Investigation on Asphaltenes Structures during Low Temperature Coal

Apr 17, 2017 - ... and Technology Star Project of Shaanxi Province (2016KJXX-32). ...... S. In 2002 International Conference on Heavy Organic Depositi...
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Investigation on Asphaltenes Structures during Low Temperature Coal Tar Hydrotreatment under Various Reaction Temperatures Liangjun Pei,†,‡ Dong Li,*,†,‡ Xu Liu,†,‡ Wengang Cui,†,‡ Ruitian Shao,†,‡ Fengfeng Xue,†,‡ and Wenhong Li†,‡ †

School of Chemical Engineering, Northwest University, Xi’an 710069, China Shaanxi Research Center of Northwest University Technology for Resource Utilization, Xi’an 710069, China

‡

ABSTRACT: In order to convert low temperature coal tar (LCT) into clean transportation fuels by hydrotreating (HDT), the changes of asphaltenes properties during HDT of LCT should be paid more attention. The HDT experiments were carried out in a trickle-bed reactor unit under various reaction temperatures, and asphaltenes were extracted from the liquid products. The properties and structure of asphaltenes were performed by X-ray diffraction, elemental analysis, size-exclusion chromatography, X-ray photoelectron spectroscopy, and nuclear magnetic resonance. Structural units analysis of average molecules and the variation trend of change in the structural parameters were revealed. The results showed that, with the temperature increase, the diameter of the aromatic sheet (La), interaromatic layer distance (dm), total carbon number (CT), and average alkyl chain length (n) decrease, while the aromaticity factor ( fA) and aromatic carbon number (CA) increase. Moreover, the mechanism of HDT was studied to make the molecular structure of asphaltenes easier to understand. At the same time, the larger condensation degree of the asphaltenes molecules, the larger aromatic degrees, the removal of the alkyl side chain was more significant.

1. INTRODUCTION Due to the stubbornly high petroleum dependence degree of China, coal-to-liquids (CTL) which could replace crude oil imports as a kind of clean coal technology, has attracted increasing interest.1 Coal will be the chief energy source in China over the next few decades. The Action Plan for Clean and Eff icient Utilization of Coal (2016−2020) which advocated to promote the modern coal chemical industry was published by the National Energy Administration of the People’s Republic of China on May 5, 2016. As of September 2016, more than 300 chemical plants are using the Low Temperature Pyrolysis Process (LTPP) to produce low temperature coal tar (LCT), and the yield of LCT has reached 10 million tons each year.2,3 LCT is a sophisticated dark brown fluid, composed of aliphatic, alicyclic, aromatic, and heterocyclic micelles and molecular aggregates. A typical feature of LCT is its large content of asphaltenes and impurities, namely, sulfur, nitrogen, oxygen, and metals. It is widely acknowledged that asphaltenes are coke precursors and that impurities are contaminants.4,5 A large amount of low-value LCT can be used as an alternative source for producing valuable clean liquid fuels through catalytic hydrotreating (HDT). After catalytic HDT, the quality of raw coal tar was improved, and high-quality fuels were obtained due to that a lot of impurities were removed.2,3,6−11 As is known to us, asphaltenes are the heaviest aromatic component as well as the most refractory portion of LCT, and the contents are in the range of 8−15 wt %.12 Abundant researches have discussed the properties of asphaltenes, but the exact molecular structure is still unknown. Asphaltenes are complex compounds, consisting of condensed polynuclear aromatics (low H/C molar ratio) which are linked by aliphatic chains. In general, asphaltenes precipitate from oil to form a solid and can be defined as the fraction of oil which is insoluble © 2017 American Chemical Society

in n-heptane or n-pentane but soluble in aromatic hydrocarbons, such as benzene or toluene.13−18 During HDT of heavy oils, asphaltenes cause the following problems to occur:14,18−21 (1) Asphaltenes not only affect the overall rate of HDT reaction adversely but also limit the largest degree of conversion. (2) Asphaltenes precipitate to solids on the catalyst surface and then block the pore mouth. (3) Asphaltenes are the precursors of coke formation, and coke is the main cause of catalysts deactivation. (4) Asphaltenes result in the formation of high viscosity residue. The composition of asphaltenes can be determined by elemental analysis (EA), and the molecular structure can be determined by a great variety of analytical techniques, such as nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), size-exclusion chromatography (SEC), and so on. Particularly, NMR has been widely used to analyze the average molecular structural parameters of asphaltenes, such as naphthenic rings, aromatic rings, aromatic carbon fraction, and length of alkyl side chains. The SEC method has been employed to analyze the molecular weight of asphaltenes.22,23 Recently, XRD has been used to analyze the reflex on diffractograms in asphaltenes samples, which is caused by the presence of coherent scattering zones, to obtain structural parameters such as interchain spacing, layer diameter, interlamellar spacing, and number of layers.24,25 The conversion mechanisms of asphaltenes during HDT are complex and still unknown because of the complex nature of asphaltenes and different simultaneous chemical reactions. Studies on asphaltenes, obtained from various petroleum residues, indicate that the condensed polyaromatic rings are Received: November 30, 2016 Revised: April 11, 2017 Published: April 17, 2017 4705

