Pd-containing magnetic periodic mesoporous organosilica nanocomposite as an efficient and highly recoverable catalyst

A novel magnetic ionic liquid based periodic mesoporous organosilica supported palladium (Fe3O4@SiO2@IL-PMO/Pd) nanocomposite is synthesized, characterized and its catalytic performance is investigated in the Heck reaction. The Fe3O4@SiO2@IL-PMO/Pd nanocatalyst was characterized using FT-IR, PXRD, SEM, TEM, VSM, TG, nitrogen-sorption and EDX analyses. This nanocomposite was effectively employed as catalyst in the Heck reaction to give corresponding arylalkenes in high yield. The recovery test was performed to study the catalyst stability and durability under applied conditions.

Procedure for Heck coupling using Fe 3 O 4 @SiO 2 @IL-PMO/Pd nanocatalyst. For this purpose, 0.48 mol% of Fe 3 O 4 @SiO 2 @IL-PMO/Pd was added to a DMF solution of Ar-X (1 mmol), alkyl acrylate (2 mmol) and base (2 mmol). This was stirred at 105 °C. After completion of the reaction, ethyl acetate (10 mL) and water (10 mL) were added and the catalyst was separated by a magnet. The mixture was decanted and the organic phase was separated and dried over Na 2 SO 4 . The desired products were obtained after evaporation of solvent and/or recrystallization.

Results and discussion
The Fe 3 O 4 @SiO 2 @IL-PMO/Pd nanocomposite was prepared according to Fig. 1. As shown, Fe 3 O 4 @SiO 2 was first prepared by coating a silica layer over the Fe 3 O 4 surface. Then, the IL-PMO shell was created over Fe 3 O 4 @SiO 2 via hydrolysis and co-condensation of TMOS and ionic liquid in the presence of pluronic p123 template. The Fe 3 O 4 @ SiO 2 @IL-PMO/Pd nanocomposite was finally obtained via treatment of Fe 3 O 4 @SiO 2 @IL-PMO with Pd(OAc) 2 . Figure 2 shows the FT-IR spectra of prepared materials. For all samples, the bands appeared at 586 and 3400 cm -1 are, respectively, assigned to Fe-O and O-H bonds (Fig. 2). For the Fe 3 O 4 @SiO 2 and Fe 3 O 4 @SiO 2 @ IL-PMO/Pd materials, the peaks at 823 and 1077 cm -1 are assigned to Si-O-Si bands indicating successful coating of amorphous silica on Fe 3 O 4 (Fig. 2b). Moreover, for the Fe 3 O 4 @SiO 2 @IL-PMO/Pd material, the peaks appeared at 2923, 1420, and 1625 cm −1 are, respectively, due to the vibrations of aliphatic C-H, C=C and C=N bands of IL rings (Fig. 2c). These results confirm the successful coating of silica and IL-based periodic mesoporous organosilica shells over magnetite NPs. Figure 3 shows the wide-angle PXRD analysis of Fe 3 O 4 , Fe 3 O 4 @SiO 2 , Fe 3 O 4 @SiO 2 @IL-PMO and Fe 3 O 4 @ SiO 2 @IL-PMO/Pd nanoparticles. The signals at 30.3, 35.7, 43.4, 53.8, 57.7 and 63.0 are, respectively, due to the reflections of 220, 311, 400, 422, 511 and 440. This confirms high stability of crystalline structure of magnetite NPs during catalyst preparation. It is also important to note that, for Fe 3 O 4 @SiO 2 , Fe 3 O 4 @SiO 2 @IL-PMO and Fe 3 O 4 @SiO 2 @IL-PMO/Pd materials, the intensity of PXRD peaks is decreased, indicating the successful modification of magnetite NPs with SiO 2 , IL-PMO and palladium moieties.
The low-angle PXRD analysis of the Fe 3 O 4 @SiO 2 @IL-PMO/Pd nanocomposite demonstrated a sharp peak at 2θ≈1 corresponding to the IL-PMO shell (Fig. 4).
The N 2 adsorption-desorption isotherm of the Fe 3 O 4 @SiO 2 @IL-PMO/Pd showed a type IV isotherm with a H1 hysteresis loop, which is characteristic of ordered mesostructures with high regularity (Fig. 5). Also, the BET surface area, average pore size and total pore volume of the designed Fe 3 O 4 @SiO 2 @IL-PMO/Pd nanocomposite were found to be 496.29 m 2 /g, 4.64 nm and 0.76 cm 3 /g, respectively. These results are in good agreement with low-angle PXRD analysis proving well formation of an ordered PMO shell for Fe 3 O 4 @SiO 2 @IL-PMO/Pd. The VSM analysis was performed to investigate the magnetic properties of Fe 3 O 4 @SiO 2 @IL-PMO/Pd (Fig. 6). This showed a saturation magnetization about 45 emu·g −1 , which is lower than that of pure magnetic iron oxide NPs (60 emu g −1 ) 44 . This proves the successful coating of SiO 2 and PMO shells over magnetite NPs and also confirms the high magnetic properties of the catalyst which is an excellent characteristic in the catalytic fields.
The EDX pattern of Fe 3 O 4 @SiO 2 @IL-PMO/Pd demonstrated the signals of Fe, O, Si, C, Cl, Pd and N elements, conforming successful coating/immobilization of SiO 2 , ionic liquid and Pd moieties on magnetite NPs (Fig. 7).
According to TG analysis, a weight loss of about 9% was observed corresponding to the immobilized/incorporated ionic liquid groups onto/into material framework (Fig. 10).
The Heck reaction was selected as a valuable coupling reaction to evaluate the catalytic activity of Fe 3 O 4 @ SiO 2 @IL-PMO/Pd as a heterogeneous catalyst. The Heck reaction between iodobenzene and ethyl acrylate was selected as a test model. The effect of solvent showed that DMF is the best giving an excellent yield of 98% (Table 1, entries 1-5). The study also showed that the rate of reaction is affected by the amount of the catalyst. As shown, the reaction yield is increased with increasing catalyst loading from 0.24 to 0.48 mol% (      After optimization, the catalyst was employed in the Heck-coupling reaction for the preparation of some styrene derivatives. As shown in Table 2, all aryl halides bearing both electron-withdrawing and electron-donating substituents reacted effectively with acrylates to give corresponding Heck products in high yield. This demonstrates high efficiency of Fe 3 O 4 @SiO 2 @IL-PMO/Pd nanocomposite for the preparation of a wide-range of important arylalkenes. The recovery of Fe 3 O 4 @SiO 2 @IL-PMO/Pd was also investigated under optimum conditions. For this, after each reaction cycle, the catalyst was removed magnetically and after washing and drying, it was reused in the next run. The results showed that the catalyst could be recovered and reused for four times with no important reduction in its performance (Fig. 11).

Conclusion
In this study, a novel core-shell structured Fe 3 O 4 @SiO 2 @IL-PMO/Pd nanocomposite was synthesized and characterized. The well immobilization/incorporation and high stability of ionic liquid and palladium moieties over magnetite NPs were confirmed by FT-IR, TG and EDX analyses. The VSM and PXRD showed good magnetic properties of Fe 3 O 4 @SiO 2 @IL-PMO/Pd. The nitrogen-sorption and low-angle PXRD showed a mesoporous shell for the designed material. This nanocomposite was catalytically employed in the Heck reaction giving high yield of corresponding coupling products. The recovery test demonstrated high stability and durability of active catalytic species during applied conditions.     Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.