Microwave rehydrated Mg-Al-LDH as base catalyst for the acetalization of glycerol

Article abstract of DOI:10.1039/c5cy00347d

Acetalization of glycerol with aldehydes to form cyclic acetals is an industrially important reaction and is generally carried out using acid catalysts. Base catalysts such as LDH can bring about microwave-assisted acetalization of glycerol and aldehydes to form 5-membered and 6-membered cyclic acetals. Among the different LDHs used, Mg-Al-LDH was found to exhibit maximum conversion of glycerol into 5-membered cyclic acetal. Modification of Mg-Al-LDH involving calcination at 450°C and subsequent microwave-assisted rehydration showed improved glycerol conversion rate under similar reaction conditions. Rehydration of calcined Mg-Al-LDH by microwave irradiation was found to result in LDH regaining its layered structure with higher basicity possibly exposing more hydroxyl ions responsible for basicity. Multiple use of methanol washed spent catalyst showed good repeatability for conversion up to three cycles which subsequently showed a marginal decrease in the conversion. Further dehydration followed by rehydration of the spent LDH catalyst under microwave irradiation was found to rejuvenate the catalytic activity to its initial level.

Full text of DOI:10.1039/c5cy00347d

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Page 1 of 25
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DOI: 10.1039/C5CY00347D
Microwave rehydrated Mg-Al-LDH as base catalyst for the acetalization of
glycerol
H.R. Prakruthi, B.M. Chandrashekara, B.S. Jai Prakash, Y.S. Bhat*
Chemistry Research Centre, Bangalore Institute of Technology, KR Road,
Bangalore 560 004, India
*Corresponding author: Eꢀmailꢀbhatys@yahoo.com,
Tel.: +91 80 26615865; fax: +91 80 26426796
Abstract:
Acetalization of glycerol with aldehydes to form cyclic acetals is an industrially important
reaction and is generally carried out using acid catalysts. Base catalysts such as LDH can bring
about microwave assisted acetalization of glycerol and aldehydes to form 5ꢀmembered and 6ꢀ
membered cyclic acetals. Among the different LDHs used, MgꢀAlꢀLDH was found to exhibit
maximum conversion of glycerol into 5ꢀmembered cyclic acetal. Modification of MgꢀAl LDH
involving calcination at 450oC and subsequent microwave assisted rehydration showed improved
glycerol conversion rate under similar reaction conditions. Rehydration of calcined MgꢀAlꢀLDH
by microwave irradiation was found to result in LDH regaining its layer structure with higher
basicity possibly exposing more hydroxyl ions responsible for basicity. Multiple use of methanol
washed spent catalyst showed good repeatability for conversion up to three cycles which
subsequently showed a marginal decrease in the conversion. Further dehydration followed by
rehydration of the spent LDH catalyst under microwave irradiation was found to rejuvenate the
catalytic activity to its initial level.
Keywords: MgꢀAlꢀLDH, glycerol, acetalization, rehydration, microwave irradiation
1. Introduction:
Glycerol (Gly) is a major byꢀproduct of biodiesel synthesis leading to large accumulation of
crude glycerol. This necessitates the conversion of crude glycerol into valuable products. Some
potential uses of glycerol include, synthesis of acrolein, ethanol, epichlorhydrin, production of
citric and lactic acids, as components in antiꢀfreezing agents and soaps, additives in fuels,
composite materials and as cosmetic bonding agents. The different reaction procedures for

