Hydrotreating Performance of FCC Diesel and ... - ACS Publications

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Kinetics, Catalysis, and Reaction Engineering

Hydrotreating performance of FCC diesel and Dibenzothiophene over NiMo supported zirconium modified Al-TUD-1 catalysts Bo Wang, Chengkun Xiao, Pengfei Li, Zhenshan Zhao, Chunming Xu, Zhen Zhao, Qian Meng, Jianmei Li, Aijun Duan, and Zhentao Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01214 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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Industrial & Engineering Chemistry Research

Hydrotreating performance of FCC diesel and Dibenzothiophene over NiMo supported zirconium modified Al-TUD-1 catalysts Bo Wang†, Chengkun Xiao†, Pengfei Li, Zhenshan Zhao, Chunming Xu, Zhen Zhao, Qian Meng, Jianmei Li, Aijun Duan*, Zhentao Chen* 1 State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, 102249, P. R. China [†] These authors contributed equally to this work *Corresponding author: [email protected]; [email protected]

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Industrial & Engineering Chemistry Research 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

Abstract Zr modified mesoporous alumina Al-TUD-1 with different Zr contents were successfully synthesized via sol-gel method and the optimal thermal treatment time was 4 h. The Characterization results of HRTEM, XPS and pyridine FT-IR revealed that Zr atoms acted as electronic promoters, not only enhanced the sulfidation degree and modulated the morphology of MoS2 active phases, but also brought more acidities into the alumina framework. Additionally, the sulfided catalysts were evaluated using dibenzothiophene (DBT) and FCC diesel as feedstocks respectively. NiMo/ZrAT-100 catalyst with the largest pore size, the highest active metal sulfidities, as well as the highest total acid and B/L value exhibited the highest HDS (99.1 %) and HDN efficiencies (98.7 %) in diesel hydrotreating reaction; moreover, it possessed the largest reaction rate constant in DBT HDS reaction. Furthermore, a possible reaction network was also proposed, in which DDS route was the predominant pathway for DBT HDS. Keyword: Zr modification; Al-TUD-1; Diesel; Dibenzothiophene; Hydrotreating

1. Introduction Due to the urgency of oil products upgrading and the increasingly stringent environmental regulations, the sulfur contents of transportation fuels are limited to no more than 10 µg/g.1 For achieving ultra-deep hydrodesulfurization, catalytic hydrotreating technique is thought to be the most efficient process in comparison with other industrial-scale technologies, such as adsorption and oxidative desulfurization,2 meanwhile some highly refractory sulfides such as dibenzothiophene (DBT) and alkyl-substituted dibenzothiophene must be effectively removed.3 Thus, the design and application of novel catalyst with high activity and selectivity but low cost are crucial. Since the catalytic activity is intimately related to the physicochemical properties of the support, the design of catalysts arouses much interest of researchers; therefore, some novel silicon-based (SBA-15, FDU-12, MCFs) and carbon-based (activated

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carbon, carbon nanotubes) catalysts were successfully synthesized and utilized.4-8 Although their tunable pore structures and high specific surface areas seemed beneficial to the reaction performance, the poor mechanical strength and high cost as well as regeneration problem still remained being solved.9 Porous alumina is by far the most widely used support material for the hydrotreating catalyst. However, the broad pore size distribution derived from the agglomeration of Al2O3 particles is not conducive to the hydrotreating reaction. One solution might introduce mesoporous channel into Al2O3 particles, which can be achieved by the self-assembly behavior of organic and inorganic species.10 Davis et al11 successfully synthesized the mesoporous alumina with surface areas and pore sizes of 710 m2/g and 2 nm using carboxylic acids to be the template agent. Kim et al12 reported a synthetic route using stearic acid to be the structure-directing agent, through varying the H2O/Al ratio to obtain mesoporous aluminas with various surface areas and pore diameters (300-500 m2/g, 7.7-3.5 nm). Unfortunately, these materials encountered a collapse of mesostructured channels after the surfactant removal which resulted from the possible strong interaction between surfactant and alumina skeleton. Recently, nonionic surfactant was proved to be a potential template for the synthesis of mesoporous aluminas. Grant et al13 reported a pathway using F127 to be the template to obtain a cage-like ordered mesoporous alumina. Liu et al9 prepared a series of ordered mesoporous NiMo/Al2O3 catalysts via a modified solvent evaporation-induced self-assembly (EISA) method by using triblock copolymer P123 as structure directing agent and ethanol as solvent, and then they were compared with the industrial γ-Al2O3 supported catalyst in DBT hydrodesulfurization reaction. The results showed that all the ordered mesoporous alumina catalysts possessed higher activity and stability, which was attributed to the effect of ordered channels as well as the interaction of active phases and the solvent. However, the high cost of such surfactant and the relatively long crystallization period greatly limited their industrial application. Al-TUD-1, a kind of stable amorphous mesoporous alumina material with the

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TUD-1 structure type, was firstly synthesized by Shan et al14. Its three-dimensional pore structure and relatively narrow pore size distribution efficiently reduced the diffusion resistance of large molecules and increased the accessibility of active sites and reactants15. Additionally, the specific surface area could be controlled via changing the H2O/Al ratio in the initial synthesis process, which varies in the range of 250-530 m2/g15. It’s well-known that the high surface area can supply more space for a better dispersion of active metals;16 therefore, Al-TUD-1 is thought to be an alternative support for hydrotreating catalysts. However, it’s worth noting that the strong metal support interaction (MSI) of alumina supported catalysts makes active metal oxides hard to be sulfided, resulting in the formation of Ni-Mo-S (I) phases, which possess lower stacking layers and expose less brim and edge sites, thus result in lower HDS activity.17 Gutiérrez18 found that the addition of Ti and Zr species could enhance the intrinsic hydrotreating activity of alumina supported catalyst. Zhang et al19 prepared a series of Al2O3-ZrO2 composite supports via chemical precipitation method, and they found that the corresponding NiMo supported catalysts possessed better performance than traditional NiMo/Al2O3 catalyst in FCC diesel hydrotreating reaction as loading suitable Zr species. The ZrO2 overlayers resulted in a decrease of MSI, thus the reducibility and sulfidity of the active components were correspondingly modified. Moreover, the incorporation of Zr could significantly increase the Lewis acidity and improved the surface acid distributions, which favored to the HDS performance.20 Combining the prominent physicochemical properties derived from the Zr incorporation with the excellent textual properties of Al-TUD-1, a series of zirconium modified Al-TUD-1 supports with various Zr contents were synthesized via direct synthetic method in this research. Then the corresponding NiMo supported catalysts were evaluated with dibenzothiophene as the probe molecule and FCC diesel as the feedback. Various characterizations were performed to investigate the influence of the Zr incorporation on the catalytic activities, which will be indicative to the design of HDS catalyst with high activity. Moreover, the effects of reaction temperature and

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WHSVs on the DBT HDS activities were systematically investigated, then the possible network of dibenzothiophene HDS over NiMo/Zr-Al-TUD-1 catalysts was proposed as well.

2. Experimental Section 2.1 Al-TUD-1 and Zr-Al-TUD-1 The mesoporous structure Al-TUD-1 was synthesized according to the literature14 using non-surfactant organic molecules tetraglycol (TEG) as template, aluminium isopropoxide as Al source. After stirring and drying, the xerogel was thermally treated under different heat treatment times (2 h, 4 h, 6 h and 10 h) to investigate the best synthetic procedure, and the corresponding Al-TUD-1 materials were named as AT-x, among which x means the heat treatment time. Zr modified TUD-Al2O3 materials were simply prepared via direct-synthetic method by aging, drying, hydrothermal treating and calcining the suspension mixture with a molar ratio composition of 1(Al(i-PrO3)): 1(TEG): 8(EtOH): 6(i-PrOH): 20(H2O): x(ZrOCl2•8H2O). Typically, aluminium isopropoxide (Al(i-PrO3)) was dissolved into a certain amount of anhydrous ethanol (EtOH) and isopropanol (i-PrOH) mixture with stirring at 313-323 K. Then, a certain amount of zirconium oxychloride (ZrOCl2•8H2O) was dropped into the solution and the mixture was maintained agitation for 4 h. After that, the template agent TEG was added, following by dribbling the mixture of water and the residual alcohol. The suspension obtained above was aged under room temperature for 24 h, and then dried at 371 K to form a xerogel. Afterward, the xerogel were thermally treated in Teflon bottle for 4 h under the condition of 433 k. Finally, the zirconium aluminate precursor was calcined at 873 K for 10 h with a temperature rising rate of 1℃/min. Zr-containing Al-TUD-1 materials were noted as ZrAT-x, where x means the Al/Zr ratio (x=100, 50, 25, 10 and 5, corresponding to the weight percentage of ZrO2 of 2.4 %, 4.6 %, 8.8 %, 19.5 % and 32.6 %). For comparison, the commercial γ-Al2O3 support was taken as the reference, which was obtained from Aluminum Corporation of China Limited. 5



