Transformation of Nickel Octaethylporphine in Hydrodemetallization

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Transformation of Nickel Octaethylporphine in Hydrodemetallization Reactions Linqing Ju,† Tingting Liu,† Jincheng Lu, Yasong Zhou,* Qiang Wei, Shiyi Li, Sijia Ding, Yahe Zhang, and Quan Shi State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing), Beijing 102249, People’s Republic of China S Supporting Information *

ABSTRACT: The transformation of nickel (Ni) porphyrins in hydrodemetallization (HDM) was investigated using 2,3,7,8,12,13,17,18-nickel-octaethylporphine (Ni-OEP) as the model compound, which was dissolved in a white oil. The HDM reactions were carried out in a high-pressure trickle-bed reactor over the oxidic and sulfided NiMo/Al2O3 catalyst. The molecular formulas of Ni compounds in the feed and hydrotreated products were characterized by positive-ion electrospray ionization (+ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The hydrotreated products were separated into two subfractions by silica gel chromatography. Molecular structures of Ni-containing intermediates were investigated by ultraviolet−visible (UV−vis) spectrometry, hydrogen nuclear magnetic resonance (1H NMR), and density function theory (DFT) calculations. The major Ni-containing intermediates of Ni-OEP were dihydrogenated (Ni-OEPH2) and tetrahydrogenated (Ni-OEPH4) porphyrins during the hydrotreating. The structure of Ni-OEPH2 was speculated as 1,2-dihydroNi-OEPH2, which could be further hydrogenated into Ni-OEPH4. Ni-OEPH4 was first detected in the Ni-OEP hydrotreated products, which should be much more unstable than Ni-OEPH2 and could be easily demetallized. The main pathway of Ni-OEP conversion during the hydrotreating was the hydrodemetallization route, and Ni was removed with the porphyrin ring rapid fragmentation. The results indicate that the catalyst with high hydrogenation and hydrogenolysis activities on hydrocarbons as well as sulfur and nitrogen compounds could have a high activity on the HDM of Ni porphyrins.

1. INTRODUCTION Heavy petroleums, especially the vacuum residuum, are highly enriched in heteroatoms, such as sulfur (S), nitrogen (N), oxygen (O), vanadium (V), and nickel (Ni). Among these heteroatoms, the most abundant and problematic are V and Ni. The vanadium concentration varies from 8 to 1200 ppm in the petroleum, whereas the Ni concentration varies from 4 to 150 ppm.1 These metal compounds have a detrimental effect on the catalysts in the hydrotreating process, especially for the hydrodemetallization (HDM) catalysts.2−5 Therefore, the study of metal compound removal is essentially important for the heavy oil processing and catalyst design. It is difficult to investigate the transformation of metal compounds in the heavy oil directly, because the composition of heavy oil is complex and the detection methods for the metal compounds in the petroleum are limited.6−11 Therefore, many scientists studied the HDM mechanism by model compounds. Three types of metal compounds (Ni and V) were common in the HDM kinetics studies: metal etioporphyrins (Etio type), metal tetraphenylporphyrins (TPP type), and metal tetra(3methylphenyl)porphyrins (T3MPP type).12−23 On the basis of the characterization of mass spectra and ultraviolet−visible (UV−vis) spectra, the sequential HDM mechanisms had been proposed by the team of Wei.12−21 It is found that, for the Etio type metal compounds (Ni and V), the major hydrotreated intermediates were Ni-etiochlorin (Ni-EPH2) and VO-etiochlorin (VO-EPH2), while for the TPP type and T3MPP type (Ni and V), more hydrotreated intermediates have been found, such as Ni-PH2, Ni-PH4, Ni-PHx, VO-PH2, and VO-PH4.12−21 It is significant to identify the hydtrotreated porphyrin © XXXX American Chemical Society

