Characterization of Naphthenic Acids in Thermally ... - ACS Publications

Jan 31, 2018 - Naphthenic acids (NAs), present in a typical Brazilian acid crude oil and its thermal degradation products, were investigated using two...
1 downloads 4 Views 2MB Size
Subscriber access provided by READING UNIV

Article

Characterization of naphthenic acids in thermally degraded petroleum by ESI(-)-FT-ICR MS and 1H NMR after solid phase extraction (SPE) and liquid/liquid extraction Eliane V. Barros, Heloisa P Dias, Fernanda E Pinto, Alexandre de Oliveira Gomes, Robson R Moura, Alvaro Cunha Neto, Jair C. C. Freitas, Gloria M. F. V. Aquije, Boniek Gontijo Vaz, and Wanderson Romão Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03099 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

Graphical Abstract

2

3 4

5

6

7 8 9 10 11

1 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 32

12 13 14

Characterization of naphthenic acids in thermally degraded petroleum by

15

ESI(-)-FT-ICR MS and 1H NMR after solid phase extraction (SPE) and

16

liquid/liquid extraction

17

Eliane V. Barros,a,b Heloisa P. Dias,a Fernanda E. Pinto,a Alexandre O. Gomes,c Robson R.

18

Moura,c Alvaro C. Neto,a Jair C. C. Freitas,d Gloria M. F. V. Aquije,e Boniek G. Vaz,f Wanderson

19

Romão,a,e†

20

a

21

Santo, 29075-910 Vitória, ES, Brazil.

22

b

23

ES, Brazil.

24

c

CENPES, Petrobras, 21941-598 Rio de Janeiro, RJ, Brazil.

25

d

Laboratory of Carbon and Ceramic Materials, Department of Physics, Federal University of

26

Espírito Santo, 29075-910, Vitória, ES, Brazil.

27

e

28

Velha, ES, Brazil.

29

f

30

Correspondent author: †[email protected] / Phone: + 55-27-3149-0833

Petroleomic and Forensic Laboratory, Chemistry Department, Federal University of Espírito

Federal Institute of Education, Science and Technology of Espírito Santo, 29056-255 Vitória,

Federal Institute of Education, Science, and Technology of Espírito Santo, 29106-010 Vila

Chemistry Institute Federal University of Goias, 74690-900, Goiânia, GO, Brazil.

31 32 2 ACS Paragon Plus Environment

Page 3 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

33 34

ABSTRACT. Naphthenic acids (NAs), present in a typical Brazilian acid crude oil and its

35

thermal degradation products, were investigated using two separation methodologies: solid phase

36

extraction (SPE) and liquid-liquid extraction (LLE). Fractions produced were characterized by

37

proton nuclear magnetic resonance spectroscopy (1H NMR) and negative-ion mode electrospray

38

ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI(-)-FT-ICR MS).

39

Among the NAs extraction methods studied, the SPE was more efficient about LLE. Besides, the

40

ESI(-)FT-ICR MS results showed that the SPE method with eluent phase variation covered the

41

detection of a larger amplitude of NAs compounds (m/z 200-1200), reducing the occurrence of

42

ion suppression on the NAs of higher average molecular weight distribution (Mw). It was possible

43

to note that the aromaticity or double bond equivalent (DBE) of these produced collective

44

fractions as well as their Mw values increased as a function of the polarity of the extraction

45

system (DCM → DCM:MeOH:FA). Also, 1H NMR analysis revealed the alkyl predominance

46

evidenced by the presence of high Hβ content in fractions, suggesting that the NAs compounds

47

have long and unbranched chains. The behavior of NAs species during the thermal degradation

48

process was also evaluated, and the results showed the presence of them in only five SPE extracts

49

out of six, containing different Mw values (Mw = 366, 417, 531, 662, and 836 Da). This suggests

50

that in the last SPE fraction (named SF6: m/z 700-1150; carbon number of C52-C72 and DBE = 0-

51

15; detected only in virgin crude oil), the NAs were selectively cracked during the thermal

52

degradation process.

