Liu Di; Liu Chenguang

(1. College of Chemical and Enνironmental Engineering, Shandong Uniνersity of Science and Technology, Qingdao, Shandong 266590; 2. State Key Laboratory of Heaνy Oil Processing, Key Laboratory of Catalysis, CNPC, China Uniνersity of Petroleum, Qingdao, Shandong 266555)

Deep Hydrodesulfurization of Diesel Fuel over Diatomite-dispersed NiMoW Sulfide Catalyst

Liu Di1; Liu Chenguang2

(1. College of Chemical and Enνironmental Engineering, Shandong Uniνersity of Science and Technology, Qingdao, Shandong 266590; 2. State Key Laboratory of Heaνy Oil Processing, Key Laboratory of Catalysis, CNPC, China Uniνersity of Petroleum, Qingdao, Shandong 266555)

Diatomite-dispersed NiMoW catalyst was prepared and characterized, and the activity of catalyst samples was tested during the HDS reaction of FCC diesel. Sulfur compounds in the feedstock and the hydrogenated products obtained over different catalysts were determined by GC-PFPD. The test results showed that the diatomite-dispersed NiMoW catalyst had high hydrodesulfurization activity for FCC diesel, which could be contributed to the excellent hydrogenation performance of the said catalyst. Characterization of catalyst by TEM and XRD indicated that the diatomite-dispersed NiMoW catalyst possessed higher layer stacking, larger curvature of MoS2or WS2, and segregated Ni3S2crystals relative to the supported catalyst. This kind of structure leads to high hydrogenation activity of the diatomite-dispersed NiMoW catalyst.

NiMoW catalyst; diatomite; hydrodesulfurization; FCC diesel; GC-PFPD

1 Introduction

Environmental concerns lead to increasingly stringent regulations on sulfur, nitrogen and aromatics content in fuels. Utilization of more active catalysts for hydrotreating and particularly hydrodesulfurization (HDS) of liquid fuel would be the less costly and the most effective way for the refineries, because it would allow for avoiding modifications of plant installations. Usually the HDS catalysts contain molybdenum sulfide promoted with cobalt or nickel and supported on a high surface area alumina[1-2], but it is difficult to prepare a kind of catalysts with higher activity by this way because of the limitation of porous structure of conventional catalyst carrier. We have reported a silica-dispersed NiMo HDS catalyst with high activity[3], herein a NiMoW catalyst dispersed by diatomite was prepared, which was characteristic of high active metals content compared with conventional supported catalyst. Diatomite was employed to disperse active metals for obtaining high catalytic activity in the catalyst system. Although such catalysts have studied, the reason for excellent catalytic performance of such catalysts compared with conventional supported catalyst has been rarely reported in the literature.

It is known that since sulfur species exist in various types and amounts in diesel fuel, the catalyst must have the ability to remove all these sulfur compounds, especially 4-methyldibenzothiophene (4-MDBT) and 4, 6-dimethyldibenzothiophene (4, 6-DMDBT) upon achieving the goal of ultra-deep HDS for diesel fuel. Therefore, the catalyst structure and its performance for removal of various sulfur compounds in FCC diesel were investigated precisely to explore the cause of high catalytic activity of diatomite-dispersed NiMoW catalyst in this paper. In addition, the supported catalyst was also evaluated under the same operating conditions for comparison.

2 Experimental

2.1 Catalyst preparation

All reagents were of analytically pure grade that were purchased from the Sinopharm Chemical Reagent Co., Ltd., and were used without further purification.

(1) Diatomite-dispersed NiMoW catalyst was prepared according to the following procedure: The diatomite was added to a water solution containing urea, nickel nitratehexahydrate, ammonium molybdate and ammonium metatungstate. The solution was heated to 95 ℃ and was maintained at that temperature for 8 h under vigorous stirring. After the reaction was completed, a precipitate was formed. The obtained precipitate was filtered, washed with water and dried at 90 ℃ for 12 h. Finally, the diatomitedispersed NiMoW catalyst was obtained by calcining the precipitate at 385 ℃ for 4 h and was labeled as NiMoW/ diatomite. The composition of the starting water solution was specified at a molar ratio of Ni/Mo/W/urea=4:1:1:10.

(2) The supported catalyst was prepared by wet impregnation of trilobal Al2O3support (including 10 wt% of molecular sieve) with an aqueous solution containing nickel nitrate hexahydrate, ammonium molybdate and ammonium metatungstate (at a molar ratio of Ni/Mo/W=4:1:1), followed by drying at 60 ℃ for 16 h and then at 100 ℃ for 24 h. This reference catalyst was labeled as NiMoW/Al2O3.

