Highly Ordered Hybrid Mesoporous Organosilica: An Exceptional Anchor for Pd Nanoparticles and Catalyst Nitroaromatic Reduction

Hybridization of mesoporous organosilica (MO) to reinforce the surface capability in adsorption and stabilization of noble metal nanoparticles are of great attention in generating noble metal-based catalysts. Here, we used a unique hybrid of organic-inorganic mesoporous silica which in pore prole pattern was similar to the well-known mesoporous silica, SBA-15. This hybrid mesoporous silica was further incorporated in the synthesis and stabilization of Pd nanoparticles on its surface and then, the obtained Pd supported MO, was employed as a heterogeneous green catalyst in the conversion of p-nitrophenol (PNP) to p-aminophenol (PAP) at room temperature with ecient recyclability.


Introduction
Mesoporous silicas modi ed/constructed by organosiloxanes (MO) [1][2][3][4] are a class of hybrid silica-based materials that are widely used in the diverse applications including catalysis 4,5 , microextraction 6 , water treatment 7 , molecular recognition 8 , photocatalysis 9 , optical thermometry 10 , sensors 11,12 , CO 2 capture 13,14 . These materials can be synthesized by incorporating organosiloxane bridges and usually have amorphous structure but with ordered/disordered pore channels. Some of these materials can have microporosity alongside the mesoporosity which can increase the surface area and subsequently e ciency of the material in the applications 15,16 . Since these materials are synthetic, there should be a synthetic bottom-up strategy for the reaching to such materials in which the presence of template (mostly a soft template), e.g., F127 and P123, is inevitable [17][18][19] .
When an organosiloxane bridge with signi cant exibility is being used in a MO's structure, the organosiloxane bridge should co-condense with a silica source (e.g., TEOS) to generate a uniform and robust mesoporous structure from viewpoint of mechanical toughness and porosity 4 . Several advantages belong to MOs, which cannot be found in conventional hybrid mesoporous silica materials 20 . For instance, in MOs, depending on the type of bridge, a higher ratio of organosilica can be embedded in the structure of MO, while in the case of conventional mesopores, e.g., SBA-15, it is an overwhelming process usually deals with pore blocking or unsuccessful process 21 . In MOs, the pore transfer for guest molecules are more e cient and more promising than conventional hybrid silica mesopores [22][23][24] . This e ciency in molecules transfer improves the lifetime of the MO and catalytic activity by minimizing the pore-blocking possibility by guest molecules [25][26][27] . This is because the organosilica motif of conventional mesoporous silicas stay on the external surface of pore channels while in MOs, it can be embedded in the pore wall 28,29 .
The use of mesoporous silica materials are of great importance among the candidates for synthesis of heterogeneous catalysts [30][31][32] . These materials can provide excellent heterogeneous surface for immobilization of catalytically active metal species for various reactions, such as cross coupling and reduction reactions [33][34][35][36][37] . These materials can also provide an excelling recyclability by tuning the ratio and type of the organosilica. Pd, among the noble metals, have played a pivotal role in the catalysis since Pd is an active catalytic species for a broad domain of reactions, e.g., cross-coupling 38 , oxidation 39 , reduction 40,41 , and dehydrogenation 42 . Here, we integrated the isocyanurate and carbamate functional groups in the MO and used it for Pd nanoparticles supporting. This was further employed as a heterogeneous green catalyst for the aqueous room temperature reduction of PNP to PAP using NaBH 4 .

