UC Berkeley

A series of alkali metal- and nickel-exchanged Al-MCM-41 catalysts were prepared via aqueous ion exchange and then investigated for gas-phase oligomerization of propene at 453 K and near ambient pressures. All catalysts were active and produced oligomers with >98% selectivity. The highest activities per Ni2+ cation were observed when the cations were highly dispersed as a consequence of either lowering the Ni loading for a fixed MCM-41 Si/Al ratio or by decreasing the concentration of exchangeable sites within the material by increasing the MCM-41 Si/Al ratio at a fixed Ni loading. The identity of the alkali metal cation had no significant effect on the catalytic activity or degree of dimer branching, except for the sample containing Cs + cations, where the decreased pore volume resulted in a lower catalyst activity and slightly more linear dimer products. Comparison of Ni-MCM-41 prepared with and without Na+ cations showed that a higher yield of oligomers could be achieved when Na+ cations are present because of partial removal of strong Brønsted acid sites. For the same reaction conditions, Ni-Na-MCM-41 was more than twice as active as smaller-pored Ni-Na-X zeolites, demonstrating that the activity of Ni2+ cations increases with the increasing free volume near the site. This effect of free volume on the activity of Ni2+ cations was further confirmed by comparing the activities of Ni-Na-X, Ni-Na-MCM-41, Ni-Na-MCM-48, and Ni-Na-SBA-15 with respect to pore size. © 2013 American Chemical Society.