Preparation and Evaluation of the Composite Containing USL Zeolite

Dec 14, 2009 - †State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, ... of Catalysis, China National Petrol...
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Energy Fuels 2010, 24, 796–803 Published on Web 12/14/2009

: DOI:10.1021/ef901098m

Preparation and Evaluation of the Composite Containing USL Zeolite-Supported NiW Catalysts for Hydrotreating of FCC Diesel Aijun Duan,† Zhenyong Gao,† Quan Huo,‡ Chengyin Wang,† Dengqian Zhang,† Mingcheng Jin,† Guiyuan Jiang,† Zhen Zhao,*,† Huifang Pan,‡ and Keng Chung§ †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, P.R. China, ‡The Key Laboratory of Catalysis, China National Petroleum Corp. (CNPC), China University of Petroleum, Beijing 102249, P.R. China, and § Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, T2N 1N4, Canada Received September 29, 2009. Revised Manuscript Received November 19, 2009

Two different ways, including an in situ synthetic method and a mechanical mixing method, were used to combine zeolite USL (ultra stable L) with alumina for preparation of a new composite support material of hydrotreating catalyst. The physicochemical properties of samples were characterized by means of XRD, N2 physisorption, SEM, FT-IR, 27Al MAS NMR, NH3-TPD, H2-TPR, and UV-vis DRS. Composite supports containing different contents of zeolite USL and Al2O3 were prepared by in situ synthetic method based on a modified pH-swing method, which showed a higher specific surface area, pore volume, as well as average pore diameter compared with the supports prepared via a mechanical mixing method. Corresponding NiW/γ-Al2O3-USL series catalysts were obtained by the incipient-wetness impregnation method, and the activities of these catalysts for FCC diesel hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) were evaluated in a high-pressure microreactor system. The assessment results indicated that the catalyst with 10 m% USL in the support prepared by the in situ method showed the highest HDS and HDN conversions, which reached a maximum of 99.3% and 94.1% for HDS and HDN, respectively. In addition, the swing pH method plays an important role in preparation of the Al2O3 support, and NiW/γ-Al2O3(P) (prepared by the swing pH method) catalyst also gave better performance for the HDS and HDN of diesel oil. These activities were much higher than those over a kind of industrial catalyst of RN10 and were also better than the corresponding catalyst in which the support was obtained by the mechanical mixing method.

activity and selectivity of HDS catalysts are obviously susceptible to their supports. The catalyst support plays a key role in the dispersion of the active components and promoters, and also affects the morphology and catalytic performance through metal-support interaction. So, increasing attention worldwide is being paid to the development and design of new supports for hydrotreating catalysts.7,8 Zeolites are well-known for their high specific surface areas and high activities in many catalytic reactions. However, high acidity of zeolites will result in deep cracking or secondary reactions in the catalytic processes. For the deep desulfurization reactions, for example, it should be required a suitable degree of acidic functionality in order to improve the ringopening of heterocyclic aromatic components, which is prior to the conversions of organic heteroatomic compounds to inorganic sulfur and nitrogen species.9 It has been reported that the incorporation of acidic zeolites in catalysts could promote the hydroisomerization and dealkylation activities of a catalyst.10,11 On the other hand, these acidic materials tended to possess either only micropores (less than 2 nm) or

1. Introduction With the increasing awareness of environmental protection, more stringent legislation was introduced throughout the world to limit the sulfur content of transportation fuels.1-3 On the contrary, crude oil and feedstocks for refinery processes to produce vehicle fuels became more and more heavy and deteriorative, leading to higher sulfur contents in FCC diesels and gasoline. The properties of those fuel products were far beyond the standards of clean fuel specifications. The traditional hydrotreating process is one of the most important techniques to remove the sulfur and nitrogen heteroatomic compounds for producing clean transportation fuels.4 However, it would be a great challenge to obtain the ultra-low sulfur fuels only depending on the common traditional hydrotreating techniques.5,6 Thus, how to improve the efficiency of hydrotreating catalysts is one of the most urgent choices for the ultra deep hydrodesulfurization (HDS) process. The *To whom correspondence should be addressed. Telephone: (þ8610) 89731586. Fax: (þ8610) 69724721. E-mail: [email protected]. (1) Duan, A. J.; Li, R. L.; Jiang, G. Y.; Gao, J. S.; Zhao, Z.; Wan, G. F.; Zhang, D. Q.; Huang, W. Q.; Chung, K. H. Catal. Today 2009, 140, 187–191. (2) Huang, W. Q.; Duan, A. J.; Zhao, Z.; Wan, G. F.; Jiang, G. Y.; Dou, T.; Chung, K. H.; Liu, J. Catal. Today 2008, 131, 314–321. (3) Li, X.; Wang, A. J.; Egorova, M.; Prins, R. J. Catal. 2007, 250, 283–293. (4) Landau, M. V.; Herskowitz, M.; Hoffman, T.; Fuks, D.; Liverts, E.; Froumin, D.V. N. Ind. Eng. Chem. Res. 2009, 48, 5239–5249. (5) Saih, Y.; Segawa, K. Appl. Catal., A 2009, 353, 258–265. (6) Song, C. S. Catal. Today 2003, 86, 211–263. r 2009 American Chemical Society

