physical parameter of a mixture (or solution) on the concentration of the substance of interest [12]. During hydrocarbon oxidation, the continuous accumulation of different oxygen-containing products (hydroperoxides, alcohols, carboxylic acids, multifunctional compounds, etc.) takes place and the ratio also varies with time. In this connection, it is almost impossible to obtain strict physicochemical evidence for the formation of inverse micelles by the product analysis, nor can the critical micelle concentration be measured. The oxidation time also cannot be a reliable criterion for the extent of oxidation because the concentration of different oxidation products varies in a nonlinear manner. We suppose that the extent of oxidation of hexadecane can be evaluated with sufficient confidence by such an integrated quantity as the refractive 20 index n D of the oxidate. The refractive index of oxidized hexadecane reflects the changes produced in the system and is to a certain degree a net characteristic of the oxidation products. The dependence of the logarithmic concentration of the solubilized dye on the refractive index (Fig. 3) comPETROLEUM CHEMISTRY

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MeOr concentration × 105 mol/l 1.6 1.2 0.8 0.4 0

10

20

30

40 50 60 Oxidation time, min

Fig. 2. The amount of solubilized methyl orange as a function of the hexadecane oxidation time (170°ë, oxygen).

1.4355

nD20 1.4365

1.4375

–4.5

–4.2

1

–4.4

–5.0 –5.5

–4.0

–4.6 2

–6.0

–4.8 –5.0

–6.5

–5.2

–7.0

–5.4

1.435 1.436 1.437 1.438 1.439 1.440 nD20

log[MeOr]

1.4345

log[MeOr]

prises two intersecting straight lines. Figure 3 also shows the dependence of the logarithmic concentration of the solubilized dye on the refractive index of the solution of the typical surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in hexadecane in the AOT concentration range 0.01–0.2 mol/l. AOT is known to form inverse micelles in hydrocarbonaceous solvents at a concentration higher that 0.058 mol/l [13]. As in the case of oxidized hexadecane, the refractive index of the examined mixtures is nonlinearly related to 20 the amount of the solubilized dye; however, the n D – log [ MeOr ] plot yields a distinct break corresponding to a concentration of 0.05 mol/l, which agrees well with the CMC reported for AOT solutions in hydrocarbons (0.05 mol/l) [13]. Comparing the data presented in Fig. 3, we may also conclude that the changes in the methyl orange solubilizing ability in the case of hexadecane samples having different extents of oxidation are also due to the formation of inverse micelles at certain degrees of conversion. A model of oxidized hexadecane prepared by mixing corresponding amounts of carboxylic acids (shortand long-chain), alcohols, and diols showed insignificant solubilization of methyl orange. Thus, it is not the complex composition alone that ensures the required solubilization effect; the presence of self-organizing entities, such as inverse micelles based on the products of severe oxidation of hydrocarbons, seems to be a necessary factor. The position of the methyl orange absorption band in the visible region is known to be strongly pH-dependent (which is the basis for its use as an acid–base indicator) and, moreover, to vary depending on the solvent nature, which is in particular used to study the composition of the polar core of inverse micelles [11]. We also measured the position of the absorption band maximum for methyl orange in different solvents and compared the results with the position of this band for the dye solubilized with oxidized hexadecane. The obtained results are shown in the table. As follows from the presented data, the position of the methyl orange absorption band strongly depends on the solvent nature and seems to be determined either by different solvation of the dye chromophoric groups or by difference in polarity of the solvation environment of a dye molecule. As mentioned above, pure hexadecane does not dissolve methyl orange; so, we failed to detect the dye spectrum (a methyl orange solution in heptane reportedly has an adsorption band at 396 nm [14]). The absorption band maximum in oxidized hexadecane occurs at 420 nm regardless of the extent of oxidation. An AOT-based micelle solution solubilizes methyl orange, yielding the solution with dye absorption peaked at 410 nm. It is obvious that the solvation of methyl orange in inverse micelles formed by an anionic surfactant may differ from the conditions created in oxidized hydrocarbons.

