Conversion of platform chemical glycerol to cyclic acetals promoted by acidic ionic liquids

Article abstract of DOI:10.1039/c4ra01443j

The condensation of glycerol, a platform chemical from renewable materials, with benzaldehyde to generate cyclic acetals was investigated using acidic ionic liquid as catalyst. Evidence was presented that the product mixture of 4-hydroxymethyl-2-phenyl-1,3-dioxolane and 5-hydroxyl-2-phenyl-1,3-dioxane, with cis and trans two stereo-isomers for each one identified by ¹H NMR were obtained. Further modification of reaction conditions promoted by N-butyl-pyridinium bisulfate ([BPy]HSO₄) led to the totally cyclic acetals with 99.8% yield at room temperature. A micro water-removal reactor constituted by ionic liquids was proposed, which favourably shifted the condensation equilibrium to the product side by transferring the produced water out of the organic phase in time, so that the water-carrying agent or reactive distillation was avoided. Moreover, the product separation made this methodology more accessible to sustainable green biomass chemistry.

Full text of DOI:10.1039/c4ra01443j

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Conversion of platform chemical glycerol to cyclic
acetals promoted by acidic ionic liquids
Cite this: RSC Adv., 2014, 4, 18917
a
Bo Wang, Yue Shen,a Jiankui Sun,a Feng Xua and Runcang Sunab
*
The condensation of glycerol, a platform chemical from renewable materials, with benzaldehyde to
generate cyclic acetals was investigated using acidic ionic liquid as catalyst. Evidence was presented that
the product mixture of 4-hydroxymethyl-2-phenyl-1,3-dioxolane and 5-hydroxyl-2-phenyl-1,3-dioxane,
with cis and trans two stereo-isomers for each one identified by 1H NMR were obtained. Further
modification of reaction conditions promoted by N-butyl-pyridinium bisulfate ([BPy]HSO4) led to the
totally cyclic acetals with 99.8% yield at room temperature. A micro water-removal reactor constituted
by ionic liquids was proposed, which favourably shifted the condensation equilibrium to the product side
by transferring the produced water out of the organic phase in time, so that the water-carrying agent or
reactive distillation was avoided. Moreover, the product separation made this methodology more
accessible to sustainable green biomass chemistry.
Received 19th February 2014
Accepted 31st March 2014
DOI: 10.1039/c4ra01443j
www.rsc.org/advances
added chemicals from glycerol is benet for the industry chain
extension, comprehensive economic improvement, environ-
1. Introduction
mental protection and biomass & bio-energy development,
which have attracted much attention of the scientic commu-
nity in recent years.7 Particularly, the condensation of glycerol
with aldehyde/ketone to cyclic acetals8 has become one of
the hot topics because the produced cyclic acetals are widely
used as avours, disinfectants, surfactants, fuel additives9 and
important reactants for synthesis of enantiomerically pure
compounds, which were widely applied for the preparation of
steroids, pharmaceuticals and fragrances.10 Moreover, the
glycerol acetals could offer another method for the preparation
of novel green platform chemicals 1,2-propanediol,11 1,3-dihy-
droxyacetone12 and 1,3-propanediol.13 Especially, it is a prom-
ising and economically viable way when the cyclic acetals are
used as the fuel additives, since the yield of bio-fuel could be
increased up to 16% with 10 wt% of the cyclic acetals added.14
Generally, cyclic glycerol acetals are synthesized in the
presence of acidic catalysts,15 including mineral acids, organic
acids,16 Lewis acid17 or solid acids,18 which are usually associ-
ated with one of the following disadvantages such as high
reaction temperature (e.g. 130 ꢀC),19 long reaction time,20
tedious isolation procedure, equipment corrosion and envi-
ronmental problems derived from the toxic organic solvents.21
In order to abate these deciencies, several alternative hetero-
geneous catalysts such as Amberlyst,14,22 ion exchange resins,23
heteropolyacids,24 zeolites,25 even sulfonated hollow sphere
