Ag2CO3 containing magnetic nanocomposite as a powerful and recoverable catalyst for Knoevenagel condensation

In this paper, the synthesis, characterization and catalytic application of a novel magnetic silica-supported Ag2CO3 (MS/Ag2CO3) with core–shell structure are developed. The MS/Ag2CO3 nanocomposite was prepared through chemical modification of magnetic MS nanoparticles with AgNO3 under alkaline conditions. The structure, chemical composition and magnetic properties of MS/Ag2CO3 were investigated by using VSM, PXRD, FT-IR, EDX and SEM techniques. The MS/Ag2CO3 nanocomposite was used as an effective catalyst for the Knoevenagel condensation under solvent-free conditions at 60 °C in an ultrasonic bath. The recovery and leaching tests were performed to study the nature of the MS/Ag2CO3 catalyst under applied conditions.

Magnetic nanoparticles (NPs) have wide applications in various fields of medicine, catalysis, environment, materials science and biotechnology due to their unique magnetic properties and ability to respond to external magnetic fields. Therefore, in recent years, many researchers have focused on making different types of these NPs. The cobalt, iron, nickel elements and their chemical compounds are precursors that commonly used to prepare magnetic NPs [1][2][3][4][5][6][7][8][9] . The use of nickel and cobalt is limited due to their toxicity and high tendency to oxidize. Among these, magnetic iron oxide NPs, especially superparamagnetic Fe 3 O 4 NPs, have been considered by researchers due to their non-toxicity, good biocompatibility and good magnetic properties 10,11 . Magnetic NPs, despite having many advantages, suffer from a series of inherent disadvantages such as high chemical activity and a high tendency to aggregate due to their high surface area. Therefore, the development of effective strategies to improve the stability of these NPs is an essential need. Coating the surface of Fe 3 O 4 NPs is one of the effective methods to stabilize them. The species including organic polymers such as dextran, chitosan, polyethylene glycol, polyaniline; organic surfactants such as CTAB, DTAB, DPB and SOS; metals such as Au and Ag; mineral oxides such as carbon and silica; biological molecules and structures such as ligands/receptors, peptides, liposomes have been used as coating shell for Fe 3 O 4 NPs to form core-shell structured MNPs 10,[12][13][14][15][16][17][18][19][20][21][22][23][24] 24 . Among the various protective shells, silica has attracted more attention between many researchers. The silica shell can reduce the magnetic dipole adsorption between nanoparticles, helping to diffuse magnetic NPs in aqueous and organic environments. Also due to poor chemical permeability, silica can prevent the destruction of MNPs in different chemical environments. Moreover, the abundant silanol groups on the silica surface provide suitable conditions for different types of modification 10,[25][26][27][28] 38 , catalyst support 10,29,[32][33][34]39,40 , ion exchanger 41 and so on. Especially, in the field of catalysis, magnetic silicas with a core-shell structure have been considered by many researchers due to their magnetic recoverability, high hydrophobicity and ability to modify their surface [42][43][44][45][46] . Some of recently developed nanocatalysts are Fe 3 O 4 @SiO 2 /Schiff-base/Cu(II) 42 46 . Transition metal catalysts have been useful in modern synthetic organic chemistry due to their diverse reactivity in enabling various molecular conversions 47 . The reactions performed using these catalysts can be classified into three groups based on the role of the metal: 1. catalytic reactions based on the oxidation/reduction cycle of the transition metal, 2. catalytic reactions in which the transition metal acts as a Lewis acid and 3.  48 . In recent years, silver metal has been more considered by researchers as an effective transition metal catalyst, due to the processes catalyzed by silver perform under mild conditions and silver is cheaper and environmentally friendly than many rare metals (Pd, Pt, Rh, Ru, etc.). Among the various silver species, silver carbonate (Ag 2 CO 3 ) can be employed as a Lewis acid, an inorganic base and a good oxidant in different organic reactions. Also, Ag 2 CO 3 can be coordinated with various unsaturated systems (carbonyls, imines, isocyanides, alkynes and alkenes) and create very stable intermediates in the course of various processes [48][49][50][51][52][53] .
On the other hand, the Knoevenagel condensation of active methylene and carbonyl compounds is among the most commonly used methods in organic chemistry for the synthesis of low-electron olefins. In recent years, many catalysts were used for Knoevenagel condensation, in which heterogeneous ones have received much attention due to the easy recovery of the catalyst and also the easy separation of the products 54 61 . In view of the above, especially the advantages mentioned for Ag 2 CO 3 , our motivation in this study is the design and preparation of a novel core-shell structured MS/Ag 2 CO 3 nanocomposite as a powerful, effective, recyclable and reusable nanocatalyst for the Knoevenagel condensation.

