Synthesis of 2,5-Diformylfuran and Furan-2,5-Dicarboxylic Acid by Catalytic Air-Oxidation of 5-Hydroxymethylfurfural. Unexpectedly Selective Aerobic Oxidation of Benzyl Alcohol to Benzaldehyde with Metal/Bromide Catalysts

Article abstract of DOI:10.1002/1615-4169(20010129)343:1<102::AID-ADSC102>3.0.CO;2-Q

The alcohol group of hydroxymethylfurfural (compound 1, HMF) is preferentially oxidized by dioxygen and metal/bromide catalysts [Co/Mn/ Br, Co/Mn/Zr/Br; Co/Mn=Br/(Co+Mn) = 1.0 mol/ mol] to form the dialdehyde, 2,5-diformylfuran (compound 2, DFF) in 57% isolated yield. HMF can be also oxidized, via a network of identified intermediates, to the highly insoluble 2,5-furandicarboxylic acid (compound 5, FDA) in 60% yield. For comparison, benzyl alcohol gives benzaldehyde in 80% using the same catalyst system. Over-oxidation (to CO₂) of HMF is much higher than that of the benzyl alcohol but can be greatly reduced by increasing catalyst concentration.

Full text of DOI:10.1002/1615-4169(20010129)343:1<102::AID-ADSC102>3.0.CO;2-Q

FULL PAPERS
Synthesis of 2,5-Diformylfuran and Furan-2,5-
Dicarboxylic Acid by Catalytic Air-Oxidation
of 5-Hydroxymethylfurfural. Unexpectedly Selective
Aerobic Oxidation of Benzyl Alcohol
to Benzaldehyde with Metal/Bromide Catalysts**
Walt Partenheimer,* Vladimir V. Grushin
Central Research and Development, E. I. DuPont de Nemours & Co., Inc., Experimental Station, Wilmington, Delaware
1
9880±0328, USA
Fax: +1 3 02-6 95±83 47; e-mail: Walter.Partenheimer@usa.dupont.com
Received August 3, 2000; Accepted October 16, 2000
Keywords: cobalt; di-
oxygen; green chem-
istry; homogeneous
catalysis; hydroxyme-
thylfurfural; oxidation
Abstract: The alcohol group of hydroxymethylfur- comparison, benzyl alco-
fural (compound 1, HMF) is preferentially oxidized hol gives benzaldehyde in
by dioxygen and metal/bromide catalysts [Co/Mn/ 80% using the same cata-
Br, Co/Mn/Zr/Br; Co/Mn=Br/(Co+Mn) = 1.0 mol/ lyst system. Over-oxida-
mol] to form the dialdehyde, 2,5-diformylfuran tion (to CO ) of HMF is
2
(compound 2, DFF) in 57% isolated yield. HMF can much higher than that of the benzyl alcohol but can
be also oxidized, via a network of identified inter- be greatly reduced by increasing catalyst concentra-
mediates, to the highly insoluble 2,5-furandicar- tion.
boxylic acid (compound 5, FDA) in 60% yield. For
acid (FDA) and/or 2,5-diformylfuran (DFF; com-
Introduction
pound 2), monomers for furan-containing polymers
[
4]
and materials with special properties. While a vari-
ety of oxidants have been used for oxidation of HMF
to 2,5-furandicarboxylic acid and DFF, only few re-
ports describe catalytic oxidations of HMF with oxy-
gen or air, the most economical oxidants. Thus, HMF
At the current rate of consumption, proven crude oil
reserves are estimated to last for less than four dec-
[1]
ades. Therefore, in recent years serious considera-
tion has been given, in both academia and industry, to
alternative feedstocks for the chemical industry of the
future. The use of renewable resources, i. e., natu-
rally occurring carbohydrates and oils produced by
various plants, would result in the development of be-
nign, environmentally friendly processes, the so-
has been oxidized with O to 2,5-furandicarboxylic
2
acid in the presence of heterogeneous Pt catalysts
[
5,6]
with stoichiometric amounts of alkali
and to DFF
or supported vanadium cat-
Although homogeneously catalyzed oxida-
tion reactions of alcohols have received much atten-
[
7,8]
with TEMPO radicals
[
9,10]
[
2]
alysts.
called green chemistry.
-Hydroxymethylfurfural (HMF; compound 1) is
5
[
11±18]
tion in recent years,
no reports have appeared
one of the few individual organic compounds that
can be prepared directly from various carbohydrates
in up to 98% yield. While the best yields of HMF have
been obtained from fructose, other abundant, low-
cost mono-, di-, and polysaccharides can be used,
such as glucose, sucrose, and starch.
