Al2O3 catalysts for hydrodesulfurization of dibenzothiophene

Accepted Manuscript Hierarchically macro-mesoporous Ni-Mo/Al2O3 catalysts for hydrodesulfurization of dibenzothiophene

Lei Yang, Chong Peng, Xiangchen Fang, Zhenmin Cheng, Zhiming Zhou PII: DOI: Reference:

S1566-7367(18)30593-4 https://doi.org/10.1016/j.catcom.2018.12.020 CATCOM 5583

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

25 October 2018 24 December 2018 31 December 2018

Please cite this article as: Lei Yang, Chong Peng, Xiangchen Fang, Zhenmin Cheng, Zhiming Zhou , Hierarchically macro-mesoporous Ni-Mo/Al2O3 catalysts for hydrodesulfurization of dibenzothiophene. Catcom (2018), https://doi.org/10.1016/ j.catcom.2018.12.020

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ACCEPTED MANUSCRIPT Hierarchically macro-mesoporous Ni-Mo/Al2O3 catalysts for hydrodesulfurization of dibenzothiophene

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Lei Yanga, Chong Pengb, Xiangchen Fangb,* , Zhenmin Chenga, Zhiming Zhoua,*

State Key Laboratory of Chemical Engineering, East China University of Science and Technology,

Dalian Research Institute of Petroleum and Petrochemicals, SINOPEC, Dalian 116000, China

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Shanghai 200237, China

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* Corresponding Author

Phone: +86 21 6425 2230; Fax: +86 21 6425 3528

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Email: [email protected] (Z. Zhou); [email protected] (X. Fang)

ACCEPTED MANUSCRIPT ABSTRACT:

Ni-Mo catalysts supported on hierarchically macro-mesoporous alumina were prepared and applied to the hydrodesulfurization (HDS) of dibenzothiophene (DBT). The alumina supports with different macroporous structures (either parallel macropores or interconnected macropores) were

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synthesized by two methods, hydrolysis of aluminum tri-sec-butoxide and dual- template approach.

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All hierarchically porous structured Ni-Mo/Al2 O 3 showed higher HDS activity when compared to a

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Ni-Mo/Al2 O 3 with only mesopores, which was mainly due to the enhanced diffusion by macropores. In particular, a dual-template derived catalyst with highly interconnected macropores and large

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porosity displayed the best activity, which remained stable during a long-term test of 100 h.

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Keywords:

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Hierarchically macro-mesoporous structure; Ni-Mo/Al2 O 3 ; Hydrodesulfurization; Diffusion;

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Dibenzothiophene

ACCEPTED MANUSCRIPT 1. Introduction The increasing environmental awareness during the last decade has led to a drastic reduction of the allowed sulfur content in gasoline and diesel fuels [1-4]. The most important sulfur-containing compounds in crude oil-derived fuels are thiophene-based molecules such as thiophene,

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benzothiophene, dibenzothiophene, and alkyldibenzothiophene, which are often difficult to remove,

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even by the hydrodesulfurization (HDS) technology. One of the main difficulties lies in the

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relatively large molecular sizes of the thiophene-based compounds, which usually result in large internal diffusion resistance inside the catalyst particles. For example, the effect of the internal

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diffusion is significant on the overall HDS rate of thiophene over a commercial catalyst with a

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mesoporous structure [5,6]. Therefore, how to increase the diffusion of species from the outer surface of catalyst to the active sites on the inner surface is a challenge for development of new

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HDS catalysts.

Compared to the conventional HDS catalysts that usually possess a unimodal mesopore size

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distribution, bimodal macro-mesoporous structured catalysts usually have much higher activities for the HDS of thiophene-based compounds, owing to the improved diffusivities of species provided by

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the macropores [6-10]. Of particular interest among the bimodal porous catalysts are those with hierarchically porous structures, which have macroporous channels with mesoporous walls [6-8]. By using a template method, Hussain et al. [7] prepared macro- mesoporous carbon supported Co-Mo catalysts, which showed much higher activities for the HDS of thiophene than commercial activated carbon and alumina supported catalysts. However, carbon is seldom used as the catalyst support for the HDS reactions, and alumina is the most widely used support in industry. We prepared a Co-Mo-Ni catalyst supported on hierarchically macro- mesoporous Al2 O3 [11,12], and found that this catalyst had a superior activity in thiophene HDS as compared to a commercial

ACCEPTED MANUSCRIPT catalyst (LY-9802, Petrochemical Research Institute of Lanzhou Petrochemical Company) without the hierarchically porous structure. It should be noted that, although the hierarchically porous structure gives rise to an improved catalytic activity in the HDS of thiophene, the molecular size of thiophene is smaller than other unreactive sulfur compounds such as dibenzothiophene (DBT). In

macro-mesoporous catalysts can be enhanced for the HDS of DBT.

