Molecular Simulation of the Catalytic Cracking of Hexadecane on ZSM

Aug 30, 2017 - ... of the Catalytic Cracking of Hexadecane on ZSM-5 Catalysts Based on Reactive Force Field (ReaxFF) ... *Telephone: +86 21 64253027...
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Molecular Simulation of the Catalytic Cracking of Hexadecane on ZSM‑5 Catalysts Based on Reactive Force Field (ReaxFF) Zhuojun Chen, Peng Zhao, Ling Zhao, and Weizhen Sun* State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China S Supporting Information *

ABSTRACT: Fluid catalytic cracking (FCC) is one of the most dominant processes for heavy feedstock conversion. By using ReaxFF dynamic simulations, the catalytic pyrolysis of hexadecane was investigated with the presence of ZSM-5, hydrated ZSM5, and hydrated Al/ZSM-5 catalysts under high temperatures. Multimolecular simulation results showed that the hydrated ZSM-5 catalyst has good catalytic reactivity at higher temperatures, and the surface hydroxyl group could promote the yield of ethylene. The hydrated Al/ZSM-5 catalyst was more suitable for the production of small molecules under lower temperatures, and the introduction of aluminum would increase the yield of C3∼C4 and prevent the formation of C−O bonds. The unimolecular simulations confirmed that the introduction of aluminum in the hydrated Al/ZSM-5 catalyst would be beneficial to the dehydrogenation of reactant molecules. Thermal stability simulations of catalysts revealed that the introduction of aluminum into the ZSM-5 catalyst could stabilize the Si−O structure and inhibit the formation of a C−O bond.

1. INTRODUCTION With the rapid development of technology and sharp increase in global population, world energy is facing new challenges. According to reports, fossil fuels occupied 81% of primary sources in 2010, and the percentage will reach 75% in 2035.1 Hence, fossil fuel would still be the main source for future decades. Compared to thermal cracking, fluid catalytic cracking (FCC), the major treatment for heavy feedstocks, could effectively convert heavy compounds to more valuable products, like gaseous fuel, naphtha, etc.,2 which occupies a leading position in petrochemical engineering.3−6 Great efforts have been made with the catalysts used in FCC processes.7−11 Early in 1915, an aluminum chloride catalyst was developed for FCC, and then an activated clay catalyst and silica/aluminum catalyst were produced subsequently. In 1945, the first commercial FCC catalyst was developed by Davision Company. Several decades later, there had been various catalysts, such as zeolite Y, USY, REUSY, and others, which had been modified in pore structure and grain size. New technologies like Ni-passivation and SOx reduction were also matured.12,13 Among all kinds of catalyst matrixes, ZSM-5 zeolite catalyst, which has been industrialized, is the most used catalyst in the petroleum refining industry14 because of its special porous channel structure, high selectivity, and thermal stability. Over the past several decades, ZSM-5 zeolites have aroused pervasive experimental research in petrochemical industries.15−17 Park and Ihm18 explored the properties of platinum-containing bifunctional ZSM-5, ZSM-22, SAPO-11, Al-MCM-41, H-Y, and H-β, showing that the Pt/ZSM-5, Pt/ ZSM-22, and Pt/H-β catalysts exhibited higher hydrocracking activity due to their strong acid sites, while others performed better in isomerization selectivity. Marcilla et al.19 studied the pyrolysis of polypropylene using ZSM-5, revealing that the ZSM-5 catalyst could decrease the temperature of the maximum decomposition rate. López et al.20 carried out the © XXXX American Chemical Society

pyrolysis of a plastic mixture in a semibatch reactor at 713 K to explore the regeneration and reuse of ZSM-5 zeolite, showing that the deactivation was reversible after heating up to 823 K in an oxygen atmosphere and could recover its initial activity, which was similar to that of a fresh catalyst. Fanchiang and Lin21 studied the catalytic pyrolysis of furfural in a continuous fixed bed system using a ZSM-5 catalyst, finding that low temperatures favored the formation of PAHs and oxygenated coke, whereas high temperatures led to monoring aromatics, light gases, and graphite-like coke. Inagaki et al.22 improved the catalytic performance of ZSM-5 zeolite through bead-milling, subsequent recrystallization, and acid treatment without diminution of their acidity. Additionally, ZSM-5 zeolite displayed good catalytic ability on coal cracking,23,24 fast pyrolysis of bio-oil,25,26 and even biomass.27−30 As shown in the above-mentioned examples, the experimental approach proved to be a more straightforward way to investigate catalytic cracking reactions in many aspects. However, the experimental methods are not enough to explain the reaction pathways and mechanism at the atomic level, which are significant for catalytic cracking processes. Additionally, the computational chemistry could provide much complementary information on reaction pathways and mechanisms on the atomic level. Modern quantum chemistry (QC) could accurately describe energies, vibrations, and geometries for small molecules, but it is not practical for researching the dynamics of larger systems. Generic force fields like DREIDING31 and UFF32 could predict properties of compounds especially coupled with charge equilibration33 but cannot explore the formation and dissociation of bonds. An improved force field was Brenner potential,34 which was Received: May 29, 2017 Revised: August 28, 2017 Published: August 30, 2017 A

