The role of the reactor wall in hydrothermal biomass conversions

Article abstract of DOI:10.1002/asia.201200556

The processing of renewable feedstocks to platform chemicals and, to a lesser degree, fuels is a key part of sustainable development. In particular, the combination of lignocellulosic biomass with hydrothermal upgrading (HTU), using high temperature and pressure water (HTPW), is experiencing a renaissance. One of the many steps in this complicated process is the in-situ hydrogenation of intermediate compounds. As formic acid and related low-molecular-weight oxygenates are among the species generated, it is conceivable that they act as a hydrogen source. Such hydrogenations have been suggested to be catalyzed by water, by bases like NaOH, and/or to involve reactive/nascent hydrogen . To achieve the temperatures and pressures required for HTU, it is necessary to conduct the reactions in high-pressure vessels. Metals are typical components of their walls and/or internal fittings. Here, using cyclohexanone as a model compound for more complex biomass-derived molecules, iron in the wall of high-pressure stainless steel reactors is shown to be responsible for the hydrogenation of ketones with low-molecular-weight oxygenates acting as a hydrogen source in combination with water. Copyright

Full text of DOI:10.1002/asia.201200556

DOI: 10.1002/asia.201200556
The Role of the Reactor Wall in Hydrothermal Biomass Conversions
[a]
Viktꢀria Fꢁbos, Alexander K. L. Yuen, Anthony F. Masters, and Thomas Maschmeyer*
th
Dedicated to Prof. Herman van Bekkum on the occasion of his 80 birthday
Abstract: The processing of renewable
feedstocks to platform chemicals and,
to a lesser degree, fuels is a key part of
sustainable development. In particular,
the combination of lignocellulosic bio-
mass with hydrothermal upgrading
among the species generated, it is con-
ceivable that they act as a hydrogen
source. Such hydrogenations have been
suggested to be catalyzed by water, by
bases like NaOH, and/or to involve
“reactive/nascent hydrogen”. To ach-
ieve the temperatures and pressures re-
quired for HTU, it is necessary to con-
duct the reactions in high-pressure ves-
sels. Metals are typical components of
their walls and/or internal fittings.
Here, using cyclohexanone as a model
compound for more complex biomass-
derived molecules, iron in the wall of
high-pressure stainless steel reactors is
shown to be responsible for the hydro-
genation of ketones with low-molecu-
lar-weight oxygenates acting as a hydro-
gen source in combination with water.
(
HTU), using high temperature and
pressure water (HTPW), is experienc-
ing a renaissance. One of the many
steps in this complicated process is the
in-situ hydrogenation of intermediate
compounds. As formic acid and related
low-molecular-weight oxygenates are
Keywords: biomass · formic acid ·
high temperature and pressure
water · reactor wall · reduction
Introduction
but also partially degrades into corrosive low-molecular-
weight oxygenates like formic acid. Therefore, a particularly
attractive cascade to investigate is the decomposition of
these problematic small oxygenates into carbon dioxide and
hydrogen in such a way as to incorporate the hydrogen gen-
erated into the liquid products. Since one of the key targets
in the conversion of biomass into synthetic fuels is the re-
duction in oxygen content of the liquid products, using the
corrosive acids to do so would eliminate them as a problem
and improve the quality of the fuels or fuel precursors si-
multaneously.
The processing of lignocellulosic biomass to platform chemi-
cals and fuels is likely to provide a pivotal resource stream
in the near future. Aqueous media are becoming increasing-
ly attractive to induce “green” chemical conversions as
water is an environmentally benign solvent. The use of high
temperature and pressure water (HTPW) as a medium is
particularly useful because of its unique properties in terms
of heat- and mass-transfer, solvation power, and raised acid-
[1,2]
ity/basicity.
Near the supercritical point, the physiochemi-
cal properties of water can be finely tuned by changing the
temperature and/or pressure. Recently, many new reaction
systems have been developed using such a water phase as
Within this context, a major reaction cascade is the reduc-
tion of aldehydes or ketones to the corresponding alcohols
[6]
in the presence of small acids, for example, formic acid,
followed by their dehydration. Such reactions have been
[3,4]
a reaction medium and/or catalyst.