2.2 Characterization of catalyst samples

The N2adsorption and desorption isotherms were measured on a Micromeritics Tristar 3000 instrument. The specific surface area determination was carried out using the BET isotherms. The X-ray powder diffraction (XRD) analysis was carried out with a X’Pert PRO MPD diffractometer using a CuKα radiation (λ=0.154 060 nm) operating at 40 kV and 40 mA. PANalytical Axios X-ray fluorescence (XRF) spectrometer was used to perform elemental analysis. The transmission electron microscopy (TEM) analysis was performed on a JEOL JEM-2100UHR transmission electron microscope. A small amount of sample was ground with a mortar and pestle. The sample was then suspended in ethanol and sonicated. A drop of the suspension was put on a lacey carbon film supported by a Cu grid.

2.3 Catalytic tests

The HDS of FCC diesel was carried out in a fixed bed flow reactor. Before the reaction, the catalysts were presulfurized with a 3 m% of carbon disulfide (CS2) in cyclohexane solution at 340℃ and 4 MPa for 12 h. After the pretreatment was completed, H2and FCC diesel were routed into the reactor. The reaction was carried out at a reaction temperature in the range of between 320℃ and 360℃, a H2pressure of 6.0 MPa, a H2flow rate of 0.225 dm3/min, and a LHSV of 1.5 h-1. NiMoW/Al2O3catalyst was also evaluated under the same operating conditions for comparative purpose. The liquid products collected from a gas-liquid separator were analyzed qualitatively with a Jena Multi EA 3100 Elementary Analysis Systems. The distribution of sulfur compounds in diesel was analyzed quantitatively using a Varian CP 3800 gas chromatograph with a pulsed flame photometric detector (GC-PFPD). The density of diesel was determined by the hydrometer method. The specific HDS activity (R) was calculated by the following equation:

whereSpandSfstand for the sulfur content in product and feedstock when using the NiMoW catalyst, respectively, andSprandSfrfor the sulfur content in product and feedstock when using the reference catalyst, respectively.

3 Results and Discussion

When the diatomite-dispersed NiMoW catalyst samples were prepared, the diatomite was employed to disperse active metals for the full utilization of active catalyst components. Elemental analysis results showed the content of active components in final sample to be 63.8% (based on metallic oxide), and the active metal loading in the supported NiMoW catalyst to be 29.4%, so the diatomite-dispersed NiMoW catalyst featured high active metals content compared with conventional supported catalyst.

Table 1 lists the activity testing results of the diatomitedispersed NiMoW catalyst and NiMoW/Al2O3catalyst at different reaction temperature for FCC diesel fraction. It is shown from Table 1 that the HDS rates of the two catalysts increased with an increasing reaction temperature , whereas the diatomite-dispersed NiMoW catalyst required a nearly 20℃ lower reaction temperature to reach the same HDS rate compared to the supported catalyst, indicating to greater catalytic activity of diatomite-dispersed NiMoW catalyst. And we can also conclude by calculation that the specific HDS activity of NiMoW/diatomite is about 2.18 times higher than that of the supported catalyst (Table 1), but upon considering the different contents of active metals in NiMoW/diatomite and NiMoW/Al2O3(63.8% vs 29.4%), both catalysts seemed to have nearly the same specific HDS activity on the basis of the same weight of active metals. However, the utilization rate ofactive metals for NiMoW/diatomite catalyst was much lower compared with the supported catalyst due to the low surface area and pore volume of NiMoW/diatomite (see Table 2), thus it is clear that the diatomite-dispersed NiMoW catalyst had higher intrinsic HDS activity as compared to NiMoW/Al2O3catalyst.

