Synthesis and characterization of N-alkyl-2

Egyptian Journal of Petroleum 27 (2018) 1337–1344

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Synthesis and characterization of N-alkyl-2-aminopyridinum oligomers as pour point depressants for crude oil Eid M.S. Azzam a,⇑, Sayed A. Ahmed b, Hanafy M. Abd El-Salam b, Osama A. Abd Allha b, Elshafie A.M. Gad a a b

Applied Surfactants Laboratory, Petrochemicals Department, Egyptian Petroleum Research Institute, 11727 Nasr City, Cairo, Egypt Department of Chemistry, Faculty of Science, Beni-Suef University, 62514 Beni-Suef City, Egypt

a r t i c l e

i n f o

Article history: Received 30 June 2018 Revised 17 September 2018 Accepted 24 September 2018 Available online 19 October 2018 Keywords: N-alkyl-2-aminopyridinum Oligomers Crude oil Pour point dispersants

a b s t r a c t The synthesis of N-decyl- (C10P), N-dodecyl- (C12P) and N-cetyl-2-aminopyridinum (C16P) oligomers was achieved by oxidative polymerization of N-decyl- (C10), N-dodecyl- (C12) and N-cetyl-2aminopyridinium bromide monomers respectively in aqueous acid medium. Density function theory (DFT) was applied to recognize the reactive center in monomers used. The result of quantum calculations indicated that the oligomerization could proceed via open ring reaction. The obtained oligomers were characterized using FTIR and 1H NMR spectroscopy. In addition, the XRD and SEM were used to investigate the morphology of the prepared oligomers. The thermal stability of the obtained oligomers was followed by thermogravimetric analysis (TGA). The conversion of these oligomers is ranged from 61 to 68% and the weight average molecular weights are closed to 3000 g mol1. The surface tension of the synthesized oligomers was measured at different concentrations and temperatures. The efficiency of the synthesized oligomers in depressing the pour point of crude oil was investigated. The synthesized oligomers showed high efficiency as pour point dispersants. Ó 2018 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction Waxy crude oils always suffer serious problems during transportation and storage, particularly in cold environments; this arises from the presence of significant amounts of paraffin wax in the crude oil that impede the flow of crude oil due to wax precipitation [1]. Regarding the composition of the waxy crudes, the high molecular weight n-alkanes (n-paraffins) are the main components in wax deposits [2,3]. Waxes are heavy paraffinic solids that settle out of a crude oil to form a gel structure. Wax formation is a liquid–solid phase transition from a liquid mixture which is largely sensitive to a drop in temperature [4]. In the petroleum industry, wax precipitation is undesirable because it may cause plugging of pipelines and process equipment. Wax precipitation is an old problem [5–8]. The crude oil constituents have pronounced effect on its flow characteristics with variation of the surrounding temperature. Egyptian crude oil contains different amounts of paraffin wax depending on the field and area of production. At low temperatures, crude oil containing high amounts of paraffin shows high pour points due to paraffin deposition; that is, paraffins tend to crystallize forming wax crystals [9]. The wax

Peer review under responsibility of Egyptian Petroleum Research Institute. ⇑ Corresponding author. E-mail address: [email protected] (E.M.S. Azzam).

deposition is a result of cooling down the crude oil below certain temperatures during transportation or storage. This temperature depends upon the constituents of crude oil and is called pour point temperature (PPT) [9]. There are many kinds of polymers that are used as PPDs to influence the behavior of the paraffin crystallites formation [10–12]. Theoretical analyses explained the interactive mechanism, co-crystallization, nucleation, or improved wax solubility [13,14]. The wax deposition inhibitors are polymeric compounds constituted by a hydrocarbon chain which provides the interaction between the additive and paraffin, and a polar segment that is responsible for the wax crystals morphology modification necessary to inhibit the aggregation stage. The development of new additives that can solve or minimize such problems is of great interest for the petroleum industry in the whole world. According to Kumar’s investigation on pour point depressants [15], an efficient polymeric additive for paraffin oils should be a linear polymer or copolymer that has pendant hydrocarbon chain groups and/or presents hydrocarbon chains in the polymeric backbone. In order to synthesize a polymer that can perform as an agent to reduce the pour point, the following characteristics should be considered: a sufficient number of pendant alkyl groups; alkyl groups with sufficiently long chains; a convenient distance between the hydrocarbon pendant chains; a medium molar mass; in the case of a copolymer, a suitable ratio between the co-monomers; a high stability of the additive and, the amorphous or crystalline nature

