Available online at www.sciencedirect.com ScienceDirect
Energy Procedia 75 ( 2015 ) 847 – 852
The 7th International Conference on Applied Energy – ICAE2015 Characterisation of Asphaltenes Extracted from an Indonesian Oil Sand Using NMR, DEPT and MALDI-TOF
Ce Zheng, Mingming Zhu* and Dongke Zhang
Centre for Energy (M473), The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
Abstract
This paper reports the findings of a preliminary study into the molecular structure and properties of asphaltenes extracted from an Indonesian oil sand, Buton Oil Sand. The molecular structural parameters of the asphaltenes were determined using liquid 13C NMR spectrum in combination with DEPT. The molecular weight distribution of the asphaltenes was measured using MALDI-TOF mass spectrometer. The results indicated that the representative molecular structure of the asphaltenes from Buton Oil Sand predominately contained only one PAH core of 6 rings with an average molecular weight around 800 Da. The results also showed that the use of 13C NMR spectrum/DEPT provided a powerful tool and useful method for characterising and predicting the molecular structures of asphaltenes.
©© 2015 2015 The The Authors. Authors. Published Published by Elsevier by Elsevier Ltd. This Ltd. is an open access article under the CC BY-NC-ND license (Selectionhttp://creativecommons.org/licenses/by-nc-nd/4.0/ and/or peer-review under responsibility). of ICAE Peer-review under responsibility of Applied Energy Innovation Institute Keywords: Asphaltenes; DEPT; MALDI-TOF; Moleculer structure; NMR; Oil sand
1. Introduction
Oil sand comprises a significant proportion of the world’s oil reserves [1, 2]. The extraction and processing of hydrocarbons from oil sand are very challenging due to the high viscosity of the bitumen and the chemical impurities such as sulphur. Asphaltenes are the most refractory species in the oil sand bitumen causing most of the processing problems. Although there has been a remarkable progress in the understanding of several fundamental properties of asphaltenes [3, 4], the molecular structures of asphaltenes are still poorly understood. Extensive studies [5-17] have been carried out on coal-derived asphaltenes and petroleum asphaltenes but very little effort has been made to study oil sand asphaltenes, especially using liquid nuclear magnetic resonance (NMR) technique. In this work, 13C NMR in combination with distortionless enhancement by polarisation transfer (DEPT) was adopted to study the structure of the asphaltenes extracted from an Indonesian oil sand, Buton Oil
* Corresponding author. Tel.: +61-8-6488 5528; fax: +61-8-6488 7622. E-mail address: [email protected].
1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Applied Energy Innovation Institute doi: 10.1016/j.egypro.2015.07.176 848 Ce Zheng et al. / Energy Procedia 75 ( 2015 ) 847 – 852
Sand. A set of average structural parameters were estimated based on the NMR spectrum in combination with DEPT. The matrix-assisted laser desorption ionisation-time-of-flight mass spectrometer (MALDI- TOF) was also employed to measure the molecular weight distribution (MWD) of the asphaltenes. It was expected that the use of the three techniques would provide some new insights into the molecular architecture of the oil sand asphaltenes.
2. Experimental
2.1. Materials The “run of mine” Buton Oil Sand was first crushed and pulverized into fine powders with sizes less than 200 µm which was then subjected to Soxhlet extraction with toluene to obtain the organic fraction. The organic fraction extracted was then fractioned to obtain the asphaltenes according to ASTM D-3279. Briefly, the organic fraction was firstly subjected to Soxhlet extraction using n-heptane as the solvent for at least 48 hours until no discoloration. The extraction was then continued by replacing n-heptane with toluene for a further period of at least 8 hours. The final extract was filtered, evaporated and dried. The elemental compositions of the asphaltenes are listed in Table 1.
