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ZrxTi0.9−xAl0.1O2 mixed oxides catalysts
Applied Thermal Engineering 91 (2015) 417e425
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Research paper
Catalytic cracking of RP-3 jet fuel over wall-coated Pt/ZrxTi0.9xAl0.1O2 mixed oxides catalysts Yi Jiao a, b, Shanshan Li a, Bin Liu a, Yongmei Du b, Jianli Wang a, *, Jian Lu b, Yaoqiang Chen a a b
Sichuan University, Chengdu 610064, Sichuan, China Xi'an Modern Chemistry Research Institute, Xi'an 710065, Shaanxi, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Pt/ZrxTi0.9xAl0.1O2 mixed oxides were introduced into cracking of jet fuel. The surface acidity provided a guarantee for activity of cracking reaction. The centralized strong acidic sites provided a guarantee for selectivity. The catalyst (nZr ¼ nTi) shows the best activity and high-temperature stability. The catalyst (nZr ¼ nTi) still holds higher activity after calcining at 800 C.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 April 2015 Accepted 25 July 2015 Available online 28 August 2015
This study involves a new series Pt-loaded ZrxTi0.9xAl0.1O2 (PZTA) mixed oxides with different molar ratios of ZrO2:TiO2 (ZrO2:TiO2:Al2O3 ¼ x:(0.9 x):0.1, where x ¼ 0, 0.225, 0.45, 0.675, 0.9) catalysts with high temperature resistant and large surface acidity for catalytic cracking of RP-3 jet fuel under hightemperature and high pressure conditions. PZTA catalysts were prepared by using a specially designed that co-precipitation technique subsequent incipient wetness impregnation, and were coated on the wall of stainless-steel microchannels. It was found that gas yield and heat sink of catalytic cracking over the as-prepared catalysts were noteworthy augmented compared with the thermal cracking. Furthermore, different molar ratios of ZrO2:TiO2 could greatly affect the catalytic cracking activities of RP-3 jet fuel. To be more specific, Cat3 (ZrO2:TiO2 ¼ 0.45:0.45) performed the excellent cracking activity and hightemperature stability, which is in accordance with its bigger amount of strong acid, larger surface area and good dispersion effect of Pt. The gas yield, the alkenes content within the gas phase product and the heat sink of Cat3 at 700 C are 40.7%, 51.5% and 3.41 MJ/kg. SEM/TEM measurements further demonstrated that the active phase (Pt) is fairly well dispersed with a particle size of 3e6 nm. The valuable information indicated in this work is that based on the special characters of PZTA, ternary composite oxides supported Pt, which should be an effective approach to maintain both catalytic activities and thermal stability. © 2015 Published by Elsevier Ltd.
Keywords: ZrxTi0.9xAl0.1O2 mixed oxides Coating catalyst Catalytic cracking Coking Acidity
* Corresponding author. Tel./fax: þ86 28 85418451. E-mail address: [email protected] (J. Wang). http://dx.doi.org/10.1016/j.applthermaleng.2015.07.085 1359-4311/© 2015 Published by Elsevier Ltd.
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1. Introduction “Thermal barrier” on the surface of an engine is becoming currently the greatest challenge associated with hypersonic flight. The large amount of heat is generated dramatically by air friction when the aircraft is in flight at the speed of supersonic or hypersonic regimes. It would cause the temperature of the fuselage and power equipment to rise sharply, potentially leading to significant damage [1e6]. Under such situations, hydrocarbon fuels as an ideal coolant can offer heat absorption capacity (heat sink) for supersonic aircrafts, through both significant physical sensible heat and heat-adsorbing by chemical cracking reactions [7e12]. However, the physical sensible heat has a fixed and limited value under a certain condition. So it is necessary to enhance the chemical endothermic capability through the chemical cracking reactions. Now, intense research efforts have been made towards the heat absorption of chemical cracking reactions over the hydrocarbon fuel, which include both thermal and catalytic cracking. Unfortunately, thermal cracking reactions require much higher temperatures and are readily subject to coking, people eventually focus on the study of catalytic cracking reactions in this field. In addition to reduce the initial cracking temperature and generate much smaller heat-absorbents, such as alkenes, the catalytic cracking can provide sufficient absorption capability and is therefore becoming an attractive option [13e16]. The frequently-used cracking catalysts are mainly composed of noble metals, zeolites, and mixed oxides catalysts [17e22]. Although the noble metal catalysts have good effect on catalytic dehydrogenation, they still have some obvious shortcomings, such as a susceptibility to poisoning, short lifetimes, more expensive and deactivation by coking [23]. Many works have been performed on the catalytic cracking of supercritical hydrocarbon over kinds of zeolite catalysts [17e19,24], which exhibit superior catalytic activity/selectivity, and provide preferable heat absorption. However, porous zeolites tend to collapse and losing acidity under high temperature (above 650 C), which makes their high-temperature stabilities relatively poor. So the catalytic cracking of zeolites above 650 C was rarely reported. Therefore, it is very necessary to study the catalysts which keep outstanding catalytic cracking activities and stabilities under high temperature. It is well known that mixed oxides catalysts not only combine the advantages of individual oxide supports, but also hold excellent acidity and textural properties. Among them, TiO2eZrO2 mixed oxides have been widely used as catalytic supports due to their superior performance which usually including larger surface area, mechanical strength as well greater surface acidity, and their performance is discrepant with different ZrO2:TiO2 molar ratios [25e29]. However, ZrO2eTiO2 still lacks of thermal stability, easy to crystallization above 650 C [29,30]. Al2O3 has the advantages of large surface area, and lack of crystallization at high temperatures, so Al2O3 has also been introduced to ZrO2eTiO2 mixed oxides supports to get ternary mixed oxides catalysts, which is also one of the important method for developing excellent catalysts with excellent activity and stability [13,30,31]. Therefore, the use of ZrO2eTiO2eAl2O3 (ZTA) mixed oxides supports would be a good way to solve the poor thermal stability of porous zeolites, and hold excellent catalytic cracking activities and stabilities under hightemperature. The objective of this work is to investigate the influence of Ptloaded ZrO2eTiO2eAl2O3 (ZTA) catalysts with varying molar ratio of ZrO2:TiO2 on the catalytic cracking of hydrocarbons under hightemperature and high pressure conditions. By introducing ZrO2eTiO2eAl2O3 with different ZrO2:TiO2 molar ratios, the structure and texture performance of these mixed oxides could be tuned, along with their surface acidity and thermal stability, so as
high temperature resistant and large specific surface area for hydrocarbon fuel catalytic cracking. The purpose of the present work is to develop an effective catalytic support and to provide some fundamental suggestions for the catalytic cracking mechanism of supercritical hydrocarbons for the catalytic heat exchangers for the advanced aircrafts. 2. Experimental 2.1. Materials The feedstock RP-3 jet fuel was purchased from Chengdu Kelong Chemical Reagent Company. The physical properties are described in Table 1. 2.2. Preparation of ZrxTi0.9xAl0.1O2 mixed oxides and Pt-loaded catalysts ZrxTi0.9xAl0.1O2 mixed oxides were prepared by coprecipitation method. Zr(NO3)4$3H2O, TiOSO4$2H2O and Al(NO3)3$9H2O were dissolved in high-purity water (the impurities content is less than 0.1 mg/L and salinity in 0.3 mg/L) with different molar ratios of ZrO2:TiO2 (ZrO2:TiO2:Al2O3 ¼ x:(0.9 x):0.1, where x 0, 0.225, 0.45, 0.675, 0.9) and precipitation was induced by the addition of a pH ¼ 10 buffer solution composed of NH3$H2O and (NH4)2CO3. The as-prepared precipitate was aged, dried for 2 h at 120 C and calcined for 3 h at 600 C or calcined for 3 h at 800 C, denoting as ZTA600y and ZTA800y, where y ¼ 1, 2, 3, 4, 5 corresponding to the different molar ratios of ZrO2:TiO2 (ZrO2:TiO2:Al2O3 ¼ x:(0.9 x):0.1, x ¼ 0, 0.225, 0.45, 0.675, 0.9). The as-prepared ZTA600y and ZTA800y were then impregnated by chloroplatinic acid as the Pt precursor (Pt content 0.70%), and then the impregnated catalysts were calcined for 2 h at 550 C. The catalysts were subsequently ball-milled with water and then coated on the inner walls of stainless-steel pipes (304#, 80 cm F3 0.5 mm, 2.51 cm3) treated by thermal oxidation at high temperatures by a way of suction using vacuum pump, controlled loads and uniformity of catalyst by adjusting the suction pressure and slurry viscosity, and finally dried. The wall-coated catalysts produced from the PZTA600y (y ¼ 1e5) and PZTA800y (y ¼ 1e5) are denoted as Cat1eCat5 and Cat6eCat10 respectively. The catalyst loading on the inner surface of tube is 0.2 ± 0.005 g/2.51 cm3, which lies in each catalyst. 2.3. Catalytic activity evaluation The self-designed experimental apparatus was used to evaluate the catalytic cracking of fuels, introducing a feed pump, mass flow meter, temperature-control device, pressure system, reactor, condenser, and an analysis system as shown in Fig. 1. The stainless-
Table 1 Specification properties of Chinese RP-3 jet fuel used in this work. Properties
Values
Density, kg/m3 Boiling range, initial point, C 10%, C 50%, C 90%, C Dry point, C n-Paraffins, % Isoparaffins, % Cycloparaffins, % Aromatics, % Sulfur content, ppm
0.7958 135 162.5 182.7 213.3 230 57.72 15.72 15.2 11.81 65.5
Y. Jiao et al. / Applied Thermal Engineering 91 (2015) 417e425
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Fig. 1. Schematic diagram of apparatus. 1 e feed tank; 2 e high pressure metering pump; 3 e check valve; 4 e mass flow meter; 5 e pressure system; 6 e heating system; 7 e cold trap; 8 e filter; 9 e backpressure valve; 10 e gaseliquid separator; 11 e liquid receiver; 12 e gas chromatograph; 13 e wet gas flow meter.
