Graphene-promoted acetalisation of glycerol under acid-free conditions

Article abstract of DOI:10.1039/c5gc02316e

Serendipity led us to unveil unexpected and uncovered properties of graphene for the acetalisation of glycerol with both aldehydes and ketones, furnishing an acid-free process for fuel bio-additive candidates. Mechanistic studies ruled out the intervention of residual acidic species or metallic cations at the surface of graphene, and therefore, the peculiar electronic properties of graphene are most probably responsible for this unforeseen reactivity. Recycling studies revealed the robustness of graphene under the experimental conditions since only a marginal erosion of the reaction yield was observed after six cycles.

Full text of DOI:10.1039/c5gc02316e

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Graphene-Promoted Acetalisation of Glycerol under Acid-Free
Conditions
Nicolas Oger,a Yuting F. Lin,a Erwan Le Grognec,a Franck Rataboul,b François-Xavier Felpin*a,c
Received 00th January 20xx,
Accepted 00th January 20xx
Serendipity led us to unveil unexpected and uncovered properties of graphene for the acetalisation of glycerol with both
aldehydes and ketones, furnishing an acid-free process for fuel bio-additive candidates. Mechanistic studies ruled out the
intervention of residual acidic species or metallic cations at the surface of graphene, and therefore, the peculiar electronic
properties of graphene are most probably responsible of this unforeseen reactivity. Recycling studies revealed the
robustness of graphene in the experimental conditions since only a marginal erosion of the reaction yield was observed
DOI: 10.1039/x0xx00000x
www.rsc.org/
after
six
cycles.
reaction. While the use of homogeneous acids (e.g., sulfuric acid,
hydrogen chloride, p-toluenesulfonic acid…) can, in some extend,
be attractive since they are inexpensive and highly active for the
condensation of glycerol with carbonyl compounds,28 extra
neutralization steps and both recovery and corrosive issues
disqualified them for sustainable processes when high loadings are
required. On the other hand, heterogeneous solid acids have shown
great potential as recyclable liquid acid surrogates for the
transformation of biomass feedstocks, especially for biodiesel
1. Introduction
The exhaustion of non-renewable fossil fuel reserves and the rapid
climate change induced by the emissions of greenhouse gases have
emphasized the urgent need for sustainable technologies.1, The
2
transformation of biomass has been recognized as the most
promising sustainable technology to provide chemical
intermediates, biodiesel and biomaterials.3-6 The deconstruction of
biopolymers and the transformation of biomass feedstocks into
small molecules of high-added value have been achieved using production,8, 9, 14, 16, 29 but for most of them, the catalytic activity
acids, bases, enzymes, transition metals as well as supercritical
conditions.7-16
results from non-easily quantifiable contributions, of Brønsted and
Lewis acid properties. This, led us to consider graphene (GR) as a
non-Lewis acidic platform for heterogeneous sulfonic acids. GR is
certainly one of the most exciting material known to date due to its
unique properties and is easily functionalized to graphene-based
materials30-36 that have been extensively used as catalysts for
various reactions,37-41 including biomass transformation.42 This
honeycomb-structured two dimensional material of one-atomic
thickness, consisting of sp2-hybridized carbons, presents a high
electron mobility (10 000 cm2.V-1.s-1), a thermal conductivity of 5
000 W.m-1.K-1, an exceptional mechanical resistance (~1 TPa, 100
times stronger than steel) and a large theoretical surface area of
2630 m2.g-1.43-46 In a recent report we described a scalable synthesis
of sulfonated graphene (GR-SO3H) that could find applications in
acid-catalyzed transformations.47 However, we describe herein how
serendipity led to unveil unexpected and uncovered properties of
GR that surpass the activity of GR-SO3H for the acetalisation of
glycerol in neutral conditions.
