Metal Porphyrin Adsorption onto Asphaltene in Pentane Solution: A

Article pubs.acs.org/EF

Metal Porphyrin Adsorption onto Asphaltene in Pentane Solution: A Comparison between Vanadyl and Nickel Etioporphyrins Feifei Chen,† Qiushi Zhu,† Zhiming Xu,‡ Xuewen Sun,‡ and Suoqi Zhao*,‡ †

College of Chemistry and Materials Science, Huaibei Normal University, Huaibei 235000, China State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China

‡

ABSTRACT: In the solvent deasphalting process, it is necessary to study the interaction between metal porphyrins and asphaltene for improving demetallization efficiency (vanadyl or nickel). Therefore, the main aim of this study is to investigate the adsorption kinetics and thermodynamics of nickel etioporphyrins onto Canadian oil sands bitumen vacuum tower bottom (VTB) asphaltene, and to compare the results with that of vanadyl etioporphyrins onto VTB asphaltene in pentane. Asphaltene was characterized by transmission electron microscopy (TEM), N2 adsorption, and X-ray diffraction technique (XRD). The results showed that vanadyl/nickel porphyrins were adsorbed onto the VTB asphaltene. The adsorption rate varied as the dosage of asphaltene, the concentration of vanadyl/nickel porphyrins, and the adsorption temperature changed. By comparison of the pseudo-first-order adsorption kinetics model of the two adsorption processes, the adsorption rate for nickel octaethylporphyrin (Ni-OEP) was faster than that for vanadyl octaethylporphyrin (VO-OEP). Furthermore, the equilibrium adsorption capacity of Ni-OEP was greater than that of VO-OEP. Moreover, the adsorption equilibriums of vanadyl/nickel porphyrins both wonderfully fitted to the Freundlich isotherm. In addition, the ΔG° and ΔH° values of two adsorption processes had regressed at different temperatures. Compared with Ni-OEP, in the same conditions it was easier for VO-OEP to be adsorbed and more heat was released in the process.

1. INTRODUCTION Metal content is significantly lower than C, H, S, N, and O content in petroleum as its mass fraction is only from 10−6 to 10−9. Unfortunately, the presence of metals, especially nickel and vanadium, which could deactivate the catalysts of both hydrocracking and catalytic cracking as well as cause a bed plug, will lead to decreased yield of light oil and inferior products during the petroleum processing.1−3 The main nickel/ vanadium forms are porphyrin-like compounds, and the rest exist as metal nonporphyrin compounds.4−7 Both are oilsoluble and hard to remove. Therefore, it is essential to efficiently remove metal from heavy oils to upgrade heavy oils. There are many ways for demetallization, and the solvent deasphalting technique is one of them.8−10 It takes advantage of selective solubility of low-molecular-weight solvents, such as pentane, propane, and butane, for different constituents to remove asphalt from vacuum residue.11 Meanwhile, most of the metals adsorbed or intercalated into asphalt are also eliminated. However, the remaining metallic impurities in the deasphalted oil (DAO), especially nickel and vanadium porphyrin-like compounds, result in poor processability of DAO. Therefore, it is necessary to study the interaction between metal porphyrins and asphaltene to improve the demetallization efficiency. Asphaltenes are the fraction isolated from the crude oil which is insoluble in normal alkanes but soluble in aromatic solvents.12 They have very complex structures and consist of polyaromatic nuclei with aliphatic side chains and rings. It is agreed that asphaltenes have high surface activities because of the presence of polar surface functional groups. Consequently, asphaltenes could strongly adsorb onto different surfaces, such as steel,13 NiO,14 TiO2, Al2O3, clay,15 calcite, and kaolin.16 Moreover, the high active surface of asphaltene may enhance © 2017 American Chemical Society

adsorption of metal porphyrins. Then, metal porphyrins could be more efficiently transferred from DAO to de-oiled asphalt (DOA) during the solvent deasphalting technique. Therefore, the interaction (porphyrin−asphaltene) should be studied to develop such a technology. We have reported the adsorption kinetics and thermodynamics of vanadyl porphyrins on asphaltene.17 In the present study, the adsorption kinetics and thermodynamics of nickel etioporphyrins on VTB asphaltene were discussed. The interaction between them was deduced from experimental results. The results can also be compared with that of vanadyl etioporphyrins adsorbed on vacuum tower bottom (VTB) asphaltene to improve the selectivity and efficiency of demetallization. The ultimate goal of providing high quality DAO for fluid catalytic cracking (FCC) and hydrocracking will be achieved. As a consequence, the metallic impurities in the petroleum will become less harmful to the subsequent processing.