(7) Breysse, M.; Afanasiev, P.; Geantet, C.; Vrinat, M. Catal. Today 2003, 86, 5–16. (8) Murali Dhar, G.; Srinivas, B. N.; Rana, M. S.; Kumar, M.; Maity, S. K. Catal. Today 2003, 86, 45–60. (9) Arias, P. L.; Cambra, J. F.; G€ uemez, B.; Legarreta, J. A.; Pawelec, B.; Fierro, J. L. G. Bull. Soc. Chim. Belg. 1995, 104, 197–204. (10) Isoda, T.; Nagao, S.; Ma, X. L.; Korai, Y.; Mochida, I. Energy Fuels 1996, 10, 1078–1082. (11) Bataille, F.; Lemberton, J. L.; Perot, G.; Leyrit, P.; Cseri, T.; Marchal, N.; Kasztelan, S. Appl. Catal., A 2001, 220, 191–205.

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only mesopores (2-50 nm), in most cases only micropores. The small microporous configurations limited the diffusion of external molecules to access the catalytic active sites inside the micropores, and also weakened the following diffusion processes of product molecules from the catalytic active sites to the outside of the pores. Therefore, the reasonable combination of zeolite with other materials would not only adjust the acidic functionality and metal-support interaction but also improve the pore structures of catalysts. Generally, after HDS process, the sulfur molecules remaining in the fuel products are mainly refractory dialkyldibenzothiophenes, such as 4,6dimethyldibenzothiophene (4,6-DMDBT), which are difficult to be removed from the fuels via hydrogenolysis route.12-15 The highly active catalysts with suitable acidity and adaptable surface physicochemical properties should be facilitated to ameliorate the mass-transfer steps, and will also be propitious to increase the efficiency of the catalyst utilization. As a kind of special zeolite, zeolite L possesses a LTL type framework topology and hexagonal crystalline structure with one-dimensional configuration, which is bounded by 12-membered-ring pores with an aperture of about 0.71 nm, leads to the formation of cavities of about 0.48  1.24  1.07 nm. The Si/Al ratio of framework is typically 3.0. Hence, a typical chemical composition of zeolite L is K9(AlO2)9(SiO2)27 per unit cell (puc).16-19 This type of zeolite L was mostly used in the processes such as Fischer-Tropsch (F-T) Synthesis,20 dehydrogenation and cyclization,21,22 catalytic reforming,23 and other processes.24 However, there are few published research reports on using zeolite L in HDS process,25,26 especially no open report to use zeolite L as acidic components of catalyst for upgrading the low grade diesel distillates in the hydrotreating process. The design and composite method of zeolite L combine with conventional NiMo or NiW/Al2O3 catalysts should be further investigated to analyze the potential utilization in the catalytic HDS process. In this work, with the aim to increase the hydrogenation activity of NiW catalysts, some novel type of hydrotreating catalyst supports containing zeolite USL were prepared by using the mechanical mixing method and in situ synthetic method based on a modified pH-swing method. The catalytic activity measurements were carried out to investigate the