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this respect, there are some doubts about whether the catalytic activities of the hierarchically

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To this end, three Ni-Mo/Al2 O3 catalysts with hierarchically macro- mesoporous structures are

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prepared and applied to the HDS of DBT. The physicochemical properties of catalysts are

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characterized and correlated with the HDS activities observed. In addition, these catalysts are compared with the catalyst without the hierarchically porous structure, which demonstrates the

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impact of the hierarchically macro- mesoporous structure on the HDS of refractory sulfur

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compounds.

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2. Experimental

2.1. Preparation of Al 2 O 3 supports

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Hierarchically macro-mesoporous Al2 O3 supports were synthesized using three methods. The first one was a soft-template route using surfactant as template, which normally yielded paralleled macroporous structure [11,12], while the latter two through a dual-template technique using polystyrene (PS) beads as hard template and surfactant as soft template, which usually resulted in interconnected macroporous structure [13]. The first one was hydrolysis of aluminum tri-sec-butoxide (Al(OsBu)3 ) in an aqueous solution with the aid of cetyltrimethylammonium bromide (CTAB) [11,12]. Typically, 0.4 g of CTAB was added into a mixture of 15 mL of ethanol and 35 mL of twice-distilled water with slow stirring at

ACCEPTED MANUSCRIPT room temperature. Then, 12.0 g of NH3 ·H2 O was added to adjust to the pH to 10. Finally, 2.0 g of Al(OsBu)3 was slowly added to the above solution with stirring for 1 h. After that, the precipitate was separated by centrifugation, washed by Soxhlet extraction for 24 h, dried at 110 °C for 24 h, and calcined at 500 °C for 5 h. For simplicity, the as-prepared macro- mesoporous Al2 O3 was

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denoted A(MM1). The second one was a dual-template approach [13]. First, monodispersed PS beads were

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synthesized by an emulsifier-free emulsion polymerization method [14]. Next, 3.0 g of Pluronic

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P123 triblock copolymer, 4.5 ml of HNO 3 (65 wt%) and 6.1 g of aluminum isopropoxide (Al(OiPr)3 )

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were added to 60 ml of ethanol with vigorous stirring for 5 h. Then, 3.1 g of the as-prepared PS particles was added to the above solution and stirred for another 5 min, after which the beaker

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containing the solution was placed in a water bath at 60 °C for drying under static conditions. Finally, the resulting solid was calcined at 700 °C for 5 h. The as-prepared Al2 O3 was denoted

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A(MM2).

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The third one was also a dual-template method but with some modification. First, an alumina gel was prepared as follows: 3.4 g of Al(OiPr)3 was added to 60 ml of deionized water with the aid of

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vigorous stirring for 1.5 h at 85 °C, after which the solution pH was adjusted to 3 by acetic acid and heated at 95 °C under reflux for 5 h. Next, 1.0 g of P123 was added to the above gel with stirring until complete dissolution, followed by cooling to room temperature. Then, 1.0 g of PS beads was added to the above solution under ultrasonication for 15 min. Finally, the combined solution was dried at 60 °C for 4 days under static conditions before calcination at 700 °C for 5 h. This Al2 O3 support was denoted A(MM3). For comparison, a mesoporous Al2 O 3 support without macropores was synthesized through hydrolysis of AlCl3 . A NaOH solution was first added slowly into an aqueous solution of AlCl3 at

ACCEPTED MANUSCRIPT room temperature with vigorous stirring until the solution pH was equal to 9.5, and then the resulting suspension was aged at 70 °C for 24 h, followed by filtering, washing, drying and calcination at 500 °C for 5 h. The mesoporous Al2 O3 was denoted A(M). 2.2. Preparation of Ni-Mo/Al 2 O3 catalysts

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Ni-Mo/Al2 O3 catalysts supported on different Al2 O3 were prepared by a successive incipient

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wetness impregnation method Nickel nitrate hexahydrate and ammonium molybdate tetrahydrate

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were used as precursors for Ni and Mo, respectively. Impregnation of the Mo precursor was the first step, followed by that of the Ni precursor. After each impregnation step, the sample was dried at

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120 °C for 12 h, and calcined at 450 °C for 5 h. The obtained powder was granulated by pressing at

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about 20 MPa, followed by crushing and sieving to particles with a diameter between 0.45 and 0.90 mm (20-40 mesh) prior to the HDS reaction.