Experimental section
Preparation of MS/Ag 2 CO 3 . For this, the Fe 3 O 4 NPs (0.6 g) 29 were dispersed in deionized water (25 mL) and EtOH (75 mL) for 0.5 h. After adding NH 3 (3.5 mL, 25% wt), the mixture was stirred at RT for 20 min. Then, tetramethoxysilane (TMOS, 0.5 mL) was added and stirring was continued at RT for 16 h. After that, the resulting solid material was magnetically collected, washed with deionized water and EtOH, dried at 80 °C for 6 h and defined as MS. For preparation of MS/Ag 2 CO 3 , MS (0.6 g) was well-dispersed in deionized water (30 mL). After 0.5 h, NaHCO 3 (2.5 mmol) was added and stirring was continued at RT for 2 h. Then, AgNO 3 (5 mmol) was added under lightless conditions. After that, the reaction combination was stirred for 12 h in an ice bath. The resulted material was collected using a magnetic field, washed with deionized water, dried and designated as MS/Ag 2 CO 3 .
Procedure for the Knoevenagel reaction using MS/Ag 2 CO 3 . For this, MS/Ag 2 CO 3 (0.015 g), ethyl cyanoacetate (1 mmol) and aldehyde (1 mmol) were added in a reaction flask and the resulted mixture was sonicated at 60 °C under solvent-free conditions. After completing of the process, EtOH (10 mL) was added and MS/ Ag 2 CO 3 was magnetically separated. Finally, the solvent was evaporated and pure Knoevenagel products were resulted after recrystallization in EtOH and n-hexane solvents. (Table 2,