2. EXPERIMENT AND THEORY 2.1. Materials. Canadian oil sands bitumen VTB was utilized to prepare the C7 asphaltene. The detailed process could be found in previous study.17 SARA (saturates, aromatics, resin, and asphaltene) analysis of Canada VTB and properties of Canada VTB-asp are shown in Table 1. Vanadyl octaethylporphyrin (VO-OEP) and nickel octaethylporphyrin (Ni-OEP) were purchased from J&K Scientific Ltd. According to the manufacturer, their main characteristics are listed in Table 2. They have structural similarities with VO etioporphyrin and Ni Received: November 22, 2016 Revised: March 14, 2017 Published: March 15, 2017 3592

DOI: 10.1021/acs.energyfuels.6b03100 Energy Fuels 2017, 31, 3592−3601

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Energy & Fuels

Asphaltenes were characterized by transmission electron microscopy (TEM), gel permeation chromatography (GPC), element contents (C/H/N), N2 adsorption, and X-ray diffraction (XRD). An FEI Tecnai G2 F20 field emission TEM was utilized for surface analysis operated at 200 kV. The point resolution and information resolution were 0.24 nm and 0.14 nm, respectively. The element contents (C/H/N) were tested by a Flash EA1112 (organic trace element analyzer). Relative molecular weights were tested by a GPC515-2410 system from Waters Corp. (USA). Tetrahydrofuran was used as mobile phase with 1 mL/ min flow rate. The detector temperature was 30 °C. The standard sample was polystyrene. The specific surface area was analyzed by N2 adsorption using an American Quadrasorb-SI and calculated by the BET method. The X-ray diffraction was performed with a D8 from Bruker Corp. (Germany) with a Cu target source and measurement angle from 5 to 90°. The scanning speed was 1 deg/min and 0.05 deg/ step. Before each test, the asphaltene should be ground to fine powder. 2.2. Adsorption Test. The VO-OEP or Ni-OEP was adsorbed on VTB asphaltene in n-pentane solution. The adsorption capacity and adsorption isotherm were obtained. Thus, simple interaction between them was determined and compared with experimental results. The full detailed experimental procedure (screening of asphlatene dosage and porphyrin concentration; standard curves of porphyrins; blank experiment; measurement of the adsorption kinetics; measurement of adsorption thermodynamics) can be found in our previous study.17 The adsorption capacities and equilibrium adsorption capacities of VO-OEP and Ni-OEP can also be calculated according to our previous study.17 2.3. Kinetic Models. The adsorption kinetics is significant for an adsorption process as it illustrates the adsorption rate of adsorbate. The adsorption mechanism could be discovered by fitting the experimental data with kinetic models. In recent work, we compared several kinetic models (Table 3) for vanadium porphyrin and pseudo first order was found to be the best fitting model.17 In this paper, the

Table 1. SARA Analysis of Canada VTB and Properties of Canada VTB-asp feed Canada VTB (>524 °C)

saturate

aromatic

8.3% 40.7% Sample: VTB-asp

C (mass %) H (mass %) N (mass %) H/C Mn metal (μg/g) Ni V

resin

asphaltene

32.5%

18.3%

76.02 7.35 1.26 1.160 1861 337.11 1039.75

Table 2. Characteristics of Ni-OEP and VO-OEP name

nickel octaethylporphyrin

vanadyl octaethylporphyrin

appearance (color) molecular formula molecular weight chemical family purity

red to purple C36H44N4Ni 591.45 nickel complex >99%

red to purple C36H44N4OV 599.71 vanadium complex >99%

etioporphyrin (VO-ETIO, Ni-ETIO; Figure 1). The other chemicals were obtained from Beijing Chemical Works and were used as received. The concentrations of VO-OEP and Ni-OEP in pentane were determined by an UV-1201 ultraviolet visible (UV−vis) spectrophotometer (Beifen-Ruili Analytical Instrument Corp., Ltd. Beijing).

