Co3O4 Nanosheet Wrapped Commercial HZSM-5 for Promoting

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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Co3O4 Nanosheet Wrapped Commercial HZSM‑5 for Promoting Catalytic Cracking of n‑Decane and Anticoking Activities Lin Long,†,∥ Zhenzhong Lan,†,∥ Zhixiong Han,† Yunfeng Qiu,*,§ and Weixing Zhou*,‡ †

ACS Appl. Energy Mater. Downloaded from pubs.acs.org by 185.14.192.147 on 08/20/18. For personal use only.

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, People’s Republic of China ‡ Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin, Heilongjiang 150001, People’s Republic of China § Key Lab of Microsystem and Microstructure of Ministry of Education, Harbin Institute of Technology, Harbin, 150080, People’s Republic of China S Supporting Information *

ABSTRACT: Endothermic hydrocarbon fuel has many advantages as a cooling medium in hypersonic aircraft; however substantial application has been greatly hindered due to easy coking, uncontrollable gas products, and low heat sink issues. Catalytic cracking of hydrocarbon fuel has been regarded as one of the most effective ways to improve the heat sink via modulating the cracking pathway. The incipient-wetness impregnation method was selected to load Co salts in commercial ZSM-5 in a large-scale manner; subsequent calcination in Ar gas resulted in Co3O4 nanosheet wrapped HZSM-5 composites. Salinization treatment was adopted to facilitate the dispersion of catalyst in a nonpolar solvent. The ratio of Brönsted/Lewis acidity decreased from 8.00 to 2.79 after modification by Co3O4 nanosheets. The synergistically catalytic effect between Co3O4 nanosheets and ZSM-5 was beneficial to the generation of a larger gas production rate with higher content of alkene, and thus resulting in a higher heat sink than benchmarked fuels. Catalytic cracking of n-decane (C10) in the presence of 0.1 wt % Co3O4 nanosheets@ZSM-5 could yield a heat sink as high as 4.64 MJ/kg at 758 °C, much higher than those of bare ZSM-5 (2.99 MJ/kg at 687 °C) and thermal cracking of C10 (3.77 MJ/kg at 728 °C). Meanwhile, the smart combination of Co3O4 nanosheets and commercial ZSM-5 could effectively suppress the coke deposition on the external surface of composites, thus resulting in efficient catalytic cracking at elevated temperatures for obtaining a higher heat sink. KEYWORDS: catalytic cracking, Co3O4 nanosheets, HZSM-5, synergistic effect, heat sink

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INTRODUCTION

combination of physical and chemical heat sink might be an appealing way to improve the overall heat sink. However, the pyrolysis heat sink of JP-7 is about 0.7 MJ/kg and the percentage conversion is also very low.9 Thus, catalytic cracking of fuel could significantly improve the chemical heat sink via the generation of a large amount of alkene. Inspired from previous work, catalytic cracking of hydrocarbon fuel was effective in improving the heat sink via modulating the cracking path and controlling the distribution of products. It is well-known that catalysts can promote the fuel to produce carbocations and free radicals to drive the pyrolysis, mainly due to catalytic dehydrogenation and catalytic pyrolysis. Catalytic dehydrogenation has the properties of a low triggering temperature and a large caloric receptivity, while the catalysts are almost always precious metals or precious

The scramjet engine is a kind of power source device of hypersonic aircraft. The temperature of the combustor will reach 4950 K when the scramjet engine works at a 12 Mach number.1 Obviously, such extreme aerodynamic heat will bring a great challenge to the structure materials.2 To this end, regenerative active-cooling technology for hydrocarbon fuel, designed by Tsioikovsky,3 has made tremendous progress in overcoming the above-mentioned issue in recent years. The endothermic hydrocarbon fuels will provide a physical and chemical heat sink to adsorb the extra heat to protect the structure material. Meanwhile, the absorbed heat drives the cracking of the hydrocarbon fuel into smaller hydrocarbon molecules or hydrogen that has a higher calorific value and shorter firing delay time.4,5 Basically, most hydrocarbon fuel could yield a physical heat sink of 1.6−1.8 MJ/kg at 810 K, which cannot fulfill the harsh requirements at a high Mach number of 5. Particularly, the heat sink will be above 4.6 MJ/ kg at a Mach number of 8.6−8 It is obvious that the © XXXX American Chemical Society

Received: May 16, 2018 Accepted: August 1, 2018 Published: August 1, 2018 A

DOI: 10.1021/acsaem.8b00780 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Energy Materials

transition metal ions into ZSM-5 will modulate the overall acidity, thus rendering the effective catalytic cracking of hydrocarbon fuel under supercritical conditions. It is also worth noting that the coke formation on the external surface of HZSM-5 can be effectively restrained, which is beneficial for maintaining the catalytic properties at elevated temperatures, thus finally resulting in maximizing the gas production and heat sink. Given that the highly exposed catalytic sites on both sides of nanosheets will provide ideal reaction sites for reagents, fast electronic and ionic transport rates as well as synergistic effects with HZSM-5 will be maximized due to the interfacial coupling effects, thus the catalytic cracking of C10 will be significantly boosted. If one looks more carefully, one may find that there are few works focused on the smart combination of 2D transition metal oxide nanosheets and commercial HZSM-5 for the catalytic cracking of hydrocarbon fuels. Herein, we successfully prepared 2D Co3O4 nanosheet wrapped ZSM-5 via the incipient-wetness impregnation method and thermal annealing, and its catalytic cracking of C10 was systematically investigated. Various loading amounts of Co3O4 from 1 to 9 wt % content were prepared and evaluated in a catalytic cracking test. XRD, SEM, EDS, and XPS characterizations were performed to disclose the morphologies and compositions. Temperature-programmed desorption of ammonia (NH3-TPD) and pyridine-absorption FTIR were used to disclose the acidic properties. As seen in Scheme S1, catalytic cracking of supercritical C10 was carried out at 3.5 MPa in the homemade cracking stage in terms of the gas products, reactor temperature, heat sink, coke evaluation, etc. The present work developed a facile, effective, and largescale preparation method for catalytic cracking of hydrocarbon fuel using commericial ZSM-5 loaded 2D transition metal oxide nanosheets and provided deep insight into the cracking mechanism for designing promising catalysts in a future study.

