Modeling catalytic effect of zeolite on alkane cracking using MAPS software platform

Introduction

Alkane cracking is a process of crucial importance in petroleum industry that involves breaking C-C bonds in order to reduce the size of complex organic molecules (such as heavy hydrocarbons or asphaltenes) into simpler molecules (such as light hydrocarbons). The initial compounds directly extracted from oil fields need to be broken down into smaller fragments before it can be used as fuels (gasoline, jet fuel, or diesel fuel for example). While thermal cracking involves very high temperatures (500 – 900 °C) and pressures (7000 kPa), catalytic cracking uses eco-friendly zeolites and much less energy for alkane dissociation reaction. In the present case study we modeled the zeolite cluster and calculated the reaction profile of simple propane dissociation into ethylene and methane molecules using MAPS-NWChem module[1]. We focus here on the chemical transformation itself in order to understand better the origin of the zeolite catalytic effect during the reaction and the influence of different parameters (such as the temperature and pressure) on the catalytic rate of the reaction.
 
Method

All calculations were performed using MAPS-NWChem module. B3LYP[2,3] density functional was used together with Grimme dispersion correction[4]. The Def2-SVP[5] basis set was used. Structures from Swisher et al[6] were used to build the initial reactant and product states. A geometry optimization was performed followed by a frequency calculation to validate the presence of a stable state (no negative frequency).
 
The Transition States (TS) were obtained using the following procedure. In a first step the NEB method within MAPS-TSL module[7] was used to obtain a good initial TS guess. A frequency calculation was then performed and one imaginary eigenvalue was selected to set up an IRC calculation. At the end, a final frequency calculation was performed to validate the presence of the real transition state (only one negative frequency). During the reaction profile search process, a stable intermediate state was found. This state was optimized starting from slightly modified TS structures. For each state several different structures were generated in order to try to find the lowest reaction path profile. In the following we will only focus on the most stable structures
 
In order to better understand the origin of the zeolite catalytic effect, ESP charges were computed for all different structures. Finally MAPS Thermodynamic analysis tool was used to compute the evolution of the entropy and enthalpy correction with temperature and pressure.
 
Structures and energies 
 

Figure 1: Propane cracking reaction path free energies (in kJ.mol-1). The reaction, intermediate product and transition states structures are also reported.
Figure 1: Propane cracking reaction path free energies (in kJ.mol-1). The reaction, intermediate product and transition states structures are also reported.

The optimized reaction mechanism shown in Figure 1 is involving two different steps. The first step is a concerted reaction that allows the creation of a methane molecule through the transfer of the proton from the zeolite to one of the propane carbon atoms and the breaking of the corresponding C-C bond of the propane. The remaining unsaturated carbon atom is stabilized by the zeolite through the creation of a covalent bond with the deprotonated oxygen. In a second step, the C-O bond is broken to allow the creation of the ethylene molecule and the regeneration of the zeolite catalyst.
 
The analysis of the reaction energies show that the first step of the reaction is the rate limiting one (with the barrier of 246 kJ.mol-1) while the second barrier is about 100 kJ.mol-1 lower. These results are in good agreement with previous studies[2,4,5,8,9].
 
The catalytic effect of the zeolite appears therefore mainly due to the formation of a stable intermediate that induces a strong diminution of the reaction barrier: the more stable the intermediate, the lower the barrier. In order to improve the catalytic rate of a zeolite for such reaction, one would need to improve the proton transfer from the zeolite in the initial step.
 
Charge analysis

In order to understand better the nature of the catalytic effect of the zeolite on the cracking reaction, an analysis of the evolution of the atomic charges (ESP charges) during the initial step of the reaction was performed. It shows that a part of the catalytic effect of the zeolite comes from the acidification of the hydrogen atom transferred during the initial step.
 
When approaching the catalytic center, the propane molecule gets polarized by the field generated by the atomic charges of the zeolite cluster. The polarization of propane increases during the early stages of the first steps (C10 negative charge and C12 positive charge increase, see Figure 2). This allows a better interaction of C10 with the proton from the zeolite and C12 with the basic oxygen. Note that the charge on C12 at the intermediate state remains non negligible. This exposes the polarization of the C12-O14 bond and therefore the reactivity for the second step of the catalytic reaction. An improvement of the catalytic effect of the zeolite could therefore be possible through an increase of the hydrogen acidity. Indeed such procedure would increase the polarization of the O14-H4 bond, make it more reactive and therefore more willing to interact with the C-C bond.
 
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Figure 2: a- Evolution of the partial charges of Al13, O14, H4, C10 and C12 during the first step of the catalytic reaction. B- Representation of the different atoms label in the reactant state.
Figure 2: a- Evolution of the partial charges of Al13, O14, H4, C10 and C12 during the first step of the catalytic reaction. B- Representation of the different atoms label in the reactant state.

Influence of Temperature and Pressure

In order to analyze the influence of the temperature and pressure on the barrier free energy we used the MAPS-Thermodynamic tool to compute the evolution of the enthalpy and entropy with these two parameters for the two steps of the reaction. The results are shown in Figure 3. The reaction barrier for entropy and free energy appears to be independent of the pressure variation.
 
Similarly the enthalpy of the reaction barrier does not vary with temperature. On the contrary the barrier entropy, which is slightly negative, changes when temperature increases. This, in turn, induces a small increase of the barrier free energy. These results do not take into account the influence of temperature and pressure on the rate of the adsorption of propane in the zeolite which was shown in a previous study to be dependent on the temperature[4].
 

Figure 3: Evolution of enthalpy, entropy and free energy with temperature and of entropy and free energy with pressure
Figure 3: Evolution of enthalpy, entropy and free energy with temperature and of entropy and free energy with pressure

Conclusion
 
In this study we used MAPS interface to model the cracking reaction of propane in a zeolite. The structure and energy of the different states were optimized using MAPS-NWChem plugin, transition states were determined using the MAPS-TSL tool, and MAPS-Thermodynamic tool was used to compute the influence of both temperature and pressure on the different reaction energies. The results obtained show that the zeolite induces the formation of a stable intermediate from which a methane molecule is created and the C2H5 group is bonded to the zeolite. This intermediate appears very stable and therefore induces a lowering of the two barriers. The atomic partial charges show that the catalytic effect of the zeolite is due to the proton acidity that induces a polarization of the C-C bond that, in turn, stabilizes the creation of the intermediate state.
 
In this study, MAPS platform was used to build the system, simulate the reactions and analyze the structures, energies and properties. MAPS platform is a powerful tool to model, understand and predict molecular reactivity for complex systems which can be crucial in catalysis, oil & gas and chemical industries.
 
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