DOI: 10.1021/acs.energyfuels.6b03180 Energy Fuels 2017, 31, 4705−4713

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Energy & Fuels

Process (LTPP). The main properties of the feedstock are listed in Table 1.

hardly hydrogenated or dealkylated during HDT. Seki and Kumata26 reported that the aliphatic side chains of asphaltenes were removed during HDT. They also found that the quality rather than the quantity of asphaltenes is the main cause of coke deactivation, and that the aromaticity of asphaltenes has an influence on catalysts deactivation. Bartholdy and Andersen27 observed that condensed products, which effect the denitrification reaction adversely, are generated during HDT. It is considered that nitrogen removal is hard to achieve due to its location in heteroaromatic rings, of which HDS is easier to occur than nitrogen removal. They also demonstrated that hydrocracking-dominated replaced hydrogenation-dominated to be the overall apparent reaction mechanism at about 370 °C, and that the reaction products of asphaltenes during HDT changed when the reaction temperature reached 380 °C. Merdrignac et al.28 observed that, when the reaction severity changes, the composition of asphaltenes during HDT also changes. When the reaction temperature increase, heteroatoms (nitrogen, metal) content and asphaltenes aromaticity increase, while asphaltenes molecular weight decreases. Asaoka et al.29 discussed the influence of reaction temperature on asphaltenes properties during HDT. They concluded that the destruction of asphaltenes aggregates and the depolymerization of asphaltenes molecules, caused by the removal of vanadium and sulfur, respectively, were the main reactions. Carolina et al.30 studied the effect of alumina and silica−alumina supported NiMo catalysts on the properties of asphaltenes in heavy petroleum hydroprocessing. The results indicate that the asphaltenes are smaller in size and lower in metals content after hydrogenation by using the NiMo/SiO2−Al2O3 catalyst than those using the NiMo/c-Al2O3 catalyst, which are more concentrated in nitrogen and the asphaltenes structure is more aromatic. The NiMo/SiO2−Al2O3 catalyst enhances the removal of metal in liquid products. Trejo et al.31 studied the changes of asphaltene structure during the hydrogenation of crude oil by scanning and transmission electron microscopy (SEM and TEM). They concluded that, by analyzing the TEM results, the removal of the alkyl chains in the hydrogenation process makes the asphaltene molecules rearranged in the solid state, which favors the stacking of aromatic cores. Through the SEM microscopy, one can see different fractions of asphaltenes as they are constituted of agglomerated particles, porous structures, and smooth surfaces. The increased output of LCT in China has attracted the attention of the scientific and petroleum-production factories, who hope to convert these materials into clean transportation fuels by HDT. Therefore, the study of the LCT asphaltenes properties changes during HDT causes widespread concern nowadays. Although many studies have been carried out to determine the structural changes of petroleum derived asphaltenes,13−18,26−29 the investigation of the changes of LCT asphaltenes properties during HDT is rare. In this work, the characteristics of asphaltenes, obtained from Shenmu (Shaanxi Province, China) LCT and from various HDT products, were analyzed. The effects of reaction temperature on asphaltenes properties were examined by XRD, NMR, XPS, and so on, and the mechanism of asphaltenes during HDT has also been discussed.

Table 1. Properties of LCT property density (20 °C) carbon hydrogen nitrogen sulfur atomic H/C ratio viscosity (50 °C) carbon residue ash SARA mass fractionsa saturated hydrocarbon aromatic hydrocarbon resin asphaltene error

