Fuel 138 (2014) 111–117

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Fuel

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Effect of alumina and silica–alumina supported NiMo catalysts on theproperties of asphaltenes during hydroprocessing of heavy petroleum

http://dx.doi.org/10.1016/j.fuel.2014.08.0230016-2361/� 2014 Published by Elsevier Ltd.

⇑ Corresponding author.E-mail address: jancheyt@imp.mx (J. Ancheyta).

Carolina Leyva a, Jorge Ancheyta b,⇑, Guillermo Centeno b

a Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada – IPN – Unidad Legaria, Legaria 694, Col. Irrigación, Mexico City 11500, Mexicob Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas 152, Col. San Bartolo Atepehuacan, Mexico City 07330, Mexico

h i g h l i g h t s

� Characterization of asphaltenesrecovered at different temperatures(380–410 �C) using two differentcatalysts.� Smaller size of asphaltenes and less

concentrated in metals with NiMo/SiO2–Al2O3.� Asphaltenes from NiMo/Al2O3 are

more aromatic and concentrated innitrogen.� NiMo/SiO2–Al2O3 catalyst enhances

metal removal in liquid products.

g r a p h i c a l a b s t r a c t

40

50

60

70

20

30

40

50

Feed 380 400 410 380 -IIReaction temperature for asphaltenes precipitation, °C

% of substitution of arom

aticrings (A

s )

Num

ber o

f aro

mat

icca

rbon

s (Car

)

A: AluminaSA: Silica-alumina

Car-A

Car-SA

As-SA

As-A

a r t i c l e i n f o

Article history:Received 3 April 2014Received in revised form 6 August 2014Accepted 7 August 2014Available online 23 August 2014

Keywords:AsphaltenesHeavy petroleumHydrotreating

a b s t r a c t

Asphaltenes were precipitated from a heavy residue and from its hydrotreated products at constant pres-sure, liquid-hourly space-velocity and hydrogen-to-oil ratio (100 kg/cm2, 0.25 h�1, and 5000 ft3/bbl,respectively). The effect of reaction temperature was studied in the range of 380–410 �C. The experimentswere carried out in a two fixed-bed reactor unit. The first test used a NiMo/c-Al2O3 catalyst and the sec-ond test a NiMo/SiO2–Al2O3 catalyst. It was observed that the molecules of asphaltenes formed after reac-tion from the NiMo/SiO2–Al2O3 catalyst are smaller in size and less concentrated in metals than thoseoriginated from NiMo/c-Al2O3 catalyst, which are more concentrated in nitrogen and the asphaltenesstructure is more aromatic. The NiMo/SiO2–Al2O3 catalyst enhances metal removal in liquid products.

� 2014 Published by Elsevier Ltd.

1. Introduction

To develop suitable catalyst for hydroprocessing of heavy oils,characterization of the reaction products must be accompaniedby precipitation and detailed analysis of the corresponding asphal-tene fraction [1,2]. It is inside the asphaltene molecule where mostof the metals, particularly nickel and vanadium, as well as otherimpurities (sulfur, nitrogen) are concentrated [3,4]. This means

that any change in the asphaltenes structure will impact directlyon the properties of the upgraded oil, such as reduction of the con-tents of sulfur, nitrogen and asphaltenes, increase in the API gravityand in the distillate content, and diminution of the viscosity andresidue fraction content. It is then of high importance to character-ize the asphaltenes by means of proper techniques to determinewhat one wants to follow during reaction.

There are various reports in the literature dealing with the char-acterization of asphaltenes precipitated from products of hydro-treating of a wide variety of crude oils, having also a wide range ofphysical and chemical properties. Different analytical and

112 C. Leyva et al. / Fuel 138 (2014) 111–117

spectroscopic methodologies have been reported to elucidate aboutthe possible relationships among the structural parameters of theasphaltenes and the transformation that they suffer during hydro-processing of heavy petroleum. Limitations, advantages and disad-vantages of the analytical techniques frequently used forasphaltene characterization can be found elsewhere [5,6].

