Oxidative Alkenylation of Arenes Using Supported Rh Materials: Evidence that Active Catalysts are Formed by Rh Leaching

Article abstract of DOI:10.1002/cctc.202001526

This work focuses on the synthesis of supported Rh materials and study of their efficacy as pre-catalysts for the oxidative alkenylation of arenes. Rhodium particles supported on silica (Rh/SiO₂; ～3.6 wt% Rh) and on nitrogen-doped carbon (Rh/NC

Full text of DOI:10.1002/cctc.202001526

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Oxidative Alkenylation of Arenes Using Supported Rh
Materials: Evidence that Active Catalysts are Formed by Rh
Leaching
[a]
[b]
[b, c]
[a]
[a]
Zhongwen Luo, Colby A. Whitcomb, Nicholas Kaylor,
Robert J. Davis,* and T. Brent Gunnoe*
Yulu Zhang, Sen Zhang,
[b]
[a]
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This work focuses on the synthesis of supported Rh materials
and study of their efficacy as pre-catalysts for the oxidative
alkenylation of arenes. Rhodium particles supported on silica
using Rh/SiO or Rh/NC as catalyst precursor, the conversion of
2
benzene and propylene or toluene and 1-pentene yields a ratio
of anti-Markovnikov to Markovnikov products that is nearly
identical to the same ratios as the molecular catalyst precursor
(
Rh/SiO ; ~3.6 wt% Rh) and on nitrogen-doped carbon (Rh/NC;
2
2
~
1.0 wt% Rh) are synthesized and tested. Heating mixtures of
[Rh(μ-OAc)(η -C H ) ] . These results and other observations are
2
4 2 2
Rh/SiO or Rh/NC with benzene and ethylene or α-olefins and
consistent with the formation of active catalysts by leaching of
soluble Rh from the supported Rh materials.
2
CuX {X=OPiv (trimethylacetate) or OHex (2-ethyl hexanoate)}
2
to 150°C results in the production of alkenyl arenes. When
Introduction
Alkyl and alkenyl arenes are used as precursors for a range of
high-value chemicals, including detergent precursors, plastics,
[
1–5]
elastomers and pharmaceuticals.
benzenes, which are primarily 2-aryl alkanes, serve as precursors
for alkylbenzene sulfonates active components in
For example, linear alkyl-
–
[6]
detergents. Styrene, one of the most important commodity
chemicals, is used for polystyrene plastics and synthetic rubber
manufacturing. The current industrial production of alkylben-
[7]
[8]
zenes uses Friedel-Crafts or zeolite acid-based technologies.
Two general technologies for styrene production, the ethyl-
benzene/styrene monomer process (EB/SM) and the propylene
oxide/styrene monomer process (PO/SM), have been commer-
[3,9–11]
cialized (Scheme 1).
Although EB/SM and PO/SM processes
have been commercialized, existing industrial processes for
styrene and alkyl arene syntheses have some disadvantages,
including: 1) multi-step processes; for example, the EB/SM
process includes benzene alkylation, transalkylation, and ethyl-
benzene dehydrogenation, while the PO/SM process involves
benzene alkylation, ethylbenzene oxidation, oxygen transfer,
and dehydration; 2) energy-intensive operations such as trans-
Scheme 1. Industrial processes for styrene monomer production: EB/SM
process (top) and PO/SM process (bottom).
alkylation and distillation; and 3) the inability to produce anti-
[
12]
Markovnikov products (i.e., 1-aryl alkenes or alkanes). Thus,
there is increased interest in developing new catalytic processes
for direct arene alkenylation at a low operating temperature
[
[
[
a] Z. Luo, Y. Zhang, Prof. S. Zhang, Prof. T. B. Gunnoe
Department of Chemistry
University of Virginia
Charlottesville, VA 22904 (USA)
E-mail: tbg7h@virginia.edu
b] C. A. Whitcomb, Dr. N. Kaylor, Prof. R. J. Davis
Department of Chemical Engineering
University of Virginia
Charlottesville, VA 22904 (USA)
E-mail: rjd4f@virginia.edu
c] Dr. N. Kaylor
Southwest Research Institute
[13–15]
that offer anti-Markovnikov selectivity.
Transition metal-catalyzed arene alkenylation (i.e., oxidative
olefin hydroarylation) that functions by a pathway involving
arene CÀ H activation and olefin insertion offers possible
advantages over traditional acid-catalyzed arene alkylation
[14,16–20]
(Scheme 2).
These potential advantages include: a) direct
arene alkenylation (rather than alkylation) via β-hydride elimi-
nation after the olefin insertion step, b) selective production of
1
-aryl alkane/alkene by circumventing carbocationic intermedi-
San Antonio, TX 78238 (USA)
ates that lead to Markovnikov selectivity, c) conversion of
electron-deficient arenes, d) new regioselectivity for alkenyla-
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.202001526
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Scheme 3. General proposed reaction mechanism for Rh mediated styrene
monomer synthesis.
Scheme 2. Generic catalytic cycles for transition metal-catalyzed arene
alkylation and alkenylation.
[14]
Scheme 4. The proposed induction period pathway for Rh(I) mediated
styrene synthesis using Cu(OAc) as the oxidant, (X=carboxylate groups).
2
[37]
[42–49]
tion or alkylation of substituted arenes, and e) inhibition of
polyalkylation since alkylated or alkenylated products can be
reported.
For example, the Davis, Kohler and Jones groups
used Pd catalysts supported on silica (Pd/SiO ), zeolite (Pd/NaY),
2
[14]
less reactive than starting arenes. Molecular catalysts involv-
titania (Pd/TiO ), and aluminum oxide (Pd/Al O ) for Heck
2
2
3
ing Ni, Ir, Ru, Pt, Pd and Rh have been studied for catalytic CÀ H
coupling reactions and found that dissolution of palladium is
[15,16,21–35]
[42,46,49]
alkylation or alkenylation of arenes with α-olefins.
correlated with reaction rate.
The solid Pd catalyst
Recently, we reported Rh-catalyzed arene alkenylation to
directly synthesize styrene and 1-aryl alkenes using molecular
functions as a reservoir for soluble Pd species (Scheme 5A). The
agglomeration and redeposition of dissolved Pd species can
also occur in the presence of supports such as SiO , Al O and
[13,14,36–41]
catalysts in homogeneous processes.
