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Reactivity and Structure Changes of Coal Tar Asphaltene during Slurry-phase Hydrocracking Juntao Du, Wenan Deng, Chuan Li, Zailong Zhang, Tengfei Yang, and Ruilong Guo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02992 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 20, 2017
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Keywords: coal tar asphaltene; slurry-phase hydrocracking; structure; XPS; NMR
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1. Introduction Coal tar asphaltene (CT-asp) accounted for about 30% of atmospheric coal tar residue. Compared to traditional petroleum asphaltene, it presented significant differences in molecular structure and hydrocracking reactivity.1,2 Moreover, CT-asp had caused serious problems during the fixed bed hydrocracking process of coal tar, such as reactor fouling, catalyst deactivation and low conversion,3,4 all of which were closely related to its molecular structure and reactivity.1,5 In addition, the coal tar atmospheric residue had an inferior quality in comparison to traditional fossil fuels.6, 7 Nevertheless, slurry-phase hydrocracking is considered as an effective method8,9 for the transformation of coal tar atmospheric residue into valuable products,10 which is considered particularly promising.11–13 However, the structural changes and reactivity of CT-asp in slurry-phase hydrocracking have not been well expounded in the reported literature.14,15 Thus, it is necessary to explore the information on the reactivity of CT-asp for the explanation of molecular structure changes during the hydrocracking process. Some analytical techniques commonly applied to oil and coal, such as Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), 1H / nuclear magnetic resonance (1H-NMR and
13
13
C
C-NMR), have been utilized in the study of structure
and reactivity of asphaltene. Functional groups and structure composition of asphaltene were semi-quantitatively determined by FTIR and XPS spectroscopy. FTIR spectroscopy was used to distinguish the specific functional groups present and to reveal information about the molecular structure of asphaltene,14,16 such as the presence of saturated and aromatic hydrocarbons.17,18 The functional groups on the asphaltene surface were characterized by XPS, a surface-sensitive technique in which most of the signal originates from the sample surface.19 The surface composition of asphaltene was quantified using XPS. Generally speaking, surface concentration of each element was slightly lower than the ACS Paragon Plus Environment
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corresponding value in the bulk asphaltene. In addition, C - C and C - H groups were the primary components of asphaltene, XPS did not allow distinguishing between aromatic and non-aromatic carbon.20,21 Information concerning the microstructure and crystallite parameters of asphaltene was provided by XRD analysis. The aliphatic layers (aliphatic chains or saturated rings) corresponded to the γ band that appeared around 2θ=20°, while the aromatic layers corresponded to the graphene band (or 002 band), which appeared around 2θ=25°.22,23 The structural parameters for asphaltene were determined from the deconvoluted XRD patterns, using the methods in reported literature.23,24 It could be noticed that the 002 peak intensity of CT-asp was higher than that of the γ peak. This result is very different from the one obtained for petroleum asphaltene and indicates that the aromaticity of CT-asp should be higher than the one of petroleum asphaltene.24 The average structural parameters and carbon type of asphaltene have been studied by 1H and 13
C-NMR.25 The 1H-NMR and
13
C-NMR spectra provide qualitative information regarding the
different types of hydrogen and carbon in coal and petroleum, respectively.26 The structural parameters of asphaltene, such as the content of aromatic and naphthenic rings and the aromatic degree, were calculated by the improved Brown-Ladner (B-L) method on the basis of the 1H-NMR data. The type and content of aromatic and aliphatic carbon, such as methyl, bridgehead or alkyl-substituted aromatic carbon, were determined by 13C-NMR. Moreover, the type and content of oxy-carbons, such as methoxyl, ether, phenoxyl, carboxyl, carbonyl and aldehyde, was evaluated. The combination of 13C-NMR and XPS was used to determine the main functional groups present in asphaltene, including C-C, C=O and C-O.27 The combined XRD-NMR procedure allowed an estimate of the size of the average aromatic sheet structure, the number of sheets in each stack unit and the amount of aromatic rings per sheet.24 It should be mentioned that, for all asphaltene samples, the aromaticity factor (fa) evaluated ACS Paragon Plus Environment
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through XRD analysis was smaller than the fa obtained from either 1H-NMR or
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13
C-NMR. The
