(640e) Effects of Synthesis Conditions and Dispersion On the Propene Oligomerization Activity of Ni-Containing Microporous Catalysts
The synthesis of transportation fuels from processes such as fluid catalytic cracking (FCC) and Fischer-Trospch synthesis can produce large quantities of light olefins (C2-C4) that cannot be directly blended into the final liquid fuel due to their high vapor pressures. Increasing the molecular weight of the olefins by oligomerization is one method by which these byproducts can be reincorporated into the liquid fuel. Supported heterogeneous nickel materials are known to be highly selective and active ethene oligomerization catalysts producing greater than 94% yield to butene and larger oligomeric products ,. Work on extending these materials to oligomerize other light olefins, including propene, has been limited due to the relatively poor stability of the catalyst in the presence of olefins larger than C2. To better understand why these catalysts deactivate in the presence of propene, we have synthesized a series of nickel exchanged zeolite catalysts, namely Na-X, and tested them for activity as a function of time-on-stream. It was observed that the synthesis method can drastically change the type of catalyst produced. When water is used as an ion exchange solvent, with Ni(NO3)2 as the precursor salt, the zeolite undergoes cation exchange leading to well-dispersed divalent nickel sites that each compensate two Al atoms in the zeolite framework. This catalyst has an initial propene conversion of approximately 5% at 5 bar but decreases to less than 1% conversion after one hour time-on-stream. In contrast, when the catalyst is synthesized using acetone as an exchange solvent, the Ni deposits as NiO on the zeolite. This technique causes the catalyst to possess almost no initial activity, but instead leads to an induction period where maximum activity of 5% is observed after 1.5 hours time-on-stream. After the 1.5 hour activation period, the catalyst deactivates in a similar fashion to the water exchanged catalyst. This suggests a common deactivation mechanism that we attribute to adsorbed hydrocarbons on the nickel sites instead of irreversible reduction of the nickel sites as was seen in another study . Impregnation of silica with nickel nitrate and nickel citrate leads to a catalyst that possesses lower activtiy but a qualitatively similar activity profile to the Na-X sample exchanged in acetone. Temperature-programmed reduction experiments with all catalysts suggests that partial exchange with either the exchange sites on Na-X or the silanol groups on silica could correlate with the activity of the catalyst. These results imply that Brønsted acid sites or Ni-O-Al linkages may not be required for catalyst activity, as was suggested by other studies ,. Furthermore, these experiments imply that the dispersion of nickel may be the critical factor in determining the activity of a heterogeneous nickel-based oligomerization catalyst and work is ongoing to validate this conclusion.
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