Table 1 Elemental compositions of the Buton oil sand asphaltenes
C H N S O (by difference) H/C Elemental analysis (wt%) 75.06 7.86 0.83 8.35 7.91 1.26 2.2. Materials characterisation The liquid 13C NMR spectroscopy analysis was performed using a Varian 400 NMR spectrometer operating at 100.55 MHz. Chromium acetylacetonate (Cr(acac)3) was added into the CDCl3 at a concentration of 0.01M to assure complete nuclear magnetic moment relaxation between pulses. The inverse gated decoupling technique was applied to suppress Nuclear Overhauser Effect. The spectrum was collected with a 45° flip angel, 6.50 seconds recycle delay and 30,000 scans. DEPT-135 was performed with the same sample on the same Varian spectrometer, using a polarisation transfer delay J = 140 Hz, a relaxation delay of 6.50 seconds and 7,500 scans. The MWD of the asphaltene sample was measured using a Bruker Ultraflex III MALDI-TOF mass spectrometer. Experiments with/without matrix were carried out in parallel to examine the effect of matrix on results. In the case of matrix employed, α-cyano-4-hydroxycinnamic acid (CHCA) was used. The detection range was executed from 0 to 4,000Da, while signals below m/z = 200 were suppressed to eliminate the background noise.
3. Results and discussion
3.1. 13C NMR A typical 13C NMR spectrum of the asphaltenes is shown in Figure 1. It is seen that the 13C NMR spectra are well separated into the aromatic region with the chemical shift ranging from 100 to 160 ppm and the aliphatic region with the chemical shift ranging from 0 to 60 ppm. The aromaticity is calculated using Dickinson’s equation [7]: (1)
where far represents aromaticity; Car and Cal are the amount of aromatic and aliphatic carbon, respectively. Ce Zheng et al. / Energy Procedia 75 ( 2015 ) 847 – 852 849
Figure 1 13C NMR spectrum of the Buton oil sand asphaltenes
Figure 2 (a) DEPT-135 NMR spectrum of the Buton oil sand asphaltenes and (b) subdivision of aliphatic region of the DEPT-135 NMR spectrum of the asphaltenes 3.2. DEPT Figure 2 (a) shows a typical DEPT-135 spectrum of the Buton oil sand asphaltenes and Figure 2 (b) shows the subdivision of the aliphatic region of the DEPT-135 spectrum. The assignments of the chemical shift regions according to different carbon types are summarised in Table 2. A large body of literature [6, 10-13, 15] has indicated similar assignments.
Table 2 13C NMR chemical shift regions for the Buton oil sand asphaltenes
Regions in ppm Types of Protons
5.0-18.0 CH3 Terminal or branched methyl groups in aliphatic chains, except where branched at α or β position from an aromatic ring or where two methyl groups are terminal 18.0-22.0 CH3 α,β Methyl groups in aliphatic chains branched at α or β position from an aromatic ring, plus where two methyl groups are terminal
Aliphatic 22.0-32.0 CH2 Methylene groups in aliphatic chains and in naphthenic rings, except where branched at α or β position from an aromatic ring, or at α position from a naphthenic ring 32.0-34.5 CH Methine groups in naphthenic rings
34.5-42.0 CH2 α,β Methylene groups branched at α or β position from an aromatic ring, or at α position from a naphthenic ring 42.0-60.0 CHnaph Methine groups in naphthenic rings
100-137 Cbr, Cpr Bridgehead aromatic carbon, overlapping with protonated/hetero aromatic carbon Aromatic 137-150 Csub Substituted aromatic carbon 150-160 Cphen Phenolic carbon
It is difficult to clearly distinguish the types of aromatic carbons in the aromatic region where the 13C NMR spectrum suffer from severe overlap. In the literature, researchers tend to simply resolve it into two 850 Ce Zheng et al. / Energy Procedia 75 ( 2015 ) 847 – 852
regions at a cut-off point of around 126.5 ppm [9, 12, 17]. Here, we adopt the method proposed by Andrews et al. [15] to determine the amount of protonated aromatic carbon using DEPT-135. Briefly, a quantitative ratio between paraffinic methyl carbon to protonated aromatic carbon in the DEPT spectrum was obtained and then applied into the quantitative 13C NMR spectrum to calculate the intensity of the protonated aromatic carbon in the overlapping region. The results are discussed in the following section. 3.3. Average Structural Parameters The average structural parameters are defined [5, 10] and calculated from the 13C NMR in combination with DEPT spectra with the results listed in Table 3. In order to estimate the aromatic carbon per cluster C, we espouse the theory developed by Solum et al. [18]. The percentage of bridgehead aromatic carbon to total aromatic carbon is defined as Xb determined following the method proposed by Andrews et al. [15]. The relationship between Xb and the number of aromatic carbon per cluster C is governed by:
(2)
where C0 and m are adjusting parameters to give the best fit of the dependence of Xb on C; and represent the mole fraction of bridgehead carbon in catacondensation and in pericondensation. From the structural parameters calculated, it is shown that the Buton oil sand asphaltene has a relatively low aromaticity and a high percentage of phenolic aromatic carbon, which matches the relatively high H/C atomic ratio and the high oxygen content of the asphaltenes as shown in Table 1. The average number of the aromatic carbon is around 22, indicating an average PAH size of 6 rings in a form of pericondensation. A great deal of publication has delivered similar findings on asphaltenes isolated from both coal and petroleum [3, 4, 15, 19, 20]. 3.4. MALDI-TOF The MWD spectrum of the asphaltene measured using MALDI-TOF is shown in Figure 3. The result presented here was obtained without the use of matrix as no difference was observed with and without the use of matrix. It is seen that the average molecular weight on the spectrum is around m/z = 800Da. Considering that MALDI-TOF mainly generates single-charged species (z=1), the average molecular weight of the asphaltene is believed to be ca. 800Da. This is in agreement with the average molecular weight calculated using the 13C NMR/DEPT method shown in Table 3, thus accrediting the validity and accuracy of the 13C NMR/DEPT method. Moreover, the island molecular architecture with only one PAH cluster per molecule is found predominating for this sample, as the average molecular weight obtained from MALDI-TOF spectrum can only accommodate a single PAH.
4. Conclusions
The present work demonstrated that the combined use of 13C NMR/DEPT and MALDI-TOF allowed the chemical structure of the Buton oil sand asphaltenes to be revealed. The main conclusions may be drawn as follows: (1) this combined elemental analysis/13C NMR/DEPT method was shown to be reliable in predicting the average structural parameters of the asphaltene; (2) the Buton oil sand asphaltenes were shown to contain a high level of oxygen and sulphur; (3) the average PAH size of the Buton oil sand asphaltenes was 6 fused rings; (4) the average molecular weight of the asphaltenes was found to be about 800Da; and (5) the island architecture with only one PAH cluster per molecule was shown to predominate in the Buton asphaltenes. Ce Zheng et al. / Energy Procedia 75 ( 2015 ) 847 – 852 851
Figure 3 Profile of the MALDI-TOF spectrum of the Buton oil sand asphaltene
Table 3 Structural parameters defined and calculated using 13C NMR and DEPT spectra
Average Molecular Parameters* Asphaltenes Average Molecular Parameters* Asphaltenes
Carbon aromaticity Car % 44.44 Phenolic aromatic Cphen % 1.22
Carbon aliphaticity Cal % 55.56 Xb – mole fraction of bridgehead C 0.453
Terminal methyl group CH3 % 6.25 C – average aromatic PAH size 22.11
Methyl group at α, β position CH3 α,β % 5.35 Substitutions per PAH 9.15
Methylene group CH2 % 26.17 Average length of substitutions 3.02 Methine group CH % 4.77 Molecular carbon atoms* 49.75
Methylene group at α, β position CH2 α,β % 9.06 Molecular Hydrogen atoms* 62.47
Methine group in naphthenic rings CHnaph % 3.97 Molecular nitrogen atoms* 0.48
Protonated/hetero aromatic Cpr % 13.62 Molecular sulphur atoms* 2.07
Bridgehead aromatic Cbr % 11.17 Molecular oxygen atoms* 3.89
Substituted aromatic Csub % 18.40 Average molecular weight* 795 * assuming that each molecule only contains one PAH cluster.
Acknowledgements
Partial financial support for this research has been received from the Australian Research Council under the ARC Linkage Projects Scheme (Project # ARC LP100200135). The authors acknowledge the facilities, and the technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation & Analysis, The University of Western Australia, a facility funded by the University, State and Commonwealth Governments. The authors also thank Dr. Reza Zareie of the School of Chemistry and Biochemistry, The University of Western Australia for setting up the MALDI-TOF and data analysis. Ce Zheng also acknowledges the University Postgraduate Awards scholarship provided by UWA.
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Biography The biography of the corresponding author. (50 words)
Dr Mingming Zhu is a research fellow at the Centre for Energy at the University of Western Australia. His research interests include diesel and biofuels combustion in diesel engines, pyrolysis of biomass and wastes, combustion of biogas in gas engines, anearobic digestion, and characterisation and utlisation of heavy oil.