(771e) Porous Polymer/Nanocrystal Composites with Wide Tunability and Shape-Selective Catalysis

Porous
polymer/nanocrystal composites with wide tunability and shape-selective
catalysis

Authors:
Andrew Riscoe, Cody Wrasman, Adi Menon, Larissa Kunz, Angel Yang, Matteo
Cargnello

Enzymes are highly efficient catalysts that accelerate
desired reaction rates by several orders of magnitude while suppressing
undesired reactions.  This effect stems from their functional amino acid
placement in specific locations in both a catalytic binding pocket and
molecular transport channels.  The evolutionarily determined placement of these
amino acids allows their pendant functional groups to stabilize transition
states, lowering activation energy barriers and accelerate desired molecular
transport while suppressing untargeted molecular transport, effectively “gating”
the active site to unwanted molecules.  In contrast, traditional heterogeneous
catalysts use extended metal surfaces of nanoparticles on a support matrix
where molecular transport is not through any controlled functional channels and
transitions states are stabilized solely on the metals and their supports rather
than in binding pockets.  Traditional heterogeneous catalysts have the
advantage of superior thermal stability, however, being stable at temperature of
up to 900°C.  Imparting some of the control of enzymatic catalysts onto
thermally stable heterogeneous catalysts has been a goal of the field of
catalysis for some time, given the potential advantages in reactivity and
selectivity in industrial reactions. 

In this work we aim to impart some of the unique structural
characteristics of enzymes into hybrid organic-inorganic catalysts with
microporous Porous Organic Frameworks (POFs) and colloidally synthesized metal
nanocrystals (NCs).  We synthesize hybrid catalysts by supporting NCs onto POF,
then remove ligands with a thermal treatment and finally grow an additional
layer of a POF utilizing pendant functional groups remaining on the support
POF.  Analogous to amino acids in enzymes, we incorporate functional monomers to
change POF functionality.  We demonstrate that this synthetic approach can be
applied to several chemistries of both POF and NC, leading to a multitude of
potential catalyst systems.  We show thermal stability of the hybrid systems microporous
pore networks with TGA and N₂ adsorption measurements and verify synthetic
control with TEM, XPS and EDS measurements.  For a Palladium-POF hybrid
catalyst, we demonstrate size-selective oxidation activity with small molecules
(CO 3.4 Å kinetic diameter (KD), propylene 4.4 Å KD) capable of fitting within
the 5.5-6 Å micropores combusting to CO₂ with O₂ present while
cyclohexane, with a 6 Å kinetic diameter is left unreacted. Further, we demonstrate
this Pd-POF catalyst can also perform size selective reductions with H₂
observing high activity with propylene and near zero activity with cyclohexene.