Supported Aluminum Catalysts for Olefin Hydrogenation

Article abstract of DOI:10.1021/acscatal.6b02771

Three-coordinate alkylaluminum sites were developed on a catechol-containing porous organic polymer support (CatPOP A₂B₁). The CatPOP-based alkylaluminum sites were characterized by solid-state attenuated total reflectance IR spectroscopy, ¹H and ²⁷Al magic-angle-spinning NMR spectroscopy, pair-distribution function X-ray absorption spectroscopy, and elemental analysis. The low-coordinate organoaluminum sites can hydrogenate and isomerize a range of mono- and disubstituted alkenes and alkynes under mild conditions (75-100 °C, 5-14 bar H₂, 20 h). Results of experimental and computational mechanistic investigations suggest a heterolytic mechanism for the observed hydrogenation-isomerization activity.

Full text of DOI:10.1021/acscatal.6b02771

Research Article
pubs.acs.org/acscatalysis
Supported Aluminum Catalysts for Olefin Hydrogenation
Jeffrey Camacho-Bunquin,*,† Magali Ferrandon,† Ujjal Das,† Fulya Dogan,† Cong Liu,† Casey Larsen,†
Ana E. Platero-Prats,†,‡ Larry A. Curtiss,† Adam S. Hock,†,§ Jeffrey T. Miller,∥ SonBinh T. Nguyen,†,⊥
Christopher L. Marshall,† Massimiliano Delferro,† and Peter C. Stair*,†,⊥
†Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States
‡X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States
§Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois 60616, United States
∥Davidson School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907, United States
⊥Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
S
* Supporting Information
ABSTRACT: Three-coordinate alkylaluminum sites were
developed on a catechol-containing porous organic polymer
support (CatPOP A2B1). The CatPOP-based alkylaluminum
sites were characterized by solid-state attenuated total
1
reflectance IR spectroscopy, H and 27Al magic-angle-spinning
NMR spectroscopy, pair-distribution function X-ray absorption
spectroscopy, and elemental analysis. The low-coordinate
organoaluminum sites can hydrogenate and isomerize a range
of mono- and disubstituted alkenes and alkynes under mild
conditions (75−100 °C, 5−14 bar H2, 20 h). Results of
experimental and computational mechanistic investigations
suggest a heterolytic mechanism for the observed hydro-
genation−isomerization activity.
KEYWORDS: alkene hydrogenation, organoaluminum, porous organic polymer, surface organometallics, heterogeneous catalysis
conditions (>200 °C, >100 bar H2); however, the nature of
the active catalysts remains ill-defined.8a The breadth of
literature on aluminum-mediated transfer hydrogenation
reactions and other Al−H-mediated reductive transformations
suggests that purposeful design of Al coordination could result
in active reduction catalysts.9 Stephan and co-workers recently
reported the catalytic hydrogenation of imines with freely
soluble AliBu3 under comparable conditions (10−20 mol % Al,
> 100 °C, 100 bar H2) and proposed a two-step mechanism
proceeding through (1) CN bond hydroalumination and (2)
Al−N bond hydrogenolysis.8b
In this report, we present an active-site design motif that
imparts transition-metal-like hydrogenation activity to main-
group Lewis acids. Specifically, we report the development of
isolated, low-coordinate alkylaluminum sites supported on a
catechol-containing porous organic polymer (CatPOP A2B1)
that catalytically hydrogenate alkyl- and aryl-functionalized
alkenes and alkynes under relatively mild conditions (75 °C, 5
bar H2).10 These well-defined alkylaluminum sites exhibit
recyclability and higher reactivity compared with their
ational design of catalytic sites for the hydrogenation of
Runsaturated functionalities has been largely based on
established metal-mediated hydrogenation mechanisms.1 While
purposeful design of hydrogenation catalysts is more
established with base2 and precious3 transition metals, such
cannot be said for main-group elements because of the scarcity
of representative strategies. The emergence of frustated Lewis
pair (FLP) catalysts resulted in increased attention to main-
group-element-mediated hydrogenation.4 FLPs, operating via
bifunctional activation of hydrogen, hydrogenate a range of
unsaturated functional groups, including electron-rich/-poor
alkenes5 and polar unsaturated moieties.6 Despite the broad
scope of substrates, FLPs are mostly unreactive toward purely
aliphatic alkenes. A few non-FLP main-group catalysts have
been shown to be active toward aliphatic alkene hydro-
genation.7,8 Wang and co-workers reported that electrophilic
organoboranes (e.g., (F5C6)2B−H) hydrogenate aliphatic
alkenes (10−20 mol % B, 6 bar H2, 140 °C) via a two-step
mechanism involving (1) alkene hydroboration and (2)
C(alkyl)−B bond hydrogenolysis.7
Compared with boron, the manipulation of aluminum
coordination environments for catalytic hydrogenation with
molecular hydrogen as the reductant is far less explored.
