On the drastic reduction of organic structure directing agent in the steam-assisted crystallization of zeolite with hierarchical porosity

Abstract

The absence of organic structure directing agent (OSDA) in the synthesis of zeolite has shown progress recently, nevertheless the formation mechanism is still not clearly understood and the technique cannot be fully generalized for the synthesis of various types of zeolite frameworks. In this research, the drastic reduction of OSDA in the synthesis of ZSM-5 employing the steam-assisted crystallization (SAC) was studied. The used tetrapropylammonium bromide as OSDA in the synthesis of ZSM-5 was one-fifth and one-twentieth of typical amount reported in literature. Upon OSDA reduction, the crystallization leads to the formation of ZSM-5 with hierarchical pore structure revealed by SEM and TEM images in which it is different compared to the parent spherical-like crystal morphology of ZSM-5 synthesized without reducing the OSDA amounts. A quantitative analysis of the X-ray diffractograms suggests that 100% crystallinity could not be achieved in the sample synthesized under the OSDA reduction confirming that the orientation of crystal growth is not similar to the conventional coffin-type ZSM-5 crystal. The deviancy in crystal growth is further associated with the formation of mesopores originated from severe defect planes of ZSM-5. On the verge of gradual disappearance, a broadened distribution of mesoporosity could return the crystallinity to 100% in which faulty elongated crystal shape, close to typical coffin-type growth orientation appeared as the product after commencing the Oswald ripening of longer period of synthesis time. The conventional hydrothermal synthesis may not be used for crystallizing ZSM-5 when the OSDA was one-twentieth of typical starting amount making the SAC to be an appropriate technique for synthesizing ZSM-5 under drastically reduced OSDA, in particular to simultaneously perform the hierarchically porous zeolite.