Glycerol is an abundant low-value chemical, synthesized from
propylene oxide or obtained from renewable resources as a by-
product from the saponification of triglycerides in the soap industry
and biodiesel production. Although it has recently been considered
as a green solvent,17-21 glycerol itself has a low interest as chemical
or fuel additive since it is a highly hygroscopic and viscous
compound that polymerizes at high temperature and oxidizes to
hazardous acrolein. By contrast, chemically-modified glycerol
derivatives by etherification22 and esterification are excellent
candidates for oxygenated fuel additives with anti-freezing, anti-
detonant and octane-improving properties.23 Glycerol acetals have
also been used in other valorization strategies as flavors,24
surfactants,25 and low-toxic solvent for both pharmaceutical26 and
veterinarian formulations.27
The acetalisation of glycerol is a well-known acid-catalyzed
2. Experimental
2.1. Materials
Graphite, glycerol, benzaldehyde, acetone, anisaldehyde, trans-
cinnamaldehyde,
furfural,
acetophenone,
cyclopentanone,
cyclohexanone and cellobiose were purchased from Sigma-Aldrich,
Fischer Scientific Ltd, Alfa Aesar and TCI Europe companies. All
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2.5. Recycling studies
reagents and solvents were used as received without further
purification with the exception of furfural which was distilled before
use.
Upon completion of the reaction, the catalyst was recovered by
filtration, washed with acetone and dried under reduced pressure
at 25 °C for 6 hours, prior to its reuse in another run.
2.2. Preparation of graphene (GR)
GR was prepared according to our previously reported procedure.47
Graphite powder (2 g) was mixed with sulfuric acid (4.6 wt. equiv., 5
mL), potassium persulfate (0.5 wt. equiv., 1 g) and phosphorus
pentoxide (0.5 wt. equiv., 1 g). The resulting mixture was heated at
80 °C for 2 hours. Then, the solid was filtered through a fritted glass
and washed with deionized (DI) water (100 mL), MeOH (200 mL)
and Et2O (200 mL). The resulting black paste (2.15 g, 108% in mass)
was dried at 40 °C under vacuum overnight. The resulting material
(2.15 g) was mixed with sulfuric acid (43.9 wt. equiv., 54 mL) at 0 °C
and potassium permanganate (3.5 wt. equiv., 7.46 g) was then
added by portion with careful attention. The reaction mixture was
stirred at 35 °C for 2 hours and then the reaction mixture was
cooled to 0 °C, followed by the addition of 30% hydrogen peroxide
(2.84 wt. equiv., 5.9 mL) in DI water (80 mL). The solid was
centrifuged (4000 rpm, 20 minutes) and the liquid phase was
removed. DI water (3 x 50 mL) was added and the centrifugation
(4000 rpm, 15 minutes) was performed three times. The same
procedure was carried out with MeOH and Et2O. The resulting
brown solid graphene oxide (GO), 3.54 g, 164% in mass) was dried
at 40 °C under vacuum overnight.
2.6. Hydrolysis of cellobiose
Cellobiose (200 mg, 0.58 mmol), GR-SO3H (20 mg), and deionized
water (40 mL) were introduced in a batch-type autoclave (60 mL)
equipped with an internal temperature control. The system was
purged and pressurized to 20 bar with Ar. The suspension was
heated to 150 °C (10 °C.min-1) and agitated using a magnetic stirrer.
After reaction and cooling, the catalyst was separated over
Millipore membrane (Teflon filter 0.45 µm, 47 mm) and dried at
room temperature overnight. The colorless filtrate was analyzed on
a Shimadzu HPLC apparatus (COREGEL 107H column, 0.5 mL.min-1
of 0.001% H2SO4, 70 °C, RI detector) for cellobiose and glucose
quantification. Yields are molar yields and are calculated as follows:
yield (%) = 100 x (nglucose/ninitialcellobiose)/2.
2.7. Analytical methods
Transmission Electron Microscopy (TEM) was performed on a
Hitachi HF2000 (cold field emission gun, accelerating voltage: 100
kV). Scanning Electron Microscopy (SEM) was performed on a JEOL
JSH7600F (field emission gun, accelerating voltage: 5 kV, in lens
detector of secondary electrons). X-ray photoelectron spectroscopy
GO (500 mg) was sonicated in DI water (500 mL) for 2 hours. A
solution of 5% Na2CO3 (12 mL) was added in order to increase the
pH up to 9-10. Then, 64% hydrazine hydrate (41.2 wt. equiv., 20 mL)
was added to the suspension and the mixture was refluxed for 24
hours. The solution was cooled down to room temperature and
filtered through Millipore membrane (nylon filter 0.45 µm, 47 mm)
and washed with 1N HCl (100 mL) and acetone (300 mL). GR (black
powder, 284 mg, 57% in mass) was dried at 40 °C under vacuum
overnight.