Figure 1. Structure of vanadyl/nickel porphyrins: (a) VO-ETIO, Ni-ETIO; (b) VO-OEP, Ni-OEP. 3593


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Energy & Fuels Table 3. Kinetic Models and Equationsa kinetic model

expression formula

pseudo-first-order equation18−21 pseudo-secondorder equation22−25 Elovich equation26−28 diffusion equation29−31

dqt/dt = k1(qe − qt)

qt = qe(1 − e−k1t)

dqt/dt = k2(qe − qt)2

qt = k2qe2t /(1 + k2qet)

dqt/dt = Ae−Bqt

qt = 1/B ln(AB) + ln(t)/B

changed after adsorption. This makes the increase of Mn become a possibility. It can be other evidence of adsorption of porphyrins onto the asphaltene. From Table 6, the specific surface area of VTB asphaltene after adsorption slightly decreases implied adsorption of porphyrins onto the VTB asphaltene, which agrees with the TEM, GPC, and element content (C/H/N) results. On the other hand, the surface area of VTB-asp+V drops from 5.3 to 3.8 m2/g and the surface area of VTB-asp+Ni drops from 5.3 to 3.5 m2/g. Maybe the adsorption capacity of Ni-OEP is greater than that of VO-OEP. The reason may be the differences in the structures and polarities of VO-OEP and Ni-OEP. The ordered structure of asphaltene can be analyzed by the X-ray diffraction technique. Meanwhile, the structure parameters of asphaltene can be calculated. A typical spectrum of Canada VTB asphaltene, characterized by XRD and shown in Figure 3, exhibits several characteristics. Three apparent peaks can be obviously found in the XRD patterns located at 20, 25, and 45°. The peaks at 20 and 25° could be identified as the γ peak and the 002 peak which were attributed to the accumulation structure of the saturated side chain and the accumulation structure of aromatic slices of asphaltene, respectively. A method was put forward in numerous references to calculate the aromatic carbon rate ( fA). The equation is displayed as follows, where CS, CA, and C are the saturated carbon number, aromatic carbon number, and total carbon number in the accumulation structure of asphaltene, respectively.

linear form

qt = kpt0.5 + C

a

qe and qt: adsorption capacities (mg/g) at equilibrium and at time t. k1, k2, and kp: rate constants of adsorption (min−1). A, B, and C: constants. pseudo first order was introduced in detail for fitting the data of adsorption capacity and time to describe the adsorption process. 2.4. Adsorption Isotherm and Thermodynamics. The experimental data were fit by Langmuir and Freundlich adsorption isotherm equations32−34 in this study (Table 4). The correlation coefficients could be used for determining which isotherm equation is suitable. The energy variation of interaction can be determined by the thermodynamic parameters35−37 such as standard free energy (ΔG°; kJ/mol), enthalpy (ΔH°; kJ/mol), and entropy (ΔS°; kJ·mol−1·K−1), which could be calculated by the thermodynamic results.

3. RESULTS AND DISCUSSION 3.1. Characterization of Asphaltene. The VTB asphaltene was used as adsorbent and VO-OEP/Ni-OEP as adsorbate in pentane with a constant adsorption temperature of 20 °C for 48 h. The fresh and saturated VTB asphaltenes were analyzed by TEM, GPC, element contents (C/H/N), N2 adsorption, and XRD. From the TEM images (Figure 2), the surface morphology change of asphaltene after adsorption of VO-OEP/Ni-OEP can be obviously found. As shown in Figure 2, black spots can be clearly observed as a result of porphyrin agglomeration after adsorption. It is clearly stated that porphyrins were truly absorbed onto the VTB asphaltene surface, which had been already mentioned in previous studies.17 However, the difference of adsorption between VO-OEP and Ni-OEP cannot be distinctly distinguished from the TEM images. As shown in Table 5, the nitrogen content of VTB asphaltene gets higher after adsorption. It can be deduced that metal porphyrins were absorbed onto VTB asphaltene as a porphyrin molecule has four nitrogen atoms, which agrees with the TEM results. Furthermore, the relative molecular mass (Mn) of VTB asphaltene also increases slightly after adsorption. This may be caused by the association of asphaltene and porphyrin after adsorption, which is formed association aggregates. According to Zhang et al.,38 the Mn of association aggregates is related to the state and mode of association such as the size of aggregates. The size of asphaltene aggregates is

fA = CA /C = CA /(CA + CS) = A(002)/(A(002) + A(γ )) (1)

