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Today and destined circumstances, with expeditiously building petroleum reservoirs, demands broadened consideration to be given to the adaption of petroleum residues into the corresponding distillate products. The extended interest in the study of chemistry and investigation of petroleum residues has started from 1973 during analysis of oil crisis. Curiosity has been maintained in establishing processes for elevate residues by reduction of various minerals like sulphur and other metals in the petroleum into corresponding distillate products. Both, chemical structural and also constitutional data on the chemical residues of the petroleum are needed for minimizing the altering conditions, for accomplishing combined designation and for establishing efficient methods of unit designs. It is thus advantageous to summarize the recent inventions in the petroleum residues studies in terms of both analytical and structural chemistry. There are many analytical techniques used to explore study the chemical and structural analysis of o] petroleum residues, among which FT-IR, NMR, etc commonly used methods. The present report is sought to focus on the on NMR spectrometric techniques used in the petroleum residual, structural analysis in addition to the molecular weight and also experimental data of the petroleum residues. Due to focus on heteroatom compounds and wide range of molecular weights in the petroleum residues, it is compulsory to separate them into fractions that are suitable for obtaining significant results by NMR spectroscopic techniques. The main review of the NMR spectroscopic technique used in exploration of the petroleum residues are been discussed in detail in this report.
Characterization and analysis of the structure of various carbon/coal materials precursors, like crude oil, petroleum residues etc is a great challenge that should be addressed in a way to appropriately superlative/optimize the production process. The chemical intricacy of these petroleum residues, where more than a millinery chemical compounds are present.1 Nonetheless, has meant that achieving comprehensive information about its chemical constitution is not practical and a range of average structural criterion has been employed alternatively. A variety of these criterion has been approximately calculated from nuclear magnetic resonance (NMR) spectroscopic technique analysis, with few understandable ones achieved from infrared (IR) spectroscopy, regardless of in many cases the esteemed amount/calculation depend on expectations that brings out appreciable delusion in the results.2 These criterion are most commonly exert in average chemical molecule, which assist as a way of anticipating the differences in their structures within various samples, although these structures are less depictive as the instance samples analyzed becomes highly amalgamate/heterogeneous, so that there is some argument beholding their advantages.3 The easy, and most hired, criteria is the aromaticity index, achieved by NMR or FT-IR, which is characteristic of the increase of the polyaromatic systems, a basic process in the compilation of petroleum residues in coke and pitches.4, 5 With the help of NMR spectroscopic analysis, the aromaticity index is achieved as the proportion of aromatic carbon/hydrogen to total carbon/hydrogen respectively. (In FTIR method of analysis, the proportion of the excitement (or intensities) of the peaks at more than 3000 cm-1 for aromatic C-H and 2700-3000cm-1 ranges for aliphatic C-H were used. 6
Aromaticity indexes are exemplarily achieved from 13C NMR spectra, which dispense direct knowledge/information about the residual carbon structures seen, but this NMR analysis is highly time consuming method. 1H NMR spectra is achieved in less time, and a semi observational correlation between the 1H and 13C spectra derived aromaticity indexes values (which are actually corresponding aromaticity indexes of hydrogen and carbon ) was achieved for a large variety of petroleum and coal residues.7 FTIR is the easy and economical analysis method and its use in replacement of 1H and 13C NMR has been extensively investigated for various coal and petroleum residues. 8-10
Chemistry of other elements (sulfur) in the petroleum - a recent perceptive:
An extensive research has been carried out in case of convergence and assessment of various organic sulfur and sulfur derived residues in the coke and petroleum residues, which has drawn great assiduity/attention in the recent years of petroleum analysis study.11 A common advent of many scientists has been derivatization of the various sulfur and related properties, especially thiols, with discrete chemical used in the NMR analysis with the goal of obtaining nuclei of the molecules which are NMR-sensitive. For instance, trimethylsilyl derivatives of the sulfur/thiol functional groups for the characterization and analysis of the samples of thiols in various coal/cola derivative, petroleum residues are obtained using 29 Si NMR. It has been reported that NMR method of analysis for thiols which in its native state compared to the sample chemical analyzed or sulfides and/or thiophenes which can be formed upon reduction are limited for their characterization methods. None of the major attempts couldn't give the desired results; it is due to the reason of the major sulfur possessing/constituents in petroleum. 12
There has been an extensive research carried by G. S. Walso et al which explained the methods for recognizing and evaluate/quantify various forms of sulfur products including sulfides, thiophenic groups etc in the natural petroleum residues sample. Waldo et al used 13C-enriched methyl iodide and silver tetrafluoroborate treated samples of sulfide compounds of the petroleum residues (forms, methyl sulfonium complexes). 13C NMR spectroscopy analysis study were carried for the methylated samples, and revealed that the carbon of the externally supplemented methyl group was found to be very cognizant or sensitive to the makeup (or nature) of the bounded sulfur atom. The novelty of Waldo group lies in the achievement of developing a method for differentiating various sulfides and thiophenes using NMR. This method is ultimately adapted to a study the crude petroleum samples to demonstrate the implication of this technique for thiophenes and sulfides characterization in crude oils/petroleum.
