Kinetic Insights into the Tandem and Simultaneous Mechanisms of Propylene Epoxidation by H₂ and O₂ on Au–Ti Catalysts

Abstract

Au−Ti proximity is thought to be responsible for propylene epoxidation by H 2 and O 2 to PO formation (HOPO). However, we find that the Ti sites intimate with Au accounts for small proportion relative to Ti sites remote from Au, based on the semi-quantification of the number of Au nanoparticles and Ti sites on the uncalcined TS-1-supported Au nanoparticle (Au/TS-1-B) catalysts. Whether those remote Ti sites are functioning in PO generation remains unclear. Herein, the abundant remote Ti sites are proposed to catalyze PO formation by the tandem mechanism with the help from the migration of active oxidant species. We isolated the Au sites and Ti sites by physically mixing uncalcined silicalite-1-supported Au (Au/S-1-B) and TS-1-B as a tandem catalyst with much less Au−Ti proximity sites. Comparable performance for HOPO of the mixed catalyst was observed relative to the Au/TS-1-B catalyst, suggesting that the tandem mechanism is also responsible for PO formation. The H 2 /D 2 exchange experiment, in situ diffuse reflectance infrared Fourier transform spectroscopy, and kinetic modeling were combined to further elucidate the mechanism. Only the kinetic model including both the tandem mechanism on remote Ti sites and the simultaneous mechanism on the Au−Ti proximity can well describe the experimental results, and the tandem mechanisms have a larger contribution to PO formation than the simultaneous mechanism on the Au/TS-1-B catalyst with Au nanoparticles on the external surface. However, for the Au/TS-1 catalyst with intraporous tiny Au clusters, the PO formation proceeds by the simultaneous mechanism. This work unveils the function of remote Ti sites in PO formation. Moreover, the kinetic behaviors are connected with the catalyst structure (micropore volume and Au location), which can provide guidance for enhancing the catalytic performance by designing efficient tandem catalysts for the HOPO process.

Highly Efficient Epoxidation of Propylene with In Situ-Generated H2O2 over a Hierarchical TS-1 Zeolite-Supported Non-Noble Nickel Catalyst

“…Next, we isolated the Ni sites and Ti sites by physically mixing S-1-supported Ni (Ni/S-1) and TS-1 as a tandem catalyst. A slight decrease in the activity of the mixed catalyst was observed relative to the Ni/TS-1 catalyst (Table S3), suggesting that Ni/TS-1 is consistent with a tandem mechanism of diffusion of gas-phase H 2 O 2 between the Ni sites and the Ti sites …”

“…A slight decrease in the activity of the mixed catalyst was observed relative to the Ni/TS-1 catalyst (Table S3), suggesting that Ni/TS-1 is consistent with a tandem mechanism of diffusion of gas-phase H 2 O 2 between the Ni sites and the Ti sites. 47 2.3. Catalytic Mechanism.…”

Phosphate-Modified Effects on Uncalcined TS-1-Immobilized Au Catalysts for Propylene Epoxidation with H2 and O2

“…For a deep understanding of the several effects of P modification on the Au and Ti active sites over the bifunctional catalyst, the physical mixing method was employed referring to our previous works, − which applies different combinations of uncalcined Ti-free silicalite-1 zeolite immobilized Au catalysts (i.e., Au/S-1-B) and TS-1-B zeolite samples for the HOPO reaction test. The corresponding results of catalytic performance of the physically mixed conventional Au/S-1-B catalyst with pristine TS-1-B sample are presented in Figure , which shows a similar PO activity of ca.…”

Supported AgCuCl Nanoclusters as Bimetal Catalysts for Propylene Epoxidation with Molecular Oxygen

“…Propylene oxide (PO) is an important organic chemical raw material mainly used in the production of polyether polyol, propylene carbonate, propylene glycol, and other chemicals. − Most of the industrial PO has been manufactured via the chlorohydrin or Halcon process, which poses significant environmental risks owing to the utilization of toxic chemicals, the generation of excessive wastewater, and the requirement of complex processes to separate and purify the coproducts. − Although the direct epoxidation of propylene using hydrogen peroxide (H 2 O 2 ) as an oxidizing agent (HPPO process) can achieve high PO selectivity (>90%), these this process suffers from low propylene conversion and low hydrogen efficiency . Recently, the direct epoxidation of propylene with molecular oxygen (DEP) has attracted considerable attention due to its environment-friendly and 100% atomic utilization. , Despite numerous attempts, the lack of efficient catalysts with high selectivity remains a significant challenge …”