39

–5.6

Fig. 3. Dependence of the logarithmic concentration of solubilized methyl orange on the refractive index of (1) the AOT solution in hexadecane (0.001–0.2 mol/l) and (2) oxidized hexadecane.

We attempted to model a possible environment for methyl orange solubilized in oxidized hexadecane (see table); for this purpose, solutions containing (a) the nonionic surfactant Tween 85 and a mixture of ë4 and ë14 alcohols and (b) a mixture of short- and long-chain alcohols and carboxylic acids with pentaerythritol ë7 dicarboxylate [15] modeling 1,3-bifunctional oxidation products were prepared. The concentrations of oxygencontaining components were selected on the basis of data on the product composition (carboxylic acids, alcohols, and bifunctional compounds) of oxidized hexadecane samples. Both mixtures solubilized methyl orange; the dye absorption band maximum in the former occurred at 410 nm but was observed at even a shorter wavelength of 408 nm in the latter. The best fit of the maximum absorption wavelength of methyl orange solubilized in oxidized hexadecane is observed for a dye solution in the cyclic ether tetrahydrofuran (THF). Its noncyclic analogue dimethyl ether of ethylene glycol (1,4-dimethoxyethane) shifts the dye

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Position of the methyl orange absorption band in different solvents No.

Solvent

Absorption band maximum, nm

1 2 3 4 5 6 7 8 9 10 11 12

CH3COOH H2O CH3OH Oxidized hexadecane1) Hexadecane + Tween 85 + C12H29OH + C4H9OH2) Hexadecane + AOT (0.2 mol/l) Hexadecane + CH3COOH + C8H17COOH + C14H29OH + C4H9OH + pentaerythritol di(C7)ester3) Hexadecane + C14H29OH (0.4 mol/l) Ethylene glycol Tetrahydrofuran Ethylene glycol monomethyl ether Ethylene glycol dimethyl ether

520 465 423 420 413 410 408 408 450 419 423 417

1)

The position of the band does not depend on the hydrocarbon oxidation time up to 120 min. The concentration of each component of the mixture in hexadecane is 0.2 mol/l. 3) The concentration of each component of the mixture in hexadecane is 0.2 mol/l, except the pentaerythritol di(C )ester (0.1 mol/l). 7 2)

absorption band to shorter wavelengths (417 nm), and the hydroxyl group substituted for a methyl group in the monomethyl ether of ethylene glycol leads to the red shift of the band (423 nm). Most probably, the results that we obtained for the position of the methyl orange absorption band in oxidized hexadecane and in different ethereal solvents suggest that the methyl orange dye is solubilized by the micelles formed from hexadecane oxidation products, wherein the dye molecules occur in a molecular layer comprising the micelle shell, rather than in the most polar part of the micelle [16]. It is likely that obstacles to the entrance of the polar dye to the pretty polar core are diffusional restraints which are inevitable if assuming that the core of such an inverse micelle includes highly polar multifunctional products of oxidative condensation of various oxygen compounds produced already in the early steps of high-temperature oxidation. This assumption, in particular, agrees well with the substantial rise in the UV absorption by oxidized hexadecane samples (see Fig. 1b). Evidently, both hexadecane and primary (and even secondary) products of its oxidation do not exhibit any detectable absorption at 360 nm which can be associated only with the products that contain a large number of conjugated C=C and C=O bonds. On the other hand, as noted above, the formation of multifunctional oxidation products was observed even in the early steps of high-temperature hydrocarbon oxidation [2], and the high-molecularweight fractions showed the presence of conjugated C=C and C=O bonds [3]. Thus, the data obtained in this work on solubilizing the methyl orange dye by hexadecane samples having different extents of oxidation show that the formation of structural analogues of inverse micelles (SAIM),