The structure parameters of asphaltene were calculated and are presented in Table 7. The results demonstrate that the interlayer spacing of asphaltene (d002) has shortened after adsorption, indicating the adsorption of VO-OEP or Ni-OEP onto the VTB asphaltene. Therefore, the average diameter of the aromatic plate (La) gets higher. Meanwhile, the attraction among aromatic piece layers has also been strengthened. It would result in reduction of the thickness of the accumulation structure of aromatic slices (Lc). The accumulation structure of asphaltene may be destroyed by the adsorption. That is why the number of aromatic piece layers (M) goes down. In addition, there are differences in the structures and polarities of VO-OEP and Ni-OEP. The structure of VO-OEP is nonplanar, whereas the structure of Ni-OEP is planar. It makes a difference in the state and mode of association (asphaltene and porphyrin) after adsorption. It is easy to understand that the changes in the structure parameters of VTB asphaltene after adsorption of VO-OEP are at different levels compared with those after adsorption of Ni-OEP. 3.2. Adsorption Rate and Kinetics. Experiments were carried out to probe the influences of temperature, adsorbent

Table 4. Isotherm and Thermodynamic Equationsa adsorption isotherm Langmuir isotherm

Freundlich isotherm

adsorption thermodynamics

Ce/qe = 1/(qmaxKL) + Ce/qmax

log qe = log Ce/n + log KF

ΔG° = ΔH° − TΔS° ΔG° = −RT ln K ln K = ΔS°/R − ΔH°/RT

a

Ce: equilibrium concentration (μg/mL). qe: adsorption capacity (μg/g). KL: Langmuir constant (L/mg). KF: constant indicating adsorption capacity of the adsorbent (μg/g). 1/n: intensity of the adsorption. 3594


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Figure 2. TEM images of Canada VTB asphaltene: (a) VTB-asp; (b) VTB-asp+V; (c) VTB-asp+Ni.

Table 5. Properties of Canada VTB asphaltene sample

C (mass %)

H (mass %)

N (mass %)

H/C

Mn

before adsorption of VTB asphaltene after adsorption (VO-OEP) of VTB asphaltene after adsorption (Ni-OEP) of VTB asphaltene

76.02

7.35

1.26

1.160

1861

76.05

7.37

1.47

1.164

1971

76.17

7.39

1.48

1.164

1936

Table 6. Surface Area of Canada VTB Asphaltenes Calculated by BET sample

surf. area (BET) (m2/g)

before adsorption of VTB asphaltene after adsorption (VO-OEP) of VTB asphaltene after adsorption (Ni-OEP) of VTB asphaltene

5.3 3.8 3.5

dosage, and initial porphyrin concentration on the adsorption capacity of VO-OEP/Ni-OEP on asphaltene. The dynamics of the adsorption process was also described by processing and discussing the kinetic adsorption data. 3.2.1. Effect of Temperature. The experiments were conducted to estimate the influence of the adsorption temperature. During these experiments, the concentration of

Figure 3. XRD patterns of Canada VTB asphaltene.

porphyrin in pentane and the asphaltene dosage were fixed at 15 μg/mL and 0.01 g, respectively. The results are shown in Figure 4. 3595