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Figure 1. Initial structure of ZSM-5 catalyst cluster from (a) front view, (b) left view, (c) top view.

capable of describing chemical reactivity, whereas it did not include the necessary nonbond interactions, i.e., Coulomb and van der Waals. ReaxFF (reactive force field) is a new computational method developed by van Duin et al.35 Compared to all these methods, ReaxFF shows excellent predictive capability, including energies and geometries, as well as bond formation and cleavage for larger systems, and provides similar accuracy to quantum chemistry. On the basis of these advantages, ReaxFF has been widely used in extensive fields such as high-energy materials, 36−38 hydrocarbon compounds,39−41 coal,42,43 and other organic compounds44,45 since it was developed. Castro-Marcano and van Duin46 researched the catalytic cracking of 1-heptene using ReaxFF, finding that thermal cracking of 1-heptene was predominantly initiated by C−C cleavage while catalytic cracking mainly started with C−C dissociation, protonation, and dehydrogenation. In this work, three kinds of catalyst models were built, i.e., ZSM-5, hydrated ZSM-5, and hydrated Al/ZSM-5, whose catalytic performances on cracking of hexadecane were simulated and compared with thermal cracking, in order to investigate the effect of the surface hydroxyl group and the introduction of aluminum. Hexadecane, the surrogate of diesel, was studied because of its wide existence in heavy oil and low cost, and the relative molecular mass of hexadecane is similar to that of diesel. Many researchers conducted experiments to study the pyrolysis process of hexadecane,47−49 but the information regarding pathways and the mechanism on an atomic level still need to be explored. We carried out unimolecular and multimolecular simulations to investigate initial mechanisms, production distribution, thermal deactivation of catalysts, and the effect of introduction of aluminum.

Table 1. Geometry Information of ZSM-5, Hydrated ZSM-5, and Hydrated Al/ZSM-5 Catalyst Models property rSi−O (Å) rO−H (Å) θSi−O−Si (deg) θO−Si−O (deg) θSi−O−H (deg)

ZSM-5 (Si96O208)

hydrated ZSM-5 (H16Si96O208)

hydrated Al/ZSM-5 (Al4H16Si92O208)

1.64−1.65 145

1.64−1.65 0.95−0.96 148

1.64−1.65 0.95−0.96 155

109

110

109

124

117

ZSM-5, hydrated ZSM-5, and hydrated Al/ZSM-5, respectively, resulting in boxes with a length of 45 Å under periodic boundary conditions. A noncatalytic simulation box containing 20 hexadecane molecules was also constructed for comparison. These systems first underwent an energy minimization and then equilibrated at 100 K for 10 ps with a time step of 0.1 fs suggested by previous work.39,44,46 After that, the temperatures of systems were increased to 1500 K, 1750 K, and 2000 K from 100 K with a heating rate of 50 K/ps. Finally, a Berendsen thermostat was employed to equilibrate the temperatures of systems at 1500, 1750, and 2000 K for 2 ns (the range of investigated temperatures was determined according to our previous work50). All these procedures were controlled by the NVT ensemble. Information such as temperature, pressure, all kinds of energies, molecular species, intermediates, trajectories, etc. were collected. In order to explore the mechanisms of catalytic cracking of hexadecane, unimolecular simulations were carried on three catalytic systems: the ZSM-5 catalyst, hydrated ZSM-5 catalyst, and hydrated Al/ZSM-5 catalyst. One hexadecane molecule and catalyst cluster was randomly put into a periodic boundary box and then underwent an energy minimization, followed by a low temperature NVT simulation at 100 K for 5 ps to prevent reactions from occurring. Next, the system was heated up to 2000 K with a heating rate of 50 K/ps and kept at 2000 K for 2 ns. The time step was 0.1 fs. Temperature was controlled using a Berendsen thermostat in all the procedures. By analyzing the trajectory files, catalytic pyrolysis mechanisms could be obtained. The thermal stability of three catalysts was also investigated. Cubic boxes containing different catalysts were built with a length of 45 Å. All the systems first underwent a process of energy minimization, after which a low-temperature equilibrium stage was applied in all systems for a short time of 10 ps. Next, systems were heated to 2000 K with a heating rate of 50 K/ps using an NVT ensemble. Finally, all the systems were equilibrated at target temperatures for 2 ns. These simulations were all done with a time step of 0.1 fs and temperature damping constant of 100 fs. ReaxFF simulations were conducted through the “reax” package of LAMMPS,51 using C/H/O/Si/Al ReaxFF parameters. These optimized parameters were validated and successfully applied in previous work.52