2.2 Catalyst The corresponding NiMo supported catalysts were prepared via a two-step incipient impregnation method with firstly dropping heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O) aqueous solution and secondly impregnating nickel nitrate hexahydrate (Ni(NO3)2·6H2O) aqueous solution.1 After each impregnation step, the obtained materials were dried at 353 K for 12 h, followed by calcined at 823 K for 6 h under the air condition. All the as-synthesized catalysts had the same active metal loadings (15.5 wt% MoO3 and 3.5 wt% NiO). 2.3 Characterization The small-angle XRD (2θ=0.5-5o) characterization were performed using a powder X-ray diffractometer (Shimadu XRD 6000) with the Cu kα radiation. The wide-angle XRD (2θ=5-35o) profiles were recorded on a Bruker D8 Advance Powder diffractometer using the Cu kα radiation. The N2 adsorption isotherms were obtained on a Micromeritics ASAP 2010 system at liquid nitrogen temperature. The pore sizes distributions were obtained using the Barrett–Joyner–Halenda (BJH) method derived from the adsorption branch. And the specific surface areas were calculated from Brunauer-Emmett-Teller (BET) method. The total volumes of micro and mesopores were estimated from the adsorbed amounts of nitrogen at the relative pressure of P/P0=0.99. The surface contents of Al, Zr, Mo and Ni were obtained through SEM-EDS analyses on a Quanta 200F instruments with the accelerate voltages of 5 kV, in combination with an EDAX genesis 4000 energy-disperative X-ray spectrometer (EDS). And the surface acid distributions and amounts were analyzed using pyridine as probe molecule on a MAGNAIR 560 FTIR spectrophotometer with a resolution of 1 cm-1. The Raman spectra of the catalysts were characterized on a Renishaw Invia Raman spectrometer with the laser line at 325 nm of a He/Cd laser. And X-ray photoelectron spectra (XPS) characterization of the sulfided catalysts were measured on a Thermo Fisher K-Alpha spectrometer with Alkα (hν = 1484.6 ev) X-ray source.

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Before the spectra testing, all binding energies were corrected by taking C 1s spectrum (Binding energy = 284.6 ev) as a standard. Transmission electron microscopy (TEM) and the high-resolution transmission electron microscopy (HRTEM) images were observed on a Philips Tecnai G2 F20 STWIN microscope with an accelerating voltage of 300 kv. The average slab length (Lav) and stacking number (Nav) were calculated based on the statistical information of MoS2 phases, which determined from 20 micrographs containing more than 300 slabs for each sulfided catalyst. The equations were listed as follow: n

n

Lav = (∑ ni li ) / ∑ ni i =1

(1)

i =1

n

n

Nav = (∑ ni Ni ) / ∑ ni i =1

(2)

i =1

2.4 Catalytic performance Hydrotreating performace of FCC diesel (the typical properties of the feedstock are listed in Table S7 of the Supporting Information), follows the procedure of 2 g catalyst with the mesh number of 40-60 loaded into the middle section of a fixed bed high pressure micro-reactor (8mm inner diameter and 400 mm in length), diluted by quartz sand at both ends. Then, they were in-situ presulfided for 4 h using 2.5 wt% CS2 in cyclohexane solution to be the sulfiding agent under the conditions of 613 K, 4 MPa and H2/Oil of 600 ml·ml-1. The HDS performance was evaluated at 623 K, 5 MPa, H2/Oil of 600 ml·ml-1 and WHSV of 1.0 h-1. In DBT HDS reaction, 1 g catalyst (40-60 mesh) were loaded in the reactor, followed by in-situ presulfidation for 4 h at 613 K and 4 MPa with the mixture of H2 and CS2 in cyclohexane solution. After sulfidation, the DBT in decalin solution which contains 500 ppm S was pumped into the reactor and the reaction were underwent under the conditions of 573 K, 6MPa, and H2/Oil kept at 200 ml·ml-1, while WHSV varied from 20 to 120 h-1. In addition, the effect of reaction temperatures (573-633 K) on the sulfur removal efficiency was also investigated. After reaction, the sulfur and nitrogen contents of reactants and products were

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analyzed by RPP-2000 SN sulfur and nitrogen instrument, the deviation of which is within 2 µg·ml-1. The corresponding HDS and HDN efficiencies were calculated as follow: HDS efficiency (%) = (Sf - Sp)/Sf × 100%

(3)

HDN efficiency (%) = (Nf - Np)/Nf × 100%

(4)

Where Sf and Nf mean the sulfur and nitrogen contents of feedstocks, and Sp and Np mean those of products. Phase equilibrium experiment was performed on the phase equilibrium instrument (Eurotecnica), which is a tank reactor with the perspective window. According to the H2/oil ratio, the molar fractions of solvent and H2 were calculated, followed by filling specific amounts of solvent and H2 into the tank and stirring for a better interaction. After heating, the phase state of solvent was observed from the perspective window and the corresponding image was recorded by the camera. To study the product compositions and reaction pathways, the liquid products were analyzed by offline gas chromatography with a mass spectrometer (Thermo-Finnigan Trace DSQ) with a HP-5 (60 m × 0.25 mm × 0.25 µm) column. And the composition of sulfides in FCC diesel was analyzed by GC-SCD instrument (Agilent 355) with a HP-5 (30 m × 0.25 mm × 0.25 µm) column. 2.5 Kinetic analysis The pseudo-first-order kinetic model (Equation (5)) was applied in DBT hydrodesulfurization reaction based on the following three hypotheses: (1) the HDS reaction is irreversible; (2) the hydrogen content is far larger than the sulfur content; (3) there are no apparent deactivation of catalyst during the reaction.21 k HDS =

1 F ln( ) m 1−τ

(5)

Where kHDS is the reaction rate contant of DBT hydrodesulfurization reaction (mol g-1 h-1), F means the molar flow rate of feedstock (mol h-1), m is the loading amount of catalyst (gram), and τ is the HDS efficiency of sulfur. Moreover, the catalytic activities of different NiMo supported catalysts were compared in terms of turn-over frequency (TOF), which was defined as the number of

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DBT molecules reacted per second and per Mo atoms located on the edge of MoS2 slabs.22 It can be calculated using the equation (6).23

TOF = (F × x) / (nMo × fMo )

(6)

fMo means the fraction of Mo atoms located on the edge of MoS2 stackings, which can be calculated by the equation (7) and (8).24 t

∑ (6 n

i

fMo =

− 6)

i =1

(7)

t

∑ (3n

2 i

− 3ni + 1)

i =1

ni =

L + 0.5 6.4

(8)

Where ni is the number of Mo atoms on the edge sites of MoS2 stacking, and L is the length of MoS2 slabs, which are determined from HRTEM images.

3. Results 3.1 Characterizations of AT-x material with different thermal treatment times The pore formation of Al-TUD-1 material depends on the condensation of aluminum species and aggregation of TEG, which are cooperative as well as competitive and indispensable. Shan et al14 reported that thermal treatment was the external driving force to accelerate the organic aggregation and the inorganic condensation, and therefore facilitated to the motivation of mesoporosity. During the experiment, they found that in the case of lower temperature (393 K), the mesopore size was obviously smaller than those thermally treated under 433 K, which derived from the relatively lower organic aggregation and the inorganic condensation rates. It’s well known that for the hydrotreating catalysts, large pore channel is favorable to the diffusion of reactant molecules and the accessibility between the molecules and the active sites. Therefore, a series of Al-TUD-1 materials were thermally treated under 433 k, and the different heat treatment times were investigated in the present research. 3.1.1 XRD Figure S1 (in Supporting information) shows the powder XRD pattern of AT-x

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and the conventional γ-Al2O3 materials. In the wide angle domain, only a hump at 2θ = 67.3° is observed for all the AT-x materials, which is totally different from the characteristic peaks of γ-Al2O3 material (2θ = 37.6° and 45.84°, JCPDS 10-0425), confirming the different topological structure of Al-TUD-1 material. 3.1.2 BET The N2 isotherms and the corresponding pore size distributions of Al-TUD-1 materials with different thermal treatment times are shown in Figure S2 (in Supporting information). It is obvious that all the materials exhibit similar type IV isotherms with H2 hysteresis loops, meaning that the mesoporous structures are well-remained after thermal treatment. The detailed physical properties are summarized in Table S1 (in Supporting information). It can be found that with the extension of heating time, the pore sizes increase at first and obtain the peak value of 9.6 nm at the thermal treatment time of 4 h, and then decrease with the further heating. Therefore, the optimal heating time should be 4 h and the corresponding AT-4 material possesses the largest specific surface area and pore volume. 3.2 Characterizations of ZrAT-x supports 3.2.1 XRD analysis

m

A m

m t

B t

ZrO2

Intensity/a.u.

