Elemental reductants for the deoxydehydration of glycols

Article abstract of DOI:10.1021/cs500461v

The elements zinc, iron, manganese, and carbon are demonstrated to be practical reductants for the oxorhenium catalyzed deoxydehydration (DODH) of biomass model polyols. These reductants and their oxidization products remain heterogeneous throughout the reaction, which aids in their separation. Their effectiveness is shown with the catalysts ammonium perrhenate and trans-[(Py)₄Re(O)₂]⁺Z^- (1). Stoichiometric experiments with the Re(V) complex indicate a likely rhenium 5?7 oxidation cycle for the deoxydehydration of polyols reported herein.

Full text of DOI:10.1021/cs500461v

Letter
pubs.acs.org/acscatalysis
Elemental Reductants for the Deoxydehydration of Glycols
J. Michael McClain, II and Kenneth M. Nicholas*
Department of Chemistry and Biochemistry, Stephenson Life Sciences Research Center, University of Oklahoma, 101 Stephenson
Parkway, Norman, Oklahoma 73019, United States
*
S Supporting Information
ABSTRACT: The elements zinc, iron, manganese, and carbon are demonstrated
to be practical reductants for the oxorhenium catalyzed deoxydehydration
(
DODH) of biomass model polyols. These reductants and their oxidization
products remain heterogeneous throughout the reaction, which aids in their
separation. Their effectiveness is shown with the catalysts ammonium perrhenate
+
−
and trans-[(Py) Re(O) ] Z (1). Stoichiometric experiments with the Re(V)
4
2
complex indicate a likely rhenium 5↔7 oxidation cycle for the deoxydehydration
of polyols reported herein.
KEYWORDS: deoxydehydration, DODH, oxorhenium, elemental reductant, polyol, alkene
ustainable energy and chemical sources are receiving great
interest worldwide. Sugars and polyols derived from
Separation of the alkene product from the reaction mixture,
S
which includes the oxidized reductant and oxometal catalyst, is
a practical issue that needs to be addressed for large-scale
application of DODH reactions. An approach to solving this
problem is to heterogenize one or more of the components
the catalyst, the reductant, and/or the oxidized reductant. This
has been accomplished to an extent using sulfite salts as
cellulosic biomass are particularly attractive as a renewable
1
2
source of hydrocarbons and of refunctionalized chemicals. As
of 2011, the U.S. was estimated to be able to potentially
displace 30% of the current petroleum consumption through
3
biomass feedstocks. The removal of oxygen from biomass
10
polyols is crucial to increase the energy content and allow their
reductant (sulfate salt product), or with the sparingly soluble
4
11c 9
use as “drop-in” biofuels. The production of commodity and
ammonium perrhenate (APR) or APR-on-C as catalysts for
DODH.
specialty chemicals through controlled deoxygenation of
biomass feedstocks is likely to be achieved more practically in
the near term. Among various oxygen-removing reactions, for
example, dehydration, hydrogenolysis, and deoxydehydration
The search for new, economical, and recyclable reagents with
variable reduction potentials has led us to investigate zerovalent
elements as reductants, which would produce insoluble or
(
DODH), the latter involves net deoxygenation and
volatile element-oxides. Metals, such as zinc, have been
−
dehydration of a glycol (Scheme 1) to produce an alkene or
reported to reduce common oxometallates, such as MnO
4
−
12
and ReO , typically in aqueous acidic solutions. Similarly,
4
Scheme 1. General Deoxydehydration of Polyols
elemental carbon has been employed in the reduction of
MnO4− and Re O . However, elemental reagents have not
13
14
2
7
been reported as terminal reductants in reactions catalyzed by
oxo−metal species.
To test the viability of elemental zinc as a reductant for the
15
5
DODH of glycols, an exploratory reaction was carried out
combining (+)-diethyl L-tartrate (DET), Zn (2 equiv) and
unsaturated alcohol. The unsaturated DODH products are
proven precursors to saturated and aromatic hydrocarbons
fuels) and to other useful chemicals and materials via addition
16
ammonium perrhenate (APR, 16 mol %) in benzene. After
heating a nitrogen-flushed reactor tube containing these
components overnight at 150 °C, analysis of the reaction
(
reactions (e.g., hydroformylation) or oligomerization/polymer-
ization.