effects of zeolite contents in composite supports on HDS and HDN performances of NiW/Al2O3-USL series catalysts with diesel feedstock. 2. Experimental Section 2.1. Preparation of Al2O3-USL Composite Supports. Zeolite USL (SiO2/Al2O3 molar ratio = 12.9) in the protonic form was supplied by the Key Laboratory of Catalysis, China National Petroleum Corp. of China University of Petroluem.27 Two types of composite supports were used in this work. One was a physical mixture of alumina and zeolite L that was denoted as Al2O3-USL(M) and obtained by mechanical mixing method, and the other was a micromesoporous composite prepared by in situ overgrowing mesoporous alumina over USL zeolite crystals. The obtained composite support was denoted as Al2O3-USL (C). 2.1.1. In Situ Synthetic Method. Alumina support and Al2O3-USL(C) composite supports were all synthesized by a modified pH-swing method28 in this study. All preparation operations were carried out at a constant temperature of 70 °C. The initial pH-swing cycle was as follows: (1) 60 mL of aqueous Al2(SO4)3 3 18H2O (acid pH) was contained in a glass vessel, denoted as solution A; (2) an aqueous NaAlO2 solution (basic pH), denoted as solution B; was dropped into solution A until the mixture reached pH 9, and aged for 5 min; (3) then another 60 mL of aqueous Al2(SO4)3 3 18H2O solution was added dropwise until the whole solution reached to about pH 3. (4) A white precipitate appeared and kept for 5 min, and was continually dropping with NaAlO2 solution until the pH was 9. This stage was the first pH-swing cycle. In this work, the above process was repeated for 4 times to obtain the support materials. After completing pH-swing cycles for four times, the final solution was aged for 60 min at pH of 9. The resulting slurry was filtered and washed by deionized water at 60 °C, then the obtained cake was dried at 120 °C and calcined at 550 °C for 4 h. In the mentioned procedure above, zeolite USL were added into the composite at pH of 3 in the first cycle. Hereby the obtained supports were labeled as Al2O3-USL-x(C), where x represented the contents of USL in the total supports, and is equal to 0, 10, 20, 30, and 40 m% respectively. 2.1.2. Mechanical Mixing Method. For comparison purposes, Al2O3-USL(M) were also obtained. The mechanical mixture of USL, pseudoboehmite powder, some amounts of nitric acid, and deionized water were first extruded, then dried at 120 °C for 4 h, and finally calcined at 550 °C for 4 h. 2.2. Preparation of Supported NiW Catalysts. The Al2O3USL or Al2O3-supported NiW catalysts were prepared by coimpregnation and the incipient-wetness method with an aqueous solution of the appropriate amounts of ammonium metatungstate hydrate [(NH4)6W12O39 3 H2O] and nickel nitrate hexahydrate [Ni(NO3)2 3 6H2O]. After the impregnation step, the obtained precursors were dispersed in an ultra sonic bath for 20 min. The prepared catalyst samples were dried at 120 °C for 12 h and calcined at 550 °C for 4 h. All the catalysts were loaded with the constant amounts of W and Ni contents (27 m% WO3, 3.5 m% NiO). A kind of commercial catalyst RN-10 was used as the reference catalyst. 2.3. Catalyst Characterizations. The composite supports and the catalysts were characterized by means of XRD, SEM, N2 physisorption, FT-IR, 27Al MAS NMR, NH3-TPD, H2-TPR, and UV-vis DRS, etc. X-ray powder diffraction (XRD) profiles were recorded in an XRD-6000 diffractometer using Cu KR radiation under 40 kV, 30 mA, scan range from 5 to 75° at a rate of 4° 3 min-1. BET surface area, average pore volume, and pore diameter were determined by using the ASAP 2020 automated

(12) Meng, X. C.; Wu, Y. X.; Li, Y. D. J. Porous Mater. 2006, 13, 365– 371. (13) Zhang, H. J.; Meng, X. C.; Li, Y. D.; Lin, Y. S. Ind. Eng. Chem. Res. 2007, 46, 4186–4192. (14) Song, C.; Ma, X. Appl. Catal., B 2003, 41, 207–238. (15) Bataille, F.; Lemberton, J. L.; Michaud, P.; Perot, G.; Vrinat, M.; Lemaire, M.; Schulz, E.; Breysse, M.; Kasztelan, S. J. Catal. 2000, 191, 409–422. (16) Pichat, P.; Franco Parra, C.; Barthomeuf, D. J. Chem. Soc., Faraday Trans. I 1975, 71, 99. (17) Bhat, S. D.; Niphadkar, P. S.; Gaydhankar, T. R.; Awate, S. V.; Belhekar, A. A.; Joshi, P. N. Microporous Mesoporous Mater. 2004, 76, 81–89. (18) Koa, Y. S.; Ahn, W. S. Powder Technol. 2004, 145, 10–19. (19) Ohgushi, T.; Matsuo, T.; Satoh, H.; Matsumoto, T. Microporous Mesoporous Mater. 2009, 117, 472–477. (20) Bengoa, J. F.; Alvarez, A. M.; Cagnoli, M. V.; Gallegos, N. G.; Yeramian, A. A.; Marchetti, S. G. Mater. Lett. 2002, 53, 6–11. (21) Bernard, J. R. Proc. Int. Conf. Zeolites, 5th. Naples, Italy, June 2-6, 1980; Rees, L. V. C., Ed.; Heyden: London, Philadelphia, pp 686-695. (22) Fang, X. G.; Li, F. Y.; Luo, L. T. Appl. Catal., A 1996, 146, 297–304. (23) Triantafillou, N. D.; Deutsch, S. E.; Alexeev, O.; Miller, J. T.; Gates, B. C. J. Catal. 1996, 159, 14–22. (24) Ban, T.; Morimoto, J.; Ohya, Y. Mater. Chem. Phys. 2008, 109, 347–351. (25) Taniguchi, M.; Imamura, D.; Ishige, H.; Ishii, Y.; Murata, T.; Hidai, M.; Tatsumi, T. J. Catal. 1999, 187, 139–150. (26) Tatsumi, T.; Taniguchi, M.; Ishige, H.; Ishii, Y.; Murata, T.; Hidai, M. Appl. Surf. Sci. 1997, 121-122, 500–504.

(27) Huo, Q.; Li, Q.; Xu, Q. H.; Dou, T.; Pan, H. F. J. Fuel Chem. Technol. 2007, 35, 302–307. (28) Ono, T.; Ohguchi, Y.; Togari, O. Preparation of Catalysts III, Elsevier Science Publisher: The Netherlands, 1983; pp 631-641.

