Effect of vitamin C template on morphology and structure of alumina: emerging application in enantiomer separation

Article abstract of DOI:10.1007/s11696-019-00744-7

Mesoporous nano-aluminas were prepared using aluminium isopropoxide, vitamin C (VC as a chiral template and a co-structural directing agent with molar ratios of 50:50 and 90:10) and water. These compositions when dried at 120?°C and calcined at 250, 350 and 500?°C for 8?h under air atmosphere transformed to boehmite and γ-alumina depending on the concentration of VC. The influence of vitamin C and its degradation products on the morphology and texture of alumina was investigated by the means of XRD, FT-IR, SEM, EDX, TEM, TGA, DSC, BET and N₂ adsorption–desorption isotherm. Mesoporous aluminas possess excellent characteristics such as large surface area (ca. 394?m²/g), 0.4?cm³/g pore volume and narrow pore size distributions that can be synthesized through a facile and green procedure. In addition, the enantiomers of propranolol hydrochloride were successfully resolved from its racemate using Al₅₀VC₅₀T₃₅₀ (Al_{Molar ratio of the Aluminium isopropoxide}, VC_{Molar ratio of the Vitamin C,}T_Temperature) (R_s = 2.5) as chiral stationary phase. The synthesis of chiral stationary phase was accomplished in a green, facile and inexpensive procedure that is of paramount importance in food and pharmaceutical industries.

Full text of DOI:10.1007/s11696-019-00744-7

Chemical Papers
https://doi.org/10.1007/s11696-019-00744-7
ORIGINAL PAPER
Efect oꢀ vitamin C template on morphology and structure oꢀ alumina:
emerging application in enantiomer separation
1
1
2
2
Hossein A. Dabbagh · Mahdieh Mozafari Majd · Fahimeh Bahrami · Ali Gholami
Received: 17 March 2018 / Accepted: 9 March 2019
©
Institute of Chemistry, Slovak Academy of Sciences 2019
Abstract
Mesoporous nano-aluminas were prepared using aluminium isopropoxide, vitamin C (VC as a chiral template and a co-struc-
tural directing agent with molar ratios of 50:50 and 90:10) and water. These compositions when dried at 120 °C and calcined
at 250, 350 and 500 °C for 8 h under air atmosphere transformed to boehmite and γ-alumina depending on the concentration
of VC. The inꢀuence of vitamin C and its degradation products on the morphology and texture of alumina was investigated
by the means of XRD, FT-IR, SEM, EDX, TEM, TGA, DSC, BET and N adsorption–desorption isotherm. Mesoporous
2
2
3
aluminas possess excellent characteristics such as large surface area (ca. 394 m /g), 0.4 cm /g pore volume and narrow
pore size distributions that can be synthesized through a facile and green procedure. In addition, the enantiomers of pro-
pranolol hydrochloride were successfully resolved from its racemate using Al VC T (Al ,
Molar ratio of the Aluminium isopropoxide
5
0
50 350
VC
T
) (R =2.5) as chiral stationary phase. The synthesis of chiral stationary phase was accom-
Molar ratio of the Vitamin C, Temperature s
plished in a green, facile and inexpensive procedure that is of paramount importance in food and pharmaceutical industries.
Keywords Alumina · l-Ascorbic acid · Vitamin C · Template synthesis · Chiral separation · Propranolol hydrochloride
Introduction
pore size and high surface area by applying templates. Jiao
and his co-workers used the mixture of cationic and anionic
surfactant as template for synthesizing organized γ-alumina
(Jiao et al. 2012). The pore volume of the γ-alumina pre-
Polymorphic aluminium oxides, generally known as alu-
mina, have paramount importance because of their stabil-
ity, high surface area and being inexpensive in a variety of
industrial applications including pharmaceuticals, catalysts,
ceramics, etc. (Boumaza et al. 2009; Murali and Thirumoor-
thy 2010; Sanchez-Valente et al. 2004), as well as adsorp-
tive material in separation processes (Čejka 2003; Janoso-
vits et al. 1997; Morterra and Magnacca 1996; Oberlander
3
pared in this research was 0.79 cm /g, and its pore size was
12.1 nm, which represented excellent textural properties of
this γ-alumina. Dabbagh et al. deployed alanine as a tem-
plate to synthesize γ-alumina with a speciꢁc surface area
2
3
of 422 m /g and a pore volume of 0.59 cm /g (Dabbagh
et al. 2011). Dabbagh and Shahraki synthesized mesoporous
nano-rod-shaped γ-alumina with narrow pore size distribu-
tion by utilizing phenol formaldehyde polymeric resin as a
template and aluminium isopropoxide as a precursor (Dab-
bagh and Shahraki 2013). This method led to γ-alumina with
1
985).
