Amine-containing yolk-shell structured magnetic organosilica nanocomposite as a highly efficient catalyst for the Knoevenagel reaction

The yolk-shell structured silica nanocomposites have been considered by many researchers due to their specific physical and chemical properties. These materials have been widely used in adsorption and catalysis processes. Especially, the void space of yolk−shell nanostructures can provide a unique environment for storage, compartmentation, and confinement in host−guest interactions. In this paper, for the first time, the preparation, characterization, and catalytic application of a novel amine-containing magnetic methylene-based periodic mesoporous organosilica with yolk-shell structure (YS-MPMO/pr-NH2) are developed. The magnetic periodic mesoporous organosilica nanocomposite was synthesized through surfactant-directed co-condensation of bis(triethoxysilyl)methane (BTEM) and tetraethoxysilane around Fe3O4 nanoparticles. After Soxhlet extraction, the surface of YS-MPMO nanocomposite was modified with 3-aminopropyl trimethoxysilane to deliver YS-MPMO-pr-NH2 nanocatalyst. This catalyst was characterized by using EDX, FT-IR, VSM, TGA, XRD, nitrogen-sorption, and SEM analyses. The catalytic activity of YS-MPMO/pr-NH2 was studied in the Knoevenagel reaction giving the corresponding products in a high yield and selectivity. The YS-MPMO/pr-NH2 nanocatalyst was recovered and reused at least four times without a significant decrease in efficiency and activity. A leaching test was performed to study the nature of the catalyst during reaction conditions Also, the catalytic performance of our designed nanocomposite was compared with some of the previous catalysts used in the Knoevenagel reaction.


Introduction
In recent years, silica-based nanocomposites have received much attention between researchers in various fields of chemistry.These materials have been extensively employed in chemical processes due to the good properties of silica such as high chemical and thermal stability, high colloidal stability, biocompatibility and easy surface modification (Maleki et al., 2015;Purbia and Paria, 2015;Sun et al., 2015;Cheng et al., 2017;Wang et al., 2019;Gopalan Sibi et al., 2020).Among these, yolk-shell (YS) structured silica nanocomposites have been considered and studied by many researchers (Nagaraju et al., 2017;Bai et al., 2018;Du et al., 2018).These nanocomposites have many applications in the areas of drug delivery, catalysis, charge transfer and storage in batteries, solar cells and supercapacitors, adsorbents for gases and pollutants, gene therapy, etc (Nagaraju et al., 2017;Xie et al., 2017;Bai et al., 2018;Du et al., 2018).For example, recently, the YS-structured nanocomposites have been used as catalyst in the synthesis of pyranopyrazoles (Neysi and Elhamifar, 2023), the Chan-Lum coupling reaction (Shaker and   Elhamifar, 2021), and the reduction of nitrobenzenes (Wang et al., 2018).
On the other hand, the Knoevenagel reaction (Gordel-Wojcik et al., 2022) is one of the most famous carbon-carbon coupling process to synthesize α,β-unsaturated compounds.In recent years, the synthesis of the Knoevenagel products in the presence of heterogeneous and homogeneous catalysts has been investigated under different conditions.Due to difficulty in the separation of homogeneous catalysts, the use of magnetic heterogeneous catalysts is a good option to improve the efficiency of the catalytic processes.Some of recently reported studies in this matter are Fe 3 O 4 @SiO 2 @propyl@DBU (Zhang et al., 2021), L-proline-Cu/TCT@NH2@Fe3O4 (Kalantari et al., 2022), MgFe 2 O 4 (Ghomi and Akbarzadeh, 2018) and Fe 3 O 4 -cysteamine hydrochloride (Maleki et al., 2017).
In view of the above, in this research, a novel magnetic yolk-shell structured PMO supported propylamine (YS-MPMO/pr-NH 2 ) is prepared, characterized and its catalytic application is developed in the Knoevenagel reaction under green conditions.
2 Experimental section 2.1 Synthesis of Fe 3 O 4 nanoparticles Fe 3 O 4 NPs were firstly prepared according to our previous procedure (Neysi et al., 2020).According to this method, FeCl 2 .4H 2 O (1.5 g) and FeCl 3 .6H 2 O (3 g) were dissolved in 160 mL of deionized water.Then, aqueous ammonia (40 mL, 28% wt) was slowly added and the obtained mixture was stirred at room temperature (RT) for 60 min under argon atmosphere.The resulting product was collected using an external magnet and it was washed completely with distilled water and EtOH.This product was dried at 70 °C for 12 h under vacuum and called Fe 3 O 4 nanoparticles.