probably by association or condensation of the hydrocarbon oxidation products, takes place during oxidation of hexadecane. These structures are capable of solubilizing methyl orange, the dye molecules occurring in the medium that corresponds in polarity to the environment composed of tetrahydrofuran molecules, rather than entering into the composition of the polar SAIM core. In order to check whether the molecular products of the reaction can influence the methyl orange solubilization, they were isolated from oxidized samples with methanol. The isolation was performed by two methods which differed in that the oxidate was used without a dye admixture in one case and with dissolved methyl orange in the other case, where its concentration was 3.2 × 10–6 mol/l. The product isolation scheme (Fig. 4) includes the following steps: (1) Mixing the oxidate with methanol in the aforementioned ratio followed by separation into two fraction: HS (about 80%) and MS (about 20%) and removal of the solvent. (2) Assessing the ability of the HS fraction to solubilize methyl orange. (3) Thin-layer chromatographic determination of the composition of the HS and MS fractions. (4) Combining the HS and MS fractions and determining the solubilizing ability. The analysis of the HS fractions in the two cases has shown that they do not dissolve the dye and, hence, do not contain micelle-forming components. The thinlayer chromatography data indicate that almost the same products, although in different concentrations, are present in the HS and MS fractions. For example, the concentration of acids in the HS and MS fractions was PETROLEUM CHEMISTRY

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41

OXIDATE Methyl orange Oxidate + methyl orange CMeOr = 3.2 × 10 –6 mol/l

Methanol

Methanol Oxidate + methanol

HS fraction

Oxidate + methyl orange methanol

MS fraction

HS fraction

MS fraction

Methyl orange HS fraction + Methyl orange

Methyl orange Oxidate + methyl orange CMeOr = 5.4 × 10 –6 mol/l

Methyl orange Oxidate + methyl orange CMeOr = 5.4 × 10 –6 mol/l

Fig. 4. Scheme for isolation of oxidation products from hexadecane and for solubilization of methyl orange.

0.066 and 0.725 mol/l, respectively, and that of hydroperoxides in the same fractions was 0.058 and 0.583 mol/l. When the HS and MS fractions were combined, solutions that solubilized methyl orange were obtained to have the dye concentration as high as 5.4 × 10–6 mol/l. The solubilization of methyl orange with the combined mixture of products indicates that the isolated polar products are capable of repeating micelle formation in the hydrocarbon medium. In addition, the identical values for the concentration of the dissolved dye in the two procedures of isolation of micelle-forming products show that the differences in the procedures do not affect the ultimate result. The increased concentration of methyl orange in the combined fraction (as compared to the initial value) is explained by incomplete removal of methanol. The obtained data show that SAIM can be isolated from the oxidate as a polar fraction soluble in methanol (MS fraction). The MS fraction comprises oxygen-containing products typical of most processes of high-temperature chain oxidation of hydrocarbons: hydroperoxides, alcohols, carboxylic acids, carbonyl compounds, and multifunctional compounds. When the MS fraction is dissolved in hexadePETROLEUM CHEMISTRY

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cane, the ability of the obtained solution to solubilize methyl orange is restored to the previous level, thus suggesting the absence of any changes in the structure and composition of the MS fraction upon isolation. It may be assumed that the structure of the MS fraction after dissolving in the hydrocarbon medium is an analogue of inverse micelles and includes a polar core stabilized by the shell of less polar, most likely, monoand bifunctional oxygen compounds, wherein the shell molecules are in dynamic equilibrium with the same molecules dissolved in the bulk of the hydrocarbon. At present, the composition of the polar core of structural analogues of inverse micelles remains an open question; it is either composed of low-molecular-weight severe-oxidation products, such as lower mono- and dicarboxylic acids and polyhydric alcohols, or the SAIM core is made of multifunctional compounds or condensation products of the multifunctional oxidation products containing the original carbon chain [2, 3]. In summary, structures analogous to inverse micelles are formed during hexadecane oxidation at 170°ë by as early as the 20th min. The presence of inverse micellar aggregates is proved by the solubiliza-

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tion of methyl orange, a dye that is insoluble in hydrocarbons. It was shown that it is the polar oxidation products that are responsible for micellization.

8.

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