metal molten salts, which are expensive and deactivated easily.10 After the 1990s, great efforts have been devoted to catalytic pyrolysis via the design of nanocatalysts with various acidities. Generally, Lewis acid on a catalyst can trigger a carbocation of alkane, and Brönsted acid can trigger a carbocation of olefin.11,12 The carbocation is the active intermediate, leading to easy cracking at the beta position, followed by generating a new carbocation. Previous papers have proved that nanocatalysts play an important role in the generation of carbocations; thus the design and synthesis of novel nanocatalysts with controllable acidities became a hot topic in past decades.13,14 Among all the reported nanocatalysts, the modified HZSM-5 materials have attracted special attention due to their unique microporous nanostructure, adjusted Brönsted/Lewis acidity, chemical engineering at the internal or external surface, etc. Desilication of ZSM-5 could increase the mesopores and decrease the strong Brönsted acid sites, contributing to improve the catalytic durability in methanol to aromatic reactions.15 Surface modification, such as silylation, was used to reduce the concentration of Brönsted acid and Lewis acid sites.16 Introducing phosphorus into ZSM-5 was also found to readily decrease the acid amounts, and 0.5 wt % phosphorus content gave rise to the best gas generation rate and heat sink due to the presence of a reasonable amount of acid.17 Very recently, Lyu et al. used phosphate modification to control the acidity and acid distribution of hierarchical porous ZSM-5, showing that external surface acid site passivation played an important role in breaking the thermodynamic equilibrium distribution of xylene isomers.18 Transitional metal ion modifications were reported to be effective at adjusting the acid properties of HZSM-5, thus providing a lot of room for maximizing the catalytic performance. Previous work found that silver ion exchange and impregnation could alter the acid properties of HZSM-5 zeolite to improve the catalytic cracking of C10.19 Liu et al. proved that the bilayer HZSM-5 zeolite of Zr-ZSM-5 possessed a favorable mass transport path and Brönsted acid sites, synergistically promoting light olefin production. They also found that Ag-ZSM-5 and ZSM-5(R2) deactivated rapidly due to severe coke formation.20 The Lewis and Brönsted acids of ZSM-5 zeolite were successfully modulated via introducing Ga, showing improved selectivity for aromatic compounds because of the presence of GaO+ species.21 Gd modified HZSM-5 catalysts not only improved the selectivity of olefins and aromatics via controlling the amount of Lewis acid sites but also reduced the carbon deposition by 50%.22 Mg-ZSM-5 showed fewer Brönsted acid sites, while it possessed slightly more Lewis acid sites, resulting in significantly improved initial activity and lifetime.23 Recently, [Mo5O12]6+ units were proved to interact with the framework of ZSM-5 to decrease the Brönsted acid sites remarkably, while increasing the Lewis acid sites.24 Clearly, the intensity of acid centers is associated with the catalytic process of the hydrocarbon fuel. Lewis acid absorbs the electron, causing alkane to lose H+ at the acidic site to become a carbocation. Meanwhile, some of the strategies have proven that the increase of Lewis acid and decrease of Brönsted acid amount in ZSM-5 will drive the catalytic cracking of supercritical C10 more efficiently. Transition metal ions possess a unique redox state and could absorb or release electrons from the carbocation or hydrocarbon during the reaction process, leading to promote the formation of carbocations and free radicals. Additionally, introducing

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EXPERIMENTAL SECTION

Materials. The 0.5−0.7 A ZSM-5 (mSiO2/nAl = 25) with a diameter of ∼2.35 ± 0.56 μm was bought from Nankai catalyst factory. Co(NO3)2·6H2O, toluene, and ethanol were bought from Shanghai Aladdin Biological Technology Company Limited. Dodecyltriethoxysilane (C16H30O2) was bought from Tokyo Chemical Industry. Millipore Milli-Q water (resistivity >18.2 MΩ cm) was used to rinse all bottles and to prepare salt solutions. All chemical were used without further purification. Preparation of Catalysts. A total of 272 mg of Co(NO3)2·6H2O was dissolved into 0.4 mL of H2O under stirring. The salt solution was slowly added into 1 g of ZSM-5 in 5 min by mechanical stirring. The whole mixing process should be gentle to maintain the solid state according to the basic principle of the incipient-wetness impregnation method. The as-obtained powder was dried in a vacuum at 60 °C for 24 h, then transferred into a tubular furnace for calcination. A total of 1 g of powder was placed in a porcelain boat at the center of the tubular furnace, and the whole system was elevated to 500 °C with a heating rate of 5 °C min−1 and maintained at 500 °C for 2 h under 100 sccm flowing Ar gas. At the end of the experiment, the tube was cooled down to room temperature naturally. The calcined powder was kept in a vacuum for the next salinization. Salinization of Catalysts. Dodecyltriethoxysilane has three oxethyls at the ends of chains and can react with hydroxyl groups on ZSM-5. The typical process is listed as follows: First, put 2 g of powder into 30 mL of toluene under magnetic stirring at 900 rev/min for 30 min at room temperature. The mixture was sonicated for 30 min at a frequency of 59 kHz. B