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conversion of glycerol to valueꢀadded chemicals reported in literature include oxidation,
transesterification, dehydration, hydrogenolysis, pyrolysis, etherification, esterification, steam
1
reforming, acetalization, and polymerization by catalytic processes . Among these, the
acetalization of glycerol is recognized as the most important process for the synthesis of green
and commercially important chemicals from glycerol.
Glycerol reacts with aldehydes and ketones giving 1,3ꢀdioxalane (5ꢀmembered) and 1,3ꢀdioxane
(6ꢀmembered) cyclic acetals which find industrial applications 2ꢀ6. These cyclic acetals are used
as precursors for the production of green platform compounds 1,3ꢀpropanediol and 1,3ꢀ
dihydroxyacetone 7.
Acetalization of glycerol has been studied with various acid catalytic systems like H2SO4, AlCl3,
FeCl3, ZnCl2, pꢀTSA and HCl 8ꢀ11 in homogenous medium. Anchored silicontungstates on MCMꢀ
41, K10, supported metal oxides, heteropolyacids, zeolites, momtmorillonite, Amberlystꢀ15,
mesoporous MoO3/SiO2, SBAꢀ15, sulfated zirconia, ZrꢀSBAꢀ16, ion exchange resins,
nanocrystalline SnO2ꢀbased solid acids, aluminium triflate grafted MCMꢀ41, organo iridium derivatives,
HfꢀTUDꢀ1, ZrꢀTUDꢀ1, SnꢀMCMꢀ41, USYꢀ2 , Mo and Wꢀpromoted SnO2, mesoporous cellular foams
(MCFs) modified by niobium or tantalum, acidic ionic liquids, TiO2–SiO2 mixed oxides and Ni–Zr
supported on mesoporous activated carbon have been used as heterogeneous catalysts 12ꢀ32
.
Layered double hydroxides (LDHs) are layered minerals and are also called anionic clays. These
constitute a class of compounds with positively charged layers and to maintain the electrical
neutrality
possess
interlayer
anions.
They
are
represented
by
X
general

x+
.
Xx−
formula, M1−x MIxII (OH)
,
M
representing layer cations and
interlayer
II
(
)

2 
x/q
anion. MII and MIII represent divalent and trivalent metal ions such as Mg, Cu, Ni, Co, Zn and
2ꢀ
represents interlayer anion of charge q such as CO3 , Clꢀ,
Xx−
Al, Zr, Fe, Cr, Ga respectively,
(
)
x/q
ꢀ
2ꢀ
NO3 , and SO4 . A wide range of composition of LDH can be prepared by coꢀprecipitation
method. LDHs have potential applications such as adsorption materials, carriers for drugs,
electrode modifiers and catalysts. LDH itself can be used as catalyst or can be calcined to get
decomposed LDH containing mixture of mixed oxides. The decomposed LDH (mixed oxides) is
known to regain its original layered structure through a structural memory effect upon
rehydration.

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There are many reports in the literature for the acetalization of glycerol with carbonyl
compounds carried out by using various solid acid catalysts. To the best of our knowledge, use of
base catalysts in the glycerol acetalization reaction has not received much attention in the
literature. The main objective of the present investigation is to carry out acetalization of glycerol
with different as synthesized, calcined and rehydrated LDH samples as catalysts with different
aldehydes to produce cyclic acetals. The reactions have been conducted in presence of
microwave irradiation with an intention to know the efficiency of acetalization and rehydration
of the LDHs in presence of microwaves.
Heterogeneously catalyzed liquid phase organic reactions are usually carried out by conductive
heating with an external heat source like heating in a hot air oven or oil bath. In this method,
transfer of heat from source to reaction mixture depends on thermal conductivity of various
materials which must be penetrated. This is a comparatively slow and inefficient method for
transferring energy from the source into the reaction mixture. And also, reaction mixture is not
uniformly heated, resulting in the temperature of the reaction vessel being higher than that of the
reaction mixture. In contrast, in microwave irradiation, heat is directly transferred from the
source to the reaction mixture by direct coupling of microwave energy with the molecules
(solvents, reagents, catalysts) that are present in the reaction mixture. This produces more rapid,
efficient, uniform internal heating of reaction mixture. The advantageous effects of microwave
irradiation compared to conventional heating are mainly because of this inꢀcore volumetric
heating (Kappe et al.) 33.
2. Experimental section
2.1. Catalysts and chemicals
Glycerol AR grade, cinnamaldehyde, benzaldehyde, anisaldehyde, chlorobenzaldehyde,
nitrobenzaldehyde, 1ꢀpentanol, Mg (NO3)2.3H2O, Zn (NO3)2.6H2O, Al (NO3)3.9H2O, Cu
(NO3)2.3H2O, Co (NO3)2.6H2O, NaOH and Na2CO3 were procured from SD fine chemicals, and
CDH chemicals India and used without purification.
2.2 Catalyst preparation
All the LDHs were prepared by coꢀprecipitation method 34ꢀ38. An aqueous solution of

