Synthesis of α-aminophosphonates using a mesoporous silica catalyst produced from sugarcane bagasse ash

Article abstract of DOI:10.1039/c5ra23233c

A new green synthesis route is proposed for obtaining a mesoporous material using sugarcane bagasse ash (SCBA) as the silica source. The material obtained was denoted by SBA-16 and its mesostructure was characterized by low-angle X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nitrogen adsorption techniques. Sulfonic acid groups were introduced to the as-synthesized material, resulting in an acid catalyst denoted by SBA-16/SO₃H. The catalytic activities of SBA-16 and SBA-16/SO₃H were investigated in Kabachnik-Fields reactions, where α-aminophosphonate compounds were produced. The results show that both products can be considered as promising catalysts, where SBA-16/SO₃H showed a slightly better performance than SBA-16.

Full text of DOI:10.1039/c5ra23233c

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Synthesis of a-aminophosphonates using
a mesoporous silica catalyst produced from
Cite this: RSC Adv., 2016, 6, 23981
sugarcane bagasse ash†
Arthur F. Boza,a Vicente L. Kupfer,a Aline R. Oliveira,b Eduardo Radovanovic,a
Andrelson W. Rinaldi,a Joziane G. Meneguin,a Nelson L. C. Domingues,b
ac
Murilo P. Moisesad and Silvia L. Favaro
*
´
A new green synthesis route is proposed for obtaining a mesoporous material using sugarcane bagasse ash
(SCBA) as the silica source. The material obtained was denoted by SBA-16 and its mesostructure was
characterized by low-angle X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy,
scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nitrogen adsorption
techniques. Sulfonic acid groups were introduced to the as-synthesized material, resulting in an acid
catalyst denoted by SBA-16/SO3H. The catalytic activities of SBA-16 and SBA-16/SO3H were investigated
in Kabachnik–Fields reactions, where a-aminophosphonate compounds were produced. The results
show that both products can be considered as promising catalysts, where SBA-16/SO3H showed
a slightly better performance than SBA-16.
Received 4th November 2015
Accepted 2nd February 2016
DOI: 10.1039/c5ra23233c
www.rsc.org/advances
Im3m space group.6 The synthesis of this material is usually
performed under acidic conditions using a non-ionic surfac-
1 Introduction
tant, Pluronic F127, as a structure-directing agent and tetrae-
thylorthosilicate (TEOS) as the silica source.6–9 The cage
structure of SBA-16 has potential advantages for many appli-
cations in different elds, including catalysis,7a–c adsorption,7d,e
electronic devices,7f,g and drug delivery,7h and the successful
modication of mesoporous silica surfaces with organic func-
tionalities has been reported.10a–e In this context, sulfonic acid
groups have been reported as an efficient acid catalyst in
organic reactions, e.g. the synthesis of a-aminophosphonate.10e
The a-aminophosphonates belong to a class of compounds that
have biological activities, which has drawn attention from
researchers because of the vast variety of applications, such as
pesticides, enzyme inhibitors, antiviral agents, antibiotics,
drugs for the treatment of cancer and HIV proteases. Their
synthesis has received considerable interest due to their struc-
ture being similar to that of amino acids.11
Brazil has the largest area of sugarcane plantations in the world
and is a leading exporter of their main products – sugar and
ethanol. According to the available data, in 2014 Brazil pro-
cessed over 630 million tons of raw material, producing about
35.5 million tons of sugar and 28.3 billion liters of ethanol.1
Sugarcane bagasse ash (SCBA) is generated as a by-product
from power plants burning biomass. For each ton of sugar-
cane bagasse burnt, about 6.2 kg of SCBA is generated,2 which is
composed mainly of silica.3a Some researchers have reported
the use of this by-product in Portland cement,3a,b ceramic
materials4 and the synthesis of zeolites.5 Another use for this
cheap silica source is reported in this work, it is the synthesis of
SBA-16 type mesoporous silica.