For asphaltenes conversion, the catalysts must possess suitableproperties, i.e. optimal pore size distribution to allow the complexmolecules to access the catalytic sites with minimal diffusion lim-itations and to enhance metal retention capacity, as well as appro-priate acidity to hydrocrack the asphaltene molecule and producelow-molecular weight valuable hydrocarbons with reduced cokeformation.

The objective of this research is to evaluate the effect of two cat-alysts, alumina and silica–alumina NiMo supported catalysts, onthe properties of asphaltenes precipitated from heavy crude oilsthat have been subjected to hydroprocessing.

2. Experimental

2.1. Hydrotreating experiments

An atmospheric residue 343 �C+ (API gravity of 4.4, 119 ppm Ni,572 ppm V, 6.24 wt% S, 25.19 wt% nC7 insolubles) obtained from aheavy crude oil (API gravity of 12.87, 83.4 ppm Ni, 445 ppm V,5.35 wt% S, 18.03 wt% nC7 insolubles) distillation was used forhydrotreating experiments. The residue was recovered from atmo-spheric distillation of the crude oil in a bench-scale fractionationunit. The crude oil was obtained from production facilities in Mex-ico. The experiments were conducted in a two-reactor in seriesbench-scale unit. The two fixed-bed reactors (500 cm3 per-reactorcapacity) were loaded with a triple layer of catalyst extrudates(nominal size of 1/18 inch) consisting of a front-end HDM catalyst,a mid-end catalyst with balanced HDM/HDS activity, and a tail-endHCR catalyst. It should be remember that during hydroprocessingof heavy oils, it is common to use various catalysts to achieve dif-ferent objectives, e.g. metal removal, sulfur removal and asphalt-enes hydrocracking. That is why more than one catalyst is themost-frequently employed approach. The first catalyst mustprovide suitable metal retention capacity, so that the followingcatalysts are protected for premature deactivation.

Each reactor consists of a tubular vessel with an internal diam-eter of 2.94 cm and a total length of 175.88 cm. The catalysts weresulfided in situ with straight-run gasoil containing 1 wt% dimethyldisulfide. The fresh catalyst system was stabilized at 380 �C by pro-cessing the feed during 100 h. Afterwards, the experiments werecarried out at temperature in the range of 380–410 �C, while keep-ing constant the pressure, liquid-hourly space-velocity (LHSV) andhydrogen-to-oil ratio at 100 kg/cm2, 0.25 h�1, and 5000 ft3/bbl,respectively. Hydrogen make-up was supplied between reactorsat a rate of 151 std. m3/m3 to compensate for hydrogen consump-tion in the first reactor. The tests were performed consecutively byfirst adjusting and stabilizing process conditions and then collect-ing product samples at each condition. At the end, catalyst activitywas monitored with a check-back test, which is a repetition of theexperiment done at 380 �C but with larger time-on-stream. Theimportance of this check-back test is to observe if there is or notsignificant catalyst deactivation. The duration of the programwas less than 250 h so as to minimize the influence of catalystdeactivation. A more detailed description of the experimentalprocedure can be found elsewhere [7,8].

Two series of experiments were designed. The first two cata-lysts were kept the same while the tail-end catalyst was changed.In the first test a NiMo/c-Al2O3 catalyst was employed while in thesecond test a NiMo/SiO2–Al2O3 catalyst was utilized. For these two

experiments, eight samples of liquid products were collected at380 �C, 400 �C, 410 �C and 380 �C II at different time-on-stream.

The asphaltenes samples recovered from these two catalysts atdifferent reaction temperatures (380–410 �C) are labeled as:Al-380, Al-400, Al-410, Al-380-II for NiMo/c-Al2O3 catalyst andSA-380, SA-400, SA-410, SA-380-II, for NiMo/SiO2–Al2O3 catalyst.