unit

value

g/mL wt % wt % wt % wt %

1.043 83.32 8.25 1.05 0.53 1.20 14.29 7.81 0.173

mm2 s−1 wt % wt % wt % wt % wt % wt % wt % %

28.56 35.49 23.25 12.7 ≤5

a

The method to obtain polar components (saturated hydrocarbon, aromatic hydrocarbon, resin and asphaltene) is based on the Chinese petroleum industry standard method (SH/T 0509).The main conditions are as follows: Isolation of asphaltene: coal tar:n-heptane = 1 g:50 mL; isolation of soluble fractions: 75−150 μm neutral Al2O3 columns. Compared with fossil fuels, LCT is an inferior oil due to its great density and high asphaltenes content.32 The density of LCT is in the range of 0.97−1.04 g/cm3 (20 °C), which is higher than that of traditional crude oil.12 The oxygen content in LCT (6−8 wt %) is generally much higher than that in crude oil and other mineral oil (less than 2 wt %).33 Moreover, the differences of SARA (Saturated hydrocarbon, Aromatic hydrocarbon, Resin and Asphaltenes) mass fractions are immediately apparent and the asphaltenes content in LCT (8−15 wt %) is generally much higher than that in crude oil (less than 2 wt %).34 In our recent studies, the composition and chemical structure of LCT asphaltenes have been discussed.35 2.2. HDT Procedure. HDT experiments were conducted in a trickle-bed reactor unit. The inside diameter of the reactor tube was 2.6 cm, and the length was 140 cm. The 40 cm long constant temperature zone of the reactor contains a 200 mL HDT catalyst packing. The commercially available catalyst supported on γ-alumina sample is made of nickel molybdenum (186 m2/g specific surface area, 0.45 cm3/g pore volume, 21.76 wt % MoO3, 6.38 wt % NiO, 5.48 wt % SiO2, and 2.58 wt % P2O5). Because the secrecy agreements were signed with the catalyst supplier, model number of the catalysts cannot be provided. The diagram of the trickle-bed reactor system and the experimental procedures have been described in previous papers.2,33 When catalyst sulfidation is completed, LCT is introduced at the desired feed rate and other conditions are adjusted to the set values. Previous studies2,6−12,36 indicated that severe reaction conditions favored the HDT of LCT and the obtaining of coal-derived liquids. According to these studies, the experimental parameters, including the hydrogen-to-oil volume ratio, pressure, and liquid hourly spacevelocity (LHSV), were fixed at the level of 1500 LN/L, 8 MPa, and 0.5 h−1, respectively. The reaction temperature changed from 350 to 410 °C. 2.3. Precipitation and Characterization of Asphaltenes. Asphaltenes were precipitated from LCT and HDT products by using n-heptane as solvent. The details of this experiment were reported elsewhere.35 The definition of asphaltenes content in the samples is asphaltenes content (wt %) = [100 × asphaltenes obtained (g)]/LCT or HDT products (g). The contents of carbon, hydrogen, sulfur, and nitrogen in asphaltenes were examined by VarioEL III analyzer. Structural

2. MATERIALS AND METHODS 2.1. Materials. The LCT feed material was obtained from Shenmu (Shaanxi Province, China) coal using the Low Temperature Pyrolysis 4706

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Energy & Fuels Table 2. Effects of Reaction Temperature on Composition of Productsa reaction temperature (°C) properties −1

density (20 °C) (g·mL ) carbon (wt %) hydrogen (wt %) nitrogen (wt %) sulfur (wt %) oxygen (wt %) atomic H/C ratio a

LCT

350

365

380

395

410

1.043 83.32 8.25 1.05 0.53 7.62 1.20

0.950 86.79 9.95 0.59 0.28 1.92 1.38

0.945 86.73 10.14 0.52 0.25 1.84 1.41

0.936 86.64 10.32 0.45 0.21 1.78 1.43

0.934 86.71 10.46 0.39 0.15 1.62 1.43

0.928 86.83 10.51 0.37 0.15 1.55 1.45

Other experimental conditions: P = 8 MPa, LHSV = 0.5 h−1, and H2/oil = 1500 LN/L.

Table 3. Effects of Reaction Temperature on Product Distribution of HDTa reaction temperature (°C) properties

LCT

350

365

380

395

410

error (%)

pyrolysis gas saturated hydrocarbon aromatic hydrocarbon resin asphaltene coke

28.33 35.21 23.06 12.60 0.80

1.02 42.68 26.52 20.97 7.88 0.81

1.08 45.96 27.43 17.46 7.14 0.83

1.17 49.32 28.43 13.26 6.50 1.32

1.32 50.07 29.06 11.73 6.31 1.51

1.5 50.88 29.77 10.23 5.98 1.64

≤5 ≤5 ≤5 ≤5 ≤5 ≤5

a

The method to obtain polar components (saturated hydrocarbon, aromatic hydrocarbon, resin and asphaltene) is based on the Chinese petroleum industry standard method (SH/T 0509). Other experimental conditions: P = 8 MPa, LHSV = 0.5 h−1, and H2/oil = 1500 LN/L.