Based on the
2
2
3,
mechanistic studies, our initial proposed catalytic cycle involves
TiO2,
(Scheme 5B).
which
forms
Pd(0)/MOx
catalyst
precursors
[
46,48,49]
(
Scheme 3): a) Rh-carboxylate assisted arene CÀ H activation, b)
In the presence of phosphine ligands, it was
ethylene coordination and insertion into a Rh-aryl bond, c) β-
proposed that aryl halides likely oxidize Pd(0) to form a
II
hydride elimination, and d) alkenyl arene dissociation and
molecular L Pd (Ar)(X), (X=Br or Cl, L=phosphine) complex,
n
oxidation of a RhÀ H intermediate with CuX (X=carboxylate) to
which is proposed to participate in a Pd(II)/Pd(0) catalytic cycle
2
[48,49]
regenerate the starting catalyst. Under some conditions, we
(Scheme 5C).
[13,36,37]
observed an induction period,
which was proposed to be
As part of our efforts to delineate the dynamic between
heterogeneous Rh and soluble Rh species for the oxidative
alkenylation of arenes, we have prepared supported Rh species
due to the rapid decomposition of the soluble Rh catalyst
precursor to form insoluble Rh(s) followed by formation of the
active catalyst through dissolution of Rh from the inactive Rh(0)
species (Scheme 4).
on silica (SiO ) and nitrogen-doped carbon (NC) and studied
2
their reactivity as catalyst precursors. Herein, we report the use
of these supported Rh materials as a catalyst precursors for the
synthesis of styrene and other alkenyl arenes with evidence
In various Pd-catalyzed CÀ C coupling reactions, dissolution
and re-adsorption of supported Pd pre-catalysts have been
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[55]
detail in the experimental section. The catalyst labeled as Rh/
NC-HCl was treated with aqueous HCl (in an attempt to remove
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metal particles) followed by a thermal treatment with H . The
2
Rh/NC-IWI material was prepared using an incipient wetness
impregnation (IWI) method for deposition of the Rh precursor
Rh(NO ) . According to ICP-OES analysis, the Rh loading was
3
3
approximately 1.0 wt% for Rh/NC-HCl material and 1.5 wt% for
Rh/NC-IWI. The Rh detected by ICP-OES analysis is associated
with only the fraction of metal that could be removed from the
sample for analysis. TEM characterization of the Rh/NC-HCl
material showed Rh nanoparticles (Figure 2) with size of
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~
15(5) nm. The recovered Rh/NC-HCl catalysts after microwave
digestion showed that some Rh nanoparticles could not be
removed from the NC support, which suggests that some of the
Rh nanoparticles are likely embedded inside the NC matrix
(Figure S2). For the Rh/NC-HCl sample, XPS analysis of the N1s
core level shows a broad envelop associated with multiple N
species in the carbon matrix. Assuming only two types of sites
for simplicity, quantitative analysis reveals about a 2:1 ratio of
pyridinic sites (binding energy of ~398.3 eV) and graphitic sites
Scheme 5. General leaching/dissolution process (oxidation of Pd) to liberate
soluble, active molecular palladium species (A), agglomeration or redeposi-
tion of Pd(0) (B), and main catalytic cycle (C).
[55]
that catalytic activity is a result of leaching of soluble Rh from
the supported materials.
(binding energy of ~400.8 eV) (Figure 3). TEM analysis of Rh/
NC-IWI showed the size of the Rh nanoparticles was ~20(4) nm
(Figure 4).
Catalytic Arene Alkenylation using Rh/SiO2 as Catalyst
Results and Discussion
Precursor. We began our studies of catalysis using supported
Rh materials with the conversion of benzene and ethylene to
styrene using conditions that are similar to our previously
Synthesis of supported Rh materials. We prepared Rh nano-
particles supported on silica (Rh/SiO ) using a modified ion-
2
[50,51]
exchange method.
by inductively coupled plasma optical emission spectrometry
ICP-OES) analysis and found to be 3.6 wt%. Further character-
The weight loading of Rh was measured
(
ization of Rh/SiO was performed using transmission electron
2
microscopy (TEM), dihydrogen chemisorption, and X-ray photo-
electron spectroscopy (XPS). Dihydrogen chemisorption of the
Rh/SiO2 material indicated a metal dispersion of ~0.7 (H/Rh
ratio), and the average metal particle size measured by TEM
was found to be 2.2(5) nm (Figure 1, Table S1). The presence of
metallic Rh(0) was confirmed with XPS, which showed a binding
[37,52,53]
energy at 307.3 eV (Figure S1).
Nitrogen-doped carbon (NC) has been used as a support for
[
54–56]
single-atom catalysts with transition metals such as Co,
Figure 2. HAADF-STEM images of the synthesized Rh/NC-HCl material.
[54]
[54]
[54]
[57]
[58]
Mn, Ni, Fe, Rh, and Pd. Supported Rh on nitrogen-
doped carbon (Rh/NC-HCl and Rh/NC-IWI) were prepared using
a modified high-temperature thermal treatment as discussed in
Figure 3. XPS characterization of N1s from Rh/NC-HCl showing at least two
types of sites: 1) pyridinic sites at binding energy ~398.3 eV and 2) graphitic
sites with binding energy ~400.8 eV.
2
Figure 1. HAADF-STEM image of synthesized Rh/SiO .
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Figure 4. HAADF-STEM images of the synthesized Rh/NC-IWI catalyst.
Figure 5. Catalytic performance using Rh/SiO
filtration (green), with addition of 500 eq. of PVPy (relative to surface Rh
atoms) at time of zero for catalysis using Rh/SiO (blue), and using the filtrate
solution) collected from a Maitlis filtration test at 12 h (red). Catalytic
2
for styrene synthesis without
2
(
reported
precursors.
catalysis
using
molecular
Rh
catalyst
conditions: 5.3 mg Rh/SiO
2
material (0.00112 mmol), 15.5 mg PVPy, 10 mL
(240 eq. relative to surface Rh atoms), 20 eq.
[13,14,36–38]
All reactions were performed at least in
benzene, 72 mg Cu(OPiv)
2
hexamethylbenzene as the internal standard, 40 psig ethylene and 150
°
C.
triplicate with results reported with standard deviations.
Heating mixtures of Rh/SiO , benzene, ethylene and Cu(OPiv)
2
2
(
OPiv=trimethylacetate) to 150°C results in the production of
styrene (note: assuming 2 equivalents of CuX (X=carboxylate)
are consumed per equivalent of styrene, the maximum catalytic
SiO , we observed only a small amount of styrene production
2
2
(Figure 5, red plot). The minimal catalytic activity of the filtrate
is consistent with ICP-OES measurement of the filtrate (after
Maitlis filtration) indicating that only approximately 1 mol% of
turnovers (TOs) with y equivalents of CuX as limiting reagent is
2
[13]
y/2). As these experiments were performed under anaerobic
conditions, Cu(I) should not be reoxidized to Cu(II) in situ, so
that Cu(II) can be considered the limiting reagent. Monitoring
the styrene production after 26 hours reveals ~60(3)% con-
version based on Cu(II) as the limiting reagent (Figure S3). The
the original amount of Rh from Rh/SiO leached into the filtrate.