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Mg (NO3)2.6H2O (0.3 mole) and Al (NO3)3.9H2O (0.1 mole) were prepared and the above
mixture was added drop wise into an alkaline solution mixture of Na2CO3 (0.23 mole) and NaOH
(0.875 mole) at room temperature for 3 h with constant stirring. It was then allowed to age for 30
minutes at 80oC under microwave heating in a microwave reactor described below. The solid
material obtained was centrifuged and washed thoroughly with hot water until the pH of the
washing becomes 7.0. The washed material was dried at 120 oC for 12h. Similarly, LDHs of Znꢀ
Al, CoꢀAl, CuꢀAl were synthesized under microwave irradiation.
Synthesized MgꢀAl LDH was calcined in air at 450 oC for 3 h. Calcination results in mixed metal
oxide formation as shown by XRD patterns. Rehydration of decomposed catalyst samples were
carried out under microwave irradiation. 5 g calcined samples were mixed with 100 ml of water,
the mixture was subjected to microwave irradiation at 100 oC with continuous stirring for 2h in
presence of nitrogen atmosphere. The irradiated mixture was cooled, centrifuged, the solid was
separated and dried in hot air oven at 120oC. The rehydrated LDH was designated as RHꢀMgꢀAl
LDH.
2.3 Characterization
All the prepared catalyst samples, including the calcined and rehydrated ones, were characterized
using different techniques viz. XRD, FTIR, and total basicity by benzoic acid titration and TPDꢀ
CO2. The structural integrity of the catalysts was analysed by Powder Xꢀray diffraction patterns.
The data were recorded with PAN analytical X’Pert PRO MPD Xꢀray diffractometer using
graphite monochromatized Cu Kα radiation (λ= 1.5406 Ǻ) with step scanning at 0.020o per
second and in the 2θ range of 4o to 80o.
39
The total basicity of LDH samples was estimated by benzoic acid titration method . 0.5 g of
LDH sample was dispersed in 5ml of toluene as solvent and stirred for 30 minutes. To the
suspension, 5 drops of dilute phenolphthalein indicator solution were added and the suspension
was titrated against 0.01M benzoic acid in toluene till the pink color of the indicator on the
catalyst sample sharply disappears. The amount of benzoic acid consumed was used for the
calculation of total basicity.
Temperature programmed desorption of (TPDꢀCO2) was also done for the measurement of
basicity of the catalyst. Preꢀtreatment of the catalyst samples was carried out in He flow at 300oC
for 1h, and then cooled to 50oC prior to the adsorption of CO2 at this temperature. The sample

