Article pubs.acs.org/IECR
Oleylamine-Protected Metal (Pt, Pd) Nanoparticles for Pseudohomogeneous Catalytic Cracking of JP-10 Jet Fuel Xiu-tian-feng E,† Yu Zhang,† Ji-Jun Zou,*,†,‡ Li Wang,†,‡ and Xiangwen Zhang†,‡ †
Key Laboratory for Green Chemical Technology of the Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China ‡ Collaborative Innovative Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China ABSTRACT: Catalytic cracking of hydrocarbon fuels is an effective way to cool aircraft materials under hypersonic flight. Pseudohomogeneous catalysis is an alternative to overcome the problems of traditional catalyst coatings. Herein, we employed the Brust−Schiffrin method to synthesize Pt and Pd nanoparticles (NPs) using oleylamine as the protecting ligand. The particle size can be controlled by tuning the ratio of protecting ligand, and uniform NPs can be obtained at an oleylamine/NP molar ratio of 2, with Pt and Pd NPs of 1−3 and 2−5 nm, respectively. IR and TG characterizations confirmed that the amine group of oleylamine is chelated on the metal surface whereas the hydrophobic carbon chain is exposed in the hydrocarbon fuel. As a result, the NPs are highly dispersible in jet fuel JP-10 without any precipitation after standing 12 months, providing the possibility of pseudohomogeneous catalysis. Suspensions containing Pt and Pd NPs (50 ppm) exhibited markedly enhanced cracking performance, with cracking conversions, gas yields, and heat sinks at 680 °C that were, respectively, 4.5, 4.4, and 1.3 and 3.1, 3.6, and 1.2 times of pure JP-10. In particular, Pt NPs can reduce the onset temperature of the cracking reaction from 650 to 600 °C. This work demonstrates the potential of fuel-dispersible NPs in hypersonic applications. supercritical cracking9−11 and further impregnated Pd on the membrane to increase the heat sink.12 However, such coating catalysts still have some limitations in practical applications, because the cracking conversion, and thus the heat sink, is not high enough at low temperature. Moreover, the coating suffers from rapid deactivation, easy scratching, and high heat resistance. An alternative to overcome the drawbacks of catalyst coatings is to mix nanosized catalysts with fuel to form stable suspensions, because suspended nanoparticles (NPs) can provide large specific surface areas and abundant active sites. Wickham et al. synthesized various metal oxides that can be dispersed in fuels and directly injected into the combustor.13 Van Devener and Anderson added CeO2 and Fe2O3 nanoparticles to JP-10 jet fuel as combustion catalysts.14 Shimizu et al. and Van Devener et al. used in situ generated Pd and Pd/ PdO NPs to catalyze methane combustion.15,16 To realize this technology, a key point is to ensure that the NPs are welldispersed in the liquid fuel, generally by modifying the surface using an amphoteric agent.14,17 For example, Bao et al. synthesized trichlorosilane-modified HZSM-5 nanocrystals and realized pseudohomogeneous catalytic cracking of hydrocarbon fuels.18,19 Li et al. fabricated gemini-surfactant-protected gold nanofluids for the cracking of fuels.20 Guo et al. prepared resorcinarene-encapsulated Ni−B for the cracking of JP-10.21 However, these catalytic cracking techniques were conducted in batch reactors, and no results in a simulated cooling system (that is, in a tubular flow reactor) have been reported.