parameters of asphaltenes were measured by liquid-state 1H NMR in a Varian-FT-80A spectrometer operating at a 1H resonance frequency of 400 MHz. Deuterated DMSO (DMSO-d6) was used as solvent, and tetramethylsilane (TMS) was used as an internal standard. Size-exclusion chromatography (SEC) analyzes the molecular weights (MWs) of the asphaltenes sample, these analyses were implemented adopting a 300 mm long, 7.5 mm i.d. polystyrene/ polydivinylbenzene-packed Mixed-D column supplied. A mixture of 1methyl-2 pyrrolidinone (NMP) and chloroform (CHCl3) in a 5:1 vol/ vol ratio at a flow rate of 0.4 mL min−1 was used as eluent. The column was operated at 75 °C. Detection was made by UV-absorbance using a PerkinElmer LC290 variable wavelength set at 285, 300, 345, and 370 nm. Injected sample solutions were in the concentration range between 0.3 and 2.4 mg mL−1.30 X-ray photoelectrom spectroscopy (XPS) is used to examine the surface outer layers of asphaltenes in recent years.35 XPS can identify the chemical environment over a few nanometers depth from binding energies (BEs). XPS was performed on a PHI 5400 instrument which uses an Al Kα source (hv = 1486.6 eV). The source was operated at 150 W. The experimental pressure was always lower than 8 × 10−9 Torr. The binding energy of C−C species (BE = 284.8 eV) is used as a standard to correct the spectra. At the pass energies of 70 and 20 eV, survey and region spectra were collected, respectively. X-ray diffraction (XRD) was used to identify the variations of asphaltenes lamellar structures. This analysis was performed on a SIEMENS D-5000 apparatus equipped with rotating and Cu Kα1 (k = 1.5318 Å) radiation at 35 kV and 25 mA by diffraction of powders using the Bragg method. The samples were analyzed in a range of 1.5 h from 0° to 65°.37 The asphaltenes of HDT product at different temperatures were analyzed by XRD, and it can obtain the crystalline parameters as follows:25,38,39 diameter of aromatic sheet (La), interaromatic layer distance (dm), interchain or internaphthene layer distance (dr), average height of the stack of aromatic sheets perpendicular to the plane of the sheet (Lc), and average number of aromatic sheets associated in a stacked cluster (M). The following is the calculation of crystalline parameters:

interchain or internaphthene layer distance

dr = 5λ /8 sin θ diameter of aromatic sheet

La = 1.84λ /ω cos θ = 0.92/B1/2 average height of the stack of aromatic sheets Lc = 0.9λ /ω cos θ = 0.45B average number of aromatic sheets M = (Lc /dm) + 1 average ring number at any orientation

R a = La /2.667 θ is the diffraction angle in which the peak is centered, λ is the wavelength of the Cu Kα radiation, ω is the peak width, B1/2 is the full width half-maximum, and 2.667 Å is the width of a single aromatic nucleus. Bragg angle 2θ = 42.48°; λ = 1.5406 nm.

3. RESULTS AND DISCUSSION 3.1. Influence of Temperature on LCT Properties during HDT. Temperature control is regarded as the most cost-effective method to control the HDT process. With the reaction temperature increase, the rate and conversion of HDT increase. The influence of temperature on product properties were studied by varying the temperature from 350 to 410 °C, and other reaction conditions, such as pressure, LHSV, and hydrogen-to-oil volume ratio, were fixed at 8 MPa, 0.5 h−1, and 1500 LN/L, respectively. The results given in Table 2 show that the density of the HDT products decreased from 1.043 to 0.928 and that the density was at the lowest value when the reaction temperature was the highest, which suggests that LCTs were converted to lighter products. It also shows that the conversions of HDS, HDO, and HDN increased with temperature increased and that the degree of nitrogen removal was lower than that of sulfur and oxygen at all temperatures. This phenomenon can be