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capacity of VO-OEP is less than the one of Ni-OEP in the same conditions. Meanwhile, k1(Ni‑OEP) is slightly more than k1(VO‑OEP). For this difference, there are two reasons at least. One reason is the differences in the structures and polarities of VO-OEP and Ni-OEP. The structure of VO-OEP is nonplanar, whereas the structure of Ni-OEP is planar; it decreases the steric hindrance effect. The other reason is the difference of metal element content in VTB-asp. The content of vanadyl element in VTB-asp is higher than the content of nickel element. 3.2.2. Effects of Adsorbate Dosage. The experiments were performed to study the influence of asphaltene amount. The adsorption temperature and concentration of porphyrin in pentane were fixed at 20 °C and 15 μg/mL, respectively. The results are illustrated in Figure 6. From Figure 6, it can be also concluded that in the same conditions Ni-OEP uptake at equilibrium conditions was slightly more than that of VO-OEP. As the asphaltene dosage decreased from 0.02 to 0.01 g, the equilibrium adsorption capacity increased from 2.261 to 4.676 mg/g for VO-OEP and from 2.238 to 4.836 mg/g for Ni-OEP, respectively. By the same token, the trend of these curves with different dosages of asphaltene is similar to that of the curves in Figure 4. The experimental data (Figure 6) were also fitted with the pseudo-first-order model. The results are illustrated in Figure 7, and the fitting parameters are outlined in Table 9. From Figure 7 and Table 9, it can be found that the pseudofirst-order model could fit the adsorption data well. The rate constant of adsorption (k1) increases with increasing the asphaltene dosage. This phenomenon could be attributed to the enlarged interface between asphaltene and the solution with increasing asphaltene dosage. Moreover, increasing the asphaltene dosage could also provide more combining sites for metal porphyrins to accelerate the combination of porphyrins and asphaltene. Meanwhile, k1(Ni‑OEP) is slightly greater than k1(VO‑OEP), which is in agreement with the results in section 3.2.1. 3.2.3. Effects of Initial Porphyrin Concentration. The experiments were performed to study the effect of initial porphyrin concentration. The adsorption temperature and asphaltene dosage were fixed at 20 °C and 0.01 g, respectively. The results are illustrated in Figure 8. From Figure 8, the similar phenomenon of adsorption capacity of Ni-OEP and VO-OEP is observed. Ni-OEP uptake at equilibrium conditions is slightly greater than that of VOOEP in the same conditions. The equilibrium adsorption

Table 7. Parameters of Canada VTB Asphaltene Obtained by XRD sample before adsorption of VTB asphaltene after adsorption (VO-OEP) of VTB asphaltene after adsorption (Ni-OEP) of VTB asphaltene

fA

dγ (nm)

d002 (nm)

Lc (nm)

La (nm)

M

0.13

4.60

3.50

26.73

11.98

8.6

0.15

4.62

3.32

14.45

16.09

5.9

0.15

4.67

3.39

14.16

18.03

5.7

As depicted in Figure 4, in the same conditions, Ni-OEP uptake at equilibrium was slightly more than that of VO-OEP. The equilibrium adsorption capacity increased from 4.562 to 4.956 mg/g for VO-OEP and from 4.450 to 5.011 mg/g for NiOEP, when the adsorption temperature decreased from 25 to 15 °C. However, it can be seen that these curves have similar trends at different adsorption temperatures. Each curve can be divided into three sections, which have been already discussed in a previous study.17 In the first step (t < 200 min), rapid increase of adsorption capacity was observed. As the adsorption rate was relatively fast, it could be inferred that adsorption of porphyrin on asphaltene may be classified as physical adsorption at the beginning of the adsorption process. During the second step (200 min < t < 750 min), the adsorption capacity slowly increased over time. In the final step (t > 750 min), the adsorption appeared to attain equilibrium because the uptake of porphyrins came to a plateau as the time changed. A relatively long adsorption time of 1800 min was used to allow the adsorption to reach equilibrium; however, reversibility was not confirmed experimentally and therefore the isotherms and thermodynamic values are given as apparent results, assuming reversible equilibrium adsorption. In recent work, we compared four kinetic models (Table 3) for adsorption of vanadium porphyrin.17 The pseudo-first-order model was considered to be the best fitting model. Therefore, in this work, we compared the adsorption kinetics between NiOEP and VO-OEP using the pseudo-first-order model to fit the experimental data (Figure 4). The results are illustrated in Figure 5, and the fitting parameters are given in Table 8. From Table 8, the pseudo-first-order model fits the whole process of adsorption well. The rate constant of adsorption (k1) increases at higher adsorption temperature. The data imply that the rate of adsorption for porphyrins becomes faster as the temperature is raised. Furthermore, the equilibrium adsorption

Figure 4. Effect of temperature on adsorption of porphyrins. The initial VO-OEP/Ni-OEP concentration was 15 μg/mL. The asphaltene dosage was 0.01 g. The volume of pentane solution was 3.5 mL. 3596


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Figure 5. Pseudo-first-order kinetic plots for adsorption of porphyrins on VTB-asp at different temperatures. Data were taken from Figure 4.