Molecular organoaluminum species (e.g., AliBu3, HAliBu2)
hydrogenate mono- and dialkylated alkenes under severe
Received: September 28, 2016
Revised: November 29, 2016
Published: December 5, 2016
© XXXX American Chemical Society
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Research Article
pyrophoric molecular congeners. Mechanistic studies revealed a
heterolytic hydrogenation mechanism.
out via X-ray photoemission spectroscopy (XPS), X-ray pair
distribution function (PDF) studies, solid-state attenuated total
reflectance Ir (ATR-IR) spectroscopy, 1H and 27Al magic-angle-
spinning (MAS) NMR spectroscopy, and ligand-displacement
experiments.
XPS characterization confirmed the trivalent state of
aluminum in both systems (SI, Figure S6). X-ray differential
pair distribution function (dPDF) studies on 2 and 3 confirmed
the alumination of the CatPOP support. PDF features
corresponding to Al−X bonds (X = O and/or C) at ∼1.95 Å
were observed (SI, Figures S11−S12). Additionally, PDF data
showed no new distances corresponding to Al−Al dimers,
consistent with the single-site nature of the Al(III) centers in
both systems. Infrared spectroscopy confirmed the presence of
Al−C(alkyl) groups in both systems. Specifically, 2 and 3 show
strong absorptions at 685 and 687 cm−1, respectively (Figure
2a), characteristically indicative of terminal Al−C(alkyl)
Our study was motivated by the results of gas-phase catalyst
synthesis and testing using an integrated atomic layer
deposition−catalysis (I-ALD-CAT) tool developed in our
group.11 One-cycle grafting of trimethylaluminum (TMA) on
silica at 175 °C afforded supported methylaluminum sites
(SiO2-Al-Me) that hydrogenate propylene under plug-flow
conditions at 75 to 100 °C (see Supporting Information (SI)
section 1). During the hydrogenation catalysis, the period of
active propylene hydrogenation coincides with the detection of
methane in the product stream, presumably via the hydro-
genolysis of Al−CH3 groups. Prior reports on the grafting of
TMA on oxide supports reveal the formation of a multitude of
sites and the dynamic nature of these supported alkylaluminum
species,12 making structure−hydrogenation activity correlation
extremely challenging.13 Coper
́
et and co-workers recently
reported that tricoordinate Al(III) defect sites on activated γ-
Al2O3 activate X−H bonds (X = H, CH3).14 These low-
coordinate Al sites on γ-Al2O3 surfaces could be the active
catalytic sites responsible for the ethylene hydrogenation
activity that Hindin and Weller reported six decades ago.15
Alternatively, Ozin and co-workers recently demonstrated that
coordinatively unsaturated surface indium sites adjacent to a
Lewis-basic surface hydroxide could form FLP sites that
photocatalytically hydrogenate CO2.16 These studies suggest
that supported coordinatively unsaturated main-group sites may
be responsible for the observed reactivity of SiO2-Al-Me.
Hence, our group purposefully developed low-coordinate
alkylaluminum sites on a single-site support, CatPOP A2B1.
