Plasma-chemically brominated single-walled carbon nanotubes as novel catalysts for oil hydrocarbons aerobic oxidation

Applied Catalysis A: General 454 (2013) 115–118

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Plasma-chemically brominated single-walled carbon nanotubes as novel catalysts for oil hydrocarbons aerobic oxidation Eldar Zeynalov a , Joerg Friedrich b,∗ , Asmus Meyer-Plath b , Gundula Hidde b , Lyatif Nuriyev a , Aygun Aliyeva a , Yutta Cherepnova a a b

National Academy of Sciences of Azerbaijan, Institute of Petrochemical Processes, Khojaly Avenue 30, AZ 1025 Baku, Azerbaijan Federal Institute of Materials Research and Testing (BAM), Division 6.6 “Polymer Surfaces”, Unter den Eichen 87, 12205 Berlin, Germany

a r t i c l e

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Article history: Received 17 August 2012 Received in revised form 10 December 2012 Accepted 1 January 2013 Available online xxx Keywords: Brominated carbon nanotubes Plasma-chemical technique Model cumene oxidation Oxidation rate Oxidation catalysts Oil hydrocarbons liquid-phase oxidation Manganese naphthenate Synthetic petroleum acids

a b s t r a c t Brominated single-walled carbon nanotubes [(Br)n -SWCNT) produced by the plasma-chemical technique were involved in the liquid-phase process of hydrocarbons aerobic oxidation. The significant catalytic effect of the (Br)n -SWCNT was revealed at first by the cumene initiated model oxidation and then in experiments on profound aerobic oxidation of petroleum naphthenic fraction derived from the commercial Azerbaijan (Baku) oils blend diesel cut. The ability of (Br)n -SWCNT to accelerate the aerobic oxidation of the hydrocarbons was found out for the first time. Obviously this phenomenon originates from the peculiarities of electronic configuration of the (Br)n -CNT patterns. The plausible mechanism of (Br)n -SWCNT catalytic action is inclined to the formation of reactive oxygen species. The catalytic activity of (Br)n -SWCNT markedly exceeds the activity of the industrial catalysts, manganese salt of indigenous petroleum acids, used for the liquid phase petroleum hydrocarbons oxidation process. © 2013 Elsevier B.V. All rights reserved.

1. Introduction An efficient bromination of graphitic materials, including highly oriented pyrolytic graphite, natural graphite, multi-walled carbon nanotubes (MWCNT), and graphitized carbon fibres has been recently achieved by the plasma-chemical technique [1,2]. Bromine concentrations of up to 50 bromine atoms per 100 C-atoms were obtained in elemental bromine and bromoform vapour under low-pressure plasma conditions using low-energetic inductively coupled radio-frequency plasma excitation. The resulting covalent C Br bonds were shown to be well-suited for transfer to hydroxyl functionalization and for grafting of organic molecules by nucleophilic substitution. Thus, the new compounds will be first applied for the preparation of amino group-containing patterns of MWCNT to increase the compatibility with polymer composites [1,3], and as active additives for the aerobic oxidation of hydrocarbons [4]. The present paper describes new data obtained on behaviour of the brominated SWCNT added into the systems of low-temperature aerobic oxidation of individual aromatic hydrocarbon (cumene)

∗ Corresponding author. Tel.: +49 30 8104 1630; fax: +49 30 8104 1637. E-mail address: [email protected] (J. Friedrich). 0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2013.01.003

and mean temperature aerobic oxidation of naphthenic–paraffinic petroleum hydrocarbons. The oxidation of petroleum hydrocarbons is an important process for synthesis of synthetic petroleum acids (SPA). The acids, serving as alternative substitutes of indigenous petroleum acids, are valuable products of petrochemical and oil refining industry. SPA, their esters and salts have found a wide variety of application as improvers of concrete water resistance and adhesion, increasers of high pressure resistance of drilling oils, preventers of foaming in jet fuel and fungus growth in wood, preservers and flame-retardant agents in fabrics, emulsifiers for increasing insecticide solubility, catalysts of rubber vulcanization and obtaining alkyl and polyester resins, stabilizers for vinyl resins, wood wet rot preservatives, etc. [5–11].

2. Experimental 2.1. Objects (Br)n -SWCNTs were obtained from the SWCNT commercial sample “Elicarb” produced by the Thomas Swan & Co. Ltd., UK. The brominated samples were prepared with the plasma-chemical technique using bromine vapour at 100 W power input (cwcontinuous wave) and pressure of 9 Pa. The percentage of grafted

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Table 1 Physico-chemical features of the diesel cut and naphthenic concentrate of Azerbaijan (Baku) crude oils blend. Fraction (boiling/freezing points range, T in ◦ C)

Crude [216 − 360/(−35 ◦ C)] Naphthenic [(220 − 350/(−52)◦ C]

Hydrocarbons content (wt%) Aromatic

Paraffinic

Naphthenic

16–18 0.6–0.8

20–22 6–8

60–62 90–92

MW (molecular weight)