In order to synthesize mesopore and macropore alumi-
nas, several methods have been devised to achieve diverse
2
large surface area (ca. 382 m /g), large pore volume and
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11696-019-00744-7) contains
supplementary material, which is available to authorized users.
narrow pore size distributions.
Herein, aluminas were prepared using vitamin C as a
chiral template. A question was raised here: Is it possible
to make asymmetric chiral alumina after total or partial
elimination of vitamin C? According to the work by Raman
et al., elimination of the template from central structure can
lead to morphological and/or stereo-chemical characteristics
related to those of the template (Raman et al. 1996). There-
fore, it is expected that the extraction of a chiral template
*
Hossein A. Dabbagh
dabbagh@cc.iut.ac.ir
1
2
Catalysis Research Laboratory, Department of Chemistry,
Isfahan University of Technology, Isfahan 8415483111, Iran
Department of Analytical Chemistry, Faculty of Chemistry,
University of Kashan, Kashan, Iran
Vol.:(0
1
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Chemical Papers
like l-ascorbic acid produces a chiral mesoporous or nano-
porous material.
Experimental
To examine this, we have used the prepared aluminas as
chiral stationary phase and the stock solution (1 mg/mL)
of propranolol hydrochloride was passed over them using
the high-performance liquid chromatography. Propranolol
hydrochloride is β-adrenergic antagonists used clinically
as the racemate. However, these enantiomers have diꢂer-
ent pharmacological properties and the S-(–) isomer con-
tains the β-adrenergic blocking activity (Barrett and Cullum
Materials
Aluminium isopropoxide (AIP), >98.0%, was supplied by
Merck, and l-(+)-ascorbic acid (99%) from Acros Organics
Co. They deployed on arrival. The deionized double distilled
water was prepared as the solvent in all procedures.
All reagents and solvents used for enantiomer resolution
were HPLC or analytical grade. Acetonitrile and methanol
of HPLC grade were supplied by Merck. Propranolol hydro-
chloride reference standard (RS) was purchased from USP
(United States Pharmacopeia).
1
968). Since pure enantiomers achieve more eꢂective treat-
ment, much eꢂort has been made to develop methods that
resolve enantiomers and determine their contents eꢃciently.
In order to resolve enantiomers, high-performance liquid
chromatography (HPLC) with chiral stationary phase (CSP)
is utilized. Cellulose chiral stationary phases and macro-
cyclic antibiotics chiral stationary phases are two kinds of
CSPs used frequently for propranolol enantioseparation.
These chiral stationary phases suꢂer from limited applica-
tion in normal phase and high price (Ren-Dan et al. 2014).
To the best of our knowledge, this is the ꢁrst publication
which deploys vitamin C to synthesize mesoporous chiral
alumina and also investigated its potential characteristic
as chiral stationary phase in enantiomer resolution. Hav-
ing uniform pore structures, large surface areas and narrow
pore size distributions make it an ideal candidate in a wide
range of catalytic processes such as adsorbents, catalysts,
high-performance ceramics and catalysts supports. Due to
its potential for resolving racemic mixture, it can be used as
a new inexpensive home-made chiral stationary phase for
propranolol enantioseparation.
Synthesis of aluminas
Aluminium isopropoxide and l-ascorbic acid were dissolved
in an excess amount of deionized distilled water (H
O/
2
Al=10) with the molar ratios of 50:50 and 90:10, respec-
tively, and stirred at room temperature for 1 h. The alcohol
and excess water were removed at 120 °C to produce alu-
minium hydroxide powders.
The powders were divided into three samples. Each
part was placed in the furnace at 250, 350 and 500 °C
for 8 h, respectively. These samples were assigned as
Al
, VC
Molar ratio of the Vitamin C
Molar ratio of the Aluminium isopropoxide
and T
. For example, the sample containing 90% alu-
Temperature
minium isopropoxide and 10% vitamin C calcined at 250 °C
was named Al VC T
.
