English

• Nowadays, because of the inferior diesel and severe environmental restrictions, the refineries have been demanded to improve the performance of HDS catalysts to produce high quality fuel. The refractory sulfur compounds, such as 4-methyldibenzothiophene (4-MDBT) and 4, 6-dimethyldibenzothiophene (4, 6-DMDBT) contained in diesel, have steric hindrance by the alkyl groups. An effective method to decrease this steric hindrance and improve the HDS activity of catalyst is the introduction of zeolite into the catalyst^{[1, 2]}. Zeolites, such as SBA, MCM, Beta and HY, as the support or co-support for HDS catalysts could promote 4, 6-DMDBT to become many high reactive molecules and significantly improve their HDS properties^[3-7]. Compared with microsized zeolite, nanosized zeolite exhibited a higher activity, lower coke content and longer life in many reactions. In addition, many other properties of HDS catalyst, such as acid properties, microstructure of surface active phase and valence state of reactive metal, were influenced by the zeolite size. These difference, in return, surely had great effect on deep HDS in diesel^[8]. To this day, the research on the application of NYA composite as the HDS catalyst support was limited^{[9, 10]}. In our previous study^[3], NiMo catalysts containing nanosized HY zeolite were prepared to evaluate the size effect of zeolite crystal on diesel HDS activities and the result showed that the prepared catalysts had higher HDS activities than the reference catalyst.

  It is generally known that the preparation method of hybrid matrix is important because it can influence the interaction between the active phase and matrix support^{[11, 12]}. Besides the mechanical mixing, the sol-gel method is an important method to prepare support. Barrera et al^{[13, 14]} prepared a series of ZrO₂-TiO₂ mixed oxide by the low-temperature sol-gel method followed by solvo-thermal treatment as HDS catalyst supports. Higher surface area and porosity were registered. MoS₂ catalysts supported on binary oxides solvo-treated at 353 K showed increased intrinsic activity (by a factor of 3.3) in dibenzothiophene hydrodesulfurization (593 K and 5.6 MPa), as to that impregnated on conventional ZrO₂-TiO₂.

  When the hybrid oxides of zeolite-Al₂O₃ prepared by sol-gel method were used as the support, the interaction between the active phase and the support might be substantially different from the catalyst prepared by mechanical mixing method. In order to evaluate the influence of preparation method (sol-gel and mechanical mixing method) of NYA composite on the HDS activity of FCC diesel, two different NiMo catalysts supported on NYA composite were prepared by sol-gel and mechanical mixing method in the present work. The catalysts were characterized by XRD, BET, TPD, H₂-TPR, HRTEM and FT-IR technologies and their HDS activity was evaluated using real FCC diesel.

  1.   Experimental

  1.1   Support and catalyst preparation

  Nanosized zeolite NaY was prepared using hydrothermal method described in the previous study^[9]. The FCC diesel used for HDS activity measurement was provided from Sinopec Qingdao Refining and Chemical Co., Ltd..

  1.1.1   Preparation of NYA composite

  17.25 g pseudo-boehmite (≥70 %, Shandong Baida Chemicals Co., Ltd) was dispersed in 150 mL 1 mol/L HNO₃ solution, stirred at 353 K for 3 h and aged for 12 h at room temperature, and then the Al-sol was obtained. 50 mL zeolite emulsion containing 2.25 g of zeolite NaY was slowly added, stirring vigorously for 1 h. After that, a certain amount of ammonia (AR, Sinopharm Chemical Regant Co., Ltd) was added and the solution changed from the sol-type to a gel-type when its pH value was 10. The mixture was aged for 12 h and dried at 373 K for 24 h.

  1.1.2   Preparation of support

  The prepared NYA composite material was added into 1.0 mol/L NH₄NO₃ (AR, Sinopharm Chemical Regant Co., Ltd) aqueous solution for three ion-exchange with the ratio of NYA/NH₄NO₃ of 1:10, and then stirred at 363 K for 2 h. After filtering, washing, drying at 393 K for 12 h and calcining at 823 K for 4 h, the H-type NYA composite material was obtained. The support was obtained by mechanically mixing the NYA composite and sesbania powder at a mass ratio of 100:3, extruding to cylindrical form, drying at 393 K for 12 h and calcining at 823 K for 4 h, labeled as Support-S.