steel tubes (inner diameter 2 mm, 80 cm length) and as-prepared wall-coated tubes (0.2 g) were used as reaction center. The catalytic performance measurement was carried out from 550 C to 700 C (the fuel temperature) and pressure was kept at 3.5 MPa. RP3 jet fuel as a probe reaction was pumped into the reactor with a high pressure constant flow pump at the flow rate of 76 mL/min. The wall-coated tubes were heated by direct current (DC) power, and the backpressure was adjusted as a constant value. The reaction was quenched by dropping temperature through a water condenser and was separated by a gaseliquid separator. The volume of the gaseous products was quantified by water displacement method and the liquid residues were collected with a conical flask and weighed. The gas phase product which comes from the gaseliquid separator was pulsed into the gas chromatograph (GC2000 III, Shanghai institute of technology and computing), equipped with an HP-Al/S separation capillary column (Agilent Technologies Co., Ltd., 50 m * 0.53 mm) and FID for analyzing small organic molecules. The content of H2 was analyzed with external standard method, 2 m packed column (stationary phase, TDX-01) and TCD detector. The liquid products were identified by HP-6890/5973 GCeMS (Agilent Technologies, Inc., Santa Clara, CA) with an FID and an HP-5/MS column (50 m * 0.25 mm). Gas yield and heat sink of several temperature spots were evaluated in the experiment. The mass closure was over 97.5% between feeds and products involving gas, liquid products, and coke.
coatings were performed using an Oxford-IE-250 energy dispersive spectrometer (EDS). The surface acidity of each catalyst was measured by NH3etemperature programmed desorption (NH3eTPD) using a TP5076 TPD instrument. During these measurements, a sample of 100 mg was heated to 400 C at a rate of 8 C/min in a flow of N2, held 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 over the temperature was increased from ambient to 700 C at a heating rate of 8 C/min, employing a thermal conductivity detector. 3. Results and discussion 3.1. Catalytic activity 3.1.1. Gas yield The thermal cracking and catalytic cracking procedures of RP-3 were performed, respectively. The observed gas yield of RP-3, defined as
Gas yield ¼
Inital mass of ðRP3Þ Final mass of residues 100% Inital mass of RP3 (1-1) G1 G 100% G
2.4. Catalysts characterization
The increasing rate ¼
The textural properties of the catalysts were detected using a Quadrasorb SI Automated Surface Area analyzer (Quantachrome Instruments, USA). Samples were initially kept under vacuum for 1 h at 300 C, then cooled to 196 C using liquid N2, at which point their N2 adsorption was measured. X-ray diffraction (XRD) patterns of the as-prepared materials were obtained using a DX-2500 X-ray diffractometer (China Dandong Fangyuan Instrument Co., Ltd.) with a graphite monochromator and Ni filter with Cu Ka radiation, operated at 40 kV and 25 mA. The samples were scanned over a 2q range of 10 e80 at a step rate of 0.03 /s. XPS was performed using an XSAM-800 spectrometer (KRATOS Co., UK) with MgKa excitation under a high voltage (13 kV) and current (20 mA), calibrated internally by the carbon deposit C1s binding energy at 284.8 eV. The micro-morphological characteristics of the catalyst coatings were observed using a Hitachi-S-4800 scanning electron microscope (SEM) and a Tecnai G2 F20 S-TWIN (FEI, USA) Transmission electron microscopy (TEM). Qualitative and quantitative analyses of elemental distributions in microscopic regions of the catalyst
where G1 and G are the gas yield of catalytic cracking and thermal cracking, respectively, and the results under two series different cracking catalysts are shown in Fig. 2. The results manifest that the gas yields of catalytic cracking over PZTA catalysts are higher than thermal cracking at the same experimental condition, in which the increasing rate of gas yield of PZTA600y (Fig. 2a) is increased by the following order: Cat3(65.7%) > Cat2(50.2%) > Cat4(28.4%) > Cat5(25.0%) > Cat1(7.9%) > thermal cracking z ZTA6003 at 600 C and Cat3(63.2%) > Cat4 (59.5%) > Cat2(57.7%) > Cat1(35.9%) > Cat5(28.1%) > thermal cracking z ZTA6003 at 650 C with the varying molar ratio of ZrO2:TiO2. Although some deactivation occurs at 700 C, Cat3 also holds the excellent increasing rate of gas yield (34.4%) compared with thermal cracking. The increasing rate of gas yield of PZTA800y (Fig. 2b) followed the same order as PZTA600y coatings: Cat8(16%) > Cat9(15.4%) > Cat7(14.0%) > Cat6(12.3%) > Cat10 (0.3%) > thermal cracking z ZTA6003 at 650 C, However, the gas yield of PZTA800y behaves worse than that of PZTA600y. Above all,
(1-2)
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Y. Jiao et al. / Applied Thermal Engineering 91 (2015) 417e425
Fig. 2. The gas yield of thermal cracking and catalytic cracking.
the gas yields of catalytic cracking of the Pt supported ternary mixed oxides (ZrO2, TiO2, Al2O3) are much better than that of the Pt supported binary mixed oxides (ZrO2 and Al2O3, TiO2 and Al2O3), which lie in PZTA600y and PZTA800y. And when the molar ratio of ZrO2:TiO2 is 0.45:0.45, the gas yield is higher than the others. Although the gas yield of PZTA800y is decreased after calcining at 800 C compared with the PZTA600y, Cat8 also hold a preferable gas yield among PZTA800y series. The introduction of the Pt supported ternary mixed oxides ZTA improves the activity and hightemperature stability of the catalyst, and the catalyst with molar ratio of ZrO2:TiO2 in 0.45:0.45 exhibits the best the activity and high-temperature stability [13,31]. What's more, the addition of catalysts thus reduces the starting temperature of the cracking reaction effectively while enhancing the depth of cracking reaction obviously, decreasing the activation energy of reactant molecule, and increasing the reaction rate [32] and yields of many kinds of small molecule hydrocarbon. The increasing of gas yield is the prerequisite and guarantee for the improvement of heat capacity of fuel in catalytic cracking. It should be noted that the gas yield of the catalyst-ZTA6003 without Pt is less than PZTA catalysts, and is approximately equal to the value of thermal cracking. The Pt acted as the active sites of the catalyst has an intimate relationship with the catalytic activity. 3.1.2. The gaseous-liquid phase distribution and the ratio of alkene to alkane The ratio of alkenes to alkanes is used to represent the alkenes selectivity of catalysts. Gas phase product distribution and the ratio of alkene to alkane in catalytic cracking and thermal cracking at each temperature spots are shown in Table 2. Gas phase products of RP-3 cracking were mainly compose of hydrogen, low-carbon alkanes (methane, ethane, propane and butane) and low-carbon alkenes (ethylene, propylene, isobutylene and butadiene); and the components were varied by changing cracking temperature and introducing different catalysts. It can be seen from Table 2, H2 content from catalytic cracking over PZTA catalysts was higher than thermal cracking and the catalytic cracking over ZTA6003-catalyst without Pt due to the dehydrogenation role of Pt [33,34]. Thermodynamic analysis shows that if unsaturated hydrocarbons such as ethylene, propylene and butylene, and hydrogen are produced during the cracking reaction, the heat absorption of the reaction is large. The lower the molar mass is, the larger the heat absorption is, and an excellent chemical heat sink will be obtained which results in good ignition performance of the cracking gas [14,15]. So, the alkene selectivity also plays an important role in increasing heat sink of fuel. Compared to the results of alkene selectivity in Table 2, the alkenes selectivity of PZTA600y catalysts is
Table 2 Gas phase product distribution of thermal cracking and catalytic cracking. Temperature Samples Mol fraction/%
ene/ane
H2
CH4
C2H6 C2H4 C3H8 C3H6 C4
550 C
None ZTA Cat1 Cat2 Cat3 Cat4 Cat5 Cat6 Cat7 Cat8 Cat9 Cat10
8.4 3.4 8.9 9.3 7.7 13.1 4.9 16.1 10.7 7.4 10.5 11.2
17.5 28.2 26.5 28.6 24.3 23.3 27.7 26.2 29.1 27.2 27.9 27.9
