(721b) Microkinetic Model of Light Olefin Oligomerization on Acidic Zeolites

1. Introduction

Large scale upgrading of shale gas has been considered in the last decade a major issue in the US energy industry and has, for this reason, become a very popular research area. Shale gas contains high concentrations of hydrocarbons other than methane, such as ethane and propane. Conversion of these light gases to liquid products would open up new opportunities for their use as valuable chemicals or fuels [1]. A typical process for the conversion of light olefins to higher molecular weight products is based on the use of shape-selective zeolites, such as H-ZSM-5. This family of zeolites is a key element in the olefins-to-gasoline process developed by Mobil to convert light olefins from fluid catalytic cracking (FCC) [2]. Significant research efforts are directed toward the kinetic modeling of oligomerization processes, which is a pre-requisite for designing and operating efficient industrial reactors. However, building a kinetic model to describe an oligomerization process is a challenging task due to the exponential growth of the reaction network with the carbon number of the products.

In this work, a detailed reaction network was automatically generated including each elementary reaction taking place on the surface of the acidic catalyst. The reaction rate of each step was expressed by an elementary rate law containing specific kinetic coefficients. All the involved kinetic parameters have a clear theoretical background and are, for this reason, independent of operating conditions and feed.

The resulting model can describe with good accuracy the intrinsic kinetics of light alkene oligomerization on H-ZSM-5 at different ranges of conversion. The dominant reaction pathways of formation of the oligomeric products are revealed based on net rate analysis.

2. Methods

The oligomerization process of light olefins over acidic zeolites can be rationalized in terms of alkylation chemistry where the first step is the protonation of a physisorbed olefin by the Brønsted acid site of the zeolite with formation of an ionic intermediate, followed by addition of an olefin, and deprotonation. This process is highly exothermic and occurs with a strong reduction of the number of molecules [3]. Additionally, the oligomers can undergo skeletal isomerization and cracking via β-scission mechanism, resulting in a mixture of olefins not multiple of the initial monomer. These reactions contribute to randomize the molecular weight distribution of the products resulting in large and highly interconnected kinetic networks. Due to the complexity of the process we applied an “adaptive-chemistry” approach by building different kinetic mechanisms that are valid for specific ranges of conditions [4]. This method results in a high efficiency of the simulation because mechanisms with high level of detail can be used only when it is strictly necessary.

Despite the large dimension of the network, the number of reactions that occur with similar chemistry can be considered relatively small. For this reason, the chemistry of the system was organized into reaction families (e.g., protonation/deprotonation, oligomerization/β-scission), and a mathematical operator was specified for each family. The species identified in the reacting system were represented using bond and electron (BE) matrices based on graph theory. The reaction mechanism was then automatically created by applying the reaction operators to the different reactants and their progeny [5]. The rate coefficients for the elementary steps were expressed as function of the temperature following a typical Arrhenius dependence and the Evans-Polanyi relationship was used to express the activation energy of each reaction step as a function of the reaction enthalpy. The pre-exponential factors were estimated using the transition state theory, assuming that every elementary step proceeds through the formation of a transition state activated complex.

3. Results and discussion

The reaction network is initiated via formation of an alkoxide by protonation of the double bond of the reacting alkene. The reverse reaction, deprotonation, represents a termination step that desorbs the alkenes from the surface of the catalyst returning a proton to the acid site. Oligomerization proceeds through addition of an alkene to an ionic intermediate and consequent formation of a new sigma bond. β-Scission breaks the bond between the carbon atom in the β-position with respect to the charged carbon atom, representing the reverse step of oligomerization.

The developed reaction network and the estimated kinetic coefficients were coupled with the design equations of a PFR to build a continuum kinetic model. The resulting system of differential equations was then integrated using a numerical solver to simulate reaction kinetics, product yields and selectivity. Two versions of the mathematical model were validated depending on the range of experimentally observed conversion. Both experimental and modeling results indicated that increasing propylene conversion results in a lower selectivity of the process to species and in a higher selectivity to secondary oligomerization products and to the cracking species , and .

The model revealed the presence of bound alkoxides at near saturation coverages during the oligomerization process at low conversion. Analysis of resultant net rates showed that, at low conversion, the addition of propoxides to physisorbed alkenes drives the oligomerization process toward species.

4. Conclusions

The microkinetic analysis presented in this paper unravels mechanistic details of acid-catalyzed oligomerization chemistry of alkenes with high industrial relevance. The developed mathematical model represents a powerful tool to reproduce and predict the product distribution obtained from experimental activities.

References

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