Selective oxidation reactions of HMF are presently
viewed as attractive routes to 2,5-furandicarboxylic
in the literature, describing the oxidation of HMF with
O and soluble metal complex catalysts.
2
In this paper, we report the first examples of aero-
bic HMF oxidation reactions, catalyzed with homoge-
neous metal/bromide systems. The easily prepared,
low-cost metal/bromide catalysts, the most common
being a mixture of Co/Mn/Br, are widely used for the
selective and efficient autoxidation reactions of hy-
[
3]
[
19]
drocarbons,
e. g., the large scale industrial synth-
*
* Contribution No. 8095
02
esis of terephthalic, isophthalic, and trimellitic acids
1
Ó WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2001
1615-4150/01/34301-102±111 $ 17.50-.50/0
Adv. Synth. Catal. 2001, 343, No. 1

FULL PAPERS
from p-xylene, m-xylene, and pseudocumene respec-
[1,19]
tively.
Surprisingly little is known, however,
about oxidation of alcohols using the metal/bromide
[19]
catalysts.
In this work, we found that, depending
on reaction conditions, hydromethylfurfural can be
oxidized to DFF or 2,5-furandicarboxylic acid with
unexpectedly high selectivity. Furthermore, the se-
lective formation of DFF in the metal bromide-cata-
lyzed oxidation of HMF prompted us to study the oxi-
dation of benzyl alcohol under similar conditions.
Remarkably, it was found that under controlled con-
ditions this oxidation can afford benzaldehyde in high
yield.
Figure 2. Products during autoxidation of benzyl alcohol
Formation of Diformylfuran from Hydromethyl-
furfural and Benzaldehyde from Benzyl Alcohol at
Atmospheric Pressure (Table 1)
Results
Products Formed
In experiments 1 and 5, the attempt to initiate the re-
action at the lower temperature failed, hence the
temperature was raised to the higher given value.
The formation of the reaction products, as deter-
mined by GC and LC, from HMF and benzyl alcohol
is illustrated in Figures 3 and 4. Maximum observed
yield of the aldehydes is 57% for DFF and 80% for
benzaldehyde. Maximum aldehyde yields occur as
the conversion of the alcohol approaches 100%. The
selectivity decreases as the conversion of the alcohol
increases with the values for HMF (51±90%) being
lower than for benzyl alcohol (80±93%), see Figure 5.
Doubling the catalyst concentration during the oxy-
genation of HMF (i) increases the reaction rate by a
factor of 2.1, (ii) increases the yield and selectivity to
DFF by 5 and 10%, respectively, and (iii) decreases
GC/MS studies were performed on two selected sam-
ples during HMF autoxidation at 70 bar, which are
consistent with the products and pathways given on
Figure 1. In addition, the usual products from the
autoxidation of acetic acid were observed, i. e., formic
acid, acetoxyacetic acid, glycolic acid, maleic acid, fu-
maric acid, succinic acid, and bromosuccinic acid in
trace amounts. A side reaction is the esterification of
the alcohols to form the more oxidatively stable acet-
ate, see compounds 6 and 7 in Figure 1 and benzyl
acetate in Figure 2. DFF and FDA have been isolated
and characterized by elemental analysis and NMR
spectra. The 2-carboxy-5-formylfuran was identified
1
and quantified by H NMR spectroscopy of isolated so-
lid samples that were either 2,5-furandicarboxylic
acid or 2,5-furandicarboxylic acid/2-carboxy-5-for-
mylfuran mixtures. The oxidation of benzyl alcohol
gives the expected benzaldehyde, benzyl acetate,
and benzoic acid products (see Figure 2).
the `overoxidation' to CO and CO by a factor of 4
2
(see experiments 2, 3, and 4 in Table 1 and Figure 6).
Further increase in catalyst concentration does not
further improve DFF yield (see experiments 4, 5, and
6
). For benzyl alcohol, the maximum benzaldehyde
yield of 80% occurs at a conversion of 85% with only
.6% benzoic acid and 3.8% benzyl acetate formed
Table 1, experiment 7). Based on their rates of disap-
4
(
pearance, benzyl alcohol is 2.0 times more reactive
than HMF (see examples 4, 7, and 8 in Table 8).