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ACS Applied Energy Materials Second, 200 mg of dodecyltriethoxysilane was added in the above toluene solution and sonicated for 30 min. The mixture was refluxed at 100 °C for 24 h, then cooled down to room temperature naturally. Third, the salinized catalysts were centrifuged at a speed of 8000 rpm/min for 5 min and washed with toluene five times to remove the excess dodecyltriethoxysilane. The as-prepared catalysts were dried at 70 °C for 12 h, then stored in a vacuum for testing. Characterization Methods and Catalytic Test Apparatus. The morphologies and energy-dispersive spectroscopy (EDS) of ZSM-5 and transition metal oxide modified samples were carried out on an FEI Quanta 200 scanning electronic microscope. ZSM-5 and Co3O4 nanosheets were characterized using field emission TEM (TecnaiG2F30, FEI, US). Typically, 1 mg of the sample was dispersed in 1 mL of deionized water under sonication for 30 min; then the ink was dropped on a Cu grid for TEM measurements. The AFM images were recorded on a Digital Instrument Nanoscope IIIa Multimode system (Santa Barbara, CA) using the tapping mode. The Raman spectra were measured on a confocal microscope-based Raman spectrometer (LabRAM XploRA, incident power of 1 mW, pumping wavelength of 532 nm). X-ray diffraction (XRD) was performed on a DIFFRACTOMETER-6000 with Cu Ka radiation (λ = 0.1542 nm). The compositions and chemical states of Co, Si, Al, C, and O elements were tested using X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha XPS, using Al (Ka) radiation as a probe). The binding energies were calibrated by using the containment carbon peak (C 1s: 284.6 eV). The deconvolution was analyzed using a Gaussian−Lorentzian peak shape after subtracting the baseline. The nitrogen adsorption−desorption isotherms were carried out on an ASAP 2020-Physisorption Analyzer (Micrometritics, USA) at 77 K. Conventional Brunauer−Emmett−Teller (BET) and density functional theory (DFT) methods are used to determine the specific surface area and pore-size distributions. FTIR spectra were performed in transmission mode in a Bruker IFS28 spectrophotometer. The critical temperature and pressure of C10 are 617.7 K and 2.11 MPa, which was often used to investigate the combustion and flow problems instead of the kerosene.25 The experimental unit in Scheme S1 was developed for all the experiments. The fluid flowed from a fuel pump to get enough flow dynamics and pressure. The mass flow rate was set as 0.43 g/s. The pressure and mass flow rates were measured by a MicroMotion CMF 010 mass flow meter (uncertainty: ±0.1%) and Rosemount 3051 transducer (uncertainty: ±0.075%), respectively. The gas products were analyzed with a gas chromatograph (GC).26 The fuel will be heated and cracked when flowed through the testing pipe, which is electrically heated at DC voltage. The temperature of the pipe wall and inlet and outlet oil temperature and pressure were tested. The maximum operating pressure of the system is 8 MPa, and the system can make the fuel temperature get to 1000 K. The experimental back pressure was 3 MPa to make sure that the fuel was under a supercritical state, because the fuels which work in the hypersonic aircraft must be at a supercritical state.5 The fuel pressure was regulated by the back pressure tank. This research used Φ3×1 mm testing pipes. The length was 800 mm, and the material type was GH3128. The pipes were purchased from China Iron & Steel Research Institute Group (Beijing, China). GH3128 has an excellent material strength and good antioxidation between 650 and 2000 °C, which was often used on aircraft engines. The elemental composition of GH3128 tubes used in this work can be seen in Table S1. The pipe was placed horizontally. It was heated by DC power; electrodes were placed at both ends of the pipe. Forty K-type thermocouples with a diameter of 0.1 mm were welded to the upper surface of the pipe (pixel pitch arrangement of thermocouples was 20 mm) to reduce the error which was caused by the heat transfer of the thermocouples. The thermocouples were proofread before the experiment. The proofreading method was to heat the testing pipe without fuel in the environment. After disposing of the first and the end thermocouple data, the displayed values of thermocouples should be nearly equal, and the thermocouples which had the greatest difference points abandoned. We need to test the thermocouples

twice with the opposite direction of current. Get the arithmetic mean value of both tests as the ultimate result to eliminate the step voltage influence. The measured surface temperature profiles (uncertainty: ±2 K) were used to calculate the heat loss before the experiment. The presence of oxygen in the pipe would influence the fuel pyrolysis significantly. So, purging the testing pipe with nitrogen to remove oxygen is necessary.27 Calculation of Heat Sink. The endothermic power of fuel equals the power consumption minus the heat loss of the testing pipe, which was caused by air convection and radiation to the environment, as seen in Scheme S2. Q̇ oil = UI − Q̇ loss

(1)

where U is voltage of the testing pipe at both ends, I is current passed through the pipe, Q̇ oil is the endothermic power of fuel, and Q̇ loss is the heat dissipation to air. When the fuel flows in the pipe, temperature distribution along the pipe is different from the heating situation without fuel. The temperature of the pipe wall must be higher along the direction of fuel flow. The emission heat flux to the environment can be calculated at the different pipe wall temperatures. The relationship between the pipe wall temperature and the heat loss can be calculated as a fitted curve: qloss ̇ (Two) =

