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Improvement of the heats of reaction in endothermic reactions of methylcyclohexane with zeolites
Catalysis Today 185 (2012) 47–53
Contents lists available at SciVerse ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Improvement of the heats of reaction in endothermic reactions of methylcyclohexane with zeolites Joongyeon Kim a , Sun Hee Park a , Byung-Hee Chun a , Byung Hun Jeong b , Jeong Sik Han b , Sung Hyun Kim a,∗ a b
Department of Chemical and Biological Engineering, Korea University, 1 Anam-Dong, Sungbuk-Ku, Seoul 136-701, Republic of Korea Agency for Defense Development, Jochiwongil 462, Yuseong, Daejon, Republic of Korea
a r t i c l e
i n f o
Article history: Received 31 May 2011 Received in revised form 19 August 2011 Accepted 16 September 2011 Available online 10 October 2011 Keywords: Heat of reaction Methylcyclohexane Zeolites Low-molecular-weight hydrocarbons Olefins
a b s t r a c t Endothermic reactions of methylcyclohexane (MCH) with zeolites were investigated in a batch reactor to increase the heats of reaction. Conversion and product distribution affected the heat of reaction. The heat of reaction with HZSM-5, which led to the formation of low-molecular-weight hydrocarbon products, was larger than the heats of reaction with other zeolites. To improve the heat of reaction with HZSM-5, Pt/HZSM-5 (1 wt.% Pt) was used in endothermic reactions. In endothermic reactions with Pt/HZSM-5, the heats of reaction increased, and there was a high yield of olefins, which were formed by dehydrogenation of paraffin on metal sites. A high conversion and high yield of low-molecular-weight hydrocarbons and olefins led to an increase in the heat of reaction. © 2011 Published by Elsevier B.V.
1. Introduction Aircraft operation at hypersonic flight speeds causes severe heat loads generated by engines or combustors with heat loads increasing with flight speeds. Because heat loads lead to structural changes in engines and aircraft failures, cooling technologies are necessary [1–3]. One cooling technology is the use of endothermic fuels. Endothermic fuels are liquid hydrocarbon fuels that undergo endothermic reactions at high temperatures and pressures. Endothermic fuels can absorb heat loads as sensible heat and heat of reaction before injection to the combustor [4–7]. Endothermic fuel cooling technology is advantageous because fuels that undergo endothermic reactions have better combustion properties [8]. To improve heat absorption by endothermic fuels, it is necessary to investigate endothermic reactions. There have been only a limited number of previous studies focused on increasing the heat of reaction for endothermic fuels. Sobel et al. [9] examined the endothermic reaction of Norpar 12 with SAPO-5 zeolite. With SAPO-5, the heat of reaction increased with an increase in the yield of low-molecular-weight and unsaturated hydrocarbons. Huang et al. [10] examined the endothermic reaction of n-octane, JP-7 and JP-8 + 100 with
∗ Corresponding author. Tel.: +82 2 3290 3297; fax: +82 2 926 6102. E-mail address: [email protected] (S.H. Kim). 0920-5861/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.cattod.2011.09.020
P-octacat (commercial zeolite) and investigated the relationship between the heat of reaction and fuel composition. Hai-long et al. [11] selected n-dodoecane as a model endothermic fuel and conducted endothermic reactions with Pd/HZSM-5. Endothermic reactions with Pd/HZSM-5 showed an increase in cracking products and heats of reaction. Sicard et al. [12] examined the endothermic reaction of n-dodecane with Y and ZSM-5. The heat of reaction and reaction mechanism were investigated through the analysis of cracking products. Daniau et al. [13] performed endothermic reactions with Norpar-12. The effects of temperature and fuel flow rate on the heat of reaction were investigated. Kerosene fuels (hydrocarbon mixtures) are widely used as endothermic fuels for aircraft systems. It is difficult to investigate the endothermic reactions of these fuels, due to their complicated compositions. For this reason, methylcyclohexane (MCH) was selected as a model endothermic fuel in this study because it is contained in aircraft fuels, such as kerosene fuels and JP-9 [14,15]. There have been many reports on the catalytic reactions with MCH [16–18]. However, there have been few investigations of heats of reaction with MCH. The aim of this study was to increase the heats of reaction of endothermic reactions with MCH. HZSM-5, HY, HBeta, and Pt/HZSM-5 were used. Heat of reaction was related to conversion and product distribution, which changed with different zeolite properties, such as pore structure, acidity and loaded metal.
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Fig. 1. Ammonia TPD for the zeolites.
2. Experimental 2.1. Catalyst preparation Three different types of zeolites with medium (HZSM-5) and large (HY, HBeta) pore sizes were investigated. Raw materials were obtained from the Zeolyst Company. HZSM-5 and HBeta were prepared by the calcination of NH4 + -form samples (CBV 8014 and CP814E) at 550 ◦ C for 3 h. HY was prepared by drying H+ -form samples (CBV780) at 120 ◦ C for 12 h. Pt/HZSM-5 (1 wt.% Pt) catalyst was prepared by incipient wetness impregnation of the HZSM-5 zeolite with tetraammineplatinum(II) nitrate (>99.9%, Sigma–Aldrich). After impregnation, the catalyst was dried at 120 ◦ C for 12 h and was calcined at 550 ◦ C for 3 h. 2.2. Catalyst characterization 2.2.1. Pore structures of the catalysts Surface areas, pore volumes and pore size distributions were determined from nitrogen adsorption–desorption isotherms carried out in a BELSORP-max (MP). Physical properties of the three zeolites are shown in Table 1. Mesopore volumes and diameters were obtained using the Barrett–Joyner–Halenda (BJH) method, and micropore volumes and diameters were obtained using the Horvath–Kawazoe (HK) method.
Fig. 2. Ammonia-FTIR spectra of zeolites: (a) HY, (b) Pt/HZSM-5, (c) HZSM-5, (d) HBeta.
