New Method of Catalytic Cracking of Hydrocarbon Fuels Using a

Mar 23, 2010 - Industrial & Engineering Chemistry Research 2014 53 (47), 18104- .... of Mechanical Engineers, Part G: Journal of Aerospace Engineering...
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New Method of Catalytic Cracking of Hydrocarbon Fuels Using a Highly Dispersed Nano-HZSM-5 Catalyst Shiguo Bao, Guozhu Liu,* Xiangwen Zhang, Li Wang, and Zhentao Mi Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, PR China

A pseudohomogeneous method of catalytic cracking of hydrocarbon fuels using a highly dispersed nanoHZSM-5 catalyst is developed. Hydrophilic nano-HZSM-5 is transformed into a hydrophobic form via organic silanization of the zeolite surface, which makes it dispersible in a model endothermic fuel such as n-dodecane. Compared with thermal cracking, catalytic cracking of n-dodecane with highly dispersed nano-HZSM-5 catalyst exhibits remarkably enhanced conversion. 1. Introduction Catalytic cracking of hydrocarbons over zeolite catalysts in packed beds reactors or fluidized beds reactors are widely used in refining industry to improve the yields of light oil.1 Currently, catalytic cracking of hydrocarbon fuels is becoming a potential technology in active cooling of hypersonic aircraft due to higher heat sink during cracking.2 Coating zeolite on the cooling channels, as a new catalyst application method, is developed for this application in hypersonic aircraft and significantly improves the heat sink of hydrocarbon fuels compared with thermal cracking.3 However, rapid deactivation and high heat resistance resulting from coke deposition over zeolite coatings are still a crucial challenge in developing thermal management system for hypersonic aircrafts. To overcome those drawbacks of catalyst coating, we attempt to develop a new catalytic cracking method to catalytic cracking of hydrocarbon fuels in hypersonic aircraft with a highly dispersed nanozeolite, i.e., dispersing nanocrystal of zeolites into hydrocarbon fuels. This new idea offers a promising method to improve catalytic activities, as well as excellent application performances of hydrocarbon fuels.4,5 In particular, the external surface area of NaZSM-5 with a particle size of 15 nm is 200 m2/g, which accounts for 37% of the total surface area, compared to less than 10 m2/g (or 3% of the total surface area) for most commercial ZSM-5 samples.5 The larger external surface area makes it possible to expose more reactive surface or acid sites, which is preferred in the processes requiring a higher reaction rate and catalytic activity.6 Another advantage of nanocrystalline zeolites is the decreased diffusion path length relative to micrometer-sized zeolites. This makes it relatively easy for the molecules to diffuse in or out the zeolite and, thus, brings higher efficiency and a lower deactivation rate to catalytic processes.7 A highly dispersed (or pseudohomogeneous) zeolite can be synthesized via surface modification of nanozeolites and play a role similar to that of a heterogeneous catalyst. To date, organically functionalized zeolite nanoparticles have gained considerable attention as a convenient tool in the design of new hybrid materials, and dispersible zeolites have also been synthesized.8-14 Larsen et al. have successfully functionalized nanocrystalline NaZSM-5 (15-200 nm) with organosilane in toluene and found that not only the hydrophobicity was increased dramatically but also the dispersibility in hexane was obtained.8 Vuong and Do reported a new route for the * To whom correspondence should be addresed. Tel.: +86-2227892340. Fax: +86-22-27402604. E-mail address: [email protected].

