Molecular size of asphaltene fractions obtained

from residuum hydrotreatmentq

Louise Bucha, Henning Groenzinb, Eduardo Buenrostro-Gonzalezc, Simon I. Andersena,Carlos Lira-Galeanac, Oliver C. Mullinsb,*

aDepartment of Chemical Engineering, Technical University of Denmark, DK-2800 Lyngby, DenmarkbSchlumberger-Doll Research, Ridgefield, CT 06877, USA

cMolecular Engineering Research Program, Instituto Mexicano del Petroleo, Eje Central Lazaro Cardenas 152, C.P. 07730 Mexico, DF, Mexico

Received 14 June 2002; revised 3 December 2002; accepted 12 December 2002; available online 29 January 2003

Abstract

Previously, fluorescence depolarization techniques (FD) have been shown to measure asphaltene molecular size, thereby establishing the

substantial difference between asphaltenes derived from crude oil vs from coal. Here, FD is used to track the changes of the asphaltenes from

a petroleum atmospheric resid feedstock that has been subjected to increasing thermal severity of catalytic hydrothermal cracking. Changes

in asphaltene properties with increasing cracking are readily observed and understood. In addition, asphaltene molecular size is measured for

various asphaltene solubility fractions in binary solvent mixtures of toluene with either n-heptane or acetone; a strong dependence is found of

asphaltene properties on the particular solvent mixtures in accord with recent publications.

q 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Asphaltene fraction; Fluorescence depolarization; Asphaltene molecular size and weight; Asphaltene subfractions; Asphaltene solubility fractions;

Catalytic Hydrothermal treatment

1. Introduction

Understanding asphaltene molecular structure is critical

in several economically important arenas. Asphaltenes are a

solubility classification (e.g. soluble in toluene, insoluble in

n-heptane); this definition captures the most aromatic

component of crude oil and thus is a very useful definition

[1–4]. In the Deepwater production of oil, asphaltene

precipitation is a problem that must be addressed within the

Flow Assurance context. Understanding asphaltene phase

behavior is key, especially the pressure-induced destabiliza-

tion of the naturally occurring asphaltene colloidal suspen-

sion in crude oil. First principles development of phase

behavior necessitates knowledge of asphaltene molecular

structure as does development of chemical treatments to

retard asphaltene precipitation. Optimization of the refining

of crude oil requires an understanding both of asphaltene

molecular structure and the corresponding changes as

refining proceeds. There are other important applications

that make significant use asphaltenes such as paving and

coating materials.

In spite of the importance of asphaltenes, there has

persisted a long standing controversy in asphaltene science

over one of the most fundamental properties, a molecule (or

mixture of molecules) can have molecular weight [5]. For

more than 20 years, there has been an order of magnitude

disagreement over asphaltene mean molecular weight [5].

Field ionization mass spectroscopy (FIMS) gave a mean

value of 700 amu with a width in the mass distribution of

roughly a factor of 2 [6]. More recent mass spectroscopy

results on petroleum asphaltenes are in rough agreement

including laser desorption mass spectroscopy (LDMS) [7],

atmospheric pressure chemical ionization (APCI) mass

spectroscopy [8] and electrospray ionization mass spec-

troscopy (ESI) [9]. Other laser desorption mass spec-

troscopy studies have yielded comparable results [10], but

baseline issues can lead to uncertainty for measurements on

asphaltenes [11]. Advocates of much higher asphaltene

molecular weights raise concerns as to whether some

selection of light ends or fragmentation might be occurring

0016-2361/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0016-2361(03)00006-1

Fuel 82 (2003) 1075–1084

www.fuelfirst.com

q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com

* Corresponding author. Tel.: þ1-203-431-5572; fax: þ1-203-438-3819.

E-mail address: omullins@ridgefield.oilfield.slb.com (O.C. Mullins).

in these mass spectrometry studies. Techniques that yield

much larger asphaltene molecular weights include gel

permeation chromatography (GPC) and colligative methods

such as vapor pressure osmometry. However these tech-

niques suffer from high requisite concentrations where

asphaltene aggregation is known to occur. GPC also suffers

from the lack of standards for comparison [12].

Recently, fluorescence depolarization (FD) has been

shown to measure asphaltene molecular size [5,13]. In FD,

one creates a polarized ensemble of excited state asphaltene

molecules utilizing a polarized beam of excitation photons.

