Petroleum Science and Technology, 33:622–628, 2015
Copyright C© Taylor & Francis Group, LLC
ISSN: 1091-6466 print / 1532-2459 online
Cracking Performance of Supercritical n-Decane with Mo-promoted Pt/CeO2-Al2O3 Catalysts
X. Li,1 Q. Zhu,2 J. Wang,3 Y. Jiao,3 X. Li,1 and Y. Chen3 1College of Chemical Engineering, Sichuan University, Chengdu, China 2School of Aeronautics & Astronautics, Sichuan University, Chengdu, China 3Key Laboratory of Green Chemistry and Technology of the Ministry of Education, College of
Chemistry, Sichuan University, Chengdu, China
The catalytic performances of Pt/MoO3/CeO2-Al2O3 (PCA-M), Pt/V2O5/CeO2-Al2O3 (PCA-V), and
Pt/MoO3/CeO2-ZrO2-Al2O3 (PCZA-M) are investigated using n-decane cracking in a plug flow reactor.
The catalytic cracking activities of the catalysts are found to increase with the order of the strong acid amount: PCA-M > PCZA-M > PCA-V. Hence, it is concluded that the larger the strong acid amount the better the cracking activities. The gas yield of PCA-M increase by 5.8% at 700◦C compared to that of PCA-V, and its heat sink increases by 0.17 MJ/kg correspondingly. PCA-M has the greatest effects on the gas yields and heat sinks.
Keywords: hydrocarbon fuel, n-decane, solid acid, catalytic cracking, supported catalysts
Hydrocarbon fuel for supersonic combustion ramjet is designed not only as propellant but also as coolant to reduce the engine heat load (Kay and Peschke, 1992). However, the fuel sensible heat due to the increased temperature is not sufficient for cooling. Hence, the chemical heat absorption (known as chemical heat sink) is needed to improve the cooling ability.
Nowadays, the catalysts for n-decane cracking mainly include two types of noble metal and zeolite. However the stability of zeolite catalysts is poor at high temperatures and the ability of regeneration is difficult. While strong solid acids (e.g., Cr2O3/Al2O3) possess the characteristics of good stability and good texture properties (Liu et al., 2006), which are good candidates for n-decane cracking.
This work aims to study the relationship between the catalytic activities and the catalysts acidity.
The textural and structural properties of the catalysts are also examined. The catalytic activities for supercritical n-decane cracking are evaluated.
Address correspondence to Q. Zhu, School of Aeronautics & Astronautics, Sichuan University, Chengdu 610065, China.
E-mail: email@example.com 622
CRACKING PERFORMANCE OF SUPERCRITICAL N-DECANE 623
Preparation of Supports and Catalysts
CeO2–Al2O3 and CeO2–ZrO2–Al2O3 (the molar ratio of n(Ce)/n(Al) = 1:1 and n(Ce)/ n(Zr)/n(Al) = 1:1:2) were prepared by the coprecipitation method. The precipitates were dried at 120◦C, and calcined at 600◦C for 3 h. Then the materials were impregnated using the (NH4)6MO7O24·4H2O and NH4VO3 solution, dried at 120◦C, denoted as CA-M, CZA-M, and CA-V (MoO3 and V2O5 were both 10.0 wt%). Similarly, the PCA-M, PCA-V, and PCZA-M catalysts (Pt was 0.50 wt%) were prepared by the impregnation method.
The catalysts were subsequently ball milled with water to obtain a pot of slurry, then coated on the inner wall of a stainless steel tubes (2 mm inner diameter × 700 mm) by vacuum. The coating amount (0.2 ± 0.005 g/700 mm) and uniformity for each catalyst were controlled by adjusting the vacuum pressure and slurry viscosity, then dried at 120◦C, calcined at 300◦C for 1 h and 500◦C for 2 h to obtain a coating tube with the thickness of the catalyst in the range of 4–6 μm measured by the metallurgic microscope.
Characterization of Catalysts
The surface acidity of each catalyst was measured by NH3–temperature programmed desorption (NH3–TPD; Tianjin Xianquan Industry and Trade Development Co., Ltd., China). During this process, 100 mg sample was heated to 400◦C at a rate of 8◦C/min in a flow of N2, maintained at 400◦C for 45 min and then cooled to room temperature. The gas was then switched to 2% NH3 in
N2 (20 mL/min) for absorption for 60 min and then the temperature was increased from ambient to 900◦C at a heating rate of 8◦C/min in the flow of N2.
The textural properties of the catalysts were assessed using a Quadrasorb specific surface area analyzer (Quantachrome Instruments, USA). Samples were firstly evacuated at 300◦C for 1 h, then cooled to –196◦C using liquid N2, at which point their adsorption of N2 was measured.
X-ray diffraction (XRD) patterns of the prepared materials were obtained using a DX-2500 X-ray diffractometer (Dandong Fangyuan Instrument Co., Ltd., China) with a graphite monochromator,
Ni filter, and Cu Kα radiation, operating at 40 kV and 25 mA. The samples were scanned over 2θ range of 10–80◦ at a step rate of 0.03◦/sec.
Evaluation of Catalytic Activity
The homemade experimental apparatus of supercritical cracking is established as shown in Figure 1.
It is composed of a feed system, reaction system, condensing system and an analysis system.
The fuel is input into the electric heated reaction tube by high pressure liquid constant flow pump, and the current and voltage on both ends of the tube are used simultaneously to calculate heat sink. Reacted fuel flows through the condensing system and filter into the gas-liquid separator to collect gas phase product as well as liquid phase product. Gas phase product is analyzed by the gas chromatograph and GC/MS is adopted to analyze the recycled liquid. The experimental pressure was 2.5 MPa and the mass flow of n-decane was strictly controlled at 1.0 g/sec. 624 X. LI ET AL.
FIGURE 1 Schematic diagram of apparatus. (1) feed tank, (2) pump, (3) check valve, (4) mass flow meter, (5) manometer, (6) electrical heater, (7) condenser, (8) filter, (9) back pressure controller, (10) separator, (11) liquid collector, (12) gas chromatography, (13) gas flow meter.
RESULTS AND DISCUSSION
Surface acidity has a great influence on the catalysts activities. Figure 2 shows the NH3-TPD profiles of the catalysts. The profiles for all catalysts show two peaks, namely, the strong acid desorption peak (400–650◦C) and the weak-medium acid desorption peak (100–400◦C) (Shali and Suguman, 2007). The strong acid amount of PCA-M is 3.2 times than that of PCA-V and 1.5 times than that of PCZA-M. The addition of MoO3 obviously increases the strong acid amount, which can enhance the cracking reactions of C–C bonds. The larger strong acid amount may result in higher alkene contents and larger heat sinks of the fuel (Dimitris et al., 1992).