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Table 1. Typical Properties of Diesel Feedstock properties

data -3

density @ 20 °C/g 3 cm S (μg 3 g-1) N (μg 3 g-1)

0.8798 1290 907

Distillation (°C) (ASTM D-86) IBP 30% 50% 70% FBP

158 212 238 279 374

Hydrocarbon Compositions (% vol) aromatics olefin saturated hydrocarbon

56.9 8.4 34.7

Figure 1. N2 adsorption-desorption isotherms of Al2O3-USL40(M) and Al2O3-USL-40(C).

gas adsorption system under vacuum prior to N2 adsorption at -196 °C. Scanning electron microscopy (SEM) observations of samples were performed using a Cambridge S-360 apparatus. FT-IR spectra were obtained in the wavenumber range from 1200 to 400 cm-1 via an FTS-3000 spectrophotometer manufactured by American Digilab company. For the transmission IR experiments under ambient conditions, the measured wafer was prepared as KBr pellet with the weight ratio of sample to KBr for 1/100. The resolution was set at 4 cm-1 during measurement. 27Al NMR spectra were acquired on a Bruker AVANCE 500 spectrometer operated at a resonance frequency of 130 MHz, and 10 000 scans. Acidic amounts of the zeolite were measured by temperature-programmed desorption (TPD) of ammonia (NH3-TPD) method. A 0.2 g sample with 40-80 mesh was pretreated in helium at 500 °C for 2 h, cooled to 120 °C, and adsorbed NH3 for 45 min. After flushing by pure helium gas at 120 °C for 1 h, TPD started at a rate of 5 °C 3 min-1 from 120 to 600 °C, and the signal was monitored with a thermal conductivity detector (TCD). H2-TPR was carried out using 10% hydrogen in helium at a constant flow rate of 40 mL 3 min-1, from 100 to 1000 °C, at a heating rate of 10 °C 3 min-1. The hydrogen consumption signal was monitored by a TCD. All the samples were degassed at 300 °C and before the outlet gases entering the TCD, a cooling trap and a filter packed with molecular sieve 5A (60-80 meshes) were used to remove H2O and CO2. The UV-vis diffuse reflectance spectra (DRS) experiments were performed on a Hitachi U-4100 UV-vis spectrophotometer with the integration sphere diffuse reflectance attachment, using BaSO4 as reference. The powder samples were loaded in a transparent quartz cell and were measured in the region of 200-800 nm at room temperature.29,30 2.4. Catalytic Activity Measurement. Catalytic performances were evaluated in a high-pressure fixed-bed reactor with 2 g of catalyst (grain size of 0.3-0.5 mm). All catalysts were presulfided for 4 h with 2 m% of CS2-cyclohexane mixture under the conditions of LHSV (liquid hourly space velocity) of 1.0 h-1, temperature of 320 °C, total pressure of 4 MPa, and a H2/cyclohexane ratio of 600 mL 3 mL-1. The feedstock used in this work was Daqing FCC diesel with 1290 ppm of S and 907 ppm of N. The properties of the diesel oil feedstock are shown in Table 1. Hydrodesulfurization and hydrodenitrogenation tests were carried out under the conditions of 350 °C, 5.0 MPa, 600 mL 3 mL-1, and 1.0 h-1. Catalytic activities were measured at steady state after 13 h on-stream. The total sulfur content and nitrogen contents in the feed and products were measured by using a LC-4 coulometric sulfur analyzer system. The instrumental errors in N and S analysis were less than 3%. The catalytic activities under investigation were estimated by HDS and HDN conversions. The conversions are defined as

follows: HDS% conversion =[(Sf - Sp)/Sf]  100%; HDN% conversion = [(Nf - Np)/Nf]  100%, where Sf and Nf are the nitrogen and sulfur contents in feed (m%), and Sp and Np are the sulfur and nitrogen contents in products (m%), respectively. The rate constants for the removal of total sulfur or nitrogen were calculated using eq 1 as shown below:31 " # LHSV 1 1 ð1Þ k ¼ n -1 Cpn -1 Cfn -1 where k is the apparent rate constant for conversion of sulfur or nitrogen presented in feedstock, h-1 (ppm)1-n, n is the order of reaction, Cp is the total sulfur or nitrogen present in product (ppm), Cf is the total sulfur or nitrogen present in feed (ppm), and LHSV is the liquid hourly space velocity (h-1). The reaction order n depends on the boiling range of the petroleum fraction. The previous studies showed that the reaction orders of HDN and HDS of diesel could fit well with 1.5 and 2.0, respectively.32,33 Taken the activity of RN-10 as a standard, which is denoted as k0, the relative activities of other catalysts are defined as follows: RAV = k/k0  100.

