Available online at http://www.urpjournals.com
International Journal of Green Chemistry and Bioprocess
Universal Research Publications. All rights reserved
ISSN 2277-7199
Original Article
Sustainable transfer-hydrogenations in glycerol-based solvents
Adi Wolfson*, Christina Dlugy and Dorith Tavor
Green Processes Centre, Chemical Engineering Department, Sami Shamoon College of Engineering, Bialik/Basel Sts.
Beer-Sheva, 84100 Israel. *[email protected].
Received 01 January 2013; accepted 01 February 2013
Abstract
Transfer-hydrogenations of nitrobenzene and benzaldehyde were performed in various alcoholic donor solvents to select the
most sustainable reaction process. The sustainability of the solvent was based on the whole process life-cycle, from solvent
production process and solvent characteristic, through reaction performance to product separation and solvent and catalyst
recycling procedures. It was found that polyols, and especially glycerol-based solvents, are more sustainable solvents for
transfer-hydrogenations than simple short chain alcohols. Though products yields in propandiols were higher than in glycerol
it was selected as the most sustainable solvent due to its renewable origin, biodegradability and recyclability as well as its
perfect characteristics that includes high boiling temperature, low vapor pressure and low toxicity.
© 2013 Universal Research Publications. All rights reserved
Keywords: Transfer-hydrogenation, sustainable solvents, green chemistry, glycerol, glycerol-based.
1. Introduction
Environmental awareness and concern have driven the
chemical industry to find new methods for the efficient
utilization of resources while minimizing of air, water, and
land pollution and of the amount of waste disposed, e.g.
green chemistry [1, 2]. Inherent to the goal of actualizing
green chemistry solutions is use of renewable and
environmentally friendly raw materials and auxiliaries as
well as catalysts that increase reaction performance and atom
efficiency. Yet, besides the reaction step, the sustainability
of a chemical process is also derived from materials and
energy consumption during pre-manufacturing processes
and separation procedures (Fig. 1 [3]). Moreover, since
organic chemistry is traditionally carried out in solution and
solvents are responsible for a large part of the waste and
pollution generated by chemical processes, a key factor to
enabling a sustainable chemical process is solvent selection
[4-6]. However, besides the greenness of a solvent, which is
primary derived from its physicochemical properties that
dictate its environmental impact as well as its recyclability
and reusability, the sustainability of a solvent depends also
on the solvent life-cycle that accounts for material and
energy utilization as well as chemical emission during its
production, use, and disposal (Fig. 2 [7]).
The reduction of unsaturated organic compound is a
fundamental transformation in organic synthesis [8]. It can
be accomplished via various catalytic or non-catalytic routes
using different hydrogen sources and different reaction
conditions such as: (i) reduction with metal hydrides like
sodium borohydride and lithium aluminum hydride [9]; (ii)
44
catalytic hydrogenation using gaseous hydrogen with
homogeneous or heterogeneous metal catalysts [10, 11]; (iii)
enzymatic reduction or hydrogenation with different
hydrogen sources [12]; (iv) and catalytic transferhydrogenation using various organic molecule as hydrogen
source [13, 14].
Among all the above mentioned pathways, Transferhydrogenation is beneficial has it does not require high
amount of reduction agents that generate high amount of
waste or hydrogen pressure which necessitate special
equipment and precautions. Moreover, in many cases
transfer-hydrogenation is performed under milder conditions
and offers much more selective route.
There are many organic molecules that can be used as
hydrogen donors, part of them like isopropyl alcohol and
tetralin are also used simultaneously as solvents, i.e. donor
solvent. Isopropyl alcohol (2-propanol) that dissolves a wide
range of organic compounds, evaporates quickly and is
relatively non-toxic compared to alternative solvents, is one
of the most commonly used donor solvent. However, its
normal boiling point of 82.5°C restricts its application in
atmospheric pressure, requires substantial energy for
product separation by evaporation and limits its ability to be
re-used.
Recently, we introduced glycerol, a renewable, recyclable
and reusable organic solvent with high boiling temperature
and low vapor pressure as sustainable solvent for organic
reactions in general [15-19] and in particular as donor
solvent in transfer-hydrogenations of various unsaturated
organic compounds [20-25]. Glycerol also tolerated the use
International Journal of Green Chemistry and Bioprocess 2013, 3(4): 44-48
the mixture was placed in a preheated oil bath and heated to
65 °C, after which it was magnetically stirred for 5 h. At the
end of the reaction, the reaction mixture was cooled and
extracted with 3×5 mL petroleum ether. The organic phase
was concentrated under reduced pressure, and the resulting
crude product was analyzed by GC analysis using an HP-5
column (30 m × 0.25 mm, 0.25 μm thick).