https://doi.org/10.1016/j.ejpe.2018.09.001 1110-0621/Ó 2018 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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E.M.S. Azzam et al. / Egyptian Journal of Petroleum 27 (2018) 1337–1344

of the additive [16–18]. Polyanilines have attracted great interest due to their excellent properties in addition to their widely applications. The methods of preparation of polyanilines chemically and/or electrochemically are well known procedures. Although several chemical properties of aminopyridine are like aniline, polyaminopyridines (PAPy) have received much less attention than polyanilines for study due to the difficulty of their preparation. The chemical yields of PAPy by electrochemical methods not exceed 5%. Although that, polyaminopyridines are used in the construction of batteries [19] and modified electrodes [20]. Due to the low reactivity of the aminopyridine in the oxidative chemical polymerization [21], a few works concerning with the synthesis of PAPy by the electrochemical methods are the most used [19–23]. To increasing the reactivity of aminopyridine, substitution of carbon ring by function groups was achieved [24]. The main target of the present work is the synthesis and characterization of N-alkyl2-aminopyridine oligomers (C10P, C12P and C16P). The synthesized oligomers were investigated as pour point dispersants for ASL-24 and Bakr-35 crude oils from the Egyptian General Petroleum Company. 2. Materials and experimental techniques 2.1. Materials 2-Aminopyridine, decyl bromide, dodecyl bromide, and cetyl bromide were obtained from Sigma Aldrich Chemical Co. Concentrated hydrochloric acid and sodium hydroxide were chemically pure grade products provided by Prolabo-Chemical Co. (U.K.). Ammonium persulphate (APS), methanol and DMF were products of Aldrich chemical company (Germany). 30% aqueous solution of sodium hypochlorite (NaOCl) was supplied by El-Nasr Chemical Co. (Egypt). Twice distilled water used as a medium for all the polymerization reactions. 2.1.1. Synthesis of N-alkyl-2-aminopyridinum bromide monomers N-decyl(C10), N-dodecyl(C12) and N-cetyl-2aminopyridinum bromide (C16) monomers were prepared by the equimolar reaction of 2-aminopyridine with decyl bromide, dodecyl bromide, and cetyl bromide, respectively, in the presence of sodium ethoxide as a medium as mentioned by Azzam et al. [25]. 2.1.2. Synthesis of N-alkyl-2-aminopyridine oligomers N-decyl-2-aminopyridinum (C10P), N-dodecyl-2aminopyridinum (C12P) and N-cetyl-2-aminopyridinum (C16P) oligomers were prepared as follow: 0.01 mol of each monomer was dissolved in 50 ml acidic (0.500 mol.L1 hydrochloric acid) solution of 30% NaOCl; 0.0125 mol of APS (2.75 g dissolved in 20 ml distilled water) is added to the monomer solution. The polymerizations were performed in 250 ml three necked round bottom flasks which were fitted with a condenser, thermometer and magnetic stirrer at 70 °C for 7 h. The formed brown acidic solution is neutralized using equal equivalent of sodium hydroxide solution to HCl acid. The neutral polymeric solutions are centrifuged to obtain the polymers. The formed oligomers are filtered, washed with water, methanol, then dried under vacuum at 60 °C. The conversion is 68% for C10P, C12P and C16P oligomers [26]. 2.2. Characterization of the synthesized oligomers FTIR and 1HNMR spectroscopy were used to confirm the structures of the prepared oligomers. The infrared measurements were carried out using Shimadzu FTIR Vertex 70 Bruker Optics technique. The 1HNMR measurements of the prepared polymers were carried out using Varian EM360L 60 MHz NMR spectrometer, the