CatPOP A2B1, a catecholate-containing porous organic
polymer, is a polymeric support with oxygenated binding
sites that allow to support10,17 isolated low-coordinate,
monomeric organometallic species that are typically unstable
in solution. The isolation of the catecholate site on CatPOP
provides a unique ligand coordination environment that
prevents bimolecular decomposition pathways.
Figure 2. (a) Fingerprint region of the infrared spectra of 2 and 3 and
the latter’s catechol and Bpy derivatives. Also shown are the respective
solid-state (b) 1H and (c) 27Al MAS NMR spectra of 3 and its
derivatives (mass normalized). The 1H MAS NMR chemical shifts for
3 at δ = 0.8 ppm (CH2) and 0.3 ppm (CH3) are due to solvent
molecules entrapped in the CatPOP framework. A more detailed
discussion of the spectral data is provided in the Supporting
Information.
i
Exposure of a hexanes solution of AlR3 (R = Bu, CH3) to a
stoichiometric amount of CatPOP A2B1 (1) affords precatalysts
2 [CatPOP-Al-iBu] and 3 [CatPOP-Al6(CH3)8] (Figure 1).
groups18 and consistent with density functional theory
(DFT)-predicted terminal Al−C stretching frequencies
(Table S2). The presence of Al−alkyl groups was also
confirmed via protonolysis experiments; treatment with excess
molecular catechol and exposure to air (Figure 3) resulted in
the disappearance of the terminal Al−alkyl IR features (Figure
2a). Moreover, treatment with the polar aprotic ligand 2,2′-
bipyridine (Bpy) resulted in a derivative in which the Al−alkyl
IR feature is conserved.
Figure 1. Metalation reaction between AlR3 and CatPOP A2B1 (1).
Complete metalation of 1 is achieved with 1−1.3 equiv of AlR3 per
catechol unit (for details, see the Supporting Information).
1
The solid-state H MAS NMR spectra of 2 and 3 show
Self-limiting, quantitative metallation of catechol units in 1 is
achieved with 1 and 1.3 equiv of AliBu3 and AlMe3, respectively
(see SI section II). Elemental analyses confirmed monoalumi-
nation of each catechol unit with AliBu3, while metallation with
TMA affords a ∼ 4:1 mixture of monoaluminated (3a) and
dialuminated (3b) catecholates. This difference in metallation
selectivity is attributed to the respective solution-phase
nuclearities of the precursors: AliBu3 is monomeric13a while
AlMe3 is oligomeric,13b with the latter kinetically favoring the
formation of dialuminum sites (3b). Elucidation of the
aluminum coordination environment in 2 and 3 was carried
signals attributable to Al−iBu (δ = 0.8 ppm (CH2) and 0.3 ppm
(CH3)) and Al−CH3 (δ = 0 to −3 ppm (CH3)) groups,
respectively (Figure 2b), consistent with the chemical shifts
reported by Basset and co-workers on well-defined silica-
supported isobutylaluminum species.12 The 27Al MAS NMR
spectra of both precatalysts do not show any pronounced 27Al
peak (Figure 2c), consistent with the presence of tricoordinate
aluminum. DFT calculations of the NMR isotropic chemical
shifts and quadrupolar coupling constants were carried out for
selected Al model compounds (SI, Table S6). The tricoordinate
Al sites in 2 and 3 were predicted to occur at 160 and 165 ppm,
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Figure 3. Formation of higher-coordinate aluminum centers 4 (red) and 5 (blue) upon exposure of 3 to excess molecular catechol and air,
respectively. The 27Al chemical shift for the catechol derivative 4 (δ = 30 ppm) is consistent with the predicted value (see SI section II). The air-
exposed material 5 shows an 27Al chemical shift at 5 ppm, characteristic of hexacoordinate Al(III) sites.
