Nickel Nanoparticles Immobilized on a Porous Triazine-Thiourea-Sulfonamide as an Ecient Heterogeneous Catalyst for Reduction of Carbonyl Compounds

In order to investigate the role of silica template, polymers and nickel nanoparticles on the reduction of aldehydes/ketones, a novel porous triazine-thiourea-sulfonamide polymeric organic support (TTSA) was prepared via in-situ polymerization of melamine (1,3,5-triazine-2,4,6-triamine), thiourea and benzene-1,3-disulfonyl chloride in the presence of silica nanoparticles as a template led to the synthesize silica TTSA nanocomposite. Next, after removal of the template, the nanocomposite was transformed into mesoporous poly triazine-thiourea-sulfonamide. Ni nanoparticles (Ni NPs) were then decorated on the designed mesoporous polymer support and the resulting TTSA@Ni NPs nanospheres were employed as heterogeneous catalyst in the construction of alchohols from reduction of aldehydes/ketones using formic acid/triethyl amine as a reducing agent in water as solvent. The catalyst is low-cost, non-toxic, and can be reused for several catalytic cycles without decreasing the activity.


Introduction
Recently, great emphasis is given to the role of environment-friendly technologies in chemical reactions. In relation to environment-friendly technologies, catalytic processes (e.g. toxic and expensive reagents removal, minimization of by-products, and simplifying of workup procedures) are particularly important factors in uencing the e ciency of the organic reaction 1 . Among the signi cant novel synthetic protocols developed in organic chemistry, the selective reduction of aldehydes and ketones to alchohol is most interesting one 2 . Alcohols can be converted to the various valuable chemicals using etheri cation, condensation, esteri cation, and oxidation [3][4][5] . In this regard, the importance of alcohol production will enforce the development of novel technologies for the production of alcohols using nanocomposites.
Recently, synthesis of metallic nanoparticles (M NPs) for use as catalyst have gained particular interest in different organic reactions [6][7][8][9] . The nano shape, size and a large surface area by volume ratio provides unique characterstics to nanocatalysts due to the structural and electronic changes which differentiates them from the original material 10 . However, the self-agglomeration and leaching problems can be negatively effect on catalytic or recycling performance of M NPs [11][12] . Selection or preparation of ideal catalyst supports, which have high surface area and ligation sites to strongly interact with M NPs, can be regarded as an e cient way to minimize these problems 13 . Up to now, different organic or inorganic solid catalyst supports have used to immobilize various M NPs [14][15][16] .
In this regard, porous cross-linked polymers consisting of different functional group, used as powerful heterogeneous catalysts used to generate several novel organic reactions under mild conditions, providing high selectivity. Their ease of handling and processing, recyclability, high thermal and chemical stabilities, low-cost production and large surface areas are important factors in uencing the e ciency of the catalyst application of porous cross-linked polymers 17 . Although different methods have been reported for the creation of porousity in polymers (self-assembly, block copolymer, template techniques, and a combination of templating and self-assembly techniques), in fact, silica template technique has gained a great attention in catalysis as it improves the e ciency of catalyst by high metal loading, high surface area and pore volume which may favorable for easy sorption of reactant and products [18][19][20] .
To date, bifunctional organocatalysts which contains sulfonamide and a basic amine moiety have attracted remarkable attention of organic chemists due to its numerous synthetic applications 21 .
Polysulfonamides are synthesized by the reaction of disulfonyl chloride and various amines. Cross-linked polysulfonamides, which are novel category of polysulfonamides, has the ability to immobilize different MNPs 22 . Cross-link is a bond which links one polymer chain to another. In this study, porous cross-linked triazine-thiourea-sulfonamide (TTSA) is considered as a novel support which has the ability to immobilize Ni nanoparticles. Porous TTSA was prepared from insitu polymerization of benzene-1,3-disulfonyl chloride, thiourea and melamine and silica nanoparticles. Melamine was used as a cross-linker and silica was used as a template. In the second step, removal of the silica template led to the synthesis of porous triazine-thiourea-sulfonamide (TTSA). Finally, porous TTSA was used as a mesoporous substrate for the stabilization of Ni nanoparticles (Scheme 1). The presence of sulfonamide, thiourea and melamine groups lead to facile immobilization of Ni NPs with high loading capacity, high chemical and thermal stability. Thus, the development of multifunctional polysulfonamide bearing sulfonamide, melamine and thiourea groups along with hydrophilicity and high surface area, as a novel porous support, can revolutionize the polysulfonamide chemistry.
In continuation of our interest to design of polysulfonamide-based catalyst for different organic reactions 23 , herein, we synthesized a novel stabilizer composed of triazine-thiourea-sulfonamide (TTSA) as a novel category of polysulfonamides. Nickel nanoparticles were then successfully immobilized on prepared support (Scheme 1) and fully characterized by FT-IR, FE-SEM, EDS, TGA, N 2 -adsorptiondesorbtion and WDX. Next, catalytic role of TTSA@Ni nanocomposite were studied in the green synthesis of alchohols from the reduction of aldehydes and ketones. The presence of thiourea and melamine as chelating groups plays an important role in the immobilization of Ni NPs. Moreover, the amphiphilicity property of polymer substrate, which is due to the presence of hydrophilic monomers (thiourea and melamine), lead to the green reduction of aldehydes/ketones in aqueous solution. Additionally, we have tested recyclability/reusability of TTSA@Ni nanocomposite (Scheme 2). Catalytic studies indicate that TTSA@Ni nanocomposite has signi cant catalytic performance by producing various alchohols with high reaction yields. More importantly, used TTSA@Ni nanocomposite easily recollected via centifusion and were reused several times in consecutive reactions by negligible leaching.