signals have been recorded in dimethyl-sulfoxide (DMSO) by using tetramethylsilane as an internal reference. 2.2.1. X-ray diffraction patterns (XRD) and scanning electron microscope (SEM) The X-ray diffraction patterns of the synthesized oligomers were characterized with the help of Pananlytical Empryan X-ray diffractometer 202964. The scan rang was (50–1400). The electron microscope analysis was carried out using JSM-6510LA Scanning electron microscopy, JEOL, Japan. 2.2.2. Thermogravimetric analysis (TGA) Thermogravimetric analysis of the synthesized oligomers was performed using a SHIMAZU DT-30 thermal analyzer. The weight loss was measured from ambient temperature up to 1000 °C, at the rate of 10 °C per minute to determination the degradation rate of the polymeric samples. 2.2.3. Surface and interfacial tension measurements The Surface tension of the freshly prepared solutions for synthesized oligomers (C10P, C12P and C16P) was measured using Du Nouy K6 tensiometer (KRUSS Type 8451), GmbH, Hamburg, Germany. The surface tension was measured at different concentrations (from 0.01 to 0.00001 mol.L1) and different temperatures (25, 40, 55 °C) as mentioned in previous publication [27]. The interfacial tension between the aqueous solution of the synthesized oligomers (C10P, C12P and C16P) at concentration of 0.1% by weight and light paraffin oil at 25, 40, 55 °C was measure as follow. 10 ml of the oligomer solution was added to a glass container and the platinum ring was adjusted to touch the solution surface. 10 ml of the light paraffin oil was then added slowly on the surface, and the readings were recorded at which the ring detached from the aqueous to the organic layer (paraffin oil). The ring was washed with acetone followed by distilled water and the measurements were repeated three times for each sample. The interfacial tension values are the average of the three readings [27]. 2.2.4. Pour point test A crude oil sample from well ASL-24 and well Bakr-35 at The Egyptian General Petroleum Company was used for evaluating the performance of the synthesized oligomers (C10P, C12P and C16P) as pour point dispersants. The Pour points of the crude oil samples were measured according to ASTM D-97 method [28]. The synthesized oligomer solution in xylene (5 ml) at 500, 1000 and 2000 ppm was added to 50 ml of crude oil sample and heated in a thermostatic bath maintained at 50 °C. The physicochemical properties of the used crude oils are listed in Table 1. 3. Results and discussions The synthesized C10P, C12P and C16P samples were checked for average molecular weight determination. The Gel Permeation

Table 1 Physicochemical properties of the crude oils. Test

Water content vol. % Specific gravity at 60/60 F A.P.I gravity at 60 F Kinematic viscosity. at 100 F/C. St. Asphaltine content Wt. % Wax content Wt. % Total sulfur content Wt. % Pour point, °C