2. SIMULATION DETAILS 2.1. Construction and Thermal Stability of Catalysts. The ZSM-5 (Si96O208) catalyst cluster model was initially constructed in which five-membered, six-membered, and 10-membered rings could be seen from left view and top view in Figure 1. For the hydrated ZSM-5 (H16Si96O208) catalyst, 16 hydrogen atoms were introduced on oxygen atoms on the basis of the ZSM-5 catalyst, generating 16 silanol groups on the surface of the catalyst. Furthermore, the hydrated Al/ZSM-5 (Al4H16Si92O208) cluster model was built by replacing four superficial silicon atoms with four aluminum atoms. To ensure the accuracy of geometric structures of catalysts, the bond length, i.e., rSi−O and rO−H, and angle, i.e., θSi−O−Si and θSi−O−H, were measured and compared with previous work (shown in Table 1). The construction of catalyst models was done with Material Studio software (Accelrys Inc.) 2.2. Computational Methods. To study the catalytic decomposition of hexadecane, three cubic simulation boxes were initially built containing 20 random hexadecane molecules for all and the cluster of B

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3. RESULTS AND DISCUSSION 3.1. Evaluation of Catalysts Models. Geometry information on ZSM-5, hydrated ZSM-5, and hydrated Al/ZSM-5 catalyst models generated in this work is shown in Table 1, including bond lengths of Si−O and O−H and angle distributions of Si−O−Si, O−Si−O, and Si−O−H. For Si−O bond length, all the catalysts show the same results, i.e., 1.64− 1.65 Å, which is consistent with the computational research of microporous silica and aluminum silica materials by Sokol et al.53 The O−H bond lengths in hydrated ZSM-5 and hydrated Al/ZSM-5 catalyst models both peak at 0.95−0.96 Å, which is the same as the 0.95 Å O−H bond length in the study of the molecular structure of hydroxyls in silica and zeolites by Sauer.54 Different from the consistency of bond length distribution, the angle distributions slightly change. The Si− O−Si angle peaks at 145° in the ZSM-5 catalyst model and reaches a maximum value of 148° in the hydrated ZSM-5 catalyst model, and these data are in good accordance with previous work on computational techniques and molecular models for ab initio calculations of solid silicates by Teppen et al.55 In the hydrated Al/ZSM-5 catalyst model, the Si−O−Si angle peaks at 155°. The O−Si−O angle centers at 109−110° in three systems and is in good agreement with the 110° of the O−Si−O angle in the ReaxFF molecular simulation of hyperthermal silicon oxidation by Khalilov et al.56 and ReaxFF molecular dynamic simulation of the structure of silica by Chowdhury et al.57 The Si−O−H angle reaches a maximum proportion at 124° in hydrated ZSM-5 and 117° in the hydrated Al/ZSM-5 model, which is very close to the result 121° of Si−O−H in the molecular dynamics research of silanols by Kobayashi et al.58 The geometry information obtained in this work could also be validated by other similar previous works.59−61 3.2. Main Product Distribution. Multimolecular simulations were conducted under temperatures of 1500, 1750, and 2000 K to investigate the main product distribution. The extra simulations have been conducted to validate the repeatability of simulations. Figure 2 shows the snapshots of simulation boxes in a noncatalytic system (a and b), ZSM-5 system (c and d), hydrated ZSM-5 system (e and f), and hydrated Al/ZSM-5 system (g and i) at 0 ps and 2 ns, as well as a snapshot of the hydrated Al/ZSM-5 system at 8 ps. It can be seen that small molecules such as C2, C3, etc. were produced at 2 ns, and some larger molecules remained in a noncatalytic system rather than catalytic systems. Additionally, the hydrated Al/ZSM-5 catalyst showed strong reactivity among all the catalysts, and it was shown in part h that many hexadecane molecules were adsorbed on the surface of the catalyst at 8 ps. This phenomenon was not observed in other catalytic systems. Also, there were more oxides generated in ZSM-5 and the hydrated ZSM-5 system, shown in parts d and f, which will be discussed in section 3.4. To validate the ReaxFF simulation results, the products with low molecular weight detected in our work were compared with previous experimental results, presented in Table 2. It can be seen that the products observed in our work are in good accordance with previous experiments for the catalytic system or noncatalytic system. The time evolution of the main products/species distribution including H2, CH2, C2H4, C3H6, and C2H3 is shown in Figure 3, indicating that ethylene was the main product. In noncatalytic systems, no reaction occurred at 1500 K according to part a, since the number of reactant

Figure 2. Snapshots of noncatalytic, ZSM-5, hydrated ZSM-5, hydrated Al/ZSM-5 systems at 0 ps and 2 ns, 2000 K (light yellow, oxygen; green, silicon; magenta, carbon; blue, hydrogen; orange, aluminum).

