Modern solid state double resonance NMR strategies for the structural characterization of adsorbate complexes involved in the MTG process

Abstract

The reaction pathways in the methanol to gasoline conversion process are still not well understood. Although recent years have witnessed remarkable progress in recent years in the in situ characterization of these reactions, the nature of the bonding of the intermediates to the Brønsted acid sites of the zeolite catalyst is still the subject of intense debate. Our approach is to study the geometry of the binding of the methanol molecule to the zeolite surface using modern double resonance NMR spectroscopy. As model systems we chose the host/guest complexes formed in the systems zeolite NaX loaded with ¹³CH₃I and zeolite HZSM-5 loaded with ¹³CH₃OH, treated at various temperatures. By exploring the dipolar coupling strength between ¹³C and ²⁷Al or ¹H and ²⁷Al using ¹H–{²⁷Al} and ¹³C–{²⁷Al} rotational echo adiabatic passage double resonance (REAPDOR) NMR and simulation of the resulting dipolar evolution curves we have been able to determine carbon–aluminium and hydrogen–aluminium internuclear distances and thus to identify the nature of the adsorbate–zeolite complexes formed as a function of the processing conditions.