Development of Intracrystalline Mesoporosity in Zeolites through Surfactant-Templating

Abstract

Novel insights into the surfactant-templating process leading to the formation of tailored intracrystalline mesoporosity in USY zeolite are presented in the light of the changes in the textural, morphological, and chemical properties of this zeolite produced during its treatment in a basic solution of cetyltrimethylammonium bromide (CTAB). The inability of analogous surfactants with bulkier heads to produce mesoporosity suggests that individual CTAB molecules can actually enter the zeolite through its microporosity. Once inside, the surfactant molecules self-assemble to produce the micelles responsible for the formation of mesoporosity causing the expansion of the zeolite crystals, as evidenced by He pycnometry measurements. The analysis of ultramicrotomed samples by transmission electron microscopy evidenced the formation of uniform intracrystalline mesoporosity throughout the entire crystals. In order to investigate an alternative method, namely, the dissolution and reassembly of zeolites, this was performed in USY leading to the formation of composite materials, which are distinctly different from the zeolite with intracrystalline mesoporosity obtained by surfactant-templating. Finally, it was proved that the presence of mesoporosity in the initial zeolite is not needed for the surfactant-templating to occur. This was verified by surfactant-templating of a NaY zeolite, which does not present the large mesopores found in USY.

Thermochemistry of Surfactant‐Templating of USY Zeolite

“…Interestingly, this apparent activation energy is lower than the one we have recently reported for the mesostructuring of CBV 720 . The lower activation energy for a zeolite with a higher Si/Al ratio (CBV 780, Si/Al=40, E aVmeso =34 kJ mol −1 ) versus a less dealuminated zeolite (CBV 720, Si/Al=15, E aVmeso CBV 720 =43 kJ mol −1 ) is expected due to the higher stability towards basic cleavage of zeolites with higher Al content in the framework …”

“…Different structure‐directing agents (SDAs) have been used to drive the interconversion of one zeolite into the other . Building on this idea, we have transformed a USY (FAU) zeolite into its hierarchical mesoporous version, while keeping its crystalline structure, by the use of a cationic surfactant, namely cetyltrimethylammonium bromide (CTAB) . Some of the advantages of using surfactants to introduce mesoporosity in zeolites are: 1) the ability to tailor the mesoporosity generated and 2) to preserve the crystallinity, strong acidity, and excellent hydrothermal stability of the original zeolite as evidenced, not only in lab‐scale experiments, [7] but also at commercial scale in a number of refineries where they are currently used as fluid catalytic cracking (FCC) catalysts.…”

“…[1, 2] Buildingo nt his idea, we have transformed aU SY (FAU) zeolite into its hierarchicalm esoporousv ersion, while keeping its crystalline structure, by the use of ac ationic surfactant, namely cetyltrimethylammonium bromide (CTAB). [3][4][5][6] Someo ft he advantages of using surfactants to introduce mesoporosity in zeolites are:1 )t he ability to tailort he mesoporosity generated and 2) to preservet he crystallinity,s trong acidity,a nd excellent hydrothermals tabilityo ft he originalz eolite as evidenced, not only in lab-scale experiments, [7] but also at commercial scale in an umber of refineries where they are currently used as fluid catalytic cracking (FCC) catalysts.. [8] Recently,w eh aves tudied the kinetics of the different steps involved in the generation of surfactant-templated mesopores in USY (CBV 720, Si/Al = 15) zeolite.B yc ombiningi ns itu and ex situ techniques, we determined the apparenta ctivation energies of those processes, resulting in values in the same range as those involved in the crystallization of zeolites (30-65 kJ mol À1 ). [9] Chem.…”

“…The mesostructured zeolitesw ere prepared by mixinga basic surfactant solution ([NH 4 OH] = 0.20 m;[ CTAB] = 0.03 m) with the USY zeolite (CBV 780, Si/Al = 40) in sealedr eactors, the temperature of whichw as set at 298, 308, 318 and 328 K and carefully controlled (AE 0.1K). Calcinationo ft he samples was carried out in ad ry air flow at 823 Kf or 5h (1.5Kmin À1 ).…”

“…[9] The lower activation energy for az eolite with ah igherS i/Al ratio (CBV 780, Si/Al = 40, E aVmeso = 34 kJ mol À1 )v ersus al ess dealuminated zeolite (CBV 720, Si/Al = 15, E aVmeso CBV 720 = 43 kJ mol À1 )i s expected due to the highers tability towards basic cleavage of zeolites with higher Al content in the framework. [4,9] [a] Dr.N . Linares In addition to the activation energies, which provide valuable information about the energy barrier and kinetics of generating mesoporosity,t he thermodynamic driving force to generate the mesoporosity,g iven by the enthalpy differenceo f products and reactants, is critically important to understand the transformation.T op rovide am ore complete picture of the energetics of the formation of mesoporosity in USY zeolites by surfactant-templating, the enthalpyo fformation of the zeolites were determined by high-temperature oxide melt solution calorimetry.T he apparatus and methodology used can be found in the ESI and elsewhere.…”