Characterization of TTSA@Ni NPs
The FT-IR spectrum of SiO 2 @TTSA (A), porous TTSA (B), TTSA@Ni (C), are shown in Fig. 1. Initially, FT-IR analysis of SiO 2 @TTSA displayed obvious peaks at 1172 cm -1 and 1462 cm -1 are related to the Si-O stretching vibrations of silica nanoparticles. The broadening peak at 3200-3400 cm -1 con rmed the presence of the N-H and NH 2 bond. In the FT-IR of SiO 2 @TTSA, the stretching vibration at 1184 and 1411 cm -1 for the SO 2 bands, indicating the presence of sulfonamide groups. In addition, the adsorption peak at 1716 cm -1 belong to the C=S band in the thiourea groups. The peaks at 1668 cm -1 approved the presence of melamine in the structure of SiO 2 @TTSA nanocomposite (Fig. 1A). There is no signi cant change in the position of peaks, after silica etching (Fig. 1B). In the nal step (Fig. 1C), the bending vibration absorption peak of C=N shifted to 1640 cm -1 , that this shift con rms the Ni NPs coordination of the C=N of melamine groups.
The size and morphology of porous substrate (TTSA) and the prepared catalyst TTSA@Ni were investigated using FE-SEM, which was shown the mesoporous structure of TTSA ( Fig. 2A). FE-SEM image of TTSA@Ni demonstrated spherical shape morphology and uniform distribution of Ni NPs on porous polymer matrix (Fig. 2B).
The EDS analysis of the TTSA@Ni NPs nanocatalyst reveals the signals of The Ni, O, S, C and N (Fig. 3). Elemental mapping for the prepared catalyst exhibits the homogeneous distribution of the elements (C, N, Ni, O, and S) in the structure of the prepared catalyst as represented in the Fig. 4. The element Ni content in the TTSA@Ni NPs catalyst was con rmed to be 1.794 mmol g −1 , which was recorded by Inductively Coupled Plasma Mass Spectrometry. TGA curve of TTSA@Ni NPs shown in Fig. 5, illustrated a small lose weight at 100 ℃, which could be related to the physically absorbed water and organic solvents. The mass loss at higher temperatures (200-600 ℃) can be correlated to the decomposition of organic group of the polymeric structure (Fig. 5).
The Brunauer-Emmett-Teller (BET) surface areas were determined by N 2 adsorption (Fig. 6), the surface areas for porous TTSA was found to be 22.23 m 2 /g and total pore volume is 0.07 cm 3 g −1 . The porosity of TTSA is important to adsorb the Ni NPs in it. These important surface properties of TTSA can strongly help to the present active catalytic sites for selective hydration of carbonyl compounds into corresponding benzylalchohols.
In order to investigate the amphiphilicity of porous TTSA@Ni, contact angle measurements were applied for water and oil droplets (Fig. 7.). Beacause the contact angle between TTSA@Ni and water (Fig. 7 A) or TTSA@Ni and oil (Fig. 7. B) were below 90°, the good amphiphilicity was observed in the reaction medium.