The total acid densities of HZSM-5, HY, HBeta and Pt/HZSM5 were 0.075, 0.039, 0.146 and 0.067 mmol NH3 /g, respectively. HBeta exhibited a large peak at low temperature (182 ◦ C) and a shoulder shaped curve at 284 ◦ C. HZSM-5 and Pt/HZSM-5 exhibited one peak at low temperature (179 ◦ C) and another peak at moderate temperature (370 ◦ C). HY exhibited two small peaks below 400 ◦ C and one large peak at high temperature (634 ◦ C). When Pt was loaded onto HZSM-5, both the weak acid site peak at 179 ◦ C and the moderate acid site peak at 370 ◦ C decreased. Although total acid density was low in HY, there were strong acid sites (at 634 ◦ C). 2.2.3. Ammonia-FTIR of catalysts The properties of the acid sites were characterized using a MIDAC model 2000 Ammonia-FTIR spectrometer. IR spectra were obtained using an IR cell with CaF2 windows. The sample zeolites were pressed into a self-supporting disc and placed in the cell. Ammonia (50 Torr) was introduced into the cell at 30 ◦ C. The FTIR spectra of ammonia absorbed on HZSM-5, HY, HBeta, and Pt/HZSM5 are shown in Fig. 2. Peaks at 1470 cm−1 and 1690 cm−1 are attributed to ammonium ions formed by interaction with Bronsted acid sites. Peaks at 1620 cm−1 are assigned to ammonia coordinated on Lewis Acid sites [19]. Bronsted acid sites (at 1470 cm−1 ) were predominant in all catalysts. 2.3. Catalytic reaction
2.2.2. Temperature programmed desorption (TPD) of adsorbed ammonia The acidity of catalysts was determined by TPD of adsorbed ammonia using a BEL-CAT-B (Fig. 1). BEL-CAT-B is a commercial catalyst analyzer (BEL JAPAN Inc.) used for NH3 temperature programmed desorption (NH3 -TPD). Detailed information about the acid sites of catalysts can be determined through NH3 -TPD. NH3 TPD was performed with a BEL-CAT-B equipped with a thermal conductivity detector (TCD). A 0.3 g sample was loaded into a quartz tube and pretreated with He gas of 50 ml/min at 500 ◦ C for 1 h. Adsorption of NH3 was performed at 100 ◦ C in an NH3 –He (5% NH3 ) mixture for 0.5 h, and the physically adsorbed NH3 was purged with He gas. NH3 -TPD was performed in the He flow by raising the temperature to 700 ◦ C at a rate of 10 ◦ C/min, and the desorbed NH3 was detected with an online thermal conductivity detector. The total amount of desorbed NH3 (total acid density) and the desorption temperature (strength of acid site) were determined through NH3 -TPD.
Endothermic reactions were performed in a stainless steel batch reactor at 325 ◦ C and 50 bar for 9 h. Metal may affect the decomposition of hydrocarbons [20]. To understand better the endothermic reactions of MCH, it is necessary to prevent the MCH from contacting the metal. Thus, the inside of the reactor was covered with quartz to prevent any effects caused by the metal. For each test, 60 ml of MCH was placed in the reactor with 0.2 g of zeolite. Before the reaction, the inside of the reactor was purged with N2 gas for 10 min to prevent the oxidation of MCH by air, especially oxygen. After purging, the reactor was pressurized to an initial pressure of 15 bar with N2 . Next, an electrical heating jacket was used to heat the MCH to the reaction temperature (325 ◦ C) with a heating rate of 10 ◦ C/min. When the temperature of MCH reached 325 ◦ C for each test, the pressure of the reactor reached 50 bar. Thus, endothermic reactions of MCH (Tc : 298.95 ◦ C, Pc : 34.8 bar) with zeolites were conducted at 325 ◦ C and 50 bar, which are conditions similar to the hypersonic flight conditions of endothermic fuels.
J. Kim et al. / Catalysis Today 185 (2012) 47–53
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Scheme 1. Schematic of the batch reactor and process flow diagram.
During each reaction (9 h), 0.5 ml of the product in the reactor was sampled seven times at intervals of 1.5 h. The 0.5 ml samples were small enough not to affect the concentration and pressure inside the reactor. Sampled products were sent to a separator that was directly connected to the reactor. Because the temperature of the sampled products was notably high, it was impossible to analyze their composition. In the separator, the hot samples were cooled to 25 ◦ C and separated into gas and liquid phases. Gaseous products were sent directly to a gas chromatograph (GC) connected to a separator. After analysis of the gaseous products, liquid products remaining in the separator were collected and analyzed by GC/MSD. Schematic of the batch reactor and process flow diagram is provided in Scheme 1. Products of the endothermic reactions were identified by GC. The composition of the gas phase products was analyzed by GC/TCD (thermal conductivity detector) and GC/FID (flame ionization detector). Hydrogen in gaseous products was analyzed using a GC/TCD that was equipped with a stainless steel column packed with a molecular sieve 5A. The oven temperature for GC/TCD was increased from 40 ◦ C to 90 ◦ C at a heating rate of 10 ◦ C/min. Hydrocarbons in gaseous products from C1 to C4 were analyzed using a GC/FID equipped with a capillary column. The oven temperature for GC/FID was increased from 40 ◦ C to 100 ◦ C at a heating rate of 10 ◦ C/min. The compositions of liquid phase products were analyzed by GC/MSD (Agilent 7890A Series GC Custom, 5975C inert MSD Standard Turbo EI) using an HP-5ms column. The oven temperature was increased from 35 ◦ C to 290 ◦ C at a heating rate of 10 ◦ C/min. 3. Results and discussion Heat absorption from heat of reaction is dependent on conversion and product distribution. To increase heats of reaction, conversion and product distribution in endothermic reactions with zeolites were investigated.
Fig. 3. Conversion of MCH during endothermic reaction with HBeta, HZSM-5 and HY at 325 ◦ C and 50 bar.
3.1. Effects of acidity on conversion Endothermic reactions of MCH were performed at 325 ◦ C and 50 bar. Conversions after 9 h were 52.3, 34.6 and 34.0% for HBeta, HZSM-5 and HY, respectively (Fig. 3). Endothermic reactions of hydrocarbons with zeolites were initiated on the acid sites [21]. Therefore, the differences in conversion between endothermic reactions were related to acidities of the zeolites. HBeta had the highest total acid density (0.146 mmol NH3 /g). It is supposed that the high total acid density of HBeta led to the high level of conversion. However, conversion was nearly the same in the endothermic reaction with HY and HZSM-5 despite a lower total acid density of HY (HY: 0.039 mmol NH3 /g, HZSM-5: 0.075 mmol NH3 /g). Although
Table 1 Physical properties of the zeolites.
