Comparative study of size-selected gold clusters (Au38) and gold nanoparticles over porous cerium-based metal–organic frameworks with UiO-66 architecture for aerobic oxidation of cinnamyl alcohol

Abstract

The catalytic activity of the size-selected gold clusters Au38(SCH2CH2Ph)24 (abbreviated with Au38) in 3-D nanoporous Ce-metal–organic framework (Ce–UiO-66) was studied. The Au38 nanoclusters were in situ immersed in Ce–UiO-66 with a loading percentage of 1 wt%. The Au38@Ce–UiO-66 catalyst demonstrated excellent catalytic activity for aerobic oxidation of cinnamyl alcohol. The oxidation reaction was tested from 40 to 100 °C utilizing O2 gas (30 ml min−1) as an oxidizing agent in atmospheric pressure. 0.1 mol% of the Au38@Ce–UiO-66 catalyst achieved 65% cinnamyl alcohol conversion at 100 °C within an hour with 100% selectivity toward cinnamaldehyde in toluene. In the case of using an aqueous solvent instead of an organic solvent (toluene), the conversion was increased to 75% under the same reaction condition. The cinnamyl alcohol conversion was increased gradually by increasing the reaction time over Au38@Ce–UiO-66 catalyst, where the conversion was enhanced to 93% after 5 h of heating at 100 °C in water. The catalytic activity of the Au38@Ce–UiO-66 catalyst was largely enhanced after oxygen thermal pretreatment at 100–200 °C, where the catalyst achieved 990 h−1 turnover frequency (TOF) after O2-pretreatment at 175 °C with 92% cinnamaldehyde and 8% cinnamic acid selectivity. The unique atomic packing structure and electronic properties of the size-selected gold nanoclusters (Au38) are responsible for their extraordinary catalytic activity. Moreover, the support (Ce–UiO-66) has a big role in the catalyst activity, since it has a high ability to absorb large quantities from oxygen on the Ce(III)/Ce(IV) centers and generate active oxygen species such as superoxo and peroxo species on the catalyst surface, which help in oxidation of cinnamyl alcohol. For comparison, gold nanoparticles were loaded over the same support, where the Au(III) ions were reduced by two different reduction methods utilizing NaBH4 in the absence of the ligand and hydrogen reduction at 200 °C, 4 h. The particle size of the Au38 clusters and gold nanoparticles over Ce–UiO-66 was investigated by a high-resolution transmission electron microscope (HR-TEM). The charge of the gold clusters and nanoparticles was zero as confirmed by X-ray photoelectron spectroscopy (XPS). The crystallinity and surface texture properties of the prepared catalysts and the pure support were measured by powder X-ray diffraction analysis and N2 gas sorption isotherm at − 196 °C, respectively.