20.9 17.6 15.7 9.5 12.1 11.2 17.1 14.1 14.6 15.2 14.3 12.8
28.8 21.5 20.6 19.2 23.4 21.9 21.9 19.1 20.2 20.9 21.3 18.5
8.8 7.1 6.3 5.7 5.3 5.8 6.7 5.9 4.9 5.3 5.7 5.5
3.4 14.5 14.6 18.1 17 16.4 14.8 15.5 14.3 16.5 17.1 15.7
12.2 7.6 7.4 9.6 10.2 8.3 6.9 3.1 6.2 7.5 3.2 8.4
1.09 0.79 0.84 1.02 1.17 1.11 0.81 0.77 0.8 0.9 0.83 0.88
600 C
None ZTA Cat1 Cat2 Cat3 Cat4 Cat5 Cat6 Cat7 Cat8 Cat9 Cat10
3.9 6.7 4.1 6.5 10.2 6.5 3.5 12.4 10.1 5.6 6.9 12.8
32.2 29.8 28.5 29.8 21.8 23.5 28.2 29.7 30.1 28 28.9 32.6
20.9 16.0 17.3 13.1 10.3 12.9 17.6 15.9 15.1 17.2 16.7 14.1
27.2 21.2 21.7 22.7 25.6 24.8 21.5 20.1 20.2 21.6 20.3 17.8
7.1 6.1 5.9 6 7.1 7.3 7.1 5.9 5.3 7.2 6.4 5
7.5 13.6 14.3 14.2 16.3 17.2 14.5 13.1 13 16.1 16.3 11.6
1.2 6.5 8.2 7.7 8.7 7.8 7.6 2.9 6.2 4.3 4.5 6.1
0.59 0.77 0.83 0.87 1.25 1.1 0.79 0.67 0.75 0.76 0.75 0.66
650 C
None ZTA Cat1 Cat2 Cat3 Cat4 Cat5 Cat6 Cat7 Cat8 Cat9 Cat10
8.5 4.8 4.6 3.4 6.8 8.6 6.8 4.8 7.9 9 8.9 6.6
34.5 28.9 28.5 24.4 21.3 22.4 23.9 28.9 26.7 26.7 26.5 29.6
26.3 16.0 16.2 14.4 12.3 11.4 14.2 16 15.7 16.8 15.7 16.7
17.8 20.8 21.1 24.8 24.7 25 23.8 20.8 20.5 21.3 20.6 20.4
4.8 6.5 6.1 7.8 7.8 7.6 7.5 6.5 6.4 7 6.3 6.4
6.6 15.4 15.4 16.8 18.2 16.8 15.4 15.4 15.3 16 14.6 15
1.5 7.4 8.1 8.4 8.9 8.2 8.4 7.6 7.5 3.2 7.4 5.3
0.39 0.81 0.85 1.02 1.21 1.16 1.04 0.81 0.85 0.76 0.84 0.74
700 C
None ZTA Cat1 Cat2 Cat3 Cat4 Cat5 Cat6 Cat7 Cat8 Cat9 Cat10
8.5 5.1 8.5 3.5 8.2 9.0 6.8 7.8 6.8 8.6 5.1 9.3
34.2 29.3 28.1 27 25.8 26.1 33.3 29.6 29.8 27.7 29.3 30
17.8 15.6 12.8 13.7 12.2 11.9 15.1 16.3 16 14.4 15.6 14.9
23.9 21.2 23.9 25.8 24.6 24.3 23.8 21.6 21.2 20.2 21.2 20.1
4.3 6.2 5.3 6.3 5.8 5.5 5.0 5.7 6.1 5.2 6.2 4.9
9.8 15.2 15.8 17.1 16.5 16.1 11.8 16.3 13.6 16.3 15.2 14.3
1.5 7.4 5.6 6.6 6.9 7.1 4.2 2.7 6.5 7.6 7.4 6.5
0.62 0.82 1.03 1.01 1.06 1.06 0.71 0.75 0.77 0.89 0.82 0.78
ene/ane means the molar content of alkenes to alkanes, ZTA refers to the ZTA6003 without Pt.
Y. Jiao et al. / Applied Thermal Engineering 91 (2015) 417e425
increased by the following order: Cat3(55.6%) > Cat4 (52.4%) > Cat2(46.5%) > Cat1(45.0%) > Cat5(44%) at 600 C and Cat3(54.8%) > Cat4(53.7%) > Cat2(50.5%) > Cat5(46.0%) > Cat1(44%) at 650 C with the varying molar ratio of ZrO2:TiO2. The alkenes selectivity of PZTA800y coatings catalysts is decreased to some extends compared with PZTA600y, but it is still larger than thermal cracking at 600 C and 650 C. The alkenes selectivity of Cat8 is maintained 47.1% after calcining at 800 C. It also can be seen the alkenes selectivity of ZTA6003-catalyst is larger than thermal cracking, but is less than the PZTA600y catalysts. Above all, PZTA owns the remarkable alkenes selectivity and thermal stability when the catalyst with molar ratio of ZrO2:TiO2 lies in 0.45:0.45. Meanwhile, optimal alkenes selectivity can meet the requirement for the increasing heat capacity of fuel in catalytic cracking. The addition of catalysts enhanced the yields of lowcarbon alkenes, and weakened the yield of low-carbon alkanes of cracking reaction, it is due to the addition of catalyst increased the probability of cracking reaction with the mechanism of carbon cation, making the reaction occur to the direction of cracking prolific alkenes [35,36]. Table 3 compares the major liquid products distribution of RP-3 jet fuel from thermal cracking and catalytic cracking at different temperatures, and the major liquid products were divided into four categories: n-paraffins, olefins, cycloparaffins, aromatics. It can be seen that the major liquid products of thermal and catalytic cracking of RP-3 both are n-paraffins at 600 C, 650 C and 700 C. It is worth noting that the amount of aromatics for thermal cracking was gradually increased with the temperature, and was much more than that for the catalytic cracking of Cat3 at 650 C and 700 C. So, the addition of PZTA catalysts improved the alkenes selectivity and reduced the rate of carbon deposition. 3.1.3. Heat sink Heat absorption of fluid (Heat Sink) is defined as heating power (W) multiplied by the thermal efficiency (h). The measurement of voltage U and current I was needed. The heat sink (Qm) can be concluded by heat absorption divided by the mass flow rate, computational formula was shown in
Qm ¼
W h UIh ¼ G 1000 G
(1-3)
Qm e heat sink (kJ/kg); G e mass flow rate (kg/s); W e heating power (kW); h e thermal efficiency; U e voltage (V); I e current (A). The error of method was less than 3.0% compared with the heat absorption of standard material, and the error of repetitive experiments of each group catalysts at different temperature spots was less than 2.5%. It is obvious that the heat sink is evidently heightened by adding catalysts as shown in Fig. 3, and compared to the results of thermal cracking, the heat sink over Cat3 is higher than the other type of catalysts, increasing by 0.18, 0.24, 0.43, 0.36 MJ/kg at 550, 600, 650, 700 C, respectively. The increasing rate of heat
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sink over PZTA600y coatings (Fig. 3a) catalysts follows by such order: Cat3(11.9%) > Cat2(10.8%) > Cat4(10.3%) > Cat1 (6.4%) > Cat1 (3.4%) at 600 C, and Cat3(17.8%) > Cat2(14.9%) > Cat1(13.1%) > Cat4(10.8%) > Cat5(9.0%) at 650 C compared with thermal cracking. Along with the variation of molar ratio of ZrO2:TiO2, the heat sink is summated at its appropriate composition, i.e. ZrO2:TiO2 ¼ 0.45:0.45, this is attributed to the optimal gas yield and alkenes selectivity of the catalyst. The heat sink of PZTA800y (Fig. 3b) had a little decrease compared with PZTA600y caused by the lower of gas yield and alkenes selectivity. In spite of this, the heat sink of Cat6eCat10 had increased by 0.15, 0.16, 0.26, 0.18, 0.21 MJ/kg at 700 C. By the experimental results of two series catalysts, although the PZTA800y was calcined at 800 C, they still had splendid gas yield and heat sink compared with thermal cracking, explaining that the catalyst has better thermal stability and catalytic activity. And Cat3 and Cat8 with the appropriate composition (the molar ratio of ZrO2:TiO2 ¼ 0.45:0.45) showed the best cracking activity and cracking selectivity in their respective series. 3.2. Textural properties and microstructure of PZTA Table 4 summarizes the specific surface area, pore volume and average pore diameter of the PZTA600y and PZTA800y. The surface area of the ZTA series initially decreases with the molar ratio of ZrO2:TiO2, followed by a transition point with maximized surface area and pore volume (275.1 m2/g and 0.45 mL/g obtained from Cat3, 113.0 m2/g and 0.30 mL/g obtained from Cat8) at 0.45:0.45. A larger surface area is beneficial with regard to uniform dispersion of active sites and thus plays a positive role in reducing the sizes of the active centers, preventing active sites from agglomerating during sintering. The pore size of ZTA600y as calculated by the BJH method shows that only pores with openings of less than 4 nm exist [37,38], and the size becomes larger at 800 C. As noted above, the specific surface area of ternary mixed oxides (ZTA) is larger than the binary mixed oxides (ZA or TA), and the molar ratio of ZrO2:TiO2 ¼ 0.45:0.45 reached the biggest. Fig. 4a and b displays the SEM morphology of Cat3 and Cat8, Fig. 4a shows that the catalyst particles are 2e4 mm in size, and present some sparklet on the surface of catalyst, where the active component Pt confirmed by EDS detection exist more concentrated; And it can be seen from Fig. 4b that the catalyst particles become bigger from 4 to 8 mm in size, the sparklet is faintly visible on the surface of Cat8. Fig. 4c and d shows TEM images of Cat3. The presence of Pt nanoparticles cannot be detected by SEM images, so TEM analysis was used to determine Pt nanoparticles. The presence of Pt species (Pt0 and PtO) is confirmed by d-spacing measurements (Fig. 4d) [39]. Also, it is observed that the active phase (Pt) is fairly well dispersed with a particle size of 3e6 nm. Therefore, apart from the mutual interaction of Pt and ZTA, their existence in form of nanoparticles helps to provide catalyst with much more reactive sites which correlates with its superior catalytic activity.
