Synthesis, characterization, and fabrication of silver nanoparticles in 1-vinyl imidazole-based hydrogels and their use in olefin oxidation, hydrogen generation, and oxo-anion adsorption

Poly(1-vinylimidazole), (p(1-VIm)), hydrogels were synthesized via free radical polymerization reaction and modified with different alkyl dihalides of variable chain lengths such as 1,2-dibromoethane (1,2-BE) and 1,4-dibromobutane (1,4-BB) to obtain polymeric ionic liquid. The chemical structure of synthesized p(1-VIm) macroporous hydrogel was confirmed by Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy. P(1-VIm)-Ag, 1,2-BE-p(1-VIm)-Ag, and 1,4-BB-p(1-VIm)-Ag were prepared by reducing Ag(CH3COO) metal salts loaded into p(1-VIm) hydrogels and then reducing with NaBH4. Metal nanoparticles embedded p(1-VIm)-Ag were visualized by transmission electron microscopy. The metal content of all composites was estimated via atomic absorption spectroscopy, and its amount was 4.44 mmol/g hydrogel, 5.44 mmol/g hydrogel, and 8.39 mmol/g hydrogel, for p(1-VIm), 1,2-BE-p(1-VIm), and 1,4-BB-p(1-VIm), respectively. According to the results, metal content of quaternized hydrogels is higher than nonquaternized. P(1-VIm)-Ag, 1,2-BE-p(1-VIm)-Ag, and 1,4-BB-p(1-VIm)-Ag composites were used as catalyst in the aerobic oxidation of olefins by emphasizing the effects of different parameters such as temperature, substituent effect, etc. Olefin oxidation reaction was carried out in higher oxidation conversion by 1,2-BE-p(1-VIm)-Ag and 1,4-BB-p(1-VIm)-Ag than p(VIm)-Ag, due to high hydrophobicity of quaternized composites. The prepared porous hydrogel composites were also used as catalyst in H-2 generation from hydrolysis of sodium borohydride (NaBH4). The activation energies, enthalpy, and entropy for NaBH4 hydrolysis catalyzed by composites were determined. According to the results, p(VIm)-Ag showed higher catalytic activity. Low performance of 1,4-BB-p(1-VIm)-Ag than 1,2-BE-p(1-VIm)-Ag and p(1-VIm)-Ag is due to high hydrophobicity of this hydrogel than the other catalytic systems whiles reaction medium is water. Furthermore, the p(1-VIm), 1,2-BE-p(1-VIm), and 1,4-BB-p(1-VIm) were utilized in the adsorption of oxo-anions such as Cr2O7-2, CrO4-2 and MnO4-. Between hydrogels, 1,4-BB-p(1-VIm) has better performance than 1,2-BE-p(1-VIm) and p(1-VIm). Because of that, ion absorption capacity is increased with an increase in the number of carbon atoms in the chain of the alkylation agents. In addition, pH dependency of adsorbents was investigated and p(VIm) showed high pH dependency. Indeed, hydrophilic/hydrophobic character and permanent positive charges of Q-p(VIm) hydrogels made them independent of the solution pH. Finally, the catalyst was easily recovered from the reaction medium, and it could be reused for other four runs without significant loss of activity. Durability of catalyst structure without leaching confirmed by FT-IR spectra and atomic absorption spectroscopy, respectively.

Automated summary

Poly(1-vinylimidazole) hydrogels modified with alkyl dihalides were used as catalysts for olefin oxidation and hydrogen generation, demonstrating that composites with higher metal content, hydrophilicity, and hyrophobicity exhibited better catalytic performance.