UI A jh

= a(Two − T0)3 + b(Two − T0)2 + c(Two − T0) + d (2) where Two is the temperature of the pipe wall; Ajh is the external surface of the pipe area; and a, b, c, and d are the parameters of fitted curves. The practical testing pipe was divided into many segments. The thermocouple was located in the middle of every segment. The temperature tested by the wall thermocouples can be seen as the average temperature of this segment. Total loss of heat can be expressed as the sum of every segment: Q̇ loss = A jh · ∑ n

qloss ̇ (Two) n

(3)

Calculation of the Fuel Outlet Temperature. Heat loss occurs between the heating electrode and outlet fuel temperature thermocouple. So, the thermocouple will check the local temperature of the fuel ,which is lower than the real outlet temperature. In fact, repeatability of the experiment will have a high error rate if heat preservation cotton is added on a section of pipe. So the heat loss of this section can be written as eq 4 after verification: Q̇ − = UI − Q̇ loss − mh ̇ 3(T3 , P3) + mh ̇ 1(T1, P1)

(4)

where T3 and P3 mean the outlet temperature and pressure of fuel and T1 and P1 mean the inlet temperature and pressure of fuel; enthalpy parameters were from the NIST database. The loss of this section is subtracted when calculating the outlet fuel temperature; the method of least-squares has been used to fit the fuel temperature (checked by the outlet thermocouple) and heat sink. The results indicated that this method can get a more accurate temperature than the situation without this processing by comparing the experimental physical parameters of fuel with the NIST database. Calculation of Gas Ratio. Gas ratio means the gas quality fraction of the total fuel which can produce these gases. Molar ratio means the percentage of one gas component in moles accounting for the total mixed gas moles under the same conditions. Expressions from eqs 5 to 7 are listed as follows:

C

aCH4 = V /22.4/ ∑ Ci

(5)

ai = Ci × aCH4

(6) DOI: 10.1021/acsaem.8b00780 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 1. Characterizations of SEM, TEM, and EDS of ZSM-5 and ZSM-5@CoM. (a) Low- and (b) high-magnification SEM images of ZSM-5@ CoM. (c) AFM height image with 1.5 × 1.5 μm of ZSM-5@CoM. Inset at the top right corner is the corresponding 3D image. Inset at the bottom right corner is the corresponding height profile. (d) Low- and (e) high-resolution TEM image of ZSM-5@CoM. Crystalline structure of ZSM-5 is illustrated in the inset of b. (f) HRTEM image of the Co3O4 nanosheet. (g) STEM-HAADF image and corresponding EDS mapping of elemental distribution.

mi = Mi × ai

(7)

for different concentrations of salt solutions. After being thoroughly dried, subsequent thermal annealing in Ar at 500 °C was carried out to convert salt into oxides. In the following part, ZSM-5@CoM was selected as a representative for systematic characterizations. As seen in Figure 1a, a lowmagnification SEM image of ZSM-5@CoM demonstrated that the average diameter is 2.35 ± 0.56 μm. Some Co3O4 nanosheets peeled off from the ZSM-5 support as confirmed by the high-magnification SEM image in Figure 1b, showing a layered nanostructure. An AFM height image was used to determine the thickness of Co3O4 nanosheets in Figure 1c; corresponding height profiles in the inset indicated that the average thickness was about 15 nm. Meanwhile, the inset at the top right corner in Figure 1c demonstrated the 3D image of broken nanosheets after ultrasonication, indicating the sheetlike nanostructure. Low-resolution TEM images in Figure 1d further confirmed the nanosheet wrapped ZSM-5 hybrid structure. High-resolution TEM (HRTEM) images in Figure 1e illustrated the well-resolved lattice fringes, indicating the highly crystalline structure of ZSM-5. In addition, the HRTEM image of Co3O4 nanosheets is illustrated in Figure 1f, showing poor crystallinity. The energy-dispersive X-ray spectrometry (EDS) mapping verified the uniform Co elemental distribution around the ZSM-5 particle in Figure 1g. In addition, 2.7 wt % (medium) Co3O4 content was confirmed by EDS in Figure S1. XRD in Figure 2a further confirmed the crystalline structure of ZSM-5@CoM in comparison to ZSM-5. In order to confirm

where V is the volume flow rate of the gaseous product; Ci is mole number of H2, CH4, C2H6, C2H4, C3H8, C3H6, C4H10, or C4H8, which is divided by CH4 mole number; aCH4 is the mole number of CH4; ai is the component mole number; Mi is the molar mass of the component; and mi is the mass flow rate of the component. Coke Deposit Analysis. The used tube was thoroughly washed by dichloromethane to remove residual n-decane. For better comparison with the coke thickness, the tube was cut into eight segments with an average length of 10 cm. All segments were dried in a vacuum at 60 °C for 6 h. The amount of coke was measured using the temperature-programmed oxidation (TPO) method in a CO2 infrared analyzer (GXH-1050, Beijing Junfang Research Institute). The segments were burned in the presence of ultrapure O2 with a flow rate of 500 sccm at 800 °C.