The rate of decay of this molecular polarization is monitored

by measuring the polarization of the emitted fluorescence as

a function of time (in the nanosecond range). Time

dependent depolarization occurs via rotational diffusion,

smaller molecules diffuse faster. Then by comparison with

known compounds or by use of well-established equations,

the molecular size can be used to determine asphaltene

molecular weight. FD is used with very dilute solutions, thus

does not suffer from aggregation problems, and in addition,

FD is performed on solutions obviating concern about

volatility. FD precludes fragmentation concerns. FD has

shown that typical mean asphaltene molecular weights are

,750 amu with a factor of 2 in the width of the distribution

[5,13]. Interestingly, these results are in close agreement

with the FIMS measurement 20 years ago [6]. In addition,

the FD results showed that (most) asphaltene molecules

possess one chromophoric group, that is, one fused aromatic

ring system per molecule [5,13]. If the aromatic ring is

linked even to one other chromophoric group, two out of

three degrees of rotation become hindered, in part because

the rotational correlation time corresponds to rotation by

1 rad. The result ‘one ring per molecule’ derives from that

small fused ring systems are imbedded in small asphaltene

molecules whereas large fused ring systems are imbedded in

large asphaltene molecules. This follows provided there is

only one fused ring system per molecule. A simple way to

picture individual asphaltene molecules is that they are ‘like

your hand’ where the palm represents the fused aromatic

ring system and the fingers represent peripheral alkane chain

substituents. Recent studies of carbon Raman X-ray

spectroscopy applied to asphaltenes indicate that the fused

ring systems are pericondensed (circular arrangement of the

fused rings) [14].

In order to test the FD results, several studies have been

performed in part to check consistency. FD methods were

combined with 13C NMR and IR applied to a series of

asphaltenes [15]. The results show that asphaltenes that lack

long alkane side chains are composed of small molecules

with small fused ring systems. Remembering that asphalt-

enes are defined by a solubility classification, the idea is that

asphaltene molecular identity is determined by competition

between two competing effects: steric hindrance from

alkane chains vs intermolecular binding due to van der

Waals interaction of stacks of aromatic ring systems [15].

The balance between these competing effects determines

solubility. These concepts are rather simple making at least

one aspect of asphaltenes, molecular identity, very tractable.

If the binding is excessive (large ring systems coupled with

little alkane substitution), then the molecules stack effi-

ciently and will not dissolve in toluene, thus are not

asphaltene. High resolution transmission electron

microscopy (HRTEM) on asphaltenes and model com-

pounds supports this contention by imaging the disruption

of aromatic ring stacking due to alkane substituents [16].

Even melting point data of alkyl aromatics clearly shows

this same competition of steric hindrance of alkane

substituents vs stacking of fused ring systems via van der

Waals interaction [15]. It is important to note that these

conclusion from these diverse techniques all rely on the

accuracy of the FD results on the molecular weight of

asphaltenes. We believe that the congruence of FIMS

(some), LDMS, APCI and ESI mass spectroscopy results

along with FD results, especially when coupled with 13C

NMR, IR and HRTEM overwhelmingly point to the mean

asphaltene molecular weight being ,750 amu. Theoretical

modeling of the phase behavior of asphaltenes has

employed an asphaltene molecular weight of 1000 amu,

comparable to our results; albeit the models require

extensive data fitting [17]. We believe it is time to view

this controversy over asphaltene molecular weight as

resolved (while maintaining vigilance) and further develop

the field of asphaltene science. Without resolution of this

issue, the basic understanding of asphaltenes and their

properties is retarded.

It is instructive to examine asphaltene subfractions and

asphaltenes from different source materials. A recent FD

study was performed on solubility fractions of asphaltenes

obtained from different ratios of toluene and n-heptane [11].

One important conclusion in this study is that the different

asphaltene subfractions appear to consist of the same

molecules but in different distributions. A recent study

utilizing IR and 1H NMR (the EBG study after the first

author) also obtained the same conclusion on a different set

of n-heptane–toluene subfractions of asphaltenes [18]. A

similar fractionation study has also been reported [19]. Both

of these studies showed a smooth variation of molecular

properties in the different subfractions interpretable in terms

of the changing solvating power of the different solutions

employed. In a gross sense, these studies can be understood

within the context of the changing dielectric constant of the

solution, which has been shown to govern the onset of

asphaltene flocculation from this solvent system [20,21].