Results And Discussion
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 1 H-and 13 C-NMR, FTIR and mass spectroscopy was developed 14 , which all have been discussed in our previous work 14 . 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 N 2 adsorption-desorption isotherms of the synthesized MO-ISO was evaluated and represented in Fig. 2. Accordingly, the surface area was obtained around 697 m 2 .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 m 2 .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 13 C-NMR of MO-ISO related to the carbonyls (one to carbamate (156 ppm) and another to the isocyanurate (148 ppm) which can con rm 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-eld scanning transmission electron microscopy (HAADF-STEM) image of MO-ISO con rms 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 con rms 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 con rmed 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 con rms 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, NaBH 4 as reducing agent was used in H 2 O as the best solvent in terms of green chemistry principles. The catalyst, Pd@MO-ISO was used in different loadings to nd 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 con rmed 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 NaBH 4 . The results show that Pd@MO-ISO catalyst is easily recoverable and reusable at least for ve consecutive cycles (Fig. 7A). In addition, the analysis of the reaction solution, once the catalyst is ltered, through atomic absorption spectroscopy (AAS) in the fth cycle, con rmed that the Pd leaching is negligible (< 1%). SEM-based EDS spectra of recovered Pd@MO-ISO after ve cycles also shows the presence of Pd species, further con rming 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). The SEM images were observed with a HITACHI SU-8230 scanning SEM. TEM images were taken with a JEOL JEM-2100F microscope (operated at 300 kV). N 2 adsorption-desorption, BJH, and BET analyses were carried out at 77 K using a Microtrac Bel BEL-mini. Prior to the measurements, the samples were evacuated at 90°C for 20-24 h. ICP-OES was performed by Pekin-Elmer (USA) model.

Synthesis of Pd@MO-ISO
For the synthesis of this organosiloxane bridge, 1,3,5-tris(2-hydroxyethyl)-1,3,5-triazinane-2,4,6-trione (THEIC) (3 mmol, 0.783 g) was reacted with (3-isocyanatopropyl)triethoxysilane (1 mmol, 0.5 mL) at 135°C for 3 h and then, cooled to 80°C and stirred at that temperature for another 3 h. Then, a a colorless oily product was obtained at the end which was used without further puri cations 14 . The synthesised isocyanurate-based organosiloxane bridge was further employed in the synthesis of the corresponding MO through co-condensation of TEOS. Accordingly, the pluronic triblock copolymer P123 (2 g, MW = 5800 g.mol -1 ), was dissolved in HCl aqueous solution (10 − 4 M, 75 mL) and stirred for 3 h. Then, the assynthesised organosiloxane bridge from the previous step was mixed with TEOS with the molar ratio of A white powder as nal product after drying at 60°C for 4 h in an oven was obtained. For simplicity, the product obtained in this stage was named MO-ISO.
Further, MO-ISO was dispersed in acetonitrile and then, Na 2 PdCl 4 (39 µmol, 0.0116 g) was added to the mixture under the vigorous stirring. After 4 h, the reaction was stopped, and the creamy product was collected by centrifugation and washing for three times with EtOH (10 mL). After drying in an oven at 60°C for 3 h, it was re-dispersed in MeOH and then, NaBH 4 was added to the dispersion and allowed to stir for 0.5 h. Finally, the nal product was centrifuged and washed for three times with ethanol (10 mL) and dried in oven at 60°C for 3 h. For simplicity, the product obtained in this step was named Pd@MO-ISO. The wt% of Pd loaded in MO-ISO was analysed by ICP-OES technique, indicating that the Pd content is 0.5 wt% in the MO-ISO.
General procedure for reduction of p-nitrophenol in the presence of Pd@MO-ISO In the catalytic test to reduce p-nitrophenol (PNP) into p-aminophenol (PAP), Pd@MO-ISO (5 mg) was dispersed in the aqueous PNP solution (3 mM, 40 mL) by sonicating for 5 min. Then, the sodium borohydride (5 mmol, 125 mg) was added to the solution and continued to stir vigorously. During the reaction, every 5 min, the sampling from reaction progress was achieved by a syringe equipped with syringe lter to separate the catalyst from the reaction media. Then, the obtained samples from each minute were analysed by UV-Vis spectrometer to monitor the reaction progress.

Conclusion
Here, we presented a heterogeneous green catalyst on the basis of a new Pd-based hybrid mesoporous organosilica which could successfully adsorb Pd ions and support it on the surface. The Pd supporting process had not a signi cant destructive effect on the mesoscopic structure of the MO-ISO and had an e cient catalytic activity in the chemical reduction of PNP to PAP by using NaBH 4 as reducing agent. The catalyst also showed a high rate of recyclability and negligible Pd species leaching over the recycling the catalyst. The catalytic activity of Pd@MO-ISO was excellent in comparison to other previously reported catalysts with the similar textural structure.    Scanning the UV-Visible spectra of PNP solution catalytically reduces to PAP.