Shen Zhibing; Ke Ming; Ren Tao; Zhang Juntao; Liang Shengrong

(1. School of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an 710065; 2. State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249; 3. The Northwest Research Institute of Chemical Industry, Xi’an 710054)

Effect of Sulfurization Temperature on Thioetherification Performance of Mo-Ni/Al2O3Catalyst

Shen Zhibing1,2; Ke Ming2; Ren Tao3; Zhang Juntao1; Liang Shengrong1

(1. School of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an 710065; 2. State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249; 3. The Northwest Research Institute of Chemical Industry, Xi’an 710054)

The Mo modified Ni/Al2O3catalysts were prepared and sulfided at different temperatures, and their catalytic activity for thioetherification of mercaptans and olefins (or dienes), hydrogenation of dienes and olefins in the thioetherification process using fluidized catalytic cracking (FCC) naphtha as the feedstock was investigated. In order to disclose the correlation between the physicochemical characteristics of catalysts and their catalytic activity, the surface structures and properties of the catalysts sulfided at different temperatures were characterized by the high resolution transmission electronic microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and H2-temperature programmed reduction (H2-TPR) technique. The results showed that an increase of sulfurization temperature not only could promote the sulfurization degree of active metals on the catalysts, but also could adjust the micro-morphology of active species. These changes could improve the catalytic performance of thioetherification, and hydrogenation of dienes and olefins. However, an excess sulfurization temperature was more easily to upgrade the ability of the catalyst for hydrogenation of olefins, which could lead to a decrease of the octane number of the product. It was also showed that a moderate sulfurization temperature not only could improve the catalytic performance of thioetherification and hydrogenation of dienes but also could control hydrogenation of olefins.

sulfurization temperature; thioetherification; Mo-Ni/Al2O3catalysts; active structure; Ni-Mo-S phase

1 Introduction

Thioetherification reaction is an addition reaction of mercaptans with olefins or dienes, which can convert light mercaptans into heavy thioethers. This process can be used to remove mercaptans in the fluidized catalytic cracking (FCC) naphtha streams[1-2]. It has been reported that the olefins are mainly contained in the low boiling fraction of the FCC naphtha and they are major contributors to the octane number of gasoline. On the other hand, the predominant low-boiling sulfur compounds are mercaptans (RSH) contained in the LCN. Hence, thioetherification can be used to transform mercaptans in the light cracked naphtha (LCN) fraction into thioethers. The heavier molecular-weight thioethers can go into the heavy cracked naphtha (HCN) fraction. Then, the full gasoline fraction is routed into a distillation tower, in which LCN with abundant olefins and low sulfur contents is separated from HCN, which contains heavy boiling sulfur compounds, including thioethers. HCN is usually treated by hydrodesulfurization process for deep desulfurization of this fraction. Meanwhile, the di-olefins that have not been converted to thioethers can be simultaneously selectively hydrogenated to mono-olefins during the thioetherification process. Therefore, the combined process can also avoid polymerization of diolefins over the hydrodesulfurization catalysts to improve the stability of this fraction. Thioetherification, hydrogenation of dienes and hydrogenation of mono-olefins take place in the thioetherification process. And hydrogenation of mono-olefins leads to the loss of the octane rating of the gasoline product. Therefore, the catalysts for thioetherification process should be modified to improve their ability for thioetherification and hydrogenation of dienes and reduce their ability for hydrogenation of olefins.

The reaction mechanism and reaction performance ofthioetherification over Ni/Al2O3and Mo modified Ni/Al2O3catalysts were researched in our recent study[3-6]and other works[7-8]. These sulfided transition metals were usually considered to have good catalytic activity for thioetherif ication and hydrotreating reactions. The micro-structures and properties of these types of catalysts have also been studied extensively for the hydrodesulfurization and hydrodenitrogenation process. And some structural models have been proposed properly. For example, a famous Ni(Co)-Mo(W)-S model was presented by Tops?e, et al.[9-10], who believed that sulfurization of the oxide phases can lead to the formation of stacks of MoS2slabs over the support surface and the Ni or Co species are located primarily on the edges of these stacks to form Ni(Co)-Mo-S model. Then the sulfided Co–Mo or Ni–Mo (W)-based HDS catalysts are further studied. There are two types of the Ni(Co)-Mo(W)-S phase, namely the type I Ni(Co)-Mo(W)-S and the type II Ni(Co)-Mo(W)-S[11-14]. Although the origin of the two types of the Ni (Co)-Mo (W)-S phase is still controversial, it is suggested that the type I Ni (Co)-Mo (W)-S phase is related to highly dispersed single slab of MoS2particles with strong interaction with the support, while the type II Ni (Co)-Mo (W)-S is related to the weakly dispersed MoS2particles that are mainly stacked and not linked with the support. It is considered that the latter has a higher intrinsic activity for HDS than the former[10,15-16].

