(745b) Atomically Dispersed Supported Metal Catalysts: Tuning Catalytic Performance with Supports and Ionic Liquid Coatings

Atomically Dispersed Supported Metal Catalysts:
Tuning Catalytic Performance with Supports and Ionic Liquid Coatings

 
Atomically dispersed supported metals are
an emerging class of catalysts. These catalysts often require stabilizing
ligands, and so it is challenging to tune their catalytic performance.¹ Varying the electron-donor properties of the
supports (which are ligands) offers opportunities to adjust the electronic
environment of the active sites. However, this approach is limited in the
number of supports that can stabilize the single-sites. Another approach is to
coat the surface of a single-site supported metal catalyst with ionic liquids
(ILs). In this approach, the combined ligand effect of both the support and IL
offers a broad potential for tuning the catalytic performance. In this investigation,
we illustrate a family of single-site iridium catalysts precisely synthesized
on various metal oxides with various electron-donor properties and coated with various
1,3-dialklyimidazolium type ILs.
We synthesized
atomically dispersed iridium gem-dicarbonyl complexes on high surface
area supports, SiO₂, TiO₂, Fe₂O₃,
CeO₂, MgO, and La₂O₃, and coated them with 1-n-butyl-3-methylimidazolium
tetrafluoroborate, [BMIM][BF₄], characterizing the species by  infrared
(IR) and extended X-ray
absorption fine structure (EXAFS) spectroscopies. Changes in electron density on iridium sites were investigated
by the differences in high-energy resolution fluorescence detection X-ray absorption
near edge structure (HERFD XANES) measurements and IR fingerprints of carbonyl
ligands on the iridium. The results demonstrate that as the electron-donor
character of the metal oxide becomes stronger, electron density on iridium increases,
evidenced by a linear decrease in Ir L_III edge energy and ν(CO)_sym
band positions. The data further indicate that the IL coatings donate electron
density to the iridium sites, much as well-known electron-donating ligands do. The
type of support strongly influences the degree of electron donation from the
IL. Accordingly, the ligand effect of the IL becomes more prominent when the
support has a weaker electron-donor character. The catalytic consequences of
this combined ligand effect of the support and IL coating were evaluated for
partial hydrogenation of 1,3-butadiene (BD) (at 333 K, atmospheric pressure,
differential conversions). The data show that the atomically dispersed
supported iridium catalysts become more selective for partial hydrogenation when
the active sites become electron rich as a result of electron donation by support
and the IL; the conclusion is demonstrated by a strong correlation between the
partial hydrogenation selectivity and Ir L_III edge energy, given in
Figure 1.
To understand
the individual ligand effect of IL coatings in greater detail, we coated γ-Al₂O₃-supported
iridium complexes with various 1,3-dialkylimidazolium ILs. The results show
that an increase in interionic interaction energy in ILs (probed by the
stretching frequency of the most strongly acidic group on the imidazolium ring,
ν(C2H)), leads to a stronger electron-donor character of the IL, as illustrated
by HERFD XANES result.^2,3 Results of this study offer a new class of atomically
dispersed supported metal catalysts with tunable electronic environments by the
choice of support and ILs, providing a broad potential for ultimate control of
the catalytic performance.

Figure
1. Variation
of and product selectivities for BD hydrogenation with the Ir L_III
edge energy of the corresponding uncoated and IL-coated iridium complexes
supported on different metal oxides. Data demonstrate the variation of
selectivity (1B (○),
C2B (◇),
T2B (△) and
total butenes (sum of 1B, T2B and C2B)). Black and blue symbols indicate
uncoated and IL-coated supported Ir complexes, respectively.

Acknowledgments

This work was supported by TUBITAK
National Young Researchers Career Development Program (CAREER) (113M552) and by
Koç University TÜPRAÞ Energy Center (KUTEM) and by the U.S. Department of
Energy (DOE), Office of Science, Basic Energy Sciences (BES), Grant
DE-FG02-04ER15513 (C-YF, ASH). AU acknowledges the TUBA-GEBIP Award and TARLA
and ASH a Chevron Fellowship. We acknowledge the Stanford Synchrotron Radiation
Lightsource (SSRL) for access to beam time on Beamline 6-2 and 9-3. SSRL, SLAC
National Accelerator Laboratory, is supported by DOE, BES, under Contract No.
DE-AC02-76SF00515.

 

 

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