7396
J. Phys. Chem. C 2007, 111, 7396-7402
A Comparison of NiMo/Al2O3 Catalysts Prepared by Impregnation and Coprecipitation Methods for Hydrodesulfurization of Dibenzothiophene Fang Liu,† Shaoping Xu,*,† L. Cao,‡ Y. Chi,*,‡ T. Zhang,‡ and Dongfeng Xue† State Key Laboratory of Fine Chemicals, Department of Chemical Engineering, Dalian UniVersity of Technology, 158 Zhongshan Road, Dalian, 116012, People’s Republic of China, and LuknoVa Incorporated, P. O. Box 5055, Somerset, New Jersey 08875 ReceiVed: December 10, 2006; In Final Form: March 16, 2007
NiMo/Al2O3 catalysts prepared by impregnation and coprecipitation methods have been compared in catalytic activity for the hydrodesulfurization of dibenzothiophene (DBT) and have been characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) techniques. The combined results of XRD and XPS indicate that the coprecipitation catalysts contained active β-NiMoO4 phase while NiAl2O4 and the mixing oxidation state of Mo6+ and Mo5+ species were observed on the impregnation catalyst. The formation of β-NiMoO4 precursor in the unsulfided NiMo/Al2O3 catalyst prepared by coprecipitation is attributed to the enhanced catalytic activity. BET surface area and N2 adsorption isotherm measurement demonstrate the characteristic of mesopores for the impregnation catalyst and of macropores for the coprecipitation catalyst. SEM images show that the impregnation method partially caused aggregated crystals while coprecipitation produced homogeneous distribution of Ni and Mo active metal species. As compared to the impregnation catalyst, the reaction temperature over the coprecipitation catalyst has been reduced by 40 K at the same DBT conversion (>99.7%). The coprecipitation catalyst is expected to show a superior durability to the impregnation catalyst because of its macroporous structure as well as the decrease in reaction temperature. The remarkable performance of the coprecipitation catalyst results from the formation of β-NiMoO4 precursor, high concentration of surface active sites, and uniform dispersion of NiMo components through optimizing the mixing between Ni and Mo species.
1. Introduction With the steady increase in the use of vehicles globally, sulfur, one of the most severe air pollutants produced from the combustion of transportation fuels, has become a big concern in environmental protection. In recent years, many countries have launched new environmental legislation aiming at reducing the release of sulfur to the atmosphere. For instance, the U.S. Environmental Protection Agency (EPA) has adopted a stringent sulfur regulation which requires refineries to reduce the gasoline sulfur level from 500 to 30 ppm by 2007.1 The new regulation has posed a technological challenge for current commercial desulfurization processes. The most commonly used process for the removal of organic sulfur from crude oil is the hydrodesulfurization (HDS) reaction which involves catalytic conversion of various sulfur compounds to hydrogen sulfide in the presence of hydrogen. The vast majority of industrial HDS catalysts are derived from aluminasupported Mo or W sulfides coupled with Co or Ni sulfides.2-10 Although the nature of the active sites in Co-Mo and Ni-Mo catalysts remains ambiguous, the Co(Ni)-Mo-S structures are currently accepted most widely to be active sites with the presence of Co(Ni) atoms on the edge of MoS2 phase.11-13 Fluorides14,15 and boron16 have been reported to improve the catalyst stability. Other catalyst modifications to promote * To whom correspondence should be addressed. Fax: +86 (411) 83683467; e-mail:
[email protected] (S. X.). Fax: +1(781) 917-0161; e-mail:
[email protected] (Y. C.). † Dalian University of Technology. ‡ Luknova Incorporated.