Considering the porous materials, SBA-16 type mesoporous
silica (a member of the Santa Barbara Amorphous family) has
been reported with a cubic arrangement corresponding to the
The functionalization of environmentally friendly SBA-16
with sulfonic acid groups and the synthesis of a-amino-
phosphonates were investigated in this work.
a
´
Laboratory of Materials Chemistry and Sensors – LMSen, State University of Maringa
´
– UEM, 5790 Colombo avenue, 87020-900, Maringa, PR, Brazil. E-mail: slrosa@uem.
br; Tel: +55-44-30111393
bLaboratory of Catalysis and Biocatalysis Organic – LACOB, Federal University of the
2 Experimental section
2.1 Synthesis of SBA-16
Grande Dourados – UFGD, Km 12 Dourados Itahum road, 79.804-970, Dourados, MS,
Brazil
c
´
Department of Mechanical Engineering, State University of Maringa – UEM, 5790
Firstly, the sugarcane bagasse ash (SCBA) was collected from the
sugar–alcohol plant located in the region of Maringa City, Par-
´
Colombo avenue, 87020-900, Maringa, PR, Brazil
´
d
´
´
Federal Technological University of Parana – UFTPR, 635 Marcılio street, 86812-460,
´
ana, Brazil. The alkali process was carried out to improve the
Apucarana, PR, Brazil
extraction of silicon from the quartz structure. The SCBA was
placed in a horizontal furnace and heated at a rate of 20 ꢀC
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra23233c
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minꢁ1 from room temperature up to 600 ꢀC, and it was then
held at this temperature for 4 h (SCBA600). Furthermore, the
compositions (weight percents) in SCBA found by X-ray uo-
rescence (XRF) were 86.2% SiO2, 2.8% Al2O3, 1.6% P2O5, 2.4%
K2O, 1.9% TiO2, 1.5% CaO, 2.9% Fe2O3 and 0.7% of trace
elements, and in SCBA600 the compositions previously re-
ported were 92.0% SiO2, 1.6% Al2O3, 1.1% P2O5, 1.0% K2O, 0.6%
TiO2, 0.8% CaO, 1.5% Fe2O3, and 1.4% of trace elements.5 The
SCBA600 was used as the silica precursor to synthesize SBA-16,
where 4.0 g of SCBA600 were mixed with 6.0 g of NaOH (ratio of
1.5 w/w) to obtain a homogeneous mixture. Then, the mixture
was heated in a nickel crucible at atmospheric pressure at 550
ꢀC for 40 minutes. The resultant fused mixture was dissolved in
50 mL of deionized water (solution 1). Then, 4.0 g of surfactant
Pluronic F127 (EO106PO70EO106, EO ¼ ethylene oxide, PO ¼
propylene oxide, Sigma Aldrich) were dissolved in 120 mL of
HCl solution, 2 mol Lꢁ1, at room temperature. The mixture was
stirred at room temperature until a homogeneous gel solution
formed (solution 2). Aerwards, solution 2 was added to solu-
tion 1, and this was stirred moderately at room temperature for
20 h. The solid product was ltered, washed with deionized
water, and air-dried at 80 ꢀC for 4 h. Finally, the as-prepared
white powder was calcinatedꢀ in air at 550 ꢀC for 6 h, with
Fig. 1 General scope for SBA-16 and SBA-16/SO3H catalyzing the
Kabachnik–Fields reaction.
2.4 Characterization techniques
Low-angle X-ray diffraction (XRD) patterns were obtained using
Bruker D8-Advance equipment operated at 40 kV and 35 mA
ꢀ
˚
with Cu-Ka radiation, a wavelength of 1.5406 A, a 0.01 step size
and 2 s per step over the range 0.5ꢀ < 2q < 5ꢀ.
Nitrogen adsorption–desorption isotherms were measured
at ꢁ196 ꢀC on Quantachrome NOVA-1200E Surface and
PoroAnalizer equipment. The specic surface areas were eval-
uated using the Brunauer–Emmett–Teller (BET) method and
the pore size distributions were calculated using the Barrett–
Joyner–Halenda (BJH) method. Fourier transform infrared
(FTIR) spectra were recorded with Thermo Fisher Scientic
Nicolet IZ10 equipment in the range of 400–4000 cmꢁ1 using
the KBr disc method. Scanning Electron Microscopy (SEM)
images were obtained with Shimadzu SSX-550 Superscan
equipment and Transmission Electron Microscopy (TEM)
images were recorded using JEOL JEM-1400 equipment oper-
ated at 120 kV.
a heating ramp to reach 550 C of 1 C minꢁ1, to remove the
ꢀ
surfactant. The sample obtained was denoted as SBA-16.
