Triazole‐based ligands functionalized silica: Effects of ligand denticity and donors on catalytic oxidation activity of Pd nanoparticles

Triazole‐based ligands, tris (triazolyl)methanol (Htbtm), bis (triazolyl)‐phenylmethanol (Hbtm), and phenyl (pyridin‐2‐yl)(triazolyl)methanol (Hpytm), with differences in ligand denticity (i.e., bidentate and tridentate) and type of N donors (i.e., triazole and pyridine) were functionalized onto a silica support to produce the corresponding SiO₂‐ L ( L  = tbtm, btm, pytm). Subsequent reactions with Pd (CH₃COO)₂ in CH₂Cl₂ yielded Pd/SiO₂‐ L . ICP‐MS reveals that Pd loadings are higher with increased N loadings, resulting in the following trend: Pd/SiO₂‐tbtm (0.83 mmol Pd g^?1) > Pd/SiO₂‐btm (0.65 mmol Pd g^?1) ~ Pd/SiO₂‐pytm (0.63 mmol Pd g^?1). Meanwhile, TEM images of the used Pd/SiO₂‐ L catalysts after the first catalytic cycle show that the mean size of Pd NPs is highest with Pd/SiO₂‐pytm (8.5 ± 1.5 nm), followed by Pd/SiO₂‐tbtm (6.4 ± 1.6 nm) and Pd/SiO₂‐btm (4.8 ± 1.3 nm). Based on TONs, catalytic studies toward aerobic oxidation of benzyl alcohol to benzaldehyde at 60 °C in EtOH showed that Pd/SiO₂‐pytm possessed the most active surface Pd(0) atoms, most likely as a result of more labile properties of the pyridine–triazole ligand compared to tris‐ and bis (triazolyl) analogs. ICP‐MS and TEM analysis of Pd/SiO₂‐btm indicate minimal Pd leaching and similar average Pd NPs sizes after 1^st and 5^th catalytic runs, respectively, confirming that SiO₂‐btm is an efficient Pd NPs stabilizer. The Pd/SiO₂‐btm catalyst was also active toward aerobic oxidation of various benzyl alcohol derivatives in EtOH and could be reused for at least 7 reaction cycles without a significant activity loss.