catalytic activity include the addition of phosphorus17-20 and chelating agents21-24 to the Co(Ni)-Mo-S system. A variety of alternative methods for catalyst preparation, such as the decomposition of metal sulfur compounds25,26 and metathesis reaction,27 have been developed to make high-performance HDS catalysts, but these methods are still in the developing stage because of the complication of preparation procedure. Sonochemical preparation of HDS catalysts has been employed to make nanoscale Co-Mo-S catalysts with small particle size and high surface area.28,29 However, this method involves the use of toxic chemicals, such as Mo(CO)6 and Co2(CO)8, and serious problems in hazard handling. HDS catalyst is the core hydrotreating catalyst used in the hydrotreater, which typically includes catalysts for hydrodesulfurization, hydrodenitrogenation, hydrogenation of aromatics, and hydrodemetallization. In 2001, Akzo Nobel (now Albemarle), Nippon Ketjen, and ExxonMobil jointly commercialized NEBULA, which was claimed currently as the world’s most active hydrotreating catalyst.30 NEBULA is predominantly an unsupported metallic catalyst with porous structure. Compared to the conventional alumina-supported HDS catalyst, the elimination of alumina support in the NEBULA catalyst leads to the increase in catalyst cost31 and the catalyst structure alteration. As a conventional catalyst preparation method, impregnation has been extensively used by commercial catalyst manufacturers32-38 for the preparation of mainstream HDS catalysts, such as KF 756 and KF 757 (Co-Mo/Al2O3) and KF 848 (NiMo/Al2O3). The general procedure for making the HDS catalyst involves the simultaneous or successive impregnation of γ-Al2O3 with an aqueous solution of Mo and Co(Ni) salts, followed by
10.1021/jp068482+ CCC: $37.00 © 2007 American Chemical Society Published on Web 05/01/2007
Comparison of NiMo/Al2O3 Catalysts drying and calcination to produce oxides, which are subsequently sulfided prior to use with a mixture of H2S and H2 or a feed containing sulfur compounds and H2. There are two intrinsic disadvantages resulting from the impregnation method: (1) the lack of uniform particle and active species distribution because of the forced condensation of metal precursors on the support surface during the drying process and (2) limited activity because of the restrained amount of active metals deposited on support surface. In contrast, precipitation as a well-established method to commercially manufacture catalysts39 has a potential to avoid the drawbacks resulting from impregnation method, leading to the development of high performance and cost-effective HDS catalyst. Ni-Mo/Al2O3 catalysts prepared by the precipitation from homogeneous solution have been reported to be more active for the HDS of thiophene than the impregnation Ni-Mo catalysts and commercial Ketjenfine and Harshaw Ni-Mo catalysts.40 However, there are very few studies conducted to reveal the effect of preparation methods on the HDS catalyst surface and bulk structure by using catalyst surface characterization techniques, such as X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). In this paper, a NiMo catalyst prepared by the Nippon Ketjen impregnation method33,34 is compared with two coprecipitation catalysts provided by Luknova Inc. Similar metal loading on alumina in the prepared catalysts has been used to compare the catalytic activity. Dibenzothiophene (DBT), one of the most refractory thiophenic compounds found in heavy oil, has been selected as a probe molecule to demonstrate the catalyst activity. Various catalyst characterization techniques were used to investigate the HDS catalysts, including X-ray diffraction (XRD) to identify the bulk-phase composition; Brunauer-Emmett-Teller (BET) to measure the catalyst surface area, pore volume, and particle size distribution via N2 isothermal adsorption and desorption; XPS to analyze the surface chemical bonding states and structure; and SEM to examine the catalyst morphology. The objective of this research is to reveal the effect of preparation methods on the catalytic activity and structure properties of the alumina-supported Mo-Ni catalysts prepared by the two different methods to provide an insight for the future NiMo catalyst design. 2. Experimental Section 2.1. Catalyst Preparation. Catalyst A was prepared by incipient wetness impregnation of the γ-Al2O3 support with aqueous solutions of molybdenum trioxide and nickel nitrate.33,34 Aqueous ammonia was added into the solution of molybdenum trioxide and nickel nitrate under intensive stirring conditions until the metal precursors were completely dissolved. The mixed solution was used to impregnate the γ-Al2O3 support (surface area 180 m2/g, from Alfa Aesar) at room temperature. After an overnight drying at room temperature, the resultant sample was further dried at 393 K for 12 h in air and then was calcined at 673 K for 2 h. The obtained catalyst sample was denoted as catalyst A, MoO3-NiO/Al2O3 consisting of 16.5% Mo and 4.4% Ni. Catalysts B and C supplied by Luknova Inc. were prepared by the coprecipitation method in aqueous media. The metal loading and ratio in both catalysts B and C were similar to catalyst A. Catalyst B was calcined in air at 673 K prior to sulfidation while catalyst C was untreated. All the catalysts were sulfided in situ prior to the reaction data collection. 2.2. Reaction Procedure. Catalyst samples were crushed and sieved to 40-60 mesh granules. One gram of the granules was homogeneously mixed with 1 g of SiC with the same mesh and then was loaded into the isothermal section of the stainless
