(594f) Supported Rhodium Complexes on Highly Dealuminated Zeolite Y: Characterization during Catalysis of Ethylene Hydrogenation

Typical supported metal catalysts consist of
nonuniform nano-structures dispersed on supports. The nonuniformity has led to
difficulties in the determination of structure-catalytic property relationships
and understanding of the catalysts. 
In contrast, supported catalysts with virtually molecular structures,
incorporating supported metal complexes, offer the advantages of simplicity
and, in prospect, uniformity of structure to facilitate fundamental
understanding.  Our goals were to
prepare and investigate well-defined and nearly uniform supported rhodium
complex that is an active and stable catalyst and to determine relationships
between catalyst structure (including the structure of the metal–support
interface) and activity for a simple test reaction, hydrogenation of
ethylene.  The supported rhodium
complex was synthesized from Rh(C₂H₄)₂(acac)
[acac is the bidentate ligand CH₃COCHCOCH₃] and highly
dealuminated zeolite Y.  The
supported sample was characterized by IR, EXAFS, and ¹³C MAS NMR
spectroscopies, both in the as-synthesized form and in the working state. 

The EXAFS spectrum of the as-synthesized sample gives
no evidence of a Rh–Rh contribution, consistent with the presence of the
rhodium as site-isolated mononuclear species, consistent with the IR data.  The EXAFS data show that the Rh center
was bonded to the support via 2 Rh–O bonds; the distance is 2.19 Å,
consistent with a strong polar bond. 
The EXAFS Rh–C contribution (coordination number N_Rh-C = 3.9, distance R_Rh-C = 2.09 Å)
indicates two ethylene ligands bonded to the Rh atoms, in agreement with the IR
spectra.  Thus, the supported
rhodium complex is among the best-defined supported metal species.

When
the initially formed sample was treated in flowing H₂ at 298 K, gas
was evolved and analyzed with a mass spectrometer; the data indicated ethane
formation even in the absence of ethylene in the feed.  This result indicates that ethylene
ligands on the rhodium were hydrogenated on the site-isolated Rh centers bonded
to the support; these ligands are candidate reaction intermediates.  The supported Rh complex was tested as
a catalyst for ethylene hydrogenation at 298 K and 760 Torr in a flow
reactor/EXAFS cell.  The EXAFS
spectrum after the H₂ treatment indicates that the rhodium was
slightly aggregated, forming clusters with a Rh–Rh bond distance of 2.66
Å and coordination number of 2.3. 
The rhodium in these minute clusters was bonded to two oxygen atoms of
the support (N_Rh–O
= 2.0, R_Rh–O =
2.10 Å).  EXAFS spectra of the
working catalyst in ethylene + H₂ show that the clusters had
fragmented to again form mononuclear rhodium complexes; no Rh–Rh
contribution was found, and the data indicated only bonding of the rhodium to
the support and to hydrocarbon ligands (N_Rh–O = 2.1, R_Rh–O = 2.06 Å; N_Rh–C = 2.9, R_Rh–C = 2.25 Å).