Energy & Fuels 2007, 21, 2517-2524
2517
Coking and Deactivation Behavior of HZSM-5 Zeolite-Based FCC Gasoline Hydro-Upgrading Catalyst Xiuying Lin,† Yu Fan,‡ Gang Shi,‡ Haiyan Liu,‡ and Xiaojun Bao*,†,‡ State Key Laboratory of HeaVy Oil Processing, China UniVersity of Petroleum, Beijing 102249, P. R. China, and The Key Laboratory of Catalysis, China National Petroleum Corporation, China UniVersity of Petroleum, Beijing 102249, P. R. China ReceiVed February 1, 2007. ReVised Manuscript ReceiVed June 10, 2007
In order to further understand the deactivation behavior of a NiMo/HZSM-5 catalyst system used for fluid catalytic cracking (FCC) gasoline hydro-upgrading, fresh and coked catalysts after operation for different timeson-stream (TOS) with FCC gasoline as a feedstock were characterized by X-ray diffraction, Fourier transformed infrared (FTIR) spectroscopy, nitrogen adsorption, and temperature-programmed desorption of ammonia, as well as FTIR analysis of adsorbed pyridine. The results showed that the amount, nature, and location of coke formed in the catalysts depended upon TOS. Coke was preferentially formed on the strong acid sites, especially on the strong Lewis acid sites in the pore channels and/or on the external surface of the catalysts, resting with the size of the reactant molecules in FCC gasoline. Coke formation led to increases in the selectivities to C8, C9, and C9+ aromatics and decreases in the selectivities to benzene and toluene in the aromatics products. Using real FCC gasoline as a feedstock, this work showed that the deactivation behavior of the HZSM-5 zeolite-based catalyst was different from that obtained using model compounds as feedstocks due to the wide size distribution of the hydrocarbon molecules in FCC gasoline and to the complex reaction mechanisms among these components. The results provided some clues for finely tuning the physicochemical properties of the catalyst to further enhance its on-stream stability.
1. Introduction In Asian countries, especially in China, one of the most urgent challenges encountered in producing clean gasoline is to reduce the olefin content in fluid catalytic cracking (FCC) gasoline that takes a share of 40-60% in the commercial gasoline pool. For this purpose, many methods (such as the optimization of FCC processes, the use of novel olefin reduction FCC catalysts, and the application of selective hydrogenation technologies) have been attempted, but their poorer performance either in olefin reduction, in product research octane number (RON) preservation, or in gasoline yield limits their application in industry.1 To reduce the olefin content of FCC gasoline while preserving the octane number of the upgraded gasoline product, a novel technology to upgrade FCC gasoline via hydroisomerization and aromatization has been recently raised. By this way, the gasoline RON loss caused by olefin reduction can be compensated for by the increased contents of isoparaffins and aromatics that have higher RONs. So far, two kinds of catalysts have been reported by two research groups in China. The first catalyst system uses nanosize HZSM-5 zeolite hydrothermally treated with ammonia water as a support and La2O3 and NiO, La2O3 and ZnO, or mixed rare earth oxides and Ga2O3 as active components, as described by Zhang et al.2-4 The other catalyst system developed * Corresponding author. Tel: + 86 (0)10 8973 4836. Fax: + 86 (0)10 8973 4979. E-mail:
[email protected]. † State Key Laboratory of Heavy Oil Processing. ‡ The Key Laboratory of Catalysis. (1) Zhang, Z.; Shi, Y.; Fu, J. Shiyou Lianzhi Yu Huagong 2003, 34, 2832. (2) Zhang, P.; Wang, X.; Guo, H.; Zhao, L.; Hu, Y. Catal. Lett. 2004, 92, 63-68. (3) Zhang, P.; Guo, X.; Guo, H.; Wang, X. J. Mol. Catal. A: Chem. 2007, 261, 139-146.
by the present researching group uses HZSM-5 zeolite modified by a novel post-treatment method that consists of combined steaming and citric acid leaching and γ-Al2O3 as a support and Ni and Mo as active metals.5-7 Both of the catalyst systems have superior olefin reduction ability, excellent RON preservability, and a high liquid product yield, demonstrating themselves as promising catalyst systems for olefin reduction. For the latter catalyst system, however, it was noted that the gradual deactivation of the catalyst becomes one of the main restricting factors for the development of the catalyst based upgrading process. The formation and retention of carbonaceous compounds that will be referred to as “coke” in this article for simplification was thought to be responsible for the catalyst deactivation.6 Numerous research endeavors have been carried out on the coking and deactivation behavior of HZSM-5 zeolite-derived catalysts.8-12 It has been widely accepted that coking occurs preferentially on strong acid sites, and the formation and (4) Zhao, X.; Guo, X.; Wang, X. Fuel Process. Technol. 2007, 88, 237241. (5) Fan, Y.; Bao, X.; Lin, X.; Shi, G.; Liu, H. J. Phys. Chem. B 2006, 110, 15411-15416. (6) Fan, Y.; Lin, X.; Shi, G.; Liu, H.; Bao, X. Microporous Mesoporous Mater. 2007, 98, 174-181. (7) Lin, X.; Fan, Y.; Shi, G.; Bao, X. The 19th Canadian Symposium on Catalysis, Saskatoon, Canada, May 14-17, 2006. (8) Bibby, D. M.; Milestone, N. B.; Patterson, J. E.; Aldridge, L. P. J. Catal. 1986, 97, 493-502. (9) de Lucas, A.; Canizares, P.; Dura´n, A.; Carrero, A. Appl. Catal., A 1997, 156, 299-317. (10) Nagamori, Y.; Kawase, M. J. Mol. Catal. A: Chem. 1998, 21, 439445. (11) Sahoo, S. K.; Viswanadham, N.; Ray, N.; Gupta, J. K.; Singh, I. D. Appl. Catal., A 2001, 205, 1-10. (12) de Lucas, A.; Can˜izares, P.; Dura´n, A. Appl. Catal., A 2001, 206, 87-93.