The rate of disappearance of the alcohol and the
rate of disappearance of the aromatic aldehyde are
consistent with first order kinetics and the rate con-
stants are given on Table 1. The rate of disappearance
of HMF is 8.1 times faster than the rate of disappear-
ance of 2-carboxy-5-formylfuran, suggesting that the
dialdehyde is quite stable, as seen from the kinetic
data on Table 1. This is consistent with the subse-
quent work-up of the reaction mixture and isolation
of DFF in a yield close to that previously determined
by GC. The benzyl alcohol, however, reacts to form
benzaldehyde faster by only a factor of 1.1 than ben-
zaldehyde reacting to benzoic acid (Table 1). Re-
Figure 1. Products from the autoxidation of hydroxymethyl-
furfural
Adv. Synth. Catal. 2001, 343, 102±111
103

asc.wiley-vch.de
Table 1. Oxygenation of hydroxymethylfurfural and benzyl alcohol at ambient atmospheric pressure
Exp.
1
2
3
4
5
6
7
HMF
HMF
HMF
HMF
HMF
HMF
benzyl
alcohol
75
0.793
13.4
Temp, °C
Reagent, M
Co, mM
50 then 95
0.725
2.6
0.0
±
75
0.794
6.6
0.15
75
0.804
6.6
0.15
8.12(0.61)
[0.972]
142
±
75
50 then 75
0.796
26.8
75
0.797
13.5
0.15
16.6(1.4)
[0.999]
69
±
0.806
27.3
0.15
15.1(0.5)
[0.994]
76.5
Zr, mM
0.15
0.15
±
1 [a]
Rate, s
9.68(0.18)
0.997]
10.8(0.5)
[0.992]
106
40.4(3.9)
[0.988]
28.5
30(3)
[0.943]
100
80
85
93
5.6
[
Alcohol, half-life, min
±
±
119
1.22(0.34)
0.862]
±
1 [b]
Rate, s
±
±
[
[
c]
Time,min
414
41
98
42
8.4
±
450
51
92
55
5.9
7.4
642
50
95
53
7.5
8.5
310
57
91
63
7.2
2.1
550
51
95
54
6.1
1.8
430
52
97
54
5.7
2.6
[c]
Yield
Conv.,%
[
c]
[
c]
Select.,%
[c
Acetate,% ]
[d]
Alcohol to CO
x
0.05
[a]
[b]
[c]
[d]
5
Rate of disappearance of aromatic alcohol ´ 10 . Standard deviation in parenthesis ( ), correlation coefficient in brackets [ ].
Rate of disappearance of aromatic aldehyde.
When maximum alkylaromatic aldehyde is observed.
Loss of alcohol due to carbon monoxide and carbon dioxide formation. Assumes no CO formation from the solvent.
x
Figure 3. Autoxidation of hydroxymethylfurfural at 75 °C
Figure 5. Benzaldehyde selectivity as function of catalyst
concentration and type of alkylaromatic alcohol
benzyl alcohol is close to completion, despite the fact
that PhCH OH and PhCHO exhibit very similar meas-
2
ured reactivities in the same experiment.
Formation of DFF from HMF at 70 Bar Air (Table 2)
Experimental error, as determined by 5 replicate ex-
periments, is given in entry 7 of Table 2. As can be
seen, 50 and 75 °C for 2 h are sufficient conditions for
obtaining good yields of DFF, up to 63%. By compar-
ing experiments 1 and 2, 3 and 4, 5 and 6, and 8 and
Figure 4. Autoxidation of benzyl alcohol at 75 °C
9
, one finds that increasing catalyst concentration
leads to (i) increased activity as evidenced by higher
markably, the oxidation is catalyzed in such a way conversions, (ii) higher selectivity for DFF (except
that essentially all the benzyl alcohol reacts first. The for experiments 5 and 6), and (iii) higher yield. Com-
benzaldehyde formed starts to undergo further oxida- paring experiments 1 and 3, 2 and 4, 5 and 8, and 6
tion to benzoic acid only after the oxidation of the and 9, one finds that the Co/Mn/Zr/Br catalyst is
1
04
Adv. Synth. Catal. 2001, 343, 102±111

FULL PAPERS
Formation of 2-Carboxy-5-formylfuran and 2,5-
Furandicarboxylic Acid at 70 Bar (Table 3)
The initial amount of HMF used was only 0.2±0.75 g
and the yields are based on isolated and washed so-
lids which were analyzed by NMR. When the tem-
perature is increased from 75 to 100±125 °C, precipi-
tation of poorly soluble 2,5-furandicarboxylic acid
commences. 2-Carboxy-5-formylfuran is also either
fairly insoluble or is prone to co-crystallization with
2
,5-furandicarboxlic acid, which results in their co-
precipitation. The yield increases with catalyst con-
centration (Figure 7), with temperature (entries 1
and 2 and 3 and 4 of Table 3), but not with the addition
of Zr to the Co/Mn/Br catalyst (entries 1 and 3 and 2
and 4). Extrapolation from Figure 7 suggests that the
maximum obtainable 2,5-furandicarboxylic acid
yield is about 70% using the Co/Mn/Zr/Br catalyst at
the specified molar ratios of these elements. It is be-
lieved that variation of the molar amounts of the Co,
Mn, Zr, and Br could well improve the yield of 2,5-
furandicarboxylic acid. Since the oxidation proceeds
through three steps from HMF to 2,5-furandicar-
boxylic acid (steps 1, 3, and 5 in Figure 1) and the re-
activity of the HMF is probably higher than 2-car-
boxy-5-formylfuran one would expect that staging
Figure 6. Carbon oxide selectivity as function of catalyst
concentration and type of alkylaromatic alcohol
more active, giving higher conversion, than Co/Mn/
Br, with the only exception being experiments 2 and
3
where the conversions are similar. The addition of
zirconium not only affects conversion, but can also
profoundly increase the selectivity (Table 2). This
point is illustrated by experiments 1 and 3 where the
addition of Zr results in a much higher yield of DFF
[
19]
the temperature would increase yield.