ZrAT-5 AT

Intensity/a.u.

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

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ZrAT-100 ZrAT-50

ZrAT-10

ZrAT-25 ZrAT-50

ZrAT-25

ZrAT-100

ZrAT-10

AT

ZrAT-5 0

1

2

3

4

5

6

7

8

20

25

2θ /degree

30

35

40

45

50

2θ /degree

Figure 1. Small-angle (A) and Wide-angle (B) XRD patterns of ZrAT-x materials

Small-angle and wide-angle XRD patterns of pure Al-TUD-1 and Zr modified supports are displayed in Figure 1A&B. In Figure 1A, all the samples exhibit a broad Bragg diffraction peak at around 1.0º, confirming that the incorporation of Zr has few

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influence on the mesoporous structure of AT material.25 According to the previous literature,26 this single diffraction peak is related to the randomly packed pores rather than the ordered arrangement of channels. Additionally, from Figure 1A, compared with AT materials, all the Zr containing supports exhibit relatively lower diffraction degrees, corresponding to the increased length of unit cell.5 However, for the ZrAT-x materials, it is obvious that the diffraction peaks become less-intensified as well as blue shift with the increase of Zr contents, meaning that suitable amount of Zr species contributes to the structure optimization.27 From the wide-angle XRD results (Figure 1B), for the Al-TUD-1 material, no discernable diffraction peaks is observed, suggesting the amorphous aluminum framework.28 Additionally, in Figure 1B, ZrO2 sample possesses five intense diffraction peaks at 2θ = 24.2°、28.19° and 31.48° (JCPDS. 36-0420), which are coincident with the monoclinic ZrO2 phases, while those at 30.17° and 34.20° (JCPDS. 37-1413) are the characteristic peaks of the tetragonal ZrO2 phases. For Zr-containing materials, when the Al/Zr ratios vary from 100 to 50, no peak belonging to ZrO2 appears, demonstrating that Zr atoms are completely fabricated into the TUD-1 framework or the sizes of ZrO2 nanoparticles are less than the XRD detection limit (4 nm).29 However, when loading excessive Zr species, tetragonal ZrO2 appears, indicating that only appropriate content of Zr species is suitable to realize a high dispersion of Zr species on the surface of the supports. 3.2.2 BET The N2 physisorption isotherms and the corresponding pore size distributions derived from the adsorption branches of the pure Al-TUD-1 and the Zr modified supports are displayed in Figure 2A&B. All the materials exhibit typical type IV adsorption isotherms with H2 hysteresis loops, indicating the existence of mesoporous structure with ink-bottle like pore, which correspond well to the XRD characterization results.30

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B

A Volume adsorbed/cm3g-1

ZrAT-5

ZrAT-5

Pore volume/cm3g-1

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

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ZrAT-10 ZrAT-25 ZrAT-50 ZrAT-100

ZrAT-10

ZrAT-25

ZrAT-50 ZrAT-100

AT 0.0

AT 0.2

0.4

0.6

0.8

1.0

0

Relative Pressure (P/P0)

20

40

60

80

100

Pore Diameter (nm)

Figure 2. N2 isotherm curves (A) and pore size distributions (B) of ZrAT-x materials Table 1. Textual properties of the AT and ZrAT-x materials

Materials ZrAT-5 ZrAT-10 ZrAT-25 ZrAT-50 ZrAT-100 AT

Weight percentage of ZrO2 (%) 32.6 18.5 8.8 4.6 2.4 0

SBET (m2 g-1)

VBJH (cm3 g-1)

Pore diameter (nm)

344 348 356 348 358 409

0.82 0.85 1.04 1.03 1.08 0.86

10.1 11.7 12.7 14.6 14.7 9.6

The detailed structural properties are summarized in Table 1. It can be found that all the modified materials exhibit larger pore diameters and volumes in comparison with the pure Al-TUD-1. This phenomenon may be resulted from the fabrication of Zr into the alumina framework since the atomic radius of Zr4+ (r=0.084 nm) is apparently larger than that of Al3+ (r=0.054 nm).31 Additionally, the broader pore size distribution may be caused by the change of unit cell size, which is derived from the substitution of Al atoms with the Zr atoms and has been proven by the peak shift in XRD characterization. Moreover, with the Al/Zr ratios vary from 100 to 25, the pore sizes and volumes decrease slightly and the specific surface areas are maintained around 350 m2/g. However, with the excessive adding of Zr species, the pore sizes and volumes decrease sharply. The pore blockage may be resulted from the aggregation of ZrO2, which is confirmed by the wide-angle XRD characterization. Comparably, ZrAT-100 support possesses the best textual and structure properties with the pore size of 14.7 nm, the surface area of 358 m2/g and the pore volume of 1.08 cm3/g.

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3.2.3 TEM characterization The TEM images of AT (Figure S3A in Supporting information) and ZrAT-25 materials (Figure S3B in Supporting information) show a typical 3D worm-like structure with irregular morphology, which is in accordance with the XRD characterizations (Figure 1A). Compared with the pure AT material, the pore structure of ZrAT-25 support retains the sponge-like structure, indicating that the in-situ incorporation of Zr species doesn’t destroy the topology of the parent Al-TUD-1 material. Furthermore, no apparent ZrO2 crystallites is observed, suggesting that the Zr species exhibit a high distribution in the alumina framework. 3.3 Characterizations of NiMo/ZrAT-x catalysts 3.3.1 Wide-angle XRD characterization Figure S4 (in Supporting information) shows the wide-angle XRD patterns of NiMo oxide catalysts. For the NiMo/ZrO2 catalyst, the peaks of tetragonal and monoclinic ZrO2 appear, meanwhile the peaks corresponded to the MoO3 and NiMoO4 phases (2θ = 25.7o and 28.7o respectively) can also be observed, revealing the nonuniform distribution of active metal oxides. This may be resulted from the relatively small specific surface area of ZrO2 support,32 which could not supply enough places for high dispersion. However, as to the series of NiMo/ZrAT-x catalysts, no peak attributed to bulk MoO3 is detected, indicating that the active metals are uniformly dispersed on the external surface and in the mesopores of the ZrAT supports. Suitable dispersed Mo species can promise the exposure of more brim and corner sites for the desulfurization reaction, and therefore conducive to the enhancement of intrinsic hydrotreating activity. 3.3.2 BET In order to obtain the physical properties of the NiMo/ZrAT-x catalysts, N2 adsorption-desorption characterization is performed, meanwhile the results are shown in Figure S5A&B (in Supporting information) and Table S2 (in Supporting information). From N2 isotherms (Figure S5A), the corresponding Zr-containing catalysts retain the parent shape characteristics of Al-TUD-1 materials, indicating that