interaromatic layer distance

dm = λ /2 sin θ 4707

DOI: 10.1021/acs.energyfuels.6b03180 Energy Fuels 2017, 31, 4705−4713

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Energy & Fuels attributed to many reasons. On the one hand, compared to S  species, nitrogen compounds were more difficult to be removed no matter in conventional crude or LCT. Ho40 also observed that, compared to nitrogen compounds, sulfur compounds were easier to convert. On the other hand, LCT contains more nitrogen than other petroleum heavy fractions and the reaction of nitrogen compounds in LCT are significant. The removal rate of oxygen is higher than nitrogen and sulfur at all temperatures. The oxygen-containing compounds of LCT are predominantly low-boiling phenols (phenol, xylenols, and cresols) which can be HDT at the moderate conditions.4,33 As presented in Table 3, with the temperature increased, the content of saturated hydrocarbon increased. This shows that, under the condition of catalytic hydrogenation reaction, the catalytic hydrogenation and cracking reaction rate of coal tar were increased with the reaction temperatures increasing. The contents of resin and asphaltenes decreased with the temperature increased; it indicates that increasing the reaction temperature in a certain range contributes to the upgrading of LCT heavy fractions. The main reason was that, under a certain hydrogen pressure condition, the coal tar macromolecules are easy to be activated when the reaction temperature is high, which is favorable to the occurrence of hydrogenolysis and cracking reaction. When the reaction temperature reached 410 °C, nearly 60% of the asphaltenes in the feedstock were converted. This not only indicates that high reaction temperature favored hydrocracking of asphaltenes, but also implies that the composition of asphaltenes changed when the reaction temperature increased. At high temperature, the contents of pyrolysis gas and coke increased rapidly; the main reason was that the hydrogenation reaction was exothermic and the cracking reaction was endothermic, so the higher the temperatures, the deeper the degree of cracking and the higher the cracked gas production. At higher temperatures, the rate of cracking reaction was higher than that of the hydrogenation reaction, the alkyl side chains of some asphaltene macromolecules were removed, so the remaining fused aromatic rings reacted with each other to form larger molecules by dehydrogenation and condensation reaction, and then generated coke. 3.2. Analysis of Elemental and Molecule Weight of Asphaltenes. The elemental analysis (C, H, O, N, S) of asphaltenes, extracted from HDT products under different temperatures, is presented in Table 4. It shows clearly that, with the reaction temperature increased, carbon content in asphaltenes increased steadily, while the content of hydrogen, sulfur, nitrogen, and oxygen decreased. When the original reaction temperature increased, it is quite true that the molecular weight reduced, mainly due to the

aromatics which located at the external part of the asphaltenes would be saturated and the breaking of alkyl chains. This is consistent with that the length of the alkyl side chain was reduced in 1HNMR and the diameter becomes smaller in XRD. The increase of temperature favored the breakage of alkyl chains and the obtaining of aromatic asphaltenes. Ancheyta et al.41 found that, when the temperature was higher than 400 °C, C−C bonds were broken easily and saturated hydrocarbons cracked profitably; this results in the formation of polyaromatic rings. The phenomenon that the H/C atomic ratio of asphaltenes reduced when increasing temperature also indicates that more aromatic structures remained after HDT. With the reaction temperature increased, the conversion of sulfur and oxygen nearly followed a linear trend. Sulfur contents reduced because the sulfur bridges on alkyl chains were destroyed. However, nitrogen content in asphaltenes decreased at first and then it did not change significantly. Nitrogen compounds, present in the asphaltenes structure, are one of the most refractory heteroatoms because they are located in aromatic rings and associated with metal complexes. The HDT of these compounds is thermodynamically unfavorable, which confers high stability.42 Some researchers have found that the content of asphaltenes during HDT is constant,28 but this regularity is not applicable for all types of feed stocks. Related to this, Ancheyta14 found that there is an increasing tendency of nitrogen content in asphaltenes during HDT, because the removal of nitrogen from aromatic rings is hard to achieve. 3.3. XRD Analysis of Asphaltenes. When asphaltenes precipitated to solid, they begin to form aromatic ring piles and further form the similar crystal structure. XRD can be used to measure the internal structure of asphaltenes by drawing a hypothetical crystallite. Some researchers obtained many parameters from this technique.25,38,39 A schematic view of a probable stacking of asphaltenes is shown in Figure 1. The

Figure 1. Cross-sectional view of asphaltenes model: zigzag structures represent the configuration of alkyl chains or naphthenic rings, and the straight lines represent the edge of flat sheets of condensed aromatic rings.

Table 4. Elemental Analysis and Molecule Weight of Asphaltenes reaction temperature (°C) properties

LCT

350

365

380

395

410

carbon (wt %) hydrogen (wt %) nitrogen (wt %) sulfur (wt %) oxygen (wt %) atomic H/C ratio molecular weight

77.82 5.86 2.74 1.93 11.21 0.90 698

82.90 6.15 2.01 0.92 8.02 0.89 605

83.31 5.87 1.97 0.84 8.01 0.85 598

83.67 5.70 1.95 0.71 7.97 0.83 582

84.46 5.58 1.93 0.62 7.42 0.79 573

84.97 5.35 1.92 0.54 7.22 0.75 565

XRD spectrogram of LCT asphaltenes of the HDT product at 410 °C is presented in Figure 2. The crystalline parameters of asphaltenes of the LCT and HDT products at different temperatures are summarized in Table 5, which shows the variation tendency of La, Lc, dm, dr, M, and Ra of asphaltenes. As shown in Table 5, with the temperature increased, the following rules can be obtained: dm, La, Lc, M, Ra, and dr decreased with the increase of temperature. It shows that high temperature favors the structure changes in asphaltenes. 4708