calculation of fa in XRD was based upon the stack cluster of aromatic carbon causing the graphene peak (002 peak), and was not based on all the aromatic carbon (eg. 1H-NMR and 13C-NMR) in the asphaltenes. Therefore the fa from XRD was not representative of the true aromaticity of the asphaltenes.23 Compared to the fa evaluated from 1
13
C-NMR, the slightly higher fa resulting from
H-NMR should be due to the presence of a certain amount of oxygen (such as carboxyl and
carbonyl) in asphaltenes.28 The purpose of this paper is to explore the hydrogenation reactivity of CT-asp and to determine the structural changes occurring during the slurry-phase hydrocracking. This was done by means of FTIR, XPS, NMR and XRD analyses. The finding of this study could contribute to advancements in the understanding of the coal tar hydrocracking mechanism, as well as in the application and development of atmospheric coal tar residue in the hydrocracking process. 2. Experimental 2.1. Precipitation of Coal Tar Asphaltene Coal tar asphaltene (CT-asp) was extracted from low-temperature coal tar in the Shanxi Province, China, using n-heptane solvent extraction. The ratio of solvent / coal tar adopted was 500 ml of solvent to 100 g of coal tar, and the extraction was carried out at 60°C for 2 h. The solution was then allowed to stand for 24 h at room temperature.29 CT-asp was collected as the n-heptane insoluble fraction, which was then dissolved in toluene at 60°C for 4 h, and filtered through suction filtration. After that, the toluene was removed from the toluene-soluble fraction by distillation. The residue as CT-asp, was dried in vacuum at 100°C for 2 h. 2.2. Hydrocracking experiments The CT-asp hydrocracking experiments were performed in a 30-mL batch reactor heated by a tin bath. The experiments were conducted at 430°C, an H2 pressure of 9 MPa (initial pressure, reaction ACS Paragon Plus Environment
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pressure 16 MPa) for reaction times (20, 40, 50, 60, and 80 min). A mixture consisting of molybdenum naphthenate and nickel naphthenate with a specific metal mass ratio of Mo to Ni, was used as the catalyst during the slurry-bed hydrocracking process. The autoclave was loaded with 5 g of CT-asp, 8 mL of toluene as solvent, 500 µg·g-1 of catalyst (expressed in terms of metal mass), and a certain amount of sulfur. It was then subjected to ultrasonic oscillation for 10 minutes. After reaction, the reactor was rapidly cooled using cold water. Each hydrocracking experiment produced four products: gas, liquid product, secondary CT-asp and coke. The mass of the gas product was determined by weighing the reactor before and after venting the off-gases, and its composition was determined by gas chromatography. The toluene-insoluble product, separated as the coke, was washed out of the residue using toluene. Then removing toluene from toluene-soluble fraction by distillation, a residue containing liquid product and secondary CT-asp was obtained. After that, the residue was mixed with n-heptane to separate the secondary CT-asp (heptane-insoluble fraction) and the heptane-soluble product after removing heptane by distillation was collected as the liquid product. The latter three products were all dried in vacuum at 110°C for 2 h. The liquid CT-asp product at 60 min was analyzed by GC-MS. 2.3. Characterization of CT-asp Carbon, hydrogen, nitrogen and sulfur contents in CT-asp were determined through the combustion method in a Varil EL-Ⅲ analyzer. The molecular weight was measured by vapor pressure osmometry (VPO) with a Knauwer molecular weight apparatus. A Bruker Equinox-55 FTIR spectrophotometer using a KBr pellet with a scanning range from 400 cm−1 to 4000 cm−1 was used. The X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos Axis Ultra DLD, equipped with an Al Kα source at 20 eV pass energies and calibrated with the main C1s peak at 284.8 eV. A D/MAX-ⅢA X-ray diffractometer was utilized using a Cu target as the X-ray source. The 1H-NMR and
13
C-NMR analyses were conducted with a Bruker Avance DMX500-type ACS Paragon Plus Environment
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spectrometer operating at 400 MHz and 75 MHz, respectively. 1H-NMR and 13C-NMR spectra were obtained as CDCl3 solution with a flip angle of 45°, tube diameter of 5 mm and a spectral width of 220 ppm. In addition, tetramethylsilane (TMS) was employed as the internal reference. The Liquid product of CT-asp at 60 min was analyzed on an Agilent 7890A gas chromatograph and 5977C mass detector with an HP-5MS column using toluene as solvent. The mass detector was scanned from 30 to 500 Da. 3. Results and discussion 3.1. Hydrocracking of CT-asp Figure 1 shows the product yields and conversion of CT-asp after hydrocracking at 430°C with the reaction time from 0 min to 80 min. As the reaction time increased, the gas yield increased to 8.3 wt %, and the liquid yield increased significantly to 46.3 wt %, while the coke yield quickly increased to 9.1 wt % within the first 20 minutes then slowly increased to 11.9 wt %. The gas products were mainly comprised of methane and ethane, whose concentrations were determined by gas chromatography. This result could be explained by the dealkylation of CT-asp. Meanwhile, the conversion of CT-asp increased to 66.5 wt % after 80 min. In short, the hydrocracking of CT-asp showed a slightly lower gas yield, moderate coke yield and decent liquid yield compared with petroleum asphaltene previously reported.30
