Nickel-Exchanged Zincosilicate Catalysts for the Oligomerization of Propylene

Two nickel-containing zincosilicates (Ni-CIT-6 and NiZn-MCM-41) and two nickel-containing aluminosilicates (Ni-HiAl-BEA and Ni-USY) are synthesized and used as catalysts to oligomerize propylene into C(3)n (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 NiZn-MCM-41 enable the comparison of a microporous to a mesoporous zincosilicate. Ni2+ 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 degrees 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 degrees C than at 180 degrees C. Both *BEA materials exhibit similar propylene conversions at each temperature, but Ni-HiAl-BEA is not as selective to C(3)n products as Ni-CIT-6. Zincosilicates demonstrate higher average selectivities to C3n 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-C3n products. The reduced acidity of the zincosilicates relative to the aluminosilicates likely accounts for higher C3n product selectivity of the zincosilicates. Zincosilicates also exhibited higher linear-to-branched hexene isomer ratios (typically 1.01.5) when compared with the aluminosilicates, which had ratios on the order of 0.3. The mesoporous zincosilicate shows the best reaction behavior (including C3n product selectivity: 99% at both temperatures for NiZn-MCM-41) of the catalytic materials tested here.