Upgrading light hydrocarbons via tandem catalysis: A dual homogeneous Ta/Ir system for alkane/alkene coupling

Article abstract of DOI:10.1021/ja405191a

Light alkanes and alkenes are abundant but are underutilized as energy carriers because of their high volatility and low energy density. A tandem catalytic approach for the coupling of alkanes and alkenes has been developed in order to upgrade these light hydrocarbons into heavier fuel molecules. This process involves alkane dehydrogenation by a pincer-ligated iridium complex and alkene dimerization by a Cp*TaCl₂(alkene) catalyst. These two homogeneous catalysts operate with up to 60/30 cooperative turnovers (Ir/Ta) in the dimerization of 1-hexene/n-heptane, giving C₁₃/C₁₄ products in 40% yield. This dual system can also effect the catalytic dimerization of n-heptane (neohexene as the H₂ acceptor) with cooperative turnover numbers of 22/3 (Ir/Ta).

Full text of DOI:10.1021/ja405191a

Communication
pubs.acs.org/JACS
Upgrading Light Hydrocarbons via Tandem Catalysis: A Dual
Homogeneous Ta/Ir System for Alkane/Alkene Coupling
David C. Leitch, Yan Choi Lam, Jay A. Labinger,* and John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, California 91125,
United States
S
* Supporting Information
Scheme 1. Idealized Tandem Catalytic Approach toward
Alkane/Alkene Upgrading
ABSTRACT: Light alkanes and alkenes are abundant but
are underutilized as energy carriers because of their high
volatility and low energy density. A tandem catalytic
approach for the coupling of alkanes and alkenes has been
developed in order to upgrade these light hydrocarbons
into heavier fuel molecules. This process involves alkane
dehydrogenation by a pincer-ligated iridium complex
and alkene dimerization by a Cp*TaCl2(alkene) catalyst.
These two homogeneous catalysts operate with up to
60/30 cooperative turnovers (Ir/Ta) in the dimerization of
1-hexene/n-heptane, giving C13/C14 products in 40% yield.
This dual system can also effect the catalytic dimerization
of n-heptane (neohexene as the H2 acceptor) with
cooperative turnover numbers of 22/3 (Ir/Ta).
ight hydrocarbons, especially those with 4−6 carbons,
Lconstitute a significant but underutilized fraction of carbon-
based energy carriers.1 Because of their volatility and low energy
density, these materials cannot be used as transportation fuels
on a large scale. While pure light alkenes such as ethylene and
propylene are extensively used in polymer and chemical
synthesis,2 mixed alkane/alkene streams, which are unsuitable
as monomers or precursors, are abundant byproducts of
catalytic cracking3 and Fischer−Tropsch synthesis.4 Further-
more, with increased exploitation of natural gas,5 heavy oils from
bitumen and kerogen,6 and lignocellulose,7 the proportion of
light hydrocarbons in the global energy mix will only increase.
One method to turn light hydrocarbons into useful fuels would
be to upgrade these feedstocks to higher-molecular-weight
compounds. In this vein, alkane metathesis has emerged as a
possible technology for upgrading light alkanes via combined
alkane dehydrogenation and alkene metathesis; however, such
a process tends to afford a statistical distribution of products,
with little selectivity for a particular desired weight fraction.8−10
While this chemistry may hold promise, a more selective
method would simplify purification processes and thus increase
the product yield and efficiency.
a second catalyst would convert the alkane component to a
1-alkene while hydrogenating the C2n product to an alkane. The
1-alkene thus formed could then be catalytically dimerized with
a second equivalent of 1-alkene, and the cycle would continue;
the net reaction would be coupling of the alkane and alkene to
give the higher alkane without generation of byproducts. Calcula-
tions indicate that such a reaction should be thermodynamically
favored below ∼250 °C;11 therefore, the catalysts for dimeriza-
tion and transfer hydrogenation must operate with appreciable
rates at relatively mild temperatures.