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the impregnation of active metals doesn’t affect the configuration of alumina skeleton. From Table S2, the surface area, pore volume and pore size of the catalysts decrease to some extent after the impregnation of active metals, which can be attributed to the metal coverage and pore blockage. Furthermore, compared with NiMo/AT catalysts, the Zr containing catalysts with the suitable Al/Zr ratio (100 and 50) possess larger pore size. As shown in Figure S5B, this phenomenon may be derived from two respective reasons. One is the decrease of the small pore volumes (diameter ranging from 2 to 8.5 nm) caused by the nano-sized ZrO2 particles deposit in the pores of Al-TUD-1. The other is the increase of the unit cell size resulted from the incorporation of Zr, which forms larger pore.19 However, the textual properties of the materials exhibit a downtrend with the further increase of Zr content, comparably, NiMo/ZrAT-100 catalyst possesses the best pore structure, with the surface area of 268.0 m2•g-1, the pore volume of 0.41 cm3•g-1 and the BJH pore diameter of 15.9 nm 3.3.3 SEM-EDS To analyze the distributions of the surface atoms quantitatively and qualitatively, the energy-dispersive X-ray spectrometer (EDX) technique is performed to record the metals information of NiMo/ZrAT-25 and NiMo/AT catalysts, and the EDX results are displayed in Figure S6A&B (in Supporting information) respectively. From the quantitative results (the inserted table), NiMo/ZrAT-25 catalyst exhibits the analogous Mo & Ni contents with NiMo/AT catalyst, meaning that the active metals are homogeneously loaded on the surface of the supports. Additionally, it’s found that NiMo/ZrAT-25 catalyst exhibits the similar Al/Zr ratio with the theoretical value, indicating that Zr species are successfully fabricated into the alumina framework and have a relatively uniform dispersion 3.3.4 Raman Raman characterization is chosen to identify the states of active metal oxides presented on the NiMo/ZrAT-x catalysts, and the results were shown in Figure 3. The peak centered at 331 cm-1 is assigned to terminal vibration of Mo=O band in tetrahedral molybdate species, while the band at 943 cm-1 is the characteristic peak of Mo=O band in octahedral polymolybdate species. The latter Mo species reflect the

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relatively weak interaction between the active components and the support and can cause the formation of MoS2 phases with relatively higher stacking layer after the presulfidation, thus facilitate to the enhancement of intrinsic catalytic activity.29,32 Moreover, the absence of well-defined peaks at 667 cm-1, 818 cm-1and 994 cm-1, which are the characteristic peaks of MoO3 crystallites, demonstrates the homogeneously distribution of the active metal oxides on all the prepared NiMo/ZrAT-x catalysts.33

943cm-1 331cm-1 NiMo/ZrAT-5

Intensity/a.u.

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


NiMo/ZrAT-10 NiMo/ZrAT-25 NiMo/ZrAT-50 NiMo/ZrAT-100 NiMo/AT 200

400

600

800

1000

1200

Raman shifts (cm-1)

Figure 3. Raman characterization of NiMo/ZrAT-x catalysts

From Figure 3, it can be found that the series of NiMo/ZrAT-x catalysts display more intensified peaks at 943 cm-1 in comparison with the NiMo/AT catalyst, which should be resulted from the formation of ZrAl(Mo7O24)x species during the incipient impregnation process.34 Since all the catalysts prepared in this research were impregnated using an aqueous solution of ammonium heptamolybdate with a pH value of 5.6, according to the previous literature,35 the dominant Mo species exists in the form of HMo7O245-. Additionally, after Zr incorporation, the electronegative difference between Al and Zr (Pauling values are 1.61 and 1.33 eV respectively) makes the Zr sites more positive in comparison with the framework Al sites. Therefore, the positive nature of the Zr site can attract the aqueous HMo7O245- anion and contributes to a suitable Mo species distribution.34 This phenomenon reflects that the incorporation of Zr can modulate the MSI and affect the dispersion states of Mo

15



Page 16 of 42

species. Finally, NiMo/ZrAT-100 catalyst with the suitable amount of Zr displays the most intensified characteristic peak at 943 cm-1, thus exhibits the best hydrotreating performance in the catalytic evaluation procedure. 3.3.5 XPS In order to elucidate the chemical state of ZrOx and ZrAlOx surface species of the supported NiMo catalysts, XPS technique was performed. According to the previous literature,36 the peak positions of Zr4+ 3d5/2 and 3d3/2 are located at 182.2 eV and 184.6 eV respectively for the tetragonal ZrO2 material. However, as shown in Figure S7 (in Supporting material), all the Zr3d peak positions of the NiMo/ZrAT-x catalysts shift to lower binding energy region, indicating that the valence state of partial Zr species is less than 4. According to the peak shape, curve fitting is calibrated to confirm the valence state of Zr species and to calculate their proportions quantitatively. The detailed peak deconvolution is completed in accordance with the binding energy values (Zr 3d5/2) for Zr4+ and Zr3+, which are 182.2 eV and 181.1 eV respectively, and the gap between the binding energy value of Zr 3d3/2 and Zr 3d5/2 is kept at 2.4 eV; moreover, the peak area ratio of Zr 3d5/2 and Zr 3d3/2 is approximately fixed at 3:2. The presence of Zr3+ species confirms that the coordination difference between Al and Zr atoms in the sol-gel procedure causes the chemical environment change of Zr atoms and promotes the reduction of Zr4+, which is consistent with the previous study of Barrtra et al.37 The detailed fitting results are shown in Table 2, in which the values of Zr3+/(Zr4++Zr3+) in the oxidation state catalysts follow the order of NiMo/ZrAT-100

>

NiMo/ZrAT-50

>

NiMo/ZrAT-25

>

NiMo/ZrAT-10

>

NiMo/ZrAT-5. After the presulfidation procedure, all the Zr species exist in quadrivalence and the presentative pattern can be found in Figure S7F. The valence transition phenomenon confirms that Zr atoms could be the electronic donator and modulate the sulfidation of Mo and Ni active phases during the presulfidation process.

Table 2. XPS fitting results of Zr 3d spectra of the oxidation state catalysts

Catalysts

Zr4+

Zr3+ 16


Zr3+/Zrtotal[a]

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NiMo/ZrAT-100 NiMo/ZrAT-50 NiMo/ZrAT-25 NiMo/ZrAT-10 NiMo/ZrAT-50 [a]

ar.% (182.2 ev) 55.4 55.6 55.8 56.7 57.7

ar.% (184.6 ev) 36.9 37.1 37.2 38.0 38.4

ar.% (181.0 ev) 4.6 4.4 4.8 3.3 2.3

ar.% (183.4 ev) 3.1 2.9 2.2 2.0 1.6

7.7 7.3 7.0 5.3 3.9

Zrtotal=Zr4++Zr3+ To investigate the effect of Zr incorporation on the sulfidation degrees of Ni and

Mo species, the freshly sulfided catalysts are analyzed by XPS technique. The binding energies obtained for Zr 3d, Mo 3d, and Ni 2p photoelectron peaks were corrected for charge shifts, with the use of the C 1s = 285 eV peak as reference. Figure 4 shows the Mo 3d XPS spectra of NiMo/ZrAT-x catalysts, which are decomposed into three well-resolved peaks by using the software of XPSPEAK version 4.1.38 The detailed deconvolution standards are as follow: the binding energies of 3d5/2 and 3d3/2 spectra of Mo4+ (corresponding to MoS2) are at around 228.7 ± 0.1 eV and 231.8 ± 0.1 eV; while those at 230.5 ± 0.1 eV and 233.6 ± 0.1 eV are assigned to the Mo5+ species of oxysulfide phases (MoSxOy); Mo6+ species under oxidic form (MoO3) are at 232.5 ± 0.1 eV and 235.6 ± 0.1 eV; and the S2s binding energy is 226.1 ± 0.1 eV.1 All the intensity ratio of Mo 3d5/2 and Mo 3d3/2 is approximately fixed at 3:2.6 The detailed sulfidities are shown in Table S3 (in Supporting information), which are calculated using the fraction of Mo4+ phases in the total Mo species. It can be seen that the sulfidation degrees of Mo species follow the order of NiMo/ZrAT-100 > NiMo/ZrAT-50 > NiMo/ZrAT-25 > NiMo/ZrAT-10 > NiMo/ZrAT-5 > NiMo/AT, which exhibit the similar downward trend to the values of Zr3+/(Zr4++Zr3+). These phenomena further confirm the promotion effect of surface Zr3+ species. Therefore, NiMo/ZrAT-100 with suitable Zr contents and the highest Zr3+ fraction possesses the highest Mo sulfidity.

17



4+ Mo (5/2),(3/2)

B 4+ Mo (5/2),(3/2) 5+ Mo (5/2),(3/2)

Intensity/a.u.

6+ Mo (5/2),(3/2)

S2s

245

240

235

230

225

220

S2s

245

240

Binding energy/ev

D

5+ Mo (5/2),(3/2)

Intensity/a.u.

S2s

245

240

235

230

225

220

245

240

230

225

220

F 4+ Mo (5/2),(3/2)

Intensity/a.u.

5+ Mo (5/2),(3/2)

235

235

Binding energy/ev

S2s

240

220

S2s

4+ Mo (5/2),(3/2)

6+ Mo (5/2),(3/2)

245

225

5+ Mo (5/2),(3/2)

6+ Mo (5/2),(3/2)

Binding energy/ev

E

230

4+ Mo (5/2),(3/2)

4+ Mo (5/2),(3/2) 6+ Mo (5/2),(3/2)

235

Binding energy/ev

Intensity/a.u.