1. INTRODUCTION Fuel technology and material development remain two of the main challenges to the realization of hypersonic flight.1−4 At hypersonic speeds, or more than 5 times faster than sound, aerodynamic heat will raise the heat load beyond the range that the structural materials of vehicles could bear. In this case, hydrocarbon fuels are required not only to provide propulsion power through combustion, but also to play a crucial role as the coolant in an advanced cooling system. The cooling capacity of fuel, also called the heat sink, can be divided into two categories: physical sensible heat due to heating of the fuel from room temperature to high temperature and chemical heat due to the endothermic cracking of hydrocarbons under supercritical conditions. The endothermic cracking of fuel is particularly important because physical heat is insufficient to cool the aircraft. Additionally, the small molecular products derived from the cracking of hydrocarbon fuels, such as hydrogen and light hydrocarbons, can benefit ignition and combustion under hypersonic conditions. Catalytic cracking and/or dehydrogenation is an important pathway to achieve a high heat sink, because catalysis can accelerate the cracking reaction significantly. For example, Cooper and Shepherd examined the catalytic cracking of JP-10 in a benchtop reactor.5 Up to now, many technologies for loading catalysts in the microchannels of heat exchangers have been studied, and an extensive literature is focused on wallcoated catalysts. Huang et al. studied the catalytic cracking of JP-7 and JP-8+100 over wall-coated SAPO-34 and Y-zeolite.3,6 Fan et al. used an HZSM-5-coated tubular reactor for the cracking of China No. 3 aviation kerosene.7 Wu and Li coated HZSM-5 crystals on a Ni-based tubular reactor as a structured catalyst for endothermic cracking.8 Our group prepared a hierarchical ZSM-5 membrane on a tubular reactor for © 2014 American Chemical Society
Received: Revised: Accepted: Published: 12312
June 9, 2014 July 13, 2014 July 15, 2014 July 15, 2014 dx.doi.org/10.1021/ie502311x | Ind. Eng. Chem. Res. 2014, 53, 12312−12318
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Figure 1. XRD patterns of oleylamine-protected (a) Pt NPs and (b) Pd NPs.
evaporation and dried at room temperature for 10 h. The dosages of oleylamine tested were 160, 320, 480, and 640 μL, denoted as x = 1−4, respectively. Unless specified otherwise, the NPs were prepared using x = 2 (320 μL). 2.3. Characterizations of Pt and Pd NPs. Transmission electron microscopy (TEM) images were obtained at room temperature on a Tecnai G2 F20 field-emission transmitting electron microscope (Philips) operating at 30 kV. X-ray diffraction (XRD) data were collected on a Rigaku D-max 2500 V/PC X-ray diffractometer (Rigaku Corporation) using a Cu Kα radiation source (40 kV, 200 mA). Infrared spectra were recorded on a Vertex 70 Fourier transform infrared (FT-IR) spectrometer. Thermogravimetric analysis (TGA) was conducted on a TGA-50 instrument under a N2 atmosphere. The particle size distributions of NPs in JP-10 were recorded using a Zetasizer Nano ZS instrument. 2.4. Catalytic Cracking of JP-10. An electrically heated tube was utilized to simulate the heat-exchanger microchannel in which hydrocarbon fuels flow and crack into small molecules. The details of the experiments and analysis were reported in previous works.18,19 The electrically heated tube reactor (3.0mm o.d. and 2.0-mm i.d.) was made of nickel-based super alloy GH3128. A direct-current stabilized power supply was used to provide direct-current voltage with two copper bars outside the tube. A suspension of JP-10 with 50 ppm of Pt or Pd NPs was fed at a flow rate of 1 g/s, and the pressure was kept at 4.0 MPa by setting a back-pressure valve behind the reactor. The residence time of the fuel in the reactor was ca. 0.7 s. The cracked effluent was separated into gas and liquid in a separator. The outlet temperature of the fuel was measured with K-type thermocouples inserted into a union cross-junction and set as the reaction temperature. The heat sink was calculated by using the real-time electric power consumed to subtract the heat loss. The electricity and temperature signals were collected each second. The setup was calibrated using hexane, and the heat sink was obtained with a reproducibility of greater than 98.5%. For each type of fuel, a temperature-programmed test was conducted in the same tube. At each temperature point, the reaction lasted at least 5 min to ensure sufficient weights of gas and liquid samples for analysis, and then the temperature was increased to another point. The temperature-programmed process took 50 min. The reaction temperature and pressure were higher than the critical point of JP-10 (425 °C and 3.7 MPa),30 so the fuel was cracked under supercritical conditions. The volume of gas product was measured using the drainage method. The gaseous products were analyzed online with a
Actually, noble metals such as Pt and Pd are very active catalysts for the cracking and dehydrogenation of hydrocarbons, and thus, they can increase the heat sink of hydrocarbon fuels.12 Therefore, it is expected that monodisperse Pt and Pd nanoparticles should be ideal catalysts for the catalytic cracking of fuels. The Brust−Schiffrin (B−S) method for preparing ligand-protected gold NPs opened a new paradigm for the facile fabrication of metal NPs22 that can also be used to synthesize monodisperse Pt and Pd NPs, as in the present case. In the B−S method, dodecanethiol is typically used as the protecting ligand, but it is not suitable in the present work because the content of sulfur in fuels is strictly restricted. In this work, we employed the B−S method to synthesize Pt and Pd NPs using oleylamine, not dodecanethiol, as the protecting ligand. These NPs were nanosized and could be stably dispersed in hydrocarbon fuel for quite a long time. The pseudohomogeneous catalytic cracking of JP-10, a widely used high-density aerospace fuel,23−28 was conducted in a simulated heat exchanger (tubular flow reactor) to demonstrate the potential of NPs. It was found that the NPs, especially Pt NPs, can significantly promote the cracking conversion and heat sink of the fuel.
