Enhancing Gel Strength and Catalytic Performance of a Nanocomposite Catalyst by Incorporating a Reinforcing Hydrogen-Bonding Component

Abstract

The gelator molecule triethylammonium 5-(3,5-bis((1H-tetrazol-5-yl)carbamoyl)benzamido)tetrazol-1-ide (G7) has been studied in our previous work, where G7 formed an organogel, metallogels, and silver nanocomposite which was used as a catalyst for the reduction of nitroaromatics. During the catalytic study of the nanocomposite M 1 G7AgNPs, it was found that the recyclability of the catalyst decreases with the increasing number of catalytic cycles due to the aggregation of silver nanoparticles. From there, the motivation is to increase the strength of the gel network so that the synthesis of a more stabilized nanocomposite can be obtained. To increase the strength of organogel G7 and study the effect of this modification on the catalytic activity of its nanocomposite catalyst, a molecule with H-bonding donor and acceptor sites, N 2,N 4,N 6-tri(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine (TPTT), has been synthesized and characterized by spectroscopic techniques. TPTT (or T) and G7 formed a reinforced gel in a DMSO:H2O mixture and exhibited an effect of strong hydrogen bond formation on the rheological and catalytic properties of the organogel and metallogel of G7T. The combined gelator system fabricated three metallogels: M 1 G7T, M 2 G7T, and M 3 G7T (M1 = Fe(III), M2 = Cu(II), and M3 = Ag(I)). Among these, M 1 G7T has been used for the production and stability of silver nanoparticles and formed M 1 G7TAg nanocomposites. This dual metal system has been used as a catalyst to reduce nitro substituents, i.e. 4-nitrophenol, 4-nitroaniline, 2-amino-5-nitrobenzonitrile, etc., into their respective amine products. Here, during the reaction the substrate which contained adjacent nitrile and amine functional groups shows indazole formation, which is a very important part of organic transformations. The recyclability of the catalyst has been done with the conversion of 4-nitrophenol to 4-aminophenol and it was found that it can be unchanged even with five repeating reactions. The organogels and metallogels of the combined G7 and T molecule show enhancement in storage modulus from 586 to 3091 Pa, showing that after the combination of the T molecule with G7, a strong hydrogen-bond interaction increases; therefore, the strength of organogels and metallogels also increases. Additionally, M 1 G7TAg demonstrates the fabrication of metallogels using G7 and T, exhibiting thixotropic behavior that is validated by time oscillation sweep studies. Therefore, the work shows major changes in the rheological properties of G7T as compared to the G7 organogel. Due to increasing strength, the stabilization of silver nanoparticles inside the metallogel network (M 1 G7T) increases, increasing the recyclability of nanocomposite catalyst M 1 G7TAg.