Method

ASTM D4007-08 ASTM D1298-99 ASTM D1298-99 ASTM D445-09 ASTM D6560-00 UOP 46/64 ASTM D4294-08 ASTM D-97

Crude oils ASL-24

BAKR-35

0.4 0.9212 22.1 85 2.0 0.6 2.1 7.22

60 0.9752 13.6 553 8.9 1.1 4.55 18.33

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anism for N-alkyl-2-aminopyridinium bromide monomers (C10, C12 and C16). ACD/ChemSktch version 11.02 (2008) was used to design the investigated molecules then geometrically optimized under no constraint using DFT (density functional theory) with the Beck’s three parameter exchange functional along with the Lee–Yang–Parr nonlocal correlation functional (B3LYP) [29] with 6-311G* basis set implemented in Gaussian 09 program package, using PC with processor Core i7 (8 CPU 1.7 GHz). Density function theory (DFT) was applied to recognize the reactive center of Nalkyl 2-aminopyridinium bromide monomers (where alkyl group = C10, C12 and C16). Various parameters were EHOMO; energy of the highest occupied molecular orbital, ELUMO; energy of the lowest unoccupied molecular orbital, DE; the energy gap, g; Global hardness, S; global softness, I: ionization potential, A: electron affinity, X: absolute electronegativity, f+, f; Fukui indices for local nucleophilic and electrophilic attacks and s+, s local softness [30]. The result of quantum calculations indicated that the oligomerization could proceed via open ring reaction. Both EHOMO; energy of the highest occupied molecular orbital, and ELUMO; energy of the lowest occupied molecular orbital was listed in Table 2 and graphically represented in Fig. 1. The following parameters; Ionization potential (I), electron affinity (A), the electronegativity (v), global hardness (g) and softness (S), can be explained in terms of the energy of the HOMO and the LUMO [31,32]

Chromatography (GPC) analysis of synthesized samples was carried out using Waters 515/2410 Gel Permeation Chromatograph (GPC, Waters, America) and a ultrahydrogel column calibrated with poly (ethylene glycol) standards and series 2410 refractive index detector, mobile phase: water, sodium nitrate (0.10 M), solvent: Water, sodium azide 0.05%, Flow rate: 1 ml/min and at Temperature: 25 °C. The data reveals that the molecular weight for each sample is low (1026 g) which indicate that the synthesized samples are oligomers. The formation of oligomers was confirmed in this work using Density function theory (DFT) as follow. 3.1. Structure of the synthesized oligomers The quantum calculations were performed using density functional theory (DFT) to confirm the open ring polymerization mechTable 2 Calculated quantum chemical parameters of the investigated molecules.

HOMO LUMO DE Gap I A X

C10

C12

C16

0.12117 0.06031 0.06086 0.12117 0.06031 0.09074

0.12090 0.06050 0.06040 0.12090 0.06050 0.09070

0.15859 0.09970 0.05889 0.15859 0.09970 0.12915

HOMO

C10

C12

C16

LUMO

Fig. 1. Graphical representation of FMO of N-alkyl-2-aminopyridinium Bromide monomers.

Table 3 Calculated Mulliken atomic charge distribution and Fukui indices for investigated oligomers. N

N+1

N1

f+

f

C10

1C 2C 3C 4N 5C 6C 17N

0.2796 0.6138 0.0082 0.6132 0.2062 0.2545 0.7405

0.0297 0.0243 0.0292 0.0252 0.0563 0.5009 0.0008

0.140 0.226 0.478 0.166 0.080 0.258 0.009

0.250 0.589 0.021 0.5880 0.150 0.2464 0.741

0.140 0.387 0.470 0.779 0.126 0.004 0.731

C12

1C 2C 3C 4N 5C 6C 17N

0.2796 0.6136 0.0082 0.6131 0.2061 0.2544 0.7406

0.0297 0.0243 0.0292 0.0252 0.0563 0.5009 0.0008

0.128 0.214 0.486 0.202 0.069 0.241 0.001

0.250 0.589 0.021 0.588 0.150 0.246 0.741

0.152 0.399 0.478 0.815 0.138 0.013 0.741

C16

1C 2C 3C 4N 5C 6C 17N

0.2586 0.6295 0.0008 0.6402 0.3007 0.3012 0.7346

0.0024 0.0000 0.0012 0.0001 0.0037 0.0009 0.0021

0.088 0.172 0.503 0.179 0.157 0.355 0.002

0.261 0.629 0.002 0.640 0.304 0.302 0.737

0.171 0.457 0.503 0.819 0.144 0.054 0.733

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Scheme 1. The formation of the synthesized oligomers via open ring mechanism.

I : ionization potential ¼ EHOMO

ð1Þ

A : electron affinity ¼ ELUMO

ð2Þ

DEgap ; the energy gapðeVÞ ¼ ELUMO EHOMO

ð3Þ

X : absolute electronegativity ¼

ð4Þ

g; Global hardness ¼ 5

IþA 2

IþA 2

5

S; global softness ¼ g1

Fig. 2a. IR-Spectrum of the synthesized oligomers.