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RESULTS AND DISCUSSION 1. Synthesis and Characterizations of Catalysts. Co3O4 was selected as the main active species for modifying the catalytic performance of ZSM-5, because of its high thermal stability, widely available sources, and adjusting capability for acidic properties of ZSM-5. The Co3O4 contents in ZSM-5@Cox (x = L, M, H, representing low, medium, and high weight percentage) have been synthesized by changing the salt concentration during the incipient-wetness impregnation method. As is known, the volume is critical during the incipient-wetness impregnation process, which is unchanged D


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Emmett−Teller (BET) specific surface area of ZSM-5@CoM was 275.8 m2g−1, slightly higher than that of pure ZSM-5 (269.3 m2g−1), indicating that the wrapped Co3O4 nanosheets did not compromise the specific surface area. The characteristic bands near 1215 and 542 cm−1 in Figure S5a are ascribed to the double five-member rings in HZSM-5. Two bands at 1090 and 790 cm−1 correspond to the internal asymmetric stretching vibration and the external symmetric stretching of Si−O−T (T = Si, Al, or other heteroatom) linkage. The peak of 450 cm−1 is assigned to the T−O bending vibration of internal tetrahedral SiO4 and AlO4. The positions of those characteristic peaks are almost unchanged after salinization, indicating the maintenance of the ZSM-5 zeolite framework.32 As confirmed by FTIR in Figure S5b, the intensities of two peaks at 2852 and 2924 cm−1, corresponding to symmetric and asymmetric stretching of CH2 groups, greatly increased after salinization, implying the successful modification of alkyl chains.33,34 2. Catalytic Cracking of C10. With the large-scale preparation of ZSM-5@CoM in hand, we first investigated the catalytic cracking of C10 in the presence of various amounts of catalysts. As seen in Figure S6a, the heat sink was plotted as a function of the outlet temperature of oil. As enlarged in the inset of Figure S6a, the addition of 0.1 wt % ZSM-5@CoM could deliver the largest heat sink above a temperature of ∼600 °C. Meanwhile, the largest gas ratio was also obtained in the case of 0.1 wt % among all tested usages. We further collected the gas products as a function of outlet temperature of oil in the cases of 0.05, 0.1, and 0.15 wt % in Figure S6b. Clearly, the molar ratios of C2H4 and C3H6 were ranging from 0.2 to 0.32 in all cases. Furthermore, more gas products containing alkene still can be generated above 680 °C in the case of 0.1 wt %. Low, medium, and high Co3O4 contents in composites were compared in catalytic cracking of C10 in Figure S7. Clearly, ZSM-5@CoM gave rise to the largest heat sink and gas ratio in comparison to low and high Co3O4 contents. Meanwhile, alkene products are retained at an elevated temperature up to 758 °C. Given the superior catalytic cracking activity of ZSM-5@CoM, the catalytic details were evaluated using ZSM-5@CoM as an example with 0.1 wt % addition in oil in the following part. We first monitored the wall temperature, which not only was used to calculate the heat loss but also was applied to evaluate the heat sink of hydrocarbon fuel. The pipe wall temperature was tested by the thermocouples welded on the outer wall. As shown in Figure 3, all pipe wall temperature points along the pipeline were tested at the moment when the coke initially

Figure 2. Characterizations of XRD and XPS of ZSM and ZSM-5@ CoM. (a) XRD patterns of ZSM-5 and ZSM-5@CoM. (b) XRD pattern of the cubic Co3O4. (c) XPS fine spectrum of Co 2p. (d) N2 absorption/desorption isotherms of ZSM-5 and ZSM-5@CoM.

the presence of Co3O4, ZSM-5@CoM was dissolved in 5 M hot KOH for 12 h to remove the ZSM-5 and was subjected to XRD measurement. The XRD pattern in Figure 2b illustrated the characteristic diffraction peaks at 31.27, 36.85, 38.54, 44.81, 55.66, 59.38, and 65.24°, corresponding to the (220), (311), (222), (400), (422), (511), and (440) planes of a cubic Co3O4 (PDF# 42-1467), respectively. Such a result solidly confirmed the presence of cubic Co3O4 wrapped on the surface of ZSM-5. XRD patterns of the other two loading amounts of ZSM-5@CoL and ZSM-5@CoH were illustrated in Figure S2, demonstrating similar peaks in comparison to pure HZSM-5 and ZSM-5@CoM, thus confirming the intact crystalline zeolite framework.28 Low- and high-magnification SEM images of ZSM-5@CoL and ZSM-5@CoH are shown in Figure S3a and b, as well as S3d and S3e, respectively. Sheet-like Co3O4 was observed on the outer surface of ZSM-5 microsized particles. EDS results in Figure S3c and f indicated the loading amounts were ∼1.3 wt % (low) and ∼8.3 wt % (high), respectively. Full X-ray photoelectron spectra (XPS) in Figure S4 indicate that ZSM-5@CoM contains Co, Si, Al, and O elements without other impurities and show the Co3O4 weight content was about 14.8 wt %; this value is much higher than the value obtained in EDS. This was caused by XPS measuring depth on the sample, which is generally 5 nm. The Co3O4 nanosheets were wrapped on the surface of ZSM-5; as a result, the XPS showed much higher Co3O4 contents. The XPS fine spectrum of Co 2p in Figure 2c was deconvoluted into six species. Two pairs of spin−orbit doublets centered at 781.1 and 797.1 eV, as well as 783.0 and 798.6 eV, ascribed to the coexistence of Co2+ and Co3+, respectively. The binding energies of two satellite peaks of Co3O4 in ZSM-5@CoM shifted to 801.6 and 790.2 eV in comparison to pure Co3O4 (803.7 and 789.2 eV), indicating the strong coupling interactions with HZSM-5.29 In order to improve the dispersibility in C10, ZSM-5@CoM was further salinized according to Scheme S3. Nitrogen adsorption− desorption isotherms are displayed in Figure 2d, showing typical type I isotherms for both ZSM-5 and ZSM-5@CoM. A sharp uptake was observed at low relative pressure, and a very slow increase of adsorbed N2 amount was found as a function of the increase of relative pressure up to 1.0, displaying the characteristic microporous framework.30,31 The Brunauer−