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far lower than the corresponding value in petroleum asphaltene (7.5 Å), proving that the molecular size of asphaltenes was lower than that of petroleum asphaltene.43,46 When increasing the temperature, HDT reaction could occur in the carbon which located on the peripheral of the aromatic ring system; it results in the decrease of the number of aromatic sheets and layer diameter of asphaltenes. The results are in agreement with Trejo,44 who discovered that the stacking of asphaltenes molecules diminished when reaction conditions were more severe. 3.4. 1HNMR Analysis of Asphaltenes. The structural parameters of asphaltenes of the LCT and HDT products at different temperatures were analyzed by a liquid-state 1H NMR spectrometer. Many structural parameters, such as the asphaltenes carbon types, aromaticity factor (fA), condensation degree parameter of the aromatic ring system (HAU/CA), replacement rate of periphery hydrogen in the aromatic ring system (σ), total carbon number (CT), aromatic carbon number (CA), naphthenic carbon number (CN), saturated carbon number (CS), total ring number (RT), naphthenic ring number (RN), aromatic ring number (RA), and average alkyl chain length (n), have been calculated by the improved Brown− Ladner (B-L).45,46 The structural parameters of asphaltenes of the LCT and HDT products at different temperatures are summarized in Table 6. Figure 3 shows the 1H NMR spectrogram of asphaltenes of the product at 410 °C. It indicates that temperature has a strong influence on the structure of asphaltenes. As shown in Table 6, with the reaction temperature increased, the following rules can be obtained: fA, CA, RT, and RA increased, while CT, σ, HAU/CA, CS, and n decreased; moreover, CN and RN remained almost at the same value. In general, the molecular weight, CT, and RT of LCT asphaltenes sample are lower than those of petroleum derived asphaltenes. This presents that the molecular size of LCT asphaltenes is smaller than that of virgin petroleum asphaltenes. The molecular size of asphaltenes during HDT decreased when the reaction temperature increased. The decrease of the molecular weight (Table 4) may be ascribed to the rupture of asphaltenes molecules. This shows that asphaltenes converted to lighter molecules with smaller ring systems during HDT. Compared to the petroleum derived asphaltenes, another important feature of LCT asphaltenes is their high fA of 0.715, which demonstrates that the LCT asphaltenes contain more condensed aromatic structures. As shown in Table 6, the fA increased when temperature increased. This phenomenon indicates that more aromatic structures were remained. When temperature increases, a shortening of the alkyl side chains occurs, and the total number of carbons gradually

Figure 2. XRD spectrogram of asphaltenes from the HDT product at 410 °C.

The dm was 2.039 Å at 350 °C; however, it was 1.963 Å at the highest temperature. The dm was proportional to the dr, which decreased from 2.548 to 2.453 Å. The result shows that, with the temperature increase, the interlamellar and interchain space of the product asphaltenes decreased, which has been interpreted in previous works as indication that the asphaltenes layer-by-layer structure interation is strengthened and the layers are more condensed with higher temperature. The interchain or inter-naphthene layer distance (dr) decreased, which was probably caused by the cracking of alkyl side chains or the opening of naphthenic rings, during HDT, aliphatic carbons were removed and condensation reactions occurred, which result in the decrease of alkyl chains and the increase of aromatic structures in asphaltenes. It eventually leads to the reduction of asphaltenes molecular structures. Therefore, during precipitation, smaller molecules may stack, and the interaromatic layer distance (dm) decreased. The severe conditions in HDT favored the obtaining of lighter asphaltenes and the stacking of small molecules due to their small cores and short alkyl side chains. As the reaction temperature increased, the structural parameters of La, Lc, M, and Ra gradually decreased. The decrease in Lc was mainly attributed to the opening of aromatic rings or the decrease in the number of aromatic sheets per stack (M); the average diameter of the aromatic sheet (La) gradually decreased from 5.227 to 4.767 Å as the cracking severity increased. The obvious variations of La were mainly attributed to the loss of aliphatic chains and mainly due to the fact that the aromatic rings of asphaltenes had not been strongly assembled. In addition, the La of asphaltenes was Table 5. Crystalline Parameters of Asphaltenes by XRD

reaction temperature (°C) crystalline parameters

LCT

350

365

380

395

410

error (%)

