4,6-dimethyldibenzothiophene Hydrodesulfurization on Nickel

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4,6-dimethyldibenzothiophene Hydrodesulfurization on Nickel modified USY supported NiMoS Catalysts: Effect of Modification Method Wenwu Zhou, Meifang Liu, Yasong Zhou, Qiang Wei, Qing Zhang, Sijia Ding, Yanan Zhang, Tingting Yu, and Qingxiang You Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01113 • Publication Date (Web): 13 Jun 2017 Downloaded from http://pubs.acs.org on June 17, 2017

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4,6-dimethyldibenzothiophene Hydrodesulfurization on

Nickel

modified

USY

supported

NiMoS

Catalysts: Effect of Modification Method Wenwu Zhou, Meifang Liu, Yasong Zhou*, Qiang Wei, Qing Zhang, Sijia Ding, Yanan Zhang, Tingting Yu, Qingxiang You State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, PR China

KEYWORDS: 4,6-DMDBT HDS; Ni modified USY; NiMo/USY catalyst; NiMoS active phase; catalytic activity; HDS pathway

ABSTRACT

The

effect

of

nickel(Ni)

modification

methods

of

USY

zeolite

on

4,6-

dimethyldibenzothiophene(4,6-DMDBT) hydrodesulfurization(HDS) has been investigated. USY zeolites were modified by a small proportion of Ni via three different methods: in-situ synthesis method, ion exchange method and impregnation method, and these Ni modified USY zeolites were used as supports for NiMo sulfide catalysts. Ni modification methods have influences on the textural properties, acidity of Ni modified USY zeolites, Ni-Mo-S active phase morphology, dispersion of active metals, structure of Ni-Mo-S active phase, sulfidation degree of

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active metals, 4,6-DMDBT HDS reaction rate constant, turn over frequencies and selectivity of 4,6-DMDBT HDS pathways. Although the impregnation modification method promotes the dispersion of active metals most profound and the ion exchange modification method enhances the sulfidation degree of both Ni and Mo the most profound, these changes didn’t enhance the catalyst activity the most significantly. The superior catalytic activity was observed over catalyst NiMo/USNiY (in-situ synthesis method), which is closely related to the proportion of NiMoS structures in the Ni-Mo-S active phase. Among the investigated three modification methods, insitu synthesis method is the most suitable for the purpose of preparing highly active HDS catalysts for 4,6-DMDBT.

1. INTRODUCTION Complete reduction of organosulfur compounds from transportation fuels has received much attention due to the emitted SO2 has negative impacts on the atmosphere and our human being’s health and due to the more and more stringent environmental legislations all around the world, especially for the allowed sulfur content of as low as 10 µg·g-1 in some big oil consumption areas 1-4

. And HDS is considered as the most effective method in complete removing of sulfur from

transportation fuels 5-7. Literatures 4, 8 have reported that Ni(Co) promoted MoS2(WS2) supported on Al2O3 are the most commercially used HDS catalysts, which is considered effective in the removing of organosulfur compounds such as thiophene(T), benzothiophene(BT) and dibezothiophene(DBT) which are less refractory, but less effective in the reduction of organosulfur compounds such as 4,6-DMDBT because of the steric hindrance caused by the two methyl groups located at the 4 and 6 positions which would prevent the adsorption of the sulfur atom at the active sites of the catalysts through σ-adsorption9. And 4,6-DMDBT is reported to undergo HDS by direct desulfurization (DDS) pathway and hydrodesulfurization (HYD)


