Effect of Synthesis Trajectories on the Si/Al ratio of Hierarchical Faujasite Used for the Adsorption of Methylene Blue

Abstract

The use of hierarchical zeolites has attracted popularity over the past decade due to their ability to allow large molecules to enter and exit their pores. These hierarchical materials can enhance the adsorption of bulky molecules and can show enhanced activity and stability in catalysis. A novel morphology of zeolite Faujasite, called the house-of-cards assembly of Faujasite sheets, was synthesized for the purpose of inducing meso/macropores into the Faujasite structure. However, the synthesized zeolite X, which has low framework Si/Al ratio, is unstable in the acidic form at high temperatures. One of the objectives of this work is to increase the Si/Al ratio of the house-of-cards morphology of Faujasite to make it viable for a wide range of industrial applications such as acid catalysis. This is explored by using direct synthesis approaches, with and without organic and inorganic additives, and by a top down approach. The zeolites produced by these methods were characterized by XRD, SEM, EDX, N2 physisorption and CO2 TPD, to study crystallinity and phase purity, morphology, Si/Al ratio, mesoporosity, and basicity. The highest Si/Al ratio that could be obtained in this work for the house-of-cards Faujasite is 1.36, from a synthesis gel of composition 5 SiO2: 1 Al2O3: 7.3 Na2O: 300 H2O. Further dealuminating this material with AHFS increases the Si/Al ratio to 1.53. However, the material was partially amorphized and micropore volume dropped to 0.10 cm3/g. Another objective of this work is to study the effect of hierarchical porosity of house-of cards Faujasite on the adsorption of methylene blue, a common pollutant in wastewater. Three samples, commercial zeolite 13X, house-of-cards zeolite X with Si/Al of 1.36 prepared from bottom up synthesis, and consequently dealuminated house-of-cards zeolite X with Si/Al of 1.53, were tested for their adsorption capacities. The highest adsorption capacity of 137.74 mg MB/g zeolite was obtained using the dealuminated sample. This capacity is twice that of the commercial microporous zeolite 13X (70.35 mg MB/g zeolite). The performance of the dealuminated hierarchical zeolite X in this work is comparable to the best material found in literature, Polyoxometalate (140.84 mg MB/g zeolite), and could potentially replace it for the adsorption of methylene blue.

Date of Award	Dec 2021
Original language	American English