Table 3 The liquid phase product distribution of the cracking reaction of RP-3 jet fuel. Liquid products
Paraffins/%
RP-3
72.99
Temperature/ C
600 C 650 C 700 C
None Cat3 None Cat3 None Cat3
45.97 49.32 43.68 55.47 24.43 49.17
Olefins/% 0 12.54 5.51 13.42 6.22 20.25 6.12
Cycloparaffins/%
Aromatics/%
15.2
11.81
13.58 13.79 11.59 18.62 4.73 10.82
27.91 31.38 31.31 19.69 50.59 33.89
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Y. Jiao et al. / Applied Thermal Engineering 91 (2015) 417e425
Fig. 3. The heat sink distribution of thermal cracking and catalytic cracking.
Table 4 Textural properties, crystallinity and surface concentration of catalysts. Samples
Cat1/Cat6 Cat2/Cat7 Cat3/Cat8 Cat4/Cat9 Cat5/Cat10
Surface area (m2/g)
Pore volume (mL/g)
Average pore diameter (nm)
Crystallinity/%
Surface concentration/%
A
B
A
B
A
B
A
B
A
B
251.7 261.1 275.1 249.1 145.1
78.2 70.7 113 89.1 61.6
0.37 0.42 0.45 0.35 0.25
0.25 0.19 0.30 0.28 0.25
2.94 3.24 2.73 2.80 3.50
6.50 7.08 5.31 6.30 8.52
49.5% 32.3% 23.6% 24.4% 32.6%
73.9% 70.6% 58.9% 62.3% 64.7%
0.48 0.51 0.66 0.47 0.32
0.26 0.31 0.34 0.29 0.19
A refers to PZTA-600, B refers to PZTA-800.
Fig. 4. SEM/TEM micrographs of the catalysts.
Y. Jiao et al. / Applied Thermal Engineering 91 (2015) 417e425
3.3. XRD and XPS results Fig. 5 presents the XRD patterns of the as-prepared catalysts, showing that the crystallization of ZTA600y (Fig. 5a) catalysts all exists as basic amorphous structures but Cat1, neither the anatase TiO2 crystal phase nor ZrTiO4 was observed in Cat2eCat5, this may be due to the addition of Al2O3. The addition of Al2O3 into the ZrO2eTiO2 material will generate a mixture, which suppressed the formations of ZrTiO4 crystals and the anatase TiO2. Crystallization of the mixed oxides will decrease both the surface area and surface acidity with concurrent lessening of the catalytic activity. The addition of Al2O3 inhibits the crystallization temperature of the TiO2 and ZrTiO4 crystal phases to a higher temperature, however, preventing this degradation of the surface area and surface acidity and thus improving the activity of the catalysts partly. And Cat3 appeared slightly promoter CeO2 microcrystalline, this is contributed to the CeO2 dispersed on the catalyst, and the peaks at 2q value of 28.4 was knows as (111) crystal plane of cubic phase CeO2. 25.5 , 48.1, 55.0 spectrum peaks of Cat1 was known as the anatase TiO2 crystal phase and the crystallinity is larger (49.5%) than others. It shows that Cat1 is easy to crystallization under lower temperature (600 C), and thermal stability is poorer. It can be seen from Fig. 5b that all of the catalysts appear more complete crystal structure under high temperature calcining, crystallinity raised, crystal species increased, and the crystallinity of Cat8 is minimal. After calcining at 800 C the crystallization of catalyst with ZrO2:TiO2 ¼ 0.45:0.45 is minimized, indicating that the thermal stability of such catalyst is optimized. The molar contents of Pt dispersed on the surfaces of Cat1eCat5 were 0.48%, 0.51%, 0.66%, 0.48% and 0.32%, respectively, showing that Pt dispersion on the catalyst surface is related to the BET surface area, forming a shell-type catalyst with a uniform dispersion of the active component. Because Pt acts as the active center of the catalyst, better dispersion of the Pt results in better catalytic activity. The contents of Pt dispersed on the Cat6eCat10 catalyst surfaces have a decrease after calcination at 800 C. Small surface area reduces the dispersion of active sites and can lead to agglomeration, affecting the catalytic activity. As noted above, the larger surface area of PZTA600 correlates with its superior catalytic activity and high-temperature stability. These results are in agreement with the catalytic activities for the cracking reaction results. 3.4. NH3eTPD Fig. 6 shows the NH3eTPD plots obtained from ZTA600y and ZTA800y. NH3eTPD techniques were used to study the acidities of
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the catalysts, such as total amount, nature, and distributions, and to find the possible explanations for the above experimental results. NH3 adsorbing on the surface of the catalysts is continuous stripping out with the rising temperatures, showing that the surface acidic sites of these catalysts have a homogeneous, continuous distribution and that there is significant surface acidity [13,40], adsorption of weak and medium acid in the curve of the lower temperature zone (50e350 C) is stripping out easily, strong acid adsorption in the curve of the higher temperature zone (400e700 C) are difficult to stripping [19,40]. Desorbed NH3 was titrated again after the dilute hydrochloric acid absorption, and after calculation can be concluded that the surface of the catalyst acid amount (Table 5). In Fig. 6a, it also depicts that each one had a strong desorption peak in the temperature region 50e350 C, and the Cat2, Cat3 and Cat4 desorption peak areas are larger in this region. In the 400e700 C region, strong acid desorption peaks are observed and the peak area of Cat3 is much larger than the others. As shown in Fig. 6b the peak intensity of catalysts decreased significantly after calcining at 800 C, and mainly in the weak-medium acid with larger acidic amount. However the desorption peaks intensity of Cat8 is still outstanding. Table 5 summarizes the acid distributions of these catalysts and shows that the total amount of acid increases in the order of Cat3 > Cat2 > Cat4 > Cat1 > Cat5 and that Cat3 has the greatest concentration of strong acid. Results show that the acidity of ternary ZrO2eTiO2eAl2O3 mixed oxides is greater than the amount of TiO2eAl2O3 than ZrO2eAl2O3, and changes with the molar ratio of ZrO2:TiO2, the amount of acid catalyst increased first and then decreased, and reached the best at the ratio of 0.45:0.45 (total acid amount of 1.003 mmol/g and the strong acid amount of 0.395 mmol/g). The proportion of strong acid in Cat3 and Cat8 is also much larger than the others, demonstrating relatively concentration of acidic center density in Cat3. Reports [41e43] have stated that CeC bond rupture occurs at strongly acidic sites on the catalyst while hydrogen transfer reactions proceed at different acidic centers. Therefore, increasing the surface acidity of the catalysts is good for increasing the ratio of CeC fractures to hydrogen transfer reactions, thus improving the alkenes selectivity and reducing carbon deposition. In summary, the NH3eTPD plots, along with the above data concerning acid distributions, explain why Cat3 has the greatest surface acidity and the most concentrated acid center density, which is helpful for increasing of cracking activity and selectivity, and Cat8 after calcining at 800 C still keeps a larger surface acidity with total acid amount of 0.599 mmol/g and the strong acid amount of 0.181 mmol/g, explain why Cat3 possesses the thermal stability, which is also advantageous to cracking reaction.
Fig. 5. XRD patterns of different ZTA600y and ZTA800y.
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Fig. 6. NH3eTPD profiles for different ZTA600y and ZTA800y.
Table 5 Acidity of catalysts. Samples
Acidity (mmol/g) Weak acid sites
Medium acid sites
Strong acid sites
Total acid sites
Cat1 Cat2 Cat3 Cat4 Cat5 Cat6 Cat7 Cat8 Cat9 Cat10
0.181 0.247 0.225 0.211 0.212 0.125 0.092 0.223 0.107 0.071
0.264 0.365 0.383 0.336 0.194 0.223 0.132 0.195 0.173 0.098
0.191 0.193 0.395 0.232 0.133
0.636 0.805 1.003 0.779 0.539 0.348 0.224 0.599 0.402 0.169
4. Conclusions Catalytic cracking of RP-3 was examined in stainless-steel microchannels coating catalysts (PZTA) with a different molar ratios of ZrO2:TiO2. It is obvious that Cat3 coatings with the same molar ratio of ZrO2:TiO2 exhibit significantly optimal catalytic cracking activities than existing research in this field due to its larger amount of strong acid and surface area. The gas yield, the alkenes content within the gas phase product and the heat sink of Cat3 at 700 C are 40.7%, 51.5% and 3.41 MJ/kg respectively. Considering the gas yield of RP-3, the alkenes selectivity and deactivation of the catalyst under high temperature, PZTA with same mole ratio of ZrO2:TiO2 might be a potential catalyst for the cracking of RP-3. It should be noted that the increasing rate of gas yield and heat sink of Cat8 has a drawback compared with Cat3, and the thermal stability of catalysts could be further improved. Moreover, the inhibition of carbon deposition of cracking reaction should be figured out. Current studies are still under way in our lab to further optimize the PZTA catalyst by the way of improving the content of Al2O3 and adding additives, so that the catalyst would be anticipated
0.181 0.122
remarkable thermal stability and carbon inhibition performance in the future. Acknowledgements The research was performed in the College of Chemistry, Sichuan University (91016002, J1103315) and Xi'an Modern Chemistry Research Institute. The research was supported by the National Natural Science Foundation of China (91016002, J1103315). References [1] T. Edwards, Liquid fuels and propellants for aerospace propulsion: 1903e2003, J. Propul. Power 19 (2003) 1089e1107. [2] I.W. Kay, W.T. Peschke, Hydrocarbon-fueled scramjet combustor investigation, J. Propul. Power 18 (1992) 507e512. [3] A. Niaei, J. Towfighi, S.M. Sadrameli, R. Karimzadeh, The combined simulation of heat transfer and pyrolysis reactions in industrial cracking furnaces, Appl. Therm. Eng. 24 (2004) 2251e2265. [4] W.X. Zhou, B. Yu, J. Qin, D. Yu, Mechanism and influencing factors analysis of flowing instability of supercritical endothermic hydrocarbon fuel within a small-scale channel, Appl. Therm. Eng. 71 (2014) 34e42.