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pathway over HDS catalysts

3, 7, 10

: DDS produces 3,3’-dimethylbiphenyl (3,3’-DMBP) as the

desulfurized product, and HYD produces tetrahydro- and hexahydro- 4,6-DMDBT as intermediates, which can be further desulfurized to yield 3,3’-dimethylcyclohexylbenzenes (3,3’DMCHB) and 3,3’-dimethylbicyclohexyls (3,3’-DMBCH), respectively. And HYD is considered as the main HDS pathway because 4,6-DMDBT molecules are mainly adsorbed to the active sites via π-coordination. However, the HYD pathway is less effective in the removal of S atoms from 4,6 DMDBT compared with the DDS pathway, and the H2 consumption of the HYD pathway is relative high, thus it is more costly. To eliminate the steric hindrance and enhance the HDS efficiency of 4,6-DMDBT, zeolite supported Ni(Co)Mo(W) catalysts have received more and more attention in recent years because of their high cracking activity and hydrogenation ability10-13. Among all the investigated zeolites, Y zeolite is considered the most suitable for high refractory sulfur-containing compounds such as 4,6-DMDBT because its microporosity(with pore diameter of about 0.73*0.73 nm) is comparable to the size of 4,6-DMDBT molecules. However, the acid properties of Y zeolite is not appropriate for 4,6-DMDBT HDS and it should be modulated before it is employed as support of HDS catalysts5. And metals such as Pd, Pt, Rh, K, Mg, Ga, La, Bi and Re incorporation into Y zeolite have been reported as the most promising methods14-18. And because it is one of the most commonly used promoters in HDS catalyst, nickel is also often considered as one of the most promising metals in such modifications19. Lin and coauthors20 found that incorporation of nickel into Y zeolites has a significant influence on its acidity and pore structure properties. Prasanth and coworkers21 reported that Ni modified zeolites via conventional cation exchange method can benefit the adsorption, especially the chemical adsorption of H2, which should be beneficial for the hydrogenation of unsaturated compounds, including DBTs, in FCC diesels. Deimund and coauthors22 also modified BEA


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zeolite via ion exchange method, and they reported that because the aluminum content in zeolite, especially on the surface of it, is relatively high, and the aluminum species can interact with Ni2+ ions to counterbalance their positive charges and the acidity properties of BEA zeolite is thus modulated. Thus it is supposed that the interaction between MoS2 and Y zeolite, which is usually via the formation of Mo-O-Al bond23, would be weakened and the morphology of the active MoS2 slabs would be modulated. And the selectivity of 4,6-DMDBT HDS pathways is related to the morphology of the active Ni(Co) promoted MoS2 slabs and the interaction between the active metals and support

5, 9, 24, 25

. However, to the best of our knowledge, barely any papers have

focused on the topic that modification of Y zeolite with nickel has an effect on the active phase morphology and catalytic performances of the corresponding HDS catalysts in the HDS of inferior diesels and neither does the effect of Ni modification method has been reported so far. At present, we have synthesized NaY zeolite according to literatures reported elsewhere[] and the synthesized materials were ion exchanged by NH4NO3 aqueous solution and modified by nickel via impregnation method and ion exchange method, respectively. Additionally, NaNiY zeolite was also synthesized via insitu-synthesis method and ion exchanged by NH4NO3 aqueous solution. The corresponding Ni-Mo-S HDS catalysts employing the mentioned materials as supports were prepared. After that, the effects of these modification methods on both acidic and textural properties of the zeolites were investigated. Afterward, the effects of the modification methods on the active phase morphology were also investigated. Finally, the HDS performances of all the prepared catalysts were evaluated by employing a model diesel and the effects of Ni modification method on the HDS pathway selectivity over different catalysts were also investigated. 2. EXPERIMENTAL SECTION