Synthesis of YS-MPMO/pr-NH 2
For this, firstly, the YS-MPMO nanocomposite (1 g) was dispersed in toluene (25 mL) at RT.Then, APTMS (3aminopropyltrimethoxysilane, 98%, 1 mmol) was added and the resulting mixture was stirred at 100 °C for 24 h.In the following, the product was magnetically separated, washed with EtOH and H 2 O, dried at 60 °C for 12 h and called YS-MPMO/pr-NH 2 nanocomposite.According to the CHN and EDX analyses the loading of amine groups on the designed nanocomposite surface was found to be 0.5 mmol/g.

Result and discussion
Firstly, core-shell structured magnetic periodic mesoporous organosilica (MPMO) was synthesized via hydrolysis and cocondensation of BTEM and TEOS around Fe 3 O 4 NPs in the presence of CTAB and pluronic P123 surfactants.After Soxhlet extraction of surfactants, the YS-MPMO was produced.This material was then modified with 3-aminopropyltrimethoxysilane (APTMS) to give YS-MPMO/pr-NH 2 nanocomposite (Figure 1).

FIGURE 10
Recoverability and reusability of the YS-MPMO/pr-NH 2 catalyst.Figure 2 shows the FT-IR spectra of Fe 3 O 4 , YS-MPMO and YS-MPMO/pr-NH 2 nanoparticles.For all materials, the characteristic peaks of Fe−O and O-H bonds are, respectively, appeared at 588 and 3,400 cm −1 (Figures 2A-C).In the FT-IR spectra of YS-MPMO and YS-MPMO/pr-NH 2 , the peaks at 940 and 1,090 cm −1 are, respectively, assigned to symmetric and asymmetric vibrations of the Si-O-Si bonds proving the successful formation of silica layer around the Fe 3 O 4 NPs.Also, for YS-MPMO and YS-MPMO/pr-NH 2 nanocomposits, the C-H signals of aliphatic moieties are appeared at 2,880-2,911 cm −1 (Figures 2B,C).
The SEM analysis of YS-MPMO/pr-NH 2 demonstrated a morphology with spherical particles and an average size of about 45 nm (Figure 3).These type nanoparticles are very important in the fields of catalysis and adsorption processes.
The EDX analysis of YS-MPMO/pr-NH 2 nanocomposite successfully confirmed the presence of Fe, O, C, N and Si elements in its framework (Figure 4).Also, the EDX mapping analysis revealed the well distribution of aforementioned elements in the framework of the YS-MPMO/ pr-NH 2 nanocomposite (Figure 5).These are in good agreement with the FT-IR results confirming well immobilization/ incorporation of methylene and propylamine moieties on/in the material framework.
The magnetic properties of YS-MPMO/pr-NH 2 nanocomposite were evaluated by using VSM analysis.The result of this study showed that YS-MPMO/pr-NH 2 nanocomposite has a superparamagnetic behavior.Also, the amount of magnetic saturation of this nanocomposite was about 43 emu/g (Figure 6).
In the wide angle PXRD pattern of YS-MPMO/pr-NH 2 , the presence of 6 peaks at 2θ: 30.3, 36, 43.5, 54.5, 57.5 and 63 °, corresponding to the crystalline structure of Fe 3 O 4 NPs, affirms the high stability of these nanoparticles during the preparation of the YS-MPMO/pr-NH 2 nanocomposite (Figure 7).
The TGA curve of YS-MPMO/pr-NH 2 nanocomposite showed three weight losses.The first one (about 2%) in the range of 25 °C-130 °C is assigned to removal of water and organic solvents.The second one (about 2%) at 150 °C-280 °C is due to the elimination of remained CTAB and pluronic P123 surfactants.The third one at 300 °C-700 °C (about 11%) is corresponded to the removal of grafted propylamine moieties on the shell surface and also incorporated methylene groups in the shell framework (Figure 8) (Neysi and Elhamifar, 2023).
The N 2 adsorption-desorption analysis of the YS-MPMO/pr-NH 2 showed a type IV isotherm with a H 2 hysteresis loop, corresponding to ordered mesostructured PMO shell (Figure 9).According to this analysis, the BET surface area and pore volume of the nanocomposite were found to be 470.67 m 2 /g and 0.973 cm 3 /g, respectively.
After characterization of the YS-MPMO/pr-NH 2 nanocomposite, its catalytic activity was examined in the Knoevenagel condensation under ultrasonic conditions.To optimize the reaction conditions, the condensation between malononitrile and benzaldehyde was selected as a model reaction.Examination of the amount of catalyst in this reaction showed that the best yield is obtained in the presence of 2.25 mol% of YS-MPMO/pr-NH 2 (Table 1, entries 1-4).Next, the catalytic activity of YS-MPMO/pr-NH 2 was investigated in different solvents of H 2 O, EtOH and toluene and also solvent-free media.This study showed that the best result is obtained under solvent-free conditions.(Table 1, entry 3 vs.entries 5-7).The H-bonding between protic EtOH and water solvents and malononitrile is a parameter which prevents and restricts the activity of this nucleophile in these solvents.Finally, the catalytic activity of Proposed mechanism for the Knoevenagel condensation using YS-MPMO/pr-NH 2 .
Frontiers in Chemistry frontiersin.org07 amine-free Fe 3 O 4 and YS-MPMO materials were studied, in which only a little yield of the desired product was obtained confirming that the designed Knoevenagel reaction is catalyzed by supported propylamine groups (Table 1, entry 3 vs.entries 8, 9).Accordingly, the use of 2.25 mol% of catalyst, RT and solvent-free media were selected as optimal conditions (Table 1, entry 3).
In the following, the catalytic activity of YS-MPMO/pr-NH 2 nanocatalyst was investigated in the condensation of various aldehydes with malononitrile under the optimal conditions.The study demonstrated that all aldehydes, bearing both electron withdrawing and electron donating substituents in various positions, give the corresponding Knoevenagel products in high yield and selectivity (Table 2).This confirms the high efficiency of the designed catalyst for the preparation of a wide range of important Knoevenagel products.
In the following, the recoverability and reusability of the YS-MPMO/pr-NH 2 nanocatalyst were investigated in the condensation of malononitrile with benzaldehyde under optimal condition.After completion of the reaction, the catalyst was magnetically removed and reused in the next run under the same conditions as the first run.Based on this study, it was found that YS-MPMO/pr-NH 2 can be recycled and reused for four runs without a significant decrease in its performance (Figure 10).
Next, a leaching test was performed to study the nature of catalyst under applied conditions.For this, the YS-MPMO/pr-NH 2 nanocatalyst was added to a flask containing benzaldehyde and malononitrile at RT.After the reaction progressed about 50%, the catalyst was separated using an external magnet and the reaction of residue was monitored for 60 min under optimal conditions.The result demonstrated no further progress of the reaction, confirming no leaching and also heterogeneous nature of active catalytic species under applied conditions.
Finally, a comparison study was performed between the present catalyst and a number of former catalysts applied in the Knoevenagel reaction (Table 3).This showed that YS-MPMO/pr-NH 2 is better than others in parameters of recovery times, reaction temperature and stability.
The mechanism of the Knoevenagel reaction is shown in Figure 11.As seen, firstly, one of the active hydrogens of malononitrile methylene is taken by YS-MPMO/pr-NH 2 nanocatalyst to deliver anion I.Then, this anion, as a nucleophile, reacts with carbonyl carbon of aldehyde to give anion II.Next, this anion takes a proton from protonated catalyst to deliver intermediate III.Finally, the desired product is formed after elimination of a water molecule.

