Isolated Pt Atoms Stabilized by Ga2O3 Clusters Confined in ZSM-5 for Nonoxidative Activation of Ethane

Abstract

Selective activation of light alkanes is an essential reaction in the petrochemical industry for producing commodity chemicals, such as light olefins and aromatics. Because of the much higher intrinsic activities of noble metals in comparison to non-noble metals, it is desirable to employ solid catalysts with low noble metal loadings to reduce the cost of catalysts. Herein, we report the introduction of a tiny amount of Pt (at levels of hundreds of ppm) as a promoter of the Ga₂O₃ clusters encapsulated in ZSM-5 zeolite, which leads to ∼20-fold improvement in the activity for ethane dehydrogenation reaction. A combination of experimental and theoretical studies shows that the isolated Pt atoms stabilized by small Ga₂O₃ clusters are the active sites for activating the inert C-H bonds in ethane. The synergy of atomically dispersed Pt and Ga₂O₃ clusters confined in the 10MR channels of ZSM-5 can serve as a bifunctional catalyst for the direct ethane-benzene coupling reaction for the production of ethylbenzene, surpassing the performances of the counterpart catalysts made with PtGa nanoclusters and nanoparticles.

Figure 1

Illustration of the structural features of typical Pt-based catalysts for dehydrogenation of light alkanes. In this figure, we take the combination of Pt and Ga as the main active component as an exemplary description. (a) Bimetallic and trimetallic Pt-based nanoparticles. The Pt atoms are segregated by the Ga or a third metal, resulting in the formation of single-atom sites in the bimetallic and trimetallic nanoparticles. (b) Combination of subnanometer Pt sites (single Pt atoms or clusters) and Ga₂O₃ nanoparticles supported on Al₂O₃. (c) Zeolite-confined subnanometer PtGa clusters made with metallic Pt species and reduced GaO_x or metallic Ga species. (d) Pt-GaO_x sites made with a single Pt atom and a GaO_x cluster. The Pt-GaO_x clusters are confined in the microporous channels of the zeolite.

Figure 2

HAADF-STEM images of metal-zeolite materials with different compositions. (a) 3Ga-ZSM5, (b) 0.04Pt-ZSM5, (c) 0.011Pt-3Ga-ZSM5, (d) 0.04Pt-3Ga-ZSM5, (e) 0.15Pt-3Ga-ZSM5, and (f) 0.43Pt-3Ga-ZSM5 samples. In the 3Ga-ZSM5 sample, we can observe the presence of GaO_x clusters within the ZSM-5 crystallites. In the 0.04Pt-ZSM5 sample, small Pt nanoparticles are observed. Regarding the 0.011Pt-3Ga-ZSM5 and 0.04Pt-3Ga-ZSM5 samples, they show a morphology similar to that of the 3Ga-ZSM5 sample because the Pt species predominantly exist as atomically dispersed species, which are not visible in these low-magnitude HAADF-STEM images. In the 0.15Pt-3Ga-ZSM5 and 0.43Pt-3Ga-ZSM5 samples, some Pt clusters and nanoparticles with high contrast (brightness in the HAADF-STEM images) are observed due to the high Pt loadings.

Figure 3

Characterization of the 0.04Pt-3Ga-ZSM5 sample by high-resolution electron microscopy. (a,b) Low-magnitude HAADF-STEM images of the 0.04Pt-3Ga-ZSM5 sample, showing the presence of GaO_x clusters. Due to the low magnitude, the single Pt atoms are not visible in these images. (c,e) High-resolution HAADF-STEM images of Pt-GaO_x clusters and (d,f) corresponding intensity profiles of the Pt-GaO_x clusters. The baselines of the contrast profiles are influenced by the thickness of the zeolite support. Nevertheless, based on the working principle of the HAADF-STEM imaging technique, the positions of the single Pt atoms can be determined due to their much higher contrasts than the neighboring atoms (Ga or O atoms). (g–j) Representative HAADF-STEM and the paired iDPC-STEM images of the Pt-GaO_x clusters in the 0.04Pt-3Ga-ZSM5 sample, which indicate their locations in the sinusoidal 10MR channels of the ZSM-5 zeolite. The sinusoidal and straight 10MR channels are marked in the iDPC-STEM images.

Figure 4

Characterization of PtGa-ZSM5 materials by X-ray absorption spectroscopy. Pt L₃-edge XANES (a) and EXAFS (b) spectra and references (Pt foil and PtO₂). Ga K-edge XANES (c) and EXAFS (d) spectra and references (Ga foil and Ga₂O₃).

Figure 5

Catalytic tests of metal-zeolite catalysts for EDH. (a) Catalytic performances of Pt-ZSM5, Ga-ZSM5, and PtGa-ZSM5 catalysts with different Pt contents for EDH. (b) Catalytic performances of 0.04Pt-xGa-ZSM5 catalysts with different Ga loadings. Reaction conditions: 600 °C, 200 mg of the catalyst, C₂H₆/N₂ with a ratio of 1/9 and a total flow of 50 mL/min as the feed gas. (c) Initial specific activities of Pt-ZSM5, Ga-ZSM5, and PtGa-ZSM5 catalysts for EDH. These results were obtained under the kinetic regime and normalized to the total number of Pt atoms in the sample. The conversion of C₂H₆ is controlled to be in the kinetic regime (ethane conversions below 15%) by varying the weight hourly space velocity. It should be noted that, under our reaction conditions, the thermodynamic equilibrium conversion of ethane is 44.2% by assuring that ethylene is the only product. (d) Specific forward reaction rate k_f and productivity of 0.04Pt-3Ga-ZSM5 and other catalysts reported in the literature for EDH at 600–660 °C. The specific forward reaction rates of different catalysts were calculated based on the catalytic results presented in the literature works. Detailed information about the catalysts is summarized in Table S3.

Figure 6

Theoretical studies on ethane dehydrogenation over PtGa-based catalysts. (a) Reaction mechanism for ethane dehydrogenation. (b) Activation energy for the elementary reaction steps during ethane dehydrogenation over different catalytic models. (c) Schematic diagram of the catalytic mechanism for ethane dehydrogenation over PtGa-ZSM5 catalysts. (d) Trend diagrams of the reaction activity and durability over different catalytic models.

Figure 7

Catalytic performances for the ethane–benzene coupling reaction. (a) Schematic diagram of the reaction of ethane and benzene to ethylbenzene. (b) Conversion rates of ethane and benzene on different metal-zeolite catalysts. (c) Stability test of the 0.04Pt-3Ga-ZSM5 catalyst for the ethane–benzene coupling reaction. The reaction rates of ethane and benzene as well as the selectivity to ethylbenzene are displayed. (d) Product distribution of different metal-zeolite catalysts calculated based on ethane. (e) Product distribution of different metal-zeolite catalysts calculated based on benzene. Reaction conditions: 400 °C, a mixture of ethane and benzene (ethane/benzene molar ratio of 9) as the reaction feed, and weight hourly space velocity at 0.71 h^–1 based on ethane. The other products shown in panels (e–d) are mainly made by trimethylbenzene and other heavy aromatics.