2
In other published work, insoluble poly(vinylpyridine) (PVPy)
has been used as a Pd(II) trap to confirm the leaching of Pd(II)
[42,61–63]
from Pd catalyst on silica support.
We performed similar
number of surface Rh atoms was determined from H chem-
trapping experiment to probe the possible leaching from
2
isorption, which indicated ~70% of Rh atoms in the metal
particles were located on the surface (Table S1). If the surface
Rh atoms are considered to be active for catalysis, then an
apparent turnover frequency (TOF) calculation can be made
from the observed rate of styrene production normalized by
catalysis using Rh/SiO . The addition of 500 equivalents of PVPy
2
to the catalytic reaction using Rh/SiO prevented the formation
2
of styrene (Figure 5, blue plot), which is consistent with the
hypothesis that the observed reactivity of Rh/SiO might be
2
from the leached and soluble Rh species. If it is assumed that
catalytic styrene production using fresh Rh/SiO2 is due to
leached Rh, using the calculated apparent TOFs (~1.8(3)×
surface Rh atoms in the reactor, which gives an apparent TOF of
À 4 À 1
5
.0(3)×10 s . This TOF is only meaningful if the heteroge-
À 2 À 1
2
À 4 À 1
neous Rh surface atoms are responsible for catalysis. Thus, we
define this value only as a point of comparison of the per Rh
10
s
for [Rh(μ-OAc)(η -C H ) ] versus 5.0(3)×10
s
for Rh/
2
4 2 2
SiO ), we can estimate that approximately 3 mol% of the Rh
2
atom activity using Rh/SiO versus the molecular precursor [Rh
from Rh/SiO is solubilized. Given that this is an approximation,
2
2
2
(
μ-OAc)(η -C H ) ] . Under the same reaction conditions using
the molecular Rh(I) complex [Rh(μ-OAc)(η -C H ) ] as catalyst
complicated by the dynamic nature of leaching, potential
readsorption/trapping by the filter and standard deviations for
TOF determinations, this estimate fits well with the 1% leaching
determined by ICP-OES.
Using an α-olefin such as propylene for the arene
alkenylation can result in multiple products depending on the
regioselectivity of olefin insertion and selectivity of β-hydride
elimination (Scheme 6). Using propylene and benzene coupling
as a probe reaction, three “linear” products (or anti-Markovnikov
products) and one “branched product” (Markovnikov product)
are observed (Scheme 6). For benzene and propylene, we
2
4 2 2
2
2
4 2 2
precursor, the styrene production measured after 0.5 h shows
7
1
0(2)% conversion of Cu(II) (Figure S4) and an apparent TOF (~
.8(3)×10 s ), which is approximately 36 times greater than
À 2 À 1
the apparent TOF using the Rh/SiO2 catalyst precursor. The
difference in apparent TOFs is consistent with either slower
catalysis using Rh/SiO (compared to soluble and molecular Rh
2
catalysis) or leaching of a small amount of Rh from the Rh/SiO2
to give soluble catalyst that is comparable in activity, per Rh
2
atom, to [Rh(μ-OAc)(η -C H ) ] .
2
4 2 2
Next, we performed experiments to probe Rh leaching
using ICP-OES measurement and the Maitlis’ filtration test.
compared the regioselectivity using Rh/SiO as catalyst precur-
2
[
49,59,60]
2
sor with [Rh(μ-OAc)(η -C H ) ] . Using Cu(OPiv) as an oxidant,
2
4 2 2
2
During the styrene synthesis mediated by Rh/SiO (conditions
the L/B ratio (i.e., the ratio of anti-Markovnikov to Markovnikov
products) for Rh/SiO2 mediated benzene and propylene
coupling reactions was 10(2):1, which is statistically similar to L/
2
listed above), a Maitlis filtration using a 200 μm filter at room
temperature was performed after 12 h of reaction, which
corresponded to 50(3)% conversion (based on Cu(II) as limiting
reagent) of the reaction (Figure 5). We then tested the filtrate
for catalytic activity. Under the same conditions used to test Rh/
2
B selectivity (14(2):1) using [Rh(μ-OAc)(η -C H ) ] as a catalyst
2
4 2 2
precursor under the identical conditions (Table S2). When Cu
(OHex)2 was used as an oxidant, statistically identical L/B
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Figure 6. Catalytic performance (turnovers versus time plots) of styrene
synthesis using fresh Rh/SiO without isolation (black) and using recovered
Rh/SiO solid (centrifuge/separation methods used; orange). Reaction
2
2
conditions: 10 mL benzene, 40 psig ethylene, 3.6 wt% Rh/SiO
.00112 mmol surface Rh atoms), 400 eq. Cu(OHex) (relative to surface Rh
2
(5.3 mg,
0
2
atoms), 20 eq. hexamethylbenzene as the internal standard (relative to
surface Rh atoms) at 150°C. Note, another 400 eq. Cu(OHex) is added at 3 h.
2
Catalyst isolation and separation were conducted at 3 h and fresh benzene,
2
400 eq. Cu(OHex) , and ethylene were recharged.
orders of magnitude faster than when using Rh/SiO . During
2
the Rh/SiO2 and Cu(OHex)2 mediated styrene production, a
filtration test was conducted after 3 h of reaction. ICP-OES
analysis of the filtrate from the reaction mixture confirmed that
approximately 34% of Rh (compared with overall Rh amount on
the silica support) leached from the silica support into solution.
Evidently, more Rh leaches from Rh/SiO when Cu(OHex) is
Scheme 6. Linear/branched product selectivity is dictated by the olefin
insertion step and, in some circumstances, the relative rates of subsequent
[14]
reactions (i.e., CurtinÀ Hammett conditions).
selectivity was observed with 8.0(2):1 for Rh/SiO and 7.4(1):1
2
2
2
2
for [Rh(μ-OAc)(η -C H ) ] (Table S3). These results are consistent
used as the oxidant (34% leaching of Rh) compared to the use
of Cu(OPiv)2 (~1%). The enhanced leaching of Rh with Cu
2
4 2 2
with heterogeneous materials leaching an active catalyst similar
to the species produced by the catalyst precursors Rh(μ-OAc)
(OHex) likely explains the increased rate of styrene production
2
2
(
η -C H ) ] and Rh/SiO .
using Cu(OHex) versus Cu(OPiv) when using Rh/SiO as the
2
4 2 2
2
2
2
2
Since CuX and HX (X=carboxylate) can be air-recycled to
CuX2 and H O, the Rh-catalyzed arene alkenylation has the
catalyst precursor. Thus, the extent of Rh leaching appears to
be dependent on the identity of the carboxylate ligand, which
could be due to the difference in basicity or a steric effect.
