Reactivity of alcohols in chemoselective transfer hydrogenation of acrolein over magnesium oxide as the catalyst

Article abstract of DOI:10.1007/s10562-010-0497-7

This paper presents the first full thermodynamic description of the hydrogen transfer between acrolein, the simplest α,β-unsaturated aldehyde, and a set of aliphatic alcohols, both primary and secondary. The vapour phase transfer hydrogenation of acrolein into allyl alcohol with various primary and secondary aliphatic alcohols used as hydrogen donors in the presence of MgO as the catalyst has been studied. Despite differentiated reactivity exhibited by these alcohols, a high chemoselectivity (>80%) to allyl alcohol has been observed for all of them. On the basis of thermodynamic calculations it has been found that secondary alcohols as hydrogen donors are more reactive than primary ones. However, ethanol or butan-1-ol have shown the highest reactivity. In their presence yields of allyl alcohol higher than 60% have been noted, which greatly exceed those predicted by thermodynamic calculations based on the following equation: acrolein + ethanol (butan-1-ol) → allyl alcohol + acetaldehyde (butyraldehyde). Although similar yields have been reported in literature, no subsequent nor side reactions have been discussed even though the attained yield cannot be accounted for by this reaction alone. As a possible explanation of the discrepancy the occurrence of a disregarded reaction, for which ΔG < 0, has been considered. It has been shown that aldol condensation fulfills these thermodynamic requirements, however, the products of this reaction are noted only at the beginning of the process and the decrease of their amount does not influence the yield of allyl alcohol.

Full text of DOI:10.1007/s10562-010-0497-7

Catal Lett (2011) 141:293–299
DOI 10.1007/s10562-010-0497-7
Reactivity of Alcohols in Chemoselective Transfer Hydrogenation
of Acrolein over Magnesium Oxide as the Catalyst
Marek Gli n´ ski
Urszula Ulkowska
•
Received: 13 April 2010 / Accepted: 3 November 2010 / Published online: 20 November 2010
The Author(s) 2010. This article is published with open access at Springerlink.com
ꢀ
Abstract This paper presents the first full thermody-
namic description of the hydrogen transfer between acro-
lein, the simplest a,b-unsaturated aldehyde, and a set of
aliphatic alcohols, both primary and secondary. The vapour
phase transfer hydrogenation of acrolein into allyl alcohol
with various primary and secondary aliphatic alcohols used
as hydrogen donors in the presence of MgO as the catalyst
has been studied. Despite differentiated reactivity exhibited
by these alcohols, a high chemoselectivity ([80%) to allyl
alcohol has been observed for all of them. On the basis of
thermodynamic calculations it has been found that sec-
ondary alcohols as hydrogen donors are more reactive than
primary ones. However, ethanol or butan-1-ol have shown
the highest reactivity. In their presence yields of allyl
alcohol higher than 60% have been noted, which greatly
exceed those predicted by thermodynamic calculations
based on the following equation: acrolein ? ethanol
Keywords Acrolein ꢀ Allyl alcohol ꢀ Hydrogen transfer ꢀ
Magnesia ꢀ Chemoselectivity ꢀ Thermodynamics
1 Introduction
Unsaturated alcohols are very important and versatile
intermediates for pharmaceutical, polymer, fragrance, and
food industries. The main route for their synthesis is the
selective hydrogenation of a,b-unsaturated carbonyl com-
pounds. Conventional reduction of these compounds by
gaseous dihydrogen over many metal catalysts results in
preferable hydrogenation of the C=C bond [1–5]. Recently,
metallic gold dispersed on FeOOH or c-Fe O has shown a
2
3
remarkable chemoselectivity in the reduction of the con-
jugated C=O bond in the a,b-unsaturated carbonyl com-
pounds [6, 7].