intermediates for the HDM kinetics study. However, as a result of the limitation of analytical techniques before, the hydrotreated porphyrin intermediates were not fully understood. Other hydrotreated intermediates could be present in the products, which have been suspected by many authors previously,12−21,24 and it is unclear whether there was a direct demetallization route for the Ni-OEP conversion. Besides, the pretreatment of the separation method for the hydrotreated products reported may result in the loss or conversion for the hydrotreated prophyrin intermediates.12−21 In this paper, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was used to detect the hydrotreated porphyrin intermediates directly. FT-ICR MS25−39 and density functional theory (DFT) calculations30,40−58 have been used to study the S, N, O, Ni, and V compounds in petroleum and their transformations over the catalytic surface in the hydrotreating. FT-ICR MS could provide high broadband mass resolving power and mass accuracy, which enables the assignment of a unique elemental composition to each peak in the mass spectra. The hydrotreated products can be characterized directly by FT-ICR MS coupled with a selective ionization source, such as electrospray ionization (ESI) without pretreatment, which can provide more information on the HDM reaction.34−37 The structures of heteroatoms could be analyzed by hydrogen nuclear magnetic resonance (1H NMR).59−61 In addition, the DFT calculation Received: April 20, 2016 Revised: August 18, 2016

A

DOI: 10.1021/acs.energyfuels.6b00952 Energy Fuels XXXX, XXX, XXX−XXX

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280 °C, total pressure of 5 MPa, H2/oil of 1:200, and LHSV of 30 h−1. After a stabilization period of 6 h, the product oil, which was signed as P2, was obtained as well. 2.2. Chromatographic Separation. To identify the structures of Ni-OEP hydrotreated intermediates, product oils (P1 and P2) were further separated into two subfractions by the silica gel chromatographic method and then the subfractions were analyzed by FT-ICR MS, UV−vis, and 1H NMR. The separation was carried out on a glass column (1 × 80 cm), which was packed with 100/200-mesh silica gel (about 50 cm high). First, 50 mL of n-hexane was used to wet the silica gel. Then, 10 mL of hydrotreated product of P1 or P2 was added on the top of the column, until it was fully dispersed into the silica gel. Solvents with various polarity were used for the elution. First, 200 mL of n-hexane was used to wash the white oil in the products, leaving NiOEP and its products on the silica gel, and then the mixed solvent of dichloromethane and n-hexane (1:1) was used to elute the first subfraction (the light green fraction on the silica gel was washed down), which was signed as S1, and then eluted with dichloromethane to obtain the second subfraction (the red fraction on the silica gel), which was signed as S2. It was reported that compounds in S1 were mainly the hydtrotreated porphyrin intermediates with a green color, and compounds in S2 were mainly Ni-OEP, which were not converted in the feed, with a red color.12 2.3. Chemical Characterization. An UV/2102PCS UV−vis spectrophotometer (Shanghai UNICO Instruments Co., Ltd.) was used to characterize the feed and its hydrotreated products. Highperformance liquid chromatography (HPLC)-grade toluene (Productor) was used as a reference solvent. The content of Ni-OEP in the feed, P1, and P2 were analyzed by the UV−vis spectroscopy at the wavelength of 552 nm by the standard curve method using white oil as a solvent. The calibration factor was 1.75 ppm/Abs for Ni-OEP (552 nm). The total content of Ni in the feed, P1, and P2 was determined by an atomic emission spectrometer (ICP, optima 7000 of PerkinElmer Company), and the detailed steps were as follows: About 1 g of oil was ashed in a muffle furnace at 550 °C for 12 h after ignition. The ash was dissolved in 2 mL of nitric acid (7.4 mol/L) and 0.5 mL of hydrochloric acid (6 mol/L) and then transferred to a 25 mL volumetric flask, where it was evenly mixed for measurement. The solution from the volumetric flask was introduced into the spectrometer to determine the Ni content. Total N was measured by oxidative combustion and chemiluminescence at hightemperature combustion in an oxygen-rich atmosphere. The 1H NMR spectra was measured in CDCl3 at room temperature (295 K) on a Bruker AV400 spectrometer (400.13 MHz for 1H) and referenced to residual solvent protons of tetramethylsilane-free CDCl3 (δ 7.25). 2.4. ESI FT-ICR MS Analysis and Data Processing. The feed, P1, P2, and their subfractions were diluted with toluene to 10 mg/mL solution, and then they were diluted with toluene/methanol (1:1, v/v) solution to yield 0.0002−0.0010 mg/mL solutions (resolved by the

could provide the detail structural information on these heteroatom compounds in the petroleum. All of these methods have become important and efficient ways to study the transformation of heteroatom compounds in the vacuum residuum hydrotreating.40−61 In this work, the combination of FT-ICR MS with the DFT calculation method was used to study the transformations of Ni-OEP model compounds in the HDM reaction. The molecular formulas of Ni compounds were determined in the hydrotreated products by FT-ICR MS. To identify the structures of the hydrotreated intermediates, the products were also further separated into subfractions by silica gel chromatography and were characterized by 1H NMR and UV− vis spectra. In combination of the results of FT-ICR MS, 1H NMR, and UV−vis spectra with the DFT calculation, the possible transformation route in the Ni-OEP HDM reaction was well-developed.