carbon (HSC-SO3H)26 etc. have been investigated. However, the
relative instability of catalytic system unquestionably restrained
their extensive applications and large-scale production. In
addition, the thermodynamic and kinetic study of the acetali-
zation of glycerol with acetaldehyde also were carried out in
Presently, global research is focused on the development of
sustainable and renewable resources, driven by declining
petroleum reserves and growing concerns about atmospheric
greenhouse gas concentrations.1 Biodiesel, as one of the
renewable energy, has attracted worldwide attention and is
considered as one of the best candidates for the substitution of
petroleum energy in the future.2 It is generally known that the
most traditional route to biodiesel production involves trans-
esterication of vegetable oils, in which the glycerol is produced
as a by-product. For each 90 m3 of biodiesel produced,
approximately 10 m3 of glycerol is generated.3
With the development of biodiesel, the price of glycerol
inevitably drop signicantly due to the excessive crude glycerol
supply,4 even some glycerol plants were shut down. For
example, the glycerol plant of Dow Chemical in Texas was
closed in January 2006, and P & G Co. did same thing to the
natural glycerol plant in US. Apart from the economic factors,
too much crude glycerol would result in a new round of envi-
ronmental pollution if it was not utilized instrumentally and
efficiently. Therefore, it is imperative to explore new applica-
tions of glycerol and release environmental and economic
pressures.5
Generally, glycerol can be used as reagent, additive and basic
chemical for a broad range of uses,6 and the synthesis of value-
aMOE Key Laboratory of Wooden Material Science and Application, Beijing Key
Laboratory of Lignocellulosic Chemistry, College of Materials Science and
Technology, Beijing Forestry University, Beijing 100083, China. E-mail: mzlwb@
bjfu.edu.cn; mzlwbb@126.com; Fax: +86-10-62336592; Tel: +86-10-62336592
bState Key Laboratory of Pulp and Paper Engineering, South China University of
Technology, Guangzhou 510640, China
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Scheme 1 Acetalization of glycerol with PhCHO catalyzed by acidic ionic liquids.
order to explain the reaction mechanism and determine the triuoromethanesulfonic acid were supplied by J&K chemical
value of the equilibrium constant.27
Ltd. and used as received.
On the other hand, the acetalization is a reversible reaction.
The timely removal of by-produced water is favourable for cyclic
acetals production. Based on this, the traditional methods
usually need reactive distillation,16,18a azeotropic distillation
with organic solvents (benzene or acetone)21 or excess of reac-
tant28 to shi the reversible acetalization to cyclic glycerol
acetals side. Undoubtedly, these methods not only raised the
technical difficulty and complexity, but also imported extra
impurities (water-carrying agent) which obviously increased the
energy consumption and production cost.
2.2. Acetalization of glycerol with PhCHO
Ionic liquids were prepared according to the methods in the
literatures.34 The condensation of glycerol with PhCHO was
carried out under batch conditions. In a round-bottomed ask
equipped with a magnetic stirrer and a reux condenser, were
placed glycerol (2.6 g), PhCHO (2.0 g) and [BPy]HSO4 ionic
liquid (0.88 g, 20 mol% based on PhCHO). The reaction mixture
was heated to 70 ꢀC with vigorous stir in an oil-bath under
atmospheric pressure, and kept at this temperature for 2 h to
ensure completeness of the reaction. Aer cooling to room
temperature, the biphasic system was formed automatically.
The upper phase containing cyclic glycerol acetal A, B and
PhCHO was isolated by simple decantation. The ionic liquid in
the lower phase can be recovered aer extraction with ether and
treatment under vacuum at 80 ꢀC, and used directly for the next
run. Aer combining organic extract and upper phase followed
by the solvent removed, the samples were detected by 1H NMR
spectra, which was recorded on a MERCURY-PLUS 400 NMR
spectrometer.