(
butan-1-ol) ? allyl alcohol ? acetaldehyde (butyralde-
Catalytic transfer hydrogenation (CTH) has been found
to be an efficient and selective method to synthesize
unsaturated alcohols [8–10]. Metal oxides, MgO [11, 12],
ZrO [13], double layered hydroxides [14], MgO–B O
2 3
hyde). Although similar yields have been reported in lit-
erature, no subsequent nor side reactions have been
discussed even though the attained yield cannot be
accounted for by this reaction alone. As a possible expla-
nation of the discrepancy the occurrence of a disregarded
reaction, for which DG \ 0, has been considered. It has
been shown that aldol condensation fulfills these thermo-
dynamic requirements, however, the products of this
reaction are noted only at the beginning of the process and
the decrease of their amount does not influence the yield of
allyl alcohol.
2
[15], and Al O –AlPO [16] have been used as catalysts.
2
3
4
According to literature data, MgO is one of the most active
catalysts in CTH [17, 18].
Acrolein, the simplest a,b-unsaturated aldehyde, is
considered one of the most difficult compounds to be
reduced to the unsaturated alcohol [19]. The first study on
CTH of acrolein and other a,b-unsaturated carbonyl com-
pounds was performed more than 50 years ago [20]. Oxi-
des of magnesium, calcium, aluminum, zinc and cadmium,
and a mixture of MgO–ZnO have been used as catalysts. It
has been shown that the catalysts which contain MgO were
the most active and chemoselective towards allyl alcohol.
Moreover, a differentiated reactivity of primary and sec-
ondary alcohols as hydrogen donors to acrolein has been
M. Gli n´ ski (&) ꢀ U. Ulkowska
Department of Catalysis and Organometallic Chemistry, Faculty
of Chemistry, Warsaw University of Technology (Politechnika),
Noakowskiego 3, 00-664 Warsaw, Poland
e-mail: marekg@ch.pw.edu.pl
123

2
94
M. Gli n´ ski, U. Ulkowska
found. Very little is known about the reasons of their
reactivities besides the fact that they are weakly influenced
by the structure of substituents surrounding the carbinol
carbon atom. A higher reactivity of ethanol towards acro-
lein than propan-2-ol has been noted [20]. Moreover, in the
CTH of a,b-unsaturated aldehydes other than acrolein, a
much higher reactivity of propan-2-ol compared to that of
ethanol has been observed [15, 21].
dry nitrogen before all measurements. Powder diffraction
data were collected on a D-5000 diffractometer (Siemens)
equipped with a scintillation counter and Ni-filtered Cu Ka
radiation. The surface areas of the samples were measured
using a Micromeritics ASAP 2020 instrument. The
TG-DTA measurements were performed using a NET-
ZSCH STA 449C thermobalance. The samples were heated
3
-1
up to 873 K in Ar flow (10 cm min , heating ramp
-
1
In this work a systematic study of the reactivity of var-
ious aliphatic alcohols as hydrogen donors to acrolein in its
vapour-phase transfer hydrogenation over pure magnesium
oxide as the catalyst has been performed. A description of
low and high reactivities of alcohols has been given and the
obtained values have been compared with those resulted
from the thermodynamic calculations. Very high reactivity
of ethanol as the hydrogen donor to acrolein observed for
MgO [20, 22], significantly exceeding that of propan-2-ol
10 deg min ). The data was processed with NETZSCH
Proteus Thermal Analysis software. The strength of the
surface acid–base sites of MgO was determined by the
Hammett method using a sequence of indicators in anhy-
drous toluene as the solvent [24]. The concentrations of
acidic and basic sites of MgO were determined using
solutions (0.01 M) of triethylamine or benzoic acid in
anhydrous toluene according to the procedure described
elsewhere [25]. The measurements were performed at
ambient temperature under dry nitrogen after 24 h contact
of MgO with these solutions in grease-less glass reactors.
Acrolein (90%, Aldrich) was dried over anhydrous
[
21], has been confirmed by us. Moreover, we have also
found that butan-1-ol is a very reactive hydrogen donor and
the observed reactivities of ethanol and butan-1-ol are
higher than those predicted by the thermodynamic calcu-
lations based on the following equation: acrolein ? ethanol
MgSO at 273 K and distilled under normal pressure in
4
nitrogen. The fraction that boils at 325–326 K was col-
lected. This procedure was repeated with the obtained
fraction. The distillate (b.p. 325.5–326.0 K) was collected
in a Schlenk-type container and kept at 243 K in a freezer.
