Understanding Your Support System: The Design of a Stable Metal–Organic Framework/Polyazoamine Support for Biomass Conversion

Introducing oligomeric or polymeric
units into metal–organic
frameworks (MOFs) can result in composites that have significantly
improved properties when compared with the individual MOF or polymer
building blocks. With such synergy in mind, this work presents the
design of a novel MOF/polyazoamine support that is used to stabilize
Pd nanoparticles (NPs). The resulting composite catalyst, tested in
the reductive amination of levulinic acid, is found to have a markedly
improved lifetime when compared to just the MOF or polymer support
containing Pd. It is demonstrated, for the first time, that the lifetime
enhancement stems directly from the polymer, which plays a dual role:
(i) the oligomer stabilizes the MOF support through the elimination
of certain vibrational modes associated with the framework ligand
and likely pore-filling effects in the largest MOF pore and (ii) the
Lewis base functionality on the oligomer backbone binds to the surface
of the Pd NPs, thus, increasing their activity and inhibiting their
aggregation. Several complementary spectroscopic (IR, X-ray photoelectron
spectroscopy, Raman spectroscopy) and computational tools (pore space
and topology analysis, molecular mechanics, and density functional
theory simulations) are used to describe the nature of the MOF–oligomer
interaction and identify the most likely location of the polymer within
the MOF pore.