5 research outputs found
Determination of the Dominant Catalyst Derived from the Classic [RhCp*Cl<sub>2</sub>]<sub>2</sub> Precatalyst System: Is it Single-Metal Rh<sub>1</sub>Cp*-Based, Subnanometer Rh<sub>4</sub> Cluster-Based, or Rh(0)<i><sub>n</sub></i> Nanoparticle-Based Cyclohexene Hydrogenation Catalysis at Room Temperature and Mild Pressures?
Determining the kinetically dominant
catalyst in a given catalytic
system is a forefront topic in catalysis. The [RhCp*Cl<sub>2</sub>]<sub>2</sub> (Cp* = [Ī·<sup>5</sup>-C<sub>5</sub>(CH<sub>3</sub>)<sub>5</sub>]) system pioneered by Maitlis and co-workers is a classic
precatalyst system from which homogeneous mononuclear Rh<sub>1</sub>, subnanometer Rh<sub>4</sub> cluster, and heterogeneous polymetallic
Rh(0)<i><sub>n</sub></i> nanoparticle have all arisen as
viable candidates for the true hydrogenation catalyst, depending on
the precise substrate, H<sub>2</sub> pressure, temperature, and catalyst
concentration conditions. Addressed herein is the question of whether
the prior assignment of homogeneous, mononuclear Rh<sub>1</sub>Cp*-based
catalysis is correct, or are trace Rh<sub>4</sub> subnanometer clusters
or possibly Rh(0)<i><sub>n</sub></i> nanoparticles the dominant,
actual cyclohexene hydrogenation catalyst at 22 Ā°C and 2.7 atm
initial H<sub>2</sub> pressure? The observation herein of Rh<sub>4</sub> species by in operando-X-ray absorption fine structure (XAFS) spectroscopy,
at the only slightly more vigorous conditions of 26 Ā°C and 8.3
atm H<sub>2</sub> pressure, and the confirmation of Rh<sub>4</sub> clusters by ex situ mass spectroscopy raises the question of the
dominant, room temperature, and mild pressure cyclohexene hydrogenation
catalyst derived from the classic [RhCp*Cl<sub>2</sub>]<sub>2</sub> precatalyst pioneered by Maitlis and co-workers. Ten lines of evidence
are provided herein to address the nature of the true room temperature
and mild pressure cyclohexene hydrogenation catalyst derived from
[RhCp*Cl<sub>2</sub>]<sub>2</sub>. Especially significant among those
experiments are quantitative catalyst poisoning experiments, in the
present case using 1,10-phenanthroline. Those poisoning studies allow
one to distinguish mononuclear Rh<sub>1</sub>, subnanometer Rh<sub>4</sub> cluster, and Rh(0)<i><sub>n</sub></i> nanoparticle
catalysis hypotheses. The evidence obtained provides a compelling
case for a mononuclear, Rh<sub>1</sub>Cp*-based cyclohexene hydrogenation
catalyst at 22 Ā°C and 2.7 atm H<sub>2</sub> pressure. The resultant
methodology, especially the quantitative catalyst poisoning experiments
in combination with in operando spectroscopy, is expected to be more
broadly applicable to the study of other systems and the āwhat
is the true catalyst?ā question
Mononuclear Zeolite-Supported Iridium: Kinetic, Spectroscopic, Electron Microscopic, and Size-Selective Poisoning Evidence for an Atomically Dispersed True Catalyst at 22 Ā°C
This work addresses the question of what is the true
catalyst when
beginning with a site-isolated, atomically dispersed precatalyst for
the prototype catalytic reaction of cyclohexene hydrogenation in the
presence of cyclohexane solvent: is the atomically dispersed nature
of the zeolite-supported, [IrĀ(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub>]/zeolite Y precatalyst retained, or are possible alternatives including
Ir<sub>4</sub> subnanometer clusters or larger, Ir(0)<sub><i>n</i></sub>, nanoparticles the actual catalyst? Herein we report
the (a) kinetics of the reaction; (b) physical characterizations of
the used catalyst, including extended X-ray absorption fine structure
spectra plus images obtained by high-angle annular dark-field scanning
transmission electron microscopy, demonstrating the mononuclearity
and site-isolation of the catalyst; and the (c) results of poisoning