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2.1. Materials Preparation. NaY zeolite with a crystallite size of about 100 nm and SiO2/Al2O3 of about 5.6 was synthesized via a hydrothermal method according to our earlier work26. NiY zeolite was also synthesized via the similar synthesis method, in a typical synthesis procedure, 2.0 g of NaOH(Beijing Modern Oriental Fine Chemical Co. Ltd., ≥99.8 %), 77 g of water glass(28 wt% SiO2, 9.1 wt% Na2O, and 62.9 wt% H2O), 3.25 g of NaAlO2(Beijing Yili Fine Chemistry Co. Ltd., ≥99 %) and 20 g of distilled water and 10 g of presynthesized inorganic structure directing agent were mixed and agitated for about 20 min, after that, 2.0 g of Al2(SO4)3(Beijing Modern Oriental Fine Chemical Co. Ltd., ≥99.8 %) and 2.5 g of NiSO4·6H2O(Beijing Modern Oriental Fine Chemical Co. Ltd., ≥98.0 %) dissolved in 10 g of distilled water was dropwisely added under vigorous stirring, and the precursor gel composition was 7.5:1:16:240 Na2O/Al2O3/SiO2/H2O. The gel was then aged at 40 °C for 18 h and then precrystallized at 65 °C in a teflon autoclave for 24 h. Finally, the pre-crystallized system was recrystallized at 95 °C for another 24 h, washed with distilled water, and calcined at 550 °C for 5 h. The as synthesized materials were firstly ion exchanged by NH4NO3(Beijing Yili Fine Chemistry Co. Ltd., ≥99 %) aqueous solution and then dealuminated by citric acid(Beijing Modern Oriental Fine Chemical Co. Ltd., ≥99.8 %) via the same method that we reported earlier and the obtained materials are denoted as USY and USNiY26, respectively. The obtained USY zeolite was modified by nickel via two different methods: ion exchange and impregnation, respectively. For the ion exchange method, 10 g of USY zeolite was added into 100 g of Ni(NO3)2 aqueous solution with a concentration of 0.3 mol/L at 95 °C under continuous agitation for about 3 h, then filtrated and calcined at 550 °C for about 4 h, such obtained material is denoted as NiUSY. For the impregnation method, 1.15 g of nickel nitrate hexahydrate(Beijing Modern Oriental Fine Chemical Co. Ltd., ≥99.8 %) was firstly dissolved in 11 g of distilled water and then


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impregnated to 9.7 g of USY zeolite, the sample was dried at room temperature and calcined at 550 °C for about 4 h, and such obtained material is denoted as Ni@USY. The mentioned USY, USNiY, NiUSY and Ni@USY zeolites were pelleted at 30 MPa for 30 min and then crushed into 20 to 40 mesh, and the Ni-Mo catalysts were prepared via an incipient wetness co-impregnation method with an aqueous solution of appropriate amounts of nickel nitrate hexahydrate and ammonium heptamolybdate tetrahydrate, then dried at 120 °C for 6 h after vaporization at room temperature overnight and calcined at 550 °C for 5 h. The corresponding catalysts are denoted as NiMo/USY, NiMo/USNiY, NiMo/NiUSY and NiMo/Ni@USY, respectively. For all the synthesized catalysts, the total concentration of NiO in the catalysts is 4 wt%, and the total MoO3 concentration was 12 wt% in the catalysts. 2.2. Materials Characterization. The powder X-ray diffraction (XRD) patterns of all the synthesized USY zeolite and nickel modified USY zeolites were carefully performed on a PANALYRICALl advance powder diffractometer by using Cu Kα radiation(40 kV, 40 mA)in the 2θ range of 5~35° with an interval of 0.1°. The crystallite size of the synthesized zeolite was observed by field emission scanning electron microscopy (SEM) on a Quanta 200F instrument, and the crystallite size was statistically analyzed by counting at least 500 crystals. The contents of SiO2 and Al2O3 in the synthesized zeolites and the contents of NiO and MoO3 in the catalysts were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a PE OPTIMA 5300DV instrument. The Ni contents on the zeolite surface and the covalent states of Ni and Mo in the catalysts were detected by X-ray photoelectron spectroscopy (XPS) on a VG ESCA Lab 250 spectrometer using Al Kα radiation as the excitation light source. Prior to the test, the C 1s peak with a binding energy of 284.6 eV and the Al 2p peak with a binding energy of 74.6 eV were used to calibrate the binding energy scale. N2 adsorption-desorption analysis on the


6

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mentioned zeolites after degassed at 200 °C with a pressure of 10-1 mbar for at least 4 h was also performed on a Micromeritics ASAP 2010 analyzer at -196 °C to observe the effect of modification method on the physical properties of USY zeolites, and the surface areas of all the mentioned zeolites are calculated according to the Brunauer-Emmett-Teller (BET) equation. To assess the acidity properties of all the mentioned zeolites, H2 temperature-programmed reduction (H2-TPR) and NH3 temperature-programmed desorption (NH3-TPD) were performed on an Autochem 2920 instrument equipped with a mass spectrometer, and pyridine-adsorbed Fourier transform infrared measurements (Py-FTIR) were conducted on a Magna 560 FT-IR analyzer. And finally, the HRTEM images of all sulfide catalysts were taken on a Philips Tecnai G2 F20 instrument with an operating electron voltage of 200 kV. At least 400 slabs taken from different parts of each catalyst were counted to determine the size distribution of molybdenum sulfide crystallites. We calculated the average length and stack number of MoS2 slabs using equations reported elsewhere9 as follows:

∑ nl Average slab length: L = ∑ n

= ∑ n N Average stack number: N ∑ n

(1)

(2)

where l is the length of the slab, n is the number of slabs with length l , and N is the number of layers in slab i. The MoS2 dispersion was also calculated using the following equation reported in the literature3: =

∑ 6( − 1)

= ∑ (3" − 3 + 1)

(3)

where is the number of Mo atoms located on the edges of NiMoS slabs, is the total number of Mo atoms, is the number of Mo atoms along one edge of a MoS2 slab


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determined from its length (L = 3.2(2ni - 1)Å), and $ is the total number of slabs determined by at least 400 MoS2 slabs taken from HRTEM images of different catalysts. 2.3. Catalytic Evaluation. 4,6-DMDBT HDS reactions catalyzed by different catalysts were carried out on a fixed-bed reactor with 8 mm inner diameter and 500 mm length loaded with 0.5 g of catalyst with a particle size of 0.59~0.84 mm in diameter diluted to 2 mL by quartz sand with the similar particle size. Before the assessment, the catalyst was presulfided by a CS2 cyclohexane solution with CS2 concentration of 2 %(volume) at 320 °C and 4 MPa for 5 h with a weight hourly space velocity (WHSV) of 10 h-1 and H2/oil(v:v) of 100. After the catalyst was sulfided, the reactor temperature was decreased to 280 °C, and a mixture of 0.5 wt% of 4,6DMDBT dissolved in cyclohexane was pumped to the reactor using an SZB-2 double-piston pump. The 4,6-DMDBT HDS reactions were performed with different WHSVs in the range 10 h1

to 160 h-1 and the H2/oil(v:v) was fixed at 200. The liquid products was carefully collected after

a stabilization period of 15 h(overnight) and immediately off-line analyzed on an Agilent 4890D gas chromatograph equipped with a 60-m capillary Rtx-1 column (0.25 mm, RESREK) with a N2 pressure of 0.3 MPa and flow velocity of 30 mL min-1 as the column temperature increased from 50 °C to 320 °C with a heat rate of 15 °C min-1 after staying at 50 °C for 1 min. To further identify each of the compounds in the liquid products, a Finnigan Trace GC–MS consisting of a Trace Ultral gas chromatograph (60 m - 0.25 mm - 0.5 µm) capillary column and an MS detector HP 5973 was used to analyze the collected products. The global rate constant (kHDS) of 4,6-DMDBT HDS reactions were calculated by employing the pseudo-first order reaction according to the following equation27: %&'( = −ln (1 − *) +-,

(4)


8

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where %&'( stands for the reaction rate constant for the pseudo-first order reaction in mol g-1 h-1, which is calculated by changing the reactant feeding rate, + is the reactant feed to the reactor in mol h-1, m is the mass of the loaded catalyst in grams, and * is the total conversion of 4,6DMDBT. 3. RESULTS AND DISCUSSION 3.1 Textural Properties.

Relative Intensity, a.u.

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d c b a 10

20

30

2 Theta, degree Figure 1. XRD patterns of Ni modified USY zeolites: (a) USY, (b) USNiY, (c) NiUSY, (d) Ni@USY. XRD patterns of the Ni modified USY zeolites via different methods were carried out and the details are shown in Figure 1. All materials present the typical diffraction character of Y zeolite in the 2θ degree range of 5~35°, indicating that after Ni modification, the zeolite units maintained intact. And no diffraction peaks related to NiO was observed, indicating that NiO species are highly dispersed on the surface of zeolites(Ni@USY); existing as framework Ni


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species(USNiY) since the Al species can be replaced by Ni species in the zeolite Y framework during the crystallization process28; or located in the super cages and double six-membered rings of Y zeolites(NiUSY)

29, 30

. However, the intensities of the diffraction peaks decrease in the

following order: USNiY > USY > NiUSY > Ni@USY, indicating that the addition of small proportion of Ni(NO3)2 into Y precursor gel can promote the formation of FAU structures and the framework Ni species can stabilize the FAU structures; FAU structures get a little destroyed during Ni2+ cation exchange since Ni(NO3)2 aqueous solution is an acid solution; and the decrease in the diffraction peak intensity of Ni@USY is mainly caused by the existence of amorphous NiO species on the surface of USY zeolites. Table 1. Structural properties of the Ni modified USY zeolites.