Purity 99.4% (GC). Commercial alcohols: methanol (p.a),
ethanol (p.a, anhydrous 99.8%), propan-2-ol (p.a.) and
butan-1-ol (p.a.) all from POCh Gliwice Poland, as well as
heptan-2-ol (98%) and octan-2-ol (97%), both from
Aldrich, were used as hydrogen donors. All alcohols were
distilled over metallic sodium and kept dry in Schlenk-type
containers under nitrogen.
(butan-1-ol) ? allyl alcohol ? acetaldehyde (butyralde-
hyde). This discrepancy has not yet been accounted for.
2
Experimental
Catalyst. To a suspension of 180 g of MgO (purum p.a.,
3
Reachim) in 2 dm of redistilled water nitric acid (68%, p.a.,
POCh Gliwice, Poland) was slowly added with stirring (c.a.
3
40 cm ). After cooling a slightly turbid solution was filtered
5
and purified by partial precipitation of Mg(OH) using
2
Catalytic activity measurements were carried out in a
fixed-bed tubular glass reactor into which a sample of the
catalyst (0.250 ± 0.005 g) was loaded in a stream of dry
nitrogen. A solution of acrolein in a hydrogen donor (at a
given molar ratio) was dosed using a microdosing pump with
3
30 cm of ammonia solution (25%, p.a., POCh Gliwice,
1
Poland). The resulted suspension was stirred for 72 h at room
temperature and filtered off. To this clear solution an excess
3
of 25% ammonia solution (1.2 dm ) was slowly added with
3
-1 -1
stirring. The precipitate of Mg(OH)2 was washed by
3
decantation with redistilled water (25 times, 1 dm in each
a LHSV (Liquid Hourly Space Velocity): 3 cm g
h
3
-1
into a stream of nitrogen (50 cm min ) which was passed
through the catalyst bed. The reaction products were col-
lected in glass receivers, cooled to 213–223 K with a propan-
2-ol–dry ice mixture. Before activity measurements, the
catalyst was maintained at 473 K in the stream of reactants
for 60 min. This procedure was omitted in the time-on-
stream experiment in which the activity of catalyst was
measured directly after the introduction of substrates.
The reaction products were analyzed by GC (HRGC
4000B KONIK) equipped with a TRACER wax capillary
column (length 30 m, 0.25 mm i.d.) and a flame ionization
detector. t-Butylbenzene was used as an internal standard.
The compounds were identified by GC–MS (HP-6890N
with a 5973N mass detector) and by comparison of the
retention time with that of a standard sample.
washing) and dried under normal pressure at 313, 353 and
3
93 K for 24 h at each temperature. The dried powder of
purified Mg(OH) (165 g) was pelletized and the pellets
2
were crushed. A sieved fraction of 0.16–0.40 mm was cal-
cined in a tubular quartz reactor at 873 K for 1 h in a stream
of air and for 5 h in a stream of dry deoxygenated nitrogen.
After cooling in a stream of nitrogen, MgO was transferred to
a Schlenk-type container and stored under nitrogen. The
same batch of Mg(OH) has been used by us before [23].
2
Magnesium oxide and its precursor—Mg(OH) , were
2
characterized by a number of techniques, such as: XRD,
nitrogen physisorption, TG–DTA, as well as the Hammett
method for determining acidic-basic properties of surfaces.