HZSM-5 HY HBeta Pt/HZSM-5
BET surface area (m2 /g)
Mesopore volume (cm3 /g)
Mesopore diameter (nm)
Micropore volume (cm3 /g)
˚ Micropore diameter (A)
429 772 633 417
0.219 0.243 0.550 0.203
76.5 21.3 76.5 66.5
0.25 0.22 0.19 0.23
5.6 7.8 6.5 5.6
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HY had the smallest total acid density, it had an NH3 desorption peak at high temperature (634 ◦ C). The peak at high temperature indicates the existence of strong acid sites. The strengths of the acid sites as well as the total acid densities affected conversion. Heat absorption from heat of reaction increased with conversion. High total acid density and strong acid sites led to an increase in conversion. Zeolites need to have high total acid densities and strong acid sites to increase heats of reaction. 3.2. Effects of pore structure on product distribution The compositions of products of the endothermic reactions with MCH classified by the numbers of carbons are shown in Fig. 4. The compositions of products gradually increased without any decrease during the reactions for the three zeolites, but product distributions were different. For HBeta and HY, C7 hydrocarbons were the major products for endothermic reactions (32.7% for HBeta, 25.4% for HY). The C7 hydrocarbons were mostly composed of isomers of MCH; the fractions of isomers in C7 hydrocarbons were 75% for HBeta and 95% for HY. The isomers of MCH were 1,1-dimethylcyclopentane (1,1-DMCP), 1,2-dimethylcyclopentane (1,2-DMCP), 1,3-dimethylcyclopentane (1,3-DMCP) and ethylcyclopentane (ECP). The molecular structures of MCH and the four isomers are shown in Scheme 2. As shown in Fig. 4(a) and (c), the difference in product composition between C7 hydrocarbons and the others was significant. For HZSM-5, C3 hydrocarbons, mostly propane, were major products (8%) of the endothermic reaction. However, the difference in product percentages between major products (C3 hydrocarbons) and the other products was not significant. Products were classified as cycloalkanes/cycloalkenes, aromatic hydrocarbons or linear/branched hydrocarbons for each zeolite, and the relative abundances of each type of product (measured at the end of each 9 h reaction) are presented in Fig. 5. For HBeta and HY, cycloalkanes/cycloalkenes, which were mostly composed of MCH isomers, were the most abundant products. For HZSM-5, the abundances of aromatic hydrocarbons and linear/branched hydrocarbons were higher than those of cycloalkanes/cycloalkenes. The differences in composition were consequences of the different pore structures of the three zeolites. As described previously, endothermic reactions of hydrocarbons (MCH) with zeolites are initiated on the acid sites, and carbenium ions are formed [21]. These carbenium ions are primarily converted to MCH isomers (1,1-DMCP, 1,2-DMCP, 1,3-DMCP and ECP), and these isomers undergo beta-scission reactions leading to C7 olefinic hydrocarbons. The C7 olefinic hydrocarbons are able to undergo further endothermic reactions, such as dehydrogenation, cracking and aromatization in the pores. A significant number of endothermic reactions (including dehydrogenation, cracking and aromatization) occur; therefore, it is difficult to describe the mechanisms of all of these reactions. As a result, only generic reaction mechanism that explains endothermic reaction of MCH is provided with Scheme 3. The sizes of the zeolite micropores were 6.5, 7.8 and 5.6 A˚ for HBeta, HY and HZSM-5, respectively (Table 1). The sizes of the micropores of HBeta and HY were relatively higher than that of the micropores of HZSM-5. After they are formed, the MCH isomers (primary products of endothermic reactions) can diffuse out quickly, due to the large pore sizes. Because the rate of diffusion was faster than the rates of further reactions, the yield of isomers was large in HBeta and HY. In contrast, HZSM-5 had narrow pores, and once the MCH isomers were formed, the diffusion of the isomers out of the pores was restricted, and the isomers were able to stay in them for a longer time. Therefore, further reaction of the isomers occurred, and the percentage
Fig. 4. Composition of products classified by number of carbons during endothermic reaction of MCH with (a) HBeta, (b) HZSM-5 and (c) HY.
composition of C3 hydrocarbons (propane) was high. Because heat of reaction is dependent on product distribution, the zeolite pore structure is important in increasing heat absorption for aircraft cooling.
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Scheme 2. Methylcyclohexane (MCH, C7 H14 ) and the four isomers of MCH.
Scheme 3. Generic reaction mechanism for endothermic reaction of MCH.
3.3. Heat of reaction by catalytic decomposition Heat absorption by endothermic fuels comes from sensible heat and heat of reaction and was calculated by NIST SUPERTRAPP program. This program offers thermophysical properties of hydrocarbon mixtures, such as enthalpy and entropy, at a specific temperature and pressure. In this study, sensible heat was obtained from the difference between MCH enthalpy at the reaction temperature (325 ◦ C) and at room temperature (25 ◦ C). Sensible heat was the same for the endothermic reactions with the three
Fig. 5. Classification of endothermic reaction products of MCH with HBeta, HY and HZSM-5 sampled at the end of the reaction (10 h).
zeolites because they were carried out at the same reaction temperature (81.0 kJ/mol). The heat of reaction was obtained from the enthalpy difference between the product mixture and MCH. All of the species in the product mixture were identified by gas chromatography (GC) and were accounted for in the heat of reaction calculation except for species that were not identified by GC/MSD analyses. From every experiment, 95 total species were detected by GC, but 6 species were unidentified. The fraction of the unidentified species in the total products was below 1% for every experiment. It is assumed that the exclusion of unidentified species did not significantly affect heat of reaction calculations. The enthalpy of product mixture could be calculated after species name and their concentrations were entered in NIST SUPETRAPP program because it had each species thermophysical properties. Then, heat of reaction was obtained from enthalpy difference between product mixture and MCH. Heats of reaction were 22.7, 10.8 and 6.2 kJ/mol for HZSM-5, HBeta and HY, respectively. Due to high conversion to products, the heat of reaction for HBeta was larger than the heat of reaction for HY, even though they had similar product distributions. Although conversion of catalytic decomposition was lower in HZSM-5 than in HBeta, the heat of reaction was higher with HZSM-5 than with HBeta. The heat of reaction is the difference in enthalpy (or state of energy) between the reactants and products in a reaction. In an endothermic reaction, the products are unstable (high state of energy) compared to the reactants; therefore, the heat of reaction increases with the instability of the products. As molecular weights (or carbon numbers) of hydrocarbons decrease, they become unstable. Cracking leads to the production of low-molecular-weight hydrocarbons. It is supposed that the heat of reaction is larger for cracking than for isomerization. Because C3 hydrocarbons formed
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◦
Fig. 6. Conversion of MCH during endothermic reaction with Pt/HZSM-5 at 325 C and 50 bar.
by cracking were the major products in the HZSM-5 reaction, the heat of reaction was the largest with HZSM-5 among the three zeolites. Based on these results, the formation of low-molecularweight hydrocarbons leads to an increase in the heat of reaction. 3.4. Increase in the heat of reaction using Pt/HZSM-5 As the molecular weights (or carbon numbers) of products decreased, the heats of reaction of the endothermic reactions increased; HZSM-5 resulted in the largest heat of reaction. In addition, unsaturated hydrocarbons (olefins) are unstable compared to saturated hydrocarbons (paraffins) because the high electron densities of multiple bonds lead to instability. Based on this theory, the heat of reaction will increase when the formation of olefins increases. Because olefins are produced by the dehydrogenation of paraffin on metal, Pt/HZSM-5, which has acid sites and metal sites, was prepared by impregnation. HZSM-5 was selected, due to its capability to lead to high heats of reaction. The conversion of the endothermic reaction with Pt/HZSM-5 was lower than that of the reaction with HZSM-5 (Fig. 6, 28% for 9 h). As described previously, the conversion of the endothermic reaction was dependent on the total acid densities and the acid
Fig. 7. Composition of products during endothermic reaction of MCH with Pt/HZSM5 classified by number of carbons.