Compared to other
[1,5,7]
thermochemical methods, hydrothermal treatments of bio-
mass offer an advantage as they eliminate the need to dry
wet biomass and as HTPW may simultaneously act not only
as a solvent but also as a reactant and effective acid–base
suggested to be catalyzed by water,
by bases like
and/or to involve “reactive/nascent hydro-
in HTPW. However, there is currently no consen-
[8–10]
NaOH
,
[
11–13]
gen”
sus in the literature as to the precise way in which formic
acid is decomposed and how any hydrogen generated is sub-
sequently incorporated during the reduction of, for example,
ketones.
[5]
catalyst. However, this approach gives rise to complex cas-
cading reaction systems in which (ligno-)cellulosic biomass
not only converts into a liquid biocrude oil and various
water-soluble organic species (e.g., furans and catechols),
The thermal decomposition of formic acid in dilute aque-
ous solutions involves two competitive reaction paths: decar-
boxylation, producing CO and H , and decarbonylation/de-
2
2
[1,5,7,14–16]
hydration, producing CO and H O.
In contrast to
[
a] Dr. V. Fꢀbos, A. K. L. Yuen, Prof. Dr. A. F. Masters,
Prof. Dr. T. Maschmeyer
2
the gas-phase decomposition by decarbonylation, the main
route in the liquid phase under sub- or supercritical condi-
School of Chemistry
The University of Sydney
NSW 2006 (Australia)
Fax : (+61)293513329
E-mail: th.maschmeyer@chem.usyd.edu.au
Homepage: www.chem.usyd.edu.au/research/maschmeyer.html
[15–17]
[18]
tions is decarboxylation.
first that water catalyzes the liquid-phase decarboxylation
Ruelle et al. have suggested
[19]
path (Figure 1). Later, Melius et al. considered the poten-
tial catalytic effect of water in both the decarboxylation and
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FULL PAPER
sults obtained from the two different reactors to the surface
of the ꢂfrequently usedꢃ reactor being rough and corroded
and thereby influencing the reaction.
By examining the water–gas shift reaction under hydro-
[35]
thermal conditions, Brill and Miksa have shown that the
extremely toxic Ni(CO)4 was formed by extraction of Ni
from a slightly corroded 316 stainless steel (SS316) tube.
These results are consistent with metals leaching from the
reactor.
The first studies of the water–gas shift reaction under su-
percritical conditions and in the absence of any metals were
[
36a]
[36b]
performed by Potic
and DiLeo,
using small sealed
examined the non-
Figure 1. Molecular elimination mechanisms for the decomposition of
formic acid; curved dotted lines indicate the eventual decomposition
products.
[34,37,38]
quartz capillaries. Later, Savage
catalytic gasification of cellulose and lignin using capillary
quartz reactors as a metal-free environment, employing the
[
20]
[36]
same technique as had Potic and co-workers. These results
consistently show that the yields of the gases generated are
strongly influenced by the presence of stainless steel, as the
yields are lower in metal-free quartz reactors.
the dehydration reactions. The literature provides many
[20–25]
studies on this topic,
taking into account the possible ef-
[39]
fects of density, surface-catalyzed reactions, and ionic or rad-
ical mechanisms. However, although all of the experimental
results have confirmed that the decarboxylation is the main
reaction path under supercritical or near-supercritical condi-
tions, several inconsistencies regarding the mechanism of
formic acid decomposition in the presence of water have not
been fully resolved.
Shen et al.
recently published a hydrogen-transfer re-
duction process for ketones using formic acid as a hydrogen
donor in HTPW. They attributed the catalytic effect to the
water, as they found no difference in the conversion of ke-
tones using a Teflon-lined batch reactor with and without
added pieces of SS316 (their reactorꢃs material). However,
contact of the reaction mixture with metal parts of the reac-
tor (stirrer, sampling tube, cap of the vessel, etc.) needs to
be considered and was not mentioned in that paper. By con-
Hydrogen produced by the thermal decomposition of
formic acid or HCOOX (X=Na or NH ) via the decarboxy-
4
[40]
lation route can be used for the hydrogenation of different
trast, Mondo et al. more recently published findings that
a 316 stainless steel reactor body is effective in the deoxyge-
nation of sugar alcohols or levulinic acid and that the addi-
tion of 316 stainless steel powder to the reactor led to the
formation of insoluble inorganic precipitates. The authors
suggested that this effect is specific to aqueous Brønsted
acidic solutions under a reducing atmosphere such as hydro-
gen. A promoting effect of the stainless steel has also been
shown in hydrogenation reactions using supercritical carbon
[11–13,26,27]
functional groups.