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Liquld Gas Coke Asphaltene conversion
80
Yield / wt %
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0 0
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Figure 1. Conversion of CT-asp at different reaction times in a slurry-phase reactor The elemental composition of the CT-asp as a function of reaction time is listed in Table 1. The H/C ratio decreased more within the first 20 minutes then the reduction got slower in general except at 80 min where an increase was observed without explanation. The result implied an increase in the aromatic degree of CT-asp as the reaction time increased.1 Sulfur content in CT-asp decreased from 0.36 wt % to 0.12 wt %, oxygen content slowly decreased from 13.14 wt % to 9.18 wt % except at 80 min, and nitrogen content initially decreased from 1.68 wt % to 1.50 wt % and then increased up to 1.71 wt % as the severity of cracking increased. These results implied that the effective removal of heteroatoms in CT-asp was not easy, particularly nitrogen and oxygen. Table 1. Elemental composition of CT-asp under different reaction times Elemental composition C / wt% H / wt% N / wt% S / wt% O / wt% H / C (Atomic ratio)
0 78.38 6.17 1.68 0.36 13.41 0.945
Reaction times / min 20 40 60 81.86 82.54 83.37 5.97 5.76 5.68 1.54 1.50 1.62 0.27 0.16 0.15 10.36 10.04 9.18 0.876 0.838 0.818
80 82.96 5.89 1.71 0.12 9.32 0.853
Figure 2 shows the number average molecular weight (MW) of CT-asp under different reaction times. The MW increased from 515.4 to 764.5, which indicated the formation of molecules having a ACS Paragon Plus Environment
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larger sized after hydrocracking. This suggested that there was poly-condensation reaction during the hydrocracking of CT-asp. In addition, the MW of CT-asp was far lower than that of petroleum asphaltene.31
800
Molecular weight (VPO)
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700
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Reaction time / min
Figure 2. Molecular weight (MW) of CT-asp under different reaction times Summarizing the above results, it could be assumed that the possible reactions of CT-asp mainly consisted in the dealkylation, poly-condensation and removal of heteroatomic functional groups (O and S). The process was attributed to the free radical thermal cracking and hydrogenation. However, to further illustrate the evolution of CT-asp, particularly the reactivity and structure changes, a detailed investigation of the chemical structure of CT-asp as a function of reaction time was needed. 3.2. Functional groups analyses of CT-asp The characteristic functional groups and structure compositions of CT-asp were explored by FTIR and XPS. The FTIR spectra of selected CT-asp (0, 40, 60 and 80 min) are shown in Figure 3. The change of peaks in correspondence of the 3000–2800 cm−1 and 1370 cm−1 wavenumbers clearly indicated that the saturated hydrocarbons in CT-asp gradually reduced. The change of peaks near 3050 cm−1, 1450–1610 cm−1 and 700–900 cm−1, representing aromatic hydrocarbons, could demonstrate that condensed aromatic ring structures appear in CT-asp after hydrocracking. The variation of peaks near 3380 cm−1 indicates that the majority of the OH functional groups were ACS Paragon Plus Environment
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gradually broken off the aromatic rings. Finally, the variation of the pronounced peak in the region of 1272 cm−1 and 1227 cm−1, could confirm that C-O, C-O-C and phenolic groups were still present in CT-asp after hydrocracking, these results were consistent with the above analysis of elemental
75 0
13 70 12 27
16 05
30 50 29 20
composition.14,16 100 80
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Transmittance / %
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60 40 100 80
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0min 60 40 3500
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Figure 3. FTIR spectra of the CT-asp under different reaction times In order to further study the structure of CT-asp, the types and contents of functional groups containing O, N and S had been determined by XPS.32 Figure 4 presents the deconvolution of the XPS spectrum relative to the O, N and S functional groups. Calculated parameters for the selected CT-asp from XPS are listed in Table 2. As shown, oxygen was mostly presented as C-O, representing ether and hydroxyl groups and constituting the largest portion of heteroatomic groups. Their content was slightly reduced as the reaction time increased. These results are consistent with FTIR analysis. The oxygen containing groups C=O and COO- clearly decreased after hydrocracking because of their relatively high hydrogenation activity. Moreover, the content of pyridinic-N increased significantly, while the content of pyrrolic-N and quaternary-N clearly decreased. Results showed that, as the severity of cracking increased, pyrrolic-N and quaternary-N could be removed through hydrogenation, except pyridinic-N.33 In addition, the content of alkyl sulfides and sulfur oxides ACS Paragon Plus Environment