Catalytic studies
The effect of various reaction parameters, including the amount of catalyst, solvents, hydrogen doner agent, and temperature were investigated in the transfer hydrogenation of benzaldehyde which was selected as test substrate ( Table 1). As Table 1 shows, the model reaction gave negligible yields in the absence of any catalyst at room temperature (entry 1). As a starting point, the effect of different hydrogen doner including formic acid, ammonium format, formic acid/ammonium format and formic acid/triethyl amine was investigated on the model reaction in the presence of 0.03 g (5.4 mol%) TTSA@Ni as the catalyst. Formic acid and ammonium format were found ineffective system (entry no. 2 and 3). Then, the effect of formic acid/ammonium format was investigated in various pH including 0.5, 1.55, 2, 3, 4, 4.5, and 5. Notably, pH plays crucial role in the reaction and the superior activity of the TTSA@Ni was vividly observed in pH of 2.5 (entry 4). Then, the catalytic activity of TTSA@Ni was investigated in the presence of HCOOH/NEt 3 in various pH (entries 5-10). As seen in Table 1, replacement of formic acid/ammonium format with formic acid/triethyl amine in pH of 3.2 signi cantly increases the yield (entry 8). It should be noted, that decreasing the temperature has a negative effect on the progress of the model reaction (entry 11). Furthermore, the model reaction was then performed in the presence of different solvents (entries 12-15) and solvent-free condition (entry 16), and H 2 O provided the desired product in the best yield (entry 8). Finally, the model reaction was examined in the presence of 0.05 g (9 mol%) TTSA@Ni, initially yield % is increased with increasing TTSA@Ni up to 0.03 g (5.4 mol%) but afterward reaction was found independent to amount of catalyst (entry 17).

Proposed mechanism
A proposed mechanism for synthesizing benzyl alcoholes was shown in Scheme 3. In the rst stage, TTSA@Ni facilitates the decomposition of HCOOH in order to produce hydride species in the presence of triethyl amine. Finally, primary and secondary alchohol was obtained through hydrogenation 2 (Scheme 3).
Lastly, reusability/recoverability of TTSA@Ni nanocomposite was studied on the reduction of benzaldehyde. First cycles were conducted in the presence of fresh TTSA@Ni nanocomposite. Following the catalytic tests, used catalyst was recollected from the reaction mixture via centrifusion and washed with water, dried and it was then directly used in consecutive catalytic runs. Reusability/recoverability tests displayed that TTSA@Ni nanocatalyst remained active even after six successive cycle (98, 96, 94, 93, 88, 84) (Fig. 8).

Synthesis of porous triazine-thiourea-sulfonamide (TTSA):
In order to create porosity in the structure of SiO 2 @TTSA nanocomposite, the silica nanoparticles of the SiO 2 @TTSA nanocomposite was selectively removed through etching of the SiO 2 nanoparticles. To SiO 2 @TTSA nanocomposite (0.5 g), HF solution (10 mL, 10 wt%), and deionized water (10 mL) was added and the mixture was stirred at room temperature for 4.5 h. The porous TTSA was ltered off, washed well with water, and dried in air (weight: 0.3 g) (Scheme 1).
Preparation of benzyl alchohol derivatives using TTSA@Ni: In a 10 mL ask, a combination of HCOOH-NEt 3 (1: 0.7) was dissolved in water (1 mL) to prepare a solution with pH of 3.2. Then, the benzaldehyde or acetophenone derivatives (1 mmol) and TTSA@Ni nanocomposites catalyst (0.03 g) were added to the reaction mixture and stirred at 80°C. The development of the reaction was followed by TLC (n-hexane/ethyl acetate, 10:3). After the completion of the reaction, mesoporous TTSA@Ni were separated via centrifusion and the reaction mixture was extracted EtOAc (20 mL). The solvent was evaporated to obtain benzyl alchohol derivatives (Scheme 2).
In some products, the crude product was puri ed by column chromatography, eluting with hexane/EtOAc, to obtain the desired product (Scheme 2).

Conclusion
The main goal of the current study was to synthesis a novel mesoporous polysulfonamide-based catalyst via silica template technique. The results of this investigation show that the favorable surface properties, amphiphilicity and high nickel loading led to high catalyst performance for hydrogenation of aldehydes/ketones in aqueous solution. Moreover, it was found that mesoporous TTSA@Ni were reused six time without any signi cant loss of its activity.

Declarations
Competing interest: No a Isolated yield Table 2: Reduction of different aldehydes and ketones to alcohols in the presence of TTSA@Ni nanocatalyst using formic acid-triethylamine as reducing agent. a        Recycling of the TTSA@Ni for the reduction of benzaldehyde.

Figure 11
Proposed reaction mechanism for the reduction of carbonyl compounds using TTSA@Ni catalyst.

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