with quadrupolar coupling constant values (Cq) of 34 and 35
MHz, respectively (see models I and II in SI, Table S6). The
lack of observable 27Al peaks in the NMR experiments can be
attributed to the low-symmetry coordination and strong
quadrupolar interactions arising from the quadrupolar nature
Table 1. Catalytic Hydrogenation of Aliphatic and Aromatic
Alkenes and Alkynes Using 3 as the Precatalyst
19
of 27Al nuclei (S = /2). Stephan and co-workers reported a
molecular three-coordinate carbene−alane complex that is
similarly 27Al NMR-silent.20 Exposure of 2 and 3 to air or an
excess of molecular catechol “healed” the low-symmetry three-
coordinate aluminum sites to form six-coordinate Al(III)
derivatives with intense 27Al peaks centered at 5 and 30 ppm,
respectively (Figures 2c and SI, Figure S17), consistent with the
DFT calculations (see model IV in SI, Table S6). The 27Al
MAS NMR findings, coupled with IR data and the NMR-scale
alumination reaction between CatPOP A2B1 and AliBu3, are the
basis for the proposed Al coordination in 2 and 3: the Al(III)
center is stabilized by a dianionic catecholate unit and an anionic
alkyl group.
5
Both 2 and 3 are active for hydrogenation of aliphatic and
aryl-substituted alkenes and alkynes under mild conditions.
Using a high-throughput batch reactor system and 1-octene (in
dodecane) as the substrate, it was observed that 3 is more active
than 2 over the range of conditions explored (1−15 mol % Al,
5−14 bar H2, 75−100 °C; for details, see SI section III).
Optimum hydrogenation with 3 was observed with Al loadings
of 8 mol % and higher at 75 °C and 14 bar H2 (96%
conversion; octane (90%) and internal octenes (6%)).
Hydrogenation is slower at lower H2 pressures (85%
conversion, 8 mol % Al with 3 or 5 bar H2) and, surprisingly,
at higher temperatures. Experiments at 100 °C (14 bar H2, 8
mol % Al) effected 75% conversion to a mixture of internal
octenes (60%) and octane (15%). The lower hydrogenation
rate and increased selectivity to internal olefins at elevated
temperatures are presumably due to higher β-H transfer/
elimination rates. The lower activity of 2 (SI, Figures S18−S19)
is attributed to the steric encumbrance of the Al by bulkier
isobutyl groups. Under the conditions explored, the non-
metallated CatPOP support does not react, while the molecular
precursors AlMe3 and AliBu3 effect stoichiometric conversions;
molecular catecholate−AlMe,21 which is generally oligomeric in
solution, is completely unreactive. Replacement of the H2
atmosphere with N2 gave only isomerization products (20 to
30% conversion) in the presence of either 2 or 3 (Table S1).
Both precatalysts can be stored in an air- and moisture-free
environment for 6 months with very little decrease in catalytic
activity. Exposure to air results in loss of catalytic activity.
The relative reactivities of the more active catalyst 3 toward
alkenes and alkynes with varying degrees of substitution were
evaluated under identical testing conditions (9 mol % Al, 75 °C,
14 bar H2, 20 h; Table 1). Catalyst 3 hydrogenates aliphatic
alkenes (entries A−C). 1-Octene is quantitatively reduced to a
a
Each hydrogenation experiment was carried out under batch reactor
conditions using 9 mol % Al and 0.25 M substrates in dodecane at 75
°C and 14 bar H2 for 20 h.
mixture mainly composed of n-octane (97%). tert-Butyl-
ethylene, a nonisomerizable alkene, is hydrogenated to the
corresponding alkane with 62% conversion and 100%
selectivity (entry B). The lower conversion is attributed to its
high volatility (bp = 41 °C), which limits its liquid-phase
solubility. 2,3-Dimethyl-2-butene (entry C), a tetrasubstituted
alkene, was hydrogenated at a much lower rate because of the
sterically encumbered CC bond. Both 2 and 3 hydrogenate
aryl-substituted alkenes and alkynes (see SI, Table S1 for
hydrogenation studies with 2). Phenylacetylene, a terminal
alkyne (entry D), is completely hydrogenated to styrene (73%)
and ethylbenzene (27%), while the internal alkynes 1-
phenylpropyne and diphenylacetylene (entries E and F) are
converted to mixtures composed of cis/trans-alkenes and the
corresponding C2 alkane products. Substrates with a disub-
stituted vinyl carbon (1-phenylcyclohexene and 1,1-diphenyl-
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ethylene; entries G and H) undergo substoichiometric
hydrogenation, presumably because of the larger barriers to
Al coordination with the sterically hindered vinyl carbons. Polar
unsaturated groups (ketones and imines, entries I and J) do not
react at all, consistent with the Lewis acidic nature of the active
sites. In addition, catalytic hydrogenation is completely
suppressed in aromatic hydrocarbon solvents such as toluene.