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Research paper
Catalytic cracking of RP-3 jet fuel over wall-coated Pt/ZrxTi0.9xAl0.1O2 mixed oxides catalysts Yi Jiao a, b, Shanshan Li a, Bin Liu a, Yongmei Du b, Jianli Wang a, *, Jian Lu b, Yaoqiang Chen a a b
Sichuan University, Chengdu 610064, Sichuan, China Xi'an Modern Chemistry Research Institute, Xi'an 710065, Shaanxi, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Pt/ZrxTi0.9xAl0.1O2 mixed oxides were introduced into cracking of jet fuel. The surface acidity provided a guarantee for activity of cracking reaction. The centralized strong acidic sites provided a guarantee for selectivity. The catalyst (nZr ¼ nTi) shows the best activity and high-temperature stability. The catalyst (nZr ¼ nTi) still holds higher activity after calcining at 800 C.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 April 2015 Accepted 25 July 2015 Available online 28 August 2015
This study involves a new series Pt-loaded ZrxTi0.9xAl0.1O2 (PZTA) mixed oxides with different molar ratios of ZrO2:TiO2 (ZrO2:TiO2:Al2O3 ¼ x:(0.9 x):0.1, where x ¼ 0, 0.225, 0.45, 0.675, 0.9) catalysts with high temperature resistant and large surface acidity for catalytic cracking of RP-3 jet fuel under hightemperature and high pressure conditions. PZTA catalysts were prepared by using a specially designed that co-precipitation technique subsequent incipient wetness impregnation, and were coated on the wall of stainless-steel microchannels. It was found that gas yield and heat sink of catalytic cracking over the as-prepared catalysts were noteworthy augmented compared with the thermal cracking. Furthermore, different molar ratios of ZrO2:TiO2 could greatly affect the catalytic cracking activities of RP-3 jet fuel. To be more specific, Cat3 (ZrO2:TiO2 ¼ 0.45:0.45) performed the excellent cracking activity and hightemperature stability, which is in accordance with its bigger amount of strong acid, larger surface area and good dispersion effect of Pt. The gas yield, the alkenes content within the gas phase product and the heat sink of Cat3 at 700 C are 40.7%, 51.5% and 3.41 MJ/kg. SEM/TEM measurements further demonstrated that the active phase (Pt) is fairly well dispersed with a particle size of 3e6 nm. The valuable information indicated in this work is that based on the special characters of PZTA, ternary composite oxides supported Pt, which should be an effective approach to maintain both catalytic activities and thermal stability. © 2015 Published by Elsevier Ltd.
Keywords: ZrxTi0.9xAl0.1O2 mixed oxides Coating catalyst Catalytic cracking Coking Acidity
* Corresponding author. Tel./fax: þ86 28 85418451. E-mail address: [email protected] (J. Wang). http://dx.doi.org/10.1016/j.applthermaleng.2015.07.085 1359-4311/© 2015 Published by Elsevier Ltd.
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1. Introduction “Thermal barrier” on the surface of an engine is becoming currently the greatest challenge associated with hypersonic flight. The large amount of heat is generated dramatically by air friction when the aircraft is in flight at the speed of supersonic or hypersonic regimes. It would cause the temperature of the fuselage and power equipment to rise sharply, potentially leading to significant damage [1e6]. Under such situations, hydrocarbon fuels as an ideal coolant can offer heat absorption capacity (heat sink) for supersonic aircrafts, through both significant physical sensible heat and heat-adsorbing by chemical cracking reactions [7e12]. However, the physical sensible heat has a fixed and limited value under a certain condition. So it is necessary to enhance the chemical endothermic capability through the chemical cracking reactions. Now, intense research efforts have been made towards the heat absorption of chemical cracking reactions over the hydrocarbon fuel, which include both thermal and catalytic cracking. Unfortunately, thermal cracking reactions require much higher temperatures and are readily subject to coking, people eventually focus on the study of catalytic cracking reactions in this field. In addition to reduce the initial cracking temperature and generate much smaller heat-absorbents, such as alkenes, the catalytic cracking can provide sufficient absorption capability and is therefore becoming an attractive option [13e16]. The frequently-used cracking catalysts are mainly composed of noble metals, zeolites, and mixed oxides catalysts [17e22]. Although the noble metal catalysts have good effect on catalytic dehydrogenation, they still have some obvious shortcomings, such as a susceptibility to poisoning, short lifetimes, more expensive and deactivation by coking [23]. Many works have been performed on the catalytic cracking of supercritical hydrocarbon over kinds of zeolite catalysts [17e19,24], which exhibit superior catalytic activity/selectivity, and provide preferable heat absorption. However, porous zeolites tend to collapse and losing acidity under high temperature (above 650 C), which makes their high-temperature stabilities relatively poor. So the catalytic cracking of zeolites above 650 C was rarely reported. Therefore, it is very necessary to study the catalysts which keep outstanding catalytic cracking activities and stabilities under high temperature. It is well known that mixed oxides catalysts not only combine the advantages of individual oxide supports, but also hold excellent acidity and textural properties. Among them, TiO2eZrO2 mixed oxides have been widely used as catalytic supports due to their superior performance which usually including larger surface area, mechanical strength as well greater surface acidity, and their performance is discrepant with different ZrO2:TiO2 molar ratios [25e29]. However, ZrO2eTiO2 still lacks of thermal stability, easy to crystallization above 650 C [29,30]. Al2O3 has the advantages of large surface area, and lack of crystallization at high temperatures, so Al2O3 has also been introduced to ZrO2eTiO2 mixed oxides supports to get ternary mixed oxides catalysts, which is also one of the important method for developing excellent catalysts with excellent activity and stability [13,30,31]. Therefore, the use of ZrO2eTiO2eAl2O3 (ZTA) mixed oxides supports would be a good way to solve the poor thermal stability of porous zeolites, and hold excellent catalytic cracking activities and stabilities under hightemperature. The objective of this work is to investigate the influence of Ptloaded ZrO2eTiO2eAl2O3 (ZTA) catalysts with varying molar ratio of ZrO2:TiO2 on the catalytic cracking of hydrocarbons under hightemperature and high pressure conditions. By introducing ZrO2eTiO2eAl2O3 with different ZrO2:TiO2 molar ratios, the structure and texture performance of these mixed oxides could be tuned, along with their surface acidity and thermal stability, so as
high temperature resistant and large specific surface area for hydrocarbon fuel catalytic cracking. The purpose of the present work is to develop an effective catalytic support and to provide some fundamental suggestions for the catalytic cracking mechanism of supercritical hydrocarbons for the catalytic heat exchangers for the advanced aircrafts. 2. Experimental 2.1. Materials The feedstock RP-3 jet fuel was purchased from Chengdu Kelong Chemical Reagent Company. The physical properties are described in Table 1. 2.2. Preparation of ZrxTi0.9xAl0.1O2 mixed oxides and Pt-loaded catalysts ZrxTi0.9xAl0.1O2 mixed oxides were prepared by coprecipitation method. Zr(NO3)4$3H2O, TiOSO4$2H2O and Al(NO3)3$9H2O were dissolved in high-purity water (the impurities content is less than 0.1 mg/L and salinity in 0.3 mg/L) with different molar ratios of ZrO2:TiO2 (ZrO2:TiO2:Al2O3 ¼ x:(0.9 x):0.1, where x 0, 0.225, 0.45, 0.675, 0.9) and precipitation was induced by the addition of a pH ¼ 10 buffer solution composed of NH3$H2O and (NH4)2CO3. The as-prepared precipitate was aged, dried for 2 h at 120 C and calcined for 3 h at 600 C or calcined for 3 h at 800 C, denoting as ZTA600y and ZTA800y, where y ¼ 1, 2, 3, 4, 5 corresponding to the different molar ratios of ZrO2:TiO2 (ZrO2:TiO2:Al2O3 ¼ x:(0.9 x):0.1, x ¼ 0, 0.225, 0.45, 0.675, 0.9). The as-prepared ZTA600y and ZTA800y were then impregnated by chloroplatinic acid as the Pt precursor (Pt content 0.70%), and then the impregnated catalysts were calcined for 2 h at 550 C. The catalysts were subsequently ball-milled with water and then coated on the inner walls of stainless-steel pipes (304#, 80 cm F3 0.5 mm, 2.51 cm3) treated by thermal oxidation at high temperatures by a way of suction using vacuum pump, controlled loads and uniformity of catalyst by adjusting the suction pressure and slurry viscosity, and finally dried. The wall-coated catalysts produced from the PZTA600y (y ¼ 1e5) and PZTA800y (y ¼ 1e5) are denoted as Cat1eCat5 and Cat6eCat10 respectively. The catalyst loading on the inner surface of tube is 0.2 ± 0.005 g/2.51 cm3, which lies in each catalyst. 2.3. Catalytic activity evaluation The self-designed experimental apparatus was used to evaluate the catalytic cracking of fuels, introducing a feed pump, mass flow meter, temperature-control device, pressure system, reactor, condenser, and an analysis system as shown in Fig. 1. The stainless-
Table 1 Specification properties of Chinese RP-3 jet fuel used in this work. Properties
Values
Density, kg/m3 Boiling range, initial point, C 10%, C 50%, C 90%, C Dry point, C n-Paraffins, % Isoparaffins, % Cycloparaffins, % Aromatics, % Sulfur content, ppm
0.7958 135 162.5 182.7 213.3 230 57.72 15.72 15.2 11.81 65.5
Y. Jiao et al. / Applied Thermal Engineering 91 (2015) 417e425
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Fig. 1. Schematic diagram of apparatus. 1 e feed tank; 2 e high pressure metering pump; 3 e check valve; 4 e mass flow meter; 5 e pressure system; 6 e heating system; 7 e cold trap; 8 e filter; 9 e backpressure valve; 10 e gaseliquid separator; 11 e liquid receiver; 12 e gas chromatograph; 13 e wet gas flow meter.