This was
not observed however, since staging the temperature
from an initial value of 50 °C for 1 h and then 125 °C
for 2 h gave no better results than the oxygenation at
(
67 vs. 38%) at the same conversion of ca. 60%. Ex-
periments 6 and 8, for which the conversions vary sig-
nificantly, represent the only exception. Under com-
parable conditions, the conversion increases with
temperature, as expected.
1
25 °C for 3 h (Figure 7).
Table 2. Oxidation of hydroxymethylfurfural (HMF) to diformylfuran (DFF) at 70 bar air
Exp.
Catalyst
[Co], mM
HMF, M
Temp, °C
Time, h
HMF, conv. % DFF select. % DFF, yield %
1
2
3
4
5
6
7
8
9
Co/Mn/Br/Zr
Co/Mn/Br/Zr
Co/Mn/Br
3.44
6.82
3.44
6.82
3.44
6.82
6.82
3.44
6.82
0.375
0.372
0.375
0.377
0.375
0.375
1.12
50
50
50
50
75
75
75
75
75
2
2
2
2
2
2
2
2
2
60.4
69.2
60.6
61.7
82.5
99.7
74.1(1.0)
71
66.6
65.3
38.4
54.6
73.2
61.6
67.5(1.4)
54.3
68.3
40.2
45.2
23.3
33.7
60.4
61.4
49.9(0.6)
38.6
63.0
Co/Mn/Br
Co/Mn/Br/Zr
Co/Mn/Br/Zr
Co/Mn/Br/Zr
Co/Mn/Br
0.377
0.377
Co/Mn/Br
92.2
Table 3. Oxidation of hydromethylfurfural to 2-carboxy-4-formylfuran (CFF) and furan-2,5-dicarboxyfuran (FDA) at 70 bar
air
Exp.
catalyst
[Co], mM
[HMF], M
temp, C
time, h
CFF, mol %
FDA, mol %
1
2
3
4
5
6
7
8
9
Co/Mn/Br/Zr
Co/Mn/Br/Zr
Co/Mn/Br
3.44
3.44
3.44
3.44
3.44
6.82
13.7
20.5
3.41
6.82
13.7
20.5
0.377
0.371
0.377
0.374
0.758
0.753
0.749
0.755
0.781
0.774
0.0753
0.768
100
125
100
125
50, 125
50, 125
50, 125
50, 125
125
125
125
2
2
2
2
1, 2
1, 2
1, 2
1, 2
3
3
3
3.1
2.1
4.1
1.8
1.6
2.5
0.0
0.0, 0.0
1.7
18.7
36.5
29.7
35.2
28.3
28.1
55.4
58.4, 63.1
27.7
Co/Mn/Br
Co/Mn/Br/Zr
Co/Mn/Br/Zr
Co/Mn/Br/Zr
Co/Mn/Br/Zr
Co/Mn/Br/Zr
Co/Mn/Br/Zr
Co/Mn/Br/Zr
Co/Mn/Br/Zr
1
1
1
0
1
2
0.0
0.0
0.0, 0.0
41.6
54.6
60.9, 58.6
125
3
Adv. Synth. Catal. 2001, 343, 102±111
105

asc.wiley-vch.de
the corresponding aromatic alcohols the catalytic
process should be carefully monitored, so that subse-
quent oxidation of the aldehyde formed can be
avoided.