2.2. Precipitation of asphaltenes

The feed and the hydrotreated products at different conditionswere used for asphaltenes precipitation, which was carried outwith n-heptane using a solvent-to-oil ratio of 5:1 (v/w). A mixtureof 250 g of oil sample and 1250 mL of n-heptane was stirred in aParr batch reactor at 700 rpm, 60 �C and 25 kg/cm2 of pressure ina nitrogen atmosphere during 30 min [9]. Then, the mixture wassettled for 30 min and after that period a vacuum system and aWhatman 943 AH filter paper with 1.5 lm pore size wereemployed in order to separate the n-heptane soluble fraction fromthe insoluble fraction. This latter was washed using n-heptaneuntil the filtered liquid was colorless.

Afterwards, the n-heptane insoluble fraction was also washedby Soxhlet method with a mixture of toluene/heptane (2:1) at96 �C during 8 h. By this purification step, two fractions can beobtained, one insoluble (whole asphaltenes fraction) and anothersoluble (n-heptane and adsorbed resins on the asphaltenes). Theremaining insolubles in n-heptane were dried at 100 �C for 12 h.Finally asphaltenes were recovered for their characterization.

2.3. Characterization of Asphaltenes

2.3.1. Elemental analysisThe carbon, hydrogen, sulfur and nitrogen contents of the

asphaltene samples were determined with a Perkin–Elmer 240analyzer. Oxygen was calculated by difference. The analyses wereperformed with 1 mg of sample ground and sieved to <0.2 mm.The results were quoted as the mean of values from four determi-nations. In all cases, the experimental error was <0.5% of the abso-lute value.

2.3.2. Atomic absorptionNickel and vanadium contents were measured by atomic

absorption using a SOLAAR AA Series Spectrometer analyzer. Solidsamples were heated to 550 �C for elimination of possible organicmaterial. After that, the samples were digested in a mixture ofacids (HCl/HNO3) with heating until total dissolution. Finally, theywere filtered and analyzed.

2.3.3. Size exclusion chromatographySize exclusion chromatography (SEC) was used to determine

the molecular weight distribution of asphaltenes. These analyseswere carried out employing a 300 mm long, 7.5 mm i.d. polysty-rene/polydivinylbenzene-packed Mixed-D column supplied byPolymer Laboratories, Church Stretton, UK. A mixture of1-methyl-2-pyrrolidinone (NMP) and chloroform (CHCl3) in 6:1vol/vol ratio at a flow rate of 0.5 mL min�1 was used as eluent.The column was operated at 80 �C. Detection was made byUV-absorbance using a Perkin–Elmer LC290 variable wavelengthset at 280, 300, 350, and 370 nm, connected in series to a PolymerLaboratories ELS1000 evaporative light scattering detector (ELS).Injected sample solutions were in the concentration range between0.4 and 2.5 mg mL�1.

2.3.4. UV-fluorescence spectroscopyUV-Fluorescence spectra were measured in a Perkin–Elmer

LS50 luminescence spectrometer, which was set to scan at240 nm min�1 with a slit width of 5 nm. A quartz cell with 1 cm

C. Leyva et al. / Fuel 138 (2014) 111–117 113

path length was used. The spectrometer featured automatic correc-tion for changes in source intensity as a function of wavelength.Emission, excitation, and synchronous spectra of the samples wereobtained in CHCl3 solution for all of the samples. In the synchro-nous mode, spectra were acquired at a constant wavelengthdifference of 20 nm. Solutions were diluted with CHCl3 to avoidself-absorption effects: dilution was increased until the fluores-cence signal intensity began to decay. UV-F provides informationon the relative concentrations and sizes of fused aromatic ringsystems. It gives enough resolution for aromatic structures withoverlapping absorption wavelengths.

2.3.5. X-ray diffraction (XRD)This analysis was performed on a SIEMENS D-5000 apparatus

equipped with rotating and Cu Ka (k = 1.5418 Å) radiation at35 kV and 25 mA by diffraction of powders using Bragg method.The samples were analyzed in a range of 2h from 0� to 70�. X-raydiffraction is employed to obtain structural information of asphal-tene aggregates. XRD provides internal structural information andthe crystallite parameters of the molecules associated in the aggre-gates, and it gives quantitative intensity curves, and structuralparameters can be obtained from the shape as well as the positionof the peaks [10].