The flocculation studies conclude that the primary (attrac-

tive) intermolecular interaction of asphaltenes is van der

Waals forces, thus dipole–dipole interactions are not

significant here [20,21]. However, the EBG study also

employed a very different solvent system to obtain a

different set of asphaltene subfractions, acetone and toluene.

Very different behavior was obtained for this solvent system

[18]. The analysis of this set of asphaltene subfractions

showed nonmonotonic asphaltene molecular properties as

L. Buch et al. / Fuel 82 (2003) 1075–10841076

the solvent ratios were changed indicating a more complex

interaction than simply van der Waals. This is not surprising

as acetone is polar with strong hydrogen bonding. Recently,

it has been reported that trace water influences the

association of asphaltenes in toluene solution indicating

that aggregation either amongst asphaltene sheets or

particles (stacked or formed in another way) is greatly

enhanced by the presence of these very small amount of

water [22]. This implies that polar forces are (also)

important in the intermolecular binding of asphaltenes.

Another conclusion of the EBG study is that almost

all of the porphyrin ring systems contained in the

asphaltene end up in the asphaltene fraction that is most

soluble in acetone. This interesting result can only be

true if the porphyrins are not cross-linked to other

aromatic fused ring systems as also concluded in several

other studies [19,23–25]. This is precisely the result of

the FD studies, asphaltenes have a single fused ring

system per molecule. In addition, the FD studies show by

direct analysis that the smallest molecules in asphaltenes

are porphyrin sized [5,13].

In addition to analysis of solubility subfractions of

asphaltenes, it is desirable to explore the effects of high

temperature, catalytic hydrotreatment of hydrocarbon feed-

stocks. It has been shown that this process results in the

cleavage of alkane substituents off of asphaltenes [26]. It has

also been shown by FD that asphaltene molecules that lack

alkane side chains have very small molecular size while

asphaltene molecules with alkane side chains have larger

molecular size [15]. Thus, we can test whether we obtain

consistency, the hydrotreated asphaltene samples are

predicted to yield smaller asphaltene molecular sizes. This

prediction if obtained (and it is) provides further verification

that the FD results are correct corroborating the central FD

result, that the asphaltene mean molecular weight is

,750 amu. There is some uncertainty in this result; much

less than the previous order of magnitude; perhaps we have

an error bar of ^15%.

In this study, we perform FD and fluorescence spec-

troscopy studies on asphaltene samples obtained from

atmospheric residua (AR) that have been subjected to

varying degrees of high temperature, catalytic hydrotreat-

ment. Asphaltenes were isolated from the original AR

feedstock (from Arabian Heavy) and from three oil samples

resulting from increasing the temperature of the hydrotreat-

ing reactor successively higher, 359, 379 and 389 8C.

Substantial changes in asphaltene molecular size are

recorded in concert with expectations based on known

chemical changes in this process. NMR analysis reveals

subtleties of the process. In addition, we study solubility

subfractions of asphaltenes acquired with two different

solvent systems, n-heptane–toluene, and acetone–toluene.

The asphaltene subfractions from the two different solvent

systems are shown to exhibit quite different behavior

consistent with previous recent publications. Time resolved

fluorescence spectra of asphaltenes are also shown to obey

predictable results.

2. Experimental section

The FD system has been described in detail elsewhere

[13]. Briefly, time-dependent FD data were collected with

the PTI C-72 system that employs a PTI GL-3300 nitrogen

laser source along with a PTI GL-302 high-resolution dye

laser with a fiber optic coupling to the measurement cell to

excite the fluorescence. A right angle fluorescence detection

system was employed. To obtain the fluorescence decay

curve, the four possible combinations of vertical (V) and

horizontal (H) polarization for the excitation and emission

Fig. 1. The time dependent data for the FD measurement showing the four combinations of vertical and horizontal polarizations for the incident and

fluorescence photon. Also shown are the fluorescence polarization decay curve (the difference curve) and its fitted curve.

L. Buch et al. / Fuel 82 (2003) 1075–1084 1077

photons are collected thereby allowing for polarization data

to be collected while accounting for throughput effects [13].