Ionic liquids are in the focus of interests in various elds of
research and development based on their interesting chemical
and physical properties.29 Some acidic ionic liquids have been
proved to be the excellent catalytic media and applied to many
kinds of organic reactions due to their inherent strong acidity and
thermal stability.30 On the other hand, it has been reported that
1-butyl-3-methylimidazolium hexauorophosphate ([Bmim]PF6)
could constitute a micro-reactor system with H2O, and improve
the oxidation of benzene to phenol beneted from its tri-functions
as the solvent, catalyst and transfer reagent.31 Moreover, metal
chlorides (InCl3, ScCl3, etc.) immobilized 1-butyl-1-methyl-
pyrrolidinium bis(triuoromethylsulfonyl)-imide ionic liquid
were used for acetalization reaction between cyclohexanone and
triethyl orthoformate, and 1,1-diethoxycyclohexane were formed
2.3. Recycling experiments
Aer cooling to room temperature, the biphasic system was
formed automatically. The upper phase containing cyclic glyc-
erol acetal and PhCHO was isolated by simple decantation to be
detected by 1H NMR. One-third of volume ether was poured into
the lower phase to extract organic components twice. Then, the
ionic liquid in the lower phase was extracted with CH2Cl2, can
be recovered aer simple evaporation of CH2Cl2 under vacuum,
with high yield and the selectivity at 0 C within 5 minutes.32 In
ꢀ
addition, N-ethyl imidazole-N(p-sulfo)benzalhydantoin-chloride
imidazole salt ionic liquids was synthesized to catalyze acetaliza-
tion of benzaldehyde.33 This prompted us to investigate the
synthesis of cyclic glycerol acetals catalyzed by acidic ionic liquids,
and the condensation of glycerol with benzaldehyde (PhCHO)
was designated as the probe reaction. As shown in Scheme 1,
two isomers 4-hydroxymethyl-2-phenyl-1,3-dioxolane (A) and
5-hydroxyl-2-phenyl-1,3-dioxane (B) were formed, depended on
the condensation position of glycerol. Besides, the inuence of
reaction parameters was also screened. Unlike the previous
studies on acetalization of glycerol,16,18a,21a this method proceeded
in a clean, simple system without water-carrying agent or reactive
distillation in the absence of organic solvents.
ꢀ
and nally dried at 80 C under reduced pressure. Aer these
treatments, the recovery ionic liquid was used directly for the
next run.
3. Results and discussion
As shown in Fig. 1, the formation of acetal A or B depend on
the different condensation position of hydroxyl groups of
glycerol.14,22 In addition, B has cis–trans isomerism from the
aromatic group on the C-2 position in relation to the secondary
hydroxyl group on the C-5 position in the dioxane ring
(1H NMR spectral peaks of B-cis and B-trans are d 5.54 and
d 5.39). In regard to acetal A, there are two asymmetric carbon
2. Experimental
2.1. Materials
All chemicals were analytical grade reagents and used as atoms in the dioxolane ring, one at the C-2 position, another
received without further purication. Pyridine, glycerol, PhCHO one at the C-4 position, which should thus give rise to two
were purchased from Sinopharm Chemical Reagent Beijing Co., pairs of DL-isomers (1H NMR spectral peaks of A-cis and A-trans
Ltd. N-Methylimidazole, butyl chloride, 1,4-butane sultone and are d 5.95 and d 5.81).21b,35
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Fig. 1 The cis–trans isomers of glycerol acetals.
was the kinetics product in the reversible acetalization of glyc-
erol. When extra powers were introduced (catalysts or energy),
the thermodynamics product B become the dominant product.
Previous reports described that a slow equilibrium exists
between A and B as shown in Scheme 2, and the transformation
of A into B is extremely sensitive towards acids.19 Correspond-
ingly, acidic catalysts not only improved the condensation of
glycerol with PhCHO, but also increased the transforming rate
of A into B. Therefore, B was the predominant product when
acidic ionic liquids were used. In fact, the acidity of the glass
wall of the reaction vessel is sufficient to promote the isomer-
isation of the most labile dioxolane isomer to dioxane isomer.19
It is generally known that in the chair conformation of 1,3-
dioxane B, the large groups in C-2 and C5 positions are usually
inclined to the stable equatorial state in view of the axial
repulsion between the large group and the axial hydrogen atoms
in the C-4 and C-6 positions. On these grounds, B-trans should
have been obtained more than B-cis. However, Fig. 2 indicated
the opposite results. This was probably on account of the
stability of intramolecularly hydrogen-bond from the axial
hydroxyl groups exclusively with the oxygen atom 1-position and
3-position as shown in Fig. 3(1). In contrast to B, A-cis was
shown to be less than A-trans. The reason is most probably that
A-trans (see Fig. 3(2)) had the relative less steric hindrance and
more stability compared with A-cis when the intramolecularly
hydrogen-bond of primary hydroxyl group with oxygen atom
3-position was formed.19
3.1. Screening of ionic liquids
In a continuation of our earlier studies on ionic liquid-catalyzed
organic reactions,36 a comparative study on the catalytic activi-
ties of several acidic ionic liquids and conventional acidic
catalysts was carried out under the same conditions, in order to
ascertain if the reaction process and products isomerization
were inuenced by ionic liquids, and the results were listed in
Fig. 2. The weight of the catalysts varied to maintain the mole
amount in every reaction.