steel tubes (inner diameter 2 mm, 80 cm length) and as-prepared wall-coated tubes (0.2 g) were used as reaction center. The catalytic performance measurement was carried out from 550 C to 700 C (the fuel temperature) and pressure was kept at 3.5 MPa. RP3 jet fuel as a probe reaction was pumped into the reactor with a high pressure constant flow pump at the flow rate of 76 mL/min. The wall-coated tubes were heated by direct current (DC) power, and the backpressure was adjusted as a constant value. The reaction was quenched by dropping temperature through a water condenser and was separated by a gaseliquid separator. The volume of the gaseous products was quantified by water displacement method and the liquid residues were collected with a conical flask and weighed. The gas phase product which comes from the gaseliquid separator was pulsed into the gas chromatograph (GC2000 III, Shanghai institute of technology and computing), equipped with an HP-Al/S separation capillary column (Agilent Technologies Co., Ltd., 50 m * 0.53 mm) and FID for analyzing small organic molecules. The content of H2 was analyzed with external standard method, 2 m packed column (stationary phase, TDX-01) and TCD detector. The liquid products were identified by HP-6890/5973 GCeMS (Agilent Technologies, Inc., Santa Clara, CA) with an FID and an HP-5/MS column (50 m * 0.25 mm). Gas yield and heat sink of several temperature spots were evaluated in the experiment. The mass closure was over 97.5% between feeds and products involving gas, liquid products, and coke.
coatings were performed using an Oxford-IE-250 energy dispersive spectrometer (EDS). The surface acidity of each catalyst was measured by NH3etemperature programmed desorption (NH3eTPD) using a TP5076 TPD instrument. During these measurements, a sample of 100 mg was heated to 400 C at a rate of 8 C/min in a flow of N2, held 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 over the temperature was increased from ambient to 700 C at a heating rate of 8 C/min, employing a thermal conductivity detector. 3. Results and discussion 3.1. Catalytic activity 3.1.1. Gas yield The thermal cracking and catalytic cracking procedures of RP-3 were performed, respectively. The observed gas yield of RP-3, defined as
Gas yield ¼
Inital mass of ðRP3Þ Final mass of residues 100% Inital mass of RP3 (1-1) G1 G 100% G
2.4. Catalysts characterization
The increasing rate ¼
The textural properties of the catalysts were detected using a Quadrasorb SI Automated Surface Area analyzer (Quantachrome Instruments, USA). Samples were initially kept under vacuum for 1 h at 300 C, then cooled to 196 C using liquid N2, at which point their N2 adsorption was measured. X-ray diffraction (XRD) patterns of the as-prepared materials were obtained using a DX-2500 X-ray diffractometer (China Dandong Fangyuan Instrument Co., Ltd.) with a graphite monochromator and Ni filter with Cu Ka radiation, operated at 40 kV and 25 mA. The samples were scanned over a 2q range of 10 e80 at a step rate of 0.03 /s. XPS was performed using an XSAM-800 spectrometer (KRATOS Co., UK) with MgKa excitation under a high voltage (13 kV) and current (20 mA), calibrated internally by the carbon deposit C1s binding energy at 284.8 eV. The micro-morphological characteristics of the catalyst coatings were observed using a Hitachi-S-4800 scanning electron microscope (SEM) and a Tecnai G2 F20 S-TWIN (FEI, USA) Transmission electron microscopy (TEM). Qualitative and quantitative analyses of elemental distributions in microscopic regions of the catalyst
where G1 and G are the gas yield of catalytic cracking and thermal cracking, respectively, and the results under two series different cracking catalysts are shown in Fig. 2. The results manifest that the gas yields of catalytic cracking over PZTA catalysts are higher than thermal cracking at the same experimental condition, in which the increasing rate of gas yield of PZTA600y (Fig. 2a) is increased by the following order: Cat3(65.7%) > Cat2(50.2%) > Cat4(28.4%) > Cat5(25.0%) > Cat1(7.9%) > thermal cracking z ZTA6003 at 600 C and Cat3(63.2%) > Cat4 (59.5%) > Cat2(57.7%) > Cat1(35.9%) > Cat5(28.1%) > thermal cracking z ZTA6003 at 650 C with the varying molar ratio of ZrO2:TiO2. Although some deactivation occurs at 700 C, Cat3 also holds the excellent increasing rate of gas yield (34.4%) compared with thermal cracking. The increasing rate of gas yield of PZTA800y (Fig. 2b) followed the same order as PZTA600y coatings: Cat8(16%) > Cat9(15.4%) > Cat7(14.0%) > Cat6(12.3%) > Cat10 (0.3%) > thermal cracking z ZTA6003 at 650 C, However, the gas yield of PZTA800y behaves worse than that of PZTA600y. Above all,
(1-2)
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Fig. 2. The gas yield of thermal cracking and catalytic cracking.
the gas yields of catalytic cracking of the Pt supported ternary mixed oxides (ZrO2, TiO2, Al2O3) are much better than that of the Pt supported binary mixed oxides (ZrO2 and Al2O3, TiO2 and Al2O3), which lie in PZTA600y and PZTA800y. And when the molar ratio of ZrO2:TiO2 is 0.45:0.45, the gas yield is higher than the others. Although the gas yield of PZTA800y is decreased after calcining at 800 C compared with the PZTA600y, Cat8 also hold a preferable gas yield among PZTA800y series. The introduction of the Pt supported ternary mixed oxides ZTA improves the activity and hightemperature stability of the catalyst, and the catalyst with molar ratio of ZrO2:TiO2 in 0.45:0.45 exhibits the best the activity and high-temperature stability [13,31]. What's more, the addition of catalysts thus reduces the starting temperature of the cracking reaction effectively while enhancing the depth of cracking reaction obviously, decreasing the activation energy of reactant molecule, and increasing the reaction rate [32] and yields of many kinds of small molecule hydrocarbon. The increasing of gas yield is the prerequisite and guarantee for the improvement of heat capacity of fuel in catalytic cracking. It should be noted that the gas yield of the catalyst-ZTA6003 without Pt is less than PZTA catalysts, and is approximately equal to the value of thermal cracking. The Pt acted as the active sites of the catalyst has an intimate relationship with the catalytic activity. 3.1.2. The gaseous-liquid phase distribution and the ratio of alkene to alkane The ratio of alkenes to alkanes is used to represent the alkenes selectivity of catalysts. Gas phase product distribution and the ratio of alkene to alkane in catalytic cracking and thermal cracking at each temperature spots are shown in Table 2. Gas phase products of RP-3 cracking were mainly compose of hydrogen, low-carbon alkanes (methane, ethane, propane and butane) and low-carbon alkenes (ethylene, propylene, isobutylene and butadiene); and the components were varied by changing cracking temperature and introducing different catalysts. It can be seen from Table 2, H2 content from catalytic cracking over PZTA catalysts was higher than thermal cracking and the catalytic cracking over ZTA6003-catalyst without Pt due to the dehydrogenation role of Pt [33,34]. Thermodynamic analysis shows that if unsaturated hydrocarbons such as ethylene, propylene and butylene, and hydrogen are produced during the cracking reaction, the heat absorption of the reaction is large. The lower the molar mass is, the larger the heat absorption is, and an excellent chemical heat sink will be obtained which results in good ignition performance of the cracking gas [14,15]. So, the alkene selectivity also plays an important role in increasing heat sink of fuel. Compared to the results of alkene selectivity in Table 2, the alkenes selectivity of PZTA600y catalysts is
Table 2 Gas phase product distribution of thermal cracking and catalytic cracking. Temperature Samples Mol fraction/%
ene/ane
H2
CH4
C2H6 C2H4 C3H8 C3H6 C4
550 C
None ZTA Cat1 Cat2 Cat3 Cat4 Cat5 Cat6 Cat7 Cat8 Cat9 Cat10
8.4 3.4 8.9 9.3 7.7 13.1 4.9 16.1 10.7 7.4 10.5 11.2
17.5 28.2 26.5 28.6 24.3 23.3 27.7 26.2 29.1 27.2 27.9 27.9
20.9 17.6 15.7 9.5 12.1 11.2 17.1 14.1 14.6 15.2 14.3 12.8
28.8 21.5 20.6 19.2 23.4 21.9 21.9 19.1 20.2 20.9 21.3 18.5
8.8 7.1 6.3 5.7 5.3 5.8 6.7 5.9 4.9 5.3 5.7 5.5
3.4 14.5 14.6 18.1 17 16.4 14.8 15.5 14.3 16.5 17.1 15.7
12.2 7.6 7.4 9.6 10.2 8.3 6.9 3.1 6.2 7.5 3.2 8.4