Fig. 8. Classification of endothermic reaction products of MCH with HZSM-5 and Pt/HZSM-5 sampled at the end of the reaction (10 h).
strengths of the zeolites. Pt/HZSM-5 had peaks at the same temperature as HZSM-5 (Fig. 2), but the areas of the peaks were smaller. Therefore, the low total acid density of Pt/HZSM-5 led to the low level of conversion. The composition of the products of the endothermic reaction with Pt/HZSM-5 classified by number of carbons is shown in Fig. 7. As with HZSM-5, C3 hydrocarbons were the major products (5.5%) followed by C7 (4.8%), C9 (3.3%) and C8 (3.3%) hydrocarbons. It is supposed that the pore structures of the zeolites affected product distributions as classified by number of carbons. However, there were difference in the types of products from reactions with Pt/HZSM-5 and HZSM-5. Products from the reactions, classified by hydrogen, olefin and paraffin, are compared in Fig. 8. The yields of products were measured at the ends of the reactions (9 h). Due to the dehydrogenation by metal sites of Pt, the yield of hydrogen (25.1%) and olefin (9.2%) was higher with Pt/HZSM-5. Olefins were mostly composed of propylene. It is supposed that propane, which was the major product of the HZSM-5 reactions, underwent dehydrogenation on the metal sites of Pt and was converted to propylene. Heat absorption by Pt/HZSM-5 was also calculated using the NIST SUPERTRAPP program (Fig. 9). An increase in heat of reaction was observed when Pt/HZSM-5 (31.7 kJ/mol) was used for the endothermic reaction. It is supposed that the formation of olefins
Fig. 9. Heat of reaction for endothermic reaction of MCH with HZSM-5, HBeta, HY and Pt/HZSM-5.
J. Kim et al. / Catalysis Today 185 (2012) 47–53
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(mostly composed of propylene) affected the heat of reaction. Based on these results, a high yield of low-molecular-weight hydrocarbons and olefins leads to an increase in the heat of reaction.
Acknowledgement
4. Conclusion
References
To improve heats of reaction for aircraft cooling, endothermic reactions of MCH with zeolites were investigated in this study. Heat of reaction was dependent on conversion and product distribution, which changed with different zeolites. Due to its high total acid density, HBeta led to a higher conversion and heat of reaction than HY; product distributions were similar. Although the conversions of endothermic reactions were lower with HZSM-5, the heat of reaction with HZSM-5 was higher than with other zeolites. As the molecular weights of hydrocarbons decrease, instability increases. Heat of reaction increases with the instability of products. HZSM-5 led to an increase in the formation of low-molecular-weight hydrocarbons. Therefore, HZSM-5 led to a higher heat of reaction than HBeta. Additionally, unsaturated hydrocarbons, which have multiple bonds, are unstable compared to saturated hydrocarbons, which have single bonds, because the higher electron density of multiple bonds leads to instability. Therefore, the heat of reaction increases with the yield of olefins. To increase the yield of olefins through the dehydrogenation of paraffin, Pt/HZSM-5 was prepared. An increase in the heat of reaction with Pt/HZSM-5 was observed for the endothermic reaction. Based on these results, a high conversion and high yield of low-molecular-weight hydrocarbons and olefins led to an increase in the heat of reaction.
[1] N. Cascoin, P. Gillard, S. Bernard, G. Abraham, M. Bouchez, E. Daniau, Y. Toure, AIAA-2006-8005, 2006. [2] N. Cascoin, P. Gillard, E. Dufour, Y. Toure, J. Thermophys. Heat Transfer 21 (2007) 86–94. [3] J.A. Gasner, R.C. Foster, C. Fujimura, AIAA-92-3721, 1992. [4] D.T. Wickham, J.R. Engel, S. Rooney, B.D. Hitch, J. Propul. Power 24 (2008) 55–63. [5] D. Petley, S. Jones, W. Dziedzic, AIAA-91-5063, 1991. [6] E. Daniau, M. Bouchez, AIAA-2003-6920, 2003. [7] G. Yong-sheng, L. Rui-sen, J. Zhejiang, Univ. Sci. 6A (2005) 632–635. [8] D. Petley, S. Jones, AIAA-90-3284, 1990. [9] D.R. Sobel, L.J. Spadaccini, J. Eng. Gas Turbine Power 119 (1997) 344–351. [10] H. Huang, L.J. Spadaccini, D.R. Sobel, J. Eng. Gas Turbine Power 126 (2004) 284–293. [11] Z. Hai-long, M. Fan-xu, G. Wei, Z. Ji-jun, Z. Xiang-wen, J. Fuel Chem. Technol. 36 (2008) 462–467. [12] M. Sicard, M. Grill, B. Raepsaet, F. Ser, AIAA-2008-2622, 2008. [13] E. Daniau, M. Sicard, AIAA-2005-3404, 2005. [14] G.Y. Gerasimow, S.A. Losev, J. Eng. Phys. Thermophys. 78 (2005) 1059–1070. [15] A. Osmont, I. Gokalp, L. Catoire., Prop. Exp. Pyrotech. 31 (2006) 343–354. [16] S. Yolcular, O. Olgun, Catal. Today 138 (2008) 198–202. [17] P. Castano, J.M. Arandes, M. Olazar, J. Bilbao, B. Pawelec, U. Sedran, Catal. Today 150 (2010) 363–367. [18] M.P. Lazaro, E.G. Bordeje, D. Sebastian, M.J. Lazaro, R. Moliner, Catal. Today 138 (2008) 203–209. [19] N. Khandan, M. Kazemeini, M. Aghaziarati, Appl. Catal. A: Gen. 349 (2008) 6–12. [20] T. Edwards, Combust. Sci. Technol. 178 (2006) 307–334. [21] H.S. Cerqueira, P.C.M. Koumba, P. Magnoux, M. Guisnet, Ind. Eng. Chem. Res. 40 (2001) 1032–1041.