2. EXPERIMENTAL SECTION 2.1. HDM Reaction. The feed was prepared by dissolving the NiOEP model compounds in the white oil. The white oil was marketed under the name Nujol purchased from China Petrochemical Group Great Wall Lubricating Oil Group Co., Ltd. Nujol is a mixture of saturated hydrocarbons consisting primarily of naphthenes. There were no S and N compounds in the white oil, and it was essentially inert at the reaction conditions.12−21,24 Ni-OEP was purchased from J&K Scientific, Ltd. [American Chemical Society (ACS) 24803-99-4]. The required amount of Ni-OEP was dissolved in the white oil, and then the solution was warmed under argon flow from room temperature to 80 °C and held at 80 °C for 1 h to eliminate dissolved oxygen in the solvent. The solution was then warmed to 300 °C, maintained at this temperature for 4 h, then cooled to room temperature, and filtered. The resultant solution contained 12 ppm (weight) of Ni. The scheme of HDM reaction processing and hydrotreated product pretreatment was shown in Scheme 1. The HDM reactions were carried out in the high-pressure fixed-bed down-flow microreactor (20 mL) packed with 5 mL of commercial HDM catalysts. The reaction over the oxidic NiMo/Al2O3 catalyst was as follows: the feed was fed continuously to the reactor at a temperature of 280 °C, total pressure of 5 MPa, H2/oil of 1:200, and liquid hourly space velocity (LHSV) of 30 h−1. After a stabilization period of 6 h, the product P1 was sampled. Another reaction over the sulfide NiMo/Al2O3 catalyst was performed as follows: prior to the reaction, the oxidic NiMo/Al2O3 catalyst was presulfided with a solution of 2 wt % CS2/cyclohexane for 6 h at a temperature of 320 °C, H2/oil of 1:200, LHSV of 10 h−1, and total pressure of 4 MPa. After the sulfidation, the flow was switched to the feed and the reaction conditions were switched to the temperature of B