13C NMR Spectroscopy in petroleum analysis:
The one dimensional carbon-13 nuclear resonance spectroscopy (NMR) experiment is comparatively susceptible or sensitive than proton 1H NMR, but possess huge chemical shift ranges. 13C possess less sensitive nucleus that results in distinct signals (peaks) and has a wide distribution of their chemical shifts. A routine analysis of 13C NMR spectrum includes matching familiar chemical shifts to the anticipated i.e., expected chemical moieties /groups.
Chemical shift ranges of carbon NMR
Figure - Ranges of chemical shift ranges of various carbon types depending on the environment of the carbon.
Integration is not useful in a typical 13C NMR spectrum due to the uneven nuclear Overhauser effect denoted as NOE, NOE intensifying of the signals by its decoupling and longitudinal relaxation times (T1's). Inverse gated decoupling and also huge delay in the region of nearly ten minutes in between the pulses can amplify or quantify the spectra. An example of ethylbenzene decoupling spectrum is shown in figure 2, where the enhancement due to nuclear overhauser effect (NOE) and under explainable conditions to the spectrum quantitatively. The NOE enhancement is higher than the one at typical conventional conditions where much lesser reproduction or repetition times and using sensitivity enriching window functions were used.
NOE enhancment in 13carbon NMR
Figure - Ethylbenzene NOE enhancement in a 13C NMR spectrum.
The chemical shifts of the carbon are used for the starting designation of the spectrum. In case of ethylbenzene, from the NMR data it can be seen that there are peaks in the regions of 15.6 and 28.9 ppm which fall in the regions of aliphatic nature, therefore belonging to the alakne and alkene carbons i.e, CH3 and CH2 carbons. Alkane group carbons commonly has lower chemical shift compared to CH2, therefore it can be temporarily ( or provisionally) assigned to the regions 15.6 and 28.9 ppm, respectively. The rest of the remaining signals of the chemical shifts falling in the aromatic regions with peaks at 125.6, 127.8, 128.3 and 144.2 chemical shifts. The 'quaternary' carbon is the one that has no hydrogen or protons attached to it regardless of being a misnomer for the carbon atom which is unsaturated such as in the present example where the carbon is attached to the three other neighboring carbon atoms. The quaternary carbons attached to three other carbon atoms commonly gives sharper signal peaks than the other carbon types and generally gives poor signals under generally used acquisition parameters- decoupling and relatively short repetition times as shown in figure 3 below.
Quaternary carbons appear lower than the others
Figure - NOE enhancement concept used in the distinction of the quaternary from proton-attached carbons.
This is due to the reason of sloe relaxation and also dearth in the NOE intensification. The aromatic carbons chemical shifts for the quarternay carbons with no hydrogen attached to it in case of the ethylbenzene i.e., C1 usually higher than the other carbons C2, C3 and C4 signals. Consequently C1 chemical shift is designated to the signal corresponding to 144.2 ppm The two pairs of carbons including chemical shifts of C2, C6 and C3, C5 equivalent carbons and there with the integral of two in the determinable spectrum while integral is 1 for the C4.