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(sulfoxides, sulfonic and sulfones) obviously decreased, while thiophene content showed scarcely any change. This indicated that most of the sulfur was removed via effective hydrogenation reaction, while thiophenes were not affected because of their steric hindrance.34, 35 As discussed above, the aromaticity of CT-asp increased with the severity of cracking. XPS analysis revealed that, among the heteroatom groups, cracking occurred in the carbonyl group, pyrrolic nitrogen, and alkyl sulfides but not in the ether group and pyridinic nitrogen. 0 min O 1s
0 min N 1s
0 min S 2p
Alkyl sulphides
Pyrrolic-N Thiophenes C-O
Quaternary-N
Sulphoxides
Pyridinic-N
Sulphones
O-C=O C=O
Sulphonic 536
534
532
530
402
400
398
171
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Binding energy (eV)
Figure 4. Deconvolution XPS patterns of O 1s, N 1s and S 2p from 0 min CT-asp Table 2. XPS curve-resolution results for O, N, and S per 100 carbons (atomic ratio %) Binding energy (eV) Oxygen type C=O 531.8 and content C-O-C,C-OH,C-O 532.9 COO534.1 total Nitrogen type pyridinic-N 398.8 and content pyrrolic-N 399.9 quaternary-N 401.1 total Sulfur type alkyl sulfides 163.4 and content thiophenes 164.8 sulfoxides 165.6 sulfones 168.1 sulfonic 170.0 total Atom type
Reaction time / min 0 60 80 2.10 1.41 1.15 4.72 4.12 4.03 1.65 0.87 0.96 8.47 6.40 6.13 0.33 0.48 0.82 1.20 0.91 0.58 0.76 0.77 0.37 2.29 2.16 1.77 0.13 0.01 0.02 0.09 0.10 0.08 0.04 0.02 0 0.08 0.02 0.01 0.02 0 0.01 0.37 0.15 0.12
3.3. Structure changes of CT-asp Information concerning the microstructure parameters and molecular structures of CT-asp were ACS Paragon Plus Environment
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provided by XRD and NMR analyses. The parameters calculated from the XRD pattern of CT-asp are shown in Figure 5 and were determined through the methods in reported literature.23,24 As seen in Figure 5(a), the average inter-aromatic layer distance (dm) was slightly affected by the increase in cracking severity. The Lc initially decreased from 3.6 Å to 3.0 Å and then increased from 3.0 Å to 4.0 Å. 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); on the contrary, the increase of Lc was mainly due to the number of aromatic sheets per stack (M).24 As seen in Figure 5(b), the inter-chain or inter-naphthene layer distance (dγ) decreased from 4.5 Å to 4.2 Å, which was probably caused by the cracking of alkyl side chains or the opening of naphthenic rings. The average diameter of the aromatic sheet (La) gradually increased from 3.1 Å to 4.4 Å as the cracking severity increased. This result was associated with the increase of NOar (the number of aromatic rings per aromatic sheet). The obvious variations of La and NOar were mainly attributed to the loss of aliphatic chains, and were mainly due to the fact that the aromatic rings of CT-asp had not been strongly assembled. In addition, the La of CT-asp was far lower than the corresponding value in petroleum asphaltene (7.5 Å), proving that the molecular size of CT-asp was lower than petroleum asphaltene.23,24 6.0
3.3
Lc, Ǻ M
3.9
5.5
La, Ǻ
5.0
NOar
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dr & La Ǻ
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fa
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dm, Ǻ
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dm & Lc Ǻ
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Figure 5. Crystalline parameters derived from XRD data for selected CT-asp (dm, inter-aromatic layer distance. dγ, inter-chain or inter-naphthene layer distance. La, average diameter of the aromatic sheet. Lc average height of the stack aromatic sheets. M, number of aromatic sheets ACS Paragon Plus Environment
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per stacked cluster. NOar, number of aromatic rings per aromatic sheet.) Additional information on the structure could be obtained through the structural parameters of CT-asp which could be calculated by the improved Brown-Ladner (B-L) method through 1H-NMR data.1 As seen in Figure 6(a), RA sharply increased from 5.5 to 9.7, but RN slightly increased from 1.4 to 2.0. This observed change suggested that the poly-condensation and hydrogenation did not occur strongly, at the same time the opening of naphthene rings and the breaking of alkyl side chains proceeded. As seen in Figure 6(b), CA and Cl obviously increased with the increase of reaction time, but CP was reduced clearly. As seen in Figure 6(c), the aromaticity factor (fA) slightly increased from 0.71 to 0.78, and HA/CA as well as σ were gradually reduced. It should be mentioned that either the alkyl side chains of CT-asp were slightly cracked or the aromatic rings grew closer together as the cracking severity increased.2 50