synthesis of MFI and faujasite nanozeolites, which can stably dispersed in organic phase such as toluene.10 Dutta and coworkers functionalization of NaY using n-octadecyltrichlorosilane (OTS), and the concentration of C18-Y in toluene is more than 0.025 g/mL.9 Herein, we demonstrate a new method for catalytic cracking of hydrocarbon fuels with highly dispersed nano zeolites. Nanoscale HZSM-5 instead of conventional microscale zeolite is used, and further functionalized by trichlorosilane which grafting linear alkyls onto the surface. 2. Experimental Section 2.1. Materials. Commercial water glass, aluminum sulfate, sodium hydroxide, and n-butylamine were used as starting materials to synthesize nanoscale ZSM-5 zeolite. Trichlorosilanes containing different linear alkyls (analytical reagent, Fluorochem Ltd.), including ethyltrichlorosilane, propyltrichlorosilane, butyltrichlorosilane, hexyltrichlorosilane, dodecyltrichlorosilane, hexadecyltrichlorosilane, were used as modification reagent to react with the zeolite. Absolute ether obtained from distilling diethyl ether (analytical reagent, Tianjin Standard Science And Technology co., Ltd.) and further dehydrating by Na was used as solvent. 2.2. Preparation of Catalyst. Nanoscale NaZSM-5 (Si/Al ) 30) with a crystal size of 50 nm was synthesized according to the method reported by Wang et al.15 A precursor zeolite solution was prepared with the following molar composition: Al2O3:SiO2:Na2O:n-C4H9NH2:NaCl:H2O ) 1.0:31.2:2.0:12.2: 18:833.3. The precursor solution was first crystallized in Teflonsealed stainless steel autoclave at autogenous pressure with stirring at 100 °C for 24 h. Then, the temperature was increased to 170 °C at a heating rate of 0.6 °C/min and maintained for 40 h. The solid products obtained were separated by filtration, washed several times with distilled water, and finally dried overnight at 110 °C. Nanoscale NH4ZSM-5 was obtained by exchange 3 times of nanoscale NaZSM-5 with 0.4 mol/L NH4NO3 solution at 80 °C for 2 h with stirring. After each treatment, the product was filtered, washed with distilled water, and then dried at 110 °C. The obtained nanoscale NH4ZSM-5 was calcinated at 540 °C for 6 h in air to form the nanoscale HZSM-5. Prior to modification, nanoscale HZSM-5 was pretreated at 140 °C in vacuum overnight to remove water and physical adsorbed substances. Predesorbed nanoscale HZSM-5 was then quickly dispersed by ultrasonic power in absolute ether, followed

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by dropwise addition of a solution of trichlorosilane in absolute ether. After 2 h reaction at room temperature (around 25 °C), the product was separated and purified by a series of high speed centrifugation and ultrasonic redispersion in diethyl ether. Finally, the modified nanoscale HZSM-5 was kept at 130 °C in vacuum overnight to remove physically adsorbed substances during the silylation. 2.3. Characterization. X-ray diffraction (XRD) data in the 2θ range of 5°-40° were collected on a Rigaku D-max 2500 V/PC X-ray diffractometer (Rigaku Corporation) using Cu KR radiation source (40 kV, 200 mA). Solid-state 29Si nuclear magnetic resonance (NMR) was conducted on a Varian Infinity plus 300 MHz spectrometer (Varian inc.) under conditions of magic angle spinning and cross-polarization. The spectra was collected at room temperature with a frequency of 59.6 MHz, a pulse delay of 5.0 s, an acquisition time of 20.0 ms, a spin rate of 3.0 kHz, and a contact time of 5.0 ms. Transmission electron microscopy (TEM) images were obtained at room temperature on a Tecnai G2 F20 field-emission transmitting electron microscope (Philips). Prior to the observation, the samples were dispersed in ethanol under ultrasonic power and finally deposited over a carbon-coated copper grid. Dynamic light scattering (DLS) measurements were performed at 25 °C and 514 nm using a BI200SM dynamic light scattering apparatus (Brookhaven). The hydrodynamic radius distribution was determined from the Laplace inversion of the measured intensityintensity time correlation function using the CONTIN program on the basis of the Stokes-Einstein equation. 2.4. Catalytic Tests. The catalytic properties of functionalized HZSM-5 nanocrystals were tested for the cracking of n-dodecane using the method reported by Yu and Eser.2 The reaction was carried out in a static stainless steel batch reactor with a volume of 25 mL. In a typical experiment, 0.01 g of functionalized HZSM-5 nanocrystals, which was used in powder, were dispersed into 10 g of n-dodecane under ultrasonic power for 10 min, and then introduced into the reactor. After purging several times in N2 to remove O2 followed by heating to 427 °C in 10 min, a cracking reaction was performed at this temperature for a certain time. The pressure of the reaction was gradually increased from 1.83 MPa (Pro ) 1.81) during the test. Finally, the reactor was cooled down to room temperature quickly in water to stop the reaction. The liquid products were collected and analyzed with a gas chromatograph (Agilent 7890A) equipped with a capillary column (PONA) and flame ionization detection (FID). The gas products were analyzed with a gas chromatograph (BEIFEN 3420) equipped with a capillary column (Al2O3/S) and thermal conductivity detector (TCD). The content of hydrogen was analyzed with a gas chromatograph (Agilent 4890) equipped with a capillary column (C-2000) and TCD. Both the liquid and gas products were identified by gas chromatography-mass spectrometry (GC-MS) using an Agilent 6890N GC connected with an Agilent 5975 inert mass selective detector. All samples used to analysis were obtained after stopping the tests. 3. Results and Discussion The XRD patterns of nanoscale HZSM-5 silylated with different trichlorosilane are shown in Figure SI-1 (of the Supporting Information). There is no significant change in the patterns of HZSM-5 nanocrystal before and after silylation, indicating that silylation had small effects on its crystalline structure. The 29Si CP/MAS NMR spectra of nanoscale HZSM-5 samples silylated with different trichlorosilane are shown in