3. Results and Discussion 3.1. N2 Physisorption. The nitrogen adsorption-desorption isotherms of Al2O3-USL composite supports starting from low to high pressures are shown in Figure 1. The N2 adsorption-desorption isotherms showed a typical hysteresis loop in which isotherms were ascribed to type IV classification (Figure 2), indicating the presence of frameworkconfined mesopores in spite of the different preparation methods. As for the case of Al2O3-USL-40(M), the hysteresis loop was not very broad (0.6 < P/P0 < 0.95), whereas the hysteresis loop of Al2O3-USL-40(C) was relatively extended, which was the characteristic of a solid with higher specific area and mesopores.34 The BET surface areas of supports and catalysts are summarized in Table 2. Wide ranges of differences were found for specific surface areas (310-200 m2 3 g-1), total pore volumes (1.02-0.32 mL 3 g-1), and average diameters (13.8-9.15 nm), as the zeolite USL contents were variable in the supports. It was easier to see that Al2O3-USL-40(C) had a larger average pore diameter, and higher pore volume and specific surface area than (31) Ferdous, D.; Dalai, A. K.; Adjaye, J. Applied Catalysis A: General 2004, 260, 153–162. (32) Bej, S. K.; Dalai, A. K.; Adjaye, J. Energy Fuels 2001, 15, 377– 383. (33) Yumoto, M.; Usui, K.; Watanabe, K.; Idei, K.; Yamazaki, H. Catal. Today 1997, 35, 45–50. (34) Leyva, C.; Rana, M. S.; Ancheyta, J. Catal. Today 2008, 130, 345–353.

(29) Wan, G. F.; Duan, A. J.; Zhao, Z.; Jiang, G. Y.; Zhang, D.; Li, R.; Dou, T.; Chung, K. H. Energy Fuels 2009, 23, 81–85. (30) Wan, G. F.; Duan, A. J.; Zhang, Y.; Zhao, Z.; Jiang, G.; Zhang, D.; Gao, Z.; Liu, J.; Chung, K. H. Energy Fuels 2009, 23, 3846–3852.

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Figure 3. The XRD patterns of the NiW/Al2O3-USL(C) catalysts.

Figure 2. The XRD patterns of the Al2O3-USL-40 supports prepared with different methods. Table 2. Textural Properties of Supports and Catalysts samples USL Al2O3(B)a Al2O3(P)b Al2O3-USL-40(M) Al2O3-USL-40(C) NiW/Al2O3 NiW/Al2O3-USL-10(C) NiW/Al2O3-USL-40(C) NiW/Al2O3-USL40(M)

SBET (m2 3 g-1)

pore volume (cm3 3 g-1)

average diameter (nm)

110.9 234.5 310.8 202.7 242.2 261.1 237.8 178.0 140.2

0.092 0.946 1.02 0.32 0.71 0.75 0.74 0.41 0.28

3.12 16.14 13.80 9.15 11.80 12.76 11.61 9.89 8.11

a The commercial Al2O3 bought from Shandong. b Al2O3 prepared by the swing pH method.

Al2O3-USL-40(M). Similar results were obtained after tungsten and nickel were supported. Therefore, it could be found that composite support based on the pH-swing method had much better textural properties than those based on the mechanical mixing method, which would be favorable for the hydrotreating performance of diesel. 3.2. XRD. Figure 2 shows the XRD patterns of composite supports obtained from different preparation methods of Al2O3-USL-40. Strong diffraction peaks located at 2θ of 5.5, 19.4, 22.7, 28.0, 29.1, and 30.7° correspond to the LTL framework topology that existed in the XRD patterns of Al2O3-USL-40(M) and Al2O3-USL-40(C),35 indicating that the structure of zeolite USL were well kept. The crystalline structure of USL was retained even after the loadings of nickel and tungsten (from Figure 3), but unfortunately the intensities of USL characteristic peaks were slightly weakened in the XRD patterns of NiW/Al2O3-USL(C). The loss of crystallinity was probably ascribed to the dilution effect or distortion caused by the interaction of Ni or W species with the zeolite framework.36,37 Additionally, no obvious peak of metal oxides was observed in the XRD patterns of all catalysts, indicating that Ni and W species existed as little crystallites smaller than the XRD limitation of 4 nm or were well dispersed on the surface of the supports. 3.3. SEM. The SEM images of Al2O3, zeolite USL, Al2O3-USL-40(M), and Al2O3-USL-40(C) are shown in Figure 4. From the Figure 4b, it could be illustrated that the

Figure 4. SEM images of (a) Al2O3; (b) zeolite USL; (c) Al2O3-USL-40(M); and (d) Al2O3-USL-40(C).