RT
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0.7 0.6 0.5 0.4 0.3 0
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Reaction time (min)
Figure 6. Structural parameters of CT-asp from 1H-NMR (RT, total rings per average molecule. RA, aromatic rings per average molecule. RN, naphthenic rings per average molecule. CA, aromatic carbons per average molecule. Cl, peripheral carbons of the aromatic system. CN, naphthenic carbons ACS Paragon Plus Environment
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per average molecule. CP, alkyl carbons per average molecule. fA, aromatic degree. HA/CA, aromatic rings condensation degree. σ, aromatic rings substitution degree.) The carbon types of CT-asp were provided by 13C-NMR spectra and the results are shown in Table 3. The low content of methyl and methylene clearly proved the presence of shorter methyl side chains as well as a lower number of naphthenic rings in CT-asp. The content of oxy-carbons was also determined by
13
C-NMR, which revealed the presence of oxy-aromatic carbon, methoxyl, ether,
phenoxyl, carboxyl, and ketone. In addition, FTIR and XPS analyses showed that most oxy-aromatic carbon bonds present oxo-bridged oxygen bonds such as ether or ester, whereas only a small portion was represented by phenolic hydroxyl. The measured content of aromatic and aliphatic carbon was in good agreement with 1H-NMR and XRD results. In addition, the fa determined through NMR was higher than the corresponding value determined through XRD as the latter method evaluated the aromaticity factor only on the basis of the stack cluster aromatic carbon, causing the graphene peak, not based on all the aromatic carbon in asphaltenes. Thus, the fa resulting from NMR is more representative of the true aromaticity of CT-asp.23 Briefly speaking, the CT-asp possesses a structure in which a number of aromatic rings were condensed with each other through diphenyl-like structures or oxo-bridged linkages. An average of two aromatic rings per aromatic sheet was present in CT-asp. Furthermore, it could be concluded that the main reaction occurring during the slurry-phase hydrocracking process of CT-asp was the cracking of alkyl side chains, and secondary reactions were the ring-opening and polycondensation of aromatic rings. Table 3. Carbon distribution of CT-asp selected (per 100 carbons) from 13C-NMR Chemical shift (ppm) 0–25 25–50 50–70
Type of carbon Methyl Methylene Methoxy, ether, alcohol ACS Paragon Plus Environment
Reaction time / min 0 40 60 80 12.9 11.0 13.6 12.3 13.1 9.8 12.5 11.2 3.7 2.9 3.0 3.9
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90–135 135–148 148–171 171–220 0–90 90–220
Aromatic carbon bound to hydrogen Bridgehead or alkyl-substituted aromatic carbon Oxy-aromatic carbon Carboxyl, ester, carbonyl, ketone Aliphatic carbon (Cal) Aromatic carbon (Car) Aromaticity fa= Car/ (Cal + Car)
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47.3 9.6 7.2 6.1 29.7 70.3 0.70
54.6 9.4 7.1 5.2 23.7 76.3 0.76
54.5 7.5 4.1 4.7 29.1 70.9 0.71
54.4 7.3 5.0 5.8 27.4 72.6 0.73
3.4. Reaction pathway of CT-asp Based on the main compounds in products and the changes of molecular structures, the reaction pathway of CT-asp in slurry-phase hydrocracking was proposed. The dominant reaction pathway of CT-asp was confirmed by the analysis of the major products. Gas products of CT-asp were mainly comprised of methane and ethane, which were determined by gas chromatography. Liquid products of CT-asp hydrocracking carried out for 60 min have been measured by GC-MS, as shown in Figure 7. The liquid products were very complex and could be divided into three main group types, as labeled in Figure 7.
Figure 7. GC-MS of liquid products of CT-asp hydrocracking under reaction time 60 min. The slurry bed hydrocracking of CT-asp followed the free radical reaction mechanism.36,37 On the basis of thermodynamic principles, it was reasonable to say that the free radicals of methane and ethane were derived from the fracture of alkyl side chains and naphthene rings, because the energy of the C - C bond was considerably lower than that of C = C. The free radicals were saturated by the activated hydrogen.38 Simultaneously, because of their relatively weak steric hindrance and electronic structure, the peripheral carbons of the aromatic rings with side chains were subject to ring ACS Paragon Plus Environment