As shown in Fig. 2, it is worthy noted that the yields of cyclic
glycerol acetals were increased obviously when extra acidic
catalysts introduced (II–X) compared with that without catalyst
(I). Particularly, under the catalyst-free condition, A was formed
dominantly, and A-trans was produced a bit more than the A-cis
isomer. Instead, B was obtained much more than A in the
reactions in the presence of acidic catalysts (II–X), and the yields
of B-trans gave a little less than B-cis isomer.
Generally, the stability of ve-membered acetal is usually
less than six-membered one in the view of the molecular ther-
modynamics.21b Moreover, taking the better reactivity of the
primary hydroxyl than secondary hydroxyl into account, the B
should have been the major product when PhCHO reacted with
glycerol. However, column I indicated that A was dominantly
produced. Hence, the easier formation of the A manifested it
From Fig. 2, it also can be seen that, H2SO4 and HCl gave rise
to the parallel yields (57.4% and 54.9%), better than that by
organic acidic catalysts such as p-toluenesulphonic acid (p-TSA,
30.3%), and similar to sulfamic acid (NH2SO3H, 58.9%) under
the same reaction conditions. However, the toxicity, corrosivity
and contamination of these catalysts forced people to use more
benign alternatives. As shown in Fig. 2, the condensation of
glycerol with PhCHO gave the better yields (66.1–73.4%) using
several different acidic ionic liquids as catalysts (column II–VII).
As for the difference of catalytic activity of these ionic liquids,
their molecular structures should be considered. For example,
the conjugated pyridinium cations in both [BPy]HSO4 and
[BSPy]HSO4 maybe provide the synergistic effect with aromatic
ring of PhCHO. Nevertheless, the hydrogen bond interaction
between –SO3H attached to [BSPy]HSO4 and –OH of glycerol
perhaps retarded its catalytic activity, which could account for
the better result by [BPy]HSO4. With regard to [BÀSMim]CF3SO3
Fig. 2 Condensation of glycerol with PhCHO catalyzed by different
acidic catalysts. (A: 4-hydroxymethyl-2-phenyl-1,3-dioxolane, B: 5-
hydroxyl-2-phenyl-1,3-dioxane. I: no catalyst; II: [BSPy]HSO4; III:
[BSMim]CF3SO3; IV: [BSMim]HSO4; V: [Bpy]HSO4; VI: [HMim]HSO4; VII:
H2SO4; VIII: HCl; IX: NH2SO3H; X: 4-toluenesulfonic acid. Reaction
conditions: 70 ꢀC, 2 h, molar ratio of glycerol to PhCHO is 1.5 : 1, 20
mol% of catalyst based on PhCHO.)
À
and [BSMim]HSO4, the higher acidity of CF3SO3 than HSO4
could explain the higher yield derived from the former. In the
case of [HMim]HSO4, although the lower acidity than other
ionic liquids, the yield of 66.1% was still better than that of
inorganic acids and p-TSA. Hence, the cyclic acetal yields
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Scheme 2 Equilibrium between 1,3-dioxolane and 1,3-dioxane.
state. In the case of the composition of product, A was more
than B in the rst 40 min. From 40 min to 150 min, the yield of B
exceeded gradually A. Generally, the acetalization of glycerol
could be divided into two stages. At the initial stage, the kinetics
product of A went on forming at a higher rate than that of the B.