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hydrogenolysis of SP3 C−N bonds;63−65 therefore, it is difficult for the N removal during the HDM reaction. The feed, P1, and P2 were analyzed by +ESI FT-ICR MS after the HDM reaction within 12 h, which ensures that more hydrotreated Ni-containing intermediates can be detected before their conversion or decomposition. Broadband and expanded +ESI FT-ICR MS spectra of the feed, P1, and P2 are shown in Figure 2. Ni-OEP in the feed can be detected in the form of the molecular ion [C36H44N4Ni]+ at m/z 590.291 43 and the protonated ion [C36H44N4Ni + H]+ at m/z 591.299 22, which was in agreement with the previous reports that there were two formations of Ni porophyrins in the mass spectra (protonated and molecular ions).26,37 Other peaks in the feed at the mass range from 592 to 595 were mainly the isotope peaks of C36H44N4Ni, and the detailed information was shown in Table S1 of the Supporting Information. Isotope peaks could be used to further identify the compounds in the mass spectra.26,34,37 In this paper, by the analysis of isotope peaks, it can be sure that the peak of m/z 591.299 22 was identified as Ni-OEP with the molecular formula of [C36H44N4Ni + H]+. In P1, the mass peak of m/z 592.307 05 shows a relatively high intensity and the detailed mass peak information was shown in Table S1 of the Supporting Information. The mass spectra of both [C36H46N4Ni]+ and [C36H46N4Ni + H]+ and their isotope peaks can be found accordingly in the mass spectra of P1 from m/z 592 to 597, which indicates that the probably hydrotreated Ni-containing intermediate molecule with two more H atoms on C36H44N4Ni was present, and its molecular formula was C36H46N4Ni. At the same time, the mass peaks of [C36H48N4Ni]+ and its isotope peaks were also found in P1, which implied that another hydrotreated Ni-containing intermediate with four more H atoms on Ni-OEP were also present, and it was signed as Ni-OEPH4. It was reported that Ni-EPH2 was the major hydrotreated intermediates for Nietioprophyrin, and it was just speculated that there may be NiEPH4 and other hydrotreated Ni-containing intermediates in the products.12−21 In our study, it is the first time to find NiOEPH4, which is significant for us to further understand the Niporphyrin HDM reaction mechanism. Figure 2 and Table S1 of the Supporting Information show the measured mass spectra of P2. Over the sulfide NiMo/Al2O3 catalyst, Ni-OEPH2 and Ni-OEPH4 were still present in the HDM reaction, although the sulfide NiMo/Al2O3 catalyst shows a higher HDM reactivity than the oxidic NiMo/Al2O3 catalyst. It can be inferred that Ni-OEPH2 and Ni-OEPH4 were the hydrotreated intermediates in the Ni-OEP HDM reaction, despite the different activities over the oxidic and sulfide NiMo/Al2O3 catalyst surfaces. By analysis of the mass peaks in P1 and P2, it can be seen that there were no other hydrotreated Ni-containing intermediates in the product oils, such as NiOEPH6, etc. However, the relative content of Ni-OEPH4 at m/ z 594.322 75 was 2.39% compared to that of Ni-OEPH2 in P1 and so was that in P2. It may be the reason that Ni-OEPH4 was not stable and converted or hydrodemetallized easily. Figure 3 shows the broadband and expanded spectra of feed, Ni-P1, and Ni-P2 for the theoretical demetallized porphyrins. It is very significant to know how the metal (Ni) was removed, because most porphyrins are synthesized by making the ring structure and then adding the metal atom. However, there are no mass spectra peaks in Figure 3 for the theoretical demetallized porphyrins (C 36 H 44 N 4 , C 36 H 46 N 4 , and C 3 6 H 4 8 N 4 ) from the metal-containing porphyrins (C36H44N4Ni, C36H46N4 Ni, and C36H48N4 Ni), which indicates

vanadium/porphyrin content), which can minimize the molecular aggregation. To enhance the ESI ionization efficiency, 5 μL of formic acid was added for +ESI analysis in the diluted samples and then the samples were analyzed in the +ESI FT-ICR MS analysis. The condition of +ESI FT-ICR MS was reported elsewhere.35 Mass spectra were internally calibrated from N1 homologous series ([CnH2n − 10N1 + H]+) of high relative abundance in a heavy oil mixture within the mass range of 500−800 Da and recalibrated by a single-point correction method with the peak of [C36H44N4Ni1 + H]+ from the mass spectra, which exhibited high relative abundant +ESI mass spectra. The details of the data analysis procedure used have been described elsewhere.62 2.5. DFT Computational Details. It is well-established that an accurate geometry structure and spectroscopic description of the calculation result strongly depend upon many factors, such as density functional, basis sets, and solvent effect. For metal porphyrins, the structures were often optimized using Becke’s three-parameter hybrid functional using the Lee−Yang−Parr correlation functional (B3LYP). For central metal Ni, the LANL2DZ basis set was selected as the appropriate basis set for optimization calculation and the 6-311G** basis set was performed great for the other non-metal atoms. The optimized geometry structures could be obtained by the calculation, and all of the computations in this study were performed by the Gaussian 09 program package.

3. RESULTS AND DISCUSSION 3.1. Characterization of Ni Compounds in the Hydrotreated Products. The concentrations of total Ni, Ni-OEP, and N in the feed, P1, and P2 were shown in Figure 1. In the

Figure 1. Concentration of total Ni, Ni-OEP, and total N in the feed, Ni-P1, and Ni-P2.

feed, the concentrations of total Ni, Ni-OEP, and total N were 11.82, 11.82, and 11.50 ppm, respectively. In P1, the concentrations of total Ni and Ni-OEP decrease to 6.53 and 5.71 ppm, respectively, indicating that the hydrotreated Nicontaining intermediates were present in P1. The concentrations of total Ni and Ni-OEP in P2 were 6.46 and 5.52 ppm, which indicates that the hydrotreated Ni-containing intermediates were also present. However, in comparison to the N concentration in the feed (11.50 ppm), the N contents in P1 and P2 were in a relatively high concentration (11.20 and 10.39 ppm, respectively). It may be the reasons that most N compounds cannot be converted in the HDM because Ni-OEP has a relatively higher hydroconversion. Except for N in NiOEP, these N compounds may be in the fragments of the broken porphyrins. It is reported that hydrodenitrogen (HDN) has required extensive hydrogenation of aromatic C prior to C


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Figure 2. Mass spectra of feed, Ni-P1, and Ni-P2.