C

5+ Mo (5/2),(3/2)

6+ Mo (5/2),(3/2)

Intensity/a.u.

A

Intensity/a.u.

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 18 of 42

230

225

S2s

245

220

5+ Mo (5/2),(3/2)

6+ Mo (5/2),(3/2)

240

235

230

225

220

Binding energy/ev

Binding energy/ev

Figure 4. Mo 3d XPS spectra of the sulfided NiMo/ZrAT-x catalysts (A) NiMo/ZrAT-5, (B) NiMo/ZrAT-10, (C) NiMo/ZrAT-25, (D) NiMo/ZrAT-50, (E) NiMo/ZrAT-100, (F) NiMo/AT

The Ni 2p XPS spectra of the sulfided NiMo/ZrAT-x catalysts and their corresponding deconvolution results are shown in Figure 5. According to the previous literatures,6, 29, 39 the binding energy of NiMoS is located at 855.3 ± 0.2 ev, the peak appears at 861.2 ± 0.1 ev is attributed to the NiO species, and the peak centered at 852.2 ± 0.1 ev should be the characteristic peak of NiSx species. The detailed fitting results are summarized in Table S4 (in Supporting information), among which the total fractions of NiMoS and NiSx reflect the sulfidation degree of Ni species. It can be found that the Ni sulfidities follow the order of NiMo/ZrAT-100 > NiMo/ZrAT-50 > NiMo/ZrAT-25 > NiMo/ZrAT-10 > NiMo/ZrAT-5 > NiMo/AT, in accordance with the

18


Page 19 of 42

Zr and Mo fitting results. The promoting effect of Ni can be explained by NiMoS model,40 in which Ni is atomically distributed at the edge of the MoS2 structure. Since the Ni metal is able to generate a “spillover” of hydrogen, which transmits from the donor phase (NiSx) to the MoS2 recipient phase, creating or modifying the catalytic active site.40 Therefore, the Zr species incorporated act as electronic donator promote the sulfidities of Ni and Mo species, and the enhanced Ni reducibility further increase the Mo sulfidation degree through “spillover” hydrogen effect.

NiMoS

NiSx

NiO NiMo/ZrAT-100

NiMo/ZrAT-50

Intensity/a.u.

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


NiMo/ZrAT-25

NiMo/ZrAT-10 NiMo/ZrAT-5 NiMo/AT

866

864

862

860

858

856

854

852

850

Binding energy/ev

Figure 5. Ni 2p XPS spectra of the sulfided NiMo/ZrAT-x catalysts

3.3.6 HRTEM To investigate the existence of nano-sized ZrO2 crystallites on the surface of ZrAT-x materials, HRTEM images of NiMo/ZrAT-25 and NiMo/ZrAT-5 are shown in Figure 6A&B and the insets are the corresponding fast Fourier transform (FFT) images of the marked regions. Figure 6A shows lattice spacing of 0.318 nm, in good agreement with the d-spacing of the (101) plane of tetragonal ZrO2,41 indicating the existence of nano-sized tetragonal ZrO2. From the scale of HRTEM image, the particle sizes of ZrO2 are ~ 6 nm, demonstrating the slight aggregation existed as the Al/Zr ratio of 25. This result is in accordance with the wide-angle XRD characterization results (Figure S4), in which less-intensified tetragonal ZrO2 peak was observed. From Figure 6B, it is obvious that the ZrO2 particle with the size of 15 nm appears as the Al/Zr ratio as high as 10, confirming the further aggregation of

19



nano-sized tetragonal ZrO2, even to further produce the isolated ZrO2 phases consequently. The nano-sized ZrO2 phases reduce the surface exposed Zr species to be the electronic promoters and may also cause the slight blockage of the catalyst pore. Therefore, NiMo/ZrAT-100 catalyst possesses the highest Zr3+ fraction, the suitable sulfidity and the best textual properties.

Figure 6. HRTEM images of (A) NiMo/ZrAT-25, and (B) NiMo/ZrAT-10 at 101 plane; the inset images show the corresponding FFT patterns of the marked regions

To further investigate the influence of Zr incorporation on the dispersion of MoS2 crystallites, HRTEM characterization is carried out to obtain the representative images of NiMo/ZrAT-x catalysts, as shown in Figure 7A-F. To make a quantitative comparison, the slab lengths and layer numbers of MoS2 slabs of the sulfided catalysts are collected through the statistical analysis based on at least 25 micrographs with approximate 700 slabs taken from different parts of each catalyst, and the results are also summarized in Table S5 (in Supporting information). In Figure 7A, the MoS2 slabs of NiMo/AT catalyst are less stacked with the stacking number of 1 or 2, which are derived from the relatively strong interaction between the active phases and the Al-TUD-1 material. The intense force makes the active components hard to be sulfided, which implies that the majority of active sites keeps at NiMoS-I type,24 thus, the catalyst has a lower catalytic activity. However, all the Zr incorporated catalysts show comparably higher stacking numbers around 2-4 layers compared with NiMo/AT catalyst, indicating that the addition of Zr can weaken the metal-support 20


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interaction and modulate the dispersions of Mo active phases.

Figure 7. HRTEM micrographs of the sulfided catalysts (A) NiMo/AT, (B) NiMo/ZrAT-100, (C) NiMo/ZrAT-50, (D) NiMo/ZrAT-25, (E) NiMo/ZrAT-10, (F) NiMo/ZrAT-5

From Table S5, the average lengths of MoS2 crystallites over the sulfided catalysts increase in the order of NiMo/ZrAT-100 (3.11 nm) < NiMo/ZrAT-50 (3.18 nm) < NiMo/ZrAT-25 (3.22 nm) < NiMo/ZrAT-10 (3.29 nm) < NiMo/ZrAT-5 (3.35 nm) < NiMo/AT (3.39 nm). The average stacking numbers follow the order of NiMo/ZrAT-5

(3.02) >

NiMo/ZrAT-10

(2.89) > NiMo/ZrAT-25

(2.67) >

NiMo/ZrAT-50 (2.65) > NiMo/ZrAT-100 (2.50) > NiMo/AT (2.26), demonstrating that the incorporation of Zr species facilitates to the decrease of slab lengths and the increase of average stacking layers. Moreover, the dispersion degrees of MoS2 slabs which are reflected by the fMo values follow the order: NiMo/ZrAT-100 > NiMo/ZrAT-50 > NiMo/ZrAT-25 > NiMo/ZrAT-10 > NiMo/ZrAT-5 > NiMo/AT, in accordance with the XPS results. NiMo/ZrAT-x catalysts display shorter slab lengths and higher stacking degrees, corroborating that the incorporation of Zr is favorable to the formation of more NiMoS-II active phases. A suitable dispersion degree of MoS2 slabs is necessary for the catalysts to expose enough brim and corner sites for the perpendicular adsorption

21



of the reactant molecules through the S atoms, thus improving the sulfur elimination.41,42 Therefore, all the Zr containing catalysts possess higher sulfidities and better hydrotreating performance compared with NiMo/AT catalyst. However, it should be noticed that only the top layer of the multi-stacks can expose more brim and edge sites, which benefit to the hydrotreating reaction.5 Thus, if the stacking layers are too high, the exposure ratio of brim sites Mo will reduce as the Mo loading kept the same, and then the catalytic activity will correspondingly decrease. Therefore, NiMo/ZrAT-100 catalyst with the shortest MoS2 slab lengths and suitable stacking degrees exhibits a better dispersion of Mo and an appropriate sulfidation degree.

