Beyond the Active Site: Tuning the Activity and Selectivity of a Metal–Organic Framework-Supported Ni Catalyst for Ethylene Dimerization

To
modify its steric and electronic properties as a support for
heterogeneous catalysts, electron-withdrawing and electron-donating
ligands, hexafluoroacetylacetonate (Facac–) and
acetylacetonate (Acac–), were introduced to the
metal–organic framework (MOF), NU-1000, via a process akin
to atomic layer deposition (ALD). In the absence of Facac– or Acac–, NU-1000-supported, AIM-installed Ni­(II)
sites yield a mixture of C4, C6, C8, and polymeric products in ethylene
oligomerization. (AIM = ALD-like
deposition in MOFs). In contrast, both Ni-Facac-AIM-NU-1000
and Ni-Acac-AIM-NU-1000 exhibit quantitative catalytic selectivity
for C4 species. Experimental findings are supported by density functional
theory calculations, which show increases in the activation barrier
for the C–C coupling step, due mainly to rearrangement of the
siting of Facac– or Acac– to partially
ligate added nickel. The results illustrate the important role of
structure-tuning support modifiers in controlling the activity of
MOF-sited heterogeneous catalysts and in engendering catalytic selectivity.
The results also illustrate the ease with which crystallographically
well-defined modifications of the catalyst support can be introduced
when the node-coordinating molecular modifier is delivered via the
vapor phase.