(44e) Structure, Coordination, and Reactivity of Stable Ni-MIL-127 for Propylene Oligomerization with Active Sites Situated in the Framework

Heterogenous and homogeneous nickel catalysts for olefin oligomerization purportedly require two open sites in a catalytic sequence to follow the postulated Cossee-Arlman mechanism. The as synthesized Ni-MIL-127, a bimetallic metal organic framework (MOFs) containing one five-coordinate nickel atom and two iron atoms situated in the inorganic node, undergoes decarboxylation, as evinced by temperature programmed oxidation and ¹H nuclear magnetic resonance spectroscopy, upon thermal treatment in helium (493 K, 0.83 cm³ s^-1) to facilitate the loss of an organic linker and engender an additional coordination site on nickel. The activated Ni-MIL-127 oligomerizes propylene devoid of cocatalysts and is stable for ~24 hours on stream, unprecedented for propylene oligomerization. Chemical titration experiments with NO in situ (500 kPa, 493 K) during propylene oligomerization enumerate the number of active sites to confirm the degree of decarboxylation on Ni-MIL-127. Steady state kinetic studies reveal an apparent activation energy of 60 kJ mol^-1 and first order dependence in propylene pressure suggesting a Ni-propyl species is the most abundant surface species during propylene oligomerization, with its presence confirmed after propylene oligomerization (101.3 kPa, 473 K) with in situ infrared spectroscopy upon pulling vacuum on the sample (< 3 Pa). Density functional theory (DFT) on cluster models of Ni-MIL-127 is employed to show that decarboxylation and Cossee-Arlman mechanisms are consistent with the structure and steady state kinetics, respectively, for propylene oligomerization on the catalyst.