Fig. 1 shows a typical decay curve (and the data used to get

the decay curve). The effect of depolarization is readily

apparent in the data. The HH and HV curves are seen to be

the same allowing quality control for the data. For both the

HH and VH curves, a 908 molecular rotation optimizes

throughput (for a given molecule). And for both curves, the

horizontal polarization transmission throughput is the

required (thus the same) for the emission monochromator.

The VV and HV curves differ in terms of molecular rotation

required but both these curves employ vertical polarization

transmission on the emission monochromator. Proper curve

fitting gives the polarization decay rate (not to be confused

with the fluorescence decay rate) [13]. For collection of

steady-state fluorescence spectra, we employed the (Photon

Technology International) PTI C-72 þ A-720 fluorescence

spectrometer using a 75 W Xe compact arc lamp source.

Subfractionation. The procedure is similar to the one

described EBG [18]. In short: a solid heptane precipitated

asphaltene sample was contacted with solvents of

increasing solvent power either in heptane–toluene or

acetone–toluene mixtures. The asphaltene subfraction

precipitates at the listed % in the figures. The A-Sol

and H-Sol samples (marked as the 100% points in the

figures) refer to the asphaltene subfraction that remained

soluble in the 90% solution. Samples were recovered by

centrifugation at 3000 rpm and a stronger solvent

solution was added to the solids left. All samples were

dried under nitrogen and stored in the dark. The initial

asphaltene sample was obtained using a modification of

the IP 143 based on 30 cm3 precipitant/g oil, precipi-

tation over night, filtration on sintered glass filters, and

followed by toluene dissolution of the solids on the filter.

The sample was concentrated using a rotoevaporator and

dried under nitrogen. The samples were washed with

heptane using sonication.

Hydrotreating was performed as described previously

[26] in a hydrodesulfurization pilot plant. The feedstock is

the resid of atmospheric distillation. Here, metals are first

removed followed by deep desulfurization. Product samples

were collected at different time intervals from the start of the

reactor. During the process the reactor temperature is

increased in order to compensate for catalyst deactivation

with an aim to keep the sulfur removal on a specified level.

Hence samples represent increasing reaction temperatures

or lifetime of the reactor bed. Cracking reactions starts to

dominate around 380 8C. The cracking leads to increased

product instability seen as asphaltenic sludge formation.

Asphaltenes from the feed and the product samples were

precipitated by addition of excess heptane (30 cm3/g oil)

following the IP 143 standard. The oil used was an Arabian

heavy atmospheric resid. The asphaltene samples were

obtained from the feedstock (TR453-00) and from three

process temperatures 359 8C (TR453-62), 379 8C (TR453-

181) and 389 8C (TR453-253).

The C13-NMR measurements were performed on a

JEOL Eclipse model 300 NMR apparatus operating at a 13C

frequency of 75 MHz. The asphaltene samples were

prepared as 100 mg/cm3 CDCl3 solutions. TMS was used

as a zero-shift reference. In order to use the 13C NMR

spectra in a quantitative manner, the spectra were subjected

to an inverse gated decoupling technique to suppress the

NOE effect, and chromium acetyl-acetonate (0.01 M in the

final solution) was added to assure complete nuclear

magnetic moment relaxation between pulses.

3. Results and discussion

Fig. 2 shows the steady-state fluorescence spectra of the

different asphaltenes. TR453 series refers to the feedstock

(-00) and successive heat treatments as described in Section

2. The TH coal asphaltene is shown for comparative

purposes. This coal sample has been previously analyzed

[15] and shown to consist primarily of aromatic carbon

(88%) with very little alkane substitution. The overall trend

for the hydrotreated asphaltenes is a blue shift in the

fluorescence emission spectrum which in general corre-

sponds to smaller fused ring systems [13,27]. Fig. 3 shows

the variation of the rotational correlation time, tr, for the

feedstock, the hydrotreated samples, and the coal asphalt-

ene. High temperature, hydrotreatment clearly results in a

decrease in the size of asphaltene molecules. Thus, the

fluorescence data shows that the hydrotreating of petroleum

source material causes the corresponding asphaltenes to

consist of smaller molecules with smaller ring systems. The

effect of hyrdrotreatment is seen to cause the asphaltenes to

become more similar to the coal asphaltene.