The precursor was heated to 473 K for 6 h in a stream of
1
23

Reactivity of Alcohols in Chemoselective Transfer Hydrogenation
295
Thermodynamic calculations. All calculations leading to
the evaluation of the Gibbs function (DG), the equilibrium
constant (K) and the thermodynamic yield (a) of the corre-
sponding products at 673 K were based on data (enthalpies,
entropies and molar heat capacities) taken from the National
Institute of Standards and Technology (NIST) Standard
Reference Database [26]. The list of equations that have been
used to calculate thermodynamic data for all reactants par-
ticipating in the studied reactions are given below:
DGðTÞ ¼ DHðTÞ ꢁ TDSðTÞ
T
Z
0
98
DHðTÞ ¼ DH2
þ
DCpðTÞdT
2
98
T
Z
DCpðTÞ
0
2
DSðTÞ ¼ DS
þ
dT
98
T
2
98
2
3
DC ðTÞ ¼ a þ bT þ cT þ dT
p
T
Z
Fig. 1 X-ray diffraction patterns of Mg(OH)
2
(a) and MgO (b)
0
0
298
DGðTÞ ¼ DH298
þ
DCpðTÞdT ꢁ TDS
þ
298
T
Z
DCp
T
0,2
ꢁ
T
ðTÞdT
9
8
5
5
0,0
2
98
ꢀ
ꢁ
DGðTÞ
-
-
-
-
0,2
0,4
0,6
0,8
K ¼ exp ꢁ
RT
3
Results and Discussion
.1 Characterization of Mg(OH) and MgO
75
3
2
6
5
-1,0
300
400
500
600
700
800
900
Structural examination by powder X-ray diffraction
T [K]
showed that Mg(OH) dried at 473 K comprises the brucite
2
2 -1
= 27 m g , pore volume
Fig. 2 TG–DTA analysis of Mg(OH)
2
phase only (Fig. 1a). S
-1
BET
3
V = 0.200 cm g
and average pore diameter 24 nm.
p
The TG-DTA analysis revealed that Tmax for the decom-
position of Mg(OH) is 692.9 K (Fig. 2) and weight loss
3.2 Catalytic Activity Measurements
2
(
320–870 K) 30.41% (exp), 30.89% (calc).
The reactivity of six aliphatic alcohols, either primary or
secondary, as hydrogen donors to acrolein has been studied
in the presence of MgO as the catalyst (Table 1). Only
straight-chained alcohols have been used in order to
reduce the influence of steric effects on their reactivity. The
reaction occurs according to the following scheme:
Samples of MgO prepared by calcination of Mg(OH) at
2
8
73 K comprise of the periclase phase only (Fig. 1b).
2
-1
3
SBET = 100 m g , V = 0.529 cm g and average pore
-1
p
diameter 17 nm. Acid–base strength (H ) 7.2 B H \ 33.0;
-
-
-
1
concentration of acidic sites 10 lmol g , concentration of
-
1
basic sites 1785 lmol g
.
O
OH
R'
O
O
+
+
+
+
OH
OH
ð1Þ
H
R
H
R
R'
UOL
SAL
SOL
123

2
96
M. Gli n´ ski, U. Ulkowska
Table 1 Transfer hydrogenation of acrolein with studied alcohols
over MgO as the catalyst
•
ethanol and butan-1-ol are the most reactive hydrogen
donors; 43 and 49% conversions of acrolein have
already been obtained at 623 K, respectively. For the
former donor higher conversions (*65%) have been
even noted [22];
a
Donor
T (K) Conv. Moles from 100 mol of acrolein
(
%)
UOL
SAL
SOL
Others
MeOH
573
3
8
3
7
0
1
0
0
0
0
1
1
1
3
1
2
4
0
1
2
2
4
8
0
3
6
•
•
propan-2-ol is only a moderate hydrogen donor,
whereas other secondary alcohols (C , C ) are much
6
6
23
73
7
8
13
22
43
64
36
49
72
10
19
31
20
30
63
19
32
65
8
3
1
more reactive;
EtOH
573
20
38
50
32
42
49
9
1
0
at 673 K the reactivity of butan-1-ol exceeded the
reactivities shown by ethanol and heavy secondary
6
6
23
73
3
1
alcohols (C , C ), yet the chemoselectivities towards
7
7
4
8
allyl alcohol were higher for both primary alcohols;
methanol is the least reactive hydrogen donor.
1
2
2
2
-BuOH
-PrOH
573
2
0
•
6
6
23
73
4
2
11
1
8
Due to the fact that in the presence of ethanol or butan-
-ol equally high yields of allyl alcohol in the chemose-
573
0
1
6
6
23
73
16
22
18
24
1
1
lective transfer hydrogenation of acrolein have been
observed (49–50%), further studies have been undertaken
to explore optimum reaction conditions (Table 2) and the
durability of the catalyst with a chosen donor (Fig. 3).