Figure 1: Considerations for sustainable organic process [3].
of more efficient and cleaner heating techniques such as
microwave and ultrasound irradiation that enhanced reaction
rate [26]. Yet, the high polarity and viscosity of glycerol, led
to the use of glycerol-based solvents, which preserve their
sustainable character but offer tunable polarity, among them
also propandiols that can be also serve as donor solvent [2729].
In this study the transfer-hydrogenation of two
representative unsaturated organic compounds, nitrobenzene
and benzaldehyde (Fig. 3), with representative homogeneous
and heterogeneous catalysts, was performed in several short
chain alcohols as donor solvent while comparing the solvent
sustainability as expressed by its production procedure and
physicochemical properties as well as its effect on reaction
performance and product separation procedure.
Figure 2: Schematic life-cycle diagram of solvent [7].
2. Experimental
2.1 Transfer-hydrogenation of nitrobenzene
In a typical procedure, 1 g of nitrobenzene, 0.1 g of Raney
nickel, and 0.2 g of NaOH were added to a vial or homemade stainless-steel reactor (for the more volatile solvents)
with 5 g of solvent (all purchased from Aldrich). The mixture
was placed in a preheated oil bath and heated to 100 °C, after
which it was magnetically stirred for 24 h. At the end of the
reaction, the reaction mixture was cooled and extracted with
3×5 mL petroleum ether. The organic phase was
concentrated under reduced pressure, and the resulting crude
product was analyzed by GC analysis using an HP-5 column
(30 m × 0.25 mm, 0.25 μm thick).
2.2 Transfer-hydrogenation of benzaldehyde
In a typical procedure, 2.2 mmol of benzaldehyde (0.22 g)
and 270 mol of KOH was added to a vial with 4g of solvent
(all purchased from Aldrich). A corresponding amount of
Ru(p-cumene)Cl2-dimer catalyst was added (S/C=100) and
45
Figure 3: Transfer hydrogenations of nitrobenzene (a) and
benzaldehyde (b).
2.3 Extraction tests
Extraction experiments were performed by mixing 5 g of the
solvent, which contained 1 g of aniline with 25 mL of the
extracting solvent in 5 extraction step, each step contained 5
mL of extracting solvent. The extracting solvent was then
evaporated under reduced pressure and the resulting product
was analyzed by GC and was used to calculate extraction
yield.
2.4 Recycling tests
Catalyst and solvent recycling experiments were done for the
transfer-hydrogenation of nitrobenzene after running the
first reaction cycle as illustrated above at 100 °c for 24 h. at
the end of the reaction, the product and the residual substrate
were extracted with 5×5 mL petroleum ether, and the
catalyst was recycled by adding fresh nitrobenzene, with the
addition of an extra 0.2 g of NaOH.
3. Results and discussion
As previously mentioned, solvent selection is a key step in
green chemistry while in catalytic transfer-hydrogenation of
unsaturated organic compounds the solvent, usually 2propanol, is also simultaneously used as hydrogen donor.
Thus, the first step of the investigation was the selection of
possible green donor solvents for the two reactions based on
their chemical, biological and physical properties. Several
short chain alcohols were proposed for this purpose and their
characteristics, including vapor pressure and median lethal
dose, LD50, which represents their air emission potential and
toxicity, as well as their flammability rating (FR), health
rating (HR) and reactivity (R) are illustrated in Table 1.
The numbers which are presented in Table 1 illustrate that
from environmental and operational points of view all
polyols are advantageous over simple alcohol, due to their
high boiling point, low volatility and low flammability as
well as their lower toxicity and health rating. In addition,
though increasing the organic chain of the alcohol, as
represented by comparing between 1-pentanol and 1propanol, decreases the volatile of the solvent it increases at
the same time its toxicity and health impact.