3.3.7 Py-FTIR Pyridine-FTIR characterization is used to analyze the strength and types of acid sites of NiMo/ZrAT-x catalysts, and the spectra are displayed in Figure S8 (in Supporting information). From previous researches,24, 29 the bands at about 1540 and 1640 cm-1 are assigned to the Brønsted acid sites, while the adsorption peaks at 1450 and 1610 cm-1 confirm the existence of Lewis acid sites. Moreover, the band centered at 1492 cm-1 is attributed to the pyridine molecules bound to both of Brønsted and Lewis acid sites. Table S6 (in Supporting information) lists the detailed acid amounts and distributions of NiMo/ZrAT-x catalysts, in which the total acid quantities are calculated based on the Py-FTIR spectra after degassing at 473 K, and the amounts of medium and strong acid sites are obtained from the IR pyridine adsorption spectra after degassing at 623 K. For NiMo/AT catalyst, there is a very small band of Brönsted acid sites observed in Figure S8 and the quantitative results in Table S6 show only 0.88 µmol g-1 Brönsted acid amounts, indicating the absence of Brönsted acidity over the porous alumina supported catalyst. However, with the incorporation of Zr species, the Brönsted and Lewis acidities increase significantly. The presence of Brönsted acidity may be resulted from the electronegativity difference between the Zr and Al atoms. When fabricating Zr atoms into the alumina skeleton, the existence of Zr atoms changes the electronic density around Al, causing the strength weakness of AlO-H bond on the surface of the catalysts, then benefiting to the formation of 22


Page 22 of 42

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Brønsted acid sites.43 Additionally, zirconia species are usually in a coordination number of 7 or 8 but the aluminum ions in alumina material are coordinated in 3, 4 or 5 states,44 when the octahedral coordinated zirconium atoms are incorporated into the alumina skeleton, the coordination unsaturated positions produce the unoccupied electronic orbits, which result in the formation of Lewis acid sites.45 As shown in Table S6, the total acid quantities and the medium & strong acid amounts decrease in the following order: NiMo/ZrAT-100 > NiMo/ZrAT-50 > NiMo/ZrAT-25 > NiMo/ZrAT-10 > NiMo/ZrAT-5 > NiMo/AT. Therefore, the moderate acidity and the synergetic effect of Brönsted and Lewis acidities make NiMo/ZrAT-100 catalyst exhibit a better hydrotreating performance. 3.4 Hydrotreating performance of FCC diesel

Figure 8. Hydrotreating performance of NiMo/ZrAT-x and NiMo/γ-Al2O3 catalysts

NiMo/ZrAT-x and the reference NiMo/γ-Al2O3 catalysts (the textual properties and acid properties are shown in Table S8&9 (in Supporting information)) are evaluated in a high pressure fixed bed reactor using FCC diesel as feedstock, and the evaluation results are shown in Figure 12 and Table S10 (in Supporting information). From Figure 8, the commercial NiMo/γ-Al2O3 exhibits relatively lower hydrotreating activities with the HDS efficiency 94.4 % and HDN efficiency of 93.5 %, while the NiMo/AT catalyst displays a better hydrotreating performance. This 23



difference may be resulted from the outstanding textual properties and relatively weaker acidity of NiMo/AT catalyst. As shown in Table S2 and Table S8, NiMo/AT catalyst possesses a more concentrated pore size distribution and larger average pore size in comparison with NiMo/γ-Al2O3 catalyst, which greatly reduce the diffusion resistance of reactants and enhances the active phase accessibility. Additionally, the larger surface area promises a suitable dispersion of active components on the surface of NiMo/AT catalyst, thus facilitates to a better catalytic activity. Furthermore, it can be found that after incorporating Zr species into the NiMo/AT catalyst, the HDS and HDN efficiencies are further enhanced, demonstrating the promoting effect of the Zr addition. As shown in Figure 8, NiMo/ZrAT-100 catalyst possesses the best hydrotreating activity with the HDS efficiency of 99.1 % and HDN efficiency of 98.7 %, in accordance with the Raman, XPS and py-FTIR characterizations. Figure 9 shows the GC-SCD pattern of FCC diesel. The X-axis is the polarity-based retention time, and the marked peaks are assigned to various sulfides listed in Figure 9. It is observed that the typical sulfides existed in FCC diesel feed are mainly composed of C1-C4 alkyl-BTs and C1-C3 alkyl-DBTs. After the HDS process over the NiMo/ZrAT-100 catalyst, an ultralow sulfur diesel (S < 10 ppm) is obtained, indicating that Zr modified Al-TUD-1 supported catalyst can be a potential alternative catalyst for the efficient removal of the alkyl substituted BTs and DBTs.

24


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C2-BT

C1-DBT DBT

C1-BT C3-BT C2-DBT

BT C4-BT

C3-DBT

0

2

4

6

8

10

12

14

16

18

20

22

Time (min)

Figure 9. GC-SCD image of FCC diesel feed

3.5 HDS results of DBT For further investigating the sulfur removal mechanism of NiMo/ZrAT-x catalysts, DBT, a representative highly refractory sulfide in the diesel fraction, is chosen to be the probe molecule.46 Moreover, in order to simulate the real reaction states of hydrotreating process, a solvent of decalin with high boiling point (cis-decalin: 496.97 K at 101.3 kPa; tran-decalin: 460.46 k at 101.3 kPa) was utilized to dissolve the reactant since under the traditional hydrotreating conditions, diesel is in the coexistence states of gas-liquid phases. With the utilization of phase equilibrium instrument, the phase statement of decalin under the actual reaction conditions (573 K, 6 MPa and H2/oil=200 ml·ml-1) can be actually observed and the image is exhibited in Figure S9 (in Supporting information). It should be noted that decalin shows the gas-liquid coexistent states. The HDS activities of DBT obtained over the series of NiMo/ZrAT-x and NiMo/AT catalysts at 573 K and different weight hour space velocities (WHSV) ranging from 20 h-1 to 120 h-1 are shown in Figure 10A&B. From Figure 10A, it can be found that the HDS efficiencies of all the catalysts increase with the decrease of WHSV, since WHSV is inversely proportional to the reaction time. Additionally, all the Zr containing catalysts display better sulfur removal performances, and the HDS efficiencies increase as the order of NiMo/AT < NiMo/ZrAT-5 < NiMo/ZrAT-10 < 25



NiMo/ZrAT-25 < NiMo/ZrAT-50 < NiMo/ZrAT-100. It is worth noticing that NiMo/ZrAT-100 catalyst under the condition of 573 K exhibits an outstanding HDS efficiency (99.2 %) at 20 h-1, which may be derived from its narrower pore size distribution, suitable stacking degree, high sulfidation degree and moderate acidity. 100

6

A

NiMo/AT NiMo/ZrAT-5 NiMo/ZrAT-10 NiMo/ZrAT-25 NiMo/ZrAT-50 NiMo/ZrAT-100

B

5

4

ln(1/1-x)

90

HDS activity/%

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 26 of 42

80


3

70 2

60

1

20

40

60

80

100

0

120

500

WHSV/h-1

1000

1500

2000

2500

3000

3500

m/F

Figure 10. (A) HDS results of DBT over NiMo/ZrAT-x and NiMo/AT catalysts, (B) First-order kinetic plots of DBT HDS (573 K, 6MPa, 200 ml/ml)

Moreover, for further comparing the catalytic activities of NiMo/ZrAT-x catalysts with the NiMo/Zr-SBA-15 and the commercial NiMo/γ-Al2O3 catalysts, the reaction temperature was increased from 573 K to 613 K while the pressure and H2/oil ratio were kept at 6 MPa and 200 ml/ml respectively. NiMo/ZrAT-5 catalyst with the relatively lower HDS efficiencies in the series of NiMo/ZrAT-x catalysts was chosen as the reference catalyst. The corresponding DBT HDS results of NiMo/ZrAT-5, NiMo/Zr-SBA-15 and NiMo/γ-Al2O3 obtained at different WHSVs are shown in Figure S10 (in Supporting information). It should be noticed that NiMo/ZrAT-5 catalyst exhibits the DBT HDS activity of 89.5 % at the WHSV of 120 h-1, which is around 1.2 and 1.5 time as those over NiMo/Zr-SBA-15 (73.5 %) and NiMo/γ-Al2O3 catalyst (61.1 %), respectively; furthermore, with the WHSV decreasing, the DBT HDS activity increases gradually and reaches as high as 99.9 % at 20 h-1 over the NiMo/ZrAT-5 catalyst. Therefore, NiMo/ZrAT-x catalysts possess the ultra-high sulfur removal ability. Comparing with the catalytic results of NiMo/ZrAT-5 catalyst in Figure 10A and Figure S10, it can be found that reaction temperature significantly influences the DBT

26


Page 27 of 42

HDS activity. Therefore, to systematically investigate the impact of the reaction temperature on the HDS activity, DBT HDS reactions were performed under various temperatures ranging from 573 K to 633 K with WHSV fixed at 100 h-1, and the results are shown in Figure 11. The HDS efficiencies follow the order of NiMo/ZrAT-100