Al2O3 particles were presented as loose aggregates of amorphous morphology. The zeolite USL had typical sheet crystals (around 700-1000 nm) with smooth surface(Figure 4a), and the particles of zeolite USL and Al2O3 existed independently in Al2O3-USL-40(M) (Figure 4c). In Figure 4d, few Al2O3 aggregates of particles could be found in Al2O3-USL-40(C), but some particles, in which the morphologies were similar to zeolite USL (generally above 1000 nm), were observed. That meant that the presence of zeolite USL particles enwrapped by a film of Al2O3 in the Al2O3-USL-40(C) composite supports. 3.4. FT-IR. The FT-IR spectra of the Al2O3, zeolite USL, and Al2O3-USL composite supports are shown in Figure 5. For the USL sample, the peak at 990 cm-1 could be assigned to TO4 asymmetric stretching vibration. The peaks at 793 and 678 cm-1 could be assigned to TO4 symmetric stretching vibration, and peaks at 616 cm-1 to the bending vibration of the inner tetrahedral. The peak at 454 cm-1 was ascribed to the double-annular vibration of the inner tetrahedral.38 For the support of Al2O3-USL-40(C), the peak attributed to the double-annular vibration of the inner tetrahedral shifted to

(35) Ko, Y. S.; Ahn, W. S. Bull. Korean Chem. Soc. 1999, 20, 1–6. (36) Bendez u, S.; Cid, R.; Fierro, J. L. G.; L opez Agudo, A. Appl. Catal., A 2000, 197, 47–60. (37) Cid, R.; Neira, J.; Godoy, J.; Palacios, J. M.; Mendioroz, S.; L opez Agudo, A. J. Catal. 1993, 141, 206–218.

(38) Ko, Y. S.; Ahn, W. S. Bull. Korean Chem. Soc. 1999, 20 (2), 1–6.

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Figure 5. FT-IR spectra of Al2O3 (a); zeolite USL (b); and Al2O3-USL composite supports (c).

Figure 7. NH3-TPD profiles of Al2O3-USL-x composite supports.

Figure 6. 27Al MAS NMR of zeolite USL, Al2O3-USL-40(M), Al2O3-USL-40(M), and Al2O3.

coordinated framework and octahedrally coordinated extra-framework Al species, respectively. There were two distinct peaks with chemical shifts at 7.865 and 66.60 ppm, which were assigned to octahedral Al and tetrahedral Al, respectively. According to Lowensteins’ rule,42 Al-O-Al linkages in zeolitic frameworks were forbidden. As a result, all Al-O tetrahedral sites must be linked to four Si-O tetrahedral sites. Meanwhile, silica had a greater electronegativity than aluminum, so the chemical environments between zeolite USL and alumina were different. The 27Al MAS NMR spectra results showed that these supports obtained by two preparation methods were obviously dominated by signals in the range of 60-7.865 ppm, which typically represented the characteristics of the tetrahedrally coordinated and octahedrally coordinated Al sites. Moreover, a peak splitting with chemical shifts could be found at around 60 ppm in Al2O3-USL-40(M), which was similar to the peaks assigned to tetrahedral Al sites of zeolite USL and alumina, whereas there is no obvious peak splitting existed at 60 ppm in Al2O3-USL-40(C).

561 cm-1, which was lower than that of Al2O3-USL-40(M). In addition to this shift, a new peak at 1012 cm-1 appeared in the FT-IR spectra of the composite support, which could be ascribed to Si-O asymmetric stretching vibration. These variations were related to the interfacial interactions between the two phases in the composite support based on the reference results in a MFI/MCM-41 system of Karlsson et al.39 The analysis results were similar to the observations by Liu et al.40 and Fan et al.41 3.5. 27Al MAS NMR. 27Al MAS NMR experiments were carried out to study the effects of different composite methods on the chemical environments of aluminum atoms in the corresponding samples, and the results are illustrated in Figure 6. The chemical shifts at about 58.624 and 1.362 ppm in spectra of zeolite USL were assigned to the tetrahedrally (39) Karlsson, A.; St€ ocker, M.; Schmidt, R. Microporous Mesoporous Mater. 1999, 27, 181–192. (40) Liu, H. T.; Bao, X. J.; Wei, W. S.; Shi, G. Microporous Mesoporous Mater. 2003, 66, 117–125. (41) Fan, Y.; Lei, D.; Shi, G.; Bao, X. J. Catal. Today 2006, 114, 388– 396.

(42) L€ owenstein, W. Am. Mineral. 1954, 39, 92–96.

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Table 3. Integrated Areas of Desorption Peak of Al2O3-USL Composite Supports in NH3-TPD Profiles samples

weak acid

USL Al2O3 Al2O3-USL-10 Al2O3-USL-40

1.27  10 (46) 3.44  104 (25) 5.27  104 (29) 7.16  104 (33)

a

5

a

moderate acid

strong acid

total acid

1.33  10 (48) 4.21  104 (31) 6.92  104 (39) 1.01  105 (46)

1.51  10 (6) 6.12  104 (44) 5.66  104 (32) 4.57  104 (21)

2.76  105 1.38  105 1.79  105 2.18  105

5

4

The value in the parentheses is the percentage of various strength of acid.