Recent investigations of the DODH reaction have focused on
establishing the substrate scope of the reaction with various
oxo−metal catalysts and reductants and on its catalytic
mechanism. The reports of DODH catalyzed by expensive
1
solution by H NMR and GC-FID showed the formation of the
trans-alkene, diethyl fumarate (DEF, 58%), with some
remaining diol, DET (42%). Metallic zinc clearly remains
after the reaction, indicating all 2 equiv of zinc were not used
up. The absence of other significantly detectable organic
products, especially ones derived from reduction of the carboxy
group of the substrate/products, is noteworthy. A control
6
oxo−rhenium species have thus far predominated. Recently,
more earth-abundant (and inexpensive) vanadium− and
molybdenum−oxo complexes have been reported with
7
DODH activity. Additionally, the range of viable reductants
6
a,d,7b,c,8
has grown from the originally reported phosphines
include more economical and benign molecular hydrogen,
to
Received: April 8, 2014
Revised: May 26, 2014
6
c,9
7b,10
6e,10b,11
sulfite,
and aliphatic and benzylic alcohols.
©
XXXX American Chemical Society
2109
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ACS Catalysis
Letter
reaction conducted under the same conditions without APR
showed only the starting DET by GC and NMR.
110 °C) with DET/Zn/APR showed alkene formation in low
yield and conversion after 2−4 days. Employing higher boiling
anisole (PhOMe, bp 154 °C) as the reaction solvent with 1,2-
decanediol as substrate (Table 1, entry 7) with APR as catalyst
for 21 h yielded 64% of 1-decene and nearly complete
conversion of the diol with a rate of alkene formation of 0.014
M/h and a rate of diol disappearance nearly twice that of the
formation of alkene at 0.034 M/h. The long GC-FID retention
products were also noted with an increasing integration over
time. When the loading of APR was lowered to 1 mol % with 2
M 1,2-decanediol in anisole (zinc reductant), a lower
conversion and an alkene yield of 34% resulted (Table 1,
entry 8).
With this encouraging result in hand, we turned to aliphatic
substrates, which typically are less reactive but are closer
models for carbohydrate-derived polyols. Choosing 1,2-
decanediol as the substrate (0.1 M in benzene) under similar
conditions used for the DET/APR/Zn reaction, a moderate
yield of 1-decene (56%) and high conversion (99%) was
obtained with 11 mol % APR and 1.1 equiv of Zn at 150 °C
after 24 h (Table 1, entry 1). A partner reaction (Table 1, entry
Table 1. DODH of 1,2-Decanediol to 1-Decene Using
a
Oxorhenium Catalysts and Elemental Reductants
The activity of other oxorhenium compounds with the
elemental reductants was briefly examined using the cationic
+
−
rhenium(V) complexes, trans-[(Py) ReO ] X [Py = pyridine;
X = Cl (1a) PF6 (1b)].
4
2
−
−
20,21
We have obtained good yields of
entry
el (eq)
LReOx
conv. (%)
yield (%)
alkenes from aliphatic diols using benzyl alcohol as reductant
22
b
1
2
3
4
5
6
7
8
9
Zn (1.1)
Zn (1.1)
Zn (2)
Fe (2)
Mn (2)
C (2)
APR
APR
APR
APR
APR
APR
APR
APR
99
76
56
with complexes 1a,b. The chloride salt 1a catalyzed the
DODH of 1,2-decanediol and zinc under the standard
conditions, affording a high conversion and yield (90%) of 1-
decene (Table 1, entry 9). The hexafluorophosphate salt (1b)
under standard conditions gave a somewhat diminished yield of
1-decene (67%) and very high conversion (Table 1, entry 10).
It does not appear that the room temperature solubility of the
oxorhenium compounds relates to their catalytic proficiency at
the reaction temperature because APR and 1a are practically
insoluble in benzene at room temperature, although 1b is
noticeably more soluble.
b
,
c
8
≥99
≥99
≥99
≥99
≥99
69
68
68
64
69
d,e
Zn (2)
Zn (2)
Zn (2)
Zn (2)
64
34
d,f
[(Py) ReO ]Cl
≥99
≥99
90
67
4
2
10
[(Py) ReO ]PF
4 2 6
a
Reactions were typically carried out in benzene (0.2 M) with ca. 10
mol % oxorhenium catalyst, ca. 2 equiv of reductant, degassed with N2
To further assess the substrate scope of the DODH
reactions, we tested the effectiveness of the elemental
reductants on diols with different electronic properties. The
and heated in a sealed pressure tube at 150 °C for approximately 24 h;
1
conversion (conv.) and yield (%) determined by GC-FID and/or H
b
c
d
NMR with internal standard. 0.1 M reaction Air atmosphere. Reflux
activated and acid-sensitive substrate 1-phenyl-1,2-ethanediol
e
with anisole as solvent. Within 21 h reaction time, full conversion was
noted with no increase in yield with 24 more hours of reaction time 1
6a,8
(
styrene diol) was selected for evaluation.
Under the
f
standard reaction conditions with Zn/APR, styrene diol gave
a moderate yield of styrene at high conversion (Table 2, entry
mol % APR and 2 M 1,2-decanediol. For further experimental details,
see the SI.