The layer distance between aromatic sheets (dm) is calculatedfrom the maximum of the graphene band by the Bragg relation,and h is the angle in which the peak is centered (Eq. (1)).

dm ¼k

2 sin hð1Þ

The distance between the saturated portions of the moleculesor interchain layer distance (dc) is given by Eq. (2).

dc ¼5k

8 sin hð2Þ

The average height of the stack of aromatic sheets perpendicu-lar to the plane of the sheet (Lc) was calculated using Eq. (3). Theanalysis of the data was carried out by measuring the full-widthhalf maximum (FWHM) of graphene band.

Lc ¼ 0:9x cos h ð3Þ

where x is the FWHM value obtained using the 002 band. The aver-age diameter of the aromatic sheets (La) on the bases of the Scherrercrystalline size formula was computed with Eq. (4).

La ¼ 1:84k=ðx cos hÞ ð4Þ

The number of aromatic sheets in a stacked cluster (M) is givenby the values of Lc and dm.

M ¼ Lc

dm

� �þ 1 ð5Þ

Table 1Grams of converted species/grams of active metals of fresh catalyst.

Temperature (�C) 380 400 410 380Time-on-stream (h) 144 192 240 288

NiMo/Al2O3

Nickel 0.005 0.007 0.007 0.005Vanadium 0.030 0.038 0.039 0.029Sulfur 4.0 4.6 4.7 3.7Nitrogen 0.13 0.22 0.29 0.12Insolubles in nC7 7.3 11.1 13.4 7.2

NiMo/SiO2–Al2O3

Nickel 0.005 0.007 0.008 0.005Vanadium 0.030 0.040 0.044 0.029Sulfur 4.2 5.0 5.3 3.9Nitrogen 0.26 0.30 0.35 0.24Insolubles in nC7 10.8 12.4 14.1 10.6

2.3.6. Nuclear magnetic resonance (NMR)Structural parameters of asphaltenes were measured by liquid

state 1H and 13C NMR in a JEOL Eclipse 300 spectrometer operatingat a 1H resonance frequency of 300 and 75 MHz for 13C. For 1H NMRspectra, deuterated chloroform (CDCl3) was used. For 13C NMR,chromium acetylacetonate [Cr(acac)3] in the final solution wasadded to ensure complete nuclear magnetic moment relaxationbetween pulses and tetramethylsilane (TMS) was employed as aninternal reference. NMR analysis leads to evaluate structuralchanges, and has been found useful in identifying and quantifyingstructural features in complex mixtures beyond the range of GC.This is best achieved through average structural parameter calcula-tions by using data from liquid-state NMR, combined with elemen-tal analysis and measurements of the molecular weight.

The measurements were performed for 30,000 scans in a gatedproton-decoupled mode. The aromaticity factor (fa) and the substi-tuted aromatic ring percentage (As) were evaluated according toCalemma et al. [11] using the following equations:

f a ¼Caromatic

Caromatic þ Caliphaticð6Þ

As ¼Percent of substituted aromatic carbon

Percent of non� bridge aromatic carbon� 100 ð7Þ

3. Results and discussion

3.1. Hydrotreated products

The conversion of metals (Ni, V), heteroatoms (S, N) and insol-ubles in n-C7 are reported in Table 1. The values are expressed asgrams of converted species divided by grams of active metals(Ni + Mo) of the fresh catalyst. Conversion was calculated in thisway because the amount of catalysts loaded to the reactor for eachexperiment was different as well as their densities. If this is notdone so, the comparison would be unfair due to the exposure ofthe same feed to different weight of catalyst and consequently dif-ferent amount of active metals.