1.09 0.79 0.84 1.02 1.17 1.11 0.81 0.77 0.8 0.9 0.83 0.88
600 C
None ZTA Cat1 Cat2 Cat3 Cat4 Cat5 Cat6 Cat7 Cat8 Cat9 Cat10
3.9 6.7 4.1 6.5 10.2 6.5 3.5 12.4 10.1 5.6 6.9 12.8
32.2 29.8 28.5 29.8 21.8 23.5 28.2 29.7 30.1 28 28.9 32.6
20.9 16.0 17.3 13.1 10.3 12.9 17.6 15.9 15.1 17.2 16.7 14.1
27.2 21.2 21.7 22.7 25.6 24.8 21.5 20.1 20.2 21.6 20.3 17.8
7.1 6.1 5.9 6 7.1 7.3 7.1 5.9 5.3 7.2 6.4 5
7.5 13.6 14.3 14.2 16.3 17.2 14.5 13.1 13 16.1 16.3 11.6
1.2 6.5 8.2 7.7 8.7 7.8 7.6 2.9 6.2 4.3 4.5 6.1
0.59 0.77 0.83 0.87 1.25 1.1 0.79 0.67 0.75 0.76 0.75 0.66
650 C
None ZTA Cat1 Cat2 Cat3 Cat4 Cat5 Cat6 Cat7 Cat8 Cat9 Cat10
8.5 4.8 4.6 3.4 6.8 8.6 6.8 4.8 7.9 9 8.9 6.6
34.5 28.9 28.5 24.4 21.3 22.4 23.9 28.9 26.7 26.7 26.5 29.6
26.3 16.0 16.2 14.4 12.3 11.4 14.2 16 15.7 16.8 15.7 16.7
17.8 20.8 21.1 24.8 24.7 25 23.8 20.8 20.5 21.3 20.6 20.4
4.8 6.5 6.1 7.8 7.8 7.6 7.5 6.5 6.4 7 6.3 6.4
6.6 15.4 15.4 16.8 18.2 16.8 15.4 15.4 15.3 16 14.6 15
1.5 7.4 8.1 8.4 8.9 8.2 8.4 7.6 7.5 3.2 7.4 5.3
0.39 0.81 0.85 1.02 1.21 1.16 1.04 0.81 0.85 0.76 0.84 0.74
700 C
None ZTA Cat1 Cat2 Cat3 Cat4 Cat5 Cat6 Cat7 Cat8 Cat9 Cat10
8.5 5.1 8.5 3.5 8.2 9.0 6.8 7.8 6.8 8.6 5.1 9.3
34.2 29.3 28.1 27 25.8 26.1 33.3 29.6 29.8 27.7 29.3 30
17.8 15.6 12.8 13.7 12.2 11.9 15.1 16.3 16 14.4 15.6 14.9
23.9 21.2 23.9 25.8 24.6 24.3 23.8 21.6 21.2 20.2 21.2 20.1
4.3 6.2 5.3 6.3 5.8 5.5 5.0 5.7 6.1 5.2 6.2 4.9
9.8 15.2 15.8 17.1 16.5 16.1 11.8 16.3 13.6 16.3 15.2 14.3
1.5 7.4 5.6 6.6 6.9 7.1 4.2 2.7 6.5 7.6 7.4 6.5
0.62 0.82 1.03 1.01 1.06 1.06 0.71 0.75 0.77 0.89 0.82 0.78
ene/ane means the molar content of alkenes to alkanes, ZTA refers to the ZTA6003 without Pt.
Y. Jiao et al. / Applied Thermal Engineering 91 (2015) 417e425
increased by the following order: Cat3(55.6%) > Cat4 (52.4%) > Cat2(46.5%) > Cat1(45.0%) > Cat5(44%) at 600 C and Cat3(54.8%) > Cat4(53.7%) > Cat2(50.5%) > Cat5(46.0%) > Cat1(44%) at 650 C with the varying molar ratio of ZrO2:TiO2. The alkenes selectivity of PZTA800y coatings catalysts is decreased to some extends compared with PZTA600y, but it is still larger than thermal cracking at 600 C and 650 C. The alkenes selectivity of Cat8 is maintained 47.1% after calcining at 800 C. It also can be seen the alkenes selectivity of ZTA6003-catalyst is larger than thermal cracking, but is less than the PZTA600y catalysts. Above all, PZTA owns the remarkable alkenes selectivity and thermal stability when the catalyst with molar ratio of ZrO2:TiO2 lies in 0.45:0.45. Meanwhile, optimal alkenes selectivity can meet the requirement for the increasing heat capacity of fuel in catalytic cracking. The addition of catalysts enhanced the yields of lowcarbon alkenes, and weakened the yield of low-carbon alkanes of cracking reaction, it is due to the addition of catalyst increased the probability of cracking reaction with the mechanism of carbon cation, making the reaction occur to the direction of cracking prolific alkenes [35,36]. Table 3 compares the major liquid products distribution of RP-3 jet fuel from thermal cracking and catalytic cracking at different temperatures, and the major liquid products were divided into four categories: n-paraffins, olefins, cycloparaffins, aromatics. It can be seen that the major liquid products of thermal and catalytic cracking of RP-3 both are n-paraffins at 600 C, 650 C and 700 C. It is worth noting that the amount of aromatics for thermal cracking was gradually increased with the temperature, and was much more than that for the catalytic cracking of Cat3 at 650 C and 700 C. So, the addition of PZTA catalysts improved the alkenes selectivity and reduced the rate of carbon deposition. 3.1.3. Heat sink Heat absorption of fluid (Heat Sink) is defined as heating power (W) multiplied by the thermal efficiency (h). The measurement of voltage U and current I was needed. The heat sink (Qm) can be concluded by heat absorption divided by the mass flow rate, computational formula was shown in
Qm ¼
W h UIh ¼ G 1000 G
(1-3)
Qm e heat sink (kJ/kg); G e mass flow rate (kg/s); W e heating power (kW); h e thermal efficiency; U e voltage (V); I e current (A). The error of method was less than 3.0% compared with the heat absorption of standard material, and the error of repetitive experiments of each group catalysts at different temperature spots was less than 2.5%. It is obvious that the heat sink is evidently heightened by adding catalysts as shown in Fig. 3, and compared to the results of thermal cracking, the heat sink over Cat3 is higher than the other type of catalysts, increasing by 0.18, 0.24, 0.43, 0.36 MJ/kg at 550, 600, 650, 700 C, respectively. The increasing rate of heat
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sink over PZTA600y coatings (Fig. 3a) catalysts follows by such order: Cat3(11.9%) > Cat2(10.8%) > Cat4(10.3%) > Cat1 (6.4%) > Cat1 (3.4%) at 600 C, and Cat3(17.8%) > Cat2(14.9%) > Cat1(13.1%) > Cat4(10.8%) > Cat5(9.0%) at 650 C compared with thermal cracking. Along with the variation of molar ratio of ZrO2:TiO2, the heat sink is summated at its appropriate composition, i.e. ZrO2:TiO2 ¼ 0.45:0.45, this is attributed to the optimal gas yield and alkenes selectivity of the catalyst. The heat sink of PZTA800y (Fig. 3b) had a little decrease compared with PZTA600y caused by the lower of gas yield and alkenes selectivity. In spite of this, the heat sink of Cat6eCat10 had increased by 0.15, 0.16, 0.26, 0.18, 0.21 MJ/kg at 700 C. By the experimental results of two series catalysts, although the PZTA800y was calcined at 800 C, they still had splendid gas yield and heat sink compared with thermal cracking, explaining that the catalyst has better thermal stability and catalytic activity. And Cat3 and Cat8 with the appropriate composition (the molar ratio of ZrO2:TiO2 ¼ 0.45:0.45) showed the best cracking activity and cracking selectivity in their respective series. 3.2. Textural properties and microstructure of PZTA Table 4 summarizes the specific surface area, pore volume and average pore diameter of the PZTA600y and PZTA800y. The surface area of the ZTA series initially decreases with the molar ratio of ZrO2:TiO2, followed by a transition point with maximized surface area and pore volume (275.1 m2/g and 0.45 mL/g obtained from Cat3, 113.0 m2/g and 0.30 mL/g obtained from Cat8) at 0.45:0.45. A larger surface area is beneficial with regard to uniform dispersion of active sites and thus plays a positive role in reducing the sizes of the active centers, preventing active sites from agglomerating during sintering. The pore size of ZTA600y as calculated by the BJH method shows that only pores with openings of less than 4 nm exist [37,38], and the size becomes larger at 800 C. As noted above, the specific surface area of ternary mixed oxides (ZTA) is larger than the binary mixed oxides (ZA or TA), and the molar ratio of ZrO2:TiO2 ¼ 0.45:0.45 reached the biggest. Fig. 4a and b displays the SEM morphology of Cat3 and Cat8, Fig. 4a shows that the catalyst particles are 2e4 mm in size, and present some sparklet on the surface of catalyst, where the active component Pt confirmed by EDS detection exist more concentrated; And it can be seen from Fig. 4b that the catalyst particles become bigger from 4 to 8 mm in size, the sparklet is faintly visible on the surface of Cat8. Fig. 4c and d shows TEM images of Cat3. The presence of Pt nanoparticles cannot be detected by SEM images, so TEM analysis was used to determine Pt nanoparticles. The presence of Pt species (Pt0 and PtO) is confirmed by d-spacing measurements (Fig. 4d) [39]. Also, it is observed that the active phase (Pt) is fairly well dispersed with a particle size of 3e6 nm. Therefore, apart from the mutual interaction of Pt and ZTA, their existence in form of nanoparticles helps to provide catalyst with much more reactive sites which correlates with its superior catalytic activity.
Table 3 The liquid phase product distribution of the cracking reaction of RP-3 jet fuel. Liquid products
Paraffins/%
RP-3
72.99
Temperature/ C
600 C 650 C 700 C
None Cat3 None Cat3 None Cat3
45.97 49.32 43.68 55.47 24.43 49.17
Olefins/% 0 12.54 5.51 13.42 6.22 20.25 6.12
Cycloparaffins/%
Aromatics/%
15.2
11.81
13.58 13.79 11.59 18.62 4.73 10.82
27.91 31.38 31.31 19.69 50.59 33.89
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Fig. 3. The heat sink distribution of thermal cracking and catalytic cracking.