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2.3. Activity test of catalysts

The HDS of DBT was carried out in a continuous fixed-bed reactor (10 mm inner diameter and

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450 mm in length) [6]. A mixture of 0.1 g catalyst and 0.5 g inert silicon carbide was placed in the middle of the reactor, and the top and the bottom of the reactor were packed with inert glass beads.

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Prior to the HDS reaction, the catalyst was presulfided in situ with 5 wt% CS 2 -decalin and H2 at 350 °C and 4 MPa for 12 h. The liquid feed composed of 5000 μg/g DBT and decalin was introduced by a HPLC pump to a preheater, in which the liquid was mixed with hydrogen, vaporized and preheated. The reaction product was analyzed with a HP-6890 gas chromatograph using a HP-5 capillary column (30 m × 0.32 mm × 0.25 μm) and a flame ionization detector.

3. Results and discussion

ACCEPTED MANUSCRIPT 3.1. SEM and HRTEM analysis The four Ni-Mo/Al2 O 3 catalysts have different morphologies, as shown in Fig. 1. No macropores are found on the surface of Ni-Mo/A(M). Ni-Mo/A(MM1) displays macroporous channels that are parallel to each other and perpendicular to the tangent of the outer surface. Ni-Mo/A(MM2) and

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Ni-Mo/A(MM3) possess interconnected spherical macropores, but the latter exhibits more highly

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connected macropores than the former. It can be seen that there exist almost no open macropores in

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some areas of Ni-Mo/A(MM2), as indicated by the dashed lines. The average macroporous diameter of Ni-Mo/A(MM2) and Ni-Mo/A(MM3) is around 300 nm, which is about 25% smaller

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than that of the PS beads about 400 nm. It is mainly caused by the shrinkage of PS microspheres

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during calcination. A comparison of the unsulfided and sulfided Ni-Mo/A(MM3) catalysts clearly shows typical layer-like structures assigned to MoS2 in the sulfided sample [15].

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[Fig. 1]

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3.2. BET surface area and pore diameter analysis Fig. 2 presents the N 2 adsorption-desorption isotherms and the corresponding pore-size distribution curves of the catalysts. All catalysts exhibit type IV isotherms with a hysteresis loop,

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characteristic of mesopores [16]. Figs. 1 and 2 together reveal that Ni-Mo/A(M) possesses only mesopores, while other three catalysts have macropores with accessible mesopores in the walls. Therefore, Ni-Mo/A(MM1), Ni-Mo/A(MM2) and Ni-Mo/A(MM3) exhibit the hierarchically macro- mesoporous structure. The hysteresis loop of Ni-Mo/A(MM3) shifts to a higher relative pressure compared to other catalysts, indicating larger mesopores. As listed in Table 1, the average mesopore size of Ni-Mo/A(MM3) is 10.9 nm, which is almost twice that of other catalysts. In addition, Ni-Mo/A(MM3) has a much large pore volume than others although its BET surface area is not the highest. This is mainly due to the highly interconnected macropores of Ni-Mo/A(MM3),

ACCEPTED MANUSCRIPT as evidenced by the SEM observation, which contribute greatly to the large porosity. An exceptional case is Ni-Mo/A(MM2) whose specific surface area and pore volume are the smallest although it has the hierarchically porous structure, which is probably associated with the relatively poorly connected pore spaces. The above analysis shows that Ni-Mo/A(MM3) has the best connectivity of

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the hierarchical pore network among all catalysts. The actual Ni and Mo loadings of the catalysts are presented in Table 1, which are very close to the nominal loadings, indicating the effectiveness

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[Table 1]

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[Fig.2]

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of the successive incipient wetness impregnation method used to prepare Ni-Mo/Al2 O3 .

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3.3. XRD, TPR and NH3 -TPD analysis

The XRD patterns of the oxidic catalysts are shown in Fig. 3. All catalysts display diffraction

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peaks (2θ = 37, 46 and 67°) assigned to the γ-Al2 O 3 phase (JCPDS 10-0425), except

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Ni-Mo/A(MM2) exhibiting amorphous Al2 O3 . Note that no peaks belonging to crystalline Ni and Mo oxide phases are present in all catalysts, implying that the Ni and Mo species are highly

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dispersed or very fine crystallites are formed [17]. [Fig.3] [Fig.4]