The reductions of aldehydes, ke-
tones, and nitroaromatics in HTPW using HCOOH,
HCOONa, and HCOONH have been performed in both
4
[26]
[27]
batch-
and continuous-flow tubular reactors
without
any co-solvent or added catalyst. Similarly, 2-propanol has
been proposed as hydrogenating agent in different reactions
under hydrothermal conditions without any additional metal
[28]
[11–13]
catalyst.
Furthermore, Kleinert and Barth
reported
[41]
a novel pyrolysis/solvolysis process of lignin conversion to
liquid “oils”. They showed that formic acid can serve as
both hydrogen donor and reaction medium in the liquifica-
tion process and that using alcohols as co-solvent can im-
prove the yields and the hydrogen-to-carbon ratio of the
products. The authors assumed that the high reactivity of
the in situ-formed hydrogen is responsible for the lignin con-
version into hydrogen-rich products without added catalyst.
However, these and similar reports have not excluded the
catalytic wall effect of the reactor experimentally.
dioxide as a solvent at 508C.
Thus, the literature is still unresolved in this field. While
most attribute a catalytic effect to the HTPW under hydro-
thermal conditions, others do not exclude the effect of the
reactor wall, yet are unable to define its role clearly. This
[42]
study and the accompanying report aim to resolve these
issues and the role of the reactor wall more thoroughly.
Through systematic study of a simple model compound
system, it is demonstrated how the reactor wall plays an es-
sential role in both formic acid decomposition and the even-
tual hydrogenation reactions. For simplicity and to allow
straightforward reproduction of these results by other labo-
ratories, cyclohexanone (1) was used as model compound
and its hydrogenation to cyclohexanol (2), followed by the
subsequent dehydration of the product 2 to cyclohexene (3)
was investigated, using formic acid as a stoichiometric re-
ductant and acid dehydration catalyst (Figure 2). The reac-
tion was performed in both metal and metal-free reactors in
HTPW.
Although many reports on HTPW processing can be
[
17,20,29–34]
found in the literature, fewer than ten publications
hint that the observed conversions are to some degree
[17]
a result of wall-effects. Bjerre and Sørensen
scribed experimental results using two different autoclaves
a new and a frequently used one) for the oxidative and
have de-
(
non-oxidative decomposition of formic acid. They have
found that the ꢂusedꢃ vessel resulted in a higher conversion
of formic acid, higher yields of CO and H , and a lower
2
2
yield of CO. The authors attributed the differences in the re-
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Role of the Reactor Wall in Hydrothermal Biomass Conversions
Figure 2. Formation of cyclohexanol (2) and cyclohexene (3) from cyclo-
hexanone (1).
Results and Discussion
Cyclohexanone can be easily converted into cyclohexanol
and cyclohexene in an aqueous solution of formic acid when
using stainless steel (SS316) reactors. The variation of pres-
sure (see Table 1, entries 1–10) by pre-pressurizing at the re-
action temperature to 60–100 bar of nitrogen did not have
any influence on the conversion or selectivity, whereas the
variation in temperature, in addition to favoring cyclohexa-
nol dehydration, results in a maximum cyclohexanone con-
version apparently between 225 and 2508C. This observation
is consistent with the trends observed regarding the yields of
gases during the decomposition of formic acid in supercriti-
Figure 3. Effect of the amount of HCOOH on the conversion of 1. Con-
ditions: 2508C, 80 bar, 4 h.
stainless steel shavings (SSS). Both, the formation of cyclo-
hexanol from cyclohexanone and noticeable additional pres-
sure build-up inside the reactor, were observed when using
formic acid, but no conversion was detected when using hy-
drogen gas as hydrogen source (Table 2, entries 3 and 4).
[27]
cal water.
