An organosilica porous material which was synthesized by co-condensation method, where the organic and inorganic are homogeneously mixed and dissolved to afford a new material in the presence of P123. Since this hybrid mesoporous silica material has been obtained by co-condensation of a synthetic organosiloxane, a facile one step solvent-free approach to synthesize the organosiloxane precursor was developed in our lab, as represented in Fig. 1.
The successful synthesis of this organosiloxane bridge using various techniques such as 1H- and 13C-NMR, FTIR and mass spectroscopy was developed14, which all have been discussed in our previous work14. We used this isocyanurate-carbamate organosiloxane bridge (ISO bridge) to synthesize the mesoporous organosilica (MO-ISO) with a high surface area and mechanically stable properties. Since in the previous study, we showed that the ratio of TEOS to organosiloxane precursor has a critical effect on the surface area and morphology, we selected the molar ratio of 1:15 (organosiloxane to TEOS, respectively). For studying the surface area, the N2 adsorption-desorption isotherms of the synthesized MO-ISO was evaluated and represented in Fig. 2. Accordingly, the surface area was obtained around 697 m2.g-1 with type IV isotherm and the average pore size is 6.2 nm. Going further, the Pd supported MO-ISO (Pd@MO-ISO) has exhibited a relative loss in the surface area (389 m2.g-1), however, the surface area is still high, compared to other porous materials (Fig. 2).
The FTIR spectrum for the heterocyclic starting material of ISO bridge was compared with the FTIR spectrum of the MO-ISO to see if the main structure of ISO has undergone no change (Fig. 3). This can be judged by the existence of two sharp bands at 1467 and 1700 cm-1 related to the stretching vibrations of the isocyanurate carbonyl located in the ring. A small shift in this regard can be observed to these band positions which can be attributed to the change in the intermolecular hydrogen bondings in the pure form and when embedded in the MO structure. Some peaks in the range of 2900–3000 cm-1 are also related to the aliphatic chains (ethylene) of the ISO bridge.
Since the generated carbamate groups are sensitive to the acidic and basic media, we carefully examined appeared two peaks the 13C-NMR of MO-ISO related to the carbonyls (one to carbamate (156 ppm) and another to the isocyanurate (148 ppm) which can confirm that the ISO bridge has been retained intact in the structure 14.
SEM micrographs of MO-THEIC morphology exhibits micro-sized particles, showing that the mesopores are assembled and aggregated into the large particles (Fig. 4A). Furthermore, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of MO-ISO confirms the presence of Pd nanoparticles distributed in the MO-ISO’s matrix. Note that the Pd nanoparticles are with lighter color since the more condense matter, the lighter appearance it takes in HAADF-STEM image (Fig. 4B). looking at the TEM image of MO-ISO reveals that the structure of MO-ISO is the hexagonally ordered with uniform pore size distribution (Fig. 4C). Moreover, the Pd supported MO-ISO confirms the presence of Pd nanoparticles are formed on the MO-ISO (Fig. 4D). The calculated sizes of Pd nanoparticles according to the TEM image are found to be sub-20 nm.
Further, the Pd nanoparticles and the MO-ISO structure using elemental TEM-mapping and TEM-based electron dispersive spectroscopy (EDS) were studied. It was confirmed that the Pd nanoparticles have been supported in MO-ISO structure using scanning the related elements of O, N, C, and Si elements by TEM-mapping (Fig. 5A). Also, Fig. 5B shows the TEM-EDS spectra of Pd@MO-ISO. This analysis confirms the presence of Pd element in the structure as well as other critical elements, e.g., C, N, and Si.
Catalytic test of Pd@MO-ISO for the reduction of nitroaromatics
Further, the catalytic activity of Pd@MO-ISO in the reduction of nitroaromatics was examined by testing PNP in the aqueous media at room temperature. In this regard, NaBH4 as reducing agent was used in H2O as the best solvent in terms of green chemistry principles. The catalyst, Pd@MO-ISO was used in different loadings to find which ratio of Pd to the reactant, PNP, has the higher TON. Accordingly, the results indicated that 5 mg catalyst in 3 mM PNP solution (40 mL) has a higher activity in terms of turnover frequency (TON). The calculated TON was found to be 513 for the PNP reduction in the presence of 5 mg Pd@MO-ISO (with 0.5 wt% of Pd content) and 0.12 mmol PNP. Indeed, higher catalyst loadings than 5 mg do not have enough effect on the improvement of the reaction yeild from viewpoint of Pd to reactant molar ratio. Furthermore, the reduction reaction in the presence of optimal loading of Pd@MO-ISO versus the time was examined by taking a sample every 5 min to record its UV-Visible spectrum. It was realized that the major fraction of the reaction progress occurs at early 10 min. The obtained results confirmed the conversion of PNP to PAP by fading an adsorption peak at 410 nm and appearing a related peak at 317 nm over the reaction time (Fig. 6).
Also, the recyclability of the Pd@MO-ISO catalyst was studied since the recyclability is one of the pivotal features for using heterogeneous catalysts in different organic transformations. Accordingly, we used the optimized reaction conditions, i.e., 3 mM PNP, room temperature, aqueous conditions, and 5 mmol NaBH4. The results show that Pd@MO-ISO catalyst is easily recoverable and reusable at least for five consecutive cycles (Fig. 7A). In addition, the analysis of the reaction solution, once the catalyst is filtered, through atomic absorption spectroscopy (AAS) in the fifth cycle, confirmed that the Pd leaching is negligible (< 1%). SEM-based EDS spectra of recovered Pd@MO-ISO after five cycles also shows the presence of Pd species, further confirming the resistance of the material against Pd leaching (Fig. 7B).
The catalytic performance of our catalyst and method with the previously reported catalytic systems was compared for PNP reduction to PAP. It is obvious that this new introduced catalyst is superior to several catalysts and methods in terms of TON (Table 1).
Table 1
Comparing the PNP reduction through our synthesized catalyst and the previously reported catalysts.
Catalyst
|
t (min)
|
Reductant
|
Solvent
|
Temp. (oC)
|
TON
|
Ref.
|
TA@Fe3O4-AgNPs
|
6
|
NaBH4
|
Water
|
r.t.
|
25
|
[43]
|
[PdClx(ACN)y(NH2-CS)z]2-
|
120
|
NaBH4
|
Water
|
22
|
10
|
[44]
|
Pd@MO-Urea
|
26
|
NaBH4
|
Water
|
r.t.
|
64
|
[4]
|
Pd@MO-ISO
|
20
|
NaBH4
|
Water
|
r.t.
|
513
|
This work
|