(XPS) was performed on
a ThermoFisher Scientific K-ALPHA
spectrometer for disk surface analysis with a monochromatized
AlKα source (hν = 1486.6 eV) and a 200 micron spot size. A pressure
of 10−7 Pa was maintained in the chamber during analysis. The full
spectra (0–1150eV) were obtained at a constant pass energy of
200eV and high resolution spectra at a constant pass energy of 40
eV. Charge neutralization was required for all insulating samples.
High resolution spectra were fitted and quantified using the
AVANTAGE software provided by ThermoFisher Scientific. X-Ray
diffraction (XRD) patterns were recorded on a Bruker D8 Advance X-
Ray Diffractometer.
2.3.Preparation of sulfonated graphene (GR-SO3H)
GR (273 mg) was sonicated in DI water (40 mL) for 2 hours. Then,
sodium nitrite (3.47 wt. equiv., 947 mg) and sulfanilic acid (2.89 wt.
equiv., 789 mg) were added to the resulting solution, allowing the
formation of the diazonium salt in situ, and the reaction was
conducted at 25 °C for 24 hours. The solution was filtered through
Millipore membrane (nylon filter 0.45 µm, 47 mm) and washed with
1N HCl (100 mL) and acetone (300 mL). The black powder (352 mg,
GR-SO3H) was dried at 40 °C under vacuum overnight. The loading
of –SO3H was calculated to be 1.79 mmol SO3H.g-1 by elemental
analysis (C : 64.54%, H : 1.87%, N : 1.84%, S : 5.72%).
3. Results and discussion
3.1. Graphene synthesis
Graphene-based materials used in this study, i.e. GR and GR-SO3H
were prepared according to our recently published procedure.47 In
short, graphite was oxidized in two sequential steps and exfoliated
into graphene oxide (GO) which was subsequently reduced to GR by
hydrazine and then, functionalized with benzene sulfonic groups to
give GR-SO3H (Scheme 1).
2.4. Acetalisation of glycerol
In a typical experiment, GR (25 mg), benzaldehyde (10 mmol) and
glycerol (1 mmol) were placed in a sealed tube and stirred at 100 °C
for 2 hours. The resulting mixture was filtered through Millipore
membrane (nylon filter 0.45 µm, 47 mm) and the solid was
thoroughly washed with acetone. The filtrate was concentrated
under vacuum and analysed by 1H NMR using 1,3,5-
trimethylbenzene as external standard.
2 | J. Name., 2012, 00, 1-3
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O
1. K2S2O8, P2O5
H2SO4, 80 °C, 2 h
COOH
HOOC
NH2-NH2, H2O
reflux, 24 h
O
O
O
2. KMnO4, H2SO4
H2O, 0-35 °C, 3 h
HO
Graphene oxide (GO)
SO3
HO
SO3H
H
Graphite
H2N
SO3H
NaNO2, H2O
25 °C, 24 h
Graphene (GR)
SO3
H
SO3H
Sulfonated graphene (GR-SO3H)
Scheme 1 Preparation of GR and GR-SO3H.
Although the process cannot be considered as sustainable, one
should remain that graphene is used as a catalyst limiting the
impact of its synthesis on the whole process.
Transmission electron microscopy (TEM) analyses showed
randomly oriented sheets for both graphene materials (i.e., GR and
GR-SO3H) and the high transparency observed was attributed to the
successful exfoliation (Fig. 1).
Fig. 1 TEM images of a) GR and b) GR-SO3H.
Fig. 2 High resolution of C 1s spectra of a) GR and b) GR-SO3H.
Survey scan spectra of c) purified GR.
The acid group density of GR-SO3H used in this study was
determined to be 1.79 mmol H+.g-1 using both acid-base titration
and NH3-TPD methods and this value was correlated by the loading
of SO3H calculated by elemental analysis. Surface analyses by X-ray
photoelectron spectroscopy (XPS) revealed
a low content of
oxygenated species and a high level of sp2 carbon consistently with
the expected honeycomb-structured material (Fig.s 2a-c).