Prior reports on trialkylaluminum grafting on oxide supports
revealed that supported alkylaluminum fragments do not
undergo protonolysis in ether and toluene as a result of the
formation of stable Lewis acid−π adducts.13b
The hydrogenation activity of 2 and 3 validates our active-
site design motif for catalytic hydrogenation: coordinatively
unsaturated monocatecholato monoalkyl Al(III) centers. We
attribute the hydrogenation activity to low Al coordination
that is not easily achieved with freely soluble monocatecholate
Al(III) species. This is further confirmed by the inactivity of the
higher-coordinated catechol, Bpy, and air-exposed derivatives.22
No aromatic ring hydrogenation was observed for phenyl-
substituted substrates, thus ruling out metal-nanoparticle-
catalyzed hydrogenation. Overall, alkene and alkyne dimeriza-
tion/oligomerization products from radical-mediated pathways
were not observed. At this point, the contributions of the
redox-active catecholato ligand to the activity are still unclear;
however, the hydrogenation activity of the SiO2-Al-R catalysts
(vide supra) suggests that redox activity of the support is not a
prerequisite (SI, Figures S20−S21).
Catalytic deuteration of 1-octene was carried out to gain
insights into the hydrogenation mechanism. Deuteration of 1-
octene with 3 (1 mol %, 75 °C, 14 bar D2) gave
dideuterooctane C8H16D2 (10%; M+ = 116 by GC−MS) and
a mixture of internal octenes (24%). Octene dideuteration is
consistent with a two-step hydrogenation mechanism similar to
the hydroalumination/C−N bond hydrogenolysis mechanism
proposed by Stephan and co-workers.8b On the other hand, 1-
octene deuteration at 100 °C resulted in near-quantitative
isomerization to monodeuterated internal octenes, consistent
with the observed lower rate of 1-octene hydrogenation at 100
°C due to the competitive isomerization.
Figure 4. Probable hydrogenation mechanisms.
mol, nearly 16 kcal/mol lower than the corresponding barrier
for H2 activation over the Al−C(alkyl) bond (50 kcal/mol),
suggesting that the heterolytic H2 activation mechanism is
kinetically more favorable. IR analysis of spent catalysts 2 and 3
showed no active aluminum hydride species; the characteristic
IR features of both spent catalysts are very similar to those of 2,
indicating that an aluminum-bound alkyl species (d) is the
catalyst resting state (Figure 4; see the detailed IR spectra in SI,
Figures S9 and S10), indicating potential catalyst recyclability.
A three-cycle 1-octene hydrogenation test using both systems
confirmed their recyclability (see SI, Figure S22), a feature that
cannot be easily achieved with their pyrophoric and highly
unstable molecular congeners.
To gain further insight into the H2 activation mechanism, we
carried out natural-bond-orbital (NBO) analyses of the
corresponding transition states TS1 and TS2 (Figure 5).
One intriguing question is the mechanism of activation of
hydrogen by precatalysts 2 and 3. The tricoordinate
alkylaluminum sites in both systems can potentially activate
hydrogen via σ-bond metathesis-like direct hydrogenolysis of
the Al−C(alkyl) bond to give the corresponding alkane and an
active Al−H intermediate (a → c; Figure 4). The importance of
Al−C groups in the precatalyst to generate supported
hydrogenation-active Al−H species was earlier demonstrated
by Basset and co-workers on isobutylaluminum species
supported on Al2O3, where the hydrogenolysis product Al−H
was shown to be active for ethylene hydrogenation and
polymerization.23 Wang et al.7 have shown this to be the main
H2 activation pathway for highly electrophilic organoboranes
with a B−C(alkyl) moiety. Alternatively, heterolytic H2
activation over an Al−O bond can generate a monoprotic
catecholato backbone and a hydrido alkylaluminum species (a
→ b). The Al−C bond in b can then undergo protonation to
generate the corresponding alkane and an active Al−H species.