2.2 Synthesis of the functionalized SBA-16 sulfonic groups
The anchor of the organic group on the mesostructured silica
surface occurred via a post-synthesis procedure.10a Initially, the
SBA-16 was functionalized with thiol (RSH) groups. This was
carried out under batch reaction conditions using a 150 mL
ask tted with a stirrer, a thermometer and a reux condenser
at 60 ꢀC. In a typical reaction, 1.0 g of SBA-16 was dissolved in 30
mL of toluene, and then 1.0 mL of 3-mercaptopropyl-
trimethoxysilane (MPTMS) was added. Aer stirring for 24 h,
the SBA-16 powder functionalized with thiol groups was recov-
ered, as described in section 2.1. Then, the thiol functionalized
SBA-16 was oxidized by slowly adding 30 mL of 30 wt% H2O2
solution dropwise over 24 h with moderate stirring at room
temperature. The solid product was recovered, as described in
section 2.1, and was denoted as SBA-16/SO3H.
The acid capacities of SBA-16 and SBA-16/SO3H were indi-
cated by temperature-programmed ammonium desorption
(NH3-TPD) and were carried out with AutoChem II 2920
equipment. The a-aminophosphonate products were
2.3 a-Aminophosphonate synthesis
Initially, the reactions were carried out varying the amount of
catalyst (0.003, 0.006, 0.008, 0.01 and 0.02 g). In a typical reac-
tion, 2.0 mmol of aniline, 2.2 mmol of benzaldehyde and 2.0
mmol of diphenylphosphite were mixed in a blank reaction and
in the presence of SBA-16 and SBA-16/SO3H in the amounts
indicated above.
Next, the aminophosphonates were synthesized in a similar
way, where 0.01 g of catalyst was used in the presence of
different solvents (toluene, dichloromethane, chloroform, THF,
ethanol and methanol). Finally, aer establishing the standard
Kabachnik–Fields reaction conditions, other a-amino-
phosphonates were synthesized following the same protocol,
but with benzaldehyde and aniline, as described in Fig. 1.
Fig. 2 Powder XRD patterns (a), FTIR spectra of the calcined SBA-16
and SBA-16/SO3H (b), and nitrogen adsorption/desorption isotherms
and pore size distribution of the SBA-16 sample (c).
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Table 1 Textural and structural properties of SBA-16a
Sample
d110 (nm)
a0 (nm)
SBET (m2 gꢁ1
)
W (nm)
Vt (cm3 gꢁ1
)
Vm (cm3 gꢁ1
)
Vm/Vt (%)
Dp (nm)
Ref.
SBA-16
SBA-16 STD
8.48
9.99
11.99
14.13
576
443
6.78
9.14
0.34
0.24
0.10
0.16
pffiffiffi
29
67
3.27
3.1
This work
8
a
d110: the plane spacing at Miller indices; a0: the cubic lattice parameter ða0 ¼ 2d110Þ; SBET: BET surface area; W: wall thickness
pffiffiffi
ðW ¼ 3a0=2 ꢁ DpÞ; Vt: total pore volume; Vm: micropore volume calculated using the BJH method; Dp: pore diameter.
Table 2 Performance with different catalyst amounts in the a-ami-
nophosphonate reactionsa
Yieldb (%)
Catalyst
Entry
(g)
Time (min)
SBA-16
SBA-16/SO3H
1
2
3
4
5
0.003
0.006
0.008
0.01
20
20
20
20
20
74
85
87
90
91
89
95
95
100
100
0.02
a
Reaction conditions: aniline (2.0 mmol), benzaldehyde (2.2 mmol),
and diphenyl phosphite (2.0 mmol) in 10 mL of solvent at room
temperature. b Isolated yields.
Table
3 Performance with different solvents in the a-amino-
phosphonate reactionsa
Fig. 3 SEM images (top) and TEM images (bottom) of SBA-16 showing
the characteristic plane for a cubic porous structure: (left) [111] and
(right) [100] directions.
Entry
Solvents
Yieldb (%)
1
2
3
4
5
6
Toluene
95
100
98
87
70
Dichloromethane
Chloroform
THF
Ethanol
Methanol
79
a
Reaction conditions: aniline (2.0 mmol), benzaldehyde (2.2 mmol),
diphenylphosphite (2.0 mmol), SBA-16 (0.01 g), solvent (10 mL), room
temperature, 20 minutes. b Isolated yields.
3 Results and discussion
3.1 Structure characterization and morphology
Fig. 2(a) shows the powder X-ray diffraction patterns of SBA-16
and SBA-16/SO3H, prepared as described previously.