  1.1.3   Preparation of catalyst

  The catalyst was obtained by co-impregnation of an solution of appropriate amount of Ni₂(OH)₂CO₃(AR, Sinopharm Chemical Regant Co., Ltd), MoO₃(AR, Sinopharm Chemical Regant Co., Ltd) and H₃PO₄(AR, Sinopharm Chemical Regant Co., Ltd). The catalyst was dried overnight at 393 K and calcined at 773 K for 4 h, labeled as Cat-S. The other catalyst was prepared following the mechanical mixing method described elsewhere^[11], labeled as Cat-M. And the corresponding support was also labeled as Support-M. MoO₃+NiO content (24%), Ni/(Mo+Ni) molar ratio (0.26) and P/MoO₃ mass ration (0.063), separately, were equal in these two catalysts and zeolite content (15%) was equal in these two supports.

  1.2   Catalysts characterization

  XRD patterns was obtained using a Rigaku D/max-IIA diffractometer at room temperature and Cu Kα radiation (λ= 0.15406 nm). Textural characteristics was measured on a Micromeritics ASAP 2020 automated gas adsorption system. FT-IR of pyridine adsorbed was measured on a Nexus spectrometer (Nicolet, USA). HRTEM photograph was selected from a JEM 2100 microscope operated at 200 kV. H₂-TPR and NH₃-TPD experiments of these two catalysts were conducted on a ChemBET-3000 apparatus (Quantachrome Instruments, USA) equipped with a thermal conductivity detector (TCD).

  1.3   Catalytic activity measurement

  The diesel HDS tests were carried out in a fixed-bed microreactor under the conditions of temperature of 633 K, total pressure of 6 MPa, H₂/toluene volume ratio of 500 and LHSV of 1.0 h^-1. Before the test, these two catalysts (10 mL, respectively) were sulfided for 6 h with 3% of CS₂-toluene mixture under the following conditions: 593 K temperature, 4 MPa total pressure, 300 H₂/toluene volume ratio and 2.0 h^-1 LHSV. An Analytikjena's elemental analyzer (Mutli EA 3100) was used to decide the sulfur content in the feedstock and product, and a Varian 3800 GC was used to analyze the sulfur-containing compound type.

  2.   Results and discussion

  2.1   XRD analysis

  To study the internal structure of these samples, XRD characterization of these two sulfide catalysts was recorded. Figure 1 shows that the diffraction peaks of MoS₂ (JCPDS 37-1492) are observed in these two sulfided catalysts.

  Figure 1

  

  Figure 1.  XRD patterns of two sulfide catalysts and Al₂O₃

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  The (002) peak at 2θ = 14° is representative for the stacking slabs along the c-axis in MoS₂ crystal. It can be observed that the (002) peak intensity in Cat-S is stronger than that in Cat-M. The result shows that the preparation procedure of sulfided catalyst using NYA composite prepared by sol-gel method as support is likely to form MoS₂ with high stacking slabs along the c-axis compared with the sample prepared from the mechanical mixing method. But the peaks of HY zeolite have not been observed, which implies that the structure of nanosized HY zeolite has been destroyed in the preparation procedure and this phenomina was explained in our previous study^[15]. Figure 2 shows that the crystallization degree of synthesized nanosized Y zeolite decreases rapidly after two calcinations, and remarkably, the calcination numbers in this work is more than 3 times. So, in this work, the destruction of nanosized Y zeolite structure is more serious.

  Figure 2

  

  Figure 2.  XRD patterns of nano Y zeolite after calcinations

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  2.2   Textural property analysis

  The textural properties of these two catalysts are shown in Table 1.