10 250
9
0
The synthesis diagram of the composites is shown in
Fig. 1.
Fig. 1 Synthesis diagram of the composites
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Chemical Papers
Characterization
separate enantiomers. These adsorbents were grinded ꢁnely
and sieved for assurance of uniform mesh size. Then, the
stainless-steel HPLC columns (150×4.6 mm column) were
packed with the ꢁne composite. Chromatographic analysis
was carried out using a high-performance liquid chroma-
tography instrument (Younglin HPLC YL9000 series) with
YL9110 Pump and with autochrome 3000 software and
UV–Visible detector YL9120. The ꢁltering of mobile phase
was performed by a 0.45-μm pore size ꢁlter (Merck Milli-
pore, USA) and then passed through vacuum to be degassed.
The elutions were carried out at ambient temperature with
UV detection at 290 nm. The monograph calls for 20 µL of
sample are injected with isocratic chromatographic separa-
tion using the mobile phase as prepared above at a ꢀow rate
of 1.0 mL/min. The resulting retention times indicated the
potential chirality of the column.
The phase development of the samples was conducted by
XRD technique (Philips Xpert instrument) with the CuKα
radiation source operating at 40 kV, 30 mA, and was scanned
on an angular step of 0.05° over a 2Ө range between 10 and
1
00°.
FT/IR-680 plus spectrometer manufactured by JASCO
provided the FT-IR spectra ranged from 400 to 4000/cm.
The SEM micrographs of the nano-composite powders
were carried out through a scanning electron microscope
(
SEM-Philips XL 30, LEO 1430VP-Carl Zeiss & Tescan
Vega-3 LMU). The ampliꢁcation of secondary electron sig-
nals was achieved by covering the samples with gold.
Energy-dispersive X-ray spectroscopy (EDX-Tescan
Vega-3 LMU) was deployed to characterize the chemical
composition of synthesized composites.
The stock solution (1 mg/mL) was prepared by dissolving
an accurately weighted amount of USP propranolol hydro-
chloride in minimum methanol and diluted with water. The
optimized mobile phase composed of n-hexane/ethanol/DEA
(70/30/0.3, v/v/v) was achieved at a ꢀow rate of 1.0 mL/min
and 25 °C. This solution (n-hexane/ethanol/DEA) was mixed
and ꢁltered (≤0.5 μm porosity).
The TEM images were taken via transmission electron
microscope (TEM-LEO-912AB, Carl Zeiss) to study par-
ticle size and structure of synthesized products, operating
at accelerating voltage of 80 kV. Prior to TEM imaging, the
samples dispersed in ethanol were placed on a carbon ꢁlm
supported by Cu mesh grid.
STA system, model 409 PC Luxx, was utilized to perform
thermal analysis including TGA and DSC for samples under
a ꢀow of air at a rate of heating of 20 up to 1100 °C/min.
Nitrogen adsorption–desorption isotherms were obtained
at − 196 °C using a BELSORP Mini II apparatus (BEL
Japan company). Prior to any measurements, all the sam-
ples were passed through a ꢀow of dry nitrogen at 150 °C
for 5 h. The pore volume and speciꢁc surface area of the
samples were calculated with the help of the multipoint
Brunauer–Emmett–Teller (BET) procedure where the
The most important parameter, which is considered in
this section, is resolution (R ). Resolution in chromatography
s
refers to the ability of column in separating two peaks. It is
deꢁned as follows:
2ꢀ
ꢁ
t
− t
R1
R2
Rs = ꢀ
ꢁ ,
W + W
1
2
where t and t are the retention time of peak 1 and peak 2.
R
R
2
1
W and W are width of peak 1 and peak 2.
1
2
relative pressure (P/P ) was 0.98. The pore size distribu-
In this case, these two peaks are the peaks of R- and
0
tions were determined by the Barrett–Joyner–Halenda
S-enantiomers of 2-propranolol. Usually, when R is greater
s
(
BJH) method using the adsorption branch of the nitrogen
than 1, it indicated the good separation of peaks.
Typical diagram for enantiomer separation using HPLC
is shown in Fig. 2.
isotherms.
Chromatographic analysis was carried out using a high-
performance liquid chromatography instrument (Young-
lin HPLC YL9000 series) with YL9110 Pump and with
autochrome 3000 software and UV–Visible detector
YL9120. The ꢁltering of mobile phase was performed by
a 0.45-μm pore size ꢁlter (Merck Millipore, USA) and then
passed through vacuum to be degassed.