Fig. 2. The fluorescence emission spectra of a series of asphaltenes from

hydrotreated samples and a coal asphaltene sample. The -00 sample is from

the feedstock, the asphaltene samples -62, -181, and -253 were processed at

increasing temperatures and the coal asphaltene is shown for comparison.

Note the evolving blue shift of the progressively hydrotreated samples

indicating a reduction in the size of the fused ring systems.

L. Buch et al. / Fuel 82 (2003) 1075–10841078

The molecular size is related to the tr by Eq. (1) derived

form the Stokes–Einstein equation

tr ¼Vh

kTð1Þ

where V is the molecular volume; h; viscosity; T is the

temperature. This applies for spherical molecules but the

effect of molecular anisotropy is not large [5,11].

One of the chemical effects of hydrotreatment is the

cracking off of peripheral alkyl substituents [26]. The

temperatures associated with the hydrotreatment (,380 8C)

are sufficient to cause cleaving of chemical bonds in a

relatively short period of time. As alkyl chains are cleaved

off the fused ring systems, this corresponding source

material for the asphaltene preparation becomes more like

the TH coal sample with decreased alkyl substitution on the

aromatic ring systems. As has been described previously

[15], the reduction of alkyl substitution decreases solubility

of the fused ring systems due to decreased steric hindrance

in ring stacking. This stabilizes the solid, hence decreases

solubility. This is also explained by thermodynamics as an

increase in the heat of fusion. For a given solubility

classification, the reduction of steric hindrance due to loss of

alkane substitution (with cracking) is balanced by a

reduction in the size of the fused ring system; the smaller

ring systems exhibit smaller intermolecular binding. The

TH coal asphaltene is an extreme example of this balance in

that this sample contains almost no alkyl carbon and also

contains very small fused ring systems and very small

asphaltene molecules [15]. Thus, the trend in the hydro-

treated samples is expected: as cracking proceeds the

asphaltene fraction contains smaller fused ring systems as

evidenced by the blue shift in the fluorescence spectra, and

contains smaller molecules as shown by the reduction of the

rotational correlation times. The TH coal asphaltene

represents a limit for this trend in that it contains almost

no alkane. However, as can be seen in Table 1 the

hydrotreating also causes the actual asphaltene content to

decrease significantly. We point out that it is not clear in our

data how best to account for the reduction of the aromatic

ring size in asphaltenes; (1) the aromatic rings become

hydrogenated (with subsequent cleavage alkyl cleavage, (2)

smaller aromatics with alkane substitution lose peripheral

groups becoming asphaltenes.

The asphaltenes of the hydrotreated samples are seen to

behave according to very simple principles involving

known molecular changes in this process coupled with

established solubility principles. The fluorescence emission

and FD results are readily interpretable relying on these

simple principles. These conclusions rely on the accuracy of

the FD technique to give molecular size. If the asphaltene

molecular weights were really 10 times bigger than

determined by FD, then the line of reasoning above would

not follow for understanding FD results of the hydrotreated

samples. Consequently, the findings here corroborate the FD

results. The FD results have been used to generate

predictions for diverse asphaltenes for 13C NMR [15],

HRTEM [16], solubility fractionation [11] and now on

hydrotreated samples. In every case, the FD results have

been corroborated and/or confirmed; petroleum asphaltene

molecular weights are ,750 amu with a width to the

distribution of about a factor of 2 for the bulk of the sample

[5,13]. Moreover, the results also indicate that the main

source of product instability in hydrotreating is not

condensation of molecules into large fused ring systems as

pointed out in the past, but indeed the decrease in solubility

due to changes in molecular structure by removal of

peripheral chains.

In the steady-state emission spectra specific peaks at ca.

450, 470 and 490 nm are observed in the hydrotreated

samples while the feed is seen to be featureless. This is

believed to be due to the increased concentration of smaller

ring systems after cracking of peripheral alkanes; the larger

Fig. 3. The asphaltene rotational correlation times tr vs emission

wavelength for a series of asphaltenes from hydrotreated samples and for

a coal asphaltene. The tr is related to the molecular size. The red-emitting

(large) and blue-emitting (small) chromophores are not attached to each

other. If they were, they would have large, similar correlation times. The -

00 sample is from the feedstock, the asphaltene samples -62, -181, and -253

were processed at increasing temperatures and the coal asphaltene is shown

for comparison. Note significant reduction in asphaltene molecular size

with progressive hydrotreatment, presumably related to the increased

cracking of alkane chains off the ring systems. The feedstock asphaltene has

alkyl substituents and the coal asphaltene is known to lack alkyl

substituents [15].