Then, the thermodynamics controlled stage of reaction was
initiated gradually, which is the temperature-dependent equi-
librium between A and B, gave rise to the relative high yield of
B.18a If B is separated out from product mixture, the le A will go
on being transformed into B till the same stable ratio obtained
based on its transformation equilibrium constant.12 On the
basis of the above viewpoints, 120 min was reasonable for the
optimal reaction time.
With these data in hand, the inuence of catalyst amount on
the acetalization of glycerol with PhCHO was investigated. It can
be observed from Fig. 5, as the proportion of ionic liquid
increased (less than 10 mol% based on PhCHO), a signicant
rise of reaction rate was observed. Further adding ionic liquid to
40 mol%, the growth trends of product yield started to decrease
slightly. When increasing the amount of ionic liquid from
40 mol% to 80 mol%, the yield of total acetal just only increased
about 1.9%. This demonstrated that too much ionic liquids
were not effective for product yield increase.
Fig. 3 Hydrogen-bonds in cyclic glycerol acetals.
obtained by ionic liquids decreased in the following order: [BPy]
HSO4 > [BSPy]HSO4 > [BSMim]CF3SO3 > [BSMim]HSO4
[HMim]HSO4. Certainly, [BPy]HSO4 ionic liquid was selected as
the representative catalyst in the following sections.
>
3.2. Optimization of the reaction conditions
We set out to examine the effect of reaction time, temperature,
catalyst amount on the condensation of glycerol with PhCHO
and on the ratio of A to B in the presence of [BPy]HSO4. Initially,
the kinetics of condensation of glycerol was estimated by
withdrawing samples at specic time intervals under the same
1
conditions followed by H NMR analysis.
As the data shown in Fig. 4, the reaction proceeded fast in
the beginning and 68.4% of yield was obtained aer 10 min at
70 ꢀC. Then the increased trends tended to be gentle. For
further 140 min prolonged, only 6.6% of yield was added,
indicating that the reaction is about to arrive at the equilibrium
As for the proportion of A and B, in the wake of ionic liquid
amount increased to 40 mol%, A in the products descended
correspondingly, and B increased dramatically from 9.6% to
Fig. 4 Effect of reaction time on the acetalization of glycerol with Fig. 5 The influence of [BPy]HSO4 ionic liquid amount (mol% based
PhCHO using [BPy]HSO4 as catalyst. 70 ꢀC, molar ratio of glycerol to on PhCHO) on the acetalization and the proportion of isomers. 70 ꢀC,
PhCHO is 1.5 : 1, 20 mol% of [BPy]HSO4 based on PhCHO.
2 h, molar ratio of glycerol to PhCHO is 1.5 : 1.
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52.3%. That is to say, B increased signicantly at the expense of
A in the course of the reaction. However, it looked like that too
much ionic liquids had a detrimental effect on the trans-
formation of A to B, which was probably due to the relatively
poor dispersion of ionic liquid in the reaction mixture. As far as
the reusability of ionic liquid is concerned, 20 mol% proportion
of ionic liquid was chosen as the best option.
In order to evaluate the inuence of reaction temperature on
the reaction even the isomer ratio, the condensation of glycerol
with PhCHO catalyzed by [BPy]HSO4 was monitored over time.
According to Fig. 6, it seemed like that the temperature showed
little effect on the acetalization of glycerol with PhCHO, because
only less than 3% of glycerol acetals were added when the
reaction temperature increased from 25 ꢀC to 120 ꢀC. Based on
these abovementioned, we believed that at the kinetically
controlled stage, A was produced rstly and then was consumed
gradually along with the formation of B due to the acid-cata-
lyzed temperature-dependent equilibrium between A and B.16
However, the acetalization proceeded so quick that the two
reaction stages have completed and total cyclic acetals yield was
nearly stable aer 120 min. Anyway, the change of isomer ratio
was still detected at the different temperature. Along with the
temperature increased, the ratio of A to B gradually deceased. It
is consistent with the research by Aksnes et al.,19 who have
established an equilibrium experiment which clearly showed
that the yield of every acetal isomer was strongly temperature
dependent. The higher temperature is benet for the acetali-
zation of glycerol with PhCHO and lower temperature is in
favour of the transformation of A to B.