Therefore, a relative increase of formic acid
should increase cyclohexanone conversion. As can be seen
in Figure 3, this is indeed the case, with a plateau being
reached at a formic acid/cyclohexanone molar ratio
of about five. Notably, when using 2-propanol as
Table 1. The conversion of cyclohexanone (1) in aqueous formic acid in a 316 stainless
steel reactor.
hydrogen donor instead of formic acid, no conver-
sion was observed, thus ruling out the Meerwein–
Ponndorf–Verley (MPV) type hydrogenation mech-
Entry
T [8C]
p [bar]
H-source
Conversion [%]
Selectivity [%]
1
2
3
[43]
anism.
Hydrogen gas is also not active under
1
2
3
4
5
6
7
8
9
200
225
250
200
225
250
200
225
250
225
225
60
60
60
80
80
80
100
100
100
80
HCOOH
HCOOH
HCOOH
HCOOH
HCOOH
HCOOH
HCOOH
HCOOH
HCOOH
–
72.5
89.0
50.6
73.5
82.2
85.2
71.0
89.8
81.7
–
10.2
4.4
3.7
10.0
4.7
2.5
9.4
3.6
2.6
–
89.8
95.6
96.3
90.0
95.3
97.5
90.6
96.4
97.4
–
these conditions (Table 1, entry 11).
To investigate possible catalytic wall-effects of
the stainless steel reactor in this system, the reac-
tions were performed in a metal-free reactor,
namely in a high-pressure sapphire NMR tube
(
(
with a titanium top) under the same conditions
taking care of the liquid-to-reactor volume ratios
1
1
0
1
to ensure comparable pressures, see the Experimen-
tal Section). As no noticeable reaction in this
metal-free environment was observed (Table 2, en-
tries 1 and 2), neither using formic acid nor hydro-
gen gas as a hydrogen source, it is clear that the
stainless steel in conventional autoclaves plays a cru-
cial role under these conditions. Importantly, in the
80
H
2
–
–
–
Conditions: 0.2 mL of 1, 0.2 mL of n-decane in 3 mL of aqueous acid solution, treaction
h. Molar ratio of H-source/water/1=5:84:1. No formation of cyclohexane was de-
tected.
=
4
Table 2. Reduction of cyclohexanone (1) with HCOOH in the presence of stainless
subsequent analysis by H and C NMR spectrosco- steel shavings (SSS).
1
13
py, using the still-sealed sapphire NMR tube, reso-
nances due to formic acid could be observed, but
none that were due to the formation of neither dis-
solved hydrogen, carbon dioxide nor carbon mon-
oxide. The fact that a noticeable pressure build-up
when opening the sapphire tube was also not ob-
served is consistent with these NMR measurements.
To study the promoting effect of the reactorꢃs
material more fully, the metal-free sapphire reactor
Entry H-source
Amount of SSS [mg] Conversion [%] Selectivity [%]
1
2
3
1
2
3
4
5
HCOOH
0
0
550
550
1
100
–
66
–
–
–
34
–
–
H
2
0.0
20
0.0
0.0
HCOOH
H
2
2-propanol 550
–
Conditions: Molar ratio of H-source/water/1=3:84:1, 2508C, 4 h; although a direct
measurement of the pressure was not possible, the conditions and volume ratios used
mirror those used in the SS reactors, so approximately 80 bar pressure is assumed in
experiment was repeated in the presence of 316 each case.
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Thomas Maschmeyer et al.
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These observations clearly demonstrate that the presence of
a metal alloy, such as 316 SS or one or more of its compo-
nents, is crucial for the reactions to occur. Importantly, de-
spite the presence of 316 SS shavings, no conversion of cy-
clohexanone could be observed when using hydrogen gas as
hydrogen source. This shows beyond doubt that 316 SS does
not act as a hydrogenation catalyst under the reaction condi-
tions, but that the metal surface and protons are involved in
ketone reduction. Furthermore, these observations throw
doubt on the role of water as a catalyst for formic acid de-
composition. Although many studies have suggested water
conversions that are observed in the presence of dilute acid,
which is present when using ’additional water’ (see Table 3).
This enhanced reactivity may be explained either by a great-
er supply of “fresh surface” to promote the reaction and/or
by a greater concentration of reactive dissolved species orig-
inating from the wallꢃs surface.
To examine whether only one metal component of the
stainless steel acts as an active species or whether a mixture
of these metals is needed, the effects of the common constit-
uents of 316 stainless steel were tested (Table 5) by repeat-
[18,19]
as a catalyst
for the decomposition of aqueous formic
Table 5. The principle components of 316 stainless steel.
acid, these reports appear questionable now, at least for
temperatures of up to 2508C, as no decomposition of aque-
ous formic acid occurs in a metal-free environment.
