Tailoring the microenvironment of Ti sites in Ti-containing materials for synergizing with Au sites to boost propylene epoxidation

doi:10.1016/S1872-2067(24)60083-5

Abstract

Abstract:

Au sites supported on Ti-containing materials (Au/Ti-containing catalyst) are currently considered as a promising catalyst for the propylene epoxidation owing to the synergistic effect that hydrogen peroxide species formed on Au sites diffuses to the Ti sites to form the Ti-hydroperoxo intermediates and contributes to the formation of propylene oxide (PO). In principle, thermal treatment will significantly affect the chemical and physical structures of Ti-containing materials. Consequently, the synergy between tailored Ti sites with different surface properties and Au sites is highly expected to enhance the catalytic performance for the reaction. Herein, we systematically studied the intrinsic effects of different microenvironments around Ti sites on the PO adsorption/desorption and conversion, and then effectively improved the catalytic performance by tailoring the number of surface hydroxyl groups. The Ti^VI material with fewer hydroxyls stimulates a remarkable enhancement in PO selectivity and H₂ efficiency compared to the Ti^VI material that possessed more hydroxyls, offering a 7-fold and 4-fold increase, respectively. As expected, the Ti^VI+IV and Ti^IV materials also exhibit a similar phenomenon to the Ti^VI materials through the same thermal treatment, which strongly supports that the Ti sites microenvironment is an important factor in suppressing PO conversion and enhancing catalytic performance. These insights could provide guidance for the rational preparation and optimization of Ti-containing materials synergizing with Au catalysts for propylene epoxidation.

Key words: Propylene epoxidation, Microenvironment, Ti-containing material, Au-Ti synergy, Hydroxyl group

Shudong Shi, Zhihua Zhang, Yundao Jing, Wei Du, Xuezhi Duan, Xinggui Zhou. Tailoring the microenvironment of Ti sites in Ti-containing materials for synergizing with Au sites to boost propylene epoxidation[J]. Chinese Journal of Catalysis, 2024, 63: 133-143.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(24)60083-5

https://www.cjcatal.com/EN/Y2024/V63/I8/133

Figures/Tables 7

Fig. 1. (a) Illustration for the PO isomerization over Ti-containing samples. PO conversion (b,c), and propanal and acetone selectivity (d) over different Ti-containing samples after introducing of PO. Conditions for PO conversion: PO/N2 = 0.5/99.5 vol%, temperature = 200 °C, GHSV = 14000 mL·g-1·h-1.

Fig. 2. Arrhenius plots for PO conversion (a) and the logarithm of multiplication of concentration effect (f(c)) and pre-exponential factor (A) (b) of different TiVI-containing samples after introducing of PO. Conditions for PO conversion: PO/N2 = 0.5/99.5 vol%, temperature = 200 °C, GHSV = 14000 mL·g-1·h-1.

Fig. 3. XRD patterns (a), Raman spectra excited at 532 nm (b) of different TiVI-containing samples and the enlarged view of HRTEM images and the corresponding lattice fringe images of TiVI-L (c,f), TiVI-M (d,g) and TiVI-H (e,h).

Fig. 4. HRTEM images of TiVI-L (a), TiVI-M (b) and TiVI-H (c). N2 adsorption-desorption isotherms (d), BJH pore size distribution (e), BET surface area and pore volume (f), NH3-TPD spectra (g), Py-IR spectra at different evacuation temperatures (h), the corresponding quantitative results of different acid sites (i), the corresponding quantitative results of different acid strengths (j) and FT-IR spectra (k) of different TiVI-containing samples.

Fig. 5. In situ PO-DRIFTS spectra collected at different times on TiVI-L (a), TiVI-M (b) and TiVI-H (c) and flushed with Ar on different TiVI-containing samples for 0 min (d), 15 min (e), and 30 min (f). Conditions for DRIFTS: T = 15 °C, Ptotal = 0.1 MPa, PO/N2 = 15 mL·min-1 (1.5 vol%), and TiVI samples weight 30 mg. The spectra were collected every 10 min.

Fig. 6. (a) Schematic diagram for the catalyst test of propylene epoxidation in the presence of H2 and O2 using Au-Ti bifunctional catalysts. PO selectivity (b) and PO formation rate (c,d) of different Ti-containing samples synergizing with Au catalysts. Conditions for HOPO: H2/O2/C3H6/N2 = 10/10/10/70 vol%, temperature = 200 °C, GHSV = 14000 mL·g-1·h-1.

Fig. 7. (a) PO rate and selectivity, and H2 efficiency of Au sites synergizing with TiIV-L catalyst obtained by physical mixture and separated bed. (b) PO selectivity and rate of Au sites synergizing with TiIV-L catalysts obtained by different weight ratios. Conditions for HOPO: H2/O2/C3H6/N2 = 10/10/10/70 vol%, temperature = 200 °C, GHSV = 14000 mL·g-1·h-1.

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