Sulfurization process of the supported Ni (Co)-Mo (W) catalysts is usually considered to have a key impact on the structures and the hydrotreating reaction performance. Okamoto, et al. showed that the sulfurization temperature could decide the structures of the type II Ni(Co)–Mo(W)–S catalysts and their intrinsic hydrodesulfurization activity[15]. Therefore, it is necessary to investigate the effect of modified Ni/Al2O3catalysts (Mo-Ni/Al2O3) sulfided at different temperatures on their structure and catalytic performance. In this work, the catalysts were also characterized by X-ray fluorescence (XRF) spectrometry, high resolution transmission electronic microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and H2-temperature programmed reduction (H2-TPR) techniques. The experiments would contribute to disclosing the relation between the physico-chemical properties and micro-structures of catalysts and their reaction performance of thioetherification and hydrogenation of dienes and olefins.

2 Experimental

2.1 Catalyst preparation and evaluation

The Mo-Ni/Al2O3catalysts were prepared via the co-impregnation of the appropriate amounts of Ni (NO3)2·6H2O and (NH4)6Mo7O24·4H2O precursors onto the commercial γ-Al2O3support. The catalysts were obtained by drying at 100 ℃ for 6 h followed by calcination at 550 ℃ for 4 h. The textural properties of Mo-Ni/Al2O3catalysts are depicted in Table 1. And other physico-chemical properties of the oxidized and sulfided Mo-Ni catalysts were presented in our previous papers[4-5].

Nickel nitrate hexahydrate (with a purity of 98%), ammonium molybdate tetrahydrate (with a purity of 97%), n-hexane (with a purity of 95%) and carbon disulfide (with a purity of 99%) were purchased from the Sinopharm Chemical Reagent Co., Ltd., and hydrogen was purchased from the Beijing Haipu Gas Company.

Catalytic performance of the catalysts sulfided at different temperatures was tested in a fixed bed flow reactor system, using FCC naphtha (containing 20.1 μg/g of mercaptan-sulfur with a diene value of 0.56 gI2/100g) as the feedstock. The FCC naphtha was provided by the Dagang Oil Refinery, PetroChina Company, Ltd. Five ml of catalysts sample were loaded in a stainless steel tube reactor for evaluating its activity. Firstly, the catalysts were in situ sulfurized with n-hexane containing 2% of CS2. The sulfurization temperature ranged from 290 ℃ to 320 ℃. After sulfurization, the liquid feed and H2were charged into the reactor by a plunger pump and a mass flow controller, respectively. The experiments were carried outunder the operating conditions covering a pressure of 1 MPa, a reaction temperature of 90 ℃, a volume ratio of H2versus liquid feed of 10, and a liquid hourly space velocity (LHSV) of 4 h-1.

2.2 Analysis

The hydrocarbon composition of the feedstocks and products were determined with an Agilent 1790 gas chromatograph (GC) equipped with a flame ionization detector (FID) and a HP-PONA capillary column (50 m×0.2 mm). The temperature in the detector and the injector was both 250 ℃. The split ratio was 1/80. The column oven temperature was increased from 40 ℃ to 150 ℃ at a heating rate of 8 ℃/min.

The mercaptan-sulfur contents of the feed and products were analyzed by the potentiometric titration method according to ASTM D3227-83.

The diolefins content was expressed by the diene value and was measured following the UOP method 326-82.

2.3 Characterization

The chemical composition of the Mo-Ni/Al2O3catalysts was measured with a XRF spectrometer (Rigaku’s ZSX Primus equipment, Rigaku Co., Japan).

The HRTEM micrographs of the sulfided catalysts were obtained on a Philips Tecnai G2 F20 transmission electron microscope (Philips Co., Netherlands). The solid powders were ultrasonically dispersed in cyclohexane and the test samples were prepared by dropping the dispersed suspensions on carbon-coated copper grids.

XPS of the samples was obtained using an ESCALab250 electron spectrometer made by the Thermo Scientific Corporation (USA) with monochromatic 150W Al Kα radiation. The pass energy for the narrow scan was 30 eV. The base pressure was about 6.5×10-8Pa. The binding energies were referenced to the Al2p line at 74.7 eV.

The H2-TPR measurements were performed in a selfmade instrument equipped with a thermal conductivity detector. About 0.2 g of sulfided catalyst was placed in a quartz sample tube. Prior to TPR studies, each catalyst sample was pre-treated in an inert gas (at a flowrate of 40 mL/min) at 500 ℃ for 2 h. After the pre-treatment, the sample was cooled down to ambient temperature and the carrier gas (a mixture composed of 5 vol% of hydrogen and 95 vol% of argon) at a flow rate of 40 mL/min was allowed to pass over the sample. Then, the analytical temperature of the sample was increased from 110 ℃to 800 ℃ at a heating rate of 10 ℃/min.