Table 1

Data on hydrotreating of Arabian Heavy [22]

Asphaltene in oil %w/w H/C atomic

Feed (-00) 5.91 1.10

HR453-62 2.87 1.14

HR453-181 2.04 1.10

HR453-253 1.35 0.98

L. Buch et al. / Fuel 82 (2003) 1075–1084 1079

aromatic ring systems without peripheral alkanes would not

dissolve in toluene. In addition, partial hydrogenation of the

original aromatic core will also result in a smaller number of

(fused) aromatic rings. These smaller ring systems have

smaller numbers of permutations than do large ring systems,

so individual chromophoric moieties can represent a

significant fraction of the total chromophores, thus be

detected in the mixture. This is the probable explanation for

fluorescence spectral structure for coal asphaltenes. Further

work will be performed to resolve this issue.

Fig. 4 shows the fluorescence emission spectra for a

series of asphaltene subfractions precipitated from acetone–

toluene mixed solvent solutions. The acetone fraction is

indicated on the figure. The A-Sol sample remained

dissolved in the 90% acetone–10% toluene solvent, so it

is different from the other samples here in that it never

precipitated from the mixed solvent system. The two

extreme samples, A-Sol and 40%, exhibit the bluest

fluoresce emission while the other samples are rather

similar in their emission spectra. FD provides a better

probe to differentiate these subfractions.

Fig. 5 shows the trend of tr vs emission wavelength for

three of the acetone–toluene subfractions. The trend of

smaller correlation times for blue-emitting chromophores

has been observed in all previous FD asphaltene analyses [5,

11,13,15]. The smaller fused ring systems generally have

shorter wavelength transitions [27]; which is based on the

familiar quantum particle in a box. Thus, for asphaltenes,

the trend is that small (blue) chromophores are in small

molecules (small tr) while large (red) chromophores are in

big molecules (big tr). As pointed out previously [5,11,13,

15], this can only be true if small chromophores are not

covalently linked to big chromophores, otherwise, they

would rotate together giving large, wavelength independent

values for tr instead of the observed small, wavelength

dependent values of tr. Observation of familiar trends in the

acetone–toluene subfractions indicates that general trends

of asphaltenes apply [15].

Fig. 4. The fluorescence emission spectra of a series of asphaltene subfractions precipitated from acetone–toluene mixed solvent. The %-acetone that was used

to obtain the corresponding precipitant is listed in the figure caption. The A-Sol fraction remained soluble in the 90% acetone solvent mixture.

Fig. 5. The rotational correlation times tr vs emission wavelength for three

asphaltene subfractions obtained with different %-acetone. The trend seen

here for the asphaltene subfractions has been reported before for related

samples [5,15]; the large, red-emitting chromophores exhibit much slower

correlation times than the small, blue-emitting chromophores. Thus, the

red-emitting (large) and blue-emitting (small) chromophores are not

attached to each. If they were, they would have large, similar and larger

correlation times. Asphaltene molecules have one chromophoric group.

L. Buch et al. / Fuel 82 (2003) 1075–10841080

Fig. 6 probes the molecular size of molecules containing

blue emitting chromophores (450 nm emission) and red

emitting chromophores (635 nm) for the different solubility

subfractions. The chromophore size is related to the

emission wavelength, but the ring geometries (e.g. degree

of katacondensed or pericondensed) also have an impact on

emission wavelength [27]. The molecular size does not

depend monotonically on the acetone fraction used to

precipitate the fractions. For the blue emitting chromohores,

very small molecules are found at the two solubility end

points (40% and A-Sol), while the intermediate subfractions

have much larger molecules for the blue emitting chromo-

phores. Ring geometry effects might account for this

different molecular size for the same optical properties

[27]. Differing alkane substitution can also explain some

differences in molecular size; the toluene–acetone samples

with largest molecular size have also been shown to

correspond to samples with the largest alkyl fraction by13C NMR on comparable samples [18].