This work was supported by a Korea University Grant.
Contents lists available at SciVerse ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Improvement of the heats of reaction in endothermic reactions of methylcyclohexane with zeolites Joongyeon Kim a , Sun Hee Park a , Byung-Hee Chun a , Byung Hun Jeong b , Jeong Sik Han b , Sung Hyun Kim a,∗ a b
Department of Chemical and Biological Engineering, Korea University, 1 Anam-Dong, Sungbuk-Ku, Seoul 136-701, Republic of Korea Agency for Defense Development, Jochiwongil 462, Yuseong, Daejon, Republic of Korea
a r t i c l e
i n f o
Article history: Received 31 May 2011 Received in revised form 19 August 2011 Accepted 16 September 2011 Available online 10 October 2011 Keywords: Heat of reaction Methylcyclohexane Zeolites Low-molecular-weight hydrocarbons Olefins
a b s t r a c t Endothermic reactions of methylcyclohexane (MCH) with zeolites were investigated in a batch reactor to increase the heats of reaction. Conversion and product distribution affected the heat of reaction. The heat of reaction with HZSM-5, which led to the formation of low-molecular-weight hydrocarbon products, was larger than the heats of reaction with other zeolites. To improve the heat of reaction with HZSM-5, Pt/HZSM-5 (1 wt.% Pt) was used in endothermic reactions. In endothermic reactions with Pt/HZSM-5, the heats of reaction increased, and there was a high yield of olefins, which were formed by dehydrogenation of paraffin on metal sites. A high conversion and high yield of low-molecular-weight hydrocarbons and olefins led to an increase in the heat of reaction. © 2011 Published by Elsevier B.V.
1. Introduction Aircraft operation at hypersonic flight speeds causes severe heat loads generated by engines or combustors with heat loads increasing with flight speeds. Because heat loads lead to structural changes in engines and aircraft failures, cooling technologies are necessary [1–3]. One cooling technology is the use of endothermic fuels. Endothermic fuels are liquid hydrocarbon fuels that undergo endothermic reactions at high temperatures and pressures. Endothermic fuels can absorb heat loads as sensible heat and heat of reaction before injection to the combustor [4–7]. Endothermic fuel cooling technology is advantageous because fuels that undergo endothermic reactions have better combustion properties [8]. To improve heat absorption by endothermic fuels, it is necessary to investigate endothermic reactions. There have been only a limited number of previous studies focused on increasing the heat of reaction for endothermic fuels. Sobel et al. [9] examined the endothermic reaction of Norpar 12 with SAPO-5 zeolite. With SAPO-5, the heat of reaction increased with an increase in the yield of low-molecular-weight and unsaturated hydrocarbons. Huang et al. [10] examined the endothermic reaction of n-octane, JP-7 and JP-8 + 100 with
∗ Corresponding author. Tel.: +82 2 3290 3297; fax: +82 2 926 6102. E-mail address: [email protected] (S.H. Kim). 0920-5861/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.cattod.2011.09.020
P-octacat (commercial zeolite) and investigated the relationship between the heat of reaction and fuel composition. Hai-long et al. [11] selected n-dodoecane as a model endothermic fuel and conducted endothermic reactions with Pd/HZSM-5. Endothermic reactions with Pd/HZSM-5 showed an increase in cracking products and heats of reaction. Sicard et al. [12] examined the endothermic reaction of n-dodecane with Y and ZSM-5. The heat of reaction and reaction mechanism were investigated through the analysis of cracking products. Daniau et al. [13] performed endothermic reactions with Norpar-12. The effects of temperature and fuel flow rate on the heat of reaction were investigated. Kerosene fuels (hydrocarbon mixtures) are widely used as endothermic fuels for aircraft systems. It is difficult to investigate the endothermic reactions of these fuels, due to their complicated compositions. For this reason, methylcyclohexane (MCH) was selected as a model endothermic fuel in this study because it is contained in aircraft fuels, such as kerosene fuels and JP-9 [14,15]. There have been many reports on the catalytic reactions with MCH [16–18]. However, there have been few investigations of heats of reaction with MCH. The aim of this study was to increase the heats of reaction of endothermic reactions with MCH. HZSM-5, HY, HBeta, and Pt/HZSM-5 were used. Heat of reaction was related to conversion and product distribution, which changed with different zeolite properties, such as pore structure, acidity and loaded metal.
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Fig. 1. Ammonia TPD for the zeolites.
2. Experimental 2.1. Catalyst preparation Three different types of zeolites with medium (HZSM-5) and large (HY, HBeta) pore sizes were investigated. Raw materials were obtained from the Zeolyst Company. HZSM-5 and HBeta were prepared by the calcination of NH4 + -form samples (CBV 8014 and CP814E) at 550 ◦ C for 3 h. HY was prepared by drying H+ -form samples (CBV780) at 120 ◦ C for 12 h. Pt/HZSM-5 (1 wt.% Pt) catalyst was prepared by incipient wetness impregnation of the HZSM-5 zeolite with tetraammineplatinum(II) nitrate (>99.9%, Sigma–Aldrich). After impregnation, the catalyst was dried at 120 ◦ C for 12 h and was calcined at 550 ◦ C for 3 h. 2.2. Catalyst characterization 2.2.1. Pore structures of the catalysts Surface areas, pore volumes and pore size distributions were determined from nitrogen adsorption–desorption isotherms carried out in a BELSORP-max (MP). Physical properties of the three zeolites are shown in Table 1. Mesopore volumes and diameters were obtained using the Barrett–Joyner–Halenda (BJH) method, and micropore volumes and diameters were obtained using the Horvath–Kawazoe (HK) method.
Fig. 2. Ammonia-FTIR spectra of zeolites: (a) HY, (b) Pt/HZSM-5, (c) HZSM-5, (d) HBeta.