Figure 3. Pipe wall temperature along the pipe length in the presence of ZSM-5 and ZSM-5@CoM during the catalytic cracking process, as well as without any catalyst under thermal cracking conditions. E


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catalytic cracking in the presence of ZSM-5@CoM would generate more gas products in comparison with the other two cases. As indicated by the red arrow in Figure 4, the gas ratio was improved by 37.4% and 5.4% for ZSM-5@CoM in comparison to pure C10 and the bare ZSM-5 system at 685 °C, indicating the stronger catalytic cracking reaction of C10 in the presence of ZSM-5@CoM. It has been observed that the gas ratio in all cases increased as a function of outlet temperature of oil. The details of gas products were systematically tested using GC in Figure 5.

clogged the pipeline. The distribution of wall temperature in three cases are varied, showing different mechanisms. In all cases, the wall temperature increased rapidly at the beginning of the pipe ranging from 0 to 20 mm, while it decreased slightly in the range of 20 to 100 mm along the pipeline. Such results were ascribed to the heat exchange capability increase due to the supercritical entrance effect, consistent with previous work.35,36 The slight variations of pipe wall temperature at the entrance might be related to the presence of ZSM-5@CoM in oil in comparison to pure C10, even though the catalytic cracking reaction did not occur at such a low temperature. In both cases of pure C10 and C10 containing ZSM-5, the pipe wall temperature increased gradually along the pipeline in the range from 100 to 800 nm. The slight fluctuation can be explained by the manually anchored thermocouples. In contrast, a smaller slope value can be found in the range from 300 to 800 mm, indicating the positive effect of ZSM-5@ CoM during catalytic cracking. The relatively slow increase in pipe wall temperature in the latter part of the pipe might be related to the involvement of catalysts, which effectively drove the catalytic cracking of C10 to generate more alkene gas products and finally improved the heat transfer ability. We next calculate the heat sink with or without catalysts according to eqs 1 and 2. As shown in Figure 4, similar heat

Figure 5. Gas products as a function of the outlet temperature of oil in the presence of ZSM-5 and ZSM-5@CoM during the catalytic cracking process, as well as without any catalyst under thermal cracking conditions.

Clearly, the gas molar ratios were varied greatly, agreeing well with previous articles.37,38 ZSM-5 delivered the lowest heat sink, which is not only ascribed to the relatively low gas ratio but also due to the specific gas products. The molar ratio of propylene was higher than that of the other gas products in the presence of bare ZSM-5. According to the heat absorption capacity of each gas product in Table S2,39 propylene possesses lower heat absorption capacity in comparison to ethylene. However, the molar ratio of ethylene in the presence of bare ZSM-5 throughout the whole working temperature range from 568 to 686 °C was below 0.2. In pure C10 without any catalyst, the molar ratios of ethylene were around 0.3, obviously higher than that in the presence of ZSM-5 and comparable to that in the presence of ZSM-5@CoM. In addition, the molar ratio of propylene is around 0.24, slightly lower than that (>0.26) in the case of ZSM-5@CoM. The alkene products possess the highest heat exchange capability; thus the relatively higher molar ratios of ethylene and propylene in the case of ZSM-5@ CoM are, highly probably, responsible for the high heat sink. Meanwhile, the coke formation was restrained at elevated temperatures in the presence of ZSM-5@CoM; therefore, more gas products are generated at outlet temperatures of oil higher than 727 °C, leading to the largest gas ratio. Importantly, the molar ratios of ethylene and propylene are almost unchanged at such high temperature, some fluctuations corresponded to the complex catalytic cracking reaction process. Taken together, the addition of ZSM-5@CoM in C10 could generate the largest gas ratio involving the highest molar ratios of ethylene and propylene among all catalytic systems. When the outlet temperature of oil reached 560 °C, gas products were initially generated, which can be seen in Figure 4. The above results confirmed that the coke formation was suppressed, and the catalytic cracking of C10 effectively proceeded in the unclogged pipe, thus leading to the smooth flowing of the supercritical liquid/gas mixture inside the pipe

Figure 4. Change of heat sink and gas ratio with the increase of the outlet temperature of oil in the presence of ZSM-5@CoM. Inset is the enlarged region above an oil temperature of 600 °C.