1
1) within 12 h, accompanied by the formation of a side
a
2
1
) under an air atmosphere showed a much lower yield (8%) of
-decene with incomplete conversion (76%) of the diol under
Table 2. DODH of Non-Aliphatic Glycols with APR
otherwise identical conditions. This is likely the result of
molecular oxygen competitively oxidizing the Zn. The yield
from the nitrogen-flushed reaction was further improved (68%
yield) when two equivalents of zinc were used in a 0.2 M
benzene solution of 1,2-decanediol (Table 1, entry 3). The
remainder of the diol is largely converted into unidentified side
products having long GC retention times and low intensity
1
peaks in the H NMR spectra near the starting diol. These long
retention peaks are not seen in control reactions where APR or
+
trans-[ReO (Py) ] is not present; see Supporting Information
2
4
(
SI) for further details.
These conditions were then used with other abundant
metals, namely, iron and manganese (Table 1, entries 4 and 5
respectively), which achieved similar yields and conversions.
Interestingly, elemental carbon (Darco G-60) also proved to be
17
an effective reductant (Table 1, entry 6), giving a 69% yield of
-decene with very high conversion. When the reactor tube was
a
Reactions were typically carried out in benzene (0.2 M) with ca. 10
1
mol % NH ReO4 (APR), ca. 2 equiv of reductant, degassed at room
temperature with N and heated in a sealed pressure tube at 150 °C for
approximately 24 h; conversion and yield (%) determined by GC-FID
4
cooled to room temperature and opened, a gas pressure
buildup was noted. This gas was collected and tested positive
2
18
19
for carbon monoxide but negative for carbon dioxide.
1
b
and/or H NMR with an internal standard. 12 h reaction time.
c
d
For operational convenience, reactions that could be
conducted at atmospheric pressure under reflux were desired.
Experiments using refluxing benzene (at 78 °C) and toluene (at
Solventless melt conditions. Isolated yield from 3.9 mmol diol
e
reaction in benzene. 42 h reaction time. See the SI for experimental
details.
2
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ACS Catalysis
Letter
product with a long GC retention (ca. 15.5 min) and a mass
pathway, because the formation of alkene with only one
(
by GCMS) that corresponds to a condensed, unsaturated
dimer of the diol, tentatively assigned as the α,β-unsaturated-
,4-diketone 2 (Figure 1) on the basis of comparison of MS
and H NMR data (ca. 7.3−8.1 ppm) with those of an authentic
turnover of 1a indicates a Re(V) to Re(VII) oxidation from
+
glycol DODH. Work is currently underway using [(Py) ReO ]
4 2
1
for kinetic and mechanistic studies of the DODH reaction.
2
3,24
ASSOCIATED CONTENT
Supporting Information
sample.
■
*
S
Experimental procedures and gas chromatograms, NMR and
mass spectra of the reaction mixtures. This material is available
free of charge via the Internet at http://pubs.acs.org
AUTHOR INFORMATION
Corresponding Author
E-mail: knicholas@ou.edu.
■
Figure 1. α,β-Unsaturated-1,4-diketone (2) side product.
*
Under similar conditions used for the aliphatic diols with zinc
as reductant and 10 mol % APR catalyst, polyfunctional diethyl
tartrate (DET) yielded 85% of (trans) DEF (Table 2, entry 12).
Similarly, with iron as the reductant, a good yield (68%) of
DEF was obtained after 24 h with very high conversion (Table
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful for the support provided by the U.S.
Department of Energy (DE-11ER16276).
■
2, entry 13). The yields of alkene in the iron-driven reactions
may suffer from magnetic agglomeration of this reductant to
the stir bar, limiting contact with substrate and catalyst. Because
DET and its DODH product, DEF, are both high boiling
liquids (280 and 218 °C, respectively), solventless reactions
were conducted at 150 °C for 16 h combining DET, carbon,
and APR under nitrogen. A 60% yield of DEF was obtained
using these solventless conditions (Table 2, entry 14). A scaled
up experiment with DET (3.9 mmol)/Zn/APR was conducted
and provided an 84% isolated yield of DEF after 24 h simply by
triturating the heterogeneous postreaction residue with
benzene and ethyl acetate (Table 2, entry 15). The glycerol
derivative batyl alcohol gave a 47% yield of the corresponding
olefin in 24 h at 150 °C (entry 16).
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)
.
(
16) This substrate with benzyl alcohol as reductant gives diethyl
11c
fumarate (DEF) nearly quantitatively.
(
17) ΔH for the DODH of ethylene glycol with C is calculated to
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(
955; pp 22−23. (See SI.)
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(
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4
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(
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(
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(
25) If the reaction involved a Re(iii)↔Re(v) cycle, then one would
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these were not observed.
(
26) With added elemental reductants the possibility of forming a
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out.
2
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