From the table the following observations can be pointed out:

� Conversion of all species increased as temperature increased, asexpected.� NiMo/SiO2–Al2O3 catalyst showed in general higher selectivity

toward all reactions than NiMo/Al2O3 catalyst.� Insolubles in nC7, a direct indicator of asphaltenes content, were

strongly converted by the catalytic bed containing the NiMo/SiO2–Al2O3 catalyst as compared with that having the NiMo/Al2O3 catalyst, which is clearly due to the more acidic characterof the former by the presence of SiO2 in the support.� The repetition of the test at 380 �C at longer time-on-stream

indicates that both catalytic systems were not exposed tosevere loss of activity, and most probably the predominantmechanism of catalyst deactivation is by coke deposition dueto the short time-on-stream during the experiments.

3.2. Characterization of asphaltenes

3.2.1. Elemental analysisThe elemental analysis (C, H, N, S) of asphaltenes and the H/C

atomic ratio are reported in Table 2. The H/C atomic molar ratiowas calculated using the following equation:

Table 3Metals concentration in asphaltene before and after hydrotreating.

Metals Feed Catalyst 380 400 410 380-II

Ni (ppm) 300 A 226 251 292 277SA 224 232 239 231

V (ppm) 1524 A 1077 1104 1198 1089SA 1051 1089 1169 1066

V/Ni 5.1 A 4.8 4.4 4.1 3.9SA 4.8 4.7 4.9 4.6

Niaspa (%) 63.5 A 48.9 53.1 61.8 58.6

SA 48.2 50.6 52.0 49.3Vasp

a (%) 67.1 A 47.4 48.6 52.7 47.9SA 46.2 48.3 51.5 46.9

a Asphaltenic nickel and asphaltenic vanadium.

114 C. Leyva et al. / Fuel 138 (2014) 111–117

HC¼ wt% of Hydrogen

wt% of Carbon

� �Atomic weight of Carbon

Atomic weight of Hydrogen

� �ð8Þ

From these values together with asphaltenes, sulfur and nitro-gen contents of the feed, the percentages of asphaltenic sulfur(Sasp) and nitrogen (Nasp) were calculated. Firstly, the grams ofnitrogen, sulfur and asphaltenes that are contained in the feedare obtained. The same calculation is done but now for the amountof nitrogen and sulfur that are contained in the correspondingasphaltenes. Finally, the percentage of asphaltenic sulfur and nitro-gen are computed by dividing their amount in the asphaltenes bytheir amount in the feed.

The following changes are observed:

� For both catalysts the H/C ratio decreases as the temperature ofreaction increases, having the lowest values at 410 �C.� With the value of the H/C ratio the aromatic character of the

asphaltenes can be established that is the higher the H/C ratiothe lower the aromaticity [12–14], this indicates that theasphaltene structure becomes more aromatic as the reactiontemperature is increased, which is further confirmed by the aro-maticity factor.� Asphaltenes obtained from the silica–alumina catalyst are less

aromatic than those recovered from the NiMo/Al2O3 catalyst.This indicates that the nature of NiMo/SiO2–Al2O3 catalyst isnot only promoting the breakage of side alkyl chains but alsothe hydrogenation of aromatic rings.� The calculated percentage of asphaltenic sulfur decreases as the

temperature increases, which is mainly due to the scission ofthe side chains in which this heteroatom is mostly located inthe form of sulfur bridges.� The contrary effect is observed for the asphaltenic nitrogen,

which is attributed to its localization in the asphaltene mole-cule, that is it remains almost unchanged and thus concentrat-ing in asphaltene.� The percentage of asphaltenic sulfur resulted to be higher with

the NiMo/SiO2–Al2O3 catalyst while the asphaltenic nitrogenexhibited the opposite behavior, that is it was higher for theNiMo/Al2O3 catalyst. This means that the silica–alumina sup-ported catalyst presented more selectivity toward the removalof asphaltenic nitrogen as compared with the alumina sup-ported catalyst, indicating that the presence of silica enhancesthe hydrogenation of aromatic rings where the asphaltenicnitrogen is more concentrated.� The check-back test using the conditions of the first experiment

(380 �C) clearly confirms that deactivation is not taking place atgreat extent.

