Nucleation and Crystal Growth Features of EMT-Type Zeolite Synthesized from an Organic-Template-Free System
Résumé
The process of formation of ultrasmall EMT-type zeolite from organic-template-free homogeneous suspensions is presented. The formation of transparent uniform suspension utilizing sodium aluminate, sodium silicate, and sodium hydroxide under controlled mixing is found to be of primary importance to control the nucleation and growth process of EMT-type crystals. The investigation of zeolite intermediates reveals the formation of uniformly sized gel particles (5–10 nm in size). The mean hydrodynamic diameter of the crystalline EMT-type zeolite corresponds to the size of the amorphous particles formed after preparation of the clear precursor suspension. Controlled formation of uniform precursor particles predetermines, to great extent, the following nucleation and growth steps and, thus, the characteristics of the ultimate product. The amorphous particles are transformed to single EMT-type crystals 6–15 nm in size at 303 K within 36 h. Small changes in the initial composition or the preparation procedure lead to the formation of other sodalite-cage-containing zeolites. Thus, it is of critical importance to control the nucleation kinetics in order to obtain the EMT-type material as pure phase. Besides the EMT zeolite, the crystallization fields of other zeolites upon low-temperature synthesis conditions are studied. The careful control of gel chemistry, combined with slow nucleation kinetics at low temperature, can provide access to important nanoscale zeolites while avoiding the use of expensive organic templates.