A basic kinetic model of the relationship between ethylene oligomerization and aromatization on ZSM-5 molecular sieve acidity and activity

Research Background
For complex petrochemical hydrocarbon conversion systems, reliable molecular scale and reaction mechanism reaction kinetic models are the key to molecular management. Unit steps (single-event kinetic), structure-oriented (Structure Oriented Lumping)
Dynamic methods such as models are being studied in depth. The unit step kinetic theory proposed by Professor Froment is considered to be an important development direction in this field because it is based on the detailed reaction mechanism network generated by the detailed digital representation of hydrocarbons. The kinetic model does not change with the composition of the reaction materials, but how to obtain a universal reaction kinetic model corresponding to different solid acid catalysts is a key scientific problem for studying the acidic activity structure-activity relationship and product distribution prediction of molecular sieve catalysts.
After adjusting the energy structure and the sources of petrochemical raw materials, ethylene oligomerization and aromatization are key steps in the production of high hydrocarbons from low-carbon olefins. Industrially, it is suitable for the production of olefins, aromatics and gasoline components using natural gas, coal-based methanol and other small molecules as raw materials. The acidity of the catalyst affects the ethylene oligomerization and aromatization product distribution and the reaction rate of each unit step of hydrocarbon interconversion.
Taking this complex reaction system as the research goal, and using ZSM-5 molecular sieve as the catalyst, a quantitative kinetic model of the relationship between acidity, reaction rate and product distribution is established, which can provide catalyst design, screen optimal reaction conditions, and maximize the target product. Theoretical support. In view of the complex reaction network, the kinetic theory of extended unit steps combined with Brönsted equation and linear free energy theory was studied to construct a quantitative structure-activity relationship between the acidity and activity of molecular sieve catalysts.
The basic steps and reaction network of olefin oligomerization and aromatization
According to the experimental results, methane, ethane, propane, propylene, butene, pentene, cyclopentene, hexene, heptene, benzene, cyclohexene, toluene, cycloheptene and xylene are included in the continuous parameter estimation Sex equation. Cycloolefins and aromatic hydrocarbons with the same carbon number are classified as a post-lumped GOI (Group Of Isomers).
As shown in the figure, the primary step level single-event kinetic model of ethylene oligomerization and aromatization is based on a detailed reaction network generated by a self-made calculation program algorithm using Boolean relationship matrix operations. The algorithm includes 10,669 basic steps, including alkyl groups. As the basic steps of rate control, de/protonation and isomerization as pseudo-equilibrium steps.

Calculation results
The kinetics of ethylene oligomerization and aromatization (OA) on ZSM-5 with different Si/Al ratios were simulated by applying the concept of unit step kinetics and Brönsted kinetic model to establish a quantitative acid activity relationship. On the basis of linear free energy theory, by introducing γ and δ as kinetic parameters to evaluate the sensitivity of OA reaction to acid strength, the activation energy of NH3 desorption is correlated with the activation energy of OA reaction and the heat of deprotonation.
The paper implements a hybrid genetic optimization algorithm to solve the kinetic model parameters, and the established model can efficiently and accurately fit the effects of three ZSM-5 molecular sieve catalysts with different silicon-to-aluminum ratios on the product distribution in the reaction network. In this paper, the intrinsic Brönsted-type kinetic parameters of different acid strength centers in the ZSM-5 catalysts with Si/Al ratios of 25, 60, and 70 are fitted, and the ZSM-5 molecular sieve with Si/Al ratio of 19 is returned to the ethylene glycol. The kinetic parameters of the unit steps in the polymerization and aromatization reactions. The model was used to predict the experimental yield and compared with the actual yield. It shows that the lumped kinetic model after reasonable product components can better fit the changes of experimental data over time. The regression kinetic model can predict the influence of molecular sieves with the same topological structure and different acidity on product distribution.


The figure shows that the predicted product yield is consistent with the experimental yield, and the error is less than 2.

The above figure shows a parity diagram comparing the experimental data of the product and the predicted data of the kinetic model.

in conclusion
Based on the carbocation mechanism, the linear free energy theory was used to establish a microscopic reaction kinetic model describing the sensitivity of the elementary step to acid strength, and the ethylene oligomerization and aromatization reaction on ZSM-5 with different Si/Al ratios was established. The quantitative relationship of acid activity. The discontinuous temperature-programmed ammonia desorption method and the NH3-TPD differential curve deconvolution method were used to quantitatively determine the acid strength distribution of the ZSM-5 series.
By introducing γ and δ as the kinetic parameters of the reaction's sensitivity to acid strength, based on the linear free energy theory, NH3 DAE is related to the reaction activation energy and heat of deprotonation. At the same time, these two parameters have clear physical meanings in the catalytic reaction kinetic model.
The reaction rate constant and the single-particle kinetic rate constant obtained by the Brönsted kinetic model. According to the quantitative acid strength distribution, the micro kinetic parameters and (to acid strength) sensitivity factors of each basic step are deduced. The linear free energy theory is used to establish a quantitative acid activity relationship. Considering all the basic steps, the sensitivity factor (γ) of the alkylation reaction is lower than the sensitivity factor (γ) of the corresponding aliphatic β cleavage, indicating that the acid strength has a greater influence on β cleavage. Alkylation reactions with lower activation energy can be catalyzed by medium-strength acid centers, while aliphatic β cleavage (higher activation energy) occurs at stronger acid centers.
In ZSM-5 molecular sieves with Si/Al ratios of 25, 60, and 70, the intrinsic kinetic parameters fitted by Brönsted equations of different acid strength centers can accurately predict the ZSM-5 molecular sieves with Si/Al ratio of 19 Product distribution.