Results and discussion
l-ascorbic acid possesses six Lewis base sites, including
a lactonic oxygen, C=O group and four hydroxyl groups.
There is a possibility for each of six basic sites to interact
with acidic sites through a covalent bond (coordinate cova-
lent bond) as well as strong hydrogen bonding (quasi-cova-
lent bond) in alumina (both clusters of aluminium hydrox-
ide and aluminium oxyhydroxide), resulting in an acid–base
complex. More research is needed to rationalize the role of
the template in the formation of pore system in each speciꢁc
case. Here, l-ascorbic acid interacts with alumina via each of
The application of synthesized alumina
in enantiomer separation
In this work, the synthesized Al VC T , Al VC T ,
50 500
5
0
50 350
50
Al VC T and Al VC T composites were evaluated
9
0
10 350
90
10 500
for enantioselective separation of propranolol hydrochloride.
For this purpose, these composites were used as station-
ary phase in a high-performance liquid chromatography to
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Chemical Papers
Fig. 2 Typical diagram for enantiomer separation using HPLC
six basic sites and/or acidic hydrogens, having the minimum
distance to the alumina surface, and interacts and/or forms
bond with alumina species where mesopores and their size
are originated.
equation, respectively (Ibrahim and Abu-Ayana 2009)
(Table 1). The phase analyses patterns of samples cal-
cined at diꢂerent temperatures of 250, 350 and 500 °C are
shown in supplementary Fig. S1 (Electronic Supplementary
Information).
X‑Ray powder diꢀraction
FT‑IR spectra
The XRD pattern was used to study the structure of com-
posites and the grain size of crystals as shown in Fig. 3. The
XRD patterns of Al VC (Fig. 3a) reꢀect completely amor-
The FT-IR spectra of Al/vitamin C composites were used
to further characterize alumina–VC transitions at diꢂerent
calcination temperature (Figs. 4, 5). The FT-IR spectrum
of Al VC calcined at 120 °C (Fig. 4) shows a footprint
50
50
phous properties at all temperatures. Probably, the excess
VC prevents the formation of a crystalline pattern. The broad
diꢂraction peaks of these samples lead to small particle sizes
and poor crystallinity. This eꢂect was reduced for Al VC .
5
0
50
of vitamin C at 1700/cm, corresponding to the stretching
vibrations of C=O (Panicker et al. 2006). This indicates that
heating the mixture to values higher than 120 °C incorporate
and/or degrade the vitamin C in the synthesized compos-
ites. The broadband at 3350–3650/cm is attributed to the
stretching vibration of the OH-group-adsorbed molecular
90
10
The nano-size nature of these composites reveals broad dif-
fraction peaks (Potdar et al. 2007). The characteristics of
XRD patterns of Al VC (Fig. 3b) conꢁrm γ-Al O at
9
0
10
2
3
calcination temperatures of 350 and 500 °C (according to
JCPDS card number 01-075-0921), AlO(OH) (boehmite)
at 120 °C and Al O .H O at 250 °C. The composite heated
H O (Bowen et al. 1990; Zhang and Binner 2002). The spec-
2
tra of sample calcined at 120 and 250 °C show increased
intensity of the new bands at 1500–1700/cm. These bands
are attributed to coordinative bond between physically
adsorbed water molecules and incompletely coordinated
aluminium ions on the oxide surface (Vlaev et al. 1989).