Electron charge distribution on the surfactant molecules were determined which can be used to calculate Fukui indices [33,34] þ (f and f ) for local nucleophilic and electrophilic attacks and s+, s local softness. þ

ð7Þ

ð8Þ

f ¼ qðNþ1Þ qN ðNucleophilic attackÞ f ¼ qN qðN1Þ ðElectrophilic attackÞ

ð5Þ ð6Þ

Fig. 2b. 1HNMR Spectrum of the synthesized oligomers.

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The local reactivity of molecule is analyzed using condensed Fukui indices. The f+, measures the changes of density while the molecules receive electrons and it corresponds to reactivity with respect to nucleophilic attack. As vice versa, f denote to reactivity with respect to electrophilic attack or when the molecule loss electrons. The electrophilic and nucleophilic attacks of molecules of C10P, C12P and C16P are listed in Table 3. For nucleophilic attack the most reactive site is N(17) atoms for each C10P, C12P and C16P, for electrophilic attack the most reactive site is C (2). The local softness indices also explain the comparison between reactivity of similar atoms of each part of different molecules. Accordingly, the proposed mechanism for this type of reaction can be illustrated as shown in Scheme 1 [36]. 3.2. Infrared and 1HNMR spectroscopy Fig. 3a. TGA of the synthesized oligomers.

Frontier molecule orbital density distributions of the investigated compounds: EHOMO and ELUMO are represented in Fig. 1. It shows that the electron density of the HOMO distributed over pyridine ring, it is observed that EHOMO increases in the following order; C10 > C12 > C16. High values of EHOMO are likely to indicate a tendency of the molecule to donate electrons to appropriate acceptor molecules with low-energy, empty molecular orbitals [35]. According to the values of EHOMO in Table 2 the tendency for the formation of a feedback bond would depend on the value of ELUMO. The lower the ELUMO, the easier is the acceptance of electrons from electron donating group. Based on the values of ELUMO, the order obtained for the decrease in ELUMO: C10 > C12 > C16. The energy band gap (DEgap), the difference of ELUMO and EHOMO is an indication to the reactivity of molecules towards open ring oligomerization [36]. According to the value of the energy gap as shown in Table 2, DEgap decreases, in order C16 > C12 > C10. So, C16 is susceptible to be more reactive toward open ring oligomerization.

The infrared spectra of the synthesized oligomers are presented in Fig. 2a. Form the which, it can conclude that, the absorption band characteristic for the main vibration is present. The stretching vibration of NH appears at 3320 cm1. In addition, both C@C stretching vibration and bending NH are appeared at 1630 cm1. CAN stretching vibration absorption band appears at 1270 cm1. CH bending pyridine ring appears at 690 cm1. In case of Nalkyl-2-aminopyridine oligomer the stretching vibration of alkyl CH2 appears at 1450 cm1. The 1H NMR of these synthesized oligomers is present in Fig. 2b. The Figure reveals that, 1H NMR (DMSO, d ppm) for N-alkyl-2-aminopyridine oligomers are 7.95 (s, 1H, NH), 7.45 (d, 1H), 6,58 (t,2H), 6.01 (d,1H), 3.34 (bs, 2H, NH2). In case of C10P oligomer the 1H NMR signals are (DMSO, d ppm): 8.76 (s, 1H, NH), 8.35 (d, 1H, CH@N pyridine), 7,87 (d, 1H, CH@N conjugated double bond), 7,21(m, 3H, CH pyridine), 6.92(d, 1H, CH, conjugated double bond), 4.5–3-85 (m, 3H, CH conjugated double bond), 3.22 (bs, 2H, NH2), 0.9–1.8 (m, 21H, decyl protons). the 1H NMR (DMSO, d ppm) of C12P and C16P oligomers are: 8.62 (s, 1H, NH),

1000 800 600

C10P

400 200 0 20000

Intensity

15000

C12P

10000 5000 0

2000 1500 1000

C16P

500 0 0

10

20

30

40

2Theta Fig. 3b. XRD of the synthesized oligomers.