The present work represents our initial efforts to develop such
an upgrading scheme, involving a dual homogeneous catalytic
system in which the tantalum catalyst Cp*TaCl2(alkene) (1)
(Cp* = C5Me5) effects alkene dimerization12 and alkane/alkene
transfer hydrogenation is carried out by an iridium catalyst
(2).8b,10,13 We demonstrate that these two species function in
tandem to effect alkane/alkene coupling, although the complete
synthetic cycle has not yet been fully achieved.
Often the major challenge in developing a tandem catalytic
process is catalyst compatibility, particularly in homogeneous
reactions.8b,14 In the present case, alkane/alkene transfer hydro-
genation requires high temperatures and relatively long reaction
times to achieve high conversion. Pincer-ligated iridium catalysts
are the current state-of-the-art homogeneous systems for these
transformations and have demonstrated applicability in tandem
reactions, namely, alkane metathesis with group-6 cocatalysts.8b,10
Herein we describe a complementary approach for upgrading
light hydrocarbons based on a tandem alkane dehydrogenation/
alkene dimerization sequence. This process takes advantage of
the mixed nature of many light byproduct streams by in-
corporating both alkanes and alkenes as substrates. In an ideal
system (shown for a linear alkane and 1-alkene in Scheme 1),
one catalyst would dimerize the alkene component of the mixed
feedstock to a C2n alkene. Subsequent transfer hydrogenation by
Received: May 23, 2013
Published: June 25, 2013
© 2013 American Chemical Society
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While these Ir catalysts are reported to be active at temperatures
as low as 100 °C,13 many alkene dimerization catalysts are
temperature-sensitive with short lifetimes.15 Furthermore, Ir−pincer
catalysts are known to be very sensitive to trace impurities,
including Lewis acids such as BR3 (and even N2!),13 whereas
many alkene dimerization systems require strong Lewis or
Brønsted acid activators, such as methylaluminoxane (MAO) or
other group-13 compounds.15,16 Finally, deactivation of one
or both catalysts by ligand transfer is a concern; for example, Ir−
pincer catalysts are known to undergo chloride ligand transfer
from Grubbs-type Ru alkene metathesis catalysts, rendering
them inactive.8b
In light of these potential complications, we chose the Ta-
based alkene dimerization catalyst 1, developed by Schrock,12
for our first trials. Cp*TaX2(alkene) complexes are reported to
be “indefinitely active” for the selective dimerization of 1-alkenes
to two regioisomers at temperatures up to 100 °C (eq 1); they
are inert to internal alkenes and the product 1,1-disubstituted
alkenes. Importantly, no cocatalyst or activator is required.
In contrast, use of the known terminal-selective13c Ir catalyst
2b (10 mM) with 1 (16 mM) afforded n-hexane, internal
hexenes, and C12, C13, and C14 alkenes; the latter two products
were formed in a combined yield of 22% (Table 1, entry 1).
[The identities of the C12/C13/C14 products were confirmed by
comparison to a product mixture resulting from codimerization
of 1-hexene and 1-heptene by 1 (Figure S4).11] Thus, this dual
catalyst system can incorporate the n-alkane solvent into the
dimerization cycle, demonstrating the feasibility of the proposed
tandem catalytic process. Under these reaction conditions, the
catalysts exhibited a “cooperativity”19 of 35%. The full synthetic
cycle of Scheme 1 was not completed, however, as no C12/C13/
C14 alkanes were observed; separate control experiments
showed that the 1,1-disubstituted alkene isomers afforded by
1 are poor acceptors for hydrogen transfer promoted by 2b.
To assess the factors that influence the efficiency of this
tandem Ta/Ir system, a number of reaction parameters were
varied (Table 1). Reducing the catalyst loadings to 8 and 5 mM
for 1 and 2b, respectively, improved the overall turnover
number (TON) without sacrificing product yield or coopera-
tivity (entry 2). Decreasing the reaction temperature to 100 °C
dramatically improved the cooperativity (63%) with a slight
reduction in yield (entry 3), while altering the 1/2b ratio
(entries 4 and 5) was detrimental to the cooperativity. Notably,
increasing [1-hexene] to 1000 mM merely resulted in the produc-
tion of more C12 dimer (entry 6); the absolute amounts of C13
and C14 were nearly identical to those obtained with 250 mM
1-hexene (entry 3). Another intriguing observation is that in most
cases the amounts of C13 and C14 were very similar. One would
expect the C13 codimer to be the major C7-containing species in
view of the large excess of 1-hexene present relative to the
amount of 1-heptene generated.