These bands were reduced at 350 and 500 °C. Characteristic
bands of boehmite are represented in the spectra of sample
calcined at these temperatures. As the peak at 1068/cm is
2
3
2
at 120 °C somewhat resembles boehmite structure (reduced
crystallinity) with an orthorhombic unit cell (JCPDS card
number 01-083-2384). This sample calcined at 250 °C
shows transition of AlO(OH) to Al O .H O (JCPDS card
2
3
2
number 00-001-0774). The average size of crystallite was
calculated as 5.20, 3.95, 2.38 and 4.33 nm for Al VC T
,
10 120
9
0
Al VC T , Al VC T and Al VC T by Scherrer
9
0
10 250
90
10 350
90
10 500
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Chemical Papers
Fig. 3 XRD patterns of alumi-
nas samples, a Al VC and b
50
50
Al VC
10
9
0
Table 1 Crystal sizes of the
composites calculated by
Scherrer equation
regarded as the Al–OH–Al bending vibration, it corresponds
to the crystallinity of the boehmite (Barzegar-Bafrooei and
Ebadzadeh 2011) However, this peak is not very obvious
at 250 °C. The bands from 400 to 750/cm are related to the
Sample
Scherrer
equation/
nm
Al VC T
10 120
5.20
3.95
2.38
4.33
9
0
bending and stretching vibration modes of AlO . The spectra
6
Al VC T
10 250
9
0
of calcined powders at temperatures 350 and 500 °C conꢁrm
the presence of γ-alumina (Dabbagh et al. 2010; Petrovic
et al. 2003) in accordance with developing peaks between
Al VC T
10 350
9
0
Al VC T
10 500
9
0
5
00 and 1000/cm. The peak in the range 500–650/cm was
ascribed to υ-AlO , and the peak in the range of 700–900/cm
6
was attributed to υ-AlO (Urretavizcaya et al. 1998). These
4
Fig. 4 FT-IR spectra of
Al VC composites and VC
50
50
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3

Chemical Papers
Fig. 5 FT-IR spectra of
Al VC composites and VC
90
10
results indicate the presence of tetrahedral and octahedral
structures in γ-alumina phase. However, these results do not
comply with those XRD.
SEM and TEM studies
SEM and TEM analyses were recorded to characterize mor-
phologic changes and the sizes of the synthesized samples.
The SEM images of mesoporous aluminas for Al VC T
FT-IR spectrum of Al VC (Fig. 5) at 120 °C similar
9
0
10
to Al VC shows smaller footprint of vitamin C at 1700/
5
0
50
50
50 350
cm, corresponding to the stretching vibrations of C=O. This
trace shows a steady decrease at higher temperatures. The
stretching vibration of the OH group connected to Al–OH
are depicted in Fig. 6. The SEM images of mesoporous alu-
minas for Al VC T and Al VC under thermal treat-
5
0
50 350
90
10
ment at diꢂerent temperatures of 120, 250, 350 and 500 °C
are shown in Figs. S2 and S3 in (Electronic Supplementary
Information). Due to topotactic nature of the solid transi-
tion of boehmite to alumina, the morphology of boehmite is
kept identical to the morphology of γ-alumina (Volpe and
Boudart 1985). Since the aluminium isopropoxide was dis-
solved in a polar solvent like water, the spherical particles
could not be attained (Liu et al. 2009). Elongated and ꢀat-
shaped particles with a porous surface are witnessed in the
mesoporous alumina powders, in which interparticle voids
are discernible in SEM micrographs. The platy crystallites
with small elongated pores coalesced in a particular shape.
group and that of adsorbed molecular H O and/or VC
2
hydroxyl groups appear as the broadband at 3350–3650/cm.
The peak at 1067/cm is characteristic band revealing the
crystallinity of the boehmite and is represented in the spectra
of sample calcined at 120 °C attributed to the Al–OH–Al
bending vibration. This peak is not observed at 250 °C,
since in accordance with XRD result, it is in Al O ∙H O
2
3
2
form not AlO(OH). The advent of alumina at temperatures
3
50 and 500 °C occurs as this peak disappears in the cal-
cined samples. These ꢁndings match with those of XRD.
As mentioned for Al VC , the bands from 400 to 750/cm
5
0
50
are related to the bending and stretching vibration modes
of AlO . Formation of γ-alumina is carried out by dehy-
6
droxylation due to calcination of boehmite. The spectra of
calcined powders at temperatures 350 and 500 °C conꢁrm
the presence of γ-alumina in accordance with the develop-
ing peaks between 500 and 1000/cm. The peak in the range
5
00–650/cm was ascribed to υ-AlO , and the peak in the
6
range of 700–900/cm was attributed to υ-AlO . These results
4
indicate the presence of tetrahedral and octahedral structures
in γ-alumina phase.
The ꢁndings of TGA and DSC analysis strongly comply
with those of the above-mentioned results, leading to sig-
niꢁcant water loss at 250–600 °C, which will be explained
in detail in the corresponding section. In addition, absorp-
tion patterns are in agreement with the results of previous
research (Dabbagh and Zamani 2011; Gandhi et al. 2010;
Kim et al. 2010; Priya et al. 1997).