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opening, dealkylation and isomerization, or were saturated by hydrogenation to generate aliphatic rings, as shown in Figure 7 (type A). The rings containing free radicals, such as polycyclic aromatic rings and phenyl group, were directly saturated by hydrogen to produce liquid products. Meanwhile, a rearrangement of methyl, carboxyl and carbonyl groups occurred, as shown in Figure 7 (type B and type C). Furthermore, rings containing free radicals were assembled through low linear polymerization, forming new stable aromatic compounds, such as secondary asphaltene and coke. The activated hydrogen acted as terminator in the polymerization-like association reactions.39, 40 The main molecular structural parameters (per average molecule) of selected CT-asp are shown in Table 4. The experimental average molecular formulae of selected CT-asp, under reaction time 0 min and 60 min, were C33.64H31.55N0.62O4.32S0.06 and C48.52H39.38N0.81O4.01S0.03, respectively, as evaluated through elemental composition and 1H-NMR. Moreover, based on the above analysis, it was possible to propose hypothetical average model structures for CT-asp, as shown in Figure 8. Sulfur was not considered in the hypothetical average model structures because of its negligible content.41 The hypothetical molecular formulae for selected CT-asp at 0 min and 60 min was shown in Figure 8 and was in good agreement with the experimental average molecular formulae. Table 4. Model chemical structural parameters (per average molecular) for CT-asp Reaction time / min 0 60 1 H NMR fa 0.71 0.77 CT 33.64 48.52 RT 6.89 10.97 RA 5.485 8.93 XRD M 1.90 2.06 NOar 1.18 1.46 13 C NMR Methyl 4.34 6.60 Methoxy 1.25 1.47 Aromatic atoms bound to hydrogen 15.90 26.47 Bridgehead and alkyl-substituted aromatic 3.24 3.64 Oxy-aromatic carbon 2.42 1.98 Carboxyl and carbonyl 2.08 2.29 XPS Oxygen 2.85 3.11 Parameters
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(Surface)
Nitrogen Sulfur
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0.77 0.12
1.05 0.07
Figure 8. Schematic representation of CT-asp slurry bed hydrocracking reaction pathway In Figure 8, a dominant free radical reaction pathway for CT-asp hydrocracking was proposed, based on the major products and the molecular structural changes involved. The proposed pathway involved primary and secondary reactions. The primary reactions were β secession of alkyl side chains and ring-opening of aromatic ring with side chains, which were dominated by thermal cracking.42 The secondary reactions were aggregation, linear polymerization and coke formation. The aggregation of some macromolecules became incompatible with the liquid phase, eventually giving rise to coke formation, which was most likely formed by random repolymerization and rearrangements of the ring radicals.43,44 In addition, with regards to the O, N and S heteroatoms in asphaltene, the breaking of C-O bonds was not easy to occur compared to that of C-C bonds, because of the conjugate of lone pair electrons and benzene rings. The removal of nitrogen present as pyrrole nitrogen was also difficult, unless the heterocyclic was saturated. Sulfur present in the form of alkyl sulfides and sulfur oxides could be easily removed, contrarily to thiophene.45 4. Conclusions The reactivity and structural changes of CT-asp were studied during the slurry-phase ACS Paragon Plus Environment
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hydrocracking. The conversion of CT-asp after hydrocracking gradually increased as the reaction times from 0 to 80 min, reaching up to 66.5 wt % with the coke yield of 11.9 wt %. The removal of sulfur and oxygen in CT-asp was relatively efficient, with the exception of thiophenes and oxy-aromatic groups. As the reaction time increased, it was found that the aromaticity of CT-asp slightly increased, along with the size of aromatic sheets and the number of sheets per stacked cluster. A structure of average two aromatic rings per aromatic sheet was present in the average molecule of CT-asp, and the aromatic sheets were condensed with each other through diphenyl-like structures or oxo-bridged linkages during the hydrocracking. In addition, the descriptive reaction pathway of CT-asp hydrocracking in a slurry-phase process was proposed based on the major products and the structural changes. Acknowledgement This work was supported by National Natural Science Foundation Young Investigator Grant Program of China (No. 21106186), the Fundamental Research Funds for the Central Universities (No. 14CX05032A) and the Graduate Student Innovation Project Funding for the Central Universities (YCX2015020). References (1) Ancheyta, J.; Centeno, G.; Trejo, F.; Marroquín, G. Changes in asphaltene properties during hydrotreating of heavy crudes. Energy & Fuels 2003, 17(5), 1233-1238. (2) Han, L.; Zhang, R.; Bi, J.; Cheng, L. Pyrolysis of coal-tar asphaltene in supercritical water.
Journal of Analytical and Applied Pyrolysis 2011, 91(2), 281-287. (3) Kazemzadeh, Y.; Malayeri, M. R.; Riazi, M.; Parsaei, R. Impact of Fe3O4 nanoparticles on asphaltene precipitation during CO2 injection. Journal of Natural Gas Science and Engineering 2015, 22, 227-234. (4) Bartholdy, J.; Andersen, S. I. Changes in asphaltene stability during hydrotreating. Energy & ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Fuels 2000, 14(1), 52-55. (5) Asaoka, S.; Nakata, S.; Shiroto, Y.; Takeuchi, C. Asphaltene cracking in catalytic hydrotreating of heavy oils. 2. Study of changes in asphaltene structure during catalytic hydroprocessing.