53

Keywords: petroleomics, naphthenic acids, extraction methods, SPE, ESI(-)FT-ICR MS, 1H

54

NMR.

55 3 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

56

Page 4 of 32

1. Introduction

57

The presence of naphthenic acids (NAs) in petroleum implies in a significant impact on its

58

productivity and economic value, since under certain conditions it may favor the formation of

59

emulsions and the corrosion of materials.1 In addition, NAs can precipitate as metal salts

60

(naphthenates), where the most common being calcium naphthenate.1,2 This type of deposit is

61

problematic, being formed in oil production systems with high total acid number (TAN).2

62

The NAs represent an average of around 2 to 4 wt. % of the crude oils, being composed of

63

a complex mixture of monocarboxylic acids of chemical formula defined as CnH2n+zO2, where n

64

represents the number of carbon (CN) atoms and z indicates the deficiency of hydrogen elapsed

65

from the cyclization of the structure, where z ≤ 0.3 Despite this definition, the term NA is also

66

commonly used in a broader sense, encompassing all organic acids (cyclic, acyclic and aromatic)

67

found in crude oil. The identification and characterization of NAs in an oil matrix have shown to

68

be quite challenging since a single crude oil sample can contain about 1,500 different NAs with a

69

range of their molecular weight from 115 to 1500 Da.3,4,5

70

The literature reports some concern on the behavior of NAs present in crude oils subjected

71

to high temperatures. This is because crude oils with TAN above 0.5 mg of KOH g-1 oil are

72

considered unsuitable for processing in traditional refineries. Although the corrosion does not

73

present a linear relation with the TAN; in the most cases, the TAN directly influences in

74

corrosion rate of petroleum.6 Based on classical and high-resolution analytical techniques, Barros

75

et. al.7 concluded that when heavy and acidic crude oils are exposed to long periods of thermal

76

degradation (t ≥ 24 h), they have characteristics of "lighter" oils due mainly to the disaggregation

77

of resins and asphaltenes and decarboxylation of NAs, thus favoring, the TAN reduction in a 4 ACS Paragon Plus Environment

Page 5 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

78

level of 68% after 36 h of degradation at 350 ° C. 6, 7 Therefore, at high degradation temperatures

79

(T ≥ 280 oC), the crude oil undergoes thermochemical reactions favoring the decarboxylation of

80

NAs (CO2 elimination) and the formation of light hydrocarbons (HCs).7 In this context, Fu et al.8

81

proposed a possible mechanism of thermal decarboxylation of NAs, which can be explained in

82

three steps, as shown in Figure 1a. Initially, NAs are deprotonated to produce carboxylate anions

83

(reaction 1), which are stabilized by resonance. Then, a heterolytic cleavage occurs on the CO2

84

group forming negatively charged alkyl radicals (reaction 2). Finally, the radicals combine with

85

H+ species to form the HCs (reaction 3). Another possible pathway is via [1,4]H rearrangement

86

as illustrated in Figure 1b. By this mechanism pathway, carboxylic acids with low CNs are

87

formed.

88

5 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 32

89 90 91

Figure 1. Mechanism of thermal decarboxylation of NAs, adapted from Fu et al.8 (a) and

92

proposed by the present study (b).

93

Some studies highlight advances in the use of analytical instrumentation for analysis of

94

NAs,9-21 where the main techniques to characterize them are: gas chromatography coupled to 6 ACS Paragon Plus Environment

Page 7 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

95

mass spectrometry,9,10 Fourier transform infrared spectroscopy,11 nuclear magnetic resonance

96

(NMR) spectroscopy12,7 and high-performance liquid chromatography.9 Besides these techniques,

97

some modern methods have been applied due to their high selectivity, such as two-dimensional

98

gas chromatography associated with time-of-flight mass spectrometry,10 tandem mass

99

spectrometry with electrospray ionization source,17 high-performance liquid chromatography

100

coupled with time mass spectrometry,18,19 and Fourier transform ion cyclotron resonance mass

101

spectrometry (FT-ICR MS).7,20,21

102

FT-ICR MS provides an ultra-high mass resolution power and mass accuracy that results