Figure 3. Broadband and expanded spectra of feed, Ni-P1, and Ni-P2 for the theoretical demetalated porphyrins: (a) molecular ion and (b) protonated ion.

3.2. Molecular Structure of Ni-OEPH2. To further identify the Ni-containing intermediates, the products (P1 and P2) were separated into two subfractions by silica gel chromatography (as shown in Scheme 1). The UV−vis spectra of feed, P1, and P2 using white oil as a solvent are shown in Figure 4a. The characteristic Ni-OEP peaks were at 394, 518, and 553 nm. However, there was a new peak at 616 nm in P1 and P2. It was reported that the peak at 616 nm belongs to NiOEPH2.12−24 The UV−vis spectra of sub1 and sub2 were shown in Figure 4b, and it can be seen that the characteristic

that there is no direct removal of Ni prior to Ni-OEP hydrogenation, and the main demetallized route for Ni-OEP was the HDM route reported by the team of Wei. Ni-OEPH2 and Ni-OEPH4 were the main hydrotreated Ni-containing intermediates during the HDM reaction rather than hydrogenated byproducts of Ni-OEP. It also indicates the main pathway of Ni-OEP conversion: Ni-OEP was first hydrogenated to Ni-OEPH2 and further hydrogenated to Ni-OEPH4. Finally, Ni-OEPH4 decomposed via a series of fast reactions, which were ending in demetallization and ring fragmentation rapidly. D


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peaks of Ni-OEPH4 in the UV−vis spectra, and it may be the reason for the low concentration of Ni-OEPH4 in the hydrotreated products. Broadband and expanded +ESI FT-ICR MS spectra for NiP1, Ni-sub1, and Ni-sub2 in Figure 5 show that [C36H46N4Ni]+ and [C36H46N4Ni + H]+ are the major mass peaks in the spectra of sub1, indicating that Ni-OEPH2 was the mainly hydrotreated Ni-containing intermediate in Ni-sub1. Ni compounds in the Ni-sub2 fraction were mainly Ni-OEP, which was not converted in the HDM reaction. However, there were no mass peaks of [C36H48N4Ni]+ in the subfractions, although it can be found in the mass spectra of P1. It may be the reason that Ni-OEPH4 was in a relatively low concentration in P1, and it may be lost or converted in the silica gel separation pretreatment, although silica gel chromatography was an efficient way to separate different polar fractions in the oil pretreating.29,34,35 Figure 6 shows 1H NMR of Ni-OEP and Ni- P1-sub1 in CDCl3, and Table 1 shows the 1H NMR data for Ni-OEP and Ni-OEPH2 in CDCl3. According to the previous reports by Chicarelli et al.,60 it can be seen that the characteristic 1H NMR spectra peaks for Ni-OEP were in the position of 9.79 ppm (singlet), 3.96 ppm (singlet), and 1.84, 1.82, and 1.81 ppm (triplet). 1H NMR peak at 9.79 ppm represents H-5, H-10, H15, and H-20 in the Ni-OEP molecule, shown in Figure 6. The 1 H NMR peak at 3.96 ppm is the representation of H atoms in CH2-b, and the H atoms of CH3-a have a characteristic triplet peak at 1.84, 1.82, and 1.81 ppm. These 1H NMR peaks were calculated, and the relative area values were shown in Table 1. According to the relative area, we can obtain the relative content of these different H atoms in the molecule. It can be seen that the relative contents of three types of H atoms in NiOEP were corresponding (close to the ratio of 1:4:6). 1 H NMR spectra of Ni-P1-sub1 show characteristic peaks of both Ni-OEP and Ni-OEPH2. Data of 1H NMR in Table 1 show that there were eight group peaks in Ni-P1-sub1. The singlet peak at the position of 9.77 ppm represents both the H5, H-10, H-15, and H-20 in the Ni-OEP molecule and H-10 and H-15 in Ni-OEPH2. Similarly, the group peaks of 1.83, 1.82, and 1.80 ppm in the 1H NMR spectra were the representation of H atoms of CH3-a in both Ni-OEP and NiOEPH2. However, the two new H atoms (CH-1 and CH-2) in