Table 2. Comparison of Low Molecular Weight Products from Experiments and ReaxFF Simulations products

ref 62a

this workb

ref 48c

this workd

H2 CH4 C2H4 C2H6 C3H6 C3H8 C4H8 C4H10 C5H10 C5H12 C6H12 C6H14

√ √ √ √ √ √ √ √ √ √ √

√ √ √ √ √ √ √ √ √ √ √ √

√ √ √ √ √ √ √ √ √ √ √ √

√ √ √ √ √ √ √ √ √ √ √ √

a Experiment: T = 645 K, silica−alumina catalyst, in helium atmosphere. bReaxFF: T = 1750 K, hydrated Al/ZSM-5 catalyst, vacuum. cExperiment: T = 648 K, noncatalytic, in argon atmosphere. d ReaxFF: T = 2000 K, noncatalytic, vacuum.

molecules remained constant during the total simulation time. This could be attributed to the lower cracking temperature and absence of a catalyst. Different from 1500 K, some reactions occurred after 1375 ps at 1750 K in a noncatalytic system, and small species like C2H4 and C2H3 were produced. Finally, there still remained about 10 reactant molecules. Part c indicates that cracking reactions took place from approximately 100 ps and dramatically proceeded at 2000 K, evidenced by the number of ethylenes, whose number reached almost 60 while the number of other species also increased to different extents. The final number of hexadecane molecules fluctuated around five. When treated with catalysts, more reactions and products were observed. For temperature-dependent analysis, according to parts d∼f, g∼i, and j∼l, it can be seen that there were fewer reactant molecules and more small species existing at higher C

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Figure 3. Time evolution of main product distribution at (a) 1500 K of noncatalytic system, (b) 1750 K of noncatalytic system, (c) 2000 K of noncatalytic system, (d) 1500 K of ZSM-5 system, (e) 1750 K of ZSM-5 system, (f) 2000 K of ZSM-5 system, (g) 1500 K of hydrated ZSM-5 system, (h) 1750 K of hydrated ZSM-5 system, (i) 2000 K of hydrated ZSM-5 system, (j) 1500 K of hydrated Al/ZSM-5 system, (k) 1750 K of hydrated Al/ZSM-5 system, (l) 2000 K of hydrated Al/ZSM-5 system.

molecules were equilibrated around 10 at 1500 K and 20 at 1750 K, which were higher than other catalysts. It can be deduced that aluminum may be more effective at lower temperatures. When at 2000 K, ZSM-5 and hydrated ZSM-5 showed better catalytic reactivity on ethylene production. Parts c, f, i, and l show us the product distribution at 2000 K. From the comparison of the number of ethylenes at 750 ps (shown as a green dashed line), it can be seen that there about 25 ethylene molecules generated in the noncatalytic system; nevertheless, about 45 ethylenes were produced in ZSM-5 and the hydrated ZSM-5 system, and about 35 in the hydrated Al/ZSM-5 system, revealing their good catalytic capability on accelerating the rate of reactions, which is in good agreement with Castro-Marcano and van Duin’s work.46 However, the hydrated Al/ZSM-5 may be not applicative for the production of ethylene at high

temperatures corresponding to earlier initial times. The number of ethylene molecules peaked in the ZSM-5 and hydrated ZSM5 catalytic system at 2000 K, whose number reached nearly 70, and the number of hexadecane molecules decreased to zero in hydrated ZSM-5 and the hydrated Al/ZSM-5 system at 2000 K. This demonstrates that temperature plays an important role in the pyrolysis of hexadecane. (The quantitative relationship between the number of surface hydroxyls and the number of ethylene is shown in Figure S1 of the Supporting Information.) For catalyst-dependent analysis, shown in parts d∼j, e∼k, and f∼ l, during the initial time the relative reactivity of these three catalysts decreases in the following sequence: hydrated Al/ ZSM-5 > ZSM-5 > hydrated ZSM-5. In particular, the good catalytic ability of the hydrated Al/ZSM-5 catalyst was achieved at lower temperatures, and it can be seen that ethylene D