420 , g/cm3 (density)

n20  (refraction index)

247 235

0.8555 0.8330

1.4695 1.4620

(Br)n -SWCNT samples were being added into the oxidation reaction mixture in concentrations up to 4 g/l.

groups in the (Br)n -SWCNT patterns was found by X-ray photoelectron spectroscopy as follows: 1) Sample 1/Br-80-120504 (1 h exposure): C = 94.8, O = 3.3, Br = 2.0%, O/C = 0.035, Br/C = 0.021 2) Sample 2/Br-81-120509 (2 h exposure): C = 87.3, O = 6.3, Br = 6.4%, O/C = 0.072, Br/C = 0.073 3) Sample 3/Br-82-120514 (untreated): C = 96.9, O = 3.1%, O/C = 0.032. The aromatic hydrocarbon used as model was cumene with 98% purity (“Sigma–Aldrich”). 2,2 -azobisisobutyronitrile (AIBN) was used as initiator for the model oxidation reaction. Before experiments were started the initiator was purified by recrystallization from benzene and ethanol solutions. Petroleum hydrocarbons were carried from the naphthenic–paraffinic concentrate obtained by the diesel cut of Azerbaijan (Baku) crude oils blend (Oil Refinery “Azernefteyag”) after de-aromatization and de-paraffination [12,13]. Data and indices of the crude diesel cut and the naphthenic concentrate are given in the Table 1.

means of differentiating curves in the case of observed induction periods. Experiments were carried out at least in triplicate and the accuracy in determining the kinetic parameters was within the range 3–7%. Oxidation of the petroleum naphthenic fraction, containing additions of (Br)n -SWCNT, was conducted in a glass laboratory setup at the temperature range of 135–140 ◦ C and for several hours under incessant air current, passing through the liquid reaction mixture with rates 100–120 l/h. Schematic design of the flow reactor for the oxidation experiments is shown in Fig. 2. Criteria of the oxidative conversion of oil hydrocarbons were values of acid number (AN) of a reaction mixture (oxidate) and yields of synthetic (SPA) and oxy-synthetic (OSPA) petroleum acids. The acidic components (SPA + OSPA) were being isolated from an oxidate by the

2.2. Methods, equipments and procedures The model cumene oxidation was carried out in the presence of different additions of (Br)n -SWCNTs at the initiation rate Wi = 6.8 × 10−8 M s−1 , temperature = 60 (±0.02) ◦ C and oxygen pressure PO2 = 20 kPa (air). From existing experience – this condition is most suitable for the preliminary correct determination of activity of any involved additives [7,8]. The volume of the reaction mixture was 10 cm3 (25 ◦ C). In order to get the assigned initiation rate 10 mg of AIBN had to be added [14–16]. The rate of oxidation was evaluated from the amount of oxygen consumed, which was measured volumetrically with the simple laboratory device shown schematically in the Fig. 1. Oxidation rates (WO2 ) were assessed both from the slopes of the kinetic curves of oxygen consumption in the case of steady rate values and also by

Fig. 1. Schematic diagram of measuring equipment used for oxygen uptake at constant pressure.

Fig. 2. Schematic drawing of laboratory glass reactor for oxidation of liquid hydrocarbons. 1: upper reaction zone of bubbling liquid hydrocarbons; 2: water trap; 3, 4: condensers; 5, 6: absorbents; 7: bibcock for sampling and draining; 8: holders for contact thermometer and temperature indicator; 9: hole for a stock feeding; 10, 11: devices for air cleaning and air stream regulation; 12: filter glass.

E. Zeynalov et al. / Applied Catalysis A: General 454 (2013) 115–118

117

140

Consumed oxygen volume, V x 104(M)

Wo = 3.0 / 10 Ms

120 100 Wo = 1.65 / 10 Ms

Wo = 6.1 / 10 Ms Wo = 1.36 / 10 Ms

80 60

1 40

2 3 4

20 0 0

5

10

15

20

25

Time (min.)

Fig. 3. Kinetic dependences of oxygen uptake for cumene initiated oxidation in the presence of brominated single-walled carbon nanotubes (Br)n -SWCNT. Initiator is 2,2 -azobis-isobutyronitrile (AIBN). Reaction mixture volume: 10 ml, initiation rate: Wi = 6.8 × 10−8 M s−1 , temperature: 60 ◦ C, oxygen pressure: PO2 = 20 kPa (air). [(Br)n -SWCNTs]: 1: 0 g/l; 2: 4.0 g/l (sample Br-82-120514 (untreated)); 3: 2.0 g/l (sample Br-81-120509); 4: 4.0 g/l (sample Br-81-120509); 5: 4.0 g/l (sample Br-80120504).

alkali (NaOH or KOH) treatment followed by reverse recovering SPA + OSPA with the acidizing (mineral acids). Next step was the separation of SPA from OSPA and un-saponifiables by light benzene or petroleum ether processing [17,18]. Infrared (IR) spectral analysis of SPA was performed on the spectrometer “Specord M-80”, Carl Zeiss, Jena, Germany. 3. Results and discussion The kinetic measurements on the model oxidation reaction are presented in Fig. 3. As seen, the oxidation line profiles display the catalytic action of the added (Br)n -SWCNT under the condition of the model experiments. The rates of the oxidation with added (Br)n -SWCNT (Fig. 3, lines 3–5) are increased in comparison with those for the reference and untreated sample oxidation (Fig. 3, lines 1 and 2).