Figure 9. UV-vis DRS spectra of bulk WO3, W/Al2O3, and NiW/ Al2O3-USL-x catalysts.

Figure 8. TPR profiles of the supported NiW catalysts.

the reduction of dispersed WO42- species of various reducibilities or probably small clusters of WO3 that were not detected by XRD. However, TPR profiles of all NiW catalysts also showed mainly two reduction peaks in the temperature ranges of 600-700 °C and 700-1000 °C, respectively. Clearly, the high-temperature peak shifted toward lower temperature, indicating that some of the W species were easier to be reduced by introducing Ni species to catalyst, which was in agreement with the results of Scheffer.49 Figure 8 shows that the peak shifts to lower temperature direction followed the orders of W/Al2O3 < NiW/Al2O3 < NiW/Al2O3-USL-10 < NiW/Al2O3-USL-40, implying that W reducibilities also obeyed this order. That is, with the increase of zeolite USL contents, the locations of two intense peaks of NiW/Al2O3-USL-x catalysts shifted toward lower temperatures, demonstrating that the active metal oxide species had weaker interactions with the Al2O3-USL-x supports than that with Al2O3 support. In brief, the incorporation of zeolite USL would be beneficial to tuning the metal-support interactions, and resulted in easier reductions of metal oxide species. Similar findings were also reported by Solı´ s et al.51 3.8. UV-vis DRS. The UV-Vis DRS analysis of the oxide precursor was performed to study the effects on coordination of metal oxide species after the introduction of zeolite USL. The UV-Vis DRS spectra in the region from 200 to 800 nm are shown in Figure 9. It can be observed that bulk WO3 presented the characteristic behavior of WOx species in an extended three-dimensional crystalline network.52 Two adsorption bands at 220-350 and 400-500 nm were detected. The intense adsorption band at about 220-350 nm was attributed to the ligand-metal charge transfer (LMCT): O2f W6þ in W-O-W bridge bonds in polymeric structures.53,54 The band at around 250-280 nm was assigned to tetrahedron

3.6. NH3-TPD. Acidic zeolites added to the aluminasupported CoMo and NiMo catalysts would be favorable for the dealkylation and isomerization reactions of the alkyl substituents, which might transform the refractory 4,6DMDBT into more reactive species and thus should accelerate HDS reactions.43-47 NH3-TPD patterns of the examined supports are presented in Figure 7. The supports showed similar distributions of the acid site strength in the range between 120 and 550 °C. The main peaks of ammonia desorption were located at about 220 °C and a shoulder peak at 350 °C for all supports. When 10% of USL was introduced to Al2O3 the amounts of weak and moderate acid sites, which were sited at 120220 °C, increased slightly, and it further increased with zeolite USL contents. Moreover, the maximal peak temperatures were also slightly increased with the introduction of USL and its content. The quantitative evaluations of weak (120-220 °C), moderate (220-350 °C), and strong (350550 °C) acid sites based on Gaussian curve fittings are presented in Table 3. It could be observed clearly that zeolite USL had moderate strength acid sites in all above cases. 3.7. H2-TPR. To investigate the types and the reducibility of active metals existing in the catalyst precursors, TPR experiments were performed over the oxide catalyst samples. The TPR profiles are given in Figure 8. On the basis of the TPR spectra of W/Al2O3, a well-developed asymmetric peak could be seen at 950 °C, which resembled those reported for W/Al2O3 catalysts.36,48-50 This peak should be attributed to (43) Landau, M. V.; Berger, D.; Herskowitz, M. J. Catal. 1996, 159, 236–245. (44) Isoda, T.; Nagao, S.; Ma, X. L.; Korai, Y.; Mochida, I. Energy Fuels 1996, 10, 1078–1082. (45) Lecrenay, E.; Sakanishi, K.; Mochida, I.; Suzuka, T. Appl. Catal., A 1998, 175, 237–243. (46) Kwak, C.; Lee, J. J.; Bae, J. S.; Choi, K.; Moon, S. H. Appl. Catal., A 2000, 200, 233–242. (47) Abu, I. I.; Smith, K. J. Appl. Catal., A 2007, 328, 58–67. (48) Thomas, R.; van Oers, E. M.; de Beer, V. H. J.; Medema, J; Moulijn, J. A. J. Catal. 1982, 76, 241–253. (49) Sheffer, B.; , Molhoek, P.; Moulijn, J.A., Appl. Catal. (1989), 46 pp.11. (50) Cordero, R. L.; Solis, J. R.; Ramos, J. V. G.; Patricio, A. B.; Agudo, A. L. Stud. Surf. Sci. Catal. 1993, 75, 1927–1930.

(51) Solı´ s, D.; L opez Agudo, A.; Ramı´ rez, J.; Klimova, T. Catal. Today 2006, 116, 469–477. (52) Lizama, L.; Klimova, T. Appl. Catal., B 2008, 82, 139–150. (53) Vissenberg, M. J.; van der Meer, Y.; Hensen, E. J. M.; de Beer, V. H. J.; van der Kraan, A. M.; van Santen, R. A.; van Veen, J. A. R. J. Catal. 2001, 198, 151–163. (54) Gutierrez-Alejandre, A.; Ramı´ rez, J.; Busca, G. Langmuir 1998, 14, 630–639.