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was flushed with He for 1h at 100oC to remove the physisorbed CO2 from the catalyst surface.
The desorption of CO2 was measured by heating the sample from 100oC to 850oC at a heating
rate of 10oC minꢀ1 and the desorbed CO2 was measured with a TCD detector 40.
FTIR spectra of the catalyst samples were recorded on Shimadzu IR Affinityꢀ1
spectrophotometer, in the range of 400ꢀ4000 cmꢀ1 using the KBr pellet method.
2.4 Catalytic tests:
Microwave heating: Ten mmol of glycerol (Gly) and thirty mmol of cinnamaldehyde (CA)
were mixed with 1g catalyst in a microwave reactor vessel with magnetic stirring. Reactor vessel
was kept in microwave lab station ‘STARTꢀS’. Variable power up to 1200W was applied by
microprocessorꢀcontrolled singleꢀmagnetron system. Initial power of 900 W was applied for 1
minute to attain the reaction temperature of 140oC. The reaction temperature was then
maintained for 59 minutes to achieve maximum conversion. The reaction mixture (scheme 1)
was then cooled, followed by addition of 5ml of pentanol, stirred for about 20 minutes to extract
adsorbed reactants and products. The used catalyst was separated by centrifugation and the
filtrate was analysed in Chemito GCꢀ1000 gas chromatograph with TRꢀWAX capillary column
(30 m length, 0.32 mm thickness and 0.5ꢁm internal diameter) attached with flame ionization
detector. The products were analyzed and confirmed by GCꢀMS 41.
Conventional heating:
Same amount of catalyst and reactants were taken as in microwave heating in a 50ml autoclave
placed in a hot air oven at 1400C for 2,4,8,14,24 hours. After the completion of the reaction,
analysis was done as in microwave heating.
O
OH
H
O
H
O
LDH
MW
H
OH
O
+
O
+
OH
OH
Cinnamaldehyde
Gycerol
OH
{2-[-2-phenylethenyl]-1,3-dioxolan-4-yl}methanol
2-[-2-phenylethenyl]-1,3-dioxan-5-ol
Scheme.1: Acetalization reaction of glycerol with cinnamaldehyde.

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3. Results and discussion:
Figure 1 shows the powder Xꢀray diffraction patterns of ZnꢀAl, MgꢀAl, CuꢀAl and CoꢀAl as
synthesized. Catalyst samples showed sharp and symmetric peaks at 2θ values 11o, 24o and 37o
corresponding to the typical LDH reflections (003), (006) and (012) planes respectively.
(003)
(006)
(012)
(015)
d
c
b
a
0
20
40
60
2
θ
(degree)
Fig.1 XRD pattern of different LDHs. (a) ZnꢀAl (b) MgꢀAl (c) Cu–Al (d) CoꢀAl
The FTIR spectra of LDH samples (Fig.2) showed a broad, asymmetric peak around 3400 cmꢀ1
generally ascribed to the OH groups attached to the metal ions in the bruciteꢀlike layer. The
bending mode of interlayer water molecule was observed at 1640 cmꢀ1 and the antisymmetric
band at 1365 cmꢀ1 corresponded to interlayer carbonate anion.

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d
c
b
%T
a
4000
3500
3000
2500
2000
1500
1000
500
Wave number (cm-1)
Fig.2. FTꢀIR Spectra of different LDHs (a) ZnꢀAl,(b) MgꢀAl, (c) CuꢀAl, (d) CoꢀAl
3.1 Comparison of Conventional and Microwave mode of heating:
The effects of variation of reaction conditions during conventional and microwave modes of
heating on acetalization of glycerol with CA are compared in Fig.3 (a) and (b). Reaction time
was varied from 5 to 120 min and 2 to 24 h under microwave and conventional heating
respectively.
Glycerol (MW)
5-membered
6- membered
80
70
60
50
40
30
20
10
0
(a)
0
20
40
60
80
100
120
Reaction time (minutes)

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70
60
50
40
30
20
10
0
Glycerol (CH)
5-membered
6-membered
(b)
0
240
480
720
960
1200
1440
Reaction time (minutes)
Fig.3 Acetalization of Gly with CA under (a) microwave and (b) conventional heating.
Reaction condition: catalyst amount, 1 g; mole ratio Gly: CA, 1:3; temperature, 1400C.
Under conventional heating, reaction proceeded very slowly and showed conversion of 68% in
24 h. On the other hand, under microwave heating, conversion of glycerol was 77% in 60
minutes. This is attributed to intensive, localized and uniform heating of reaction mixture by
microwaves. Microwave irradiation processes are known to decrease reaction time from hours to
minutes enhancing the rate of reaction 42. In the present study, glycerol and cinnamaldehyde
used as reactants for acetalization reaction gives higher rate of conversion under MW irradiation.
The heating characteristics of a particular material (for example, a solvent) under microwave
irradiation conditions are dependent on its dielectric properties. The ability of a specific
substance to convert electromagnetic energy into heat at a given frequency and temperature is
determined by the soꢀcalled loss factor tan d. This loss factor is expressed as the quotient tan d =
e′′/e′, where e′′ is the dielectric loss, which is indicative of the efficiency with which
electromagnetic radiation is converted into heat, and e′ is the dielectric constant describing the
ability of molecules to be polarized by the electric field. A reaction medium with a high tan d
value is required for efficient absorption and, consequently, for rapid heating [Kappe et al.]. In
the present work, enhancement in the rate of product formation under microwave irradiation is
attributed to relatively high tan d values of the reactants.
Furthermore, catalyst surface also plays a major role. In the absence of catalyst, though the
reactants are polar, microwave radiations fail to initiate the reaction. Microwave irradiation,