To demonstrate catalytic alkane/alkene coupling, we examined
the conversion of a 1-hexene/n-heptane mixture in the presence
of the Ir transfer hydrogenation catalyst (t-Bu4[POCOP]-
Ir(C2H4) (2a)13d or t-Bu4[PCP]IrH4 (2b)13a and Cp*TaCl2-
(C2H4) (1)12c (Figure 1); the formation of C13 and C14 products
here would signal the operation of tandem catalysis.
Together, these two observations suggest that most of the
tandem catalytic productivity occurs only after high conversion,
when [1-hexene] is low. To confirm this, the reaction from
entry 3 of Table 1 was monitored over time (Figure 2). Initially,
1-hexene was rapidly consumed and transformed mainly into
C
12 by homodimerization. The C13 product formed slowly, and
Figure 1. Precatalysts investigated for tandem catalysis.
the C14 dimer did not form to a significant degree until nearly
three half-lives had passed; more than half the amount of C14
was generated after >98% of the 1-hexene was consumed.
These results indicate that the relative rates of dimerization and
dehydrogenation are best matched at low [1-alkene], in accord
with previous observations that high alkene concentrations
inhibit transfer hydrogenation.13 In addition, competitive alkene
isomerization is minimized at low [1-alkene]; isomerization
reduces the catalyst cooperativity by converting 1-heptene into
internal heptenes that do not undergo dimerization (eq 3).
Heating a solution of [POCOP]Ir catalyst 2a and 1 in
1-hexene/n-heptane (250−1300 mM alkene) under a range of
reaction conditions (100−150 °C, 1−48 h, Ta:Ir = 1−3:1) gave
no measurable amount of C13 or C14 products; however, it is
evident from GC analysis of the resulting hydrocarbon mixture
that both catalysts did operate: nearly all of the 1-hexene was
consumed, with n-hexane and C12 hexene dimers as the major
observed products (eq 2 and Figure S511). n-Hexane resulted
Importantly, control experiments showed that the initial rates of
dimerization and transfer hydrogenation in the tandem catalytic
reaction are identical to the catalytic rates exhibited by
the corresponding catalysts operating separately (Figure S8).11
This indicates that the two catalysts truly operate independently
in solution under these conditions, with no mutual inhibition or
deactivation.
The reaction profile in Figure 2 indicates that better C13/C14
yield, catalyst cooperativity, and TONs might be achieved
by maintaining a steady, low concentration of 1-hexene.
from Ir-catalyzed transfer hydrogenation, while the C12 pro-
ducts formed via Ta-catalyzed 1-hexene dimerization. Hence,
deactivation caused by catalyst incompatibility is not respon-
sible for the lack of cooperation. Instead, we hypothesize, as
have others,8b,17 that catalyst 2a is not selective for 1-alkene
formation, resulting in an overall concentration of 1-heptene
that was too low for catalyst 1 to incorporate; internal heptenes,
which are not dimerized by catalyst 1, were therefore the other
hydrogen transfer products.18
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a
Table 1. Evaluation of Catalyst Cooperativity in 1-Hexene/n-Heptane Coupling by 1 and 2b
b
b
b
c
c
d
entry [1-hexene]0 (mM) [1]/[2b] (mM) T (°C) n-hexane (mM)
C12 (mM)
C13/C14 (mM)
TON for 1
TON for 2b
coop. (%)
1
2
3
4
5
6
7
250
250
16/10
8/5
125
125
100
100
100
100
98
112
106
52
57
58
14/13
15/13
13/10
13/9
5 (2)
11 (4)
13 (3)
19 (4)
8 (1)
11 (4)
21 (8)
10 (6)
15 (6)
11 (5)
17 (8)
35
39
63
41
47
45
250
8/5
81
250
5/5
78
75
250
12/5
8/5
53
85
9/8
1000
1200
87
432
293
18/10
181/59
58 (4)
67 (30)
e
f
g
g
g
8/5
329
66 (60)
91
a
b
See the Supporting Information for an expanded table. Determined by GC/FID using adamantane as an internal standard; values are averages of at
c
d
least two runs. TONs in parentheses are for the production of C13 + C14 (1) and the total 1-heptene incorporated into C13 and C14 (2b). Defined
e
f
as in ref 19. 1-Hexene was added at ∼50 mM h−1 over 24 h, and the total reaction time was 32−36 h; ∼95% conversion. The reaction was run at
g
reflux (n-heptane, bp ∼98 °C) using an oil bath at 120 °C. Estimated on the basis of mass balance (see the Supporting Information for details).