Table 8. Calculated Parameters of Pseudo-First-Order Kinetics for VO-OEP/Ni-OEP at Different Temperaturesa pseudo-first-order equation VO-OEP

Ni-OEP

a

T (°C)

qe,cal (mg/g)

k1 (min−1)

R2

qe,exp (mg/g)

15 20 25 15 20 25

4.850 4.594 4.473 5.041 4.882 4.378

0.00596 0.00620 0.00651 0.00603 0.00637 0.00874

0.992 0.995 0.982 0.982 0.982 0.986

4.956 4.676 4.562 5.011 4.836 4.450

Co(VO‑OEP/Ni‑OEP) = 15 μg/mL, masp = 0.01 g.

Figure 7. Pseudo-first-order kinetic plots for adsorption of porphyrins on VTB-asp with different asphaltene dosages. Data were taken from Figure 6.

Table 9. Calculated Parameters of Pseudo-First-Order Kinetics for VO-OEP/Ni-OEP with Different Asphaltene Dosagesa pseudo-first-order equation VO-OEP Ni-OEP

Figure 6. Effects of adsorbate dosage on adsorption of porphyrins. The initial VO-OEP/Ni-OEP concentration was 15 μg/mL. The temperature was 20 °C. The volume of pentane solution was 3.5 mL.

a

m (g)

qe,cal (mg/g)

k1 (min−1)

R2

qe,exp (mg/g)

0.01 0.02 0.01 0.02

4.594 2.235 4.882 2.200

0.00620 0.00773 0.00637 0.00781

0.995 0.987 0.982 0.987

4.676 2.261 4.836 2.238

T = 20 °C, Co(VO‑OEP/Ni‑OEP) = 15 μg/mL.

probability of porphyrins in solution is increased with increased initial porphyrin concentration, which will extend the combination of porphyrins and asphaltene. Meanwhile, k1(Ni‑OEP) is slightly greater than k1(VO‑OEP), which agrees with the results in sections 3.2.1 and 3.2.2. 3.3. Apparent Isotherms of Adsorption. Studies of adsorption isotherms and thermodynamics were carried out to obtain more information about the adsorption process. In each test, 0.05 g of asphaltene was used as adsorbent. The concentrations of porphyrins in pentane were 4, 6, 8, 10, and 12 μg/mL, and the adsorption temperatures were 15, 20, and 25 °C during different tests. As mentioned above, uptake of porphyrins will remain constant as the adsorption time is increased from 750 to 1800 min. Therefore, a total adsorption time of 48 h was used to ensure achievement of equilibrium.

capacity increased from 3.248 to 4.676 mg/g for VO-OEP and from 3.307 to 4.836 mg/g for Ni-OEP, respectively, as the initial porphyrin concentration increased from 10 to 15 μg/mL. Meanwhile, at different concentrations of the pentane solutions containing VO-OEP or Ni-OEP, the adsorption data have similar trends to that in Figure 4. The experimental data (Figure 8) were fitted with the pseudo-first-order model. The results are illustrated in Figure 9, and the fitting parameters are shown in Table 10. From Figure 9 and Table 10, it can also be observed that the pseudo-first-order model can describe the process fairly well with varied initial porphyrin concentration. The increase of initial porphyrin concentration will decrease the rate constant of adsorption (k1). The reason is that the molecular collision 3597