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increased first and then decreased. The possible reason is the smaller secondary reaction rate at lower temperatures, resulting in more C7−C15 species being produced, while these species cracked into smaller ones at higher temperatures of 2000 K. In addition, the noncatalytic system revealed the fewest products and the most reactants regardless of temperature. For sort C1− C2, the hydrated Al/ZSM-5 catalyst showed good catalytic performance at lower temperatures of 1500 and 1750 K, while the hydrated ZSM-5 catalyst gave the best catalytic capability at 2000 K. For sort C3−C4, the hydrated Al/ZSM-5 catalyst could obviously promote its yield. (The quantitative relationship between the number of aluminum and the number of C3−C4 is displayed in Figure S2 of the Supporting Information.) The hydrated Al/ZSM-5 catalyst also increased the number of methane (CH4) from two for the ZSM-5 and hydrated ZSM-5 systems to 11. Particularly, compared to products obtained in ZSM-5 and hydrated ZSM-5 systems, there were hardly any oxides generated in the hydrated Al/ZSM-5 system, which will be discussed in section 3.4. 3.3. Initial Mechanisms. Unimolecular simulations of the ZSM-5 system, hydrated ZSM-5 system, and hydrated Al/ZSM5 system were carried out to investigate the initial mechanisms of catalytic decomposition of hexadecane molecules. We put one hexadecane molecule and one catalyst cluster into a periodic boundary simulation box with a length of 35 Å. The simulation box underwent an energy minimization and equilibrated at 100 K for 10 ps, then was heated to 2000 K. In order to study and compare the initial catalytic pyrolysis mechanism of hexadecane, we repeated the former 100 ps of the unimolecular simulation of each system 10 times and found that 100% of the reactant molecules in the hydrated Al/ZSM-5 system reacted through dehydrogenation within 20 ps, while almost no reactions occurred in hydrated ZSM-5 system. In the ZSM-5 system, 70% of the reactants were initiated by

temperatures. In conclusion, for ethylene production, ZSM-5 and hydrated ZSM-5 catalysts may be useable at higher temperatures, while ther hydrated Al/ZSM-5 catalyst could be applicative at lower temperatures. In order to further investigate the detailed composition of products, species were categorized into five groups, C1−C2, C3−C4, C5−C6, C7−C15, and C16, presented in Figure 4.

Figure 4. Distribution of Cn at the end of cracking simulation in noncatalytic, ZSM-5, hydrated ZSM-5, and Al-HZSM-5 systems.

Different filling patterns represent different kinds of products, and the colors green, red, blue, and orange represent noncatalytic, ZSM-5 catalyst, hydrated ZSM-5 catalyst, and hydrated Al/ZSM-5 systems, respectively. The number of reactant molecules decreased with the rising temperature, and there were fewer hexadecane molecules remaining in catalytic systems than in the noncatalytic system. For the amount of each product category, sort C1−C2 and sort C3−C4 showed a similar trend, whose number was increasing with the ascending temperature, although the number of category C7 −C 15

Table 3. Detailed Mechanism of Unimolecular Catalytic Cracking of Hexadecane within 1000 ps at 2000 K on ZSM-5 Catalyst

E

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Figure 5. Snapshots in unimolecular simulation of the hydrated ZSM-5 system at (a) 68 ps, (b) 79 ps, (c) 91 ps (light yellow, oxygen; green, silicon; magenta, carbon; blue, hydrogen; red, the carbon atom which lost a hydrogen atom at 68 ps).

observed in other catalytic systems. Unsaturated oxygen of the ZSM-5 catalyst could be the possible reason for the high yield of carbon dioxide, whereas in hydrated ZSM-5 and hydrated Al/ZSM-5 systems, there were not so many unsaturated oxygen atoms. For the hydrated ZSM-5 system, there were fewer reactions occurring. Figure 5 shows the snapshots of the hydrated ZSM-5 system at 68 ps and 91 ps, corresponding to three main reactions. The light yellow, green, magenta, and blue represent oxygen, silicon, carbon, and hydrogen, respectively. First, a hydrogen atom on the 8-C atom marked in red (with arrow) was abstracted by the catalyst, and the 8-C was connected to the catalyst at 79 ps. At 91 ps, a •CH3 group was formed through decomposition and bonded with the catalyst. In the unimolecular system of the hydrated Al/ZSM-5 catalyst, reactions were much different from the abovementioned two catalysts. The reaction pathways of unimolecular simulation of the hydrated Al/ZSM-5 catalytic system was shown in Figure 6. The carbon atoms that lost the hydrogen atom are marked in red, and the position was defined as 1 to 16 corresponding to atoms from left to right. The first reaction occurred at 14 ps when a hydrogen atom dropped from the 5-C atom, which produced a hexadecyl. Then, the 4-C atoms and 10-C atom lost hydrogen atoms