It has been shown that an increase in the donor–acceptor
molar ratio to a value 6 resulted in an increase in the con-
version of acrolein up to 86 and 89% at 673 K for ethanol
and butan-1-ol, respectively. When D/A = 9, slightly
higher conversions of acrolein (89–91%) for both alcohols
have been noted only at 673 K. For D/A equal to 6 and 9, a
decrease of the temperature of the maximum yield of allyl
alcohol from 673 to 623 K for both alcohols as hydrogen
donors has been observed. This maximum reached the value
of 63–64% (D/A = 6) at chemoselectivities 88 and 81% for
4
3
-HepOH 573
0
0
6
6
23
73
1
1
b
41
5
9
-OctOH
573
18
26
1
0
6
6
23
73
2
1
c
40
7
12
D/A = 3 (mol/mol)
a
UOL allyl alcohol, SAL propionaldehyde and SOL propan-1-ol
b
c
8
% of UOL was observed at 723 K
3
% of UOL was observed at 723 K
where: R = H, Me, n-Pr, n-Am or n-Hex; R’ = H or Me.
The CTH of acrolein with alcohols results in the for-
mation of allyl alcohol (UOL), propionaldehyde (SAL),
and propan-1-ol (SOL) as the reaction products. For all
hydrogen donors allyl alcohol was the main reaction
product. At 623 K the chemoselectivity of its formation
higher than 80% (81–88%) has been noted. For all donors
the highest yields of allyl alcohol were achieved at 673 K,
although the reaction chemoselectivity was lowered to
Table 2 Transfer hydrogenation of acrolein with ethanol or butan-1-
ol over MgO as the catalyst
a
Donor
EtOH
b
T (K) Conv. (%) Moles from 100 mol of acrolein
UOL
SAL
SOL
Others
573
51
73
86
52
70
89
62
78
89
59
72
91
47
64
59
44
52
49
55
63
51
46
49
43
2
4
1
3
1
2
3
1
3
4
2
4
5
3
4
5
623
673
573
623
673
8
16
3
6
2–78% only.
4
In general it has been stated that the reactivity of acro-
6
9
lein as the hydrogen acceptor from alcohols in the presence
of MgO is very low compared to the reactivities deter-
mined for saturated carbonyl compounds [17, 18, 23, 27–
9
27
2
1
-BuOH 573
3
6
6
5
6
6
23
73
73
23
73
4
7
2
9]. At 473 K the conversion of acrolein (results not
10
5
23
5
shown) did not exceed 10%, whereas for saturated carbonyl
compounds the conversions higher than 60% have been
reported [17, 18, 23, 27, 29]. However, pronounced dif-
ferences in the reactivities of alcohols as hydrogen donors
in the transfer hydrogenation of acrolein have been
observed. The results of studies performed within this work
indicate that:
b
7
13
32
11
D/A = 6 and 9 (mol/mol)
a
UOL allyl alcohol, SAL propionaldehyde and SOL propan-1-ol
D/A = 9
b
1
23

Reactivity of Alcohols in Chemoselective Transfer Hydrogenation
297
conversion
UOL
SAL
SOL
done for 673 K since this was the temperature at which the
maximum yield of allyl alcohol had been observed
(Tables 3, 4, 5, 6). For the purpose of analysis, the
description of a course of the following reactions has been
assumed:
1
00
8
6
4
2
0
0
0
0
0
OH
R'
O
ð2Þ
ð3Þ
ð4Þ
ð5Þ
+
H2
R
R
R'
O
OH
O
0
30
60
90
120 150 180 240 300 360 420
t [min]
+
+
+
+
OH
H
H
H
R
R
R'
OH
R'
R
R'
Fig. 3 Activity of MgO versus time (time-on-stream) in the reduc-
tion of acrolein with ethanol. D/A = 6 (mol/mol). T = 623 K
O
O
O
O
+
H
R
R'
ethanol and butan-1-ol, respectively. Only a slightly lower
yield of allyl alcohol (55%) has been reported in the first
publication on CTH of acrolein [20]. A much higher yield
of allyl alcohol (73%) has been attained, however the cat-
alyst was MgO–B O with propan-2-ol as the hydrogen
OH
R'
O
2
+
2
OH
R
R
R'
0
2
3
where: R = H, Me, n-Pr, n-Am, n-Hex and R = H, Me.