103

in high degree of confidence in the molecular weight assignments and, consequently, being

104

suitable for the complex mixtures analysis in petroleomics.3

105

negative ion-mode electrospray ionization (ESI(-)), is often used for polar compounds analysis,

106

generating deprotonated molecules, which are heteroatom species such as NAs, phenols, and

107

species with carbazole core structure. Another powerful analytical technique for the analysis of

108

petrochemicals is NMR spectroscopy, which is widely used for structure elucidation and

109

confirmation of organic compounds.3,5,7,13,14 Its coverage ranges of oil samples, from the

110

characterization of lighter fractions (jet fuels and gas oils) to heavier fractions of oil (vacuum

111

residues, crude oils, tar pitches, resins, and asphaltenes),13,15,16 allowing a fast, non-destructive

112

and quantitative analysis.

FT-ICR MS, associated with

113

However, prior to the characterization of petroleum fractions, such as NAs, three

114

separation methods have been commonly employed: liquid-liquid extraction (LLE), solid phase

115

extraction (SPE), and the preparative liquid chromatography.9,11,23-26 In the LLE, alkaline

116

solutions are usually employed to obtain NAs fractions.11,27,28 Colati et al.11 proposed, to isolate

7 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 32

117

NAs by LLE, washing with alkaline hydroalcoholic solutions, achieving selectivity and acid

118

removal efficiency with reductions in the TAN of around 90% at pH 14. On the other hand, the

119

SPE method allows the separation of NAs using a wide range of materials, which, the literature

120

highlights: zeolites, clays, aluminosilicates, silica gel, granulated activated carbon, and ion-

121

exchange resins.28 Rowland et al.21 employed an SPE methodology to obtain NAs subfractions

122

using aminopropyl silica, reaching a separation of NAs by their polarity differences and by

123

distinct Mw ranges, being characterized by ESI-FT-ICR MS.21

124

In the present work, we investigate NAs species present in an acid crude oil and its

125

thermal degradation products obtained by two separation methodologies, which are SPE and

126

LLE, where the produced fractions were characterized by 1H NMR and ESI(-)-FT-ICR MS. The

127

1

128

NAs corresponding to linear, naphthenic and aromatic chain molecules.

H NMR spectra of some acid fractions were compared to those of the commercial standards of

129

130

2. Materials and methods

131

2.1 Chemicals

132

All chemicals employed were of analytical grade (purity higher than 99.5%). Potassium

133

hydroxide, toluene, diethyl ether, chloridric acid, n-heptane, and dichloromethane were supplied

134

by Vetec Fine Chemicals Ltda, Brazil. Petroleum ether, ethanol, and methanol were purchased by

135

Dinâmica Química Contemporânea LTDA, Brazil. Formic acid, dichloromethane-d2 (99.5 atom

136

% D, contains 0.03 wt%, tetramethylsilane, TMS), ammonium hydroxide, sodium trifluoroacetate

137

and NAs standards (decanoic acid, pentadecanoic acid, stearic acid, arachidic acid, cyclopentane 8 ACS Paragon Plus Environment

Page 9 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

138

carboxylic acid, cyclohexane-1,1-dicarboxylic acid, 3-cyclohexane propionic acid, naphthoic acid

139

and naphthenic acetic acid) were purchased from Sigma-Aldrich Chemicals, USA. A mega bond

140

elut-NH2 cartridge, 2g x 12 mL, was supplied by Agilent Technologies, USA.

141

142

2.2 Characterization and thermal degradation of petroleum

143

In this study, crude oil (A0) was selected from a production field located in a sedimentary

144

basin on the Brazilian coast. It was characterized in the facilities of the Petroleum

145

Characterization Laboratory of the Federal University of Espírito Santo (Labpetro/UFES),

146

following the American Society of Tests and Materials (ASTM). Thus, the API gravity was

147

obtained according to ISO 12185 (1996)29 and TAN was measured according to ASTM D664

148

(2011) by potentiometric titration (Metrohm 836 automatic titrator).30 The thermal degradation

149

assays were performed using an autoclave containing approximately 1000 mL of the oil and

150

heated to 320 °C during degradation times of 24 and 72 h, producing two degraded samples

151

named A24 and A72.

