(492a) Facile, High-Yield Synthesis of Nanocrystalline ZSM-5 Rich in Framework Aluminum
Highly crystalline nanosize ZSM-5 particles, rich in framework Al content (Si/Al=25) and low in the degree of intergrowth were synthesized with only 2 h hydrothermal treatment at 165 C by using NaCl as the precursor for Na ion, in a solution where the Na/Al ratio was unity. The product yield was 75%. In contrast, replacing NaCl with the same concentration of NaOH resulted in no solid formation in 2 h. Solids were formed by extending the hydrothermal synthesis time to 24 h, but particles of extensive intergrown nanocrystalline ZSM-5 were formed, and the product yield was low, in part caused by quartz formation.
For synthesis with NaCl, the time of addition also has a dramatic effect on the morphology, uniformity, yield and time of formation of ZSM-5 nanocrystallites. When added at the beginning of the synthesis, 60 nm primary particles (as determined by TEM, SEM, and X-ray line broadening), with high aluminum contents (Si/Al = 25) that aggregated into discrete 100-150 nm cubic clusters were obtained. If NaCl was added 20 min later, no solid could be collected after 2 h crystallization.
The short duration of crystallization of ZSM-5 using the synthesis procedure where NaCl was added at the beginning of the synthesis suggested a possible atypical crystallization mechanism. In order to gain insight on this, solids were collected at different stages of the synthesis for detailed spectroscopic characterization. 27Al MAS NMR characterization of the solids showed that Al ions remained almost exclusively in a tetrahedral coordination throughout the first 2 h of synthesis. However, based on 29Si MAS NMR, only Q2 Si species was found up to 1.5 h of synthesis. At 1.75 h, small amounts of Q3 and Q4 species could be detected, and at 2 h, the dominant species became Q4. Similar rapid changes at around 1.75 h of synthesis were also observed in IR spectra, which showed vibrations associated with the framework and crystalline pentasil rings rapidly increased in intensity from 1.75h to 2h. The data suggested that crystallization of these ~60 nm ZSM-5 nanocrystallites occurred via an abrupt, unconventional solid phase transformation.