The lack of monotonic behavior of acetone–toluene

subfractions of asphaltenes has been recorded previously

with regard to aromatic carbon content and alkyl chain

content [18]. For the acetone–toluene mixed solvent

system, the minimum in aromatic carbon occurs for the

asphaltene subfraction precipitated with 60% acetone/40%

toluene; the aromatic carbon fraction increases with

increasing and decreasing acetone in the precipitating

solvent. Correspondingly, the aliphatic carbon in the

asphaltene subfraction is maximized at this same inter-

mediate solvent system, 60% acetone/40% toluene. In other

words, a large asphaltene molecular size correlates with a

large alkane fraction in accordance with results obtained for

coal and petroleum asphaltenes [15]. The FD results shown

in Fig. 6 exhibit a very similar profile in support of the

previous findings [18]. The large deviation from monotonic

behavior of asphaltene subfractions on %-acetone indicates

Fig. 6. The rotational correlation times tr vs %-acetone for the different

asphaltene subfractions obtained from the acetone–toluene mixtures. Two

emission wavelengths were investigated probing the smallest and largest

chromophores for the solubility subfractions. Note the complex, non-

monotonic dependence of tr (thus molecular size) on acetone fraction.

Similar systematics have been reported for the fraction of aromatic carbon

in similar asphaltene subfractions [18]. (The A-Sol sample is listed as

100%.)

Fig. 7. The fluorescence emission spectra of a series of asphaltene

subfractions precipitated from the n-heptane–toluene mixed solvent. The

%-n-heptane that was used to obtain the corresponding precipitant is listed

in the figure caption. The H-Sol fraction remained soluble in the 90% n-

heptane solvent mixture. The fluorescence emission curves are quite

similar; FD is a better probe.

Fig. 8. The rotational correlation time tr for the different asphaltene

subfractions obtained from acetone–toluene mixtures and from n-heptane–

toluene mixtures. The acetone-derived subfractions exhibit complex

dependence on the %-acetone, the n-heptane fractions exhibit nearly

monotonic behavior. This is in accord with recent papers on asphaltene

subfractions from both mixed solvent systems [11,18]. (The A-Sol and H-

Sol samples remained soluble in the 90% solvent mixture; they are listed as

100%.)

L. Buch et al. / Fuel 82 (2003) 1075–1084 1081

that the precipitation dependence is complex. In particular,

acetone is polar and can hydrogen bond as a hydrogen

acceptor. In our view it is probable that the polar and

hydrogen bond interactions between the asphaltene mol-

ecules and acetone give rise to complex (nonmonotonic)

solubility behavior of the asphaltenes in the acetone–

toluene mixed solvent system. This behavior is in contrast to

the behavior previously reported for asphaltene subfractions

with n-heptane/toluene. In this solvent system, monotonic

behavior of the asphaltene subfractions on %-n-heptane

have been observed for molecular size by FD [11].

Fig. 7 shows the plot of the fluorescence spectra and

Fig. 8, tr for the different heptane fractions. Fig. 8 shows

that the asphaltene subfractions from the n-heptane/toluene

system exhibit nearly monotonic behavior while the

acetone–toluene asphaltene subfractions also shown in

Fig. 8 are not monotonic. In addition, precipitation onset of

asphaltenes in this mixed solvent system has been success-

fully interpreted in terms of molecular polarizability alone

[19,20]. It appears that the particular solvent system

determines whether effects from polarity are observed or

not.

Fig. 9 shows the time-resolved fluorescence emission

spectra for several delay times for the TR453-253 hydro-

treated asphaltene. Only for the zero time delay, a Rayleigh

scattering peak is observed at ,370 nm corresponding to

the solvent CH stretch mode. For longer delay times, the

fluorescence spectra undergo an increasing blue shift. This

effect is probably related to the Exponential Energy Gap

Law that has been shown to apply for crude oils [28].

Essentially, radiationless decay mechanisms for electro-

nically excited states compete more effectively against

fluorescence emission at smaller excitation energies

thereby producing faster decay and smaller lifetimes for

Fig. 9. Time-resolved fluorescence emission spectra of the TR453-253 asphaltene sample. At longer times, there is a blue shift in the fluorescence emission

spectrum corresponding to longer lifetime for shorter wavelength emission and may be related to the exponential energy gap law [28]; smaller bandgaps yield

larger decay rates. Growth of spectral structure at 440 nm shows the increasing importance of small chromophores emission at long times.