152

2.3 Extraction and characterization of NAs

153

The acid fractions were extracted from samples A0, A24, and A72 by the LLE method

154

proposed by Colati et al.,11 and by SPE method proposed by Rowland et al.21 Figure 2a-c shows

155

an overall scheme of the extractions performed in this study. In the SPE procedure, fractions

156

resulting from clean up steps – CU1 and CU2 (Figure 2b); as well as CU-I to CU-IV (Figure 2c)

157

– were not included in the characterization analysis. Acid fractions of the NAs were characterized

158

by 1H NMR and ESI(-)FT-ICR MS. 9 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 32

159 160

Figure 2. Scheme of NA extractions: (a) LLE; (b) SPE and; (c) SPE as a function of Mw. (AE –

161

aqueous extract, CU – Clean up, EtOH – Ethanol, MeOH – Methanol, FA – formic acid, SF –

162

acids subfractions). (Adapted from refs. 11 and 21)

163

164

2.3.1 1H NMR

165

1

166

H NMR spectra were acquired on a Varian (VNMRS 400) spectrometer, operating at a

9.4 T magnetic field using a 5mm Broad Band 1H /19F /X probe at 25.0 ºC.

167

For the analysis of the nine commercially available NA standards, a mass of 20-30 mg of

168

each standard was dissolved in 600 µl of dichloromethane-d2 (CD2Cl2). The analyses were

169

performed under the following experimental conditions: 1H NMR frequency of 399.73 MHz;

10 ACS Paragon Plus Environment

Page 11 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

170

6410.3 Hz spectral window; 90º pulse with a duration of 12.2 µs; 3.8339 s acquisition time; 64

171

scans; and TMS as the reference standard.

172

The crude oil samples and their respective NAs fractions were also dissolved in CD2Cl2,

173

according to Barros et al.7 The analyses were carried out under the following experimental

174

conditions: frequency of 399.73 MHz; 6410.3 Hz spectral window; 90º pulse with duration of

175

9.32 µs; 2.5559 s acquisition time; 512 scans; and TMS as the reference standard.

176

177

2.3.2 ESI(-)-FT-ICR MS

178

The analyses were carried out on a Bruker Daltonics® mass spectrometer (model 9.4 T

179

Solarix, Bremen, Germany), equipped with a commercially available ESI source, configured to

180

operate over a mass range of m/z 200-2000. The FT-ICR mass spectra of samples were acquired

181

using the negative ionization mode, ESI(-).

182

For the analysis of the samples obtained from LLE, 2 mg of the acid fraction were

183

dissolved in 1 mL of toluene. Then, 0.5 mL of this solution was mixed with 0.5 mL of methanol

184

(1:1) at 0.1% m/v of ammonium hydroxide (NH4OH), similar to the procedure described by

185

Colati et al.11 The acid fractions obtained by SPE were dissolved up to 1 mg mL-1 in a toluene:

186

methanol solution (1:1) containing 0.25% m/v of NH4OH.

187

The ESI(-) conditions were: nebulizer gas pressure of 1.5 bar, a capillary voltage of 3.9

188

kV and transfer capillary temperature of 250 ºC. The ion accumulation time in the hexapole was

189

0.1 s, and each spectrum was acquired by accumulating 200 scans of time domain transient

190

signals of 4 mega-point, transient length of 2.936 s, and a magnitude mode processing. The 11 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 32

191

trapping voltage of the ICR cell was -0.60V. All mass spectra were externally calibrated using

192

0.05 mg mL-1 of a sodium trifluoroacetate (NaTFA) solution (m/z from 200 to 1200), followed by

193

internal recalibration using a set of more abundant homologous alkylated compounds for each

194

sample. The resolving power (m/∆m50% ≈ 550 000, where ∆m50% is the full peak width at half-

195

maximum peak height) of m/z = 400 and mass accuracy