sink profiles are observed below ∼450 °C, which is the outlet fuel temperature, including pure C10, C10+ZSM-5, and C10+ZSM-5@CoM systems, respectively. Above point A, namely higher than 645 °C, the heat sink rapidly increased in the presence of ZSM-5@CoM. Clearly, the heat sink could be distinguished above 645 °C for three systems. Catalytic cracking of C10 in the presence of 0.1 wt % Co oxides@ZSM5 could yield a heat sink as high as 4.64 MJ/kg at 758 °C, much higher than those of bare ZSM-5 (2.99 MJ/kg at 687 °C) and thermal cracking of C10 (3.77 MJ/kg at 728 °C). Briefly, the heat sink of C10 in the presence of ZSM-5@CoM has improved 54.78% and 23.11% in comparison to ZSM-5 and pure C10, respectively. The complete blocking temperature for pure C10 was 728 °C and decreased to 687 °C after the addition of ZSM-5, whereas it increased to 758 °C in the presence of ZSM-5@CoM. The addition of bare ZSM-5 did not result in improving the heat sink in comparison to pure C10, but it showed the lowest pipe wall temperature, indicating the relative high heat exchange capability. A severe coking effect on the pipe wall in the presence of bare ZSM-5 was observed at relatively lower fuel temperatures, showing the lowest gas ratio with less ethylene, and thus finally resulting in poor heat sink. It is assumed that the combination of thermal cracking and F


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might be formed via a metal-triggered mechanism, which is widely reported in previous work.42,43 We also measured SEM images at the boundary between coke and inner wall of pipe in Figure S10, and fibers are observed at the interface, indicating the possibility of a metal-triggered mechanism. Inspired from previous work, Ni metal has good solubility for carbon at higher temperatures and can be used as a catalyst for the formation of carbon nanotubes, hexagonal boron nitride, graphene, etc.44−46 Thus, the formation of filamentous carbon might start on the surface of alloy pipes, which contains 61.3 wt % Ni. Another important factor should be considered, that is, the stability of the catalyst at elevated temperatures. Commonly, nanocatalysts are always poisoned by coke deposition during catalytic cracking at high reaction temperatures. If serious coke was formed on the surface of catalysts, the active sites such as Lewis or Brönsted acid sites will be blocked, and in turn the deactivation of catalysts occurs. As seen in Figure 7, D and G

for heat exchange. As illustrated in Figure 6a, the pipe was cut into eight parts with equivalent lengths after performing the

Figure 6. SEM images of cokes inside the pipe as a function of pipe temperature, corresponding to various pipe lengths. (a) The pipe length and temperature distribution. (b) Coke thickness distribution (indicated by red arrow) and amount of coke (indicated by blue arrow) along the pipe length. (c−f) SEM images of coke film at ∼100 mm, ∼300 mm, ∼500 mm, and ∼700 mm, respectively.

test in the presence of ZSM-5@CoM. SEM images in Figure S8b−i demonstrated that the coke was uniformly covered on the inner surface of the pipe wall. As seen in Figure S8j−q, the coke thickness and the amount of coke were measured and plotted in Figure 6b. The thickness increased gradually from 0 to 300 mm, whereas an about 110 μm thickness of coke was suddenly formed at the ∼400 mm position. The sudden increase of coke thickness appeared at ∼400 mm, corresponding to the temperature range from 800 to 810 °C. In the final part of the pipe, the coke thickness first increased slowly from 110 to 140 μm, showing gradual carbon morphology transitions from amorphous carbon spheres to filamentous carbon in Figure 6c−e. Then, the ultrahigh temperature at the end of the pipe was >900 °C, and the coke thickness fluctuated greatly, and the majority of the carbon was fibers, as in Figure 6f. However, the pipe was completely blocked for pure C10 when the end of the pipe temperature reached 728 °C, leading to complete blocking. It is also found that the amount of coke increased along the pipe length and became more severe at the ∼800 mm position, similar to the thickness change along the pipe length. The amount of coke near the end of the pipe fluctuated greatly, which might be ascribed to the uneven coverage of coke on the final part of the pipe due to the complex coke formation routes at ultrahigh temperatures. The addition of ZSM-5@CoM in C10 has a positive effect to suppress the coke formation at elevated temperatures. We further used Raman to analyze the coke on the inner wall of the pipe at various positions. As shown in Figure S9a, two peaks centered at 1360 and 1580 cm−1 were ascribed to D and G peaks at pipe positions of 300 mm, respectively. After deconvolution, a subpeak was found at ∼1500 cm−1, corresponding to the presence of oxidized amorphous carbon.40,41 In contrast, at a pipe position of 500 mm in Figure S9b, this subpeak was missing, and the majority of carbon nanostructures are filamentous carbon. This subpeak appeared again at the pipe position of 700 mm in Figure S9b, confirming the catalytic cracking reaction became drastic, and the coke might be formed in a relatively fast and complicated process. It is assumed that the amorphous carbon was, highly probably, related to the initially formed polycyclic aromatic hydrocarbon (PAH) compounds. The filamentous carbon

Figure 7. Raman spectra of ZSM-5@CoM at various outlet oil temperatures.

peaks corresponding to the carbon were negligible below 624 °C, and a very small peak is observed at 660 °C, indicating the weak coke formation on the surface of catalysts. This result is very important for the catalytic cracking of C10 in the presence of ZSM-5@CoM at high temperatures above 600 °C. Taken together, ZSM-5@CoM exhibited intriguing activity for catalytic cracking of C10. BET results indicated that the specific surface area was almost unchanged after wrapping Co3O4 nanosheets on the ZSM-5; thus the improved catalytic activity was not directly related to this factor. Another important factor is the change of acidities after incorporation of Co3O4 nanosheets, as seen in Figure 8. NH3-TPD experiments in Figure 8a showed two peaks at about 100− 200 °C and 350−450 °C, which can be assigned to the desorption of NH3 at weak acidic sites and strong acidic sites, respectively.17,47 After modification by Co3O4 nanosheets, the intensities of these two peaks were both lower than bare ZSM5, indicating that weak and strong acidities both decreased. Pyridine-absorbed FT-IR spectra of the catalysts were also measured at temperatures of 200 and 300 °C, as in Figure 8b and c. L and B represent Lewis acid sites and Brönsted acid sites, respectively. The peak at 1540 cm−1 corresponds to the pyridinium ion (PyH+), which is formed after pyridine interacting with Brönsted acid sites. Pyridine adsorbed on Lewis acid sites has a characteristic band of 1450 cm−1. The band at 1490 cm−1 arises from both types of pyridine.30,48,49 Both PyH+ and Lewis acid sites exist on the catalysts’ surface. For ZSM-5, the peak intensity of Lewis acid sites obviously decreases at higher temperatures, suggesting that the strength of Lewis acid sites is weaker than that of Brönsted acid sites. As G