J. Phys. Chem. C, Vol. 111, No. 20, 2007 7397 steel tubular reactor with 10 mm in inner diameter and 350 mm in length. Both ends of the catalyst section were packed with 40-60 mesh SiC to provide a well-mixed feed at the reactor inlet and to ensure efficient heat transfer along the catalyst bed. The reactor was heated inside a tube furnace equipped with a PID controller. A thermal couple was placed in the middle of the catalyst bed to monitor reaction temperature, which was raised from room temperature to 573 K at a rate of 5 K/min under 45 mL/min of H2 flow at 3.0 MPa. The system pressure was controlled with a back pressure regulator. Catalyst sulfidation was carried out by flowing 3% carbon disulfide in decahydronaphthalene at 573 K, 3.0 MPa, oil liquid hourly space velocity (LHSV) of 2.7 h-1, and H2/oil volumetric ratio of 500 for 5 h. After sulfidation, the catalyst activity for hydrodesulfurization was evaluated by lowering the reaction temperature to 533 K and by feeding 1 wt % dibenzothiophene (DBT) in decahydronaphthalene into the reactor. Although 1.0 wt % DBT in the feed was lower than typical ∼1.4 wt % sulfur in the current commercial feed stock,9 this served as a comparative testing approach to screen high-performance catalyst in this research. Other testing conditions fell within the nominal range for commercial operation. First, liquid sample was collected at the 60th minute and later samples were taken every 30 min interval. Collected liquid samples were analyzed by a GC-920 gas chromatography (Haixin Instruments Company) equipped with a flame ionization detector and an OV1701 capillary column. Main hydrodesulfurization products were cyclohexylbenzene and biphenyl. Products were identified and quantified by using standard compounds. The catalyst performance was found stable after 4 h of on-stream testing. For each continued testing, the reaction system was heated to the target temperature, and 1% DBT in decahydronaphthalene was fed into the reactor without further catalyst sulfidation. Reagents used in this work included DBT (98%, SigmaAldrich), carbon disulfide (98%, Xilong Chemical Factory), decahydronaphthalene (98%, Sinopharm Chemical Reagent Co. Ltd.), cyclohexylbenzene (99.5%, Shengqi Chemical Factory), and biphenyl (99.5%, Jinmaotai Chemical Factory). All these chemicals were used as received without further purification. H2 (99.9%) and N2 (99.999%) were obtained from Dalian University of Technology and were purified with activated carbon. 2.3. Catalyst Characterization. X-ray diffraction (XRD) data were collected using Cu KR radiation and a Rigaku Miniflex diffractometer from 10 to 90° 2θ with a step width of 0.02°. Samples were ground to fine powders for analysis. BET surface area and N2 adsorption isotherms for pore size distribution were conducted using an ASAP 2010 instrument supplied by Micromeritics. After being degassed for 2 h at 373 K and 10 h at 393 K under vacuum, samples were cooled to ambient temperature and were transferred to the adsorption port. X-ray photoelectron spectroscopy (XPS) was carried out by Rocky Mountain Laboratories, Inc. with a Kratos HSi 165 X-ray photoelectron spectrometer. One small piece of each catalyst sample was loaded onto a double stick tape for introduction into the vacuum chamber for analysis. The analytical chamber vacuum prior to testing was ∼10-10 Torr. Measurements were made on the exterior of the catalyst pieces. The analysis depth for XPS was ∼100 Å. Samples were measured with a monochromatic Al KR X-ray source in an area of ∼0.5 mm in diameter. After a survey analysis of all elements (except for H and He) at concentrations above 0.05-1.0 atom %, high-energy resolution analysis was performed on the C, O, Al, Ni, and Mo regions of the spectrum for each sample to provide chemical
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Figure 1. DBT conversion as a function of reaction temperature over catalysts A, B, and C at 2.98 MPa, H2/oil 500, and LHSV 2.76 h-1.
Figure 2. DBT conversion as a function of reaction time over catalysts A, B, and C at 553 K, 2.98 MPa, H2/oil 500, and LHSV 2.76 h-1.
Figure 3. Steady-state selectivity of (a) cyclohexylbenzene (CHB) and (b) biphenyl (BP) as a function of reaction temperature over catalysts A, B, and C at 2.98 MPa, H2/oil 500, and LHSV 2.76 h-1.
environment information. The samples were charge neutralized during analysis and were charge corrected using C 1s hydrocarbon signal at 284.5 eV. Scanning electron microscopy (SEM) was used to examine the morphology of the catalyst surface by using a KYKY-2800B instrument. The catalyst samples were cut and the cross sections were coated with a gold film in sample preparation. 3. Results 3.1. Reaction Data for the HDS of DBT. Figure 1 shows the comparison of three samples, catalysts A, B, and C, for the DBT conversion at different reaction temperatures. Catalyst A prepared by the impregnation method showed lower catalytic activity than catalysts B and C prepared by the coprecipitation method at low-temperature range (