Fig. 6 SEM images of Al VC T350
5
0
50
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Chemical Papers
As the vitamin C molar ratio increased, the corrugations on
the external surface of agglomerates got rather rough, lead-
ing to a smaller surface area. Also, the surface structure is
maintained with the less porous surface.
and 20 nm widths. The closely packed platelet-like particles
are with the mean diameter of 20 nm. In Al VC sample
5
0
50
at 500 °C (Fig. 7b), the rod-like crystallites with a length of
roughly 300 nm and width of 100 nm are clearly discernible
and the aggregates of overlapping plates are at higher mag-
niꢁcation. For Al VC sample (Fig. 7c), the crystallites
The chemical compositions of Al/VC products were iden-
tiꢁed by EDX analysis (Table 2). These analyses indicate
that the remains of vitamin C destruction products disap-
peared as the temperature increased. The EDX patterns of
mesoporous aluminas at diꢂerent temperatures are shown
in Figs. S4 and S5 (Electronic Supplementary Information).
The TEM micrographs (Fig. 7) show the morphology of
mesoporous alumina for Al VC at 350 °C and 500 °C
9
0
10
resemble the morphology of agglomerates of closely packed
platelet-like particles and ink-bottle pores.
Thermal gravimetric analysis
The thermal gravimetric behaviour of the boehmite was uti-
lized to gather information about the pattern of VC loss.
The TGA–DSC curves of as-prepared Al/VC composite for
Al VC and Al VC are presented in Fig. 8. The ther-
5
0
50
and for Al VC under a calcination temperature of 500 °C.
90
10
Both rod-like and closely packed platelet-like particles in
Al VC sample at 350 °C (Fig. 7a) could be observed. The
50
50
50
50
90
10
dimension of rod-like crystallites is about 100 nm length
mogram for Al VC (Fig. 8a) demonstrates the weight
50 50
Table 2 Relative percentage
T/ °C
Al VC
Al VC
5
0
50
90
10
of chemical composition of
the synthesized composites
calculated by EDX (all data
are reported in terms of mass
fraction)
Carbon
Oxygen
Aluminium
Carbon
Oxygen
Aluminium
120
250
29.22
12.22
3.26
58.70
71.51
72.89
75.06
12.08
16.27
23.85
22.13
5.87
3.34
1.90
1.60
72.84
70.13
71.32
67.82
21.29
26.53
26.78
30.58
3
5
50
00
2.81
ink-bottle pores
(a)
(b)
(c)
Fig. 7 TEM images of a Al VC T , b Al VC T , c Al VC T
10 500
5
0
50 350
50
50 500
90
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Chemical Papers
Fig. 8 TG–DSC curves of the as-prepared Al/vitamin C composites: a Al VC T , b Al VC T
10 120
5
0
50 120
90
loss occurred in three stages. The initial weight loss (from
room temperature to 250 °C) was related to a major loss of
adsorbed water (associated with the endothermic peak at
α-Al O (Bokhimi et al. 2001; Jayaseelan et al. 1998). There
2 3
was no signiꢁcant weight loss for calcination temperatures
above 700 °C, which implies fully elimination of all organic
materials, and the stable residue can be determined as pure
single-phase α-alumina.
1
36 °C in the DSC curves) and vitamin C decomposition
(
Ergu et al. 2008; Ibrahim and Abu-Ayana 2009; Juhász et al.