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Figure 1. Distribution of particle sizes for nanoscale HZSM-5 silylated with dodecyl measured using DLS.

Figure 2. TEM images of nanoscale HZSM-5 (A) before and (B) after C12 functionalization.

Figure SI-2 (of the Supporting Information). Each NMR spectrum has a prominent resonance at -112.92 ppm that is assigned to framework tetrahedral silicon sites (Si(OSi)4, Q4). The intensity of this resonance is gradually increased with increasing length of linear alkyls grafted after silylation (except for C2-HZSM-5), which could be due to transformation of portion of Q3 species into Q4 ones. The higher intensity of C2-HZSM-5 resonance at -112.92 ppm than that of C3-HZSM-5 can be ascribed to the smaller molecular size of ethyltrichlorosilane, which can graft on the inner surface of micropores. Moreover, a wide overlapping resonance at about -105.8 ppm, which consist of overlapping resonances of Si(1Al), Si(2Al), Si(3Al), HOSi(OSi)3, and (HO)2Si(OSi)2, can also be observed.16,17 However, an additional broad resonance at about -50 to -70 ppm for silylated nanoscale HZSM-5 assigned to Tn (n ) 1-3) species which are the result of the reaction between the silicon in the organosilane and one to three silanol groups of zeolite during silylation respectively. This result suggests successful silanization on the surface of nanoscale HZSM-5. Similar results can also be obtained in the work done by Vuong and Do10 and Larsen and co-workers.8 A representative size distribution of silylated nanoscale HZSM-5 obtained using DLS is shown in Figure 1. The distribution of particle size is from 100 to 500 nm, and the average particle size is about 330 nm. TEM images of HZSM-5 and C12-HZSM-5 nanoparticles are shown in Figure 2. Smaller particles with diameters of about 300 nm are observed for C12-HZSM-5, while large aggregates of nanocrystals with diameters of about 2 µm for HZSM-5. In order to compare the dispersibility of parent and modified nanoscale HZSM-5 in hydrocarbon fuel, the samples are introduced into a model fuel, n-dodecane, under ultrasound, and the result is shown in Figure 3. Obviously, the parent HZSM-5 is hard to disperse in n-dodecane and deposite on the bottom. On the contrary, the

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Figure 5. Conversion versus reaction time from thermal and nanoscale C4-HZSM-5 catalyzed decomposition of n-dodecane at 427 °C. Figure 3. Nanoscale HZSM-5 silylated with different linear alkyls dispersed in n-dodecane after sonicating for 10 min (0.01 g zeolite in 10 g n-dodecane).

Figure 4. Effects of different linear alkyls to the catalytic activity of silylated nanoscale HZSM-5 on cracking of n-dodecane at 427 °C (40 min).