Table 4 Textural properties, crystallinity and surface concentration of catalysts. Samples
Cat1/Cat6 Cat2/Cat7 Cat3/Cat8 Cat4/Cat9 Cat5/Cat10
Surface area (m2/g)
Pore volume (mL/g)
Average pore diameter (nm)
Crystallinity/%
Surface concentration/%
A
B
A
B
A
B
A
B
A
B
251.7 261.1 275.1 249.1 145.1
78.2 70.7 113 89.1 61.6
0.37 0.42 0.45 0.35 0.25
0.25 0.19 0.30 0.28 0.25
2.94 3.24 2.73 2.80 3.50
6.50 7.08 5.31 6.30 8.52
49.5% 32.3% 23.6% 24.4% 32.6%
73.9% 70.6% 58.9% 62.3% 64.7%
0.48 0.51 0.66 0.47 0.32
0.26 0.31 0.34 0.29 0.19
A refers to PZTA-600, B refers to PZTA-800.
Fig. 4. SEM/TEM micrographs of the catalysts.
Y. Jiao et al. / Applied Thermal Engineering 91 (2015) 417e425
3.3. XRD and XPS results Fig. 5 presents the XRD patterns of the as-prepared catalysts, showing that the crystallization of ZTA600y (Fig. 5a) catalysts all exists as basic amorphous structures but Cat1, neither the anatase TiO2 crystal phase nor ZrTiO4 was observed in Cat2eCat5, this may be due to the addition of Al2O3. The addition of Al2O3 into the ZrO2eTiO2 material will generate a mixture, which suppressed the formations of ZrTiO4 crystals and the anatase TiO2. Crystallization of the mixed oxides will decrease both the surface area and surface acidity with concurrent lessening of the catalytic activity. The addition of Al2O3 inhibits the crystallization temperature of the TiO2 and ZrTiO4 crystal phases to a higher temperature, however, preventing this degradation of the surface area and surface acidity and thus improving the activity of the catalysts partly. And Cat3 appeared slightly promoter CeO2 microcrystalline, this is contributed to the CeO2 dispersed on the catalyst, and the peaks at 2q value of 28.4 was knows as (111) crystal plane of cubic phase CeO2. 25.5 , 48.1, 55.0 spectrum peaks of Cat1 was known as the anatase TiO2 crystal phase and the crystallinity is larger (49.5%) than others. It shows that Cat1 is easy to crystallization under lower temperature (600 C), and thermal stability is poorer. It can be seen from Fig. 5b that all of the catalysts appear more complete crystal structure under high temperature calcining, crystallinity raised, crystal species increased, and the crystallinity of Cat8 is minimal. After calcining at 800 C the crystallization of catalyst with ZrO2:TiO2 ¼ 0.45:0.45 is minimized, indicating that the thermal stability of such catalyst is optimized. The molar contents of Pt dispersed on the surfaces of Cat1eCat5 were 0.48%, 0.51%, 0.66%, 0.48% and 0.32%, respectively, showing that Pt dispersion on the catalyst surface is related to the BET surface area, forming a shell-type catalyst with a uniform dispersion of the active component. Because Pt acts as the active center of the catalyst, better dispersion of the Pt results in better catalytic activity. The contents of Pt dispersed on the Cat6eCat10 catalyst surfaces have a decrease after calcination at 800 C. Small surface area reduces the dispersion of active sites and can lead to agglomeration, affecting the catalytic activity. As noted above, the larger surface area of PZTA600 correlates with its superior catalytic activity and high-temperature stability. These results are in agreement with the catalytic activities for the cracking reaction results. 3.4. NH3eTPD Fig. 6 shows the NH3eTPD plots obtained from ZTA600y and ZTA800y. NH3eTPD techniques were used to study the acidities of
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the catalysts, such as total amount, nature, and distributions, and to find the possible explanations for the above experimental results. NH3 adsorbing on the surface of the catalysts is continuous stripping out with the rising temperatures, showing that the surface acidic sites of these catalysts have a homogeneous, continuous distribution and that there is significant surface acidity [13,40], adsorption of weak and medium acid in the curve of the lower temperature zone (50e350 C) is stripping out easily, strong acid adsorption in the curve of the higher temperature zone (400e700 C) are difficult to stripping [19,40]. Desorbed NH3 was titrated again after the dilute hydrochloric acid absorption, and after calculation can be concluded that the surface of the catalyst acid amount (Table 5). In Fig. 6a, it also depicts that each one had a strong desorption peak in the temperature region 50e350 C, and the Cat2, Cat3 and Cat4 desorption peak areas are larger in this region. In the 400e700 C region, strong acid desorption peaks are observed and the peak area of Cat3 is much larger than the others. As shown in Fig. 6b the peak intensity of catalysts decreased significantly after calcining at 800 C, and mainly in the weak-medium acid with larger acidic amount. However the desorption peaks intensity of Cat8 is still outstanding. Table 5 summarizes the acid distributions of these catalysts and shows that the total amount of acid increases in the order of Cat3 > Cat2 > Cat4 > Cat1 > Cat5 and that Cat3 has the greatest concentration of strong acid. Results show that the acidity of ternary ZrO2eTiO2eAl2O3 mixed oxides is greater than the amount of TiO2eAl2O3 than ZrO2eAl2O3, and changes with the molar ratio of ZrO2:TiO2, the amount of acid catalyst increased first and then decreased, and reached the best at the ratio of 0.45:0.45 (total acid amount of 1.003 mmol/g and the strong acid amount of 0.395 mmol/g). The proportion of strong acid in Cat3 and Cat8 is also much larger than the others, demonstrating relatively concentration of acidic center density in Cat3. Reports [41e43] have stated that CeC bond rupture occurs at strongly acidic sites on the catalyst while hydrogen transfer reactions proceed at different acidic centers. Therefore, increasing the surface acidity of the catalysts is good for increasing the ratio of CeC fractures to hydrogen transfer reactions, thus improving the alkenes selectivity and reducing carbon deposition. In summary, the NH3eTPD plots, along with the above data concerning acid distributions, explain why Cat3 has the greatest surface acidity and the most concentrated acid center density, which is helpful for increasing of cracking activity and selectivity, and Cat8 after calcining at 800 C still keeps a larger surface acidity with total acid amount of 0.599 mmol/g and the strong acid amount of 0.181 mmol/g, explain why Cat3 possesses the thermal stability, which is also advantageous to cracking reaction.
Fig. 5. XRD patterns of different ZTA600y and ZTA800y.
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Fig. 6. NH3eTPD profiles for different ZTA600y and ZTA800y.
Table 5 Acidity of catalysts. Samples
Acidity (mmol/g) Weak acid sites
Medium acid sites
Strong acid sites
Total acid sites
Cat1 Cat2 Cat3 Cat4 Cat5 Cat6 Cat7 Cat8 Cat9 Cat10
0.181 0.247 0.225 0.211 0.212 0.125 0.092 0.223 0.107 0.071
0.264 0.365 0.383 0.336 0.194 0.223 0.132 0.195 0.173 0.098
0.191 0.193 0.395 0.232 0.133
0.636 0.805 1.003 0.779 0.539 0.348 0.224 0.599 0.402 0.169
4. Conclusions Catalytic cracking of RP-3 was examined in stainless-steel microchannels coating catalysts (PZTA) with a different molar ratios of ZrO2:TiO2. It is obvious that Cat3 coatings with the same molar ratio of ZrO2:TiO2 exhibit significantly optimal catalytic cracking activities than existing research in this field due to its larger amount of strong acid and surface area. The gas yield, the alkenes content within the gas phase product and the heat sink of Cat3 at 700 C are 40.7%, 51.5% and 3.41 MJ/kg respectively. Considering the gas yield of RP-3, the alkenes selectivity and deactivation of the catalyst under high temperature, PZTA with same mole ratio of ZrO2:TiO2 might be a potential catalyst for the cracking of RP-3. It should be noted that the increasing rate of gas yield and heat sink of Cat8 has a drawback compared with Cat3, and the thermal stability of catalysts could be further improved. Moreover, the inhibition of carbon deposition of cracking reaction should be figured out. Current studies are still under way in our lab to further optimize the PZTA catalyst by the way of improving the content of Al2O3 and adding additives, so that the catalyst would be anticipated
0.181 0.122
remarkable thermal stability and carbon inhibition performance in the future. Acknowledgements The research was performed in the College of Chemistry, Sichuan University (91016002, J1103315) and Xi'an Modern Chemistry Research Institute. The research was supported by the National Natural Science Foundation of China (91016002, J1103315). References [1] T. Edwards, Liquid fuels and propellants for aerospace propulsion: 1903e2003, J. Propul. Power 19 (2003) 1089e1107. [2] I.W. Kay, W.T. Peschke, Hydrocarbon-fueled scramjet combustor investigation, J. Propul. Power 18 (1992) 507e512. [3] A. Niaei, J. Towfighi, S.M. Sadrameli, R. Karimzadeh, The combined simulation of heat transfer and pyrolysis reactions in industrial cracking furnaces, Appl. Therm. Eng. 24 (2004) 2251e2265. [4] W.X. Zhou, B. Yu, J. Qin, D. Yu, Mechanism and influencing factors analysis of flowing instability of supercritical endothermic hydrocarbon fuel within a small-scale channel, Appl. Therm. Eng. 71 (2014) 34e42.
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