Abstract

Zeolites are indispensable heterogeneous catalysts in chemical and petrochemical transformations. Their crystalline microporous frameworks impose confinement effects that dictate how molecules are adsorbed, stabilized, and transformed within the pore space. Central to these processes are intermolecular interactions that control reactant binding, transition-state stabilization, intermediate lifetimes, and product desorption. Three classes of interactions dominate in zeolite catalysis: (i) host–guest and (ii) guest–guest interactions involving the zeolite framework and confined organics and (iii) environmental effects, such as water and carrier gases, that reshape adsorption equilibria and reaction dynamics. Despite their importance in zeolite catalysis, their structures, properties, and quantification remain unresolved.

Solid-state NMR (ssNMR) spectroscopy has emerged as a powerful tool to characterize intermolecular interactions, providing atomic-level insights into local structure and dynamics. Changes in chemical shifts and lineshapes provide information on adsorption environments, whereas dipolar couplings directly quantify spatial proximity and host–guest interactions. However, the characterization of these systems remains experimentally challenging due to the complexity of zeolite-catalyzed reaction networks and the frequent involvement of low-sensitivity nuclei, such as metal active sites. In this Account, we describe our efforts to study intermolecular interactions. By applying or developing advanced one-dimensional (1D) and two-dimensional (2D) NMR experiments, in combination with magnetic resonance imaging (MRI), we enable the mapping of spatial distributions, differentiation between binding sites, and even tracking of reactive intermediates within working catalysts. We highlight how ssNMR has been applied to probe three representative dimensions of intermolecular chemistry in zeolites: the assembly of supramolecular reaction centers where organic intermediates and Brønsted acid sites interact cooperatively; the guest–guest interactions among confined hydrocarbon species that dictate selectivity in catalytic processes such as methanol-to-olefins conversion; and the regulatory role of microsolvated water, which alters diffusion barriers, stabilizes transition states, and mediates proton-transfer steps. These studies reveal how subtle variations in polarity, hydrophilicity, and confinement drastically alter catalytic performance by reshaping the microenvironment of the active sites.

Our goal of this work is to illustrate how ssNMR serves as a direct experimental tool for probing weak but critical intermolecular forces, thereby linking molecular-scale interactions to macroscopic catalytic performance. The methodologies described, encompassing the characterization and quantification of weak intermolecular interactions, extend beyond zeolites to offer versatile approaches that can be applied to heterogeneous catalysis, porous materials, and biomimetic systems.

Link:https://pubs.acs.org/doi/10.1021/acs.accounts.5c00652?utm_source=SendGrid_ealert&utm_medium=ealert&utm_campaign=ASAP_achre4_v0_i0