Materials

SBETa(m2)

Vtotal(cm3g-

Vmeso(cm3g-

1

)

1

Cryst.e NiContentb(wt.%)

Nisurfacec(wt.%)

)

(%)

USY

623

0.72

0.23

--

--

100

USNiY

648

0.76

0.20

2.5

1.7

108

NiUSY

615

0.69

0.22

2.4

0

94

Ni@USY

526

0.58

0.18

2.5

2.9

89

a

Determined by the BET method.

b

Determined by ICP-AES.

c

Determined by Ni 2p XPS.

d

Derived from XRD. The SEM images of the synthesized zeolites were displayed in Figure S1 and the statistical

results were summarized in Table S1(see supporting information). The results show that after


10

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introducing small amounts of NiO to the zeolite framework, the crystallite size of the synthesized NaNiY increases from about 268 nm of sample NaY to that about 326 nm of sample NaNiY. The surface areas and pore volumes of support materials also play important roles in the corresponding hydrotreating catalysts, thus N2 adsorption-desorption isotherms of the mentioned Ni modified USY zeolites were carried out and the results were summarized in Table 1. The surface area and total pore volume of different Ni modified zeolites increase in the following order: Ni@USY < NiUSY < USY < USNiY while the mesopore volume increase in the following order: Ni@USY < USNiY < NiUSY < USY. This is possible because that some micropores were blocked by the introduced NiO via impregnation method; for the ion-exchange method, Seo and Tim 29, 30 confirmed that Ni species is mainly existing as Ni2+ ions in the super cages and the double six-membered rings, they also confirmed that there is a small proportion of Ni species existing as (Ni4AlO4)3+ clusters(NiO combined with non-framework Al3+) on the inner surface of USY zeolite and they observed the dealumination of the zeolite framework in the Ni2+ exchange process, all these caused decreases in surface area and pore volume of the exchanged Y zeolite; and for the insitu-synthesis method(USNiY), the introduced framework Ni species is beneficial for the formation and stabilization of FAU structures(Figure 1) and thus there are less micropores destroyed and there are less mesopores created during the hydrothermal treatment of HY zeolites. The Ni contents in each of the modified USY zeolites were detected by ICP-AES and Ni 2p XPS and the results were also summarized in Table 1, from the results we can see that the Ni contents are similar in the modified USY zeolites obtained via different modification methods. 3.2. Acidity Properties.


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d c b

a

100

200

300

400

500

Temperature, ℃

Figure 2. NH3-TPD profiles of Ni modified USY zeolites: (a) USY, (b) USNiY, (c) NiUSY, (d) Ni@USY. Since acidity of supports is crucial to the activities of bi-functional hydrotreating catalysts, the acidities of the Ni modified USY zeolite via different modification methods were investigated by NH3-TPD and pyridine-adsorbed FTIR techniques and the results are shown in Figure 2 and Figure 3, and the details are summarized in Table 2. The NH3-TPD profiles displayed in Figure 2 reveal that there are two NH3 desorption peaks centered at about 200 °C and about 350 °C, demonstrating that there are two types of acid sites with different acid strength existing for USY zeolites. The peak centered at 200 °C is usually attributed to the weak acid sites while the peak centered at about 350 °C is usually attributed to the strong acid sites. Compared with that of USY, the lower temperature desorption peak of USNiY shifts to an even lower temperature and the intensity of this desorption peak decreased a lot, indicating a slight decrease in the acid strength and a profound decrease in the amount of weak acid sites. This is possible due to that the acid sites in USY zeolites are considered to be the Si-OH-Al bonds or the unsaturated