Figure 4. (a) UV−vis spectra of feed, Ni-P1, and Ni-P2 using white oil as a solvent and (b) UV−vis spectra of Ni-P1-sub1 and Ni-P1-sub2 using toluene as a solvent.

peaks of Ni-OEPH2 were at 397, 554, and 616 nm. These findings were in agreement with the results from the team of Wei, who found that 390 and 610 nm were the characteristic peaks for Ni-EPH2.12−21 However, there were no characteristic

Figure 5. Broadband and expanded +ESI FT-ICR MS spectra for Ni-P1, Ni-P1-sub1, and Ni-P1-sub2. E


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Figure 6. 1H NMR of Ni-OEP and Ni-P1-sub1 in CDCl3.

Table 1. 1H NMR Data for Ni-OEP and Ni-OEPH2 in CDCl3 name

δ (ppm)

Ni-OEP

Ni-OEPH2

a

multiplicitya

assignment

relative area

9.79 3.96 1.84, 1.82, 1.81 9.77

s s t s

1.00 3.98 5.91 6.45

7.69−7.73 7.50−7.54 4.32, 4.30, 4.24, 4.23, 3.95, 3.93, 1.83, 1.82, 1.67−1.74

m m t q q t m

H-5, H-10, H-15, H-20 CH2-b CH3-a H-5, H-10, H-15, H-20 (Ni-OEP) H-10, H-15 (Ni-OEPH2) H-5 H-20 CH-1 CH-2 CH2-b, CH2-c, CH2-d CH3-a CH3-e, CH3-f

4.29 4.22, 4.20 3.92, 3.90 1.80

1.00 1.03 0.99 0.99 27.21 38.89 6.44

s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; and b, broad.

[dihedral (Ni, N1, N2, and N3) = 0.019°; dihedral (C1, N2, N3, and N4) = 0.275°; dihedral (C2, N2, N3, and N4) = 0.030°; dihedral (C3, N2, N3, and N4) = 0.480°; and dihedral (C4, N2, N3, and N4) = 0.694°], and the bond length of Ni−N was 1.995 Å, with eight ethyls attached to the porphyrin core in the different spatial positions. The C−C bond lengths in the porphyrin core were between 1.381 and 1.455 Å, which was between the normal bond length of C−C (1.54 Å) and CC (1.34 Å). It indicates that the porphyrin core was a conjugate system with a strong coordination bond of Ni−N. In Ni-OEPH2, the bond lengths of C−C, C−N, and N−Ni were changed as a result of two more H atoms added in the porphyrin core. It can be seen from Table 2 that the bond length of R1 increased from 1.455 to 1.564 Å and that of R2 also changed from 1.381 to 1.501 Å. The bond length of R5 and R6 also increased by 0.095 and 0.102 Å, respectively. However, other bond lengths, such as R3, R4, R7, R8, R9, and R10, decreased in comparison to the bond lengths of Ni-OEP, and they decreased by 0.049, 0.021, 0.051, 0.040, 0.023, and 0.031 Å, respectively. Besides, it can be seen from Figure 7 that the structure of Ni-OEPH2 was distorted in comparison to that of Ni-OEP. Ni was almost still in a planar structure with four N atoms [dihedral (Ni, N1, N2, and N3) = 4.984°], but the C1, C2, and C3 atoms in Ni-OEPH2 were in a different plane with