The thermodynamic analysis of the dehydrogenation of
alcohols and the transfer hydrogenation reactions of acro-
lein can be summarized as follows:
donor [15]. At D/A = 9, significant yields of propan-1-ol up
to 27–32% have been attained, at the expense of allyl
alcohol, lowering the yield of the latter (49–52%). Ethanol
was the most chemoselective hydrogen donor in the whole
range of temperature (573–673 K). Hence, this alcohol has
been chosen for the time-on-stream measurement.
•
•
•
•
for all alcohols except methanol the dehydrogenation at
73 K is thermodynamically favorable (DG \ 0);
secondary alcohols are more prone to release hydrogen
6
At the beginning of the time-on-stream test (0–30 min)
high conversions of acrolein (78–79%) were accompanied
by relatively low chemoselectivities (44–71%) of its
hydrogenation into allyl alcohol. The main side product
was propan-1-ol, its yield reached 25% and decreased
sharply with time.
than the primary ones;
butan-1-ol is slightly more susceptible to dehydroge-
nation than ethanol;
the CTH of acrolein into allyl alcohol is not thermo-
dynamically favorable (DG [ 0), irrespective of the
type of the alcohol used as the hydrogen donor;
the formation of propionaldehyde is favored thermo-
dynamically (DG \ 0), irrespective of the type of
alcohol used as the hydrogen donor;
It has also been found that in the first 30 min of the test
noticeable amounts of crotonaldehyde were formed. This
compound is a product of the aldol condensation reaction
of acetaldehyde which occurs on the basic sites of mag-
nesia, according to Eq. 6 presented in Sect. 3.3. Its yield
decreased sharply with time reaching 11%, 5% and 1%
after 0, 30, and 60 min, respectively.
•
•
•
allyl alcohol is unstable in comparison with propion-
aldehyde; the magnitude of the instability is expressed
-
1
by a value of 51.65 kJ mol
;
the formation of propan-1-ol is thermodynamically
favored (DG \ 0) for all alcohols as hydrogen donors
except methanol;
The yield of allyl alcohol increased steeply in the first
period of the test (0–60 min) from 34 to 64% mainly at the
expense of propan-1-ol. The catalyst reached stable activity
after 60 min in the stream of reactants. After that the
chemoselectivity towards allyl alcohol reached 90%,
whereas the yield of alcohol was in the range of 60–56%.
Table 3 Calculated thermodynamic data for the dehydrogenation at
73 K of the studied alcohols
6
-
1
)
Alcohol
DG (kJ mol
K
a (%)
3
.3 Thermodynamic Description of the Process
MeOH
EtOH
8.25
-8.35
0.2289
18.6
81.6
88.2
97.3
98.1
97.7
4.4933
7.4841
In order to elucidate the reasons of such big differences in
the reactivities of alcohols, as well as the low reactivity of
acrolein, a thermodynamic analysis of the dehydrogenation
of alcohols and the transfer hydrogenation reactions of
acrolein has been performed. The calculations have been
1
2
2
2
-BuOH
-PrOH
-11.26
-20.09
-21.96
-20.95
36.2611
50.6280
42.3038
-HepOH
-OctOH
123

2
98
M. Gli n´ ski, U. Ulkowska
Table 4 Calculated thermodynamic data for the transfer hydroge-
nation of acrolein by studied alcohols at 673 K with the formation of
allyl alcohol
alcohol with those calculated in the thermodynamic anal-
ysis of the process it has been found that the used hydrogen
donors, except ethanol and butan-1-ol, follow the thermo-
dynamic model of CTH of acrolein described by Eq. 3. In
the case of the above-mentioned exceptions, the experi-
mentally determined yields of allyl alcohol greatly excee-
ded the thermodynamic yields for both donor–acceptor
molar ratios (Table 7). The results obtained in the past for
ethanol as hydrogen donor [20, 22] also exceeded those
predicted by thermodynamic calculation based on Eq. 3.