>

NiMo/ZrAT-50

>

NiMo/ZrAT-25

>

NiMo/ZrAT-10

>

NiMo/ZrAT-5 > NiMo/AT, which are in good agreement with the HDS results under various WHSVs. It can be found that the HDS activities are kinetically enhanced with the temperature increasing, in accordance with the previous study.47 Comparably, the HDS efficiencies of all the NiMo/ZrAT-x catalysts are higher than 97.5 % at the WHSV of 20 h-1, among which NiMo/ZrAT-100 catalyst exhibits a highly efficient removal of sulfides with the HDS efficiency of 99.9 %, revealing its outstanding HDS potential. 100

90

HDS activity/%

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


80


70

60 300

320

340

360

°

T/ C

Figure 11. DBT HDS activities over NiMo/ZrAT-x catalysts at different temperatures (100 h-1, 6MPa, 200 ml/ml)

27



80

80

A 70 BP

70

Selectivity/%

60

Selectivity/%

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 28 of 42

50 40 CHB

30 20

BP

60 CHB 30 20

10

10

DCH

0 120

100

80

60

40

DCH

0

THDBT 20

THDBT 300

WHSV/h-1

320

340

360

T/ ° C

Figure 12. Selectivity of the final products over NiMo/ZrAT-100 catalysts (A) at different WHSVs (300 ℃, 6MPa, 200 ml/ml) and (B) at different temperatures (100 h-1, 6MPa, 200 ml/ml)

Figure 12A shows the product distributions of DBT HDS over NiMo/ZrAT-100 catalyst at different WHSVs at the temperature of 573 K and the correlated GC-MS patterns are shown in Figure S11. It can be found that the selectivity of biphenyl (BP) decreases with the increase of reaction time, while that of cyclohexylbenzene (CHB) exhibits an upward trend, indicating that for the NiMo/ZrAT-100 catalyst, low WHSV is conducive to the hydrogenation (HYD) pathway. However, the selectivity of BP is still higher than 50 % with the lowest WHSV, so the direct desulfurization (DDS) pathway is still the predominant route for DBT HDS. Therefore, it can be concluded that the value of HYD/DDS is greatly enhanced with the WHSV decreasing and these two pathways become almost of equally importance when the WHSV is 20 h-1. Additionally, the selectivity to form tetrahydrodibenzothiophene (THDBT) decreases with the decrease of WHSV, but the trend of dicyclohexyl (DCH) shows an inverse direction. Additionally, the relationship between the product distributions (Figure S12 (in Supporting information)) and the reaction temperatures is clearly displayed in Figure 12B, in which the selectivity to BP decreases, while CHB increases with the reaction temperature rising. These phenomena suggest that the high temperature can favor the HDS reaction via the HYD route more than the DDS route for the NiMo/ZrAT-100 catalyst. With the reaction temperature changing from 573 to 633 K, BP is always the primary product, demonstrating that DDS pathway is the preferential reaction route of

28


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the DBT HDS. In order to evaluate the influence of Zr fabrication on the reaction pathway of DBT HDS, the hydrotreated products obtained at the similar total DBT conversions of 50 % are analyzed using a GC-MS chromatograph (Figure S13 (in Supporting information)), and the possible reaction network of DBT HDS over the series NiMo/ZrAT-x catalysts is then depicted in Figure 13. It’s obvious that the network is composed of direct desulfurization (DDS) and hydrogenation (HYD) routes, in accordance to the previous literatures.5, 48 In the DDS pathway, sulfur atom is directly eliminated via hydrogenolysis of C-S bonds, forming biphenyl (BP) as the final product. As in the HYD pathway, one of the aromatic rings of DBT molecule is firstly hydrogenated to produce tetrahydrodibenzothiophene (THDBT), and then the sulfur atom is removed to produce cyclohexylbenzene (CHB) or the intermediate is further hydrogenated to form dicyclohexyl (DCH) [46]. DDS S

DBT

BP

HYD

Slow HYD

HYD

S

THDBT

DCH

CHB

Figure 13. Possible reaction network of DBT HDS over NiMo/ZrAT-x catalysts

The detailed DBT HDS product selectivities over the NiMo/ZrAT-x catalysts are listed in Table 3, in which the DDS/HYD ratios increase in the following order: NiMo/AT (3.84) > NiMo/ZrAT-100 (2.94) > NiMo/ZrAT-50 (2.78) > NiMo/ZrAT-25 (2.56) > NiMo/ZrAT-10 (2.27) > NiMo/ZrAT-5 (2.04). The NiMo/ZrAT-100 catalyst possesses the large DDS/HYD ratio of DBT HDS, which may be attributed to the highest B/L [49] and more MoS2 stacking degree.50 Then, according to the equation 5, first-order kinetic plots of DBT HDS are fitted and displayed in Figure 13B. The linear

regression

curves

indicate

that

DBT

HDS

reaction

follows

the

pseudo-first-order kinetic model, in accordance with the previous research.51 The 29



Page 30 of 42

reaction rate kHDS which reflected by the slope of the fitting curves are listed in Table 3. Furthermore, the TOF values of the DBT HDS over the NiMo/ZrAT-x series catalysts are also calculated using the equation 6. From Table 3, the TOF and kHDS values of the series catalysts follow the order of NiMo/ZrAT-100 (4.77 h-1, 13.5×10-4 mol•g-1•h-1) > NiMo/ZrAT-50 (4.59 h-1, 12.6×10-4 mol•g-1•h-1) > NiMo/ZrAT-25 (4.46 h-1, 11.9×10-4 mol•g-1•h-1) > NiMo/ZrAT-10 (3.58 h-1, 8.1×10-4 mol•g-1•h-1) > NiMo/ZrAT-5 (3.49 h-1, 7.9×10-4 mol•g-1•h-1) > NiMo/AT (3.01 h-1, 6.8×10-4 mol•g-1•h-1). It should be noticed that the kHDS of DBT HDS over NiMo/ZrAT-100 catalyst is more than 1.9 times of NiMo/AT catalyst, confirming the promoting effect of Zr incorporation. Table 3. Catalytic performance of DBT HDS over NiMo/ZrAT-x catalysts

Catalyst NiMo/ZrAT-100 NiMo/ZrAT-50 NiMo/ZrAT-25 NiMo/ZrAT-10 NiMo/ZrAT-5 NiMo/AT

kHDS×104 mol•g-1•h-1

R2 fit for kHDS

TOFa h-1

13.5 12.6 11.9 8.1 7.9 6.8

0.99 0.99 0.99 0.98 0.98 0.97

4.77 4.59 4.46 3.58 3.49 3.01

THDBT 0.58 0.44 0.41 0.36 0.31 0.59

Product Selectivityb HYD CHB DCH 22.67 1.95 23.85 2.17 25.32 2.49 27.21 2.91 29.45 2.99 18.40 1.66

DDS BP 74.7 73.54 71.79 69.52 67.24 79.36

a.

Number of reacted DBT molecules per hour and per Mo atom at the edge sites. Determined at about 50% of total DBT conversion by changing WHSV. c. HYD: THDBT + DCH+ CHB; DDS: BP

b.

To further investigate the catalytic stability of the catalysts, all the used catalysts after HDS reactions are characterized by BET characterization, the N2 isotherms and the detailed textual properties are shown in Figure S14 and Table S11. It can be found that all the used catalysts largely preserve the structural properties of the fresh catalysts since the textual parameters are just slightly decrease in comparison with the BET results of the fresh NiMo catalysts (Figure S5 and Table S2). Additionally, NiMo/ZrAT-100 and NiMo/AT catalysts are chosen to analyze the deactivation rate as a function of time. The overall performance is evaluated in DBT HDS reaction as the operating conditions are 573 K, 6 MPa, H2/oil ratio of 200 ml/ml, WHSV of 20 h-1 and the on-stream time of 100 h. The as-obtained samples were analyzed by the sulfur

30


DDS/HYDc 2.94 2.78 2.56 2.27 2.04 3.84

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and nitrogen analyzer for every 4 h, and the HDS activities are shown in Figure S15 (in Supporting information). From the reaction data, both of NiMo/ZrAT-100 and NiMo/AT catalysts exhibited the good stability after 100 h on-stream reaction. Finally, the re-usability of NiMo/ZrAT-100 catalyst is performed by batch feeding in 6 runs. The detailed evaluation is performed as follow: firstly, the NiMo/ZrAT-100 catalyst is evaluated at 20 h-1 with the operating conditions of 573 K, 6 MPa and H2/oil ratio of 200 ml/ml; then, the feed is stopped and the temperature is decreased to the ambient temperature; thirdly, re-increasing the temperature to 573 K and evaluating the catalyst with the WHSV of 20 h-1 again; fianlly, repeating the previous operations to continuously 6 runs. The corresponding HDS results are shown in Figure S16 (in Supporting information), it is clearly seen that HDS conversions are kept similar after operating 6 runs, indicating that the NiMo/ZrAT-100 catalyst is stable and reusable. This result is consistent with the time-on-stream result.