Industrial & Engineering Chemistry Process Design and Development 1983, 22(2), 242 - 248. (6) Han, J.; Wang, X.; Yue, J.; Gao, S.; Xu, G. Catalytic upgrading of coal pyrolysis tar over char-based catalysts. Fuel Processing Technology 2014, 122, 98-106. (7) Chang, N.; Gu, Z. Kinetic model of low temperature coal tar hydrocracking in supercritical gasoline for reducing coke production. Korean Journal of Chemical Engineering 2014, 31(5), 780-784. (8) Han, J.; Liu, X.; Yue, J.; Xi, B.; Gao, S.; Xu, G. Catalytic upgrading of in situ coal pyrolysis tar over Ni-char catalyst with different additives. Energy & Fuels 2014, 28(8), 4934-4941. (9) Kusy, J.; Andel, L.; Safarova, M.; Vales, J.; Ciahotny, K. Hydrogenation process of the tar obtained from the pyrolisis of brown coal. Fuel 2012, 101, 38-44. (10) Gu, Z.; Chang, N.; Hou, X.; Wang, J.; Liu, Z. Experimental study on the coal tar hydrocracking process in supercritical solvents. Fuel 2012, 91(1), 33-39. (11) Ortiz-Moreno, H.; Ramírez, J.; Sanchez-Minero, F.; Cuevas, R.; Ancheyta, J. Hydrocracking of Maya crude oil in a slurry-phase batch reactor. II. Effect of catalyst load. Fuel 2014, 130, 263-272. (12) Martinez-Grimaldo, H.; Ortiz-Moreno, H.; Sanchez-Minero, F.; Ramírez, J.; Cuevas-Garcia, R.; Ancheyta-Juarez, J. Hydrocracking of Maya crude oil in a slurry-phase reactor. I. Effect of reaction temperature. Catalysis Today 2014, 220-222, 295-300. (13) Sánchez, S.; Ancheyta, J. Effect of pressure on the kinetics of moderate hydrocracking of Maya crude oil. Energy & Fuels 2007, 21(2), 653-661. (14) Sun, M.; Ma, X.; Cao, W.; Du, P.; Yang, Y.; Xu, L. Effect of polymerization with paraformaldehyde on thermal reactivity of >300°C fraction from low temperature coal tar. ACS Paragon Plus Environment
Page 18 of 23
Page 19 of 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Thermochimica Acta 2012, 538, 48-54. (15) Dai, F.; Gao, M.; Li, C.; Xiang, S.; Zhang, S. Detailed description of coal tar hydrogenation process using the kinetic lumping approach. Energy & Fuels 2011, 25(11), 4878-4885. (16) Wang, Z.; Ge, Y.; Shui, H.; Ren, S.; Pan, C.; Kang, S.; Lei, Z.; Zhao, Z.; Hu, J. Molecular structure and size of asphaltene and preasphaltene from direct coal liquefaction. Fuel Processing
Technology 2015, 137, 305-311. (17) Wang, P.; Jin, L.; Liu, J.; Zhu, S.; Hu, H. Analysis of coal tar derived from pyrolysis at different atmospheres. Fuel 2013, 104, 14-21. (18) Gauthier, T.; Danialfortain, P.; Merdrignac, I.; Guibard, I.; Quoineaud, A. Studies on the evolution of asphaltene structure during hydroconversion of petroleum residues. Catalysis Today 2008, 130(2-4), 429-438. (19) Siskin, M.; Kelemen, S. R.; Eppig, C. P.; Brown, L. D.; Afeworki, M. Asphaltene molecular structure and chemical influences on the morphology of coke produced in delayed coking. Energy &
Fuels 2006, 20(3), 1227-1234. (20) Xia, W.; Yang, J. Reverse flotation of Taixi oxidized coal. Energy & Fuels 2013, 27(12), 7324-7329. (21) Kelemen, S. R.; Afeworki, M.; Gorbaty, M. L.; Cohen, A. D. Characterization of organically bound oxygen forms in lignites, peats, and pyrolyzed peats by X-ray photoelectron spectroscopy (XPS) and solid-state. Energy & Fuels 2002, 16(6), 1450-1462. (22) Sadeghi, M.; Chilingarian, G. V.; Yen, T. F. X-ray diffraction of asphaltenes. Energy Sources 1986, 8(2-3), 99-123. (23) AlHumaidan, F. S.; Hauser, A.; Rana, M. S.; Lababidi, H. M. S.; Behbehani, M. Changes in asphaltene structure during thermal cracking of residual oils: XRD study. Fuel 2015, 150, 558-564. (24) Jayaraj, C.; Sarpal, A. S.; Kapur, G. S.; Krishna, A.; Tyagi, B. R.; Jain, M. C.; Jain, S. K.; ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Bhatnagar, A. K. Chemical structure of bitumen derived asphaltenes by nuclear magnetic resonance spectroscopy and X-ray diffractometry. Fuel 1996, 75(8), 999-1008. (25) Ahmad, I.; Shakirullah, M.; Ur Rehman, H.; Ishaq, M.; Khan, M. A.; Shah, A. A. NMR analysis of cracking products of asphalt and assessment of catalyst performance. Energy 2009, 34(2), 127-133. (26) Yan, L.; Bai, Y.; Zhao, R.; Li, F.; Xie, K. Correlation between coal structure and release of the two organic compounds during pyrolysis. Fuel 2015, 145, 12-17. (27) Tong, J.; Han, X.; Wang, S.; Jiang, X. Evaluation of structural characteristics of Huadian oil shale kerogen using direct techniques (solid-state