The total acid densities of HZSM-5, HY, HBeta and Pt/HZSM5 were 0.075, 0.039, 0.146 and 0.067 mmol NH3 /g, respectively. HBeta exhibited a large peak at low temperature (182 ◦ C) and a shoulder shaped curve at 284 ◦ C. HZSM-5 and Pt/HZSM-5 exhibited one peak at low temperature (179 ◦ C) and another peak at moderate temperature (370 ◦ C). HY exhibited two small peaks below 400 ◦ C and one large peak at high temperature (634 ◦ C). When Pt was loaded onto HZSM-5, both the weak acid site peak at 179 ◦ C and the moderate acid site peak at 370 ◦ C decreased. Although total acid density was low in HY, there were strong acid sites (at 634 ◦ C). 2.2.3. Ammonia-FTIR of catalysts The properties of the acid sites were characterized using a MIDAC model 2000 Ammonia-FTIR spectrometer. IR spectra were obtained using an IR cell with CaF2 windows. The sample zeolites were pressed into a self-supporting disc and placed in the cell. Ammonia (50 Torr) was introduced into the cell at 30 ◦ C. The FTIR spectra of ammonia absorbed on HZSM-5, HY, HBeta, and Pt/HZSM5 are shown in Fig. 2. Peaks at 1470 cm−1 and 1690 cm−1 are attributed to ammonium ions formed by interaction with Bronsted acid sites. Peaks at 1620 cm−1 are assigned to ammonia coordinated on Lewis Acid sites [19]. Bronsted acid sites (at 1470 cm−1 ) were predominant in all catalysts. 2.3. Catalytic reaction
2.2.2. Temperature programmed desorption (TPD) of adsorbed ammonia The acidity of catalysts was determined by TPD of adsorbed ammonia using a BEL-CAT-B (Fig. 1). BEL-CAT-B is a commercial catalyst analyzer (BEL JAPAN Inc.) used for NH3 temperature programmed desorption (NH3 -TPD). Detailed information about the acid sites of catalysts can be determined through NH3 -TPD. NH3 TPD was performed with a BEL-CAT-B equipped with a thermal conductivity detector (TCD). A 0.3 g sample was loaded into a quartz tube and pretreated with He gas of 50 ml/min at 500 ◦ C for 1 h. Adsorption of NH3 was performed at 100 ◦ C in an NH3 –He (5% NH3 ) mixture for 0.5 h, and the physically adsorbed NH3 was purged with He gas. NH3 -TPD was performed in the He flow by raising the temperature to 700 ◦ C at a rate of 10 ◦ C/min, and the desorbed NH3 was detected with an online thermal conductivity detector. The total amount of desorbed NH3 (total acid density) and the desorption temperature (strength of acid site) were determined through NH3 -TPD.
Endothermic reactions were performed in a stainless steel batch reactor at 325 ◦ C and 50 bar for 9 h. Metal may affect the decomposition of hydrocarbons [20]. To understand better the endothermic reactions of MCH, it is necessary to prevent the MCH from contacting the metal. Thus, the inside of the reactor was covered with quartz to prevent any effects caused by the metal. For each test, 60 ml of MCH was placed in the reactor with 0.2 g of zeolite. Before the reaction, the inside of the reactor was purged with N2 gas for 10 min to prevent the oxidation of MCH by air, especially oxygen. After purging, the reactor was pressurized to an initial pressure of 15 bar with N2 . Next, an electrical heating jacket was used to heat the MCH to the reaction temperature (325 ◦ C) with a heating rate of 10 ◦ C/min. When the temperature of MCH reached 325 ◦ C for each test, the pressure of the reactor reached 50 bar. Thus, endothermic reactions of MCH (Tc : 298.95 ◦ C, Pc : 34.8 bar) with zeolites were conducted at 325 ◦ C and 50 bar, which are conditions similar to the hypersonic flight conditions of endothermic fuels.
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Scheme 1. Schematic of the batch reactor and process flow diagram.
During each reaction (9 h), 0.5 ml of the product in the reactor was sampled seven times at intervals of 1.5 h. The 0.5 ml samples were small enough not to affect the concentration and pressure inside the reactor. Sampled products were sent to a separator that was directly connected to the reactor. Because the temperature of the sampled products was notably high, it was impossible to analyze their composition. In the separator, the hot samples were cooled to 25 ◦ C and separated into gas and liquid phases. Gaseous products were sent directly to a gas chromatograph (GC) connected to a separator. After analysis of the gaseous products, liquid products remaining in the separator were collected and analyzed by GC/MSD. Schematic of the batch reactor and process flow diagram is provided in Scheme 1. Products of the endothermic reactions were identified by GC. The composition of the gas phase products was analyzed by GC/TCD (thermal conductivity detector) and GC/FID (flame ionization detector). Hydrogen in gaseous products was analyzed using a GC/TCD that was equipped with a stainless steel column packed with a molecular sieve 5A. The oven temperature for GC/TCD was increased from 40 ◦ C to 90 ◦ C at a heating rate of 10 ◦ C/min. Hydrocarbons in gaseous products from C1 to C4 were analyzed using a GC/FID equipped with a capillary column. The oven temperature for GC/FID was increased from 40 ◦ C to 100 ◦ C at a heating rate of 10 ◦ C/min. The compositions of liquid phase products were analyzed by GC/MSD (Agilent 7890A Series GC Custom, 5975C inert MSD Standard Turbo EI) using an HP-5ms column. The oven temperature was increased from 35 ◦ C to 290 ◦ C at a heating rate of 10 ◦ C/min. 3. Results and discussion Heat absorption from heat of reaction is dependent on conversion and product distribution. To increase heats of reaction, conversion and product distribution in endothermic reactions with zeolites were investigated.
Fig. 3. Conversion of MCH during endothermic reaction with HBeta, HZSM-5 and HY at 325 ◦ C and 50 bar.
3.1. Effects of acidity on conversion Endothermic reactions of MCH were performed at 325 ◦ C and 50 bar. Conversions after 9 h were 52.3, 34.6 and 34.0% for HBeta, HZSM-5 and HY, respectively (Fig. 3). Endothermic reactions of hydrocarbons with zeolites were initiated on the acid sites [21]. Therefore, the differences in conversion between endothermic reactions were related to acidities of the zeolites. HBeta had the highest total acid density (0.146 mmol NH3 /g). It is supposed that the high total acid density of HBeta led to the high level of conversion. However, conversion was nearly the same in the endothermic reaction with HY and HZSM-5 despite a lower total acid density of HY (HY: 0.039 mmol NH3 /g, HZSM-5: 0.075 mmol NH3 /g). Although
Table 1 Physical properties of the zeolites.