These results demonstrate that the oxidant {i.e., Cu(II)} and pro-
ligand (carboxylate) are important for the Rh leaching process.
2
3
9
potential to use catalytic amounts of Cu(II). We have examined
the reactivity of the Rh/SiO2 catalyst precursor for aerobic
styrene synthesis (1 atm air) and compared its aerobic catalytic
performance in the presence and absence of Cu(OPiv) (Fig-
Recycling of Rh/SiO . We tested the catalytic activity of
2
2
ure S5). Heating Rh/SiO2 in a mixture of benzene, 40 psig
recycled Rh/SiO that was recovered from a styrene synthesis
2
ethylene, 1 atm air, 100 eq. Cu(OPiv) and 1000 eq. HOPiv at
reaction after 3 h of reaction (Figure 6). Using this isolated Rh/
2
1
50°C, the supported catalyst precursor Rh/SiO exhibits
SiO material, we probed the conversion of benzene, ethylene
2
2
catalytic activity for styrene production. Compared with
catalysis using stoichiometric Cu(II) oxidant, a 185(10) TO of
styrene production was achieved at 100 h when 1 atm air was
added (Figure S5). In the absence of Cu(II) salt using air as the
sole oxidant, only 35(5) TO of styrene could be achieved after
and Cu(OHex) to styrene at 150°C. Figure 6 shows comparative
2
reaction profiles using freshly made Rh/SiO and the recovered
2
Rh/SiO material. A reaction using the recovered Rh/SiO shows
2
2
a longer induction period and slower rate of styrene production
(Figure 6). In contrast, using the filtrate after 3 h of reaction
(Figure 7) shows comparable catalysis to the catalytic reaction
1
04 h (Figure S5).
Impact of Carboxylate. Using Rh/SiO2 as
a
catalyst
using fresh Rh/SiO . Again, these results are consistent with the
2
precursor, catalysis using Cu(OHex)2 is approximately three
times more rapid than using Cu(OPiv) . Using Cu(OHex) as an
active catalyst being formed by leaching of Rh into solution.
Compared with an induction period of ~1 h when new Rh/
2
2
oxidant, we found that Rh/SiO2 mediated styrene synthesis
gave an overall yield of about 70(4)% {based on stoichiometric
and limiting Cu(II)} after 8 h (Figure 6). Under similar conditions
using Cu(OPiv) , Rh/SiO catalysts gave a 65(3)% yield after 26 h
SiO is used for the styrene synthesis, the recovered Rh/SiO2
2
material showed a longer induction period of ~12 h (Figure 6,
note: the start time point for the recycled catalytic reaction is at
3 h in Figure 6). Although approximately 66% of the overall Rh
2
2
2
(
Figure S3). For the molecular precursor [Rh(μ-OAc)(η -C H ) ] ,
species in the Rh/SiO catalyst did not leach into the solution
2
4 2 2
2
the apparent TOFs using Cu(OHex) and Cu(OPiv) are 3.0(2)×
during the catalysis at 3 h (based on ICP-OES analysis, see
2
2
À 2 À 1
À 2 À 1
1
0
s
and 1.5(2)×10 s , respectively, which is one to two
above), the reactivity of recovered Rh/SiO catalysts is substan-
2
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2 2
Figure 8. TEM images of Rh/SiO (a) recovered after 3 h of catalysis (Rh/SiO ,
Figure 7. Catalytic performance (turnovers versus time plots) for styrene
synthesis using fresh Rh/SiO without isolation (black) and using the
recovered filtrate after centrifuge/separation test (red). Reaction conditions:
10 mL benzene, 40 psig ethylene, 3.6 wt% Rh/SiO (5.3 mg, 0.00112 mmol
Cu(OHex) , ethylene, benzene, 150°C) and energy-filtered TEM map of
2
2
copper at the recovered Rh/SiO (b). Red box and blue box are the regions
2
that energy-filtered TEM map of copper was examined with the bright spots
on the right indicating copper.
2
2
surface Rh atoms), 400 eq. Cu(OHex) (relative to surface Rh atoms), 20 eq.
hexamethylbenzene as the internal standard and 150°C. Note, another
4
00 eq. Cu(OHex) is added at 3 h. Catalyst separation and isolation were
2
conducted at 3 h. Then, the filtrate was recharged with 400 eq. of Cu(II)
oxidant, and ethylene. The catalysis using the filtrate solution is shown in
red with the starting time at 3 h.
tially reduced. One explanation for the long induction period
observed with the recovered Rh/SiO material is oxidation of Rh
2
during the isolation of Rh/SiO (which requires transferring out
2
of and back into the glove box) following the initial catalytic
reaction, which could inhibit leaching and/or catalysis. Thus, we
probed dihydrogen reduction of the recovered Rh/SiO material
2
Figure 9. Catalytic performance (turnovers versus time plot) of styrene
[64,65]
at 150°C.
The dihydrogen-treated, recovered Rh/SiO2 ex-
2 2 2
synthesis using fresh Rh/SiO (black), and Cu(II)-pretreated Rh/SiO (blue), H /
hibited similar reactivity compared with the recovered and
untreated Rh/SiO2 (Figure S6), which suggests that dioxygen
Cu-Rh/SiO (yellow) and ethylene-benzene treated Rh/SiO (orange). Reaction
2
2
2
conditions: 10 mL benzene, 40 psig ethylene, 3.6 wt% Rh/SiO (5.3 mg,
0.00112 mmol surface Rh atoms), 400 eq. Cu(OHex) (relative to surface Rh
2
does not deactivate the recovered Rh/SiO catalysts.
