Enhanced chemical activity and wettability at adjacent Bronsted acid sites in HZSM-5

The adjustable acid sites in zeolites and its well-defined pore structure allows for a fine-tuning of the catalytic performance. The activity and selectivity of several reactions have been shown to be dependent on the location and distribution of the acid sites in the zeolite. However, the underlying mechanisms responsible for this dependence remain to be explored. Here, we report density functional theory calculations, through which we investigate the impact of proximity of Bronsted acid sites in the HZSM-5. We find that Bronsted sites with close spatial proximity can significantly strengthen the adsorption of water, which is used as a molecular probe for the local activity. We find that a water molecule can form H-bonds with two adjacent sites with increased adsorption energy. This increase is attributed to enhanced polarization of the water molecule through pronounced interfacial charge transfer. In some case, we observe that the proton becomes detached from the zeolite framework and forms a hydronium ion. We further show that hybrid functional calculations are essential for accurately describing the structural stability when two sites are located in close proximity of each other. Both, the enhanced polarization and proton delocalization may affect activity and selectivity for zeolite-catalyzed reactions.