50

60

70

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8.30 (d, 1H, CH@N pyridine), 8.01 (d, 1H, CH@N conjugated double bond), 7,96 (d,1H, CH@NH conjugated double bond), 7.25 (t, 1H, CH pyridine), 7.03 (t, 1H, CH pyridine), 6.91 (t,1H, CH@C, conjugated double bonds), 6.84 (t, 1H, CH pyridine), 6.55 (m, 2H, Conjugated double bond), 4.27 (bs, 2H, NH2), 1.2–3.2 (m, 25H, dodecyl protons and/or 34H cetyl protons). 3.3. Thermal analysis (TGA) The Thermogravimetric analysis of the prepared oligomers is present in Fig. 3a. The first thermal degradation step of both C10P and C12P start at 60 °C and end at about 105 °C which attributed to the losing of moisture and water of hydration; then from 110 to 250 °C the bonded water and doping molecules are lost. For both C10P and C12P a sequence of steps started from 250 to 600 °C which could be to degradation of alkyl group on different ranges on temperature. Above 600 °C about 38% of C12P is remained tell 1000 °C but C10P is completely decomposed at 730 °C. In case of C16P oligomer, the moisture is lost at 150 °C and the main decomposition step start at 250 and ended at 350 °C with residual weight of 20%. Above 350 °C this residual weight started to decomposed to 750 °C and about 10% C1P is remained without degradation up to 1000 °C. From the obtained TGA data it is clear that the alkylation of 2-minopyridine with C10 and C12 is more thermal stable than C16. Above 750 °C, the most stable one is C12P up to 1000 °C with residual weight of about 38%. 3.4. Morphological studies of the prepared oligomers (XRD and SEM pictures) XRD patterns of the prepared oligomers are present in Fig. 3b. The XRD patterns reveal that the prepared oligomers are crystalline materials but the alkylation decreases this crystallinity. Also, the peaks concentrated in 2h ranged from 2 to 70°. Scanning electron microscopic images of these oligomers are present in Fig. 4. The SEM images show that C10P oligomer have surface like compacted sheets but C12P oligomer have surface composed from grains and compact sheets. The morphology of C16P oligomer is different from the above three oligomers; it has a surface of different structures ranged from grains to sheets. Also, in comparison between C16P and the other oligomers it seems pores one. 3.5. Surface tension and interfacial tension of the synthesized oligomers The relation of surface tension with the concentration of the synthesized oligomers (C10P, C12P and C16P) at different temperatures (25, 40, and 55 °C) was represented in Fig. 5. The surface tension reduction is the most commonly measured properties of surfactants in solution which depends directly on the replacement of molecules of solvent at the interface by molecules of surfactants. The results in Fig. 5 show that the surface tension decreases as the alkyl chain in the hydrophobic part of the synthesized oligomers increases from C10 to C16. The higher reduction in surface tension appeared with the synthesized oligomer C16P than the others oligomers (C10P, C12P). It can be noticed that, as the temperature increases the surface tension of the synthesized oligomers solution decrease. The interfacial tension values of the synthesized oligomers against light paraffin oil at different temperatures (25, 40, and 55 °C) are shown in Fig. 6. These values ranged from 6 to 2 dyne/cm. The results in Fig. 6 show that as the alkyl chain in the hydrophobic part increase (from C10 to C16) the interfacial tension decrease which related to the increase in the hydrophobicity of the molecules and consequently increases their efficiency at the liquid–liquid interface. It was observed also that the interfacial

Fig. 4. SEM of the synthesized oligomers.

tension of the synthesized oligomers against light paraffin oil decreased as the temperature increase from 25 to 55 °C as shown in Fig. 6. The decrease in the surface and interfacial tension with the increase in temperature is related to the increase in kinetic energy of molecules which make the molecules apart from each other and the surface of the liquid become less stable. 3.6. Effect of the synthesized oligomers on pour point of crude oil samples (ASL-24 and Bakr-35) The data in Tables 4 and 5 shows the pour point of the crude oil samples (ASL-24 and Bakr-35) before and after addition of the synthesized oligomers at different concentrations (500, 1000 and 2000 ppm). The results in Tables 4 and 5 shows that the synthesized oligomers can decrease the pour point temperatures from