2
atoms), 20 eq. hexamethylbenzene as the internal standard (relative to
We hypothesized that the Cu(II) salts might influence the
catalysis in two roles (beyond serving as the oxidant): 1) Cu(II)
likely oxidizes Rh(0) to form soluble catalyst, and 2) reduced Cu
surface Rh atoms) and 150°C. Note: another 400 eq. Cu(OHex) is added at
2
3
h for the black plot.
might be deposited on the Rh/SiO material as the reaction
2
proceeds, which could inhibit continued leaching of Rh into
solution. TEM and XPS characterization of recovered Rh/SiO2
material after 3 h of reaction were used to analyze possible
copper deposition. Indeed, energy-filtered TEM mapping of
copper on recovered Rh/SiO2 (red and blue box regions in
Figure 8) showed Cu deposition (bright spots within the blue
box, Figure 8b) on the recovered solid (Figure 8). Moreover, XPS
results are consistent with the hypothesis that Cu deposition
might inhibit Rh leaching from the Rh/SiO material. Moreover,
2
when a pretreatment of Rh/SiO2 with 40 psig ethylene and
benzene was carried out at 150°C for 3 h before the addition of
400 eq. Cu(OHex) salts (relative to the number of surface Rh
2
atoms), catalytic reactivity was reduced substantially (Figure 9,
orange). This result indicated that ethylene and/or benzene
analysis of recovered Rh/SiO confirmed Cu deposition with a
might also deactivate the Rh/SiO catalyst.
2
2
binding energy of approximately 933 eV, which was attributed
Catalytic Arene Alkenylation using Rh/NC catalysts. In our
effort to compare the impact of support on arene alkenylation
using Rh nanomaterials as catalyst precursors, we prepared Rh
materials supported by NC. The NC support has been demon-
[66–68]
to Cu(I)/Cu(0) species (Figure S7).
When Rh/SiO2 is pretreated in a benzene solution with
4
4
00 eq. Cu(OHex) at 150°C for 3 h, followed by charging with
2
[
55,58,69–71]
0 psig ethylene, the catalytic performance of the Cu(II) pre-
strated to stabilize supported transition metal catalysts.
treated Rh/SiO (Cu(II)-pretreated Rh/SiO ) showed about one-
The synthesis and characterization of Rh-NC materials have
been discussed above.
2
2
third of the activity compared to catalysis using fresh Rh/SiO2
Figure 9, blue). Separately, we treated Rh/SiO2 with 2 bar
(
Using 1 wt% Rh/NC-HCl as the catalyst precursor with Cu
(OHex)2 as the oxidant for the conversion of benzene and
ethylene (40 psig) to styrene at 150°C, a short induction period
of ~4 h was observed (Figure 10). After 28 h, a 35(3)% yield
(relative to Cu(II) oxidant) was achieved. In the absence of the
dihydrogen and 200 eq. of Cu(OHex) (relative to the amount of
2
surface Rh) at 150°C for 3 h assuming that reduced Cu(s) would
be formed. Using this H /Cu-Rh/SiO material for styrene
2
2
synthesis, the rate of styrene formation is dramatically reduced
compared with using fresh Rh/SiO2 (Figure 9, yellow). These
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SiO (see above), the soluble materials in the filtrate from cycle
2
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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0
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0
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9
0
1
2
3
4
5
6
7
1
were not active for the styrene production (Figure S9).
Compared with overall Rh loading at the beginning of the
catalysis, ICP-OES measurement of the recovered Rh/NC-HCl
catalysts at 116 h (cycle 3) and the soluble filtrate Rh species/
solution (116 h) showed approximately 47% of the starting Rh
loading and 0.4% of Rh in the solution (i.e., the filtrate),
respectively. The minimal leaching from the Rh/NC-HCl material
is consistent with the result that the filtrate from Rh/NC-HCl is
less active for styrene synthesis (Figure S9). In contrast, 34% Rh
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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3
4
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4
5
5
5
5
5
5
5
5
leaching was observed from Rh/SiO mediated catalysis, which
2
explains that the filtrate from Rh/SiO catalyst is more active
2
Figure 10. Turnovers versus time plot using 1 wt% Rh/NC-HCl mediated
styrene synthesis. Conditions: 10 mL benzene, 40 psig ethylene, 1 wt% Rh/
NC-HCl (13 mg, 0.00112 mmol Rh), 400 eq. Cu(OHex) (relative to Rh
2
measured through ICP-OES), and 20 eq. hexamethylbenzene (0.0224 mmol)
as the internal standard at 150°C.
than the filtrate from Rh/NC-HCl.
The 1.5 wt% Rh/NC-IWI material was synthesized and
characterized as described above. Large Rh nanoparticles (~
2
0 nm) were observed in the Rh/NC-IWI material. We tested Rh/
NC-IWI as a catalyst precursor at conditions similar to those
used to test Rh/NC-HCl (see above). The Rh/NC-IWI precursor
showed no discernable induction period at the time scale
studied (Figure 12). However, as shown in Figure 12, an
isolation of solid Rh/NC-IWI catalysts were conducted in cycle 2
and 3, followed by recharging of fresh benzene, copper(II) and
ethylene. The catalyst performance for styrene synthesis
decreased in cycles 2 and 3 (Figure 12), which is qualitatively
comparable with Rh-NC-HCl in Figure 11. An induction period
of 2–4 h was also observed in these two cycles. This might
indicate a possible continuous leaching from the solid Rh/NC-
IWI. Compared with Rh/NC-HCl, catalysis normalized by Rh
atom (measured by ICP-OES) using Rh/NC-IWI was much faster
producing an 87(3)% yield (relative to Cu(II) oxidant) of styrene
at 2 h (Figure 12) versus a 53(7)% yield (relative to Cu(II)
oxidant) of styrene at 40 h (Figure 10).
Rh, the NC support showed no catalytic reactivity for styrene
production through 38 h (Figure S8).
As described above for catalysis using Rh/SiO2, we
performed centrifuge and decanting experiments, which in-
volved the isolation of solid Rh/NC-HCl material after 28 h and
7
2 h of reaction. We then probed catalysis with the recovered
Rh/NC-HCl material for styrene production (Figure 11). After
specific time points, these experiments involved centrifuging
reaction solutions and decanting to separate insoluble materials
from soluble filtrate species. Then, the recovered solid material
was recharged with Cu(II), benzene and ethylene, heated to
1
50°C and styrene production was quantified. In a representa-
tive experiment, we recovered and recycled insoluble Rh/NC-
HCl twice (Figure 11). Catalytic cycles 2 and 3 show a slightly
reduced styrene production rate (Figure 11). Compared with
the ~4 h induction period when fresh Rh/NC-HCl is used,
slightly longer induction periods (~8 h) were observed for both
cycles 2 and 3 (Figure 11). In contrast to observations with Rh/
In contrast to Rh/SiO material, the insoluble solid materials
2
using Rh/NC precursors could be recycled without substantial
loss in activity or increased induction periods. The differences
between Rh/SiO2 and Rh/NC materials could possibly be
explained by 1) the Rh/NC materials are active heterogeneous
Figure 11. Catalytic turnovers versus time plots using Rh/NC-HCl as catalyst
precursor for styrene synthesis: cycle 1 (0–28 h), cycle 2 (32 h–68 h), and
cycle 3 (84 h–116 h). Reaction conditions: 10 mL benzene, 40 psig ethylene,
Figure 12. Catalytic turnovers versus time plots using Rh/NC-IWI precursor
mediated styrene synthesis: cycle 1 (0–2 h), cycle 2 (2 h–6 h), and cycle 3
(6 h–14 h). Reaction conditions: 10 mL benzene, 40 psig ethylene, 1 wt% Rh/
1
wt% Rh/NC-HCl (39 mg, 0.00336 mmol Rh), 400 eq. Cu(OHex)
2
(relative to
2
NC-IWI (13 mg, 0.00112 mmol Rh), 400 eq. Cu(OHex) , (relative to Rh
Rh measured through ICP-OES), and 20 eq. hexamethylbenzene as an
internal standard at 150°C. Insoluble solid material was separated and
recycled between each cycle with fresh benzene, Cu(II) oxidant, ethylene,
and internal standard recharged.
