Time-Resolved Dynamics of Intracrystalline Mesoporosity Generation in USY Zeolite

Abstract

The treatment of zeolites with surfactants in alkaline media is an effective and versatile technique to impart intracrystalline well-defined mesoporosity in these materials. In this study, the dynamics of surface reconstruction that occurs during the treatment of USY zeolite by surfactant-templating was monitored in situ by atomic force microscopy. The development of surfactant-templated mesoporosity and the concurrent healing of defects that are characteristic of steamed zeolites occur in less than one hour at room temperature, which emphasizes the low energy barriers needed to reorganize the crystalline structure of this zeolite. This transformation was also followed by X-ray diffraction, N 2 adsorption, and TEM analysis of ultramicrotomed samples to confirm that the rapid formation of surfactant-templated mesoporosity and the reconstruction of the zeolite crystals occur not only on the surface of the zeolite, but homogeneously throughout the whole zeolite. This process involves a significant and rapid breaking and re-formation of bonds; however, the zeolite does not dissolve during this process as solids recovery at any given time of the treatment is approximately 100% and the concentration of soluble Si or Al species in the liquid is negligible. Parametric analysis revealed that excessive NaOH leads to the partial transformation of zeolite into an amorphous mesoporous solid,

Surfactant‐Templated Zeolites: From Thermodynamics to Direct Observation

“…In a more recent report, [ 38 ] similar results were obtained by using in situ AFM, but in this case, the development of surfactant‐templated mesoporosity was systematically observed and analyzed. Moreover, a rapid reconstruction of surface defects during mesostructuring was observed with great resolution (few nanometers) in a time‐resolved manner.…”

“…c) Adapted with permission. [ 38 ] Copyright 2019, American Chemical Society. d) Adapted with permission.…”

“…Later on, the effect of the amount of base in the incorporation of the mesoporosity was also reported. [ 18,38 ] Figure 1c shows how, at a given time, the amount of mesoporosity increases with the initial concentration of base; which determines the quantity of mesoporosity that will be developed in the zeolite. Therefore, the mesopore volume in surfactant‐templated zeolites can be conveniently controlled by adjusting the concentration and strength of the base used during the treatment.…”

“…The use of weaker bases in the surfactant‐templating process, such as NH 4 OH, entails higher temperatures of treatment, i.e., 150 °C, to have appreciable reaction rates for the transformation; however, stronger bases (MOH, M = K, Na) can lead to similar results at lower temperatures (≈80 °C). [ 18,24,38 ] The relationship between the OH − and Si concentrations during the treatment was also independently analyzed by Garlarneau and co‐workers using octadecyltrimethylammonium bromide as surfactant for the mesosotructuration. [ 27 ] The authors found that the use of higher OH − /Si ratios during the surfactant‐templating produces a larger volume of mesoporosity incorporated that, consequently, decreases the micropore volume of the original zeolite.…”

Surfactant-templated zeolites for the production of active pharmaceutical intermediates

“…The formation of mesoporosity in the zeolite was also studied by N 2 physisorption at 77 K. As shown in Figure 2C, the parent zeolite (CBV 720) produces a type I isotherm with some contribution of large mesopores, which are due to the steaming process performed by the supplier. 12 After the surfactant-templating treatment, the obtained zeolite displays a type I+IV isotherm, with a sharp N 2 uptake at a relative pressure of approximately 0.4, indicating the presence of very narrow and well-defined mesoporosity (see inset in Figure 2). 10,11 The incorporation of mesoporosity does not compromise the microporous volume neither the crystallinity of the zeolite (see Figure 2D for the XRD patterns).…”