Nickel-Exchanged Zincosilicate Catalysts for the Oligomerization of Propylene

Abstract

Two nickel-containing zincosilicates (Ni-CIT-6 and Ni–Zn-MCM-41) and two nickel-containing aluminosilicates (Ni-HiAl-BEA and Ni-USY) are synthesized and used as catalysts to oligomerize propylene into C_(3n) (C_6 and C_9) products. Both Ni-CIT-6 and Ni-HiAl-BEA have the *BEA topology and are investigated to assess the effects of framework zinc versus aluminum because the former gives two framework charges per atom, whereas the latter, only one. Ni-CIT-6 and Ni–Zn-MCM-41 enable the comparison of a microporous to a mesoporous zincosilicate. Ni^2+ ion-exchanged into zeolite Y has been previously reported to oligomerize propylene and is used here for comparison. Reaction data are obtained at 180 and 250 °C, atmospheric pressure, and WHSV = 1.0 h^–1 in a feed stream of 85 mol % propylene (in inert). At these conditions, all catalysts are capable of oligomerizing propylene with steady-state conversions ranging from 3 to 16%. With the exception of Ni-HiAl-BEA, all catalysts have higher propylene conversions at 250 °C than at 180 °C. Both *BEA materials exhibit similar propylene conversions at each temperature, but Ni-HiAl-BEA is not as selective to C_3n products as Ni-CIT-6. Zincosilicates demonstrate higher average selectivities to C_3n products than the aluminosilicates at both reaction temperatures tested. Hexene products other than those expected by simple oligomerization are present, likely formed by double-bond isomerization catalyzed at acid sites. Additionally, both of the aluminosilicate materials catalyzed cracking reactions, forming non-C_3n products. The reduced acidity of the zincosilicates relative to the aluminosilicates likely accounts for higher C_(3n) product selectivity of the zincosilicates. Zincosilicates also exhibited higher linear-to-branched hexene isomer ratios (typically 1.0–1.5) when compared with the aluminosilicates, which had ratios on the order of 0.3. The mesoporous zincosilicate shows the best reaction behavior (including C_(3n) product selectivity: ∼99% at both temperatures for Ni–Zn-MCM-41) of the catalytic materials tested here.