H.R. Prakruthi et al. / Journal of Molecular Catalysis A: Chemical 408 (2015) 214–220
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and hence, LDHs can behave as potential solid heterogeneous base
2.3. Characterization
catalysts for organic synthesis [1,20].
Microwave irradiation has been used for the synthesis of many
include more uniform and rapid heating resulting in reduced reac-
tion time for synthesis of chemicals. To the best of our knowledge,
such as, in the rehydration of LDHs has not received much attention
in the literature [23,24]. Moreover, glycerol is a very good absorber
of microwave radiations and hence, microwave could play a greater
role in the synthesis of GC. There are very few reports of LDHs as
catalysts for the synthesis of GC [1,14,15,25]. The main objective of
the present study is to investigate the effect of microwave heating
in the synthesis and rehydration of LDHs followed by evaluation of
the catalytic activity of the resulting LDHs in the transesterification
of glycerol with DMC.
The prepared samples were characterized using various tech-
niques, such as BET, XRD, FTIR, TGA and total basicity by benzoic
acid titration and temperature programmed desorption of CO2
(TPD- CO2). BET surface area measurements were carried out using
Quantachrome Nova-1000 surface analyzer under liquid nitrogen
temperature. X-ray diffraction (XRD) patterns were recorded by
Shimadzu MAXima X XRD-7000 X-ray diffractometer with step
scanning at 2ꢀ = 0.02 per step from 3◦ to 80◦ on graphite monochro-
matized Cu K␣ radiation (ꢁ = 1.5406 Å). XRD patterns were also
confirmed using files from the Joint Committee on Powder Diffrac-
tion Standards (JCPDS).
The total basicity of LDH samples were estimated by benzoic
acid titration method [12,29]. Suspension of 0.5 g of sample in 2 ml
toluene solvent was stirred for 30 m followed by addition of 2–5
drops of dilute phenolphthalein solution as indicator. The mixture
was titrated against 0.01 M benzoic acid in toluene. The benzoic acid
consumed was used for the calculation of total basicity. FTIR spectra
of the samples were recorded by KBr pellet method using Shimadzu
IR Affinity-1 spectrophotometer, in the range of 400–4000 cm−1
with 4 cm−1 resolution of 40 scans. The basicity of catalysts were
determined by temperature programmed desorption of CO2 (TPD-
CO2) in an indigenously built unit equipped with a furnace, quartz
U tube and thermal conductivity detector using helium as a carrier
gas. The catalyst sample was pre-treated in He flow at 100 ◦C for
1 h, and then cooled to room temperature prior to the adsorption
of CO2 at this temperature. After the adsorption of CO2 for 30 min
the sample was flushed with He for 1 h at 100 ◦C in order to remove
physisorbed CO2 from catalyst surface. The desorption profile was
recorded at a heating rate of 10 ◦C min−1 from 100 to 900 ◦C and the
evolved CO2 was monitored with a thermal conductivity detector.
2. Experimental
2.1. Catalysts and chemicals
Analytical reagent grade glycerol, dimethyl carbonate, N,N-
dimethyl formamide, Mg (NO3)2·3H2O, Zn (NO3)2·6H2O, Al
(NO3)3·9H2O, Cu (NO3)2·3H2O, Ni (NO3)2·6H2O, NaOH and Na2CO3
were procured from SD fine chemicals, India and used without
purification.
Zn–Al layered double hydroxides were prepared by the co-
precipitation method following the procedure described elsewhere
[26–28]. Zn (NO3)2·6H2O and Al (NO3)3·9H2O were dissolved in
distilled water with a Zn:Al molar ratio of 2:1(solution A). Solu-
tion A was mixed with a mixture of Na2CO3:NaOH molar ratio
3:7 (solution B) under vigorous stirring at ambient temperature
with controlled pH between 10 and 11. The resultant mixture was
allowed to age at 80 ◦C for 30 min under microwave heating. The
mixture was centrifuged and repeatedly washed with hot water
until the centrifugate was neutral. The solid separated was dried
at 120 ◦C for 12 h. Sample was designated as Zn–Al LDH. Similarly,
LDHs of Mg–Al, Ni–Al, Cu–Al were synthesized under microwave
irradiation.
Synthesized Zn–Al LDH sample was thermally treated in air at
430 ◦C for 3 h. This resulted in mixed oxide formation with the
loss of the layered structure. Rehydration of thermally treated cat-
alysts was carried out in different modes. One of the methods
of rehydration of thermally treated Zn–Al LDH catalyst sample
involved microwave irradiation. Five grams of thermally treated
sample were mixed with 200 ml (40 ml/g) of water and the mixture
was subjected to microwave irradiation at 100 ◦C with contin-
uous stirring for 1 h in presence of nitrogen atmosphere. The
irradiated mixture was cooled, centrifuged, the obtained solid
was separated and dried in hot air oven at 120 ◦C. In the proce-
dure adopted using mechanical stirring, five grams of thermally
treated LDH sample were mixed with 200 ml water in a beaker
and stirred at room temperature for 24 h. In the sonication pro-
cess of rehydration, thermally treated LDH (5 g) was rehydrated
in water under sonication for 2 h at 60 Hz. Vapour phase rehy-
dration of thermally treated catalyst sample was carried out
in a quartz fixed bed reactor. Catalyst was packed at the cen-
ter of the reactor between two plugs of quartz wool. Water
vapour carried by nitrogen gas at 130 ◦C was passed over the
sample for 7 h for rehydration to occur. The rehydrated solid
was cooled, taken out and dried in hot air oven. These were
designated as RH-MW, RH-MS, RH-US and RH-VP LDHs respec-
tively.
2.4. Catalytic tests
Catalytic tests were carried under microwave irradiation and
conventional heating modes. Microwave irradiation reactions were
carried out in a microwave lab station for synthesis ‘START-S’ model
milestone, Italy, having software which enables on-line control of
temperature of the reaction mixture with the aid of infrared sen-
sor by regulation of microwave power output. All the reactions
were carried out in a 50 ml glass vessel and the reaction mixture
was stirred with the help of in-built automatic magnetic stirrer
using teflon stirring bar. Reactor vessel was kept in such a way
that the reaction mixture was exactly in line with infrared sensor
which monitors the temperature. Variable power up to 1200 W was
applied by microprocessor controlled single magnetron system.
Glycerol (10 mmol, 0.92 g) and DMC (30 mmol, 2.7 g) were
mixed with the catalyst sample in a 50 ml reaction vessel. The reac-
tion mixture was subjected to microwave irradiation by initially
mixture was stirred with 5 ml of dimethyl formamide (DMF) sol-
vent (used in the separation of catalyst after the reaction from the
mixture) and then the used catalyst was filtered off. The reaction
mixture (Scheme 1) was analysed in Chemito GC-1000 gas chro-
matograph with TR-WAX capillary column (30 m length, 0.5 m
internal diameter and column thickness 0.32 mm) and attached
with flame ionization detector.
The catalytic activities of the rehydrated (RH) samples were also
found out. The reactions were also carried in conventional heating
and product formation was confirmed by procedure already given
for comparison.