Element
%
Fe
Cr
Ni
Mo
2–3
Mn
2
C
60–70
16–18
10–14
0.08
However, water was observed to strongly promote the re-
action of cyclohexanone with formic acid, either concentrat-
ed (water content 1.85%, measured by Karl-Fischer titra-
tion) or dilute (water content 91.6%), under otherwise iden-
tical conditions (Table 3), showing that additional water, al-
though not necessary, is clearly beneficial.
ing the sapphire tube experiments in the presence of pow-
dered samples of each individual element under the same
conditions as used to test the SSS. None of these powders
(chromium, nickel, molybdenum, and carbon) were active in
the formic acid-based reduction of cyclohexanone at 2508C;
however, the reaction did occur in the presence of iron or
manganese (Table 6). As the ratio of Fe/Mn in 316 SS is ap-
proximately 35:1, it is reasonable to conclude that iron,
rather than manganese, is the main reactive metal (although
the involvement of manganese cannot be discounted com-
pletely). Again, regardless of the metal present, the use of
Table 3. Comparison of the reactions with and without additional water.
Entry
Water
H-source
Conversion [%]
Selectivity [%]
1
2
3
[
[
a]
b]
1
2
+
HCOOH
HCOOH
53
29
8.0
7.9
92.0
92.2
À
[
a] Molar ratio of H-source/water/1=3:84:1. [b] Molar ratio of H-source/
gaseous H as hydrogen source did not result in any reduc-
2
1
=3:1. Conditions: 2508C, 80 bar, 4 h.
tion, which is consistent with the results obtained in the
presence of SSS (Table 2).
In the experiments above, the reaction mixture
became pale green in the presence of stainless steel.
To check whether the leaching of metals was influ-
enced by the concentration of acid, 700 mg SSS
were soaked in 3 mL diluted (2m) and in concen-
trated formic acid (98%) at room temperature.
After 4 hours, no coloration could be observed in
the case of the concentrated formic acid; however,
a bluish-green color developed in the dilute solu-
Table 6. Reduction of cyclohexanone (1) with HCOOH or H
or manganese powder in the sapphire reactor.
2
in the presence of iron
Entry Metal H-source Amount of metal
Conversion
Selectivity
[%]
[
mg]
[%]
1
2
3
1
2
3
Fe
Fe
Mn
HCOOH 570
530
HCOOH 250
55.4
0.0
4.8
37.5
–
62.5
–
H
2
100.0
0.0
tion. The control reaction carried out in the absence Conditions: Molar ratio of HCOOH/water/1=3:84:1, [HCOOH]aq =2m, 2508C,
8
0 bar, 4 h.
of formic acid did not result in any coloration of the
reaction mixture. The corresponding trace metal
analyses of the acid solutions are shown in Table 4.
The much more extensive leaching of metals in dilute
acidic solutions is mirrored to some extent in the greater
It has to be noted that the formation of by-products, such
as iron formate(s) and oxide(s), were observed after the re-
action when either SSS or iron powder was used under
HTPW conditions. Analyzing the precipitates by X-ray dif-
fraction (XRD) revealed that iron(II) formate hydrate, Fe-
Table 4. The leaching of the most abundant metal components from 316
SS in concentrated and dilute formic acid solutions.
A
H
U
G
R
N
N
(OOCH) ·2H O, which is easily oxidized to iron
A
T
N
G
T
R
E
N
N
U
N
G
(III) for-
2
2
Metal
c
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(metal) [ppm]
c AHCTUNGTERNNUN(G metal) [ppm]
mate hydrate under air, and magnetite (Fe O ) were formed
3
4
in conc. HCOOH
in 2 M HCOOH
[44]
during the reaction. Moreover, according to the literature
Fe(OOCH) ·2HCOOH is a green powder and can easily be
converted into iron(II) formate dihydrate under these condi-
Fe
Cr
Ni
61.7
6.03
1.13
17270
4068
1350
AHCTUNGTRENNUNG
2
[45,46]
tions.