Fig. 4 displays the TPR profiles of four Ni-Mo/Al2 O3 catalysts. The low temperature reduction peak (403 °C for Ni-Mo/A(M), 395 °C for Ni-Mo/A(MM1), 436 °C for Ni-Mo/A(MM2) and 397 °C for Ni-Mo/A(MM3)) is assigned to the reduction of octahedrally coordinated polymeric Mo species [8,18-20], which are recognized as the precursors of the active phase MoS 2 . It is generally accepted that the lower the reduction peak, the weaker the interaction between the octahedral Mo species and the catalyst support, and the easier the sulfidation of the Mo oxide phase [8]. Therefore,

ACCEPTED MANUSCRIPT the higher reduction peak of the octahedral Mo species for Ni-Mo/A(MM2) makes relatively difficult to sulfidize this catalyst, which is probably unfavorable for the HDS activity. The NH3 -TPD profiles of four catalysts are presented in Fig. S1 (Supplementary data), which shows that the total acidity of catalyst follows the order Ni-Mo/A(M) > Ni-Mo/A(MM3) > Ni-Mo/A(MM2) >

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Ni-Mo/A(MM1).

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3.4. HDS performance of catalysts

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Fig.5 shows the variation of the conversion of DBT with the reaction temperature over four Ni-Mo/Al2 O 3 catalysts, from which we can compare the apparent activity of different catalysts. In

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general, the activity follows the order Ni-Mo/A(MM3) > Ni-Mo/A(MM1) > Ni-Mo/A(MM2) >

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Ni-Mo/A(M), which is different from the order in total acidity, indicating that the catalyst activity has no direct relationship with the catalyst acidity. However, it is notable that the catalysts with the

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hierarchically macro-mesoporous structure have higher HDS activity than the catalyst with only

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mesopores. In particular, Ni-Mo/A(MM2) is superior to Ni-Mo/A(M) although the former has smaller surface area and pore volume as well as higher metal-support interaction. This demonstrates that the hierarchically porous structure significantly contributes to the improved HDS activity. The

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macropores are expected to reduce the diffusion resistance of reactants from the outer surface to the inner surface of catalyst [21-24]. Indeed, the data shown in Fig. 5 agree with this expectation. It is observed that the difference in conversion between Ni-Mo/A(M) and Ni-Mo/A(MM2) (indicated by the shadow area) is small at lower reaction temperature, but becomes larger with increasing the temperature. Considering that the reaction rate increases more rapidly with temperature than the diffusion rate, the diffusion limitation tends to be significant at high temperature [25]. Therefore, the increased difference in conversion with reaction temperature is ascribed to the enhanced diffusion by the presence of macropores in Ni-Mo/A(MM2). A comparison between Ni-Mo/A(M)

ACCEPTED MANUSCRIPT and Ni-Mo/A(MM1) also arrives at this result. It should be pointed out that when the conversion of DBT is above 90%, the reaction rate is greatly decreased as a result of a very low concentration of DTB. In this case, the HDS of DBT might be reaction controlled. Hence, only the data with the conversion lower than 90% are used in the above analysis.

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[Fig.5] As regards the catalysts with a hierarchically porous structure, Ni-Mo/A(MM2) has the lowest

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activity due to the higher metal-support interaction and the less porosity, while Ni-Mo/A(MM3)

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with highly interconnected macropores possesses large porosity and relatively low metal-support

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interaction, thus resulting in the highest activity. The HDS product of DBT is predominantly composed of biphenyl (BP) and cyclohexylbenzene (CHB), irrespective of the catalysts used (Fig.

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S2 in the Supplementary data), suggesting that the conversion of DBT over Ni-Mo/Al2 O 3 mainly

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proceeds via the direct desulfurization pathway instead of the hydrogenation route [26]. [Fig.6]

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Ni-Mo/A(MM3) is further submitted to a long-term run. As shown in Fig. 6, the conversion of DBT is kept almost unchanged (94% on average) throughout the run, and the selectivity to BP or

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CHB is also maintained. Moreover, the morphology of the catalyst is well preserved after 100 h on stream. Therefore, Ni-Mo/A(MM3) exhibits good stability in the HDS of DBT.

4. Conclusions We have prepared three Ni-Mo/Al2 O3 catalysts with different hierarchically macro-mesoporous structures, which show higher activity in the HDS of DBT than the catalyst with only mesopores. The improved activity is mainly attributed to the enhanced diffusion of DBT in the hierarchically porous catalysts. In particular, a hierarchically macro- mesoporous Ni-Mo/Al2 O3 with highly

ACCEPTED MANUSCRIPT interconnected macropores exhibits the best activity and long-term stability in the HDS of DBT.

Acknowledgements Financial supports from the National Natural Science Foundation of China (20706018 and 21776088) are gratefully acknowledged.