4. Discussion It’s well known that the catalytic activities of FCC diesel hydrotreating and DBT HDS are intimately related to the textual properties, acidity, MSI effect, sulfidation degree of catalysts as well as the morphologies of MoS2 active phases. The pore structure properties affected the accessibility of active phases and the diffusion of reactants. As shown in Figure 5, compared with the NiMo/AT catalyst, NiMo/ZrAT-x catalysts had relatively larger pore sizes and more concentrated pore size distributions, furthermore, NiMo/ZrAT-100 catalyst possessed the largest pore size (15.9 nm) as well as the highest surface area and pore volume (268 m2/g and 0.41 cm3/g). The open channels provided enough space for the uniform dispersion of active metals and a good diffusion of reactants and products, which made the MoS2 active phases more accessible. The enhancements of the active phase dispersion and sulfidity were the essential factors to improve the catalytic activity.52 The electronegative difference between the incorporated Zr atoms and the framework Al atoms resulted in the relatively positive Zr sites, which attracted HMo7O245- species during the impregnation process and 31



promoted the formation of ZrAl(Mo7O24)x species,34 thus, the incorporation of Zr modulated the MSI and the dispersion of active components. This explanation was confirmed by the Raman characterization results, in which the 943 cm-1 peak was more intensified after adding Zr. Additionally, the interaction between Zr and Al atoms through sol-gel process caused the formation of Zr3+ species,37 which acted as electronic promoters to improve the sulfidation degrees of Mo and Ni species. However, due to the aggregation of partial Zr species, the tetragonal ZrO2 sizes became increasingly larger with the increase of Zr incorporation, as shown in wide-angle XRD and HRTEM characterizations, thus the Zr3+/(Zr4++Zr3+) values decreased. Therefore, from XPS results, NiMo/ZrAT-100, which possessed the highest Zr3+/(Zr4++Zr3+) value (7.7 %), exhibited the greatest Mo sulfidity (57.5 %) and Ni sulfidity (69.9 %). Moreover, the morphologies of MoS2 affected the activity and DBT HDS selectivity. From HRTEM results, the strong MSI over the sulfided NiMo/AT catalyst resulted in the longest slab length (3.39 nm) and the lowest stack number (2.26), facilitating the formation of more “type I” active phases that possessed less stacks with insufficient brim and edge sites, and therefore contributed to a lower intrinsic catalytic activity. While with the addition of Zr, the active metal dispersion and MoS2 morphology were modulated and NiMo/ZrAT-100 catalyst exhibited the shortest Lav (3.11 nm) and a suitable Nav (2.50), confirming the existence of more “type II” NiMoS active sites with more stack layer numbers. Moreover, the fMo parameter of NiMo/ZrAT-100 catalyst (0.35) was obviously larger than that of NiMo/AT (0.30), suggesting the exposure of more edge Mo atoms and thus improved the catalytic activity. Therefore, NiMo/ZrAT-100 catalyst had the highest HDS activity with the TOF value of 4.77 h-1 and kHDS of 13.5×10-4 mol•g-1•h-1. Besides, an appropriate acidity and the synergetic of B and L acid sites are the essential factors for improving the catalytic activity and the product selectivity.49 The addition of Zr changed the electronic density and the coordination states around the framework Al atoms, thus greatly enhanced the acidities of Zr containing catalysts,

32


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which was confirmed by the pyridine FT-IR results. Compared with NiMo/AT catalyst, NiMo/ZrAT-x catalysts with stronger total acidities exhibited better catalytic activities, indicating that the number of acid sites had an influence on the activity. Furthermore, as shown in Table S6 and Table 3, the catalysts with higher B/L ratio displayed high DDS/HYD ratio, meaning that the acid site distributions affected the selectivity of the HDS pathways. Among all the catalysts, NiMo/ZrAT-100 possessed the strongest total acidity (59.47µmol g-1) and the highest B/L ratio (0.072), thus showed the best catalytic activity and the high DDS/HYD ratio (2.94). In all, the synergistic effects of the appropriate textual properties, proper acidity, desirable

sulfidity

and

moderate

MoS2

active

phase

morphology

made

NiMo/ZrAT-100 to be the best catalyst, which supplied a potential catalyst candidate for the industrial hydrotreating technique.

5. Conclusion Mesoporous alumina Al-TUD-1 materials were thermally treated under various times to investigate the optimal synthetic procedure, and the period of 4 h was confirmed to be the best. Then a series of Zr modified Al-TUD-1 were successfully synthesized under the optimal process via a sol-gel method using ZrOCl2•8H2O as Zr resources. After impregnating Ni and Mo active components, various characterizations were used to evaluate the effect of Zr incorporation on the physicochemical properties of the catalysts. The Raman characterization results showed that Zr incorpotation facilitated to the formation of polymolybdate species, and the pyridine FT-IR identified that the addition of Zr species enhanced the Brønsted and Lewis acidities. The XPS and HRTEM characterizations confirmed that the surface Zr3+ species could act as electronic promoters and modulated the MSI, which greatly enhanced the sulfidity and also improved the morphology of MoS2 active phases to expose more brim and edge sites. Among all the catalysts, NiMo/ZrAT-100 catalyst possessed the best diesel hydrotreating performance with the HDS efficiency of 99.1 % and HDN efficiency of 98.7 % with the conditions of 1 h-1, 623 K, 5 Mpa and 600 ml/ml. For DBT HDS 33



reaction, it exhibited the highest catalytic activity with the kHDS of 13.5×104 mol•g-1•h-1 and TOF value of 4.77 h-1, which were attributed to the synergetic effect of the appropriate textual properties, relatively high B/L ratio, desirable sulfidity and high stacking degree of the MoS2 active phases. Moreover, NiMo/ZrAT-100 catalyst displayed an outstanding sulfur removal ability of DBT with the HDS activity of 99.2 % under the operating conditions of 573 K, 6 MPa, 200 ml/ml and WHSV of 20 h-1, indicating that it would be a high efficient catalyst for industrial application. The possible reaction network for DBT HDS over NiMo/ZrAT-x catalysts were proposed, in which two reaction pathways were involved: DDS and HYD. The increase of temperature and the reduction of WHSV were conducive to the HYD selectivity, but the DDS route was truly the predominant reaction pathway. Furthermore, NiMo/ZrAT-100 possessed high DDS/HYD ratio of 2.94 due to the highest total acid amounts and the highest B/L ratio.

6. Acknowledges This work was financially supported by the National Science Foundation of China (U1463207, No.21676298, 21476257); a CNOOC project (CNOOC-KJ 135 FZDXM 00 LH 003 LH-2016); Opening Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology (2015K003); and CNPC Key Research Project and KLGCP (GCP 201401).

Associated Contents The typical property of diesel feedstock (Table S1); TEM image of AT and ZrAT-25 (Figure S1); Wide-angle XRD patterns of NiMo/ZrAT-x and NiMo/ZrO2 catalysts (Figure S2); Zr3d XPS spectra of the sulfide NiMO/ZrAT-x catalysts (Figure S3); The pyridine IR spectra of NiMo/ZrAT-x catalysts at 473 K and 623 K (Figure S4); Textual properties of NiMo/γ-Al2O3 catalyst (Table S2); Acid properties of NiMo/γ-Al2O3 catalyst (Table S3); HDS activities of NiMo/ZrAT-x series catalysts (Table S4); The phase equilibrium image of decalin under the reaction conditions (Figure S5); HDS result of NiMo/ZrAT-5 and NiMo/Al2O3 catalysts at different

34


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WHSVs (613 K, 6 MPa, 200 ml/ml) (Figure S6); The product distribution in the HDS of DBT over NiMo/ZrAT-100 at different WHSVs (573 K, 6 MPa, 200 ml/ml) (Figure S7); The product distribution in the HDS of DBT over NiMo/ZrAT-100 at different temperatures (100 h-1, 6 MPa, 200 ml/ml) (Figure S8); The product distributions in the HDS of DBT over different catalysts at about 50 % of the total DBT conversion (Figure S9); Time on stream over NiMo/ZrAT-100 and NiMo/AT catalysts (573 K, 6 MPa, 200 ml/ml) (Figure S10).

35



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Hydrotreating Performance of FCC Diesel and ... - ACS Publications

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