3 Results and Discussion

3.1 Effects of sulfurization temperatures on the catalytic activity of Mo-Ni/Al2O3catalyst

The reaction performance of the thioetherification process was tested over the Mo-Ni/Al2O3catalysts sulfided at different temperatures using FCC naphtha as the feedstock. The results are displayed in Figure 1. The test results showed that as sulfurization temperature of the catalysts was increased, the catalytic activity for thioetherification reaction, selective hydrogenation of dienes and hydrogenation of mono-olefins could be improved. When the sulfurization temperature increased to 300 ℃, the catalyst demonstrated good reaction performance for thioetherification reaction and selective hydrogenation of dienes. The conversion rate of mercaptans and dienes was 96% and 91%, respectively. Under this condition, only a small amount of olefins was hydrogenated over the sulfided Mo-Ni/Al2O3catalysts. When the temperature continued to increase to 310 ℃, the performance for thioetherification reaction and selective hydrogenation of dienes was synchronously promoted and their conversion rates reached nearly 99%. The hydrogenation rate of olefins was still less than 8%. However, as temperature was further in-creased, there had been much fewer margins to facilitate performance of thioetherification reaction and selective hydrogenation of dienes. Conversely, the performance of olefins hydrogenation was significantly increased. The hydrogenation rate of olefins was 17% and 33% at 320 ℃and 330 ℃, respectively.

Figure 1 Reaction performance of the catalysts sulfided at different temperatures■—Conversion of Mercaptan;■—Selective hydrogenation of diene;■—Hydrogenation of olefinReaction conditions: temperature=90 ℃, pressure= 1 MPa, SV=4 h-1, H2/oil volume ratio=10

These results showed that the sulfurization temperature had an important impact on catalytic performance. An appropriate sulfurization temperature not only could promote thioetherification and selective hydrogenation of dienes, but also could avoid an excess of olefins being hydrogenated. Therefore, the optimal sulfurization temperature of Mo-Ni/Al2O3catalyst for the thioetherification process should be specified as 310 ℃.

3.2 Effects of sulfurization temperature on the micro-structure and properties of Mo-Ni/Al2O3catalysts

The sulfurization temperature of catalysts could decide the type of active species, their micro-structure and physico-chemical properties, which would also affect the catalytic performance in the thioetherification process. Therefore, it is necessary to verify that the catalysts which were sulfided at different temperatures were characterized to show the relationship between the structure and activity of the catalysts.

The H2-TPR technique of the sulfided catalysts at different temperatures was performed to investigate the type of the metal species of catalysts and the interaction between Ni, Mo and the support. Figure 2 shows the H2-TPR profiles of the sulfided Ni-Mo/Al2O3catalysts. In our previous study[4], we had found out that the reduction peak of the sulfided Ni-Mo/Al2O3catalysts at around 483 ℃ could be assigned to the NiO species in the catalysts. The signal at around 778 ℃ could be caused by a strong reduction peak of the Ni–Al spinel phase. And the peak at around 248 ℃ was considered as the Ni-Mo-S phase related with the active sites of hydrodesulfurization (or hydrodenitrogenation)[17-18].

Figure 2 shows that when the sulfurization temperature was increased from 290 ℃ to 330 ℃, the amount of Ni-Mo-S phase at a reduction temperature of between 200 ℃and 250 ℃ was gradually increased and the Ni–Al spinel phase was significantly decreased. This phenomenon clearly indicated that an increase of sulfurization temperature could improve the sulfurization degree of Ni, Mo and Ni-Al spinel species. Therefore, the amount of active sites could be increased with a rising temperature, which would improve the catalytic activity for the related reactions in the thioetherification process.

Figure 2 H2-TPR profiles of the catalysts sulfided at different temperatures

Figure 3 XPS spectra of the catalysts sulfided at different temperatures: (a) Ni 2p3/2, (b)Mo 3d

The XPS spectra of Ni 2p3/2and Mo 3d regions of Mo-Ni/Al2O3catalysts sulfided at different temperatures are given in Figure 3 and Table 2. The spectra of the catalystssulfided at 290 ℃ were deconvoluted by the Gaussian-Lorentzian curve-fitting method of XPS PEAK software. The peak at around 853.7eV (in Figure 3a) was ascribed to NiS or sulfided Ni species existing in the Ni-Mo-S phase[17,19]. The Ni-Mo-S phase was composed of MoS2and sulfided Ni species located in the edges or corners of needle-stacks of MoS2, which was considered as active sites in the hydrogenation reaction. Another peak between 856.4 eV and 858.0 eV was considered as different stoichiometric Ni-Al spinel species. And the peak at around 862.8 eV was a satellite peak of Ni species. It can be seen from Figure 3a and Table 2 that the sulfidation degree of Ni atoms was improved with the sulfurization temperature. Meanwhile, the amount of Ni-Al spinel species was decreased in this changing process. This experimental results were consistent with results of H2-TPR analysis.