This mechanism is similar to the heterolytic activation of H−H
and C−H bonds over a metal−oxygen bond proposed for
supported transition (Cr3+, Fe2+, Co2+)24 and post-transition
(Zn2+, Ga3+)25 metals, where the metal site acts as a Lewis acid
catalyst. DFT calculations predict that the free energy barrier
(ΔG⧧) for H2 activation over the Al−O bond in 3a is 34.2 kcal/
Figure 5. Natural bond orbitals involved in charge transfer in the
transition states for H2 activation over (a) the Al−O bond (TS1) and
(b) the Al−C bond (TS2).
NBO analysis revealed significant two-way charge transfer in
the transition states, from H2 to Al in 3 and vice versa, resulting
in transition state stabilization. Charge transfer from H2 to 3
involves the σ bonding orbital of H2 (donor) and an empty 3p
orbital of Al (acceptor), both observed in the transition states
for Al−O and Al−C bond hydrogenolysis. The reverse charge
transfer from 3 to H2 involves the σ* antibonding orbital of H2
(acceptor); however, the donor orbital is different for the two
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transition states. In TS2, the Al−C σ orbital acts as the electron
donor (σ-bond metathesis), while in the case of TS1, an oxygen
nonbonding p orbital is the donor (direct hydrogenolysis of the
Al−O bond). An additional charge-transfer stabilization
mechanism is also observed in TS1 involving a filled
nonbonding 2p orbital of O donating charge to the empty 3p
orbital on Al. This explains the more facile H2 activation over
an Al−O bond. By contrast, this additional stabilization is
missing in TS2 since the Al-bound carbon center does not have
a nonbonding electron pair. Additionally, DFT calculations also
suggest that the energy barrier for H2 scission over an Al−O
bond in the dialuminum species 3b is lower by ∼15 kcal/mol
compared with the monoaluminum sites 3a (SI, Figures S24
and S25). At this point, it is still unclear whether the dinuclear
Al moieties in 3 significantly contribute to the observed higher
reactivity.
In summary, well-defined CatPOP-supported organoalumi-
num precatalysts were synthesized and rigorously characterized.
The hydrogenation activity displayed by the Al(III) sites in 2
and 3 is rare in the area of organoaluminum catalysis, validating
the effectiveness of the catalyst design strategy: isolated,
monomeric, and coordinatively unsaturated organoaluminum sites
that are not easily formed in solution can mediate organic
transformations that are unprecedented compared with their
dimeric congeners. More rigorous mechanistic studies are
currently underway to gain insights into the mechanism of
hydrogenation and the nature of the active hydrogenation
species.
ANL. High-energy X-ray scattering data suitable for PDF
analyses were collected at beamline 11-ID-B at the Advanced
Photon Source (APS) at ANL. APS is a U.S. DOE User Facility
operated for the DOE Office of Science by ANL under
Contract DE-AC02-06CH11357. We also thank Dr. Javier
Bareno for the assistance with the air-sensitive XPS experi-
̃
ments.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b02771.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: bunquin@anl.gov.
*E-mail: pstair@northwestern.edu.
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ORCID
Jeffrey Camacho-Bunquin: 0000-0003-2297-3404
Adam S. Hock: 0000-0003-1440-1473
SonBinh T. Nguyen: 0000-0002-6977-3445
Christopher L. Marshall: 0000-0002-1285-7648
Massimiliano Delferro: 0000-0002-4443-165X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the U.S. Department of Energy
(DOE), Office of Basic Energy Sciences, Division of Chemical
Sciences, Geosciences, and Biosciences, under Contract DE-
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