θ (deg) B1/2 (Å) dm (Å) dr (Å) La (Å) Lc (Å) M Ra

21.520 0.091 2.099 2.624 10.110 4.945 3.356 3.791

22.190 0.176 2.039 2.548 5.227 2.557 2.254 1.960

22.34 0.181 2.034 2.539 5.172 2.523 2.236 1.927

22.680 0.187 1.997 2.496 4.920 2.406 2.205 1.845

22.83 0.189 1.976 2.482 4.833 2.365 2.189 1.803

23.100 0.193 1.963 2.453 4.767 2.332 2.188 1.787

≤5 ≤5 ≤5 ≤5 ≤5 ≤5

4709

DOI: 10.1021/acs.energyfuels.6b03180 Energy Fuels 2017, 31, 4705−4713

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Energy & Fuels Table 6. Average Structural Data of Asphaltenes Obtained by 1H NMR reaction temperature (°C) parameter symbol

parameter name

LCT

350

365

380

395

410

fA σ HAU/CA CT CA CN CS RT RN RA n

aromaticity factor replacement rate of periphery hydrogen in the aromatic ring system condensation degree parameter of the aromatic ring system total carbon number aromatic carbon number naphthenic carbon number saturated carbon number total ring number naphthenic ring number aromatic ring number average alkyl chain length

0.715 0.47 0.575 45.37 34.52 7.97 10.85 10.26 2.13 8.13 1.58

0.726 0.356 0.711 41.80 30.34 6.23 11.45 9.03 1.95 7.08 1.52

0.742 0.342 0.673 41.04 30.71 6.25 10.38 9.14 1.94 7.06 1.49

0.767 0.327 0.638 40.48 31.03 6.29 9.45 9.21 1.95 7.26 1.48

0.789 0.301 0.622 39.85 31.52 6.24 8.37 9.59 2.04 7.29 1.43

0.802 0.289 0.616 39.72 31.85 6.18 7.88 9.81 2.13 7.46 1.42

3.5. XPS Analysis of Asphaltenes. The elements of the outer 7 nm layer in asphaltenes of the LCT and HDT products at different temperatures were analyzed by XPS. Figure 4 shows

Figure 3. 1H NMR spectrogram of asphaltenes from the HDT product at 410 °C.

decreased. For this reason, there were fewer numbers of naphthenic carbons and saturated carbons in asphaltenes, and aromatic carbons remain constant, so that the H/C atomic ratio tends to decrease. H/C reduction was also attributed to the cracking of asphaltenes molecules. The substituted rate of the aromatic protons gradually decreased with the increase of temperature, which was caused by the alkyl side chains broken off of periphery aromatic rings of asphaltenes. These results are roughly consistent with the XRD analysis. Compared to LCT, the RA and CA of HDT asphaltenes decreased, but at high temperature, the RA and CA increased. It shows that, as the temperature increased, the polycondensation reaction increased. As the HDT condition become severer, asphaltenes convert into large aromatic units, of which the structure is highly condensed, and even a rearrangement of cata-condensed aromatic rings to pericondensed-like structures occur. The aspheltenes value of n in LCT is 1.58 carbons, whereas, after HDT at 410 °C, it was 1.42 carbons. This reduction demonstrates that the average length of alkyl chains decreased; the interaromatic layer distance decreased also. These results can be reflected in XRD. More condensed asphaltenic structures were obtained because of the dealkylation. When the temperature increased, the alkyl chains which are the most labile points broke and C−C bonds ruptured to form free radicals.

Figure 4. XPS survey spectrum of asphaltenes from the HDT product at 410 °C.

the XPS survey spectrum of asphaltenes of the product at 410 °C. The surface of asphaltenes is composed of carbon atoms and a small amount of O, N, and S atoms. The atomic surface elemental concentrations (atomic percent: at. %) for asphaltenes in the LCT and HDT products at different temperatures are calculated in Table 7. As shown in Table 7, when the reaction temperature increased, the carbon content in asphaltenes increased steadily, while the content of sulfur, nitrogen, and oxygen decreased. These results are roughly compatible with the elemental analysis in Table 4. The deconvolution of the C 1s peak envelope into different functionalities of asphaltenes in the LCT and HDT product at 410 °C are shown in Figures 5 and 6, respectively. Five peaks were used to curve-resolve the C 1s signal of asphaltenes with binding energy (BE) corresponding to the appropriate peaks at 283.2, 284.3, 285.4, 286.3, and 289.1 eV, respectively.47,48 The relative content of the C-containing components is summarized in Table 7. In consideration of the unavoidable differential charging effects of the XPS instrument, an extra “ghost” component should be added to the genuine C 1s peak. The 283.2 eV peak corresponds to differential charging (dc peaks). The 284.3 eV 4710

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Energy & Fuels Table 7. Atomic Percentage Determined by XPS of Asphaltenes reaction temperature (°C) properties

LCT

350

365

380

395

410

error (%)

carbon (at. %) oxygen (at. %) nitrogen (at. %) sulfur (at. %)