samples after silylation can be stably dispersed for at least 1 day and form homogeneous systems. This shows that silylation with linear alkyls on the surface of nanoscale HZSM-5 can dramatically increase its hydrophobicity and protect against aggregation. The catalytic activity of silylated nanoscale HZSM-5 is investigated to catalyze decomposition of n-dodecane, and the results are shown in Figure 4. The conversions of n-dodecane catalyzed by different linear alkyls grafted nanoscale HZSM-5 are all higher than that of thermal cracking, indicating that nanoscale HZSM-5 silylated with different linear alkyls still shows high catalytic activity. As the length of alkyls grafted increased, the n-dodecane conversion first increases and then decreases possibly as a mutual result of both the steric hinder effect and dispersion behavior. The H2 content of gas products catalyzed by different linear alkyls grafted nanoscale HZSM-5 for 40 min is shown in Figure SI-3 (of the Supporting Information). A certain amount of H2 is obtained in the products of catalytic cracking and exhibits a similar discipline with the n-dodecane conversion discussed above. However, little H2 is obtained in the products of thermal cracking. Figure 5 presents the catalytic cracking performance of C4-HZSM-5 as a function of time, compared with the thermal cracking of n-dodecane. For thermal cracking, the conversion of n-dodecane linearly increases with increasing reaction time and reaches 35 mol % after reacting 2 h, which is in good accordance with the previous

observation of Yu and Eser.2 While, for the catalytic cracking of n-dodecane in presence of C4-HZSM-5, the conversion of n-dodecane increases dramatically during the first 34 min and reaches 20 mol %, which is about 4 times as high as that obtained in thermal cracking. However, the increment gradually decreases from 34 to 120 min and similar to that of thermal cracking. After reacting 2 h, the conversion of n-dodecane reaches 55 mol %, which is about 1.6 times as high as that in thermal decomposition. This result indicates that C4-HZSM-5 catalyst show high catalytic activity and can remarkably enhance the conversion of n-dodecane compared with thermal cracking. Moreover, deactivation of this catalyst (become brown at the end of the test) occurs at about 34 min, which may be caused by coke deposition. The H2 content of gas products as a function of reaction time catalyzed by C4-HZSM-5 is shown in Figure SI-4 (of the Supporting Information). For thermal cracking, H2 content increases slowly with the reaction time. However, for catalytic cracking, the H2 content is larger than that of thermal cracking and exhibits a tendency of first increases and then decreases. Thermal cracking and catalytic cracking in this experiment can be distinguished via the distribution of liquid products and gas products. As shown in Figures SI-5 and SI-6 (of the Supporting Information), the products of thermal cracking are primarily saturated and high-molecular-weight species (n-C11, n-C10, n-C9, n-C8, etc.), corresponding to a low heat sink. However, for the catalytic cracking, the products are substantially lower in molecular weight and are primarily unsaturated, making them more desirable from the point of view of high endotherm characteristics.18 In addition, aromatic compounds, such as toluene, are observed in catalytic cracking. For the gaseous products distribution in Figures SI-7 and SI-8 (of the Supporting Information), the amount of C1 and C2 components is larger than that of C3 and C4 components for thermal cracking, while the opposite result is obtained for catalytic cracking. Additionally, iso-C4H10, a typical gas product of catalytic cracking, is also observed. A similar product distribution was also obtained in the work done by Sicard and co-workers.19 4. Conclusions A new method has been developed to catalytic cracking of hydrocarbon fuels with a highly dispersed nanozeolite. The nanoscale HZSM-5 silylated using organosilane shows excellent catalytic activity and dispersibility into the hydrocarbon fuels. With this method, other nanozeolites, such as Y, can also be used as pseudohomogeneous catalysts to catalytic cracking of

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hydrocarbon fuels. Moreover, this work also provided a facile and general method to decompose hydrocarbons, which may find wider applications besides the refining industry. Acknowledgment The authors gratefully acknowledge financial support from National Natural Science Foundation of China (Grant No. 20806058). Supporting Information Available: Some representative characterization results such as XRD, 29Si CP/MAS NMR, catalyst test results, and chromatograms, including Figures SI-1-8. This information is available free of charge via the Internet at http://pubs.acs.org/. Literature Cited (1) Cooper, M.; Shepherd, J. E. Experiments studying thermal cracking, catalytic cracking, and pre-mixed partial oxidation of JP-10. In 39th AIAA/ ASME/SAE/ASEE Joint Propulsion conference and exhibit, Huntsville, Alabama, July 20-23, 2003; p 4687. (2) Yu, J.; Eser, S. Thermal Decomposition of C10-C14 Normal Alkanes in Near-Critical and Supercritical Regions: Product Distributions and Reaction Mechanisms. Ind. Eng. Chem. Res. 1997, 36 (3), 574–584. (3) Grill, M.; Sicard, M.; Ser, F.; Potvin, C.; Djega-Mariadassou, G. Preparation of zeolite Y and ZSM-5 coatings for cracking fuel in a cooling system for hypersonic vehicles. In Studies in surface science and catalysis; Elsevier: New York, 2007; Vol. 170, Part 1, pp 258-266. (4) Larsen, S. C. Nanocrystalline Zeolites and Zeolite Structures: Synthesis, Characterization, and Applications. J. Phys. Chem. C 2007, 111 (50), 18464–18474. (5) Song, W.; Justice, R. E.; Jones, C. A.; Grassian, V. H.; Larsen, S. C. Synthesis, Characterization, and Adsorption Properties of Nanocrystalline ZSM-5. Langmuir 2004, 20 (19), 8301–8306. (6) Wang, K. Y.; Wang, X. S. Comparison of catalytic performances on nanoscale HZSM-5 and microscale HZSM-5. Microporous Mesoporous Mater. 2008, 112 (1-3), 187–192. (7) Tosheva, L.; Valtchev, V. P. Nanozeolites: Synthesis, Crystallization Mechanism, and Applications. Chem. Mater. 2005, 17 (10), 2494–2513.