2. EXPERIMENTAL SECTION 2.1. Materials. H2PtCl6·6H2O, NaBH4, and oleylamine were reagent-grade and were purchased from Tianjin Guangfu Fine Chemical Research Institute. PdCl2 was purchased from Alfa Aesar. Tetra-n-octylammonium bromide (TOAB) was purchased from J&K Chemical. Toluene was reagent-grade and was purchased from Tianjin Jiangtian Chemical Technology Company. Deionized water was used in all experiments. All reagents were used as received. 2.2. Preparation of Pt and Pd NPs. Pt and Pd NPs were synthesized at room temperature by a typical B−S procedure.22,29 For each synthesis, 0.795 g of TOAB dissolved in 250 mL of toluene was added to 1.0 g of H2PtCl6·6H2O dissolved in 200 mL of H2O or 0.086 g of PdCl2 dissolved in 50 mL of H2O under stirring for 1.5 h. Then, the mixture was separated into two phases, and the bottom water layer was removed. Protecting ligand (oleylamine) was added to the organic phase, and stirring was applied for 2 h. Subsequently, NaBH4 (0.183 g) dissolved in 20 mL of H2O was added within 10 min under stirring. After another 10 min of stirring, the particles were recovered by phase separation and rotatory 12313
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Figure 2. TEM images of (a−d) Pt NPs and (e−h) Pd NPs synthesized using different amounts of oleylamine.
3000A micro GC gas analyzer (Agilent Technologies, Palo Alto, CA), which has three thermal conductivity detectors (TCDs) and three multichannel analytical columns, namely, molecular sieve (10 m × 12 μm), Plot U (10 m × 30 μm), and alumina (10 m × 8 μm). The liquid products were analyzed on an Agilent 7890A gas chromatograph using a flame ionization detector (FID) and a PONA column (50 m × 0.20 mm × 0.50 μm). On this basis, the cracking conversion of JP-10 was calculated by considering both the gas and liquid compositions. After the test, the tube was cut into 4-cm-length segments, and the carbon deposited on the tube wall was analyzed by temperature-programmed oxidation. The formed CO2 was detected using a carbon dioxide IR analyzer with a repeatable error of less than 1.0%, and the amount of deposit is reported in mg/cm2.
3. RESULTS AND DISCUSSION 3.1. Structure of Pt and Pd NPs. Figure 1 shows XRD patterns of Pt and Pd NPs. For Pt NPs, the diffraction peaks at 39.7°, 46.2°, 67.4°, and 81.2° were indexed to the {111}, {200}, {220}, and {311} planes, respectively, of metallic Pt (JCPDS no. 04-0802),31 confirming that H2PtCl6 was reduced to Pt particles. Similarly, for Pd NPs, the diffraction peaks at 40.1°, 46.6°, 68.1°, 82.1°, and 86.6° were indexed to the {111}, {200}, {220}, {311}, and {222} planes, respectively, of metallic Pd (JCPDS no. 46-1043),32,33 again suggesting the formation of Pd particles. According to the Scherrer equation, the average grain sizes of the Pt and Pd NPs were 3.2 and 4.5 nm, respectively. Figure 2 shows TEM images of the Pt and Pd NPs obtained using different amounts of oleylamine. For Pt NPs, the particle size first decreased and then increased when the amount of ligand rose. At x = 2, the NPs had the smallest and most
Figure 3. IR spectra of (a) oleylamine, (b) Pt NPs, and (c) Pd NPs.