3.2. Acetalisation of glycerol
We carried out our preliminary studies with the acetalisation of
glycerol with benzaldehyde at 100 °C in a sealed tube (Table 1).
Table 1
Acetalisation of glycerol with benzaldehyde.
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accelerating or stabilizing reaction intermediates. However, this
effect was also ruled out since the use of graphite and coronene, as
simplistic mimics of GR, did not promote the acetalisation process
(entries 10-11). The presence of metal impurities arising from the
oxidation step, e.g., Mn, could be responsible of the activity of GR.
However, the surface analysis of purified GR did not show
impurities in a detectable extent (Fig. 2c) and, more importantly,
the use of unpurified GR, having detectable Mn impurities, as
determined by XPS analyses (Fig. S1, ESI), displayed a reduced
activity, even after prolonged reaction times (entry 12). At this time,
it is premature to definitively conclude on the specific role of GR
but its unique electronic properties are likely responsible for this
uncovered reactivity under acid-free conditions. The acetalisation of
Entry
1
Catalyst
--
Time
(h)
2
Selectivity
3a/3b
Yield
(%)b
26
85/15
2
GR-SO3H
GR-SO3H
GR
2
33/67
62/38
63/37
66/34
63/37
64/36
65/35
100/0
87/13
81/19
79/21
49
92
91
68
82
91
91
25
23
16
81
3
14
2
4
5
GR
0.5
1
glycerol with benzaldehyde provided
a
mixture of (2-
6
GR
phenyl[1,3]dioxolan-4-yl)methanol 3a and 2-phenyl[1,3]dioxan-5-ol
3b, as previously mentioned in the literature.28 Analysis by 1H NMR
showed the formation of four isomers corresponding to two pairs of
diastereoisomers arising from a nearly equimolar cis/trans mixture
of five and six-membered acetals. The chemical shift values (δ) of
signals accounting for the acetal proton [-OCH(Ph)O-] of 3a-b are
depicted in Table 3 along with the characteristic proton signals of
the compounds prepared in this study. The formation of the five-
membered ring 3a was kinetically favored with GR in our reaction
conditions, with a 3a/3b selectivity of 63/37 and a GR loading of 25
mg mmol-1 of glycerol. Deutsch and Lieske observed a progressive
evolution of five to six-membered acetals upon extended reaction
times with Amberlyst-36 as catalyst, an outcome rationalized by an
acid-catalyzed equilibrium between 3a and 3b.48 The same behavior
was also reported by Adam et al. using sulfonic acid immobilized
onto silica.49 This trend was not observed with GR, even after 14
hours of stirring; the absence of any acidic function in our system
obviously precluding such an equilibrium (entry 4 vs. 8). By contrast,
an equilibrium leading to an evolution of the selectivity was indeed
observed with GR-SO3H (entry 2 vs. 3), but in these cases, isomer 3b
was kinetically favored, while 3a was the thermodynamic acetal.
Actually, the survey of recent literature results on the acetalisation
of glycerol with benzaldehyde did not provide any rational
explanation and the 1,3-dioxolane/1,3-dioxane ratio likely depends
on both the nature of the catalyst and torsional effects in acetal
rings.
7
GR
3
8
GR
14
14
2
9
GO
10
11
graphite
coronene
GRc
2
12
14
a
Reaction conditions: glycerol 1 (1 mmol), benzaldehyde 2 (10
mmol) and the catalyst (25 mg) were stirred at 100 °C for the
indicated time. 1H NMR yield using 1,3,5-trimethylbenzene as
b
internal standard. c Unpurified GR.