Results and discussion
The synthesis of MS/Ag 2 CO 3 is shown in Fig. 1 Fig. 2. For all samples, the characteristic signals at 3397 and 583 cm −1 are, respectively, due to O-H and Fe-O bonds. Also, the band cleared at 1662 cm −1 is due to bending vibration of O-H bonds 32,65 . For MS and MS/Ag 2 CO 3 , the signals at 823 and 1069 cm −1 are assigned to Si-O-Si, confirming the construction of SiO 2 shell around the Fe 3 O 4 core (Fig. 2b,c) 26 . Importantly, for the MS/Ag 2 CO 3 nanocomposite, the observed peaks at 705, 884, 1381 and 1448 cm −1 are attributed to the absorption bands of CO 3 2 (Fig. 2c), indicating successful immobilization of Ag 2 CO 3 particles on the surface of MS 66 . The wide-angle PXRD pattern of Fe 3 O 4 and MS/Ag 2 CO 3 nanomaterials are shown in Fig. 3. As shown, for both samples, six characteristic peaks are observed at 2θ of 30.10, 35.58, 43.29, 53.81, 57.44 and 63.24 degree, corresponding to the crystal planes of (220), (311), (400), (422), (511) and (440), respectively. These are related to the crystalline structure of magnetite NPs confirming high stability of Fe 3 O 4 during catalyst preparation. The pattern of MS/Ag 2 CO 3 nanocomposite also showed two sharp peaks at 2θ of 33.2 and 38.5 degree corresponding to the Ag 2 CO 3 NPs (Fig. 3b) 10,66,67 . This proves successful construction of Ag 2 CO 3 NPs on MS core.
The VSM analysis was done to investigate the magnetic property of MS/Ag 2 CO 3 nanocomposite (Fig. 4). As shown, the saturation magnetization of 17.5 emu/g was found for this material. Also, the VSM curve showed that this material has a superparamagnetic behavior.
The surface morphology of MS/Ag 2 CO 3 nanocomposite was studied by using SEM analysis. This showed that the MS/Ag 2 CO 3 nanocomposite has a uniform spherical structure (Fig. 5).
The EDX spectrum showed that the designed MS/Ag 2 CO 3 is composed of Fe, Si, O, Ag and C elements confirming the successful incorporation/immobilization of expected species in the material framework (Fig. 6).
In the following, the catalytic activity of MS/Ag 2 CO 3 was evaluated in the Knoevenagel reaction. To obtain the optimum conditions, the reaction of benzaldehyde with ethyl cyanoacetate was chosen as a model ( Table 1). The study showed that the catalyst loading is a very important factor in the reaction progress, in which the best result was obtained using 0.015 g of catalyst (Table 1, entries 1-4). The solvent screening demonstrated that solvent-free condition is the best for the reaction (Table 1, entries [4][5][6][7][8]. Evaluation of temperature showed that the highest activity of MS/Ag 2 CO 3 is resulted at 60 °C (Table 1, entry 4 versus entries 9, 10). In the next, the activity of Ag 2 CO 3 -free Fe 3 O 4 and MS nanomaterials was compared with that of MS/Ag 2 CO 3 showing that the presence of Ag 2 CO 3 particles as active catalytic centers are necessary for the development of the reaction (Table 1, entry 4 versus entries 11, 12). To prove the effect of both Ag and CO 3 species in the reaction progress, the catalytic activity of MS/Ag 2 CO 3 was compared with AgCl, Na 2 CO 3 and NaNO 3 salts (Table 1, entry 4 versus entries 13-15). The  www.nature.com/scientificreports/ results showed that AgCl and Na 2 CO 3 deliver a low to moderate yield of desired product. While, using NaNO 3, no progress was observed in the reaction. Interestingly, in the presence of designed MS/Ag 2 CO 3 the best result was obtained. These confirm that both Ag and CO 3 species are necessary for the development of reaction. According to these results, it can be concluded that the designed MS/Ag 2 CO 3 acts as a bifunctional catalyst in the reaction. With the optimal conditions in hand that are bolded in Table 1 (entry 4), a variety of aldehydes were employed as substrate (Table 2). Generally, for aromatic aldehydes bearing both electron-withdrawing or electron-donating substituents, electronic nature or substitution pattern had little effect on this process and MS/Ag 2 CO 3 was able to effectively catalyze the reaction to give the Knoevenagel products in high to excellent yields. Also, terephthalaldehyde, hetero-aromatic aldehydes such as thiophene-2-carbaldehyde and furan-2-carbaldehyde, 1-naphthaldehyde and hexanal also gave the corresponding Knoevenagel adducts in good to high yield at relatively short time. It is important to note that, as previously summarized by Tietze et al., all condensations of cyanoacetate with aromatic and aliphatic aldehydes give E-isomer almost exclusively 68 . In the present study, all synthesized Knoevenagel products were identified as E-isomer by comparing their melting points, IR, and NMR spectra with valid samples [62][63][64][69][70][71][72] .  www.nature.com/scientificreports/ The ability to recycle and reuse of catalysts are important issues that should be considered in heterogeneous catalytic systems. In this regard, the recyclability and reusability of MS/Ag 2 CO 3 catalyst were evaluated in the condensation of benzaldehyde with ethyl cyanoacetate as a test model. For this, in the end of reaction, the catalyst was magnetically separated and reused in the next reaction run at conditions the same as first run. As it is clear in Fig. 7, MS/Ag 2 CO 3 can be recovered and reused at least 6 times without the significant loss in its activity and productivity.
For determining the nature of the MS/Ag 2 CO 3 catalyst, a leaching test was done at optimal conditions. For this, after progress of about 50% of the process, the MS/Ag 2 CO 3 catalyst was collected using a magnet and the reaction progress of the catalyst-free residue was monitored. Importantly, after 1 h, no progress was observed in converting the starting material to product. This proves the heterogeneous nature of the MS/Ag 2 CO 3 catalyst and also confirms no-leaching of active Ag 2 CO 3 particles during the applied conditions.
Although the exact reaction pathway for the Knoevenagel condensation with the MS/Ag 2 CO 3 catalyst is not clear for us, however, based on the results presented for the Ag 2 CO 3 catalyst in other organic reactions, a plausible mechanism for this reaction is presented in Fig. 8. Since Ag 2 CO 3 has a dual role as a base and a one-electron  Finally, the desired Knoevenagel product 5 is resulted after dehydration of the β-hydroxyl compound. At the end, the performance of the MS/Ag 2 CO 3 catalyst was compared with the previous catalysts in the Knoevenagel condensation (Table 3). As demonstrated, the study showed that MS/Ag 2 CO 3 is a catalyst with higher efficiency, stability and durability time than other catalysts. These findings are attributed to the magnetic nature and the chemically immobilized Ag 2 CO 3 particles. In fact, the high performance of Ag 2 CO 3 NPs in the catalytic processes is due to its bifunctional role as both inorganic base and Lewis acid.

Conclusion
In summary, a novel magnetic silica-supported Ag 2 CO 3 (MS/Ag 2 CO 3 ) was successfully prepared and its catalytic performance was studied. The FT-IR and EDX techniques showed the well immobilization of Ag 2 CO 3 particles on the MS nanomaterial. The wide-angle PXRD analysis demonstrated the high stability of Fe 3 O 4 NPs during steps of catalyst preparation. The PXRD pattern also confirmed the well formation of Ag 2 CO 3 NPs on the MS nanocomposite. The superparamagnetic behavior of MS/Ag 2 CO 3 was confirmed by the VSM analysis. The SEM image also demonstrated a uniform spherical structure for this catalyst. The MS/Ag 2 CO 3 nanocatalyst was efficiently employed in the Knoevenagel condensation under moderate conditions and delivered the desired products in high to excellent yield. Also, MS/Ag 2 CO 3 could be recycled and reused with maintaining its activity in several times.    Mg@PS-anthra Cat. 0.05 g, solvent-free, r. t., 2 h 5 77 MS/Ag 2 CO 3 Cat. 0.015 g, solvent-free, 60 °C, 20 min, ))) 6 This work