measured through ICP-OES), 20 eq. hexamethylbenzene as the internal
standard (relative to Rh loading) at 150°C. Insoluble solid material was
separated and recycled between each cycle with fresh benzene, Cu(II)
oxidant, ethylene, and internal standard recharged.
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catalysts or 2) for Rh/NC materials the catalytic styrene
production occurs by reversible leaching of soluble Rh into the
solution under catalytic conditions. To further compare Rh/NC
materials with soluble and molecular Rh catalysis, we compared
regioselectivities for arene alkenylation using the α-olefins
propylene and 1-pentene to quantify Markovnikov vs. anti- Conclusion
Markovnikov selectivity (Schemes 6 and 7). We assumed that if
the Rh/NC material is a heterogeneous catalyst then different
Markovnikov vs. anti-Markovnikov selectivity likely would be
observed. Also, the use of toluene provides an opportunity to
determine ortho/meta/para selectivity (Scheme 7).
active Rh species, in which leaching and dissolution of
supported Rh catalysts precursors forming soluble Rh catalysts
possibly occur.
1
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Supported Rh nanoparticles on silica (3.6 wt% Rh/SiO ) and
2
nitrogen-doped carbon (1 wt% Rh/NC-HCl and 1.5 wt% Rh/NC-
IWI) were synthesized and characterized by TEM, XPS, and ICP-
OES. The catalytic performances of supported Rh nanoparticles
for styrene formation were tested and compared. In this work,
we have demonstrated the leaching of 34% of the Rh/SiO2
catalyst during styrene synthesis is likely due to the oxidation of
reduced Rh upon reaction with Cu(II) salt. Further, the leached
and soluble Rh is most likely the active catalyst for arene
alkenylation. Although our studies indicate that the leaching of
Rh forms the soluble and active catalyst, the nitrogen-doped
carbon supports facilitate catalyst recycling. Compared with Rh/
SiO2 catalyst, nitrogen-doped carbon supported Rh catalysts
underwent a possible dissolution and re-adsorption process.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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5
Under identical catalytic conditions, the L/B selectivities
2
induced from Rh/SiO , Rh/NC-HCl and [Rh(μ-OAc)(η -C H ) ] for
2
2
4 2 2
both benzene and propylene coupling and toluene and 1-
pentene coupling were examined. The L/B selectivities for
benzene and propylene coupling are statistically identical for
Rh/SiO , Rh/NC-HCl and the molecular Rh complex [Rh(μ-OAc)
2
2
(
η -C H ) ] (Table 1). For coupling of benzene and propylene,
2
4 2 2
the L/B selectivity observed with Rh/NC-HCl is 7.5(2), while Rh/
2
SiO and molecular complex Rh(μ-OAc)(η -C H ) ] yield 8.0(2)
2
2
4 2 2
and 7.4(1), respectively. Likewise, arene alkenylation using
toluene and 1-pentene for Rh/NC-HCl, Rh/SiO and [Rh(μ-OAc)
2
2
(
η -C H ) ] catalysts gave statically identical L/B selectivities,
2
4 2 2
8
.2(9):1, 10.4(2):1, and 9.9(6):1, respectively. Also, the meta/ Experimental Section
2
para ratios for Rh/NC-HCl, Rh/SiO2 and [Rh(μ-OAc)(η -C H ) ]
2
4 2 2
General Methods. All reactions were performed under inert
conditions employing standard Schlenk techniques or in
catalysts are statistically identical at 1.7(1):1, 1.8(1):1, and
.7(1):1, respectively (Table S5). Again, the statistically identical
a
1
dinitrogen-filled glovebox unless specified otherwise. Glovebox
purity was maintained by periodic dinitrogen purges and was
L/B selectivities observed from both supported Rh catalysts and
2
monitored by a dioxygen analyzer (O concentration was <15 ppm
molecular salt [Rh(μ-OAc)(η -C H ) ] likely indicates the same
2
2
4 2 2
for all reactions). Tetrahydrofuran (THF) was dried over potassium
benzophenone ketyl under dinitrogen. Benzene was dried by
passage through columns of activated alumina. Pentane was dried
over sodium benzophenone ketyl. Benzene-d was used as received
6
and stored under a dinitrogen atmosphere over 4 Å molecular
1
sieves. H NMR spectra were acquired on a Varian Mercury 600 MHz
1
13
spectrometer. All H and C spectra are referenced against residual
1
proton signals ( H NMR) of deuterated solvents. GC/FID was
performed using a Shimadzu GC-2014 system with a 30 m×
9
0.25 mm HP5 column with 0.25 μm film thickness. GC-MS was
performed using a Shimadzu GCMS-QP2010 Plus instrument with a
0 m×0.25 mm SHRXI-5MS column with a 0.25 μm thickness.
3
Electron impact (EI) ionization was used.
For sampling reaction mixtures in heated Fisher-Porter reactors, the
reactors were allowed to cool to room temperature, sampled under
dinitrogen, recharged with olefin, and reheated. Aliquots of the
reaction mixture (<100 μL) were analyzed by GC/FID using relative
peak areas versus the internal standard (hexamethylbenzene).
Styrene, allyl benzene, α-methyl styrene, trans-β-methyl styrene,
and cis-β-methyl styrene production were quantified using linear
Scheme 7. Rh/NC mediated linear/branched product selectivity for coupling
reactions between toluene and 1-pentene with different ortho, meta, and
para selectivity.