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shows that the correlation coefficients of Freundlich isotherm equation (R2, 0.997−0.999) are higher than those of the Langmuir isotherm equation (R2, 0.959−0.990), indicating that the Freundlich isotherm model is more suitable for describing the adsorption process. It is known that the Freundlich isotherm model includes monolayer and/or multilayer adsorption. By the same token, a complicated adsorption of metal porphyrins on asphaltene is implied. It could also be deduced that monolayer adsorption may occur in some areas, but in other areas, multilayer adsorption may take place. Moreover, the aggregation of metal porphyrins on the surface of asphaltene which has been observed by TEM analysis could also be illuminated by the multilayer adsorption. The Freundlich isotherm equation was applied to calculate the adsorption capacity (KF) and adsorption intensity (1/n) of metal porphyrins. The results are summarized in Table 11. The adsorption capacity and adsorption intensity decrease with increasing temperature, indicating that a lower temperature is beneficial for adsorption of metal porphyrins. Furthermore, KF(VO‑OEP) is slightly higher than KF(Ni‑OEP) whereas 1/n(VO‑OEP) is lower than 1/n(Ni‑OEP) in the same conditions. This indicates the adsorption capacity of VO-OEP is greater than that of NiOEP. But the adsorption intensity of VO-OEP is weaker than that of Ni-OEP. It is not correspondent with the experiment results. Thus, it may be assumed that a certain amount of VOOEP is desorbed from the asphaltene. Therefore, the equilibrium adsorption capacity of Ni-OEP is greater than that of VO-OEP, which is in accordance with the results in section 3.2. 3.4. Apparent Thermodynamics of Adsorption. The KF shown in Table 11 was used to estimate the thermodynamic parameters including ΔG°, ΔH°, and ΔS° according to equations in Table 4. From the linear relationship between ln KF and 1/T, the ΔH° and ΔS° of metal porphyrins absorbed on asphaltene surface were determined; the relevant slope was −ΔH°/R and the intercept was ΔS°/R. The results are presented in Figure 12 and Table 12. ΔG° is always used for evaluating the spontaneity of a process. In Table 12, negative values of ΔG° indicate that the adsorption processes are spontaneous no matter which metal porphyrins are used. For all tests, increasing the adsorption temperature will decrease the value of ΔG°, indicating that the driven force of adsorption of metal porphyrins was enhanced at low temperature, which is consistent with the results in section 3.2.1. Physical and chemical adsorption processes can be approximately differentiated using the value of ΔG°.39 Generally, the ΔG° value of physical adsorption is varied between 0 and −20 kJ·mol−1, while the value of the latter process is between −80 and −400 kJ·mol−1. In the present work, the value of ΔG° is in the range −20 to 0 kJ·mol−1. Therefore, it can be concluded that physical adsorption exists during adsorption of metal porphyrins, which is in accordance with the results in section 3.2. ΔH° is always used for determining whether an adsorption process is exothermic or endothermic. From Table 12, it can be clearly seen that adsorption of metal porphyrins is an exothermic process as the value of ΔH° is negative, which is in accordance with that lower temperature is beneficial for the adsorption. A strong adsorbate−adsorbent interaction is indicated as the absolute value of ΔH° is more than 40 kJ· mol−1. Moreover, the enthalpy change of adsorption (∼50 kJ· mol−1) is significantly larger than that of hydrogen bonding (5−30 kJ·mol−140), suggesting that a strong secondary

Figure 8. Effects of initial porphyrin concentration on adsorption of porphyrins. The asphaltene dosage was 0.01 g. The temperature was 20 °C. The volume of pentane solution was 3.5 mL.

Figure 9. Pseudo-first-order kinetic plots for adsorption of porphyrins on VTB-asp with different initial porphyrin concentrations. Data were taken from Figure 8.

Table 10. Calculated Parameters of Pseudo-First-Order Kinetics for VO-OEP/Ni-OEP with Different Initial Porphyrin Concentrationsa pseudo-first-order equation

VO-OEP Ni-OEP a

Co (μg/mL)

qe,cal (mg/g)

k1 (min−1)

R2

qe,exp (mg/g)

10 15 10 15

3.176 4.594 3.234 4.882

0.0100 0.00620 0.0103 0.00637

0.986 0.995 0.990 0.982

3.248 4.676 3.307 4.836

T = 20 °C, masp = 0.01 g.

Langmuir and Freundlich adsorption isotherm equations were used for processing of adsorption data in this section to describe the interaction between adsorbate and adsorbent. The fitting results are illustrated in Figure 10 and Figure 11, and the fitting parameters are listed in Table 11. Interestingly, Figures 10 and 11 show satisfactory results for fitting the experimental data with the two adsorption isotherm equations at different temperatures. However, Table 11 also 3598


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Figure 10. Apparent Langmuir isotherms of VO-OEP/Ni-OEP. The dosage of asphaltene was 0.05 g. The initial VO-OEP or Ni-OEP concentrations were 4, 6, 8, 10, and 12 μg/mL. The volume of pentane solution was 20 mL.

Figure 11. Apparent Freundlich isotherms of VO-OEP/Ni-OEP. The dosage of asphaltene was 0.05 g. The initial VO-OEP or Ni-OEP concentrations were 4, 6, 8, 10, and 12 μg/mL. The volume of pentane solution was 20 mL.