12

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coordinated Al caves31, and some of the Al precursors are substituted by Ni precursors during the synthesis of USNiY zeolite, thus the amount of acid sites decreased. The higher temperature desorption peak of USNiY presents almost the same desorption temperature compared with that of USY, however, the intensity of this peak increased a lot, indicating that the amount of strong acid sites increased. This can be explained by the theory that the average electronegativity value of 1.91 for Ni is higher than the value for Al (1.71) and Ni is more covalent than Al, implying a stronger acid strength of both Brønsted protons and Lewis caves for Ni-substituted zeolites, thus there are more strong acid sites for USNiY than that for USY. The lower temperature desorption peaks for NiUSY and Ni@USY shift to higher temperature and the intensity of the lower temperature peak increased compared with that of USY, indicating that both the acid strength and the amounts of weak acid sites for these samples increased. However, the higher temperature desorption peak shifts to lower temperatures and the intensities of high temperature desorption peaks decreased, and the two peaks become overlapped, indicating that both the acid strength and amount of strong acid sites decreases after Ni modification. More importantly, another type of acid sites(with acid strength between the aforementioned two types of acid sites) are formed after Ni modification. This can be explained by the theory proposed by Johnson et al32: The dispersion of the metal atoms on the support increased with higher concentration of acid sites, particularly the strong Lewis acid sites present in USY. The amount of both weak acid sites and strong acid sites were calculated by detecting the desorbed NH3 on a mass spectrometer and the results were summarized in Table 2. The results show that the amount of the weak acid sites of the modified zeolites increases in the order: USNiY < USY < Ni@USY < NiUSY and the amount of the strong acid sites increases in the order: Ni@USY < NiUSY < USY < USNiY.


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To differentiate Brønsted acid sites (BAS) and Lewis acid sites (LAS) and observe the changes in BAS and LAS over the Ni modified USY zeolites via different modification methods, pyridine adsorbed FTIR(Py-FTIR) was performed and the corresponding Py-FTIR spectrums after pyridine desorbed at 200 °C and 350 °C are displayed in Fig. 3 and the amounts of BAS and LAS were calculated and summarized in Table 2. The results show that the weak LAS of the modified zeolites increase in the following order: USNiY(223 µmol g-1) < NiUSY(243 µmol g-1) < Ni@USY (298 µmol g-1) < USY(326 µmol g-1) and the weak BAS increase in the following order: Ni@USY(259 µmol g-1) < USY(288 µmol g-1) < NiUSY(364 µmol g-1) < USNiY(438 µmol g-1); the strong LAS of the modified zeolites increase in the following order: USNiY(52 µmol g-1) < Ni@USY(109 µmol g-1) < USY(116 µmol g-1) < NiUSY(128 µmol g-1) and the strong BAS increase in the following order: Ni@USY(84 µmol g-1) < USY(105 µmol g-1) < NiUSY(116 µmol g-1) < USNiY(206 µmol g-1). It is because that the loaded NiO on USY zeolites(Ni@USY) covered some strong LAS(aforementioned) and some new LAS were formatted in the NiO granulars; for the ion exchange process, Ni2+ cation mainly substitutes the protons located in the super cages of Y zeolite and lead to the change in the acidic site density and acidic strength. These results don’t quite agree on those observed from NH3-TPD because the

NH3 molecules can adsorb on Ni2+ species to form a complex. More importantly, the least strong LAS over USNiY zeolite lead to a lowered interaction between active metals 33, 34 and the


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A

Absorbance, a.u.

d

c

b

a

1440

1480

1520

1560

-1

Wavenumber, cm B

d

Absorbance, a.u.

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c

b

a

1440

1480

1520

1560

-1

Wavenumber, cm

Figure 3. Py-FTIR spectra of Ni Modified USY zeolites: (a) USY, (b) USNiY, (c) NiUSY, (d) Ni@USY; (A) after desorbed at 200 °C and (B) after desorbed at 350 °C. supports(H2-TPR results), which results in higher stacking of NiMoS slabs and enhances Ni and Mo sulfidation over the corresponding catalyst(HRTEM results and XPS analysis). The ratios of


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both weak and strong BAS/LAS of USY, USNiY, NiUSY and Ni@USY zeolites also changed, and a moderate BAS/LAS ratio is also considered very important to the catalytic activity of the corresponding catalyst. Table 2. Acidity properties of Ni modified USY. Weak acid sites (µmol g-1)

Strong acid sites (µmol g-1)

Samples

a

Ama

LASb

BASb

BAS/LASb

Ama

LASb

BASb

BAS/LASb

USY

1053

326

288

0.88

415

116

105

0.91

USNiY

863

223

438

1.96

524

52

206

3.96

NiUSY

1269

243

364

1.50

389

128

116

0.91

Ni@USY

1248

298

259

0.87

335

109

84

0.77

Am refers to the amount of acid sites determined by NH3-TPD, which is not accurate since the

NH3 molecules can interact with Ni2+ species to form a complex. b

Determined by pyridine-FTIR.