Fig. 7 Effect of molar ratio of substrates on the acetalization of glycerol
with PhCHO using [BPy]HSO4 as catalyst. 25 ꢀC, 2 h, [BPy]HSO4 20 wt%
based on PhCHO. (The yield is based on glycerol when benzaldehyde is
excess, otherwise, the yield is based on benzaldehyde.)
glycerol when PhCHO was stoichiometric excess. At the same
time, the yield of cyclic acetals was improved with increasing
molar ratio of glycerol to PhCHO when excessive glycerol was
used. Undoubtedly, acetalization is a reversible equilibrium
reaction which generates water as by-product. The increase of
any one of the substrates could shi the reaction to product
side. Therefore, the yield of cyclic acetals could be improved.
Moreover, the better result (ꢁ100% of yield at the ratio of 1 : 3)
could be achieved than that (92.6% of yield at the ratio of 3 : 1
and 96.6% of yield at the ratio of 5 : 1). In the case of the
utilization of renewable resource, the excessive glycerol seemed
more available for the preparation of glycerol acetals, which has
been dened as the platform chemical derived from biodiesel
production.5b Anyway, these results were comparative with or
better than the results reported with water-carrying agent or
instant evaporation device.16,18a
The inuence of molar ratio of glycerol to PhCHO on the
acetalization and isomer ratio was also studied at ambient
because the cost-efficient ratio is favourable for the practical
application. Fig. 7 showed that the yield of cyclic acetals
increased with the increase in the molar ratio of PhCHO to
3.3. Mass transfer mechanism of acetalization of glycerol
with PhCHO catalyzed by [BPy]HSO4
It is well known that the water formed in the acetalization of
glycerol with PhCHO could weaken the catalyst activities and
favor the reverse reaction. Most of the acetalizations of glycerol
with alcohols need Dean-Strap with organic solvents (benzene
or acetone) to shi the reaction to product side. However, these
methods are not environmentally recommended since most of
these solvents are hazard to humans, and extra water-removing
device inevitably raises the energy consumption and cost. In
this study, all of the reactions proceeded in a clean and simple
system without azeotropic distillation or reactive distillation
device in the absence of organic solvents.
Fig. 6 The influence of temperature on the acetalization of glycerol
with PhCHO and proportion of isomers using [BPy]HSO4 as catalyst.
2 h, molar ratio of glycerol to PhCHO is 1.5 : 1, [BPy]HSO4 20 mol%
based on PhCHO.
As we know glycerol is immiscible with PhCHO, but soluble
in [BPy]HSO4 ionic liquid. When the reaction mixture was stirred
vigorously, we proposed that a micro water-removing reactor was
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formed as shown in Fig. 8. In this micro water-removing reactor, reactivity of aldehyde than ketone in the acetalization. As for the
the condensation was carried out at the interface between polar isomer, acetalization of furfural with glycerol provided the
phase and organic phase catalyzed by [BPy]HSO4. Then the cyclic higher 1,3-dioxane proportion in the products. The steric
acetals were produced and transported to organic phase fol- hindrance of furan ring may account for this result similar as
lowed by inverted transport of H2O to polar phase. It looked like benzene ring in benzaldehyde. Acetaldehyde and furfural may
a micro reactor equipped with a micro water-removing device, be prepared from the renewable resources. The former can be
which could separate out the produced water from the organic obtained from bioethanol following a dehydrogenation or
phase in time to shi the reaction to products side. Thus, it partial oxidation process,37 and the latter is achieved through
maybe can explain why the ionic liquids could nally achieve the the dehydration of sugar and is being regarded as one of the
satisfactory product yields compared with other methods. platform chemicals for the petroleum based products alterna-
Moreover, ionic liquid could be reused four times and nearly tive.38 From abovementioned, it can be said [BPy]HSO4 ionic
kept its initial activity (98.5% of yield, 2 h, at ambient, PhCHO to liquid as catalyst has the acceptable application for the prepa-
glycerol is 2 : 1), which demonstrated the good stability and ration of 1,3-dioxolane and 1,3-dioxane by the acetalization of
operability of ionic liquid.
various carbonyl compounds with glycerol, especially of some
bio-based aldehydes.