International Journal of Green Chemistry and Bioprocess 2013, 3(4): 44-48
Table 1: Characteristics of potential alcoholic donor solvents
Solvent
TNBP (°C)
P° -20°C (mmHg)
LD50 (mg/Kg)
FR a
HRa
-4
Glycerol
290.0
7.95*10
12,600
1
2
1,2-Propanediol
187.6
1.27
20,000
1
2
1,3-Propanediol
214.0
0.08
15,000
1
2
Ethylene glycol
197.3
0.06
4,700
1
2
1-Propanol
97.5
15.00
1,870
3
2
2-Propanol
82.5
31.50
5,054
3
1
1-Pentanol
138
1.50
200
3
3
a
FR-Fire Rating, HR-Hazard Rating, R-Reactivity: 0-least, 1-sligth, 2-moderate, 3-high, 4- extreme.
Ra
0
0
0
0
0
0
0
Ranking
1
2
2
3
5
4
6
Table 2: Transfer-hydrogenation in alcoholic solvents
Viscosity
Nitrobenzenea
Aniline extraction
Banzaldehydec
Solvent
Log P
Ranking
b
20-30°C (cP) conversion (%)
yield (%)
Conversion (%)
Glycerol
-4.15
629
47.2
92
19.1
3
1,2-Propanediol
-0.92
52
88.9
85
36.9
1
1,3-Propanediol
-1.093
56
88.5
84
35.9
1
Ethylene glycol
-1.36
21
88.2
81
36.7
1
1-Propanol
0.25
1.94
38.4
11.9
4
2-Propanol
0.05
1.77
76.5
32.0
2
1-Pentanol
1.4
4.3
18.2
3
a
Reaction conditions: 5 g solvent, 1 g nitrobenzene, 0.2 g NaOH, 0.1 g Raney nickel, 100°C, 24 h.
b
Extraction conditions: 5 g glycerol, 1 g nitrobenzene, 5x5 mL petroleum ether, room temperature.
c
Reaction conditions: 4 g solvent, 2.2 mmol benzaldehyde, Ru(p-cumene)Cl2-dimer -S/C=100, 072 mol KOH, 65°C, 5 h.
Table 3: Extraction solvents characteristics
Solvent
TNBP (°C)
P° -20°C
(mmHg)
LD50
(mg/Kg)
Log P
FRb
HRb
Rb
Aniline extraction
yield (%)
Diethyl ether
34.6
439
1215
0.83
4
2
2
98
Petroleum ether
90-100
20
~2000
3.8
4
2
0
92
Dichloromethane
39.6
355
1600
1.4
1
3
0
78
Ethyl acetate
77.1
93
5620
0.68
3
2
1
FR-Fire Rating, HR-Hazard Rating, R-Reactivity: 0-least, 1-sligth, 2-moderate, 3-high, 4-extreme
a
Extraction conditions: 5 g glycerol, 1 g nitrobenzene, 3x5 mL petroleum ether, room temperature.
87
b
Comparing between the four polyols show that ethylene
glycol is the most toxic, as expressed by its lowest LD50,
while glycerol has to highest boiling point and lowest
volatility. Yet, besides the solvent properties, which
determine their environmental impact, their production
process, which also involves the use of materials and energy,
should also be considered (Fig. 2). While glycerol origin is
from renewable source all other alcohols are mainly
manufacture from petroleum based products such as
ethylene and propylene, although propandiols can be also
produced from glycerol by hydrogenolysis. 30,31 This aspect
also makes glycerol production advantageous from energy
consumption point of view. In addition, glycerol is a byproduct of a simple and efficient transesterification of oils
and fats in the production of fatty acids derivatives for
cosmetics and biofuel, i.e. biodiesel, uses. Moreover, as the
production of biodiesel and hence of glycerol is annually
increases, glycerol price decreases and it is essential to find
alternative uses for its utilization. Based on this
characterization, the solvent presented in Table 1 were
ranked in a decrease order according to their greenness:
glycerol>propanediols>ethylene glycol>2-propanol>1propanol>1-pentanol.
46
The second step of the investigation was performing the
transfer-hydrogenation of the two representative organic
molecules (Fig. 3) in the seven donor solvents under similar
conditions. Comparison of the product yields is summarized
in Table 2.
