Preparation and modification of Au/TS-1 catalyst in the direct epoxidation of propylene with H2 and O2
Graphical abstract
Introduction
Propylene oxide (PO), an important propylene based chemical intermediate, is mainly used in the production of organic raw materials like polyether polyols and propylene glycol, which are widely applied in the fields of food, tobacco, cosmetics, medicine and etc. The development of daily necessities has raised increasing demand for PO. Nowadays, PO is mainly produced by the chlorohydrin method and the hydroperoxide process, as shown in Fig. 1. Although the chlorohydrin process shows the advantages of high product selectivity and low construction investment, the use of highly toxic chlorine reagents leads to equipment corrosion and the generation of harmful byproducts. In view of people's increasing awareness of environmental protection and hence more strict laws, the chlorohydrin process is to be abandoned eventually. The hydroperoxide process avoids the drawbacks of environmental pollution and serious corrosion, but it suffers from long process, high investment and strict requirements on the quality of raw materials. Most importantly, the ethylbenzene/isobutane hydroperoxide process co-produces a large amount of styrene/tert-butyl alcohol, which strongly influences the economy of this process. To solve the problems of the above industrial PO production routes, hydrogen peroxide (H2O2) oxidized PO preparation (HPPO) catalyzed by titanium silicate-1 zeolites (TS-1) was developed, with water as the only co-product in theory, as illustrated in Fig. 1. At present, researchers have obtained a deep understanding of this reaction’s mechanism and have scaled it up to industrial production [1,2]. However, in view of the high cost and risk of H2O2 transportation, a H2O2 manufacture plant is usually built close to the PO plant, to obtain the highest economic benefit [3]. A great deal of efforts had been made to remove this limitation before the route of gas-phase propylene epoxidation was found, in which H2O2 was first synthesized by H2 and O2 and then consumed in-situ to oxidize propylene to PO. Compared with the HPPO process, this gas-solid reaction shows obvious merits of easy operation, simple product separation and high economic benefit.
The realization of gas-phase epoxidation of propylene is mainly driven by the development of Au based catalyst. In 1998, Haruta's group proposed for the first time that Au/TiO2 could catalyze propylene to produce PO with high selectivity under H2 and O2 atmosphere. Since then, a variety of Au-Ti catalysts, such as TS-1, TS-2, Ti-Beta, Ti-MCM-41, Ti-MCM-48, Ti-SBA-15 and etc, have been prepared and their catalytic performance have been investigated, as shown in Table 1. It was found that, compared with metal oxide support, Ti-containing zeolites showed better catalytic property, proposed explanation being that their regular pore structure could impose geometric constraints on Au particles to prevent them from migrating and aggregating [4]. Among them, TS-1 zeolite has attracted extensive attention after its success in industrial production. Au/TS-1 catalyst has been optimized to further improve its catalytic performance by changing pH value of deposition solution and adding promoter in Au loading process, as well as modifying the pore structure of TS-1 support [5]. These modification strategies of Au/TS-1 catalysts have been reviewed in this paper and the development direction has been proposed in order to promote the large-scale utilization of the gas phase epoxidation process.
Section snippets
Reaction mechanism and related side reaction
Au/TS-1 catalyst shows excellent catalytic performance for gas-phase epoxidation of propylene, attributed to the synergistic effect between Au particles and TS-1 zeolites [18]. Au active sites catalyze H2 and O2 to generate H2O2, which is utilized in-situ in the epoxidation of propylene adsorbed on the framework Ti atoms of TS-1 support [19]. However, there are two different views of the specific route: the sequential mechanism and the simultaneous mechanism.
The sequential reaction mechanism
Active sites of TS-1 zeolites for the epoxidation reaction
Delgass et al. found that Au supported on TS-1 zeolites had the best catalytic performance compared with rutile, anatase and amorphous titanium species, indicating that tetra-coordination Ti atoms were active sites for gas phase epoxidation of propylene [29]. Non-framework Ti species such as TiO2 were responsible for the cracking of propylene to ethanal and the deep oxidation of PO to CO2 [30,31]. Furthermore, DFT calculation suggested that the [Ti(OSi)3OH] species were more active than fully
Au active sites for the generation of H2O2
According to the mechanism of gas phase epoxidation of propylene, Au nanoparticles are the key active sites to generate H2O2. Studies have shown that their catalytic activity is greatly affected by particle size, location and chemical state [49].
Au species generally exists in the form of single atoms, bilayers of Au, sub-nm clusters, clusters (1−2 nm) or nanoparticles (NPs, 2−5 nm), as shown in Fig. 4. Researchers have different views on the influence of Au species’ size on their catalytic
The pretreatment of Au/TS-1 catalyst
It is well known that the acidic sites induced by excessive hydroxyl groups are detrimental to the selectivity of PO, which is necessary to be diminished. Kanungo et al. modified Au/TS-1 catalyst by silylation to convert the SiOH (TiOH) group into Si-O-SiR3 (Ti-O-SiR3), which enhanced the hydrophobicity of catalyst, promoting the rapid desorption of PO and preventing its ring-opening reaction [81,82]. However, diminished TiOH groups are adverse to the conversion of propylene. In order to
Conclusion
In view of serious pollution problem and poor economic benefit of the chlorohydrin and hydroperoxide process to produce PO in industry, the greener, more efficient and flexible gas-phase epoxidation of propylene catalyzed by Au/TS-1 is seizing more attention. In this process, small Au particles catalyze H2 and O2 to generate H2O2, and then propylene adsorbed on the framework Ti atoms of TS-1 zeolites is oxidized to PO. This review highlighted the synthesis and modification methods to obtain Au
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
This work was supported by the Natural Science Foundation of Shandong Province (No. ZR2020KB006).
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