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however, has been found by many workers to be more effective in initiating and bringing the
reaction to equilibrium at a faster rate than the thermal radiation43ꢀ54
.
In the present work, the initiation involves the active base sites (ꢀOH and –Oꢀ) attached to the
Mg and Al ions which accept a proton from the glycerol molecule. Apparently, proton accepting
property is influenced by the vibrational (thermal) and rotational (microwave) motions of both
the active sites on the catalyst surface and the proton donating property of glycerol molecule. It
appears that rotational motions in some way facilitate the removal of protons from the polar
glycerol molecule than the thermal vibrations 55.
3.2 Catalyst Screening:
The results of glycerol acetalization with cinnamaldehyde over different LDHs at Gly : CA mole
ratio of 1:3 at 140oC for 60 minutes under microwave heating are shown in Fig. 4. It can be
observed that, among the different LDHs, MgꢀAl showed a maximum conversion of 56% which
is attributed to its higher basicity as determined by benzoic acid titration method (Table 1). All
the LDH samples showed higher selectivity towards 1,3 dioxalane (5ꢀmembered) than 1,3
dioxane (6ꢀmembered) product with MgꢀAl LDH exhibiting a maximum selectivity of 96% for
the 5ꢀmembered cyclic acetal. Further studies were carried out with MgꢀAl LDH through
modification to achieve higher conversion and product yield.
80
Glycerol
70
60
50
40
30
20
10
0
5-membered
6-membered
Zn-Al
Mg-Al
Cu-Al
Co-Al
Fig.4 Acetalization of Glycerol with cinnamaldehyde (CA) with different LDH catalysts.
Reaction condition: reaction temperature, 140oC; mole ratio Gly:CA, 1:3; catalyst amount, 1g,
reaction time, 60min; (MW)

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Table.1 Basicity of as synthesized and modified LDH.
Catalyst Basicity density (µmol/g) % Conversion of
of Gly
112
124
105
98
46
56
42
39
ZnꢀAl
MgꢀAl
CuꢀAl
CoꢀAl
3.3 Catalyst modification
LDHs are known to regain their structures upon calcination and rehydration. Rehydration of the
calcined LDH samples is achieved by several methods. In the present study, rehydration by
microwave irradiation and its effect on the catalytic activity of MgꢀAlꢀLDH was studied. MgꢀAl
LDH catalyst was calcined at 450 oC for 3 h, and then rehydrated under microwave irradiation in
o
N2 atmosphere at 100 C for 2h. Fig 5 shows the XRD patterns of the MgꢀAlꢀLDH, calcined
MgꢀAlꢀLDH and the RHꢀMgꢀAlꢀLDH. The XRD pattern of calcined sample (Fig 5b) showed the
reflections which are ascribed to the oxides of Mg and Al and that of the rehydrated sample (Fig
5c) shows all the peaks corresponding to MgꢀAlꢀLDH. Acetlization results are graphically
represented in Fig. 6 for the as synthesized, calcined and the rehydrated samples. The catalytic
activity of RHꢀMgꢀAlꢀLDH for acetalization of glycerol showed increased conversion of 77 %
with 96% selectivity towards 5ꢀmembered cyclic acetal.
(003)
(006)
(012)
(015)
c
b
a
10
20
30
40
50
60
2θ (degree)
Fig.5 XRD pattern of (a) asꢀsynthesized LDH (b) Calcined LDH (c) RHꢀMgꢀAl LDH

