Structure and Site Evolution of Framework Ni Species in MIL-127 MOFs for Propylene Oligomerization Catalysis

A mixed-valence oxotrimer metal–organic framework
(MOF),
Ni-MIL-127, with a fully coordinated nickel atom and two iron atoms
in the inorganic node, generates a missing linker defect upon thermal
treatment in helium (>473 K) to engender an open coordination site
on nickel which catalyzes propylene oligomerization devoid of any
cocatalysts or initiators. This catalyst is stable for ∼20
h on stream at 500 kPa and 473 K, unprecedented for this chemistry.
The number of missing linkers on synthesized and activated Ni-MIL-127
MOFs is quantified using temperature-programmed oxidation, 1H nuclear magnetic resonance spectroscopy, and X-ray absorption spectroscopy
to be ∼0.7 missing linkers per nickel; thus, a majority of
Ni species in the MOF framework catalyze propylene oligomerization.
In situ NO titrations under reaction conditions enumerate ∼62%
of the nickel atoms as catalytically relevant to validate the defect
density upon thermal treatment. Propylene oligomerization rates on
Ni-MIL-127 measured at steady state have activation energies of 55–67
kJ mol–1 from 448 to 493 K and are first-order in
propylene pressures from 5 to 550 kPa. Density functional theory calculations
on cluster models of Ni-MIL-127 are employed to validate the plausibility
of the missing linker defect and the Cossee–Arlman mechanism
for propylene oligomerization through comparisons between apparent
activation energies from steady-state kinetics and computation. This
study illustrates how MOF precatalysts engender defective Ni species
which exhibit reactivity and stability characteristics that are distinct
and can be engineered to improve catalytic activity for olefin oligomerization.