While the background reaction, carried out in the absence of
catalyst, gave
a mixture of five- and six-membered acetals
(compounds 3a and 3b respectively) with 26% combined yield
(entry 1), the use of GR-SO3H (3 mol% H+) gave a much improved,
but still unsatisfactory yield of 49% (entry 2). In order to evaluate
the impact of graphene-grafted sulfonic acid functions on the
reaction outcome, we carried out the acetalisation with
unfonctionalized graphene. To our great surprise, a very high yield
(91%) was obtained for a mixture of five- and six-membered acetals
(entry 4). Such a high yield could be reached with GR-SO3H, but only
after 14 hours of stirring (entry 3). Interestingly, with GR-SO3H the
six membered acetal 3b appeared as the kinetically favored isomer
being the major product after 2 hours of stirring, but upon
extended reaction times, a reversed selectivity in favor of acetal 3a
was observed. Incomplete conversions were observed upon
reduced reaction times with GR (entries 5-6), while extended
stirring times proved to be useless (entries 7-8). Intrigued by this
unexpected high reactivity of GR, we reasoned that it could arise
from both residual oxygenated and acidic functions remaining at
the surface of the material. While the surface analysis by XPS
indeed provided evidences for C-O and C=O bonds at 286.8 and
287.8 eV respectively (Fig. 2a), the absence of peak at ~288.5 eV
accounting for carboxylic acids combined to the absence of acidic
protons as determined by acid-base titration ruled out the presence
of acidic functions at the surface of GR. Moreover, GO, which
contains many oxygenated and acidic functions, was found inactive
for this transformation, even after prolonged reaction times (entry
9). All these results, suggest that the high activity observed for GR
cannot be attributed to the surface acidity of the material. The high
hydrophobicity of GR compared to GO and, to a lower extent GR-
With these promising results in hand we further explored the
behavior of GR for the acetalisation of glycerol with a collection of
carbonyl compounds (Table 2).
Table 2
Acetalisation of glycerol with carbonyl compounds.
Entry Carbonyl
compound
Catalyst Time Selectivity Yield
(h)
5-mem/6-
mem
(%)b
1
2
Benzaldehyde
Benzaldehyde
GR
2
2
63/37
91
49
GR-
SO3H
GR
33/67
63/37
SO3H, could favor
a hydrophobic environment susceptible of
3
Anisaldehyde
2
99
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4
trans-
GR
2
66/34
89
trans-
-OCH(Ar)O-
-OCH(Fur)O-
5.59 (s), 5.45 (s) / 5.19
(s), 5.07 (s)
Cinnamaldehyde
Furfural
Cinnamaldehyde
Furfural
5
6
7
8
GR
GR
GR
GR-
SO3H
--
2
2
68/32
85c
76
85d
59d
6.04 (s), 5.89 (s) / 5.61
(s), 5.50 (s)
1.45 (d)a, 1.38 (d)a / ND
Acetone
>99/<1
>99/<1
>99/<1
Acetone
-OC(CH3)2O-
Acetone
2
Acetophenone
Cyclopentanone
-OC(CH3)(Ph)O-
-OCH(R)CH2-OH
1.72 (s), 1.69 (s) / ND
Acetone
14
3.48 (dd)b, 3.56 (dd)c
/
/
0d
ND
9
Acetone
14
2
--
Cyclohexanone
-OCH(R)CH2-OH
3.52 (dd)d, 3.59 (dd)e
10
11
12
Acetophenone
Acetophenone
Cyclopentanone
Cyclohexanone
GR
GR
GR
GR
>99/<1
>99/<1
>99/<1
>99/<1
25
72d
99
99
ND
2
a J = 0.6 Hz. b J = 4.8, 11.2 Hz. c J = 4.8, 11.6 Hz. d J = 4.8, 11.6 Hz. e J =
2
4.8, 11.6 Hz
13
a
2
Reaction conditions: glycerol 1 (1 mmol), carbonyl compound (10
3.3. Recycling studies
mmol) and the catalyst (25 mg) were stirred at 100-120 °C for the
b
indicated time. 1H NMR yield using 1,3,5-trimethylbenzene as
The reusability of the catalyst is a crucial feature when developing a
sustainable process. The reuse of GR was explored for the
acetalisation of glycerol with benzaldehyde on four consecutive
runs (Fig. 3). After completion of each run, the reaction mixture was
filtered on a nylon membrane, the catalyst was washed with water
and acetone to remove adsorbed reagents and then dried under
vacuum. Only a slight decrease was observed upon successive
reuses, suggesting that GR was very stable in the reaction
conditions. We also note that the selectivity in favor of the five-
membered acetal, with respect to the six-membered one, slightly
increased from 65/35 for the first run to 80/20 for the fourth one. A
close examination of the yields obtained for each isomer (dioxolane
vs dioxane), revealed that this increased selectivity was attributed
to the slight erosion of the yield for the six-membered acetals while
the yield for the five-membered isomers remained constant over
the successive reuses. This unexpected behavior is still under
investigation in our laboratory. The yield significantly decreased
after the fifth run. Neither the elemental composition, nor the
aggregation state of GR were affected after the fifth reuse as
supported by XPS and XRD analyses (Fig. S2 and S3, ESI). Therefore,
the catalyst deactivation was attributed to the surface fouling by
polyoxygenated organic residues, disrupting the contact of reagents
with the GR surface. This was supported by TEM images showing GR
sheets covered by a cloudy material (Fig. S4, ESI). To address this
issue, GR was thoroughly washed with water and ethanol in an
ultrasonic bath after the fifth run. Gratifyingly, the catalytic activity
for the five-membered acetal was fully restored in the sixth reuse,
confirming that the GR integrity was conserved. A similar study was
conducted with acetone as substrate and confirmed the good
recyclability of GR, since after the third run the yield for solketal
reached 81% (vs. 85% for the first run).