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In the calcined sample of MgꢀAl LDH absence of peaks at 11o, 24o and 37o indicates collapse of
the LDH structure and formation of oxide phase at 32o, 43o and 52o respectively (Fig.5b). RHꢀ
MgꢀAl LDH showed similar reflections of LDH as synthesized (Fig. 5 a and c).
80
Glycerol conversion (%)
% yield of 5-membered
70
% yield of 6-membered
60
50
40
30
20
10
0
Mg-Al
Calcined Mg-Al
RH Mg-Al
Fig.6. Acetalization of glycerol with different catalysts.
Reaction condition: reaction temperature, 140oC; mole ratio Gly:CA, 1:3; catalyst amount, 1g, reaction
time, 60min; (MW).
In order to optimize the reaction conditions, various parameters like effect of reaction
temperature, catalyst amount, mole ratio of the reactants and reaction time for acetalization of
Gly with CA over RHꢀMgꢀAl LDH were studied. Under optimized reaction conditions the
reactions were carried out with 1 g RHꢀMgꢀAlꢀLDH, Gly: aldehyde; ratio of 1:3 at temperature
140oC with microwave energy 900W for 1h. Results for various aldehydes are shown in Table.2.
Acetalization of glycerol with cinnamaldehyde showed higher activity (about 77% conversion)
with 96 % selectivity for 1,3ꢀdioxalane and 4% for 1,3ꢀdioxane. Higher reactivity of
cinnamaldehyde is attributed to the presence of conjugated carbonyl group. Methoxy group in
the anisaldehyde causes less conversion because of its electron donating nature rendering the
carbonyl carbon less electrophilic.

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Table.2. Acetalization of glycerol with various aldehydes.
Carbonyl Compounds Conversion (%) 1,3ꢀdioxalane
(% Yield)
1,3ꢀdioxane
(% Yield)
Cinnamaldehyde
Benzaldehyde
Anisaldehyde
chloroꢀbenzaldehyde
nitroꢀbenzaldehyde
77
56
44
52
59
71
51
34
43
49
6
8
10
9
7
Reaction condition: reaction temperature, 140oC; mole ratio Gly : CA, 1:3; catalyst amount, 1g,
reaction time, 60min; RHꢀMgꢀAlꢀLDH, MW
3.4 Basicity studies
The increased activity of the rehydrated sample is attributed to the recovery of its layered
structure with increase in accessibility to significant extent of OHꢀ ions present on the surface
and edge sites of the catalyst sample. This is supported by the TPD CO2 profiles of asꢀ
synthesized and RHꢀMgꢀAl LDH [Fig 7 and 8].
The deconvoluted TPD profiles of catalyst samples showed three peaks. The total basicity and
basic strength distribution of the catalysts samples were assigned based on peak appeared with
respect to temperature. The peak at 400ꢀ580oC, related to desorption of CO2 from the weak basic
strength sites. The desorption peaks at 620ꢀ790oC and 800ꢀ900oC are attributed to medium and
strong basic strength sites respectively. In the as synthesized sample of LDH, desorption peak at
400ꢀ580oC is more intense compared to other peaks responsible for weak basic sites. However,
after modification through rehydration, the intensity of this peak decreased and the peaks at 620ꢀ
790oC and 800ꢀ900oC becomes more intense compared to the LDH sample as synthesized. The
latter can be attributed to the partial conversion of weak basic sites into medium and strong basic
sites. This could be due to the redistribution of OH groups during the rehydration process. Even
the basicity as measured by the benzoic acid titration for the rehydrated sample was appreciably
higher (181µmol/g) than the LDH sample as synthesized (124 µmol/g).Our observations are in
line with study done by Alvarez et al., for rehydration of MgꢀAl LDH under ultrasonication
resulting in increased basicity of the LDH sample 56.