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Energy Fuels 2010, 24, 796–803

: DOI:10.1021/ef901098m

Duan et al.

Table 4. Hydrotreating Activity Results of Diesel over Supported NiW Catalysts series number 1 2 3 4 5 6 7 8 9 a

catalysts

S contents /μg 3 g-1

HDS (%)

RVA(S)

N contents (μg 3 g-1)

HDN (%)

RVA(N)

feedstock RN-10 NiW/Al2O3(B)a NiW/Al2O3(P)b NiW/Al2O3-USL-5(C) NiW/Al2O3-USL-10(C) NiW/Al2O3-USL-20(C) NiW/Al2O3-USL-30(C) NiW/Al2O3-USL-40(C) NiW/Al2O3-USL-10(M)

1290 39.75 12.10 9.82 14.54 9.10 27.82 40.17 49.21 19.76

96.9 99.06 99.3 98.87 99.3 97.8 96.9 96.2 98.5

100 199 223 179 232 124 99 88 151

907 71.25 55.19 65.90 61.93 53.34 77.74 85.38 116.80 66.96

92.1 93.29 92.7 93.17 94.1 91.4 90.6 87.1 92.6

100 132 109 116 136 91 82 58 107

The commercial Al2O3 bought from Shandong. b Al2O3 prepared by the swing pH method.

or octahedron tungstenic species. The band at around 280-340 nm was ascribed to tungstenic oxide and octahedron tungstenic species. These octahedron tungstenic species were more easy for the formation of coordinatively unsaturated or sulfur vacancies and thus should be favorable for hydrodesulfurization reaction.55,56 From the spectra in Figure 9, peaks at 590 and 630 nm attributed to tetrahedrally coordinated Ni2þ ions in NiAl2O4 were not found, which is consistent with the previous research results.57-59 However, a new visible band at around 420 nm, a typical characteristic of octahedrally coordinated Ni2þ ions, shifted to higher wavelength with the zeolite USL content increasing, indicating that an enhanced incorporation of nickel with tungsten favored for producing Ni-W-O species, the precursor of Ni-W-S active phases. 3.9. Catalytic Activity. Sulfur and nitrogen contents as well as HDS and HDN efficiencies of diesel products over NiW series catalysts are presented in Table 4 and Figure 10. As shown in Table 4, compared with the commercial Al2O3(B), the Al2O3(P) prepared by the swing-pH method has a higher catalytic activity, this is due to the higher surface area, larger pore volume, and suitable pore diameter distribution of the Al2O3(P). So it is obvious that the textural properties were the key point to affect the catalytic performance in the same type of materials. Furthermore, the addition of zeolite USL to the catalyst support could improve the hydrotreating performances, and the assessment results indicated that the catalyst of NiW/Al2O3-USL-10(C) with 10 m% USL in the support exhibited the highest HDS and HDN conversions of 99.3 and 94.1%, respectively. Meanwhile, its relative catalytic activities of HDS and HDN were 2.32 and 1.36 times higher than those of the commercial catalyst of RN-10, and they were also better than the corresponding catalyst of NiW/Al2O3-USL-10(M) in which the support was prepared by mechanical mixing method. The higher catalytic activities of NiW/Al2O3-USL (C) catalysts might be caused by the following reasons: (i) zeolites combined closely with alumina in the in situ prepared composite supports, which might be favorable for the cooperative effects of L and B acid. (ii) From Table 4, compared with NiW/Al2O3-USL(M) catalysts, the corresponding NiW/Al2O3-USL(C) had a higher specific surface area, total pore volume, and average pore diameter, which should be of benefit to the higher dispersion of active metals and to the diffusion of large

Figure 10. Sulfur and nitrogen contents in hydrotreated diesel products obtained over catalysts containing zeolite USL. 1: RN10; 2: NiW/Al2O3(B); 3: NiW/Al2O3(P); 4: NiW/Al2O3-USL5(C); 5: NiW/Al2O3-USL-10(C); 6: NiW/Al2O3-USL-20(C); 7: NiW/Al2O3-USL-30(C); 8: NiW/Al2O3-USL-40(C); 9: NiW/ Al2O3-USL-10(M).

molecule like the sterically hindered alkyl-substituted DBTs, for example, 4,6-DMDBT.15,60,61 (iii) The combination of zeolite USL with Al2O3 weakened the interaction between the active metals and supports, and resulted in the higher reducibility of tungsten speices on the composite supports. Simultaneously, Ni-W-O active phase precursors were easy to be formed, which would be good for the enhancement of hydrotreating activity. Sulfur content of the optimal HDS product in Table 4 accorded with the sulfur regulation of Euro IV specification (