Effects of TiO₂ in Low Temperature Propylene Epoxidation Using Gold Catalysts

Abstract

Propylene epoxidation with molecular oxygen has been proposed as a green and alternative process to produce propylene oxide (PO). In order to develop catalysts with high selectivity, high conversion, and long stability for the direct propylene epoxidation with molecular oxygen, understanding of catalyst structure and reactivity relationships is needed. Here, we combined atomic layer deposition and deposition precipitation to synthesize series of well-defined Au-based catalysts to study the catalyst structure and reactivity relationships for propylene epoxidation at 373 K. We showed that by decorating TiO2 on gold surface the inverse TiO2/Au/SiO2 catalysts maintained ∼90% selectivity to PO regardless of the weight loading of the TiO2. The inverse TiO2/Au/SiO2 catalysts exhibited improved regeneration compared to Au/TiO2/SiO2. The inverse TiO2/Au/SiO2 catalysts can be regenerated in 10% oxygen at 373 K, while the Au/TiO2/SiO2 catalysts failed to regenerate at as high as 473 K. Combined characterizations of the Au-based catalysts by X-ray absorption spectroscopy, scanning transmission electron microscopy, and UV–vis spectroscopy suggested that the unique selectivity and regeneration of TiO2/Au/SiO2 are derived from the site-isolated Ti sites on Au surface and Au–SiO2 interfaces which are critical to achieve high PO selectivity and generate only coke-like species with high oxygen content. The high oxygen content coke-like species can therefore be easily removed. These results indicate that inverse TiO2/Au/SiO2 catalyst represents a system capable of realizing sustainable gas phase propylene epoxidation with molecular oxygen at low temperature.

Theoretical Studies on the Direct Propylene Epoxidation Using Gold-Based Catalysts: A Mini-Review

“…According to Figure 10, it is the OOH species formed on Au nanoparticles that directly transfers to the neighboring Ti sites (interfacial Ti sites at the dual Au/Ti sites) to form active Ti-OOH species and epoxidize propylene to form an oxametallacycle OMP (OMC' or OMC", depending on which carbon of propylene is connected with O). Their KMC simulations closely reproduce the experimental findings [140]. For instance, it was found that the O 2 feed concentration has a slight effect on PO selectivity (since O 2 adsorption on Au particles is very weak), which is in good agreement with the kinetic tests of Taylor and Chen [142,143].…”

“…In their model, H 2 O 2 is first generated from coadsorbed H 2 and O 2 on Au nanoparticles, and H 2 O 2 can then either degrade into water or diffuse to TiO 2 sites supported on SiO 2 to epoxidize propylene molecules [139]. The PO formation rates predicted by KMC simulations are consistent with experimental reports corresponding to different temperatures and feed concentrations of O 2 [140]. In their later kinetic modeling studies [141], several key side reactions encountered during PO formation are taken into account (acrolein formation, propanal and acetone formation from OMP isomerization, oxidative cracking into CO 2 , etc.…”

Precisely Engineered Supported Gold Clusters as a Stable Catalyst for Propylene Epoxidation

“…It is well known that small Au nanoparticles, and particularly those with a core diameter below 5 nm, are crucial to achieve catalytic activity in propylene epoxidation. [ 7a , 22 ] Thus, in an effort to avoid harsh ligand removal methods, we demonstrate a simple way to remove ligands by using a non‐thermal O 2 plasma technique that does not induce an increase in the size of the Au nanoparticles. [23] Figure 2 d schematically illustrates the O 2 plasma removal procedure for Au/TS‐1 materials.…”

Precisely Engineered Supported Gold Clusters as a Stable Catalyst for Propylene Epoxidation

“…This makes it very difficult to control catalytic activity through a specific particle size. It is well known that small Au nanoparticles, and particularly those with a core diameter below 5 nm, are crucial to achieve catalytic activity in propylene epoxidation [7a, 22] . Thus, in an effort to avoid harsh ligand removal methods, we demonstrate a simple way to remove ligands by using a non‐thermal O 2 plasma technique that does not induce an increase in the size of the Au nanoparticles [23] .…”