dehydrogenation; the other reactants remained unreacted. Therefore, the initial reactivity obtained in our unimolecular simulation decreases as follows: hydrated Al/ZSM-5 > ZSM-5 > hydrated ZSM-5, which is in good agreement with our multimolecular simulation results. To investigate the detailed mechanism, we prolonged the simulation time to about 1 ns for each system. Table 3 presents the pathways of unimolecular cracking of hexadecane on the ZSM-5 catalyst with a function of time, which was observed in one simulation. The capital R represents the original catalyst cluster, and R1and R2 through R13 represent the catalyst clusters, which adsorbed different kinds of atoms or groups. The 10 ps of low temperature simulation at 100 K was ignored in Table 3 because no reactions occurred during this stage, and hence the simulation time started from the heating stage. The unimolecular simulation started from the dehydrogenation of the hexadecane molecule occurring at 72 ps, in which the hydrogen atom on the 4-C atom was abstracted by the catalyst, producing a hexadecyl. Here, we defined that the 1-C atom was the carbon atom that directly bonded with the catalyst, and the neighbor carbon was the 2-C atom and so on. The first C−C bond dissociation was observed at 139 ps, when the hexadecyl radical cracked into a pentene molecule and an undecyl radical. And then, the undecyl radical was adsorbed on the catalyst cluster at 170 ps. During 389−394 ps, the pentene molecule lost two hydrogen atoms in succession and produced the •C5H8 group, which bonded with the catalyst after collision. Almost at the same time, the 4-C atom of the •C5H8 group formed a single bond with the catalyst. From 400 to 438 ps, a methyl radical was dropped from the •C5H8 structure and combined with hydroxyl, producing a methanol molecule, which decomposed into formaldehyde and hydrogen at 438 ps. At 478 ps, the catalyst cluster released an oxygen molecule. Afterward, the oxygen atom which bonded with the 4-C atom decomposed from the catalyst cluster and produced the C4H5O structure. At 529 ps, the 10-C atom of the •C11H23 group lost a hydrogen atom, and at 534 ps, the 10-C atom bonded with the catalyst. During the period of 810 to 812 ps, the hydrogen molecule cracked into two hydrogen radicals, of which one combined with oxygen and finally converted to a water molecule, and the other was seized by the catalyst cluster. At 864 ps, a CHO2 group was generated and dissociated from the catalyst at 894 ps. It finally lost a hydrogen atom and produced a dioxide molecule. Simulations during 864 and 1041 ps displayed the production process of carbon dioxide, in which a carbon atom bonded with two oxygen atoms in succession and finally lost the hydrogen atom. This process has not been

Figure 6. Unimolecular reaction pathways of hydrated Al/ZSM-5 system at 2000 K (magenta, carbon; blue, hydrogen; red, the carbon atom which lost hydrogen atoms). F

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aldehyde (CH2O) and acetaldehyde (C2H4O). Other oxides such as acrolein (C3H4O), propanol (C3H6O), and some radicals were also formed. However, oxide products were quite different in hydrated Al/ZSM-5 system. There were only one methanol molecule produced at 1750 K, otherwise neither alcohols nor aldehydes were generated, except oxygen radical, hydroxyl, and water molecules. This indicated that the existence of aluminum could stabilize the Si−O structure and inhibit the formation of C−O bonds for its high thermal stability, which has been proved by Castro-Marcano et al.46 To further validate the properties of hydrated Al/ZSM-5, which was discussed above, hexadecane molecules were removed to eliminate the effect of reactant molecules. We put the catalyst into a periodic boundary simulation box and explored the thermal stability of these three catalysts. The catalysts first underwent an energy minimization and then equilibrated at 100 K for 10 ps, after which the system was heated up to 2000 K at a rate of 50 K/ps. Finally, the system was maintained at 2000 K. Figure 7 shows us the snapshots of ZSM-5, hydrated ZSM-5, and hydrated Al/ZSM-5 catalyst systems at the end of simulations. Shown in Figure 7, there were two oxygen molecules in the ZSM-5 system and three water molecules in the hydrated ZSM-5 system. On the contrary, nothing was released from the hydrated Al/ZSM-5 catalyst cluster. The differences between the three systems exhibit the highest thermal stability of the hydrated Al/ZSM-5 catalyst, which is in accordance with previous work.46 The possible explanation is the higher melting point caused by the presence of aluminum, which makes the catalyst structure more stable. The high thermal stability of the hydrated Al/ZSM-5 catalyst could reasonably be interpreted from the oxide analysis of multimolecular simulations, in which the hydrated Al/ZSM-5 system produced almost no alcohols or aldehydes. Bond dissociation energy was also calculated to verify the effect of the introduction of Al atoms on catalyst structures. The simplified structures were constructed to calculate the bond dissociation energy of the Si−O bond at different positions. Shown in Table 5, it can be concluded that the introduction of Al atoms could increase the bond dissociation energy of the Si−O bond, especially the adjacent Si−O bond, which further indicates that the introduction of Al atoms could stabilize the Si−O bond.