3.2.2. Atomic absorptionBefore the alumina or silica-alumina catalysts, two other cata-

lysts were loaded to the reactor in which most of the metals areretained, so that the third catalyst faces a feed with low amount

Table 2Elemental analysis of asphaltenes.

Feed A-380 A-400 A-410 A

C (wt%) 82.7 87.4 87.7 89 8H (wt%) 8.1 8.5 8.3 7.3N (wt%) 1.1 0.8 0.8 1.0O (wt%) 0.1 0.1 0.1 0.1S (wt%) 8.0 3.2 3.1 2.6H/Ca 1.17 1.16 1.13 0.97Sasp

b (%) 32.44 13.04 12.68 10.37 1Nasp

b (%) 47.42 33.15 35.25 41.54 4

a Atomic molar ratio.b Asphaltenic sulfur and asphaltenic nitrogen.

of metals. According to the results of metals content in asphaltenesshowed in Table 3, the following observations can be done:

� The silica–alumina catalyst improves the metal removal in theliquid products than alumina catalyst (Table 1), which is inagreement with the metal content in the asphaltene structure,that is the asphaltenes obtained with the NiMo/Al2O3 catalystare more concentrated in metals.� Metal content increases as the reaction temperature increases,

this suggests that the metal porphyrin complexes tend to con-centrate in the asphaltene structure, due to dealkylation of sidealkyl chains because of aliphatic carbon removal, whereas met-als tend to remain unchanged and concentrated at highertemperature.

3.2.3. Size exclusion chromatographyThe SEC chromatograms of asphaltenes are illustrated in Fig. 1a

and 1b. A bimodal profile is observed for all the samples. Theretained peak (roughly eluting between 15 and 25 min) consistsof material resolved by the column while the excluded peak (elut-ing between 11 and 15 min) represents material excluded from thecolumn porosity. In these chromatograms a valley is observedbetween the two peaks, which has been interpreted in previousworks as indication of a change in molecular conformation to arigid three-dimensional structure [15], suggesting that theexcluded material may be significantly different from the retainedmaterial. From Fig. 1a and 1b the following comments can behighlighted:

� For the asphaltenes obtained from products hydrotreated withboth catalysts, it is appreciated that the excluded material issimilar for all samples, however, the retained peak is different,which indicates that asphaltene molecules are reduced in theirmolecular size as the reaction temperature is increased.� The elution time and the intensity of the peaks indicate that

there is a formation of small molecules at 410 �C due to theseverity of the reaction.

-380-II SA-380 SA-400 SA-410 SA-380-II

6.2 86.5 87.2 87.4 85.77.9 8.5 8.5 8.3 8.51.0 0.8 0.8 0.8 0.80.1 0.1 0.1 0.1 0.14.8 4.1 3.4 3.4 4.91.10 1.18 1.16 1.13 1.199.22 16.67 13.81 13.56 19.902.80 32.73 33.99 34.83 34.41

SA-410SA-400

SA-380-II

SA-380

Feed

0.0000

0.0004

0.0008

0.0012

0.0016

0.0020

0.0024

0.0028

0.0032

Inte

nsity

10 12 14 16 18 20 22 24 26

Elution time, min

A-380

A-380-IIA-400

A-410

Feed

0.0000

0.0004

0.0008

0.0012

0.0016

0.0020

0.0024

0.0028

0.0032

Inte

nsity

(a)

(b)

Fig. 1. Size exclusion chromatography of (a) asphaltenes obtained from NiMo/Al2O3

and (b) asphaltenes obtained from NiMo/SiO2–Al2O3.

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016SA - retained material A - retained material

Are

a of

reta

ined

pea

k

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002SA - excluded material A - excluded material

Are

a of

exc

lude

d pe

ak

0

2

4

6

8

10

12

14

16

Feed 380 °C 400 °C 410 °C 380 °C - II

SA - ratio A - ratio

Temperature, °C

Rat

io (R

etai

ned/

excl

uded

) are

as' p

eaks

(a)

(b)

(c)

Fig. 2. (a) Area of the retained peaks, (b) area of the excluded peaks and (c) ratio ofretained/excluded areas of peaks.