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Supplementary data

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Supplementary material

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References

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ACCEPTED MANUSCRIPT [26] M. Houalla, N.K. Nag, A.V. Sapre, D.H. Broderick, B.C. Gates, AIChE J. 24 (1978) 1015-1021.

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Graphical abstract

ACCEPTED MANUSCRIPT Figure captions Fig. 1.

SEM images of different Ni-Mo/Al2 O3 catalysts (top four panels) and PS beads (bottom

left two panels) as well as HRTEM images of unsulfided and sulfided Ni-Mo/A(MM3) catalysts (bottom right two panels). N 2 adsorption-desorption isotherms and corresponding pore-size distribution curves of

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Fig. 2.

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different Ni-Mo/Al2 O3 catalysts. XRD patterns of different Ni-Mo/Al2 O3 catalysts.

Fig. 4.

TPR profiles of different Ni-Mo/Al2 O3 catalysts.

Fig. 5.

Comparison of the activity of different Ni-Mo/Al2 O 3 catalysts in the HDS of DBT.

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Fig. 3.

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Reaction conditions: inlet liquid flow rate of 1.93 mmol·min-1 , pressure of 3 MPa, and H2 /liquid of

Conversion of DBT and selectivity to BP and CHB over Ni-Mo/A(MM3) during 100 h on

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Fig. 6.

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stream. The inset shows the SEM image of the 100 h- used catalyst. Reaction conditions: inlet liquid

of 11.

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flow rate of 1.93 mmol·min-1 , temperature of 300 °C, pressure of 3 MPa, and H2 /liquid molar ratio

ACCEPTED MANUSCRIPT Table1 Textural properties and metal loadings of Ni-Mo/Al2 O 3 catalysts BET surface area (m2 ·g-1 )

Pore volume (cm3 ·g-1 )

Average mesoporous sizea (nm)

Average macroporous sizeb (μm)

Ni-Mo/A(M)

197

0.32

5.7

Ni-Mo/A(MM1)

224

0.28

Ni-Mo/A(MM2)

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Ni-Mo/A(MM3)

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Mo

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2.9

11.9

5.5

0.51

2.8

11.6

0.20

4.7

0.30

2.9

11.7

0.52

10.9

0.31

2.8

11.9

BJH pore diameter determined from the adsorption branch.

b

Average macroporous diameter obtained from analysis of SEM images. Determined by the ICP-OES analysis.

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a

c

c

Ni

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Catalysts

Actual loading (wt%)

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Ni-Mo/A(M)

2 μm

500 nm

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1 μm

5 μm

5 μm

5 μm Unsulfided Ni-Mo/A(MM3)

PS beads

5 μm

Sulfided Ni-Mo/A(MM3)

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PS beads

Ni-Mo/A(MM3)

Ni-Mo/A(MM2)

Ni-Mo/A(MM1)

20 nm

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500 nm

10 nm

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800

600

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4

500

3

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400 300 200

1

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0 0.0 0.2 0.4 0.6 0.8 P/Po

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100

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dV/dlog(dp) / cm3g-1nm-1

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Volume adsorbed / (cm3g-1)

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6 Ni-Mo/A(M) Ni-Mo/A(MM1) Ni-Mo/A(MM2) Ni-Mo/A(MM3)

10 dp / nm

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0 100

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Intensity / a.u.

Ni-Mo/A(M)

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Ni-Mo/A(MM1)

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Ni-Mo/A(MM2)

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40 50 2 / degree

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20

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10

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Ni-Mo/A(MM3)

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70

80

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Figure 4

Intensity / a.u.

Ni-Mo/A(M)

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Ni-Mo/A(MM1)

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Ni-Mo/A(MM2)

Ni-Mo/A(MM3)

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400 600 Temperature / oC

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0

800

1000

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100

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80

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70

Ni-Mo/A(M) Ni-Mo/A(MM1) Ni-Mo/A(MM2) Ni-Mo/A(MM3)

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Conversion of DBT / %

90

40 240

260

280

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50

300

320 o

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Temperature / C

340

360

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100

100

80

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100 h-used

60

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90

40

85

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Conversion of DBT /%

95

500 nm

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80 CHB

75 20

40 60 Time on stream / h

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0

Selectivity to BP or CHB / %

BP

80

20

0 100

ACCEPTED MANUSCRIPT Highlights

Hierarchically macro-mesoporous Ni-Mo/Al2O3 catalysts showed high HDS activity.

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The improved activity was mainly ascribed to the enhanced diffusion by macropores.

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The metal-support interaction affected to some extent the HDS activity of catalyst.

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The best catalyst developed showed good stability during 100 h of test.

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