Effective Regulation of the Au Spatial Position in a Hierarchically Structured Au/HTS-1 Catalyst: To Boost the Catalytic Performance of Propene Epoxidation with H₂ and O₂

Abstract

The traditional deposition–precipitation (DP) method is common but not universal in manipulating Au nanoparticles’ spatial locations in propene epoxidation catalysts, especially for hierarchical hydrophilic TS-1 supports. Directional loading of Au nanoparticles to hierarchical hydrophilic TS-1 pores is challenging because the strong hydrophilicity of the hierarchical TS-1 zeolite would make Au nanoparticles loaded by the DP method tend to be deposited on the hydrophilic support surface, thus blocking the pore mouths and leading to deactivation. Therefore, manipulating Au nanoparticles’ spatial locations on hierarchical hydrophilic TS-1 supports is of great significance to enhance the catalytic performance but is a bottleneck problem. In this work, taking hierarchical hydrophilic HTS-1 supports as an example, we propose a new modified isometric impregnation (NIMG) method to manipulate the spatial location of Au nanoparticles. Moreover, the Au spatial location inside HTS-1 pores is quantitatively reflected by introducing the V na parameter. Combined with the N2 physisorption, high-angle annular dark-field scanning transmission electron microscopy, inductively coupled plasma, and X-ray photoelectron spectroscopy, it is found that the Au nanoparticles loaded by the NIMG method tend to be uniformly distributed inside the pores of HTS-1 due to the capillary effect. As expected, the Au/HTS-1­(NIMG) catalyst (with the Au nanoparticles inside the pores) exhibits a much higher propylene oxide (PO) formation rate, PO selectivity, and H2 efficiency than the Au/HTS-1­(DP) catalyst (with the Au nanoparticles on the outer surface). This is possibly because loading Au nanoparticles inside the pores is conducive to the transfer of H2O2 from Au to the nearby abundant Ti active sites, thus weakening the decomposition of H2O2 to H2O, avoiding the ring-opening side reactions, and promoting propene epoxidation into PO. This work sheds new light for controlling the spatial location of Au in hierarchically structured hydrophilic catalysts for direct propene epoxidation.

Review and perspectives on TS-1 catalyzed propylene epoxidation

“…Au SG, Hydrothermal method 7.4 85.0 119 21 Huang et al., 228 Au/HTS-1(NIMG) 0.10% wt. Au NIMG, Hydrothermal method 6 91.2 150 25 Sheng et al., 229 Au/TS-1-B 0.03% wt. Au IWI, Hydrothermal method – 95.0 88 42 Zhang et al., 226 Au/TS-1 0.12% wt.…”

“…Highly dispersed (<5nm) gold nanoparticles were loaded onto amorphous or crystalline oxides containing Ti to catalyze the epoxidation of olefins with O 2 as an oxidant. 229 , 291 Au nanoparticles adsorb H 2 /O 2 to produce hydrogen peroxide and migrate to nearby isolated tetrahedral Ti 4+ sites to form Ti-OOH for propylene epoxidation. 292 Hayashi et al.…”

“… 228 When TS-1 molecular sieves with the above hierarchical structure were used as carriers, Au nanoparticles prepared by the DP method tended to deposit on the hydrophilic outer surfaces of the carriers, thereby clogging the pore mouths. 229

Figure 14 Design of catalysts (A) Schematic diagram of Au locations of Au/TS-1-O and Au/TS-1-B catalysts. 294 Copyright 2013, Elsevier B.V. (B) Au loading and (C) PO formation rate of Au/TS-1 catalysts prepared at different temperatures as a function of aging time.

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“…This shows that the capillary effect of the novel impregnation method can prevent Au nanoparticles from accumulating on the Au/HTS-1(NIMG) catalyst surface. 229 , 320 Lei et al. developed a modified incipient wetness impregnation (Miwi) method for the immobilization of Au NPs on TS-1 through hydrolysis (at a Ph of ca.…”

TS-1 with Abundant Micropore Channel-Supported Au Catalysts toward Improved Performance in Gas-Phase Epoxidation of Propylene

“…It is efficient to increase the PO selectivity by promoting PO desorption and diffusion in the TS-1 channel; hence, a variety of hierarchical TS-1 zeolites were developed. − TS-1 zeolites were synthesized using F127 with a mesopore size of 3.3 nm (MTS-1). The epoxidation of propylene catalyzed by Au/MTS-1 yielded a PO selectivity of 94% and C 3 H 6 conversion of 7.7% .…”