Arthur F. Bozaa,
Vicente L. Kupfera,
Aline R. Oliveirab,
Eduardo Radovanovica,
Andrelson W. Rinaldia,
Joziane G. Meneguina,
Nelson L. C. Dominguesb,
Murilo P. Moisésad and
Silvia L. Favaro*ac
aLaboratory of Materials Chemistry and Sensors – LMSen, State University of Maringá – UEM, 5790 Colombo avenue, 87020-900, Maringá, PR, Brazil. E-mail: slrosa@uem.br; Tel: +55-44-30111393
bLaboratory of Catalysis and Biocatalysis Organic – LACOB, Federal University of the Grande Dourados – UFGD, Km 12 Dourados Itahum road, 79.804-970, Dourados, MS, Brazil
cDepartment of Mechanical Engineering, State University of Maringá – UEM, 5790 Colombo avenue, 87020-900, Maringá, PR, Brazil
dFederal Technological University of Paraná – UFTPR, 635 Marcílio street, 86812-460, Apucarana, PR, Brazil
First published on 17th February 2016
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.
Sugarcane bagasse ash (SCBA) is generated as a by-product from power plants burning biomass. For each ton of sugarcane 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 Im3m space group.6 The synthesis of this material is usually performed under acidic conditions using a non-ionic surfactant, Pluronic F127, as a structure-directing agent and tetraethylorthosilicate (TEOS) as the silica source.6–9 The cage structure of SBA-16 has potential advantages for many applications in different fields, including catalysis,7a–c adsorption,7d,e electronic devices,7f,g and drug delivery,7h and the successful modification of mesoporous silica surfaces with organic functionalities 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 α-aminophosphonate.10e The α-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 structure being similar to that of amino acids.11
The functionalization of environmentally friendly SBA-16 with sulfonic acid groups and the synthesis of α-aminophosphonates were investigated in this work.
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, after establishing the standard Kabachnik–Fields reaction conditions, other α-aminophosphonates were synthesized following the same protocol, but with benzaldehyde and aniline, as described in Fig. 1.
Nitrogen adsorption–desorption isotherms were measured at −196 °C on Quantachrome NOVA-1200E Surface and PoroAnalizer equipment. The specific surface areas were evaluated 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 Scientific Nicolet IZ10 equipment in the range of 400–4000 cm−1 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 operated at 120 kV.
The acid capacities of SBA-16 and SBA-16/SO3H were indicated by temperature-programmed ammonium desorption (NH3-TPD) and were carried out with AutoChem II 2920 equipment. The α-aminophosphonate products were 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 shifts are expressed in parts per million (ppm) and coupling constants (J) are reported in Hertz (Hz).
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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). |
Both show a single intense diffraction peak at 2θ values of 1.01 and 1.04°, respectively, which belongs to the cubic phase of the (1 1 0) plan. The slight shift to a higher 2θ value is due to the grafting of sulfonic acid groups into the SBA-16 pores.12 From Bragg’s law (nλ = 2dsin
θ) and the relationships (1/dhkl2 = (h2 + k2 + l2)/a02) and
between the cubic lattice parameter (a0) and the plane distance (dhkl) at different Miller indices (hkl), if the first peak belongs to the (110) diffraction peak of the cubic phase, the diffraction peaks should appear at 2θ200 = 1.433° and 2θ211 = 1.755° for SBA-16 and 2θ200 = 1.47° and 2θ211 = 1.80° for SBA-16/SO3H. Our second diffraction peaks in the experimental data match the theoretical results very well with 2θ = 1.425° and 1.471°, respectively, and a third small peak for SBA-16/SO3H at 1.79° only. Therefore, the XRD patterns of SBA-16 can be indexed as (1 1 0), (2 0 0), and (2 1 1) reflections, 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 cm−1. The bands around 460 cm−1 and 810 cm−1 are assigned to the symmetric stretching and rocking modes of the Si–O–Si vibrations. The broad band at 1300–1000 cm−1 (with the peak around 1080 cm−1) is assigned to the asymmetric stretching mode of the Si–O–Si moiety.13a,b The shoulder at 960 cm−1 is assigned to silanol groups existing in the structure of the material. The broad band at 3100–3700 cm−1 is associated with the –OH stretching vibration mode of the silanol groups and the water hydroxyl groups and the other band observed at 1640 cm−1 is assigned to the vibrations of adsorbed water molecules.13c The characteristic IR signals of the sulfonic acid group in the SBA-16/SO3H sample overlap at 920 cm−1 (νS–O), 1425 cm−1 (νasSO), and 3000–3222 cm−1 (ν(O–H)) and the clear peak at 2340–2360 cm−1 is described as the first overtone of the O–H bending vibration mode of the SO3H groups engaged in a strong hydrogen bond.13d
The adsorption/desorption isotherms for SBA-16 are shown in Fig. 2(c). The sample exhibits isotherms of type IV with a hysteresis adsorption/desorption of type H2, according to the IUPAC classification, characteristic of mesoporous materials.14 The textural and structural properties are shown in Table 1. The pore wall thickness of SBA-16 is calculated by ,9 where a0 and Dp are the cubic lattice parameter and the pore diameter, respectively.
Sample | d110 (nm) | a0 (nm) | SBET (m2 g−1) | W (nm) | Vt (cm3 g−1) | Vm (cm3 g−1) | Vm/Vt (%) | Dp (nm) | Ref. |
---|---|---|---|---|---|---|---|---|---|
a d110: the plane spacing at Miller indices; a0: the cubic lattice parameter ![]() ![]() |
|||||||||
SBA-16 | 8.48 | 11.99 | 576 | 6.78 | 0.34 | 0.10 | 29 | 3.27 | This work |
SBA-16 STD | 9.99 | 14.13 | 443 | 9.14 | 0.24 | 0.16 | 67 | 3.1 | 8 |
The specific surface area of 576.11 m2 g−1 is close to the values obtained by the conventional synthesis of SBA-16, i.e. using a conventional silica source (338–526 m2 g−1).8,9 Moreover, the value of the pore diameter (3.27 nm) and the percentage of micropore volume (29%) reaffirm the mesostructure of SBA-16.
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 left 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 physisorption (Fig. 2(c)).
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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. |
A further increase in the catalyst loading up to 0.02 g did not result in any significant change in the yield.
Table 3 shows the influence of solvents in the synthesis of the α-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.
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 unmodified SBA-16, which confirms 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
Entry | Catalyst (g) | Time (min) | Yieldb (%) | |
---|---|---|---|---|
SBA-16 | SBA-16/SO3H | |||
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. | ||||
1 | 0.003 | 20 | 74 | 89 |
2 | 0.006 | 20 | 85 | 95 |
3 | 0.008 | 20 | 87 | 95 |
4 | 0.01 | 20 | 90 | 100 |
5 | 0.02 | 20 | 91 | 100 |
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.
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, specifically in the Brønsted acid sites (intermediate I). Secondly, the aniline will attack intermediate I and, after the removal of water, the imine intermediate is obtained (intermediate II). After this step, intermediate II will undergo an attack from the phosphite, which will produce the α-aminophosphonate. This plausible mechanism is in accordance with the literature and it is shown in Fig. 5.17a,b
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Fig. 5 Plausible mechanism for the synthesis of α-aminophosphonates over a SBA-16 or SBA-16/SO3H catalyst. |
The products and results of the synthesis of the α-aminophosphonates using heterogeneous catalysts SBA-16 and SBA-16/SO3H are given in Table 4.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23233c |
This journal is © The Royal Society of Chemistry 2016 |