uniform particle size, predominantly 1−3 nm. For x > 2, some irregular-shaped NPs, such as triangles, truncated triangles, and quadrilaterals, appeared. Previously, we observed the shape evolution of Au NPs in B−S synthesis. When the protecting agents and synthesizing time are tuned, the shape and particle size of the NPs will change under a kinetic-to-thermodynamic control.29 It is possible to control the facets and shapes of NPs when the synthetic conditions are finely tuned. However, such particles have large sizes with insufficient active sites and unsatisfactory suspension stabilities in fuels. Therefore, the particle size is critical for the present fuel application. The same tendency was observed for Pd NPs, but the effect of ligand 12314
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Figure 6. (a) Gaseous product distributions and (b) chromatogram spectra of liquid products obtained at 680 °C. Figure 4. TG patterns of (a) Pt NPs and (b) Pd NPs synthesized using different amounts of oleylamine. Insets: Actual amounts of protecting agents.
flexural vibration in the range of 1580−1650 cm−1, as well as a C−H (C ≥ 7) flexural vibration in the range of 720 cm−1. It is noted that the Pt and Pd NPs also showed an obvious C−H flexural vibration, indicating that oleylamine was chelated on the surface of the metal particles. However, the N−H peaks of the Pt and Pd NPs were very weak, suggesting that it is the amine groups of oleylamine that interact with the metal surface. Actually, oleylamine is also frequently used as a protecting ligand, and the interaction between the N atom and metal atoms has been reported.36
amount was smaller compared with the case of Pt NPs. In addition, the Pd NPs were larger than the Pt NPs and had a relatively uniform size of 2−5 nm at x = 2. It is well-known that, in a typical B−S synthesis, the protecting agent (dodecanethiol) is chelated on the NP surface through a metal−S bond.34,35 Figure 3 shows the IR spectra of oleylamine-protected NPs. Pure oleylamine showed a N−H stretching vibration in the range of 3250−3330 cm−1 and a
Figure 5. Particle size distributions and photographs of NP (800 ppm)/JP-10 suspensions after standing for 12 months. 12315
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Figure 7. Cracking performances of NP (50 ppm)/JP-10 suspensions: (a) cracking conversion, (b) gas yield, (c) heat sink, and (d) coke deposition.
tetrahydrodicyclopentadiene (exo-THDCPD, C10H16). As shown in Figure 6, JP-10 cracking produces many small molecules in the gas and liquid phases. The major gaseous products were H2, CH4, C2H4, C2H6, C3H8, C3H6, 1-C4H8, trans-C4H8, iso-C4H8, and cis-C4H8, whereas the major liquid products were C5−C7 alkanes, alkenes, and cycloalkanes. The product distribution is very similar to those reported in the literature,37−40 and catalytic cracking does not affect the product selectivity. Figure 7 shows the cracking behavior of JP-10 with increasing temperature. At low temperature, the cracking was negligible, and the heat sink was completely determined by the physical sensible heat, so the heat sink increased linearly with temperature. For pure JP-10, thermal cracking initiated at approximately 650 °C, after which the conversion and gas yield increased successively. Compared with the base line of physical sensible heat, the heat sink rose greatly because of the occurrence of endothermic reactions. At 680 °C, the cracking conversion, gas yield, and heat sink were 12.1%, 3.2%, and 2086 kJ/g, respectively. The suspension containing Pd NPs also began to crack at 650 °C. At this temperature, the protective ligands have been removed from the surface of the NPs through thermal decomposition, and the exposed NPs serve as a catalyst for the cracking of JP-10. Therefore, the reaction rate was obviously higher than that of pure JP-10, and the cracking conversion, gas yield, and heat sink were 37.6%, 11.5%, and 2488 kJ/g at 680 °C, respectively. The influence of oleyamine on the reaction can be excluded because it had decomposed completely under the reaction conditions. Importantly, in the presence of Pt NPs, JP-10 began to crack at 600 °C, so Pt NPs can significantly reduce the onset temperature of cracking reaction. TGA already showed that the protective ligands on Pt NPs are easier to decompose than those on Pd NPs, so it is reasonable that Pt NPs can exhibit catalytic activity at lower