Table 2

NMR raw data normalized to total signal

1H NMR (ppm) 13C NMR (ppm) or intensity n 13C n 1H

Hgþb Ha Haro Cali Caro CH2 (long) CH3

HR453 0.5–2.0 2–4 6–9 10–60 110–160 I(14.1) I(29.7)

-00 75 13 11 45 55 0.22 1.16 5.3 6.8

-62 83 8 8 54 46 0.14 0.89 6.4 11.4

-181 78 12 10 44 56 0.17 0.99 5.8 7.5

-253 79 11 10 45 55 0.17 1.03 6.1 8.2

L. Buch et al. / Fuel 82 (2003) 1075–10841082

the red-emitting chromophores. Note too that at long delay

times, fluorescence spectral structure is seen to increase at

440 and 390 nm. This effect is due to the small number of

configurations of small ring systems (which are the blue

emitters) as discussed above.

3.1. NMR

Both 1H and 13C NMR were recorded on the four

samples related to hydrotreating. No real trend was

obtained in the spectra indicating the overall complex

chemistry of the hydrotreating process. As previously

noted, catalytic hydrotreating cleaves peripheral alkane

chains, producing a trend for increasing aromatic fraction

with increasing hydrotreatment. However, the process of

catalytic hydrogenation (of aromatic rings) produces a

trend towards decreasing the aromatic fraction. Those

molecules with aromatic cores devoid of alkane side

chains may have better access to the catalyst surface so

the two opposite trends may be coupled. The net result is

that no trend is observed in the aromatic fraction as seen

in Table 2 and is consistent with the lack of a trend in the

CH ratio in Table 1.

The last two columns give the 13C alkyl carbon chain

length (n 13C) while the same can be generated from H

spectra (n 1H); there is a qualitative agreement between the

two independent measurements. However, there is appar-

ently no trend that tells us that we get more aromatic

material. The increase in aliphatic H and C also confirmed

between the two techniques shows that the 360 8C product

mainly is changed due to hydrogenation. Apparently from

the asphaltene content about 50% of the original asphalt-

enes are converted into species soluble in heptane (non-

asphaltenes) if we assume that the laydown on the catalyst

is not significant (but this must be considered for a true

mass balance made over the reactor). There is no direct

indication of large changes between peri and kata-

condensed (110–160 ppm range), but there are more H-

Car bonds than substituted (C or other groups) aromatics

indicating a somewhat open structure of the katacondensed

type [29]. It is expected that the loss of heteroatomic

content such as sulfur would reduced the intermolecular

attraction of molecules, thereby increasing solubility in

toluene. Perhaps this accounts for some of the loss of the

asphaltene fraction.

4. Conclusions

Various asphaltenes and asphaltene subfractions have

been investigated by FD methods. The results reported here

present a test for the validity of the FD method and

consistency with many previously reported results is found.

Thus, we have corroboration that the primary FD result, that

mean petroleum asphaltene molecular weight is ,750 amu

is correct. Analysis of high temperature hydrotreated

samples shows that the loss (cracking off) of alkane chains

results in a reduction of size of the fused ring systems as

well as a reduction of molecular size for the asphaltene

fraction. Thus, the asphaltenes remaining after hydrotreat-

ing consist of more compact molecules. Also, it is observed

that a large part of the feed asphaltenes is converted to resin

type material soluble in heptane. This is assumed to be due

to hydrogenation of structures enhancing the solubility in

alkane solvents.

For acetone–toluene subfractions of asphaltenes, large

nonmonotonic variations of molecular size on %-acetone

are found in concert with previous findings and in contrast to

the nearly monotonic variation observed for the n-heptane–

toluene mixed solvent subfractions. The complex behavior

observed for the acetone–toluene subfractions is probably

related to dipolar interactions between the asphaltenes and

the solvent. Van der Waals interaction may dominate

asphaltene intermolecular interaction, but polarity affects

can be found as well.

Acknowledgements

We are indebted to Haldor-Topsøe for the preparation

of the hydrotreated petroleum samples. We are indebted to

Professor Iino of Tohoku University for the TH coal

sample. SIA thanks the Danish Technical Research

Council for financial support under the Talent Program.

We are pleased to thank J.A. Garcia-Martinez for

technical assistance in the 1H and 13C studies. EB-G

and CL-G acknowledge IMP for permission to publish

this work.

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