All the three chemical shifts signals are of C-H type so that their line-widths and nuclear overhauser effect enhancement upon decoupling are analogous. As a consequence, the ratio of 2:2:1 in the heights of the peaks is admissibly well conserved in the general decoupled spectrum and can be utilized to designate or assigning C4 corresponding to the 125.6 ppm signal. If the two dimensional experiments were used to obtain a full designation then no supplementary one dimensional experiments are required. Nevertheless, if it is a simple molecule or only a fractional assignments is essential then one or additional one dimensional experiments including gated coupling and DEPT spectrum could be beneficial. When the 13C spectrum is not responsive enough, the projection utilizing long and short ranges of heteronuclear correlation spectrum can be used in order to enhance 13C spectrum sensitivity. Further detailed analysis can be achieved with two dimensional NMR spectrums.
13C-NMR spectroscopy in comparison with the IH-NMR spectroscopy, the former spectroscopy is a method which has been refined or developed only during the last decennium. The major drawbacks of 13C-NMR spectroscopy is, 13 C is less abundant and its negligible magnetogyric ratio resulting in the less susceptible or sensitiveness.20 Later on, these less sensitivity has been overcome by the accession of I-'C-pulse Fourier transform technique. 20 The conquering significance of the 13C-NMR spectroscopy as an interpretive tool is based on its capacity to provide precisely both statistical and compositional knowledge on the composite organic chemical molecules possessing carbon skeleton in its structures.21 Differences in the chemical shielding, coupling constants and also relaxation convention are the major physical criterion used for illustrating the structures of various organic carbon molecules with 13C-NMR spectroscopy.21 chemical shielding and relaxation behaviors are the most important among these parameters which are of special importance in the petroleum residual spectroscopic analysis. 2
The chemical characterization analysis of the complex mixtures has always played a key role in the crude oil, petroleum and petrochemical manufactories so as to get enrich the understanding of the relationship characteristic properties associated with its chemical structures and process its development. Among all the above mentioned analytical techniques in studying the petroleum products, NMR spectroscopy has been proved to be have significant importance for giving detailed and discriminating knowledge on its structure about the constituents of various petroleum distillates fractions and process course.22 The recent advances in spectroscopy where, diffusion-ordered NMR spectroscopy (DOSY) has been used as one of the main method in petroleum complex analysis using NMR spectroscopy. The DOSY is mainly used to establish the constituents of the mixtures is dependent on the variations in the diffusion coefficients of the distinctive components. The diffusion coefficient is measured using spin echo experiment concept with DOSY, commonly named after pulsed field gradient (PFG) in its two dimensional DOSY version. The DOSY results are always demonstrated as one dimension and diffusion invariable in the other. 23,-26 Consequently, the results suggests the researchers to use separation methods using chromatography, but in an NMR spectroscopy tube.
In modern times, the DOSY experiments have gained significance and been implemented by overcoming the determination disagreements (resolution problems) by heteronuclear and three dimensional variants criterion introduced in its DOSY experiments.27, 28, 29 The 13C NMR and 1H NMR spectroscopy of the fractional petroleum distillates are increasing overlapped and also compacted due to the occupation of various to aromatic, naphthenic and liner and branched paraffinic mixture of molecules. 30 In the initial days of the research in petroleum analysis, various separation methods in association with DEPT, GASPE were used in addition to the NMR spectral techniques. 30 These allow the researcher to get an detailed appointment of the signlas of the chemical shifts followed by the certainty of the average structural statuses. DOSY offers an alternative relevant and efficient technique for the petroleum and oil complexes and distillates with quantitative NMR analysis. 30
Qualitative Aspects of 13C-NMR spectroscopy:
One of the characteristic advantages of the 13C-NMR spectroscopy is its many-fold deflection (or variation) in the shielding of the carbon when compared with the protons. Acquisition of 13C chemical shifts including aliphatic, alkanes, aromatic hydrocarbon, cyclic structures of alkanes and also other heterocyclic chemical moieties, esters, ether, carbonyl groups etc are the most characteristic components of the petroleum and oil residues. 31-34 In 13C-NMR spectroscopy, aliphatic hydrocarbons are seen to usually absorbed at slightly higher ranges of about 10 to 50 ppm. For aromatic hydrocarbons, the range is between 118 to 160 ppm. Thus it can be noted and allows a understandable characteristic distinction between the aliphatic and aromatic hydrocarbon residues in the petroleum (even with the weal S/N proportion). The saturated carbons can be differentiated into two main categories; aliphatic and naphthenic carbons. In turn, aliphatic carbons can exist in alkanes and their aromatic rings with alkyl substituent's in the petroleum residues. The naphthenic carbons in the aromatic regions were designated with a frequencies resonating ranging between 20 to 45 ppm in case of mono- and poly-cycloalkanes.35
The 3C NMR spectrum of the petroleum residues with saturated moieties are very complex in their nature and so provisionally considered to have a extended envelop because of the naphthenic carbons which are overlapped proportionately sharp lines because of the open chain alkyl moieties, thus accounting for the burdensome to conclude aliphatic and naphthenic carbons from the basic NMR data obtained from the petroleum distillates. There are many reports explaining various attempts made to determine the naphthenic carbons from the frequencies ranges between 20 to 45 ppm after takeoff of the frequency contributions from the sharp signals due o the general and isoparaffins in this area, accommodating exceptional agreement with n-d-M agreements. 36
Dalling and group reported that the alkyl carbons (methyl groups) attached to the aromatic ring was seen to absorb at 14.6 to 16.9 ppm and 18.5 to 21.0 ppm. 37 Nonetheless, some obscure supplements from the other aliphatic carbons in these areas may announce contingency (uncertainty) in the chemical residues. Maekawa et al. have reported that the CH3 alpha carbon adjacent to the aromatic ring commonly absorbed and frequencies ranges of these groups are found to be 19 to 22 ppm, which could be distantly broaden to higher and lower frequencies because of the presence of neighboring heterocyclic residues with nitrogen, hydroxyl phenolic groups. The 13C-NMR spectrum with the aliphatic carbon signals have been designated by Snape et al. 38
Fischer et al reported the spectral peaks for arylic alkyl (methyl) groups in sterically unhindered and ortha-substituted references in the chemical complexes to 21.5 and 19.7 ppm frequencies respectively.39 The CH2 carbons i.e., the aromatic residues with the methylene carbon linkages noted to resonate in the absorption frequency ranges from 34 to 42 ppm. Whereas, highly sterically hindered CH2 groups were seen to be at 2.5 to 8ppm at a slightly higher frequency range when compared to the possessed methylene functions.39 Takegami et al. have reported that the internal CH2 carbons of longer paraffinic or alkyl groups shown to absorb the frequencies at 29.7 ppm, while resonance in CHz-C2H2-CH3 of bounding methyl and methylene residues were assigned at 14.1 ppm for C1, 22.7 ppm for C2 and 32.0 ppm for C3 respectively.40 The proportion of the intensities of the frequencies at 29.7 ppm and 14.1 ppm has been utilized to note the paraffinic to alkyl chain length in the petroleum and coal extracted distillates.41, 42 The carbons which are nearer to the extended (branching point) centre of the distant chains have been assigned frequencies of 27.1 ppm ,30.1 ppm, 34.0 ppm and 37.0 ppm . 40
Hajek et al attributed the frequencies for naphthenic carbons at a range between 35 to 53 ppm, β- carbon attached to the aromatic rings at 23 t0 31 ppm and terminal alkyl (methyl) carbons with frequencies at 13 to 23 oom for the demonstration of relaxation times of the various carbon complexes and residues in the coal derive oil and petroleum.43 Ozubko et al reported an chemical shifts designated to the carbons in the aromatic regions depending on their comparative single compounds. 44 The aromatic compounds bounded to the neighboring strong electron detaching hydroxyl groups in supplement of low filed, succeeded at a inconsiderably higher filed by the alkykl group bounded carbons, internal aromatic and then hydrogen ric (protonated ) carbons. Maekawa et al. attributed the spectral frequency ranges for the sterically counteracted internal carbon (hindered) at 137 ppm to 139 ppm ranges.45 For the internal and other carbons attached to the three distinct internal carbons with the frequencies of 129 pm tp 134 ppm and 123 ppm to 126 ppm range respectively. 45 The characteristic coal gasification moiety has been designated to the carbonyl functional group with a notable sharp and well distinctive peak at 187.7 ppm was reported.