simultaneously at 17 ps, generating an ethylenic radical. An ethylenic biradical was formed after the hydrogen of the 3-C atom was seized by the oxygen atom of the catalyst cluster at 19 ps, and the hydrogen atom on the 15-C atom was also captured 1 ps later. At 21 ps, the 14-C atom lost a hydrogen atom, which was followed by the loss of hydrogen on the 16-C atom at 24 ps. From 24 to 32 ps, the 16-C atom lost three hydrogen atoms in succession. At 41 ps, the 1-C atom lost a hydrogen, and the rest cracked into two C8 structures at 42 ps, which was the first C−C bond cleavage reaction observed in the unimolecular simulation of the hydrated Al/ZSM-5 system. At the same time, the 7-C atom lost a hydrogen atom. Both C8 species dissociated into C3 and C5, during which two hydrogen transfer reactions occurred at 45 and 68 ps (marked with green arc arrows). Among all the unimolecular pathways, the dehydrogenation reactions took place frequently, which occupied 85.7% of all the reactions, except for three C−C bond dissociation reactions at 42, 64, and 68 ps, respectively. At the end of the unimolecular simulation, C5H8, C3H2, C3H5, and C5H4 were obtained. These products were in good agreement with the product distribution of multimolecular simulations, in which a large number of sort C3−C4 products were generated. 3.4. Thermal Stability of Catalysts. In the catalytic systems, some oxides such as alcohols, aldehydes, and water molecules were generated due to the high temperature and long simulation time, which demonstrated the thermal deactivation of catalysts. Table 4 presents the type and the number of oxides at the end of simulations of the noncatalytic system and catalytic systems at 1500, 1750, and 2000 K. Oxides generated in ZSM-5 and hydrated ZSM-5 catalytic systems were quite similar including alcohols such as methanol (CH4O), ethanol (C2H6O), propanol (C3H8O), and aldehydes such as formTable 4. Summary of Oxide Products at the End of Hexadecane Catalytic Cracking Simulations system temperature/K 1500

1750

2000

ZSM-5 1 1 1 1 1 1 2 1 1 1 1 2 1 1

C3H8O CH4O HO C11H24O C2H6O CH2O CH2O C3H6O C4H9O C2H5O C2H2O H2O C2H3O C3H4O

1 1 4 1 2 1 1 1 1

CH4O CO CH2O CHO H2O CH3O C3H6O C8H18O C2H6O

hydrated ZSM-5 1 1 1 1 1

C5H12O O C10H21O C16H33O H2O

1 7 1 1 1 1 1 2 1 1 6 3 3 1 1 1 1 1

C11H24O H2O C12H26O C2H2O C7H14O CH4O HO O C4H7O C4H6O H2O CH4O CH4O CH2O C2H4O CH3O HO C2H5O

hydrated Al/ZSM-5 2O 1 HO 1 H2O

4. CONCLUSIONS The ReaxFF force field was applied to investigate the catalytic cracking of hexadecane using ZSM-5, hydrated ZSM-5, and hydrated Al/ZSM-5 catalysts. In multimolecular simulations, the catalytic systems exhibited higher catalytic reactivity than the noncatalytic one. The hydrated Al/ZSM-5 showed good reactivity at lower temperatures. The hydrated ZSM-5 catalyst was more applicable at higher temperatures, indicating that surface hydroxyl could promote the yield of light products, i.e., ethylene. Additionally, the introduction of aluminum could increase the yield of C3∼C4. The unimolecular simulation results proved that the hydrated Al/ZSM-5 showed the highest reactivity. The introduction of aluminum would abstract hydrogen atoms from reactant molecules. The thermal stability simulation indicated that the hydrated Al/ZSM-5 catalyst could stabilize the Si−O structure and inhibit the formation of C−O bonds. The simulation results obtained in this work were in good agreement with previous experimental results, which validates that the ReaxFF force field could be used in catalytic pyrolysis and oxidation processes of hydrocarbons.