Structure of the Catalyst
Addition of the simple acetate salts into acetic acid re-
sults in a complex mixture which is only partially un-
derstood. A brief synopsis based on available infor-
mation follows. The structures of Co(II) and Mn(II)
in acetic acid/water mixtures can be summarized by
the equation:
Figure 7. Effect of catalyst concentration and temperature
staging on FDA yield. See details in Table 3.
where the square brackets indicate the ligands in the
inner coordination sphere. In acetic acid, the cationic
metal species are largely associated, with the small
quantities of the dissociated species existing as ion
Discussion
[
24]
General Considerations
pairs
in both monomeric and dimeric forms
[
25±27]
The reaction network in Figure 1 is consistent with (n = 1, 2).
Upon addition of water, equilibrium
the detailed studies of the oxidation reactions of is established between various metal aquo acetic acid
[
28]
many substituted methylaromatic species, aromatic complexes. Using reported equilibrium constants
alcohols, and benzaldehydes using metal/bromide one can calculate the distribution of these complexes
[19]
catalysts.
modified free radical chain mechanism (see be- species exist in 10% water/acetic acid mixtures.
The latter is thought to operate via a and demonstrate that these aquo/acetic acid metal
[
29]
low). The free radical chain mechanism gives the The weakly bound AcOH ligand (5.9 kcal/mol) is la-
oxidizability of toluene, benzyl alcohol, and benzal- bile, exchanging with water and acetic acid instanta-
[22]
[30]
dehyde as 0.05, 0.85, and 290 respectively.
It is neously at room temperature.
The addition of per-
clear from these values that the steady state con- acids, peroxy radicals, oxygenated intermediates, etc.
centration of benzaldehyde is expected to remain to a mixture of Co(II)/Mn(II) in acetic acid may there-
low in metal/bromide catalyzed systems. We find fore result in fast ligand exchange to form the transi-
however that the oxygenation of HMF gives prefer- ent catalytic species. Addition of hydrogen bromide to
entially DFF rather than 5-(hydroxymethyl)furan- Co(II) or Mn(II) or a Co(II)/Mn(II) mixture in anhy-
2
-carboxylic acid (compare steps 1 and 2 on Fig- drous acetic acid results in the majority of the brom-
ure 1). Extending this work to benzyl alcohol gave ide being coordinated to the metal. However, addition
even higher yields and selectivity to aldehydes. The of water (5% or greater) results in almost complete
[
29]
kinetics of the Co/Br catalyzed oxygenation of ben- ionization of the M±Br bond.
zyl alcohol has been reported
In the presence of
albeit without a water the addition of bromide results in outer-sphere
comment on potentially high selectivities and ligand exchange processes, as shown in the equation
[23]
yields of benzaldehyde.
below.
The advantages of the catalytic oxidation described
herein is that the catalyst is composed of inexpensive,
simple metal acetate salts and a source of ionic bro-
mide (NaBr, HBr, etc.). The reaction times are within
a few hours at easily accessible temperatures. The It is possible that the ion-paired bromide forms hy-
acetic acid solvent is inexpensive and nearly all alco- drogen bonds to the aquo ligands (Figure 8, struc-
hols are highly soluble in it. Although acetoxylation of ture b). The lability of the ligands, the known di-
the alcohols with the acetic acid solvent does occur, meric
structure
of
Co(II)
acetate,
and
this side-reaction results in only a 5±8% yield loss. It polynuclearity of Zr(IV) in water suggests that poly-
is noteworthy that there are other solvents available nuclear Co(II)±Mn(II) and Co(II)±Zr(IV)±Mn(II)
for metal/bromide catalyzed systems, which could may exist (Figure 8). Such mixed-metal polymeric
[
19]
[31]
potentially eliminate this problem.
Due to the high species have been isolated from acetic acid.
Re-
suggest that acetic acid/water
[
32]
activity of the metal/bromide catalysts the aldehyde cent observations
formed in high yield can undergo further oxidation. solutions may be more complex, containing water-
To obtain high yields of aromatic benzaldehydes from rich microphases. It is proposed that Co(III) aquo
1
06
Adv. Synth. Catal. 2001, 343, 102±111

FULL PAPERS
Figure 8. Suggested structures for Co,Mn,Br mixtures in 5% H
2
O/HOAc. M'=M''=Co(II), Co(III), Mn(II), Mn(III)
acetate and Co(III) aquo acetate bromide have Rationale for High Yields of Aromatic Benzalde-
structures similar to those shown in Figure 8 (a and hydes from Aromatic Alcohols
b, respectively).