Such a significant difference in yields is not within
experimental error. The origin of this discrepancy must be
the occurrence of a disregarded reaction for which DG \ 0,
which influences the CTH reaction. After careful inspec-
tion of the possible reactions the aldol condensation of
acetaldehyde or butyraldehyde, the products of the transfer
hydrogenation reaction between ethanol or butan-1-ol and
acrolein has been chosen (Eqs. 6–7):
-
1
)
a
Donor
DG (kJ mol
K
a (%)
MeOH
EtOH
34.62
18.02
15.11
6.28
0.0021
0.0400
0.0672
0.3257
0.4548
0.3800
7.5
b
28.0
c
1
2
2
2
a
-BuOH
-PrOH
34.3
57.7
63.1
60.2
-HepOH
-OctOH
4.41
5.41
D/A = 3
b
a = 37.5% for D/A = 6
a = 45.2% for D/A = 6
c
Table 5 Calculated thermodynamic data for the transfer hydroge-
nation of acrolein by studied alcohols at 673 K with the formation of
propionaldehyde
-1
)
a
Donor
DG (kJ mol
K
a (%)
O
O
MeOH
EtOH
-17.03
-33.63
-36.54
-45.37
-47.24
-46.24
20.98
407.8
686.0
97.7
99.9
ð6Þ
ð7Þ
2
2
+ H O
2
H
H
1
2
2
2
a
-BuOH
[99.9
[99.9
[99.9
[99.9
O
O
-PrOH
3 324
4 640
3 878
+ H O
2
-HepOH
-OctOH
H
H
D/A = 3
The aldol condensation of both aldehydes is strongly
favored thermodynamically, e.g. for acetaldehyde
Table 6 Calculated thermodynamic data for the transfer hydroge-
nation of acrolein by studied alcohols at 673 K with the formation of
propan-1-ol
-
1
DG (673 K) = -20.34 kJ mol . However, its products
are present only at the beginning of the CTH of acrolein
with ethanol or butan-1-ol. Therefore, the participation of
this reaction in the preliminary period of the test has no
influence on the yield of allyl alcohol observed after
60 min of the reaction. Hence, the aldol condensation of
the above-mentioned aldehydes cannot be considered the
reason of such a significant difference in yields of allyl
-
1
)
a
Donor
DG (kJ mol
K
a (%)
MeOH
EtOH
1.48
-31.73
-37.55
-55.21
-58.94
-56.93
0.7678
290.2
821.1
61.5
98.7
99.5
99.9
99.9
99.9
1
2
2
2
a
-BuOH
-PrOH
19 275
37 575
26 235
-HepOH
-OctOH
Table 7 Theoretical and experimental yields of allyl alcohol
obtained in the transfer hydrogenation of acrolein by ethanol or butan-
D/A = 3
1
-ol at 673 K
•
•
propan-1-ol is the most stable product of the transfer
hydrogenation of acrolein for butan-1-ol and all
secondary alcohols as hydrogen donors;
Donor
D/A = 3, a (%)
D/A = 6, a (%)
calc
28.0
exp
calc
37.5
exp
propan-1-ol is unstable in comparison with propional-
dehyde only for ethanol as the hydrogen donor, the
magnitude of the instability is expressed by a value of
a
c
c
b
EtOH
47
50
49
55
64
33
cd
b
-
1
1
.9 kJ mol
.
1-BuOH
34.3
45.2
cd
3
6
Therefore, it can be concluded that the CTH of acrolein
a
into allyl alcohol with alcohols as hydrogen donors must be
kinetically controlled due to the instability of the main
product in comparison to the others. Moreover, upon
comparing the experimental values of the yield of allyl
At 653 K [22]
b
[20]
c
This work
d
At 623 K
1
23

Reactivity of Alcohols in Chemoselective Transfer Hydrogenation
alcohol as the one observed by us, as well as by others [20,
299
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