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(8) Song, W.; Woodworth, J. F.; Grassian, V. H.; Larsen, S. C. Microscopic and Macroscopic Characterization of Organosilane-Functionalized Nanocrystalline NaZSM-5. Langmuir 2005, 21 (15), 7009–7014. (9) Singh, R.; Dutta, P. K. Use of surface-modified zeolite Y for extraction of metal ions from aqueous to organic phase. Microporous Mesoporous Mater. 1999, 32 (1-2), 29–35. (10) Vuong, G.-T.; Do, T.-O. A New Route for the Synthesis of Uniform Nanozeolites with Hydrophobic External Surface in Organic Solvent Medium. J. Am. Chem. Soc. 2007, 129 (13), 3810–3811. (11) Smaihi, M.; Gavilan, E.; Durand, J. O.; Valtchev, V. P. Colloidal functionalized calcined zeolite nanocrystals. J. Mater. Chem. 2004, 14 (8), 1347–1351. (12) Zhan, B. Z.; White, M. A.; Lumsden, M. Bonding of organic amino, vinyl, and acryl groups to nanometer-sized NaX zeolite crystal surfaces. Langmuir 2003, 19 (10), 4205–4210. (13) Li, D.; Yao, J. F.; Wang, H. T.; Hao, N.; Zhao, D. Y.; Ratinac, K. R.; Ringer, S. P. Organic-functionalized sodalite nanocrystals and their dispersion in solvents. Microporous Mesoporous Mater. 2007, 106, 262– 267. (14) Vassylyev, O.; Hall, G. S.; Khinast, J. G. Modification of zeolite surfaces by Grignard reagent. J. Porous Mater. 2006, 13 (1), 5–11. (15) Wang, X.; Guo, X. Ultrafine, granular five-element circular type zeolite preparation - by mixing water glass, silica sol and aluminium sulfate, in presence of organic amine, with alkali, salt, water and seed crystals. CN1240193-A; CN1260126-C, 2000. (16) Maciel, G. E.; Sindorf, D. W. Silicon-29 NMR study of the surface of silica gel by cross polarization and magic-angle spinning. J. Am. Chem. Soc. 1980, 102 (25), 7606–7607. (17) Fyfe, C. A.; Gobbi, G. C.; Kennedy, G. J. Investigation of the conversion (dealumination) of ZSM-5 into silicalite by high-resolution solidstate silicon-29 and aluminum-27 MAS NMR spectroscopy. J. Phys. Chem. 1984, 88 (15), 3248–3253. (18) Sobel, D. R.; Spadaccini, L. J. Hydrocarbon fuel cooling technologies for advanced propulsion. J. Eng. Gas Turbine Power 1997, 119, 344– 351. (19) Sicard, M.; Grill, M.; Raepsaet, B.; Ser, F. Comparison between thermal and catalytic cracking of a model endothermic fuel. In 15th AIAA International Space Planes and Hypersonic Systems and Technologies Conference, Dayton, Ohio, April 28-May 1, 2008; p 2622.

ReceiVed for reView November 13, 2009 ReVised manuscript receiVed March 8, 2010 Accepted March 11, 2010 IE901801Q