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300
250
200
150
100
50
Mg-Al LDH
0
400
600
800
1000
Temperature (degree)
Fig.7 TPD CO2 profile of MgꢀAl LDH
250
RH-Mg-Al LDH
200
150
100
50
0
400
600
800
1000
Temperature (oC)
Fig.8 TPD CO2 of RHꢀMgꢀAl LDH
3.5 Plausible reaction mechanism:
According to literature, all the acetalization of glycerol reactions are carried out by using various
Bronsted and Lewis solid acids with different carbonyl compounds 8ꢀ32. In presence of solid
acids, acetalization takes place through protonation of carbonyl oxygen of carbonyl compound
followed by the nucleophilic addition of hydroxyl groups of the glycerol forming glycerol acetals
through dehydration.

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Present study shows that base catalysts such as LDHs are also effective in bringing out formation
of glycerol acetals. MgꢀAl LDH acts as base catalyst by removing the hydroxyl hydrogen of
glycerol leading to generation of alkaoxy nucleophile as shown in scheme 2. It then readily
attacks the available electrophilic carbonyl carbon of aldehyde forming cyclic acetals through
dehydration.
O
H
R
H
R
OH
-
O
OH
OH
OH
O
LDH
O
O
R
-
O
H
OH
OH
OH
O
OH
H
R
O
O
O
H
O
O
R
H
R- C6H5C2H2 group
H
H
OH
Scheme.2 Possible reaction mechanism of LDH catalyzed acetals formation
3.6 Regeneration of spent catalyst:
To study the catalyst deactivation, RHꢀMgꢀAl LDH was reused several (three) times under
optimized conditions. After carrying out the reaction the used catalyst was separated by
centrifugation after every cycle. It was then washed with methanol (5ml) and acetone to remove
the adsorbed reactants and products on the surface of the catalyst, dried at 100oC for 2 h and then
used for fresh batch of reaction. The structure of the catalyst was found to be unchanged after its
use which was confirmed by XRD. Further the catalyst was used multiple times, the results of
which are shown in the table.3. Conversion was found to decrease slightly for the fourth cycle.
The partially deactivated catalyst was calcined at 450oC for 3 h and then rehydrated under MW
irradiation in presence of N2 atmosphere (2h, 100oC). The results (Table 3) show that the
catalyst, apparently, regains its original layered structure with its initial activity (*Fifth cycle).
Table.3 Catalyst recyclability Studies
Cycles
Glycerol Conversion (%)
First cycle
77
77
76
64
76
Second cycle
Third cycle
Fourth cycle
Fifth cycle*

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Reaction condition: reaction temperature, 140oC; mole ratio Gly:CA, 1:3; catalyst amount, 1g,
reaction time, 60min; (MW) *Fourth cycle catalyst sample calcined and rehydrated.
4. Conclusion: Acid catalyzed acetalization of glycerol into value added products can be
brought about by base catalysts such as LDHs with equal ease. LDHs, being solid base catalysts,
produce 5ꢀ and 6ꢀ membered acetals when glycerol is reacted with aldehydes. Hydroxyl groups,
apparently present on the surface and edge sites of LDHs, act as basic sites for the acetalization.
Calcination of LDH, leads to corresponding mixed metal oxides resulting in the loss of LDH
structure. Microwave assisted rehydration is found to be very effective in restoring LDH
structure of the calcined sample with improved basicity. The results show an improvement in the
rate of acetalization by rehydrated MgꢀAlꢀLDH compared to asꢀsynthesized LDH sample. This is
attributed to enhanced basicity after rehydration as supported by the TPDꢀCO2.
Acknowledgement: The authors would like to thank the Principal and Governing Council of
Bangalore Institute of Technology for the facilities provided and VTU for financial support.
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Figures:
(003)
(006)
(012)
(015)
d
c
b
a
0
20
40
60
2θ (degree)
Fig.1 XRD pattern of different LDHs. (a) ZnꢀAl (b) MgꢀAl (c) Cu–Al (d) CoꢀAl
d
c
b
%T
a
4000
3500
3000
2500
2000
1500
1000
500
Wave number (cm-1)
Fig. 2. FTꢀIR Spectra of different LDHs (a) ZnꢀAl,(b) MgꢀAl, (c) CuꢀAl, (d) CoꢀAl

