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 SiO 2 ‐ L ( L  = tbtm, btm, pytm). Subsequent reactions with Pd (CH 3 COO) 2 in CH 2 Cl 2 yielded Pd/SiO 2 ‐ L . ICP‐MS reveals that Pd loadings are higher with increased N loadings, resulting in the following trend: Pd/SiO 2 ‐tbtm (0.83 mmol Pd g −1 ) > Pd/SiO 2 ‐btm (0.65 mmol Pd g −1 ) ~ Pd/SiO 2 ‐pytm (0.63 mmol Pd g −1 ). Meanwhile, TEM images of the used Pd/SiO 2 ‐ L catalysts after the first catalytic cycle show that the mean size of Pd NPs is highest with Pd/SiO 2 ‐pytm (8.5 ± 1.5 nm), followed by Pd/SiO 2 ‐tbtm (6.4 ± 1.6 nm) and Pd/SiO 2 ‐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/SiO 2 ‐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/SiO 2 ‐btm indicate minimal Pd leaching and similar average Pd NPs sizes after 1 st and 5 th catalytic runs, respectively, confirming that SiO 2 ‐btm is an efficient Pd NPs stabilizer. The Pd/SiO 2 ‐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.