3.4. Catalytic activity of [BPy]HSO4 for acetalization of other
aldehydes/ketone
4. Conclusion
It is well known that glycerol acetals can be used to reduce
particulate emissions, to increase octane number in gasoline, to
improve the properties of biodiesel (viscosity, oxidation
stability, ash point, etc.), or as antifreezing agents of biodiesel.
Especially the glycerol acetals from short chain aldehydes, such
as acetaldehyde and butyraldehyde, present ash points close to
the diesel specications, combine positive environmental
effects through lower emissions and suitable octane numbers.
Thus, the acetalizations of several carbonyl compounds suitable
to prepare the fuel additives were carried out in [BPy]HSO4
catalytic system. Table 1 shows the results for some aldehydes
or ketone investigated.
Glycerol, a platform chemical from renewable materials, has
been used as the starting material for the preparation of cyclic
glycerol acetals catalyzed by acidic ionic liquids. The probe
reaction carried out using PhCHO as reactant has indicated that
acidic ionic liquids gave the satisfactory yields (66.1–73.4%)
which derived from their relative special structures, and the
yield of 99.8% for total cyclic glycerol acetals was obtained
under the optimized conditions catalyzed by [BPy]HSO4. It was
probably attributed to the micro water-removing system
constituted by ionic liquid in virtue of its special solubility, in
which the water could be transferred out from the organic phase
in time to shi the condensation equilibrium to products side.
In addition, the acetalization of bio-based aldehydes such as
furfural and acetaldehyde with glycerol also indicated the
applicability of [BPy]HSO4 ionic liquid.
Obviously, the rst three aldehydes gave relative better yields
of acetals compared with acetone, which based on the higher
The product mixture A and B were identied as cis and trans
two stereo-isomer for each one checked by 1H NMR. The acetal
A and B could constitute the excellent compound for the addi-
tives of gasoline, diesel and biodiesel fuels. B is of particular
interest as precursor for 1,3-propanediol derivatives. In the
catalyst-free condition, A (A-cis and A-trans) was formed domi-
nantly, and A-trans was produced more than A-cis. In contrast, B
took part in the majority catalyzed by acidic catalysts, in which,
B-trans was given less than B-cis. Higher temperature could be
benet for the acetalization of glycerol with PhCHO and lower
temperature was in favour of the transformation of A to B.
Unlike the previous studies on the acetalization of glycerol, this
reaction proceeded in a clean and simple system without water-
carrying agent or reactive evaporation device in the absence of
organic solvents. Because [BPy]HSO4 ionic liquid has combined
the advantages of friendly benign acidic catalyst with an excel-
lent product separation methodology, as well as the cost effi-
ciency and green aspect, which are in line with the demand of
sustainable chemistry.
Fig. 8 Mass transfer mechanism of acetalization of glycerol with
PhCHO catalyzed by [BPy]HSO4 in the micro water-removing reactor.
Table 1 Acetalization of several carbonyl compounds in [BPy]HSO4
ionic liquida
Yield of
Yield of
Yield of
acetals (%)
1.3-dioxolane (%)
1.3-dioxane (%)
Acetaldehyde
Butyraldehyde
Furfural
95.6
96.8
92.5
76.7
39.4
38.7
28.2
33.5
56.2
58.1
64.3
43.2
Acetone
Acknowledgements
a
Reaction condition, [BPy]HSO4 20 mol% based on carbonyl
compound, 25 ꢀC, 2 h, molar ratio of glycerol to carbonyl compound
is 1 : 2.
The authors are grateful for nancial supports from the Beijing
Higher Education Young Elite Teacher Project (YETP0765),
18922 | RSC Adv., 2014, 4, 18917–18923
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Fundamental Research Funds for the Central Universities 17 B. M. Smith, T. M. Kubczyk and A. E. Graham, RSC Adv.,
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