2
012; Sarkar et al. 2007). Second weight loss was observed
The thermogram for Al VC (Fig. 8b) shows similar
9
0
10
in the range of 250–400 °C which corresponds to an exo-
thermic peak at about 300 °C (in DSC curve) due to the
dehydration of the aluminium hydroxides and continuation
of vitamin C decomposition. The most noticeable weight
loss, between 400 and 700 °C, corresponds to an endother-
mic peak in the range of 550–650 °C associated with the
removal of organic residues, dehydroxylation and forma-
tion of alumina and water residue vaporization (Othman and
Sahadan 2006). The broad exothermic peak between 700 and
three stages of weight loss. The ꢁrst weight loss (from room
temperature to 200 °C) was ascribed to the elimination of
physically and chemically adsorbed water (associated with
the endothermic peak at 102 °C in the DSC curves) and vita-
min C decomposition. The second weight loss was observed
in the range of 200–400 °C, and the continuation of vitamin
C decomposition, which corresponds to an exothermic peak
at about 386 °C (in DSC curve), was a result of the dehy-
dration of the aluminium hydroxides. The next weight loss,
between 400 and 650 °C, follows the same trend described
1200 °C indicates the transformation of transition alumina to
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Chemical Papers
in the third stage of weight loss as in Al VC . The pattern
calcination temperatures of 250, 350 and 500 °C for 8 h. It
was expected that the surface area increases by templating
5
0
50
of curves in Al VC samples was similar to Al VC sam-
90
10
50
50
ples in which there was no observation of paramount weight
loss for temperatures higher than 800 °C and the residue
was identiꢁed to be pure α-Al O . A total weight loss of
vitamin C. The N adsorption–desorption isotherms, corre-
2
sponding BJH pore size distribution curves and BET plots
of mesoporous alumina are recorded in Fig. 9. Relying on
the IUPAC classiꢁcation of physisorption isotherms, the
isotherm of all samples is classiꢁed as type IV. However,
the characteristic features of these samples vary in terms of
its hysteresis loop. Generally, type IV isotherms are related
to complex pore interconnectivity. Many characteristics of
mesoporous materials are observed in this type of isotherm
where capillary condensation occurs in mesopores (Sing
2
3
8
6.06 and 53.70% was observed for Al VC and Al VC
50 50 90 10
during the thermal process, respectively. The excess weight
loss for Al VC is because of more vitamin C content. The
50
50
maximum decomposition rate of vitamin C is 191–221 °C
(
Juhász et al. 2012).
In the presence of vitamin C, it was expected to see the
endothermic peak around 197 °C (melting point of vitamin
C). The absence of this peak in thermograms corroborates
the decomposition of vitamin C before 197 °C as mentioned
in FT-IR section. Clearly, these ꢁndings are consistent with
the IR results, as it shows the decomposition of vitamin C
below 200 °C and conꢁrms the formation of amorphous
γ-alumina by dehydroxylation due to calcination of boehmite
in accordance with second weight loss observed in the range
of 200–400 °C.
1985). The N adsorption hysteresis loops are indications
2
of textural porosity (Tanev and Pinnavaia 1996). Al VC
5
0
50
samples at 250 and 350 °C (Fig. 9a, green and blue curves)
exhibit a type IV isotherm with H3-type hysteresis loop.
The appearance of type H3 hysteresis loops is attributed to
plate-like particles slit-shaped pores. These results resem-
ble greatly with as-prepared composites in their mesoporous
appearance. The observed isotherm for Al VC at 500 °C
5
0
50
is categorized as type IV isotherm with H2-shaped hyster-
N adsorption–desorption isotherm
esis loops (Fig. 9a, red curve). In Al VC samples at all
2
90
10
temperatures, the isotherm curves (Fig. 9b) show a type IV
with H2 hysteresis loops, which implies to ink-bottle pores.
The BET speciꢁc surface area, the pore volumes and
the average pore diameters measurements of the Al/VC
Brunauer–Emmett–Teller (BET) analysis was performed
to evaluate the precise speciꢁc surface area, the pore size
distribution and porous nature of the Al O samples at
2
3
Fig. 9 N adsorption–desorp-
2
tion isotherms of a Al VC
50
50
and b Al VC
10
9
0
1
3

Chemical Papers
composites in the molar ratios of 50:50 and 90:10 at cal-
cination temperatures of 250, 350 and 500 °C are listed in
Table 3. In the synthesis of Al O process, the data show that
High‑pressure liquid chromatography
The chirality of the columns was investigated for propranolol
hydrochloride. On ꢁrst successful attempt with the acetoni-
trile as mobile phase, the results showed the separation of
enantiomers using Al VC T as stationary phase. The
2
3
as the proportion of vitamin C was decreased, the S
was
BET
2
increased, ranging from 4.9 to 480.5 m /g. This was related
to the fact that the internal spaces of the pores are ꢁlled
with vitamin C. This assumption is justiꢁed by a decrease
in the pore volume. Regarding the Al VC T , the largest
5
0
50 350
chromatogram (Fig. 10a) showed two peaks for R- and S-iso-
mers with R (resolution) about 1 (Table 4), conꢁrming the
9
0
10 350
s
2
surface area for γ-Al O is calculated to be 394 m /g. The
chirality of this composite. However, the band broadening
(and hence low eꢃciency) indicated the necessity of opti-
mization of the process. The chromatograms of other com-
posites showed only one peak, indicating no separation for
propranolol hydrochloride enantiomers (Figs. 10b, c and d).