Figure 3b shows that the Mo3d-S2s region of XPS of Mo-Ni/Al2O3catalysts was sulfided at different temperatures. The deconvolution results of the spectra for the catalysts sulfided at 290 ℃ clearly presented three different states of Mo (IV, V, VI), indicating that the sulfidation process was not complete[20-25]. It has been reported that Mo (IV) species exist as the sulfided phase (MoS2), and Mo (VI) species are in the oxidized phase and Mo (V) species are in an intermediate state of Mo oxysulfide (MoOxSy). The S2s region was at 226 eV, suggesting that the catalysts had been subject to sulfidation[21]. The quantitative results (listed in Table 2 based on the deconvoluted spectra) showed that the percentage of the sulfided Mo species was gradually improved with an increase in temperature. Upon combining the characterization result of Ni species with the reaction performance of the catalysts sulfided at different temperatures in Figure 1, it was inferred that improvement of sulfided Mo species could increase the amount of the Ni-Mo-S sites, which could improve significantly the reaction performance in the thioetherification process, especially for the hydrogenation of olefins.

The HRTEM micrographs of Mo-Ni/Al2O3catalysts sulfided at different sulfidation temperature are shown in Figure 4. The classical needle-stacks of MoS2could be observed in all micrographs. When the sulfurization temperature was 290 ℃, the sulfidation degree was only 52% (Table 2) and most of MoS2species formed the monolayer stack. This lamellar crystal of MoS2had a high degree of dispersion and was ascribed to the type I Ni-Mo-S phase[11-13]. This phase was considered to have weak activity for hydrodesulfurization. However, the results of Figure 1 show that the conversion rate of mercaptans was 86% and the selective hydrogenation rate of dienes was 80%, indicating that the type I Ni-Mo-S phase possessed a good performance for thioetherification and selective hydrogenation of dienes. It is also identified that the phase exhibited weak ability to hydrogenation of olefins.

As the sulfurization temperatures and the sulfidation degree of metal composition continued to rise, the layers of needle-stacks were gradually increased. When the temperature was 300 ℃ and 310 ℃, the thickness of needlestacks was increased to 2—3 layers and the type I Ni-Mo-S phase also began to be transformed to the type II Ni-Mo-S phase. The catalytic activity for thioetherification and selective hydrogenation of dienes reached a higher level. Their conversion rates were 97% and 96%, respectively. Meanwhile, the hydrogenation rate of olefins was still less than 7%. However, when the temperature rose to 320 ℃ and 330 ℃, the sulfidation degree of Mo species had been beyond 70% and the thickness of needlestacks started to be more than 4 layers. On account of its reaction performance, this structure had good activity for thioetherification and selective hydrogenation of dienes, while the side reaction — hydrogenation of olefins — was also sharply improved. Especially, the catalyst which was sulfided at a temperature of 330 ℃ could result in a 33% percent of hydrogenation rate of olefins, which had severely affected the octane number of product. Therefore, the ideal sulfurization temperature should be 310 ℃,which could make the catalysts retain their appropriate active structures, catalytic activity and selectivity.

Table 2 Sulfidation degree of Ni and Mo species in MN-3 catalysts conducted at different sulfidation temperatures

Figure 4 HRTEM micrographs of the catalysts sulfided at different temperatures

3 Conclusions

The sulfurization temperature has an important impact on structures and properties of Mo-Ni/Al2O3catalysts and their catalytic activity for thioetherification reaction. An increase of sulfurization temperature not only could promote the sulfurization degree of the catalysts and the distribution of active site species, but could also adjust the micro-morphology of active species. The results showed that the type I Ni-Mo-S phase with a monolayer stack has good catalytic performance for thioetherification and hydrogenation of dienes, which had weak ability for hydrogenation of olefins. As the sulfurization temperature was increased, the type I Ni-Mo-S phase was transformed to the type II Ni-Mo-S phase with multilayer stacks, which could further improve the catalytic activity for thioetherification, hydrogenation of dienes and hydrogenation of olefins, especially for hydrogenation of olefins. When the sulfurization temperature was 310 ℃, the 2—3 layers of short stacks of MoS2contributed to forming the type II Ni-Mo-S phase with Ni atoms. The catalyst also displayed a superior catalytic performance for thioetherification and hydrogenation of dienes with weak activity for hydrogenation of olefins.

Acknowledgements:The financial support provided by the National Natural Science Foundation of China (Granted No. 21276276) is gratefully acknowledged.

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date: 2015-04-08; Accepted date: 2015-09-24.

Dr. Shen Zhibing, E-mail: szb@xsyu. edu.cn, shen_zhibing@163.com.