High yields of benzaldehydes are observed despite
the fact that benzyl alcohol and benzaldehyde react
at nearly the same rate in the metal/bromide cata-
Theory and Models of Metal/Bromide Catalysis
Different aspects of metal/bromide catalysis have lyzed system. In particular, for benzyl alcohol oxida-
been discussed with emphasis on high reactiv- tion the benzoic acid yield remains under 1% at 60%
[
19,27,33±36]
ity,
superior selectivity over a broad tem- conversion and is only 4.6% when the maximum
[19,34]
perature range,
of the metals.
this field have been recently reported.
and the synergy and antagonism yield of benzaldehyde is obtained (80%) at 85% con-
Important new observations in version. These observations are certainly unexpected
[34,36,37]
[
38]
Kinetic stu- and hence merit a comment. There are at least three
dies suggest that oxidation of Co(II) by peracids (to factors which would account for the clean and selec-
[
27,37]
give carboxylic acids)
give peroxides)
and peroxy radicals (to tive formation of benzaldehyde under the conditions
initiates the series of reactions employed.
shown in Figure 9. The rapidity of the peroxide reac- There may be a rapid, preferential bonding of the
[33]
tions with Co, followed by the subsequent redox cas- aromatic alcohol with either or both Co(II) and
cade leading to the generation of the selective bro- Mn(II), which initiates their oxidation in preference
[
38]
mide atom or the dibromide radical
accounts for to the benzaldehyde. The formation of benzyl alcohol
the properties of these catalysts. Initiation of the hy- metal species might occur via replacement of the la-
.
drocarbon RH to the radical R via the Co/Mn/Br re- bile, weak acetic acid or aquo ligands or hydrogen
dox cascade is faster than Co/Br, which in turn is fas- bonding to the coordinated AcOH or water molecules
a
s
c
ter than Co. Co(III) , Co(III) , Co(III) are different (similar to the bromide in Fig. 9). Once all of the aro-
Co(III) compounds, with structures suggested in Fig- matic alcohol has been oxidized the catalyst initiates
[
26]
ure 8 possessing different reactivity.
the benzaldehyde oxidation.
There is experimental evidence that acetic acid re-
tards autoxidation by hydrogen bonding to the peroxy
Adv. Synth. Catal. 2001, 343, 102±111
107

asc.wiley-vch.de
Figure 9. Summary of chemistry of Co, Co/Br, and Co/Mn/Br autoxidation catalysts. Half-lifes are at 60 °C in 10% water/
acetic acid
termining step because the q values in a Hammett
plot of Co/Mn/Zr/Br and Co/Mn/Br are the same
[
35]
within experimental error.
However, there is a
brief report that addition of Zr to a Co/Br catalyst de-
creases the rate of benzyl alcohol formation and in-
[
42]
creases the rate of benzaldehyde formation.
We
have duplicated this effect with a Co/Mn/Zr/Br cata-
lyst in 10% water/acetic acid at 95 °C with the oxyge-
nation of p-xylene. The increase in the rate of reac-
Figure 10. Suggested structure of hydrogen bonded acetic
acid to benzyl alcohol intermediates
tion is proportional to the Zr concentration which in
[
39]
radicals at the a-position (Figure 10).
It is concei- turn is directly proportional to the observed reduction
[
40]
vable that the acetic acid is more effective at inhibit- of the benzyl alcohol/benzaldehyde ratio.
One pos-
ing the carbonyl functionality than the benzylic hy- sibility is that the rate of alcohol oxidation (Figure 2,
droxy group. This explanation seems less likely step 3) is increasing relative to step 2. It is also possi-
because the oxidation of the aromatic alcohol and ble that step 1 is becoming more important than
benzaldehyde would have been already initiated, step 2. We suggest that the new catalytic species form
and even if they proceed further at different rates, sig- when Zr is added to Co/Mn/Br (see Figure 8), which
nificant amounts of aromatic acids should be formed goes through similar redox cascades as shown in Fig-
from the benzaldehyde. However, the actual amount ure 9, changing some of the relative rates presented
of benzoic acid formed from benzaldehyde does not in Figure 2. More details on the function of Zr are
[
19]
exceed 1±5% (see above).
available.