HZSM-5 HY HBeta Pt/HZSM-5
BET surface area (m2 /g)
Mesopore volume (cm3 /g)
Mesopore diameter (nm)
Micropore volume (cm3 /g)
˚ Micropore diameter (A)
429 772 633 417
0.219 0.243 0.550 0.203
76.5 21.3 76.5 66.5
0.25 0.22 0.19 0.23
5.6 7.8 6.5 5.6
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HY had the smallest total acid density, it had an NH3 desorption peak at high temperature (634 ◦ C). The peak at high temperature indicates the existence of strong acid sites. The strengths of the acid sites as well as the total acid densities affected conversion. Heat absorption from heat of reaction increased with conversion. High total acid density and strong acid sites led to an increase in conversion. Zeolites need to have high total acid densities and strong acid sites to increase heats of reaction. 3.2. Effects of pore structure on product distribution The compositions of products of the endothermic reactions with MCH classified by the numbers of carbons are shown in Fig. 4. The compositions of products gradually increased without any decrease during the reactions for the three zeolites, but product distributions were different. For HBeta and HY, C7 hydrocarbons were the major products for endothermic reactions (32.7% for HBeta, 25.4% for HY). The C7 hydrocarbons were mostly composed of isomers of MCH; the fractions of isomers in C7 hydrocarbons were 75% for HBeta and 95% for HY. The isomers of MCH were 1,1-dimethylcyclopentane (1,1-DMCP), 1,2-dimethylcyclopentane (1,2-DMCP), 1,3-dimethylcyclopentane (1,3-DMCP) and ethylcyclopentane (ECP). The molecular structures of MCH and the four isomers are shown in Scheme 2. As shown in Fig. 4(a) and (c), the difference in product composition between C7 hydrocarbons and the others was significant. For HZSM-5, C3 hydrocarbons, mostly propane, were major products (8%) of the endothermic reaction. However, the difference in product percentages between major products (C3 hydrocarbons) and the other products was not significant. Products were classified as cycloalkanes/cycloalkenes, aromatic hydrocarbons or linear/branched hydrocarbons for each zeolite, and the relative abundances of each type of product (measured at the end of each 9 h reaction) are presented in Fig. 5. For HBeta and HY, cycloalkanes/cycloalkenes, which were mostly composed of MCH isomers, were the most abundant products. For HZSM-5, the abundances of aromatic hydrocarbons and linear/branched hydrocarbons were higher than those of cycloalkanes/cycloalkenes. The differences in composition were consequences of the different pore structures of the three zeolites. As described previously, endothermic reactions of hydrocarbons (MCH) with zeolites are initiated on the acid sites, and carbenium ions are formed [21]. These carbenium ions are primarily converted to MCH isomers (1,1-DMCP, 1,2-DMCP, 1,3-DMCP and ECP), and these isomers undergo beta-scission reactions leading to C7 olefinic hydrocarbons. The C7 olefinic hydrocarbons are able to undergo further endothermic reactions, such as dehydrogenation, cracking and aromatization in the pores. A significant number of endothermic reactions (including dehydrogenation, cracking and aromatization) occur; therefore, it is difficult to describe the mechanisms of all of these reactions. As a result, only generic reaction mechanism that explains endothermic reaction of MCH is provided with Scheme 3. The sizes of the zeolite micropores were 6.5, 7.8 and 5.6 A˚ for HBeta, HY and HZSM-5, respectively (Table 1). The sizes of the micropores of HBeta and HY were relatively higher than that of the micropores of HZSM-5. After they are formed, the MCH isomers (primary products of endothermic reactions) can diffuse out quickly, due to the large pore sizes. Because the rate of diffusion was faster than the rates of further reactions, the yield of isomers was large in HBeta and HY. In contrast, HZSM-5 had narrow pores, and once the MCH isomers were formed, the diffusion of the isomers out of the pores was restricted, and the isomers were able to stay in them for a longer time. Therefore, further reaction of the isomers occurred, and the percentage
Fig. 4. Composition of products classified by number of carbons during endothermic reaction of MCH with (a) HBeta, (b) HZSM-5 and (c) HY.
composition of C3 hydrocarbons (propane) was high. Because heat of reaction is dependent on product distribution, the zeolite pore structure is important in increasing heat absorption for aircraft cooling.
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Scheme 2. Methylcyclohexane (MCH, C7 H14 ) and the four isomers of MCH.
Scheme 3. Generic reaction mechanism for endothermic reaction of MCH.
3.3. Heat of reaction by catalytic decomposition Heat absorption by endothermic fuels comes from sensible heat and heat of reaction and was calculated by NIST SUPERTRAPP program. This program offers thermophysical properties of hydrocarbon mixtures, such as enthalpy and entropy, at a specific temperature and pressure. In this study, sensible heat was obtained from the difference between MCH enthalpy at the reaction temperature (325 ◦ C) and at room temperature (25 ◦ C). Sensible heat was the same for the endothermic reactions with the three
Fig. 5. Classification of endothermic reaction products of MCH with HBeta, HY and HZSM-5 sampled at the end of the reaction (10 h).
zeolites because they were carried out at the same reaction temperature (81.0 kJ/mol). The heat of reaction was obtained from the enthalpy difference between the product mixture and MCH. All of the species in the product mixture were identified by gas chromatography (GC) and were accounted for in the heat of reaction calculation except for species that were not identified by GC/MSD analyses. From every experiment, 95 total species were detected by GC, but 6 species were unidentified. The fraction of the unidentified species in the total products was below 1% for every experiment. It is assumed that the exclusion of unidentified species did not significantly affect heat of reaction calculations. The enthalpy of product mixture could be calculated after species name and their concentrations were entered in NIST SUPETRAPP program because it had each species thermophysical properties. Then, heat of reaction was obtained from enthalpy difference between product mixture and MCH. Heats of reaction were 22.7, 10.8 and 6.2 kJ/mol for HZSM-5, HBeta and HY, respectively. Due to high conversion to products, the heat of reaction for HBeta was larger than the heat of reaction for HY, even though they had similar product distributions. Although conversion of catalytic decomposition was lower in HZSM-5 than in HBeta, the heat of reaction was higher with HZSM-5 than with HBeta. The heat of reaction is the difference in enthalpy (or state of energy) between the reactants and products in a reaction. In an endothermic reaction, the products are unstable (high state of energy) compared to the reactants; therefore, the heat of reaction increases with the instability of the products. As molecular weights (or carbon numbers) of hydrocarbons decrease, they become unstable. Cracking leads to the production of low-molecular-weight hydrocarbons. It is supposed that the heat of reaction is larger for cracking than for isomerization. Because C3 hydrocarbons formed
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◦
Fig. 6. Conversion of MCH during endothermic reaction with Pt/HZSM-5 at 325 C and 50 bar.
by cracking were the major products in the HZSM-5 reaction, the heat of reaction was the largest with HZSM-5 among the three zeolites. Based on these results, the formation of low-molecularweight hydrocarbons leads to an increase in the heat of reaction. 3.4. Increase in the heat of reaction using Pt/HZSM-5 As the molecular weights (or carbon numbers) of products decreased, the heats of reaction of the endothermic reactions increased; HZSM-5 resulted in the largest heat of reaction. In addition, unsaturated hydrocarbons (olefins) are unstable compared to saturated hydrocarbons (paraffins) because the high electron densities of multiple bonds lead to instability. Based on this theory, the heat of reaction will increase when the formation of olefins increases. Because olefins are produced by the dehydrogenation of paraffin on metal, Pt/HZSM-5, which has acid sites and metal sites, was prepared by impregnation. HZSM-5 was selected, due to its capability to lead to high heats of reaction. The conversion of the endothermic reaction with Pt/HZSM-5 was lower than that of the reaction with HZSM-5 (Fig. 6, 28% for 9 h). As described previously, the conversion of the endothermic reaction was dependent on the total acid densities and the acid
Fig. 7. Composition of products during endothermic reaction of MCH with Pt/HZSM5 classified by number of carbons.