experiments, including those with the size-selective poisons PĀ(C<sub>6</sub>H<sub>11</sub>)<sub>3</sub> and PĀ(OCH<sub>3</sub>)<sub>3</sub> determining the location of the catalyst in the zeolite pores. Also
reported are quantitative poisoning experiments showing that each
added PĀ(OCH<sub>3</sub>)<sub>3</sub> molecule poisons one catalytic
site, confirming the single-metal-atom nature of the catalyst and
the lack of leaching of catalyst into the reactant solution. The results
(i) provide strong evidence that the use of a site-isolated [IrĀ(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub>]/zeolite Y precatalyst allows a
site-isolated [Ir<sub>1</sub>]/zeolite Y hydrogenation catalyst to
be retained even when in contact with solution, at least at 22 Ā°C;
(ii) allow a comparison of the solidāsolution catalyst system
with the equivalent one used in the solidāgas ethylene hydrogenation
reaction at room temperature; and (iii) illustrate a methodology by
which multiple, complementary physical methods, combined with kinetic,
size-selective poisoning, and quantitative kinetic poisoning experiments,
help to identify the catalyst. The results, to our knowledge, are
the first identifying an atomically dispersed, supported transition-metal
species as the catalyst of a reaction taking place in contact with
solution
Agglomerative Sintering of an Atomically Dispersed Ir<sub>1</sub>/Zeolite Y Catalyst: Compelling Evidence Against Ostwald Ripening but for Bimolecular and Autocatalytic Agglomeration Catalyst Sintering Steps
Agglomerative
sintering of an atomically dispersed, zeolite Y-supported
catalyst, Ir<sub>1</sub>/zeolite Y, formed initially from the well-characterized
precatalyst [IrĀ(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub>]/zeolite Y
and in the presence of liquid-phase reactants, was monitored over three
cycles of 3800 turnovers (TTOs) of cyclohexene hydrogenation at 72
Ā°C. The catalyst evolved and sintered during each cycle, even
at the relatively mild temperature of 72 Ā°C in the presence of
the cyclohexene plus H<sub>2</sub> reactants and cyclohexane solvent.
Post each of the three cycles of catalysis, the resultant sintered
catalyst was characterized by extended X-ray absorption fine structure
spectroscopy and atomic-resolution high-angle annular dark-field scanning
transmission electron microscopy. The results show
that higher-nuclearity iridium species, Ir<sub><i>n</i></sub>, are formed during each successive cycle. The progression from the
starting mononuclear precursor, Ir<sub>1</sub>, is first to Ir<sub>ā¼4ā6</sub>; then, on average, Ir<sub>ā¼40</sub>; and finally, on average, Ir<sub>ā¼70</sub>, the latter more
accurately described as a bimodal dispersion of on-average Ir<sub>ā¼40ā50</sub> and on-average Ir<sub>ā¼1600</sub> nanoparticles. The size distribution and other data disprove Ostwald
ripening during the initial and final stages of the observed catalyst
sintering. Instead, the diameter-dispersion data plus quantitative
fits to the cluster or nanoparticle diameter vs time data provide
compelling evidence for the underlying, pseudoelementary steps of
bimolecular agglomeration, B + B ā C, and autocatalytic agglomeration,
B + C ā 1.5C, where B represents the smaller, formally Ir(0)
nanoparticles, and C is the larger (more highly agglomerated) nanoparticles
(and where the 1.5 coefficient in the autocatalytic agglomeration
of B + C necessarily follows from the definition, in the bimolecular
agglomeration step, that 1C contains the Ir from 2B). These two specific,
balanced chemical reactions are of considerable significance in going
beyond the present state-of-the-art, but word-only, āmechanismāīøthat
is, actually and instead, just a collection of phenomenaīøfor
catalyst sintering of āParticle Migration and Coalescenceā.