It is possible that one or more coordination com-
pounds in the reaction mixture specifically inhibits Overoxidation to COx
the benzaldehyde oxidation or promotes the aromatic A weakness of most published work on oxygenations
alcohol reaction. We have found that sodium bromide using air as the primary oxidant is the lack of meas-
strongly inhibits the oxidation of benzaldehyde, urement of CO formation during the reaction. The
x
whereas Co enhances this reaction. The rate of oxy- potential for the formation of the highly reactive per-
gen uptake is 12.0 mL/min without catalyst, 0.3 mL/ oxides and consequently peroxy, hydroxyl, etc. radi-
min with sodium bromide and 13.2 mL/min with cals always exists when mixtures of transition metals,
[
40]
Co(II) acetate at 80 °C in acetic acid.
Obviously, further experimentation is required to present and hence `overoxidation' to CO nearly al-
dioxygen, hydrocarbons, and organic solvents are
x
confirm these conjectures.
ways occurs.
Much higher amounts of carbon monoxide and car-
Effect of Zirconium on Selectivity
bon dioxide (CO ; x = 1, 2) form during HMF oxyge-
x
In the high pressure experiments, we found that the nations as compared with the benzyl alcohol oxida-
selectivity to DFF increased in the presence of Zr in tion under similar conditions (Table 1, Figure 6).
the Co/Mn/Br catalyst. The effect of Zr on cobalt me- Tracer studies indicate that the origin of CO is from
x
tal/bromide catalysts is generally thought to increase both the aromatic substrates and the acetic acid sol-
[19,35,41]
[43]
its activity
and Zr does not affect the rate de- vent.
The formation of CO in the HMF reaction is
x
1
08
Adv. Synth. Catal. 2001, 343, 102±111

FULL PAPERS
Figure 11. Important pathways in hydroxymethylfurfural oxidation
apparently not predominately from the solvent be- Effect of Catalyst Concentration on Activity and
cause only trace amounts of acetoxyacetic acid, an Overoxidation to COx
oxidatively stable by-product formed from the acetic We find that increasing catalyst concentration in-
acid, is observed by GC. We determined the amount creases activity at the early stages, but then remains
of CO formed by numerical integration and then as- constant or decreases slightly (Table 1), consistent
x
[
19]
sumed that 100% came from the total destruction of with previous observations.
Kinetic studies show a
HMF. A significant yield loss of 1.8 to 8.5% is calcu- second order dependence of cobalt concentration for
[
33]
lated. This significant loss, as compared to the benzyl a Co/Br catalyst.
alcohol oxidation (ca. 0.05% lost to CO ) is accounted
Remarkably, overoxidation to CO is suppressed at
x
x
for by at least two reasons.
higher catalyst concentrations. This has been ob-
Decarbonylation. By-product formation during p- served in metal/bromide catalyzed systems pre-
[44]
[19]
[47]
xylene
and alkylnaphthalene oxidation
is con- viously.
The non-selective, thermal pathways are
sistent with hydrogen atom abstraction from the alde- (i) decarbonylation in step 5 and subsequent by-pro-
hyde followed by decarbonylation and eventual aro- duct formation in step 6, (ii) peroxy radical attack on
matic ring loss via the formation of phenol the furan ring in step 7, and (iii) thermal dissociation
(
Figure 11). HMF is initially a di-functional molecule of the peroxide (step 9), leading to the carboxylate ra-
already containing a formyl functionality in contrast dical and the highly reactive OH radical. Step 9 is fol-
to benzyl alcohol. Hence higher HMF loss via this me- lowed by ring addition of the hydroxyl radical to furan
chanism is anticipated.
(step 10), which will eventually lead to by-products
Enhanced ring attack due to reduced resonance en- including CO . The carboxylate radical can decar-
x
ergy. The resonance energy values for benzene, boxylate (step 11), leading to the same products as in
naphthalene, and furan are 36, 31, and 17 kcal/ the decarbonylation process (step 12). As the catalyst
[
45]
mol.
The difference between metal/bromide cata- concentration increases at least two selective, metal
lyzed alkylbenzene and alkylnaphthalene oxygena- catalyzed pathways become increasingly important.
