Toluene methylation with syngas to para-xylene by bifunctional ZnZrOx-HZSM-5 catalysts

doi:10.1016/S1872-2067(21)63975-X

Abstract

Abstract:

Toluene methylation with methanol on H-ZSM-5 (Z5) zeolite for the directional transformation of toluene to xylene has been industrialized. However, great challenges remain because of the high energy barrier of methanol deprotonation to the methoxy group, the side reaction of methanol to olefins, coke formation, and the deactivation of zeolites. Herein, we report the toluene methylation coupled with CO hydrogenation to showcase an enhancement in para-xylene (PX) selectivity by employing a bifunctional catalyst composed of ZnZrO_x (ZZO) and modified Z5. The results showed that a PX selectivity of up to 81.8% in xylene and xylene selectivity of 64.8% in hydrocarbons at 10.3% toluene conversion can be realized over the bifunctional catalyst on a fixed-bed reactor. The selectivity of gaseous hydrocarbons decreased to 10.9%, and approximately half of that was observed in methanol reagent route where the PX selectivity in xylene was 38.8%. We observed that the acid strength, the quantity ratio of Brönsted and Lewis acid sites, and the pore size of zeolites were essential for the PX selectivity. The investigation of the H₂/D₂ kinetic isotope effect revealed that the newborn methyl group in xylene resulted from the hydrogenation of CO rather than toluene disproportionation. Furthermore, the catalyst showed no evident deactivation within the 100 h stability test. The findings offer a promising route for the production of value-added PX with high selectivity via toluene methylation coupled with syngas conversion.

Key words: Toluene methylation, Syngas conversion, Para-xylene, Bifunctional catalyst, ZnZrO_x-ZSM-5

Xiaoqin Han, Jiachang Zuo, Danlu Wen, Youzhu Yuan. Toluene methylation with syngas to para-xylene by bifunctional ZnZrO_x-HZSM-5 catalysts[J]. Chinese Journal of Catalysis, 2022, 43(4): 1156-1164.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(21)63975-X

https://www.cjcatal.com/EN/Y2022/V43/I4/1156

Figures/Tables 12

Fig. 1. Catalytic performance of toluene methylation with syngas over ZZO-nSZ5 (a) and ZZO-3SZ5 (b) with different ZZO mass percentages. Reaction conditions: T = 340 °C, P = 3.0 MPa, H2/CO = 2 (V/V), GHSV = 12000 mL g-1h-1, WHSVtoluene = 0.9 h-1, and time on stream (TOS) = 15 h. Xyl.: xylene, C1-C5: gaseous hydrocarbons, Benz.: benzene, PX: PX in xylene, ET: 4-ethyltoluene, TMB: 1,2,4-trimethylbenzene, C9+: other aromatics, Conv.: toluene conversion.

Fig. 2. Comparison of CO hydrogenation and CH3OH routes. (a) Catalytic performance; (b) Apparent activation energy. Reaction conditions: T = 340 °C, P = 3.0 MPa, and TOS = 15 h. CO hydrogenation route: H2/CO = 2 (V/V), ZZO-3SZ5 mass ratio = 3/7, GHSV = 12000 mL g-1h-1, and WHSVtoluene = 0.9 h-1. CH3OH route: H2/N2 = 3/2 (V/V), GHSV = 12000 mL g-1h-1, WHSVtoluene = 0.9 h-1, and C7H8/CH3OH = 6 (mol/mol).

Fig. 3. Catalytic performance of ZZO and 3SZ5 in different intimacy modes. Reaction conditions: T = 340 °C, P = 3.0 MPa, H2/CO = 2 (V/V), ZZO-3SZ5 mass ratio = 3/7, GHSV = 12000 mL g-1h-1, WHSVtoluene = 0.9 h-1, and TOS = 15 h.

Fig. 4. (a) H2/D2 kinetic isotope effect of toluene methylation with CO + H2 (D2) over ZZO-3SZ5. Reaction conditions: T = 340 °C, P = 3.0 MPa, H2/CO = 2 (V/V), ZZO-3SZ5 mass ratio = 3/7, GHSV = 12000 mL g-1h-1, and WHSVtoluene = 0.9 h-1. Mass spectra in H/D exchange reaction: (b) toluene; (c) xylene.

Fig. 5. SEM images of 3SZ5 (a) and ZZO-3SZ5 (b).

Fig. 6. XRD patterns of ZZO, Z5, and ZZO-nSZ5.

Fig. 7. Pore width distribution of nSZ5 based on the nonlocal density functional theory method.

Fig. 8. (a) Py-FTIR spectra of ZZO-nSZ5; (b) NH3-TPD profiles of nSZ5.

Fig. 9. CO-TPD profiles of ZZO, 3SZ5, and ZZO-3SZ5.

Fig. 10. In situ DRIFTS spectra of ZZO exposed to CO + H2 and then changed to CO + D2 (a), ZZO-3SZ5 exposed to CO + H2 + C7H8 and then changed to CO + D2 + C7H8 (b).

Fig. 11. A possible reaction mechanism for syngas conversion coupled with toluene methylation over the bifunctional catalyst ZZO-nSZ5.

Fig. 12. Stability of ZZO-3SZ5. Reaction conditions: T = 340 °C, P = 3.0 MPa, H2/CO = 2 (V/V), ZZO-3SZ5 mass ratio = 3/7, GHSV = 12000 mL g-1h-1, and WHSVtoluene = 0.9 h-1.

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