6 H2O 1 HO 1 CH4O

16H2O 7 HO

G

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Figure 7. Snapshots of (a) ZSM-5, (b) hydrated ZSM-5, (c) hydrated Al/ZSM-5 systems at the end of simulations at 2000 K (light yellow, oxygen; green, silicon; blue, hydrogen; orange, aluminum). (2) Cerqueira, H. S.; Caeiro, G.; Costa, L.; Ramôa Ribeiro, F. Deactivation of FCC catalysts. J. Mol. Catal. A: Chem. 2008, 292 (1− 2), 1−13. (3) Sadrameli, S. M. Thermal/catalytic cracking of liquid hydrocarbons for the production of olefins: A state-of-the-art review II: Catalytic cracking review. Fuel 2016, 173, 285−297. (4) Vazquez, S. A.; Martinez-Nunez, E. HCN elimination from vinyl cyanide: product energy partitioning, the role of hydrogen-deuterium exchange reactions and a new pathway. Phys. Chem. Chem. Phys. 2015, 17 (10), 6948−6955. (5) Szymanski, J. K.; Temprano-Coleto, F.; Perez-Mercader, J. Unusual kinetics of poly(ethylene glycol) oxidation with cerium(IV) ions in sulfuric acid medium and implications for copolymer synthesis. Phys. Chem. Chem. Phys. 2015, 17 (10), 6713−6717. (6) Chen, Z.; FitzGerald, P. A.; Warr, G. G.; Atkin, R. Conformation of poly(ethylene oxide) dissolved in the solvate ionic liquid Li(G4) TFSI. Phys. Chem. Chem. Phys. 2015, 17 (22), 14872−14878. (7) Anil Kumar, V.; Pant, K. K.; Kunzru, D. Potassium-containing calcium aluminate catalysts for pyrolysis of n-heptane. Appl. Catal., A 1997, 162 (1), 193−200. (8) Lemonidou, A. A.; Vasalos, I. A.; Hirschberg, E. J.; Bertolacini, R. J. Catalyst Evaluation and Kinetic Study for Etylene Production. Ind. Eng. Chem. Res. 1989, 28, 524−530. (9) Rahimi, N.; Karimzadeh, R. Catalytic cracking of hydrocarbons over modified ZSM-5 zeolites to produce light olefins: A review. Appl. Catal., A 2011, 398 (1−2), 1−17. (10) Guo, K.; Li, H.; Yu, Z. In-situ heavy and extra-heavy oil recovery: A review. Fuel 2016, 185, 886−902. (11) Mohanty, S.; Kunzru, D.; Saraf, D. N. Hydrocracking: a review. Fuel 1990, 69 (12), 1467−1473. (12) Scherzer, J. Octane-enhancing zeolitic FCC catalyst. Catal. Rev.: Sci. Eng. 1989, 31 (3), 215−354. (13) Harding, R. H.; Peters, A. W.; Nee, J. R. D. New developments in FCC catalyst technology. Appl. Catal., A 2001, 221 (1−2), 389− 396. (14) Primo, A.; Garcia, H. Zeolites as catalysts in oil refining. Chem. Soc. Rev. 2014, 43 (22), 7548−7561. (15) Abbot, J.; Wojciechowski, B. The mechanism of catalytic cracking of n-alkenes on ZSM-5 zeolite. Can. J. Chem. Eng. 1985, 63 (3), 462−469. (16) Zhu, X.; Liu, S.; Song, Y.; Xu, L. Butene catalytic cracking to propene and ethene over potassium modified ZSM-5 catalysts. Catal. Lett. 2005, 103 (3−4), 201−210. (17) Liu, C.; Deng, Y.; Pan, Y.; Gu, Y.; Qiao, B.; Gao, X. Effect of ZSM-5 on the aromatization performance in cracking catalyst. J. Mol. Catal. A: Chem. 2004, 215 (1), 195−199. (18) Park, K. C.; Ihm, S. K. Comparison of Pt/zeolite catalysts for nhexadecane hydroisomerization. Appl. Catal., A 2000, 203 (2), 201− 209. (19) Marcilla, A.; Gomez, A.; Reyes-Labarta, J.; Giner, A.; Hernández, F. Kinetic study of polypropylene pyrolysis using ZSM-5 and an

Table 5. Bond Dissociation Energy of Si−O Bond in Simplified Catalyst Structures (green, Si; pink, Al; yellow, O; blue, H)

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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/acs.energyfuels.7b01519. The quantitative relationship between the number of surface hydroxyls and the number of ethylenes as well as the relationship between the number of aluminums and the number of C3−C4 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86 21 64253027. E-mail: [email protected]. ORCID

Weizhen Sun: 0000-0002-9957-3620 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support by the National Natural Science Foundation of China (91434108), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, and the Shanghai Excellent Technical Leaders Program (14xd1425500) is gratefully acknowledged. The authors also would like to acknowledge Professor Adri C. T. van Duin from Pennsylvania State University for providing force field parameters.

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