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65

C10P

Surface tension (mNm-1)

60 55

25 °C 40 °C 55 °C

50 45 40 35

0.00001 0.000025 0.00005

0.0001

0.00025 0.0005 Conc. (mol / l)

0.001

0.0025

0.005

0.01

60 55


25 °C 40 °C 55 °C

C12P

50 45 40 35 30

25

0.00001 0.000025 0.00005

0.0001

0.00025 0.0005 0.001 Conc. (mol / l)

0.0025

0.005

0.01

60


55

25 °C 40 °C 55 °C

C16P

50 45 40 35 30 25

0.00001 0.000025 0.00005

0.0001

0.00025

0.0005

0.001

0.0025

0.005

0.01

Conc. (mol / l)

Fig. 5. Surface tension of the synthesized oligomers at different concentrations and temperatures.

Table 4 Pour point of crude oil sample (ASL-24) before and after addition of the synthesized oligomers at 2000, 1000 and 500 ppm.

7.0 C10P C12P C16P

-1

Interfacial tension (mNm )

6.5 6.0 5.5

Sample

Pour point (°C)

Blank

7.22 °C 2000 (ppm) 5.55 °C 6.66 °C 1.11 °C

PC10 PC12 PC16

5.0 4.5

1000 (ppm) 3.88 °C 3.88 °C 1.66 °C

500 (ppm) 1.66 °C 1.66 °C 7.22 °C

4.0 3.5 Table 5 Pour point of crude oil sample (Bakr-35) before and after addition of the synthesized oligomers at 2000, 1000 and 500 ppm.

3.0 2.5 2.0 25

30

35

40

45

50

55

0

Temperature ( C) Fig. 6. Interfacial tension of the synthesized oligomers at different temperatures.

Sample

Pour point (°C)

Blank

18.33 °C 2000 (ppm) 4.44 °C 7.22 °C 10.00 °C

PC10 PC12 PC16

1000 (ppm) 7.22 °C 10.00 °C 18.33 °C

500 (ppm) 7.22 °C 10.00 °C 18.33 °C

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7.22 to 6.66 °C for ASL-24 and from 18.33 to 4.44 °C Bakr-35 crude sample at concentration 2000 ppm onto the crude under investigations. The pour point of the crude oil samples under investigation is decreased as the concentration of additive oligomers increases from 500 to 2000 ppm as shown in Tables 4 and 5. The alkyl chain could improve the compatibility of the oligomers in the crude oil, and could destroy the formed interlocking network of waxes [37]. It is clear from Tables 4 and 5 that oligomers PC10 and PC12 have more efficiency on reducing the pour point of the crude oil samples than PC16 oligomer. 4. Conclusions N-alkyl-2-aminopyridinum oligomers were synthesized. Their chemical structure was confirmed using FTIR and 1H NMR spectroscopy. The Density function theory (DFT) calculations confirmed the formation of the synthesized oligomers via open ring mechanism. The surface and interfacial tension measurements confirmed the surface activity of the synthesized oligomers. The synthesized oligomers were investigated as Pour Point Depressants (PPDs) for crude oils (ASL-24 and Bakr-35). The oligomers under investigation showed high efficiency as PPDs. The maximum pour point reduction was obtained when the crude oil was treated by the 2000 ppm of C12P for ASL-24 and 2000 ppm of C10P for Bakr-35 crude oil. While the minimum pour point reduction was obtained with the 2000 ppm of C16P for ASL-24 and 500 ppm of C12P for Bakr-35 crude oil. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ejpe.2018.09.001. References [1] P. Kriz, S.I. Andersen, Energy Fuels 19 (2005) 948. [2] A.L. Machado, E.F. Lucas, Petrol. Sci. Technol. 19 (2001) 197.

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