Fig. 8. Classification of endothermic reaction products of MCH with HZSM-5 and Pt/HZSM-5 sampled at the end of the reaction (10 h).
strengths of the zeolites. Pt/HZSM-5 had peaks at the same temperature as HZSM-5 (Fig. 2), but the areas of the peaks were smaller. Therefore, the low total acid density of Pt/HZSM-5 led to the low level of conversion. The composition of the products of the endothermic reaction with Pt/HZSM-5 classified by number of carbons is shown in Fig. 7. As with HZSM-5, C3 hydrocarbons were the major products (5.5%) followed by C7 (4.8%), C9 (3.3%) and C8 (3.3%) hydrocarbons. It is supposed that the pore structures of the zeolites affected product distributions as classified by number of carbons. However, there were difference in the types of products from reactions with Pt/HZSM-5 and HZSM-5. Products from the reactions, classified by hydrogen, olefin and paraffin, are compared in Fig. 8. The yields of products were measured at the ends of the reactions (9 h). Due to the dehydrogenation by metal sites of Pt, the yield of hydrogen (25.1%) and olefin (9.2%) was higher with Pt/HZSM-5. Olefins were mostly composed of propylene. It is supposed that propane, which was the major product of the HZSM-5 reactions, underwent dehydrogenation on the metal sites of Pt and was converted to propylene. Heat absorption by Pt/HZSM-5 was also calculated using the NIST SUPERTRAPP program (Fig. 9). An increase in heat of reaction was observed when Pt/HZSM-5 (31.7 kJ/mol) was used for the endothermic reaction. It is supposed that the formation of olefins
Fig. 9. Heat of reaction for endothermic reaction of MCH with HZSM-5, HBeta, HY and Pt/HZSM-5.
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(mostly composed of propylene) affected the heat of reaction. Based on these results, a high yield of low-molecular-weight hydrocarbons and olefins leads to an increase in the heat of reaction.
Acknowledgement
4. Conclusion
References
To improve heats of reaction for aircraft cooling, endothermic reactions of MCH with zeolites were investigated in this study. Heat of reaction was dependent on conversion and product distribution, which changed with different zeolites. Due to its high total acid density, HBeta led to a higher conversion and heat of reaction than HY; product distributions were similar. Although the conversions of endothermic reactions were lower with HZSM-5, the heat of reaction with HZSM-5 was higher than with other zeolites. As the molecular weights of hydrocarbons decrease, instability increases. Heat of reaction increases with the instability of products. HZSM-5 led to an increase in the formation of low-molecular-weight hydrocarbons. Therefore, HZSM-5 led to a higher heat of reaction than HBeta. Additionally, unsaturated hydrocarbons, which have multiple bonds, are unstable compared to saturated hydrocarbons, which have single bonds, because the higher electron density of multiple bonds leads to instability. Therefore, the heat of reaction increases with the yield of olefins. To increase the yield of olefins through the dehydrogenation of paraffin, Pt/HZSM-5 was prepared. An increase in the heat of reaction with Pt/HZSM-5 was observed for the endothermic reaction. Based on these results, a high conversion and high yield of low-molecular-weight hydrocarbons and olefins led to an increase in the heat of reaction.
[1] N. Cascoin, P. Gillard, S. Bernard, G. Abraham, M. Bouchez, E. Daniau, Y. Toure, AIAA-2006-8005, 2006. [2] N. Cascoin, P. Gillard, E. Dufour, Y. Toure, J. Thermophys. Heat Transfer 21 (2007) 86–94. [3] J.A. Gasner, R.C. Foster, C. Fujimura, AIAA-92-3721, 1992. [4] D.T. Wickham, J.R. Engel, S. Rooney, B.D. Hitch, J. Propul. Power 24 (2008) 55–63. [5] D. Petley, S. Jones, W. Dziedzic, AIAA-91-5063, 1991. [6] E. Daniau, M. Bouchez, AIAA-2003-6920, 2003. [7] G. Yong-sheng, L. Rui-sen, J. Zhejiang, Univ. Sci. 6A (2005) 632–635. [8] D. Petley, S. Jones, AIAA-90-3284, 1990. [9] D.R. Sobel, L.J. Spadaccini, J. Eng. Gas Turbine Power 119 (1997) 344–351. [10] H. Huang, L.J. Spadaccini, D.R. Sobel, J. Eng. Gas Turbine Power 126 (2004) 284–293. [11] Z. Hai-long, M. Fan-xu, G. Wei, Z. Ji-jun, Z. Xiang-wen, J. Fuel Chem. Technol. 36 (2008) 462–467. [12] M. Sicard, M. Grill, B. Raepsaet, F. Ser, AIAA-2008-2622, 2008. [13] E. Daniau, M. Sicard, AIAA-2005-3404, 2005. [14] G.Y. Gerasimow, S.A. Losev, J. Eng. Phys. Thermophys. 78 (2005) 1059–1070. [15] A. Osmont, I. Gokalp, L. Catoire., Prop. Exp. Pyrotech. 31 (2006) 343–354. [16] S. Yolcular, O. Olgun, Catal. Today 138 (2008) 198–202. [17] P. Castano, J.M. Arandes, M. Olazar, J. Bilbao, B. Pawelec, U. Sedran, Catal. Today 150 (2010) 363–367. [18] M.P. Lazaro, E.G. Bordeje, D. Sebastian, M.J. Lazaro, R. Moliner, Catal. Today 138 (2008) 203–209. [19] N. Khandan, M. Kazemeini, M. Aghaziarati, Appl. Catal. A: Gen. 349 (2008) 6–12. [20] T. Edwards, Combust. Sci. Technol. 178 (2006) 307–334. [21] H.S. Cerqueira, P.C.M. Koumba, P. Magnoux, M. Guisnet, Ind. Eng. Chem. Res. 40 (2001) 1032–1041.
This work was supported by a Korea University Grant.