Yoshida et al. classified the aromatic carbon areas into three main regions; namely protonated carbon, bridge-head carbon and substituted carbons. 46 The corresponding spectral frequencies were assigned to all these three aromatic carbons at 115.0 ppm to 129.2 ppm, 129.2 ppm to 132.5 ppm, 132.5 ppm to 149.2 ppm for the protonated, bridge-head and substituted carbons respectively. 46 The substituted carbon is extendely bbem referenced to the substituted carbon attached to the alkyl group (methyl) or the cycloparaffinic α -CH2 (frequencies between 132.5 ppm to 137.2 ppm) and to the substituted carbons attached to the methyl and the methylene connection between carbons attached to the aromatic rings (whose frequency ranges is between 137.2 ppm to 149.2 ppm). 46 The Car -hydroxyl group (i.e., phenolic carbon) frequency resonance eas seen to exist in ranges between 149.2 ppm to 158.0 ppm. 46 Mashimo et al. have later reported and explained the characterization and quantification of the bridge-head aromatic carbons in coal derived chemical residues and petroleum distillates. 47
Gillet et al. have given an explanation to assigning of the different carbons aromatic regions including; aromatic carbon, methyl and alkyl substituted aromatic carbon, where the substitution at the connection of the two aromatic rings (C ar, ar) or three aromatic rings (Car,ar.ar) or substitution of the aromatic and naphthenic rings (Car,n). 48 From the reported literature on the frequency of the above mentioned substituted carbons has reported that the only two areas of 123.5 ppm to 126.0 ppm because of the Car.ar.ar and Car.CH3 in the frequency regions 129.0 ppm to 137.0 ppm are evidently significant. 49 It has been asserted that the characteristic unique distinction between the carbons which are substitute and not substituted at the frequencies ranges of 130 ppm to 160 ppm and 118 ppm to 130 ppm is fractionally deviate (erroraneous).50 The aromatic alkyl substituted carbon has been assigned at the spectral frequency of 137 ppm tp 160 ppm is definitive. The 13 C NMR spectral frequencies correlation for the variety of hydrocarbons in the oil and petroleum residuses are assigned based on above discussed Car.alk explination as listed in tables shown below. Bartle et at. and Maekawa et al has reported the designation of aromatic carbons for the quaternary and tertiary ones bounded to the heteroatoms. 51, 52
Quantitative Aspects of 13C-NMR spectroscopy:
As the spin lattice relaxation times denoted as T1 of the 13- carbon nuclei in the molecules differs from few to several hundreds of seconds, a distinctive equilibrium will composedly been developed for each 13- carbon surroundings after the excitation by a pulse in the pft spectroscopy. Pft is pulsed Fourier transform spectroscopic technique in petroleum residual analysis. The spin lattice relaxation times to the pulse repetition proportion is the criteria based on which these equilibrium is allowed to become imbalanced after a series of the pulses treatment, thus resulting false results when the signal peaks regions are integrated. An another significant problem with the 13C-pft spectroscopy is the nuclear overhauser enhancement (NOE) during the determination of the chemical residues in oil and petrol quantitatively.
The elaborated decoupling of protons are generally utilized in order to explain the spectral resulting an deviation of population in the nuclei spin of the carbons and subsequently an increased in the signals intensities, which regrettably is not the actual ones for the carbon moieties in the same molecule. These restrictions of the 13 C NMR spectroscopy have been elucidated by various methods. The disagreement of the NOE has been explained by the following:
Gating the decoupler meanwhile the attainment of data but turning it off in the course of the pulse delay; and
The relaxation assistants like paramanagnetic materials are added that can quench all nuclear overhauser enhancement effects to a considerable extent.
The consequences due to unequal relaxation times can be overwhelemed by the following methods:
The pulse flip should be decreased to disturb the spin population inconsiderably.
The T1 of the carbons on the molecules can be shorten considerably by treating of the solutions with relaxation agents like paramanagnetic materials.