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00780. Scheme S1, schematic diagram of experimental setup; Table S1, elemental composition of GH3128 tubes; Scheme S2, schematic diagram of the reactor; Figure S1, EDS spectrum of ZSM-5@CoM; Figure S2, XRD patterns of ZSM-5@CoL, ZSM-5@CoM, ZSM-5@CoH; Figure S3, low- and high-magnification SEM images of ZSM-5@CoL and ZSM-5@CoH; Figure S4, full X-ray photoelectron spectra (XPS) of ZSM-5@CoM; Scheme S3, schematic illustration of catalysts preparation process; Figure S5, FTIR spectra of the pristine and salinization modification ZSM-5@CoM; Figure S6, the change of heat sink and gas ratio with the increase of the outlet temperature of oil in the presence of 0.05, 0.1, and 0.15 wt % ZSM-5@CoM and corresponding gas products in different catalyst concentrations; Figure S7, the change of heat sink and gas ratio with the increase of the outlet temperature of oil in the presence of 0.1 wt % ZSM-5@CoL, ZSM-5@CoM, and ZSM-5@CoH and corresponding gas products in using low, medium, and high loading catalysts; Table S2, heat absorption capacity of the gas pyrolysis products; Figure S8, SEM images of cokes along pipe length; Figure S9, Raman spectra of coke at various pipe positions; Figure S10, SEM images at the boundary between coke and the inner wall of the pipe; Table S3, acid properties of ZSM5 and ZSM-5@CoM catalysts (PDF)

Figure 8. Acid property of catalysts. (a) NH3-TPD profiles of ZSM-5 and ZSM-5@CoM. (b) Pyridine-absorbed FT-IR spectra of ZSM-5. (c) Pyridine-absorbed FT-IR spectra of ZSM-5@CoM.

summarized in Table S3, B, L, and total acid amount decreased after Co3O4 modification, highly possibly ascribed to the strong interaction between Co ions and the oxygen of the bridging hydroxyl groups. It is also implied that the ratio of Brönsted/Lewis acidity was adjusted and decreased from 8.00 to 2.79 after modification in comparison to ZSM-5. Carbon deposition could be suppressed due to the relatively lower acidic sites in our catalysts; thus micropores will be exposed during harsh reaction conditions, consistent with previous work.17 And particularly, appropriate Lewis acidic density adjusted by the synergistic effects between Co3O4 and ZSM-5 is favorable for the production of carbocations to promote supercritical C10 cracking.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *Phone: +86(0)451 86403142-307. Fax: +0451 86403142. Email: [email protected].

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CONCLUSIONS Co3O4 nanosheets with a ∼15 nm thickness have been successfully wrapped on the surface of ZSM-5 using the incipient-wetness impregnation method and subsequent calcination. The catalysts can be readily synthesized in a large-scaled manner and were salinized for better dispersibility in C10. BET measurements confirmed that the smart combination of Co3O4 nanosheets and ZSM-5 did not compromise the specific surface area and thus possessed highly exposed acidic sites and a favorable mass transport path for reagents. NH3-TPD and pyridine-absorbed FT-IR implied that the ratio of Brönsted/Lewis acidity was adjusted and decreased from 8.00 to 2.79 after modification in comparison to ZSM-5. The as-prepared ZSM-5@CoM could effectively catalyze the cracking of C10 in supercritical conditions, giving rise to a larger gas production rate with a higher content of alkene than ZSM-5 and pure C10. ZSM-5@CoM not only could yield a high heat sink of 4.64 MJ/kg at 758 °C but also effectively suppressed the coke deposition on the external surface of catalysts and the inner wall of the pipe at elevated temperatures. Consequently, the present work opens a new avenue for the development of low-cost, large-scale, facile yet effective catalysts for catalytic cracking of endothermic hydrocarbon fuel with promising heat sink and anticoking properties.

ORCID

Yunfeng Qiu: 0000-0002-0163-4908 Weixing Zhou: 0000-0002-4138-7657 Author Contributions ∥

These authors contributed equally to this work.

Author Contributions

W.Z. and Y.Q. conceived the experiments. L.L. and Z.L. designed and conducted the preparation, salinization, characterizations, and experiments. Z.L. analyzed the results. Z.H. made some preparation of catalysts. Y.Q. and L.L. analyzed the results. Y.Q. and L.L. wrote the manuscript. All the authors reviewed and approved the manuscript. L.L. and Z.L. made equal contributions to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (91741204 and U1432130), Key Laboratory of Microsystems and Microstructures Manufacturing of Ministry of Education, Harbin Institute of Technology (No. 2015KM006), and Heilongjiang Postdoctoral Foundation. H


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Co3O4 Nanosheet Wrapped Commercial HZSM-5 for Promoting

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