This suggests that the green color observed during
Concentrations of each metal were determined after leaching for 19 h at
RT by inductively coupled plasma–atomic emission spectroscopy (ICP–
AES).
the treatment of the SSS with acid can be attributed to the
presence of this compound. To test whether the iron for-
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Role of the Reactor Wall in Hydrothermal Biomass Conversions
À1
mates and/or oxides formed during the reaction are active
for the hydrogenation of cyclohexanone, they were prepared
and tested separately in a metal-free NMR tube. No reac-
tion was observed in any of these cases under HTPW condi-
tions; thus, neither iron oxides nor iron formates promote
the hydrogenation under the conditions outlined here. Nota-
bly, iron formate can decompose at higher temperature
by an increase to 1308C at a rate of 108Cmin . Retention times were:
cyclohexene 5.7; cyclohexanol 11.1; cyclohexanol 11.4; and n-decane
1
4.5 min. All mass balances (relative to cyclohexanone) were Æ10% or
better, most were Æ5% or better. Powder X-ray diffraction (XRD)
measurements were made using a PANalytical X-Pert PRO MRD X-ray
diffractometer equipped with a PIXcel detector, and using Ni-filtered
CuKa radiation (l 1.5419 ꢅ) for the identification of the solid by-products
formed during the reaction.
[45,46]
(
above 2508C)
which might explain the observations of
Hydrogenation of Cyclohexanone under HTPW Conditions in a Batch
Reactor
CO and/or CO during the investigation of the formic acid
2
[15–19,25]
decomposition by other authors.
Experiments were carried out in 15 mL 316 stainless steel standard batch
reactors (AMTEC slurry phase reactor SPR-16) with individual pressure,
temperature, and stirring control. Aqueous formic acid solutions (0.5–2m,
In the case of manganese, a very vigorous reaction oc-
curred (intensive gas production) when the metal was added
to the acid solution. This rapid decomposition of formic acid
to gaseous products in the presence of manganese results in
the depletion of formic acid from the solution and, there-
fore, little formic acid is left to react with cyclohexanone,
consistent with the low yields of cyclohexanol observed.
3
mL) of cyclohexanone (0.2 mL) and of n-decane (0.2 mL) as an internal
standard were measured into the vessels. The reactors were purged with
nitrogen before being closed to remove the residual air, then heated up
to 200–2508C, reaching pressures of 40–80 bar. Additional nitrogen was
added at the reaction temperature if the pressure had not reached the set
value of 60, 80, or 100 bar. The reaction mixtures were stirred at the reac-
tion temperature and pressure for 4 h. The reactors were cooled to ambi-
ent temperature, the residual pressure (20–40 bar) was released, and the
reactors were opened. The reaction mixtures were separated by extrac-
tion with dichloromethane (2ꢄ45 mL) and dried over sodium sulfate.
After filtration, the samples were analyzed by GC.
Conclusions
When H
pressurized with H
tures (200–2508C), yielding pressures in the 40–80 bar range.
2
was used instead of formic acid, the reactors were purged and
to 20 bar and then heated to the desired tempera-
Cyclohexanone can be easily converted into cyclohexanol
and mainly cyclohexene in an aqueous solution of formic
acid in any of several common 316 stainless steel batch reac-
tors, but no such reaction was observed to any degree in
a metal-free sapphire reactor. However, reactivity can be in-
duced in the metal-free reactor by adding metal shavings of
stainless steel. Iron has been shown to be the likely principle
actor in this reaction cascade. Furthermore, a dilute formic
acid solution works more powerfully than a concentrated so-
lution, corresponding to the extent of leaching observed. Al-
though iron formates were isolated from the reaction, nei-
ther they nor common iron oxides promote these reactions.
This clearly illustrates that the metal surface of the auto-
clave plays a crucial role under these conditions.
2
HTPW Treatment of Cyclohexanone in the Presence of Formic Acid in
a Sapphire Tube
The metal-free experiments were carried out in a 7 mL sapphire high-
pressure NMR tube (with a titanium top). Aqueous formic acid solutions
(
0.5–2m, 2 mL) of cyclohexanone (120 mg) and n-decane (72 mg) as an
internal standard were measured into the tube. It was purged with nitro-
gen, closed, and heated to the desired temperature in a sand bath. After
a reaction time of 4 h, the sand bath was removed and the tube was air-
cooled to ambient temperature. No pressure build-up was observed upon
opening. The reaction mixture was extracted with dichloromethane (3ꢄ
5
mL), separated, and the organic phase was dried over sodium sulfate.