  Table 1

  

  Table 1.  Textural properties of two catalysts

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  Sample	SBET/ (m2·g-1)	Smi/ (m2·g-1)	Pore volume v/(cm3·g-1)	Pore size d/nm
  Cat-S	124.9	15.3	0.27	9.1
  Cat-M	207.6	27.6	0.41	9.2

  As seen in Table 1, BET surface area, pore size, and pore volume of Cat-S are smaller than that of Cat-M, which implies that the sol-gel method has a negative effect on the textural property of the catalyst compared with the mechanical mixing method. Barrera et al^{[13, 14]} has reported the feasibility of improving ZrO₂-TiO₂ texture by solvo-thermal post-treatments during sol-gel synthesis. Contrary to their results, the texture of the NYA composite prepared in our work is poor, which should be attributed to the absence of either solvent treatment or vacuum driness for the prepared NYA composite. It can be seen from Figure 3 that the nitrogen adsorption/desorption isotherms of these two catalysts are of type Ⅳ pattern. From Figure 4, it is found that there is no small pore in Cat-S, which implies that the sol-gel preparation procedure is likely to destroy the pore structure of nanosized zeolite.

  Figure 3

  

  Figure 3.  N₂ adsorption-desorption isotherms of two catalysts

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  Figure 4

  

  Figure 4.  Pore size distribution of two catalysts

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  2.3   NH₃-TPD analysis

  NH₃-TPD patterns of these two catalysts are shown in Figure 5.

  Figure 5

  

  Figure 5.  NH₃-TPD profiles of two catalysts

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  These two catalysts represent a similar distribution of acid site in the range from 100 to 600 ℃. The NH₃ desorption peaks are located at about 220 ℃, indicating these two catalysts mainly contain weak acid sites. The weak acid site possibly has an important effect on HDS activity of these catalysts, because it could decrease the rapid deactivation caused by the strong acid sites^{[11, 16]}. From Figure 5, it can also be seen that the acid amount of Cat-S is smaller than that of Cat-M.

  2.4   FT-IR analysis

  FT-IR spectra of pyridine adsorption on these two oxide catalysts are given in Figure 6. It is well known that the characteristic bands at 1450 and 1540 cm^-1 are assigned to pyridine adsorbed on the Lewis (L) acid sites and Br∅nsted (B) acid sites, respectively^{[16, 17]}. The amount of L and B acid sites is calculated^[18] and presented in Table 2, and it can be seen that the sol-gel method results in the decrease of B acid sites and L acid sites. The sol-gel method almost has not taken effect on the B/L ratio.

  Figure 6

  

  Figure 6.  FT-IR profiles of two catalysts

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  Table 2

  

  Table 2.  Amounts of the acid sites of two catalysts

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  Sample	Amount of acid sites/(mmol·g-1 catalyst)		B/L (ratio)
  	L	B
  Cat-S	0.32	0.27	0.84
  Cat-M	0.53	0.45	0.85

  2.5   H₂-TPR analysis

  H₂-TPR profiles of these two oxide catalysts are given in Figure 7. Both the samples show two main reduction peaks in low temperature range (400-600 ℃) and high temperature range (800-1000 ℃), respectively. Tops∅e et al^[19] proposed a NiMoS active phase model that this phase includes the single-slab structure (type-Ⅰ) and the multiple-slab structure (type-Ⅱ). The type-Ⅰ structure interacts strongly with the support, while the type-Ⅱ has weak interaction with support and exhibits greater hydrogenation activity^[20]. In general, the reduction of MoO₃ could be divided into two steps: low-temperature reduction and high-temperature reduction, and MoO₃ with low-temperature reduction were generally accepted to be the precursor of the active type-Ⅱ Ni-Mo-S phase^[21]. As seen in Figure 7, the low-temperature peak location of Cat-S shifts toward higher temperature, demonstrating that the metal species have stronger interaction with the support in Cat-S than that in Cat-M. But it is worth noting that the low-temperature peak area in Cat-S is higher than Cat-M, which illustrates that there is more MoO₃ phases to be converted to type-Ⅱ Ni-Mo-S phases.