2
Table 1. L/B selectivity comparison of Rh/NC-HCl, Rh/SiO
2
, and Rh(I) salt [Rh(μ-OAc)(η -C
2
H
4
)
2
]
2
mediated propylene and 1-pentene arene alkenylation.
[
a]
[b]
Catalysts
L/B selectivity for benzene and propylene coupling
L/B selectivity for benzene and propylene coupling
Rh/NC-HCl
Rh/SiO2
7.5(2)
8.0(2)
7.4(1)
8.2(9)
10.2(2)
9.9(6)
2
2 4 2 2
[Rh(μ-OAc)(η -C H ) ]
[a] Catalytic conditions: 10 mL benzene or toluene, 25 psig propylene, or 1000 eq. 1-pentene, 1 wt% Rh/NC-HCl (13 mg, 0.00112 mmol Rh), Rh/SiO (5.3 mg,
2
2
0
2 4 2 2 2
.00112 mmol Rh) or [Rh(μ-OAc)(η -C H ) ] (0.00112 mmol Rh), 400 eq. Cu(OHex) (relative to Rh), 20 eq. hexamethylbenzene as the internal standard (relative
to Rh) and 150°C. For the pentyl-products quantification, synthesized products are hydrogenated with 10 wt% Pd/C (10 mg) catalyst at 100 psig H
at room
2
temperature overnight.
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À 1
À 1
regression analysis of gas chromatograms of standard samples of
the authentic product. A plot of peak area ratios versus molar ratios
gave a regression line using hexamethylbenzene (HMB) as the
internal standard. For the GC/FID instrument, the slope and
correlation coefficient of the regression lines were 1.67 and 0.99
purity N flow (100 mLmin ) with a ramp rate of 10°Cmin . A
2
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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2
3
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vigorously stirred, room-temperature, 1 M HCl solution was then
used to remove any formed Rh nanoparticles from the surface.
After the acid treatment, the solid was thoroughly washed with DI
water and dried overnight at 120
°
C. Before use, the sample was
À 1
À 1
(
styrene), 0.87 and 0.99 (trans-stilbene), 1.40 and 0.99 (allylbenzene),
heated at 10°Cmin to 450°C in ultrahigh-purity H (100 mLmin )
2
1
.23 and 0.99 (α-methyl styrene), 1.47 and 0.99 (cis-β-methyl
and held at that temperature for 2 h. This catalyst has been labeled
as Rh/NC-HCl. A metal-free nitrogen carbon catalyst (NC) was
synthesized using an acetone solution of 0.4 g dicyandiamide
impregnated into 0.5 g of carbon black using the same initial
thermal treatment as the metal catalyst.
[37]
styrene), 1.38 and 0.99 (trans-β-methyl styrene), respectively. The
turnover (TO) is defined as the product (i.e. styrene or linear alkenyl
arenes) per catalyst site at any given time. The quantification of
various tolyl-pentane products is performed by a reported GC-MS
[40]
method. Briefly, for the 1-tolyl-1-pentane products, the slope and
correlation coefficient of the regression lines were 1.34 and 0.998
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
Additionally, a 1.5 wt% Rh/NC-IWI catalyst was synthesized using
the NC support. A solution of 5 wt% Rh metal from an aqueous Rh
(NO ) 6H O solution was impregnated onto the NC and subsequen-
(1-m-tolyl-1-pentane), 1.85 and 0.997 (1-p-tolyl-1-pentane) and 1.84
3
2
2
and 0.998 (1-o-tolyl-1-pentane), respectively. For decane, the slope
and correlation coefficient were 2.08 and 0.998, respectively. The
production of 2-tolyl-2-pentenes was quantified using the slope
and correlation coefficient for a fit of cumene:n-propylbenzene,
which enabled an approximation of the ratio of 1-tolyl-1-pentenes
to 2-tolyl-2-pentenes. The slope and correlation coefficient of the
regression line were 1.24 and 0.98 for cumene:n-propylbenzene,
respectively. Ethylene and propylene were purchased in gas
cylinders from GTS-Welco and used as received. Copper(II) 2-
tially dried overnight at 120
°
C. The solid was then heated to 450°C
À 1
with a ramp rate of 10°Cmin
in ultrahigh-purity H2 flow
À 1
(
100 mLmin ) and held at that temperature for 2 h.
Transmission electron microscopy (TEM). Samples analyzed by
transmission electron microscopy (TEM) were prepared by dispers-
ing the powders in cyclohexane or hexanes (99.5%, anhydrous,
Sigma-Aldrich) and sonicating for 1 minute before mounting on
Au-supported holey carbon grids. The catalyst samples were
imaged using an FEI Titan 80–300 operating at 300 kV. The Rh/SiO2
sample was also characterized by an EDAX energy dispersive
spectrometer (EDS) system in the scanning mode of TEM for single-
nanoparticle composition analyses and elemental mapping. Lattice
spacings were determined from selected-area electron diffraction
and Fourier transforms of high-resolution TEM images.
ethylhexanoate was purchased from Sigma-Aldrich and used as
2
received. Copper (II) pivalate and Rh(μ-OAc)(η -C
H
)
4
]
2
was pre-
2
2
72
pared according to literature procedures. All other reagents were
purchased from commercial sources and used as received. Before
catalysis, the supported Rh catalysts were treated with 2 bar of
dihydrogen at 150°C for 1 h. Caution: Mixtures of hydrocarbons and
oxidants are potentially explosive. In these experiments, we took
precautions to ensure safe mixtures and used pressure relief valves.
X-ray photoelectron spectroscopy (XPS). The XPS was performed
using a Phi VersaProbe III with a monochromatic Al Kα X-ray source
(1486.7 eV) and a hemispherical analyzer; instrument base pressure
Rh/SiO synthesis. Silica-supported Rh nanoparticle catalysts (Rh/
2
[50,51]
À 7
SiO ) were prepared via an ion exchange method
of the Rh
2
was ~10 Pa. Due to the air-sensitivity of Rh catalysts, a PHI
precursor using Davisil 636 silica (Sigma-Aldrich) as support. RhCl3
3 h O precursor (0.250 g, 99%, Sigma-Aldrich) was dissolved in a
solution of aqueous ammonia (5.5 mL ammonium hydroxide
14.8 M, 28%–30% of NH in water), ACS plus, Fisher Scientific, in
vacuum transfer vessel (Model 04-111) was used to protect the
sample from exposure to the ambient atmosphere during the
transfer from the glovebox to the XPS analysis chamber. Thus, the
entire procedure was performed without exposing the catalyst to
air or moisture. An X-ray beam of 100 μm was utilized and was
rastered over 1.4 mm to reduce the X-ray flux on the target. The
electron-energy analyzer was operated with a pass energy of 55 eV
for high-resolution scans with a 50 ms per step dwell time. Dual-
2
(
3
2
82 mL distilled deionized water). The RhCl3 solution was added
dropwise over 10 min to 4.75 g of acid-washed Davisil 636 silica in
1
14 mL of distilled deionized water at 70 C. The mixture was stirred
°
for 60 min at 70°C, and then cooled to room temperature. The
mixture was washed with water and vacuum dried overnight. After
calcination in flowing air (medical grade, GTS-Welco) at 400°C for
charge compensation, using a low-energy flood gun with a bias of
+
1
eV and a low-energy Ar beam, was utilized during data
2
h, the sample was reduced in flowing dihydrogen (99.999%, GTS-
acquisition. The Si 2p3/2 peak for SiO (103.5 eV) and C1s (284.6 eV)
2
Welco) at 250°C for 2 h. After reduction, the system was evacuated
and cooled to 30°C for analysis. At 30°C, the stoichiometric ratio of
H to surface Rh is assumed to be 1:1. Using dihydrogen
peak were used as a binding energy reference for the spectra.