After filtration, samples were taken and analyzed by GC.
When H was used instead of formic acid solution, the tube was purged
2
Therefore, any discussions regarding hydrogenations of
then pressurized with 20 bar of H before closing.
2
(
at least) ketones under HTPW conditions in the presence
Hydrogenation of Cyclohexanone under HTPW Conditions in a Sapphire
Tube
of small organic acids need to consider the wall-effect of the
reactor. Furthermore, it can be surmised that in more com-
plex systems, as are present in the case of the treatment of
biomass under HTPW conditions, the reactor wall is likely
to play a significant role. The role of the water, the fate of
the iron, the composition of the hydrogen generated, and as-
sociated mechanistic considerations will be discussed more
Dilute aqueous formic acid solution (0.5–2m, 2 mL), cyclohexanone
(
120 mg), and n-decane (72 mg) as an internal standard were measured
into the sapphire high-pressure NMR tube. Solid (250–550 mg) samples
of stainless steel shavings or of iron, manganese, chromium, nickel, mo-
lybdenum, or carbon powder were added to the reaction mixture. The
tube was purged with nitrogen, closed quickly, and heated to the desired
temperature in a sand bath. The tube was not purged with nitrogen in
the case of iron or manganese addition, as gas evolution was observed
immediately after addition of the metal to the acidic solution. After a re-
action time of 4 h, the sand bath was removed and the tube was air-
cooled to ambient temperature. Significant pressure build-up was ob-
served when using iron or manganese when it was opened. The reaction
mixture was extracted with dichloromethane (3ꢄ5 mL), separated, and
the organic phase was dried over sodium sulfate. After filtration, samples
were taken and analyzed by GC.
[42]
in detail in the accompanying report.
Experimental Section
Cyclohexanone, cyclohexanol, n-decane, and formic acid (99% purity)
(
all Sigma–Aldrich) were used without further purification. Quantitative
analysis was performed on a Shimadzu GC-17 A Gas Chromatograph
GC) fitted with a Restek column (RTX5, 30 mꢄ0.25 mmꢄ0.25 mm) and
(
Treatment of the SSS with Acid for ICP Analysis of the Metal Leaching
equipped with an FID detector. Quantification was achieved by compar-
ing integration values to those derived from standard curves (5 different
concentrations) for the starting material and products, using authentic
samples. In each GC run, n-decane was used as an internal standard. The
injector and detector ports were maintained at 2508C, and the analytes
separated using a heating profile of 408C for 5 min (isothermal), followed
Two parallel experiments were carried out: SSS (700 mg) was soaked in
diluted (2m, 3 mL) and concentrated formic acid (98 wt%, 3 mL) at
room temperature. After 4 h, no coloration was observed in the case of
the concentrated formic acid; however, a bluish-green color developed in
the dilute acid solution. The acid solutions were filtered and diluted to
Chem. Asian J. 2012, 00, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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Thomas Maschmeyer et al.
FULL PAPER
2
5 mL with water. Quantitative analysis for the elemental composition of
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ACHTUNGTNERNUNG( III)
7
639.
A
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Testing Iron Formates and Oxides for the Hydrogenation of
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8
9
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Received: June 22, 2012
Published online: && &&, 0000
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 0000, 00, 0 – 0
ÝÝ These are not the final page numbers!

FULL PAPER
Biomass goes to the wall: The wall-
effect of metal reactors is crucial in the
high temperature and pressure water
Biomass Conversion
Viktꢀria Fꢁbos, Alexander K. L. Yuen,
Anthony F. Masters,
(
HTPW) hydrogenation of cyclohexa-
none using formic acid as a reductant.
Although it is possible to convert
cyclohexanone into cyclohexanol and
cyclohexene in a standard 316 stainless
steel autoclave using HCOOH, it is
impossible to do so in a completely
metal-free environment, such as a sap-
phire high-pressure reactor. However,
reactivity can be induced by adding
metal shavings of stainless steel or
iron.
Thomas Maschmeyer*
&&&& — &&&&
The Role of the Reactor Wall in
Hydrothermal Biomass Conversions
Chem. Asian J. 2012, 00, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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