The complete repolarization of all nuclei can be achieved by slowing the pulses.
Nevertheless, the inclusion of paramagnetic agents will results in the improving line and hence get involved with the explanatory elemental analysis quantitatively, while the descending of flip angle and gated decoupling commands long accession times, particularly in less soluble chemical residues. Despite of all the above mentioned disabilities, 13C-pft NMR quantitative spectral technique of analysis of the petroleum distillates are usually determined by the gated decoupling and including of relaxation agents, mainly paramagnetic materials such as chromium and iron complexes. 43
Hajek et al. explained the reasons for the relaxation agents like paramagnetic materials inclusions in their metal complexes in the crude oils and petroleum residues like 13C-pft NMR quantitative spectral technique of analysis can be achieved with no external inclusion of the relaxation agents. 42, 43, 53 It is further been proved that these paramagnetic agents seen in the traces have the potential of decreasing NOE effect much adequately than the relaxation agents, which usually when introduced in huge concentrations, in turn, generates signals line enlargement. Thiault et al. later explained different expressions of 13C-NMR spectroscopy analysis quantitatively using Fourier transform. Gillet et al. explained the minimized experimental parameters for quantitative 13C-NMR quantitative analysis of the petroleum distillate and crude oil residues. 2 13C-NMR quantitative analysis has been given a special focus in all the petroleum research with emphasis on the quantitative e analysis to calculate the aromaticity of the chemical residues. Aromaticity (fa) is the ratio of the aromatic carbons to the sum of carbons constituents of the petroleum residues and oil crudes.
The 13C-NMR where compared with many other quantitative analysis methods by determining the aromatic carbons in case of the petroleum crude and moieties by Shoolery et al.54 The 13C - FT NMR applications and uses for analyzing the aromatic samples, where there are no other possible determining methods available in the quantitative analysis of the petroleum samples have been explained by Vercier et al. 55 There are many reports from the literature in petroleum NMR analysis where, 13C - FT NMR spectroscopy method was used to analyze the proportionate amounts of aromatic and aliphatic carbons in the petroleum crude and residues possessing paramagnetic ions. The differences in the aromaticity values determined using 13C and 1H NMR quantitative spectroscopy, in case of vacuum chemical constituents, has been attributed in terms of non-responsibility of the aromatic carbons which are quaternary in the proton NMR method of petroleum analysis.
Gillet et al discussed on the basis of the 1H-NMR spectroscopy and classified aromatic carbons in the petroleum residues and oil crude complexes and their residues into alkyl and methyl substituted carbons, protonated and bridge-head carbons with immense conviction. 56 Stating a few prediction, the subscription of the saturated carbons with aliphatic and naphthenic carbons have been studies successively with the characterization and analyzing familiar structural units in the methyl (alkyl) regions of the 13C NMR spectrum analysis. Exploiting these results, some of the significant moderate (average) structural features such as C to H ratio (C/H), aromatic regions compactness index (fc), ration of carbon and hydrogen's saturated (C sat/H sat), the petroleum and oils aromaticity is thus assessed. 56, 57
The extensive research carried out in crude oil characterization using 13C - FT NMR spectroscopy, where some fundamental derived properties or conditions have been reported with the aim of emphasis on the related physico-chemical experimental data of the crude oils collected from the regions of Middle-East and the been studies by not transforming (converting) into their distillates of the simpler fractions. 58 Finally, the detailed explanation for these sample oils was given to note the waxy nature by determining the fraction of nature of alky chain groups and the naphthenic carbons.
1H - NMR Spectroscopy in petroleum analysis:
The atomic nucleus is charged particles whose upon spinning generates a magnetic field. The nuclear spin are random and spins with no definite direction when an magnetic field is created around it, also the nuclei align it selves can ne wither in or opposite to the applied magnetic field around the nuclei.
Figure (a) - No external magnetic field schematic
Figure 4 (b) Schematic with the nuclei in presence of applied magnetic field.
Figure - Schematic showing differences in the energies of α- and β- spin states of the nuclei.