ICP-OES measurement. Quantitative elemental analyses for the
supported Rh catalysts are carried out with inductively coupled
plasma optical emission spectrometry (ICP-OES) on a PerkinElmer
Avio-200 ICP spectrometer, and a microwave digestion method was
5
1
chemisorption on a Micromeritics ASAP 2020 adsorption system,
the number of available metal sites was determined by extrapolat-
ing the linear portion of the isotherm to zero pressure with the
assumption of no dihydrogen uptake on the support.
[57]
developed based on a modified literature method. In a typical
ICP sample preparation procedure, a certain amount of Rh sample
Rh/NC synthesis. Rhodium supported on nitrogen-doped carbon
(i.e. 10 mg Rh/SiO or 13 mg of Rh/NC) and aqua regia (6 mL) are
2
(
Rh/NC-HCl) was prepared similar to
a
method reported
added in a quartz vessel. With the assistance of microwaves, the
[55,57,73]
previously.
Carbon Black Pearls 2000 from Cabot Corporation
°
solution was heated at 180 C for 7 min with a ramp rate of about
was used as the carbon support. All of the other chemicals were
purchased from Sigma-Aldrich Corporation. An aqueous solution of
°
1
0 C/min. After cooling, the solution was transferred into a 10 mL
tube. A fraction of the above solution (0.2 mL) was diluted in a
Rh(NO3) 3H O was mixed with an ethanol solution of 1,10-
2
2
10 mL volumetric flask by the addition of 0.2 wt% HNO solution.
3
phenanthroline (Rh:phenanthroline molar ratio=1:2) for 20 min at
The concentration of Rh in the diluted solution was analyzed by
ICP-OES. To certify the accuracy of this method, the digestion was
compared to a reference sample analyzed by Galbraith Laboratories
8
0°C. The mixture was added dropwise to a vigorously stirred slurry
of carbon black in a 0.1 M NaOH aqueous solution at 80°C for two
hours. The slurry was then cooled, filtered, and thoroughly washed
with DI water. The complex was then impregnated with an acetone
solution of 80 wt% dicyandiamide relative to the complex, with
vigorous stirring followed by drying at 70°C overnight. The solid
(
2323 Sycamore Drive, Knoxville, TN 37921). The analytic results we
measured are consistent with the results reported from Galbraith
Laboratories.
was then thermally treated at 700
°
C for two hours under ultrahigh-
ChemCatChem 2020, 12, 1–12
www.chemcatchem.org
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© 2020 Wiley-VCH GmbH
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doi.org/10.1002/cctc.202001526
ChemCatChem
Catalytic alkenylation of benzene with ethylene and propylene
using Cu(X)2 {(X=OPiv (trimethylacetate) or OHex (2-ethyl hex-
anoate)}. Representative catalytic reactions are described here. A
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4 2
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(0.046 g, 0.23 mmol), and benzene (10 mL) was prepared in a
[
volumetric flask. Fisher-Porter reactors were charged with 10 mL
solution of benzene and copper salt (400 eq. per Rh, 4.4 mmol). The
vessels were sealed, pressurized with ethylene (40 psig) or
propylene (25 psig), and subsequently stirred and heated to 150°C.
For the catalysis with ethylene, the reactions were sampled every 2
or 4 h. With propylene, the reactions were sampled when the
solution turned yellowish brown and Cu oxidant was consumed.
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Catalytic alkenylation of toluene with 1-pentene using Cu(OHex)
2
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(OHex (2-ethyl hexanoate)). A stock solution containing [Rh(η -
2
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4 2
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or equivalent amount solid Rh catalysts: Rh/SiO , Rh/NC-HCl or Rh/
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NC-IWI, hexamethylbenzene (0.046 g, 0.23 mmol), and toluene
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(
The vessels were sealed and pressurized with nitrogen (50 psig),
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[
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[
This work was primiarly supported by the U.S. Department of
Energy, Office of Basic Energy Sciences, Chemical Sciences, Geo-
sciences, and Biosciences Division (DE-SC0000776) and the UVA-
MAXNET Energy effort. CW acknowledges support from the U.S.
National Science Foundation, Award#: CBET-1802482 for the
preparation and characterization of Rh materials.
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81.
The authors declare no conflict of interest.
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Keywords: Rhodium · Styrene · Alkenyl Arenes · Leaching · Re-
adsorption
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Manuscript received: September 19, 2020
Revised manuscript received: October 12, 2020
Version of record online: ■■■, ■■■■
[
ChemCatChem 2020, 12, 1–12
www.chemcatchem.org
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© 2020 Wiley-VCH GmbH
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FULL PAPERS
Z. Luo, C. A. Whitcomb, Dr. N. Kaylor,
Y. Zhang, Prof. S. Zhang, Prof. R. J.
Davis*, Prof. T. B. Gunnoe*
1
– 12
Oxidative Alkenylation of Arenes
Using Supported Rh Materials:
Evidence that Active Catalysts are
Formed by Rh Leaching
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Alkenylation of arenes: For the
oxidation of reduced Rh upon
reaction with Cu(II) carboxylate.
oxidative arene alkenylation reactions,
our study using various supported Rh
catalysts indicates possible leaching
of oxidized Rh into solution to form
the soluble and active catalyst. The
leaching of Rh is likely due to the
Compared with Rh/SiO catalyst,
2
nitrogen-doped carbon (NC)
supported Rh catalysts (Rh/NC)
underwent a possible dissolution and
reabsorption process.