C. Leyva et al. / Fuel 138 (2014) 111–117 115

� The peak of SA-410 has less intensity but long elution time indi-cating the presence of small molecules at 410 �C.� The chromatograms of both 380 and 380-II asphaltenes have

almost the same shape and intensity, this confirms that theasphaltene structure is not strongly affected by time-on-stream,and hence catalyst deactivation is not considerable.� Both chromatograms show a sequence with the elution time

and the formation of small molecules is enhanced accordingto the increased reaction temperature, which is confirmed bythe values of the retained and excluded peak areas as well asthe ratio of retained/excluded material of all asphaltenes asshown in Fig. 2a–c. It is then possible to establish that theasphaltenes from silica-alumina catalyst possess smaller mole-cules due to the higher ratio of retained material and excludedmaterial as compared with alumina catalyst.� The retained/excluded material ratio increases as the reaction

severity increases (Fig. 2c). The effect of acidity of NiMo/SiO2–Al2O3 catalyst over NiMo/Al2O3 catalyst is then given by theretained/excluded ratio of the former being higher than the lat-ter. The aggregate molecular weight was reduced as reactiontemperature was increased, which indicates that the size ofthe asphaltene molecule is also decreased.

3.2.4. UV-fluorescence spectroscopyThe relative magnitudes of aromatic ring systems were studied

by UV-Fluorescence spectroscopy as is illustrated in Fig. 3a and b. Itis observed that:

� The spectra of asphaltenes obtained from NiMo/Al2O3 catalyst(Fig. 3a) reveal an increase in fluorescence intensity at longerwavelengths, which indicates the presence of large aromaticring systems in this asphaltene sample [16], specially at hightemperature.� The spectra of asphaltenes obtained from NiMo/SiO2–Al2O3

catalyst (Fig. 3b) presents a similar trend that those from alu-mina catalyst, however, the intensity and wavelength aresmaller.� These results suggest that there are differences in the aromatic

cluster sizes as function of reaction temperature, and theasphaltenes from NiMo/Al2O3 catalyst are more aromatic thanthose from NiMo/SiO2–Al2O3 catalyst. These results correlatewith those obtained from elemental analysis.

3.2.5. X-ray diffraction (XRD)The crystalline parameters values are presented in Table 4,

whereby the following observations can be made:

� From the XRD diffractograms (not shown), some differences areobserved as the reaction temperature increases, for instance thepeak at 26� corresponding to aromatic rings (graphene peak) ofthe asphaltenes is slightly changed, which indicates the modifi-cation of the aromatic coke as the severity of the reactionincreases.

A-380

A-400A-410

A-380-II

Feed

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Inte

nsity

(a)

SA-410

SA-380-II

SA-400

SA-380

Feed

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Inte

nsity

300 350 400 450 500 550 600 650 Wavelength

(b)

Fig. 3. Synchronous spectra of (a) asphaltenes obtained from NiMo/Al2O3 and (b)asphaltenes obtained from NiMo/SiO2–Al2O3.

Table 4Crystalline and structural parameters of asphaltenes.

dm (Å) dc (Å) Lc (Å) La (Å) M f a As

Feed 3.65 4.79 17.84 13.42 5.9 0.29 70.0A-380 3.63 4.76 16.81 12.63 5.9 0.37 54.5Al-400 3.63 4.74 15.93 12.55 5.4 0.41 51.7Al-410 3.63 4.71 14.94 12.51 5.1 0.49 46.3A-380-II 3.64 4.76 15.43 12.56 5.4 0.41 52.2

SA-380 3.63 4.75 15.93 12.16 5.4 0.36 55.2SA-400 3.63 4.73 14.96 12.14 5.1 0.38 54.0SA-410 3.62 4.73 13.80 12.13 4.8 0.44 49.5SA-380-II 3.63 4.74 14.89 12.14 5.3 0.39 53.8

40

50

60

70

20

30

40

50

Feed 380 400 410 380-II

Reaction temperature for asphaltenes precipitation, °C

% of substitution

of aromatic

rings( As )

Num

bero

f aro

mat

icca

rbon

s(Car

)

Fig. 4. Number of aromatic carbons (Car) and percent of substitution of aromaticrings of asphaltenes (As). (}) Car–A, (D) Car–SA, (s) As–A and (+) As–SA.

