Ethylene Oligomerization Catalyst Optimization Using Fundamental Kinetic Modeling
Kenneth Toch1, Joris W. Thybaut1,*, Maria A. Arribas2, Agustin Martinez2, Guy B. Marin1
1 Laboratory for Chemical Technology, Ghent University, Krijgslaan 281 – S5, 9000 Ghent, Belgium
2 Instituto de Tecnologica Quimica, Universidad Politecnica de Valencia, Av. de los Naranjos, E-46022 Valencia, Spain
Homogeneously catalyzed ethylene oligomerization is an established industrial process, mainly resulting in even carbon atom numbered alpha alkenes. Heterogeneous catalysis is preferred, however, e.g., because of environmental reasons and for avoiding a catalyst separation step from the product. Moreover, heterogeneous catalysis offers more opportunities to tune the product distribution and corresponding yields to the market demands, i.e., even-numbered, linear α-alkenes versus a high octane fuel blend hydrocarbon mixture .
The present work aims at developing an active, stable and selective catalyst for ethylene oligomerization. This challenge is addressed via fundamental modeling using Single-Event MicroKinetics (SEMK) . Model parameters are determined which can be classified as kinetic and catalyst descriptors. The first type of parameters is specific to the reaction family considered and independent from the catalyst, while the latter specifically accounts for the effect of catalyst properties, such as Si/Al ratio, metal-ion site concentration… on the kinetics, e.g., via chemisorption enthalpies, sterical constraint factors,… Through simulation, the catalyst descriptors can be varied in order to identify optimal catalyst behavior defined in terms of product selectivity or yield.
Experimental ethylene oligomerization data have been gathered on three different catalysts, i.e. 1.8wt% Ni-SiO2-Al2O3, 2.7wt% Ni-Beta and 1.7wt% Ni-USY. At 393 K, a total pressure of 3.5 MPa and a space time equal to 48 kgcat s moleth-1, see Figure 1 a, stable behavior up to 9h time on stream was exhibited by Ni-SiO2-Al2O3 and Ni-Beta, whereas Ni-USY suffered a significant activity loss. Ni-SiO2-Al2O3 was more active than Ni-Beta and, hence, was selected as the benchmark catalyst for modelling purposes. As observed in Figure 1b, reaction products found mainly comprised even-numbered olefins in the C4-C12 range. Only traces of odd-numbered alkenes were founded at the reaction conditions studied. A different product distribution was observed over the silica-alumina-based and microporous Beta-based catalysts. A Schulz Flory distribution was obtained with the Ni-SiO2-Al2O3 catalyst. The C10+ fraction products (diesel products) was 38 wt% and 17wt % for the Ni-Beta and Ni-SiO2-Al2O3 catalysts, respectively, probably because the oligomerization activity over the acid sites is more important over the more acidic zeolite-based catalyst.
Figure 1a: Ethylene conversion on 1.8wt% Ni-SiO2-Al2O3, 2.7% Ni-Beta and 1.7 wt%Ni-USY as function of time on stream at 393 K, 3.5MPa and 48.0 kgcat s moleth-1. Figure 1b. Olefin distribution obtained on Ni-SiO2-Al2O3 and Ni-Beta catalysts.
A more extended ethylene oligomerization kinetic data set has been acquired on the 1.8wt%Ni-SiO2-Al2O3 within a temperature range from 323 K to 503 K, total pressures between 1.0 and 3.5 MPa with a molar ethylene content in the feed of 60%. The space time was varied from 3.0 to 45.0 kgcat s moleth-1. At these conditions, intrinsic kinetics are measured. The reaction network considered in the SEMK model was limited to molecules with a maximum carbon number of 12 and contained over 3000 species and over 10000 elementary steps. Physical adsorption of the components inside the catalyst pores, prior to any chemical elementary step was also accounted for.
As previously reported in the literature [1;3], two distinct activity regions were observed as a function of the temperature, see Figure 2. At low temperatures mainly linear α-alkenes are obtained through a coordinated ethylene insertion mechanism on an active Ni cation site. Between 373 and 403 K, in the presence of ethylene, an irreversible transformation of Ni occurs which leads to a significantly lower activity of these Ni sites . As a result, higher temperatures, i.e., above 423 K, are required reach the same ethylene conversion through reaction on the acid sites after an initial dimerization on the Ni cation sites. The absence of odd carbon numbered alkenes in the product spectrum shows that no cracking occurred, see Figure 1b.
Figure 2: Experimental conversion of ethylene as function of temperature at 3.5MPa and 45.0 kgcat s moleth-1.
Because these higher operating temperatures offer more possibilities for tuning the product distribution, the corresponding data are assessed first using the microkinetic model. Simulations have been performed using initial parameter values based on literature data . The C4 fraction obtained is entirely built up out of linear alkenes. C6 and heavier fractions also contain branched isomers. Metal-ion catalyzed oligomerization leads to Anderson Schulz Flory product distribution. The acid catalyzed oligomerization disturbs this ASF distribution, however, see Figure 3, left. This is more pronounced with increasing temperatures, see Figure 3 right. The decrease with the temperature of the linear alkene content in the C8 fraction from 43.2 % at 443 K to 25.2 % at 503 K, also illustrates the increase of the relative importance of acid catalyzed reactions at higher temperatures.
Figure 3: Left: natural logarithm of the molar outlet flow rates as function of the carbon number; Right: product distribution as a function of temperature, acid catalyzed oligomerization included at 3.5 MPa and 6.7 kgcat s moleth-1.
The research leading to these results has received funding from the European Community's Seventh Framework Program FP7/2007-2013 under grant agreement n° 228953.
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