10.1021/ef0700634 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/01/2007
2518 Energy & Fuels, Vol. 21, No. 5, 2007
deposition of coke on HZSM-5 zeolite can result in a gradual decrease in catalyst activity or even cause complete catalyst deactivation by covering acid sites or by blocking the access of reactant molecules to acid sites. However, most of the studies were performed with model single hydrocarbons or their mixture with known compositions, which means that the results obtained may not be directly applied to complex systems involving molecules of different sizes, such as those encountered in FCC gasoline upgrading. For example, coke formed from isobutene was located within the internal pore structure and was mainly of paraffinic nature, whereas coke from toluene was found on both the external and internal surface of the zeolite and was polyaromatic in nature. It was also noted that coke derived from different compounds had different influences on catalyst activity; for example, in toluene disproportionation, coke formed from toluene resulted in a much more significant decrease in catalyst activity than that from isobutene with the same coke content.13 To understand the deactivation behavior of the modified HZSM-5 zeolite-based catalyst developed by the present group and thus to provide insight for further prolonging the catalyst life for FCC gasoline hydro-upgrading, a series of catalysts after operation for different durations with FCC gasoline as a feedstock were characterized by various techniques and compared with the fresh catalyst, and a possible deactivation route was proposed in the present investigation. 2. Experimental Section 2.1. Catalyst Preparation. The parent HZSM-5 zeolite with a molar SiO2/Al2O3 ratio of 51.2 and an average size of 1 µm (Shanghai Huaheng Chemical Plant, P. R. China) was first steamed in 100% water vapor flowing at a weight hourly space velocity (WHSV) of 1 h-1 at 480 °C for 4 h; then was leached with a 1.2 M citric acid solution (10 mL/g zeolite) at 65 °C for 6 h; and finally was filtered, dried, and calcined at 520 °C for 5 h for use. The HZSM-5 zeolite obtained was then mixed with a binder pseudoboehmite (usually referred to as SB powders, Condea, Germany) in a mass ratio of 7:3 and extruded into Φ1.5 mm × 2∼3 mm sticks. After being dried at 110 °C overnight and calcined at 520 °C for 5 h, the sticks were impregnated with aqueous solutions of ammonium molybdate (AR, Beijing Chemical Reagents Company, P. R. China) and nickel nitrate (AR, Beijing Chemical Reagents Company, P. R. China) successively. After each impregnation step, the sticks were dried at 120 °C for 5 h and calcined at 480 °C for 4 h to obtain the NiO(1 wt %)MoO3(3 wt %)/HZSM-5-γ-Al2O3 catalyst. 2.2. Catalytic Performance Test. Using a FCC gasoline from a refinery in China as a feedstock (Table 1), we assessed the catalytic performance of the catalyst in a 10 mL continuously flowing fixed bed microreactor (Φ8 mm) for different lengths of time-on-stream (TOS) under the following conditions: catalyst loading of 4 g, 390 °C, 1.8 MPa, H2/gasoline volumetric ratio of 200, and gasoline WHSV of 2 h-1. The catalyst samples to be tested were presulfurized for 1 h at 150, 230, and 260 °C, respectively, and for 4 h at 290, 320, and 340 °C, respectively, with a CS2(3 wt %)/n-octane mixture as a vulcanizer and hydrogen as a carrier gas. The feedstock and the products obtained were analyzed by an Angilent gas chromatograph equipped with a flame ionization detector and a 50-m-long PONA (paraffins, olefins, naphthenes, aromatics) capillary column. After being run for a given period of TOS, the reactor was flushed with a nitrogen flow for 30 min at the reaction temperature and then cooled down to room temperature in a nitrogen atmosphere, and the deactivated catalyst was taken out for different analyses. The catalysts after being run for 3, 18, 100, and 400 h TOS were denoted as C-3, C-18, C-100, and C-400, respectively. (13) Uguina, M. A.; Serrano, D. P.; Grieken, R. V.; Ve`nes, S. Appl. Catal., A 1993, 99, 97-113.
Lin et al. Table 1. Properties of the Feeding FCC Gasoline and Liquid Product Items Lumped composition (v%)
S (µg/g) RON Liquid yield (wt %)
n-Paraffins i-Paraffins Olefins Naphthenes Arenes Benzene Toluene C8 aromatics C9 aromatics C9+ aromatics
Feedstock
Product (average)
5.32 32.26 39.13 6.06 17.24 0.61 2.61 5.25 5.03 3.75 76 91.4 -
11.35 39.57 18.54 8.43 22.11 0.49 3.60 7.19 6.67 4.16