c
d
internal standard. 12.5 mg of GR was used. Reaction carried out
at 120 °C.
A similar selectivity, still in favor of the kinetically controlled five-
membered cyclic acetal, was observed with both aldehydes and
ketones. Tuning the electronic properties of aldehydes did not
significantly modify the reaction outcome (entries 1-5). We
underline the remarkable reactivity of GR that allows a lower
catalyst loading (12.5 mg mmol-1 of glycerol) when furfural was
used as acetalisation reagent (entry 5). This high selectivity
favorably competes with those extracted from literature
precedents.50-53 The synthesis of 2,2-dimethyl-1,3-dioxolan-5-ol,
also known as solketal, from glycerol and acetone is a relevant
process for the gasoline industry. Solketal has been used as a highly
soluble fuel additive increasing the octane number by 2.5 units.
Usually, the condensation of glycerol with acetone under acidic
conditions gives a mixture of five- and six-membered acetals, with a
high selectivity in favor of solketal (>90%). Although acetone was
expected to be less reactive than aldehyde, GR exhibited a
comparable activity, producing solketal as the only detectable
compound in 76% yield at 100 °C and even 85% yield at 120 °C
(entries 6-7). By contrast, GR-SO3H displayed a much lower activity
since solketal was selectively produced in only 59% after 14 hours
of stirring (entry 8), while no product could be detected in the
absence of catalyst (entry 9). A perfect selectivity in favor of the
five-membered acetal was observed with other ketones (entries 6-
13). Acetophenone proved to be less reactive in the reaction
conditions, likely due to steric hindrance, though a good yield was
obtained by increasing the temperature from 100 to 120 °C (entries
10 vs 11). By contrast, steric hindrance did not affect the reactivity
of cyclic ketones that gave quantitatively and selectively the
corresponding five-membered acetals at 100 °C (entries 12-13).
Table 3. Identification of characteristic chemical shifts by 1H NMR
(400 MHz).
Reactant
Proton signal
-OCH(Ph)O-
-OCH(Ar)O-
Chemical shift (ppm)
5-mem / 6-mem
5.97 (s), 5.83 (s) / 5.55
(s), 5.43 (s)
Benzaldehyde
Anisaldehyde
5.90 (s), 5.77 (s) / 5.50
(s), 5.38 (s)
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In summary, we described in this contribution how serendipity led
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acetalisation of glycerol with aldehydes and ketones in neutral
conditions. Importantly, graphene features high catalytic activity
and excellent recyclability properties for the production of high
added value acetals. Preliminary mechanistic studies suggested that
the high activity unveiled for graphene could not be ascribed to
residual acids nor metallic cations and could rather be attributed to
the peculiar electronic properties of the honeycomb-structured 2D
material. We are currently exploring in depth the exact role of
graphene in this process and we expect that this uncovered activity
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Uncovered properties of graphene for the acetalisation of glycerol with both aldehydes and ketones
in acid-free conditions allow the synthesis of fuel bio-additive candidates.