Optimization eꢂorts resulted in mobile phase composed
of n-hexane/ethanol/DEA (70/30/0.3, v/v/v) at a ꢀow rate
of 1.0 mL/min and 25 °C. This solution (n-hexane/ethanol/
DEA) was mixed and ꢁltered (≤0.5 μm porosity) (Fig. 11).
The chromatogram (Fig. 12) showed two peaks for R- and
2
3
average pore diameter of the Al/VC samples is in the range
of 2.7–6.8 nm range. This narrow pore size distribution is
of paramount value in catalytic and adsorption application.
Table 3 Textural properties of samples
2
Sample
SBET (m /g)
Pore size (nm)
Pore
volume
3
(
cm /g)
S-isomers with R (resolution) about 2.5 (Table 5), conꢁrm-
s
ing the chirality of Al VC T composite.
5
0
50 350
Al VC T
50 250
4.89
480.5
6.67
393.77
136.21
316.12
5.81
2.69
6.75
3.48
3.67
4.52
0.01
0.32
0.01
0.34
0.12
0.36
50
Validation of reproducibility for the columns packed with
Al VC T
10 250
90
Al VC T was performed via repeating the separation
5
0
50 350
Al VC T
50 350
50
of propranolol hydrochloride enantiomers on Al VC T
5
0
50 350
Al VC T
10 350
90
as CSP under the same separation condition (Fig. 12). It
Al VC T
50 500
50
is observed that the retention time, selectivity, resolution
Al VC T
10 500
90
Fig. 10 Chromatograms of resolution of propranolol hydrochloride enantiomers by HPLC using a Al VC T , b Al VC T500, c
5
0
50 350
50
50
Al VC T , d Al VC T as a chiral stationary phase and acetonitrile as mobile phase
90
10 350
90
10 500
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3

Chemical Papers
Table 4 Obtained results from
HPLC and the surface area
of the composite (n-hexane/
ethanol/DEA as mobile phase)
Racemate
Propranolol hydrochloride Al VC T
50 350
Composite
RT (min) Area (mV×s) Height (mV) RS
Pore size (nm)
0.85
816.2073
25.2729
25.7603
0.95 6.75
5
0
1
–
–
–
0.2333 18975.6777
1.3483 28183.7875
Al VC T
50 500
905.7872
–
–
–
3.67
3.48
4.52
5
0
–
–
Al VC T
10 350
1.7983 20106.1797
1546.9004
9
0
–
–
Al VC T
10 500
1.4567 63407.95
–
1379.1127
–
1
0
Fig. 11 Chromatograms of
resolution of propranolol hydro-
chloride enantiomers by HPLC
Al VC T as a chiral station-
50
50 350
ary phase and n-hexane/ethanol/
DEA as mobile phase
and the peak shape are kept well during the experiments,
conꢁrming the reliability of this home-made CSP (Table 6).
By considerations of the BET results, probably the pore
size of the composites dominated the ability to resolving
enantiomers. As Al VC T has larger pore size (6.75 nm)
50
50 350
in comparison with the other three (Table 3), it is successful
in enantioseparation. Also, our calculation with GAUSS-
IAN G09 showed that R- and S-enantiomers of 2-propran-
olol have diꢂerent molar volumes. The molar volume for
3
R-enantiomer is 190.943 cm /mol, and the molar volume
3
for S-enantiomer is 234.675 cm /mol. Therefore, one of the
enantiomers ꢁts better in the pore and the resolution occurs.
Above all, as shown in Fig. 13, propranolol hydrochloride
has aromatic group that is connected to an OCH group and
2
this group (OCH ) is connected to the chiral centre. Also,
2
this molecule is composed of hydroxyl and amino functional
groups. These functional groups have more interactions with
the surface leading to chiral separation.
In conclusion, Al VC T obtained in facile and green
5
0
50 350
procedure is successful in resolving propranolol hydro-
Fig. 12 Duplicate enantiomer recognition of propranolol hydrochlo-
chloride enantiomers. The potential in resolving β-blockers
ride enantiomers on Al VC T as CSP
5
0
50 350
1
3

Chemical Papers
Table 5 Obtained results from HPLC using Al VC T as CSP and
5
0
50 350
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