  Figure 7

  

  Figure 7.  H₂-TPR profiles of two catalysts

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  2.6   HRTEM analysis

  Figure 8 shows the TEM images of these two catalysts. It is found that the layer number of MoS₂ slab in Cat-S is higher than that in Cat-M. To investigate directly the stacking numbers and lengths of the MoS₂ slabs in these two catalysts, the statistical results, on the basis of enough MoS₂ slabs (no less than 300) from 10 TEM images selected in different areas of one sample, are calculated and shown in Table 3 and Figure 9. The average slab length, layer number and fraction available Mo are calculated from the following equations described in the previous report^[15].

  Figure 8

  

  Figure 8.  HRTEM images of (a) Cat-S and (b) Cat-M

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  Figure 9

  

  Figure 9.  MoS₂ stacking number distribution of two catalysts

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  Table 3

  

  Table 3.  Average stacking degree (N_A), average slab length (L_A) and fraction available Mo (f_Mo) of two catalysts

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  Catalyst	NA	LA /nm	fMo
  Cat-M	3.1	3.52	0.32
  Cat-S	3.6	4.32	0.27

  $ {L_{\rm{A}}} = \frac{{\sum\limits_{i = 1}^n {{n_i}{L_i}} }}{{\sum\limits_{i = 1}^n {{n_i}} }} $

  (1)

  $ {N_{\rm{A}}} = \frac{{\sum\limits_{i = 1}^n {{n_i}{L_i}} }}{{\sum\limits_{i = 1}^n {{n_i}} }} $

  (2)

  $ {f_{{\rm{Mo}}}}{\rm{ = }}\frac{{\sum\limits_{i = 1....t} {6{m_i}-6} }}{{\sum\limits_{i = 1....t} {3m_i^2-3{m_i} + 1} }} $

  (3)

  where, L_i is the length of slab, N_i is the number of layers in the particles i, f_Mo is the average fraction of Mo atoms at the edge and corner of MoS₂ particle surface, n_i is the number of the particles with L_i length or N_i layers, m_i is the number of Mo atoms along one side of a MoS₂ slab determined from its length (L= 3.2(2m_i-1) (nm)), and t is the total number of slabs shown by the HRTEM images.

  For Cat-S, the average slab length is 4.32 nm and the average layer number is 3.6. Compared with Cat-M, Cat-S exhibits a broader slab length and higher layer number. High MoS₂ stacking degree in these two sulfided catalysts illustrates that MoS₂ phase in them are mainly type-Ⅱ NiMoS type. Higher layer number of MoS₂ particle implies that Cat-S forms more reductive type-Ⅱ NiMoS type than Cat-M. But it only provides a smaller density of multivacancies and weaker HYD activity because of longer MoS₂ slab and smaller f_Mo, which is the average fraction of Mo atoms at the edge and corner of MoS₂ particle surface, compared with Cat-M^{[22, 23]}.

  2.7   HDS activity evaluation

  The sulfur compounds in the diesel used in this study are mainly alkyl-DBT, described elsewhere^[15]. The hydrotreating activity results of FCC diesel and S removal rate of DBT, 4-MDBT and 4, 6-DMDBT in the diesel over these two catalysts are showed in Table 4 and Table 5, respectively. From Table 4 and Table 5, it can be seen that Cat-S exhibits a lower HDS conversion compared with Cat-M. The decrease in HDS activity for Cat-S is potentially associated with the lower hydrogenation activity. It was reported that the MoS₂ slabs length influenced the hydrogenation performance of catalyst because its corner and edge sites were believed to be the active sites of hydrogenation^[24]. The hydrogenation activity of catalyst increases when the amount of its corner and edge sites increases. Cat-S provides less corners and edges leading to its weak hydrotreating activity.

