A series of ZSM-5 zeolites with different acidities and b-axis diffusion distances were synthesized under hydrothermal conditions using the seed-induced method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), ammonia temperature-programmed desorption (NH3-TPD), and infrared spectroscopic analysis of the adsorption of pyridine (Py-IR). The structure-activity relationships of PtFe@S-1&ZSM-5 tandem catalysts in the propane dehydro-aromatization reaction (PDA) were investigated, especially focusing on the impact of ZSM-5 acidity and b-axis diffusion distance. The results indicate that the ZSM-5 zeolite with a low Si/Al ratio (similar to 25) and suitable b-axis diffusion distance (similar to 100 nm) coupled with PtFe@S-1 is more conducive on improving the PDA catalytic performance. When the mass ratio of PtFe@S-1/ZSM-5 was 1:1, the propane conversion and aromatics selectivity of PtFe@S-1&ZSM-5-25 remained above 78.9% and 40.5% within 500 min on stream, respectively.

Unraveling the structure-activity relationship and improving the catalytic performance is paramount in propane dehydro-aromatization reactions. Herein, a tandem catalyst with high propane dehydro-aromatization reaction performance was prepared via coupling the PtFe@S-1 with Zn/ZSM-5 zeolites (PtFe@S-1&1.0Zn/ZSM-5), which exhibits high dehydrogenation activity, aromatics selectivity (~60% at ~78% propane conversion), and stability. The addition of zinc inhibits the cleavage of C6= intermediates on ZSM-5 and promotes the aromatization pathway by weakening zeolite acid strength, significantly improving the selectivity to aromatics. This understanding of the structure-activity relationship in propane dehydro-aromatization reaction helps develop future high-performance catalysts.

ZSM-5 zeolite modified by molybdenum is one of the promising catalysts for methane dehydroaromatization (MDA). The introduction of methanol to couple with methane over metal-modified ZSM-5 can facilitate the MDA reaction, but the reaction mechanism, optimal energy pathways, and kinetic and selectivity controlling factors remain to be clarified. In this study, periodic density functional theory (DFT) calculations were performed to investigate the mechanism of methane–methanol coupling and aromatization over Mo/ZSM-5. The calculation results showed that the process of methane–methanol coupling to light olefins (mainly ethylene and propylene) was determined by the C–C coupling step, while further aromatization of the ethylene and propylene intermediate was kinetically controlled by the dehydrogenation step involved in the regeneration of the Brønsted acid site over Mo/ZSM-5. The co-adsorption of H2O produced from methanol dehydration had little effect on methane–methanol coupling to ethylene but increased the rate-limiting barrier for ethylene aromatization to benzene. To further improve the catalytic performance of Mo/ZSM-5, we found that introducing a second metal component such as Co, Ni, or Nb into Mo/ZSM-5 could promote the C–C coupling process and enable these bimetallic combinations to be promising candidates for methane–methanol coupling reactions.

Direct conversion of methane into other value-added chemicals remains as a grand challenge due to its stable non-polar tetrahedral structure. Coupling methane with methanol over Mo-modified HZSM-5 is promising to activate methane at lower temperatures than for pure methane. This work investigates the effect of Si/Al ratio (25-250) and reaction conditions (reaction temperatures from 500 to 650 degrees C and methane-to-methanol molar ratios of 10 to 1 on methane conversion into light olefins (ethene, propene, butene)) using methanol as a co -reactant over 3Mo/HZSM-5. It was found that at 600 degrees C and CH4/CH3OH = 5 the 3Mo/HZSM-5 (Si/Al = 160) catalyst delivers the best performance of 0.72 mmol g -1 h- 1 methane conversion rate and 50 % light olefins selectivity. Through 13C-labeled methane, this study also provided the direct evidence on the involvement of methane carbon in the reaction products. The results of this study enabled us to propose a possible reaction pathway for the methane-methanol coupling reaction.

Zn/ZSM-5 exhibits superior catalytic activity for reactant activation and C–C coupling in methane–methanol coupling revealed by DFT calculations. Introducing Co, Ni or Pd into Zn/ZSM-5 further promotes the reaction by reducing C–C coupling barriers.