3.3. Active Phase Characterization. It is believed that the reducibility of the active metals can effectively reflect the interaction between the active metals and the supports, which is closely related to the formation of NiMoS active phase23. Therefore, the H2-TPR characterization of the investigated catalysts was performed and the H2-TPR profiles are displayed in Figure 4. The results revealed that two main H2 consumption peaks with reduction temperature of about 450 °C and 800 °C, the lower peak is attributed to the reduction of NiO to Ni and MoO3 to MoO2 while the higher one is attributed to the reduction of MoO2 to Mo, respectively. The reduction temperature of the lower H2 consumption peak slightly decreases in the following order: NiMo/USY > NiMo/Ni@USY ~ NiMo/NiUSY > NiMo/USNiY, and the reduction termperature of the higher H2 consumption


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peak decrease in the following order: NiMo/USY ~ NiMo/Ni@USY > NiMo/NiUSY > NiMo/USNiY, suggesting that the interaction between active metals and supports decrease in the same order. However, the area of the H2 consumption peak, especially the higher one, increases in the following order: NiMo/USY ~ NiMo/Ni@USY < NiMo/NiUSY < NiMo/USNiY, indicating that the dispersion of active metals increases in the same order, which can also be supported by the HRTEM results.

d


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

c

b a

400

800

Temperature, ℃

Figure 4. H2-TPR profiles of the catalysts (a) NiMo/USY, (b) NiMo/USNiY, (c) NiMo/NiUSY and (d) NiMo/Ni@USY. To visualize the morphologies of the NiMoS active phase and compare the sulfide metal dispersion over different supports, HRTEM studies were performed and the representative HRTEM images of the corresponding sulfide NiMo supported catalysts are displayed in Figure 5. The average slab length and stacking layers of Ni promoted MoS2 slabs were obtained via a statistical analysis of at least 300 slabs from different parts of the sulfide catalysts. The slab length and layer stacking distribution of MoS2 slabs of the series catalysts are displayed in Figure


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S1 and Figure S2. The average slab length, average layer number and Mo species dispersion-fMo were calculated according to equations Eq. 1, Eq. 2 and Eq. 3, respectively. The results demonstrate that the morphology of the so-called Ni-Mo-S phase changes with the Ni modification method. The average length of the NiMoS slabs increases in the following order: NiMo/Ni@USY < NiMo/USNiY < NiMo/NiUSY < NiMo/USY, the average stacking number of the Ni-Mo-S slabs increases in the following order: NiMo/USY < NiMo/Ni@USY < NiMo/NiUSY < NiMo/USNiY and the fMo of the investigated catalysts increases in the following order: NiMo/USY < NiMo/NiUSY < NiMo/USNiY < NiMo/Ni@USY. The average length(3.42 nm) of Ni-Mo-S slabs over NiMo/USY is the longest, however, there are about 36 % of monolayered NiMoS slabs which are considered to be less active NiMoS phase for 4,6-DMDBT HDS, leading to the lowest average stacking number(1.68) of the NiMoS slabs over NiMo/USY catalyst. This is because of the interaction between the active metals and the supports-usually considered through the formation of strong Mo-O-Al bonds, is too strong and the active metal precursors are less likely to be sulfide. Thus for catalyst NiMo/USNiY, due to the substitution of the framework Al species by Ni species, there are less Mo-O-Al bonds formed at the zeolite surface, the average stacking number of NiMoS slabs is about 2.84, which is much higher than that over catalyst NiMo/USY, and there are up to 70 % NiMoS slabs with stacking number of 3 over this catalyst. The average length of the NiMoS slabs is about 2.88 nm over catalyst NiMo/USNiY, which is much shorter than that of catalyst NiMo/USY. Due to the relative high surface area of Ni modified USY zeolites, the dispersions of Mo species over all the four investigated catalysts are relative high, which is believed to be benefit for the formation of the real active sites-NiMoS phases. These results agree well with those observed from the H2-TPR