TGA was conducted to determine the actual amount of protection agents chelated on the Pd and Pt NPs. From Figure 4, it can be seen that the mass loss of the Pt NPs occurred from 200 to 350 °C, attributed to the decomposition of surface oleylamine. However, the mass loss of Pd NPs was completed at about 500 °C, which suggests that the metal−ligand interaction is stronger for the Pd NPs than for the Pt NPs. The weight increased at higher temperature was due to the oxidation of the metal surface after removal of the protective layer. According to the amount of weight loss, we calculated the actual amount of protective agent on the NP surface. For both Pt and Pd NPs, for x > 2, the actual amount of ligand on the surface did not increase significantly, possibly because the NP surface was already fully covered by ligand molecules. Considering that the molar ratio of x = 2 also gave uniform and small NPs, in the following sections, we synthesized NPs with this composition for further fuel applications. 3.2. Stability of NP Suspension. The dispersion behaviors of Pt and Pd NPs in JP-10 jet fuel were investigated. NPs at a concentration of 800 ppm were dispersed in JP-10 under ultrasonication to form a very stable suspension. First, highspeed centrifugation was used to test the stability of the suspension. After centrifugation at 8000 rpm for 30 min, no precipitation was observed on the bottom of the liquid. Figure 5 shows that, after standing for 12 months, the suspensions still exhibited no precipitation. Moreover, according to zeta analysis, Pt (2 nm) and Pd (4 nm) NPs dispersed in JP-10 are agree with the TEM result in Figure 2, which confirming that the NPs exist in form of single particles without any aggregations. As mentioned above, the amine group of oleylamine is chelated on metal surface, and the long hydrophobic carbon chain exposed in hydrocarbon fuel can anchor the particles in the fuel very stably. 3.3. Pseudohomogeneous Catalytic Cracking of JP10. JP-10 is a high-purity fuel consisting of 98.5% exo12316
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ACKNOWLEDGMENTS The authors appreciate the support from the National Natural Science Foundation of China (21222607) and the Program for New Century Excellent Talents in Universities (NCET-090594).
temperature. Moreover, in the temperature range studied, Pt NPs were found to be more active than Pd NPs, and the cracking conversion, gas yield, and heat sink were 55.2%, 14.2%, and 2705 kJ/g at 680 °C, respectively. Because the catalyst concentration in the present work was at least 1 order of magnitude less than those of reported pseudohomogeneous catalysts,14,17−21 the particle size that determines the concentration of active sites is critical for the catalytic performance. Therefore, the high activity of the Pt NPs can be attributed to the very small particle size. It should be noted that the NPs were no longer protected by the ligand under reaction conditions. Nevertheless, the NPs still mixed well in fuel carrier and flowed along the tube. Although the deposition of NPs on the tube wall during the reaction seems inevitable,14 most of the NPs should be suspended in the fuel because the residence time is very short. In addition, a previous study on Pd/HZSM-5 coating showed that, when the atomic Pd concentration reaches 0.9%, the heat sink is increased by only ca. 10% compared with that of the HZSM5 coating.12 Therefore, the significant increase in heat sink in the present case cannot be attributed to trace NPs deposited on the wall. During cracking, carbon deposition is inevitable, and the higher the cracking conversion, the greater the carbon deposition. Figure 7d shows the carbon deposited along with the tubular reactor. It can be seen that most of the carbon was deposited in the latter part and that the amount deposited was in the order Pt-catalytic > Pd-catalytic > thermal cracking, in agreement with the cracking activity. Heavy carbon deposition can increase the flow resistance or even block the tube. In the present test (50 min), however, the system worked well without blockage of the fluid flow passage.
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REFERENCES
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4. CONCLUSIONS Pd and Pt NPs protected by oleylamine have been successfully prepared using a modified Brust−Schiffrin method. The particle size was the most uniform when the oleylamine/NP molar ratio was 2, with Pt and Pd NPs of 1−3 and 2−5 nm, respectively. These NPs can be stably dispersed in jet fuel JP-10 for at least 12 months without any precipitation or aggregation. In the pseudohomogeneous cracking of JP-10 in a tubular reactor, the presence of NPs can significantly increase the cracking conversion, gas yield, and heat sink, compared with the thermal cracking of pure JP-10, which can be attributed to the occurrence of catalytic cracking. In particular, Pt NPs can reduce the onset temperature of the cracking reaction from 650 to 600 °C and exhibit the best cracking behavior, with a cracking conversion, gas yield, and heat sink of 55.2%, 14.2%, and 2705 kJ/g at 680 °C, respectively. At this temperature, the heat sink of the Pt-containing suspension is 1.3 times higher than that of pure JP-10. Consequently, this work demonstrates the potential of fuel-dispersible Pt NPs in hypersonic applications with acceptable added costs (ca. $3 per kilogram of fuel).
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