The difference between the two types of spins, α-spin and β-spin is denoted by the energy difference, Δ E as shown in above figure 5. The Δ E id dependent on the magnitude of the applied magnetic field around the nucleus. It should be noted that the applied magnetic filed potency (strength) is directly proportional to the difference in the α and β energy spin states. The radiation has with the same energies is placed above the sample, thus the flipping of spin is seen from one to the other states. Thus, the relaxation of the nucleus is achieved. In simple terms, relaxation is obtained when the excited nuclei come back to its original condition. During this process of relaxation, frequency of the emitted electromagnetic signals are highly dependent on the Δ E.
The H-NMR spectroscopy is the technique that allows researchers to read these signals and align (plot) them on the intensity versus frequency of the signals graph. In other terms, the proton NMR signal is seen when there is a noticed radiation matches with the Δ E. This energy which is necessary to spin the flip is dependent on the nucleus environment particularly magnetic field's properties.
1H - NMR spectroscopy is one of the convenient quantitative analysis tool used in exploring the elemental and compositional data of the petroleum and crude oil residues.60 The relaxation time (T1), which is a major problem in proton NMR analysis ca be overwhelmed by the scanning of the spectrum frequencies rapidly, such that the relaxation time T1 is comparatively lesser than the time taken for it to pass through it. Thiault et al have discussed the different parameters in order to quantitatively analyze and determine the 1H - NMR spectrum in FT mode.61 The major absorptions of the chemical residues in the petroleum and crude residues are broadly classified into four regions as listed in the table 1 below.
Type of chemical residue
Frequency range (in ppm)
9.30 - 6.30
Protons attached to a saturated α-carbon
4.50 - 1.85
protons of CH 2 plus CH groups on side chains (or saturated compounds) β- farther from the ring and CH -'groups β the ring
1.85 - 1.00
to protons of methyl groups on side chains y or farther from the aromatic ring or methyls of saturated compounds
1.00 - 0.50
*protons attached to olefinic carbons
6.30 - 4.50
Table 1- The type of chemical residue and corresponding frequencies are shown.
* The protons bounded to the carbons of olefinic groups are generally not seen in the samples of the petroleum that are not treated before the study or NMR analysis.
Shift range parts (in ppm)
Har : Aromatic protons
H (sat) : CH3, CH2 and CH α to the ring
CH3 α to the ring
CH2 + CH groups on side chains
H (sat) : CH2 + CH groups on side chains β
H (sat) : CH2 + CH groups on side chains γ
Table 2- Proton chemical shift correlation for hydrocarbons
The hydrogen's attached to the aromatic carbons of the petroleum are further classified into mono-, di- and polynuclear aromatics.
Nature of aromatic carbons
6.78 (6.60 - 6.81)
8.3 (7.7 - 8.3)
Table 3- The nature of aromatic carbons and corresponding frequency ranges are shown.
The distinctive region of the signals at 4.50 ppm to 1.85 ppm into a frequency signal peak at 2.12 ppm to 2.20 ppm α- methylhydrogens and a signal peak at 2.41 ppm to 2.53 ppm of the α- methylhydrogens along with an frequency absorbed in the 6.60 ppm to 6.81 ppm regions can be considered as an direct proof of the nature being mono aromatic.62
Statistical Structural Analysis of NMR Spectroscopy in petroleum analysis:
The 13C NMR and 1H-NMR spectroscopic analysis for the carbon and hydrogen arrangements in the chemical residues of the sample, i.e., petroleum residues have been studies combined with a number of average molecular weight and the expressions corresponding to the elemental analysis in order to obtain a variety of average structural parameters for exploring petroleum chemical residues. 40, 42 Shenkin explained the average structure criterion in details with explanation to the structural hidden suggestions and also listed direction for comparing and correlating the understandings from the experimental and also average structure methods. 63 Later, Boduszynski studied in depth focusing the average structure assessment and analysis for the crude heavy oils extracted from the fossil fuels. 64
Car.alk+Car. CH3) / Car
Car.H/Car = Har/Car(C/H)
(Csat + Car)/(2 xCsat+Car)
CH3 γ/Car.alk=H(sat)/(C/H) Car.alk
Hnx(sat)/2=(Hn into Hx(sat))2
Table 4- General equations for calculating various structural parameters are shown.