[
19], [46]
tions has been discussed
and is largely due to At [Co] > 0.01 M, kinetic and chemiluminescence data
the enhanced ring bromination and enhanced peroxy provide evidence that the direct oxidation of Co(II) by
radical ring attack that occurs in the naphthalene de- peroxy radicals becomes important
[
33]
(step 13). This
rivatives. This forms intermediates (e. g., phenols) reaction will increasingly supplant step 7 as the cata-
which quickly undergo exhaustive oxidation to COx. lyst concentration increases, hence reducing ring at-
Since the resonance energy of furan is even lower tack and deaccelerating the CO formation. Because
x
than naphthalene (31 vs. 17 kcal/mol), the higher rate step 14 is 400,000 times faster than step 9, displaying
[
27]
of CO formation is not surprising.
a 18 kcal/mol lower activation energy barrier,
hy-
x
droxyl radical formation and decarboxylation are
109
Adv. Synth. Catal. 2001, 343, 102±111

asc.wiley-vch.de
greatly diminished as the cobalt concentration in-
creases. The reason both selectivity and activity are
often enhanced in metal/bromide systems is that re-
actions 13 and 14 produce Co(III) which quickly goes
through the redox cascade shown in Figure 9 to con-
tinue to initiate the reaction.
In the future, it is planned to extend the methodol-
ogy described herein to substituted benzylic alcohols,
aliphatic alcohols, and a variety of other alkylaro-
matic systems such as naphthalene, pyrrole, and thio-
phene derivatives.
NMR (CDCl
3
, 20 °C): ꢀ = 7.4 (s, 2 H, furan H), 9.8 (s, 2 H,
CHO); C NMR (CD Cl , 20 °C): d = 120.4 (s, CH), 154.8 (s,
qC), 179.7 (s, CHO); Mass spectrum: m/z = 124.
,5-Furandicarboxylic acid was isolated in the following
manner. The solubility of 2,5-furandicarboxylic acid is
1
3
2
2
2
±4
6
.6 ´ 10 g/g in 3% H
when the reaction solutions are cooled to room temperature
9% of the 2,5-furandicarboxylic acid precipitates. The so-
2
O/HOAc at room temperature. Hence
9
lids after reaction were filtered, washed with acetic acid,
then water, and air-dried. If insufficiently oxidized, the so-
lids contained both 2-carboxy-5-formylfuran and 2,5-furan-
dicarboxylic acid in varying amounts. All of the reported 2-
carboxy-5-formylfuran and 2,5-furandicarboxylic acid yields
are based on the precipitated solids only. The composition of
the isolated solids containing 2-carboxy-5-formylfuran and
2,5-furandicarboxylic acid in the solids were determined
from their NMR spectra in DMSO.
Experimental Section
1
Aldrich cobalt(II) and Fluka manganese(II) acetate tetrahy-
drates, Alfa cerium(III) acetate hydrate, EM Science sodium
bromide, benzyl alcohol and acetic acid, Baker hydrobromic
acid, Aldrich zirconium(IV) acetate and biphenyl, and Lan-
caster hydromethylfurfural were used as received. Catalysts
were prepared by dissolving the above compounds into
acetic acid in the amounts specified on Table 1±3.
For 2-carboxy-5-formylfuran; H NMR (DMSO): d = 7.4 (s,
13
1 H, furan CH), 7.7 (s, 1 H, furan CH), 9.8 (s, 1 H, CHO);
C
NMR (DMSO): d = 122.3 (s, CH), 153.2 (s, CH), 172.0 (s,
COOH), 179.7 (s, CHO).
1
For 2,5-furandicarboxylic acid; H NMR (DMSO): d = 7.3
1
3
(s, 2 H, furan CH); C NMR (DMSO): d = 118.5 (s, CH), 148.1
(s, C), 158.8 (s, COOH); anal. calcd. for C , %: C, 46.16;
6
4 5
H O
H, 2.59; found for solids obtained in experiments 6, 7, 11, 12
in Table 3, %: C, 45.93; 45.93; 45.45; 45.79; H, 2.57; 2.43; 2.44;
2.43.
Autoxidation at Ambient Atmospheric Pressure
A glass cylindrical reactor, as previously described,
[20]
was
used. Initial weight of acetic acid was 100 g, with Co/
Mn = 1.0 mol/mol, Br/(Co+Mn) = 1.0 mol/mol in all cases.
We found that HMF, but not benzylic alcohol, required addi-
tion of 0.5±1.0 g of acetaldehyde to initiate the reaction at
Acknowledgments
7
5 °C. The rate of oxygen uptake was continually monitored
by measuring the flow rate into the reactor and the concen-
tration of dioxygen in the vent gases. The vent gases (O , N
CO, CO ) were measured using an automated GC system.
We thank Dr. James R. Valentine for the GC-MS results. Ri-
chard C. Newton, David E. Rothfuss, and Robert J. Young,
Jr. are thankfully acknowledged for experimental assistance.
2
2
,
2
Liquid samples were removed during the reaction and ana-
lyzed via GC as soon as possible.
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