Table 11. Isotherm Apparent Parameters and Correlation Coefficients Langmuir isotherm

Freundlich isotherm

sample

T (°C)

qmax (mg/g)

KL (L/mg)

qmaxKL (L g−1)

R2

KF

1/n

R2

VTB+V

15 20 25 15 20 25

16.667 6.250 3.571 20.000 9.091 5.882

0.0526 0.0829 0.100 0.0276 0.0426 0.0433

0.877 0.518 0.357 0.552 0.388 0.254

0.959 0.988 0.990 0.971 0.982 0.983

1.035 0.650 0.463 0.621 0.465 0.316

0.745 0.637 0.588 0.84 0.769 0.749

0.999 0.999 0.999 0.997 0.999 0.999

VTB+Ni

Table 12. Thermodynamic Parameters of VO-OEP/Ni-OEP ΔG° (kJ/mol) sample

ΔH° (kJ/mol)

ΔS° (J/mol·K)

15 °C

20 °C

25 °C

VTB+V VTB+Ni

−54.91 −48.13

−133.33 −113.51

−16.54 −15.40

−15.78 −14.96

−15.21 −14.26

adsorption may also exist during the adsorption process. As deduced from the analysis of ΔG° and ΔH°, it can be concluded that adsorption of metal porphyrins can be attributed to chemistry as well as physical adsorption. This means that both monolayer adsorption and multilayer adsorption take place, which has already been discussed above. ΔS° could be used to estimate the chaos of the solid−liquid interface during the adsorption process. Negative values of ΔS° in Table 12 indicate that free states of porphyrins are orderly transferred to adsorbed states. The comparison of two processes was performed in which asphaltene adsorbs VO-OEP and Ni-OEP separately in npentane solution. The ΔG°, ΔH°, and ΔS° of VO-OEP

Figure 12. Van’t Hoff plots of VO-OEP and Ni-OEP.

adsorbate−adsorbent bond exists (probably metal-coordination interaction), which needs further study. Therefore, chemical 3599


Article

Energy & Fuels

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adsorbed on the asphaltene are higher than the parameters of Ni-OEP. This means that it is easier for VO-OEP to be adsorbed, and in the process more heat is released. According to the literature,41 the adsorption behavior was influenced by the polarities of adsorbate and adsorbent. Molecules of asphaltene and VO-OEP are polar ones, while Ni-OEP is a nonpolar one. It can also be expected that adsorption of VOOEP is easier than Ni-OEP.

4. CONCLUSIONS Adsorption of VO-OEP or Ni-OEP on VTB asphaltene was determined by characterization of VTB asphaltene after adsorption. Adsorption capacity was affected by several factors such as asphaltene dosage, initial porphyrin concentration, and adsorption temperature. Higher equilibrium adsorption capacity could be achieved at lower temperature, smaller adsorbent usage, and higher initial porphyrin concentration for each metal porphyrin. The overall experimental data showed similar trends and could be described by the pseudofirst-order adsorption kinetics model. Rapid adsorption of metal porphyrins implied that physical adsorption may occur at the beginning of the adsorption. Moreover, the equilibrium uptake and rate of adsorption for Ni-OEP were higher than those for VO-OEP. The adsorption equilibriums of vanadyl/nickel porphyrins were both wonderfully fitted by the Freundlich isotherm equation. By evaluating thermodynamic parameters, the adsorption process was considered to be spontaneous and exothermic. Higher value of ΔH° indicated that chemical adsorption may occur. Furthermore, in the same conditions it was easier for VO-OEP to be adsorbed and more heat was released in the process. Meanwhile, the adsorption intensity of VO-OEP is weaker than that of Ni-OEP. Thus, it may be assumed that a certain amount of VO-OEP was desorbed from the asphaltene. Moreover, it can be confirmed that the strong interaction between VO-OEP and asphaltene was different from that of between Ni-OEP and asphaltene. As the adsorption of metal porphyrins was confirmed, the demetallization efficiency during solvent deasphalting could be increased by enhancing the interaction between asphaltene and metal porphyrins in further study.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +8610-8973-9015. E-mail: [email protected]. com or [email protected]. ORCID

Feifei Chen: 0000-0003-4512-1415 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for the financial support from the National “Twelfth Five-Year” Plan for Science & Technology Support (2012BAE05B06) and the National Natural Science Foundation of China (NSFC) (U1162204 and 21176254).

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REFERENCES

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Metal Porphyrin Adsorption onto Asphaltene in Pentane Solution: A

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