83.91 13.32 1.72 1.05

86.61 11.55 1.31 0.53

87.03 11.19 1.29 0.49

87.55 10.89 1.18 0.38

88.07 10.56 1.03 0.34

88.36 10.31 1.01 0.32

≤5 ≤5 ≤5 ≤5

atoms of sp3 hybridization. The 286.3 eV peak represents carbon bound to one oxygen by a single bond (e.g., C−O, C− OH, etc.). The 289.1 eV peak corresponds mainly to carbon bound to oxygen by three bonds (COO−). It is assumed that a π aromatic character, which correlates with carbon atoms of sp2 hybridization, confers by condensed, polynuclear, aromatic-ring systems in the LCT-derived asphaltenes, is available, as inferred from CC π-conjugation in highly oriented pyrolytic graphite (HOPG). In contrast, the asphaltenes aliphatic components associates with the availability of carbon atoms of sp3 hybridization, as approximated from the C−C bond in diamond.49,50 Hence, the larger the sp2/(sp2 + sp3) percentage is, the more aromatic asphaltenes will be obtained. The sp2/(sp2 + sp3) percentage of asphaltenes, which is extracted from Doba (southeastern Chad) heavy crude, is 41%.51 Asphaltenes from LCT exhibit a higher sp2 percentage of 45.7% (Table 8), which also indicates that the asphaltenes of LCT contain more condensed aromatic structures. As shown in Table 8, the carbon atoms of sp2 hybridization of asphaltenes increased from 43 at. % at 350 °C to 48 at. % at 410 °C. At the same time, carbon atoms of sp3 hybridization decreased from 36 to 31 at. %. Furthermore, the sp2/(sp2 + sp3) percentage of asphaltenes increased from 54.4% to 60.8%. The data show that the aromaticity of asphaltenes increased when the temperature increased; this result is roughly consistent with the XRD and 1 H NMR analysis.

Figure 5. XPS C 1s spectrum and fitted curves of asphaltenes from LCT.

4. CONCLUSIONS By analyzing the experimental results under various reaction temperatures, the following regularities can be obtained: When the reaction temperature increases: (1) Sulfur contents and molecular weight of asphaltenes decrease, and nitrogen content decreases at first, but it does not change significantly later. (2) The alkyl chains and the structural parameters of asphaltenes, such as La, Lc, dm, and dr, decrease, and the piling of aromatic layers is destructed so the molecular weight of asphaltenes decreases. (3) The fA, CA, RT, and RA increase, while σ, HAU/CA, and n decrease, and the values of CN and RN remain almost constant. The increase of fA indicates that more aromatic structures are obtained and aliphatic carbons in alkyl chains are broken; the decrease of σ and HAU/ CA means that a larger condensation degree of the aromatic

Figure 6. XPS C 1s spectrum and fitted curves of asphaltenes from the HDT product at 410 °C.

peak represents contributions from carbon atoms of sp2 hybridization. The 285.4 eV peak corresponds to carbon

Table 8. Carbonous Forms of C7-Asphaltene Analyzed by XPS C ls Peak reaction temperature (°C) carbonous form

BE/eV

LCT

350

365

380

395

410

error (%)

dc (at. %) sp2 (at. %) sp3 (at. %) C−O (at. %) COO− (at. %) sp2/(sp2 + sp3) (%)

283.2 284.3 285.4 286.3 289.1

9 37 44 7 3 45.7

10 43 36 8 3 54.4

11 44 34 6 5 56.41

9 46 34 7 4 57.5

11 47 32 8 2 59.49

10 48 31 8 3 60.8

≤5 ≤5 ≤5 ≤5 ≤5 ≤5

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Energy & Fuels rings was obtained. (4) The aromaticity and the sp2 carbons of asphaltenes increase, while the sp3 carbons decrease; this result is roughly the same with the XRD and 1H NMR analysis. It can be observed that the structural parameters change significantly and the changes disappear with the increase of the cracking intensity. By using XRD and 1H NMR measurements to support XPS results further, more quantitative analysis can be obtained to study the influence of reaction conditions on the HDT process.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-18681859699 (mobile), +86-029-88373425 (home). Fax: +86-029-88305825. E-mail: [email protected]. ORCID

Dong Li: 0000-0002-4578-0595 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of this work is provided by the National Natural Science Foundation of China (21646009), the National Natural Science Foundation of China (21206136), The Doctoral New Teacher Fund Project of the Ministry of Education of China (20126101120013), the Shaanxi Province Science and Technology Co-ordination Innovation Project Planned Program (2014KTCL01-09), the Shaanxi Province Department of Education Industrialization Training Project (14JF026; 15JF031), and the Young Science and Technology Star Project of Shaanxi Province (2016KJXX-32).

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