Study On Synthesis, Characterization, And Crystal Growth Mechanism Of Porosity-Controlled Molecular Sieves

Molecular sieves (zeolites) are representative of crystalline microporous solids. Their main characteristic is that the tetrahedral primary building blocks are linked through oxygens, producing a three-dimensional network containing channels and cavities of molecular dimensions. They have been widely studied in catalysis as well as in separation and purification fields. However, because of their interpenetrating pore channels in the molecular size range of 0.3-1.5 nm, these materials are subject to diffusion limitations that restrict substrate accessibility to active sites on the interior surface as well as a high back-pressure on flow systems. Although uniformly structured mesoporous molecular sieves are synthesized in a way that a tunable pore size in the range of 2-30 nm can be achieved, the relatively weak acidity and poor hydrothermal properties of these invariably amorphous or paracrystalline materials have resulted in limited practical applications. Thus, the preparation of porosity-controlled zeolites of mesopores is the subject of increasing interest.In this study, different siliceous and alumina materials, such as kaolin clay, an alumia-pillared clay, natural zeolites from several different mineral resources of China, and chemicals, have been tried as synthesis resources, and resorcinol-formaldehyde aerogels and carbon aerogels of different mesostructures have been used as templates for synthesizing porosity-controlled zeolites of mesopores. Samples were thoroughly characterized with X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), thermogravimetric analysis (TG), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption at 77 K, as well as FT-IR spectroscopy and 29Si nuclear magnetic resonance spectroscopy (NMR). The formation of the highly crystalline zeolitic forms of synthesized samples is supported by XRD and FT-IR spectra, and their invariant basic cell structure is indicated by 29Si MAS NMR spectra. The porosity-controlled ZSM-5 zeolites of mesopores have additional mesopores of 9-25 nm in widths and 0.07-0.2 cm3/g in volumes, and porosity-controlled A-type zeolites of mesopores have additional mesopores of around 15 nm in widths and 0.43 cm3/g in volumes, besides their perfect inherent micropores. Experimental results show the mesoporous systems of the finally obtained zeolites can be influenced by proper preparation of resorcinol-formaldehyde aerogels and carbon aerogels through solution chemistry. Consequently, porosity-controlled zeolites of mesopores can be synthesized with this unique methodology. Also crystal growth mechanism of porosity-controlled molecular sieves has been discussed in details.These synthesized porosity-controlled zeolites of mesopores have independent and intricately interwoven three-dimensional networks of mesopore channels, which can result in a reduction of the diffusion path length and an increase of the external surface area. Hence, these porosity-controlled zeolites of mesopores are attractive for use as adsorbents and catalyst supports in catalytic applications such as fluid catalytic cracking (FCC) and hydrocracking of gas oil. The preparation porosity-controlled zeolites of mesopores is progress in advanced nanoporous crystallinte materials.