Conclusion
A novel amine-containing magnetic periodic mesoporous organosilica with yolk-shell structure (YS-MPMO/pr-NH 2 ) was successfully synthesized and characterized.The TGA, EDX and FT-IR analyses showed the successful immobilization/ incorporation of propylamine and methylene groups into/onto material framework.The SEM image confirmed that the YS-MPMO/pr-NH 2 has a spherical morphology.Also, the superparamagnetic behavior of the YS-MPMO/pr-NH 2 nanocomposite was proved by VSM analysis.The nitrogensorption analysis showed the presence of a shell with high surface area for the designed nanocamposite.The PXRD analysis demonstrated high stability of Fe 3 O 4 NPs during the catalyst preparation.Examination of the catalytic activity of YS-MPMO/ pr-NH 2 in the Knoevenagel reaction showed that this catalyst has an excellent performance in this process.The leaching test confirmed the heterogeneous nature of active catalytic sites under applied conditions.The catalyst was also recovered and reused several times with maintaining its efficiency.

TABLE 1
The effect of solvent and catalyst loading in the Knoevenagel reaction of malononitrile with benzaldehydea.

TABLE 2
Synthesis of the Knoevenagel products in the presence of the YS-MPMO/pr-NH 2 nanocatalyst.

TABLE 3
Comparison of the catalytic activity of YS-MPMO/pr-NH 2 with former catalysts.