The broad molecular weight distribution, as the result of considerable differences in the predictability between the petroleum and oil residues complexes has been found to be the one of the major limitation. Few of the basic important formulae for calculating the general structural parameters as shown in the table 4 as below.
Fraction of aromatic carbon
Aromatic substitution ratio
Fraction of unsubstituted aromatic carbon atoms.
Fraction of aromatic carbon substituted by alkyl(not methyl)group.
Fraction of aromatic carbon substituted by methyl group.
Compactness index of the aromatic part of the sample
Overall carbon and hydrogen atomic ratio.
C/H ratio of saturated part of the sample.
Branching index of alphabetic chains
Number of terminal methyl carbons
Number of alkyl substituent's
Number of carbon atoms in condensed points.
Number of aliphatic carbon atoms.
Number of carbon atoms in average molecular formula
Number of atoms in average one unit
Table 5- General average structural parameters for petroleum distillates from NMR spectroscopic analysis are shown.
The nomenclature of some of these familiar structural parameters computed for the various residues of the petroleum are listed in table 5 as shown above. Sebor et al. extensively studies and quantitatively analyzed various asphalt - resin fractions using NMR spectroscopy techniques and has given a detailed maltenes, asphalt and asphaltenes structural features expressions. 65 Dereppe et al. has given a derivative of the factors (or parameters) of an average molecule to analyze asphaltenes and the petroleum fractions and residues using 13C- and 1H - NMR spectroscopic methods. 66 The pyrolysis tars molecular structure was discussed by Shmirs et al. where they used lH-NMR and IR spectroscopy methods. 67
Yashina et al. reported the extensive study on the n-alkanes content, nature of aliphatic branching, degree of aromatic compounds substitution in assembly heavy and crude oils and petroleum fraction was studied using l3C - NMR spectroscopy. 68 The analysis of Iranic heavy, crude and taching residues were carried by Takegami et al in order to note the average structural patterns in combination with the knowledge of aliphatic areas of the quantitative analysis using 13C-NMR spectroscopy.40 Later, an explanation to details of the novel method for analyzing fossil fuels in terms of goal to study its structural actors (or parameters) were studies by Cookson and Smith group employing IH- and 13C-NMR spectroscopy techniques.69 Suzuki et al. in recent times reported IH- and 13C-NMR spectroscopy characterization for analyzing the compositional and elemental quantitative analysis of tarsand bitumens. 70
Advantages of NMR in petroleum analysis: 14
The main advantages of NMR in petroleum analysis are listed briefly as below:
In case of expedition or investigation of oils and petroleum manufactories, low field NMR spectroscopic method of analysis is majorly with the aim of examining T2 relaxation time scattering (or distribution) of various liquids in core plugs.
These administration of distribution can be explained in order to analys pore size distribution, porosity, bound water volumes, permeability etc.
Instrument tools used for general readings of relaxation scaterring or distributions generally maintained at 2MHz so as to achieve optimized errorors or artefacts cue to the ferromagnetic complexes or materials in the petroleum or oil samples and ultimately to match the NMR logging tools functioning.
Practical experimental (laboratory) base instruments are used to measure downhole logging devices for more appropriate and also elevated calibration on cores.
13C and 1H-NMR spectroscopy has facilitated considerable breakthrough to be made in the perspective of the elemental constituents of the petroleum residues and distillates, which allows us to make more beneficial uses and applications of these petroleum products possible. The concepts in understanding of the significant heteroatom functionality could be achieved by increasing the 13C-NMR sensitivity techniques. In asphalts compounds, where the enhancement of the sensitivity can be achieved by addition of the high amounts of the relaxation agents like paramagnetic materials. Furthermore, the reduction in the declaration could be rewarded by using superconducting spectrometers tools which can operate at range of 400-600 MHz. Research in understanding of the concepts is not still clear in the areas of differentiating the connections of the aromatic rings with bridgehead carbons and the bridged junctions of the aromatic and naphthenic carbons. More work is essential to get decisive data on the naphthenic and aliphatic carbons compounds from the 13C NMR spectrum of the saturated regions in crude oils and petroleum.