◆
Figure 2. Reaction progress for entry 3 of Table 1. Legend: blue
,
■
◇
▲
1-hexene; red , n-hexane; green , internal hexenes; purple , C12;
20
□
○
blue , C13; brown , C14. Lines are drawn as visual guides.
Longer reaction times did not increase the amount of C14,
signaling catalyst decomposition.
In summary, we have shown that a homogeneous dual Ta/Ir
catalyst system can effect both selective coupling of alkane/
alkene mixtures and dimerization of alkanes to branched alkene
products with a high degree of cooperation. Hydrogen transfer
from alkane to dimeric alkene is the only missing step in the full
upgrading process shown in Scheme 1. This final step might be
accomplished by modifying the Ir catalyst and/or by selecting a
different dimerization catalyst to produce less-branched alkene
isomers; these latter products may be more desirable from a
fuels standpoint as well. Investigations along all of these lines
are currently in progress.
Indeed, when 1-hexene was added gradually via syringe pump
to a refluxing n-heptane solution of 1 (8 mM) and 2b (5 mM),
the outcome improved dramatically (Table 1, entry 7). When
the amount of 1-hexene added was equivalent to 1200 mM,
the yield of C13 + C14 was 40%, corresponding to cooperative
TONs of 60 for 2b and 30 for 1 and an estimated cooperativity
of 91%.11 These results are in stark contrast to those obtained
when 1000 mM 1-hexene was added all at once, where the yield
of C13 + C14 was only 6% (entry 6).
A variant of this tandem catalysis is the catalytic coupling of
alkanes using a sacrificial hydrogen acceptor that is inert to alkene
dimerization. Neohexene is a sterically encumbered hydrogen
acceptor commonly used in Ir-catalyzed alkane dehydrogen-
ation;13 control experiments indicated that neohexene is not
dimerized by catalyst 1 at 100 °C and that its presence does not
shut down catalytic dimerization of 1-heptene. A combination
of the more active Ir catalyst 2c (2 mM) with 1 (8 mM) and
250 mM neohexene in n-heptane at 100 °C for 18 h resulted in
∼50% conversion of neohexene and the generation of C14
n-heptane dehydrogenation/dimerization products in 18% yield
(eq 4). Additionally, a small amount of a C13 product was
observed (∼1% yield, mass confirmed by GC/MS), correspond-
ing to the coupling of neohexene and 1-heptene.11 In terms of
catalyst efficiency, cooperative TONs of 22 and 3 were achieved
for 2c and 1, respectively, with an overall cooperativity of 38%.19
ASSOCIATED CONTENT
* Supporting Information
■
S
Thermodynamic analysis, detailed experimental procedures,
figures containing GC traces of product mixtures, expanded
product tables, and conversion plots. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
jal@caltech.edu; bercaw@caltech.edu
■
Notes
The authors declare the following competing financial interest(s):
A provisional patent application partially based on this work has
been filed.
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Communication
hexane]. For the reaction in eq 4, the cooperativity is equal to ([C13] +
2[C14])/[neohexane].
ACKNOWLEDGMENTS
This work was funded by BP through the XC2 Program. D.C.L.
thanks NSERC for a PDF.
■
(20) The initial [n-hexane] was not zero because of activation of
precatalyst 2b. Because the Ir tetrahydride was used, 2 equiv of 1-
hexene per equivalent of catalyst was hydrogenated immediately upon
mixing; since the catalyst loading was 5 mM, the initial [n-hexane] was
∼10 mM.
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