As illustrated in Table 2, employing 2-propanol, which is
commonly used as donor solvent, resulted in higher
conversions of both substrates compare to 1-propanol and 1pentanol, probably as the oxidation potential of secondary
alcohols is higher than these of primary alcohols. Employing
the three diols, ethylene glycol, 1,2- and 1,3-propandiol,
yielded comparable conversions, which were slightly higher
than in 2-propanol, and might be attributed to the higher
amount of hydroxyl groups that were available as hydrogen
donors. However, glycerol which bare three hydroxyl
groups, was less active toward transfer-hydrogenation than
2-propanol and the three diols. It might be attributed to its
high polarity, as expressed by its lowest log P -the
logarithms of the partition coefficient of a compound
between octanol and water, that leads to lower miscibility of
the relatively non-polar substrates in glycerol as well as to
the high viscosity of glycerol that effects mass and heat
transfer. It is worth to mention that glycerol was
International Journal of Green Chemistry and Bioprocess 2013, 3(4): 44-48
dehydrogenated to dihydroxyacetone, which means that the
secondary hydroxyl was reacted. Based on the results in
Table 2 the various donor solvents were ranked according to
their performance as donor solvent in both reactions in a
decrease order: propanediols=ethylene glycol>2-propanol>
glycerol>1-propanol>1-pentanol.
Finally, product separation and catalyst and solvents
recycling were also tested. Separation of a product from
simple alcohols like 2-propanol is usually done by
evaporation of the solvent under reduced pressure and
washing of the catalyst. On contrary, using polyols as donor
solvents allowed product separation by extraction with
polyols immiscible solvents such as ethers or
dichloromethane and although it required addition of an
extraction solvent it also allowed catalyst recycling and
solvent re-use. Thus, the extractions of neat aniline from the
various tested polyols were performed with petroleum ether
as representative extraction solvent following by the
evaporation of the extraction solvent under reduced pressure,
as illustrate in Table 2. It was found that the extraction yield
of aniline from glycerol was higher than these of the three
diols, probably due to the higher polarity of glycerol that
leads to higher partition coefficient of aniline between the
hydrophobic petroleum ether phase and the more polar
reaction solvent.
Yet, as addition of hazardous and non-environmentally
friendly extraction solvent reduces the sustainability of the
overall reaction process, selection of the extraction solvent
is also important. Employing polyols as donor solvent allow
using various hydrophobic solvents as extraction solvents,
and their characteristics are summarized in Table 3. It can be
seen that diethyl ether and dichloromethane have the highest
vapor pressures and lowest LD50, which suggest on their
higher air pollution potential and toxicity, while among all
the selected extraction solvents ethyl acetate and petroleum
ether are more environmentally friendly. On the other hand
employing diethyl ether and dichloromethane as extraction
solvents require lower energy for their removal, due to their
low boiling points. However, the extraction solvent
properties affect not only its environmental impact, but also
the effectiveness of the extraction. From the results in Table
3 it can be seen that all the solvent showed comparable
aniline extraction yields from glycerol, with minor
advantage to both ethers. Yet, based on the entire
considerations, it seems that ethyl acetate is the most
sustainable extraction solvent.
Finally, the recycling of Raney nickel together with Glycerol
or 1,2-propandiol was tested in the transfer-hydrogenation of
nitrobenzene to aniline, using ethyl acetate as extraction
solvent. It was previously published that the base plays a key
role in the reaction mechanism and performing the second
reaction step without addition of a base led to huge decrease
in conversion while addition of fresh base to the second
reaction step increased the conversion.23 It was suggested
that activation of the catalyst in the first cycle and the
presence of higher total amount of base, the fresh base that
was added together with the leftovers from the first cycle are
the reasons for the increased conversion. Indeed recycling of
the solvent and the catalyst within while adding fresh sodium
hydroxide increased the reaction conversion in the second
47
reaction step with both solvents, from 19.1 to 28.5 in
glycerol and from 36.9 to 51.1 in 1,2-propanediol, showing
the ability of both the solvent and the catalyst to be recycled
Conclusions
Selected alcoholic donor solvents were compared regarding
their greenness and their performance in catalytic transferhydrogenation of nitrobenzene and benzaldehyde. Based on
the life-cycle analysis of the different donor solvents and on
their characteristics as well as their effects on reaction
performance, product separation and catalyst recycling
procedure it is clear that all polyols are advantageous over
simple primary or secondary alcohols. In addition,
comparing between glycerol and the three tested diols
showed the superiority of glycerol as the greenest solvent
due to its renewable origin, simple and efficient synthesis
and ideal psychochemical properties as well as its
biodegradability. On the other hand the reaction
performance in the three diols was almost double than in
glycerol. Nevertheless, it seems that based on the whole
reaction process glycerol is the most sustainable solvent for
the tested transfer-hydrogenation reactions and the lower
reaction activity in glycerol can be compensated by
prolonging the reaction time.
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Source of support: Nil; Conflict of interest: None declared
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International Journal of Green Chemistry and Bioprocess 2013, 3(4): 44-48