13
C-NMR, XPS, FT-IR, and XRD). Energy &
Fuels 2011, 25(9), 4006-4013. (28) Hao, N.; Ben, H.; Yoo, C. G.; Adhikari, S.; Ragauskas, A. J. Review of NMR characterization of pyrolysis oils. Energy & Fuels 2016, 30(9), 6863-6880. (29) Trejo, F.; Ancheyta, J.; Centeno, G.; Marroquín, G. Effect of hydrotreating conditions on Maya asphaltenes composition and structural parameters. Catalysis Today 2005, 109(1-4), 178-184. (30) Zhao, Y.; Wei, F.; Zhang, S.; Yu, Y. Kinetics and selectivity of asphaltene hydrocracking. Fuel 2011, 90(5), 1900-1906. (31) Liu, D.; Li, Z.; Fu, Y.; Zhang, Y.; Gao, P.; Dai, C.; Zheng, K. Investigation on asphaltene structures during Venezuela heavy oil hydrocracking under various hydrogen pressures. Energy &
Fuels 2013, 27(7), 3692-3698. (32) Domazetis, G.; Raoarun, M.; James, B. D.; Liesegang, J.; Pigram, P. J.; Brack, N.; Glaisher, R. Analytical and characterization studies of organic and inorganic species in brown coal. Energy &
Fuels 2006, 20(4), 1556-1564. (33) Nowicki, P.; Pietrzak, R.; Wachowska, H. X-ray photoelectron spectroscopy study of nitrogen-enriched active carbons obtained by ammoxidation and chemical activation of brown and ACS Paragon Plus Environment
Page 20 of 23
Page 21 of 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
bituminous coals. Energy & Fuels 2010, 24(2), 1197-1206. (34) Kelemen, S. R.; Afeworki, M.; Gorbaty, M. L.; Kwiatek, P. J.; Sansone, M.; Walters, C. C.; Cohen, A. D. Thermal transformations of nitrogen and sulfur forms in peat related to coalification.
Energy & Fuels 2006, 20(2), 635-652. (35) Sun, Z.; Li, D.; Ma, H.; Tian, P.; Li, X.; Li, W.; Zhu, Y. Characterization of asphaltene isolated from low-temperature coal tar. Fuel Processing Technology 2015, 138, 413-418. (36) Zhang, S.; Liu, D.; Deng, W.; Que, G. A review of slurry-phase hydrocracking heavy oil technology. Energy & Fuels 2007, 21(6), 3057-3062. (37) Deng, W.; Luo, H.; Gao, J.; Que, G. Stability change of asphaltene in the residue during slurry-phase hydrocracking. Energy & Fuels 2011, 25(11), 5360-5365. (38) Cardozo, S. D.; Schulze, M.; Tykwinski, R. R.; Gray, M. R. Addition reactions of olefins to asphaltene model compounds. Energy & Fuels 2015, 29(3), 1494-1502. (39) Marinov, N. M.; Pitz, W. J.; Westbrook, C. K.; Vincitore, A. M.; Castaldi, M. J.; Senkan, S. M.; Melius, C. F. Aromatic and polycyclic aromatic hydrocarbon formation in a laminar premixed n-butane flame. Combustion and Flame 1998, 114(1–2), 192-213. (40) Hosseini-Dastgerdi, Z.; Tabatabaei-Nejad, S. A. R.; Khodapanah, E.; Sahraei, E. A comprehensive study on mechanism of formation and techniques to diagnose asphaltene structure; molecular and aggregates: a review. Asia-Pacific Journal of Chemical Engineering 2015, 10(1), 1-14. (41) Nomura, M.; Artok, L.; Murata, S.; Yamamoto, A.; Hama, H.; Gao, H.; Kidena, K. Structural evaluation of Zao Zhuang coal. Energy & Fuels 1998, 12(3), 512-523. (42) Deng, W.; Lu, J.; Li, C. Hydrogenation behavior of bicyclic aromatic hydrocarbons in the presence of a dispersed catalyst. Energy & Fuels 2015, 29(9), 5600-5608. (43) Li, G.; Li, L.; Shi, L.; Jin, L.; Tang, Z.; Fan, H.; Hu, H. Experimental and theoretical study on ACS Paragon Plus Environment
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 22 of 23
the pyrolysis mechanism of three coal-based model compounds. Energy & Fuels 2014, 28(2), 980-986. (44) Perry, S. T.; Fletcher, T. H.; Solum, M. S.; Pugmire, R. J. Modeling nitrogen evolution during coal pyrolysis based on a global free-radical mechanism. Energy & Fuels 2000, 14(5), 1094-1102. (45) Rocha, A.; Farojr, A.; Oliviero, L.; Vangestel, J.; Mauge, F. Alumina-, niobia-, and niobia/alumina-supported
NiMoS
catalysts:
Surface
properties
and
activities
in
the
hydrodesulfurization of thiophene and hydrodenitrogenation of 2,6-dimethylaniline. Journal of
Catalysis 2007, 252(2), 321-334
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