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Glycerol (MW)
5-membered
6- membered
80
70
60
50
40
30
20
10
0
(a)
0
20
40
60
80
100
120
Reaction time (minutes)
70
60
50
40
30
20
10
0
Glycerol (CH)
5-membered
6-membered
(b)
0
240
480
720
960
1200
1440
Reaction time (minutes)
Fig.3 Acetalization of Gly with CA under (a) microwave (MW) and (b) conventional (CH)
heating. Reaction condition: catalyst amount, 1 g; mole ratio Gly: CA, 1:3; temperature,
1400C.

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80
70
60
50
40
30
20
10
0
Glycerol
5-membered
6-membered
Zn-Al
Mg-Al
Cu-Al
Co-Al
Fig.4 Acetalization of Glycerol with cinnamaldehyde(CA) with different LDH catalysts.
Reaction condition: reaction temperature, 140oC; mole ratio Gly:CA, 1:3; catalyst amount, 1g,
reaction time, 60min; (MW)
(003)
(006)
(012)
(015)
c
b
a
10
20
30
40
50
60
2θ (degree)
Fig.5 XRD pattern of (a) asꢀsynthesized LDH (b) Calcined LDH (c) RHꢀMgꢀAl LDH

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80
70
60
50
40
30
20
10
0
Glycerol conversion (%)
% yield of 5-membered
% yield of 6-membered
Mg-Al
Calcined Mg-Al
RH Mg-Al
Fig.6. Acetalization of glycerol with different catalysts.
Reaction condition: reaction temperature, 140oC; mole ratio Gly:CA, 1:3; catalyst amount, 1g, reaction
time, 60min; (MW).
300
Mg-Al LDH
250
200
150
100
50
0
400
600
800
1000
Temperature (degree)
Fig.7 TPD CO2 profile of MgꢀAl LDH

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250
200
150
100
50
RH-Mg-Al LDH
0
400
600
800
1000
Temperature (oC)
Fig.8 TPD CO2 of RHꢀMgꢀAl LDH

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Tables:
Table.1 Basicity of as synthesized and modified LDH.
Catalyst Basicity density (µmol/g) % Conversion of
of Gly
112
124
105
98
46
56
42
39
ZnꢀAl
MgꢀAl
CuꢀAl
CoꢀAl
Table.2. Acetalization of glycerol with various aldehydes.
Carbonyl Compounds Conversion (%) 1,3ꢀdioxalane
(% Yield)
1,3ꢀdioxane
(% Yield)
Cinnamaldehyde
Benzaldehyde
Anisaldehyde
chloroꢀbenzaldehyde
nitroꢀbenzaldehyde
77
56
44
52
59
71
51
34
43
49
6
8
10
9
7
Reaction condition: reaction temperature, 140oC; mole ratio Gly : CA, 1:3; catalyst amount, 1g,
reaction time, 60min; RHꢀMgꢀAlꢀLDH, MW

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Schemes:
O
OH
OH
H
O
H
LDH
H
O
+
O
O
MW
+
OH
OH
Cinnamaldehyde
Gycerol
OH
{2-[-2-phenylethenyl]-1,3-dioxolan-4-yl}methanol
2-[-2-phenylethenyl]-1,3-dioxan-5-ol
Scheme.1: Acetalization reaction of glycerol with cinnamaldehyde.
O
H
R
H
R
OH
-
O
OH
OH
OH
O
LDH
O
O
-
R
O
H
OH
OH
OH
O
OH
H
R
O
O
O
H
O
O
R
H
R- C6H5C2H2 group
H
H
OH
Scheme.2 Possible reaction mechanism of LDH catalyzed acetals formation