  Table 4

  

  Table 4.  Hydrotreating activity results of FCC diesel over two catalysts

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  	Feedstock	Cat-M	Cat-S
  d20 ℃/(g·cm-3)	0.970	0.930	0.935
  S concentration, w/(μg·g-1)	7216	652	981
  HDS rate/%	-	91.7	86.4
  Distillation range t/℃
  IBP	222	198	201
  5%	234	214	219
  10%	245	221	226
  50%	288	249	255
  90%	357	319	306
  FBP	380	326	327

  Table 5

  

  Table 5.  S removal rate of DBT, 4-MDBT and 4, 6-DMDBT over different catalysts

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  Removal rate	Cat-M	Cat-S
  DBT/%	99.93	95.47
  4-MDBT/%	81.11	76.15
  4, 6-DMDBT/%	63.42	61.22

  It is well known that many factors influence jointly the HDS performance of catalyst. Cat-S possesses less acid amount, which is favorable for the decrease of deep-cracking of feedstock and increase of liquid yield as proved by distillation range data in Table 4. The catalytic reaction takes place through accessible active sites on the surface areas in pore of catalyst. The poor textural property is partly responsible for the weak HDS activity of Cat-S.

  3.   Conclusions

  The catalyst prepared by mechanical mixing method has many advantages compared with the catalyst prepared by sol-gel method, such as better textural property, more acid amount, superior metal reducibility and higher Mo atoms dispersion, which make for the HDS performance of catalyst. The sol-gel method makes more MoO₃ phases to be converted to active type-Ⅱ NiMoS phase and leads to higher layer number of MoS₂ slabs, which is in favor of the formation of more active NiMoS phase. Furthermore, its less acid amount is favorable for the decrease of deep-cracking for diesel. The inferior textural property of the catalyst prepared by sol-gel method is presumed to be the main reason for the low HDS activity because there is no any optimization measure to improve its textural characteristics. Thus, the next research is suggested to focus on the improvement of the catalyst structure by the solvo-thermal treatment or the addition pore-expanding agents.

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• 

  Figure 1  XRD patterns of two sulfide catalysts and Al₂O₃

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  Figure 2  XRD patterns of nano Y zeolite after calcinations

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  Figure 3  N₂ adsorption-desorption isotherms of two catalysts

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  Figure 4  Pore size distribution of two catalysts

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  Figure 5  NH₃-TPD profiles of two catalysts

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  Figure 6  FT-IR profiles of two catalysts

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  Figure 7  H₂-TPR profiles of two catalysts

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  Figure 8  HRTEM images of (a) Cat-S and (b) Cat-M

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  Figure 9  MoS₂ stacking number distribution of two catalysts

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  Table 1.  Textural properties of two catalysts

  Sample	SBET/ (m2·g-1)	Smi/ (m2·g-1)	Pore volume v/(cm3·g-1)	Pore size d/nm
  Cat-S	124.9	15.3	0.27	9.1
  Cat-M	207.6	27.6	0.41	9.2

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  Table 2.  Amounts of the acid sites of two catalysts

  Sample	Amount of acid sites/(mmol·g-1 catalyst)		B/L (ratio)
  	L	B
  Cat-S	0.32	0.27	0.84
  Cat-M	0.53	0.45	0.85

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  Table 3.  Average stacking degree (N_A), average slab length (L_A) and fraction available Mo (f_Mo) of two catalysts

  Catalyst	NA	LA /nm	fMo
  Cat-M	3.1	3.52	0.32
  Cat-S	3.6	4.32	0.27

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  Table 4.  Hydrotreating activity results of FCC diesel over two catalysts

  	Feedstock	Cat-M	Cat-S
  d20 ℃/(g·cm-3)	0.970	0.930	0.935
  S concentration, w/(μg·g-1)	7216	652	981
  HDS rate/%	-	91.7	86.4
  Distillation range t/℃
  IBP	222	198	201
  5%	234	214	219
  10%	245	221	226
  50%	288	249	255
  90%	357	319	306
  FBP	380	326	327

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  Table 5.  S removal rate of DBT, 4-MDBT and 4, 6-DMDBT over different catalysts

  Removal rate	Cat-M	Cat-S
  DBT/%	99.93	95.47
  4-MDBT/%	81.11	76.15
  4, 6-DMDBT/%	63.42	61.22

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