Dual Active Sites on Molybdenum/ZSM-5 Catalyst for Methane Dehydroaromatization: Insights from Solid-State NMR Spectroscopy

Abstract

Methane dehydroaromatization (MDA) on Mo/ZSM-5 zeolite catalyst is promising for direct transformation of natural gas. Understanding the nature of active sites on Mo/ZSM-5 is a challenge for applications. Herein, using ¹ H{⁹⁵ Mo} double-resonance solid-state NMR spectroscopy, we identify proximate dual active sites on Mo/ZSM-5 catalyst by direct observation of internuclear spatial interaction between Brønsted acid site and Mo species in zeolite channels. The acidic proton-Mo spatial interaction is correlated with methane conversion and aromatics formation in the MDA process, an important factor in determining the catalyst activity and lifetime. The evolution of olefins and aromatics in Mo/ZSM-5 channels is monitored by detecting their host-guest interactions with both active Mo sites and Brønsted acid sites via ¹ H{⁹⁵ Mo} double-resonance and two-dimensional ¹ H-¹ H correlation NMR spectroscopy, revealing the intermediate role of olefins in hydrocarbon pool process during the MDA reaction.

Keywords: active sites; host-gust interactions; methane dehydroaromatization; solid state NMR; zeolites.

Figure 1.

¹H MAS NMR (left) and ¹H-¹H DQ-SQ (right) MAS NMR spectra (acquired at 9.4 T) of H-ZSM-5, fresh 5Mo/ZSM-5 and reacted 5Mo/ZSM-5 samples for 10–120 min at 973 K.

Figure 2.

Experimental and deconvoluted ⁹⁵Mo QCPMG MAS NMR spectra (acquired at 35.2 T) of fresh 5Mo/ZSM-5 and 5Mo/ZSM-5 reacted for 60 min at 973 K. Solid (―): experimental, solid (―): simulated, dash (―, ―): deconvoluted.

Figure 3.

¹H{⁹⁵Mo} S-RESPDOR NMR spectra (acquired at 18.8 T) of (a) fresh 5Mo/ZSM-5, (b) 5Mo/ZSM-5 reacted for 30 min and (c) for 120 min of MAD reaction at 973 K. Recoupling time is 5.12 ms. Normalized ΔS of (d) Brønsted acid, (e) olefins and (f) aromatics versus MDA reaction time. ΔS= S₀-S.

Figure 4.

Experimental ¹H{⁹⁵Mo} S-RESPDOR dephasing fraction of Brønsted acidic protons in 5Mo/ZSM-5 as a function of recoupling time. The best fit of the experimental results was determined to be f=0.15 and D_IS=120 Hz, corresponding to a ¹H-⁹⁵Mo internuclear distance of 4.0 Å. f denotes the pre-factor parameter and D_IS is the heteronuclear dipolar interaction between two spins used in the analytical formula. Sim. and exp. represent simulated and experimental curves, respectively.

Figure 5.

2D ¹H-¹H spin diffusion NMR spectra (acquired at 9.4 T) of 5Mo/ZSM-5 reacted at 973 K for (a) 10 min, (b) 30 min, (c) 60 min and (d) 120 min. Mixing time is 300 ms.