Ni-OEPH2 respond to the new group peaks at 4.32, 4.30, and 4.29 ppm and at 4.24, 4.23, 4.22, and 4.20 ppm in the 1H NMR spectra. As a result of the effect of the two new H atoms in NiOEPH2, the group peaks of 7.73−7.69 ppm in the 1H NMR spectra belong to H-5 and the group peaks of 7.54−7.50 ppm belong to H-20 in Ni-OEPH2 accordingly. The H atoms of CH2-b, CH2-c, and CH2-d have four peaks at 3.95, 3.93, 3.92, and 3.90 ppm in the 1H NMR spectra. The group peaks in the range of 1.74−1.67 ppm belong to the H atoms of CH3-e and CH3-f. Relative area data show that the H content ratio of H-5, H-20, CH-1, and CH-2 was close to 1:1:1:1, which was corresponding with the structure of 1,2-dihydro-Ni-OEPH2. Thus, by the analysis of the 1H NMR spectrum for Ni-OEP and its hydrotreated intermediates, it comes to the conclusion that the structure of Ni-OEPH2 was probably 1,2-dihydro-NiOEPH2, shown in Figure 6, which was very important for us to further understand the properties of Ni-containing intermediates in the Ni-OEP hydrotreating. 3.3. DFT Simulation on Molecular Structures of NiOEPH2 and Ni-OEPH4. The molecular structures of Ni compounds were simulated by the DFT calculation method shown in Figure 7. Table 2 shows the bond lengths (Å) of NiOEP and Ni-OEPH2. It can be seen that the C, N, and Ni atoms of Ni-OEP were almost in the same planar structure F


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Figure 7. Optimized structural schematics of (a and b) Ni-OEP and (c and d) Ni-OEPH2.

DFT for Ni-OEP (a) and Ni-OEPH2 (b). Frontier molecular orbitals are very important in the theoretical calculation because they can provide more information on the molecular activity. The energy gap (ΔEL−H) between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) can reflect the molecular activity and redox properties.66,67 Furthermore, the local regions where the molecular orbitals are localized represent the most probable reactive sites. In most of the studies, the HOMO (a1u), HOMO − 1 (a2u), LUMO, and LUMO + 1 (eg) orbitals of metal porphyrins are localized on the ligand macrocycle rather than on the central metal atom; that is, the oxidation and reduction processes are almost carried out on the macrocycle. As shown in Figure 8, the introduction of hydrogen will cause slight modification of HOMO and LUMO orbitals. As a consequence, the calculated frontier molecular orbitals of Ni-OEP are localized on the carbon atoms of the whole macrocycle, which are almost composed of Pz orbitals of carbon atoms. However, the frontier molecular orbitals of Ni-OEPH2 changed the shape and distribution. This change means that the active site on the macrocycle may focus on the hydrogenated pyrrole ring; thus, further hydrogenation probably occurs on the ring. From the energetic point of view, as shown in Table 3, the introduction of hydrogen in NiOEP will cause the degeneracy decrease of LUMO and LUMO

Table 2. Bond Lengths (Å) of Ni-OEP and Ni-OEPH2 by DFT Calculation bond length (Å) bond R1 R2 R3 R4 R5 R6 R7 R8 R9 R10

C1−C2 C2−C3 C3−C4 C4−N1 C1−N1 C1−C20 N1−Ni N2−Ni N3−Ni N4−Ni

Ni-OEP

Ni-OEPH2

1.455 1.381 1.455 1.389 1.389 1.391 1.995 1.995 1.995 1.995

1.564 1.501 1.406 1.368 1.484 1.493 1.944 1.955 1.972 1.964

the porphyrin plane [dihedral (C1, N2, N3, and N4) = 17.327°; dihedral (C2, N2, N3, and N4) = 15.561°; dihedral (C3, N2, N3, and N4) = 13.567°; and dihedral (C4, N2, N3, and N4) = 3.967°]. It can be inferred that the properties of NiOEPH2 were not as stable as those of Ni-OEP, and it can be further converted in the hydrotreating. DFT simulation can also provide the chemical properties of Ni-OEP and Ni-OEPH2. Figure 8 shows the schematic representation of the frontier molecular orbitals calculated by G


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Figure 8. Schematic representation of the frontier molecular orbitals calculated by DFT for Ni-OEP and Ni-OEPH2.

could improve the conversion of Ni-OEP to Ni-OEPH2 and NiOEPH4, while the high hydrogenolysis property could contribute to the ring fragmentation of Ni-containing intermediates in the hydrotreating. Therefore, a catalyst with high hydrogenation and hydrogenolysis properties could be more appropriate for Ni removal, which was in agreement with our previous study.5

Table 3. DFT Calculated Energy Data of the Frontier Molecular Orbitals for Ni-OEP and Ni-OEPH2 molecular formula Ni-OEP