Table 1 Analysis results of FCC diesel samples treated by different catalysts

In order to ascertain the cause of the difference between their catalytic activities, the removal of various sulfur compounds on diatomite-dispersed NiMoW and NiMoW/Al2O3catalysts was studied. Firstly, sulfur compounds in the feedstock and the hydrogenated products on different catalysts were determined by GC-PFPD (see Figure 1). And based on pertinent literature[4-5], the qualitative and quantitative analyses of individual sulfides were performed on the basis of the GC-PFPD chromatograms (Figure 2). Figure 1 shows that sulfur compounds in FCC diesel fraction include benzothiophene (BT), dibenzothiophene (DBT), and their alkyl-substituted derivatives, which can be grouped as C1BT—C6BT, C1DBT—C4DBT, and the contents of DBT and their alkyl-substituted derivatives cover above 80% of the total sulfur compounds in oils (Figure 2). In a comparison on the reactivity of various sulfur compound groups over different catalysts as depicted in Figure 3, it is found out that BT and their alkyl-substituted derivatives exhibit similar reactivity regardless of the type of the catalysts, whereas the conversion of DBT and alkyl DBTs over NiMoW/Al2O3is dramatically lower than that of CnBT over NiMoW/diatomite, and the differencebetween these catalysts on their ability to convert DBT derivatives widens as the number of carbon atoms in alkyl substituents increases, indicating that diatomite-dispersed NiMoW catalyst possesses higher catalytic activity for conversion of alkyl DBTs. Current study has showed that the removal of alkyl DBTs is mainly achieved through the hydrogenation route due to the hindrance effect of alkyl groups at the 4 and/or 6 positions of DBTs[6-7], so the above results can be explained by the reasoning that the diatomite-dispersed NiMoW catalyst has much higher hydrogenation activity than the supported catalyst. This result could be supported by the density variations of oil products, for example, the density of diesel fraction at 320 ℃ over the diatomite-dispersed NiMoW catalyst is almost on a similar level with that of diesel at 360 ℃over the supported catalyst (Table 1), which also reveals the excellent hydrogenation activity of diatomite-dispersed NiMoW catalyst.

Table 2 Textural properties of NiMoW/diatomite and NiMoW/Al2O3catalysts

Figure 1 GC-PFPD charts of the feedstock and the hydrogenated products obtained on different catalysts at 360℃

Figure 2 All kinds of sulfides content in FCC diesel fraction

Figure 3 HDS conversion of each group of sulfur compounds in FCC diesel fraction over different catalysts at 360℃

It is well-known that the catalyst performance depends on catalyst structure. Based on the structure–performance relationship, HRTEM characterization of two types of catalysts was carried out (Figures 4 and 5). Over the diatomite-dispersed NiMoW catalyst, MoS2or WS2can be observed with layers denoting generally stacking values between 2 and 5, and the supported catalyst displays lower average number of layers (1—2 layers) in the stacks. On the other hand, it is identified by XRD patterns of two catalysts in Figure 6 that the peak at 2θ=14.4° of diatomite-dispersed NiMoW catalyst, the intensity of which is representative of thecdirection layer stacking of the (002) basal planes of crystalline MoS2or WS2, is sharper than that of the supported catalyst, which demonstrates that the diatomite-dispersed NiMoW catalysts possesses higher layer stacking of MoS2or WS2. This result is in agreement with the TEM results. It was proposed and generally accepted that the stacking number of Mo(W)-sulphide slabs is a crucial structure parameter that determines the hydrogenation activity of MoS2or WS2[8-9]. Multilayered MoS2or WS2slabs provide a higher density of multiple vacancies compared with single-layered or thin slabs facilitating the π-complexation of the aromatic ring. This enhances the hydrogenation activity of multilayered MoS2or WS2slabs[9-12]. In addition, it is found from HRTEM images that the segregated Ni3S2crystals are in some cases “coated’’ with MoS2/WS2layers. On one hand theNi3S2crystals in the catalyst structure are serving as support for highly dispersed MoS2or WS2; on the other hand this structure results in a large curved MoS2or WS2stacks with numerous defects coupled with intergrowth, which can provide more coordinated unsaturated sites[13], and ultimately improve hydrogenation activity of catalyst. Consequently, we confirm that larger curvature of multilayered MoS2or WS2slabs, and the segregation of Ni3S2on diatomite-dispersed NiMoW catalyst are responsible for its excellent performance relating to HDS of diesel fraction.

Figure 4 HRTEM images of sulfided NiMoW/diatomite catalyst

Figure 5 HRTEM image of sulfided NiMoW/Al2O3catalyst

Figure 6 XRD patterns of NiMoW/diatomite and NiMoW/ Al2O3catalysts

4 Conclusions

In this paper, we prepared a kind of HDS catalyst—the NiMoW catalyst dispersed by diatomite. According to HDS test results for FCC diesel fraction, it is concluded that the diatomite-dispersed NiMoW catalyst has higher intrinsic HDS activity than the NiMoW/Al2O3catalyst. Characterization showed that the diatomite-dispersed NiMoW catalyst possessed higher layer stacking, larger curvature of MoS2or WS2, and the segregated Ni3S2crystals relative to supported catalyst. Thus, it is confirmed that the multilayered structure of active metals is the root cause of the excellent performance of diatomite-dispersed NiMoW catalyst in deep hydrodesulfurization of diesel fuels.

Acknowledgments:The authors are thankful to Shandong University of Science and Technology for the partial support of this research and for financial support of National Natural Science Foundation of China (Grant No.21306106), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No.2012371812012), and China Postdoctoral Science Foundation (Grant No.2012M541941).

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Recieved date: 2013-08-06; Accepted date: 2013-08-13.

Liu Di, E-mail: Ld002037132@163. com.