Both show a single intense diffraction peak at 2q values of
1.01 and 1.04ꢀ, respectively, which belongs to the cubic phase of
the (1 1 0) plan. The slight shi to a higher 2q value is due to the
graing of sulfonic acid groups into the SBA-16 pores.12 From
Fig. 4 NH3-TPD curves for SBA-16 and SBA-16/SO3H.
2
Bragg’s law (nl ¼ 2d sin q) and the relationships (1/dhkl ¼ (h2 +
pffiffiffi
2
k2 + l2)/a0 ) and ða0 ¼ 2d110Þ between the cubic lattice
parameter (a0) and the plane distance (dhkl) at different Miller
indices (hkl), if the rst peak belongs to the (110) diffraction
peak of the cubic phase, the diffraction peaks should appear at
2q200 ¼ 1.433ꢀ and 2q211 ¼ 1.755ꢀ for SBA-16 and 2q200 ¼ 1.47ꢀ
and 2q211 ¼ 1.80ꢀ for SBA-16/SO3H. Our second diffraction
peaks in the experimental data match the theoretical results
very well with 2q ¼ 1.425ꢀ and 1.471ꢀ, respectively, and a third
characterized using 1H and 13C nuclear magnetic resonance
(NMR) spectroscopy, using CDCl3 as the solvent and a 300 MHz
Bruker AVANCE III HD spectrometer. Chemical shis are
expressed in parts per million (ppm) and coupling constants (J)
are reported in Hertz (Hz).
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Table 4 Synthesis of different a-aminophosphonates in the presence of SBA-16 and SBA-16/SO3H
Entry
Aldehyde
Amine
Product
SBA-16 (yielda/timeb)
SBA-16/SO3H (yielda/timeb)
1
100/20
100/20
2
3
90/30
92/30
75/45
93/30
98/30
78/45
4
5
87/30
93/45
90/30
95/30
6
a
Isolate yields (%). b Time reaction (min).
small peak for SBA-16/SO3H at 1.79ꢀ only. Therefore, the XRD
patterns of SBA-16 can be indexed as (1 1 0), (2 0 0), and (2 1 1)
reections, corresponding to a cubic Im3m structure.7h
Fig. 2(b) shows the Fourier transform infrared (FTIR) spectra
of SBA-16 and SBA-16/SO3H. They show the typical absorption
peaks related to groups of a mesoporous silica network around
3700–3100, 1300–1000, 1640, 960, 810, and 460 cmꢁ1. The
bands around 460 cmꢁ1 and 810 cmꢁ1 are assigned to the
symmetric stretching and rocking modes of the Si–O–Si vibra-
tions. The broad band at 1300–1000 cmꢁ1 (with the peak around
1080 cmꢁ1) is assigned to the asymmetric stretching mode of
the Si–O–Si moiety.13a,b The shoulder at 960 cmꢁ1 is assigned to
silanol groups existing in the structure of the material. The
broad band at 3100–3700 cmꢁ1 is associated with the –OH
stretching vibration mode of the silanol groups and the water
hydroxyl groups and the other band observed at 1640 cmꢁ1 is
assigned to the vibrations of adsorbed water molecules.13c The
Fig.
5 Plausible mechanism for the synthesis of a-amino-
phosphonates over a SBA-16 or SBA-16/SO3H catalyst.
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characteristic IR signals of the sulfonic acid group in the SBA- gradually until the catalyst’s best performance with 0.01 g. The
16/SO3H sample overlap at 920 cmꢁ1 (nS–O), 1425 cmꢁ1 (nasS] data for the other reactions are presented in Table 4 with
O), and 3000–3222 cmꢁ1 (n(O–H)) and the clear peak at 2340–2360 a comparison between SBA-16 and SBA-16/SO3H as the hetero-
cmꢁ1 is described as the rst overtone of the O–H bending geneous catalyst (Tables 3 and 4). We could observe that the
vibration mode of the SO3H groups engaged in a strong catalyst produced the a-aminophosphonates in good and
hydrogen bond.13d
excellent yields. Therefore, regarding the structural differences
The adsorption/desorption isotherms for SBA-16 are shown of the SBAs, the presence of the sulfonic group did not result in
in Fig. 2(c). The sample exhibits isotherms of type IV with any considerable improvement in the yields for the Kabachnik–
a hysteresis adsorption/desorption of type H2, according to the Fields reaction. Through the analysis of these data, we
IUPAC classication, characteristic of mesoporous materials.14 concluded that the catalysis was preferentially performed in the
The textural and structural properties are shown in Table 1. The Brønsted acid sites. The presence of the silanol group in SBA-16
pore wall thickness of SBA-16 is calculated by causes a positive charge density, attributed to the hydroxyl
pffiffiffi
W ¼ 3a0=2 ꢁ Dp,9 where a0 and Dp are the cubic lattice
group, that can be considered as a very weak Brønsted acid
site.16
parameter and the pore diameter, respectively.