Ni-OEPH2

orbit

molecular orbital eigenvalue

LUMO + 1 LUMO HOMO HOMO − 1 ΔEL−H (eV) LUMO + 1 LUMO HOMO HOMO − 1 ΔEL−H (eV)

−2.147 −2.149 −5.116 −5.365 2.967 −1.758 −2.716 −4.112 −5.217 1.396

4. CONCLUSION In this work, the transformation of Ni porphyrins in the HDM was investigated using 2, 3,7,8,12,13,17,18-nickel-octaethylporphine (Ni-OEP) as the model compound. The major Nicontaining intermediates in the Ni-OEP hydrotreating were dihydrogenated (Ni-OEPH2) and tetrahydrogenated (NiOEPH4) porphyrins. Ni-OEPH4 was first detected in the hydrotreated products, which was significant to the kinetic study of the metal porphyrin hydrotreating. The structure of Ni-OEPH2 was probably 1,2-dihydro-NiOEPH2 and was distorted in comparison to the structure of NiOEP, and its orbital energy gap of ΔEL−H shifts down from 2.967 eV for Ni-OEP to 1.396 eV for Ni-OEPH2, which shows an unstable property, and could be further hydrogenated to NiOEPH4. Ni-OEPH4 was first detected in the Ni-OEP hydrotreated products, which should be much more unstable than Ni-OEPH2 and could be easily demetalized. The main pathway of Ni-OEP conversion during the hydrotreating was the HDM route, and Ni was removed with the porphyrin ring rapid fragmentation. This work indicates that a catalyst with both high hydrogenation and hydrogenolysis activities could have a high activity on the hydrogenation of Ni porphyrins.

+ 1 orbits, in which the LUMO + 1 orbital energy shifts up from −2.147 to −1.758 eV but the LUMO orbital energy shifts down from −2.149 to −2.716 eV; at the same time, the HOMO orbital energy shifts up from −5.116 to −4.112 eV. As a result, the energy gap of ΔEL−H shifts down from 2.967 eV for Ni-OEP to 1.396 eV for Ni-OEPH2, which shows that the molecular activity was enhanced obviously, while the molecular stability was decreased. As a consequence, the characteristic peaks in the electronic absorption spectra for Ni-OEPH2 should have a longer wavelength. This is indeed found in the experimental UV−vis spectra for Ni-OEPH2 in Figure 4. It can be inferred that Ni-OEPH4 may be the structure of two more H atoms in Ni-OEPH2. There may be various structures for Ni-OEPH4 because two more H atoms may be added to different positions of Ni-OEPH2. We just give three possible structures of Ni-OEPH4 calculated by the DFT method, which were shown in Figure S1 and Table S2 of the Supporting Information. However, it was definite that the structures of NiOEPH4 were much more distorted than that of Ni-OEP and Ni-OEPH2, and they have a low orbital energy gap of ΔEL−H (2.1396, 2.4174, and 2.6150 eV, respectively), making NiOEPH4 more unstable and leading to their fast reactions ending in demetallization and ring fragmentation. That is why NiOEPH4 was in a relatively low concentration in the hydrotreated products. Therefore, it can be inferred that Ni-OEP may be first hydrogenated to Ni-OEPH2 in the hydrotreating, and this hydtrotreated Ni -containing intermediate was then further hydrogenated to Ni-OEPH4 over both oxidic and sulfide NiMo/Al2O3 catalysts. Finally, Ni-OEPH4 reacts via a series of fast reactions ending in demetallization and ring fragmentation. This study also indicates that, for the Ni-OEP kinetic study, the major hydrotreated intermediates were Ni-OEPH2 and NiOEPH4 in the hydrotreating. For the HDM catalyst design, it can be inferred that a catalyst with high hydrogenation activity

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b00952. Three possible structures of Ni-OEPH4 by the DFT method (Figure S1), measured mass spectra and their isotope peaks in feed, Ni-P1, and Ni-P2 (Table S1), and DFT optimized energy values for three possible NiOEPH4 (Table S2) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-10-8973-3501. E-mail: [email protected]. Author Contributions †

Linqing Ju and Tingting Liu contributed equally to this work.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC, U1362203 and 21376262). REFERENCES

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Transformation of Nickel Octaethylporphine in Hydrodemetallization

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