Fig. 4. Kinetic dependences of oxygen uptake for cumene initiated oxidation in the presence of brominated multi-walled carbon nanotubes (Br)n -MWCNT (content of Br-groups is 17.8%). Initiator is 2,2 -azobis-isobutyronitrile (AIBN). Reaction mixture volume: 10 ml, initiation rate: Wi = 6.8 × 10−8 M s−1 , temperature: 60 ◦ C, oxygen pressure: PO2 = 20 kPa (air). [(Br)n -MWCNT]: 1: 0 g/l; 2: 1.0 g/l; 3: 2.0 g/l; 4: 5.0 g/l.

The plausible mechanism of the catalytic action may be explained through formation of reactive oxygen species (ROS). It is known, that the accumulation of Br-groups on CNT surface proceeds in an unusual order perpendicularly to the nanotube surface [19]. As a result, the area on which bromine is accumulated possesses the shifted electron density due to the exceeding inductive effect that can in turn provoke the formation of ROS. It was interesting to compare the results obtained with those of the oxidation in the presence of MWCNT. Kinetic data on MWCNT in the model oxidation were obtained by us earlier [4]. Fig. 4 represents these data. Comparing the data shown in Figs. 3 and 4 one can assert that the acceleration in the model oxidation is observed for both kinds of CNTs and is proportional to the content of grafted Br-groups. As next step of the present investigation the testing of (Br)n SWCNT in oxidation of the oil naphthenic fractions was fulfilled to make assessment of the additive applicability for synthesis of artificial (synthetic) petroleum acids, i.e. SPA.

Table 2 Oxidation of naphthenic fraction 220–350 ◦ C of the commercial diesel cut of Azerbaijan (Baku) oils blend. Amount of the feedstock = 128 g (150 ml), [(Br)n -CNTs] = 1.6 g/kg (1.3 g/l). Comparative data of the oxidation in the presence of industrial catalysts Mn naphthenate (salts of IPA, Mn-Naft.) are also given. [Mn-Naft.] = 15 g/kg [9,11,18]. Reaction temperature = 140 ◦ C, air stream = 100–120 l/h, reaction time is 6 h. Catalyst type

Oxidate AN, mg KOH/g

Yield of SPA, % (to raw stock)

Yield of OSPA, % (to raw stock)

(Br)n -SWCNT (Br)n -MWCNT Mn-Naft.

60 75 55

25 30 14.5

3.0 7.0 11.0

Fig. 5. IR-spectrum of the synthetic petroleum acids obtained in the presence of (Br)n -CNTs.

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Results of the oxidation of naphthenic fraction of the commercial Azerbaijan (Baku) oils blend in the presence of (Br)n -SWCNT and (Br)n -MWCNT are given in Table 2. Thus, the table data specify that (Br)n -CNTs at much lower concentrations exhibit much more catalytic activity in comparison with the industrial catalyst. However, the found experimental facts require further thorough investigations with engagement other hydrocarbon oxidizing systems. The SPA obtained was analyzed by the IR spectroscopy (Fig. 5). The spectrum displays the absorption peaks in the region 925–950 cm−1 responsible for deformation vibrations of C H bonds in naphthenic structures, 1250 cm−1 – valence vibration of oxygen grouping (C O) of cyclic ester, 1400–1480 cm−1 – deformation vibrations of C H bonds of CH2 and CH3 groups in linear and naphthenic structures, including those of connected with carbonyl groups as well as stretching vibrations of CH2 groups at 2920 cm−1 , 1750 and 1780 cm−1 are stretching vibrations of carbonyl groups, including the deformation vibrations at 4–5th membered naphthenic rings, 2960 cm−1 – stretching vibrations of as –CH3 and OH along with broad absorption >3000 cm−1 groups at naphthenic rings. 4. Conclusions Brominated carbon nanotubes(Br)n -CNTs exhibit a strong catalytic effect in reactions of aerobic oxidation of hydrocarbons. This fact, for the first time, was unequivocally proved in the systems comprising of the radical reaction of model cumene initiated (2,2 azobisisobutyronitrile, AIBN) oxidation and naphthenic–paraffinic fraction of liquid oil hydrocarbons aerobic high temperature oxidation. The results obtained are considered as preliminary and require further thorough investigations. Acknowledgments This work was supported by the Science Development Foundation under the President of the Republic of Azerbaijan – Grant No EI˙ F-2011-1(3)-82/62/4-M-59 and the German Federal Institute for

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