The specic surface area of 576.11 m2 gꢁ1 is close to the
On the other hand, the SBA-16/SO3H catalyst gives a better
performance compared to SBA-16, which is expected because of
its strong acid sites attributed to the sulfonic acid groups, in
accordance with the TPD-NH3 analysis. Besides, to the best of
our knowledge, the imine synthesis (intermediate II) is the slow
step for Kabachnik–Fields reactions.
values obtained by the conventional synthesis of SBA-16, i.e.
using a conventional silica source (338–526 m2 gꢁ1).8,9 More-
over, the value of the pore diameter (3.27 nm) and the
percentage of micropore volume (29%) reaffirm the meso-
structure of SBA-16.
Therefore, we may infer that the presence of the catalyst
resulted in an improvement in the reaction rate for the
synthesis of this intermediate. With this information, we
devised a plausible mechanism for these reactions. Firstly, the
benzaldehyde will interact with SBA, specically in the Brønsted
acid sites (intermediate I). Secondly, the aniline will attack
intermediate I and, aer the removal of water, the imine
intermediate is obtained (intermediate II). Aer this step,
intermediate II will undergo an attack from the phosphite,
which will produce the a-aminophosphonate. This plausible
mechanism is in accordance with the literature and it is shown
in Fig. 5.17a,b
SEM and TEM were employed for the investigation of the
morphological structure of SBA-16 (Fig. 3). The SEM images
show the irregular morphology for SBA-16, synthesized without
hydrothermal treatment. The TEM images show the porous
structure along the [111] direction (bottom le image) and
along the [100] direction (bottom right image). A well-ordered
cubic porous structure 3–5 nm in diameter can be observed,
corresponding to a cubic Im3m structure.8 This value is in good
agreement with the analysis from XRD (Fig. 2(a)) and N2 phys-
isorption (Fig. 2(c)).
A further increase in the catalyst loading up to 0.02 g did not
result in any signicant change in the yield.
The products and results of the synthesis of the a-amino-
phosphonates using heterogeneous catalysts SBA-16 and SBA-
16/SO3H are given in Table 4.
Table 3 shows the inuence of solvents in the synthesis of
the a-aminophosphonates. Toluene, dichloromethane and
chloroform presented the highest yields (Table 3, entries 1–3).
Therefore, we established DCM as the standard solvent for
further Kabachnik–Fields reactions with starting materials of
diphenylphosphite (2.0 mmol), aniline 4-derivatives (2.0 mmol)
and benzaldehyde 4-derivatives (2.2 mmol), and 0.01 g of the
catalyst.
4 Conclusions
In the present work, we have proposed a new approach for the
synthesis of SBA-16 type mesoporous silica from sugarcane
bagasse ash. The as-synthesized material has good textural and
structural properties, and this is a good alternative use for
SCBA. Furthermore, the material obtained was successfully
functionalized with sulfonic acid groups, allowing an
improvement in its catalytic activity for the synthesis of a-
aminophosphonates.
Fig. 4 shows the NH3-TPD curves for SBA-16 and SBA-16/
SO3H. A large desorption peak centered around 190 ꢀC was
observed for SBA-16/SO3H. Note that no desorption peak
appeared in the NH3-TPD curve of the unmodied SBA-16,
which conrms that the peak for SBA-16/SO3H can be
assigned to NH3 desorption from the sulfonic acid groups. In
addition, another peak between 250 and 500 ꢀC occurs, but this
indicates the decomposition of sulfonic acid groups in
temperatures above 200 C.15
ꢀ
Acknowledgements
We gratefully acknowledge the Brazilian agencies (CAPES/
3.2 Catalyst tests
CNPQ) for nancial support.
A blank reaction was carried out without any catalyst for 60
minutes and the compound of interest was not obtained. Table
2 shows the performance of the a-aminophosphonate synthesis
with various catalyst loadings. The amount of SBA-16 and SBA-
16/SO3H varied from 0.003 to 0.02 g. The yields increased
Notes and references
´
˜
´
´
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