The steps of bimolecular plus autocatalytic agglomeration provide
two specific, balanced chemical equations useful for fitting sintering
kinetics data, as is done herein, thereby quantitatively testing proposed
sintering mechanisms. These two pseudoelementary reactions also define
the specific words and concepts for sintering of bimolecular agglomeration
and autocatalytic agglomeration. The results are also significant
as the first quantitative investigation of the agglomeration and sintering
of an initially atomically dispersed metal on a structurally well-defined
(zeolite) support and in the presence of liquid reactants (cyclohexene
substrate and cyclohexane solvent) plus H<sub>2</sub>. A list of additional
specific conclusions is provided in a summary section
Synthesis and Characterization of [Ir(1,5-Cyclooctadiene)(Ī¼-H)]<sub>4</sub>: A Tetrametallic Ir<sub>4</sub>H<sub>4</sub>-Core, Coordinatively Unsaturated Cluster
Reported herein is the synthesis of the previously unknown
[IrĀ(1,5-COD)Ā(Ī¼-H)]<sub>4</sub> (where 1,5-COD = 1,5-cyclooctadiene),
from commercially available
[IrĀ(1,5-COD)ĀCl]<sub>2</sub> and LiBEt<sub>3</sub>H <i>in the
presence of excess 1,5-COD</i> in 78% initial, and 55% recrystallized,
yield plus its unequivocal characterization via single-crystal X-ray
diffraction (XRD), X-ray absorption fine structure (XAFS) spectroscopy,
electrospray/atmospheric pressure chemical ionization mass spectrometry
(ESI-MS), and UVāvis, IR, and nuclear magnetic resonance (NMR)
spectroscopies. The resultant product parallelsīøbut the successful
synthesis is different from, vide infraīøthat of the known and
valuable Rh congener precatalyst and synthon, [RhĀ(1,5-COD)Ā(Ī¼-H)]<sub>4</sub>. Extensive characterization reveals that a black crystal
of [IrĀ(1,5-COD)Ā(Ī¼-H)]<sub>4</sub> is composed of a distorted
tetrahedral, <i>D</i><sub>2<i>d</i></sub> symmetry
Ir<sub>4</sub> core with two long [2.90728(17) and 2.91138(17) Ć
]
and four short IrāIr [2.78680 (12)ā2.78798(12) Ć
]
bond distances. One 1,5-COD and two edge-bridging hydrides are bound
to each Ir atom; the IrāHāIr span the shorter IrāIr
bond distances. XAFS provides excellent agreement with the XRD-obtained
Ir<sub>4</sub>-core structure, results which provide both considerable
confidence in the XAFS methodology and set the stage for future XAFS
in applications employing this Ir<sub>4</sub>H<sub>4</sub> and related
tetranuclear clusters. The [IrĀ(1,5-COD)Ā(Ī¼-H)]<sub>4</sub> complex
is of interest for at least five reasons, as detailed in the Conclusions
section
Synthesis and Characterization of [Ir(1,5-Cyclooctadiene)(Ī¼-H)]<sub>4</sub>: A Tetrametallic Ir<sub>4</sub>H<sub>4</sub>-Core, Coordinatively Unsaturated Cluster
Reported herein is the synthesis of the previously unknown
[IrĀ(1,5-COD)Ā(Ī¼-H)]<sub>4</sub> (where 1,5-COD = 1,5-cyclooctadiene),
from commercially available
[IrĀ(1,5-COD)ĀCl]<sub>2</sub> and LiBEt<sub>3</sub>H <i>in the
presence of excess 1,5-COD</i> in 78% initial, and 55% recrystallized,
yield plus its unequivocal characterization via single-crystal X-ray
diffraction (XRD), X-ray absorption fine structure (XAFS) spectroscopy,
electrospray/atmospheric pressure chemical ionization mass spectrometry
(ESI-MS), and UVāvis, IR, and nuclear magnetic resonance (NMR)
spectroscopies. The resultant product parallelsīøbut the successful
synthesis is different from, vide infraīøthat of the known and
valuable Rh congener precatalyst and synthon, [RhĀ(1,5-COD)Ā(Ī¼-H)]<sub>4</sub>. Extensive characterization reveals that a black crystal
of [IrĀ(1,5-COD)Ā(Ī¼-H)]<sub>4</sub> is composed of a distorted
tetrahedral, <i>D</i><sub>2<i>d</i></sub> symmetry
Ir<sub>4</sub> core with two long [2.90728(17) and 2.91138(17) Ć
]
and four short IrāIr [2.78680 (12)ā2.78798(12) Ć
]
bond distances. One 1,5-COD and two edge-bridging hydrides are bound
to each Ir atom; the IrāHāIr span the shorter IrāIr
bond distances. XAFS provides excellent agreement with the XRD-obtained
Ir<sub>4</sub>-core structure, results which provide both considerable
confidence in the XAFS methodology and set the stage for future XAFS
in applications employing this Ir<sub>4</sub>H<sub>4</sub> and related
tetranuclear clusters. The [IrĀ(1,5-COD)Ā(Ī¼-H)]<sub>4</sub> complex
is of interest for at least five reasons, as detailed in the Conclusions
section