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Energy & Fuels

Figure 5. HRTEM images of the sulfide NiMo supported catalysts: (a) NiMo/USY, (b) NiMo/USNiY, (c) NiMo/NiUSY, (d) NiMo/Ni@USY. analysis. As is believed that slabs with shorter length and lower stacking number are beneficial to the formation of “type Ⅱ active phase”. However, for highly refractory reactants such as 4,6DMDBT, since the methyl groups at 4 and 6 positions can prevent the reactant molecules adsorbing to the active sites via σ-adsorption, Ni promoted MoS2 slabs with too small sizes are less active because the planar adsorption of the reactants to the small active metal slabs also


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become difficult. Thus the catalyst NiMo/USNiY with proper NiMoS slab length and stacking number exhibits the highest catalytic activity. Table 3. Average length, layer number and fMo of MoS2 slabs of all catalysts. Catalysts

NiMo/USY NiMo/USNiY NiMo/NiUSY NiMo/Ni@USY

Average length (nm)

3.42

2.88

3.26

2.68

Average layer number

1.68

2.84

2.21

1.94

fMo

0.26

0.29

0.28

0.30

To identify the covalent states of Mo species, the catalysts prepared from the Ni modified USY zeolites which were prepared by different modification methods were characterized by XRay photoelectron spectroscopy(XPS) method after the catalysts were sulfided. Figure 6 displays the Mo 3d XPS spectra and the deconvolution results of the investigated catalysts and the details are summarized in Table 4. The decompositions with binding energies of about 228.9±0.2 eV and 231.7±0.2 eV are correspond to the Mo 3d5/2 and Mo 3d3/2 levels for Mo4+ species, respectively; while those of about 230.5±0.1 eV and 233.6±0.1 eV are correspond to the Mo 3d5/2 and Mo 3d3/2 levels for Mo5+ species and those of about 232.7±0.1 eV and 236.0±0.1 eV are correspond to the Mo 3d5/2 and Mo 3d3/2 levels for Mo6+ species34. The binding energies of each Mo species among the investigated catalysts slightly increased in the following order: NiMo/Ni@USY ~ NiMo/NiUSY ~ NiMo/USY < NiMo/USNiY, indicating the interaction between Mo and the supports is the weakest for NiMo/USNiY. This is possible because the concentration of strong Lewis acid sites over the corresponding zeolites increased in the


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a

b 4+

Mo

4+

Mo

Mo

S

222

228

234

Intensity, cps

Intensity, cps

Mo5+ 6+

240

Mo S

222

5+

6+

Mo

228

Binding Energy, eV

234

240

Binding Energy, eV

c

d 4+

Mo

4+

Mo

S

5+

Mo

222

228

234

Intensity, cps

Mo Intensity, cps

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

6+

240

Mo S

222

Binding Energy, eV

5+

Mo

228

6+

234

240

Binding Energy, eV

Figure 6. XPS spectra of Mo 3d for the sulfide NiMo supported catalysts: (a) NiMo/USY, (b) NiMo/USNiY, (c) NiMo/NiUSY, (d) NiMo/Ni@USY. following order: USNiY < Ni@USY ~ USY ~ NiUSY. Since the sulfidation degree of Mo species(Mosulfidation), defined as the percentage of Mo4+ species(Mo4+/(Mo4+ + Mo5+ + Mo6+)), has an important influence on the catalytic performances of the hydrotreating catalysts, Mosulfidation for the investigated catalysts were calculated and summarized in Table 4. The Mosulfidation for NiMo/USY, NiMo/USNiY, NiMo/NiUSY and NiMo/Ni@USY are 56 %, 70 %, 74 % and 68 %, respectively. The surface molar ratio of Ni and Mo to Al and Si has also been calculated, it increases in the following order: NiMo/NiUSY(0.11) < NiMo/USY(0.13)

4,6-dimethyldibenzothiophene Hydrodesulfurization on Nickel

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