116 C. Leyva et al. / Fuel 138 (2014) 111–117

� There is a rupture of the alkyl chains and the fragmentation ofmolecules, thus causing a modification in the crystallineparameters.� The average layer distance is not strongly modified during the

reaction. The values of dm for the asphaltenes from alumina cat-alyst are slightly larger than those of silica–alumina catalyst.� In the case of the average interchain layer distance (dc), this

parameter also diminishes as the severity of reaction increases,being the dc for the A-asphaltenes set a little bit larger thanSA-asphaltenes set.� The average stack height of the aromatic sheets perpendicular

to the plane of the sheets (Lc) parameters is in the range of13.79–16.81 Å. Both cases present a decreasing tendency simi-larly to other parameters, having the asphaltenes from aluminacatalyst higher values than silica–alumina asphaltenes.� The average diameter of the aromatic sheets (La) is in between

12.13 and 12.62 Å, this parameter indicates that the diameter ofaromatic sheets of SA-asphaltenes is shorter than those ofA-asphaltenes.� The number of aromatic sheets in a stacked cluster (M) oscil-

lates between 5.1 and 5.9. The asphaltenes from silica-aluminacatalyst have lower values than alumina asphaltenes whichindicates that the latter are less compact than the former.

� In general, the comparison of the hydrotreated asphaltenes withthe asphaltene from the feed, it is possible to state that thosefrom the silica–alumina catalyst present significant modifica-tion in their crystalline structures than samples from aluminacatalyst.

3.2.6. Nuclear magnetic resonance (NMR)The quantitative comparisons of structural parameters (fa, As) of

the asphaltenes are presented in Table 4, from which the followingis observed:

� The set of asphaltenes from alumina catalyst possesses highervalues of fa than those of silica due to the presence of more aro-matic systems, for instance the A-410 and SA-410 samples cor-responding to the high reaction temperature show aromaticityfactor of 0.49 and 0.44, respectively. This is mainly due to thedealkykation of aromatic ring systems, and in lesser extent tothe aromatization of naphthene ring system, cyclodehydroge-nation of alkyl chains to form other naphthene ring systems,and dehydrogenation of the new naphthene ring system to formmore aromatic rings.� As the reaction temperature is increased, the substituted aro-

matic rings percentage (As) is reduced due to the increase inthe aromatization of the asphaltene constituents and in thenumber of aromatic ring systems reflected by the increase inaromatic carbon atoms as can be seen in Fig. 4. Thus, the newspecies formed by the original asphaltenes have high aromatic-ity and less number of alkyl chains.

4. Conclusions

The characterization of liquid products demonstrated that thesilica-alumina catalyst enhances the removal of more metals thanalumina catalyst, which is attributed to its acidic nature that pro-vokes deep hydrogenation of the aromatic structure thus releasingthe metal that is located at the interior of the asphaltene.

The asphaltenes obtained from NiMo/SiO2–Al2O3 catalyst aremore compacted and have smaller molecular size than those fromNiMo/Al2O3 catalyst, however, the latter have higher concentrationof metals in their structures, and are more aromatic than thosefrom NiMo/SiO2–Al2O3.

The X-ray diffraction information shows that the layer distancebetween aromatic sheets and the interchain layer distance of satu-rated portions remain almost constant. The values of the average

C. Leyva et al. / Fuel 138 (2014) 111–117 117

stack height of the aromatic sheets perpendicular to the plane ofthe sheets confirm that the asphaltenes molecules from NiMo/SiO2–Al2O3 catalyst are more compacted than asphaltenes mole-cules of alumina catalyst.

The samples collected at 380 �C and long time-on-streamindicate that the catalyst deactivation is not significant at theconditions studied here.

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